Paired Electrosynthesis at Interdigitated Microband Electrodes: Exploring Diffusion and Reaction Zones in the Absence of a Supporting Electrolyte

Electrosynthesis traditionally requires dedicated reactor systems and an added electrolyte, although some paired electrosynthesis processes are possible at interdigitated microband electrodes simply immersed in solution and without an intentionally added electrolyte. Here, 1,1'-ferrocenedimethanol oxidation and activated olefin electro-hydrogenation reactions are investigated as model processes at a Pt-Pt interdigitated microband array electrode with 5 μm width and with 5 μm interelectrode gap. Voltammetric responses for electro-hydrogenation are discussed, and product yields are determined in methanol (MeOH) in the presence/absence of an added electrolyte (LiClO4). An isotope effect is observed in CH3OD solvent, leading to olefin monodeuteration linked to a fast EC-type process close to the cathode surface (in the cathode reaction zone) rather than to charge annihilation in the interelectrode zone. A finite element simulation is employed to visualize/discuss reaction zones and to contrast the rate of charge annihilation processes with/without a supporting electrolyte.

Application of Nanostructured Carbon-Based Electrochemical (Bio)Sensors for Screening of Emerging Pharmaceutical Pollutants in Waters and Aquatic Species: A Review

Pharmaceuticals, as a contaminant of emergent concern, are being released uncontrollably into the environment potentially causing hazardous effects to aquatic ecosystems and consequently to human health. In the absence of well-established monitoring programs, one can only imagine the full extent of this problem and so there is an urgent need for the development of extremely sensitive, portable, and low-cost devices to perform analysis. Carbon-based nanomaterials are the most used nanostructures in (bio)sensors construction attributed to their facile and well-characterized production methods, commercial availability, reduced cost, high chemical stability, and low toxicity. However, most importantly, their relatively good conductivity enabling appropriate electron transfer rates-as well as their high surface area yielding attachment and extraordinary loading capacity for biomolecules-have been relevant and desirable features, justifying the key role that they have been playing, and will continue to play, in electrochemical (bio)sensor development. The present review outlines the contribution of carbon nanomaterials (carbon nanotubes, graphene, fullerene, carbon nanofibers, carbon black, carbon nanopowder, biochar nanoparticles, and graphite oxide), used alone or combined with other (nano)materials, to the field of environmental (bio)sensing, and more specifically, to pharmaceutical pollutants analysis in waters and aquatic species. The main trends of this field of research are also addressed.

Electrochemical immunosensors - A powerful tool for analytical applications

Immunosensors are biosensors based on interactions between an antibody and antigen on a transducer surface. Either antibody or antigen can be the species immobilized on the transducer to detect antigen or antibody, respectively. Because of the strong binding forces between these biomolecules, immunosensors present high selectivity and very high sensitivity, making them very attractive for many applications in different science fields. Electrochemical immunosensors explore measurements of an electrical signal produced on an electrochemical transductor. This signal can be voltammetric, potentiometric, conductometric or impedimetric. Immunosensors utilizing electrochemical detection have been explored in several analyses since they are specific, simple, portable, and generally disposable and can carry out in situ or automated detection. This review addresses the potential of immunosensors destined for application in food and environmental analysis, and cancer biomarker diagnosis. Emphasis is given to the approaches that have been used for construction of electrochemical immunosensors. Additionally, the fundamentals of immunosensors, technology of transducers and nanomaterials and a general overview of the possible applications of electrochemical immunosensors to the food, environmental and diseases analysis fields are described.

Effective Method for Micro-Patterning Arene-Terminated Monolayers on a Si(111) Electrode

Microstructured electrodes are significant to modern electrochemistry. A representative aromatic group, 4-ferrocenylphenyl one, was covalently bound to a micropatterned silicon electrode via the arylation of a hydrogen-terminated silicon(111) surface formed selectively on a Si wafer. Starting from a silicon(100)-on-insulator (SOI) wafer, the aromatic monolayer was attached sequentially by spin-coating a resist, electron beam lithography, Cr/Au deposition, lift-off, anisotropic etching with aqueous KOH solution, and Pd-catalyzed arylation. Cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) are used to characterize the coupling reaction between 4-ferrocenyl group and silicon substrate, and to confirm performance of the final modified microsized electrode. These data show that this synthetic protocol gives chemically well-defined and robust functionalized monolayers on a silicon semiconducting surface with a small electrode.

