Xanthene dyes are common fluorophores which have been widely used as molecular probes. The xanthene fluorophores can be used as highly selective optical sensors to detect disease biomarkers. A new fluorogenic dye containing an alpha, beta-unsaturated aldehyde moiety exhibits selective fluorescent signal enhancement in the presence of cysteine or peptides containing N-terminal cysteine residues. The mechanism is based on synergistic covalent and supramolecular interactions. A unique rhodamine boronic acid indicator is used in an optimized data collection protocol for wavelength- and time-dependent selectivity of various saccharides and nucleosides. One indicator is thereby capable of selectively distinguishing structurally related analytes in mixtures. Moreover, the rhodamine-based boronic acid responds linearly to increasing riboside concentrations in urine samples, potentially enabling the screening for inborn purine metabolism disorders.