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

This work aims to study the interaction between cationic metal complexes (M ⁺) and gold nanoparticles (AuNPs ). The M ⁺ 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 ⁺ and AuNPs 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 ⁺ and AuNPs 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 ⁺, and the Gibbs free energy of reaction (ΔG ), which was negative for each pair of M ⁺ and AuNPs , 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 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 was used in the presence of M ⁺ as an evaluation test to show how the M ⁺ species will disturb the 4-NP^- binding site on the surface of gold nanoparticles.