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Biomedical subjects

D R Walt

Publications and source records attributed to D R Walt.

At least 19 recordsLinked to original sources
PubMed1998

Optical sensor arrays for odor recognition.

Optical sensor arrays containing fluorescent solvatochromatic dyes immobilized in a plurality of polymers generate information-rich responses upon exposure to organic vapors. The response profiles are used to train a variety of computational networks such that subsequent exposure of the array to the vapors enables them to be classified and/or quantified. A number of strategies can be taken to enhance sensitivity and to increase sensor diversity.

Animals↗
PubMed1998

Randomly ordered addressable high-density optical sensor arrays.

Array-based sensors provide an architecture for multianalyte sensing. In this paper, we report a new approach for array fabrication. Sensors are made by immobilizing different reactive chemistries on the surfaces of microspheres. Sensor arrays are prepared by randomly distributing a mixture of microsphere sensors on an optical substrate containing thousands of micrometer-scale wells. The sensors occupy a different location from array to array; thus the identity of each sensor is ascertained and registered on the detector using encoding schemes, rather than by a predetermined location in the array. The approach thereby shifts the demand from fabrication to signal processing. The availability of commercial image analysis software makes such a shift both cost-effective and time efficient.

Cost-Benefit Analysis↗
PubMed1998

An olfactory neuronal network for vapor recognition in an artificial nose.

Odorant sensitivity and discrimination in the olfactory system appear to involve extensive neural processing of the primary sensory inputs from the olfactory epithelium. To test formally the functional consequences of such processing, we implemented in an artificial chemosensing system a new analytical approach that is based directly on neural circuits of the vertebrate olfactory system. An array of fiber-optic chemosensors, constructed with response properties similar to those of olfactory sensory neurons, provide time-varying inputs to a computer simulation of the olfactory bulb (OB). The OB simulation produces spatiotemporal patterns of neuronal firing that vary with vapor type. These patterns are then recognized by a delay line neural network (DLNN). In the final output of these two processing steps, vapor identity is encoded by the spatial patterning of activity across units in the DLNN, and vapor intensity is encoded by response latency. The OB-DLNN combination thus separates identity and intensity information into two distinct codes carried by the same output units, enabling discrimination among organic vapors over a range of input signal intensities. In addition to providing a well-defined system for investigating olfactory information processing, this biologically based neuronal network performs better than standard feed-forward neural networks in discriminating vapors when small amounts of training data are used.

Animals↗
PubMed1998

Current trends in 'artificial-nose' technology.

Basic principles derived from biological olfaction, such as combining semiselective sensor arrays with pattern recognition, have been used to develop instrumentation capable of broad-band chemical detection and quantification. Commercially available instruments are useful in areas including quality control in the food, beverage and fragrance industries, environmental monitoring, chemical-purity and -mixture analysis, and medical diagnostics. Ongoing research is aimed at the development of more-advanced instruments that are smaller, cheaper, faster and more stable and reliable. These second-generation instruments are likely to find an increasing number of applications, including the on-line monitoring of fermentation and other bioprocesses.

Electronic Data Processing↗
PubMed1997

Fiberoptic DNA sensor array capable of detecting point mutations.

The first integrated fiberoptic DNA sensor array capable of simultaneously monitoring multiple hybridization events has been prepared. In this study, 5'-amino-terminal DNA probes were covalently immobilized by reaction with succinimidyl ester residues in acrylamide-based polymer matrices. DNA sensor arrays of three different oligonucleotide probes, p(dA)(18-mer), H-ras wild-type (10-mer), and H-ras mutant (11-mer) were fabricated, and real-time hybridization of 5'-fluorescein isothiocyanate (FITC)-labeled target oligonucleotides to the array was monitored. H-ras wild-type and Eta-ras mutant differ by a one-base substitution (point mutation) and were used to demonstrate the sensor's ability to distinguish point mutations. Thermal studies indicated a 13 degrees C decrease in the Tm of a duplex with a single-base mismatch. The fiberoptic DNA sensor array was used to discriminate a point mutation by monitoring the real-time hybridization of FITC-labeled target oligonucleotides at 54 degrees C and detected labeled-target oligonucleotides in the range 0.2-196 nM. The lower detection limit is approximately an order of magnitude lower than previously reported DNA biosensors. The DNA sensor array was used to positively identify a point mutation of a biotin-primer-labeled (109 bp) PCR product of the H-ras oncogene. The unitary fiberoptic sensor array is highly sensitive, has the ability to determine point mutations, and has the potential to sample submicroliter volumes due to the small volumes of the individual array elements (20 pL).

