A novel fluorescent aptasensor for thrombin detection: using poly(m-phenylenediamine) rods as an effective sensing platform

Conjugated Polymers for Aptasensing Applications

Rapid and specific assaying of molecules that report on a pathophysiological condition, environmental pollution, or drug concentration is pivotal for establishing efficient and accurate diagnostic systems. One of the main components required for the construction of these systems is the recognition element (receptor) that can identify target analytes. Oligonucleotide switching structures, or aptamers, have been widely studied as selective receptors that can precisely identify targets in different analyzed matrices with minimal interference from other components in an antibody-like recognition process. These aptasensors, especially when integrated into sensing platforms, enable a multitude of sensors that can outperform antibody-based sensors in terms of flexibility of the sensing strategy and ease of deployment to areas with limited resources. Research into compounds that efficiently enhance signal transduction and provide a suitable platform for conjugating aptamers has gained huge momentum over the past decade. The multifaceted nature of conjugated polymers (CPs), notably their versatile electrical and optical properties, endows them with a broad range of potential applications in optical, electrical, and electrochemical signal transduction. Despite the substantial body of research demonstrating the enhanced performance of sensing devices using doped or nanostructure-embedded CPs, few reviews are available that specifically describe the use of conjugated polymers in aptasensing. The purpose of this review is to bridge this gap and provide a comprehensive description of a variety of CPs, from a historical viewpoint, underpinning their specific characteristics and demonstrating the advances in biosensors associated with the use of these conjugated polymers.

Highly sensitive analysis of tetrodotoxin based on free-label fluorescence aptamer sensing system

Tetrodotoxin (TTX) specifically can bind to its nucleic acid aptamer (TTX-aptamer) and cause the conformation of TTX-aptamer to be switched from the single-strand random coiling form to the compact neck ring structure. Based on the microenvironment difference of the fluorescence reporter, berberine in between the single-stranded coil oligonucleotides and the structure of the neck ring, a simple, rapid and sensitive label-free fluorescence aptamer sensing system for detection of TTX was developed. Various factors affecting the analysis of TTX were optimized, including the concentration of berberine, ion strength, pH, reaction time, the concentration of TTX-aptamer. Under the optimal experimental conditions, the fluorescence intensity of the sensing system and the concentration of TTX showed a good linear relationship in the range of 0.1 nM to 500 nM, with the detection limit of 0.074 nM. The standard recovery test result exhibited that the recoveries of TTX in serum samples were 96.54%-106.40%. The established method has the advantages of high specificity, good sensitivity, quickness and convenience, low cost, and can be used for the detection of TTX in serum samples.

Porous Silicon-Based Aptasensors: The Next Generation of Label-Free Devices for Health Monitoring

Aptamers are artificial nucleic acid ligands identified and obtained from combinatorial libraries of synthetic nucleic acids through the in vitro process SELEX (systematic evolution of ligands by exponential enrichment). Aptamers are able to bind an ample range of non-nucleic acid targets with great specificity and affinity. Devices based on aptamers as bio-recognition elements open up a new generation of biosensors called aptasensors. This review focuses on some recent achievements in the design of advanced label-free optical aptasensors using porous silicon (PSi) as a transducer surface for the detection of pathogenic microorganisms and diagnostic molecules with high sensitivity, reliability and low limit of detection (LoD).

A dual-channel homogeneous aptasensor combining colorimetric with electrochemical strategy for thrombin

In this protocol, a dual-channel homogeneous aptasenor was proposed for protein molecule determination, employing thrombin as target analyte. The colorimetric and electrochemical transducers were combined in a single analytical system for signal readout. In this dual-channel sensing strategy, the G-quadruplex sequence was released and incorporated with hemin to form DNAzyme for naked-eye colorimetric detection. Meanwhile, the hydroxyapatite nanoparticle as signal probe was combined with magnetic nanoparticles to construct sandwich-type structure for generating the electrochemical current when thrombin was present in solution. By introducing two kinds of reporter probes and transducers, this dual-channel sensor produced two different kinds of signal to improve the analytical accuracy and diversity. The results revealed that the dual-channel sensor achieved the quantatitive determination of thrombin with low limit of detection (0.40 fM) and wide range (0.1 fM to 1 nM), which offer a promise for rapid and accurate detection of biomolecule.

