Microfluidic devices are advantageous for a wide variety of chemical and biological applications because they offer a way to manipulate small sample volumes, thus decreasing cost of potentially expensive reagents and enabling high throughput by harnessing the unique properties of fluids on the microscale.¹ 3D printing of microfluidic chips has sparked tremendous interest because commercial bench-top printers are becoming commonplace at reasonably low cost and devices can be made outside of a cleanroom, thus opening the field for more widespread development.² 3D printing has a distinct advantage over photolithography when fabricating chips with complex channel geometries and internal voids, because these types of devices can be fabricated in a single-step due to bottom-up, layer-by-layer nature of the technique.³

One of the challenges for microfluidics is the integration between the microfluidic fluid handling with a suitable analysis technique that can be performed “on chip”. For this reason, many analysis techniques used in microfluidics employ optical-based detection schemes using colorimetric or fluorescent labels, because light can easily pass through the microfluidic substrate. In addition, there are no additional fabrication steps required in order to make chips that can perform on chip analyses using these techniques. However, when coupling capillary electrophoresis or electrochemistry on chip, integrating the detection with the fluid handling can be a challenge. In order to integrate electrodes into microfluidic devices, one of the following approaches are typically utilized: epoxying metal wires or mesh into a flow channel,⁴ potting metallic wires in microfluidic fittings that are interfaced using screws and ferrules,⁵ or by evaporation of metal films (or carbon bands) onto flat wafers followed by bonding with the PDMS chip.^6,7

Here we present the single-step fabrication of monolithic microfluidic devices with embedded composite-carbon electrodes suitable for electrochemical detection of a variety of chemical species prepared by 3D printing. In this approach, we employ a dual-extrusion fused deposition modeling printer is used in order to simultaneously print the microfluidic body and electrodes, which eliminates multiple steps in the fabrication and greatly simplifies device manufacturing. We demonstrate the utility of this approach using two exemplar microfluidic systems for hydrodynamic electrochemistry: in a simple channel flow configuration and for the detection of water-in-oil droplets. We show that the carbon electrodes employed here may be modified for different applications by electrodepositing metals such as platinum or gold on the carbon electrode surface in situ. We envision these devices to have broad applications for chemical and biological sensing.

References

(1) Whitesides, G. M. The Origins and the Future of Microfluidics. Nature 2006, 442, 368–373.

(2) Gross, B. C.; Lockwood, S. Y.; Spence, D. M. Recent Advances in Analytical Chemistry by 3D Printing. Anal. Chem. 2017, 89, 57–70.

(3) Channon, R. B.; Joseph, M. B.; Macpherson, J. V. Additive Manufacturing for Electrochemical (Micro)Fluidic Platforms. Electrochem. Soc. Interface 2016, 25, 63–68.

(4) O’Neil, G. D.; Christian, C. D.; Brown, D. E.; Esposito, D. V. Hydrogen Production with a Simple and Scalable Membraneless Electrolyzer. J. Electrochem. Soc. 2016, 163, F3012–F3019.

(5) Bishop, G. W.; Satterwhite-warden, J. E.; Bist, I.; Chen, E.; Rusling, J. F. Electrochemiluminescence at Bare and DNA-Coated Graphite Electrodes in 3D-Printed Fluidic Devices. ACS Sensors 2016, 1, 197–202.

(6) Anderson, M. J.; Crooks, R. M. Microfluidic Surface Titrations of Electroactive Thin Films. Langmuir 2017, 10.1021/acs.langmuir.7b01542.

(7) Channon, R. B.; Joseph, M. B.; Bitziou, E.; Bristow, A. W. T.; Ray, A. D.; Macpherson, J. V. Electrochemical Flow Injection Analysis of Hydrazine in an Excess of an Active Pharmaceutical Ingredient: Achieving Pharmaceutical Detection Limits Electrochemically. Anal. Chem. 2015, 87 (19), 10064–10071.