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.

We demonstrate the fabrication, characterization and application of microfluidic chips capable of continuous electrophoretic separation via free flow isoelectric focussing (FFIEF). By integration of a near-infrared (NIR) fluorescent pH sensor layer under the whole separation bed, on-line observation of the pH gradient and determination of biomolecular isoelectric points (pI) was achieved within a few seconds. Using an optical setup for imaging of the intrinsic fluorescence of biomolecules at 266 nm excitation, labelling steps could be avoided and the native biomolecules could be separated, collected and analysed for their pI. The fabricated microchip was successfully used for the monitoring of the separation and simultaneous observation of the pH gradient during the isoelectric focussing of the proteins α-lactalbumin and β-lactoglobulin, blood plasma proteins and the antibiotics ampicillin and ofloxacin. The obtained pIs are in good agreement with literature data, demonstrating the applicability of the system. Mass spectra from the separated antibiotics taken after 15 minutes of continuous separation from different fractions at the end of the microchip validated the separation via microfluidic isoelectric focussing and indicate the possibility of further on- or off-chip processing steps.

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Microreactors have gained increasing attention in their application toward continuous micro flow synthesis. An unsolved problem of continuous flow synthesis is the lack of techniques for continuous product purification. Herein, we present a micro free-flow electrophoresis device and accompanying setup that enables the continuous separation and purification of unlabeled organic synthesis products. The system is applied to the separation and purification of triarylmethanes. For imaging of the unlabeled analytes on-chip a novel setup for large area (3.6 cm) deep ultra violet excitation fluorescence detection was developed. Suitable separation conditions based on low conductivity electrophoresis buffers were devised to purify the product. With the optimized conditions, starting materials and product of the synthesis were well separated (R > 1.2). The separation was found to be very stable with relative standard deviations of the peak positions smaller than 3.5% over 15 min. The stable conditions enabled collection of the separated compounds, and purity of the product fraction was confirmed using capillary electrophoresis and mass spectrometry. This result demonstrates the great potential of free-flow electrophoresis as a technique for product purification or continuous clean-up in flow synthesis. Graphical Abstract Micro free-flow electrophoresis (μFFE) allows continuous separation and purification of small organic synthesis products. Enabled by a novel deep-UV imaging setup starting materials and product of a recently developed synthesis for triarylmethanes could be purified. Thereby demonstrating the potential of μFFE as continuous purification technique for micro flow synthesis.

The continuous separation mechanism of micro free-flow electrophoresis (μFFE) is a straightforward, suitable tool for microscale purification of reaction mixtures. However, aqueous separation buffers and organic reaction solvents limit the applicability of this promising combination. Herein, we have explored nonaqueous micro free-flow electrophoresis for this purpose and present its suitability for a continuous workup of organic reactions performed in acetonitrile. After successful nonaqueous FFE separation of organic dyes, the approach was applied to continuously recover the photocatalyst [Ru­(bpy)3]2+ from a homogeneous, acetonitrile-based reaction mixture. This approach opens up possibilities for further downstream processing of purified products and is also attractive for recycling of precious catalyst species.