Gold Nanoparticles Decorated Hematite Photoelectrode for Sensitive and Selective Photoelectrochemical Aptasensing of Lysozyme

Photoelectrochemical aptasensor (PECAS) is a new and promising detection platform with both high sensitivity and good selectivity. Exploration of new photoelectrode materials and establishment of effective charge transfer channel between photoelectrode and aptamer are the main challenges in this field. In this work, an efficient PECAS based on Au nanoparticles (NPs) decorated Fe₂O₃ nanorod photoelectrode is rationally designed, fabricated, and exhibited excellent sensitivity and selectivity for detection of lysozyme (Lys) with an ultralow detection limit of 3 pM and wide detection range from 10 pM to 100 nM. The Au NPs not only act as anchor to establish an efficient charge transfer channel between the photoelectrode and the aptamer, but also help to enhance the PEC performance through adjusting the carrier density of Fe₂O₃. The rationally designed photoelectrode opens up a distinctive avenue for promoting the PECAS to be a versatile analysis method.

Fe-nitrilotriacetic acid coordination polymer nanowires: an effective sensing platform for fluorescence-enhanced nucleic acid detection

The determination of specific nucleic acid sequences is key in identifying disease-causing pathogens and genetic diseases. In this paper we report the utilization of Fe-nitrilotriacetic acid coordination polymer nanowires as an effective nanoquencher for fluorescence-enhanced nucleic acid detection. The detection is fast and the whole process can be completed within 15 min. This nanosensor shows a low detection limit of 0.2 nM with selectivity down to single-base mismatch. This work provides us with an attractive sensing platform for applications.

Proximity Binding and Metal Ion-Dependent DNAzyme Cyclic Amplification-Integrated Aptasensor for Label-Free and Sensitive Electrochemical Detection of Thrombin

Thrombin plays important roles for the diagnosis of neurodegenerative and cardiovascular diseases. By integrating proximity binding-induced strand displacement and metal ion-dependent DNAzyme recycling amplification, we demonstrate here the development of a simple and sensitive strategy for the detection of thrombin in human serums. The binding of the two distinct aptamers to the thrombin targets increases the local concentration of the aptamers and facilitates the release of the enzymatic sequences through proximity binding-induced strand displacement. The liberated enzymatic sequences further hybridize with the G-quadruplex containing and hairpin-structured substrate sequences on the sensor electrode to form the metal-ion dependent DNAzymes. Subsequently, the metal ions catalyze the cleavage of the substrate sequences to unlock the G-quadruplex forming sequences and to release the enzymatic sequences to trigger another cleavage cycle. Such metal ion-dependent DNAzyme recycling amplification leads to the formation of many active G-quadruplex forming sequences, which associate with hemin to form G-quadruplex/hemin complexes on the electrode surface. Direct electron transfer of hemin to the electrode during the potential scan can thus generate significantly amplified current for sensitive detection of thrombin at the low picomolar level. The work demonstrated here can thus offer new opportunities for the development of convenient signal amplification strategies for detecting various protein targets.

Dual-stabilizer-capped CdSe quantum dots for “Off–On” electrochemiluminescence biosensing of thrombin by target-triggered multiple amplification

A novel CdSe QDs was used to design a promising ECL biosensor for thrombin assay by target-triggered multiple amplification.

Solid phase synthesis of a thrombin binding aptamer on macroporous silica for label free optical quantification of thrombin

Human α-thrombin (TB) is a serine protease with a crucial role in coagulation and hemostasis.

