Electrified Interactions of Polyzwitterions with Charged Surfaces: Role of Dipole Orientation and Surface Potentials.

The zwitterionic groups possess strong dipole moments, leading to inter- or intrachain interactions among zwitterionic polymers. This study aims to demonstrate the interaction of polyzwitterions poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), and poly(carboxybetaine methacrylate) (PCBMA) with electrified surfaces, despite their electrically neutral nature. We studied the adsorption of polyzwitterions and their monomers on electrified surfaces by using an electrochemical quartz crystal microbalance with dissipation (EQCM-D). The interaction between zwitterionic molecules and charged surfaces is explored by adjusting the surface potentials. Interestingly, the adsorption of polyzwitterions can be influenced by external potential, primarily due to the formation of polyzwitterions restricting the mobility of zwitterionic groups, affecting the adsorption behavior of polyzwitterions based on the surface potential. The impact is determined by the arrangement of positive and negative ions within the zwitterionic groups, which are the dipole orientation. Additionally, surface potentials determine the adsorption rate, amount, and chain conformation of the adsorbed thin polyzwitterion layers. The effect of ionic strength was investigated by introducing electrolytes into the aqueous solutions to assess the range of influenced surface potentials.

Zwitterionic Conducting Polymers: From Molecular Design, Surface Modification, and Interfacial Phenomenon to Biomedical Applications.

Conducting polymers (CPs) have gained attention as electrode materials in bioengineering mainly because of their mechanical softness compared to conventional inorganic materials. To achieve better performance and broaden bioelectronics applications, the surface modification of soft zwitterionic polymers with antifouling properties represents a facile approach to preventing unwanted nonspecific protein adsorption and improving biocompatibility. This feature article emphasizes the antifouling properties of zwitterionic CPs, accompanied by their molecular synthesis and surface modification methods and an analysis of the interfacial phenomenon. Herein, commonly used methods for zwitterionic functionalization on CPs are introduced, including the synthesis of zwitterionic moieties on CP molecules and postsurface modification, such as the grafting of zwitterionic polymer brushes. To analyze the chain conformation, the structure of bound water in the vicinity of zwitterionic CPs and biomolecule behavior, such as protein adsorption or cell adhesion, provide critical insights into the antifouling properties. Integrating these characterization techniques offers general guidelines and paves the way for designing new zwitterionic CPs for advanced biomedical applications. Recent advances in newly designed zwitterionic CP-based electrodes have demonstrated outstanding potential in modern biomedical applications.

Construction of a nano-phase-separated structure on a hydrogel surface.

A hydrogel surface with a nano-phase-separated structure was successfully fabricated by grafting a fluorine-containing polymer using activators regenerated by electron transfer atom transfer radical polymerisation (ARGET ATRP). The modified hydrogel surface exhibits water repellency and high elasticity with maintaining transparency.

Combination of AFM and Electrochemical QCM-D for Probing Zwitterionic Polymer Brushes in Water: Visualization of Ionic Strength and Surface Potential Effects.

The surface modification of soft zwitterionic polymer brushes with antifouling properties represents a facile approach to enhancing the performance of bioelectronics. Ionic strength and applied potentials play a crucial role in controlling polymer brushes' conformation and hydration states. In this study, we quantitatively investigated and compared poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) and poly(sulfobetaine methacrylate) (PSBMA) brushes at different salt concentrations and applied surface potentials. Initiator-containing poly(3,4-ethylenedioxythiophene) films (poly(EDOT-Br)) were prepared by electropolymerization. After the conducting polymer was deposited, polymer brushes grew from the electrode surface through surface-initiated atom-transfer radical polymerization (SI-ATRP). Polymer brushes were carefully characterized for their surface morphologies using an atomic force microscope (AFM). The force volume method measured using AFM enabled the analysis of the Young's modulus of the two polymer brushes. Hydration states and protein binding behaviors of polymer brushes were examined using quartz crystal microbalance with dissipation (QCM-D). We further integrated a potentiostat with the QCM-D to conduct an electrochemical QCM-D study. The energy dissipation and frequency changes corresponded to the ion adsorption on the film surface under different ionic strengths. The results of both hydration states and nonspecific protein binding behavior indicate that PMPC brushes have greater ionic strength independency, implying the conformation of the unchanged PMPC brushes. Moreover, we illustrated how the surface potential influences nonspecific and specific binding behavior on PMPC brushes on PEDOT films compared with electrified poly(EDOT-PC) electrodes. We concluded that PMPC brushes exhibit unique behaviors that are barely affected by ion concentration, and that the brushes' modification results in less influence by surface potential due to the finite Debye length influencing the electrode surface to outer environment in an NaCl aqueous solution.

