Recent progress of metal nanoclusters in electrochemiluminescence

Metal nanoclusters (MeNCs), composed of a few to hundreds of metal atoms and appropriate surface ligands, have attracted extensive interest in the electrochemiluminescence (ECL) realm owing to their molecule-like optical, electronic, and physicochemical attributes and are strongly anticipated for discrete energy levels, fascinating electrocatalytic activity, and good biocompatibility. Over the past decade, huge efforts have been devoted to the synthesis, properties, and application research of ECL-related MeNCs, and this field is still a subject of heightened concern. Therefore, this perspective aims to provide a comprehensive overview of the recent advances of MeNCs in the ECL domain, mainly covering the emerged ECL available MeNCs, unique chemical and optical properties, and the general ECL mechanisms. Synthesis strategies for desirable ECL performance are further highlighted, and the resulting ECL sensing applications utilizing MeNCs as luminophores, quenchers, and substrates are discussed systematically. Finally, we anticipate the future prospects and challenges in the development of this area.

Monitoring single Au38 nanocluster reactions via electrochemiluminescence

Herein, we report for the first time single Au₃₈ nanocluster reaction events of highly efficient electrochemiluminescence (ECL) with tri-n-propylamine radicals as a reductive co-reactant at the surface of an ultramicroelectrode (UME). The statistical analyses of individual reactions confirm stochastic single ones influenced by the applied potential.

Spooling electrochemiluminescence spectroscopy: development, applications and beyond

One of the most widely used techniques to generate light through an efficient electron transfer is called electrochemiluminescence, or electrogenerated chemiluminescence (ECL). ECL mechanisms can be explored via 'spooling spectroscopy' in which individual ECL spectra showing emitted light are collected continuously during a potentiodynamic course. The obtained spectra are spooled together and plotted along the applied potential axis; because the potential sweep occurs at a defined rate, this axis is directly proportional to time. Any changes in the emission spectra can be correlated to the corresponding potentials and/or times, leading to a deeper understanding of the mechanism for light generation-information that can be used for efficiently maximizing ECL intensities. The formation of intermediates and excited states can also be tracked, which is crucial to interrogating and drawing electron transfer pathways (i.e., understanding the chemical reaction mechanism). Spooling spectroscopy is not limited to ECL; we also include instructions for the use of related methodologies, such as spooling photoluminescence spectroscopy during an electrolysis procedure, which can be easily set up. The total time required to complete the protocol is ~49 h, from making electrodes and an ECL cell, fabricating light-tight housing, to setting up instruments. Preparing the lab for an individual experiment (making an electrolyte solution of a targeted luminophore, cooling down the CCD camera, calibrating the spectrometer and surveying electrochemistry) takes ~1 h 15 min, and performing the spooling ECL spectroscopy experiment itself requires ~10 min.

Atomically precise alloy nanoclusters: syntheses, structures, and properties

Metal nanoclusters fill the gap between discrete atoms and plasmonic nanoparticles, providing unique opportunities for investigating the quantum effects and precise structure-property correlations at the atomic level. As a versatile strategy, alloying can largely improve the physicochemical performances compared to the corresponding homo-metal nanoclusters, and thus benefit the applications of such nanomaterials. In this review, we highlight the achievements of atomically precise alloy nanoclusters, and summarize the alloying principles and fundamentals, including the synthetic methods, site-preferences for different heteroatoms in the templates, and alloying-induced structure and property changes. First, based on various Au or Ag nanocluster templates, heteroatom doping modes are presented. The templates with electronic shell-closing configurations tend to maintain their structures during doping, while the others may undergo transformation and give rise to alloy nanoclusters with new structures. Second, alloy nanoclusters of specific magic sizes are reviewed. The arrangement of different atoms is related to the symmetry of the structures; that is, different atoms are symmetrically located in the nanoclusters of smaller sizes, and evolve into shell-by-shell structures at larger sizes. Then, we elaborate on the alloying effects in terms of optical, electrochemical, electroluminescent, magnetic and chiral properties, as well as the stability and reactivity via comparisons between the doped nanoclusters and their homo-metal counterparts. For example, central heteroatom-induced photoluminescence enhancement is emphasized. The applications of alloy nanoclusters in catalysis, chemical sensing, bio-labeling, and other fields are further discussed. Finally, we provide perspectives on existing issues and future efforts. Overall, this review provides a comprehensive synthetic toolbox and controllable doping modes so as to achieve more alloy nanoclusters with customized compositions, structures, and properties for applications. This review is based on publications available up to February 2020.

