An iridium oxide reference electrode for use in microfabricated biosensors and biochips

Realization of a PEDOT:PSS/Graphene Oxide On-Chip Pseudo-Reference Electrode for Integrated ISFETs

A stable reference electrode (RE) plays a crucial role in the performance of an ion-sensitive field-effect transistor (ISFET) for bio/chemical sensing applications. There is a strong demand for the miniaturization of the RE for integrated sensor systems such as lab-on-a-chip (LoC) or point-of-care (PoC) applications. Out of several approaches presented so far to integrate an on-chip electrode, there exist critical limitations such as the effect of analyte composition on the electrode potential and drifts during the measurements. In this paper, we present a micro-scale solid-state pseudo-reference electrode (pRE) based on poly(3,4-ethylene dioxythiophene): poly(styrene sulfonic acid) (PEDOT:PSS) coated with graphene oxide (GO) to deploy with an ion-sensitive field-effect transistor (ISFET)-based sensor platform. The PEDOT:PSS was electropolymerized from its monomer on a micro size gold (Au) electrode and, subsequently, a thin GO layer was deposited on top. The stability of the electrical potential and the cross-sensitivity to the ionic strength of the electrolyte were investigated. The presented pRE exhibits a highly stable open circuit potential (OCP) for up to 10 h with a minimal drift of ~0.65 mV/h and low cross-sensitivity to the ionic strength of the electrolyte. pH measurements were performed using silicon nanowire field-effect transistors (SiNW-FETs), using the developed pRE to ensure good gating performance of electrolyte-gated FETs. The impact of ionic strength was investigated by measuring the transfer characteristic of a SiNW-FET in two electrolytes with different ionic strengths (1 mM and 100 mM) but the same pH. The performance of the PEDOT:PSS/GO electrode is similar to a commercial electrochemical Ag/AgCl reference electrode.

Biocompatible reference electrodes to enhance chronic electrochemical signal fidelity in vivo

In vivo electrochemistry is a vital tool of neuroscience that allows for the detection, identification, and quantification of neurotransmitters, their metabolites, and other important analytes. One important goal of in vivo electrochemistry is a better understanding of progressive neurological disorders (e.g., Parkinson's disease). A complete understanding of such disorders can only be achieved through a combination of acute (i.e., minutes to hours) and chronic (i.e., days or longer) experimentation. Chronic studies are more challenging because they require prolonged implantation of electrodes, which elicits an immune response, leading to glial encapsulation of the electrodes and altered electrode performance (i.e., biofouling). Biofouling leads to increased electrode impedance and reference electrode polarization, both of which diminish the selectivity and sensitivity of in vivo electrochemical measurements. The increased impedance factor has been successfully mitigated previously with the use of a counter electrode, but the challenge of reference electrode polarization remains. The commonly used Ag/AgCl reference electrode lacks the long-term potential stability in vivo required for chronic measurements. In addition, the cytotoxicity of Ag/AgCl adversely affects animal experimentation and prohibits implantation in humans, hindering translational research progress. Thus, a move toward biocompatible reference electrodes with superior chronic potential stability is necessary. Two qualifying materials, iridium oxide and boron-doped diamond, are introduced and discussed in terms of their electrochemical properties, biocompatibilities, fabrication methods, and applications. In vivo electrochemistry continues to advance toward more chronic experimentation in both animal models and humans, necessitating the utilization of biocompatible reference electrodes that should provide superior potential stability and allow for unprecedented chronic signal fidelity when used with a counter electrode for impedance mitigation.

Iridium Oxide Enabled Sensors Applications

There have been numerous studies applying iridium oxides in different applications to explore their proton-change-based reactions since the 1980s. Iridium oxide can be fabricated directly by applying electrodeposition, sputter-coating method, or oxidation of iridium wire. Generally, there have been currently two approaches in applying iridium oxide to enable its sensing applications. One was to improve or create different electrolytes with (non-)electrodeposition method for better performance of Nernst Constant with the temperature-related system. The mechanism behind the scenes were summarized herein. The other was to change the structure of iridium oxide through different kinds of templates such as photolithography patterns, or template-assisted direct growth methods, etc. to improve the sensing performance. The detection targets varied widely from intracellular cell pH, glucose in an artificial sample or actual urine sample, and the hydrogen peroxide, glutamate or organophosphate pesticides, metal-ions, etc. This review paper has focused on the mechanism of electrodeposition of iridium oxide in aqueous conditions and the sensing applications towards different biomolecules compounds. Finally, we summarize future trends on Iridium oxide based sensing and predict future work that could be further explored.

