Controllable synthesis and electrocatalytic applications of atomically precise gold nanoclusters

The fate of nanoparticle surface chemistry during reductive electrosynthesis in aprotic media

Reductive electrochemical coupling of carbon dioxide with organic molecules (electrocarboxylation, EC) represents a green route towards value-added carboxylic acids and serves as a promising strategy for carbon footprint mitigation. Despite the industrial prospects of this synthetic process, little has been done towards the optimization of cathode materials at the nanoscale. Herein, we pave the way towards the use of metal nanoparticles (NPs) as electrocatalysts in EC by demonstrating the effects of NP surface chemistry on electroorganic transformations and the evolution of surface functionalization in the course of reductive electrosynthesis in aprotic media. Using spherical Au NPs capped with citrate or cetylpyridinium chloride (CPC) as our study subjects, we examined the effect of Au NP surface chemistry on the selectivity of EC of benzyl bromide in acetonitrile and determined the fate of the surface adsorbates of Au NPs in the course of the reaction using Raman spectroscopy and X-ray photoelectron spectroscopy. We show that the CPC-stabilized Au NPs outperform the citrate-stabilized NPs at a low applied potential of -1.5 V vs. Ag/Ag+ with the former showing an almost two-fold increase in the faradaic efficiency towards phenylacetic acid. This higher selectivity is attributed to the reaction on the liberated Au surface stemming from the stripping of CPC molecules. In contrast to the CPC-functionalized NPs, the citrate-stabilized Au NPs retain their adsorbates during the reaction, which undergo electrochemical transformations during EC.

Heteroatom number-dependent cluster frameworks in structurally comparable Pd-Au nanoclusters

Investigating the impact of heteroatom alloying extents on regulating the cluster structures is crucial for the fabrication of cluster-based nanomaterials with customized properties. Herein, two structurally comparable PdxAu12 (x = 1, 2) nanoclusters with a uniform surface environment but completely distinct kernel configurations were controllably synthesized and structurally determined. The single Pd-alloyed Pd1Au12 nanocluster retained an icosahedral metal framework, while the Pd2Au12 nanocluster with two Pd heteroatoms exhibited a unique toroidal configuration. The additional Pd heteroatom not only led to significant changes in the cluster frameworks but also profoundly affected their electrocatalytic CO2 reduction performance. The Pd1Au12 nanocluster demonstrated enhanced catalytic performance, exhibiting a higher current density, a lower onset potential, and greater CO faradaic efficiency compared to the Pd2Au12 nanocluster. This work offers new insights into the customization of the structures and properties of gold nanoclusters by regulating the doping degree of Pd heteroatoms.

A comprehensive review of atomically precise metal nanoclusters with emergent photophysical properties towards diverse applications

Atomically precise metal nanoclusters (MNCs) composed of a few to hundreds of metal atoms represent an emerging class of nanomaterials with a precise composition. With the size approaching the Fermi wavelength of electrons, their energy levels are well-separated, leading to molecule-like properties, like discrete single electronic transitions, tunable photoluminescence (PL), inherent structural anisotropy, and distinct redox behavior. Extensive synthetic efforts and electronic structure revelation have expanded applicability of MNCs in catalysis, optoelectronics, and biology. This review highlights the intriguing photophysical and electrochemical behaviors of MNCs and their regulatory parameters and applications. Initially, we present a brief discussion on the evolution of MNCs from gas-phase naked metal clusters to monolayer ligand-protected MNCs along with representative studies on their electronic structure. Due to their quantized molecular orbitals, they often exhibit PL, which can be regulated based on their capping ligands, number of atoms, crystal packing, presence of heterometal, and surrounding environment. Apart from PL, the relaxation pathways of MNCs on an ultrafast time scale have been extensively studied, which significantly differ from that of plasmonic metal nanoparticles. Moreover, their interaction with high-intensity light results in unique non-linear optical properties. The synergy between MNCs in a hierarchical self-assembled structure has been exploited to enhance their PL by precisely tuning their non-covalent interactions. Moreover, several NC-based hybrids have been designed to exhibit efficient electron or energy transfer in the photoexcited state. In the next section, we briefly focus on the redox behavior of NCs and facile electron transfer to suitable substrates, which result in enzyme-like catalytic activity. Utilizing these photophysical and electrochemical behaviors, NCs are widely employed in catalysis, optical sensing, and light-harvesting applications, which are also discussed in this review. In the final section, conclusions and open questions for the NC research community are included. This review will provide a comprehensive view of the emerging physicochemical properties of MNCs, thereby enabling an understanding for their precise modulation in future.

Rational design and synthesis of atomically precise nanocluster-based nanocomposites: a step towards environmental catalysis

Atomically precise metal nanoclusters (NCs) and metal-organic frameworks (MOFs) possess distinct properties that can present challenges in certain applications. However, integrating these materials to create new composite functional materials has gained significant interest due to their unique characteristics through a range of applications, particularly in catalysis. Considering MOFs as hosts and NCs as guests, several synergistic effects have been observed in composites, particularly in environmental catalytic reactions. However, the precise role of encapsulated NCs within the MOF pore structure is still in its infancy. Besides, stabilizing NCs, whether through intact ligands or without ligands via the MOF host, presents challenges that are currently being investigated. This feature article reviews recent advancements in the synthesis of NC@MOF composites, focusing on cutting-edge strategies for selecting MOFs and the roles of NC ligands, as well as characterization and catalytic applications.

