Copper-based nanoparticles (CuNPs) have attracted great attention in the fields of energy conversion, catalysis, and biomedicine due to their facile preparation procedures, abundant raw materials, low toxicity, tunable ultra-small size, tailorable surface chemistry, and desirable physical and chemical performances. In particular, the CuNPs luminescent materials have greatly boosted the development of optical sensors. However, for crystalline metal-based nanomaterials, there are fewer reactive sites due to the long-range ordering of their lattice structure, and the existence of crystal defects resulting from its impossibility to reach absolute zero degree would inhibit photo-generated electron transfer. Hence, exploring novel microstructures of CuNPs is a stringent requirement for both luminescent materials and optical detection. In recent years, the amorphous metal-based nanoparticles have been validated that their disordered structure could not only facilitate the charge transfer between the metal core and surface ligand by mitigating the recombination of electron and hole in the field of energy conversion, but also expose more reactive sites by its low-coordination atoms in the field of catalysis. Combination of the easy charge transfer and rich reactive sites, the amorphous CuNPs are reasonably expected to be an ideal material for photoluminescence as well as optical detection. Whereas, due to the factor that amorphous microstructure is the thermodynamic sub-stable state of CuNPs, it is a great challenge to inhibit its formation of stable crystal. Thus, whether the amorphous CuNPs with necessary photoluminescence performance for optical detection could be achieved remains enigmatic and is vitally important for ultrasensitive and robust detection.

The researchers prepared amorphous glutathione functionalized copper-based nanoclusters (GSH-CuNPs) based on through-space conjugation (TSC) with excellent phosphorescence properties by clusterization-triggered emission (CTE). The formation of this amorphous aggregated state structure originated from inhibiting the interatomic metallic bonding by organic small molecule ligands and subtly tailoring the solvent polarity. Compared with the previously reported copper-based nanostructure phosphorescent materials, the material designed in this work has significant improvements in phosphorescent emission properties including high quantum yield (13.22%), long phosphorescence lifetime (21.7 μs), large Stokes shift (298 nm), as well as anti-mechanochromic luminescence characteristic, all of which facilitate the optical detection. The surface ligands of amorphous GSH-CuNPs exposed a large number of carboxyl and amino groups to provide rich recognition sites for 2,4,6-trinitrotoluene (TNT), achieving ultra-sensitive and specific phosphorescence quenching detection of TNT as a typical explosive, and proposing a triplet-state phosphorescence quenching mechanism based on photo-induced electron transfer (PET) through theoretical calculations combined with relevant experimental data. In addition, the solid-state luminescence of GSH-CuNPs was extended to establish a CuNPs-paper chip, which realized the on-site visualized sampling and detection of solid TNT residues with excellent recoverable cycle detection performance; the extended CuNPs-sponge sensor realized the ultra-sensitive detection of trace airborne TNT microparticulates, which laid a research foundation for integrating into portable in-field detector applicable for searching hidden TNT explosives. This is the first time that the amorphous Cu-based nanoparticles induced by the aggregation of Cu-based complex have been achieved and attempted to optical detection, which fundamentally helps understanding different existence forms of metal-based nanomaterials as well as stretching their application potentials.

More information: Xincun Dou, Amorphous Copper-Based Nanoparticles with Clusterization-Triggered Phosphorescence for Ultrasensing 2,4,6-Trinitrotoluene, Advanced Materials, 2023, https://doi.org/10.1002/adma.202300526.