The small normal Raman cross-section of glucose is a major challenge in its detection by surface enhanced Raman spectroscopy (SERS) for medical applications, such as blood glucose level monitoring of diabetic patients and evaluation of patients with other medical conditions, since glucose is a marker for many human diseases. Here we will discuss the use of commercially available multilayer graphene sheets as substrates on which gold nanoparticles are chemically assembled by reduction of sodium citrate.
Results show that these substrates are capable of providing SERS enhancement factors up to 1010 with a lower limit of detection (LOD) of 10-8M in aqueous solutions of glucose. The LODs on graphene are many orders of magnitude lower than values obtained on gold-coated chemically etched Klarite silicon substrates, marketed by Renishaw Diagnostics, which are widely-used commercial SERS substrates. The glucose spectra over a range of concentrations in the 400-1500cm-1 fingerprint region were recorded using 532nm laser excitation, 10mW laser power and a 50x microscope objective. The intensity of the 1,340cm-1 line of glucose varied linearly with glucose concentration and can be used as a calibration for samples of unknown concentrations. Chemometric methods were used to provide improved spectra at very low concentrations. Graphene can also provide fractional charge transfer (CT) effects to glucose to provide secondary enhancement of the Raman spectra.
The discovery of SERS in the late 1970s enabled enhancement of the Raman scattering cross-section of adsorbed pyridine1 on a silver electrode by five to six orders of magnitude,1-4 which was explained in terms of the local plasmonic electromagnetic (EM) field on a rough surface.5,6 There have been heated debates over the details of the SERS mechanism; however, it is now generally accepted to be a combination of two mechanisms – charge transfer (CT) and electromagnetic (EM) enhancement. The origin of EM, a key contributor to enhancement of Raman signals, is in the magnified electromagnetic field with light-excited surface plasmon resonance.7 A typical location of strong electromagnetic fields is in nano-gaps between the metal nanostructures (so-called ‘hotspots’) and at their sharp corners.8 Coexistence of CT with EM creates a typical SERS system on the basis of the transfer of charge between SERS substrate and the analyte molecules that lead to high enhancement factors.
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Source: European Pharmaceutical Review