pH Sensor Applications Research Articles

A convenient purification method for silver nanoclusters and its applications in fluorescent pH sensors for bacterial monitoring

Herein, we report a convenient approach to purify water-soluble dihydrolipoic acid (DHLA)-capped Ag nanoclusters (Ag NCs) by pH-induced precipitation under acidic conditions. The fluorescence of Ag NCs could be completely recovered by re-dispersing the precipitate into a basic solution using DHLA and NaBH4 as stabilizing ligands and etching reagent. DHLA-Ag NCs-doped agarose hydrogels have been prepared to monitor pH with a wide range from 8.0 to 4.0. When pH decreased, the fluorescence of the hydrogels under a UV lamp decreased and completely disappeared after pH 5. The DHLA-Ag NCs-doped agarose hydrogels biosensor showed low cytotoxicity and long stability. Accordingly, a fluorescent pH sensor for bacterial monitoring has been employed based on the “OFF–ON” signal switch of the Ag NCs-agarose hydrogel.

Multifunctional N,S co-doped carbon quantum dots with pH- and thermo-dependent switchable fluorescent properties and highly selective detection of glutathione

Smart and multifunctional materials that can be responsive to the environment change have aroused extensive attention in the last few years, because they can be suitable for various applications, such as biosensing, biotechnology and drug delivery. Herein, multifunctional N,S co-doped carbon quantum dots (N,S-CQDs) with pH dependent and color-switchable fluorescent property were synthesized directly from L-cysteine and NH3·H2O by a one-step hydrothermal route at 100 °C. The N,S-CQDs are responsive to pH and exhibit color-switchable fluorescence performance between alkaline and acidic environments with good reversibility. The reasonable mechanism was also proposed. The N,S-CQDs exhibit well ionic stability, good biocompatibility and temperature sensitive fluorescent properties. In addition, the N,S-CQDs show a highly selective detection towards glutathione from other bithiols such as Cys and Hcy, which make the N,S-CQDs be sensor reagents for glutathione detection. Given these excellent performances, the as-synthesized N,S-CQDs have great potential to be used for pH sensor, temperature sensor and bioengineering applications in the near future.

(Invited) Oxide Neuron Transistor for High-Performance Biochemical Sensors

Synergic integration of presynaptic inputs from the dendrites plays an important role for sensory information process and cognitive computation, and the idea of building bio-inspired solid-state devices has been around for decades. In 1992, Shibata et al proposed the Si-based neuron transistors with multiple input gates that are capacitively coupled to a floating gate . The “on” or “off” state of the neuron transistors depends on the integrated effect of the multiple input gates. One of the unique features of the neuron transistors is the ultralow power dissipation during the calculation due to the gate-level sum operation in a voltage mode. From then on, Si-based neuron transistors have attracted much attention for chemical and biological detection due to the easy adjustment of threshold voltage. But, up to now, electrolyte-gated neuron transistors with amorphous oxide channel layers for biochemical sensing applications are not reported. For portable applications, low-voltage operation is preferred. Electrolyte gated electric-double-layer (EDL) transistors can act as potential candidates with a low operation voltage due to the strong EDL modulation at the electrolyte/channel interface. Recently, oxide-based EDL transistors gated by solid-state inorganic electrolytes were proposed by our group. At the same time, artificial synapses and neuron transistors with low power consumption and fundamental biological functions were mimicked in individual device. The sensitivity of a standard ion-sensitive field-effect transistor is limited to be 59.2 mV/pH (Nernst limit) at room temperature. Here, inspired by the dendritic integration and spiking operation of a biological neuron, oxide-based neuron transistors with multiple input gates are fabricated on flexible plastic substrates for pH sensor applications. When the neuron transistor sensor is operated in a quasi-static dual-gate synergic sensing mode, it shows a high pH sensitivity of ~105 mV/pH, which is higher than the Nernst limit. Our results also demonstrate that single-spike dynamic mode can remarkably improve pH sensitivity and reduce response/recover time and power consumption. At last, we find that appropriate depression applied on the sensing gate electrode can further enhance the pH sensitivity and reduce the power consumption. Our flexible neuron transistors provide a new-concept sensory platform for biochemical detection with high sensitivity, rapid response and ultralow power consumption.

