The interplay of chromophore-spacer length in light-induced gold nanocluster self-assembly.

The light-induced self-assembly of chromophore-tethered precision nanoclusters (NCs) has recently received significant attention due to their facile control over structure, function, and reversibility under ambient conditions. However, the magnitude of assembly depends on the photoswitching efficiency, chemical structure, and proximity of the chromophore to the NC surface. Herein, using azobenzene alkyl monothiol (AMT)-capped gold NCs with two different spacer lengths (denoted as C3-NC and C9-NC), we show that reversible cis ↔ trans isomerization efficiency can be readily tuned to control the self-assembly kinetics of NCs. Irrespective of the chain length, the time required for trans-to-cis (140 s) and cis-to-trans (260 s) isomerization of individual C3-AMT and C9-AMT is identical in dichloromethane solution. When a similar experiment was performed using a solution of C3-NCs and C9-NCs, it resulted in self-assembled disc-like superstructures. Notably, the trans-to-cis photoswitching in C3-NC could reach only 65% even after 460 seconds of irradiation. On the other hand, C9-NC completed this process within 160 seconds of irradiation. The low photoswitching efficiency of the C3-NC analog is due to the short and rigid spacer length of C3-AMT ligands, which are in close proximity to the NC surface, resulting in steric hindrance experienced at the NC-chromophore interface. Importantly, the slow photoswitching in C3-NCs helps isolate and investigate the intermediates of assembly. Using high-resolution electron microscopy, atomic force microscopy, and 3D reconstruction, we show that the discs are made up of densely packed arrays of NCs. The prolonged illumination of C9-NCs results in a chain-like assembly due to the dipolar attraction between the previously assembled superstructures. The efficient photoisomerization of chromophores located away from the nanocluster surface has been identified as the key element to speed up the light-induced assembly in chromophore-tethered nanoclusters. Such information will be useful while developing nanoscale photoswitches for electrochemistry, biosensors, and electronic devices.

Platinum-Grafted Twenty-Five Atom Gold Nanoclusters for Robust Hydrogen Evolution.

A robust hydrogen evolution is demonstrated from Au25(PET)18]- nanoclusters (PET = 2-phenylethanethiol) grafted with minimal platinum atoms. The fabrication involves an electrochemical activation of nanoclusters by partial removal of thiols, without affecting the metallic core, which exposes Au-sites adsorbed with hydrogen and enables an electroless grafting of platinum. The exposed Au-sites feature the (111)-facet of the fcc-Au25 nanoclusters as assessed through lead underpotential deposition. The electrochemically activated nanoclusters (without Pt loading) show better electrocatalytic reactivity toward hydrogen evolution reaction than the pristine nanoclusters in an acidic medium. The platinum-grafted nanocluster outperformed with a lower overpotential of 0.117 V vs RHE (RHE = Reversible Hydrogen Electrode) compared to electrochemically activated nanoclusters (0.353 V vs RHE ) at 10 mA cm-2 and is comparable with commercial Pt/C. The electrochemically activated nanoclusters show better reactivity at higher current density owing to the ease of hydrogen release from the active sites. The modified nanoclusters show unique supramolecular self-assembly characteristics as observed in electron microscopy and tomography due to the possible metallophilic interactions. These results suggest that the post-surface modification of nanoclusters will be an ideal tool to address the sustainable production of green hydrogen.

Luminescent quantum dots: Synthesis, optical properties, bioimaging and toxicity.

Luminescent nanomaterials such as semiconductor nanocrystals (NCs) and quantum dots (QDs) attract much attention to optical detectors, LEDs, photovoltaics, displays, biosensing, and bioimaging. These materials include metal chalcogenide QDs and metal halide perovskite NCs. Since the introduction of cadmium chalcogenide QDs to biolabeling and bioimaging, various metal nanoparticles (NPs), atomically precise metal nanoclusters, carbon QDs, graphene QDs, silicon QDs, and other chalcogenide QDs have been infiltrating the nano-bio interface as imaging and therapeutic agents. Nanobioconjugates prepared from luminescent QDs form a new class of imaging probes for cellular and in vivo imaging with single-molecule, super-resolution, and 3D resolutions. Surface modified and bioconjugated core-only and core-shell QDs of metal chalcogenides (MX; M = Cd/Pb/Hg/Ag, and X = S/Se/Te,), binary metal chalcogenides (MInX2; M = Cu/Ag, and X = S/Se/Te), indium compounds (InAs and InP), metal NPs (Ag, Au, and Pt), pure or mixed precision nanoclusters (Ag, Au, Pt), carbon nanomaterials (graphene QDs, graphene nanosheets, carbon NPs, and nanodiamond), silica NPs, silicon QDs, etc. have become prevalent in biosensing, bioimaging, and phototherapy. While heavy metal-based QDs are limited to in vitro bioanalysis or clinical testing due to their potential metal ion-induced toxicity, carbon (nanodiamond and graphene) and silicon QDs, gold and silica nanoparticles, and metal nanoclusters continue their in vivo voyage towards clinical imaging and therapeutic applications. This review summarizes the synthesis, chemical modifications, optical properties, and bioimaging applications of semiconductor QDs with particular references to metal chalcogenide QDs and bimetallic chalcogenide QDs. Also, this review highlights the toxicity and pharmacokinetics of QD bioconjugates.

