Monolayer-functionalized microfluidics devices for optical sensing of acidity

This paper describes the integration of opto-chemosensors in microfluidics networks. Our technique exploits the internal surface of the network as a platform to build a sensing system by coating the surface with a self-assembled monolayer and subsequently binding a fluorescent sensing molecule to the monolayer. Fluorescent molecules were used that can switch between a fluorescent and a non-fluorescent state, depending on the acidity of the surrounding solution. Two systems were investigated. The first employs surface confinement of a Rhodamine B dye in a glass micro channel that serves as a molecular switch in organic solutions. Upon rinsing the micro channels with acidic or basic solutions it was possible to switch between the fluorescent and non-fluorescent forms reversibly. Moreover, this system could be used to monitor the mixing of two solutions of different acidity along the micro channel. To widen the scope of optical sensing in micro channels an Oregon Green dye derivative was immobilized, which functions as a sensing molecule for pH differences in aqueous solutions. In this case, a hybrid system was used consisting of a glass slide and PDMS channels. The fluorescence intensity was found to be directly correlated to the pH of the solution in contact, indicating the possibility of using such a system as a pH sensor. These systems allow real-time measurements and can be easily implemented in micro- and nanofluidics systems thus enabling analysis of extremely small sample volumes in a fast and reproducible manner.     

Novel sol–gel reversible thermochromic materials for environmental sensors

Anhydrous cobalt(II) chloride has been encapsulated in several inorganic sol–gel matrices with different solvent/water ratios. Sols were cast into cuvettes and hermetically closed. Such sol–gel materials were found to be sensitive to temperature in the 10–50 °C range showing a change of colour. General characterisation of the sensitive materials was made by immersion into a thermostatic water bath and recording of the corresponding visible spectra. The optical response consisted of a change in colour from light pink to deep blue as the temperature increases. These temperature detectors behave as sensors showing good optical sensitivity in the range mentioned above and reversibility for more than 30 cycles. The sensors response time is at about 15 min and their lifetime is 2 months at least. These sol–gel materials have been designed to be applied for preservation and conservation purposes. High temperatures and cyclical temperature changes can yield severe consequences for the correct preservation of cultural heritage materials (textiles, archaeological ceramics and other remains, metallic objects and statues, stained glass windows, etc.) both in museums and outdoors.     

Optochemical Sensor for Ammonia Based on a Lipophilized pH Indicator in a Hydrophobic Matrix

Abstract

An optical sensor for the determination of ammonia in water based on ion pairing has been investigated. A pH-sensitive dye is immobilized as an ion pair in a silicone matrix. The colour of the dye changes from yellow to blue depending on the concentration of ammonia in the sample solution. This change is reversible. The concentration of ammonia can be determined by measuring the transmittance at a given wavelength.

All measurements were performed with a dual-beam optical meter. The measurement range was from 5.9 × 10^?7 to 1 × 10^?3 M (0.01 to 17 mg/l) in 0.1 M phosphate buffer of pH 8. The detection limit was 10 μg/l. The response times at a flow rate of 2.5 ml/min were 4 min for t₉₀ and 10 min for t₁₀₀ at a change from 41.9 to 82.5 μM ammonia and 12 min for t₉₀ and 48 min for t₁₀₀ at a change from 160 to 0 μM ammonia. The operational lifetime of the ammonia sensor was limited to a period of a few days only. A continuous decrease in baseline signal and relative signal change was observed over the whole measurement. The storage stability was more than 10 months (dry). With respect to possible application of the ammonia sensor to environmental analysis, the influence of pH, typical interferences, such as amines and various detergents on the sensor response was investigated. No interference due to pH was observed in the range from pH 5 to pH 9. With methyl- and ethylamine the response was not completely reversible. The sensor was affected by cationic detergents, but not by anionic or neutral detergents.     

