Integrated Deep-Probe Optical Waveguides for Label Free Bacterial Detection

Rapid, specific, and sensitive detection of pathogenic bacteria is very important within areas like food safety, medical diagnostics, hospital infection and biological warfare. Optical evanescent wave sensors are evolving to meet these challenges. Evanescent wave biosensors generate an electromagnetic wave at the sensor surface that penetrates 100-200 nm into the surrounding medium, which have proven to be a highly sensitive tool to monitor interactions in the close vicinity of the sensor surface. However, the use of such waveguides for bacterial detection is problematic for several reasons. These include the short penetration depth of the evanescent field of these waveguides (100-250 nm) compared to a typical size of a bacterium (1-5 μm), which places the majority of the bound cell outside the evanescent field. In addition, the low refractive index contrast between the bacterium cytoplasm and aqueous environments in which detection is usually performed, and availability and accessibility of antigens on the bacterium surface binding to the biorecognition elements. Finally, the sensor performance can for example be limited due to (i) mass transport of large analytes like bacteria which limits the binding to the immobilized recognition receptors, (ii) non-specific binding, and (iii) long analysis time. This article will focus on the development of deep-probe optical evanescent wave sensor such as metal clad leaky waveguide sensor for bacterial detection. In addition, two complete detection systems integrated with physical force fields to overcome these problems will be presented. These sensor systems are based on MCLW sensors and integrated with respectively an electric field and ultrasound standing waves as physical force to concentrate and enhance capturing of bacteria spores into immobilized antibodies on the sensor surface. The integration improves the detection limit by a few orders of magnitude and shortens the analysis time significantly.