Polymeric nanofilm-coated optical fibre sensor for speciation of aromatic compounds

An optical fibre sensor has been shown to be suitable for monitoring of benzene, toluene and o-xylene (BTX) with both high selectivity and sensitivity. The sensing principle underlying this experimental device is based on the changes of the reflected optical power when BTX vapours are present in the analytical tube containing an optical fibre coated with a thin film of poly[methyl(3, 3, 3-trifluoropropyl)siloxane]. The interaction of organic vapour with the sensitive surface promotes a variation of the light power, proportional to the amount of adsorbed BTX vapour. A set of experiments concerning different operational conditions was performed in order to promote a higher analytical performance and the newly developed BTX sensor showed higher sensitivity and shorter analytical time than a method based on gas chromatography–flame ionisation detector. Furthermore, the proposed sensor also provides the basis for an inexpensive analytical technique with adequate specificity for measurements of BTX at trace levels with appropriate reversibility, repeatability, and reproducibility. Finally, the analytical performance of the developed sensor was also evaluated and found adequate for industrial air samples.     

Carbon nanotube field-effect transistor detector associated to gas chromatography for speciation of benzene,toluene, ethylbenzene, (o-, m- and p-)xylene

An analytical methodology based on a field-effect transistor detector using carbon nanotubes (NTFET) coupled to a gas chromatograph has been developed for the speciation of the following aromatic compounds: benzene, toluene, ethylbenzene, m-xylene, p-xylene and o-xylene (BTEX). This methodology combines the proven separation capability of gas chromatography (GC) with the potential for detection of a NTFET. The developed analyzer shows a high and stable analytical response upon repeated analysis of BTEX during 4 weeks, with detection limit less than 4 μg/L. The GC–NTFET system also shows a great suitability for actual monitoring of indoor atmospheres and no significant difference was observed between the results obtained by the developed analyzer and a more classical analytical methodology, namely gas chromatography–flame ionization detection (GC–FID).     

Breath analysis by optical fiber sensor for the determination of exhaled organic compounds with a view to diagnostics

Breath analysis constitutes a promising tool in clinical and analytical fields due to its high potential for non-invasive diagnostics of metabolic disorders and monitoring of disease status. An optical fiber (OF) sensor has been developed for determination of volatile organic compounds (ethane, pentane, heptane, octane, decane, benzene, toluene and styrene) in human breath for clinical diagnosis.The analytical system developed showed a high performance for breath analysis, inferred for the analytical signal intensity and stability, linear range, and detection limits ranging from 0.8 pmol L⁻¹, for heptane, and to 9.5 pmol L⁻¹, for decane. The OF sensor also showed advantageous features of near real-time response and low instrumentation costs, besides showing an analytical performance equivalent to the breath analysis by gas chromatography-mass spectrometry (GC-MS), used as the reference method.     

Comparison of a gas chromatography-optical fibre (GC-OF) detector with a gas chromatography-flame ionization detector (GC-FID) for determination of alcoholic compounds in industrial atmospheres

An analytical methodology based on an optical fibre detector coupled to gas chromatograph has been developed for the speciation of some volatile alcoholic compounds. This methodology combines the separation capability of gas chromatography with an optical fibre detector made of an optical fibre sensitized with a thin polymeric film of poly[methyl(3,3,3-trifluoropropyl)siloxane] (PMTFPS). The response of the detector has been characterized at 650 nm for nine different alcohols (allyl alcohol, n-propyl alcohol, sec-butyl alcohol, isobutyl alcohol, n-butyl alcohol, isoamyl alcohol, methyl isobutyl carbinol, cyclohexanol and diacetone alcohol). An alternative method based on gas chromatography-flame ionization detector (GC-FID) was also used in order to evaluated the performance and compare the analytical results with the proposed method. The time of analysis, the analytical error and the analytical performance were similar for both methods. However, the analytical apparatus based on the GC-OF detector is much less expensive than the GC-FID and show high accuracy and suitability for actual monitoring on indoor atmospheres.     

