Dual nanoparticle amplified surface plasmon resonance detection of thrombin at subattomolar concentrations

Machine Learning-Assisted High-Throughput Identification and Quantification of Protein Biomarkers with Printed Heterochains

Advanced in vitro diagnosis technologies are highly desirable in early detection, prognosis, and progression monitoring of diseases. Here, we engineer a multiplex protein biosensing strategy based on the tunable liquid confinement self-assembly of multi-material heterochains, which show improved sensitivity, throughput, and accuracy compared to standard ELISA kits. By controlling the material combination and the number of ligand nanoparticles (NPs), we observe robust near-field enhancement as well as both strong electromagnetic resonance in polymer-semiconductor heterochains. In particular, their optical signals show a linear response to the coordination number of the semiconductor NPs in a wide range. Accordingly, a visible nanophotonic biosensor is developed by functionalizing antibodies on central polymer chains that can identify target proteins attached to semiconductor NPs. This allows for the specific detection of multiple protein biomarkers from healthy people and pancreatic cancer patients in one step with an ultralow detection limit (1 pg/mL). Furthermore, rapid and high-throughput quantification of protein expression levels in diverse clinical samples such as buffer, urine, and serum is achieved by combining a neural network algorithm, with an average accuracy of 97.3%. This work demonstrates that the heterochain-based biosensor is an exemplary candidate for constructing next-generation diagnostic tools and suitable for many clinical settings.

An Overview on Gold Nanorods as Versatile Nanoparticles in Cancer Therapy

Gold nanorods (GNRs/AuNRs) are a group of gold nanoparticles which their simple surface chemistry allows for various surface modifications, providing the possibility of using them in the fabrication of biocompatible and functional nano-agents for cancer therapy. AuNRs, moreover, exhibit a maximum absorption of longitudinal localized surface plasmon resonance (LSPR) in the near-infrared (NIR) region which overlaps with NIR bio-tissue 'window' suggesting that they are proper tools for thermal ablation of cancer cells. AuNRs can be used for induction of mono or combination therapies by administering various therapeutic approaches such as photothermal therapy (PTT), photodynamic therapy (PDT), chemotherapy (CT), radiotherapy (RT), and gene therapy (GT). In this review, anticancer therapeutic capacities of AuNRs along with different surface modifications are summarized comprehensively. The roles of AuNRs in fabrication of various nano-constructs are also discussed.

Anisotropic CdSe Tetrapods in Vortex Flow for Removing Non-Specific Binding and Increasing Protein Capture

Non-specific binding (NSB) is one of the important issues in biosensing performance. Herein, we designed a strategy for removing non-specific binding including anti-mouse IgG antibody and bovine serum albumin (BSA) by utilizing anisotropic cadmium selenide tetrapods (CdSe TPs) in a vortex flow. The shear force on the tetrapod nanoparticles was increased by controlling the rotation rate of the vortex flow from 0 rpm to 1000 rpm. As a result, photoluminescence (PL) signals of fluorescein (FITC)-conjugated protein, anti-mouse IgG antibody-FITC and bovine serum albumin (BSA)-FITC, were reduced by 35% and 45%, respectively, indicating that NSB can be removed under vortex flow. In particular, simultaneous NSB removal and protein capture can be achieved even with mixture solutions of target antibodies and anti-mouse IgG antibodies by applying cyclic mode vortex flow on anisotropic CdSe TPs. These results demonstrate successfully that NSB can be diminished by rotating CdSe TPs to generate shear force under vortex flow. This study opens up new research protocols for utilization of anisotropic nanoparticles under vortex flow, which increases the feasibility of protein capture and non-specific proteins removal for biosensors.

