New trends in affinity sensing

A sensitive and simple competitive nanozyme-linked apta-sorbent assay for the dual-mode detection of ochratoxin A

The enzyme-linked apta-sorbent assay (ELASA) is widely used for the detection of small-molecule compounds as a result of low cost and reagent stability of aptamers. However, enzyme labels used in ELASA still suffer from some drawbacks, such as high production cost and limited stability. To overcome the drawbacks, we reported a nanozyme-linked apta-sorbent assay (NLASA) coupled with surface-enhanced Raman scattering (SERS)-colorimetric dual-mode detection. For nanozyme labels, Pd-Pt bimetallic nanocrystals (Pd-Pt NRs) could catalyze 3,3',5,5'-tetramethylbenzidine (TMB) to blue TMB²⁺, whose color variation could not only be distinguished by naked eyes but also had a strong SERS signal. The NLASA method was employed to detect ochratoxin A (OTA) with a limit of detection values of 0.097 nM (0.039 ppb) and 0.042 nM (0.017 ppb) via the colorimetric and SERS methods, respectively. This method was applied for the determination of OTA in wine and grape samples, and the detection results were in a satisfied agreement with those determined by the high performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) method. The proposed NLASA method provided a rapid and sensitive detection for OTA and could also be broadened for other small-molecules.

Functional nucleic acid biosensors utilizing rolling circle amplification

Functional nucleic acids regulate rolling circle amplification to produce multiple detection outputs suitable for the development of point-of-care diagnostic devices.

2D Materials-Based Aptamer Biosensors: Present Status and Way Forward

Current advances in constructing functional nanomaterials and elegantly designed nanostructures have opened up new possibilities for the fabrication of viable field biosensors. Two-dimensional materials (2DMs) have fascinated much attention due to their chemical, optical, physicochemical, and electronic properties. They are ultrathin nanomaterials with unique properties such as high surface-to-volume ratio, surface charge, shape, high anisotropy, and adjustable chemical functionality. 2DMs such as graphene-based 2D materials, Silicate clays, layered double hydroxides (LDHs), MXenes, transition metal dichalcogenides (TMDs), and transition metal oxides (TMOs) offer intensified physicochemical and biological functionality and have proven to be very promising candidates for biological applications and technologies. 2DMs have a multivalent structure that can easily bind to single-stranded DNA/RNA (aptamers) through covalent, non-covalent, hydrogen bond, and π-stacking interactions, whereas aptamers have a small size, excellent chemical stability, and low immunogenicity with high affinity and specificity. This review discussed the potential of various 2D material-based aptasensor for diagnostic applications, e.g., protein detection, environmental monitoring, pathogens detection, etc.

DNA building blocks for AFM tip functionalization: An easy, fast and stable strategy

Biosensing atomic force microscopy (AFM) offers the unique feature to determine the energy landscape of a bimolecular interaction at the real single molecule level. Furthermore, simultaneous and label-free mapping of molecular recognition and the determination of sample topography at the nanoscale gets possible. A prerequisite and one of the major parts in biosensing AFM are the bio-functionalized AFM tips. In the past decades, different approaches for tip functionalization have been developed. Using these functionalization strategies, several biological highly relevant interactions at the single molecule level have been explored. For the most common approach, the use of a heterobifunctional poly(ethylenglycol) crosslinker, a broad range of linkers for different chemical coupling strategies is available. Nonetheless, the time consuming functionalization protocol as well as the broad distribution of rupture length reduces the possibility of automation and may reduce the accuracy of the results. Here we present a stable and fast forward approach based on tetra-functional DNA tetrahedra. A fast functionalization and a sharp defined distribution of rupture length gets possible with low effort and high success rate. We tested the performance on the classical avidin biotin system by using tetrahedra with three disulfide legs for stable and site directed coupling to gold coated tips and a biotinylated end at the fourth vertex. A special advantage appears when working with a DNA aptamer as sensing molecule. In this case, the fourth strand can be extended by a certain DNA sequence complementary to the linkage part of an aptamer. This AFM tip functionalization protocol was applied on thrombin using DNA aptamers directed against the fibrinogen binding side of human thrombin.

Selection and characterization of structure-switching DNA aptamers for the salivary peptide histatin 3

In this study, single-stranded DNA aptamers that switch structural conformation upon binding to the salivary peptide histatin 3 have been reported for the first time. Histatin 3 is an antimicrobial peptide that possesses the capability of being a therapeutic agent against oral candidiasis and has recently been linked as a novel biomarker for acute stress. The aptamers were identified through a library immobilization version of an iterative in vitro process known as the Systematic Evolution of Ligands by EXponential enrichment (SELEX). Through the SELEX process, four unique aptamer candidates sharing a consensus sequence were identified. These selected sequences exhibited binding affinity and specificity to histatin 3 and in order to further characterize these aptamers, a direct format enzyme-linked aptamer sorbent assay (ELASA) was developed. The best performing candidate demonstrated an equilibrium dissociation constant (K_d) value of 1.97 ± 0.48 μM. These novel aptamers have the potential to lead to the further development of refined sensing assays and platforms for the detection and quantification of histatin 3 in human saliva and other biological media.

