Sensitive pH Sensor Research Articles

Highly Sensitive Potentiometric pH Sensor Based on Polyaniline Modified Carbon Fiber Cloth for Food and Pharmaceutical Applications.

This study introduces a potentiometric pH sensor that is extremely sensitive and specifically designed for food and pharmaceutical applications. The sensor utilizes a pH-sensitive interface fabricated by electropolymerizing polyaniline (PANI) on carbon fiber cloth (CFC). Structural and morphological analyses of PANI-CFC and CFC have been conducted by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS). The investigation of the functional groups was conducted by using Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The electrochemical characteristics were assessed by utilization of cyclic voltammetry (CV) and open-circuit potential (OCP) measurements in a three-electrode configuration. The sensor exhibited a sensitivity of 60.9 mV/pH, while retaining consistent performance within the pH range of 4 to 12. The repeatability and robustness of the sensors were verified. The accuracy of the PANI-CFC sensor was confirmed by validation using real samples, demonstrating its compatibility with commercially available pH sensors. The application of density functional theory (DFT) calculations revealed an interaction energy of -173.2886 kcal/mol, indicating a strong affinity of H+ ions towards PANI-CFC electrode. Further investigation was conducted to examine the chemical reactivity of PANI, revealing a HOMO-LUMO energy gap of -0.98 eV. This study highlights the PANI-CFC sensor as a reliable and efficient pH-sensing platform for food and pharmaceuticals applications, performing robustly in both laboratory and real-world settings.

A hydrazone-linked covalent organic framework as a dual-mode colorimetric and fluorescence pH sensor

Covalent organic frameworks (COFs) have emerged as a class of crystalline porous materials with stable structures and tuneable functionalities, attracting great interest in recent years. Benefiting from regular pore structure, adjustable functional unit, and excellent photophysical properties, the fluorescent COFs exhibit promising prospects for sensing application. In this study, a hydrazone-linked COF (MOH-Tf12Tf21) was synthesized by Schiff-base reaction of 2-hydroxy-1,3,5-benzenetricarbaldehyde (Tf1), 2,4-dihydroxy-1,3,5-benzenetricarbaldehyde (Tf2) and 2,5-dimethoxybenzene-1,4-dicarbohydrazide (MOH) under solvothermal conditions. The resulting MOH-Tf12Tf21 exhibits high crystallinity, excellent porosity, and good thermal and chemical stability. N2 adsorption tests assessed the surface area, further determined by the BET with a value of 507 m2 g−1. The MOH-Tf12Tf21 could also be found to be responsive to the pH values by fluorometry and colorimetry. The results suggested that MOH-Tf12Tf21 should be suitable as a sensitive, visual, and reusable fluorescence pH sensor. Moreover, the fluorescence intensity of MOH-Tf12Tf21 showed an excellent linear relationship with pH values in the range of 1.0–6.0. The nonlinear relationship between the fluorescence intensity and high pH values (7.0–14.0) was well established. Theoretical calculations were further performed to illustrate the remarkable color change and fluorescence turn-on/off response upon MOH-Tf12Tf21 interaction with different pH values. This work should provide new opportunities for developing COFs-based sensing platforms.

Negative Capacitance Dual-Gated ISFETs as Ultra-Sensitive pH Sensors.

Ion-sensitive field-effect transistors (ISFETs) are promising candidates for next-generation pH sensors, enabling highly sensitive and label-free biomolecular and chemical detection. Emerging FETs based on the negative capacitance (NC) effect offer steep-subthreshold switching and higher drive current simply by integrating a ferroelectric (FE) material into the gate stack. Here, we propose a novel NC dual-gated ISFET (NC-DG-ISFET)-based pH sensor, with FE layers integrated into both the top and the bottom gate stacks. The current and voltage sensitivities of the proposed device are extracted from its transfer characteristics, obtained by combining the numerical solutions of the one-dimensional (1D) Landau-Khalatnikov (L-K) equation with three-dimensional (3D) technology computer-aided design (TCAD) simulations. Results show that the NC-DG-ISFET can surpass the sensitivity of some of the state-of-the-art DG-ISFET pH sensors. The inclusion of the FE layers into the gate stacks of a baseline DG-ISFET leads to 51% reduction in subthreshold swing (SS), causing a 5× increase in current sensitivity (SI) in the subthreshold region of operation and a 2× increase in voltage sensitivity (SV). The influence of channel thickness and channel length on the sensor performance is also invesitgated. The findings presented here provide a new pathway to leverage the steep-switching behavior of NCFETs for the next generation of highly sensitive and label-free DG-ISFET pH sensors.

