Shiming Zhang

Solvent-induced changes in PEDOT:PSS films for organic electrochemical transistors

Organic electrochemical transistors based on the conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) are of interest for several bioelectronic applications. In this letter, we investigate the changes induced by immersion of PEDOT:PSS films, processed by spin coating from different mixtures, in water and other solvents of different polarities. We found that the film thickness decreases upon immersion in polar solvents, while the electrical conductivity remains unchanged. The decrease in film thickness is minimized via the addition of a cross-linking agent to the mixture used for the spin coating of the films.

Water stability and orthogonal patterning of flexible micro-electrochemical transistors on plastic

Water-stable, flexible and micro-scale organic electrochemical transistors (OECTs) based on poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) were fabricated on a plastic substrate using a new process based on a fluorinated photoresist. The PEDOT:PSS films, mixed solely with a biocompatible conductivity enhancer, show robust adhesion on plastic substrates, and exhibit unchanged electrical properties under extreme bending. This work simplifies the fabrication of high-performance OECTs and places them in a highly competitive position for flexible electronics and healthcare applications.

Conducting Polymer Transistors Making Use of Activated Carbon Gate Electrodes

The characteristics of the gate electrode have significant effects on the behavior of organic electrochemical transistors (OECTs), which are intensively investigated for applications in the booming field of organic bioelectronics. In this work, high specific surface area activated carbon (AC) was used as gate electrode material in OECTs based on the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonate) (PSS). We found that the high specific capacitance of the AC gate electrodes leads to high drain-source current modulation in OECTs, while their intrinsic quasi-reference characteristics make unnecessary the presence of an additional reference electrode to monitor the OECT channel potential.

Melanin-based flexible supercapacitors

Biocompatible and biodegradable materials that store electrochemical energy are attractive candidates for applications in bioelectronics and electronics for everywhere. Eumelanin is a ubiquitous biopigment in flora and fauna. It exhibits strong broad-band UV-visible absorption, metal chelation as well as good thermal and photo-stability. Eumelanin is based on 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole carboxylic acid (DHICA) building blocks, present in different redox forms (hydroxyquinone, semiquinone and quinone). The synergy between the redox activity of the building blocks and the capability of several of their functionalities to reversibly bind cations constitutes the foundation for the use of melanin in pseudocapacitive energy storage systems. In this work, we report on the energy storage properties of eumelanin in supercapacitor configuration. Initially, a gravimetric specific capacitance as high as 167 F g−1 (specific capacity of 24 mA h g−1) was observed for eumelanin on carbon paper electrodes, in aqueous electrolytes. A maximum power density of up to 20 mW cm−2 was deduced for the corresponding melanin supercapacitors. Capitalizing on these results, we used an unconventional patterning approach to fabricate binder-free flexible micro-supercapacitors on plastic substrates. Our results demonstrate that melanin is a valid candidate for future supercapacitor electrodes. The biocompatibility and biodegradability featured by eumelanin, combined with its easy availability and room temperature processing, make it an extremely attractive material for environmentally and human friendly energy storage solutions.

Water-Enabled Healing of Conducting Polymer Films

The conducting polymer polyethylenedioxythiophene doped with polystyrene sulfonate (PEDOT:PSS) has become one of the most successful organic conductive materials due to its high air stability, high electrical conductivity, and biocompatibility. In recent years, a great deal of attention has been paid to its fundamental physicochemical properties, but its healability has not been explored in depth. This communication reports the first observation of mechanical and electrical healability of PEDOT:PSS thin films. Upon reaching a certain thickness (about 1 µm), PEDOT:PSS thin films damaged with a sharp blade can be electrically healed by simply wetting the damaged area with water. The process is rapid, with a response time on the order of 150 ms. Significantly, after being wetted the films are transformed into autonomic self-healing materials without the need of external stimulation. This work reveals a new property of PEDOT:PSS and enables its immediate use in flexible and biocompatible electronics, such as electronic skin and bioimplanted electronics, placing conducting polymers on the front line for healing applications in electronics.

Effect of channel thickness, electrolyte ions, and dissolved oxygen on the performance of organic electrochemical transistors

We investigated the device characteristics of organic electrochemical transistors based on thin films of poly(3,4-ethylenedioxythiophene) doped with poly(styrene-sulfonate). We employed various channel thicknesses and two different electrolytes: the micelle forming surfactant cetyltrimethyl ammonium bromide (CTAB) and NaCl. The highest ON/OFF ratios were achieved at low film thicknesses using CTAB as the electrolyte. Cyclic voltammetry suggests that a redox reaction between oxygen dissolved in the electrolytes and PEDOT:PSS leads to low ON/OFF ratios when NaCl is used as the electrolyte. Electrochemical impedance spectroscopy reveals that doping/dedoping of the channel becomes slower at high film thickness and in the presence of bulky ions.

Resistive switching controlled by the hydration level in thin films of the biopigment eumelanin

Melanins are biopigments ubiquitous in flora and fauna, exhibiting a range of interesting functional properties such as UV-Vis photoprotection, thermoregulation, hydration-dependent electrical conduction and metal chelation. In the human body, melanins can be found in the skin, hair, middle ear, retina and heart. The metal chelation properties of neuromelanin, a subclass of melanins found in the dopaminergic neurons of the brain, may be involved in Parkinsons disease. Considering synthetic and natural (from the ink sac of the cuttlefish) thin films of eumelanin, the subclass of melanins most investigated by materials scientists, we report on two types of resistive switching (standard and hybrid), with different ON/OFF ratios, observed in planar gold/eumelanin/gold structures. The resistive switching process, based on the formation of conductive filaments, was studied by means of transient electrical measurements, scanning electron microscopy, atomic force microscopy and time of flight-secondary ion mass spectrometry. We observed an extended correlation among the factors affecting the two different types of switch: primarily the eumelanins hydration level as well as the presence of chelating groups in the eumelanin molecular structure and the electrical bias. This work contributes to the development of environmentally benign organic resistive switching devices, namely electrochemical metallization memory cells making use of the biocompatible, biodegradable and abundant eumelanin biopigment as the ion conductive layer.

Highly stretchable electrospun conducting polymer nanofibers

Biomedical electronics research targets both wearable and biocompatible electronic devices easily adaptable to specific functions. To achieve such goals, stretchable organic electronic materials are some of the most intriguing candidates. Herein, we develop highly stretchable poly-(3,4-ethylenedioxythiphene) (PEDOT) doped with tosylate (PEDOT:Tos) nanofibers. A two-step process involving electrospinning of a carrier polymer (with oxidant) and vapor phase polymerization was used to produce fibers on a polydimethylsiloxane substrate. The fibers can be stretched up to 140% of the initial length maintaining high conductivity.

Flexible self-powered biosensors

Current biological sensors require bulky external power sources. Ultrathin solar cells have now been fabricated that can power flexible, wearable sensors for the precise and continuous monitoring of biological signals.Integration of organic photovoltaic cells and electrochemical transistors.