Redox Hydrogel Research Articles

On-Chip Enzymatic Microbiofuel Cell-Powered Integrated Circuits

In situ energy harvesting is a major limitation for the development of implantable biomedical digital devices. Considerable efforts have been made to improve Glucose/O2 Enzymatic Biofuel Cells (EBFCs), able to generate electrical power in the physiological environment, in order to match the demands of biomedical electronics [1]. A key advance was the development of redox hydrogels. They permit efficient connection of high amounts of oxidoreductases to electrodes, leading to easily miniaturizable BFCs with power densities in the range of 100 μW.cm-2 suitable for application [2]. However, their open circuit potentials still fall below the requirements of conventional electronics. On the other hand, the power and voltage demands have decreased thanks to tremendous progress in low-power electronics [3], rendering devices compatible with the performance of EBFCs. Here we demonstrate that common data processing functions such as binary counting can be powered by a glucose/O2 micro-EBFC directly integrated on an Application Specific Integrated Circuit (ASIC) at the die level. An EBFC based on Glucose Oxidase and Bilirubin Oxidase hydrogels immobilized on 400 μm diameter electrodes was designed on the Si chip of the 4-bit ripple-counter. The system could be continuously powered during several hours from a single 50 μL droplet of aerated 5mM glucose solution. 1] S. Cosnier, A. Le Goff, M. Holzinger, Electrochem. Comm., 2014, 38, 19-23. [2] N. Mano, F. Mao, A. Heller, JACS, 2003, 125, 6588-6594. [3] A. Chandrakasan, N. Verma, D.C. Daly, Annual Rev. Biomed. Eng., 2008, 10, 247-274.

Analysis of photosystem II electron transfer with natural PsbA-variants by redox polymer/protein biophotoelectrochemistry

Redox polymer/protein biophotoelectrochemistry was used to analyse forward electron transfer of isolated PSII complexes with natural PsbA-variants. PsbA1- or PsbA3-PSII was embedded in a redox hydrogel that allows diffusion-free electron transfer to the electrode surface and thus measurement of an immediate photocurrent response. The initial photocurrent density of the electrode is up to ~2-fold higher with PsbA1-PSII under all tested light conditions, the most prominent under high-light [2,300 μmol(photon) m-2 s-1] illumination with 5 μA cm-2 for PsbA3-PSII and 9.5 μA cm-2 for PsbA1-PSII. This indicates more efficient electron transfer in low-light-adapted PsbA1-PSII. In contrast, the photocurrent decays faster in PsbA1-PSII under all tested light conditions, which suggests increased stability of high-light-adapted PsbA3-PSII. These results confirm and extend previous observations that PsbA3-PSII has increased P680+*/QA-* charge recombination and thus less efficient photon-to-charge conversion, whereas PsbA1-PSII is optimised for efficient electron transfer with limited stability.

Facile synthesis of polyaniline-polythionine redox hydrogel: Conductive, antifouling and enzyme-linked material for ultrasensitive label-free amperometric immunosensor toward carcinoma antigen-125

Sensitivity enhancement and proteins adsorption are the common challenges faced in protein immunoassays. In this work, an ultrasensitive and protein-resistant label-free amperometric immunosening platform for carcinoma antigen-125 (CA125) based on redox polyaniline-polythionine hydrogel (PANI-PThi gel) was developed. The as-prepared hydrogel, which was facilely synthesized by electropolymerization, exhibited good conductivity and strong hydrophilicity while the sensitivity and specificity of the immunosensor can be enhanced. Furthermore, the as-prepared AuNPs functionalized PANI-PThi gel exhibited strong current signal and H2O2 electrocatalytic ability, which guaranteed a large current variable range. Based on these, the prepared immunosensor revealed a wide linear range from 0.0001 U mL−1 to 1 kU mL−1, a limit of detection of 0.00125 U mL−1 and its sensitivity was at least three-fold higher than previous works. More importantly, the prepared immunosensor exhibited excellent specificity, making it capable of assaying CA125 in human serum.

Improved Performance of Glucose Bioanodes Using Composites of (7,6) Single-Walled Carbon Nanotubes and a Ferrocene-LPEI Redox Polymer.

