UV and Sunlight Driven Photoligation of Quantum Dots: Understanding the Photochemical Transformation of the Ligands

Abstract

We have recently reported that photoinduced ligation of ZnS-overcoated quantum dots (QDs) offers a promising strategy to promote the phase transfer of these materials to polar and aqueous media using multidentate lipoic acid (LA)-modified ligands. In this study we investigate the importance of the underlying parameters that control this process, in particular, whether or not photoexcited QDs play a direct role in the photoinduced ligation. We find that irradiation of the ligand alone prior to mixing with hydrophobic QDs is sufficient to promote ligand exchange. Furthermore, photoligation onto QDs can also be carried out simply by using sunlight. Combining the use of Ellman's test with matrix-assisted laser desorption/ionization and electrospray ionization mass spectrometry, we probe the nature of the photochemical transformation of the ligands. We find that irradiation (using either a UV photoreactor or sunlight) alters the nature of the disulfide groups in the lipoic acid, yielding a different product mixture than what is observed for chemically reduced ligands. Irradiation of the ligand in solution generates a mixture of monomeric and oligomeric compounds. Ligation onto the QDs selectively favors oligomers, presumably due to their higher coordination onto the metal-rich QD surfaces. We also show that photoligation using mixed ligands allows the preparation of reactive nanocrystals. The resulting QDs are coupled to proteins and peptides and tested for cellular staining. This optically controlled ligation of QDs combined with the availability of a variety of multidentate and multifunctional LA-modified ligands open new opportunities for developing fluorescent platforms with great promises for use in imaging and sensor design.

Ultraefficient Cap-Exchange Protocol To Compact Biofunctional Quantum Dots for Sensitive Ratiometric Biosensing and Cell Imaging

“…46 By contrast, the photoligation method developed by the Mattoussi group can work directly with the air-stable LA form of ligands. 46−49 However, the requirement for a large excess of ligands (LQMR = 2–4 × 10 4 , Table 1) can limit its application with precious or expensive ligands. Thus, an efficient cap-exchange method that uses the minimum amount (ca.…”

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

“…47 Indeed, this photoligation strategy has provided high quality hydrophilic nanocrystals, while preserving the photophysical properties of the QDs and the integrity of sensitive functional groups. 47,48 We have also found that photoirradiation of the lipoic acid generates several byproducts, including thiol radicals, monomeric dihydrolipoic acid, and oligomers. 48 Here, we build on those developments and report the use of photochemically modified LA-PEG ligands, combined with a one-pot aqueous growth, to prepare fluorescent AuNCs with tunable properties.…”

“…47,48 We have also found that photoirradiation of the lipoic acid generates several byproducts, including thiol radicals, monomeric dihydrolipoic acid, and oligomers. 48 Here, we build on those developments and report the use of photochemically modified LA-PEG ligands, combined with a one-pot aqueous growth, to prepare fluorescent AuNCs with tunable properties. The reaction is borohydride-free and relies on the photochemical modification of LA-PEG ligand prior to mixing with the Au precursors.…”

“…It indicates that the amine groups may be strongly interacting with radical sulfur groups formed during UV-irradiation of the dithiolane ring. 48 At this point we do not have a solid chemical rationale to what exactly happens to the reaction and how the presence of amine groups promotes reduction in cluster size with growth time, instead of the "expected" increase in size and red-shift in the PL emission. We are exploring ideas and possible rationales to understand what exactly happens to the growth when amine-modified ligands are used.…”

Toward Biocompatible Semiconductor Quantum Dots: From Biosynthesis and Bioconjugation to Biomedical Application

“…With the replacement of primary hydrophobic ligand (e.g., TOPO) by DHLA-PEG on the QD surface, the QDs can be easily transferred from hexane to methanol. 156 After the purification and removal of organic solvents, the obtained QDs demonstrate good water solubilization. Notably, both UV and sunlight can initiate the reaction, and the addition of a small amount of tetramethylammonium hydroxide may shorten the period of ligand exchange.…”