Fully integrated optofluidic SERS platform for real-time and continuous characterization of airborne microorganisms.

An optofluidic, surface-enhanced Raman spectroscopy (SERS) platform was developed to detect airborne microorganisms, continuously and in real time. The platform consists of an on-chip analysis system integrated with an aerosol sampler and Raman spectrometer. A stratified two-phase flow, consisting of the sampled air stream and a stream of collection medium, is formed in the curved channel. The inertia of collected particles, such as bacterial cells, carries them across the phase boundary in the curved channel such that they impact the liquid stream directly. The collection efficiency of the microchannel was evaluated using different-sized standard polystyrene-latex particles. A collection efficiency of 99.6% was attained for particles with an average aerodynamic diameter of 1 µm, a typical size for bacterial aerosols, by optimizing the flow rates of the sample air and liquid medium. A silver colloid in the collection medium was used as the SERS adsorbent. After passing through a serpentine mixing channel, bacterial particles were detected by SERS in real time using the custom Raman spectroscopy system. The detection system was evaluated with five test bacteria: S. epidermidis, M. luteus, E. hirae, B. subtilis, and E. coli. The concentration of airborne S. epidermidis corresponded to a Raman peak at 732.5 cm-1. The limit of detection was approximately 102 CFU/mL and the total bacterial aerosol concentration was determined in real time based on the ratio of sampling air to SERS colloid.

Development of an automated wet-cyclone system for rapid, continuous and enriched bioaerosol sampling and its application to real-time detection.

We present a novel bioaerosol sampling system based on a wet-cyclone for real-time and continuous monitoring of airborne microorganisms. The Automated and Real-time Bioaerosol Sampler based on Wet-cyclone (ARBSW) continuously collects bioaerosols in a liquid medium and delivers the samples to a sensing device using a wireless remote control system. Based on a high air-to-liquid-flow-rate ratio (∼ 1.4 × 105) and a stable liquid thin film within a wet-cyclone, the system achieved excellent sampling performance as indicated by the high concentration and viability of bioaerosols (> 95% collection efficiency for > 0.5-µm-diameter particles, > 95% biological collection efficiency for Staphylococcus epidermidis and Micrococcus luteus). Furthermore, the continuous and real-time sampling performance of the ARBSW system under test-bed conditions and during a field test demonstrated that the ARBSW is capable of continuously monitoring bioaerosols in real time with high sensitivity. Therefore, the ARBSW shows promise for continuous real-time monitoring of bioaerosols and will facilitate the management of bioaerosol-related health and environmental issues.

Highly Enriched, Controllable, Continuous Aerosol Sampling Using Inertial Microfluidics and Its Application to Real-Time Detection of Airborne Bacteria.

We report a novel microfluidic technique for sampling of aerosols into liquids. The two-phase fluid, sampling air and collecting liquid, forms a stratified flow in the curved microchannel. By passing fluids through the curved region, the particles are transferred from air into the liquid phase by the particle centrifugal and drag forces. This microfluidic-based aerosol-into-liquid sampling system, called the MicroSampler, is driven by particle inertial differences. To evaluate the physical particle collection efficiency of the MicroSampler, we used standard polystyrene-latex (PSL) particles ranging in size from 0.6 to 2.1 µm and measured particle concentrations upstream and downstream of the MicroSampler with an aerodynamic particle sizer. The cutoff diameter of particle collection was selected controlling the air flow velocity (microfluidic air flow of 0.6 L/min showed a particle collection efficiency of ∼98% at a particle diameter of 1 µm), and continuous enriched particle sampling was possible for real-time postprocessing application. With regard to biological collection efficiency, the MicroSampler showed superior microbial recovery (Staphylococcus epidermidis) compared to the conventional BioSampler technique. These results indicate that our MicroSampler can be used as a portable, cost-effective, simple, and continuous airborne microorganism collector for applications in real-time bioaerosol detection.

Novel electrostatic precipitator using unipolar soft X-ray charger for removing fine particles: Application to a dry de-NOX process.

The novel electrostatic precipitator (ESP), consisting of a soft X-ray charger and a collection part, was demonstrated and applied to a dry de-NOX process to evaluate its performance in by-product particle removal. NOX gas was oxidized by ozone (O3) and neutralized by ammonia (NH3) sequentially, and finally converted to an ammonium nitrate (NH4NO3) aerosol with ∼ 100-nm peak particle diameter. The unipolar soft X-ray charger was introduced for charging the by-product particles in this dry de-NOX process. For the highest particle collection efficiency, the optimal operating conditions of the soft X-ray charger and collection part were investigated by adjusting the applied voltage of each device. The results showed that ∼ 99% of NOX was removed when the O3/NOX ratio was increased to 2 (i.e., the maximum production conditions of the NH4NO3 by-product particles by the gas-to-particle conversion process). The highest removal efficiency of particle (∼ 90%) was observed with operating conditions of positive polarity and an applied voltage of ∼ 2-3 kV in the charger chamber. The unipolar soft X-ray charger has potential for particle removal systems in industrial settings because of its compact size, ease of operation, and non-interruptive charging mechanism.

Integrated micro-optofluidic platform for real-time detection of airborne microorganisms.

We demonstrate an integrated micro-optofluidic platform for real-time, continuous detection and quantification of airborne microorganisms. Measurements of the fluorescence and light scattering from single particles in a microfluidic channel are used to determine the total particle number concentration and the microorganism number concentration in real-time. The system performance is examined by evaluating standard particle measurements with various sample flow rates and the ratios of fluorescent to non-fluorescent particles. To apply this method to real-time detection of airborne microorganisms, airborne Escherichia coli, Bacillus subtilis, and Staphylococcus epidermidis cells were introduced into the micro-optofluidic platform via bioaerosol generation, and a liquid-type particle collection setup was used. We demonstrate successful discrimination of SYTO82-dyed fluorescent bacterial cells from other residue particles in a continuous and real-time manner. In comparison with traditional microscopy cell counting and colony culture methods, this micro-optofluidic platform is not only more accurate in terms of the detection efficiency for airborne microorganisms but it also provides additional information on the total particle number concentration.

Herbal Extract Incorporated Nanofiber Fabricated by an Electrospinning Technique and its Application to Antimicrobial Air Filtration.

Recently, with the increased attention to indoor air quality, antimicrobial air filtration techniques have been studied widely to inactivate hazardous airborne microorganisms effectively. In this study, we demonstrate herbal extract incorporated (HEI) nanofibers synthesized by an electrospinning technique and their application to antimicrobial air filtration. As an antimicrobial herbal material, an ethanolic extract of Sophora flavescens, which exhibits great antibacterial activity against pathogens, was mixed with the polymer solution for the electrospinning process. We measured various characteristics of the synthesized HEI nanofibers, such as fiber morphology, fiber size distribution, and thermal stability. For application of the electrospun HEI nanofibers, we made highly effective air filters with 99.99% filtration efficiency and 99.98% antimicrobial activity against Staphylococcus epidermidis. The pressure drop across the HEI nanofiber air filter was 4.75 mmH2O at a face air velocity of 1.79 cm/s. These results will facilitate the implementation of electrospun HEI nanofiber techniques to control air quality and protect against hazardous airborne microorganisms.