A sensitive electrochemiluminescent sensor chip based on ssDNA-Ru (II) complex and aptamer for the determination of thrombin

In this work, an electrochemiluminescence (ECL) sensor chip for sensitive detection of thrombin (TB) was prepared using a screen-printed electrode (SPE) as a working electrode and an aptamer as a specific recognition moiety. To produce an ECL sensor chip, a layer of pL-Cys was immobilized on the surface of SPE by the cyclic voltammetry scanning method, a layer of AuNPs was assembled through an Au-S bond and hairpin DNA was further immobilized on the electrode surface. Ru (bpy)₂ (mcpbpy)²⁺ , as a luminescent reagent, was covalently bound to ssDNA to prepare a luminescent probe ssDNA-Ru. The probe hybridized with TB aptamer to form a capture probe. In the presence of TB, the TB aptamer in the capture probe bound to TB, causing the release of ssDNA-Ru that could bind to hairpin DNA on the electrode surface. Ru (II) complex as a luminescent reagent was assembled onto the electrode, and pL-Cys was used as a co-reactant to. enhance the ECL efficiency. The ECL signal of the sensor chip generated based on the above principles had a linear relationship with log TB concentration at a range of 10 fM-1 nM, and the detection limit was 0.2 fM. Finally, TB detection by this method was verified using real blood samples. This work provides a new method using an aptamer as a foundation and SPE as a material for the detection of biological substances.

Emerging Biosensors to Detect Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2): A Review

Coronavirus disease (COVID-19) is a global health crisis caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) is the gold standard test for diagnosing COVID-19. Although it is highly accurate, this lab test requires highly-trained personnel and the turn-around time is long. Rapid and inexpensive immuno-diagnostic tests (antigen or antibody test) are available, but these point of care (POC) tests are not as accurate as the RT-PCR test. Biosensors are promising alternatives to these rapid POC tests. Here we review three types of recently developed biosensors for SARS-CoV-2 detection: surface plasmon resonance (SPR)-based, electrochemical and field-effect transistor (FET)-based biosensors. We explain the sensing principles and discuss the advantages and limitations of these sensors. The accuracies of these sensors need to be improved before they could be translated into POC devices for commercial use. We suggest potential biorecognition elements with highly selective target-analyte binding that could be explored to increase the true negative detection rate. To increase the true positive detection rate, we suggest two-dimensional materials and nanomaterials that could be used to modify the sensor surface to increase the sensitivity of the sensor.

Recent Advancements in Aptamer-Based Surface Plasmon Resonance Biosensing Strategies

Surface plasmon resonance (SPR) can track molecular interactions in real time, and is a powerful as well as widely used biological and chemical sensing technique. Among the different SPR-based sensing applications, aptamer-based SPR biosensors have attracted significant attention because of their simplicity, feasibility, and low cost for target detection. Continuous developments in SPR aptasensing research have led to the emergence of abundant technical and design concepts. To understand the recent advances in SPR for biosensing, this paper reviews SPR-based research from the last seven years based on different sensing-type strategies and sub-directions. The characteristics of various SPR-based applications are introduced. We hope that this review will guide the development of SPR aptamer sensors for healthcare.

Gold Nanoparticle-Enhanced and Roll-to-Roll Nanoimprinted LSPR Platform for Detecting Interleukin-10

Localized surface plasmon resonance (LSPR) is a powerful platform for detecting biomolecules including proteins, nucleotides, and vesicles. Here, we report a colloidal gold (Au) nanoparticle-based assay that enhances the LSPR signal of nanoimprinted Au strips. The binding of the colloidal Au nanoparticle on the Au strip causes a red-shift of the LSPR extinction peak, enabling the detection of interleukin-10 (IL-10) cytokine. For LSPR sensor fabrication, we employed a roll-to-roll nanoimprinting process to create nanograting structures on polyethylene terephthalate (PET) film. By the angled deposition of Au on the PET film, we demonstrated a double-bent Au structure with a strong LSPR extinction peak at ~760 nm. Using the Au LSPR sensor, we developed an enzyme-linked immunosorbent assay (ELISA) protocol by forming a sandwich structure of IL-10 capture antibody/IL-10/IL-10 detection antibody. To enhance the LSPR signal, we introduced colloidal Au nanocube (AuNC) to be cross-linked with IL-10 detection antibody for immunogold assay. Using IL-10 as a model protein, we successfully achieved nanomolar sensitivity. We confirmed that the shift of the extinction peak was improved by 450% due to plasmon coupling between AuNC and Au strip. We expect that the AuNC-assisted LSPR sensor platform can be utilized as a diagnostic tool by providing convenient and fast detection of the LSPR signal.