Transformations of methyl orange dimers in aqueous–acid solutions,according to UV–Vis spectroscopy data

The effect acidity has on the UV–Vis absorption spectra of azo dye methyl orange (MOD) in aqueous solutions of hydrochloric acid in the pH range of 1.7 to 7 and sulfuric acid in the 0.24 to 18 mol/L range of concentrations is investigated. The spectral transformations of MOD solutions are compared to the corresponding spectral transformations of solutions of dimethylaminoazobenzene (DAB), which is an azo dye akin to MOD. A close resemblance between the spectral transformations of MOD and dimers DAB₂ is revealed. It is concluded that the ground state of MOD, like the ground state of DAB, consists of not individual molecules but of supramolecular dimers MOD₂. It is found that dimers MOD₂ in aqueous low-acidic solutions are reversibly protonated with the formation of di- and triprotonated forms, which reversibly dissociate into diprotonated monomers upon an increase in acidity. The structural formulas of the chromogenic groups responsible for the spectral transformations, and the mechanisms of their reversible transformations, are given.     

Single-molecule spectroscopy studies of microenvironmental acidity in silicate thin films

Single-molecule (SM) spectroscopic methods were employed to study single site variations in the acidity properties of sol-gel-derived silicate films. The pH-sensitive dye Carboxy SNARF-1 (C.SNARF-1) was used to sense film acidity. Its concentration in the films was maintained at nanomolar levels to allow for SMs to be spectroscopically interrogated. The ratio of C.SNARF-1 fluorescence at 580 nm (protonated form) and 640 nm (deprotonated form) was used to characterize local film pH. SM data were acquired both for "untreated" films and for those treated by immersion for either 1 or 8 h in phosphate solutions of different pH. The SM results prove that the spectral variability observed is dominated by static variations in the local matrix acidity. Shorter immersion times lead to relatively broad histograms and broad "titration" curves, providing clear evidence for kinetic limitations to access of certain film environments by the immersion solutions. Films subjected to longer immersion times generally exhibit narrower histograms. Particularly narrow distributions were obtained for films treated near pH 8-9, while much broader histograms were produced near pH 7. These results are attributed to the buffering effects of surface silanols near pH 9 and enhanced pH sensitivity of the dye near pH 7.     

LED-compatible fluorosensor for measurement of near-neutral pH values

We report on an optical sensor material suitable for fluorimetric measurement of pH in the 6–9 range using a new, fully LED-compatible fluorescent dye. Its base form has a strong absorption between 580 and 630 nm that matches the emission band of conventional yellow or orange light-emitting diodes. Two kinds of dye immobilization are reported. The first is based on covalent binding to a cellulosic matrix and the resulting material is intended for use in sensing membranes. The second involves physical entrapment of the dye in a sol-gel matrix which can be used for optical fiber tip coating as well as in evanescent wave type sensors. Both kinds of sensor materials are studied with respect to dynamic pH ranges, response times, sensitivity toward ion strength, and stability.Dedicated to Professor G. Werner, Leipzig, on the occasion of his 60th birthday.     

Preparation of Silica-Gel Film with pH Indicators by the Sol–Gel Method

Gel films with various pH indicators were prepared by the sol–gel method without catalysts. The obtained gel films showed good response for various pH solutions as optical pH sensors and no leaching of the indicators was observed. The feature of absorption spectra of the indicators in the gel films was almost the same as that in the aqueous solutions. pKa of the indicators in the gel films was shifted with the increase of the TMOS content in the starting solutions. These results suggest that the pH indicators in the gel films were trapped in similar environment as in the solutions.     

