Microfluidic platforms employing integrated fluorescent or luminescent chemical sensors: A review of methods, scope and applications

Herein we critically review microfluidic platforms that contain integrated fluorescent or luminescent chemical sensor assemblies. These were employed in particular for miniaturized oxygen and pH sensing. Microchips with optical temperature sensing capability are also covered since these share many concepts and applications. Other analytes and derived parameters from the above analytes are found in some sensing approaches in microfluidics. After an introduction, the work is structured into three main chapters dealing with the fabrication and microintegration of these sensors, readout and detection strategies, and applications of these microsystems. The fabrication is discussed with a focus on soft lithography-based approaches in polydimethylsiloxane (PDMS) or PDMS and glass hybrid devices that form the majority of work so far. Alternative approaches, particularly using glass or quartz as the main chip material are also covered. Detection techniques employed to date are the subject of the next chapter, where simple intensity as well as lifetime- or wavelength-referenced schemes are presented and the utility of image-based sensing on the microscale is discussed. Lastly, exciting applications of these microfluidic chips are highlighted. Luminescent oxygen and pH sensing has been of particular interest in the field of microbioreactors but other areas are also of interest, particularly chemical reactors and electrophoresis. Optical temperature sensing is discussed and its use in fundamental studies as well as in enzyme reactors. Integrated microsystems with biosensing capabilities and some for monitoring of metal ions and other analytes are also presented.