Highly selective mercury detection at partially oxidized graphene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) nanocomposite film-modified electrode

Partially oxidized graphene flakes (po-Gr) were obtained from graphite electrode by an electrochemical exfoliation method. As-produced po-Gr flakes were dispersed in water with the assistance of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). The po-Gr flakes and the po-Gr/PEDOT:PSS nanocomposite (po-Gr/PEDOT:PSS) were characterized by Raman spectroscopy, Fourier transform-infrared spectroscopy (FT-IR), UV–Vis spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). In addition, we demonstrated the potential use of po-Gr/PEDOT:PSS electrode in electrochemical detection of mercury ions (Hg²⁺) in water samples. The presence of po-Gr sheets in PEDOT:PSS film greatly enhanced the electrochemical response for Hg²⁺. Cyclic voltammetry measurements showed a well-defined Hg²⁺ redox peaks with a cathodic peak at 0.23 V, and an anodic peak at 0.42 V. Using differential pulse stripping voltammetry, detection of Hg²⁺ was achieved in the range of 0.2–14 μM (R² = 0.991), with a limit of detection of 0.19 μM for Hg²⁺. The electrode performed satisfactorily for sensitive and selective detection of Hg²⁺ in real samples, and the po-Gr/PEDOT:PSS film remains stable on the electrode surface for repeated use. Therefore, our method is potentially suitable for routine Hg²⁺ sensing in environmental water samples.

Introduction

Mercury (Hg) is an essential element in the industry; however, the metal and its compounds are extremely dangerous to human health and to the environment. Although the use of Hg is regulated in many countries, it is still used in several domestic and industrial applications, which has led to the accumulation of Hg residues in landfills, soils, and streams (Seco-Reigosa et al., 2014). As a result, the focus of numerous investigations has been on closely monitoring Hg present in the environment (Pesavento et al., 2009). Standard methods for Hg analysis include: cold vapor atomic absorption spectrometry (CVAAS) (EPA, 2007a) and inductively coupled plasma-mass spectrometry (ICP-MS) (EPA, 2007b). Some Hg-monitoring applications, without requiring sample pretreatment, depend upon point-of-use sensors that are simple, rapid, stable, reliable, and inexpensive. Accordingly, various methods have been developed such as colorimetric (Liu et al., 2010), fluorometric (Wang et al., 2014a), magnetic (Najafi et al., 2013), electrochemical (Martin-Yerga et al., 2013), etc. The electroanalytical techniques have played a major role in simplified testing for Hg and have been approved by many regulatory bodies (EPA, 1996). In general, many electroanalytical or colorimetric methods rely upon a change in electrical signal following a reaction (amalgamation) of gold (Au) or other precious metals with Hg, either at the electrode or within the sample solution to selectively bind with the target mercury ions (Hg²⁺) (Welch et al., 2004; Martin-Yerga et al., 2013). The major drawbacks in using Au for sensing Hg are the significant effect of the sample matrix (Botasini et al., 2013) and the structural changes on the sensor material caused by the amalgam formation (Welch et al., 2004; Martin-Yerga et al., 2013), which require some additional chemical, electrochemical, and mechanical pretreatment of the sample (Anandhakumar et al., 2012).

Some electrochemical methods for sensing Hg use the conjugation of functional groups, such as the donor ligands of nitrogen (N) or sulfur (S) present in amino acids or conducting polymers, such as poly (3,4-ethylenedioxythiophene) (PEDOT), which have a strong binding preference for Hg²⁺ (Chow and Gooding, 2006; Giannetto et al., 2011; Anandhakumar et al., 2012). In the case of PEDOT, although it contains S, which can endow two unpaired electrons, it may not be possible to use PEDOT by itself due to the low signal sensitivity, high insolubility, and intractability (Martin-Yerga et al., 2013). Therefore, to produce a stable and flexible polymer, a conventional poly(styrenesulfonate) (PSS) is incorporated into PEDOT to form PEDOT:PSS (Vacca et al., 2008), which is an excellent copolymer because of its high conductivity, environmental stability (Wang et al., 2014b). To date, the potentials of PEDOT:PSS for electrode modification and electrochemical sensing of toxic metal ions have been seldom reported (Anandhakumar et al., 2011; Yasri et al., 2011; Rattan et al., 2013).

