Application of Electrochemically Reduced Graphene Oxide on Screen

Feb 29, 2012 - Internal Calibration Potentiometric Aptasensors for Simultaneous Detection of Hg, Cd, and As Based on a Screen-Printed Carbon Electrode...
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Application of Electrochemically Reduced Graphene Oxide on Screen-Printed Ion-Selective Electrode Jianfeng Ping, Yixian Wang, Yibin Ying, and Jian Wu* College of Biosystems Engineering and Food Science, Zhejiang University, Hangzhou 310058, PR China S Supporting Information *

ABSTRACT: In this study, a novel disposable all-solid-state ionselective electrode using graphene as the ion-to-electron transducer was developed. The graphene film was prepared on screen-printed electrode directly from the graphene oxide dispersion by a one-step electrodeposition technique. Cyclic voltammetry and electrochemical impedance spectroscopy were employed to demonstrate the large double layer capacitance and fast charge transfer of the graphene film modified electrode. On the basis of these excellent properties, an allsolid-state calcium ion-selective electrode as the model was constructed using the calcium ion-selective membrane and graphene film modified electrode. The mechanism about the graphene promoting the ion-to-electron transformation was investigated in detail. The disposable electrode exhibited a Nernstian slope (29.1 mV/decade), low detection limit (10−5.8 M), and fast response time (less than 10 s). With the high hydrophobic character of graphene materials, no water film was formed between the ion-selective membrane and the underlying graphene layer. Further studies revealed that the developed electrode was insensitive to light, oxygen, and redox species. The use of the disposable electrode for real sample analysis obtained satisfactory results, which made it a promising alternative in routine sensing applications.

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polyaniline, polythiophene, and their derivatives.8−10 Sensors with these CPs as the intermediate layers could significantly improve the potential stability by unblocking the charge transfer at the sensor’s interfaces. However, the redox side reaction with dissolved oxygen, the light sensitivity, and the unavoidable water layer at the interface between ion-selective membrane and CPs has limited practical application of these CPs-based SC-ISEs.2 Recently, an approach based on the use of nanostructured materials as the solid-contact ion-to-electron transducers has been proven to be greatly promising and effective. Gold nanoparticles11 and carbon-based nanomaterials like fullerene,12 single-walled, and multiwalled carbon nanotubes (CNTs),13−15 have been employed to fabricate SC-ISEs. Especially, the carbon-based nanomaterials with the large double layer capacitance could remarkably minimize the potential drift (less than 20 μV h−1) of the SC-ISEs. Furthermore, with the high hydrophobic properties of carbon materials, no significant interfacial aqueous film is formed between the polymeric sensing membrane and the underlying solid-contact, which make them more attractive as solid-contact transducers in the SC-ISEs than the other materials such as CPs.15,16 Until now, the carbon nanomaterials-based films on

olid-contact ion-selective electrode (SC-ISE), a new generation of ISE, is a promising substitute for conventional liquid-contact ISE, drawing tremendous attention from both the experimental and theoretical scientific communities since its discovery in 1971.1−3 The coated-wire electrode (CWE) is the initial form of SC-ISE, in which a sensing membrane is coated directly onto a metallic conductor.1 However, although CWE may be useful in some practical applications, it exhibits low long-term potential stability due to the “blocked” interface between the ionically conducting sensing membrane and electronically conducting contact.4,5 In the past decades, many efforts have been made to improve the potential stability and reproducibility of SC-ISEs. One effective approach is the use of an intermediate layer with suitable redox and ion-exchange properties that are able to convert effectively the ionic signal through the ion-selective membrane into an electronic signal between the metal contact and the sensing member.2,6 SC-ISE with a self-assembled monolayer of a lipophilic redox-active compound between the ion-sensing membrane and gold surface could provide a well-defined pathway for charge transfer resulting in a stable system.7 However, the low redox capacitance of the monolayer limited the application. In order to minimize the polarizability of the solid-contact, a sufficiently high redox capacitance of the intermediate layer is required.6 Conducting polymers (CPs) that possess high redox capacitance have been introduced into the SC-ISE as the internal ion-to-electron transducers, including polypyrrole, © 2012 American Chemical Society

Received: December 28, 2011 Accepted: February 29, 2012 Published: February 29, 2012 3473

