Interferometric refractive index detection methods for microfluidic devices - A review

The introduction of microfluidic platforms for chemical analysis has placed new demands on the development of detectors to track chemical processes and quantify assays in small volumes. Methods based on detecting refractive index offer intriguing possibilities for developing compact, inexpensive detectors that are readily integrated with microfluidic devices. Since refractive index is a universal signal, analytes from inorganic ions to large proteins can be detected without the addition of external labels or contrast agents and the variety of approaches developed provides flexibility in platform design.

Herein we summarize the development of Mach Zehnder Interferometry (MZI), Young Interferometry (YI), Fabry-Pérot Interferometry (FPI), Fiber Bragg Gratings (FBG), and Back-Scatter Interferometry (BSI) for refractive index measurements in microfluidic devices. For each technique, we discuss the mechanism of the measurement and present examples meant to illustrate the breadth and capabilities of the approach. Given the large volume of work in this area, the examples are meant to highlight the various ways each approach has been implemented. Stand-off detection versus immersive sensing designs are discussed and the figures of merit for both bulk refractive index measurements and specific sensing applications are summarized. These results are discussed in terms of future potential for developing inexpensive, scalable detectors for microfluidic analysis platforms.

Refractive index detection in microfluidics can be implemented using integrated waveguides, optical fibers, structured optical fibers, through-space beams and other configurations, making it a highly flexible approach for microscale detection. This review highlights recent applications for selected interferometric methods, compares performance metrics, and discusses future potential for scaling and integration with microfluidics.