Electrochemical and Electrical Biosensors for Wearable and Implantable Electronics Based on Conducting Polymers and Carbon-Based Materials.

Bioelectronic devices are designed to translate biological information into electrical signals and vice versa, thereby bridging the gap between the living biological world and electronic systems. Among different types of bioelectronics devices, wearable and implantable biosensors are particularly important as they offer access to the physiological and biochemical activities of tissues and organs, which is significant in diagnosing and researching various medical conditions. Organic conducting and semiconducting materials, including conducting polymers (CPs) and graphene and carbon nanotubes (CNTs), are some of the most promising candidates for wearable and implantable biosensors. Their unique electrical, electrochemical, and mechanical properties bring new possibilities to bioelectronics that could not be realized by utilizing metals- or silicon-based analogues. The use of organic- and carbon-based conductors in the development of wearable and implantable biosensors has emerged as a rapidly growing research field, with remarkable progress being made in recent years. The use of such materials addresses the issue of mismatched properties between biological tissues and electronic devices, as well as the improvement in the accuracy and fidelity of the transferred information. In this review, we highlight the most recent advances in this field and provide insights into organic and carbon-based (semi)conducting materials' properties and relate these to their applications in wearable/implantable biosensors. We also provide a perspective on the promising potential and exciting future developments of wearable/implantable biosensors.

Grafting of Porous Conductive Fiber Mats with an Antifouling Polymer Brush by Means of Filtration-Based Surface Initiated ATRP.

This work addresses the challenge of surface modification of porous, electrospun fiber mats containing an insoluble conducting polymer coating. Herein, a novel methodology of grafting a polymer brush onto conducting polymer fiber mats is developed that employs filtering of the polymerization solution through the fiber mat. An electrospun sulfonated polystyrene-poly(ethylene-ran-butylene)-polystyrene (sSEBS) fiber mat is first coated with a layer of conducting copolymer bearing an Atom Transfer Radical Polymerization (ATRP) initiating functionality (PEDOT-Br). The surface-initiated ATRP from the fibers' surface is then carried out to graft a hydrophilic polymer brush (poly(ethylene glycol) methyl ether methacrylate) by means of filtering the polymerization solution through the fiber mat. Scanning electron microscopy (SEM) images reveal a progressive change in the morphology of the fiber mat surface with the increasing volume of the filtrated polymerization solution, while energy dispersive X-ray spectrosdcopy (EDX) spectra show a change in the atomic oxygen to sulfur (O/S) ratio, therefore confirming the successful grafting from the fibers' surface. The conductive fiber mat grafted with hydrophilic brushes shows a 20% reduction in the non-specific adsorption of bovine serum albumin (BSA) compared to a pristine fiber mat. This study is a proof-of-concept for this novel, filtration-based, surface-initiated polymerization methodology.

Toward cheaper light harvesting systems: Using earth-abundant metal oxide nanoparticles in self-assembled peptide-porphyrin nanofibers.

Cheap artificial light harvesting systems, which competently harvest solar energy and promote efficient energy transfer, are highly sought after in the renewable sector. We report the synthesis of self-assembled peptide-porphyrin fibers (SJ 6) fabricated with iron(III) oxide (Fe3 O4 ) nanoparticles as feasible electron acceptors. Charge-complementarity between the negatively charged peptide (20E) and the protonated Zn-tetraphenyl porphyrin (ZnTPyP) led to an ordered assembly of the ZnTPyP molecules, enabling efficient light harvesting. X-ray diffraction data indicates a more ordered structure in SJ 6 compared to 20E and ZnTPyP. The incorporation of Fe3 O4 nanoparticles into SJ 6 showed significant fluorescence quenching, indicating efficient electron flow from the donor to the acceptor. The SJ 6-nFe3 O4 system performed the light reaction of photosynthesis as confirmed by the reduction of 1 mM NAD+ to 0.180 mM NADH upon exposure to visible light (Xe lamp λ > 420 nm) for 1 h. The photochemical regeneration of NADH using the SJ 6-nFe3 O4 system was coupled to glutamate dehydrogenase-catalyzed conversion of α-ketoglutarate to L-glutamate. These results confirm the successful synthesis of an artificial light harvesting peptide-porphyrin system with Fe3 O4 nanoparticles as promising low-cost electron separators.

