Versatile supramolecular pH-tolerant hydrogels which demonstrate pH-dependent selective adsorption of dyes from aqueous solution

Article abstract of DOI:10.1039/c3cc45969a

We report a novel gelator functionalised with hydrazides (as replacements for carboxylic acids) which, as a result, is able to assemble into hydrogels across a wide pH range-this gelator exhibits pH-switchable dye adsorption-desorption dependent on protonation of the target dyes and their resulting interactions with the self-assembled gel nanofibres.

Full text of DOI:10.1039/c3cc45969a

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Versatile supramolecular pH-tolerant hydrogels which
demonstrate pH-dependent selective adsorption of
dyes from aqueous solution†
Cite this: Chem. Commun., 2013,
49, 11164
Received 5th August 2013,
Accepted 8th October 2013
Babatunde O. Okesola and David K. Smith*
DOI: 10.1039/c3cc45969a
www.rsc.org/chemcomm
We report a novel gelator functionalised with hydrazides (as interactions.6 Other groups have also recently begun to explore
replacements for carboxylic acids) which, as a result, is able to this area.7
assemble into hydrogels across a wide pH range – this gelator
It is noteworthy that, in general terms, most low-molecular-
exhibits pH-switchable dye adsorption–desorption dependent on weight hydrogelators are strongly pH dependent. Most commonly,
protonation of the target dyes and their resulting interactions with the protonation of a carboxylic acid group (pKa ca. 4–5) plays an
the self-assembled gel nanofibres.
integral role in hydrogelation, which only then occurs at lower
pH values.6,8 Furthermore, many of the syntheses are multi-step
Supramolecular gels are fascinating colloidal soft materials in and not easily amenable to scale-up. We therefore became inter-
which molecular-scale information programmed into a gelator gets ested in developing hydrogels capable of pollutant uptake, and set
translated up to nanoscale fibres via self-assembly and then directs the following criteria for their design: (i) simple synthetic proce-
the macroscopic performance of the material.1 These materials dures, (ii) pH-tolerant gels, (iii) selective pollutant uptake. This
have great potential for applications – there has been significant communication reports our initial progress.
interest in self-assembling hydrogels, which may find uses in
We decided to employ a gelator based on dibenzylidene sorbitol
aqueous environments ranging from nanomedicine to environ- (DBS). DBS is a ‘butterfly-amphiphile’ well-known to gelate organic
mental science.2 In recent times, some attention has begun to fluids and synthesised by simple one-step condensation between
focus on using low-cost supramolecular gels to adsorb pollutants sorbitol and two equivalents of benzaldehyde.9,10 However, DBS does
from water.3 Self-assembled gels are intriguing materials for this not dissolve well in aqueous solvents. We recently reported that DBS–
purpose, as they are highly porous, and their internal nano- CO2H is an effective, but pH-dependent, hydrogelator.11 With this
structuring should ensure they can bring high surface areas into in mind, we considered potential replacements for the carboxylic
contact with the pollutant, giving rise to effective adsorption. Das acid which might be more pH tolerant, yet still provide sufficient
and co-workers have done significant work in this area,4 reporting hydrophilicity to support hydrogelation. As such, we alighted on the
a dried organogel which adsorbed crystal violet and other dyes, hydrazide functional group. Hydrazides have only rarely been noted as
with dye removal being dependent on the gelator structure,4a and a potential organogelators.12 We therefore synthesised DBS–hydrazide
more effective system which formed ionogels.4b They also devel- (Scheme 1) using a simple, high yielding, two-step synthetic procedure
oped pH-responsive hydrogels which could be recycled and adapted from those known for DBS. Condensation of two equivalents
re-used.4c Banerjee and co-workers have reported metallo-hydrogels of methyl-ester-functionalised benzaldehyde with sorbitol gave the
and tri-peptide hydrogels, demonstrating that they can act as 1,3,2,4-substituted product (DBS–CO2Me) in 77% yield after simple
re-usable dye extraction agents.5 Hayes, Escuder and co-workers washing for purification. Subsequent reaction with hydrazine hydrate
used a series of urea-derived gels to get extremely effective led to DBS–CONHNH2 product in excellent 91% yield.
uptake of dyes from aqueous solution – these gels formed at
Initially, we probed the ability of this compound to form gels
low pH on protonation of a carboxylic acid and then adsorbed in water and discovered that it formed hydrogels. Gelation was
amine-based dyes, such as methylene blue, through acid–amine
Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK.
