Whenever you read an article about nano this or nano that, chances are you come across a large number of confusing three-letter acronyms - AFM, SFM, SEM, TEM, SPM, FIB, CNT and so on. It seems scientists earn extra kudos when they come up with a new three-letter combination. One of the most important acronyms in nanotechnology is AFM - Atomic Force Microscopy. This instrument has become the most widely used tool for imaging, measuring and manipulating matter at the nanoscale and in turn has inspired a variety of other scanning probe techniques. Originally the AFM was used to image the topography of surfaces, but by modifying the tip it is possible to measure other quantities (for example, electric and magnetic properties, chemical potentials, friction and so on), and also to perform various types of spectroscopy and analysis. Today we take a look at one of the instruments that has it all made possible. So far, over 20,000 AFM-related papers have been published; over 500 patents were issued related to various forms of scanning probe microscopes (SPM); several dozen companies are involved in manufacturing SPM and related instruments, with an annual worldwide turnover of $250-300 million, and approx. 10,000 commercial systems sold (not counting a significant number of home-built systems).
The need for 3D visualization and analysis at high spatial resolution is likely to increase as nanoscience and nanotechnology become increasingly important and nanotomography could play a key role in understanding structure, composition and physico-chemical properties at the nanoscale. Scientists from the Electron Microscopy Group at the University of Cambridge in the UK report that nanotomography is becoming an important tool in the study of the size, shape, distribution and composition of various materials, including nanomaterials.
There is a significant and growing need across the research and medical communities for low-cost, high throughput DNA separation and quantification techniques. The isolation of DNA is a prerequisite step for many molecular biology techniques and experiments. Although single molecule techniques afford extremely high sensitivity, to date, such experiments have remained within the confines of academic and research laboratories. The primary reasons for this state of affairs relate to throughput, detection efficiencies and analysis times. For example, in a conventional solution-based single molecule detection experiment, one can only detect approximately 10,000 molecules per minute, or one molecule every 6 milliseconds. While this may sound a lot, consider that a small drop of water (ca. 5 ml) contains approx. 1.67 x 10e23 molecules, that is 1.67 followed by 23 zeros. At that speed you need over 100 trillion years to detect all the water molecules in this single drop. Using a novel nanopore array developed by researchers in the UK, expect to be able to detect up to 1 million molecules simultaneously in the same 6 millisecond time window, representing an improvement in throughput of over six orders of magnitude (and bringing the timeframe for analyzing the molecules in a single water drop down to some 60 billion years - about five to six times the estimated age of the universe).
In the early 1870s, the German physicist Ernst Karl Abbé formulated a rigorous criterion for being able to resolve two objects in a light microscope. According to his equation, the best resolution achievable with visible light is about 200 nanometers. This theoretical resolution limit of conventional optical imaging methodology was the primary factor motivating the development of recent higher-resolution scanning probe techniques. The interaction of light with an object results in the generation of what is called 'near-field' and 'far-field' light components. The far-field light propagates through space in an unconfined manner and is the visible light utilized in conventional light microscopy. The near-field (or evanescent) light consists of a nonpropagating field that exists near the surface of an object at distances less than a single wavelength of light. So called near-field microscopy beats light's diffraction limit by moving the source very close to the subject to be imaged. When the first theoretical work on a new technique called "scanning near-field optical microscopy" (SNOM or NSOM) appeared in the 1980's, Abbé's classical diffraction limit was overcome, and resolution even down to single molecule level became feasible. However, light microscopy is still the only way to observe the interior of whole, or even living, cells. The use of fluorescent dyes makes it possible to selectively obtain images of individual cell components, for example, proteins. Today, the wavelength dogma has been overcome with the development of the stimulated emission depletion (STED) microscope. Now, the German team that developed STED is reporting layer-by-layer light microscopic nanoscale images of cells and without having to prepare thin sections with a technique called optical 3D far-field microscopy. They use a chemical marker for fluorescence nanoscopy that relies on single-molecule photoswitching.