A Conductive Microfiltration Membrane for In Situ Fouling Detection: Proof-of-Concept Using Model Wine Solutions.

Cross-flow microfiltration, using a microporous membrane, is a well-established technique for wine clarification in oenology because of its cost-effectiveness and high-throughput. However, membrane fouling remains a significant issue for wine filtration in high-throughput systems. Herein, an approach for in situ real-time monitoring of fouling in filtration systems using a conductive filtration membrane and a model fluid for filtration is reported. The membrane is fabricated by embedding poly(3,4-ethylenedioxythiophene) into an electrospun sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene microporous membrane, producing a conductive microfiltration membrane. Measurement of the resistance of the conductive membrane during filtration with the fouling solutions containing pectin, as one of the major foulants in unfiltered wine and pre-fermentation grape juice, shows a time- and concentration-dependent response. This work opens a door to new methodology for in situ monitoring of fouling processes in wine and juice filtration systems.

The Applications of Solid-State NMR to Conducting Polymers. The Special Case on Polyaniline.

Polyaniline is one of the most well studied conducting polymers due to its advanced electrical, chemical, redox and morphological properties. The high conductivity of regular polyaniline, when partially oxidized and doped under acidic conditions, has been associated with the formation of unique electronic states known as polarons and bipolarons. Alternative aniline oxidation products and interesting nanotube and nanorod forms have been observed as the synthesis conditions are varied. Solid-state NMR has offered great opportunities for structural investigations and the determination of molecular dynamics in such a complex and diverse material. This review summarizes various applications of solid-state NMR techniques to polyaniline and its derivatives and the information that can be obtained by solid-state NMR.

Molecular Approach to Conjugated Polymers with Biomimetic Properties.

The field of bioelectronics involves the fascinating interplay between biology and human-made electronics. Applications such as tissue engineering, biosensing, drug delivery, and wearable electronics require biomimetic materials that can translate the physiological and chemical processes of biological systems, such as organs, tissues. and cells, into electrical signals and vice versa. However, the difference in the physical nature of soft biological elements and rigid electronic materials calls for new conductive or electroactive materials with added biomimetic properties that can bridge the gap. Soft electronics that utilize organic materials, such as conjugated polymers, can bring many important features to bioelectronics. Among the many advantages of conjugated polymers, the ability to modulate the biocompatibility, solubility, functionality, and mechanical properties through side chain engineering can alleviate the issues of mechanical mismatch and provide better interface between the electronics and biological elements. Additionally, conjugated polymers, being both ionically and electrically conductive through reversible doping processes provide means for direct sensing and stimulation of biological processes in cells, tissues, and organs. In this Account, we focus on our recent progress in molecular engineering of conjugated polymers with tunable biomimetic properties, such as biocompatibility, responsiveness, stretchability, self-healing, and adhesion. Our approach is general and versatile, which is based on functionalization of conjugated polymers with long side chains, commonly polymeric or biomolecules. Applications for such materials are wide-ranging, where we have demonstrated conductive, stimuli-responsive antifouling, and cell adhesive biointerfaces that can respond to external stimuli such as temperature, salt concentration, and redox reactions, the processes that in turn modify and reversibly switch the surface properties. Furthermore, utilizing the advantageous chemical, physical, mechanical and functional properties of the grafts, we progressed into grafting of the long side chains onto conjugated polymers in solution, with the vision of synthesizing solution-processable conjugated graft copolymers with biomimetic functionalities. Examples of the developed materials to date include rubbery and adhesive photoluminescent plastics, biomolecule-functionalized electrospun biosensors, thermally and dually responsive photoluminescent conjugated polymers, and tunable self-healing, adhesive, and stretchable strain sensors, advanced functional biocidal polymers, and filtration membranes. As outlined in these examples, the applications of these biomimetic, conjugated polymers are still in the development stage toward truly printable, organic bioelectronic devices. However, in this Account, we advocate that molecular engineering of conjugated polymers is an attractive approach to a versatile class of organic electronics with both ionic and electrical conductivity as well as mechanical properties required for next-generation bioelectronics.

