Electrochemical Glucose Sensors Enhanced by Methyl Viologen and Vertically Aligned Carbon Nanotube Channels

Free-standing, vertically aligned carbon nanotubes (VACNTs) were patterned into 16 µm diameter microchannel arrays for flow-through electrochemical glucose sensing. Non-enzymatic sensing of glucose was achieved by the chemical reaction of glucose with methyl viologen (MV) at an elevated temperature and pH (0.1 M NaOH), followed by the electrochemical reaction of reduced-MV with the VACNT surface. The MV sensor required no functionalization (including no metal) and was able to produce on average 3.4 electrons per glucose molecule. The current density of the MV sensor was linear with both flow rate and glucose concentration. Challenges with interference chemicals were mitigated by operating at a low potential of -0.2 V vs. Ag/AgCl. As a comparison, enzymatic VACNT sensors with platinum nano-urchins were functionalized with glucose oxidase by covalent binding (EDC/NHS) or by polymer entrapment (PEDOT) and operated in phosphate buffered saline (PBS). With normalization by the overall cross-sectional area of the flow (0.713 cm2), the sensitivity of the MV, enzyme-in-solution, and covalent sensors were 45.93, 18.77, and 1.815 mA cm-2 mM-1, respectively. Corresponding limits of detection were 100, 194, and 311 nM glucose. The linear sensing ranges for the sensors were: 250 nM – 200 µM glucose for the MV sensor, 500 nM – 200 µM glucose for the enzyme-in-solution sensor, and 1 µM – 6 mM glucose for the covalent sensor. The flow cell and sensor cross-sectional area were scaled down (0.020 cm2) to enable detection from 200 µL of glucose with MV by flow injection analysis (FIA). The sensitivity of the small MV sensor was 5.002 mA cm-2 mM-1, with a limit of detection of 360 nM glucose and a linear range up to at least 150 µM glucose. The small MV sensor has the potential to measure glucose levels found in 200 µL of saliva

Printed Graphene Electrochemical Biosensors Fabricated by Inkjet Maskless Lithography for Rapid and Sensitive Detection of Organophosphates

Solution phase printing of graphene-based electrodes has recently become an attractive low-cost, scalable manufacturing technique to create in-field electrochemical biosensors. Here, we report a graphene-based electrode developed via inkjet maskless lithography (IML) for the direct and rapid monitoring of triple-O linked phosphonate organophosphates (OPs); these constitute the active compounds found in chemical warfare agents and pesticides that exhibit acute toxicity as well as long-term pollution to soils and waterways. The IML-printed graphene electrode is nano/microstructured with a 1000 mW benchtop laser engraver and electrochemically deposited platinum nanoparticles (dia. ∼25 nm) to improve its electrical conductivity (sheet resistance decreased from ∼10 000 to 100 Ω/sq), surface area, and electroactive nature for subsequent enzyme functionalization and biosensing. The enzyme phosphotriesterase (PTE) was conjugated to the electrode surface via glutaraldehyde cross-linking. The resulting biosensor was able to rapidly measure (5 s response time) the insecticide paraoxon (a model OP) with a low detection limit (3 nM), and high sensitivity (370 nA/μM) with negligible interference from similar nerve agents. Moreover, the biosensor exhibited high reusability (average of 0.3% decrease in sensitivity per sensing event), stability (90% anodic current signal retention over 1000 s), longevity (70% retained sensitivity after 8 weeks), and the ability to selectively sense OP in actual soil and water samples. Hence, this work presents a scalable printed graphene manufacturing technique that can be used to create OP biosensors that are suitable for in-field applications as well as, more generally, for low-cost biosensor test strips that could be incorporated into wearable or disposable sensing paradigms

