Monitoring the antioxidant effects of catechin using single-walled carbon nanotubes: Comparative analysis by near-infrared absorption and near-infrared photoluminescence.

We measured the optical responses of single-walled carbon nanotubes (SWNTs) after adding Japanese green tea or catechin. SWNTs were covered with DNA in aqueous solution, and tea or catechin solution was added to the DNA-SWNT suspension. The antioxidant effects of tea and catechin were detected as changes in the near-infrared (NIR) absorption (ABS) and NIR-photoluminescence (PL) spectra of the SWNTs. Commercial Japanese tea, diluted 100 times and containing 15μg/mL catechin, was sufficient for recovering NIR-ABS and NIR-PL spectra when the DNA-SWNT suspension was pre-treated with 0.03% hydrogen peroxide(H2O2). Similar results were obtained with 15μg/mL of pure catechin solution. SWNTs with specific chirality were sensitive to the NIR-ABS and NIR-PL changes. The (10, 5)/(8, 7) and (9, 4) SWNTs showed the highest recovery in NIR-ABS and NIR-PL, respectively. NIR-PL recovery was higher than that of NIR-ABS for (10, 5)/(8, 7) and (9, 4). Spectral changes could be monitored thoroughly at pH 8.0, contrary to pH 6.0 and 7.3. However, the most dynamic recovery of NIR-ABS and NIR-PL was observed at pH 6.0. Furthermore, time-lapse measurements revealed that recovery was faster with tea or catechin addition than H2O2-induced oxidation.