Single-Particle Tracking and Trajectory Analysis of Fluorescent Nanodiamonds in Cell-Free Environment and Live Cells

Effects of Thermal Oxidation and Proton Irradiation on Optically Detected Magnetic Resonance Sensitivity in Sub-100 nm Nanodiamonds

In recent decades, nanodiamonds (NDs) have emerged as innovative nanotools for weak magnetic fields and small temperature variation sensing, especially in biological systems. At the basis of the use of NDs as quantum sensors are nitrogen-vacancy center lattice defects, whose electronic structures are influenced by the surrounding environment and can be probed by the optically detected magnetic resonance technique. Ideally, limiting the NDs' size as much as possible is important to ensure higher biocompatibility and provide higher spatial resolution. However, size reduction typically worsens the NDs' sensing properties. This study endeavors to obtain sub-100 nm NDs suitable to be used as quantum sensors. Thermal processing and surface oxidations were performed to purify NDs and control their surface chemistry and size. Ion irradiation techniques were also employed to increase the concentration of the nitrogen-vacancy centers. The impact of these processes was explored in terms of surface chemistry (diffuse reflectance infrared Fourier transform spectroscopy), structural and optical properties (Raman and photoluminescence spectroscopy), dimension variation (atomic force microscopy measurements), and optically detected magnetic resonance temperature sensitivity. Our results demonstrate how surface optimization and defect density enhancement can reduce the detrimental impact of size reduction, opening to the possibility of minimally invasive high-performance sensing of physical quantities in biological environments with nanoscale spatial resolution.

Free radical detection in precision-cut mouse liver slices with diamond-based quantum sensing

Free radical generation plays a key role in many biological processes including cell communication, maturation, and aging. In addition, free radical generation is usually elevated in cells under stress as is the case for many different pathological conditions. In liver tissue, cells produce radicals when exposed to toxic substances but also, for instance, in cancer, alcoholic liver disease and liver cirrhosis. However, free radicals are small, short-lived, and occur in low abundance making them challenging to detect and especially to time resolve, leading to a lack of nanoscale information. Recently, our group has demonstrated that diamond-based quantum sensing offers a solution to measure free radical generation in single living cells. The method is based on defects in diamonds, the so-called nitrogen-vacancy centers, which change their optical properties based on their magnetic surrounding. As a result, this technique reveals magnetic resonance signals by optical means offering high sensitivity. However, compared to cells, there are several challenges that we resolved here: Tissues are more fragile, have a higher background fluorescence, have less particle uptake, and do not adhere to microscopy slides. Here, we overcame those challenges and adapted the method to perform measurements in living tissues. More specifically, we used precision-cut liver slices and were able to detect free radical generation during a stress response to ethanol, as well as the reduction in the radical load after adding an antioxidant.

Cellular uptake and fate of cationic polymer-coated nanodiamonds delivering siRNA: a mechanistic study

Gene silencing using small interfering RNAs (siRNAs) is a selective and promising approach for treatment of numerous diseases. However, broad applications of siRNAs are compromised by their low stability in a biological environment and limited ability to penetrate cells. Nanodiamonds (NDs) coated with cationic polymers can enable cellular delivery of siRNAs. Recently, we developed a new type of ND coating based on a random copolymer consisting of (2-dimethylaminoethyl) methacrylate (DMAEMA) and N-(2-hydroxypropyl) methacrylamide (HPMA) monomers. These hybrid ND-polymer particles (Cop+-FND) provide near-infrared fluorescence, form stable complexes with siRNA in serum, show low toxicity, and effectively deliver siRNA into cells in vitro and in vivo. Here, we present data on the mechanism of cellular uptake and cell trafficking of Cop+-FND : siRNA complexes and their ability to selectively suppress mRNA levels, as well as their cytotoxicity, viability and colloidal stability. We identified clathrin-mediated endocytosis as the predominant entry mechanism for Cop+-FND : siRNA into U-2 OS human bone osteosarcoma cells, with a substantial fraction of Cop+-FND : siRNA following the lysosome pathway. Cop+-FND : siRNA potently inhibited the target GAPDH gene with negligible toxicity and sufficient colloidal stability. Based on our results, we suggest that Cop+-FND : siRNA can serve as a suitable in vivo delivery system for siRNA.

