Exfoliated near infrared fluorescent silicate nanosheets for (bio)photonics

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Fluorescence imaging is one of the most important methods in the natural sciences and especially biomedical research. Compared to other wavelengths, the near infrared (NIR) range of the electromagnetic spectrum, which roughly extends from 800 to 1700 nm, provides many advantages. Due to reduced NIR light scattering and absorption, there are several major benefits such as higher contrast, lower phototoxicity or higher tissue penetration. In past years, diverse players have been established as NIR fluorophores, however their abundance is still quite low compared to visible fluorophores. Besides that, issues such as low biocompatibility, low quantum yields, low optical tunability and bleaching have to be kept in mind. These affect the use of most of the currently existing NIR fluorophores such as the organic dye indocyanine green (ICG), and nanomaterials such as InAs quantum dots, lanthanide-doped nanoparticles and semiconducting single-walled carbon nanotubes (SWCNTs). For this reason, it is of great scientific interest to discover and study new NIR fluorophores for sophisticated applications in (bio)photonics.

In our work we developed a novel approach to transform the most ancient pigment made by man into a nanomaterial for NIR fluorescence (bio)imaging. The pigment is Egyptian Blue (CaCuSi4O10, EB), a calcium copper silicate (Fig. a) whose origins are dated back to Ancient Egypt (2500 BC). Bulk EB powder has been used in Egyptian art but only recently it was found that it displays NIR fluorescence at ≈ 910 nm (Fig. b) and a remarkably high quantum yield > 10 %. When looking at its crystal structure, one could speculate that this phyllosilicate can be easily exfoliated into 2D nanomaterials (i.e. nanosheets, EB-NS) due to the presence of weak Van der Waals bonds between the silicate layers. However, it was so far not clear how the NIR fluorescence changes with the size of the nanosheets and if demanding imaging applications are possible.

In our manuscript we present a novel exfoliation approach, which comprises ball milling and tip sonication. Diameters down to few tenths of nanometers and thicknesses down to the monolayer regime have been obtained. Even in the nanosheet form, the NIR fluorescence is retained and is not showing any bleaching nor peak shift in the emission wavelength, thus underlining how photostable this material is. One important question was how fluorescence intensity scales with size. To shed light in this direction, correlative methods including SEM-NIR imaging (Fig. c) and microrheology measurements on the single nanosheet level were performed. These analyses further confirmed that even the smallest EB-NS far below Abbe’s optical resolution limit (≈ 500 nm) display strong NIR fluorescence.

In vivo imaging of single EB-NS in Drosophila embryos showed the great advantages of a non-bleaching NIR nanoscale fluorophore compared to typical organic dyes. Stand-off (remote) detection of EB-NS in plants (Fig. d) further demonstrated how the lower autofluorescence of biological materials in the NIR leads to high contrast, and how bright EB-NS are compared to other state-of-the-art fluorophores. Furthermore, viability assays carried out on cells also demonstrated high biocompatibility. In summary, we created a novel NIR fluorescent nanomaterial with highly interesting optical properties and large potential for demanding bioimaging applications. 

An ancient pigment becomes a novel NIR fluorophore
An ancient pigment as novel NIR fluorophore. a Crystal structure of Egyptian Blue (EB). Ca is depicted as green, Cu as bronze, O as red spheres, whereas Si is represented in its typical tetrahedral geometry. b 2D excitation-emission spectrum of Egyptian Blue nanosheets (EB-NS). c A single resolution-limited EB-NS observed at our near infrared (NIR) setup and under a scanning electron microscope (SEM). The size of the orange frame roughly corresponds to the resolution limit (i.e. ≈ 500 nm). d Stand-off detection of EB-NS (frame 1), single-walled carbon nanotubes (SWCNTs, frame 2) and buffer (control, frame 3) in plants of Arabidopsis thaliana.

To read more about this novel NIR nanomaterial for photonics and (bio)imaging, please check the results of our paper: 
Selvaggio, G. et al. Exfoliated near infrared fluorescent silicate nanosheets for (bio)photonics. Nat Commun 11, 1495 (2020). https://rdcu.be/b28ZZ

Poster image: NIR image of exfoliated EB-NS.

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Sebastian Kruss

Group Leader, Nanobiotechnology Group, Institute of Physical Chemistry, University of Göttingen

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