image: Figure 1 | Schematic illustration of the concept of rapid and localized singlet-oxygen generation enabled by quasi-BIC metasurfaces. Left: In the absence of photoexcitation, the region near the metasurface is dominated by triplet oxygen (blue spheres), and the tumor cells remain viable. Right：Under photoexcitation at a designated wavelength, the Au–TiO₂ metasurface enhances optical absorption and initiates interfacial reactions that convert triplet oxygen into singlet oxygen (red spheres), producing a high-concentration, spatially confined reactive-oxygen environment in the vicinity of the device surface and thereby inducing tumor-cell damage/death (black cells).
Credit: Xing Fu et al.
Singlet oxygen, the lowest excited state of molecular oxygen, exhibits high cytotoxicity against tumor cells and plays a central role in photodynamic therapy. Singlet oxygen is typically produced by molecular photosensitizers, which, however, suffer from limitations such as photobleaching, poor wavelength selectivity, and biocompatibility issues. As alternative sensitizers, metallic and semiconductor nanostructures offer better photostability, but their singlet-oxygen quantum yield is often too low to achieve rapid and precise therapeutic effects under low-dose illumination.
In a new paper published in Light: Science and Applications, a team led by Prof. Xing Fu, and Prof. Qiang Liu from Tsinghua University, China, reports the generation of singlet oxygen at molar-level concentrations within seconds under continuous-wave excitation, which demonstrates a surprisingly six-orders-of-magnitude enhancement over conventional methods. By integrating Bound states in the continuum (BIC) into photocatalysis, the team achieved strong visible-light absorption with minimal metal loading. This approach overcomes the classic trade-off in photocatalysis: while higher metal content traditionally increases light absorption and carrier generation, it also significantly shortens carrier lifetimes.
Specifically, the team designed a metasurface consisting of TiO₂ elliptical nanopillars capped with a 7 nm gold layer. A slight in-plane symmetry breaking (implemented by rotating neighboring ellipses) converts an ideal BIC into a radiative quasi-BIC, achieving optical absorption as high as 45% at 532 nm within a 100-nm optical length. Fig. 2 shows the resulting dramatic near-field enhancement, which raises the electronic temperature in the gold. This enables hot carriers to inject into the conduction band of TiO₂ and initiate interfacial redox reactions that generate singlet oxygen. Importantly, the minimal metal volume simultaneously ensures high carrier activity while effectively suppressing charge recombination.
Beyond high efficiency, the approach offers programmable selectivity. By tuning the geometric scaling of the metasurface, the resonance wavelength can be designed, enabling wavelength-addressable activation. Since singlet oxygen is generated and confined near the patterned regions, the method enables position- and pixel-selective cytotoxicity without additional molecular sensitizers, demonstrated by localized cell death that follows resonance-matched pixels under different illumination wavelengths.
These scientists summarize the advancements of their work:
“BIC-engineered metasurfaces can serve as a general route for efficient photon-to-chemical conversion at solid–liquid interfaces, with immediate opportunities in rapid-acting photodynamic therapy, selective oxidation, and on-chip or flow microreactors where low dose and geometric precision are critical. ”
Article Title
Quasi-BIC metasurfaces enable rapid, localized singlet-oxygen generation