Demonstration of a simple, economical and practical technique utilising an imprinted polymer for metal ion sensing

This paper describes a simple, economical and practical optical sensor that has been developed using an ion imprinted polymer for detecting copper(II) ion using reflectance spectrometry. The imprinted polymer was synthesised in the presence of copper(II) ion using 4-vinylpyridine as monomer, 2-hydroxyethyl methacrylate as co-monomer, and ethylene glycol dimethacrylate as cross-linker in methanol via free radical polymerisation. The polymer formed was physically ground into fine particles, fabricated into a sensor probe and attached at the tip of an optical fibre bundle. The sensor operates optimally at pH 5 under constant stirring condition and has a linear dynamic range for copper(II) ion determination of 0.4–4.0 mM. The limit of detection for the sensor was 123 µM. The sensor has high selectivity for monitoring copper (II) ion in a multi-analyte environment, while also exhibiting better sensitivity compared to non-imprinted polymer. The response of the sensor can be regenerated fully without significant loss in its analytical signal for re-use. This reduces the consumable cost and at the same time making the operational process of such sensor simpler and more practical to be employed in real applications.     

Optical fibre sensor for hydrogen sulphide monitoring in mouth air

A reversible optical fibre chemical sensor for hydrogen sulphide monitoring in mouth air based on reflectance measurements has been developed. The active sensing phase has been prepared by immobilising the colorimetric reagent 2,6-dichlorophenolindophenol (DCPI) in a silica gel support. The principle of the determination is based on the increase of reflectance of such solid sensing phase when hydrogen sulphide reduces the colorimetric reagent with the subsequent decolouration process. The addition of 1.26 μg of Cu(II) per gram of solid support improved the response time and reversibility of the sensing phase.The detection limit is 10 ppb (v/v) of hydrogen sulphide. The linear range using the Kubelka-Munk function extends at least up to 1000 ppb (v/v). The sensor exhibits a response time of less than 2 min for hydrogen sulphide concentrations in the linear range and the signal is reversible.The optical sensor has been successfully tested for human malodour monitoring and the results validated by comparison with those obtained for the same individuals using a commercially available electrochemical instrument.     

Fluorescence resonance energy transfer disposable sensor for copper(II)

A disposable sensor has been developed for the measurement of copper(II) concentration in aqueous solution based on a change in the fluorescence of porphyrazine 2,7,12,17-tetra-tert-butyl-5,10,15,20-tetraaza-21H,23H-porphine (TP). The sensor was constructed by spin-coating a polyester support with a PVC solution containing TP, a plasticizer, the chelating agent Zincon and the ion-pairing benzetonium chloride. The measurement principle is based on the radiationless resonance energy transfer (RET) from TP immobilized in membrane, and acting as fluorescence donor, to Zincon acting as an acceptor induced by copper(II). The absorption spectrum of the Zincon-Cu(II) complex presents adequate overlapping with the emission spectrum of TP, producing a useful analytical signal by the RET process.The disposable sensor responds to copper(II) irreversibly over a dynamic range from 0.039 to 14 μmol L⁻¹ (2.5-890 μg L⁻¹) with a sensor-to-sensor reproducibility (relative standard deviation RSD) of 1.9%, as log aCu2+, at the medium level of the range and a response time of 10 min. The performance of the optical disposable sensor was tested for the analysis of copper in different types of natural waters (river, well, spring and swimming pool), validating results against a reference procedure.     