Fluorescence Polarization-Based Bioassays: New Horizons

Fluorescence polarization holds considerable promise for bioanalytical systems because it allows the detection of selective interactions in real time and a choice of fluorophores, the detection of which the biosample matrix does not influence; thus, their choice simplifies and accelerates the preparation of samples. For decades, these possibilities were successfully applied in fluorescence polarization immunoassays based on differences in the polarization of fluorophore emissions excited by plane-polarized light, whether in a free state or as part of an immune complex. However, the results of recent studies demonstrate the efficacy of fluorescence polarization as a detected signal in many bioanalytical methods. This review summarizes and comparatively characterizes these developments. It considers the integration of fluorescence polarization with the use of alternative receptor molecules and various fluorophores; different schemes for the formation of detectable complexes and the amplification of the signals generated by them. New techniques for the detection of metal ions, nucleic acids, and enzymatic reactions based on fluorescence polarization are also considered.

Electrochemical dual-aptamer biosensors based on nanostructured multielectrode arrays for the detection of neuronal biomarkers

Multielectrode arrays (MEAs) have been increasingly used for the development of biosensors due to their capability to record signals from multiple channels, fast mass transfer rates, and high spatial resolution. Alzheimer's disease (AD) is often associated with mitochondrial dysfunction, which is closely related to reduced levels of adenosine triphosphate (ATP). Therefore, simultaneous detection of ATP together with amyloid-β oligomers (AβO), a reliable biomarker for AD, can potentially advance the early detection of Alzheimer's disease. In this work, a dual-aptamer modified MEA chip was developed that consists of microelectrodes modified with electrodeposited 3D nanostructures (3D-GMEs). Electrodeposition methods, deposition potential, and deposition time were systematically altered and the active surface areas as well as the electrode morphologies were characterized by cyclic voltammetry and scanning electron microscopy. The nanostructured microelectrodes were sequentially modified with AβO and ATP specific aptamer receptors. To achieve the modification of different aptamer receptors at different 3D-GMEs of the same MEA chip, electrochemical cleaning was applied to individual 3D-GMEs. Ferrocene labels were attached to the aptamer receptors to enable amperometric signaling after target-aptamer binding. The developed aptasensor showed a linear detection range from 1 pM to 200 nM for the detection of AβO and from 0.01 nM to 1000 nM for the detection of ATP. Finally, ATP and AβO were detected simultaneously in the same analyte solution by the same sensor chip, which could support the early detection of AD, provide comprehensive information about the health status of the patient, and be helpful for pathological studies of neurodegenerative diseases.

A rapid, easy, and sensitive method for detecting His-tag-containing chitinase based on ssDNA aptamers and gold nanoparticles

Chitooligosaccharides are oligosaccharides with many biological activities that can be used in food production for sweeteners, preservatives and humectants, among other products. Chitin, a long-chain polymer of N-acetylglucosamine and a derivative of glucose, can be hydrolyzed by applying chitinase to break down glycosidic bonds to form chitooligosaccharides. Chitinases arising from heterologous gene expression are usually linked to a 6 × His-tag to facilitate easy purification. Heterologously expressed chitinase linked to a 6 × His-tag is a transgenic element, but enzyme activity tests cannot be used to distinguish transgenic elements from natural elements. In this study, we established a rapid and easy method to detect His-tag-containing chitinase using gold nanoparticles (AuNPs) and ssDNA aptamers. Using this method, His-tag-containing chitinase could be detected at concentrations as low as 0.136 nM within 5 min. Color changes of AuNPs showed a positive correlation with His-tag-containing chitinase concentrations.

Aptamer-based ellipsometric sensor for ultrasensitive determination of aminoglycoside group antibiotics from dairy products

BACKGROUND

Residual antibiotics taken along with food consumed through the food chain are the main cause of the super-bacteria and may damage organs such as liver and kidney. Therefore, monitoring residual antibiotic levels of products in the food chain is both important and a requirement. Maximum residual limits for kanamycin and neomycin are 150 ng mL^-1 and 500 ng mL^-1 respectively, which are challenging for most sensor platforms. In this paper, a novel method is presented for the determination of antibiotics residues in animal-derived foods.

RESULTS

Aptamer-based kanamycin and neomycin biosensors based on the spectroscopic ellipsometer and the surface plasmon resonance-enhanced total internal reflection ellipsometer methods as transducing element were developed. Detection limits of both sensor platforms were in the 0.1-1 nmol L^-1 ranges, and the detection range was between the detection limit and 1000 nmol L^-1 .

