Multi-Modal Multi-Array Electrochemical and Optical Sensor Suite for a Biological CubeSat Payload.

CubeSats have emerged as cost-effective platforms for biological research in low Earth orbit (LEO). However, they have traditionally been limited to optical absorbance sensors for studying microbial growth. This work has made improvements to the sensing capabilities of these small satellites by incorporating electrochemical ion-selective pH and pNa sensors with optical absorbance sensors to enrich biological experimentation and greatly expand the capabilities of these payloads. We have designed, built, and tested a multi-modal multi-array electrochemical-optical sensor module and its ancillary systems, including a fluidic card and an on-board payload computer with custom firmware. Laboratory tests showed that the module could endure high flow rates (1 mL/min) without leakage, and the 27-well, 81-electrode sensor card accurately detected pH (71.0 mV/pH), sodium ion concentration (75.2 mV/pNa), and absorbance (0.067 AU), with the sensors demonstrating precise linear responses (R2 ≈ 0.99) in various test solutions. The successful development and integration of this technology conclude that CubeSat bio-payloads are now poised for more complex and detailed investigations of biological phenomena in space, marking a significant enhancement of small-satellite research capabilities.

Preparation and application of graphene-poly (diallyldimethylammoniumchloride)-iron oxide nanoparticles buckypaper for hydrogen peroxide detection.

We have reported the preparation and characterization of a novel, freestanding, paper-like graphene (G)-poly(diallyldimethylammoniumchloride) (PDDA)-Fe3O4 nanoparticles (NPs) composite. This G-based flexible buckypaper (BP) composed of stacked G-PDDA-NP platelets exhibited excellent mechanical properties, superior electrical properties, and enzyme mimetic activity, making it potentially suitable for in electrochemical sensor applications. The negatively charged NPs were immobilized on positively charged G-PDDA through the electrostatic interaction to form nanoscale G-PDDA-NP platelets, which were further assembled by flow-directed assembly to form BP. The resulting BP has macroscopic flexibility and stiffness due to the van der Waals forces between nanoscale G-PDDA-NP platelets and interlocking-tile arrangement of the platelets. The morphology and structure of the individual G-PDDA-NP platelets and the resulting BP were analyzed by using AFM, SEM and EDX. The BP was attached to an Au or Pt electrode to construct a non-enzyme H2O2 chemical sensor. The NPs acted as a "spacer" to increase the distance between the G sheets and decrease the chances of formation of a stacked graphitic structure, thereby increasing the surface area of the G electrode. The Fe3O4 NPs immobilized and embedded in the BP have intrinsic enzyme mimetic activity like natural peroxidase. The high surface area and excellent electrical conductivity of G improved the catalytic properties of NPs. The obtained H2O2, chemical sensor exhibited prominent electrocatalytic activity towards H2O2, with a wide linear range from 10 ppm (approximately 0.3 mM) to 800 ppm (approximately 23 mM), correlation coefficient of 0.986, and a high sensitivity of 218 microA mM(-1) x cm(-2). Such low-cost G-PDDA-NP composite BPs prepared by facile methods pave way towards novel sensors with better performance.

Immunosensor based on the ZnO nanorod networks for the detection of H1N1 swine influenza virus.

This paper presents an immunosensor fabricated on patterned zinc oxide nanorod networks (ZNNs) for detecting the H1N1 swine influenza virus (H1N1 SIV). Nanostructured ZnO with a high isoelectric point (IEP, approximately 9.5) possesses good absorbability for proteins with low IEPs. Hydrothermally grown ZNNs were fabricated on a patterned Au electrode (0.02 cm2) through a lift-off process. To detect the H1N1 SIV, the sandwich enzyme-linked immunosorbent assay (ELISA) method was employed in the immunosensor. The immunosensor was evaluated in an acetate buffer solution containing 3,3',5,5'-tetramethylbenzidine (TMB) via cyclic voltammetry at various H1N1 SIV concentrations (1 pg/mL-5 ng/mL). The measurement results of the fabricated immunosensor showed that the reduction currents of TMB at 0.25 V logarithmically increased from 259.37 to 577.98 nA as the H1N1 SIV concentration changed from 1 pg/mL to 5 ng/mL. An H1N1 SIV immunosensor, based on the patterned ZNNs, was successfully realized for detecting 1 pg/mL-5 ng/mL H1N1 SIV concentrations, with a detection limit of 1 pg/mL for H1N1 SIV.

Aptamer-based immunosensor on the ZnO nanorods networks.

