High pH Sensitivity Research Articles

Structural properties and sensing performance of high- k Sm 2O 3 membrane-based electrolyte–insulator–semiconductor for pH and urea detection

For high sensitive pH sensing, an electrolyte–insulator–semiconductor (EIS) device with a samarium oxide (Sm 2O 3) sensing membrane fabricated by reactive sputtering and the subsequent post-deposition annealing (PDA) treatment was proposed. In this work, the effect of thermal annealing (700, 800, and 900 °C) on the structural characteristics of Sm 2O 3 membranes was investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and atomic force microscopy. The observed structural properties were then correlated with the resulting pH sensing performances. For enzymatic field-effect-transistors-based urea biosensing, a hybrid configuration of the proposed Sm 2O 3 membrane with urease-immobilized alginate film attached was established. The EIS device with the Sm 2O 3 membrane annealed at 700 °C exhibited a higher detection sensitivity of 56.2 mV/pH, a lower hysteresis voltage of 6.2 mV, and a lower drift rate of 1.29 mV/h compared to those at other annealing temperatures. These results are attributed to the formation of a thinner low- k interfacial layer at the oxide/Si interface and the higher surface roughness. Furthermore, the presented urea biosensor was proved to be able to detect urea with good linearity ( R 2 = 0.99) and reasonable sensitivity of 2.457 mV/mM in the urea concentration range of 5–40 mM.

Preparation and characterization of the titanium dioxide thin films used for pH electrode and procaine drug sensor by sol–gel method

We used titanium dioxide (TiO 2) as the sensing layer of an ion selective pH sensor electrode, and as the substrate for a procaine drug sensitive membrane sensor. The TiO 2 thin films were prepared using sol–gel spin coating technology. We adopted the Ti(OC 4H 9) 4 as the precursor, and added an ethanol solute to obtain the TiO 2 sol. The sol–gel was spun coated onto the indium tin oxide (ITO) substrate. The drug sensitive membrane was coated on the TiO 2 film. We then measured the I DS–V G curves of the TiO 2 ion selective electrode (ISE) pH sensor in pH buffer solutions that had different pH concentrations using a Keithley 236 Semiconductor Parameter Analyzer instrument. The procaine concentration was measured from 10 −2 M to 10 −6 M with the drug sensitive membrane using a HP 34401A Digital Multimeter. We prepared the TiO 2 ISE pH sensor and obtained a high pH sensitivity of 58.73 mV/pH. Uniform TiO 2 films surface structures, with an average roughness (Ra) of 10.211 nm and root mean square roughness (Rms) of 13.01 nm were obtained. The drift effect of the titanium dioxide ion selective pH sensor electrode is 1.97 mV h −1. The sensitivity of the procaine drug sensor is 55.03 mV pC −1 between 1.0 × 10 −2 mol L −1 and 1.0 × 10 −6 mol L −1 procaine concentrations. The detection limit is 5.0 × 10 −6 mol L −1. The response time to reach 90% output voltage is 16 s. Forty seconds are required to reach 95% output voltage. The procaine drug sensor 1 × 10 −3 mol L −1 drift test is 3.64 mV h −1 and the variation in output voltage of the repeated measurement is less than 7.4 mV.

Photophysical Properties of the Prefluorescent Nitroxide Probes QT and C343T

The photophysical properties of the nitroxide prefluorescent probes 4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethyl-piperidin-4-yl) ester free radical (QT) and 2,3,4,6,7,8-hexahydro-quinolizino [1,10,9-gh] coumarin-3-carboxylic acid (1-oxyl-2,2,6,6-tetramethyl-piperidin-4-yl) ester free radical (C343T) were evaluated as a function of pH and solvent properties. The absorbance of QT showed high pH sensitivity. The pKa values for the different ionization forms involved in the acid-base equilibrium of the quinoline chromophore were determined in the ground and excited states. The fluorescence lifetimes of QT, and N-hydroxylamine (QTH) and quinoline methyl ester (QMe) derivatives, showed that the intramolecular quenching efficiency by the nitroxide moiety is independent of the quinoline ionization form. The fluorescence and absorbance of C343T were highly sensitive to solvent polarity in agreement with a charged transfer excited state of the chromophore. However, we noted a decrease in the intramolecular fluorescence quenching efficiency by the nitroxide moiety when changing the polarity of the solvent from hexane to water. This behavior has been attributed to a suppression of an energy transfer mechanism in the nitroxide quenching process in very polar solvents. The results obtained in micelles allow us to propose QT and C343T as sensors for pH and micropolarity, respectively, in addition to their role as monitors for free radicals or hydrogen transfer from phenols.

