Selective sensing of Hg2+ via sol–gel transformation of a cholesterol-based compound

Article abstract of DOI:10.1080/10610278.2018.1461219

A cholesterol-based dithioacetal functionalised compound 1 has been designed, synthesised and successfully applied for the selective detection of Hg²⁺ ion. The specific Hg²⁺-induced deprotection of the thioacetal functionality of 1 resulted in sol-to-gel transition in DMF-H₂O (1:1, v/v) via the formation of precursor aldehyde 3. This Hg²⁺-prompted rapid sol to gel conversion is insensitive to the nature of Hg²⁺-salt. In the study, other metal ions did not show any change. To the best of our knowledge, this is a first time report of a chemodosimeter that functions as a selective ‘naked-eye’ Hg²⁺-detector by showing in situ sol-to-gel conversion.

Full text of DOI:10.1080/10610278.2018.1461219

Supramolecular Chemistry
ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20
Selective sensing of Hg2+ via sol–gel
transformation of a cholesterol-based compound
Atanu Panja & Kumaresh Ghosh
To cite this article: Atanu Panja & Kumaresh Ghosh (2018) Selective sensing of Hg2+ via sol–gel
transformation of a cholesterol-based compound, Supramolecular Chemistry, 30:8, 722-729, DOI:
10.1080/10610278.2018.1461219
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Supramolecular chemiStry, 2018
Vol. 30, No. 8, 722–729
https://doi.org/10.1080/10610278.2018.1461219
Selective sensing of Hg2+ via sol–gel transformation of a cholesterol-based
compound
Atanu Panja and Kumaresh Ghosh
Department of chemistry, university of Kalyani, Kalyani, india
ARTICLE HISTORY
ABSTRACT
received 19 January 2018
accepted 25 march 2018
A cholesterol-based dithioacetal functionalised compound 1 has been designed, synthesised and
successfully applied for the selective detection of Hg2+ ion. The specific Hg2+-induced deprotection
of the thioacetal functionality of 1 resulted in sol-to-gel transition in DMF-H2O (1:1, v/v) via the
formation of precursor aldehyde 3. This Hg2+-prompted rapid sol to gel conversion is insensitive to
the nature of Hg2+-salt. In the study, other metal ions did not show any change. To the best of our
knowledge, this is a first time report of a chemodosimeter that functions as a selective ‘naked-eye’
Hg2+-detector by showing in situ sol-to-gel conversion.
KEYWORDS
Supramolecular gelator;
mercury ion sensing; sol–gel
transformation; cholesterol-
based compound; naked-eye
detection
Selective sensing of Hg2+ via sol-gel transformation
of a cholesterol-based compound
Atanu Panja and Kumaresh Ghosh*
A cholesterol-based dithioacetal functionalised compound 1
has been designed, synthesised and successfully applied for
the selective detection of Hg2+ ion. The specific Hg2+-induced
deprotection of the thioacetal functionality of 1 resulted in
sol-to-gel transition in DMF-H2O (1:1, v/v).
A cholesterol-based dithioacetal functionalised compound 1 has been designed, synthesised and
successfully applied for the selective detection of Hg2+ ion. The specific Hg2+-induced deprotection
of the thioacetal functionality of 1 resulted in sol-to-gel transition in DMF-H2O (1:1, v/v).
Introduction
externally added chemical analytes and offers a familiar
approach in designing molecular sensors (1, 3).
Over the last few decades, the design and synthesis of stim-
uli responsive low molecular weight gelators (LMWG) have
gained interest in supramolecular gel chemistry owing to
their promising applications from medicine to material
science (1). Supramolecular gelators are special class of
molecules that are self-assembled into three dimensional
cross-linked architectures through the implication of vari-
ous non covalent forces (hydrogen bonding, π-π stacking,
van der Waals interactions etc.) and resulted in trapping of
solvent molecules to form viscoelastic semisolid materials
or gels (2). Sometime, the self-organisation of the gela-
tor molecules is stimulated in the presence of different
Of the different chemical analytes, metal ions receive
attention because of their significance in both biology and
environment. In this relation, the sensing and detection of
Hg2+ ions is an important aspect. Mercury ions are consid-
ered to be one of the most hazardous and toxic element
as its excess consumption in human body can damage
several body functions like brain, heart, kidney, stomach
etc. and thereby lead to variety of harmful effects includ-
ing digestive, cardiovascular and neurological disorders
(4). Because of its fatal hazardousness, mercury has been
strictly banned in electrical and electronic equipment by
CONTACT Kumaresh Ghosh
ghosh_k2003@yahoo.co.in
Figures showing the gelation profile, hrmS spectrum, binding constant and detection limit curves, characterization spectra. available at https://doi.org/10.
1080/10610278.2018.1461219.
