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

Graphical abstract

Highlights

• •Airborne microorganisms have adverse effects on human health, livestock, crops, and the environment.
• •We developed a novel bioaerosol sampling system based on a continuous wet cyclone (ARBSW).
• •The ARBSW system is capable of highly enriched sampling of bioaerosols in a short time with superior microbial recovery.
• •The ARBSW system enables continuous real-time bioaerosol monitoring in the field.

Abstract

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.

1. Introduction

Concerns over airborne microorganisms, called bioaerosols, have increased due to their adverse effects on the human body and the environment [[1], [2], [3], [4]]. Bioaerosols, such as pathogenic viruses, bacteria, and fungal spores, are associated with infectious diseases, allergies, and asthma [2,[5], [6], [7]]. For example, the Middle East respiratory syndrome (MERS) and severe acute respiratory syndrome (SARS) coronaviruses, which have recently killed hundreds of people in East Asia, are transmitted by direct inhalation of contaminated bioaerosols [8]. Therefore, the World Health Organization (WHO) recommends that the bioaerosol concentration within indoor air be less than 500 CFU/m³ _air [9,10].

A technique enabling rapid real-time detection of airborne microorganisms would be advantageous for public health research and bioterrorism defense; however, the development of such a system is at an early stage. A major reason for the slow pace of development is the difficulty of determining the concentration and type of bioaerosols in the air stream [[11], [12], [13], [14], [15], [16]]. Bioaerosols are typically detected by first collecting them in a liquid or on a surface and analyzing the collected particles using culture-based techniques, biochemical assays (e.g., polymerase chain reaction (PCR)) and/or enzyme-linked immunosorbent assay (ELISA) [14,[17], [18], [19], [20]]. Recently, the development of real-time detection systems for microorganisms in liquid medium has made considerable progress [14,[21], [22], [23]]; however, the performance of bioaerosol sampling systems with regard to biological stability, particle collection efficiency, and particle enrichment is still insufficient for use as a real-time system. In addition, integration of the bioaerosol sampling system and the detection system, including rapid and stable sample transfer, would be desirable for continuous real-time monitoring of bioaerosols.

The ideal bioaerosol sampler should have the following characteristics: (i) rapid and continuous sampling, (ii) high collection efficiency and stable microbial recovery (e.g., liquid or semi-liquid collection medium), (iii) high particle enrichment, (iv) integration with the detection system (e.g., continuous and consistent sample delivery) and (v) portability and automated operation. Tan et al. developed an automated electrostatic sampler that involves collection of bioaerosols in a liquid reservoir and their delivery to sensing devices [24]. Liu et al. and other researchers described an airborne pathogen direct analysis system based on microfluidic enrichment [[11], [12], [13]]; however, these systems require prolonged sampling for enrichment. A novel bioaerosol sampling system, the MicroSampler, based on two-phase fluid control in a microchip can be used with a real-time bioaerosol sensor [25]; however, due to the low throughput, adequate sampling is difficult in the presence of bioaerosol concentrations < 500 CFU/m³ _air.

A wet-cyclone collects aerosols into a liquid film on the inner wall of the cyclone using the particle centrifugal force and the liquid surface tension. Such systems achieve high sampling performance and enable the concentration of samples due to the high flow rate ratio between the incoming air and drainage liquid. Considerable research has focused on the development of wet-cyclone bioaerosol samplers [[26], [27], [28], [29], [30]]; however, these have a large particle cut-off diameter, and a low particle collection efficiency and aerosol-to-liquid transfer rate; furthermore, the two-phase flow operation is unstable and few fully integrated bioaerosol sampling systems are capable of continuous real-time sampling.

Here, the Automated and Real-time Bioaerosol Sampler based on Wet-cyclone (ARBSW) system for continuous real-time monitoring of bioaerosols is presented. The ARBSW system continuously collects airborne microorganisms and automatically delivers them to an analytical sensor. The aerosol collection efficiency, particle air-to-liquid transfer efficiency, sample enrichment, and microbial recovery of the ARBSW are evaluated. In addition, the ARBSW was subjected to sampling sensitivity tests of bioaerosol detection under test-bed conditions, as well as real-time bioaerosol monitoring in a real atmosphere environment. The results demonstrate that the ARBSW facilitates continuous bioaerosol monitoring with high sensitivity in real-world environments.

