A Novel Highly Sensitive Zeolite-Based

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A Novel Highly Sensitive ZeoliteBased Conductometric Microsensor for Ammonium Determination O. Y. Saiapina

a b

Jaffrezic-Renault

, S. V. Dzyadevych

a c

d

, A. Walcarius & N.

b

a

Laboratory of Biomolecular Electronics, Institute of Molecular Biology and Genetics, National Academy of Sciences of Ukraine, Kyiv, Ukraine b

University of Lyon, Laboratory of Analytical Sciences, University Claude Bernard Lyon 1, Villeurbanne Cedex, France c

Institute of High Technologies, Taras Shevchenko Kyiv National University, Kyiv, Ukraine d

LCPME, CNRS-University Henri Poincare Nancy 1, Villers-les-Nancy, France Accepted author version posted online: 10 Apr 2012. Version of record first published: 02 Aug 2012

To cite this article: O. Y. Saiapina, S. V. Dzyadevych, A. Walcarius & N. Jaffrezic-Renault (2012): A Novel Highly Sensitive Zeolite-Based Conductometric Microsensor for Ammonium Determination, Analytical Letters, 45:11, 1467-1484 To link to this article: http://dx.doi.org/10.1080/00032719.2012.675487

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Analytical Letters, 45: 1467–1484, 2012 Copyright # Taylor & Francis Group, LLC ISSN: 0003-2719 print=1532-236X online DOI: 10.1080/00032719.2012.675487

Sensors


A NOVEL HIGHLY SENSITIVE ZEOLITE-BASED CONDUCTOMETRIC MICROSENSOR FOR AMMONIUM DETERMINATION O. Y. Saiapina,1,2 S. V. Dzyadevych,1,3 A. Walcarius,4 and N. Jaffrezic-Renault2 1

Laboratory of Biomolecular Electronics, Institute of Molecular Biology and Genetics, National Academy of Sciences of Ukraine, Kyiv, Ukraine 2 University of Lyon, Laboratory of Analytical Sciences, University Claude Bernard Lyon 1, Villeurbanne Cedex, France 3 Institute of High Technologies, Taras Shevchenko Kyiv National University, Kyiv, Ukraine 4 LCPME, CNRS-University Henri Poincare Nancy 1, Villers-les-Nancy, France Natural zeolite clinoptilolite was successfully applied in the sensing technology for electrochemical detection of ammonium. A novel ammonium-selective sensor was developed based on clinoptilolite, possessing intrinsic ammonium-sieving and ion exchange capacity. The sensor design allowed measurements in both differential mode and requiring no classical reference electrode. The sensor selectivity towards Naþ, Kþ, Ca2þ, Mg2þ, and Al3þ was studied. The limit of detection and the dynamic range of the ammonium-selective conductometric microsensor, determined in the phosphate buffer solution, were 1.0 108 M and 0–8 mM, respectively. The ammonium sensor presented high operational and storage stability. Keywords: Ammonium detection; Clinoptilolite; Conductometric microsensor; Ion exchange

INTRODUCTION Ammonium as a major constituent of deep ground waters (NHþ 4 N up to 39 mg=L) has received a tremendous amount of attention, due to effects of N loading in part on health and potentially on ecosystem (Howarth and Marino 2006). Ammonium is present in groundwater naturally due to anaerobic degradation of Received 14 December 2011; accepted 21 January 2012. The authors would like to thank the European Commission for their funding of the Project PIRSES-GA-2008-230802, as well as National Academy of Sciences of Ukraine (complex scientifictechnical program ‘‘Sensor systems for medical-ecological and industrial purposes’’), NATO (Project CBP.NUKR.CLG 984221), and Rhoˆne-Alpes Re´gion for the MIRA project. Address correspondence to O. Y. Saiapina, Laboratory of Biomolecular Electronics, Institute of Molecular Biology and Genetics, National Academy of Sciences of Ukraine, 150 Zabolotnogo St., Kyiv 03680, Ukraine. E-mail: [email protected] 1467