Quantum Sensing of Free Radicals in Primary Human Granulosa Cells with Nanoscale Resolution

Cumulus granulosa cells (cGCs) and mural granulosa cells (mGCs), although derived from the same precursors, are anatomically and functionally heterogeneous. They are critical for female fertility by supporting oocyte competence and follicular development. There are various techniques used to investigate the role of free radicals in mGCs and cCGs. Yet, temporospatial resolution remains a challenge. We used a quantum sensing approach to study free radical generation at nanoscale in cGCs and mGCs isolated from women undergoing oocyte retrieval during in vitro fertilization (IVF). Cells were incubated with bare fluorescent nanodiamonds (FNDs) or mitochondria targeted FNDs to detect free radicals in the cytoplasm and mitochondria. After inducing oxidative stress with menadione, we continued to detect free radical generation for 30 min. We observed an increase in free radical generation in cGCs and mGCs from 10 min on. Although cytoplasmic and mitochondrial free radical levels are indistinguishable in the physiological state in both cGCs and mGCs, the free radical changes measured in mitochondria were significantly larger in both cell types, suggesting mitochondria are sites of free radical generation. Furthermore, we observed later occurrence and a smaller percentage of cytoplasmic free radical change in cGCs, indicating that cGCs may be more resistant to oxidative stress.

Fluorescent Nanodiamonds for Tracking Single Polymer Particles in Cells and Tissues

Polymer nanoparticles are widely used in drug delivery and are also a potential concern due to the increased burden of nano- or microplastics in the environment. In order to use polymer nanoparticles safely and understand their mechanism of action, it is useful to know where within cells and tissues they end up. To this end, we labeled polymer nanoparticles with nanodiamond particles. More specifically, we have embedded nanodiamond particles in the polymer particles and characterized the composites. Compared to conventional fluorescent dyes, these labels have the advantage that nanodiamonds do not bleach or blink, thus allowing long-term imaging and tracking of polymer particles. We have demonstrated this principle both in cells and entire liver tissues.

Mesoporous Polydopamine-Encapsulated Fluorescent Nanodiamonds: A Versatile Platform for Biomedical Applications

Fluorescent nanodiamonds (FNDs) are versatile nanomaterials with promising properties. However, efficient functionalization of FNDs for biomedical applications remains challenging. In this study, we demonstrate mesoporous polydopamine (mPDA) encapsulation of FNDs. The mPDA shell is generated by sequential formation of micelles via self-assembly of Pluronic F127 (F127) with 1,3,5-trimethyl benzene (TMB) and composite micelles via oxidation and self-polymerization of dopamine hydrochloride (DA). The surface of the mPDA shell can be readily functionalized with thiol-terminated methoxy polyethylene glycol (mPEG-SH), hyperbranched polyglycerol (HPG), and d-α-tocopheryl polyethylene glycol 1000 succinate (TPGS). The PEGylated FND@mPDA particles are efficiently taken up by, and employed as a fluorescent imaging probe for, HeLa cells. HPG-functionalized FND@mPDA is conjugated with an amino-terminated oligonucleotide to detect microRNA via hybridization. Finally, the increased surface area of the mPDA shell permits efficient loading of doxorubicin hydrochloride. Further modification with TPGS increases drug delivery efficiency, resulting in high toxicity to cancer cells.

Intracellular Relaxometry, Challenges, and Future Directions

Nitrogen vacancy (NV) centers change their optical properties on the basis of their magnetic surroundings. Since optical signals can be detected more sensitively than small magnetic signals, this technique allows unprecedented sensitivity. Recently, NV center-based relaxometry has been used for measurements in living cells with subcellular resolution. The aim of this Outlook is to identify challenges in the field, including controlling the location of sensing particles, limitations in reproducibility, and issues arising from biocompatibility. We further provide an outlook and point to new directions in the field. These include new diamond materials with NV centers, other defects, or even entirely new materials that might replace diamonds. We further discuss new and more challenging samples, such as tissues or even entire organisms, that might be investigated with NV centers. Then, we address future challenges that have to be resolved in order to achieve this goal. Finally, we discuss new quantities that could be measured with NV centers in the future.