Key words
polyampholyte hydrogels - charge tuning - pH-responsive hydrogels - catch-and-release
Introduction
Polymer gels are three-dimensional networks composed of a linear polymer chain crosslinked
by chemical bonds or physical interactions that attract great interest due to their
unique properties and potential applications in various fields from energy harvesting
and additive manufacturing to tissue engineering and drug delivery.[2] Hereby, examples of both hydrogels and organogels have been reported, where hydrogels
are typically formed from hydrophilic polymers with a high affinity for water and
can absorb and retain large amounts of water.[3] Contrarily, organogels are formed from hydrophobic polymers, which have a low affinity
for water and instead interact with organic solvents to form a gel-like structure.[4] Due to their excellent water-holding capability, hydrogels are mostly used as superabsorbent
materials for baby diapers and hygiene articles such as femcare and incontinence,[5] and as water retention granules for agriculture.[6] Their biocompatibility and ability to mimic biological environments render them
interesting materials for biomedical applications such as cell regeneration and repair;
moreover, the mechanical strength, elasticity, and degradation rate can be adjusted
according to the targeted tissues and organs.[7] Organogels, on the other hand, are widely utilized in applications where organic
media are needed for instance in cosmetics[8] or oral and topical drug delivery.[9] Due to their selectivity towards the organic phase in water, organogel adsorbents
have also been a promising solution for the removal of hard-to-degrade organic pollutants
in wastewater.[10] Swelling refers to the change in the volume of a gel as it absorbs a compatible
fluid (water or organic solvent) which relies on the crosslinking density, solvent
nature, and polymer–solvent interaction.[11] Besides, the swelling behavior of hydrogels can also be affected by some environmental
factors such as the presence of certain ions, pH, and temperature.[12] For instance, the degree of ionization of the polymer network can be altered upon
changes in pH-value, resulting in a change in swelling behavior.[13] Therefore, controlling the polymer chemistry and the synthesis conditions allows
us to tailor the properties of crosslinked gels. Chemically crosslinked gels are relatively
durable under large deformation and their mechanical strength is correlated with the
degree of swelling, which can be enhanced by incorporating filler materials[14] and other monomers that can form their own network interpenetrating with polymer
gels.[15]
The nature of the polymer and the desired properties of the resultant gels are decisive
to choose the synthesis method of hydrogels and organogels. One typical approach for
hydrogel synthesis is the chemical or physical crosslinking of hydrophilic pre-formed
polymers.[16] For example, natural polymers such as cellulose, chitosan, and alginate can be crosslinked
by ionic or covalent binding.[17] Also, synthetic amphiphilic block copolymers such as poly(ethylene glycol)-block-poly(ε-caprolactone)-block-poly(ethylene glycol) (PEG-PCL-PEG) in solution can form a gel by temperature change.[18] A similar principle is applied in the synthesis of organogels in which a solution
of the hydrophobic polymer such as polystyrene in hexane or toluene can be heated
and slowly cooled leading to gel formation.[19] Alternatively, adding low-molecular-weight organic gelators such as fatty acids,
steroids, and anthryl derivatives or polymeric organic gelators such as poly(ethylene),
poly(methacrylic acid-co-methyl methacrylate), and polyurethane promotes gelation upon cooling.[20] Besides all these, the free-radical crosslinking copolymerization of hydrophilic
or hydrophobic monomers with bifunctional monomers used as a crosslinker is still
commonly performed for gel formation due to a number of advantages such as versatility
across different monomers and initiators, in combination with scalability for industrial
applications.[21] Typical examples are combinations of acrylic acid with N,N′-methylenebisacrylamide (MBAA) for hydrogels,[22] or styrene with divinylbenzene in case of organogels.[23] Initiating systems are crucial for performing the synthesis of gels and determining
whether the polymerization can be initiated by heating or light irradiation. For instance,
poly(vinyl alcohol) macromers modified with pendant acrylamide groups can be photopolymerized
using Irgacure 2959 as a photoinitiator towards hydrogels.[24] Alternatively, 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO) is an appealing
water-soluble photoinitiator for polymerization of acrylic monomer and PEG-diacrylate
crosslinker.[25] As thermal initiators in the synthesis of gels, azo compounds such as 2,2′-azobis(isobutyronitrile)
(AIBN)[26] and organic peroxides such as benzoyl peroxide[27] are mostly preferred. Using redox initiators allows performing the synthesis of
gels under milder conditions such as at room temperature or slightly elevated temperatures.[28] Moreover, redox-initiating systems such as ammonium persulfate (APS)/N,N,N′,N′-tetramethyl ethylenediamine (TEMED) are often used in aqueous systems, rendering
this approach suitable for the synthesis of hydrogels that are designed for biomedical
applications.[29] However, the potential toxicity of initiators and the lack of control over the network
architecture should be taken into consideration.[30] Some crosslinked polymers obtained by this method include poly(2-hydroxyethyl methacrylate),[31] poly(acrylic acid),[32] or poly(acrylamide) as hydrophilic polymers, and poly(methyl methacrylate)[33] and poly(octadecyl acrylate)[34] as hydrophobic examples. Interestingly, amino acid-based acrylic monomers protected
