Aqueous Analytes Research Articles

Nanoparticle functionalised small-core suspended-core fibre - a novel platform for efficient sensing.

Detecting small quantities of specific target molecules is of major importance within bioanalytics for efficient disease diagnostics. One promising sensing approach relies on combining plasmonically-active waveguides with microfluidics yielding an easy-to-use sensing platform. Here we introduce suspended-core fibres containing immobilised plasmonic nanoparticles surrounding the guiding core as a concept for an entirely integrated optofluidic platform for efficient refractive index sensing. Due to the extremely small optical core and the large adjacent microfluidic channels, over two orders of magnitude of nanoparticle coverage densities have been accessed with millimetre-long sample lengths showing refractive index sensitivities of 170 nm/RIU for aqueous analytes where the fibre interior is functionalised by gold nanospheres. Our concept represents a fully integrated optofluidic sensing system demanding small sample volumes and allowing for real-time analyte monitoring, both of which are highly relevant within invasive bioanalytics, particularly within molecular disease diagnostics and environmental science.

Squeezed hollow-core photonic Bragg fiber for surface sensing applications

We propose to use squeezed hollow-core photonic bandgap Bragg fibers for surface sensing applications. We demonstrate theoretically and confirm experimentally that squeezing a section of the Bragg fiber core increases overlap between the optical fields of the core guided modes and the modes bound to the sensing layer, thus, significantly enhancing their interaction via anticrossing phenomenon, which, in turn, enhances surface sensitivity of the fiber sensor. As a practical demonstration, we apply our fiber sensor to in situ monitoring of the dissolution dynamics of a sub-micron-thick polyvinyl butyral (PVB) film coated on the surface of the liquid-filled Bragg fiber core. Strong spectral shift is observed during the dissolution of the PVB film, and a surface spectral sensitivity of ~0.07nm/nm is achieved experimentally with aqueous analytes. The proposed fiber sensor offers a new sensing modality and opens new sensing applications for photonic bandgap fibers, such as real-time detection of binding and affinity, study of kinetics, etc. for a range of chemical and biological samples.

Mid-infrared surface plasmon polariton chemical sensing on fiber-coupled ITO coated glass.

A novel fiber-coupled indium tin oxide (ITO) coated glass slide sensor for performing surface plasmon polariton chemical monitoring in the ∼3.5 μm mid-infrared (IR) range is reported. Efficient mid-IR fiber coupling is achieved with 3D laser written waveguides, and the coupling of glass waveguide modes to ITO surface plasmon polaritons (SPPs) is driven by the varying phase matching conditions of different aqueous analytes across the anomalous dispersion range determined by their molecular fingerprints. By means of using both a mid-IR fiber supercontinuum source and a diode laser, the excitation of SPPs is demonstrated. The sensor sensitivity is tested by discriminating CH from OH features of ethanol in water solutions, demonstrating an instrumental ethanol limit of detection of 0.02% in a wide concentration range of at least 0%-50%. The efficient optical monitoring of mid-IR SPPs in smart glass could have a broad range of applications in biological and chemical sensing.

Highly Sensitive, Bloch Surface Wave D-Type Fiber Sensor

A novel sensor based on a D-type optical fiber coated with a specially designed dielectric multilayer forming a one-dimensional photonic bandgap structure is proposed. The characteristics of its propagation modes are numerically analyzed at a wavelength of 785 nm to evaluate the sensor's performance. The results show that its sensitivity can drastically exceed that of its metal-coated, surface plasmon resonance counterpart, due to the sharp resonance and low wave-propagation loss of its Bloch surface wave. The novel sensor can also be easily adapted for all kinds of aqueous analytes of different refractive indices by modifying the photonic bandgap structure. This could lead to the development of compact, ultra-sensitive biochemical sensing devices.

