Mercaptoacetic acid-capped silver nanoparticles colloid: Formation, morphology, and SERS activity

Mercaptoacetic acid-capped spherical silver nanoparticles with a diameter of about 17 nm were prepared by a simple chemical reaction. The formation process of the silver nanoparticles was investigated by UV-visible (UV-vis) spectroscopy and transmission electron microscopy. The results show that the spherical and rodlike particles were formed at the beginning of the reaction, and then the rodlike particles were gradually converted into spherical particles with the reaction continuing. Finally, the content of the rodlike particles was less than 3% in the silver colloid. Thus, the final obtained silver nanoparticles were uniform in their shapes and showed little variation in their sizes. This silver colloid can remain stable for several weeks, which makes it convenient for use in practice. We also investigated the effect of Cl- on the rate of changes in the optical properties of the silver colloid by UV-vis absorption. The results indicate that Cl- accelerates the aggregation of the colloidal particles by effectively screening the repulsive electrostatic interactions between the negatively charged silver particles. We used the self-assembled technique to transfer the silver nanoparticles onto solid substrates from the colloid with and without Cl- in the solution. The UV-vis spectra show that the absorption band red shifts and a new band appears at a longer wavelength when the silver nanoparticles are transferred onto the substrate from the colloid with Cl- in the solution, indicating that the silver nanoparticles can grow and aggregate on the substrate. This was further confirmed by the atomic force microscopy measurements. Both the silver colloid and the substrates prepared by transferring the silver nanoparticles from the silver colloidal solution onto the quartz slides can serve as surface-enhanced Raman spectroscopy (SERS)-active substrates. It was found that the SERS enhancement depends on the sizes and aggregation of the silver particles, and the addition of Cl- generates much stronger SERS signals.