Signal amplification for thrombin impedimetric aptasensor: sandwich protocol and use of gold-streptavidin nanoparticles

The influence of charge on the translation of the sandwich ELISA approach to electronic biosensors

The sandwich approach, whereby an antigen is captured by a primary antibody and detected by a secondary antibody, is commonly used to improve the selectivity and sensitivity of enzyme-linked immunosorbent assays (ELISA). This work details the experimental factors that impact the reliable translation of this sandwich approach to two commonly used electronic biosensors, namely potentiometric and impedimetric biosensors. Previous studies have demonstrated the Debye screening limitations associated with potentiometric biosensors. However, the correlation between the ionic strength of the measurement buffer and the impedimetric biosensing response has not been studied. Potentiometric biosensors were able to successfully detect the primary antibody and the target antigen by decreasing the ionic strength of the phosphate buffered saline (PBS) measurement buffer from 1x PBS to 0.01x PBS. However, the secondary antibody used for the selective signal amplification was not reliably detected. Therefore, the sandwich approach is not viable for potentiometric sensing at biologically relevant ionic strengths, due to the Debye screening effect. Alternatively, decreasing the ionic strength of the measurement buffer allowed for the successful translation of the sandwich approach to impedimetric biosensors. Impedimetric biosensing literature typically attributes a measured increase in the charge transfer resistance to an increase in the thickness of the immobilized biolayer. However, this work highlights the influence that both the charge and thickness of the biolayer have on the transport of the redox couple. Decreasing the ionic strength of the measurement buffer lowers the molecular charge screening effect. This permits the transport of a positively charged redox probe through a negatively charged immobilized biolayer via migration and diffusion. The results demonstrate that the use of a buffer at a lower, yet biologically relevant ionic strength allows for the successful translation of the sandwich approach to impedimetric biosensors.

Dummy Molecularly Imprinted Polymers Using DNP as a Template Molecule for Explosive Sensing and Nitroaromatic Compound Discrimination

This work reports a rapid, simple and low-cost voltammetric sensor based on a dummy molecularly imprinted polymer (MIP) that uses 2,4-dinitrophenol (DNP) as a template for the quantification of 2,4,6-trinitrotoluene (TNT) and DNP, and the identification of related substances. Once the polymer was synthesised by thermal precipitation polymerisation, it was integrated onto a graphite epoxy composite (GEC) electrode via sol–gel immobilisation. Scanning electron microscopy (SEM) was performed in order to characterise the polymer and the sensor surface. Responses towards DNP and TNT were evaluated, displaying a linear response range of 1.5 to 8.0 µmol L−1 for DNP and 1.3 to 6.5 µmol L−1 for TNT; the estimated limits of detection were 0.59 µmol L−1 and 0.29 µmol L−1, for DNP and TNT, respectively. Chemometric tools, in particular principal component analysis (PCA), demonstrated the possibilities of the MIP-modified electrodes in nitroaromatic and potential interfering species discrimination with multiple potential applications in the environmental field.

Impedimetric Aptamer-Based Biosensors: Applications

Impedimetric aptamer-based biosensors show high potential for handheld devices and point-of-care tests. In this review, we report on recent advances in aptamer-based impedimetric biosensors for applications in biotechnology. We detail on analytes relevant in medical and environmental biotechnology as well as food control, for which aptamer-based impedimetric biosensors were developed. The reviewed biosensors are examined for their performance, including sensitivity, selectivity, response time, and real sample validation. Additionally, the benefits and challenges of impedimetric aptasensors are summarized.

An electrochemical sandwich-type aptasensor for determination of lipocalin-2 based on graphene oxide/polymer composite and gold nanoparticles

In this work, we reported an electrochemical aptasensor based on the poly-3-amino-1,2,4-triazole-5-thiol/graphene oxide composite (P(ATT)-GO) and gold nanoparticles (AuNPs) modified graphite screen-printed electrode (GSPE) (GSPE/P(ATT)-GO/AuNPs) for determination of lipocalin-2 (LCN2) (neutrophil gelatinase-associated lipocalin). A sandwich based strategy was utilized to enhance the electrochemical signal. First, a thiol tethered DNA aptamer was immobilized onto the composite electrode. Then, the LCN2 solution was incubated with the aptamer modified GSPE/P(ATT)-GO/AuNPs. Secondary aptamer (Apt2) peculiar to the LCN2 and labeled with biotin was interacted with the LCN2. A streptavidin-alkaline phosphatase conjugate was then applied to the surface. The determination of LCN2 was performed by using the electroactive property of α-naphthol which is acquired the product from the interaction between alkaline phosphatase and α-naphthyl phosphate. The constructed electrode was characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The aptamer modified GSPE/P(ATT)-GO/AuNPs showed the superior electrocatalytic performance towards the voltammetric determination of LCN2 with a wide linear range (1.0-1000.0 ng/mL) and a low limit of detection (LOD) (0.3 ng/mL). The proposed aptasensor revealed the excellent sensitivity, anti-interference ability and reproducibility which approved that the GSPE/P (ATT)-GO/AuNPs is a promising composite for the sensitive detection of LCN2. The fabricated aptasensor was applied for the determination of LCN2 in fetal bovine serum samples using the standard addition method and the recovery values were in the range of 99.2% and 103.22%.

