Chemosensors, Volume 4, Issue 3 (September 2016) – 9 articles

We use a set of three resistive sensors based on undoped multi-walled carbon nanotubes, B-doped multi-walled carbon nanotubes, and N-doped multi-walled carbon nanotubes to study fungal infection in strawberries inoculated with Rhizopus sp. or with Aspergillus sp. section Nigri. We apply tristimulus analysis using the conductance variation of the sensors when exposed to the infected strawberries to distinguish between uninfected strawberries and strawberries infected with Rhizopus sp. or with Aspergillus sp. section Nigri, and to obtain a graphical representation providing a tool for the simple and fast detection and identification of the fungal infection. Full article

The availability of devices for cancer biomarker detection at early stages of the disease is one of the most critical issues in biomedicine. Towards this goal, to increase the assay sensitivity, device miniaturization strategies empowered by the employment of high affinity protein binders constitute a valuable approach. In this work we propose two different surface-based miniaturized platforms for biomarker detection in body fluids: the first platform is an atomic force microscopy (AFM)-based nanoarray, where AFM is used to generate functional nanoscale areas and to detect biorecognition through careful topographic measurements; the second platform consists of a miniaturized electrochemical cell to detect biomarkers through electrochemical impedance spectroscopy (EIS) analysis. Both devices rely on robust and highly-specific protein binders as aptamers, and were tested for thrombin detection. An active layer of DNA-aptamer conjugates was immobilized via DNA directed immobilization on complementary single-stranded DNA self-assembled monolayers confined on a nano/micro area of a gold surface. Results obtained with these devices were compared with the output of surface plasmon resonance (SPR) assays used as reference. We succeeded in capturing antigens in concentrations as low as a few nM. We put forward ideas to push the sensitivity further to the pM range, assuring low biosample volume (μL range) assay conditions. Full article

This work describes the investigation of a label-free immunosensor for the detection of aflatoxin B₁ (AFB₁). CD-trodes (electrodes obtained from recordable compact disks) were used as low-cost and disposable transducers after modification with a self-assembled monolayer (SAM) of lipoic acid. The anti-aflatoxin B₁ antibody was immobilized via EDC/NHS activation, followed by blocking with bovine serum albumin and immunoassays with AFB₁. The optimization of analytical parameters and the detection were carried out using electrochemical impedance measurements. Using chemometric tools, the best conditions for the immunosensor development were defined as: anti-AFB₁ antibody at 1:2000 dilution and surface blocking with 0.5% bovine serum albumin, both incubated for 1 h, and antibody–antigen immunoreaction for 30 min. The impedimetric immunosensor showed a linear range from 5 × 10⁻⁹ to 1 × 10⁻⁷ mol·L⁻¹ (1.56–31.2 ng·mL⁻¹), limit of detection and limit of quantification, respectively, 3.6 × 10⁻¹⁰ and 1.1 × 10⁻⁹mol·L⁻¹ (0.11 and 0.34 ng·mL⁻¹). The proposed immunosensor was applied to analyze peanut samples. Full article

In this article, it is shown that the efficiency of an electrochemical aptasensing device is influenced by the use of different nanoparticles (NPs) such as gold nanoparticles (Au), silver nanoparticles (Ag), hollow gold nanospheres (HGN), hollow silver nanospheres (HSN), silver–gold core shell (Ag@Au), gold–silver core shell (Au@Ag), and silver–gold alloy nanoparticles (Ag/Au). Among these nanomaterials, Ag@Au core shell NPs are advantageous for aptasensing applications because the core improves the physical properties and the shell provides chemical stability and biocompatibility for the immobilization of aptamers. Self-assembly of the NPs on a cysteamine film at the surface of a carbon paste electrode is followed by the immobilization of thiolated aptamers at these nanoframes. The nanostructured (Ag@Au) aptadevice for Escherichia coli as a target shows four times better performance in comparison to the response obtained at an aptamer modified planar gold electrode. A comparison with other (core shell) NPs is performed by cyclic voltammetry and differential pulse voltammetry. Also, the selectivity of the aptasensor is investigated using other kinds of bacteria. The synthesized NPs and the morphology of the modified electrode are characterized by UV-Vis absorption spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis, and electrochemical impedance spectroscopy. Full article

