Abstract
The layered silicates Egyptian Blue (CaCuSi4O10, EB), Han Blue (BaCuSi4O10, HB) and Han Purple (BaCuSi2O6, HP) emit as bulk materials 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 monodisperse nanosheets, report their physicochemical properties and use them for (bio)photonics. The approach uses ball milling followed by tip sonication and centrifugation steps to exfoliate the silicates into NS with lateral size and thickness down to ≈ 16–27 nm and 1–4 nm, respectively. They emit at ≈ 927 nm (EB-NS), 953 nm (HB-NS) and 924 nm (HP-NS), and single NS can be imaged in the NIR. The fluorescence lifetimes decrease from ≈ 30–100 μs (bulk) to 17 μs (EB-NS), 8 μs (HB-NS) and 7 μs (HP-NS), thus enabling 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 monodisperse NIR fluorescent silicate nanosheets, determine their size-dependent photophysical properties and showcase the potential for NIR photonics.
Highlights
To gain access to nanosheets of de ned height and lateral size, we developed a multi-step exfoliation procedure, which was optimized for Egyptian blue (EB), Han blue (HB) and Han purple (HP) (Fig. 1)
The exfoliated EB into mm-sized nanosheets (EB-NS), HB-NS and HP-NS show lateral sizes of few tenths of nm and heights ranging from monolayers to multilayers (Fig. 2 and S12a–f†)
As expected from the fragmentation of crystal structures like silicates undergoing sonication and/or milling, a log-normal function can best describe the overall trend of both lateral size and thickness of EB-NS, HB-NS and HP-NS.[55]
Summary
Two-dimensional (2D) nanomaterials have attracted considerable interest in light of their exceptional photophysical properties and their potential for multiple applications.[1,2,3,4] Following graphene, other 2D materials such as transition metal dichalcogenides (TMDs) have been explored.[4,5,6] Single layers of 2D TMDs possess a bandgap and show versatile chemistry.[1,4,7,8,9] Because of the very high surface area displayed by these nanostructures, TMD nanosheets (NS) have been employed in catalysis, energy storage, sensing, and electronics.[1,4,5,7,9,10,11,12,13,14]. We exfoliate EB, HB and HP into nanosheets (EB-NS, HBNS and HP-NS) via a mixed approach of planetary ball milling (PB) and tip sonication (TS) in water This route enables us access to NS of de ned size distribution, of which we report properties including uorescence spectra and lifetimes. We show the potential of these materials for biophotonic applications such as lifetime-encoded imaging or deep-tissue imaging
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