Type Ib diamonds emit bright fluorescence at 550 -800 nm from nitrogen-vacancy point defects, (N-V) 0 and (N-V) ؊ , produced by high-energy ion beam irradiation and subsequent thermal annealing. The emission, together with noncytotoxicity and easiness of surface functionalization, makes nano-sized diamonds a promising fluorescent probe for single-particle tracking in heterogeneous environments. We present the result of our characterization and application of single fluorescent nanodiamonds as cellular biomarkers. We found that, under the same excitation conditions, the fluorescence of a single 35-nm diamond is significantly brighter than that of a single dye molecule such as Alexa Fluor 546. The latter photobleached in the range of 10 s at a laser power density of 10 4 W/cm 2 , whereas the nanodiamond particle showed no sign of photobleaching even after 5 min of continuous excitation. Furthermore, no fluorescence blinking was detected within a time resolution of 1 ms. The photophysical properties of the particles do not deteriorate even after surface functionalization with carboxyl groups, which form covalent bonding with polyL-lysines that interact with DNA molecules through electrostatic forces. The feasibility of using surface-functionalized fluorescent nanodiamonds as single-particle biomarkers is demonstrated with both fixed and live HeLa cells.blinking ͉ photobleaching ͉ single-molecule detection ͉ single-particle tracking ͉ live cell O ne of the key avenues to understanding how biological systems function at the molecular level is to probe biomolecules individually and observe how they interact with each other directly in vivo. Laser-induced fluorescence is a technique widely adopted for this purpose owing to its ultrahigh sensitivity and capabilities of performing multiple-probe detection (1-3). However, in applying this technique to imaging and tracking a single molecule or particle in a biological cell, progress is often hampered by the presence of ubiquitous endogenous components such as flavins, nicotinamide adenine dinucleotides, collagens, and porphyrins that produce high fluorescence background signals (4-6). These biomolecules typically absorb light at wavelengths in the range of 300-500 nm and fluoresce at 400-550 nm (Fig. 1). To avoid such interference, a good biological fluorescent probe should absorb light at a wavelength longer than 500 nm and emit light at a wavelength longer than 600 nm, at which the emission has a long penetration depth through cells and tissues (5, 7). Organic dyes and fluorescent proteins are two types of molecules often used to meet such a requirement (1,8,9); however, the detrimental photophysical properties of these molecules, such as photobleaching and blinking, inevitably restrict their applications for long-term in vitro or in vivo observations. Fluorescent semiconductor nanocrystals (or quantum dots), on the other hand, have gained considerable attention in recent years because they hold a number of advantageous features including high photobleaching thresholds a...

We report the direct observation of the thermalization of electrons in gold following 180 fs optical pulse excitation. The evolution of the electron energy distribution from the nascent (as photoexcited) to a hot Fermi-Dirac distribution was measured by time-resolved photoemission spectroscopy. Depending on the excitation density, thermalization times as long as =1 ps were observed. A model incorporating both electron-electron and electron-phonon scattering, and using Fermi-liquid theory to properly account for screening is found to reproduce the main features of the experiment.Electron-electron (e-e) scattering in metals has usually been studied by transport' measurements. The contribution of e-e scattering to resistance can only be observed at low temperature, because above the Debye temperature electron-phonon (e-p) scattering completely dominates the resistivity.According to Landau s Fermi-liquid theory, the resistance due to e-e scattering is p, , = AT, where T is the temperature and A is a constant. However, even at low temperature, extraction of p, , from the measured resistivity is complicated by electron-phonon (e-p) and defect scattering. ' Observation of the thermalization of electrons excited by ultrafast optical pulses provides an alternative means to study e-e scattering. The relaxation of an optically excited, non-Fermi-Dirac distribution to a hot Fermi-Dirac distribution is mainly through e-e scattering due to the large momentum exchange and large phase space available for the process which involves quasiparticle energies in the range of an electron volt.In this paper, we report the first direct measurement of the thermalization process in an optically excited metal. We are able to observe the nascent (as photoexcited) electron energy distribution, and the time evolution from the nascent distribution to a Fermi-Dirac distribution. The thermalization process is found to take up to -1 ps for low optical excitation levels, and proceeds more rapidly for higher optical excitation levels. Because thermalization and electron-phonon energy relaxation occur on similar time scales {on the order of ps), we find that even in this regime it is necessary to simultaneously include both e-e and e-p scattering to fully understand the dynamics. A model based on the Boltzmann transport equation under the relaxation-time approximation is pro-t=0 fs 130 fs 400 fs 670 fs 1300 fs 0.1 0 C ENERGY (eV) FIG. 1. Electron energy distribution function vs energy with 120 pJ/cm absorbed laser fluence at five time delays. The dashed line is the best Fermi-Dirac fit and the corresponding electron temperature T,, is shown. The vertical scale is in units of the density of states.posed to explain the experiment. Fermi-liquid theory is used to properly account for Coulomb screening. Time-resolved photoemission spectroscopy was used to measure the time evolution of the electron energy distribution following ultrashort laser pulse excitation of a gold sample. The sample was a room temperature 300-A-thick gold film held in vacuum at 5X10...

Fluorescent nanodiamond is a new nanomaterial that possesses several useful properties, including good biocompatibility, excellent photostability and facile surface functionalizability. Moreover, when excited by a laser, defect centres within the nanodiamond emit photons that are capable of penetrating tissue, making them well suited for biological imaging applications. Here, we show that bright fluorescent nanodiamonds can be produced in large quantities by irradiating synthetic diamond nanocrystallites with helium ions. The fluorescence is sufficiently bright and stable to allow three-dimensional tracking of a single particle within the cell by means of either one- or two-photon-excited fluorescence microscopy. The excellent photophysical characteristics are maintained for particles as small as 25 nm, suggesting that fluorescent nanodiamond is an ideal probe for long-term tracking and imaging in vivo, with good temporal and spatial resolution.