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O. Y. SAIAPINA ET AL.

organic matter and artificially as a result of organic waste disposal. Anthropogenic ammonium is one of the major dissolved components in some types of groundwater contaminant plumes. NHþ 4 concentrations of the order of 1–10 mmol=L have been observed in aquifers contaminated by landfill leachate and concentrated wastewater disposal practices (Bo¨hlke, Smith, and Miller 2006). Septic systems and agricultural practices also may result in locally elevated recharge rates of NHþ 4 . The potential danger of high ammonium concentrations in aquifers is degradation of groundwater quality and limitations of its usability, it can have substantial effects on water-rock interactions, and it can be a substantial source of nitrogen in surface waters due to groundwater discharge. Thus, identification of the sources of NHþ 4 origination and its quantitative determination is of environmental importance. During several previous decades, reports have been conducted on a range of quantitative methods for ammonium determination [fluorometric (Ke´rouel and Aminot 1997) and colorimetric (Aminot, Ke´rouel, and Birot 2001) methods and isotope-ratio mass spectrometry (Benson et al. 2009)]. However, these methods, in the case of low-level ammonium determination, suffer from low sensitivity and high contamination; some of them are resource-intensive and time-consuming. Meanwhile, the use of electrochemical sensors, and potentiometric ion-selective electrodes (ISEs), in particular, for different cations and anions has already become practically indispensable. There are numerous publications on the detection of heavy metals, transition and alkaline earth metals (Gupta and Kumar 1999; Gupta et al. 1997; Gupta, Jain, Maheshwari, et al. 2006; Gupta, Jain, Kumar, et al. 2006; Gupta, Jain, Khurana, et al. 1999; Gupta, Singh, and Gupta 2007; Gupta, Singh, Khayat, et al. 2007; Gupta, Singh, et al. 2006; Gupta et al. 2009; Gupta, Mangla, and Agarwal 2002; Gupta, Goyal, and Sharma 2009a; Gupta, Goyal, and Sharma 2009b; Gupta, Prasad, and Kumar 2003; 2004; Gupta et al. 2000; Gupta, Chandra, and Lang 2005; Gupta, Chandra, and Mangla 2002; Jain, Gupta, and Singh 1996; Jain, Gupta, Sahoo, et al. 1995; Jain, Gupta, Singh, et al. 2006; Jain, Gupta, Singh, et al. 1997; Singh, Gupta, and Gupta 2007; Srivastava, Gupta, and Jain 1996; Srivastava et al. 1995). Reported potentiometric ion-selective electrodes for chloride, acetate, phosphate, carbonate, arsenite, chromate, uranyl, and molibdate ions comprised poly(vinyl chloride) based membranes of modified calixarenes (Gupta, Ludwig, and Agarwal 2005; Gupta, Mangla, et al. 1999; Jain et al. 2005), hydrogen bonding diamide or disubstituted phenylhydrazone-based receptors (Gupta, Goyal, and Sharma 2008; Gupta, Goyal, and Sharma 2009c; Jain, Gupta, and Raisoni 2006a), neutral carrier and organic resin (Jain, Gupta, and Singh 1995), porphyrins (Gupta and Agarwal 2005; Gupta, Chandra, Chauhan, et al. 2002; Gupta, Jain, Singh, et al. 1999; Prasad, Gupta, and Kumar 2004), macrocyclic dithioxamide receptors (Jain, Gupta, and Raisoni 2006b), and poly(vinyl chloride) and polystyrene based membranes (Jain, Gupta, Khurana, et al. 1997). Among electrochemical techniques for ammonium determination in aqueous media, the ISEs based on enzymes, ion exchangers, and neutral molecules, acting as ion carriers (e.g., nonactin and liquid membranes, ionophores, synthetic receptors) appeared to be one of the largest group (see Table 1). Although the aforementioned ion-selective components play a substantial role in the sensor performance, the synthesis indeed may limit their wide application due to requirement of experienced chemists; otherwise, it requires considerable financial investment. Leakage

