97
5.4.1.3 Particle Size Analysis
Dynamic light scattering (DLS) and laser diffraction are usually used to determine
the size-distribution profile of small particles (from a few nanometers to a few
micrometers) in suspension. DLS also known as photon correlation spectroscopy
(PCS), is based on the measurement of the intensity of scattered monochromatic
laser light, operating on the principle that particles move randomly in gas or liquid,
experiencing Brownian motion. It is able to characterize particles in the range from
5 nm to 3 μm or <5 μm, depending on their weight. The laser diffraction is based on
the scattering angle of the laser beam and provides information about volume size
distribution data in the range from 0.5 to 3500 μm (Foerter-Barth and Teipel 2000).
Because of its better resolution, DLS has been more widely applied to MIP characterization. Garcia-Mutio et al. (2016) measured the hydrodynamic diameter of
molecularly imprinted NPs grafted onto the surface of Au electrodes. Long et al.
(2011) also applied DLS to monitor the growth of polymer particles in precipitation
polymerization systems.
DLS measurements are generally used to obtain data complementary to microscopic analysis. One of the main drawbacks of this technique is that aggregates can
be confused with large particles and, unlike electron microscopy, does not provide
visual information to discard aggregation phenomena (De Middeleer et al. 2016), in
any case, modern DLS instruments are capable of recognizing particle aggregates,
ignoring them for particle size determination.
5.4.2 Physicochemical Characterization
Due to their insoluble nature of MIPs, physicochemical characterization generally
involves solid-state techniques. Solid-state nuclear magnetic resonance (NMR) and
Fourier transform infrared (FTIR) spectroscopy are often used to estimate the
degree of polymerization, as well as the incorporation of functional groups into the
polymer matrix (Hosier et al. 2004; Spivak 2005).
NMR spectroscopy is based on spin orientation of individual atoms such as
1
H or
13
C in a high-intensity magnetic field. NMR spectra have been widely used for
molecular structure elucidation and the characterization of binding interactions of
MIPs. NMR analysis during the prepolymerization stage provides information on
template-monomer interactions (Molinelli et al. 2005; Scorrano et al. 2015;
Sánchez-González et al. 2019), monomer-crosslinker interactions (Shoravi et al.
2010) and the successful removal of the template (Abbate et al. 2011; Deng et al.
2016). NMR deficiencies with respect to sensitivity limit its use in the MIP characterization, especially for those MIPs that present a low number of binding sites.
Skogsberg et al. (2007) combined solid-state cross-polarization/magic-angle spinning NMR and suspended-sate high resolution/magic angle spinning NMR to obtain
molecular level details about the formation process of binding sites of a
9- ethyladenine MIP.
5 Plastic Receptors Developed by Imprinting Technology as Smart Polymers Imitating…
5.4.1.3 Particle Size Analysis
Dynamic light scattering (DLS) and laser diffraction are usually used to determine
the size-distribution profile of small particles (from a few nanometers to a few
micrometers) in suspension. DLS also known as photon correlation spectroscopy
(PCS), is based on the measurement of the intensity of scattered monochromatic
laser light, operating on the principle that particles move randomly in gas or liquid,
experiencing Brownian motion. It is able to characterize particles in the range from
5 nm to 3 μm or <5 μm, depending on their weight. The laser diffraction is based on
the scattering angle of the laser beam and provides information about volume size
distribution data in the range from 0.5 to 3500 μm (Foerter-Barth and Teipel 2000).
Because of its better resolution, DLS has been more widely applied to MIP characterization. Garcia-Mutio et al. (2016) measured the hydrodynamic diameter of
molecularly imprinted NPs grafted onto the surface of Au electrodes. Long et al.
(2011) also applied DLS to monitor the growth of polymer particles in precipitation
polymerization systems.
DLS measurements are generally used to obtain data complementary to microscopic analysis. One of the main drawbacks of this technique is that aggregates can
be confused with large particles and, unlike electron microscopy, does not provide
visual information to discard aggregation phenomena (De Middeleer et al. 2016), in
any case, modern DLS instruments are capable of recognizing particle aggregates,
ignoring them for particle size determination.
5.4.2 Physicochemical Characterization
Due to their insoluble nature of MIPs, physicochemical characterization generally
involves solid-state techniques. Solid-state nuclear magnetic resonance (NMR) and
Fourier transform infrared (FTIR) spectroscopy are often used to estimate the
degree of polymerization, as well as the incorporation of functional groups into the
polymer matrix (Hosier et al. 2004; Spivak 2005).
NMR spectroscopy is based on spin orientation of individual atoms such as
1
H or
13
C in a high-intensity magnetic field. NMR spectra have been widely used for
molecular structure elucidation and the characterization of binding interactions of
MIPs. NMR analysis during the prepolymerization stage provides information on
template-monomer interactions (Molinelli et al. 2005; Scorrano et al. 2015;
Sánchez-González et al. 2019), monomer-crosslinker interactions (Shoravi et al.
2010) and the successful removal of the template (Abbate et al. 2011; Deng et al.
2016). NMR deficiencies with respect to sensitivity limit its use in the MIP characterization, especially for those MIPs that present a low number of binding sites.
Skogsberg et al. (2007) combined solid-state cross-polarization/magic-angle spinning NMR and suspended-sate high resolution/magic angle spinning NMR to obtain
molecular level details about the formation process of binding sites of a
9- ethyladenine MIP.
5 Plastic Receptors Developed by Imprinting Technology as Smart Polymers Imitating…