Novel Wood Resistance Measurement Method Reducing the Initial Transient Instabilities Arising in DC Methods Due to Polarization Effects

A novel method for measuring the electrical resistance in wood is presented. It is based on applying an Alternating Current (AC) to two electrodes rammed into the wood. The method reduces the transient time for value stabilization. In case of Direct Current (DC) resistance measurement methods, typically used in wood measurement, an initial transient exists, invalidating the measured value during an initial transient period. This measurement method uses an electronic circuit based on a relaxation oscillator where the wood automatically sets the oscillation frequency depending on its electrical resistance. Compared to other AC methods, this circuit greatly simplifies the measurement process, not requiring any previous analysis for wood AC frequency estimation. Experimental results for four different wood species showed that, in all cases, the transient response of the measured wood resistance is improved when compared to the DC resistance method, reducing the stabilization time from 10-12 min in DC measurements to almost zero for the novel measurement method. The deviation between the initial wood resistance measurement and the stable value resulted in a range between 0.2% and 5% with the proposed method, while a range between 6% and 10% was obtained when using the DC method. Moreover, the proposed circuit is able to detect changes in wood resistance produced by small variations in temperature and environmental relative humidity during continuous long term measurements. For repeatability analysis, it was verified that similar ambient conditions of temperature and relative humidity (variations lower than 1 °C and 1%, respectively) in different moments provided variations lower than 1.5%. The method can also be applied to other fiber materials affectd by polarization effects when an electrical current is applied into them

High temperature supercapacitors

The scientific objective of this research program was to determine the feasibility of manufacturing an ionic liquid-based supercapacitor that could operate at temperatures up to 220 °C. A secondary objective was to determine the compatibility of ionic liquids with other cell components (e.g. current collectors) at high temperature and, if required, consider means of mitigating any problems. The industrial motivation for the present work was to develop a supercapacitor capable of working in the harsh environment of deep offshore boreholes. If successful, this technology would allow down-hole telemetry under conditions of mechanical vibration and high temperature. The obstacles, however, were many. All supercapacitor components had to be stable against thermal decomposition up to T ≥ 220 °C. Volatile components had to be eliminated. If possible, the finished device should be able to withstand voltages greater than 4 V, in order to maximise the amount of stored energy. The internal resistance should be as low as possible. Side reactions, particularly faradaic reactions, should be eliminated or suppressed. All liquid components should be gelled to minimise leakage in the event of cell damage. Finally, any emergent problems should be identified. [Continues.

New Approach for Making Standard the Development of Biosensing Devices by a Modular Multi-Purpose Design

The fast widening of biosensing applications, such as healthcare, drug delivery, food, and military industries, is increasing the need for generality and compatibility among different sensors. To address this challenge, we present here an innovative approach for the fast development of new electronic biosensing systems, linking a custom-designed front-end with a multi-purpose system. We envision an open tool to help designers to focus on the target molecule and related detection method instead of designing each time a dedicated electronic device. The architecture of the proposed system is based on a modular approach, where only the front-end and the software need to be custom re-designed according to the application. Considering current research and applying a rigorous definition of the technical requirements, the core of the system is designed to fit the highest number of biosensing methods. The flexibility of this approach is successfully demonstrated with three different types of biosensors, i.e., amperometric, ion-sensitive, and memristive

Characterisation of electrode microarrays produced photolithographically and with thiol self-assembled monolayers on gold electrodes

The macroscale electrochemical theory breaks down with microstructures smaller than the dimensions of the diffusion layer, showing that such electrodes have significant quantitative effects and qualitative advantages. The great advantage of microelectrodes over macroelectrodes is the minimisation of interference, which gives rise to much lower detection limits. Microelectrodes have much reduced ohmic drops and capacitive effects and can be used in the absence of supporting electrolyte. These features have opened a growing interest in the fabrication and application of microelectrodes in various areas. There are different microelectrode geometries, but disc type is the most used. Microelectrode arrays have been proposed as a way of increasing the magnitude of the current (produced for a single microelectrode), while maintaining the advantages of the single microelectrode. Although the inlaid microdisc microelectrode can be considered as one the most popular microelectrode geometry, there is also a need to consider conical recessed, recessed and protruding microdiscs as photolithographic microfabrication techniques often result in non-ideal geometries. It has been proved using surface imaging techniques such as scanning Kelvin nanoprobe (SKN), scanning electron microscopy (SEM) and white light interferometer microscopy that conical recessed electrodes with gradient potential along the recessed walls are formed during standard photolithographic methods for producing microelectrode arrays. Microarrays are ubiquitously used for high-throughput measurements using various signal transduction techniques. Ideally each sensor in a microarray platform should perform optimally to ensure an error free response. In this thesis, a simple method for designing a microelectrode array platform (MEA) is described, allowing a ‘defective’ cluster of sensing arrays to be easily identified. It is possible to extend this concept for multiple analyses on a single chip. Molecular electronic is a promising technology which would be an alternative. The concept of molecular electronics is the use of single molecules or arrays, or layers of molecules for the fabrication of electronic components such as wires, switches, and storage elements. Indeed, functionalised thiol monolayer-based microelectrode array may provide unique possibilities, facilitating electrochemical measurements involving electron transfer via electron tunnelling. The conjugated structure of rigid, linear molecule increases greatly the rate of electron transfer across the monolayer. Charge transfer and self-assembly characteristics of novel fully conjugated molecules molecular wires (synthesised at the Department of Chemistry and Centre for Molecular and Nanoscale Electronics, Durham University) assembled on flat gold electrodes are evaluated using Marcus model of electron-transfer and tunnelling theory. The behaviour of these wires is compared with heptanethiol and dodecanethiol SAMs. A preliminary study for application of self-assembled monolayer of molecular wires in microelectrode arrays for multiple analyses on a single chip has been successfully reached

