A new strategy for the detection of adenosine triphosphate by aptamer/quantum dot biosensor based on chemiluminescence resonance energy transfer

Recent Applications of Quantum Dots in Pharmaceutical Analysis

Nanotechnology has emerged as one of the most potential areas for pharmaceutical analysis. The need for nanomaterials in pharmaceutical analysis is comprehended in terms of economic challenges, health and safety concerns. Quantum dots (QDs)or colloidal semiconductor nanocrystals are new groups of fluorescent nanoparticles that bind nanotechnology to drug analysis. Because of their special physicochemical characteristics and small size, QDs are thought to be promising candidates for the electrical and luminescent probes development. They were originally developed as luminescent biological labels, but are now discovering new analytical chemistry applications, where their photo-luminescent properties are used in pharmaceutical, clinical analysis, food quality control and environmental monitoring. In this review, we discuss QDs regarding properties and advantages, advances in methods of synthesis and their recent applications in drug analysis in the recent last years.

Recent achievements and advances in optical and electrochemical aptasensing detection of ATP based on quantum dots

The design and fabrication of high sensitive and selective biosensing platforms areessential goals to precisely recognize biomaterials in biological assays. In particular, determination of adenosine triphosphate (ATP) as the main energy currency of the cells and one of the most important biomolecules in living organisms is a pressing need in advanced biological detection. Recently, aptamer-based biosensors are introduced as a new direct strategy in which the aptamers (Apts) directly bind to the different targets and detect them on the basis of conformational changes and physical interactions. They can also be conjugated to optical and electronic probes such as quantum dot (QD) nanomaterials and provide unique QD aptasensing platforms. Currently, these Apt-based biosensors with excellent recognition features have attracted extensive attention due to the high specificity, rapid response and facile construction. Therefore, in this review article, recent achievements and advances in aptasensing detection of ATP based on different detection methods and types of QDs are discussed. In this regard, the optical and electrochemical aptasensors have been categorized based on detection methods; fluorescence (FL), electrochemiluminescence (ECL) and photoelectrochemical (PEC) and they have been also divided to two main groups based on QDs; metal-based (M-based) and carbon-based (C-based) materials. Then, their advantages and limitations have been highlighted, compared and discussed in detail.

Recent advances in the biosensing of neurotransmitters: material and method overviews towards the biomedical analysis of psychiatric disorders

Neurotransmitters are the most important messengers of the nervous system, and any changes in their balances and activities can cause serious neurological, psychiatric and cognitive disorders such as schizophrenia, Alzheimer's disease and Parkinson's disease.

A label-free hairpin aptamer probe for colorimetric detection of adenosine triphosphate based on the anti-aggregation of gold nanoparticles

A facile and rapid colorimetric approach was described for selective and sensitive determination of adenosine triphosphate (ATP) based on a hairpin aptamer probe and the anti-aggregation of AuNPs. Poly(diallyldimethylammonium chloride) (PDDA) can induce the aggregation of AuNPs due to the electrostatic interaction causing a red to blue color change. Upon the addition of ATP, aptamer-based hairpin probe is opened and releases flexible ssDNA ends. The released flexible ssDNA ends can interact with PDDA and prevent PDDA-induced AuNPs aggregation. Thus, a visible color change from blue to red and a decrease in the absorption ratio (A₆₁₀/A₅₂₀) are observed. Under the optimal conditions, the hairpin aptamer-based colorimetric assay exhibits high sensibility and selectivity for the detection of ATP with a detection limit of 1.7nM. Moreover, this assay is successfully used in the rapid determination of ATP in spiked human serum samples with good recoveries in the range of 102.88 to 104.07%.