A new optical pH sensor based on a mixture of Alizarin and Orange dyes immobilized on an agarose membrane

Two techniques, including chemical immobilization of a combination of two indicators of Alizarin and Orange dyes and the epoxy activation of the agarose membrane, were used for the preparation of a new optic pH sensor. For this purpose, the mentioned dyes were immobilized on agarose support activated by an epoxy, followed by optimizing the impacts of the coupling pH, as well as the ratio and concentration of the two dyes. The sensor was set up in a flow cell and effectively employed for online pH calculations. The new optic pH sensor could be applied between 4.5 and 11pH values. The sensor could quickly respond to pH alterations in nearly 25 s. The sensor's response is adjustable and replicable. Ionic strengths of up to 0.5 mol L−1 could have no meaningful impact on the response signal. In addition, no proof of any signal drift or dye leaching was detected over a three-month period.•The chemical immobilization of two indicators on agarose membranes activated by an epoxy could lead to a sensitive optic pH sensor for a wide range of pH.•The intended sensor was mounted in a flow cell and effectively utilized for the purpose of online pH measurement.•The suggested optode was employed to determine pH in real samples of water.

Study of the pH effect on the optical and morphological properties of S, N self-doped carbon dots applied as fluorescent anti-counterfeiting ink and pH sensor

The pH value is an important parameter as it is part of several processes, whether environmental or biological. In this report, S, N self-doped carbon dots (CDs) were synthesized by a simple hydrothermal method using cysteine (cys) and citric acid as precursors for a detailed investigation of size, morphological, photoluminescent, and structural changes at different pH values and its use as pH sensor and fluorescent ink. The fluorescence intensity of cys-CDs was dependent on the pH, presenting a linear relationship with pH values in the range of 2.0–9.0. Using spectroscopic techniques, a mechanism for the pH-dependent fluorescence is proposed, based on the aggregation of cys-CDs and also protonation/deprotonation of surface functional groups that change the excited state. The cys-CDs were found to be efficient as fluorescent pH sensors using real samples (distilled water and tap water). Furthermore, the pH changes in cys-CDs can be used for the visual enhancement of anti-counterfeiting technologies. Thus, the results of this study show that cys-CDs can act as an efficient and pH sensitive fluorescent sensor, which can be used to measure the pH value of water samples, due to its high fluorescence intensity, and can be applied successfully as a fluorescent ink.

A Compact, Highly Sensitive pH Sensor Based on Polymer Waveguide Bragg Grating

Accurate probing, monitoring, and controlling the pH level is critical in different fields. In the last two decades, optical fiber/waveguide-based pH sensors have been widely investigated with the aim of further enhancement of sensitivity and miniaturization. Here, we propose a novel and cost-effective pH sensor based on polymer waveguide Bragg grating, addressing main aspects like sensitivity enhancement, miniaturization, narrow band response, and stable performance. The large evanescent field due to metal under cladding and high index coating combined with excellent penetration depth match with pH-sensitive polymer coating of poly-acrylic acid/poly-allylamine hydrochloride (PAA/PAH) in an aqueous medium, ensures a sensitivity of 10.4 nm/pH for a pH range of 4 to 7, two orders higher than all reported Bragg grating pH sensors to our knowledge. A narrow band (∼1 nm) response has been ensured by low propagation loss of the TE mode. Further, the variations in sensitivity and peak reflectivity are within 3% for a ±10% inaccuracy in various structural parameters, reflecting excellent fabrication tolerances. The proposed sensor is an important development, especially for the perishable food industry in which a high sensitivity in a pH range of 4 to 7 is highly desirable to accurately determine the freshness of foods.

