Larry L. Miller

Variable temperature film and contact resistance measurements on operating n-channel organic thin film transistors

We report structural and electrical properties in thin films of an n-channel organic semiconductor, N,N′-dipentyl-3,4,9,10-perylene tetracarboxylic dimide (PTCDI–C5). The structure of polycrystalline thin films of PTCDI–C5 was studied using x-ray diffraction and atomic force microscopy. Films order with single crystal-like packing, and the direction of π-π overlap is in the substrate plane. Organic thin film transistors (OTFTs) based on PTCDI–C5 were fabricated on hydrophobic and hydrophilic substrates. OTFTs showed effective mobility as high as 0.1 cm2/V s. Contact resistance of operating OTFTs was studied using resistance versus length plots and a four-probe method for three different contact metals (Au, Ag, Ca). Typical OTFTs had a specific contact resistance of 8×104 Ω cm at high gate voltage. There was no dependence of contact resistance with contact metal. Variable temperature measurements revealed that film resistance in the OTFT was activated in the temperature range 100–300 K, with typical activa...

Voltammetric properties of electrodes modified by poly-[N-(9,10-anthraquinone-2-carbonyl)ethylenimine] and used in aqueous solution

Abstract Three differently loaded anthraquinone polymers have been prepared by condensation of 2-anthraquinonecarbonylchloride and poly(ethylenimine). They have been adsorbed on vitreous carbon or mercury electrodes by dipping the electrode in a solution of pyridine or methylene chloride containing 0.01–0.05% polymer. The influence of adsorption parameters and of electrochemical variables on the voltammetric behavior of the polymer-coated electrodes in aqueous buffer solutions is described. Changing the dip time in pyridine from 30 s to 5 min and changing the polymer concentration have very little effect on the apparent coverage (=1.3-1.8×10−9 mol cm−2) and peak shape. In contrast, an increase of the loading (the number of monomer units loaded with quinone) or the use of methylene chloride as a dip-coating solvent instead of pyridine affected the shape of the cathodic and anodic peaks which broaden and tend to separate. The peak shape is characterized by a tailing which tends to disappear at slow scan rate. The modifications of the peak shape and position when the scan rate is changed, have been shown to fit qualitatively with a proposed polylayer model in which electron (proton) transfer to the sublayer nearest the underlying conductor is slow. It has been observed that the weak acid dissociation constants for the dihydroanthraquinone and hydroanthraquinone anion are both different for the corresponding monomer in solution (pKA=8.65 and 11.6) and the adsorbed polymer (pKA=9 and 13).

Electrochemical oxidation of NADH: Kinetic control by product inhibition and surface coating

The electrochemical oxidation of NADH to NAD+ was reexamined using aqueous buffers and a glassy carbon electrode. Using cyclic voltammetry, it was shown that NAD+ was an inhibitor of the oxidation process. This makes the reaction self-inhibited and that accounts for several anomalous observations which were previously made. In the presence of excess NAD+, the NADH oxidation is first-order in NADH. Rate limiting one-electron transfer in an ECE mechanism is indicated. Previous chronoamperometric data which indicated that NADH+ had a lifetime of about 10 ms, were also reinterpreted. It was shown that the previous result was achieved only if the electrode surface was precoated. Therefore, the result cannot be used to calculate the lifetime of NADH+. It is suggested, however, that this lifetime is less than 1 μs.

Electrochemically controlled release

Abstract In vitro experiments on iontophoretic delivery of model anions: acetate, benzoate, hexanoate and dodecanoate, at constant current through hairless mouse skin show that smaller ions are delivered more efficiently and that flux-concentration −1 current −1 is nearly constant. Electrochemically controlled drug-ion delivery from conducting polymer films is described. This method allows release rates to be controlled by the current passed .

Electrochemically Controlled Release of Drug Ions from Conducting Polymers

Abstract The binding and release of organic ions from conducting polymer films can be controlled electrochemically.

Spectroscopic studies of bipolarons from oligomerized 3-methoxythiophene in solution

Abstract 3-Methoxythiophene was anodically polymerized, then cathodically reduced to provide a neutral oligomeric product (1) composed of 3-methoxythiophene units. 1 and its oxidation product 2 were soluble in N,N-dimethylformamide, acetronitrile and benzonitrile and the solution phase redox process was studied as a model for reactions of insoluble polythiophenes. Oxidation to form 2 was accomplished using iodine or electrochemistry. The strong acids, nitric, perchloric or trifluoroacetic also oxidized 1 to 2. Rereduction of 2 to 1 used triethylamine. Quantitative studies showed that I 2 or anodic oxidation at 0.6 V gave 2 with the same extent of oxidation. Ultraviolet-visible spectroscopy showed that 2 was composed of at least two different types of species, each type showing a pair of absorption bands between 630 and 860 nm. E.s.r. showed that 2 was mainly diamagnetic, and it is proposed that soluble dications (bipolarons) are produced.