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Glutamate oxidase and glutamate dehydrogenase Polyanilinepoly(styrene sulfonate-comaleic acid) composite films

Copper ion-doped clinoptilolite

SiO2=ZrO2= phosphate-NH4 composite Poly(vinyl chloride) membrane with nonactin= monactin Zirconium titanium phosphate ion exchanger

Amperometric

Amperometric

Potentiometric

Potentiometric

Potentiometric

Amperometric

Glutamate dehydrogenase (type III)

Sensing probe

Amperometric

Type of transducer

1 106–1 102

1 105–1 101, slope 56.3 mV=decade

1 104–1, slope 42 mV=decade

7.7 107–4 102

1 106–1 101

1.2 105–1

2 105–1 103, slope 191 mA mol1 L

0–1.25 102, slope 12 mA mM1 and 0–1 102, slope 2.55 mA mM1 depending on the preparation procedure

0–1.4 102 and 0–1 101 depending on the preparation procedure of conducting composite films 2 105–1 102

Up to 2 10

4

1 105–3 104, slope 325.87 nA s1 mM1

4

Linear range, M

Up to 3.1 104

1 10 –3 10

5

Dynamic range, M

Table 1. Sensors for ammonium determination

1 105

n=a

1.58 107

5 106

n=a

2.06 106

1 10

5

LOD, M


Response time 30 s; lifetime over 2 years (if stored under controlled conditions)

Response time 1 min; lifetime 6 months Response time 5 s

Response time about 1 min

Lifetime 4–20 days depending on enzyme immobilization support Response time 2 s; lifetime 18 days n=a

Response time, lifetime

(Continued )

Hassan et al. 2001

Do et al. 2001

Walcarius, Vromman, and Bessiere, 1999 Coutinho et al. 2007

Kwan, Hon, and Renneberg 2005 Luo and Do 2006

Bertocchi, Compagnone, and Palleschi 1996

Ref.

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Poly(vinyl chloride) membrane with nonactin Poly(vinyl chloride) membrane with nonactin

Poly(vinyl chloride) membrane with nonactin Photocured poly(nbutyl acrylate) membrane with nonactin Poly(vinyl chloride) membrane with nonactin Poly(vinyl chloride) membrane with nonactin

Sensing probe

data are not available.

Potentiometric

Potentiometric

Potentiometric

Potentiometric

Potentiometric

Potentiometric

Type of transducer

1 106

8 106

2 105–1 101, slope 56.2 mV=decade 1 105–1 101, slope 56 mV=decade

1 104–1 102, slope 58.4 mV=decade 6 103–1 101, slope (55.6 5) mV=decade

n=a

n=a

1 106–1 102

1 103–1 101

1 103

1 106

(1.0–4.0) 106

1 105–1 101, slope (57–58) mV=decade

LOD, M 5 105

Linear range, M n=a

1 104–1 101, slope 56 mV= decade n=a

Dynamic range, M

Table 1. Continued


n=a

Response time about 20 s; lifetime over 6 months Response time several minutes

Response time 6 min; lifetime over 120 days Response time 2–3 min; lifetime over 3 months Response time 5 min

Response time, lifetime

Shen, Cardwell, and Cattrall 1997 Wells and Miller 2000

Schwarz, Kaden, and Pausch 2000.

Liu and Sun 1997

Lee, Sagir, and Musa 2004

Ikeda et al. 1998

Ref.


