Ultrasensitive ratiometric fluorescent probes for Hg(II) and trypsin activity based on carbon dots and metalloporphyrin via a target recycling amplification strategy

A convenient and ultrasensitive ratiometric fluorescent probe was innovatively developed for Hg(II) detection and trypsin activity evaluation based on carbon dots (CDs) and tetraphenylporphyrin tetrasulfonic acid (TPPS) using bovine serum albumin (BSA) as the substrate of trypsin. The ratiometric fluorescence signal arises from CDs (λ_em = 506 nm) and TPPS (λ_em = 645 nm) via an inner filter effect. Hg²⁺ can trigger the formation of TPPS-Mn²⁺ metalloporphyrin for target Hg²⁺ recycling amplification, while both TPPS-Hg²⁺ and TPPS-Mn²⁺ metalloporphyrins do not affect the fluorescence of CDs. Small amino acids and peptide fragments, which are the products of BSA under the digestion of trypsin, bind stronger with Hg²⁺ than with TPPS. The decomposition of both TPPS-Hg²⁺ and TPPS-Mn²⁺ metalloporphyrins leads to a variation in the ratiometric fluorescence signal. Under optimized conditions, this probe provided an inspiring detection limit of 0.086 nM for Hg²⁺ and 0.013 ng mL^-1 for trypsin, which possessed acceptable sensitivity for Hg²⁺ detection and trypsin activity evaluation in authentic samples. This unprecedented CD-based ratiometric fluorescence proposal for ultrasensitive quantification of Hg²⁺ concentration and selective assessment of trypsin activity gives a new insight for designing metal ion assays or enzymatic activity bioassays under different enzymatic substrates in the near future.