We’re pretty sure that people were always fascinated with things that are out of view. However, we’re not only talking about the far and undiscovered continents, unavailable galaxies, but also – our nearest, yet undiscovered surroundings: an invisible micro- and nanoworld.

In 1980s, it led to the construction of a tunnel and an atomic force microscope invented by Binning and Rohrer which can show things invisible to a human eye…

This intensively developed technique has come a long way until today and despite the same principle of operation, the technical capabilities of currently used microscopes go far beyond scanning modes of microscopes from 30 years ago.

Nevertheless, this technique is still one of the tools for both quantitative and qualitative examination of materials in the nanoscale.

Thanks to this, it’s possible to measure morphology of various objects in the nanometer scale, meaning shape, roughness, and lots of other parameters such as mechanical, electrical, or thermal properties of the newly developed materials (and living matter), especially related with their surface.

For modern micro- and nanotechnologies an atomic force microscope plays a role of a versatile caliper (it’s a basic measuring device used in workshops).

It allows to study such complex phenomena as the elasticity of cell membranes or other soft biological objects!

The key element of an atomic force microscope is a scanning probe that scans an examined surface with nanometric resolution.

A very distant analogy of this idea can be a needle sliding over a vinyl record groove…

The irregularities of the groove read by the needle are converted into an electrical signal which – after appropriate reinforcement – let us hear the recorded music.

In our microscope, the „atomically” sharp needle, follows the shape of the surface, (and depending on the scanning mode) providing us with information about its shape and many other parameters such as temperature, current, voltage, and hardness of the surface of tested material.

A very precise nanoprobe is the heart of the atomic force microscope. Devices of this kind have been developed and offered to our customers at Łukasiewicz – IMiF in Piaseczno near Warsaw for many years.

The offer of this kind of nano-measuring devices is very extensive, but to find out what they can do, you will need to keep reading…

Typical AFM nanoprobe made in our Institute. This pyramid is so sharp that there is a countable number of atoms (1-5nm) at the end of it!

What else such microscope tools are capable of?

It may be boldly argued that vibrating and flexing probe is one of the most sensitive and the most universal measuring tools of modern times.

The frequency of vibrations or degree of deflection will change if an external force acts on this device or when some additional mass is embedded in it.

An example known by everyone is a weight suspended on a string. The greater the mass suspended at the end of the spring, the less frequency the entire system will vibrate. That’s the whole philosophy because when it comes to our probe-based sensor, the role of the spring is played by a probe cantilever (a micro-metric size flexible arm ended with probe`s tip) characterized with a thickness less than of a human hair.

What can be embedded? I’m about to explain…

If at the end of this kind of tiny cantilever we will make a chemically or biologically active area to which some kind of substance gets attached (e.g. particles or bio-chemical objects like viruses, bacteria, proteins, or DNA fragments), as a result we will obtain an ultra-sensitive weight that can register, for instance, the fact of embedding of a single Escherichia coli bacterium. And its weight is just about 600 femtograms (0.0000000000006 grams!).

I must underline that this is not the world record as current world reports say about the result at the level of zeptograms (it’s 21 decimal places, which corresponds to a countable number of atoms).

So the challenge is to measure the frequency of vibrations of such a micrometer or nanometer structure. Luckily, there are a few methods, and measuring frequency changes is one of the best grasped by humankind. And this is how we get the weight of a measured micro-object expressed in frequency, and after conversion (which is not trivial, but feasible) we can weigh or detect the presence of biological substances or objects (even single viruses, proteins, or chemical compounds).

What’s interesting, when producing a greater number of micro-weights in one sensor and making them sensitive to various substances, we approach the idea of a peculiar concept of an “artificial nose.”

Silicon sensor for 2 and 4 different substances made in our Institute

What does it mean?

A human being, thanks to receptors in olfactory epithelium, can identify around 10 000 substances on the basis of the differences in their odor. The complex biochemical process of identifying fragrance leads from the registration of particles of a substance in the receptor to an electrical impulse in the olfactory cortex of the brain.

“The artificial nose”, even though it’s not perfect, works in a similar way. The fact of occurrence of a specific particle is registered by a micromechanical sensor, and the electrical information about the amount of a substance is the result of signal processing by special electronic systems.

We are also working on this type of sensors in our Piaseczno laboratory!

Author: Paweł Janus, Ph.D., Eng., pawel.janus@imif.lukasiewicz.gov.pl