This week I am away because I am attending the IQEC CLEO Pacific Rim conference in Sydney (international laser physics, optics and quantum optics conference). This morning I went along to a talk (by one Varun Sreenivasan) about imaging within live cells using colour centres in nanodiamonds, and thought it was rather interesting and neat. The basic concept is to replace organic dyes and fluorescent proteins with luminescent diamonds!
Diamonds naturally contain some concentration of impurity atoms which are captured during formation of the crystal. For artificial diamonds, usually synthesised by methane vapour deposition or detonation of an explosive compound, the principle impurity is nitrogen. If the crystal is irradiated with ionising radiation (gamma or alpha particles in particular) carbon atoms are displaced from the crystal, leaving vacancies: at high temperatures these are able to diffuse. If a vacancy is `captured' by a nitrogen impurity (which are covalently incorporated into the crystal) the new compound entity is referred to as a NV centre.
NV centres are neat because they essentially behave like an atom. By this I mean that they have transitions which are in the optical frequency range, allowing for optical detection of the centres. Specifically, excitation in the green (533nm) gives fluorescence in the red (630nm). They also have an interesting electronic structure in that, instead of singlet ground and excited states with a triplet intermediate, they have triplet ground and excited states with a singlet intermediate. A singlet state involves two electrons with a total spin angular momentum of zero, for a total of one spin projection state, whereas the triplet has net spin one, so has three spin projection states. This means that the NV centre has interesting spin properties which can be exploited for imaging and magnetometry (as it turns out, they might also be useful in nanothermometry and single-spin sensing, or miniaturised NMR/MRI!).
The advantages of NV nanodiamonds over traditional fluorophores are threefold. Firstly, the nanodiamond is very inert chemically and biologically, meaning that they are not cytotoxic or carcinogenic like existing options. Secondly, the surface chemistry of nanodiamonds is flexible, so that there are opportunities to attach specific proteins, functional groups or the like to them. Finally, the nanodiamonds are small, bright and quite photostable: they do not bleach under continued exposure, like a fluorescent protein, or blink (have irregular variations in emission intensity) to the same extent as a quantum dot (it is thought that reduced blinking in NV diamonds is because electronic excitations are localised to the NV site, whereas in a quantum dot the excitation is delocalised across the whole dot).
This presentation concerned labelling nanodiamonds with somatostatin (a regulator which interacts with GPCRs to help drive blood pressure homeostasis) to cause specific cells to endocytose them. The way in which this was done was to use a `lego-like' approach that can be readily extended to other functionalising groups/compounds/regulatory molecules. Rather than rely on covalent attachment or weaker adsorption, a protein-protein interaction was used to attach molecules to the crystal surface. The proteins barstar and barnase interact quite strongly (for a non-ionic, non-covalent bonding interaction) and `clip together', forming the basis of a method allowing attachment of different compounds to the diamond surface. This is quite stable and can be extended relatively easily to a variety of compounds of interest.
When somatostatin binds to the cell membrane of the target and initiates endocytosis the entire diamond is drawn inside (these are 30-40nm in size, although many people are now looking at sizes of 4-5nm) and the three-dimensional position of the crystal can be tracked. In the presence of a magnetic field the spin-field interactions mean that even the orientation can be tracked! Another talk expanded on this... but I will leave that for another time!
As a final comment: I thought it was impressive that the body will actually renally clear these nanodiamonds so long as they are below 8nm in length!
The whole idea is quite interesting and provides a neat quantum/biology interface too. Watch this space!
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