Due to high electrical conductivity, biocompatibility, and the exceptional surface-to-volume ratio, graphene (Gr) received more attention in various applications, including chemical and biosensors (Hill et al., 2011; Sundramoorthy and Gunasekaran, 2014). Recently, incorporating Gr on the working electrode surface to enhance the electrochemical signal for analytical applications has become fairly common (Mikolaj and Zbigniew, 2012; Sundramoorthy and Gunasekaran, 2014). Generally, Gr or reduced graphene oxide (rGO) are used for sensor applications (Yang and Gunasekaran, 2013; Yang et al., 2013). Gr can be obtained from graphite by mechanical cleavage (Jayasena and Subbiah, 2011), chemical exfoliation (Zhang et al., 2010), thermal decomposition (Wang et al., 2012), or electrochemical exfoliation (Low et al., 2013). Among other methods, electrochemical exfoliation of graphite electrode is considered a simple, rapid, and “green” method, as the use of toxic or corrosive reducing reagents or stabilizers are avoided in this method (Su et al., 2011; Singh et al., 2012; Chang et al., 2013; Gee et al., 2013; Mao et al., 2013).

The electrochemical exfoliation of graphite can be achieved by a one or two-step process. In a two-step process, in the first step, graphite electrode is activated in an electrolyte solution at a relatively low bias voltage. During this part of the process, the electrode expands due to the intercalation of electrolyte (usually Li⁺ or Na⁺) ions into the graphite lattice (Zhong and Swager, 2012). The second step involves applying a higher bias voltage to ensure exfoliation of the expanded graphite electrode and separating Gr flakes into the solution (Qi et al., 2011; Su et al., 2011; Zhang et al., 2012; Gee et al., 2013; Li et al., 2013). For example, when lithium ion (Li⁺) is used as an electrolyte, −3.0 V of static bias voltage is applied to intercalate Li⁺ into graphite electrode (Wang et al., 2011). In this work, we have electrochemically synthesized partially oxidized Gr (po-Gr) flakes to study its potential application in detecting mercury (Hg²⁺) ions with PEDOT:PSS. The po-Gr/PEDOT:PSS conducting film readily conjugates with Hg²⁺ in water and allowed us to do selective detection of Hg²⁺ in real samples.

Materials and Methods

Reagents

We used analytical-grade chemicals from Fisher, Acros Organics, and Sigma-Aldrich (USA). Supporting electrolytes were prepared using HNO₃ or NaCl. Stock solution of 10 × 10⁻⁴ M Hg²⁺ was prepared using mercury nitrate [Hg(NO₃)₂] and used after further dilution. All aqueous solutions were prepared using deionized water with 18.2 MΩ cm (EMD, Millipore). The PEDOT:PSS sample was received from CIDETEC research group (San Sebastian, Spain) (Istamboulie et al., 2010).

Electrochemical Synthesis of po-Gr

The po-Gr flakes were obtained by electrochemical exfoliation of graphite sheet. Briefly, a two-electrode cell was used with a piece of flexible graphite sheet (Graphitestore, Inc., USA) as a working electrode and a platinum (Pt) wire were placed parallel to and about 10 mm away from the graphite electrode which served as a counter electrode. The electrodes were connected to a DC power supply (Tektronix PS 280). About 10 mm of the working and counter electrodes were immersed into 25 mL containing 0.1 M HClO₄ and 0.1 M NaCl which served as an electrolyte. The exfoliation was performed by applying DC bias on the working electrode. Initially, a potential of −2.5 V was applied for 60 min to facilitate the electrochemical expansion of graphite electrode by intercalation of Na⁺ ions into graphite layers; after which, a potential of +10 V was applied for another 60 min to achieve exfoliation. The electrolyte solution containing the exfoliated po-Gr flakes was vacuum filtrated (using membrane with a pore size 0.4 μm) and washed with deionized water many times to remove the residual electrolyte.