Characterization and application of a new pH sensor based on magnetron sputtered porous WO3 thin films deposited at oblique angles

In this communication we report about an outstanding solid-state pH sensor based on amorphous nanocolumnar porous thin film electrodes. Transparent WO3 thin films were deposited by reactive magnetron sputtering in an oblique angle configuration to enhance their porosity onto indium tin oxide (ITO) and screen printed electrodes (SPE). The potentiometric pH response of the nanoporous WO3-modified ITO electrode revealed a quasi-Nernstian behaviour, i.e. a linear working range from pH 1 to 12 with a slope of about ⿿57.7mV/pH. pH detection with this electrode was quite reproducible, displayed excellent anti-interference properties and a high stable response that remained unaltered over at least 3 months. Finally, a pH sensor was developed using nanoporous WO3-modified screen printed electrode (SPE) using a polypyrrole-modified Ag/AgCl electrode as internal reference electrode. This full solid state pH sensor presented a Nernstian behaviour with a slope of about ⿿59mV/pH and offered important analytical and operation advantages for decentralized pH measurements in different applications.

Flexible Sensory Platform Based on Oxide-based Neuromorphic Transistors.

Inspired by the dendritic integration and spiking operation of a biological neuron, flexible oxide-based neuromorphic transistors with multiple input gates are fabricated on flexible plastic substrates for pH sensor applications. When such device is operated in a quasi-static dual-gate synergic sensing mode, it shows a high pH sensitivity of ~105 mV/pH. Our results also demonstrate that single-spike dynamic mode can remarkably improve pH sensitivity and reduce response/recover time and power consumption. Moreover, we find that an appropriate negative bias applied on the sensing gate electrode can further enhance the pH sensitivity and reduce the power consumption. Our flexible neuromorphic transistors provide a new-concept sensory platform for biochemical detection with high sensitivity, rapid response and ultralow power consumption.

Potentiometric RuO2–Ta2O5 pH sensors fabricated using thick film and LTCC technologies

The paper reports on the preparation, properties and application of potentiometric pH sensors with thick film RuO2–Ta2O5 sensing electrode and Ag/AgCl/KCl reference electrode screen printed on an alumina substrate. Furthermore, it presents fabrication procedure and characterization of a new miniaturized pH sensor on LTCC (low temperature cofired ceramics) substrate, destined for wireless monitoring. The crystal structure, phase and elemental composition, and microstructure of the films were investigated by X-ray diffractometry, Raman spectroscopy, scanning electron microscopy and energy dispersive spectroscopy. Potentiometric characterization was performed in a wide pH range of 2–12 for different storage conditions and pH loops. The advantages of the proposed thick film pH sensors are: (a) low cost and easy fabrication, (b) excellent sensitivity close to the Nernstian response (56mV/pH) in the wide pH range, (c) fast response, (d) long lifetime, (e) good reproducibility, (f) low hysteresis and drift effects, and (g) low cross-sensitivity towards Li+, Na+ and K+ as interfering ions. The applicability of the sensors for pH measurement of river, tap and distilled water, and some drinks was also tested.

Lateral-Coupled Junctionless IZO-Based Electric-Double-Layer Thin-Film Transistors Gated by Solid-State Phosphosilicate Glass Electrolyte**Supported by the National Natural Science Foundation of China under Grant Nos 51302276 and 51301043, and the Ningbo Natural Science Foundation under Grant No 2013A610129.

We describe the lateral-coupled junctionless indium-zinc-oxide (IZO) thin-film transistors (TFTs) in which there are no junctions between channel and source/drain electrodes and with solid-state phosphosilicate glass electrolyte (PSG) gating. Due to the three-dimensional high proton conduction and lateral coupled electric-double-layer (EDL) capacitance (>1 μF/cm2) of the PSG, the low voltage (2.0 V) junctionless IZO TFTs and the dual coplanar gate devices are obtained. An AND logic function is demonstrated on the basis of the junctionless EDL-TFTs. Such devices are promising for applications in pH sensors, humidity sensors, biosensors, and neuron network simulation.

Apparent pH sensitivity of solution-gated graphene transistors.

