Understanding stimuli-responsive oligomer shell of silver nanoclusters with aggregation-induced emission via chemical etching and their use as sensors

Abstract

We reported new findings, i.e., stimuli-responsivity, on the oligomer shell of aggregation-induced emission (AIE) type silver nanoclusters via thiol-induced chemical etching. The intriguing AIE properties of the lipoic acid-protected silver nanoclusters were initially determined with their large Stokes shift, microsecond-scale lifetime and a high content of Ag(I) components. Additionally, likely owing to shell-provided protection, they exhibited desired aqueous storage stability (over 3 months). By modulating solution pH and salinity, the oligomer shell rigidified/softened, resulting in enhancement/quenching of the AIE-based photoluminescence of the silver nanoclusters, accordingly. However, their featured UV–vis absorption remained relatively intact, demonstrating that the changes in photoluminescence are attributed to the transition of radiative and non-radiative energy of the silver nanoclusters. Interestingly, by protonation at low solution pH and electrostatic shield at high salinity, the shrunk shell suppressed the well-established cysteamine-induced etching of the silver nanoclusters, significantly inhibiting the quenching of emission and the bleaching of absorption induced by cysteamine. Such unique stimuli-responsive shell also could serve as a selective window, which could prevent larger size and negatively charged etchants, e.g., glutathione and cysteine, to access and etch the Ag(0) core. Based on this finding, a novel luminescent sensor for cysteamine using the AIE-type silver nanoclusters was then developed with three more advantages. First, owing to relatively active metallic Ag, etching reaction could be fully completed with minutes at ambient condition. Second, specific thiol-Ag interactions also provided superior selectivity for cysteamine over the other 19 natural amino acids (nonthiol-containing) and ascorbic acid. Third, due to high affinity of thiol-Ag interactions, the sensor was endowed with high sensitivity for cysteamine detection with a detection of limit of 15.7 nM. This study not only depicts a new design strategy for other AIE-type metal nanoclusters in sensing applications, but also provides an effective and facile method for understanding structural details of ultrasmall core-shell metal nanoclusters.