ZEOLITE-BASED AMMONIUM CONDUCTOMETRIC MICROSENSOR

1471

of these receptors to the solution and their limited stability reduce the shelf lifetime of these sensors. In addition, a measuring potentiometric cell in this case has to include a reference electrode of a macroscopic size which requires artisanal manufacturing. Therefore, the cost of fabrication of potentiometric ion-selective electrodes remains very high, and it undermines a large progress achieved in microelectrodes and conductometric microsensors, in particular (Arkhypova et al. 2005). Zeolite-modified electrodes (ZMEs) emerged in the early 1980s (Murray 1980; Murray 1984) and offered a numerous possibilities of intelligently designing the surface of conventional electrodes. A capability to improve the electrical response by combining the intrinsic properties of the modifier to a selected electrochemical reaction was likely an impulse for the further elaboration of ZMEs. A particular structural feature of zeolites is the existence of channels and=or cavities linked by channels. This provides a porous structure of molecular dimensions and a large specific surface area. A characteristic that distinguishes zeolites from other porous materials is the combination of a variety of pore sizes and shapes with an ion exchange capacity in a single material (Breck 1974; Meyer and Olson 1992). Thus, the valuable properties of zeolites for their application in the development of new electroanalytical devices (sensors) are essentially determined by their structures. For example, sorption characteristics depend upon the size of pore openings and void volume; ion exchange selectivity depends upon the number and nature of cation sites and their accessibility; catalytic behavior depends upon the pore openings, dimensionality of channel system, the cation sites, and the space available for reaction intermediates; and host applications depend on the size and spacing of the cages (McCusker and Baerlocher 2001). The economic feasibility of the zeolite application could be derived from the following facts. Depending on the application purposes (catalysts, adsorbents or detergents), the price of zeolite varies considerably: from about $ 0.02 to dozens of dollars per pound. Natural zeolites in bulk applications are sold for about $ 0.10=pound, and in industrial adsorbent applications for $1–1.50=pound (McCusker and Baerlocher 2001). In the current paper, we report the application of the natural zeolite clinoptilolite for the conductometric detection of ammonium. Generally, a natural zeolite clinoptilolite is known as a commonly suggested zeolite in the field of wastewater and water treatment (Woinarski et al. 2003). However, there has been very little work investigating the effect of the clinoptilolite ion exchange selectivity on the ion specie determination (Hamlaoui, Reybier, et al. 2002; Hamlaoui, Kherrat, et al. 2002). In the work, the quantitative detection of ammonium has been accomplished by measurement of the bulk conductance GB of a thin zeolitic layer, where the magnitude of GB was related to the primary ion contents in the solution analyzed. The zeolite-modified microsensors were studied using conductometric and impedimetric transduction modes. MATERIAL AND METHODS Chemicals Ammonium nitrate, sodium nitrate, potassium nitrate, magnesium nitrate, calcium nitrate tetrahydrate, and aluminum (III)-nitrate nonahydrate were supplied

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by Sigma-Aldrich (France). A phosphate buffer solution (PB) used was KH2PO4= Na2HPO4 (Acros Organics, Belgium) unless otherwise stated. All chemicals were of analytical reagent grade and ultra-pure (UP) water used was obtained from a Millipore purification system (France).


Origination and Characteristics of the Natural Zeolite Clinoptilolite A powdered sample clinoptilolite was obtained from the Mediterranean Society of Zeolites (SOMEZ, France) and its unit cell formula was (Na0.10K0.57) (Ca0.47Mg0.15)(Al1.97Fe0.12)(Si9.96Ti0.02)O24, 7H2O (chemical composition determined by elemental analysis using fluorescence). This natural zeolite (ZN-C1BF-R from SOMEZ) originated from Romania. The average size of clinoptilolite particles was 0.4 mm (90% between 0.2 and 1.0 mm). The sample was microporous, and its specific surface area was 101 m2 g1, with total pore volume of 0.036 cm3 g1, as determined by BET analysis, performed from N2 adsorption isotherms. The cation exchange capacity (CEC) was determined as 2.6 meq g1. The purity was checked by X-ray diffraction, thermogravimetric analysis and 29Si and 27Al nuclear magnetic resonance. The structure was typical of the Heulandite family of tectosilicates (sheet-like) and its morphology revealed monoclinic crystal form with platelets of 10–20 nm thick (see Fig. 1). SEM images were obtained using FEI Quanta 50. Zeolites are generally defined as aluminosilicates, possessing three-dimensional frameworks of linked silicon–aluminum–oxygen tetrahedra, their general formulation is ðCnþ Þx ½ðAlO 2 Þnx ðSiO2 Þy , m(H2O). Clinoptilolite is distinguished from other zeolites of the heulandite group by lower void volume and higher silica content (Si=Al, >4). The framework contains a network of channels defined by two eight-ring pores (0.26 0.47, 0.33 0.46 nm) and a ten-ring pore (0.30 0.76 nm) (Woinarski et al. 2003). The presence of aluminum in the framework induces an excess negative charge, requiring the introduction of charge-compensating cations into the structure. These extra-framework cations are not covalently bound to the zeolite structure; they have considerable freedom of movement and can be readily substituted with a variety of other cations (Walcarius 1999). This provides high

