A guiding light: spectroscopy on digital microfluidic devices using in-plane optical fibre waveguides

Abstract

We present a novel method for in-plane digital microfluidic spectroscopy. In this technique, a custom manifold (.stl file available online as ESM) aligns optical fibres with a digital microfluidic device, allowing optical measurements to be made in the plane of the device. Because of the greater width vs thickness of a droplet on-device, the in-plane alignment of this technique allows it to outperform the sensitivity of vertical absorbance measurements on digital microfluidic (DMF) devices by ∼14×. The new system also has greater calibration sensitivity for thymol blue measurements than the popular NanoDrop system by ∼2.5×. The improvements in absorbance sensitivity result from increased path length, as well as from additional effects likely caused by liquid lensing, in which the presence of a water droplet between optical fibres increases fibre-to-fibre transmission of light by ∼2× through refraction and internal reflection. For interrogation of dilute samples, stretching of droplets using digital microfluidic electrodes and adjustment of fibre-to-fibre gap width allows absorbance path length to be changed on-demand. We anticipate this new digital microfluidic optical fibre absorbance and fluorescence measurement system will be useful for a wide variety of analytical applications involving microvolume samples with digital microfluidics.

Integration of World-to-Chip Interfaces with Digital Microfluidics for Bacterial Transformation and Enzymatic Assays

“…One of the fundamental applications of DMF is the implementation of enzymatic reactions. ,− The protocol typically consists of precise metering of reactants by dispensing droplets from reservoirs and merging and mixing to create a droplet that represents the microreactor. The microreactors on the DMF device have been analyzed using integrated in-line detectors ,, or with offline detectors (e.g., fluorescence-based plate readers). ,, However, most enzymatic assays on DMF are either performed at room temperature or without refilling of reservoirs for dispensing droplets. ,, These corresponding factors give rise to two limitations: (1) prevents the study of most industrial-based and thermoresistant enzymes, which are active at higher temperatures (>25 °C), and (2) prevents the study of different conditions on the device that requires droplets (containing substrate enzyme, or buffer) to be dispensed multiple times. These limitations motivate the development of our world-to-chip for digital microfluidics for the implementation of enzymatic assays that require heating and testing of multiple conditions in parallel.…”

“…20,45−48 The protocol typically consists of precise metering of reactants by dispensing droplets from reservoirs and merging and mixing to create a droplet that represents the microreactor. The microreactors on the DMF device have been analyzed using integrated in-line detectors 19,49,50 or with offline detectors (e.g., fluorescence-based plate readers). 5,14,51 However, most enzymatic assays on DMF are either performed at room temperature or without refilling of reservoirs for dispensing droplets.…”

Quantitative Affinity Determination by Fluorescence Anisotropy Measurements of Individual Nanoliter Droplets

“…In addition, the FA signals from the lysates were found to be highly specific to HumRadA-BRC4 fl interactions, potentially paving the way for diagnostic applications from more complex sample matrices (e.g., bodily fluids). 25 The ability to determine precise FA signals in single droplets also provides the basis for coupling detection with library selections in formats in which droplets cocompartmentalize genotype and phenotype. 26 Anisotropy detection would expand the range of assays that can be used for library selections in protein engineering by directed evolution 27 or metagenomic screening, 8c to enable not only assays that lead to production of fluorophores (e.g., as leaving groups 8b ), but also, assays that instead detect size changes by fluorescence anisotropy (e.g., for enzymatic breakdown of macromolecular targets by proteases of glycosidases).…”

“…We showed that extracting the two key parameters, K d and level of protein expression, was possible via acquisition of just two high-resolution titrations. In addition, the FA signals from the lysates were found to be highly specific to HumRadA-BRC4 fl interactions, potentially paving the way for diagnostic applications from more complex sample matrices (e.g., bodily fluids) . The ability to determine precise FA signals in single droplets also provides the basis for coupling detection with library selections in formats in which droplets cocompartmentalize genotype and phenotype .…”

UV–Vis Spectra-Activated Droplet Sorting for Label-Free Chemical Identification and Collection of Droplets

“…To address this bottleneck, there has been tremendous progress in recent years in integrating droplet microfluidics with a number of label-free analytical measurements. ,,,− − ,− Notably, Holland-Moritz et al developed a mass spectrometry-activated droplet sorter that circumvents the sample-destructive nature of mass spectrometry by splitting droplets into two streams, analyzing a portion with electrospray ionization mass spectrometry in real-time, and sorting the partner droplet stream based on the mass spectra. While a very promising approach, its overall throughput of 0.7 samples per second leaves much to be desired for high-throughput screens.…”