Water-soluble conjugated polymer as a platform for adenosine deaminase sensing based on fluorescence resonance energy transfer technique

Fluorogenic target competitors for developing label-free and sensitive folding-unswitching aptamer sensors

BACKGROUND

Aptamers have aroused tremendous applications in sensors, drug deliveries, diagnosis, and therapies. In particular, target-induced global structure switching of aptamers has been widely used to develop selective sensors. However, fluorophore and/or quencher modification, sequence elongation, and nano-interface adsorption are required to design such global structure-switching aptamer sensors (SSAS) in order to signal target binding events. Accordingly, these requirements make SSAS at a high cost and expense of sensors' sensitivity. In this aspect, efforts should be made to overcome these drawbacks of SSAS.

RESULTS

Herein, we tried to develop label-free folding-unswitching aptamer sensors (FUAS) by searching fluorogenic target competitors. Using adenine nucleoside/nucleotide as the proof-of-concept model targets, we screened out berberine (BER) from natural isoquinoline alkaloids (having rings comparable to targets) as the best fluorogenic target competitor. Binding of BER at the conserved nucleotides of intact aptamer foldings turned on this fluorogenic target competitor' fluorescence. Targets then competed with this fluorogenic target competitor over the same conserved nucleotides to cause its release in favor of a resultant fluorescence change. We found that the developed FUAS are much more sensitive than the previously reported SSAS. The FUAS were successfully applied to assays of ATP and adenosine deaminase in serums, and to screening of the adenosine deaminase's inhibitor, verifying the reliability and applicability of this FUAS platform in variant fields.

SIGNIFICANCE

We demonstrate that by designing fluorogenic target competitors, FUAS can be alternatively developed in a label-free manner and with a higher sensitivity than the previously developed SSAS. This work opens a new way to develop high-performance aptamer-based sensors. Furthermore, our developed FUAS should inspire more interest for wide applications incluidng target-triggered drug deliveries when therapeutic fluorogenic target competitors are used.

Sensing, Imaging, and Therapeutic Strategies Endowing by Conjugate Polymers for Precision Medicine

Conjugated polymers (CPs) have promising applications in biomedical fields, such as disease monitoring, real-time imaging diagnosis, and disease treatment. As a promising luminescent material with tunable emission, high brightness and excellent stability, CPs are widely used as fluorescent probes in biological detection and imaging. Rational molecular design and structural optimization have broadened absorption/emission range of CPs, which are more conductive for disease diagnosis and precision therapy. This review provides a comprehensive overview of recent advances in the application of CPs, aiming to elucidate their structural and functional relationships. The fluorescence properties of CPs and the mechanism of detection signal amplification are first discussed, followed by an elucidation of their emerging applications in biological detection. Subsequently, CPs-based imaging systems and therapeutic strategies are illustrated systematically. Finally, recent advancements in utilizing CPs as electroactive materials for bioelectronic devices are also investigated. Moreover, the challenges and outlooks of CPs for precision medicine are discussed. Through this systematic review, it is hoped to highlight the frontier progress of CPs and promote new breakthroughs in fundamental research and clinical transformation.

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.

Integration of phosphate functionalized Pt/TiO2 and Ru(bpy)32+ sensitization for ultrasensitive assay of adenosine deaminase activity on a novel split-typed PEC aptasensor

To date, it is still a challenge for high-performance photoelectrochemical (PEC) assay of low-abundance adenosine deaminase (ADA) in fundamental research and clinical diagnosis. Herein, phosphate-functionalized Pt/TiO₂ (termed PO₄^3-/Pt/TiO₂) was prepared as ideal photoactive material to develop a split-typed PEC aptasensor for detection of ADA activity, coupled by a Ru(bpy)₃²⁺ sensitization strategy. We critically studied the effects of the PO₄^3- and Ru(bpy)₃²⁺ on the detection signals, and discussed the signal-amplified mechanism. Specifically, hairpin-structured adenosine (AD) aptamer was splited into single chain via ADA-induced catalytic reaction, and subsequently hybridized with complementary DNA (cDNA, initially coating on magnetic beads). The in-situ formed double-stranded DNA (dsDNA) was further intercalated by more Ru(bpy)₃²⁺ to amplify the photocurrents. The resultant PEC biosensor showed a broader linear range of 0.05-100 U L^-1 and a lower limit of detection (0.019 U L^-1), which can fill the blank for analysis of ADA activity. This research would provide some valuable insights for building advanced PEC aptasensors in ADA-related research and clinical diagnosis.

