Anthroneamine based chromofluorogenic probes for Hg2+ detection in aqueous solution

Article abstract of DOI:10.1016/j.tetlet.2012.01.134

Anthroneamine derivatives 1-3 (H₂O:DMSO; 9:1, HEPES buffer, pH 7.0 ± 0.1) undergo highly selective fluorescence quenching with Hg ²⁺. The observed linear fluorescence intensity change allows the quantitative detection of Hg²⁺ between 200 nM/40 ppb - 12 μM/2.4 ppm even in the presence of interfering metal ions viz. Na⁺, K ⁺, Mg²⁺, Ca²⁺, Ba²⁺, Cr ³⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu ²⁺, Zn²⁺, Ag⁺, Cd²⁺, Pb ²⁺. Probes 1-3 and their Hg²⁺ complexes also show the broad pH resistance for their practical applicability.

Full text of DOI:10.1016/j.tetlet.2012.01.134

Tetrahedron Letters 53 (2012) 2030–2034
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Tetrahedron Letters
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Anthroneamine based chromofluorogenic probes for Hg2+ detection
in aqueous solution
⇑
Ashwani Kumar, Subodh Kumar
Department of Chemistry, UGC-Center for Advance Studies, Guru Nank Dev University, Amritsar 143 005, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Anthroneamine derivatives 1–3 (H2O:DMSO; 9:1, HEPES buffer, pH 7.0 0.1) undergo highly selective
fluorescence quenching with Hg2+. The observed linear fluorescence intensity change allows the quanti-
Received 6 December 2011
Revised 30 January 2012
Accepted 31 January 2012
Available online 14 February 2012
tative detection of Hg2+ between 200 nM/40 ppb—12
lM/2.4 ppm even in the presence of interfering
metal ions viz. Na+, K+, Mg2+, Ca2+, Ba2+, Cr3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Pb2+. Probes 1–3 and
their Hg2+ complexes also show the broad pH resistance for their practical applicability.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Fluorescence
Anthroneamine
Hg2+ chemosensor
Aqueous buffer
The development of artificial chemosensors for both qualitative
and quantitative recognition of environmentally and biologically
important species is of great interest. In this regard, the chemosen-
sors which can selectively sense the heavy metal ions such as mer-
cury, lead, and copper are of great interest.1 Mercury ions can
easily pass through biological membranes and cause serious dam-
age to the central nervous and endocrine systems.2 Hg2+ ions are
considered highly dangerous because both elemental and ionic
mercury can be converted into methyl mercury by bacteria in the
environment, which subsequently bio-accumulates through the
food chain.3 So, considerable efforts have been made to synthesize
the fluorescent chemosensors that can sense the mercury ions in a
selective and sensitive manner under the physiological
conditions.4
Among, the various molecular architectures, the thioether con-
taining crown ethers/acetals,5 podands,6 thioureas,7 amines/
amides,8 spirolactones,9 heterocycles based moieties10 etc. appro-
priately appended with chromogenic and fluorescent moieties
have found applications in developing Hg2+ sensors. Most of these
probes sense Hg2+ ion in a non-reversible (chemodosimeters) man-
ner selectively and others in a reversible (chemosensors) manner
usually non-selectively, and show some interference from either
Cu2+/Ag+. The sensitivity of such types of probes is significantly
affected by the percentage of water or effect of pH. Even though
a few fluorescent Hg2+ probes with good water solubility, high
selectivity and sensitivity are reported. As far as quantitative prac-
tical Hg2+ detection is concerned, a linear fluorescence response,
uniform fluorescence output at a broad pH range, compatibility
with aqueous medium, higher selectivity, sensitivity, fast response
and easy synthetic procedures of probes are most important. The
Hg2+ probes reported so far have achieved only a few of the above
mentioned criteria and do not satisfy all the desired features.So
keeping in mind all these facts, new anthroneamine based probes
1–3 have been synthesized which show highly selective and sensi-
tive reversible colorimetric and fluorescence behavior to Hg2+ ions
under aqueous conditions (H2O:DMSO; 9:1; HEPES buffer, pH
7.0 0.1). The presence of electron-withdrawing bromine atoms
on aromatic ring and alkyl chain on amide nitrogen tunes the sen-
sitivity and pH applicability ranges of probes toward Hg2+ ions.