2. Materials and methods

2.1. Test particles and microorganisms

Standard-size particles and bacteria were used as test aerosols to evaluate the performance of the ARBSW system. Monodisperse and spherical standard polystyrene-latex (PSL) particles (0.1, 0.3, 0,5, 0.6, 0.7, 0.8, 0.9, 1.0 and 2.0 μm in diameter; 1.06 g/cm³ density; Duke Scientific Corp., Palo Alto, CA, USA) and red fluorescent PSL (FPSL) particles (0.3, 0.48, 0.8, 1.0 and 2.1 μm in diameter; 1.05 g/cm³ density; Fluoro-Max™, Thermo Scientific, Waltham, MA, USA) were used to evaluate the aerosol collection efficiency and air-to-liquid particle transfer efficiency of the ARBSW. Staphylococcus epidermidis (ATCC 12228) and Micrococcus luteus (ATCC 9341) were used as the test airborne microorganisms. These Gram-positive bacteria are commonly found in indoor environments and on human skin [31,32], and are used widely in bioaerosol research [21,[33], [34], [35]]. In particular, S. epidermidis is an important opportunistic pathogen and is the most common source of infections on indwelling medical devices [36]. The bacteria were incubated in nutrient broth (Becton Dickinson, Franklin Lakes, NJ, USA) at 37 °C for 24 h. Upon reaching an optical density at 600 nm of 0.6, bacterial suspensions were harvested by centrifugation (5000 × g, 10 min), washed three times with sterilized deionized water (SDW) and diluted with 20 mL of SDW. Subsequently, a 30 mL aliquot (∼ 10⁸ colony forming units (CFU)/mL) was poured into the nebulizer.

2.2. Test aerosol generation

Figure S1(a) shows a schematic diagram of the test aerosol generation. Compressed clean air was passed into a six-jet Collison nebulizer (BGI Inc., Waltham, MA, USA) via a mass flow controller (MFC, FC-280S; Mykrolis Corp., Billerica, MA, USA) at a flow rate of 5 L/min. To remove moisture and electrical charge from the aerosols, the nebulized particles were sequentially passed through a diffusion dryer and ²¹⁰Po neutralizer. The test aerosol flow was diluted with an additional clean air flow (7–11 L/min) in a mixing chamber and inserted into the ARBSW system.

2.3. Real-time aerosol measurement

As shown in Figure S1(b), the size distribution and number concentration of the test aerosols before and after passing through the ARBSW system were measured in real-time using a wide-range particle spectrometer (WPS) (1000XP; MSP Corp., Shoreview, MN, USA; particle size range 10 nm to 10 μm) and an aerodynamic particle sizer (APS) (3314; TSI Inc., St. Paul, MN, USA; particle size range 0.5–20 μm), respectively.

2.4. The wet-cyclone module operation

Figure S1(c) shows the schematic diagram of the wet-cyclone module operation. There is one aerosol inlet and three sampling liquid inlets in the side of the wet-cyclone, and the outlets for the exhausted air and hydrosol liquid (including particle sample) are in the upper and bottom side. During operation, the liquid sampling medium (e.g., SDW) was injected at 5–13 mL/h through the three ports of the wet-cyclone using a syringe pump (KD200; KD Scientific Inc., Holliston, MA, USA). The liquid drainage flow rate was controlled at 2–13 mL/h using a peristaltic pump (T60-WX10; Longer Corp., Hebei, China). The liquid sampling medium can be transferred continuously and directly to the particle detection part or storage container array for later processing.

2.5. Particle characterization

Figure S1(d) shows a schematic diagram of the particle characterization after sample collection. Airborne bacterial particles were deposited onto copper transmission electron microscopy (TEM) grids (carbon film on a copper mesh; CF300-Cu; Electron Microscopy Sciences, Hatfield, PA, USA) using an electrostatic precipitating nanoparticle collector (model 4650; HCT Inc., Icheon, Republic of Korea). A field emission-scanning electron microscope (FE-SEM; Teneo VolumeScope, FEI, Hillsboro, OR, USA) was used to visualize the structure and morphology of surface-deposited airborne bacterial particles.