An ''off-on'' phosphorescent aptasensor switch for the detection of ATP

An "off-on" phosphorescent aptasensor based on the 3-mercaptopropionic acid (MPA) capped Mn-doped ZnS quantum dots (MPA-Mn:ZnS QDs)/aptamer hybrid system was developed to detect adenosine triphosphate (ATP) in biological fluids. The phosphorescence of MPA-Mn:ZnS QDs was obviously quenched when ATP aptamer was added due to the aggregation induced effect. ATP aptamer, adsorbed on the surface of the phosphorescent MPA-Mn:ZnS QDs, has a high affinity for ATP. And then, with the addition of ATP, phosphorescence was gradually recovered because of the stronger special binding interaction between ATP and ATP aptamer than that between QDs and ATP aptamer. In this case, a high sensitivity and selectivity of phosphorescent aptasensor for the detection of ATP has constructed with a low detection limit of 0.9 nM and a wide linear range from 2 nM to 9 µM. What's more, the phosphorescent aptasensor does not require complex pretreatments and can effectively eliminate the interference from auto fluorescence and scattering light.

A simple and sensitive aptasensor for colorimetric detection of adenosine triphosphate based on unmodified gold nanoparticles

A simple and sensitive colorimetric aptasensor for rapid and facile detection of adenosine triphosphate (ATP) has been demonstrated here based on aptamer-based hairpin probes and unmodified gold nanoparticles (AuNPs). The hairpin probe is constructed by adding another five nucleotides to the 5'-end of an anti-ATP aptamer which can hybridize to nucleotides at the 3'-end of the aptamer, forming a hairpin-shaped structure. In the absence of ATP, the hairpin probes are rigid, and the AuNPs are susceptible to salt-induced aggregation. Conversely, upon binding with target ATP, the hairpin probes undergo conformational changes, forming aptamer-ATP complexes and exposing flexible ends which coat the surface of AuNPs to inhibit their aggregation in the high salt solution. Subsequently, a blue-to-red color change can be recognized by the naked eye. The aptasensor achieved selective responses toward ATP with a detection limit of 0.1μM, and exhibited high-quality detection performance in biological samples. In addition, this detection method is simple, rapid and cost-effective, holding great potential for further applications in point-of-care research.

Hybridization chain reaction-based colorimetric aptasensor of adenosine 5'-triphosphate on unmodified gold nanoparticles and two label-free hairpin probes

This work designs a new label-free aptasensor for the colorimetric determination of small molecules (adenosine 5'-triphosphate, ATP) by using visible gold nanoparticles as the signal-generation tags, based on target-triggered hybridization chain reaction (HCR) between two hairpin DNA probes. The assay is carried out referring to the change in the color/absorbance by salt-induced aggregation of gold nanoparticles after the interaction with hairpins, gold nanoparticles and ATP. To construct such an assay system, two hairpin DNA probes with a short single-stranded DNA at the sticky end are utilized for interaction with gold nanoparticles. In the absence of target ATP, the hairpin DNA probes can prevent gold nanoparticles from the salt-induced aggregation through the interaction of the single-stranded DNA at the sticky end with gold nanoparticles. Upon target ATP introduction, the aptamer-based hairpin probe is opened to expose a new sticky end for the strand-displacement reaction with another complementary hairpin, thus resulting in the decreasing single-stranded DNA because of the consumption of hairpins. In this case, gold nanoparticles are uncovered owing to the formation of double-stranded DNA, which causes their aggregation upon addition of the salt, thereby leading to the change in the red-to-blue color. Under the optimal conditions, the HCR-based colorimetric assay presents good visible color or absorbance responses for the determination of target ATP at a concentration as low as 1.0nM. Importantly, the methodology can be further extended to quantitatively or qualitatively monitor other small molecules or biotoxins by changing the sequence of the corresponding aptamer.

Optical Aptasensors for Adenosine Triphosphate

Nucleic acids are among the most researched and applied biomolecules. Their diverse two- and three-dimensional structures in conjunction with their robust chemistry and ease of manipulation provide a rare opportunity for sensor applications. Moreover, their high biocompatibility has seen them being used in the construction of in vivo assays. Various nucleic acid-based devices have been extensively studied as either the principal element in discrete molecule-like sensors or as the main component in the fabrication of sensing devices. The use of aptamers in sensors - aptasensors, in particular, has led to improvements in sensitivity, selectivity, and multiplexing capacity for a wide verity of analytes like proteins, nucleic acids, as well as small biomolecules such as glucose and adenosine triphosphate (ATP). This article reviews the progress in the use of aptamers as the principal component in sensors for optical detection of ATP with an emphasis on sensing mechanism, performance, and applications with some discussion on challenges and perspectives.

