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 heightto-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's office. Table of Content (ToC) Image Description: (Left) Schematic diagram showing antibody functionalized (anti-CIP2A) vertically aligned carbon nanotubes (VANTAs) arrayed in an interdigitated electrode (IDE) footprint. (Left Inset) An optical image of a VANTA IDE immunosensor fabricated on silicon wafer. (Right) Electrochemical impedance sensing of CIP2A antigen concentrations with the biofunctionalized VANTA IDEs.

High-aspect-ratio, porous membrane of vertically-aligned carbon nanotubes (CNTs) were developed through a templated microfabrication approach for electrochemical sensing. Nanostructured platinum (Pt) catalyst was deposited onto the CNTs with a facile, electroless deposition method, resulting in a Pt-nanowire-coated, CNT sensor (PN-CNT). Convective mass transfer enhancement was shown to improve PN-CNT sensor performance in the non-enzymatic, amperometric sensing of hydrogen peroxide (H 2 O 2). In particular, convective enhancement was achieved through the use of high surface area to fluid volume structures and concentration boundary layer confinement in a channel. Stir speed and sensor orientation especially influenced the measured current in stirred environments for sensors with through-channel diameters of 16 µm. Through-flow sensing produced drastically higher signals than stirred sensing with over 90% of the H 2 O 2 being oxidized as it passed through the PN-CNT sensor, even for low concentrations in the range of 50 nM to 500 µM. This effective utilization of the analyte in detection demonstrates the utility of exploiting convection in electrochemical sensing. For through-flow at 100 µL s-1 , a sensitivity of 24,300 µA mM-1 cm-2 was achieved based on the frontal projected area (871 µA mM-1 cm-2 based on the nominal microchannel surface area), with a 0.03 µM limit of detection and a linear sensing range of 0.03-500 µM.

Freestanding, 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 (1-ethyl-3-(-3-dimethylaminopropyl)carbodiimide/ N-hydroxysuccinimide) or by polymer entrapment [poly(3,4-ethylene-dioxythiophene)] and operated in phosphate buffered saline. With normalization by the overall cross-sectional area of the flow (0.713 cm), the sensitivity of the MV, enzyme-in-solution, and covalent sensors were 45.93, 18.77, and 1.815 mA cm mM, respectively. Corresponding limits of detection were 100, 194, and 311 nM glucose. The linear sensing ranges for the sensors were 250 nM to 200 μM glucose for the MV sensor, 500 nM to 200 μM glucose for the enzyme-in-solution sensor, and 1 μM to 6 mM glucose for the covalent sensor. The flow cell and sensor cross-sectional area were scaled down (0.020 cm) to enable detection from 200 μL of glucose with MV by flow injection analysis. The sensitivity of the small MV sensor was 5.002 mA cm mM, 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.

Advances in nanomaterials, combined with electrochemical impedance spectroscopy (EIS), have allowed electrochemical biosensors to have high sensitivity while remaining label-free, enabling the potential for portable diagnosis at the point-of-care. We report porous, 3D vertically aligned carbon nanotube (VACNT) electrodes with underlying chromium electrical leads for impedance-based biosensing. The electrodes are characterized by electrode height (5, 25, and 80 μm), gap width (15 and 25 μm), and geometry (interdigitated and serpentine) using scanning electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). The protein streptavidin is functionalized onto VACNT electrodes for detection of biotin, as confirmed by fluorescence microscopy. EIS is used to measure the change in impedance across electrodes for different biotin concentrations. The impedance data shows two distinct semi-circular regions which are modeled by an equivalent electrical circuit. VACNT electrode height, gap width, and geometrical pattern each have an impact on sensor sensitivity, with tall, closelyspaced VACNT interdigitated electrodes (IDEs) having the highest sensitivity. With an electroactive surface area that is 15x the 2D geometric area, 80 μm tall VACNT IDEs with a gap width of 15 μm are 4.3x more sensitive than short (5 μm) IDEs and 1.6x more sensitive than serpentine electrodes (SEs). The biosensors obtain a limit of detection of 1 ng/mL biotin with two linear sensing regions (0.001-1 μg/mL and 1-100 μg/mL). Although this biosensing platform is shown with streptavidin and biotin, it could be extended to other proteins, antibodies, viruses, and bacteria.

This work investigates the electrochemical dynamics and performance of additively manufactured composite electrolytes for resistive switching. Devices are comprised of a Ag/AgI–Al2O3/Pt stack, where the solid state electrolyte is additively manufactured using extrusion techniques. AgI–Al2O3 composite electrolytes are characterized by x-ray diffraction and electrochemical impedance spectroscopy. The ionic conductivities of the electrolytes were measured for different concentrations of Al2O3, observing a maximum conductivity of 4.5 times the conductivity of pure AgI for composites with 20 mol. % Al2O3. There was little change in activation energy with the addition of Al2O3. Setting the Ag layer as the positive electrode and the Pt layer as the negative electrode, a high conductivity state was achieved by applying a voltage to electrochemically establish an electrically conducting Ag filament within the solid state AgI–Al2O3 electrolyte. The low conductivity state was restored by reversing this applied voltage to electrochemically etch the newly grown Ag filament. Pure AgI devices switch between specific electrical resistivity states that are separated by five orders of magnitude in electrical conductivity. Endurance tests find that the AgI resistive switches can transition between a low and high electrical conductivity state over 8500 times. Composite AgI–Al2O3 resistive switches formed initial Ag filaments significantly faster and also demonstrated two orders of magnitude separation in resistivity when cycling for 1600 cycles.