The hydroxyl radical is far more reactive than superoxide anions. Its half-life is too short for detection via NBT or XTT. 

Regards, Christian

But do you think some of the Oh radicals may still react with XTT or NBT? Are these scavengers 100% accurate?

As far as I know reactions between hydroxyl radicals and XTT or NBT, respectively, have not been described in the literature. Maybe other RG members can help with your question. I would assume that the steady state concentration of hydroxyl radicals is too low for NBT or XTT detection.

Regards, Christian

Yeah, I too didn't find any literature on this part or maybe I may have missed it. I really want to understand the mechanism by which pCBA would consume OH radicals or NBT would consume superoxide radical. Maybe by looking at their structure, one may deduce the actual reason why pCBA only reacts with OH radicals and not with superoxide or singlet oxygen.

I agree, it is definitely an interesting question. If I find out more I will update you here.

Cheers, Christian

Due to half life it cannot be detected with NBT. 

Regards,

Parameswari

Is this is a proven fact about half life being short? What if there is only deionized water, NBT and OH radicals, what would happen to OH radicals then? What made NBT to react only with superoxide instead of OH radicals?

If you would have a system that constantly produces hydroxyl radicals you would probably see NBT reduction. NBT reduction is not very specific. Other compounds, like folic acid, can also lead to formazan formation. The short half life of hydroxyl radicals has been proved. Please see references in the attached paper.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3460889/

Environmental Toxicology and Chemistry of Oxygen Species By Dr Irena Kruk also mentions that formazon from NBT can also be produced by OH radicals. NBT scavenger is not reaction specific. So, what would be a specific superoxide scavenger which will not react with OH and singlet oxygen?

Specificity is usually a problem when it comes to detect reactive oxygen species. Furthermore, they are converted in the cell to other forms (superoxide is also non-enzymatically dismutated to give rise to hydrogen peroxide, for example). As such it can be more informative to measure oxidative stress in a more holistc way. 

Please have a look at the publication in the attachment. It can be a helpful resource.

Article Measurement of Reactive Oxygen Species, Reactive Nitrogen Sp...

Thank you, I read the paper and it's about the biological system. My system would contain UV light-induced TiO2 nanoparticles with humic acid. I want to measure OH radicals, superoxide anions separately with high accuracy. Plus we can not use EPR and other chemiluminescence techniques.