Detection of airborne Bacillus anthracis spores by an integrated system of an air sampler and a cantilever immunosensor

A near real-time, sensitive and reliable method of detecting airborne Bacillus anthracis spores by air sampling (wet mode) and then exposing the collected sample to an antibody-functionalized piezoelectric-excited millimeter-sized cantilever (PEMC) sensor is presented. Using a commercial air sampler, a 10 min air sample at 267 L/min captured the airborne particulates containing Bacillus anthracis (BA) spores and concentrated them into 5 mL of phosphate buffered saline (PBS). This sample was then injected into a flow cell containing an antibody-functionalized PEMC sensor. The resonant frequency of the PEMC sensor at 925.1 kHz decreased exponentially as the BA spores attached to the sensor surface producing a positive response well beyond the noise level in 2 min and reached a steady state value in 20 min. In liquid phase, the sensor response correlated well (R2 = 0.99) with spore concentration and was shown to follow: [Response in Hz] = (0.0637) × (spore concentration in #/mL). Our results show that detection of 38 BA spores/L of air is achievable in near real-time with an estimated lower limit of detection of ∼5 spores/L of air in the configuration tested.