A New Sol-Gel Derived Optical Fiber Sensor for High Acidity Measurements: Application in Nuclear Fuel Reprocessing

Nuclear fuel reprocessing requires high acidity levels that should be accurately and rapidly controlled in order to optimize extraction yields and process efficiency. A new optical fiber sensor based on a sol-gel film doped with an acid-sensitive indicator has been developed to improve remote and on-line monitoring.The organic molecule was physically incorporated in an acid-catalyzed sol-gel matrix and coated onto the core of a denuded optical fiber. The sensor response was evaluated in synthetic solutions of nitric acid, concentrations ranging from 1 to 10 mol · l^–1. Changes in absorption properties and response characteristics of the sensor, when compared to the indicator in solution, have been discussed. Analytical performances of the sensor, i.e. sensitivity, response time, reversibility and repeatability are very encouraging for the on-line analysis. Viability of the sensor for process measurements depends on the stability which might be improved by a strict control of the sensitive film porosity.     

Near infrared (NIR) spectroscopy for in-line monitoring of polymer extrusion processes

In recent years, near infrared (NIR) spectroscopy has become an analytical tool frequently used in many chemical production processes. In particular, on-line measurements are of interest to increase process stability and to document constant product quality. Application to polymer processing e.g. polymer extrusion, could even increase product quality. Interesting parameters are composition of the processed polymer, moisture, or reaction status in reactive extrusion. For this issue a transmission sensor was developed for application of NIR spectroscopy to extrusion processes. This sensor includes fibre optic probes and a measuring cell to be adapted to various extruders for in-line measurements. In contrast to infrared sensors, it only uses optical quartz components. Extrusion processes at temperatures up to 300 degrees C and pressures up to 37 MPa have been investigated. Application of multivariate data analysis (e.g. partial least squares, PLS) demonstrated the performance of the system with respect to process monitoring: in the case of polymer blending, deviations between predicted and actual polymer composition were quite low (in the range of +/-0.25%). So the complete system is suitable for harsh industrial environments and could lead to improved polymer extrusion processes.     

Phase Selective Alternating Current Voltammetric Signalling Protocol: Application in Dendritic Co‐polymer Sensor for Anthracene

A sensitive anthracene (AN) sensor was developed with a gold electrode modified with a dendritic star‐copolymer film by in situ electrochemical co‐polymerization of generation 3 propylenethiophenoimine and 3‐hexylthiophene. The sensor's {Au/G3PPT‐co‐P3HT: i. e. gold‐generation 3 poly(propylenethiophenoimine)‐co‐poly(3‐hexylthiophene)} analytical response for anthracene was obtained by phase selective alternating current voltammetric (PSACV) signal transduction. An in‐phase angle of 0° yielded the most sensitive stripping signal and produced the best discrimination between the Faradaic and capacitive currents. The PSACV sensor exhibited a linear range (LR) of 3.48–56.4 nmol/L AN and a limit of detection (LOD) of 2.62 nmol/L AN. The LOD is comparable to the value of 4.4 nmol/L AN reported for glassy carbon electrode modified with graphenated polyaniline sensor. The low LOD value suggests that the AN sensor has promise for monitoring compliance to World Health Organisation (WHO) approved limit for polyaromatic hydrocarbons (PAHs) in wastewater (3.93 nmol/L). The Au/G3PPT‐co‐P3HT sensor is not as sensitive as gas chromatography coupled to tandem mass spectrometry (GC‐MS/MS) and reversed‐phase high performance liquid chromatography (RP‐HPLC) methods. However, the advantage of the PSACV signalling protocol is that real sample test results indicate that the sensor can be used for the determination of AN in oil‐polluted wastewater.     

Development of a versatile,easy and rapid atmospheric monitor for benzene,toluene, ethylbenzene and xylenes determination in air

A new procedure for the passive sampling in air of benzene, toluene, ethylbenzene and xylene isomers (BTEX) is proposed. A low-density polyethylene layflat tube filled with a mixture of solid phases provided a high versatility tool for the sampling of volatile compounds from air. Several solid phases were assayed in order to increase the BTEX absorption in the sampler and a mixture of florisil and activated carbon provided the best results. Direct head-space-gas chromatography–mass spectrometry (HS-GC–MS) measurement of the whole deployed sampler was employed for a fast determination of BTEX. Absorption isotherms were used to develop simple mathematical models for the estimation of BTEX time-weighted average concentrations in air. The proposed samplers were used to determine BTEX in indoor air environments and results were compared with those found using two reference methodologies: triolein-containing semipermeable membrane devices (SPMDs) and diffusive Radiello samplers. In short, the developed sampling system and analytical strategy provides a versatile, easy and rapid atmospheric monitor (VERAM).     

