Solvent-Induced Aggregation of Self-Assembled Copper-Cysteine Nanoparticles Reacted with Glutathione: Enhancing Linear and Nonlinear Optical Properties

Copper-thiolate self-assembly nanostructures are a unique class of nanomaterials because of their interesting properties such as hierarchical structures, luminescence, and large nonlinear optical efficiency. Herein, we synthesized biomolecule cysteine (Cys) and glutathione (GSH) capped sub-100 nm self-assembly nanoparticles (Cu-Cys-GSH NPs) with red fluorescence. The as-synthesized NPs show high emission enhancement in the presence of ethanol, caused by the aggregation-induced emission. We correlated the structure and optical properties of Cu-Cys-GSH NPs by measuring the mass, morphology, and surface charge as well as their two-photon excited fluorescence cross-section (sigma(2PEPL)), two-photon absorption cross-section (sigma(TPA)) and first hyperpolarizability (beta) of Cu-Cys-GSH NPs in water and water-ethanol using near-infrared wavelength. We found a high beta value as (77 +/- 10) x 10(-28) esu (in water) compared to the reference medium water. The estimated values of sigma(2PEPL) and sigma(TPA) are found to be (13 +/- 2) GM and (1.4 +/- 0.2) x 10(4) GM, respectively. We hope our investigations of linear and nonlinear optical properties of copper-thiolate self-assemblies in water and its solvent-induced aggregates will open up new possibilities in designing self-assembled systems for many applications including sensing, drug delivery, and catalysis.