Process Variability in Top-Down Fabrication of Silicon Nanowire-Based Biosensor Arrays

Silicon nanowire field-effect transistors (SiNW-FET) have been studied as ultra-high sensitive sensors for the detection of biomolecules, metal ions, gas molecules and as an interface for biological systems due to their remarkable electronic properties. "Bottom-up" or "top-down" approaches that are used for the fabrication of SiNW-FET sensors have their respective limitations in terms of technology development. The "bottom-up" approach allows the synthesis of silicon nanowires (SiNW) in the range from a few nm to hundreds of nm in diameter. However, it is technologically challenging to realize reproducible bottom-up devices on a large scale for clinical biosensing applications. The top-down approach involves state-of-the-art lithography and nanofabrication techniques to cast SiNW down to a few 10s of nanometers in diameter out of high-quality Silicon-on-Insulator (SOI) wafers in a controlled environment, enabling the large-scale fabrication of sensors for a myriad of applications. The possibility of their wafer-scale integration in standard semiconductor processes makes SiNW-FETs one of the most promising candidates for the next generation of biosensor platforms for applications in healthcare and medicine. Although advanced fabrication techniques are employed for fabricating SiNW, the sensor-to-sensor variation in the fabrication processes is one of the limiting factors for a large-scale production towards commercial applications. To provide a detailed overview of the technical aspects responsible for this sensor-to-sensor variation, we critically review and discuss the fundamental aspects that could lead to such a sensor-to-sensor variation, focusing on fabrication parameters and processes described in the state-of-the-art literature. Furthermore, we discuss the impact of functionalization aspects, surface modification, and system integration of the SiNW-FET biosensors on post-fabrication-induced sensor-to-sensor variations for biosensing experiments.

An implantable multifunctional neural microprobe for simultaneous multi-analyte sensing and chemical delivery

A multifunctional chemical neural probe fabrication process exploiting PDMS thin-film transfer to incorporate a microfluidic channel onto a silicon-based microelectrode array (MEA) platform, and enzyme microstamping to provide multi-analyte detection is described. The Si/PDMS hybrid chemtrode, modified with a nano-based on-probe IrOx reference electrode, was validated in brain phantoms and in rat brain.

Miniaturized Planar Room Temperature Ionic Liquid Electrochemical Gas Sensor for Rapid Multiple Gas Pollutants Monitoring

The growing impact of airborne pollutants and explosive gases on human health and occupational safety has escalated the demand of sensors to monitor hazardous gases. This paper presents a new miniaturized planar electrochemical gas sensor for rapid measurement of multiple gaseous hazards. The gas sensor features a porous polytetrafluoroethylene substrate that enables fast gas diffusion and room temperature ionic liquid as the electrolyte. Metal sputtering was utilized for platinum electrodes fabrication to enhance adhesion between the electrodes and the substrate. Together with carefully selected electrochemical methods, the miniaturized gas sensor is capable of measuring multiple gases including oxygen, methane, ozone and sulfur dioxide that are important to human health and safety. Compared to its manually-assembled Clark-cell predecessor, this sensor provides better sensitivity, linearity and repeatability, as validated for oxygen monitoring. With solid performance, fast response and miniaturized size, this sensor is promising for deployment in wearable devices for real-time point-of-exposure gas pollutant monitoring.

