Delayed Reactive Oxygen Species Production by UV Pre-irradiated Orthovanadate Nanocrystals

Reactive oxygen species (ROS) like superoxide anions $\left(\mathrm{O}_{2}{ }^{\circ}\right)$, hydroxyl radicals $(\cdot \mathrm{OH})$, and hydrogen peroxide $\left(\mathrm{H}_{2} \mathrm{O}_{2}\right)$ are crucial for cellular functions. At low physiological concentrations, ROS act as important redox messengers involved in intracellular signaling and regulation. In contrast, cancer cells maintain ROS levels near the threshold of cell death, making them more susceptible to additional oxidative stress caused by ROS-generating agents. Over the past decade, research into cancer treatments focused on modulating ROS has expanded significantly. Many researchers have dedicated efforts to exploring diverse strategies for inducing ROS production using various nanomaterials. This approach holds promise for developing effective therapies that exploit ROS to damage cancer cells. That is why, nanomaterials that facilitate the generation of ROS and induce oxidative stress have shown promising potential for various nanomedicine applications. In this study, we report delayed pro-oxidant activity of UV pre-irradiated orthovanadate nanocrystals in water solutions without any external stimuli. Our experiments have revealed that orthovanadate nanocrystals demonstrate delayed pro-oxidant activity, specifically the generation of ROS, after UV pre-irradiation and subsequently keeping in darkness. This delayed pro-oxidant activity has been linked to the generation of $\mathrm{O}_{2}^{*}$, $\cdot$ $\cdot \mathrm{OH}$, and ${H}_{2} {O}_{2}$. We hypothesize that the primary source responsible for the effective delayed generation of $\mathrm{O}_{2}^{\circ-}$ and $\cdot \mathrm{OH}$ by orthovanadate nanocrystals in the dark is attributed to the abundance of $\mathrm{V}^{4+}$ ions and oxygen vacancies within the crystal lattice. In dark conditions, oxygen adsorption can lead to the generation of $\mathrm{O}_{2}^{--}$ through a reaction involving the electrons stored in $\mathrm{V}^{4+}$ ions. Simultaneously, photo-induced holes generated by UV pre-irradiation may become trapped within local metastable levels. The presence of these metastable levels serves to localize the holes, delaying their transfer to the surface of the nanocrystal and generation of $\cdot \mathrm{OH}$. The delayed redox activity observed in orthovanadate nanocrystals holds great promise for biomedical applications. This distinctive characteristic offers a novel strategy for advancing the treatment of malignant cells, potentially revolutionizing approaches in cancer therapy.