π-π-Anchored sorafenib on carbon black as a stable molecular redox electrocatalyst for thiol oxidation and point-of-care sensing in cancer cells.

Sorafenib (SF), a first-line targeted therapy for hepatocellular carcinoma, induces tumor cell death partly through modulation of redox processes via interactions with free thiol groups. Immobilization of such redox-active molecules on electrode surfaces provides a powerful platform for real-time monitoring of thiol chemistry, which is highly relevant for biomedical diagnostics. In this study, an in situ electrochemical strategy was developed to functionalize SF as a surface-confined redox-active species (CB@SF-Redox) on carbon black (CB)-modified electrodes. The resulting interface exhibited a well-defined formal potential (E0 ≈ 0.1 V vs Ag/AgCl) and a high surface coverage (Γ ≈ 16 nmol cm-2).

Strong π-π interactions between SF and the graphitic domains of CB facilitated stable immobilization. In situ electrochemical quartz crystal microbalance (EQCM) and high-resolution mass spectrometry (HR-MS) confirmed the formation of a dimeric intermediate (SF-NH-NH-SF) during the electrochemical process. Electrochemical studies revealed that the CB@SF-Redox interface efficiently mediated thiol oxidation and reduction at a remarkably low potential (∼ -0.1 V vs Ag/AgCl), which was 300-1000 mV lower than that of most reported electrocatalysts. This platform was further employed for point-of-care thiol detection using differential pulse voltammetry with a three-in-one screen-printed electrode and a single-drop sample approach, achieving a detection limit of 2 μM and a sensitivity of 0.96 μA μM-1.

Furthermore, real-time monitoring of intracellular thiol levels in HCT 116 colorectal cancer cells was demonstrated. Overall, this work presents a robust electrochemical strategy for transforming pharmaceutical molecules into functional redox interfaces, offering significant potential for next-generation diagnostic tools in clinical and therapeutic applications.