A Poly(carbazole-alt-triazole) with Thiabendazole Side Groups as an "On-Off-On" Fluorescent Probe for Detection of Cu(II) Ion and Cysteine

A novel conjugated polymer PCZBTA-TBZ containing thiabendazole as recognition unit was synthesized via Suzuki coupling reaction, and its structural characterization, spectroscopic analysis and photophysical properties were investigated. In the metal ion response study, the addition of Cu²⁺ led to the occurrence of the photoinduced electron transfer (PET) mechanism, which significantly quenched the fluorescence of the polymer PCZBTA-TBZ with a quenching effect of 98%. Furthermore, I^- can significantly quench the fluorescence of the polymer, but other anions have no such effect. According to the density functional theory calculation, compared with other polycarbazoles or other alternative copolymers containing carbazole, with alternating carbazole and triazole enhances the electron mobility and reduces the energy band gap of the polymer. Due to the strong coordination ability between Cu²⁺ and Cys, the adding Cys competes the Cu²⁺ in the [PCZBTA-TBZ-Cu²⁺] complex, blocking the occurrence of PET, and the fluorescence intensity of PCZBTA-TBZ is restored. The addition of other amino acids caused almost no change. The polymer is expected to be used for dual fluorescence detection of specific metal ions and Cys.

Gold nanoparticle-based signal amplified electrochemiluminescence for biosensing applications

Since the content levels of biomarkers at the early stage of many diseases are generally lower than the detection threshold concentration, achieving ultrasensitive and accurate detection of these biomarkers is still one of the major goals in bio-analysis. To achieve ultrasensitive and reliable bioassay, it requires developing highly sensitive biosensors. Among all kinds of biosensors, electrogenerated chemiluminescence (ECL) based biosensors have attracted enormous attention due to their excellent properties. In order to improve the performance of ECL biosensors, gold nanoparticles (Au NPs) have been widely utilized as signal amplification tags. The introduction of Au NPs could dramatically enhance the performance of the constructed ECL biosensors via diverse ways such as electrode modification material, efficient energy acceptor in ECL resonant energy transfer (ECL-RET), reaction catalyst, surface plasmon resonance (SPR) enhancer, and as nanocarrier. Herein, we summarize recent developments and progress of ECL biosensors based on Au NPs signal amplification strategies. We will cover ECL applications of Au NPs as a signal amplification tag in the detection of proteins, metal ions, nucleic acids, small molecules, living cells, exosomes, and cell imaging. Finally, brief summary and future outlooks of this field will be presented.

Detection of Streptavidin Based on Terminal Protection and Cationic Conjugated Polymer-Mediated Fluorescence Resonance Energy Transfer