Strategy to Immobilize Peptide Probe Selected through In Vitro Ribosome Display for Electrochemical Aptasensor Application.

In this study, we demonstrated an electrochemical aptasensor for calmodulin (CaM) detection and the peptide sequence (YWDKIKDFIGG) is obtained from in vitro ribosome display selection. To immobilize this peptide probe on the electrode surface, cystine was incorporated at the end of this peptide sequence. After a maleimide-functionalized poly(3,4-ethylenedioxythiophene), poly(EODT-MI), film was electropolymerized on the electrode, the peptide probe was immobilized through thiol-ene conjugation with the cystine end. Four peptides with different linkers were used for the binding test of bovine serum albumin and CaM using a quartz crystal microbalance. The zwitterionic linker EKEKEKEKEKEK provided good antifouling properties and the highest CaM binding. Furthermore, the immobilization of the peptide with this zwitterionic linker resulted in a minimal increase in the electrochemical impedance. By immobilizing the peptide with the selected zwitterionic linker, we successfully demonstrated an electrochemical aptasensor with a linear detection range for CaM from 0.01 to 10 mg/L and a detection limit of 0.001 mg/L.

Electrochemical SERS on 2D Mapping for Metabolites Detection.

Surface-enhanced Raman scattering (SERS) has been widely used for bioanalysis because it provides a high sensitivity for detecting analytes of ultralow concentrations. However, the clinical application of a 2D SERS-active substrate remains challenging because of the difficulty of obtaining accurate quantification, especially at low concentration. In this study, we proposed an analytical method that integrates an optimized sample mapping strategy with an electrochemical SERS (EC-SERS) technique to resolve this problem. We adopted this method to detect two metabolites of azathioprine, namely 6-thioguanine nucleotides (6-TGNs) and 6-methylmercaptopurine (6-MMP), as our proof-of-concept experiment. We first prepared a conductive SERS-active substrate by electrochemically depositing Au nanoparticles (AuNPs) on indium tin oxide glass. The two metabolites were then randomly absorbed on the surface of the AuNPs of the SERS-active substrates. When we applied a negative potential on the substrate, we observed a large enhancement of Raman intensity for both metabolites, which was attributed to both the charge transfer effect and reorientation of metabolites on the substrate surface, leading to the formation of Au-S bonds. In addition, by optimizing the mapping range, we were able to efficiently reduce the standard deviation of SERS intensity and achieve a consistent standard deviation lower than 10%. With these two features, we were able to achieve quantitative analysis of 6-TGNs and 6-MMP with a detection limit of 10 and 100 nM, respectively. The integration of EC-SERS and the mapping method provided a reliable and quantitative analytical platform for analytes, which can be electrochemically modulated, like 6-TGNs and 6-MMP.

Synergistic Effects of Ions and Surface Potentials on Antifouling Poly(3,4-ethylenedioxythiophene): Comparison of Oligo(Ethylene Glycol) and Phosphorylcholine.

