Chirality luminescent properties of single-walled carbon nanotubes during redox reactions

The optical measurement of single-walled carbon nanotubes (SWNTs) is a useful approach for understanding chirality characteristics because the excitation and emission wavelengths that produce the maximum emission intensity are unique to the chirality of SWNTs. The emission properties of SWNT chirality are useful in various biological applications and are envisaged to be applicable in nanobiosensors. Therefore, it is necessary to detect even small environmental changes, such as redox reactions, and it is important to identify optimal chirality according to the applications. In this study, we focused on the presence of SWNTs with different chiralities within the same SWNT powder, and we investigated the correlation between the rate of change of the emission intensity due to redox reactions and SWNT chirality. We measured the redox change rate of chirality, which was difficult to detect because of the low emission intensity, by increasing the exposure time during photoluminescence (PL) measurement. 0.5 mg of SWNT powder and 1 mL of double-stranded DNA (dsDNA) stock solution were mixed and sonicated using a probe-type sonicator on ice. The supernatant of the prepared dsDNA-SWNT dispersion was stored after centrifugation. Hydrogen peroxide (H₂O₂; final concentration: 0.03%) was added to this dispersion for oxidation, and then, a catechin aqueous solution (final concentration 1.5 μg/mL) was added to the solution to measure PL. The exposure time was 90 s to obtain sufficient emission intensity. The measurement results showed that the magnitude of the rate of change of the PL intensity due to redox reaction was different for each chirality. Focusing on the (8,6) and (9,4) chiralities, which showed a large rate of change, the PL intensity decreased by 57.4% and 54.6% from the initial state, respectively, when H₂O₂ was added. Subsequently, these values increased by 1024% and 558% with the addition of a catechin aqueous solution, respectively. Furthermore, from the comparison of the PL detection results and optical response characteristics of each chirality, it was observed that the rate of change of the PL intensity of SWNTs during redox reactions using H₂O₂ and catechin had the strongest correlation with the SWNT diameter.