Nafion (5% wt in low aliphatic alcohols), was purchased from Aldrich, and then diluted to 1% Nafion with isopropyl alcohol. All chemicals used in this study were analytical reagent grade and used without further purification. Zn²⁺, Cd²⁺, Pb²⁺, Cu²⁺ and Hg²⁺ standard stock solutions (1,000 mg L⁻¹, atomic absorption standard solution) were obtained from Sigma Aldrich and diluted as required. Sodium acetate and acetic acid were purchased from Aldrich. 0.1 M acetate buffer (pH 4.6) was used as supporting electrolyte. Ultra pure water (Millipore) was used for all preparations.

Square-wave anodic stripping voltammetry (SWASV) measurements were performed using a 797 VA COMPUTRACE instrument interfaced with a personal computer. The Nafion-G nanocomposite drop coated onto the glassy carbon electrode, served as the working electrode, with the Ag/AgCl (saturated KCl) and platinum electrode acting as the reference and auxiliary electrode respectively. All electrochemical experiments were carried out in a one compartment cell. The surface chemistries of graphite, graphite oxide and graphene were characterized on a Fourier Transform InfraRed spectrometer (Perkin Elmer Spectrum 100) and the structural properties were evaluated through X-ray diffraction (Phillips X-ray diffractometer) with Cu-Kα radiation. A tapping-mode atomic force microscope (Veeco Nanoman V) was employed to evaluate the morphology of graphite oxide and graphene, with special emphasis on estimating its thickness. The silicon tip [antimony (n) doped] had a curvature radius of 2.5–3.5 μm, a force constant of 1–5 N m⁻¹ and a resonance frequency of 60–100 kHz. The samples for AFM were prepared by drop coating the graphene/water and graphene oxide/water (5 μL) dispersion onto a silicon wafer. Transmission electron microscopy images were taken on a Tecnai F20 HRTEM and the Raman spectra were recorded on a Dilor XY Raman spectrometer with a Coherent Innova 300 Argon laser with a 514.5 nm laser excitation.

The graphite oxide was synthesized from graphite powder according to the literature with little modification [19,20]. Graphite oxide (100 mg) was dispersed in 100 mL of water and sonicated for 1 h, followed by the addition of 200 mg of NaBH₄ to the dispersion. The mixture was stirred at 125 °C for 3 h. The black solid was isolated by centrifugation, washed with water and then dried.

A 0.5 mg mL⁻¹ graphene solution (100 μL) was mixed with an equal volume of 1.0 wt% Nafion-isopropyl alcohol solution by ultrasonication for ca. 30 min. or until fully dispersed. Then, a 5 μL aliquot of the mixture was coated onto the glassy carbon electrode (GC) to obtain the Nafion-G modified electrode [18].

The three electrodes were immersed into the electrochemical cell, containing 20 mL 0.1 M acetate buffer (pH 4.5), 10 mg L⁻¹ Hg²⁺, and the target metal ions. The Nafion-G modified GC electrode with mercury film was plated in situ by spiking the sample with the required concentration of Hg²⁺ and simultaneously depositing Hg and the target metals on the surface of the electrode at −1.3 V for 120 s. Following the conditioning step, the stirring was stopped and after 10 s the voltammogram was recorded by applying a continuously changing square-wave potential (with a voltage step of 5 mV, amplitude of 25 mV, and frequency of 50 Hz). Prior to the next cycle, a cleaning step (60 s at 0.3 V, with solution stirring) was used to remove the target metals and metal film.

Figure 1 shows the Fourier Transform InfraRed (FT-IR) spectra of graphite, graphite oxide (GO) and graphene. For graphite, no distinct peaks are detected. GO however, showed a rich collection of transmission bands corresponding to C=O (1,719 cm⁻¹), aromatic C=O (1,597 cm⁻¹), carboxy C–O (1,411 cm⁻¹, epoxy (1,283 cm⁻¹) and O–H (3,400 cm⁻¹) groups. After reduction with NaBH₄ most of the functional groups were eliminated. These results concur with those reported by Chen et al. [21].

The XRD patterns of graphite, graphite oxide (GO) and graphene are shown in Figure 2. Graphite showed a very strong 002 peak at 26.37°, GO a 001 peak at 9.88° and graphene, 002 peak at 24.88°. The GO peak shift is due to the formation of hydroxyl, epoxy and carboxyl groups. After reduction to graphene some of the oxygen-containing functional groups are removed and this causes the graphene peak to shift to 24.88°. This suggests the conjugated graphene network (sp² carbon) is reestablished during the reduction process, which is associated with the ring-opening of the epoxides.

The Raman spectrum of graphite, GO and graphene is shown in Figure 3. The Raman spectra of the materials simply confirm the observations of the XRD patterns i.e., the changes of structure during the reduction process from GO to graphene. The intensity ratio (I_D/I_G) of D band and G band of GO is about 0.946, while the I_D/I_G of graphene is 1.23 due to the presence of unrepaired defects that remained after the removal of large amounts of oxygen-containing functional groups. This I_D/I_G ratio value is consistent with most chemical reduction reports by Fan et al. [22].

