Colorimetric recognition of d10 metal ions through an adenine-based ICT probe

Article abstract of DOI:10.1246/bcsj.82.813

We report hereby use of p-nitrophenyltriazenopurine, (PNTP) as a colorimetric receptor for recognition of d¹⁰ metal ions in DMSO through visible color changes. All the chosen metal ions showed 1:1 stoichiometry with PNTP. Among the chosen metal

Full text of DOI:10.1246/bcsj.82.813

Short Articles
Bull. Chem. Soc. Jpn. Vol. 82, No. 7, 813–815 (2009)
813
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Colorimetric Recognition of
d10 Metal Ions through an
Adenine-Based ICT Probe
-0.213
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N
0.016
N
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N
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K. K. Upadhyay,* Ajit Kumar, Rakesh K. Mishra,
-0.588
N
N
and Rajendra Prasad
0.219
Figure 1. Charges obtained from NBA using B3LYP/
6-31G** optimized geometry of PNTP.
Department of Chemistry, Faculty of Science,
Banaras Hindu University, Varanasi-221005, India
tains triazene as its ³ unit in its D-³-A system hence it was
thought worthwhile to explore the possibility of interactions of
this reagent with ZnII, CdII, and HgII. The sensing ability of
PNTP for cations and anions both may be attributed to the si-
multaneous presence of triazene17 as well as imidazole moiety.18
The structural optimization and computation of atomic
charges from natural bond orbital analysis (NBA) for PNTP
with B3LYP/6-31G** through density functional theory (DFT)
clearly proved that it possesses electron-rich and -deficient
pockets simultaneously within the same framework. That is why
PNTP is a fit case for its potential application as an ICT probe.19
The optimized structure with NBA charges on different atoms of
PNTP has been given in Figure 1. The sum total of NBA
charges on various fragments of PNTP indicated that the purine
moiety is electron rich in comparison to the p-nitrophenyl
group. Both of these units are attached with triazene moiety.
Hence there will be a natural flow of electron density from pu-
rine (D) to p-nitrophenyl (A) group through triazene (D-³-A).
The absorption spectrum of the PNTP was characterized by
the presence of absorption maxima at 398 nm along with a
broad band at 575 nm in DMSO while in nujol mull the broad
band at 575 nm was not observed. The peak at 398 nm is
assigned to the p-nitrophenyl moiety of PNTP while the broad
band at 575 nm is due to intramolecular charge transfer (ICT)
from electron-rich purine to electron-deficient p-nitrophenyl
through triazene. It is worthy to mention that the ICT band of
PNTP at 575 nm was observed only in polar aprotic solvents
like DMSO, DMF and not in other non-polar aprotic solvents
like chloroform. Hence, the polarity of the solvent may be
taken as responsible for decreasing the energy barrier for ICT
which ultimately may lead to an increase in the solvent
relaxation time and finally giving rise to a new peak at
575 nm in the light of a recent literature report.20 Nevertheless,
intermolecular hydrogen bonding between PNTP (through
imidazolic -NH-) and polar aprotic solvents through X = O
(where X = S for DMSO and X = C for DMF) may be yet
another probable reason behind the genesis of the 575 nm peak
of PNTP in its UV-vis spectrum in DMSO.21
Received October 14, 2008; E-mail: drkaushalbhu@yahoo.
co.in
We report hereby use of p-nitrophenyltriazenopurine,
(PNTP) as a colorimetric receptor for recognition of d10 metal
ions in DMSO through visible color changes. All the chosen
metal ions showed 1:1 stoichiometry with PNTP. Among the
chosen metal ions HgII was most preferred by PNTP.
Highly accurate sensing of cations and anions is an urgent
need of ours today for many areas of technology, including
biological, clinical, environmental, and waste management
applications.1,2 The colorimetric chemosensors have attracted
considerable attention since they provide immediate qualitative
signal, which allows direct naked-eye detection of cations/
anions due to some specific color changes of solution.3 The
selection of metal ions in this communication is based on their
important roles in enzymatic/environmental areas.4-6 Zinc acts
as a co-factor in various enzymes and plays an important role
in protein synthesis and cell division besides its role in the
functioning of the immune system.7 The environmental havoc
produced by CdII and HgII are well known in the form of itai-
itai8 and minamata9 disease respectively. The selective sensing
protocols for heavy metal ions such as CdII and HgII are critical
owing to their high toxicity.10,11 These heavy metal ions have
detrimental effects on humans, causing impairment in brain
development, renal dysfunction, calcium metabolism disorders,
chronic inflammation to the heart and kidney, and impairing
reproductive systems.12,13 Extensive studies have been carried
out on the development of protocols for the determination
of biologically relevant metal ions.14,15 Most of them are
disadvantageous in terms of cost and hence not suitable
for routine analysis. One accurate available method for low
level determination of heavy metals is AAS, but it involves
expensive instrumentation and sample pretreatment that is time
consuming and inconvenient also. Hence, there is a critical
need for the development of selective, portable, and inex-
pensive diagnostic tools for determination of heavy metals.
In our previous communication16 we reported synthesis,
characterization, and application of p-nitrophenyltriazenopurine
Addition of 1 equivalent of ZnII, CdII, and HgII as their
chloride salts respectively to the 5 © 10¹5 M solution of PNTP
in DMSO produced visible color changes as well as changes in
its UV-vis spectral bands (Figures 2a and 2b). This observa-
tion clearly proved the sensitivity of PNTP towards chosen
metal ions.
¹
(PNTP) for the binding of some anions, particularly PF6 as its
tetrabutylammonium salt. The application of triazenes for the
spectrophotometric determination of d10 metal ions particularly
CdII and HgII is well documented in the literature.17 PNTP con-
In order to have a deeper insight of the sensing phenomenon
the UV-vis titrations were performed by the interaction of
Published on the web July 10, 2009; doi:10.1246/bcsj.82.813

