## 3 Results and Discussion
### Nanoparticle Synthesis and Characterization
#### 3.1.1 Silver Nanoparticles Synthesis
In Figure 3, we present the spectra recorded for a sample submitted to an ultrasound field of 60 W (30% from the maximum power) for different durations. The sample consisted of a 50 mL solution containing 0.5 mM AgNO\({}_{3}\), 1 mM \(\beta\)-CD, and 10 mM K\({}_{2}\)CO\({}_{3}\). After 5 min of irradiation, we recorded a well-defined colloid peak at around 405 nm with an increase in its intensity after another period of 5 min irradiation. After the sample was left undisturbed for 15 or 30 min, we noticed a significant increase in the peak's intensity. This means that after the exposure of the sample to ultrasound, unreacted chemical species lead to this evolution. Extending the ultrasound irradiation times leads to unstable samples in which the colloid settles on the bottom of the vial after several hours, as we will present below for Au colloids. Nevertheless, ultrasound irradiation produces AgNPs with larger sizes and a higher polydispersity index (19.5 \(\pm\) 4.5 nm) as seen in Figure 3b,c. * Room temperature synthesis  
One possible explanation for the continuation of colloid formation even after the ultrasound irradiation stopped is related to the capacity of CDs to reduce Ag ions in alkaline environments, even at lower temperatures. During the ultrasound exposure, the samples are heated by the energy dissipated, reaching around 55-60 \({}^{\circ}\)C. Most of the papers published in the literature for the CD-mediated synthesis of coinage metal NPs make use of such elevated temperatures (or higher). However, there are some reports that Ag ions can be reduced by natural \(\beta\)-CD even at room temperature in the presence of NaOH [34]. We also tested synthesizing AgNPs in an alkaline medium induced by potassium carbonate using \(\beta\)-CDs. In an exploratory experiment, we prepared a 20 mL solution with  
Figure 3: (a) UV-Vis absorption spectra for AgNPs synthesized in an ultrasound field of 60 W, after 5 min of sonication (black), after 10 min of sonication (red), after 10 min of sonication, and 15 min of the undisturbed sample (blue), and after another 15 min of the sample left undisturbed. The colloids were diluted 3 times before the measurement. (b) TEM image of the AgNPs and (c) their size distribution. an Ag:\(\beta\)-CD ratio of 1:1 (1 mM:1 mM) and 10 mM K\({}_{2}\)CO\({}_{3}\), then allowed it to stand for 12-24 h without any external energy input. The absorption spectrum (Figure 2d) showed a plasmonic peak at 407 nm with a half-width of 80-90 nm. We systematically investigated the synthesis using two ratios of Ag:\(\beta\)-CD, namely, 1:1 and 1:2, and two different concentrations of K\({}_{2}\)CO\({}_{3}\). The UV-Vis absorption spectra of the colloids synthesized in this manner are presented in Figure 4.  
All colloids present a strong plasmonic peak, characterized by an FWHM \(<\) 70 nm indicating a low polydispersity with the absorbance value at the peak correlated with the amount of silver precursor. Moreover, there is a slight redshift in the peak wavelengths as the concentration of Ag increases, which might be related to an increase in the NPs' size. TEM images of some samples together with their size distribution are presented in Figure S7. Like in the case of US-assisted synthesis, the AgNPs synthesized at room temperature present larger sizes with means of 22-23 nm and a higher polydispersity (SD of 6-7 nm). While in many applications, uniform-sized NPs are preferred, a significant increase in the mean size leads to an increased amplification of the electromagnetic field and, consequently, to an increased enhancement in SERS.