Ion-pair induced supramolecular assembly formation for selective extraction and sensing of potassium sulfate

Article abstract of DOI:10.1039/c9sc02923k

Selective extraction of sulfates in the form of alkali metal salts using charge-neutral molecular receptors is one of the holy grails of supramolecular chemistry. Herein we describe, for the first time, a squaramide-based ion pair receptor equipped with a

Full text of DOI:10.1039/c9sc02923k

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DOI: 10.1039/C9SC02923K
Journal Name
ARTICLE
Ion-pair induced supramolecular assembly formation for selective
extraction and sensing of potassium sulfate
Damian Jagleniec, Łukasz Dobrzycki, Marcin Karbarz and Jan Romański*
Received 00th January 20xx,
Accepted 00th January 20xx
Selective extraction of sulfates in the form of alkali metal salts using charge-neutral molecular receptors is one of the holy
grails of supramolecular chemistry. Herein we describe, for the first time, a squaramide-based ion pair receptor equipped
with a crown ether site that is able to extract potassium sulfate from aqueous to organic phase (the analogous monotopic
DOI: 10.1039/x0xx00000x
www.rsc.org/
1
anion receptor lacking the crown ether unit lacks this ability). H NMR, UV-vis, DOSY-NMR, DLS, MS experiments and solid-
state single crystal structure provided evidence of the formation of a supramolecular core-shell like assembly upon
interaction of the receptor with potassium sulfate. The presence of monovalent potassium salts, in contrast, promoted the
formation of simple 1:1 complexes. Unlike the 4:1 assembly, the 1:1 complexes are poorly soluble in organic media. This
feature was utilized to overcome the Hofmeister series and allow for selective extractions of extremely hydrophilic sulfates
over lipophilic nitrate anions, which was unambiguously proved by quantitative AES and ion chromatography
measurements. A simple modification of the receptor structure led to a “naked eye” optical sensor able to selectively detect
sulfates under both SLE and LLE conditions.
or charged anion receptors can be used as sulfate extractants.[5]
However, in such cases a bulky counterion is needs to be
present and the process has the nature of an exchange rather
Introduction
Due to the numerous roles played by sulfates in the
environment and biological systems, the design of artificial
receptors able to selectively recognize sulfates is of great
interest in supramolecular chemistry.[ Sulfates have a very
high hydration energy (−1090 kJ/mol),[ making strong and
selective binding of this anion by neutral receptors a very
challenging task, especially in aqueous or interfacial conditions.
On the other hand, selective binding of extremely hydrophilic
than extraction. To the best of our knowledge, simple extraction
of sulfates associated with alkali metals from aqueous to
organic phase has not yet been reported in the literature. We
envisioned that utilizing the advantage of ion pair receptors, as
compounds which can bind ion pairs cooperatively and are able
to interact with salts under interfacial conditions, may fill this
gap in methodology.[ Nature solved the problem of sulfate
binding by means of sulfate binding protein (SBP), where four
sulfate oxygen atoms are bound through seven hydrogen
bonding interaction donated by specific residues of the
protein.[ Thus we addressed the question of whether several
relatively simple and rigid non-multimacrocyclic ion pair
receptors can form hydrogen bonding inorganic-organic core-
shell like assemblies and enable selective extraction of sulfates
from aqueous to organic phase. The basis for distinguishing
nitrate and sulfate anions should be the formation of complexes
of various stoichiometry depending on the valency and
geometry of the anions tested, which should affect the
solubility of the complexes in the organic phase. To verify our
hypothesis we designed and synthesized squaramide based ion
pair receptors in which the anion binding site is directly linked
with an electron deficient phenyl ring and with the benzo 18-
crown-6 unit (Scheme 1).
1]
2]
6]
sulfate anions over lipophilic anions, such as nitrate anion
hydration energy −306 kJ/mol),[ is highly desired and requires
2]
(
[
3]
the Hofmeister bias to be overcome. One field of potential
application is in environment protection, for instance in the
disposal of the sulfate- and nitrate-containing high-level liquid
waste (HLLW) stored at the Hanford site.[ The presence of
sulfate in HLLW and its low solubility limit in borosilicate glass is
a serious issue and is problematic upon vitrification of HLLW.
