Exfoliation and Optical Properties of Near-Infrared Fluorescent Silicate Nanosheets

The silicates Egyptian Blue (CaCuSi4O10, EB), Han Blue (BaCuSi4O10, HB) and Han Purple (BaCuSi2O6, HP) emit in bulk bright and stable fluorescence in the near-infrared (NIR), which is of high interest for (bio)photonics due to minimal scattering, absorption and phototoxicity in this spectral range. So far the optical properties of nanosheets (NS) of these silicates are poorly understood. Here, we exfoliate them into nanosheets and report their physicochemical properties. The approach uses ball milling followed by tip sonication and centrifugation steps to exfoliate the silicates into NS with a lateral size ≈ 16-27 nm and thickness ≈ 1-4 nm. They emit at ≈ 927 nm (EB-NS), 953 nm (HB-NS) and 924 nm (HP-NS) and single NS can be resolved in the NIR. Fluorescence lifetimes decrease from ≈ 30-100 μs (bulk) to 17 μs (EB- NS), 8 μs (HB-NS) and 7 μs (HP-NS). NS of different composition/size can be imaged by fluorescence lifetime imaging, which enables lifetime-encoded multicolor imaging both on the microscopic and the macroscopic scale. Finally, remote imaging through tissue phantoms reveals the potential for bioimaging. In summary, we report a procedure to gain NIR fluorescent silicate nanosheets, characterize their photophysical properties and show their potential for NIR photonics.