Insightful Analysis of Phenomena Arising at the Metal|Polymer Interphase of Au-Ti Based Non-Enzymatic Glucose Sensitive Electrodes Covered by Nafion

This paper focuses on the examination of glucose oxidation processes at an electrode material composed of gold nanoparticles embedded in a titanium template. Three different conditions were investigated: the chloride content in the electrolyte, its ionic conductivity and the presence of a Nafion coating. The impact of the provided environment on the oxidation reaction was evaluated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Two models, namely: chemisorption and incipient hydrous oxide/adatom mediator (IHOAM), were applied to explain the complex voltammetric responses of the electrodes exposed to solutions of varied glucose concentrations. Three different phenomena were observed for the studied cases. The first is related to the transition between the dominant mechanism of glucose oxidation from the IHOAM model to the chemisorption model. This happens only in an electrolyte containing chlorides after exceeding a certain amount of glucose. The second effect exhibits a bottleneck nature resulting from the presence of Nafion on the electrode’s surface. In this case, mass transport through the semi-permeable polymer is hampered, due to the blocking of channels and physical internal cross-linking. This leads to a preconcentration of glucose inside the pores resulting in an increase in both the material sensitivity and the linear range of the calibration curve. Lastly, the third effect is manifested in a low concentration of the supporting electrolyte. It is based on the fact that mass transport of hydroxyl ions is governed not only by diffusion, but also by migration. These three effects have a tremendous impact on the glucose oxidation mechanism and reveal its very complex nature.

Novel approach to interference analysis of glucose sensing materials coated with Nafion

We focus here on a novel approach to analysing the mechanisms of interference phenomena in glucose sensing, taking into account the changes within the Nafion layer deposited on the active surface. Several electrochemical techniques were used to verify the sustainability of catalytic properties of the electrode material after exposure to different compounds, i.e. ascorbic acid (AA), glycine, urea, acetylsalicylic acid (AsA), and acetaminophen (AAp). Through analysis of impedance data, we concluded that AAp and AsA were trapped permanently in the Nafion membrane, which significantly affected results repeatability. These observations were also confirmed by FT-IR investigations of the membrane after its immersion in solutions containing different interfering species. Moreover, after exposure to AsA and, unexpectedly, large concentrations of urea, the catalytic properties were completely lost, which, in consequence, make sensor reuse impossible. Such behaviour was justified by the chain reorganisation and swelling. Mechanisms involving adsorption onto the interphase and absorption in the membrane were proposed as key factors responsible for deterioration of membrane functionality and were confronted with FT-IR investigations. Following that, application of Nafion for non-invasive glucose sensor protection is unsatisfactory and cannot be considered for multiple detection procedures, especially taking into account biological fluids full of different interfering species.

Improved Adhesion of Nafion™-Coated Separator to Water-Processable LiNi0.5Mn1.5O4 Electrodes

The adhesion between electrode and separator is a key feature in cell assembly. Nafion™-coated separators for water-processed LiNi0.5Mn1.5O4 (LNMO) electrodes are here proposed as an alternative to the polyolefin separators. Specifically, polyolefin separators are modified with Nafion™ solutions and their adhesion to high-potential LNMO electrodes is investigated. The physicochemical properties of the Nafion™-coated separator and its electrochemical performance in Li/LNMO cells are discussed and compared to those obtained with polyolefin Celgard® (Charlotte, NC, USA) PP2075 separator. Improved adhesion and cycling stability, which could be further enhanced by a mild lamination process, were demonstrated with a thin layer of Nafion™ (0.1 mg cm−2).

Bioelectrochemical remediation of phenanthrene in a microbial fuel cell using an anaerobic consortium enriched from a hydrocarbon-contaminated site

