Self-assembled gold nanoparticles and amphiphile peptides: a colorimetric probe for copper(ii) ion detection

Abstract

Morphological, spectroscopic and scattering studies of the self-assembly and aggregation process of hybrids containing gold nanoparticles (AuNPs) and the amyloid peptides [RF]₄ and P[RF]₄ (where R = arginine; F = phenylalanine; P = proline) in aqueous solution were performed. Two methodologies were tested for the AuNP nucleation, using sodium borohydride (NaBH₄) or epigallocatechin gallate (EGCG) as a reducing agent. This led to remarkable distinct modes of assembly, AuNP decorated fibrils with NaBH₄ reduction or isolated AuNPs with EGCG reduction. For both methodologies, the presence of spherical AuNPs was observed by plasmonic resonance bands in absorption spectra at ∼520 nm. Zeta potential measurements confirmed stable systems, with a similar aggregation state. Circular dichroism spectra revealed an antiparallel β-sheet conformation of the peptides. The transmission electron microscopy (TEM) images showed the coexistence of nanometer fibers and globular nanoparticles with 20 nm size. The small-angle X-ray scattering (SAXS) results show that the NaBH₄ systems presented large cylindrical structures, while with increasing P[RF]₄ content, a decrease in radius was observed. However, the EGCG-AuNPs were characterized by spherical particles, with a radius of 10–20 nm. Also, the colorimetric efficiency of the hybrids in the capture of Cu²⁺ ions in solution was monitored. Raman spectroscopy data confirmed the conformation/structure of self-assembled samples. Moreover, there are indications for a surface-enhanced Raman spectroscopy (SERS) effect for Cu²⁺ sites. The set of results indicates that these systems could act as a promising sensitive metal concentration probes.