High pH Sensitivity Research Articles

Analytical Modeling and Simulation of AlGaN/GaN MOS-HEMT for High Sensitive pH Sensor

This paper presents an analytical model of AlGaN/GaN MOS-HEMT based pH sensor for the first time to determine pH of different electrolyte solutions. Gouy-chapman stern model has been used to calculate the surface charge density. The results obtained using analytical model have been verified and show good agreement with the simulated results. Cavity length, thickness of AlGaN barrier layer and Al composition have been optimized to improve the sensitivity of the device. It has been observed that the drain current and threshold voltage decreases with increase in pH of the electrolytic solutions. Proposed AlGaN/GaN MOS-HEMT sensor demonstrate a quick response to the pH changes. The maximum drain-on-sensitivity of the device is 132mA/mm-pH. Surface potential and threshold voltage sensitivity of 0.95mV/pH and 950mV/pH respectively have been obtained which is much higher than the Nernstian limit(59.2mV/pH).

PH Sensitivity Estimation in Potentiometric Metal Oxide pH Sensors Using the Principle of Invariance

A numerically solvable engineering model has been proposed that predicts the sensitivity of metal oxide- (MOX-) based potentiometric pH sensors. The proposed model takes into account the microstructure and crystalline structure of the MOX material. The predicted pH sensitivities are consistent with experimental results with the difference below 6% across three MOX (RuO2, TiO2, and Ta2O5) analysed. The model distinguishes the performance of different MOX phases by the appropriate choice of surface hydroxyl site densities and dielectric constants, making it possible to estimate the performance of MOX electrodes fabricated through different high-temperature and low-temperature annealing methods. It further addresses the problem, cited by theoreticians, of independently determining the C1 inner Helmholtz capacitance parameter while applying the triple-layer model to pH sensors. This is done by varying the C1 capacitance parameter until an invariant pH sensitivity across different electrolyte ionic strengths is obtained. This invariance point identifies the C1 capacitance. The corresponding pH sensitivity is the characteristic sensitivity of MOX. The model has been applied across different types of metal oxides, namely, expensive platinum group oxides (RuO2) and cheaper nonplatinum group MOX (TiO2 and Ta2O5). High temperature annealed, RuO2 produced a high pH sensitivity of 59.1082 mV/pH, while TiO2 and Ta2O5 produced sub-Nernstian sensitivities of 30.0011 and 34.6144 mV/pH, respectively. Low temperature annealed, TiO2 and Ta2O5 produced Nernstian sensitivities of 59.1050 and 59.1081 mV/pH, respectively, illustrating the potential of using cheaper nonplatinum group MOx as alternative sensor electrode materials. Separately, the usefulness of relatively less investigated, cheap, and readily available MOX, viz. Al2O3, as the electrode material was analysed. Low-temperature-annealed Al2O3 with a Nernstian sensitivity of 59.1050 mV/pH can be considered as a potential electrode material. The proposed engineering model can be used as a preliminary prediction mechanism for choosing potentially cheaper alternative sensor electrode materials.

PHmScarlet is a pH-sensitive red fluorescent protein to monitor exocytosis docking and fusion steps

pH-sensitive fluorescent proteins (FPs) are highly advantageous for the non-invasive monitoring of exocytosis events. Superecliptic pHluorin (SEP), a green pH-sensitive FP, has been widely used for imaging single-vesicle exocytosis. However, the docking step cannot be visualized using this FP, since the fluorescence signal inside vesicles is too low to be observed during docking process. Among the available red pH-sensitive FPs, none is comparable to SEP for practical applications due to unoptimized pH-sensitivity and fluorescence brightness or severe photochromic behavior. In this study, we engineer a bright and photostable red pH-sensitive FP, named pHmScarlet, which compared to other red FPs has higher pH sensitivity and enables the simultaneous detection of vesicle docking and fusion. pHmScarlet can also be combined with SEP for dual-color imaging of two individual secretory events. Furthermore, although the emission wavelength of pHmScarlet is red-shifted compared to that of SEP, its spatial resolution is high enough to show the ring structure of vesicle fusion pores using Hessian structured illumination microscopy (Hessian-SIM).

