G. Gauglitz

Chemical and biochemical sensors based on interferometry at thin (multi-) layers☆

Abstract Spectral interferometry is presented as a tool to monitor the swelling of polymers caused by organic gases or hydrocarbons in waste water as well as the adsorption and interaction of antigens and antibodies in immunoreactions. Modern diode-array technology allows the consequent observation of changes in optical pathlength on a fractional nanometer scale with subsecond repetition times. The theory of multiple-reflection principles in white-light interferometry determines the possibilities and limitations of this method. The optical set-up and some applications in gas sensing and label-free immunosensing are discussed with respect to the sensitivity, selectivity and limits of detection at present.

Immunosensors for pesticide determination in natural waters

Abstract The development and application of immunosensors for environmental monitoring has grown steadily in recent years. In this review, immunosensors developed in the last few years are highlighted regarding their importance and practical contribution to environmental analysis with particular emphasis on monitoring of pesticide levels. Different transduction elements and mechanisms used for the detection of the physicochemical change(s) produced by the biological interaction are shown, as well as their application to the development of real-time measurement devices or systems. In addition, examples concerning the analysis of pesticides in natural water samples obtained using a recently developed optical immunosensor are given. Validation of immunosensor measurements with conventional chromatographic techniques is also reported. The immunosensor system detection mechanism is based on a solid-phase fluoroimmunoassay combined with an optical transducer chip chemically modified with an analyte derivative. Applications to the analysis of atrazine, simazine, paraquat, alachlor, 2,4-D and isoproturon in water are also reviewed.

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.

Affinity detection of low molecular weight analytes.

In this paper we report attempts to detect directly the binding of a low molecular weight substance to a protein binding site. An optical transducer based on reflectometric interference spectroscopy (RIFS) was used to detect the binding of biotin (244 g/mol) to a thin silica film surface coated with streptavidin. RIFS allows measurement of changes in the optical thickness of thin transparent films with high resolution. During immobilization of streptavidin, an increase in layer thickness of about 5 nm was detected. Subsequent incubation with biotin (4 μM) resulted in a thickness increase of about 70 pm. Repeated incubation with biotin gave no further increase in layer thickness. The lowest biotin concentration showing significant effects was 40 nM. Incubation with benzoic acid (40 μM) gave no thickness change. The setup allowed significant detection of thickness increases of 2 pm and above. Therefore, the thickness effects observed in the study could be unambiguously and clearly identified.

A gas sensor based on an integrated optical Mach-Zehnder interferometer

Abstract In this paper we demonstrate a sensor system for measuring gaseous compounds using an integrated optical Mach-Zehnder interferometer (IO-MZ chip). One of the surface waveguide arms of the IO device is covered with a polysiloxane layer, sensitive to organic solvents. The refractive index of the polymer layer changes continuously when exposed to the vapours of such solvents. The influence on the evanescent field of the guided mode causes a change in the optical pathlength in the covered arm. Incoupled light of a semiconductor laser diode results in an interference pattern at the output of the IO-MZ chip. The phase shift of the signal is dependent on the gas concentration and the solvent type. We have determined the values for some hydrocarbons, chlorohydrocarbons and aromatic compounds. The extremely short response time of the system allows it to be used like a gas chromatographic detector. The applicationn of Maxwells equations for the propagation of light in waveguides offers a simulation of the interference signals. By this means, the experimental signal obtained can be correlated to changes in refractive index of the polymer layer. The dependence of the interference patterns on wavelength is explained. White light interference is obtained by use of a xenon lamp.

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.

Biosensors using integrated optical devices

SummaryA label-free, homogeneous immunoassay can be realised using surface single-mode wave guide structures. The evanescent field of the guided mode penetrates the adjacent vicinity of the wave guide, the biochemical layer. Thus, changes in its thickness and refractive index during an antigen-antibody interaction cause a change in the effective refractive index of the wave guide itself. Such small changes can be transformed into intensity modulations using an interferometric set-up. Investigations are described using an integrated optical Mach-Zehnder interferometer (IO-MZ-chip) and the experimental results are compared with model calculations. The theoretical sensitivity of the IO-MZ-chips used was determined applying Maxwells equations to the propagation of light in stripe wave guides. For the experimental formation of a protein adlayer a three-step biotinavidin protocol for the immobilisation of biotinylated immunoglobulins was employed.

Integrated optical Mach-Zehnder interferometers as simazine immunoprobes

Abstract Immunoassay has become a versatile tool in several fields of analytical chemistry. We describe the characterization and the application of different integrated optical channel waveguide Mach-Zehnder interferometers (MZIs) as label-free immunoprobes. The performance of the classical MZI is compared with that of a modified structure which incorporates a 3 × 3 coupler. Characterization of the devices demonstrates a dramatic improvement gained by using the 3 × 3 coupler. Two main advantages are achieved by the modified device. First, the possibility of referencing the output signal allows the elimination of signal fluctuations due to coupling and light-source instabilities. An increase of the signal-to-noise ratio by a factor of up to 10 is achieved. Secondly, the phase shift between the three outputs allows unambiguous detection with optimum sensitivity. For the detection of the herbicide simazine, the functional properties of the transducer surface are optimized by an appropriate chemical modification. Using this improved device, a simazine immunoassay has been carried out with a test midpoint of 0.3 ppb and a detection limit of approximately 0.1 ppb. The excellent performance, established manufacturing techniques and the potential for simplification and parallelization make the device attractive for further development.

Compact surface plasmon resonance-transducers with spectral readout for biosensing applications

Abstract Surface plasmon resonance (SPR) has become an important tool in sensing applications. The most abundant set-up records surface plasmon excitation for monochromatic light as a function of the angle of incidence. This requires imaging systems and makes the overall set-up bulky. An alternative is spectrally resolved SPR, which allows to separate sensor and signal analysis. One approach is based on the excitation of surface plasmon waves on the surface of a declad optical fibre. This allows an elegant probe-type geometry, but is less desirable under manufacturing and liquid handling aspects. We have investigated the possibility to develop compact SPR-probes combining spectral SPR and a planar sensor geometry. Two miniaturised set-ups were compared to a classical optical-bench device. Performance was assessed by use of the devices as a refractometer. Results were confirmed by two bioassays: (i) by a protein multilayer system consisting of a biotin-albumin-conjugate and polystreptavidin; and (ii) by monitoring the interaction of thrombin and an immobilised thrombin-inhibitor. All set-ups showed good long term stability. Noise values from refractometry were in good agreement with bioassays, for the classical set-up, but increased noise levels were observed for the miniaturised set-ups indicating a requirement for improved mechanical device stability.