The commercially available 1-aminoanthracene-9,10-dione (4)
on reaction with bromine in acetic acid at rt gave the respective di-
bromo derivative 5, (98%), red solid, mp 240 °C. Compound 6 was
obtained by heating 1-chloro-anthracene-9,10-dione with n-butyl-
amine in DMSO. The stirring of suspension of 4–6 in acetonitrile
with 2-chloroacetylchloride (2 equiv) at 45–50 °C in the presence
of K2CO3 gave the respective 1-chloroacetamideanthracene-9,10-
diones 7–9, yellow colored solid, (>90%). The reaction of 7 with pyr-
idine (2 equiv) in DMF on water bath for 6 h followed by hydrolysis
with ethanol-morpholine mixture under reflux conditions gave yel-
lowish-green solid 1 (85%)11 (Scheme 1). Similarly, reactions of
compounds 8 and 9 with pyridine followed by heating with etha-
nol-morpholine mixture gave respective probes 212 and 313 in
respective 60% and 85% yields. Probes 1–3 and intermediates were
characterized using 1H NMR, 13C NMR, IR, HRMS, and elemental
analysis techniques. All the intermediate compounds and the final
probes could be purified by crystallization and did not require col-
umn chromatography.
⇑
Corresponding author.
E-mail address: subodh_gndu@yahoo.co.in (S. Kumar).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.01.134

A. Kumar, S. Kumar / Tetrahedron Letters 53 (2012) 2030–2034
2031
The presence of electron-withdrawing halogen atoms in probe 2
(kmax = 430 nm; = 10800) causes hypsochromic shift of its elec-
tronic spectrum in comparison to that of probes 1 (kmax = 449 nm;
= 11950) and 3 (kmax 450 nm; = 13100). However, probes 1 and
2 on excitation at 435 nm show kem at 533 nm and 540 nm, respec-
tively. Probes 1 and 3 have same high quantum yields ( = 0.53),
but probe 2 ( = 0.067) probably due to the heavy atom effect of
bromine atoms has a poor quantum yield.
Probe 1 (20 M H2O: DMSO; (9: 1), HEPES buffer pH 7.0 0.1),
in its UV–vis spectrum showed absorbance maxima at 449 nm
= 11950) which on addition of Hg2+ ions (20 ppm/100
M)
ꢀ
ꢀ
ꢀ
U
U
l
(ꢀ
l
underwent bathochromic shift at 522 nm associated with the
naked eye visible color change from yellow-green to pink (Fig. 1).
The addition of other metal ions viz. Na+, K+, Mg2+, Ca2+, Ba2+
,
Figure 1. Effect of addition of Hg2+ and other metal ions on UV–vis spectrum and
Cr3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Pb2+ even up to
200 ppm/1 mM did not show any change in the UV-Vis spectrum
or color of the solution of probe 1.
visible color change in 1 (20
l
M, H2O: DMSO (9:1), HEPES buffer (pH 7.0 0.1) on
addition of Hg2+
.
On gradual addition of aliquots of Hg2+ ions, the absorbance of
0.12
450 nm
probe 1 (10 lM, H2O: DMSO (9:1); HEPES buffer pH 7.0 0.1) at
kmax = 450 nm decreased gradually with concomitant increase in
1
0.8
0.6
0.4
0.2
0
0.09
0.06
absorbance at 522 nm (Fig. 2a) and achieved a plateau above
12 ppm/60 l
M of Hg2+ with isobestic point at 500 nm. Spectral fit-
ting of the absorbance data using the nonlinear regression analysis
program SPECFIT- 32 shows the formation of 1:1 stoichiometric
complex with logbML = 5.18 0.06. Ratiometrically probe 1 can
522 nm
0.03
detect Hg2+ ions from 1
DMSO solution (Fig. 2b).
lM to 60 lM in 90% aqueous buffered
0
0
10 20 30 40 50 60
Hg2+ (µM)
The addition of different metal ions, viz Na+, K+, Mg2+, Ca2+, Ba2+
,
430 510
270 350
590 670
Cr3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Pb2+ to the solution of
[1+Hg2+ (5 equiv)] complex did not alter the absorbance intensity
obtained for [1 + Hg2+ (5 equiv)] the complex and points that these
metal ions do not interfere in the estimation of Hg2+ (Fig. SI 1). On
Wavelength (nm)
Figure 2. (a) Effect of gradual addition of Hg2+ ions on UV–vis spectrum of 1
(10 M, H2O: DMSO (9:1), HEPES buffer pH 7.0 0.1). (b) Absorbance ratiometric
response (A522/A450 of probe (10 M, H2O: DMSO (9:1), HEPES buffer pH
7.0 0.1) toward [Hg2+].
l
)
1
l
excitation at 435 nm, probe 1 (10
buffer pH = 7.0 0.1) gave an emission band with kmax 533 nm
= 0.53), which on addition of Hg2+ (4 ppm/20
M) was
lM, H2O: DMSO (9:1), HEPES
(U
l
quenched by >90%. Concomitantly, under illumination at 365 nm,
the bright green solution of probe 1 became dark on addition of
Hg2+ ions. The addition of other metal ions viz., Na+, K+, Mg2+
,
Ca2+, Ba2+, Cr3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Pb2+ even up
to 200 ppm/1 mM did not quench the fluorescence intensity of
probe 1 (Figs. 3 and 5). Therefore, probe 1 recognizes only Hg2+
among various metal ions studied here.
