Protonated water clusters, H + (H 2 O) n (n ) 5-8), from a supersonic expansion have been investigated by vibrational predissociation spectroscopy and ab initio calculations. The experimental spectra were obtained at an estimated cluster temperature of 170 ( 20 K. Recorded absorption bands at the frequency range of 2700-3900 cm -1 are attributed to the free-and hydrogen-bonded-OH stretches of the ion core and the surrounding solvent molecules. Ab initio calculations, performed at the B3LYP/6-31+G* level, indicate that geometries of the H + (H 2 O) 5-8 isomers are close in energy, with the excess proton either localized on a single water molecule, yieldingSystematic comparison of the experimental and computed spectra provides compelling evidence for both cases. The unique proton-transfer intermediate H 5 O 2 + (H 2 O) 4 was identified, for the first time, by its characteristic bonded-OH stretching absorptions at 3178 cm -1 . The existence of five-membered-ring isomers at n ) 7 is also evidenced by the distinct bonded-OH stretches at 3500-3600 cm -1 and by the free-OH stretch of threecoordinated H 2 O at 3679 cm -1 . Among these H + (H 2 O) 7 isomers is a newly discovered H 5 O 2 + -containing pentagonal ring, which is computed to be lowest in Gibbs free energy at 170 K. Its spectroscopic signature is the splitting of two equivalent bonded OH stretches into a doublet (3544 and 3555 cm -1 ) by vibrational coupling through ring closure. No profound spectral evidence, however, was found for the formation of four-membered rings although it is predicted to be favorable in terms of total interaction energy for H + (H 2 O) 7 . Six-membered ring and three-dimensional cagelike structures are also stable isomers but they are less strongly bound. The preference of five-membered-ring formation at n ) 7 appears to be the result of a delicate balance between entropy and enthalpy effects at the presently investigated cluster temperature. The correlation of this investigation with other studies of neutral water clusters and of the hydration of biological macromolecules is emphasized.

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.