The effect of incorporating different types of carbon nanotubes into composite films of a redox polymer (FcMe2-C3-LPEI) and glucose oxidase (GOX) was investigated. The composite films were constructed by first forming a high-surface area network film of either single-walled carbon nanotubes (SWNTs) or multiwalled carbon nanotubes (MWNTs) on a glassy carbon electrode (GCE) by solution casting of a suspension of Triton-X-100 dispersed SWNTs. Next a glucose responsive redox hydrogel was formed on top of the nanotube-modified electrode by cross-linking FcMe2-C3-LPEI with glucose oxidase via ethylene glycol diglycidyl ether (EGDGE). Electrochemical and enzymatic measurements showed that composite films made with (7,6) SWNTs produced a higher response (3.3 mA/cm2) to glucose than films made with (6,5) SWNTs (1.8 mA/cm2) or MWNTs (1.2 mA/cm2) or films made without SWNTs (0.7 mA/cm2). We also show that the response of the composite films could be systematically varied by fabricating SWNT films with different weight ratios of (7,6) and (6,5) SWNTs. Optimization of the (7,6) SWNTs loading and the redox polymer-enzyme film produced a glucose response of 11.2 mA/cm2. Combining the optimized glucose films with a platinum oxygen breathing cathode into a biofuel cell produced a maximum power density output of 343 μW/cm2.

Interrogation of a PS1-Based Photocathode by Means of Scanning Photoelectrochemical Microscopy.

In the development of photosystem-based energy conversion devices, the in-depth understanding of electron transfer processes involved in photocurrent generation and possible charge recombination is essential as a basis for the development of photo-bioelectrochemical architectures with increased efficiency. The evaluation of a bio-photocathode based on photosystem 1 (PS1) integrated within a redox hydrogel by means of scanning photoelectrochemical microscopy (SPECM) is reported. The redox polymer acts as a conducting matrix for the transfer of electrons from the electrode surface to the photo-oxidized P700 centers within PS1, while methyl viologen is used as charge carrier for the collection of electrons at the reduced FB site of PS1. The analysis of the modified surfaces by SPECM enables the evaluation of electron-transfer processes by simultaneously monitoring photocurrent generation at the bio-photoelectrode and the associated generation of reduced charge carriers. The possibility to visualize charge recombination processes is illustrated by using two different electrode materials, namely Au and p-doped Si, exhibiting substantially different electron transfer kinetics for the reoxidation of the methyl viologen radical cation used as freely diffusing charge carrier. In the case of p-doped Si, a slower recombination kinetics allows visualization of methyl viologen radical cation concentration profiles from SPECM approach curves.

Conductive catalytic redox hydrogel composed of aniline and vinyl-ferrocene for ultrasensitive detection of prostate specific antigen

In this work, a conductive catalytic redox hydrogel composed of aniline and vinyl-ferrocene was prepared which showed good conductivity and strong electrochemical redox signal at 0.04V (vs. Ag/AgCl). The hydrogel was simply generated in situ on the glassy carbon electrode to fabricate an amperometric immunosensor. The redox hydrogel provided a three dimensional porous structure, which improved the specific surface area of modified electrode. Based on the catalytic activity of ferrocene toward the oxidation of ascorbic acid, the electrochemical signal can be amplified. Gold nanoparticles were modified on the redox hydrogel by electrochemical deposition to promote electron transfer, which also can immobilize antibody. Prostate-specific antigen (PSA), a significant indicator of prostate cancer, had been measured as a model tumor marker. Under the optimized conditions, a linear range of the prostate specific antigen concentrations from 0.001 to 200ngmL−1 with a detection limit of 0.54pgmL−1 ((S/N)=3) was obtained. Importantly, the detection of PSA in the human blood serum samples showed satisfactory accuracy with the enzyme linked immunosorbent assay results, which showed the redox hydrogel possessed huge potential for the detection of other tumor markers.

Electrospun Carbon Nanofibers as Supports for Bioelectrodes

A wide range of carbon nanofiber (CNF) mats with controlled properties of fiber diameter, surface area, porosity, and conductivity were fabricated via a facile and economical electrospinning method. CNFs are used as supports for bioelectrodes to enhance enzyme utilization. This study employed glucose oxidase (GOx) in a redox hydrogel system, which mediates electron transfer via osmium metal center, to create a glucose bioelectrode. CNFs reduces the hydrogel thickness and enhances the electron transport. CNFs exhibit promising glucose oxidation current density reaching 10 mA cm−2 that is higher than those of commercial carbon materials such as multiwalled carbon nanotubes (MWCNT), Bucky paper, Pyrograf, and Toray paper.

