Design and preparation of a simplified microdroplet generation device for nanoliter volume collection and measurement with liquid microjunction–surface sampling probe–mass spectrometry

Abstract

Given recent interest in laboratory automation and miniaturization, the microdroplet research space has expanded across research disciplines and sectors. In turn, the microdroplet field is continually evolving and seeking new methods to generate microdroplets, especially in ways that can be integrated into diverse (microfluidic) workflows. Herein, we present a convenient, low-cost, and re-usable microdroplet generation device, termed as the “NanoWand,” which enables microdroplet formation in the nanoliter volume range through modulated surface energy and roughness, that is, an open surface energy trap (oSET), using commercially available and readily assembled coating and substrate materials. A wand-like shape is excised from a microscope glass cover slip via laser-micromachining and rendered hydrophobic; a circle is then cut-out from the hydrophobically modified wand's tip using laser-micromachining to create the oSET. By adjusting the size of the oSET with laser-micromachining, the volume of the microdroplet can be similarly controlled. Using liquid microjunction–surface sampling probe–mass spectrometry (LMJ-SSP-MS), specific NanoWand droplet capture volumes were estimated to be 117 ± 23.6 nL, 198 ± 30.3 nL, and 277 ± 37.1 nL, corresponding to oSET diameters of 0.75, 1.00, and 1.25 mm, respectively. This simple approach provides a user-friendly way to form and transfer microdroplets that could be integrated into different liquid handling applications, especially when combined with the LMJ-SSP and ambient ionization MS as a powerful and rapid analytical tool.