To calculate the FPSL particle concentration in the drainage liquid medium, aliquots (∼ 10 μL) from the wet-cyclone outlet were injected into a disposable hemocytometer (DHCN01; INCYTO, Cheonan, Republic of Korea). Next, the FPSL particles were counted using a fluorescence microscope (BX51; Olympus, Tokyo, Japan) with a U-MWG2 filter set (excitation, 510–550 nm; emission, > 590 nm). Images of at least nine microscopic fields were captured using a charge-coupled device (CCD) array camera. Particles were enumerated using ImageJ software (National Institutes of Health, Bethesda, MD, USA).

For the colony counting assay, the collected bacterial suspensions were serially diluted in SDW. Aliquots of 100 μL of the suspensions were spread on the surface of nutrient agar (Becton Dickinson) in Petri dishes. Colonies were counted after incubating the Petri dishes for 24 h at 37 °C.

2.6. Characteristics and performance evaluation of the ARBSW system

The prototype ARBSW system automatically controls the air sampling, liquid supply, liquid drainage flow rate and sample delivery using software based on Arduino and MATLAB. The system can be remotely controlled using the wireless control panel and is 25 × 15 × 20 cm in size. Aerosols are entered through the inlet of the ARBSW by an air pump and continuously collected in the sampling liquid flow. Two peristaltic pumps enable a continuous supply of liquid to the ARBSW system and a transfer of liquid to the analytical detection part at the appropriate flow rate.

For evaluating the bioaerosol collection efficiency of the ARBSW, the number concentrations of the test bioaerosols before and after transmission through the ARBSW system were measured using an APS. Simultaneously, the drainage liquid was sampled (∼ 100 μL) and the colony number was determined using the colony counting assay. Finally, the culturable bioaerosol number concentration (CFU/mL) was determined according to the total liquid volume.

The BioSampler (SKC Inc., Eighty Four, PA, USA) was used for performance evaluation in terms of the relative microbial recovery (%) and sample enrichment ratio of bioaerosol. The BioSampler was formed of glass and consisted of three parts: an inlet, a nozzle section with three tangential sonic nozzles and a collection vessel. The collection vessel was filled with a liquid collection medium (20 mL). The nozzles of the BioSampler create a swirling air flow (12.5 L/min_air of air supply) that maintains microorganism viability by gently moving particles onto the collection surface without re-aerosolization [25,[37], [38], [39]]. Under the same bioaerosol exposure condition (∼ 100 particles/cm³ _air of total aerosol number concentration (TANC)), we sampled the bioaerosols using the ARBSW and BioSampler and obtained the culturable bioaerosol number concentration, respectively. Finally, we compared the relative microbial recovery (%) and the sample enrichment ratio between the ARBSW and BioSampler under various sampling times.

2.7. Experimental setup for sensitivity test and real-world field test

Figure S2 shows a schematic diagram of the sensitivity test setup to evaluate the real-time sampling responsivity of the ARBSW under the abrupt bioaerosol exposure conditions. The S. epidermidis suspension was nebulized for 40 s at 10-min intervals in a biosafety cabinet. While the ARBSW system samples surrounding bioaerosols in a real-time manner, the APS monitors the change of TANC at 1-min intervals. The real-time colony concentration of sampled particles using ARBSW was determined using the colony counting assays and compared with the real-time data of APS.

Figure S3 shows a schematic diagram of the real-world field test setup. The temperature, relative humidity and wind speed were monitored using a hot-wire anemometer with thermohygrometer (TES‐1341; TES Electrical Electronic Corp., Taipei, Taiwan). The atmospheric particulate matter (PM) concentration and TANC were monitored using an optical particle counter (OPC) (model 1.109; GRIMM Aerosol Technik Ainring GmbH & Co. KG, Ainring, Germany). The ARBSW also sampled the atmospheric bioaerosols continuously and the liquid samples were stored every 5 min. These samples were tested using colony counting assays to obtain the bioaerosol colony concentration.

3. Results and discussion

3.1. Principle and design of the wet-cyclone module for the ARBSW

Fig. 1 (a) shows the aerosol collection principle of the wet-cyclone module for the ARBSW system. The cyclone is a conical device that creates an internal helical air stream and collects aerosols on the inner wall through centrifugal force. In our wet-cyclone module, a stable liquid thin film is formed on the inner wall of the cyclone by centrifugal force and the shear force generated by the high air-flow rate. At the same time, the liquid film slowly and continuously flows down the cyclone according to the balance between the supply and drainage liquid flow for delivering samples. Due to the large difference between the air and liquid flow rates, aerosols entering the wet-cyclone are completely collected in the liquid film, rapidly concentrated to a high enrichment ratio and continuously delivered to an analytical sensor.