"Children begin to learn by seeing, hearing, tasting and, above all, by touching. In a very similar approach, we are currently learning to orient ourselves in the nanoworld by 'feeling' materials - not with our fingers, but with microscopes that allow us to probe these materials with atomic resolution." (Robert W. Stark, LMU Munich in "Getting a feeling for the nanoworld"). Researchers' ability to engineer materials and achieve superior electronic, thermal, magnetic, and mechanical properties depends on tools that can identify and characterize material components and their spatial arrangement at the nanoscale. Equally important, understanding structure and function relationships in biological systems also demands tools that can probe structural properties with molecular resolution. Atomic force microscopes (AFM) are the most widely used tools to image matter at the nanoscale. Due to its mechanical operation, the AFM can in principle also perform nanomechanical measurements. This aspect of the AFM has been explored by researchers over the past two decades. However, current state-of-the-art techniques are very slow (it takes about one second for the AFM tip to approach, push into and retract from the surface of a material) and they apply rather large forces during the measurement process that damage the tip and the sample. Researchers at Harvard and Stanford universities have developed a specially designed AFM cantilever tip, the torsional harmonic cantilever (THC), which offers orders of magnitude improvements in temporal resolution, spatial resolution, indentation and mechanical loading compared to conventional tools. With high operating speed, increased force sensitivity and excellent lateral resolution, this tool facilitates practical mapping of nanomechanical properties.
You might remember our Spotlight from a few months ago (25 years of scanning probe microscopy and no standards yet) where we gave an overview of how scanning probe microscopy has flourished over the past 25 years. The most versatile implementation of the scanned probe principle is the atomic force microscope (AFM). It has become one of the foremost tools for imaging, measuring and manipulating matter at the nanoscale. The essential part of an AFM is a microscale cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law. A multi-segment photodiode measures the deflection via a laser beam, which is reflected on the cantilever surface. Because there are so many promising areas in nanotechnology and biophysics which can be examined by AFM (force spectroscopy on DNA, muscle protein titin, polymers or more complex structures like bacteria flagella, 3-D imaging, etc. ) the availability of instruments is crucial, especially for new groups and young scientists with limited funds. The price tag of AFMs runs in the hundreds of thousand s of dollars, though. Until now, AFM heads are made of metal materials by conventional milling, which restricts the design and increases the costs. German researchers have shown that rapid prototyping can be a quicker and less costly alternative to conventional manufacturing.
Cells are the smallest 'brick' in life's building structures. Every living organism is made of cells. Individual cells carry their own DNA and have their own life cycle. Considering that larger organisms, such as humans, are basically huge, organized cell cooperatives, the study of individual live cells is a hugely important scientific task. Among the most significant technical challenges for performing successful live-cell imaging experiments is to maintain the cells in a healthy state and functioning normally on the microscope stage while being illuminated. Especially if scientists want to look into cellular processes that occur in cells in their natural state and that cannot be observed by traditional cytological methods. It is well known that cells move, grow, duplicate, and move from point A to point B. Up to now people studied these mechanical properties with optical microscopes because it is the most common and simple method, very efficient, a very well developed and advanced technology. However, with optical microscopes detection is limited to objects no smaller than the wavelengths of the visible region of light, roughly between 400 and 700 nanometers. Distances or movement smaller than this range cannot be seen with these instruments. Researchers in Kyoto, Japan have applied a near-field optical approach to measure cell mechanics and were able to show intriguing data of nanoscale cell membrane dynamics associated with different phenomena of the cell's life, such as cell cycle and cell death.
It has been 25 years since the scanning tunneling microscope (STM) was invented, followed four years later by the atomic force microscope, and that's when nanoscience and nanotechnology really started to take off. Various forms of scanning probe microscopes based on these discoveries are essential for many areas of today's research. Scanning probe techniques have become the workhorse of nanoscience and nanotechnology research. Given the 25-year development timeframe, it is surprising that even today there is no generally accepted standard for scanning probe microscopy (SPM). There is no unified SPM terminology, nor is there a standard for data management and treatment, making access and processing of SPM data collected by different types of instruments an error-prone exercise. SPM standardization has only recently begun as part of an effort by the International Organization for Standardization (ISO) the largest developer of industrial standards. Meanwhile the development of SPM instruments and analysis software continues, increasing the already large family of scanning probe microscopy.