May 26, 2016
Atomic force microscopy – 30 years on
Thirty years since its first inception, the atomic force microscope has proved a hugely versatile tool. Applications range from quantifying dopant distributions in electronics and the analysis of dust particles in space, to characterizing biopsies for cancer diagnostics. More than simply bringing atomic-scale resolution to non-conducting surfaces, modifications of the technology have provided important tools for sensing chemical entities and mechanical properties, with force sensitivities so great they can be used to study and control mitosis in the proliferation of life itself. nanotechweb.org visited Basel in Switzerland, home to some of the pioneers in AFM technologies, to find out how far the field has come in the past three decades.
The development of scanning probe technologies began with the scanning tunnelling microscope (STM), and was driven by the semiconductor industry in the late 1970s. Christoph Gerber, co-inventor of the atomic force microscope, points out that although electronics feature sizes were coming close to the nanometre scale in the 1970s, there was no way of obtaining spectroscopic information of such small features. “We thought that if we established a tip very close – so that due to the proximity there would be tunnelling - we would have an instrument that could do this kind of spectroscopic work.”
From there came the idea of scanning the tip and keeping the quantum tunnelling current constant. This would effectively trace a topography of the surface with a lateral resolution that could image atoms. “The big breakthrough for STM came when we were able to image the 7 × 7 reconstruction of silicon (1,1,1),” explains Gerber. As the arrangement of atoms at the surface differs from the bulk, glimpsing this reconstruction in a real image was a powerful demonstration of the instrument’s potential.
It was another 3-4 years before the seminal results from IBM’s Zurich lab were later verified at a workshop in Oberlach. “Everyone was extremely excited as you can imagine,” says Gerber. Yet the technique had one major constraint – it was limited to conducting surfaces. At the same workshop in Oberlach ideas began to emerge with the aim of achieving similar resolution on a non-conducting surface. “Gerd Binnig came up with the idea to measure interactive forces between the tip and the sample surface and maybe this could be done by introducing a cantilever with an integrated tip,” says Gerber. “We took it from there and designed and developed the first AFM based on the latest development of the STM.”
The original AFM is now part of an exhibition called “The Making of the Modern World” in the London Science Museum in London, and a replica is on display at the University of Basel, where Gerber is currently affiliated. Here also the Swiss Nanoscience Institute (SNI), now headed by Christian Schönenberger, first emerged from the National Centre of Competence for Nanoscience, establishing the first curriculum for nanoscience a year later. Basel has a rich heritage of discovery and invention, from its pre-eminence in the Middle Ages for paper production – the bedrock of scholarly communication at the time – to the development of calculus by Basel-born Johann Bernoulli. It is also where Gerber grew up and developed a love of engineering, just streets from the University’s science buildings where he works today.
Lab on a tip
The world’s first student to work on an AFM was Ernst Meyer, also a researcher at the SNI. Despite great success with the tool since, he admits he was actually quite happy working on superconductors at the time. “My supervisor was really excited about this new technique and persuaded me to work on it,” he recalls.
Meyer leads the way into the basement at the SNI, where vibrations can be minimized to allow ultrasensitive measurements. “We found out recently that the resolution of AFM is getting better than STM,” says Meyer, describing how you can now see nanoribbons with a resolution that can detect single hydrogen atoms and carbon rings.
The large shiny instrument behind him – maintained at 4K where even thermal vibrations start to grind to a halt - is in fact a combination of an AFM and an STM, so it can measure both forces and power. Meyer and his colleagues recently demonstrated the extraordinary sensitivity of the tool in measurements of friction between a graphene nanoribbon sliding along a gold surface. It turns out there is barely any friction at all, with thermal activation at room temperature alone being enough to cause movement of the superlubricated nanoribbons across the surface.
Inside is a quartz tuning fork not dissimilar to those found in Switzerland’s famed watches. Franz Giessibl, now based at the University of Regensberg, first transformed a quartz tuning fork taken from a watch into a force sensor for AFM. He made the breakthrough from a makeshift lab in his room where he studied the tool at weekends during a brief period working as a management consultant. The changes in the oscillation frequency of the tuning fork indicate changes in the forces exerted between tip and surface, and the stiffness of the fork allows high-resolution imaging without the large-amplitude oscillations previously required.
Despite the incredible resolution the tool can achieve, nowadays AFM offers much more than just high-precision imaging. In another lab Meyer shows a machine that can scan surfaces as large as 100 × 100 micrometres, measuring electrostatic surfaces with enough sensitivity to quantitatively measure dopant distribution. As the electronics industry works to smaller and smaller footprints, the distribution of dopants in materials starts to play a significant role in device performance. “So the AFM is not just a tool to measure topography,” says Meyer. “It’s like a lab on a tip to measure all kinds of forces – in this case electrostatic forces.”
