# Fast One-Step Synthesis of Anisotropic Silver Nanoparticles
## 3 Results and Discussion
It is known that, in the presence of light, citrate is able to reduce Ag\({}^{+}\) to Ag\({}^{0}\)[33]:  
\[\text{Citrate}^{3-}+\lambda\rightarrow\text{Acetone-1,3-dicarboxylate}^{2-}+\text{CO}_{2}+\text{H}^{+}+2\text{e}^{-} \tag{1}\]  
\[\text{Ag}^{+}+\text{e}^{-}\rightarrow\text{Ag}^{0} \tag{2}\]  
At the same time, H\({}_{2}\)O\({}_{2}\) can both oxidize and reduce silver [17,18]:  
\[2\text{Ag}+\text{H}_{2}\text{O}_{2}\to 2\text{Ag}^{+}+2\text{OH}^{-} \tag{3}\]  
\[\text{H}_{2}\text{O}_{2}+2\text{Ag}^{+}+2\text{OH}^{-}\to 2\text{Ag}+2 \text{H}_{2}\text{O}+\text{O}_{2} \tag{4}\]  
Figure 3: (**a**,**b**) SEM images of Ag NPTs; (**c**) size distribution of Ag NPTs obtained from SEM image analysis. Besides reducing Ag in the presence of light, citrate also has the function of being a capping agent for Ag NPs, as it easily binds to the (111) facets of silver [34,35]. This allows both NP stabilization in solution and their preferential growth along the Ag (100) direction, which is less preferred by citrate. Hence, Ag NPs initially produced by the laser ablation process were first oxidized by H2O2, being partially dissolved in the liquid medium, and then reduced back by citrate, supported by H2O2, adding preferentially to the uncapped (100) Ag facet. It is interesting to note that the absorption spectrum shown in Figure 2 did not dramatically change its shape after about 10 min of this process, while the intensity increased and then stabilized over time (Figure S4). This suggests that an equilibrium was reached within the solution between the production of NPs by laser ablation and their conversion into NPT and that after reaching this equilibrium, the process only increased the NPT concentration in the solution. Simulations of the extinction spectra were performed to support and complement the experimental results. Figure 5 shows simulated extinction spectra for NPTs of a round and hexagonal shape, with a size of 70 nm and 82.5 nm and a thickness of 10 and 20 nm, representative of the structures observed by SEM and AFM. Figure 6 shows the experimental absorption spectrum, already shown in Figure 2, with the simulated ones. It can be observed that the simulated spectra were mainly in agreement with the longer wavelength part of the experimental spectrum. This partial disagreement might be due to the fact that the simulated spectra were obtained for isolated NPTs, thus not taking into account the interaction among the particles. However, this aspect is still unclear and will be subject to further investigation. The obtained NPTs were finally tested for plasmon sensitivity (\(S\)), i.e., the variation of the plasmon resonance peak position due to a change in the refractive index (\(S=\Delta\lambda/\Delta n\)), which is typically reported in terms of nm/RIU (refractive index unit). To change the refractive index, we used sucrose solutions at set concentrations, since the relation between sucrose concentration in water and the refractive index is well established [31]. Figure 7 shows the plasmon resonance peak position of a Ag NPT solution produced by our process by varying the refractive index, along with the data simulated for disks of 70 nm in size and 20 nm in thickness. Fitting the data yielded a plasmon sensitivity \(S\) of 216 nm/RIU, close to that obtained by the simulations. This value is comparable to the others, typically below 250 RIU, reported in the literature for Ag NPTs with a similar plasmon resonance [7, 8, 9, 10, 11], with the advantage of obtaining the nanostructures in a fast and simple one-step process, despite the poor SPR tunability. If we compare the experimental data with those obtained from the simulation of discs and hexagons (70 nm \(\times\) 20 nm), we observe that the simulated data showed a higher plasmon sensitivity. This was probably due to a higher SPR wavelength in the simulated spectra, which was expected to exhibit higher plasmon sensitivity, in agreement with our previous work [36].