Figure 1. SEM images of the clinoptilolite sample used.


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ion exchange properties and high affinity for transition metal cations, while low affinity for anions and non-polar organics (Haggerty and Bowman 1994). In particular, clinoptilolite exhibits high CEC and extraordinary affinity for ammonium. As it was shown in the work (Barrer, Papadopoulos, and Rees 1967), the clinoptilolite selectivity to ammonium was provided by a replacement of all sodium ions from the zeolitic exchange sites with ammonium (see Fig. 2). The ion exchange between Naþ and NHþ 4 in the clinoptilolite matrix was also reported by Ming and Dixon (1987) and Jha and Hayashi (2009).


Ion-Exchange Selectivity of Clinoptilolite-Based Sensor The selectivity of any ion-selective sensor is one of its most important characteristics as this property often determines whether the sensor may be used for particular purposes. Generally, in multi-component systems, cation selectivity is determined by a complex relationship between cation charge and electronic structure, and sometimes temperature (Ames 1960). In the work carried out by Ames (1960) it was shown that the order of selectivity varied considerably and was determined for clinoptilolite as 2þ Csþ > Rbþ > Kþ > NHþ Sr2þ > Naþ > Ca2þ > Fe3þ > Al3þ > Mg2þ > Liþ . 4 > Ba Thus, the clinoptilolite pore diameters dictate the size of the molecule that can enter the channels. In this molecular sieve process, ions that are too large to fit into the channels are excluded from internal surfaces and can only exchange with sites on the external surface. In addition, as it was shown by Palmer and Gunter (2001), the clinoptilolite structure is not completely rigid and the effective pore size can vary according to the exchangeable cation incorporated into the framework channels. Thus, eventually it was argued that selectivity can be interpreted from Eisenmann’s theory whereby the cation selectivity of zeolites with weak ionic fields, like clinoptilolite, is related to the cation’s free energy of hydration: it is energetically more favorable for cations with higher hydration energies to remain in solution (Sherry 1969). However, experimental work tends to indicate that selectivity can also be dictated by the hydrated radius of the cation (Ouki and Kavannagh 1997).

Figure 2. Ion exchange mechanism in the zeolitic layer: A and B are the evictor ion (i.e., NHþ 4 ) and resident ion (i.e., Naþ), respectively, in zeolite and solution phases; nB and nA represent the charge of ion B and ion A, respectively.