Novel sensing platform based on gold nanoparticle-aptamer and Fe-metal-organic framework for multiple antibiotic detection and signal amplification

The development of a feasible antibiotic detection method is important in water quality analysis. In this study, we developed a metal-organic framework (MOF)-aptamer-3,3',5,5'-tetramethylbenzidine (TMB)-H₂O₂-based sensing platform composed of the reaction variable of TMB catalytic oxidation as the label (from colorless to blue) and aptamer as the target recognition element for antibiotic detection. The platform works by calculating the relation between the antibiotic concentration and the resultant decrease in MOF's catalytic activity. Basing from the comparison of typical iron-based MOF materials (Fe-MIL-53, Fe-MIL-88A, and Fe-MIL-100), we selected Fe-MIL-53 to obtain an improved signal amplification effect. The outstanding performance of the Fe-MIL-53-based sensing platform can be attributed to its topological flexibility and small electron transfer impedance. In addition, a signal increment of up to 86% was obtained with an intensified gold nanoparticle (AuNP)-supported aptamer. The inhibitory catalytic activity stemmed from the coating of antibiotic-(AuNP-aptamer) conjugates onto the outer surface of the MOF material, which increased the impedance and decreased the electron transfer efficiency. Validation results indicated that the platform showed high selectivity and sensitivity (i.e., wide linearity range of 50-200 nM, detection limit up to 8.1 ng/mL, and recovery rate of 106%-110%) for chloramphenicol detection and universal applicability for other antibiotics, including ampicillin, tetracycline, and oxytetracycline. In general, the detection reliability and easy operation of this platform render it a promising candidate for antibiotic detection in future water quality monitoring practices.

Selection of DNA aptamers against penicillin G using Capture-SELEX for the development of an impedimetric sensor

This paper describes for the first time the selection of aptamers selective to penicillin. Aptamers were selected using a specific process called Capture-SELEX (Systematic Evolution of Ligands by Exponential Enrichment). This technique is based on the selection of DNA aptamers using penicillin G in solution while the ssDNA library is fixed on a support. One aptamer showing a good affinity to penicillin was finally selected and tested in electrochemical sensor configuration, using electrochemical impedance spectroscopy as detection technique. The developed aptasensor allowed the detection of penicillin in a wide concentration range, comprised between 0.4 and 1000µgL^-1 Such performance was compatible with milk analysis, as the maximum residue limit tolerated in this matrix is 4µgL^-1. The selectivity of the developed sensor was also studied, showing that the sensor was also able to bind other beta-lactam antibiotics, although with a weaker affinity. Finally the sensor was used for detection of penicillin G in milk. It was shown that a simple sample treatment with isopropanol followed by filtration was sufficient to eliminate matrix effects, allowing the determination of penicillin in milk at concentrations compatible with legislation requirements.

Enhancement of aptamer immobilization using egg shell-derived nano-sized spherical hydroxyapatite for thrombin detection in neuroclinic

In the present study, we describe the sonochemical isolation of nano-sized spherical hydroxyapatite (nHA) from egg shell and application towards thrombin aptasensing. In addition to the sonochemical method, two conventional methods present in literature were carried out to perform a comparative study. Various analysis methods including Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD), Energy-Dispersive Analysis of X-Rays (EDAX), and Thermal Gravimetric Analysis (TGA) have been applied for the characterization of nHA and its nanocomposite with marine-derived collagen isolated from Rhizostoma pulmo jellyfish. TEM micrographs revealed the sonochemically synthesized nHA nanoparticles to have a unique porous spherical shape with a diameter of approximately 60-80nm when compared to hydroxyapatite nanoparticles synthesized using the other two methods which had a typical needle shaped morphology. EDAX, XRD and FTIR results demonstrated that the obtained patterns belonged to hydroxyapatite. Electrochemical impedance spectroscopy (EIS) is the main analyzing technique of the developed thrombin aptasensor. The proposed aptasensor has a detection limit of 0.25nM thrombin. For clinical application of the developed aptasensor, thrombin levels in blood and cerebrospinal fluid (CSF) samples obtained from patients with Multiple Sclerosis, Myastenia Gravis, Epilepsy, Parkinson, polyneuropathy and healthy donors were analyzed using both the aptasensor and commercial ELISA kit. The results showed that the proposed system is a promising candidate for clinical analysis of thrombin.

