Ion-selective electrode biochip for applications in a liquid environment

Physiological sensing conducted in a liquid environment requires electrodes with long lifetime. The development of a robust ion-selective electrode–based biochip in a lab-on-a-chip platform is described. To compare electrode lifetime, which is driven by the transducer layer, electrochemical measurements were performed in a custom-made flow-cell chamber. The results of potentiometric measurement of cationic analytes demonstrate the electrodes to have a near-Nernstian slope profile even after they are stored for almost a month in liquid medium. The electrodes also achieved H2O2 amperometric sensitivity (1.25 and 3.32 µAmM-1cm-2 for PEDOT:PSS and PEDOT:CaSO4 respectively) and lower detection limit (2.21 µM, 8.4 µM, 3.44 µM, for H+, NH4+, Ca2+ respectively) comparable to that of wire-type electrodes. Furthermore, the lifetime is dependent on the electrodeposition method of the conductive polymer, and the transducer layer must be modified to fit the analyte types. These results indicate that extended lifetime of microfabricated ion-selective electrodes in a multiplex format can be realized by optimizing the microfabricated electrode surface functionalization

Lab-on-a-chip based biosensors for fundamental space biology research

The space environment poses significant challenges to the development and survival of biological organisms. Particularly the altered gravity of space is known to have an adverse affect on development of animals, plants and human beings. How organisms react to different gravity regimes such as the micro-g environment faced during space flight or the reduced gravity on Moon and Mars is an exciting area of research. One way to characterize physiological changes particularly in cultured cells and microorganisms is to measure the concentrations of extracellular biomolecules that play a central role in growth, development, form and function. Such studies are possible using modern electrochemical biosensors. The reduced payload requirements of spaceflight pose a constraint on the size of these biosensors. With the advent of Micro-Electro-Mechanical-Systems (MEMS) based biosensors or BioMEMS, it is now possible to produce miniaturized biosensors that can easily address this requirement. This work focuses on two such MEMS fabricated lab-on-a-chip based biosensors for understanding the fundamental space biology of model organisms. The first device is called the Cell Electrophysiology Lab-on-a-chip or the CEL-C biochip. This biochip was designed with the specific science objective of studying the gravity-sensing dynamics of the spore of the fern Ceratopteris richardii. The CEL-C biochip combines calcium sensing chemistries with microfabricated electrodes. After integration with signal processing electronics and an automated data acquisition system the biochip can perform simultaneous measurements on 16 spores simultaneously. The CEL-C biochip served as the enabling technology for ground based and reduced gravity studies on the C. richardii system. The results unearthed a previously unknown mechanism of gravity sensing in the spores possibly involving mechanosensory ion channels and pumps. The second lab-on-a-chip is called the CHO biochip and was developed with the goal of studying gravitational physiology of cyanobacteria in a space environment. This is a multianalyte sensing biochip which integrates sensors for pH, carbonate/bicarbonate and O2 on the same device. These three parameters play a central role in photosynthesis and carbon fixation in cyanobacteria. While both these biochips were designed to address a specific scientific problem, they can serve as general purpose tools for fundamental research, biological and biomedical applications. These foundation technologies have now opened doors for new lab-on-a-chip devices for neurophysiology research, biomedical diagnostics, environmental monitoring and agricultural applications

Lab-on-a-chip approaches for space-biology research

Lab-on-a-chip (LOC) systems with electrochemical sensing capability can provide real-time physiological measurements in spaceflight environments. They are easily miniaturized and integrated with existing space hardware systems. To reduce crew time during spaceflight research, the systems can be made autonomous and simple to use. Research and development of electrochemical-sensing LOC systems are still in progress for fundamental space-biology research in microgravity. Ion-selective electrodes as electrochemical sensors are miniaturized in an all-solid-state format for easier packaging and handling. The design, fabrication, and application of these sensors are discussed, with examples from those developed at the Physiological Sensing Facility (PSF) at Purdue University. The objective of this paper is not to provide an exhaustive review of current LOC systems, but to describe research developments made for the purpose of conducting physiological measurements in microgravity with examples of patents that support space missions

Large naturally-produced electric currents and voltage traverse damaged mammalian spinal cord

