95
used during polymerization (Santora et al. 2002; Esfandyari-Manesh et al. 2011).
The surface-to-volume ratio also plays a key role in MIP particles. It involves having a greater availability of the binding site, which results in a greater binding
capacity (De Middeleer et al. 2016).
The surface area is most commonly determined by nitrogen adsorption porosimetry, exposing a fixed mass of dry polymer to an inert gas such as nitrogen.
Adsorption isotherms are constructed by measuring the amount of adsorbed gas as
a function of pressure. Subsequently, the application of the Brunauer-Emmett-Teller
(BET) equation allows to obtain the information about the specific surface area
(Naderi 2015). The pore diameter and the pore size distribution (the relative abundance of each pore size in the whole tested imprinted polymer) can be elucidated
from the nitrogen adsorption data analyzed by the Barrett-Joyner-Halenda (BJH)
method (Spivak 2005). In general terms, larger BET surface areas and higher BJH
pore sizes lead to greater absorption capacity (Cheng et al. 2014).
While nitrogen adsorption porosimetry is more reliable when measuring micro(diameters below 2 nm) and mesopores (diameters between 2 and 50 nm), mercury
intrusion porosimetry is more sensitive for macropores (greater than 50 nm)
(Roquerol et al. 1994). In the latter, mercury is forced into a fixed mass of dry polymer under controlled pressurization (Tan et al. 2012). Although BET and BJH isotherms and mercury porosimetry have proven their usefulness in elucidating surface
area and pore size distribution, they are indirect approaches with data susceptible to
over-analysis and inaccurate outcomes (Neusser et al. 2017).
Ellipsometric measurements have become an interesting tool to characterize
polymer thickness in MIP membranes and films, covering a wide range of applications including molecularly imprinted nanofilms on surface plasmon resonance sensors (Yola et al. 2015), imprinted quartz crystal microbalance (QCM) sensors (Battal
et al. 2018) and MIP coated electrodes for potentiometric biosensors (Kajisa
et al. 2018).
5.4.1.2 Microscopic Analysis
Microscopic analysis of MIPs such as atomic force microscopy (AFM), scanning
electron microscopy (SEM) and transmission electron microscopy (TEM) are the
main techniques used for the morphological MIP characterization (Fig. 5.8). SEM
analysis allows to obtain the information about the geometry and size of the MIP
particles, as well as establishing differences between morphologies of imprinted
and non-imprinted polymers (NIP) (De Middeleer et al. 2016).
Yoshimatsu et al. (2007) developed a new method to obtain precise control of
molecularly imprinted NPs and microspheres in the range of 100 nm to 2.5 μm by
precipitation polymerization. Particle size and morphology were analyzed using
SEM, but previously, polymer microspheres were coated with Au by sputtering.
More recently, Neusser et al. (2017) introduced the focused ion beam/scanning
electron beam (FIB/SEM) tomography for analyzing the nanoscale porosity of
5 Plastic Receptors Developed by Imprinting Technology as Smart Polymers Imitating…
used during polymerization (Santora et al. 2002; Esfandyari-Manesh et al. 2011).
The surface-to-volume ratio also plays a key role in MIP particles. It involves having a greater availability of the binding site, which results in a greater binding
capacity (De Middeleer et al. 2016).
The surface area is most commonly determined by nitrogen adsorption porosimetry, exposing a fixed mass of dry polymer to an inert gas such as nitrogen.
Adsorption isotherms are constructed by measuring the amount of adsorbed gas as
a function of pressure. Subsequently, the application of the Brunauer-Emmett-Teller
(BET) equation allows to obtain the information about the specific surface area
(Naderi 2015). The pore diameter and the pore size distribution (the relative abundance of each pore size in the whole tested imprinted polymer) can be elucidated
from the nitrogen adsorption data analyzed by the Barrett-Joyner-Halenda (BJH)
method (Spivak 2005). In general terms, larger BET surface areas and higher BJH
pore sizes lead to greater absorption capacity (Cheng et al. 2014).
While nitrogen adsorption porosimetry is more reliable when measuring micro(diameters below 2 nm) and mesopores (diameters between 2 and 50 nm), mercury
intrusion porosimetry is more sensitive for macropores (greater than 50 nm)
(Roquerol et al. 1994). In the latter, mercury is forced into a fixed mass of dry polymer under controlled pressurization (Tan et al. 2012). Although BET and BJH isotherms and mercury porosimetry have proven their usefulness in elucidating surface
area and pore size distribution, they are indirect approaches with data susceptible to
over-analysis and inaccurate outcomes (Neusser et al. 2017).
Ellipsometric measurements have become an interesting tool to characterize
polymer thickness in MIP membranes and films, covering a wide range of applications including molecularly imprinted nanofilms on surface plasmon resonance sensors (Yola et al. 2015), imprinted quartz crystal microbalance (QCM) sensors (Battal
et al. 2018) and MIP coated electrodes for potentiometric biosensors (Kajisa
et al. 2018).
5.4.1.2 Microscopic Analysis
Microscopic analysis of MIPs such as atomic force microscopy (AFM), scanning
electron microscopy (SEM) and transmission electron microscopy (TEM) are the
main techniques used for the morphological MIP characterization (Fig. 5.8). SEM
analysis allows to obtain the information about the geometry and size of the MIP
particles, as well as establishing differences between morphologies of imprinted
and non-imprinted polymers (NIP) (De Middeleer et al. 2016).
Yoshimatsu et al. (2007) developed a new method to obtain precise control of
molecularly imprinted NPs and microspheres in the range of 100 nm to 2.5 μm by
precipitation polymerization. Particle size and morphology were analyzed using
SEM, but previously, polymer microspheres were coated with Au by sputtering.
More recently, Neusser et al. (2017) introduced the focused ion beam/scanning
electron beam (FIB/SEM) tomography for analyzing the nanoscale porosity of
5 Plastic Receptors Developed by Imprinting Technology as Smart Polymers Imitating…