Multifunctional Water Sensors for pH, ORP, and Conductivity Using Only Microfabricated Platinum Electrodes

Monitoring of the pH, oxidation-reduction-potential (ORP), and conductivity of aqueous samples is typically performed using multiple sensors. To minimize the size and cost of these sensors for practical applications, we have investigated the use of a single sensor constructed with only bare platinum electrodes deposited on a glass substrate. The sensor can measure pH from 4 to 10 while simultaneously measuring ORP from 150 to 800 mV. The device can also measure conductivity up to 8000 μS/cm in the range of 10 °C to 50 °C, and all these measurements can be made even if the water samples contain common ions found in residential water. The sensor is inexpensive (i.e., ~$0.10/unit) and has a sensing area below 1 mm², suggesting that the unit is cost-efficient, robust, and widely applicable, including in microfluidic systems.

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.

On-chip metal/polypyrrole quasi-reference electrodes for robust ISFET operation

To operate an ion-sensitive field-effect transistor (ISFETs) it is necessary to set the electrolyte potential using a reference electrode. Conventional reference electrodes are bulky, fragile, and too big for applications where the electrolyte volume is small. Several researchers have proposed tackling this issue using a solid-state planar micro-reference electrode or a reference field-effect transistor. However, these approaches are limited by poor robustness, high cost, or complex integration with other microfabrication processes. Here we report a simple method to create robust on-chip quasi-reference electrodes by electrodepositing polypyrrole on micro-patterned metal leads. The electrodes were fabricated through the polymerization of pyrrole on patterned metals with a cyclic voltammetry process. Open circuit potential measurements were performed to characterize the polypyrrole electrode performance, demonstrating good stability (±1 mV), low drift (∼1 mV h(-1)), and reduced pH response (5 mV per pH). In addition, the polypyrrole deposition was repeated in microelectrodes made of different metals to test compatibility with standard complementary metal-oxide-semiconductor (CMOS) processes. Our results suggest that nickel, a metal commonly used in semiconductor foundries for silicide formation, is a good candidate to form the polypyrrole quasi-reference electrodes. Finally, the polypyrrole microelectrodes were used to operate foundry fabricated ISFETs. These experiments demonstrated that transistors biased with polypyrrole electrodes have pH sensitivity and resolution comparable to ones that are biased with standard reference electrodes. Therefore, the simple fabrication, high compatibility, and robust electrical performance make polypyrrole an ideal choice for the fabrication of outstanding microreference electrodes that enable robust and sensitive operation of multiple ISFET sensors on a chip.

A solid-state thin-film Ag/AgCl reference electrode coated with graphene oxide and its use in a pH sensor

In this study, we describe a novel solid-state thin-film Ag/AgCl reference electrode (SSRE) that was coated with a protective layer of graphene oxide (GO). This layer was prepared by drop casting a solution of GO on the Ag/AgCl thin film. The potential differences exhibited by the SSRE were less than 2 mV for 26 days. The cyclic voltammograms of the SSRE were almost similar to those of a commercial reference electrode, while the diffusion coefficient of Fe(CN)₆³⁻ as calculated from the cathodic peaks of the SSRE was 6.48 × 10⁻⁶ cm²/s. The SSRE was used in conjunction with a laboratory-made working electrode to determine its suitability for practical use. The average pH sensitivity of this combined sensor was 58.5 mV/pH in the acid-to-base direction; the correlation coefficient was greater than 0.99. In addition, an integrated pH sensor that included the SSRE was packaged in a secure digital (SD) card and tested. The average sensitivity of the chip was 56.8 mV/pH, with the correlation coefficient being greater than 0.99. In addition, a pH sensing test was also performed by using a laboratory-made potentiometer, which showed a sensitivity of 55.4 mV/pH, with the correlation coefficient being greater than 0.99.

Electrochemical artifacts originating from nanoparticle contamination by Ag/AgCl quasi-reference electrodes

Electrochemical techniques rely on the stability of a defined reference potential. Due to the need for miniaturization, electrochemical lab-on-a-chip platforms often employ Ag/AgCl quasi-reference electrodes for this purpose. Here, we report on electrochemical artifacts resulting from nanoparticle-electrode collisions originating from standard chlorinated silver wires.