Polycyclic aromatic hydrocarbons (PAH) are organic pollutants that require remediation due to their detrimental impact on human and environmental health. In this study, we used a novel approach of sequestering a model PAH, phenanthrene, onto a solid carbon matrix bioanode in a microbial fuel cell (MFC) to assess its biodegradation coupled with power generation. Here, the bioanode serves as a site for enrichment of electroactive and hydrocarbon-degrading microorganisms, which can simultaneously act to biodegrade a pollutant and generate power. Carbon cloth electrodes loaded with two rates of phenanthrene (2 and 20 mg cm^-2) were compared using dual chamber MFCs that were operated for 50 days. The lower loading rate of 2 mg cm^-2 was most efficient in the degradation of phenanthrene and had higher power production capacities (37 mW m^-2) as compared to the higher loading rate of 20 mg cm^-2 (power production of 19.2 mW m^-2). FTIR (Fourier-Transform Infrared Spectroscopy) analyses showed a depletion in absorbance peak signals associated with phenanthrene. Microbes known to have electroactive properties or phenanthrene biodegradation abilities like Pseudomonas, Rhodococcus, Thauera and Ralstonia were enriched over time in the MFCs, substantiating the electrochemical and FTIR analyses. The MFC approach taken here thus offers great promise towards PAH bioelectroremediation.

Engineering Shelf-Stable Coating for Microfluidic Organ-on-a-Chip Using Bioinspired Catecholamine Polymers

The conceptualization of body-on-a-chip in 2004 resulted in a new approach for studying human physiology in three-dimensional culture. Despite pioneering works and the progress made in replicating human physiology on-a-chip, the stability, reliability, and preservation of cell-culture-treated microfluidic chips remain a challenge. The development of a reliable surface treatment technique to more efficiently and reproducibly modify microfluidic channels would significantly simplify the process of creating and implementing organ-on-a-chip (OOC) systems. In this work, a new flow-based coating technique using bioinspired polymers was implemented to create reliable, reproducible, ready-to-use microfluidic cell culture chips for OOC studies. Single-channel polydimethylsiloxane microfluidic chips were coated with the bioinspired catecholamine polymers, polydopamine (PDA) and polynorepinephrine (PNE), using a flow-based coating technique. The functionality of the resulting microfluidic chips was evaluated by extensive surface characterizations, at 130 °C, in the presence of various cleaning and culture media in static and flow conditions regularly used in OOCs and tested for shelf life by storing the coated microfluidic chips for 4 months at room temperature. Microfluidic chips coated with polycatecholamine were then seeded with the mouse cancer cell line Cath.a.differentiated (CAD) and with the normal human cerebral microvascular endothelial cell line human cerebral microvascular endothelial cells (hCMEC)/D3. Cell viability, cell phenotype, and cell functionality were assessed to evaluate the performance of both the coatings and the surface treatment technique. Both PDA- and PNE-coated microfluidic chips maintained high viability, phenotype, and functionality of CAD cells and hCMEC/D3 cells. In addition, CAD cells retained high viability when they were cultured in both the polymer-coated chips, which were stored at room temperature for up to 120 days. These results suggest that flow-based techniques to coat surfaces with polycatecholamines can be used to generate ready-to-use microfluidic OOC chips that offer long-term stability and reliability for the culture of cell types with application in pathophysiological studies and drug screening.

Interesting Facets of Surface, Interfacial, and Bulk Characteristics of Perfluorinated Ionomer Films

Ion-containing perfluorinated polymers possess unique viscoelastic properties, excellent proton conductivity, and nanophase-segregated structure all arising from the clustering of hydrophilic sulfonic acid groups within a matrix of hydrophobic fluorocarbons. When these ionomers are confined to nanothin films, a broad swathe of structural organization imparting a rich variety of surface, interfacial, and bulk characteristics can be expected. However, our understanding of perfluorinated ionomer thin film behavior is still in a rudimentary stage, and much of the research focus to date has been on its hydration-related structure and properties pertinent to electrochemical applications. Thus, many hidden gems-their interesting surface and interfacial properties-have been overlooked. In this Invited Feature Article, which is a summary of the key contributions by the author's group, including several collaborative publications on ionomer thin films, we unravel many of these facets. In addition, the article attempts to integrate knowledge acquired from a variety of investigations of different aspects of the ionomer thin films to refine and develop a consistent picture of their structure and behavior. First, we focus on the self-assembly of ionomers and show that dispersion media and hydrophobicity/hydrophilicity of the substrate can result in partial or even no coverage of substrates, shedding light on the complexity of polymer-substrate, polymer-solvent, and polymer-polymer interactions, an insight completely obscured when the spin-coating method is adopted for film creation. We demonstrate that the same ionomer can be used to create a variety of surfaces ranging from superhydrophilic to highly hydrophobic by controlling the film thickness or through the choice of substrate material. The ultrathin, hydrophilic surfaces of self-assembled Nafion ionomer films exhibit wettability switching behavior which opens the door to creating stimuli-responsive smart surfaces. The thermoresponsive behavior of the films is discussed in the context of surface (wettability) and bulk (thermal expansion) characteristics as well as a newly discovered vibrational mode. The substrate- and film thickness-dependent thermal expansion coefficients reinforce the importance of interfacial interactions and confinement on the structure/properties of these films. They also open up the potential of tuning ionomer bulk properties via substrate chemistry. The discovery of a vibrational mode that becomes thermally activated at high temperature has provided new insights into the origins of the molecular motions responsible for the α-relaxation of the Nafion ionomer as well as the underlying reason for wettability switching. Our recent neutron reflectometry study of different ionomers varying in side-chain composition/length on a platinum substrate shows that the interfacial hydration level is correlated to the side-chain length, which opens up the possibility of the controlling the interfacial electrochemistry. Finally, a systematic analysis of factors affecting proton conduction is presented to elucidate the yet-unresolved origins of the suppressed conduction of nanothin ionomer films compared to that of the bulk membrane. By revealing these interesting yet poorly understood facets of ionomer thin films, the article aims to stimulate further scientific pursuit on this topic.