Stable nitroxyl radicals as pH, thiol and electron transfer probes

In this review article original applications of stable nitroxyl radicals (SNRs) developed by the author with his students and colleagues are presented. ESR (electron spin resonance) spectra of SNRs of the imidazoline and imidazolidine types display high pH sensitivity. Mechanisms of proton exchange in such SNRs were studied in detail and made possible their wide application as pH probes (range of pH 0-13.5) in a variety of chemical and biological systems, including in vivo assays. A stable biradical, bis (2,2,5,5-tetramethyl-3-imidazoline-1-oxyl-4-il)-disulfide, was synthesized and used to monitor thiol-disulfide exchange with free thiols in solution. This approach permitted us to develop an extremely sensitive, quantitative and noninvasive method for determining the concentrations of glutathione and cysteine in living cells. The biradical was used to clarify the topography of cysteines in enzymes and to study their involvement in folding and stability of some proteins. We proposed and implemented a novel ESR approach to study electron transfer: the kinetics of photooxidation of an SNR (with concomitant production of an ESR-silent oxoammonium ion) by a Ru(III)-bipyridyl complex. Recovery of the ESR signal was observed when illumination was terminated. The rate of recovery depends strongly on the presence of organic electron donors, including some amino acids, and on pH. We used site-directed spin-labeled mutants of bacteriorhodopsin, to which a Ru(III) complex was also bound at a well-defined locus, to show the applicability of the proposed method to study the electron transfer and effects of local neighboring and water penetration in proteins. *** DIRECT SUPPORT *** A04RK041 00002

Noninvasive monitoring of intracellular pH change induced by drug stimulation using silica nanoparticle sensors

We have synthesized and applied a nanoparticle-based pH sensor for noninvasive monitoring of intracellular pH changes induced by drug stimulation. The pH sensor is a two-fluorophore-doped nanoparticle sensor (2DFNS) that contains a pH-sensitive indicator (fluorescein isothiocyanate, FITC) and a reference dye (tris(2,2'-bipyridyl)dichlororuthenium(II) hexahydrate, RuBPY). The nanoparticles have an average diameter of 42 +/- 3 nm and can easily be taken up by cells for noninvasive intracellular pH measurement. The 2DFNS exhibited excellent pH sensitivity, reversibility, and a dynamic range of pH 4-7 for biological studies. We have used 2DFNS to monitor pH changes in living cells by drug stimulation. Both lysosomal pH changes in murine macrophages stimulated by chloroquine and intracellular acidification in apoptotic cancer cells were monitored in real time and with high pH sensitivity. Hela cells underwent intracellular acidification with a drop in pH from 7.2 to 6.5 after 8 h of treatment with 2 mumol/L dexamethasone, and this intracellular pH drop in the apoptotic cells was not influenced by the addition of zinc ions. The application of 2DFNS to intracellular pH measurements yields some important advantages: excellent pH sensitivity, little environmental effect on the pH dye, excellent quantification, high stability and excellent reversibility.

