Unlocking NIR-II Photoluminescence in 2D Copper Tetrasilicate Nanosheets Through Flame Spray Pyrolysis

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. Two-dimensional 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, we introduce flame-spray-pyrolysis (FSP) 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. Group-II ion incorporation shifts emission into the NIR-II range, with Ba0.33Sr0.33Ca0.33CuSi4O10 peaking at 1007 nm, while minor Mg-doping induces a hypsochromic shift and extends fluorescence lifetimes. The engineered CTS achieve quantum yields 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.