In Situ Chemical Monitoring and Imaging of Contents within Microfluidic Devices Having a Porous Membrane Wall Using Liquid Microjunction Surface Sampling Probe Mass Spectrometry

Abstract

The ability to observe dynamic chemical processes (e.g., signaling, transport, etc.) in vivo or in situ using nondestructive chemical imaging opens a new door to understanding the complex dynamics of developing biological systems. With the advent of “biology-on-a-chip” devices has come the ability to monitor dynamic chemical processes in a controlled environment, using these engineered habitats to capture key features of natural systems while allowing visual observation of system development. Having the capability to spatially and temporally map the chemical signals within these devices may yield new insights into the forces that drive biosystem development. Here, a porous membrane sealed microfluidic device was designed to allow normal microfluidic operation while enabling continuous, location specific sampling and chemical characterization by liquid microjunction surface sampling probe mass spectrometry (LMJ-SSP MS). LMJ-SSP was used to extract fluids with nL-to-μL/min flow rates directly from selected areas of the microfluidic device without negatively impacting the device function. These extracts were subsequently characterized using MS. This technique was used to acquire MS images of the entirety of several multi-input microfluidic devices having different degrees of fluid mixing. LMJ-SSP MS imaging visualized the spatial distribution of chemical components within the microfluidic channels and could visualize chemical reactions occurring in the device. These microfluidic devices with a porous membrane wall are wholly compatible with the construction of biology-on-a-chip devices. This ultimately would enable correlation of biosystem physical structure with an evolving chemical environment

Hotspots of root-exuded amino acids are created within a rhizosphere-on-a-chip

“…The LMJ-SSP-MS system used here was previously described by Cahill, et al 18 The probe consists of three coaxially aligned capillaries. The outer capillary consisted of a stainless-steel tube (∼0.686 mm i.d.…”

“…17 Device assembly for exudate sampling Microfluidic devices were attached to polyester track-etched (PETE) membranes (0.4 μm pore size, 12 μm thickness, 2 × 10 6 pores per cm 2 , 90 mm diameter, Sterlitech Corp.) to analyse the amino acids exuded from Brachypodium roots using LMJ-SSP-MS. To bond the PETE membrane, a published wet stamping method was used with minor modifications. 5,18 A clean silicon wafer was placed on a spin coater with 10 : 1 PDMS to curing agent and was spun at 5000 rpm for 15 minutes to create an approximately 6 μm thick layer of PDMS film. The rhizosphere-on-a-chip PDMS device was laid, design side down, onto the wet PDMS layer, gently peeled off, then pressed against the PETE membrane, taking care to avoid wrinkling the membrane.…”

“…The LMJ-SSP-MS system used here was previously described by Cahill, et al 18 The probe consists of three coaxially aligned…”

“…To experimentally validate the formation of exudate hotspots within the rhizosphere-on-a-chip, we used an established liquid micro-junction surface sampling probe mass spectrometry (LMJ-SSP-MS) method to spatially sample the chemical microenvironment. 18 After germinating Brachypodium seeds overnight, three seedlings were transferred individually to three rhizosphere-on-a-chips as previously described and grown for 12 days. Two hours prior to sampling, Optima™ LC/MS grade water was added to the device inlets to wash away accumulated exudates and re-fill the pore spaces.…”

Spatially resolved absolute quantitation in thin tissue by mass spectrometry

“…On-tissue LE methods can be adapted for MS imaging without much issue. Since the LMJ-SSP is a continuous-flow system, the LMJ-SSP can collect MS images by simply rastering the probe across the sample surface [145][146][147]. However, analyte of LXY in the whole-body section.…”