In this paper, a fast and simple strategy for sensitive detection of streptavidin (SA) was proposed based on terminal protection of small molecule-linked DNA and cationic conjugated polymer-mediated fluorescence resonance energy transfer (FRET). In principle, we designed a biotin-labelled DNA probe (P1) as the recognitive probe of SA, along with a complementary DNA probe (P2) to form double-stranded DNA (dsDNA) with P1. SYBR Green I (SG I) as a fluorescent dye was further used to specifically bind to dsDNA to emit stronger fluorescence. The cationic poly[(9,9-bis(6′-N,N,N-triethy-lammonium)hexyl) fluorenylene phenylene dibromide] (PFP) acted as the donor to participate in the FRET and transfer energy to the recipient SG I. In the absence of SA, P1 could not hybridize with P2 to form dsDNA and was digested by exonuclease I (Exo I); thus, only a weak FRET signal would be observed. In the presence of SA, biotin could specifically bind to SA, which protected P1 from Exo I cleavage. Then, P1 and P2 were hybridized into dsDNA. Therefore, the addition of SG I and PFP led to obvious FRET signal due to strong electrostatic interactions. Then, SA can be quantitatively detected by monitoring FRET changes. As the whole reagent reaction was carried out in 1.5 mL EP and detected in the colorimetric dish, the operation process of the detection system was relatively simple. The response time for each step was also relatively short. In this detection system, the linear equation was obtained for SA from 0.1 to 20 nM with a low detection limit of 0.068 nM (S/N = 3). In addition, this strategy has also achieved satisfactory results in the application of biological samples, which reveals the application prospect of this method in the future.

Energy transduction through FRET in self-assembled soft nanostructures based on surfactants/polymers: current scenario and prospects

The self-assembled systems of surfactants/polymers, which are capable of supporting energy funneling between fluorophores, have recently gained significant attraction. Surfactant and polymeric micelles form nanoscale structures spanning a radius of 2-10 nm are generally suitable for the transduction of energy among fluorophores. These systems have shown great potential in Förster resonance energy transfer (FRET) due to their unique characteristics of being aqueous based, tendency to remain self-assembled, spontaneous formation, tunable nature, and responsiveness to different external stimuli. This review presents current developments in the field of energy transfer, particularly the multi-step FRET processes in the self-assembled nanostructures of surfactants/polymers. The part one of this review presents a background and brief overview of soft systems and discusses certain aspects of the self-assemblies of surfactants/polymers and their co-solubilization property to bring fluorophores to close proximity to transduce energy. The second part of this review deals with single-step and multi-step FRET in the self-assemblies of surfactants/polymers and links FRET systems with advanced smart technologies including multicolor formation, data encryption, and artificial antenna systems. This review also discusses the diverse examples in the literature to present the emerging applications of FRET. Finally, the prospects regarding further improvement of FRET in self-assembled soft systems are outlined.

Developed a novel sensor based on fluorescent graft conjugated polymer for the determination of aristolochic acid in traditional Chinese medicine

A novel fluorescent graft conjugated polymer (poly (2, 5-bis (Polyethylene glycol oxybutyrate)-1, 4-phenylethynylene-alt-1, 4-phenyleneethynylene, PPE-OB-PEG) has been designed and synthesized for the determination of aristolochic acid (AA). The detection conditions and detection characters of PPE-OB-PEG were systematically explored in this work. The fluorescence intensity of PPE-OB-PEG changes with the different concentration of AA. PPE-OB-PEG has a good linear range towards AA from 1.00 × 10^-7 to 8.00 × 10^-5 mol L^-1 and the limit of detection (LOD) is 3.00 × 10^-8 mol L^-1 (S/N = 3). PPE-OB-PEG have been applied to detect AA in traditional Chinese medicine samples and the results are satisfactory. The experimental results show that PPE-OB-PEG can be used as a fluorescence probe for rapid and sensitive detection of AA.

Development of a Simple Assay Method for Adenosine Deaminase via Enzymatic Formation of an Inosine-Tb3+ Complex

Adenosine deaminase (ADA), which catalyzes the irreversible deamination of adenosine to inosine, is related to various human diseases such as tuberculous peritonitis and leukemia. Therefore, the method used to detect ADA activity and screen the effectiveness of various inhibitor candidates has important implications for the diagnosis treatment for various human diseases. A simple and rapid assay method for ADA, based on the enzymatic formation of a luminescent lanthanide complex, is proposed in this study. Inosine, an enzymatic product of ADA with stronger sensitization efficiency for Tb³⁺ than adenosine, produced a strong luminescence by forming an inosine-Tb³⁺ complex, and it enabled the direct monitoring of ADA activity in real-time. By introducing only Tb³⁺ to adenosine and ADA in the buffer, the enhancement of luminescence enabled the detection of a low concentration of ADA (detection limit 1.6 U/L). Moreover, this method could accurately determine the inhibition efficiency (IC₅₀) of the known ADA inhibitor, erhythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), and the inhibition of ADA could be confirmed by the naked eye. Considering its simplicity, this assay could be extended to the high-throughput screening of various ADA inhibitor candidates.

