Enzyme-free and label-free fluorescence aptasensing strategy for highly sensitive detection of protein based on target-triggered hybridization chain reaction amplification

UIO66 low background signal and fluorescence synergism strategy for highly sensitive detection of Salmonella typhimurium

Successful construction of a detection method for Salmonella typhimurium (S. typhimurium) based on the synergy of hybridization chain reaction (HCR) and fluorescence was realized in this paper. First, the aptamer modified with the quenching group Black Hole Quencher-1 acid (BHQ1) was immobilized on the magnetic beads in combination with the complementary chain of the aptamer modified with 6-carboxyfluorescein (6-FAM). Second, S. typhimurium and cDNA-6-FAM immobilized on magnetic beads competitively bound to the aptamer. Finally, the cDNA-6-FAM was released after magnetic separation acted as a promoter to trigger HCR amplification when the target presented. The fluorescence signal could be significantly improved by the combination of green SYBR Green I (SGI) and HCR long double-stranded DNA and the fluorescent synergy of 6-FAM and SGI. Because of the separation of target and its aptamer, the trigger strand was abstracted by magnetic separation. There was no HCR to generate long double-stranded DNA, and the fluorescence of excess hairpin/SGI could be adsorbed through UIO66 so that only a very low background signal was detected. This fluorescent sensor was capable of monitoring S. typhimurium in the range of 10-3.2 × 107 CFU mL-1 with a limit of detection as low as 1.5 CFU mL-1. Because of the excellent properties of the aptasensor and the validity of SGI fluorescence synergy, this HCR enzyme-free amplification strategy could be generalized to other areas.

Soft interface confined DNA walker for sensitive and specific detection of SARS-CoV-2 variants

Nucleic acid detection is conducive to preventing the spread of COVID-19 pandemic. In this work, we successfully designed a soft interface confined DNA walker by anchoring hairpin reporter probes on cell membranes for the detection of SARS-CoV-2 variants. In the presence of target RNA, the cyclic self-assembly reaction occurred between hairpin probes H1 and H2, and the continuous walking of target RNA on cell membranes led to the gradual amplification of fluorescence signal. The enrichment of H1 on membranes and the unique fluidity of membranes promoted the collision efficiency between DNA strands in the reaction process, endowing this method with high sensitivity. In addition, the double-blind test of synthetic RNA in 5% normal human serum demonstrated the good stability and anti-interference in complex environment of this method, which exhibited great potential in clinical diagnostics.

DNA framework signal amplification platform-based high-throughput systemic immune monitoring

Systemic immune monitoring is a crucial clinical tool for disease early diagnosis, prognosis and treatment planning by quantitative analysis of immune cells. However, conventional immune monitoring using flow cytometry faces huge challenges in large-scale sample testing, especially in mass health screenings, because of time-consuming, technical-sensitive and high-cost features. However, the lack of high-performance detection platforms hinders the development of high-throughput immune monitoring technology. To address this bottleneck, we constructed a generally applicable DNA framework signal amplification platform (DSAP) based on post-systematic evolution of ligands by exponential enrichment and DNA tetrahedral framework-structured probe design to achieve high-sensitive detection for diverse immune cells, including CD4+, CD8+ T-lymphocytes, and monocytes (down to 1/100 μl). Based on this advanced detection platform, we present a novel high-throughput immune-cell phenotyping system, DSAP, achieving 30-min one-step immune-cell phenotyping without cell washing and subset analysis and showing comparable accuracy with flow cytometry while significantly reducing detection time and cost. As a proof-of-concept, DSAP demonstrates excellent diagnostic accuracy in immunodeficiency staging for 107 HIV patients (AUC > 0.97) within 30 min, which can be applied in HIV infection monitoring and screening. Therefore, we initially introduced promising DSAP to achieve high-throughput immune monitoring and open robust routes for point-of-care device development.

Robust tag-free aptasensor for monitoring of tobramycin: Architecting of rolling circle amplification and fluorescence synergism

With the unpredictable risks on human health and ecological safety, tobramycin (TOB) as an extensively applied antibiotic has embraced global concern. Herein, a label-free fluorescent aptasensor was developed that opened up an innovative sensing strategy for monitoring trace TOB levels. Based on the rolling circle amplification (RCA) process, a giant DNA building was established by the catalytic action of T4 DNA ligase and Phi 29 DNA polymerase with the cooperation of the specific aptamer as a primer skeleton. By having the role of signal amplifier template, the RCA product with the G-quadruplex sequence duplications was decorated by a high number of the thioflavin T (ThT) fluorescent dyes. The aptasensor with good selectivity toward TOB achieved a detection limit as low as 150 pM. Thanks to its accurate target quantification, ease of operation, economic manufacture, as well as high potency for real-time and point-of-care testing, the represented aptasensor is superb for clinical application and food safety control.

Interactions of Amino Group Functionalized Tetraphenylvinyl and DNA: A Label-Free "On-Off-On" Fluorescent Aptamer Sensor toward Ampicillin

As a type of aggregation-induced emission (AIE) fluorescent probe, tetraphenylvinyl (TPE) or its derivatives are widely used in chemical imaging, biosensing and medical diagnosis. However, most studies have focused on molecular modification and functionalization of AIE to enhance the fluorescence emission intensity. There are few studies on the interaction between aggregation-induced emission luminogens (AIEgens) and nucleic acids, which was investigated in this paper. Experimental results showed the formation of a complex of AIE/DNA, leading to the quenching of the fluorescence of AIE molecules. Fluorescent test experiments with different temperatures proved that the quenching type was static quenching. The quenching constants, binding constants and thermodynamic parameters demonstrated that electrostatic and hydrophobic interactions promoted the binding process. Then, a label-free "on-off-on" fluorescent aptamer sensor for the detection of ampicillin (AMP) was constructed based on the interaction between the AIE probe and the aptamer of AMP. Linear range of the sensor is 0.2-10 nM with a limit of detection 0.06 nM. This fluorescent sensor was applied to detect AMP in real samples.

A tag-free fluorescent aptasensor for tobramycin detection using a hybridization of three aptamer strands and SYBR Green I dye

In this study, a sensitive fluorescent method is designed to detect tobramycin (TOB) drug applying a hybrid structure of three aptamer strands and SYBR Green I (SGI) fluorescent dye as the bioreceptor segment and signal indicator, respectively. The preferential binding of the aptamers to TOB resulted in the collapse of the hybridized aptamer skeleton to the single strands. So, the intercalation of SGI molecules reduced that quenched the fluorescence response. The aptasensing assay provided the superior target specificity with a detection limit (LOD) of 0.153 pM and a wide linear dynamic range over 0.5 pM-300 μM. The aptasensor could successfully quantify TOB in human serum samples. The tag-free sensor with the remarkable advantages of simplicity, easy-to-use, cost-effectiveness, and high sensitivity is superior to be applicable for clinical samples.

