Hakhyun Nam

All-solid-state electronic tongue and its application for beverage analysis

Disposable all-solid-state planar-type potentiometric electronic tongue has been developed with the carbon paste electrode array screen-printed on a polymeric substrate. Highly cross-sensitive solvent polymeric membranes based on different matrices [e.g. poly(vinyl chloride) (PVC), aromatic polyurethane, and polypyrrole (Ppy)] and doped with common electroactive components for potentiometric measurements (e.g. various plasticizers, and cation- and anion-selective ionophores) were deposited on the screen-printed carbon paste electrodes (SCPEs). It was observed that an incorporation of 10 wt.% of Prussian Blue (PB; Fe4(III)[Fe(II)(CN)6]3) into a commercially available carbon paste and electrochemical preanodization of SCPEs in KCl solution at 1.6 V provide the all-solid-state planar-type electrodes with significantly improved potentiometric stability. The proposed fabrication method gives possibility for simple and reproducible mass-production of low-cost disposable electronic tongue microsystems. The practical utility of all-solid-state disposable electronic tongue chips has been demonstrated with a flow injection cell for the analysis of potable waters, soft drinks, and beers. It is shown that the potentiometric measurements with the SCPE-based all-solid-state chips and the combined use of chemometric methods (e.g. principal components analysis, partial least regression (PLS), and principal component regression (PCR)) for the analysis of obtained data sets successfully discriminate various types of samples according to their tastes.

Multicomponent analysis of Korean green tea by means of disposable all-solid-state potentiometric electronic tongue microsystem

Abstract All-solid-state ‘electronic tongue’ microsystem comprised of polymeric sensors of different types such as highly cross-sensitive sensors based both on PVC and aromatic polyurethane (ArPU) matrices doped with various membrane active components, electrochemically deposited conductive films of polypyrrole (PPy) and polyaniline (PAn) and potentiometric glucose biosensors has been developed and applied for the analysis of beverages: natural coffee, black tea and different sorts of green teas. The system can discriminate different kinds of teas (black and green) and natural coffees. Components that are responsible for giving unique taste such as caffeine, catechines, sugar, amino acid l -arginine have been determined for green tea samples with unknown manufacturer specifications.

Selective electrochemical sensing of glycated hemoglobin (HbA1c) on thiophene-3-boronic acid self-assembled monolayer covered gold electrodes.

We report a novel concept of sensing glycated hemoglobin, HbA 1c, which is now the most important index for a long-term average blood glucose level, by first selectively immobilizing it on the thiophene-3-boronic acid (T3BA) self-assembled monolayer (SAM)-covered gold electrode by a selective chemical reaction with boronic acid. HbA 1c thus immobilized is then detected by the label-free electrochemical impedance spectroscopic (EIS) measurements with a redox probe, an equimolar mixture of K 3Fe(CN) 6 and K 4Fe(CN) 6, present. The rate of charge transfer between the electrode and the redox probe is shown to be modulated by the amount of HbA 1c in the matrix hemoglobin solution due to the blocking effect caused by the binding of HbA 1c with boronic acid. Both the formation of a well-defined T3BA-SAM on the gold surface and the chemical binding of its boronic acid with HbA 1c in solution were confirmed by quartz crystal microbalance, atomic force microscopy, and EIS experiments.

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.

Solid-state ion sensors with a liquid junction-free polymer membrane-based reference electrode for blood analysis

Abstract A set of ion-selective membranes for the measurement of clinically relevant electrolytes, i.e., K+, Na+, Ca2+, H+ and Cl−, and a solvent-processible polyurethane (PU)-based reference membrane were cast on metal electrodes patterned on small ceramic plates, and on silicon-based sensors. Since the liquid junction-free PU membrane-based reference electrode provides a constant potentiometric signal for an extended period of time, accurate determination of blood electrolytes could be made without using a separate reference flow. The PU membrane-based reference electrodes not only improve the mass producibility of miniaturized solid-state ion sensors but also simplify the configuration of flow cell cartridges without compromising their analytical performance.

Calixazacrown ethers for copper(II) ion-selective electrode

Five novel 1,3-alternate calix[4]azacrown ethers having 2-picolyl, 3-picolyl, and benzyl unit on the nitrogen atom were synthesized and used as ionophores for transition metal-selective polymeric membrane electrodes. The electrode based on 2-picolyl armed 1,3-alternate calix [4] azacrown ether exhibited Nernstian response toward copper (II) ion over a concentration range (10(-4.5) M-10(-2.5) M). The detection limit was determined as 10(-5) M in pH 7 and the selectivity coefficients for possible interfering cations were evaluated. Anions in the sample solution strongly affected the electrode response.

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.

Potentiometric properties of ion-selective electrode membranes based on segmented polyether urethane matrices.

Potentiometric responses of polyurethane (PU)-based membranes containing valinomycin and varying amounts of plasticizer (DOA) and/or lipophilic additive (KTpClPB) were examined as a function of soft segment [poly(tetramethylene ether glycol)] contents in aromatic diisocyanate-based PU matrices. Upon increasing the weight percentages (w(soft)) of soft segments, which in part behave like a built-in plasticizer, providing the matrices with rubbery structure (glass transition temperature below -58 °C), the amounts of DOA and/or KTpClPB necessary to result in near-Nerntian response (e.g., slope > 50 mV/decade) to potassium were substantially lowered. The apparent effect of adding plasticizer to PU-based membranes was comparable to that resulting from an increase of free carrier concentration in normal PVC-based membranes. Owing to the chemical interaction between mobile anionic sites and urethane chains, plasticizer-free PU membranes could be prepared with the PU matrices with high soft segment contents (w(soft) ≥ 60 wt %). PUs composed of 60 ≤ w(soft) < 80 wt % were recommended as the matrix for fabricating ISE membranes with no or low plasticizer content.

Ion-selective electrodes based on molecular tweezer-type neutral carriers

Potentiometric properties of cholic and deoxycholic acid derivatives substituted with various ion-recognizing moieties, such as dithiocarbamate, bipyridyl, glycolic and malonic diamides, urea and thiourea, and trifluoroacetophenons (TFAP), have been studied using solvent polymeric membranes. The dithiocarbamate and bipyridyl group containing ionophores exhibit high silver ion selectivity. The cholic acid derivatized with glycolic diamides exhibited high calcium selectivity, but its complex formulation constant was 10(5) times smaller than that of ETH 1001. The reduced calcium binding ability of the glycolic diamide-substituted ionophore was advantageous for eliminating anionic interference. The bi- or tripodal malonic diamide-substituted ionophores exhibited substantially increased magnesium selectivity. Anion-selective ionophores have been designed by substituting urea and thiourea group containing chains to the hydroxyl linkers of chenodeoxycholic acid frames; their selectivity closely followed the sequence of Hoffmeister series, except the unusually large response of the thiourea-substituted ionophore to sulfate. The most successful examples of cholic or deoxycholic acid frame-based ionophores are those functionalized with two carbonate-selective TFAP groups: bipodal TFAP groups behaves like a tweezers for the incoming carbonate, and exhibit analytically interference free and quantitative responses to the carbonate in serum and seawater samples.