Flexible Laser-Induced Graphene for Nitrogen Sensing in Soil

Flexible graphene electronics are rapidly gaining interest, but their widespread implementation has been impeded by challenges with ink preparation, ink printing, and postprint annealing processes. Laser-induced graphene (LIG) promises a facile alternative by creating flexible graphene electronics on polyimide substrates through the one-step laser writing fabrication method. Herein, we demonstrate the use of LIG, created with a low-cost UV laser, for electrochemical ion-selective sensing of plant-available nitrogen (i.e., both ammonium and nitrate ions: NH₄⁺ and NO₃^-) in soil samples. The laser used to create the LIG was operated at distinct pulse widths (10, 20, 30, 40, and 50 ms) to maximize the LIG electrochemical reactivity. Results illustrated that a laser pulse width of 20 ms led to a high percentage of sp² carbon (77%) and optimal peak oxidation current of 120 μA during cyclic voltammetry of ferro/ferricyanide. Therefore, LIG electrodes created with a 20 ms pulse width were consequently functionalized with distinct ionophores specific to NH₄⁺ (nonactin) or NO₃^- (tridodecylmethylammonium nitrate) within poly(vinyl chloride)-based membranes to create distinct solid contact ion-selective electrodes (SC-ISEs) for NH₄⁺ and NO₃^- ion sensing, respectively. The LIG SC-ISEs displayed near Nernstian sensitivities of 51.7 ± 7.8 mV/dec (NH₄⁺) and -54.8 ± 2.5 mV/dec (NO₃^-), detection limits of 28.2 ± 25.0 μM (NH₄⁺) and 20.6 ± 14.8 μM (NO₃^-), low long-term drift of 0.93 mV/h (NH₄⁺ sensors) and -5.3 μV/h (NO₃^- sensors), and linear sensing ranges of 10^-5-10^-1 M for both sensors. Moreover, soil slurry sensing was performed, and recovery percentages of 96% and 95% were obtained for added NH₄⁺ and NO₃^-, respectively. These results, combined with a facile fabrication that does not require metallic nanoparticle decoration, make these LIG electrochemical sensors appealing for a wide range of in-field or point-of-service applications for soil health management.

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.

Enhanced enzymatic activity from phosphotriesterase trimer gold nanoparticle bioconjugates for pesticide detection

The rapid detection of organophosphates (OPs), a class of strong neurotoxins, is critically important for monitoring acute insecticide exposure and potential chemical warfare agent use. Herein, we improve the enzymatic activity of a phosphotriesterase trimer (PTE₃), an enzyme that selectively recognizes OPs directly, by conjugation with distinctly sized (i.e., 5, 10, and 20 nm diameter) gold nanoparticles (AuNPs). The number of enzymes immobilized on the AuNP was controlled by conjugating increasing molar ratios of PTE₃ onto the AuNP surface via metal affinity coordination. This occurs between the PTE₃-His₆ termini and the AuNP-displayed Ni²⁺-nitrilotriacetic acid end groups and was confirmed with gel electrophoresis. The enzymatic efficiency of the resultant PTE₃-AuNP bioconjugates was analyzed via enzyme progress curves acquired from two distinct assay formats that compared free unbound PTE₃ with the following PTE₃-AuNP bioconjugates: (1) fixed concentration of AuNPs while increasing the bioconjugate molar ratio of PTE₃ displayed around the AuNP and (2) fixed concentration of PTE₃ while increasing the bioconjugate molar ratio of PTE₃-AuNP by decreasing the AuNP concentration. Both assay formats monitored the absorbance of p-nitrophenol that was produced as PTE₃ hydrolyzed the substrate paraoxon, a commercial insecticide and OP nerve agent simulant. Results demonstrate a general equivalent trend between the two formats. For all experiments, a maximum enzymatic velocity (V_max) increased by 17-fold over free enzyme for the lowest PTE₃-AuNP ratio and the largest AuNP (i.e., ratio of 1 : 1, 20 nm dia. AuNP). This work provides a route to improve enzymatic OP detection strategies with enzyme-NP bioconjugates.

A Soft Wearable Microfluidic Patch with Finger-Actuated Pumps and Valves for On-Demand, Longitudinal, and Multianalyte Sweat Sensing

Easy sample collection, physiological relevance, and ability to noninvasively and longitudinally monitor the human body are some of the key attributes of wearable sweat sensors. Examples typically include reversible sensors or an array of single-use sensors embedded in specialized microfluidics for temporal analysis of sweat. However, evolving this field to a level that truly represents "lab-on-skin" technology will require the incorporation of advanced functionalities that give the user the freedom to (1) choose the precise time for performing sample analysis and (2) select sensors from an array embedded within the device for performing condition-specific sample analysis. Here, we introduce new concepts in wearable microfluidic platforms that offer such capabilities. The described technology involves a series of finger-actuated pumps, valves, and sensors incorporated within soft, wearable microfluidics. The incoming sweat collects in the inlet chamber and can be analyzed by the user at the time of their choosing. On-demand sweat analyte assessment is achieved by pulling a thin tab to activate a pump which opens a valve and allows the pooled sweat to enter a chamber embedded with sensors for the desired analytes. The article describes a thorough characterization of the platform that demonstrates the robustness of the pumping, valving, and sensing aspects of the device under conditions mimicking real-life scenarios. A two-day-long human pilot study validates the system and illustrates the device's ability to offer on-demand, longitudinal, and multianalyte sensing. Our work represents the first example of a wearable system with such on-demand sensing capabilities and opens exciting avenues in sweat sensing for acquiring new insights into human physiology.