For electrified surfaces, ions and applied potentials play major roles in controlling the surface properties. Antifouling materials such as poly(ethylene glycol) and zwitterionic polymers that resist nonspecific protein binding and cell adhesion play a key role in various biomedical applications. In this study, we investigated and compared the antifouling properties of conducting polymers grafted with oligo(ethylene glycol) groups and phosphorylcholine (PC) groups in the presence of different anions and applied potentials. Considerable effort has been made to illustrate the different effects of manipulating the antifouling properties of these two surfaces. We prepared polymer films by applying electropolymerization to two functionalized (3,4-ethylenedioxythiophene) polymers containing triethylene glycol and PC groups, respectively. A quartz crystal microbalance with dissipation (QCM-D) was employed to characterize the negatively charged bovine serum albumin and positively charged lysozyme adsorption as a function of ionic concentration in the presence of various Hofmeister anions. The frequency changes corresponded to the protein or ion adsorption/desorption behavior on the surface. The anions adsorbed on polymer films to effectively enhance the hydration layer of the polymer surface and reduce nonspecific protein binding. We further integrated a potentiostat with the QCM-D to control the protein adsorption/desorption behaviors by applying potentials, and we conducted an electrochemical QCM-D study. Most importantly, with the synergistic effect of ions and surface potential, a nearly fresh polymer surface was regenerated. This study describes principles to maintain and regenerate the antifouling properties of electrified surfaces, which are critical for implanted bioelectronics applications.

Critical Study of the Recognition between C-Reactive Protein and Surface-Immobilized Phosphorylcholine by Quartz Crystal Microbalance with Dissipation.

C-reactive protein (CRP), a biomarker for cardiovascular disease, has been reported to have a strong affinity to zwitterionic phosphorylcholine (PC) groups in the presence of calcium ions. In addition, PC-immobilized surfaces have been used as a nonfouling coating to prevent nonspecific protein binding. By appropriately using the features of PC-immobilized surfaces, including specific recognition to CRP and nonfouling surface, it is reasonable to create an antibody-free biosensor for the specific capture of CRP. In this study, PC-functionalized 3,4-ethylenedioxythiophene (EDOT) monomers were used to prepare PC-immobilized surfaces. The density of PC groups on the surface can be fine-tuned by changing the composition of the monomer solutions for the electropolymerization. The density of PC group was confirmed by X-ray photoelectron spectroscopy (XPS). The specific interaction of CRP with PC groups was monitored by using a quartz crystal microbalance with dissipation (QCM-D). The amount of protein binding could be estimated by the reduction in frequency readout. Through the QCM-D measurement, we revealed the nonfouling property and the specific CRP capture from our PC-immobilized surfaces. Notably, the dissipation energy also dropped during the binding process between CRP and PC, indicating the release of water molecules from the PC groups during CRP adsorption. We anticipate that surface-bound water molecules are mainly released from areas near the immobilized PC groups. Based on Hofmeister series, we further examined the influence of ions by introducing four different anions including both kosmotrope (order maker) and chaotrope (disorder maker) into the buffer for the CRP binding test. The results showed that the concentration and the type of anions play an important role in CRP binding. The present fundamental study reveals deep insights into the recognition between CRP and surface-immobilized PC groups, which can facilitate the development of CRP sensing platforms.

Nanoscale analysis of a functionalized polythiophene surface by adhesion mapping.

Functionalized ethylenedioxythiophene (EDOT) monomers, hydroxymethyl EDOT (EDOT-OH), and zwitterionic phosphorylcholine EDOT (EDOT-PC) were electropolymerized to prepare the homopolymers poly(EDOT-OH) and poly(EDOT-PC), and mixtures of these monomers were used to produce the copolymer poly(EDOT-OH)-co-poly(EDOT-PC). Force-extension-curve-based atomic force microscopy (AFM) was utilized to analyze the surfaces of the films. The PEDOT-OH film yielded force-extension curves for short stretching, and the PEDOT-PC film yielded curves for long stretching. A dendron-modified AFM tip with anthracene groups tethered at the end resulted in adhesion maps with the highest contrast. The analytical data for the copolymer films correlated with the corresponding monomer composition, and the maps revealed that the average size for the copolymer nanodomains ranged from 10-14 nm. This approach can be applied to studies aimed at understanding the surface structure of other relevant polymers and copolymers at the nanoscale level.

3D bioelectronic interface: capturing circulating tumor cells onto conducting polymer-based micro/nanorod arrays with chemical and topographical control.

