The synthesis and development of a dual-analyte colorimetric sensor: Simultaneous estimation of Hg2+ and Fe3+

Article abstract of DOI:10.1016/j.dyepig.2010.07.012

The selectivity and sensitivity of a pH stable hetarylazo dye equipped with binding sites consisting of N, S and N, O combinations, towards Hg²⁺ and Fe³⁺ over a large number of other cations including lanthanides, is described. Hg²⁺ and Fe³⁺ ions coordinate to the dye through N, S binding site forming 2:1 and 1:1 complexes, respectively. Distinct naked-eye color changes for both Hg²⁺ (yellow to purple) and Fe ³⁺ (yellow to red) were quantified for simultaneous estimation of these ions. To further establish the binding and sensing phenomenon, another analogous hetarylazo dye lacking N, O combination was evaluated and found to give similar results. For practical applicability in routine life, very handy and ready-to-use paper strips coated with the dye have been prepared for the detection of Hg²⁺, just like the pH paper strips.

Full text of DOI:10.1016/j.dyepig.2010.07.012

Dyes and Pigments 88 (2011) 296e300
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The synthesis and development of a dual-analyte colorimetric sensor:
Simultaneous estimation of Hg2þ and Fe3þ
*
Paramjit Kaur , Divya Sareen
Department of Chemistry, Guru Nanak 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:
The selectivity and sensitivity of a pH stable hetarylazo dye equipped with binding sites consisting of N, S
and N, O combinations, towards Hg2þ and Fe3þ over a large number of other cations including lanthanides,
is described. Hg2þ and Fe3þ ions coordinate to the dye through N, S binding site forming 2:1 and 1:1
complexes, respectively. Distinct naked-eye color changes for both Hg2þ (yellow to purple) and Fe3þ
(yellow to red) were quantified for simultaneous estimation of these ions. To further establish the binding
and sensing phenomenon, another analogous hetarylazo dye lacking N, O combination was evaluated and
found to give similar results. For practical applicability in routine life, very handy and ready-to-use paper
strips coated with the dye have been prepared for the detection of Hg2þ, just like the pH paper strips.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 20 April 2010
Received in revised form
17 July 2010
Accepted 27 July 2010
Available online 6 August 2010
Keywords:
Hetarylazo dye
pH stable
Colorimetric sensor
Naked-eye
Simultaneous
Lanthanides
1. Introduction
analytes using such systems is generally straightforward as the
analysis of differential response of the analytes at different wave-
The field of molecular sensing has witnessed considerable
advancement as a myriad of different types of molecular sensors
have been designed and demonstrate a high degree of predictability
and selectivity towards a variety of ions and neutral species, from
aqueous as well as non-aqueous environments [1e5]. Such sensing
events are imperative for applications including environmental,
biological, clinical and industrial waste management regimes [6].
Recently, this area of investigation has observed a paradigm shift [7]
in the interest from selective receptors to differential receptors
supporting multi-parameter functionality on a single platform
(multi-analyte sensing) that has allowed rapid, sensitive, specific,
non-expensive, analysis of several identical analytes suitable for
realtime on-sight sensing which has been accomplished by such
mathematical systems as PCA, PLS, ANNs etc [8]. A very interesting
molecular architect “lab on a molecule” has been described for the
analysis of Pb2þ, Hg2þ and Cu2þ using a quadruple-channel sensing
[9]. However, inspite of the advantages in the simultaneous esti-
mation of the analytes using minimal sensing system, the multi-
analyte single molecular probes employing colorimetric [10] as well
as fluorescent [11] detection are only limited. Further, detection of
lengths does not require intricate mathematical systems, however,
detection of a multianalyte mixture, where the optical responses
are not fully resolved poses a challenge and such examples have
eluded the chemical literature. In continuation of our interest in the
development of colorimetric chemosensors [12], we envisaged, if
differential binding sites comprising of NS (for soft metal ions [13])
and NOO (for hard metal ions [14] and lanthanides [15]) are incor-
porated in a chromophore of the type 3, multianalyte sensing might
result. Recently, Suzuki et al. [14] have reported a unique molecular
system with similar binding sites which allowed determination
of Cu2þ, Fe3þ, Pb2þ, Al3þ and Cr3þ based on the nature of the
heteroatoms and their interaction with the cations.
