Graphene oxide-based electrochemical sensor: a platform for ultrasensitive detection of heavy metal ions

Abstract

Facile functionalization of graphene oxide sheets on gold surface results in complexation-enhanced electrochemical detection of heavy metal ions, shown here for Pb²⁺, Cu²⁺ and Hg²⁺, with improved detection limits by two orders of magnitude relative to the control electrode.

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The applications of graphene in nanotechnology-based devices have received unceasing interests since its discovery, attributed to its unique and often outstanding physicochemical and electronic properties.¹ For electronic devices, extremely high quality graphene with little defects and oxygenated groups is necessary, and this is commonly synthesized through very delicate chemical vapor synthesis often with low yield.² As to chemical applications, the trends in recent years have been focusing on the adaptation of reduced-graphene oxide (rGO), a lower quality variant of the graphene analogue, which can be obtained from the chemical reduction of oxidized and exfoliated graphite.³ For an even simpler and cheaper processing, graphene oxide (GO) sheets, which are obtained right after the oxidation/exfoliation of graphite without a further reduction step, can be explored for various applications such as hybrid functional materials,⁴ drug-delivery vehicles,⁵ and fluorescence-based sensors.⁶ This is despite the much lower electrical conductivity (at least two order of magnitudes) of GO compared to rGO.⁷ Nevertheless, given its large specific surface area and strong hydrophilic nature, GO shows great potential in the removal of aqueous pollutants,⁸ especially for a wide range of heavy metal ions.⁹ For example, the strong interactions of Cu²⁺ with GO surface make it an excellent adsorbent material,¹⁰ while at the same time enhancing the electronic conductivity through the binding of metal ions to the oxygen moieties on GO surface.¹¹ In the same way, Ca²⁺ and Mg²⁺ can be deliberately added as cross-linkers to enhance the mechanical strength of graphene oxide composites.¹²

Following this train of thoughts, we design a GO-based electrochemical sensor on the basis of two important characteristics, that is, high adsorption of analytes and good conductivity thereafter. The GO was synthesized from the oxidation and exfoliation of purified graphite powders, following the well-established Hummers' method¹³ (see ESI†). The washed, filtered and dried brownish GO consists of densely mixed oxygenated groups, such as hydroxyl and epoxide (mostly on the basal surface), and carboxyl (mostly at the sheet edges).¹⁴ These oxygenated moieties provide avenues for direct modification by surface covalent functionalization.^4,5,15 Using the ethyl(dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide (EDC/NHS) mediated coupling strategy, the edges of GO, that is through the carboxyl groups, are attached directly onto a L-cysteine modified gold electrode surface (Scheme 1, see ESI† for detailed procedures). To the best of our knowledge, this is the first report of fabrication of a GO-based sensor using the facile EDC/NHS coupling strategy for the ultrasensitive detection of heavy metal ions. The attached GO sheets (see electron microscopy images, Fig. S2†) form the extended heterogeneous sites for the adsorption of metal ions, predominantly through the oxygenated sites as discussed below.

Scheme 1 Stepwise modification of gold electrode with L-cysteine functionalization followed by the EDC/NHS-mediated coupling of activated GO. The oxygenated sites of attached GO provide avenues for the capturing and complexation of metal ion analytes.

Wide scan X-ray photoelectron spectroscopy (XPS) confirms the presence of Au, S, C, N and O upon self-assembled monolayer attachment of L-cysteine on the bare gold surface (Fig. 1a). The carboxyl (O=C–O) and amide (–NH₂) moieties are clearly evident from the C 1s binding energy peaks at 288.8 eV (Fig. 1b) and N 1s peak at 399.6 eV (Fig. 1c), under the respective narrow scans. Although not shown, the thiol group is also present, as measured from the S 2p binding energy peak at 162.0 eV. Subsequent attachment of GO resulted in the increased C and O intensity seen from the full scan in Fig. 1d, arising predominantly from the GO sheets. In the C 1s narrow scan (Fig. 1e), the peaks at 284.6, 286.0 and 288.5 eV were ascribed to the C=C, C–O and C=O of graphene oxide, respectively.¹⁶ Importantly, the peak at 287.6 eV which belongs to the O=C–N shows the successful coupling of activated GO by EDC/NHS with cysteine.¹⁷ The weak binding energy peak at 284.8 eV was assigned to the C–S bond of the cysteine self-assembled monolayer on gold surface.¹⁸ The corresponding N 1s narrow scan is shown in Fig. 1f, where the amide nitrogen from the coupling reaction of –COOH and –NH₂ is evident at 400.2 eV.¹⁹ The highest binding energy peak in the N 1s spectrum at 402.5 eV can be assigned to the nitrogen of unreacted NHS esters on the surface.²⁰ The attachment of GO on gold surface was further confirmed electrochemically via the cyclic voltammogram of ferricyanide (Fig. S3†). The electron transfer resistance increases from 950 Ω for the L-cysteine modified gold electrode to 1360 Ω for the modified electrode after GO attachment (Fig. S4†).