Sensitive monitoring of RNA transcription by optical amplification of cationic conjugated polymers

We reported a new strategy for sensitive monitoring in vitro RNA synthesis in real time based on fluorescence resonance energy transfer (FRET) from water-soluble conjugated polymer poly (9, 9-bis (6'-N, N, N,-trimethylammonium) hexyl) fluorene-co-alt-1,4-phenylene) bromide (PFP) to fluorogenic RNA aptamer/fluorophore (Spanich2/DFHBI and Broccoli/DFHBI) system. In this strategy, RNA of interest was transcribed accompanied by the Spanich2 or Broccoli. Then the 3, 5-difluoro-4-hydroxybenzylidene imidazolinone (DFHBI) bound to the RNA aptamer sequence and thereby induced a fluorescence signal. PFP was used as the fluorescence energy donor, and Spanich2/DFHBI was the fluorescence energy acceptor. The fluorescence signal of Spanich2/DFHBI was amplified by light-harvesting and fluorescence amplification ability of PFP via FRET. And the limit of detection (LOD) (0.29 nM) was near 10-fold lower than that of RNA aptamer/DFHBI (LOD is 2.8 nM) alone by measuring the FRET ratio, which greatly reduced the variation of background signals. Most importantly, the addition of PFP did not interfere with RNA transcription in vitro, so this method was successfully applied to sensitively monitor RNA transcription and effect of T7 RNA polymerase inhibitor in real time, supplying a sensitive and simple method to study the modulation and inhibitor of RNA polymerase in vitro.

Fluorogenic probes for disease-relevant enzymes

Traditional biochemical methods for enzyme detection are mainly based on antibody-based immunoassays, which lack the ability to monitor the spatiotemporal distribution and, in particular, the in situ activity of enzymes in live cells and in vivo. In this review, we comprehensively summarize recent progress that has been made in the development of small-molecule as well as material-based fluorogenic probes for sensitive detection of the activities of enzymes that are related to a number of human diseases. The principles utilized to design these probes as well as their applications are reviewed. Specific attention is given to fluorogenic probes that have been developed for analysis of the activities of enzymes including oxidases and reductases, those that act on biomacromolecules including DNAs, proteins/peptides/amino acids, carbohydrates and lipids, and those that are responsible for translational modifications. We envision that this review will serve as an ideal reference for practitioners as well as beginners in relevant research fields.

A cationic conjugated polymer coupled with exonuclease I: application to the fluorometric determination of protein and cell imaging

A strategy is described for the detection of protein by using a cationic fluorescent conjugated polymer coupled with exonuclease I (Exo I). Taking streptavidin (SA) as model protein, it is observed that Exo I can digest single-stranded DNA conjugated with biotin and carboxyfluorescein (P1) if SA is absent. This leads to the formation of small nucleotide fragments and to weak fluorescence resonance energy transfer (FRET) from the polymer to P1. If, however, SA is present, the high affinity of SA and biotin prevents the digestion of P1 by Exo I. This results in the sorption of P1 on the surface of the polymer through strong electrostatic interaction. Hence, efficient FRET occurs from the fluorescent polymer to the fluorescent label of P1. Fluorescence is measured at an excitation wavelength of 370 nm, and emission is measured at two wavelengths (530 and 425 nm). The ratio of the two intensities (I₅₃₀/I₄₂₅) is directly related to the concentration of SA. Under the optimal conditions, the assay has a detection limit of 1.3 ng·mL^-1. The method was also applied to image the folate receptor in HeLa cells, thus demonstrating the versatility of this strategy. Graphical abstract A fluorometric strategy is described for protein detection and cell imaging based on a cationic conjugated polymer (PFP) coupled with exonuclease I (Exo I) trigged fluorescence resonance energy transfer (FRET).

