Surface-enhanced Raman spectroscopic sensing of glucose
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.
SERS can be used to sense molecules in trace amounts for biochemical and chemical analysis. Molecular fingerprint specificity is combined with potential sensitivity down to the single molecule level in SERS. Chemists have therefore used this powerful technique in a wide range of applications and in particular biosensing, such as that of glucose, which plays a significant role in metabolic activities in the human body. Glucose has a relatively small Raman cross-section of 5.6×10-30cm2/(molecule-sr),9 which can be enhanced by SERS for sensing purposes. Further optimisation of SERS can be facilitated by molecule adsorption on a nanostructured gold- or silver-coated substrate to ensure enhancement by chemical and electro-magnetic mechanisms.10
Graphene is an emerging substrate for SERS because of its compatibility with a variety of biological and chemical species, its chemical inertness, and the novel presence of a single to few layers of sp2 bonded carbon atoms forming a network sheet.11,12 Graphene, however, has some inherent disadvantages because of its domain structure due to the presence of defects at the edges and variations in the number of layers that can lead to irreproducibility in electronic and optical data. Commercial graphene sheets with uniform multilayers from Graphene Laboratories Inc were found to provide reproducible SERS results.
Here, the use of commercial multilayer graphene sheets as SERS substrates on which gold nanoparticles were assembled by citrate reduction,13,14 is discussed. A Thermo Scientific DXR micro-Raman spectrometer was used to obtain the Raman spectra with 532nm laser excitation at a spatial resolution of 10μm and a spectral resolution of 2cm-1. Improved Raman spectra at very low concentrations down to 10-8M were obtained after chemometric smoothing of the data (Figure 1).
Figure 2a shows the conventional Raman spectrum of pristine multilayered graphene where the graphitic mode at 1,580cm-1 is extremely sharp, indicating highly-ordered carbon layers whereas the so-called G line at 2,720 cm-1 is broadened with many features due to multilayering. Figure 2b shows the conventional spectrum of crystalline glucose with Raman lines in cm-1 together with…