Construction of Plasmonic Core-Satellite Nanostructures on Substrates Based on DNA-Directed Self-Assembly as a Sensitive and Reproducible Biosensor

We report the successful construction of plasmonic core-satellite nanostructured assemblies on two-dimensional substrates, based on a strategy of combining DNA-functionalized plasmonic nanoparticles (NPs) with the specific recognition ability toward target to enable satellite NPs to self-assemble around the core immobilized on substrates. A strongly coupled plasmonic resonance band was observed because of the close proximity between core and satellite NPs, which presented significant red-shift and enhanced extinction with respect to the local surface plasmon resonance (LSPR) band of individual core NPs on the substrate. The functionality of this core-satellite nanostructured assembly as a biosensor was further explored, and the changes in extinction intensity and the peak shift of the plasmonic coupling resonance band arising from the probe-target DNA binding event all proved to be useful criteria for target DNA detection. Moreover, high selectivity down to single-base mismatched DNA was achieved using this strongly coupled plasmonic core-satellite nanostructured assembly on a substrate. Such substrate-based detection was advantageous, and its reusability and high cycle stability were demonstrated after five cycles of disassembly and reassembly. Our work demonstrates the biosensing capacity of this DNA-functionalized plasmonic nanoassembly model system on two-dimensional substrate, which is also applicable to the detection of numerous DNA-recognized biomolecules. Likewise, the presented construction method can be extended to fabricate other compositional core-satellite nanoassemblies.

Utilizing Intrinsic Properties of Polyaniline to Detect Nucleic Acid Hybridization through UV-Enhanced Electrostatic Interaction

Detection of specific RNA or DNA molecules by hybridization to "probe" nucleic acids via complementary base-pairing is a powerful method for analysis of biological systems. Here we describe a strategy for transducing hybridization events through modulating intrinsic properties of the electroconductive polymer polyaniline (PANI). When DNA-based probes electrostatically interact with PANI, its fluorescence properties are increased, a phenomenon that can be enhanced by UV irradiation. Hybridization of target nucleic acids results in dissociation of probes causing PANI fluorescence to return to basal levels. By monitoring restoration of base PANI fluorescence as little as 10(-11) M (10 pM) of target oligonucleotides could be detected within 15 min of hybridization. Detection of complementary oligos was specific, with introduction of a single mismatch failing to form a target-probe duplex that would dissociate from PANI. Furthermore, this approach is robust and is capable of detecting specific RNAs in extracts from animals. This sensor system improves on previously reported strategies by transducing highly specific probe dissociation events through intrinsic properties of a conducting polymer without the need for additional labels.

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.

Recent advances in carbon nanodots: synthesis, properties and biomedical applications

Herein, a mini review is presented concerning the most recent research progress of carbon nanodots, which have emerged as one of the most attractive photoluminescent materials. Different synthetic methodologies to achieve advanced functions and better photoluminescence performances are summarized, which are mainly divided into two classes: top-down and bottom-up. The inspiring properties, including photoluminescence emission, chemiluminescence, electrochemical luminescence, peroxidase-like activity and toxicity, are discussed. Moreover, the biomedical applications in biosensing, bioimaging and drug delivery are reviewed.

Label-free fluorescent aptasensor for potassium ion using structure-switching aptamers and berberine

A simple, rapid and label-free fluorescent aptasensor was fabricated for the detection of potassium ion (K(+) ion) in aqueous solution using K(+) ion-stabilized single stranded DNA (ssDNA) with G-rich sequence as the recognition element and a fluorescent dye, berberine, as the fluorescence probe. In the presence of K(+) ion, the G-rich ssDNA is promoted to form the aptamer-target complex with a G-quadruplex conformation, and berberine binding to the G-quadruplex structure results in the enhancement of its fluorescence. The fluorescence intensity of the sensing system displayed a calibration response for K(+) ion in the range of 0-1600μM with a detection limit of 31nM (S/N=3) and a relative standard deviation (RSD) of 0.45%. This label-free fluorescence aptasensor is conveniently and effectively applicable for analysis of K(+) ion in blood serum samples with the recovery range of 81.7-105.3%. The assay for detection of potassium ion is easy, economical, robust, and stable in rough conditions.

