Organoclay-modified electrodes: preparation

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Jan 18, 2015 - Differential pulse voltammetry. EASA. Electro-assisted self-assembly. EIS. Electrochemical Impedance Spectroscopy. EG. Expanded graphite.
Organoclay-modified electrodes: preparation, characterization and recent electroanalytical applications Ignas K. Tonle, Emmanuel Ngameni, Francis M. M. Tchieno & Alain Walcarius Journal of Solid State Electrochemistry Current Research and Development in Science and Technology ISSN 1432-8488 Volume 19 Number 7 J Solid State Electrochem (2015) 19:1949-1973 DOI 10.1007/s10008-014-2728-0

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Author's personal copy J Solid State Electrochem (2015) 19:1949–1973 DOI 10.1007/s10008-014-2728-0

REVIEW

Organoclay-modified electrodes: preparation, characterization and recent electroanalytical applications Ignas K. Tonle & Emmanuel Ngameni & Francis M. M. Tchieno & Alain Walcarius

Received: 28 November 2014 / Revised: 21 December 2014 / Accepted: 29 December 2014 / Published online: 18 January 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract Amperometric sensors dedicated to the determination of pollutants and other compounds of interest face daily a great challenge: the development of sensitive, reproducible and low-cost devices allowing fast analyses. This review deals with the beneficial role and application of organoclays exploited as sensing materials in various fields of electroanalysis, for the past 15 years (period 2000–2014). After a description of different preparation methods leading to clay minerals chemically modified by organic compounds, the common methods used for their characterization are exposed; then, their application as electrode modifiers is described, covering several approaches that were developed to enhance either the sensitivity or the selectivity of the indexed organoclay-based sensors. Finally, a brief description of voltammetric methods frequently used for electroanalytical purposes is given, followed by an update of recent and salient results achieved for the detection of inorganic and organic electroactive compounds or ions. I. K. Tonle (*) : F. M. M. Tchieno Electrochemistry and Chemistry of Materials, Department of Chemistry, Faculty of Science, University of Dschang, P.O. Box 67, Dschang, Cameroon e-mail: [email protected] I. K. Tonle : E. Ngameni Laboratory of Analytical Chemistry, Department of Inorganic Chemistry, Faculty of Science, The University of Yaounde 1, P.O. Box 812, Yaounde, Cameroon A. Walcarius Laboratoire de Chimie-Physique et Microbiologie pour l’Environnement, UMR 7564, CNRS - Université de Lorraine, 405, rue de Vandoeuvre, 54600 Villers-lès-Nancy, France Present Address: I. K. Tonle Fakultät für Chemie, Elektroanalytik und Sensorik, Ruhr Universität Bochum, 44801 Bochum, Germany

Keywords Organoclays . Chemically-modified electrodes . Electroanalysis . Amperometric sensors . Review Abbreviations 2,4-D 2,4-DCP 2Mpy 4MPy APTES AQ ASCV ASDPV ATTA BHIC BT BTMA CEC CMC CPE CPTES CTA DDA DDBA DDMA DDTMA DO2+ DPV EASA EIS EG FHT GA GCE

2,4-Dichlorophenoxyacetic acid 2,4-Dicholorophenol o-(2-Mercaptopyridine) p-(4-mercaptopyridine) γ-Aminopropyltrimethoxysilane Anthraquinone Anodic stripping cyclic voltammetry Anodic stripping differential pulse voltammetry Attapulgite 1-Benzyl-3-(2-hydroxyethyl) imidazolium chloride Bentonite Benzyltrimethylammonium Cation exchange capacity Carboxymethylcellulose Carbon paste electrode 3-Chloropropyltriethoxysilane Cetyltrimethylammonium Dodecylamine Dodecyl dimethylbenzylammonium Didodecyldimethyl ammonium Dodecyltrimethylammonium P-Phenylenedimethylene bis dodecyl N,N dimethylammonium dibromide Differential pulse voltammetry Electro-assisted self-assembly Electrochemical Impedance Spectroscopy Expanded graphite Fluorohectorite Gallic acid Glassy carbon electrode

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HDTBP HDTMA HEPC HV/RDE Im KT MAS-NMR MAT MC MCC MMT MPTMS MCV nd OME OP PANI PCH-SHn%

PMB PPMA PPTA PTMPA PVA Ru(bpy)32+ SA SCE SM SSA SWV TBAB TDD TEA TEOS TMA TMPA

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Hexadecyltributhylphosphonium Hexadecyltrimethylammonium 1-(2-Hydroxyethyl)-pyridiniumchloride Hydrodynamic voltammetry/rotating disk electrode 1-(2-Hydroxyethyl)-3-methylimidazolium chloride Kaolinite Magic angle spinning-nuclear magnetic resonance 2-Mercapto-5-amino-1,3,4-thiadiazole Methylcellulose Methylcarboxycellulose Montmorillonite 3-Mercaptopropyltrimethoxysilane Multisweep cyclic voltammetry Not determined Organoclay-modified electrode 1,10-Phenanthroline Polyaniline Thiol-functionalized porous clay heterostructures (with n varying between 4 and 35) Poly(methyleneblue) Phosphomolybdic acid Phosphotungstic acid Phenyltrimethylammonium Poly(vinyl alcohol) Tris(2,2′bipyridyl)ruthenium(II) Salicylic acid Saturated calomel electrode Smectite Specific surface area Square wave voltammetry Tetrabutylammonium bromide 1,3,4-Thiadiazole-2,5-dithiol Triethanolamine Tetraethoxysilane Tetramethylammonium Trimethylpropylammonium

Introduction During the past 15 years, scientific research devoted to the implementation of amperometric sensors useful in the analysis and determination of various electroactive compounds has gained growing interest. In fact, the presence in natural media of micro pollutants such as drugs, pesticides, heavy metals, cosmetics and associated by-products represents a great environmental and health problem. Yet, the inherent toxic character of these compounds, combined to the relative low

degradation rate of some of them and the non-biodegradable character of others made them persistent in living organisms where they participate in metabolism processes, thereby inducing damages. The daily release of the four abovementioned classes of compounds in aqueous media arises from variable anthropogenic sources including mining processes [1, 2], agricultural activities [3, 4], chemical industry and others. To identify and prevent the pollution of various environmental compartments, international regulations have been established that define the threshold concentrations of common toxic compounds in surrounding water or in drinking water as well as in other natural matrices. Thus, the monitoring and traceability of toxic pollutants is an ethic duty that concerns the whole scientific community and challenges researchers working in several areas covering analytical chemistry, environmental science, pollution control and chemistry of materials. For the detection and quantification of hazardous compounds, a wide variety of analytical methods are nowadays available, which can be divided into two main categories: (i) spectrometric techniques such as atomic absorption, atomic fluorescence and inductively coupled spectrometry convenient for inorganic species and (ii) chromatographic methods (high-performance liquid chromatography, ionic chromatography or thin-layer chromatography) mostly exploited for organic compounds. These techniques are quite excellent from an analytical point of view but practically, they require expensive, sophisticated and heavy equipment [5]. Additionally, they need highly trained operators and often tedious sample pre-treatments and are no longer suitable for the detection of analytes at ultra-trace levels. Consequently, the development of in situ, real-time and highly sensitive sensors remains a permanent and relevant challenge. Electrochemical methods represent a convenient alternative to these techniques for several reasons: they are reagentless, require simple operational procedures and lowcost equipment [6]. Moreover, they can allow quite fast analyses leading to experimental data obtained in real time and with a high sensitivity. On the other hand, a relevant problem in modern electrochemistry dealing with the identification of a given species in a natural or environmental matrix concerns the selectivity. Yet, many interfering compounds are most often present with the target analyte. To solve this problem, the selectivity of sensors has been improved through the functionalization of conventional electrode surfaces and the design of new electrode configurations incorporating a material bearing a selective functional group which displays affinity towards the desired analyte. In this context, research on the synthesis of clay-based organic–inorganic materials has received considerable attention due to the following inherent characteristics of pristine clay minerals: (i) these materials are characterized by a well-known ordered structure consisting in few exceptions of regular polyhedrals that form a 3D stable and regular network; (ii) this flexible structure furnish

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interesting chemical intercalation properties, convenient for the entrapment of active functional organic guest molecules and (iii) they possess hydroxyl groups on their platelets that are prominent candidates for the grafting of other organofunctional groups [7, 8]. The chemical modification of clay minerals usually leads to “organoclays” that could be defined as a family of materials obtained by modifying clays and clay minerals with various organic moieties through intercalation process and surface grafting [9–11]. The removal of pollutants by batch sorption studies using organoclays is widely described in literature that have demonstrated that these materials can be used for the uptake from aqueous media of undesirable compounds at relatively low concentrations. This could probably explain the interest in such materials in the field of electrochemistry where they are increasingly exploited as electrode materials. The modification of electrodes aims to circumvent some limitations of unmodified electrodes such as low sensitivity, slow kinetics and in some cases high overpotential. Organoclaybased composite electrodes are used for preconcentration electroanalysis, electrocatalysis, permselective coatings and amperometric sensors or biosensors [12]. The two main types of electrodes most often exploited for electroanalytical purposes are carbon paste electrodes incorporating a functionalized clay mineral and film-coated electrodes issued upon drop-coating or spin-coating of a thin film of organoclay on the surface of a conventional solid electrode. Some uncommon other electrodes configurations exist that will also be exposed in this paper which comprehensively reviews the pathways used for the preparation of organoclays and their potential applications for the electrochemical analysis of various compounds. It is important to mention that biosensors, i.e. chemical sensors based on the recognition system using a biological mechanism instead of a chemical process, are not included in this review, while electrochemical sensors and biosensors based on clay modified electrodes (i.e. dealing not specifically with organoclays) have been the subject of some previous reviews [13, 14].