The three-dimensional (3D) poly(3,4-ethylenedioxythiophene) (PEDOT)-based bioelectronic interfaces (BEIs) with diverse dimensional micro/nanorod array structures, varied surface chemical pro-perties, high electrical conductivity, reversible chemical redox switching, and high optical transparency are used for capturing circulating tumor cells (CTCs). Such 3D PEDOT-based BEIs can function as an efficient clinical diagonstic and therapeutic platform.

Engineering Superaerophobic Electrodes Using Hydrophilic PEDOT and Colloidal Lithography for Enhanced Bubble Release and Efficient Hydrogen Evolution Reaction.

The efficient removal of gas bubbles is essential to reduce the reaction overpotential and improve the electrode stability in the hydrogen evolution reaction (HER). To address this challenge, the current study combines hydrophilic functionalized poly(3,4-ethylenedioxythiophene) (PEDOT) with colloidal lithography to create superaerophobic electrode surfaces. The fabrication process involves the use of polystyrene (PS) beads with varying sizes (100, 200, and 500 nm) as hard templates and the electropolymerization of EDOTs with hydroxymethyl (EDOT-OH) and sulfonate (EDOT-SuNa) functional groups. The surface properties and HER performances of the electrodes are investigated. The electrode modified with poly(EDOT-SuNa) and 200 nm PS beads (SuNa/Ni/Au-200) exhibits the best hydrophilicity with a water contact angle of 37°. Moreover, the overpotential required at -10 mA cm-2 is substantially reduced from -388 mV (flat Ni/Au) to -273 mV (SuNa/Ni/Au-200). This approach is further applied to commercially available nickel foam electrodes, showing improved HER activity and electrode stability. These results highlight the potential for promoting catalytic efficiency by constructing a superaerophobic electrode surface.

Unraveling the Adhesion Behavior of Different Cell Lines on Biomimetic PEDOT Interfaces: The Role of Surface Morphology and Antifouling Properties.

The poly(3,4-ethylenedioxythiophene) (PEDOT) interface, renowned for its biocompatibility and intrinsic conductivity, holds substantial potential in biosensing and cellular modulation. Through strategic functionalization, PEDOT derivatives can be adaptable for multifaceted applications. Notably, integrating phosphorylcholine (PC) groups into PEDOT, mimicking the hydrophilic headgroups from cell membranes, confers exceptional antifouling properties on the coating. This study systematically investigated biomolecule interactions with distinct forms of PEDOT, incorporating variations in surface modifications and structure. Zwitterionic PEDOT-PC was electropolymerized on smooth and nanostructured surfaces using various feeding ratios in electrolytes to finely control the antifouling properties of the interface. Precise electropolymerization conditions governed the attainment of smooth and nanostructured filamentous surfaces. The study employed a quartz crystal microbalance with dissipation (QCM-D) to assess protein binding behavior. Bovine serum albumin (BSA), lysozyme (LYZ), cytochrome c (cyt c), and fibronectin (FN) were used to evaluate their binding affinities for PEDOT films. FN, a pivotal extracellular matrix component, was included for connecting to cell adhesion behavior. Furthermore, the cellular adhesion behaviors on PEDOT interfaces were evaluated. Three cell lines─MG-63 osteosarcoma, HeLa cervical cancer, and fibroblast NIH/3T3 were examined. The presence of PC moieties significantly altered the adhesive response, including the number of attached cells, their morphologies, and nucleus shrinkage. MG-63 cells exhibited the highest tolerance for PC moieties. A feeding ratio of PEDOT-PC exceeding 70% resulted in cell apoptosis. This study contributes to understanding biomolecule adsorption on PEDOT surfaces of diverse morphologies and degrees of the antifouling moiety. Meanwhile, it also sheds light on the responses of various cell types.

Recent Advances and Biomedical Applications of Peptide-Integrated Conducting Polymers.

