<p>In this study, we present a reversible, fluorescence turn-on sensor for noninvasive detection of serine using copper–cobalt nanoclusters (Cu–CoNCs) quenched by Cu²⁺ ions. The Cu²⁺ ions suppress the intrinsic fluorescence of Cu–CoNCs primarily through static quenching and the inner filter effect (IFE), both of which contribute to the observed fluorescence suppression. Serine chelates Cu²⁺ through its amine and hydroxyl groups, sequestering the quencher and reinstating the excited-state properties of the nanoclusters. Fluorescence lifetime measurements, spectral analysis and Stern-Volmer plots support the proposed mechanism. The sensor demonstrates strong selectivity and specificity toward serine in biological matrices such as saliva and urine, where fluorescence recovery is both significant and reproducible. Optimal performance was observed at a near neutral to slightly basic pH, with extreme acidic or alkaline conditions adversely affecting sensor response. Real sample analysis confirms high 92–101% recovery rates from spiked saliva samples. This economical, portable sensing platform provides a promising route for point of care diagnostics. The approach underscores the potential of serine as a noninvasive biomarker for cancer and related metabolic disorders, paving the way for early screening using easily accessible biological fluids.</p>