Sulfates cause corrosion of the glass melter, the constituent
electrodes, and are responsible for decreasing of durability of
glass logs, directly posing a safety hazard during vitrification and
storage. One of the ways of sequestering sulfate from its
aqueous solutions involves anion or ion pair receptor facilitated
liquid−liquid extraction (LLE). In this context it has been
demonstrated that properly preorganized macrocyclic, tripodal
7]
4]
a. Faculty of Chemistry, University of Warsaw, Pasteura 1, PL 02-093 Warsaw,
Poland. E-mail: jarom@chem.uw.edu.pl
Electronic Supplementary Information (ESI) available: [General information,
synthetic details, 1H-NMR and UV-vis titration data, extraction experiments details, Results and discussion
X-ray crystal data for 1×Na
2
SO
4
(CCDC 1877168), 1×KNO
3
(CCDC 1877169) and
1
2
×H O (CCDC 1877170).]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
R3
O
O
to an increased acidity of squaramide protons upon interaction
View Article Online
DOI: 10.1039/C9SC02923K
O
R2
of the benzocrown unit with cations, which reinforce
O
interactions with anions.[
8]
O
N
H
N
H
R1
O
O
O
Table 1. Association constants (Ka) for interactions between receptor 1 and selected
anions and apparent association constants for interaction of 1 with anions in the
presence of one equivalent of sodium perchlorate or potassium hexafluorophosphatea.
1
3
2
1
: R , R = CF ; R = H
3
1
3
2
2
: R , R = H; R = NO
2
Scheme 1. Squaramide-based receptor 1 and sensor 2.
1 + 1 equiv. Na+
1 + 1 equiv. K+
1
Cl-
176000b
100900
49800
2500
492500 b
363200
142000
5900
766000 b
509600
217000
7100
The synthesis of receptors 1 and 2 is outlined in the Scheme 2.
Briefly, receptor 1 was obtained by sequential amidation of
dimethyl squarate with 3,5-trifluoromethylaniline followed by
-
-
NO
Br-
NO
I-
2
4
-aminobenzo-18-crown-6. In the case of receptor 2, which is
3
1940
3850
4500
designed as a simple optical sensor, 4-nitroaniline was used in
the first step of synthesis. All intermediates and receptors can
be simply purified by crystallization and are thus easily
accessible on a large scale.
K11= 15000
K21= 108900
K11= 58600
K21= 63300
-
-c
-c
HSO
4
2-
-c
-c
SO
4
CF3
F3C
O
O
O
O
O
O
O
−5
ii)
[a] UV-Vis, solvent CH
3
CN, temperature 293 K, [1] = 3.14 × 10 M, anions added as TBA
salts [TBAX] ∼3 × 10−3 M; M , Errors <10%. [b] titrations performed in 0.5% water in
acetonitrile; [c] the data obtained could not be fitted to a 1:1 or 2:1 binding mode; 4:1
fitting produced no reliable association constants (see ESI).
−1
N
H
F3C
NH2
F3C
iii)
S1
1
As expected, for receptor 1 possessing benzo-18-crown-6 unit,
the highest enhancement in anion binding was observed for
titrations performed in the presence of potassium rather than
sodium cations. This correlates well with the association
constants obtained for complexes of receptor 1 with sodium
O
O
O
O
O
O
i)
O
O
O
O2N
H2N
O
O
O
iii)
2
(
=
added as NaClO
48700 M and KK+= 133300 M ). This is also supported with
4
) and potassium (added as KPF
6
) cations (KNa+
O
O
O
-1
-1
O
O
O
O2N
ii)
O2N
titrations performed in the presence of 5% of water in
acetonitrile using NaCl and KCl instead of salts generated in situ.
We found that in the presence of water, receptor 2 is still able
N
H
NH2
O
S2
to recognize these salts with stability constants of 2300 and
Scheme 2. Synthesis of receptor 1 and 2. Reagents and conditions: i) H
2
, Pd/C,
MeOH-THF, 12h, r.t., quantitative; (ii) methanol, 72h, r.t., 95% for S1 and 84% for
-1
4
300 M for NaCl and KCl, respectively.
S2; (iii) methanol, 48h, r.t., 90% for 1 and 75% for 2.