Fig. 1 XPS of L-cysteine modified gold electrode: (a) full scan, deconvoluted narrow scans of (b) C 1s binding energy and (c) N 1s binding energy; and of GO-modified gold electrode: (d) full scan, deconvoluted narrow scans of (e) C 1s binding energy and (f) N 1s binding energy.

Cyclic voltammetry provides the initial qualitative analysis of as-prepared GO-modified gold electrode before and after adsorption of aqueous metal ions, hereby using Pb²⁺, Cu²⁺ and Hg²⁺ as the model analytes (Fig. S5†). We confirm that no redox peaks of the ions could be measured in the NH₄Ac buffer electrolyte (pH 7.0) containing 50 mM KCl prior to the accumulation of heavy metal ions. After accumulation in an aliquot containing aqueous metal ions for 10 min (see Fig. S6†) followed by rinsing with metal-free NH₄Ac buffer, the redox peaks for Pb²⁺, Cu²⁺ and Hg²⁺ appeared at the expected positions of −0.11 V/−0.23 V vs. Ag/AgCl,²¹ 0.25 V/0.17 V vs. Ag/AgCl,^19,21b and 0.55 V/0.51 V vs. Ag/AgCl,²² respectively (Fig. S5†).

For quantitative analysis, the square-wave voltammetry (SWV), which has a higher sensitivity compared to the CV technique, is employed under the same electrolyte condition as mentioned above. It is well known that the increased sensitivity of SWV arises from the larger net current (the difference between forward current and reverse current) with very little non-Faradaic and charging currents.²³ Fig. 2 shows the calibration curve, measured on the anodic sweep, for (a) Pb²⁺, (b) Cu²⁺ and (c) Hg²⁺, over the GO-modified sensor. Here, regions of linear detection range are evident for all three metal ions, with detection limits as low as sub-ppb levels for the GO-modified electrode, that is, 0.4 ppb for Pb²⁺, 0.8 ppb for Hg²⁺ and 1.2 ppb for Cu²⁺. These minimum limits are much lower than the guideline values (10 ppb Pb²⁺, 6 ppb Hg²⁺ and 2000 ppb Cu²⁺) for drinking water given by the World Health Organization (WHO),²⁴ and in fact, two orders of magnitude lower than that of the L-cysteine/gold electrode control, that is, 20 ppb for Pb²⁺, 10 ppb for Hg²⁺ and 50 ppb for Cu²⁺ (Table 1). The ultrahigh sensitivity is attributed to the accumulation of metal ions on the GO surface (shown below), giving rise to an improved Faradaic to capacitive current ratio (signal-to-noise, S/N ratio).²⁵ It is interesting to note that the maximum limits of the linear range, 12.8 ppb for Hg²⁺, 51.2 ppb for Pb²⁺ and 200 ppb Cu²⁺, are inversely proportional to the adsorption capacity of GO, suggesting electronic interactions between the adsorbed metal ions when brought to close proximity on GO.²⁶

Fig. 2 Calibration plots of SWV peak current of (a) Pb²⁺, (b) Cu²⁺ and (c) Hg²⁺ over GO-modified gold electrode. Insets show the corresponding SWV sweep. See Fig. S8† for reproducibility measurements for both minimum and maximum linear range detection limits.