© 2018 informa uK limited, trading as taylor & Francis Group

SUPRAMOLECULAR CHEMISTRY
723
the European Union and also its concentration in drink-
ing water has been strictly regulated by the World Health
Organization (WHO) and the Environmental Protection
Agency (EPA) (5). Despite such limitations, mercury pol-
lution still arises from natural or industrial effluents (6).
Therefore, the development of an easy and efficient
detection system for mercury ions is of great significance.
There are many methods of Hg2+ ion sensing and detec-
tion in the literature that includes AAS (Atomic Absorption
Spectroscopy) (7), ICP-MS (Inductive Coupled Plasma-Mass
Spectroscopy), electrochemical, and fluorometric or color-
imetric sensors (8). In addition, for development of Hg2+
sensing probes, a different strategy, i.e. implication of reac-
tion based sensors, specially the Hg2+-promoted depro-
tection of thioacetal group has been considerably used
(9). However, these methods are time consuming, require
complicated sample preparation and sophisticated instru-
ments. In fluorometric sensing of Hg2+, sometimes inter-
ference of other metal ions cannot be ignored. Detection
of Hg2+ via thioacetal/thioketal deprotection in solution is
often perturbed by other metal ions (10) such as Ag+, Cu2+
or Cd2+ and thereby limits the efficacy and selectivity of
the process. In this regard, supramolecular gels are bene-
ficial in overcoming these limitations (11) although such
an easy-to-handle naked-eye discriminating tool has the
selectivity dilemma (11).
for amplification of van der Waals interaction to induce
the self-aggregation.
Considering all these aspects, we initially investigated
the gel forming abilities of 1 and its precursor aldehyde 3
in different solvents and solvent mixtures, and compared
the results (Table 1S). In the study while compound 1 was
insensitive toward gel formation, the precursor aldehyde
3 formed gel from a number of solvents such as CH3CN,
hexane, DMF/H2O. Despite the presence of large cho-
lesterol surface and long alkyl chains for strong van der
Waal interaction, intermolecular aggregation of 1 to form
cross-linked network for gelation was unsuccessful. Thus,
in the present study, the Hg2+-induced deprotection of the
thioacetal part in 1 to form 3 will be of immense interest
in gel chemistry for visual tracing of Hg2+. Considering the
solubility of metal salts and also the nucleophilic action
of water in Hg2+-endorsed deprotection reaction of thio-
acetal groups, we took DMF-H2O (1:1, v/v) as the desired
medium over CH3CN or hexane.
Figure 2 displays the effect of different metal ions on
sol to gel phase transition of 1. The metal responsive
behaviour of 1 was investigated by adding aqueous solu-
tion of different metal ions (1 equiv. c = 0.2 M) to the DMF
solution of 1, and the volume was adjusted on further
addition of water. Instant gel formation occurred for Hg2+
ion (as acetate salt), whereas under identical condition,
all other metal ions (Cu2+ and Ag+ as nitrate salts, Zn2+ as
acetate salt, Cd2+, Pb2+, Al3+, Fe3+, Fe2+, Mg2+, Ni2+ and Co2+
as perchlorate salts) resulted in precipitation only. These
results corroborated compound 1 as highly sensitive and
selective chemodosimeter for Hg2+ ion, rather than any
As part of our ongoing research programme in design-
ing stimuli responsive supramolecular gelators (11, 12),
herein, we report a new cholesterol-based dithioacetal
functionalized compound 1 that demonstrates naked eye
detection of Hg2+ ions in DMF-H2O (1:1, v/v) involving sol-
to-gel phase transformation (Figure 1).
10
Results and discussion
H
S
O
H
Synthesis
H
recognition
unit
O
O
S
Compound 1 was obtained according to the Scheme 1.
Initially, cholesterol was converted to the chloro ester 2
(12) which upon reaction with 4-hydroxy benzaldehyde
in the presence of K2CO3 in CH3CN gave the cholesterol
appended aldehyde 3. Protection of the aldehyde group
with 1-dodecanethiol in the presence of BF3.OEt2 afforded
compound 1 in appreciable yield. All the compounds were
fully characterized by usual spectroscopic methods.
The different components of the designed compound
1 are highlighted in Figure 1. The cholesterol motif in the
structure, due to its large surface area, is incorporated to
assist the self-assembly of the molecules through hydro-
phobic interaction (3). The thiol part that is obtained from
the precursor aldehyde is used as the reaction-based rec-
ognition unit of the molecule. In this aspect, 1-dodecan-
ethiol that bears a long alkyl chain has been considered
steroidal group
1
10
Figure 1. Structure of compound 1.
CHO
O
(iii)
74%
(ii)
78%
(i)
1
R
OH
R =
Cl
R
O
96%
O
O
2
3
R
O
H
H
H
Scheme 1. (i) chloroacetylchloride, pyridine, drychcl3, rt, 10 h; (ii)
4-hydroxybenzaldehyde, K2co3, ch3cN, 5 h; (iii) 1-dodecanethiol,
BF3.oet2, dry ch2cl2, 0 °c, 30 min.

