Real-Time Fluorescence Measurement of Airborne Bacterial Particles Using an Aerosol Fluorescence Sensor with Dual Ultraviolet- and Visible-Fluorescence Channels

Abstract

In the search for methods by which an ambient environment can be continuously monitored for potentially harmful biological aerosols, particle fluorescence methods have received considerable attention over the past decade. The aerosol fluorescence sensor (AFS) is a monitoring device for continuous, real-time detection of airborne biological particles. The AFS method is based on the principle of ultraviolet (UV) light-induced fluorescence targeting the intrinsic fluorescence of common amino acids found in living matter. The sensor uses a UV optical excitation source to illuminate an airstream flowing continuously through the sensor detection volume. Fluorescence from all particles present within the sensing volume is measured using two photomultiplier detectors optically filtered to detect radiation in the UV and visible (Vis) bands, enabling generic discrimination between different aerosol populations. In this study, we investigated the real-time fluorescence characteristics of airborne bacterial particles (Escherichia coli and Bacillus subtilis) and nonbacterial particles (polystyrene latex [PSL] spheres) using an AFS. The UV fluorescence intensity of both bacterial bioaerosols was higher than that of PSL particles at the same total particle concentration. In particular, the ratio of UV- to Vis-fluorescence, which can differentiate between bacterial bioaerosols and PSL particles, was significantly higher for bacterial bioaerosols (E. coli: 5.836; B. subtilis: 6.023; PSL: 4.073). To optimize the AFS, the variation in the fluorescence intensity characteristics was evaluated under various sensor gain settings for the two fluorescence channels and the flash frequency of the excitation light. The amplitude of the measured fluorescence offset and the dynamic measurement range depended on the gain of the system. The coefficient of variance increased with decreasing flash frequency. These experimental results provide basic information about the feasibility of AFS for real-time detection of bioaerosols and may contribute to the development of new bioaerosol detection systems.