Dual-colored graphene quantum dots-labeled nanoprobes/graphene oxide: functional carbon materials for respective and simultaneous detection of DNA and thrombin

Convenient and simultaneous detection of multiple biomarkers such as DNA and proteins with biocompatible materials and good analytical performance still remains a challenge. Herein, we report the respective and simultaneous detection of DNA and bovine α-thrombin (thrombin) entirely based on biocompatible carbon materials through a specially designed fluorescence on-off-on process. Colorful fluorescence, high emission efficiency, good photostability and excellent compatibility enables graphene quantum dots (GQDs) as the best choice for fluorophores in bioprobes, and thus two-colored GQDs as labeling fluorophores were chemically bonded with specific oligonucleotide sequence and aptamer to prepare two probes targeting the DNA and thrombin, respectively. Each probe can be assembled on the graphene oxide (GO) platform spontaneously by π-π stacking and electrostatic attraction; as a result, fast electron transfer in the assembly efficiently quenches the fluorescence of probe. The presence of DNA or thrombin can trigger the self-recognition between capturing a nucleotide sequence and its target DNA or between thrombin and its aptamer due to their specific hybridization and duplex DNA structures or the formation of apatamer-substrate complex, which is taken advantage of in order to achieve a separate quantitative analysis of DNA and thrombin. A dual-functional biosensor for simultaneous detection of DNA and thrombin was also constructed by self-assembly of two probes with distinct colors and GO platform, and was further evaluated with the presence of various concentrations of DNA and thrombin. Both biosensors serving as a general detection model for multiple species exhibit outstanding analytical performance, and are expected to be applied in vivo because of the excellent biocompatibility of their used materials.

A sensitive electrochemical aptasensor based on palladium nanoparticles decorated graphene-molybdenum disulfide flower-like nanocomposites and enzymatic signal amplification

In the present study, with the aggregated advantages of graphene and molybdenum disulfide (MoS2), we prepared poly(diallyldimethylammonium chloride)-graphene/molybdenum disulfide (PDDA-G-MoS2) nanocomposites with flower-like structure, large surface area and excellent conductivity. Furthermore, an advanced sandwich-type electrochemical assay for sensitive detection of thrombin (TB) was fabricated using palladium nanoparticles decorated PDDA-G-MoS2 (PdNPs/PDDA-G-MoS2) as nanocarriers, which were functionalized by hemin/G-quadruplex, glucose oxidase (GOD), and toluidine blue (Tb) as redox probes. The signal amplification strategy was achieved as follows: Firstly, the immobilized GOD could effectively catalyze the oxidation of glucose to gluconolactone, coupling with the reduction of the dissolved oxygen to H2O2. Then, both PdNPs and hemin/G-quadruplex acting as hydrogen peroxide (HRP)-mimicking enzyme could further catalyze the reduction of H2O2, resulting in significant electrochemical signal amplification. So the proposed aptasensor showed high sensitivity with a wide dynamic linear range of 0.0001 to 40 nM and a relatively low detection limit of 0.062 pM for TB determination. The strategy showed huge potential of application in protein detection and disease diagnosis.

An amplified electrochemical aptasensor for thrombin detection based on pseudobienzymic Fe3O4-Au nanocomposites and electroactive hemin/G-quadruplex as signal enhancers

A sensitive and selective electrochemical aptasensor for thrombin detection was constructed based on hemin/G-quadruplex as the signal label and Fe3O4-Au nanocomposites with glucose oxidase (GOx-) and peroxide-mimicking enzyme activity as the signal enhancers. Due to their large surface area and good biocompatibility, Fe3O4-Au nanocomposites were employed to immobilize electroactive hemin/G-quadruplex, which was formed by the conjugation between a single-stranded guanine-rich nucleic acid and hemin. Based on the GOx-mimicking enzyme activity, Au nanoparticles on the surface of the Fe3O4-Au nanocomposites effectively catalyzed the oxidization of glucose in the presence of dissolved O2, accompanied by the production of H2O2. Both the Fe3O4 cores of Fe3O4-Au nanocomposites and hemin/G-quadruplex with H2O2-mimicking enzyme activity could catalyze the reduction of the generated H2O2, which promoted the electron transfer of hemin and amplified the electrochemical signal. The proposed electrochemical aptasensor had a wide dynamic linear range of 0.1 pM to 20 nM with a lower detection limit of 0.013 pM, which provided a promising method for a sensitive assay for the detection of proteins in electrochemical aptasensors.