Au25(SR)18: the captain of the great nanocluster ship

Noble metal nanoclusters are in the intermediate state between discrete atoms and plasmonic nanoparticles and are of significance due to their atomically accurate structures, intriguing properties, and great potential for applications in various fields. In addition, the size-dependent properties of nanoclusters construct a platform for thoroughly researching the structure (composition)-property correlations, which is favorable for obtaining novel nanomaterials with enhanced physicochemical properties. Thus far, more than 100 species of nanoclusters (mono-metallic Au or Ag nanoclusters, and bi- or tri-metallic alloy nanoclusters) with crystal structures have been reported. Among these nanoclusters, Au25(SR)18-the brightest molecular star in the nanocluster field-is capable of revealing the past developments and prospecting the future of the nanoclusters. Since being successfully synthesized (in 1998, with a 20-year history) and structurally determined (in 2008, with a 10-year history), Au25(SR)18 has stimulated the interest of chemists as well as material scientists, due to the early discovery, easy preparation, high stability, and easy functionalization and application of this molecular star. In this review, the preparation methods, crystal structures, physicochemical properties, and practical applications of Au25(SR)18 are summarized. The properties of Au25(SR)18 range from optics and chirality to magnetism and electrochemistry, and the property-oriented applications include catalysis, chemical imaging, sensing, biological labeling, biomedicine and beyond. Furthermore, the research progress on the Ag-based M25(SR)18 counterpart (i.e., Ag25(SR)18) is included in this review due to its homologous composition, construction and optical absorption to its gold-counterpart Au25(SR)18. Moreover, the alloying methods, metal-exchange sites and property alternations based on the templated Au25(SR)18 are highlighted. Finally, some perspectives and challenges for the future research of the Au25(SR)18 nanocluster are proposed (also holding true for all members in the nanocluster field). This review is directed toward the broader scientific community interested in the metal nanocluster field, and hopefully opens up new horizons for scientists studying nanomaterials. This review is based on the publications available up to March 2018.

Red-emitted electrochemiluminescence by yellow fluorescent thioglycol/glutathione dual thiolate co-coated Au nanoclusters

This study reports the occurrence of a special red-emitted anodic electrochemiluminescence (ECL) emission at +1.4 V (vs. Ag/AgCl) on a glass carbon electrode (GCE) after the addition of thioglycol (TG) to surface-unsaturated glutathione (GSH)-coated Au nanoclusters (NCs), with an emission peak at ∼630 nm. Compared to the ECL at a potential of +1.8 V (vs. Ag/AgCl) and an emission peak at 580 nm (corresponding to fluorescence) for only GSH-coated Au NCs, this ECL emission not only exhibits a lower ECL potential but also shows a significantly red-shifted emission wavelength up to ∼50 nm. We demonstrated that the formation of TG/GSH dual ligand-coated Au NCs is responsible for the red-shifted ECL emission. Other common thiol compounds cannot result in similar effect on GCE, and no ECL is observed on other electrodes such as indium tin oxide and platinum electrodes. This finding offers a great possibility to design novel feasible ECL systems for different complicated applications.