Preparation of po-Gr/PEDOT:PSS Dispersion

The obtained po-Gr flakes were dried at 60°C for 30 min, and then dispersed in 10 mL PEDOT:PSS solution by a probe sonicator (Sonics, VibraCell VCX130) for 15 min. Later, the po-Gr/PEDOT:PSS dispersion was centrifuged for 30 min at 2,000 rpm (Sorvall Super T21) and the supernatant was used for further characterization. The po-Gr flakes were also dispersed in water (without PEDOT:PSS) for 15 min by probe sonicator and centrifuged at 2,000 rpm for 30 min to collect supernatant for control studies. All experiments were performed at room temperature (25 ± 3°C).

Preparation of po-Gr/PEDOT:PSS-Modified Electrode

A 10 μL sample of po-Gr/PEDOT:PSS or po-Gr dispersion was placed on a well cleaned (after mirror-like polishing with alumina powder) glassy carbon electrode (GCE) surface and dried in an air-oven for 30 min to evaporate solvents. By this method, po-Gr/PEDOT:PSS or po-Gr film-coated electrode was obtained and gently washed by immersing in water for about 5 min to remove unbounded materials from the electrode surface. For Raman measurements, the po-Gr film prepared on glass substrate was reduced with hydrazine in pH 9.0 water solution at 80°C for 2 h (Park et al., 2011).

Electrochemical Measurements and Characterization

Electrochemical measurements were performed by using an electrochemical workstation (660D, CH Instruments). A 10-mL volume, three-electrode system was used with GCE, Ag/AgCl (3 M KCl), and Pt wire as working, reference, and counter electrode, respectively. The GCE was used either bare or after modification with po-Gr film, PEDOT:PSS film, or po-Gr/PEDOT:PSS film. The electrochemical responses of the bare GCE and modified GCE’s toward Hg²⁺ were examined with cyclic voltammetry (CV) in 0.05 M HNO₃ as an electrolyte.

Electrochemical impedance spectroscopy (EIS) measurements were performed in a solution containing 2.5 mM [Fe(CN)₆]^4-/3- and 0.1 M KCl supporting electrolyte in the frequency range of 1–106 Hz. Differential pulse stripping voltammetry (DPSV) was performed by applying deposition and then stripping steps with the following parameters: initial potential, −0.2 V; final potential, 0.8 V; amplitude, 50 mV; pulse width, 0.2 s; pulse period, 0.5 s; sample width, 0.0169 s; and deposition time, 2 min. During the deposition period, the solution was stirred at 800 rpm, and the potential was held at −0.30 V. The DPSV voltammograms were recorded upon injection of Hg²⁺ ions in the range of 0.2–14.0 μM in 0.05 M HNO₃. The interferences of some metal ions (Ca²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cd²⁺, Zn²⁺, Cr⁶⁺, and Pb²⁺) on the determination of 6.0 μM Hg²⁺ in 0.05 M HNO₃ were also investigated by adding their respective nitrate salts.

The surface morphology of po-Gr flakes was studied by scanning electron microscopy (SEM) (LEO1530, Gemini FESEM, Carl Zeiss). Further characterizations were performed on dry samples using Raman spectroscopy (LabRAM Aramis Horiba Jobin Yvon Confocal Raman Microscope, wavelength: 532 nm) and attenuated total reflectance-Fourier transform-infrared spectroscopy (ATR-FT-IR) (Spectrum 100, PerkinElmer). UV–Vis spectra of po-Gr and po-Gr/PEDOT:PSS dispersed in water were obtained using a spectrophotometer (Lambda 25, PerkinElmer). X-ray diffraction (XRD) pattern of samples were measured using Bruker D8 Discover diffractometer.

