Self-Assembled Metal Nanoclusters: Driving Forces and Structural Correlation with Optical Properties

Studies on self-assembly of metal nanoclusters (MNCs) are an emerging field of research owing to their significant optical properties and potential applications in many areas. Fabricating the desired self-assembly structure for specific implementation has always been challenging in nanotechnology. The building blocks organize themselves into a hierarchical structure with a high order of directional control in the self-assembly process. An overview of the recent achievements in the self-assembly chemistry of MNCs is summarized in this review article. Here, we investigate the underlying mechanism for the self-assembly structures, and analysis reveals that van der Waals forces, electrostatic interaction, metallophilic interaction, and amphiphilicity are the crucial parameters. In addition, we discuss the principles of template-mediated interaction and the effect of external stimuli on assembly formation in detail. We also focus on the structural correlation of the assemblies with their photophysical properties. A deep perception of the self-assembly mechanism and the degree of interactions on the excited state dynamics is provided for the future synthesis of customizable MNCs with promising applications

Core-Size Dependent Fluorescent Gold Nanoclusters and Ultrasensitive Detection of Pb²⁺ Ion

Gold nanoclusters (Au NCs) are new class of fluorescent nanomaterials with widespread applications in energy, water and healthcare. Here, we report a green synthesis of Au NCs with tunable emission wavelength from 590 to 510 nm in aqueous medium by core etching and ligand exchange method. Investigation reveals that the number of Au atoms present in the core of nanoclusters controls the emission wavelength. The quantum yield (QY) of nanoclusters increases from 0.57 to 3.15% with changing core from Au₁₂ to Au₆. Time resolved spectroscopic study reveals that the emission with higher lifetime (>100 ns) originates from ligand to metal charge transfer (LMCT; S to gold core of NCs). It is demonstrated that the highly green emitting NCs (Au-510) are more sensitive than orange emitting NCs (Au-590) toward Pb²⁺. The detection limit of Pb²⁺ is found to be 10 nM which is much lower than allowed concentration of Pb²⁺ in drinking water. Thus, Au NCs based optical sensor is promising for the selective detection of Pb²⁺ in drinking water

Ultrafast Relaxation Dynamics of Luminescent Copper Nanoclusters (Cu₇L₃) and Efficient Electron Transfer to Functionalized Reduced Graphene Oxide

Luminescent copper nanoclusters (Cu NCs) have emerged as fascinating nanomaterials for potential applications in optoelectronics, catalysis, and sensing. Here, we demonstrate the synthesis of l-cysteine-capped Cu NCs in aqueous medium having a bright cyan emission (489 nm) with a quantum yield of 6.2%. The structure of the Cu NCs (Cu₇L₃) is investigated by using density functional theory (DFT) calculation and mass spectrometric study. Further, time-dependent density functional theory (TD-DFT) calculations provide the insights of electronic transitions, and it is correlated with experimental data. With near-HOMO–LUMO gap excitation, Cu NCs are excited to the S₄ state and subsequently relaxed to the S₁ state through an internal conversion process with a time scale in the ultrafast region (326.8 ± 6.5 fs). Furthermore, the structural relaxation in S₁ takes place at a picosecond time scale, and the radiative relaxation occurs from S₁ to S₀. Finally, Cu NCs are attached with imidazole-functionalized partially reduced graphene oxide (ImRGO) via electrostatic attraction. A dramatic photoluminescence (PL) quenching and shortening of the decay time of the Cu cluster in the presence of ImRGO indicate the photoinduced electron transfer process, which is confirmed from ultrafast spectroscopic study. The photoinduced electron transfer in a Cu NC–ImRGO nanocomposite should pave the way for potential applications in light harvesting