Optical Diagnostics of Porous Silicon Nanoparticles Biodegradation
Authors: Gongalsky Maxim Bronislavovich , Osminkina Liubov , Tolstik Elena , Bezsudnova Yulia , Sivakov Vladimir , Timoshenko Victor 
Published: February 2020

SUMMARY

Optical methods such as Raman micro-spectroscopy and photoluminescence were employed for diagnostics of biodegradation of porous silicon nanoparticles (SiNPs) in phantom suspensions and breast cancer cells (MCF-7 cell line) in-vitro.Two types of SiNPs, with and without photoluminescence in the visible spectral range, were investigated. The biodistribution of photoluminescent SiNPs within the cells obtained by Raman imaging was additionally verified by high-resolution structured-illumination optical microscopy (HR-SIM). Almost complete degradation of SiNPs inside the living cells and aqueous suspensions was observed after 2 weeks of storage. The results reveal new prospects of multi-modal optical characterization of SiNPs during their dissolution in vitro.

1. INTRODUCTION

SiNPs are well-known to be degradable in physiological fluids and in living tissues due to dissolution process resulting in formation of orthosilicic acid [1]. Usually several week are sufficient for complete release of SiNPs from the body [1]. Nonetheless, the SiNP degradation rate depends on the particle size, porosity, surface coverage, pH value and ambience and can vary from a few days up to several months [2]. The SiNP biodegradation is associated usually with the disappearance of their PL property. It can also be assessed by histological methods or by the analysis of silicon content in the tissues by inductively coupled plasma optical emission spectrometry. However, these methods of analysis are not very precise, fast or/and straightforward. Mesoporous SiNPs do not possess efficient PL, therefore we used two optical methods: PL and Raman spectroscopy for characterization of different types of PSiNPs.

2. EXPERIMENTAL RESULTS

We have prepared 100 nm SiNPs by grinding of meso- and microporous silicon films (PSi). Both types of the films were obtained by electrochemical etching of Si wafers in HF-based solutions. Micro PSiNPs have demonstrated efficient PL properties, whereas meso PSiNPs have not, but they provided better contrast in Raman imaging.
Degradation of micro PSiNPs in suspensions was investigated during 2 week in order to determine influence of size, concentration, pH level influence on degradation rate. We used centrifugation to obtain micro PSiNPs samples with different size of NPs in order to find size dependency of degradation rate. Meso PSiNPs were tested in the same regime by Raman spectroscopy. Dialysis was applied to enhance degradation. Both methods showed size shrinking of nanoparticles, since PL peak positions is determined by quantum confinement in nanocrystals and Raman peak shifts in small nanocryslals due to the confinement of phonons.
Then degradation was examined in vitro. The uptake of silicon nanoparticles by MCF-7 breast cancer cells was observed for time intervals between 6 hours and 2 weeks. The 3D HR-SIM revealed the localization of PL-SiNPs inside cells. Staining of the cell organelles proved the accumulation of nanoparticles mostly in the cytoplasm and in the nucleus periphery (see left Fig.). The Raman images for SiNPs are shown in central Fig. SiNPs exhibited a characteristic peak of Raman scattering at 520 cm-1, which is due to the presence of nanocrystalline silicon in the cells (see right Fig.). The strongly highlighted signal from SiNPs shows presence of SiNPs inside the cell structure without being overlapped by the surrounding signals from the cell organelles.
To investigate time-depending modifications of SiNPs the detailed analyses of Raman imaging data for the timeframe of 2 weeks were performed in-vitro. The chemical changes of SiNPs in MCF-7 cell interior determining the biodegradation processes of nanoparticles were evaluated from the corresponding shifts and changes of the Raman spectra and intensity during increased incubation time. Based on the performed investigations the model of SiNPs uptake by cancer cells and biodegradation is proposed.

3. CONCLUSIONS

The photolumescent and Raman spectroscopies were used to visualize the intracellular localization and subsequent biodegradation of SiNPs in cancer cells for the first time. The obtained results are important for the development of new methods of ex-vivoexpress-diagnosis of early stages of cancer.

REFERENCES

Park J.-H., Gu L., von Maltzahn G., Ruoslahti E., Bhatia S.N., Sailor M.J. Nat. Mater. 8(4), 331–336 (2009).

Shabir Q. Biodegradability of Porous Silicon. In: Handbook of Porous Silicon. Canham L. (Ed.). Springer Int. Publ., 395–401 (2014).

Figure 1