Influence of pH and aurate/amino acid ratios on the tuneable optical features of gold nanoparticles and nanoclusters

Blue-and green-emitting gold nanostructures have been synthesized by spontaneous interactions ofthe tetrachloroaurate anions (AuCl4-) withL-histidine (His) and L-tryptophan (Trp) at 37 degrees C in aqueousmedium. The dominant role of AuCl4-/amino acid molar ratios as well as the pH in the formation ofgold nanoparticles (Au NPs) or nanoclusters (Au NCs) was confirmed by numerous spectroscopic techniques. We demonstrate that without additional reducing agents the His and Trp show different reductionbehaviour with AuCl4-which supports the fact that the unique optical features originate from diversestructures. Because of the pH-dependent complexation and reduction capability of the studied aminoacids the dominant presence of polynuclear fluorescent gold(I) complexes (quantum yield (QY%) similar to 4%) with a well-ordered structure is presumable for His-directed Au system confirmed by mass spectrometry, X-ray diffraction and infrared experiments. The existence of d = 8 nm-sized Au NPs or subnanometer-sized(d < 2 nm) Au NCs (Au3-Au5) that have tuneable blue or green photoluminescence (lambda(emission)= 470-500 nm) was identified in the samples containing Trp depending on the applied AuCl4-/Trp ratios. The blueemitting Au/His nanohybrid system as potential bioimaging agent was used to visualize fluorescentlyalbuminand chitosan-based carrier composite particles. (C) 2017 Elsevier B. V. All rights reserved.