Indian Journal of Chemistry
Vol. 51A, June 2012, pp. 816-820
2,7-Diferrocenyl-3,6-diazaocta-2,6-diene:
A highly selective dual fluorescent sensor
for Zn2+ and Ag+ and electrochemical
sensor for Zn2+
Kaku Dutta & Diganta Kumar Das*
Department of Chemistry
Gauhati University, Guwahati 781 014, Assam, India
Email: [email protected]
Received 23 May 2011; revised and accepted 15 May 2012
2,7-Diferrocenyl-3,6-diazaocta-2,6-diene (DFDD), obtained by
condensation reaction of acetylferrocene and ethylenediamine,
exhibits Zn2+ ion-induced enhancement of fluorescence intensity
and Ag+ ion-induced quenching of fluorescence intensity. The Zn2+
ion shifts the redox potential of DFDD from +0.660 V ± 0.005 V to
+ 0.615 ± 0.005 V as observed by cyclic voltammetry and square
wave voltammetry. Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+,
Cd2+, Pb2+ and Hg2+ have very little effect on fluorescence
intensity of DFDD. These metal ions together with Ag+ ion have
no effect on the electrochemistry of DFDD. Both, fluorescence
and UV-visible spectroscopic data show a 1:1 interaction between
DFDD and Zn2+ with binding constant 75 M -1. The applicability
of DFDD as fluorescence sensor for Zn2+ ion has been shown in
its estimation in commercial tablets.
Keywords: Sensors, Electrochemical sensors, Fluorescent
sensors, Fluorescence, Cyclic voltammetry, Square
wave voltammetry, Zinc, Silver
Design and synthesis of molecular systems which can
selectively detect biologically important ions is one of
the most challenging areas of research. Zinc is an
essential element in all biological systems and is
present in more than 70 enzymes1. Among its many
roles in the human body, zinc regulates gene
expression2, is involved in cellular apoptosis3 and is a
constituent of RNA and DNA polymerases4. Zinc also
find a role in immune system, endocrine system and
gastroenterological system5. Deficiency in Zn2+ or
disorder in Zn metabolism leads to neurological
problems and diseases such as Alzheimer’s disease,
amyotrophic lateral sclerosis, Guan ALS-Parkinsonism
dementia and hypoxia-ischemia and epilepsy6. Due to
its importance in various cellular functions, its
detection is of particular interest.
Fluorescence measurement of specific biological
molecules and ions by artificial chemosensors is a
versatile technique having high sensitivity, rapid
response, and easy performance7-10. Development of
new fluorescent sensors for zinc has gained current
research interest. A novel pyrazoline derivative
synthesized starting from a chalcone and 3-chloro-6hydrazinylpyridazine is reported to determine Zn2+ ion
with high selectivity and a low detection limit in
CH3CN:EtOH (90/10, v/v)11. New bis(pyrrol-2-ylmethyleneamine) ligands are also reported to act as
selective sensor12 for Zn2+. [N,NO-di(quinoline-2methylene)-1,2-phenylenediimine] is reported to
exhibit high selectivity toward Zn2+ over other metal
ions including Cd2+ due to the formation of a 1:1
metal:ligand complex13. A ratiometric fluorescent
sensor for zinc ions based on covalently immobilized
derivative of benzoxazole is also known14. Schiff base
derived from benzil and ethylenediamine has been very
recently reported as fluorescent Zn2+ ion sensor15. There
are, however, very few reports on electrochemical
sensing of Zn2+ ion. Reports are available for Zn2+ ion
determination based on adsorptive stripping
voltammetry16-18 and cyclic voltammetry19 techniques.
One single molecule capable of selectively sensing
Zn2+ ion both by fluorescence and electrochemically is
probably not reported.
Although silver is widely used in the electrical
industry, photographic and imaging industry, and
pharmacy, it has adverse biological effects20.
Biotoxicity of silver includes inactivation of
sulfhydryl enzymes, combination with amine,
imidazole, and carboxyl groups of various
metabolites, causing serious environmental and health
problems21. A number of reports are available on
fluorescence detection of Ag+ ion22-26.
