Nanodiamond (ND) with nitrogen vacancy (NV-) color centers has emerged as an important material for quantum sensing and imaging. Fluorescent, carboxylated ND (140 nm) is investigated for the detection of dopamine (DA), caffeine (CA), and ascorbic acid (AA). Over a 200 nM range, DA and CA quenched the ND fluorescence by 7.1 and 9.8%, respectively. For AA, fluorescence was quenched (2.9%) at nM concentrations and enhanced at μM concentrations. The quenching fit well to Langmuir adsorption isotherms. For DA-CA mixtures, the CA at nM concentrations did not affect DA quenching but interfered when at μM concentrations. The DA at nM or μM concentrations lessened CA quenching. For DA-AA mixtures with DA at mM concentrations, AA quenched fluorescence throughout the nM and μM range, with increased quenching in the nM range. These studies support ND fluorescence modulation as a possible sensor modality for bioanalyte detection.