Arguably, humans will soon become the first life-size nanotechnology built from the ground up to return the favor — by creating a nanoscale machinery from the top down. As the world awaits fulfillment of this promise, a less glamorous nano- and micro- particle technology is slowly stealing the show. Among the many kinds of materials now used to make finely divided powders and spheres, gold has emerged as the premier element, primarily because of its unique opto-electronic properties. Researchers from Munich have recently used this so-called ‘plasmonic gold’ to build what they are dubbing a microscale photonic elevator. There are lots of things that can be done by hitting flecks of gold with various beams of energy — precision thermal control, fluorescence, or movements in a 2D plane for example — but the elevator itself is control in the third axis.

Plasmons are basically oscillations of free electrons. The electron density in a plasmonic nanoparticle can couple with electromagnetic radiation of wavelengths that are far larger than the particle itself. What differentiates these particles from plasmons that exist just on the surface of an interface is that they have unique scattering and absorbance properties that derive from their physical size and shape. In other words, their interaction with light or other radiation is not just defined by what they are elementally, but by their geometry and position relative to other particles.

One device which has been able to transform our existing nanotechnology, still scarcely less crude than bulk-solution phase chemistry, is the laser trap. This device, along with the various kinds of force microscopes out there are the micro-sized hands with which we can feel and fumble the nano. As countless other researchers have done before, the authors used such a trap to confine silica spheres barely a micron in diameter to a plane. The little bit of magic that they introduced was to create in their words ‘Janus’ face particles by coating just the top half of the silica beads with a thermally deposited gold vapor. The plasmon scattering effects created by the asymmetric gold coating let them orient the particles with respect to the axis of the laser, and also drag along that dimension — the ‘elevator’ we spoke of.

This new facility with gold is but the surface of a larger gold technology ecosystem. High-end audio cables don’t have a gold coating for fashion, or even cleanliness, but rather for conduction. If you can conduct electrons well, you can probably also be trusted to ferry heat. Now many of you have probably heard about optogenetics, stimulating and recording neurons using genetically encoded goodies that interact with light. With the electronic finesse afforded by gold, you can potentially skip some of the genetic misadventures one might expect in trying to transfect and interact with a human brain using foreign genes. In other words, you can have all the benefits of optogenetics, but need none of the actual genetics.

The way to do this is to couple (slightly smaller) 50nm gold particles to something that could bind to the natural ion channels that activate neurons, and warm them with cool green light. Granted, the first researchers to succeed hooked their gold to a scorpion neurotoxin called Ts1, which unfortunately may not yet be the most desirable alternative to genetics. We are zeroing in on the brain here because from what we have seen with these kinds of nanotools, biological applications have been perceived as the most exciting, and therefore attractive to funding bodies. The authors have already envisioned first applications in remote sensing, force measurement, controlled heating, and even microswimming.

However these are precisely the kinds of things that the national BRAIN Initiative scientists are looking to use to engage with the entire brain at once on the smallest possible scales. One thing neuroscientists have been able to do already, at least in vitro, is add trapping spheres to growing nerves and trawl the neurites along toward desired targets. For something like peripheral nerve repair, having a volume for your work zone instead of just a plane could be the deal maker. Getting a laser trap inside the central nervous system however, is probably not realistic. Fortunately, any good remotely energized smartdust technology is not just limited to optical interactions.

If the heating energy is delivered electromagnetically as opposed in the optical regime, what we would have would be nothing short of full-blown super-accurate wireless magnetothermal deep brain stimulation. For this to work you first need get yourself some good heat-sensitive capsaicin receptor TRPV1 ion channels. You can use your own naturally existing ones of course, but might also want to introduce special versions using the kinds genetic tools we mentioned above.

The gold effects will be good for local calibrations and feedback, but to interact magnetically, you need to add a magnetically active element like iron. The heating principle is similar to the gold concept, only now we have a beautiful thing: You don’t need to restrict yourself to just those areas you can reach from above (or perhaps thread a fiber optic catheter to) — you can instantly target the entire brain non-invasively from without to fire off your fav neurons.

The best quality iron to use is a 22nm spherical ‘Fe3O4’ particle. It has the best heating rate per gram, is minimally toxic, lasts for months, and you don’t need any pesky exogenous targeting transgenes to get it in or localize it. When we begin to talk about superparamagnetic iron particles, encapsulated in special shielding membranes and linked to targeting molecules, we have already a fairly sophisticated nanodevice on our hands. In an upcoming post we will say a little more about new work in this area, and take a look at just how accurate magnetic probes and stimulators can be.

In particular, we will talk about new MRI and MRM (Magnetic Resonance Microscopes), which might now access regions of the brain as tight as 10 microns. Of note, the convergence of these new electromagnetic technologies at the hardware level will mean that the same basic instrument, perhaps with a few minor component swaps, can be optimized to image, stimulate, physically position and orient, or therapeutically treat. When commodified, this baby will be the kit that every budding neuronaut will covet.