Molecular "Building Block" and "Side Chain Engineering": Approach to Synthesis of Multifunctional and Soluble Poly(pyrrole phenylene)s.

Here, the synthesis of a novel poly(pyrrole phenylene) (PpyP) that is both modular in ways of functionalization and soluble in organic solvents is reported, and therefore solution processable. This is achieved through the functionalization of the side-chain substituents in pyrrole phenylene (PyP) repeating units. t Butyl acrylate brushes are first grafted through atom transfer radical polymerization from one type of PyP, followed by oxidative chemical co-polymerization of the grafted PyP with a PyP bearing different side chains-either an azide or a methoxy moiety, resulting in a soluble PpyP where solubility is not dopant-dependent. Successful post-polymerization modification through "click" chemistry and post-polymerization processing via electrospinning are also demonstrated. Additionally, performed computational calculations indicate planarity of the novel polyrrole phenylene monomers and ionisation potentials that favor α-α bond formation during their polymerization.

Electrospun Polythiophene Phenylenes for Tissue Engineering.

This research focuses on the design of biocompatible materials/scaffold suitable for use for tissue engineering. Porous fiber mats were produced through electrospinning of polythiophene phenylene (PThP) conducting polymers blended with poly(lactide- co-glycolic acid) (PLGA). A peptide containing an arginylglycylaspartic acid (RGD) fragment was synthesized using solid phase peptide synthesis and subsequently grafted onto a PThP polymer using azide-alkyne "click" chemistry. The obtained RGD functionalized PThP was also electrospun into a fiber mat. The electrospun mats' morphology, roughness and stiffness were studied by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM) and their electroactivity by cyclic voltammetry. The fibers show excellent cytocompatibility in culture assays with human dermal fibroblasts-adult (HDFa) and human epidermal melanocytes-adult (HEMa) cells. The electrospun fibers' roughness and stiffness changed after exposing the fiber mats to the cell culture medium (measured in dry state), but these changes did not affect the cell proliferation. The cytocompatibility of our porous scaffolds was established for their applicability as cell culture scaffolds by means of investigating mitochondrial activity of HDFa and HEMa cells on the scaffolds. The results revealed that the RGD moieties containing PThP scaffolds hold a promise in biomedical applications, including skin tissue engineering.

Chain shape and thin film behaviour of poly(thiophene)-graft-poly(acrylate urethane).

Electronic graft copolymers with conjugated polymer backbones are emerging as promising materials for various organic electronics. These materials combine the advantages of organic electronic materials, such as molecular tunability of opto-electronic and electrochemical properties, with solution processability and other 'designer' physical and mechanical properties imparted through the addition of grafted polymer side chains. Future development of such materials with complex molecular architecture requires a better understanding of the effect of molecular parameters, such as side chain length, on the structure and, in turn, on the electronic properties. In this study, poly(thiophene)-graft-poly(acrylate urethane) (PTh-g-PAU) was examined as a model system and we investigate the effect of side chain length on the overall shape and size in solution. Furthermore, the changes in the swelling behaviour of the graft copolymer thin films help in understanding their electrochemical redox properties.

PNA versus DNA in electrochemical gene sensing based on conducting polymers: study of charge and surface blocking effects on the sensor signal.

In this communication, we present an in-depth study of DNA/DNA, DNA/PNA and PNA/PNA hybridisation on a conducting polymer-modified electrode, measured by means of electrochemical impedance spectroscopy (EIS). DNA or PNA nucleic base sequence probes (where DNA stands for deoxyribonucleic acid and PNA for peptide nucleic acid) were covalently attached onto the sensor surface. As PNA is a non-charged variant of DNA, we investigate the effects of the surface charge and surface blocking by the surface confined probe/target nucleic bases complexes onto the kinetics of redox reaction of Fe(CN)63-/4- couple occurring at the electrode/solution interface that provides electrochemical readout for hybridisation. A range of hybridisation detection experiments were performed, where the surface charge and surface charge density were varied, through varying the charged nature of the probe and the target (i.e. PNA or DNA) and the density of surface-bound PNA and DNA probes. To further the understanding of these effects on the measured electrochemical signal, kinetic studies of the hybridisation reactions were undertaken, and the equilibrium binding constants and binding rate constants for the hybridisation reactions were obtained. The study provides valuable insights to guide future designs of biosensors.

