Abstract
To monitor airborne nano-sized particles (NPs), a single-chip differential mobility particle sizer (DMPS) based on resonant micro cantilevers in defined micro-fluidic channels (µFCs) is introduced. A size bin of the positive-charged fraction of particles herein is separated from the air stream by aligning their trajectories onto the cantilever under the action of a perpendicular electrostatic field of variable strength. We use previously described µFCs and piezoresistive micro cantilevers (PMCs) of 16 ng mass fabricated using micro electro mechanical system (MEMS) technology, which offer a limit of detection of captured particle mass of 0.26 pg and a minimum detectable particulate mass concentration in air of 0.75 µg/m3. Mobility sizing in 4 bins of a nebulized carbon aerosol NPs is demonstrated based on finite element modelling (FEM) combined with a-priori knowledge of particle charge state. Good agreement of better than 14% of mass concentration is observed in a chamber test for the novel MEMS-DMPS vs. a simultaneously operated standard fast mobility particle sizer (FMPS) as reference instrument. Refreshing of polluted cantilevers is feasible without de-mounting the sensor chip from its package by multiply purging them alternately in acetone steam and clean air.
Highlights
Air quality is a most important factor of a healthy life with high current relevance.For example, long-term exposure to a polluted ambient can be a similar hazard to life expectancy as inhaling tobacco smoke and about 15% of the worldwide deaths from the current coronavirus disease 2019 (COVID-19) pandemic might be related to a longlasting exposure to air polluted by anthropogenic sources (e.g., particulate matter (PM)from combustion sources) [1,2]
Based on mobility separation of charged particles flowing in the μFC, we describe a procedure for analyzing the size distribution of polydisperse aerosols at different sampling voltages, i.e., the micro electro mechanical system (MEMS) device is operated as a differential mobility particle sizer (MEMS-DMPS)
Carbon nano-sized particles (NPs) captured during separation/sampling are clearly visible in the scanning electron microscopy (SEM) images of the piezoresistive micro cantilevers (PMCs) in Figure 5 taken at the end of the measurement run, whereas they do not appear on the reference structure
Summary
Air quality is a most important factor of a healthy life with high current relevance. Particle size distributions have not been measured so far using such M/NEMS resonant sensors, which would be necessary for a more comprehensive evaluation of air quality. Owing to their sensitive dynamic response, micro cantilevers have attracted considerable interest for high-resolution mass sensing [11]. Based on mobility separation of charged particles flowing in the μFC, we describe a procedure for analyzing the size distribution of polydisperse aerosols at different sampling voltages, i.e., the MEMS device is operated as a differential mobility particle sizer (MEMS-DMPS) For validating this concept we use carbon engineered NPs, which can be considered as a model aerosol for environmental.
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