Aptamer-Based Probes for Cancer Diagnostics and Treatment

Aptamers are single-stranded DNA or RNA oligomers that have the ability to generate unique and diverse tertiary structures that bind to cognate molecules with high specificity. In recent years, aptamer researches have witnessed a huge surge, owing to its unique properties, such as high specificity and binding affinity, low immunogenicity and toxicity, and simplicity of synthesis with negligible batch-to-batch variation. Aptamers may bind to targets, such as various cancer biomarkers, making them applicable for a wide range of cancer diagnosis and treatment. In cancer diagnostic applications, aptamers are used as molecular probes instead of antibodies. They have the potential to detect various cancer-associated biomarkers. For cancer therapeutic purposes, aptamers can serve as therapeutic or delivery agents. The chemical stabilization and modification strategies for aptamers may expand their serum half-life and shelf life. However, aptamer-based probes for cancer diagnosis and therapy still face several challenges for successful clinical translation. A deeper understanding of nucleic acid chemistry, tissue distribution, and pharmacokinetics is required in the development of aptamer-based probes. This review summarizes their application in cancer diagnostics and treatments based on different localization of target biomarkers, as well as current challenges and future prospects.

Allosteric aptasensor-initiated target cycling and transcription amplification of light-up RNA aptamer for sensitive detection of protein

The early detection of biomarker proteins in clinical samples is of great significance for the diagnosis of diseases. However, it is still a challenge to detect low-concentration protein. Herein, a label-free aptamer-based amplification assay, termed the ATC-TA system, that allows fluorescence detection of very low numbers of protein without time-consuming washing steps and pre-treatment was developed. The target induces a conformational change in the allosteric aptasensor, triggers the target cycling and transcription amplification, and ultimately converts the input of the target protein into the output of the light-up aptamer (R-Pepper). It exhibits ultrahigh sensitivity with a detection limit of 5.62 fM at 37 ℃ and the accuracy is comparable to conventional ELISA. ATC-TA has potential application for the detection of endogenous PDGF-BB in serum samples to distinguish tumor mice from healthy mice at an early stage. It also successfully detects exogenous SARS-CoV-2 spike proteins in human serum. Therefore, this high-sensitive, universality, easy-to-operate and cost-effective biosensing platform holds great clinical application potential in early clinical diagnosis.

Label-free ratiometric homogeneous electrochemical aptasensor based on hybridization chain reaction for facile and rapid detection of aflatoxin B1 in cereal crops

Aflatoxin B1 (AFB1) contamination has raised global concerns in agricultural and food industry; thus, sensitive, accurate and rapid AFB1 sensors are essential in many circumstances. Herein, we developed a label-free and immobilization-free ratiometric homogeneous electrochemical aptasensor based on hybridization chain reaction (HCR) for facile and rapid determination of AFB1. Methylene blue (MB) and ferrocene (Fc) were used as label-free probes to produce a response signal (I_MB) and a reference signal (I_Fc) in solution phase, respectively. The ratio of I_MB/I_Fc was used as a yardstick to quantify AFB1. HCR was exploited to enlarge the intensity of I_MB as well as ratiometric signal. By combining label-free homogeneous assay and ratiometric strategy, the resulting aptasensor offered sensitive, rapid, and reliable determinations of AFB1 with a detection limit of 38.8 pg mL^-1. The aptasensor was then used to determine AFB1 in cereal samples with comparable reliability as HPLC-MS.

Simultaneous Determination of Tetrodotoxin in the Fresh and Heat-Processed Aquatic Products by High-Performance Liquid Chromatography-Tandem Mass Spectrometry

Tetrodotoxin (TTX) was simultaneously detected in the fresh and heat-processed aquatic products by high-performance liquid chromatography-tandem mass spectrometry method. The detection conditions were investigated, including the chromatography column and mobile phase. Based on the optimized parameters, a sensitive determination method of TTX was established. The proposed method featured the merits of a good linear relationship between signal and TTX concentration (R2 = 0.9998), a wide detection matrix-based range of 0.2-100 ng/g, and a low detection limit of 0.2 ng/g, etc. The spiked assays evidenced its accuracy and reliability with recoveries of 90.5-107.2%. Finally, the developed method was simultaneously successfully applied in the determination of TTX in various fresh and heat-processed aquatic products.

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.

Graphene Oxide Biosensors Based on Hybridization Chain Reaction Signal Amplification for Detecting Biomarkers of Radiation-Resistant Nasopharyngeal Carcinoma and Imaging in Living Cells

Since microRNA-205 (miRNA-205) is a predictive biomarker for antiradiation of nasopharyngeal carcinoma (NPC), quantitative detection of miRNA-205 is important for developing personalized strategies for the treatment of NPC. In this investigation, based on the graphene oxide (GO) sensor and hybridization chain reaction (HCR) for fluorescence signal amplification, a highly sensitive and selective detection method for miRNA-205 was designed. A target-recycling mechanism is employed, where a single miRNA-205 target triggers the signal amplification of many DNA signal probes. The biosensor shows the ability to analyze miRNA-205 in solution, and it can detect miRNA-205 at concentrations as low as 311.96 pM. Furthermore, the method is specific in that it distinguishes between a target miRNA and a sequence with single-, double-, and three-base mismatches, as well as other miRNAs. Considering its simplicity and superior sensitivity, it was also verified in 1‰ serum with a detection limit of 111.65 pM. Importantly, the method successfully demonstrated that miRNA-205 could be imaged in living cells, which provided the possibility of localizing target molecules in live cell imaging applications. This method has great clinical application potential in the determination of miRNA-205, a biomarker for radiation-resistant NPC.

Target-induced activation of polymerase activity for recycling signal amplification cascades for sensitive aptamer-based detection of biomarkers

It is of great importance to develop biosensing methods for the sensitive and selective analysis of biomarkers at very low levels in biological samples. Using a new target-induced activation of the DNA polymerase activity for recycling amplification cascades, we describe an aptamer-based method for highly sensitive detection of platelet-derived growth factor BB (PDGF-BB) in human serums. The polymerase activity is initially inhibited by the binding of the polymerase to the enzyme aptamer sequence. PDGF-BB associates with and switches a PDGF-BB binding aptamer to trigger the release of an active polymerase, which further initiates the simultaneous recycling of the target PDGF-BB molecules and the enzyme aptamer sequence for the subsequent displacement of the fluorescently quenched probes to recover the fluorescence. Due to two recycling cascades, substantial fluorescence magnification is obtained for the highly sensitive detection of PDGF-BB with a low detection limit of 5.1 pM. Moreover, the potential applicability of this method for real samples was verified by determining PDGF-BB in diluted human serums, relying on the excellent specificity and selectivity of the aptamer. The demonstration of the PDGF-BB assay method here thus can be expanded for the construction of diverse sensing platforms for detecting different trace biomarkers with the integration of an elaborate design of the aptamer probes.

