A magnetostatic-coupling based remote query sensor for environmental monitoring.

A new type of in situ, remotely monitored magnetism-based sensor is presented that is comprised of an array of magnetically soft, magnetostatically-coupled ferromagnetic thin-film elements or particles combined with a chemically responsive material that swells or shrinks in response to the analyte of interest. As the chemically responsive material changes size the distance between the ferromagnetic elements changes, altering the inter-element magnetostatic coupling. This in turn changes the coercive force of the sensor, the amplitude of the voltage spikes detected in nearby pick-up coils upon magnetization reversal and the number of higher-order harmonics generated by the flux reversal. Since the sensor is monitored through changes in magnetic flux, no physical connections such as wires or cables are needed to obtain sensor information, nor is line of sight alignment required as with laser telemetry; the sensors can be detected from within sealed, opaque or thin metallic enclosures.

A remotely interrogatable sensor for chemical monitoring.

A new type of continuously operating, in-situ, remotely monitored sensor is presented. The sensor is comprised of a thin film array of magnetostatically coupled, magnetically soft ferromagnetic thin film structures, adhered to or encased within a thin polymer layer. The polymer is made so that it swells or shrinks in response to the chemical analyte of interest, which in this case is pH. As the polymer swells or shrinks, the magnetostatic coupling between the magnetic elements changes, resulting in changes in the magnetic switching characteristics of the sensor. Placed within a sinusoidal magnetic field the magnetization vector of the coupled sensor elements periodically reverses directions, generating magnetic flux that can be remotely detected as a series of voltage spikes in appropriately placed pickup coils. one preliminary sensor design consists of four triangles, initially spaced approximately 50 micrometers apart, arranged to form a 12 mm x 12 mm square with the triangle tips centered at a common origin. Our preliminary work has focused on monitoring of pH using a lightly crosslinked pH sensitive polymer layer of hydroxyethylmethacrylate and 2-(dimethylamino) ethylmethacrylate. As the polymer swells or shrinks the magnetostatic coupling between the triangles changes, resulting in measurable changes in the amplitude of the detected voltage spirits.

A fiber optic sensor for water in organic solvents based on polymer swelling.

The sensing element is a bead of commercially available anion exchange resin formed by introducing a quaternary ammonium group onto porous crosslinked polystyrene. The diameter of the bead is 20-40% larger in water than in organic solvents investigated in this study including acetone, ethanol, methyl ethyl ketone, 3-pentanone, 4-methyl-2-pentanone and 3-heptanone. The size of the bead varies continuously with the activity of water. In the sensor bead size changes are coupled to movement of a reflecting diaphragm. Movement of the diaphragm changes the intensity of light reflected into an optical fiber. Light intensity is measured as a function of water activity. The results demonstrate that it is possible to sense reversibly and continuously the activity of water in an organic solvent using the phenomenon of the polymer swelling. Problems encountered with cracking of the polymer bead with use and swelling of the diaphragm in the sensor can be addressed by modifying the polymer formulation and changing the sensor design.

Single fiber-optic pH sensor based on changes in reflection accompanying polymer swelling.

We have prepared fiber-optic pH sensors consisting of a small drop of aminated polystyrene on the tip of a single optical fiber with a core diameter of 100 microns. The sensor is prepared by dipcoating a partially polymerized solution and then completing the polymerization on the fiber. This is followed by amination with diethanolamine. The polymer formulation includes xylene/dodecane to introduce porosity and Kraton G1652, a styrene-ethylene, butylene-styrene, triblock copolymer as a toughening agent. The polymer swells as the amine groups are protonated. This is accompanied by an increase in clarity of the polymer and a decrease in the intensity of light reflected back into the optical fiber. Intensity decreases by over a factor of 2 as the pH is decreased from 8.0 to 6.5. The resulting sensor is small and mechanically stable with a response time of several minutes.

Transducer mechanisms for optical biosensors. Part 1: The chemistry of transduction.

The first stage of optical biosensor transduction involves a chemical interaction between the analyte and an indicator phase to produce an optically detectable signal. This stage is critical because it determines stability, selectivity and sensitivity as well as establishing the wavelengths required for the optical measurement. Several types of analyte/indicator interaction are possible. Direct indicators such as those used in optical pH sensors are in equilibrium with the analyte in the sample. They may be used for continuous measurements and can be coupled to the sensing of other analytes such as acidic and basic gases like carbon dioxide and ammonia. Integrating reagents react irreversibly with analyte and require that the rate of product formation be measured. Sensors based on catalysis by an immobilised enzyme involve a steady-state measurement of optically detectable substrate or product. High selectivity can be achieved using antibodies as reagents and basing sensing on competitive binding; however, response time is a serious problem. A variety of methods have been employed to immobilize the indicator phase including adsorption on solid substrates, covalent bonding to a substrate and confinement by membranes permeable to analyte but not to indicator.

On the nature of immobilized tris(carboxymethyl)ethylenediamine.

