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

## TL;DR

This paper studies how cationic metal complexes interact with gold nanoparticles, revealing a spontaneous self-assembly process and how these complexes affect catalytic reactions.

## Contribution

The study introduces a detailed analysis of the self-assembly mechanism and binding behavior of various cationic metal complexes on gold nanoparticles.

## Key findings

- The interaction process involves three steps: induction time, flocculation, and agglomeration.
- The Gibbs free energy of reaction is negative, indicating a spontaneous agglomeration process.
- Most complexes show independent agglomeration, but complex 5 exhibits a positive binding propensity.

## Abstract

This work aims to
study the interaction between cationic
metal
complexes (Mz+) and gold nanoparticles
(AuNPsz–). The Mz+ 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 Mz+ and AuNPsz– 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 Mz+ and AuNPsz– 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 (Kf), which is dependent on the temperature and geometric structure
of each group of Mz+, and the
Gibbs free energy of reaction (ΔGr), which was negative for each pair of Mz+ and AuNPsz–, 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 AuNPsz– 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 AuNPsz– was used in the presence of Mz+ as an evaluation test to show how the
Mz+ species will disturb the
4-NP– binding site on the surface of gold nanoparticles.