Upon gradual addition of Hg2+ to 1 (10
lM, H2O: DMSO (9:1),
HEPES buffer pH 7.0 0.1), the intensity of the emission band at
kmax 533 nm decreased gradually with an increase in the concen-
tration of Hg2+ (Fig. 4). The spectral fitting of the titration data
shows the formation of 1:1 stoichiometric complex (logbML
=
5.85 0.06). For the practical quantitative detection, fluorescence
spectral changes should vary linearly with the concentration of
Hg2+. A linear response of the fluorescence intensity as a function
Figure 3. Effect of addition of Hg2+ and other metal ions on fluorescence spectrum
of [Hg2+] was observed from 1–12
l
M (R = 0.997) (inset Fig. 4).
The competitive experiments conducted in the presence of Hg2+
(12
M) mixed with Na+, K+, Mg2+, Ca2+, Ba2+, Cr3+, Fe2+, Co2+, Ni2+
and visible color change of 1 (10
lM, H2O: DMSO (9:1), HEPES buffer pH 7.0 0.1)
under illumination at 365 nm.
l
,
Cl
O
O
H2N
R
N
O
O
NHR
X
O
O
RN
X
(i) Pyridine, DMF
X
X
ClCOCH2Cl
K2CO3, acn, rt
100°C
(ii) morpholine, ethanol
reflux
7, X = H, R = H (95%)
8, X = Br, R = H (95%)
9, X = H, R = n-Bu (90%)
X
O
X
1, X = H, R = H (85%)
2, X = Br, R = H (60%)
3, X = H, R = n-Bu (80%)
4, X = H, R = H,
5, X = Br, R = H,
6, X = H, R = n-Bu,
Scheme 1. Synthesis of probes 1–3.

2032
A. Kumar, S. Kumar / Tetrahedron Letters 53 (2012) 2030–2034
Further in order to evaluate the role of amine NH2 and amide
NH in Hg2+ binding, probe 3 was synthesized. Probe 3 (20
M,
900
750
600
450
300
150
0
850
700
550
400
250
533 nm
l
H2O:DMSO; 9:1, HEPES buffer pH = 7.0 0.1), where amidic NH
of 1 has been converted to N-butyl moiety, on gradual addition
of Hg2+ ions showed similar absorbance and emission spectral
changes as shown by probe 1 (Figs. SI 7–9). These results clearly
point that the N-alkylation of amidic N does not affect the Hg2+ rec-
ognition behavior of probe 1 and thus amine and amide carbonyl
group mainly participate in Hg2+ recognition
R = 0.9975
0
2
4
6
8
10 12
Hg2+ (µM)
For the rapid monitoring of aqueous Hg2+ in environmental or
biological samples, the fluorescence intensity should be resistant
to changes in pH that may occur in unbuffered natural systems
or in the analysis of samples from acidic or basic environments.
The changes in the fluorescence intensity of probes 1-3 were mon-
itored at solution pH values ranging 2–13, within which most bio-
logical samples can be tested. An analysis of pH induced changes in
the fluorescence intensity of probe 1 shows that 1 undergoes
deprotonation (monoanion) at pH P11 and protonation at pH 64
([1.H]+) both associated with fluorescence quenching. Therefore,
probe 1 exists as a neutral molecule between pH ranges of 4-11
(Figs. SI 10–11). Similar analysis of pH induced changes in the fluo-
rescence of probe 2 showed that it remains in the neutral form be-
tween pH 4-9 (Figs. SI 14–15). Probe 3, due to the absence of amide
NH moiety, showed enhanced basic pH stability as well as its prac-
tical applicability between the pH ranges of 4-12.5 (Fig. 7). There-
fore, the protection of amide NH with N-butyl group stabilizes the
probe over the broader pH range of 4-12 and the presence of elec-
tron-withdrawing bromo group in probe 2 facilitates the deproto-
nation of amide NH and thus affects their applicability over a
relatively lower pH range of 4-9.
