Advanced nanodiamonds as a versatile probe for biological applications

Most researches show that Nanodiamonds (NDs) possess several significant advantages, such as low cytotoxic, high biocompatibility and good thermal conductivity. Fluorescent nanodiamonds (FNDs), produced by ion beam irradiation and subsequent annealing, emit bright and stable fluorescence. NDs are promising for applications in many fields owing to their outstanding properties.
In our study, we made a further discussion on how production method, particle size, nitrogen content and the crystallographic structure of diamond affect the fluorescence property of FNDs. First we measured nitrogen content for all kinds of samples and reduced the particle size from micrometer to submicrometer level by ball milling. We used different ion sources to irradiate NDs to form FNDs. A systematic investigation of fluorescence performance and lifetime of as-prepared nanodiamonds was done. We found that fluorescence intensity increased with the nitrogen density ranging from 100 to 200 ppm. This trend, however, failed to continue for NDs with higher nitrogen density (up to 390 ppm) but poorer crystallinity. Our results indicate that it is possible to produce smaller and brighter FNDs in the future.
In order to demonstrate FNDs as a fluorescent probe for biological applications, we use FNDs as a cell tracker for cell long-term tracking and cell exocytosis studies. FNDs (~100 nm) were introduced into cell by endocytosis through incubation. No significant alteration in growth or proliferation of FND-labeled cells is observed for up to 8 day. Flow cytometric analysis, in combination with parallel cell doubling-time measurements, indicates that there are about 15% excretion of the endocytosed FNDs after 6 days of labeling for both HeLa and 489_2.1 cell, and about 30 % for 3T3-L1 cell. A comparative experiment with FND and the widely used dye, CFSE (Carboxy-fluorescein diacetate succinimidyl ester), demonstrates that the nanoparticle platform is a promising alternate probe for long-term cell labeling and tracking applications.
In addition to fluorescent nanodiamonds, we make efforts in developing other features of nanodiamonds. Irradiated nanodiamonds (INDs) are a new type of nanomaterial served as a potential photoacoustic (PA) contrast agent. This work characterized in detail the photophysical properties of these materials prepared by ion irradiation of natural diamond powders using various spectroscopic methods. Compared with gold nanorods of similar dimensions, the INDs have a substantially smaller molar extinction coefficient and can generate stronger photoacoustic signal. This phenomenon may be attributed by INDs having much higher thermal stability, stronger hydrophilic interaction with water, and a lower nanobubble formation threshold. Besides, no sign of photodamage was detected after high-energy illumination of the INDs for hours. Cell viability assays with 100 μg/mL INDs concentration showed that the nanomaterial is non-cytotoxic and potentially useful for long-term PA bioimaging application.