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ISE) was constructed as the model using the calcium ionselective membrane (Ca-ISM) and graphene-based electrode substrate, which exhibited satisfactory potentiometric characteristics. Furthermore, the developed solid-contact Ca-ISE was employed to selective determination of calcium ion (Ca2+) levels in real food samples.

the electrode surface are mostly prepared by drop-casting the relevant carbon nanomaterials dispersion solution.12−16 However, such a preparation methodology is tedious and timeconsuming.17,18 Moreover, since the pure carbon materials are not soluble in most kinds of solvents, a suitable dispersion method must be used to obtain homogeneous carbon materials dispersion solution before the coating process, particularly in the preparation of CNTs-based films. Functionalization of CNTs or the use of common surfactants could obtain welldispersed CNTs solution.13,19,20 However, these dispersion methods can adversely influence the electronic properties of CNTs.21 Additionally, the purification of CNTs before the use should be taken into account since the CNTs always contain amorphous carbon particles and metal oxide catalyst residuals. However, the purification process using strong acid or high temperature may introduce pH sensing groups on the tube wall of CNTs (like −COOH) that interferes with the potentiometric response of CNTs-based SC-ISEs. Graphene, a sheet of sp2 bonded carbon atoms arranged into a honeycomb structure, has given considerable attention in the field of analytical chemistry in view of its outstanding electrical conductivity, mechanical strength, and chemical stability.22 Recently, our group found that graphene could be used as the solid-contact ion-to-electron transducer in the SC-ISE with excellent potentiometric performance, such as fast response, long-term potential stability, and light insensitivity.23 However, the synthesis of graphene using the chemical reduction method involved excessive toxic reducing agents, and the formation of graphene film on the electrode surface needed a multistep coating process.24 More recently, a promising strategy in graphene synthesis based on electrochemical method to produce electrochemically reduced graphene oxide (ERGNO) has been reported.25,26 This method offers several advantages over other graphene fabrication techniques, including being green, efficient, inexpensive, and rapid. More importantly, the ERGNO could form a stable film on the electrode surface without any further treatment.27 On the other hand, as the demand for point-of-care testing and on-spot monitoring in clinical, environmental, and industrial analysis increases, both practical and economic interests have driven the development of various kinds of disposable electrochemical sensors based on screen-printing technology.28 Screen-printed potentiometric all-solid-state sensors with a straightforward fabrication process, high reproducibility, and cost-effectiveness have received extensive interest in the field of potentiometric analysis.29 Disposable planar SC-ISEs using CPs and functionalized CNTs as transducers have been successfully applied to determine various analytes, such as Ag+, K+, Cu2+, NH4+, and NO3−.20,30,31 However, the use of one-step electrodeposited graphene film as the solid-contact transducer in disposable SC-ISEs has never been explored until now. In this work, a novel disposable potentiometric sensor using graphene film as the intermediate layer acting as the ion-toelectron transducer was proposed. The graphene film modified electrode was prepared by one-step electrodeposition of the exfoliated graphene oxide onto the surface of screen-printed electrode (SPE) which was printed on the ceramic substrate in order to prolong the useful life in aqueous solution. Such an electrode platform possesses large double layer capacitance, fast charge transfer, and high hydrophobicity, which make it quite attractive for the fabrication of a disposable potentiometric sensor. An all-solid-state calcium ion-selective electrode (Ca-

