MicroRNA (miRNA) profile-based point-of-care (POC) diagnostic methods have attracted considerable attention. In our laboratory, singleplex miRNA detection on a power-free poly(dimethylsiloxane) (PDMS) microfluidic chip with laminar flow-assisted dendritic amplification (LFDA) has been developed. In this study, to obtain the miRNA profile and to improve the reliability of the diagnosis, multiplex miRNA detection on the same system is demonstrated without compromising any advantages of the singleplex miRNA detection. The limit of detection (LOD) was at the femto- to picomolar level and the assay time was 20 min. The sensitivity, rapidity, and portability of the microfluidic chip are adequate for POC diagnosis.

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A simple evaluation method has been developed for a metal thin layer on a sphere-like plastic microstructure, based on its light-scattering property since, according to our results, the light-scattering intensity of silver-coated microbeads depends significantly on the silver coverage. Our attempt was carried out by using a dark-field microscopy coupled with a spectrometer.

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A dual-electrochemical sensor based on a test-strip assay with immunochemistry and enzyme reactions has been developed for the determination of albumin and creatinine. Each nitrocellulose membrane with an immobilization area of an anti-albumin antibody or three enzymes was prepared in the device with three working electrodes for measuring albumin, creatinine, and ascorbic acid, as well as an Ag/AgCl electrode used as a counter/pseudo-reference electrode. The reactions of three enzymes were initiated by flowing a solution containing creatinine to detect an oxidation current of hydrogen peroxide. A sandwich-type immunocomplex was formed by albumin and antibody labeled with glucose oxidase (GOx). Captured GOx catalyzed the reduction of Fe(CN)₆³⁻ to Fe(CN)₆⁴⁻, which was oxidized electrochemically to determine the captured albumin. The responses for creatinine and albumin increased with the concentrations in millimolar order and over the range 18.75 – 150 μg mL⁻¹, respectively. The present sensor would be a distinct demonstration for producing quantitative dual-assays for various biomolecules used for clinical diagnoses.

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A microdevice for coulometric detection of organophosphate pesticides (OPs) was fabricated based on the measurement of the inhibition of an enzyme, acetylcholinesterase (AChE), by OPs. Thiocholine (TCh) produced in the enzymatic reaction of AChE with acetylthiocholine (ATCh) as a substrate was oxidized on a microelectrode array formed in a main flow channel. Volumes of plugs of necessary solutions were measured using a structure consisting of a row of rhombuses formed in an auxiliary flow channel. The plugs were merged and solution components were mixed at a T-junction formed with the main and auxiliary flow channels. A linear relationship was confirmed between the generated charge and the logarithm of the OP (malathion) concentration in a concentration range between 10⁻⁶ and 10⁻³ M with a correlation coefficient of 0.951. The lower limit of detection was 412 nM.

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Fundamental aspects of the attachment of gold nanoparticles (AuNPs) onto a 3-aminopropyltrimethoxysilane (APTMS) modified indium tin oxide (ITO) electrode were explored using commercially available Au colloid solutions of 5, 10 and 20 nm. In particular, competitive attachments of AuNPs were observed using mixed solutions of two Au colloids. Consequently, it was found that smaller AuNPs are easily attached on an APTMS modified ITO. On the other hand, the result of the stepwise attachments showed that after the surface was modified by the first AuNPs, the second AuNPs have difficulty attaching. This means, if surface connecting −NH₂ terminals of APTMS are once occupied, further modification or exchange of the attached AuNPs would not be easy. From the present results, a contaminant amount of smaller AuNPs is considered to be a practical problem in modifying ITO surfaces by AuNPs.

