A novel pH sensing membrane based on an ionic liquid-polymer composite

A novel pH sensing membrane was developed that consists of the ionic liquid n-cetylpyridinium hexafluorophosphate (CPFP), poly(vinyl chloride), and quinhydrone. The membrane is stable and flexible and can be easily deposited on the electrode. Electrochemical impedance spectroscopy was used to study the interfacial charge transfer of this membrane. Compared to a traditional plasticizer-based membrane electrode, the new electrode possesses excellent potentiometric characteristics for monitoring pH, such as a response time of less than 10 s, high sensitivity, stability, and reproducibility. The response is almost Nernstian, with a slope of ?57.5?±?0.2 mV pH^?1 in the pH range from 2 to 9.5. The new electrode was used for direct monitoring of pH in real food samples.

Phase transition of microcapsule membranes induced by pH changes

The phase transition of poly(L-lysine-alt-terephthalic acid) (PPL) microcapsules induced by changes in the pH of the medium was investigated. The total number of ionizable groups in one capsule or the charge density of the microcapsule membrane was found to be important for the phase transition phenomenon to occur. Thus, the existence of a minimum was suggested, for the total number of ionizable groups in one capsule or for the charge density of the microcapsule membrane, if a pH-induced phase transition of PPL microcapsules is to be caused.The presence of SDS at low concentrations brought about the phase transition of PPL microcapsules, even when the microcapsules were poorly charged at low pH values, through enhanced adsorption of the surfactant unions to the constituent polymers of the microcapsules at high ionic strengths of the medium. Addition of 1,4-dioxane to the microcapsule suspension prevented the phase transition phenomenon from taking place.     

Naphthalimide-rhodamine based fluorescent probe for ratiometric sensing of cellular pH

The pH values of lysosomes in cancer cells is slightly lower than that in normal cells, which can be used to distinguish cancer cells from normal cells. According to this, a naphthalimide-rhodamine based fluorescent probe(hereafter referred to as RBN) with a pK_a of 4.20 was designed and synthesized for ratiometric sensing of cellular pH via fluorescence resonance energy transfer(FRET), which can respond to different pH precisely through ratiometric fluorescence intensity(Ⅰ_(577)/Ⅰ_(540)). RBN can be employed to distinguish cancer cells from normal cells on the basis of different fluorescent response, in particular, RBN showed excellent water solubility and low cell toxicity, all these are quite significant for potential application in cancer diagnose and therapy.     

Sugar sensing with synthetic multifunctional pores

Recently, synthetic multifunctional pores have been identified as "universal" detectors of chemical reactions. In this report, we show that with the assistance of enzymes as variable co-sensors, synthetic multifunctional pores can serve as similar universal sensors of variable components in mixed analytes. Sugar sensing in soft drinks is used to exemplify this new concept. This is achieved using invertase and hexokinase as co-sensors and a new synthetic multifunctional pore capable of discriminating between ATP and ADP in an "on-off" manner as sensor. The on-off discrimination between ATP as good and ADP as poor pore blocker is shown to be reasonably tolerant of changing experimental conditions. These results identify universal sensing with synthetic multifunctional pores as a robust, sensitive, and noninvasive method with appreciable promise for practical applications.     

A wide pH range optical sensing system based on a sol-gel encapsulated amino-functionalized corrole

The synthesis of a new compound, 10-(4-aminophenyl)-5,15-dimesitylcorrole, and its application for the preparation of optical chemical pH sensors is described. The dye materials were immobilized in a sol-gel glass matrix and characterised upon exposure to aqueous buffer solutions. The response of the sensor is based on the fluorescence intensity changing of corrole owing to multiple steps of protonation and deprotonation. Due to its containing several proton sensitive centers, the 10-(4-aminophenyl)-5,15-dimesitylcorrole based optode shows a wider response range toward pH than that of tetraphenylporphyrin (TPPH(2)) and 5,10,15-tris(pentafluorophenyl)corrole (H(3)(tpfc)). It shows a linear pH response in the range of 2.17-10.30. The effect of the composition of the sensor membrane has been studied and the experimental conditions were optimized. The optode showed good reproducibility and reversibility, and common co-existing inorganic ions did not show obvious interference to its pH measurement.     

