Lloyd W. Burgess

Single fibre optic fluorescence pH probe

Fibre optic sensing is a growing technology in analytical chemistry. Scattering, absorbance, reflectance and luminescence spectroscopic measurements have been made using fibre waveguides. Fluorescence is particularly suited for use in fibre sensing because of its sensitivity and versatility, and the ease with which this type of measurement may be implemented with a single fibre optic.The probe configuration used in this study for pH measurement consists of fluorescein isothiocyanate (FITC) covalently bonded to a porous glass bead. This bead is attached to a single, multi-mode optical fibre, which conducts both the excitation and emission radiations. The increase in fluorescence intensity, as the acid form of the immobilised FITC is converted to its basic form, is related to the pH. The dynamic range of this miniature probe is pH 3–7 and this can be extended by decreasing the dye loading. The response time is between 20 and 35 s, depending on the capacity.

A Fiber-Optic FT-NIR Evanescent Field Absorbance Sensor

A polymer-clad fiber optic is used as an evanescent field absorbance sensor for solvents that penetrate into the polymer cladding. The sensor is coupled to an FT-NIR spectrometer for spectral measurements from 1.0 to 2.2 μm. A 400-μm core fiber optic, 1.5 m in length, was coiled with a 1.5-cm radius on a Teflon® support. The coiled sensor was then immersed in various nonpolar organic liquids which partitioned into the hydrophobic polysiloxane cladding. The evanescent-field near-infrared spectra of pure hexane, chloroform, and carbon tetrachloride are shown along with ethanol in chloroform. Various amounts of chloroform in carbon tetrachloride and toluene in cyclohexane were used to test the quantitative response. This sensor data are compared with data from a conventional NIR spectrometer using principal component regression.

Evanescent fiber-optic chemical sensor for monitoring volatile organic compounds in water.

The transport of trichloroethylene, 1,1,1-trichloroethane, and toluene in aqueous solutions through a polydimethylsiloxane film was modeled using a Fickian diffusion model to fit data obtained from an evanescent fiber-optic chemical sensor (EFOCS). The resultant diffusion coefficients for these analytes were respectively 3 × 10(-)(7), 5 × 10(-)(7), and 1 × 10(-)(7) cm(2)/s. Inclusion of an interfacial conductance term, defined as the ratio of the mass transport coefficient across the polymer surface and the analyte diffusion coefficient in the polymer, was required to accurately model the data. It was determined that the interfacial conductance terms were generally of the same order of magnitude for the analytes examined, suggesting a constant transport mechanism for the analytes. Linear chemometric algorithms were used to model the EFOCS response to aqueous mixtures of the three analytes with individual analyte concentrations between 20 and 300 ppm. Both partial least-squares and principal component regression algorithms performed comparably on the calibration sets, with cross-validated root-mean-squared errors of prediction for trichloroethylene, 1,1,1-trichloroethane, and toluene of approximately 26, 29, and 22 ppm, respectively. The resultant prediction model was then used to determine analyte concentrations in an independent data set with comparable precision.

A New Approach for Measuring Single-Cell Oxygen Consumption Rates

A novel system that has enabled the measurement of single-cell oxygen consumption rates is presented. The experimental apparatus includes a temperature controlled environmental chamber, an array of microwells etched in glass, and a lid actuator used to seal cells in the microwells. Each microwell contains an oxygen sensitive platinum phosphor sensor used to monitor the cellular metabolic rates. Custom automation software controls the digital image data collection for oxygen sensor measurements, which are analyzed using an image-processing program to yield the oxygen concentration within each microwell versus time. Two proof-of-concept experiments produced oxygen consumption rate measurements for A549 human epithelial lung cancer cells of 5.39 and 5.27 fmol/min/cell, closely matching published oxygen consumption rates for bulk A549 populations.

Development of a positive pressure driven micro-fabricated liquid chromatographic analyzer through rapid-prototyping with poly(dimethylsiloxane) : Optimizing chromatographic efficiency with sub-nanoliter injections

A rapid and low-cost means of developing a working prototype for a positive-displacement driven open tubular liquid chromatography (OTLC) analyzer is demonstrated. A novel flow programming and injection strategy was developed and implemented using soft lithography, and evaluated in terms of chromatographic band broadening and efficiency. A separation of two food dyes served as the model sample system. Sample and mobile phase flowed continuously by positive displacement through the OTLC analyzer. Rectangular channels, of dimensions 10 mum deep by 100 mum wide, were micro-fabricated in poly-dimethylsiloxane (PDMS), with the separation portion 6.6 cm long. Using a novel flow programming method, in contrast to electroosmotic flow, sample injection volumes from 0.5 to 10 nl were made in real-time. Band broadening increased substantially for injection volumes over 1 nl. Although underivatized PDMS proved to be a sub-optimal stationary phase, plate heights, H, of 12 mum were experimentally achieved for an unretained analyte with the rectangular channel resulting in a reduced plate height, h, of 1.2. Chromatographic efficiency of the unretained analyte followed the model of an OTLC system limited by mass-transfer in the mobile phase. Flow rates from 6 nl min(-1) up to 200 nl min(-1) were tested, and van Deemter plots confirmed plate heights were optimum at 6 nl min(-1) over the tested flow rate range. Thus, the best separation efficiency, N of 5500 for the 6.6 cm length separation channel, was achieved at the minimum flow rate through the column of 6 nl min(-1), or 3 ml year(-1). This analyzer is a low-cost sampling and chemical analysis tool that is intended to complement micro-fabricated electrophoretic and related separation devices.

