Aqueous Growth of Gold Clusters with Tunable Fluorescence Using Photochemically Modified Lipoic Acid-Based Ligands

Abstract

We report a one-phase aqueous growth of fluorescent gold nanoclusters (AuNCs) with tunable emission in the visible spectrum, using a ligand scaffold that is made of poly(ethylene glycol) segment appended with a metal coordinating lipoic acid at one end and a functional group at the other end. This synthetic scheme exploits the ability of the UV-induced photochemical transformation of LA-based ligands to provide DHLA and other thiol byproducts that exhibit great affinity to metal nanoparticles, obviating the need for chemical reduction of the dithiolane ring using classical reducing agents. The influence of various experimental conditions, including the photoirradiation time, gold precursor-to-ligand molar ratios, time of reaction, temperature, and the medium pH, on the growth of AuNCs has been systematically investigated. The photophysical properties, size, and structural characterization were carried out using UV-vis absorption and fluorescence spectroscopy, TEM, DOSY-NMR, and X-ray photoelectron spectroscopy. The hydrodynamic size (RH) obtained by DOSY-NMR indicates that the size of these clusters follows the trend anticipated from the absorption and PL data, with RH(red) > RH(yellow) > RH(blue). The tunable emission and size of these gold nanoclusters combined with their high biocompatibility would make them greatly promising for potential use in imaging and sensing applications.

Gold nanoclusters with bright near-infrared photoluminescence

“…8 In recent years, various approaches have been developed to prepare highly photoluminescent AuNCs with emission in the UV-vis region, for example: (1) engineering the particle surface by using different ligands, such as dendrimers, 9 DNA, 10,11 peptides, and proteins; [12][13][14][15][16] (2) controlling the metal core size; 17 (3) aggregation-induced emission; [18][19][20][21] and (4) rigidification of the Au(I)-thiolate shell. 22,23 Recent advances in the synthesis of AuNCs have enabled the development of NIR-photolumines-cent (NIR PL) AuNCs functionalized with a variety of thiol-containing ligands, including tiopronin, 24 thioctic (lipoic) acid, [25][26][27] polymers, 28 11-mercaptoundecanoic acid, 29 zwitterionic structures, 30 glutathione, [31][32][33] mercaptosuccinic acid, 16 and proteins. 12,[34][35][36] The increase in nonradiative pathways with decreasing energy of emitted light (i.e., the "energy gap law") 37 makes it extremely challenging to prepare AuNCs that emit in the NIR region with high quantum yields (QYs).…”

Shortwave Infrared in Vivo Imaging with Gold Nanoclusters

“…2–5 Au NCs with tunable emission from ultraviolet to near-IR (NIR) region have been reported in the last several decades, and utilization of those Au NCs for in vitro cell labeling and in vivo fluorescence imaging applications is an active research area. 6–10 In earlier reports, Au NCs with emission in the NIR region are particularly being investigated for in vivo imaging applications owing to the enhanced tissue penetration enabled by NIR light. 9,11,12 As in vivo imaging in the SWIR range has further reduced background noise from tissue scattering than traditional visible and NIR imaging, we focus our effort on obtaining Au NCs for SWIR in vivo imaging applications in this work.…”

Gold nanocluster mediated delivery of siRNA to intact plant cells for efficient gene knockdown

“…We synthesized the PEI-AuNCs with a one-step reduction of gold precursor in the presence of three types of lipoic acid-PEI (~2.5K g/mol linear PEI, ~800 g/mol and ~25K g/mol branched PEI) 28, 29 . Lipoic acid groups are utilized because they can strongly coordinate onto the inorganic surfaces of gold nanocrystals and can be easily combined with tunable length polymers 30 .…”