Intracavity microfluidic dye laser droplet absorption
Introduction
Microdroplet formation techniques play an important role due to the potential applications of droplets. Passive [1], [2] as well as active systems [3], [4] are now able to produce monodisperse droplets of different sizes and shapes in a controlled way. Numerous applications have been developed based on this novel possibility. Droplets can be used in chemistry as microreactors to synthesise precise emulsions [5] or particles [6], study protein crystallization [7]. Droplets can also serve in biology to design miniaturized assays for blood analysis [8] or for performing polymerase chain reaction of DNA [9].
Fluorescence experiment is a sensitive method to analyze the content of droplets [10], [11]. For that, an additional procedure is needed to encapsulate a fluorescing element into droplets. Photobleaching is also a common difficulty in a fluorescence experiment. Measuring concentration via optical absorption technique is attractive because it is fast, non destructive and label free. However the sensitivity of on-chip absorbance measurement systems is fundamentally limited by the short optical path lengths that can be achieved. Using an optical resonator can increase efficiently the light-sample interaction length. It has been demonstrated an intracavity laser absorption technique using a microfluidic dye laser with a continuous flux channel [12]. Applying intracavity techniques to droplets can be useful to follow the dynamic of reaction, quantify a component by measuring its absorbance or detect tracks of a biological entity. As neither label nor specialization of the detector surface is required for intracavity technique, measurement does not change the nature of the entity and is not necessarily the last step of a lab-on-chip process.
We demonstrate in present work such a sensitive intracavity dye laser system suitable for measuring the absorption of droplets.
Section snippets
Experimental
The microfluidic devices used in this work were fabricated with a classical Glass-PDMS technology. Fig. 1 shows the microfabrication process. First, the spacers and microfluidic channels were patterned in 125 μm thick photoresist layer (Microchem SU-8 2100). Sacrificial fibers were aligned on the mould to frame the future spot of the laser cavity (Fig. 2). Spacers were useful to maintain the optical fibers in position. The mould was coated with trichloromethylsilane (TMCS) vapor as anti-adhesion
Results and discussion
Droplets are produced by adjusting the flow rates of the two immiscible liquids. There are different droplet morphologies in function of the flow rates. What matters is the ratio between the flow rates of the dispersed phase and the continuous phase. Changing the ratio allows to produce either unconfined droplets or confined droplets.
In the case of unconfined droplet, a sphere propagates along the channel. Measuring the absorption of such an element is not trivial. The spherical shape deflects
Conclusion
In present work, we have proposed a microdroplet production system combined with a microfluidic dye laser. The microdroplet generator delivers nanoliter samples. Lasing effect is obtained with confined droplets, taking advantage of their elasticity. The laser output intensity varies clearly when a droplet passes through the cavity. This integrated intracavity laser system is versatile and suitable for sensitive absorption measurement on few nanoliter volumes. The technique benefits of the
Acknowledgments
The authors would like to thank the LPN clean-room staff and the LPPM optical instrumental group for technical assistance. G. Aubry is financially supported by the French Foundation RTRA-Triangle de la Physique under contract No. 68.
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2016, Analytica Chimica ActaCitation Excerpt :Other demonstrated applications include protein crystallization [20] and enzymatic kinetic assays [21], emulsion-based polymerase chain reaction [22], chemical synthesis [23,24], and single cell-based analysis [25]. For detection in droplet microfluidics, various detection methods have been used including laser-induced fluorescence [26], mass spectrometry [27], Raman spectroscopy [28], absorption spectroscopy [29], and bright-field microscopy [30]. Although these techniques offer highly selective, sensitive and accurate quantification, the main disadvantage are high equipment cost and challenge of coupling them with miniaturized systems for on-site applications.