Conducting polymers (CPs) are of great interests to researchers around the world in biomedical applications owing to their unique electrical and mechanical properties. Besides, they are easy to fabricate and have long-term stability. These features make CPs a powerful building block of modern biomaterials. Peptide functionalization has been a versatile tool for the development of CP-based biomaterials. With the aid of peptide modifications, the biocompatibility, target selectivity, and cellular interactions of CPs can be greatly improved. Reflecting these aspects, an increasing number of studies on peptide-integrated conducting polymers have been reported recently. In this review, various kinds of peptide immobilization strategies on CPs are introduced. Moreover, the aims of peptide modification are discussed in three aspects: enhancing the specific selectivity, avoiding nonspecific adhesion, and mimicking the environment of extracellular matrix. We highlighted recent studies in the applications of peptide-integrated CPs in electrochemical sensors, antifouling surfaces, and conductive biointerfaces. These studies have shown great potentials from the integration of peptide and CPs as a versatile platform for advanced biological and clinical applications in the near future.

One-step electrochemical deposition of antifouling polymers with pyrogallol for biosensing applications.

Electrochemical techniques are highly sensitive and label-free sensing methods for the detection of various biomarkers, toxins, or pathogens. An ideal sensing element should be electroconductive, nonfouling, and readily available for conjugation of ligands. In this work, we have developed a facile, one-step electrodeposition method based on pyrogallol polymerization for preparation of a nonfouling and biotinylated surface on indium tin oxide (ITO). A copolymer of sulfobetaine methacrylate and aminoethyl methacrylate (pSBAE) was synthesized and deposited on ITO in the presence of pyrogallol via cyclic voltammetry. The deposition took less than 15 minutes to sufficiently inhibit cell adhesion. Using biotinylated pSBAE, the modified surface resisted nonspecific protein adsorption from the fetal bovine serum solution and detected added avidin concentrations. The results show an efficient platform to fabricate an electrochemical biosensor for the detection of biomarkers. We expect that this facile one-step technology could be applied to conjugate various biosensing elements for nonfouling biosensors.

Conducting polymer-based sensors for food and drug analysis.

Conducting polymers (CPs) are a category of polymeric materials with conjugated main chains. The characteristic electrical and optical properties of CPs can be fine-tuned through controlling the doping states of CPs. Because of their long-term stability in water, CPs have been demonstrated as electroactive biointerfaces and electrode materials especially in aqueous environments. Serving as multifunctional interfaces and organic electrodes for the integration bioelectronics and devices, CPs have been studied and applied in various biological applications. This paper provides a review of conducting polymer-based electrochemical sensors, particularly those used in biological fields. General conducting polymers and derivatives and their main electrochemical sensing platforms with different design of devices are introduced. Cyclic voltammetry, differential pulse voltammetry, chronoamperometry, electrochemical impedance spectroscopy, and quartz crystal microbalance methods and their features are then explored as detection methods for the analysis of drugs and food. To enhance the sensitivity and lower the detection limit of sensing platforms, various CP-based nanocomposites have been designed and developed. Although the electrodes made of CP-based nanocomposites usually outperform those made of pristine CPs, more systematic studies are required to provide insights into the design of nanocomposite-based electrodes. More applications of CP-based sensors for advanced food and drug analyses are expected.

Electropolymerized Poly(3,4-ethylenedioxythiophene)/Screen-Printed Reduced Graphene Oxide-Chitosan Bilayer Electrodes for Flexible Supercapacitors.

An electropolymerized poly(3,4-ethylenedioxythiophene) (PEDOT)/screen-printed reduced graphene oxide (rGO)-chitosan (CS) bilayer material was coated on carbon cloth to form electrodes for gel-electrolyte flexible supercapacitors. The conductive polymer and carbon-based materials mainly contribute pseudocapacitance (PC) and electrical double-layer capacitance (EDLC), respectively. The high porosity and hydrophilicity of the PEDOT/rGO-CS bilayer material offers a large contact area and improves the contact quality for the gel electrolyte, thereby enhancing the capacitive performance. Cyclic voltammetry (CV) under a potential scan rate of 2 mV/s revealed that a maximum areal capacitance of 1073.67 mF/cm2 was achieved. The capacitance contribution ratio PC/EDLC was evaluated to be ∼67/33 by the Trasatti method. A 10,000-cycle CV test showed a capacitance retention rate of 99.3% under a potential scan rate of 200 mV/s, indicating good stability. The areal capacitance remains similar under bending with a bending curvature of up to 1.5 cm-1.