Fiber optic lifetime pH sensing based on ruthenium(II) complexes with dicarboxybipyridine

This review will focus on the role of mass spectrometry in the emerging field of lipidomics. Particular emphasis will be placed on recent developments in the use of tandem mass spectrometry methods in lipid analysis using low-energy collision induced dissociation (CID). After a brief discussion on ionization techniques, novel ion-activation methods that allow for increased sensitivity and selectivity will be critically discussed. Examples will be drawn from the analysis of higher order lipids, specifically triacylglycerols (TAGs) and glycerophospholipids, as the numerous positional isomers and head groups present in these classes of lipids continue to pose a significant analytical challenge to the field of lipidomics. The role of bioinformatics in the development of lipidomics will also be discussed.     

Stability of new optical pH sensing material based on cross-linked poly(vinyl alcohol) copolymer

Cross-linked poly(vinyl alcohol) (PVA)-silica gel copolymer has been employed as a optical pH sensor substrate for immobilisation of fluorescein. Cross-linking was carried out by the sol-gel process incorporating PVA in initial sol-gel solution of tetra-methoxysilane (TMOS) under acidic conditions. Three dimensional network formation could be achieved using compositions of PVA/TMOS=80-90/20-10 vol.% to result in crack-free films. The fluorescent sensor layers were prepared by dip-coating of gel solution onto glass slides. The dynamic fluorescence response towards different pH values was investigated in terms of the influence of sample ionic strength, membrane composition as well as age of sol-gel layers. Depending on the composition of the matrix pK_a values of 6.50, 6.68 and 7.06 were found 18 days after continues storage in buffer.     

pH sensing using boron doped diamond electrodes

The boron doped diamond (BDD) electrode is presented as an appropriate candidate for next generation glass-free, highly stable and accurate pH sensors. The method used in this study is based on the potential change related to the hydrogen evolution reaction following a current step, which is pH dependent. Alkali cations in the solution have no influence on the accuracy of the pH calibration curve, which provides an advantage with respect to the conventional pH glass electrode. The unwanted influence of electrochemically active compounds in solution can be avoided by adjusting the current density applied during chronopotentiometric measurements. The accuracy of the pH measurements is due to the excellent stability as well as the wide potential window and low background current of BDD electrodes. This faculty was not observed when using conventional electrode materials. The efficacy of this new type of pH sensor has been tested using tap water as a typical real sample.     

Study on low-cost calibration-free pH sensing with disposable optical sensors

As labor costs become more expensive, less labor-intensive disposable devices have become more ubiquitous. Similarly, the disposable optical pH sensor developed in our lab could provide a convenient yet cost-effective way for pH sensing in processes that require stringent pH control. This optical pH sensor is prepared in uniform individual lots of 100–200 sensors per lot. Calibration is accomplished on a few randomly selected sensors out of each lot. We show that all others in the same lot can then be used directly without requiring individual calibration. In this paper, a calibration model is derived to include all the factors that affect the signal of the disposable sensor. Experimental results show that the derived calibration model fits the experimental data. The readings of 28 randomly selected disposable sensors with 4 sensors from each of the 7 lots show an error less than 0.1 pH units in the useful sensing range of the sensor. The calibration model indicates that if further improvement on precision is desired, more uniform porous material and more advanced coating techniques will be required. When it comes to the effects of the varying coasters, house-made low-cost fluorometers, the variability in the brightness ratio of the blue-to-violet LEDs is the primary reason for the lack of precision. Other factors like LED light intensity distribution, optical properties of the filters and electronics also contribute to the coaster-to-coaster difference, but to a lesser extent. Two different methods for correcting the instrument variations were introduced. After correction, the collective reading errors for all the tested instruments were reduced to less than 0.2 pH units within the sensor's useful sensing range. Based on this result, our lab is currently implementing further improvements in modifying the coasters to equalize the ratios of blue-to-violet LED brightness.     

