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A novel colorimetric and fluorescent dual-mode sensing system based on molybdophosphoric heteropoly acid (PMA) and fluorescent microspheres (FMs) was established for monitoring the activity of alkaline phosphatase (ALP). In the presence of ALP, L -ascorbic acid-2-phosphate (AAP) could be hydrolyzed catalytically to ascorbic acid (AA), which could reduce PMA to phosphorus molybdenum blue (PMB), accompanied by the generation of colorimetric signals depending on the level of ALP. Meanwhile, the fluorescence of FMs was quenched markedly by the PMB produced due to the inner-filter effect, which constituted the response mechanism for the dual-mode sensing systems of ALP. On this basis, a PMA-FMs based dual-mode sensing system was used for the detection of ALP, which not only possessed remarkable sensitivity, with a limit of detection of 0.27 U L−1 and 0.11 U L−1, but also exhibited good analytical performance in biological samples with satisfactory results. Moreover, a simple and portable test kit for the visual detection of ALP in real serum samples was fabricated utilizing a smartphone with image-recognition and data-processing capabilities as a visual-detection platform. [ABSTRACT FROM AUTHOR]

Light-up aptamers have attracted growing attention due to their advantages of being label-free and having low fluorescence background. In this work, we developed a light-up fluorescence assay for label-free detection of tumor cells based on a bifunctional split aptamer (BFSA) that contained two DNA strands (BFSA-a and BFSA-b). BFSA-a and BFSA-b were constructed by combining aptamers ZY11 and ThT.2-2, which could specifically bind to the tumor cell SMMC-7721 and activate the fluorescence of thioflavin T (ThT). A Helper strand was introduced to hybridize with BFSA-b, and then BFSA-a and BFSA-b were separated if the target cell was absent. Only when the target cell is present can BFSA-a approach and hybridize with BFSA-b due to the ‘induced-fit effect’, which made the Helper strand dissociate. Then ThT bound to BFSA and the fluorescence of ThT was activated. The results indicated that this fluorescence assay had a good linear response to the target cells in the range of 250–20 000 cells in 100 μL binding buffer; the lowest cell number actually detected was 125 cells in 100 μL buffer. This assay also displayed excellent selectivity and was successfully applied to detect target cells in 20% human serum samples. The design of bifunctional split aptamers realized no-washing, label-free, low-cost, one-step detection of tumor cells, which could generate detectable fluorescence signals just by mixing nucleic acid aptamers and fluorescent reporter molecules with target cells. Such a design of aptamer probes also has the potential to construct stimuli-responsive controlled drug delivery systems. [ABSTRACT FROM AUTHOR]

Herein, we proposed an enzyme-free strategy for the amplified detection of DNA by combining the efficient fluorescence enhancement capability of a β-cyclodextrin-tethered cationic polymer (cationic polyβ-CD) to pyrene with the amplification capability of target DNA triggered hybridization chain reaction (HCR). Cationic polyβ-CD with positive charge was synthesized. Two hairpin probes, H1 and H2, were employed in the system and the pyrene-labelled H2 was chosen as the signal unit. The pyrene attached on the sticky end of H2 was flexible and there was strong electrostatic interaction between cationic polyβ-CD and negatively-charged H2, so pyrene could easily enter the cavity of CD that is tethered on the cationic polymer, accompanied by significant fluorescence enhancement. Once target DNA was introduced, HCR was triggered to form a rigid long dsDNA polymer with pyrene attached on it. The pyrene was hardly able to enter the cavity of cationic polyβ-CD because of steric hindrance, leading to a weak fluorescent signal. Owing to the efficient pyrene fluorescence enhancement of cationic polyβ-CD and the amplified capability of HCR, an enzyme-free sensitive detection of target DNA was achieved with a detection limit of 0.1 nM and high selectivity. [ABSTRACT FROM AUTHOR]

Nowadays, enzyme-free nucleic acid-based signal amplification strategies are frequently utilized in the design of biosensors due to their excellent sensitivity. Developing more extended analytical methods is fundamental for basic studies in the biological and biomedical fields. Herein, we introduce an enzyme-free amplified detection strategy for the small molecule adenosine. The approach is based on adenosine–aptamer binding triggered catalyzed hairpin assembly and host–guest interactions between β-cyclodextrin polymer (β-CDP) and pyrene. Two hairpin probes (probe H1 and probe H2) and an aptamer-trigger/inhibitor duplex probe were employed in the system and the pyrene-labeled probe H1 was chosen as the signal unit. In the absence of adenosine, the aptamer-trigger was inhibited by the inhibitor strand. The hairpin probes were in the closed hairpin formation without activation of the trigger strand. Pyrene labeled at the 5′-termini of the single-stranded stem of probe H1 could be easily trapped in the hydrophobic cavity of β-CDP because of weak steric hindrance, leading to a significant fluorescence enhancement. Once the hairpin assembly was catalyzed by the adenosine–aptamer binding event, a hybridized DNA duplex H1/H2 was created continuously. Pyrene labeled at the 5′-termini of the DNA duplex H1/H2 finds it difficult to enter the cavity of β-CDP due to steric hindrance, leading to a weaker fluorescence signal. Thus, the target could be detected by this simple mix-and-detect amplification method without a need for expensive and perishable protein enzymes. As low as 42 nM of adenosine was detected by this assay, which is comparable to that of some reported colorimetric methods. Meanwhile, the proposed method was further successfully applied to detect adenosine in human serum samples, showing great potential for adenosine detection from complex fluids. [ABSTRACT FROM AUTHOR]

A highly specific recognition-before-labeling strategy has been developed for sensitive detection of non-small cell lung cancer A549 cells, by using fluorescent QDs as signal units and DNA aptamers as recognition elements. A QD–aptamer system used for cell imaging and bioanalysis mostly relies on the recognition-after-labeling strategy in which aptamers were firstly labeled with QDs and then the QD–aptamer conjugates as a whole were utilized for specific recognition. Here in our strategy, aptamers were used firstly to recognize target cells, and then fluorescent QDs were sequentially added to bind the aptamers and light the target cells. The proposed recognition-before-labeling strategy didn't require the complex process of QD functionalization, and avoided the possible impact on the aptamer configuration from steric hindrance. Meanwhile, QDs, with strong fluorescence and good photostability, also give this method a high signal-to-background ratio (S/B). The recognition-before-labeling strategy is simple and sensitive, suggesting a new method for in vitro diagnostic assays of cancer cells. [ABSTRACT FROM AUTHOR]

A multiple amplification strategy has been developed for nucleic acid detection based on host–guest interaction between the β-cyclodextrin polymer (β-CDP) and pyrene. Briefly, the detection system consists of three parts: the polymerase and nicking enzyme-assisted isothermal strand displacement amplification (SDA) activated by a target DNA or microRNA; the exonuclease III-aided cyclic enzymatic amplification (CEA); and the fluorescence enhancement effect based on host–guest interaction between β-CDP and pyrene. This strategy showed a good positive linear correlation with target DNA concentrations in the range from 75 fM to 1 pM with a detection limit of 41 fM. Significantly, our amplification platform was further validated and evaluated successfully by assaying miRNA-21 in human serum. The proposed assay has great potential as a nucleic acid quantification method for use in biomedical research, clinical analysis and disease diagnostics. [ABSTRACT FROM AUTHOR]

A highly sensitive dynamic light scattering (DLS) assay for glucose detection without interference from ascorbic acid was demonstrated based on mercaptophenylboronic acid functionalized gold nanoparticles (MPBA–AuNPs). The stable, water-solubility MPBA–AuNPs were synthesized using facile and mild method. In the presence of glucose, specific binding between glucose and boronic acids resulted in the aggregation of MPBA–AuNPs. Thus, glucose could be detected by monitoring the average particle size change of the assay solution using DLS analysis. The detection limit was estimated to be around 10 nM. This assay also showed good selectivity, and glucose could be detected without interference even if 1 mM ascorbic acid was present. Furthermore, glucose in human serum was successfully detected here. Therefore, this sensitive and selective approach has the potential to become a new key for glucose detection. [ABSTRACT FROM AUTHOR]

Living cell studies can offer tremendous opportunities for biological and disease studies. Due to their high sensitivity and selectivity, minimum interference with living biological systems, ease of design and synthesis, fluorescent nucleic acid probes (FNAPs) have been widely used in living cell studies, such as for intracellular detection, cell detection, and cell-to-cell communication. Here, we review the general requirements and the recent developments in FNAPs for living cell studies. We broadly classify these designs as hybridization probes and aptamer probes. For hybridization probes, we describe recently developed designs, such as nanomaterial-based and amplification-based hybridization probes. For aptamer probes, we discuss four general paradigms that have appeared most frequently in the literature: nanomaterial-based, nanomachine-based, cell surface-anchored and activatable aptamer probe designs in vivo. FNAPs promise to open up new and exciting opportunities in biological marks detection for a wide range of biological and medical applications. [ABSTRACT FROM AUTHOR]

Herein, we proposed an enzyme-free strategy for the amplified detection of DNA by combining the efficient fluorescence enhancement capability of a β-cyclodextrin-tethered cationic polymer (cationic polyβ-CD) to pyrene with the amplification capability of target DNA triggered hybridization chain reaction (HCR). Cationic polyβ-CD with positive charge was synthesized. Two hairpin probes, H1 and H2, were employed in the system and the pyrene-labelled H2 was chosen as the signal unit. The pyrene attached on the sticky end of H2 was flexible and there was strong electrostatic interaction between cationic polyβ-CD and negatively-charged H2, so pyrene could easily enter the cavity of CD that is tethered on the cationic polymer, accompanied by significant fluorescence enhancement. Once target DNA was introduced, HCR was triggered to form a rigid long dsDNA polymer with pyrene attached on it. The pyrene was hardly able to enter the cavity of cationic polyβ-CD because of steric hindrance, leading to a weak fluorescent signal. Owing to the efficient pyrene fluorescence enhancement of cationic polyβ-CD and the amplified capability of HCR, an enzyme-free sensitive detection of target DNA was achieved with a detection limit of 0.1 nM and high selectivity.