Heavy metal cytotoxicity has become a mainstay in scientific research due to the vast range of detrimental effects on living organisms. Cd 2+ in particular can activate a wide range of physiological pathways that can cause significant increases in oxidative stress, cancer formation, or cellular death. This is partly due to its ability to bioaccumulate in the body with a long half-life for excretion. Here, we used hydrophilic redox mediators, ferrocenecarboxylate, 1′1-ferrocene dicarboxylate and hexaamineruthenium, as molecular probes in scanning electrochemical microscopy (SECM) to determine the membrane permeability in single live human bladder cancer (T24) cells affected by Cd 2+ . Hydrophilic and charged sensing agents should be impermeable to the cells. However, when T24 cells were treated with 25 μM Cd 2+ (t ≤ 6 h), the membrane permeability to these sensing agents can be induced to approximately 7.0 × 10 −5 m/s. For acute mM Cd 2+ exposure, the membrane permeability can be induced to 2.0 × 10 −4 m/s within an hour. This increase in membrane permeability is indicative of the loss in membrane integrity, where species can now enter the cell through nonspecific diffusion channels, pore formation, or structural collapse of the membrane. Using the MTT proliferation assay, we were able to confirm that the low dosage Cd 2+ treatment had very minor effect on cellular viability, while the acute treatments decreased viability to 68%. Our SECM results coupled with the MTT assays show the sufficient permeation of these sensors and compromised cell viability. These findings by means of SECM demonstrate that hydrophilic redox mediators are ideal in sensing the effects of toxins, which may induce permeability through detrimental effects on the membrane integrity of the cells.

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The effects of exposure to toxic heavy metals, such as chromium, are of interest in scientific research, owing to its association with oxidative stress, cytotoxicity, and carcinogenicity. This study aims to explore the effects of Cr (VI) on live cell responses. Herein, scanning electrochemical microscopy (SECM) is employed by using depth scan imaging and feedback mode to monitor the membrane permeability of single live human bladder cancer (T24) cells following 1 h incubations with Cr (VI) stimuli. By using membrane‐permeable and impermeable redox mediators, ferrocenemethanol and ferrocenecarboxylic acid, respectively, SECM depth scans yield both electrochemical and topographic information. This provides insights into the relative changes in membrane homeostasis with increased exposure to Cr (VI). Here, SECM has shown great power in determining membrane response to Cr (VI) exposure. Dependent on the level of exposure, transition between three distinct trends was observed. At low incubation concentrations of Cr (VI), the cell membrane permeability coefficients were relatively unaffected. With moderate increases in Cr (VI) concentrations, membrane permeability coefficients of the incubated cells were observed to decrease. Finally, with the higher incubation concentrations, membrane permeability coefficients were found to increase toward values similar to control cells. The Cr (VI) toxicity was further investigated by means of a MTT cell viability study, which exhibited a similar decreasing trend to the cell membrane permeability. These findings further demonstrate the strength of SECM as a bioanalytical technique for monitoring cellular homeostasis.

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