Integrated microlenses and micromirrors for high-throughput, high-sensitivity fluorescence detection

A novel platform integrating micro-fabricated optical components enables highly parallelized single droplet analysis with exquisite sensitivity

A current challenge in the field of droplet microfluidics lies in the transition from bulky and complex optical detection systems to miniaturized, portable devices, whilst retaining the high sensitivity required for precise chemical and biological assays. To this end, Dr. Xiaobao Cao from the deMello group has developed a novel optofluidic approach where miniaturized optical components – mirrors and lenses – are directly integrated around the fluidic channel allowing ultrahigh throughput droplet analysis with high sensitivity.

Optofluidics is a field that aims to combine optical detection components with microfluidics by directly incorporating both into integrated devices. This is an important step towards miniaturization and is necessary to move analysis platforms away from the lab and into smaller and lower-cost devices. Fluorescence detection, which relies on the collection of light emitted by a sample following excitation, is a gold-standard analysis tool in droplet microfluidics due to its high sensitivity and versatility. However, when concentrations are extremely low, highly sensitive optical set-ups and detectors are usually required, which are bulky and costly, and limit parallelization. The challenge for miniaturization is double: optical components are needed to efficiently guide the excitation light onto the microfluidic channel, but also to collect the maximum number of photons emitted by fluorescence and guide them toward the detector.

In this work, Xiaobao and his collaborators have designed a novel optofluidic platform that integrates microlens and micromirror units positioned on opposites sides of a microfluidic channel. The microlenses ensure precise delivery of the excitation light into the flowing droplets, while the mirrors efficiently collect the emitted photons and reflect them toward the detector. With this approach, the amount of fluorescence light collected from flowing droplets is greatly enhanced, by two orders of magnitude, even for ultra-fast droplets. This system brings another key advantage by enabling extensive parallelization. Indeed, because the optical signal is enhanced at the scale of each droplet, it is possible to use a lower magnification objective with a larger field of view, so many parallel microchannels can be imaged simultaneously. The authors demonstrate the utility of this platform by performing a protein expression assay at the single cell level, a challenge known to necessitate both fast and sensitive methods. Here, they assayed the activity of adenylate kinase from a single-cell bacterial encapsulation, with a throughput of 40 000 droplets per second.

This work highlights the effectiveness of micro-fabricated optical components to achieve high-sensitivity and high-throughput optofluidic platforms that can be successfully used for droplet analysis with single-cell capability.

Written by Julie Probst
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