Fig. 1.

Continuous liquid-based aerosol sampling using the wet-cyclone. (a) Schematic diagram of the operating principle of the wet-cyclone. The aerosol is collected into a thin liquid film on the inner wall of the wet-cyclone using the particle centrifugal force and the liquid surface tension. The stable thin liquid film allows the wet-cyclone to achieve a superior particle collection efficiency and high enrichment particle sampling. (b-1) Front- and (b-2) top-view photographs of the wet-cyclone module. One aerosol inlet and three sampling liquid inlets are located on the side of the wet-cyclone; the exhaust air and hydrosol liquid (including particles) outlets are located on the top and bottom, respectively.

Our bioaerosol sampling system has a cut-off diameter of 0.3 μm and collection efficiency of > 99% for aerosols > 1 μm in diameter with high-throughput operation. The cut-off diameter (d₅₀), defined as the diameter of the aerosol having a collection efficiency of 50%, is expressed as follows [40]:

$$d_{50}=\sqrt{\frac{9\mu \bullet b}{2\pi {\bullet \rho }_p{\bullet U}_{\text{a}}{\bullet C}_c{\bullet N}_t}}$$ (1)

where μ is the air dynamic viscosity, b is the width of the air inlet, ρ_p is the particle density, U_a is the air velocity at the inlet, C_c is the Cunningham slip correction factor calculated using C_c = 1 + 0.5Kn_p [2.34 + 1.05 exp(-0.195Kn_p)], Kn_p is the Knudsen number of the particle and N_t is the number of turns of the air stream in the cyclone.

As shown in Eq. (1), the characteristics of the air flow inside the cyclone play a decisive role in the cut-off diameter. The behavior of the air stream directly depends on the geometry of the cyclone; for this reason, we analyzed the air flow and aerosol behavior in the wet-cyclone to optimize the cyclone design parameters using commercial computational fluid dynamics software (Fluent 16.0; ANSYS Inc., Canonsburg, PA, USA). The finite volume method was employed to solve the governing equations and the discrete phase model in Fluent code was used for particle tracking. Figure S4 shows the optimal cyclone geometry and dimensions and Figure S5(a) shows the trajectories of aerosols entering the cyclone at the optimal inlet air flow rate of ∼ 16 L/min. The particle collection position gradually lowers with decreasing particle size; all particles ≥ 0.75 μm diameter were captured in the liquid film (Fig. S5(c)) indicated a cut-off diameter of ∼ 0.3 μm. Fig. 1(b) shows photographs of the wet-cyclone module based on the simulation results.

3.2. Stabilization of two-phase flow in the wet-cyclone module

For real-time continuous particle sampling with a high concentration ratio, the liquid sampling medium should form a stable film on the wall of the cyclone under the high air flow rate condition. The stable liquid film means that the liquid covers the entire surface of the cyclone wall without being sprayed out of the film, to prevent sampling loss in the cyclone. The drag force of the fluid on the particle is proportional to the cube of the particle size, whereas the force between the particle and the wall is proportional to the particle size. Therefore, if fine aerosols directly adhere to the cyclone wall, there is no way to remove the particles, resulting in sampling loss [41]. The influence of air flow on a liquid film can be expressed by the modified Webber number, We_m. This can be conceived of as a measure of the relative importance of the inertia of a fluid compared to its surface tension [25]:

$${We}_m=\frac{{\rho }_l{{\bullet U}_a}^2{\bullet t}_l}{{\sigma }_l}$$ (2)

where ρ_l is the liquid density, t_l is the thickness of the liquid film and σ_l is the surface tension of the liquid.

When We_m increases the inertia of the liquid due to the shear force generated at the interface between the liquid film, the air flow overwhelms the surface tension of the film surface and the liquid is separated from the surface and sprayed into the surrounding air stream. To maintain a stable liquid film the We_m should be lowered by adjusting the air flow rate and the liquid flow rate. However, reducing the air flow rate to lower We_m hampers particle concentration and decreases the collection efficiency of fine aerosols (see Eq. (1)). Therefore, it is important to optimize the liquid flow inside the cyclone by controlling the liquid supply and drainage flow rates.