Synthesis of an alginate–poly(sodium acrylate‐co‐acrylamide) superabsorbent hydrogel with low salt sensitivity and high pH sensitivity

AbstractThis article describes the synthesis and swelling behavior of a superabsorbing hydrogel based on sodium alginate (NaAlg) and polyacrylonitrile (PAN). The physical mixture of NaAlg and PAN was hydrolyzed with a solution of NaOH to yield an alginate–poly(sodium acrylate‐co‐acrylamide) [Alg–poly(NaAA‐co‐AAm)] superabsorbent hydrogel. A proposed mechanism for hydrogel formation was suggested, and the structure of the product was established with Fourier transform infrared spectroscopy. The effects of reaction variables were systematically optimized to achieve a hydrogel with a swelling capacity as high as possible. Under the optimized conditions concluded, the maximum capacity of swelling in distilled water was 610 g/g. The absorbency of the synthesized hydrogels was also measured in various salt solutions. The swelling ratios decreased with an increase in the ionic strength of the salt solutions. In addition, the swelling capacity was determined in solutions with pHs ranging from 1 to 13. The Alg–poly(NaAA‐co‐AAm) hydrogel exhibited pH responsiveness, so a swelling–deswelling pulsatile behavior was recorded at pHs 2 and 8. This on–off switching behavior made the hydrogel as a good candidate for the controlled delivery of bioactive agents. Finally, the swelling kinetics of the hydrogels with various particle sizes were preliminarily investigated as well. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2927–2937, 2006

A novel fluorescent pH probe for expression in plants

BackgroundThe pH is an important parameter controlling many metabolic and signalling pathways in living cells. Recombinant fluorescent pH indicators (pHluorins) have come into vogue for monitoring cellular pH. They are derived from the most popular Aequorea victoria GFP (Av-GFP). Here, we present a novel fluorescent pH reporter protein from the orange seapen Ptilosarcus gurneyi (Pt-GFP) and compare its properties with pHluorins for expression and use in plants.ResultspHluorins have a higher pH-sensitivity. However, Pt-GFP has a broader pH-responsiveness, an excellent dynamic ratio range and a better acid stability. We demonstrate how Pt-GFP expressing Arabidopsis thaliana report cytosolic pH-clamp and changes of cytosolic pH in the response to anoxia and salt-stress.ConclusionPt-GFP appears to be the better choice when used for in vivo-recording of cellular pH in plants.

Modified carrageenan. 6. crosslinked graft copolymer of methacrylic acid and kappa-carrageenan as a novel superabsorbent hydrogel with low salt- and high pH-sensitivity

A novel, polysaccharide-based, superabsorbent hydrogel was synthesized through crosslinking graft copolymerization of methacrylic acid (MAA) onto kappa-carrageenan (κC), using ammonium persulfate (APS) as a free radical initiator in the presence of methylenebisacrylamide (MBA) as a crosslinker. A proposed mechanism for κC-g-polymethacrylic acid (κC-g-PMAA) formation was suggested and the hydrogel structure was confirmed using FTIR spectroscopy. The effect of grafting variables, including MBA, MAA, and APS concentration, was systematically optimized to achieve a hydrogel with the maximum possible swelling capacity. The swelling kinetics in distilled water and various salt solutions were preliminarily investigated. Absorbency in aqueous salt solutions of lithium chloride, sodium chloride, potassium chloride, calcium chloride, and aluminum chloride indicated that the swelling capacity decreased with increased ionic strength of the swelling medium. This behavior can be attributed to the charge screening effect for monovalent cations, as well as ionic crosslinking for multivalent cations. The swelling of superabsorbing hydrogels was measured in solutions with pH ranging from 1 to 13. In addition, the pH reversibility and on-off switching behavior, at pH levels of 3.0 and 8.0, give the synthesized hydrogels great potential as an excellent candidate for the controlled delivery of bioactive agents.

Study on Light and Temperature Properties of AlN pH-Ion-Sensitive Field-Effect Transistor Devices

Aluminum nitride (AlN) thin films were deposited by rf sputtering in an investigation on surface potential properties and hydrogen ion sensing characteristics. The pH sensitivity and surface potential of the produced AlN thin films were analyzed. AlN pH-ion-sensitive field-effect transistor (ISFET) devices have a high pH sensitivity (∼53 mV/pH) at pHs of 1–11. However, light exposure and temperature effects are very critical for ISFET applications. In this study, the light effect and temperature characteristics for the pH response based on AlN/SiO2 sensing gate ISFET are emphasized. The influence of light and temperature on AlN pH-ISFET behavior under various light intensities (0–4000 lx) and ambient temperatures (5–65°C) is examined. The results show that incident light results in an increased leakage current and a decreased pH sensitivity interference. The pH response of the AlN pH-ISFET was in the 48–57 mV/pH range that was also influenced by the ambient temperature.