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EXPERIMENTAL SECTION Reagents. Graphite oxide (GO) was synthesized from graphite (spectral pure, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) by a modified Hummers method.32,33 Exfoliation of GO to graphene oxide (GNO) was achieved by ultrasonication of GO dispersion (0.5 wt %) using a supersonic cleaner (SK3300HP, 180 W, Shanghai, China). Single-walled carbon nanotubes (SWCNTs, purity > 95%, diameter of 1−2 nm, length of 5−30 μm) was obtained from Beijing Nachen S&T Ltd. (Beijing, China). The commercially available conductive graphite ink (Electrodag 423SS) and insulating ink (Electrodag 452SS) were obtained from Acheson Co., Ltd. (USA). Calcium ionophore N,N,N′,N′-tetracyclohexyl3-oxapentanediamide (ETH 129), poly(vinyl chloride) (PVC), potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (KTFPB), and o-nitrophenyl octyl ether (o-NPOE) were purchased from Sigma. Calcium chloride and tetrahydrofuran (THF) were obtained from Sinopham Chemical Reagent Co., Ltd. (Shanghai, China). All other chemicals not mentioned here were of analytical reagent grade and were used as received. Double distilled water was used throughout. Electrode Preparation and Modification. Home-made screen-printed electrode (SPE) was prepared on an automatic screen printer (Z-C3050A, Zheng Ting Screen Printing Machine Co. Ltd., Shanghai, China). Prior to the printing, the ceramic substrate (5 mm thickness) was photoetched into the designed size (40 mm × 15 mm) and cleaned with ethanol and double distilled water and dried at room temperature. The working electrode was printed with the conductive graphite ink (Electrodag 423SS). Then, the prepared electrode was heated in an oven for 30 min at 120 °C to evaporate the solvents. Finally, an insulating layer made from insulating ink (Electrodag 452SS) was printed on the surface of carbon film and then solidified by irradiating with 254 nm ultraviolet. The working area of the prepared SPE was calculated as 0.07 cm2 (3 mm diameter) with a 8 mm × 1 mm connecting strip. The electrochemically reduced graphene oxide (ERGNO) modified SPE (denoted as SPE/ERGNO) was fabricated according to the published work.27 Prior to the modification, the electrode surface was washed with distilled water and dried in N2 atmosphere. Electrochemical reduction of GNO on the electrode surface was performed in the N2 purged GNO dispersion solution (0.5 mg L−1) with a magnetic stirrer at a working potential of −0.8 V (vs Ag/AgCl) applied for 600 s. For comparison, SWCNTs modified SPE (denoted as SPE/ SWCNTs) was made by repeatedly spraying an aqueous suspension (10−2 wt %) of the purified SWCNT containing 1 wt % sodium dodecyl sulfate onto the electrode surface and drying after each spray process. The composition of the calcium ion-selective membrane (Ca-ISM) cocktail was composed of ca. 1.0% (w/w) calcium ionophore ETH 129, 0.2% (w/w) KTFPB, 65.8% (w/w) oNPOE, and 33.0% (w/w) PVC dissolved in THF solvent.34 The dry fraction of the membrane cocktail was ca. 15% (w/w). For the fabrication of Ca-ISE, 100 μL of cocktail was deposited onto the working electrode surface. Then, all the membrane 3474

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electrodes were left to dry for 24 h at room temperature and subsequently conditioned in 10−3 M CaCl2 solution for at least 1 day before further measurements and also between the measurements. Apparatus and Measurements. The scanning electron microscopy (SEM) experiment was made on a Philips XL-30 ESEM. Tapping-mode atomic force microscopy (AFM) was conducted on the mica with a Dimension Icon AFM equipped with a ScanAsyst (Bruker AXS, Germany). Electrochemical impedance spectroscopy (EIS) measurements were performed using a Solartron Analytical model 1260 Impedance-Gain-Phase Analyzer in combination with a model 1287 Electrochemical Interface (Solartron Analytical, Farnborough, UK). Cyclic voltammetry (CV) was carried out using a CHI 440 electrochemical workstation (CH Instruments, USA). All the electrochemical experiments were performed in a one-compartment electrochemical cell. A conventional three-electrode system, consisting of the prepared SPE as the working electrode, a saturated Ag/AgCl electrode as a reference electrode, and a platinum wire as an auxiliary electrode, was employed. Potentiometry was carried out using a PalmSens (Palm Instrument BV, Houten) that consists of a portable potentiostat interfaced with a palmtop PC. All the experiments were performed at around 25 °C.