A numerical analysis was conducted to identify the optimal air flow and liquid flow conditions for the stable film. The volume-of-fluid (VOF) model, a two-phase flow model in Fluent code, was developed to calculate the hydrodynamics of liquid film formation inside the cyclone under various operating conditions. For the reasons mentioned above, the air flow rate was set at > 12 L/min and the liquid drainage flow rate was limited to 12 mL/h to attain an enrichment ratio of > 10⁴. Within the cyclone the surface of the thin liquid film rapidly evaporates due to the high air flow rate; therefore, if the liquid supply flow rate was lower than or equal to the drainage flow rate, a stable liquid film would not be formed and the liquid would split into multiple streams (Fig. 2 (a)(i); supplied liquid flow rate (Q_s) = 9 mL/h, drainage liquid flow rate (Q_d) = 10 mL/h). In contrast, if the supply flow rate were larger than the drainage flow rate, the liquid would float inside the cyclone and tend to clog (Fig. 2(a)(iii); Q_s = 9 mL/h, Q_d = 5 mL/h). The optimal supply and drainage flow rate ratio is around 1.1 (Fig. 2(a)(ii); Q_s = 9 mL/h, Q_d = 8.2 mL/h).

Fig. 2.

Characteristics of the stabilization of two-phase flow in the wet-cyclone. (a) Numerical analysis for optimization of the air and liquid flow conditions in the wet-cyclone. The air flow rate (Q_a) is 16 L/min. The supplied liquid flow rate (Q_s) is 9 mL/h and the drainage liquid flow rate (Q_d) is (i) 10, (ii) 8.2 and (iii) 5 mL/h. The dark area within the wet-cyclone indicates the liquid film. (b) Optimized operating conditions for the supplied- and drainage-liquid flow rate to obtain the stable status of the two-phase flow at the wet-cyclone. The intake air flow rate is 12, 14 and 16 L/min. (c) Photographs of the status of the wet-cyclone according to the operating conditions [(i), (ii), and (iii)] indicated in Fig. 2(b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

The conditions for the formation of a stable liquid film in the real wet-cyclone were optimized using the numerical analysis. Fig. 2(b) shows the optimized operating domains, where stable liquid films are formed at air flow rates of 12, 14 and 16 L/min. For 12 L/min, the optimal liquid supply-todrainage-flow rate ratio is around 1.12, similar to the numerical analysis result (Fig. 2(c)(ii)). The optimal ratio slightly increased with an increasing air flow rate due to faster evaporation (1.17 and 1.22 at air flow rates of 14 and 16 L/min, respectively). If the amount of supplied liquid was too low (Fig. 2(c)(i)) or too high (Fig. 2(c)(iii)) unstable films were formed, similar to the numerical analysis result. These cases are also shown in Movie S1.

3.3. Performance evaluation of the aerosol sampling in the wet-cyclone module

We evaluated the aerosol collection performance of the wet-cyclone module at liquid supply flow rates of 0, 5, 7, 9, 11 and 13 mL/h and sampling air flow rates of 12, 14 and 16 L/min using standard PSL particles (Fig. 3 (a)). The total aerosol collection efficiency (η_T) was defined as the fraction of the total number concentration of the entering PM retained by the wet-cyclone:

$${\eta }_T=\frac{C_{in}-C_{out}}{C_{in}}$$ (3)

where, C_in and C_out represent the aerosol number concentration at the air inlet and outlet, respectively.

Fig. 3.

Aerosol sampling performance of the wet-cyclone system. The aerosol collection efficiency of the standard polystyrene latex (PSL) particle for the wet-cyclone system with various sampling air flow rates of (a1) 12 L/min, (a2) 14 L/min and (a3) 16 L/min is evaluated at various supplied liquid flow rates of 0, 5, 7, 9, 11 and 13 mL/h. The aerosol-to-liquid transfer efficiency of the wet-cyclone system is evaluated using fluorescent PSL particles under sampling air flow rates of (b1) 12 L/min, (b2) 14 L/min and (b3) 16 L/min.