Solution-gated graphene transistors were developed recently for use in pH sensor applications. The device operation is understood to rely on the capability of hydronium and hydroxide ions in solution to change the electrical properties of graphene. However, hydronium and hydroxide ions are accompanied by other ionic species in a typical acidic or basic solution and, therefore, the roles of these other ionic species must be also considered to fully understand the pH response of such devices. Using series of pH buffer solutions designed carefully, we verified that the magnitude and even the direction of pH-dependent Dirac voltage (VDirac) shift (the typical pH sensing indicator) depend strongly on the concentration and composition of the buffers used. The results indicate that the interpretation of the apparent pH-dependent VDirac response of a solution-gated graphene transistor must include the contributions of the additional ions in the solution.

Application of Optical pH Sensors in the Microfluidic Free-flow Isoelectric Focusing of Biomolecules

We present methods for fabrication and application of microfluidic chips for free-flow electrophoresis (FFE) that allow integrated pH sensing. Monitoring of the pH gradient in microfluidic free-flow isoelectric focusing (FFIEF) allows on-line adjustment of the parameters for electrophoresis and evaluation of the isoelectric point (pI) of focused analyte bands almost in real time. The fabrication of these assemblies is demonstrated via multistep photopolymerization or a laser cutting and lamination technique. Integration of pH sensing layers is accomplished via photopolymerization or inkjet printing of hydrogels that contain covalently attached pH probes. These microchips were characterized and employed for the FFIEF of proteins, peptides and plasma. Multispectral microscopic imaging was used to separate the pH sensor and the biomolecule fluorescence. We also show pH sensing in FFE based on time-domain dual lifetime referencing (t-DLR), a microreactor for integrated labelling prior to the FFIEF and the isoelectric focusing of unlabeled compounds.

Morphology controllable fabrication of poly-o-phenylenediamine microstructures tuned by the ionic strength and their applications in pH sensors

We report a one-recipe-various-structure method to prepare poly(o-phenylenediamine) (PoPD) microstructures. The assembly units, i.e., PoPD oligomers, were formed in the presence of a low redox potential oxidant of CuCl2. The further assembly process of the oligomers was triggered by the introduction of concentrated NaCl solution. The belts, bow-tie-shaped microstructures and clusters of PoPD were fabricated in a controlled manner by varying the concentration of NaCl and oPD. The introduction of NaCl is believed to influence the hydrogen bonding behaviour of PoPD oligomers. The morphology and optical properties were influenced by acidic or basic treatment. Interestingly, the belts after treating with NaOH produce strong fluorescence emission. A pH sensor based on this property was developed.

바이오센서로 응용을 위한 양극산화알루미늄의 양극산화 온도에 따른 제작 및 전기적 특성

We fabricated the electrolyte-dielectric-metal (EDM) sensor on the base of AAO (anodic aluminum oxide) template with variation of the anodizing temperature. When a surface is immersed or created in an aqueous solution, a discontinuity is formed at the interface where such physicochemical variables as electrical potential and electrolyte concentration change significantly from the aqueous phase to another phase. Because of the different chemical potentials between the two phases, charge separation often occurs at the interfacial region [1]. This interfacial region, togeter with the charged surface, is usually known as the electrical double layer (EDL) [2]. The structural and electrochemical properties of AAO sensor were investigated for applications in capacitive pH sensors. To change the thickness of the AAO template, the anodizing temperature was varied from <TEX>$5^{\circ}C$</TEX> to <TEX>$20^{\circ}C$</TEX>, the thickness of the AAO template invreased from 300 nm to 477 nm. The pH sensitivity of sensors with the anodizing temperature of <TEX>$20^{\circ}C$</TEX> showed the highest value of 56.4 mV/pH in the pH range of 3 to 11. The EDM sensor with the anodizing temperature of <TEX>$20^{\circ}C$</TEX> exhibited the best long-term stability of 0.037 mV/h.

Synthesis, Cellular Delivery and &lt;em&gt;In vivo&lt;/em&gt; Application of Dendrimer-based pH Sensors

The development of fluorescent indicators represented a revolution for life sciences. Genetically encoded and synthetic fluorophores with sensing abilities allowed the visualization of biologically relevant species with high spatial and temporal resolution. Synthetic dyes are of particular interest thanks to their high tunability and the wide range of measureable analytes. However, these molecules suffer several limitations related to small molecule behavior (poor solubility, difficulties in targeting, often no ratiometric imaging allowed). In this work we introduce the development of dendrimer-based sensors and present a procedure for pH measurement in vitro, in living cells and in vivo. We choose dendrimers as ideal platform for our sensors for their many desirable properties (monodispersity, tunable properties, multivalency) that made them a widely used scaffold for several biomedical devices. The conjugation of fluorescent pH indicators to the dendrimer scaffold led to an enhancement of their sensing performances. In particular dendrimers exhibit reduced cell leakage, improved intracellular targeting and allow ratiometric measurements. These novel sensors were successfully employed to measure pH in living HeLa cells and in vivo in mouse brain.

