Although a lot of work has been done in developing microfluidic channels, components and techniques for labs-on-chips, their applications have so far been limited to laboratory prototypes without widespread use in practical applications. One important reason is that the detection of the result of a chemical analysis is still a weak point in many lab-on-a-chip devices. Indeed, decreasing the channel dimensions to nanoliter sample volumes with low amounts of analytes, requires highly sensitive detection techniques. Optical detection is one of the techniques capable of providing sufficiently high sensitivity; however it is commonly still accomplished by bulky and expensive microscopes located off-chip. These large instruments are incompatible with the concept of miniaturization and integration in microfluidic devices. Therefore miniaturized and integrated optical detection systems are needed to fully exploit the potential advantages of labs-on-chips.
We have developed such a detection system combining both laser-induced fluorescence and absorbance analysis in fused silica capillary microfluidic channels. The system consists of a plastic microfabricated part with integrated optics which are directly aligned with the capillary. An additional GRIN lens which is positioned in a micromilled base plate, filters, an excitation source and two PMT detectors complete the detection unit. We have designed the detection system by means of nonsequential ray tracing simulations and the microfabricated part is prototyped by means of Deep Proton Writing. The performance of the system is tested in a proof-of-concept demonstration setup. With the current setup we achieve a concentration measurement range from 0.6µM to 12mM for absorbance measurementsvand from 6pM up to 0.6mM for fluorescence. This research is done in collaboration with the Department of Chemical Engineering of the Vrije Universiteit Brussel (Prof. G. Desmet).
In addition to the optical design, the DPW-prototyping and the proof-of-concept demonstration, we have done a complete tolerance analysis to investigate the manufacturability of the system. A set of tolerances was defined using non-sequential optical ray tracing simulations combined with statistical design methodology. Unlike a sensitivity analysis for one parameter at a time, this method allowed to consider interactions between different parameters. We have investigated experimentally that for most parameters the misalignment errors in the prototyped setup are bound to the tolerance limits defined in the simulated tolerance analysis and that the observed fabrication errors are acceptable such that the DPW-technology is an adequate prototyping technology to manufacture a master component applicable for mass replication through e.g. high-precision hot embossing. We have experimentally demonstrated a first step towards replication, by replicating the DPW-prototype using the elastomeric molding and vacuum casting technique.
Currently we are investigating the use of this system for two applications: the characterization of lubricant oils and the detection of chromatographic separations.
Different types of lubricant oils from different types of turbines can be characterized with this micro-optical detection unit and can be distinguished by means of both absorbance and fluorescence measurements. This type of measurements is not new: the use of optical techniques to characterize lubricant oil samples has been demonstrated already in the past. However the micro-optical system used here has two advantages compared to conventional lubricant oil characterization equipment. It consumes only low, nanoliter amounts of sample, while conventional methods need milliliter volumes. Moreover it has a small size and a relatively low cost as it contains plastic mass manufacturable micro-optics and standard available light sources and detectors. Therefore this system is an ideal candidate for on-line quality monitoring of lubricant oils, which is not possible with the conventionally used bulky and expensive instruments. The lubricant oil samples used in this study are provided by Mecoil Diagnosi Meccaniche srl and the monitoring is done in collaboration with the “Nello Carrara” Institute of Applied Physics (CNR-IFAC) in Firenze, Italy (Dr. A. Mignani).
A second application is the characterization of chromatographic separations. We have characterized the separation of coumarin and rhodamine dyes with our detection system. The ability to measure both coumarines, which are excited at 405nm and which have an emission peak between 420 and 500nm, and rhodamines, which are excited at 488nm and which have an emission peak between 500nm and 600nm, with the same micro-optical unit, illustrate the versatility of our system. For this application we collaborate with the Pfizer Analytical Research Centre (PARC), Department of Organic Chemistry of the University of Ghent (Prof. P. Sandra).
|Sara Van Overmeire|
|+32 2 629 36 58|
|+32 2 629 34 51|
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