Study on transport of molecules in gel by surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS)-based biosensors have recently been extensively developed because of their high sensitivity and nondestructive nature. Conventional SERS substrates are unsuitable for detecting biomolecules directly from human skin. As a result, considerable effort is being devoted on developing a gel-based SERS sensor capable of segregating and detecting biomolecules because of differences in molecular transport phenomena within the gel. However, no comprehensive studies on the transport processes of molecules in gels have been published for gel-type SERS sensors. This paper reports the differences in the transport phenomena of different molecules based on the time change of SERS spectrum intensity. The Au nanorod array substrate was coated with HEC gel to prepare a sample cell to study diffusion. The SERS spectra of aqueous solutions of 9 types of molecules were measured using the prepared sample cells. The rate at which each molecule diffuses into the gel differs depending on the molecule. The time variation of the characteristic SERS peak of each molecule was investigated on the basis of a one-dimensional diffusion model, and the diffusion coefficient D was calculated for each molecule. The diffusion coefficient was compared with the molecular weight and size, and it was discovered that the larger the molecular weight and size, the slower the diffusion, which is consistent with molecular motion theory and the inhibitory effect of the gel substance.