Prospects of Nanomaterials in Biosensors

Point-of-care (POC) devices by means of advanced MEMS

Microelectromechanical systems (MEMS) have become an invaluable technology to advance the development of point-of-care (POC) devices for diagnostics and sample analyses. MEMS can transform sophisticated methods into compact and cost-effective microdevices that offer numerous advantages at many levels. Such devices include microchannels, microsensors, etc., that have been applied to various miniaturized POC products. Here we discuss some of the recent advances made in the use of MEMS devices for POC applications.

Application of nanomaterials in the bioanalytical detection of disease-related genes

In the diagnosis of genetic diseases and disorders, nanomaterials-based gene detection systems have significant advantages over conventional diagnostic systems in terms of simplicity, sensitivity, specificity, and portability. In this review, we describe the application of nanomaterials for disease-related genes detection in different methods excluding PCR-related method, such as colorimetry, fluorescence-based methods, electrochemistry, microarray methods, surface-enhanced Raman spectroscopy (SERS), quartz crystal microbalance (QCM) methods, and dynamic light scattering (DLS). The most commonly used nanomaterials are gold, silver, carbon and semiconducting nanoparticles. Various nanomaterials-based gene detection methods are introduced, their respective advantages are discussed, and selected examples are provided to illustrate the properties of these nanomaterials and their emerging applications for the detection of specific nucleic acid sequences.

Biosensors for Food Toxin Detection: Carbon Nanotubes and Graphene

There is increased interest towards the application of carbon based nanomaterials to biosensors since these can be used to quickly detect presence of the toxins in food, agricultural and environmental systems. The accurate, faster and early detection of food toxins is presently very important for ensuring safety and shelf life of agricultural commodities resulting from food contamination. The carbon materials (CNTs) and recently discovered graphene have been predicted to be promising candidates in the development of electrochemical biosensor owing to their exceptionally large surface area and interesting electrochemical properties. We focus on some of the recent results obtained in our laboratories pertaining to the development of biosensors based on multi-walled carbon nanotubes and graphene for mycotoxin(aflatoxin ) detection.

Functional layer-by-layer design of xerogel-based first-generation amperometric glucose biosensors

Xerogel-based first-generation amperometric glucose biosensors, constructed through specific layer-by-layer assembly of films featuring glucose oxidase doped xerogel, a diffusion-limiting xerogel layer, and capped with both electropolymerized polyphenol and blended polyurethane semipermeable membranes, are presented. The specific combination of xerogels formed from specific silane precursors, including propyl-trimethoxysilane, isobutyl-trimethoxysilane, octyl-trimethoxysilane, and hydroxymethyl-triethoxysilane, exhibit impressive dynamic and linear ranges of detection (e.g., ≥24-28 mM glucose) and low response times, as well as significant discrimination against common interferent species such as acetaminophen, ascorbic acid, sodium nitrite, oxalic acid, and uric acid as determined by selectivity coefficients. Additionally, systematic electrochemical and contact angle studies of different xerogel silane precursors, varying in structure, chain length, and/or functional group, reveal that sensor performance is more dependent on the tunable porosity/permeability of the layered interfaces rather than the hydrophobic character or functional groups within the films. While the sensing performance largely exceeds that of existing electrochemical glucose sensing schemes in the literature, the presented layered approach establishes the specific functionality of each layer working in concert with each other and suggests that the strategy may be readily adaptable to other clinically relevant targets and is amenable to miniaturization for eventual in situ or in vivo sensing.

Laccase biosensor based on phytic acid modification of nanostructured SiO₂ surface for sensitive detection of dopamine

In this work, three kinds of nanostructured silica-phytic acid (SiO2-PA) materials with diverse morphologies including spherical SiO2-PA (s-SiO2-PA), rod-like (r-SiO2-PA), and helical SiO2-PA (h-SiO2-PA) were prepared with the help of electrostatic interaction. The SiO2-PA nanomaterials with different morphologies were characterized by using transmission electron microscopy (TEM), Fourier transform infrared (FTIR), electrochemical impedance spectroscopy (EIS), and circular dichroism spectrum (CD). Diverse morphologies of SiO2-PA were used as electrode decorated materials to achieve a high efficiency for electrochemical dopamine (DA) detection. The laccase biosensors were fabricated by immobilizing different morphologies of SiO2-PA nanomaterials and laccase onto the glassy carbon electrode (GCE) surface, successively. Then the electrochemical responses of the different morphologies of nanostructured SiO2-PA nanomaterials to laccase were discussed. Results indicated that compared to laccase/s-SiO2-PA and laccase/r-SiO2-PA, the laccase/h-SiO2-PA-modified electrode showed the best electrochemical performances.

