Interaction of N ‐benzylamino(boronphenyl)methylphosphonic acid analogs with the gold colloidal surface under different concentration and pH conditions

In this study, we present surface‐enhanced Raman spectroscopy (SERS) investigations of five analogs of a novel group of N ‐benzylamino(boronphenyl)methylphosphonic acids: N ‐benzylamino‐(3‐boronphenyl)‐ S ‐methylphosphonic acid ( m ‐Ph S ), N ‐benzylamino‐(4‐boronphenyl)‐ S ‐methylphosphonic acid ( p ‐Ph S ), N ‐benzylamino‐(2‐boronphenyl)‐ R ‐methylphosphonic acid ( o ‐Ph R ), N ‐benzylamino‐(3‐boronphenyl)‐ R ‐methylphosphonic acid ( m ‐Ph R ), and N ‐benzylamino‐(4‐boronphenyl)‐ R ‐methylphosphonic acid ( p ‐Ph R ) deposited onto 10‐nm gold nanoparticles in an aqueous solution at physiological pH (pH = 7). In addition, for the p ‐Ph R molecule, the SERS spectra in the various conditions of pH levels of the solutions (from pH = 3 to 11) and phosphonic acid concentration (10 −3 –10 −5  M) were measured. In general, the SERS spectral profiles indicate that at pH = 7, all of the aforementioned molecules interact with the colloidal gold surface via a boronphenyl ring. However, the orientation of the boronphenyl ring onto the substrate surface is different for each of the studied molecules. The boronic acid group of p ‐Ph S and p ‐Ph R binds to the gold nanoparticles, whereas the phosphonic acid group assists in the interaction with the substrate for p ‐Ph R only. For all the molecules, the –C L C α (P)N– fragment distinctly influences the molecule/gold interactions, especially in the case of o ‐Ph R . The previous differences in the compound behavior at the gold/liquid interface underline the role of an absolute configuration (– R and – S ) and a type of isomer ( orto ‐, meta ‐, and para ‐) on the SERS signals, which means their influence on the adsorbate geometry. Additionally, based on the SERS results for p ‐Ph R in various environmental conditions, we draw conclusions about spectral changes (adsorbate geometry changes) as a result of the pH of solution and molecule concentration alternations. Copyright © 2014 John Wiley & Sons, Ltd.