Interactions Between Hemin-Binding DNA Aptamers and Hemin–Graphene Nanosheets: Reduced Affinity but Unperturbed Catalytic Activity

Surface Nanosieving Polyether Sulfone Particles with Graphene Oxide Encapsulation for the Negative Isolation toward Extracellular Vesicles

Extracellular vesicles (EVs) contain specific biomarkers for disease diagnosis. Current EV isolation methods are hampered in important biological applications due to their low recovery and purity. Herein, we first present a novel EV negative isolation strategy based on surface nanosieving polyether sulfone particles with graphene oxide encapsulation (SNAPs) by which the coexisting proteins are irreversibly adsorbed by graphene oxide (GO) inside the particles, while EVs with large sizes are excluded from the outside due to the well-defined surface pore sizes (10-40 nm). By this method, the purity of the isolated EVs from urine could be achieved 4.91 ± 1.01e¹⁰ particles/μg, 40.9-234 times higher than those obtained by the ultracentrifugation (UC), size-exclusion chromatography (SEC), and PEG-based precipitation. In addition, recovery ranging from 90.4 to 93.8% could be obtained with excellent reproducibility (RSD < 6%). This was 1.8-4.3 times higher than those obtained via SEC and UC, comparable to that obtained by PEG-based precipitation. Taking advantage of this strategy, we further isolated urinary EVs from IgA nephropathy (IgAN) patients and healthy donors for comparative proteome analysis, by which significantly regulated EV proteins were found to distinguish IgAN patients from healthy donors. All of the results indicated that our strategy would provide a new avenue for highly efficient EV isolation to enable many important clinical applications.

Highly-sensitive and fast detection of human telomeric G-Quadruplex DNA based on a hemin-conjugated fluorescent metal-organic framework platform

The formation of G-quadruplex (G4) structures in Human telomeric DNA (H-Telo) has been demonstrated to inhibit the activity of telomerase enzyme that is associated with the proliferation of many cancer cells. Accordingly, G-quadruplex structures have become one of the well-established targets in anticancer therapeutic strategies. And, the development of simple and selective detection platforms for G4 structures has become a significant focus of research in recent years. In this study, a simple "off-on" fluorometric method was developed for the selective detection of picomolar quantities of H-Telo G4 DNA based on a fluorescent cerium-based metal organic framework (Ce-MOF) conjugated with hemin to form the sensing probe, Hemin@Ce-MOF. The solvothermal synthesis of the Ce-MOF took advantage of 5-aminoisophtlalic acid (5AIPA) as the organic bridging ligand, (Ce₂(5AIPA)₃(DMF)₂). Characterization of Ce-MOF and Hemin@Ce-MOF was performed by XRD, XPS, TEM, SEM, BET and FTIR techniques. The detection and quantification of the H-Telo was carried out through the adsorption/incorporation of hemin molecules on the pores and surface of Ce-MOF resulting in the fluorescent quenching of the system followed by the restoration of the fluorescence upon addition of H-Telo probably due to a competition between H-Telo and Ce-MOF to bind to hemin. The impact of the key variables including MOF quantity, hemin concentration and detection time was investigated and optimized. Under the optimized conditions, the developed probe provides a limit of detection (LOD) of 665 pM, linear dynamic range (LDR) of 1.6-39.7 nM and excellent selectivity towards H-Telo. Taken together, these results present a simple, novel and superior platform for the selective detection of H-Telo G4 DNA.

Controlling dopamine binding by the new aptamer for a FRET-based biosensor

Dopamine is one of the most important neurotransmitters. A high-quality DNA aptamer for dopamine was reported in 2018. However, fundamental understanding of its binding and folding is lacking, which is critical for related biosensor design. Herein, we performed careful assays using a label-free technique called isothermal titration calorimetry (ITC) to study its secondary structure. We divided this aptamer into four regions and individually examined each of them. We confirmed two stems, but the third stem is believed to be part of a loop. The aptamer was then truncated. The original aptamer had a Kd of 2.2 ± 0.3 μM at 25 °C. Shortening the structure by one or two base pairs increased the Kd to 6.9 and 44.4 μM, respectively. Dopamine binding was promoted by both increasing the Mg2+ concentration and decreasing the temperature. At 5 °C, a Kd of 0.4 μM was achieved. Based on this understanding, we designed two fluorescence resonance energy transfer (FRET) quenching biosensors that differ only by a base pair. The shorter sensor had 3-fold higher sensitivity and a detection limit of 0.9 μM. In 1% fetal bovine serum, the sensor retained a similar limit of detection of 1.14 μM. A two-fluorophore ratiometric FRET sensor was also demonstrated with a low detection limit of 0.12 μM. This work indicated the feasibility of designing folding-based sensors for dopamine, and this design can be extended to other sensing modalities such as electrochemistry and colorimetry.