Integrated optical NIR-evanescent wave absorbance sensorfor chemical analysis

A new, long-path integrated optical (IO) sensor for the detection of non-polar organic substances is described. The sensing layer deposited on a planar multimode IO structure is built by a suitable silicone polymer with lower refractive index (RI). It acts as a hydrophobic matrix for the reversible enrichment of non-polar organic contaminants from water or air. Light from the near-infrared (NIR) range is coupled into the planar structure and the evanescent wave part of the light field penetrating into the silicone layer interacts with the enriched organic species. As a result, light is absorbed at the characteristic frequencies of the corresponding C-H, N-H or O-H overtone and combination band vibrations of the analytes. To perform evanescent field absorbance (EFA) measurements, the arc-shaped strip waveguide structure of 172 mm interaction length was adapted to a tungsten-halogen lamp and an InGaAs diode array spectrograph over gradient index fibers. Dimethyl-co-methly(phenyl)polysiloxanes with varying degrees of phenylation were prepared and used as sensitive coating materials for the IO structure. Light attenuation in the arc-shaped waveguides is high and typical insertion losses in the range of 14–18 dB were obtained. When the coated sensors were brought in contact with aqueous samples, the light transmission decreases, which is due to the formation of H₂O micro-emulsions in the silicone superstrates. Nevertheless, after reaching constant light transmissions, absorbance spectra of aqueous trichloroethene samples were successfully collected. For gas measurements, where water cross sensitivity problems are absent, the sensitivity of the IO device for trichloroethene was tested as a function of the RI of the silicone superstrate. The slope of the TCE calibration function increases by a factor of 10 by using a poly(methylphenylsiloxane) layer with a RI of 1.449 instead of poly(dimethylsiloxane) (RI: 1.41). A comparison of the IO-EFA and an earlier developed fiber-optic EFA sensor for trichloroethene measurements in the gas phase showed an increase in sensitivity per unit length of the waveguide by a factor of up to 120.     

Optimisation of an integrated optical evanescent wave absorbance sensor for the determination of chlorinated hydrocarbons in water

The suitability of an integrated optical chemical sensor for the determination of highly volatile chlorinated hydrocarbons in aqueous solutions has been proven. The analytes are detected by NIR absorption spectrometry in the evanescent field of an integrated optical strip waveguide generated in a BGG31 (Schott, Germany) glass substrate, which is coated with a hydrophobic polymer superstrate as sensing layer. It has been shown that the sensitivity increases when the refractive index of the superstrate is increased from 1.333 up to 1.46. Different UV-cured polysiloxanes with low cross sensitivity to water have been prepared. Due to the good light transmission properties of the IO-sensors prepared by this method, quantitative measurements have been performed with the model system trichloroethene (TCE) in water. A detection limit of 22 ppm has been found and the sensor response times (t(90)-value) are between five and fourteen minutes for a coating thickness of around 30 microm. The sensor response is totally reversible. The analyte desorbes in air within 2 min. The enrichment of trichloroethene in the polysiloxane coating can be described by film diffusion through the aqueous boundary layer as rate determining step.     

Optimisation of an integrated optical evanescent wave absorbance sensor for the determination of chlorinated hydrocarbons in water

The suitability of an integrated optical chemical sensor for the determination of highly volatile chlorinated hydrocarbons in aqueous solutions has been proven. The analytes are detected by NIR absorption spectrometry in the evanescent field of an integrated optical strip waveguide generated in a BGG31 (Schott, Germany) glass substrate, which is coated with a hydrophobic polymer superstrate as sensing layer. It has been shown that the sensitivity increases when the refractive index of the superstrate is increased from 1.333 up to 1.46. Different UV-cured polysiloxanes with low cross sensitivity to water have been prepared. Due to the good light transmission properties of the IO-sensors prepared by this method, quantitative measurements have been performed with the model system trichloroethene (TCE) in water. A detection limit of 22 ppm has been found and the sensor response times (t₉₀-value) are between five and fourteen minutes for a coating thickness of around 30 m. The sensor response is totally reversible. The analyte desorbes in air within 2 min. The enrichment of trichloroethene in the polysiloxane coating can be described by film diffusion through the aqueous boundary layer as rate determining step.     

