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Technical Note
Polyurethane Ionophore-Based Thin Layer Membranes for Voltammetric Ion Activity Sensing Maria Cuartero, Gaston A. Crespo, and Eric Bakker Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b01085 • Publication Date (Web): 17 May 2016 Downloaded from http://pubs.acs.org on May 19, 2016
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Analytical Chemistry
Polyurethane Ionophore-Based Thin Layer Membranes for Voltammetric Ion Activity Sensing Maria Cuartero, Gaston A. Crespo* and Eric Bakker* Department of Inorganic and Analytical Chemistry, University of Geneva, Quai Ernest-Ansermet 30, CH1211 Geneva, Switzerland. Corresponding Author:
[email protected];
[email protected] ACS Paragon Plus Environment
Analytical Chemistry
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Abstract We report on a plasticized polyurethane ionophore-based thin film material (of hundreds of nanometer thickness) for simultaneous voltammetric multianalyte ion activity detection triggered by the oxidation/reduction of an underlying poly(3-octylthiophene) film. This material provides excellent mechanical, physical and chemical robustness compared to other polymers. Polyurethane films did not exhibit leaching of lipophilic additives after rinsing with a direct water jet and exhibited resistance to detachment from the underlying electrode surface, resulting in a voltammetric current response with less than 250 nm) was found to deteriorate the analytical performances of the sensor and to result in a poor multianalyte discrimination.4 Unfortunately, recent work in our laboratory found that thin layer PVC sensing films exhibited limited robustness. By rinsing the electrode with ultrapure water, thereby directing the water jet onto the membrane surface, it was found that the integrated charge and the peak height decreased significantly.4 To minimize this undesired effect, their rinsing was subsequently avoided and a soft paper was instead used to dry the membranes. While this procedure still allows one to make fundamental progress, the stated materials limitation must be overcome to make this exciting new technology suitable for real world applications. While PVC membranes have undoubtedly been the core of polymeric membrane ISEs since the early days,8,9 other polymers (e.g., polystyrene, acrylates, poly(vinyl butyral), polyamide, polyimide, polyurethanes and Teflon) have also been evaluated to identify possible matrix materials.10 Among these, polyurethane and acrylate membranes have been particularly promising. Fiedler and Ruzicka11 explored a
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series of polyurethane membranes based on valinomycin and varying plasticizer loadings (from pure polyurethane to an equimolar polyurethane/plasticizer mixture). A membrane containing 50 wt-% polyurethane behaved similarly to traditional PVC membranes with 30% PVC (sensitivity, selectivity and robustness for potassium detection).11 One key drawback of polyurethane is its demonstrated pH cross-sensitivity originating from the protonatable urethane groups, and ionophore-free polyurethane membranes have even been used as pH sensors and as “intelligent polymers” to detect pH changes.12,13 This natural pH response is known to be sufficiently suppressed in membranes containing ionophores such as valinomycin.11 Importantly, it was subsequently demonstrated the excellent adhesion, bio and hemocompatibility, and structural and mechanical adaptability of the polyurethane matrix.14-17 The improved biocompatibility of the material was evidenced with intra-arterial implantable sensors for potassium detection.16 More recently, other polymers have also been identified to develop robust ISEs, including poly(n-butylmethylmethacrylate)18,19,
poly(methylmethacrylate-decylmethacrylate)19 (plasticizer-free membranes)
and fluorous amorphous perfluoropolymers.20 However, their compatibility with biological fluids have typically not yet been established. Here, thin layer sensing films based on alternate polymers are characterized in view of improving their robustness relative to PVC based films. The films are backside contacted with POT and interrogated by cyclic voltammetry. As the principal motivation is to develop biocompatible and robust thin layer membrane sensors for multi-analyte detection, the performance of the proposed membranes was compared to PVC membranes both in artificial and clinical samples (undiluted human blood and serum).
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Analytical Chemistry
Experimental Section Reagents, Materials and Equipment. Aqueous solutions were prepared by dissolving the appropriate salts in deionized water (>18 MΩ cm). All experiments were carried out at room temperature of 22 ± 1 °C. Lithium perchlorate (>98%, LiClO4), 3-octylthiophene (97%, OT), high molecular weight poly(vinyl chloride) (PVC), polyurethane (selectophore, PU), polystyrene (analytical grade, PS), bis(2ethylhexyl)sebacate (DOS), sodium tetrakis[3,5-bis-(trifluoromethyl)phenyl]borate (NaTFPB), lithium ionophore VI (Li-I), potassium ionophore I (K-I), sodium chloride (NaCl), lithium chloride (LiCl), potassium chloride (KCl), calcium chloride (CaCl2), magnesium chloride (MgCl2), hydrochloric acid (HCl), acetonitrile (anhydrous, >99.8%, ACN) and tetrahydrofuran (>99.9 %, THF) were purchased from Sigma Aldrich. Citrated human plasma and blood samples were provided by Hôpitaux Universitaires de Genève (HUG). Poly(methylmethacrylate-decylmethacrylate) (MMA-DMA) copolymer was prepared as previously described.19 Lipophilic multi-walled carbon nanotubes (f-MWCNTs) were synthetized from commercial MWCNTs (>95% wt. purity, outer diameters of 10-20 nm, length~50 µm purchased from HeJi, Inc., Zengcheng City, China) as reported.21 Cyclic voltammograms were recorded with a PGSTAT 128N (Metrohm Autolab B.V., Utrecht, The Netherlands) controlled by Nova 1.11 software (supplied by Autolab) running on a PC. A double-junction Ag/AgCl/3 M KCl/1 M LiOAc reference electrode (6.0726.100 model, Metrohm, Switzerland) and a platinum electrode (6.0331.010 model, Metrohm, Switzerland) were used in a three-electrode cell. Preparation of the electrodes. POT was electrochemically polymerized on GC surface as described elsewhere (see Supporting Information for more details).4,5 Then, a volume of 25 µL of the corresponding membrane cocktail was spin coated on the POT-based electrode. Table SI-1 shows the composition of the membrane cocktails used through the paper. The thickness of both the POT and sensing films have already been optimized in earlier work.4
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Results and Discussion Figure 1 shows the cyclic voltammograms for membranes (MI-IV, Table SI-1) based on different polymers (PVC, PU, PS and MMA-DMA) before and after rinsing with ultrapure water. Note that the presence of cation-exchanger (Na+R-) was shown to promote the cation-transfer process between membrane and solution while anion transfer in the opposite direction was effectively supressed within the potential window.4 As observed, only the membranes containing PU exhibited a negligible change in the signal after rinsing. In the case of PVC and PS (Figures 1b and 1c), the position of the peak shifted to less positive potentials (from 353 to 289 mV, and from 347 to 265 mV for PVC and PS) and the peak intensity as well as the integrated charge decreased drastically (~4.5 times). The MMA-DMA film (Figure 1d) was somewhat more robust. While the peak position shifted to more positive potentials (from 400 to 418 mV) the peak intensity was influenced to a lesser extent than the other materials (~50%). As the PU films were rinsed repeatedly, they showed excellent reproducibility of the peak potential (variation of