PH and Procaine Sensing Characteristics of Extended-Gate Field-Effect Transistor Based on Indium Tin Oxide Glass

The pH sensing properties of an extended-gate field-effect transistor (EGFET) based on indium tin oxide (ITO) glass are investigated in this study. The separating structures in the EGFET are examined using a current–voltage measurement system to measure I–V curves. A readout circuit is applied to measure the pH sensing EGFET output voltage, and nonideal factors (drift and hysteresis effects) are monitored in different buffer solutions (pH=1–11). It was found that separating structures for ITO glass sensors have a high pH sensitivity (∼55 mV/pH), a low drift rate and a small hysteresis width in the concentration range between pH=1 and 11 in buffer solutions. The EGFET was used in fabricating a drug-sensitive field-effect transistor (DrugFET) sensor for procaine hydrochloride. The DrugFET was prepared using a combination of the EGFET and a drug-sensitive membrane that measured the procaine concentration in the 1×10-2–1×10-6 mol/L range. The separating structure was used with a procaine DrugFET with a good response (∼200 s) at room temperature.

Physiological characterization of Saccharomyces cerevisiae kha1 deletion mutants

Maintenance of intracellular K+ homeostasis is one of the crucial requisites for the survival of yeast cells. In Saccharomyces cerevisiae, the high K+ content corresponds to a steady state between simultaneous influx and efflux across the plasma membrane. One of the transporters formerly believed to extrude K+ from the yeast cells (besides Ena1-4p and Nha1p) was named Kha1p and presumed as a putative plasma membrane K+/H+ antiporter. We prepared kha1 and tok1-kha1 deletion strains in the B31 and MAB 2d background. Both the strains contain the ena1-4 and nha1 deletions; that means they lack the main active sodium and potassium efflux systems. MAB 2d has additional trk1 and trk2 deletions, i.e. is impaired in active K+ uptake as well. We performed a large physiological study with these strains to specify the phenotype of kha1 deletion. In our experiments, no difference in K+ content or efflux was observed in strains lacking the KHA1 gene compared with control strains. Two main phenotype manifestations of the kha1 deletion were growth defect on high external pH and hygromycin sensitivity. The correlation between these phenotypes and the kha1 deletion was confirmed by plasmid complementation. Fluorescence microscopy of green fluorescent protein (GFP)-tagged Kha1p showed that this antiporter is localized preferentially intracellularly (in contrast to the plasma membrane Na+/H+ antiporter Nha1p). Based on these findings, Kha1p is probably not localized in plasma membrane and does not mediate efflux of alkali metal cations from cells, but is important for the regulation of intracellular cation homeostasis and optimal pH control, similarly as the Nhx1p.

Apoplastic pH and FeIIIreduction in young sunflower (Helianthus annuus) roots

The relationship between the apoplastic pH in young sunflower roots (Helianthus annuusL.) and the plasmalemma ferric chelate reductase (FC‐R; EC 1.16.1.7) activity in roots was investigated. The hypothesis was tested that a high apoplastic pH depresses FC‐R activity, thereby restricting the uptake of Fe2+into the cytosol. Until recently, little has been known about this relationship, because pH and redox reaction measurements are difficult to perform within the confines of the root apoplast. We recorded the apoplastic pH by means of the fluorescence ratio in conjunction with video microscopy by covalently tagging fluorescein boronic acid to OH groups of the root cell wall. FeIIIreduction was measured using a similar approach by tagging ferrozine diboronic acid with OH groups of the cell wall. Ferrozine forms an Fe2+complex, thus indicating the reduction of ferric iron. In roots bathing in buffered outer solutions of different pH, a high pH sensitivity of apoplastic FeIIIreduction was found, with the highest ferric iron reduction rates at an apoplastic pH of 4.9; above an apoplastic pH of 5.3, no reduction was observed. Nitrate in the bathing solution increased the apoplastic pH and hence depressed the FeIIIreduction; ammonium had the reverse effect. Nitrate together with HCO3–, a combination which is typical of calcareous soils, had the strongest depressing effect. From the results, it can be concluded that the main reason for the frequently occurring iron deficiency chlorosis of plants grown on calcareous soils is the inhibition of FeIIIreduction in the apoplast, and hence Fe2+uptake into the cytosol.