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RESULTS AND DISCUSSION Basic Characterizations. The thickness of as-prepared ERGNO was first examined by tapping-mode AFM (see Figure S1 in Supporting Information), which shows a thickness of ∼1.0 nm that matches well with the reported apparent thickness of ERGNO25,26 and is comparable to the reported apparent thickness of single-sheet chemically reduced graphene oxide,35,36 suggesting the single-sheet nature of ERGNO obtained in this work. The morphology of ERGNO film on the SPE surface was characterized by SEM. As shown in Figure S2 (Supporting Information), the ERGNO film presents a uniform surface topography. It should be noted that the graphene electrodeposition can occur on any conducting surfaces, and moreover, the graphene coating on the electrode surface is very stable as a result of its poor insolubility in common solvents. CV and EIS Characterizations. Figure 1A shows the cyclic voltammograms of the bare SPE, SPE/SWCNTs, and SPE/ ERGNO in 0.1 M CaCl2 solution. It is clear that the background current at the bare SPE is quite low, while the distinct capacitive current is obtained at both SPE/SWCNTs and SPE/ERGNO. Moreover, the capacitive current obtained at SPE/ERGNO is ca. 2 times higher than the one obtained at SPE/SWCNTs, indicating the large double layer capacitance at the graphene film modified electrode. Since the solid-contact with high redox or double layer capacitance is one of the stability conditions required in the all-solid-state potentiometric ion sensors, graphene could be used as an effective solidcontact transducer.13,17 The effect of oxygen on the cyclic voltammogram of SPE/ ERGNO was shown in Figure 1B. The oxygen reduction occurs at −0.1 V with a maximum at −0.2 V which is more positive than that at the bare SPE (maximum at −0.4 V, data not shown), probably due to the catalytic activity of ERGNO.27 In the potential range between 0.1 and 0.5 V, the signals show no obvious change in the absence and presence of oxygen, suggesting the presence of oxygen does not affect the capacitive process of SPE/ERGNO.

Figure 1. (A) Cyclic voltammograms for bare SPE (solid line), SPE/ SWCNTs (dotted line), and SPE/ERGNO (dashed line). (B) Cyclic voltammograms for SPE/ERGNO in the absence of oxygen (solid line) and in the presence of oxygen (dashed line). Supporting electrolyte: 0.1 M CaCl2. Scan rate: 100 mV s−1.

Figure 2A shows the impedance plots of the SPE/ERGNO recorded in 0.1, 0.05, and 0.01 M CaCl2 solutions. As shown, all the impedance spectra are dominated by a 90° capacitive line. A slight deviation from the capacitive line is found at the high frequencies, indicating the fast charge transfer at the SPE| ERGNO and the ERGNO|solution interfaces as well as the ERGNO layer. Furthermore, the values of high-frequency intersection with the Z′ axis depend strongly on the concentration of electrolyte, suggesting the measured impedance is mainly the solution resistance and not the ohmic resistance of the ERGNO film. The influence of dc potential (Edc) on the impedance spectra of SPE/ERGNO in the presence of air was investigated. As shown in Figure 2B, the variations of Edc produce a profound effect on the impedance behavior. Especially when the Edc is −0.2 V, a semicircle is found, indicating the presence of the charge-transfer process. This is in well agreement with the oxygen reduction peak observed in the cyclic voltammograms (Figure 1B). On the other hand, when the Edc is 0.2 V, the signals exhibit no obvious change in the absence and presence of O2 (Figure 2C) which also coincides with the voltammetric results. It should be mentioned that these results were similar with those obtained at the CNTs film modified electrode under the same experimental conditions.37 The impedance behaviors of the CWE (SPE/Ca-ISM) and SC-ISE (SPE/ERGNO/Ca-ISM) were compared in Figure 2D. As shown, the resistance, which is equal to the bulk membrane resistance coupled with the contact resistance between the electrode substrate and the polymeric membrane, decreases from 13.87 to 8.21 MΩ with the introduction of the solidcontact graphene film between the SPE and polymeric membrane that makes the charge transport process easier.23 3475

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Figure 2. (A) Impedance plots of SPE/ERGNO in different concentrations of supporting electrolyte (CaCl2): (a) 0.1 M, (b) 0.05 M, (c) 0.01 M. Frequency range, 10 kHz to 0.3 Hz; Edc, 0.20 V; ΔEac, 10 mV. (B) Influence of Edc (frequency range, 10 kHz to 0.3 Hz; ΔEac, 10 mV) on impedance plots of SPE/ERGNO in 0.1 M CaCl2 solution: (a) 0.2 V, (b) 0 V, and (c) −0.2 V. (C) Effect of O2 on impedance plots at Edc = 0.2 V (frequency range, 10 kHz to 0.3 Hz; ΔEac, 10 mV). (D) Impedance plots of SPE/ERGNO/Ca-ISM (a) and SPE/Ca-ISM (b) in 0.1 M CaCl2 solution. Frequency range, 100 kHz to 0.3 Hz; Edc, 0.20 V; ΔEac, 10 mV.

