Daniel A. Bernards

Solid-state electroluminescent devices based on transition metal complexes

Transition metal complexes have emerged as promising candidates for applications in solid-state electroluminescent devices. These materials serve as multifunctional chromophores, into which electrons and holes can be injected, migrate and recombine to produce light emission. Their device characteristics are dominated by the presence of mobile ions that redistribute under an applied field and assist charge injection. As a result, an efficiency of 10 lm/W--among the highest efficiencies reported in a single layer electroluminescent device--was recently demonstrated. In this article we review the history of electroluminescence in transition metal complexes and discuss the issues that need to be addressed for these materials to succeed in display and lighting applications.

Enzymatic sensing with organic electrochemical transistors

Since their development in the 1980s organic electrochemical transistors (OECTs) have attracted a great deal of interest for biosensor applications. Coupled with the current proliferation of organic semiconductor technologies, these devices have the potential to revolutionize healthcare by making point-of-care and home-based medical diagnostics widely available. Unfortunately, their mechanism of operation is poorly understood, and this hinders further development of this important technology. In this paper glucose sensors based on OECTs and the redox enzyme glucose oxidase are investigated. Through appropriate scaling of the transfer characteristics at various glucose concentrations, a universal curve describing device operation is shown to exist. This result elucidates the underlying device physics and establishes a connection between sensor response and analyte concentration. This improved understanding paves the way for rational optimization of enzymatic sensors based on organic electrochemical transistors.

Observation of Electroluminescence and Photovoltaic Response in Ionic Junctions

Electronic devices primarily use electronic rather than ionic charge carriers. Using soft-contact lamination, we fabricated ionic junctions between two organic semiconductors with mobile anions and cations, respectively. Mobile ionic charge was successfully deployed to control the direction of electronic current flow in semiconductor devices. As a result, these devices showed electroluminescence under forward bias and a photovoltage upon illumination with visible light. Thus, ionic charge carriers can enhance the performance of existing electronic devices, as well as enable new functionalities.

All-Plastic Electrochemical Transistor for Glucose Sensing Using a Ferrocene Mediator

We demonstrate a glucose sensor based on an organic electrochemical transistor (OECT) in which the channel, source, drain, and gate electrodes are made from the conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS). The OECT employs a ferrocene mediator to shuttle electrons between the enzyme glucose oxidase and a PEDOT:PSS gate electrode. The device can be fabricated using a one-layer patterning process and offers glucose detection down to the micromolar range, consistent with levels present in human saliva.

Organic light-emitting devices with laminated top contacts

We demonstrate the fabrication of organic light-emitting devices based on a ruthenium complex with indium tin oxide anodes and laminated Au cathodes. Light emission was uniform over the whole device area, indicating a high-quality mechanical and electrical contact. The devices showed no rectification, indicating that the laminated contact was ohmic and caused no damage to the ruthenium complex. Comparison with devices using evaporated Au cathodes confirmed the quality of the lamination process.

Microfluidic gating of an organic electrochemical transistor

A microfluidic-based organic electrochemical transistor is reported. The integrated microfluidic channel not only confines and directs the flow of liquid electrolyte over the active layer of the transistor but also provides the gate electrode for the transistor. The active layer employed in this work is poly(3, 4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), which results in a transistor that is inherently “on” but that can be turned “off” through application of a positive gate voltage. The transistor behavior is understood in terms of an electrochemical mechanism and is shown to depend on the ionic strength of the electrolyte. The applicability of the device to microfluidic-based chemical and biological sensing is discussed.

Direct 120 V, 60 Hz operation of an organic light emitting device

We report on lighting panels based on ruthenium(II) tris-bipyridine complexes that can be sourced directly from a standard US outlet. With the aid of the ionic liquid 1-butyl-3-methylimidazolium, the conductivity of the light emitting layer was enhanced to achieve device operation at a 60Hz frequency. Lighting panels were prepared using a cascaded architecture of several electroluminescent devices. This architecture sustains high input voltages, provides fault tolerance, and facilitates the fabrication of large area solid-state lighting panels. Scalability of the drive voltage, radiant flux, and external quantum efficiency is demonstrated for panels with up to N=36 devices. Direct outlet operation is achieved for panels with N=16, 24, and 36 devices.

Contact issues in electroluminescent devices from ruthenium complexes

We report on the temporal evolution of the current, radiance and efficiency of electroluminescent devices based on films of [Ru(bpy)3]2+(PF6−)2 (bpy is 2,2′-bipyridyl) with various electrodes. Under forward bias (with the bottom electrode wired as the anode) the device characteristics were independent of the electrodes used. The situation was different under reverse bias, where differences were observed in the steady-state as well as in the transient characteristics of devices with different electrodes. The origin of this asymmetry is discussed.