724
A. PANJA AND K. GHOSH
Figure 2. (colour online) photograph showing the phase changes of 1 (25 mg/ml) in presence of equiv. amount of different metal ions
in DmF-h2o (1:1, v/v) [from left to right: hg2+, cu2+, ag+, Zn2+, cd2+, pb2+, al3+, Fe3+, Fe2+, mg2+, Ni2+ and co2+].
other metal ions tested. Hg2+ ion, due to its strong affinity
towards sulphur atoms, involves rapid dethioacetalization
of 1 leading to the formation of the precursor aldehyde 3
that undergoes instant gelation in the medium.
anions in the gelation (Figure 4). Moreover, the gelation
resulting in from Hg2+-induced dethioacetalization of 1
was achieved in the presence of Cd2+, Cu2+, Ag+, and Fe2+
ions and thereby confirmed their non interference in the
gelation process (Figure 2S).
To investigate the strength of interaction of 1 with Hg2+
ion, we carefully measured the minimum gelator concen-
tration (mgc) and the gel melting temperature (Tgel) of the
respective gels of 1 derived from the addition of Hg2+ ions
and 3 itself. The mgc value for gelation of 1 (12 mg/mL) in
the presence of Hg2+ was greater than the mgc value of gel
3 in DMF-H2O (7 mg/mL). Thus to compare the Tgel values,
a common concentration (12 mg/mL) of each for gelation
was considered. Under this condition, two gels were found
to have almost similar gel melting temperatures (52 and
54 °C for 1 on interaction with Hg2+ ions and 3, respec-
tively). The small difference in Tgel is presumably due to
the presence of other chemical analytes in the gel state
of 1. Like 3, the Hg2+-induced gel of 1 was also thermor-
eversible in nature and gave distinct gel-sol phase transi-
tion upon successive heating and cooling (Figure 1S). The
aggregated morphologies of the xerogels derived from 3
and 1 in presence of Hg2+ ions were investigated by SEM
images that revealed almost identical overlapped rugate
layer structure (Figure 3).
The gelation kinetics of 1 was dependent upon the
concentrations of both the compound and Hg2+ ions.
The higher concentration of 1 (25 mg/ mL) in presence
of one equiv. of Hg2+ ions resulted in gelation immedi-
ately. Similarly, lower concentration of 1 (12 mg/mL) in the
presence of 10 equiv. amounts of Hg2+ ions gave instant
gelation. On contrary, compound 1 at its mgc (12 mg/mL)
underwent gelation in the presence of one equiv. of Hg2+
ions after almost 2 h. These observations clearly indicated
that the gel forming ability of 1 was undoubtedly related
to its interaction with Hg2+ ion. Stronger interaction with
Hg2+ ion (when both the gelator and Hg2+ ion concentra-
tions were high) resulted in rapid formation of precursor
3, for which the required mgc level was reached rapidly
and shortened the gelation time.
The in situ formation of aldehyde 3, on reaction of 1 with
Hg2+ ion was initially established by FTIR analysis (Figure
5). The FTIR spectrum of 1 in the gel state, obtained from
treatment of Hg2+, showed a prominent new broad band
at 1658 cm−1. This stretching band was absent when the
spectrum of 1 was recorded in its amorphous state. This
new broad peak at 1658 cm−1 is attributed to the character-
istic stretching signal for formyl group (–CH=O) obtained
through Hg2+-promoted dethioacetalization reaction. FTIR
spectrum of aldehyde 3 in gel state was almost identical to
that of 1 in presence of Hg2+. In the amorphous state, com-
pound 3 showed stretching at 1701 cm−1 for –CHO group.
This moved to the broad region at 1655 cm−1 in gel state
due to participation in forming network. Importantly, the
signal for amide carbonyl of DMF that remains as less vol-
atile solvent in the gel (not dried), also shows stretching in
this region. Therefore, the broad peak at 1658 cm−1 for the
gel derived from compound 1 with Hg2+ ions is undoubt-
edly comprised of stretching frequencies for –CHO of
compound 3 (generated from compound 1 through
deprotection) and amide carbonyl of DMF solvent. This
corroborated the successful deprotection of thioacetal
moiety and regeneration of aldehyde functionality.
1
For further confirmation, we studied the H NMR of
1, in the absence and presence of Hg2+ ions (Figure 6).
Presence of equiv amount of Hg2+ ion (as perchlorate salt)
caused disappearance of the signal with a large down-
field chemical shift of 5.13 ppm of the proton of type Ha.