Absorption-based sensors☆

Many chemical sensors based on fiber optics and absorption spectroscopy have been reported in applications ranging from biomedical and environmental monitoring to industrial process control. In these diverse applications, the analyte can be probed directly, by measuring its intrinsic absorption, or by incorporating some transduction mechanism such as a reagent chemistry to enhance sensitivity and selectivity. Physical and performance requirements are placed on a device depending on its intended use. In applications such as chemical process monitoring, survivability and the assurance of the long-term quality of the analytical data are paramount. The above needs have resulted in devices that now employ multivariate data analysis, complex sampling interfaces, and reagent renewal mechanisms. The response from such systems can provide information not only about target analyte(s), but can also signal the presence of interferences, and may potentially be used to follow sensor degradation. Examples are given of devices currently being investigated along with a discussion of some of the remaining material, chemical, and optical challenges.

Characterization and use of a Raman liquid-core waveguide sensor using preconcentration principles

A novel Raman sensor using a liquid-core optical waveguide is reported, implementing a Teflon-AF 2400 tube filled with water. An aqueous analyte mixture of benzene, toluene and p-xylene was introduced using a 1000 microl sample loop to the liquid-core waveguide (LCW) sensor and the analytes were preconcentrated on the inside surface of the waveguide tubing. The analytes were then eluted from the waveguide using an acetonitrile-water solvent mixture injected via a 30 microl eluting solvent loop. The preconcentration factor was experimentally determined to be 14-fold, in reasonable agreement with the theoretical preconcentration factor of 33 based upon the sample volume to elution volume ratio. Raman spectra of benzene, toluene and p-xylene were obtained during elution. It was found that analytically useful Raman signals for benzene, toluene and p-xylene were obtained at 992, 1004 and 1206 cm(-1), respectively. The relative standard deviation of the method was 3% for three replicate measurements. The limit of detection (LOD) was determined to be 730 ppb (parts per billion by volume) for benzene, exceptional for a system that does not resort to surface enhancement or resonance Raman approaches. The Raman spectra of these test analytes were evaluated for qualitative and quantitative analysis utility.

STUDY OF ANALYTE DIFFUSION INTO A SILICONE-CLAD FIBER-OPTIC CHEMICAL SENSOR BY EVANESCENT WAVE SPECTROSCOPY

The diffusion rates of various polar and nonpolar analytes in dimethylsiloxane were examined with the use of a commercially available 200-μm silica-core/300-μm silicone-clad fiber as the optical element for evanescent wave spectroscopy in the near-infrared spectral region. An analytical solution to Ficks second law was used to model the time-dependent analyte concentration at the core/cladding interface. Successful fit of the analytical solutions to infrared data verifies the assumption of constant diffusion coefficients that is necessary to solve the equation. Transport rates of polar analytes in silicone can be estimated with the use of a single-parameter model that results in diffusion coefficients of 3.2 × 10−1, 1.6 × 10−1, 8.1 × 10−7, and 3.9 × 10−7 cm2/s for methanol, ethanol, 2-propanol, and n-butanol, respectively. Estimating the transport of larger nonpolar analytes in the silicone cladding requires a two-parameter model that includes a diffusion coefficient and an interfacial conductance term. For pentane, hexane, heptane, and cyclohexane the resultant diffusion coefficients and interfacial conductance parameters are 6.9 × 10−7, 4.6 × 10−7, 4.4 × 10−7, and 2.3 × 10−7 cm2/s and 2500, 2000, 2000, and 600 μm−1, respectively.

An absorbance-based fiber-optic sensor for CO2(aq) measurement in porewaters of sea floor sediments

Abstract We present results of the construction, testing and calibration of a sensor designed for in situ measurement of CO 2(aq) concentrations in porewaters of sea floor sediments. The sensor relies on pH-dependent absorbance changes of a dye-containing solution enclosed within a gas-permeable membrane. It is rugged enough to withstand insertion several centimeters in sediments without damage, and small enough to make measurements separated spatially by ≤ 2 mm. Response time at 20 °C is a few minutes. Calibration results imply that the sensor is capable of resolving differences in CO 2(aq) concentration of ± 8%, or about 2–3 × 10 −6 M, and is stable over time scales of several hours. Preliminary results from two sea floor locations in the equatorial Atlantic of depths 3300 and 4700 m are qualitatively consistent with simultaneous, adjacent pH electrode measurements. Sensor performance could be improved by adding a system for in situ reagent delivery, and by monitoring a non-absorbing wavelength in the dye absorbance spectrum to decrease dependence on the dye concentration of the internal solution.

Liquid-core waveguides for chemical sensing

A renewable reagent liquid core waveguide (LCW) chemical sensor which used a membrane material as both the sampling element and waveguiding component has been developed. Both aqueous and nonaqueous reagent chemistries were used. The aqueous chemistries were modified by using an ethylene glycol/water mixture (refractive index approximately equals 1.38) which allowed light to be guided inside the tubular membrane material. Several fluoropolymer membrane materials (e.g., PTFE, PFA, FEP) as well as temperature effects on waveguiding were also examined. Two different applications were used to demonstrate LCW chemical sensing. Ammonia was detected using a bromothylmol blue reagent while trichloroethylene (TCE) vapor was detected using the Fujiwara (a base catalyzed pyridine) reagent chemistry.