A Highly Sensitive Miniaturized Impedimetric Perchlorate Chemical

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Feb 15, 2018 - microelectrodes, chemical sensor, electrochemical impedance spectroscopy, perchlorate. ... Research on Microelectronics and Nanotechnology, Sousse 4034, Tunisia. (e-mail: ...... E. Baker, University of. Purdue, Lafayette, IN ...
IEEE SENSORS JOURNAL, VOL. 18, NO. 4, FEBRUARY 15, 2018

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A Highly Sensitive Miniaturized Impedimetric Perchlorate Chemical Sensor Najib Ben Messaoud, Abdoullatif Baraket, Cherif Dridi, Naglaa M. Nooredeen, Mohammed Nooredeen Abbas, and Abdelhamid Errachid

Abstract— In this paper, we have developed a miniaturized a chemical sensor based on a new nanostructured Co-phthalocyanine (Co(II)Pc-PAA) derivative functionalized Au microelectrodes for perchlorate ClO− 4 detection. The morphological properties of the sensitive layer have been characterized by contact angle measurement. The response of the obtained sensor-based CoPc/Au microelectrodes has been investigated by electrochemical impedance spectroscopy measurements. The experimental impedance data of the sensor device were analyzed by an equivalent electrical circuit using a modified Randles model for better understanding the phenomena present at the sensing membrane/electrolyte interface. Therefore, under optimized working conditions in terms of polarization and frequency, best performances have been achieved when compared with those obtained in the literature for Au electrodes-based devices functionalized with the same molecule. The present chemical sensor has provided a lower detection limit (17.3 pM), the lowest achieved until now to our knowledge, with a larger linear range from 1.73 10−11 to 10−1 M. The selectivity of the sensor has been also studied by evaluating the response towards ClO− 4 with other interfering anions. The measurement were stable after ten days of the chemical sensor storage at room temperature. This is very promising for environmental application using rapid analyses and low-cost chemical sensors. Perspectives for a potentiometric sensor at higher concentrations were also assessed. Index Terms— Cobalt(II) phthalocyanine derivative, microelectrodes, chemical sensor, electrochemical impedance spectroscopy, perchlorate.

I. I NTRODUCTION ERCHLORATE (ClO− 4 ) is regarded as one of the major sources of this environmental contamination and adding to the drinking water contaminant candidate list toxic chemicals, such as high water solubility, mobility, considerable stability

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Manuscript received July 9, 2017; revised September 25, 2017 and October 27, 2017; accepted November 21, 2017. Date of publication December 6, 2017; date of current version January 18, 2018. This work was supported in part by SMARTCANCERSENS Project under Grant FP7-PEOPLE-2012-IRSES N 318053 and in part by NATO Science for Peace Project under Grant CBP.NUKR.SFP 984173. The associate editor coordinating the review of this paper and approving it for publication was Prof. Venkat R. Bhethanabotla. (Corresponding author: Cherif Dridi.) N. Ben Messaoud and C. Dridi are with the Nanomaterials, Microsystems for Health, Environment and Energy Laboratory, LR16CRMN01, Centre for Research on Microelectronics and Nanotechnology, Sousse 4034, Tunisia (e-mail: [email protected]). A. Baraket and A. Errachid are with the Institut des Sciences Analytiques, Université Lyon, Université de Claude Bernard Lyon 1, UMR 5280, 69100 Villeurbanne, France. N. M. Nooredeen is with the National Research Centre, Polymer and Pigment Department, Cairo 12311-Dokki, Egypt. M. Nooredeen Abbas is with the Analytical Laboratory, National Research Centre, Cairo 12311-Dokki, Egypt. Digital Object Identifier 10.1109/JSEN.2017.2780445

and persistence [1], [2]. The existence of ClO− 4 in the environment represents a potential negative effect on human health who consumes water containing ClO− 4. This can interfere with the ability of the thyroid gland to utilize iodine to produce thyroid hormones. These effects caused to abnormalities in child development and the occurrence of thyroid cancer. Moreover, ClO− 4 ions pose the greatest threat in the drinking water of expectant mothers, children under 12 years and persons with malfunctioning thyroids [3]. Therefore, the determination of ClO− 4 ions has been carried out directly or indirectly by a variety of classical and instrumental methods, such as volumetric titrations [4], gravimetry [5], spectrophotometry [6], atomic absorption spectrophotometry [7], Electrochemiluminescence [8] and fluorescence [9]. However, most of these methods are also suffer from various interferences and are relatively expensive and timeconsuming procedures. Therefore, a simple, rapid, sensitive and selective method for the determination of ClO− 4 ions is required. Electrochemical impedance spectroscopy (EIS) is a powerful diagnostic tool for analyzing the electrical response of chemical sensors and it is considered as extremely sensitive to a weak surface changes of microelectrodes surface [10]. This technique has found increasing application in the field of analytical sciences [11] in recent years. It has been intensively used for the characterization of charge transport across membranes and membrane/solution interfaces [12]. Thus, it has become increasingly popular as a label-free detection tool for many different types of bio-, chemical and electrochemical sensors for the detection of binding events on the transducer surface. EIS had attracting and considerable attention in recent years due to advances made in electrochemical instrumentation. Besides, an electrochemical sensor is derived from the coupling of a ligand-receptor binding reaction to a signal transducer. They are used in clinical diagnostics, environmental analysis, and food technology [13]–[15]. In the last few years, the use of miniaturized transducers fabricated by thin-film technology gained importance in sensor research. Generally electrochemical and chemical sensors were based on miniaturized microelectrodes to reduce both size and cost of electronic devices. The fabrication process of these microelectrodes is based on different technologies regarding the material of the transducer and the substrate. These fabrication techniques offer the advantages of the possibility of mass production and the use of small size transducers [16].