CONCLUSION

Both ellipsometry-based aptasensors can be used as an alternative to the existing enzyme-linked immunosorbent assay-based method in terms of assay time (10 min), detection limit (0.22 ng mL^-1 for neomycin and 0.048 ng mL^-1 for kanamycin), and detection range. © 2020 Society of Chemical Industry.

Spectrophotometric ellipsometry based Tat-protein RNA-aptasensor for HIV-1 diagnosis

Rapid and reliable diagnosis of Human Immunodeficiency Virus (HIV) Type I that causes autoimmune deficiency syndrome (AIDS) is still important today. In this study, the HIV-I Tat (trans-activator of transcription) protein-specific RNA-aptamer (antiTat) and spectroscopic ellipsometer were preferred to increase specificity and sensitivity in the diagnosis. The ellipsometry is a well-known characterization tool for the ultra-thin films, where polarization state changes show surface deposition in terms of the ellipsometric angles, psi (Ψ) and delta (Δ). Here, we reported the HIV-Tat protein detection performance of antiTat aptamers both for the spectroscopic ellipsometry (SE) and for the surface plasmon resonance enhanced total internal reflection ellipsometry (SPReTIRE), first time. Detection limits for antiTat aptamers with various configurations were in the range of nM-pM protein in the buffer solution. For instance, SPRe-TIRE configuration revealed a detection limit of 1 pM (or about 1.5 pg/mL) for HIV-Tat protein in the range of 1.0-500 nM.

An aptamer affinity column for purification and enrichment of aflatoxin B1 and aflatoxin B2 in agro-products

We have developed an aptamer affinity column (AAC) for the purification and enrichment of trace aflatoxin B₁ (AFB₁) and aflatoxin B₂ (AFB₂) in genuine agro-products through the covalent conjugation of amino modified aptamer and NHS-activated Sepharose. The coupling and working conditions found to be suitable for this AFB-AAC were examined in regard to coupling time (2 min), loading volume (30 mL), and the methanol concentration (< 10%) used in the loading step. The performance of AFB-AAC was then further evaluated in terms of capacity (329.1 ± 13.7 ng for AFB₁ and 162.5 ± 8.9 ng for AFB₂), selectivity (excellent), reusability (twenty-three times for AFB₁ and twelve times for AFB₂), and repeatability (92.7% ± 2.9% for AFB₁ and 71.5% ± 3.4% for AFB₂). Furthermore, the AAC clean-up combined with HPLC-FLD demonstrated excellent linearity over a wide range, good sensitivity with an LOD of 50 pg mL^-1 for AFB₁ and 15 pg mL^-1 for AFB₂, and acceptable recovery with different spiking levels in different matrices. Finally, the AAC was successfully applied to analyte AFB₁ and AFB₂ in four types of agro-products as well as a maize flour reference material, and the results were found to be in accordance with those of commercial IACs. This study provides a reference for the analysis of other trace analytes by merely changing the corresponding aptamer and represents a strong contender for immune affinity columns. Graphical abstract An aptamer affinity column for purification and enrichment of aflatoxin B₁ and aflatoxin B₂ in agro-products with the aid of HPLC-FLD and a post-column photochemical derivatization reactor.

An aptamer-based colorimetric lateral flow assay for the detection of human epidermal growth factor receptor 2 (HER2)

An aptamer-based colorimetric lateral flow assay was developed for the detection of human epidermal growth factor receptor 2 (HER2). In this study, two approaches were examined using HER2 binding aptamers and gold nanoparticles. The first method used was a solution-based adsorption-desorption colorimetric approach wherein aptamers were adsorbed onto the gold nanoparticle surface. Upon the addition of HER2, HER2 binds specifically with its aptamer, releasing the gold nanoparticles. Addition of NaCl then induces the formation of gold nanoparticle aggregates. This leads to a color change from red to blue and a detection limit of 10 nM was achieved. The second method used an adsorption-desorption colorimetric lateral flow assay approach wherein biotin-modified aptamers were adsorbed onto the gold nanoparticle surface in the absence of HER2. In the presence of HER2, HER2 specifically binds with its aptamer leading to release of the gold nanoparticles. These solutions were applied to the lateral flow assay format and a detection limit of 20 nM was achieved. Both colorimetric and lateral flow assays are inexpensive, simple, rapid to perform and produce results visible to the naked-eye.