A universal equivalent circuit for carbon-based supercapacitors

A universal equivalent circuit is proposed for carbon-based supercapacitors. The circuit, which actually applies to all porous electrodes having non-branching pores, consists of a single vertical ladder network in series with an RC parallel network. This elegant arrangement explains the three most important shortcomings of present-day supercapacitors, namely open circuit voltage decay, capacitance loss at high frequency, and voltammetric distortion at high scan rate. It also explains the shape of the complex plane impedance plots of commercial devices and reveals why the equivalent series capacitance increases with temperature. Finally, the construction of a solid-state supercapacitor simulator is described. This device is based on a truncated version of the universal equivalent circuit, and it allows experimenters to explore the responses of different supercapacitor designs without having to modify real supercapacitors

Facile Synthesis of Bio-templated Tubular Co3O4 Microstructure and its Electrochemical Performance in Aqueous Electrolytes

Template-assisted facile synthesis of tubular Co3O4 microstructures and its electrochemical performance was studied to understand its use as a potential electrode material for supercapacitors. Tubular porous Co3O4 microstructures were synthesized using cotton fibers as bio-template. The as-obtained templated Co3O4 structure inherits the morphology and microstructure of cotton fiber. The electrochemical performance of the electrode made up of tubular Co3O4 structure was evaluated in 3M KOH, NaOH, and LiOH aqueous electrolytes. The large-surface-area of tubular Co3O4 microstructure has a noticeable pseudocapacitive performance with a capacitance of 401 F/g at 1 A/g and 828 F/g at 2 mV/s, a Coulombic efficiency averaging ~100%, and excellent cycling stability with capacitance retention of about 80% after 5,000 cycles. Overall, the tubular Co3O4 microstructure displayed superior electrochemical performance in 3M KOH electrolyte with peak power density reaching 5,500 W/kg and energy density exceeding 22 Wh/kg. The superior performance of the tubular Co3O4 microstructure electrode is attributed to its high surface area and adequate pore volume distribution, which allows effective redox reaction and diffusion of hydrated ions. The facile synthesis method can be adapted for preparing various metal oxide microstructures for possible applications in catalysis, electrochemical, sensors, and fuel cell applications

Electrochemical noise limits of femtoampere-sensing, CMOS-integrated transimpedance amplifiers

Low-noise operational amplifiers are an important tool in the life sciences. Biosensor measurements typically rely on low-noise transimpedance amplifiers to record biological signals. Two different techniques were used to leverage the advantages of low-noise circuitry for bioelectronics. A CMOS-integrated system for measuring redox-active substrates using electrochemical read-out at very low noise levels is presented. The system incorporates 112 amplifier channels capable of current sensing with noise levels below 1 fArms in a 3.5-Hz bandwidth. The amplifier is externally connected to a gold microelectrode with a radius of 15 µm. The amplifier enables measurement of redox-couples such as potassium ferrocyanide/ferricyanide with concentrations down to 10 nM at current levels of only 300 fA. The electrochemical noise that sets the limits of detection is also measured and analyzed based on redox mass transfer equation and electrochemical impedance spectroscopy. Secondly, CMOS-integrated low noise junction field-effect transistors (JFETs) were developed in a standard 0.18-µm CMOS process. These JFETs reduce input referred flicker noise power by more than a factor of 10 when compared with equally sized n-channel MOS devices by eliminating oxide interfaces in contact with the channel. We show that this improvement in device performance translates into a factor-of-10 reduction in the input-referred noise of integrated CMOS operational amplifiers when JFET devices are used at the input

