Smart Porous Monoliths for Chromatographic Separations
3.7 Conclusions and Outlook
A wide variety of monolithic columns based on organic polymers, silica, and hybrid organic–silica materials has been described as supports for smart polymers. This vari- ety can be explained by the large availability of functional monomers and crosslinkers, as well as organosilanes. On the other hand, most of the stimuli‐response materials are still based on the PNIPAAm, and this chapter presented several approaches by which to grow PNIPAAm brushes in different monolithic structures. As in conventional mono- lithic chromatography, organic monoliths work better for macromolecules, whereas silica and hybrid columns have better efficiency for separation of small molecules.
Potentially interesting applications of monolithic materials affording stimuli–
response characteristics are now using membranes, remote control technologies, and optical sensors based on thermal stimulus to explore the properties of PNIPAAm com- bined with the optical properties of Au layers [62–64]. Light is a simple and noninvasive
Smart Porous Monoliths for Chromatographic Separations 97
Br Br
Br
NIPAAm, AAm, PMEDTA, CuBr protein template
ATRP O
O
O O
H2N NH2
NH2 O
O
O
(a) (b)
O OH
O
OH H
N
HN
NH 2-bromoisobutyryl
bromide
Removal of template H
H H
H N
N O C
H H H
O O
O C
C N
H H
H H N
N O C
H H H
O O
O C
C N Protein imprinted
cavity Adsorption of template
Figure 3.13 Synthetic approach for the surface modifications of P(GMA‐co‐EDMA) monolith with ethylenediamine, 2‐bromoisobutyryl bromide, and surface grafting of protein‐imprinted PNIPAAm layer. The scanning electron micrographs show the internal structures of the P(GMA‐co‐EDMA) monolith (a), and its counterpart grafted with protein‐imprinted PNIPAAm layer (b) with significantly larger nonporous microglobules and more dense morphology, confirming the successful grafting of imprinted PNIPAAm. Source: Adapted from Reference [18]. Reproduced with permission of the John Wiley & Sons.
signal that can be explored together with photosensitive molecules that undergo revers- ible isomerization when irradiated with light of different wavelength to create gas sepa- ration technologies. For instance, nanoporous, photoswitchable metal–organic framework (MOF) monolithic layers containing azobenzene moieties as side groups can be used for selective separations of mixtures of H2 and CO2 or N2 and CO2 by controlling the isomerization state of azobenzene groups by light. By adjusting the intensities of the irradiation beams of 365 or 455 nm, well‐defined cis : trans ratios of azobenzene can be achieved on the MOF surface. The separation factor for a mixture of H2 and CO2 can be increased for the trans state, which is the predominant isomer pro- duced by irradiation with ultraviolet‐light. On the other hand, the separation factor of the cis membrane can be decreased by the 455 nm irradiation [65].
The works described in this chapter show that smart monoliths are still in the
“proof‐of‐concept” phase. The stimuli–response of most of the presented materials was demonstrated with synthetic mixtures of steroids or proteins. Some materials were tested for separations of phenols, alkylbenzenes, and ketones, but in all cases the applications were made with synthetic samples. Practically, no application of the smart monoliths to complex samples in biological, environmental, and food matrices was demonstrated, being thus a field yet to be explored by analytical chemists. Flow analysis is a field that can benefit from these smart monoliths, since these devices can be operated with pressures compatible with syringe pumps and selection/injection valves, as already demonstrated with the creation of sequential/flow injection chro- matography [66, 67].
Acknowledgements
Research on monolithic materials has been funded by grants 2013/18507‐4 from the São Paulo Research Foundation (FAPESP) and 306075/2013‐0 from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).
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Handbook of Smart Materials in Analytical Chemistry, First Edition. Edited by Miguel de la Guardia and Francesc A. Esteve-Turrillas.
© 2019 John Wiley & Sons Ltd. Published 2019 by John Wiley & Sons Ltd.
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