Highly sensitive colorimetric and fluorescent pH sensor of physiological interest

Highly sensitive colorimetric and fluorescent pH sensor of physiological interest

Preparation of nanopillar array electrode of iridium oxide for high performance of pH sensor and its real‐time sweat monitoring

AbstractA highly sensitive potentiometric pH sensor was developed based on nanopillar array electrodes of iridium oxide. The as‐prepared pH sensor exhibited a high pH sensitivity (69.43 mV/pH), fast response time (8.1 s), and good durability (0.76 mV/h). The sensing performance of the pH sensor was maintained under mechanical bending and even after 1000 repetitive bending/releasing cycles. As a proof‐of‐concept, a wearable headband sensor was fabricated by integrating the pH sensor with a wireless electronic module based on a printed circuit board. The on‐body test indicated that the wearable pH sensor provides reliable and stable data in the real‐time monitoring of pH changes in human sweat during stationary indoor cycling. The pH sensors based on nanopillar array electrodes of iridium oxide have great potential in many portable or wearable applications in healthcare systems, non‐invasive diagnostics, environmental analysis, and food sensors.

Fully Transparent and Highly Sensitive pH Sensor Based on an a-IGZO Thin-Film Transistor with Coplanar Dual-Gate on Flexible Polyimide Substrates

In this paper, we propose a fully transparent and flexible high-performance pH sensor based on an amorphous indium gallium zinc oxide (a-IGZO) thin-film transistor (TFT) transducer with a coplanar dual-gate structure on polyimide substrates. The proposed pH sensor system features a transducer unit consisting of a floating gate (FG), sensing gate (SG), and control gate (CG) on a polyimide (PI), and an extended gate (EG) sensing unit on a separate glass substrate. We designed a capacitive coupling between (SG) and (CG) through the FG of an a-IGZO TFT transducer to contribute to sensitivity amplification. The capacitance ratio (CSG/CCG) increases linearly with the area ratio; therefore, the amplification ratio of the pH sensitivity was easily controlled using the area ratio of SG/CG. The proposed sensor system improved the pH sensitivity by up to 359.28 mV/pH (CSG/CCG = 6.16) at room temperature (300 K), which is significantly larger than the Nernstian limit of 59.14 mV/pH. In addition, the non-ideal behavior, including hysteresis and drift effects, was evaluated to ensure stability and reliability. The amplification of sensitivity based on capacitive coupling was much higher than the increase in the hysteresis voltage and drift rate. Furthermore, we verified the flexibility of the a-IGZO coplanar dual-gate TFT transducer through a bending test, and the electrical properties were maintained without mechanical damage, even after repeated bending. Therefore, the proposed fully transparent and highly sensitive a-IGZO coplanar dual-gate TFT-based pH sensor could be a promising wearable and portable high-performance chemical sensor platform.

Dual fluorescent hollow silica nanofibers for in situ pH monitoring using an optical fiber.

This study reports a sensitive and robust pH sensor based on dual fluorescent doped hollow silica nanofibers (hSNFs) for in situ and real-time pH monitoring. Fluorescein isothiocyanate (FITC) and tris(2,2'-bipyridyl)dichlororuthenium(ii) hexahydrate (Ru(BPY)3) were chosen as a pH sensitive dye and reference dye, respectively. hSNFs were synthesized using a two-step method in a reverse micelle system and were shown to have an average length of 6.20 μm and average diameter of 410 nm. The peak intensity ratio of FITC/Ru(BPY)3 was used to calibrate to solution pH changes. An optical-fiber-based fluorescence detection system was developed that enabled feasible and highly efficient near-field fluorescence detection. The developed system enables fully automated fluorescence detection, where components including the light source, detector, and data acquisition unit are all controlled by a computer. The results show that the developed pH sensor works in a linear range of pH 4.0-9.0 with a fast response time of less than 10 s and minimal sample volume of 50 μL, and can be stored under dark conditions for one month without failure. In addition, the as-prepared hSNF-based pH sensors also have excellent long-term durability. Experimental results from ratiometric sensing confirm the high feasibility, accuracy, stability and simplicity of the dual fluorescent hSNF sensors for the detection of pH in real samples.

