Benzene layer-aligned electrochemical transformation of SWCNTs to redox-active macro-walled CNTs: enabling oxygen interference-free monitoring of ROS release from HeLa cancer cells

Abstract

The search for novel carbon allotropes with unique electrochemical properties remains a key area of research in materials science. Here, we introduce a novel carbon nanotube material termed macro-walled CNTs, synthesized through an in situ benzene (BZ) electrochemical reaction on a glassy carbon electrode modified with single-walled carbon nanotubes (GCE/SWCNTs) in a pH 2 KCl–HCl electrolyte solution. The modified electrode, denoted as GCE/SWCNT@BZ-Redox (where BZ-Redox represents redox-active benzene species), exhibited a well-defined redox peak at E°′ = 150 ± 10 mV vs. Ag/AgCl, along with a surface excess value of 5.1 nmol cm⁻². The physicochemical characterization of SWCNT@BZ-Redox, conducted using TEM, FTIR, Raman spectroscopy, and various electrochemical methods including scanning electrochemical microscopy (SECM) imaging, revealed a significant modification involving highly redox-active benzene multilayers with a diameter of approximately ∼200 nm, which is about 10 times larger than that of pristine SWCNTs (∼10–15 nm). Of particular interest is the stability of the redox peak under physiological pH conditions, as well as its ability to mediate hydrogen peroxide reduction reactions at a potential of −0.25 V vs. Ag/AgCl, akin to the reaction catalyzed by horseradish peroxidase enzymatic systems. For practical applications, continuous monitoring of reactive oxygen species (ROS), specifically H₂O₂ release kinetics from stressed HeLa cancer cells under simulated conditions, without any dissolved oxygen interference, was demonstrated using a home-made bath-injection analysis system coupled with a screen-printed electrode modified with SWCNT@BZ-Redox as a detector.