Structural changes of purple membrane and bacteriorhodopsin during its denaturation induced by high pH

Bacteriorhodopsin (bR) trimers naturally form two-dimensional hexagonal crystals in purple membrane (PM), which make it very stable. However, the dnaturation of bR was found to occur during a very narrow pH range when the pH was increased above 12.0, as indicated by inactivation of the photochemical cycle observed by flash photolysis kinetic spectra. Here, atomic force microscopy was used to study the surface structural changes of PM during the denaturation process induced by high pH. Together with the absorption and fluorescence spectra, it was found that the structural changes could be divided into three steps. First, some hydrophobic amino acids of bR become exposed to the aqueous environment and PM loses its 2D crystalline structure, transforming into the so-called "nonisland" structure. Second, bR molecules are extracted out of membrane and form protrusions on the surface like islands in the sea; therefore, the "nonisland" structure transforms into the "island" structure. Finally, most bRs break off from the membrane and form large depositions.     

Isothermal titration calorimetric studies of the pH induced conformational changes of bovine serum albumin

Bovine serum albumin (BSA) is a soft globular protein that undergoes conformational changes through several identified transition steps in the pH range 2–13.5. The ability to change conformation makes BSA capable of complexing different ligands from fatty acids to cations or drugs and carries them in the bloodstream. Microcalorimetric titration of BSA with NaOH solution was performed to measure the enthalpy of conformational changes. Two exothermic enthalpy changes were found in the course of the titration between pH 3 and 9.5, which can be identified with the E–F, and the F–N transitions. The enthalpy change at pH 3.5 (transition from the E to the F form of BSA, folding of intra-domain helices in domain I) is independent of the protein concentration. The second transition (F–N, folding of domain III) was observed at pH 4.8 for the 0.1% BSA solution, but it shifted to higher pH values as the protein concentration increased to 0.2% and 0.3%. The tightening of the protein structure with increasing pH was verified measuring intrinsic fluorescence of tryptophan residues. At even higher pH value, pH 10.5, fluorescence measurements revealed protein expansion. The BSA conformational changes were also measured by dynamic light scattering. The hydrodynamic diameter was smaller at the i.e.p. of BSA (5–7 nm at pH ~5) and larger at the two ends of the pH range (17.5 nm at pH 2 and 8.3 nm at pH 10).     

Crosslinked fibrous composites based on cellulose nanofibers and collagen with in situ pH induced fibrillation

Collagen and cellulose nanofiber based composites were prepared by solution casting followed by pH induced in situ partial fibrillation of collagen phase and crosslinking of collagen phase using gluteraldehyde. Microscopy studies on the materials confirmed the presence of fibrous collagen and cellulose nanofibers embedded in the collagen matrix. The cellulose nanofiber addition as well as collagen crosslinking showed significant positive impact on the nanocomposite’s mechanical behaviour. The synergistic performance of the nanocomposites indicated stabilization and reinforcement through strong physical entanglement between collagen and cellulose fibres as well as chemical interaction between collagen matrix and collagen fibrils. The mechanical performance and stability in moist conditions showed the potential of these materials as implantable scaffolds in biomedical applications. The collagen-cellulose ratio, crosslinking agent and crosslinking level of collagen may be further optimised to tailor the mechanical properties and cytocompatibility of these composites for specific applications such as artificial ligament or tendon.     

Biomimetic sensing based on chemically induced assembly of a signaling DNA aptamer on a fluid bilayer membrane

The adenosine aptamer was split into two halves and linked to a fluid liposome surface; addition of adenosine resulted in aptamer assembly, which did not occur if the split aptamer was attached to silica nanoparticles, demonstrating the feasibility of using aptamer probes to study diffusion within lipid membranes.     

