X-ray chemical enhancement

28 January 2012 - Catalysis

x ray catalysis Cheng 2012  Neal N. Cheng of the University of California introduces a new concept he calls chemical enhancement as the increased yield of a chemical reaction due to the chemical properties of the added materials. The context is the hydroxylation of coumarin-3-carboxylic acid (3CCA) in water catalysed by gold nanoparticles (AuNP). The focus is the effect of X-ray irradiation during this reaction. Due to its high atomic number gold is efficient in X-ray absorption. In biology these irradiated gold particles manifest themselves as producers of reactive oxygen species by radiation re-emission but in this particular reaction Cheng argues gold is doing more than just that.
In a typical experiment 3CCA was dissolved in phosphate buffer and DMSO, A AuNP solution (30 nm diameter, PEG coated, Turkevich_method, Sodium borohydride assisted) was added and the mixture irradiated at 3 to 40 Gy per minute. Formation of hydroxylated product was easily monitored by fluorescence as the 3CCA starting material is only weakly fluorescent. Sure enough the enhancement of the fluorescence signal (against control without AuNP added) increases with increased radiation dose. Making the gold particles smaller will increase the signal but adding more gold will only enhance the signal up to a certain concentration. According to Cheng this implies that a rate-limiting agent is in play. Sodium azide does not affect the enhancement ruling out singlet oxygen but superoxide dismutase known to scavenge superoxide does.
The resulting mechanistic picture on display here featuring this superoxide was pieced together without much assistance from the Cheng article. The little cartoon accompanying the abstract contains more clues provided you have incredible eyesight. Both water and oxygen bring about the hydroxylation with the superoxide molecule activated by the AuNP surface. Adding a negative charge to the nanoparticle is a prerequisite. The ultimate second reaction product is hydrogen peroxide
But how are we to understand chemical enhancement? Is the phenomenon really new? This blog is having some doubts. No explanation is given for chemical enhancement other than that it is the difference between the observed enhancement and the physical enhancement you can predict to result from simple radiation (ROS yield). A more reasonable explanation is a simple synergy effect between various oxygen species and available metal surface. Each modification of the experimental setup will bring about a change in the ROS line-up and the mechanism completely changes. And by the way, from the supporting information: will the enhancement increase or decrease when you remove oxygen? Answer: increase!