Rotational and translational diffusion of anisotropic gold nanoparticles in liquid crystals controlled by varying surface anchoring

Abstract

We study translational and rotational diffusion of anisotropic gold nanoparticles (NPs) dispersed in the bulk of a nematic liquid crystal fluid host. Experimental data reveal strong anisotropy of translational diffusion with respect to the uniform far-field director, which is dependent on shape and surface functionalization of colloids as well as on their ground-state alignment. For example, elongated NPs aligned parallel to the far-field director translationally diffuse more rapidly along the director whereas diffusion of NPs oriented normal to the director is faster in the direction perpendicular to it while they are also undergoing elasticity-constrained rotational diffusion. To understand physical origins of these rich diffusion properties of anisotropic nanocolloids in uniaxially anisotropic nematic fluid media, we compare them to diffusion of prolate and oblate ellipsoidal particles in isotropic fluids as well as to diffusion of shape-isotropic particles in nematic fluids. We also show that surface functionalization of NPs with photosensitive azobenzene groups allows for in situ control of their diffusivity through trans-cis isomerization that changes surface anchoring.

Optically and elastically assembled plasmonic nanoantennae for spatially resolved characterization of chemical composition in soft matter systems using surface enhanced spontaneous and stimulated Raman scattering

“…The random sharp edges of NB particles allow them to efficiently localize the electromagnetic field and thereby provide high field enhancement for the incident electromagnetic wave [Fig 2(b)]. When dispersed in LCs, the NB particles possessed homeotropic anchoring of LC molecules at their surfaces, which led to quadrupolar elastic distortions 12,13 of director field n(r) with a half-integer disclination around particle's perimeter (inset in Fig. 1).…”

“…3 Short (20×40nm) gold nanorods (SR) with capping ligands polystyrene (PS) or methoxy poly(ethylene glycol) thiol (mPEG) and disc-shaped gold nanoparticles with sharp irregular edges called 'Nanobursts' (NB) 12,13 were obtained from Nanopartz Inc. The relatively poly-disperse NSOL-functionalized gold NB nanoparticles had average lateral size ~ 500 nm, as indicated by the TEM images shown in Fig 2(a).…”

“…9 However, manipulating such particles in different media and controlling the position within the sample where the enhancement occurs has been a challenge, although certain metal nanoparticles can be manipulated with optical tweezers and can be localized in topological defects in liquid crystalline (LC) host media. 12,13 In this work, we combine SERS with optical manipulation and LC-mediated self-assembly of metal nanoparticles to demonstrate chemical identification of molecules and individual nanoparticles with the help of surface-enhanced Raman techniques, including Raman micro-spectroscopy, coherent antiStokes Raman scattering (CARS), and stimulated Raman scattering (SRS) polarizing microscopies (PM).…”

Magnetically responsive gourd-shaped colloidal particles in cholesteric liquid crystals

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Two- [7][8][9][10][11] and three-dimensional 14 crystals were designed in nematics using different elastic dipoles and quadrupoles, and singular defects were tailored using spherical 16 and topologically non-trivial 17 colloidal particles. Recent studies also showed the importance of size [18][19][20][21][22][23][24] and shape [24][25][26][27][28][29][30][31] in elastic interactions and ensuing assemblies of colloidal particles in nematics. On the other hand, colloidal dispersions in twisted nematic and cholesteric LCs (CLCs) have received less attention [32][33][34][35][36][37][38][39][40][41][42][43]<...>…”

Highly Sensitive and Practical Detection of Plant Viruses via Electrical Impedance of Droplets on Textured Silicon-Based Devices

“…In particular, in a suspended liquid with viscosity η s , and in a highly-diluted regime, the particles are able both to rotate freely without interacting each other due to the large distance and to diffuse due to the Brownian forces only (~ K B T ). The Brownian rotational and translation diffusion coefficients are defined as D r,B and D t,B ; for nearly spherical particles of diameter d , given by: $D_{\mathrm{r},\mathrm{B}}=\frac{k_{B}T}{\left(\pi {\eta }_s{d}^3\right)}$ $D_{\mathrm{t},\mathrm{B}}=\frac{k_{B}T}{\left(6\pi {\eta }_{s}R\right)}$ while for a rod-like particle of length L and diameter d , and if the aspect ratio L / d is in the range 2 < L / d < 30, they are given by [36,37]: $D_{r,B}=\frac{3k_{B}T\left(ln\left(\frac{2L}{d}\right)-0.8\right)}{\left(\pi {\eta }_s{L}^3\right)}$ $D_{t,B}=\frac{3k_{B}T\left(ln\left(\frac{2L}{d}\right)-0.8\right)}{\left(3\pi {\eta }_{s}L\right)}$ (with k B being the Boltzmann constant, and T the absolute temperature). The Equations (7a,b) and (8a,b) indicate the dependence of the diffusion on the geometrical radius and aspect ratio.…”