FridayAFM - stinging hairs

Héctor here, your AFM expert at Nanosurf calling out for people to share their Friday afternoon experiments. Today I put the mechanical properties of the nettle plant stinging hairs to the test.

I grew up in the countryside, and one of the first things my sister and I learnt is to stay away from nettle plants (Urtica dioica). A brief touch and that's it, it is going to itch for half an hour at least. This is why while I explored the other things around me to see the details, the nettle plants were a big unknown to me (same for snakes, but better not to touch that topic...).

So, this week I bought a nice nettle plant (with stinging hairs, but not the Urtica dioica), the Coleus or painted nettle. 

My original idea was too ambitious (yet I still think is possible, I just need more tries). Let me explain.

Our DriveAFM system has a readout laser for detecting cantilever movement, and a photothermal excitation laser to heat up the cantilever and produce movement. One of the beauties of this way of exciting cantilevers is that AFM probes don't need to necessarily be mounted on the AFM for the lasers to operate. This means that we can do things such as those shown in the next figure.

1 Using the readout and photothermal excitation lasers to characterize a probe that is mounted onto the AFM (Multi-75E) and one that is lying on the sample holder (Tap-150).

I mounted a probe onto the AFM (Multi-75E), and put another probe on top of the sample stage, mounted onto a detached probe holder (the probe, Tap-150 is mounted upside down in respect to the detached probe holder so it's reflective side is facing the AFM).

Using the approaching motors and the automated stage, I brought the two cantilevers in close proximity, and once they were there, I used the lasers to characterize them. First I performed a thermal tune on the Multi-75E, showing its resonant peak at about 73 kHz (and a minor peak at about 470 kHz), then, using the lasers motors, to position the lasers over the Tap-150, I repeated the thermal tune and found the resonant peak of the probe at about 158 kHz. You can see the whole process in video once more here:

Once I characterized both probes independently, I brought them close together to see what happens to the whole system once they are "coupled".

1 Using the readout and photothermal excitation lasers to characterize two probes pressing against each other, one that is mounted onto the AFM (Multi-75E) and one that is lying on the sample holder (Tap-150).

This time I used the photothermal laser and sweep its frequency to see at which frequency I find resonant peaks. First I repeat the Multi-75E on its own, finding the same peak I found with the thermal tune. Then, I pushed one probe against the other (we see this by monitoring the photodiode and noticing that when the two probes are coupled together, there is some deflection on the Multi-75E probe). Then I performed the same frequency sweep and I found that this time we don't see the 73 kHz but one at 142 kHz. 

I went even further, and excited the Tap-150 instead while keeping the readout laser on the Multi-75:

I lack sufficient knowledge to interpret these results properly, but it seems that for this case, when we monitor the soft probe with low resonant frequency, we see (almost unaltered) the mechanical properties of the stiffer probe. Can this be applied to gain knowledge about other systems? Can we obtain some knowledge about the nettle plant stings? As we can see in the next figure, the stings are really tiny and look flimsy enough for an AFM probe to be able to move them without breaking. So I cut a branch from the painted nettle plant and mounted it in the sample holder. I used the clips to hold it in place and a gold coated glass slide to provide some contrast.

1 Painted nettle plant, cutting one branch and mounting it onto a sample holder with a gold coated glass slide to add background contrast.

It took great care to bring the Multi-75E probe close to one of the stings, but with the help of the approach motors and the automated stage (both allowing to move only a few micrometers at a time when needed), eventually I managed to bring things together. 

1 Testing the mechanical properties of the painted nettle plant by pressing it with a Multi-75E AFM probe.

Once they are close together, by monitoring the deflection it is possible to determine when they are in "contact", although, in this occasion I was not measuring the force (which could be a very interesting experiment).

As before with the Tap-150, I performed a frequency sweep, interestingly, I found three peaks. Probably the one at about 62 kHz is the original peak from the Multi-75E probe and the other two belong to the sting, one at about 95 kHz and another at a much lower frequency, about 37 kHz. At this point, there isn't much more we can do without a numerical model of the sting structure, however, hopefully, somebody with the capability to do the modelling part will take this as inspiration and continue where I left off.

However, there is something else we can do apart from mechanical properties. We can image the surface of the sting and map the nanomechanical properties.

1 Painted nettle plant sting's surface imaged with a Multi-75E (topography) in Static Force mode and a WM06 (nanomechanical) in WaveMode.

The surface of the sting is dotted with protuberances. According to literature, this could be a trick to reinforce the structure without having to add a lot of mass to the system, or to modify the hydrophobicity, or to create and antibacterial effect. It is interesting to note from the elastic modulus map that the protuberances have different stiffness than the rest of the surface. However, by looking at the adhesion it's possible to see that there are areas covered in some residue (water? wax?), and this modifies the behaviour of the stiffness map. In the areas exposed, the stiffness of the surface is closer to that of the protuberances (which are exposed), while in the areas where the surface is not exposed, the stiffness is much more different than that of the protuberances. Probably the best is to compare both in the case where they are exposed, but I don't know if the layer penetrates underneath and affects the plant tissue underneath in addition to adding a soft layer on the surface.

So, what was my initial plan? To take a sting with tweezers, clamp it onto the probe holder and use it to image. It will be less sharp than an AFM probe, but it will an interesting experience. Or at least, glue it somehow to the cantilever and use the end of the sting instead of the probe apex. I didn't managed, but I still think it can be done and quite interesting.

I hope you enjoyed what I showed you, that it brings you new ideas to experiment yourselves, and that this information about the painted nettle plant is useful to somebody.

Stay in touch, and if you have more info, please share.

... and if you made it this far, here are some discarded images. Exciting the sting with the laser (see how the peak at high frequency increases its amplitude), and probing a probe resting on a gel-pack box (a ContGD probe).

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