Quantitative Detection of the Disappearance of the Antioxidant Ability of Catechin by Near-Infrared Absorption and Near-Infrared Photoluminescence Spectra of Single-Walled Carbon Nanotubes.

We succeeded in quantitatively detecting the disappearance of catechin antioxidant ability as a function of time using near-infrared (NIR) absorbance and NIR photoluminescence (PL) spectra of single-walled carbon nanotubes (SWNTs) wrapped with DNA molecules (DNA-SWNT hybrids). When 15 μg/mL of catechin was added to the oxidized hybrid suspension, the absorbance of SWNTs increased, according to the antioxidant ability of catechin, and the effect was maintained at least for 30 min. When catechin concentrations were less than 0.3 μg/mL, SWNT absorbance gradually decreased, although it increased when catechin is added. The results revealed that disappearance of the catechin effects could be quantitatively detected by NIR absorbance spectra. When NIR PL was employed, the disappearance of PL intensity was also observed in the case of low catechin concentrations. However, time-lapse measurement of the disappearance was difficult because the PL intensity was rapidly quenched. In addition, the optical responses were different due to different chirality of SWNTs. Our results suggested that both NIR absorbance and PL can detect disappearance of catechin antioxidant effects; in particular, slow response of NIR absorbance was effective to detect time dependence of the disappearance of the catechin effects. Contrarily, PL revealed huge and rapid responses in contrast to NIR absorbance. PL might be effective for reversible use of DNA-SWNT hybrids as a nanobiosensor.