A fiber optic evanescent field absorption sensor for monitoring organic contaminants in water

Summary A fiber optic chemical sensor for determination of organic compounds in aqueous solution has been developed. Based on the evanescent field principle, a quartz glass fiber with a polysiloxane cladding is used as in-situ measuring probe. A compact sensor built from a 6-m coiled fiber has been connected to a commercially available fast scanning dispersive NIR spectrometer. The siloxane cladding fulfils two functions: on the one hand, it acts as protecting layer of the fragile silica fiber core, and on the other hand, it is selective with respect to non-polar organic compounds due to its organophilic properties. Hence, interactions of the evanescent field at the core/cladding interface with organic species penetrating into the cladding can be measured without interferences from broad water OH absorption bands. Aqueous solutions of chlorinated hydrocarbon solvents (CHS) have been used to test the sensor response. NIR evanescent field absorbance spectra of methylene chloride, chloroform and trichloroethylene diffused into the fiber cladding are shown in the 900–2100 nm spectral range. Different amounts of CHCl₃ dissolved in water have been determined in order to evaluate the quantitative sensor response. A linear absorbance/concentration relationship has been found for solutions between 80–6800 mg l^–1. Kinetic experiments performed with CHCl₃ solutions resulted in sensor response times of 5–10 min. The sensor seems to be promising for the remote monitoring of organic contaminants, e.g. CHS, in drainage waters of contaminated areas.     

Investigations of the optical properties of thin, highly absorbing films under attenuated total reflection conditions: Leaky waveguide mode distortions

Spectra of thin highly absorbing Nafion films doped with Ru(bpy)₃²⁺ on SF11 glass substrates were studied by internal reflection spectroscopy using a single reflection configuration. For the system under study, two modes of light interaction with the film are available: attenuation due to evanescent wave penetration and light propagation within the absorbing film. Unlike evanescent wave spectroscopy, light propagation within the film causes distortions in the measured spectra due to leaky waveguide propagation modes. Upon light propagation in a film doped with Ru(bpy)₃²⁺ spectral shifts up to 50 nm to longer wavelengths can occur and additional absorbance peaks can appear in the spectra. These film-based distortions depend on the complex refractive index, the thickness of the film and the angle of incidence. These effects become significant for an extinction coefficient above 0.01 and a film thickness above 200 nm. It is shown that spectral distortions can lead to quite complex dynamics in the internal reflection spectra upon analyte preconcentration in the film. Ru(bpy)₃²⁺ partitioning into the Nafion film causes significant refractive index changes that in turn alter leaky waveguide mode conditions in the film and, can even lead to a reduction of measured absorbance despite the increase in the extinction coefficient of the film.     

Determination of chlorinated hydrocarbons in water by fiber-optic evanescent wave spectroscopy and partial least-squares regression

An application of the multivariate calibration technique of partial least-squares (PLS) regression to near-infrared spectra of a fiber-optic sensor based on the evanescent wave principle is presented. The sensing element consists of a quartz glass fiber with a silicone cladding which enriches nonpolar water contaminants. Due to the interaction of the extracted molecules with the part of the light which is transmitted in the evanescent wave zone of the cladding, absorbance spectra of the contaminants can be collected. In view of a sensor application for in-situ environmental analysis, aqueous solutions of chlorinated hydrocarbon solvents (CHS), which often can be found as major water contaminants, have been measured. PLS regression was applied to three sets of CHS samples, representing typical features of NIR evanescent wave spectral data. These are, e.g., strong overlapping of the absorption bands of different CHS components, peak distortions due to temperature variations between reference and sample measurement and noisy data at analyte concentrations near to the limit of detection, respectively. For trichloroethene and 1,1-dichloroethene, where the calibration model was built for samples within a small concentration range of 1–9 mg l^–1, satisfactory prediction results could be obtained with a relatively small root-mean-square error of 0.3 mg l^–1 compared to analytical reference measurements. In contrast to this, for a three component system of dichloromethane, trichloromethane and trichloroethene with strongly overlapping absorption bands, where samples over a very broad concentration range from 3–4940 mg l^–1 were included in the PLS model, the prediction accuracy decreased enormously and for some samples strong deviations between real and predicted data occurred. Nevertheless, applying multivariate calibration to this difficult system with similar spectral features and huge differences in the concentration of the species allowed an acceptable spectral distinction and at least a semi-quantitative determination of the CHS species.     