Porous platinum nanotubes modified with dendrimers as nanocarriers and electrocatalysts for sensitive electrochemical aptasensors based on enzymatic signal amplification

A highly sensitive electrochemical aptasensor for thrombin detection is developed and demonstrated by using porous platinum nanotubes modified with polyamidoamine dendrimers as nanocarriers and electrocatalysts. The proposed strategy affords a low detection limit of 0.03 pM based on enzyme-based signal amplification.

"Clickable" LNA/DNA probes for fluorescence sensing of nucleic acids and autoimmune antibodies

Herein we describe fluorescent oligonucleotides prepared by click chemistry between novel alkyne-modified locked nucleic acid (LNA) strands and a series of fluorescent azides for homogeneous (all-in-solution) detection of nucleic acids and autoimmune antibodies.

Aptamer binding assays for proteins: the thrombin example--a review

Experimentally selected single-stranded DNA and RNA aptamers are able to bind to specific target molecules with high affinity and specificity. Many analytical methods make use of affinity binding between the specific targets and their aptamers. In the development of these methods, thrombin is the most frequently used target molecule to demonstrate the proof-of-principle. This paper critically reviews more than one hundred assays that are based on aptamer binding to thrombin. This review focuses on homogeneous binding assays, electrochemical aptasensors, and affinity separation techniques. The emphasis of this review is placed on understanding the principles and unique features of the assays. The principles of most assays for thrombin are applicable to the determination of other molecular targets.

Planar Hall magnetoresistive aptasensor for thrombin detection

The use of aptamer-based assays is an emerging and attractive approach in disease research and clinical diagnostics. A sensitive aptamer-based sandwich-type sensor is presented to detect human thrombin using a planar Hall magnetoresistive (PHR) sensor in cooperation with superparamagnetic labels. A PHR sensor has the great advantages of a high signal-to-noise ratio, a small offset voltage and linear response in the low-field region, allowing it to act as a high-resolution biosensor. In the system presented here, the sensor has an active area of 50 µm × 50 µm with a 10-nm gold layer deposited onto the sensor surface prior to the binding of thiolated DNA primary aptamer. A polydimethylsiloxane well of 600-µm radius and 1-mm height was prepared around the sensor surface to maintain the same specific area and volume for each sensor. The sensor response was traced in real time upon the addition of streptavidin-functionalized magnetic labels on the sensor. A linear response to the thrombin concentration in the range of 86 pM-8.6 µM and a lower detection limit down to 86 pM was achieved by the proposed present method with a sample volume consumption of 2 µl. The proposed aptasensor has a strong potential for application in clinical diagnosis.

Target-induced structure switching of DNA for label-free and ultrasensitive electrochemiluminescent detection of proteins

Highly sensitive and label-free detection of thrombin is achieved via a target-induced DNA structure switching strategy and Exo III-assisted recycling amplification.

Functionalized aptamers as nano-bioprobes for ultrasensitive detection of bisphenol-A

A novel functionalized aptamer based 'turn-off' fluorescent biosensor for ultra-sensitive detection of small molecules like bisphenol-A in water samples.

Sensitive electrochemical aptasensor for thrombin detection based on graphene served as platform and graphene oxide as enhancer

A sensitive electrochemical aptasensor was developed with conductive graphene served as platform and inert graphene oxide (GO) as enhancer. An electrodeposited nano-Au layer was firstly formed on conductive graphene modified glass carbon electrode surface for further immobilizing of electrochemical redox probe hexacyanoferrates nanoparticles (NiHCFNPs). Subsequently, another nano-Au layer was formed for immobilizing of thrombin aptamer (TBA). In the presence of thrombin, the TBA on the electrode surface could bind with thrombin, which made a barrier for electrons and inhibited the electro-transfer, resulting in the decreased electrochemical signals of NiHCFNPs. Owing to the non-conductivity property of graphene oxide, further decreased electrochemical signals of NiHCFNPs could be obtained via the sandwich reaction with GO-labeled TBA. According to the signal changes before the thrombin recognition and after sandwich reaction, trace detection of thrombin could be achieved. As a result, the proposed approach showed a high sensitivity and a wider linearity to thrombin in the range from 0.005 nM to 50 nM with a detection limit of 1 pM.