A Grand Avenue to Au Nanocluster Electrochemiluminescence

In most cases of semiconductor quantum dot nanocrystals, the inherent optical and electrochemical properties of these interesting nanomaterials do not translate into expected efficient electrochemiluminescence or electrogenerated chemiluminescence (ECL) because of the surface-state induction effect. Thus, their low ECL efficiencies, while very interesting to explore, limit their applications. As their electrochemistry is not well-defined, insight into their ECL mechanistic details is also limited. Alternatively, gold nanoclusters possess monodispersed sizes with atomic precision, low and well defined HOMO-LUMO energy gaps, and stable optical and electrochemical properties that make them suitable for potential ECL applications. In this Account, we demonstrate strong and sustainable ECL of gold nanoclusters Au₂₅^z (i.e., Au₂₅(SR)₁₈^z, z = 1-, 0, 1+), Au₃₈(SR)₂₄, and Au₁₄₄(SR)₆₀, where the ligand SR is 2-phenylethanethiol. By correlation of the optical and electrochemical features of Au₂₅ nanoclusters, a Latimer-type diagram can be constructed to reveal thermodynamic relationships of five oxidation states (Au₂₅²⁺, Au₂₅⁺, Au₂₅⁰, Au₂₅^-, and Au₂₅^2-) and three excited states (Au₂₅^-*, Au₂₅⁰*, and Au₂₅⁺*). We describe ECL mechanisms and reaction kinetics by means of conventional ECL-voltage curves and novel spooling ECL spectroscopy. Notably, their ECL in the presence of tri-n-propylamine (TPrA), as a coreactant, is attributed to emissions from Au₂₅^-* (950 nm, strong), Au₂₅⁰* (890 nm, very strong), and Au₂₅⁺* (890 nm, very strong), as confirmed by the photoluminescence (PL) spectra of the three Au₂₅ clusters electrogenerated in situ. The ECL emissions are controllable by adjustment of the concentrations of TPrA· and Au₂₅^-, Au₂₅⁰, and Au₂₅⁺ species in the vicinity of the working electrode and ultimately the applied potential. It was determined that the Au₂₅^-/TPrA coreactant system should have an ECL efficiency of >50% relative to the Ru(bpy)₃²⁺/TPrA, while those of Au₂₅⁰/TPrA and Au₂₅⁺/TPrA reach 103% and 116%, respectively. Au₂₅^-* is the main light emission source for Au₂₅^z in the presence of benzoyl peroxide (BPO) as a coreactant, with a relative efficiency of up to 30%. For Au₃₈, BPO leads to the Au₃₈^-* excited state, which emits light at 930 nm. In the Au₃₈/TPrA coreactant system, we find that highly efficient light emission at 930 nm is mainly from Au₃₈⁺* (and also Au₃₈³⁺*), with an efficiency 3.5 times that of the Ru(bpy)₃²⁺/TPrA reference. We show that the ECL and PL of the various Au₃₈ charge states, namely, Au₃₈^2-, Au₃₈^-, Au₃₈⁰, Au₃₈⁺, Au₃₈²⁺, and Au₃₈⁴⁺, have the same peak wavelength of 930 nm. Finally, we demonstrate ECL with a peak wavelength of 930 nm from the Au₁₄₄/TPrA coreactant system, which is released from the electrogenerated excited states Au₁₄₄⁺* and Au₁₄₄³⁺*. In our opinion, these gold nanoclusters represent a new class of effective near-IR ECL emitters, from which applications such as bioimaging, biological testing, and medical diagnosis are anticipated once they are made water-dispersible with hydrophilic capping ligands.

Efficient electrochemiluminescence of a boron-dipyrromethene (BODIPY) dye

Electrochemiluminescence of a boron-dipyrromethene dye–tri-n-propylamine system was found to be efficient at a unique peak wavelength of 707 nm.

Cathodic electrochemiluminescence behavior of an ammonolysis product of 3,4,9,10-perylenetetracarboxylic dianhydride in aqueous solution and its application for detecting dopamine

The preparation process and cathodic electrochemiluminescence behavior of an ammonolysis product of 3,4,9,10-perylenetetracarboxylic dianhydride in an aqueous solution.

Thermodynamic and kinetic origins of Au25(0) nanocluster electrochemiluminescence

Au clusters with protecting organothiolate ligands and core diameters less than 2 nm are molecule-like structures, suitable for catalysis, optoelectronics and biology applications. The spectroscopy and electrochemistry of Au25(0) (Au25[(SCH2CH2Ph)18], SCH2CH2Ph = 2-phenylethanethiol) allowed us to construct a Latimer-type diagram for the first time, which revealed a rich photoelectrochemistry of the cluster and the unique relationship to its various oxidation states and corresponding excited states. The occurrence of cluster electrochemiluminescence (ECL) was examined in the presence of tri-n-propylamine (TPrA) as a co-reactant and was discovered to be in the near-infrared (NIR) region with peak wavelengths of 860, 865, and 960 nm, emitted by Au25(+*), Au25(0*), and Au25(-*), respectively. The light emissions, with an efficiency up to 103% relative to that of the efficient Ru(bpy)3(2+)/TPrA system, depended on the kinetics of the reactions between the electrogenerated TPrA radical and Au25(z) (z = 2+, 1+, 1-, and 2-) in the vicinity of the electrode or the bulk Au25(0). These thermodynamic and kinetic origins were further explored by means of spooling ECL and photoluminescence spectroscopy during a sweep of the potential or at a constant potential applied to the working electrode. NIR-ECL emissions of the cluster can be tuned in wavelength and intensity by adjusting the applied potential and TPrA concentration based on the above discoveries.

Au NPs driven electrochemiluminescence aptasensors for sensitive detection of fumonisin B1

A simple gold nanoparticles–Ir complex driven electrochemiluminescence aptasensors was fabricated for the sensitive detection of fumonisin B1.