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 9 . 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.

As shown elsewhere 12 , the photoacoustic signal in transmission configuration, assuming sample thickness L as the only variable (with fixed modulation frequency f) in the liquid’s sample thermally thick regime, can be expressed as follows:

δ P = E exp − σ s L ,

where E is a complex constant and σ _s = (1 + i)(πf/α _s)^1/2, with α _s as the sample’s thermal diffusivity. The amplitude ∣δP∣ and phase Ф, of this equation can be written as follows:

δ P = E exp − π f α s L ,

Φ = Φ 0 − π f α s L .

The signal phase Ф is a linear function of the sample thickness with the slope given by the following:

B = π f a s 1 2 ,

from which the sample’s thermal diffusivity can be obtained.

A transversal section of the photoacoustic experimental setup for thermal diffusivity measurements is shown in Figure 1. The photoacoustic cell consisted of a cylindrical cavity made in a stainless steel body and communicated with a commercial electret microphone (Panasonic, Kadoma, Osaka, Japan), model WM-61A. The resultant signals detected by the microphone were processed by a lock-in amplifier model SR830 (Stanford Research, Menlo Park, CA, USA) for amplification and de-modulation; the transistor-transistor logic output of the lock-in was used for the modulation control of a 660-nm laser diode system (Qioptiq Photonics Ltd. (formerly Point Source Ltd.), Hamble, Hampshire, UK) model IFLEX-2000 at a fix modulation frequency of 2 Hz.

The photoacoustic signal was recorded as a function of the sample thickness, taking 16 experimental points from a relative sample thickness l ₀ 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/B ². Measurements were performed at room temperature 25 ± 2°C.

The particle size, shape/morphology and distribution of nanoparticles in the base medium were determined by transmission electron microscopy (TEM) with a JEOL JEM2010 equipment (JEOL Ltd., Akishima, Tokyo, Japan) at 200 kV and 106 μA. Samples were prepared by adding 50 μl of the stock nanoparticle solution onto 200-mesh carbon-coated copper grids to obtain TEM images. In Figure 2, TEM images show a larger diameter of the gold nanoparticles by decreasing the number of seeds, as it was expected. The dispersion of gold nanoparticles has a median size distribution near 12% for all the samples, including large-diameter gold nanoparticles. This is an advantage if compared with the citrate method, in which it is only possible to produce monodispersed and spherical gold nanoparticles of up to 50 nm in diameter.

The absorption spectra of synthetized nanoparticles were measured as 1:2 dilutions (stock to H₂O) using an ultraviolet–visible (UV–vis) spectrophotometer (8453 Agilent Technologies, Inc., Santa Clara, CA, USA). Figure 3A shows the UV–vis absorption spectra of the different size gold nanofluids studied. As expected, the increase in gold nanoparticle diameters resulted in a longer absorption λ _max values (Figure 3B), according to Mie’s theory 13 14 .