Porous fluorine-doped tin oxide as a promising substrate for electrochemical biosensors—demonstration in hydrogen peroxide sensing

Conducting porous substrates of high hydrophilicity are advantageous in applications of electrochemical biosensors as host electrodes, offering not only fast charge transport and large sensing surface areas but also necessary wettability in aqueous analytes. In this study, inexpensive, highly hydrophilic (contact angle < 5°), conducting (sheet resistance of 17 Ω□-1) porous fluorine-doped tin oxide (FTO) glass is fabricated from commercial FTO glass with a novel, simple, one-step Sn4+-based anodic treatment process, and used as a host electrode for electrochemical biosensors. We demonstrate its superior performance with hydrogen peroxide sensing. The hydrogen peroxide sensor is fabricated by simply depositing Pt nanoparticles onto the surface of the porous FTO substrate (PFTO) with a polyol process. Pt-decorated commercial FTO (CFTO) is also investigated as a control. The sensitivity achieved with the Pt-decorated PFTO is almost one order of magnitude higher than that of the Pt-decorated CFTO (25.8 vs. 2.69 mA M-1), and the response time is shortened from 36 s for the Pt-decorated CFTO to 1 s for the Pt-decorated PFTO. The PFTO proves to be a promising electrode substrate for host functional materials in electrochemical biosensors and can be readily applied to sensing of a wide variety of biosubstances.

Multi-hole fiber based surface plasmon resonance sensor operated at near-infrared wavelengths

We propose a surface plasmon resonance (SPR) sensor based on a single mode optical fiber with six air holes. A thin gold film and a TiO2 film are deposited on the walls of air holes. Due to high refractive index of TiO2 the proposed sensor can operate at near-infrared (IR) wavelengths. The characteristics of the sensor were numerically investigated based on the finite-element method (FEM). The numerical results indicate that the optical loss spectrum of SPR sensor can be tuned easily by changing the thicknesses of gold and TiO2 layers. The refractive-index resolution of 2.7×10−5 (sensitivity S≈370/RIU) for aqueous analytes can be achieved.

All photonic bandgap fiber spectroscopic system for detection of refractive index changes in aqueous analytes

We propose and experimentally demonstrate a sensor of refractive index (RI) of aqueous analytes based on a hollow-core low-refractive-index-contrast Bragg fiber. Variations in refractive index of liquid analytes filling the hollow core of Bragg fiber could modify the resonant condition of the modal confinement, thus leading to both spectral shifts and intensity changes in the fiber transmission. As a proof-of-principle demonstration, we characterize sensor performance using a set of NaCl solutions with different concentrations. The experimental sensitivity of the Bragg fiber sensor is found to be ∼1400 nm/RIU (refractive index unit), which is comparable to those of surface plasmon resonance sensors. Moreover, we demonstrate the integration of a Bragg fiber bundle spectrometer in the liquid-core Bragg fiber sensing system to replace the grating-based monochromator, which could potentially lead to significant cost-saving and increased sensing speed. In summary, we demonstrate an all-photonic-bandgap-fiber RI sensing system that uses a hollow core Bragg fiber to hold and probe the analyte, and a solid-core Bragg fiber bundle for spectral interpretation of the transmitted light.

Optoelectrical Detection System Using Porous Silicon-Based Optical Multilayers

Porous silicon-based multilayer structures for optical sensors have been simulated, fabricated and tested. The properties of optical sensors using porous silicon multilayers can be adjusted by appropriate substrate material, morphology, process parameters in the pore formation process and by surface treatment (thermal oxidation). Heavily and lightly doped p-doped substrates have been used to realize porous silicon layers with different morphology, porosity (30%-80%), pore size (mesoporous range) and specific surface area (200-700 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ). Thermal oxidation stabilizes the surface and results in hydrophilic surfaces for effective adsorption of liquid analytes. Oxidation reduces the porosity and the pore size but improves the wetting behavior of liquid analytes in the porous volume. Different multilayer structures using native and oxidized porous silicon and corresponding concepts of optical sensor systems have been proved for aqueous and organic analytes. Sensors using small pore size (2-4 nm) and high porosity (70%-80%) have been realized and characterized. A simple, low cost optical sensor system based on multilayer, a LED-based illumination system providing discrete wavelengths (RGB) and a wide band detector has been realized and tested.