Reports on dual fluorescent sensors for more than one
metal ions are somewhat rare and is known for a few
pairs of metal ions27-33, e.g., Cu2+, Ni2+; Zn2+, Ni2+; Cu2+,
Zn2+ and Pb2+, K+. To the best of our knowledge there is
no report of fluorescent dual sensor for Zn2+ and Ag+.
The molecule, 2,7-diferrocenyl-3,6-diazaocta-2,6diene (DFDD, I) has been reported34 for preparing
metal complexes and studying their properties. DFDD
is the product of condensation of acetylferrocene and
ethylenediamine in minimum amount of methanol and
exhibited fluorescence of high intensity in the
emission range 380 – 680 nm when excited at
360 nm.
NOTES
DFDD
(I)
Herein, we report DFDD as a selective dual
fluorescent sensor for Zn2+ ion (by switch “on” mode)
and for Ag+ (by switch “off” mode). The redox
potential of DFDD undergoes a 0.045 ± 0.005 V
negative shift on interaction with Zn2+ ion. Several
metal ions like Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+,
Cu2+, Cd2+, Pb2+ and Hg2+, singly or together, do not
interfere in the fluorescence property of DFDD while
these metal ions along with Ag+ ion do not effect the
electrochemical properties of DFDD.
Experimental
All chemicals were from Merck (all metal salts
were sulphates except for AgNO3 and PbNO3) and
used without any further purification. 1H-NMR
spectra were recorded using a Bruker Ultrashield
300 MHz NMR spectrometer. Chemical shifts are
expressed in ppm (in CDCl3, with TMS as internal
standard) and coupling constants (J) in Hz. The
UV-visible spectrum was recorded on a Shimadzu
UV-vis-1800
spectrophotometer.
Fluorescence
experiments were performed on a Hitachi F-2500
spectrophotometer at room temperature. FTIR data
were collected using KBr pellet, on Perkin Elmer
spectrophotometer (RX1).
Electrochemical experiments were carried out at
room temperature using CHI 600B electrochemical
analyzer (USA) on a conventional three electrode
system with Ag/AgCl (3 M KCl) as the reference
electrode, a platinum wire as the counter electrode
and a glassy carbon disc (GC) as working electrode.
Tetrabutylammonium perchlorate (0.1 M) solution
was used as supporting electrolyte. Nitrogen gas was
purged through the electrolytic solution for at least
5 minutes to remove any dissolved oxygen before
every experiment. Nitrogen atmosphere was
maintained over the electrolytic solutions during each
experiment. Prior to every experiment the GC electrode
was cleaned as reported earlier35. For square wave
voltammetry experiments, the square wave amplitude
817
was 25 mV, the frequency was 15 Hz and the potential
step height for base stair case wave front was 4 mV.
2,7-Diferrocenyl-3,6-diazaocta-2,6-diene (DFDD)
was prepared as per reported procedure34 by refluxing
a mixture of 1 mmol (0.230 g) acetylferrocene and
0.5 mmol (0.4 mL) ethylenediammine in 10 mL
methanol for 2 hours. A brownish red crystalline
compound was obtained, which was filtered and
recrystallized from methanol. Yield: 68.9 %;
m. pt: 90 ºC; soluble in MeOH.
UV-visible spectra of the compound in methanol
showed λmax at 336 nm and 457 nm, originating in the
ferrocenes in DFDD. FTIR spectra of the compound
recorded in KBr pellet showed the peak for (νC=N) at
1589 cm-1. 1H NMR of the compound in CDCl3 was
recorded which showed δ H values at 1.656 (s) ppm
(2H, CH2); 2.406 (s) (3H, Me); 4.212 (s)
(5H, ferrocene); 4.513, 4.780 (2H, s, ferrocene).
Results and discussion
Methanol-water solution, 1:1 (v/v) of DFDD
(10-4 M) shows fluorescence emission in the range
380 - 600 nm with λmax at 450 nm when excited with
360 nm radiation. The fluorescence intensity remains
unaffected for over a period of 1 week and also when
pH was changed from 2 - 10.
