Heterobimetallic-doped carbon dots for colorimetric, fluorescent and photothermal sensing of total antioxidant capacity in fruits.

A heterobimetallic-doped carbon dot (Mn, Ce-CDs) based triple‑signal strategy integrating colorimetric, fluorescent, and photothermal signals for TAC determination in fruits is reported. The Mn, Ce-CDs are synthesized via a one‑step hydrothermal route and exhibit intrinsic oxidase‑mimetic activity, catalyzing the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by dissolved oxygen to generate blue oxidized TMB (oxTMB). This process simultaneously quenches the CDs' fluorescence (via inner filter effect) and the generated oxTMB produces a photothermal effect under near‑infrared irradiation. Upon introduction of antioxidants (ascorbic acid, glutathione, cysteine), the high‑valence metal centers (Mn3+, Ce4+) are reduced to lower states (Mn2+, Ce3+), interrupting the bimetallic redox cycle. Consequently, oxTMB formation is suppressed, fluorescence is restored, and the photothermal response decreases. Under optimized conditions, the colorimetric mode achieved detection limits of 38.98, 49.63 and 47.65 nM for AA, GSH, and Cys, respectively, the fluorescent mode gave 27.04, 25.89 and 32.84 nM, respectively, and the photothermal mode provided 3.74, 1.36 and 2.67 µM, respectively. The colorimetric and fluorescent modes exhibit good linearity in the range of 0.5-50 µM, while the photothermal mode shows linear response from 5 to 50 µM. Recoveries from spiked fruit samples range from 96.7% to 101.4%, demonstrating good inter-method agreement. This work presents Mn and Ce heterobimetallic-doped carbon dots as a versatile triple‑mode sensing platform for TAC assessment in fruits, with potential applications in fruit quality control.