A novel versatile microbiosensor for local hydrogen detection by means of scanning photoelectrochemical microscopy

The development of a versatile microbiosensor for hydrogen detection is reported. Carbon-based microelectrodes were modified with a [NiFe]-hydrogenase embedded in a viologen-modified redox hydrogel for the fabrication of a sensitive hydrogen biosensor By integrating the microbiosensor in a scanning photoelectrochemical microscope, it was capable of serving simultaneously as local light source to initiate photo(bio)electrochemical reactions while acting as sensitive biosensor for the detection of hydrogen. A hydrogen evolution biocatalyst based on photosystem 1-platinum nanoparticle biocomplexes embedded into a specifically designed redox polymer was used as a model for proving the capability of the developed hydrogen biosensor for the detection of hydrogen upon localized illumination. The versatility and sensitivity of the proposed microbiosensor as probe tip allows simplification of the set-up used for the evaluation of complex electrochemical processes and the rapid investigation of local photoelectrocatalytic activity of biocatalysts towards light-induced hydrogen evolution.

Multiple signal amplification strategies for ultrasensitive label-free electrochemical immunoassay for carbohydrate antigen 24-2 based on redox hydrogel

In this work, multiple signal amplification strategies for ultrasensitive label-free electrochemical immunoassay for carbohydrate antigen 24-2 (CA242) were developed using redox sodium alginate-Pb2+-graphene oxide (SA-Pb2+-GO) hydrogel. The SA-Pb2+-GO hydrogel was synthesised by simply mixing SA, GO, and Pb2+ and then implemented as a novel redox species with a strong current signal at −0.46V (vs. Ag/AgCl). After the three-dimensional and porous SA-Pb2+-GO hydrogel was in situ generated on a glassy carbon electrode (GCE), chitosan was adsorbed on the obtained electrode to further enrich Pb2+. When chitosan-Pb2+/SA-Pb2+-GO/GCE was incubated with anti-CA242 using glutaraldehyde and blocked by bovine serum albumin, the immunoassay platform for CA242 was obtained. Owing to the addition of GO, the obtained conductive SA-GO/GCE was beneficial for signal amplification. After incubating SA-GO/GCE with excessive amounts of Pb2+, the resistance of SA-Pb2+-GO/GCE further decreased and a strong redox signal was obtained. The chitosan fixed by electrostatic adsorption resulted in further adsorption of Pb2+, behaving as further amplifying the signal and improving conductivity. In this case, multiple signal amplification strategies were involved in the proposed immunosensor for the ultrasensitive detection of CA242. Under the optimal conditions, the proposed immunosensor exhibited a wide linear range from 0.005UmL−1 to 500UmL−1 with an ultralow detection limit of 0.067mUmL−1. In comparison to previous works, the sensitivity of this method was 32.98μA (log10CCA242)−1, which was a five-fold increase from the previous works.

Facilitated Electron Hopping in Nanolayer Oxygen‐Insensitive Glucose Biosensor for Application in a Complex Matrix

AbstractElectrochemical experimental evidence of facilitated electron transfer in a sub‐micrometer biosensor is presented. Layer‐by‐layer self‐assembled deposition provides the unique advantage to specifically control the thickness of the biosensors, allowing an oxygen‐insensitive device with a film thickness of 70 nanometers to be obtained. The biosensor is based on a poly(allylamine) osmium redox mediator and glucose 1‐oxidase. The immobilized enzyme contributes to the signal generation through the full biosensor thickness, with no loss of “active enzyme” in the outer layer of the biosensor through reaction with oxygen, as reported in the case of thick redox hydrogels. The application of the biosensor in complex matrices was approached with tests in wastewater. Encapsulation of the biosensor with Nafion® membrane ensured the protection of the enzyme molecules from the external environment, allowing the successful application of the sensor in a complex matrix.