Lateral flow assays for Ochratoxin A using metal nanoparticles: comparison of "adsorption-desorption" approach to linkage inversion assembled nano-aptasensors (LIANA)

Nano-aptamer probes were prepared and used in lateral flow colorimetric assays for the detection of Ochratoxin A (OTA). In this study, two approaches were examined using 5'-biotin-modified OTA aptamers and silver or gold nanoparticles (AgNP or AuNP). The first method used an "adsorption-desorption" approach wherein aptamers were adsorbed onto the metal nanoparticle surface. Upon the addition of OTA, the aptamer binds specifically to the target, releasing the NPs. The above solutions were applied on a lateral flow assay (LFA) and a detection limit of 6.3 nM was achieved with both metal nanoparticles. The second method used a labelled approach based on Linkage Inversion Assembled Nano-Aptasensors (LIANAs) using a DNA linker containing a 5'-5' linkage inversion (5'-5' linker) to assemble biotinylated aptamer-functionalized metal nanoparticles. In the presence of target, OTA specifically binds with its aptamer leading to release of the linker and disassembly of LIANA aggregates into dispersed nanoparticles. The same solutions were applied in LFA format and the lowest detection limit of 0.63 nM was achieved. The results indicated that the LIANA-based LFA strips were more sensitive than the "adsoprtion-desorption" LFAs. Both lateral flow assays are inexpensive, simple, and rapid to perform and produces results visible to the naked-eye.

Targeted imaging of breast cancer cells using two different kinds of aptamers -functionalized nanoparticles

Breast cancer which is the most commonly diagnosed cancer among women; have been known as a serious threat for health and life around the world. So development of an approach for early-stage diagnosis of breast cancer is vital. In this study, we designed a double aptamer-nanoparticle conjugates-based (DANP) complex for specific detection and visualization of MCF-7 cells using Mucin 1 (MUC 1) aptamer-conjugated gold nanoparticles (MUC1 apt - GNPs) and adenosine triphosphate (ATP) aptamer-conjugated CdTe quantum dots (ATP apt-QDs). The ATP apt-QDs was attached onto MUC1 apt - GNPs surface through Van der Waals forces and electrostatic interactions between ATP aptamer and GNPs leading to the formation of DANP complex. Atomic force microscopy asserted DANP complex formation. The imaging process was based on the recognition of MUC1 protein on the surface of MCF-7 cells by MUC1 aptamer and specific internalization of DANP complex into target cells (MCF-7). Existence of abundant amounts of ATP in lysosome led to release of ATP apt-QDs from the MUC1 apt-GNPs surface resulting in strong fluorescence emission. The flow cytometry analysis and fluorescence microscopy confirmed significant internalization of DANP complex into MCF-7 cells (target) in comparison with CHO cells (non-target). Based on the obtained results, the DANP complex possesses high potency for efficient detection and monitoring of breast cancer cells (MCF-7).

Lysozyme Aptamer-Functionalized Magnetic Nanoparticles for the Purification of Lysozyme from Chicken Egg White

Lysozyme is in high demand due to its many favorable characteristics such as being naturally occurring, non-toxic, and easy to digest and absorb. Recently, superparamagnetic nanoparticles with strong magnetic responsiveness have attracted significant interest for enzyme purification. The aptamer of the enzyme can be chemically synthesized rapidly at a large scale using simple and low-cost preparation methods. Therefore, Fe₃O₄ nanoparticles (Fe₃O₄ NPs) were prepared by chemical co-precipitation and were then functionalized with amino groups to produce NH₂-Fe₃O₄ NPs. The specific reaction of aldehyde and amino groups was used to attach lysozyme aptamers with specific sequences to NH₂-Fe₃O₄ NPs to produce Apt-NH₂-Fe₃O₄ NPs. The synthesized materials were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), hysteresis loop analysis, and thermogravimetric analysis (TGA). The optimal experimental conditions for adsorption of lysozyme were investigated. The effects of initial lysozyme concentration, adsorption time, pH, reaction temperature, and ionic strength were determined. The maximum adsorption capacity and relevant activity of Apt-NH₂-Fe₃O₄ NPs was 460 mg·g^-¹ and 16,412 ± 55 U·mg^-¹ in an aqueous lysozyme solution. In addition, as demonstrated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) electrophoresis analysis, lysozyme could be separated from crude fresh egg white using Apt-NH₂-Fe₃O₄ NPs with an amount up to 113 ± 4.2 mg·g^-¹ and an activity up to 16,370 46 U·mg^-¹.

Development of an in Vitro Autocatalytic Self-Replication System for Biosensing Application

Molecular self-replication is a fundamental function of all living organisms with the capability of templating and catalyzing its own synthesis, and it plays important roles in prebiotic chemical evolution and effective synthetic machineries. However, the construction of the self-replication system in vitro remains a great challenge and its application for biosensing is rare. Here, we demonstrate for the first time the construction of an in vitro enzymatic nucleic acid self-replication system and its application for amplified sensing of human 8-oxoguanine DNA glycosylase (hOGG1) based on autocatalytic self-replication-driven cascaded recycling amplification. In this strategy, hOGG1 excises 8-oxoguanine (8-oxoG) to unfold the hairpin substrate, activating the autonomous biocatalytic process with molecular beacons (MBs) as both the fuels for producing nucleic acid templates and the generators for signal output, leading to the continuous replication of biocatalytic nucleic acid templates and the repeated cleavage of MBs for an enhanced fluorescence signal. This strategy exhibits an extremely low detection limit of 4.3 × 10^-7 U/μL and a large dynamic range of 5 orders of magnitude from 1 × 10^-6 to 0.05 U/μL. Importantly, it can be applied for the detection of enzyme kinetic parameters, the screening of hOGG1 inhibitors, and the quantification of hOGG1 activity in even 1 single lung cancer cell, providing a new approach for biomedical research and clinical diagnosis.