Simultaneous analysis of 23 priority volatile compounds in water by solid-phase microextraction–gas chromatography–mass spectrometry and estimation of the method's uncertainty

Most water contaminations with volatile organic compounds (VOCs) are traceable to leaking underground fuel reservoirs, solvent storage vessels, agricultural practices, industrial residues, and deficient wastewater treatment and disposal. In order to perform effective monitoring of such organic micropollutants in a straightforward manner, a multiresidue method for the determination of 23 VOCs (trihalomethanes (THMs), BTEX and chlorinated solvents) in water has been developed using solid-phase microextraction (SPME) and capillary gas chromatography–mass spectrometry (GC–MS). This group includes also methyl-tert-butyl ether, epichlorhydrine, and vinyl chloride which present additional analytical difficulties. Three different fibres were assayed: 7-µm polydimethylsiloxane (PDMS), 100-µm PDMS, and 75-µm Carboxen-PDMS, and the extraction conditions were optimized. The best results for the majority of the analytes and mainly for those with the lowest signals were obtained using the Carboxen-PDMS fibre after 15 min of extraction in the headspace mode at a room temperature of 20 ± 2°C. The analytical sensitivity, linearity, precision, accuracy, and uncertainties have been studied for method validation in agreement with the international standard ISO/IEC 17025:2005. The limits of detection achieved with the proposed method (0.06–0.17 µg L^?1) are adequate to determine the VOCs at the restrictive levels established by the European legislation. This was a decisive achievement to enable the analysis of all VOCs listed under the drinking-water directive in a single assay. The method exhibits performance capabilities suitable for routine analysis of VOCs in drinking-water by quality-control laboratories as enforced by EU Directives. The method is currently being used for this purpose, and participation in proficiency tests was assessed, with encouraging results.     

Determination of benzene, toluene, ethylbenzene and xylenes in air by solid phase micro-extraction/gas chromatography/mass spectrometry

The aim of the study was to analyse BTEX compounds (benzene, toluene, ethylbenzene, xylenes) in air by solid phase micro-extraction/gas chromatography/mass spectrometry (SPME/GC/MS), and this article presents the features of the calibration method proposed. Examples of real-world air analysis are given. Standard gaseous mixtures of BTEX in air were generated by dynamic dilution. SPME sampling was carried out under non-equilibrium conditions using a Carboxen/PDMS fibre exposed for 30 min to standard gas mixtures or to ambient air. The behaviour of the analytical response was studied from 0 to 65 g/m³ by adding increasing amounts of BTEX to the air matrix. Detection limits range from 0.05 to 0.1 g/m³ for benzene, depending on the fibre. Inter-fibre relative standard deviations (reproducibility) are larger than 18%, although the repeatability for an individual fibre is better than 10%. Therefore, each fibre should be considered to be a particular sampling device, and characterised individually depending on the required accuracy. Sampling indoor and outdoor air by SPME appears to be a suitable short-delay diagnostic method for volatile organic compounds, taking advantage of short sampling time and simplicity.     

