Atomically precise metal nanoclusters are increasingly exploited in electrocatalysis such as water electrolysis and CO2 reduction. The catalytic effects are generally evaluated by conventional ...electrochemical methods or end‐product analysis. Herein, near infrared electrochemiluminescence (ECL) of the nanoclusters is introduced as a new signaling mechanism to complement classical voltammetric analysis for elucidating essential electrocatalytic parameters. Coreactant ECL from an aqueous soluble Au22 nanocluster (NCs) enhanced by a common pH buffer materials 4‐(2‐hydroxyethyl)‐1‐piperazineethanesulfonic acid (HEPES) and by a piperazine drug hydroxyzine (HDZ) is studied under oxidative reduction pathways. Foot of the wave analysis on voltametric features and the kinetics profiles in potential‐step experiments are studied in different nanocluster and coreactant concentrations. Benchmark parameters such as rate constants are determined under EC (Electron‐transfer‐Chemical‐reaction) mechanism as feasibility validations. Because the luminescence properties of AuNCs are sensitive to the surface ligands or adsorbents, ECL is proposed as a new signal readout for operando studies providing insights for governing reaction mechanism and kinetics during catalytic reactions. Retrospectively, mechanistic understanding of the complex multi‐step reactions during ECL generation can provide guidance for the evaluation and optimization of ECL property for better quantitative applications.
As shown in the TOC, electrochemiluminescence (ECL) of the nanoclusters is introduced as a new signaling mechanism for elucidating essential electrocatalytic parameters and to complement foot of the wave analysis on voltametric current features. ECL provides kinetic insights during catalytic reactions for operando studies. Retrospectively, mechanistic understanding of the multi‐step ECL reactions offer guidance for the evaluation and optimization of ECL property.
Current rectification is well known in ion transport through nanoscale pores and channel devices. The measured current is affected by both the geometry and fixed interfacial charges of the ...nanodevices. In this article, an interesting trend is observed in steady-state current–potential measurements using single conical nanopores. A threshold low-conductivity state is observed upon the dilution of electrolyte concentration. Correspondingly, the normalized current at positive bias potentials drastically increases and contributes to different degrees of rectification. This novel trend at opposite bias polarities is employed to differentiate the ion flux affected by the fixed charges at the substrate–solution interface (surface effect), with respect to the constant asymmetric geometry (volume effect). The surface charge density (SCD) of individual nanopores, an important physical parameter that is challenging to measure experimentally and is known to vary from one nanopore to another, is directly quantified by solving Poisson and Nernst–Planck equations in the simulation of the experimental results. The flux distribution inside the nanopore and the SCD of individual nanopores are reported. The respective diffusion and migration translocations are found to vary at different positions inside the nanopore. This knowledge is believed to be important for resistive pulse sensing applications because the detection signal is determined by the perturbation of the ion current by the analytes.
The impacts of Au-thiolate bonding on the near infrared (IR) luminescence of Au nanoclusters are studied by designing two types of monolayer reactions. Firstly, 1,4-dithiol durene (durene-DT) is ...reacted with Au
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monolayer protected clusters (MPCs) stabilized by phenylethanethiolate (PhC2S) ligands. Upon the addition of durene-DT, the near IR luminescence of Au MPCs intensifies while the well-defined absorbance bands diminish. The optical transition is associated with the ligand exchange process monitored by proton NMR. In the second approach, PhC2S monothiols are reacted with durene-DT stabilized Au nanoclusters (DTCs). The addition of PhC2S to the Au DTCs induces the gradual decrease of the near IR luminescence. Mass spectrometry and NMR analysis reveal similar final products of mixed thiolate Au nanoclusters from both reactions. The results suggest that the 1,4-dithiolate-Au bonding interaction is a promising factor to further enhance the near IR luminescence of Au nanoclusters for biomedical applications.
Near infrared luminescence of gold nanoclusters is enhanced by the bonding between the Au core and 1,4-dithiolate ligands.
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.
When is a metal not a metal? The ultrafast electron and phonon dynamics of Au246(SR)80 nanoclusters reveal that Au246 is still non‐metallic despite its large size (2.2 nm in diameter). A prominent oscillatory feature with frequency of 0.5 THz is observed indicating the presence of a small band‐gap and thus non‐metallic or molecular‐like behaviour.
Rich and discrete energy states in gold nanoclusters enable different combinations of electronic transitions and correspondingly electrochemical and optical properties for a variety of applications. ...The impacts on those electronic transitions by the emergence and magnitude/alignment of a band gap and by the contributions from different atomic/molecular orbitals require further study. Au nanoclusters with 130 core Au atoms are of interest in this report because they are at the transition size regime where a small yet well-defined band gap can be resolved along with continuous quantized frontier core orbitals. Here, electrochemical analysis is combined with UV–vis–near infrared optical measurements to unveil previously unresolved electronic transitions. Finite changes in the steady-state optical absorption spectrum are captured by spectroelectrochemistry when the Au nanoclusters are charged to different states via electrolysis. Multiple previously unresolved peaks and valleys as well as isosbestic “points/regions” are observed in the differential spectrum. The detailed spectral features are explained by the respective electronic transitions to those affected energy states. Key features are also well correlated with ultrafast absorption analysis which provides additional insights, such as the lifetime of the corresponding transitions. The experimentally measured energy states and transitions could serve as references for future theoretical study to learn the respective contributions from different atomic orbitals and, importantly, to explore routes to enhance or suppress certain transition so as to modulate the corresponding electrochemical and optical properties for better applications.
