Histidine–dialkoxyanthracene dyad for selective and sensitive detection of mercury ions

Article abstract of DOI:10.1080/10610278.2017.1412438

Histidine-dialkoxyanthracene (HDA) was synthesised as a turn off type fluorescent sensor for fast and sensitive detection of mercury ions (Hg²⁺) in aqueous media. The two histidine moieties act as ‘claws’ to selectively complex Hg²⁺. The binding ratio of HDA to Hg²⁺ was 1:1 (metal-to-ligand ratio). The association constant for Hg²⁺ towards the receptor HDA obtained from Benesi–Hildebrand plot was found to be 3.22?×?10⁴?M^?1 with detection limit as low as 4.7?nM (0.94?μg/L).

Full text of DOI:10.1080/10610278.2017.1412438

Supramolecular Chemistry
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Histidine–dialkoxyanthracene dyad for selective
and sensitive detection of mercury ions
Sachin Patil, Widad Belhajjame, Basem Moosa & Niveen M. Khashab
To cite this article: Sachin Patil, Widad Belhajjame, Basem Moosa & Niveen M. Khashab
(2017): Histidine–dialkoxyanthracene dyad for selective and sensitive detection of mercury ions,
Supramolecular Chemistry, DOI: 10.1080/10610278.2017.1412438
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Date: 08 January 2018, At: 19:03

Supramolecular chemiStry, 2017
https://doi.org/10.1080/10610278.2017.1412438
Histidine–dialkoxyanthracene dyad for selective and sensitive detection of
mercury ions
Sachin Patil, Widad Belhajjame, Basem Moosa and Niveen M. Khashab
Smart hybrid materials(Shms) laboratory, advanced membranes and porous materials center (ampmc), King abdullah university of Science
and technology(KauSt), makkah, Kingdom of Saudi arabia
ARTICLE HISTORY
ABSTRACT
received 27 april 2017
accepted 27 November 2017
Histidine-dialkoxyanthracene (HDA) was synthesised as a turn off type fluorescent sensor for fast
and sensitive detection of mercury ions (Hg2+) in aqueous media. The two histidine moieties act as
‘claws’ to selectively complex Hg2+. The binding ratio of HDA to Hg2+ was 1:1 (metal-to-ligand ratio).
The association constant for Hg2+ towards the receptor HDA obtained from Benesi–Hildebrand plot
was found to be 3.22 × 104 M−1 with detection limit as low as 4.7 nM (0.94 μg/L).
KEYWORDS
Fluorescent probe; mercury
ions; anthracene; histidine
1. Introduction
Design of fluorescent receptors for mercury has
attracted much attention due to the ease of detection
by simple analysis on a common fluorescence spectro-
photometer (7–9). Fluorescence spectroscopy has several
advantages over other methodologies due to its high sen-
sitivity, ease of visualization and short response time for
detection (10). A number of receptors that reliably detect
low concentrations of Hg+2 have been utilized (11). Some
of these receptors, however, show insufficient selectivity,
or need long time for efficient detection. Therefore, it is
important to explore new molecules or chemo-sensors
for the detection of Hg2+ by utilizing their chromogenic,
fluorogenic, or electrochemical properties (12–18).
Mercury ions are very toxic to the human body as they can
damage the brain, nervous system, endocrine system, and
other biological systems quite severely at high concen-
trations (1, 2). Mercury safety limit (in any form) in drink-
able water is set at 0.002 mg L−1 by the US Environmental
Protection Agency (EPA) (3). Mercury pollution is a world
wild problem and many sources of this pollution have
been reported (4). Inorganic mercury, Hg(0) and Hg2+, is
released into the environment by different means such
as coal and gold mining, solid waste incineration, wood
pulping, fossil fuel combustion, and chemical manufactur-
ing (5, 6). Several analytical methods with low detection
limits and excellent selectivity are known to trace mercury
but require bulky instruments and need tedious sample
pre-treatments (4).
Due to its strong luminance and chemical stability,
the anthracene unit has been widely used in the devel-
opment of fluorescent sensors (19–22). Several groups
CONTACT Niveen m. Khashab
niveen.khashab@kaust.edu.sa
this supplemental data for these article can be accessed at https://doi.org/10.1080/10610278.2017.1412438.
© 2017 informa uK limited, trading as taylor & Francis Group