Electronics and sensors based on pyroelectric AlGaN/GaN heterostructures – Part B: Sensor applications

In the present article recent results concerning sensor applications of AlGaN layers and AlGaN/GaN heterostructures are summarized. The piezoresistive effect in piezoelectric AlGaN layers is investigated and the dependence of the piezoresistive gauge factor on the Al content is attributed to the influence of strain induced piezoelectric fields. An enhancement of this effect is observed in AlGaN/GaN heterostructures resulting in high longitudinal gauge factors. The response of gas sensitive Pt:GaN Schottky diodes to hydrogen and hydrogen containing gases is analyzed up to temperatures of 600 °C and employed to realize gas sensitive field effect transistors which are demonstrated to operate up to 400 °C. In addition, ion sensitive field effect transistors (ISFETs) have also been fabricated on the basis of AlGaN/GaN heterostructures. The GaN surface shows a high pH sensitivity which is attributed to the presence of a thin native metal oxide layer on the surface. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Highly pH-sensitive ion pair amphiphile vesicle

Several ion pair amphiphiles (IPA) that are composed of two positive parts (ammonium surfactants) and one negative part (dicarboxlyate surfactant) are prepared to test whether the IPA vesicles can sense the change in pH of surrounding solution and respond by reassembling their aggregation structures. Among IPA candidates, one with asymmetric chain length of positive parts showed high and reliable pH-sensitivity as compared with IPA with symmetric chain length, which can be utilized in switching-on of release of encapsulated fluorescence marker and design of pH-sensitive drug delivery vehicle. Reasons for such high pH-sensitivity are sought from combined physical properties such as turbidity, zeta potential, surface tension, and release rate of marker through the bilayer membrane.

Chemical and Functional Characterization of Products Obtained from Yam Tubers

Aiming to find alternative uses of yam (Dioscorea alata) in the food industry, three products (flour, starch and mucilage) were obtained form yam tubers and characterized. Flours were obtained from peeled fresh tubers, dried at two different temperatures (30 and 55 °C) and then milled. Starch and mucilage were extracted from mashed yam before drying. Products were characterized by their chemical, structural, morphological, rheological and thermal properties. Flours showed differences in pasting, thermal and water absorption properties as a function of drying temperature. Starch contained 30% amylose. The analysis of monosaccharide composition of starch showed traces of mannose, xylose and rhamnose. The onset gelatinization temperature was 74.4 °C for starch, whereas those for flours varied between 71.5 and 74.4 °C. The relatively high gelatinization temperature and the absence of breakdown in the amylograph test evidenced the stable structure of starch when submitted to heat and shear. The mucilagenous material contained 55.36% protein and 43.05% starch, and also an acidic polysaccharide fraction linked to a neutral fraction. Flow curves of 1% mucilage dispersions showed a pseudoplastic behavior with high pH sensitivity. Results demonstrated that yam products have promising applications in the food industry.

The effect of pH on the alternative oxidase activity in isolated Acanthamoeba castellanii mitochondria.

Mitochondria of Acanthamoeba castellanii possess a cyanide-resistant GMP-stimulated ubiquinol alternative oxidase in addition to the cytochrome pathway. In a previous work it has been observed that an interaction between the two ubiquinol-oxidizing pathways exists in intact A. castellanii mitochondria and that this interaction may be due to a high sensitivity of the alternative oxidase to matrix pH. In this study we have shown that the alternative oxidase activity reveals a pH-dependence with a pH optimum at 6.8 whatever the reducing substrate may be. The GMP stimulation of alternative oxidase is also strongly dependent on pH implicating probably protonation/deprotonation processes at the level of ligand and protein with an optimum pH at 6.8. The ubiquinone redox state-dependence of alternative oxidase activity is modified by pH in such a way that the highest activity for a given ubiquinone redox state is observed at pH 6.8. Thus pH, binding of GMP, and redox state of ubiquinone collaborate to set the activity of the GMP-stimulated alternative oxidase in isolated A. castellanii mitochondria. The high pH sensitivity of the alternative oxidase could link inactivation of the cytochrome pathway proton pumps to activation of the alternative oxidase with acceleration of redox free energy dissipation as a consequence.

