Label-free tomographic imaging of nanodiamonds in living cells

Nanodiamonds have acknowledged growing attention due to their facile functionalization, stable fluorescence, low toxicity, and decent biocompatibility. However, despite advances applications of nanodiamonds in drug delivery, catalysis and bio-sensing, laser irradiation still limits long-time tracking of nanodiamonds in living cells due to phototoxicity. Here, using optical diffraction tomography, we performed quantitative morphological and biophysical analysis of living cells via endocytosis or electroporation of nanodiamonds (5 nm, 35 nm, and 100 nm) without the need for a fluorescence label. Optical diffraction tomography is an inexpensive and noninvasive microscopy technique, which images cells and subcellular structures as a function of their refractive index. The laser excitation power is much weaker than in the case of fluorescence microscopy, which reduces phototoxicity. Thanks to the very high refractive index of diamond, nanodiamonds in HeLa cells can be clearly discriminated from cellular structures using optical diffraction tomography. As an application, we show that aggregation and deaggregation of internalized nanodiamonds can be detected via changes in the refractive index distribution of the entire cell. Optical diffraction tomography successively images prevention of in-cell particle aggregation through polyglycerol coating of nanodiamonds. In the case of endocytosis, optical diffraction tomography shows deaggregation of nanodiamonds after a prolonged incubation time. Together, our findings implicate that refractive index measurements are a favorable tool to track nano diamonds, without a fluorescent label, inside living cells. This could be useful to study real-time therapeutic or metabolic activities in living cells using very weak laser irradiation. Finally, the elaborate creation of fluorescent defects in nanodiamonds becomes redundant.