Biosensing Techniques↗
PubMed1997

Fast temporal response fiber-optic chemical sensors based on the photodeposition of micrometer-scale polymer arrays.

Fiber-optic chemical sensor microarrays for the detection of pH and O2 have been developed with subsecond response times. Sensor microarrays are fabricated by the covalent immobilization (pH sensor arrays) or the physical entrapment (O2 sensor arrays) of fluorescent indicators in photodeposited polymer matrices on optical imaging fibers. Polymer microarrays are comprised of thousands of individual elements photodeposited as hemispheres such that each element of the sensor array is coupled directly to a discrete optical element of the imaging fiber and is not in contact with other neighboring elements. Because of the hemispherical shape and the individuality of the array elements, diffusion of analyte to the sensor elements is dominated by radial diffusion, resulting in a rapid response time. pH-sensitive arrays based on fluorescein respond to a 1.5-unit pH change within 300 ms, while the O2-sensitive arrays respond to O2 changes within 200 ms (90% of steady state response).

Biosensing Techniques↗
PubMed1997

In situ fluorescence imaging of localized corrosion with a pH-sensitive imaging fiber.

A fiber-optic pH-imaging sensor array capable of both visualizing remote corrosion sites and measuring local chemical concentrations at these sites was applied to realtime corrosion monitoring. The imaging fiber's distal face, containing an immobilized pH-sensitive fluorescent dye, was brought into contact with metal surfaces submerged in aqueous buffers and fluorescence images were acquired as a function of time. Heterogeneous fluorescence signals were observed due to both pH increases at cathodic surface sites and pH decreases at anodic surface sites. These fluorescence signals showed both localization and rates of corrosion activity. Three corrosion processes were investigated, galvanic corrosion at a copper/aluminum interface and crevice corrosion and pitting at a stainless steel surface. The spatial resolution of the technique was limited by proton/hydroxide diffusion and the diameter of the individually clad optical fibers comprising the imaging bundle.

Corrosion↗
PubMed1997

Multianalyte biosensors on optical imaging bundles.

We present an optical biosensor design that expands the utility of enzyme biosensors. These biosensors are fabricated by site-selective photodeposition of analyte-sensitive polymer matrices on optical imaging fibres. These dual-analyte arrays allow for the simultaneous, independent measurement of the analyte of interest and the transducing analyte. The first integrated optical-biosensors using this design have been prepared that allow both the dependent and independent analytes to be measured simultaneously, for example penicillin and pH (Healey & Walt, 1995) or glucose and O2 (Li & Walt, 1995). Independent measurement of the transducing analyte allows penicillin or glucose to be quantitated in the presence of a concurrent pH or O2 change, respectively. Penicillin can be measured in the range 0.25-10.0 mM in the pH range 6.2-7.5. Glucose can be measured in the range 0.6-20.0 mM in the O2 range 20-100%. The utility of the sensor design was demonstrated by using the penicillin-dual-analyte biosensor to quantitate penicillin produced during a Penicillium chrysogenum fermentation.

Biosensing Techniques↗
PubMed1996

A chemical-detecting system based on a cross-reactive optical sensor array.

