This is an interesting sort of weapon concept I thought of. I imagine it to be useful for remotely causing pain to someone in an area. Essentially, you'd have police or maybe drones spray or apply by some means nanoparticles to someone, which would either sit on someone's skin or perhaps penetrate some distance, while remaining in the skin. They would absorb radiation such as microwaves, which would allow them to heat up remotely. That way, a strong microwave field would cause, say protesters to be burned while others around the emitter would be mostly unharmed. Microwaves do interact with human tissue, so perhaps if the nanoparticles could absorb/attenuate radio raves, that would work better. I suppose that one issue that occurs to me is that maybe the power needed to remotely heat up the particles might automatically fry any electronics in the area. Although maybe a dish could be used to focus the radiation a limited amount? To the extent that it wouldn't harm people without the spray. Thoughts on the plausibility of this?
We already use this concept to kill cancer
Trying to kill a cancer in the human body is almost an exact scaled-down analog of what you're trying to do--selectively attack only the "bad guys" while sparing "innocent civilians".
One way people have done this is using Photothermal Therapy with gold nanoparticles. Gold nanoparticles are delivered to the cancer site, and light of a certain wavelength is used to heat up the nanoparticles while affecting the surrounding non-nanoparticle impregnated flesh much less. Yep. They can BURN THE CANCER OUT OF YOU.
Here are some of the challenges in doing this.
How do we hit the bad guys without hitting the good guys?
This technique works by using surface plasmon resonance (SPR), which is a phenomenon unique to noble metal nanoparticles. In short, SPR makes the electrons at the surface of a nanostructure resonate, sort of "concentrating" them at the surface.
For this to happen, the nanoparticle has to be of the correct shape and size, and the light hitting it has to be the right frequency to resonate with it WITHOUT being the right frequency to resonate with water (like microwaves).
Cancer therapy also has a bit of an advantage over you here. It turns out, the blood vessels in tumors are kind of crappy, and they leak everywhere. (This is called the enhanced permeability and retention effect.) So nanoparticles will tend to selectively accumulate in them. That's great for us. Doesn't really help you though.
How big can we make these particles?
This is a bit tricky. If you make the particles too big, your immune system is going to notice them and clear them out. You probably want them to be 100s of nanometers or smaller.
What electromagnetic radiation do we need to hit them with?
Now we run into another problem--not with the particles, but with the light we need to activate them.
Here's the info page on gold nanoparticles from your friendly local lab-supplies-and-dangerous-chemicals store: A list of the different sizes of nanoparticles they have and their SPR wavelengths.
5nm ---- 520nm (green light)
50nm ---- 535nm (mildly yellower green light)
100nm ---- 572nm (yellow light)
As it turns out, however, green light isn't so great at shining through human flesh. Neither is red, yellow, blue, or purple light. If this happens not to be true for you in particular, please let us know whether you are planning to use your powers of invisibility for good or for evil.
The wavelengths of light that ARE good at getting through human skin are 650-900nm, with 800nm being the best. However, "best" is relative--in most cases it can only penetrate a couple cm at best.
You mentioned radio waves in your question, but radio waves have a wavelength the size of about a building, so by my understanding much bigger things than nanoparticles would be needed. But if you can get the guy to swallow a car antenna you might be all set.
How do we make nanoparticles that work with this wavelength?
Turns out if you make the nanoparticles have certain fancy shapes (rods, hollow spheres, what have you), you can tune their resonant wavelength a bit so that it's in the near IR.
Could we make nanoparticles that worked at other wavelengths, say radio waves?
Well, no. Not with what we have now, ate least.
However, there is a small glimmer of hope for you because other noble metals like silver have slightly different SPRs than gold. As far as I know though, they are not drastically different, so it's mostly near the IR-to-UV wavelengths.
So if you're okay with a bit of hand waving you might be able to claim that due to some as-yet-undiscovered mega special surface plasma resonance effect on some as-yet-discovered unobtanium, they are activated by radio waves. There are probably a lot of other problems with this, but your average readers might not care.
The most convincing thing would be to say that for some reason the nanoparticles only get trapped in the top couple mms of skin, and that because of (insert science here) they resonate super well with near IR. Then shine a near IR laser of high intensity at them. Non-criminals might also get a bit toasty though.
It's unlikely however that you would be able to get them through walls, so I'm not sure what advantage this would have over, say, shooting them with a gun.
Hope that helps!
We have that already
You don't actually need to embed particles in the skin, since the microwaves will excite molecules in the tissue, creating the sensation of burning. It's colloquially referred to as a "heat ray".
Simply put, the easiest way of applying something like this would be by integrating it with a civil database or monitoring device, and having the nanoparticles attach to, and send signals down, their nerve endings based on their behavior at the moment.
For instance, a guy waving a stick around, yelling things that quite frankly aren't nice, gets pain and possibly even paralysis, whereas dear old granny who got caught up in all this remains perfectly fine.