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The fabrication of ultrathin-ring electrodes with a diameter of 2 mm and a thickness of 100 nm is established. The ultrathin-ring electrodes provide a large density of pseudo-steady-state currents, and realize pseudo-steady-state amperometry under quiescent conditions without a Faraday cage. Under the limiting current conditions, the current response at the ultrathin-ring electrode can be well explained by the theory of the microband electrode response. Cyclic voltammograms at the ultrathin-ring electrode show sigmoidal characteristics with some hysteresis. Numerical simulation reveals that the hysteresis can be ascribed to the time-dependence of pseudo-steady-state current. The performance of amperometry with the ultrathin-ring electrode has been verified in its application to redox enzyme kinetic measurements.

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A One-step technique for depositing gold nanoclusters (GNCs) onto the surface of a glassy carbon (GC) plate was developed by using pulse laser deposition (PLD) with appropriate process parameters. The method is simple and clean without using any templates, surfactants, or stabilizers. The experimental factors (pulse laser number and the pressure of inert gas (Ar)) that affect the morphology and structure of GNCs, and thus affect the electrocatalytic oxidation performance towards glucose were systematically investigated by means of transmission electron microscopy (TEM) and electrochemical methods (cyclic voltammograms (CV) and chronoamperometry methods). The GC electrode modified by GNCs exhibited a rapid response time (about 2 s), a broad linear range (0.1 to 20 mM), and good stability. The sensitivity was estimated to be 31.18 μA cm⁻² mM⁻¹ (vs. geometric area), which is higher than that of the Au bulk electrode. It has a good resistance to the common interfering species, such as ascorbic acid (AA), uric acid (UA) and 4-acetaminophen (AP). Therefore, this work has demonstrated a simple and effective sensing platform for the nonenzymatic detection of glucose, and can be used as a new material for a novel non-enzymatic glucose sensor.

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A novel non-enzymatic oxalic acid (OA) sensor was developed using a nanocrystal PtPd loaded reduced graphene nanosheets (PtPdNCs/RGO)-modified electrode. PtPdNCs/RGO were successfully achieved by a facile, one-step and template-free method, in which PtPd nanoparticles with 100 nm-scale were assembled from polyhedral PtPd nanocrystals of various shapes and dispersed on the graphene nanosheets. Resulting PtPdNCs/RGO were characterized and used for PtPdNCs/RGO-modified electrodes. Electrochemical oxidation of OA on the modified electrode was investigated by cyclic voltammetry and differential pulse voltammetry (DPV). Well-defined peaks of OA oxidation could be obtained using an electrode that indicated its high electrochemical activity. The concentration of OA and the current responses could be obtained in the ranges of 0.5 – 10 and 10 – 35 mM with correlation coefficients of 0.9994 and 0.9952; the detection limit (S/N = 3) was found to be 0.05 mM. The modified electrode presented good characteristics in terms of both stability and reproducibility, promising its applicability in practical analysis.

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This work presents for the first time the electrochemical determination of europium using cyclic voltammetry at gold electrodes modified with 2-pyridinol-1-oxide. A well-defined oxidation peak was observed in cyclic voltammetry as a result of the oxidation of the europium at ∼1100 mV in phosphate buffer at pH 7.0. The peak current increased linearly with the increase of concentration of the europium over the range from 1 to 80 μM and detection limit (based on 3-sigma) and quantification were found to be 0.3 and 0.549 μM, respectively. The analytical utility of the developed protocol was evaluated by performing the detection of the europium in river water. Europium is also linear over the concentration range 10 to 150 μM. (I_p/μA = 0.7239x + 108.19, R² = 0.9981 and n = 9) with a detection limit of 6.5 μM (based on 3-sigma). This simple and effective protocol exhibited good sensitivity, precision and reliability towards the detected analyte.