A triangular three-dye DNA switch capable of reconfigurable molecular logic

Structural DNA nanotechnology has developed profoundly in the last several years allowing for the creation of increasingly sophisticated devices capable of discrete sensing, locomotion, and molecular logic. The latter research field is particularly attractive as it provides information processing capabilities that may eventually be applied in situ, for example in cells, with potential for even further coupling to an active response such as drug delivery. Rather than design a new DNA assembly for each intended logic application, it would be useful to have one generalized design that could provide multiple different logic gates or states for a targeted use. In pursuit of this goal, we demonstrate a switchable, triangular dye-labeled three-arm DNA scaffold where the individual arms can be assembled in different combinations and the linkage between each arm can be physically removed using toehold-mediated strand displacement and then replaced by a rapid anneal. Rearranging this core structure alters the rates of Förster resonance energy transfer (FRET) between each of the two or three pendant dyes giving rise to a rich library of unique spectral signatures that ultimately form the basis for molecular photonic logic gates. The DNA scaffold is designed such that different linker lengths joining each arm, and which are used as the inputs here, can also be used independently of one another thus enhancing the range of molecular gates. The functionality of this platform structure is highlighted by easily configuring it into a series of one-, two- and three-input photonic Boolean logic gates such as OR, AND, INHIBIT, etc., along with a photonic keypad lock. Different gates can be realized in the same structure by altering which dyes are interrogated and implementation of toehold-mediated strand displacement and/or annealing allows reconfigurable switching between input states within a single logic gate as well as between two different gating devices

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 (Vmax) increased by 17-fold over free enzyme for the lowest PTE3–AuNP ratio and the largest AuNP (i.e., ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1, 20 nm dia. AuNP). This work provides a route to improve enzymatic OP detection strategies with enzyme–NP bioconjugates

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 post-print annealing processes. Laser-induced graphene (LIG) promises a facile alternative by creating flexible graphene electronics on polyimide substrates through a one-step laser writing fabrication method. Herein we demonstrate the use of LIG, created through 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 rates (10, 20, 30, 40, and 50 ms) in order to maximize the LIG electrochemical reactivity. Results illustrated that a laser pulse rate of 20 ms led to a high percentage of sp2 carbon (77%) and optimal peak oxidation current of 120 uA during ferricyanide cyclic voltammetry. Therefore, LIG electrodes created with a 20 ms pulse rate were consequently functionalized with distinct ionophores specific to NH4+(nonactin) or NO3- (tridodecylmethylammonium nitrate) within polyvinyl chloride (PVC)-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/decade (NH4+) and -54.8 ± 2.5 mV/decade (NO3-), detection limits of 28.2 ± 25.0 uM (NH4+) and 20.6 ± 14.8 uM (NO3-), low long-term drift of 0.93 mV/hr (NH4+) sensors and -5.3 μV/hr (NO3-) sensors and linear sensing ranges within 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

Platinum-paper micromotors: An urchin-like nanohybrid catalyst for green monopropellant bubble-thrusters

Platinum nanourchins supported on microfibrilated cellulose films (MFC) were fabricated and evaluated as hydrogen peroxide catalysts for small-scale, autonomous underwater vehicle (AUV) propulsion systems. The catalytic substrate was synthesized through the reduction of chloroplatinic acid to create a thick film of Pt coral-like microstructures coated with Pt urchin-like nanowires that are arrayed in three dimensions on a two-dimensional MFC film. This organic/inorganic nanohybrid displays high catalytic ability (reduced activation energy of 50-63% over conventional materials and 13-19% for similar Pt nanoparticle-based structures) during hydrogen peroxide (H2O2) decomposition as well as sufficient propulsive thrust (\u3e0.5 N) from reagent grade H2O2 (30% w/w) fuel within a small underwater reaction vessel. The results demonstrate that these layered nanohybrid sheets are robust and catalytically effective for green, H2O2-based micro-AUV propulsion where the storage and handling of highly explosive, toxic fuels are prohibitive due to size-requirements, cost limitations, and close person-to-machine contact

Enabling Inkjet Printed Graphene for Ion Selective Electrodes with Postprint Thermal Annealing

Inkjet printed graphene (IPG) has recently shown tremendous promise in reducing the cost and complexity of graphene circuit fabrication. Herein we demonstrate, for the first time, the fabrication of an ion selective electrode (ISE) with IPG. A thermal annealing process in a nitrogen ambient environment converts the IPG into a highly conductive electrode (sheet resistance changes from 52.8 ± 7.4 MΩ/□ for unannealed graphene to 172.7 ± 33.3 Ω/□ for graphene annealed at 950 °C). Raman spectroscopy and field emission scanning electron microscopy (FESEM) analysis reveals that the printed graphene flakes begin to smooth at an annealing temperature of 500 °C and then become more porous and more electrically conductive when annealed at temperatures of 650 °C and above. The resultant thermally annealed, IPG electrodes are converted into potassium ISEs via functionalization with a poly(vinyl chloride) (PVC) membrane and valinomycin ionophore. The developed potassium ISE displays a wide linear sensing range (0.01–100 mM), a low detection limit (7 μM), minimal drift (8.6 × 10–6 V/s), and a negligible interference during electrochemical potassium sensing against the backdrop of interfering ions [i.e., sodium (Na), magnesium (Mg), and calcium (Ca)] and artificial eccrine perspiration. Thus, the IPG ISE shows potential for potassium detection in a wide variety of human fluids including plasma, serum, and sweat