This paper presents the fabrication and characteristics of a new aptamer-based electrochemical immunosensor on the patterned zinc oxide nanorod networks (ZNNs) for detecting thrombin. Aptamers are single-stranded RNA or DNA sequence that binds to target materials with high specificity and affinity. An antibody-antigen-aptamer sandwich structure was employed to this immunosensor for detecting thrombin. First, hydrothermally grown ZNNs were patterned on the patterned 0.02 cm2 Au/Ti electrodes on a glass substrate by lift-off process. The high isoelectric point (IEP, approximately 9.5) of nanostructured ZnO makes it suitable for immobilizing proteins with low IEP. Then 5 microL of the 500 nM antibody was immobilized on the ZNNs electrode. 5 micro/L of the mixture of 1 microM aptamer labeled by ferrocene (Fc) and thrombin was dropped on the electrode for antibody-antigen binding. The peak oxidation currents of the immunosensors at various thrombin concentrations were measured by using cyclic voltammetry. The peak oxidation current was observed at 340 mV versus Ag/AgCl electrode, and the peak oxidation current increased linearly from 62.26 nA to 354.13 nA with the logarithmic concentration of thrombin in the range from 100 pM to 250 nM. Fabrication of an aptamer-based immunosensor for thrombin detection is a new attempt and the characteristics of the fabricated immunosensors showed that the fabricated aptamer-baded immunosensor worked electrochemically well and had a low detection limit (approximately 91.04 pM) and good selectivity.

An electrochemical route to graphene oxide.

Large-scale graphene oxide (GO) with adjustable resistivity was synthesized from graphite via an electrochemical method using KCl solution as an effective electrolyte. During the exfoliation process, electrostatic force intercalates chloride ions between the expanded graphite layers on the anode. These chloride ions form small gas bubbles between the graphite layers in the electrochemical reaction. It is believed that the gas bubbles expand the gap between graphite sheets and produce a separating force between adjacent graphene layers. This separating force overcomes the Van der Waals force between adjacent sheets and exfoliates graphene layers from the starting graphite. Because the graphene is electrochemically oxidized by chorine during the exfoliation, the exfoliated GO sheets are hydrophilic and easily dispersed in the electrolyte solution. The GO solution prepared by the electrochemical exfoliation can be simply sprayed or spin-coated onto any substrate for device applications. The measured average thicknesses of a monolayer, bilayer, and trilayer exfoliated GO on SiO2 substrate were 1.9, 2.8, and 3.9 nm, respectively. It was observed that the measured resistance of the exfoliated GO sheets increases due to electrochemical oxidation in the solution. This electrochemical approach offers a low-cost and efficient route to the fabrication of graphene based devices.

Wettability control of a transparent substrate using ZnO nanorods.

This paper presents a simple way of controlling the wettability of a structured surface with ZnO nanorods on a transparent substrate. A combination of ZnO nanostructures and stearic acid was used to create superhydrophobic surfaces with the potential properties of being self-cleaning, waterproof, and antifog. ZnO nanorods were uniformly covered on glass substrates through a simple hydrothermal method with varying growth time which affects the surface morphology. When a substrate is dipped into 10 mM stearic acid in ethanol for 24 h, chemisorption of the stearic acid takes place on the ZnO nanorod surface, after which the hydrophilic ZnO nanorod surfaces are modified into hydrophobic ones. The contact angle of a water droplet on this superhydrophobic ZnO nanorod surface increased from 110 degrees to 150 degrees depending on the growth time (from 3 to 6 h) with a high transparency of above 60%. In addition, the water contact angle can be made to as low as 27 degrees after exposing the substrate to 10-mW/cm2 UV for 1 h.

Investigation on fabrication of nanoscale patterns using laser interference lithography.

Nanoscale patterns are fabricated by laser interference lithography (LIL) using Lloyd's mirror interferometer. LIL provides a patterning technology with simple, quick process over a large area without the usage of a mask. Effects of various key parameters for LIL, with 257 nm wavelength laser, are investigated, such as the exposure dosage, the half angle of two incident beams at the intersection, and the power of the light source for generating one or two dimensional (line and dot) nanoscale structures. The uniform dot patterns over an area of 20 mm x 20 mm with the half pitch sizes of around 190, 250, and 370 nm are achieved and by increasing the beam power up to 0.600 mW/cm2, the exposure process time was reduced down to 12/12 sec for the positive photoresist DHK-BF424 (DongJin) over a bare silicon substrate. In addition, bottom anti-reflective coating (DUV-30J, Brewer Science) is applied to confirm improvements for line structures. The advantages and limitations of LIL are highlighted for generating nanoscale patterns.

Flexible and Conductive Graphene-Poly (diallyldimethylammoniumchloride) Buckypaper.

This paper describes the fabrication and characterization of flexible, conductive reduced graphene oxide (rGO)-poly(diallyldimethylammoniumchloride) (PDDA) buckypaper (BP). PDDA acts as a reducing agent to prepare an rGO-PDDA nanosheet dispersion from graphite oxide. The incorporation of PDDA as a "glue" molecule successfully binds rGO nanosheets into BPs with strong interlayer binding. The resulting BPs were characterized by scanning electronic microscopy (SEM), Raman, energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and resistivity measurements. The sp2 structure was greatly restored by the PDDA-induced reduction. Moreover, rGO was chemically doped from the adsorbed PDDA, which causes the Raman G band to shift from ~1585 to ~1610 cm(-1). This chemical doping substantially increased the density of the free charge carriers in rGO and thereby further enhanced the electrical conductivity of the rGO-BP. Good inter-layer connection in the rGO percolating network was obtained after thermal annealing at higher than ~250 °C. The resulting rGO-PDDA-BPs exhibited an isotropic sheet resistance as low as ~100 Ω/sq, which indicates a reduction by six orders of magnitude compared to the GO-BPs resistance before annealing. This PDDA-induced reduction with a low-temperature annealing process preserved the BPs' structural integrity and mechanical flexibility, thus overcoming the fragility problems with high-temperature annealing.

