Optical Trapping and Gram-Type Differentiation of Living Bacteria in 2D Hollow Photonic Crystal Cavities

Thanks to their small footprint and to their ability to manipulate objects using low powers, optofluidic systems, based on the integration of photonic structures with microfluidic layers, were shown to be promising tools for biological analysis. Here, we report on the optical trapping and Gram-type differentiation of seven types of living bacteria in 2D hollow photonic crystal cavities. Photonic crystals on silicon and a polydimethylsiloxane (PDMS) frame were respectively processed using standard and soft lithography techniques. A solution of diluted bacteria in de-ionized water was injected in the PDMS frame, letting the bacteria move in unconstrained Brownian motion close to the hollow defect. Excitation of the resonant cavity with a tuneable laser permitted to record multiple trapping events. The analysis of the membrane-dependent resonance frequency shift due to the electromagnetic field-bacteria coupling allowed for the Gram-type differentiation of bacteria in a fast, label-free, and non-destructive way.