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This paper for the first time reports on novel and non-enzymatic method for studying the free radical-scavenging properties of phenolic compounds against superoxide anion radicals (O₂·⁻) by using the cathodic electrochemiluminescence (ECL) of lucigenin (Luc²⁺). The ECL of Luc²⁺ at a glassy carbon (GC) electrode is observed in an aeration electrolytic solution (pH 7), which is believed to be due to the reaction of a one-electron reduced form of Luc²⁺ (i.e. a radical cation, Luc·⁺) with in situ electrogenerated O₂·⁻. The ECL intensity is dependent on the concentration of dissolved oxygen, and is suppressed dramatically by superoxide dismutase (SOD), a typical O₂·⁻ scavenger. Since the coexisting hydrogen peroxide (H₂O₂) has no influence on the cathodic ECL of Luc²⁺, it is thus suggested that the ECL signal specifically reflected the O₂·⁻ concentration level generated at the electrode surface. When phenolic compounds were added into the solution, this resulted in the inhibition of ECL signals due to the elimination of O₂·⁻. The ECL inhibition rate measured at each concentration was compared against the SOD equivalent (U mL⁻¹), and the relative antioxidant efficiency, K_ao (U mmol⁻¹ equivalent SOD), was used to evaluate the antioxidant activity of some phenolic compounds, including flavonoids, in this study. Structurally different water-soluble phenols were compared, and those compounds containing to catechol skeletal structure are found to present the higher antioxidant capacity.

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The effect of the sp²/sp³ ratio in an unbalanced magnetron sputtered nanocarbon film electrode was studied for determining Cd²⁺ and Pb²⁺ by anodic stripping voltammetry (ASV). The signal-to-noise ratio in the ASV measurement improved as the sp³ concentration in the carbon film increased because the noise current decreased with the increasing sp³ concentration. The detection limits with a carbon film containing 50% sp³ were 0.25 and 1.0 μg L⁻¹ for Cd²⁺ and Pb²⁺ with high repeatability (Cd: 4.6% and Pb: 6.4%, n = 3). For a real sample measurement, a pretreatment system combining a photooxidation reactor and a cation exchange column was used to eliminate the interference from EDTA and Cu²⁺, which forms a stable complex or alloy with Cd²⁺ and Pb²⁺. More than 99% of the interference was eliminated, and accurate signal currents for Cd²⁺ and Pb²⁺ were successfully obtained with the pretreatment system.

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A gold nanoparticle modified boron-doped diamond electrode was developed as a transducer for biochemical oxygen demand (BOD) measurements. Rhodotorula mucilaginosa UICC Y-181 was immobilized in a sodium alginate matrix, and used as a biosensing agent. Cyclic voltammetry was applied to study the oxygen reduction reaction at the electrode, while amperometry was employed to detect oxygen, which was not consumed by the microorganisms. The optimum waiting time of 25 min was observed using 1-mm thickness of yeast film. A comparison against the system with free yeast cells shows less sensitivity of the current responses with a linear dynamic range (R² = 0.99) of from 0.10 mM to 0.90 mM glucose (equivalent to 10 – 90 mg/L BOD) with an estimated limit of detection of 1.90 mg/L BOD. However, a better stability of the current responses could be achieved with an RSD of 3.35%. Moreover, less influence from the presence of copper ions was observed. The results indicate that the yeast-immobilized BOD sensors is more suitable to be applied in a real condition.

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We studied a nitrogen-doped nanocarbon film electrode with a nitrogen concentration of lower than 10.9 at% formed by the unbalanced magnetron (UBM) sputtering method. The sp³ content in the nitrogen-doped UBM sputtering nanocarbon film (N-UBM film) slightly increases with increasing nitrogen concentration. The nitrogen-containing graphite-like bonding decreases and pyridine-like bonding increases with increasing nitrogen concentration. The N-UBM film has a very smooth surface with an average roughness of 0.1 to 0.3 nm, which is almost independent of nitrogen concentration. The N-UBM film electrode shows a wider potential window (4.1 V) than a pure-UBM film electrode (3.9 V) due to its slight increase in the sp³ content. The electrocatalytic activity increased with increasing nitrogen concentration, suggesting that the electroactivity is maximum when the nitrogen concentration is around 10.9 at%, which is confirmed by the peak separation of Fe(CN)₆⁴⁻. The hydrogen peroxide (H₂O₂) reduction potentials at the N-UBM film electrode shifted about 0.1 V, and the peak current of H₂O₂ increased about 4 times.

