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Leiden physicists detect single nanoparticles in motion

It's pretty hard to spot a single 5 nanometer particle. Leiden physicists pulled it off by turning a 120 nm gold rod into a detector.

It's the tiniest of signals: a small dip in an otherwise noisy graph, lasting for a couple of microseconds. The blip signals that a 5 nanometer gold nanoparticle, has just passed by. 

'It's a proof of principle', says physicist Martin Baaske of the Michel Orrit Single Molecule group, who published a paper in ACS Nano about the new technique. Now, the hunt is on for using the technique in detecting natural nanoparticles, such as small proteins near membranes. 'That is harder to do, but we think may be possible', says Baaske.

The crucial detector is a 120 nanometer long gold rod, sitting on the bottom of a glass plate. Gold is a conductor, which means that electrons can move freely within it, a like sloshing water in a bathtub. The electron waves, called plasmons, prefer to oscillate at a frequency that is determined by the size of the gold particle: the plasmon frequency.

A nanoparticle moves in the vicinity of a gold nanorod (Image Baaske et al)

Plasmon frequency

This can be detected by shining a beam of light on the rod. The incoming electromagnetic waves cause the electrons to oscillate, and this in turn causes another electromagnetic wave to come out. The closer the incoming light waves matches the optimal sloshing frequency, the stronger the outgoing beam.

Now, the exact oscillation frequency is also influenced by nanoparticles close by, in the rod’s electromagnetic near field. So when a 5 nanometer gold nanoparticle passes by, submerged in water, the rod’s plasmon frequency will temporarily change. This way, the passing nanoparticle will betray itself in an ever so slight dip in the detected intensity.

Using this technique, the group managed to detect 5 nanometer small spheres of gold passing through the near field of the nanorod. It's possible to infer the particles sizes and speeds from the size and time characteristics of the dips. 

Two dips in the intensity signal a nanoparticle passing by (Image Baaske et al)

Small dips

'The crucial thing was to get the signal to noise ratio up', says Baaske. Although some predictions said that couldn't be done easily, the group pulled it off, and detected the slight dips, corresponding to nanoparticles passing by the gold rod.

Detecting proteins is much harder, however, since they don't influence the nanorod's plasmon frequency as strongly. 

Nanoparticle concentrations inside biological complex objects like cells are extremely high. To test the sensors performance in such an environment the group performed measurements using a highly concentrated solution of 8 nanometer oil droplets.  While individual particles cannot be distinguished at high concentrations, their collective influence could be teased out from the intensity variations of the light beam over microseconds. 'The particles cause correlations in the intensity that wouldn't be there in pure noise', says Baaske. It's like hearing a soft rain on the roof without being able to distinguish the individual droplets.

The next step would be increasing the sensitivity, and then move on to detecting real proteins, instead of artificial oil particles. 'That may be even harder, since proteins tend to cling together, or cling to the glass walls or the nanorod', says Baaske. 

Martin Dieter Baaske, Peter Sebastian Neu, and Michel Orrit, Label-Free Plasmonic Detection of Untethered Nanometer-Sized Brownian Particles, 
ACS Nano 2020 14 (10), 14212-14218

DOI: 10.1021/acsnano.0c07335

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