At a 12 L/min air flow rate (Fig. 3(a1)), the collection efficiency of aerosols over 0.5 μm in diameter was > 95%, similar to the simulation result. As the air flow rate was increased, the particle collection performance improved due to the increased centrifugal force; the collection efficiency of aerosols > 0.5 μm in diameter was ∼ 98.98% at 14 L/min (Fig. 3(a2)) and ∼ 99.72% at 16 L/min (Fig. 3(a3)). However, the liquid supply flow rate had little effect on the aerosol collection performance.

The proportion of the input aerosols delivered to the analytical sensor part was also assessed. In Eq. (3), the η_T is equal to the sum of the air-to-liquid particle transfer efficiency and the fraction of particles lost to the inner wall of the cyclone. This air-to-liquid particle transfer efficiency (η_AL) is defined as the transfer fraction of aerosol in the air to the liquid medium in the cyclone. We measured η_AL by comparing the total number of FPSL particles collected with that in the liquid drainage medium:

$${\eta }_{AL}=\frac{N_d}{N_{in}{\bullet \eta }_T}=\frac{C_d\bullet Q_d}{C_{in}\bullet Q_a\bullet {\eta }_T}$$ (4)

where N_d is the total number of particles in the liquid drainage medium, N_in is the total number of aerosols in the inlet air, C_d is the number concentration of particles in the liquid drainage medium and Q_a is the air flow rate.

As shown in Fig. 3(b), η_AL increased as the liquid flow rate and air flow rate were increased; however, the particle size had little effect on the η_AL of the wet-cyclone. These results are related to the uniformity and coverage of liquid film on the inner wall of the wet-cyclone. The higher air and liquid flow rates yield a more uniform liquid film with greater coverage in the wet-cyclone, which can decrease the particle loss therein. In this study, the optimum flow rates (η_AL > 98.5%) were determined to be: air, 16 L/min; liquid supply, 9 mL/h; and liquid drainage, 7 mL/h (air-to-liquid enrichment ratio of ∼ 1.4 × 10⁵).

3.4. Characteristics and performance evaluation of the ARBSW system

The ARBSW system was developed using the above numerical and experimental results and all parts are operated automatically. Fig. 4 is a photograph of the ARBSW and wireless remote-control panel. The real-time bioaerosol sampling performance of the ARBSW system was evaluated using test microorganisms (S. epidermidis and M. luteus). Fig. 5 (a) shows the size distributions of the test bioaerosols as unimodal curves. The specific geometric mean diameter (GMD) of S. epidermidis and M. luteus was 0.86 ± 0.01 μm and 1.10 ± 0.01 μm, respectively. The GMD is defined as Ʃn_jlndj/N, where n_j is the number of particles in the j^th group, d_j is the diameter of an individual particle and N is the total number of particles. The maximum and minimum aerodynamic diameters of S. epidermidis and M. luteus were ∼ 0.55 and ∼ 1.3 μm, and ∼ 0.55 and ∼ 2.1 μm, respectively. SEM showed that cells of both taxa are spherical (Fig. 5(b)).

Fig. 4.

Photograph of the Automated and Real-time Bioaerosol Sampler based on the Wet-cyclone (ARBSW) system and the wireless remote-control panel.

Fig. 5.

Bioaerosol sampling performance of the ARBSW system. (a) Size distributions of the test bioaerosols (S. epidermidis and M. luteus) and (b) scanning electron microscopy (SEM) images. The geometric mean diameter (GMD) of S. epidermidis and M. luteus bioaerosols is 0.86 ± 0.016 μm and 1.10 ± 0.009 μm, respectively. (c-1) Collection efficiency according to particle diameter and (c-2) total collection efficiency of the test bioaerosols. (d) Photographs of the cultured colonies of S. epidermidis and M. luteus after collection using the BioSampler and the ARBSW system. (e) Comparison of the relative recovery of S. epidermidis and M. luteus bioaerosols sampled using the BioSampler and the ARBSW system. (f) Comparison of the S. epidermidis colony concentration between the ARBSW system and the BioSampler according to sampling time. The aerodynamic particle sizer (APS) is used for measurement of the total bioaerosol concentration of sampled air during the sampling period.