Highly-sensitive full-scale organic pH sensor using thin-film transistor topology

Here, we demonstrated a high sensitive pH sensor using organic thin-film as the sensing layer. It works as a low-voltage operated ion-sensitive organic thin-film transistor incorporating analyte drop as gate dielectric for which pH needs to be measured as the gate dielectric. The analytes of varying pH were prepared in an aqueous medium and used as gate dielectric to an OTFT with bottom-contact top-gated (BCTG) configuration operated at voltages below 1.0 V. It effectively senses pH values ranging from 3 to 12 with a dual sensitivity of about 1.2 μA/pH and 53 mV/pH with a response speed of a few milliseconds. Therefore, this depicts rapid, calibration-free, and stable performance over a period of 300 s without any signs of degradation. Our device also shows a slow degradation rate of −2.2 mV/h at neutral pH for three days. Hence, the results obtained in this report alter the concept of cheaper and eco-friendly broad-range organic pH sensor design. • Demonstrated broad-range, ultrafast, and calibration-free organic pH sensor. • Organic channel of TFT topology used as the sensing layer without any protection. • Our pH sensor can show dual sensitivity mode in terms of V TH and I D modulation. • Redox reactions in the P3HT channel and analyte are responsible for the operation. • This is promising for single-use and flexible rapid pH detection technology.

Highly sensitive pH sensor based on flexible polyaniline matrix for synchronal sweat monitoring

The design of a flexible pH sensor with high sensitivity, excellent stability, and low cost has great potential in the applications of the medical care and health fields. In this study, a high-performance flexible pH sensor with a two-electrode configuration was fabricated. This flexible pH sensor integrated the three-dimensional polyaniline (3D PANI) working electrode and Ag/AgCl reference electrode for monitoring the pH value of sweat in real-time. Polyaniline with phytic acid as dopant and crosslinker with a three-dimensional porous matrix structure provided a larger contact area for hydrogen ions in solution and a wider transmission path for charges, which improves the pH response performance of the 3D PANI sensor. The results demonstrated that the 3D PANI sensor exhibited linear super-Nernstian behavior (69.33 mV/pH) in the physiological range (pH 4.00 ∼ 9.00) of sweat, excellent performances in terms of response time (7.75 s), reversibility (3.80 mV), repeatability (0.44 % RSD), drift rate (0.10 mV/h), and temperature drift (0.06 mV/pH/oC). Particularly, the 3D PANI sensor in the bending state showed identical performances to the unbent condition, which makes it a promising candidate in the field of in-situ real-time sweat pH sensors for wearable electronics.

Wide-range and highly sensitive pH sensor based on a figure-eight fiber structure coated with copper/polyvinyl alcohol hydrogel

Construction of pH sensors based on optical fiber encapsulated copper (Cu-NPs) has been accomplished. Briefly, a length of single-mode fiber (SMF) of about 25 cm has been wrapped in two rings by twisting one side of the fiber on the other from both ends to form a figure-eight shape. To upsurge the sensitivity, the sensor configuration has been immobilized with copper nanoparticles/polyvinyl alcohol hydrogel (Cu/PVA) composite. The Cu/PVA composite was employed to shape a membrane structure on the sensing active length by laminating it using the dip-coating method. The wavelength interrogation method was employed to evaluate the sensitivity of the fabricated pH sensor. The fabricated fiber pH sensor exhibits a sigmoidal response above a wide range of pH from 1 to 14. The result displays a superior sensitivity of ∼4.8 nm/pH for a range of 1-7 pH and 3.86 nm/pH for the range of 8-14 pH with an excellent linear response. Besides the great sensitivity, the dual-parameters measurement of pH and refractive index was effectively accomplished with perfect stability. The proposed fiber sensor possesses a superior performance compared with the other sensors.

