Appel Ingo, Nádasi Hajnalka, Reitz Christian, Sebastián Nerea, Hahn Horst, Eremin Alexey, Stannarius Ralf, Behrens Silke S
Institut für Katalyseforschung und -technologie, Karlsruher Institut für Technologie (KIT), Postfach 3640, 76021 Karlsruhe, Germany.
Phys Chem Chem Phys. 2017 May 17;19(19):12127-12135. doi: 10.1039/c7cp01438d.
Magnetic nanoparticles (MNPs) functionalized with (pro-)mesogenic ligands are implemented into a nematic liquid crystal (LC) and studied regarding both colloidal stability and magneto-optical behavior. In this study, the particle surface is specifically engineered to tune the MNP interactions with the LC host. For this purpose, four types of (pro-)mesogenic ligands (ML) are synthesized, which are composed of three structural parts, i.e., a rigid, LC motif (i.e., cyanobiphenyl) and a functional group for nanoparticle binding, both linked via a flexible spacer of different alkyl chain lengths. Electrostatically stabilized CoFeO and γ-FeO nanoparticles with narrow size distribution and sizes below 3 nm are obtained via co-precipitation and subsequently functionalized to yield MNP@ML nanoparticles. Studies on the behaviour of the MNP@ML nanoparticles in the commercial LC host (i.e., 4-pentyl-4'-cyanobiphenyl (5CB)) in the bulk and in thin films in LC test cells, reveal the initial formation of some heterogeneities after transition from the isotropic to the nematic phase. Homogenous MNP@ML-5CB hybrids with long-term, colloidal stability, however, are obtained after magnetic separation of initially formed particle aggregates. In particular, MLs with carboxy groups and high structural flexibility (i.e., long linker lengths) are shown to be well suited to form stable MNP colloids, allowing for high MNP doping levels. As compared to undoped 5CB, the CoFeO@MLx-5CB hybrids show an increased sensitivity to the magnetic field, affecting the Fréedericksz transition. The strongest effect, however, is observed in magnetic and electric fields. The coupling of the ultrasmall, spherical MNPs with the LC director in the magnetic field suggests the formation of LC-induced, anisometric MNP clusters.
用（前）介晶配体功能化的磁性纳米颗粒（MNP）被引入向列型液晶（LC）中，并对其胶体稳定性和磁光行为进行了研究。在本研究中，对颗粒表面进行了特殊设计，以调节MNP与LC主体的相互作用。为此，合成了四种类型的（前）介晶配体（ML），它们由三个结构部分组成，即刚性的LC基序（即氰基联苯）和用于纳米颗粒结合的官能团，两者通过不同烷基链长度的柔性间隔基相连。通过共沉淀获得尺寸分布窄且尺寸小于3 nm的静电稳定的CoFeO和γ-FeO纳米颗粒，随后进行功能化以制备MNP@ML纳米颗粒。对MNP@ML纳米颗粒在商用LC主体（即4-戊基-4'-氰基联苯（5CB））中的本体行为以及在LC测试盒中的薄膜行为的研究表明，从各向同性相转变为向列相后最初会形成一些不均匀性。然而，通过对最初形成的颗粒聚集体进行磁分离后，可获得具有长期胶体稳定性的均匀MNP@ML-5CB杂化物。特别是，具有羧基和高结构灵活性（即长连接基长度）的ML被证明非常适合形成稳定的MNP胶体，允许高MNP掺杂水平。与未掺杂的5CB相比，CoFeO@MLx-5CB杂化物对磁场的敏感性增加，影响了弗雷德里克斯转变。然而，在磁场和电场中观察到的效果最强。超小的球形MNP与磁场中LC指向矢的耦合表明形成了LC诱导的各向异性MNP簇。