Graft Copolymers with Conducting Polymer Backbones: A Versatile Route to Functional Materials.

Graft copolymers with a conducting polymer backbone are a promising class of materials for diverse applications including, but not limited to, organic electronics, stimuli-responsive surfaces, sensors, and biomedical devices. These materials take advantage of the unique electrochemical and optoelectronic properties of conducting polymers, complemented by chemical and/or physical properties of the grafted sidechains. In this Personal Account, we discuss our work in designing functional surfaces based on graft copolymers with a conducting polymer backbone, in the context of broader developments in the field. We review the synthetic approaches available for the rational design of conducting-polymer-based graft copolymers, and examine the types of functional surfaces and soluble materials that may be engineered using these techniques.

Conducting polymer based electrochemical biosensors.

Conducting polymer (CP)-based electrochemical biosensors have gained great attention as such biosensor platforms are easy and cost-effective to fabricate, and provide a direct electrical readout for the presence of biological analytes with high sensitivity and selectivity. CP materials themselves are both sensing elements and transducers of the biological recognition event at the same time, simplifying sensor designs. This review summarizes the advances in electrochemical biosensors based on CPs. Recognition probe immobilisation techniques, transduction mechanisms and detection of various target biomolecules have been discussed in detail. Efforts to miniaturize CP-based electrochemical biosensors and fabrication of sensor arrays are also briefly reviewed.

A Label-Free, Sensitive, Real-Time, Semiquantitative Electrochemical Measurement Method for DNA Polymerase Amplification (ePCR).

Oligonucleotide hybridization to a complementary sequence that is covalently attached to an electrochemically active conducting polymer (ECP) coating the working electrode of an electrochemical cell causes an increase in reaction impedance for the ferro-ferricyanide redox couple. We demonstrate the use of this effect to measure, in real time, the progress of DNA polymerase chain reaction (PCR) amplification of a minor component of a DNA extract. The forward primer is attached to the ECP. The solution contains other PCR components and the redox couple. Each cycle of amplification gives an easily measurable impedance increase. Target concentration can be estimated by cycle count to reach a threshold impedance. As proof of principle, we demonstrate an electrochemical real-time quantitative PCR (e-PCR) measurement in the total DNA extracted from chicken blood of an 844 base pair region of the mitochondrial Cytochrome c oxidase gene, present at ∼1 ppm of total DNA. We show that the detection and semiquantitation of as few as 2 copies/µL of target can be achieved within less than 10 PCR cycles.

Ultrasensitive colorimetric detection of 17ß-estradiol: the effect of shortening DNA aptamer sequences.

We report a strategy enabling ultrasensitive colorimetric detection of 17ß-estradiol (E2) in water and urine samples using DNA aptamer-coated gold nanoparticles (AuNPs). Starting from an established sensor format where aggregation is triggered when target-bound aptamers dissociate from AuNP surfaces, we demonstrated that step-change improvements are easily accessible through deletion of excess flanking nucleotides from aptamer sequences. After evaluating the lowest energy two-dimensional configuration of the previously isolated E2 binding 75-mer aptamer (KD ∼25 nM), new 35-mer and 22-mer aptamers were generated with KD's of 14 and 11 nM by simply removing flanking nucleotides on either side of the inner core. The shorter aptamers were found to improve discrimination against other steroidal molecules and to improve colorimetric sensitivity for E2 detection by 25-fold compared with the 75-mer to 200 pM. In comparing the response of all sequences, we find that the excess flanking nucleotides suppress signal transduction by causing target-bound aptamers to remain adhered to AuNPs, which we confirm via surface sensitive electrochemical measurements. However, comparison between the 22-mer and 35-mer systems show that retaining a small number of excess bases is optimal. The performance advances we achieved by specifically considering the signal transduction mechanism ultimately resulted in facile detection of E2 in urine, as well as enabling environmental detection of E2 at levels approaching biological relevance.

