Ligand effect on the size, valence state and red/near infrared photoluminescence of bidentate thiol gold nanoclusters

Abstract

Synthesis and characterization of gold nanoclusters (Au NCs) stabilized by a zwitterion ligand (Zw) at different Au : Zw ratios are demonstrated. Au NCs exhibit photoluminescence (PL) emission which is tunable from the near infrared (805 nm) to the red spectral window (640 nm) and strongly influenced by the ligand shell size. Optical and chemical investigations suggest the presence of gold polymeric species and large nanoclusters for a molar ratio of Au : Zw = 1 : 1. For 1 : 5 < Au : Zw < 1 : 1, Zw induces etching of the large clusters and the formation of a monolayer of the bidentate ligands on the Au NCs (cluster size ∼7 to 10 kDa) accompanied by red PL emission at λ = 710 nm. A second organic layer starts to form for larger Zw fractions (Au : Zw < 1 : 5) as a result of electrostatic and covalent interactions of the zwitterion leading to an enhancement and a blue-shift of the PL emission. The effect of temperature and pH on the optical properties of gold clusters is strongly dependent on the ligand shell and demonstrates the importance of defining gold nanoclusters as supramolecular assemblies with a complex environment.

Gold nanoclusters as a contrast agent for image-guided surgery of head and neck tumors

“…The nature of the biocompatible ligands protecting the gold metal core and their density have been tuned to construct 4 different structures that were compared to the well-defined Au25SG18 NCs. An increase of zwitterionic or pegylated coating on the gold core leads a photoluminescence enhancement of the AuNCs due to a more efficient energy transfer of the gold shell, and a better protection from the environment (28,29). As expected, a thicker ligand coating on AuNCs that still remains quite small (<22 kDa) seems to enhance the interaction with cell surface, through stronger cell internalization with zwitterionic ligand and antifouling effect for charged pegylated ligand.…”

“…All types of AuNCs emit in the red/near-infrared region with a blue-shift and an increase of their brightness for thick organic shell ( Figure S3, Table 1). This behavior has been extensively reported and is correlated to the nature, the density, and the rigidity of the ligands stabilizing the metal core of AuNCs (28)(29)(30).…”

Shortwave Infrared in Vivo Imaging with Gold Nanoclusters

“…21–25 Zwitterionic and bidentate thiol molecules in particular, have been explored as surface ligands and were shown to result in a higher QY and longer wavelength emission. 26,27 We hypothesized that these zwitterionic and bidentate thiol ligands may enable expansion of the emission spectrum of Au NCs to the SWIR range. Here we report that Au NCs capped with a suitable shell of zwitterionic ligands (Scheme 1) can have an emission peak in the range of 900–1000 nm with acceptable PL QYs and excellent stability under physiological conditions.…”

“…28–30 In this study, we first synthesized lipoic acid-based sulfobetaine (LA-sulfobetaine)-capped SWIR-emitting Au NCs in water by modifying a literature method that reduces chloroauric acid in the presence of LA-sulfobetaine. 26 These LA-sulfobetaine capped Au NCs have an average inorganic core size of 1.6 nm (Figure S1). Their emission peak is centered at ~1000 nm (Figure 1a) with an initial QY of 0.6%.…”

Multivalent Glycosylation of Fluorescent Gold Nanoclusters Promotes Increased Human Dendritic Cell Targeting via Multiple Endocytic Pathways

“…. In the latter example, extensive experimental studies supported by theoretical models 30,35 demonstrate that both the NC core and the thiolated gold (I) shell contribute to their photoluminescence properties, with the shell being especially important for red-NIR region emissions [36][37][38][39] . For instance, the Mattoussi lab reported the preparation of red-emitting Au NCs with a quantum yield of up to 14% when stabilised with bidentate thiolated ligands containing a zwitterionic group 40 .…”