In this work, we developed a sensitive and efficient ratiometric electrochemical method for lipopolysaccharide (LPS) detection using Cu-based metal-organic frameworks (Cu-MOFs) as a catalyst and target-triggered quadratic cycles for signal amplification. First, in the presence of target LPS, the conformation change of the specifically designed hairpin probes 1 (HP1) triggered the target cyclic-induced polymerization to produce the output DNA with the aid of phi29 DNA polymerase (phi29). Then, the obtained output DNA hybridized with ferrocene-labeled hairpin probes 2 (Fc-HP2, which were immobilized on the electrode) to generate a nicking endonuclease (N.BstNBI) cleavage site. Thus, with N.BstNBI, the original signal molecules of Fc left from the electrode, and the single-stranded capture-probe-modified sensing interface was obtained. At this time, signal probes conducted by Au-nanoparticles-functionalized Cu-MOFs and labeled hairpin probes 3 (HP3/AuNPs/Cu-MOFs) were hybridized with capture probes for hairpin assembly. Herein, AuNPs/Cu-MOFs were not only used as nanocarriers for immobilizing HP3 but also acted as electroactive materials for signal reporting. With the proposed target-triggered quadratic cycles, the cleavage sites of Fc-HP2 were cut, and capture probes were obtained to hybridize with HP3/AuNPs/Cu-MOFs, which caused the signal decrease of Fc. Then Cu-MOFs were closed to the electrode for the signal increase of Cu-MOFs. Furthermore, when glucose was present in the detection solution, AuNPs/Cu-MOFs catalyzed the oxidation of glucose to realize the enzyme-free signal amplification. By measuring the peak currents ratio of the Cu-MOFs and Fc, the proposed aptasenor for LPS detection showed a low detection limit (0.33 fg/mL) and a wide linear range from 1.0 fg/mL to 100 ng/mL with high accuracy and sensitivity. This ratiometric electrochemical approach is expected to be a valuable strategy for detection of other analytes.
An enzyme-free electrochemical immunosensor based on the host-guest nanonets of N,N-bis(ferrocenoyl)-diaminoethane/β-cyclodextrins/poly(amidoamine) dendrimer-encapsulated Au nanoparticles (Fc-Fc/β-CD/PAMAM-Au) for procalcitonin (PCT) detection has been developed in this study. The signal probe was constructed as follows: amine-terminated β-CD was adsorbed to PAMAM-Au first, and then the prepared Fc-Fc was recognized by the β-CD to form stable host-guest nanonets. Next, secondary antibodies (Ab2) were attached into the formed netlike nanostructure of Fc-Fc/β-CD/PAMAM-Au by chemical absorption between PAMAM-Au and -NH2 of β-CD. Herein, the PAMAM-Au act not only as nanocarriers for anchoring large amounts of the β-CD and Ab2 but also as nanocatalysts to catalyze the oxidation of ascorbic acid (AA) for signal amplification. Moreover, the Fc-Fc could be stably immobilized by the hydrophobic inner cavity of β-CD as well as improving solubility by the hydrophilic exterior of β-CD. With the unique structure of two ferrocene units, Fc-Fc not only affords more electroactive groups to make the electrochemical response more sensitive but also plays a role of combining dispersive β-CD-functionalized PAMAM-Au to form the netlike nanostructure. Furthermore, Fc-Fc exhibits good catalytic activity for AA oxidation. When the detection solution contained AA, the synergetic catalysis of PAMAM-Au and Fc-Fc to AA oxidation could be obtained, realizing enzyme-free signal amplification. The proposed immunosensor provided a linear range from 1.80 pg/mL to 500 ng/mL for PCT detection and a detection limit of 0.36 pg/mL under optimal experimental conditions. Moreover, the immunosensor has shown potential application in clinical detection of PCT.
In this paper, a facile approach for the synthesis of stimuli-responsive fluorescent elastomer was developed. The photoluminescence should partly be attributed to the hampered intramolecular rotation of the aryl rotors of the dye molecules by wrapping and coiling of the flexible polymer chains. In addition, the reversibility of this responsiveness was also confirmed.
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