2
S. PATIL ET AL.
have been involved in the synthesis and photochemical
characterisation of chemosensors containing a anthracene
fluorophore and amino acid receptor units (23). Among
the amino acids, L-histidine is one of the strongest met-
al-coordinating ligands and plays an important role in the
binding of metal ions to proteins. L-Histidine has amino,
imidazole, and carboxylate groups as three potential metal
binding sites. At physiological pH, the tridentate chelation
of L-histidine ligand has been established for bis-metal
complexes of Co+2, Ni+2, Zn+2, and Cd+2 (24–26). Herein, we
report a histidine derivative bearing an anthracene unit,
Histidine-dialkoxyanthracene (HDA), which functions as a
cheap, sensitive and efficient fluorescence sensor for Hg2+.
that HDA crystallized in monoclinic space group P21 (Table
S1) (27). The corresponding details of the structure deter-
mination and refinement data are summarised in Table S1
(Supplementary Information, available online). However,
all our efforts to obtain a suitable single crystal of HDA
and Hg2+ complexes for X-ray diffraction analysis were
unsuccessful. Refinement parameters of HDA (Cambridge
Crystallographic Data Centre as entry CCDC 1442746):
2.3. Competitive ion titration
We then proceeded to examine the selectivity of the che-
mosensor. The selectivity of HDA to the various metal ions
was tested as selectivity, which is an important character-
istic feature of an ion-selective chemosensor. HDA shows
strong fluorescence emission at 431 nm when excited at
365 nm. The selectivity of the chemosensor HDA for the
recognition of Hg2+ was studied by fluorescence spectros-
copy (Figure 2). The titration of HDA by Na+, K+, Ca2+, Zn2+,
Mn2+, Fe2+, Co2+, Hg2+, Ni2+, Cu2+, Cd2+, Pb2+, Al3+, Ag+ and
Mg2+ in HEPES at pH 7.34 containing 0.1% DMSO exhib-
ited no change in the fluorescence intensity of HDA in the
presence of different ions studied except for Hg2+. Blue
emission color of HDA was converted into colorless by
addition of Hg2+ (Figure 3). These results suggest that the
HDA recognises Hg2+ in the presence of all the other ions.
A small fluorescence quenching observed when HDA
was associated with Cu2+ > Zn2+ > Al+3 > Bi+2 > Pb+2 ions can
be considered less significant as none of the other transi-
tion metals ions interfered in the detection of mercury in
aqueous medium. The quenching effects could be attrib-
uted to enhanced spin–orbit coupling and electron trans-
fer mechanisms, respectively (28). The other tested metal
ions did not induce significant change or little change. The
interference experiments established reinforced impres-
sive selectivity for Hg2+ ions over other metal ions.
2. Result and discussion
2.1. Synthesis
Fluorescent HDA was synthesised starting from non-flu-
orescent anthraquinone (AQ) (Scheme 1) (27). Briefly, AQ
was first reduced by sodium dithionite and alkylated in
situ with t-butyl bromoacetate in an immiscible mixture of
water and CH2Cl2 in the presence of a phase transfer cata-
lyst. The resulting t-butyl protected ester was hydrolysed
with trifluoroacetic acid (TFA) to obtain diacid 2 (Scheme 1)
with 55% yield over two steps. Then diacid 2 was coupled
with two equivalents of histidine methyl ester 1 to obtain
HDA (detailed synthesis is described in experimental sec-
tion). Chemical structure of HDA was confirmed by 1H and
13C NMR spectra.
2.2. X-ray crystallography
Crystals of HDA were obtained by recrystallization from
N,N-dimethylformamide. ORTEP plot derived from the sin-
gle X-ray crystal structure of HDA is presented in Figure 1.
Single-crystal X-ray diffraction (SCXRD) analysis showed
Figure 1. (colour online) ortep plots of hDa with 50% probability ellipsoids. crystals are obtained in DmF.