Copolymers of Gelatin-graft-poly(3-hexylthiophene) for Transient Electronics.

As transient electronics continue to advance, the demand for new materials has given rise to the exploration of conducting polymer (CP)-based electronic materials. The big challenge lies in balancing conductivity while introducing controlled degradable properties into CP-based transient materials. In response to this, we present in this work a concept of using conducting polymers attached to an enzymatically biodegradable biopolymer to create transient polymer electronics materials. Specifically, poly(3-hexyl thiophene) (P3HT) is covalently grafted onto biopolymer gelatin, affording graft copolymer gelatin-graft-poly(3-hexyl thiophene) (termed Gel-g-P3HT). The thin films of Gel-g-P3HT that were produced by optimized processing solvent (THF/H2O cosolvent) showed enhanced π-π stacking domains of P3HT, resulting in semiconducting thin films with good electroactivity. Due to the presence of amide bonds in the gelatin backbone, Gel-g-P3HT underwent degradation over a period of 5 days, resulting in the formation of amphiphilic micellar nanoparticles that are biocompatible and nontoxic. The potential of these conductive and degradable graft copolymers was demonstrated in a pressure sensor. This research paves the way for developing biocompatible and enzymatically degradable polymer materials based on P3HT, enabling the next generation of transient polymer electronics for diverse applications, such as skin, implantable, and environmental electronics.

Electrochemical biosensing platform based on AuNWs/rGO-CMC-PEDOT:PSS composite for the detection of superoxide anion released from living cells.

Detection of superoxide anion (O2·-) levels holds significant importance for the diagnosis and even clinical treatments of oxidative stress-related diseases. Herein, we prepared a composite electrode material to encapsulate copper-zinc superoxide dismutase (SOD1) for biosensing of O2·-. The sensing material consists of gold nanowires (AuNWs), reduced graphene oxide (rGO), carboxymethyl cellulose (CMC) and PEDOT:PSS. CMC provides abundant -COOH to bind SOD1, with a high adsorption coverage of 1.499 × 10-9 mol cm-2 on the sensor surface. rGO and PEDOT endow the composite with significant conductivity, whereas PSS has antifouling capability. Moreover, AuNWs exhibit excellent electrical conductivity and a high aspect ratio, which promotes electron transfer, and ultimately enhances the catalytic performance of the enzyme. Meanwhile, SOD1(Cu2+) catalyzes the dismutation of O2·- to O2 and H2O2, and H2O2 is then electrochemically oxidized to generate amperometric signals for determination of O2·-. The sensor demonstrates outstanding detection performance for O2·- with a low detection limit of 2.52 nM, and two dynamic ranges (14.30 nM-1.34 µM and 1.34 µM-42.97 µM) with corresponding sensitivity of 0.479 and 0.052 µA µM-1cm-2, respectively. Additionally, the calculated apparent Michaelis constant (Kmapp) of 1.804 µM for SOD1 demonstrates the outstanding catalytic activity and the surface-immobilized enzyme's substrate affinity. Furthermore, the sensor shows the capability to dynamically detect the level of O2·- released from living HepG2 cells. This study provides an inovative design to obtain a biocompatible electrochemical sensing platform with plenty of immobilization sites for biomolecules, large surface area, high conductivity and flexibility.

Direct writing of conducting polymers.

Described herein is a new printing method-direct writing of conducting polymers (CPs)-based on pipette-tip localized continuous electrochemical growth. A single barrel micropipette containing a metal wire (Pt) is filled with a mixture of monomer, supporting electrolyte, and an appropriate solvent. A droplet at the tip of the pipette contacts the substrate, which becomes the working electrode of a micro-electrochemical cell confined to the tip droplet and the pipette. The metallic wire in the pipette acts as both counter and reference electrode. Electropolymerization forms the CP on the working electrode in a pattern controlled by the movement of the pipette. In this study, various width poly(pyrrole) 2D and 3D structures are extruded and characterized in terms of microcyclic voltammetry, Raman spectroscopy, and scanning electron microscopy.

From energy storage to pathogen eradication: unveiling the antibacterial and antiviral capacities of flexible solid-state carbon cloth supercapacitors.