A water-soluble polythiophene-Au nanoparticle composite for pH sensing

In this paper, we report the development of a reversible pH sensor in aqueous medium based on the fluorescence properties of a polythiophene-gold nanoparticle (Au NP) composite. The composite was synthesized in water by simultaneous reduction of HAuCl(4) to Au NPs and polymerization of thiophene in the presence of no additional reagents. It was stable for weeks and had characteristic emissions, which changed in the pH range of 3.0 to 6.0, thus providing a mean for probing the pH of an aqueous solution. Measurement of the pH could be performed over several cycles of titrations, pointing to the robustness of the materials for such sensing applications. The mass spectra of the composite at two extreme pH values were identical, indicating that the primary structure of the polymer was not affected due to changes in pH of the medium. Transmission electron microscopic (TEM) measurements indicated the presence of small sized Au NPs with the polymer in the milieu. The composite could be titrated by acid (or base) and considering the acid-base equilibria at different pHs, we have been able to calculate the pK(eq) of the composite, which was further used in calculating the pH of an aqueous solution from the emission spectrum of the composite. Our approach took advantage of redox chemistry in synthesizing the water-soluble composite and the optical behavior of a conjugated polymer in developing an important pH sensor, which may form the basis of further development of versatile pH or other sensors by suitably modifying the backbone of the monomer.     

Colorimetric and fluorescent sensing of biologically important fluoride in physiological pH condition based on a positive homotropic allosteric system

A novel positive homotropic allosteric system 1 based on 3-methylpyrozole-5-one-4-one-2′,4′-dinitrophenylhydrazone was designed, synthesized and characterized. Colorimetric and fluorescent sensing of anions was achieved in physiological condition (pH 7.4), resulting from the positive homotropic allosterism of 1 induced by anions tested. In particular, the compound 1 exhibited a two-step response to the strong basic anions such as F⁻. In the first step, the hydrazone form of 1 interacted with anions through hydrogen bonding with an obvious color change from yellow to orange upon addition of 0.3 equiv. of anions. In the second step, with further addition of anions, the hydrazone form of 1 was shifted to the azophenol form, whose anion binding was accompanied with an orange-to-purple color change. In addition, the receptor 1 exhibited a fluorescent enhancement response to anions exploiting two possible signaling transduction mechanisms: (1) inhibition of photoinduced electronic transfer (PET) and (2) binding-induced rigidity of the host molecule.     

Studies on thermally induced structural changes in silicon based polymers by positron annihilation

Temperature dependence of positron annihilation lifetime spectra of polysilanes such as, poly(methyl-n-propylsilane) (PMPrS) and poly(di-n-hexylsilane) (PDHS) has been investigated. The τ₃ in PMPrS is seen to increase monotonically around the solid–liquid transition temperature. The transition temperature and free volume are observed to depend on the molecular weight and/or packing of the backbones. For PDHS, a sharp change in τ₃ and I₃ is seen at the solid–solid transition temperature. Free volume radius probability density functions, above and below the transition temperature, are presented in PMPrS and PDHS. Positron studies are complimented by conventional thermal analysis studies.     

Aminated polystyrene membranes for a fiber optic pH sensor based on reflectance changes accompanying polymer swelling

