1. WHAT IS ES-DMA?
ES-DMA is an analytical technique that uses an electrospray to aerosolize nanoparticles before characterizing their electrical mobility by DMA at ambient conditions. Quantitative characterization of bio-macromolecules and nanoparticles from 10 to hundreds nm can be achieved by ES-DMA.
Figure 1: Schematic of ES-DME system. (D-H. Tsai, R. A. Zangmeister, L. F. Pease III, M. J. Tarlov, and M. R. Zachariah Langmuir 2008 24 (16), 8483-8490)
As shown in FIGURE 1, ES-DMA consists of an ES generator, a differential mobility analyzer column, and a condensation particle counter. The ES generator aerosolizes nanoparticles through a nozzle
First of all, the nanoparticle dispersion is added to a volatile buffer and placed inside a pressure chamber. Second, the solution is delivered to the nozzle through a capillary to aerosolize multiply charged droplets. Then, the droplets are mixed with air and carbon dioxide before passing through the neutralizer. After that, the solvent is evaporated and there is a decrease in the droplets charge distribution. As a result, more and more droplets contain single net charge nanoparticles which continue to pass through the DMA. DMA separates positively or negatively charged particles by applying a negative or positive potential. Inside a DMA is a complicated process, but I will try to explain it shortly. DMA will exist in an electric field to collect the charged particles based on their electrical mobility, the fluid flow rate, and the DMA geometry. Therefore, the electrical mobility of particles will be identified by DMA. By applying Stoke's law, the particle diameter can be calculated. To identify the number concentration of the size-selected particles, the CPC is used.
Figure 2: Condensation Particle Counter (CPC) Working Principle. (Suvajyoti Guha, Mingdong Li, Michael J. Tarlovand Michael R. Zachariah -Trends in Biotechnology May 2012, Vol. 30, No. 5)
2. Which information can be provided through ES-DMA?
- Mobility Diameter and Number concentration per unit volume (as shown in Figure 3).
- Kinetic Stability of polymeric conjugated-particles in different pH can be also observed through Mobility Diameter Distribution Change (as shown in Figure 3).
Figure 3: BSA-AgNPs. (a) Particle size distributions of BSA-AgNPs at pH 6.7 measured by ES-DMA. (b) Particle size distributions of BSA-AgNPs at pH 2.7 measured by ES-DMA. (c) Particle size distributions of BSA-AgNPs at pH 2.3 measured by ES-DMA. (d) TEM analysis of monomers and finite-sized clusters of BSA-AgNPs. Samples were at pH 2.5. CBSA= 2 μmol/L. The scale bars were 20 nm
- Aerosol particle mass analyzer (APM) coupled with ES-DMA can provide Mobility Diameter + Number concentration + Particle Mass.
Figure 4: Cartoon depiction of electrostatic-directed assembly of GO with surface functionalized AgNPs (Step 1), characterization of GO-AgNP aqueous colloid using a stand-alone ES-DMA (Step 2), and a coupled instrument, ES-DMA/APM (Thai Phuong Nguyen, Wei-Chang Chang, Yen-Chih Lai, Ta-Chih Hsiao & De-Hao Tsai - Analytical and Bioanalytical Chemistry, 409,5933–5941, 2017)