In Situ Generation and Consumption of H2O2 by Bienzyme-Quantum Dots Bioconjugates for Improved Chemiluminescence Resonance Energy Transfer

Exploration of quantum dots (QDs) as energy acceptors revolutionizes the current chemiluminescence resonance energy transfer (CRET), since QDs possess large Stokes shifts and high luminescence efficiency. However, the strong and high concentration of oxidant (typically H2O2) needed for luminol chemiluminescence (CL) reaction could cause oxidative quenching to QDs, thereby decreasing the CRET performance. Here we proposed the use of bienzyme-QDs bioconjugate as the energy acceptor for improved CRET sensing. Two enzymes, one for H2O2 generation (oxidase) and another for H2O2 consumption (horseradish peroxidase, HRP), were bioconjugated onto the surface of QDs. The bienzyme allowed fast in situ cascaded H2O2 generation and consumption, thus alleviating fluorescence quenching of QDs. The nanosized QDs accommodate the two enzymes in a nanometric range, and the CL reaction was confined on the surface of QDs accordingly, thereby amplifying the CL reaction rate and improving CRET efficiency. As a result, CRET efficiency of 30-38% was obtained; the highest CRET efficiency by far was obtained using QDs as the energy acceptor. The proposed CRET system could be explored for ultrasensitive sensing of various oxidase substrates (here exemplified with cholesterol, glucose, and benzylamine), allowing for quantitative measurement of a spectrum of metabolites with high sensitivity and specificity. Limits of detection (LOD, 3σ) for cholesterol, glucose, and benzylamine were found to be 0.8, 3.4, and 10 nM, respectively. Furthermore, multiparametric blood analysis (glucose and cholesterol) is demonstrated.

Detection of adenosine triphosphate in HeLa cell using capillary electrophoresis-laser induced fluorescence detection based on aptamer and graphene oxide

A method for ATP quantification based on dye-labeled aptamer/graphene oxide (aptamer/GO) using capillary electrophoresis-laser induced fluorescence (CE-LIF) detecting technique has been established. In this method, the carboxyfluorescein (FAM)-labelled ATP aptamers were adsorbed onto the surface of GO, leading to the fluorescence quenching of FAM; after the incubation with a limited amount of ATP, stronger affinity between ATP aptamer and ATP resulted in the desorption of aptamers and the fluorescence restoration of FAM. Then, aptamer-ATP complex and excess of aptamer/GO and GO were separated and quantified by CE-LIF detection. It was shown that a linear relation was existing in the CE-LIF peak intensity of aptamer-ATP and ATP concentration in range of 10-700 μM, the regression equation was F=1.50+0.0470C(ATP) (R(2)=0.990), and the limit of detection was 1.28 μM (3S/N, n=5), which was one order magnitude lower than that of detection in solution by fluorescence method. The approach with excellent specificity and reproducibility has been successfully applied to detecting concentration of ATP in HeLa cell.

A sensitive aptasensor for colorimetric detection of adenosine triphosphate based on the protective effect of ATP-aptamer complexes on unmodified gold nanoparticles

Adenosine triphosphate (ATP) is the most direct source of energy in organisms. This study is the first to demonstrate that ATP-aptamer complexes provide greater protection for unmodified gold nanoparticles (AuNPs) against salt-induced aggregation than either aptamer or ATP alone. This protective effect was confirmed using transmission electron microscopy, dynamic light scattering, Zeta potential measurement, and fluorescence polarization techniques. Utilizing controlled particle aggregation/dispersion as a gauge, a sensitive and selective aptasensor for colorimetric detection of ATP was developed using ATP-binding aptamers as the identification element and unmodified AuNPs as the probe. This aptasensor exhibited a good linear relationship between the absorbance and the logarithm concentration of ATP within a 50-1000 nM range. ATP analogs such as guanosine triphosphate, uridine triphosphate and cytidine triphosphate resulted in little or no interference in the determination of ATP.