The fluorescence titration reaction of 1:1 (v/v)
methanol-water solution of DFDD by a host of M2+
ions was carried out. A steady and smooth
enhancement in intensity in the case of only Zn2+ ion
was observed which saturates at 1.0 equivalent (Fig. 1).
Fig. 1  Changes in fluorescence spectra of DFDD (10-4 M) as a
function of added Zn2+ ion concentration in 1:1 (v/v) CH3OH-H2O
solution. [Inset: Plot of I/I0 as a function of Zn2+ ion
concentration].
818
INDIAN J CHEM, SEC A, JUNE 2012
The overall enhancement was found to be 2.4 fold at
saturation. The plot of I/I0 increases linearly with
Zn2+ ion concentration as shown in inset of Fig. 1
(I/I0 = 1.277[Zn2+] + 0.9599). The enhancement in
fluorescence intensity may be attributed to the
reversal of the photoinduced electron transfer (PET)
of the ethylenediamine N lone pairs upon
Zn2+ binding.
A least squares fitting of the linear plot of
log {(I-I0)/(Imax -I)} versus log [Zn2+] yielded the slope
as 1.17 ± 0.02, indicating a 1:1 binding between
DFDD and Zn2+. The binding constant was calculated
to be 75 M -1.
The effect of various divalent ions on fluorescence
intensity of DFDD was studied. Titrations carried out
with Mg2+, Mn2+, Fe2+, Cd2+, Hg2+ and Ni2+ ions
registered a very small increase in fluorescence
intensity. A moderate enhancement in the intensity of
the emission peak was observed when Pb2+ and Ca2+
ions were added. Titrations carried out with metal
ions Cu2+ and Co2+ exhibited small quenching in
fluorescence intensity of DFDD, while titration with
Ag+ ion quenched the fluorescence considerably.
Quenching effect of Ag+ ion on the fluorescence
spectra of DFDD (0.5 × 10-4 M) has been shown in
Fig. 2. The plot of I/I0 versus [Ag+] ion showed a
smooth decrease in intensity (I/I0 = 0.1126 [Ag+] +
0.9963) with co-efficient of linearity as 0.9872
(Fig. 2, inset). Addition of NaCl to the already
quenched DFDD was found to regenerate the original
intensity of DFDD. Hence, fluorescence quenching of
Fig. 2  Changes in fluorescence spectra of DFDD (0.5 × 10-4 M)
as a function of added Ag+ ion concentration in 1:1 (v/v) CH3OH-H2O
solution. [Inset: Plot of I/I0 as a function of Ag+ ion
concentration].
DFDD by Ag+ is reversible with respect to chloride
ion. Thus, the steady-state fluorescence data obtained
from the titration of DFDD with various M2+ ions
clearly suggests that it can detect Zn2+ ion by
fluorescence enhancement and Ag+ ion by fluorescence
quenching. The lower detection limits were found to be
10-6 M for Zn2+ and 0.5 × 10-5 M for Ag+ ion.
In acetonitrile, the fluorescence intensity of the
sensor molecule is less as compared to that in
methanol-water. On addition of Zn2+, a three-fold
enhancement in fluorescence intensity was observed.
For the other metal ions, the results were similar to
those obtained in methanol-water.
In order to confirm the binding and stoichiometry
of Zn2+ with DFDD, electronic spectra of DFDD was
recorded at varying added concentration of Zn2+ ion.
The absorbance at λmax 336 nm and 457 nm was found
to increase with Zn2+ ion concentration with an
isosbestic point at 300 nm, indicating a transition
between free species and complexed species. The
increase in the 336 nm band is indicative of the
interaction of Zn2+ ion with nitrogen of
ethylenediamine32.
The absorbance at 457 nm was recorded as a
function of [Zn2+] ion concentration and the plot of
log {(Ao-As)/(As-Aα)} versus log [Zn2+] was found to
be linear. The least squares fitting of data showed that
the slope was 1.112 ± 0.002, indicating 1:1
stoichiometric binding between DFDD and Zn2+
which is in agreement with data obtained from
fluorescence. The binding constant 70.795 M-1 was
similar to that obtained from fluorescence intensity
plot. The 1H NMR of DFDD in presence of
1 equivalent of Zn2+ ion was recorded and the peaks
due to -CH2- and –CH3 protons were found to shift
downfield while the peaks due to protons in C5H5
rings were unaffected. This result confirms the
binding of Zn2+ to the two immine N of DFDD.