The bioaerosol collection efficiency of the ARBSW system was initially assessed. The collection efficiency of S. epidermidis and M. luteus was more than 95% over their entire size range (Fig. 5(c1)), similar to the standard PSL particles. The total collection efficiency of both bioaerosols was > 99% (Fig. 5(c2)). Next, the microbial recovery of bioaerosols sampled by the ARBSW system was assessed. A comparative test was conducted with the conventional verified bioaerosol sampler, called BioSampler, which has a collection efficiency of > 90% for bioaerosol sizes of > 0.5 μm and a microbial recovery of > 99%. According to the BioSampler’s operational manual, 12.5 L/min of air enters 20 mL of sampling liquid in a reservoir and the enrichment of the collected particles is proportional to the sampling time (typically 20 min to reduce the desiccation effect) [25,[37], [38], [39]]. In contrast, the ARBSW system maintains a high particle enrichment ratio regardless of the sampling time, due to the constant air-to-liquid flow rate ratio (∼ 1.4 × 10⁵; use of the continuous entered air flow (16 L/min) and drainage liquid flow (7 mL/h)). Therefore, if there is no biological or physical loss in the ARBSW system, the concentration of microbes captured by the ARBSW during 20 min is theoretically ∼ 11-fold higher than that of the BioSampler under the same environmental conditions. Fig. 5(d) shows photographs of colonies of S. epidermidis and M. luteus after collection. The sampling time was 20 min and the TANC of test bacterial bioaerosols was kept constant at ∼ 100 particles/cm³ _air. The colony concentrations of S. epidermidis (460 ± 24.1 CFU/mL) and M. luteus (338 ± 19.5 CFU/mL) sampled by the ARBSW system were ∼ 11.2- and ∼ 10.6-fold higher than those by the BioSampler, which were 41 ± 6.0 and 32 ± 3.5 CFU/mL, respectively. These values correspond to the theoretical predictions. As shown in Fig. 5(e), the relative microbial recovery of S. epidermidis and M. luteus using the ARBSW system was 102 ± 4.2% and 96 ± 5.4%, respectively (those of the BioSampler were fixed at 100%), showing that the ARBSW system has comparable microbial recovery to the BioSampler.

It is important that the ARBSW system obtains highly concentrated samples consistently and rapidly, so that real-time sensing of bioaerosols can be achieved. We assessed the S. epidermidis colony concentration at sampling times of 0.5, 1, 2, 5 and 10 min using the ARBSW and BioSampler (Fig. 5(f)). The TANC from the nebulized bacterial medium was kept constant at ∼ 20 particles/cm³ _air. The concentration of the sampled particles was too low to be measured with the BioSampler when the sampling time was less than 1 min. A concentration of 7.8 × 10 CFU/mL was measured for a sampling period of 2 min, and it was observed that the concentration increased proportionally with sampling time. In contrast, the ARBSW system yielded a highly enriched sample (∼ 9.8 × 10⁴ CFU/mL) irrespective of the sampling time. Therefore, the ARBSW system is capable of highly enriched sampling of bioaerosols in a short time, with superior microbial recovery compared to a conventional instrument.

3.5. Real-time and continuous sampling performance evaluation using a test-bed condition

Real-time airborne microorganism monitoring systems should be capable of responding to abrupt changes in bioaerosol concentrations. To evaluate the responsivity of the ARBSW system, a test-bed environment that allows for sudden changes in bioaerosol concentration was used, as shown in Fig. 6 (a) (details of the experimental setup are in Fig. S2). Fig. 6(b) shows the TANC and colony concentration of the bioaerosols at 1-min intervals during the test. APS showed that the TANC rapidly increased from 0 to 1.6 particles/cm³ _air during the first nebulization, and decreased to 0.4 particles/cm³ _air after 1-min of nebulization. After 2 min of nebulization, the TANC was < 0.1 particles/cm³ _air, indicating removal of > 90% of bioaerosols. The colony concentration increased to 0.24 CFU/cm³ _air during the nebulization and decreased to 0.06 CFU/cm³ _air within 1 min. After 2 min of nebulization, the concentration was < 0.01 CFU/cm³ _air. In the second nebulization, the TANC increased from 0 to 1.2 particles/cm³ _air and the colony concentration increased from 0 to 0.2 CFU/cm³ _air. Thus, the colony concentration results of the ARBSW correspond well to the TANC variation measured by APS. Additionally, the results demonstrate that the ARBSW system is capable of continuous and accurate real-time bioaerosol monitoring, even in the presence of rapid changes in concentration.