This happened due to the formation of –CHO function-
ality for which the proton Ha became deshielded. The
newly appeared peaks in the NMR spectrum of 1 in the
presence of Hg2+ ion resembled to the proton signals
of 3, which undoubtedly suggested the proposed Hg2+-
induced dethioacetalization reaction (13) (Figure 6 (right
side)). The signal for water that is inherited either from
moisture in CDCl3 or from the metal salt is presumably
in the region ~6.5 ppm. The greater downfield chemical
It is further to be mentioned that gel formation from 1
also occurred from the addition of other Hg2+-salts such
−
−
as Cl−, ClO4 and NO3. This ruled out any role of counter

SUPRAMOLECULAR CHEMISTRY
725
Figure 3. (colour online) Sem images of the xerogels of (a) 1 derived from addition of hg(oac)2 and (b) 3 itself.
Figure 4. (colour online) preparation of DmF-h2o gel of 1 in
presence of different hg-salts.
Figure 5. partial Ftir spectra of (a) 1 in amorphous state, (b) 1 in
gel state resulting from addition of hg(oac)2, (c) 3 in gel state and
(d) 3 in amorphous state.
shift of this signal is due to the presence of Hg2+ ions that
may be involved in coordination. However, addition of
small amount of D2O to the CDCl3 solution of 1 contain-
ing Hg(ClO4)2 facilitated the deprotection. HRMS analysis
of the isolated product from the reaction of 1 with Hg2+
ion showed the prominent peak at 549.3937 which was
assignable to the mass of compound 3 [549.3938 (M+H)+]
(Figure 3S).
To test the reversibility of the sol–gel process, we
added 2 equiv. amounts of aq. solution of EDTA to the gel
state. The gel state remained unperturbed for longer time
(Figure 4S). The stability of the gel in presence of EDTA even
after 30 min undeniably suggested the irreversibility of the
Hg2+-mediated deprotection reaction of thioacetal moiety.
To understand the stability of compound 1 in the pres-
ence of acids, aqueous DMF solution of 1 was treated with
trifluoroacetic acid and acetic acid separately. As can be
seen from Figure 5S, compound 1 under the conditions
was stable and did not undergo transformation to alde-
hyde 3 that introduces gel in the medium.
The selective response of 1 towards Hg2+ ion was realized
in solution also. UV–vis titration of 1 (c = 2.50 × 10−5 M) with
different metal ions (as perchlorate salts, c = 1.0 × 10−3 M)
was conducted in DMF: H2O (1:1, v/v). Addition of 5 equiv.
amounts of Hg2+ ion to the solution of 1 resulted in drastic
changes in the absorption spectra of 1. A gradual incre-
ment of the absorption band at 280 nm for n-π* transition
in –CHO group (Figure 7(a)) was noted while all other metal
ions in the study did not bring such change (Figure 7(b)).
In order to realize the kinetics of Hg2+-induced depro-
tection reaction, UV–vis spectral changes with time were

726
A. PANJA AND K. GHOSH
Figure 6. (colour online) 1h Nmr spectra of (a) 1, (b) 1 with hg(clo4)2 (after 6 h), (c) 1 with hg(clo4)2 in cDcl3 containing 2% D2o (after
1 h) and (d) 3 itself in cDcl3. mass values of the aldehyde 3 obtained through reaction (c) and itself (d) are highlighted (left); pictorial
representation of the hg2+ ion induced hydrolysis of dithioacetal in 1 (right). asterisk protons in the Nmr spectra represent signals of the
cholesteryl group.
Figure 7. change in absorbance of 1 (c = 2.50 × 10−5 m) upon gradual addition of 5 equiv. amounts of (a) hg2+, (b) other metal ions
(c = 1.0 × 10−3 m) in DmF-h2o (1:1, v/v) and (c) time dependent change in absorption of 1 at 280 nm in the presence of 5 equiv. amounts
of hg2+ ions.
followed. In this regard, Figure 7(c) demonstrates the
increase in absorption intensity of 1 at 280 nm in presence
of 5 equiv. amount of Hg2+ ions. This increase in absorption
was due to gradual formation of the aldehyde compound
3. The absorption reached plateau within 4 min and indi-
cated the completion of the dethioacetalization reaction.
The Benesi-Hildebrand plot (14) indicated good linear
stoichiometric interaction of 1 with Hg2+ ion (Figure 6S)
and the detection limit (15) of 1 for detection of Hg2+ ion
was determined to be as low as 5.71 × 10−6 M (Figure 7S).
functionality in DMF: H2O (1:1, v/v) resulted in the forma-
tion of aldehyde 3 which exhibited nice gel and validated
the visual sensing of Hg2+. In the event, other metal ions
did not exhibit any marked change. It is fact that Hg2+-
induced gelation of 1 was independent of nature of the
mercury salts. Addition of other Hg2+-salts such as Cl−, ClO−4
−
and NO3 also induced gelation. To the best of our knowl-
edge, this is a first time report of a chemodosimeter that
works as selective‘naked-eye’Hg2+-detector by exhibiting
in situ sol-to-gel phase transformation.