by tert-butyloxycarbonyl (Boc) groups can be transformed into organogels, and subsequently
to hydrogels by deprotection.[35] Hydrogels can be formed by not only homopolymers but also by different types of
copolymers such as block, graft, alternating, and random.[36] This allows designing novel tailor-made hydrogels, for example, adding hydrophobic
monomers into the hydrogel composition results in an amphiphilic polymer co-network
that has an enhanced toughness as a result of interactions between the hydrophobic
segments.[37] Hydrogels may have smart functions such as phase transition or stiffness change,
swelling/deswelling, and release of a cargo molecule controlled by external stimuli
such as pH, temperature, light, magnetic field, or biological triggers including enzymes,
glucose, or antigens.[38] When polyelectrolytes constitute the hydrogel, the corresponding net charge is one
way to classify these materials.[39] In case of weak polyelectrolytes, the net charge depends on the surrounding pH value
and examples for such functional units include carboxylates, phosphates, and sulfonates
as anions or primary and secondary amines as cations.[40] The charged nature of these hydrogels is not only an essential factor in physical
properties but also promotes electrostatic interactions, which opens a venue for a
wide range of applications such as tissue engineering, delivery devices, actuators,
and electronics.[41]
Polydehydroalanine is a polyampholyte featuring both carboxylic acids and amines in
each repeating unit and thereby provides pH-dependent charge, as polyanion at high
pH, polyzwitterion at neutral pH, and polycation at low pH.[42] PDha has an isoelectronic point at around pH 5,[42] and despite being hydrophilic in general, solubility is best at basic pH (> pH 10)
due to columbic attractions between the oppositely charged species at lower pH values.[43] We have recently shown that PDha can be used as a coating for magnetic nanoparticles,
providing a platform for the reversible adsorption of charged dyes or macromolecules.[44] Due to its captodative nature, the direct polymerization of dehydroalanine is not
possible so alternative approaches including deprotection strategies are needed, e.g.,
2-acetamidoacrylic acid can be polymerized by free-radical polymerization (FRP) and
subjected to concentrated HCl to cleave the amide groups.[45]
Recently, our group reported the synthesis of PDha through FRP of precursor monomers,
including tert-butoxycarbonylaminomethylacrylate (tBAMA), benzyl 2-tert-butoxycarbonylaminoacrylate (tBABA), and methyl 2-benzyloxycarbonylaminoacrylate (BOMA) by utilizing their selectively
removable protective groups.[42],[46] PtBAMA holds promise as a parent polymer of PDha because of its suitability for radical
polymerization. We have outlined the controlled radical polymerization of tBAMA including atom transfer radical polymerization[47] and nitroxide-mediated polymerization.[42] We have also utilized well-defined PtBAMA as a starting material to create polyampholyte hydrogels comprising PDha and
poly(ethylene glycol) (PEG).[48]
In this work, we describe free-radical crosslinking copolymerization of tBAMA with MBAA and study three different initiating systems including AIBN as a thermal
initiator, TPO as a photoinitiator, and APS/TEMED as a redox initiator to identify
suitable conditions for the formation of a gel. The presence of hydrophobic protective
groups enabled the crosslinked PtBAMA to form organogels with more apolar solvents. We investigate the mild and effective
deprotection of -NH2 and -COOH using TFA/water mixtures to obtain PDha hydrogels. Finally, we demonstrate
pH-sensitive swelling of hydrogels and their potential as a catch-and-release platform
using positively charged methylene blue (MB+) and negatively charged methyl blue (MB−) dyes.
Results and Discussion
Herein, we develop an alternative strategy for polyampholyte hydrogels based on PDha
via FRP of tBAMA and subsequent deprotection ([Scheme 1]). The choice of initiating system would significantly influence the formation of
a gel. Our objective is to identify the most suitable initiating systems by evaluating
various parameters such as solvent, crosslinker ratio, initiator ratio, and duration.
The monomer tBAMA used in all experiments was synthesized in a two-step procedure starting from
serine (see Figure S1 for 1H NMR).[42] FRP was carried out in the presence of MBAA as a crosslinker using photo-, thermal
and redox-initiating systems. This synthetic approach differs from previous attempts
for PDha-based hydrogels not only with the crosslinking method used but also with
achieving hydrogels mainly consisting of PDha. Hydrogels in prior studies contain
predominantly PEG (up to 90%) in addition to PDha as polyampholytic stickers.[48] Cleavage of protecting groups on the crosslinked PtBAMA was possible using stepwise aqueous acid and base solutions, simultaneously leading
to a conversion from an organogel to a hydrogel. Successful deprotection was followed
by changes in swelling behavior of the gel, where the swelling first started at the
edges of the material during deprotection, and finally leading to a fully swollen
PDha hydrogel ([Figure 1]). After complete conversion, the developed PDha hydrogels exhibited pH-dependent
swelling behavior in water and could be utilized for the reversible adsorption of
oppositely charged small molecules, which is of interest in the field of water purification.