Fig. 6.

Real-time bioaerosol sampling experiment in a test-bed environment. (a) Photograph of the test-bed experimental setup to evaluate the real-time sampling performance of the ARBSW system. (b) Variation of real-time bioaerosol and colony concentrations monitored by the APS and the ARBSW system during bioaerosol exposure events.

3.6. Real-world field test

A field test of the ARBSW system was conducted, where real-time monitoring of bioaerosols at a pond was performed at the Korea Institute of Science and Technology (KIST, Seoul, Republic of Korea) on August 23, 2017 (14:00–17:00) (Fig. 7 (a)). Fig. 7(b) shows the variations in atmospheric PM and total culturable bioaerosol concentrations during the field test (details of the experimental setup are in Fig. S3). PM₁₀ and PM_2.5 are defined as the mass fractions (μg/m³ _air) of aerosols with an aerodynamic diameter smaller than 10 and 2.5 μm, respectively [40]. The PM₁₀ concentration decreased from 13 to 10 μg/m³ _air and the PM_2.5 decreased from 11 to 9 μg/m³ _air. During the field test, the TANC also decreased from ∼ 5.5 × 10⁵ to ∼ 3.7 × 10⁵ particles/m³ _air at a particle size range > 1.0 μm (˜ 33% reduction) (Fig. S6), similar to the trends of PM concentrations (Fig. 7(b)). The total bioaerosol concentration monitored by the ARBSW decreased from 220 to 180 CFU/m³ _air at a rate similar to those of PM₁₀ and PM_2.5. These results indicate that the bioaerosol concentration was influenced predominantly by the PM concentration. More environmental monitoring data are shown in the supporting information (Fig. S6). These results demonstrate that the ARBSW system can perform continuous and real-time bioaerosol monitoring in real-world environments.

Fig. 7.

Real-world field test. (a) Photograph of the real-time continuous monitoring of atmospheric air quality nearby Korea Institute of Science and Technology (KIST). (b) Variations in atmospheric PM₁₀, PM_2.5, and total culturable bioaerosol concentrations during the field test.

4. Conclusion

The ARBSW system enables real-time monitoring of bioaerosols in real environments. This system has superior particle collection and particle transfer efficiency into a liquid medium. Compared with a conventional bioaerosol sampler, the ARBSW system enables marked particle enrichment and stable microbial recovery. Furthermore, the ARBSW not only has an excellent sampling performance with rapid responsivity, but is also suitable for the real-time monitoring of bioaerosols in real-world environments. The continuous liquid-based particle sampling used by the ARBSW system, when integrated with a real-time particle analysis system (e.g., microfluidic flow cytometer), enables continuous quantitative characterization of airborne microorganisms and facilitates analysis of the physicochemical and biological properties of bioaerosols, for example using markers for specific aptamers or antibodies.

Author contributions

J.H.J. conceived the initial idea; Y.S.C., J.C., and S.C.H. designed and implemented the experiments; and J.H.J. and S.C.H. developed and refined the concept and wrote the paper.

Notes

The authors declare no competing financial interests.

Biographies

Yu Sung Cho is currently a researcher in the center for environment, health, and welfare research at the Korea Institute of Science and Technology (KIST) Seoul, Republic of Korea. Also, he is a M.S. candidate in the Green School at Korea University, Seoul, Republic of Korea.

Seung Chan Hong has received B.S. in mechanical engineering from Yonsei University, Seoul, Republic of Korea (2013), and M.S. in mechanical engineering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea (2015). He is currently a Ph.D. candidate in the school of mechanical and aerospace engineering at Seoul National University, Seoul, Republic of Korea. His research has been focused on the aerodynamic motion control of airborne particulate matters.

Jeongan Choi has received B.S. in mechanical engineering from Hanyang University, Seoul, Republic of Korea (2013), and M.S. in mechanical engineering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea (2015). He is currently pursuing the Ph.D. degree with the department of mechanical science and engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA.

Jae Hee Jung has received Ph.D. in mechanical engineering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea (2008). He is currently a senior research scientist in the center for environment, health, and welfare research at the Korea Institute of Science and Technology (KIST), Seoul, Republic of Korea. His research has been focused on the detection and control of airborne biological particulate matters.

Appendix A. Supplementary data

The following are Supplementary data to this article: