Electrochemical Cross-linking Research Articles

Electrochemical Crosslinking of Alginate-Towards Doped Carbons for Oxygen Reduction.

Electrochemical crosslinking of alginate strands by in situ iron oxidation was explored using a potentiostatic regime. Carbon-based materials co-doped with iron, nitrogen, and/or sulfur were prepared via electrolyte composition variation with a nitrogen-rich compound (rivanol) or through post-treatments with sodium sulfide. Nanometer-sized iron particles were confirmed by transmission and field emission scanning electron microscopy in all samples as a consequence of the homogeneous dispersion of iron in the alginate scaffold and its concomitant growth-limiting effect of alginate chains. Raman spectra confirmed a rise in structural disorder with rivanol/Na2S treatment, which points to more defect sites and edges known to be active sites for oxygen reduction. Fourier transform infrared (FTIR) spectra confirmed the presence of different iron, nitrogen, and sulfur species, with a marked difference between Na2S treated/untreated samples. The most positive onset potential (-0.26 V vs. saturated calomel electrode, SCE) was evidenced for the sample co-doped with N, S, and Fe, surpassing the activity of those with single and/or double doping. The mechanism of oxygen reduction in 0.1 M KOH was dominated by the 2e- reduction pathway at low overpotentials and shifted towards complete 4e- reduction at the most negative explored values. The presented results put forward electrochemically formed alginate gels functionalized by homogeneously dispersed multivalent cations as an excellent starting point in nanomaterial design and engineering.

Combining a protein-targeting small molecule and a thiol-targeting small molecule for detecting a serum risk marker of liver tumor recurrence

This work proposes a novel bioassay designed to detect the 2B receptor of serotonin in serum samples, which can serve as a risk marker for cancer recurrence after surgical resection. Traditional methods for detecting this marker are often costly and time-consuming, requiring specialized reagents and equipment. The new bioassay is designed to enable direct and reagent-less detection of the 2B receptor in serum samples, without the need of antibodies or enzymes. The assay uses a small molecule ligand for the 2B receptor combined with a thiol-targeting fluorescent dye on a compact peptide-based molecular frame. This design allows for a rapid and specific readout of the fluorescent signal upon probe-protein interaction. In addition, the covalent biosensing process used in the assay allows for signal enhancement by electrochemical cross-linking of serum proteins. The bioassay was successfully used to detect the 2B receptor in serum samples from hepatocarcinoma patients, indicating its potential as a powerful tool for early cancer detection and monitoring.

Electrodeposition induced covalent cross-linking of chitosan for electrofabrication of hydrogel contact lenses

To meet the requirement of personalization, there is an urgent need to develop a simple, efficient and versatile manufacturing method for customized contact lens manufabrication. Here, we report a novel electrofabrication methodology (i.e., electrodeposition) for the fabrication of hydrogel contact lenses, which can induce covalent cross-linking between chitosan and epichlorohydrin simultaneously. The transmittance and toughness of hydrogels are improved by electrochemical cross-linking without affecting their oxygen permeability. Furthermore, the geometry of the chitosan based hydrogel contact lenses can be customized simply by the electrode template, and its characteristics can be regulated by electrical signals and electrochemical cross-linking. The electrodeposited hydrogel contact lenses have good optical properties, mechanical properties and biocompatibility, and their anti-adhesion properties to Staphylococcus aureus are close to commercial contact lenses. This work reveals the mechanism of electrochemical cross-linking between chitosan and epichlorohydrin and provides an alternative method for contact lens fabrication.

Electrochemical Characterization of Redox Probes Confined in 3D Conducting Polymer Networks.

In this manuscript we present a versatile platform for introducing functional redox species into tailor‐made 3D redox polymer networks. Electrochemical characterization based on cyclic voltammetry is applied to verify the immobilization of the redox species within the conducting networks. Ultimately this strategy shall be extended to (photo)electrocatalytic applications which will profit from the conducting polymer matrix. Soluble precursor copolymers are synthesized via radical copolymerization of vinyltriphenylamine (VTPA) with chloromethylstyrene (CMS) in different ratios, whereas CMS is subsequently converted into azidomethylstyrene (AMS) to yield poly(VTPA‐co‐AMS) copolymers. Spin‐coating of poly(VTPA‐co‐AMS) on gold electrodes yields thin films which are converted into stable polymer network structures by electrochemical crosslinking of the polymer chains via their pendant triphenylamine groups to yield N,N,N′,N′‐tetraphenylbenzidine (TPB) crosslinking points. Finally, the resulting redox‐active, TPB‐crosslinked films are functionalized with ethynylferrocene (EFc) as a representative redox probe using a click reaction. Main experimental tools are polarization modulation infrared reflection absorption spectroscopy and scan rate dependent cyclic voltammetry. Especially the latter proves the successful conversion and the immobilization of redox probes in the polymer matrix. The results are compared with the reference system of azide‐terminated self‐assembled monolayers on gold substrates, allowing to distinguish between free and immobilized EFc species.