Simulation and Experimental Study of the pH-Sensing Property for AlN Thin Films

The aluminum nitride (AlN) thin film has been investigated for using in pH-ISFET (ion-sensitive field-effect transistor) devices. The AlN thin film was used as the sensitive membrane for simulating and analyzing the pH sensing properties. According to the simulation, we find that the AlN/SiO2 gate ISFET has high pH sensitivity and has a linear response for the current–voltage (I–V) and capacitance–voltage (C–V) curves. In the simulation, the pH sensitivity is about 52–59.8 mV/pH for adapting in all testing solutions (pH=1–14). In addition, in the experiment, the I–V and C–V curves of AlN/SiO2 gate ISFET were also measured and the pH-sensing property was discussed in the different buffer solutions.

Chemical imaging sensor and its application to biological systems

The chemical imaging sensor is a semiconductor-based chemical sensor that can visualize the two-dimensional distribution of chemical species in the specimen. One of the prospective applications of the chemical imaging sensor is the visualization of biochemical activities in biological systems. The chemical imaging sensor was successfully applied for the detection of Escherichia coli colonies. For further applications to biological systems, enhanced adhesion of the sample to the sensing surface seems to be indispensable. As a candidate material for biocompatible sensors, porous Si was tested and was found to have a high pH sensitivity of 57.3 mV/pH.

Alternative sensor materials for light-addressable potentiometric sensors

Ta 2O 5 and porous Si were investigated as alternative sensor materials for the light-addressable potentiometric sensor (LAPS). The Ta 2O 5 LAPS, prepared by the pulsed laser deposition (PLD) technique, showed a high pH sensitivity of 57.5 mV/pH and a good linearity. Porous Si was prepared by anodic etching of Si in a mixture of HF and ethanol. The porous Si LAPS with a Si 3N 4 layer showed a pH sensitivity of 57.3 mV/pH. As an application of the porous Si LAPS, a penicillin LAPS was fabricated by immobilization of penicillinase on the surface. The pH change due to the enzymatic reaction was detected for the penicillin concentration between 250 μM and 10 mM.

Polyanion/gelatin complexes as pH‐sensitive gels for controlled protein release

AbstractPolyanion/gelatin complexes including poly(methacrylic acid) (PMAA)/gelatin, poly(acrylic acid) (PAA)/gelatin, and heparin/gelatin are investigated as pH‐sensitive gels for controlled protein release. Polyanions can interact with gelatin and form amorphous precipitates within a certain pH range, which is affected by the polyanion nature. The entrapment efficiency of model proteins (myoglobin, cytochrome c, and pepsin) into the complexes is rather high (>80%). By using a modified colloid titration that mixes a solution of gelatin and model proteins titrated with polyanion solution, myoglobin and cytochrome c are found to interact with polyanions by electrostatic forces at low pH, while pepsin either interacts with the polyanion when the pH is below its isoelectric point (IEP) or complexes with gelatin at a pH above IEPpepsin. At pH 7.4 all the complexes dissociate and proteins are rapidly released within a few hours. The complexes are stable and the proteins are retained within a certain pH range, which is related to the polyanion type (e.g., 5.0–2.0 for PMAA, 4.6–1.2 for PAA, and <4.3 for heparin). The three processes of complex formation, dissociation, and protein release have a good correlation. In addition, the protein release transition takes place within a rather narrow pH range (ca. 0.5 units) and the protein nature has little effect on the protein release profile. The high protein entrapment efficiency and good pH sensitivity of the protein release can be mainly attributed to the electrostatic attractive interactions between proteins and polyanion or gelatin. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1416–1425, 2001