As we continue to push the boundaries of innovation in fields ranging from electronics to medicine, the importance of nanoparticles (NPs) cannot be overstated. These tiny particles, measuring just a few nanometers in size, hold the key to unlocking new properties and functionalities that can revolutionize industries. However, the properties of NPs are not just determined by their chemical composition, but also by their size, shape, and morphology.
The challenge lies in producing NPs with a defined and narrow particle size distribution (PSD), as even a small change in PSD can have a significant impact on their properties. For instance, the interaction of NPs with light is strongly size- and shape-dependent. To address this challenge, researchers have developed various techniques for producing NPs, including top-down and bottom-up synthesis methods. However, these techniques often produce size-distributed NPs, and a classification step is necessary to adjust or modify the obtained PSD according to the needs of the later product.
In recent years, several classification techniques have been developed, including deflector wheel classifiers, gas cyclones, centrifuges, and hydrocyclones. However, these techniques are limited to particle sizes above 1 μm and are not suitable for the classification of NPs. The classification of NPs is particularly challenging due to their small size and the need to balance the centrifugal force and drag force acting on the particles.
To address this challenge, researchers have turned to alternative techniques, including electrophoresis, mobility analysis, analytical ultracentrifugation (AUC), and size-selective precipitation (SSP). Electrophoresis, for instance, is based on the size- and shape-dependent mobility of charged particles in a stationary electrical field. AUC, on the other hand, is a powerful technique that enables the separation of NPs to analyze disperse properties with outstanding resolution.
However, these techniques are mainly restricted to the laboratory scale and cannot be used for industrial separation of NPs. To overcome this limitation, researchers have turned to chromatographic techniques, such as size-exclusion chromatography (SEC), which have been successfully used for the separation of macromolecules, including proteins and viruses.
SEC is based on the hydrodynamic volume of particles or molecules, where smaller species diffuse further into the porous structure of the stationary phase and thus elute the column later than the larger fraction. This technique has been successfully applied to the analytical separation of NPs, including SiO2 NPs, metal NPs, and semiconductor NPs, in a size range of 2 nm to 200 nm.
In a recent study, researchers demonstrated the feasibility of using SEC for the classification of AuNPs with different sizes and their mixtures. By using an additional fraction collector, they were able to collect size-selected fractions in separate vials, which allowed for further analysis of the obtained fractions with respect to mass fractions, PSDs, and size-dependent separation efficiencies.
The results of this study are promising, and they pave the way towards the development of advanced classification techniques that can be used for the preparative and continuous separation of NPs. By combining chromatographic methods with evaluation methods from particle technology, researchers can gain a deeper understanding of the complex size-dependent classification process and unlock the full potential of NPs.
As we move forward, it is essential that we continue to invest in research and development in this area, as the potential benefits of NPs are vast and varied. By unlocking their potential, we can create new materials and technologies that can transform industries and improve our daily lives.