The low-frequency part of the EIS for SPE/Ca-ISM displays a large semicircle, indicating the large charge-transfer resistance with a small capacitance at the “blocked” interface (SPE|CaISM).8 In the case of SC-ISE, the negligibility of low-frequency semicircle indicates that the graphene contact increases lowfrequency capacitance and the ion-to-electron transduction that occurs properly between the electronically conducting SPE substrate and ionically conducting ion selective membrane.4 Chronopotentiometry. Reversed chronopotentiometry is used to evaluate the electrical capacity of the solid-contact and the short-time potential stability of the developed electrode. Constant current of ±1 nA was applied on the Ca-ISEs, while the potential of the electrodes was measured in 0.1 M CaCl2 solution. Figure 3 shows the typical chronopotentiograms recorded for the SPE/Ca-ISM and SPE/ERGNO/Ca-ISM. The potential jump in the response is used to calculate the total resistance of the electrode. The estimated total resistance for the ERGNO-based SCISE is 8.34 MΩ; that is in good agreement with the resistance obtained from the EIS measurement. From the same experimental plots, the shortterm stability of the potentials can be derived from the ratio ΔE/Δt. The calculated potential drift value of the SPE/ ERGNO/Ca-ISM is about 13.2 μV s−1, which is much lower than the value (1782 μV s−1) of the CWE, i.e., SPE/Ca-ISM. According to the equation ΔE/Δt = i/CLF (CLF, low-frequency capacitance), the CLF of the solid-contact layer ERGNO in the membrane electrode is estimated to be 75.8 μF, which is larger than the CLF value (51.2 μF) of SWCNTs-based SC-ISE (data

Figure 3. Chronopotentiograms for SPE/Ca-ISM and SPE/ERGNO/ Ca-ISM recorded in 0.1 M CaCl2 solution. The applied current is +1 nA for 60 s and −1 nA for 60 s.

not shown) and similar with those CPs-based solid-contact transducers.17 Potentiometric Characterizations. Figure 4A illustrates the potentiometric electromotive force (EMF) signal recorded after successive addition of the Ca2+ to the test solution. It can be seen that the signal has a stable behavior, with no perturbations or random noise after each addition. The response is almost Nernstian displaying a slope of 29.1 mV/ decade (standard deviation of the slope is 0.2 mV/decade, R = 0.9990) and a linear range from 10−5.6 to 10−1.6 M of Ca2+ (Figure 4B). The limit of detection is calculated as 10−5.8 M. Furthermore, the response is much faster (≤10 s) than those of electrodes that contain a similar membrane but with liquid 3476

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Figure 4. (A) EMF measurements recorded for increasing the concentration of Ca2+ in the solution; inset: expansion of the selected range. (B) Calibration curve of the SPE/ERGNO/Ca-ISM, conditioned in 10−3 M CaCl2 solution.

contact. Such an advantage could be attributed to the absence of an inner solution that favors the response time of the electrode. The potentiometric selectivity coefficients (denoted as log CapotCaM) of the developed SPE/ERGNO/Ca-ISM were determined using the separated solutions method (SSM). The selectivity coefficients were calculated by measuring the duplicate the EMF of three different electrodes in different concentrations of interfering ions obtaining the following results: log CapotCaMg = −4.7 ± 0.2, log CapotCaLi = −2.4 ± 0.3, log CapotCaK = −2.3 ± 0.2, and log CapotCaNa = −2.9 ± 0.3. These results are in good agreement with those liquid-contact counterparts, suggesting the selectivity is solely determined by the ion-selective membrane and not by the solid-contact layer. For long-term measurement, the formation of aqueous layer inside the electrodes has been proved to be the main reason of instability because its composition changes due to permeability of the membrane to ions and CO2, leading to changes in pH. For investigating the probable formation of water film between the Ca-ISM and the solid-contact layer, a potentiometric water layer test was employed and the potential response was recorded. As shown in Figure 5A, the response of SPE/Ca-ISM exhibits a substantial potential drift when changing back from Mg2+ to Ca2+, confirming the presence of a water film which is the key sticking point for the restriction of CWE application.15 On the contrary, no such potential drift is observed when the ERGNO film as the solid-contact layer is used. This demonstrates that the water layer is absent in the developed SC-ISE due to the highly hydrophobic character of ERGNO materials.23 In addition, the intermediate term stability is calculated with the signal recorded in the third step of the test. The obtained potential drift is 14.7 μV h−1 in the 20 h