814
Bull. Chem. Soc. Jpn. Vol. 82, No. 7 (2009)
© 2009 The Chemical Society of Japan
0.7
0.7
0.6
0.5
0.4
0.3
0.2
0.1
(a)
(a)
(b)
(c)
PNTP
0.6
ZnII
CdII
HgII
0.5
0.4
0.3
0.2
0.1
300 350 400 450 500 550 600 650 700
Wavelength/nm
300 350 400 450 500 550 600 650 700
Wavelength/nm
0.7
0.6
0.5
0.4
0.3
0.2
0.1
(b)
300 350 400 450 500 550 600 650 700
Wavelength/nm
Figure 2. (a) UV-vis spectra of PNTP and corresponding
changes on addition of 1 equivalent of ZnII, CdII, and HgII.
(b) Visible color changes of DMSO solution of PNTP on
addition of 1 equivalent of ZnII, CdII, and HgII. (I) The
counter ions in all the cases were chloride ions. (II) 5 ©
10¹5 M DMSO solution of PNTP was used for 2a and 2b.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
5 © 10¹5 M DMSO solution of PNTP with ZnII, CdII, and HgII
in the range of 0.20-2 equivalents each as their chloride salts.
The corresponding UV-vis titration curves have been shown in
Figures 3a-3c.
300 350 400 450 500 550 600 650 700
Wavelength/nm
Clear changes in the UV-vis absorption characteristics of
PNTP as its DMSO solution on the addition of d10 metal ions
can be noted from the above titration curves. The addition of
HgII leads to a clear vanishing of the 575 nm peak of PNTP
with the origin of a new peak at 436 nm and an isosbestic point
at 418 nm. On the other hand the addition of ZnII and CdII to
PNTP under similar conditions lead only partial vanishing of
575 nm peak with the origin of a new peak at 446 and 483 nm
respectively. The isosbestic points in these two cases were
marked on 424 and 438 nm respectively. The occurrence of
isosbestic points for all the three metal ions clearly proved their
binding with PNTP and existence of only one species i.e.,
corresponding metal complexes at respective equilibria. Inter-
estingly in all the cases the absorption peak of PNTP at 398 nm
suffered a regular hypochromic shift on concomitant addition of
0.2-2 equivalents of ZnII, CdII, and HgII. This very hypochromic
shift in the 398 nm peak of PNTP was chosen for the calculation
of the corresponding binding constants. Satisfactory results for
the binding constants with the correlation coefficients over 0.99
were obtained (Table 1) for all three analytes. The nonlinear
curve fitting of the titration data on 1:1 metal/ligand binding
confirmed 1:1 host/guest ratio between PNTP with all the
chosen metal ions. The binding constants (Kass) and correlation
coefficients (R) were obtained by a nonlinear least-squares
analysis of A versus CH and CG using the following equation
from the literature22 in the ORIGIN 7.0 software:
Figure 3. UV-vis titration curves of PNTP with d10 metal
ions: (a) ZnII, (b) CdII, and (c) HgII.
¹1
Table 1. Binding Constants Ka/M of PNTP with Chosen
Metal Ions
¹1 b)
Cationsa)
Ka/M
Rc)
ZnII
CdII
HgII
1.38 « 0.24 © 105
2.28 « 0.42 © 105
6.09 « 1.25 © 105
0.99117
0.99090
0.99244
a) The cations were used as their chloride salts. b) The data were
calculated from UV-vis titration in DMSO. c) The data values
of R were obtained by the results of nonlinear curve fitting.
Where, A is the absorption intensity and CH and CG are the
corresponding concentrations of the host and guest; C0 is the
initial concentration of the host. The corresponding results are
listed in Table 1.
A perusal of the binding constants indicated the order of
preference of PNTP among chosen metal ions as:
HgII > CdII > ZnII
ð2Þ
Since the guest species are the d10 metal ions hence the
triazene unit of PNTP is expected to be the most susceptible
site for their binding on the similar line of earlier literature
reports.23 The results of DFT calculations given in Table 2
categorically established that out of the various possibilities of
the chelate formation the six-membered one is the most
A ¼ A0 þ ðAlim ꢀ AÞ=2C0fCH þ CG þ 1=Kass
2
1=2
ꢀ ½ðCH þ CG þ 1=KassÞ ꢀ 4CHCGꢁ
g
ð1Þ

K. K. Upadhyay et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 7 (2009)
815
Table 2. Total Energies and M-N Bond Lengths on the
Basis of DFT Calculations for Various Possible MCl2-
PNTP Chelates
respectively. At the same time the PNTP is a unique type of
chemosensor having ability to bind with anions16 and cations.
Authors are thankful to the coordinator SAP for enabling us
to perform DFT calculations using Gaussian 03. AK and RKM
are thankful to UGC & CSIR New Delhi for providing financial
assistance in the form of fellowships.
Chelate
Energy/au
Bond distances/¡
2.06690 2.13324
(N2-Zn) (N13-Zn)
2.01795 3.81824
(N8-Zn) (N11-Zn)
2.05321 3.12582
Six membered
¹3711.99977491
Five membered
¹3711.99282258
¹3711.97998709
Supporting Information
Four membered
Competition experiments of PNTP with d10 metal ions. This
material is available free of charge on the web at http://www.csj.jp/
journals/bcsj/.
(N13-Zn) (N11-Zn)
O2N
18
17
16
19
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