Complex Fluids Group, Instituto de Física, Universidade de Brasília, Caixa Postal 04455, 70919-970 Brasília, Federal District, Brazil.
Departamento de Física, Universidade Federal de Santa Catarina, 88040-900 Florianópolis, Santa Catarina, Brazil.
Langmuir. 2021 Jan 26;37(3):1129-1140. doi: 10.1021/acs.langmuir.0c03052. Epub 2021 Jan 14.
The heat produced by magnetic nanoparticles, when they are submitted to a time-varying magnetic field, has been used in many auspicious biotechnological applications. In the search for better performance in terms of the specific power absorption (SPA) index, researchers have studied the influence of the chemical composition, size and dispersion, shape, and exchange stiffness in morphochemical structures. Monodisperse assemblies of magnetic nanoparticles have been produced using elaborate synthetic procedures, where the product is generally dispersed in organic solvents. However, the colloidal stability of these rough dispersions has not received much attention in these studies, hampering experimental determination of the SPA. To investigate the influence of colloidal stability on the heating response of ferrofluids, we produced bimagnetic core@shell NPs chemically composed of a ZnMn mixed ferrite core covered by a maghemite shell. Aqueous ferrofluids were prepared with these samples using the electric double layer (EDL) as a strategy to maintain colloidal stability. By starting from a proper sample, ultrastable concentrated ferrofluids were achieved by both tuning the ion/counterion ratio and controlling the water content. As the colloidal stability mainly depends on the ion configuration on the surface of the magnetic nanoparticles, different levels of nanoparticle clustering are achieved by changing the ionic force and pH of the medium. Thus, the samples were submitted to two procedures of EDL destabilization, which involved dilution with an alkaline solution and a neutral pH viscous medium. The SPA results of all prepared ferrofluid samples show a reduction of up to half the efficiency of the standard sample when the ferrofluids are in a neutral pH or concentrated regime. Such results are explained in terms of magnetic dipolar interactions. Our results point to the importance of ferrofluid colloidal stability in a more reliable experimental determination of the NP heat generation performance.
磁性纳米粒子在时变磁场中产生的热量已在许多有前途的生物技术应用中得到应用。为了提高比功率吸收（SPA）指数的性能，研究人员研究了化学成分、尺寸和分散性、形状以及形态化学结构中的交换刚度的影响。使用精细的合成程序生产了单分散的磁性纳米粒子组装体，其中产物通常分散在有机溶剂中。然而，这些粗糙分散体的胶体稳定性在这些研究中并没有得到太多关注，这阻碍了 SPA 的实验测定。为了研究胶体稳定性对铁磁流体加热响应的影响，我们制备了由 ZnMn 混合铁氧体核包覆的磁性核壳 NPs，其化学组成由 maghemite 壳组成。使用这些样品通过双电层（EDL）策略制备了水基铁磁流体，以保持胶体稳定性。通过从适当的样品开始，通过调整离子/抗衡离子比和控制水含量，实现了超稳定的浓缩铁磁流体。由于胶体稳定性主要取决于磁性纳米粒子表面的离子构型，因此通过改变离子力和介质的 pH 值可以实现不同程度的纳米粒子团聚。因此，将样品进行 EDL 去稳定化的两个程序，涉及用碱性溶液和中性 pH 粘性介质进行稀释。所有制备的铁磁流体样品的 SPA 结果表明，当铁磁流体处于中性 pH 或浓缩状态时，效率降低了标准样品的一半。这些结果可以用磁偶极相互作用来解释。我们的结果表明，在更可靠地确定 NP 发热性能的实验中，铁磁流体胶体稳定性非常重要。