Liquid phase ion mobility spectrometry

A novel analytical method, called Liquid Phase Ion Mobility Spectrometry (LiPIMS) was demonstrated, where aqueous phase analytes were ionized and introduced into non-aqueous liquids, transported by an external electric field from the point of generation to a collection electrode. Ions were produced from a unique liquid phase ionization process, called Electrodispersion Ionization. Spectra of analyte ions illustrated the potential of LiPIMS as a new separation technique. Experimental data showed that electrodispersion ionization was effective in generating nanoampere level of ion current in hexane and benzene from aqueous samples. By controlling the ionization voltage in relation to the sample flow rate, it was possible to operate the electrodispersion ionization source in both continuous and pulsed ionization modes. Unique LiPIMS spectra of aqueous samples of tetramethylammonium bromide, tetrabutylammonium bromide and bradykinin were presented and their respected liquid phase ion mobility values were determined.

Development of dispersive liquid-phase microextraction based on new ionic liquid 1,3-diisooctylimidazolium hexafluorophosphate as solvent for the extraction and determination of dicofol and its degradation products in water samples

In the current paper we describe a novel sample preparation technique termed dispersive liquid-phase microextraction for the preconcentration and determination of 2,2,2-trichloro-1,1-bis(4-chlorophenyl)ethanol (dicofol) and its degradation products in water samples that includes 2-(2-chlorophenyl)-2-(4-chlorophenyl)-1,1-dichloroethene(2,4′-DDE), 1,1-dichloro-2,2-bis(4-chlorophenyl)ethane(4,4′-DDE) and 1,1,1-trichloro-2-(2-chlorophenyl)-2-(4-chlorophenyl)ethane (2,4′-DDT) coupled with gas chromatography mass spectrometry (GC-MS), in which a new ionic liquid 1,3-diisooctylimidazolium hexafluorophosphate abbreviated as [D(i-C8)IM][PF6] was used as extraction solvent. For each one extraction, 1.00 mL of the methanol solution containing 40 µL of the ionic liquid was sprayed into 25.00 mL of water sample. In the meantime the ionic liquid was finely dispersed into the aqueous phase and analytes were rapidly migrated into the ionic liquid. After the solution was centrifuged for 2 min at 5000 rpm, the droplets of the ionic liquid are subsided in the bottom of the conical test tube (30.0 ± 0.2 µL). Moreover, the factors relevant to extraction efficiencies were investigated and optimised including the volume of the ionic liquid, disperser solvent, extraction time, sample pH and ionic strength. Under optimal conditions, the enrichment factors of the extraction were between 550 and 725 with an extraction efficiency ranging from 66% to 87% for each different analyte. Finally, 1.0 µL of the ionic liquid collected from above extraction was injected into the injector block of GC-MS instrument for analysis. The detection limit (S/N = 3), the relative standard deviations for 2.0 µg L−1 of the standard analyte (n = 5) and linearity in a calibration range were found to be 3–8 ng L−1, 1.0–2.7% and 10–3000 ng L−1, respectively. Good spiked recoveries over the range of 92.0–13.5% were obtained. The proposed method offers the advantages of simplicity of operation, rapidity, good extraction efficiency and enrichment factor; it has been successfully applied to determination of dicofol and its degradation products in environmental water samples.

