Stability of mixed PEO-thiol SAMs for biosensing applications

The secret of a successful affinity biosensor partially hides in the chemical interface layer between the transducer system and the biological receptor molecules. Over the past decade, several methodologies for the construction of such interface layers have been developed on the basis of the deposition of self-assembled monolayers (SAMs) of alkanethiols on gold. Moreover, mixed SAMs of polyethylene oxide (PEO) containing thiols have been applied for the immobilization of biological receptors. Despite the intense research in the field of thiol SAMs, relatively little is known about their biosensing properties in correlation with their long-term stability. Especially the impact of the storage conditions on their biosensing characteristics has not been reported before to our knowledge. To address these issues, we prepared mixed PEO SAMs and tested their stability and biosensing performance in several storage conditions, i.e., air, N2, ethanol, phosphate buffer, and H2O. The quality of the SAMs was monitored as a function of time using various characterization techniques such as cyclic voltammetry, contact angle, grazing angle Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. In addition, the impact of the different storage conditions on the biosensor properties was investigated using surface plasmon resonance. Via the latter technique, the receptor immobilization, the analyte recognition, and the nonspecific binding were extensively studied using the prostate specific antigen as a model system. Our experiments showed that very small structural differences in the SAM can have a great impact in their final biosensing properties. In addition it was shown that the mixed SAMs stored in air or N2 are very stable and retain their biosensor properties for at least 30 days, while ethanol appeared to be the worst storage medium due to partial oxidation of the thiol headgroup. In conclusion, care must be taken to avoid SAM degradation during storage to retain typical SAM characteristics, which is very important for their general use in many proposed applications.     

Properties of mixed alkanethiol-dendrimer layers and their applications in biosensing

We studied the properties of mixed alkanethiol-dendrimer layers on a gold support and their application in biosensing. We showed that properties of glucose sensor can be modified using a different ratio of 1-hexadecanethiol (HDT) and poly(amidoamine) dendrimer of first generation (G1). The cyclic voltammetry in the presence of the redox couple, Fe(CN)(6)(3-)/Fe(CN)(6)(4-), was used for estimating how effectively the layer blocks the redox probe's access to the electrode surface. A scanning electrochemical microscope (SECM) was used to image the resulting distribution of the organic compounds. We found that with increasing content of dendrimers, the integrity of the layers was improved.     

Heat-stabilized glycosphingolipid films for biosensing applications

We have investigated a means of producing thin, oriented lipid monolayers which are stable under repeated washing and which may be useful in biosensing or surface-coating applications. Phosphatidylcholine and the glycosphingolipid GM1 were used as representative lipids for this work. Initially, a mixed self-assembled monolayer of octanethiol and hexadecanethiol was produced on a gold surface. This hydrophobic monolayer was then brought into contact with a thin lipid film that had been assembled at the liquid/air interface of a solution, allowing the lipid to deposit on the gold surface through hydrophobic interactions. The lipid layer was then heated to cause intermingling of the fatty acid and alkanethiol chains and cooled to form a highly stable film which withstood repeated rinsing and solution exposure. Presence and stability of the film were confirmed via ellipsometry, Fourier transform infrared spectroscopy, and quartz crystal microbalance (QCM), with an average overall film thickness of approximately 3.5 nm. This method was then utilized to produce GM1 layers on gold-coated QCM crystals for affinity sensing trials with cholera toxin. For these sensing elements, the lower detection limit of cholera toxin was found to be approximately 0.5 microg/mL, with a logarithmic relationship between toxin concentration and frequency response spanning over several orders of magnitude. Potential sites for nonspecific adsorption were blocked using serum albumin without sacrificing toxin specificity.     

Carbon nanotube transistors for biosensing applications

Electronic detection of biomolecules, although still in its early stages, is gradually emerging as an effective alternative to optical detection methods. We describe field effect transistor devices with carbon nanotube conducting channels that have been developed and used for biosensing and biodetection. Both transistors with single carbon nanotube conducting channels and devices with nanotube network conducting channels have been fabricated and their electronic characteristics examined. The devices readily respond to changes in the environment, and such effects have been examined using gas molecules and coatings with specific properties. Device operation in (conducting) buffer and in a dry environment--after buffer removal--is also discussed. Applications in the biosensing area are illustrated with three examples: the investigation of the interaction between devices and biomolecules, the electronic monitoring of biomolecular processes, and attempts to integrate cell membranes with active electronic devices.     

