Narrow Results

A biomimetic and facile approach for integrating Fe₃O₄ and Au with polydopamine (PDA) was proposed to construct gold-coated Fe₃O₄ nanoparticles (Fe₃O₄@Au–PDA) with a core–shell structure by coupling in situ reduction with a seed-mediated method in aqueous solution at room temperature. The morphology, structure and composition of the core–shell structured Fe₃O₄@Au–PDA nanoparticles were characterized by transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and X-ray photoelectron spectrometry (XPS). The formation process of Au shell was assessed using a UV-Vis spectrophotometer. More importantly, according to investigating changes in PDA molecules by Fourier transform infrared spectroscopy (FTIR) and in preparation process of the zeta-potential data of nanoparticles, the mechanism of core–shell structure formation was proposed. Firstly, PDA-coated Fe₃O₄ are obtained using dopamine (DA) self-polymerization to form thin and surface-adherent PDA films onto the surface of a Fe₃O₄ "core". Then, Au seeds are attached on the surface of PDA-coated Fe₃O₄ via electrostatic interaction in order to serve as nucleation centers catalyzing the reduction of Au³⁺ to Au⁰ by the catechol groups in PDA. Accompanied by the deposition of Au, PDA films transfer from the surface of Fe₃O₄ to that of Au as stabilizing agent. In order to confirm the reasonableness of this mechanism, two verification experiments were conducted. The presence of PDA on the surface of Fe₃O₄@Au–PDA nanoparticles was confirmed by the finding that glycine or ethylenediamine could be grafted onto Fe₃O₄@Au–PDA nanoparticles through Schiff base reaction. In addition, Fe₃O₄@Au–DA nanoparticles, in which DA was substituted for PDA, were prepared using the same method as that for Fe₃O₄@Au–PDA nanoparticles and characterized by UV-Vis, TEM and FTIR. The results validated that DA possesses multiple functions of attaching Au seeds as well as acting as both reductant and stabilizing agent, the same functions as those of PDA.

Polydopamine (PDA) capsule and core–shell structures with tailored structures and properties are of particular interests due to their multifunctions and potential applications as new colloidal structures in diverse fields. Among the available fabrication methods, PDA film onto colloidal particles followed by selective template removal has attracted extensive attention due to its advantages of precise control over the size, wall thickness and functions of the obtained capsules. The past several years has witnessed a rapid increase of research concerning the new fabrication strategies, functionalization and applications of this kind of capsules and core–shell structures, particularly in many fields such as drug delivery, catalysis, antibacterial, etc. In this review, the very recent progress of the capsule and core–shell structures based on PDA are summarized. There are basically two sections, including the fabrication process of PDA capsules, core–shell structures, and the various applications based on PDA.

Polydopamine-coated Fe₃O₄ (Fe₃O₄@PDA) nanoparticles (NPs) were prepared as synergistic redox mediators for the catalytic reduction, by NaBH₄, of azo dyes such as methyl orange (MO) and methyl red (MR). Transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were applied to determine their surface morphology, surface chemistry and detailed chemical composition, respectively. The latter technique confirmed the presence of quinone moieties. Moreover, a vibrating sample magnetometer (VSM) was used to confirm the superparamagnetic properties of these NPs. The characteristic optical absorption maximum of MO at 462nm was used to monitor the decolorization process. This was employed to determine the catalytic activity in the reaction. An enhancement of the catalytic activity of the magnetic-separable Fe₃O₄@PDA nanocatalyst over that of PDA microspheres (MPs) was observed. Moreover, their reusability and stability were also investigated. A synergistic electron transfer mechanism involving both Fe₃O₄ and PDA moieties was proposed as follows: the quinone moieties and Fe (III) species in Fe₃O₄@PDA NPs served as systematic redox mediators, with quinone receiving an electron from NaBH₄. The reduced quinone next transfers an electron to the Fe (III) moiety, generating an Fe (II) species that in turn transfers an electron to the azo dye. We determined that this process resulted in enhanced reductive degradation of azo dyes when compared with PDA MPs. Moreover, Fe₃O₄@PDA NPs could be magnetically separated and recycled. We therefore concluded that these NPs show great potential in the immobilization of homogeneous catalysts in the chemical reduction processes of azo dyes.

Halloysite nanotubes@polydopamine (HNTs@PDA) nanocomposites were successfully synthesized, and Au nanoparticles (Au NPs) were loaded on HNTs@PDA by electrostatic adsorption. The electrochemical sensor based on HNTs@PDA–Au nanocomposites was constructed. The effect of the composition, structure and property of HNTs@PDA–Au on the response performance of the sensor was explored. The morphology and composition of the nanocomposites were characterized by field emission scanning electron microscopy, transmission electron microscopic, X-ray diffraction, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. Electrochemical results revealed HNTs@PDA–Au as the sensing interface with good effect on electrocatalytic oxidation of N₂H₄. This sensor detected N₂H₄ in the range from 0.75$\mu$mol$\cdot$L${}^{-1}$ to 2.8mmol$\cdot$L${}^{-1}$, with a sensitivity of 171.7$\mu$A (mmol$\cdot$L${}^{-1}{)}^{-1}$cm${}^{-2}$ and a detection limit of 0.25$\mu$mol$\cdot$L${}^{-1}$ (S/N $=$ 3).

Nanofiber mats produced by electrospinning, with the advantages of specific surface area, porosity and chemical tenability, are an ideal support material for deposition of metal$-$organic framework (MOF) crystals. In this study, four types of MOFs (MIL-53(Al), ZIF-8, UiO-66-NH₂ and NH₂-MIL-125(Ti)) were deposited on polydopamine (PDA)-modified electrospun polyvinyl alcohol (PVA)/SiO₂ organic$-$inorganic hybrid nanofiber mats by bulky synthesis. Because of the formation of Si–O–C–O–Si bridges between PVA chains and silica network, electrospun PVA/SiO₂ organic$-$inorganic hybrid nanofiber mats are quite stable in water or organic solvents and at high temperature are suitable as supports for MOFs deposition. The PDA layer, which exhibits a powerful adhesive ability to attach foreign objects, can effectively improve growth of MOFs on the surface of PVA/SiO₂ nanofiber mats. The obtained MOF composites combining the unique properties of electrospun nanofibers mats and MOFs particles become flexible and tailorable, greatly expanding the application range of MOFs materials. The synthesized MOF composites were used to adsorb chloramphenicol (CAP) in water. It was found that the four MOF composites could remove CAP from water effectively and MIL-53(Al) composite had the highest adsorption capacity due to the higher specific surface area.