Large enhancement of sensitivity and a wider working range of glass pH electrode with amperometric and potentiometric responses

Increasing the Sensitivity of pH Glass Electrodes with Constant Potential Coulometry at Zero Current

It has recently become possible to increase the sensitivity of ion-selective electrodes (ISEs) by imposing a constant cell potential, allowing one to record current spikes with a capacitor placed in series in the circuit. The approach requires a transient current to pass through the measurement cell, which unfortunately may introduce measurement errors and additionally excludes the use of high-impedance indicator electrodes, such as pH glass electrodes. We present here an electronic circuit that overcomes these limitations, where the cell is measured at zero current in combination with a voltage follower, and the current spike and capacitor charging occur entirely within the instrument. The approach avoids the need for a counter electrode, and one may use any electrode useful in potentiometry regardless of its impedance. The characteristics of the circuit were found to approach ideality when evaluated with either an external potential source or an Ag/AgCl electrode. The current may be linearized and extrapolated to further reduce the measurement time. The circuit is further tested with the most common yet very challenging electrode, the pH glass electrode. A precision of 64 μpH was obtained for 0.01 pH change up to 0.05 from a reference solution. Similar pH changes were also measured reliably further away from the reference solution (0.5-0.55) and resulted in a precision of 377 μpH. The limitations of this experimental setup were explored by performing pH calibrations within the measuring range of the probe.

Iron-porphyrin-based covalent-organic frameworks for electrochemical sensing H2O2 and pH

Here, a novel iron-porphyrin-based covalent organic framework (COF_{p-Fepor NH2-BTA}) was synthesized and applied for electrochemical sensing H₂O₂ and pH which involved in many biological processes. The COF_{p-Fepor NH2-BTA} was obtained by post-modification of porphyrin-based COF (COF_{p-por NH2-BTA}) which was firstly synthesized by aldehyde-ammonia condensation reaction between 1,3,5-benzenetricarboxaldehyde and 5,10,15,20-tetrakis(4-aminophenyl)-21H,23H- porphine. The COF_{p-por NH2-BTA} was proved to be regular and uniform spherical particles with diameter about 1 μm, as well as possessed good crystalline structure and abundant micropores of about 1.4 nm. The resulted COF_{p-Fepor NH2-BTA} after post-modification with Fe²⁺ maintained the original shape and crystalline structure of COF_{p-por NH2-BTA}, while the micropores decreased to be about 0.89 nm. Electrochemical results indicated that the synthesized COF_{p-Fepor NH2-BTA} had good electrochemical redox and proton activity owing to iron-porphyrin, enabling to simultaneously be used as mimic peroxidase to catalyze the reduction of hydrogen peroxide (H₂O₂) and evaluate pH using current and potential as signal, respectively. The prepared sensor showed good performance for H₂O₂ detection from 6.85 nM to 7 μM with the detection limit of 2.06 nM (S/N = 3), and pH test from 3.0 to 9.0. This work demonstrated that the iron-porphyrin-based COF could be used as a mimic peroxidase to apply in biological fields.

3D Co-Ni Nanocone Array Shielded with Conducting Amorphous Carbon Used as Fused, Separable, and Stable Mimicking Peroxidases for RGB-Color Intensiometric pH Indication

We demonstrate an in situ synthesis for preparing a carbon-shielded three-dimensional Co-Ni nanocone array on Ni foam (CoNi NCs/NF@C) via the solvothermal and thermal annealing processes. It is found that the easily separable CoNi NCs/NF@C possesses high peroxidase/catalase dual-mimic activity and good catalytic stability. The fusion of the amorphous carbon sheath with the Co-Ni nanocones (1) effectively improves interfacial electron transfer and catalytic stability of the Co-Ni nanocone array because of the excellent conductivity of amorphous carbon and (2) protects the Co-Ni nanocone array in the catalysis process from exposing to the harsh chemical environment, dramatically escaping the catalytic activity loss of Co-Ni (hydro)oxide. Interestingly, when CoNi NCs/NF@C mimics peroxidase using 3,3',5,5'-tetramethylbenzidine (TMB) as a substrate, the color of the TMB-H₂O₂-CoNi NC/NF@C system changes at different pH values. Based on this property, a facile strategy was developed for semiquantitative and qualitative determination of pH using the Eyedropper function in Microsoft's PowerPoint software, where the RGB (red, green, and blue) value of the sample can be conveniently measured by using a standard colorimetric card without the requirement of complicated instrumentation. Moreover, the relationship between the color of the reaction system and the pH was investigated, which was demonstrated by the total Euclidean distance (ED), that is, the square root of the sum of the squares of the ΔRGB values. The ED change of the reaction system is reversible and occurs in the pH range from 0.64 to 8.4, which is useful for indicating the pH of strongly acidic environments. The colorimetric system exhibits a linear range from 0.64 to 2.38 and 2.5 to 6.5. A colorimetric card was designed based on the color changes of this system as a function of pH values. This work provides a colorimetric assay method for the simple, rapid, and visual indication of pH which can be used to understand the biological processes in physiology and pathology fields.

A novel chemical sensor with multiple all-solid-state electrodes and its application in freshwater environmental monitoring

Freshwater quality detection is important for pollution control. Three important components of water quality are pH, ammonia and dissolved H₂S and there is an urgent need for a high-precision sensor for simultaneous and continuous measurement. In this study, all-solid-state electrodes of Eh, pH, NH₄ ⁺ and S^2- were manufactured and mounted to a wireless chemical sensor with multiple parameters. Calibration indicated that the pH electrode had a Nernst response with slope of 53.174 mV; the NH₄ ⁺ electrode had a detection limit of 10^-5 mol/L (Nernst response slope of 53.56 mV between 10^-1 to 10^-4 mol/L). Ag/Ag₂S has a detection limit of 10^-7 mol/L (Nernst response slope of 28.439 mV). The sensor was cylindrical and small with low power consumption and low storage demand to achieve continuous in-situ monitoring for long periods. The sensor was tested for 10 days in streams at Trawsgoed Dairy farm in Aberystwyth, UK. At the intensively farmed Trawsgoed, the concentration of NH₄ ⁺ in the stream rose sharply after the application of slurry to adjacent fields. Further, the stream was overhung with extensive vegetation and exhibited changes in pH, which correlated with photosynthetic activity. Measurements of S^2- were stable throughout the week. Our data demonstrate the applicability of our multiple electrode sensor.

Integrated multi-ISE arrays with improved sensitivity, accuracy and precision

Increasing use of ion-selective electrodes (ISEs) in the biological and environmental fields has generated demand for high-sensitivity ISEs. However, improving the sensitivities of ISEs remains a challenge because of the limit of the Nernstian slope (59.2/n mV). Here, we present a universal ion detection method using an electronic integrated multi-electrode system (EIMES) that bypasses the Nernstian slope limit of 59.2/n mV, thereby enabling substantial enhancement of the sensitivity of ISEs. The results reveal that the response slope is greatly increased from 57.2 to 1711.3 mV, 57.3 to 564.7 mV and 57.7 to 576.2 mV by electronic integrated 30 Cl⁻ electrodes, 10 F⁻ electrodes and 10 glass pH electrodes, respectively. Thus, a tiny change in the ion concentration can be monitored, and correspondingly, the accuracy and precision are substantially improved. The EIMES is suited for all types of potentiometric sensors and may pave the way for monitoring of various ions with high accuracy and precision because of its high sensitivity.