NIR-Mediated Nanohybrids of Upconversion Nanophosphors and Fluorescent Conjugated Polymers for High-Efficiency Antibacterial Performance Based on Fluorescence Resonance Energy Transfer

A novel nanohybrid comprised of upconversion nanophosphors (UCNPs) and fluorescent conjugated polymers (PFVCN) is rationally fabricated. The new UCNP/PFVCN nanohybrids combine the excellent antibacterial ability of PFVCN and the near IR (NIR) absorbing property of UCNPs, which allows for NIR-mediated antibacterial through the effective fluorescence resonance energy transfer from UCNPs to PFVCN accompanied with generation of reactive oxygen species to kill bacteria.

Photophysical Diversity of Water-Soluble Fluorescent Conjugated Polymers Induced by Surfactant Stabilizers for Rapid and Highly Selective Determination of 2,4,6-Trinitrotoluene Traces

The increasing application of fluorescence spectroscopy in development of reliable sensing platforms has triggered a lot of research interest for the synthesis of advanced fluorescent materials. Herein, we report a simple, low-cost strategy for the synthesis of a series of water-soluble conjugated polymer nanoparticles with diverse emission range using cationic (hexadecyltrimethylammonium bromide, CTAB), anionic (sodium dodecylbenzenesulfonate, SDBS), and nonionic (TX114) surfactants as the stabilizing agents. The role of surfactant type on the photophisical and sensing properties of resultant polymers has been investigated using dynamic light scattering (DLS), FT-IR, UV-vis, fluorescence, and energy dispersive X-ray (EDS) spectroscopies. The results show that the surface polarity, size, and spectroscopic and sensing properties of conjugated polymers could be well controlled by the proper selection of the stabilizer type. The fluorescent conjugated polymers exhibited fluorescence quenching toward nitroaromatic compounds. Further studies on the fluorescence properties of conjugated polymers revealed that the emission of the SDBS stabilized polymer, N-methylpolypyrrole-SDBS (NMPPY-SDBS), is strongly quenched by 2,4,6-trinitrotoluene molecule with a large Stern -Volmer constant of 59 526 M(-1) and an excellent detection limit of 100 nM. UV-vis and cyclic voltammetry measurements unveiled that fluorescence quenching occurs through a charge transfer mechanism between electron rich NMPPY-SDBS and electron deficient 2,4,6-trinitrotoluene molecules. Finally, the as-prepared conjugated polymer and approach were successfully applied to the determination of 2,4,6-trinitrotoluene in real water samples.

Water-Soluble Nonconjugated Polymer Nanoparticles with Strong Fluorescence Emission for Selective and Sensitive Detection of Nitro-Explosive Picric Acid in Aqueous Medium

Water-soluble nonconjugated polymer nanoparticles (PNPs) with strong fluorescence emission were prepared from hyperbranched poly(ethylenimine) (PEI) and d-glucose via Schiff base reaction and self-assembly in aqueous phase. Preparation of the PEI-d-glucose (PEI-G) PNPs was facile (one-pot reaction) and environmentally friendly under mild conditions. Also, PEI-G PNPs showed a high fluorescence quantum yield in aqueous solution, and the fluorescence properties (such as concentration- and solvent-dependent fluorescence) and origin of intrinsic fluorescence were investigated and discussed. PEI-G PNPs were then used to develop a fluorescent probe for fast, selective, and sensitive detection of nitro-explosive picric acid (PA) in aqueous medium, because the fluorescence can be easily quenched by PA whereas other nitro-explosives and structurally similar compounds only caused negligible quenching. A wide linear range (0.05-70 μM) and a low detection limit (26 nM) were obtained. The fluorescence quenching mechanism was carefully explored, and it was due to a combined effect of electron transfer, resonance energy transfer, and inner filter effect between PA and PEI-G PNPs, which resulted in good selectivity and sensitivity for PA. Finally, the developed sensor was successfully applied to detection of PA in environmental water samples.