Polyaniline coated micro-capillaries for continuous flow analysis of aqueous solutions

The inner walls of fused silica micro-capillaries were successfully coated with polyaniline nanofibres using the “grafting” approach. The optical response of polyaniline coatings was evaluated during the subsequent redoping–dedoping processes with hydrochloric acid and ammonia solutions, respectively, that were passed inside the micro-capillary in continuous flow. The optical absorbance of the polyaniline coatings was measured and analysed in the wavelength interval of [300–850 nm] to determine its optical sensitivity to different concentrations of ammonia. It was found that the optical properties of polyaniline coatings change in response to ammonia solutions in a wide concentration range from 0.2 ppm to 2000 ppm. The polyaniline coatings employed as a sensing material for the optical detection of aqueous ammonia have a fast response time and a fast regeneration time of less than 5 s at room temperature. The coating was fully characterised by scanning electron microscopy, Raman spectroscopy, absorbance measurements and kinetic studies. The response of the coatings showed very good reproducibility, demonstrating that this platform can be used for the development of micro-capillary integrated sensors based on the inherited sensing properties of polyaniline.     

A Dual Colorimetric/Fluorescence System for Determining pH Based on the Nucleophilic Addition Reaction of an o‐Hydroxymerocyanine Dye

Owing to their ability to monitor pH in a precise and rapid manner, optical probes have widely been developed for biological and nonbiological applications. The strategies thus far employed to determine pH rely on two types of processes including reversible protonation of amine nitrogen atoms and deprotonation of phenols. We have developed a novel dual, colorimetric/fluorescence system for determining the pH of a solution. This system utilizes an o‐hydroxymerocyanine dye that undergoes a nucleophilic addition reaction that subsequently causes reversible structural changes interconverting a merocyanine to a spirocyanine and a spirocyanine to a spiropyran. It was demonstrated that the dye can be employed to measure the pH of solutions in the 2.5–5.75 and 9.6–11.8 ranges with color changes from yellow to dark blue and then to lavender. Moreover, the fluorescence response associated with the spirocyanine–spiropyran transformation of the dye occurring in alkaline solutions provides a precise method.     

pH‐Tunable Hydrogelators for Water Purification: Structural Optimisation and Evaluation

A focused library of potential hydrogelators each containing two substituted aromatic residues separated by a urea or thiourea linkage have been synthesised and characterized. Six of these novel compounds are highly efficient hydrogelators, forming gels in aqueous solution at low concentrations (0.03–0.60 wt %). Gels were formed through a pH switching methodology, by acidification of a basic solution (pH 14 to ≈4) either by addition of HCl or via the slow hydrolysis of glucono‐δ‐lactone. Frequently, gelation was accompanied by a dramatic switch in the absorption spectra of the gelators, resulting in a significant change in colour, typically from a vibrant orange to pale yellow. Each of the gels was capable of sequestering significant quantities of the aromatic cationic dye, methylene blue, from aqueous solution (up to 1.02 g of dye per gram of dry gelator). Cryo‐transmission electron microscopy of two of the gels revealed an extensive network of high aspect ratio fibers. The structure of the fibers altered dramatically upon addition of 20 wt % of the dye, resulting in aggregation and significant shortening of the fibrils. This study demonstrates the feasibility for these novel gels finding application as inexpensive and effective water purification platforms.     

pH-Tunable hydrogelators for water purification: structural optimisation and evaluation

A focused library of potential hydrogelators each containing two substituted aromatic residues separated by a urea or thiourea linkage have been synthesised and characterized. Six of these novel compounds are highly efficient hydrogelators, forming gels in aqueous solution at low concentrations (0.03-0.60 wt%). Gels were formed through a pH switching methodology, by acidification of a basic solution (pH 14 to ≈ 4) either by addition of HCl or via the slow hydrolysis of glucono-δ-lactone. Frequently, gelation was accompanied by a dramatic switch in the absorption spectra of the gelators, resulting in a significant change in colour, typically from a vibrant orange to pale yellow. Each of the gels was capable of sequestering significant quantities of the aromatic cationic dye, methylene blue, from aqueous solution (up to 1.02 g of dye per gram of dry gelator). Cryo-transmission electron microscopy of two of the gels revealed an extensive network of high aspect ratio fibers. The structure of the fibers altered dramatically upon addition of 20 wt% of the dye, resulting in aggregation and significant shortening of the fibrils. This study demonstrates the feasibility for these novel gels finding application as inexpensive and effective water purification platforms.     