We have prepared toughened, porous, aminated polystyrene membranes that undergo an increase in reflectance as the pH increases from 6.8 to 8.0. Vinylbenzyl chloride (VBC) is copolymerized with divinylbenzene (DVB) in the presence of a toughening agent, Kraton G1652, a styrene-ethylene, butylene-styrene triblock copolymer, and a porogenic solvent, xylene/ dodecane. The optimum formulation for sensing is 2% DVB (mol DVB/mol VBC), 2% Kraton (g Kraton/g VBC) and 40% (v/v) 21 xylene: dodecane. Benzoyl peroxide is used as the initiator. The components are partially polymerized at 85 °C to a viscosity of 600–800 centipoise. The polymerization is then stopped by reducing the temperature. A drop of the partially polymerized solution is confined between two microscope slides and the polymerization reaction is completed. The resulting membrane is then swollen in 1,4-dioxan and reacted with diethanolamine. These membranes have been incorporated into a pH sensor based on changes in reflected intensity measured through a bifurcated bundle of twenty unbuffered 50/55 core/cladding glass-on-glass optical fibers with numerical apertures of 0.57. The resulting sensor is stable and requires inexpensive optical components, a red-emitting LED as the source and a silicon photodiode as the detector.     

Electrochemiluminescence induced photoelectrochemistry for sensing of the DNA based on DNA-linked CdS NPs superstructure with intercalator molecules

A novel detection protocol of DNA was developed using electrochemiluminescence (ECL) induced photoelectrochemistry (PEC) synthesis based on DNA-linked CdS NPs superstructure with methylene blue as the intercalator molecule.     

Reorganization of immobilized horse and yeast cytochrome c induced by pH changes or nitric oxide binding

The redox properties of horse and yeast cytochrome c electrostatically immobilized on carboxylic acid-terminated self-assembled monolayers (SAMs) have been determined over a broad pH range (pH 3.5-8) in the absence and presence of nitric oxide. Below pH 6, both proteins exhibit comparable midpoint potentials, coverages, and electron-transfer rate constants, which suggests that they are adsorbed on the SAM in a similar fashion. Above pH 6, a sharp decrease in electron-transfer rate constants is observed for immobilized yeast cytochrome c, which is indicative of a change in the electron tunneling pathway between the heme and the electrode and hence suggests that the protein reorients on the surface. Such a decrease is not observed for horse cytochrome c and therefore must be related to the specific charge distribution on yeast cytochrome c. Apart from the charge distribution on the protein, the reorientation also seems to be related to the charge on the SAM surface. The presence of nitric oxide causes a decrease in electron-transfer rate constants of both yeast and horse cytochrome c at low pH. This is probably due to the fact that nitric oxide induces a conformational change of the protein and also changes the reorganization energy for electron transfer.     

Multi-stimuli programmable FRET based RGB absorbing antennae towards ratiometric temperature,pH and multiple metal ion sensing

A red-green-blue (RGB) multichromophoric antenna 1 consisting of energy donors naphthalimides and perylenediimides and a central aza-BODIPY energy acceptor along with two subchromophoric red-blue (RB 6) and green-blue (GB 12) antennae was designed that showed efficient cascade Förster resonance energy transfer (FRET). RGB antenna 1 showed pronounced temperature-dependent emission behaviour where emission intensities in green and red channels could be tuned in opposite directions by temperature giving rise to unique ratiometric sensing with a temperature sensitivity of 0.4% °C. RGB antenna 1 showed reversible absorption modulation selectively in the blue region (RGB ↔ RG) upon acid/base addition giving rise to pH sensing behaviour. Furthermore, RGB antenna 1 was utilized to selectively sense metal ions such as Co²⁺ and Fe³⁺ through a FRET turn-off mechanism induced by a redox process at the aza-BODIPY site that resulted in the selective spectral modulation of the red band (i.e., RGB → GB). Model antenna RB 6 showed white light emission with chromaticity coordinates (0.32, 0.33) on acid addition. Antennae 1, 6 and 12 also exhibited solution state electrochromic switching characterized by distinct colour changes upon changing the potential. Finally, antennae 1, 6 and 12 served as reversible fluorescent inks in PMMA/antenna blends whereby the emission colours could be switched or tuned using different stimuli such as acid vapour, temperature and metal ions.

RGB antennae consisting of naphthalimides, perylenediimides and aza-BODIPY with efficient FRET show unique ratiometric temperature sensing, metal sensing (FRET-off) and pH sensing through various stimuli sensitive band tuning.