Caffeine's antioxidant potency optically sensed with double-stranded DNA-encased single-walled carbon nanotubes.

There is great interest in developing a sensitive method being able to quantitatively measure and compare antioxidant potencies of samples of interest against multiple reactive oxygen species (ROS) whose imbalance could cause oxidative stress. Here, a sensitive nanoprobe, double-stranded DNA encased single-walled carbon nanotubes (SWNTs) has been developed to determine antioxidant potencies of selected samples (caffeine, regular coffee, and decaffeinated coffee) against ROS, hydrogen peroxide, and hydroxyl radicals. Antioxidant vitamin C and uric acid are used as standards. The method focuses on unique dual optical sensing capability of SWNTs, the rate of spectral suppression when exposed to ROS, and the magnitude of spectral recovery of the ROS-suppressed SWNTs when an antioxidant is added. It is found that the dual sensing capability of SWNTs is still sustained when reacting with the reactive hydroxyl radicals. The results show that caffeine's antioxidant potency is weak, about one millionth of those of vitamin C and uric acid. It is a better scavenger of hydrogen peroxide and a little less effective for hydroxyl radicals. In comparison, coffee, regardless of regular or decaffeinated, is a more efficient antioxidant than caffeine, having an antioxidant potency about ten thousand times stronger. This work provides a versatile detection method for evaluating the antioxidant potencies of samples of interest against various ROS for chemical, biological, and medical applications.