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reviewed by de Paiva et al. [15] while some of their unresolved issues were also critically reviewed by He et al. [16]. Briefly, the chemical modification of clay minerals comprises the simple binding (adsorption) of inorganic and organic anions at the edges of clay minerals, chemical grafting of organic compounds and pillaring using various types of poly(hydroxo metal) cations followed by grafting as well as intercalation and ion exchange processes. In this review however, only those of these methods leading to stable and functionalized clay minerals that were successfully exploited to date for the preparation of modified electrodes shall be exposed. Ion exchange (class 1) Ion exchange is the most popular method described in available literature for the preparation organoclays based on smectites. Yet, smectites are swelling 2:1 clay minerals in which the unit layer consists of an octahedral sheet sandwiched between two opposing tetrahedral sheets. Due to inherent isomorphous substitution arising by the replacement of structural cations by other cations of lower valency (e.g. Al3+ by Mg2+or Mg2+ by Li+, in the octahedral sheet, and Si4+ by Al3+ in the tetrahedral sheet), the layer structure becomes negatively charged. This charge is naturally balanced by inorganic cations (such as Na+, K+, Ca2+ or Mg2+) in the interlayer space [17]. These interlayer cations can be easily replaced in aqueous solutions by more voluminous ones, and quaternary alkylammonium cations (also known as surfactants) are the mostly used to that purposes. Within the interlayer region of the hosting clay mineral, the surfactant molecules can adopt a monolayer, a bilayer, a pseudotrimolecular or a paraffin layer depending on their concentration in relation with the cation exchange capacity of the clay mineral [18, 19]. It is important to mention that other kinds of organic groups can be used, the final aim being the elaboration of composite materials with both hydrophilic and hydrophobic surfaces as well as a large porosity and surface area that have been proven to be efficient sorbents for organic molecules. Organoclays obtained by ion exchange will be referred as class 1 in this review.

Organoclays Intercalation or insertion (class 2) Synthesis of organoclays This section describes various approaches used for the chemical modification of natural clay minerals. An organoclay is a composite material in which the major component is a clay mineral, combined with an organic compound. They are synthesized following chemical processes or reactions where a given organic compound can be bound or attached to the clay backbone. Depending on the clay structure and properties, several methods can be employed for the preparation of organoclays. A compilation of these methods was recently

By an intercalation process, organic molecules can be inserted in clay minerals either by a solid-state reaction (without the use of solvents) or by a long-term shaking of a solution containing the clay material in suspension along with the molecule to be intercalated. This process is convenient for the intercalation of neutral compounds into dried montmorillonite and vermiculites, for example [15], or for the intercalation of small polar molecules (urea, dimethylsulfoxide, dimethylformamide) in kaolinite sheets [20, 21]. Following this method, neutral compounds such as crown-ethers and

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cryptand [22], non-ionic surfactants [23], pyridine and derivatives [24, 25], ethylene and related polymers [26], and alkylamines [27] can be successfully intercalated in smectite and kaolinite clay minerals. Apparently, the process is not indicated for fibrous clay minerals such as sepiolite. Recently, it was reported that an alkylammonium (cetyltrimethylammonium, CTA+) ion and the neutral thiourea molecule could be coinserted by a one-step mechanism within montmorillonite [28]. Two features were thus exploited for the preparation of such an organoclay: the partial displacement of sodium ions previously introduced in the interlayer space of the clay mineral and the coinsertion of thiourea molecules within the alkyammonium sheets lying flat as monolayer or bilayer between the clay sheets (Scheme 1). Grafting (class 3)

Pillaring (class 4)

1.443 nm

Expandable clay minerals can be modified by pillaring (also known as pillarization), a method which was developed in the late 1970s [42–44]. Pillaring occurs when the naturally occurring interlayer monovalent and divalent cations (Na+ or Ca2+) are exchanged with highly charged polymeric metal species such as polyoxocations (the Keggin-like Al13 oligomer [Al 1 3 O 4 (OH) 2 4 (H 2 O) 1 2 ] 7 + is a typical example of polyoxocations), metal trischelates, organometallic complexes, metal cluster cations, metal oxide sols or with stable metal oxides [45]. The pillar agents supporting the layers are generally converted into oxides after calcination [46], leading to modified clays called pillared clays (PILCs). The interest of pillaring is in the conversion of inexpensive and thermally unstable clay minerals into highly porous and stable structures. PILCs are known to possess high surface area and permanent porosity due to the presence of robust oxide particles between the clay layers. They are attractive adsorbents for environmental applications [47]. They were also successfully used as catalysts in catalysis for advanced oxidation processes since their open microporous structures contain both Bronsted

1.28 nm

12.08Å

Another way to prepare organoclays is chemical grafting. Here, the inorganic clay mineral structure and the guest organic compound are linked via strong-type interactions (i.e. covalent or iono-covalent bonds). Contrary to materials of classes 1 and 2 as stated in this paper and for which the intercalated molecule can be reversibly desorbed, the grafting approach enables a durable immobilization of the reactive organic compound by strong binding on the clay surfaces, preventing therefore their leaching into the surrounding medium when used in solutions [29]. In the meantime, this process contributes to circumvent the inherent limitations of unmodified clays (low loading capacity, relatively weak binding strength, low selectivity) since the grafted molecule is chosen according to the functional group on its structure, which acts as a recognition element for a target analyte. Functionalized organosilanes are the best grafting agents, and the mechanism of their reaction (also known as silylation) with the hydroxyl groups of clay minerals has been fully discussed in the literature [30] on the basis of parallel works undertaken on silica [31–33]. However, some other molecules can be grafted on clay minerals. About the nature of sites useful for silylation, phyllosilicate surfaces contain two basic types: the siloxane surface and hydroxyl surface groups. The 2:1 clay minerals (e.g. smectite group minerals) only contain

siloxane surfaces while the 1:1 clay minerals (e.g. kaolinite group minerals) contain both the two kinds of surfaces [34]. For expandable 2:1 swelling clay minerals, all internal and external surface and broken edges provide possible sites for the direct silane grafting. However, the process is more complicated for 1:1 clay minerals of kaolinite group due to the hydrogen bonding between neighbouring layers [35]. To achieve a successful interlayer grafting, pre-intercalation with small polar molecules, e.g. dimethylsulfoxide (DMSO), urea and dimethylformamide (DMF), is an indispensable step to prepare an intercalate precursor, which is then be used as substrate for further organosilane intercalation and subsequent silylation. Detellier and co-workers have largely conducted a series of works on the interlayer grafting of kaolinite by using several other classes of organic compounds [20, 36–40]. Scheme 2 illustrates a typical grafting process of an ionic liquid (1-benzyl-3-(2-hydroxyethyl) imidazolium chloride (BHIC)) on kaolinite [41].

Sa(Na) :

Na+

Sa(Na,T) : Thiourea

Sa(CTA0.25,T) : Cetyltrimethylammonium ion

Scheme 1 A representation of the co-insertion process of neutral thiourea and cethyltrimethylammonium ions within the gallery of smectite (adapted with permission from reference [28])

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(H2O, 48 h)

1.58 nm

0.71 nm

BHIC

1.71 nm

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DMSO

K

K-DMSO

K/BHIC

K-BHIC

Scheme 2 Grafting of 1-benzyl-3-(2-hydroxyethyl) imidazolium chloride (BHIC) on kaolinite (K) using a dimethylsulfoxide (DMSO) precursor (K-DMSO). K/BHIC stands for kaolinite with both BHIC

intercalated and grafted while K-BHIC is used for kaolinite grafted with BHIC. Adapted from reference [41] with the permission of RSC

and Lewis acid sites [48]. Although PILCs were originally prepared using inorganic pillars, the concept has been extended to sol–gel process which involves hydrolysis and condensation of organically modified silicon alkoxides. Thus, silicon alkoxides were incorporated in the interlayers of high-chargedensity clay minerals such as fluorohectrite or vermiculite with subsequent removal of the cationic surfactants that served as a template [49]. The conventional calcination process induces the loss of the surfactants, forming micropores in the gallery of the starting clay minerals. New developments have been made in the synthesis of PILCs by incorporating surfactants into the Al13-exchanged form of clays [50]. The so-called porous clay heterostructures (PCHs) are considered as pillared clays since they are issued from the covalent bonding of organic modifier into the gallery of phyllosilicate clay minerals [51]. Most of them have been prepared by prepillaring of clay minerals with polycations followed by silylating with alkylchlorosilanes. Besides the traditional uses of PILCs previously mentioned, they have been proven to be attractive materials for the modification of solid electrodes due to features as regular mesostructure that offers a favourable environment for fast diffusion rates, resulting in great sensitivity while the layered morphology ensured good mechanical stability [52]. This promising application will be discussed in this review.

were used by Ianchis et al. for the silylation of a commercial alkylammonium-modified montmorillonite [55]. The obtained materials display high hydrophobic character resulting from a combination of the length of the organic chain and of the amount of silane groups grafted onto clay edges. Many examples can be found in specialized journals on the synthesis of other organoclay composites with applications in several domains.

Other uncommon methods Some platforms leading to organoclay nanocomposites materials are available in the literature. Laachachi et al. [53] prepared montmorillonite-based composites by mixing in appropriate ratio molten poly(methylacrylate) pellets and metal oxides (Fe2O3 and TiO2) in a mixer rheometer at 225 °C and 50 rpm. These materials showed improved thermal stability and fire retardant ability. In the same line, aqueous-layered silicate dispersions were metalized by neutral silver, palladium and copper nanoparticles that were immobilized on the nanoplatelet surfaces of bentonite. Upon blending such anionic hybrid dispersions with cationic poly(methylacrylate) (PMMA) latex, the resulting hybrid nanocomposites that were formed were successfully used as antibacterial agent against the ubiquitous and infectious bacterial strain Staphylococcus aureus [54]. Alkoxysilanes with different organosilyl groups

Desired features with respect to applications for electroanalytical purposes Organoclays are made of two components with significant different physical and chemical properties that remain separate and distinct at the microscopic scale. These composite materials are synthesized to combine the desired properties of each of their components. For example, by introducing organic functional groups to a clay mineral by grafting, the surface silanol groups are partially converted to new organofunctional surface endowed with organophilic properties. The resulting nanocomposite structure can therefore act differently from the original pristine clay mineral. Usually, the insertion or attachment of a guest organic compound to clay minerals is directed to the following pronounced advantages: (1) the improvement of complexation ability in the case the attached molecule bears functional groups that can strongly and quantitatively immobilize target analyte species by complexation; (2) the design of materials with a constant composition, enabling easy analysis and interpretation of results; (3) the enhancement of the selectivity of the pristine clay via the presence of bound ligands or other functional groups; (4) the irreversible binding of the organic function that can allow the use of the inorganic–organic material for the long-term retention of target species (e.g. metal ions) and (5) the change of the surface property of clay minerals (i.e. hydrophobic to hydrophilic balance), thereby boosting the active sites at their surface to achieve better adsorption efficiency for polar organic molecules, for instance. For example, Scheme 3 compares the possibilities for the uptake of ionic species using a smectite type clay, before and after its modification by grafting using amino and mercapto containing organosilanes [29]. For this study, based on different forms in solution of Hg(II) ions serving as pollutant model, an ion exchange mechanism can