Flexible Thermoelectric Generators with Inkjet-Printed Bismuth Telluride Nanowires and Liquid Metal Contacts

Solution phase printing of nanomaterials is becoming increasingly important for the creation of scalable flexible electronics including those associated with biomedical and energy harvesting applications. However, the use of solution-phase printed thermoelectric energy generators (TEGs) has been minimally explored. Herein we report a highly flexible inkjet-printed TEG. Bismuth telluride (Bi2Te3) and bismuth antimony telluride (Bi0.5Sb1.5Te3) nanowires (NWs) are inkjet printed onto polyimide to form n-type and p-type legs for the TEGs. A post-print thermal annealing process is used to increase the thermoelectric performance of the printed NWs while eutectic gallium-indium (EGaIn) liquid metal contacts electrically connect the TEG legs in series. Annealing conditions for the combination of p/n legs are examined to maximize the thermoelectric efficiency of the TEG prototype. The maximum power factor was found to be 180 μW m-1K-2 and 110 μW m-1K-2 for Bi2Te3 and Bi0.5Sb1.5Te3 respectively, and a maximum power of 127 nW at a 32.5 K temperature difference. The performance of the TEG device does not diminish even after multiple bending (up to 50 times) experiments around a tight radius of curvature (rod dia. 11 mm). Hence this inkjet-printed flexible TEG is a step towards a fully functional wearable TEG device

Fabrication of High-resolution Graphene-based Flexible Electronics via Polymer Casting

In this study, a novel method based on the transfer of graphene patterns from a rigid or flexible substrate onto a polymeric film surface via solvent casting was developed. The method involves the creation of predetermined graphene patterns on the substrate, casting a polymer solution, and directly transferring the graphene patterns from the substrate to the surface of the target polymer film via a peeling-off method. The feature sizes of the graphene patterns on the final film can vary from a few micrometers (as low as 5 µm) to few millimeters range. This process, applied at room temperature, eliminates the need for harsh post-processing techniques and enables creation of conductive graphene circuits (sheet resistance: ~0.2 kΩ/sq) with high stability (stable after 100 bending and 24 h washing cycles) on various polymeric flexible substrates. Moreover, this approach allows precise control of the substrate properties such as composition, biodegradability, 3D microstructure, pore size, porosity and mechanical properties using different film formation techniques. This approach can also be used to fabricate flexible biointerfaces to control stem cell behavior, such as differentiation and alignment. Overall, this promising approach provides a facile and low-cost method for the fabrication of flexible and stretchable electronic circuits

CIP2A Immunosensor Comprised of Vertically-aligned Carbon Nanotube Interdigitated Electrodes Towards Point-of-Care Oral Cancer Screening

Vertically aligned carbon nanotube array (VANTA) coatings have recently garnered much attention due in part to their unique material properties including light absorption, chemical inertness, and electrical conductivity. Herein we report the first use of VANTAs grown via chemical vapor deposition in a 2D interdigitated electrode (IDE) footprint with a high height-to-width aspect ratio (3:1 or 75:25 µm). The VANTA-IDE is functionalized with an antibody (Ab) specific to the human cancerous inhibitor PP2A (CIP2A)—a salivary oncoprotein that is associated with a variety of malignancies such as oral, breast, and multiple myeloma cancers. The resultant immunosensor is capable of detecting CIP2A label-free across a wide linear sensing range (1 –100 pg/mL) with a detection limit of 0.24 pg/mL within saliva supernatant—a range that is more sensitive than the corresponding CIP2A enzyme linked immunosorbent assay (ELISA). These results help pave the way for rapid cancer screening tests at the point-of-care (POC) such as for the early-stage diagnosis of oral cancer at a dentist\u27s office