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In numerous applications, an electronic device must be stable, portable and particularly flexible. Metallo-phthalocyanines (MPcs) have been widely used as sensitive membrane onto the transducers for different applications of chemical sensors. Recently (MPcs) become very attractive and practically alternative materials due to their intense high thermal, color and chemical stabilities, photovoltaic effect, semiconductivity, redox activity, discshaped molecular structure and highly conjugated [17]. This extraordinary versatility makes the MPc materials as attractive candidates for potential applications in fields such as photovoltaic cells, molecular metals, chemical sensors, nonlinear optics, and electrochromic display devices [18]–[21]. Moreover, MPcs has a pronounced effect on the sensing behavior of the sensors, especially in the detection of some biologically and environmentally important compounds because of the importance of their metal center [22], [23]. In the same principle, arrays of phthalocyanine based sensors with redox activity have been used to develop electronic tongues using potentiometry, voltammetry or impedance spectroscopy [24]. Cobalt phthalocyanine (CoPc) and its derivatives have been used to act as effective electrocatalyst towards a wide range of redox systems. Electrodes modified with this compound were widely used for the electrocatalytic determination of many important compounds such as ClO− 4 [25], [26]. Therefore, it is necessary to investigate these properties to facilitate the development of a chemical sensor using a new nanostructured CoPc derivative. The aim of this work was to develop a simple and sensitive electrochemical chemical sensor for the determination of ClO− 4 using a new CoPc derivative. Surface treatment was analyzed by contact angle measurements (CAM). The sensitivity of the developed material towards ClO− 4 ions was characterized by EIS and the characteristics of the micro chemical sensor in terms of sensitivity and selectivity were also evaluated.

Fig. 1. Chemical structure of the synthetized Cobalt Phthalocyanine, C,C,C,C-tetracarboxylic acid- Polyacrylamide (Co(II)Pc-PAA).

Fig. 2.

Schematic view of the impedance spectroscopy set-up.

II. M ATERIALS AND M ETHODS A. Reagent Tetrahydrofuran (THF), piranha solution (1:3 Hydrogen peroxide (H2 O2 ) + 2:3 sulfuric acid (H2 SO4 )), lithium perchlorate (LiClO4 ) and Phosphate-buffered saline solution (PBS) (0.01 M, pH 7). Carbonate, hydrogen phosphate, nitrite and sulfate used in interference tests. All of the analytical reagent were purchased from Sigma Aldrich. The ionophore, Cobalt Phthalocyanine, C,C,C,C-tetracarboxylic acid-Polyacrylamide (Co(II)Pc-PAA) (Fig.1) was synthesized and purified according to our previous work [27]. B. Instrumentation and Experimental Details Surface treatment was analyzed by CAM using ‘Digidrop’ from GBX (France) to characterize Au substrate after each chemical surface modification. The measurements were carried out by dispensing a droplet of 5 μL of ultrapure water on the sensitive film surface at 24 ± 2°C. Four values of CAM were recorded on each substrate. Impedimetric measurements were performed at room temperature in a Faraday box. A conventional three electrode

cell was used, including a saturated calomel electrode (SCE) as a reference, a platinum wire auxiliary electrode and the CoPc modified Au microelectrode was used as working microelectrode. The impedance analysis was performed by using the potentiostat/galvanostat VMP3 (BioLogic Science Instruments, (France)) controlled by Ec-Lab software. The measurements were recorded in PBS pH 7, by varying the frequency in the 100 kHz–100 MHz range by using a sinusoidal excitation signal with the amplitude of 10 mV at a potential of −250 mV. The scheme of the experimental set-up is presented in Fig. 2. Potentiometric measurements were carried out at room temperature, using a homemade data acquisition system setup with four multi-channels microelectrodes connected and controlled by a personal computer. Measurements were made relative to an Ag/AgCl double junction reference electrode under magnetic stirring. Calibration curves were obtained by adding successive aliquots of ClO− 4 solutions (prepared with a concentration range of 10−4 to 10−9 M by serial dilution), to 25 ml of PBS to increase the ClO− 4 concentration from 10−9 to 10−4 M.