Results and Discussion

Characterization of po-Gr and po-Gr/PEDOT:PSS Film

The scheme for electrochemical exfoliation of po-Gr flakes and modification of GCE surface with po-Gr/PEDOT:PSS dispersion for Hg²⁺ detection using DPSV is shown in Figure 1. After successful exfoliation, po-Gr flakes were dispersed separately in water, and PEDOT:PSS solution (Figure 2A). The po-Gr solution (Figure 2A, image a) was light yellowish brown in color and the po-Gr dispersed in PEDOT:PSS was light bluish (Figure 2A, image b). UV–Vis spectra of po-Gr solution showed a strong absorption band at 261 nm, which can be assigned to the partially oxidized graphene sheets. It has been shown that graphene oxide (GO) and rGO have absorption peaks at 230 nm, and 270 nm (π–π* transition of aromatic C–C bonds), respectively (Figure 2B) (Li et al., 2008; Choi et al., 2010).

FIGURE 1

Figure 1. Schematic representation of electrochemical synthesis of po-Gr and electrochemical detection of mercury (Hg²⁺) at po-Gr-PEDOT:PSS film-coated electrode by differential pulse stripping voltammetry (DPSV).

FIGURE 2

Figure 2. (A) Photographs of (a) po-Gr and (b) po-Gr/PEDOT:PSS solutions. (B) UV–Vis spectra of (i) po-Gr/PEDOT:PSS, (ii) po-Gr, and (iii) PEDOT:PSS solutions (Inset shows enlarged view of absorption peak of PEDOT:PSS).

The optical absorption peak of our synthesized po-Gr solution (after centrifugation) is in the range between that for GO and rGO; thus, we describe it as po-Gr. However, UV–Vis spectra of po-Gr/PEDOT:PSS solution showed two major absorption bands, first band observed at 261 nm and second broad band centered at 870 nm. The first peak corresponds to the optical absorption of the po-Gr and the second peak corresponds to the oxidized PEDOT:PSS (light blue) (Gustafsson-Carlberg et al., 1995; Pettersson et al., 1999; Tarabella et al., 2012). For comparison, we also measured UV–Vis spectra of PEDOT:PSS solution, which shows a broad optical absorption band centered at 870 nm (Figure 2B). This study provides evidence that po-Gr sheets are stabilized in PEDOT:PSS solution.

Raman spectra of the (a) pristine graphite sheet, (b) as-synthesized po-Gr flakes, (c) po-Gr film prepared from water dispersion after centrifugation, and (d) po-Gr film after reduction with hydrazine are presented in Figure 3. The spectra show D, G, and 2D bands for all samples (Table 1). For pristine graphite sheet, almost insignificant D band was observed, because of highly crystalline structure of graphite without defects (Figure 3A). However, Raman spectrum of the other three samples (Figures 3B–D) showed significant changes upon exfoliation, following redispersion in water and after reduction with hydrazine (Table 1). The G band is characteristic of sp2-hybridized C = C bonds in graphene sheets (Childres et al., 2013); whereas, the D band is associated with structural defects and partially disordered structures of the sp2 domain (Childres et al., 2013). The 2D band located at 2718 cm⁻¹ originates from a double-resonance process (Krauss et al., 2009; Yan et al., 2011). The calculated values of I_D/I_G listed in Table 1. Su et al. (2011) show that it changed significantly from 1.073 to 1.234 after reduction with hydrazine, indicating that restoration of C–C bonds in the graphene lattice, and a decrease in the average size of graphene domains (Lee et al., 2014). This study supports our understanding that as synthesized graphene flakes contain significant defects, due to functional groups generated upon electrochemical exfoliation at high voltage (Morales et al., 2011). I_D/I_G ratio (0.310) of as-synthesized graphene flakes is relatively small compared to po-Gr film (after centrifugation), which may be due to the presence of large graphene flakes without complete exfoliation (Figure 3B). Therefore, it was necessary to disperse po-Gr flakes in a suitable dispersant with a probe sonicator to achieve complete exfoliation of graphene sheets. In this work, we used PEDOT:PSS to disperse po-Gr flakes in water.