SUPRAMOLECULAR CHEMISTRY
3
120
100
80
60
40
20
0
700
600
500
400
300
200
100
0
HDA
HDA + Hg2+
400
450
500
550
600
Wavelength (nm)
100
80
60
40
20
0
Figure 2. (colour online) (a) Fluorescence spectra of hDa(7 μm) and hDa with hg(ii)(30 μm); (b) Fluorescence intensity changes upon
addition of different metal ions; (c) Fluorescence intensity changes upon addition of different metal ions with hg(ii) (16 μm). all the
experiments done in 1% DmSo with hepeS buffer at ph 7.4; excitation wavelength is 380 nm.
Figure 3. (colour online) (a) Fluorescence spectra hDa(7 μm) and hDa with hg(ii)(2–30 μm); (b) Binding curve of hDa with hg(ii and (c)
photographs of solution of hDa and hDa with hg(ii) under 365 nm uV light. all the experiments done in 1% DmSo with hepeS buffer at
ph 7.4, excitation wavelength is 380 nm. (d) Benesi–hildebrand plot from fluorescence titration of hDa with hg(ii).
To make a thorough inquiry of the utility of chemosen-
sor HDA as an ion-selective chemosensor for Hg+2, com-
petitive experiments were carried out in the presence of
fluorescence spectra in the 1 % DMSO (Figure 2). The results
of the studies have revealed that these competing ions
exerted no or little influence on the fluorescence emission
spectra (Figure 2) of chemosensor HDA with Hg+2, which
further indicated that HDA has specific selectivity to Hg+2.
+2
Hg and various other ions (Cu+2, Co+2, Ni+2, Fe+3, Fe+2,
Zn+2, Cd+2, Tl+3, In+3, Al+3, Rh+3, Cr+3, Pb+2 and Bi+3) with the

