A fluorescence "off-on" probe CBF, constructed by incorporating a dioxaborine unit into a microenvironment-sensitive fluorophore, was developed for serum albumin (SA). Upon binding to SA, the dioxaborine group in CBF was hydrolyzed into β-diketonate, which triggered dramatic fluorescence enhancement (over 1000-fold) along with a remarkable blue-shift (∼100 nm). The bioimaging results suggested that more SA were taken in by cancer cells.