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.

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.