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Scheme 3 Possible chemical interactions between a swelling clay mineral (montmorillonite) used in sodium exchanged form (a and b), and upon grafting with thiol (c) and amine (d) groups for the uptake of various forms of mercury(II). Reproduced with permission from reference [29]

occur with the clay sheets where the interlayer cations of the clay mineral are replaced by Hg2+ (pathway (a)). A mean value of 0.38 mmol per gramme of clay mineral was obtained at the operating pH (4). At pH above 4, Hg(II) existing in the form of Hg(OH)2 are accumulated by condensation with the surface hydroxyl groups of the clay (pathway (b)) but this process is not very quantitative since the maximum amount of adsorbed Hg(II) was around 0.4 mmol per gramme around pH 5. By contrast, the grafted clays gave rise to higher capacities than the unmodified ones: the binding process is essentially due to complexation of Hg(II) species by thiol (pathway (c)) or the amine functions (pathway (d)). The key feature displayed by the functionalized clays is the enhancement of Hg(II) uptake (0.68 mmol g−1 at pH 1 and 0.78 mmol g−1 at pH 6, respectively, for the material with SH and NH2 groups). This typical example illustrates the utility of functionalizing clay minerals in a manner to increase their ability towards the recognition of ionic species. In the last section of this paper, one will see how that trend can be exploited when these organoclay materials are used to modify carbon paste electrodes for electroanalysis purposes. Physico-chemical characterization techniques Infrared spectroscopy In most studies reported on the functionalization of clay minerals following the methods previously mentioned, infrared spectroscopy (IRS) has been used to confirm the presence of the functional groups on either the grafted, intercalated or bounded guest molecules. Also, information concerning their structure, composition and structural changes arising upon chemical modification can be derived [56, 57]. This technique is based on the fact that a chemical substance shows marked selective absorption in the infrared region [58]. After absorption of IR radiation, the molecules of a chemical compound vibrate at distinct frequencies, giving rise to close-packed

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absorption bands in the IR absorption spectrum, which will correspond to the characteristic functional groups and bonds present in the group. Thus, an IR spectrum of a chemical substance is a fingerprint for its identification. In fact, IR radiation is used to probe the energy levels of molecules. The total energy of a molecule can be thought of as a combination of transitional, vibrational, rotational and electronic energies. These types of molecular energies are quantized, and only discrete energy levels are permitted. Therefore, molecules absorb energy inputs only at those nonarbitrary levels. Infrared energy corresponds to rotational and vibrational transitions of molecules. Since molecules in solid rarely rotate freely, IR studies of minerals are primarily concerned with vibrational transitions [59]. For IRS analysis of organoclays, the KBr method is the most used: the sample is diluted (2– 10 %) in KBr and pressed to form a pellet; the background spectrum of a KBr pressed pellet is firstly recorded, followed by measuring the sample pellet. Also, the mid-infrared region of the spectrum (4000 to 400 cm−1) is the most interesting part of the spectrum as it contains the fundamental framework vibrations of the Si(Al)O4 and -OH hydroxyl groups of clay minerals. It can be divided into three important zones to be explored for a clay mineral before and after chemical modification: the hydroxyl stretching region (4000–3000 cm−1), the CH stretching vibrations region (3100–2800 cm−1) and the HCH bending vibrations (1550–400 cm−1) [60]. Identification of functional groups in these regions is possible because different organoclays will absorb at different frequencies. In addition to the characteristic nature of the absorptions, their magnitude in the spectrum due to a given species can be related to the concentration of that species, thereby allowing semi-quantitative analysis. X-ray diffraction X-ray diffraction (XRD) is one of the most important, wellestablished and powerful methods in the primary investigation of clay minerals since it enables to identify the variety of crystalline mineral phases present in a clay material (or even in soils) [61]. The method is based on the scattering of X-rays by the electrons of atoms. This is due to the fact that wavelengths of X-rays are similar to interatomic distances and so the X-rays scattered by different atoms will interfere destructively, giving rise to diffracted beams. Since each clay mineral has a distinct set of atomic layer spacings (called d-spacings), subsequent measurements can be used to identify the mineral. All crystalline minerals in a given sample can be identified from one XRD scan, if they are present in sufficient abundance [62]. About the theoretical background, the analysis of materials at solid state using XRD is based on Bragg’s law developed in 1914, which defines a diffraction relationship between the wavelength (λ) of an incoming ray and the dspacing by nλ=2dsinθ, where the integer n is the order of the

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diffracted beam which can be any whole number, d is the distance between adjacent planes of atoms (d-spacing) and θ is the angle of incidence of the X-ray beam [63]. In materials science as in the field of organoclays, powder XRD (PXRD) finds frequent use for several reasons: sample preparation is relatively easy, experiments are often rapid and non-destructive, the patterns of PXRD peaks can be exploited to determine the texture of a material and the changes in peak width and position are useful to yield information on the organoclay after its modification. This is quite important as the d001 basal spacing (plane perpendicular to individual layers in the clay mineral structure) is a key parameter that allows to follow the intercalation process arising within the clay mineral gallery [64]. In the case where the intercalation process has been effective, an expansion (or contraction in the case of desorption) of the interlayer is reflected by a change in the d001 value of the clay. In the corresponding diffraction pattern, this will be observed as a shift in the [00l] crystal planes in the clay, making this a useful tool for monitoring and characterizing changes in interlayer structure of the clay [65]. It is important to mention that attention should be paid during the preparation of the sample; this latter should be well ground in a mortar and/or pestles in view to create a uniform particle size that ensures that all possible crystallite orientation are present in the sample. Thermal analysis Thermal analysis of organoclays is well known and described in many papers, as notably demonstrated in a review published in 2004 by Yariv [66]. According to the ICTAC (International Confederation for Thermal Analysis and Calorimetry) recommendations, “thermal analysis forms a group of techniques in which a property of the sample is monitored against time or temperature while the temperature of the sample, in a specified atmosphere, is programmed. The program may involve heating or cooling at a fixed rate of temperature change, or holding the temperature constant, or any sequences of these” [66, 67]. Thermal analysis methods are usually associated to other techniques for the study of organoclays, such as thermo-IR spectroscopy analysis [68, 69], thermo-XRD analysis [70], evolved-gas analysis [71] and differential-scanning calorimetry (DSC), and to a more recent technique called emanation thermal analysis (ETA) [72]. For these analyses, the organoclay material is gradually heated, usually under an inert gas atmosphere. Depending on the thermal stability of the organic compound bound on the clay mineral and on the nature of the latter, exothermic or endothermic peaks are generated when the temperature is increased. Most organoclays begin to undergo thermal decomposition with heating, and three main situations are regularly observed: (i) the desorption of physisorbed water (if present) and the loss of hydration water from the interlayer cations

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arising between room temperature and 110 °C (not considered as a clay degradation), (ii) the thermal decomposition of the organic molecule and (iii) the dehydroxylation of the clay (between 400 and 700 °C depending on the clay mineral nature). The results are usually expressed by the thermal gravimetric (TG) weight loss curve and by the first derivative of this curve, i.e. the differential thermal gravimetric (DTG) curve that both give the changes in weight of a sample under controlled temperature conditions. As the temperature of a given system changes, it will undergo various thermodynamic transitions to different phases at discrete temperatures. When weighed on a balance, a dense phase (at liquid or solid state) that is vaporized will result in a change in sample weight as the transition progresses. Yariv has demonstrated the applicability of DTATG for differentiating between adsorbed and free organic matter and also between ionic and molecular adsorption [66]. Solid-state nuclear magnetic resonance spectroscopy Solid-state nuclear magnetic resonance (NMR) spectroscopy is an important tool employed to study both molecular structure and dynamic behaviour of organoclays since it can generate new support and responses on the intermolecular interaction and the structural organization and even on the dynamic behaviour of the nanocomposite clays [73]. Depending on the method used to modify a given clay mineral, NMR can give rise to information such as hydrogen relaxation data for the samples measured by low-field NMR [73] or peaks characterized by chemical shifts corresponding to a sequence of elements linked by chemical bonds. For example, during the grafting of montmorillonite by APTES (aminopropyltriethoxysilane) [74, 75], solid-state 29Si MAS NMR spectra of the silylated products showed two prominent T 2 [Si(OSi)2(OR′)R] and T3 [Si(OSi)3R] (R=CH2CH2CH2NH2, R′=H or CH2CH3), implying the formation of polysiloxane oligomers (T stands for tetrafunctional units and the superscript is the number of bridging O atoms surrounding the silicon atom) [76]. Tonle et al. [77] used solid-stated 13C and 29 Si MAS/NMR spectroscopy to elucidate the structure of the silylating agents attached to kaolinite. The spectra obtained show clear evidence of alkylsilyl groups bonded in the interlayer surface of the kaolinite. As shown by other similar works [39, 78, 79], solid-state NMR appears to be an important auxiliary tool to obtain supplementary and useful data on clay minerals chemically modified by organic compounds. N2 adsoption-desorption studies The physico-chemical properties and applications of clay minerals and organoclays depend on surface phenomena taking place at the interface of these materials considered as solid phases and liquids or gases. Gas adsorption is thus of major importance for the characterization of a wide range of porous

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materials [80]. Although physical approaches have been utilized in the determination of the porosity of porous materials, most frequently, chemical procedures are used that include selective molecular absorption in aqueous suspensions and gas adsorption under dry conditions [81]. In both cases, the amounts needed to cover the entire sample surface with a molecular monolayer are measured. The parameters necessary for the determination the surface properties and porosity of a given material are the specific surface area (SSA), the total pore volume (derived from the amount of vapour adsorbed at a relative pressure close to unity), the pore size (which can be estimated from the pore volume or BET treatment of the data) and the micropore surface area. The absorption of methylene blue dye in aqueous suspension and, more widespread, that of ethylene glycol monoethylether have been used to determine SSA for several decades [82–84]. Of all the many gas and vapour methods readily available for adsorptive studies, the BET method for N2-adsorption is nowadays universally preeminent as standard method in SSA determination. Yet, with the aid of user-friendly commercial equipment and on-line data processing, it is now possible to use nitrogen adsorption at 77 K for both routine quality control and the investigation of new materials. The BET method is based on the relationship between the applied pressure and the volume of gas forced into the investigated material sample. The determination of surface area by BET method has been prompted from a work by Benton and White [85] who realized in the early 1930s that multilayer adsorption of nitrogen can occur at liquid nitrogen temperature (77 K). This was already an extension of Langmuir’s monumental work which stipulates that the amount of adsorbed gas at the plateau corresponds to complete monolayer coverage (type I isotherm) [86]. Brunauer and Emmett [87] then found that the adsorption isotherms of nitrogen and several other gases on an iron synthetic ammonia catalyst were all of similar sigmoidal shape (later designated type II). According to a well-developed review on use of nitrogen adsorption for the characterization of porous materials [78], the exploitation of N2 adsorption for pore size analysis dates from the late 1940s. This method is based on the application of the Kelvin equation, with a correction for the multilayer thickness on the pore walls. Although the first computational procedure was proposed by Shull [88], the method devised by Barrett, Joyner and Halenda (BJH) in 1951 remains the most popular way of deriving the pore size distribution from an appropriate nitrogen isotherm [89]. For the analysis of organoclays, the recorded N2 adsorption–desorption isotherms are examined, and their characteristic indicates to which group they belong on a well-established classification. Also, the form of hysteresis loop can be exploited to derive qualitative data on the porosity of the examined layered, which is especially useful to testify porosity changes induced by the modification procedure and by the presence of the organic groups in the materials.