Nanoparticle film assemblies as platforms for electrochemical biosensing--factors affecting the amperometric signal enhancement of hydrogen peroxide

Factors affecting the enhanced amperometric signal observed at electrodes modified with polyelectrolyte-gold nanoparticle (Au-NP) composite films, which are potential interfaces for first-generation biosensors, were systematically investigated and optimized for hydrogen peroxide (H2O2) detection. Polyelectrolyte multilayer films embedded with citrate-stabilized gold nanoparticles exhibited high sensitivity toward the oxidation of H2O2. From this Au-NP film assembly, the importance of Au-NP ligand protection, film permeability, the density of Au-NPs within the film, and electronic coupling between Au-NPs (interparticle) and between the film and the electrode (interfacial) were evaluated. Using alternative Au-NPs, including those stabilized with thiols, polymers, and bulky ligands, suggests that the amperometric enhancement of H2O2 is optimized at poly-L-lysine-linked film assemblies embedded with Au-NPs possessing small, charged, and conductive (conjugated) peripheral ligands. As a potential application of these Au-NP film assemblies, an enhanced amperometric signal for H2O2 oxidation was shown for modified "needle" electrodes. The overall aim of this research is to gain a greater understanding of designing electrochemical sensing strategies that incorporate Au-NPs and target specific analytes.

Monolayer-protected nanoparticle doped xerogels as functional components of amperometric glucose biosensors

First-generation amperometric glucose biosensors incorporating alkanethiolate-protected gold nanoparticles, monolayer protected clusters (MPCs), within a xerogel matrix are investigated as model systems for nanomaterial-assisted electrochemical sensing strategies. The xerogel biosensors are comprised of platinum electrodes modified with composite films of (3-mercaptopropyl)trimethoxy silane xerogel embedded with glucose oxidase enzyme, doped with Au225(C6)75 MPCs, and coated with an outer polyurethane layer. Electrochemistry and scanning/transmission electron microscopy, including cross-sectional TEM, show sensor construction, humidity effects on xerogel structure, and successful incorporation of MPCs. Analytical performance of the biosensor scheme with and without MPC doping of the xerogel is determined from direct glucose injection during amperometry. MPC-doped xerogels yield significant enhancement of several sensor attributes compared to analogous films without nanoparticles: doubling of the linear range, sensitivity enhancement by an order of magnitude, and 4-fold faster response times accompany long-term stability and resistance to common interfering agents that are competitive with current glucose biosensing literature. Ligand chain length and the MPC/silane ratio studies suggest the MPC-induced enhancements are critically related to structure-function relationships, particularly those affecting interparticle electronic communication where the MPC network behaves as a three-dimensional extension of the working electrode into the xerogel film, reducing the system's dependence on diffusion and maximizing efficiency of the sensing mechanism. The integration of MPCs as a functional component of amperometric biosensor schemes has implications for future development of biosensors targeting clinically relevant species.

Determination of sulfite with emphasis on biosensing methods: a review

Sulfite is used as a preservative in a variety of food and pharmaceutical industries to inhibit enzymatic and nonenzymatic browning and in brewing industries as an antibacterial and antioxidizing agent. Convenient and reproducible analytical methods employing sulfite oxidase are an attractive alternative to conventional detection methods. Sulfite biosensors are based on measurement of either O2 or electrons generated from splitting of H2O2 or heat released during oxidation of sulfite by immobilized sulfite oxidase. Sulfite biosensors can be grouped into 12 classes. They work optimally within 2 to 900 s, between pH 6.5 and 9.0, 25 and 40 °C, and in the range from 0 to 50,000 μM, with detection limit between 0.2 and 200 μM. Sulfite biosensors measure sulfite in food, beverages, and water and can be reused 100-300 times over a period of 1-240 days. The review presents the principles, merits, and demerits of various analytical methods for determination of sulfite, with special emphasis on sulfite biosensors.