Entropy-driven catalytic amplification adjusted by stoichiometry for single-nucleotide variants detection with high abundance sensitivity

Single-nucleotide variants (SNV) detection with high abundance sensitivity is of great significance in clinical application, molecular diagnostics and biological research. In this study, a high abundance sensitivity SNV detection strategy based on entropy-driven catalytic (EDC) amplification adjusted by stoichiometry is proposed. In EDC, the toehold exchange reaction is used to initiate subsequent catalytic reaction and can be adjusted by stoichiometry. When the by-product concentration in the toehold exchange reaction is excessive, the forward reaction will be inhibited, which can reduce or even block the unexpected reaction between the non-target and the probe. Meanwhile, some targets can still successfully take a toehold exchange reaction with the probe, thus completing the subsequent EDC. By adjusting the EDC, the SNV identification specificity of this system was improved and is superior to any single adjusted stoichiometry or EDC. When the low abundance target is detected from the mixture, this strategy enables SNV detection at 0.1% abundance with high abundance sensitivity. And even if the mixture contains three kind of 1000-fold interference sequences, this strategy can still discriminate the target SNV. Furthermore, the practical applicability of the adjusted EDC system was verified by p53 mutation discrimination in human urine.

Highly sensitive detection of carcinoembryonic antigen using copper-free click chemistry on the surface of azide cofunctionalized graphene oxide

In this study, we reported a highly sensitive method for detecting carcinoembryonic antigen (CEA) based on an azide cofunctionalized graphene oxide (GO-N₃) and carbon dot (CDs) biosensor system. Carbon dots-labeled DNA (CDs-DNA) combined with GO-N₃ using copper-free click chemistry (CFCC), which quenched the fluorescence of the CDs via fluorescence resonance energy transfer (FRET). Upon the addition of CEA, fluorescence was recovered due to the combination of CEA and aptamer. Under optimal conditions, the relative fluorescence intensity was linear with CEA concentration in the range of 0.01-1 ng/mL (R² = 0.9788), and the limit of detection (LOD) was 7.32 pg/mL (S/N = 3). This biosensor had a high sensitivity and good selectivity for CEA detection in serum samples, indicating that the novel sensor platform holds a great potential for CEA and other biomarkers in practical applications.

Fluorometric determination of agrA gene transcription in methicillin-resistant Staphylococcus aureus with a graphene oxide-based assay using strand-displacement polymerization recycling and hybridization chain reaction

A graphene oxide (GO)-based fluorescent bioassay was developed to quantify agrA gene transcription (its mRNA) in methicillin-resistant Staphylococcus aureus (MRSA). This method is based on the use of Klenow fragment (KF)-assisted target recycling amplification and hybridization chain reaction (HCR). A triple complex was designed that contained a capture probe (CP), a trigger probe (TP), and a help probe (HP), which were partially complementary to one another. In the absence of the target, all the oligonucleotides labeled with carboxyfluorescein (FAM) are adsorbed onto the surface of GO by π-stacking interactions. This adsorption quenches the FAM signal. On the contrary, the target RNA causes the triple complex to disintegrate and initiates strand-displacement polymerization reaction (SDPR) and HCR in the presence of the appropriate raw materials, including the primer, KF, dNTPs, hairpin 1 (H1), and hairpin 2 (H2), generating double-stranded DNA (dsDNA) products. These dsDNA products are repelled by GO and produce strong fluorescence, measured at excitation/emission wavelengths of 480/514 nm. The fluorescent signal is greatly amplified by SYBR Green I (SGI) due to the synergistic effect of dsDNA-SGI. The target was assayed with this method at concentrations in the range 10 fM to 100 pM, and the detection limit (LOD) was 10 fM. This method also displayed good applicability in the analysis of real samples. It provides a new way of monitoring biofilm formation and studying the mechanisms of drug actions. Graphical abstract Schematic representation of the graphene oxide-based fluorescent bioassay for agrA gene transcription in methicillin-resistant Staphylococcus aureus by using strand-displacement polymerization recycling and hybridization chain reaction.

Development of a highly sensitive detection method for TTX based on a magnetic bead-aptamer competition system under triple cycle amplification

We have established an assay that relies on aptamer and isothermal amplification for the tetrodotoxin (TTX)detection. The method uses triple cycle amplification (strand displacement amplification combined with catalytic hairpin assembly) and fluorescent reporter as an output signal. Free TTX and cDNA compete for binding to aptamer-modified magnetic beads. The cDNA collected by magnetic separation then used as a primer to trigger triple cycle amplification to obtain more ssDNA. The ssDNA combined with the reporter probe, and the original quenched fluorescence can be recovered. In addition, a linear relationship between fluorescence spectrum and different target concentrations is revealed. This method allows TTX to be detected by fluorometry with a detection limit as low as 0.265 pg mL^-1. It was applied to clams and shellfish, achieving recoveries ranging from 100% to 107.33% and 99.67%-116.67%, respectively. The results were consistent with the commercial TTX ELISA kit. This assay is highly sensitive, reliable and has a good specificity. Therefore, it provides a better alternative to the standard method for quantitative detection of TTX.

DNA branched junctions induced the enhanced fluorescence recovery of FAM-labeled probes on rGO for detecting Pb2

The reduced graphene oxide (rGO) could strongly adsorb and quench the fluorescence of dye-labeled single-stranded DNA (ssDNA); thus, it is widely applied in fluorescent sensors. However, these sensors may suffer from a limited sensitivity due to the low fluorescence recovery when adding the complementary DNA (cDNA) sequence. In this work, the powerful DNA branched junctions were constructed to improve the fluorescence recovery of FAM-labeled probe on rGO. In the presence of target Pb²⁺, the ribonucleotide (rA) in the substrate was cleaved specifically and the catalytic hairpin assembly of three metastable hairpins was further initiated, accompanied by the formation of DNA branched junctions. Then, the liberated Pb²⁺ could be recyclable. Impressively, the DNA branched junctions not only hybridize with the FAM-labeled probes with a high efficiency, but also are significantly undesirable for the rGO. Thus, a high fluorescence recovery of FAM-labeled probe on rGO was expected. The integration of the high fluorescence recovery and dual-cycle signal amplification endows the sensing strategy with a good performance for Pb²⁺ detection, including low detection limit (0.17 nM), good selectivity, and satisfactory practical applicability. The proposed DNA branched junctions offer a novel avenue to improve the fluorescence recovery of the dye-labeled probes on rGO for biological analysis.

