Thermo-Responsive Polymer-Modified Carbon Nanofiber Electrodes with Autonomous and Heteronomous Switchable Selectivity for Catechol Derivatives
Authors: Komori Kikuo , Liu Minghao , Shiraishi Kohei 
Published: December 2024

Sophisticated switchable interfaces enable to develop advanced micromachines and biodevices. In the electrochemistry field, functional electrodes switchable by basically either environmental stimuli, such as changes in temperature and pH, or artificial input signals, such as a specific wavelength and magnetic field, have attracted great attention to develop sensors with tunable sensitivity or selectivity and fuel cells with tunable power output in the past few decades. Recently, we have developed thermo- and photo-responsive cup-stacked carbon nanofiber electrodes modified with thermo-responsive poly(N-isopropylacrylamide) (PNIPA/CSCNF electrodes) [1]. PNIPA is known to be reversibly switched between hydrophilic expanded coil structure and hydrophobic collapsed globule structure by environmental temperature changes. CSCNFs have been used as not only electroactive electrode materials but also photothermal conversion materials of near-infrared (NIR) light. Redox species, such as a hydrophilic [Fe(CN)6]3–/4– ion, easily diffused to the CSCNF electrode surface across the swollen PNIPA layer because of its hydrophobic hydration at 25 ˚C without NIR light irradiation, leading to the larger current responses. However, in an electrolyte solution at 45 ˚C and even 25 ˚C with NIR light (> 940 nm) irradiation, the contracted structure of PNIPA was formed because of its hydrophobic interaction, resulting in suppressing transport of the redox species to the CSCNF electrode surface due to its steric hindrance and hydrophobicity. Therefore, the obtained PNIPA/CSCNF electrode allowed to control the activation and inhibition of the electrochemical process for the diffusional redox probe by both alternating surrounding temperature and irradiating the electrode surface with NIR light (Figure 1). If two chemicals with similar structures but different octanol-water partition coefficient log K
ow, such as catechol (Mw = 111, Log K
ow = 0.88) and dopamine (Mw = 153, Log K
ow = –0.98) known as one of the neurotransmitters, contain in the same solution, the present electrode might enable to detect separately. In the present work, we examined autonomous and heteronomous switchable selectivity for catechol and dopamine at the thermo- and photo-responsive PNIPA/CSCNF electrode. As a result, both catechol and dopamine permeated to the CSCNF electrode surface through the swollen PNIPA layer at 25 ˚C, causing their electrochemical signal readout. In contrast, transport of dopamine was selectively suppressed to the CSCNF electrode surface by exposing PNIPA to temperature at 45 ˚C or NIR light irradiation to the CSCNF electrode surface even at 25 ˚C due to the steric hindrance and hydrophobicity of the collapsed globule structure of the PNIPA layer. The electrochemical signal for catechol was therefore observed preferentially. We thus expected that the present electrode would be applied to electrochemical sensors with autonomous and heteronomous switchable selectivity for catechol-based neurotransmitters. [1] K. Komori, R. Ihara, S. Hirao, M. Liu, Y. Toyota, M. Nakata, Y. Tani, and K. Shiraishi, J. Electroanal. Chem., 992, 116704 (2022).

Figure 1