Eco‐Friendly N, S Co‐Doped Graphene Quantum Dots for Sensing 3‐Nitro‐L‐Tyrosine via Dark State Formation: Evidence From Photoluminescence and Single‐Particle Spectroscopy

Abstract 3‐Nitro‐L‐tyrosine (3NT) is a key biomarker of oxidative stress associated with neurodegenerative and cardiovascular diseases. Here, we report a selective optical sensing strategy based on photoluminescence (PL) quenching of nitrogen and sulfur co‐doped graphene quantum dots (NS‐GQDs) synthesized from garlic, a natural and sustainable precursor. PL quenching by 3NT is driven by static interactions, as revealed by single‐particle near‐field IR spectroscopy. Time‐dependent density functional theory indicates that complexation with 3NT induces a significant reduction in the oscillator strength of key excited states in the NS‐GQDs, suppressing radiative transitions and suggesting the formation of a non‐emissive “dark states”. This interaction creates an unambiguous optical fingerprint for 3NT, enabling highly selective molecular recognition. Our findings establish a mechanistic blueprint for the rational design of biocompatible carbon‐based nanomaterials for next‐generation sensing platforms.