Hetarylazo dyes are prized for their ease in preparation as well as
intense absorptions in the visible region and are thus ideal candi-
dates for colorimetric chemosensors. Indeed 3 shows differential
behaviour towards Hg2þ and Fe3þ out of a number of compeititive
cations tested and facilitates a naked-eye as well as colorimetric
detection of these two metal ions, simultaneously. The fact that
robust methods [16] based on the use of membrane ultrafilteration
technology allow selective removal of Fe3þ ions from dilute aqueous
solutions, 3 may well be visualized as a highly selective sensor for
the detection of Hg2þ ions which pose very serious environmental
as well as biological concerns [17]. A wide array of colorimetric [18]
and fluorescent [19] molecular sensors with specific response to
* Corresponding author. Tel.: þ91 183 2258853; fax: þ91 183 2258819 20.
E-mail address: paramjit19in@yahoo.co.in (P. Kaur).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.07.012

P. Kaur, D. Sareen / Dyes and Pigments 88 (2011) 296e300
297
Hg2þ as well as Fe3þ have been reported [20]. Additionally, the
detection process is facilitated, convincingly, without resorting to
complex mathematical models inspite of the fact that the optical
responses were not completely resolved. To further widen the scope
of sensor 3, we have also evaluated the binding with a number
of lanthanide cations, as 3 possesses two ester functionalities.
To establish the fact that the binding of 3 with Hg2þ involves N and
S donor sites, the dye 4, lacking ester functionalities was also
evaluated. Finally, we have explored the possibility of preparing
handy paper strips (“dip-stick”) coated with 3 for determining
Hg2þ, similar to the ones commonly used for pH determination.
30 ꢁC and heating the stirred mixture to 60e65 ꢁC to ensure
complete dissolution. The reaction mixture was cooled to 5 ꢁC and
acetic acid (8 ml) and propionic acid (2 ml) were added and stirred.
The temperature was then reduced to ꢀ5 ꢁC, 1 (2.02 g, 0.01 mol)
was added slowly and the whole reaction mixture stirred at the
same low temperature for at least 2 h. The completion of diazoti-
zation was checked by TLC.
A solution of 2a/2b (2.93 g, 0.015 mol) in acetic anhydride and
acetic acid was added to ice/water mixture (200 g) in a beaker
equipped with mechanical stirrer and pH meter. A pinch of sul-
phamic acid was added followed by the addition of diazonium salt
liquor at 0e5 ꢁC over 1 h and the mixture stirred for 3 h at ambient
temperature. The pH was maintained in the range 1.5e2.0 during the
addition using aqueous sodium hydroxide solution. The completion
of reaction was checked by spotting the reaction mixture on a filter
paper close to a spot of an alkaline aqueous solution of H-acid and
detecting any color appearance at the interface of the two bound-
aries. No color indicated the completion of reaction. Finally the pH
was raised to 4.0e4.5. The dye was extracted with dichloromethane,
dried over anhydrous sodium sulphate, the solvent removed and the
residue purified by column chromatography with silica gel G
(60e120 mesh) using 50% ethyl acetate/hexane as eluent.
2. Experimental
2.1. General
Mass spectra (LCMS) were recorded on Bruker Daltonics esquire
3000 spectrometer. 1H NMR and 13C NMR spectra have been recorded
on JEOL-FT NMR-AL at 300 and 75 MHz, respectively, with TMS as
internal standard using CDCl3 as deuterated solvent and few on
BRUKER Avance II 400 MHz and 100 MHz, respectively using CD3CN as
deuteratedsolvent. IRspectrawere recordedonFT IR-SHIMADZU8400
Fourier-transform spectrophotometer in the range 400e4000 cmꢀ1
using KBr as medium. The purity of the solid products was checked by
elemental analysis performed on Thermoelectron FLASH EA1112,
CHNS analyzer. UV-visible spectral studies were conducted on Shi-
madzu 1601 PC spectrophotometer. The absorption spectra have been
recorded between 1100 and 200 nm. The pH titrations have been
performed with the Equip-Tronics Digital pH meter model-EQ 610.
Dye 3: Yield 70%, m.p. 90e92 ꢁC, 1H NMR (300 MHz, CDCl3),
d
(ppm): 1.28 (t, J ¼ 7.2 Hz, 6H, 2 ꢂ COOCH2CH3), 2.68 (t, J ¼ 7.2 Hz,
4H, 2 ꢂ NCH2CH2), 3.83 (t, J ¼ 7.2 Hz, 4H, 2 ꢂ NCH2CH2), 4.17
(q, J ¼ 7.2 Hz, 4H, 2 ꢂ COOCH2CH3), 6.78 (d, J ¼ 9.3 Hz, 2H, aromatic
H), 7.97 (d, J ¼ 9.3 Hz, 2H, aromatic H), 9.02 (s, 1H, NCHS). 13C NMR
(75 MHz, CDCl3),
d (ppm): 14.14, 32.45, 46.87, 61.01, 111.88, 143.39,
151.35, 151.87, 171.29. IR (KBr): nmax/cmꢀ1 3050 (aromatic CH), 1745
(ester). MS: m/z 428 (Mþ þ 23). Elemental Analysis: Calcd. for
C18H23N5SO4: C, 53.33; H, 5.68; N, 17.28; S, 7.90 Found: C, 53.32; H,
5.67; N, 17.26; S, 7.89 (S1, Supplementary data).
