## 3 Results and Discussion
### Photothermal Experiments  
The dispersion of gold nanoparticles was subjected to photothermal experiments by irradiation with a resonant wavelength of light entering the plasmon field of the nanoparticles. The temperature was monitored along the heating process. We note that the measured value of temperature in the bulk solution includes the systematic error of heat dissipation by the environment, especially by the solvent [19]. Therefore, the increase of temperature is compared within the different values of ionic strength. Additional considerations in further studies comprising plasmonic nanoparticles of different sizes and shapes should be included [20]. The wavelength of incoming light was adjusted to the maximum wavelength of the plasmon peak of the nanoparticles, see Table 1. An increase of the temperature of the dispersion of nanoparticles was observed almost immediately after the irradiation begins, see Figure 3. After an irradiation time of ca. 8 min the temperature was not increased further, instead reaching a constant value. \begin{table}
\begin{tabular}{c c c c} \hline
**NaCl/mM** & \(\lambda_{\text{max},\text{plasmon}}\)/nm & \(\lambda_{\text{irradiation}}\)/nm & \(\Delta\mathbf{T_{max}}\)/\({}^{0}\mathbf{C}\) \\ \hline
0 & 523 & 524 & 1.6 \\
10 & 523 & 524 & 1.8 \\
20 & 523 & 524 & 1.6 \\
30 & 524 & 524 & 1.7 \\
40 & 524 & 524 & 2.1 \\
50 & 526 & 525 & 2.0 \\
60 & 528 & 526 & 1.9 \\
70 & 535 & 534 & 2.0 \\
80 & 545 & 575 & 1.7 \\
90 & 613 & 617 & 1.7 \\
100 & 624 & 627 & 1.9 \\ \hline \end{tabular}
\end{table} 
Table 1: Values of maximum wavelength for the plasmon peak of the gold nanoparticles (\(\lambda_{\text{max},\text{plasmon}}\)), wavelength of incoming light during photothermal experiments (\(\lambda_{\text{irradiation}}\)), and final increase of temperature during photothermal experiments (\(\Delta\mathbf{T_{max}}\), standard deviation equal to or lower than 5%) for the different values of ionic strength. Figure 3: Increase of temperature of the dispersion of nanoparticles against the time of irradiation for different values of sodium chloride concentration: 0 mM (black circles), 40 mM (red circles), and 90 mM (blue circles). The standard deviation between measurements was in all cases equal to or lower than 5%. The kinetics of photothermal heating was similar in all cases. In a similar study published by [PERSON] et al., the kinetics of photothermal heating was similar, showing a plateau at ca. 10 min [21]. In that study, the nanoparticles were coated by polyaniline (PANI) polymer. Therefore, we conclude that the kinetics of heating is not determined by the coating of the nanoparticles, but rather by the gold nanoparticles themselves. However, the final value of increase of temperature was slightly but significantly higher for the moderate values of ionic strength, i.e., 40 mM of sodium chloride. This comparatively superior result might be ascribed to a balance between the interplay of two opposite effects. At low values of ionic strength or in the absence of salt, the plasmonic nanoparticles are at a relatively large distance to each other due to the ionic repulsion between the citrate-coated surfaces of the nanoparticles. On the other hand, at high values of ionic strength, the gold nanoparticles are highly aggregated, therefore promoting scattering of the incoming light due to large aggregates, as well as an excessive broadening of the plasmon peak, with both effects contributing to a decrease in the photothermal efficiency. The intermediate case of ionic strength appears to display a certain aggregation of the plasmonic nanoparticles that promotes the enhancement of the photothermal effect with no excessive aggregation of the nanoparticles. This limited aggregation of the nanoparticles allows for an effective plasmon coupling. Therefore, such an intermediate state of aggregation might be the optimum design when engineering plasmonic nanoparticles for in vivo application of photothermal therapy.