Abstract:

17β-Estradiol (E2) is a natural steroidal estrogen essential for a variety of physiological functions in organisms. However, external E2, which is renowned for its potent biological effects, is also considered to be an endocrine-disrupting compound (EDC) capable of disturbing the normal operation of the endocrine system, even at nanogram-per-liter (ng/L) concentrations. Studies have revealed that medical and livestock wastewater can be contaminated with E2, which poses potential risks to human health. Currently, the primary method for detecting E2 relies on liquid chromatography-mass spectrometry, which is limited with regard to on-site or large-scale sample testing due to instrumental constraints. Herein, we developed a magnetic graphene oxide (MGO)/aptamer separating material. The MGO was synthesized by creating a water-in-oil microemulsion at 90 ℃, an agarose hydrogel to load the Fe₃O₄ nanoparticles, and layered graphene oxide (GO). In contrast to conventional methods, such as chemical co-precipitation and solvothermal approaches, this method is more time-efficient and does not require high temperature or pressure. Moreover, the use of a physical encapsulation technique for enwrapping the Fe₃O₄ nanoparticles and layered GO eliminates the need for chemical modification. This approach reduces the use of harmful chemicals, ensures complete loading, and results in highly efficient encapsulation. The MGO was characterized using Fourier-transform infrared (FT-IR) spectroscopy and transmission electron microscopy (TEM), as well as dynamic light scattering (DLS) and Zeta potential analyses, which revealed that the Fe₃O₄ nanoparticles had been successfully loaded onto the GO to produce MGO particles mainly around 5 μm in size. Additionally, this study demonstrated that the aqueous MGO dispersion is highly stable.

This substance was used to develop a fluorescent biosensor that uses a “turn-on” mechanism to rapidly and highly sensitively detect E2. MGO is capable of adsorbing a fluorescently labeled E2 aptamer (FAM-Apt) in solution, resulting in fluorescence quenching through fluorescence resonance energy transfer (FRET) between the fluorescent group and graphene. However, E2 preferentially binds to FAM-Apt, resulting in the FAM-Apt separating from the MGO in the presence of E2, thereby restoring fluorescence. The developed biosensor exhibits a robust linear correlation between relative fluorescence intensity and E2 mass concentration in the 1-1000 ng/mL range, and boasts a low detection threshold of 1 ng/mL. The use of MGO as an absorbent and fluorescence quencher led to an E2-detection limit that is two orders of magnitude lower than that of a GO-based sensor. This biosensor also outperforms other aptamer-based systems in terms of detection time, linear range, and sensitivity; it also demonstrates remarkable resilience toward various interfering ions and exhibits strong selectivity among structurally similar estrogen analogs. A range of ions commonly present in water samples were introduced into the reaction system at specific concentrations to gauge the impact of interfering ions on sensor performance. With the exception of Fe³⁺ ions at 0.3 mg/L, which led to a lower fluorescence intensity, interfering ions were found to exhibit minimal effects. Biosensor specificity and selectivity were further scrutinized by introducing four estrogenic disruptors, including estriol (E3), 17β-ethynylestradiol (EE2), estrone (E1), and bisphenol A (BPA), each at a mass concentration of 1 μg/mL under the same reaction conditions used to detect E2. The recovered relative fluorescence-signal values for E1 and E3 were determined to be 33% and 23% that of E2, respectively, while EE2 and BPA hardly elicited any fluorescence signal recovery, thereby highlighting the ability of the biosensor to precisely detect E2 with minimal interference from estrogen analogs. The efficacy of the MGO-FAM-Apt biosensor was subsequently validated by testing river-water samples containing known quantities of added E2, which yielded recoveries of between 91.0% and 110.0%, thereby affirming the reliability of this biosensor for use in practical applications. The developed sensor may be somewhat limited compared to liquid chromatography-high-resolution mass spectrometry in detection limit, but the developed biosensor is cost-effective, detects rapidly, and is capable of simultaneously analyzing multiple samples, making it suitable for on-site or large-scale E2 testing of environmental water samples.

Key words: 17β-estradiol (E2), magnetic solid phase separation, fluorescence biosensor, magnetic graphene oxide (MGO)