Gang Cui

Effect of pre-treatment on the surface and electrochemical properties of screen-printed carbon paste electrodes.

The effect of various electrochemical pre-treatment methods on the surface and electrochemical properties of screen-printed carbon paste electrodes (SPCE) prepared with three different commercial products was examined. It was observed that a positively charged redox couple, e.g., hexaammineruthenium(III), exhibited quasi-reversible behavior at the untreated SPCE. However, the cyclic voltammograms (CVs) of the SPCE prepared with general-purpose carbon inks did not exhibit clear redox peaks to other representative redox couples [e.g., hexacyanoferrate(III), hexachloroiridate(IV), dopamine, and hydroquinone] without activation. Electrochemical pre-treatment methods were sought in four different aqueous solutions, i.e., sulfuric acid, potassium chloride, sodium hydrogencarbonate, and sodium carbonate, applying various activation potentials. It was found that the pre-treatment procedure in saturated Na2CO3 solution at 1.2 V provides a mild and effective condition for activating the SPCE. By measuring the water contact angles at the SPCE surfaces and recording their SEM images, it was confirmed that the electrochemical pre-treatment effectively removes the organic binders from the surface carbon particles. A prolonged period of activation (> 5 min) or the use of high potentials (> 1.2 V) increased the capacitance of the electrode over 20 microF cm(-2). The pre-treated SPCE behaved like a random array microelectrode, exhibiting a sigmoidal-shaped CV at a slow scan rate. The short pre-anodization method in Na2CO3 solution was generally applicable to most SPCE prepared with general-purpose carbon inks.

Disposable amperometric glucose sensor electrode with enzyme-immobilized nitrocellulose strip

Electrochemical properties of screen-printed carbon paste electrodes (CPEs) with a glucose oxidase-immobilized and hexamineruthenium (III) chloride ([Ru(NH(3))(6)](3+)) containing nitrocellulose (NC) strip were examined. The NC strip (2x8 mm) placed on the CPEs printed on polyester (PE) film is tightly sealed using another PE film on the top with open edges on both sides. Samples containing macromolecules and particles (e.g. proteins and blood cells) are applied at one edge of the NC strip and reach the detection area, chromatographically separating small molecules (e.g. glucose, ascorbate, acetaminophen, and uric acid) of analytical interests. Since sample volumes and the amount of catalytic reagents (mediator and glucose oxidase) are precisely predefined by the dimension and pore size (8 mum) of the NC strip, the sensor-to-sensor reproducibility and accuracy of analysis are greatly improved. The use of [Ru(NH(3))(6)](3+) mediator, which exhibits characteristic substantially lowers the applied potential (0.0 V vs Ag/AgCl) for glucose determination and eliminates the interference from other oxidizable species, providing improved analytical results.

Potentiometric pCO2 sensor using polyaniline-coated pH-sensitive electrodes

A very simple method of constructing a differential pCO2 sensor device is described, using polyaniline-based pH electrodes in place of the pH-ISFET-based system employed previously. The polyaniline film-coated Pt electrode was shown to exhibit not only an enhanced potentiometric pH response but also a greatly reduced oxygen sensitivity compared with the uncoated Pt electrode. The proposed differential system employs two identical polyaniline electrodes to make a pCO2 probe and a reference electrode. Both electrodes are made by coating the polyaniline surface with a gas-permeable silicone rubber membrane doped with valinomycin. The pCO2 electrode is covered with a hydrogel-based recipient layer inside the gas-permeable membrane whereas the reference electrode does not have the hydrogel layer. In this sensor configuration, the emf differences between the pCO2 and the reference electrodes serve as analytical signals and hence the ion responses caused by the two parts of the outer gas-permeable membranes cancel out. The polyaniline-based gas sensor system was shown to provide better emf stability than the uncoated Pt electrode-based counterpart.

Differential Thick-Film Amperometric Glucose Sensor with an Enzyme-Immobilized Nitrocellulose Membrane

A disposable glucose sensor based on a differential amperometric measurement is fabricated on activated carbon paste electrodes with the use of a glucose oxidase (GOx)-immobilized nitrocellulose (NC) membrane. Two identical three-electrode cells are screen-printed symmetrically on both faces of a single polyester (PE) substrate. Both electrodes of the two-sided sensor strip are covered with NC membranes incorporating a mediator (K3[Fe(CN)6]), one membrane with GOx and the other with bovine serum albumin (i.e., no GOx): the former serves as a glucose-sensing cell and the other as a reference cell. In the differential measurements between these two cells, the response signals toward various redox species other than glucose cancel out. Prior to placing the NC membranes, the working carbon paste electrodes are pretreated by anodizing them in saturated sodium carbonate at 1.2 V (vs. SCE): the electrochemical reversibility of the mediator is found to be enhanced significantly on such an activated electrode. The analytical performance of the proposed differential glucose sensor strip is demonstrated by measuring glucose values in normal and abnormal serum samples: 5.08±0.52 (found) vs. 5.10±0.46 (listed), and 16.56±1.71 (found) vs. 16.92±1.72 (listed).

Effect of dissolved CO2 on the potential stability of all-solid-state ion-selective electrodes

The influence of dissolved CO2 on the potentiometric responses of all-solid-state ion-selective electrodes (ISEs) was systematically examined with four different types of electrodes fabricated by pairing pH-sensitive and pH-insensitive metal electrodes (Pt and Ag/AgCl, respectively) with pH-sensitive and pH-insensitive ion-selective membranes (H+-selective membrane based on tridodecylamine and Na+-selective membrane based on tetraethyl calix[4]arenetetraacetate, respectively). The experimental results clearly showed that the carbonic acid formed by the diffused CO2 and water vapor at the membrane/metal electrode interface varies the phase boundary potentials both at the inner side of the H+-selective membrane (deltaE(in)mem) and at the metal electrode surface (deltaEelec). The potential changes, deltaE(in)mem and deltaEelec, occurring at the facing boundaries, are opposite in their sign and result in a canceling effect if both the membrane and metal surface are pH-sensitive. Consequently, the H+-selective membrane coated on a pH-sensitive electrode (Pt) tends to exhibit a smaller CO2 interference than that on a pH-insensitive electrode (Ag/AgCl). When the all-solid-state Na+ and K+ ISEs were fabricated with both pH-insensitive metal electrode and ion-selective membrane, they did not suffer from CO2 interference. It was also confirmed that plasticization of the PVC leads to increased CO2 permeation. Various types of intermediate layers were examined to reduce the CO2 interference problem in the fabrication of H+-selective all-solid-state ISEs. The results indicated that the H+-selective electrode needs an intermediate layer that maintains a constant pH unless the carbonic acid formation at the interfacial area is effectively quenched.