Ferrocene-functionalized graphene electrode for biosensing applications

A novel ferrocene-functionalized reduced graphene oxide (rGO)-based electrode is proposed. It was fabricated by the drop casting of ferrocene-functionalized graphene onto polyester substrate as the working electrode integrated within screen-printed reference and counter electrodes. The ferrocene-functionalized rGO has been fully characterized using FTIR, XPS, contact angle measurements, SEM and TEM microscopy, and cyclic voltammetry. The XPS and EDX analysis showed the presence of Fe element related to the introduced ferrocene groups, which is confirmed by a clear CV signal at ca. 0.25 V vs. Ag/AgCl (0.1 KCl). Mediated redox catalysis of H₂O₂ and bio-functionalization with glucose oxidase for glucose detection were achieved by the bioelectrode providing a proof for potential biosensing applications.     

Functionalized carbon nanotubes and nanofibers for biosensing applications

This review summarizes recent advances in electrochemical biosensors based on carbon nanotubes (CNTs) and carbon nanofibers (CNFs) with an emphasis on applications of CNTs. CNTs and CNFs have unique electric, electrocatalytic and mechanical properties, which make them efficient materials for developing electrochemical biosensors.We discuss functionalizing CNTs for biosensors. We review electrochemical biosensors based on CNTs and their various applications (e.g., measurement of small biological molecules and environmental pollutants, detection of DNA, and immunosensing of disease biomarkers). Moreover, we outline the development of electrochemical biosensors based on CNFs and their applications. Finally, we discuss some future applications of CNTs.     

Electrospun palladium (IV)-doped copper oxide composite nanofibers for non-enzymatic glucose sensors

Pd (IV)-doped CuO oxide composite nanofibers (PCNFs) have been successfully fabricated via electrospinning and then employed to construct an amperometric non-enzymatic glucose sensor. The PCNFs based glucose sensors display distinctly enhanced electrocatalytic activity towards the oxidation of glucose, showing significantly lower overvoltage (0.32 V) and ultrafast (1 s) and ultrasensitive current (1061.4 μA mM⁻¹ cm⁻²) response with a lower detection limit of 1.9 × 10⁻⁸ M (S/N = 3). Additionally, excellent selectivity, reproducibility and stability have also been obtained. These results indicate that PCNFs are promising candidates for amperometric non-enzymatic glucose detection.     

One-step electropolymeric co-immobilization of glucose oxidase and heparin for amperometric biosensing of glucose

Electropolymerization was used for the co-deposition of glucose oxidase and heparin onto metal electrode transducers. Such electropolymeric co-entrapment within a non-conducting poly(1,2-phenylenediamine) (PPD) film imparts both biocatalytic and anticoagulation activities onto the transducer, and greatly improves the performance of the sensor after exposure to whole blood. Essentially identical glucose signals are observed before and after exposure to blood samples. Scanning electron micrographs after such exposure reveal no platelet deposition or formation of a fibrin "clot". The effect of the heparin co-immobilization on the glucose response is examined. Improved biocompatibility is reported also in connection with a needle-type carbon paste biosensor configuration. The simultaneous localization of the enzyme and heparin offers great promise for simplifying the preparation of enzyme electrodes and designing biocompatible implantable glucose biosensors.     

Substrate-dependent kinetics in tyrosinase-based biosensing: amperometry vs. spectrophotometry

Despite the broad use of enzymes in electroanalytical biosensors, the influence of enzyme kinetics on the function of prototype sensors is often overlooked or neglected. In the present study, we employ amperometry as an alternative or complementary method to study the kinetics of tyrosinase, whose catalytic activity results in o-quinone products. We further compare our results for four monophenolic substrates with those obtained from ultraviolet-visible spectrophotometry and show that the results from both assays are in good agreement. We also observe large variations in the enzyme kinetics for different monophenolic substrates depending on the R-group at the para position. To further study this effect, we investigate the stability of quinone products in the enzymatic assay. This information can in principle be utilized to discriminate between different phenolic species by monitoring the reaction rate.     