Strikingly, when divalent sulfate anion was tested as a
tetrabutylammonium salt in acetonitrile the 2:1 binding mode
was better suited while the 1:1 fitting produced a high error
rate. Furthermore, the analogous titration experiment
conducted in the presence of potassium cations produced an
isotherm with a two-step binding profile, indicating the
occurrence of a more complex binding equilibrium depending
Initial evidence that squaramide 1 might act as an ion pair
receptor and is able to interact with anions in the presence of
cations in an enhanced manner came from UV-vis spectroscopy
in acetonitrile. The addition of incremental amounts of TBA salts
of various anions to a solution of 1 caused bathochromic shifts
in the absorption maximum of the receptor, enabling the
determination of apparent stability constants. We found that
receptor 1 binds these anions with moderate to very high
[
9]
on the concentration of in situ generated K
2
SO
4
.
This also
demonstrates the differing ability of free receptor 1 and its
complex with potassium cations to recognize anions. To gain
more insight into the ion pair binding mechanism and prove
disparity in monovalent and divalent anions, in particular in
-
-
-
-
-
3 2
strength, in the order I < NO < Br < NO <Cl (Table 1). Due to
the very strong interaction of receptor 1 with chlorides and the
variability of data due to water capture during titrations, we
were able to obtain reliable stability constants when we
standardized conditions and carried out experiments in the
presence of 0.5% of water. In the case of a basic anion, such as
acetate, dihydrogen phosphate or hydrogen phosphate
deprotonation was observed. Interestingly, when receptor 1
was titrated in the presence of one equivalent of sodium or
1
nitrate and sulfate binding by receptor 1, H NMR experiments
3
were conducted in CD CN. Analysis of the binding isotherms
thus obtained supported the data collected from UV-Vis
titrations. Specifically, the data collected from nitrate titration
in the absence and presence of potassium cation can be simply
fitted to the 1:1 binding mode, suggesting the formation of
[
3
1×KNO ] complex. On the other hand, titrations carried out
potassium cations (added as NaClO
4 6
or KPF ), enhancement in
2 4 2 4
with both TBA SO or in situ generated K SO resulted in
anion binding was observed in all cases. This can be attributed
2
| J. Name., 2012, 00, 1-3
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inconsistent perturbation in the titration profile. In particular, of 1 in acetonitrile, came from DOSY-NMR and dVyienwaAmrticilce Olinglihnet
upon incremental addition of sulfate anions into acetonitrile scattering (DLS) studies. In order to provDeOIt:h10e.1f0o3r9m/Ca9tSiCo0n29o2f3Ka
solution of receptor 1 containing one equivalent of potassium large complex, diffusion NMR experiments were carried out in
cations, the signals corresponding to the aromatic and crown CD CN. Formation of such assembly should cause a significant
3
ether protons were shifted inconsistently. These signals were decrease in the diffusion coefficient (D) values. The collected
initially shifted downfield, but after exceeding approx. 0.25 data clearly indicate the formation of supramolecular structures
equivalents of sulfate anions they were moved upfield, when TBA
2
SO
4
and KPF
6
were added to a solution of receptor 1
CN. The presence of in situ generated
−3
suggesting that in equilibrium the 4:1 complex (receptor:anion) (c = 3.04 × 10 M) in CD
3
is present (Figure 1).[
10]
K
promoted an important decrease in the diffusion
2
SO
4
−10
−10
2
coefficient of the ligand from D = 10.53 ×10 to 7.56 × 10
m
−1
s , with ΔD = −28%. On the other hand, the addition of TBAPF
6
to the solution of 1 did not affect its diffusion coefficient.
Furthermore, DLS measurements were performed to determine
the size of the supramolecular assembly in CH
3
CN. In the case
of the addition of TBA SO and KPF to the solution of 1, the Z-
2
4
6
average provided confirmation for the formation of a large
supramolecular assembly. The values of the hydrodynamic
diameter was found to be dH=28 nm while receptor 1 alone was
not detectable. Similarly, receptor 1 pretreated with in situ
3
Figure 1. Titration curves obtained by following crown ether and aromatic protons
upon incremental addition of in situ generated K
2
SO
4
into solution of 1 in CD
3
CN.
generated KNO was also not detectable by DLS, suggesting that
3
KNO does not promote the formation of large assembly in
solution.
Further confirmation that an inorganic/organic assembly is
2
-
+
formed in solution, after addition of SO
4
and K to the solution
Figure 2. Molecular assemblies in single crystal structures of: a) [1×Na2SO4], b) [1×KNO3] and c) [1×H2O].