Experimental
Conclusions
Materials
In summary, a new pro gelator 1, based on the choles-
terol-dithioacetal platform has been designed, synthe-
sised and effectively applied as a Hg2+ ion sensing kit.
The specific Hg2+-induced deprotection of the thioacetal
Cholesterol, chloroacetyl chloride, 1-dodecanethiol and
other chemicals were purchased from spectrochem and
were carefully handled. Metal salts used in the study were

SUPRAMOLECULAR CHEMISTRY
727
purchased from Sigma-Aldrich. All solvents used in the
synthesis were purified, dried and distilled as required.
Solvents used in NMR experiments were obtained from
Aldrich. Thin layer chromatography was performed on
Merck precoated silica gel 60-F254 plates. 1H and 13C NMR
spectra were recorded using Bruker 400 MHz instrument
using TMS as internal standard. High resolution mass
data were acquired by the electron spray ionization (ESI)
technique on XEVO GS-2 QTOf Waters mass spectrome-
ter. FTIR measurements of all compounds and dried gels
(xerogels) were carried out using a Perkin-Elmer L120-
00A spectrometer (νmax in cm−1) using KBr cell and KBr
pellets, respectively. Scanning electron microscopy (SEM)
images were obtained on EVO LS-10 ZEISS instrument.
UV–vis studies were performed using Shimadzu UV-2450
spectrophotometer.
100 MHz): δ 190.7, 167.5, 162.6, 139.0, 131.9, 130.6, 123.2,
114.9, 75.6, 65.3, 56.6, 56.1, 49.9, 42.3, 39.6, 39.5, 37.9, 36.8,
36.5, 36.1, 35.7, 31.89, 31.82, 29.7, 28.2, 28.0, 27.6, 24.2, 23.8,
22.8, 22.5, 21.0, 19.2, 18.7, 11.8; FTIR (KBr) ν cm−1: 3403,
2945, 1775, 1701, 1606, 1212; HRMS (TOF MS ES+): calcd.
549.3866 (M+1)+, found 549.3890 (M + 1)+.
Compound 1:
To an ice cold solution of compound 2 (1 g, 1.82 mmol)
and BF3.OEt2 (1 ml) in dry CH2Cl2 (15 mL), 1-dodecanethiol
(1.1 g, 5.18 mmol) was added dropwise and the reaction
mixture was stirred at room temperature. The progress of
the reaction was monitored by TLC. After completion of
the reaction, CH2Cl2 was evaporated under reduced pres-
sure and water was added to the crude mass. Compound
was extracted with 2% CH3OH in CHCl3. Evaporation of the
solvent gave the crude product which on purification by
column chromatography using 5% ethyl acetate in petro-
leum ether as eluent afforded the pure compound 3 in
74% yield (1.26 g, mp 62 °C). 1H NMR (CDCl3, 400 MHz): δ
7.29 (d, 2H, J = 8.8 Hz), 6.78 (d, 2H, J = 8.8 Hz), 5.31 (d, 1H,
J = 4 Hz), 4.76 (s, 1H), 4.67 (m, 1H), 4.51 (s, 2H), 2.49–2.38
(m, 4H), 2.27–0.60 (89H, cholesteryl and alkane protons);
13C NMR (CDCl3, 100 MHz): δ 168.2, 157.3, 139.2, 133.7,
128.9, 123.0, 114.6, 75.2, 65.8, 56.6, 56.1, 52.5, 49.9, 42.3,
39.7, 39.5, 37.9, 36.8, 36.5, 36.1, 35.7, 32.2, 31.9, 31.8, 29.68,
29.66, 29.63, 29.5, 29.3, 29.2, 29.1, 28.9, 28.2, 28.0, 27.6,
24.2, 23.8, 22.8, 22.7, 22.5, 21.0, 19.3, 18.7, 14.1, 11.8; FTIR
(KBr) ν cm−1: 2922, 2851, 1753, 1609, 1509, 1466, 1378,
1211, 1082; MALDI-TOF MS: calcd 934.727 (M)+, found
934.734 (M)+.
