Enhanced Thermal Lens Effect in Gold Nanoparticle-Doped Lyotropic Liquid Crystal by Nanoparticle Clustering Probed by Z-Scan Technique
This work presents an experimental study of the thermal lens effect in Au nanoparticles-doped lyotropic liquid crystals under cw 532 nm optical excitation. Spherical Au nanoparticles of about 12 nm were prepared by Turkevich’s method, and the lyotropic liquid crystal was a ternary mixture of SDS, 1-DeOH, and water that exhibits an isotropic phase at room temperature. The lyotropic matrix induces aggregation of the nanoparticles, leading to a broad and a red-shifted surface plasmon resonance. The thermal nonlinear optical refraction coefficient n2 increases as a power of number density of nanoparticles, being possible to address this behavior to nanoparticle clustering.
KeywordsThermal lens Gold nanoparticles Clustering Lyotropic liquid crystal
In the last years, there has been an increasing interest in the thermal and optical properties of nanocomposite materials because of the new phenomena arising from the nanoscale and the envisaged technological applications. In this field stands out the nanofluids, stable suspensions of metallic nanoparticles whose optical properties arise from the surface plasmon resonance (SPR), a collective oscillation of the free electrons in the conduction band [1, 2]. An important feature of this band is its dependence on size  and shape [4, 5] of the nanoparticles and on the presence of clusters [6, 7]. Nanofluids also exhibit enhanced thermal transport properties [8, 9, 10]. Liquids with suspended nanoparticles display thermal conductivities significantly higher than that of the base fluids. Besides the numerous researches on the linear and nonlinear optical properties of noble metal nanoparticles, the photothermal properties of nanofluids have not deserved the same attention [11, 12, 13]. In addition to an interest in basic research, the study of the photothermal properties of nanofluids is relevant for technological and medical applications. In this sense, plasmonic photothermal therapy of tumors already is a promising new field in the nanomedicine [14, 15]. An important issue in nanofluids, envisaging practical applications, is the clustering [16, 17]. It was shown that the aggregation of Au nanoparticles leads to an enhancement in the heating effect under optical excitation  but a complete model about the generation and transport of heat taking into account the complex structure of the aggregates is lacking.
The aim of this paper is to give an insight into the role of nanoparticle clustering in the thermal nonlinear optical response of a medium containing gold nanoparticles (AuNPs), proposing a phenomenological model of the thermal conductivity and of the thermal lens effect. To this end, we have explored the formation of a thermal lens by the Z-scan technique in two different media containing spherical citrated gold nanoparticles, water, and lyotropic liquid crystal, varying nanoparticle concentration, under cw optical excitation at 532 nm.
2 Experimental Details
The result was a ruby red colloid. Measurements of zeta potential reveal that the nanoparticles are highly stable and have an average surface charge of −33.4 ± 0.5 mV at pH 8.2, which remained stable, protected from the light, for at least 5 months.
The lyotropic liquid crystal used in this study is a mixture of sodium dodecyl sulfate (SDS) [23 wt%], 1-decanol (1-DeOH) [2 wt%], and water [75 wt%], which exhibits an isotropic phase at room temperature (24 °C). The basic units of these kind of liquid crystals are the micelles, anisotropic aggregates of amphiphilic molecules, whose average dimensions have a typical size of ∼100Å . In the isotropic phase, the principal axes of the different anisotropic aggregates are oriented in space in a random way, i.e., the optical axis is not defined.
For both, colloidal gold and AuNP-doped lyotropic liquid crystal samples, named after colloid (C) and lyotropic (L), respectively, for shortness, we prepared seven dilutions of nanoparticles. The concentrations or number density (N) in particles/ml were N1( 5.66×1011), N2( 8.49 × 1011), N3( 11.32 × 1011), N4( 14.15 × 1011), N5( 16.98×1011), N6(19.8×1011), and N7( 22.64 × 1011). These concentrations correspond to a filling factor of ∼10−5−10−4%.
The following equipments were employed for characterizing the nanoparticles: Cary 50 from Varian (linear optical absorption), X-ray diffractometer Ultima IV from Rigaku (crystalline structure), Zetasizer Nano ZS90 from Malvern (zeta potential), TECNAI F30 (TEM), and Abbe refractometer RTA-100 (refraction index as a function of temperature).
2.1 The Z-scan Technique
The signal of n2 coincides with that of dn/dT. In this work, the samples were conditioned in 200 μm-thick glass cells and it was used a Gaussian cw laser at λ = 532 nm (Ventus, Laser Quantum). The beam waist at focus was 26 μm and data acquisition was made via oscilloscope (Tektronix, TDS1012B). The incident power on the sample was ∼18 mW. This value is the threshold for the appearance of a thermal lens effect, within the sensitivity of our experimental apparatus, without unwanted hydrodynamical effects. It is worth to mention also that the characteristic formation time of a thermal lens in the aqueous colloid, i.e., the time to develop a stable refraction index gradient, is about 5 ms. A concomitant process in colloidal systems, in which a thermal gradient is stablished, is a mass diffusion process, named thermophoretic effect, with time scale in the order of seconds and that leads to a lens of matter. Within the sensibility of our experimental setup, it was not observed the formation of a lens of matter, so the optical response comes just from a thermal lens effect.
3 Results and Discussions
3.1 Dependence of κ and n2 on Clustering
In this work, we have showed that lyotropic liquid crystals doped with AuNPs have an optical absorption spectrum characterized by a broad red-shifted surface plasmon resonance, which can be explained by the formation of nanoparticle aggregates. The induced thermal lens for both media, aqueous dispersions of AuNPs and AuNP-doped lyotropic liquid crystals, is negative ( n2<0). It is found experimentally that n2∝N for the aqueous dispersions of AuNPs and n2∝N2.4 for the AuNP-doped lyotropic liquid crystals. The nonlinear dependence of n2 on N for the AuNP-doped lyotropic liquid crystals can be explained by a decrease of the surface to volume ratio of the loosely packed clusters when increase the number of particles in the aggregate. The theoretical value of the exponent, 2.2, is in good agreement to the experimental one.
This work had the financial support of the Brazilian agencies CAPES, CNPq, FINEP, and Fundação Araucária, and it is part of the research program of the Instituto Nacional de Ciência e Tecnologia de Fluidos Complexos (INCT-FCx). The authors also thanks to the Advanced Microscopy Laboratory (LMA) of the Institute of Nanoscience of Aragon.
- 1.S.A. Maier. Plasmonics: Fundamentals and applications (Springer, New York, 2007)Google Scholar
- 17.K.V. Wong, M.J. Castillo. Adv. Mech. Eng. 2010, 795478 (2010)Google Scholar
- 30.CRC Handbook of Chemistry and Physics (CRC Press, Boca Raton, FL, 2003), 84th edGoogle Scholar