A novel nest hybridization chain reaction based electrochemical assay for sensitive detection of circulating tumor DNA

As an ideal biomarker candidate, circulating tumor DNA (ctDNA) plays a vital role in noninvasive diagnosis of cancer. However, most traditional approaches for quantifying ctDNA are cumbersome and expensive. In the present work, a novel electrochemical biosensor based on nest hybridization chain reaction was proposed for the sensitive and specific detection of PIK3CA E545K ctDNA with a simple process. The nest hybridization chain reaction was initiated by the hybridization of two dumbbell-shaped DNA units which were assembled by two classes of well-designed DNA probes respectively, leading to the formation of a complex DNA structure. In the presence of target ctDNA, the amplified hybridization chain reaction products were captured by target ctDNA, resulting in a significant increase of electrochemical signal. Under the optimal conditions, the developed biosensor exhibited good analytical performance for the detection of target ctDNA with the linear range from 5 pM to 0.5 nM and the detection limit of 3 pM. Furthermore, this assay was successfully applied to the detection of ctDNA in spiked-in samples, pleural effusion and serum samples of malignant tumor patients. This simple and cost-effective sensing system holds great potentials for ctDNA detection and cancer diagnosis.

Affinity binding-mediated fluorometric protein assay based on the use of target-triggered DNA assembling probes and aptamers labelled with upconversion nanoparticles: application to the determination of platelet derived growth factor-BB

The target-triggered DNA assembling probe is presented for highly selective protein detection. Target-triggered DNA assembling is used in an amplification strategy based on affinity binding for identification and determination of proteins in general. Specifically, it was applied to the platelet derived growth factor-BB (PDGF-BB). A hairpin DNA (H-DNA) probe was designed containing (a) an aptamer domain for protein recognition and (b) a blocked DNAzyme domain for DNAzyme cleavage. An assistant DNA (A-DNA) probe containing aptamer and complementary domains was also employed to recognize protein and to induce DNA assembly. Once H-DNA and A-DNA recognize the same protein, H-DNA and A-DNA are in close proximity to each other. This induces DNA assembling for protein-triggered complex (Protein-Complex) with free DNAzyme domains. The free DNAzymes trigger the circular cleavage of molecular beacons for amplified signals. The assay is performed by fluorometry at an excitation wavelength of 980 nm and by collecting fluorescence at 545 nm. The platelet derived growth factor-BB (PDGF-BB) was accurately identified and selectively determined by this assay with a 22 pM detection limit (using the 3σ criterion). The responses for PDGF-BB is nearly 6-fold higher than for PDGF-AB, and 16-fold higher than PDGF-AA. This upconversion assay avoids any interference by the autofluorescence of biological fluids. Graphical abstractSchematic representation of the principle of the target-triggered DNA assembling probes mediated amplification strategy based on affinity binding for PDGF-BB. The UCNP probe is used for the quantitation of PDGF-BB with high selectivity.

A tetrahedral DNA nanoflare for fluorometric determination of nucleic acids and imaging of microRNA using toehold strands

The authors describe a tetrahedral DNA nanostructure loaded with SYBR Green (SG-TDN) for fluorometric determination of nucleic acids. After intercalating into the TDN, fluorescence of SG is enhanced by 260-fold (exc 480 nm, em 524 nm), and the resulting SG-TDN nanoflare displays >7-fold stronger fluorescence than that of FAM-labeled TDN. The SG-TDNs were coupled to magnetic microparticles and polydopamine nanoparticles to construct multi-functional nanoprobes through sequence hybridization using a toehold strand. The method was applied to detect a stretch of microRNA sequence (20 bp) in buffer and in undiluted serum with excellent selectivity, over a wide linear range and with a low limit of detection (0.2 nM). The probe was also applied for visualization of tumor-related microRNA in living cells via fluorescence imaging. Graphical abstract Schematic representation of tetrahedron-based DNA nanoflare for fluorometric nucleic acid determination in undiluted blood serum and living cells.

A Multicolor Fluorescence Nanoprobe Platform Using Two-Dimensional Metal Organic Framework Nanosheets and Double Stirring Bar Assisted Target Replacement for Multiple Bioanalytical Applications

Multicolor fluorescence probes can show fluorescence of different colors when detecting different targets, and the excellent feature can create a highly differentiated multicolor sensing platform. However, most of the previously reported multicolor luminescent materials usually suffer from high toxicity and photobleaching, complex preparation procedures, and poor water solubility, which may not be conducive to bioanalytical applications. Two-dimensional metal organic frameworks (2D MOFs), which have large specific surface areas with long-range fluorescence quenching coupled with biomolecular recognition events, have encouraged innovation in biomolecular probing. Here, we propose a 2D-MOF-based multicolor fluorescent aptamer nanoprobe using a double stirring bar assisted target replacement system for enzyme-free signal amplification. It utilizes the interaction between 2D MOFs and DNA molecules to detect multiple antibiotics quickly, sensitively, and selectively. Since 2D MOFs have excellent quenching efficiency for luminescence of fluorescent-dye-labeled single-strand DNA (ssDNA), the background fluorescence can be largely reduced and the signal-to-noise ratio can be improved. When the adsorbed ssDNA formed double helix double-stranded DNA with its complementary ssDNA, its fluorescence can be almost fully recovered. The assay was tested by detecting chloramphenicol (CAP), oxytocin (OTC), and kanamycin (KANA) in biological samples. The developed aptasensor was sufficiently sensitive to detect the antibiotic residues as low as 1.5 pM CAP, 2.4 pM OTC, and 1 pM KANA (S/N = 3). It has been preliminarily used for multicolor imaging of three different antibiotics in fish tissue slices with satisfactory results.