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Elemental analysis Elemental analysis is a routine analytical technique performed to characterize and/or prove the elemental composition of an organic sample and other types of materials, notably the organic–inorganic hybrid materials. It is used to quantify organic matter in a sample by measuring common elements used as building blocks in organic compounds: carbon, hydrogen, nitrogen and sulphur. This is achieved through the complete combustion of the sample, releasing the organic matter as gaseous forms of the individual elements of interest. During the combustion process taking place in a furnace (at ca. 1000 °C), carbon is converted to carbon dioxide, hydrogen to water, nitrogen to nitrogen gas or oxides of nitrogen and sulphur to sulphur dioxide [90]. If other elements such as halide are present, they will also be converted to their corresponding combustion products, such as hydrogen halide. The evolved gases are separated through a series of gas traps and chromatography columns and can be detected in a variety of ways including (i) gas chromatography separation followed by quantification using thermal conductivity detection, (ii) a partial separation by gas chromatography (known as frontal chromatography) followed by thermal conductivity detection (CHN but not S) and (iii) a series of separate infrared and thermal conductivity cells for detection of individual compounds [91]. The technique exploits the fact that numerous organic compounds include no additional elements besides C, H and N except oxygen, which is seldom determined separately. Concerning the principle of measurement, the sample under test is weighed in a tin capsule. After folding the capsule (looking rather like wrapped tin foil), the sample is placed in the autosampler. The tin capsule enclosing the sample falls into the reactor chamber where excess oxygen is introduced before. At about 990 °C, the material is “mineralized”. Formation of carbonmonoxide is probable at this temperature even under these conditions of excess oxygen. The complete oxidation is reached at a tungsten trioxide catalyst which is passed by the gaseous reaction products. The resulting mixture should thus consist of CO2, H2O and NOx. But also, some excess O2 passes the catalyst [92]. The product gas mixture flows through a silica tube packed with copper granules. In this zone held at about 500 °C, remaining oxygen is bound and nitric/nitrous oxides are reduced. The leaving gas stream includes the analytically important species CO2, H2O und N2. Eventually included SO2 or hydrohalogenides are absorbed at appropriate traps. High-purity helium is used as carrier gas. Finally, the gas mixture is brought to a defined pressure/volume state and is passed to a gas chromatographic system. Blank values are taken from empty tin capsules, and calibration is done by elemental analysis of standard substances supplied by the instrument’s manufacturer for this purpose. For the analysis of organoclays, this technique provides reliable elemental contents of organic components from the intercalated or

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grafted moieties. In some cases, the knowledge of these elements can allow a comparison of their theoretical content (i.e., as calculated from the composition of the synthesis medium) to propose a chemical structure for the composite material [77, 93]. Microscopic methods Further investigation of the physico-chemical properties of organoclays can be carried out using a variety of surface characterization techniques such as fluorescent microscopy (FM), confocal laser scanning microscopy (CLSM), atomic force microscopy (AFM), scanning electron microscopy (SEM) or transmission electron microscopy (TEM). Electron microscopy is an imaging technique that uses electron beams to examine samples on a very small scale, offering much better resolution and magnification than light-based sources [94]. About the fundamental principle of these methods, the sample surface is scanned using an electron beam of shorter wavelengths. The electrons interact with the sample, yielding a variety of different products that can be subsequently detected; typically using secondary electrons. This generates an image of the sample that contains breadth and depth information for a good visualization of its 3-dimensional structure. This makes these techniques useful for visualizing structural and overall changes to the visual composition of a material [95]. The choice of a particular tool or of a combination of them depends on the property to be investigated on the organoclays, and usually, a technique is used for a better understanding of the morphology and the nanotopography of the surface of clay minerals before and after their chemical modification [96]. Without going into too many details about each of these methods, it is clear to mention that they can provide useful information about the organization and aggregation of clay platelets and by a topographic visualization the changes resulting from the modification process. For example, TEM can be used to estimate the edge–surface areas of organoclays or for the characterization of the microstructure and morphology of pure organoclays [97]; in addition, the particle size and particle size distribution can easily be extracted from the AFM data [98, 99]. TEM and SEM micrographs are usually exploited to characterize the layer stacking and the surfactant packing density of organoclays of classes that are most prepared in the literature [100–102].

Organoclay-modified electrodes Carbon paste electrodes Adams was the first to propose a simple electrode preparation protocol based on the mixing of a graphite powder with a pasting liquid [103]. Since then, these modified electrodes

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have proved to be simple and reliable amperometric sensors. They are obtained when a paste obtained by mixing a carbon (graphite) powder, a pasting liquid serving as binder and usually a third component (in solid state) is inserted into a holder, most often a teflon or a glass tube. Prior to use, the surface to be pluged into the solution is polished on a weighing paper to give a smooth finish. Graphite, the major component which is usually a multicrystalline conductive powder, must be of small size (around 0.001 mm) to show lower residual currents [104]. However, other carbon materials such ethyne black [105], glassy carbon powders [106, 107], diamond [108], carbon microspheres [109], fullerenes [110], carbon nanotubes [111–114] and more recently graphene foams [115] have also been used. Concerning the pasting liquid (binder), it could be a volatile or non-volatile compound but mostly, it should be insoluble in the supporting electrolyte solution. Paraffin oil, silicon oil, nujol, ceresin wax, bromoform or bromonaphthalene are the commonly used binders. Since their introduction in the field of electroanalysis, carbon paste electrodes (CPEs) have been largely exploited to qualitative and quantitative analyses of various compounds owing to their advantages: they are easy to prepare, are inexpensive and give rise to reproducible signals, display low background and long-time stability and high polarization limits in both anodic and cathodic directions [116]. Many papers dedicated to CPEs have been published, with two well-documented reviews on their electrochemistry and electroanalytical applications, produced in 2009 on the occasion of the half-of-century anniversary of their discovery [117, 118]. Undoubtedly, CPEs remain powerful and useful tools with a wide diversity in applicability, especially for the building of low-cost and sensitive sensors for the detection of extremely low concentration of electroactive species (the lowest limits of detection achieved at CPEs lie between 10−14 and 10−16 M) [119–122]. Film-coated electrodes The coating of thin film layers on solid electrode surfaces is the ordinary way to prepare sorption-based modified electrodes. Common porous carbon substrates like glassy carbon, graphite, ordinary pyrolytic graphite and basal plane pyrolytic graphite electrodes are used to that aims, in addition to gold, platinum or SnO2 electrodes [123]. Electrodes modified by a film of clay mineral were introduced in the field of electroanalysis by Ghosh and Bard [124] to elucidate charge phenomena in clay layers and the effect of the film structure on the mobility of the electrochemical probe to the electrode surface [125–127]. These electrodes, covered by a thin film of unmodified clay material have found potential applications in electrocatalysis [128, 129]. Later, they prompted the development of solid electrodes modified by organoclays, which are nowadays extensively investigated as sensors [39, 77, 79,

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130–133] or biosensors [134–136]. Two main approaches are usually used to build clay film-coated electrodes: (i) the spin coating of a dispersion of the clay or organoclay particles and (ii) the drop coating on the active surface of the electrode of the clay dispersion. In both cases, the coating stage is followed by drying either in air or in an oven at a given temperature. However, as stated by Navratilova and Kula [137], the way of drying, the type of clay and the size of clay particles are crucial factors for obtaining a film with required stability, uniformity and thickness. Other types of clay-modified electrodes Other strategies described in the literature for the preparation of film-modified electrodes include the use of silane linkages to couple clay to the underlying electrode surface [138]. Dip coating has also been evoked, although it usually leads to unstable devices. The use of polymeric additives has been proposed to improve the stability of such electrodes [139, 140]. Moreover, Langmuir-Blodgett method was applied to prepare thin film on electrode surfaces but is limited till now to unmodified clay minerals [141–143]. Recently, the electroassisted generation of clay-mesoporous silica composite films onto glassy carbon electrodes was reported [144]. The method involved the deposition of clay particles by spin coating on glassy carbon electrode (GCE) and the subsequent growth of a surfactant-templated silica matrix around these particles by electro-assisted self-assembly. This approach offering a longterm operational stability allows the use of such composite electrode in multiple successive preconcentration electroanalyses. One can also mention in this section the in situ impregnation of a clay mineral film previously coated on solid electrode, by a solution containing an organic compound or a complexe likely to act as ionophore for a target species [145].