Figure 5. (A) Water layer test for the SPE/Ca-ISM and SPE/ ERGNO/Ca-ISM,; the measurements were switched between 0.01 M CaCl2 and 0.01 M MgCl2. (B) Effect of light on the potential stability of the SPE/ERGNO/Ca-ISM in 0.01 M CaCl2 solution. (C) Effect of O2 and CO2 on the potential stability of the SPE/ERGNO/Ca-ISM in different CaCl2 solution. (D) Redox interference test for the SPE/ ERGNO/Ca-ISM recorded in 0.01 M CaCl2 solution with 0.01 M FeCl3/FeCl2 (a) and 10−4 M CaCl2 solution with 10−4 M FeCl3/FeCl2 (b).

continuous monitoring, which is much lower than the CNTsbased solid-contact Ca-ISE (493 μV h−1).38 The light, O2, and CO2 sensitivity of the developed electrode was investigated. For the test of light sensitivity, the whole electrochemical cell was kept in the dark until the electrode was exposed either to room light or to UV light or to infrared light 3477

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Ca2+ concentration. The results obtained by this method were compared to those obtained by the laboratory standard method atomic absorption spectrometry (AAS) summarized in Table 1.

for 300 s, followed each time by a return to the dark. Electrode potential response was recorded in 0.01 M CaCl2 solution. It can be seen that no significant potential drift is found during the measurement (Figure 5B), suggesting the ERGNO transducer has no light sensitivity. The O2 and CO2 sensitivity was evaluated by measuring the potential in 0.01 M CaCl2 solution first purged with N2 for 1 h, then O2 for 1 h, later CO2 for 1 h, and last N2 for 1 h. As shown in Figure 5C, the SPE/ ERGNO/Ca-ISM is insensitive to O2, confirming the inherent advantages of ERGNO over the CPs acting as the solid-contact transducer since the CPs possess redox property and are consequently sensitive to O2.6 On the other hand, the inlet of CO2 gas into the test solution exhibits no obvious effect on the potential response of the ERGNO-based Ca-ISE. This result is similar with that obtained in chemically reduced graphene oxide-based SC-ISE.23 In the case of ERGNO, the electrodeposition process could remove some oxygen-contained groups on the graphene oxide sheet and hence the carboxylic groups commonly found on carbon materials are not existed in ERGNO.25−27 Redox interference was studied by measuring the potential stability of the proposed electrodes in solutions containing different concentration of redox-active species. As illustrated in Figure 5D, the potential of Ca-ISE based on ERGNO layer was unaffected when the electrodes were measured in a solution with constant ionic strength (0.01 or 10−4 M CaCl2) and constant total amount of the redox couple (10−2 M or 10−4 M FeCl3/FeCl2). These results demonstrate that the redox-active species in the solution barely reach the ERGNO layer and consequently no redox response is observed. The last outstanding feature of the SPE/ERGNO/Ca-ISM is the high stability in solution measurement and storage. The main problem of disposable potentiometric strip for practical application is the deformation or softening of the substrate in the long term solution condition.20 Therefore, the storage of electrode is always under dry condition, and an unavoidable preconditioning should be taken before the use. In order to solve this problem, the ceramic plate was employed as the substrate. The ERGNO-based Ca-ISE was immersed in 10−3 M CaCl2 solution when not used. Results showed that the electrode still responded linearly to Ca2+ in the same activity range (10−5.6−10−1.6 M), and the slopes calculated from the linear range of the calibration plots were 28.8 and 28.5 mV/ decade after 1 and 2 weeks, respectively. The stability of the SPE/ERGNO/Ca-ISM under dry storage condition was also examined. The disposable Ca-ISE exhibited a close to Nernstian response (28.9 mV/decade) in the linear range from 10−5.6 to 10−1.6 M of Ca2+ after 3 months of dry storage. Determination of Ca2+ in Milk and Beverage Samples. The feasibility of the developed disposable potentiometric sensor for the determination of Ca2+ in milk and beverage samples was investigated. It should be noted that the concentrations of ionic calcium in the selected samples range between 1.0 and 50 mM. Eight solid-contact Ca-ISEs based on ERGNO transducer (four under solution storage (1 week) and others under dry storage (1 week)) were used. Before the use of Ca-ISEs under dry storage, the electrodes were conditioned in 10−4 M CaCl2 solution for 30 min. The determination of Ca2+ concentration in the diluted samples using 2.0 mM phosphate buffer solution (pH 6.8) (v/v = 1:49) was performed by one point calibration procedure, in which a calibration curve was first obtained in the buffer solution and then the electrode was immersed into the diluted sample for 5 min to get a potential value (last point) used to calibrate the