Compact Grating-Coupled Biosensor for the Analysis of Thrombin

A compact optical biosensor for direct detection of thrombin in human blood plasma (HBP) is reported. This biosensor platform is based on wavelength spectroscopy of diffraction-coupled surface plasmons on a chip with a periodically corrugated gold film that carries an antifouling thin polymer layer consisting of poly[(N-(2-hydroxypropyl)methacrylamide)-co-(carboxybetaine methacrylamide)] (poly(HPMA-co-CBMAA)) brushes. This surface architecture provides superior resistance to nonspecific and irreversible adsorption of abundant compounds in the analyzed HBP samples in comparison to standard surface modifications. The carboxylate groups along the polymer brushes were exploited for the covalent immobilization of aptamer ligands. These ligands were selected to specifically capture the target thrombin analyte from the analyzed HBP sample in a way that does not activate the coagulatory process at the biosensor surface with poly(HPMA-co-CBMAA) brushes. Direct label-free analysis of thrombin in the medically relevant concentration range (1-20 nM) is demonstrated without the need for diluting the HBP samples or using additional steps for signal enhancement. The reported platform constitutes the first step toward a portable and sensitive point-of-care device for direct detection of thrombin in human blood.

Mixed monolayer decorated SPR sensing surface for thrombin detection

The development of surface plasmon resonance (SPR) based immunosensor for thrombin detection was aimed. For this purpose, 3,3' Dithiodipropionic acid di (N-hydroxysuccinimide ester) (DSP):6-mercapto-1-hexanol (MCH) mixed self-assembled monolayers (mSAMs) were formed on gold surfaces for immobilization of anti-thrombin antibody. The performance of the immunosensor was determined against the target protein thrombin at various concentrations using flow cell coupled SPR. The linear detection range of the immunosensor was 30.0-100.0 nM with an R² value of 0,992. Limit of Detection (LOD) and Limit of Quantification (LOQ) were determined to be 6.0 nM and 30.0 nM, respectively. The selectivity of the immunosensor was tested against a non-target model protein, human serum albumin (HSA) and the obtained ΔRU value was found to be below the ΔRU value corresponding to the LOQ concentration for thrombin. The immunosensor's capability to detect thrombin in diluted complex serum matrix was also tested and the obtained ΔRU value (159 ± 16) was compared with ΔRU value obtained for thrombin detection in PBS solution (137 ± 19). Based on the results, it was shown that DSP:MCH interface is a promising immobilization platform for binding biological recognition elements for the development of biosensors.

Plasmonic Selection of ssDNA Aptamers against Fibroblast Growth Factor Receptor

In this work, we describe the selection of ssDNA aptamers targeting fibroblast growth factor receptor binding protein 3 K650E, which has roles in cell division, growth, and differentiation through the kinase cascade. The selection process was based on the label-free, real-time monitoring of binding interactions by surface plasmon resonance, allowing for convenient manipulation of the selection rounds. Next generation sequencing data provided four major motif families from which nine individual sequences were selected based on their abundance levels. Electrophoretic mobility shift assays revealed binding of the selected aptamers to the target protein without significant interference from fibroblast growth factor receptor binding protein 2, indicating the selectivity of the aptamers. The dissociation constant at equilibrium for the best aptamer candidate, SU-3, was found to be (28.2 ± 19.6) × 10^-9 M (n = 5) using a single-cycle kinetic analysis method. Advantages of the experimental setup and potential applications of the selected aptamers are discussed.

Direct quantification of cancerous exosomes via surface plasmon resonance with dual gold nanoparticle-assisted signal amplification