2.2. Chemicals
Metal salts used in the spectrophotometric studies were of
analytical grade and bought from SigmaeAldrich. The solvents used
were also of analytical grade purchased from Thomas Baker.
Dye 4: Yield 70%, m.p. 138e140 ꢁC, 1H NMR(400 MHz, CD3CN),
d
(ppm): 1.24 (t, J ¼ 7.2 Hz, 6H, 2 ꢂ NCH2CH3), 3.55(q, J ¼ 7.2 Hz, 4H,
2 ꢂ NCH2CH3), 6.86 (d, J ¼ 9.6 Hz, 2H, aromatic H), 7.87 (d, J ¼ 9.6 Hz,
2H, aromatic H), 9.09 (s, 1H, NCHS). 13C NMR (100 MHz, CD3CN),
2.3. General procedure for synthesis of 3 and 4
d (ppm): 11.54, 44.56, 111.49, 141.79, 151.46, 152.77, 181.08. IR (KBr):
nmax/cmꢀ1 2950 (aromatic CH). MS: m/z 284 (Mþ þ 23). Elemental
Analysis: Calcd. for C12H15N5S: C, 55.17; H, 5.75; N, 26.82; S, 12.26
Found: C, 55.15; H, 5.75; N, 26.80; S,12.25 (S1, Supplementary data).
2.3.1. Synthesis of 1
Thiosemicarbazide (6.07 g, 1.0 mol) was added (below 25 ꢁC)
portion wise, to a stirred mixture of formic acid (4.06 g, 3.4 ml,
1.32 mol, 98e100%) and sulphuric acid (12.27 g, 6.7 ml, sp. gr. 1.84)
and the resulting mixture was heated for 3 hours at 90 ꢁC and then
drowned in ice/water mixture (150 ml). The pH was adjusted in the
range 7.5e8.0 with ammonium hydroxide, whereupon precipita-
tion occurred. The temperature was maintained in the range
10e15 ꢁC during the addition. The precipitates were filtered, dried
and crystallized from hot methanol.
3. Results and discussion
Out of the various metal ions tested, the hetarylazo dye 3
(yellow, lmax 480 nm, 3max 30,776 m2molꢀ1) detects Hg2þ (purple,
lmax 531 nm and 564 nm), and Fe3þ (red, lmax 531 nm and 564 nm)
N
N
N
N
NOHSO4, -5 oC
Diazotization
2.3.2. Synthesis of 2a
NH2
N N
Aniline (4.66 g, 4.6 ml, 0.10 mol) and ethyl acrylate (25.03 g,
27.3 ml, 0.50 mol) in acetic acid (9.01 g, 8.6 ml, 0.30 mol) with
catalytic amount of cupric chloride were heated at 130e140 ꢁC in
a sealed stainless steel container for about 12e14 h to obtain a black
tarry mixture which was neutralized with aqueous sodium bicar-
bonate solution and extracted with ethyl acetate. The extract was
washed with water, dried over anhydrous sodium sulphate and the
solvent removed under reduced pressure to give the residue which
was chromatographed over silica gel G (60-120 mesh) using 80%
ethyl acetate/hexane as eluent. 2b is commercially available as
N,N-Diethyl aniline.
S
S
R
1
Coupling, 0 oC
N
R
2a R= -CH2CH2COOC2H5
2b R= -CH2CH3
N
N
N
R
N
S
N
R
3 R= -CH2CH2COOC2H5
4 R= -CH2CH3
2.3.3. Synthesis of 3 and 4 [21] (Scheme 1)
A diazotization mixture was prepared by adding sodium nitrite
(0.83 g, 0.012 mol) to sulphuric acid (14.7 g, 8 ml, 0.12 mol, 98%) at
Scheme 1. Preparation of hetarylazo compound 3 and 4.