Optical fiber biosensor coupled to chromatographic separation for screening of dopamine, norepinephrine and epinephrine in human urine and plasma

An optical fiber biosensor has been developed for the determination of catecholamines (dopamine, norepinephrine and epinephrine) based on the recognition capacity of the enzyme laccase. In this study, a glass tube constituted by a fused silica fiber coated with a film of polystyrene/divinylbenzene resin (PS/DVB) was used for catecholamines separation. Firstly, the analyzer was tested for calibration and its analytical performance for catecholamines detection was compared with a classical analytical method, namely high performance liquid chromatography-electrochemical detector (HPLC-ED). The developed analytical device shows a high potential for catecholamines quantification with a detection limit of 2.1, 2.6 and 3.4 pg mL⁻¹ for dopamine, norepinephrine and epinephrine, respectively. The analytical sensitivity, inferred from the slope of the calibration curves established for a range of concentrations between 5 and 125 pg mL⁻¹, was found to be 0.344, 0.252 and 0.140 dB/pg mL⁻¹ for dopamine, norepinephrine and epinephrine, respectively. Furthermore, catecholamines speciation with the PS/DVB fiber was completely achieved in 3 min. The analytical performance of the reported sensor was also evaluated and found adequate for catecholamines determination in human urine and plasma samples.     

Potentiometric Surfactant Sensor Based on 1,3-Dihexadecyl-1H-benzo[d]imidazol-3-ium for Anionic Surfactants in Detergents and Household Care Products

A 1,3-dihexadecyl-1H-benzo[d]imidazol-3-ium-tetraphenylborate (DHBI-TPB) ion-pair implemented in DHBI-TPB surfactant sensor was used for the potentiometric quantification of anionic surfactants in detergents and commercial household care products. The DHBI-TPB ion-pair was characterized by FTIR spectroscopy and computational analysis which revealed a crucial contribution of the C–H∙∙∙π contacts for the optimal complex formation. The DHBI-TPB sensor potentiometric response showed excellent analytical properties and Nernstian slope for SDS (60.1 mV/decade) with LOD 3.2 × 10⁻⁷ M; and DBS (58.4 mV/decade) with LOD 6.1 × 10⁻⁷ M was obtained. The sensor possesses exceptional resistance to different organic and inorganic interferences in broad pH (2–10) range. DMIC used as a titrant demonstrated superior analytical performances for potentiometric titrations of SDS, compared to other tested cationic surfactants (DMIC > CTAB > CPC > Hyamine 1622). The combination of DHBI-TPB sensor and DMIC was successfully employed to perform titrations of the highly soluble alkane sulfonate homologues. Nonionic surfactants (increased concentration and number of EO groups) had a negative impact on anionic surfactant titration curves and a signal change. The DHBI-TPB sensor was effectively employed for the determination of technical grade anionic surfactants presenting the recoveries from 99.5 to 101.3%. The sensor was applied on twelve powered samples as well as liquid-gel and handwashing home care detergents containing anionic surfactants. The obtained results showed good agreement compared to the outcomes measured by ISE surfactant sensor and a two-phase titration method. The developed DHBI-TPB surfactant sensor could be used for quality control in industry and has great potential in environmental monitoring.     

A Mid-Infrared Sensor for Monitoring of Chlorinated Hydrocarbons in the Marine Environment

Abstract

As part of the European research project SOFIE - “Spectroscopy using Optical Fibres in the Marine Environment”, a portable sensor system for chlorinated hydrocarbons in seawater is being developed. This novel analytical tool for real-time in-situ monitoring of a particularly important class of seawater pollutants consists of a robust, miniaturised FT-IR spectrometer in a sealed aluminium pressure vessel and a fibre optic sensor head. In a laboratory set-up using an ATR-crystal as a simplified sensor head, the effect of potentially interfering substances, both of natural and anthropogenic origin, on the sensor response was tested. It was found that the sensor readings for a specific analyte are not susceptible to aliphatic and aromatic components as well as other chlorinated hydrocarbons up to concentrations well above the average levels to be encountered in the oceans. The same applies for the parameters salinity and turbidity. Consequently, the proposed sensor system should be well suited for real-world sub-sea applications.     

