Fig.1: Fluorescence from nanodiamonds.

Fluorescent nanodiamonds have several advantages over commonly used organic dyes and quantum dots:they are exceptionally photostable, do not ‘blink’ and have therefore been recently considered as superior long term imaging probes. Unlike their quantum dot counterparts, nanodiamonds are biocompatible and their surfaces can be easily functionalized. Although they are promising as next generation imaging probes, mass producing nanodiamonds remains a challenge. Now, Huan-Cheng Chang, Wunshain Fann1 and colleagues of Academia Sinica andTaiwan National University show that it is possible to produce nanodiamonds in large quantities using helium beams.

The nitrogen vacancy (N-V) defect centers in diamond are normally produced by bombarding the material with a high energy (typically 2 MeV) electron beam followed by annealing at high temperatures (~ 800°C). However, this procedure requires sophisticated and expensive equipment. The Taiwan team’s home-built device irradiates diamond powders with a high fluence, medium-energy helium ion beam (He+). Their setup—which can be easily installed and operated safely in an ordinary laboratory—increased the yield of fluorescent nanodiamonds by nearly two orders of magnitude.

Helium ions are chemically inert and so embedding them in the diamond lattice is not expected to change the photophysical properties of the nanodiamonds. Furthermore, with a 40keV He+, about 40 N-V can be generated, whereas only up to 13 vacancies can be formed using a 3MeV proton (H+) beam. This efficient generation of defect centers means that a lower ion dosage is required.

The nanodiamonds produced by the 40keV He+ beam showed similar fluorescence properties under one-photon and two-photon imaging as those formed from 3MeV H+ beams. Notably, the excellent photostability was preserved even for particles as small as 25 nm and could be tracked in cells over a long time to reveal the detailed activities inside the cells in three dimensions. “In addition to possibly using them as targeted carriers in drug delivery and for probing stem cell trafficking in tissue development, if properly doped with impurity atoms, nanodiamonds may also find use in solar cell applications,” says Chang. “For biological imaging, the next challenge is to make smaller (~10 nm) and brighter nanodiamonds.”