Unconventional Electrochemistry in Micro-/Nanofluidic Systems

Electrochemistry is ideally suited to serve as a detection mechanism in miniaturized analysis systems. A significant hurdle can, however, be the implementation of reliable micrometer-scale reference electrodes. In this tutorial review, we introduce the principal challenges and discuss the approaches that have been employed to build suitable references. We then discuss several alternative strategies aimed at eliminating the reference electrode altogether, in particular two-electrode electrochemical cells, bipolar electrodes and chronopotentiometry.

Oxygen profiling of the unsaturated zone using direct push drilling

A methodology for oxygen profile measurements in the unsaturated zone is developed based on direct push drilling using sampling liners equipped with homemade silicone septa. The oxygen measurement is carried out by puncturing the septum with a commercial retractable optode needle fitted with a fluorescent tip. Metrological characteristics and method validation were carried out under laboratory conditions using different levels of oxygen and various water contents. The relative standard deviations under dry and water saturated soil conditions were less than 0.3% and 5% for 0.5 mg L(-1) of oxygen and less than 2% and 3% for 9 mg L(-1). Field demonstrations in a calcareous-sandstone soil aquifer treatment system with a layered clayey, marl and sandstone lithology of widely different water contents provided down to 30 m deep profiles of the dissolved oxygen level with less than 1.5 m spatial resolution. A single sensor was used for over 50 field measurements, though recalibration was required after approximately 30 measurements due to the deterioration of the fluorescent tip.

Redox cycling without reference electrodes

The reference electrode is a key component in electrochemical measurements, yet it remains a challenge to implement a reliable reference electrode in miniaturized electrochemical sensors. Here we explore experimentally and theoretically an alternative approach based on redox cycling which eliminates the reference electrode altogether. We show that shifts in the solution potential caused by the lack of reference can be understood quantitatively, and determine the requirements for accurate measurements in miniaturized systems in the absence of a reference electrode.

Biochips and other microtechnologies for physiomics

This paper presents a review of microtechnologies relevant to applications in cellular physiology, including biochips, electrochemical sensors and optrodic sensing techniques. Microelectrodes have been the main tools for measuring cellular electrophysiology, oxygen, nitric oxide, neurotransmitters, pH and various ions. Optical fiber sensing methods, such as indicator-based optrodes, with fluorescence lifetime measurement, are now emerging as viable alternatives to electroanalytical chemistry. These new optrode techniques are possible because of recent advances in the optoelectronics industry and are comparably easier to miniaturize, have faster response times, do not consume the analyte and have lower operational costs. This review serves as a summary and predicts future trends for both electrochemical and optical luminescence lifetime sensing as components in lab-on-a-chip devices for physiological sensing.

Microdevice for on-site fish freshness checking based on K-value measurement

An electrochemical microfluidic device with two sensing sites in the upper and lower streams of a flow channel was fabricated to measure the K-value as a means of evaluating the freshness of fish. In this device, plugs of solutions were processed using mechanisms to place a plug at the sensing site and to merge and mix two plugs in a single flow channel. The sums of ATP-related compound concentrations used for the calculation of the K-value were measured at the first and second sensing sites. The ratio of the output currents agreed well with the K-value calculated from predetermined concentrations in standard solutions. The K-values of jack mackerel, yellow tail, and sea bream extracts were then obtained using the device and were found to agree well with those obtained by high-performance liquid chromatography (HPLC). In addition, the changes in the K-value with time were observed to depend strongly on the type of fish for these three fish extracts.

Combinatorial and high-throughput screening of materials libraries: review of state of the art

Rational materials design based on prior knowledge is attractive because it promises to avoid time-consuming synthesis and testing of numerous materials candidates. However with the increase of complexity of materials, the scientific ability for the rational materials design becomes progressively limited. As a result of this complexity, combinatorial and high-throughput (CHT) experimentation in materials science has been recognized as a new scientific approach to generate new knowledge. This review demonstrates the broad applicability of CHT experimentation technologies in discovery and optimization of new materials. We discuss general principles of CHT materials screening, followed by the detailed discussion of high-throughput materials characterization approaches, advances in data analysis/mining, and new materials developments facilitated by CHT experimentation. We critically analyze results of materials development in the areas most impacted by the CHT approaches, such as catalysis, electronic and functional materials, polymer-based industrial coatings, sensing materials, and biomaterials.

