Surface-Enhanced Raman Scattering (SERS) Spectroscopy with Borohydride-Reduced Silver Colloids: Controlling Adsorption of the Scattering Species by Surface Potential of Silver Colloid

Abstract

Formation of Ag colloid/adsorbate SERS-active systems (upon adsorption of the selected adsorbates on the surface of Ag colloidal particles) as a function of (i) NaBH₄ to AgNO₃ molar ratio in the preparation protocol of Ag colloid, and (ii) aging of the colloid has been investigated by Surface-enhanced Raman scattering (SERS) spectroscopy. Oligomeric synthetic polypeptides, bovine serum albumin, phosphate coadsorbed with CuTMePyP [copper(II) derivative of 5,10,15,20-tetrakis-(N-methylpyridinium-4-yl)porphyrin chloride] and borates in systems with N-containing bases were selected as model adsorbates. Both (i) a decrease of NaBH₄ to AgNO₃ molar ratio upon preparation and (ii) aging of Ag colloid affect adsorption of the adsorbates and consequently, their SERS spectra, in the same manner. Aging of Ag colloid is thus viewed as a slow hydrolysis of BH₄^- anions. The actual concentration of BH₄^- in the system is identified as the most important factor controlling adsorption of all the selected adsorbates on the surface of Ag colloid. As this factor can be related to the surface potential, the conditions controlling adsorption of the selected adsorbates are specified in terms of a more negative and/or more positive surface potential of Ag colloidal particles. A more positive surface potential promotes adsorption of polypeptides, bovine serum albumin and phosphate while observation of spectral features of borates in the SERS spectra of N-containing bases in alkaline solutions is conditioned by a more negative surface potential.