spectroscopy (FT-IR), zeta potential measurement, particle size analysis (PSA), and
energy dispersive X-ray spectroscopy (EDX).
The absorption of UV-visible radiation induces an excitation of electrons from
lower-energy orbital to higher-energy orbital in a molecule. The UV-visible absorption occurs at a specific wavelength for each molecular structure (Xu et al. 2014). It
is a technique widely employed in the study of nanomaterials and can be used as a
simple and reliable method for monitoring the stability of NP solutions. The optical
absorption spectra of metallic NPs are dominated by surface plasmon resonance
(Vaghari et al. 2016). Thus, position and shape of absorption peak depend mainly on
factors such as size, shape, and NP polydispersity, surrounding medium, and substances absorbed on its surface (Mahmoudi et al. 2009; Xu and Sun 2013). For
example, the appearance of an absorption peak around 400 nm is indicative of the
presence of AgNPs (Roduner 2006). Usually, the appearance of this peak is closely
associated with color changes of the reaction solution, being the first signal that
indicates qualitatively that the NPs have been produced. For the AgNPs the characteristic color change is yellow to brown (Sharma et al. 2015), and for AuNPs the
color change is from yellow to purple (Bastús et al. 2014).
The main use of FT-IR in NP characterization is to investigate the presence of
biomolecules (such as capping proteins) bound to NPs. For instance, Ahmad et al.
(2003a) using FT-IR spectroscopy plus gel electrophoresis hypothesized that certain
proteins could be enzymes that reduce chloroaurate ions and cap the AuNPs formed
by the actinobacterium Thermomonospora sp. It is well known that proteins can bind
to NPs either through free amine groups or cysteine residues in the proteins, and
therefore, stabilization of the NPs by surface-bound proteins is a possibility.
The most popular tools for the visualization of NPs are electron microscopy techniques. Depending on the technique, resolutions down to the sub-nanometer range
can be achieved. Transmission electronic microscopy, SEM, and AFM are most
extensively applied techniques not only to visualize the shape of synthesized NPs
but also the state of sorption, size, aggregation, dispersion, and structure of NPs (Liu
2006). With TEM and SEM techniques, NPs can be analyzed by its size, morphology, and shape distribution (Chauhan et al. 2016) although generally, imaging of
lighter atoms in an electron microscope is more difficult as they scatter electrons less
efficiently. Another alternative is to use AFM for analyzing the sample upon liquid
conditions. However, it has the disadvantage that particles not fixed to a substrate
will float around and eventually can stick to the cantilever, which leads to imaging
artifacts (Ghazwani 2015).
Dynamic light scattering is a widely used technique in the characterization of NPs
on a simple solvent or biological environment (Wen et al. 2016). In addition, it is one
of the methods most employed to quantify size distribution and surface charges of
NPs suspended in a liquid. The main advantage of DLS is that measurements are fast
and noninvasive when a sample is in its native colloidal state (Xue et al. 2016).
The elemental composition of NPs can be determined by EDX (Shi et al. 2015),
while XRD is utilized to determine the crystal structure of the NPs. X-ray powder
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D. Costa et al.
energy dispersive X-ray spectroscopy (EDX).
The absorption of UV-visible radiation induces an excitation of electrons from
lower-energy orbital to higher-energy orbital in a molecule. The UV-visible absorption occurs at a specific wavelength for each molecular structure (Xu et al. 2014). It
is a technique widely employed in the study of nanomaterials and can be used as a
simple and reliable method for monitoring the stability of NP solutions. The optical
absorption spectra of metallic NPs are dominated by surface plasmon resonance
(Vaghari et al. 2016). Thus, position and shape of absorption peak depend mainly on
factors such as size, shape, and NP polydispersity, surrounding medium, and substances absorbed on its surface (Mahmoudi et al. 2009; Xu and Sun 2013). For
example, the appearance of an absorption peak around 400 nm is indicative of the
presence of AgNPs (Roduner 2006). Usually, the appearance of this peak is closely
associated with color changes of the reaction solution, being the first signal that
indicates qualitatively that the NPs have been produced. For the AgNPs the characteristic color change is yellow to brown (Sharma et al. 2015), and for AuNPs the
color change is from yellow to purple (Bastús et al. 2014).
The main use of FT-IR in NP characterization is to investigate the presence of
biomolecules (such as capping proteins) bound to NPs. For instance, Ahmad et al.
(2003a) using FT-IR spectroscopy plus gel electrophoresis hypothesized that certain
proteins could be enzymes that reduce chloroaurate ions and cap the AuNPs formed
by the actinobacterium Thermomonospora sp. It is well known that proteins can bind
to NPs either through free amine groups or cysteine residues in the proteins, and
therefore, stabilization of the NPs by surface-bound proteins is a possibility.
The most popular tools for the visualization of NPs are electron microscopy techniques. Depending on the technique, resolutions down to the sub-nanometer range
can be achieved. Transmission electronic microscopy, SEM, and AFM are most
extensively applied techniques not only to visualize the shape of synthesized NPs
but also the state of sorption, size, aggregation, dispersion, and structure of NPs (Liu
2006). With TEM and SEM techniques, NPs can be analyzed by its size, morphology, and shape distribution (Chauhan et al. 2016) although generally, imaging of
lighter atoms in an electron microscope is more difficult as they scatter electrons less
efficiently. Another alternative is to use AFM for analyzing the sample upon liquid
conditions. However, it has the disadvantage that particles not fixed to a substrate
will float around and eventually can stick to the cantilever, which leads to imaging
artifacts (Ghazwani 2015).
Dynamic light scattering is a widely used technique in the characterization of NPs
on a simple solvent or biological environment (Wen et al. 2016). In addition, it is one
of the methods most employed to quantify size distribution and surface charges of
NPs suspended in a liquid. The main advantage of DLS is that measurements are fast
and noninvasive when a sample is in its native colloidal state (Xue et al. 2016).
The elemental composition of NPs can be determined by EDX (Shi et al. 2015),
while XRD is utilized to determine the crystal structure of the NPs. X-ray powder
204
D. Costa et al.