Enhanced Reactive Oxygen Species Generation by GdVO4:Eu³⁺ Nanoparticles and Methylene Blue Complexes under UV–Vis and X‑ray Irradiation

Abstract

This study investigates the generation of reactive oxygen species (ROS) in aqueous solutions containing gadolinium orthovanadate GdVO₄:Eu³⁺ nanoparticles (VNPs) and their complexes with methylene blue (MB). Using conjugated‑diene formation, hydroxyl‑radical detection with coumarin, and singlet‑oxygen monitoring via ADPA, we compared ROS production under UV–Vis and X‑ray irradiation. VNPs–MB complexes produced high levels of hydroxyl radicals and singlet oxygen when illuminated with UV–Vis light, owing to efficient non‑radiative energy transfer from VNPs to MB. In contrast, under X‑ray exposure, VNPs scavenged hydroxyl radicals, a novel observation for this material class. These findings suggest that VNPs–MB complexes could serve as effective agents for X‑ray‑induced photodynamic therapy.

Background

Radiation therapy (RT) treats roughly half of all cancer patients, relying largely on indirect DNA damage mediated by ROS generated during water radiolysis. High‑atomic‑number nanoparticles (Au, Ag, Hf, Gd, Ti) and semiconductor nanoparticles (TiO₂, ZnO, CuO, CeO₂, Al₂O₃, ZnS) enhance RT by amplifying ROS production via photoelectric and Compton interactions. Similarly, semiconductor nanoparticles generate ROS under UV irradiation through charge‑carrier separation: holes oxidize water to produce hydroxyl radicals, while electrons reduce oxygen to superoxide, which can further form singlet oxygen. Scintillating nanoparticles can also transfer X‑ray‑induced energy to photosensitizers, enabling deep‑tissue photodynamic therapy (PDT). Prior work demonstrated that GdVO₄:Eu³⁺ nanoparticles (VNPs) can transfer excitation energy to methylene blue, suggesting potential as X‑ray‑activated PDT agents.

Experimental

Chemicals

All reagents were analytical grade; GdCl₃·6H₂O, EuCl₃·6H₂O, NaVO₃, and methylene blue (MW = 373.90 g mol⁻¹) were used without further purification.

Synthesis of GdVO₄:Eu³⁺ Nanoparticles

Gd₀.₉Eu₀.₁VO₄ colloids were prepared by co‑precipitation of GdCl₃, EuCl₃, EDTA, and Na₃VO₄ under reflux (100 °C, 24 h). The resulting dispersion was dialyzed (MWCO 12 kDa) for 24 h to remove excess ions.

Characterization

Transmission electron microscopy (TEM) revealed spindle‑shaped VNPs (8 × 25 ± 5 nm). Dynamic light scattering gave an average hydrodynamic diameter of 44 ± 0.3 nm. Zeta potential was –18.75 ± 0.15 mV at pH 7.8, reflecting EDTA surface capping. UV–Vis absorption displayed a broad band (250–350 nm) from O→V charge transfer and a red‑shifted Eu³⁺ emission band. Tauc analysis yielded a band gap of 4.13 eV, higher than reported for larger‑particle powders, indicating quantum‑size effects.

Preparation of VNPs–MB Complexes

MB (10 µM) was mixed with VNPs (0.1–10 mg mL⁻¹) in aqueous solution, stirred for 1 h, and the resulting complex had a surface‑bound MB concentration of 10 µM.

ROS Detection Methods

Conjugated‑Diene Test: PC liposomes (1.2 mmol L⁻¹) were oxidized by ROS, monitored at 234 nm after 30 min of UV irradiation (310–400 nm, 43 W cm⁻²).

Hydroxyl Radical Assay: Coumarin (0.1 mmol L⁻¹) was irradiated with 325 nm laser (1 h) or 30 kV X‑rays (30 min). Fluorescence of 7‑hydroxycoumarin (λₘₐₓ ≈ 460 nm) quantified OH•.

Singlet‑Oxygen Assay: ADPA (10 µM) fluorescence (excitation 378 nm) was quenched by singlet oxygen generated under 457 nm laser irradiation (50 mW, 30 min).

Results and Discussion

VNPs Characterization

Spindle‑shaped VNPs exhibit a tetragonal zircon structure (XRD). Their small size leads to a blue‑shifted absorption edge, explaining the measured 4.13 eV band gap.

ROS Generation under UV–Vis

Conjugated‑diene formation was greatest in VNPs–MB complexes, followed by VNPs alone, MB alone, and control. This hierarchy reflects: (1) VNP‑driven hydroxyl and singlet‑oxygen production; (2) MB‑mediated singlet‑oxygen generation; and (3) synergistic energy transfer from VNPs to MB in the complex. Hydroxyl‑radical assays confirmed that VNPs alone generate significant OH• under UV, whereas VNPs–MB complexes produce roughly half that amount, likely due to MB’s adsorption shielding surface water and diverting electron‑hole recombination toward MB excitation. Singlet‑oxygen assays showed a two‑fold increase in ADPA quenching for VNPs–MB complexes versus MB or VNPs alone, confirming efficient energy transfer.

ROS Generation under X‑ray Irradiation

Contrary to UV, X‑ray exposure resulted in reduced 7‑hydroxycoumarin fluorescence for VNP solutions, indicating scavenging of OH• by VNPs. This effect intensified with higher VNP concentrations and represents the first report of GdVO₄:Eu³⁺ nanoparticles acting as ROS quenchers under X‑ray conditions. The underlying mechanism may involve electron capture by VNPs or surface‑mediated radical neutralization, akin to the known behavior of CeO₂ nanoparticles.

Conclusions

GdVO₄:Eu³⁺ nanoparticles and their methylene blue complexes exhibit pronounced ROS production under UV–Vis irradiation, combining hydroxyl radical and singlet‑oxygen pathways. Under X‑ray exposure, VNPs uniquely scavenge hydroxyl radicals. These dual capabilities position VNPs–MB complexes as promising candidates for X‑ray‑activated photodynamic therapy.

Abbreviations

• MB – Methylene Blue
• PS – Photosensitizer
• ROS – Reactive Oxygen Species
• VNPs – GdVO₄:Eu³⁺ Nanoparticles