Precision Nanocluster-Based Toroidal and Supertoroidal Frameworks Using Photocycloaddition-Assisted Dynamic Covalent Chemistry.

Atomically precise nanoclusters (NCs) have recently emerged as ideal building blocks for constructing self-assembled multifunctional superstructures. The existing structures are based on various non-covalent interactions of the ligands on the NC surface, resulting in inter-NC interactions. Despite recent demonstrations on light-induced reversible self-assembly, long-range reversible self-assembly based on dynamic covalent chemistry on the NC surface has yet to be investigated. Here, it is shown that Au25 NCs containing thiolated umbelliferone (7-hydroxycoumarin) ligands allow [2+2] photocycloaddition reaction-induced self-assembly into colloidal-level toroids. The toroids upon further irradiation undergo inter-toroidal reaction resulting in macroscopic supertoroidal honey-comb frameworks. Systematic investigation using electron microscopy, atomic force microscopy (AFM), and electron tomography (ET) suggest that the NCs initially form spherical aggregates. The spherical structures further undergo fusion resulting in toroid formation. Finally, the toroids fuse into macroscopic honeycomb frameworks. As a proof-of-concept, a cross-photocycloaddition reaction between coumarin-tethered NCs and an anticancer drug (5-fluorouracil) is demonstrated as a model photo-controlled drug release system. The model system allows systematic loading and unloading of the drug during the assembly and disassembly under two different wavelengths. The results suggest that the dynamic covalent chemistry on the NC surface offers a facile route for hierarchical multifunctional frameworks and photocontrolled drug release.

Light-Emitting Atomically Precise Nanocluster-Based Flexible QR Codes for Anticounterfeiting.

Despite tremendous progress in the field of fluorescence-based anticounterfeiting, the advanced anticounterfeiting techniques are still posing challenges all over the world due to their cost and reliability. Recently, light-emitting atomically precise nanoclusters have emerged as attractive building blocks because of their well-defined structure, function, and stable photoluminescence. Herein, we report the room temperature fabrication of a stable, flexible, nontoxic, and low-cost precision nanocluster-based luminescent ink for the stencil printing of an optically unclonable security label. Nanocluster-based printing ink shows brilliant photoluminescence owing to its extended C-H···π/π···π interactions. Spectroscopic and microscopic investigations show that intercalated nanoclusters in the printed security labels are highly stable as their optical features and molecular compositions are unaffected. The exceptional mechanical, thermal, photo, and aqueous stabilities of the printed security labels endorse to demonstrate the printing and smartphone-based electronic reading of the quick response code on a currency. Finally, confidential information protection and decryption under a precise window of light have been achieved by adopting the optical contrast illusion. The overall cost of the security label is found to be approximately 0.013 USD per stamp.

Self-Assembly of Precision Noble Metal Nanoclusters: Hierarchical Structural Complexity, Colloidal Superstructures, and Applications.

Ligand protected noble metal nanoparticles are excellent building blocks for colloidal self-assembly. Metal nanoparticle self-assembly offers routes for a wide range of multifunctional nanomaterials with enhanced optoelectronic properties. The emergence of atomically precise monolayer thiol-protected noble metal nanoclusters has overcome numerous challenges such as uncontrolled aggregation, polydispersity, and directionalities faced in plasmonic nanoparticle self-assemblies. Because of their well-defined molecular compositions, enhanced stability, and diverse surface functionalities, nanoclusters offer an excellent platform for developing colloidal superstructures via the self-assembly driven by surface ligands and metal cores. More importantly, recent reports have also revealed the hierarchical structural complexity of several nanoclusters. In this review, the formulation and periodic self-assembly of different noble metal nanoclusters are focused upon. Further, self-assembly induced amplification of physicochemical properties, and their potential applications in molecular recognition, sensing, gas storage, device fabrication, bioimaging, therapeutics, and catalysis are discussed. The topics covered in this review are extensively associated with state-of-the-art achievements in the field of precision noble metal nanoclusters.

Light-Triggered Reversible Supracolloidal Self-Assembly of Precision Gold Nanoclusters.

Monolayer thiol-protected noble metal nanoclusters are attractive nanoscale building blocks for well-defined colloidal superstructures. However, achieving facile reversible self-assembly of nanoclusters using external stimuli is still in its infancy. Herein, we report the synthesis and photon-assisted reversible self-assembly of thiolated azobenzene-stapled Au25 nanoclusters. Photoactivation of functionalized nanoclusters in dichloromethane by irradiating ultraviolet light at 345 nm results in a visual change and formation of disc-like colloidal superstructures (d ∼ 100-1000 nm). The superstructures readily disassemble into individual nanoclusters upon irradiating with visible light at 435 nm. Systematic changes in both the electronic absorption bands and nuclear magnetic resonance spectra of chromophores in solution suggest that the photoisomerization of surface ligands drives the self-assembly. High-resolution transmission electron microscopy, electron tomographic reconstruction, dynamic light scattering, and small-angle X-ray powder diffraction show that the disc-like superstructures contain densely packed nanoclusters. Long-range self-assembly and disassembly under ultraviolet and visible light, respectively, demonstrate reversible photoswitching in nanoclusters.