Having negligible toxicity compared to quantum dots based on compound semiconductors [1,2] and being capable of providing efficient imaging and therapeutic functionalities based on its unique physicochemical characteristics, [3][4][5] nanosilicon occupies a particularly important niche related to biological applications. The imaging functionality of nanosilicon typically employs photoluminescence (PL) of quantum-confined excitonic states in silicon (Si) nanocrystals (NCs) with sizes smaller than the exciton Bohr's radius (≈5 nm for the bulk crystalline Si (c-Si)), which enables tracking the presence of Si nanoparticles (NPs) in cells or tissues. [6][7][8][9][10][11][12][13] On the other hand, Si NPs can serve as efficient sensitizers of local heating under external stimuli to initiate hyperthermia-based therapies. As an example, Presenting a safe alternative to conventional compound quantum dots and other functional nanostructures, nanosilicon can offer a series of breakthrough hyperthermia-based therapies under near-infrared, radiofrequency, ultrasound, etc., excitation, but the size range to sensitize these therapies is typically too large (>10 nm) to enable efficient imaging functionality based on photoluminescence properties of quantum-confined excitonic states. Here, it is shown that large Si nanoparticles (NPs) are capable of providing two-photon excited luminescence (TPEL) and second harmonic generation (SHG) responses, much exceeding that of smaller Si NPs, which promises their use as probes for bi-modal nonlinear optical bioimaging. It is finally demonstrated that the combination of TPEL and SHG channels makes possible efficient tracing of both separated Si NPs and their aggregations in different cell compartments, while the resolution of such an approach is enough to obtain 3D images. The obtained bi-modal contrast provides lacking imaging functionality for large Si NPs and promises the development of novel cancer theranostic modalities on their basis.

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