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Final evidence of the formation of 4:1 and 1:1 complexes for
sulfate and nitrate promoted assemblies came from X-ray
analysis.[ Despite making a number of attempts to crystallize
11]
complexes of receptor 1 with K
crystals. However, slow diffusion of diethyl ether into an
MeCN/MeOH solution of 1 pretreated with excess of Na SO
2 4
SO , we were unable to obtain
2
4
enabled us to obtain crystals suitable for X-ray diffraction
analysis. The resulting structure revealed the formation of a
complex with 4:1 stoichiometry, entirely embedding the sulfate
into a shell formed by four ligands (Figure 2a). The sulfate anion
is surrounded by disordered receptor moieties oriented so as to
form eight hydrogen bonds between the oxygen atoms of
tetrahedron-shaped sulfate and the amide hydrogen atoms.
2 4
The electroneutrality of the [1×Na SO ] complex is ensured by
two sodium cations trapped in benzocrown cavities by two out
of four ligands. A different single-crystal structure was formed
after slow diffusion of diethyl ether into an MeCN/MeOH
1
Figure 3. Partial HNMR spectra receptor 1 (a) 2.0 mM in wet CDCl
3
(b) after
extraction of 50 mM K SO (c) after back extraction to distilled water.
2
4
solution of 1 obtained after solid-liquid extraction with KNO
the [1×KNO ] crystal structure the asymmetric part of the unit
cell consists of two ligands and two KNO ion pairs. Like in the
case of the previous structure, the potassium cations reside in
the benzocrown cavities and are in addition coordinated from
both sides by nitrate anions, giving an infinite 1-D polymeric
3
. In
3
This clearly indicates that receptor 1 is able to extract extremely
3
hydrophilic sulfate anion from aqueous to organic phase and
[12]
form complexes in the organic layer.
This is also supported
by electron spray ionization mass spectrometric measurements
of the extracted organic solution, which clearly shows
assembly (Figure 2b). Strong hydrogen bond interactions
characteristic peaks (m/z) appearing at 2750.7 [4×1 + K
2
SO
4
+
-
between squaramide protons and NO
3
anions results in the
+
2-
K ] and 1316.3 [4×1 + SO
4
] (see the ESI). Control experiments
formation of supramolecular layers of moieties. Interestingly, in
a salt-free environment, ligand 1 crystallizes as monohydrate
with analogous urea based ion pair receptor or squaramide
based ion pair receptor equipped with benzo-15-crown-5 ether
or anion receptor lacking a crown ether unit revealed an
[
2
1×H O] with water molecules located in the benzocrown part
and additionally coordinated by H atoms of the squaramide
moiety, giving an altogether dimeric arrangement (Figure 2c).
Taking into consideration the different properties of nitrate and
sulfate complexes with 1 formed in solution and in the solid
state, we envisioned that using a squaramide-based ion pair
receptor would be able to differentiate between these salts
under interfacial conditions and preferentially extract sulfates
into organic phase. As a first test, LLE experiments were
2 4 2 4
inability to extract either K SO or Na SO in such conditions (the
receptors denoted in the ESI as S4, S5 and S3, respectively).
Furthermore, the ability to release potassium sulfate by
1
receptor 1 was confirmed by the H NMR back extraction
experiment, which resulted in the signals returning to the initial
position after contacting with water (Figure 3c).
In order to establish the extraction efficiency in the experiments
described above, we used atomic emission spectroscopy (AES)
and quantified the potassium content in the organic phase. The
fraction of receptor 1 molecules occupied by a potassium cation
after extraction of 2 mM solution of 1 in chloroform with 50 mM
1
conducted under H NMR control using a 2 mM solution of 1 in
CDCl
KNO
3
and various aqueous 50 mM salt solutions (KCl, KBr, KNO
3
,
2
, Na PO and K SO ). With the exception of K SO
2
SO4, KH
2
4
2
4
2
4
extraction, in all cases the precipitation of solids was noted,
showing the insolubility of the supramolecular species formed
in CDCl . This opens up the unique opportunity to distinguish
3
these salts under interfacial conditions by preferentially
increasing the solubility of sulfate complexes with receptor 1
2 4
of K SO aqueous solution was determined to be 36%. After
back extraction the potassium content was found to be
negligible. This suggests that receptor 1 may act as a suitable
candidate as a sulfate transporter. By contrast, neither binary
mixture of squaramide based anion receptor (denoted as
receptor S3 in the ESI) with 4-nitrobenzo-18-crown-6-ether nor
urea based ion pair receptor (denoted as receptor S4 in the ESI)
are effective ionophores (see ESI). Efforts were then made to
prove the selectivity of receptor 1 towards sulfates and its
ability to overcome the Hofmeister series. Thus we carried out
rather than complexes of 1 with other salts. Indeed, comparison
1
of the H NMR spectrum of free 1 in water saturated CDCl
3
revealed substantial changes in the resonance signals after
contacting with aqueous solution of K SO (Figure 3b).