PEGylated DOX-coated nano graphene oxide as pH-responsive multifunctional nanocarrier for targeted drug delivery

Nano graphene oxide (NGO) has high drug-loading capacity due to its huge surface area. However, the limited stability and the poor biocompatibility of NGO hampered its application as drug delivery carrier under physiological conditions. Thereby, a new strategy of using chemical conjugation on NGO with hydrophilic polymers was adopted but currently was too complicated, low yield and costly. In this study, doxorubicin-hyd-PEG-folic acid (DOX-hyd-PEG-FA) polymers were coated on the surface of NGO via π–π stocking and the hydrophobic effect between DOX and NGO. With the PEG shell protection, the biocompatibility of NGO was significantly improved. The drug-loading capacity of nanoparticles was more than 100%. FA ligands on the nanoparticle could guide the nanoparticles actively targeting to tumour cells. The hydrazone bond between DOX and PEG was decomposed spontaneously in the weakly acidic environment, which made PEG layer dissociated from NGO. Furthermore, DOX was easily protonized at low pH conditions, which weakened the interaction between DOX and NGO. Thus, DOX could be released rapidly from the nanoparticles in tumour cells. In summary, NGO@DOX-hyd-PEG-FA is an easy-prepared nanoparticle with excellent biocompatibility, high pH-sensitivity and active tumour targeting. Therefore, it is a promising multifunctional nanocarrier effective for targeted drug delivery.

Dual stimuli-responsive nanoplatform based on core-shell structured graphene oxide/mesoporous silica@alginate

A dual stimuli-responsive nanoplatform was rationally designed for controlled drug delivery. The nanosheets of graphene oxide (GO) were first modified with aminated mesoporous silica (NH2-mSiO2), and then methotrexate (MTX) was loaded into the mesopores of mSiO2. Alginate (Alg) acted as the “gatekeeper” was then anchored to the MTX-loaded GO/NH2-mSiO2 by amidation reaction, achieving the encapsulation of MTX in the core-shell structured GO/mSiO2@Alg. Due to the high pH sensitivity of amide bond and the excellent photothermal conversion ability of GO, the constructed nanoplatform could be used for pH and near-infrared (NIR) controlled delivery of MTX. The results of cell viability test demonstrate the high inhibitory rate of the dual stimuli-responsive nanoplatform toward hepatoma (HepG2) cells.

High Sensing Performance of Nanoimprinted HfO2 Sensing Membrane for Electrode-Insulator-Semiconductor pH Sensors

In this paper, HfO2 thin films as a sensing membrane deposited on nanoimprinted Si substrates by both atomic layer deposition (ALD) and sputtering methods were investigated for an electrolyte–insulator–semiconductor (EIS) pH sensor application. X-ray diffraction, X-ray photoelectron spectroscopy and atomic force microscopy were performed to examine the crystalline structures, chemical compositions and surface morphologies of the HfO2 sensing films, respectively. The structural properties of these HfO2 films were correlated to their sensing performances. Compared with the sputtering method, the unpatterned HfO2 sensing film by the ALD method showed a higher pH sensitivity of 55.06 mV pH−1, a lower drift rate of 0.31 mV h−1 and a smaller hysteresis voltage of 0.90 mV. The high pH sensitivity and good stability may be attributed to the increase in surface roughness and the reduction of a silicate layer caused by the ALD method. Additionally, the ALD-HfO2 EIS sensor deposited on the nanoimprinted Si substrate demonstrated a Nernstian pH response (61.36 mV pH−1) than that on an unpatterned Si substrate. For compatibility with advanced complementary metal–oxide–semiconductor technology, the ALD-HfO2 sensing film deposited on the nanoimprinted Si substrate is a promising candidate for pH sensing applications.

