Determination of pH with acid—base indicators: Implications for optical fibre probes

Instant pH sensor based on the functionalized cellulose for detecting strong acid leaks

Acid spills cause large-scale environmental damage and casualties. To respond to such incidents, a sensor capable of detecting acid leaks is required. Cellulose is a useful substrate material for the fast detection of acid leaks because it has high hydrophilicity and porosity. On the other hand, methods of manufacturing cellulose-based sensors are still complicated or time-consuming. Thus, in this study, a simple and rapid synthesis method for a cellulose-based pH sensor was proposed. The functionalization of α-cellulose was achieved via chloroacetyl chloride, and Congo red was covalently immobilized to the functionalized cellulose for detecting strong acids. The manufacturing process was composed of two steps as above and finished within 8 h. The developed sensor exhibited absorbance changes in the pH range of 0.2 to 3.0, and response time was shorter than 1 s. A prototype system using this sensor was manufactured and tested, and it detected acid leaks easily and quickly.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

Effect of Solution Ionic Strength on the pKa of the Nitroxide pH EPR Probe 2,2,3,4,5,5-Hexamethylimidazolidin-1-oxyl

Spin probe and spin labeling Electron Paramagnetic Resonance methods are indispensable research tools for solving a wide range of bioanalytical problems-from measuring microviscosity and polarity of phase-separated liquids to oxygen concentrations in tissues. One of the emerging uses of spin probes are the studies of proton transfer-related and surface electrostatic phenomena. The latter Electron Paramagnetic Resonance methods rely on molecular probes containing an additional functionality capable of reversible ionization (protonation, in particular) in the immediate proximity to an Electron Paramagnetic Resonance-active reporter group, such as (N-O^•) for nitroxides. The consequent formation of protonated and nonprotonated nitroxide species with different magnetic parameters (A _iso, g _iso) could be readily distinguished by Electron Paramagnetic Resonance. Bioanalytical Electron Paramagnetic Resonance studies employing pH-sensitive paramagnetic probes typically involve determination of the equilibrium constant (pK _a) between the protonated and nonprotonated forms of the nitroxide. However, any chemical equilibrium involving charged species, such as ionization of acids and bases, and so the reversible protonation of the nitroxide, is known to be affected by an ionic strength of the solution. Currently, only scarce data for the effect of the solution ionic strength on the experimental pK _a's of the ionizable nitroxides can be found in the literature. Here we have carried out a series of Electron Paramagnetic Resonance titration experiments for aqueous solutions of 2,2,3,4,5,5-hexamethylimidazolidin-1-oxyl (HMI) nitroxide known for one of the largest differences in the isotropic nitrogen hyperfine coupling constant A _iso between the protonated and nonprotonated forms. Electrolyte concentration was varied over an exceptionally large range (i.e., from 0.05 to 5.0 M) to elucidate the effect of ionic strength on the ionization constant of this pH-sensitive Electron Paramagnetic Resonance probe and the data were compared to the Debye-Hückel limiting law. Effects of the ionic strength on the magnetic parameters of the ionizable nitroxides are also discussed.

pH sensor based on upconverting luminescent lanthanide nanorods

The pH sensor exploits the phenomenon of upconversion luminescence and is based on a hydrogel matrix containing (a) nanorods of the NaYF(4):Er,Yb type that can be excited with 980-nm laser light to give a green and red (dual) emission, and (b) a longwave absorbing pH probe that causes a pH-dependent inner filter effect.

