The Self-Assembly of Cationic Metal Complexes on Gold Nanoparticle Surface

This work aims to study the interaction between cationic metal complexes (M^z+) and gold nanoparticles (AuNPs^z–). The M^z+ complexes were chosen from previous works described in the literature and were synthesized as defined. For example, they are as follows: 1 = [RuCl(dppb)(bipy)(py)](PF₆); 2 = [RuCl(dppb)(bipy)(vpy)](PF₆); 3 = [RuCl(dppb)(bipy)(mepy)](PF₆); 4 = [RuCl(dppb)(bipy)(tbpy)](PF₆); 5 = [RuCl₂(dppb)(bipy)](PF₆); 6 = [Fe(bipy)₃]Cl₂; 7 = [Ru(bipy)₃](PF₆)₂; 8 = [TPyP{RuCl(dppb)(bipy)}₄](PF₆)₄; and 9 = [RuCl(p-cymene)(Diipmp)](PF₆). The interactions between M^z+ and AuNPs^z– were carried out using conductometry and UV–vis spectroscopy. These experiments allowed determination of kinetic parameters, revealing three different steps in the interaction process: induction time, flocculation, and agglomeration. The self-assembly between M^z+ and AuNPs^z– was investigated using three different models of binding site, namely, Langmuir or direct plot, Benesi–Hildebrand, and Scatchard. These models provide the fraction of total binding sites occupied (θ), the formation constant (K_f), which is dependent on the temperature and geometric structure of each group of M^z+, and the Gibbs free energy of reaction (ΔG_r), which was negative for each pair of M^z+ and AuNPs^z–, revealing a spontaneous agglomeration process. The Hill coefficient (n) was 1 for almost all complexes, indicating that agglomeration is an independent process, except for 5, where n = 2, suggesting a positive propensity to bind onto the AuNPs^z– surface. The models have confirmed a noncovalent interaction between these species. The relative error in site binding does not show any variation with changes in the temperature, but a fine-tuning of the n value to 1.00 was observed with the increase of the temperature. Finally, the reduction reaction of the 4-nitrophenolate anion (4-NP^–) by NaBH₄ catalyzed by AuNPs^z– was used in the presence of M^z+ as an evaluation test to show how the M^z+ species will disturb the 4-NP^– binding site on the surface of gold nanoparticles.