Magnetic-assisted self-assembled aptamer/protein hybrid probes for efficient capture and rapid detection of cancer cells in whole blood

Self-assembly of building blocks for constructing multifunctional materials has opened prospects for sensing applications in the biomedical fields. In particular, the combination of aptamer with DNA assembly-based nanotechnology has greatly improved the performance of cancer cell detection. Nevertheless, the cancer cell detection strategies of integrating aptamer with protein are relatively sparse. So we have developed a self-assembled aptamer method to realize the efficient capture and rapid detection of cancer cells by ingeniously combining aptamer modified magnetic nanoparticles as capture nanoprobes with self-assembled aptamer/protein hybrid probes (SAPPs) as signal amplification probes. By merely mixing the component materials together simultaneously, the SAPPs, integrating aptamer for cancer cell recognition with protein for amplifying signal, were fabricated by DNA-governed one-step assembly. In addition, the SAPPs-based method exhibits efficient capture, rapid (about 45 min) and specific CCRF-CEM detection performance, with limits of detection down to 75 cells/mL in buffer and 200 cells/mL in whole blood.

A hybridization chain reaction coupled with gold nanoparticles for allergen gene detection in peanut, soybean and sesame DNAs

Food allergy is an abnormal immune response of the immune system to some foods, which has caused great harm to people's health. Therefore, it is particularly important to detect allergens in food. In this article, a hybridization chain reaction (HCR) was coupled with gold nanoparticles (AuNPs) to detect the allergen genes of peanut, soybean and sesame DNAs. Two hairpin probes (H1 and H2) were designed for the allergen target genes of peanut, soybean and sesame DNAs. In the presence of target DNA, the hybridization chain reaction was triggered by it producing long double-stranded DNA (dsDNA) products. In the gold nanoparticle system, long dsDNA couldn't be adsorbed on the surface of AuNPs. When the concentration of salt ions in the solution increased, gold nanoparticles accumulated and led to a decrease of ultraviolet absorption. In the absence of target DNA, no hybridization chain reaction occurred. The hairpin probes could be adsorbed on the surface of AuNPs and no accumulation happened for gold nanoparticles even if the concentration of salt ions in the solution was increased. This method required no enzymes and had a strong specificity, so it was very easy to distinguish target DNA from non-target DNA. The detection limit of three allergens detected by this method was as low as 0.5 nM. The feasibility of this method for the detection of commercial commodities had been demonstrated by the successful detection of the DNAs extracted from commercial commodities, which were treated with extreme thermostable single-stranded binding protein (ET SSB).

Colorimetric aggregation assay for kanamycin using gold nanoparticles modified with hairpin DNA probes and hybridization chain reaction-assisted amplification

The authors describe a colorimetric method for determination of kanamycin by using gold nanoparticles (AuNPs) as the element of signal-conversion and by applying hybridization chain reaction-assisted signal amplification. The assay is carried out by monitoring the absorbance change and color change adding salt to the reaction solution containing kanamycin (analyte), hairpin DNA probe, and AuNPs. Three hairpin DNA probes with sticky ends were absorbed on the AuNPs via their sticky ends. Cating with DNA prevents them from salt-induced aggregation (which leads to a color change from red to blue) in the complete absence of kanamycin. In contrast, in the presence of kanamycin, the aptamer hairpin DNA probe binds kanamycin, and the newly exposed section of DNA triggers a cascade of hybridization chain reactions with formation of numerous dsDNAs. On addition of salt, the AuNPs form blue aggregates due to the repulsion between dsDNA and AuNPs. Under optimal conditions, the ration of absorbance at 520 and 630 nm drops with the kanamycin concentration in the range from 1 to 40 μM, and the limit of detection is 0.68 μM. The assay can selectively distinguish kanamycin from other antibiotics. The method was applied to kanamycin detection in (spiked) milk samples and gave excellent recoveries. Graphical abstract Schematic presentation of colorimetric method for kanamycin detection using gold nanoparticles modified with hairpin DNA probes and hybridization chain reaction-assisted amplification.

A highly sensitive and low-background fluorescence assay for pesticides residues based on hybridization chain reaction amplification assisted by magnetic separation

Due to the concern over food safety, it is important to detect the pesticides residues in agricultural products. Here, a highly sensitive and low background fluorescent strategy for the detection of pesticides residues has been developed. The fluorescence intensity of N-methyl mesoporphyrin IX (NMM) binding G-quadruplex could be turn off because of inhibiting effect of the pesticides on the acetylcholinesterase (AChE) activity. For that, four single-stranded DNAs (named linker, trigger, H1 and H2, respectively) are rational designed and T-Hg-T mismatches duplex DNAs as a recognizer combined with the separation of magnetic beads. The design of hybridization chain reaction (HCR) amplification strategy assisted by magnetic separation has been adopted to improve the detection sensitivity. In the presence of pesticides, the amount of the thiol group generated by hydrolysis reaction of acetylcholine (ACh) is reduced, lead to release of less trigger DNA. Therefor subsequent HCR process is retarded with decreased fluorescence intensity. The reduced fluorescence intensity has a quantitative relationship with the pesticide concentration. The limit of detection of chlorpyrifos was estimated to be 2.0 ng ml-1. It has been applied to detect the pesticides residues in real samples.

High-sensitive immunosensing of protein biomarker based on interfacial recognition-induced homogeneous exponential transcription

A novel and versatile immunosensing strategy was developed for ultrasensitive and specific detection of proteins by organically integrating interfacial specific target recognition and homogeneous transcription amplification. In principle, classic antigen-antibody sandwich structure on the microplate could realize the specific identification of target protein. Biotinylated DNA probe was subsequently introduced by streptavidin-biotin system as a bridge linking interfacial and homogeneous reaction. The biotinylated DNA initiated exponential transcription amplification in the solution, which converted per target recognition event on the interface to numerous single-stranded RNA products in solution for highly sensitive fluorescence immunosensing. The proposed immunoassay based on interfacial recognition-induced homogeneous exponential transcription (IR-HET) for vascular endothelial growth factor (VEGF) detection showed a good linear range from 0.01 to 1000 pg/mL and the limit of detection as low as 1 fg/mL, which was 3 orders lower than traditional ELISA method. The established strategy was also successfully applied to directly detect VEGF from culture supernatants of tumor cells and clinical body fluid samples, proving very high sensitivity, selectivity and low matrix effect. Therefore, IR-HET-based immunosensing strategy might become a potential powerful tool be applied in ultrasensitive detection of low abundance protein biomarker for clinical early diagnosis, treatment and prognosis.