Sensitive detection of cancerous exosomes is critical to early diseases diagnosis and prognosis. Herein, a sensitive aptasensor was demonstrated for exosomes detection by surface plasmon resonance (SPR) with dual gold nanoparticle (AuNP)-assisted signal amplification. Dual nanoparticle amplification was achieved by controlled hybridization attachment of AuNPs resulting from electronic coupling between the Au film and AuNPs, as well as coupling effects in plasmonic nanostructures. By blocking the Au film surface with 11-Mercapto-1 -undecanol (MCU), nonspecific adsorption of AuNPs onto the SPR chip surface was suppressed and regeneration of the SPR sensor was realized. This method was highly sensitive and we have achieved the limit of detection (LOD) down to 5 × 10³ exosomes/mL, which showed a 10⁴-fold improvement in LOD compared to commercial ELISA. Moreover, the SPR sensor had the capability to differentiate the exosomes secreted by MCF-7 breast cancer cells and MCF-10A normal breast cells. Furthermore, the SPR sensor could effectively detect the exosomes in 30% fetal bovine serum. The work provides a sensitive and efficient quantification approach to detect cancerous exosomes and offers an avenue toward future diagnosis and comprehensive studies of exosomes.

Electrochemiluminescent aptasensor for thrombin using nitrogen-doped graphene quantum dots

An electrochemiluminescent (ECL) aptamer based method is described for the determination of thrombin. Three-dimensional nitrogen-doped graphene oxide (3D-NGO) was placed on a glassy carbon electrode (GCE) to provide an electrode surface that displays excellent electrical conductivity and acts as a strong emitter of ECL. The modified electrode was further coated with chitosan via electrodeposition. Finally, the amino-modified aptamer was immobilized on the modified GCE. The interaction between thrombin and aptamer results in a decrease in ECL. The assay has a linear response in the 1 fM to 1 nM thrombin concentration range and a 0.25 fM lower detection limit (at an S/N ratio of 3). The method was applied to the determination of thrombin in spiked human plasma samples, and recoveries ranged between 94 and 105% (with RSDs of <3.6%). The calibration plot was recorded at potential and wavelength of fluorescence emission (wavelength: 445 nm; potential: 0 to -2 V). Graphical abstract A bare glassy carbon electrode (GCE) does not display electrochemiluminescence (ECL). If, however, nitrogen-doped graphene quantum dots, chitosan, and three-dimensional nitrogen-doped graphene oxide (NGQD-chitosan/3D-NGO) are electrodeposited on the GCE, strong ECL can be observed. The ECL intensity decreased after aptamer and bovine serum albumin (BSA) were dropped onto the electrode (curve a). However, the ECL further decreases after addition of thrombin (TB; curve b).

Optimized polydopamine coating and DNA conjugation onto gold nanorods for single nanoparticle bioaffinity measurements

The formation of a stable polydopamine layer on a nanorod surface depends on the underlying chemistry and optimization enables the formation of packed ssDNA monolayers for bioaffinity applications.

Noble metal nanoparticles in biosensors: recent studies and applications

The aim of this review is to cover advances in noble metal nanoparticle (MNP)-based biosensors and to outline the principles and main functions of MNPs in different classes of biosensors according to the transduction methods employed. The important biorecognition elements are enzymes, antibodies, aptamers, DNA sequences, and whole cells. The main readouts are electrochemical (amperometric and voltametric), optical (surface plasmon resonance, colorimetric, chemiluminescence, photoelectrochemical, etc.) and piezoelectric. MNPs have received attention for applications in biosensing due to their fascinating properties. These properties include a large surface area that enhances biorecognizers and receptor immobilization, good ability for reaction catalysis and electron transfer, and good biocompatibility. MNPs can be used alone and in combination with other classes of nanostructures. MNP-based sensors can lead to significant signal amplification, higher sensitivity, and great improvements in the detection and quantification of biomolecules and different ions. Some recent examples of biomolecular sensors using MNPs are given, and the effects of structure, shape, and other physical properties of noble MNPs and nanohybrids in biosensor performance are discussed.

