Integration of miniature, ultrasensitive chemical sensors in microfluidic devices

Simple construction, good detection limit1, very low power demand, and simple experimental setup coupled with miniaturization opportunities arising from solid-state format makes ISEs an excellent prospect for integration in autonomous sensing devices and ultimately their integration in large wireless chemo-sensing networks.2,3 Microfluidics, also known as “lab-on-a-chip” is an emerging technology that is changing the future of instrument design. Microfluidics enables small scale fluid control and analysis, allowing developing smaller, more cost-effective, and more powerful systems.4,5,6 We are working on development of miniature devices featuring sensitive yet simple sensors that could enable rapid access to important environmental information from in-situ deployed sensors, and thereby facilitate timely action to minimize the adverse impact of emerging incidents. Our work involves integration of ultra-sensitive yet simple chemical sensors into a microfluidic device that has integrated wireless communications capabilities. Our ultimate objective is to develop a microfluidic chip that will incorporate polymer-based lead-selective solid-state electrodes. We will test the series of developed chips for the best design to accommodate these sensors. Initially, we are targeting lead-selective sensors and their application to the monitoring of drinking and natural water quality. Our ultimate vision is the development of a microfluidic-based platform with fully integrated screen-printed solid-state ISEs, and the associated reference electrode, which will be suitable for use as a chemo-sensing component in a widely distributed wireless sensor network (WSN) for monitoring the quality of a fresh water system. A key challenge in the realization of this vision is to build in advanced system diagnostics, and particular, sensor status tests using simple electronic signals, in a manner similar to those used in physical transducers.7 In this way, it may be possible to assist in distinguishing sensor malfunction or signal artifacts from real events, even in relatively simple, low cost platforms

Miniature, all-solid-state ion-selective sensor as a detector in autonomous, deployable sensing device

Lowering of the detection limit of ion-selective electrodes (ISEs) as well as their simple construction, low production cost and low power requirements make ISEs an ideal candidate for detector systems that can be integrated into autonomous, deployable sensing devices. Routine analysis and early warning systems are applications that first spring to mind, however great added value can be obtained by integration of many such devices into a wireless sensing network. In this work we describe our work towards the miniaturization of ISEs and their integration of with all-solid-state reference electrode into an all-solid-state sensor with a view of integration in autonomous, deployable sensing device. This work has two avenues: 1) development of a platform that can house all-solid-state ISEs and reference electrodes and 2) development of electronic circuitry for data acquisition and wireless transmission of the data. The latter utilizes novel, in-house made motes (a node in a wireless sensor network that is capable of performing some processing, gathering sensory information and communicating with other connected nodes in the network) that operate at lower frequency and therefore consume lower power then other, commercially available ones. In addition, they are easier to program which bridges the gap of communication between chemists and computer scientists. Intensification of the work in producing all-solid-state reference electrodes has enabled us to work on development of a platform that houses all-solid-state ISEs and reference electrode. We will here describe our progress in this avenue of our research

Electro-diffusion at different length scales

The modelling of electro-diffusion in the multicomponent system in open space and time domains has been only recently addressed and made numerous applications in biology, fuel cells, electrochemical sensors and reference electrodes possible. In this work we show the numerical simulations of electrical potential over time and resulting electrochemical impedance spectra of ion-selective membrane electrodes (ISE’s). The numerical results are obtained by use of the coupled Nernst-Planck, Poisson and continuity equations (forming the NPP model). The equations are solved by means of the finite difference method, the Rosenbrock solver with the use of Matlab (by MathWorks) platform. The potential-time response of the ISEs in open- and closed-circuit conditions as a function of varying heterogeneous rate constants, ionic concentrations and membrane thickness are computed. The potential-time response to small-current perturbation is applied for simulations of the impedance spectra. The results obtained show that the membrane with Nernstian response presents only one capacitive arc in the impedance spectra, related to conductivity and dielectric properties of the membrane material. Non-Nernstian behaviour is related to slow ionic transport through the membrane|solution interfaces and is manifested by the appearance of an additional (capacitive) arc between the highfrequency bulk and the low-frequency (Warburg) arcs. The presented approach directly relates the diffusivities in the membrane and the interface properties (heterogeneous rate constants determining the transport across interfaces) to the characteristic properties of the impedance spectra (characteristic radial frequencies). It is concluded that the Matlab platform allows solving the NPP problem and simulating the non-linear effects in electrodiffusion in a convenient way

Electrochemical impedance spectroscopy as a tool for probing the functionality of ion-selective membranes

Recent success in lowering of the detection limit of ion-selective electrodes (ISEs) to part-perbillion levels have opened up the possibility for their application in environmental analysis. Its simplicity, low cost, and low power requirement coupled with excellent selectivity and sensitivity make ISEs excellent detecting system in autonomous and deployable sensing devices for routine analysis and as early warning systems. However, the necessity for calibration of detecting systems implies the use of sometimes complicated and costly systems for calibration solution and waste handling, pumps and data acquisition including the labour for system maintenance. Reducing the need for sensor calibration (or its complete elimination) would not only simplify sensing devices and reduce their costs but would allow integration of chemical sensors into the emerging area of wireless sensing networks (WSNs). It is envisioned that this integration will bring new dimensions into chemical sensing and bring benefits in many aspects of human lives. Here, we describe our attempts to address the issue of reducing the need for sensor calibration. The functionality of a typical physical transducer is probed using electrical signals testing its resistance, impedance, conductance etc. We employ a similar strategy and apply relatively simple AC signals to an ion-selective membrane in order to probe its functionality after it has been subjected to conditions that simulate in-situ long-term deployments. For example, we observe the impedance spectra of membranes that have been physically damaged, biofouled and/or have components leached out. Comparing this information with the sensor's potentiometric behaviour, we can draw conclusions regarding the functionality of the devices and their suitability to continue serving as a reliable detectors, for example, in remote locations