FIGURE 3

Figure 3. Raman spectra of (A) pristine graphite sheet, (B) po-Gr flakes (as-synthesized from electrochemical exfoliation), (C) po-Gr film (after centrifugation), and (D) po-Gr film after reduction with hydrazine (532 nm laser was used) (*peak is assigned to the glass substrate).

TABLE 1

Table 1. The D, G, 2D bands and I_D/I_G ratio were estimated from the Raman spectrum of pristine graphite sheet, po-Gr flakes, po-Gr film (prepared after centrifugation), and po-Gr film after chemical reduction using hydrazine.

The as-synthesized po-Gr flakes, po-Gr film (after centrifugation), PEDOT:PSS film, and po-Gr/PEDOT:PSS film were characterized by FT-IR (Figure 4). For as-synthesized po-Gr flakes, no significant bands were observed, which may be due to the incomplete exfoliation process. Indeed, after successful redispersion in water and following centrifugation, po-Gr film on glass showed significant bands at ~3430 (OH stretch), 2337 (CO₂ stretch), 1722 (C = O stretch), and 1637 cm⁻¹(OH bending and C = C stretch) (Pham et al., 2011). The FT-IR spectrum of PEDOT:PSS showed peaks at 1372, 1289, 1124, 1023, 1002 cm⁻¹, which are derived from PEDOT:PSS (Alemu Mengistie et al., 2013; Yoo et al., 2014). The IR bands at 1160 and 1023 cm⁻¹ are assigned to SO³⁻ of the PSS. Further, the FT-IR spectrum of po-Gr/PEDOT:PSS consists bands of PEDOT:PSS, which proves that po-Gr sheets wrapped with the polymer structure (Figure 4).

FIGURE 4

Figure 4. FT-IR Spectra of po-Gr flakes, po-Gr film (prepared after centrifugation), po-Gr/PEDOT:PSS film, and PEDOT:PSS film (highlighted area in the figure shows the –OH stretch due to partial oxidation of po-Gr sheets).

Surface morphology of po-Gr sheets was also studied by SEM. SEM images of the exfoliated po-Gr film show wrinkled or folded thin sheets with the lateral dimension of 1–3 μm (Figure 5A). The XRD spectrum of graphite sheet exhibits an intense peak at 26.5°, corresponding to d-spacing of 0.34 nm (Figure 5B). However, after electrochemical exfoliation, po-Gr flakes exhibit a broad peak at ~16 to 23° and another intense peak at 26.52°. The broad peak at ~16 to 23° indicates the presence of functional groups containing oxygen, which is formed during electrochemical exfoliation (Figure 5B) (Fang et al., 2009). There is also slight shift in the peak position of po-Gr flakes from graphite after electrochemical exfoliation from 26.50 to 26.52 (inset of Figure 5B), which may due to intercalation of ions into the graphene layers.

FIGURE 5

Figure 5. (A) SEM image of po-Gr sheets and (B) XRD patterns of pristine graphite sheet and po-Gr flakes (inset shows enlarged view of peak 002). The distorted peak in the range of 15–24 θ is assigned to the partially oxidized parts of graphene flakes.

Electrochemical Impedance Studies

Figure 6 is the Nyquist plot of the modified GCE’s in 2.5 mM [Fe(CN)₆]^4-/3- in 0.1 M KCl. The semicircular part in the high-frequency region represents electron-transfer-limiting process with its effective diameter equal to Faradaic charge transfer resistance (R_ct), which is responsible for the electron transfer kinetics of redox reactions at the electrode-electrolyte interface (Kumar et al., 2010; Yang and Gunasekaran, 2013). The R_ct values of po-Gr/GCE (R_ct = 144 Ω), PEDOT:PSS/GCE (R_ct = 65 Ω), and po-Gr/PEDOT:PSS/GCE (R_ct = 54 Ω), modified electrodes were lower than that for bare electrode (R_ct = 228 Ω), indicating higher conductivity as a result of modification processes (Figure 6). Generally poor conductivity of GO-modified electrode is due to the presence of excessive oxygenated species, which accentuates the insulating characteristics (Yang and Gunasekaran, 2013). The R_ct for po-Gr film is also higher than for PEDOT:PSS/GCE and po-Gr/PEDOT:PSS/GCE’s perhaps due to the presence of oxygenated species associated with po-Gr, which may affect conductivity of the electrode. However, when po-Gr sheets present in PEDOT:PSS, it improves the conductivity of the electrode (Figure 6).