The vertebrate olfactory system has long been recognized for its extraordinary sensitivity and selectivity for odours. Chemical sensors have been developed recently that are based on analogous distributed sensing properties, but although an association between artificial devices and the olfactory system has been made explicit in some previous studies, none has incorporated comparable mechanisms into the mode of detection. Here we describe a multi-analyte fibre-optic sensor modelled directly on the olfactory system, in the sense that complex, time-dependent signals from an array of sensors provide a 'signature' of each analyte. In our system, polymer-immobilized dye molecules on the fibre tips give different fluorescent response patterns (including spectral shifts, intensity changes, spectral shape variations and temporal responses) on exposure to organic vapours, depending on the physical and chemical nature (for example, polarity, shape and size) of both the vapour and the polymer. We use video images of temporal responses of the multi-fibre tip as the input signals to train a neural network for vapour recognition. The system is able to identify individual vapours at different concentrations with great accuracy. 'Artificial noses' such as this should have wide potential application, most notably in environmental and medical monitoring.

Biosensing Techniques↗
PubMed1996

A fiber-optic DNA biosensor microarray for the analysis of gene expression.

A fiber-optic biosensor array is described for the simultaneous analysis of multiple DNA sequences. A bundle of optical fibers was assembled with each fiber carrying a different oligonucleotide probe immobilized on its distal end. Hybridization of fluorescently labeled complementary oligonucleotides to the array was monitored by observing the increase in fluorescence that accompanied binding. The approach enables fast (< 10 min) and sensitive (10 nM) detection to multiple DNA sequences simultaneously, with the potential for quantitative hybridization analysis.

Biosensing Techniques↗
PubMed1995

Improved fiber-optic chemical sensor for penicillin.

An optical penicillin biosensor is described, based on the enzyme penicillinase. The sensor is fabricated by selective photodeposition of analyte-sensitive polymer matrices on optical imaging fibers. The penicillin-sensitive matrices are fabricated by immobilizing the enzyme as micrometer-sized particles in a polymer hydrogel with a covalently bound pH indicator. An array of penicillin-sensitive and pH-sensitive matrices are fabricated on the same fiber. This array allows for the simultaneous, independent measurement of pH and penicillin. Independent measurement of the two analytes allows penicillin to be quantitated in the presence of a concurrent pH change. An analysis was conducted of enzyme kinetic parameters in order to model the penicillin response of the sensor at all pH values. This analysis accounts for the varying activity of the immobilized penicillinase at different pH values. The sensor detects penicillin in the range 0.25-10.0 mM in the pH range 6.2-7.5. The sensor was used to quantify penicillin concentration produced during a Penicillium chrysogenum fermentation.

Biosensing Techniques↗
PubMed1995

Dual-analyte fiber-optic sensor for the simultaneous and continuous measurement of glucose and oxygen.

A fiber-optic sensor for the continuous and simultaneous determination of glucose and oxygen is described. The sensor is comprised of dual-analyte sensing sites in defined positions on the distal end of an imaging fiber (350 microns o.d.). Each sensing site is an individual polymer cone covalently attached to the activated fiber surface using localized photopolymerization. The oxygen sensor consists of a double-layer polymer cone. The inner polymer cone is a hydrophobic gas-permeable copolymer containing an oxygen-sensitive ruthenium dye, and the outer layer is a poly(hydroxyethyl methacrylate) (HEMA) polymer. The glucose sensor is an oxygen sensor with a poly-HEMA outer layer containing immobilized glucose oxidase. The fluorescence images of both sensing sites are captured with a CCD camera, and the measured fluorescence intensities are related to analyte concentrations. Oxygen quenching data for both sensing sites fit a two-site Stern-Volmer quenching model. The sensor has been used to simultaneously monitor independent changes in glucose and oxygen concentrations. Glucose calibration curves were obtained under varying oxygen tensions, and the detection limit is 0.6 mM glucose. The effect of fluctuations in oxygen partial pressure on the glucose response can be used to calibrate the sensor. The sensor response time varies from 9 to 28 s, depending on the different thicknesses of the enzyme layer. The sensor maintains the same sensitivity for 2 days. Multiple glucose sensing sites with different enzymatic activities can be immobilized on the distal end of the fiber, affording control of the linear range.