In addition, we compared the herein presented synthetic approach to the chemical crosslinking
of PDha in terms of pH-dependent swelling and adsorption abilities of resultant hydrogels.
Scheme 1 Synthesis of PtBAMA organogels by free-radical polymerization of tBAMA in the presence of MBAA as a crosslinker (a) and synthesis of the hydrogel by
deprotection of crosslinked PtBAMA with two consecutive reactions in TFA/H2O and LiOH (b).
Figure 1 PtBAMA organogel formed after CHCl3 addition to crosslinked PtBAMA and PDha hydrogel formed after deprotection reaction.
Synthesis of PtBAMA Organogels
Photocrosslinking Polymerization of tBAMA Using TPO as an Initiator
Using the UV photoinitiator TPO and in the presence of the crosslinker MBAA, tBAMA was successfully polymerized. UV curing was carried out at different exposure
times, while some of the samples were additionally treated by post-thermal-curing.
Different solvents and bulk polymerization were employed in optimization, as well
as varying crosslinker (CL%) and initiator ratios (I%) were used.
UV curing in MeOH, DMSO, and acetone successfully produced crosslinked PtBAMA resulting in a transparent solid, which turned into an organogel when exposed
to CHCl3 (2 mL) for 24 hours, while curing in dioxane was not successful. The solvent-to-monomer
ratio was fixed at 1 : 1 (w/w) and the successful parameters are listed in [Table 1]. Photocrosslinking of PtBAMA in acetone and DMSO was achieved using 1% (w/w) of initiator at the CL% ranging
between 0.25 and 7% (w/w). For curing in MeOH, the initiator ratio was reduced to
provide optimal conditions for efficient crosslinking. Both dioxane and bulk polymerization
did not result in the formation of crosslinked material. Independent of the amount
of CL (0.25%, 2.5%, 5% w/w), initiator (1% w/w), and curing methods (Table S1), polymerization
in dioxane led to non-crosslinked, branched, or partially crosslinked PtBAMA, which was dissolved in CHCl3. This resulting polymer had M
n of 70,000 g · mol−1 with a dispersity index of 3.4 according to size exclusion chromatography (SEC) analysis
(Figure S2). Such a broad molecular weight distribution is not uncommon in FRP.
Table 1 Synthetic parameters for successful crosslinking of tBAMA.
|
Initiator
|
Solvent
|
CL% (w/w)
|
I% (w/w)
|
Time
|
Temperature
|
|
*Additional thermal treatment is applied for 12 hours.
|
|
TPO
|
Acetone
|
7.7
|
1
|
3 h*
|
RT
|
|
|
2
|
1
|
1 h*
|
RT
|
|
|
7
|
1
|
3 h
|
RT
|
|
DMSO
|
5
|
1
|
3 h
|
RT
|
|
|
2
|
1
|
1 h*
|
RT
|
|
|
0.5
|
1
|
3 h
|
RT
|
|
MeOH
|
2
|
1
|
1 h*
|
RT
|
|
|
0.5
|
0.1
|
3 h
|
RT
|
|
|
0.25
|
0.2
|
3 h
|
RT
|
|
AIBN
|
DMSO
|
0.1
|
0.5
|
3 h
|
70 °C
|
|
|
0.5
|
0.5
|
3 h
|
70 °C
|
|
|
1
|
0.5
|
3 h
|
70 °C
|
|
|
2
|
0.5
|
3 h
|
70 °C
|
|
APS/TEMED
|
DMSO
|
0.5
|
0.5
|
12 h
|
37 °C
|
|
|
2
|
0.5
|
12 h
|
37 °C
|
Among the listed successful conditions, we noted that 3 hours of UV irradiation was
necessary to yield crosslinked polymers. Crosslinking by 1 hour of irradiation was
unsuccessful unless post-thermal-curing is applied, which is often used to increase
the strength of gels in photocrosslinkable resins.[49] While the polymerization occurs during this short-term irradiation, crosslinking
continues during thermal treatment at 90 °C in the following 12 hours, finally resulting
in polymer organogel in CHCl3. Besides the high conversion in relatively short times as one clear advantage of
photopolymerization using TPO, the photocrosslinking of tBAMA can be used for a quick organogel film formation, which reduces local evaporation
of solvents and thus can ensure smooth film formation.
Thermal Crosslinking Polymerization of tBAMA Using AIBN as an Initiator
Crosslinked PtBAMA was also synthesized using AIBN as a thermal initiator and MBAA as a crosslinker.