MB-硬脂酸复合薄膜光波导传感器检测氯化氢气体

利用旋转甩涂法将亚甲基蓝(MB)掺杂的硬脂酸溶液涂成薄膜固定在钾离子(K⁺)交换玻璃光波导表面上, 研制了MB-硬脂酸复合薄膜/K⁺交换玻璃光波导传感器, 并对酸性气体进行了检测. 该复合薄膜与氯化氢(HCl)气体作用时, 薄膜颜色从深蓝色变为浅蓝色, 导致薄膜对倏逝波的吸收降低, 使传感器的输出光强度增强. 结果表明, 在室温下该传感器对低浓度的氯化氢气体仍具有较好的重复性和选择性响应, 可检测到体积分数为1×10^-6%的HCl气体, 响应和恢复时间分别为7和20 s, 相对标准偏差为±6.06%. 该传感器具有灵敏度高、 响应-恢复速度快、 可逆性好、 成本低和容易制备等特点.     

Planar waveguides for ultra-high sensitivity of the analysis of nucleic acids

In the first part of this paper, the need for analytical techniques capable of highly parallel and sensitive nucleic acid analysis, with the capability of achieving very low limits of detection (LODs) and of resolving small differences in concentration, is described. Whereas the requirement for performing simultaneously multi-analyte detection is solved by the approach of nucleic acid microarrays, requirements on sensitivity can often not be satisfied by classical detection technologies. Inherent limitations of conventional fluorescence excitation and detection schemes are identified, and the implementation of planar waveguides as analytical platforms for nucleic acid microarrays, with fluorescence excitation in the evanescent field associated with the guided excitation light, is proposed. The relevant parameters for an optimization of sensitivity are discussed.In the second part of this paper, the specific formats of our planar waveguide platforms, which are compatible with established industrial standard formats allowing for integration into industrial high throughput environments, are presented, as well as the dedicated optical system for fluorescence excitation and detection that we developed. In a direct comparison with a state-of-the-art scanner, it is demonstrated that the implementation of genomic microarrays on planar waveguide platforms allows for unprecedented, direct detection of low-abundant genes in limited amounts of sample. Otherwise, when using conventional fluorescence excitation and detection configurations, the detection of such low amounts of nucleic acids requires massive sample preparation and signal or target amplification steps.     

Ultra-sensitive mid-infrared evanescent field sensors combining thin-film strip waveguides with quantum cascade lasers

We demonstrate ultra-sensitive chemical sensing in the mid-infrared spectral regime with a combination of quantum cascade lasers (QCLs) with GaAs/Al(0.2)Ga(0.8)As strip waveguides fabricated via metal-organic vapor-phase epitaxy (MOVPE) and reactive ion etching (RIE) using evanescent field absorption spectroscopy. These strip waveguides have been designed with a width of 200 μm, thereby facilitating 2-D confinement and mode-matched propagation of mid-infrared radiation emitted from a distributed feedback (DFB) QCL at a wavelength of 10.3 μm. Acetic anhydride was detected with a limit of detection (LOD) of 18 pL (19.4 ng) deposited at the waveguide surface by overlapping of the vibrational absorption of the methyl group with the emission frequency of the QCL. The obtained results indicate a remarkable enhancement in sensitivity by three orders of magnitude compared to previously reported multimode planar silver halide waveguides. Further reduction of the waveguide strip width to 50 μm resulted in an additional sensitivity enhancement yielding a calculated LOD of 0.05 pL for the exemplary analyte acetic anhydride, which is among the most sensitive evanescent field absorption measurements with a miniaturized mid-infrared sensor system reported to date.     