Recent trends in potentiometric sensor arrays--a review

Nowadays there exists a large variety of ion sensors based on polymeric or solid-state membranes that can be used in a sensor array format in many analytical applications. This review aims at providing a critical overview of the distinct approaches that were developed to build and use potentiometric sensor arrays based on different transduction principles, such as classical ion-selective electrodes (ISEs) with polymer or solid-state membranes, solid-contact electrodes (SCE) including coated wire electrodes (CWE), ion-sensitive field-effect transistors (ISFETs) and light addressable potentiometric sensors (LAPS). Analysing latest publications on potentiometric sensor arrays development and applications certain problems are outlined and trends are discussed.

DNA diagnostics: nanotechnology-enhanced electrochemical detection of nucleic acids

The detection of mismatched base pairs in DNA plays a crucial role in the diagnosis of genetic-related diseases and conditions, especially for early stage treatment. Among the various biosensors that have been used for DNA detection, EC sensors show great promise because they are capable of precise DNA recognition and efficient signal transduction. Advancements in micro- and nanotechnologies, specifically fabrication techniques and new nanomaterials, have enabled for the development of highly sensitive, highly specific sensors making them attractive for the detection of small sequence variations. Furthermore, the integration of sensors with sample preparation and fluidic processes enables for rapid, multiplexed DNA detection essential for POC clinical diagnostics.

Microfabricated reference electrodes and their biosensing applications

Over the past two decades, there has been an increasing trend towards miniaturization of both biological and chemical sensors and their integration with miniaturized sample pre-processing and analysis systems. These miniaturized lab-on-chip devices have several functional advantages including low cost, their ability to analyze smaller samples, faster analysis time, suitability for automation, and increased reliability and repeatability. Electrical based sensing methods that transduce biological or chemical signals into the electrical domain are a dominant part of the lab-on-chip devices. A vital part of any electrochemical sensing system is the reference electrode, which is a probe that is capable of measuring the potential on the solution side of an electrochemical interface. Research on miniaturization of this crucial component and analysis of the parameters that affect its performance, stability and lifetime, is sparse. In this paper, we present the basic electrochemistry and thermodynamics of these reference electrodes and illustrate the uses of reference electrodes in electrochemical and biological measurements. Different electrochemical systems that are used as reference electrodes will be presented, and an overview of some contemporary advances in electrode miniaturization and their performance will be provided.

Liquid-phase packaging of a glucose oxidase solution with parylene direct encapsulation and an ultraviolet curing adhesive cover for glucose sensors

We have developed a package for disposable glucose sensor chips using Parylene encapsulation of a glucose oxidase solution in the liquid phase and a cover structure made of an ultraviolet (UV) curable adhesive. Parylene was directly deposited onto a small volume (1 μL) of glucose oxidase solution through chemical vapor deposition. The cover and reaction chamber were constructed on Parylene film using a UV-curable adhesive and photolithography. The package was processed at room temperature to avoid denaturation of the glucose oxidase. The glucose oxidase solution was encapsulated and unsealed. Glucose sensing was demonstrated using standard amperometric detection at glucose concentrations between 0.1 and 100 mM, which covers the glucose concentration range of diabetic patients. Our proposed Parylene encapsulation and UV-adhesive cover form a liquid phase glucose-oxidase package that has the advantages of room temperature processing and direct liquid encapsulation of a small volume solution without use of conventional solidifying chemicals.