Table 1 Detection limits of GO-modified and L-cysteine (prior to GO attachment) modified Au electrodes in the sensing of Pb²⁺, Cu²⁺ and Hg²⁺

Electrode	Minimum detection limit (ppb)
	Pb^2+	Cu^2+	Hg^2+
a Calibration plot in Fig. S7.
GO-modified Au	0.4	1.2	0.8
L-cysteine modified Au^a	20	50	10

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The Langmuir adsorption isotherms of Pb²⁺, Cu²⁺ and Hg²⁺ on GO are shown in Fig. 3a. The saturation adsorption capacity was estimated at 204, 270, and 41 mg metal ions per g GO for Pb²⁺, Hg²⁺ and Cu²⁺, respectively, which is much higher than other carbon analogues including reduced graphene oxide.²⁷ Previous reports focused either on electrostatic attraction²⁶ or metal coordination¹⁰ between GO and the analyte ions as the possible mode of adsorption. The former appears straightforward given the highly negative zeta potential of GO (Fig. 3b) that would readily attract the positive divalent analyte metal ions. However, a comparison with rGO, which is also characterized by similar zeta potential profile but with reduced oxygenated moieties, shows a lower adsorption capacity of 149, 178, and 31 mg metal ions per g rGO for Pb²⁺, Hg²⁺ and Cu²⁺, respectively. Quantification of O : C ratio by XPS revealed an oxygen content three times higher for GO (O : C = 2.7) compared to rGO (O : C = 0.09). Note: purification of graphite source prior to Hummers' treatment is necessary to achieve a high O : C ratio. However, the adsorption capacities of metal ions on rGO easily exceed above 70% of that on GO. In other words, we believe a mixed electrostatic attraction and metal coordination (Fig. 3c) may have taken place, the latter especially dominant on GO. This is further evidenced by the significant aggregation of GO upon addition of metal ions as a result of metal coordination and cross-linking of GO sheets (Fig. 3d). By comparison, rGO is only slightly aggregated and the suspension remains stable.

Fig. 3 (a) Langmuir adsorption isotherms of Pb²⁺, Cu²⁺ and Hg²⁺ on GO (solid symbols) and rGO (open symbols). (b) Zeta potential of GO and rGO as a function of pH. (c) Illustration of coordination and complexation of metal ions on GO. (d) Photograph showing the aggregation of aqueous GO and rGO induced by the presence of heavy metal ions.

Using Pb²⁺ as the reference analyte, we further assess the selectivity of the GO-modified electrode. In principle, selective detection requires the covalent attachment of ion-specific antibodies, proteins or polymers²⁸ through the functionalization of the oxygen moieties, which is beyond the scope of the present study. Instead, it is interesting to investigate the selectivity of the “bare” GO as is, which provides further insights on its sensing characteristics. Fig. 4a shows the effects of the addition of various individual interfering ions, that is, Zn²⁺, Ni²⁺, Fe³⁺, Cd²⁺, Cr⁶⁺, Cu²⁺ and Hg²⁺, during the detection of 25 ppb Pb²⁺. Each test was carried out individually with one interfering ion at low (100 ppb) and high (1000 ppb) concentrations (Fig. S9†). We found that the addition of Zn²⁺, Ni²⁺, Fe³⁺, Cu²⁺ and Hg²⁺ significantly reduced the sensing response of Pb²⁺ at both low and high concentrations of the interfering ions. Essentially, these interfering divalent cations compete with Pb²⁺ ions for adsorption on GO through electrostatic attraction and coordination on the oxygenated sites. Along the same analogy, minimal interference was observed in the case of Cr⁶⁺ in the form of chromate (Cr₂O₇²⁻), where there is a lack of Cr valance d-orbital for the coordination with the lone pair electrons of oxygen on GO. In fact, the electrostatic repulsion between Cr₂O₇²⁻ and the negatively charged GO kept the anions separated. Cd²⁺ has little influence (in fact slightly positive) on the sensing response of Pb²⁺ due to the weak coordination and similar redox potential as discussed below.²⁹

Fig. 4 Interference studies in terms of (a) sensing response of 25 ppb Pb²⁺ over GO-modified electrode, and (b) adsorption of 10 ppm Pb²⁺ on GO, in the presence of interfering ions (Zn²⁺, Fe³⁺, Ni²⁺, Cu²⁺, Hg²⁺, Cd²⁺ and Cr⁶⁺).

