Tryptophanhydroxamic Acid-Stabilized Ultrasmall Gold Nanoclusters: Tuning the Selectivity for Metal Ion Sensing

Sub-nanometer-sized gold nanoclusters (Au NCs) were prepared via the spontaneous reduction of [AuCl₄]^-- ions with a hydroxamate derivative of L-tryptophan (Trp) natural amino acid (TrpHA). The prepared TrpHA-Au NCs possess intense blue emission (λ_em = 470 nm; λ_ex = 380 nm) with a 2.13% absolute quantum yield and 1.47 ns average lifetime. The Trp-stabilized noble metal NCs are excellent metal ion sensors for Fe³⁺, but in this work, we highlighted that the incorporation of the hydroxamate functional group with an excellent metal ion binding capability can tune the selectivity and sensitivity of these NCs, which is a promising way to design novel strategies for the detection of other metal ions as well. Moreover, their simultaneous identification can also be realized. By decreasing the sensitivity of our nano-sensor for Fe³⁺ (limit of detection (LOD) ~11 µM), it was clearly demonstrated that the selectivity for Cu²⁺-ions can be significantly increased (LOD = 3.16 µM) in an acidic (pH = 3-4) condition. The surface-bounded TrpHA molecules can coordinate the Cu²⁺ confirmed by thermodynamic data, which strongly generates the linking of the NCs via the Cu²⁺ ions in acidic pH, and a parallel fluorescence quenching occurs. In the case of Fe³⁺, the degree of quenching strongly depends on the metal ion concentration, and it only occurs when the NCs are not able to bind more Fe³⁺ (~10 µM) on the surface, causing the NCs' aggregation.