All the reactions were conducted in DMSO at a fixed solvent-to-monomer ratio of 1 : 1
(w/w) using a 0.5% (w/w) initiator ratio (I%), while the crosslinker ratio (CL%) was
varied for optimization at 0.1, 0.5, 1 and 2% (w/w) ([Table 1]). As a general procedure, the crosslinker and initiator were separately dissolved
in DMSO and were then added to the monomer tBAMA in a 4-mL transparent glass vial. Successful crosslinking after 3 hours at 70 °C
was confirmed by swelling in CHCl3. However, we noticed that the resulting gels were prone to disintegration upon prolonged
exposure to solvent. Those smaller gel pieces were not dissolved in the solvent, which
hints towards crosslinking, but nevertheless, we could conclude that gels were less
stable if compared to the photopolymerization.
Redox-Triggered Crosslinking Polymerization of tBAMA Using APS/TEMED
APS as an initiator and TEMED as a catalyst are often employed in FRP at room temperature
as a redox initiating system.[50] We attempted to perform crosslinking polymerization of tBAMA at 37 °C for 12 hours in DMSO at a fixed solvent-to-monomer ratio of 1 : 1 (w/w)
using a 0.5% (w/w) initiator ratio (I%), and crosslinker ratios (CL%) of 0.5 and 2%
(w/w) ([Table 1]). Under both conditions, we observed successful gel formation once the resulting
material was exposed to CHCl3.
Swelling of Crosslinked PtBAMA in Organic Solvents
The swelling of polymeric networks is crucial for their potential use as adsorbents,
with swelling kinetics being a key factor.[51] Given that the choice of organic solvents can significantly impact how much an organogel
swells, leading to different levels of expansion, we conducted an investigation into
the swelling kinetics in various solvents. These solvents have different properties,
including polarity, dielectric constant (ε), and density. The solvents we examined
included chloroform (CHCl3, ε = 4.8, d = 1.49), dichloromethane (CH2Cl2, ε = 9.1, d = 1.33), acetone (ε = 20.7, d = 0.784), methanol (ε = 33, d = 0.792), and dimethyl sulfoxide (DMSO, ε = 46.7, d = 1.1), as well as water (ε = 80.1, d = 1) ([Figure 2]). Swelling ratios (SRs) were calculated by the weight difference at regular time
intervals at room temperature for crosslinked PtBAMA (0.1 I% and 0.5 CL%), which were prepared in small portions of dried gels, immersed
in 20 mL solvent.
Figure 2 Solvent uptake kinetics of crosslinked PtBAMA (0.1 I%, 0.5 CL%); Swelling ratio (Q% w/w) vs. time.
We aimed at identifying the compatibility of solvents with the PtBAMA network using different organic solvents with dielectric constant ranging from
4 to 20 and high polarity with dielectric constant which is above 20, plus water as
the solvent with highest polarity. We herein excluded non-polar solvents with dielectric
constant below 4 like hexane as we assumed poor interaction with the PtBAMA network.[42]
These polymer networks displayed different degrees of swelling in medium polarity
solvents while the SR decreased in highly polar solvents ([Table 2]). We observed in general that the SR gradually increased over time, reaching a plateau
after 18 hours in DMSO, 24 hours in CH2Cl2, acetone, and methanol, and 72 hours in CHCl3. Hereby, the highest SR was 5274% in CHCl3, while the lowest was 157% in DMSO, recorded after 96 hours. It was also observed
that the rates of absorption in CHCl3 and CH2Cl2 were equal during the first 10 hours, but eventually CHCl3 achieved a higher overall SR. When we recalculated the SR by dividing weight-based
SR by the solvent density, we interestingly found a higher volume-based SR in acetone
than CH2Cl2 despite an increased dielectric constant. In addition, the final size of the gels
was similar in both cases (Figure S3). This variance in SRs could stem from lower
polarity of CH2Cl2 if compared to acetone although it has higher ε.
Table 2 Equilibrium swelling ratio (SR or Q) of PtBAMA organogels after immersion for 96 hours in different solvents.
|
Solvent
|
ε
|
Polarity
|
SR% (w/w)
|
SR% (v/w)
|
|
CHCl3
|
4.81
|
4.1
|
5274
|
3540
|
|
CH2Cl2
|
9.1
|
3.1
|
3309
|
2488
|
|
Acetone
|
20.7
|
5.1
|
2114
|
2695
|
|
Methanol
|
32.7
|
5.1
|
1640
|
2071
|
|
DMSO
|
46.7
|
7.2
|
157
|
143
|
|
H2O
|
80.1
|
10.2
|
70
|
70
|
The rate of solvent uptake gradually decreased from CHCl3 to methanol as the ε of the respective solvent increased, however, a decrease in
SR was not as dramatic as a decrease in dielectric constant, leading to the conclusion
that crosslinked PtBAMA can form organogels in a wide range of organic solvents. We also noted that water
was quite incompatible with the material at this stage, resulting in collapse, showing
very low SR of about 70%. This can be explained by the nature of PtBAMA which has bulky and hydrophobic protecting groups such as Boc for amine and methyl
ester for carboxylic acid. Overall, PtBAMA can form organogels in organic solvents with ε ranging from 4.81 to 32.7 and
be utilized in both encapsulation of hydrophobic molecules and some environmental
applications such as adsorbing organic wastes.[52] With this motivation in hand, we attempted to use the crosslinked PtBAMA for crude oil adsorption, resulting in 400% SR after 96 hours in spite of high
viscosity as seen in Figure S3.