Development of a chemiluminescent aptasensor for ultrasensitive and selective detection of aflatoxin B1 in peanut and milk

More and more attention about food safety leads to a research hotspot to develop new detection methods for food contaminant. To address the problems of serious interference and low sensitivity, a chemiluminescent aptasensor for the detection of aflatoxin B1(AFB1) in food was developed in this paper. It is based on horseradish peroxidase (HRP) catalyze the luminol chemiluminescence reaction. The hybridization chain reaction (HCR) signal amplification strategy has been used to improve the detection sensitivity. Magnetic separation could further reduce background signal obviously at the same time. AFB1 as a model of analyte to test the capability of our developed assay system. Under the optimal experimental conditions, CL intensity showed a good linear correlation with the concentrations of AFB1 ranging from 0.5 to 40 ng mL^-1. The limit of detection was estimated 0.2 ng mL^-1 based on 3 times of the signal-to-noise ratio which is lower than those of the previously reported sensors. It could be used to detect AFB1 content in real samples, such as peanuts and milk which were purchased in local supermarket. The results proved that the sensing system has good anti-interference and selectivity. In all, it has potential for practical application in food safety field.

A fluorescent aptasensor based on single oligonucleotide-mediated isothermal quadratic amplification and graphene oxide fluorescence quenching for ultrasensitive protein detection

In this work, we have developed a novel fluorescent aptasensor based on single oligonucleotide-mediated isothermal quadratic amplification (SOIQA) and graphene oxide (GO)-mediated fluorescence quenching for the ultrasensitive detection of proteins in a homogeneous solution. The SOIQA consists of a fluorophore-labeled aptamer hairpin probe containing T7 exonuclease (T7 Exo)-resistant 5'-protruding termini and a mismatch base at its 3'-end, DNA polymerase, T7 Exo and GO. The target analyte binds with the aptamer sequences and unfolds the fluorophore-labeled aptamer hairpin probe to form a new DNA hairpin, inducing the catalytic recycling of the target analyte (assisted by DNA polymerase) and DNA sequences (aided by T7 Exo) to achieve SOIQA, which results in the digestion of numerous fluorophore-labeled aptamer hairpin probes and the generation of a large amount of mononucleotides carrying the fluorophore. These mononucleotide products cannot be adsorbed onto the GO, leading to a dramatic increase in the fluorescence intensity for the amplified detection of the target molecules. In the absence of the target analyte, however, the SOIQA reaction is inhibited and the fluorophore-labeled aptamer hairpin probe is adsorbed onto the GO, leading to an extremely low fluorescence background signal. To test the feasibility of the SOIQA systems, a protein cancer marker, carcinoembryonic antigen (CEA) was used as the model analyte. The developed aptasensor could detect CEA with a detection limit of 28.5 fg mL-1 (∼142 aM), high specificity and a broad detection range of 6 orders of magnitude. And this one-step incubation can be completed in 60 min. In addition, the approach uses only one oligonucleotide strand, and is simple. Moreover, this SOIQA sensing method is suitable for rapid and direct quantification of proteins in complex biological samples such as clinical serum. Considering the simplicity and superior sensitivity/specificity, the developed sensing method provides a promising platform for the analysis of a variety of low-abundance biomolecules.

Ultrasensitive detection of the androgen receptor through the recognition of an androgen receptor response element and hybridization chain amplification

An ultrasensitive electrochemical detection of the androgen receptor (AR) was developed based on the protection of a DNA duplex by the AR from restriction endonuclease-mediated digestion and a subsequent hybridization chain reaction (HCR). Two partially complementary DNA probes P1 and P2 were designed to form an androgen receptor binding probe (ARBP) through hybridization. The ARBP contains a duplex at one end and two single-stranded tails at the other end. The duplex part containing the recognition sites of the AR and NspI restriction endonuclease was immobilized on an Au electrode, whereas the single-stranded parts served as capture probes to activate the HCR. In the absence of the AR, NspI can cleave the duplex and release the capture probes, and thus, no HCR occurs. However, the AR can bind to the ARBP and protect the duplex from cleavage; therefore, the capture probes can trigger the HCR between four carefully designed G-quadruplex forming hairpin probes and the capture probes, resulting in the formation of numerous G-quadruplexes. Finally, differential pulse voltammetry (DPV) was carried out to quantify the AR. The assay revealed a detection limit of 7.64 fM. The verification of its high specificity and practicability in serum samples indicated its potential applications in the fields of clinical examination and disease diagnosis.

Zero background and triple-signal amplified fluorescence aptasensor for antibiotics detection in foods

It's important to eliminate matrix interference for accurate detecting antibiotic residues in complex food samples. In this study, we designed a zero-backgrounded fluorescence aptasensor to achieve on-site detection of antibiotic residues, with chloramphenicol (CAP) as representative analyte. Moreover, a three stir-bars assisted target recycling system (TSBTR) was designed to achieve triple signal amplification and increase the sensitivity. The bars included one magnetic stir-bar modified with two kinds of long DNA chains, and two gold stir-bars modified with Y shape-duplex DNA probes respectively. In the presence of CAP, the target could recurrently react with the probes on the bars and replace a large amount of long DNA chains into supernatant. After then, the bars were taken out and SYBR green dye was added to the solution. The dye can specifically intercalate into the duplex structures of DNA chains to emit fluorescence while not emitting a signal in its free state. Under the optimized experimental conditions, a wide linear response range of 5 orders of magnitude from 0.001 ng mL^-1 to 10 ng mL^-1 was achieved with a detection limit of 0.033 pg mL^-1 CAP. The assay was successfully employed to detect CAP in food samples (milk & fish) with consistent results with ELISA's. High selectivity and sensitivity were attributed to the zero background signal and triple signal-amplification strategy. Moreover, the detection time can be shortened to 40 min due to that three signal amplified process can occur simultaneously. The fluorescent aptasensor was also label- and enzyme-free. All these ensure the platform to be rapid, cost-effective, easily-used, and is especially appropriate for detection antibiotics in food safety.

A fluorometric aptamer method for kanamycin by applying a dual amplification strategy and using double Y-shaped DNA probes on a gold bar and on magnetite nanoparticles

A simple and highly sensitive fluorometric method is described for the determination of the antibiotic kanamycin (Kana) in food. Dual signal amplification is accomplished by making use of double Y-shaped aptamer DNA probes acting as a capture probes and signal amplification probes. The DNA probes were immobilized on a gold bar and on a magnetic bar, respectively. On addition of Kana, the Y-shaped aptamer probe captures Kana and then is disassembled to release two single-stranded DNAs. These trigger target recycling and HCR between the two bars simultaneously. As a result, many long duplex DNA chains are formed in the supernatant. After pulling out the bars and adding the fluorescent intercalating probe SYBR Green I, strong fluorescence (with excitation/emission peaks at 497/525 nm) is induced. The use of such double Y-shaped DNA probes obviously overcomes the unspecific signal amplification by HCR which increases selectivity and sensitivity. This is due to the fact that the hairpin of HCR is separated in being present in different arms of the Y-shaped probe. Under the optimal conditions, the assay has a limit of 0.45 pg·mL^-1 for Kana. It was applied to analyze spiked milk, fish and pork samples. Graphical abstract The scheme represents a sensitive fluorometric aptamer-based method to detect kanamycin (Kana). It is making use of a double stirring bar-assisted dual amplification strategy with zero background. Abbreviations: apt: aptamer, AuNPs: gold nanoparticles, HCR: hybridization chain reaction.