Common electroanalytical methods Cyclic voltammetry Undoudtely, the most popular voltammetric method is cyclic voltammetry (CV), generally used to investigate the electrochemical behaviour of new redox systems [146] or as a routine technique for the monitoring of well-known redox probes. In fact, CV can rapidly offer a rapid location of redox potentials of the investigated systems in addition to the eventual effects of the surrounding medium on the redox process involved [147]. When multiple cycles are subsequently recorded, for example, in the cases where adsorption processes and coupled chemical reactions are involved, quantitative data on the kinetics of electron transfer reaction and on the thermodynamics

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of the redox reaction can be derived. For the characterization of the organoclay-modified electrodes (OMEs), this technique is mostly used to examine the processes occurring at modified electrodes incorporating an organoclay, mostly not only on the basis of selected redox probes in solution but also in a direct way for organoclays bearing redox species. Depending on the charge of the clay material, chemical interactions guided by electrostatic forces can be favoured or prevented. The appearance of peaks and their shape are then used to determine, for example, the number of electrons involved in the electrochemical reaction, or to derive data about processes that can eventually occur such as adsorption or coupled chemical reactions [148]. Figure 1 illustrates the application of CV for the collection of information about the porosity and the permselectivity properties of a glassy carbon electrode coated with a pristine swelling clay mineral (montmorillonite), before and after its intercalation with a surfactant [130]. By using [Fe(CN)6]3− and [Ru(NH3)6]3+ ions as redox probes, one can observe on the GCE coated with a film of pristine Ba-Na+ clay mineral (Fig. 1a), stable voltammograms are obtained upon multisweep scan when the supporting elecytrolyte contains [Fe(CN)6]3−. The mean intensity (3.4 μA) corresponding to these stable signals is lower than that obtained at the bare GCE (7.5 μA). Such a behaviour was attributed to electrostatic repulsions between the negatively charged clay platelets and the [Fe(CN)6]3−probe [149]. At the opposite, when the GCE was coated with the same clay mineral chemically modified by intercalation P-phenylenedimethylene bis dodecyl-N,N dimethylammonium dibromide (a gemini surfactant bearing two positive charges) and herein denoted Ba-DO4, continuous potential cycling in the same conditions leads to an increase in peak current (Fig. 1b). After 15 cycles, the electrode response reaches a steady state with a current 3.6 times higher than that recorded on the pristine sodium clay film, indicating an effective preconcentration capability for anionic species of the BaD04 material. The authors attributed this behaviour to the aggregation upon the modification of the organic cations, via both ion exchange and hydrophobic bonding, leading to the appearance of positive charges in the clay layers and on the clay surface [150, 151]. By performing the same experiment with the positively charged redox probe [Ru(NH3)6]3+, repetitive scans resulted in a slight faradic response and did not increase peak height (Fig. 1d); the steady-state currents recorded corresponded almost to those displayed by the bare GCE. The probe species enter the film mainly through small pinholes and DO2+, competing with the surfactant cation for the ion exchange sites. By covering the GCE by a film of pristine clay instead of Ba-DO4, the CV peaks increase ca. fourfold relative to that obtained at the bare GCE (Fig. 1c), due to more favourable ion exchange. So, the analysis of the results in Fig. 1 support the idea that the uptake of [Fe(CN)6]3− or [Ru(NH3)6]3+ is controlled by an ion exchange process,

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1959

Fig. 1 MCV recorded (a) in 0.1 M NaCl+1.5×10−3 M [Fe(CN)6]3− using a GCE coated with (a) Ba-Na+ and (b) Ba-DO4 materials, and (b) in 0.1 M NaCl+1.5×10−3 M [Ru(NH3)6]3+ on the GCE coated with a thin film of (c) Ba-Na+, and (d) BaDO4 . The dotted line on (a) and (d) represents the signal of the redox probes on the bare GCE. The potentials scan rate was 50 mV s−1. (Reproduced with permission from reference [130])

mainly governed by the charge of the GCE modifier and somewhat by the porosity of the film coated onto the electrode. This example illustrates how CV can be used to characterize qualitatively the permselective properties of clay and organoclay-modified electrodes. Pulse voltammetric techniques Pulse voltammetric techniques were introduced in electrochemistry by Barker and Jenkin [152], aiming at lowering the detection limits of voltammetric measurements. This was achieved significantly increasing the ratio between the faradaic and capacitive currents [153]. Basically, all pulse techniques are based on the same principle: a sequence of pulsed potentials is applied to the working electrode; after the potential is stepped, the capacitive current quite undesirable decays exponentially with time to a negligible value while the faradaic current decays more slowly for the same reaction as a function of 1/(time)½; that is, the rate of decay of the charging current is considerably faster than the decay of the faradaic current. The important parameters for pulse techniques are the height of the potential pulse (pulse amplitude) that may be constant or variable depending on the technique, the pulse width considered as the duration of the potential pulse, the sample period which is the time at the end of the pulse during which the current is measured. For some pulse techniques, the pulse period defining the time required for one potential cycle

must also be specified but this parameter is only significant for polarography (pulse experiments using a mercury drop electrode) where it corresponds to the lifetime of each drop. A number of different pulse techniques are available in the literature but square wave voltammetry and differential pulse voltammetry that differ in their potential pulse wave forms are the most used techniques in analytical determination employing organoclay-modified electrodes; their basic theories are briefly exposed in the coming sections. Square wave voltammetry Square wave voltammetry (SWV) is a powerful electrochemical technique that has received growing attention for routine quantitative analyses. It is suitable for analytical application, mechanistic study of electrode processes, electrokinetic measurements and biochemically oriented studies as well as analytical determination [154–156]. SWV utilizes a combination of a staircase potential modulation and periodic square-shaped potential function, applied at a stationary electrode. The potential wave form consists of a square wave of constant amplitude superimposed on a staircase wave form. The current is measured at the end of each half-cycle, and the current measured on the reverse half-cycle (ir) is subtracted from the current measured on the forward half-cycle (if). This difference current (if–ir) is displayed as a function of the applied potential [157]. Square wave voltammetry yields peaks for

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faradaic processes, where the peak height is directly proportional to the concentration of the species in solution. By measuring the difference in currents, the discrimination against the charging current is increased as any residual charging current is subtracted out. Additionally, the shape of the current response is a symmetric peak rather than the sigmoidal curve typically found in direct current or normal pulse techniques. Another advantage of SWV is the short determination durations since scan rates up to 1 V s−1 are employed [158]. Differential pulse voltammetry The potential wave form for differential pulse voltammetry (DPV) consists of small pulses (of constant amplitude) superimposed upon a staircase wave form. As SWV, DPV is of extreme usefulness in electroanalysis as the technique is particularly designed for measuring trace levels of organic and inorganic species [159]. Once more, the current is sampled twice in each pulse period: one just before the pulse application, and then in the pulse life when the capacitive current has decayed [153, 160]. The electrode signal which is the difference between the currents measured for each single pulse, consists of peaks the height of which is directly proportional to the concentration of the corresponding analyte. The widespread applications of DPV is also due the peak-shaped of i-E curves which results in improved resolution between two species with similar redox potentials as peaks separated by 50 mV may be measured [153]. Both SWV and DPV have been exploited for the stripping voltammetric determination at low concentrations of various species at OMEs using a procedure comprising two steps: the chemical accumulation of the analyte under open-circuit conditions and the electrochemical detection of the preconcentrated species by pulse stripping voltammetry, either in anodic or cathodic direction.

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EIS data are most often represented in Nyquist and Bode plots. Nyquist plot depicts the imaginary part versus the real part of the impedance, which is indicative of the capacitive and inductive character of the system under investigation, and each point in the Nyquist plot corresponds to impedance at one frequency [163]. Qualitatively, a Nyquist plot includes a semicircle and a linear zone, which correspond to the electron transfer-limited process and the diffusion-limited process, respectively [164]. Nyquist plots have the advantage that activation-controlled processes with distinct time constants show up as unique impedance arcs and the shape of the curve provides insight into possible mechanism or governing phenomena. However, this format of representing impedance data has the disadvantage that the frequency dependence is implicit; therefore, the AC frequency of selected data points should be indicated. Bode plots refer to representation of the impedance magnitude (or the real or imaginary components of the impedance) and phase angle as a function of frequency. Because both the impedance and the frequency often span orders of magnitude, they are frequently plotted on a logarithmic scale. Bode plots explicitly show the frequency dependence of the impedance of the device under test [163]. A typical example dealing with the use of EIS for the characterization of electron transfer properties at a glassy carbon electrode modified by an organoclay is represented on Fig. 2 [164]. The high-frequency section of Nyquist curves describe an arc, the diameter of which displays the electron transfer resistance, that increases in the presence of a clay mineral (MMT) and even more if the clay is filled with a surfactant-like cation (TBAB), from 128 Ω (for bare GCE) to 395 Ω (for GCE+ MMT+TBAB(partial)/M), indicating better electrode surface coverage with non-conductive clay film in the latter case.

During the past few years, electrochemical impedance spectroscopy (EIS) has been increasingly used for research and development of new materials as well as for product verification and quality assurance in manufacturing operations [160, 161]. It is also an efficient and semi-quantitative technique useful in various domains of electrochemistry such as corrosion, electrode structures characterization, fuel cells and sensors [162]. EIS experiments are performed by applying physically sound equivalent circuit models wherein physicochemical processes occurring within the electrode are represented by a network of resistors, capacitors and inductors. Thus, meaningful qualitative and quantitative information on electron transfer properties and regarding the sources of impedance within the electrode are extracted by analysing the data fitted into an equivalent electrical circuit model [162]. For the characterization of electrodes and electrochemical cells,

-Z'' / KΩ

Electrochemical impedance spectroscopy

Z' / KΩ Fig. 2 Nyquist plots for GCE/M (a), GCE+MMT/M (b), GC+MMT+ TBAB/M (c) and GC + MMT + TBAB (partial) /M (d), obtained in 10 mM K 3 [Fe(CN) 6 ] in PBS (0.1 M; pH 7.4). EIS conditions, amplitude 0.005; frequency range, 100 kHz to 0.01 Hz; number of frequencies, 71. Reprinted with permission from reference [164]

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The experimental results of EIS confirmed that the clay films coated well the GCE surface and the clay’s presence decreases the electron transfer rate of the redox probe.