Since the crosslinked PtBAMA forms organogels and can be synthesized using conventional initiators by means
of UV irradiation, heating and redox reaction, we believe that it holds a potential
to widespread use in organogel applications such as drug delivery media for topical
and oral pharmaceuticals.[53]
Synthesis of PDha Hydrogels
As PDha represents a polyampholyte with high charge density, we were also interested
in mainly PDha-based hydrogels. In previous work, we described the deprotection of
the Boc protective group using TFA at 50 °C or the hydrolysis of the methyl ester
in PtBAMA homopolymers at 100 °C using LiOH·H2O. Additionally, treatment with TFA at room temperature can simultaneously remove
both the Boc group and the methyl ester.[54] We therefore varied the conditions used in the deprotection step towards more mild
conditions. The crosslinked PtBAMA were successfully converted to PDha hydrogels by hydrolysis of both protecting
groups in a two-step process. We hydrolyzed crosslinked PtBAMA (0.5% CL, 1% I) using TFA solutions, leading to the formation of poly(dehydroalanine-co- aminomethylacrylate) P(Dha-co-AMA) hydrogel. Here, the Boc groups are cleaved resulting in -NH2 groups and also the methyl ester is hydrolyzed to a certain extent. Subsequently,
the resultant hydrogel is treated by a basic LiOH solution for complete deprotection
of the methyl ester resulting in -COOH groups.
[Figure 3] shows the 13C CP/MAS NMR spectra of the crosslinked PtBAMA and the dried gels obtained via the successful conditions as in 50% w/w TFA aqueous
solution for 6 h and 0.1 M LiOH solution for 2 h. The signals of the crosslinker overlapped
with the signals of PtBAMA, so they are invisible on the spectrum. We noted a complete cleavage of-Boc groups
for all reaction conditions as in TFA for 3 h and 12 h, and in 50% w/w TFA aqueous
solution for 6 h as seen in Figure S4. The signals at 29 and 155 ppm assigned to the
methyl carbons and the carbonyl carbon of-Boc group, respectively, only appeared in
the reference sample and vanished by TFA treatment. We also observed that the methoxy
groups were cleaved to a large extend, giving successful deprotection of 92% for 6 h
in 50 wt.% TFA aqueous solution, 85% for 3 h and 92% for 12 h in neat TFA. As it represents
milder conditions, 50 wt.% TFA aqueous solution was selected for the rest of our studies.
After the reaction, the swollen gel was washed six times with diethyl ether to remove
TFA as well as byproducts. It was later kept under air for 1 day and under a high
vacuum the following day to get fully dried.
Figure 3
13C CP/MAS NMR of dried PtBAMA organogel (0.5% CL, 1% I) (upper), dried P(Dha-co-AMA) hydrogel obtained after deprotection in 50% w/w TFA aqueous solution for 6 h
(middle), and dried PDha hydrogel resulting from deprotection of P(Dha-co-AMA) hydrogel in 0.1 M LiOH for 2 h (lower).
Swelling Behavior of PDha Hydrogels
The SR is an important property for hydrogels. It depends on factors like how tightly
the hydrogel is connected, how the material behaves under stress, its internal structure,
and external factors such as pH and salt concentration.[55] In this study, hydrogels begin swelling as deprotection occurs. However, to properly
assess the SR, it needs to be measured at equilibrium conditions. In this regard,
solid gels formed through the deprotection of PtBAMA (0.5% CL, 0.1% I) were immersed in deionized water for 48 hours. In cases where
not specified otherwise, experiments were conducted in triplicate, and the average
SR was determined, resulting in an equilibrium SR of 8420 ± 883%. To explore how pH
affects swelling, the gels were submerged in water with different pH levels for 76
hours. [Figure 4a] displays the pH-dependent SR of PDha hydrogels, resulting in the following values:
298 ± 70% at pH 3, 801 ± 189% at pH 5, 3231 ± 785% at pH 7, 3349 ± 733% at pH 9, and
3797 ± 549% at pH 11. The SR showed a modest increase up to pH 5. However, a significant
and abrupt increase was observed at pH 7, likely resulting from the complete deprotonation
of -COOH and the formation of a polyzwitterionic gel. As pH reached 11, a further
slight increase in SR was noted, possibly due to electrostatic repulsion between the
predominantly polyanionic chains between the crosslinks.[56] The capacity to maintain swelling properties over multiple cycles is a crucial attribute
for hydrogels that experience volume changes due to external stimuli. Consequently,
we exposed PDha hydrogels to three swelling–deswelling cycles in solutions with oscillating
pH levels ranging from 12 to 2 at room temperature. The results, illustrated in [Figure 4b], indicate that the hydrogels can be reversibly swollen. We therefore also carried
out rheological amplitude sweep measurement to ascertain the magnitude of the linear
viscoelastic (LVE) response and the storage modulus (G′) within the LVE region. Results were also compared with hydrogels comprising PDha
stickers as reported earlier[48a] (Figure S5). When G′ remains constant regardless of the strain amplitude, the sample can be deformed
within this range without irreversible damage, being comparable to the toughness of
such a sample.[57] The LVE region of the herein-developed PDha hydrogel stretches up to 2.5% strain
with an average storage modulus (G’) of 1700 Pa. The significant drop in G′ indicates that the material exhibits brittle fracturing behavior. The behavior before
the crossover point suggests that the material has a gel-like structure and behaves
as a viscoelastic solid before microcracks start forming.
