Near Infrared Electrochemiluminescence of Rod-Shape 25-Atom AuAg Nanoclusters That Is Hundreds-Fold Stronger Than That of Ru(bpy)3 Standard.

Near infrared (near-IR) electrogenerated chemiluminescence (ECL) from rod-shape bimetallic Au12Ag13 nanoclusters is reported. With ECL standard tris(bipyridine)ruthenium(II) complex (Ru(bpy)3) as reference, the self-annihilation ECL of the Au12Ag13 nanoclusters is about 10 times higher. The coreactant ECL of Au12Ag13 is about 400 times stronger than that of Ru(bpy)3 with 1 mM tripropylamine as coreactants. Voltammetric analysis reveals both oxidative and reductive ECLs under scanning electrode potentials. Transient ECL signals (tens of milliseconds) and decay profiles are captured by potential step experiments. An extremely strong and transient self-annihilation ECL is detected by activating LUMO and HOMO states sequentially via electrode reactions. The ECL generation pathways and mechanism are proposed based on the key anodic and cathodic activities arising from the energetics of this unique atomic-precision bimetallic nanocluster. Successes in the generation of the unprecedented strong near-IR ECL strongly support our prediction and choice of this nanocluster based on its record-high 40% quantum efficiency of near-IR photoluminescence. Correlation of the properties to the atomic/electronic structures has been a long-pursued goal particularly in the fast growing atomic-precision nanoclusters field. The mechanistic insights provided in this fundamental study could guide the design and syntheses of other nanoclusters or materials in general to achieve improved properties and further affirm the structure-function correlations. The high ECL signal in the less interfered near-infrared spectrum window offers combined merits of high-signal-low-noise/interference or high contrast for broad analytical sensing and immunoassays and other relevant applications.

On the Non-Metallicity of 2.2†nm Au246 (SR)80 Nanoclusters.

The transition from molecular to plasmonic behaviour in metal nanoparticles with increasing size remains a central question in nanoscience. We report that the giant 246-gold-atom nanocluster (2.2 nm in gold core diameter) protected by 80 thiolate ligands is surprisingly non-metallic based on UV/Vis and femtosecond transient absorption spectroscopy as well as electrochemical measurements. Specifically, the Au246 nanocluster exhibits multiple excitonic peaks in transient absorption spectra and electron dynamics independent of the pump power, which are in contrast to the behaviour of metallic gold nanoparticles. Moreover, a prominent oscillatory feature with frequency of 0.5 THz can be observed in almost all the probe wavelengths. The phase and amplitude analysis of the oscillation suggests that it arises from the wavepacket motion on the ground state potential energy surface, which also indicates the presence of a small band-gap and thus non-metallic or molecular-like behaviour.

Near-Infrared Electrogenerated Chemiluminescence from Aqueous Soluble Lipoic Acid Au Nanoclusters.

Strong electrogenerated chemiluminescence (ECL) is detected from dithiolate Au nanoclusters (AuNCs) in aqueous solution under ambient conditions. A novel mechanism to drastically enhance the ECL is established by covalent attachment of coreactants N,N-diethylethylenediamine (DEDA) onto lipoic acid stabilized Au (Au-LA) clusters with matching redox activities. The materials design reduces the complication of mass transport between the reactants during the lifetime of radical intermediates involved in conventional ECL generation pathway. The intracluster reactions are highly advantageous for applications by eliminating additional and high excess coreactants otherwise needed. The enhanced ECL efficiency also benefits uniquely from the multiple energy states per Au cluster and multiple DEDA ligands in the monolayer. Potential step and sweeping experiments reveal an onset potential of 0.78 V for oxidative-reduction ECL generation. Multifolds higher efficiency is found for the Au clusters alone in reference to the standard Rubpy with high excess TPrA. The ECL in near-IR region (beyond 700 nm) is highly advantageous with drastically reduced interference signals over visible ones. The features of ECL intensity responsive to electrode potential and solution pH under ambient conditions make Au-LA-DEDA clusters promising ECL reagents for broad applications. The strategy to attach coreactants on Au clusters is generalizable for other nanomaterials.

Electronic coupling between ligand and core energy states in dithiolate-monothiolate stabilized Au clusters.

Electron transfer activities of metal clusters are fundamentally significant and have promising potential in catalysis, charge or energy storage, sensing, biomedicine and other applications. Strong resonance coupling between the metal core energy states and the ligand molecular orbitals has not been established experimentally, albeit exciting progress has been achieved in the composition and structure determination of these types of nanomaterials recently. In this report, the coupling between core and ligand energy states is demonstrated by the rich electron transfer activities of Au130 clusters. Quantized electron transfers to the core and multi-electron transfers involving the durene-dithiolate ligands were observed at lower and higher potentials, respectively, in voltammetric studies. After a facile multi-electron oxidation from +1.34 to +1.40 V, several reversal reduction processes at more negative potentials, i.e. +0.91 V, +0.18 V and -0.34 V, were observed in an electrochemically irreversible fashion or with sluggish kinetics. The number of electrons and the shifts of the respective reduction potentials in the reversal process were attributed to the electronic coupling or energy relaxation processes. The electron transfer activities and subsequent relaxation processes are drastically reduced at lower temperatures. The time- and temperature-dependent relaxation, involving multiple energy states in the reversal reduction processes upon the oxidation of ligands, reveals the coupling between core and ligand energy states.

Transitions in Discrete Absorption Bands of Au130 Clusters upon Stepwise Charging by Spectroelectrochemistry.

Rich and tunable physicochemical properties make noble metal clusters promising candidates as novel nanomolecules for a variety of applications. Spectroelectrochemistry analysis is employed to resolve previously inaccessible electronic transitions in Au130 clusters stabilized by a monolayer of di- and monothiolate ligands. Well-defined quantized double-layer charging of the Au core and oxidizable ligands make this Au130 nanocluster unique among others and enable selective electrolysis to different core and ligand charge states. Subsequent analysis of the corresponding absorption changes reveals that different absorption bands originate from different electronic transitions involving both metal core energy states and ligand molecular orbitals. Besides the four discrete absorption bands in the steady-state UV-visible-near-IR absorption spectrum, additional transitions otherwise not detectable are resolved upon selective addition/removal of electrons at cores and ligand energy states, respectively, upon electrolysis. An energy diagram is proposed that successfully explains the major features observed in electrochemistry and absorption spectroscopy. Those assignments are believed applicable and effective to explain similar transitions observed in some other Au thiolate clusters.