Enzyme Adsorption on Nanoparticle Surface Probed by Highly Sensitive Second Harmonic Light Scattering

Recent developments in second harmonic light scattering technique and the associated theoretical models have provided a deeper insight of molecular interactions on micro- and nanoparticle surfaces. This technique is extended to probe the thermodynamics of protein adsorption on nanoparticle surface which is crucial for understanding the fate of nanoparticle-based formulations in biomedical applications. A modified Langmuir adsorption model has been applied to extract the thermodynamic parameters from the experimental data. The general applicability of the technique is established by extracting free energy change, association constant, and binding stoichiometry of adsorption of a moderate size protein, alcohol dehydrogenase, and a small size protein, insulin, on gold nanoparticles. The free energy change for the adsorption is found to be of the order of -55kJ/mol, which indicates that the interaction of proteins with the nanoparticle surface involves weak forces. On the other hand, the low value of the free energy change makes the detachment of the protein from the particle surface easier and guarantees reversibility of the binding process. In addition, one gets the binding stoichiometry of the proteins with the nanoparticle surface which opens up the possibility of controlling the payload of the protein- or peptide-based therapeutics in future biomedical applications.

High-sensitive surface plasmon resonance microRNA biosensor based on streptavidin functionalized gold nanorods-assisted signal amplification

Herein, a facile and sensitive microRNA (miRNA) biosensor was designed by using interfacial biotinylated thiolated DNA molecular beacon (MB) as probe and streptavidin functionalized gold nanorods (Stre-GNRs) as tag for the enhanced surface plasmon resonance (SPR) signal. The MB probe with two terminals labeled with biotin and thiol groups, respectively, was modified on the gold film via thiol-gold interaction. Upon hybridization with the target, the biotinylated group became accessible to the Stre-GNRs. The introduction of the Stre-GNRs tag to the gold film produced strong SPR signal for detection. Our work has illustrated that the plasmonic field extension generated from the gold film to GNRs and the mass increase due to the GNRs have led to drastic sensitivity enhancement. Under optimal conditions, this proposed approach allowed detection of miRNA with the limit of detection (LOD) down to 0.045 pM. The results have shown that the MB probe functionalized sensing film, together with streptavidin-conjugated GNRs, was readily served as a plasmonic coupling partner that can be used as a powerful ultrasensitive sandwich assay for miRNA detection, and GNRs were readily served as promising amplification labels in SPR sensing technology.

Critical overview on the application of sensors and biosensors for clinical analysis

Sensors and biosensors have been increasingly used for clinical analysis due to their miniaturization and portability, allowing the construction of diagnostic devices for point-of-care testing. This paper presents an up-to-date overview and comparison of the analytical performance of sensors and biosensors recently used in clinical analysis. This includes cancer and cardiac biomarkers, hormones, biomolecules, neurotransmitters, bacteria, virus and cancer cells, along with related significant advances since 2011. Some methods of enhancing the analytical performance of sensors and biosensors through their figures of merit are also discussed.

Bioinspired Assemblies and Plasmonic Interfaces for Electrochemical Biosensing

Electrochemical biosensing represents a collection of techniques that may be utilized for capture and detection of biomolecules in both simple and complex media. While the instrumentation and technological aspects play important roles in detection capabilities, the interfacial design aspects are of equal importance, and often, those inspired by nature produce the best results. This review highlights recent material designs, recognition schemes, and method developments as they relate to targeted electrochemical analysis for biological systems. This includes the design of electrodes functionalized with peptides, proteins, nucleic acids, and lipid membranes, along with nanoparticle mediated signal amplification mechanisms. The topic of hyphenated surface plasmon resonance assays is also discussed, as this technique may be performed concurrently with complementary and/or confirmatory measurements. Together, smart materials and experimental designs will continue to pave the way for complete biomolecular analyses of complex and technically challenging systems.

Large area graphene nanomesh: an artificial platform for edge-electrochemical biosensing at the sub-attomolar level

Recent advances in large area graphene growth have led to tremendous applications in a variety of areas. The graphene nanomesh with its tunable band-gap is of great interest for both fundamental research, to explore the effect of edges on both the 2D electrical conduction and its electrochemical behavior, and applications such as nanoelectronic devices or highly sensitive biosensors. Here, we report on the fabrication of a large surface graphene nanomesh by nanoimprint lithography (NIL) to produce controlled artificial edges. The electrochemical response of this high quality single graphene layer imprinted nanomesh shows an enhancement in capacitance associated with faster electron transfer which can be attributed to the high density of edges. The electrochemical performances of this nanomesh graphene platform have been also studied for label-free DNA detection from Hepatitis C virus as a model. We demonstrate that such a nanomesh platform allows direct detection at the sub-attomolar level with more than 90% of molecules located on the imprinted artificial edges. Such a graphene nanomesh electrode will find useful future applications in the field of biosensing.