Electrochemically deposited iridium oxide reference electrode integrated with an electroenzymatic glutamate sensor on a multi-electrode array microprobe

An implantable micromachined multi-electrode array (MEA) microprobe modified for utilization as a complete electrochemical biosensor for rapid glutamate detection is described. A post-fabrication method for electrochemical deposition of an iridium oxide (IrOx) film onto a designated microelectrode enabled incorporation of an IrOx reference electrode (RE) on the microprobe. The on-probe IrOx RE provides an alternative to the commonly utilized Ag/AgCl wire RE, which has been shown to be unstable and to cause an inflammatory response in living tissue. The IrOx film electrodeposited onto a platinum site was tested as part of a complete chemical sensing system that included a platinum counter electrode and enzymatic glutamate sensing electrodes all on a single silicon-based MEA platform. The thin film IrOx was mechanically robust enough to endure conditions of repeated heating and wetting during the MEA fabrication process. The pH dependence of the IrOx open circuit potential (OCP) was measured at -77±0.4 mV/pH and remained stable over a two-week period. The on-probe IrOx RE was tested in a two- and three-electrode system with glutamate biosensors. The biosensors were shown to detect a physiologically relevant range of glutamate concentrations and to reject the interferents, dopamine and ascorbic acid. By incorporating all of the electrodes onto a single device, baseline noise was reduced by an average of ∼61% in vitro and ∼71% in vivo with reduced tissue damage, since only a single probe needed to be implanted.

Brain-friendly amperometric enzyme biosensor based on encapsulated oxygen generating biomaterial

A novel first-generation Clark-type biosensor platform that can eliminate the oxygen dependence has been presented. Sufficient oxygen to drive the enzymatic reaction under hypoxic conditions was produced by encapsulated oxygen generating biomaterial, calcium peroxide. The catalase immobilized in chitosan matrix was coated on top of the groove to decompose residual hydrogen peroxide to oxygen. A glucose biosensor was developed on the proposed platform as proof of concept. Under hypoxic conditions, developed glucose biosensors maintained their sensitivity response around 84% of their response at oxygen tension of 151 mmHg. The sensitivity deviation was less than 5.3% with the oxygen tension traversed from 0 to 57 mmHg. Under oxygen tension of 8.3 mmHg, the sensitivity of 37.130 nA/mM and the linear coefficient of R(2)=0.9968 were obtained with the glucose concentration varying from 0.05 to 10mM. This new platform is particularly attractive for injured brain monitoring.

Modulation of fibroblast inflammatory response by surface modification of a perfluorinated ionomer

An ideal surface for implantable glucose sensors would be able to evade the events leading to chronic inflammation and fibrosis, thereby extending its utility in an in vivo environment. Nafion™, a perfluorinated ionomer, is the membrane material preferred for in situ glucose sensors. Unfortunately, the surface properties of Nafion™ promote random protein adsorption and eventual foreign body encapsulation, thus leading to loss of glucose signal over time. Details of the techniques to render Nafion™ nonprotein fouling are given in a previous article [T. I. Valdes et al., Biomaterials 29, 1356 (2008)]. Once random protein adsorption is prevented, a biologically active peptide can be covalently bonded to the treated Nafion™ to induce cellular adhesion. Cellular responses to these novel decorated Nafion™ surfaces are detailed here, including cell viability, cell spreading, and type I collagen synthesis. Normal human dermal fibroblasts (NHDFs) were cultured on control and modified Nafion™ surfaces. Findings indicate that Nafion™ modified with 10% 2-hydroxyethyl methacrylate and 90% tetraglyme created a nonfouling surface that was subsequently decorated with the YRGDS peptide. NHDFs were shown to have exhibited decreased type I collagen production in comparison to NHDF cells on unmodified Nafion™ surfaces. Here, the authors report evidence that proves that optimizing conditions to prevent protein adsorption and enhance cellular adhesion may eliminate fibrous encapsulation of an implant.

