Low density lipoprotein detection based on antibody immobilized self-assembled monolayer: investigations of kinetic and thermodynamic properties.

Human plasma low density lipoprotein (LDL) immunosensor based on surface plasmon resonance (SPR) and quartz crystal microbalance (QCM) was fabricated by immobilizing antiapolipoprotein B (AAB) onto self-assembled monolayer (SAM) of 4-aminothiophenol (ATP). The AAB/ATP/Au immunosensor can detect LDL up to 0.252 microM (84 mg/dL) and 0.360 microM (120 mg/dL) with QCM and SPR, respectively. The SPR and QCM measurements were further utilized to study the reaction kinetics of the AAB-LDL interaction. The adsorption process involved was explored using Langmuir adsorption isotherm and Freundlich adsorption models. The thermodynamic parameters such as change in Gibb's free energy (DeltaG(ads)), change in enthalpy (DeltaH(ads)), and change in entropy (DeltaS(ads)) determined at 283, 298, and 308 K revealed that the AAB-LDL interaction is endothermic in nature and is governed by entropy. Kinetic, thermodynamic, and sticking probability studies disclosed that desorption of the water molecules from the active sites of AAB and LDL plays a key role in the interaction process and increase in temperature favors binding of LDL with the AAB/ATP/Au immunosensor. Thus, the studies were utilized to unravel the most important subprocess involved in the adsorption of LDL onto AAB-modified ATP/Au surface that may help in the fabrication of LDL immunosensors with better efficiency.

Training effect and fatigue in polypyrrole-based artificial muscles.

Artificial muscles based on an electrochemomechanical strain (ECMS) in conducting polymers, namely polypyrrole (PPy) film, have been studied from viewpoints of training, fatigue and aging by repeat cycles under tensile loads. ECMS was approximately 2% in a saline solution, resulting from both insertion and exclusion of Na(+) with solvated water molecules as well in the film. Transient responses of ECMS and current induced by voltage stimuli were measured under tensile stresses up to 5 MPa to see the training effect, fatigue and aging of the film. At higher stresses the film showed larger creeping, which resulted from realignment or conformation change, slipping and breaking of polymer chains. After the experience of large stresses, the training effect in ECMS was appreciably observed as an increase of the strain. Without stress the conductivity of the film was stable (no fatigue) upon an electrochemical cycle; however, under high tensile stresses the conductivity decreased remarkably (fatigue and aging). It is to be noted that straightened polymer chains can be easily oxidized and degraded due to lower pi-electron energy. The conversion efficiency from electrical to mechanical energy in this system was found to be less than 0.03%.

Work behaviors of artificial muscle based on cation driven polypyrrole.

A soft actuator mimicking natural muscles (artificial muscle) has been developed using a flexible conducting polymer of polypyrrole films, which were driven by electrical stimulus in a saline solution. The work characteristics were studied under various load stresses and found to behave like natural muscles. The artificial muscles shrunk and stiffened by the positive electrical stimulus by 2-3% at the maximum force of 5 MPa, and relaxed by application of negative voltages. At larger load stresses, the artificial muscle shrunk slowly as natural muscles do. The driving current also lasted longer at larger loads, indicating that the muscle sensed the magnitude of the load stress. During contraction of the muscle, the conversion efficiency from the electrical input and mechanical output energies was estimated to be around 0.06%. The maximum volumetric work was approximately estimated to be 100 kJ m(-3). These figures are unexpectedly small compared with those of natural muscles.

Polypyrrole nanotube array sensor for enhanced adsorption of glucose oxidase in glucose biosensors.

A novel amperometric biosensor based on polypyrrole (PPy) nanotube array deposited on a Pt plated nano-porous alumina substrate and its performances are described. Glucose oxidase (GOx) enzyme was selected as the model enzyme in this study. Commercially available nano-porous alumina discs were used to fabricate electrodes in order to study the feasibility of enzyme entrapment by physical adsorption. A PPy/PF6- film comprising of nanotube array was synthesized using a solution containing 0.05 M Pyrrole and 0.1 M NaPF6 at a current density of 0.3 mA/cm2 for 90 s. The immobilization was done by physical adsorption of 5 microL of GOx (from a stock solution of 2 mg/mL of 210 U/mg) on each electrode. A sensitivity of 7.4 mA cm(-2) M(-1) was observed with PPy nanotube array where the maximum tube diameter was 100 nm. A linear range of 500 microM-13 mM and a response time of about 3 s were observed with a nanotube array where the maximum tube diameter was 200 nm. The synthesized nanotube arrays were characterized by galvanostatic electrochemical technique. Calculated value of apparent Michaelis-Menten constant (Km) was 7.01 mM. The use of nano-porous template electrodes leads to an efficient enzyme loading and provides an increased surface area for sensing the reaction. These factors contribute to increase the characteristic performances of the novel biosensor.

