Analytisk kemi – några reflektioner

Analytical chemistry is an interdisciplinary field of science related to many practical applications, such as clinical diagnostics, environmental monitoring and industrial process analysis. Over the last 50 years, chemical analytics has evolved from centralized laboratory measurements towards on-line and distributed analysis, including wearable chemical sensors for personal health monitoring. In this article I wish to reflect upon these developments in analytical chemistry, illustrated by examples from my own research. The text provides a glimpse of analytical chemistry from my personal perspective

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

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

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

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

Ion-selective electrodes in real-life applications: can we reduce or even eliminate the need for calibration?

The recent success in lowering of the low detection limit of ion-selective electrodes (ISEs) has opened up new application areas (most notably environmental analysis). The new possibilities have intensified issues that weren’t critical in other application fields. In particular, the potential of ISEs to be integrated as detector systems in autonomous, deployable devices for environmental analysis have opened the question of the measure of uncertainty in prediction of unknown concentration based on calibration as well as the necessity of reduction or even elimination of calibration itself. Here we point out the need for the maximal efficiency of using obtained results based on utilization of solid-contact ISEs for heavy metal analysis in soil. We advocate altering the current definition of detection limit and using signal-to-noise ratio as suggested earlier by Bakker and Pretsch.[1] We also suggest new method for jointly estimating the calibration curve and the unknown concentrations using all the data. This method is in statistical analysis called Bayesian analysis. It allows more accurate prediction of unknown concentration, especially near and even below the detection limit. Furthermore, it allows using of multiple sensors (without disregarding poor performing sensors) which will allow further tightening of the prediction intervals. Finally, we will present initial work on developing “calibrationless” chemical sensors where we use ISEs as model system. We are developing tests that will enable us to understand whether the surface or the bulk of the membrane has been altered (i.e. due to biofouling or poisoning of the electrode surface) thus avoiding the need to re-calibrate the sensors. [1] Bakker E. Pretsch E; Trends Anal. Chem.; 2005, 24, (3), 19

Disposable solid-contact ion-selective electrodes for environmental monitoring of lead with ppb

Solid-contact Pb2+-selective-electrodes and solid contact reference electrodes suitable for use as disposable sensing devices for environmental monitoring of lead have been prepared on screen-printed substrates. Accurate control over the fabrication procedures leads to excellent reproducibility of their calibration characteristics such as slope, offset and limit of detection. In particular, the limit of detection in the nanomolar range opens the possibility of their use for trace analysis of Pb2+ in environmental water samples. Significantly, the potentiometric measurements correlate well with data determined using inductively coupled plasma mass spectrometry (ICP-MS) in a number of real samples taken from local rivers. Ways in which these sensors might be employed in autonomous platforms for monitoring water quality in-situ are discussed. The possibility of including arrays of virtually identical sensors is highlighted as a possible route to achieve long-term deployments

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

Hand-Held Transistor Based Electrical and Multiplexed Chemical Sensing System

We describe a hand-held sensing system using a transistor based multiplexed platform and a detector that couples the electrochemical information wirelessly to a smartphone. The custom disposable platform exploits the ion-sensitive FET (ISFET) technology. Via simple surface modifications the design allows a broad range of analytes to be tested with low cost. We compared our read-out device to a commercial potentiometer using K+ as an example species analyte. The developed sensing system has a slightly better limit of detection and is notably less susceptible to external noise which is commonly observed with potentiometers. The designed platform is fabricated using standard electronic processes with gold surface and we used commercial discrete transistors as the transducing element. It can be mass produced with high yield and low cost. To circumvent the drift that typically occurs with modified solid state electrodes we incorporated a transducing layer between the electric conductor (gold pad) and the ionically conducting ion-selective membrane. The polyaniline doped with dinonylnaphtalene sulfonic acid (PANI-DNNSA) was used as a transducing layer for the first time. The PANI-DNNSA layer significantly reduces the drift of the electrodes compared to a configuration without the transducing layer. The system is easy to use with a transistor based detection that can be modified for a vast variety of existing potentiometric tests.</p