Efficient Enzymatic Oxidation of Glucose Mediated by Ferrocene Covalently Attached to Polyethylenimine Stabilized Gold Nanoparticles

AbstractBioanodes for fuel cell applications require highly efficient oxidation reactions to achieve a sufficiently large current density. In this study, gold nanoparticles have been synthesized using branched polyethylenimine (bPEI), a well‐known polymer that forms a hydrogel in water, as the stabilizer. Primary amine groups available in bPEI provide active sites for further conjugation with ferrocene propionic acid via the 1‐Ethyl‐3‐(3‐dimethylaminopropyl)carbodiimide coupling reaction, with the enzyme glucose oxidase, using glutaraldehyde as linkers. This composite material was then used for the fabrication of glucose oxidase electrodes by drop casting of aqueous solutions onto glassy carbon electrodes. The three‐dimensional structure offered by the new hydrogel facilitates communication between the enzyme and the electrode through the redox mediator ferrocene. This allows the glucose oxidase electrode to exhibit excellent activity towards electrocatalytic oxidation of glucose in phosphate buffer solutions at pH 7 with a maximum current density of approximately 800 μA cm−2, one of the highest values reported so far for redox hydrogel based glucose oxidase electrodes. Over a wide glucose concentration range, the enzyme response follows that predicted by the Michaelis‐Menten equation with a Michaelis constant of 8.4 mM. In the sensing context, this electrode also exhibits a wide linear dynamic glucose concentration range of 0.5–10 mM with a limit of detection of 0.04 mM.

Deep Oxidation of Multiple (Poly)Saccharides at a Bi-Enzymatic Bioelectrode

As perhaps the most commonly utilized fuel for enzymatic fuel cells (EFCs), glucose oxidation is largely under-exploited. Theoretically, the oxidation of glucose and its downstream metabolites is able to deliver a maximum of 24e- to a bioanode, however the vast majority of EFCs reported to date only perform a single 2e- oxidation of glucose. Commercially available glucose oxidase (GOx) from Aspergillus sp. oxidizes the anomeric (C1) carbon of glucose. Recently, mediated bioelectrocatalysis of pyranose dehydrogenase from Agaricus meleagris (AmPDH) via an osmium redox hydrogel has been demonstrated, whereby AmPDH is reported to be able to oxidize glucose and other saccharides at combinations of their respective C1/C2/C3 positions.1 Another attractive property of an EFC is the ability to utilize a single biocatalyst to oxidize a range of fuels. We recently reported the use of an engineered commercially-available glucose oxidase with broader substrate specificity at an electrode surface that resulted in an EFC configuration that was able to oxidize multiple (poly)saccharides, including lactose from dairy milk and starch.2 This study reports the ability of a ferrocene redox hydrogel to mediate bioelectrocatalytic saccharide oxidation by AmPDH. Further, AmPDH and broad substrate specificity glucose oxidase were combined to yield an EFC that was able to operate on multiple (poly)saccharides while theoretically increasing the extent of oxidation of each substrate, resulting in a more-versatile EFC with greater efficiency.

DNA Redox Hydrogels: Improving Mediated Enzymatic Bioelectrocatalysis

We present the preparation of a redox DNA hydrogel for mediated bioelectrocatalysis of oxidoreductase enzymes for biosensor and biofuel cell applications. The noncovalent functionalization of DNA with redox molecules is achieved by intercalation of aromatic redox probes into the DNA double helix or electrostatic binding of redox-active tetraalkylammonium ions to phosphate groups on DNA. Prepared DNA redox hydrogels demonstrate the capability of mediating bioelectrocatalytic glucose oxidation by oxidoreductase enzymes. This is the first evidence that redox DNA hydrogels can replace redox polymer hydrogels for self-exchange-based mediation for bioelectrocatalytic applications. This study contributes toward advances in the use of DNA, an emerging biomaterial, in enzymatic bioelectrocatalysis-based applications.

Design of an Os Complex-Modified Hydrogel with Optimized Redox Potential for Biosensors and Biofuel Cells.