Nanotechnology-Based Surface Plasmon Resonance Affinity Biosensors for In Vitro Diagnostics

In the last decades, in vitro diagnostic devices (IVDDs) became a very important tool in medicine for an early and correct diagnosis, a proper screening of targeted population, and also assessing the efficiency of a specific therapy. In this review, the most recent developments regarding different configurations of surface plasmon resonance affinity biosensors modified by using several nanostructured materials for in vitro diagnostics are critically discussed. Both assembly and performances of the IVDDs tested in biological samples are reported and compared.

Visual detection of thrombin using a strip biosensor through aptamer-cleavage reaction with enzyme catalytic amplification

A new class of strip biosensors has been established based on well-distributed thrombin aptamer-linked gold nanoparticle aggregates, which will undergo a cracking reaction when the target recognizes its homologous aptamer. Combining the aptamer-cleavage reaction with the enzyme catalytic amplification system, our proposed lateral flow strip biosensor (LFB) is capable of visually detecting 6.4 pM of thrombin without instrumentation within 12 minutes. Under the optimal conditions, the quantitative detection of thrombin by a portable strip reader exhibited a linear relationship between the peak area and the concentration of thrombin in the range of 6.4 pM-500 nM with a detection limit of 4.9 pM, which is three orders of magnitude lower than that of the aptamer-functionalized gold nanoparticle-based LFB (2.5 nM, Xu et al., Anal. Chem., 2009, 81, 669-675). As the aptamers have no special requirements and the gold nanoparticles can also be replaced by other metallic nanoparticles, this method for strip sensing is expected to be generally applicable in point of care testing, home testing, medical diagnostics, clinical diagnosis, and environmental monitoring.

Magnetic Optical Microarray Imager for Diagnosing Type of Diabetes in Clinical Blood Serum Samples

Due to rapidly rising rates of diabetes and prediabetic conditions worldwide and the associated lethal complications, it is imperative to devise new diagnostic tools that reliably and directly measure insulin levels in clinical samples. Herein, we report a simple and sensitive direct imaging of insulin levels in diabetic patient samples using a surface plasmon resonance microarray imager (SPRi). To enhance sensitivity, we utilized magnetic nanoparticles (MNPs) to capture insulin from serum samples either directly or via a capture antibody immobilized on MNPs. The insulin-captured nanoparticles were allowed to bind surface insulin-antibody for detection from pixel intensity increase using a charge coupled device (CCD) built-in with the SPRi. We have compared the analytical figures-of-merit of the SPRi immunoarray on detecting insulin prepared in various percentages of serum solutions. A four parameter logistic model was used to obtain the best fit of microarray responses with insulin concentration and indicated the cooperative binding of insulin-nanoparticle conjugates to surface antibody in both the buffer insulin and the serum insulin conjugates with MNPs. The cooperativity effect is attributed to the greater association of magnetic nanoparticle-bound insulin molecules with increasing concentration of insulin binding to surface antibody. This is the first report of an SPRi immunoarray to accomplish clinical diagnosis of diabetic and prediabetic conditions based on insulin levels with serum matrix effect analysis and comparison between direct and sandwich insulin assay formats.

Investigation of biological cell–small molecule interactions with a gold surface plasmon resonance sensor using a laser scanning confocal imaging-surface plasmon resonance system

In our work, we investigated the interactions between a small molecule, folic acid, and biological cells through the interaction of folic acid and folate receptors using a laser scanning confocal imaging-surface plasmon resonance (LSCI-SPR) system.

Gel electrophoretic analysis of differently shaped interacting and non-interacting bioconjugated nanoparticles

Gel electrophoresis is demonstrated for monitoring bioaffinity interactions between protein-functionalized nanoparticles featuring different shapes as well as for particle separation.