AFM in diagnostics
Key to AFM is the extraordinary sensitivity of the cantilevers, and in the early 2000s a different way of exploiting this attribute was developed. By functionalizing the surface of the cantilever, it can be made to absorb molecules from a gaseous or fluid sample, which then bend the cantilever or change its oscillation frequency. An array of cantilevers with different functionalizations could be used as an electronic nose to sniff out whether certain substances are present.
“We first used this to detect different solvent molecules in breath samples,” explains Hans Peter Lang, a researcher in another lab at the SNI. “In the beginning we used cantilevers, and then we found a different compact form of sensor which is similar in function but different in concept – using piezoresistive membranes.” This type of breath sensor has proved a remarkably accurate non-invasive diagnostic tool for lung as well as head and neck cancer in pilot tests, and is now close to commercialization.
Francois Huber, who shares a lab with Hans Peter Lang, highlights how the cantilever arrays have also become useful for identifying single gene mutations from biopsies. Recently introduced cancer drugs have particularly high efficacy for specific cancer genes, such as the HER2 gene for aggressive breast cancer and the BRAF mutation found in 50% of malignant melanoma incidents. “Before we treated cancers with general chemistry or radiation – everybody got the same treatment and either you were lucky or unlucky,” says Huber. “Here we can actually target the cancer directly – it goes towards personalized medicine so that you treat patients according to their genetic predisposition.”
Over in the Biozentrum building, Rod Lim is using nanoindentation with an AFM to identify healthy from malign tissue in breast biopsies. Although tumours feel stiff to the touch, researchers have found that single cancer cells are actually more compliant than normal tissue. Lim and his colleagues developed an AFM-based instrument called ARTIDIS (automated and reliable tissue diagnostics) that makes tens of thousands of indentation force measurements to derive maps of the stiffness distribution across entire biopsies, which can then reveal the nature of the tissue.
In terms of the imaging capabilities, Lim is quick to point out that AFM in biology goes beyond taking static snapshots. “High-speed AFM now enables you to look at nanostructures or molecular machines in real time as they move and interact with other molecules,” he says. Using a high-speed AFM developed by Toshio Ando at Kanazawa University, he and his team have been studying the nuclear pore complexes through which substances pass in and out of cell nuclei. “How these pores work has been a mystery for the past 20 years or so since they were discovered,” says Lim. With their high-speed AFM they have been able to image polymer-like proteins on the pores that work like tentacles, blocking non-specific proteins but grabbing hold of specific proteins – even larger proteins - that are important for cell functions.
Across the river from the NSI Basel is home to several giants in the pharma and life science industry - such as Roche and Novartis – as well as the Department of Biosystems Science and Engineering (D-BSSE) of ETH Zurich in Basel, where Daniel Müller’s group are also pushing the boundaries of AFM research in biology. “We are using this technique to investigate ligand receptor interactions, the key players being g-protein-coupled receptors,” explains Moritz Pfreundschuh. These proteins are important for several processes in the human body, including blood coagulation. By covalently bonding ligands to the AFM tip and raster scanning over the membrane protein, Pfreundschuh can quantify the strength of the bond, which provides information on the kinetic properties and ultimately the function of the protein.
Also leveraging the force sensitivity of AFM, his colleague at ETH in Basel, Cedric Cattin, has successfully used an AFM tip to control cell mitosis. This type of cellular reproduction begins with cell rounding, in which the DNA is condensed and eventually segregates into two adjacent daughter cells. “It was well known that the cell rounds up and that below a certain height of confinement this rounding does not happen anymore, but until recently it was not possible to quantify the forces of the process,” explains Cattin. He used a modified AFM cantilever to achieve the required force sensitivity and incorporated a confocal light microscope to image the rounding and mitosis processes occuring more and more slowly as the confining cantilever force increased to 200nN, until mitosis ceased altogether.
More than 300,000 academic papers have been published on research based on AFM over the past 30 years, and the tool seems to have greatly outperformed its original remit – to image non-conducting surfaces with atomic resolution. More than a passive view of the world at the nanoscale, the tool has provided a means of probing and exploiting how matter at this scale interacts.
More on the latest developments in AFM technologies coming soon in the Nanotechnology focus collection on scanning probe microscopy, which celebrates the 30th anniversary of the Nobel Prize for scanning tunnelling microscopy, and the first report on atomic force microscopy.