Bienzyme-functionalized monodispersed biocompatible cuprous oxide/chitosan nanocomposite platform for biomedical application

The ultrafine monodispersed cuprous oxide (Ufm-Cu(2)O) nanoparticles have been successfully synthesized by a facile wet chemical method using poly-N-vinylpyrrolidone (PVP) as a capping agent. This colloidal solution of Ufm-Cu(2)O and chitosan (CS) is electrophoretically deposited (EPD) onto the indium tin-oxide (ITO) glass substrate. Thus synthesized nanocomposite has been characterized by X-ray powder diffraction (XRD, ∼6 nm), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopic techniques. This novel biomedical nanocomposite platform has been explored to fabricate a cholesterol biosensor by immobilizing cholesterol esterase (ChEt) and cholesterol oxidase (ChOx) onto Ufm-Cu(2)O-CS/ITO electrode surface. The seed germination tests of these biomaterials (Ufm-Cu(2)O-CS nanocomposite and ChOx-ChEtUfm-CuO(2)-CS nanobiocomposite), conducted using the disc diffusion method, reveal strong activity against the common pathogens and crops, indicating biocompatibility of the nanocomposite. Under optimized conditions, the linearity between the current response and the cholesterol concentration has been obtained in the range of 10-450 mg/dL, with detection limit of 15.9 mg/dL cm(-2) and a high sensitivity of 0.895 μA/(mg/dL cm(-2)). The proposed biocompatible ChEt-ChOx/Ufm-Cu(2)O-CS/ITO bioelectrode shows fast response time (<5 s), good reproducibility, and long-term stability. This biocompatible biosensor has been used to determine the total cholesterol levels in human serum samples. Investigated antimicrobial activities of bienzyme-functionalized Ufm-Cu(2)O-CS nanocomposite are the potential platform for biomedical applications.

Stoichiometry of alloy nanoparticles from laser ablation of PtIr in acetone and their electrophoretic deposition on PtIr electrodes

Charged Pt-Ir alloy nanoparticles are generated through femtosecond laser ablation of a Pt₉Ir target in acetone without using chemical precursors or stabilizing agents. Preservation of the target's stoichiometry in the colloidal nanoparticles is confirmed by transmission electron microscopy (TEM)-energy-dispersive x-ray spectroscopy (EDX), high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM)-EDX elemental maps, high resolution TEM and selected area electron diffraction (SAED) measurements. Results are discussed with reference to thermophysical properties and the phase diagram. The nanoparticles show a lognormal size distribution with a mean Feret particle size of 26 nm. The zeta potential of -45 mV indicates high stability of the colloid with a hydrodynamic diameter of 63 nm. The charge of the particles enables electrophoretic deposition of nanoparticles, creating nanoscale roughness on three-dimensional PtIr neural electrodes within a minute. In contrast to coating with Pt or Ir oxides, this method allows modification of the surface roughness without changing the chemical composition of PtIr.

Vapor-liquid-solid grown silica nanowire based electrochemical glucose biosensor

Vapor-liquid-solid (VLS) grown silica nanowires (SiO(2)NWs) have been deposited electrophoretically on a gold electrode and utilized for covalent immobilization of glucose oxidase (GOx). Covalent binding has been achieved via 3-aminopropyltriethoxysilane (APTES) modification and N-ethyl-N'-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide chemistry. Scanning electron microscopy, transmission electron microscopy and cyclic voltammetry techniques have been used to characterize SiO(2)NW and GOx/APTES/SiO(2)NW/Au bioelectrode. Electrochemical studies reveal that SiO(2)NW increases the effective electro-active surface area thus resulting in higher loading of enzyme. Response characteristics show linearity in the range of interest 25-300 mg dl(-1), with a detection limit of 11 mg dl(-1), sensitivity: 0.463 µA (mg dl(-1))(-1) and regression coefficient of 0.992.

Direct electrochemistry of glucose oxidase and biosensing for glucose based on graphene

We first reported that polyvinylpyrrolidone-protected graphene was dispersed well in water and had good electrochemical reduction toward O(2) and H(2)O(2). With glucose oxidase (GOD) as an enzyme model, we constructed a novel polyvinylpyrrolidone-protected graphene/polyethylenimine-functionalized ionic liquid/GOD electrochemical biosensor, which achieved the direct electron transfer of GOD, maintained its bioactivity and showed potential application for the fabrication of novel glucose biosensors with linear glucose response up to 14 mM.