E-mail: david.smith@york.ac.uk
† Electronic supplementary information (ESI) available: Experimental details for
synthesis of DBS–CONHNH2 and characterisation data for all compounds.
Spectra from UV-vis adsorption assays, details of dye adsorption from mixtures
and pseudo second order kinetics for kinetic studies of dye adsorption. See DOI:
Scheme 1 Synthesis of novel hydrogelator DBS–CONHNH2.
10.1039/c3cc45969a
c
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Fig. 1 Scanning (A) and transmission (B) electron microscopy images of xerogel
formed (from H2O) by DBS–CONHNH2. Scale bars 100 nm.
achieved by application of a simple heat–cool cycle. To form the most
consistent and reproducible gels, a small amount of DMSO was
added to enhance solubility (4% v/v). Thermally stable gels were
always formed in the absence of DMSO, but a small amount of gelator
often remained undissolved – as such, DMSO was added to enhance
reproducibility. We monitored the effect of DMSO addition on gel
thermal stability and found that up to 5%, it was very limited (see
ESI,† Fig. S2), but at higher loadings, the Tgel value decreased.
Scanning and transmission electron microscopy on dried samples
of the xerogel indicated a well-defined nanofibrillar morphology, with
fibre diameters of ca. 10 nm (Fig. 1). This is consistent with the
formation of solvated, highly porous nanomaterials with high surface
areas, potentially capable of coming into contact with pollutant
species. Variable temperature circular dichroism (VT-CD) measure-
ments (ESI,† Fig. S3) indicated the formation of temperature sensitive
chiral nanostructures with a peak maximum at ca. 270 nm, indicative
of chiral nanoscale organisation of the aromatic ‘wings’ of the gelator
within the self-assembled gel network.
Most interestingly, DBS–CONHNH2 formed gels across a very
broad pH range from 2 to 11.5 (Fig. 2). Furthermore, the Tgel value
of the 10 mM hydrogel of DBS–CONHNH2 was constant at ca. 70 1C
across this range (ESI,† Fig. S2). We reason that this pH-independent
gelation reflects the choice of hydrazide functional group, which has a
pKa value o 3.0, meaning it should remain mainly in its neutral form
across this range. This pH-independent gelation behaviour is in
contrast to many supramolecular hydrogels which contain carboxylic
acid functional groups and are, as such, highly pH dependent, only
forming effective gels under conditions of controlled acidification,
typically with pH o 5.8,11 Unlike previous low molecular weight
hydrogelators, this system may therefore be appropriate for dye
adsorption across a wide pH range. It should also be noted that
this hydrazide-functionalised system forms gels at biologically and
environmentally relevant pH values – we suggest that hydrazides may
be effective and general replacements for carboxylic acids in other
hydrogelators, to make them more pH tolerant.
Fig. 3 Dyes tested for adsorption onto hydrogels.
Fig. 4 Maximum dye adsorption by DBS–CONHNH2 under different conditions of pH.
then pipetted onto the top of this gel, in order to determine the
maximum possible dye uptake (Fig. 4). This mixture was left to stand
and the maximum amount of removal of the dye from the aqueous
supernatant was monitored by UV-vis spectroscopy (ESI,† Fig. S4). We
performed this experiment both using pure water, and then at
controlled pH values of 2 and 12 with pH adjustment using HCl
and NaOH (both 0.1 M).
The maximum possible dye adsorption values (700–1050 mg gÀ1
,
Fig. 4) are highly competitive with the best previously reported
supramolecular gel nanomaterials – indeed it is very rare to achieve
such high loadings as those here, with reported values typically being
o100 mg gÀ1 4–7
.
The best previously reported value for dye adsorp-
tion using a supramolecular gelator was ca. 1000 mg gÀ1 for MB
uptake6 – our new system is clearly competitive with this. It is difficult
to calculate an exact molar stoichiometry of uptake, as dyes are not
provided from commercial sources in pure form. However, based on
indicated purities, we calculate that the gelator can adsorb near
stoichiometric amounts of dye. This is suggestive of the fact that
the nanoscale fibres inherent within the gel network provide a highly
accessible ‘solid-like’ system with remarkably high surface area to
maximise dye uptake. Indeed, it appears that almost every molecule
which is self-assembled into the gel network has activity. We also
found that a mixture of AB25 and NBB could be simultaneously
adsorbed from acidic solution by the gel (ESI,† Fig. S5) – reflective of
real-world conditions where mixtures of dyes may be present.