Label-Free Fluorescence Assay of S1 Nuclease and Hydroxyl Radicals Based on Water-Soluble Conjugated Polymers and WS₂ Nanosheets

We developed a new method for detecting S1 nuclease and hydroxyl radicals based on the use of water-soluble conjugated poly[9,9-bis(6,6-(N,N,N-trimethylammonium)-fluorene)-2,7-ylenevinylene-co-alt-2,5-dicyano-1,4-phenylene)] (PFVCN) and tungsten disulfide (WS₂) nanosheets. Cationic PFVCN is used as a signal reporter, and single-layer WS₂ is used as a quencher with a negatively charged surface. The ssDNA forms complexes with PFVCN due to much stronger electrostatic interactions between cationic PFVCN and anionic ssDNA, whereas PFVCN emits yellow fluorescence. When ssDNA is hydrolyzed by S1 nuclease or hydroxyl radicals into small fragments, the interactions between the fragmented DNA and PFVCN become weaker, resulting in PFVCN being adsorbed on the surface of WS₂ and the fluorescence being quenched through fluorescence resonance energy transfer. The new method based on PFVCN and WS₂ can sense S1 nuclease with a low detection limit of 5 × 10(-6) U/mL. Additionally, this method is cost-effective by using affordable WS₂ as an energy acceptor without the need for dye-labeled ssDNA. Furthermore, the method provides a new platform for the nuclease assay and reactive oxygen species, and provides promising applications for drug screening.

A sensitive biosensor with a DNAzyme for lead(II) detection based on fluorescence turn-on

In this paper, we described a new DNAzyme-based fluorescent biosensor for the detection of Pb(2+). In the biosensor, the bulged structure is formed between the substrate labeled with fluorescein amidite (FAM) and DNAzyme after being annealed. Ethidium bromide (EB), the DNA intercalator, then intercalates into the double-stranded DNA section. Once FAM is excited, the FRET takes place from FAM to EB, which leads to the fluorescence of FAM decreasing greatly. In the presence of Pb(2+), the substrate is cleaved by DNAzyme, which breaks the bulged structure. Then EB is released and the FRET from FAM to EB is inhibited. In this case, the fluorescence of FAM increases dramatically. Thus, the Pb(2+) ions can be detected by measuring the fluorescence enhancement of FAM. Under optimal conditions, the increased fluorescence intensity ratio of FAM is dependent on the lead level in the sample, and exhibits a linear response over a Pb(2+) concentration range of 0-100 nM with a detection limit of 530 pM. The sensor showed high selectivity in the presence of a number of interference ions. The river water samples were also tested with satisfying results by using the new method. This sensor is highly sensitive and simple without any additional treatments, which provides a platform for other biosensors based on DNAzyme.

Visual Detection of Multiplex MicroRNAs Using Cationic Conjugated Polymer Materials

A simple, visual, and specific method for simultaneous detection of multiplex microRNAs (miRNAs) has been developed by integrating duplex-specific nuclease (DSN)-induced amplification with cationic conjugated polymer (CCP) materials. The probe DNA with a complementary sequence to target miRNA is labeled with fluorescein dye (FAM). Without target miRNA, the single-strand DNA probe cannot be digested by DSN. Upon adding CCPs, efficient fluorescence resonance energy transfer (FRET) from CCP to FAM occurs owing to strong electrostatic interactions between CCP and the DNA probe. In the presence of target miRNA, the DNA probe hybridizes with target miRNA followed by digestion to small nucleotide fragments by DSN; meanwhile, the miRNA is released and subsequently interacts again with the probe, resulting in the cycled digestion of the DNA probe. In this case, weak electrostatic interactions between oligonucleotide fragments and CCP lead to inefficient FRET from CCP to FAM. Thus, by triggering the FRET signal from CCP to FAM, miRNA can be specially detected, and the fluorescence color change based on FRET can be visualized directly with the naked eye under an UV lamp. Furthermore, an energy transfer cascade can be designed using CCP and DNA probes labeled at the 5'-terminus with FAM and Cy3 dyes, and the multistep FRET processes offer the ability of simultaneous detection of multiplex miRNAs.