2
4
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competitive extraction experiments under ion chromatography
control. Specifically, aqueous mixture of KNO and K SO (5 mM
View Article Online
DOI: 10.1039/C9SC02923K
3
2
4
each) was extracted with 5 mM or 20 mM solution of 1 in
chloroform, respectively. In both cases a significant decrease in
concentration of sulfates was detected while nitrates were
affected only slightly. The drop in concentration was calculated
to be 25 and 74% for sulfates and 4.9 and 7.2% for nitrates,
using 5 and 20 mM of 1 in chloroform, respectively (Figure 4). Figure 5. Chromatogram obtained during back extraction experiments after
tenfold dilution. Organic phase: 20mM of receptor 1 in chloroform.
Importantly, the selectivity of receptor 1 towards sulfates was
retained even when the spectrum of potassium salts in the
aqueous mixture was extended to chlorides, bromides, nitrites
Finally, we modified the receptor structure to act as a simple
optical sensor (receptor 2) capable of “naked eye” recognition
and dihydrogen phosphates. On the other hand use of solution
of sulfates. The addition of potassium sulfate to 2.1 mM
containing of basic salts such as divalent or trivalent phosphates
solution of 2 in DMSO did indeed result in a drastic color change
cause no phase separation most likely due to the deprotonation
from orange to purple, while the addition of potassium nitrate
of the receptor. This indicates the limitations of this system and
did not affect the color change. Similarly, only aqueous solution
operating at neutral and acidic pH (see the ESI).
2 4
of K SO induced a color change from yellow to red after
contacting with 0.5 mM solution of 2 in nitrobenzene (Figure 6).
To gain more insight into the binding ability of receptor 2 and to
establish
its
mechanism
of
action,
we
added
tetrabutylammonium nitrate and tetrabutylammonium sulfate
to the solution of 2 in deuterated DMSO. We found that nitrate
anion does not influence the chemical shifts of signals
corresponding to the squaramide protons of receptor 2. This
clearly demonstrates that in such competitive media receptor 2
is not able to form complexes with this anion. On the other
hand, the addition of tetrabutylammonium sulfate to the
solution of 2 in deuterated DMSO, apart from the color change,
1
resulted in the disappearance of squaramide signals in the H
NMR spectrum, suggesting deprotonation rather than
formation of complexes. This was supported by UV-vis
measurements. Particularly, the addition of TBANO to the
3
solution of receptor 2 in DMSO does not affect its UV-vis
spectrum. Contrarily, upon addition of sulfate anions or basic
anions such as acetate or hydroxide, a new bathochromic band
in the UV-vis spectra was observed. Based on these findings we
concluded that the color change upon addition of sulfates to the
solution of 2 originates more likely from the deprotonation
event, which is unusual for these anions, making possible the
optical differentiation of nitrates and sulfates. This shows its
sensing limitations because basic anions also cause a change in
the color of the receptor 2 (see the ESI). Nevertheless, this
system can serve as a facile supportive sensor for monitoring
Figure 4. Chromatograms obtained during extraction experiments after tenfold
dilution (a) source phase (b) after extraction with 5 mM of 1 in CHCl (c) after
extraction with 20 mM of 1 in CHCl
3
3
.
In order to prove the high performance and very high selectivity
of receptor 1 towards sulfates we performed more competitive
extraction experiments using aqueous solution containing one
order of magnitude higher concentration of potassium nitrate
(50 mM) than potassium sulfate. For this reason, the
concentration of anions was monitored in aqueous phase after
back extraction. We found that in such conditions receptor 1 is
still able to preferentially extract extremely hydrophilic sulfates
rather than nitrates (Figure 5). Taking into consideration the
assumed stoichiometry of complexes formed with receptor 1 in
-
-
3 4
competitive NO /SO extraction processes using receptor 1.