Visual multiple recognition of protein biomarkers based on an array of aptamer modified gold nanoparticles in biocomputing to strip biosensor logic operations

We developed a strip biosensors array based on aptamer-modified gold nanoparticles as receptors and combined the protein-aptamer binding reaction with the streptavidin-biotin interaction as well as the sandwich format. We found that a series of protein receptors obtained a distinct response pattern to each target protein. Three proteins have been well distinguished with the naked eyes and a portable reader without mutual interference, accompanying with lower limit of detection and wider linear range. A complete set of four elementary logic gates (AND, OR, INH, and NAND) and eight combinative logic gates (AND-OR; AND-INH; OR-INH; INH-NAND; AND-OR-INH; AND-INH-NAND; OR-INH-NAND; AND-OR-INH-NAND) are thoroughly realized using this array, which could eventually be applicable to the keypad-lock system with enhanced complexity in the near future. Moreover, this array shows excellent linear relationships, anti-interference capability, real human serum samples applicability, long-term storage stability and reproducibility. All indicate that this design has very good prospects for development.

Aptamers-based sandwich assay for silver-enhanced fluorescence multiplex detection

In this work, aptamers-modified silver nanoparticles (AgNPs) were prepared as capture substrate, and fluorescent dyes-modified aptamers were synthesized as detection probes. The sandwich assay was based on dual aptamers, which was aimed to accomplish the highly sensitive detection of single protein and multiplex detection of proteins on one-spot. We found that aptamers-modified AgNPs based microarray was much superior to the aptamer based microarray in fluorescence detection of proteins. The result shows that the detection limit of the sandwich assay using AgNPs probes for thrombin or platelet-derived growth factor-BB (PDGF-BB) is 80 or 8 times lower than that of aptamers used directly. For multiplex detection of proteins, the detection limit was 625 pM for PDGF-BB and 21 pM for thrombin respectively. The sandwich assay based on dual aptamers and AgNPs was sensitive and specific.

Direct Detection of α-1 Antitrypsin in Serum Samples using Surface Plasmon Resonance with a New Aptamer-Antibody Sandwich Assay

The challenges associated with performing surface plasmon resonance (SPR) based measurements in serum and other biofluids have continued to limit the applicability of this valuable sensing technology for sensitive bioaffinity measurements of proteins in clinically relevant samples. In this paper, a new sandwich assay is introduced for the quantitative SPR analysis of α-1 antitrypsin (AAT), which is a recognized biomarker for Alzheimer's disease. Detection was performed via the specific adsorption of AAT onto a gold chip surface modified with a DNA aptamer. The measurement dynamic range and also sensitivity in serum were improved with the subsequent surface binding of antiAAT. A methodology was established to measure the target protein in serum, albumin and immunoglobulin G (IgG) solutions with the results correlated with measurements in buffer only. A comparison between SPR and enzyme-linked immunosorbent assay (ELISA) measurements was also made. The detection of AAT in serum at clinically relevant concentrations was demonstrated with target concentrations as low as 10 fM readily achievable.

Target catalyzed hairpin assembly for constructing a ratiometric electrochemical aptasensor

In this paper, we develop a novel dual-signaling amplified aptasensor for protein detection via target-catalyzed hairpin assembly. Thrombin was chosen as a model target. This aptasensor contains two DNA hairpins termed as H1 and H2. H1, which is modified at its 3' ends with a methylene blue (MB), consists of the aptamer sequence of human thrombin. Meanwhile, H2 which is modified at its 3' ends with a ferrocene (Fc), is partially complementary to H1. Upon the addition of target protein, it can facilitate the opening of the hairpin structure of H1 and thus accelerate the hybridization between H1 and H2, the target protein can be displaced from hairpin H1 by hairpin H2 through a process similar to DNA branch migration. The released target found another H1 to trigger the cycle, resulting in the multiplication of the Fc confined near the GE surface and MB away from the GE surface. When IFc/IMB is used as the response signal for quantitative determination of thrombin, the detection limit (41 fM) is much lower than that by using either MB or Fc alone. This new dual-signaling aptasensor is readily regenerated and shows good response toward the target. Furthermore, this amplified aptasensor shows high selectivity toward its target protein. The clever combination of the functional DNA hairpin and the novel device achieved a ratiometric electrochemical aptasensor, which could be used as a simple, sensitive high repeatability and selective platform for target protein detection.