For further characterization Transmission Electron Microscopy (TEM) analysis was done. TEM samples were prepared by pipetting the graphene dispersion onto a holey mesh grid. The TEM image of graphite [Figure 4(a)], shows its graphitic structure as large thick dark flakes. Graphite cannot be exfoliated even when sonicated under the same conditions as GO. For GO [Figure 4(b)] large sheets were observed to be situated on top of the grid, resembling a wavy silk veil. The sheets are transparent and entangled with one another. The structure of graphene (reduced GO) is different from that of GO [Figure 4(c)]. At low magnification, the structure of graphene looks flat, with transparent layers on top of one another. The silk-like parts as well as the restacked parts can be seen. Wrinkles and folding are also observed on the surface as well as at the edges of the structure. Corrugation and scrolling are part of the intrinsic nature of graphene nanosheets, which results from the fact that the 2-D membrane structure becomes thermodynamically stable via bending [23]. Figure 4(d) shows a high resolution TEM (HRTEM) image of graphene. Layers of graphene can be seen in this image.

Atomic force microscopy (AFM) was also performed on graphene, to characterize the degree of exfoliation. Figure 5(a) represents the AFM topography image of graphene, wherein several graphene sheets were randomly deposited on the silicon substrate. A flat graphene sheet was selected for further investigation using the 3D view [Figure 5(b)]. The graphene surface was slightly rough and this could be due to the existence of some functional groups. The cross sectional view across the plain area of the sheet gave an estimated height of 1.3 nm which is consistent with that reported by Shen et al. [24].

Figure 6 shows the SWASV analytical characteristics of different film coated GC electrodes e.g., Nafion (green line), Nafion-G (solid line), Graphene (black line) by in situ plated Hg-film, for Zn²⁺, Cd²⁺ and Pb²⁺ determination. The stripping voltammograms were obtained in a solution containing 30 μgL⁻¹ of each of the target metals, 10 mgL⁻¹ Hg²⁺ in 0.1 M acetate buffer (pH 4.6). The sharper higher peak current for the target metal ions on the five metal films were obtained at the Nafion-G-modified electrode, and is consistent with the voltammetric behaviors of carbon nanotubes/Nafion [12] and ordered mesoporous carbon/Nafion electrodes [14]. The signal enhancement may be attributed to the change of the morphologies and the structure as well as the interfacial electron-transfer properties. Each peak appearing at a certain peak potential in Figure 6, represents the point at which a particular metal strips out of the amalgam (stripping step) or is re-oxidized back into solution. The stripping potentials for Zn²⁺, Cd²⁺ and Pb²⁺ appear at approximately −1.1 V, −0.7 V and −0.5 V respectively; the redox reaction involved during stripping analysis is given by Equation (1): (1) Mn n + + n é +   Hg   → M   ( Hg )         Deposition step   ( reduction reaction ) M   ( Hg ) → M n + + n é + Hg             Stripping Step   ( oxidation reaction )

Figure 7(a) shows the effect of deposition potential on the peak current of Zn²⁺, Cd²⁺, Pb²⁺ and Cu²⁺ after 120 s, deposition was studied in the potential range from −0.2 V to −1.5 V. As the deposition potential shifts from −0.6 V to −1.2 V the stripping peak current increased and the more negative the peak potential became the peak currents started to reduce. The different trends observed for Zn²⁺, Cd²⁺, and Pb²⁺ and Cu²⁺ may be due to the different standard potentials. Thus −1.3 V was used as the optimal deposition potential for Zn²⁺, Cd²⁺ and Pb²⁺ in subsequent experiments whereas, an optimal deposition potential of −1.0 V was chosen for Cu²⁺.

The effect of deposition time on the peak currents of Zn²⁺, Cd²⁺, Pb²⁺ and Cu²⁺ was studied and the results obtained are shown in Figure 7(b). As the deposition time increased so did the stripping peak current of each metal ion. The increase occurred linearly with deposition time because of the increased amount of analyte on the Nafion-G modified electrode. A deposition time of 120 s was chosen for further analysis due to the rapid surface saturation which occurred after 120 s.

The effect of rotation speed during the pre-concentration step was also studied in the range 200–2,000 rpm [Figure 7(c)]. As the square-root of rotation speed of the stirring rod increased so did the stripping peak currents of Zn²⁺, Cd²⁺, Pb²⁺ and Cu²⁺ [28]. Establishing the optimum rotation speed facilitates the convective transport of the metal ions in solution to the working electrode surface and hence contributes towards the sensitivity of stripping analysis. A rotation speed of 1,000 rpm was chosen for further analysis.

