Zinc oxide nanostructured random lasers: A review of their potential as light sources for bioimaging and biosensing applications

Luminescent nanostructures are gaining prominence as vital probes for detection and sensing due to the growing demand for advanced imaging techniques that require superior light sources. Although conventional lasers and superluminescent diodes offer high brightness, their high spatial coherence can result in speckle patterns that compromise image quality. In contrast, random lasers (RLs) leverage disordered media and multiple light scattering to produce low-coherence emissions. Among the materials used for RLs, ZnO nanostructures are particularly promising due to their strong light emission, high refractive index, and efficient light scattering properties, making them ideal candidates for advanced sensing and imaging applications. ZnO-based RLs are known for narrow emission lines and speckle-free output. Recent advances in multiphoton excitation (MPE) of ZnO RLs have enabled upconversion ultraviolet lasing using lower-energy, near-infrared light, which offers deeper tissue penetration. However, scalability, cost, and durability challenges must be addressed to support wider adoption. This review explores how RLs are engineered to achieve low spatial coherence and lower lasing thresholds, with emphasis on MPE mechanisms that allow emission at shorter wavelengths than the excitation source, thus enabling high-quality imaging. The benefits of these innovations for advanced bioimaging are highlighted, alongside the potential of ZnO nanostructures for bioimaging and biosensing, particularly when interfaced with biological tissues. Prospects include incorporating ZnO RLs into flexible fiber systems, which could promote their commercialization in medical diagnostics and other applications, with long-term photostability and device durability also discussed.