Graphene oxide quantum dots immobilized on mesoporous silica: preparation, characterization and electroanalytical application

Abstract

Because of its high surface area and combination of various functional groups, graphene oxide (GO) is currently one of the most actively studied materials for electroanalytical applications. It is not practical to utilize self-supported GO on its own and thus it is commonly integrated with different supporting carriers. Having a large lateral size, GO can only wrap the particles of the support and thus can significantly reduce the surface area of porous materials. To achieve synergy from the high surface area and polyfunctional nature of GO, and the rigid structure of a porous support, the lateral size of GO must essentially be decreased. Recently reported graphene oxide quantum dots (GOQDs) can fulfil this task. Here we report the successful preparation of an SiO₂-GOQDs hybrid, where GOQDs have been incorporated into the mesoporous network of silica. The SiO₂-GOQDs emit a strong luminescence with a band maximum at 404 nm. The Raman spectrum of SiO₂-GOQDs shows two distinct peaks at 1585 cm^-1 (G-peak) and 1372 cm^-1 (D-peak), indicating the presence of a graphene ordered basal plane with aromatic sp²-domains and a disordered oxygen-containing structure. Covalent immobilization of GOQDs onto aminosilica via such randomly structured oxygen fragments was proven with the help of Fourier transform infrared spectroscopy, solid-state cross-polarization magic angle spinning ¹³C nuclear magnetic resonance, and X-ray photoelectron spectroscopy. SiO₂-GOQDs were used as a modifier of a carbon paste electrode for differential pulse voltammetry determination of two antibiotics (sulfamethoxazole and trimethoprim) and two endocrine disruptors (diethylstilbestrol (DES) and estriol (EST)). The modified electrodes demonstrated a significant signal enhancement for EST (370%) and DES (760%), which was explained by a π-π stacking interaction between GOQDs and the aromatic system of the analytes.

Fig. 1. Chemical formulas of medications studied in this research.

Fig. 2. N₂ adsorption–desorption isotherms of SiO₂-NH₂ and SiO₂-GOQDs (a); incremental pore size distribution by surface area (PSD) (b).

Fig. 3. Low (a and b) and high (c and d) magnification SEM images of GO (a) and SiO₂-GOQDs (b–d) with EDS element mapping of O, Si, C and N on SiO₂-GOQDs (e).

Fig. 4. Photoluminescent spectra of solid SiO₂-NH₂ (black line) and SiO₂-GOQDs (red line) under excitation at 340 nm.

Fig. 5. Raman spectra of GOQDs (—) and SiO₂-GOQDs (- -).

Fig. 6. FTIR spectra of GOQDs, SiO₂-NH₂, SiO₂-GOQDs.

Fig. 7. CP/MAS ¹³C NMR spectrum of SiO₂-GOQDs.

Fig. 8. Survey (a) and fitted XPS spectra of the C1s (b) and N1s (c) of SiO₂-GOQDs and pristine SiO₂-NH₂ with deconvoluted data.

Fig. 9. Schematic structure of surface layer in SiO₂-GOQDs.

Fig. 10. DPV curves of SMZ and TMP mixture on CPE/SiO₂-GOQDs (a) and CPE (b), and linear relationship between peak currents and the concentrations of the analytes (inset). The analytes were present in a mixture (5 : 1) with the following concentrations of SMZ: (a) 4.0, 8.0 and 20 μmol L⁻¹; (b) 4.0 μmol L⁻¹. Supporting electrolyte: 0.04 mol L⁻¹ of BRbs (pH 5.8), 0.5 mol L⁻¹ of NaNO₃.

Fig. 11. DPV curves of DES (a and b) and EST (c and d) on CPE/SiO₂-GOQDs (a and c) and bulk CPE (b and d), and linear relationship between peak currents and the concentrations of the analytes (insets). Concentration of the analytes: DES – 0.15, 0.30 and 0.52 μmol L⁻¹, EST – 0.014, 0.027, 0.062 μmol L⁻¹. Supporting electrolyte: 0.04 mol L⁻¹ of BRbs (pH 5.8), 0.5 mol L⁻¹ of NaNO₃.