470
510
550
Wavelength (nm)
590
63
0
670
Figure 4. Effect of gradual addition of Hg2+ ions on the fluorescence intensity of 1
(10 lM, H2O: DMSO 9:1, HEPES buffer pH 7.0 0.1). Inset shows the linear
relationship between FI and [Hg2+] (points refer to the experimental values).
The fluorescence-pH titration of 1-Hg2+ (1:10) complex also
2+
shows that probe 1 forms stable [1-Hg2+
]
complex between pH
4-10 (Figs. SI 12–13) and probe 2 between pH 4 - 9.0 (Figs. SI
16–17). In the case of probe 3 enhanced stability of [3-Hg2+] com-
plex between the broad pH range of 4–11.5 was observed (Fig. 8)
(Figs. SI 20–21). The uniform activity over such a wide range of
pH makes these molecules suitable for the analysis of environmen-
tal samples that would occur well within this extended range of
pH, or for use in unbuffered medium.
Figure 5. Effect of addition of individual metal ions and effect of interfering metal
ions in presence of Hg2+ on the fluorescence intensity of probe 1 (10
DMSO (9:1), HEPES buffer pH 7.0 0.1) (kmax 533 nm).
lM, H2O:
So, substituents on the aromatic ring tune the sensitivity of the
Cu2+, Zn2+, Ag+, Cd2+, Pb2+ (1 mM) show no significant variation in
the fluorescence intensity in comparison with that observed by
the addition of Hg2+ without the other metal ions (Fig. SI 2). These
results point toward the highly selective recognition of Hg2+ and its
applicability in the presence of other metal ions (Fig. 5). To exam-
ine further the reversibility of the processes, 10 equiv of EDTA was
added to the solutions of probe 1 complexed with Hg2+. The bright
green fluorescence immediately turned on (fluorescence) and the
yellow-green color (UV–vis. change) of the solution reappears
(Fig. SI 3). This result implies the reversible character of binding
probes and the sensing range of 1–12
lM for probe 1 is further en-
hanced to 200 nM–4 M for probe 2 upon the substitution of bro-
l
mo group on aromatic ring. Whereas, alkylation of amidic nitrogen
of sensor 1 with Hg2+
Probe 2 (20
.
l
M, H2O: DMSO; 9:1, HEPES buffer pH = 7.0 0.1),
on gradual addition of Hg2+ ions showed gradual decrease in absor-
bance at kmax 432 nm along with the formation of new absorption
band at 515 nm (Figs. SI 4–6). The spectral fitting of these titration
data shows the formation of only 1:1 stoichiometric complex
(logbML = 5.18 0.18). Probe 2 (10
buffer pH = 7.0 0.1), on excitation at 435 nm gave emission band
centered at 540 nm (
= 0.067). On gradual addition of Hg2+ to the
lM, H2O: DMSO; 9:1, HEPES
U
solution of 2, the intensity of the emission band at kmax 540 nm de-
creased gradually with increase in the concentration of Hg2+
(Fig. 6). The spectral fitting of these titration data shows the forma-
tion of ML2 and ML complexes (logbML2 = 13.31 0.24, logbML
=
6.85 0.10). Linear decrease in fluorescence showed that probe 2
Figure 6. Effect of gradual addition of Hg2+ ions on the fluorescence intensity of 2
can detect Hg2+ ions in the range of 200 nM (40 ppb) – 4
(0.8 ppm).
lM
(10
lM, H2O: DMSO (9:1), HEPES buffer pH 7.0 0.1). Inset shows the linear
relationship between FI and [Hg2+] (points refer to the experimental results).

A. Kumar, S. Kumar / Tetrahedron Letters 53 (2012) 2030–2034
2033
References and notes
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1
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200
100
0
2
3
4
5
6
7
8
pH
9
10 11 12 13 14
Figure 8. Effect of pH on fluorescence intensity of Hg2+ complexes of probes 1–3
(H2O:DMSO (9:1)), (points refer to the experimental results).
tunes the pH range for the practical sensing of Hg2+ ions i.e. probe 1
can detect Hg2+ within the pH range of 4–10, and probe 3 with
extended pH range of 4–11.5.