BEN MESSAOUD et al.: HIGHLY SENSITIVE MINIATURIZED IMPEDIMETRIC PERCHLORATE CHEMICAL SENSOR

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TABLE I C ONTACT A NGLES OF THE G OLD S URFACE

Fig. 3. (a) Optical image and schematic illustration of Au microelectrodes based on silicon substrate. (b) Wire bonding connection of microeletrode pads to the PCB.

C. Fabrication of the Microelectrodes Based on Silicon Technology The microelectronics fabrication process for the microelectrodes has been performed at Centro Nacional de Microelectronica (CNM) in Barcelona, Spain. The process has only two photolithographic steps. The starting material is P-type  1 0 0  silicon 100 mm diameter wafers with a nominal thickness of 525 μm. The process starts with a thermal oxidation process to grow a thick oxide layer (8000 Å). Afterwards, a 10 nm Ti layer was introduced first, and then the 250 nm of Au layer was sputtered onto the surface. Then, a photoresist layer was spincoated and was exposed in UV light with a pattern mask. Etching away the exposed photoresist was performed and the left remained photoresist is exactly the microelectrodes. The unprotected gold with the photoresist was then removed by wet-etching. Afterwards, two layers of SiO2 (4000 Å) and Si3N4 (4000 Å), acting as a passivation layer, were then deposited using PECVD (Plasma-Enhanced Chemical Vapor Deposition). Finally, the passivation was opened by using photolithographic process to define working Au microelectrodes (WE) (300 μm × 300 μm) and microelectrodes pads for electrical connections (100 μm × 100 μm). Fig. 3. shows an optical and schematic illustration of Au microelectrodes based on silicon technology. The wafer was diced and the devices were glued to a printed circuit board (PCB) using an epoxy resin (Epo-Tek H77, from Epoxy Technology). The Au microelectrodes pads were connected to the PCB using aluminum wire (25 μm Ø) which was wirebonded with Kulicke and Soffa equipment 4523A. The bonding area of the devices and the bonding wires were packaged with the same epoxy resin (Epo-Tek H77) to protect the electrical connections from the liquid environment [28]. D. Preparation of Co(II)Pc-PAA Modified Microelectrodes The Au microelectrodes were firstly sonicated at room temperature for 10 min in acetone using an ultrasonic bath, rinsed with ultrapure water and then dried under nitrogen. After that, these microelectrodes were cleaned for 1 min with a freshly prepared “piranha” mixture (1/3 H2 O2 + 2/3 H2 SO4 ), washed with water and dried with nitrogen. Finally, these microelectrodes were cleaned under UV/Ozone

(ProCleanerTM from BioForce, France) for 30 min in order to remove all organic contaminations. The membrane components (4 mg) were dissolved in 1 mL of tetrahydrofuran (THF), then the membrane solution (∼4 μL) was drop-casted on the microelectrode surface and left at room temperature for 24h for a complete THF evaporation. III. R ESULTS AND D ISCUSSION A. Contact Angle Measurements (CAM) In order to assess the effectiveness of the functionalization, orientation and the wettability properties of the gold surface with Co(II)Pc-PAA molecules, CAM were performed on bare Au, before and after activation with piranha. The obtained values were summarized in Table I. Bare Au electrode had a contact angle of (82° ± 1°) close to the values reported in the literature [29], [30]. After surface oxidation with Piranha solution, the Au surface becomes hydrophilic (53° ± 1°). After the functionalization process with the Co(II)Pc-PAA molecules, the CAM has increased again to (76° ± 1°) due to the hydrophobic character of the membrane. This confirms the adhesion of the membrane onto the Au microelectrode surface which was more important than what was reported in our previous work [25]. B. Impedimetric Measurements 1) Optimisation of Measurement Conditions: To determine the optimal experimental parameters (voltage and frequency range) the functionalized Au microelectrodes was used, in order to minimize the diffusion of ions from the bulk of the electrolyte to the electrode/electrolyte [31]. Fig. 4 shows the Nyquist plot of the modified Au microelectrodes at different applied potentials within the frequency range 100 MHz to 100 kHz. The impedance spectra obtained at −0.35 V/SCE presented a remarkable decrease of the Warburg impedance and the best semi-circle shape. For this reason, subsequent impedance analysis was estimated in terms of polarisation potential of −0.35 V/SCE versus SCE is used for all the following experiments. The parameters of the equivalent circuit were optimized to ensure the best fit of the experimental data. This fitting was performed with Ec-Lab software (Fig. 5). The impedance spectra of the modified Au microelectrodes with Co(II)Pc-PAA were modeled using Randles model equivalent circuit (RS + CPE/Rct ) (inset in Fig. 5.). Where (Rs ) is the electrolyte solution resistance, CPE (constant phase element) is related to the capacitance of the functionalized