3 2 4
organic phase (1:1 for KNO and 4:1 for K SO ) extraction
efficiency was calculated to be 6 and 64% for potassium nitrate
and potassium sulfate, respectively. From a practical point of
view, this applies to approx. 0.32 / 0.12 molar ratio of nitrate /
sulfate ions in the extract.
Figure 6. SLE and LLE experiments (A) From left: solution of 2 in DMSO, after
addition of solid KNO , after addition of solid K SO , (B) From left: solution of 2 in
nitrobenzene and water, solution of 2 in nitrobenzene and aqueous KNO , solution
of 2 in nitrobenzene and aqueous K SO
3
2
4
3
2
4
.
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Journal Name
B. P. Hay, N. L. Bill, J. L. Sessler, B. A. Moyer, CheVmiew. ACroticmlemOnulinne.
Conclusions
2
011, 47, 7611-7613; (d) C. A. Williams, N. J. Seipp, V. S.
DOI: 10.1039/C9SC02923K
In summary, a new ion pair receptor was synthesized and
characterized by standard spectroscopic protocols as well as by
DOSY, DLS and by single X-ray crystal diffraction analyses. Based
on titration experiments we evidenced the high ability of
receptor 1 to form complexes with ion pairs. In contrast to
potassium salts of the monovalent anions tested, which form
Bryantsev, B. A. Moyer, Separ. Sci. Tech. 2018, 53, 1864-1873;
e) S. K. Kim, J. Lee, N. J. Williams, V. M. Lynch, B. P. Hay, B. A.
Moyer, J. L. Sessler, J. Am. Chem. Soc. 2014, 136, 15079-
5085; (f) N. J. Williams, C. A. Seipp, K. A. Garrabrant, R.
Custelcean, E. Holguin, J. K. Keum, J. R. Ellias, B. A. Moyer,
Chem. Commun. 2018, 54, 10048-10051; (g) B. Akhuli, I. Ra-
vikumar, P. Ghosh, Chem. Sci. 2012, 3, 1522-1530.
(a) M. P. Wintererst, T. G. Levitskaia, B. A. Moyer, J. L. Sessler,
L. H. Delmau, J. Am. Chem. Soc. 2008, 130, 4129-4139; (b) D.
J. White, N. Laing, H. Miller, S. Parsons, P. A. Tasker, S. Coles,
Chem. Commun. 1999, 20, 2077-2078; (c) S. K. Kim, V. M.
Lynch, N. J. Young, B. P. Hay, C-H. Lee, J. S. Kim, B. A. Moyer,
J. L. Sessler, J. Am. Chem. Soc. 2012, 134, 2087-20843; (d) Q.
He, Z. Zhang, J. T. Brewster, V. M. Lynch, S. K. Kim, J. L. Sessler,
J. Am. Chem. Soc. 2016, 138, 9779-9782; (e) J. Romański, P.
Piątek, J. Org. Chem. 2013, 78, 4341-4347; (f) Q. He, N. J.
Williams, J. H. Oh, V. M. Lynch, S. K. Kim, B. A. Moyer, J. L.
Sessler, Angew. Chem. Int. Ed. 2018, 57, 1-6; (g) J. M.
Mahoney, A. M. Beatty, P. J. Duggan, B. D. Smith, Inorg. Chem.
(
1
6
1
:1 stoichiometry complexes with receptor 1, in the case of
potassium sulfate the 4:1 complex was evidenced. This feature
was utilized to differentiate the solubility of complexes in
organic media in favor of the 4:1 assembly and enable selective
extraction of potassium sulfate from aqueous solution to
chloroform. To the best of our knowledge, compound 1 is the
first ion pair receptor capable of extracting the extremely
hydrophilic sulfate anion in the form of an alkali metal salt from
the aqueous to the organic phase. We have demonstrated that
receptor 1 extracts potassium sulfate selectively even in the
presence of lipophilic anions such as nitrates, as shown
independently by qualitative ion chromatography analysis. A
simple modification of the receptor structure allowed us to
obtain a “naked eye” optical sensor 2 capable of detecting
sulfates under both SLE and LLE conditions and to do so with
selectivity relative to potassium nitrate.
2
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The authors declare no conflict of interests.
Acknowledgements
This work was supported by Grant no. 2018/30/E/ST5/00841
from the National Science Centre, Poland.
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Formation of a supramolecular core-shell like assembly upon interaction of the receptor with potassium sulfate
enable for its selective extraction.
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