Enabling Molecular Gapping and Bridging on a Biosensing Surface via Electrochemical Cross-Linking and Cleavage.

Protein detection in complicated biological samples requires robust design and a rigorous rinsing process. Many recently developed artificial targeting probes, however, often do not possess antibody-like binding strength that enables them to endure harsh biosensing conditions, and the classic 2-to-1 sandwich binding pattern is unavailable for many targets, often necessitating a complicated indirect signal conversion mechanism. Here, an attempt is made to provide innovative "covalent" solutions to such problems by employing peptide reactivity to form a covalent and robust biosensing structure upon target binding. Both the cross-coupling and cleavage of peptide chains are employed to achieve the classic 2-to-1 binding when only one targeting probe is available. Specifically, a targeting probe against the protein and a signaling probe are coimmobilized onto the sensing interface. The ratio between the two probes and their surface density is modulated so that direct cross-linking between the two immobilized probes is suppressed by the average distance between two such probes. Upon protein binding, the protein molecule may bridge that gap by itself. The signaling probe can carry any motif of signal amplification. And here a Cu-ion-complexed motif, which can exhibit peroxidase-like activity upon electrochemical agitation, is employed multipurposely as the catalyst for cross-linking, cleavage, and signal amplification. Three nonhomologous target proteins can be sensitively quantified in serum-spiked samples and clinical sample detection of one of them is also successful; these results suggest the potential of the proposed method in future clinical applications.

Electroluminescent Behaviors of Electrochemically Cross‐Linkable Poly(benzyl ether) Terthiophene Dendrimers

In this study, the electroluminescent (EL) properties of electrochemically cross-linkable, terthiophene terminated poly(benzyl ether) dendrimers (G0-3T and G1-3T DNs) are investigated. In noncross-linked 3T DNs, there is either no emission or a low-intensity emission due to poor charge transfer of the isolated terthiophene (3T) oligomers at the periphery of the DNs. However, increase in the length of conjugation via electrochemical cross-linking leads to the emission of a yellowish color. It is seen that the emission of this color is dependent on the dendritic structures and effectively controlled charge trapping. Thus, these results suggest that the dendritic molecular structure, which is associated with the conjugated length of the emissive peripheral units and the generation of the dendrimers, is an important factor in controlling EL properties.

Neutral state colorless electrochromic polymer networks: Spacer effect on electrochromic performance

A series of carbazole-based monomers and their corresponding polymers with various spacer units were synthesized and coated onto ITO-glass surface via an electrochemical cross-linking process. All carbazole-based polymers exhibited multi-electrochromic behavior thanks to two separate oxidation processes. All materials were characterized by FT-IR, 1H NMR, GPC, DSC, UV–vis, Florescence and CV. In addition, electrochromic properties of the cross-linked polymer films were investigated via spectro-electrochemical measurements. The colorless films were converted to yellowish green and greenish blue colors at the anodic regime. The results indicate that the spacer unit between the carbazole unit and the polymer backbone has a great impact on the optical and electrochemical properties and also on the electrochromic performance of these polymers. PVBEC with benzyloxy based spacer exhibits a high contrast ratio (ΔT% = 55 at 690 nm), a faster response time of about 2.1 s, a higher coloration efficiency (331 cm2 C−1) and a better stability (retains its performance by 94.3% even after 1000 cycles).

Surface‐tethered conjugated polymers created via the grafting‐from approach

ABSTRACTPolyacrylate brushes with pendent terthiophenes [poly(terthiophene methyl methacrylate) (PTTMM)] were successfully grown from indium tin oxide and gold with surface‐initiated atom transfer radical polymerization. The films formed on both substrates were characterized by ellipsometry and Fourier transform infrared spectroscopy. Using cyclic voltammetry, we electrochemically crosslinked the PTTMM brush to form a conjugated polymer network. The conjugation lengths in the film were increased as evidenced by ultraviolet–visible spectroscopy. Additionally, an atomic force microscopy study on the surface‐modified solid substrate revealed the formation of a smooth and uniform polymer brush with a low surface roughness, even after electrochemical crosslinking. These uniformly grafted conducting polymer brushes may find use in photovoltaic devices. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41363.

