Smart Porous Monoliths for Chromatographic Separations
3.4 Salt Responsive Monoliths
Table 3.2 summarizes typical monolithic materials currently used for separations, extraction, sensing, and construction of micro valves for flow‐based systems of analysis, showing the predominance of the thermal stimulus, especially that of PNIPAAm, on the development of smart monoliths.
Table 3.2 Representative examples of smart monoliths and their potential applications.
Modulation Responsive polymer Base monolith Dimension Potential applications References
Temperature PNIPAAm Aminated P(GMA‐ co ‐EDMA) 10 mm thick membrane Separation of proteins, thermal valves, thermal gates [22]
PNIPAAm P(NIPAAm‐ co‐ BMA) 150 × 4.6 mm i.d. column Separation of steroids by
hydrophobic interactions [21]
PNIPAAm P(NIPAAm‐ co‐ MBAAm) 30 μ m deep × 170 μ m
wide × 200 μ m long chip Thermal valves in μ ‐FIA for chemiluminescence detection of H 2 O 2
[25]
PNIPAAm P(NIPAAm‐ co‐ MBAAm) 100 μ m deep × 200 μ m
wide × 200 μ m long chip Thermal valves in microfluidic chip proved via photobleaching of Coumarin 519
[26]
PNIPAAm P(NIPAAm‐ co‐ MBAAm) 20 μ m deep × 130 μ m
wide × 200 μ m long chip Reversible immobilization and release of glucose oxidase for electrochemical determination of glucose in human blood
[29]
PNIPAAm P(NIPAAm‐ co‐ MBAAm) 100 × 4.6 mm i.d. column Separation of steroids by
hydrophobic interactions [31]
PNIPAAm P(NIPAAm‐ co‐ MBAAm) 100 × 4.6 mm i.d. column Separation of aromatic ketones by hydrophobic interactions [32]
PNIPAAm P(St‐ co‐ DVB) 10–20 cm long × 100 μ m
i.d. capillary column Separation of steroids by
hydrophobic interactions [34]
PMeO 2 MA P(MeO 2 MA‐ co ‐OEGMA) 50 × 4.6 mm i.d. column Separation of steroids by
hydrophobic interactions [35]
Carboxylated PNIPAAm Aminated silica 100 × 4.6 mm i.d. column Separation of steroids by
hydrophobic interactions [38, 39]
(Continued )
PNIPAAm CPTMS modified silica 50 ×3.2 mm i.d. Separation of steroids by
hydrophobic interactions [40]
P(NIPAAm‐ co ‐
DMAEMA‐ co ‐tBAAm CPTMS modified silica 50 × 4.6 mm i.d. Separation of adenosine
nucleotides by ion exchange [41]
P(NIPAAm‐ co ‐BMA) CPTMS modified silica 50 × 3.2 mm i.d. Separation of sodium benzoate, phenol, methylbenzene, ethyl p ‐aminobenzoate, ethyl benzoate, and methyl hydroxybenzoate by hydrophobic interactions
[43]
P(MeO 2 MA‐ co ‐OEGMA) Aminated silica 100 × 4.6 mm i.d. column Separation of steroids and two proteins (myoglobin and lysozyme) by hydrophobic interactions
[36]
Lysozyme templated
PNIPAAm Hybrid organic–silica 100 × 4.6 mm i.d. column Temperature mediated MIP for capture and release of lysozyme [45]
Myoglobin templated
PNIPAAm Aminated P(GMA‐ co ‐EDMA) 100 μ m i.d. fused silica
capillaries Temperature mediated MIP for capture and release of myoglobin
[18]
Ketoprofen templated
AAm P(AMPS‐co‐AAm‐ co ‐EDMA) 100 × 4.6 mm i.d. column Temperature mediated MIP for extraction of Ketoprofen from milk samples
[46]
PNIPAAm PNIPAAm grafted on SiO 2 NPs immobilized in fused silica capillary
100 μ m i.d. capillary In‐tube extraction of estrogens diethylstilbestrol, dienestrol and hexestrol from milk by hydrophobic interaction
[47]
Modulation Responsive polymer Base monolith Dimension Potential applications References
pH MAA or PMAA P(GMA‐ co ‐EDMA) 50 × 4.6 mm i.d. column Alkyl benzenes by hydrophobic interactions and four basic proteins by cation exchange
