Noninvasive Salivary Ketone Monitoring: A Ratiometric Pdot Biosensor toward Personalized Ketogenic Management

β-Hydroxybutyrate (BHB), a key ketone body and metabolic biomarker, enables personalized ketogenic-diet management and metabolic-disease monitoring through rapid, noninvasive salivary detection. We present an NADH-responsive semiconducting polymer dot (Pdot) biosensor utilizing enzymatic catalysis of BHB to convert NAD⁺ into NADH. This conversion triggers an intramolecular D-π-A electron transfer between triphenylamine (TPA) donors and NADH acceptors within the Pdots. Upon UV excitation, the intrinsic fluorescence of DPA-PFCN Pdots at 650 nm is quenched, while NADH fluorescence at 470 nm is concurrently enhanced. This yields a ratiometric signal (I₄₇₀/I₆₅₀) that exhibits an exponential correlation with BHB concentration across physiologically relevant ranges. Both solution- and paper-based assay formats facilitate rapid, low-cost analysis. We further demonstrate dynamic monitoring of salivary BHB in healthy volunteers via smartphone-based imaging. This ratiometric Pdot fluorescence sensor provides precise, real-time, noninvasive BHB detection suitable for early metabolic-disease screening and personalized dietary management.