As competitive adsorption has been identified, Fig. 4b shows the net adsorption of 10 ppm Pb²⁺ on GO in the presence of the abovementioned interfering ions. Since direct quantification of adsorption of Pb²⁺ on the GO-modified electrode at an ultralow sensing concentration may not be so straightforward, the studies of adsorption interference were conducted in relevance to the Langmuir isotherm. At the reference concentration of 10 ppm, adsorbed Pb²⁺ exists as a submonolayer on GO (Fig. 3a), which in principle allows for sufficient buffer adsorption sites in the presence of low concentration of foreign ions. As shown in Fig. 4b, the degree of Pb²⁺ adsorption interference is varied for the different detrimental ions Zn²⁺, Fe³⁺, Ni²⁺, Cu²⁺ and Hg²⁺. Despite being in the submonolayer range, the presence of a low concentration (10 ppm) of Fe³⁺ significantly reduced the adsorption of Pb²⁺. This is prompted by the stronger Coulombic force of attraction between Fe³⁺ and the oxygenated moieties of GO (2.1 times higher compared to Pb²⁺). On the contrary, Zn²⁺ exhibits little adsorption interference at low concentration but at high concentration (100 ppm), Pb²⁺ adsorption is retarded by more than 60%, implying competitive adsorption when the Langmuir sites become limiting. The same applies to Hg²⁺. Based on the different extent of adsorption interference, it appears that sensing response is not necessarily proportional to the net adsorbed Pb²⁺. For example, a much lower sensing response of Pb²⁺ in the presence of Fe³⁺ would be expected relative to Zn²⁺, but this is clearly not the case (Fig. 4a). Likewise, the stronger adsorption interference of Cu²⁺ relative to Ni²⁺ was not reflected in the sensing response. The results imply adsorption-induced galvanic interference from the secondary ions during the sensing of Pb²⁺ on GO. Galvanic oxidation of Pb by highly electronegative analytes, i.e., Cu²⁺ and Hg²⁺, resulted in a reduced Pb²⁺ stripping signal, more so than in the presence of electropositive interfering ions such as Zn²⁺, Fe³⁺ and Ni²⁺. Since the electrostatic repulsion of Cr₂O₇²⁻ kept Cr⁶⁺ away from the GO surface, it did not interfere with the adsorption and sensing response of Pb²⁺. Cd²⁺ shows negligible adsorption interference at low concentration due to the soft acid nature of Cd²⁺ relative to Pb²⁺, and only became obvious at high concentration. Because of the relatively weak interaction, it shows minimal interference to Pb²⁺ sensing.

Finally, the reproducibility of GO-modified electrodes was assessed by comparing the stripping peak current of 50 ppb Pb²⁺, 25 ppb Cu²⁺ and 10 ppb Hg²⁺ using electrodes fabricated on different occasions (n = 5). In all cases, excellent reproducibility of sensing response was achieved, with relative standard deviations (RSDs) of 2.1% for Pb²⁺, 3.8% for Cu²⁺ and 3.3% for Hg²⁺ respectively, revealing the high reproducibility of the electrodes (Fig. S10†).

Conclusions

In summary, we have showcased the facile fabrication of a highly reproducible GO-based electrochemical sensor for the ultrasensitive detection of heavy metal ions. The high sensitivity arises from the strong adsorption of these heavy metal ions on GO, both electrostatically and through metal coordination on oxygenated sites. In this respect, it is more beneficial than rGO, which lacks the oxygenated group active sites compared to GO (and is also simpler to process compared to rGO). In general, the showcase device provides a universal platform for which further functionalization with analyte-specific proteins, antibodies and polymers can be achieved through covalent attachment on the oxygenated moieties for high performance sensors.

Acknowledgements

The authors acknowledge the financial support of Hong Kong UGC through the General Research Fund (CityU 104812) and City University of Hong Kong through Startup Grant (7200252).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details on the preparation of GO-modified electrodes, physicochemical characterization and electrochemical measurements, CV and SWV of ferricyanide and heavy metal ions, calibration plots of heavy metal ions, SWV of Pb²⁺ in the presence of interfering ions, and reproducibility measurements of the GO-modified electrodes. See DOI: 10.1039/c4ra02247e

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