Hydrophobic laser-induced graphene potentiometric ion-selective electrodes for nitrate sensing.

Current solid-contact ion-selective electrodes (ISEs) suffer from signal-to-noise drift and short lifespans partly due to water uptake and the development of an aqueous layer between the transducer and ion-selective membrane. To address these challenges, we report on a nitrate ISE based on hydrophobic laser-induced graphene (LIG) coated with a poly(vinyl) chloride-based nitrate selective membrane. The hydrophobic LIG was created using a polyimide substrate and a double lasing process under ambient conditions (air at 23.0 ± 1.0 °C) that resulted in a static water contact angle of 135.5 ± 0.7° (mean ± standard deviation) in wettability testing. The LIG-ISE displayed a Nernstian response of - 58.17 ± 4.21 mV dec-1 and a limit-of-detection (LOD) of 6.01 ± 1.44 µM. Constant current chronopotentiometry and a water layer test were used to evaluate the potential (emf) signal stability with similar performance to previously published work with graphene-based ISEs. Using a portable potentiostat, the sensor displayed comparable (p > 0.05) results to a US Environmental Protection Agency (EPA)-accepted analytical method when analyzing water samples collected from two lakes in Ames, IA. The sensors were stored in surface water samples for 5 weeks and displayed nonsignificant difference in performance (LOD and sensitivity). These results, combined with a rapid and low-cost fabrication technique, make the development of hydrophobic LIG-ISEs appealing for a wide range of long-term in situ surface water quality applications.

Tuning the Structure, Conductivity, and Wettability of Laser-Induced Graphene for Multiplexed Open Microfluidic Environmental Biosensing and Energy Storage Devices.

The integration of microfluidics and electrochemical cells is at the forefront of emerging sensors and energy systems; however, a fabrication scheme that can create both the microfluidics and electrochemical cells in a scalable fashion is still lacking. We present a one-step, mask-free process to create, pattern, and tune laser-induced graphene (LIG) with a ubiquitous CO2 laser. The laser parameters are adjusted to create LIG with different electrical conductivity, surface morphology, and surface wettability without the need for postchemical modification. Such definitive control over material properties enables the creation of LIG-based integrated open microfluidics and electrochemical sensors that are capable of dividing a single water sample along four multifurcating paths to three ion selective electrodes (ISEs) for potassium (K+), nitrate (NO3-), and ammonium (NH4+) monitoring and to an enzymatic pesticide sensor for organophosphate pesticide (parathion) monitoring. The ISEs displayed near-Nernstian sensitivities and low limits of detection (LODs) (10-5.01 M, 10-5.07 M, and 10-4.89 M for the K+, NO3-, and NH4+ ISEs, respectively) while the pesticide sensor exhibited the lowest LOD (15.4 pM) for an electrochemical parathion sensor to date. LIG was also specifically patterned and tuned to create a high-performance electrochemical micro supercapacitor (MSC) capable of improving the power density by 2 orders of magnitude compared to a Li-based thin-film battery and the energy density by 3 orders of magnitude compared to a commercial electrolytic capacitor. Hence, this tunable fabrication approach to LIG is expected to enable a wide range of real-time, point-of-use health and environmental sensors as well as energy storage/harvesting modules.

All-graphene-based open fluidics for pumpless, small-scale fluid transport via laser-controlled wettability patterning.

Open microfluidics have emerged as a low-cost, pumpless alternative strategy to conventional microfluidics for delivery of fluid for a wide variety of applications including rapid biochemical analysis and medical diagnosis. However, creating open microfluidics by tuning the wettability of surfaces typically requires sophisticated cleanroom processes that are unamenable to scalable manufacturing. Herein, we present a simple approach to develop open microfluidic platforms by manipulating the surface wettability of spin-coated graphene ink films on flexible polyethylene terephthalate via laser-controlled patterning. Wedge-shaped hydrophilic tracks surrounded by superhydrophobic walls are created within the graphene films by scribing micron-sized grooves into the graphene with a CO2 laser. This scribing process is used to make superhydrophobic walls (water contact angle ∼160°) that delineate hydrophilic tracks (created through an oxygen plasma pretreatment) on the graphene for fluid transport. These all-graphene open microfluidic tracks are capable of transporting liquid droplets with a velocity of 20 mm s-1 on a level surface and uphill at elevation angles of 7° as well as transporting fluid in bifurcating cross and tree branches. The all-graphene open microfluidic manufacturing technique is rapid and amenable to scalable manufacturing, and consequently offers an alternative pumpless strategy to conventional microfluidics and creates possibilities for diverse applications in fluid transport.