Label-free aptasensor for adenosine deaminase sensing based on fluorescence turn-on

A label-free and fluorescence turn-on aptamer biosensor has been developed for the detection of adenosine deaminase (ADA) activity with simplicity and selectivity. Adenosine aptamer will form a tight stem-loop structure upon binding with adenosine. In the absence of ADA, only a small quantity of picagreen intercalates into the stem section of aptamer, resulting in a low fluorescence of picagreen when excited at 490 nm. Interestingly, after the addition of ADA, adenosine is hydrolyzed to inosine, and the released aptamer forms double-stranded DNA (dsDNA) with its complementary single-stranded DNAc, followed by the intercalation of picagreen to dsDNA. When the solution is excited, picagreen emits strong green fluorescence. The increased fluorescence intensity of picagreen is dependent on the concentration of ADA. The detection limit of the ADA is determined to be 2 U L(-1), which is lower than ADA cutoff value (4 U L(-1)) in the clinical requirement and more sensitive than most of the reported methods. Compared to other previous ADA sensors, the assay is not only label-free but also a turn-on signal, and possesses properties of lower cost and simpler detection system. Furthermore, this label-free strategy is also applicable to the assay of other enzymes and screening of corresponding inhibitors.

Thioflavin T as an Efficient G-Quadruplex Inducer for the Highly Sensitive Detection of Thrombin Using a New Föster Resonance Energy Transfer System

We report a new Föster resonance energy transfer (FRET) system that uses a special dye, thioflavin T (ThT), as an energy acceptor and a water-soluble conjugated polymer (CP) with high fluorescence as an energy donor. A simple, label-free, and sensitive strategy for the detection of thrombin in buffer and in diluted serum was designed based on this new system using ThT as an efficient inducer of the G-quadruplex. The difference between the blank and the positive samples was amplified due to distinctive FRET signals because thrombin has little effect on the intercalation of ThT into the G-quadruplex. In the absence of the target, ThT induces the aptamer to form a G-quadruplex and intercalates into it with strong fluorescence. The electrostatic attractions between the negatively charged G-quadruplex and positively charged CP allow a short donor-acceptor distance, resulting in a high FRET signal. However, in the presence of the target, the aptamer forms a G-quadruplex-thrombin complex first, followed by the intercalation of ThT into the G-quadruplex. A long distance exists between the donor and acceptor due to the strong steric hindrance from the large-sized thrombin, which leads to a low FRET signal. Compared with previously reported strategies based on the FRET between the CP and dye, our strategy is label-free, and the sensitivity was improved by an order of magnitude. Our strategy also shows the advantages of being simple, rapid (about 50 min), sensitive, label-free, and low-cost in comparison to strategies based on the FRET between quantum dots and dyes.

Fluorescent biosensor for sensitive analysis of oxytetracycline based on an indirectly labelled long-chain aptamer

A fluorescent assay for oxytetracycline detection is presented based on an indirectly fluorescein-labelled aptamer probe, which was fabricated through hybridization of an oxytetracycline long-chain aptamer with a FAM-labelled short-chain ssDNA.

