Atomic force microscopy uses intermolecular interactions between a sharp tip and the sample to record images with incredibly high spatial resolutions. With modern atomic force microscopes, spatial resolutions of < 1 nm are now available, meaning it is possible to resolve single molecules and even the smallest atomic-scale surface defects in materials.1
Image Credit: Microgen/Shutterstock.com
An atomic force microscope can be used with different scanning modes, but these modes typically rely on the same general principle. Tips for atomic force microscopy are often on the order of 10 nm in radius and the ‘smoothness’ of the tip is critical for achieving good quality images.
The tip is mounted on a cantilever. As the tip is moved across the surface of the substrate, the interactions between the atoms in the tip and the surface either attract or repel the tip causing the cantilever to bend. By measuring the deflection of the cantilever, which is often done by laser interferometry, the surface of the substrate can be mapped and reconstructed as an image.
What is Atomic Force Microscopy Used For?
The very high spatial resolutions achievable with atomic force microscopy have meant atomic force microscopy has become a workhorse technique in materials science for development and characterization work. In nanoscale applications, where the device's properties depend on nanoscale features, atomic force microscopy is one of the few imaging techniques with sufficient spatial resolution.2
Atomic force microscopy has also been used for imaging soft matter, including measuring proteins, polymers and solution-solute interfaces.3 Many technical developments of atomic force microscopy for soft matter and measuring samples in liquid environments have also been beneficial for applying the method to food science.4
How is Atomic Force Microscopy Used to Measure Food Quality?
Determining food quality can be a challenging task. A full analysis of food quality needs to consider the chemical composition of the food and the presence of any pathogens, as well as the surface texture of the product.5 Atomic force microscopy can address many of these challenges owing to the excellent spatial resolution meaning species such as bacteria can be detected and local inhomogeneities in the food texture can be visualized.5
One example of an application for atomic force microscopy in food quality is studying droplet-droplet interactions in the presence or absence of particular proteins or polymers.5
The interactions between droplets determine many of the physical properties in emulsions and, ultimately, the product's final mouth feel and texture. Understanding how to manipulate these interactions is essential in food design and quality control to ensure the final product has the desired texture.
Advantages of AFM for Food Quality Analysis
The key advantage of atomic force microscopy over other microscopy methods is the enhanced spatial resolution, but for food quality analysis, it is also essential that samples can be measured in a variety of states.
With developments in atomic force microscopy, it is possible to measure food samples at atmospheric, liquid and even cryo-cooled conditions.
One advantage atomic force microscopy has over many optical microscopy methods is that, while some sample preparation is still required, atomic force microscopy is a label-free technique so the sample can be monitored in ‘native’ conditions.6
One major development for food quality analysis with atomic force microscopy is the development of in situ atomic force microscopy. Through miniaturization of the microscopes, it is now possible to create portable atomic force microscopes that can be used for live monitoring of processes outside of a laboratory environment.
In situ atomic force microscopy has been used to monitor dynamically evolving processes such as membrane-mediated protein-protein interactions as the atomic force microscopy images can be captured quickly enough to visualize the movement of the biomolecules involved.8
For food science and quality analysis, atomic force microscopy is still yet to become a routine analysis technique, but the ability to monitor how aggregation and clustering occur between biomolecules is of crucial importance for food quality control.
The Importance of Using Atomic Force Microscopy for Food Quality Evaluation
Two key areas in food science of using atomic force microscopy in food quality evaluation are processing and preservation techniques. Controlling protein and polymer structures in food is an essential part of the development process and conditions such as temperature and pH will be varied to achieve the optimum conditions.9
Atomic force microscopy can interrogate how processes such as heating alter the nanoscale structures of proteins to check there are no unwanted degradation or side reactions. This type of analysis is also essential for checking the nutritional content of food and which types of chemical species are present.
As well as individual imaging biomolecules, atomic force microscopy can be used to image the larger aggregates and nanostructures that form. While compounds such as gelatin feature a more complex array of structures, from spherical to rod and fiber-like aggregates, all of these can be imaged with atomic force microscopy, alongside retrieving information such as how the addition of preservatives such as salt and sugar changes the structures formed.7
Overall, atomic force microscopy images contain rich information on the nanostructures in food and can be used for a detailed analysis to establish food quality.
With the greater availability of commercial atomic force microscopes and smaller, less expensive instruments, atomic force microscopy is likely to become more commonplace in food quality analysis.
References and Further Reading
- Miller, E. J., Trewby, W., Payam, A. F., Piantanida, L., Cafolla, C., & Voïtchovsky, K. (2016). Sub-nanometer Resolution Imaging with Amplitude-modulation Atomic Force Microscopy in Liquid. 1–10. https://doi.org/10.3791/54924
- Klimov, V. (2014). Nanoplasmonics. CRC press. https://doi.org/10.1201/b15442
- Garcia, R., & Proksch, R. (2013). Nanomechanical mapping of soft matter by bimodal force microscopy. European Polymer Journal, 49(8), 1897–1906. https://doi.org/10.1016/j.eurpolymj.2013.03.037
- Yang, H., Wang, Y., Lai, S., An, H., Li, Y., & Chen, F. (2007). Application of Atomic Force Microscopy as a Nanotechnology Tool in Food Science. Journal of Food Science, 72(4), 65–75. https://doi.org/10.1111/j.1750-3841.2007.00346.x
- Gunning, A. P., & Morris, V. J. (2018). Getting the feel of food structure with atomic force microscopy. Food Hydrocolloids, 78, 62–76. https://doi.org/10.1016/j.foodhyd.2017.05.017
- Taylor, S. (Ed.). (2003). Advances in Food and Nutrition Research: Cumulative Index: Volumes 1-45 (Vol. 46). Gulf Professional Publishing. https://www.elsevier.com/books/advances-in-food-and-nutrition-research/taylor/978-0-12-385989-1
- Ding, M., Shi, C., & Zhong, J. (2019). Atomic force microscopy for food quality evaluation. In Evaluation Technologies for Food Quality. Elsevier Inc. https://doi.org/10.1016/B978-0-12-814217-2.00028-7
- Casuso, I., Sens, P., Rico, F., & Scheuring, S. (2010). Experimental Evidence for Membrane-Mediated Protein-Protein Interaction. Biophysj, 99(7), L47–L49. https://doi.org/10.1016/j.bpj.2010.07.028
- Nwachukwu, I. D., & Aluko, R. E. (2021). Food Protein Structures, Functionality and Product Development. The Royal Society of Chemistry. https:// doi:10.1039/9781839163425-00001