Ozone-free electrostatic collection of aerosolised microspheres

The transmission of pathogenic bioaerosols poses a substantial risk not only to human health but also to animal welfare and agricultural productivity, where the spread of infections can lead to significant economic and public health system burdens. This underscores the importance of developing reliable aerosol sampling techniques that can capture airborne particles.

Electrostatic precipitation (ESP) is a promising method for aerosol collection¹. However, its dependence on corona discharge to charge particles generates ozone byproducts. The presence of ozone can compromise the integrity of bioaerosols². This is problematic in applications where preserving the viability of the collected bioaerosols is essential, such as those requiring cultivation. Therefore, to address this limitation whilst keeping the advantages of ESP-based techniques, ozone-free (i.e. corona discharge-free) electrostatic actuation was investigated as a potential alternative.

Indium tin oxide (ITO)-coated microscope slides (Diamond Coatings) were electrically connected to four different electrical conditions: negative, positive, grounded, and floating (no connection) in an 8 m³ aerosol test chamber. Voltages between -10kV to +10kV were applied. 1 µm diameter fluorescent polystyrene microspheres (PSL) were used as model aerosolised particles. Four optical particle counters (OPC-N3, Alphasense) were positioned in the vicinity of the slides to continuously monitor aerosol concentration. For each experiment, aerosols were nebulised for 15 minutes, followed by a 10-minute sampling period during which voltages were applied. Afterwards, the chamber was cleaned using an extraction system equipped with HEPA filter, and then the samples were retrieved for imaging using an EVOSM700 fluorescence microscope. Particle counts were obtained using Celeste 6 analysis software and normalised against the chamber concentration. To direct the particle flow towards the slide, aiming to enhance the collection efficiency, a fan-assisted collection device was constructed to direct airflow onto the slide (Fig 1a). Fan speed and spatial placement were varied to optimise collection efficiency.

Ozone-free electrostatic collection of PSL particles was successfully demonstrated, with both positive and negative biases collecting up to 17.0 and 8.5 times more PSL particles than the grounded and floating slides, respectively. A correlation was observed between applied voltage and collection performance, as higher voltages generated stronger electric fields, thereby enhancing the electrostatic force and particle capture. The simple fan-driven collection device achieved an initial collection efficiency of 31.5%. Investigation into fan speed and spatial positioning revealed that lower fan speeds and a closer fan-to-collection-medium distance performed better, with the highest collection efficiency at 59.1% at 10.9 L/min air flow rate (lower speed setting) (Fig 1b).

These findings demonstrate that ozone-free electrostatic collection is an effective alternative approach to the ESP-based method for aerosol collection, with the potential of maintaining bioaerosol viability, which will be tested in the near future to confirm. Overall, the results establish a foundation for advancing electrically actuated aerosol collection devices and highlight promising future applications in public health surveillance, environmental bioaerosol monitoring, and agricultural biosecurity.

This work was supported by Research England-funded Biodetection Technologies Hub and the Engineering and Physical Sciences Research Council [grant number EP/X017591/1].

References: [1] Foat et al. (2016), [2] Ouyang et al. (2023).