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Fig. 4. Nyquist impedance spectra of the Au microelectrodes based on Co(II)Pc-PAA for different polarizations vs saturated calomel reference electrode (SCE) (frequency range: 100 kHz–100 mHz and an amplitude of 10 mV sinusoidal modulation in 0.01 M PBS (pH7)).

Fig. 6. (a) Nyquist plots obtained from bare Au microelectrodes and Co(II)PcPAA functionalized Au microelectrodes. (b) Determination of the coverage rate of functionalized Co(II)Pc-PAA microelectrodes. Fig. 5. Plot of impedance spectra (in Nyquist presentation) and the fit results of μAu/Co(II) Pc-PAA polymer.

Co(II)Pc-PAA Au electrode/electrolyte interface and Rct is related to the charge transfer resistance. 2) Determination of the Coverage Rate of Au Microelectrodes: In order to quantify coverage rate of the Co(II)Pc-PAA on Au microelectrodes surface. Fig. 6(a) shows the impedance behavior before and after functionalization using the same experimental conditions. Here, the first Nyquist plot semicircle corresponds to the bare Au microelectrodes which have a weak Rtc . After functionalization, a remarkable increase of the Nyquist plot semi-circle was observed, which can be attributed to the increase of Rtc after thin film deposition. To determine the coverage rate (θ ) of the Au microelectrode surface, the real part of the impedance was plotted before and after functionalization as a function of the inverse of the square root of the pulsation (ω−1/2 ) (Fig 6 (b)). At low frequencies,

the intersection of the linear part with the real axis of the impedance (ω−1/2 → 0) correspond to the Rct before and after functionalization. The Au microelectrode coverage rate was determined from Equation 1 and was about 79%. θ = 1 − [Rct (bar e electr ode) /Rct ( f uncti onali zed electr ode)]

(1)

3) Impedance Analysis of the Functionalized Au Microelectrodes:  Analytical performances The EIS analyses of the Co(II)Pc-PAA thin film deposited on Au microelectrodes, and for increasing ClO− 4 concentrations were being performed in PBS and are presented as Nyquist plots, in Fig. 7(a). The electrode covered with the membrane was dropped in the electrolyte solution to be characterized with EIS. Each scan of EIS measurements (one Nyquist plot semi-circle) takes 46s/scan. The number of scans was at least 5 scans in order to obtain reliable and stable measurement.

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TABLE II F ITTING PARAMETERS O BTAINED F ROM THE N YQUIST D IAGRAMS O BTAINED Co(II)Pc-PAA F UNCTIONALIZED G OLD M ICROELECTRODES FOR D IFFERENT ClO− 4 C ONCENTRATIONS

Fig. 7. (a) Impedance spectra presented in Nyquist plots of Co(II)Pc-PAA modified microelectrodes for different ClO− 4 concentrations. Frequency range: 100 kHz–100 mHz, an amplitude of 10 mV sinusoidal modulation, and polarization potential of −350 mV in 0.01 M PBS (pH7) solution. (b) Variation of the −Log(Rct /Rct0 ) as a function of the cologarithm of ClO− 4 concentration of Co(II)Pc-PAA thin film.

This corresponds to 46 s × 5 = 230 s (20 years) in mentoring and training researchers from undergraduate through to post-doctoral level. His research activity has been focused since 1992 on the development of electrochemical sensors to be applied in areas, such as the environment, clinical, and pharmaceutical analysis. States Joint project entitled Highly Durable, Sensitive and Selective Chemical and Optical Sensors on the Basis of Covalently Attached Ionophores with the cooperation of Prof. E. Baker, University of Purdue, Lafayette, IN, USA. He is a Principal Investigator of the Egypt-USA joint project entitled Ionophore-Based Electrodes with Nanoporous 3DOM Carbon Solid Contacts with the cooperation of Dr. P. Buhlmann, University of Minnesota, Minneapolis, MN, USA.

Abdelhamid Errachid received the degree in physics from Université Moulay Ismaïl, Meknes, in 1992, and the Ph.D. degree in electronic engineering from the Universitat Autònoma de Barcelona, Spain, in 1998. Since 2008, he has been with the Laboratory of Analytical Sciences, Université Claude Bernard Lyon 1, as a Professor. He is currently involved in several national and European projects. His current research activity is focused on bioelectronics, biofunctionalization, and nanobiotechnology.