Microanalysis Using Acoustically Actuated Droplets Pinned Onto a Thread

Tzu-Hsien Yang,Pawel L Urban,Hao-Chun Yang,Gurpur Rakesh D Prabhu,Cheng-Hao Chang

doi:10.1109/access.2019.2948642

Tzu-Hsien Yang, Pawel L Urban + Show 3 more

Open Access

https://doi.org/10.1109/access.2019.2948642

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Abstract

An acoustic wave-assisted microscale assay platform is demonstrated. Liquid droplets containing samples and reagents are dispensed and pinned onto a hydrophobic thread, and actuated by sound waves. The transport of droplets is achieved by modulating sound emitted by two sources. The resulting sound intensity gradient pushes the droplets in one direction. The available distance of droplet actuation is related to droplet size. The moving droplets merge at the end of their actuation range, and the merged droplet travels further due to its greater size. Fluorometric analysis of the merged droplet is accomplished by positioning an excitation light source above a defined interrogation zone along the thread. The emitted light is analyzed by a miniature spectrometer positioned orthogonally to the excitation light beam. The sound sources and detection system are controlled by universal electronic modules (Raspberry Pi, Arduino). The system is operated automatically with the aid of a custom-developed graphical user interface. Here, we demonstrate its application in enzyme assays (trypsin, β-galactosidase). In-droplet enzymatic reaction progress curves are readily obtained. The actuated droplets can be put in motion and stopped without a significant delay. Only microliter-range volumes of sample and reagent are consumed in every analysis, while sub-milliliter volumes of chemical waste are produced.

Highlights

In conventional fluorometric assays, 2-3 mL sample volumes are normally required in order to fill up a standard quartz cuvette, and enable excitation of fluorophores
Microtiter plates, flow injection systems, or microfluidic chips, the samples are always surrounded by solid walls, and they assume the shape of the vessel or channel
It was previously shown that the potential of the acoustic levitation increases with the diameter of the levitated droplet [16]

Summary

INTRODUCTION

2-3 mL sample volumes are normally required in order to fill up a standard quartz cuvette, and enable excitation of fluorophores. There is a need to decrease both sample and reagent volumes in fluorometric analysis. Downscaling chemical assays is typically achieved with the aid of microtiter plates. In this case, sample volumes are in the order of tens of microliters [1]. Microtiter plates, flow injection systems, or microfluidic chips, the samples are always surrounded by solid walls, and they assume the shape of the vessel or channel. Many organic compounds (e.g. biomolecules) tend to adsorb on solid surfaces, including glass [5]. For diffusion to play a role in mixing, the surface-to-volume ratio of a liquid aliquot has to be minimized. Access to tools for performing reaction and detection in liquid droplets suspended in the air is limited. We demonstrate a facile approach for performing chemical reactions in levitating droplets moving along an inert hydrophobic thread positioned horizontally (Fig. 1)

EXPERIMENTAL

RESULTS AND DISCUSSION

CONCLUSION