Ligand-Based Shielding Effect Induced Efficient Near-Infrared Electrochemiluminescence of Gold Nanoclusters and Its Sensing Application

Preparing high-efficiency ECL gold nanoclusters (Au NCs) still faces a serious challenge due to the poor stability of co-reactant radicals in aqueous media. Herein, we report a ligand-based shielding effect induced record near-infrared (λmax = 786 nm) ECL efficiency of β-cyclodextrin-protected Au NCs (β-CD-Au NCs) with triethylamine (TEA) as co-reactant. The ligand of β-CD-Au NCs with a matched hydrophobic cavity could encapsulate TEA driven by host-guest chemistry, which not only allows the generation of TEA• in the cavity to diminish environmental exposure, thus reducing the quenching by dissolved oxygen, water, etc., but also shortens the charge transfer pathway without extensive chemical modification. Density functional theory, 1H NMR spectra, electron paramagnetic resonance, and differential pulse voltammetry studies revealed that the β-CD ligand-based shielding effect significantly increased the reactivity efficiency of TEA. More importantly, in stark contrast to those of traditional ligand-protected Au NCs, the ECL efficiency of β-CD-Au NCs enhanced 321-fold versus BSA-Au NCs, 153-fold versus ATT-Au NCs, and 19-fold versus GSH-Au NCs with 1 mM TEA. Therefore, this work provides an in-depth understanding of the crucial role of ligands in enhancing the active co-reactant radical stability for high-efficiency ECL metal NCs to immensely stimulate their promising applications. Using the β-CD-Au NCs as emitters, a "signal off" ECL sensing platform was constructed to detect noradrenaline as a model target with a lower detection limit of 0.91 nM.