Nanoscale porosity in polymer films: fabrication and therapeutic applications

This review focuses on current developments in the field of nanostructured bulk polymers and their application in bioengineering and therapeutic sciences. In contrast to well-established nanoscale materials, such as nanoparticles and nanofibers, bulk nanostructured polymers combine nanoscale structure in a macroscopic construct, which enables unique application of these materials. Contemporary fabrication and processing techniques capable of producing nanoporous polymer films are reviewed. Focus is placed on techniques capable of sub-100 nm features since this range approaches the size scale of biological components, such as proteins and viruses. The attributes of these techniques are compared, with an emphasis on the characteristic advantages and limitations of each method. Finally, application of these materials to biofiltration, immunoisolation, and drug delivery are reviewed.

Nanotemplating of Biodegradable Polymer Membranes for Constant-Rate Drug Delivery

2010 WILEY-VCH Verlag Gmb In recent years, advances in drug delivery have allowed for better control over dose and localized release, which has improved treatment efficacy and continues to lead to innovative therapies. Nanoscale materials have been at the forefront of delivery strategies and continue to have significant impact. While the majority of development has focused on various nanoparticle delivery vehicles, nanostructured substrates also have attractive properties for drug delivery associated with their structure. Initially developed theoretically and first demonstrated in zeolites, non-Fickian diffusion is possible in porous materials when the size of a diffusing species is comparable to the pore size of a material. The process is often referred to as ‘‘single-file’’ diffusion, which can lead to concentration-independent transport, and such zero-order release kinetics lead to nanostructured membranes that are particularly attractive for drug-delivery applications. Given relevant size scales, small-molecule delivery requires pores on the order of a nanometer, while macromolecule delivery requires pores on the order of a few tens of nanometers. As a result, careful material selection and design are of crucial importance in order to utilize this phenomenon for drug delivery. Inorganic oxides have been among the first nanostructured materials to receive significant attention. For several decades, anodic growth of porous alumina has produced a wide range of pore sizes and densities; high pore densities (>10 cm ) with pore sizes of the order of 10 nm can be achieved. As a result, aluminamembranes have been investigated for controlled release and immunoisolation. More recently, advances in anodic growth of nanostructured titania have gained significant momentum and have been employed for a variety of therapeutic applications. Various pore sizes have been demonstrated, including several examples down to 20–30 nm. Alternatively, nanostructured silicon membranes have also been developed using contemporary microfabrication, which allows for precise control over the pore size. While the pore density is comparatively low, sub-10-nm pores can be produced with a high degree of uniformity over large areas. Given the established knowledge base of the semiconductor manufacturing, it is straightforward to modulate pore size and density through device design and fabrication conditions. While nanostructured inorganic membranes benefit from uniform pore size and established processing techniques, these materials are not ideal for all drug-delivery applications. In particular, therapies that are not amenable to surgical implantation/excision or implant environments that require significant mechanical compliance are problematic for inorganic materials. An obvious alternative that avoids some of these drawbacks are polymeric materials: polymers have a wide range of chemical and mechanical properties, which allow design of flexible and potentially injectable devices. In addition, the use of biodegradable polymers enables devices that naturally degrade once their therapeutic payload is delivered. Some of the earliest nanoporous polymers were formed by track-etching with high-energy radiation. Established commercial examples exhibit pore sizes down to 15 nm and pore densities on the order of 10 cm . More recently, selfassembly techniques, such as block copolymers (BCP) and layer-by-layer (LbL) growth of polyelectrolytes, have emerged as popular routes to produce nanoscale features in polymers. Through appropriate design and processing, BCPs naturally phase-separate into nanoscale domains, which form nanostructured films upon selective dissolution of one phase. This approach is capable of features comparable to that of inorganic materials. Unfortunately such films are restricted to a limited materials selection and oftentimes produce sub-100-nm-thick films, although more recently thicker membranes have been demonstrated. LbL assembly of polyelectrolytes is another popular approach to form nanostructured polymers. Sequential deposition of polycations and polyanions can be used to form films of arbitrary thickness, which may form nanostructures upon exposure to particular conditions. In general, these materials have lower uniformity and pore formation is not well understood. In addition, LbL films are restricted to a bilayer structure of anionic and cationic polymers. Initially applied to antireflective coatings, these films have also been utilized for controlled release of small-molecule therapeutics. Both block copolymers and layer-by-layer assembly are attractive routes for nanostructure formation, but both approaches are restricted to a limited set of materials and application of these approaches to established commercially available materials is uncertain. Ideally a nanoporous polymer would be biodegradable and flexible, allowing for injection, therapeutic delivery, and subsequent degradation. Such a device decouples delivery kinetics and device degradation. An excellent candidate material is poly(caprolactone) (PCL), as it has been shown to degrade in vivo over tens of months and maintains its structural integrity throughout the vast majority of the degradation timeframe. Conventional microfabrication techniques have been successfully applied to PCL, but such techniques are impractical for the required nanoscale resolution. Furthermore, while PCL nanofibers and nanoparticles have been developed, little work has