Tubular linear actuators using conducting polymer, polypyrrole.

Conducting polymers show an electrochemomechanical deformation (ECMD), which is able to be utilized as soft actuators. A tubular linear actuator of polypyrrole film is fabricated and the characteristics are examined. The film was electrochemically prepared on an acryl resin rod in an aqueous electrolyte solution of pyrrole and dodecylbenzensulfonic acid (DBS), followed by removing the rod. The actuations of tubular polypyrrole film due to ECMD in various conditions have been examined to clarify the mechanism. It has been found that the tubular actuator elongates upon reduction with the strain of 7%, which is more than twice of that observed in a rectangular film. The facts indicate that cations play the role of dopants instead of large DBS anion and the tubular structure gives the better performance for large strain.

An amperometric urea biosensor based on covalent immobilization of urease onto an electrochemically prepared copolymer poly (N-3-aminopropyl pyrrole-co-pyrrole) film.

An amperometric biosensor has been developed for the quantitative determination of urea in aqueous solution. The principle is based on the use of pH-sensitive redox active dissolved hematein molecule. The enzyme, urease (Urs), was covalently immobilized on a conducting copolymer poly (N-3-aminopropyl pyrrole-co-pyrrole) film, electrochemically prepared onto an indium-tin-oxide (ITO)-coated glass plate. The covalent linkage of enzyme and porous morphology of the polymer film lead to high enzyme loading and an increased lifetime stability of the enzyme electrode. Amperometric response was measured as a function of concentration of urea, at fixed bias voltage of 0.0 V vs. Ag/AgCl in a phosphate buffer (pH 7.0). The electrode gives a linear response range of 0.16-5.02 mM for urea in aqueous medium. The response time is 40 s reaching to a 95% steady-state current value, and 80% of the enzyme activity is retained for about 2 months.

Correlation of the number of thiophene units with structural order and carrier mobility in unsubstituted even- and odd-numbered alpha-oligothiophene films.

We investigate the correlation of the number of thiophene units with the structural order and carrier mobility of the films through studies on thin-film transistors (TFTs) based on alpha-quinquethiophene (5T), alpha-sexithiophene (6T), and alpha-septithiophene (7T). The X-ray diffraction (XRD) data of the nT films deposited at low substrate temperatures present obviously different structural orders depending on the parity of the number of thiophene units. Although even-numbered nT films present well-ordered structures and large carrier mobilities, odd-numbered nT films present two different crystalline polymorphs and vastly low carrier mobilities reflecting the coexistence of two crystalline polymorphs. However, the XRD data of both even- and odd-numbered nT films deposited at high substrate temperatures indicate that the nT molecules form single well-ordered structures. Those ordered TFTs exhibit large carrier mobilities accompanying an increase in the number of thiophene units, 0.05, 0.08, and 0.13 cm2 V(-1) s(-1) for 5T, 6T, and 7T, respectively. The parity of the number of thiophene units affects the structural order intrinsically in grown thin films, and affects carrier mobilities extrinsically in their TFTs.

Poly-3-hexyl thiophene Langmuir-Blodgett films for application to glucose biosensor.

Langmuir-Blodgett films of poly(3-hexyl thiophene) have been prepared by simultaneous entrapment of glucose oxidase and transferred onto the indium-tin-oxide coated glass plates. The films have been characterized by FTIR spectroscopy and detailed response studies have been performed with respect to glucose concentration, temperature, and storage time. These P3HT/SA/GOX electrodes have been utilized for glucose estimation from 100-500 mg/dL by amperometric method. The electrodes have been found to have sensitivity of 0.75 nA/mg/dL detection limit of 50 mg/dL and shelf life of about 75 days at 4 degrees C.