Laser-synthesized oxide-passivated bright Si quantum dots for bioimaging

Crystalline silicon (Si) nanoparticles present an extremely promising object for bioimaging based on photoluminescence (PL) in the visible and near-infrared spectral regions, but their efficient PL emission in aqueous suspension is typically observed after wet chemistry procedures leading to residual toxicity issues. Here, we introduce ultrapure laser-synthesized Si-based quantum dots (QDs), which are water-dispersible and exhibit bright exciton PL in the window of relative tissue transparency near 800 nm. Based on the laser ablation of crystalline Si targets in gaseous helium, followed by ultrasound-assisted dispersion of the deposited films in physiological saline, the proposed method avoids any toxic by-products during the synthesis. We demonstrate efficient contrast of the Si QDs in living cells by following the exciton PL. We also show that the prepared QDs do not provoke any cytoxicity effects while penetrating into the cells and efficiently accumulating near the cell membrane and in the cytoplasm. Combined with the possibility of enabling parallel therapeutic channels, ultrapure laser-synthesized Si nanostructures present unique object for cancer theranostic applications.


FTIR analyses of the porosity of laser-ablated films
Fourier transform infra-red (FTIR) spectroscopy in the near-and middle IR ranges was used to determine both the effective refraction index, n, and width, d, of the obtained LA-Si films. The films were deposited on GaF 2 substrates, which were transparent in the investigated spectral region. Typical reflection spectra measured at different angles of incident are shown in Fig.1. The spectra consist of oscillations of the Fabry-Perot interference with maxima described by the following equation: where k is the wavenumber, α is the angle of incidence, m is the number of interference order.
Taking into account the same number maxima at different angles of incidence, one can estimate both n and d.
The value of n was used to estimate porosity, P, of the samples by using an effective medium approximation (EMA) based on Bruggeman approximation 1 : where is the effective dielectric function ( = 2 ); 1 and 2 are the dielectric permittivities of Si nanocrystals and air, respectively; 1 and 2 are the corresponding filling The reflection data of Fig.1S were analyzed by using Eqs.1 and 2 to estimate the porosity of LA-Si films, which accounted P=0.70±0.05.
Fig. S1. FTIR reflectance spectra of a LA-Si film deposited on CaF 2 substrate and measured at different angles of incidence.
Note that more accurate evaluation of P should take into account SiO x surrounding of Si nanocrystals as the third component of the effective medium 2 . Nevertheless, the porosity value of P=70% agrees well with the data obtained by using specular x-ray reflectivity for similar laser-ablated layers produced at 2 Torr of He 3 .

FTIR analyses of the composition of LA-Si NPs
The chemical composition of LA-Si NPs deposited from the aqueous suspension on an ATR crystal and evacuated at 10 -3 Torr was analyzed by means of FTIR spectroscopy. Fig. S2 shows transmittance spectrum of LA-Si NPs after 4 days of storage in aqueous medium. The observed absorption peak between 1000 and 1200 cm -1 corresponds to the Si-O valence vibrations. 4 The stoichiometry parameter, x, can be estimated from the absorption peak position as it is described in Ref. 5: it should be equal to 1082 cm -1 for SiO 2 and 980 cm -1 for SiO. One can obtain x = 1.95±0.05 for the spectral peak position 1080 cm -1 shown in Fig. 2S, evidencing nearly perfect dioxide composition of the NP surrounding. It is important that the FTIR spectrum in Fig. 2S contains an additional peak at 875 cm -1 , which can be attributed to SiO x layer with x=1.55. As this peak vanishes for both x=1 and x=2, one can conclude that the suboxide layer is non-uniform and its stoichiometry may vary while going to deeper layers. Nevertheless, the SiO 1.5 phase is not dominating as it is accompanied by a Si-O valence peak shifted to 1040 cm -1 . The latter peak can be interpreted as a shoulder of the main peak at 1077 cm -1 .

LA-Si NPs deposited from aqueous suspensions on metal (stainless still) and initial LA-Si
NPs layers deposited on CaF 2 substrates were investigated by using the Raman spectroscopy to estimate the mean size of Si QDs. Raman spectrum of the samples is shown in Fig. 3S.
Here, a narrow peak near 520 cm -1 corresponds to the nanocrystalline Si phase, whereas a broad band centered at 480 cm -1 corresponds to the amorphous Si phase 6  The measured peak position (Fig.3S) Fig. S3. Raman spectra of as-prepared LA-Si NPs (red curve) and those after 11 days of storage in water (blue curve). Black curve represents the reference c-Si spectrum. Arrows point main peak position.

Z-scan imaging of cancer cells with LA-Si NPs
Spatial localization of LA-Si NPs in cancer cells is illustrated by images of the confocal fluorescent microscopy with Z-step of 0.29 µm (z_scan_cells_Si_NPs.avi).