Electroanalytical applications of OMEs Determination of organic compounds Table 1 presents an overview of OMEs used for the detection of organic compounds based on a literature survey for the past 15 years. As one can observe, most studies have been focused on phenol and its derivatives. This is due to their toxic effects on humans, animals and plants and their ability to be accumulated and detected at an OME [165, 166]. Organoclays derived from the intercalation of quaternary alkylammonium ions within smectites have been proven to display great affinity towards organic compounds due to their hydrophobic character [167–170]. Thus, Jovanovic and co-workers [19, 166, 171–174] have intensively investigated the development of amperometric sensors based on organoclays of classes 1 and 2 as admitted in this review. In most of their studies, the redox behaviour of the analyte is first investigated at a GC organoclay-modified electrode using multisweep cyclic voltammetry. These authors have established from CV data that the incorporation of alkylammonium cations into smectite often enhanced the electrode stability. Also, great responses are recorded at modified electrodes in comparison to the bare GCE. Other parameters have been investigated, such as the effect of surfactant loading [171] or its nature [172], and the influence of the type of modifying agent on the electrochemical performance of the electrode [173]. For example, the current density for p-nitrophenol oxidation wave was observed to decrease with the increase of BTMA loading while the electrode stability was significantly improved with the increase of BTMA loading [171]. On the basis of electrochemical responses obtained from CV experiments, SWV or DPV is further exploited to detect the phenolic pollutants [172, 173]. The same group also reported the use of bentonite pillared by a series of poly(hydroxo metal) cations for the electrochemical oxidation of phenol in acidic solution [166]. Multisweep cyclic voltammetry was applied to analyse the behaviour of the bentonite modified GCE, followed by the effect of pillaring on the electrocatalytic properties of the electrode. Similar to the above studies, Yang et al. [175] used a CTA-MMT-modified CPE to investigate the oxidation of 4chlorophenol which was remarkably increased at the modified electrode. Taking this aspect as advantage, they elaborated a sensitive and convenient electrochemical method for the determination of 4-chlorophenol which showed a limit of detection of 2×10−8 M. The proposed sensor was successfully applied to determine 4-chlorophenol in water samples.

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Another work based on a similar electrode but oriented towards the detection of phenol was reported by Zhou and coworkers [176] who estimated under optimized conditions a detection limit of 6×10−8 M. The need of electrodes with improved porosity and sensitivity and characterized by high rates of charge transfer and mass transport has favoured the development of CPE modified by clay minerals containing in their galleries a silica matrix formed by the hydrolysis of a silane. This approach has been mostly investigated by the group of Kong [177–179] who successfully exploited the build electrodes for the detection of aminophenol [177], hydroquinone [178] and catechol [179]. Pesticides are intensively used throughout the world as defoliants or against weed and pests in a large variety of crops [180, 181]. They are known as highly toxic chemicals, and their utilization presents negative effects for the surface water and soils where they diffuse during their application [182]. Thus, in the scope of OMEs devoted to the implementation of electrochemical devices suitable for the detection and the sensing of pesticides, Tcheumi et al. [130] performed the electrochemical analysis of methylparathion (MP) pesticide by the means of a GCE modified by the coating of a gemini surfactant-intercalated clay mineral. For the sensitive detection of MP, they used a procedure involving two steps: preconcentration under open circuit followed by voltammetric detection by SWV. The electrode response obtained (after 5 min of preconcentration in 4×10−5 M MP) was more than five times higher than that exhibited by the GCE covered by a film of the pristine clay. This opens the way to the development of a sensitive method for the detection of the pesticide. After optimization of all parameters involved in both analytical steps, a linear calibration curve for MP was obtained in the concentration range from 4×10−7 to 8.5×10−6 M in acetate buffer at pH 5 (Fig. 3), with a detection limit of 7×10−8 M. The proposed method allows the stripping determination of MP in spring water. The same type of procedure was used to electrochemically determine mesotrione [133], a marketed herbicide, launched by Syngenta and largely used in Europe for maize crop protection. The smectite clay mineral used to modify the GCE was intercalated using separately CTA and DDA surfactants; however, the organoclay with DDA gives the best results in terms of sensitivity (detection limit 0.26 μM). The fabricated device was applied to the commercial formulation Callisto, used in European maize market. Organoclays functionalised by the grafting of organic compounds were also evaluated as sensing component at chemically modified electrodes. Using an organoclay functionalized with thiol group from the covalent grafting of MPTMS onto the surface of a natural smectite clay mineral, Tonle et al. [183] exploited the thiol functionalized clay as electrode modifier for the sensing of methylene blue dye and by means of a carbon paste electrode. A significant enhancement of the electrode response towards the dye was observed with the

DDBA/TEOS DDBA/TEOS DDBA/TEOS DO2+ CTA CTA MPTMS TMPA

HEPC

MMT MMT MMT SM MMT MMT SM SM

KT

PPTA/CTA

L-Cystein

PMB AQ HDTBP HDTMA β-CD

MMT

SM

ATTA MMT FHT MMT MMT

CPE: graphite, silicon, modifier, clay mineral

GCE/drop-coating: PMB-ATTA GCE/drop-coating: MMT, AQ CPE: graphite powder, mineral oil, organoclay

GCE/drop-coating: organoclay

GCE/drop-coating: clay, PPTA, CTA, MC

GCE/coating: organoclay, CMC

GCE/drop-coating: organoclay, nafion, isopropyl alcohol, carbon black CPE: organoclay, EG, solid paraffin CPE: organoclay, EG, solid paraffin CPE: organoclay, graphite, paraffin oil GCE/drop-coating: organoclay CPE: carbon powder, binder, organoclay CPE: CTAB-MMT, graphite, paraffin oil CPE: carbon powder, nujol, organoclay GCE/drop-coating: organoclay

GCE/drop-coating: organoclay, nafion, propanol GCE/drop-coating: organoclay, carbon black, nafion GCE/drop-coating: organoclay, nafion, isopropyl alcohol, carbon black GCE/drop-coating: organoclay

OME configuration

AA 4-Nitrophenol 2,4-DCP 2,4-D Isoorientin (flavonoid)

Hydroquinone

o-Aminophenol Catechol Hydroquinone Methylparathion (pesticide) Phenol 4-Chlorophenol Methylene blue AA UA SA GA Isoproturon, Carbendazim Methylparathion

p-Nitrophenol

Mesotrione (herbicide)

Phenol p-Nitrophenol p-Nitrophenol

Analyte

2.5×10−7–2.5×10−5 M (DPV)

1–300 ng mL−1 10–500 ng mL−1 20–700 ng mL−1 (SWV) 2.0×10−6–10−5 M (DPV) 1.0×10−5–5.0×10−2 M (CV) 0.3–45 mg L−1 (DPV)

0.5 μM 1.13 μM 0.57 μM 7.0×10−8 M 6.0×10−8 M 2.0×10−8 M 4×10−7 M nd

1–100 μM (DPV) 10–1000 μM 5–2000 μM 4×10−7–8.5×10−6 M 1.0×10−7–3.0×10−5 M 5.0×10−8–1.0×10−5 M 1×10−6–1.4×10−5 M (CV) 0.07–1.1 mM 0.02–0.3 mM (CV)

1.0×10−6 M 0.02 mg L−1 2.32 μg L−1 2.64 μg L−1 2.95×10−7 M

1 ng mL−1 10 ng mL−1 20 ng mL−1 8.0×10−7 M

nd

nd

0.26 μM

0.25–2.5 μM (SWV) 2–16 mM (CV)

nd 7×10−6 M 1×10−6 M

Detection limit

(CV) 0.01–0.1 mM 2×10−7–2×10−4 M (SWV)

Linear range (voltammetric method)

52

187 185 189

186

188

184

176 178 177 130 175 174 182 183

174

133

173 171 172

Ref.

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nd not determined

Pillaring and other methods

Grafting

CTA DDMA BTMA

SM

BT

HDTMA, DDTMA, TMA BTMA HDTMA, DTMA, TMA

BT SM BT

Ion exchange Intercalation or insertion

Modifier(s)

Clay

Detection of organic compounds by organoclay-modified electrodes (period 2000–2014)

Clay modification method

Table 1

Author's personal copy J Solid State Electrochem (2015) 19:1949–1973

Author's personal copy J Solid State Electrochem (2015) 19:1949–1973 7 6

(a)

Ip (µA)

5

Current

Fig. 3 a Variation of peak current with MP concentration on GCE modified with a geminisurfactant-intercalated smectite, from (a) to (k) 0.4, 0.5, 0.6, 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5 and 8.5×10−6 M. b Calibration curve corresponding to a. Adapted with permission from reference [130]

1963

2 μA

4 3 2

(k)

-0.6 -0.4 -0.2

0.0

0.2

0.4

0.6

1

0

2

Potential / V ( vs SCE)

clay bearing thiol groups in comparison with the pristine one, leading to a detection limit of 4×10−7 M. The only work on a clay mineral grafted with a surfactant group was reported by Mbouguen et al. [184] who have successfully grafted trimethylpropylammonium moieties at the surface of a smectite. The resulting organoclay deposited as thin film onto a GCE surface was afterwards exploited as a suitable matrix for the accumulation and the electrochemical detection of ascorbic acid and uric acid using cyclic voltammetry. The electroanalytical response was improved by coating the electrode surface with a first layer of sublimed ferrocene, and then overcoating with the organoclay film to avoid the mediator leaching. The resulting bilayer film exhibited good characteristics such as extended linear range and high sensitivities for AA and UA, in cyclic voltammetry and amperometry. To lower overpotentials for AA oxidation and improve the electrode response, the GCE surface was first covered by a layer of sublimed ferrocene prior to the organoclay coating. This appeared to be an interesting approach to make possible the selective detection of AA and UA. Kenne and Detellier introduced very recently a nanohybrid material obtained by the grafting of 1-(2-hydroxyethyl)-pyridinium and drop-coated on a GCE for the electroanalysis of salicylic and gallic acids [185]. This work demonstrated that by a judicious choice of modifier, kaolinite can serve as a hosting material for the building of amperometric sensors for anionic organic compounds. To prepare physorption-based GCE using organoclay, a quite simple procedure was proposed by Hu et al. [186]. The organoclay was formed in situ by drop coating a film of MMT on the GCE, on which a desired volume of anthraquinone was deposited, leading to a modified electrode which shows remarkable sensitivity for the detection of 4-paranitrphenol by DPV. Very close to that work, an aminoacid (namely Lcystein) was combined directly to a smectite-type clay mineral on a GCE, and the obtained sensor was evaluated for the detection of hydroquinone [187]. Attapulgite, a fibrous clay mineral was used as a matrix for the electropolymerization of