With the emergence of deadly viral and bacterial infections, preventing the spread of microorganisms on surfaces has gained ever-increasing importance. This study investigates the potential of solid-state supercapacitors as antibacterial and antiviral devices. We developed a low-cost and flexible carbon cloth supercapacitor (CCSC) with highly efficient antibacterial and antiviral surface properties. The CCSC comprised two parallel layers of carbon cloth (CC) electrodes assembled in a symmetric, electrical double-layer supercapacitor structure that can be charged at low potentials between 1 to 2 V. The optimized CCSC exhibited a capacitance of 4.15 ± 0.3 mF cm-2 at a scan rate of 100 mV s-1, high-rate capability (83% retention of capacitance at 100 mV s-1 compared to its value at 5 mV s-1), and excellent electrochemical stability (97% retention of the initial capacitance after 1000 cycles). Moreover, the CCSC demonstrated outstanding flexibility and retained its full capacitance even when bent at high angles, making it suitable for wearable or flexible devices. Using its stored electrical charge, the charged CCSC disinfects bacteria effectively and neutralizes viruses upon surface contact with the positive and negative electrodes. The charged CCSC device yielded a 6-log CFU reduction of Escherichia coli bacterial inocula and a 5-log PFU reduction of HSV-1 herpes virus. Antibacterial and antiviral carbon cloth supercapacitors represent a promising platform technology for various applications, including electronic textiles and electronic skins, health monitoring or motion sensors, wound dressings, personal protective equipment (e.g., masks) and air filtration systems.

A Portable and Disposable Electrochemical Sensor Utilizing Laser-Scribed Graphene for Rapid SARS-CoV-2 Detection.

The COVID-19 pandemic caused by the virus SARS-CoV-2 was the greatest global threat to human health in the last three years. The most widely used methodologies for the diagnosis of COVID-19 are quantitative reverse transcription polymerase chain reaction (RT-qPCR) and rapid antigen tests (RATs). PCR is time-consuming and requires specialized instrumentation operated by skilled personnel. In contrast, RATs can be used in-home or at point-of-care but are less sensitive, leading to a higher rate of false negative results. In this work, we describe the development of a disposable, electrochemical, and laser-scribed graphene-based biosensor strips for COVID-19 detection that exploits a split-ester bond ligase system (termed 'EsterLigase') for immobilization of a virus-specific nanobody to maintain the out-of-plane orientation of the probe to ensure the efficacy of the probe-target recognition process. An anti-spike VHH E nanobody, genetically fused with the EsterLigase domain, was used as the specific probe for the spike receptor-binding domain (SP-RBD) protein as the target. The recognition between the two was measured by the change in the charge transfer resistance determined by fitting the electrochemical impedance spectroscopy (EIS) spectra. The developed LSG-based biosensor achieved a linear detection range for the SP-RBD from 150 pM to 15 nM with a sensitivity of 0.0866 [log(M)]-1 and a limit of detection (LOD) of 7.68 pM.

Flexible 3D printed microwires and 3D microelectrodes for heart-on-a-chip engineering.

We developed a heart-on-a-chip platform that integrates highly flexible, vertical, 3D micropillar electrodes for electrophysiological recording and elastic microwires for the tissue's contractile force assessment. The high aspect ratio microelectrodes were 3D-printed into the device using a conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). A pair of flexible, quantum dots/thermoplastic elastomer nanocomposite microwires were 3D printed to anchor the tissue and enable continuous contractile force assessment. The 3D microelectrodes and flexible microwires enabled unobstructed human iPSC-based cardiac tissue formation and contraction, suspended above the device surface, under both spontaneous beating and upon pacing with a separate set of integrated carbon electrodes. Recording of extracellular field potentials using the PEDOT:PSS micropillars was demonstrated with and without epinephrine as a model drug, non-invasively, along within situmonitoring of tissue contractile properties and calcium transients. Uniquely, the platform provides integrated profiling of electrical and contractile tissue properties, which is critical for proper evaluation of complex, mechanically and electrically active tissues, such as the heart muscle under both physiological and pathological conditions.