Tailoring the luminescence of atomic clusters via ligand exchange reaction mediated post synthetic modification.

The growing prominence of atomic nanoclusters in fields of practical relevance has made modulation of their luminescent characteristics an important challenge for their future applications. Herein we report chemical reaction assisted modulation of luminescence of histidine stabilized gold nanoclusters via a ligand exchange reaction with cysteine. Upon addition of 3.8 mM cysteine, as evinced by X-ray photoelectron spectroscopy in conjunction with transmission electron microscopic analyses, histidine molecules were found to desorb from the surface of the Au NCs leading to the latter's aggregation into macroscopic units. Consequently, the luminescence of the His Au NCs underwent a large bathochromic shift from 475 nm to 500 nm with a concomitant decrease in the luminescence intensity. Thereafter, upon addition of 18 mM cysteine to a dispersion of His Au NCs, cysteine molecules by virtue of strong aurophilic interactions were found to adsorb on to the surface of the Au NCs, leading to the disaggregation of the macroscopic structures. This was accompanied by restoration of the luminescence features of the Au NCs to an emission maximum of 486 nm with partial recovery of the luminescence intensity. Thus, the work embodied herein demonstrates post-synthetic chemical reactions of nanoclusters as an effective and viable tool for tailoring the photoluminescence of atomic clusters to meet application demands.

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

Herein, we report discrimination of dicarboxylic acids - fumaric acid (FA) and maleic acid (MA) - exhibiting geometrical isomerism, using nanoclusters based luminescent probe having excitation under broad day light. The luminescent probe was designed via complexation reaction between zinc ions and ligands (mercaptopropioinc acid; MPA) stabilizing the gold nanoclusters. This resulted in formation of nanoaggregates exhibiting bright green luminescence upon excitation at 450 nm capable of discriminating between FA and MA upto nanomolar level. The basis of discrimination has been attributed to deprotonation of FA and MA following interaction with MPA moieties present on the surface of the nanoaggregates and being governed by the stability of the respective conjugate base of the geometrical isomers of the dicarboxylic acids. As a consequence of different extent of deprotonation of FA and MA upon interaction with the cluster aggregates, different effect on the luminescence of the aggregates was observed, thus enabling discernible fluorimetric discrimination between FA and MA under visible light excitation.

Photo induced chemical modification of surface ligands for aggregation and luminescence modulation of copper nanoclusters in the presence of oxygen.

Surface modification of nanoparticles has been a popular approach to tailor the properties of nanoparticles. Herein we report the unprecedented photo oxidation of cysteine moeties on the surface of copper nanoclusters (Cu NCs) leading to aggregation of Cu NCs, which further led to quenching of luminescence of the latter. Upon illumination of a dispersion of Cu NCs at 365 nm wavelength light, the luminescence of Cu NCs was completely quenched. Furthermore, the extent of luminescence quenching of Cu NCs upon photo illumination could be tuned by varying the area of exposure of light. Confirmation of photooxidation of cysteine molecules was made through Fourier transformed infrared (FTIR) studies, while the formation of submicron sized aggregates of Cu NCs as a result of photo oxidation of cysteine stabilizing the nanoclusters was evinced through transmission electron microscopy (TEM). The study embodied herein opens up new avenues for the tailoring of the chemical and optical properties of metal nanoclusters through chemical transformation of surface ligand moieties, which is envisioned to emerge as a powerful strategy for broadening the application potential of metal nanoclusters.

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

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 p Ka 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.

Crystallization-Induced Emission Enhancement of Nanoclusters and One-Step Conversion of "Nanoclusters to Nanoparticles" as the Basis for Intracellular Logic Operations.

Herein, we report the construction of intracellular logic operations using luminescent histidine stabilized gold nanoclusters (His Au NCs). The luminescence intensity of His Au NCs was found to be significantly enhanced following interaction with zinc ions, owing to "Crystallization induced emission enhancement". Further, the luminescence intensity of His Au NCs was found to be effectively quenched in presence of sulphide ions, owing to transformation of emissive His Au NCs to non-emissive gold nanoparticles. Thus, the collective and individual effects of zinc ions and sulphide ions causing significant variation in the luminescence intensity of His Au NCs, were used as input parameters for construction of intracellular logic operations such as Tri state buffer, "on-off" switch and INHIBIT gate within mammalian cells.