Copolymerization of Electroactive Poly(benzyl ether) Dendrimer and Terthiophene Monomer

In this study, we report the copolymerization, film properties, and HOMO/LUMO levels of the first generation (G1) poly(benzyl ether) dendrimer functionalized on its periphery with terthiophene and terthiophene monomers. Pure G1 dendrimer (DN1), 1:1 and 1:18 molar ratios of the G1 dendrimer and the monomer (e.g., DNTT11 and DNTT118, respectively) are used to compare film formation and HOMO/LUMO levels. The conjugated polymer network (CPN) film obtained from the highest molar ratio of terthiophene monomers shows smooth film formation. Addition of the terthiophene monomer for electrochemical cross-linking controls the surface morphology and the tunability of energy levels of the CPN films. Furthermore, in situ electrochemical quartz crystal microbalance (EQCM) measurements are performed to characterize frequency change (corresponding to mass change) per sweep cycle and relative viscoelastic properties through resonant resistance-resonant frequency (R-F) diagram during the formation of each CPN film. The DN1 film has relatively higher eleastic film and mass loading on the QCM surface than the copolymeried DNTT11 and DNTT118 films. Furthermore, the addition of terthiophene monomers increases the viscosity of the film due to the effect of introducing linear species on the CNP film formation in solution. Thus, the copolymerization of eletroactive dendrimers with the same electroactive monomers enables control on surface morphological features and energy bandgap for future optoelectronic device applications and physical properties (i.e., viscoelasticity) of the films.

Electrochemically Active Cross‐Linking Reaction for Fluorescent Labeling of Aliphatic Alkenes

Although cross-linking reactions serve as a valuable tool for the integration of two or more functionalities or properties, the application of electrochemical synthesis to cross-linking reactions is restricted due to the difficulty of mass transfer. Thus, the primary purpose of this research is to explore electrochemical cross-linking systems to construct a fluorescent probe, triggered by the formation of a covalent linkage. The second purpose is to apply the probe to insoluble targets. Towards these goals, a combination of electrochemically active phenol derivatives and aliphatic alkenes were employed to form polycyclic compounds. Several of the dihydrobenzofuran derivatives formed through [3+2] cyclization reactions exhibited fluorescence. Furthermore, this approach allowed the effective modification of alkene-modified silica gel with electrochemically active species, which enables the construction of fluorescent probes that are triggered by C-C bond formation.

Monitoring in situ Electrochemical Crosslinking in Nanostructured Precursor Polymer Films by EC‐SPR Spectroscopy

AbstractThe optical/electrochemical and crosslinking properties of polycarbazole LbL films is studied using EC‐SPR spectroscopy. The in situ electrochemical crosslinking and doping/de‐doping properties are monitored for several different layered film structures. P4VPCBZ and PCEMMA are used as precursor polyelectrolytes, PAA and PDADMAC as non‐crosslinkable polyelectrolytes, which are effective barrier layers for electron transfer. The changes in reflectivity and charge during CV illustrates that quantitative control of electrochemical crosslinking and doping/de‐doping can be achieved by varying the composition of the precursor and counter‐polyelectrolyte layers. The method may provide new opportunities for basic studies of model conjugated polymer systems and for electrooptic device applications.magnified image

Interfacial Behavior of OEG–Linear Dendron Monolayers: Aggregation, Nanostructuring, and Electropolymerizability

We report on the interesting interfacial behavior of oligoethylene glycol or OEGylated linear dendron monolayers at the air-water interface as a function of (a) carbazole dendron generation, (b) the length of the OEG units, and (c) the surface pressure applied upon compression. Surface pressure-area isotherms, hysteresis studies, and isobaric creep measurement revealed a structure-property relationship consistent with the hydrophilic-lipophilic balance of a linear dendron with the OEG group serving as the surface anchor to the water subphase. AFM studies revealed that all the OEGylated carbazole dendrons self-assemble into spherical morphology at low surface pressures but form ribbonlike structures as the surface pressure is increased. This nanostructuring is primarily imparted by the increase in van der Waals forces with increasing amount of carbazole units per dendron generation on a hydrophilic mica surface. Further, electrochemical cross-linking of the carbazole molecules by cyclic voltammetery (CV) on doped Si wafer has enabled the formation of an LB film monolayer with a secondary level of organization in the monolayer imparted by the inter- and intramolecular cross-linking among the carbazole units. This study should provide a basis for monolayer film materials based on combining the LB technique and electrochemical cross-linking for nanostructuring superstructures at the air-water interface.