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Electrode Design and Preparation of Ion-Selective Coating Each conductometric transducer consisted of two identical pairs of interdigitated thin film electrodes (150 nm thick), fabricated by gold vapor deposition onto a non-conducting pyroceramic substrate (5 30 mm). A 50-nm thick intermediate chromium layer was used to improve adhesion of gold to the substrate. Both the digit width and interdigital distance were 10 mm, and their length was 1.5 mm. The sensitive area of each pair of electrodes was 2.9 mm2. The transducers were manufactured at the V. Ye. Lashkaryov Institute of Semiconductor Physics (Kyiv, Ukraine). In further manipulations, one pair of electrodes, covered with a zeolitic membrane, constituted a working sensor. For the sensor fabrication, the powdered sample of clinoptilolite was preliminarily suspended in UP water to the concentration of 50 mg mL1. Before application, the zeolite suspension was sonicated three times for 15 min each. To improve adhesion of the clinoptilolite particles to the sensitive electrode surface, the conductometric transducer was treated with piranha solution and thoroughly rinsed in UP water. Afterward, electrodes were carefully degreased with ethanol. A zeolite coating for the working sensor was elaborated by spreading 1.2 mL of clinoptilolite suspension onto the sensitive electrode surface. The second pair of electrodes was left bare and served as a reference sensor. After drying (time 1 hour), the sensor chip was thoroughly rinsed in UP water.

Electrochemical Measurements The following conditions were used for conductometric measurements. Each pair of interdigitated electrodes was connected to a fixed block of holders of a portable conductometric set-up developed in collaboration with Institute of Electrodynamics of National Academy of Sciences of Ukraine and reported in the work (Dzyadevych et al. 2009). While working, the sensors were supplied with a sinusoidal potential Uin, at the frequency of 30 kHz and amplitude of 10 mV. These conditions allowed us to avoid faradaic processes, double-layer charging and polarization of microelectrodes. Illumination and temperature variations had practically no influence on characteristics of the sensors. Simulation of the processes in the measuring cell with the conductometric transducer is shown in Fig. 3. Measurements were carried out in a glass cell filled with electrolyte solution (volume 3 mL), under vigorous magnetic stirring. An output potential Uout of each conductometric transducer was proportional to the difference between impedances of the working and reference sensors. Thus, a steady-state response of the ammonium-selective microsensor was plotted as a function of the analyte concentration. Electrochemical impedance spectroscopy (EIS) measurements were performed in a frequency range of 0.05 Hz to 100 kHz using VoltaLab 80, Model PGZ 301 (Radiometer Analytical, France), in a two-electrode configuration. Measurements were carried out in a glass cell filled with electrolyte solution, under vigorous magnetic stirring. EIS measurements were performed 1–2 min after injection of the analyte into the cell to attain a steady-state situation at the (zeolite-modified electrode)= solution interface.



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Figure 3. Schematic representation of the working sensor of ammonium-selective microsensor and the equivalent electric circuit simulating processes in a cell with a conductometric transducer (Dzyadevych and Soldatkin 2008): 1 – Cdl, capacity of double layer (determines the oriented dipoles at the electrode surface and does not depend on current frequency); 2 – ZW, diffusion, or Warburg impedance (responsible for concentration polarization due to ion diffusion from interface into electrolyte volume); 3 – Rp, penetration resistance (responsible for chemical polarization due to electrochemical reactions at the electrode surface and is frequency-independent); 4 – Cox, capacity of oxide of electrode material; and 5 – Rs, electrolyte resistance (simulates the resistance of solution inside the membrane).

RESULTS AND DISCUSSION Design Features of the Zeolite-Modified Conductometric Microsensor for Ammonium Determination For successful measurements of ions, a well-balanced optimization of the electrode characteristics, relevant for the application, should be discussed rather than the superiority of a single property. Considering the ion exchange to be the main mechanism of zeolite-based sensor, such relevant property foremost is a selectivity of detection.

Characterization of Electrical Properties of the Zeolite-Modified Electrode Using EIS Measurements Initially, the admittance shifts in the zeolite-based membrane were studied in a salt-free electrolyte. Output signals of the sensor were measured at the ammonium concentration in the cell in a range of 1.0 108–1.0 103 M.


