Article Review: Nanotech Biosensors

Designing Electrochemical Interfaces with Functionalized Magnetic Nanoparticles and Wrapped Carbon Nanotubes as Platforms for the Construction of High-Performance Enzyme Biosensors

Marcos Eguilaz, Reynaldo Villalonga, Paloma Yanez-Sedeno, and José M. Pingarront

Analytical Chemistry, Article ASAP
DOI: 10.1021/ac201466m
http://pubs.acs.org/doi/full/10.1021/ac201466m

Structure of enzyme biosensor.

This paper describes the construction and use of electrodes for multi-enzyme biosensors, that could be used to detect the presence and concentrations of medically relevant biomolecules. The authors focused on using magnetic ferrite nanoparticles (MNPs) functionalized with coated carbon nanotubes for the determination of cholesterol. This involved coimmobilization of cholesterol oxidase and horseradish peroxidase onto the electrode. These enzymes react with and breakdown cholesterol into electroactive products. The chemicals used for coating the nanotubes were glutaraldehyde and a form of ammonium chloride called PDDA. Detection was characterized using the electrochemical techniques of amperometry and cyclic voltammetry. In cyclic voltammetry, an applied voltage causes oxidation of electroactive molecules at the electrode surface, producing increased current flow that is specific for different analytes.

Cyclic Voltammetry.

Specifically, the electrode potential is ramped linearly versus time. When a set potential is reached, the potential ramp is inverted, causing the analyte to be reduced at its characteristic reduction potential. A cyclic voltammogram is then produced, which shows the current at the working electrode plotted versus the applied voltage. A subclass of voltammetry is amperometry, in which the electrode is held at constant potentials.

Carbon Nanotubes.

The use of carbon nanotubes to bring about efficient biomolecule immobilization with useful electroanalytical capabilities has been well documented, but magnetic nanoparticles offer promising performance in this area as well. MNPs have high protein load capacities, are nontoxic, easy to prepare compared to metal nanoparticles, and can provide enhanced sensitivity. They have been used in enzyme electrodes for the detection of glucose and tyrosinase, as well as in electrochemical immunosensors for the detection of antibodies. PDDA is a cationic polyelectrolyte that is useful for its ability to form films and induce adhesion. The positively charged PDDA coats negatively charged CNT surfaces, providing them with better solubility and stability in solution. Ultimately, the researchers found that this platform gave increased sensitivity for cholesterol determination as compared to methods employing CNTs alone.

The article mentions that MNPs are non-toxic and that they have high biocompatibility, but this is not expanded on or referenced. A quick search for magnetic nanoparticle toxicity reveals that this is not as cut-and-dried as the paper suggests, with many conflicting viewpoints being presented.

The authors reported a higher reduction peak current for the MNP containing electrodes, which they attributed to higher enzyme loading, perhaps due to their greater surface-to-volume ratio.The cyclic voltammograms show the potentials shifting toward more positive values and gradual increases in reduction peak current. The authors ruled out the presence of impurities as the cause after imaging the electrode surfaces with SEM and TEM (Scanning & Tunneling electron microscopy), blaming instead the oxidized nanotube surfaces for the shifting values.