Localized surface plasmon effect on 3-mercaptopropionic acid and citrate stabilized gold nanoparticles for biosensor application

Herein, we attempt to study the synthesis process of 3-mercaptopropionic acid (3-MPA) capped gold nanoparticles (Au-MPA) and to optimize the preparation of colorimetric sensor based on the localized surface plasmon (LSP) phenomenon compared to the citrate stabilized gold nanoparticles (Au-Citrate). Both Au-Citrate and Au-MPA were prepared using the reduction method. In our case, the product of Au-MPA shows red-wine color of solution with a plasmonic peak at 527[Formula: see text]nm, while Au-Citrate has a plasmonic peak at 519[Formula: see text]nm with the same color of the solution. Both particles have a spherical shape as validated by the TEM image with the average size of Au-Citrate and Au-MPA being 34[Formula: see text]nm and 51[Formula: see text]nm, respectively. Different peak values can be explained by the different size distributions and modified local refractive indices due to the adsorbed ligand on the metal surfaces. Vibrational frequency characterization shows shifting and broadening especially in the region of CO stretching as the result of carboxyl group interaction. Functionalization of AuNPs by the use of biocytin (Biotinylated-AuNPs) shows good stability as revealed with no significant change in the absorbance spectra compared to the AuNP capped by citrate or 3-MPA. The addition of various concentrations of avidin into the assay results in the color changes and the red-shift of the absorbance spectra due to the cross-link aggregation with an intermediate value in the sensing characteristic. This research provides preliminary studies for colorimetric colloidal sensor performances tunability by modifying the adsorbed ligand as the linker between the metal and the bioreceptor.