10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Deposition Characteristics of Bioaerosols: Towards Black Silicon-Based MEMS Bioaerosol Detection
UGUR SOYSAL, Evelyne Géhin, Frédéric Marty, Emmanuelle Algré, Charles Motzkus, Université Paris-Est, CERTES
Abstract Number: 125 Working Group: Low-Cost and Portable Sensors
Abstract As excessive exposure to bioaerosols results in adverse health effects, the development of real-time bioaerosol detection is highly relevant. Widely used systems are bulky, time-consuming or expensive to maintain. Over the past decade, sensor developments have shown the following features: portable, inexpensive, and suitable for monitoring real-time measurements. With respect to the semiconductor industry, airborne particle monitoring systems can be easily accessed through the development of silicon-based nano/micro scale systems and can be achieved by combining an appropriate sampling method with miniature sensors. Nano and micro electromechanical (NEMS/MEMS) sensors could be promising tools for bioaerosol detection, and provide real-time mass concentration measurements, particularly in terms of sensitivity. However, reducing the size of the fundamental aspects of aerosol sampling methods reveals strengths and limitations in the systems. Unlike typically used sampling methods in the case of traditional particle detectors, the collection efficiency cannot be the only concern to characterize the utility of nano/micro scale detection systems. Due to the arbitrarily shaped and sized NEMS/MEMS sensors, the deposition characteristics such as deposition shape and size should be investigated and adapted to the developed sensors in order to obtain a high sampling efficiency.
In this work, we have designed and fabricated a multiple round nozzle single-stage bio-impactor based on classical impactor design theory. While the design value of cut-off diameter is 0.4 μm, the operating flow rate and Reynolds number are 10 LPM and 2825, respectively. To study the deposition characteristics of bioaerosols, we have generated monodispersed particles with various sizes (aerodynamic median diameter: 0.91 μm ± 0.01 μm, 2.07 μm ± 0.01 μm, 2.5 μm ± 0.03 μm and 3.8 μm ± 0.004 μm) of fluorescence particles as test particles, Aspergillus niger spores, Staphylococcus epidermidis, and Pseudomonas fluorescens with aerodynamic median diameters of 3.00 μm ± 0.01 μm, 0.73 μm ± 0.01, and 0.66 μm ± 0.007 μm, respectively. These particles have been impacted on silicon and nanostructured silicon (i.e. black silicon, pillars a few nm dia and ~ 3.00 μm length) surfaces as an impaction plate of the designed impactor. Although the properties of the biological and non-biological test particles used in this study are different, their deposition characteristics show similarities and the deposition patterns correlated with the size of each type of particles.
Preliminary results show that the halo-shaped, ring-shaped or only primary deposit (underneath the nozzle) have been observed for silicon and black silicon impaction plates. A halo type pattern consists of a primary deposit in the center that is surrounded by a secondary deposit. A ring-shaped pattern forms a particle-free area underneath the nozzles, thus the deposit encloses this area. There have been only a few study discussing the reason for the deposit shapes and those are contradictory. Herein, we show that the halo and ring-shaped deposits occur due to rebound or re-entrainment of the particles on a flat silicon surface. As the particle diameter is increased from sub-μm to micron size, the deposits have changed from the halo to ring shape. All the sub-μm size particles have demonstrated a similar halo type pattern in turn an enlarged deposition area. Although black silicon has not significantly affected the deposition characteristics of sub-μm particles, it breaks all the micron size particles into fragments and provides a localized circular deposition pattern (dia < 500μm) underneath the nozzle which is smaller than the nozzle diameter (500 μm). The sharp nano pillars have enabled to reduce bouncing effect, thus have provided a confined deposition pattern, which can be easily aligned with the micro sensor. Though porous substrates are often avoided in impactors due to the penetration of the impinging jet that causes excess particle collection, which leads non-ideal collection efficiency curves, it can be negligible for large cut-off diameters as in this work. Therefore, we propose that black silicon implemented MEMS mass sensors are favorable for the detection of micron size particles like Aspergillus niger spores.