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Among semiconductor quantum dots, silicon nanocrystals (SiNCs) are gaining interest due to their high biocompatibility and natural abundance of silicon. The optical properties make SiNCs optimal candidates for luminescent bioprobes: (i) emission energy tuneable to the red and NIR spectral region, compatible with the biological window; (ii) high photoluminescence quantum yield; (iii) no sensitivity to molecular oxygen despite the long lifetime of emission; (iv) long-lived luminescence that enables time-gated detection in the hundreds of μs timescale, which allows removal of scattered excitation light and autofluorescence of the biological sample with a low-cost equipment (gating times of the order of hundreds of μs). SiNCs can be passivated through covalent bond formation between Si and C atoms giving extremely robust systems. The two major issues for SiNCs are the poor absorption outside the UV spectral region and the difficulty to obtain water suspendable SiNCs that maintain their photo physical properties. Our group addressed the first problem by introducing light absorbing units on the SiNCs surface; these dyes can be excited and transfer the energy to the silicon core behaving like a light-harvesting antenna. Regarding the second issue, several attempts are reported in literature to make SiNCs water dispersible, but these approaches often suffer of scarce stability in aqueous environment, and loss of SiNCs photophysical properties. Our approach is based on covalent functionalization of SiNCs via two-step synthesis involving a first coating step in organic solvent and a post-functionalization through thiol-ene click chemistry in order to introduce poly(ethyleneglycol) (PEG) as a water soluble group. SiNCs obtained with this approach are colloidally stable and retain NIR emission with long emission lifetimes (tens of microsecond).
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