Polythiophene precursor electrochemical nanolithography: highly local thermal and morphological characterization

We describe the formation of nanopatterns on an electroactive polythiophene precursor (polyterthiophene methylmethacrylate, P3T) film using a current-sensing atomic force microscopy (CS-AFM) technique. Patterning parameters such as applied bias voltages, sweep duration, and scan rate were adapted for the nanolithographic electrochemical patterning process. The formation of nanopatterns on the film was mainly attributed to the electrochemical cross-linking and oxidation process of the terthiophene units. This is related to the electron transport process under an applied electric field and mass transfer due to Joule heating generated in a localized area from the tip. To further highlight the role of Joule heating, thermal AFM lithography of various blend film compositions (P3T:PMMA) was made using the heated tip AFM (HT-AFM) method. The thermal effects on local morphology were then studied not only as a function of sequential increase in local temperature, but also by nano-scale direct thermal-writing.

An Ethanol/O2 Biofuel Cell Based on an Electropolymerized Bilirubin Oxidase/Pt Nanoparticle Bioelectrocatalytic O2‐Reduction Cathode

An effective O2-reducing bioelectrocatalytic electrode is prepared by the electrochemical crosslinking of thioaniline-modified Pt nanoparticles (NPs) and thioaniline-functionalized bilirubin oxidase (BOD). An O2/ethanol biofuel cell element is constructed by integrating the Pt NP/BOD cathode and an electrically contacted alcohol dehydrogenase (AlcDH)-based anode.

Carbazole Peripheral Poly(benzyl ether) Dendrimers at the Air−Water Interface: Electrochemical Cross-Linking and Electronanopatterning

A Langmuir film of a third-generation carbazole-terminated poly(benzyl ether) (G3-CtPBE) dendrimer was investigated at the air-water interface. Langmuir-Blodgett (LB) films were deposited on gold substrates and investigated by atomic force microscopy (AFM), followed by electrochemical and electronanopatterning studies. For the G3-CtPBE dendrimer aggregates, variable concentration and surface pressure gave control over aggregate size and shape at the air-water interface. At a lower concentration C1, aggregate-spherical nanoparticles were observed with a face-on or overlapped orientation with increasing surface pressure. However, at a higher concentration C2, their surface morphologies exhibited circular and rod-shaped aggregates with respect to increasing surface pressure attributed to an edge-on configurational change. Moreover, in situ simultaneous interfacial potentiostatic electrodeposition with LB transfer at the air-water interface was employed for the first time with the G3-CtPBE dendrimers onto a hydrophilic surface under constant voltage (i.e., close to the oxidation potential of G3-CtPBE for electrochemical cross-linking). Electrochemical cross-linking on G3-CtPBE dendrimer LB films was also performed ex situ to investigate electrochemical and optical properties. Finally, as an application of a cross-linkable LB film, electronanolithography was carried out to prepare nanopatterns using the current sensing atomic force microscopy (CS-AFM) technique.

Synthesis and Properties of a Novel Cross-Linked Electroactive Polymer Formed from a Bipolar Starburst Monomer

This Article describes the synthesis and physical properties of two bipolar starburst monomers: 1,3,5-tris{5-(7-(carbazol-9-yl)-(9,9′-spirobifluoren-2-yl)-1,3,4-oxadiazol-2-yl}benzene (OXD-CBZ) and 1,3,5-tris[7-diphenyl-(9,9′-spirobifluorene)-1,3,4-oxadiazoyl]benzene (OXD-DPA) featuring an electron-deficient tris(1,3,4-oxadiazole)phenylene ring as an interior core bridged by rigid spirobifluorene units to terminal electroactive carbazole (CBZ) and diphenylamino (DPA) groups. The electronic absorption spectra of OXD-CBZ and OXD-DPA depend only slightly on the solvent polarity, revealing that weak electronic coupling existed between the donor (CBZ and diphenylamino groups) and acceptor (1,3,4-oxadiazole) moieties in the ground state. In contrast, the emission spectra of these starburst bipolar molecules were strongly dependent on the solvent polarity, a phenomenon that we attribute to the polarized excited states generated through intramolecular charge transfer. The starburst monomer OXD-CBZ exhibited a redox gradient, where the peripheral CBZ moieties exhibited lower oxidation potentials than those of the fluorene units in the interior, allowing us to obtain a new electropolymerized macromolecule. The polymer films derived from OXD-CBZ exhibited good conductivity, reversible electrochemical processes, and stable color changes (from transparent to green to light blue) with high coloration efficiency upon electro-oxidation. The radical cations of OXD-CBZ were localized within the CBZ rings; the lack of substituents at the C3 and C6 positions of the CBZ unit led to the effective electrochemical cross-linking process. In contrast, electro-oxidation of OXD-DPA, which was end-capped with DPA groups, produced stable radical cations that did not undergo associated electrochemical reactions. The higher stability of the OXD-DPA radical cations through resonance stabilization with the fluorene ring impeded the progress of typical triphenylamine dimerization reactions.