FIGURE 6

Figure 6. Nyquist (Z_re vs. Z_img) impedance spectra collected using bare GCE, po-Gr/GCE, PEDOT:PSS/GCE, and po-Gr/PEDOT:PSS/GCE electrodes in 2.5 mM [Fe(CN)₆]^4-/3- and 0.1 M KCl. Inset: enlarged high-frequency region.

Detecting Hg²⁺

Linear sweep voltammetry

The linear sweep voltammograms (LSVs) of Hg²⁺ recorded on bare, po-Gr/PEDOT:PSS-, PEDOT:PSS-, and po-Gr-modified GCE’s in 0.05 M HNO₃ show two electrochemical oxidation peaks for Hg²⁺ (Figure 7). These peaks are attributed to two well-defined one-electron steps, according to the two-step equation: Hg²⁺ + e⁻ = Hg⁺ + e⁻ = Hg⁰ (Orlik and Galus, 2007). The presence of po-Gr on both bare GCE and PEDOT:PSS-modified GCE improved the peak current (I_pa) compared to that in the absence of po-Gr. For example, 3-fold, 23-fold, and 100-fold increases in oxidation currents were obtained with po-Gr-, PEDOT:PSS-, and po-Gr-PEDOT:PSS-modified GCE’s, respectively compared to 23.4 μA obtained with bare GCE (Figure 7). These I_pa increases are attributed to the combined effects of sulfonic (Pillay et al., 2013) and thiol (Chandrasekhar et al., 2007; Kadarkaraisamy et al., 2011; Mandal et al., 2013) functional groups on PEDOT:PSS matrix/po-Gr nanocomposite (Shao et al., 2010; Anandhakumar et al., 2012).

FIGURE 7

Figure 7. Linear sweep voltammograms recorded for 0.8 mM Hg²⁺ in 0.05 M HNO₃ solution on (i) PEDOT:PSS-, (ii) po-Gr-, (iii) bare-, and (iv) po-Gr/PEDOT:PSS/modified GCE’s. Inset: enlarged bare and po-Gr-modified GCE’s. Scan rate = 100 mV/s.

The effect of scan rate on the voltammograms of Hg²⁺ at 0.4 mM concentration is shown in Figure 8A. Significant increases in I_pa with increasing scan rate from 1 to 100 mV/s were obtained using po-Gr/PEDOT:PSS-modified GCE (Figure 8A). The relation between I_pa vs. square root of scan rate was linear, which indicated that the electrode reaction was diffusion-controlled (Bard and Faulkner, 2001). However, at lower scan rates of up to 20 mV/s, the I_pa showed only single oxidation peak, which may be due to a slower process, as the reaction occurs in one step: Hg²⁺ + 2e− = Hg⁰ (Orlik and Galus, 2007). The po-Gr/PEDOT:PSS-modified GCE showed a linear response for various Hg²⁺ concentrations from 0.3 to 1.6 mM (R² = 0.997) (Figure 8B).

FIGURE 8

Figure 8. (A) A linear sweep voltammograms recorded using po-Gr/PEDOT:PSS-modified GCE in 0.4 mM Hg²⁺ + 0.05 M HNO₃ at various scan rates (1–100 mV/s) and (B) cyclic voltammograms of different concentration of Hg²⁺. (i) 0.0, (ii) 0.1, (iii) 0.2, (iv) 0.4, (v) 0.6, (vi) 0.8, (vii) 1.0, (viii) 1.2, (ix) 1.4, and (x) 1.6 mM (inset shows the corresponding calibration curve) in 0.05 M HNO₃ medium using po-Gr/PEDOT:PSS-modified GCE at a scan rate of 20 mV/s.