Figure 4 Swelling properties of PDha hydrogels (0.5% CL, 0.1% I); pH-dependent swelling (a)
and oscillatory swelling cycles at pH 12 and 2 (b).
Dynamic Catch-and-Release of Anionic and Cationic Model Dyes
Polyampholytic hydrogels are appealing to relevant fields including wastewater treatment
because they supply enhanced removal and separation of pollutants and delivery systems
providing the controlled release of carried cargo molecules on target.[58] We hypothesize that the hydrogels designed in this study, characterized by a high
density of dynamic charge, have the potential to demonstrate controlled adsorption
and release of charged molecules by adjusting the pH levels. Those used as anionic
and cationic models are extensively employed in textile, leather, and paper industries
as colorants (Figure S6). By PDha hydrogels (0.5% CL, 0.1% I), cationic methylene
blue (MB+) was captured at basic pH and released at acidic pH. In contrast, anionic methyl
blue (MB−) was captured at acidic pH and released at basic pH.
In the case of cationic model, as demonstrated in [Figure 5a], the pre-equilibrated hydrogel (48 hours at pH 12) was immersed in MB+ solution (0.025 mg · mL−1, in water at pH 12) for 48 hours and the dye adsorption was monitored by UV-Vis spectra
([Figure 5b]). The cationic dye molecules are supposed to be electrostatically attracted by anionic
groups of PDha hydrogel at this basic pH. The MB+-adsorbed hydrogel was then washed with pH 12 solution and placed in pH 2 solution.
After 48 hours, we observed that the hydrogel shrank and released MB+ molecules. Meanwhile, the variance of the absorption band of MB+ as seen when pH is changed to 2 was probably due to the disturbance of the self-aggregated
dimer formation.[59] We noted that the hydrogel remained slightly blue after desorption of MB+, although the overall net charge is supposably positive under these conditions. We
explain this by the fact that some MB+ is physically entrapped inside hydrogel meshes. We suppose this phenomenon was due
to the H-bonding between the carboxylic acid groups of PDha at acidic pH.[60] The remaining dyes were somewhat released after the second round of washing with
acid solution ([Figure 5b]). Adsorption and desorption of MB+ were quantified by UV-Vis absorbance, and the calibration curve developed earlier
is used for the latter.[44a] The adsorption capacity (w/w%) per mg of dry gel followed the decreasing regime
by increasing the amount of hydrogel used, being on average 2.8% for 6 mg, 2.4% for
15 mg and 1.9% for 29 mg ([Figure 5c]). We noted that 98.5% of the adsorbed dyes were released only after the first washing
of the hydrogel having 2.4% MB+ with acid solution. In case of the anionic model, we similarly performed pH-controlled
adsorption/desorption but this time by using MB− and going from acidic to basic pH ([Figure 6a]). In a similar fashion, the pre-equilibrated hydrogel (48 hours at pH 2) was immersed
in MB− solution (0.075 mg · mL−1, in water at pH 2) for 48 hours leading to the decrease in absorbance due to the
adsorption of dye ([Figure 6b]). Here we believe electrostatic attraction is again the driving force of adsorption,
however, this time it occurs between cationic groups of PDha hydrogels and anionic
MB− molecules. The adsorbed MB− were then released out of hydrogel when it is subject to a basic solution (pH 12)
for 48 hours, resulting from the electrostatic repulsion between anionic charges of
PDha hydrogel and dyes. We also noted the resultant solution needs to be acidified
to pH 2 as the dye is colorless in alkaline media. Quantification of adsorption and
desorption was done according to UV absorbance of MB− at pH 2 and revealed that the adsorption capacities were 9.9% for 11 mg, 5.3% for
15 mg and 3.1% for 20 mg dry gel ([Figure 6c]). We further found that the hydrogel with 5.4% MB− released almost ca. 99% of the adsorbed MB− after the first washing. From these findings, we believe that PDha hydrogels with
a pH-triggered two-way catch-and-release feature might be utilized to develop a facile,
selective and reversible adsorption/desorption platform. Further studies may include
the reversible adsorption of other small molecules with different charge density,
the investigation of the adsorption isotherms and the upscaling of catch-and-release
platform for wastewater treatment.