Surface-plasmon-resonance sensor based on three-hole microstructured optical fiber

We propose a novel surface-plasmon-resonance sensor design based on coating the holes of a three-hole microstructured optical fiber with a low-index dielectric layer on top of which a gold layer is deposited. The use of all three fiber holes and their relatively large size should facilitate the fabrication of the inclusions and the infiltration of the analyte. Our numerical results indicate that the optical loss of the Gaussian guided mode can be made very small by tuning the thickness of the dielectric layer and that the refractive-index resolution for aqueous analytes is 1x 10(-4).

Sol–gel immobilized cyano-polydimethylsiloxane coating for capillary microextraction of aqueous trace analytes ranging from polycyclic aromatic hydrocarbons to free fatty acids

Sol–gel coating containing highly polar cyanopropyl and nonpolar poly(dimethylsiloxane) components (sol–gel CN-PDMS coating) was developed for capillary microextraction (CME). The sol–gel chemistry provided an efficient means to immobilize the CN-PDMS coating by establishing chemical anchorage between the coating and the fused silica capillary inner surface. This chemical bond provided excellent thermal and solvent stability to the created sol–gel coating. For the extraction of polar and nonpolar analytes, the upper allowable conditioning temperatures were 330 °C and 350 °C, respectively. To our knowledge, this is the first time when a CN-PDMS thick coating survived such a high operation temperature. The prepared sol–gel CN-PDMS coating provided effective extraction of polar and nonpolar analytes simultaneously from aqueous samples. The cyanopropyl moiety in sol–gel CN-PDMS coatings provided effective extraction of highly polar analytes such as free fatty acids, alcohols, and phenols without requiring derivatization, pH adjustment or salting out procedures. The PDMS moiety, on the other hand, provided efficient extraction of nonpolar analytes. The extraction properties of the sol–gel CN-PDMS coatings can be fine tuned via manipulation of relative proportions of 3-cyanopropyltriethoxysilane and hydroxy-terminated PDMS in the sol solution used to create the coatings. Detection limits of nanogram/liter (ng/L) were achieved for both highly polar and nonpolar analytes directly extracted from aqueous media using sol–gel CN-PDMS coated microextraction capillaries followed by GC analysis.

ATR-FTIR spectroscopic analysis of sorption of aqueous analytes into polymer coatings used with guided SH-SAW sensors

Attenuated total internal reflectance Fourier transform infrared (ATR-FTIR) spectroscopy was used for the investigation of sorption of aqueous solutions of analytes into polymer coatings. A series of simple model polymers, such as poly(dimethylsiloxane), poly(epichlorhydrin), and poly(isobutylene), and films and analytes, such as aqueous solutions of ethylbenzene, xylenes, toluene, and nitrobenzene, were used to evaluate the use of ATR-FTIR spectroscopy as a screening tool for sensor development. The ratios of integrated infrared absorption bands provided a simple and efficient method for predicting trends in partition coefficients. Responses of polymer-coated guided shear horizontal surface acoustic wave (SH-SAW) sensor platforms to the series of analytes, using polymer coatings with similar viscoelastic properties, were consistent with ATR-FTIR predictions. Guided SH-SAW sensor responses were linear in all cases with respect to analyte concentration in the tested range. Comparison of ATR-FTIR data with guided SH-SAW sensor data identifies cases where mass loading is not the dominant contribution to the response of the acoustic wave sensor. ATR-FTIR spectra of nitrobenzene, coupled with computational chemistry, provided additional insight into analyte/polymer interactions.

Performance of a photothermal detector with turbid liquids

A closed-cell photothermal detector for aqueous analytes has been evaluated at 254 and 678 nm. We used a detector with a water meniscus as a pressure sensor, whose periodic deflection was measured using a low-finesse optical fiber Fabry-Perot interferometer. Performance was compared with a commercial diode array spectrometer and found to be similar for absorption measurements in nonturbid samples, but the results were affected up to 60 times less by scattered light. Finally the photothermal cell was converted into an integrating cavity using ceramic inserts, showing freedom from scattering-related errors at 678 nm but a degradation in performance at 254 nm.