Blu-ray based optomagnetic aptasensor for detection of small molecules

This paper describes an aptamer-based optomagnetic biosensor for detection of a small molecule based on target binding-induced inhibition of magnetic nanoparticle (MNP) clustering. For the detection of a target small molecule, two mutually exclusive binding reactions (aptamer-target binding and aptamer-DNA linker hybridization) are designed. An aptamer specific to the target and a DNA linker complementary to a part of the aptamer sequence are immobilized onto separate MNPs. Hybridization of the DNA linker and the aptamer induces formation of MNP clusters. The target-to-aptamer binding on MNPs prior to the addition of linker-functionalized MNPs significantly hinders the hybridization reaction, thus reducing the degree of MNP clustering. The clustering state, which is thus related to the target concentration, is then quantitatively determined by an optomagnetic readout technique that provides the hydrodynamic size distribution of MNPs and their clusters. A commercial Blu-ray optical pickup unit is used for optical signal acquisition, which enables the establishment of a low-cost and miniaturized biosensing platform. Experimental results show that the degree of MNP clustering correlates well with the concentration of a target small molecule, adenosine triphosphate (ATP) in this work, in the range between 10µM and 10mM. This successful proof-of-concept indicates that our optomagnetic aptasensor can be further developed as a low-cost biosensing platform for detection of small molecule biomarkers in an out-of-lab setting.

Quantum dot-DNA aptamer conjugates coupled with capillary electrophoresis: A universal strategy for ratiometric detection of organophosphorus pesticides

Based on the highly sensitivity and stable-fluorescence of water-soluble CdTe/CdS core-shell quantum dots (QDs) with broad-specificity DNA aptamers, a novel ratiometric detection strategy was proposed for the sensitive detection of organophosphorus pesticides by capillary electrophoresis with laser-induced fluorescence (CE-LIF). The as-prepared QDs were first conjugated with the amino-modified oligonucleotide (AMO) by amidation reaction, which is partial complementary to the DNA aptamer of organophosphorus pesticides. Then QD-labeled AMO (QD-AMO) was incubated with the DNA aptamer to form QD-AMO-aptamer duplex. When the target organophosphorus pesticides were added, they could specifically bind the DNA aptamer, leading to the cleavage of QD-AMO-aptamer duplex, accompany with the release of QD-AMO. As a result, the ratio of peak height between QD-AMO and QD-AMO-aptamer duplex changed in the detection process of CE-LIF. This strategy was subsequently applied for the detection of phorate, profenofos, isocarbophos, and omethoate with the detection limits of 0.20, 0.10, 0.17, and 0.23μM, respectively. This is the first report about using QDs as the signal indicators for organophosphorus pesticides detection based on broad-specificity DNA aptamers by CE-LIF, thus contributing to extend the scope of application of QDs in different fields. The proposed method has great potential to be a universal strategy for rapid detection of aptamer-specific small molecule targets by simply changing the types of aptamer sequences.

A cascade amplification strategy based on rolling circle amplification and hydroxylamine amplified gold nanoparticles enables chemiluminescence detection of adenosine triphosphate

A highly sensitive and selective chemiluminescent (CL) biosensor for adenosine triphosphate (ATP) was developed by taking advantage of the ATP-dependent enzymatic reaction (ATP-DER), the powerful signal amplification capability of rolling circle amplification (RCA), and hydroxylamine-amplified gold nanoparticles (Au NPs). The strategy relies on the ability of ATP, a cofactor of T4 DNA ligase, to trigger the ligation-RCA reaction. In the presence of ATP, the T4 DNA ligase catalyzes the ligation reaction between the two ends of the padlock probe, producing a closed circular DNA template that initiates the RCA reaction with phi29 DNA polymerase and dNTP. Therein, many complementary copies of the circular template can be generated. The ATP-DER is eventually converted into a detectable CL signal after a series of processes, including gold probe hybridization, hydroxylamine amplification, and oxidative gold metal dissolution coupled with a simple and sensitive luminol CL reaction. The CL signal is directly proportional to the ATP level. The results showed that the detection limit of the assay is 100 pM of ATP, which compares favorably with those of other ATP detection techniques. In addition, by taking advantage of ATP-DER, the proposed CL sensing system exhibits extraordinary specificity towards ATP and could distinguish the target molecule ATP from its analogues. The proposed method provides a new and versatile platform for the design of novel DNA ligation reaction-based CL sensing systems for other cofactors. This novel ATP-DER based CL sensing system may find wide applications in clinical diagnosis as well as in environmental and biomedical fields.