A simple method for quantifying biomass cell and polymer distribution in biofilms

Biofilms are ubiquitous and play an essential role in both environmental processes and hospital infections. Standard methods are not capable of quantifying biomass concentration in dilute suspensions. Furthermore, standard techniques cannot differentiate biomass composition. In this study, a user-friendly technique was developed for measuring biomass cell and polymer content in detached biofilms using a standard coulter counter. The method was demonstrated for an environmentally relevant strain of Pseudomonas aeruginosa (Schroeter) Migula grown in a bioreactor and also for a medically relevant strain of P. aeruginosa (PAO1) grown on standard growth pegs. Results were compared and validated by standard assays, including EPA method 1684 for measuring biomass, microscopic direct counts, and a crystal violet staining assay. The minimum detection limit for the coulter counter method (0.07 mg-biomass L(-1)) was significantly lower than the EPA method 1684 (1.9 ± 0.4 mg/L) and the crystal violet assay (1.1 ± 0.2 mg L(-1)). However, the coulter counter method is limited to dilute biomass samples (below 204 ± 16 mg L(-1)) due to clogging of the aperture tube. While biomass measurements are useful, the major advantage of the coulter counter method is the ability to directly determine EPS, cell, and aggregate fractions after mild chemical treatment. The rapid technique (4-5 min per sample) was used to measure biomass fractions in dispersed P. aeruginosa (Schroeter) and PAO1 biofilms. This technique will be critical for understanding biofilm formation/dispersal.

A Miniaturized, Battery-Free, Wireless Wound Monitor That Predicts Wound Closure Rate Early

Diabetic foot ulcers are chronic wounds that affect millions and increase the risk of amputation and mortality, highlighting the critical need for their early detection. Recent demonstrations of wearable sensors enable real-time wound assessment, but they rely on bulky electronics, making them difficult to interface with wounds. Herein, a miniaturized, wireless, battery-free wound monitor that measures lactate in real-time and seamlessly integrates with bandages for conformal attachment to the wound bed is introduced. Lactate is selected due to its multifaceted role in initiating healing. Studies in healthy and diabetic mice reveal distinct lactate profiles for normal and impaired healing wounds. A mathematical model based on the sensor data predicts wound closure rate within the first 3 days post-injury with ≈76% accuracy, which increases to ≈83% when pH is included. These studies underscore the significance of monitoring biomarkers during the inflammation phase, which can offer several benefits, including short-term use of wound monitors and their easy removal, resulting in lower risks of injury and infection at the wound site. Improvements in prediction accuracy can be achieved by designing mathematical models that build on multiple wound parameters such as pro-inflammatory and metabolic markers. Achieving this goal will require designing multi-analyte wound monitors.

Biofluid-Activated Biofuel Cells, Batteries, and Supercapacitors: A Comprehensive Review

Recent developments in wearable and implanted devices have resulted in numerous, unprecedented capabilities that generate increasingly detailed information about a user's health or provide targeted therapy. However, options for powering such systems remain limited to conventional batteries which are large and have toxic components and as such are not suitable for close integration with the human body. This work provides an in-depth overview of biofluid-activated electrochemical energy devices, an emerging class of energy sources judiciously designed for biomedical applications. These unconventional energy devices are composed of biocompatible materials that harness the inherent chemistries of various biofluids to produce useable electrical energy. This work covers examples of such biofluid-activated energy devices in the form of biofuel cells, batteries, and supercapacitors. Advances in materials, design engineering, and biotechnology that form the basis for high-performance, biofluid-activated energy devices are discussed. Innovations in hybrid manufacturing and heterogeneous integration of device components to maximize power output are also included. Finally, key challenges and future scopes of this nascent field are provided.

Wearable Flexible Perspiration Biosensors Using Laser-Induced Graphene and Polymeric Tape Microfluidics

Wearable biosensors promise real-time measurements of chemicals in human sweat, with the potential for dramatic improvements in medical diagnostics and athletic performance through continuous metabolite and electrolyte monitoring. However, sweat sensing is still in its infancy, and questions remain about whether sweat can be used for medical purposes. Wearable sensors are focused on proof-of-concept designs that are not scalable for multisubject trials, which could elucidate the utility of sweat sensing for health monitoring. Moreover, many wearable sensors do not include the microfluidics necessary to protect and channel consistent and clean sweat volumes to the sensor surface or are not designed to be disposable to prevent sensor biofouling and inaccuracies due to repeated use. Hence, there is a need to produce low-cost and single-use wearable sensors with integrated microfluidics to ensure reliable sweat sensing. Herein, we demonstrate the convergence of laser-induced graphene (LIG) based sensors with soft tape polymeric microfluidics to quantify both sweat metabolites (glucose and lactate) and electrolytes (sodium) for potential hydration and fatigue monitoring. Distinct LIG-electrodes were functionalized with glucose oxidase and lactate oxidase for selective sensing of glucose and lactate across physiological ranges found in sweat with sensitivities of 26.2 and 2.47 × 10-3 μA mM-1 cm-2, detection limits of 8 and 220 μM, and linear response ranges of 0-1 mM and 0-32 mM, respectively. LIG-electrodes functionalized with a sodium-ion-selective membrane displayed Nernstian sensitivity of 58.8 mV decade-1 and a linear response over the physiological range in sweat (10-100 mM). The sensors were tested in a simulated sweating skin microfluidic system and on-body during cycling tests in a multisubject trial. Results demonstrate the utility of LIG integrated with microfluidics for real-time, continuous measurements of biological analytes in sweat and help pave the way for the development of personalized wearable diagnostic tools.