Electroactive planar waveguide studies of Ru(bpy)3(2+) intercalated in a thin clay film: I. Transport and electrochemical phenomena

Electroactive planar waveguide (EAPW) instrumentation was used to perform potential modulated absorbance (PMA) experiments at indium tin oxide (ITO) electrodes coated with 0-, 300-, 800-, and 1200-nm-thick SWy-1 montmorillonite clay. PMA experiments performed at low potential modulation monitor mass transport events within 100 nm of the ITO surface and, thus, when used in conjunction with cyclic voltammetry (CV), can elucidate charge transport mechanisms. The data show that at very thin films electron transfer is controlled by electron hopping (sensitive to the anion species in the electrolyte) in an adsorbed Ru(bpy)(3)(2+) layer. As the thickness of the clay film grows, electron transfer may become controlled by mass transfer of Ru(bpy)(3)(2+) within the clay film to and from the electrode surface, a mechanism that is affected by the swelling of the film. Film swelling is controlled by the cation of the electrolyte. Films loaded with Ru(bpy)(3)(2+) while being subjected to evanescent wave stimulation demonstrate a large hydrophobic layer. The growth of the hydrophobic layer is attributed to the formation of Ru(bpy)(3)(2+*), which has negative charge located at the periphery of the molecule enhancing clay/complex repulsion. The results suggest that the structure of the film and the mechanism of charge transport can be rationally controlled. Simultaneous measurements of the ingress of Ru(bpy)(3)(2+) into the clay film by CV and PMA provide a means to determine the diffusion coefficient of the complex.     

Two-photon fluorescence excitation using an integrated optical microcavity: a promising tool for biosensing of natural chromophores

Application of an integrated optics (IO) microcavity (MC) for evanescent excitation of two-photon excited fluorescence (TPF) is demonstrated. The MC provides a high local intensity, which is required for the TPF, because of resonant enhancement of the intracavity power and a strong two-dimensional confinement of the guided mode. Numerical estimations show a large increase, by more than a factor of 10⁴ of the TPF intensity at the MC compared to a conventional straight waveguide. This will lead to a significant improvement of the detection limits of UV-absorbing chromophores (down to 10⁻⁸ M) when using the MC as a biosensor. Feasibility of TPF excitation using an IO MC is confirmed experimentally for the first time.     

Optical biosensors based on integrated polymer light source and polymer photodiode

Evanescent coupling is used to couple light from an organic Lambertian emitter into a single‐mode planar waveguide. A polymer light emitting diode pumps a photoluminescent layer located directly on top of the waveguide. At the out‐coupling grating stage, a fully organic mini‐spectrometer compatible with monolithical integration on optical bio chips has been developed. It consists of a single‐mode waveguide with integrated diffraction grating and a dense array of polymer photodiodes as sensing element. An overall spectral resolution of down to 5 nm has been achieved with the integrated optoelectronic system. As a proof of principle the fully organic optical device has been used in combination with a fluidic system to demonstrate an absorption‐based bio‐test with mouse immunoglobulin G. In a further step towards low‐cost and disposable lab‐on‐chip biosensors, the mentioned organic building blocks have been combined with a surface plasmon stack integrated directly onto the single mode waveguide. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Graphene-Based Waveguides: Novel Method for Detecting Biological Activity

We demonstrate the fabrication of a biosensor based on graphene coupled with polydimethylsiloxane (PDMS) waveguide. Biosensors work on the principle of local evanescent graphene-coupled wave sensor. It is observed that the evanescent field shifts in the presence of chemical or biological species as evanescent waves are extremely sensitive to a change in refractive index. This method helps to monitor the target analyte by attaching the selective receptor molecules to the surface of the PDMS optical waveguide resulting in its optical intensity distribution shift. We monitor the electrical properties of graphene in the dark and under illumination of PDMS waveguide. The changes in photocurrent through the graphene film were monitored for blue, green, and red light. We observed that the fabricated graphene-coupled PDMS optical waveguide sensor is sensitive to visible light for the used bioanalytes.     

