Studying biological cell function demands the design of tools for cell imaging and tracking which combine water-dispersibility, photostability, fluorescence and biocompatibility. Among these tools, fluorescent organic dyes and cadmium-based semiconductor nanoparticles are promising as cellular probes. However, the biocompatible fluorescent organic dyes suffer from severe photobleaching—a decrease in fluorescence intensity upon ultraviolet light exposure. Semiconductor nanoparticles are strongly fluorescent and photostable, but release toxic heavy metals under ultraviolet irradiation.

Now, to overcome these problems, Shuit-Tong Lee from City University of Hong Kong and Chun-Hai Fan from the Chinese Academy of Sciences, Shanghai, and their collaborators1 have created a new family stable and non-toxic fluorescent cellular probes using silicon-based nanostructures.

Fig. 1: Schematic illustration of the silicon nanoparticles combining with biomolecules.

The silicon-based nanostructures were developed based on computational assessment of the reactivity of silicon nanoparticles to acrylic acid—an organic compound which reacts with silicon. The calculations revealed acrylic acid to preferably bind with silicon via silicon-oxygen bonds upon irradiation with blue-light and to form a polymer under ultraviolet light.

Through highly selective photochemical reactions, the researchers first functionalized the silicon nanoparticles with an oxygen-bound acrylic acid layer using blue light, and then linked the nanoparticles to one another using ultraviolet light, generating water-dispersible and biocompatible nanospheres. “The size of the nanospheres and the number of silicon quantum dots incorporated into each nanosphere are both increased along with the elongation of irradiation time,” says Fan.

In addition to dramatically enhancing their fluorescence intensity, the incorporation of the silicon nanoparticles into nanospheres widens their absorption spectra, enabling the detection of fluorescent signals under different excitation wavelengths. “This provides versatility and convenience for biological applications, such as cell imaging and long-term monitoring of cellular events,” says Fan. Cell labeling tests using human kidney cells showed the silicon-based nanospheres to surpass the photostability of both traditional organic dyes and cadmium-based nanoparticles.

The researchers are currently focusing on developing a facile biochemical strategy to synthesize silicon-nanospheres/biomolecules conjugates. “The resultant bioconjugates will be further utilized in various bioapplications, such as immunofluorescent target-cell labeling, in-vivo and in-vitro imaging, and biosensors,” says Fan.