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Graphite nanosheets prepared by thermal expansion and successive sonication were utilized for the construction of a multi-walled carbon nanotubes/graphite nanosheets based amperometric sensing platform to simultaneously determine acetaminophen and dopamine in the presence of ascorbic acid in physiological conditions. The synergistic effect of multi-walled carbon nanotubes and graphite nanosheets catalyzed the electrooxidation of acetaminophen and dopamine, leading to a remarkable potential difference up to 200 mV. The as-prepared modified electrode exhibited linear responses to acetaminophen and dopamine in the concentration ranges of 2.0 × 10⁻⁶ – 2.4 × 10⁻⁴ M (R = 0.999) and 2.0 × 10⁻⁶ – 2.0 × 10⁻⁴ M (R = 0.998), respectively. The detection limits were down to 2.3 × 10⁻⁷ M for acetaminophen and 3.5 × 10⁻⁷ M for dopamine (S/N = 3). Based on the simple preparation and prominent electrochemical properties, the obtained multi-walled carbon nanotubes/graphite nanosheets modified electrode would be a good candidate for the determination of acetaminophen and dopamine without the interference of ascorbic acid.

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A bifunctional probe (FecNC), containing a recognition part and an electrochemical active center, was applied to electrochemical detection of GG mismatch duplexes. The preparation of gold electrodes modified by mismatch and complementatry duplexes was characterized by electrochemical impedance spectroscopy (EIS) and optimized for better detection in terms of self-assembly time, hybridization time, and incubation time. The interaction between FecNC and DNA duplexes modified on the surface of a gold electrode was explored by square wave voltammetry (SWV) and EIS. The results showed that the DNA duplexes with GG mismatch on the surface of a gold electrode was easily detected by the largest electrochemical signal of the bifunctional probe because of its selective binding to GG mismatches. The bifunctional probe could offer a simple, effective electrochemical detection of GG mismatches, and theoretical bases for development of electrochemical biosensors. Further, the method would be favorable for diagnosis of genetic diseases.

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A novel photoelectrochemical strategy for the sensitive determination of p-nitrophenol (PNP) was developed using a glassy carbon electrode (GCE) modified with CdSe quantum dots (QDs) and DNA composite film (CdSe-DNA/GCE). Various surface analytical techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM), were employed to characterize the synthesized CdSe QDs and CdSe-DNA modified electrode. The interfacial behaviors of the modified electrodes were analyzed by electrochemical impedance spectroscopy (EIS), and the interaction between PNP and DNA was studied by UV-visible spectrometry. Due to the PNP-DNA interaction, CdSe-DNA/GCE exhibited a sensitive photocurrent response toward PNP under visible-light irradiation. The influencing factors, such as the concentration of DNA used for fabricating CdSe-DNA/GCE and the bias potential applied in the photoelectrochemical measurement, were investigated. Under the optimized conditions, the photocurrent on CdSe-DNA/GCE was linearly increased with the PNP concentration from 0.7 to 50 μmol L⁻¹, with a detection limit (3 S/N) of 0.27 μmol L⁻¹. The proposed photoelectrochemical strategy was successfully applied to monitoring the degradation of PNP.

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The propagation of the change in potential differences across liquid membranes from the potential-sending cell to the potential-receiving cell was investigated by use of a system combined with three liquid membrane cells, which were composed of two aqueous phases and a 1,2-dichloroethane solution phase. The ionic composition of one potential-sending cell (S) was identical to that of the receiving cell (Rec), and that of another potential-sending cell (Ap) was different from that of the Rec. When the connection of cell Rec was switched from cell S to cell Ap, the change in the membrane potential was caused by the circulating current. The greater the ratio of the interfacial area of the membrane of cell Ap to that of cell Rec, the faster the change in the membrane potential propagated from cell Ap to cell Rec.