4

6

8

10

[MP] / μM

MB on a GCE. The electroactivity and catalytic properties of MB were thus exploited by the modified electrode to detect AA by CV measurements [188]. Manisankar et al. proposed the development of electrochemical sensors for the determination of three pesticides, namely, isoproturon, carbendazim and methyl parathion using heteropolyacid montmorillonite clay-modified GCE [189]. Prior to the detection of the pollutants, they investigated their redox behaviour in the presence and absence of a surfactant (CTA was used as model) in the supporting electrolyte. The micellar effect of the surfactant was thus shown to enhance the accumulation of the pesticides in the clay pores and increases the electron transfer rate. The developed procedure allows the SWV determination of the three pesticides after optimizing all the experimental parameters, to limits of determination down to nanogrammes-permilliliter levels. Early in the 2000s, Ozkan et al. have developed another type of amperometric sensors by incorporating separately in a CPE mixed-ion FHT and a mixed-ion MMT [190]. These materials were prepared by intercalating within the galleries of clay minerals hydrophilic inorganic ions (Na+) and lipophilic onium ions. Incorporated at the 5 wt% level into a CPE, they showed great ability for the specific determination of 2,4-D (herbicide) and 2,4-DCP (a phenolic derivative pollutant). A recent study reported the use of a cyclodextrin/ MMT composite material for the investigation of the electrochemical oxidation and interfacial electron transfer behaviour of isoorientin (a flavonoid) at CPE by employing CV, DPV and EIS [52]. Detection of inorganic species Inorganic contaminants of surrounding and drinking waters originate from natural and anthropogenic factors due to industrial activities, petroleum contamination and sewage disposal [191]. They include some anionic species such as fluoride, nitrate or cyanide, but mainly heavy metal species. Over 50 elements are classified as heavy metals, including transition metals, some metalloids, lanthanides and actinides. However,

Author's personal copy 1964

17 of them are considered to be both very toxic and relatively accessible by living organisms [192]. Lead (Pb), mercury (Hg), arsenic (As) and cadmium (Cd) are generally considered as ‘leader’ elements in human poisoning, even at trace level. At the opposite, some other heavy metals including copper (Cu), zinc (Zn), nickel (Ni), cobalt (Co), selenium (Se) and bismuth (Bi) not only are known to play a vital role in physiological concentrations but can also be toxic at larger doses [192, 193]. For the electroanalysis of inorganic compounds, several OMEs were prepared. In this section, the methods found in literature for the electrochemical detection of heavy metals will be first exposed, followed by those used for anionic species. To date, about 15 papers have been published on electrodes modified using organoclays and most of them (as gathered in Table 2) were focused on mercury (53 %) and lead (47 %), followed by copper (27 %) and cadmium (20 %). Table 2 summarizes the detection of inorganic species by OMEs. The group of Filho has evaluated MMT intercalated by HDTMA and MAT [194, 195] or by HDTMA and TDD [196] as preconcentration agent in a chemically modified CPE for the determination of mercury(II) in aqueous solution. Prior to this application, the sorptive capability of such prepared organoclay particles for the removal of mercury ions was tested by batch and chromatographic column techniques, in water containing other metals such as Cd(II), Pb(II), Cu(II), Zn(II) and Ni(II). Then, they were used as CPE modifiers and the electrode response was evaluated with respect to pH, electrode composition, preconcentration time and mercury concentration as well as with the effect of other potential interfering cations. The best limit of detection achieved was 0.15 μg L−1 [196]. Similarly, Colilla et al. [197] have minutiously studied the intercalation of 2- and 4-mercaptosubstituted pyridines (2Mpy and 4Mpy, respectively) in sodic MMT, obtaining composite materials with interesting functional properties derived from the presence of the mercapto group. Upon correlating the structural arrangements of the intercalated species with the complexing ability of mercapto groups towards heavy metal ions, these materials were tested as active phase of CPEs for the amperometric determination of Cd(II), Pb(II), Cu(II) and Hg(II) ions in aqueous solutions. CPE-based modified MMT showed good electroanalytical behaviour due to the particular arrangement of the guest species in the interlayer space of the clay, while pure 2- or 4mercaptopyridines alone in CPEs did not give an electroanalytical response towards heavy metal ions. A low-cost electrochemical sensor for lead detection was designed by Bouwe et al. [198], using an organoclay obtained by the intercalation of 1,10-phenanthroline (OP) in the interlayer space of MMT. The OP-MMT intercalate was used as Pb(II) sensor via a CPE by adsorptive stripping voltammetry. The amount of accumulated Pb(II) increased with the accumulation time and remained constant after relatively long times (>15 min), i.e. after an

J Solid State Electrochem (2015) 19:1949–1973

adsorption, equilibrium was reached. Upon optimizing all parameters governing the steps involved in the electroanalysis procedure, a limit of detection in the sub-nanomolar range (4× 10−10 M) was obtained. Other works based on GCE modified by intercalated or functionalized organic compounds for electroanalytical purposes were published in the course of the year 2014. For example, MMT/polyaniline nanocomposites materials were prepared and characterized by scanning electron microscopy and electrochemical techniques, and then used to modify a GCE [199]. The modified electrode which showed great affinity towards Pb(II) ions was applied to the sensitive detection of the same pollutant by means of ASDPV. Maghear et al. [164] also described the application of a GCE modified by spin coating of smectite clay partially exchanged with tetrabutylammonium ions (TBA) to the preconcentration electroanalysis of several metal ions (Cd, Pb and Cu). The intercalation of TBA between the clay mineral sheets, although partial, induced the expansion of the interlayer spacing (as ascertained by XRD) resulting in the enhancement of mass transport rates (as pointed out by electrochemical monitoring of permeability properties of these thin (organo)clay films on GCE). The OCME was then applied to the anodic stripping square wave voltammetric analysis of metal ions after accumulation at open circuit giving rise to detection limits as low as 3.6×10−8 M for copper and 7.2×10−8 M for cadmium. Organoclays prepared by one-step co-intercalation of cetyltrimethylammonium ions (CTA+) and thiourea in various amounts within the interlayer region of smectite were used as electrode material for the detection of Pb(II) [28]. The electrochemical features and permeability properties of the organoclays, coated as thin films onto the surface of a glassy carbon electrode (GCE), were first characterized through ion exchange multisweep cyclic voltammetry. Thanks to the chelating ability of thiourea supported on the material, the modified clay mineral sample containing CTA+ ions at a concentration equivalent to 25 % of the CEC and thiourea in excess of the CEC of the pristine clay mineral were exploited to build a sensitive and selective sensor for lead detection. The optimal response was obtained for the functionalized clay, showing a sensitivity higher by ca. one order of magnitude with respect to the unmodified electrode. The experimental parameters governing the preconcentration electroanalysis of lead were optimized, and a linear calibration curve for P b(II) was obtained in the concentration range from 10−9 to 10−7 M, with a detection limit of 2.9×10−11 M was found much lower than those usually reported in the literature. Till now, very few reports are based on organoclays of class 3 (i.e. issued from grafting of organic compounds) for the elaboration of heavy metals sensors in spite of the advantageous features presented by such organoclays (durable immobilization of the reactive organic groups by strong binding on the clay surfaces which prevents their leaching in the surrounding medium when used in solutions). Tonle et al. investigated the possible use of organically modified clays deriving from the reaction of

CPE: organoclay, graphite mineral oil CPE: organoclay, graphite, binder CPE: organoclay, graphite, binder CPE: graphite, composite clay CPE: carbon, nujol and organoclay GCE/organoclay, MCC GCE/organoclay Pt electrode/clay material and polystyrene GCE/organoclay, CMC CPE: carbon, nujol and organoclay CPE: carbon, nujol and organoclay GCE/organoclay GCE/clay material, PVA, [Fe(OP)3]2+

MMT HDTMA/ MAT MMT HDTMA/TDD MMT 2Mpy, 4MPy Chitosan CPTES/NaSH Im TEA APTES BHIC APTES, MPTMS APTES, MPTMS APTES [Fe(OP)3]2+

MMT KT KT KT KT KT SM

SM SM

Pb(II) Cd(II)

MMT AQ

GCE/AQ+colloidal dispersion of MMT

Cu(II) Hg(II) Hg(II)

IO3−

Hg(II) [Fe(CN)6]3− [Ru(NH3)6]3+ NO2−

NO3− Pb(II) SCN− CN− [Ru(CN)6]4− I− Hg(II)

Hg(II) [Fe(CN)6]3− Cu(II) Cd(II) Pb(II) Pb(II) Pb(II) Pb(II) Cu(II) Hg(II) Hg(II) Cd(II), Pb(II), Cu(II), Hg(II)

Analyte

SM TEOS/CTAB GCE/clay particles+silica matrix MMT TEOS/MPTMS/CTAB (DDA) GCE/organoclay SM MPTMS/TEOS (DDA) GCE/organoclay

ATTA PANI/PPMA

GCE/organoclay, PPMA

GCE/organoclay GCE/organoclay CPE: carbon, nujol and organoclay

MMT PANI SM CTA-Thiourea MMT OP

BT

CPE: MAT-HDTMA-clay GCE: organoclay/carbon black-nafion GCE/organoclay

MMT MAT/HDTMA SM HDTMA MMT TBAB

OME preparation

2 nM–1 μM 8 nM–1 μM (DPV)

1.0 μM–25.0 mM 1.0 μM–20.0 mM (CV, chronoamperometry) 2.0×10−6–5.2×10−4 M (chronoamperometry) (CV, SWV) (HV/RDE) 4–20 nM 0.05–0.8 μM (DPV)

(ASDPV) (CV)

0.01–0.7 mg L−1 (DPV) 10−5–1.6×10−3 M (CV) 1.2×10−7– 7.5×10−6 M 2.16×10−7–2.5×10−6 M (SWV) 4×10−9–1×10−7 M (ASDPV) 10−9–10−7 M – (SWV) 0.01–0.7 mg L−1 (ASDPV) 0.01–1.0 mg L−1 (ASDPV) – CV 3×10−4–0.1 M 3×10−7–10−5 M CV CV CV 5×10−7–10−5 M (SWV) 1.0×10−7–7.0×10−7 M

1 nM 3 nM

nd nd 5×10−10 M

207

144 132 131

210

5.3×10−7 M

29 149

209 93 79 39 77 41 200

211

M M M

M M

195 196 197

199 28 198

194 208 164

nd

9.3×10−5 6.0×10−8 nd nd nd 1.5×10−7 8.7×10−8 6.8×10−8 nd nd

2.0 μg L−1 nd 3.6×10−8 M 7.2×10−8 M 1×10−9 M 2.9×10−11 M 4.0×10−10 M 4.0×10−8 M 2.0 μg L−1 0.15 μg L−1 nd

Linear range (voltammetric method) Detection limit Ref.

