Fully integrated optofluidic SERS platform for real-time and continuous characterization of airborne microorganisms

An optofluidic, surface-enhanced Raman spectroscopy (SERS) platform was developed to detect airborne microorganisms, continuously and in real time. The platform consists of an on-chip analysis system integrated with an aerosol sampler and Raman spectrometer. A stratified two-phase flow, consisting of the sampled air stream and a stream of collection medium, is formed in the curved channel. The inertia of collected particles, such as bacterial cells, carries them across the phase boundary in the curved channel such that they impact the liquid stream directly. The collection efficiency of the microchannel was evaluated using different-sized standard polystyrene-latex particles. A collection efficiency of 99.6% was attained for particles with an average aerodynamic diameter of 1 μm, a typical size for bacterial aerosols, by optimizing the flow rates of the sample air and liquid medium. A silver colloid in the collection medium was used as the SERS adsorbent. After passing through a serpentine mixing channel, bacterial particles were detected by SERS in real time using the custom Raman spectroscopy system. The detection system was evaluated with five test bacteria: S. epidermidis, M. luteus, E. hirae, B. subtilis, and E. coli. The concentration of airborne S. epidermidis corresponded to a Raman peak at 732.5 cm^-1. The limit of detection was approximately 10² CFU/mL and the total bacterial aerosol concentration was determined in real time based on the ratio of sampling air to SERS colloid.