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A combined microfluidic flow injection analysis-thermal lens microscopy ( $$\upmu $$ FIA-TLM) system was applied for determination of neutrophil gelatinase-associated lipocalin (NGAL)-a biomarker of acute kidney injury. NGAL was determined following a commercial ELISA assay and transfer of the resulting solution into the $$\upmu $$ FIA-TLM system with a 100 $$\upmu $$ m deep microchannel. At an excitation power of 100 mW, the $$\upmu $$ FIA-TLM provided about seven times lower limits of detection (1.5 pg $${\cdot }\mathrm{mL}^{-1})$$ as compared to a conventional ELISA test, and a sample throughput of six samples per minute, which compares favorably with sample throughput of the microtiter plate reader, which reads 96 wells in about 30 min. Comparison of results for NGAL in plasma samples from healthy individuals and for NGAL dynamics in patients undergoing coronary angiography measured with transmission mode spectrometry on a microtiter plate reader and with $$\upmu $$ FIA-TLM showed good agreement. In addition to improved LOD, the high sensitivity of $$\upmu $$ FIA-TLM offers possibilities of a further reduction of the total reaction time of the NGAL ELISA test by sacrificing some of the sensitivity while reducing the duration of individual incubation steps. [ABSTRACT FROM AUTHOR]

Thermal lens microscopy (TLM), due to its high temporal ( $${\sim }\mathrm{ms}$$ ) and spatial resolution ( $${\sim }\upmu \mathrm{m}$$ ), has been coupled to lab-on-chip chemistry for detection of a variety of compounds in chemical or biological fields. Due to the very short optical path length (usually below 100 $$\upmu \mathrm{m}$$ ) in a microchip, the sensitivity of TLM is unfortunately still 10 to 100 times lower than conventional TLS with 1 cm sample length. Optimization of the TLM optical configuration was made with respect to different pinhole aperture-to-beam size ratios for the best signal-to-noise ratio. In the static mode, the instrumental noise comes mainly from the shot noise of the probe beam when the chopper frequency is over 1 kHz or from the flicker noise of the probe beam at low frequencies. In the flowing mode, the flow-induced noise becomes dominant when the flow rate is high. At a given flow rate, fluids with a higher density and/or a higher viscosity will cause larger flow-induced noise. As an application, a combined microfluidic flow injection analysis ( $$\upmu \mathrm{FIA}$$ )-TLM device was developed for rapid determination of pollutants by colorimetric reactions. Hexavalent chromium [Cr(VI)] was measured as a model analyte. Analytical signals for 12 sample injections in 1 min have been recorded by the $${\upmu }$$ FIA-TLM. For injections of sub- $$\upmu $$ L samples into the microfluidic stream in a $$50\,\upmu \mathrm{m}$$ deep microchannel, a limit of detection of $$4\,\mathrm{ng}{\cdot }\mathrm{mL}^{-1}$$ was achieved for Cr(VI) in water at 60 mW excitation power. [ABSTRACT FROM AUTHOR]

The optical configuration of a thermal lens microscope (TLM) was optimized for detection in a microfluidic chip with respect to the flow velocity, and the pump and probe beam parameters (beam waists, offsets, and mode mismatching degree). It was found that an appropriate pump-probe beam offset for a certain flow velocity would provide not only a higher sensitivity but also a better response linearity of TLM over three orders of magnitude of sample concentration. Diffraction-limited pump beam excitation is advantageous for space-resolved measurement, while a larger pump beam with 10 times lower power density is favorable for higher sensitivity at given experimental conditions. As an application, TLM was used to study the diffusion of azobenzene in a microfluidic chip. Diffusion profiles at different distances from the mixing point were recorded by scanning the TL signal along the cross section of the microchannel. By fitting the diffusion profiles to a theoretical model of mass transfer in a microchannel, diffusion coefficients of azobenzene in octane and methanol were determined to be $$5\times 10^{-10}\,\hbox {m}^{2}{\cdot }\,\hbox {s}^{-1}$$ and $$6\times 10^{-10}\,\hbox {m}^{2}{\cdot }\,\hbox {s}^{-1}$$ , respectively. [ABSTRACT FROM AUTHOR]

Development of thermal lens spectrometry (TLS) as a micro space‐compatible photothermal technique and its applications for analysis of different chemical compounds in micro space and particularly in microfluidic systems are reviewed. Theoretical treatment of TLS in micro space has evolved from simply following conventional theory and predictions in macro space to employing a more accurate theory where impacts of the excitation source (Gaussian laser, top‐hat incoherent light source, beam divergence, power density), detection scheme (probe beam waist, mode‐mismatching degree), sample flow, and sample cell (top/bottom layers, side wall) on the TL signal are included. Noise sources (light sources, sample status, detector) in TLS systems have been analyzed, and optimum pinhole‐to‐beam radius ratio is suggested for the maximum signal‐to‐noise ratio. With different excitation light sources from ultraviolet, to visible, to near‐infrared regions and coupled with microfluidic devices, these TLS instruments with good temporal and spatial resolution have found many applications for highly sensitive and/or high‐throughput detection of chemical or biochemical analytes, for cell imaging or single particle/molecule detection, and for characterization of molecular diffusion in single‐ or two‐phase systems. Prospects and challenges of TLS for future applications in microchemical analysis are discussed. [ABSTRACT FROM AUTHOR]

Droplet-based liquid–liquid extraction in a microchannel was studied, both theoretically and experimentally. A full 3D mathematical model, incorporating convection and diffusion in all spatial directions along with the velocity profile, was developed to depict the governing transport characteristics of droplet-based microfluidics. The finite elements method, as the most common macroscale simulation technique, was used to solve the set of differential equations regarding conservation of moment, mass and solute concentration in a two-domain system coupled by interfacial surface of droplet-based flow pattern. The model was numerically verified and validated online by following the concentrations of a solute in two phases within the microchannel. The relative azobenzene concentration profiles in a methanol/n-octane two-phase system at different positions along the channel length were retrieved by means of a thermal lens microscopic (TLM) technique coupled to a microfluidic system, which gave results of high spatial and temporal resolution. Very good agreement between model calculations and online experimental data was achieved without applying any fitting procedure to the model parameters. [ABSTRACT FROM AUTHOR]