Self-assembly of hybridized ligands on gold nanodots: tunable photoluminescence and sensing of nitrite

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.

An overview on the current understanding of the photophysical properties of metal nanoclusters and their potential applications

Photophysics of atomically precise metal nanoclusters (MNCs) is an emerging area of research due to their potential applications in optoelectronics, photovoltaics, sensing, bio-imaging and catalysis. An overview of the recent advances in the photophysical properties of MNCs is presented in this review. To begin with, we illustrate general synthesis methodologies of MNCs using direct reduction, chemical etching, ligand exchange, metal exchange and intercluster reaction. Due to strong quantum confinement, the NCs possess unique electronic properties such as discrete optical absorption, intense photoluminescence (PL), molecular-like electron dynamics and non-linear optical behavior. Discussions have also been carried out to unveil the influence of the core size, nature of ligands, heteroatom doping, and surrounding environments on the optical absorption and photophysical properties of metal clusters. Recent findings reveal that the excited-state dynamics, nonlinear optical properties and aggregation induced emission of metal clusters offer exciting opportunities for potential applications. We discuss briefly about their versatile applications in optoelectronics, sensing, catalysis and bio-imaging. Finally, the future perspective of this research field is given.

Identification of Intermediate Au22(SR)4(SR')14 Cluster on Ligand-Induced Transformation of Au25(SR)18 Nanocluster

We report the ligand-exchange-induced transformation from an icosahedral Au₂₅(SR)₁₈ cluster (where SR = 2-phenylethanethiol (PET)) to a bitetrahedral Au₂₂(SR)₄(SR')₁₄ cluster (where SR' = 4-tert-butylbenzenethiol (TBBT)). This partial exchange of the ligands was achieved by controlling the concentration of the incoming TBBT ligand. Being a bulky and aromatic ligand, TBBT can efficiently distort the atomic structure of the Au₂₅PET₁₈ cluster, resulting in Au₂₂(PET)₄(TBBT)₁₄, which is highly stable and considered to be an intermediate with a bitetrahedral structure. Time-dependent mass spectrometry and optical spectroscopy revealed the dissociation of the parent cluster and gave a deep insight on the ligand-exchange mechanism. Theoretical calculations and extended X-ray absorption fine structure studies confirm the formation of the Au₂₂ structure. Identifying the atomic structure of the intermediate species opens a new avenue to study the transformation of one cluster to another.

Parameters affecting the synthesis of carbon dots for quantitation of copper ions

A simple, eco-friendly, and low-cost electrochemical approach has been applied to the synthesis of carbon dots (C dots) from histidine hydrochloride in the absence or presence of halides (Cl, Br, and I) at various potentials up to 10 V. The as-formed C dots refer to C dots, Cl-C, Br-C, and I-C dots. The time-evolution UV-vis absorption and photoluminescence (PL) spectra provide more detailed information about the formation of C dots. Upon increasing the reaction time from 1 to 120 min, more and more C dots are formed, leading to increased PL intensity. The halides play two important roles in determining the formation of C dots; controlling the reaction rate and surface states. When compared to chloride and bromide, iodide has a greater effect on varying surface states and inducing PL quenching through intersystem crossing. The PL intensities of the four types of C dots all decrease upon increasing Cu²⁺, Hg²⁺, and Ag⁺ concentrations. In the presence of 0.8 mM I^-, I-C dots compared to C dots, Cl-C dots, and Br-C dots are slightly better for quantitation of Cu²⁺. Fourier transform infrared spectroscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy results of I-C dots reveal the interactions of Cu²⁺ with the surface ligands (imidazole and histidine). The I-C dot probe in the presence of 0.8 mM I^- is selective toward Cu²⁺ over the tested metal ions such as Hg²⁺ and Ag⁺. The assay provides a limit of detection of 0.22 μM for Cu²⁺ at a signal-to-noise ratio of 3. Practicality of this probe has been validated by the analyses of tap, lake, and sea water samples, with negligible matrix effects.

Luminescence polarization of gold nanoclusters reveals directional self-assembly

Study of luminescence polarization of spherical nanoclusters arising purely through a self-assembly route has not been explored to date. Here we introduce a gold nanocluster system that explicitly shows luminescence polarization solely through a self-assembly pathway. The directionality involved in the self-assembly governs the luminescence polarization of the nanoclusters. The steady-state and time-resolved emission anisotropy measurements revealed the anisotropically ordered behavior of one-dimensionally self-assembled green emitting nanoclusters. On the contrary, the randomly self-assembled yellow emitting nanoclusters exhibited insignificant luminescence anisotropy.