Synthesis
Chloro-acetic acid 17-(1,5-dimethyl-hexyl)-10,13-di-
methyl-2,3,4,7,8,9,10,11,12,13,14,15,16,17-tetra-
decahydro-1H-cyclopenta[a]phenanthren-3-yl ester
(2):(12)
To a stirred solution of cholesterol (0.5 g, 1.29 mmol) in
20 mL dry CHCl3 was added chloroacetyl chloride (0.16 mL,
1.93 mmol) and pyridine (0.05 mL, 0.65 mmol) under nitro-
gen atmosphere. The mixture was allowed to stir for 10 h
at room temperature. After completion of reaction, the
reaction mixture was neutralized with NaHCO3 solution,
and then was extracted with CHCl3 (3 × 30 mL). The organic
layer was washed several times with water and separated
and dried over Na2SO4. Evaporation of the solvent gave
white solid compound. Recrystallization from CH3OH
afforded pure product 2 (0.58 g, yield 96%, mp 148 °C). 1H
NMR (400 MHz, CDCl3): δ 5.37 (m, 1H), 4.72 (m, 1H), 4.03 (s,
2H), 2.36 (m, 2H), 2.02–0.85 (m, 38H, cholesteryl protons),
0.67 (s, 3H); FTIR (KBr, cm−1): 2939, 2907, 2821, 1753, 1620,
1195.
Gelation test and SEM imaging
The required amount of compounds were first dissolved in
suitable solvents and slightly warmed to form a homoge-
neous solution. The solution was kept at the room temper-
ature undisturbed. Gel formation was tested by inversion
of vial method. Sample of gel for SEM imaging was dried
under vacuum and then coated with a thin layer of gold
metal.
Compound 3:(12)
A mixture of 4-hydroxy benzaldehyde (1 g, 8.19 mmol)
and K2CO3 (2.26 g, 16.38 mmol) was refluxed in dry CH3CN
for 2 h and then compound 2 (4.55 g, 9.82 mmol) was
added to it. The reaction mixture was refluxed for 12 h.
Then the organic solvent was evaporated under reduced
pressure and water was added to the crude mass. Then
reaction mixture was extracted with 2% CH3OH in CHCl3.
Evaporation of the solvent gave the crude product which
was purified by column chromatography using 10% ethyl
acetate in petroleum ether as eluent to afford the pure
Determination of gel-sol transition temperature
(Tgel
)
The gel-to-sol transition temperature (Tgel) was defined as
the temperature at which the gel melted and started to
flow. Tgel was measured by the dropping ball method. In
this test, a small glass ball was carefully placed on the top
of the gel to be tested, which was present in a test tube.
The tube was slowly heated in a thermostated oil bath until
the ball fell to the bottom of the test tube. The temperature
at which the ball reaches the bottom of the test tube is
taken as Tgel of that system.
1
compound 3 in 78% yield (3.50 g, mp 114 °C). H NMR
(CDCl3, 400 MHz): δ 9.90 (s, 1H), 7.85 (d, 2H, J = 8 Hz), 7.01
(d, 2H, J = 8 Hz), 5.38 (d, 1H, J = 4 Hz), 4.75 (m, 1H), 4.68 (s,
2H), 2.35–0.67 (43H, cholesteryl protons); 13C NMR (CDCl3,

728
A. PANJA AND K. GHOSH
General procedures for UV–vis titrations
References
(1) (a) Steed, J.W. Chem. Soc. Rev. 2010, 39, 3686–3699; (b) J.W.
Steed Functional Molecular Gels. In Soft Matter; Escuder,
B.; Miravet, J.F., Eds.; The Royal Society of Chemistry,
Cambridge, UK, 2014; (c) Datta, S.; Bhattacharya, S.
Chem. Soc. Rev. 2015, 44, 5596–5637; (d) Yu, G.; Yan, X.;
Han, C.; Huang, F. Chem. Soc. Rev. 2013, 42, 6697–6722;
(e) Sangeetha, N.M.; Maitra, U. Chem. Soc. Rev. 2005, 34,
821–836; (f) Amabilino, D.B.; Smith, D.K.; Steed, J.W. Chem.
Soc. Rev. 2017, 46, 2404–2420; (g) Webber, M.J.; Langer, R.
Chem. Soc. Rev. 2017, 46, 6600–6620.
Stock solution of compound was prepared in DMF/H2O
(1:1, v/v) in the concentration of 2.50 × 10−5 M. Stock solu-
tions of metal ions were also prepared in the same sol-
vent in the concentration of 1.0 × 10−3 M. Then solution of
compound (2 mL) was taken in the cuvette and different
metal ions were individually added in different amounts.
Upon addition, the change in absorbance of compounds
was recorded.
(2) (a) Weiss, R.G.; Terech, P. Molecular Gels; Springer,
Dordrecht, 2006; (b) Fages, F. Low Molecular Mass Gelators,
Topics in Current Chemistry, Springer-Verlag, Berlin, 2005;
pp 256–283; (c) Smith, D.K. Molecular Gels-Nanostructured
Soft Materials. In Organic Nanostructures; Atwood, J.L. and
Steed, J.W., Eds.; Wiley-VCH, Weinheim, 2008; (d) Babu,
S.S.; Praveen, V.K.; Ajayaghosh, A. Chem. Rev. 2014, 114,
1973–2129; (e) Gale, P.A.; Busschaert, N.; Haynes, C.J.E.;
Karagiannidis, L.E.; Kirby, I.L. Chem. Soc. Rev. 2014, 43, 205–
241.