J Solid State Electrochem (2015) 19:1949–1973

nd not determined

Pillaring and others

Grafting

Intercalation or insertion

Ion exchange

Modifier(s)

Detection of inorganic compounds by organoclay-modified electrodes (period 2000–2014)

Clay modification method Clay

Table 2

Author's personal copy 1965

Author's personal copy 1966

smectite with APTES and MPTMS, respectively, bearing aminopropyl or mercaptopropyl groups for the detection of mercury(II) using a CPE [29]. As shown on Fig. 4, the DPV curves of the electrodes modified with the grafted clays was clearly higher than that observed for the raw clay-modified carbon paste sensor, demonstrating that the organo-functional groups attached to the clay particles included in the bulk of the CPE are easily accessible to Hg(II) ions. This work was further extended to the development of sensitive devices for the detection of Hg(II) [200]. The used two-step Hg(II) detection, i.e. open circuit accumulation followed by voltammetric analysis, was fairly efficient for Hg(II) analysis. Interestingly, a linear response was obtained in the concentration range from 0.1 to 0.7 M Hg(II). In optimized conditions, the detection limits of the method were found to be 8.7×10−8 and 6.8×10−8 M, respectively, for the amine and thiol-functionalized clay minerals. In a very close approach, the same group reported in 2011 the grafting of 3-chloropropyltriethoxysilane onto the interlayer surfaces of kaolinite, followed by substitution under carefully controlled conditions of chloro by thiol groups [93]. The final functionalized kaolinite was fully characterized using several physico-chemical techniques that allowed to propose for the organoclay the following chemical formula: Si4Al4O10(OH)6[O2Si(OEt)(CH2)3SH]. The functionalized clay mineral was then evaluated as electrode modifier for lead(II) sensing, leading to a limit of detection of the same order of magnitude as those mentioned in the previous studies (6×10−8 M). The interfering effect of some ions likely to influence the stripping determination of lead(II) ions was

Fig. 4 Typical DPV curves obtained for the analysis of Hg(II) using CPEs modified with a pristine smectite clay mineral (a), and with the same clay bearing aminopropyl (b) or mercaptopropyl (c) groups. The detection of Hg(II) was performed in 0.1 M HCl after 1-min electrolysis. Preconcentration was achieved for 2 min at open circuit in aqueous solution containing 2×10−5 M Hg(II). Reproduced with permission from reference [29]

J Solid State Electrochem (2015) 19:1949–1973

evaluated, and the proposed method applied to the analysis of tap water. The group of Walcarius has extensively developed the structuration of electrode surfaces with inorganic thin films by using the sol–gel process which is suitable to generate uniform deposits of metal or semimetal oxides (mainly silica) and organic–inorganic hybrid thin films of controlled thickness, composition and porosity [200–204]. Intrinsically simple, this method can be achieved by electrolytic deposition from sol solutions [205, 206]. In 2013, this concept was successfully extended to produce clay-mesoporous silica composite films onto GCEs. Thus, the particles of smectite were spin-coated on GCE, followed by the growing of a surfactant-templated silica matrix around these particles by electro-assisted self-assembly (EASA). Typically, a cathodic potential was applied to the GCE immersed into a hydrolyzed sol (containing tetraethoxysilane, TEOS, as the silica source, and cetyltrimethylammonium bromide, CTAB, as surfactant) in order to generate the necessary hydroxyl catalysts inducing the formation of the mesoporous silica. After removal of the surfactant template, the composite film became highly porous (i.e. to redox probes) and the clay recovered its pristine interlayer distance and cation exchange properties. The organoclay obtained, shown to be prominent electrode modifier, was applied in the preconcentration electroanalysis of copper(II) [144]. Some years before that relevant work, the same concept of sol–gel process (but without the electro-assisted step) was exploited for the elaboration of thiol functionalized (using MPTMS/TEOS) porous clay heterostructures (PCHs) that were used as sensing devices for Hg(II) upon coating on GCEs [131, 132]. About their sensitivity, the limit of detection was greatly improved since a 6-nM value was reached using DPASV with only 3 min of accumulation, the linear range being from 50 to 800 nM [131]. It has to be noticed that increasing the accumulation time allowed another linear range from 4 to 20 nM yielding a remarkably low detection limit of 0.5 nM. The simultaneous determination of trace levels of cadmium (II) and lead (II) by ASDPV was reported by Yuan et al. [207], who used an anthraquinone (AQ)-improved Namontmorillonite nanoparticle (nano-SWy-2) to chemically modify a GCE. The analytical procedure used was based on a non-electrolytic preconcentration via ion exchange model, followed by an accumulation period via complex formation in the reduction stage (typically at −1.2 V vs SCE), and then by an anodic stripping process. The variables involved in that procedure were optimized, leading to linear calibration graphs in the concentration ranges from 8×10−9 M to 1×10−6 M (Cd2+) and 2×10−9 M to 1× 10−6 M (Pb2+). After 5 min of preconcentration, the lowest detectable concentration of Cd2+ was evaluated to be 3 nM and that of Pb2+was 1 nM. The applications for the detection of trace levels of Cd2+and Pb2+ in milk powder and lake water samples indicated that the developed sensor was economical for potent electroanalytical procedures.

Author's personal copy J Solid State Electrochem (2015) 19:1949–1973

Concerning the electroanalytical behaviour and detection of anionic species using OMEs, some contributions are found in literature. It has been observed that by a suitable chemical modification of smectite-type clay minerals, their inherent cation exchange properties can be tuned into anion exchange ability, favouring thereby their affinity towards anions. This so-called tuning charge selectivity has been applied in the electroanalysis of several charged species using modified electrodes [149]. For example, smectite grafted with propylamine groups exhibited a higher ability to accumulate [Fe(CN)6]3− anions in acidic media due to protonation of amine groups. Following these studies, ion exchange properties of the same clay mineral was before and after its modification by intercalation of HDTMA evaluated for the uptake of [Ru(NH3)6]3+ and [Fe(CN)6]3− ions used as redox probes [208]. By thoroughly examining key experimental parameters affecting the incorporation of the probes within the film (ionic strength, surfactant loading and the nature of supporting electrolyte), the ion exchange processes occurring at the electrode were highlighted. Closely related to these studies, Tonle et al. have developed a new family GCE coated by kaolinite organically modified by APTES [77], TEA [39] or by a ionic liquid [79] that were applied to the cyclic voltammetric preconcentration of cyanide-based anions [Ru(CN)6]4−, CN− and SCN−, respectively. Although the last mentioned reports were not extended to the detection of the investigated analytes at trace levels, they clearly demonstrated that kaolinite-based functionalized nanohybrid materials could be largely exploited as electrode modifiers in the field of electroanalysis. Nanocomposites based on the intercalation of the cationic natural surfactant, chitosan, in montmorillonite were used in the development of electrochemical sensors for the potentiometric determination of NO3− anionic species [209]. The prepared chitosan–montmorillonite nanocomposites exhibit good functional and mechanical properties, and their electronic conductivity was increased when they were combined with graphite particles. Recently, two sensing platforms were reported. The first one was prepared by associating attapulgite, polyaniline and phosphomolybdic acid for the determination of iodate anions was recently proposed by Zhang and coworkers [210]. To build such sensor, nanostructured attapulgite was utilized to reduce the agglomeration of polyaniline particles, and the as-prepared attapulgite/ polyaniline composite was used to immobilize phosphomolybdic acid. So doing, excellent electrocatalytic performance was obtained due to the synergistic effect of nanostructured attapulgite, polyaniline and phosphomolybdic acid. The electrochemical responses of as-prepared modified electrode were investigated by cyclic voltammetry and chronoamperometry. Upon coating on a GCE, the system displays many advantages such as good reproducibility, high stability and fast amperometric response. It was applied to detect iodate in a commercial table salt sample (the limit of

1967

detection achieved was 5.3×10−7 M). The second platform dedicated to nitrite sensing was developed by incorporating [Fe(OP)3]2+ (OP is 1,10 phenanthroline) ions in a bentonite film coated on a GCE [211]. The system which showed high electrocatalytic activity for nitrite oxidation, by decreasing the nitrite oxidation potential and increasing the peak current, was applied for the determination of nitrite in potable water samples.

Conclusion Clays minerals are naturally occurring materials, with attractive properties and a flexible structure which allows their chemical functionalization. They can be modified according to a wide range of pathways that can be tailored in regard to their applications although intercalation of organic molecules by exchange reactions and grafting on hydroxyl groups present on their surface remain the most described methods. The organoclays thus obtained display improved and even additional properties in comparison to their unmodified counterparts: larger specific surface areas, organophilic properties for intercalated materials and high affinity towards several pollutants present in surrounding and drinking water as well as in various aqueous environmental matrices. The availability of various physico-chemical techniques for the characterization of clay minerals before and after their modification is also exploited. Furthermore, the organic–inorganic hybrids have been shown to be active and efficient materials in the modification of conventional electrodes, which can then be used as electrochemical sensors for the determination of heavy metals, pesticides, phenolic compounds, drugs, dyes and anionic pollutants (e.g. nitrate, nitrite, iodate and thiocyanate). These devices offer the advantages of being inexpensive, simple, and rapid, and compatible with mass production of reproducible electrodes. As concluding remarks, it appears that the group of 2/1 swelling clay minerals (e.g. montmorillonite) is almost the exclusive one used for the elaboration of organoclay-based sensors dedicated to the detection of organic compounds (see Table 2), probably because of its intrinsic ability to swelling, facilitating thereby their functionalization with organic moieties that can easily fit into the interlayer galleries of the clay. Future works should be focused on the exploration of materials from kaolinite group. Otherwise, approximately 50 % of reports on organoclay-modified electrodes are used for the quantification of heavy metals, probably due to the great number and hazardous character of the latter. Important to be mentioned is that electroanalytical devices based on modified electrodes are currently dominated by both carbon paste and glassy carbon coated by film or organocalys. Finally, the field of organoclays remain very exciting since their potentialities could be explored to a large

Author's personal copy 1968

extend for the preparation of disposable tools for the on-line and real-time monitoring of pollutants. Acknowledgments This work was supported by the Alexander von Humboldt Foundation (Germany). The joint donation by the International Union of Crystallography (UICr) and Bruker AXS SAS (France) of a powdered X-Ray diffractometer (Siemens D5005) to the University of Dschang is also acknowledged.

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