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Posted: Jun 01, 2006
Superior optical labels made from diamond nanoparticles
(Nanowerk Spotlight) Optical labeling is an important tool in biological imaging because it offers superb discrimination between the sites of interest and the crowded background of a biological specimen. Diamonds nanocrystals have several advantages over other optical labels and open new opportunities in optical imaging, especially in applications where the size of optical labels represents an important parameter.
Optical labels, or markers, fall into either one of two categories: luminescent labels and scattering labels. Luminescent optical labels include fluorescent dyes and quantum dots. Scattering, by Raman spectroscopy such as e.g. Surface Plasmon Resonance microscopy, involves the coupling of incident light with the internal vibrational states of molecules. At resonance, when the exciting light is tuned to an electronic absorption band of the molecule, the intensity of Raman scattering increases by orders of magnitude and renders a bright appearance of the labels.
"None of these optical labels is ideal," Dr. Taras Plakhotnik from the School of Physical Sciences at the University of Queensland in Australia explains to Nanowerk. "Photo instability, low cross section, and toxicity of the luminescent labels present a problem, especially in the context of tracking individual labels. In the case of the scattering labels, thermal perturbation of biological environments (several degrees centigrade) and unpredictable catalytic properties of immunolabels are disturbing issues for biologists."
Plakhotnik and his group started a project to develop a novel type of luminescent labels for bio-imaging applications based on diamond nanocrystals. This recent work, titled "Imaging and sizing of diamond nanoparticles" and published in the March 1, 2006 issue of Optics Letters, was a very first step of their project on diamond based optical labels. The project's goal is to mass produce photo-activated nanodiamonds as photo-stable (practically not bleaching) and nontoxic luminescent nanoprobes for biological applications.
"Color-center nanocrystals represent a luminescent label encapsulated within a scattering optical label and display a blend of merits that are characteristic of both luminescent and scattering labels: high-contrast imaging due to the spectral separation of excitation–emission bands, photo stability, immunity to the environment, and strong elastic scattering owing to the high refractive index relative to that of the biological medium" says Plakhotnik.
Moreover, the color centers in diamonds, have a number of advantages over existing luminescent probes. Plakhotnik summarizes the advantages of luminescent diamonds over other optical labels:
Unlike gold nano spheres, diamond can be made luminescent and therefore very small crystals (<5 nm) can be optically detected.
Unlike organic dyes, some types of color centers in diamond are photo stable.
Unlike semiconductor quantum dots and many dye molecules, diamond is a non toxic material (in recent experiments, the team was able to transfer a large amount of nanodiamonds into a living cell without affecting the cell's function). Diamonds are not blinking.
Fluorescence life time of nanodiamonds is approximately 22 ns. Although this reduces the maximum number of photons which a single color center can emit, a relatively long luminescence (typical intercellular auto luminescence lasts for 1-4 ns after a pulsed excitation) allows effective time gating which substantially enhances image contrast.
"Because diamond has a very high refractive index, its scattering is very strong and almost independent on the refractive index of the environment" Plakhotnik explains. "We were able to show that scattering of 40 nm diamonds can be easily detected, even when the diamonds are embedded in a polymer, and that the intensity of the scattered light can be used for precise measurement of the sizes of individual crystals."