Solid-Contact pH Sensor with Covalent Attachment of Ionophores and Ionic Sites to a Poly(decyl methacrylate) Matrix

Abstract

With a view to improving the sensor lifetime, solid-contact ion-selective electrodes (ISEs) were prepared with a plasticizer-free and cross-linked poly(decyl methacrylate) matrix, to which only the ionic sites, only the ionophore, or both the ionic sites and ionophore were covalently attached. In earlier work with covalently attached ionophores or ionic sites, it was difficult to discount the presence of ionophores or ionic site impurities that were not covalently attached to the polymer backbone because the reagents used to introduce the ionophore or ionic sites had high hydrophobicities. In this work, we deliberately chose readily available hydrophilic reagents for the introduction of covalently attached H + ionophores with tertiary amino groups and covalently attached sulfonate groups as ionic sites. This simplified the synthesis and made it possible to thoroughly remove ionophores and ionic sites not covalently attached to the polymer backbone. Our results confirm the expectation that hydrophobic ISE membranes with both covalently attached ionophores and ionic sites have impractically long response times. In contrast, ISEs with either covalently attached H + ionophores or covalently attached ionic sites responded to pH with quick Nernstian responses and high selectivity. Both conventional plasticized poly(vinyl chloride) (PVC)-based ISEs and the new poly(decyl methacrylate) membranes were exposed to 90 °C heat for 2 h, 10% ethanol for 1 day, or undiluted blood serum for 5 days. In all three cases, the poly(decyl methacrylate) ISEs exhibited properties superior to conventional PVC-based ISEs, confirming the advantages of the covalent attachment.

Ion‐Selective Electrodes With Sensing Membranes Covalently Attached to Both the Inert Polymer Substrate and Conductive Carbon Contact

“…No attempt was made to covalently attach the ionic sites to the PDMA backbone, as it has been shown that ISEs with both the ionophore and the ionic sites covalently attached exhibit larger electrical resistances and excessive response times and are not suitable for potentiometric analysis. [19] We note that leaching of the ionic sites Angewandte Chemie Forschungsartikel from these sensing membranes is only possible by transfer of these very hydrophobic anions along with protons into the sample, a process that is also hindered by the strong affinity of the protons to the ionophore. [3] Indeed, this is the reason why in this work we chose to covalently attach the ionophore and not the ionic sites.…”

“…Because of its methacrylate group, this ionophore is incorporated into the PDMA polymer backbone in the course of the photoinitiated polymerization. [19] The ionic site tetrakis(pentafluorophenyl)borate, which is required to provide membrane permselectivity for cations, [3] was included into the photografting solution in a 1 to 3 molecular ratio to the ionophore. No attempt was made to covalently attach the ionic sites to the PDMA backbone, as it has been shown that ISEs with both the ionophore and the ionic sites covalently attached exhibit larger electrical resistances and excessive response times and are not suitable for potentiometric analysis.…”

“…[3] Indeed, this is the reason why in this work we chose to covalently attach the ionophore and not the ionic sites. [19] After overnight conditioning in 10 mM pH 7.1 sodium phosphate buffer, the response of these electrodes to pH was tested in 10.0 mM phosphate buffer solutions by pH adjustment with aliquots of 1 M NaOH and HCl. Three identically prepared electrodes showed a slope of À 58.5 � 1.2 mV decade À 1 and a working range from pH 1.4 to pH 9.7 (see Figure 2).…”

“…[15,16] The life span of these sensors can be improved by use of plasticizer-free polymers as the membrane matrix (such as polyacrylates, polymethacrylates, and silicones) and the covalent attachment of ionophores or ionic sites to this matrix polymer. [17][18][19][20] Another problem that often limits the lifetimes of such sensors is the formation of a water layer [21,22] at the interface of the sensing membrane to the underlying electron conductor and the delamination of the sensing membranes from the electrode body. Screw caps [23] and other mechanical devices can be used to prevent such delamination, but this makes miniaturization more difficult and for certain applications can increase the risk of air bubble trapping and damage to the sensing membranes by overtightening.…”

“…Efforts to overcome these drawbacks often involve ionophore‐doped sensing membranes with solid contacts to underlying electron conductors such as conducting polymers, [9, 10] high‐surface‐area carbons, [11–14] and hydrophobic redox buffers [15, 16] . The life span of these sensors can be improved by use of plasticizer‐free polymers as the membrane matrix (such as polyacrylates, polymethacrylates, and silicones) and the covalent attachment of ionophores or ionic sites to this matrix polymer [17–20] . Another problem that often limits the lifetimes of such sensors is the formation of a water layer [21, 22] at the interface of the sensing membrane to the underlying electron conductor and the delamination of the sensing membranes from the electrode body.…”