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A selective voltammetric determination of homocysteine and glutathione was applied to cell tissue culture media and human plasma via a single two-step method. The two-step method relies on the 1,4-Michael addition reaction between electro-oxidized catechol and the target thiol. Furthermore, the procedure relies on the differing reaction kinetics of the ortho-quinone with various thiol species giving different responses as a function of the scan rate. At faster scan rates homocysteine is only detected, while at slower scan rates the adduct signal reflects both homocysteine and glutathione. As a result, the quantification of both homocysteine and glutathione can be determined with a combination of both sets of data. The previous proof-of-concept (P. T. Lee, D. Lowinsohn, and R. G. Compton, Sensors, 2014, 14, 10395), is applied to the quantification of thiols in both tissue culture media and human plasma alone. Analytical parameters were determined for both homocysteine and glutathione in the respective media and the linear range. The sensitivities in tissue culture media are ca. 3 nA μM⁻¹ and ca. 1 nA μM⁻¹ and the limits of detections are ca. 2 μM and ca. 1 μM for homocysteine and glutathione, respectively. In human plasma, the sensitivities were determined to be 94 and 39 nA μM⁻¹, and the limit of detections are ca. 0.8 μM and ca. 0.8 μM for homocysteine and glutathione, respectively.

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Glucose dehydrogenase (GlDH) and ferrocene were coadsorbed on a carbon felt (CF) sheet (5 × 10 mm, 2 mm thickness), which was used to construct an electrode for the electrochemical detection of glucose. A potential of +0.3 V vs. Ag/AgCl was applied on the base CF, and the current was measured. After the addition of glucose, the current increased and reached a steady state within 50 s. The current response was proportional to the glucose concentration up to 20 μM, with a lower detection limit of 1 μM. The surface of the CF electrode was covered by layers of polystyrene sulfonate and poly-L-lysine using layer-by-layer technique. Again the current response was proportional to glucose concentration up to 20 μM, with a lower detection limit of 2 μM. The oxidation current owing to electrochemical interferents such as L-ascorbate and acetaminophen was 1/8 times of the current observed on the unprotected electrode. In addition, the protection imparted stability to the electrode. Our work demonstrates that a GlDH/ferrocene CF electrode, protected with polystyrene sulfonate and poly-L-lysine, could be used for the electrochemical detection of glucose.

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In this study, a simple, highly sensitive electrochemical biosensor for myoglobin was developed using a myoglobin-specific binding peptide as a sensing probe. A peptide (Myo-3R7, CPSTLGASC, 838 Da) identified by phage display and that specifically binds to myoglobin was covalently immobilized on a gold electrode functionalized via a dithiobis(succinimidyl propionate) (DSP) self-assembled monolayer (SAM). The peptide immobilization was confirmed with fluorescence microarray scanning and cyclic voltammetry (CV). The electrochemical performance of the biosensor with respect to myoglobin was characterized by CV and differential pulse voltammetry (DPV) using Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ as a redox probe. We successfully detected myoglobin in a broad working range of 17.8 to 1780 ng mL⁻¹ with a correlation coefficient (R²) of 0.998. The estimated limit of detection (LOD) was fairly low, 9.8 ng mL⁻¹ in 30 min. The electrochemical biosensor based on a myoglobin-specific binding peptide offers sensitivity, selectivity, and rapidity, making it an attractive tool for the early detection of cardiac infarction.

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A new glucose meter was developed employing a novel disposable glucose sensor strip comprising a nicotinamide adenine dinucleotide-glucose dehydrogenase (NAD-GDH) and a mixture of Fe compounds as a mediator. An iron complex, 5-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl)-1,10-phenanthroline iron(III) chloride (Fe-PhenTPy), was synthesized as a new mediator for the NAD-GDH system. Due to the high oxidation potential of the mediator, the detection potential was tuned to be more closely fitted toward the enzyme reaction potential, less than 400 mV (vs. Ag/AgCl), by mixing with an additional iron mediator. The impedance spectrometry for the enzyme sensor containing the mixed mediators showed an enhanced charge transfer property. In addition, a new cartridge-type glucose meter was manufactured using effective aligned-electrodes, which showed an enhanced response compared with conventional electrode alignment. The proposed glucose sensor resulted in a wide dynamic range in the concentration range of 30 – 500 mg dL⁻¹ with a reduced interference effect and a good sensitivity of 0.57 μA mM⁻¹.