298
P. Kaur, D. Sareen / Dyes and Pigments 88 (2011) 296e300
Fig. 1. UVevis spectra of 3 (3 ꢂ 10ꢀ5 M, in CH3CN) in the presence of different metal
ions (2 ꢂ 10ꢀ5 M, in H2O). The CH3CN:H2O ratio in the resulting solution is 9:1.
from their aqueous solutions (Fig.1), while other relevant metals do
not modulate the absorption spectrum of 3, or its complex with
Hg2þ and Fe3þ in any significant way.
Fig. 3. Partial 1H NMR (CD3CN) spectrum of 3 (a) before and (b) after addition of Hg2þ ion.
The absorption band at 480 nm in acetonitrile solution, corre-
sponding to yellow color of 3 is attributed to the internal charge-
transfer (ICT) of the chromophore due to the push-pull effect of
naked eye also (Fig. S3(a), Supplementary data). Further, these
changes were fully reversible as the addition of EDTA reversed the
spectroscopic as well as color changes (Fig. S4, Supplementary data).
In the titrations of Hg2þ or Fe3þ with 3, increasing the concentration
of the appropriate metal salt, a decrease in the intensity of the
absorption band of 3 at 480 nm was attended by appearance of
the twin absorption bands at 531 and 564 nm, in both the cases. The
formation of isosbestic points were noticed at 502 nm and 512 nm in
the titrations of Hg2þ and Fe3þ, respectively (Fig. 2). The observed
pattern of the changes in the absorptions during titrations continued
until 2 ꢂ10ꢀ5 M concentration of the metal ions. At this point, the
titrations were adjudged complete resulting in the formation of
respective complexes. In these titrations (metal ion concentration:
2 ꢂ10ꢀ6 M to 2 ꢂ 10ꢀ5 M), the observed change in the absorbances
were used for the quantitative estimation of concentration of both
the metal ions.
Conceptually, in accordance with the hard soft acid base (HSAB)
principle [22], complexation of Hg2þ and Fe3þ is most likely to
involve the sulfur atom of thiadiazole moiety of 3 and nitrogen
atom of the eN]Ne link resulting in the enhanced intramolecular
charge transfer causing the observed red shift in the absorption
band. This is further attested by the observed shift in the C5-H
signal of the thiadiazolyl moiety in the 1H NMR experiment
depicted in Fig. 3. However, different extents of the red shift in Hg2þ
complex [(Dl, 51 nm), dominating peak 531 nm] and Fe3þ complex
[(Dl, 84 nm), dominating peak 564 nm] may be attributed to the
difference in electronegativity [23] of both the metal ions in their
the electron-donating N,N-di(b-ethoxycarbonylethyl) aniline group
and electron-withdrawing thiadiazole moiety. To see if the che-
mosensing properties of 3 are dependent upon the pH of the
system, the effect of pH variation was noted during titration of
a solution of 3 with NaHCO3 as well as HCl in the concentration
range 0.01 M to 0.1 M (Fig. S2, Supplementary data). The charge-
transfer absorption band at 480 nm observed an insignificant shift
in its wavelength position coupled with no change in the color of 3,
over the whole pH range (1.0e11.0) tested. Thus 3 demonstrates
considerable stability to pH variation which is often considered an
advantage for rapid monitoring of aqueous analytes in environ-
mental or biological settings without resorting to buffered media.
The change in the absorption spectrum of 3 in the presence
of aqueous solutions of upto 10 equivalents of 18 metal cations (group
I, II and transition metal ions Liþ, Naþ, Kþ, Ba2þ, Mg2þ, Ca2þ, Mn2þ
,
Fe2þ, Fe3þ, Co2þ, Ni2þ, Hg2þ, Pb2þ, Cu2þ Zn2þ and Al3þ added as
perchlorates and Agþ and Cd2þ as nitrates) was read at pH 7.78. Only
in case of Hg2þ and Fe3þ, the absorption spectrum showed changes
compared to the ion-free solution (Fig. 1). In both of these cases, the
single absorption band of 3 at 480 nm was red shifted to show twin
absorptions at 531 and 564 nmwith variable intensity [Hg2þ: 531 nm
(
3max 45,415 m2molꢀ1) and 564 nm (3max 41,355 m2molꢀ1), Fe3þ
:
531 nm (3max 37,455 m2molꢀ1) and 564 nm (3max 38,845 m2molꢀ1),
Fig. 2]. The observed color changes allowed a visual distinction of
Hg2þ and Fe3þ from other metal ions, as well as from each other with
A
B
1.4
1.6
1.4
1.2
1
1.2
1
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
350
450
550
650
350
450
550
650
Wavelength (nm)
Wavelength (nm)
Fig. 2. Changes in the absorption spectrum of 3 (3 ꢂ 10ꢀ5 M, in CH3CN) upon addition of increased concentrations of (A) Hg2þ solution and (B) Fe3þ solution (2 ꢂ 10ꢀ6 M to
5 ꢂ10ꢀ5 M).