Individual analysis of Zn²⁺, Cd²⁺, Pb² and Cu²⁺

All four metals i.e., Zn²⁺, Cd²⁺, Pb²⁺ and Cu²⁺ were determined individually at the Nafion-G mercury film electrode using SWASV. Calibration plots [Figures 8(a–d)], for individual metal solutions ranging from 1–7 μg L⁻¹ for Zn²⁺, Cd²⁺, Pb²⁺ and 20 μg L⁻¹–180 μg L⁻¹ for Cu²⁺, gave the sensitivities and detection limits shown in Table 1. A slight shift in the peak potentials of the metals with increasing metal ion concentration towards positive potential was observed and suggests an IR-drop effect. Figure 8(e) is the voltammograms for 0.5–5.0 μg L⁻¹ of Zn²⁺, Cd²⁺ and Pb²⁺ at the Nafion-G-Hg film. The calibration plots [Figure 8(e)] for the simultaneous analysis of metals gave the sensitivities and detection limits shown in Table 2. Copper was not determined simultaneously with Zn²⁺, Cd²⁺ and Pb²⁺ due to the intermetallic interference which exists between Cu-Zn [25].

When comparing individual with simultaneous analysis, a difference in the sensitivities for some of the metals was observed. The sensitivity of Pb²⁺ remained relatively the same, whereas Cd²⁺ and Zn²⁺ showed a significant decrease when analyzed simultaneously. A similar trend was observed when Zn²⁺ was determined individually; here a sensitivity of 1.25 μA L μg⁻¹ was obtained in comparison to 0.758 μA L μg⁻¹ for the simultaneous determination.

In general, higher sensitivities were obtained for individual analysis, since only one of the metals binds to the limited number of active sites at the modified electrode surface and is involved in forming the amalgam film during the deposition step. However, during simultaneous analysis all metals present in solution compete for the limited number of active sites and are all co-deposited during the formation of the amalgam film. In addition, differences in sensitivities between individual and simultaneous determinations can also be attributed to the formation of intermetallic compounds between heavy metals when present together in the same solution [25]. The sensitivity of Pb²⁺ remained the same during both individual and simultaneous analysis since, it is most likely to be available for deposition whereas, the Cd²⁺ and Zn²⁺ are involved in a Zn-Cd intermetallic compound formation [26].

The detection limits (3σ blank/slope) of the metals for both individual and simultaneous analysis were determined using a deposition time of 120 s and are summarized in Tables 1 and 2. A summary of previously reported analyses for Zn²⁺, Cd²⁺ and Pb²⁺ are shown in Table 3. The detection limits are lower for reference [18] and [39] since longer deposition times are being used to pre-concentrate the metal ions. In this work a shorter deposition time is being used, and therefore offers higher detection limits in comparison to those previously studied.

The accuracy of the analysis at the Nafion-G modified electrode was evaluated through recovery studies. The electrolyte (0.1 M acetate buffer) was spiked with a known amount of metal ions and analyzed by SWASV using a deposition time of 120 s. Four replicates were run for each sample and their concentrations determined using the Standard Addition method. The recovery percentages of the metals are shown Table 4.

Individual analysis of Cd²⁺, Pb²⁺ and Cu²⁺ was done in the same solution whereas, Zn²⁺ was determined using a fresh sample. The reason being, that a very low recovery for Zn²⁺ was obtained when analyzed in the same sample as well as simultaneously. This is due to the competition of metals for the Nafion-G mercury film and also the intermetallic interaction which exists between Zn²⁺ and Cu²⁺. According to Lazar et al. [25], this interference is most pronounced when Zn²⁺ and Cu²⁺ are determined simultaneously, which is clearly shown from the results obtained (Table 4).

Results from Table 4 also show a variation in stripping voltammetry and ICP-MS analysis. This may be due to different interferences which are inherent to a particular technique and which ultimately influences the final result. Furthermore, the low ICP-MS results could also be due to the manner in which the samples were prepared.

For the purpose of practical applicability, a real water sample was collected from Edith Stephens Wetlands Park and Nafion-G HgFE was employed for the determination of Zn²⁺, Cd²⁺ and Pb²⁺ metal ions. The lake water was adjusted to pH 4.6 using sodium acetate buffer. A deposition time of 600 s was used for the analysis. The determination of metal ions in the lake water is hampered by the presence of organic compounds (e.g., humic acids) which may form stable complexes with the metal ions thus making the metal ions unavailable for analysis. In addition, these organic acids also absorb onto the surface of the working electrode thus decreasing the surface area of the electrode causing a reduction in the analytical signal. As a consequence a longer deposition time was used to obtain a signal [40]. The results obtained were compared with ICP-MS. The Nafion-G HgFE was sensitive enough to be able to detect Zn²⁺, Cd²⁺ and Pb²⁺ as shown in Table 5. However, for Cd²⁺, a higher result was obtained with ICP-MS compared to SWASV which may be due to the intrinsic difficulties of working associated with working at ultra-trace levels. The non-detection of Cu²⁺ can be attributed to the formation of stable copper complexes with organic compounds or humic acids.