Abstract
When graphene is used as SERS substrates, it contributes to the chemical mechanism (CM) of enhancement of Raman signal, owing to which the detection limit is very low (lower than mM content of probe molecules). The CM of enhancement depends largely on the interactions between the substrate and the probe molecules. Therefore, in this work, we have investigated the possibility of increasing the SERS activity of graphene by improving the interaction between the probe molecule and the graphene substrate by establishing exclusively strong covalent bonding between them. Fluorescein (Fl) was selected as a probe molecule because it is one of the most commonly used fluorophore in bioscience. As a graphene substrate, reduced graphene oxide (rGO) platelets were used. In addition, silver nanoparticles (AgNPs) were added onto the hybrids to further increase the enhancement by electromagnetic mechanism. Highly enhanced Raman signal of Fl onto neat rGO was achieved for micromolar concentration of the probe molecules. This was attributed to the covalent bonding between them, which introduced hole doping to rGO, decreasing the Fermi level of rGO and bringing it more closely to the LUMO of Fl. This induces aligning of their energy levels, resulting in higher contribution of the nonresonance effect to the charge transfer mechanism of enhancement, which, in this case, occurred intramolecularly. When AgNPs were added onto the rGO substrate, the expected enhancement performance was not observed. On the one hand, this was attributed to small size (∼20 nm) of AgNPs and lack of aggregates and, on the other, due to the unusually high contribution of CM determined.
Highlights
One or few layers of graphene[1−5] or graphene oxide (GO)[4,6] have been widely used as active substrates for surface-enhanced Raman scattering (SERS) analysis.[7,8] Graphene-based surfaces, similar to the metallic surfaces, quench the fluorescence background of the analyte.[9]
Covalent attachment of Fl onto the GO platelets was demonstrated by UV and Fourier transform infrared (FTIR) spectroscopic analyses of the thoroughly cleaned samples by 5 + 2 subsequent centrifugations, during which it was supposed that the not attached Fl was removed from the samples
The main idea was to improve the interactions between the substrate and the probe molecule (Fl) that may lead toward improving the contribution of reduced graphene oxide (rGO), that is, chemicalenhancing mechanism to the overall enhancement
Summary
One or few layers of graphene[1−5] or graphene oxide (GO)[4,6] have been widely used as active substrates for surface-enhanced Raman scattering (SERS) analysis.[7,8] Graphene-based surfaces, similar to the metallic surfaces, quench the fluorescence background of the analyte.[9] Besides that, graphene-based substrates provide a large surface area, allowing adsorption of a high number of probe molecules.[10,11] The main drawback of graphene-based SERS substrates is that they contribute only to the chemical enhancement (CE) of the signal; the maximum enhancing factors achieved are usually in the range of 102−104.8,10,12,13. Fluorescein (Fl) is a xanthene dye with a large variety of technical applications, among them as a fluorophore in bioscience. Owing to its large fluorescence quantum yield, low toxicity, and high photostability, it has been used as a tracer in groundwater, to label biological compounds, to detect corneal abrasions, and as a dye in food, drugs, and cosmetics. The combination of graphene-based substrates with Fl probe molecules is one of the ways to successfully eliminate the problems of both strong fluorescence background of Fl and weak enhancement of graphene.[5]
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