Fabrication of natural polysaccharide based hydrogel with utility to entrap pollutants

In the recent years of innovations hydrogels plays important role in various industrial applications. Hydrogels with combination of natural polymers are widely used for its biocompatibility and ecofriendly nature such as pectin and carboxymethyl cellulose (CMC). Plasticizers are utilized to enrich the physicochemical characteristics of hydrogels based on natural polymers. Polyethylene glycol 400 (PEG 400) is known to render thermostability. Whereas copper ions has wound healing properties and acts as biocide. These hydrogels due to its high porosity can have various utility to entrap small particles such as dust, pathogens and other pollutants and can act as very good face mask. Therefore the fabricated hydrogel is not only thermostable but also have high cytocompatibility, pH sensitivity, porosity and degree of swelling.

High Performance NiOx Extended-Gate Field-Effect Transistor Biosensor for Detection of Uric Acid

In this paper, we developed a high performance NiOx extended-gate field-effect transistor (EGFET) biosensor for detection of uric acid. The structural and sensing properties of the NiOx sensing film deposited on a n+-type Si substrate was examined for an EGFET pH sensor. X-ray diffraction, atomic force microscope and X-ray photoelectron spectroscopy were used to analyze the film features of the NiOx sensing film. The NiOx sensing film based on EGFET exhibited a high pH sensitivity of 58.53 mV pH−1, a small hysteresis voltage of 1.4 mV and a low drift rate of 0.30 mV h−1. Moreover, the NiOx EGFET biosensor showed a high linearity in the uric acid range between 1 and 30 mg dl−1. In addition, this NiOx EGFET biosensor demonstrated a very good selectivity to uric acid over other interfering substrates (ascorbic acid, glucose, urea).

Cyanine Dyes Containing Quinoline Moieties: History, Synthesis, Optical Properties, and Applications.

Cyanine dyes carrying quinoline moieties are an important class of organic molecules that are of great interest for applications in many fields like medicine, pharmacology, and engineering. Despite their exceptional properties, such as stability, high molar extinction coefficients, and high pH-sensitivity, this class of dyes has been less analyzed and reviewed in the last few decades. Therefore, this review article focuses on discussing the history of quinoline compounds, various synthetic routes to prepare quinolinium salts and symmetrical and asymmetrical mono-, di-, tri-, penta- and heptamethine cyanine dyes, containing quinoline moieties, together with their optical properties and applications as photosensitizers in photodynamic therapy, probes in biomolecules for labeling of nucleic acids, as well as imaging agents.

Colorimetric Textile Sensor for the Simultaneous Detection of NH3 and HCl Gases.

For the immediate detection of strong gaseous alkalis and acids, colorimetric textile sensors based on halochromic dyes are highly valuable for monitoring gas leakages. To date, colorimetric textile sensors for dual-gas detection have usually been fabricated by electrospinning methods. Although nanofibrous sensors have excellent pH sensitivity, they are difficult to use commercially because of their low durability, low productivity, and high production costs. In this study, we introduce novel textile sensors with high pH sensitivity and durability via a facile and low-cost screen-printing method. To fabricate these textiles sensors, Dye 3 and RhYK dyes were both incorporated into a polyester fabric. The fabricated sensors exhibited high detection rates (<10 s) and distinctive color changes under alkaline or acidic conditions, even at low gas concentrations. Furthermore, the fabricated sensors showed an outstanding durability and reversibility after washing and drying and were confirmed to contain limited amounts of hazardous materials. Thus, our results show that the fabricated textile sensors could be used in safety apparel that changes its color in the presence of harmful gases.

Over 59 mV pH−1 Sensitivity with Fluorocarbon Thin Film via Fluorine Termination for pH Sensing Using Boron‐Doped Diamond Solution‐Gate Field‐Effect Transistors

pH sensing facilitates many substantial aspects of the society such as chemical laboratory analysis, agriculture, or water and soil qualities. However, existing pH sensors have problems and limitations such as fragility, hysteresis, or slow responding time. In this research, a new method utilizing fluorocarbon thin film via fluorine termination and boron‐doped diamond (BDD) solution‐gate field‐effect transistors (SGFETs) for pH sensing is developed for the first time. The fluorocarbon film device demonstrates a high pH sensitivity of 67.4 and 34.9 mV pH−1 in acid and alkaline pH regions, respectively.