Abstract Background Immediately after damage to the nervous system, a cascade of physical, physiological, and anatomical events lead to the collapse of neuronal function and often death. This progression of injury processes is called "secondary injury." In the spinal cord and brain, this loss in function and anatomy is largely irreversible, except at the earliest stages. We investigated the most ignored and earliest component of secondary injury. Large bioelectric currents immediately enter damaged cells and tissues of guinea pig spinal cords. The driving force behind these currents is the potential difference of adjacent intact cell membranes. For perhaps days, it is the biophysical events caused by trauma that predominate in the early biology of neurotrauma. Results An enormous (≤ mA/cm2) bioelectric current transverses the site of injury to the mammalian spinal cord. This endogenous current declines with time and with distance from the local site of injury but eventually maintains a much lower but stable value (2). The calcium component of this net current, about 2.0 pmoles/cm2/sec entering the site of damage for a minimum of an hour, is significant. Curiously, injury currents entering the ventral portion of the spinal cord may be as high as 10 fold greater than those entering the dorsal surface, and there is little difference in the magnitude of currents associated with crush injuries compared to cord transection. Physiological measurements were performed with non-invasive sensors: one and two-dimensional extracellular vibrating electrodes in real time. The calcium measurement was performed with a self-referencing calcium selective electrode. Conclusion The enormous bioelectric current, carried in part by free calcium, is the major initiator of secondary injury processes and causes significant damage after breach of the membranes of vulnerable cells adjacent to the injury site. The large intra-cellular voltages, polarized along the length of axons in particular, are believed to be associated with zones of organelle death, distortion, and asymmetry observed in acutely injured nerve fibers. These data enlarge our understanding of secondary mechanisms and provide new ways to consider interfering with this catabolic and progressive loss of tissue.</p

A comparative study of enzyme immobilization strategies for multi-walled carbon nanotube glucose biosensors

This work addresses the comparison of different strategies for improving biosensor performance using nanomaterials. Glucose biosensors based on commonly applied enzyme immobilization approaches, including sol-gel encapsulation approaches and glutaraldehyde cross-linking strategies, were studied in the presence and absence of multi-walled carbon nanotubes (MWNTs). Although direct comparison of design parameters such as linear range and sensitivity is intuitive, this comparison alone is not an accurate indicator of biosensor efficacy, due to the wide range of electrodes and nanomaterials available for use in current biosensor designs. We proposed a comparative protocol which considers both the active area available for transduction following nanomaterial deposition and the sensitivity. Based on the protocol, when no nanomaterials were involved, TEOS/GOx biosensors exhibited the highest efficacy, followed by BSA/GA/GOx and TMOS/GOx biosensors. A novel biosensor containing carboxylated MWNTs modified with glucose oxidase and an overlying TMOS layer demonstrated optimum efficacy in terms of enhanced current density (18.3 +/- 0.5 mu A mM(-1) cm(-2)), linear range (0.0037-12 mM), detection limit (3.7 mu M), coefficient of variation (2%), response time (less than 8 s), and stability/selectivity/reproducibility. H(2)O(2) response tests demonstrated that the most possible reason for the performance enhancement was an increased enzyme loading. This design is an excellent platform for versatile biosensing applications

Multi-analyte biochip (MAB) based on all-solid-state Ion-selective electrodes (ASSISE) for Physiological Research

Lab-on-a-chip (LOC) applications in environmental, biomedical, agricultural, biological, and spaceflight research require an ion-selective electrode (ISE) that can withstand prolonged storage in complex biological media 1-4. An all-solid-state ion-selective-electrode (ASSISE) is especially attractive for the aforementioned applications. The electrode should have the following favorable characteristics: easy construction, low maintenance, and (potential for) miniaturization, allowing for batch processing. A microfabricated ASSISE intended for quantifying H+ , Ca2+, and CO32- ions was constructed. It consists of a noble-metal electrode layer (i.e. Pt), a transduction layer, and an ion-selective membrane (ISM) layer. The transduction layer functions to transduce the concentration-dependent chemical potential of the ion selective membrane into a measurable electrical signal. The lifetime of an ASSISE is found to depend on maintaining the potential at the conductive layer/membrane interface 5-7. To extend the ASSISE working lifetime and thereby maintain stable potentials at the interfacial layers, we utilized the conductive polymer (CP) poly(3,4-ethylenedioxythiophene) (PEDOT) 7-9 in place of silver/silver chloride (Ag/AgCl) as the transducer layer. We constructed the ASSISE in a lab-ona chip format, which we called the multi-analyte biochip (MAB) (Figure 1). Calibrations in test solutions demonstrated that the MAB can monitor pH (operational range pH 4-9), CO32- (measured range 0.01 mM - 1 mM), and Ca2+ (log-linear range 0.01 mM to 1 mM). The MAB for pH provides a near-Nernstian slope response after almost one month storage in algal medium. The carbonate biochips show a potentiometric profile similar to that of a conventional ion-selective electrode. Physiological measurements were employed to monitor biological activity of the model system, the microalga Chlorella vulgaris. The MAB conveys an advantage in size, versatility, and multiplexed analyte sensing capability, making it applicable to many confined monitoring situations, on Earth or in space