Recent trends in electrochemiluminescence aptasensors and their applications

Aptamers are single stranded DNA or RNA ligands which can be selected for different targets from proteins to small organic dyes. In the past few years great progress has been accomplished in the development of aptamer based bioanalytical assays with different detection techniques. Among them, electrochemiluminescence (ECL) aptasensors are very promising because they have the advantages of both electrochemical and chemiluminescence biosensors, such as high sensitivity, low background, cost effectiveness, and ease of control. In this review, we summarize the recent efforts to construct novel and improved ECL aptasensors and their application.

Bifunctional aptasensor based on novel two-dimensional nanocomposite of MoS2 quantum dots and g-C3N4 nanosheets decorated with chitosan-stabilized Au nanoparticles for selectively detecting prostate specific antigen

A novel nanostructured biosensing platform was designed based on two-dimensional (2D) nanocomposite of graphitic carbon nitride (g-C₃N₄) nanosheets and MoS₂ quantum dots (MoS₂ QDs), followed by decoration with chitosan-stabilized Au nanoparticles (CS-AuNPs) (denoted as MoS₂QDs@g-C₃N₄@CS-AuNPs), of which CS-AuNPs were prepared by plasma enhanced-chemical vapor deposition. Owning to the good surface plasmon performance of the CS-AuNPs and excellent electrochemical activity of MoS₂QDs@g-C₃N₄ nanosheets, the as-obtained 2D MoS₂QDs@g-C₃N₄@CS-AuNPs nanocomposite was simultaneously explored to construct both surface plasmon resonance spectroscopy (SPR) sensor and electrochemical aptasensor. The MoS₂QDs@g-C₃N₄@CS-AuNPs-based aptasensor shows strong bio-binding affinity toward the prostate specific antigen (PSA) targeted aptamer strands as compared to the individual component, including MoS₂ QDs, g-C₃N₄, and CS-AuNPs. When detecting PSA, the low limit of detection (LOD) of 0.71 pg mL^-1 deduced by electrochemical aptasensor is three orders of magnitude lower than that deduced by SPR sensor (0.77 ng mL^-1). As expected, both SPR sensor and electrochemical aptasensor demonstrate good selectivity, highly stability, acceptable reproducibility, and well consistent applicability in human serum. The satisfactory results suggest potential application of the MoS₂QDs@g-C₃N₄@CS-AuNPs in bifunctional biosensing fields and clinical monitoring of cancer markers.

Diagnosing human blood clotting deficiency

There are different clotting factors present in blood, carries the clotting cascade and excessive bleeding may cause a deficiency in the clotting Diagnosis of this deficiency in clotting drastically reduces the potential fatality. For enabling a sensor to detect the clotting factors, suitable probes such as antibody and aptamer have been used to capture these targets on the sensing surface. Two major clotting factors were widely studied for the diagnosis of clotting deficiency, which includes factor IX and thrombin. In addition, factor IX is considered as the substitute for heparin and the prothrombotic associated with the increased thrombin generation are taking into account their prevalence. The biosensors, surface plasmon resonance, evanescent-field-coupled waveguide-mode sensor, metal-enhanced PicoGreen fluorescence and electrochemical aptasensor were well-documented and improvements have been made for high-performance sensing. We overviewed detecting factor IX and thrombin using these biosensors, for the potential application in medical diagnosis.

Trace analysis by ion mobility spectrometry: From conventional to smart sample preconcentration methods. A review

Ion mobility spectrometry (IMS) is a rapid and high sensitive technique widely used in security and forensic areas. However, a lack of selectivity is usually observed in the analysis of complex samples due to the scarce resolution of the technique. The literature concerning the use of conventional and novel smart materials in the pretreatment and preconcentration of samples previous to IMS determinations has been critically reviewed. The most relevant strategies to enhance selectivity and sensitivity of IMS determinations have been widely discussed, based in the use of smart materials, as immunosorbents, aptamers, molecularly imprinted polymers (MIPs), ionic liquids (ILs) and nanomaterial. The observed trend is focused on the development of IMS analytical methods in combination of selective sample treatments in order to achieve quick, reliable, sensitive, and selective methods for the analysis of complex samples such as biological fluids, food, or environmental samples.