Spectrophotometric analysis of trichloroethylene in various environmental and biological samples

A simple sensitive extractive spectrophotometric method for determination of trichloroethylene is proposed. Trichloroethylene is treated with pyridine to form glutaconic aldehyde by heterolytic cleavage of the pyridine ring. Glutaconic aldehyde is further coupled with 4-aminoacetanilide to form an orange–red dye which is extractable in 3-methyl-1-butanol. The extracted dye shows absorption maximum at 520 nm. The system obeys Beer’s law in the range of 0.05–0.8 μg mL^?1. Important analytical parameters such as time, temperature, reagent concentration, acidity etc. have been optimized for complete colour reaction. Sandell’s sensitivity and molar absorptivity for the system were found to be 0.001 μg cm^?2 and 1.2 × 10⁵ L mol^?1 cm^?1, respectively. The proposed method is satisfactorily applied to micro-level determination of trichloroethylene in various environmental and biological samples.     

Atmospheric Pressure Plasma Degradation of Azo Dyes in Water: pH and Structural Effects

The degradation of aqueous solutions containing azo dyes (ortho-, meta-, and para-methyl red) was carried out by means of atmospheric pressure plasma treatment. As evidenced by optical emission spectroscopy, the metastable argon in the discharge is responsible for initiating reactions in dye solutions. The bleaching of aqueous solutions is attributed to the destruction of dye molecules as observed in the UV–visible absorption spectra. We found that the degradation pathways of methyl red critically depend on the pH values in aqueous solutions as well as isomeric structures. The reaction pathways are entirely different in basic (pH = 11), near-neutral (pH = 6), and acidic conditions (pH = 2). Kinetic analysis shows that acidic condition gives the fastest degradation rates of methyl red isomers with removal rate: ortho > meta > para among all conditions. At basic condition, the degradation rates are equally slow for all methyl red isomers.     

Synthesis and spectrophotometric studies of water-soluble amino[bis(ethanesulfonate)] azobenzene pH indicators

Three azobenzene pH indicators with amino[bis(ethanesulfonate)] substituents were synthesized and studied by UV–visible absorption spectroscopy in aqueous solution. The indicators exhibit brilliant and distinct colour changes with transitions between pH 1 and 4. Significant changes were observed in the UV–visible spectra on titration with acid with pK_a values ranging from 2.2 to 2.8. The indicators demonstrate individual changes in colour as a function of pH. These novel pH indicators complement the existing library of azobenzene indicator dyes and may be useful for environmental situations with high proton concentrations.     

Improvement of colour strength and colourfastness properties of gamma irradiated cotton using reactive black-5

The dyeing behaviour of gamma irradiated cotton fabric using Reactive Black-5 dye powder has been investigated. The mercerized, bleached and plain weaved cotton fabric was irradiated to different absorbed doses of 100, 200, 300, 400, 500 and 600 Gy using Co-60 gamma irradiator. Dyeing was performed using irradiated and un-irradiated cotton with dye solutions. The dyeing parameters such as temperature of dyeing, time of dyeing and pH of dyeing solutions were optimised. The colour strength values of dyed fabrics were evaluated by comparing irradiated and un-irradiated cotton in CIE Lab system using Spectra flash SF650. Methods suggested by International Standard Organisation (ISO) were employed to study the effect of gamma irradiation on the colourfastness properties of dyed fabric. It is found that gamma irradiated cotton dyed with Reactive Black-5 has not only improved the colour strength but also enhanced the rating of fastness properties.     

