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Browsing by Author "Liu, Xun"

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    Homogeneous assays for aptamer-based ethanolamine sensing: no indication of target binding
    (Royal Society of Chemistry, 2022-02-11) Ding, Yuzhe; Liu, Xun; Huang, Po-Jung Jimmy; Liu, Juewen
    Ethanolamine is an important analyte for environmental chemistry and biological sciences. A few DNA aptamers were previously reported for binding ethanolamine with a dissociation constant (Kd) as low as 9.6 nM. However, most of the previous binding assays and sensing work used either immobilized ethanolamine or immobilized aptamers. In this work, we studied three previously reported DNA sequences, two of which were supposed to bind ethanolamine while the other could not bind. Isothermal titration calorimetry revealed no binding for any of these sequences. In addition, due to their guanine-rich sequences, thioflavin T was used as a probe. Little fluorescence change was observed with up to 1 μM ethanolamine. Responses within the millimolar range of ethanolamine were attributed to the general fluorescence quenching effect of ethanolamine instead of aptamer binding. Finally, after studying the adsorption of ethanolamine to gold nanoparticles (AuNPs), we confirmed the feasibility of using AuNPs as a probe when the concentration of ethanolamine was below 0.1 mM. However, no indication of specific aptamer binding was observed by comparing the three DNA sequences for their color changing trends. This work articulates the importance of careful homogeneous binding assays using free target molecules.
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    Stabilization of Gold Nanoparticles by Hairpin DNA and Implications for Label-Free Colorimetric Biosensors
    (2022-04-21) Liu, Xun; Zhao, Yu; Ding, Yuzhe; Wang, Jianhua; Liu, Juewen
    With extremely high extinction coefficients and other unique optical properties, gold nanoparticles (AuNPs) have received growing interest in developing biosensors. DNA hairpin structures are very popular probes for the detection of not only complementary DNA or RNA but also aptamer targets. This work aims to understand the effect of the structure and sequence of hairpin DNA for the stabilization of AuNPs and its implications in AuNP-based label-free colorimetric biosensors. A series of hairpin DNA with various loop sizes from 4 to 26 bases and sequences (random sequences, poly-A and poly-T) were tested, but they showed similar abilities to protect AuNPs from aggregation. Using hairpin DNA with a tail under the same conditions, optimal protection was achieved with a six-base or longer tail. DNA hairpins are likely adsorbed via their tail regions or with their terminal bases if no tail is present. Molecular dynamics simulations showed that the rigidity of the hairpin loop region disfavored its adsorption to AuNPs, while the flexible tail region is favored. Finally, a DNA sensing assay was conducted using different structured DNA, where hairpin DNA with a tail doubled the sensitivity compared to the tail-free hairpin.