Highly air stable branched octithiophene oligomer for organic field effect transistor and pH sensor applications

We report the synthesis, morphology, and organic field effect transistor (OFET) characteristics of novel branched octithiophene oligomer: 5,5′″″-dihexyl-3′,5″″-bis(5-hexylthiophen-2-yl)-2,2′:5′,3″:5″,2″′:4″′,2″″:4″″,2″″′-sexithiophene (8T) prepared by the Stille coupling reaction. The effects of the surface modification and film thickness on the crystallinity, charge transport and sensing characteristics were explored systematically. 8T exhibited two absorption peaks at 300 and 350 nm with an optical band gap of 2.88 eV. It showed a highly crystalline structure with a melting point at 65 °C and had a lamellar packing structure, confirmed by grazing incidence small-angle X-ray scattering (GISAXS). Among different vapor-deposition conditions, the 8T based OFETs showed the field effect mobility up to 2.12 × 10−2 cm2 V−1 s−1 and a high on/off ratio of 5.1 × 106 on the bare substrate. The relatively low-lying HOMO energy level (−5.53 eV) of 8T led to the outstanding air stability of the OFET device. Also, the 8T sensor devices maintained the stable device performance under aqueous conditions, and the current ratio between pH values of 5 and 10 could be above 3 orders of magnitude. The above results demonstrated that the air-stable 8T could be as a promising candidate for high performance organic electronic devices.

Modeling of Hydrogel Coated Fiber Bragg Grating pHSensor

This paper present the modeling and simulation of the hydrogel coated FBG for pH sensor application. The hydrogel swelling was modeled by using Poisson-Nernst-Planck equation and mechanical equilibrium equation. The simulation was done by assuming that the hydrogel was coated on unetched fiber and etched fiber. The strain induced at the fiber core was analyzed using finite element method. It was found that at the region near the jacketed fiber, the strain is distributed nonuniformly. The non uniform area for etched fiber is shorter than that of unetched fiber. The Bragg wavelength shift due to strain of the etched fiber is much larger than that of the unetched fiber.

PH Sensing Characterization of Programmable Sm2O3/Si3N4/SiO2/Si Electrolyte–Insulator–Semiconductor Sensor with Rapid Thermal Annealing

A programmable silicon–oxide–nitride–oxide–silicon (SONOS)-like electrolyte–insulator–semiconductor (EIS) sensor with a Sm2O3/Si3N4/SiO2/Si structure is demonstrated for pH detection. This proposed programmable EIS sensor with insulator–nitride–oxide (INO) multiple sensing layers exhibits a high pH sensitivity (larger than the ideal Nernst response, ~59 mV/pH) owing to the hydrogen ion attraction by electrons trapped within the embedded Si3N4 layer after stressing. To increase the pH response, rapid thermal annealing is performed on the programmable EIS devices. The obtained results support our hypothesis that the pH response can be effectively increased after 700 °C annealing. A sensing model on the basis of the charge attraction and surface roughness is also presented. Compared with the conventional EIS device, the programmable EIS sensor functionalized with annealing and stressing can be used in future pH sensor applications owing to its high pH sensing response.

PH Sensing Characterization of Programmable Sm2O3/Si3N4/SiO2/Si Electrolyte–Insulator–Semiconductor Sensor with Rapid Thermal Annealing

A programmable silicon–oxide–nitride–oxide–silicon (SONOS)-like electrolyte–insulator–semiconductor (EIS) sensor with a Sm2O3/Si3N4/SiO2/Si structure is demonstrated for pH detection. This proposed programmable EIS sensor with insulator–nitride–oxide (INO) multiple sensing layers exhibits a high pH sensitivity (larger than the ideal Nernst response, ∼59 mV/pH) owing to the hydrogen ion attraction by electrons trapped within the embedded Si3N4 layer after stressing. To increase the pH response, rapid thermal annealing is performed on the programmable EIS devices. The obtained results support our hypothesis that the pH response can be effectively increased after 700 °C annealing. A sensing model on the basis of the charge attraction and surface roughness is also presented. Compared with the conventional EIS device, the programmable EIS sensor functionalized with annealing and stressing can be used in future pH sensor applications owing to its high pH sensing response.

