Reversible electrochemical injection of discrete numbers of electrons into sterically stabilized silicon nanocrystals (NCs) (approximately 2 to 4 nanometers in diameter) was observed by differential pulse voltammetry (DPV) in N,N'-dimethylformamide and acetonitrile. The electrochemical gap between the onset of electron injection and hole injection-related to the highest occupied and lowest unoccupied molecular orbitals-grew with decreasing nanocrystal size, and the DPV peak potentials above the onset for electron injection roughly correspond to expected Coulomb blockade or quantized double-layer charging energies. Electron transfer reactions between positively and negatively charged nanocrystals (or between charged nanocrystals and molecular redox-active coreactants) occurred that led to electron and hole annihilation, producing visible light. The electrogenerated chemiluminescence spectra exhibited a peak maximum at 640 nanometers, a significant red shift from the photoluminescence maximum (420 nanometers) of the same silicon NC solution. These results demonstrate that the chemical stability of silicon NCs could enable their use as redox-active macromolecular species with the combined optical and charging properties of semiconductor quantum dots.

Blue luminescent nanocrystals (NCs) were prepared electrochemically from multiwalled carbon nanotubes (MWCNTs) for the first time. The carbon NCs were characterized by UV−vis, photoluminescence, Raman, XRD spectroscopy, and high-resolution transmission electron microscopy. The structure evolution of the MWCNTs during electrochemical treatments was monitored by SEM ex situ. Since the MWCNTs were formed with scrolled graphene layers, we propose that tetrabutyl ammonium (TBA) cations most probably intercalate into the gaps and the defects during electrochemical cycling and break the tubes near the defects.

Electrogenerated chemiluminescence (ECL) was observed from TOPO-capped CdSe nanocrystals dissolved in CH 2 Cl 2 containing 0.1 M TBAP. The solution of monodisperse nanocrystals with an absorption maximum at 537 nm was synthesized at 330 °C using TOPO (trioctylphosphineoxide) and TOP (trioctylphosphine) as capping agents and Cd-acetate and Se powder as precursors. The photoluminescence (PL) spectrum showed an emission maximum at 545 nm. Cyclic voltammetry and differential pulse voltammetry of this solution displayed no distinctive features, but light emission was observed through the annihilation of oxidized and reduced forms electrogenerated during cyclic potential scans or steps. The oxidized species was somewhat more stable than the reduced form. The ECL spectrum was substantially red shifted by ∼200 nm from the PL spectrum, suggesting that surface states play an important role in the emission process.

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 Au (i.e., Au(SR), z = 1-, 0, 1+), Au(SR), and Au(SR), where the ligand SR is 2-phenylethanethiol. By correlation of the optical and electrochemical features of Au nanoclusters, a Latimer-type diagram can be constructed to reveal thermodynamic relationships of five oxidation states (Au, Au, Au, Au, and Au) and three excited states (Au*, Au*, and Au*). 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 Au* (950 nm, strong), Au* (890 nm, very strong), and Au* (890 nm, very strong), as confirmed by the photoluminescence (PL) spectra of the three Au clusters electrogenerated in situ. The ECL emissions are controllable by adjustment of the concentrations of TPrA· and Au, Au, and Au species in the vicinity of the working electrode and ultimately the applied potential. It was determined that the Au/TPrA coreactant system should have an ECL efficiency of >50% relative to the Ru(bpy)/TPrA, while those of Au/TPrA and Au/TPrA reach 103% and 116%, respectively. Au* is the main light emission source for Au in the presence of benzoyl peroxide (BPO) as a coreactant, with a relative efficiency of up to 30%. For Au, BPO leads to the Au* excited state, which emits light at 930 nm. In the Au/TPrA coreactant system, we find that highly efficient light emission at 930 nm is mainly from Au* (and also Au*), with an efficiency 3.5 times that of the Ru(bpy)/TPrA reference. We show that the ECL and PL of the various Au charge states, namely, Au, Au, Au, Au, Au, and Au, have the same peak wavelength of 930 nm. Finally, we demonstrate ECL with a peak wavelength of 930 nm from the Au/TPrA coreactant system, which is released from the electrogenerated excited states Au* and Au*. 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.

An overview is given on great achievements in electrogenerated chemiluminescence or electrochemiluminescence (ECL) by Allen J. Bard. The established ECL theories and fundamentals are first described. The developed analytical instrumentation and methodology for ECL are then revealed. Studies of various types of ECL luminophores are summarized. The discovered ECL applications especially in bioanalysis and bioimaging are finally depicted. Bard's presentations and publications on ECL are indispensable resources of inspiration and guides for ECL researchers worldwide, who will further advance ECL research and applications already in a prosperous stage.