Target-Specific Superparamagnetic Hydrogel with Excellent pH Sensitivity and Reversibility: A Promising Platform for Biomedical Applications.

Superparamagnetism has been widely used for many biomedical applications, such as early detection of inflammatory cancer and diabetes, magnetic resonance imaging (MRI), hyperthermia, etc., whereas incorporation of superparamagnetism in stimulus-responsive hydrogels has now gained substantial interest and attention for application in these fields. Recently, pH-responsive superparamagnetic hydrogels showing the potential use in disease diagnosis, biosensors, polymeric drug carriers, and implantable devices, have been developed based on the fact that pH is an important environmental factor in the body and some disease states manifest themselves by a change in the pH value. However, improvement in pH sensitivity of magnetic hydrogels is a dire need for their practical applications. In this study, we report the distinctly high pH sensitivity of new synthesized dual-responsive magnetic hydrogel nanocomposites, which was accomplished by copolymerization (free-radical polymerization) of two pH-sensitive monomers, acrylic acid (AA) and vinylsulfonic acid (VSA) with an optimum ratio, in the presence of presynthesized superparamagnetic iron oxide nanoparticles (Fe3O4(OH)x). The monomers contain pH-sensitive functional groups (COO– and SO3– for AA and VSA, respectively), and they have also been widely used as biomaterials because of the good biocompatibility. The pH sensitivity of the superparamagnetic hydrogel, poly(acrylic acid-co-vinylsulfonic acid), PAAVSA/Fe3O4, was investigated by swelling studies at different pH values from pH 7 to 1.4. Distinct pH reversibility of the system was also demonstrated through swelling/deswelling analysis. Thermal stability, chemical configuration, magnetic response, and structural properties of the system have been explored by suitable characterization techniques. Furthermore, the study reveals a pH-responsive significant change in the overall morphology and packing fraction of iron oxide nanoparticles in PAAVSA/Fe3O4 via energy-dispersive X-ray (EDX) elemental mapping with the field emission scanning electron microscopy (FESEM) study (for freeze-dried PAAVSA/Fe3O4, swelled at different pH values), implying a drastic change in susceptibility and induced saturation magnetization of the system. These important features could be easily utilized for the purpose of diagnosis using magnetic probe and/or impedance analysis techniques.

Liquid-crystal-droplet-based Monitoring System for Water-soluble Inorganic Acidic Gases from the Atmosphere

A liquid crystal (LC)-based pH sensor for real-time monitoring of variations in localized pH values near the LC droplets was developed, and its applicability for the detection of acidic gases was explored. It was found that 4-cyano-4′-pentylbiphenyl (5CB), when doped with hexanoic acid (HA), shows a bright-fan-shaped (planar orientation of LCs) to dark-cross (homeotropic orientation of LCs) optical response to a minimal change of the pH value (from 6.5 to 6.6). The fast and intensive pH-driven optical response can be explained mainly with the orientational transitions of 5CB, induced by the protonation and deprotonation of HA at the interface of aqueous/LC droplet. Because of its high pH sensitivity, the LC-based sensor was further exploited for monitoring the local pH changes, which were originated from concentration changes of acidic gases. The results suggested that at an industrial scale/production environment, the sensor shows excellent performance for long-term monitoring and sensing of acidic gases. This type of LC-based sensor may find to fulfill its applicability potential in the trace measurement of acidic gases.