Iontophoretic transport of acetate and carboxylate ions through hairless mouse skin. A cation exchange membrane model

Abstract As models for transdermal iontophoretic drug delivery, carboxylate anions were electrochemically driven from aqueous solutions across excised hairless mouse skin into aqueous buffer solutions. The flux of 14C-anions was measured. Using [14C]acetate, the flux increased as the current increased, as the acetate concentration in the donor solution increased, as the pH of the donor solution decreased, or if the donor solution was not stirred. The composition of the receptor solution had little effect on the acetate flux, as did turning the skin backwards. Under the same conditions, a comparison of the efficiencies of carboxylate delivery showed acetate > hexanoate > dodecanoate. Acetate delivery through the perfluorosulfonic acid cation-exchange membrane, Nafion, was studied for comparison. The mouse skin results are interpreted in terms of iontophoretically driven acetate delivery through essentially aqueous pathways with similarities to transport through cation-exchange membranes.

Electrochemically controlled binding and release of salicylate, TCNQXXX and ferrocyanide from films of oligomeric 3-methoxythiophene

Abstract Oligomerized 3-methoxythiophene ( 1 ) was prepared anodically. It was deposited from acetonitrile solution onto carbon electrodes. Upon oxidation at 0.4–0.6 V (SCE) in aqueous solutions of the anions salicylate or ferricyanide, the film of 1 was oxidized and incorporated the anions to maintain electroneutrality. These anion-loaded films on carbon were placed in aqueous NaCl and then the anions were released by stepping the electrode potential to pass cathodic current, e.g. to −0.4 V. In the case of salicylate, detection of the released anion by emission spectroscopy showed that at open circuit much of the salicylate exchanged out of the film over a period of 10 min, but release of the salicylate could be completed in only 30 s by stepping the potential to −0.4 V. There was a linear relationship between charge passed and amount of salicylate released. Ferricyanide release into aqueous NaCl was monitored electrochemically. Exchange of ferricyanide for chloride at open circuit was slow on the time scale of minutes, but release was completed in 30 s at −0.2 V. TCNQ was bound into 1 on carbon without electrochemical oxidation. A 1 /TCNQ charge transfer complex (solid state conductivity, σ ≈ 0.1 Ω −1 cm −1 ) is formed. This film showed electrochemical responses characteristic of both 1 and TCNQ. In aqueous NaCl formation of TCNQ XXX at −0.2 V did not give release, but the use of aqueous LiCl did. This behavior was closely analogous to that of a film of pure TCNQ, except that 1 increased the amount of active TCNQ.

The effects of cross-linking and anodic surface roughening on quinone polymer/carbon electrodes

A polymer in which anthraquinone-2-carbonyl groups were bound to polyethyleneimine was coated onto a glassy carbon electrode. Electrodes of this kind were studied using cyclic voltammetry and pH 7 aqueous solutions. At pH <10 only those quinone units in contact with the carbon surface are electroactive. It was shown that anodic surface roughening increased the limited number of electroactive groups in the polymer film and gave more stable activity and narrower voltammetric peaks. Above pH 10 redox propagation through the layer is more rapid but the anionic product desorbs. This desorption was inhibited by cathodically cross-linking a layer of mixed polymers on a polyethyleneimine backboned polymer containing fluorenone units as well as anthraquinone units.

Redox processes in polymeric anthraquinone layers: The importance of polymer chain motion

Abstract Anthraquinone polymers whose loadings in electroactive sites range from 20 to 75%, were adsorbed on a hanging mercury drop electrode (HMDE) which can be expanded after the adsorption. The voltammetric results obtained in buffered aqueous or aqueous—alcoholic solutions are presented. It is shown that in acid media, the charging of the coating is limited to the layer adjacent to the electrode. In neutral and alkaline solutions, the charging of some of the bulk of the coating takes place. This process is slower when polymers containing a higher loading of anthraquinone units are used. The factors which control these results are discussed.