We probed the ability of this gel to remove different pollutant dyes
(Fig. 3) from water, performing an experiment in which a gel
was formed in a cuvette from 0.5 ml of water and 4 mg of gelator
(i.e. 0.8% wt/vol sample of gel). A concentrated solution of dye was
Clearly, as illustrated in Fig. 4, the solution pH controls the
selectivity of the gel network towards dye adsorption in a remarkable
manner. In basic media, MB was selectively adsorbed, whereas in
acidic media, AB25 and NBB were selectively removed. The system
is therefore quite different to the acid-functionalised gelators
investigated previously, which only show good uptake of MB at
Fig. 2 pH independent gel formation from pH 2 to 11.5.
c
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Table 1 Data from the kinetics of dye adsorption fitted to a pseudo second
To recycle the gel, we reasoned that changing pH may be a
way of desorbing dyes from the gel. Gels with adsorbed AB25
(and NBB) were indeed shown to be cleaned when exposed to a
supernatant basic solution (NaOHaq, 0.1 M) (Fig. 5). Adsorp-
tion–desorption is therefore pH switchable – with AB25 (and
NBB) being adsorbed in acidic conditions and desorbed in
basic conditions – a simple mechanism for gel recycling.
In summary, DBS–CONHNH2 is a novel pH-tolerant hydro-
gelator. Indeed, we suggest that the hydrazide group may be a
broadly applicable replacement for carboxylic acids which can
extend the pH range over which gels form. Gels of DBS–CONHNH2
can achieve very high levels of dye adsorption, with pH controlling
dye protonation and hence selectivity. As such, DBS–CONHNH2 has
ambidextrous ability to extract different types of dye under different
conditions of pH. Furthermore, changing pH can be used to control
dye adsorption–desorption and give rise to gel recycling. In current
and future work, we are developing a range of materials to adsorb
different pollutant species under controlled conditions and explor-
ing how they cooperate and/or can have their uptake properties
tuned and controlled by nanoscale engineering.
order mechanism
Max uptake in kinetic
Rate of uptake,
pH
run (mg gÀ1
)
k2 (g mgÀ1 hÀ1
)
MB
AB25
NBB
11.5
2.4
2.4
770
876
668
0.9 Â 10À4
1.9 Â 10À4
2.5 Â 10À4
low pH values.6 We suggest that the protonation states of the dyes
play a key role in determining their ability to interact with our
nanoscale gel network. This is discussed in more detail below.
To probe the kinetics of dye adsorption onto the gel we followed
uptake over time, and fitted the data to both pseudo first order and
pseudo second order kinetics. We found a significantly better fit of
all data when applying pseudo second order kinetics (Table 1, ESI,†
Fig. S4, R2 Z 0.99). This is often the case when studying adsorption,
and supports the view that chemically-specific interactions between
the gel nanofibres and the pollutant dyes are occurring.13 We ran all
of the experiments so that the gel was becoming almost saturated
with dye by the end of the kinetic run. All dyes showed fairly similar
rates of uptake, however, at pH 2.4 AB25 and NBB were taken up
significantly more quickly than MB.
We thank the Wild fund (University of York) for partial
support of this research.
In order to consider the effect of pH on dye adsorption, it is
informative to consider the structures of the dyes. It should be
assumed that all sulfonic acids, as strong acids, exist in anionic
form across the entire pH range, however, the amine groups
(and phenols) will be protonated or deprotonated dependent
on the pKa values of the dyes. We suggest that dyes may be best
adsorbed by this hydrogel when their charge is lowest. For MB,
this will be at high pH, where the amines are not protonated
and it only has +1 charge. For the other dyes it will be at low pH,
where the amines are protonated, and can therefore somewhat
counteract the negative charge of the sulfonate(s). AB25 has two
amine groups whereas NBB only has one, and this possibly
explains why the former dye has a higher degree of uptake as it
will have a lower net charge. We suggest that when the dyes are
more highly charged, they prefer to be solvated in water rather
than interacting with the supramolecular hydrogel fibres
through hydrogen bond interactions. As such, the gel fibres
can distinguish between different families of dye (amine-based
and sulfonate-based) under different pH regimes.
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Fig. 5 Desorption of AB25 from DBS–CONHNH2 hydrogel into 0.1 M NaOH
sampling the supernatant at time 0, 20 min, 40 min and 3 h.
c
11166 Chem. Commun., 2013, 49, 11164--11166
This journal is The Royal Society of Chemistry 2013