Visible Light Excitation-Induced Luminescence from Gold Nanoclusters Following Surface Ligand Complexation with Zn²⁺ for Daylight Sensing and Cellular Imaging

Abstract

Herein, we report a complexation reaction-mediated extended aggregation of gold nanoclusters exhibiting luminescence under visible light excitation. The complexation reaction between the carboxylate groups of mercaptopropionic acid and zinc ions induced the aggregation of gold nanoclusters, which featured bright green luminescence upon excitation with visible light of wavelength 450 nm and beyond. This luminescence of aggregated Au NCs, easily discernible with bare eyes (under broad daylight excitation), was used as a probe for luminescence-based detection of molecules based on the pK a values of the latter. This aspect has been an unfilled dream of scientists pursuing research on the development of nanoscale sensors, as luminescence-based detection techniques offer a greater degree of accuracy and sensitivity compared to absorption-based methods, and was thus far an unexploited/untapped area by nanoscale materials. Moreover, facile imaging of mammalian cells was achieved using these aggregated clusters upon excitation with visible light. This study demonstrates the utility of luminescent nanoclusters, akin to organic dyes, as materials active under visible light excitation. Thus, the complexation reaction-based tailoring of the optical properties of nanoclusters served as an effective tool in pushing the absorption maxima of the nanoclusters from an ultraviolet to visible range, enabling the luminescence of nanoclusters under broad daylight excitation. Hence, the work embodied herein offers a unique route to widen the application potential of metal nanoclusters as sensors and bioimaging agents operating under visible light excitation.

Controlling the Chemistry of Nanoclusters: From Atomic Precision to Controlled Assembly

“…( c ) Schematic illustration depicting possible mode of reaction among Zn Au NCs and aforementioned molecules. Reprinted with permission from [ 81 ]. Copyright 2019 American Chemical Society.…”

“…On the other hand, amino acids with a pKa greater than that of MPA did not react with the surface MPA molecules, thereby causing no discernible effect on the luminescence of Zn Au NCs. Thus, based on the differential reactivity of the MPA molecules present on the surface of Zn Au NCs towards amino acids with varying pKa, the latter could be distinguished easily ( Figure 4 b) [ 81 ].…”

“…To a similar end, the reactivity of the MPA molecules residing on the surface of allied Zn Au NCs probes, were also used to discriminate between geometrical isomers. However, unlike the case of amino acids, the proton-transfer reaction in the current report was further governed by intra molecular hydrogen bonding interactions among the analytes ( Figure 4 c) [ 81 ]. Interestingly, in a different study, Antoine and coworkers, measured the two photon excited fluorescence cross sections of the Zn Au NCs aggregates.…”

Fullerene-Mediated Aggregation of M25(SR)18– (M = Ag, Au) Nanoclusters

“…Chandra et al reported self-assembly of water-soluble Au clusters into spherical colloidal superstructures with high photocatalytic activity and applications in bioimaging by utilizing the interaction of the clusters with Sn 2+ ions . Other cations like Zn 2+ were also used to study phenomena such as aggregation-induced emission in Au clusters. , Anionic templates like CO 3 2– were used to link Ag 17 S 8 clusters into a two-dimensional (2D) honeycomb-like structure . Nonappa et al reported the formation of spherical capsids and 2D monolayer thick sheets of clusters which were formed due to weak hydrogen bonding interactions .…”

Zinc‐Ion‐Induced Aggregation of Gold Clusters for Visible‐Light‐Excitation‐Based Fluorimetric Discrimination of Geometrical Isomers

“…Recently, our research group has developed zinc‐induced aggregates of nanoclusters as highly luminescent probes active under visible light for visual fluorimetric sensing of molecules . It has been demonstrated that molecules with varying pKa, interact differently with the ligands present on the surface of zinc‐induced nanoaggregates of clusters, eventually leading to different fluorescence response of the latter.…”