[48]
Hydrophobic and
ionizable ene reagents P(GMA‐ co ‐EDMA) Glass plate monolithic
layers Super hydrophobic/super
hydrophilic transition mediated by pH, verified by the WCA
[49]
Salt Sulfobetaines P(SPE‐ co ‐TRIM) or
P(SPE‐ co ‐EDMA) 150 × 2.7 mm i.d. glass
column Uptake and release of proteins
by electrostatic interactions [50]
PMETA P(GMA‐ co ‐EDMA) 110 mm × 100 μ m i.d. PTFE
coated fused silica F − , ClO 3 − , BrO 3 − , Cl − , NO 2 − , Br − by ion exchange using sodium benzoate as mobile phase
[51]
PNIPAAm P(CMSt‐ co ‐DVB) 100 × 4.6 mm i.d. column Hydrophobic interaction
chromatography of proteins [52]
pH/temperature PNIPAAm PSt/PAA) and PSt/PNIPAAm 12 mm diameter × 3–8 mm
thick Separation of dextrans [53]
pH/salt PDMAEMA PEDMA 50 × 4.6 mm i.d. column Separation of steroids by
hydrophobic interactions [54]
The P(GMA‐co‐EDMA) monolith has been exploited to produce ion exchangers by chemical derivatization of the epoxy groups of GMA with Na2SO3, ethylene diamine (EDA), chloroacetic acid and iminodiacetic acid (IDA) [9]. These monoliths are efficient for the separation of proteins by cation or anion exchange, but they fail to separate small cations and anions. This poor performance has been assigned to the low density of epoxy groups at the pore surface, which results in low density of ion‐
exchange functionalities after derivatization. To increase the density of functional groups, Connolly and Paull [51] photografted [2(methacryloyloxy)ethyl]trimethylam- monium chloride (META) in the presence of benzophenone on the pore surface of a generic P(GMA‐co‐EDMA) monolith prepared inside a 100 μm i.d. PTFE coated‐
fused silica capillary. The stationary phase was salt responsive, with the permeability increasing with the salt concentration. This column was able to separate fluoride, chlorite, bromate, chloride, nitrite, and bromide using sodium benzoate as mobile phase (Table 3.2).
Hydrophobic interaction chromatography (HIC) of proteins is based on the retention of proteins to the stationary phase in high salt concentration, followed by elution upon decreasing the salt concentration. Zhang et al. [52] demonstrated that a monolithic col- umn formed by copolymerizing chloromethyl styrene (CMSt) and DVB, grafted with PNIPAAm by ATRP can be both temperature and salt responsive, but exhibited a much better performance in HIC by varying the salt concentration. The retention factors of several steroids increased with the increase in salt concentrations (NaCl, Na2SO4, and (NH4)2SO4) at a constant temperature of 28 °C, with the higher retention being found for progesterone, the steroid with one of the largest log P (Table 3.1). This finding was consistent with the hydrophobic mechanism of retention. High salt concentrations shrank the PNIPAAm brushes even at temperatures <LCST. Base line separation of cytochrome c, myoglobin, bovine serum albumin (BSA), and thyroglobulin bovine was achieved by a combination of linear and stepwise gradient elution with Na2SO4 from 2.0 to 0 mol l−1 in 0.050 mol l−1 phosphate buffer, pH 7.0. The four proteins were strongly retained by the ungrafted P(CMSt‐co‐DVB) monolith, not being eluted from the column until the 0.050 mol l−1 phosphate buffer (pH 7) mobile phase was changed to 10% (v v−1) methanol : water.