4
S. PATIL ET AL.
2.4. Metal ion titrations
3. Conclusion
To understand the binding of metal ions with HDA, we
measured the fluorescent spectrum of HDA in aqueous
solution (HEPES at pH 7.34) containing 0.1% DMSO (Figure
3). Without metal ions, HDA exhibits strong fluorescence
emission at 431 nm when excited at 365 nm. Titration of
this with Hg2+ results in a gradual quenching of the fluo-
rescence emission at 431 nm.
The binding constant affinities of HDA with Hg2+ is
determined from the emission intensity data following the
modified Benesi–Hildebrand equation (29, 30), 1//ΔI = 1//
ΔI max + (1/K[C]) (1//ΔI max). Here ΔI = I–Imin and ΔI
max = Imax–Imin, where Imax, I, and Imin are the emission
intensities of HDA considered in the absence of Hg2+, at an
intermediate Hg2+ concentration, and at a concentration of
complete saturation where K is the binding constant and
[C] is the Hg2+ concentration respectively. From the plot
of [1 / (I–Imin)] against [C]–1 for Hg2+, the value of K has
been determined from the slope. The association constant
(Ka) as determined by fluorescence titration method for
Hg2+ with HDA is found to be 3.22 × 104 M−1. The minimum
detection limit of Hg2+ by HDA is found to be 4.7 nM, as
derived on the basis of fluorescence studies.
To understand the stoichiometry of the HDA and Hg2+
complex and evaluate how many Hg2+ molecules bind one
HDA structure. We carried out the fluorescence measure-
ments by varying mole fractions of Hg2+ and HDA in the
solution, keeping the sum of the Hg2+ and HDA concen-
trations constant. The fluorescence of the solutions thus
obtained was correlated according to Job’s plots. A typical
plot for a Hg2+ and HDA structure is shown 1:1 complex
stoichiometry formed between HDA with Hg2+ (Figure 4).
HDA has been synthesised and evaluated for recogni-
tion of cations. It behaves as a selective chemosensor
with excellent selectivity and sufficient sensitivity for the
detection of Hg2+ ions in aqueous system. Job’s plot con-
firms the 1:1 complex stoichiometry formed between HDA
with Hg2+ plot determined by fluorescence. The emission
studies clearly suggest that the Hg2+ binds to HDA and
the association constant is 3.22 × 104 M−1. HDA can detect
Hg2+ up to ~4.7 nM by fluorescence, suggesting its appli-
cability to detect Hg2+ ions at environmentally relevant
concentrations.
4. Experimental
4.1. Synthesis of Histidine methyl ester
To a solution of L-histidine (4 g; 26.2 mmol) in methanol,
thionyl chloride (2.4 mL; 32.1 mmol) was added dropwise
at 0 °C. The reaction was refluxed for 16 h. After completion
of the reaction the mixture was concentrated to dryness
and co-evaporated three times with methanol to yield
L-histidine methyl ester 1 as a white powder (5.9 g; 94.5%);
1HNMR (500 MHz,: δ = 8.79 (s, 1H); 7.39 (s, 1H); 4.29 (m, 1H);
3.71 (s, 3H); 3.29 (m, 2H): 13CNMR δ = 169.12, 135.06, 127.35,
54.17, 52.07, 25.87.
4.2 Synthesis of HDA-COOH
Anthraquinone (1.29 g, 6.18 mmol, 1 eq.), Adogen 464
(2.32 g, 5.00 mmol) were dissolved in degassed water
(75 mL) and CH2Cl2 (75 mL) under argon and stirred for
twenty minutes at room temperature followed by addi-
tion of sodium dithionite (2.15 g, 12.4 mmol, 2 eq.) and
stirred for an additional 30 min. After the addition of NaOH
(2.47 g, 61.8 mmol, 10 eq.) and ten more minutes of stir-
ring, tert-butyl-bromoacetate (5.89 g, 30.9 mmol, 5 eq.) was
added dropwise. The reaction was then stirred overnight.
After separating the organic phase, the aqueous phase
was extracted three times with CH2Cl2 (50 mL). The joined
organic fractions were washed with water and dried over
anhydrous MgSO4. The solvent was then removed under
reduced pressure. Subsequently, trifluoroacetic acid (TFA,
30 mL) was added and the mixture was stirred for three
hours at room temperature. After complete deprotection,
the pH of the solution was adjusted to 14 by carefully add-
ing aqueous NaOH. After washing three times with CH2Cl2
(50 mL), the pH of the aqueous phase was adjusted to 0–1
with HCl. A precipitate formed was separated by filtration
and washed with a small amount of HCl. After removing
the solvent under reduced pressure, diacid 2 was obtained
as a yellow powder (1.12 g, 3.43 mmol, 55.5 %).
Figure 4. Job’s plot of hDa complex with hg(ii) in hepeS buffer
1 % DmSo measured at 431 nm, give rise of 1:1 complexation
stoichiometry, total concentration of hDa-hg2+ was kept same as
100 mm.

SUPRAMOLECULAR CHEMISTRY
5
3.10–3.09 (m, 4H), 13C NMR (125.7 MHz, MeOD+CDCl3)
δ 171.69, 168.93, 145.97, 135.29, 126.17, 124.5, 121.87,
73.24, 52.62, 52.02, 29.02. HRMS (Orbitrap-ESI): calcd for
C32H32N6O8 [M+H]+ 629.2390, found 629.2382.
Acknowledgements
We gratefully acknowledge King Abdullah University of Science
and Technology (KAUST) for financial support.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the King Abdullah University of Sci-
ence and Technology (KAUST).
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