Chapter 7 New materials for biosensors, biochips and molecular bioelectronics

Self-assembly of acetylcholinesterase on gold nanoparticles electrodeposited on graphite

The immobilisation of AChE enzyme through chemisorption on Au-modified graphite was examined with view of its prospective application in the design of membraneless electrochemical biosensors for the assay of enzyme inhibitors. The developed immobilisation protocol has been based on a two-stage procedure, comprising i) electrodeposition of gold nanostructures on spectroscopic graphite; followed by ii) chemisorption of the enzyme onto gold nanoparticles. Both the coverage of the electrode surface with Au nanostructures and the conditions for enzyme immobilisation were optimised. The proposed electrode architecture together with the specific type of enzyme immobilisation allow for a long-term retaining of the enzyme catalytic activity. The extent of inhibition of the immobilised acetylcholinesterase enzyme by the organophosphorous compound monocrotophos has been found to depend linearly on its concentration over the range from 50 to 400 nmol mL−1 with sensitivity 77.2% inhibition per 1 µmol mL−1 of monocrotophos.

Flexible poly(amic acid) conducting polymers: effect of chemical composition on structural, electrochemical, and mechanical properties

A new approach for creating flexible, mechanically strong poly(amic acid) (PAA) hybrid copolymers is described. The reduction of gold salts to gold nanoparticles by PAA coupled with its copolymerization in the presence of various silanes (e.g., N-[3-(trimethoxysilyl)-propyl] aniline (TMOSPA), 3-aminopropyl-trimethoxysilane (APTMOS), dichlorodimethylsilane (DCMS), and tetramethoxysilane (TMOS)) has enabled the design of a series of polymeric films. The resulting poly(amic acid), silane, and gold (PSG) solutions were employed for the fabrication of flexible, ternary polymers with a minimum bend ratio of 3 mm using thermal desolvation and/or wet-phase inversion techniques. By controlling the composition and synthesis conditions, porous PSG films were produced that are flexible or rigid, transparent or opaque, and/or mechanically strong. (1)H NMR, (13)C NMR, and Fourier transform infrared spectroscopy (FTIR) characterization results showed that the carboxylic acid moieties were retained in the PSG copolymer. Thermal stabilities with degradation characteristics of the polymers were determined using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Although structurally and morphologically different from the parent PAA, copolymerization with silanes had significantly improved the mechanical and interfacial property of the PSG class of films.

NanoMonitor: a miniature electronic biosensor for glycan biomarker detection

Aim: The goal of our research is to develop an ultrasensitive diagnostic platform called ‘NanoMonitor’ to enable rapid label-free analysis of a highly promising class of biomarkers called glycans (oligosaccharide chains attached to proteins) with high sensitivity and selectivity. The glycosylation of fetuin – a serum protein – and extracts from a human pancreatic cancer line was analyzed to demonstrate the capabilities of the NanoMonitor. Material & methods: The NanoMonitor device consists of a silicon chip with an array of gold electrodes forming multiple sensor sites and works on the principle of electrochemical impedance spectroscopy. Each sensor site is overlaid with a nanoporous alumina membrane that forms a high density of nanowells on top of each electrode. Lectins (proteins that bind to and recognize specific glycan structures) are conjugated to the surface of the electrode. When specific glycans from a test sample bind to lectins at the base of each nanowell, a perturbation of electrical double-layer occurs, which results in a change in the impedance. Using the lectins Sambucs nigra agglutinin (SNA) and Maackia amurensis agglutinin (MAA), subtle variations to the glycan chains of fetuin were investigated. Protein extracts from BXPC-3, a cultured human pancreatic cancer cell line were also analyzed for binding to SNA and MAA lectins. The performance of the NanoMonitor was compared to a conventional laboratory technique: lectin-based enzyme linked immunosorbent assay (ELISA). Results & discussion: The NanoMonitor was used to identify glycoform variants of fetuin and global differences in glycosylation of protein extracts from cultured human pancreatic cancerous versus normal cells. While results from NanoMonitor correlate very well with results from lectin-based ELISA, the NanoMonitor is rapid, completely label free, requires just 10 µl of sample, is approximately five orders of magnitude more sensitive and highly selective over a broad dynamic range of glycoprotein concentrations. Conclusion: Based on its performance metrics, the NanoMonitor has excellent potential for development as a point-of-care handheld electronic biosensor device for routine detection of glycan biomarkers from clinical samples.

Status of biomolecular recognition using electrochemical techniques

The use of nanoscale materials (e.g., nanoparticles, nanowires, and nanorods) for electrochemical biosensing has seen explosive growth in recent years following the discovery of carbon nanotubes by Sumio Ijima in 1991. Although the resulting label-free sensors could potentially simplify the molecular recognition process, there are several important hurdles to be overcome. These include issues of validating the biosensor on statistically large population of real samples rather than the commonly reported relatively short synthetic oligonucleotides, pristine laboratory standards or bioreagents; multiplexing the sensors to accommodate high-throughput, multianalyte detection as well as application in complex clinical and environmental samples. This article reviews the status of biomolecular recognition using electrochemical detection by analyzing the trends, limitations, challenges and commercial devices in the field of electrochemical biosensors. It provides a survey of recent advances in electrochemical biosensors including integrated microelectrode arrays with microfluidic technologies, commercial multiplex electrochemical biosensors, aptamer-based sensors, and metal-enhanced electrochemical detection (MED), with limits of detection in the attomole range. Novel applications are also reviewed for cancer monitoring, detection of food pathogens, as well as recent advances in electrochemical glucose biosensors.