Fluorescent ovalbumin-functionalized gold nanocluster as a highly sensitive and selective sensor for relay detection of salicylaldehyde, Hg(II) and folic acid

A sensitive and selective relay-based scheme for the detection of salicylaldehyde, Hg2+, and folic acid (FA) has been demonstrated using fluorescent ovalbumin functionalized gold nanoclusters (OVA-AuNCs, λem = 655 nm) in this article. The OVA-AuNCs were conjugated to salicylaldehyde via an imine linkage to form Salic_OVA-AuNCs conjugate. The molecular docking study reveals that multiple functional groups and amino acid residues are involved in the interaction between salicylaldehyde and the OVA-AuNCs. The coupling of salicylaldehyde with OVA-AuNCs results in fluorescence quenching at 655 nm and concomitant formation of an emission band at 500 nm, which have leveraged to detect salicylaldehyde down to 2.02 µM. Following that, the Salic_OVA-AuNCs has been used for the detection of Hg2+ and FA. Several processes, such as internal charge transfer (ICT), photoinduced electron transfer (PET) and metallophilic interactions, are involved between the Salic_OVA-AuNCs nanoprobe and the analytes, which allowed to detect Hg2+ and FA down to 0.13 nM and 0.11 nM, respectively. The Salic_OVA-AuNCs nanoprobe has an additional naked-eye utility when applied to paper-strip sensing strategy for Hg2+ and FA detection.

Insights of the efficient hydrogen evolution reaction performance in bimetallic Au4Cu2 nanoclusters

The design of efficient electrocatalysts for improving hydrogen evolution reaction (HER) performance using atomically precise metal nanoclusters (NCs) is an emerging area of research. Here, we have studied the HER electrocatalytic performance of monometallic Cu6 and Au6 nanoclusters and bimetallic Au4Cu2 nanoclusters. A bimetallic Au4Cu2/MoS2 composite exhibits excellent HER catalytic activity with an overpotential (η10) of 155 mV vs. reversible hydrogen electrode observed at 10 mA cm-2 current density. The improved HER performance in Au4Cu2 is due to the increased electrochemically active surface area (ECSA), and Au4Cu2 NCs exhibits better stability than Cu6 and Au6 systems and bare MoS2. This augmentation offers a greater number of active sites for the favorable adsorption of reaction intermediates. Furthermore, by employing X-ray photoelectron spectroscopy (XPS) and Raman analysis, the kinetics of HER in the Au4Cu2/MoS2 composite were elucidated, attributing the favorable performance to better electronic interactions occurring at the interface between Au4Cu2 NCs and the MoS2 substrate. Theoretical analysis reveals that the inherent catalytic enhancement in Au4Cu2/MoS2 is due to favorable H atom adsorption over it and the smallest ΔGH* value. The downshift in the d-band of the Au4Cu2/MoS2 composite influences the binding energy of intermediate catalytic species. This new catalyst sheds light on the structure-property relationship for improving electrocatalytic performance at the atomic level.

Elucidation of the electronic structures of thiolate-protected gold nanoclusters by electrochemical measurements

Metal nanoclusters (NCs) with sizes of approximately 2 nm or less have different physical/chemical properties from those of the bulk metals owing to quantum size effects. Metal NCs, which can be size-controlled and heterometal doped at atomic accuracy, are expected to be the next generation of important materials, and new metal NCs are reported regularly. However, compared with conventional materials such as metal complexes and relatively large metal nanoparticles (>2 nm), these metal NCs are still underdeveloped in terms of evaluation and establishment of application methods. Electrochemical measurements are one of the most widely used methods for synthesis, application, and characterisation of metal NCs. This review summarizes the basic knowledge of the electrochemistry and experimental techniques, and provides examples of the reported electronic states of thiolate-protected gold NCs elucidated by electrochemical approaches. It is expected that this review will provide useful information for researchers starting to study metal NCs.

A Sensitive Fluorescence Sensor for Tetracycline Determination Based on Adenine Thymine-Rich Single-Stranded DNA-Templated Copper Nanoclusters

A sensitive fluorescent sensor has been developed for the determination of tetracycline (TC) using adenine thymine (AT)-rich single-stranded DNA (ssDNA) templated copper nanoclusters (CuNCs) as a fluorescent probe. Fluorescent ssDNA-CuNCs were synthesized by employing AT-rich ssDNA as templates and ascorbic acid as reducing agents through a facile one-step method. The as-prepared ssDNA-CuNCs exhibited strong fluorescence with a large Stokes shift (240 nm) and stable fluorescence emission. In the presence of TC, the fluorescent intensity of ssDNA-CuNCs was obviously decreased through the inner filter effect, due to the spectral overlapping between ssDNA-CuNCs and TC. Under the optimal conditions, the strategy exhibited sensitive detection of TC with a linear range from 2 nM to 30 μM and with a limit of detection of 0.5 nM. Furthermore, the sensor was successfully applied for the detection of TC in milk samples. Therefore, it provided a simple, rapid, and label-free fluorescent method for TC detection.

Programmable kernel structures of atomically precise metal nanoclusters for tailoring catalytic properties

The unclear structures and polydispersity of metal nanoparticles (NPs) seriously hamper the identification of the active sites and the construction of structure-reactivity relationships. Fortunately, ligand-protected metal nanoclusters (NCs) with atomically precise structures and monodispersity have become an ideal candidate for understanding the well-defined correlations between structure and catalytic property at an atomic level. The programmable kernel structures of atomically precise metal NCs provide a fantastic chance to modulate their size, shape, atomic arrangement, and electron state by the precise modulating of the number, type, and location of metal atoms. Thus, the special focus of this review highlights the most recent process in tailoring the catalytic activity and selectivity over metal NCs by precisely controlling their kernel structures. This review is expected to shed light on the in-depth understanding of metal NCs' kernel structures and reactivity relationships.