William R. Penrose

Sensors, Chemical Sensors, Electrochemical Sensors, and ECS

BCPS Department, Illinois Institute of Technology, Chicago, Illinois 60616, USAThe growing branch of science and technology known as sensors has permeated virtually all professional science and engineeringorganizations. Sensor science generates thousands of new publications each year, in publications ranging from magazines such asPopular Mechanics and Discover to learned journals like the Journal of The Electrochemical Society (JES). The ElectrochemicalSociety ~ECS!, which has declared itself the society for solid state and electrochemical science and technology, and its worldwidemembership, have been vitally instrumental in contributions to both the science and technology underlying sensors. This article isabout a few of the chemical sensors that have evolved, those still now evolving, and the continuing role of ECS in advancementof sensor science and engineering.© 2003 The Electrochemical Society. @DOI: 10.1149/1.1539051# All rights reserved.Available electronically January 13, 2003.

An ultrathin platinum film sensor to measure biomolecular binding.

A sensitive conductimetric immunosensor has been demonstrated based on an ultrathin platinum film on an oxidized silicon base. The film is about 25 A thick and is seen to consist of a discontinuous layer with channels 20-30 A wide. Monoclonal antibodies were bound to the sensor surface using conventional biosensor chemistry. Impedance at fixed frequencies across the film was used to track modification and binding at the surface. Impedance increased 55% at 20 Hz during the activation of the surface with anti-alkaline phosphatase (anti-AP). Binding of alkaline phosphatase (AP) to the prepared surface results in a further increase of 12%. p-Nitrophenyl phosphate hydrolysis confirmed binding and activity of the AP. About 40 amol AP were bound on the 0.5 cm(2) electrode. Non-specific binding of horseradish peroxidase caused an impedance change <6%. Control experiments showed small impedance changes and trace enzyme activity. Since the mechanism of electrical conduction of the thin film was not established, modeling of thin-film response was used to distinguish between redox processes, capacitance and tunneling mechanisms. The data fit well with the diffusion distributed elements (DE) model as well as a transmission line distribution element (DX) model. The first model, DE, is distributed elements for diffusion. The second DX model represents a transmission line. The sensors behave in a distributed network or like a transmission line.

Sensitive measurement of ozone using amperometric gas sensors

We have investigated the ability of miniature amperometric gas sensors (AGS) to detect ozone in air at concentrations as low as 5 ppb. Baseline drift of the sensor, which occurs on a time scale of hours, can be compensated by periodic correction using sample air scrubbed free of oxidants. Interfering gases expected are nitrogen dioxide and nitrous acid. The latter can be removed with a sodium carbonate-glycerol filter, but a chemical filter that can remove nitrogen dioxide in the presence of ozone is not known. However, filters containing indigo can be used to remove ozone from an air stream without affecting the NO2. Selective and sensitive measurement of ozone was achieved by using two matched sensors connected in series, with an indigo filter between. The first sensor measured ozone plus nitrogen dioxide, whereas the second sensor was exposed only to NO2. The ozone signal is the difference in the two signals.

Impedance Imaging of Chemical and Biochemical Systems

Impedance detection of chemical and biochemical events has been performed using an array of microfabricated capacitors [1]. Originally developed for fingerprint acquisition, the modified sensor has a 256 × 364 array of elements, each 50 × 50 µm in size. This sensor can be adapted for various types of sensing applications involving conductivity or impedance of samples. Spatial resolution has been studied using pollen of different size ranges. Biofilms of chicken embryo heart cells have been imaged. The sensor has several adjustable electrical parameters for different ranges of conductivity. Calibration curves have been constructed to determine the effect of each parameter on the response. This new sensor has the capacity for simultaneous optical and chemical observation of multiphase samples in real time.