Effects of Thickness Effect and Rapid Thermal Annealing on pH Sensing Characteristics of Thin HfO2 Films Formed by Atomic Layer Deposition

In this article, thin hafnium oxide (HfO2) films deposited by atomic layer deposition (ALD) were investigated as a sensing layer on an electrolyte–insulator–semiconductor (EIS) structure for pH sensor applications. Compared with sputtering, ALD provides the possibility of shrinking the thickness of the HfO2 sensing layer down to 3.5 nm with a low drift coefficient (<0.2 mV/h). To increase the pH response, an additional rapid thermal annealing was performed on a 3.5-nm-thick ALD-HfO2 film. After annealing at 900 °C, the pH sensitivity could be effectively increased to that near the Nernstian response (59.6 mV/pH), and the drift coefficient (<1 mV/h) and hysteresis width (4.26 mV) are still sufficient. On the basis of the results of atomic force microscopy (AFM) analysis, the increase in surface roughness is possibly the reason for the increase in pH response after annealing. For compatibility with advanced complementary metal–oxide–semiconductor (CMOS) technology, the thin ALD-HfO2 film is a promising candidate for pH sensor fabrications.

CuO nanoflowers as an electrochemical pH sensor and the effect of pH on the growth

Well-crystallized flower-shaped cupric oxide (CuO) nanostructures composed of thin leaves have been synthesized by simple low-temperature chemical bath method and used to fabricate pH sensor. We examined the effect of the pH on the growth of the CuO nanostructures, by changing the pH of the precursor solutions different morphologies of the CuO nanostructures were obtained. CuO nanoflowers have recently become important as a material that provides an effective surface for electrochemical activities with enhanced sensing characteristics. The proposed sensor exhibited a linear electrochemical response within a wide pH range of (2–11). The experimental results (time response, electrochemical activity, reproducibility, absorption spectra, and XRD) indicate that the CuO nanoflowers can be used in pH sensor applications with enhanced properties.

Development of Engineered Sensing Membranes for Field-Effect Ion-Sensitive Devices Based on Stacked High-$\kappa$ Dielectric Layers

We evaluated the pH-sensing properties of engineered sensing membranes fabricated by stacking high-κ dielectric layers to improve the sensitivity and long-term stability of field-effect ion-sensitive device applications. The SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (OHA) stacked layer structure was proposed as an engineered sensing membrane, and the characteristics were compared with SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (O) and SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (ON) membranes. As a result, the engineered OHA sensing membrane revealed higher capacitance and better sensitivity and stability than the O and ON membranes. The electrolyte-insulator-semiconductor device with the OHA membrane exhibited a high sensitivity level of 54.4 mV/pH, a low hysteresis voltage of 14.26 mV, and a small drift rate of 2.15 mV/h. Therefore, the use of engineered OHA sensing membranes is suitable for increasing pH sensitivity and is promising for long-term stable field-effect pH sensor applications.

Reference Electrode–Insulator–Nitride–Oxide–Semiconductor Structure with Sm2O3 Sensing Membrane for pH-Sensor Application

We investigate a reference electrode–insulator–nitride–oxide–semiconductor (RINOS) structure with a Sm2O3 sensing membrane and using silicon nitride as the charge trapping layer for pH detection. The proposed RINOS device with the oxide–nitride–oxide (ONO) structure exhibits a high pH sensitivity (larger than the ideal Nernst response, ∼59 mV/pH) owing to hydrogen ion adsorption by the trapped electrons within the embedded Si3N4 layer when applying a stress voltage. As the applied voltage and time increase, pH sensitivity increased. The possible sensing mechanism based on charge attraction was demonstrated using schematic band diagrams. To improve the retention of an increased sensitivity, an additional SiO2 layer as a blocking layer between the Sm2O3 and Si3N4 films to form the RIONOS device was proposed. Compared with the conventional electrolyte–insulator–semiconductor (EIS) structure, the proposed RIONOS device can be used to detect ultra small pH variations owing to its high pH-sensing response.