Stimuli-responsive poly(N-isopropylacrylamide) nanogel with covalently labeled rhodamine B urea derivatives (P(NIPAM-co-RhBUA)) is utilized as a sensitive fluorescent probe for Cr3+ in aqueous solution, and its thermo-induced tunable detection capacity is investigated. At 20 degrees C, non-fluorescent nanogel can selectively bind with Cr3+ over some other metal ions, leading to prominent fluorescence OFF-ON switching due to the recognition of RhBUA with Cr3+. Upon heating above the phase transition temperature, enhanced fluorescence intensity is observed (approximate to 61-fold increase at 45 degrees C) for the nanogel in the presence of Cr3+, accompanied with an improved detection sensitivity, which suggest that hydrophobic microenvironment generated in the collapsed nanogel plays an active role for their detection performance.