PH-Sensitive Sensors at Work on Poultry Meat Degradation Detection: From the Laboratory to the Supermarket Shelf

In the last twenty years, the number of publications presenting generalized pH-sensitive devices proposed for food freshness monitoring has been steadily growing, but to date, none of them have succeeded in exiting the laboratory and reaching the supermarket shelf. To reach this scope, we developed a large-scale applicable pH-sensitive sensor array to monitor perishable foods’ degradation. We ensured freshness monitoring in domestic conditions, using sales packages and during chilled storage, by simple naked-eye readout and multivariate imaging analysis, and we fully corroborated the device by (i) projection of unknown independent samples in the PCA model, (ii) TVB-N quantification and (iii) microbiological assay. The choice of commercial and cheap dye and polymeric support already employed in food packaging ensures the low-cost and scalability of the device and the promising results obtained make this device an eligible candidate for large-scale implementation.

Superhydrophobic Functionalized Ti3C2Tx MXene-Based Skin-Attachable and Wearable Electrochemical pH Sensor for Real-Time Sweat Detection.

Sweat pH is a critical indicator for evaluating human health. With the extensive attention on the wearable and flexible biosensing devices, the technology for the monitoring of human sweat can be realized. In this study, a sensitive, miniaturized, and flexible electrochemical sweat pH sensor was developed for the continuous and real-time monitoring of the hydrogen-ion concentration in human sweat. A flexible electrode was fabricated on the poly(ethylene terephthalate) (PET) substrate by a simple and low-cost screen-printing technology, which was based on the integration of fluoroalkyl silane-functionalized Ti3C2Tx (F-Ti3C2Tx) and the polyaniline (PANI) membrane technology instead of the traditional ion-sensitive membrane. The surface functionalization strategy for Ti3C2Tx with perfluorodecyltrichlorosilane can provide environmental stability. Functionalized Ti3C2Tx (F-Ti3C2Tx) was doped with PANI to obtain improved responsiveness, sensitivity, and reversibility. The constructed microsize, portable, and wearable F-Ti3C2Tx/PANI pH sensor aimed to real-time monitor the pH value of human sweat during exercise. On-body sweat pH monitoring for females and males, respectively, exhibited high accuracy and continuous stability compared with ex situ analyses. This study thus offers a facile and practical solution for developing a highly reliable MXene-based mini-type pH sensor to realize the online monitoring of human sweat pH.

The Sensitive Optical pH Sensor Based on the Complex of Nanosheet and Carbon Dots

AbstractThe requirement to non‐invasively quantify H+ is gradually increasing. In this regards, optical pH sensors based on fluorescent materials have advantages. However, the stability and sensitivity of currently used chromophores still need to be further improved to meet the continuous increase in actual demand for pH measurement. Here, we proposed to employ Montmorillonite (MMT) nanosheets as matrix and carbon dots (CD) as chromophores to fabricate a new complex. The as‐prepared MMT‐CD have the following advantages: 1) the aggregation of CD is effectively reduced, leading to the effectively improved fluorescence performance; 2) MMT‐CD has good light stability and storage stability; 3) as‐prepared the fluorescence pH sensor shows good response to pH, high sensitivity, and wide linear range. Therefore, MMT‐CD materials have great potential in the rapid and sensitive sensing of pH value.

Electropolymerised pH Insensitive Salicylic Acid Reference Systems: Utilization in a Novel pH Sensor for Food and Environmental Monitoring.

This work summarizes the electrochemical response of a salicylic acid-based carbon electrode for use as a novel solid-state reference electrode in a redox-based pH sensor. This novel reference produces a pH insensitive response over a range of pH 3–10 in solutions with low buffer concentrations, different compositions, conductivities, and ionic strengths is produced. The pH of the local environment is shown to be determined by the chemistry and the electrochemical response of the redox active species on the surface of the electrode; the local pH can be controlled by the electropolymerized salicylic acid moieties due to the acid concentration on the surface, avoiding any perturbation in environmental pH and leading to a stable novel reference system. Sensitivities of −7.1 mV/pH unit, −2.4 mV/pH unit, −0.2 mV/pH unit, and 2.5 mV/pH units were obtained for different food medias, hydroponic solution, seawater, and cell-culture media, respectively, confirming its ability to control the local pH of the electrode. This reference system is paired with a new pH sensing element based on electropolymerized flavanone to provide a calibration free, pH sensitive sensor to effectively and accurately measure the pH of various media with high viscosity, low conductivity, low/high buffer concentration or cell-culture environment, presenting a maximum error of +/−0.03 pH units.