Multistep synthesis and electrochemical characterization of an Os complex-modified redox hydrogel exhibiting a redox potential ≈+30 mV (vs. Ag/AgCl 3 M KCl) is demonstrated. The careful selection of bipyridine-based ligands bearing N,N-dimethylamino moieties and an amino-linker for the covalent attachment to the polymer backbone ensures the formation of a stable redox polymer with an envisaged redox potential close to 0 V. Most importantly, the formation of an octahedral N6-coordination sphere around the Os central atoms provides improved stability concomitantly with the low formal potential, a low reorganization energy during the Os(3+/2+) redox conversion and a negligible impact on oxygen reduction. By wiring a variety of enzymes such as pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase, flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase and the FAD-dependent dehydrogenase domain of cellobiose dehydrogenase, low-potential glucose biosensors could be obtained with negligible co-oxidation of common interfering compounds such as uric acid or ascorbic acid. In combination with a bilirubin oxidase-based biocathode, enzymatic biofuel cells with open-circuit voltages of up to 0.54 V were obtained.

Wiring of the aldehyde oxidoreductase PaoABC to electrode surfaces via entrapment in low potential phenothiazine-modified redox polymers

Phenothiazine-modified redox hydrogels were synthesized and used for the wiring of the aldehyde oxidoreductase PaoABC to electrode surfaces. The effects of the pH value and electrode surface modification on the biocatalytic activity of the layers were studied in the presence of vanillin as the substrate. The enzyme electrodes were successfully employed as bioanodes in vanillin/O2 biofuel cells in combination with a high potential bilirubin oxidase biocathode. Open circuit voltages of around 700mV could be obtained in a two compartment biofuel cell setup. Moreover, the use of a rather hydrophobic polymer with a high degree of crosslinking sites ensures the formation of stable polymer/enzyme films which were successfully used as bioanode in membrane-less biofuel cells.

Naphthoquinone Derivatives As Low-Potential Electron Mediators of Fad-Dependent Glucose Dehydrogenase

Flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) is rapidly emerging as an alternative glucose-oxidizing enzyme in enzymatic fuel cells (EFCs). FAD-GDH has the benefit over its commonly used counterpart, glucose oxidase (GOx), of not undergoing the parasitic reduction of O2 to H2O2, which can be troublesome in EFC applications. Additionally, the FAD cofactor has a relatively low redox potential in comparison to heme-based and PQQ-dependent enzymes, which can allow for a much higher theoretical cell potential. However, FAD-GDH is unable to facilitate rapid rates of direct electron transfer, and therefore requires a low-overpotential redox mediator to take full advantage of the low oxidation potential of its FAD cofactor. We have prepared a range of naphthoquinone analogues that are capable of mediating electron transfer between FAD-GDH and carbon electrodes, with onset potentials ranging between -0.425 to -0.150 V (vs. SCE), at pH 7.4. Several naphthoquinone derivatives were immobilized either onto a poly(ethylenimine) backbone or directly onto the outer shell of FAD-GDH to yield a naphthoquinone redox hydrogel and a naphthoquinone-labeled FAD-GDH. Both methods demonstrated large catalytic current densities at low onset potentials. When coupled with an O2-reducing biocathode (bilirubin oxidase, direct electron transfer-type) the glucose/O2 EFC possessed a large open-circuit potential (OCP) of 0.864 V and was able to deliver a maximum current density of 5.4 mA cm-2. The EFC reached its maximum power density (2.3 mW cm-2) at 0.55 V.

Modeling Multi-Scale Carbon Fiber Supports for Thin Film Bioelectrodes

Carbon electrodes with their super electro-catalytic properties are commonly used in electrochemistry, especially in biomolecular redox reactions1. Electrospinning as a low cost, facile technique is applied to produce uniform carbon nanofibers between 150 nm and 1µm fiber diameter by pyrolyzing electrospun Polyacrylonitrile (PAN) fiber at 250°C and 1100°C. Varying the concentration of PAN in N,N-Dimethylformamide precursor solution changes the fiber diameter and leads to diversity in materials properties, such as surface area, void fraction and conductivity. These electrospun carbon nano fiber (ECNF) properties are characterized by SEM, BET, and four-point probe resistivity measurements. Thin film Os+/2+ mediated electron in hydrogel immobilized glucose oxidase layer is coated on either single micro fiber 2 or free - standing carbon nanofiber mats (Figure 1) to study the effect of carbon fiber physical properties on electron transfer in mediated redox hydrogel bioanode. Electron transport via redox polymer Os+/2+ is enhanced by reducing the average hydrogel film thickness with uniform distribution of small fiber of high surface area (<1.6E3 cm-1 surface area per volume). A transport model is developed to give more insight into these correlations and yield more understanding of electron transfer mechanism in thin film porous electrode. Figure 1

Light Induced H2 Evolution from a Biophotocathode Based on Photosystem 1--Pt Nanoparticles Complexes Integrated in Solvated Redox Polymers Films.