Effect of mercerization and gamma irradiation on the dyeing behaviour of cotton using stilbene based direct dye

The dyeing behaviour of mercerized and gamma irradiated cotton fabric using stilbene based direct dye has been investigated. The fabric was treated with different concentrations of alkali to optimize the mercerization. The optimum mercerized cotton fabric was irradiated to absorbed doses of 2, 4, 6, 8 and 10 kGy using Cs-137 gamma irradiator. Dyeing was performed using irradiated and un-irradiated cotton with dye solutions. The dyeing parameters such as temperature, time of dyeing, pH of dyeing solutions and salt concentration were optimized. The colour strength values of dyed fabrics were evaluated by comparing irradiated and un-irradiated cotton in CIE Lab system using Spectra flash SF650. Methods suggested by International Standard Organization (ISO) were employed to study the effect of gamma irradiation on the colourfastness properties of dyed fabric. It was found that mercerized and irradiated cotton have not only improved the colour strength but enhanced the rating of fastness properties also.     

Fluorescent dye guest-host effects in advanced ferroelectric liquid crystals

Previous work has shown that guest-host ferroelectric systems incorporating dichroically absorbing dyes are suitable for use in colour display applications. These utilize either the dichroic absorption of a conventional dye, or the emission of a fluorescent dye. We present here the electrooptical properties of several advanced ferroelectric liquid crystals doped with a new fluorescent dye, coloured blue in emission. The data consists of measurements of tilt angle, response time, spontaneous polarization, and rotational viscosity, from which we conclude that certain hosts are not adversely affected by the fluorescent dopants. These results are then discussed in connection with the use of these mixtures in two novel colour display configurations, which are also presented, utilizing either the dichroic absorption or the polarized fluorescence of the fluorophore guests.     

INVESTIGATION OF POWER ABSORPTION BY METHYLENE BLUE IN A LIGHT SCATTERING MEDIUM WITH AN INTERNAL PHOTODYNAMIC ACTINOMETER

Abstract— Power absorption by aqueous solutions of methylene blue containing high concentrations of polystyrene microspheres was measured at 650 nm, using the photosensitized inactivation of subtilisin Carlsberg as an internal actinometer. The results were analyzed with the one-dimensional diffusion approximation for a finite slab. It is shown that the power absorption is determined by two macroscopic parameters, the optical penetration depth and the linear absorption coefficient. The optical penetration depth was determined independently by measuring flux profiles with an inserted fiber-optic method. The results are in satisfactory agreement with the predictions of the diffusion model, with microsphere diameters from 0.5 to 5 μm and a wide range of scatterer and dye concentrations. The presence of the scatterers diminished the power absorbed by the dye in all cases. The predictions of one-dimensional diffusion model are compared to Kubelka-Munk theory, and shown to be equivalent for optically dense systems     

Sensing Element of a Benzopurpurin 4B-Based Optical Sensor for Determining the Acidity of Etch Solutions

Deposition conditions are studied, and a Langmuir–Blodgett film (LBF) based on the benzopurpurin 4B indicator immobilized in the polyamide acid salt matrix is obtained. The protolytic properties of the indicator are studied in water, micelles of cationic and nonionic surfactants, and LBFs. It is shown that, in going from water to the cited micelles and LBFs, pK _ind is shifted to the acidic region by 3.25, 1.18, and 2.30 pH units, respectively. The possibility of using the produced LBF as the sensing element of an optical sensor for the determination of the acidity of solutions in the pH range 1–0 is demonstrated. The LBF is applied to the determination of the acidity of solutions in etch baths. The response time in the operating range is 10–20 s; the standard deviation is no more than 4%.     

Multiwavelength spectrophotometric determination of acidity constants of some azo dyes

A multiwavelength spectrophotometric titration method was applied to study the acidity constants of some azo dyes in water. The UV-vis absorption spectra of azo dye solutions were recorded in the course of their pH-metric titration with a standard base solution. The protolytic equilibrium constants, spectral profiles, concentration diagrams and also the number of components have been calculated. The quantitative effects of the substituents on the acidity of the studied azo dyes were investigated by the linear free energy relationship (LFER) using Hammet sigma constant (sigma) and field and resonance effects of Kamlet and Taft (f and Re, respectively).