Hybrid heterojunction solar cell based on organic-inorganic silicon nanowire array architecture

Silicon nanowire arrays (SiNWs) on a planar silicon wafer can be fabricated by a simple metal-assisted wet chemical etching method. They can offer an excellent light harvesting capability through light scattering and trapping. In this work, we demonstrated that the organic-inorganic solar cell based on hybrid composites of conjugated molecules and SiNWs on a planar substrate yielded an excellent power conversion efficiency (PCE) of 9.70%. The high efficiency was ascribed to two aspects: one was the improvement of the light absorption by SiNWs structure on the planar components; the other was the enhancement of charge extraction efficiency, resulting from the novel top contact by forming a thin organic layer shell around the individual silicon nanowire. On the contrary, the sole planar junction solar cell only exhibited a PCE of 6.01%, due to the lower light trapping capability and the less hole extraction efficiency. It indicated that both the SiNWs structure and the thin organic layer top contact were critical to achieve a high performance organic/silicon solar cell.     

Optical leaky waveguide biosensors for the detection of organophosphorus pesticides

Organophosphorus (OP) pesticides can be rapidly detected by integrating organophosphorus hydrolase with an optical leaky waveguide biosensor. This enzyme catalyses the hydrolysis of a wide range of organophosphorus compounds causing an increase in the pH. Thus, the direct detection of OP is possible by monitoring of the pH changes associated with the enzyme's activity. This article describes the use of an optical, leaky waveguide clad with absorbing materials for the detection of OP pesticides by measuring changes in refractive index, absorbance and fluorescence. In the most effective configuration, a thick sensing layer was used to increase the amount of immobilized enzyme and to increase the light interaction with the sensing layer, resulting in a greatly enhanced sensitivity. The platforms developed in this work were successfully used to detect paraoxon and parathion down to 4 nM concentrations.     

Design of new integrated optical substrates for immuno-analytical applications

Integrated optical Mach-Zehnder interferometers supply information on changes in refractive index and/or thickness of a film placed as a superstrate on top of one of its surface wave-guides. The internal propagation of light is influenced by the evanescent field reaching into the superstrate. This propagating light interferes with an uninfluenced wave in the second arm after recombination. The result is an intensity modulation depending on the refractive index parameters of the substrate, the waveguide itself and the properties of the superstrate. Taking an antigen layer as the superstrate, its interaction with antibodies changes its thickness by several nanometers. This can be observed by recording the change in intensity of the signal of the interferometer. The sensitivity of such a device depends on particular values of the optical parameters of substrate and waveguide with respect to the given superstrate properties. Computer calculations help to select optimum glass and waveguide fabrication conditions. The numerical results of a variety of assumed conditions have been tested experimentally. The application to the improved detection of triazines is discussed.     

Molecule‐Enhanced Surface‐Enhanced Infrared Absorption Spectroscopy (MOSEIRA)

Surface‐enhanced infrared absorption spectroscopy (SEIRA) of methanol, ethanol, 1‐propanol, and 2‐propanol in thin films on planar silver halide (AgX) fibers under slow N₂ flow using 1 sec scans reveals structure in absorbance–time plots. The absorption intensities show extra enhancements (3×) in the absorbance (O? H stretch) ascribed to oligomers present at the AgX surface (molecule enhanced, thus MOSEIRA).This is above those due to amplification (40×, 20 reflections) and enhancement (30×, image dipoles or surface phonon polaritons). In the case of ethanol an excellent initial pentamer spectrum evolves over 8–10 min to a mixture of pentamer, tetramer, and trimer spectra that within another minute forms small oligomers and monomers. We use a new type of cell for infrared spectroscopy containing an AgX planar fiber. The optical configuration leads to a vicinal region at the surface defined by evanescent waves. Within this region are surface‐induced organized species such as ethanol oligomers. The planar AgX fiber supports 20 reflections and transmits light over a wide visible–infrared wavelength range. Short scan times permit the study of volatile substrates or solvents, including the effects of solvent polarity.     