A miniaturized glow discharge applied for optical emission detection in aqueous analytes

DC glow discharge atomic emission spectroscopy of aqueous analytes has been performed in a planar glass microstructure at close to atmospheric pressure using argon as a carrier gas. Sample introduction was achieved using a technique in which the liquid sample itself is employed as the cathode for the glow discharge. To attain sufficient transport of the sample into the discharge region to allow for emission spectroscopy, acidification of the liquid sample is required. Results demonstrate the detection of copper in solution is possible with the observation of five atomic emission lines of copper (at 325 nm, 327 nm, 511 nm, 515 nm and 522 nm). It is hoped that this detection method could be integrated with other micro-fluidic systems as part of a μ-TAS device as well as being useful as a portable, stand-alone detector. However, stability issues will need to be resolved to allow for further optimization and characterization of the device.

A novel silicon-based sensor array with capacitive EIS structures

Abstract A modified C / V (capacitance/voltage) measuring set-up for the synchronous operation of several EIS (electrolyte–insulator–semiconductor) sensors was developed. Therewith, the simultaneous detection of different substances in aqueous analytes is possible by using capacitive sensors that can easily be manufactured. This novel method is exemplarily demonstrated for a sensor array based on a pH-sensitive EIS structure (Al/Si/SiO 2 /Si 3 N 4 ) and a penicillin-sensitive EIS structure (Al/Si/SiO 2 /Si 3 N 4 /penicillinase). Both sensors are connected in parallel with each other. By using two additional d.c. voltage sources the single C / V curves are shifted against each other along the voltage axis. The resulting `step-like' C / V curve allows the distinct assignment of each step fraction to the corresponding sensor, and thus the simultaneous detection of the pH and penicillin concentration in a single measurement.

Rapid, noninvasive concentration measurements of aqueous biological analytes by near-infrared Raman spectroscopy

Accurate concentration measurements of glucose, lactic acid, and creatinine in saline solution have beena chieved with near-IR Raman spectroscopy and a partial least-squares analysis. The Raman spectra were acquired remotely through optical fibers. A root-mean-squared prediction error of 1.2 mM for glucose concentration was achieved in 100 s. Concentrations of other analytes were predicted with similar accuracy.

Concentration and Optical Measurement of Aqueous Analytes in an Organic Solvent Segmented Capillary under High Electric Field

Concentration and Optical Measurement of Aqueous Analytes in an Organic Solvent Segmented Capillary under High Electric Field

An investigation of the effects of microstructure on the fatigue and fracture behavior of α2 + β forged Ti24Al11Nb : Soboyejo, W.O. Metall. Trans. A June 1992 23A (6) 1737–1750

We propose a surface plasmon resonance (SPR) sensor based on a single mode optical fiber with six air holes. A thin gold film and a TiO2 film are deposited on the walls of air holes. Due to high refractive index of TiO2 the proposed sensor can operate at near-infrared (IR) wavelengths. The characteristics of the sensor were numerically investigated based on the finite-element method (FEM). The numerical results indicate that the optical loss spectrum of SPR sensor can be tuned easily by changing the thicknesses of gold and TiO2 layers. The refractive-index resolution of 2.7×10−5 (sensitivity S≈370/RIU) for aqueous analytes can be achieved.

Total N-nitroso group analysis of foods. II. Further studies on the precision and sensitivity of the assay.

The total N-nitroso content of foods can be measured by chemical denitrosation with hydrogen bromide and chemiluminescence detection of the cleaved nitric oxide radical. The denitrosation reagent itself causes a significant detector response which has limited the application of the technique to trace analysis. A procedure is described in which the errors associated with this interference are minimized. Application of this method to the trace analysis of aqueous and solid samples is reported together with an investigation of the effects of sample size on the accuracy and sensitivity of the assay as applied to aqueous analytes. The magnitude and significance of the false-positive response from nitrate is discussed in relation to the analysis of cured meats.