Differential pulse stripping voltammetry measurements

The DPSV is a highly sensitive technique for electroanalysis of trace metals in different samples. It involves two steps for the detection of Hg²⁺ (Somerset et al., 2010) such as (i) deposition of Hg⁰ at an optimized potential for a certain duration and (ii) anodic stripping of deposited Hg⁰. As shown in Figure 8B, the reduction of Hg²⁺ occurs at 0.21 V vs. Ag/AgCl. The effects of the deposition potential and time on the Hg stripping responses were investigated accordingly. The stripping peak currents at different deposition potentials (from −0.5 to +0.2 V) showed that the best stripping signal was obtained in the range of −0.30 to 0.0 V vs. Ag/AgCl (Figure 9B). Based on these results, a deposition potential of −0.30 V was selected for further investigation. Figure 9A shows the relationship between the Hg stripping signal against the deposition time at −0.3 V vs. Ag/AgCl using po-Gr/PEDOT:PSS-modified electrode. The peak current becomes fairly stable after 120 s of deposition.

FIGURE 9

Figure 9. The effect of (A) deposition time at −0.3 V vs. Ag/AgCl and (B) deposition potential for 120 s on the DPSV stripping responses of 6 μM of Hg²⁺ at po-Gr/PEDOT:PSS-modified GCE in 0.05 M HNO₃.

The anodic DPSV response of the po-Gr/PEDOT:PSS-modified electrode with successive Hg²⁺ concentrations in 0.05 M HNO₃ solution are presented in Figure 10. A linear variation of the I_pa was observed for concentrations ranging from 0.2 to 14.0 μM (R² = 0.991), with a sensitivity of 8.72 μA/μM. The limit of detection (LOD) and limit of quantification (LOQ) were calculated as LOD = 3.3 SD/b and LOQ = 10 SD/b, where SD is the standard deviation of five reagent blank determinations and b is the slope of the calibration curve (Shrivastava and Gupta, 2011). Using po-Gr/PEDOT:PSS-modified GCE, the LOD and LOQ were found to be 0.19 and 0.58 μM for Hg²⁺, respectively. This LOD is lower than dithiodianiline-derivative-modified electrode (~2.1 μM) (Somerset et al., 2010) and silver ink screen-printed electrode (~0.5 μM) (Chiu et al., 2008). The po-Gr-PEDOT:PSS-modified GCE also exhibited excellent stability and reproducibility with relative standard deviation (RSD) of 0.93% for 10 successive measurements of 6.0 μM Hg²⁺ in 0.05 M HNO₃ solution. The RSD for six similarly prepared electrodes tested under the same conditions was 1.6%. LOD of Hg²⁺ at Au-PEDOT carbon composite film was ~5 μM (Anandhakumar et al., 2012). In addition, Au-PEDOT film-modified electrode may not be suitable for repeated use because of amalgam formation with AuNPs. Amalgam formation on the electrode surface affects reproducibility of analytical response. It is necessary to regenerate or activate the electrode surface each time with ethylenediaminetetraacetic acid (Giannetto et al., 2011). Electrochemical response of Hg²⁺ at po-Gr/PEDOT:PSS-modified GCE is not dependent on amalgam formation, so it offers reproducible measurements without the need for regeneration or electrode activation.

FIGURE 10

Figure 10. DPSV curves for different Hg²⁺ concentrations (0, 0.2, 1.0, 3.0, 5.0 7.0, 9.0, 11.0, and 14.0 μM) in 0.05 M HNO₃ using po-Gr/PEDOT:PSS-modified GCE (inset shows calibration plot of peak current vs. Hg²⁺ concentration). The reduction voltage is −0.30 V vs. Ag/AgCl for 120 s, then DPSV stripping in the range −0.2 to 0.8 V vs. Ag/AgCl at increments of 4 mV, amplitude 0.05 V, pulse width 0.2 s, and pulse period 0.5 s.

