Thermal diffusivity measurement of spherical gold nanofluids of different sizes/concentrations
© López Muñoz et al.; licensee Springer. 2012
Received: 22 June 2012
Accepted: 24 July 2012
Published: 30 July 2012
In recent times, nanofluids have been studied by their thermal properties due to their variety of applications that range from photothermal therapy and radiofrequency hyperthermia (which have proven their potential use as coadjutants in these medical treatments for cancer diseases) to next-generation thermo-fluids. In this work, photoacoustic spectroscopy for a specific study of thermal diffusivity, as a function of particle size and concentration, on colloidal water-based gold nanofluids is reported. Gold nanoparticles were synthetized in the presence of hydroquinone through a seed-mediated growth with homogenous sizes and shapes in a range of 16 to 125 nm. The optical response, size and morphology of these nanoparticles were characterized using ultraviolet–visible spectroscopy and transmission electron microscopy, respectively. Thermal characterizations show a decrease in the thermal diffusivity ratio as the nanoparticle size is increased and an enhancement in thermal diffusivity ratio as nanoparticle concentration is added into the nanofluids. Compared with other techniques in the literature such as thermal lens and hot wire method, this photoacoustic technique shows an advantage in terms of precision, and with a small amount of sample required (500 μl), this technique might be suitable for the thermal diffusivity measurement of nanofluids. It is also a promising alternative to classical techniques.
KeywordsGold nanoparticles Nanofluids Photoacoustic Thermal diffusivity
Thermal properties of nanofluids, which are mixtures of nanomaterials suspended in an organic or inorganic base fluid, are especially interesting due to their variety of applications that range from photothermal therapy and radiofrequency hyperthermia (which have proven their potential use as coadjutants in these medical treatments for cancer diseases [1, 2]) to next-generation thermo-fluids. Recently, it has been found that nanofluids exhibit higher thermal conductivity and thermal diffusivity than base fluids themselves. Thermal research in nanofluids has been mainly focused in thermal conductivity measurements [3–5], but in recent years, other techniques have been developed for thermal diffusivity measurements in nanofluids, such as the hot-wire technique and the thermal lens spectrometry [6–8]. However, these methodologies are limited due to the difficulty in achieving accurate results, the complexities in their theoretical model and the amount of sample required for measurements.
In this article, an aqueous synthesis of gold nanoparticles in the presence of hydroquinone through a seed-mediated growth is presented. Also, the study of thermal diffusivity in gold nanofluids as a function of particle size (16–125 nm) and concentration by photoacoustic spectroscopy opens new horizons for the thermal research area in nanofluids.
Hydroquinone synthesis of gold nanoparticles
Typical synthesis of gold nanoparticles by the chemical reduction of gold chloride using sodium citrate can only produce quality particles up to 50 nm in size; beyond which, they are poly-dispersed and non-spherical . The reduction of gold chloride onto nanoparticle seeds (gold nanoparticles, <20 nm) together with hydroquinone as a selective reducing agent improves these obtained particles with homogenous sizes and shapes in a size range of 50 to 200 nm [10, 11].
Gold nanoparticle seeds were synthetized by sodium citrate reduction. A solution of gold chloride is brought to boil and immediately followed by the addition of a solution of sodium citrate. The solution is removed from heat once nanoparticle maturation is completed, as indicated by the colour transition.
Large-diameter gold nanoparticles were synthetized by hydroquinone method, using consistent concentrations of gold chloride, sodium citrate and hydroquinone but decreasing the number of seeds, which results in the growth of larger nanoparticles. The addition of less seeds results in larger gold nanoparticle diameters. The different size nanofluids were centrifuged at 6,000 rpm for 30 min and re-dispersed with high-performance liquid chromatography (HPLC) water for a final concentration of 0.1 mg/ml.
For concentrated nanofluids, stock solutions of gold nanoparticles synthetized by hydroquinone method were centrifuged at 6,000 rpm for 30 min for final concentrations of 1 mg/ml. The concentrated solutions were re-dispersed with HPLC water to obtain different gold nanoparticle concentrations.
All chemicals used were of analytical grade from Sigma-Aldrich Corporation (St. Louis, MO, USA) and were used as received; HPLC water was used in all the gold nanoparticles’ synthesis.
Basic photoacoustic theoretical scheme
from which the sample’s thermal diffusivity can be obtained.
The photoacoustic signal was recorded as a function of the sample thickness, taking 16 experimental points from a relative sample thickness l0 at 10-μm steps using a micro-linear actuator model T-NA08A50 (Zaber Technologies, Inc., Vancouver, Canada). Linear fits were done for the photoacoustic phase to obtain parameter B, as described in the theoretical section, from which the sample’s thermal diffusivity was obtained by means of the relation α s = 2π/B2. Measurements were performed at room temperature 25 ± 2°C.
Results and discussion
Thermal diffusivity measurement
Thermal diffusivity of gold nanofluids for different nanoparticle diameters for constant nanoparticle concentration of 0.1 mg/ml
Particle size (nm)
α(10−4 cm2·s−1) ± 0.04
Thermal diffusivity of gold nanofluids at different nanoparticle concentrations
α55 nm(10−4 cm2·s−1) ± 0.04
α83nm(10−4 cm2·s−1) ± 0.04
The present study investigates the effect of concentration and size of gold nanoparticles on the thermal diffusivity of gold nanofluids prepared by hydroquinone method through a seed-mediated growth by photoacoustic spectroscopy. The thermal diffusivity ratio changed inversely with the nanoparticle size; this effect was studied in a size range of 16 to 125 nm for gold nanofluids. The thermal diffusivity ratio has been found to increase with concentration; this effect was studied in a range of 1 to 0.2 mg/ml for gold nanofluids. The experimental data for thermal diffusivity ratio as a function of concentration and size for gold nanofluids depict similar behaviours in enhancement ratio compared to thermal conductivity reports and other techniques in the literature such as thermal lens and hot wire method with high accuracy, and with a small amount of sample required (500 μl), photoacoustic spectroscopy might be suitable for thermal diffusivity measurement of nanofluids and being a promising alternative to classical techniques.
high-performance liquid chromatography
transmission electron microscopy
The authors acknowledge Zaber Technologies, Inc. for providing a micro-linear stage, Química Aromática SA, COFAA-IPN and CONACyT for the partial support of this work.
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