Electrostatically Gated Graphene-Zinc Oxide Nanowire Heterojunction.

This paper presents an electrostatically gated graphene-ZnO nanowire (NW) heterojunction for the purpose of device applications for the first time. A sub-nanometer-thick energy barrier width was formed between a monatomic graphene layer and electrochemically grown ZnO NWs. Because of the narrow energy barrier, electrons can tunnel through the barrier when a voltage is applied across the junction. A near-ohmic current-voltage (I-V) curve was obtained from the graphene-electrochemically grown ZnO NW heterojunction. This near-ohmic contact changed to asymmetric I-V Schottky contact when the samples were exposed to an oxygen environment. It is believed that the adsorbed oxygen atoms or molecules on the ZnO NW surface capture free electrons of the ZnO NWs, thereby creating a depletion region in the ZnO NWs. Consequentially, the electron concentration in the ZnO NWs is dramatically reduced, and the energy barrier width of the graphene-ZnO NW heterojunction increases greatly. This increased energy barrier width reduces the electron tunneling probability, resulting in a typical Schottky contact. By adjusting the back-gate voltage to control the graphene-ZnO NW Schottky energy barrier height, a large modulation on the junction current (on/off ratio of 10(3)) was achieved.

A method for stable electrical connection of a multi-channeled polyimide electrode with PCB.

We propose a novel packaging method of a thin polyimide multichannel microelectrode. For the simple electrical connection of polyimide (PI) electrodes, we made a via-hole at the interconnection pads of thin PI electrodes, and constructed a Ni ring by electroplating through the via-hole for the stable soldering and strong adhesion of the electrode to PCB. For the construction of a well-organized Ni ring, the electroplating condition was optimized, and the electrical property of the packaged electrode was evaluated. A 40 channel thin PI electrode was fabricated and packaged by the proposed method, and we performed the animal experiment with this packaged electrode for the high-resolution recording of neural signals from the skull of a rat.

Interconnection of multichannel polyimide electrodes using anisotropic conductive films (ACFs) for biomedical applications.

In this paper, we propose a method for interconnecting soft polyimide (PI) electrodes using anisotropic conductive films (ACFs). Reliable and automated bonding was achieved through development of a desktop thermocompressive bonding device that could simultaneously deliver appropriate temperatures and pressures to the interconnection area. The bonding conditions were optimized by changing the bonding temperature and bonding pressure. The electrical properties were characterized by measuring the contact resistance of the ACF bonding area, yielding a measure that was used to optimize the applied pressure and temperature. The optimal conditions consisted of applying a pressure of 4 kg f/cm(2) and a temperature of 180 °C for 20 s. Although ACF base bonding is widely used in industry (e.g., liquid crystal display manufacturing), this study constitutes the first trial of a biomedical application. We performed a preliminary in vivo biocompatibility investigation of ACF bonded area. Using the optimized temperature and pressure conditions, we interconnected a 40-channel PI multielectrode device for measuring electroencephalography (EEG) signals from the skulls of mice. The electrical properties of electrode were characterized by measuring the impedance. Finally, EEG signals were measured from the mice skulls using the fabricated devices to investigate suitability for application to biomedical devices.

Polyimide-based multi-channel arrayed electrode for measuring EEG signal on the skull of mouse.

In this paper, we have developed 40 channel multiple electrodes mounted on the surface of mouse's skull using polyimide substrate and tested its performance by measuring EEG signals. The recording site of the electrode was electroplated by Pt to enhance both contact impedance and adhesive strength by applying proper current, cleaning surface and removing H(2) gas bubbles. For in vivo test, the electrode was placed on the skull of F1 mouse and EEG signals were measured. We observed the suitability of electrode for measuring EEG signals from multiple areas on the skull. The spectrum of EEG signal to change was observed by urethane administration.

Electrical characteristics of various submucosal injection fluids for endoscopic mucosal resection.

Submucosal fluid injection, prerequisite to endoscopic mucosal resection, necessitates detailed evaluation for proper selection. We aimed to compare height of gastric tissues after submucosal injection, and to verify electrical implications of injectants. Porcine stomach pieces were cut out, and eight solutions were used: normal saline, 0.5% sodium hyaluronate (SH), 0.25% SH, hydroxypropyl methylcellulose, 10% glycerin, fibrinogen, 1% sodium alginate (SA), and 2.5% SA. Elevated heights were measured after submucosal injection of the eight fluids, and electrical impedance was measured for fluids plus a reference solution (0.01 N KCl) with a potentiostat electroimpedance spectrometry and an insulation-tipped knife. Resistivity was calculated thereafter. Normal saline and 10% glycerin solution showed greater height diminution. Resistivity were in the range of 80-110 Omega cm, except for 309.7 Omega cm for fibrinogen. Higher resistivity may improve performance of electrosurgery, probably by strengthening impedance and heat dissipation. Further studies are required to back up this basic experiment for clinical application.