Electrostatically mediated layer-by-layer assembly of a bioinspired thymine polycation and gold nanoparticles

Abstract

In this work, we built self-assembled structures of gold nanoparticles (AuNPs) using a new polyelectrolyte formed by copolymerization of the monomers 4-vinylbenzyl thymine (VBT) and 4-vinylbenzyl triethylammonium chloride (VBA). For the synthesis, a monomer ratio of 1 mol of VBT to 16 mol of VBA was used to obtain the polycation [(VBT)1:(VBA)16]16+ with a charge density suitable for the electrostatic adsorption of citrate stabilized AuNPs. The objective of this work was to explore the optical and electrochemical properties of the structure built on gold and quartz substrates with a view to its potential application as a (bio)sensor for optical and electrochemical detection. The polycation adsorption process was studied by surface plasmon resonance, which allowed us to select the appropriate concentration and adsorption time for the polymer and verify the formation of the self-assembly. Characterization of the system by atomic force microscopy allowed determining that AuNPs are distributed in isolation, such as dimers, trimers, tetramers, or bigger aggregates. The analysis of the resonance of the localized surface plasmons (LSPR) of the AuNPs, as well as the increase in the intensity of the polycation Raman signals revealed the electromagnetic coupling between close nanoparticles. In addition, the electrochemical impedance spectroscopy study of gold electrodes modified with the ([(VBT)1:(VBA)16]16+/AuNPs)n multilayer showed that incorporation of the AuNPs in the structure produced an increase in the apparent heterogeneous charge transfer constant. This fact indicates that the structure of the film and the nanoparticles distribution favor the electronic conduction mediated by AuNPs. The observed properties indicate that this multilayer represents a very interesting platform for the development of (bio)sensors based on optical or electrochemical detection.