Preparation and characterization of room temperature ionic liquid/single-walled carbon nanotube nanocomposites and their application to the direct electrochemistry of heme-containing proteins/enzymes

This work describes the formation and possible electrochemical application of a novel nanocomposite based on single-walled carbon nanotubes (SWNTs) and imidazolium-based room-temperature ionic liquids (RTILs) of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF₄, a hydrophilic RTIL) and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF₆, a hydrophobic RTIL). The nanocomposites ([bmim]BF₄-SWNTs, and [bmim]PF₆-SWNTs) were formed by simply grinding the SWNTs with the respective RTIL. The results of the X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy indicated that the nanocomposites were formed by adsorption of an imidazolium ion on the surface of SWNTs via the “cation-π” interaction. SEM images showed that [bmim]BF₄-SWNTs (or [bmim]PF₆-SWNTs) nanocomposites could uniformly cover the surface of a glassy carbon (GC) electrode resulting in a RTILs-SWNTs/GC modified electrode with a high stability. The RTILs-SWNTs composite could be readily used as a matrix to immobilize heme-containing proteins/enzymes (myoglobin, cytochrome c, and horseradish peroxidase) without undergoing denaturation, as was verified by UV-vis and circular dichroic (CD) spectroscopic results. The voltammetric results showed that heme-containing proteins/enzymes entrapped in RTILs-SWNTs composites displayed a pair of well-defined, stable redox peaks, which were ascribed to their direct electron-transfer reactions. The results of controlled experiments showed that the positive charged imidazolium ion played a significant effect on the electrochemical parameters, such as the redox peak separation and the value of the formal potentials, etc., of the electron-transfer reaction of non-neutral species dissolved in solution or immobilized on the electrode surface. Further results demonstrated that the heme-containing proteins/enzymes entrapped in RTILs-SWNTs composites could still retain their bioelectrocatalytic activity toward the reduction of oxygen and hydrogen peroxide. The results depicted in this work may pave a new avenue to electrocatalysis, proteins/enzymes electrochemistry, and bioelectrochemical synthesis, etc.     

A MEMS affinity glucose sensor using a biocompatible glucose-responsive polymer

We present a MEMS affinity sensor that can potentially allow long-term continuous monitoring of glucose in subcutaneous tissue for diabetes management. The sensing principle is based on detection of viscosity changes due to affinity binding between glucose and poly(acrylamide-ran-3-acrylamidophenylboronic acid) (PAA-ran-PAAPBA), a biocompatible, glucose-specific polymer. The device uses a magnetically driven vibrating microcantilever as a sensing element, which is fabricated from Parylene and situated in a microchamber. A solution of PAA-ran-PAAPBA fills the microchamber, which is separated from the surroundings by a semi-permeable membrane. Glucose permeates through the membrane and binds reversibly to the phenylboronic acid moiety of the polymer. This results in a viscosity change of the sensing solution, which is obtained by measuring the damped cantilever vibration using an optical lever setup, allowing determination of the glucose concentration. Experimental results demonstrate that the device is capable of detecting glucose at physiologically relevant concentrations from 27 mg/dL to 324 mg/dL. The glucose response time constant of the sensor is approximately 3 min, which can be further improved with device design optimization. Excellent reversibility and stability are observed in sensor responses, as highly desired for long-term, stable continuous glucose monitoring.     

Amperometric urea biosensors based on the entrapment of urease in polypyrrole films

Urease (Urs) was immobilized in electrochemically prepared polypyrrole (PPy) and the resulting films were characterized by cyclic voltammetry, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and ultraviolet visible spectroscopy (UV-VIS). The enzymatic activity of Urs entrapped in the PPy matrix was confirmed by the catalytic conversion of urea into carbon dioxide and ammonia, when urea was detected amperometrically at different concentrations in standard samples and commercial fertilizers. The PPy/Urs biosensors exhibited selectivity, a relatively high efficiency at urea concentrations below 3.0 mmol L⁻¹, and a sensitivity to urea of 2.41 μA cm⁻² mmol⁻¹ L.     

Amperometric cholesterol biosensors based on hybrid organic–inorganic Langmuir–Blodgett films

Preparation and characterization of a thin-film cholesterol biosensor employing an organic–inorganic hybrid system of cholesterol oxidase (ChOx) and Prussian blue (PB) is described. ChOx was immobilized in Langmuir–Blodgett (LB) films consisting of positively charged octadecyltrimethylammonium (ODTA) and nano-sized PB clusters. Immobilization was performed by simple immersion of ODTA/PB LB films into an aqueous solution of ChOx. Subsequent ChOx absorption into LB films was confirmed by infrared reflection–absorption spectroscopy. Obtained ODTA/PB/ChOx LB films clearly exhibited a response current to cholesterol under an applied potential of 0.0 V (vs. Ag/AgCl). The linearity of current density versus cholesterol concentration was confirmed for the range 0.2–1.2 mmol/L. The present study indicates that simple immersion of ODTA/PB LB films into an enzyme solution is a promising method to produce many types of thin-film biosensors comprising a hybrid system of an oxidative enzyme and PB nano-clusters that work at a very low potential range.     

Smart sensors/actuators for biomedical applications: Review

This paper reflects on review of smart sensor activities for biomedical applications. The rise of biotechnology has provided innovative development of new therapies and detection methods for life threatening diseases. As a worldwide research focus, there is especially a strong interest in the use of microsystems in health care, particularly as smart implantable devices. Recent years have seen an increasing activity of hip and knee replacement and other type of implants, which are some of the most frequently performed surgical procedures in the world. Loosening of hip prosthesis is the dominant issue for many patients who undergo a hip arthroplasty. Artificial joints are subject to chronic infections associated with bacterial biofilms, which only can be eradicated by the traumatic removal of the implant followed by sustained intravenous antibiotic therapy. This review focuses on the clinical experience using all kinds of smart implants like orthopedic implants instrumented with strain gauges, retina implant system using image sensors. Technical design improvements will enhance function, quality of life, and longevity of total knee arthroplasty and all other kind of implants. Application of biocompatible nanomaterials in implantable biosensors for continuous monitoring of metabolites is an area of sustained scientific and technological interests.     

Preparation of nanostructured thin-film interfaces applicable in biosensors

Gold nanostructured thin-film electrodes (250 nm) were formed by using radio frequency (RF) diode sputtering, bias sputtering and ion etching. Biosensors based on the deoxyribonucleic acid (DNA) film immobilized at the gold surface were prepared by double-stranded DNA (dsDNA) adsorption. The behaviour of the DNA layer was investigated by using differential pulse voltammetry (DPV) with the DNA marker Co[(phen)₃]³⁺ and cyclic voltammetry (CV) with the K₃[Fe(CN)₆] indicator and DNA-Co[(phen)₃]³⁺ dissociation rate constant (k). The best result of attachment DNA binding were obtained in case of both sputtered sample only and the sample which was etched for 10 min at 150 °C.     

Surface plasmon resonance-based biosensors: From the development of different SPR structures to novel surface functionalization strategies

Surface plasmon resonance (SPR)-based biosensors are very powerful tools for the study of biomolecular interactions, chemical detection and immunoassays. This paper reviews the performance of various SPR structures and detection schemes focusing on propagating surface plasmons generated in planar structures. Some aspects of their surface functionalization, the key element which imparts biofunctionality to these structures and hence transforming them into biosensors, will also be discussed accordingly. The ultimate performance of SPR-based biosensors will thus be determined by both their inherent optical performance and suitable surface functionalization.