Highly efficient colorimetric detection of ATP utilizing a split aptamer target binding strategy and superior catalytic activity of graphene oxide–platinum/gold nanoparticles

The specific binding of ATP and its aptamer linked the split aptamer-modified GO/PDDA/PtAuNPs and magnetic beads together. Using magnetic separation, TMB was catalyzed into a colored product by nanocomposites, which enabled rapid detection of ATP.

Composite silica coated gold nanosphere and quantum dots nanoparticles for X-ray CT and fluorescence bimodal imaging

A composite nanoparticle containing QDs and Au was fabricated, and it was used for CT and fluorescence dual mode imaging simultaneously.

PEI/Zr⁴⁺-coated nanopore for selective and sensitive detection of ATP in combination with single-walled carbon nanotubes

By virtue of a biomimetic nanopore and single-walled carbon nanotubes, a new sensor for adenosine triphosphate (ATP) detection is designed. As compared to the routine approach, the present scenario does not entail the surface modification of nanopore with analyte-specific probes. The underlying mechanism relies on a symmetric nanopore sequentially modified with polyethyleneimine (PEI) and Zr(4+) that can quantitate the concentration of ATP-bound aptamer, while other free aptamers are removed by single-walled carbon nanotubes (SWNTs). The detection limit of the nanopore sensor is 27.46 nM, and the linear range is from 50 nM to 400 nM. The biosensor with an excellent selectivity against guanosine triphosphate (GTP), uridine triphosphate (UTP), and cytosine triphosphate (CTP) can be applied in the real samples such as Hela cell.

Enhanced chemiluminescence of CdTe quantum dots-H₂O₂ by horseradish peroxidase-mimicking DNAzyme

In this study, it was found that horseradish peroxidase (HRP)-mimicking DNAzyme could effectively enhance the CL emission of CdTe quantum dots (QDs)-H2O2 system, whereas HRP could not enhance the CL intensity. The CL enhancement mechanism was investigated, and the CL enhancement was supposed to originate from the catalysis of HRP-mimicking DNAzyme on the CL reaction between CdTe QDs and H2O2. Meantime, compared with CdTe QDs-H2O2 CL system, H2O2 concentration was markedly decreased in QDs-H2O2-HRP-mimicking DNAzyme CL system, improving the stability of QDs-H2O2 CL system. The QDs-based CL system was used to detect sensitively CdTe QDs and HRP-mimicking DNAzyme (as biologic labels). This work gives a path for enhancing CL efficiency of QDs system, and will be helpful to promote the step of QDs application in various fields such as bioassay and trace detection of analyte.

Luminol-based chemiluminescent signals: clinical and non-clinical application and future uses

Chemiluminescence (CL) is an important method for quantification and analysis of various macromolecules. A wide range of CL agents such as luminol, hydrogen peroxide, fluorescein, dioxetanes and derivatives of oxalate, and acridinium dyes are used according to their biological specificity and utility. This review describes the application of luminol chemiluminescence (LCL) in forensic, biomedical, and clinical sciences. LCL is a very useful detection method due to its selectivity, simplicity, low cost, and high sensitivity. LCL has a dynamic range of applications, including quantification and detection of macro and micromolecules such as proteins, carbohydrates, DNA, and RNA. Luminol-based methods are used in environmental monitoring as biosensors, in the pharmaceutical industry for cellular localization and as biological tracers, and in reporter gene-based assays and several other immunoassays. Here, we also provide information about different compounds that may enhance or inhibit the LCL along with the effect of pH and concentration on LCL. This review covers most of the significant information related to the applications of luminol in different fields.

Fluorescence detection of adenosine-5'-triphosphate and alkaline phosphatase based on the generation of CdS quantum dots

We have developed an analytical method to detect adenosine-5'-triphosphate (ATP) and alkaline phosphatase (ALP) based on the generation of CdS quantum dots (QDs). We demonstrated that Cd(2+) cation reacts with S(2-) anion to generate fluorescent CdS QDs in the presence of some certain amount of ATP. With increase in the ATP concentration, the fluorescence intensity of CdS QDs was also enhanced. ATP can be converted into adenosine by the dephosphorylation of ALP, so that the generation of CdS QDs would be inhibited in the presence of ALP. Therefore, this novel analysis system could be applied to assay ATP and ALP based on the growth of fluorescent CdS QDs.

