For instance, a microfluidic ATP-bioluminescence sensor for the detection of airborne microbes using commercial available photo-diodes has been recently reported . Although optical absorption detection is compatible with microfluidics, they suffer from relatively poor detection limits due to the short effective path length found in microfluidic channels . Consequently, fluorescence detection remains the dominant optical detection technique in microfluidics. Here the conjugation of affinity markers (e.g. antibodies, DNA etc.) with fluorescent compounds like fluorescein isothiocyanate (FITC), phycoerythrin (PE) cyanin- or Alexa-dyes is most commonly used. Alternative approaches are based on the incorporation of two fluorescence molecules into the biosensor, using fluorescence resonance energy transfer (FRET) .
Other optical methods include chemiluminescence (CL), bioluminescence (BL) and Surface Plasmon Resonance (SPR) biosensors. While chemiluminescence describes the generation of light due to release of energy during a chemical reaction, SPR measures changes in refractive index caused by structural alterations in the vicinity of a thin film metal surface . The numerous chemiluminescence (CL) applications in microfluidic analysis systems using immobilized enzymes, antibodies or nucleic acids have been recently described [37-39]. In turn, electroanalytical methods are highly compatible with micro- and nanomachining (MEMS) technology and can be segmented into current (amperometric), potential (potentiometric) or impedance (impediometric) techniques [40-43].
Evolving from ISFETs, a recent technology combines potentiometry and optical detection, known as light addressable potentiometric sensor (LAPS), that can be used for the detection of pathogen E. GSK-3 coli . Alternative detection methods for pathogen sensing include the application of silver dots for direct optical density measurements using a scanometric reader [45,46], or biosensors using resonance light scattering (RLS) techniques based on nanometer-sized metallic particles (mostly gold) covalently linked to antibodies. These metal colloidal particles radiate energy in the form of scattered light when illuminated by a white light source . Altogether, LOC devices present themselves as a flexible technology platform that can be readily adapted to specific identification needs. A whole range of materials and mode of detection can be specifically selected for either low cost applications or high end analysis. Having reviewed the various materials and detection methods employed in lab-on-a-chip devices, we now provide a detail list of LOC studies grouped by class of target analytes.3.