Instant room temperature synthesis of self-assembled emission-tunable gold nanoclusters: million-fold emission enhancement and fluorimetric detection of Zn2+

Facile synthesis of luminescent metal nanoclusters (NCs) accompanied by emission color tuning is currently an active area of research. In this work we describe a rapid (1 s) room temperature synthesis of luminescent Au NCs from completely nonluminescent NCs through the incorporation of Zn²⁺. The nanoclusters are initially stabilized by mercaptopropionate, and the coordination of Zn²⁺ with the carboxylate groups of the ligands rigidifies the Au(i) thiolates restricting the intramolecular rotation-vibrational motion. This significantly reduces the nonradiative relaxation of the excited state to produce yellow luminescent NCs (λ_em = 580 nm, QY: 6%, τ = 0.2 ms) with almost a million-fold emission enhancement. The enhanced luminescence is due to the self-assembly mediated aggregation induced emission (AIE) of NCs. These NCs on aging for 24 hours transform to highly ordered green emitting NCs (λ_em = 500 nm, QY: 20%, τ = 20 ns). The blue shift in emission is due to the dominance of inter Au(i)-Au(i) interaction and inter-NC Zn²⁺ interaction over the intra modes. TEM images show this distinct transition, a decrease in inter NC distance with increased self-assembly. Excited state relaxation dynamics associated with Au(i) thiolate shell dynamics in yellow and green emitting NCs is explained based on the time resolved fluorescence study. The rapid formation of luminescent NCs from nl-NCs has been used for efficient visual and fluorimetric detection of Zn²⁺.

Water-Soluble Naked Gold Nanoclusters Are Not Luminescent

Here, the synthesis of water-dispersible naked gold nanoclusters (AuNC_naked ) is reported by a simple reduction of HAuCl₄ with NaOH at room temperature, and it is shown that they are non-luminescent. They are then easily passivated with different thiols and adenosine monophosphate, leading to luminescent NCs. This is an important finding because the photoluminescence of the passivated NCs can now be clearly attributed to the ligand-AuNC surface interaction. These results are also highly relevant from the point of view of the preparation of luminescent NCs from the same NC batch. This strategy can be valuable for the preparation of a broad range of nano(bio)composites.

The effect of ligand–ligand interactions on the formation of photoluminescent gold nanoclusters embedded in Au(i)–thiolate supramolecules

Photoluminescence of cysteine-capped gold nanoclusters obtained via the reduction of –[Cys–Au(i)]_n– supramolecules is highly dependent on the degree of supramolecular aggregation.

Fluorescent metal quantum clusters: an updated overview of the synthesis, properties, and biological applications

A brief history of metal quantum clusters, their synthesis methods, physical properties, and an updated overview of their applications is provided.

Synthesis, Optical Properties, and Sensing Applications of Gold Nanodots

In this Personal Account, we briefly address our journey in developing photoluminescent nanomaterials for sensing purposes, with a focus on gold nanodots (Au NDs). Their synthetic strategies, optical properties, and sensing applications are emphasized. The Au NDs can be simply prepared from the etching of small-sized Au nanoparticles (<3 nm in diameter) by thiol compounds such as 11-mercaptoundecanoic acid under alkaline conditions. This simple approach allows the preparation of various functional Au NDs by choosing different thiol compounds as etching agents. Since the optical properties of Au NDs are highly dependent on the core and shell of each Au ND, the selection of etching reagents is important. Over the years we have developed various sensing systems using Au NDs for the detection of metal ions, anions, and proteins, based on analyte-induced photoluminescence quenching/enhancement of Au NDs as a result of changes in their oxidation state, shell composition, and structure.