Integration of a sensor system into microfluidic chips

There have been considerable developments in the field of potentiometric sensors in recent years mainly with respect to lowering detection limits and making sensors smaller, solid-state, robust and less expensive.[1, 2] In potentiometric measurements two electrodes are needed, an indicator or ion-selective electrode (ISE) and a reference electrode. However, recent progress in the design and characteristics of the indicator electrodes cannot be exploited without similar progress in the design of the reference electrodes. In this paper we present development of chips with fully integrated solid-contact reference (SC-RE) and ion-selective (SC-ISEs) electrodes. In these electrodes, a conducting polymer (CP) (poly(3,4-ethylenedioxythiophene)) is used as the solid contact ion-to-electron transducer[3]. The conducting polymer is deposited using galvanostatic electropolymerization.[4, 5] The ability to produce reliable miniaturized reference electrodes, has given us the opportunity to develop several prototype versions of miniature, solid-contact sensor systems (i.e. with fully integrated ion-selective and reference electrodes) that can be further integrated into microfluidic platforms. We have prepared microchips using different designs to test for the best accommodation of the sensors and to optimise the sensor-chip platform characteristics. Our initial goal is to prepare Pb-ISEs suitable for use as a chemo-sensing component in a widely distributed wireless sensor network (WSN) for monitoring the quality of a fresh water system, together with advanced diagnostics to evaluate the on-going functionality of the sensors using simple electronic signals.[5, 6

Development of miniature all-solid-state potentiometric sensing system

A procedure for the development of a pen-like, multi-electrode potentiometric sensing platform is described. The platform comprises a seven-in-one electrode incorporating all-solid-state ion-selective and reference electrodes based on the conductive polymer (poly(3,4-ethylenedioxythiophene) (PEDOT)) as an intermediate layer between the contacts and ion-selective membranes. The ion-selective electrodes are based on traditional, ionophore-based membranes, while the reference electrode is based on a polymer membrane doped with the lipophilic salt tetrabutyl ammonium tetrabutyl borate (TBA-TBB). The electrodes, controlled with a multichannel detector system, were used for simultaneous determination of the concentration of Pb2+ and pH in environmental water samples. The results obtained using pH-selective electrodes were compared with data obtained using a conventional pH meter and the average percent difference was 0.3%. Furthermore, the sensing system was successfully used for lead-speciation analysis in environmental water samples

Nernst-Planck-Poisson Model for the Description of Behaviour of Solid-Contact Ion-Selective Electrodes at Low Analyte Concentration

All-solid-state electrodes are increasingly being used in clinical, industrial and environmental analysis. This wide range of applications requires deep theoretical description of such electrodes. This work concentrates on the development of a numerical tool for the qualitative prediction of electrochemical behaviour for solid-contact ion-selective electrodes at low analyte concentrations. For this purpose, a general approach to the description of electro-diffusion processes, namely the Nernst-Planck-Poisson (NPP) model, was applied. The results obtained from this model are verified by experimental data of lead(II)-selective electrodes based on a polymeric PVC membrane with polybenzopyrene doped with Eriochrome Black T used as the solid contact

The Water Uptake of Plasticized Poly(vinyl chloride) Solid‐Contact Calcium‐Selective Electrodes

A hyphenated method based on FTIR‐ATR and electrochemical impedance spectroscopy has been applied to simultaneously measure the water uptake, changes in the bulk resistance and potential of plasticized poly(vinyl chloride) (PVC) based Ca 2+ ‐selective coated‐wire (CaCWE) and solid‐contact electrodes (CaSCISEs). Most of the water uptake of the ion‐selective membranes (ISMs) used in both electrode types took place within the first 9 h in 10 −3  M CaCl 2 showing good correlation with the stabilization of the individual electrode potentials. The bulk resistance of the ISMs of the CaCWEs and the CaSCISEs with poly(3‐octylthiophene) (POT) as the solid‐contact (SC) increased most during the first 18 h in 10 −3  M CaCl 2 . The increase in the resistance was found to be related to the exchange of K + for Ca 2+ in the ISM and the formation of the Ca 2+ ‐ionophore (ETH 5234) complex having a lower diffusivity than the free K + ions. In contrary to previously published results on silicone rubber based SCISEs and poly(methyl methacrylate):poly( n ‐decyl methacrylate) membranes containing POT, the plasticized PVC‐based CaSCISEs with POT as the SC had a higher water uptake than the CaCWEs. The CaSCISEs had a detection limit of 2×10 −8  M Ca 2+ and a good potential reproducibility of 148.9±1.0 mV in 10 −4  M CaCl 2 .Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86920/1/2156_ftp.pd