Elimination of Undesirable Water Layers in Solid-Contact Polymeric Ion-Selective Electrodes

Abstract

This study aims to develop a novel approach for the production of analytically robust and miniaturized polymeric ion sensors that are vitally important in modern analytical chemistry (e.g., clinical chemistry using single blood droplets, modern biosensors measuring clouds of ions released from nanoparticle tagged biomolecules, lab-on-a-chip applications, etc.). This research has shown that the use of a water repellent polymethyl methacrylate/polydecyl methacrylate (PMMA/PDMA) copolymer as the ion sensing membrane, along with a hydrophobic poly(3-octylthiophene 2,5-diyl) (POT) solid-contact as the ion-to-electron transducer, is an excellent strategy for avoiding the detrimental water layer formed at the buried interface of solid-contact ISEs. Accordingly, it has been necessary to implement a rigorous surface analysis scheme employing electrochemical impedance spectroscopy (EIS), in-situ neutron reflectometry/EIS (NR/EIS), secondary ion mass spectrometry (SIMS) and small angle neutron scattering (SANS) to probe structurally the solid-contact/membrane interface, so as to identify the conditions that eliminate the undesirable water layer in all solid-state polymeric ion sensors. In this work, we provide the first experimental evidence that the PMMA/PDMA copolymer system is susceptible to water “pooling” at the interface in areas surrounding physical imperfections in the solid-contact, with the exposure time for such an event in a PMMA/PDMA copolymer ISE taking nearly twenty times longer than that for a plasticized polyvinyl chloride (PVC) ISE, and the simultaneous use of a hydrophobic POT solid-contact with a PMMA/PDMA membrane can eliminate totally this water layer problem.

A solid-contact Pb2+-selective electrode using poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene) as ion-to-electron transducer

“…To obtain a SC-ISE with satisfactory performance, three requirements must be met for the inner contact including high chemical stability, proper ion to electron transduction and good adhesion with matrix polymer. It has been reported that the buildup of well-defined interface through the selection of the conductive polymer films does not necessarily lead to good performances of SC-ISEs [21], however, a thin aqueous layer formed between the ion-selective membrane and the conducting polymer could account for the potential drifts and poor detection limits of the electrodes [22,23]. Recently, a novel approach was developed for the production of analytically robust sensors by the use of an acrylate copolymer (P(MMA-DMA)) as the ion-selective membrane matrix, and poly(3-octylthiophene) (POT) solid contact as the ion-to-electron transducer [1,19,23,24].…”

“…It has been reported that the buildup of well-defined interface through the selection of the conductive polymer films does not necessarily lead to good performances of SC-ISEs [21], however, a thin aqueous layer formed between the ion-selective membrane and the conducting polymer could account for the potential drifts and poor detection limits of the electrodes [22,23]. Recently, a novel approach was developed for the production of analytically robust sensors by the use of an acrylate copolymer (P(MMA-DMA)) as the ion-selective membrane matrix, and poly(3-octylthiophene) (POT) solid contact as the ion-to-electron transducer [1,19,23,24]. The diffusion coefficients in P(MMA-DMA) copolymer (∼10 −11 cm 2 s −1 ) are up to three orders of magnitude lower than those for conventional plasticize PVC membranes [25], which may make it relatively robust to chemical changes at the inner membrane side.…”

Pre-Polarized Hydrophobic Conducting Polymer Solid-Contact Ion-Selective Electrodes with Improved Potential Reproducibility

“…[18][19][20][21] The role of the SC is to provide a stable inner phase boundary potential 20,22,23 at the substrate/SC and SC/ISM interfaces, and to prevent the aqueous layer formation at the inner interface, which coupled to diffusion of ionic species within the ISM may lead to potential instability. 24 As all ISMs exhibit water uptake [25][26][27][28][29][30][31] , the prevention of the aqueous layer formation 32,33 with ill-defined ion activity has proven to be a major challenge.…”

A simple approach for fabricating solid-contact ion-selective electrodes using nanomaterials as transducers

“…At the low frequencies, a large semicircle is found at the EIS of the CWE, indicating a large charge-transfer resistance with a small capacitance at the "blocked" interface of the CWE. The negligible of low-frequency semicircle in the case of the solidcontact ISE shows that the ETH 500/SWCNTs contact increases the low-frequency capacitance and facilitates the ion-to-electron transduction between the electrode substrate and the ion-selective membrane [38].…”