Stable electrospray signal on a microfabricated glass chip with three-dimensional open edge and tiered depth geometries - PubMed (original) (raw)
Stable electrospray signal on a microfabricated glass chip with three-dimensional open edge and tiered depth geometries
Alexander J Schmidt et al. Microelectron Eng. 2023.
Abstract
This paper presents the microfabrication and performance of a three-dimensional electrospray ionization (ESI) emitter tip made from glass, which achieves stable current signals important for chemical analysis. Our novel microfabrication process and custom-built signal conditioning hardware provides the advantage of providing accurate features and steady signals. The fabrication process relies on standard microfabrication techniques (i.e., deposition, photolithography, and wet etching). This fabrication method involves the novel application of two layers of positive and negative photoresists in addition to Parafilm® wax tape. Open edge and tiered depth details were successfully created from a multilayer planar mask. This is a benefit for integrated miniaturized and microfluidic systems that often require micro features for their functionality but relatively large millimeter size features for their physical periphery. We demonstrate the fundamental performance of electrospray with this microfluidic chip. The emitter tip was fixed on a linear axis stage with high resolution (10 μm) to finely control the tip distance from a metal counter electrode plate. A custom printed circuit board system was built to safely control four voltages applied to the microchip ports from a single high voltage power supply. To readily form the electrospray, non-aqueous solvents were used for their low viscosity and a constant voltage of +2.7 kV was applied to the sheath electrospray microchannel. The liquid being sprayed was 80/20 (v/v) methanol/acetonitrile with 0.1% acetic acid in the sheath microchannel and with ammonium acetate (10-40 mM) in its remaining microchannels. The electrospray signal was measured in response to varying the distance (1.4 to 1.6 mm) between the electrospray emitter tip and a metal counter electrode plate in addition to the varying concentration of the background electrolyte, ammonium acetate. Stable and repeatable electrospray signal showed linear relationships with emitter tip distance and concentration (r2≥0.95).
Keywords: electrospray ionization; electrospray stability; glass etching; glass microfabrication; microfluidics; miniaturization.
Conflict of interest statement
Conflicts of Interest The authors declare no competing financial interest. The electrical schematics and LabVIEW© code for data acquisition from the system is available on GitHub for non-commercial use. Please refer to Professor Cristina Davis’ webpage for more information.
Figures
Figure 1.
(a) Schematic of the microfluidic chip layout. SI: sample inlet, SO: sample outlet, BI: buffer inlet, SLI: sheath liquid inlet. All microchannels have a depth of 100 μm. The buffer microchannel (BI to ESI tip) has a length of 25 mm and a width of 20 μm. The sample loading microchannel (SI to SO) has a length of 10 mm and a width of 20 μm. The SLI microchannel has a length of 10.8 mm and a width of 200 μm. (b) Photograph of the microfluidic chip with an integrated electrospray emitter tip, held by a tweezer after dicing. (c) Scanning electron microscope (SEM) image of drill-free reservoir port. (d) Top-down view SEM image of the ESI emitter tip. (e) Side view photo of the ESI emitter tip.
Figure 2.
Summary and illustration of the microfabrication process. The microfabrication process for the access ports wafer is excluded due to redundancy. 1. Start with new glass wafer and sputter chromium (Cr) both sides. 2. Apply SPR 220–7 on top side, S1813 on bottom side. UV exposure to define ESI tip edge on top side. UV exposure to define alignment marks on bottom side. Develop both sides. Etch Cr both sides. 3. Apply Parafilm® tape on bottom side. 4. Etch glass in HF and remove both photoresists and Parafilm® tape. Alignment marks remain on bottom side. 5. Apply SU8 3010 on bottom side. UV exposure to define microchannels on bottom side (alignment marks not illustrated for simplification). Develop SU8 3010 on bottom side. 6. Apply SPR 220–7 on bottom side. UV exposure to define ESI tip and alignment marks (not shown) on bottom side. Develop SPR 220–7. 7. Etch Cr and etch glass in HF. Remove SPR 220–7. 8. Etch Cr and etch glass in HF with Parafilm® applied on bottom side. 9. Remove SU8 3010 and Parafilm®. Etch Cr both sides. 10. Surface plasma activation and thermal fusion bonding of the access ports wafer and the microchannels wafer.
Figure 3.
(a) Expanded microchip fixture assembly consisting of the linear stage, the microchip and its top and bottom fixtures, the Pt electrode fixture, and counter electrode plate. (b) Electronics for the control of four voltages from a single high voltage source. (c) Photograph of the experimental setup. 1. National Instruments connector block. 2. Digital microscope displaying a top-down view of the glass emitter tip. 3. National Instruments Chassis. 4. DC power supply for a vacuum pump (not shown). 5. High voltage power supply. 6. Microchip fixture and Pt electrode fixture. 7. High voltage control electronics. 8. Linear stage. 9. Signal conditioning electronics.
Figure 4.
(a) Digital microscope image with a top-down view of the glass microfluidic chip emitter tip. (b) Digital microscope image with a top-down view of the electrospray Taylor cone generated at the glass emitter tip. The liquid being sprayed was 80/20 (v/v) methanol/acetonitrile with 0.1% acetic acid from the Sheath Liquid Interface (SLI) reservoir and 80/20 (v/v) methanol/acetonitrile with ammonium acetate at a concentration of 20 mM from the remaining three reservoirs.
Figure 5.
Signal response of electrospray ionization to the distance between the electrospray emitter tip and the metal counter electrode plate. The liquid being sprayed was 80/20 (v/v) methanol/acetonitrile with 0.1% acetic acid from the Sheath Liquid Inlet (SLI) reservoir and 80/20 (v/v) methanol/acetonitrile with ammonium acetate at a concentration of 20 mM from the remaining three reservoirs. A constant voltage of +2.7 kV was applied to the SLI reservoir to perform electrospray.
Figure 6.
(a) Signal response of electrospray ionization to the distance between the electrospray emitter tip and the metal counter electrode plate. The liquid being sprayed was 80/20 (v/v) methanol/acetonitrile with 0.1% acetic acid from the Sheath Liquid Inlet (SLI) reservoir and 80/20 (v/v) methanol/acetonitrile with ammonium acetate at a concentration of 20 mM from the remaining three reservoirs. A constant voltage of +2.7 kV was applied to the SLI reservoir to perform electrospray. Data are averaged over 1 min durations for n=5 replicates. (b) Signal response of electrospray ionization to the concentration of ammonium acetate at an emitter distance of 1600 μm. The liquid being sprayed, and voltage applied to the SLI reservoir were the same as was done in caption (a) with varying concentrations. Data are averaged over 1 min durations for n=5 replicates.
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