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An electrochemical route for the decoration of multiwalled carbon nanotubes (MWCNTs) with anisotropic Au nanostructures and the electroanalytical application of decorated MWCNTs are described. MWCNTs were electrochemically decorated with flowers and buds-like Au nanostructures in aqueous solution in the presence of KI. The flowers and buds-like nanostructures had an average size of 80 nm with a predominant Au(111) plane. The analytical application of the decorated MWCNTs in the electroanalysis of biologically important analytes, such as uric acid (UA), epinephrine (EN) and ascorbic acid (AA), was studied. The nanoparticles of flower-like morphology efficiently catalyze the oxidation of the bioanalytes at a less-positive potential. Simultaneous electroanalysis of AA, UA and EN have been achieved. Well separated individual voltammetric peaks were obtained in their coexistence. The decorated MWCNT modified electrode is very stable and highly sensitive towards UA and EN. It could detect micromolar levels of bioanalytes without any interference. The catalytic property of the nanostructures is superior to that of the conventional spherical nanoparticle. The morphology of the nanoparticle controls the electrocatalytic activity.

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In the present study, we monitored the alkaline phosphatase (ALP) activity of embryoid bodies (EBs) of mouse embryonic stem (ES) cells using a large-scale integration (LSI)-based amperometric device with 400 sensors and a pitch of 250 μm. In addition, a simulation analysis was performed to reveal the positional relationship between the EBs and the sensor electrodes toward more precise measurements. The study shows that simulation analysis can be applied for precise electrochemical imaging of three-dimensionally cultured cells by normalization of the current signals.

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This study is the first to report on the detection of biogenic amines in an aqueous solution using an organic field-effect transistor (OFET) device with an extended gate electrode modified with a layer of diamine oxidase and a horseradish peroxidase osmium-redox polymer. The limit of detection (LOD) for histamine was estimated to be 1.2 μM. These results reveal that extended-gate type OFET devices are highly suitable enzyme-based biosensors for the detection of biogenic amine levels.

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We herein report on the development of an extended-gate type organic field-effect transistor (OFET)-based immunosensor for the detection of human immunoglobulin A (IgA). The titration results of IgA exhibited shifts in the transfer characteristics of the OFET sensor device with increasing IgA concentration. A linear detection range from 0 to 10 μg/mL was realized with a detection limit of 2.1 μg/mL, indicating that the OFET-based immunosensor can be potentially applied to the monitoring of infectious diseases and psychological stress in daily life.

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In this study, we developed a micropyramid array electrode that facilitates contact between a nitrocellulose membrane and electrode, which is important to realize a quantitative and sensitive electrochemical detection system for immunochromatography. We evaluated the micropyamid array electrode with our newly developed detection system that can measure contact forces between a membrane and electrode, and also investigated the relationship between redox current and contact forces. By using normal pulse voltammetry, we observed higher reduction currents over a flat surface electrode at lower contact forces with the micropyramid array.

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We report on a novel voltammetric detection of oxalic acid by using glassy carbon electrodes with covalently attached nitrogen-containing functional groups prepared by stepwise electrolysis. A glassy carbon electrode electrooxidized in an ammonium carbamate solution was electroreduced at –1.0 V (vs. Ag/AgCl) in 1.0 M sulfuric acid for a long time. We found that the electrocatalytic oxidation wave of oxalic acid obtained by this modified glassy carbon electrode was moved to a more negative potential region than that obtained by a platinum electrode in an acidic medium. A good linearity for the peak current signals was observed in the concentration range from 0.1 to 50 mM.

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