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.

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.[a] Dr.

Modulation of luminescent characteristics of atomic gold clusters is an important technological challenge for broadening their futuristic applications. Post synthetic modification of histidine stabilized gold nanoclusters via ligand exchange reaction with cysteine is one such viable route towards modulation of the luminescence properties of gold nanoclusters.

Herein, we introduce a unique approach for discrimination between two important biothiols (BTs): cysteine (Cys) and glutathione (Glu), on the basis of changes in chromaticity coordinates of a dual-emitting "a few particle" nanoprobe. A novel dual-channel emission nanocomposite was developed by zinc-mediated conjugation of green-emitting fluorescein with red-emitting gold nanoclusters (Au NCs), hereafter referred to as Zn−FL−Au NCs. Cys and Glu having different interactions with Zn−FL−Au NCs were found to have different effects on the overall luminescence of Zn−FL−Au NCs, which eventually led to observation of different chromaticity indices of the latter upon interaction with these two BTs. For example, upon addition of Cys to a dispersion of Zn−FL−Au NCs having chromaticity coordinates (0.38, 0.49), the luminescence peak due to Au NCs was found to have been selectively quenched, as a consequence of which the chromaticity coordinates of Zn−FL−Au NCs changed to (0.33, 0.52). On the other hand, upon interaction with Glu, the intensity of the luminescence peak due to Au clusters as well as fluorescein (FL) enhanced to a certain extent, however, keeping the chromaticity coordinates almost similar to those of Zn−FL−Au NCs (0.38, 0.47). Allied results were obtained from super-resolution microscopic analysis wherein luminescence of "a few particle" of Zn−FL−Au NCs was used to probe the differential interaction with the aforementioned biothiols, leading to a different degree of changes in the Commission Internationale d'E ́clairage (CIE) coordinates of the former. Also, the specificity of the nanoprobe toward Cys was confirmed by monitoring the interactions of Zn−FL−Au NCs with other interfering chemical compounds. Moreover, the optical properties of Zn−FL−Au NCs were found to be retained following incubation in human blood serum, which further supports the use of this nanoprobe for discrimination of BTs in practical systems.

Photooxidation of ligands stabilizing copper nanoclusters for modulation of luminescence.