Table 1. Determination of Ca2+ Levels in Milk and Beverage Samples Using the SPE/ERGNO/Ca-ISM under Solution Storage, and Dry Storage, and the Standard Method Atomic Absorption Spectrometry (AAS) Ca2+ concentration (mM) SPE/ERGNO/Ca-ISMa sample

solution storage

milk 1 milk 2 milk 3 fruit drink 1 fruit drink 2 fruit drink 3 vegetable extract drink 1 vegetable extract drink 2 vegetable extract drink 3

± ± ± ± ± ± ± ± ±

28.73 19.28 36.47 8.26 6.15 2.78 11.63 15.27 18.72

2.16 2.04 2.32 1.18 0.97 0.42 1.32 1.69 1.98

c

dry storage 28.69 19.24 36.71 8.37 6.11 2.73 11.61 15.31 18.68

± ± ± ± ± ± ± ± ±

5.21 5.67 6.81 2.83 1.94 1.53 4.07 3.99 5.24

AASb 28.79 19.37 36.52 8.21 6.17 2.81 11.74 15.39 18.92

± ± ± ± ± ± ± ± ±

1.02 0.93 1.26 0.82 0.75 0.35 1.09 1.06 1.37

a

Potentiometric measurements were repeated twice on two different days using four electrodes (n = 8). bAAS measurements were repeated twice on two different days using the same instrument (n = 4). cMean value ± standard deviation.

The errors for the measurements of Ca-ISEs under dry storage are significantly higher than those for Ca-ISEs under solution storage and AAS. If the condition in 10−4 M CaCl2 solution was prolonged to 3 h or more, the errors would be minimized to the same with those for Ca-ISEs under solution storage. Nevertheless, the measured values (mean values) for calcium ion in these samples using the developed method are in good agreement with those obtained by the AAS method. These demonstrate that the ERGNO-based SC-ISE with easy fabrication, simple operation, and low-cost could be used as an excellent device for the determination of Ca2+ in complex samples.

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CONCLUSIONS In this work, we developed a screen-printed potentiometric sensor using one-step electrodeposited graphene as the solidcontact transducer. With large double layer capacitance, fast charge transfer, and high hydrophobicity of the graphene layer, the all-solid-state calcium ion-selective electrode exhibited excellent potentiometric performance. A Nernstian response with the slope of 29.1 mV/decade in the linear range from 10−5.6 to 10−1.6 M of Ca2+ and a detection limit of 10−5.8 M was obtained. Compared to CPs-based SC-ISE, graphene constructed Ca-ISE displayed no obvious potential drift when exposing the electrode under different lights, O2 and CO2, more importantly, no water film between the polymeric membrane and graphene layer. A long-term operation in solution could be achieved due to the use of ceramic substrate that prolongs the lifetime of the potentiometric strip in aqueous phase. Furthermore, the analytical results of real samples obtained by the proposed electrode were comparable to the standard method. This work provides a useful way for implementing graphene-based solid-contact transducer in a disposable potentiometric strip which could be used as an effective and promising tool in routine sensing applications. Further work would be performed on exploring a planar all3478

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solid-state reference electrode using graphene layer and subsequently integrating it with the above SC-ISE onto one strip to fabricate a whole potentiometric strip cell.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-571-88982180. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the Key Scientific Project of China (No. 2009ZX08012-004B) and the Key Scientific Creation Group Project of Zhejiang Province (No. 2011R09037-02).

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dx.doi.org/10.1021/ac203480z | Anal. Chem. 2012, 84, 3473−3479