Peroxidase-mimicking DNAzymes for biosensing applications: a review

DNAzymes are single stranded DNA molecules that exhibit catalytic activity and are exploited in medicine, biology and material sciences. Development in this area is related to the many advantages of DNAzymes over conventional protein enzymes, such as thermal stability and simpler preparation. DNAzymes with peroxidase-like activity have recently attracted great interest. To assure such catalytic activity, oligonucleotides have to adopt a G-quadruplex structure, which can bind the hemin molecule. This system facilitates a redox reaction between the target molecule and hydrogen peroxide, which results in the appearance of an oxidized target molecule (product). DNAzymes with peroxidase-mimicking activity have great potential in bioanalytical chemistry. This review presents fundamentals concerning the design and engineering of DNAzymes with peroxidase-like activity, describes their properties and spectral characteristics and shows how DNAzymes can contribute to bioanalytical research. Examples of bioanalytical applications of DNAzymes with peroxidase-like activity include nucleic acid probes with DNAzyme labels for the detection of specific DNA sequences in colorimetric or chemiluminescent assays. Assays for telomerase or methyltransferase activity, which are potential targets in anticancer therapy, are also described in this review. Other applications include the determination of metal cations such as Ag(+), K(+), Hg(2+), Pb(2+) or Cu(2+) and amplified detection of small molecules such as adenosine, cocaine or AMP and proteins such as lysozyme or thrombin. In the last decade, DNAzymes have become part of numerous applications in many areas of science from chemistry to biology to medicine.     

External stimulation-controlled dynamic DNA devices for biosensing and biomedical applications

DNA devices that can recognize molecular inputs and transform them into functional outputs in an autonomous manner have been actively pursued as versatile toolkits for controlled nanofabrication, molecular network regulation, biosensing and cellular function modulation. The introduction of external stimuli-responsive units not only ensures the programmability and functionality of DNA devices themselves, but also confers rapid, remote and reversible dynamic regulation capabilities. This facilitat...     

Single on-chip gold nanowires for electrochemical biosensing of glucose

The development of glucose diagnostic devices with low detection limits is of key importance in diabetes-related research. New highly sensitive sensors are required for non-invasive detection of glucose in bodily fluids, other than blood, and an electrochemical sensor based on a single gold nanowire for rapid, reliable and quantitative detection of low glucose concentrations (10 μM-1 mM), is presented in this paper. Single gold nanowire devices are fabricated at silicon chip substrates using a hybrid electron beam-photolithography approach. Critical dimensions of the nanowires are characterised using a combination of scanning electron and atomic force microscopies. Fabricated nanowire devices are characterised by direct electrical probing and cyclic voltammetry to explore functionality. The voltammetric detection of glucose was performed using ferrocene monocarboxylic acid as an oxidising mediator in the presence of glucose oxidase. The biosensor can be applied to the quantification of glucose in the range of 10 μM-100 mM, with an extremely high sensitivity of 7.2 mA mM(-1) cm(-2) and a low detection limit of 3 μM (S/N = 3). The sensor demonstrated high selectivity towards glucose with negligible interference from other oxidizable species including uric acid, ascorbic acid, mannose, fructose, salicylic acid (Aspirin) and acetaminophen (Paracetamol).     

Metal-alloy-dispersed carbon-paste enzyme electrodes for amperometric biosensing of glucose

The preferential electrocatalytic detection of hydrogen peroxide and the selective biosensing of glucose at carbon-paste enzyme electrodes dispersed with bimetallic (Ru–Pt) alloy particles are described. Unlike the marked acceleration of the redox reaction of the peroxide product (versus single metal catalysts), the signals of common interfering compounds shift to higher potentials. Such use of carbon-supported alloy particles thus results in a greatly enhanced sensitivity compared to the dispersion of pure metals, without compromising the remarkable selectivity inherent to metallized carbon biosensors.     

Renewable nanocomposite layer-by-layer assembled catalytic interfaces for biosensing applications

A novel, easily renewable nanocomposite interface based on layer-by-layer (LbL) assembled cationic/anionic layers of carbon nanotubes customized with biopolymers is reported. A simple approach is proposed to fabricate a nanoscale structure composed of alternating layers of oxidized multiwalled carbon nanotubes upon which is immobilized either the cationic enzyme organophosphorus hydrolase (OPH; MWNT-OPH) or the anionic DNA (MWNT-DNA). The presence of carbon nanotubes with large surface area, high aspect ratio and excellent conductivity provides reliable immobilization of enzyme at the interface and promotes better electron transfer rates. The oxidized MWNTs were characterized by thermogravimetric analysis and Raman spectroscopy. Fourier transform infrared spectroscopy showed the surface functionalization of the MWNTs and successful immobilization of OPH on the MWNTs. Scanning electron microscopy images revealed that MWNTs were shortened during sonication and that LbL of the MWNT/biopolymer conjugates resulted in a continuous surface with a layered structure. The catalytic activity of the biopolymer layers was characterized using absorption spectroscopy and electrochemical analysis. Experimental results show that this approach yields an easily fabricated catalytic multilayer with well-defined structures and properties for biosensing applications whose interface can be reactivated via a simple procedure. In addition, this approach results in a biosensor with excellent sensitivity, a reliable calibration profile, and stable electrochemical response.     