We report on a biophotocathode based on photosystem 1 (PS1)-Pt nanoparticle complexes integrated in a redox hydrogel for photoelectrocatalytic H2 evolution at low overpotential. A poly(vinyl)imidazole Os(bispyridine)2Cl polymer serves as conducting matrix to shuttle the electrons from the electrode to the PS1-Pt complexes embedded within the hydrogel. Light induced charge separation at the PS1-Pt complexes results in the generation of photocurrents (4.8 ± 0.4 μA cm(-2)) when the biophotocathodes are exposed to anaerobic buffer solutions. Under these conditions, the protons are the sole possible electron acceptors, suggesting that the photocurrent generation is associated with H2 evolution. Direct evidence for the latter process is provided by monitoring the H2 production with a Pt microelectrode in scanning electrochemical microscopy configuration over the redox hydrogel film containing the PS1-Pt complexes under illumination.

Electrospun Carbon Nanofiber Supports for Bioelectrodes

A high surface-area electrospun carbon nanofiber (CNF) support for bioelectrodes is introduced to enhance enzyme utilization.1 This study employed Glucose Oxidase (GOx) in redox hydrogel system, which mediated electron transfer via Osmium (Os2+/Os+) centers, to create a glucose bioelectrode. The strategy was to enhance the transport of electrons via the redox mediator by reducing the average hydrogel thickness at fixed hydrogel volume. The tunable electrospun carbon fiber enables of hydrogel film thickness of hydrogel film by uniform distribution on small fibers. Glucose oxidizing current density made from electrospun carbon nanofiber and several commercial counterparts suggest a possibility for a cheap fibers, in a diameter range below100 nm, that enable high current density (~8.5 mA cm-2) comparable to carbon nanotubes (CNTs). To adjust electrospun CNF size, varied concentrations of Polyacrylonitrile (PAN)/ Dimethylformamide (DMF) precursor solution (7 wt% to 14 wt%) was spun in a high voltage field prior to oxidation stabilization and carbonization in Argon environment at two temperatures, 850°C and 1100°C.2 Graphiticity of CNFs was improved with the increasing heat treatment temperature (shown by Raman spectroscopy), leading to higher conductivity of bioelectrode. CNFs based on 7 wt% PAN/DMF carbonized at 1100°C had a diameter of 150 nm (Figure 1b), a decrease from the 250 nm PAN fiber (Figure 1a), and showed good performance in electrochemical characterization compared to MWCNTs electrode (Figure 2). Acknowledgement We thank Buckeye Composites Inc. and Pyrograf Products Inc. for providing material samples. Reference (1) Wen, H.; Nallathambi, V.; Chakraborty, D.; Calabrese Barton, S. Carbon Fiber Microelectrodes Modified with Carbon Nanotubes as a New Support for Immobilization of Glucose Oxidase. Microchim. Acta 2011, 175, 283–289. (2) Bhardwaj, N.; Kundu, S. C. Electrospinning: A Fascinating Fiber Fabrication Technique. Biotechnol. Adv. 2010, 28, 325–347. Figure 1

Mechanism of protection of catalysts supported in redox hydrogel films.

The use of synthetic inorganic complexes as supported catalysts is a key route in energy production and in industrial synthesis. However, their intrinsic oxygen sensitivity is sometimes an issue. Some of us have recently demonstrated that hydrogenases, the fragile but very efficient biological catalysts of H2 oxidation, can be protected from O2 damage upon integration into a film of a specifically designed redox polymer. Catalytic oxidation of H2 produces electrons which reduce oxygen near the film/solution interface, thus providing a self-activated protection from oxygen [Plumeré et al., Nat Chem. 2014, 6, 822-827]. Here, we rationalize this protection mechanism by examining the time-dependent distribution of species in the hydrogenase/polymer film, using measured or estimated values of all relevant parameters and the numerical and analytical solutions of a realistic reaction-diffusion scheme. Our investigation sets the stage for optimizing the design of hydrogenase-polymer films, and for expanding this strategy to other fragile catalysts.