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

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

Ionic liquid-based, liquid-junction-free reference elctrode

In this paper, we describe a new type of polymer membrane-based reference electrode (RE) based on ionic liquids (ILs), in both liquid-contact (LCRE) and solid-contact reference electrode (SCRE) forms. The ILs used were bis(trifluoromethane sulfonyl)amid with 1-alkyl-3methyimidazolium as well as phosphonium and ammonium cations. In addition to their charge stabilisation role, it was found that the ILs also functioned as effective plasticizers in the PVC matrix. The LCREs and SCREs were prepared using the same design as their corresponding indicator electrodes. LCREs were prepared by casting in glass rings while SCREs were prepared on platforms made using screen-printing technology, with poly(3-octylthiophene-2,5 diyl) (POT) as the intermediate polymer. After potentiometric characterization of the response mechanism, the practical performance of the REs was studied using potentiometric titrations (Pb2+ and pH), and characterised using cyclic voltammetry and impedance spectroscopy. All results were compared via parallel experiments in which the novel RE was substituted by a conventional double junction Ag/AgCl reference electrode. The mechanism of response is most likely based on a limited degree of partitioning of IL ions into the sample thereby defining aquo-membrane interfacial potential. Despite their simple nature and construction, the REs showed excellent signal stability, and performed well in the analytical experiments. The identical mode of fabrication to that of the equivalent indicator (or Ion-Selective) electrode (ISE) will facilitate mass-production of both indicator and reference electrode using the same fabrication line, the only difference being the final capping membrane composition

Antimicrobial Colloidal Silver-Lignin Particles via Ion- and Solvent Exchange

Acid-precipitated lignin nanoparticles with a cationic polymer coating exhibit antibacterial activity when infused with silver. While the use of such particles would be beneficial due to their high antibacterial activity with a low silver content, their production holds steps that are difficult to scale up to inexpensive industrial manufacture. For example, the production of acid-precipitated lignin nanoparticles requires the use of ethylene glycol, which is not easily recycled. Furthermore, the binding of silver to these particles is weak, and thus the particles need to be used rapidly after preparation. Here, we show that with a deprotonation reaction of an organic solution of anhydrous lignin and subsequent ion exchange with silver nitrate and colloid formation by solvent exchange, highly spherical silver carboxylate colloidal lignin particles (AgCLPs) can be prepared. Silver is not released from the particles in deionized water but can be released in physiological conditions, shown by their high antibacterial efficacy with low silver loading. In comparison to lignin nanoparticles with weakly bound silver, AgCLPs have high antibacterial activity even without cationic polyelectrolyte coating, and they retain their antibacterial activity for days. While the rapid depletion of silver from silver-infused lignin nanoparticles can be considered beneficial for some applications, the sustained antibacterial activity of the AgCLPs with ionically bound silver will enable their use in applications where silver nanoparticles have been previously used. Our results demonstrate that CLPs, which can be produced with a closed cycle process on a large scale, can be rapidly and quantitatively functionalized into active materials.Peer reviewe

Electrochemical characterization of redox activity and stability of various tris(2,2‘-bipyridine) derived complexes of iron(II) in aqueous solutions

Tris(2,2'-bipyridine) Fe(II) complexes with different 4,4'-placed substituents were studied electrochemically in aqueous solutions. Digital simulation of the experimental cyclic voltammograms enabled the evaluation of the redox potentials, electrochemical kinetics as well as complex stability. The substituent effect on the formal potential of the complexes was investigated, showing that electron-withdrawing substituents shift the formal potential to a positive direction from the potential of the unsubstituted [Fe(II)(bpy)3]2+ complex (0.875 V vs. Ag/AgCl). Respectively electron-donating substituents shift the formal potential to a negative direction. The most positive formal potential (0.97 V vs. Ag/AgCl) was obtained with 4,4'-dicarboxyl substituted and the lowest 0.56 V vs. Ag/AgCl with 4,4'-di-OMe substituted [Fe(II)(bpy)3]2+. We show here that the stability of the compounds in the oxidized form can be evaluated by voltammetry. None of the studied complexes was stable enough for flow battery applications, but knowledge of their decomposition rates was obtained via simulations, considering that all oxidized species undergo a chemical reaction, resulting in a loss of redox-active species. The counterion of the complex affected the solubility and stability of the complex, as the presence of tetrafluoroborate resulted in faster decomposition than the presence of sulfate. Battery testing of the most stable Fe(II) complex revealed a voltage drop upon discharge, lowering the energy efficiency. Battery cycling showed a capacity decay most likely related to the chemical reaction occurring to the oxidized species. Even though the studied complexes are not suitable for aqueous flow battery applications as such, knowledge of a substituent, counterion, and electrolyte effect on their performance is needed to develop these complexes further and to improve their stability via structural design. We show here that voltammetry is a suitable tool for fast initial evaluation of the stability of the materials.peerReviewe

Capacitive Model for Coulometric Readout of Ion-Selective Electrodes

We present here a capacitive model for the coulometric signal transduction readout of solid-contact ion-selective membrane electrodes (SC-ISE) with a conducting polymer (CP) as an intermediate layer for the detection of anions. The capacitive model correlates well with experimental data obtained for chloride-selective SC-ISEs utilizing poly­(3,4-ethylenedioxythiophene) (PEDOT) doped with chloride as the ion-to-electron transducer. Additionally, Prussian blue is used as a simple sodium capacitor to further demonstrate the role of the transduction layer. The influence of different thicknesses of PEDOT as a conducting polymer transducer, different thicknesses of the overlaying ion-selective membranes deposited by drop casting and spin coating, and different compositions of the chloride-selective membrane are explored. The responses are evaluated in terms of current–time, charge–time, and charge–chloride activity relationships. The utility of the sensor with coulometric readout is illustrated by the monitoring of very small concentration changes in solution