Characterization and electrochemical response of DNA functionalized 2nm gold nanoparticles confined in a nanochannel array

Polyvalent gold nanoparticle oligonucleotide conjugates are subject of intense research. Even though 2nm diameter AuNPs have been previously modified with DNA, little is known about their structure and electrochemical behavior. In this work, we examine the influence of different surface modification strategies on the interplay between the meso-organization and the molecular recognition properties of a 27-mer DNA strand. This DNA strand is functionalized with different sulfur-containing moieties and immobilized on 2nm gold nanoparticles confined on a nanoporous alumina, working the whole system as an electrode array. Surface coverages were determined by EXAFS and the performance as recognition elements for impedance-based sensors is evaluated. Our results prove that low DNA coverages on the confined nanoparticles prompt to a more sensitive response, showing the relevance in avoiding the DNA strand overcrowding. The system was able to determine a concentration as low as 100pM of the complementary strand, thus introducing the foundations for the construction of label-free genosensors at the nanometer scale.

Detection of C-reactive protein on an integrated microfluidic system by utilizing field-effect transistors and aptamers

Cardiovascular diseases (CVDs) cause more than 17 × 10⁶ deaths worldwide on a yearly basis. Early diagnosis of CVDs is therefore of great need. The C-reactive protein (CRP) is an important biomarker for analyzing the risks of CVDs. In this work, CRP-specific aptamers with high sensitivity and specificity and field-effect-transistor (FET) devices were used to recognize and detect CRP by using an integrated microfluidic system automatically while consuming less volumes of reagents and samples (about 5 μm). In order to package the FET device into the microfluidic chip, a new method to prevent liquid leakage was proposed. Sensitive detection of CRP has been demonstrated on the developed microfluidic system. It is the first time that aptamer-FET assays could be realized on an integrated microfluidic system. Experimental results showed that the aptamer-FET assay was capable of detecting CRP with concentrations ranging from 0.625 mg/l to 10.000 mg/l, which may be promising for early diagnosis of CVDs.

A reusable aptasensor of thrombin based on DNA machine employing resonance light scattering technique

The design of molecular nanodevices attracted great interest in these years. Herein, a reusable, sensitive and specific aptasensor was constructed based on an extension-contraction movement of DNA interconversion for the application of human thrombin detection. The present biosensor was based on resonance light scattering (RLS) using magnetic nanoparticles (MNPs) as the RLS probe. MNPs coated with streptavidin can combine with biotin labeled thrombin aptamers. The combined nanoparticles composite is monodispersed in aqueous medium. When thrombin was added a sandwich structure can form on the surface of MNPs, which induced MNPs aggregation. RLS signal was therefore enhanced, and there is a linear relationship between RLS increment and thrombin concentration in the range of 60pM-6.0nM with a limit of detection at 3.5pM (3.29S_B/m, according to the recent recommendation of IUPAC). The present aptasensor can be repeatedly used for at least 6 cycling times by heat to transfer G-quadruplex conformation to single strand of DNA sequence and release thrombin. MNPs can be captured by applying the external magnetic field. Furthermore, the proposed biosensor was successfully applied to detect thrombin in human plasma.

Use of anchor protein modules in fluorescence polarisation aptamer assay for ochratoxin A determination

A new strategy for sensitive fluorescence polarisation (FP) analysis is proposed which uses aptamer as the receptor and anchor protein modules as the enhancers by including the aptamers in complexes with protein modules. This approach is based on increasing the size differences of bound and unbound fluorophores. The strategy was applied in an ochratoxin A (ОТА) assay with the competitive binding of fluorophore-labelled and free OTA with aptamer-based receptors. We showed that the binding of labelled OTA with aptamer included in complexes with anchors led to higher a FP than binding with free aptamer. This allowed the aptamer concentration to be reduced, thus lowering the limit of detection by a factor of 40, down to 3.6 nM. The assay time was 15 min. To evaluate the applicability of the FP assay with aptamer-anchor complex to real samples, we conducted OTA measurements in spiked white wine. The OTA limit of detection in wine was 2.8 nM (1.1 μg/kg), and the recoveries ranged from 83% to 113%. The study shows that the proposed anchor strategy is efficient for increasing the sensitivity of FP-based aptamer assays.

A novel colorimetric competitive aptamer assay for lysozyme detection based on superparamagnetic nanobeads

Lysozyme (Lys) commonly presents in wines and are known to cause toxicological impact on human health. The need of highly sensitive and reliable detection methods are evident in such matrix. In this work, we developed a competitive aptamer based assay for detection of Lys by employing carboxylated magnetic beads as a support to immobilize the target molecule Lys. The used aptamer sequence was biotinylated which further binds with Streptavidin-Alkaline phosphatase (Stp-ALP) in the micro wells. Colorimetric tests were performed in order to optimize different experimental parameters. The Lys assay showed a good linearity in the range of 5-140nM with a limit of detection (LOD) 10nM. The mid-point value (IC₅₀) 110nM and the analysis time (60min) validated the developed aptasensor as a promising tool for routine use. The assay displayed good recoveries of Lys in the range 99.00-99.27% and was demonstrated for the detection of Lys in wine samples.

