Tese_Tânia Vieira.pdf - Ubi Thesis

Chapter III – Results and Discussion
absorption depends on particles size, shape and the dielectric constant of the surrounding
medium (Guzmán et al. 2009).
In order to verify the concentration of AgNO 3 that produced the highest number of AgNPs,
the absorption spectra of the AgNPs were obtained and are presented in Figure 9. All samples
presented the characteristic surface plasmon of AgNPs (peak between 400 and 420 nm) with the
exception of the spectrum of high concentrations (green line) where no AgNPs were produced. It
was also observed that for lower concentrations (black line) the absorbance is reduced indicating
the presence of few AgNPs. Such was also confirmed through SEM images. Based on these results,
it was concluded that the intermediate concentration of AgNO 3 was the best to produce the
AgNPs. Recently, it was reported by Martinez-Castanon that the absorption spectrum of spherical
AgNPs present a maximum absorbance band between 420 and 450 nm with a blue or red shift
with particle size decreasing or increasing, respectively (Martinez-Castanon et al. 2008). These
observations showed that the reducing agent NaBH 4 produced smaller nanoparticles than that
produced with C 6 H 8 O 6 . In the same study, Martinez-Castanon reported that the width of each
plasmon is related to the size distribution. As referred above, large peaks indicate a large size
distribution of nanoparticles, and for irregular particles (non spherical), two or more plasmon
bands are expected depending on the symmetry of the particles (Martinez-Castanon et al. 2008).
Therefore, the nanoparticles herein produced with C 6 H 8 O 6 were less uniform in their size and less
symmetric compared to that produced, using NaBH 4 .
The spectra of the samples obtained by UV-Vis spectroscopy are in agreement with those
previously reported in literature (Kim et al. 2007; Guzmán et al. 2009; Vasilev et al. 2010;
Guidelli et al. 2011). For instance, Kim and colleagues observed an absorption band at 391 nm
for AgNPs (Kim et al. 2007). In the work of Guidelli and coworkers a characteristic absorption
peak at 435 nm of AgNPs was observed (Guidelli et al. 2011). A study performed by Guzmán and
collaborators demonstrated that the peak of the AgNPs was around 418 nm (Guzmán et al. 2009).
Vasilev and collaborators observed a peak around 410 nm for AgNPs (Vasilev et al. 2010). In own
study, an absorption peak around 400 nm was also observed, in accordance to previous studies.
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