Label-free colorimetric aptasensor based on nicking enzyme assisted signal amplification and DNAzyme amplification for highly sensitive detection of protein

Highly sensitive detection of proteins is essential to biomedical research as well as clinical diagnosis. Here, we develped a novel label-free colorimetric aptasensor based on nicking enzyme assisted signal amplification and DNAzyme amplification for highly sensitive detection of protein. The system consists of a hairpin DNA probe carrying an aptamer sequence for target, a G-riched DNA probe containing two G-riched DNAzyme segments and the recognition sequence as well as cleavage site for nicking enzyme, a blocker DNA, and the nicking enzyme. The hybridization of the G-riched DNA with the blocker DNA prohibits the formation of the activated DNAzymes in the absence of target. Upon addition of target to the system, the hairpin probe is opened by the specific recognition of the target to its aptamer. The open hairpin probe hybridizes with a G-riched DNA and forms a DNA duplex, which triggers the selective cleavage of the G-riched DNA probe by nicking enzyme, leading to the release of the aptamer-target complex and the G-riched DNAzyme segments. The released open hairpin probe then hybridizes with another G-riched DNA probe, and the cycle starts anew, resulting in the continuous cleavage of the G-riched DNA probes, generating a much of G-riched DNAzyme segments. The G-riched DNAzyme segments interact with hemin and generates the activated DNAzyme that can catalyze the H2O2-mediated oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS(2-)) to the colored ABTS(•-), thus providing the amplified colorimetric detection of target. With the use of thrombin (Tb) as a proof-of-principle analyte, this sensing platform can detect Tb specifically with a detection limit as low as 1.5 pM, which is at least 4 orders of magnitude lower over the unamplified colorimetric assay. Moreover, the assay does not involve any chemical modification of DNA, which is simple and low-cost. This sensing platform provides a promising approach for the amplified analysis of target molecules.

A sensitive gold nanoparticles sensing platform based on resonance energy transfer for chemiluminescence light on detection of biomolecules

In this article, we report a gold nanoparticles (AuNPs) sensing platform based on chemiluminescence resonance energy transfer (CRET) for light on detection of biomolecules. In designing such a CRET-based biosensing platform, the aptamer was first covalently labeled with a chemiluminescent reagent, N-(4-aminobutyl)-N-ethylisoluminol (ABEI). The ABEI labeled aptamer was then hybridized with AuNPs functionalized ssDNA which was complementary to the aptamer, obtaining the aptasensor. The CRET between ABEI and AuNPs in the aptasensor led to the CL quenching of ABEI. In the presence of a target analyte, it formed a complex with aptamer, and released ABEI-aptamer from AuNPs surface that resulted in CL recovery of ABEI. To test this design, a thrombin (used as a model analyte) aptasensor was prepared and evaluated. The results indicate that the proposed approach is simple and provided a linear range of 50-550 pM for thrombin detection with a detection limit of 15 pM. This new methodology can be easily extended to assay other biomolecules by simply changing the recognition sequence with the substrate aptamer.

Upconversion fluorescence resonance energy transfer biosensor with aromatic polymer nanospheres as the lable-free energy acceptor