(3) (a) Piepenbrock, M.-O.M.; Lloyd, G.O.; Clarke, N.; Steed, J.W.
Chem. Rev. 2010, 110, 1960–2004; (b) Zhou, Y.; Zhang, J.F.;
Yoon, J. Chem. Rev. 2014, 114, 5511–5571; (c) Busschaert,
N.; Caltagirone, C.; Rossom, W.V.; Gale, P.A. Chem. Rev. 2015,
115, 8038–8155; (d) Kobaisi, M.; Bhosale, S.V.; Latham, K.;
Raynor, A.M.; Bhosale, S.V. Chem. Rev. 2016, 116, 11685–
11796; (e) Okesola, B.O.; Smith, D.K. Chem. Soc. Rev. 2016,
45, 4226–4251; (f) Tam, A.Y.-Y.; V.W.–W Yam. Chem. Soc. Rev.
2013, 42, 1540–1567; (g) Ren, C.; Zhang, J.; Chenb, M.;Yang,
Z. Chem. Soc. Rev. 2014, 43, 7257–7266; (h) Svobodova, H.;
Noponen, V.; Kolehmainen, E.; Sieva¨nen. E. RSC Adv. 2012,
2, 4985–5007; (i) Mehdi, H.; Pang, H.; Gong, W.; Dhinakaran,
M.K.; Wajahat, A.; Kuang, X.; Ning, G. Org. Biomol. Chem.
2016, 14, 5956–5964.
(4) (a) Vupputuri, S.; Longnecker, M.P.; Daniels, J.L.; Guo, X.;
Sandler, D.P. Environ. Res. 2005, 97, 194–199; (b) Baughman,
T.A. Environ. Health Perspect. 2006, 114, 147–152; (c) Hoyle,
I.; Handy, R.D. Aquat. Toxicol. 2005, 72, 147–159; (d) Onyido,
I.; Norris, A.R.; Buncel, E. Chem. Rev. 2004, 104, 5911–5930.
(5) (a) Risk Assessment, Management and Communication
of Drinking Water Contamination, U.S. Environmental
Protection Agency, Washington, DC: US EPA625/4-89/024,
EPA, 1989; (b) Saha, S.; Mahato, P.; Baidya, M.; Ghosh, S.K.;
Das, A. Chem. Commun. 2012, 48, 9293–9295.
Binding constant determination (14)
Benesi-Hildebrand plot was adopted to determine the
binding constant value using the expression: A0/(A − A0)
−1
= [εM/(εM – εC)](Ka−1Cg + 1), where εM and εC represent
molar extinction coefficients for the receptor and the com-
plex, respectively, at a selected wavelength, A0 denotes the
absorbance of free receptor at that specific wavelength
and Cg is the concentration of the guest. The measured
absorbance A0/(A − A0) as a function of the inverse of the
guest concentration fits a linear relationship, indicating
1:1 stoichiometry of the receptor guest complex. The ratio
of the intercept to the slope was used to determine the
binding constant Ka. Instead of fluorescence intensities,
respective absorbance of the receptor was also used to
get the binding constant values.
Calculation of detection limit (15)
Detection limit was calculated using the UV–vis titration
data. The absorbance of 3 was measured 5 times, and the
standard deviation of blank measurement was achieved.
To have the slope, absorbance values were plotted against
concentrations of Hg2+. The detection limits were calcu-
lated using the equation: Detection limit = 3σ/k, where σ
is the standard deviation of blank measurement, and k is
the slope.
(6) (a) Nolan, E.M.; Lippard, S.J. Chem. Rev. 2008, 108, 3443–
3480; (b) Renzoni, A.; Zino, F.; Franchi, E. Environ. Res. 1998,
77, 68–72.
(7) (a) Cizdziel, J.V.; Gerstenberger, S. Talanta 2004, 64, 918–
921; (b) Moreton, J.A.; Delves, H.T. J. Anal. At. Spectrom.
1998, 13, 659–665.
Acknowledgements
AP thanks CSIR, New Delhi, India for a fellowship. KG thanks
SERB, DST, New Delhi.
(8) (a) Kim, H.N.; Ren, W.X.; Kim, J.S.; Yoon, J. Chem. Soc. Rev.
2012, 41, 3210–3244; (b) Saleem, M.; Lee, K.H. RSC Adv.
2015, 5, 72150–72287; (c) Kim, H.N.; Lee, M.H.; Kim, H.J.;
Kim, J.S.; Yoon, J. Chem. Soc. Rev. 2008, 37, 1465–1472; (d)
Yuan, L.; Lin, W.; Zheng, K.; He, L.; Huang, W. Chem. Soc. Rev.
