Temperature is an essential parameter in all biological systems, but information about the actual temperature in living cells is limited. Especially, in photothermal therapy, local intracellular temperature changes induce cell death but the local temperature gradients are not known. Highly sensitive nanothermometers would be required to measure and report local temperature changes independent of the intracellular environment, including pH or ions. Fluorescent nanodiamonds (ND) enable temperature sensing at the nanoscale independent of external conditions. Herein, we prepare ND nanothermometers coated with a nanogel shell and the photothermal agent indocyanine green serves as a heat generator and sensor. Upon irradiation, programmed cell death was induced in cancer cells with high spatial control. In parallel, the increase in local temperature was recorded by the ND nanothermometers. This approach represents a great step forward to record local temperature changes in different cellular environments inside cells and correlate these with thermal biology.

Nanodiamonds (NDs) with color centers are excellent emitters for various bioimaging and quantum biosensing applications. In our work, we explore new applications of NDs with silicon-vacancy centers (SiV) obtained by high-pressure high-temperature (HPHT) synthesis based on metal-catalyst-free growth. They are coated with a polypeptide biopolymer, which is essential for efficient cellular uptake. The unique optical properties of NDs with SiV are their high photostability and narrow emission in the near-infrared region. Our results demonstrate for the first time that NDs with SiV allow live-cell dual-color imaging and intracellular tracking. Also, intracellular thermometry and challenges associated with SiV atomic defects in NDs are investigated and discussed for the first time. NDs with SiV nanoemitters provide new avenues for live-cell bioimaging, diagnostic (SiV as a nanosized thermometer), and theranostic (nanodiamonds as drug carrier) applications.

Long-term tracking of nanoparticles to resolve intracellular structures and motions is essential to elucidate fundamental parameters as well as transport processes within living cells. Fluorescent nanodiamond (ND) emitters provide cell compatibility and very high photostability. However, high stability, biocompatibility, and cellular uptake of these fluorescent NDs under physiological conditions are required for intracellular applications. Herein, highly stable NDs encapsulated with Cowpea chlorotic mottle virus capsid proteins (ND-CP) are prepared. A thin capsid protein layer is obtained around the NDs, which imparts reactive groups and high colloidal stability, while retaining the opto-magnetic properties of the coated NDs as well as the secondary structure of CPs adsorbed on the surface of NDs. In addition, the ND-CP shows excellent biocompatibility both in vitro and in vivo. Long-term 3D trajectories of the ND-CP with fine spatiotemporal resolutions are recorded; their intracellular motions are analyzed by different models, and the diffusion coefficients are calculated. The ND-CP with its brilliant optical properties and stability under physiological conditions provides us with a new tool to advance the understanding of cell biology, e.g., endocytosis, exocytosis, and active transport processes in living cells as well as intracellular dynamic parameters.

Nanodiamonds (NDs) have high potential as a drug carrier and in combination with nitrogen vacancies (NV centers) for highly sensitive MR-imaging after hyperpolarization. However, little remains known about their physiological properties in vivo. PET imaging allows further evaluation due to its quantitative properties and high sensitivity. Thus, we aimed to create a preclinical platform for PET and MR evaluation of surface-modified NDs by radiolabeling with both short- and long-lived radiotracers. Serum albumin coated NDs, functionalized with PEG groups and the chelator deferoxamine, were labeled either with zirconium-89 or gallium-68. Their biodistribution was assessed in two different mouse strains. PET scans were performed at various time points up to 7 d after i.v. injection. Anatomical correlation was provided by additional MRI in a subset of animals. PET results were validated by ex vivo quantification of the excised organs using a gamma counter. Radiolabeled NDs accumulated rapidly in the liver and spleen with a slight increase over time, while rapid washout from the blood pool was observed. Significant differences between the investigated radionuclides were only observed for the spleen (1 h). In summary, we successfully created a preclinical PET and MR imaging platform for the evaluation of the biodistribution of NDs over different time scales.

Long-term tracking of nanoparticles to resolve intracellular structures and motions is essential to elucidate fundamental parameters as well as transport processes within living cells. Fluorescent nanodiamond (ND) emitters provide cell compatibility and very high photostability. However, high stability, biocompatibility, and cellular uptake of these fluorescent NDs under physiological conditions are required for intracellular applications. Herein, highly stable NDs encapsulated with Cowpea chlorotic mottle virus capsid proteins (ND-CP) are prepared. A thin capsid protein layer is obtained around the NDs, which imparts reactive groups and high colloidal stability, while retaining the opto-magnetic properties of the coated NDs as well as the secondary structure of CPs adsorbed on the surface of NDs. In addition, the ND-CP shows excellent biocompatibility both in vitro and in vivo. Long-term 3D trajectories of the ND-CP with fine spatiotemporal resolutions are recorded; their intracellular motions are analyzed by different models, and the diffusion coefficient are calculated. The ND-CP with its brilliant optical properties and stability under physiological conditions provides us with a new tool to advance the understanding of cell biology, e.g., endocytosis, exocytosis, and active transport processes in living cells as well as intracellular dynamic parameters.

Temperature is an essential factor in all biological processes and most of the biological activities are related to temperature. However, information about the temperature in living cells are limited. In photothermal therapy, an emerging cancer treatment due to its high spatial control and non-invasive nature, the local effect of intracellular temperature change is not well understood. Fluorescent nanodiamonds (ND) are a unique carbon material possessing optical defects inside the carbon lattice that enable sensing of temperature on a nanoscale completely independent of external conditions. Herein, we coat ND with polymers to form a nanogel shell around NDs that can absorb indocyanine green, a common photothermal agent, to obtain nanodiamond-nanogel-indocyanine green. Upon irradiation, we not only show successful killing of cancer cells with a high control in space but we are also able to sense the increase in temperature using single NDs as nanothermometers in cells.