Target recycling amplification for label-free and sensitive colorimetric detection of adenosine triphosphate based on un-modified aptamers and DNAzymes

Based on target recycling amplification, the development of a new label-free, simple and sensitive colorimetric detection method for ATP by using un-modified aptamers and DNAzymes is described. The association of the model target molecules (ATP) with the corresponding aptamers of the dsDNA probes leads to the release of the G-quadruplex sequences. The ATP-bound aptamers can be further degraded by Exonuclease III to release ATP, which can again bind the aptamers of the dsDNA probes to initiate the target recycling amplification process. Due to this target recycling amplification, the amount of the released G-quadruplex sequences is significantly enhanced. Subsequently, these G-quadruplex sequences bind hemin to form numerous peroxidase mimicking DNAzymes, which cause substantially intensified color change of the probe solution for highly sensitive colorimetric detection of ATP down to the sub-nanomolar (0.33nM) level. Our method is highly selective toward ATP against other control molecules and can be performed in one single homogeneous solution, which makes our sensing approach hold great potential for sensitive colorimetric detection of other small molecules and proteins.

Optical aptasensors for quantitative detection of small biomolecules: a review

Aptasensors are aptamer-based biosensors with excellent recognition capability towards a wide range of targets. Specially, there have been ever-growing interests in the development of aptasensors for the detection of small molecules. This phenomenon is contributed to two reasons. On one hand, small biomolecules play an important role in living organisms with many kinds of biological function, such as antiarrhythmic effect and vasodilator activity of adenosine. On the other hand, the concentration of small molecules can be an indicator for disease diagnosis, for example, the concentration of ATP is closely associated with cell injury and cell viability. As a potential analysis tool in the construction of aptasensors, optical analysis has attracted much more interest of researchers due to its high sensitivity, quick response and simple operation. Besides, it promises the promotion of aptasensors in performance toward a new level. Review the development of optical aptasensors for small biomolecules will give readers an overall understanding of its progress and provide some theoretical guidelines for its future development. Hence, we give a mini-review on the advance of optical aptasensors for small biomolecules. This review focuses on recent achievements in the design of various optical aptasensors for small biomolecules, containing fluorescence aptasensors, colorimetric aptasensors, chemiluminescence aptasensors and other optical aptasensors.

Homogeneous assay of target molecules based on chemiluminescence resonance energy transfer (CRET) using DNAzyme-linked aptamers

We have designed a single-stranded DNAzyme-aptamer sensor for homogeneous target molecular detection based on chemiluminescence resonance energy transfer (CRET). The structure of the engineered single-stranded DNA (ssDNA) includes the horseradish peroxidase (HRP)-like DNAzyme, optimum-length linker (10-mer-length DNA), and target-specific aptamer sequences. A quencher dye was modified at the 3' end of the aptamer sequence. The incorporation of hemin into the G-quadruplex structure of DNAzyme yields an active HRP-like activity that catalyzes luminol to generate a chemiluminescence (CL) signal. In the presence of target molecules, such as ochratoxin A (OTA), adenosine triphosphate (ATP), or thrombin, the aptamer sequence was folded due to the formation of the aptamer/analyte complex, which induced the quencher dye close to the DNAzyme structure. Consequently, the CRET occurred between a DNAzyme-catalyzed chemiluminescence reaction and the quencher dye. Our results showed that CRET-based DNAzyme-aptamer biosensing enabled specific OTA analysis with a limit of detection of 0.27ng/mL. The CRET platform needs no external light source and avoids autofluorescence and photobleaching, and target molecules can be detected specifically and sensitively in a homogeneous manner.