Signal-on electrochemiluminescence aptasensor for bisphenol A based on hybridization chain reaction and electrically heated electrode

A simple and sensitive electrochemiluminescence (ECL) aptasensor has been developed for bisphenol A (BPA) detection. The capture DNA (CDNA) was modified on the heated indium-tin-oxide (ITO) working electrode surface firstly and then hybridized with BPA aptamer to form double strand DNA (dsDNA). The presence of target can cause the releasing of aptamer from the electrode surface since the aptamer prefers to switch its configuration to combine with BPA. Subsequently, the free CDNA will induce hybridization chain reaction (HCR) to produce long dsDNA on the electrode surface. Ru(phen)₃²⁺ can integrate into the grooves of dsDNA to act as an ECL reagent, thus enhanced ECL signal can be detected. The temperature control during the processes of target recognition and HCR were realized through the heated electrode instead of the bulk solution heating. Furthermore, the performance of the ECL aptasensor can be further enhanced at elevated electrode temperature. Under the optimized conditions, the ECL intensity of the system has a linear relationship with the logarithm of BPA concentration in the range of 2.0 pM-50 nM. The limit of detection (LOD) at 55 °C (electrode surface temperature) was calculated to be 1.5 pM, which was approximately 6.5-fold lower than that at 25 °C. The proposed biosensor has been applied to detect the BPA in drink samples with satisfactory results.

Chronocoulometric aptamer based assay for staphylococcal enterotoxin B by target-triggered assembly of nanostructured dendritic nucleic acids on a gold electrode

A rapid and ultrasensitive method is described for the detection of staphylococcal enterotoxin B (SEB). It is based on the formation of a dendritic DNA superstructure by integrating (a) target-induced triggering of DNA release with (b) signal amplification by a hybridization chain reaction. Partially complementary pairing of aptamer and trigger DNA forms a duplex structure. The capture DNA is then placed on the surface of a gold electrode through gold-thiol chemistry. In the presence of SEB, the aptamer-target conjugate is compelled to form. This causes the release of trigger DNA owing to a strong competition with SEB. The trigger DNA is subsequently hybridized with the partial complementary sequences of the capture DNA to trigger HCR with three auxiliary DNA sequances (referred to as H1, H2, H3). Finally, the dendritic DNA superstructure is bound to hexaammineruthenium(III) cation by electrostatic adsorption and assembled onto the modified gold electrode. This produces an amplified electrochemical signal that is measured by chronocoulometry. Under optimal conditions, the charge difference increases linearly with the logarithm of the SEB concentrations in the range from 5 pg·mL^-1 to 100 ng·mL^-1 with a detection limit as low as 3 pg·mL^-1 (at S/N = 3). Graphical abstract An electrochemical switching strategy is presented for the sensitive detection of Staphylococcus enterotoxin B based on target-triggered assembly of dendritic nucleic acid nanostructures.

A novel fluorescence method for activity assay and drug screening of T4 PNK by coupling rGO with ligase reaction

T4 polynucleotide kinase (PNK) is the primary member of the 5′-kinase family that can transfer the γ-phosphate residue of ATP to the 5′-hydroxyl group of oligonucleotides.

Silver ion-stabilized DNA triplexes for completely enzyme-free and sensitive fluorescence detection of transcription factors via catalytic hairpin assembly amplification

A new silver ion-stabilized DNA triplex enables enzyme-free and amplified sensitive fluorescence detection of transcription factors.

Multiplexed electrochemical aptasensor for antibiotics detection using metallic-encoded apoferritin probes and double stirring bars-assisted target recycling for signal amplification

Simultaneous and sensitive detection of various antibiotic residues in one sample is essential to evaluation of food safety status. Herein, a multiplexed electrochemical aptasensor for multiplex antibiotics detection, with kanamycin (KANA) and ampicillin (AMP) as representative analytes, was designed by using metal ions encoded apoferrtin probes and double stirring bars-assisted target recycling for signal amplification. The encoded probes were prepared by apoferritin loading Cd²⁺ and Pb²⁺ ions and labeling with duplex DNAs (aptamers corresponding to KANA and AMP hybrid with its complementary DNA sequence), respectively. In the presence of KANA and AMP, the targets can recurrently react with the probes on the bars, and then replace a lot of Apo-Mencoded signal tags into supernatant. The peak currents of Cd²⁺and Pb²⁺from the tags corresponding with the concentrations of KANA and AMP were detected by square wave voltammetry in one run. As a result, KANA and AMP can be detected simultaneously within the range from 0.05 pM to 50 nM. And the detection limits were 18 fM KANA and 15 fM AMP (S/N = 3). The assay was testified to detect KANA and AMP residues with consistent results of ELISA in samples, e.g. milks and fishes. The assay was highly-sensitive, selective, cost-effective and easy-to-operate due to Apo-M encoded probes with high loading capacity of signal source substances. Moreover, double stirring bar-assisted target recycling, which was enzyme-free and could overcome matrix interference, was fabricated for signal amplification. Thus, the assay showed potential advantages for sensitively screening of antibiotic residues in foods.

Nucleic acid aptamers in diagnosis of colorectal cancer

Nucleic acid aptamers are promising recognition ligands for diagnostic applications. They are short DNA or RNA molecules isolated from large random libraries through the Systematic Evolution of Ligands by EXponential enrichment (SELEX) procedure. These molecules, with a particular three-dimensional shape, bind to a wide range of targets from small molecules to whole cells with high affinity and specificity. The unique properties of nucleic acid aptamers including high binding affinity and specificity, thermostability, ease of chemical production, ease of chemical modification, target adaptability, simple storage, resistance to denaturation, low immunogenicity, and low cost make them potential diagnostic tools for clinical use. Colorectal cancer is one of the most common types of cancer in humans and the third leading cause of cancer deaths in the world. Due to low response rate to current therapies in advanced stages of the disease, early detection of CRC can be useful in disease management. This review highlights recent advances in the development of nucleic acid aptamer-based methods for diagnosis, prognosis, and theranosis of colorectal cancer.

