Jay M. Johnson

Humidity sensor based on conductivity measurements of a poly(dimethyldiallylammonium chloride) polymer film

Abstract In order to explore further the properties of the ionically conductive polymer poly(dimethyldiallylammonium chloride (poly(DMDAAC)), a relative humidity (RH) sensor based on conductivity measurements has been fabricated. This sensor consists of a platinum conductivity cell, comprising four parallel platinum ‘finger’ electrodes on a 25.4 mm × 25.4 mm × 0.64 mm ceramic wafer. Two glass masks provide a reproducible sensing area. The wafer is spin-coated with a thin poly(DMDAAC) film and subsequently crosslinked with a 10 Mrad gamma-irradiation dosage using a 60Co source. The sensor exhibits a nonlinear conductivity response to RH in the 20–80% range. A steady-state response to an RH change of 12 to 50% occurs within 1–2 min. The conductivity varies with film thicknesses ranging from 0.5–30 μm. For example, conductivity values obtained at 97% RH are 147.9 μmho for a 30 μm film, compared to 26.5 μmho for a 0.5 μm film. Sensors show a 0.–30% decrease in conductivity after continuous exposure to ambient laboratory atmosphere for 15 months.

Metal complexes as mediator-titrants for electrochemical studies of biological systems

Various redox-active metal complexes were evaluated electrochemically for stability, reversibility, and general utility as mediator-titrants in the 0 to + 1.0 V vs standard hydrogen electrode range. Those complexes which were evaluated and appear to be useful in potentiostatic or potentiometric techniques are Ru(bpy)3(CIO4)2, Ru(bpy)2(im)2, Os(bpy)3(NO3)2, K2Fe(phen)(CN)4, Co(phen)3Cl2, and Co(terpy)2Cl2.

Manganese Detection with a Metal Catalyst Free Carbon Nanotube Electrode: Anodic versus Cathodic Stripping Voltammetry

Anodic stripping voltammetry (ASV) and cathodic stripping voltammetry (CSV) were used to determine Mn concentration using metal catalyst free carbon nanotube (MCFCNT) electrodes and square wave stripping voltammetry (SWSV). The MCFCNTs are synthesized using a Carbo Thermal Carbide Conversion method which results in a material that does not contain residual transition metals. Detection limits of 120 nM and 93 nM were achieved for ASV and CSV, respectively, with a deposition time of 60 s. CSV was found to be better than ASV in Mn detection in many aspects, such as limit of detection and sensitivity. The CSV method was used in pond water matrix addition measurements.

Oxygen sensors based on the ionically conductive polymer poly(dimethyldiallylammonium chloride)

Abstract Graphite rods coated with poly(dimethyldiallylammonium chloride), poly(DMDAAC), and subjected to immobilization by γ-irradiation have been shown to respond linearly to dissolved oxygen concentration. The oxygen is detected electrochemically by reduction at a potential of −0.40 V versus Ag/AgCl. The concentration range of linearity investigated is 1.4 to 9.3 ppm. The polymer-coated electrode has a sensitivity of 12.5 μA cm−2 ppm−1, a detection limit of 1.4 ppm and a response time of 5 min or less. Poly(DMDAAC) also provides the electrode surface with some protection against adsorbing species during the measurement. Further, poly(DMDAAC) can function as the electrolyte in a solid-state cell and allows electrochemical measurements to be made in air in the absence of a supporting electrolyte solution. The response to oxygen concentration in the surrounding atmosphere is shown.

Simplified Nitrate-Reductase-Based Nitrate Detection by a Hybrid Thin-Layer Controlled Potential Coulometry/Spectroscopy Technique

A novel method for the detection of nitrate was developed using simplified nitrate reductase (SNaR) that was produced by genetic recombination techniques. The SNaR consists of the fragments of the Mo-molybdopterin (MO-MPT) binding site and nitrate reduction active site and has high activity for nitrate reduction. The method is based on a unique combination of the enzyme-catalyzed reduction of nitrate to nitrite by thin-layer coulometry followed by spectroscopic measurement of the colored product generated from the reaction of nitrite with Griess reagents. Coulometric reduction of nitrate to nitrite used methyl viologen (MV(2+)) as the electron transfer mediator for SNaR and controlled potential coulometry in an indium tin oxide (ITO) thin-layer electrochemical cell. Absorbance at 540 nm was proportional to the concentration of nitrate in the sample with a linear range of 1-160 μM and a sensitivity of 8000 AU M(-1). The method required less than 60 μL of sample. Detection of nitrate could also be performed by measuring the charge associated with coulometry. However, the spectroscopic procedure gave superior performance because of interference from the large background charge associated with coulometry. Results for the determination of nitrate concentration in several natural water samples using this device with spectroscopic detection are in good agreement with analysis done with a standard method.

Fabrication and characterization of a platinum/ceramic electrochemical sensor

Abstract An electrochemical sensor has been fabricated by screen printing platinum electrodes onto a ceramic base material. The sensor code consists of three electrodes: a platinum auxiliary electrode, a platinum working electrode and a reference electrode made by depositing silver and then silver chloride onto a third platinum electrode. The resulting sensor, when immersed in a solution of K3Fe(CN)6/KCl and examined by cyclic voltammetry, possesses electrochemical characteristics similar to those of a commercial platinum disk electrode. The operable potential window of the sensor at various pH values is suitable to observe the redox characteristics of many analytes of interest. The platinum/ceramic sensor is also applicable to solid-state electrochemical measurements when coated with an ionically conductive polymer such as poly(ethylene oxide) doped with LiCF3SO3.

Microfabricated Electrochemical Detector for High-Performance Liquid Chromatography

A microfabricated electrochemical cell has been developed as a disposable detector for flow injection analysis (FIA) and high-performance liquid chromatography (HPLC). The simplicity of the fabrication process allows this detector to be used as a low-cost, disposable device that can be replaced easily if its performance degrades rather than disassembling the detector and polishing the electrode surface, which is the usual procedure. The detector consists of thin film-metal electrodes-platinum working electrode, platinum auxiliary electrode, and silver/silver chloride coated on Pt reference electrode-deposited on a polyimide substrate with a locking layer of chromium in between. A microfluidic cover made of polyimide directs the solution flow across the electrodes. The detector was evaluated with FIA of ferrocyanide and HPLC of ascorbic acid and acetaminophen and a mixture of two pharmaceutical compounds-dextrorphan and levallorphan-with acetaminophen internal standard. The HPLC calibration curves showed good linearity (R(2) > 0.99). Limits of detection were 1 nM for acetaminophen, 1 nM for ascorbic acid, 50 nM for dextrorphan, and 80 nM for levallorphan. When detected with a commercial detector dextrorphan and levallorphan had lower limits of detection, 3 and 5 nM, respectively. Chromatograms of the mixture were comparable to those obtained with a commercial detector. The detector could be used continuously for about 48 h with FIA and about 10-20 h with HPLC after which performance gradually degraded as the AgCl on the reference electrode dissolved causing loss of potential control.

Biosynthesis of Silver Nanoparticles

The use of biomolecules in the creation of inorganic materials offers an alternative to conventional synthetic methods. Biomolecules are currently used to control nucleation and growth of inorganic nanoparticles. Here we demonstrate the formation of silver nanoparticles in the presence of silver-binding peptides. Examination of the silver nanoparticles by transmission electron microscopy revealed a variety of crystal morphologies such as hexagons, triangles and spheres. The peptides serve to reduce the silver ions in the aqueous solution to metallic silver as well as control crystal growth. The nucleation property of peptides can be used as tool for bottom-up fabrication.