Low-cost real-time multi-parameter bio-aerosol sensors (invited paper)

Abstract

Sensors that are able to provide reagent-free, continuous monitoring for potential bio-aerosol hazards are required in many environments. In general, increasing the number of optical and spectroscopic properties of individual airborne particles that can be measured increases the level of detection confidence and reduces the risk of false-positive detection. This paper describes the development of relatively low-cost multi-parameter prototype sensors that can monitor and classify the ambient aerosol by simultaneously recording both a 2x2 fluorescence excitation-emission matrix and multi-angle spatial elastic scattering data from individual airborne particles. The former can indicate the possible presence of specific biological fluorophores in the particle whilst the latter provides an assessment of particle size and shape. The prototype sensors described continuously sample ambient air through a delivery system, designed so that particles in the sample flow are surrounded by clean, filtered sheath air. The sample particles are then drawn through the focussed light beam from a continuous diode laser. Each individual particle, down to ~0.5µm in size, produces a scattered light pattern that is recorded by a multi-pixel photodetector. The scattered light signal also provides a trigger to initiate the sequential firing of two optically filtered xenon sources that irradiate the particle with pulsed UV radiation centred upon ~280 nm and ~370 nm wavelengths, optimal for excitation of bio-fluorophores tryptophan and NADH respectively. For each excitation wavelength, fluorescence is detected across two bands embracing the peak emissions of the same bio-fluorophores. Particle classification may be achieved by evaluating these spatial scatter and fluorescence data to appropriately ‘position’ the particle within multi-parameter space. Particles are measured at rates up to ~125 particles/s (limited by the xenon recharge time), corresponding to all particles for concentrations up to 1.3 x 10 4 particles/l. Analysis of results from aerosols of BG spores and a variety of other materials are described, along with examples of the temporal fluctuations in bio-aerosols in two very different environments.