Glucose and lactate biosensors for scanning electrochemical microscopy imaging of single live cells

We have developed glucose and lactate ultramicroelectrode (UME) biosensors based on glucose oxidase and lactate oxidase (with enzymes immobilized onto Pt UMEs by either electropolymerization or casting) for scanning electrochemical microscopy (SECM) and have determined their sensitivity to glucose and lactate, respectively. The results of our evaluations reveal different advantages for sensors constructed by each method: improved sensitivity and shorter manufacturing time for hand-casting, and increased reproducibility for electropolymerization. We have acquired amperometric approach curves (ACs) for each type of manufactured biosensor UME, and these ACs can be used as a means of positioning the UME above a substrate at a known distance. We have used the glucose biosensor UMEs to record profiles of glucose uptake above individual fibroblasts. Likewise, we have employed the lactate biosensor UMEs for recording the lactate production above single cancer cells with the SECM. We also show that oxygen respiration profiles for single cancer cells do not mimic cell topography, but are rather more convoluted, with a higher respiration activity observed at the points where the cell touches the Petri dish. These UME biosensors, along with the application of others already described in the literature, could prove to be powerful tools for mapping metabolic analytes, such as glucose, lactate, and oxygen, in single cancer cells.

Integrated Electrochemical Analysis System with Microfluidic and Sensing Functions

An integrated device that carries out the timely transport of solutions and conducts electroanalysis was constructed. The transport of solutions was based on capillary action in overall hydrophilic flow channels and control by valves that operate on the basis of electrowetting. Electrochemical sensors including glucose, lactate, glutamic oxaloacetic transaminase (GOT), glutamic pyruvic transaminase (GPT), pH, ammonia, urea, and creatinine were integrated. An air gap structure was used for the ammonia, urea, and creatinine sensors to realize a rapid response. To enhance the transport of ammonia that existed or was produced by the enzymatic reactions, the pH of the solution was elevated by mixing it with a NaOH solution using a valve based on electrowetting. The sensors for GOT and GPT used a freeze-dried substrate matrix to realize rapid mixing. The sample solution was transported to required sensing sites at desired times. The integrated sensors showed distinct responses when a sample solution reached the respective sensing sites. Linear relationships were observed between the output signals and the concentration or the logarithm of the concentration of the analytes. An interferent, L-ascorbic acid, could be eliminated electrochemically in the sample injection port.

Enzyme electrode formed by evaporative concentration and its performance characterization

A highly concentrated immobilized enzyme layer was formed on a small working electrode, and the behavior of the electrode as an amperometric sensor was examined. To this end, a super-hydrophobic layer was formed in an area other than the sensitive area by using polytetrafluoroethylene (PTFE) beads. A small droplet of an enzyme solution containing glucose oxidase (GOD) and bovine serum albumin (BSA) was placed on the sensitive area, concentrated by evaporation, and crosslinked with glutaraldehyde. With the same enzyme activity per unit area, the current density increased with smaller working electrodes. Also, the current density increased with higher enzyme loadings up to a limiting value. In addition, the linear range of the calibration plot was expanded to higher glucose concentrations. The enzyme electrode fabricated by the novel method was incorporated in a micro-flow channel. Compared with large enzyme electrodes with the same enzyme activity per unit area, smaller electrodes showed a significant increase in the current density and a decrease in the flow dependence. The conversion efficiency could be improved by narrowing the flow channel and increasing the number of electrodes, which was comparable with a large electrode placed in a shallow flow channel.

Enhanced electrochemical performance at screen-printed carbon electrodes by a new pretreating procedure

A new method for the pretreatment of screen-printed carbon electrodes (SPCEs) by two successive steps was proposed. In step one, fresh SPCEs were soaked into NaOH with high concentration (e.g. 3 M) for tens to hundreds of minutes, and the resulted electrodes were called as SPCE-I. In step two, SPCE-I were pre-anodized in low concentration of NaOH, which were designated as SPCE-II. The pretreated electrodes showed remarkable enhancement in heterogeneous electron transfer rate constant (k0) increased from 1.6x10(-4) cms(-1) at the fresh SPCE to 1.1x10(-2) cms(-1) at SPCE-I for Fe(CN)6(3-/4-) couple. The peak to peak separation (deltaE(p)) in cyclic voltammetry was reduced from ca. 480 to 84 mV, indicating that the electrochemical reversibility was greatly promoted, possibly due to the removing of polymers/oil binder from the electrode surfaces. The electroactive area (A(ea)) of the electrode was increased by a factor of 17 after pretreatment in step one. Further analysis by the electrochemical impedance method showed that the electron transfer resistance (R(ct)) decreased from ca. 2100 to 1.4 ohms. These pretreated electrodes, especially SPCE-II, exhibited excellent electrocatalytic behavior for the redox of dopamine (DA). Interference from ascorbic acid (AA) in the detection of DA at SPCE-II could be effectively eliminated due to the anodic peak separation (190 mV) between DA and AA, which resulted from the functionalization of the electrode surface in the pretreatment of step two. Under optimum conditions, current responses to DA were linearly changed in two concentration intervals, one was from 3.0x10(-7) to 9.8x10(-6) M, and the other was from 9.8x10(-6) to 3.3x10(-4) M. The detection limit for DA was down to 1.0x10(-7) M.