Impact of the Solid Interface on Proton Conductivity in Nafion Thin Films

Proton conductivity of polyelectrolytes in the interfacial region with a solid is key to the performance of polyelectrolyte-based fuel cells. The proton conductivity of Nafion thin films was examined as a function of the thickness along both directions, normal and parallel to the interface. Neutron reflectivity measurements revealed that a water-containing multilamellar structure was formed at the substrate interface. The presence of the interfacial layer, or the two-dimensional proton-conductive pathway, suppressed and enhanced the out-of-plane and in-plane proton conductivities, respectively. The method of proton conductivity in the interfacial region differed from that in the bulk, namely, the Grotthuss mechanism. Using laminated films, we conclude by showing that the proton conductivity in the Nafion thin film changes on the basis of the interface-to-volume ratio. This knowledge will be helpful for the design of devices containing polyelectrolytes with solid materials.

Polyethylenimine Modified Graphene-Oxide Electrochemical Immunosensor for the Detection of Glial Fibrillary Acidic Protein in Central Nervous System Injury

Glial fibrillary acidic protein (GFAP) is as an intermediate filament protein expressed by certain cells in the central nervous system (CNS). GFAP has been recognized as a reliable biomarker of CNS injury. However, due to the absence of rapid and easy-to-use assays for the detection of CNS injury biomarkers, measuring GFAP levels to identify CNS injury has not attained widespread clinical implementation. In the present work, we developed a polyethylenimine (PEI) coated graphene screen-printed electrode and used it for highly sensitive immunosensing of GFAP. Covalent binding of GFAP antibody to the PEI-modified electrode surface along with electrochemical impedance spectroscopy was used for detecting the change in the electrical conductivity of the electrodes. A highly linear response was recorded for various GFAP concentrations. Quantitative, selective, and label-free detection was achieved in the dynamic range of 1 pg mL^-1 to 100 ng mL^-1 for GFAP spiked in phosphate buffer saline, artificial cerebrospinal fluid, and human blood serum. The performance of the immunosensor was further validated and correlated by testing samples with the commercially available enzyme-linked immunosorbent assay method. This functionalized electrode could be used clinically for rapid detection and monitoring of CNS injury.

Proton-Conductivity Enhancement in Polymer Thin Films

Highly proton conductive polymers have long attracted the attention of researchers for use in energy conversion, sensors, catalysts, and other applications. From the viewpoint of the scientific history of the creation of highly proton conductive polymers, one fundamental approach is based on the strategy of phase-segregated structures with strong acid groups. This Feature Article presents a new approach to enhancing the proton conductivity of the polymer thin films using an interface that can modify the degrees of freedom for a polymer structure through interaction between the substrate surface and polymers. I introduce suppressed proton conductivity into Nafion thin films and then specifically examine the enhancement in proton conductivity by the molecular orientation of the polymers. As the last topic, a highly proton conductive organized polyimide thin film is demonstrated using the lyotropic liquid-crystal property. Both molecular ordering and the in-plane oriented structure can enhance proton conductivity. Moreover, the optical domain and degree of molecular ordering derived from the molecular weight can contribute strongly to the proton transport property.