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.