Figure 5 Photograph of methylene blue solutions (1: MB+ aqueous solution (0.025 mg · mL−1) at pH 12, 2: after hydrogel addition (corresponding to 10 – 20 mg dry gel indicated
by dashed line), 3: MB+ adsorption after 24 hours, 4: adding pH 2 solution to MB+ adsorbed hydrogel, 5: desorbed MB+ after 6 hours) (a), absorbance spectra of methylene blue solution, the leftover solution
after hydrogel addition, the resultant solution after washing steps at pH 2 (b), and
the amount of adsorbed methylene blue with the adsorption capacities of PDha hydrogels
(c).
Figure 6 Photograph of methyl blue solutions (1: MB− aqueous solution (0.075 mg · mL−1) at pH 2, 2: after hydrogel addition (corresponding to 10 – 20 mg dry gel indicated
by dashed line), 3: MB− adsorption after 24 hours, 4: adding pH 12 solution to MB−adsorbed hydrogel, 5: resultant colorless solution and hydrogel (indicated by dashed
line) during desorption of MB− for 6 hours, 6: resultant solution with desorbed MB− at pH 2 (adjusted by HCl addition) (a), absorbance spectra of methyl blue solution,
the leftover solution after hydrogel addition, the acidified resultant solution after
washing steps at pH 12 (b), and the amount of adsorbed methyl blue with the adsorption
capacities of PDha hydrogels (c).
Conclusions
Herein we described a synthetic two-step pathway for PDha hydrogels: the FRP of tBAMA is first realized by photo-, thermo-, or redox-initiation systems in the presence
of a bifunctional crosslinker, and then the protecting groups of the resulting crosslinked
polymer are cleaved. Additionally, the crosslinked PtBAMA prior to deprotection was able to swell in common organic solvents. After deprotection,
the resulting polyampholytic PDha hydrogels show pH-responsive swelling, as demonstrated
by multiple cycles and up to 8420% of the equilibrium SR in deionized water. We demonstrate
the proof of concept for a two-way catch-and-release platform using these PDha-based
polyampholytic hydrogels that can reversibly adsorb both positive MB+ and negative MB− model compounds depending on the pH value. While hydrogels show an efficient desorption
of both dyes, adsorption depends on gel amount, and the average adsorbed amounts of
MB+ at pH 12 and MB− at pH 2 were found to be 22 and 50 µg · g−1, respectively. Reversible adsorption using larger charged compounds such as proteins
or forming film with various morphologies can be the subject of upcoming studies.
We envision that PDha hydrogels with pH-controlled dynamic charge features can find
utility in a number of areas of environmental and biological/medical science.
Experimental Section
General Methods
All chemicals and solvents were purchased from either Sigma Aldrich, TCI, CHEMSOLUTE®, or Carbolution Chemicals and used without purification if not mentioned otherwise.
tBAMA was synthesized according to an earlier described protocol.[42] For detailed experimental procedures and characterization including SEC, NMR and
UV/Vis spectroscopy, and rheology, please refer to the Supporting Information.
General Procedure for Free-Radical Crosslinking of tBAMA
PtBAMA was synthesized by radical polymerization in the presence of MBAA as a crosslinker
using TPO, AIBN, and APS as initiators in the bulk and various solvents including
1,4-dioxane, chloroform, acetone, methanol, and DMSO. Different crosslinker (CL% w/w)
and initiator (I% w/w) ratios were used during polymerization, which were defined
as follows:
Photocrosslinking
tBAMA (300 mg), MBAA (6 mg, CL 2% w/w), and TPO (3 mg, 1% w/w) were dissolved in 0.3 mL
MeOH in a 4-mL glass vial protected by an aluminium foil. After stirring the mixture
for 15 min, the reaction vial was degassed with argon for 10 min, placed in the UV
cube (250 W) for polymerization. After 1 hour, the reaction was aborted by removing
the vessel from the UV cube. The resultant gel was further treated at 90 °C for 12
hours and then placed in 3 mL CHCl3 to yield a swollen organogel.
Thermal Crosslinking
tBAMA (300 mg), MBAA (1.5 mg, CL 0.5% w/w), and AIBN (1.5 mg, 0.5% w/w) were dissolved
in 0.3 mL DMSO in a 4-mL glass vial. The reaction mixture was stirred for 15 min and
degassed with argon for 10 min. After that, the reaction vial was placed in an oil
bath at 70 °C for polymerization. The reaction was stopped after 3 hours, and the
resultant gel was subject to 3 mL CHCl3 to yield a swollen organogel.