High-basicity determination in mixed water-alcohol solutions by a dual optical sensor approach

The activity of NaOH is known to be significantly affected by the presence of an alcohol in aqueous solutions. A novel linear relationship between (deltaA/deltaC(alcohol)) and C(base) was found in the highly alkaline, mixed H2O-ROH solutions (R = Me, Et, i-Pr). The use of this linear relationship led to a dual-transducer approach to decompose the optical signals of optical base sensors and to give base and alcohol concentrations in concentrated NaOH-H2O-ROH solutions ([OH-] = 0.05-3.6 M). The scope of the new dual-sensor approach was evaluated, and errors in C(base) and C(alcohol) were analyzed. The optical base sensors consist of sol-gel SiO2-ZrO2-organic polymer composites doped with high-pKa indicators. The pKa(s) of the indicators encapsulated in the composite films were determined and found to be affected by the composition of the sol-gel composites. Optical sensors and their uses in multicomponent systems are of intense current interest.( 1-7) In the multicomponent systems, the activity of the analyte and sensor response are often affected by change in ionic strength. For optical sensors that are based on indicator equilibria involving the analyte as their transducing mechanism, such effect is particularly significant. The concentrations of both the analyte and other chemicals affect ionic strength, and the sensor response to concentration of the analyte is thus often indistinguishable from those of other chemicals. An accurate measurement of each component in these multicomponent systems is actively studied. Several approaches have been developed to correct ionic strength in optical sensing for the pH region and solutions of low-to-medium ionic strength. (1-9) We recently reported a dual-transducer approach to measure acid concentrations (2-9 M HCl) in salt-containing, concentrated strong acids such as MClx-HCl (M = Li, Ca, Al) solutions. (10) This approach was shown to reduce the error in C(acid) from, for example,

High-acidity determination in salt-containing acids by optical sensors. The scope of a dual-transducer approach and the Hammett acidity function

A dual-transducer approach based on sol-gel optical sensors was recently reported to measure acid and salt concentrations, C(acid) and C(salt), in concentrated aqueous LiCl-HCl, CaCl2-HCl, and AlCl3-HCl solutions (C(acid) at 5-6 M; C(salt) < or = 2 M). The scope of this new approach has been studied in salt-containing HCl solutions with C(acid) at 2-9 M, and factors that influence sensor responses and accuracy have been investigated. A linear relationship between (deltaA/deltaC(salt))C(acid) and (dA/dC(acid))C(salt)=0, which is the basis of this dual-transducer approach, was found to lead to an empirical linear relationship between (deltaH0)C(acid) and (deltaC(salt))C(acid) (H0: Hammett acidity function of the indicator encapsulated in the sensor).

Optical sensors and the salt effect: a dual-transducer approach to acidity determination in a salt-containing concentrated strong acid

A dual-transducer approach has been developed to decompose the optical signals of acid sensors in salt-containing concentrated acid solutions and to give acid and salt concentrations in concentrated LiCl-HCl, CaCl2-HCl, and AICl3-HCl solutions, respectively. The optical acid sensors in this approach are films of porous sol-gel SiO2 or SiO2-Nafion composite doped with low-pKa indicators. A novel linear relationship (dA/dCsalt)cCacid = beta x (dA0/dCacid)Csalt = 0 (A = absorbance of the sensor in a salt-containing HCl solution; A0 = absorbance of the sensor in a salt-free acid solution) was found, and the current approach is based on a set of nonlinear equations derived from this relationship.

Development of an optical fibre sensor for ammonia, urea, urease and IgG

An optical fibre sensor utilizing Brilliant Yellow in a thin cellulose acetate membrane as a pH sensor has been developed. By use of gas permeable membrane, urease-immobilized membrane and IgG immobilized membrane we obtained optical sensors for ammonia, urea, urease, IgG suitable for physiological and clinical applications. The system makes use of commercially available instrumentation for absorbance measurement and flow cell for fast analysis.

Polymer membranes in clinical sensor applications. III. Hydrogels as reactive matrix membranes in fibre optic sensors

The potential of hydrogel copolymer membranes in clinical sensors, based on fibre optics, is addressed. The particular properties of the membranes of relevance in this application are the ease of refractive index modulation and the potential of the hydrogel to act as a permselective barrier in which a colorimetric agent may be immobilized. The results presented illustrate the complexity of colorimetric and refractive index effects together with their dependence on pH and tonicity for hydrogels of a given composition range. The incorporation of an acryloyl-functionalized reagent (bromopyrogallol red) is used to illustrate the way in which a working pH sensor based on these combined properties may be designed and fabricated.