Thus, we have developed new anthroneamine based colorimet-
ric and fluorescent probes 1–3 for selective and sensitive Hg2+
detection in aqueous medium. The ability of these probes to func-
tion in un-buffered aqueous solution with linear and fast fluores-
cence response will be useful for the rapid and quantitative
detection of Hg2+ in environmental samples. Probes 1 and 3 find
their applicability over a broad pH range of 4–12 with lowest
detectable limits between 1 lM (0.2 ppm)––2 lM (0.4 ppm). The
presence of electronegative bromine increases the sensitivity ten
times for probe 2 toward Hg2+ with the lowest detection limit of
200 nM (40 ppb). Further, the effect of substituents and extension
of aromatic ring to increase the sensitivity toward Hg2+ ions is in
progress.
11. Anthroneamine (1): Dark yellow solid. Mp >250oC, HRMS (m/z +Na) = 285.0687,
(Theoretical (m/z + Na) = 285.0640), IR vmax (KBr) 1595, 1635, 1671, 2857,
3305, 3422. 1H NMR (DMSO-d6 + CDCl3, 300 MHz): d 6.97 (s, 2H, NH2), 7.47 (t,
J = 7.8 Hz, 1H, ArH), 7.57 (t, J = 7.5 Hz, 1H, ArH), 7.65 (d, J = 7.8 Hz, 1H, ArH),
7.83 (t, J = 7.8 Hz, 1H, ArH), 8.16 (d, J = 7.8 Hz, 1H, ArH), 8.46 (d, J = 7.8 Hz, 1H,
ArH), 8.64 (d, J = 8.1 Hz, 1H, ArH), 12.35 (s, 1H, NH); 13C NMR (CDCl3+TFA,
75 MHz): d 108.58, 112.37, 116.14, 119.82, 122.67, 125.82, 126.57, 127.02,
129.15, 129.66, 131.15, 135.05. Found: C, 73.31; H, 3.93; N, 10.72%. C20H18
N2O2 requires C, 73.27; H, 3.84; N, 10.68%.
Acknowledgments
We thank DST, New Delhi for the financial assistance and FIST;
UGC, New Delhi for CAS and fellowship to AK; CDRI Lucknow and
the IISc Bangalore for mass spectra.
12. 2,4-Dibromoanthroneamine (2): Dark brown solid. Mp >260 °C HRMS (m/z
+Na) = 440.8844, 442.8924, 444.8894 (1:2:1), (Theoretical (m/z
+
Na) = 440.8850), IR vmax (KBr) 1590, 1664, 2855, 3033, 3162, 3405. 1H NMR
(DMSO-d6, 300 MHz): d 7.54–7.59 (doublet merged with broad NH2 signal, 3H,
1ArH & NH2), 7.79 (t, J = 7.8 Hz, 1H, ArH), 7.99 (s, 1H, ArH), 8.21 (d, J = 7.8 Hz,
1H, ArH), 8.49 (d, J = 8.1 Hz, 1H, ArH), 9.78 (s,1H, NH). 13C NMR (DMSO-d6,
75 MHz): d 103.06, 112.74, 115.27, 117.42, 122.53, 122.79, 125.96, 126.90,
127.40, 128.31, 131.41, 133.11, 133.50, 139.32, 157.99, 180.56. Found: C,
45.82; H, 1.98; N, 6.72%. C20H18 N2O2 requires C, 45.75; H, 1.92; N, 6.67%.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2012.01.134.

2034
A. Kumar, S. Kumar / Tetrahedron Letters 53 (2012) 2030–2034
13. N-Butylanthrone amine (3): Yellow needle like crystalline solid. Mp = 118 °C,
HRMS (m/z +Na) = 341.1268, (Theoretical (m/z +Na) = 341.1266), IR vmax (KBr)
1581, 1649, 2866, 3329. 1H NMR (CDCl3, 400 MHz): d 1.04 (t, J = 7.2 Hz, 3H,
CH3), 1.52–1.57 (m, 2H, CH2), 1.82–1.84 (m, 2H, CH2), 4.46 (t, J = 7.6 Hz, 2H,
CH2), 5.94 (s, 2H, NH2), 7.56–7.58 (m, 2H, 2Â ArH), 7.62 (t, J = 8.4 Hz, 1H, ArH),
7.77 (d, J = 8.4 Hz, 1H, ArH), 8.36 (d, J = 8.4 Hz, 1H, ArH), 8.53 – 8.59 (m, 2H, 2 x
ArH); 13C NMR (CDCl3, 100 MHz): d 13.77, 20.39, 30.02, 43.73, 107.12, 118.24,
121.12, 123.36, 125.75, 127.79, 128.29, 128.86, 132.08, 132.57, 133.22, 135.77,
137.07, 158.65, 182.56. Found: C, 75.31; H, 5.81; N, 8.85%. C20H18 N2O2 requires
C, 75.45; H, 5.70; N, 8.80%.