Unlocking NIR-II Photoluminescence in 2D Copper Tetrasilicate Nanosheets through Flame Spray Synthesis

Expanding fluorescence bioimaging into the second near-infrared spectrum (NIR-II, 1000-1700 nm) unlocks advanced possibilities for diagnostics and therapeutics, offering superior tissue penetration and resolution. 2D copper tetrasilicate (CTS) pigments (MCuSi₄O₁₀, M = Ca, Sr, Ba) are known for their brightness and stability, yet synthetic challenges have curbed their integration into bioimaging. Here, flame-spray-pyrolysis (FSP) is introduced as a versatile and scalable synthesis approach to produce ultra-bright, metastable CTS nanosheets (NS) by annealing multi-element metal oxide nanoparticles into 2D crystals through calcination or laser irradiation. Group-II ion incorporation shifts emission into the NIR-II range, with Ba_0.33Sr_0.33Ca_0.33CuSi₄O₁₀ peaking at 1007 nm, while minor Mg-doping induces a hypsochromic shift and extends fluorescence lifetimes. The engineered CTS achieves quantum yields of up to 34%, supporting NS high-frame-rate imaging (> 200 fps). These unique properties enable CTS-NS to serve as powerful contrast agents for super-resolution NIR bioimaging, demonstrated in vivo through transcranial microcirculation mapping and macrophage tracking in mice using diffuse optical localization imaging (DOLI). This pioneering synthesis strategy unlocks wavelength-tunable NS for advanced NIR-II bioimaging applications.