International Journal of Thermophysics

, Volume 35, Issue 12, pp 2178–2186 | Cite as

Influences of Detection Pinhole and Sample Flow on Thermal Lens Detection in Microfluidic Systems

  • Mingqiang Liu
  • Mladen Franko


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.


Colorimetric reaction Microfluidic chip Optimization  Signal-to-noise ratio Thermal lens microscopy 



This work was financially supported by the Early-Career Research Grant 3330-14-509065 (to M. Liu) from the Ministry of Education, Science and Sport of Slovenia. We thank Prof. Igor Plazl at the University of Ljubljana for kindly lending us the microchip and microsyringe pumps.


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Copyright information

© Springer Science+Business Media New York 2014

Authors and Affiliations

  1. 1.Laboratory for Environmental ResearchUniversity of Nova GoricaNova GoricaSlovenia

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