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It was found that the shape of admittance curves (see Fig. 4a–b) completely corresponded to the classical view of admittance curves for electrode=electrolyte interface (Jaffrezic-Renault and Dzyadevych 2008) and indicated an increase in the sensor response with the gradual increase of ammonium concentration in the bulk solution. The operation of ammonium-selective conductometric microsensor is based on the specific ion exchange between ammonium from the aqueous solution and Naþ from the sensitive zeolitic membrane, resulting in the change of interfacial resistance. Considering the application of clinoptilolite as an ion-selective component for the sensor, it should be mentioned that the main property of the ion exchanger, which is of interest in the sensor elaboration, was in the first place its equilibrium behavior, described in terms of equilibrium isotherms and kinetics, which are dependent on the initial solution concentration and the intrinsic characteristics of the ion exchange ´ urkovic´, Cerjan-Stefanovic´, and Filipan 1997). system (C Usually, to describe quantitatively the ion exchange capacity of zeolite it is referred to the total CEC which is the number of equivalent ionogenic groups in the material. Nevertheless, not all these sites are available for exchange as exclusion of cations occurs through molecular sieve processes and some of the sites may be present in mineral impurities or at inaccessible sites of the material framework. However, it is necessary to mention that the consideration of ion exchange efficacy for electroanalytical purposes implies the ion exchange in the ‘‘narrow range,’’ and not the exchange of all ammonium species to reach an equilibrium situation in a zeolite suspension. It means, that even the ion exchange involves sole ammonium ion at the (zeolite-modified electrode)=solution interface, it is sufficient for the resistance shift. In other words, the conductometric detection of ammonium is controlled only by the ion exchange ‘‘equilibrium’’ (or at least by pseudo-steady state) which is established in small volume of solution corresponding to the diffusion layer at the electrode=solution interface.

Figure 4. a–b. Nyquist plots for the zeolite-based electrode in the ammonium solution of the following concentrations: 1–1.0 103 M, 2–5 104 M, 3–1.0 104 M, 4–1.0 105 M, 5–1.0 106 M (figure a) and those in ammonium solution of the lower concentrations: 4–1.0 105 M, 5–1.0 106 M, –1.0 107 M, . –1.0 108 M, –0 M (the response in UP water only) (figure b).


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According to the dependences shown in Fig. 5a–d, it was found that at high frequencies (Fig. 5a–b), which allow observing the intra-crystal electrical properties (Capdeville et al. 2004), at increasing ammonium concentration the admittance real part (conductivity) largely increased (up to 230 mS=mM), whereas its imaginary part was not significantly influenced. This point justifies the interest paid to conductometric transduction for ammonium detection. Contrary, at low frequencies (Fig. 5c–d), when crystal=electrolyte electrical properties are observed, the dependences of both admittance parts on ammonium concentration in the membrane are very similar and increase gradually, due to ion-exchange process. We suggest that this observation can be attributed to the impact of the zeolitic layer. Here, the conductivity variation in the solution inside the zeolitic layer was recorded due to reaching the pseudo-equilibrium (or a steady-state situation) in the diffusion layer at (zeolite-modified electrode)=solution interface.

Figure 5. a–d. Dependences of the admittance real part (a) and the admittance imaginary part (b) on the ammonium concentration obtained at the frequency of 50 kHz, and the dependences of the admittance real part (c) and the admittance imaginary part (d) on the ammonium concentration obtained at the frequency of 1 kHz.

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Selectivity Studies of Zeolite-Based Conductometric Microsensor for Ammonium Determination


Measurements of the sensor analytical signal to ammonium in the presence of probable interfering cations (Naþ, Kþ, Mg2þ, Ca2þ, Al3þ) allowed us to conclude that the sensor had pronounced selectivity to ammonium (see Fig. 6). It was observed that in the presence of background interference the sensor response was not significantly higher compared to that in the media without added interference. The obtained dependences were analogous to those for ISEs (Bakker, Pretsch, and Bu¨hlmann 2000). The conductometric selectivity coefficients of the ammonium-selective microsensor based on clinoptilolite were determined by Eq. 1, whereas the definition of KCon was shown to be analogous to KPot (Cammann 1979): Con ¼ KA;B

