Regulating the compactness of Cu nanoclusters with assembly-induced emission properties for highly sensitive optical strain sensing

Abstract

Flexible low-strain sensing materials with noncontact detection methods are urgently needed for several emerging applications such as defect monitoring, stress localization, and micro-deformation detection. Among various reported methods, correlating optical signals with stress/strain distributions has been recognized as an effective approach for monitoring material deformation without physical contact. Herein, we report a noncontact strain-responsive fluorescent sensor based on polydimethylsiloxane (PDMS) embedded with strong red-emissive copper nanocluster (CuNC) assemblies. The CuNC units, capped with hydrophobic 2-mercapto-4-methylpyrimidine (MMP), self-assembled into ordered microrods in aqueous medium with a certain amount of free MMP trapped inside (CuNCs-MMP); thus, intramolecular rotation and vibration of the surface ligands were highly restricted within the obtained dense aggregation. Owing to the compatibility with PDMS and the favorable molecular characteristics of MMP, the CuNCs-MMP were well dispersed and embedded in the PDMS matrix with less defects. When the composite elastomer underwent slight deformation under minor external strain, the free ligands in the assembled structure moved along with the PDMS matrix, leading to rearrangement of surface ligands and additional space for intramolecular rotation and vibration, thereby enhancing ligand-related nonradiative relaxation and decreasing photoluminescence (PL) intensities. The relative emission intensity at 690 nm varied systematically over the external strain range of 0% to 5%, with a minimum detectable strain of 0.5% and reasonable repeatability. This work provides a feasible strategy to construct a noncontact fluorescent sensor based on assembly-induced emission of CuNCs.