Red-Emitting Silver Nanoclusters for Dual-Mode Detection of Cu²⁺ and Vitamin B₁₂ in Living Cells

Biomolecule-stabilized metal nanoclusters (MNCs) exhibit enormous potential as unique luminescent materials in various applications. However, synthesis of highly stable and bio-friendly fluorescent MNCs is still a challenge. Here, we report a facile synthesis of red-emitting silver nanoclusters (LYS–AgNCs) within a dithiothreitol-reduced lysozyme (LYS) scaffold. The nanoclusters exhibit a uniform size distribution, excellent water solubility, and superior photoluminescence properties featuring a quantum yield of 6.1%, a massive (280 nm) Stokes shift, and solid-state emission, and pH stability augments their applicability in dual-mode sensing platforms for Cu2+ and vitamin B12 (VB12). Two contrasting fluorescence (FL)-quenching mechanisms are responsible for the sensing; Cu2+-induced FL quenching occurs via a multifaceted mechanism involving both static and dynamic quenching, whereas the inner filter effect and Förster resonance energy transfer are mainly responsible for VB12-induced FL quenching. An inexpensive portable paper strip is fabricated using LYS–AgNCs for on-site field application, enabling instrument-free fast and visual detection of Cu2+ and VB12. Moreover, LYS–AgNCs also possess favorable biocompatibility against human cervical cancer cells (HeLa), making them a suitable nanoprobe for cell imaging and an efficient agent for detecting Cu2+ and VB12 inside live cells as well. Finally, we demonstrate the applicability of LYS–AgNCs for real-sample analysis of VB12 with a satisfactory outcome.