Nematic gold nanoparticles

L. Cseh, G.H. Mehl, The design and investigation of room temperature thermotropic nematic gold nanoparticles. J. Am. Chem. Soc. 128, 13376-13377 (2006)... 

L. Cseh, G.H. Mehl, Strucmre-property relationships in nematic gold nanoparticles. 

Mixtures of a nematic liquid crystal (LC or LC ) with small quantities of gold nanoparticles coated with alkylthiolates (<5 wt%) including an alkylthiolate functionalized with a chiral group have been studied (Figure 8.29) [72]. All mixtures show nematic mesophases with transition temperatures and phase stability very similar to those oftheliquid crystal precursors LC or LC. The introduction ofachiral center into the mixtures (mixtures of Au ) produce chiral nematic mesophases. A similar result is obtained in mixtures of Au and LC doped with the chiral dopant (s)-Naproxen. 

Some important observations, which should apply de facto to many nematic systems containing dispersed nanoparticles, particularly those with metal or semiconductor cores, were reported in 2006 by Prasad et al. [297]. The authors found that gold nanoparticles stabilized with dodecanethiol decreased the isotropic to nematic phase transition of 4-pentyl-4 -cyanobiphenyl (5CB) almost linearly with increasing nanoparticle concentration (x p) and increased the overall conductivity of these mixtures by about two orders of magnitude. However, the anisotropy of the electric conductivity (Act = [Pg.349]

Yoshida et al. recently disclosed an alternative method that allowed them to produce stable suspensions of gold nanoparticles (1-2 nm in diameter) in nematic liquid crystals [315]. They used a simple sputter deposition process, which allowed them to prepare thin liquid crystal films of well-dispersed gold nanoparticles in both 5CB and E47 (available from Merck) with a nanoparticle size depending on the used nematic liquid crystal. Unfortunately, the authors did not provide any details on whether the nanoparticles were capped with a ligand or bare, non-coated particles, which makes it difficult to assess and compare the reported thermal as well as electro-optic data. However, very similar effects were found as a result of nanoparticle doping, including lower nematic-to-isotropic phase transition temperatures compared to the used pure nematics as well as 10% lower threshold voltages at nanoparticle concentrations below 1 wt% [315]. 

Our group pursued another approach of combining the properties of nanoparticles with chiral nematic liquid crystal phases. The idea was to decorate gold nanoparticles with chiral molecules known to be strong inducers of chiral nematic phases. To realize the idea, we prepared a series of alkylthiol-capped gold nanoparticles, either pure monolayer or mixed monolayer, with all or about every second of the alkylthiols end-functionalized with (5)-naproxen (e.g., 6 in Fig. 11) [349]. 

In comparison to nematic liquid crystals, examples of smectic liquid crystals doped with quasi-spherical nanoparticles became more elusive over the last few years. This is surprising especially considering recent work by Smalyukh et al., who found that nanoscale dispersion (based on /V-vinyl-2-pyrrolidone-capped gold nanoparticles with 14 nm diameter) in a thermotropic smectic liquid crystal (8CB) are potentially much more stable than dispersions of nanoparticles in nematics [367]. 

Preliminary investigations of the liquid crystal phase behavior of these gold nanoparticles initially revealed an enantiotropic nematic phase (based on polarized light optical microscopy and thermal analysis) as well as some pattern formation of the gold nanoparticles in TEM experiments [540, 541],... 

Fig. 19 Gold nanoparticles 13 and 14 capped with a mixed monolayer of hexanethiol (13) or dedecanethiol (14) and a lateral substituted nematic liquid crystal thiol attached in a side-on fashion (thiol ratio 1 1) [540, 541]...

Fig. 22 Nanoparticles decorated with pro-mesogenic dendronized or bent-core liquid crystal ligands nematic Fe304 mixed monolayer nanoparticles capped with dendronized cyanobiphenyl ligands and oleic acid (17) [132], and mixed monolayer, non-mesogenic gold nanoparticles decorated with bent-core liquid crystal and hexane thiolates (18) [547]...

H. Qi, B. Kinkead, V.M. Marx, H.R. Zhang, T. Hegmann, Miscibility and alignment eifects of mixed monolayer cyanobiphenyl liquid-crystal-capped gold nanoparticles in nematic cyanobiphenyl liquid crystal hosts. Chem. Phys. Chem. 10, 1211-1218 (2009)... 

Mraitazami R, Spillmann CM, Naciri J, Rama BR (2012) Enhanced thermomechanical properties of a nematic liquid crystal elastomer doped with gold nanoparticles. Sens Actuators A Phys 178 175-178... 

The nematic nanoparticies have been prepared by a two step synthetic process. First, gold nanopartides are covered with an alkylthiol monolayer (hexyl- and dodecylthiol) in a second step, the alkylthiol-nanoparticles are reacted with the functionalized thiol mesogen in dichloromethane at room temperature to obtain the monolayer-protected liquid crystal gold nanopartides. These materials are chemically stable and display a nematic mesophase at room temperature [67, 68]. Other examples include liquid crystal gold nanopartides functionalized by hexaalkoxy-substituted triphenylene [69]. 

Lyotropic liquid crystalline nanoparticles have also been described. Concentrated solutions of gold nanorods in water in the presence of a surfactant (cetyltrimethyl-ammonium bromide) display a nematic mesophase stable up to 200 °C [74[. The N mesophase was identified by optical microscopy by their typical nematic droplets texture. 

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