Photopatterned conjugated polymer electrochromic nanofibers on paper

Electrochromic nanofibers of conducting polymer (terthiophene) have been deposited over a conventional paper sheet by means of the electrospinning technique, and subsequently photopatterned by means of UV radiation. The synthesis of a processable precursor copolymer with a norbornylene matrix and pendant units of terthiophene makes the electrospinning process available, and allows for chemical or electrochemical crosslinking of the precursor copolymer to obtain a conducting polymer. The inclusion of photocrosslinkable units (methacrylate) in the precursor copolymer also allows for photopatterning of the material. This was applied to obtain patterns on the paper which can be chemically oxidized or reduced resulting in electrochromic characters. SEM images of the conducting polymer nanofibers together with the cellulose fibers show how these materials can be attached to textile fibers, adding new functionalities that are reminiscent of the chameleonic abilities of some living creatures.

Electrochemical Cross-Linking and Patterning of Nanostructured Polyelectrolyte−Carbazole Precursor Ultrathin Films

Nanostructured ultrathin films of pendant carbazole-modified polyelectrolytes were fabricated using the layer-by-layer deposition technique in order to correlate the importance of highly ordered structures and well-defined layer composition in electrochemical cross-linking and nanopatterning. Both UV−vis and surface plasmon resonance (SPR) spectroscopy studies indicated differences in the film structure and bilayer formation based on the degree of electrostatic interaction and carbazole content. Cyclic voltammetry (CV) and SPR studies confirmed the relationship between film structure and the amount of carbazole units per layer affecting the degree of electrochemical cross-linking. CV in particular was used to determine the mechanism of electron transport and electrochemical cross-linking phenomena. Nanopatterning was demonstrated using current sensing AFM (CS-AFM) in which different applied voltages, writing speeds, and composition of polyelectrolytes were studied together with the formation of a complex nanopattern.

Nanocomposite Films of a Polyfluorene Copolymer and Carbazole– Thiol‐Capped Gold Nanoparticles: Electrochemical Crosslinking and Energy‐Transfer Properties

AbstractThe preparation and properties of an electrochemically crosslinked Au nanoparticle/polyvinylcarbazole (PVK)/polyfluorene (PFC) nanocomposite film is described. Blends of the blue‐light emitting PFC, PVK, and the 11‐(9H‐carbazol‐9‐yl)‐undecane‐1‐thiol‐capped Au nanoparticles (CBZ‐S‐Au) were prepared as thin films by spin‐coating and subsequent crosslinking by cyclic voltammetry (CV). The electrochemical properties are then evaluated on the basis of the redox behavior (reversibility) and electron‐transport properties of the films. They are further characterized by UV‐vis spectroscopy, photoluminescence spectroscopy, and atomic force microscopy (AFM). The large differences in photoluminescence properties of the CBZ‐S‐Au/PFCVK crosslinked copolymer composite film compared to the PFCVK polymer alone result in an efficient energy‐transfer system, in which the excimer peak is almost totally quenched. This indicates a compatible match between the absorbance of the Au plasmons and the emission of the excimer energy trap from PF polymers. Furthermore, electrochemical crosslinking enhances this match both by shifting the absorbance to greater overlap and decreasing the Förster distance between the Au nanoparticle and the PFCVK polymer chains.

Investigating Carbazole Jacketed Precursor Dendrimers: Sonochemical Synthesis, Characterization, and Electrochemical Crosslinking Properties

Precursor carbazole terminated dendrons and dendrimers up to generation four (G4-D) were synthesized using a convergent approach. Sonication as a means of facilitating organic reactions in dendrimer chemistry was explored resulting in very facile and very fast (up to 50x) reaction times compared to those using traditional reflux conditions. The limits of peripheral group functionality were explored as a function of generation. The electrochemical cross-linking of the dendrimers as thin films revealed unusual cyclic voltammetry (CV) behavior depending upon the generations, which were significantly different from their linear counterpart, Poly(N-vinylcarbazole) (PVK). G1-D showed a higher extent of intermolecular cross-linking while G4-D showed a higher extent of intramolecular cross-linking. The formed films were optically clear and possess superior energy band gap properties making them an alternative candidate over PVK for future hole-transport layer materials in electro-optical devices.