Immobilized tris(carboxymethyl)ethylenediamine (TED) (also known as ethylenediamine-N,N,N'-triacetic acid) serves as a stationary phase for fractionation of proteins and the preparation of metal-free proteins by immobilized metal-ion affinity chromatography. The Cu(II) complex of commercially available immobilized TED has been characterized by elemental, electrochemical and spectroscopic techniques. There is a large discrepancy between the theoretical capacity determined from the nitrogen content and the experimental capacities determined by atomic-absorption spectroscopy and anodic stripping voltammetry (ASV), indicating that a substantial portion of the immobilized ligand is not binding Cu(II). In addition, the titration of immobilized TED with Cu(II), monitored by ASV, suggests that more than one ligand is involved in binding Cu(II). A comparison of the EPR spectrum for immobilized Cu(II)-TED with spectra for various model complexes shows that the major immobilized ligand is ethylenediamine-N,N'-diacetic acid (EDDA) with immobilized TED present only as a minor component. The complexation constant for Cu(II) is close to the value for Cu(II)-EDDA in solution. The formation of EDDA is consistent with the method for synthesizing immobilized TED.

An optical ionic-strength sensor based on polyelectrolyte association and fluorescence energy transfer.

The optical ionic-strength sensor is based on an indicator phase consisting of an aqueous solution of fluorescein-labelled dextran and polyethyleneimine labelled with Sulforhodamine 101 (Texas Red), confined behind a dialysis membrane. At low ionic strength the polymers associate and the average distance between the fluorescein and Texas Red is short enough for efficient energy transfer to occur. With increasing ionic strength the polymers dissociate and the efficiency of energy transfer decreases. The measured parameter is the ratio of the emission intensity at 520 nm, where fluorescein fluorescence is maximal, to the intensity at 620 nm, where the Texas Red emission is strong. The increase in the intensity ratio as a function of ionic strength is similar but not quite the same for different ions, suggesting that the mechanism of response involves more than a simple ionic strength effect.

Immunoassay labels based on chemiluminescence and bioluminescence.

Reagents required for reactions that produce chemiluminescence (CL) or bioluminescence (BL) may be coupled to antibodies or antigens and used as labels for immunoassay. Because methods based on CL and BL have very low detection limits, they have the potential to replace assays that currently employ radioisotopes as labels. The feasibility of several BL and CL labels has been demonstrated. To date, isoluminol derivatives have been most widely studied. Several steroid assays involving isoluminol labels have been reported, and labelled compound has been detected at levels approaching 10(17) moles. Acridinium ester-labelled compounds have also been detected at this level. In addition to systems in which the label is a reactant required for light, CL and BL can be used to analyze the amount of product generated by enzyme labels. This approach has also yielded very low detection limits. Systems have been developed using enzyme labels that catalyze formation of ATP which is then assayed by the firefly reaction or that catalyze formation of peroxide which is determined by either luminol or peroxyoxalate CL.

Determination of fluorophor-labeled compounds based on peroxyoxalate chemiluminescence.

Fluorophor-labeled compounds are determined by adding an aromatic oxalate ester and hydrogen peroxide and measuring the resulting chemiluminescence (CL). Rhodamine B is excited more efficiently than 2',7'-dichlorofluorescein and dimethylaminonapthalenesulfonic acid. Hydrogen peroxide concentration is the most important variable influencing intensity-time characteristics of the reaction. Measurements are possible in a predominantly aqueous reaction medium but precision is poor, presumably due to oxalate ester insolubility on a microscale. Increasing the percentage of organic solvent leads to improved precision and a larger signal. The detection limit for rhodamine B is around 10(-9) M for a medium containing 50% organic solvent. It is limited by variability in background CL which is associated with impurities in the solvents used for the measurement. The CL excitation efficiency decreases when rhodamine is bound to protein but increases when rhodamine is coupled to folic acid. The addition of albumin to rhodamine-labeled antialbumin causes a 30% decrease in CL intensity.

Flow-injection analysis with chemiluminescence detection.

In "flow-injection analysis," the sample is inserted into a stream of reagent by use of a sample-injection valve. Mixing occurs downstream from the valve in a coil of tubing. With chemiluminescence detection this coil is positioned in front of a photomultiplier. We have evaluated the system for detection of hydrogen peroxide, using the luminol reaction with cupric ion as a catalyst. The effects of flow rate, sample volume, and reaction kinetics on the magnitude, duration, and repeatability of the chemiluminescent response have been evaluated. Precisions of 1 to 2% relative standard deviation on replicate measurements are readily achievable. A sample throughput of six samples per minute is possible with very little peak overlap. This detection system can be coupled to any process in which peroxide is generated.

Analytical studies using iodine-luminol chemiluminescence.

Trace amounts of substances have been related to iodine consumption monitored by chemiluminescence from the iodine-luminol system. Iodine titrations of sulphite and arsenic(III) have been carried out at levels of 1.0 x 10(-7) and 5.0 x 10(-8)M with a precision and error of better than 1% on a day-to-day basis. A calibrated standard of 1.8 ppm SO(2) in air was analysed with a precision of +/-1.2%, and an error of 0.9%. A rate method has been developed based on the reaction between iodine and penicillin G; a linear range from 10(-8) to 10(-7)M was found, the precision being +/-9%.