A label-free and portable multichannel surface plasmon resonance immunosensor for on site analysis of antibiotics in milk samples

Techniques for immunosensing like surface plasmon resonance (SPR) may respond to the need for rapid screening methods to improve food safety. This paper describes the development of a novel portable six channel SPR biosensor based on the plasmon of gold diffraction grating surface for simultaneous multianalyte antibiotic detection in milk samples. Representative congeners from three important antibiotic families (FQs: fluoroquinolones, SAs: sulfonamides and CAP: phenicols) were chosen for this study. The chips are covalently biofunctionalized with haptenized proteins by means of a previously formed mixed self assembled monolayer (m-SAM) prepared using two types of mercapto alkyl reagents containing polyethyleneglycol (PEG) units. The samples or standards are mixed with specific polyclonal antibodies and injected into the sensor device. The detectability accomplished is very good (i.e. in buffer, enrofloxacin, 0.30 μg L(-1); sulfapyridine, 0.29 μg L(-1); and chloramphenicol, 0.26 μg L(-1)) and whole milk samples can be analyzed directly without clean-up steps, by just diluting the sample five times with water to remove non-specific interferences caused by the matrix. Although the detectability of CAP regarding the MRPL (minimum required performance limit) is slightly compromised by the dilution, the detectability accomplished by FQs and SAs was far below the maximum residue levels (MRLs) established by the European Union.

DNA diagnostics: nanotechnology-enhanced electrochemical detection of nucleic acids

The detection of mismatched base pairs in DNA plays a crucial role in the diagnosis of genetic-related diseases and conditions, especially for early stage treatment. Among the various biosensors that have been used for DNA detection, EC sensors show great promise because they are capable of precise DNA recognition and efficient signal transduction. Advancements in micro- and nanotechnologies, specifically fabrication techniques and new nanomaterials, have enabled for the development of highly sensitive, highly specific sensors making them attractive for the detection of small sequence variations. Furthermore, the integration of sensors with sample preparation and fluidic processes enables for rapid, multiplexed DNA detection essential for POC clinical diagnostics.

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.

Electronic anabolic steroid recognition with carbon nanotube field-effect transistors

A proof of concept of the electronic detection of two anabolic steroids, stanozolol (Stz) and methylboldenone (MB), was carried out using two specific antibodies and arrays of carbon nanotube field-effect transistors (CNTFETs). Antibodies specific for Stz and MB were prepared and immobilized on the carbon nanotubes (CNTs) using two different approaches: direct noncovalent bonding of antibodies to the devices and bonding the antibodies covalently to a polymer previously attached to the CNTFETs. The results indicated that CNTFETs bonded to specific antibodies covalently or noncovalently are able to detect the presence of steroids. Statistically significant changes in the threshold voltage and drain current were registered in the transistors, allowing the steroids to be recognized. On the other hand, it was determined that the specific antibodies do not detect other steroids other than Stz and MB, such as nandrolone (ND) because, in this case, statistically significant changes in the transistors were not detected. The polymer prevents the aggregation of antibodies on the electrodes and decreases the transistor hysteresis. Nevertheless, it is not able to avoid the nonspecific adsorption of streptavidin, meaning that nonspecific adsorption on CNTs remains a problem and that this methodology is only useful for purified samples. Regarding the detection mechanism, in addition to charge transfer, Schottky barrier, SB, modification, and scattering potential reported by other authors, an electron/hole trapping mechanism leading to hysteresis modification has been determined. The presence of polymer seems to hinder the modulation of the electrode-CNT contact.

Nanomaterials as analytical tools for genosensors

Nanomaterials are being increasingly used for the development of electrochemical DNA biosensors, due to the unique electrocatalytic properties found in nanoscale materials. They offer excellent prospects for interfacing biological recognition events with electronic signal transduction and for designing a new generation of bioelectronic devices exhibiting novel functions. In particular, nanomaterials such as noble metal nanoparticles (Au, Pt), carbon nanotubes (CNTs), magnetic nanoparticles, quantum dots and metal oxide nanoparticles have been actively investigated for their applications in DNA biosensors, which have become a new interdisciplinary frontier between biological detection and material science. In this article, we address some of the main advances in this field over the past few years, discussing the issues and challenges with the aim of stimulating a broader interest in developing nanomaterial-based biosensors and improving their applications in disease diagnosis and food safety examination.

Perspective on optical biosensors and integrated sensor systems

Optical biosensors have begun to move from the laboratory to the point of use. This trend will be accelerated by new concepts for molecular recognition, integration of microfluidics and optics, simplified fabrication technologies, improved approaches to biosensor system integration, and dramatically increased awareness of the applicability of sensor technology to improve public health and environmental monitoring. Examples of innovations are identified that will lead to smaller, faster, cheaper optical biosensor systems with capacity to provide effective and actionable information.

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.