Sustainable energy storage devices using laser-induced graphene electrodes

Premi UAB de la Fundació Autònoma Solidària (FAS) als millors Treballs de Fi de Grau sobre desenvolupament sostenible i justícia global. 5a Edició, curs 2020/2021This project presents the development of laser-induced graphene (LIG) electrodes for application on sustainable electrochemical energy storage devices. Cardboard is selected as organic recyclable substrate, upon which fire-retardant treatments and lignin coatings have been tested prior to laser scribing. Laser conditions optimization have provided a cardboard derived conductor material with sheet resistance (Rs) values in the order of tens of Ω·sq-1. Particularly, a newly developed lignin and nanocellulose coating treatment of cardboard has yielded 8.1 Ω·sq-1 Rs, close to the best value reported for LIG on an organic substrate. Electrochemical and morphological characterization of cardboard based LIG showed that it presents a highly porous 3D electroactive surface. This material has been implemented in the fabrication of cardboard derived supercapacitors (SC) and batteries. Compared with polyimide derived LIG, cardboard SCs demonstrate higher areal capacitance (1 mF·cm-2), with similar performance to literature references. On the other hand, the biopolymer-based cardboard battery is the first proof-of-concept of its kind ever reported. Although it presents high internal resistance, the battery provides almost 1 V open circuit voltage, and a maximum power density output of 19.5 µW·cm-2. Thus, this work opens a new pathway in the research of potentially recyclable, biodegradable or even compostable batteries based on cardboard derived laserinduced graphene electrode

Localization of metal ions in DNA

M-DNA is a novel complex formed between DNA and transition metal ions under alkaline conditions.  The unique properties of M-DNA were manipulated in order to rationally place metal ions at specific regions within a double-stranded DNA helix.   Investigations using thermal denaturation profiles and the ethidium fluorescence assay illustrate that the pH at which M-DNA formation occurs is influenced heavily by the DNA sequence and base composition.  For instance, DNA with a sequence consisting of poly[d(TG)•d(CA)] is completely converted to M-DNA at pH 7.9 while DNA consisting entirely of poly[d(AT)] remains in the B-DNA conformation until a pH of 8.6 is reached.  The pH at which M-DNA formation occurs is further decreased by the incorporation of 4-thiothymine (s4T).  DNA oligomers with a mixed sequence composed of half d(AT) and the other half d(TG)•d(CA) showed that only 50% of the DNA is able to incorporate Zn2+ ions at pH 7.9.  This suggests that only regions corresponding to the tracts of d(TG)•d(CA) are being transformed.   Duplex DNA monolayers were self-assembled on gold through a Au-S linkage and both B- and M-DNA conformations were studied using X-ray photoelectron spectroscopy (XPS) in order to better elucidate the location of the metal ions.  The film thickness, density, elemental composition and ratios for samples were analyzed and compared.  The DNA surface coverage, calculated from both XPS and electrochemical measurements, was approximately 1.2 x 1013 molecules/cm2 for B-DNA.  All samples showed distinct peaks for C 1s, O 1s, N 1s, P 2p and S 2p as expected for a thiol-linked DNA.  On addition of Zn2+ to form M-DNA the C 1s, P 2p and S 2p showed only small changes while both the N 1s and O 1s spectra changed considerably.  This result is consistent with Zn2+ interacting with oxygen on the phosphate backbone as well as replacing the imino protons of thymine (T) and guanine (G) in M-DNA.   Analysis of the Zn 2p spectra also demonstrated that the concentration of Zn2+ present under M-DNA conditions is consistent with Zn2+ binding to both the phosphate backbone as well as replacing the imino protons of T or G in each base pair.  After the M-DNA monolayer is washed with a buffer containing only Na+ the Zn2+ bound to the phosphate backbone is removed while the Zn2+ bound internally still remains. Variable angle x-ray photoelectron spectroscopy (VAXPS) was also used to examine monolayers consisting of mixed sequence oligomers.  Preliminary results suggest that under M-DNA conditions, the zinc to phosphate ratio changes relative to the position of the d(TG)•d(CA) tract being at the top or bottom of the monolayer.    Electrochemistry was also used to investigate the properties of M-DNA monolayers on gold and examine how the localization of metal ions affects the resistance through the DNA monolayer.  The effectiveness of using the IrCl62-/3- redox couple to investigate DNA monolayers and the potential advantages of this system over the standard Fe(CN)63-/4- redox couple are demonstrated.  B-DNA monolayers were converted to M-DNA by incubation in buffer containing 0.4 mM Zn2+ at pH 8.6 and studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) with IrCl62-/3-.   Compared to B-DNA, M-DNA showed significant changes in CV, EIS and CA spectra.  However, only small changes were observed when the monolayers were incubated in Mg2+ at pH 8.6 or in Zn2+ at pH 6.0.  The heterogeneous electron-transfer rate (kET) between the redox probe and the surface of a bare gold electrode was determined to be 5.7 x 10-3 cm/s.  For a B-DNA modified electrode, the kET through the monolayer was too slow to be measured.  However, under M-DNA conditions, a kET of 1.5 x 10-3 cm/s was reached.  As well, the percent change in resistance to charge transfer (RCT), measured by EIS, was used to illustrate the dependence of M-DNA formation on pH.  This result is consistent with Zn2+ ions replacing the imino protons on thymine and guanine residues.  Also, at low pH values, the percent change in RCT seems to be greater for d(TG)15•d(CA)15 compared to oligomers with mixed d(AT) and d(TG)•d(CA) tracts.  The IrCl62-/3- redox couple was also effective in differentiating between single-stranded and double-stranded DNA during dehybridization and rehybridization experiments.