aA ZA

ð1Þ

ZB aB where aA is the activity of the primary ion, aB is the activity of the interfering ion, ZA and ZB are the charges of ion A and ion B, respectively. The values of the selectivity coefficients are summarized in Table 2. According to Table 2, the preference of the developed microsensor for ammonium increased in the cations order Naþ > Kþ > Ca2þ > Mg2þ > Al3þ. The found dependence differed from the order of selectivity determined for the clinoptilolite by Ames (1960) and indicated that ammonium was preferred in the solutions containing other monovalent ions, i.e., sodium and potassium ions.

Figure 6. Analytical signals of zeolite-based ammonium-selective conductometric microsensor to ammonium concentration in the range of 0.5–4 mM vs. ammonium activity (in logarithmic scale), in the solutions of NaNO3, KNO3, Mg(NO3)2, Ca(NO3)2, and Al(NO3)3, with concentration of 5 mM each, and in H2O.


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Table 2. Selectivity coefficients log KCon of the clinoptilolitebased ammonium-selective microsensor measured by the fixed interference method (Cammann 1979) Selectivity coefficients log KCon KNHþ4 Naþ KNHþ4 Kþ KNHþ4 Mg2þ KNHþ4 Ca2þ KNHþ4 Al3þ

Determined values 0.56 0.57 0.59 0.595 0.6


The tendency in the gradual increment of the zeolite selectivity toward mono- and multivalent ions was, however, observed in both cases.

Analytical Characteristics of the Ammonium-Selective Conductometric Microsensor Based on Clinoptilolite With the purpose of further use of the sensor in more complex media, it was advisable to study the sensor operational suitability in the buffering system. The sensor selectivity and sensitivity were determined in 5 mM KH2PO4=Na2HPO4 (pH 6.2) as the most widely used buffer solution for the conductometric measurements. The pH value of PB was taken based on the works on determination of optimum efficacy of the ion exchange in zeolites (Koon and Kaufman 1975; Kithome et al. 1998). According to the results obtained by EIS, the zeolite-based conductometric microsensor for ammonium determination showed a pronounced ability to detect ammonium with the LOD of 1.0 108 M (see Fig. 7). Compared to the known

Figure 7. Sensitivity of zeolite-based sensor (a) and of conductometric transducer, not modified with clinoptilolite, (b) to ammonium, in 5 mM PB (pH 6.2).


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ammonium-selective ISEs (see Table 1), the developed sensor may be considered as more advantageous since the value of the LOD for the reported ammonium-selective sensors was no less than 1.58 107 M (Coutinho et al. 2007). In addition, the amplitude of the zeolite-based electrode responses, measured by EIS, was found to be much higher compared to the sensitivity of the same electrode but not modified with zeolite. These observations demonstrate that in the presence of cations of buffer solution (Kþ and Naþ), ammonium ions at low concentrations are still preferred by zeolite with high sensitivity. In differential mode of measurements, the conductometric transducer, not modified with zeolite, did not respond to ammonium at all, while the sensitivity, dynamic range and response time of the zeolite-based electrode for ammonium were 2.5 mS=mM, 0–8 mM, 10 2 s, respectively. The study on operational stability revealed the measurement relative standard deviation at the level of 4–5%. The sensor lifetime was about 5 months.

CONCLUSIONS A highly sensitive conductometric microsensor for ammonium determination was developed based on the natural zeolite clinoptilolite. It was confirmed that the pronounced affinity of zeolite to ammonium allowed high sensitivity of ammonium detection in complex media. The analytical characteristics of the developed ammonium-selective sensor were: sensitivity 2.5 mS=mM, limit of detection 1.0 108 M, response time 10 2 s, dynamic range up to 8 mM, and storage stability about 5 months. Thus, inexpensive zeolite clinoptilolite may be considered as a reliable ion-selective component for the further application with analytical purposes.

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