Differential Removal of Nanoparticles on the Surface of a Thin Film Substrate.

Purposeful identification, selection, and collection of particles are of great significance in environmental research. Microscopy is the common technique used in previous studies of particle identification. However, the microscopic technique was intricate and time-consuming. To conduct an intensive analysis of targeted particles, there is a need for the development of a simple method that can differentially abandon the nontargeted particles and only retain the targeted particles on the surface of a substrate. In the study, three methods were attempted for differential removal of nontargeted nanoparticles on the surface, including air jet, nanobubble, and ultrasonic methods. Acidic particles were taken as the targeted particles, while nonacidic particles were regarded as nontargeted particles. The results showed that regardless of methods, acidic particles were retained on the surface due to the strong particle-surface interaction. As for nonacidic particles, air jet treatment and nanobubble treatment were not able to completely remove nonacidic particles from the surface with the removal efficiencies of 5.1 ± 3.4 and 89.3 ± 4.1%, respectively, while the nonacidic particles were entirely removed in the ultrasonic treatment. Ethanol rather than deionized (DI) water was the proper solution in the ultrasonic treatment to avoid contamination. In conclusion, ultrasonic by ethanol was fully efficient for differential removal of nonacidic particles on the surface. The principle of differential removal of particles is the differences in the particle-surface interaction force between nonacidic particles (i.e., physically attached particles) and acidic particles (i.e., chemically formed particles). Nonacidic particles are removed from the surface through cavitation to form bubbles in the gap between a nonacidic particle and the surface in the ultrasonic treatment. In contrast, the space between an acidic particle and the surface is filled by the reaction, and thus bubbles cannot enter the crevice to remove the acidic particle. The developed method is useful for aerosol research.