Trends in the Design and Development of Specific Aptamers Against Peptides and Proteins

Aptamers are single stranded oligonucleotides, comparable to monoclonal antibodies (mAbs) in selectivity and affinity and have significant strategic properties in design, development and applications more than mAbs. Ease of design and development, simple chemical modification and the attachment of functional groups, easily handling and more adaptability with analytical methods, small size and adaptation with nanostructures are the valuable characteristics of aptamers in comparison to large protein based ligands. Among a broad range of targets that their specific aptamers developed, proteins and peptides have significant position according to the number of related studies performed so far. Since proteins control many of important physiological and pathological incidents in the living organisms, particularly human beings and because of the benefits of aptamers in clinical and analytical applications, aptamer related technologies in the field of proteins and peptides are under progress, exclusively. Currently, there is only one FDA approved therapeutic aptamer in the pharmaceutical market, which is specific to vascular endothelial growth factor and is prescribed for age related macular degenerative disease. Additionally, there are several aptamers in the different phases of clinical trials. Almost all of these aptamers are specific to clinically important peptide or protein targets. In addition, the application of protein specific aptamers in the design and development of targeted drug delivery systems and diagnostic biosensors is another interesting field of aptamer technology. In this review, significant efforts related to development and applications of aptamer technologies in proteins and peptides sciences were considered to emphasis on the importance of aptamers in medicinal and clinical applications.

Tetraphenylporphyrin as a protein label for triple detection analytical systems

Porphyrins and metalloporphyrins are promising new protein labels that can be detected using multiple techniques; improving the reliability of the analysis and broadening the range of the linear response. Here, we investigate the potential of 5,10,15,20-tetraphenyl-21H,23H-porphyrin (Tpp) as a hybrid protein label. The electrochemical and optical properties of porphyrin conjugated with bovine serum albumin (BSA), chicken egg albumin (CEA) and immunoglobulin G (IgG) were determined and optimal conditions for Tpp-protein conjugation established. Model conjugates of carboxylated Tpp with BSA and short peptides were characterized using differential pulse voltammetry, UV-Vis spectrophotometry and spectrofluorimetry. These results reveal that Tpp is a promising molecule to be used in a triple detection protein labelling system.

Design of nuclease-based target recycling signal amplification in aptasensors

Compared with conventional antibody-based immunoassay methods, aptasensors based on nucleic acid aptamer have made at least two significant breakthroughs. One is that aptamers are more easily used for developing various simple and rapid homogeneous detection methods by "sample in signal out" without multi-step washing. The other is that aptamers are more easily employed for developing highly sensitive detection methods by using various nucleic acid-based signal amplification approaches. As many substances playing regulatory roles in physiology or pathology exist at an extremely low concentration and many chemical contaminants occur in trace amounts in food or environment, aptasensors for signal amplification contribute greatly to detection of such targets. Among the signal amplification approaches in highly sensitive aptasensors, the nuclease-based target recycling signal amplification has recently become a research focus because it shows easy design, simple operation, and rapid reaction and can be easily developed for homogenous assay. In this review, we summarized recent advances in the development of various nuclease-based target recycling signal amplification with the aim to provide a general guide for the design of aptamer-based ultrasensitive biosensing assays.

Development of an impedimetric aptasensor for the determination of aflatoxin M1 in milk

An aptasensor was designed for the determination of aflatoxin M1 (AFM1) in milk based on DNA-aptamer recognition and electrochemical impedance spectroscopy detection. A hexaethyleneglycol-modified 21-mer oligonucleotide was immobilized on a carbon screen-printed electrode through carbodiimide immobilization, after diazonium activation of the sensing surface. Cyclic voltammetry and electrochemical impedance spectroscopy in the presence of ferri/ferrocyanide redox probe were used to characterize each step of the aptasensor development. Aptamer-AFM1 interaction induced an increase in electron-transfer resistance, allowing the determination of AFM1 in buffer in the range 2-150 ng/L (LOD=1.15 ng/L). Application to milk analysis showed that a preliminary treatment was mandatory. A simple filtration through a 0.2 µm PTFE membrane allowed determination of AFM1 in milk for concentrations ranging from 20 to 1000 ng/kg. These performances are compatible with the AFM1 levels set in European Union for milk and dairy products for adults (50 ng/kg) and infants (25 ng/kg).

Mesoporous silica nanoparticles grafted with a light-responsive protein shell for highly cytotoxic antitumoral therapy

A novel phototriggered drug delivery nanocarrier, which exhibits very high tumor cytotoxicity against human tumoral cells, is presented.

An electrochemical aptasensor for thrombin using synergetic catalysis of enzyme and porous Au@Pd core-shell nanostructures for signal amplification

In this work, a sensitive electrochemical aptasensor for thrombin (TB) based on synergetic catalysis of enzyme and porous Au@Pd core-shell nanostructure has been constructed. With the advantages of large surface area and outstanding catalytic performance, porous Au@Pd core-shell nanostructures were firstly employed as the nanocarrier for the immobilization of electroactive toluidine blue (Tb), hemin/G-quadruplex formed by intercalating hemin into the TB aptamer (TBA) and glucose oxidase (GOx). As a certain amount of glucose was added into the detection cell, GOx rapidly catalyzed the oxidation of glucose, coupling with the local generation of H2O2 in the presence of dissolved O2. Then, porous Au@Pd nanoparticles and hemin/G-quadruplex as the peroxidase mimics efficiently catalyzed the reduction of H2O2, amplifying the electrochemical signal and improving the sensitivity. Finally, a detection limit of 0.037pM for TB was achieved. The excellent performance of the aptasensor indicated its promising prospect as a valuable tool in simple and cost-effective TB detection in clinical application.