Julie Probst, a final-year Ph.D. student in the deMello group at ETH Zürich, has shown for the first time that broad-band absorption spectra can be extracted with high sensitivity from picoliter-volume droplets at high speed.
Despite the obvious attractions of using absorbance-based detection schemes in microfluidic systems, they are rarely used. The primary reason for this stems from the inherently small optical pathlengths representative of microfluidic systems (only a few tens of microns). Put simply, absorbance is directly proportional to the distance that the light travels through the sample, and thus absorbance-based schemes typically offer inferior detection limits to many other optical detection methods. Some in the field have circumvented this limitation via extended path-length geometries and cavity enhancements, but these are complex, almost always limited to single wavelength absorption measurements and operate at low throughput.
The system developed by Julie and her colleagues uses a combination of confocal illumination (a technique able to confine incident light into a narrow volume that matches droplet volumes), a fast spectrometer and a post-processing algorithm to achieve high signal-to-noise ratios and remove spectral contributions from the continuous phase. Thanks to these innovations, Julie’s platform can measure nM-µM concentrations of small molecule chromophores in single picoliter droplets. Additionally, she has applied her system to probing the kinetics of salt-induced gold nanoparticle aggregation through changes in broad-band signals, verifying theoretical predictions with experimentally measured data.
In the future, it is expected that the detection technology will be a particularly useful tool for performing label-free broadband kinetic measurements and protein engineering.
Written by Ankit Jain
Read the full paper here