The open container-used microfluidic chip using IrO(x) ultramicroelectrodes for the in situ measurement of extracellular acidification

The measurement of metabolic activity based on the extracellular acidification rate has attracted wide interests in the field of biochemical detection. In the study, the chip comprising a microfluid-controlled open container and iridium oxide (IrO(x)) pH ultramicroelectrodes (UMEs) was constructed for the purpose of in situ measurement of extracellular acidification rate. The feasible anodic depositing parameters of IrO(x) film were in the range of +0.53 to +0.8 V by means of exploring the electrochemical properties of alkaline Ir(IV) deposition solution. The IrO(x) pH UMEs electrodeposited for 300 cycles between 0 V and +0.6 V exhibited the near-super-Nernstian sensitivity of -68 to -76 mV/pH and the good stability with potential drifting of 11.7 mV within 24h. The design of the open container connected with a position-raised microchannel improved the sensing stability of IrO(x) pH UMEs, with the potential deviation of as low as 0.1 mV under the flow rate of 20 μl/min. The acidification rate of HeLa cells (2160 cells/mm(2)) repeatedly measured 5 times in the microfluidic chip showed the good reproducibility of 0.021±0.002 pH/min. Moreover, the chip can decrease the acidosis occurrence, a decrease of only 0.13-0.17 pH unit in 8 min interval, during the measurement of cellular metabolic activity.

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.

A novel lab-on-a-tube for multimodality neuromonitoring of patients with traumatic brain injury (TBI)

A novel lab-on-a-tube integrated with spirally-rolled pressure, temperature, oxygen and glucose microsensors is described for multimodal neuromonitoring of patients with traumatic brain injury. In addition to measuring various crucial parameters in real-time continuous formats, the newly developed device also works as an intraventricular catheter to lower the elevated intracranial pressure by draining cerebrospinal fluid.

Toward real-time continuous brain glucose and oxygen monitoring with a smart catheter

Oxygen and glucose biosensors have been designed, fabricated, characterized and optimized for real-time continuous monitoring on a new smart catheter for use in patients with traumatic brain injury (TBI). Oxygen sensors with three-electrode configuration were designed to achieve zero net oxygen consumption. Glucose sensors were based on the use of platinum nanoparticle-enhanced electrodes that were modified with polycation and glucose oxidase immobilized by chitosan matrix. An iridium oxide electrode was developed to work as a biocompatible reference electrode with enhanced durability and stability in the biological solutions. A study of the effect of temperature on oxygen sensor performance, and both temperature and oxygen effects on glucose sensor performance were accomplished to enhance their operative stability and provide useful information for in vivo applications. A new methodology for automatic correction of the temperature and oxygen dependence of biosensor outputs is demonstrated through programmed LabView software. In vitro experiments in both physiological and pathophysiological ranges (oxygen: 0-60 mmHg; glucose: 0.1-10 mM; temperature: 25-40 degrees C) with clinical samples of cerebrospinal fluid obtained from TBI patients have demonstrated stable measurements with enhanced accuracy, indicating the feasibility of the sensors for a real-time continuous in vivo monitoring.

Differential pH measurements of metabolic cellular activity in nl culture volumes using microfabricated iridium oxide electrodes

In this paper we describe a new approach to measure pH differences in microfluidic devices and demonstrated acidification rate measurements in on-chip cell culture systems with nl wells. We use two miniaturized identical iridium oxide (IrOx) thin film electrodes (20 micromx400 microm), one as a quasi-reference electrode, the other as a sensing electrode, placed in two confluent compartments on chip. The IrOx electrodes were deposited onto microfabricated platinum (Pt) electrodes simultaneously using electrodeposition. Incorporating the electrodes into a microfluidic device allowed us to expose each electrode to a different solution with a pH difference of one pH unit maintaining a confluent connection between the electrodes. In this configuration, we obtained a reproducible voltage difference between the two IrOx thin film electrodes, which corresponds to the electrode sensitivities of -70 mV/pH at 22 degrees C. In order to measure the acidification rate of cells in nl cell culture volumes we placed one IrOx thin film electrode in the perfusion channel as a quasi-reference electrode and the other in the cell culture volume. We obtained an acidification rate of 0.19+/-0.02 pH/min for fibroblast cells using a stop flow protocol. These results show that we can use two identical miniaturized microfabricated IrOx electrodes to measure pH differences to monitor the metabolic activity of cell cultures on chip. Furthermore, our approach can also be applied in biosensor or bioanalytical applications.