R.D. Harris

Integrated optical directional coupler biosensor

We present measurements of biomolecular binding reactions, using a new type of integrated-optical biosensor based on a planar directional coupler structure. The device is fabricated by Ag(+) - Na(+) ion exchange in glass, and definition of the sensing region is achieved by use of transparent fluoropolymer isolation layers formed by thermal evaporation. The suitability of the sensor for application to the detection of environmental pollutants is considered.

Determination of simazine in water samples by waveguide surface plasmon resonance

We assessed a new sensing device based on the monitoring of immunobinding reactions using waveguide surface plasmon resonance (WSPR) for the determination of simazine in water samples. Standard solutions between 0.1 and 1.0 μg l−1 analysed in triplicate showed a mean within-day variability of 5%. Calibration curves for the same standards conducted on five consecutive days showed a 14% mean day-to-day variability. The detection limit calculated as three standard deviations below the mean blank value was 0.2 μg l−1. The upper limit of the working range calculated as a 90% decrease in the blank signal was 2.4 μg l−1. The cross-reactivity of atrazine and terbuthylazine was 61 and 63%, respectively. The recovery from spiked natural ground- and surface-water samples ranged from 55 to 153% for spikes ranging from 0.1 to 1.0 μg l−1. For the 11 surface- and 8 ground-water samples tested, the correlation coefficient between WSPR and high pressure liquid chromatography/gas chromatography (HPLC/GC) values was significant (p<0.05) when the chromatography values were calculated as the weighted sum of simazine and atrazine, taking into account the predetermined cross-reactivity of the latter in the WSPR determination. The present system is therefore better suited for screening groups of pesticides than for the determination of a single molecule. An attempt at analysing a soil water sample proved unsuccessful due to interference probably resulting from strong non-selective polyanion-polycation binding to the transducer surface which includes a basic amino dextran. The total duration of one determination, 22 min, enables almost immediate measurements without any sample pretreatment other than 0.45 μm filtration. No significant alteration of the sensor was observed after 200 determinations.

Integrated optical surface plasmon resonance immunoprobe for simazine detection

This paper presents the detailed design and characterisation of a regenerable integrated optical surface plasmon resonance immunoprobe as a detector for the triazine herbicide simazine. A sensor design theoretically optimised for use in the aqueous environment is presented and its fabrication described. Experimental results on the sensitivity to changes in bulk refractive index of the analyte and on non-specific binding of ovalbumin are presented. Binding inhibition immunoassays were conducted for simazine and the lower limit of detection determined to be 0.16 microgram/l using anti-simazine IgG antibodies and 0.11 microgram/l using anti-simazine Fab fragments. A sample test cycle of 20 min was established.

Theory and modelling of optical waveguide sensors utilising surface plasmon resonance

A theoretical analysis of the phenomenon of excitation of surface plasma waves in integrated-optical waveguide structures is carried out. Rigorous approach to analysis of light propagation through a waveguide structure with a thin metal overlayer supporting surface plasma waves is formulated using a bi-directional mode expansion and propagation method. It is demonstrated that because the back-reflections in the structure are very weak and most of optical power is transmitted by only a limited number of modes of the sensing structure, the method can be considerably simplified.

Immunofluorescence sensor for water analysis

We demonstrated a bulk optical fluorescence based immunosensor capable of multianalyte water analysis. Calibration curves obtained for 2.3-dichlorophenoxyacetic acid (2,4-D) and simazine had detection limits of 0.035µg/l and 0.026µg/l respectively. The sensor is reusable due to its regenerability and cost effective due to the use of components customary in the trade. Ways to further enhance device sensitivity by means of a high index film deposited on the sensor surface or by employing an integrated optical waveguide as transducer are presented. A concept for the detection of a varying range of analytes on the same transducer is discussed.

Sensitivity enhancement of integrated optical sensors by use of thin high-index films

The proportion of power carried in the superstrate medium by the guided modes of integrated optical waveguides can be increased by the addition of a thin high-index film. Enhanced refractive-index sensing is demonstrated with channel waveguide Mach-Zehnder interferometers with Ta(2)O(5) overlays. Sensitivity increases by a factor greater than 50, and a detection limit better than 5 x 10(-7) is obtained. This approach is broadly applicable to sensing at waveguide surfaces where the strength of evanescent fields dictates performance.

Electrochemically-controlled waveguide-coupled surface plasmon sensing

A surface plasmon resonance sensing device has been constructed, in which excitation of a surface plasmon on a metal film is achieved by coupling from light propagating in an optical waveguide. The metal film cladding has been used as an electrode surface and it is proposed that these structures may be applied as novel optoelectrochemical sensors. Specific cationic adsorption has been monitored in potential regions where the metal free-electron contribution is small, and the sensitivity of this sensing technique has been deduced by comparison with ellipsometric measurements. The interfacial model used to analyse the ellipsometric results has been tested by calculating coverage by water in the water + ethylene glycol mixture employed in the surface plasmon device. The high sensitivity of this technique to adsorbed films is found to be greater than that predicted by the theoretical model described, and reasons for this discrepancy are discussed.

Waveguide surface plasmon resonance studies of surface reactions on gold electrodes

We describe the fabrication and characterisation of gold-coated graded index channel waveguide sensors designed for simultaneous electrochemical and surface plasmon resonance studies. The active sensing electrode area is a thin gold film between 0.5 and 5 mm in length and 200 microm wide deposited on top of a 3 microm wide waveguide which forms one arm of a Y-junction while the other arm of the Y-junction serves as a reference. Using these devices we have measured simultaneously the changes in transmittance through the device whilst carrying out cyclic voltammetry in either sulfuric or perchloric acid solution or during the deposition of an UPD layer of copper at the gold surface. In all cases we obtain stable and reproducible results which demonstrate the very high sensitivity of the devices to sub-monolayer changes occurring at the gold electrode surface. The response of these integrated optoelectrochemical devices is discussed in terms of a numerical model for the propagation of light within the waveguide structure.

Waveguide surface plasmon resonance sensor for electrochemically controlled surface reactions

Electrochemical oxidation of the surface of a thin gold film overlaid on an optical waveguide and placed in a cell containing sulfuric acid is monitored by integrated optical surface plasmon resonance (IOSPR) combined with cyclic voltammetry. Waveguide transmittance correlates well with the charge transferred to and from the electrode to oxidize and reduce the surface, with a 60% reduction in transmittance being observed for complete formation of the surface oxide. IOSPR sensors combined with electrochemical measurement and control show potential for sensitive and robust integrated multisensors for electroactive species.

Sensitivity enhancement of transducers for total internal reflection fluorescence

We have developed, modeled and optimized optical transducers for total internal reflection fluorescence (TIRF). The transducers are part of a compact and rugged immuno-analytical instrument designed for simultaneous detection of up to six analytes in aquatic samples (e.g. atrazine and 2,4-D). Binding inhibition assays, using Cy5.5 labeled antibodies to detect the target analytes, have been carried out. Calibration curves with mid-points of tests below 1 (mu) g/1 and detection limits below 0.1 (mu) g/1 have been achieved. As transducer either ion exchanged integrated optical channel waveguides or planar multimode slab waveguides have been employed. The transducer performance was significantly enhanced by incorporating thin high index films at the waveguide surface and by applying high refractive index solutions in the superstrate. Peak signal enhancement factors of more than ten have been observed and an increase in signal to noise ratio by a factor of more than four have been achieved. Strong polarization dependent effects on the enhancement by high index films have been found both theoretically and experimentally.