Dong-Hwa Yun

Comparison of Effective Working Electrode Areas on Planar and Porous Silicon Substrates for Cholesterol Biosensor

Porous silicon-based biosensors were originally developed to further stet the miniaturization of a host of devices. In this paper, we describe the relationship between the enlargement of an electrodes area and its sensitivity for the determination of cholesterol concentrations with covalent binding to immobilized enzymes. For comparison, we conducted a series of experiments using a planar silicon electrode and a porous silicon electrode. We determined the effective surface area of the electrodes using the Randles–Sevcik equation. The active surface area of the planar electrode was approximately 0.1608 cm2, and that of the porous electrode was approximately 0.5054 cm2. Cholesterol oxidase was covalently immobilized on each electrode by silanization. The sensitivities were 0.08567 µA/mM for the planar sensor and 0.2656 µA/mM for the porous sensor. The calculated effective surface area and sensitivity of the porous electrode were about 3.1-fold larger than those of the planar electrode.

An Electrochemical Biosensor Array for Rapid Detection of Alanine Aminotransferase and Aspartate Aminotransferase

An increment of alanine aminotransferase (ALT) or aspartate aminotransferase (AST) in human serum indicates an abnormal symptom of the liver. Hence, an electrochemical biosensor array that uses micro electro mechanical systems technology is required for rapid and integrated measurement of ALT/AST. Here we describe a biosensor array consisting of two glutamate sensors. It turned out that porous silicon layers formed on each working electrode were useful to increase the effective surface area. This biosensor array was constructed with platinum electrodes and a polydimethylsiloxane (PDMS) microchannel. Electrodes in sampling wells minimized a cross-interference effect and permitted multiple sampling by immobilization with glutamate oxidase using a silanization technique. The device sensitivities derived from semi-logarithmic plots were 0.145 μA/(U/l) for ALT and 0.463 μA/(U/l) for AST over a range of 1.3 U/l to 250 U/l. Hene, this ALT/AST biosensor array can be applied in diagnostic and home use.

Fabrication and Electrochemical Characterization of Nanoporous Silicon Electrode for Amperometric Urea Biosensor

We describe a new type of biosensor that employs a modified gold electrode based on nanoporous silicon (NPSi) for the electrochemical detection of urea. Urease (Urs) was covalently immobilized onto an Au/NPSi electrode functionalized with 3-mercaptopropionic acid (3-MPA). Amperometric calibration curves for both NPSi and planar silicon (PLSi)-based urea sensitive electrodes were compared in the range of 0.3 to 4.5 mM urea concentrations. The Michaelis–Menten constant (Km) was determined using the amperometric method. The electrochemical active area (Aea) of the 3-MPA/Au/NPSi electrode was evaluated using cyclic voltammetry (CV) and the result was compared with the 3-MPA/Au/PLSi electrode. Measured sensitivity of the Urs/SAMs/Au/NPSi electrode is ca. 2.05 µA mM-1 cm-2 and that of the Urs/SAMs/Au/PLSi electrode is ca. 1.10 µA mM-1 cm-2. About 1.8 times of sensitivity increase is obtained in the Au/NPSi electrode.

Electrochemical biosensor array for liver diagnosis using a silanization technique on nano-porous silicon electrode

Cholesterol, bilirubin and ALT/AST present in the serum are often used as biomarkers for liver diseases. For this study, we describe our biosensor array system consisting of cholesterol, bilirubin and glutamate sensors. We determined that porous silicon (PS) layers formed on each working electrode greatly increased the effective surface area. The electrodes in the sampling wells minimized a cross-interference effect to permit multiple sampling by immobilization of the enzymes using a silanization technique. The device sensitivities observed were 0.2656 muA/mM for cholesterol, 0.15354 mA/mM for bilirubin, 0.13698 muA/(U/l) for ALT and 0.45439 muA/(U/l) for AST.

Nano-biosensor base on protected glucose oxidase nanoparticles

Covalent modification of redox enzyme-glucose oxidase (GOx)-within porous composite organic/inorganic network was described. The polymerization of organic/inorganic network was synthesized via a three-step process, which consisted of a covalent modification of GOx followed by a polymerization between modified enzyme and methacryloxypropyltrimethoxysilane (MPS) and then hydrolysis and crosslinking. The synthesized GOx nanoparticles were less than 20 nm in size. They were observed by transmission electron microscope (TEM) and analyzed using Fourier transform infrared spectrophotometer (FT-IR). The nano-biosensor based on protected GOx nanoparticles demonstrated extension of primarily lifetime and detection of extremely limited concentration (pM) in human serum. The hybrid organic/inorganic network was determined by application of protected glucose oxidase nanoparticles in electrochemical nano-biosensor. The hybrid enzyme nanostructures were expected as a method to stabilize enzyme for other biocatalytic application.

3-Mercaptopropionic acid modified porous silicon substrate used in hyperammonemia

Amperometric urea sensor is more suitable than optical and potentiometric urea sensor to diagnose hyperammonemia. However, because sensitivity in low concentration decreases remarkably, despite amperometric urea sensor has been studied for a long time it has not been applied for clinical diagnosis. In this paper, a new structure for an amperometric urea sensor was fabricated by MEMS, electrochemical etching, and electrostatic covalent binding techniques. Until now most amperometric urea sensors have had a membrane fixed on top of the transducer. That method often leads to malfunction of the sensor, arising from problems such as inadequate membrane adhesion, insufficient mechanical stability, and low sensitivity. To solve these kinds of problems, urease (Urs) was immobilized by electrostatic covalent binding method on the porous silicon substrate coated self-assembled monolayer (SAM). Electrostatic covalent binding method was used to keep anisotropic orientation of urease on SAM.

3-Mercaptopropionic Acid modified Porous Silicon Substrate used in Hyperammonemia

Amperometric urea sensor is more suitable than optical and potentiometric urea sensor to diagnose hyperammonemia. However, because sensitivity in low concentration decreases remarkably, despite amperometric urea sensor has been studied for a long time it has not been applied for clinical diagnosis. In this paper, a new structure for an amperometric urea sensor was fabricated by MEMS, electrochemical etching, and electrostatic covalent binding techniques. Until now most amperometric urea sensors have had a membrane fixed on top of the transducer. That method often leads to malfunction of the sensor, arising from problems such as inadequate membrane adhesion, insufficient mechanical stability, and low sensitivity. To solve these kinds of problems, urease (Urs) was immobilized by electrostatic covalent binding method on the porous silicon substrate coated self-assembled monolayer (SAM). Electrostatic covalent binding method was used to keep anisotropic orientation of urease on SAM.

Sensitivity improvement of polypyrrole-based urea sensor using copper ion doping effect

The functionalization of an integrated urea sensor by a copper-poly pyrrole matrix doping urease molecules and its application to the amperometric detection of urea is described. Copper cluster was doped into polypyrrole matrix to increase electrical conductivity. Urease was electroadsorbed onto this conductive matrix. The comparison of different organic and inorganic host matrices in terms of storage and operational stabilities clearly demonstrated the advantages of the composite matrix. Pt thin film electrode fabricated on silicon substrate by the radio frequency sputtering and titanium layer is also deposited as an underlayer to increase adhesive strength. The copper-doped polypyrrole and polypyrrole films are coated on platinum thin film electrode by cyclic voltammetry, respectively. Under the optimized condition, the limiting current obtained from the copper-doped polypyrrole urea sensor is proportional to the urea concentrations with the slope of 4.5 microampere per decade, which is two times higher sensitivity value than that of the polypyrrole urea sensor. In addition, the electrode surface was analyzed by scanning electro microscopy.