Adsorption of dansylated amino acids on molecularly imprinted surfaces: a surface plasmon resonance study

Surface plasmon resonance spectroscopy (SPR) was used to measure the adsorption kinetics and isotherms of dansylated amino acids onto surface-confined molecularly imprinted polymer films (MIP-Fs) and the corresponding non-imprinted polymer control films (NIP-Fs). The surface-confined polymer films were grafted from flat gold surfaces using atom transfer radical polymerization (ATRP). This approach allowed uniform nanothin films to be grown, thereby ensuring that the amino acids see a uniform surface during adsorption. N,N'-Didansyl-l-cystine (DDC) and didansyl-l-lysine (DDK) were used as the template molecules to form the MIP-Fs. Adsorption kinetics data were analyzed using single- and dual-site Langmuir adsorption models. It was found that, within the experimental measurement range, adsorption isotherm data were well described by any of four isotherm models: Langmuir, dual-site Langmuir, Freundlich, or Langmuir-Freundlich (LF). The relatively high heterogeneity index values regressed using the Freundlich and LF isotherms suggest the formation of fairly homogeneous MIP-Fs; although Scatchard analysis reveals binding site heterogeneity does exist. Selectivity studies showed that the MIP-Fs display cross-reactivity between DDC and DDK; nevertheless, MIP-Fs prepared against one template showed selectivity for that template. Solution pH and polymer layer thickness were studied as independent parameters to determine their impacts on amino acid adsorption, as monitored by SPR.