Gas chromatography-optical fiber detector for the speciation of aromatic hydrocarbons in confined areas

An analytical method, based on separation with gas chromatography (GC) and detection with optical fiber (OF), was used for the separation, detection and quantification of benzene, toluene, ethylbenzene, p-xylene, m-xylene and o-xylene. The use of OF as a detector is based on the variations of the reflected optical power detected when the aromatic compounds eluted from the GC column are sorbed in a thin polymeric film on a single-mode OF. General figures of merit, such as the analytical time, analytical error and analytical performance of GC-OF were similar to those of the classical analytical methods, such as a gas chromatography-flame ionization detector (GC-FID). However, the developed GC-OF method constitutes a much less expensive alternative for the speciation of aromatic hydrocarbons compounds, with high accuracy, and being most suitable for actual monitoring work on confined environments.     

Analytical performance of an optical pH sensor for acid-base titration

The analytical performance of a reflectance optical fibre sensor is described for the determination of pH, particularly that of acid-base titration end-points. The sensor makes use of cresol red immobilized on the anion-exchange resin Dowex 1-X10, has a pH transition interval in the range 5.5–7.5 and uses a PTFE membrane as interface. The sensor r.s.d. is ca. 1% and the accuracy is excellent. It is useful for titrating strong and weak acids (such as HF) with bases and vice versa, and requires very small sample volumes. The sensor can titrate turbid and coloured solutions at different temperatures. The minimum response time necessary after each titrant addition is in the range 0.5–1 min.     

Nanostructured optical fibre arrays for high-density biochemical sensing and remote imaging

Optical fibre bundles usually comprise a few thousand to tens of thousands of individually clad glass optical fibres. The ordered arrangement of the fibres enables coherent transmission of an image through the bundle and therefore enables analysis and viewing in remote locations. In fused bundles, this architecture has also been used to fabricate arrays of various micro to nano-scale surface structures (micro/nanowells, nanotips, triangles, etc.) over relatively large areas. These surface structures have been used to obtain new optical and analytical capabilities. Indeed, the imaging bundle can be thought of as a “starting material” that can be sculpted by a combination of fibre drawing and selective wet-chemical etching processes. A large variety of bioanalytical applications have thus been developed, ranging from nano-optics to DNA nanoarrays. For instance, nanostructured optical surfaces with intrinsic light-guiding properties have been exploited as surface-enhanced Raman scattering (SERS) platforms and as near-field probe arrays. They have also been productively associated with electrochemistry to fabricate arrays of transparent nanoelectrodes with electrochemiluminescent imaging properties. The confined geometry of the wells has been loaded with biosensing materials and used as femtolitre-sized vessels to detect single molecules. This review describes the fabrication of high-density nanostructured optical fibre arrays and summarizes the large range of optical and bioanalytical applications that have been developed, reflecting the versatility of this ordered light-guiding platform.     

Smartphone-Assisted Protein to Creatinine Ratio Determination on a Single Paper-Based Analytical Device

Proteinuria is a condition in which an excessive amount of protein is excreted in urine. It is, among others, an indicator of kidney disease or risk of cardiovascular disease. Rapid and reliable diagnosis and monitoring of proteinuria is of great importance for both patients and their physicians. For that reason, a paper-based sensor for proteinuria diagnosis was designed, optimized, and validated utilizing smartphone-assisted signal acquisition. In the first step, a few commonly employed protein assays were optimized and compared in terms of analytical performance on paper matrix. The tetrabromophenol blue method was selected as the one providing a sufficiently low limit of detection (39 mg·L⁻¹) on the one hand and appropriate long-term stability (up to 3 months) on the other hand. The optimized assay was employed for protein-to-creatinine ratio (PCR) determination on a single paper-based sensor. For both analytes the linear ranges were within the clinically relevant range. The analytical usefulness of the developed sensors was demonstrated by a PCR recovery study in artificial urine. The obtained PCR recoveries were from ca. 80 to 150%.