Synthesis of a conductive polymer thin film having a choline phosphate side group and its bioadhesive properties.

A conductive polymer thin film having choline phosphate as the side group was prepared. Quartz crystal microbalance (QCM) was employed to evaluate the adsorption of the model protein, bovine serum albumin (BSA), on the films deposited on indium tin oxide (ITO) electrodes. Cell adsorption on the film was evaluated by a fibroblast NIH3T3.

Engineering Antifouling and Antibacterial Stainless Steel for Orthodontic Appliances through Layer-by-Layer Deposition of Nanocomposite Coatings.

In this study, a nanocomposite coating composed of polydopamine, functionalized poly(3,4-ethylenedioxythiophene) (PEDOT), and silver nanoparticles (AgNPs) was synthesized through layer-by-layer deposition. Biomimitic polydopamine and hydroxyl-functionalized PEDOT were used to enhance the adhesion strength. The deposition of PEDOT functionalized with zwitterionic phosphorylcholine can contribute to the antifouling property. After immersion in the AgNO3 solution, Ag+ ions were adsorbed on PEDOT films and further reduced to form AgNPs spontaneously, which conferred antibacterial properties on these nanocomposite films. Escherichia coli and Streptococcus mutans were chosen to represent two common Gram-negative and Gram-positive oral pathogens. We further conducted inductively coupled plasma mass spectrometry to confirm that the Ag+ ions released from these nanocomposite films did not exert adverse effects on the human body. These results suggested that, when applied to stainless steel orthodontic appliances, these durable antifouling and antibacterial coatings may be useful for avoiding bacterial infection.

Tunable Protein/Cell Binding and Interaction with Neurite Outgrowth of Low-Impedance Zwitterionic PEDOTs.

Zwitterionic poly(3,4-ethylenedioxythiophene) (PEDOT) is an effective electronic material for bioelectronics because it exhibits efficient electrical trade-off and diminishes immune response. To promote the use of zwitterionic PEDOTs in bioelectronic devices, especially for cell alignment control and close electrocoupling, features such as tunable interaction of PEDOTs with proteins/cells and spatially modulating cell behavior are required. However, there is a lack of reliable methods to assemble zwitterionic EDOTs with other functionalized EDOT materials, having different polarities and oxidation potentials, to prepare PEDOTs with the aforementioned surface properties. In this study, we have developed a surfactant-assisted electropolymerization to assemble phosphorylcholine (PC)-functionalized EDOT with other functionalized EDOTs. By adjusting compositions, the interaction of PEDOT copolymers with proteins/cells can be finely tuned; the composition adjustment has an ignorable influence on the impedance of the copolymers. We also demonstrate that the cell-repulsive force generated from PC can spatially guide the neurite outgrowth to form a neuron network at single-cell resolution and greatly enhance the neurite outgrowth by 179%, which is significantly more distinctive than the reported topography effect. We expect that the derived tunable protein/cell interaction and the PC-induced repulsive guidance for the neurite outgrowth can make low-impedance zwitterionic PEDOTs more useful in bioelectronics.

Electric-field-assisted growth of functionalized poly(3,4-ethylenedioxythiophene) nanowires for label-free protein detection.

The construction of functionalized poly(3,4-ethylenedioxythiophene) (PEDOT) nanowire devices for label-free protein detection is reported. Direct growth/assembly of PEDOT nanowires with carboxylic acid side-chain functional groups (poly(EDOT-COOH)) across the electrode junction is achieved by using an electric-field-assisted method. These functionalized PEDOT nanowire devices show typical depletion-mode p-type field-effect transistor (FET) properties. Upon conjugation with a protein-binding aptamer, the PEDOT nanowire FET devices are used for label-free electronic detection of a target protein of interest. The binding of a positively charged protein causes a substantial decrease in current flow, attributed to the specific interaction between target protein molecules and aptamer-conjugated polymer chains.