Atomic force microscopy (AFM) is an analytical tool used to investigate the character of molecules at nanoscale dimensions. Nanoemulsions are colloidal dispersions containing nanometer-scaled droplets, applied across varied disciplines of science. Recently, AFM has been used to characterize the droplet interfaces of nanoemulsions.
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The size of the dispersed droplets in the emulsions determines the physical properties and thermodynamic stability of nanoemulsions.
Scientists have stated that for designing a personalized nanoemulsion-based delivery system of bioactive compounds, it is extremely important to understand the droplet topography as well as surface interaction forces between droplets or between a droplet and the emulsifier.
Importance of Nanoemulsions
Nanoemulsions are an isotropic dispersed system that consists of two immiscible liquids, e.g., oil and water.
The droplet size of nanoemulsions varies greatly. These droplets have high stability as well as a high surface-to-volume ratio.
Scientists use nanoemulsion techniques to generate uniform nanoparticles, possessing unique properties, which have a wide range of applications, especially in the food and medicine sectors.
Nanoemulsions have been associated with the delivery of many pharmaceutical compounds, especially lipophilic ones.
Nanoemulsion of these compounds not only helps disperse them in an aqueous medium but also protects them against degradation before reaching the target site. Additionally, this technique has been used to design an improved delivery system for the controlled release of bioactive compounds.
Several scientists have studied the nanoemulsions of functional compounds, such as essential oils (e.g., eugenol, fish oil, thymol, lemon oil, etc), β-carotene, curcumin, etc., using food-grade emulsifiers.
Food technologists employ nanoemulsions to encapsulate effective active ingredients to develop functional food products and modify food structure. Additionally, nanoemulsions developed using bioactive compounds have been found to elevate the quality and shelf life of many food products.
To prepare nanoemulsions, emulsifiers are added, which are surface-active, amphiphilic molecules.
Some examples of food-based emulsifiers are maltodextrin, whey protein isolate, κ-carrageenan, sodium caseinate, and lecithin.
The interaction and rearrangements of molecules between the particles in the dispersed phase and the emulsifiers are the main contributing factor for the formation, stability, and rheology of the newly developed nanoemulsions.
Analysis of Nanoemulsions Using Atomic Force Microscopy
AFM has been widely used to examine the interfacial properties of biomolecules from micron to nanoscale dimension. This reveals the behavior of molecules at the droplet interfaces of nanoemulsions.
This technique utilizes nanostructure probes to scan material surfaces at subnanometric resolution. AFM is also used to obtain the chemical profile and physicochemical properties of dispersed and continuous phases of particles and emulsifiers.
AFM offers data about the surface interaction forces at the nanometer scale and also provides three-dimensional images of the nanostructured surface morphology under dry, wet, and vacuum conditions.
The resolution and quality of the images are determined by the configuration of the AFM tips. When scanning the surface of a sample, a photodiode detector detects any deflections in the beam introduced to the sample.
A computer system receives these electrical signals from the detector to generate feedback signals.
Depending on the experiment, the probe is maintained either at a constant force or height above the sample. This helps obtain precise topographic images and determine the interaction forces between the sample's surface and the atoms of the tips.
To obtain an accurate image of the droplet topography, nanoemulsion particles must be strongly bound in their native and intact state to a solid and smooth surface. Some of the common surfaces on which the droplets are fixed are glass, mica, and silicon oxide.
These surfaces help to withstand the lateral force generated by the AFM scanning tip. Another important factor that helps the production of accurate images by AFM is that the droplets must be dispersed well on the solid surface.
Nanoemulsions are diluted 100 to 1000 times using distilled water to avoid agglomeration and coalescence of the droplets.
This diluted sample is fixed on a solid surface via electrostatic attraction or chemical bonding.
The rate of dispersion is based on the dilution ratio of the nanoemulsions, electrostatic energy associated with the droplets, the interfacial free energy, the hydrophilic and hydrophobic forces interacting between droplets, and the surfactants and additives present in the nanoemulsions.
Advantages of Atomic Force Microscopy in Characterizing Nanoemulsions
Although scanning electron microscopy (SEM) and transmission electron microscopy have been used to study the surface topography of the droplets, they have certain limitations. These techniques are elaborate as sample preparation requires multiple steps.
Typically, droplet interfacial interactions are measured using surface force apparatus; however, this instrument cannot be used to determine nanoemulsion samples as it can only measure the interactions between flat surfaces.
One of the main advantages of the AFM technique for the analysis of nanoemulsions over conventional techniques is that it can analyze droplet interfaces under conditions that are similar to real nanoemulsion systems.
Another advantage is that this analytical tool can be operated easily, and the configuration of AFM equipment is much simpler than SEM. Additionally, AFM can accurately determine smaller droplet sizes of seaweed oil nanoemulsion with Tween 80, which was not possible by the dynamic light scattering method.
Another interesting contribution of AFM is its ability to differentiate between nanoemulsion properties prepared from different surfactant types, such as nanoemulsions stabilized by small- and large-molecule emulsifiers.
Continue reading: Synthesizing Uniform Nanoparticles with Nanoemulsion Strategies.
References and Further Reading
Ho, M.T. et al. (2021) An overview of nanoemulsion characterization via atomic force microscopy. Critical Reviews in Food Science and Nutrition, Available at: https://www.tandfonline.com/doi/full/10.1080/10408398.2021.1879727
Gurpreet, K and Singh, S.K. (2018) Review of Nanoemulsion Formulation and Characterization Techniques. Indian Journal of Pharmaceutical Sciences. Available at: https://www.ijpsonline.com/articles/review-of-nanoemulsion-formulation-and-characterization-techniques-3530.html
Preetz, C. et al. (2010) Application of atomic force microscopy and ultrasonic resonator technology on nanoscale: Distinction of nanoemulsions from nanocapsules. European Journal of Pharmaceutical Sciences. 39(1-3). pp. 141-151. Available at: https://doi.org/10.1016/j.ejps.2009.11.00