MEMS-based condensation particle growth chip for optically measuring the airborne nanoparticle concentration
To monitor airborne nanoparticles at a particular point of interest sensitively and accurately, we developed a compact and inexpensive but highly-precise nanoparticle detection system. The proposed system, based on nucleation light-scattering, consists of two components: a microelectromechanical system (MEMS)-based particle growth chip that grows nanoparticles to micro-sized droplets through condensation and a miniaturized optical particle counter (mini-OPC) that detects individual grown droplets using a light-scattering method. To minimize the dimensions and cost of this system, all elements of the particle growth chip were integrated onto a glass slide through simple photolithography and 3D printing. Moreover, a passive cooling technique was adopted, which eliminated the need for an active cooling system. Thus, our system was much more compact, inexpensive, and power-efficient than conventional nanoparticle detection instruments. Through quantitative experiments using Ag nanoparticles in the size range of 5 to 70 nm, it was found that our system could count extremely small nanoparticles (12.4 nm) by growing them to micrometer-sized droplets. Furthermore, our system could provide an accurate number concentration of nanoparticles (the maximum difference was within 15% compared to the reference instrument), regardless of high (3500 N cm−3) and low (0.05 N cm−3) concentration environments. These results indicate that our system can be applied successfully to the monitoring of nanoparticles in various kinds of fields including not only indoor and outdoor environments but also high-tech industries utilizing cleanrooms, air filtration systems, etc.