Development of an ‘OFF-ON-OFF’ colorimetric and fluorometric pH sensor for the study of physiological pH and its bioimaging application

2-hydroxyl-1-naphthaldehyde based probe 1,1′((1E,1′E)((4hydroxy1,3phenylene) bis(azanylylidene)bis(methanylylidene)bis(naphthalen-2-ol) (NAP) has been synthesized and thoroughly characterized by IR, 13C & 1H NMR, fluorescence, UV-Vis, and mass spectral studies. The detection of pH is a necessary feature of the intracellular atmosphere and probably affects all cellular systems. NAP acts as a highly sensitive pH sensor in ethanol/HEPES (7:3, v/v) medium. Upon increasing the pH of NAP solution from acidic-neutral-basic medium, a distinct color change occurs from colorless-yellow-brown followed by a subsequent bathochromic shift in UV-Vis spectra and fluorescence response from ‘OFF-ON-OFF’. Photo-induced electron transfer (PET) mechanism has been proposed for the fluorescence change on the basis of theoretical studies on protonated, neutral and deprotonated NAP. Bioimaging investigations display a pH dependent fluorescence behavior of NAP in gut tissue of drosophila 3rd instar larvae. Additionally, NAP paper strips have been successfully developed for solid state pH sensing based on colorimetric and fluorescence responses.

Stable pH sensitivity of LaAlO3/SrTiO3 interfacial electronic gas

Probing pH value is essential for many applications. The request of developing stable and highly sensitive pH sensor with small size are increased recently, this is a challenge to the traditional glass pH sensors. This work investigated the pH sensing performance of the novel two-dimensional electronic gas (2DEG) at the LaAlO3/SrTiO3 heterostructure interface discovered recently. The experimental results demonstrated that the devices host excellent sensing ability to the pH value of aqueous solutions. Quite stable output current is realized for a given pH value, and the output current is linearly dependent on pH value as required by sensor applications. This work would prompt the application research of LaAlO3/SrTiO3 heterostructure and design of new generation of advanced pH sensors.

PH-Responsive Dual-Emitting Hydroxypropyl Methylcellulose-Based Material Containing Fluorescein Isothiocyanate and CaAl2O4:Eu2+,Dy3+ Phosphors.

Herein, we prepared a dual-emitting cellulose film with pH response, which offers high transparency, good flexibility, and intense thermal stability. The color of the fluorescent film that changes from green to blue-green to cyan was achieved by covalently attaching organic dye fluorescein isothiocyanate (FITC), inorganic pigment NH2-CaAl2O4:Eu2+,Dy3+ (NH2-CAO), and organic-inorganic fluorescence species onto hydroxypropyl methylcellulose (HMPC) chains, respectively. Benefiting from the "anchoring" and "dilution" effects of the HMPC skeleton, HPMC-FITC and HPMC@NH2-CAO fluorescent solutions and solid-state films emit green and blue-green fluorescence at 535 and 480 nm, respectively. The obtained pH-responsive cellulose-based dual-emitting film can continuously emit cyan light at the two emission peaks of 480 and 535 nm for a long time and exhibits strong fluorescence intensity under exceedingly alkaline conditions. Moreover, the HPMC-based fluorescent solution coated on glass and fabric substrate shows strong fluorescence under 365 nm UV light stimulation. Compared with the existing cellulose-based fluorescent films, this work expands the emission wavelength range of cellulose-based fluorescent films and prolongs the luminescent time of environment-responsive fluorescent films. This provides a new way to prepare intelligent color-changing fabric-coating materials and sensitive pH sensors based on biomass.