Effect of Interference

The influence of various common interfering metal cations (Ca²⁺, Zn²⁺, Ni²⁺, Cr⁶⁺, As³⁺, Cd²⁺, Co²⁺, Fe²⁺, and Cu²⁺) in the presence of 6.0 μM Hg²⁺ in 0.05 M HNO₃ were tested using the po-Gr/PEDOT:PSS-modified GCE. The interference effect was calculated as:

$\mathit{\text{Interference}}\left(\%\right)=\frac{I_{\mathit{Hg}}-I_{\mathit{Hg}+\mathit{\text{interferent}}}}{I_{\mathit{Hg}}}\times 100$

where, I_Hg = peak current for Hg²⁺, I_{Hg+ interferent} = peak current for Hg²⁺ plus added interferent. The data in Table 2 indicate that the performance of the po-Gr-PEDOT:PSS-modified GCE was unaffected by the tested interferents. The stripping peak current of Hg²⁺ exhibited no change in the presence of Zn²⁺, Cd²⁺, Ca²⁺, As³⁺, or Ni²⁺ ions even at concentrations each at more than 300 times that of Hg²⁺. Assuming an acceptable interference of ±5% (Fifield and Kealey, 2000), only Cu²⁺ at 16 times that of Hg²⁺ and Fe²⁺, at 10 times that of Hg²⁺, can be considered as interferents. DPSV of a solution containing Fe²⁺, Cu²⁺, and Hg²⁺, exhibited oxidation peaks for Fe²⁺ at 0.44 V and for Cu²⁺ at −0.024 V vs. Ag/AgCl (Figure S1 in Supplementary Material).

TABLE 2

Table 2. Interference study of other metal ions on the DPSV measurement of 6.0 μM Hg²⁺ using po-Gr/PEDOT:PSS-modified GCE.

Validation with Real Sample Tests

The performance of the po-Gr/PEDOT:PSS-modified GCE was evaluated by comparing test results with those determined according to US Environmental Protection Agency (EPA) Method 245.1 (EPA, 2007a) using CVAAS performed at the Wisconsin State Laboratory of Hygiene. The data obtained by measuring Hg²⁺ content in three unknown laboratory waste samples (Table 3) provide an excellent validation of proposed method.

TABLE 3

Table 3. Comparison of Hg²⁺ content determined by using po-Gr/PEDOT:PSS/GCE and by standard cold vapor atomic absorption spectroscopy (CVAAs) method in unknown samples.

Conclusion

We synthesized po-Gr from graphite sheets by electrochemical exfoliation using HClO₄/NaCl solution as an electrolyte. The po-Gr and po-Gr-PEDOT:PSS film were characterized using SEM, Raman, FT-IR, XRD, and UV–Vis spectroscopy. Both EIS and CV measurements proved that the presence of po-Gr enhanced the electrochemical catalytic properties of PEDOT:PSS material. The po-Gr/PEDOT:PSS-modified GCE exhibited higher catalytic peak current for Hg²⁺ compared to bare and PEDOT:PSS-modified GCE. The Gr-PEDOT:PSS/GCE was stable and reproducible for determining Hg²⁺ at micromolar levels. The LOD determined by DPSV was 0.19 μM, and the detection was linear in the range of 0.2–14.0 μM (R² = 0.991). The sensor response was not affected by other metal ions. Accurate selective detection of Hg²⁺ in laboratory water samples showed that our method is suitable for routine Hg²⁺ sensing in environmental samples.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The authors are thankful to funding support to Nael G. Yasri through Scholar Rescue Fund Fellowship. Authors would also like to thank Prof. Thierry Noguer (Université de Perpignan Via Domitia) for cooperation and supplying the sensitized PEDOT:PSS sample.

Supplementary Material

The Supplementary Material for this article can be found online at http://www.frontiersin.org/Journal/10.3389/fmats.2014.00033/abstract

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