Detection of Intermolecular Interactions Based on Surface Plasmon Resonance Registration

Methods for registration of intermolecular interactions based on the phenomenon of surface plasmon resonance (SPR) have become one of the most efficient tools to solve fundamental and applied problems of analytical biochemistry. Nevertheless, capabilities of these methods are often insufficient to detect low concentrations of analytes or to screen large numbers of objects. That is why considerable efforts are directed at enhancing the sensitivity and efficiency of SPR-based measurements. This review describes the basic principles of the detection of intermolecular interactions using this method, provides a comparison of various types of SPR detectors, and classifies modern approaches to enhance sensitivity and efficiency of measurements.

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.

A simple modular aptasensor platform utilizing cucurbit[7]uril and a ferrocene derivative as an ultrastable supramolecular linker

A simple modular aptamer-based sensor (aptasensor) platform was prepared by combining the merits of the rapid and efficient preparation of a self-assembled monolayer of cucurbit[7]uril (CB[7] SAM) and the strong and specific binding affinity of CB[7] to ferrocenemethylammonium (FA), as an ultrastable supramolecular linker, to immobilize aptamers on CB[7] SAM.

Sensitive detection of bisphenol A based on a ratiometric electrochemical aptasensor

A new ratiometric electrochemical aptasensor has been developed for highly sensitive and selective detection of bisphenol A (BPA). The double-stranded DNA (dsDNA), consisting of the BPA aptamer (DNA1) and methylene blue (MB)-labeled complementary DNA (cDNA), was immobilized on a gold nanoparticle (AuNP) modified glassy carbon (GC) electrode. In the presence of BPA, the specific BPA–aptamer interaction drove the release of the MB-labeled cDNA from the electrode surface. As a result, the oxidation peak current of MB (IMB) decreased and that of BPA (IBPA) increased. The peak current ratio (IBPA/IMB) of BPA and MB was linear with the concentration of BPA in the range from 1 to 100 pmol/L with a detection limit of 0.6 pmol/L. The detection limit is much lower than that obtained by most of the reported electrochemical methods. On the other hand, the developed aptasensor possesses good selectivity, reproducibility, and stability, and the related sensing structure is very simple, showing promising practical applications in BPA assays.

Microarrays as Model Biosensor Platforms to Investigate the Structure and Affinity of Aptamers

Immobilization of nucleic acid aptamer recognition elements selected free in solution onto the surface of biosensor platforms has proven challenging. This study investigated the binding of multiple aptamer/target pairs immobilized on a commercially available microarray as a model system mimicking biosensor applications. The results indicate a minimum distance (linker length) from the surface and thymine nucleobase linker provides reproducible binding across varying conditions. An indirect labeling method, where the target was labeled with a biotin followed by a brief Cy3-streptavidin incubation, provided a higher signal-to-noise ratio and over two orders of magnitude improvement in limit of detection, compared to direct Cy3-protein labeling. We also showed that the affinities of the aptamer/target interaction can change between direct and indirect labeling and conditions to optimize for the highest fluorescence intensity will increase the sensitivity of the assay but will not change the overall affinity. Additionally, some sequences which did not initially bind demonstrated binding when conditions were optimized. These results, in combination with studies demonstrating enhanced binding in nonselection buffers, provided insights into the structure and affinity of aptamers critical for biosensor applications and allowed for generalizations in starting conditions for researchers wishing to investigate aptamers on a microarray surface.

In vitro selection of RNA aptamers against CA125 tumor marker in ovarian cancer and its study by optical biosensing

Early identification of neoplastic diseases is essential to achieve timely therapeutic interventions and significantly reduce the mortality of patients. A well-known biomarker is the Cancer Antigen 125 (CA125) or 16 mucin (MUC 16), a glycoprotein of the human family of mucins, already used for the diagnostic and prognostic evaluation of ovarian cancer. Therefore, the detection of CA125 to now remains a promising tool in the early diagnosis of this tumor. In this paper, we describe the development of RNA aptamers that bind with high affinity the tumor antigen CA125. We performed eight cycles of selection against CA125 purified protein. The selected aptamers were cloned and sequenced and the binding properties of the most promising sequences were studied by Real Time PCR and Surface Plasmon Resonance (SPR) to evaluate their ability in targeting CA125 protein with perspective applications in aptamer-based bioassays.

