Noble metal quantum dots for nanomedicine

Abstract

Fluorescent, biocompatible metal nanoclusters offer a promising platform for optical sensing. Here, glutathione-stabilized gold nanoclusters (GSH–AuNCs) were synthesized and their photophysics systematically tuned by post-synthetic metal additions. Pristine GSH–AuNCs showed a distinct UV–Vis feature near 365 nm, strong red emission centered at ~634 nm, and a multi-exponential lifetime with ⟨τ⟩ ≈ 131 ns. Optical responses were quantified by steady-state fluorescence, excitation–emission matrices (EEM), UV–Vis (500–700 nm), and time-correlated single-photon counting (TCSPC, Δ-diode 374 nm). Adding Ag⁺ (1–20 µL) produced an immediate red shift (to ~645 nm from 3 µL) and a marked intensity increase that peaked at ~16 µL, followed by a decline at 18–20 µL. Lifetimes lengthened with the growth of long-lived components (⟨τ⟩ ≈ 1.16 µs at 20 µL Ag⁺), consistent with surface passivation and formation of Au–Ag motifs that stabilize LMCT-type emissive states. In contrast, Cu²⁺ (1–6 µL) caused strong, largely monotonic quenching with only minor wavelength changes and a shorter average lifetime (⟨τ⟩ ≈ 117 ns), indicating additional non-radiative decay pathways introduced by Cu–ligand coordination. Post-synthetic Au dosing was performed on Ag-modified clusters (20 µL Ag⁺ baseline) to examine surface reorganization; this step forms mixed Au/Ag/Au nanoclusters rather than regenerating the pristine AuNCs. Upon adding 1–8 µL Au, the fluorescence increased and blue-shifted toward 616–622 nm; spectra acquired at a 2.0-nm emission slit were intensity-converted to the 3.5-nm standard with a calibrated factor (A₃․₅/A₂ = 2.7207). The lifetime for 20 µL Ag⁺ + 8 µL Au shortened to ⟨τ⟩ ≈ 89.5 ns, indicating enhancement via a higher radiative rate rather than excited-state prolongation. Across all series, UV–Vis showed no plasmon band, ruling out growth of large nanoparticles and pointing to surface- chemistry control. Together, these results demonstrate that selective coordination/alloying (Ag⁺, Cu²⁺, Au) provides a practical handle to brighten, red/blue-shift, or quench GSH– AuNC emission in a predictable way; the tunability and stability highlight Au/Ag/Au and Ag-modified GSH–AuNCs as adaptable optical probes with clear potential for analyte sensing in biologically relevant media.