Cysteamine CdS quantum dots decorated with Fe3+ as a fluorescence sensor for the detection of PPi

A new sensitive and selective fluorescence sensor for the detection of pyrophosphate (PPi) in aqueous media based on the Fe(3+) decorated cysteamine CdS QDs ([Cys-CdS QDs]-Fe(3+)) was proposed. The presence of PPi can induce the fluorescence quenching of [Cys-CdS QDs]-Fe(3+) due to the high formation constants between the phosphate group and Fe(3+). Because the complex between Fe(3+) and PPi acts as an efficient quencher, the concentration of PPi can be evaluated by tracking the degree of fluorescence quenching. The fabricated sensor was optimized to obtain the best sensor selectivity and sensitivity. Under optimal conditions, a linear relationship between the fluorescence response and the concentration of PPi was established in the range of 0.5-10 μM. The limits of detection and quantitation for PPi were found to be 0.11 and 2.78 μM, respectively. Furthermore, the proposed sensor exhibited high selectivity toward PPi relative to other common anions. The proposed sensor was successfully applied to the detection of PPi in urine samples with satisfactory results.

A label-free conjugated polymer-based fluorescence assay for the determination of adenosine triphosphate and alkaline phosphatase

A sensor was developed based on the quenching effect of Cu²⁺ on PPESO₃ and the hydrolysis of ATP by ALP.

Quantum dot approaches for target-based drug screening and multiplexed active biosensing

Biomolecule detection using quantum dots (Qdots), nanometer-sized semiconductor crystals, effectively addresses the limitations associated with conventional optical and biochemical techniques, as Qdots offer several key advantages over traditional fluorophores. In this minireview, we discuss the role of Qdots as a central nanoscaffold for the polyvalent assembly of multifunctional biomolecular probes and describe recent advances in Qdot-based biorecognition. Specifically, we focus on Qdot applications in target-based, drug screening assays and real-time active biosensing of cellular processes.

Exciton energy transfer-based fluorescent sensing through aptamer-programmed self-assembly of quantum dots

A novel exciton energy transfer-based ultrasensitive fluorescent sensing strategy for the detection of biological small molecules and protein has been established through split aptamer-programmed self-assembly of quantum dots (QDs). The signal is produced from exciton energy transfer of the self-assembled QDs. The recognition is accomplished using an aptamer sensor scaffold designed with two split fragment sequences, which specifically bind to the model analytes. The extent of particle assembly, induced by the analyte-triggered self-assembly of QDs, leads to an exciton energy transfer effect between interparticles, giving a readily detectable fluorescent quenching and red shift of the emission peak, which enables us to quantitate the target in dual signal modes. The application of the technique is well demonstrated using two representative split aptamer-based model systems for the detection of adenosine and thrombin. The sensitivity of this exciton energy transfer-based fluorescent sensing is much better than that of plasmonic coupling-based colorimetric methods. Limit of detections (LODs) down to 12 nM and 15 pM can be achieved for adenosine and thrombin, respectively. The sensing strategy is proposed as a general platform for robust and specific aptamer-target analysis which could be further developed to monitor a wide range of target analytes. The concept and methodology developed in this work shows a good promise in the study of molecular binding events in the biological and medical applications.

An ultrasensitive quantum dots fluorescent polarization immunoassay based on the antibody modified Au nanoparticles amplifying for the detection of adenosine triphosphate

In this work, an ultrasensitive fluorescent polarization immunoassay (FPIA) method based on the quantum dot/aptamer/antibody/gold nanoparticles ensemble has been developed for the detection of adenosine triphosphate (ATP). DNA hybridization is formed when ATP is present in the PBS solution containing the DNA-conjugated quantum dots (QDs) and antibody-AuNPs. The substantial sensitivity improvement of the antibody-AuNPs-enhanced method is mainly attributed to the slower rotation of fluorescent unit when QDs-labeled oligonucleotides hybridize with antibody modified the gold nanoparticle. As a result, the fluorescent polarization (FP) values of the system increase significantly. Under the optimal conditions, a linear response with ATP concentration is ranged from 8×10(-12) M to 2.40×10(-4) M. The detection limit reached as low as 1.8 pM. The developed work provides a sensitive and selective immunoassay protocol for ATP detection, which could be applied in more bioanalytical systems.