2013, 42, 622–661; (e) Du, J.; Hu, M.; Fan, J.; Peng, X. Chem.
Soc. Rev. 2012, 41, 4511–4535; (f) Mahato, P.; Saha, S.; Das,
P.; Agarwalla, H.; Das, A. RSC Adv. 2014, 4, 36140–36174.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the SERB, DST, New Delhi [grant
number EMR/2016/008005/OC].

SUPRAMOLECULAR CHEMISTRY
729
(9) (a) Tang, Y.; Lee, D.; Wang, J.; Li, G.; Yu, J.; Lin, W.; Yoon, J.
Chem. Soc. Rev. 2015, 44, 5003–5015; (b) Song, K.C.; Kim,
J.S.; Park, S.M.; Chung, K.C.; Ahn, S.; Chang, S.K. Org. Lett.
2006, 8, 3413–3416; (c) Ahamed, B.N.; Ghosh, P. Dalton
Trans. 2011, 40, 12540–12547; (d) Bhalla, V.; Roopa; Kumar,
M.; Sharma, P.R.; Kaur, T. Dalton Trans. 2013, 42, 15063–
15068; (e) Kumar, A.; Dubey, M.; Pandey, R.; Gupta, R.K.;
Kumar, A.; Kalita, A.C.; Pandey, D.S. Inorg. Chem. 2014, 53,
4944–4955; (f) Saha, S.; Agarwalla, H.; Gupta, H.; Baidya,
M.; Suresh, E.; Ghosh, S.K.; Das, A. Dalton Trans. 2013, 42,
15097–15105; (g) Hua, J.; Hub, Z.; Liuc, S.; Zhangb, Q.; Gaoa,
H.-W.; Uvdal, K. Sens. Actuators B 2016, 230, 639–644; (h)
Rao, A.S.; Kim, D.; Wang, T.; Kim, K.H.; Hwang, S.; Ahn, K.H.
Org. Lett. 2012, 14, 2598–2601; (i) Zhang, X.; Xu, Y.; Guo, P.;
Qian, X. New J. Chem. 2012, 36, 1621–1625; (j) Cheng, X.; Li,
Q.; Li, C.; Qin, J.; Li, Z. Chem. Eur. J. 2011, 17, 7276–7281; (k)
Tanga, L.; Dinga, S.; Zhangb, X.; Zhonga, K.; Houa, S.; Bian,
Y.J. Photochem. Photobiol. 2017, 340, 15–20.
(11) (a) Ghosh, B.N.; Bhowmik, S.; Mal, P.; Rissanen, K. Chem.
Commun. 2014, 50, 734–736; (b) Hemamalini, A.; Das, T.M.
New J. Chem. 2013, 37, 2419–2425; (c) Lu, J.; Hu, J.; Song, Y.;
Ju, Y. Org. Lett. 2011, 13, 3372–3375; (d) Ghosh, K.; Panja, A.;
Panja, S. New. J. Chem. 2016, 40, 3476–3483.
(12) (a) Panja, S.; Bhattacharya, S.; Ghosh, K. Langmuir 2017, 33,
8277–8288; (b) Ghosh, K.; Panja, S. Chemistry Select 2017,
2, 959–966; (c) Ghosh, K.; Panja, S. Chem. Select. 2016,
1, 3667–3674; (d) Ghosh, K.; Panja, S. RSC. Adv. 2015, 5,
12094–12099; (e) Ghosh, K.; Kar, D.; Panja, S.; Bhattacharya,
S. RSC Adv. 2014, 4, 3798–3803; (f) Ghosh, K.; Panja, S.;
Bhattacharya, S. RSC. Adv. 2015, 5, 72772–72779; (g) Ghosh,
K.; Panja, S. Supramol. Chem. 2017, 29, 350–359.
(13) Rao, A.S.; Kim, D.; Wang, T.; Kim, K.H.; Hwang, S.; Ahn, K.H.
Org. Lett. 2012, 14, 2598–2601.
(14) Chou, P.T.; Wu, G.R.; Wei, C.Y.; Cheng, C.C.; Chang, C.P.; Hung,
F.T.J. Phys. Chem B 2000, 104, 7818–7829.
(15) Sarkar, A.R.; Heo, C.H.; Park, M.Y.; Lee, H.W.; Kim, H.M. Chem.
Commun. 2014, 50, 1309–1312.
(10) (a) Lin, W.Y.; Yuan, L.; Tan, W.; Feng, J.B.; Long, L.L. Chem. Eur.
J. 2009, 15, 1030–1035; (b) Mahapatra, A.K.; Roy, J.; Sahoo,
P. Tetrahedron Lett. 2011, 52, 2965–2968; (c) Cheng, X.H.;
Li, Q.; Qin, J.G.; Li, Z. ACS Appl. Mater. Interfaces 2010, 2,
1066–1072.