Single-step laser-induced graphene on paper as a platform for monitoring pH within smart bandages for wound application

Chronic wounds and wound infection occurrences have become major public health concerns over the last years, affecting millions of people and costing billions to national healthcare systems. Many of these incidents are avoidable through proper monitoring of the wound health status. The boom of wearable sensors for in situ monitoring of relevant biomarkers like temperature, humidity, glucose concentration, and other analytes have shown to be viable pathways to continuous injury monitoring for chronic status and infection prevention. Among these analytes, pH has been a much-neglected parameter despite having a strong correlation with the wound healing status. Acute wounds initially present acidic pH levels (≈4), while chronic wounds oscillate indefinitely at higher pH levels (≈8). The discovery of laser-induced graphene (LIG) gave rise to the development of lowcost, flexible electrodes for the fabrication of biosensors on a multitude of recyclable and sustainable substrates, namely on paper. This dissertation project contributes to the research and optimization of LIG on paper as a medium for the fabrication of environmentally friendly, low-cost, and biocompatible pH sensors for future implementation within smart bandages and medical wound dressings. LIG on paper fabrication parameters were optimized for better electrical performance, having reached a sheet resistance value of 14.0 ± 1.50 Ω sq−1. LIG-based pH sensors were conceived based on a voltammetric approach by modifying the working electrode with riboflavin (vitamin B2) and monitoring its electrochemical response to different pH environments. The proposed device showed Super-Nernstian sensitivity of 78.2 ± 3.37 mV pH−1 over a 2 to 8, physiologically relevant, pH range, making it suitable for the application within a wound environment.Feridas crónicas e as ocorrências de infeção em feridas são atualmente grandes preocupações de saúde pública, afetando milhões de pessoas e custando milhares de milhões aos sistemas nacionais de saúde. Muitos destes incidentes são evitáveis através de uma monitorização adequada do estado de regeneração dos ferimentos. A expansão dos sensores wearable para monitorização localizada de biomarcadores relevantes como temperatura, humidade, concentração de glucose, e outros analitos, tem demonstrado ser uma possível via de controlo contínuo do estado destas lesões, para prevenção da evolução para situação crónica e do surgimento de infeções. Entre estes analitos, o pH tem sido um parâmetro muito negligenciado, apesar de ter uma forte correlação com a fase de cicatrização da ferida. As feridas agudas apresentam inicialmente níveis de pH ácidos (≈4), enquanto que as feridas crónicas oscilam indefinidamente entre níveis de pH mais elevados (≈8). A descoberta do grafeno induzido por laser (LIG) deu origem ao desenvolvimento de elétrodos flexíveis e de baixo custo para o fabrico de biossensores, tendo por base diversos substratos recicláveis e sustentáveis, nomeadamente o papel. Este projeto de dissertação contribui para a investigação e otimização da produção de LIG em papel como meio para o fabrico de sensores de pH ecológicos, de baixo custo e biocompatíveis. Isto visando a implementação futura em pensos inteligentes e curativos médicos. Foi realizada a otimização dos parâmetros para fabricação do LIG em papel, tendo atingido valores de resistência em folha de 14.0 ± 1.50 Ω sq−1. Os sensores de pH baseados em LIG foram concebidos com base numa abordagem voltamétrica, modificando o elétrodo de trabalho com riboflavina (vitamina B2) e monitorizando a sua resposta eletroquímica a ambientes com diferentes valores de pH. O dispositivo proposto mostrou sensibilidade Super-Nernstiana de 78.2 ± 3.37 mV pH−1 ao longo de uma janela de pH fisiologicamente relevante de 2 a 8, tornando-o adequado para a aplicação em ferimentos