Water soluble porphyrin-fluorescein triads: Design, DFT calculation and pH-change-triggered fluorescence response

This paper reports the results of the synthesis and identification of water soluble trans-diaxial complexes of Sn(IV)-5,10,15,20-tetra-(4-sulfophenyl)porphyrin with fluorescein, in which the organic ligands are bound to the tin cation of the tetrapyrrolic macrocycle via a carboxyl (triad I) or a hydroxyl (triad II) group. The structures of triads (I, II) were confirmed by the methods of one- and two-dimensional NMR and quantum-chemical modeling. A spectrophotometric method was used to study the spectral, acidic and fluorescence properties of the synthesized compounds. The paper also discusses the possible mechanisms of protolytic and tautomeric equilibria in the processes involving fluorescein ligands in the axial complexes with hydrophilic Sn(IV)porphyrin. It is found that the fractional distribution of the triads with the lactone, quinoid, and zwitterionic forms of fluorescein axial ligands depends on the medium acidity, exhibiting high pH sensitivity in the ranges where this substance and its derivatives in the free state do not exhibit fluorescence. As a result, the ability of the porphyrin-fluorescein triads to determine water acidity can be used to develop an effective method of detecting pH-dependent biological processes and environmental pollution. This method will facilitate the production of new fluorescent sensors for biomedical purposes (drug delivery triggered by pH changes or oxygen distribution in tissues) and engineering applications (wastewater detection).

Polyethylenimine wrapped mesoporous silica loaded benzotriazole with high pH-sensitivity for assembling self-healing anti-corrosive coatings

The aim of this work is to develop a self-healing coating doped with pH-sensitive nanocontainers for the active corrosion protection of mild steel. A pH-sensitive carboxyl-functionalized mesoporous silica nanomaterials (MSNs-COOH) was prepared via chemical modification of mesoporous silica nanomaterials (MSNs) which was synthesized in a tetraethyl orthosilicate (TEOS) solution via base catalyzed sol-gel process. Benzotriazole (BTA) molecules were loaded into the internal pores of the prepared MSNs-COOH. Then the BTA-loaded MSNs-COOH were wrapped by the highly branched polyethylenimine (PEI) to obtain the nanocontainers of BTA@MSNs-COOH-PEI. SEM, TEM, DLS, XRD, nitrogen adsorption-desorption isotherms, TGA and FT-IR analyses were employed to characterize the as-prepared nanocontainers. The pH-sensitive releasing property of BTA@MSNs-COOH-PEI was studied using UV–vis spectroscopy, which indicates that the releasing rate of BTA from BTA@MSNs-COOH-PEI can be accelerated under alkaline conditions. Afterwards, a self-healing coating was fabricated based on the epoxy resin primer doped with BTA@MSNs-COOH-PEI which can release BTA inhibitor once localized corrosion occurs. Finally, the active anti-corrosion performance of the as-prepared coating was confirmed by EIS measurements and salt spray tests. The self-healing coating prepared in this work brings a new sight for elaborate design of active anti-corrosion coatings.

Targeted Chemotherapy for Breast Cancer Using an Intelligent Doxorubicin-Loaded Hexapeptide Hydrogel.

Self-assembling peptide hydrogels have a high water content, good biocompatibility and have become a competitive research object in the fields of tissue engineering, cancer treatment and drug delivery. In our research, a hexapeptide with high pH sensitivity was designed and synthesized by utilizing a solid-phase synthesis method. Under physiological conditions, the peptide could self-assemble into a hydrogel. When it reached the tumor acidic microenvironment, the peptide was degraded and doxorubicin was released to exert its antitumor effect. A series of physicochemical properties were investigated, including gelling ability, secondary structure, micromorphology, rheological properties and drug release studies. The results illustrated that PIDO peptide hydrogel has good pH responsiveness and injectability. In vitro cytotoxicity experiments and in vivo antitumor experiments showed that PIDO peptide hydrogel has a highly effective therapeutic effect on tumor cells and is less toxic to normal tissues. Our research provides a promising option for targeted drug delivery and sustainable release.

