Counterexample to Luminescent Metal Nanocluster Paradigm: Reactive Au(I) Complexes from His–Au(III) Synthetic Reactions and Their Chemistry for Direct Analysis of d-Penicillamine

There is a widely accepted material characterization paradigm in the success of synthesis of luminescent metal nanoclusters (NCs) in the aqueous phase: new emission, metal reduction, and ultrasmall particles (size < 3 nm). Herein, we falsified well-known fluorescent histidine (His)-directed Au NCs and a new model of metastable His–Au(I) complexes with emissive His oxidation products has been established. The redox reaction of His and Au(III) yields His oligomers with blue-green fluorescence and reducible Au(I) self-assemblies, which can form ultrasmall particles at electron bombardment. The resultant Au(I) complexes can be further reduced by d-penicillamine (DPA) via forming anisotropic Au nanoparticles with distinct local surface plasmon resonance absorption. The emerging absorption can quench the fluorescence of the His oxidation products through the inner filter effect pathway. A facile dual-model analytical approach is thus proposed to directly detect DPA fluorometrically and colorimetrically without interference from common biothiols, including cysteine and glutathione. Thus, with the help of a smartphone app, a highly sensitive and selective point-of-care testing for DPA direct detection can be realized. Our study warrants the importance of thinking twice about characterization results and supports corrective models for finding new reactions and possible applications.