To further test the distribution of a drop of an aqueous solution of CV, we dropped a drop of 4 ml CV solution on a normal and agar coated paper. The spot of the drop on the normal filter paper dispersed larger (5 mm) than that on the agar coated filter paper (2 mm) upon drying for the same time (9 min). The hydrophobic agar film has the ability to concentrate dissolved CV solutes of much diluted aqueous solutions, thus preventing the diffusion of analyte over a large area and hence acting as an analyte concentrator. Such effect of concentrating analyte using Teflon film for enhanced SERS analysis has been studied by Lu et al. [28].The agar film on filter paper has an excellent molecular trapping capability for enhanced Raman signals as exhibited for relatively small explosive molecule, TNT, a dye CV and a pesticide,chloropyrifos. To test the stability and long term shelf life of the fabricated substrate, a freshly prepared test stripe and a one month older stripe, stored in dark, was used for SERS measurements of
103 M CV dye solution. The intensity of peaks in both the cases almost coincides with each other, indicating retaining the SERS capability of stripes even during storage (Fig. S4). This stability is attributed to the polymeric framework of agar in which silver nanoparticles are strongly trapped in its pores and hence acting as a binder. The silver nanoparticles trapped inside agar gel did not loosened even after a long time, which was reported as a major
drawback using other filter paper based SERS substrates [21]. In normal filter paper doped with Ag-NP, the particles are loosely attached to the fibrils of filter paper. Upon long storage, they detach from the surface and a significant loss in the SERS activity has been observed over time. The presence of agar film on the filter paper did not hinder its flexibility and the fabricated SERS stripes are flexible as they can be rolled easily. The observed enhanced
benefit of our fabricated SERS test stripes as compared to the reported normal paper based SERS substrates can be explained by multifunctional role played by agar film. The film acts as a reducing agent, SERS ‘‘hot-spots inducer, binder and concentrator of the analyte. The high SERS activity of the substrate is supplemented by its simple nature of synthesis, dip coating method. The facile synthesis, ultrasensitive detection and stability over a long period
of time made the fabricated SERS substrate an ideal candidate for the ultra trace detection.