Label-free chemiluminescent ATP aptasensor based on graphene oxide and an instantaneous derivatization of guanine bases

In this work, a novel label-free chemiluminescent (CL) aptasensor has been developed for rapid and facile detection of adenosine triphosphate (ATP, as model analyte) using graphene oxide (GO) nano-platform. The strategy relies on the preferential binding of GO to single-stranded DNA (ssDNA) over rigid double-stranded DNA (dsDNA) or aptamer-target complexes, and the instantaneous derivative reaction between phenylglyoxal (PGO), a special CL reagent as the signaling molecule, and guanine nucleobases (G) of aptamer strands adsorbed on the surface of GO. In the absence of ATP, the aptamers adsorbed onto the surface of GO leading to a strong background CL signal. Conversely, in the presence of ATP, the aptamers formed the aptamer-ATP complexes which had weak binding ability to GO resulting in a significant CL signal decrease. The CL intensity was adversely related to the ATP concentration in the assay solution. The biosensor's signal decreased linearly with the logarithm of the concentration of ATP from 2 to 80 nmol with a detection limit of 1.4 nmol. The aptasensor also showed high selectivity against cytosine triphosphate (CTP), guanosine triphosphate (GTP), and uridine triphosphate (UTP). The method presented here holds the advantages of being label-free, cost effective, rapid, sensitive and selective, which would shows great promise for clinical application.

Gold nanotip array for ultrasensitive electrochemical sensing and spectroscopic monitoring

A gold nanotip array platform with a combination of ultrasensitive electrochemical sensing and spectroscopic monitoring capability is reported. Adenosine triphosphate is detected down to 1 pM according to the impedance changes in response to aptamer-specific binding. Furthermore, the local molecular information can be monitored at the individual plasmonic nanotips, and hence provide the capability for a better understanding of complex biological processes.

Protein-binding aptamer assisted signal amplification for the detection of influenza A (H1N1) DNA sequences based on quantum dot fluorescence polarization analysis

We report a fluorescence polarization platform for H1N1 detection based on the construction of a DNA functional QD fluorescence polarization probe and a bi-functional protein binding aptamer (Apt-DNA). The assay has a linear range from 10 nM to 100 nM with a detection limit of 3.45 nM and is selective over the mismatched bases.

A novel aptasensor for the ultra-sensitive detection of adenosine triphosphate via aptamer/quantum dot based resonance energy transfer

We designed a novel aptamer based biosensor (aptasensor) for ultrasensitive detection of adenosine triphosphate (ATP) through resonance energy transfer (RET). The ATP aptamer was modified with Cy3 at the 3' end, and a green quantum dot (525) was attached to the 5' end of its complementary sequence respectively. The ATP aptamer and its complementary sequence could assemble into a duplex structure in the absence of target ATP, and then decrease the distance between the quantum dot and Cy3 which could produce significant RET signal. Upon ATP binding, the ATP aptamer could dissociate with its complementary sequence and then increase the distance between the quantum dot and Cy3 which would significantly decrease the RET signal. Therefore, the ATP detection could be easily achieved through detection of the fluorescence intensity ratio between 525 nm and 560 nm. The results show that the emission fluorescence intensity ratio of 525/560 is linearly related to the logarithmic concentration of ATP. The linear range of this aptasensor is from 0.1 nM to 1 μM, and the detection limit is lower down to 0.01 nM. Excellent selectivity of this aptasensor for ATP has been demonstrated through the detection of thymidine triphosphate (TTP), cytidine triphosphate (CTP), guanosine triphosphate (GTP) and adenosine diphosphate (ADP) respectively as control. The method we described here could easily detect ATP with excellent selectivity, linearity and sensitivity down to the nanomolar range, as well as avoid photobleaching.

Aptamer-modified nanoparticles as biosensors

Aptamers are short oligonucleotides that are capable of selectively binding to their corresponding target. Therefore, they can be thought of as a nucleic acid-based alternative to antibodies and can substitute for their amino acid-based counterparts in analytical applications, including as receptors in biosensors. Here they offer several advantages because their nucleic acid nature and their binding via an induced fit mechanism enable novel sensing strategies. In this article, the utilization of aptamers as novel bio-receptors in combination with nanoparticles as transducer elements is reviewed. In addition to these analytical applications, the medical relevance of aptamer-modified nanoparticles is described.