Manipulating the distribution of electric field intensity to effectively enhance the spatial and spectral fluorescence intensity of fluorescent nanodiamonds.

Fluorescent nanodiamonds (FNDs) having nitrogen-vacancy (NV) centers have drawn much attention for their biocompatibility and stable optical properties. Nevertheless, the NV centers are located in the interior of the FNDs, and it has not been possible to increase the fluorescence intensity of FNDs efficiently using previously developed enhancement methods. In this paper, we present a simple nanocavity structure that enhances the fluorescence intensity of FNDs. The designed Al/SiO2 nanocavities are stable and inexpensive, and provide a large region for efficient enhancement of fluorescence that can cover most 100 nm FNDs. By tuning the thickness of the capping SiO2 layer of the Al/SiO2 nanocavities, the distributions of both the spatial and spectral electric field intensities of the FNDs could be controlled and manipulated. In general, the FNDs were excited using a green-yellow laser; the broadband fluorescence of the FNDs comprised the emissions from neutral (NV0) and negatively charged (NV-) NV centers. To enhance the fluorescence intensity from the NV- centers of the FNDs, we designed an Al/70 nm SiO2 nanocavity to function at excitation and emission wavelengths of 633 and 710 nm, respectively, allowing the NV- centers to be excited efficiently; as a result, we achieved an enhancement in fluorescence intensity of 11.2-fold. Moreover, even when we covered 100 nm FNDs with polyglycerol (forming p-FND), the fluorescence intensities of the p-FND particles placed on the nanocavities remained greatly enhanced.

Highly stable lipid-encapsulation of fluorescent nanodiamonds for bioimaging applications.

Highly stable lipid-encapsulated fluorescent nanodiamonds (FNDs) are produced by photo-crosslinking of diacetylene-containing lipids physically attached to the FND surface. Not only is this coating method simple and fast, but also it gives the FND-lipid hybrids favorable properties for bioapplications. The hybrids are useful as fluorescent biolabels as well as fiducial markers for correlative light and electron microscopy.

Measuring Nanoscale Thermostability of Cell Membranes with Single Gold-Diamond Nanohybrids.

Much of the current understanding of thermal effects in biological systems is based on macroscopic measurements. There is little knowledge about the local thermostability or heat tolerance of subcellular components at the nanoscale. Herein, we show that gold nanorod-fluorescent nanodiamond (GNR-FND) hybrids are useful as a combined nanoheater/nanothermometer in living cells. With the use of a 594 nm laser for both heating and probing, we measure the temperature changes by recording the spectral shifts of the zero-phonon lines of negatively charged nitrogen-vacancy centers in FNDs. The technique allows us to determine the rupture temperatures of individual membrane nanotubes in human embryonic kidney cells, as well as to generate high temperature gradients on the cell membrane for photoporation and optically controlled hyperthermia. Our results demonstrate a new paradigm for hyperthermia research and application.

Fluorescent Nanodiamond: A Versatile Tool for Long-Term Cell Tracking, Super-Resolution Imaging, and Nanoscale Temperature Sensing.

Fluorescent nanodiamond (FND) has recently played a central role in fueling new discoveries in interdisciplinary fields spanning biology, chemistry, physics, and materials sciences. The nanoparticle is unique in that it contains a high density ensemble of negatively charged nitrogen-vacancy (NV(-)) centers as built-in fluorophores. The center possesses a number of outstanding optical and magnetic properties. First, NV(-) has an absorption maximum at ∼550 nm, and when exposed to green-orange light, it emits bright fluorescence at ∼700 nm with a lifetime of longer than 10 ns. These spectroscopic properties are little affected by surface modification but are distinctly different from those of cell autofluorescence and thus enable background-free imaging of FNDs in tissue sections. Such characteristics together with its excellent biocompatibility render FND ideal for long-term cell tracking applications, particularly in stem cell research. Next, as an artificial atom in the solid state, the NV(-) center is perfectly photostable, without photobleaching and blinking. Therefore, the NV-containing FND is suitable as a contrast agent for super-resolution imaging by stimulated emission depletion (STED). An improvement of the spatial resolution by 20-fold is readily achievable by using a high-power STED laser to deplete the NV(-) fluorescence. Such improvement is crucial in revealing the detailed structures of biological complexes and assemblies, including cellular organelles and subcellular compartments. Further enhancement of the resolution for live cell imaging is possible by manipulating the charge states of the NV centers. As the "brightest" member of the nanocarbon family, FND holds great promise and potential for bioimaging with unprecedented resolution and precision. Lastly, the NV(-) center in diamond is an atom-like quantum system with a total electron spin of 1. The ground states of the spins show a crystal field splitting of 2.87 GHz, separating the ms = 0 and ±1 sublevels. Interestingly, the transitions between the spin sublevels can be optically detected and manipulated by microwave radiation, a technique known as optically detected magnetic resonance (ODMR). In addition, the electron spins have an exceptionally long coherence time, making FND useful for ultrasensitive detection of temperature at the nanoscale. Pump-probe-type nanothermometry with a temporal resolution of better than 10 µs has been achieved with a three-point sampling method. Gold/diamond nanohybrids have also been developed for highly localized hyperthermia applications. This Account provides a summary of the recent advances in FND-enabled technologies with a special focus on long-term cell tracking, super-resolution imaging, and nanoscale temperature sensing. These emerging and multifaceted technologies are in synchronicity with modern imaging modalities.

Time-Resolved Luminescence Nanothermometry with Nitrogen-Vacancy Centers in Nanodiamonds.

Measuring temperature in nanoscale spatial resolution either at or far from equilibrium is of importance in many scientific and technological applications. Although negatively charged nitrogen-vacancy (NV(-)) centers in diamond have recently emerged as a promising nanometric temperature sensor, the technique has been applied only under steady state conditions so far. Here, we present a three-point sampling method that allows real-time monitoring of the temperature changes over ±100 K and a pump-probe-type experiment that enables the study of nanoscale heat transfer with a temporal resolution of better than 10 µs. The utility of the time-resolved luminescence nanothermometry was demonstrated with 100 nm fluorescent nanodiamonds spin-coated on a glass substrate and submerged in gold nanorod solution heated by a near-infrared laser, and the validity of the measurements was verified with finite-element numerical simulations. The combined theoretical and experimental approaches will be useful to implement time-resolved temperature sensing in laser processing of materials and even for devices in operation at the nanometer scale.