This paper presents the fabrication and characterization of a flexible gas sensor based on TiO₂ nanotubes membrane, onto which array interdigitated gold electrodes in one side and a common heater in the backside were obtained using conventional microfabrication techniques. This was used to detect hydrogen sulphide within a concentration range of 6–38 ppm. The response to low concentrations of H₂S at low temperature and good stability make the sensor a promising candidate for practical applications. These results support the proposal that the TiO₂ nanotubes membrane flexible sensors are promising in portable on-site detection based on low cost nanomaterials. Full article

Cell based cancer analysis is an important analytic method to monitor cancer progress on stages by detecting the density of circulating tumour cells (CTCs) in the blood. Among the existing microfluidic techniques, dielectrophoresis (DEP), which is a label-free detection method, is favoured by researchers. However, because of the high conductivity of blood as well as the rare presence of CTCs, high separation efficiency is difficult to achieve in most DEP microdevices. Through this study, we have proposed a strategy to improve the isolation performance, as such by integrating a magnetophoretic (MAP) platform into a DEP device. Several important aspects to be taken into MAP design consideration, such as permanent magnet orientation, magnetic track configuration, fluid flow parameter and separation efficiency, are discussed. The design was examined and validated by numerical simulation using COMSOL Multiphysics v4.4 software (COMSOL Inc., Burlington, MA, USA), mainly presented in three forms: surface plot, line plot, and arrow plot. From these results, we showed that the use of a single permanent magnet coupled with an inbuilt magnetic track of 250 μm significantly strengthens the magnetic field distribution within the proposed MAP stage. Besides, in order to improve dynamic pressure without compromising the uniformity of fluid flow, a wide channel inlet and a tree-like network were employed. When the cell trajectory within a finalized MAP stage is computed with a particle tracing module, a high separation efficiency of red blood cell (RBC) is obtained for blood samples corresponding up to a dilution ratio of 1:7. Moreover, a substantial enhancement of the CTCs’ recovery rate was also observed in the simulation when the purposed platform was integrated with a planar DEP microdevice. Full article

Guanine-rich nucleic acids are able to self-assemble into G-quadruplex four-stranded secondary structures, which are found at the level of telomeric regions of chromosomes, oncogene promoter sequences and other biologically-relevant regions of the genome. Due to their extraordinary stiffness and biological role, G-quadruples become relevant in areas ranging from structural biology to medicinal chemistry, supra-molecular chemistry, nanotechnology and biosensor technology. In addition to classical methodologies, such as circular dichroism, nuclear magnetic resonance or crystallography, electrochemical methods have been successfully used for the rapid detection of the conformational changes from single-strand to G-quadruplex. This review presents recent advances on the G-quadruplex electrochemical characterization and on the design and applications of G-quadruplex electrochemical biosensors, with special emphasis on the G-quadruplex aptasensors and hemin/G-quadruplex peroxidase-mimicking DNAzyme biosensors. Full article

Aptasensors based on stripping voltammetry exhibit several advantages, such as high sensitivity and multi-target detection from stripping voltammetric technology, and high selectivity from the specific binding of apamers with targets. This review comprehensively discusses the recent accomplishments in signal amplification strategies based on nanomaterials, such as metal nanoparticles, semiconductor nanoparticles, and nanocomposite materials, which are detected by stripping voltammetry after suitable dissolution. Focus will be put in discussing multiple amplification strategies that are widely applied in aptasensors for small biomolecules, proteins, disease markers, and cancer cells. Full article

In this work, a novel magnetic beads (MBs)-based immunosensing approach for the rapid and simultaneous determination of the main peanut allergenic proteins (Ara h 1 and Ara h 2) is reported. It involves the use of sandwich-type immunoassays using selective capture and detector antibodies and carboxylic acid-modified magnetic beads (HOOC-MBs). Amperometric detection at −0.20 V was performed using dual screen-printed carbon electrodes (SPdCEs) and the H₂O₂/hydroquinone (HQ) system. This methodology exhibits high sensitivity and selectivity for the target proteins providing detection limits of 18.0 and 0.07 ng/mL for Ara h 1 and Ara h 2, respectively, with an assay time of only 2 h. The usefulness of the approach was evaluated by detecting the endogenous content of both allergenic proteins in different food extracts as well as trace amounts of peanut allergen (0.0001% or 1.0 mg/kg) in wheat flour spiked samples. The developed platform provides better Low detection limits (LODs) in shorter assay times than those claimed for the allergen specific commercial ELISA kits using the same immunoreagents and quantitative information on individual food allergen levels. Moreover, the flexibility of the methodology makes it readily translate to the detection of other food-allergens. Full article