Direct SERS Detection of Folic Acid Using Modified Graphene Oxide-Gold Nanobowl Composites

Gold nanobowls (AuNBs) synthesized by the template-free method were deposited on graphene oxide (GO) to obtain an ultrasensitive surface enhanced Raman spectroscopy (SERS) platform for folic acid (FA) detection. GO was conditioned in aqueous solutions at various pH values to optimize the adsorption of the FA molecule and the intensity of the SERS signal. It was found that the conditioning procedure influences the orientation of FA on the SERS supports and the quality of the spectra in result. The limit of detection (LOD) for FA was equal to 10^-12 M for the best combination of GO conditioned at pH = 1 and AuNBs (GO-1/AuNBs). The composites of AuNBs with GO conditioned in solutions of higher pH were less efficient. The LOD values were equal to 10^-8 M for AuNBs deposited on GO conditioned at pH = 5.6 (GO-5.6/AuNBs) and 10^-7 M for AuNBs deposited on GO conditioned at pH = 13 (GO-13/AuNBs). The high sensitivity of the GO-1/AuNBs composite has been explained by the high adsorption of FA and the molecular orientation with a p-aminobenzoic ring attached to the surface. We suppose that such an orientation fosters efficient charge transfer between the adsorbed molecule and the support, which in turn enhances the SERS intensity by the chemical effect in addition to the electromagnetic enhancement yield by AuNBs. The adsorbed FA molecules attain the random conformation or adsorption through the pteridine ring on the supports with GO conditioned at higher pH. Such orientation results in less efficiency in obtaining the enhancement through the chemical mechanism.