Determination of the activities of glutamic oxaloacetic transaminase and glutamic pyruvic transaminase in a microfluidic system

A microfluidic system for the analysis of the activities of glutamic-oxaloacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT) was fabricated. The device consists of a glass chip with a micro-electrochemical L-glutamate sensor and a polydimethylsiloxane (PDMS) sheet with a Y-shaped micro-flow channel. A sample solution and a substrate solution for the enzymes were introduced from two injection ports at the end of the flow channel. When the flows were stopped, substrates in a solution mixed immediately with either of the enzymes by diffusion in a mixing channel. L-glutamate produced by the enzymatic reaction of GOT or GPT in the flow channel was detected by using the L-glutamate sensor. A distinct current increase was observed immediately after mixing, and the initial slope of the response curve varied in proportion to the activity of GOT or GPT. The relation between the slope of the response curve and the enzyme activity was linear between 7 and 228 U l-1 for GOT and 9 and 250 U l-1 for GPT. The quality of the response curve was improved with an increase in the channel height. The measurement based on the rate analysis in the micro-flow channel facilitated the reduction of the influence of interferents. The influence of the viscosity of the sample solution was also checked for the analysis of real samples. The determination of the enzyme activities was also conducted in a system with micropumps fabricated for a sample injection. Two solutions could be mixed in the mixing channel, and the activity of the enzymes could be measured as in the experiments using microsyringe pumps.

Electrochemical microfluidic biosensor for nucleic acid detection with integrated minipotentiostat

An electrochemical microfluidic biosensor with an integrated minipotentiostat for the quantification of RNA was developed based on nucleic acid hybridization and liposome signal amplification. Specificity of the biosensor was ensured by short DNA probes that hybridize with the target RNA or DNA sequence. The reporter probe was coupled to liposomes entrapping the electrochemically active redox couple potassium ferri/ferrohexacyanide. The capture probes were coupled to superparamagnetic beads that were isolated on a magnet in the biosensor. Upon capture, the liposomes were lysed to release the electrochemical markers that were detected on an interdigitated ultramicroelectrode array in the biosensor just downstream of the magnet. The current was measured, stored and displayed by miniaturized instrumentation (miniEC). The accuracy of the miniEC was evaluated by comparing its performance to a standard bench-top electrochemical workstation in static and dynamic DC amperometric experiments. In both sets of experiments, the inexpensive miniEC performance was comparable in signal strength to that of the electrochemical workstation. In fact, the miniEC achieved a detection limit of 0.01 microM combined ferri/ferrohexacyanide concentration which was 10 x lower than that of the standard lab-bench system. The response time of the miniEC system was the same for low concentrations taking about 10 s to steady state. It was, however, slower at higher concentrations, taking 5 s versus only 1 s for the bench-top system. Finally, the functionality of the miniEC was successfully demonstrated with the detection of Dengue virus RNA.

Controlled-release microchips

Efficient drug delivery remains an important challenge in medicine: continuous release of therapeutic agents over extended time periods in accordance with a predetermined temporal profile; local delivery at a constant rate to the tumour microenvironment to overcome much of the systemic toxicity and to improve antitumour efficacy; improved ease of administration, and increasing patient compliance required are some of the unmet needs of the present drug delivery technology. Microfabrication technology has enabled the development of novel controlled-release microchips with capabilities not present in the current treatment modalities. In this review, the current status and future prospects of different types of controlled-release microchips are summarised and analysed with reference to microneedle-based microchips, as well as providing an in-depth focus on microreservoir-based and nanoporous microchips.