We report a new upconversion fluorescence resonance energy transfer (UC-FRET) biosensor using poly-m-phenylenediamine (PMPD) nanospheres as the energy acceptor in this paper. A single-stranded DNA (ssDNA) tagged with a sulfydryl group at the 5'-terminus was covalently linked to poly(ethylenimine) (PEI) functionalized upconversion phosphors (UCPs, the energy donor). Because of the π-rich electronic structure of PMPD, self-assembly of the donor and the acceptor was achieved through the π-π stacking interaction between ssDNA and PMPD. The fluorescence of the donor was quenched by the acceptor in a PMPD-concentration-dependent manner. A maximum quenching degree of 90% was acquired, which was among the highest levels of all previous reports. Upon the formation of double-stranded DNA (dsDNA) between the target DNA and the probe DNA, the energy acceptor was separated from the donor due to the weakened interaction between dsDNA and PMPD. The fluorescence of UCPs was accordingly restored, and a linear response was obtained with the target concentration ranging from 0.1 to 6.0 nM. The limit of detection was calculated as 0.036 nM, which was a highly competitive sensitivity. The sensor also showed high precision, pronounced specificity, and the applicability to complicated sample matrix (human serum). The UCPs-PMPD FRET sensing platform takes advantages of both the optical merits of the upconversion donors and the superquenching ability and good water-solubility of the aromatic polymer nanoparticles. This study will open the opportunity to develop a new class of UC-FRET biosensors.

Aptamer carbon nanodot sandwich used for fluorescent detection of protein

Carbon nanodots (C-Dots) have attracted growing interest in recent years due to their low cost, ready scalability, excellent chemical stability, biocompatibility, colloidal stability, and resilience of photoluminescence. They have been employed as novel, ideal fluorescent probes for bio-imaging and smart sensing. In addition, taking advantage of their low-cytotoxicity, C-Dots have potential applications in biochemical and cell biological fields. Herein, we present the first assay with aptamer-functionalized C-Dots as a sensory platform for protein detection. The presence of thrombin can induce the aptamer-modified fluorescent C-Dots to form a sandwich structure with aptamer-functionalized silica nanoparticles through specific protein/aptamer interaction. The assay shows high specificity toward thrombin. A detection limit of 1 nM is obtained, which is significantly improved as compared to that of many previously reported fluorescence-based thrombin detection assays. Using other modified aptamers and antibodies instead of thrombin binding aptamers, this strategy may offer a suitable approach for detection of other proteins in biological, pharmaceutical and nano-mechanical applications.

Aptamer-based silver nanosensor for multiple protein detection

In the present work we design a novel aptamer-based silver nanosensor for one-spot simultaneously detection of multiple proteins. SS-DNA modified AgNPs were immobilized on the aldehyde coated glass slide to form an AgNP array. Then dye-labeled aptamer sequences were allowed to hybridize with their complementary strands assembled on the surface of AgNPs. The target proteins were introduced to associate with the corresponding aptamers to form the aptamer-target complexes. The removal of the aptamer-target complexes resulted in a remarkable decrease in fluorescent signals. This nanosensor is found to be highly sensitive for the detection of proteins. When thrombin was employed as a sample model, the limit of detection of the optimized nanosenor was 0.4 fmol with a linear response of 0.8 fmol to 0.5 pmol. We further demonstrated the multiple protein detection of IgE and thrombin using multicolor silver nanoprobes, which shows effective recognition of the relative protein individually or simultaneously. This silver nanosensor offers a unique heterogeneous approach for protein detection with several advantages, such as high sensitivity, rapidity, high throughput, and miniaturization.

A novel optical thrombin aptasensor based on magnetic nanoparticles and split DNAzyme

In this paper, we report a novel and sensitive optical sensing protocol for thrombin detection based on magnetic nanoparticles (MNPs) and thrombin aptamer, employing split HRP-mimicking DNAzyme halves as its sensing element, which can catalyze the H(2)O(2)-mediated oxidation of the colorless ABTS into a blue-green product. A single nucleotide containing the recognition element and sensing element is utilized in our protocol. The specific recognition of thrombin and its aptamer leads to the structure deformation of the DNA strands and causes the split of the DNAzyme halves. Therefore, the decrease of absorption spectra can be recorded by the UV-visible Spectrophotometer. DNA-coated MNPs are utilized to separate the interferential materials from the analyst, thus making this assay can be applied in the detection of thrombin in complex samples, such as human plasma. This original, sensitive and cost-effective assay showed favorable recognition for thrombin. The absorbance signals with the concentration of thrombin over a range from 0.5 to 20 nM and the detection limit of thrombin was 0.5 nM. The controlled experiments showed that thrombin signal was not interfered in the presence of other co-existence proteins.