Ratiometric fluorescence and colorimetric dual-mode sensing platform based on carbon dots for detecting copper(II) ions and D-penicillamine

A sensing platform with both ratiometric fluorescence and colorimetric responses towards copper(II) ions (Cu²⁺) and D-penicillamine (D-pen) was constructed based on carbon dots (CDs). o-Phenylenediamine (OPD) was employed as a chromogenic development reagent for reaction with Cu²⁺to generate the oxidation product 2,3-diaminophenazine (oxOPD), which not only emits green fluorescence at 555 nm, but also quenches the blue fluorescence of CDs at 443 nm via the inner filter effect (IFE) and Förster resonance energy transfer (FRET). Additionally, oxOPD exhibits obvious absorption at 420 nm. Since the intense chelation affinity of D-pen to Cu²⁺ greatly inhibits the oxidation of OPD, the intensity ratio of fluorescence at 443 nm to that at 555 nm (F₄₄₃/F₅₅₅) and the absorbance at 420 nm (A₄₂₀) were conveniently employed as spectral response signals to represent the amount of D-pen introduced into the testing system. This dual-signal sensing platform exhibits excellent selectivity and sensitivity towards both Cu²⁺ and D-pen, with low detection limits of 0.019 μM and 0.092 μM, respectively. In addition, the low cytotoxicity of the testing reagents involved in the proposed sensing platform facilitates its application for live cell imaging. Graphical abstract: [Figure not available: see fulltext.].