Adenosine deaminase biosensor combining cationic conjugated polymer-based FRET with deoxyguanosine-based photoinduced electron transfer

We demonstrated a sensitive and selective adenosine deaminase (ADA) detection by modulating the fluorescence resonance energy transfer (FRET) between cationic conjugated poly(9,9-bis(6'-N,N,N-trimethylammonium) hexyl)fluorine phenylene) (PFP) and the deoxyguanosine-tailored hairpin aptamer. The hairpin aptamer was labeled with a fluorophore FAM at one end and three deoxyguanosines (Gs) at the other end as a quencher. In the absence of ADA, aptamer forms hairpin-like conformation with adenosines making close affinity of Gs and FAM, which results in the weak FRET from PFP to FAM because of FAM fluorescence being quenched by Gs via photoinduced electron transfer (PET). After addition of ADA, adenosine was hydrolyzed by ADA, followed by the release of free aptamer. In this case, FAM being far away from Gs, the strong FRET thus was obtained due to the quenching process being blocked. Therefore, the new strategy based on the FRET ratio enhancement is reasonably used to detect the ADA sensitively, combining the fluorescence signal amplification of conjugated polymers with the initiative signal decreasing by Gs. The detection limit of the ADA assay is 0.3 U/L in both buffer solution and human serum, which is more sensitive than most of those previously documented methods. Importantly, the assay is rapid, homogeneous, and simple without a complicated treating process. The ADA inhibitor, erythro-9-(2-hydroxy-3-nonyl) adenine hydrochloride (EHNA), was also studied based on this assay, and the detection limit of EHNA is 10 pM. This strategy provides a new platform for the detection of other biomolecules and enzymes.

Labeling-free fluorescent detection of DNA hybridization through FRET from pyrene excimer to DNA intercalator SYBR green I

A novel labeling-free fluorescence complex probe has been developed for DNA hybridization detection based on fluorescence resonance energy transfer (FRET) mechanism from pyrene excimer of pyrene-functionalized poly [2-(N, N-dimethylamino) ethyl methacrylate] (PFP) to SYBR Green I (SG, a specific intercalator of double-stranded DNA) in a cost-effective, rapid and simple manner. The complex probe consists of the positively charged PFP, SG and negatively charged single-stranded DNA (ssDNA). Upon adding a complementary strand to the complex probe solution, double-stranded DNA (dsDNA) was formed, followed by the intercalation of SG into dsDNA. The pyrene excimer emission was overlapped with the absorption of SG very well and the electrostatic interactions between PFP and dsDNA kept them in close proximity, enabling efficient FRET from pyrene excimer to SG. The fluorescence of SG in the duplex DNA resulting from FRET can be successfully applied to detect DNA hybridization with high sensitivity for a very low detection limit of 10nM and excellent selectivity for detection of single base pair mismatch.

Ultrasensitive turn-on fluorescent detection of trace thiocyanate based on fluorescence resonance energy transfer

Thiocyanate (SCN(-)) is a small anion byproduct of cyanide metabolism. Several methods have been reported to measure SCN(-) above the micromolar level. However, SCN(-) is derived from many sources such as cigarettes, waste water, food and even car exhaust and its effect is cumulative, which makes it necessary to develop methods for the detection of trace SCN(-). In this paper, a simple and ultrasensitive turn-on fluorescence assay of trace SCN(-) is established based on the fluorescence resonance energy transfer (FRET) between gold nanoparticles (AuNPs) and fluorescein. The detection limit is 0.09 nM, to the best of our knowledge, which has been the lowest detection LOD ever without the aid of costly instrumentation. The fluorescence of fluorescein is significantly quenched when it is attached to the surface of AuNPs. Upon the addition of SCN(-), the fluorescence is turned on due to the competition action between SCN(-) and fluorescein towards the surface of AuNPs. Under an optimum pH, AuNPs size and concentration, incubation time, the fluorescence enhancement efficiency [(IF-I0)/I0] displays a linear relationship with the concentration of SCN(-) in the range of 1.0 nM to 40.0 nM. The fluorescein-AuNP sensor shows absolutely high selectivity toward SCN(-) than other 16 anions. The common metal ions, amino acids and sugars have no obvious interference effects. The accuracy and precision were evaluated based on the recovery experiments. The cost effective sensing system is successfully applied for the determination of SCN(-) in milk products and saliva samples.