Fiber Optic Technology↗
PubMed1995

Combined imaging and chemical sensing using a single optical imaging fiber.

Despite many innovations and developments in the field of fiber-optic chemical sensors, optical fibers have not been employed to both view a sample and concurrently detect an analyte of interest. While chemical sensors employing a single optical fiber or a noncoherent fiberoptic bundle have been applied to a wide variety of analytical determinations, they cannot be used for imaging. Similarly, coherent imaging fibers have been employed only for their originally intended purpose, image transmission. We herein report a new technique for viewing a sample and measuring surface chemical concentrations that employs a coherent imaging fiber. The method is based on the deposition of a thin, analyte-sensitive polymer layer on the distal surface of a 350-microns-diameter imaging fiber. We present results from a pH sensor array and an acetylcholine biosensor array, each of which contains approximately 6000 optical sensors. The acetylcholine biosensor has a detection limit of 35 microM and a fast (< 1 s) response time. In association with an epifluorescence microscope and a charge-coupled device, these modified imaging fibers can display visual information of a remote sample with 4-microns spatial resolution, allowing for alternating acquisition of both chemical analysis and visual histology.

Acetylcholinesterase↗
PubMed1995

Photodeposition of micrometer-scale polymer patterns on optical imaging fibers.

Microstructures were fabricated on optical imaging fibers with a photopolymerization technique. Monodisperse polymeric microarrays were produced containing spots of 2.5 micrometers in diameter spaced 4.5 micrometers apart. Polymer microarrays were also deposited on other substrates by using imaging fibers for light delivery. The technique allows micrometer-scale photopatterning with masks larger than the desired dimensions.

Fiber Optic Technology↗
PubMed1994

Fabrication of patterned sensor arrays with aryl azides on a polymer-coated imaging optical fiber bundle.

Arrays of sensing regions are photodeposited on the distal tip of a single imaging optical fiber. First, the distal surface of the fiber is spin-coated with a thin film of poly(hydroxyethyl methacrylate). The fluorophor is then derivatized with a photoreactive group and subsequently immobilized in a finite area of the film by discrete illumination. Dye incorporation occurs only in the illuminated areas, creating distinct regions of analyte-sensitive fluorescent dye at the fiber's distal end. This paper describes both the chemistry and the manipulations required to make an optical microarray and demonstrates the technique with pH sensors. The fabrication of a four-sensor array is described along with performance data.

Azides↗
PubMed1993

Fiber-optic sensor for continuous monitoring of fermentation pH.

We have developed a fiber-optic chemical sensor for on-line monitoring of fermentation pH. The sensor is based on a covalently bound fluorescent dye immobilized within a water-permeable polymer layer on an optical fiber. Measurements were performed on a portable fluorimeter and employed a ratiometric approach to account for system instabilities. We show that the use of this fiber-optic sensor provides fast, accurate and reliable measurements during E. coli fermentation in a complex medium.

Buffers↗
PubMed1991

Chemical sensors based on controlled-release polymer systems.

A novel chemical sensor has been developed in which the polymer ethylene-vinyl acetate is used as a controlled-release system to deliver reagents to the sensing region of an optical fiber for a homogeneous competitive immunoassay based on fluorescence energy transfer. A competition reaction is used to enable continuous measurements of the solution antigen concentration. More generally, the technique allows irreversible indicating chemistries to be used in the construction of chemical sensors that can measure continuously for long periods. Although the sensor configuration has not been optimized in all respects, data are presented for a model system in which a fluorescein-labeled antibody and Texas Red-labeled immunoglobulin G (IgG) are used.

Antibody Specificity↗