Gas chromatographic sensing on an optical fiber by mode-filtered light detection

A chemical sensor for gas phase measurements is reported which combines the principles of chemical separation and fiber optic detection. The analyzer incorporates an annular column Chromatographic sensor, constructed by inserting a polymer-clad optical fiber into a silica capillary. Light from a helium-neon laser is launched down the fiber, producing a steady intensity distribution within the fiber, but a low background of scattered light. When sample vapor is introduced to the sensor, and an analyte-rich volume interacts with the polymer cladding, Chromatographic retention is observed simultaneously with a change in the local refractive index of the cladding. An increase in cladding refractive index (RI) causes light to be coupled out of the fiber, with detection at a right-angle to the annular column length to provide optimum S/N ratio. This detection mechanism is called mode-filtered light detection. We report a gas Chromatographic separation on a 3.1 m annular column (320 microm i.d. silica tube, 228 microm o.d. fiber with a 12 microm fluorinated silicone clad) of methane, benzene, butanone and chlorobenzene in 6 min. The annular column length was reduced to 22 cm to function as a sensor, with selected organic vapors exhibiting unique retention times and detection selectivity. The detection selectivity is determined by the analyte RI and the partition coefficient into the cladding. The calculated limit of detection (LOD) for benzene vapor is 0.03% by volume in nitrogen, and several chlorinated species had LOD values less than 1%. For binary mixtures of organic vapors, the detected response appears to be the linear combination of the two organic standards, suggesting that the annular column may be useful as a general approach for designing chemical sensors that incorporate separation and optical detection principles simultaneously.     

Oriented structure of octadecyl acridine orange intercalated in the monolayer and Langmuir–Blodgett film of octadecyl adenine‐thymine base pairs

The oriented structure of acridine orange (AO) in both monolayer and Langmuir–Blodgett (LB) film has been studied by optical waveguide (OWG) spectroscopy using polarized incident light. Mixed monolayer and LB films, consisting of octadecyl acridine orange (C₁₈‐AO) incorporated in stacked base pairs of octadecyl adenine (C₁₈‐Ade) and octadecyl thymine (C₁₈‐Thy), were prepared on a quartz waveguide. Absorption of transverse electric field (TE) polarized light was about twice that of transverse magnetic field (TM) polarized light. Both OWG spectra have λ_max at 500 nm, which is characteristic of monomeric AO molecules. This result strongly suggests that C₁₈‐AO molecules were dispersed uniformly in the mixed monolayer and were excited more effectively by the TE polarized light. Since the absorption moment of AO molecules is related to their long axis, it is proposed that C₁₈‐AO molecules are incorporated in C₁₈‐Ade/C₁₈‐Thy pairs with the long axis parallel to the layer surface. The absorbance at 500 nm was proportional to the number of layers on the waveguide. The dichroic ratio of the absorbance at 500 nm for TE polarized light to that for TM polarized light was constant regardless of the number of layers. The C₁₈‐AO molecules were uniformly incorporated in each layer with the long axis relatively parallel to the layer surface. Copyright © 2004 John Wiley & Sons, Ltd.     

Grating coupler integrated photodiodes for plasmon resonance based sensing

In this work, we demonstrate an integrated sensor combining a grating-coupled plasmon resonance surface with a planar photodiode. Plasmon enhanced transmission is employed as a sensitive refractive index (RI) sensing mechanism. Enhanced transmission of light is monitored via the integrated photodiode by tuning the angle of incidence of a collimated beam near the sharp plasmon resonance condition. Slight changes of the effective refractive index (RI) shift the resonance angle, resulting in a change in the photocurrent. Owing to the planar sensing mechanism, the design permits a high areal density of sensing spots. In the design, absence of holes that facilitate resonant transmission of light, allows an easy-to-implement fabrication procedure and relative insensitivity to fabrication errors. Theoretical and experimental results agree well. An equivalent long-term RI noise of 6.3 × 10(-6) RIU/√Hz is obtained by using an 8 mW He-Ne laser, compared to a shot-noise limited theoretical sensitivity of 5.61 × 10(-9) RIU/√Hz. The device features full benefits of grating-coupled plasmon resonance, such as enhancement of sensitivity for non-zero azimuthal angle of incidence. Further sensitivity enhancement using balanced detection and optimal plasmon coupling conditions are discussed.