Redox-Triggered Crosslinking
tBAMA (300 mg), MBAA (1.5 mg, CL 0.5% w/w), (NH4)2S2O8 (1.5 mg, 0.5% w/w) and TEMED (0.3 mg, 0.1% w/w) were dissolved in 0.3 mL DMSO in
a 4-mL glass vial. After degassing the reaction mixture with argon for 10 min and
stirring for 15 min, the vial was placed in an oil bath at 37 °C. The reaction was
aborted after 12 hours and placed in 3 mL CHCl3 to yield a swollen organogel.
Deprotection of the Crosslinked PtBAMA to Yield PDha Hydrogel
30 mg of crosslinked PtBAMA were soaked in 5 mL of 50% w/w aqueous TFA solution for 6 hours. The resultant
swollen gel was washed with diethyl ether six times, during which time the gel collapsed.
The modified gel was then dried in the air for 24 hours, followed by drying under
high vacuum. After that, the dried gel was immersed in a 0.1 M aqueous LiOH solution
for 2 hours. The resultant deprotected hydrogel was washed six times with diethyl
ether and dried in air for 24 hours, followed by drying under high vacuum, to be used
in NMR analyses and swelling studies.
Solubility Test After Crosslinking
The solubility test was done for rapid testing to qualify the resultant materials
after crosslinking. The obtained materials were subjected to CHCl3 to test whether they could swell to form an organogel.
Swelling of the Crosslinked PtBAMA in Organic Solvents
Solvent uptake kinetics of crosslinked PtBAMA gels were studied gravimetrically at room temperature. Freshly prepared dry gels
(50 – 100 mg) were immersed in the respective solvents such as CHCl3, CH2Cl2, acetone, MeOH, DMSO and H2O. A measured amount of dry gel (W
d) was added into the solvent, and then gels were taken out in a specific interval
of time, blotted the surface quickly, and weighed (W
s). The SR value was determined at least from three samples and mean and standard deviation
were calculated. W
d and W
s are the mass of the dried and swollen crosslinked polymer samples, respectively.
The equilibrium SR was calculated using equation as follows:
Swelling of Hydrogels in Water
Freshly prepared dried hydrogels (10 – 20 mg) were immersed in the respective solvents
such as CHCl3 and water. A measured amount of dry gel (W
d) was soaked in the solvent for 96 hours. After that, hydrogels were taken out, blotted
the surface quickly, and weighed (W
s), to minimize errors due to the surface solvent. The SR was determined at least from
three samples, and mean and standard deviation were also calculated. The equilibrium
SR values of PDha hydrogels were calculated using the given SR% equation. For the
pH-dependent SR calculation, dry gels were placed in water at pH ranging from 2 to
12 for 48 hours. For the oscillatory swelling, a dry gel was first placed in water
at pH 12 for 24 hours and hydrogels were weighed, and subsequently this hydrogel was
placed in water at pH 2 for 24 hours and weighed again. This cycle was then repeated
with the same hydrogel two times.
Dynamic Catch-and-Release of Anionic and Cationic Model Dyes
Methylene blue (MB+) was used as the cationic model compound. Dry gels (10 – 20 mg) were placed in pH 12
water for 48 hours for equilibrium and immersed in aqueous MB+ solution (0.025 mg · mL−1) at pH 12 for 48 hours. After that, gels were taken out and the UV-Vis absorbance
of the resultant solution was recorded. The adsorbed MB+ amount was calculated according
to molar extinction constant at 579 nm. These swollen MB+-absorbed gels were then
placed in water at pH 2 for desorption. After 48 hours, gels were taken out and the
UV-Vis absorbance of the resultant solution was recorded. The released MB+ amount was calculated using an earlier-developed calibration curve.[44a] The desorption was repeated one more time with a fresh portion of pH 2 water.
Methyl blue (MB−) was used as the anionic model compound. Dry gels (10 – 20 mg) were placed in pH 2
water for 48 hours to reach equilibrium and immersed in aqueous MB− solution (0.075 mg · mL−1) at pH 2 for 48 hours. After that, gels were taken out and the UV-Vis absorbance
of the resultant solution was recorded. The adsorbed MB− amount was calculated according to the molar extinction constant at 596 nm. These
swollen MB−-absorbed gels were then placed in water at pH 12 for desorption. After 48 hours,
gels were taken out and the pH of resultant solution was adjusted to pH 2 using concentrated
HCl. The UV-Vis absorbance of this solution was recorded as for the adsorption study
of MB− and the desorbed MB− amount was calculated. All calculations were made in triplicate for both compounds.
Funding Information
This work was supported by the German Research Council (DFG) (TRR234 “CataLight,”
project ID: 364 549 901, project B05 and Z02).