Electrostatic interactions of mobile charges in solution with the fixed surface charges are known to strongly affect stochastic sensing and electrochemical energy conversion processes at nanodevices ...or devices with nanostructured interfaces. The key parameter to describe this interaction, surface charge density (SCD), is not directly accessible at nanometer scale and often extrapolated from ensemble values. In this report, the steady-state current–voltage (i–V) curves measured using single conical glass nanopores in different electrolyte solutions are fitted by solving Poisson and Nernst–Planck equations through finite element approach. Both high and low conductivity state currents of the rectified i–V curve are quantitatively fitted in simulation at less than 5% error. The overestimation of low conductivity state current using existing models is overcome by the introduction of an exponential SCD distribution inside the conical nanopore. A maximum SCD value at the pore orifice is determined from the fitting of the high conductivity state current, while the distribution length of the exponential SCD gradient is determined by fitting the low conductivity state current. Quantitative fitting of the rectified i–V responses and the efficacy of the proposed model are further validated by the comparison of electrolytes with different types of cations (K+ and Li+). The gradient distribution of surface charges is proposed to be dependent on the local electric field distribution inside the conical nanopore.
Visible−near-IR luminescence spectra of gold MPCs that are similar, irrespective of the number of core atoms (all <2 nm diameter) and different monolayers, are reported. The luminescence can be ...quantitatively invoked by introducing polar ligands into nonpolar MPC monolayers and by galvanic exchange of metal atoms on the MPC core surface with different metals. The observed emissions are believed to result from surface-localized states that depend on both the core metal of the nanoparticle and the ligands attached to the metal surface.
Skin sensitization test data are required or considered by chemical regulation authorities around the world. These data are used to develop product hazard labeling for the protection of consumers or ...workers and to assess risks from exposure to skin-sensitizing chemicals. To identify opportunities for regulatory uses of non-animal replacements for skin sensitization tests, the needs and uses for skin sensitization test data must first be clarified. Thus, we reviewed skin sensitization testing requirements for seven countries or regions that are represented in the International Cooperation on Alternative Test Methods (ICATM). We noted the type of skin sensitization data required for each chemical sector and whether these data were used in a hazard classification, potency classification, or risk assessment context; the preferred tests; and whether alternative non-animal tests were acceptable. An understanding of national and regional regulatory requirements for skin sensitization testing will inform the development of ICATM's international strategy for the acceptance and implementation of non-animal alternatives to assess the health hazards and risks associated with potential skin sensitizers.
•We reviewed regulatory requirements for skin sensitization testing, by chemical sector, of seven countries or regions.•This review summarizes data needs for hazard classification, potency classification, and risk assessment.•We identify preferred test methods and note whether non-animal alternative test methods are acceptable.•This effort will inform an international strategy for implementing non-animal approaches for skin sensitization assessment.
The interior surface of the glass nanopore electrode was modified with spiropyran moieties to impart photochemical control of molecular transport through the pore orifice (15−90 nm radius). In low ...ionic strength acetonitrile solutions, diffusion of a positively charged species (Fe(bpy)3 2+) is electrostatically blocked with ∼100% efficiency by UV light-induced conversion of the neutral surface-bound spiropyran to its protonated merocyanine form (MEH+). Transport through the pore orifice is restored by either irradiation of the electrode with visible light to convert MEH+ back to spiropyran or addition of a sufficient quantity of supporting electrolyte to screen the electrostatic field associated with MEH+. The transport of neutral redox species through spiropyran-modified glass nanopores is not affected by light, allowing photoselective transport of redox molecules to the electrode surface based on charge discrimination. The glass nanopore electrode can also be employed as a photochemical trap, by UV light conversion of surface-bound spriropyran to MEH+, preventing Fe(bpy)3 2+ initially in the pore from diffusing through the orifice.
Mass transport through an interfacial area at nanometer scale is a key process to be addressed in research and applications employing nanostructured electrodes, nanofluidic devices, and high surface ...area materials. Ionic transport through single glass nanopores is investigated by multifrequency impedance techniques. The conical glass nanopores display current rectification under controlled experimental conditions in voltammetric studies. Being inaccessible by conductivity and DC voltammetry, phase sensitive capacitive and “inductive” components in the two-dimensional impedance spectrum reveal dynamic ionic transport information. The nanopore impedance responses are strongly influenced by the concentration of electrolytes and are correlated with current rectification. Multitime-constant impedance loops are detected in different frequency ranges. The multitime-constant features are attributed to the negative charges at the glass−solution interface. The impedance data are interpreted by designed equivalent circuit models. With the correlation of experimental and modeling results established, current signals can be differentiated into two categories: those originated from ionic transport affected by the immobilized charges at the solid−solution interface and those resulted from the applied waveform with ionic transport governed by geometric factors such as the radius of the nanopore.
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