Photoassisted photoluminescence fine-tuning of gold nanodots through free radical-mediated ligand-assembly

In this study, we have developed a simple photoassisted ligand assembly to fine-tune the photoluminescence (PL) of (11-mercaptoundecyl)-N,N,N-trimethylammonium bromide-capped gold nanodots (11-MUTAB-Au NDs). The 11-MUTAB-Au NDs (size: ca. 1.8 nm), obtained from the reaction of gold nanoparticles (ca. 3 nm) and 11-MUTAB, exhibited weak, near-infrared (NIR) PL at 700 nm with a quantum yield (QY) of 0.37% upon excitation at 365 nm. The PL QY of the Au NDs increased to 11.43% after reaction with 11-mercaptoundecanoic acid (11-MUA) for 30 min under ultraviolet (UV) light, which was accompanied by a PL wavelength shift to the green region (∼520 nm). UV-light irradiation accelerates 11-MUA assembly on the 11-MUTABAu NDs (11-MUA/11-MUTAB-Au NDs) through a radical-mediated reaction. Furthermore, the PL wavelength of the 11-MUA/11-MUTAB-Au NDs can be switched to 640 nm via cysteamine under UV-light irradiation. We propose that the PL of the Au NDs with NIR and visible emissions was originally from the surface thiol-Au complexes and the Au core, respectively. These dramatically different optical properties of the Au NDs were due to variation in the surface ligands, as well as the densities and surface oxidant states of the surface Au atoms/ions. These effects can be controlled by assembling surface thiol ligands and accelerated by UV irradiation.

Luminescent Metal Nanoclusters with Aggregation-Induced Emission

Thiolate-protected metal nanoclusters (or thiolated metal NCs) have recently emerged as a promising class of functional materials because of their well-defined molecular structures and intriguing molecular-like properties. Recent developments in the NC field have aimed at exploring metal NCs as novel luminescent materials in the biomedical field because of their inherent biocompatibility and good photoluminescence (PL) properties. From the fundamental perspective, recent advances in the field have also aimed at addressing the fundamental aspects of PL properties of metal NCs, shedding some light on developing efficient strategies to prepare highly luminescent metal NCs. In this Perspective, we discuss the physical chemistry of a recently discovered aggregation-induced emission (AIE) phenomenon and show the significance of AIE in understanding the PL properties of thiolated metal NCs. We then explore the unique physicochemical properties of thiolated metal NCs with AIE characteristics and highlight some recent developments in synthesizing the AIE-type luminescent metal NCs. We finally discuss perspectives and directions for future development of the AIE-type luminescent metal NCs.

Synthesis of chiral fluorescence silver nano-clusters and study on the aggregation-induced emission enhancement and chiral flip

Using racemic GSH as ligand, AgNCs with aggregation-induced emission enhancement and chiral flip were synthesized with solid-phase synthesis method.

Nucleotide/Tb³⁺ coordination polymer nanoparticles as luminescent sensor and scavenger for nitrite ion

Newly emerged metal organic coordination polymers have aroused the great interest in designing tailored functional materials. In this study, multiple functional components, luminescent Tb(3+) ion, nucleobase and antenna molecule, were integrated in a single material and prepared into a responsive nanoparticle for nitrite. The terbium coordination polymer nanoparticles made of this kind of material have the dual functions of recognition and transduction and obey a preset sensing mechanism without a post-functionalization of common materials. As the result of the tailored, the terbium coordination polymer nanoparticles are highly sensitive and selective to nitrite by means of Dexter energy transfer between Tb(3+) ion and nitrite, and can be used for the scavenger for nitrite in aqueous solution. The detection limit, dynamic range and removal capacity of U-Tb-OBBA CPNPs for nitrite are 0.3 µM, 0.3-470 µM and 4.44 mg per gram of particles, respectively. Metal organic coordination polymers show an attractive potential in constructing smart sensing materials.

Carbon nanodots as ligand exchange probes in Au@C-dot nanobeacons for fluorescent turn-on detection of biothiols

Au nanoparticle-carbon dot core-shell (Au@C-dot) nanocomposite was synthesized in aqueous medium at room temperature using the carbon dots as reducing agents themselves. The carbon nanodots also function as an effective stabilizer by forming a thin layer surrounding Au nanoparticles (Au NPs) similar to self-assembled monolayers. Ligand exchange with thiol containing biomolecules resulted in the release of carbon dots from the Au NP surface leading to an enhancement of fluorescence. Simultaneously the agglomeration of Au NPs stimulated by the interaction of biothiols led to changes in the surface plasmon properties of Au NPs. A detailed spectroscopic investigation revealed a combination of static and dynamic quenching being involved in the process. Thus, the Au nanoparticle-carbon dot composite could be used as a dual colorimetric and fluorometric sensor for biothiols ranging from amino acids, peptides, proteins, enzymes etc. with a detection limit of 50 nM.