An imidazo[1,5-α]pyridine-derivated fluorescence sensor for rapid and selective detection of sulfite

Sulfur-containing species are essential in the composition and the metabolism of the organisms, thus developing a full set of implements to cover all of them is still a favorable choice. Herein, we chose imidazo [1,5-α]pyridine moiety as the basic fluorophore for the detection of sulfite, and preliminarily completed the toolset since biothiols (GSH, Cys, Hcy), H2S, and PhSH could be detected by sensors based on the same backbone. The designed sensor, IPD-SFT, with structural novelty and large Stokes shift (130 nm), indicated the most attractive advantages of remarkably rapid response period (within 1 min) and high selectivity for sulfite from all the sulfur-containing species. Other practical properties included high sensitivity (LOD = 50 nM) and wide pH adaptability (5.0–11.0). Furthermore, IPD-SFT could monitor both exogenous and endogenous sulfite. It not only raised a potential tool for sulfite detection, but also preliminarily completed the toolset for all the sulfur-containing species. The development of such toolsets might reveal the sulfur-containing metabolism and corresponding physiology and pathological procedures.

Facile preparation of FITC-modified silicon nanodots for ratiometric pH sensing and imaging.

A ratiometric fluorescent pH sensor was facilely constructed by covalent modification of amino-terminated silicon nanodots (SiND) with pH-sensitive fluorescein isothiocyanate (FITC). After optimization, the SiND-FITC(40:1) material with a SiND:FITC initial mass ratio of 40:1 was selected for the sensing of hydrogen ions. It was observed that the material inherits the unique features of SiND and FITC, and there is significant improvement of SiND acid-base stability, which is a favorable factor in terms of providing fluorescence reference signal. The SiND-FITC(40:1) material displays not only high pH sensitivity, but also good stability and anti-interference ability, and the response process is highly reversible. Deploying the SiND-FITC(40:1) material, we have made available a simple, sensitive, and precise approach for pH sensing. In aqueous solutions, the I517/I466 fluorescence intensity ratio of SiND-FITC(40:1) increases linearly in the pH range of 5.40-7.76. This dual emission nanosensor was successfully applied for pH sensing and cellular fluorescence imaging.

A simple acidic ‘turn-on’ fluorescent pH probe based on BOPYIN and its visual detection and cellular imaging

A simple high selective and sensitive fluorescent pH probe based on a new seven-membered BOPYIN (BOrondifluoride-3,3-dimethyl-2-[2-(2-PYrrolyl) ethenyl]INdole, Probe-NH2, Scheme 1) has been designed and synthesized. With the increase of acidity (pH from 5.51 to 2.47), the emission intensity dramatically enhanced to strong fluorescence. The linear response range is from 5.5 to 3.0 with pKa = 3.63, which is suitable for pH detection in biological system. Moreover, this probe has excellent selectivity response to H+, short response time (less than 10 s), reversibility and low cytotoxicity. Possible sensing mechanism studies demonstrated that the linear response of the probes to acidity of solution is the conjugation of H+ with response site N atom in primary amino group on BOPYIN. Besides, the fluorescent change of the Probe-NH2 relies on PET and/or ICT process which is proved by experiment and further confirmed by DFT theoretical calculation. Finally, the probe has been successfully applied for visual detection and cellular imaging.

Fabrication of Colorimetric Textile Sensor Based on Rhodamine Dye for Acidic Gas Detection.

For the immediate detection of gaseous strong acids, it is advantageous to employ colorimetric textile sensors based on halochromic dyes. Thus, a rhodamine dye with superior pH sensitivity and high thermal stability was synthesized and incorporated in nylon 6 and polyester fabrics to fabricate textile sensors through dyeing and printing methods. The spectral properties and solubility of the dye were examined; sensitivity to acidic gas as well as durability and reversibility of the fabricated textile sensors were investigated. Both dyed and printed sensors exhibited a high reaction rate and distinctive color change under the acidic condition owing to the high pH sensitivity of the dye. In addition, both sensors have outstanding durability and reversibility after washing and drying.