Size-selective template-assisted electrophoretic assembly of nanoparticles for biosensing applications

The precise, size-selective assembly of nanoparticles gives rise to many applications where the assembly of nano building blocks with different biological or chemical functionalizations is necessary. We introduce a simple, fast, reproducible-directed assembly technique that enables a complete sorting of nanoparticles with single-particle resolution. Nanoparticles are size-selectively assembled into prefabricated via arrays using a sequential template-directed electrophoretic assembly method. Polystyrene latex (PSL) nanoparticles with diameters ranging from 200 to 50 nm are selectively assembled into vias comparable to nanoparticle diameter. We investigate the effects of particle size and via size on the sorting efficiency. We show that complete sorting can be achieved when the size of the vias is close to the diameter of the nanoparticles and the size distribution of the chosen nanoparticles does not overlap. The results also show that it is necessary to keep the electric field on during the insertion and removal of the template. To elucidate the versatility and nil effects that the electrophoresis assembly technique has on the assembled nanoparticle characteristics, we have assembled cancer-specific monoclonal antibody-2C5-coated nanoparticles and have also shown that they can successfully measure low concentrations of the nucleosome (NS) antigen.     

Emerging biosensing and transducing techniques for potential applications in point-of-care diagnostics

With the deepening of our understanding in life science, molecular biology, nanotechnology, optics, electrochemistry and other areas, an increasing number of biosensor design strategies have emerged in recent years, capable of providing potential practical applications for point-of-care (POC) diagnosis in various human diseases. Compared to conventional biosensors, the latest POC biosensor research aims at improving sensor precision, cost-effectiveness and time-consumption, as well as the development of versatile detection strategies to achieve multiplexed analyte detection in a single device and enable rapid diagnosis and high-throughput screening. In this review, various intriguing strategies in the recognition and transduction of POC (from 2018 to 2021) are described in light of recent advances in CRISPR technology, electrochemical biosensing, and optical- or spectra-based biosensing. From the perspective of promoting emerging bioanalytical tools into practical POC detecting and diagnostic applications, we have summarized key advances made in this field in recent years and presented our own perspectives on future POC development and challenges.

POC diagnostics are driven by the rapid advances in CRISPR, electrochemical and optical biosensors. Related emerging strategies are described and discussed from the perspective of facilitating the practical application of biosensors in POC testing.     

Fabrication and chemical surface modification of nanoporous silicon for biosensing applications

In this work, porous silicon (PS) films with varied porosity (68–82%) were formed on the p-type, boron-doped silicon wafer (100) by the electrochemical anodisation in an aqueous hydrofluoric acid and isopropyl alcohol solution at different current densities (I _d) ranging from 20–70 mA cm^?2, respectively. Biofunctionalisation of the PS surface was carried out by chemically modifying the surface of PS by the deposition of 3-aminopropyltriethoxysilane thermally leading to high density of amine groups covering the PS surface. This further promotes the immobilisation of immunoglobulin (human IgG and goat anti-human IgG binding) on to the PS surface. Formation of nanostructured PS and the attachment of antibody–antigen to its surface were characterised using photoluminescence (PL), Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy techniques, respectively. The possibility of using these structures as biosensors has been explored based on the significant changes in the PL spectra before and after exposing the PS optical structures to biomolecules. These experimental results open the possibility of developing optical biosensors based on the variation of the PL position of the PL spectra of PS-based devices.     

Electrodeposition of a biocompatible hydroxyapatite matrix to immobilize glucose oxidase for sensitive glucose biosensing

Microchimica Acta - The electrochemical quartz crystal microbalance was used to quantitatively examine the electrodeposition of hydroxyapatite (HA) at a gold electrode. Potentiostatic...     

Bimetallic nickel and palladium complexes for catalytic applications

South Africa is a mineral-rich country and one area in which minerals can be very important is catalysis. Over an extended period of time, homogeneous catalysis has grown to become very useful, particularly in the chemical and pharmaceutical industries. The organometallic compounds required in the catalysis industry have advanced from metallocenes to an alternative in the form of a-diimines. Most a-diimines are prepared from iminopyridyl moieties and are most active with nickel and palladium transition metals. This review providesa history of homogeneous catalysis and a discussion on iminopyridines, with the main focus on the nickel and palladium complexes formed from them. There follows a discussion of the bimetallic nickel and palladium complexes in various catalytic applications such as Suzuki and Heck coupling, with the main focus on ethylene polymerisation. The limitations are addressed and possible solutions presented to overcome those challenges. Several reviews in the related topics are to be found in the literature but not the a-diimine with iminopyridines and bimetallic nickel and palladium metals.