Reaction like The Fenton can produce hydroxyl radicals even in dark conditions

I totaly agree with Kamran.

If often fenton reaction occured in aqueous solution, transforming H2O2 in hydroxyl radical, it is possible to have fenton reaction in organic or inorganic medium.

Then you can generate hydroxyl radical, and observe its effects, without suffering from the oxidative action of H2O2.

Sincerely

You can use the deoxyribose assay, in which OH radical are produced, even in dark conditions.

Here find the reference:

J M Gutteridge and B Halliwell. The deoxyribose assay: an assay both for 'free' hydroxyl radical and for site-specific hydroxyl radical production.Biochem J. 1988; 253(3): 932–933.

Best regards

You can also use the gas phase ozonolysis of isoprene under dark conditions. This is the reference:

Malkin et al.: Atmos. Chem. Phys., 10, 1441–1459, 2010

or:

http://mcm.leeds.ac.uk/MCM/browse.htt?species=C5H8

If the catalyst contains noble metallic sites (e.g., platinum (Pt)) and hydrogen is present alongside oxygen in the atmosphere, then the hydroxyl radicals can potentially be generated even under dark conditions. The hydrogen and oxygen molecules dissociatively adsorb on the active Pt sites to yield adatoms that undergo additional reactions to yield hydroxyl radicals. Similarly, if hydrogen peroxide is present in the atmosphere, then the hydroxyl radicals can be generated on the active Pt sites under dark conditions.

See the following references for more details: (i) Chen, Y., Li, D., Wang, X., Wang, X., Fu, X., 2004. H2–O2 atmosphere increases the activity of Pt/TiO2 for benzene photocatalytic oxidation by two orders of magnitude. Chemical Communications, 2304-2305, (ii) Li, D., Chen, Z., Chen, Y., Li, W., Huang, H., He, Y., Fu, X., 2008. A New Route for Degradation of Volatile Organic Compounds under Visible Light: Using the Bifunctional Photocatalyst Pt/TiO2−xNx in H2−O2 Atmosphere. Environmental Science & Technology 42, 2130-2135, and (iii) Chen, L., Xu, Z., He, C., Wang, Y., Liang, Z., Zhao, Q., Lu, Q., 2018. Gas-phase total oxidation of nitric oxide using hydrogen peroxide vapor over Pt/TiO2. Applied Surface Science 457, 821-830.