An efficient nonlinear hybridization chain reaction-based sensitive fluorescent assay for in situ estimation of calcium channel protein expression on bone marrow cells

A sensitive and highly efficient approach to monitor the expression of proteins on live cells was urgently needed to demonstrate its factor and mechanism and most important for clinical diagnostics and molecular biology. Herein, we developed a simple and highly efficient strategy, nonlinear hybridization chain reaction (nonlinear HCR), for the sensitive determination of proteins on live cells with transient receptor potential vanilloid 4 (TRPV4) and RAW264.7 cells as a model. Unlike the normal hybridization chain reaction (HCR) with multiplicative amplification, an exponential amplified fluorescent response could be obtained in theory based on the proposed nonlinear HCR. As a result, the nonlinear HCR generated a significant enhancement about 3 times compared with the normal HCR and 10 times compared with the directly immunofluorescence assay. Based on the proposed nonlinear HCR, the fluorescent signals increased with the concentration of TRPV4 in the range from 10 pg/mL to 100 ng/mL with a detection limit of 2.8 pg/mL, which would be useful for the sensitive detection of proteins in cell lysis or on cell surface. At the same time, the significant improvements via nonlinear HCR were achieved in the fluorescent imaging system compared with traditional immunofluorescence staining and normal HCR, proving the significant value of nonlinear HCR-based amplification strategy. Success in the establishment of the highly efficient nonlinear HCR strategy offered a simple and sensitive approach to demonstrate the concentration of special proteins on cell and other proteins and nucleotide potentially, revealing a simple and efficient technology for research fields of clinical diagnostics and molecular biology.

A hybridization chain reaction amplification strategy for fluorescence imaging of human telomerase activity in living cells

A hybridized chain reaction (HCR)-based biosensing method has been developed for the imaging detection of intracellular telomerase activity. The telomerase-targeting responder-transmitter DNA complex (HPT) consisting of telomerase primer sequence (HP) and a HCR initiator (trigger) is transfected into cell plasma. In the presence of telomerase, HPT can be recognized and extended, producing plenty of triggers which initiate HCR amplification reaction. Finally, a long nicked dsDNA with a lot of outstretched single chains was formed by hybridizing with Q of the reporter complex, generating an enhanced fluorescence signal. The developed biosensing approach can be used for the detection of telomerase activity in cell lysate with the detection limit of 578 cells/100 μl. In addition, this strategy has been successfully applied not only for the sensitive and specific imaging of telomerase activity in living cells but also for comparing of telomerase activity among different cell lines. Therefore, the method might become a potential alternative tool for telomerase-related cancer diagnosis and therapy in medical research and early clinical diagnosis.

Chemical and Biological Sensing Using Hybridization Chain Reaction

Since the advent of its theoretical discovery more than 30 years ago, DNA nanotechnology has been used in a plethora of diverse applications in both the fundamental and applied sciences. The recent prominence of DNA-based technologies in the scientific community is largely due to the programmable features stored in its nucleobase composition and sequence, which allow it to assemble into highly advanced structures. DNA nanoassemblies are also highly controllable due to the precision of natural and artificial base-pairing, which can be manipulated by pH, temperature, metal ions, and solvent types. This programmability and molecular-level control have allowed scientists to create and utilize DNA nanostructures in one, two, and three dimensions (1D, 2D, and 3D). Initially, these 2D and 3D DNA lattices and shapes attracted a broad scientific audience because they are fundamentally captivating and structurally elegant; however, transforming these conceptual architectural blueprints into functional materials is essential for further advancements in the DNA nanotechnology field. Herein, the chemical and biological sensing applications of a 1D DNA self-assembly process known as hybridization chain reaction (HCR) are reviewed. HCR is a one-dimensional (1D) double stranded (ds) DNA assembly process initiated only in the presence of a specific short ssDNA (initiator) and two kinetically trapped DNA hairpin structures. HCR is considered an enzyme-free isothermal amplification process, which shows substantial promise and offers a wide range of applications for in situ chemical and biological sensing. Due to its modular nature, HCR can be programmed to activate only in the presence of highly specific biological and/or chemical stimuli. HCR can also be combined with different types of molecular reporters and detection approaches for various analytical readouts. While the long dsDNA HCR product may not be as structurally attractive as the 2D and 3D DNA networks, HCR is highly instrumental for applied biological, chemical, and environmental sciences, and has therefore been studied to foster a variety of objectives. In this review, we have focused on nucleic acid, protein, metabolite, and heavy metal ion detection using this 1D DNA nanotechnology via fluorescence, electrochemical, and nanoparticle-based methodologies.

Electrochemical detection of NGF using a reduced graphene oxide- titanium nitride nanocomposite

There is a correlation between the severity of neurological impairment in patients that have suffered a cerebrovascular accident and the nerve growth factor (NGF) level. This study addressed the fabrication of a titanium nitride (TiN) and reduced graphene oxide (RGO)-based composite with remarkable electrocatalytic activity towards NGF oxidation in a phosphate buffer solution (PB, 0.1 M). The proposed electrochemical sensor was linearly related to the NGF concentration in the range of 10 nM-5 μM with a detection limit of 2.6 nM.

Recent advances on aptamer-based biosensors to detection of platelet-derived growth factor

Platelet-derived growth factor (PDGF-BB), a significant serum cytokine, is an important protein biomarker in diagnosis and recognition of cancer, which straightly rolled in proceeding of various cell transformations, including tumor growth and its development. Fibrosis, atherosclerosis are certain appalling diseases, which PDGF-BB is near to them. Generally, the expression amount of PDGF-BB increases in human life-threatening tumors serving as an indicator for tumor angiogenesis. Thus, identification and quantification of PDGF-BB in biomedical fields are particularly important. Affinity chromatography, immunohistochemical methods and enzyme-linked immunosorbent assay (ELISA), conventional methods for PDGF-BB detection, requiring high-cost and complicated instrumentation, take too much time and offer deficient sensitivity and selectivity, which restrict their usage in real applications. Hence, it is essential to design and build enhanced systems and platforms for the recognition and quantification of protein biomarkers. In the past few years, biosensors especially aptasensors have been received noticeable attention for the detection of PDGF-BB owing to their high sensitivity, selectivity, accuracy, fast response, and low cost. Since the role and importance of developing aptasensors in cancer diagnosis is undeniable. In this review, optical and electrochemical aptasensors, which have been applied by many researchers for PDGF-BB cancer biomarker detection, have been mentioned and merits and demerits of them have been explained and compared. Efforts related to design and development of aptamer-based biosensors using nanoparticles for sensitive and selective detection of PDGF-BB have been reviewed considering: Aptamer importance as recognition elements, principal, application and the recent improvements and developments of aptamer based optical and electrochemical methods. In addition, commercial biosensors and future perspectives for rapid and on-site detection of PDGF-BB have been summarized.