P. Kaur, D. Sareen / Dyes and Pigments 88 (2011) 296e300
299
1.2
1
2+
Hg at 531 nm
3+
Fe at 564 nm
0.8
0.6
0.4
0.2
0
0
0.2
0.4
0.6
0.8
1
[M]/[M]+[3]
Fig. 4. Job’s plots of Hg2þ and Fe3þ complex formation. [M]/[M]þ[3] is the mole
fraction of the respective metal ion.
Fig. 6. Demonstration of colours of (i) dye
3 M and
with (ii)10ꢀ1 M, (iii) 10ꢀ2
(iv) 5 ꢂ10ꢀ3 M Hg2þ in (a) solution and (b) on coated dip sticks.
respective oxidation states and consequent stabilization of the
LUMO (Fig. S5, Supplementary data) could be expected.
The spectral features in Job’s plots (Fig. 4) are consistent with
the 1:2 (Hg2þ: 3) and 1:1 (Fe3þ: 3) binding ratios, respectively,
for the two metal ions. The spectral fitting of the titration data of
3 using SUPERQUAD [24] programme furnished very similar log
were introduced in aqueous solutions of Hg2þ ions of varying
concentrations, an instant purple color was developed, however,
compared to solution, the detection limit on paper was limited to
5 ꢂ10ꢀ3 M. Fig. 6 shows the colors developed on paper strips vis-
à-vis corresponding standard colors obtained in solution studies.
Development of such dip-sticks is useful as instant qualitative
information is obtained without any special equipment.
b
values, 8.8572 and 8.7175 for Hg2þ and Fe3þ, respectively.
The difference in wavelengths of maximum absorptions of both
the complexes (Hg2þ complex, lmax 531 nm and Fe3þ complex, lmax
564 nm) and observed identical formation constants were used for
the simultaneous estimation of metal ions from their mixture solu-
tionwith the limiting condition that the total metal ion concentration
should not exceed 2 ꢂ10ꢀ5 M in solution in the current assay. The
detailed calculations are provided in Supporting Information (S6,
Supplementary data).
Further the possibility of binding of hard oxygen sites, generally
considered as good binding sites for lanthanides, was also explored.
Addition of upto 10 equivalents of Ho3þ, Tm3þ, Lu3þ (as nitrates in
THF), Er3þ, Tb3þ, Gd3þ (as chlorides in DMSO), Ce3þ (as sulfate in
DMSO), Eu3þ (as triflate in CH3OH) and Dy3þ (as sulfonate in DMSO)
to 3 (3 ꢂ 10ꢀ5 M, in CH3CN) did not alter the absorption spectrum of
3 (Fig. 5) (Fig. S3(b), Supplementary data). Thus, dye 3 can be
regarded as highly selective sensor for Hg2þ, even in the presence of
lanthanide cations tested herein.
When dye 4 was used in place of 3, a similar response to the metal
ions as well as selectivity pattern was noted (Fig. S7, Supplementary
data). This is suggestive of only insignificant participation of ester
moieties of 3 in the binding phenomenon. However, the choice of 3
as a selective Hg2þ sensor was guided by the clarity of color changes
which were superior to 4.
In order to see if the results obtained in the solution studies
could be visualized on paper strips to obtain ready to use “dip
sticks”, when paper strips coated with 3 (S8, Supplementary data)
4. Conclusion
In summary, we have demonstrated that the hetarylazo deriva-
tive 3 can be used for the detection of Hg2þ and Fe3þ, simultaneously
and selectively over the other thiaphilic metal ions such as Agþ and
Pb2þ as well as a number of other cations including lanthanides.
The observed spectral responses allowed simultaneous quantitative
estimation of both Hg2þ and Fe3þ ions, from their mixture solution
and this aspect makes it a rare example. The fact that 3 can be
prepared in multi gram quantities using simple laboratory prepar-
ative method, the practicability of the method is obvious. Further,
development of dip-sticks is attractive although the process is
limited by higher detection limit compared to solution study but
provides an excellent opportunity to obtain readily accessible handy
detection devices.
Acknowledgements
The authors thank UGC (SAP) scheme and CSIR, New Delhi [01
(2265)/08/EMR-II] for the financial assistance.
Appendix. Supplementary information
Characterization of 3 and 4 and additional spectral data.
Supplementary data associated with this article can be found in the
online version, at doi:10.1016/j.dyepig.2010.07.012.
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