In vitro and in vivo cell-capture strategies using cardiac stent technology - A review

Stenosis is a symptom of coronary artery disease (CAD), and is caused by narrowing of arteries in the heart. Over the last several decades, medical implants such as cardiac stents have been developed to counter stenosis. Upon implantation of a stent to open up a restricted artery, narrowing of the artery can reoccur (restenosis), due to an immune response launched by the body towards the stent. Currently, restenosis is a major health concern for patients who have undergone heart surgery for coronary artery disease. Recently, there have been new methods developed to combat restenosis, which have shown potential signs of success. One proposed method is the use of stents to capture cells, thereby reducing immune response. This review will explore the different methods for cell capture both in vitro and in vivo. Biological modifications of the stent will be surveyed, as well as the use of surface science to immobilize biological probes. Immobilization of proteins and nucleotides, as well as use of magnetic field are all methods that will be further discussed. Finally, concluding remarks and future prospects will be presented.

Deploying aptameric sensing technology for rapid pandemic monitoring

The genome of virulent strains may possess the ability to mutate by means of antigenic shift and/or antigenic drift as well as being resistant to antibiotics with time. The outbreak and spread of these virulent diseases including avian influenza (H1N1), severe acute respiratory syndrome (SARS-Corona virus), cholera (Vibrio cholera), tuberculosis (Mycobacterium tuberculosis), Ebola hemorrhagic fever (Ebola Virus) and AIDS (HIV-1) necessitate urgent attention to develop diagnostic protocols and assays for rapid detection and screening. Rapid and accurate detection of first cases with certainty will contribute significantly in preventing disease transmission and escalation to pandemic levels. As a result, there is a need to develop technologies that can meet the heavy demand of an all-embedded, inexpensive, specific and fast biosensing for the detection and screening of pathogens in active or latent forms to offer quick diagnosis and early treatments in order to avoid disease aggravation and unnecessary late treatment costs. Nucleic acid aptamers are short, single-stranded RNA or DNA sequences that can selectively bind to specific cellular and biomolecular targets. Aptamers, as new-age bioaffinity probes, have the necessary biophysical characteristics for improved pathogen detection. This article seeks to review global pandemic situations in relation to advances in pathogen detection systems. It particularly discusses aptameric biosensing and establishes application opportunities for effective pandemic monitoring. Insights into the application of continuous polymeric supports as the synthetic base for aptamer coupling to provide the needed convective mass transport for rapid screening is also presented.

C,N-bipyrazole receptor grafted onto a porous silica surface as a novel adsorbent based polymer hybrid

A simple heterogeneous synthesis of pure adsorbent based polymer hybrid made by condensing a functionalized C,N-bipyrazole with a 3-glycidoxypropyl-trimethoxysilane silylant agent, previously anchored on a silica surface was developed. The formed material (SG2P) was characterized through elemental analysis, FT-IR spectroscopy, (13)C NMR of solid state, scanning electron microscope (SEM), and was studied and evaluated by determination of the surface area using the BET equation, the adsorption and desorption capability using the isotherm of nitrogen and B.J.H. pore sizes. The new material exhibits good thermal stability determined by thermogravimetry curves and good chemical stability was examined in various acidic and buffer solutions (pH 1-7). The binding and adsorption abilities of SG2P were investigated for Hg(2+), Cd(2+), Pb(2+), Zn(2+), K(+), Na(+) and Li(+) cations and compared to the results of classical liquid-liquid extraction with the unbound C,N-bipyrazole compound. The grafting at the surface of silica does not affect complexing properties of the ligand and the SG2P exhibits a high selectivity toward Hg(2+) ion with no complexation being observed towards zinc and alkali metals. The extracted and the complexing cation percentages were determined by atomic absorption measurements.

A novel electrochemical aptamer-antibody sandwich assay for lysozyme detection

In this paper, we have reported a novel electrochemical aptamer-antibody based sandwich biosensor for the detection of lysozyme. In the sensing strategy, an anti-lysozyme aptamer was immobilized onto the carbon electrode surface by covalent binding via diazonium salt chemistry. After incubating with a target protein (lysozyme), a biotinylated antibody was used to complete the sandwich format. The subsequent additions of avidin-alkaline phosphatase as an enzyme label, and a 1-naphthyl phosphate substrate (1-NPP) allowed us to determine the concentration of lysozyme (Lys) via Differential Pulse Voltammetry (DPV) of the generated enzyme reaction product, 1-naphthol. Using this strategy, a wide detection range from 5 fM to 5 nM was obtained for a target lysozyme, with a detection limit of 4.3 fM. The control experiments were carried out by using bovine serum albumin (BSA), cytochrome c and casein. The results showed that the proposed biosensor had good specificity, stability and reproducibility for lysozyme analysis. In addition, the biosensor was applied for detecting lysozyme in spiked wine samples, and very good recovery rates were obtained in the range from 95.2 to 102.0% for lysozyme detection. This implies that the proposed sandwich biosensor is a promising analytical tool for the analysis of lysozyme in real samples.