DFDD
showed
quasi
reversible
cyclic
voltammetric response in 1:1 CH3OH:H2O on GC
electrode; both the cathodic and anodic currents were
found to increase with scan rate. The redox potential
was found to be 0.660 ± 0.005 V (∆E = 0.182 V) at
scan rate 0.050 Vs-1 versus Ag-AgCl as reference. The
plot of cathodic and anodic current against square root
of scan rate was found to be linear, indicating a
reversible redox process. As ferrocene is a well
known electrochemically reversible redox molecule36,
the observed cyclic voltammogram is due to
ferreocene/ferroceneium couple presents in DFDD.
NOTES
819
Fig. 3  Cyclic voltammetric response of DFDD in 1:1 CH3OHH2O at varying Zn2+ ion concentration. [0 mM (curve A), 0.32 mM,
0.56 mM, 0.74 mM and 1.23 mM (curve E). Scan rate: 0.05 Vs-1;
working electrode: GC; reference electrode: Ag-AgCl].
Fig. 4  Square wave voltammogram of DFDD in 1:1 CH3OH-H2O
at Zn2+ ion concentration. [0 mM (curve A), 0.32 mM, 0.56 mM,
0.74 mM and 1.23 mM (curve E). Scan rate: 0.05 Vs-1; working
electrode: GC; reference electrode: Ag-AgCl].
Figure 3 shows the effect of Zn2+ ion concentration
on the cyclic voltammogram of DFDD. Addition of
aqueous solution of Zn2+ ion into the electrolytic
medium shifted the cyclic voltammogram of DFDD in
negative direction and the redox potential became
0.615 ± 0.005 V (i.e., 0.045 V shift was observed) at
Zn2+ ion concentration 1.23 mM. The plot of redox
potential versus concentration of Zn2+ ion shown in
Fig. 3 (inset) clearly reveals that both the cathodic and
anodic currents increased due to the interaction with Zn2+
ion. The enhancement in cathodic and anodic currents of
DFDD was linear with Zn2+ ion concentration and
maximum enhancement was approximately 1.20-fold at
1 equivalent Zn2+ ion concentration.
The square wave voltammogram of DFDD at
different added concentration of Zn2+ ion is given in
Fig. 4. For a given addition of Zn2+ ion, the shift in
redox potential values obtained were in good
agreement with that obtained by cyclic voltammetry.
The plot of current versus Zn2+ ion concentration was
linear as shown in Fig. 4 (inset).
Cyclic
voltammogram
and
square
wave
voltammogram of DFDD was also recorded in presence
of Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Cd2+, Pb2+
and Hg2+ ions. No significant effects in the cyclic and
square wave voltammograms were observed when these
ions were added into the electrolytic medium singly or
together. This indicates the selectivity of the sensor
towards Zn2+ over the above mentioned metal ions. The
electrochemical behavior of DFDD remained the same
in acetonitrile as in methanol-water medium.
Table 1  Relative increase in fluorescence intensity of DFDD
solutions on addition of varying amounts of Zn+2 ion
concentration
[Zn2+] added (×104 M)
0.25
0.59
0.72
0.91
1.20
1.40
{(I-I0)/I0}
Std solution
Test solutiona
0.24
0.31
0.45
0.52
0.56
0.60
0.28
0.34
0.48
0.56
0.60
0.63
a
Supra Cal (MMC Health Care, HP, India) containing
Ca2+: 280 mg; Mg2+: 100 mg; Zn2+: 4 mg; vitamin D3: 200 IU.
Applicability of the sensor DFDD was tested in the
estimation of concentration of Zn2+ in multimineral
tablet (Supra Cal, MMC Health Care, H.P. India). The
test solution was prepared by dissolving one tablet in
50 mL phosphate buffer solution (PBS) (pH 7.0) and
the standard solution of Zn2+ ion was prepared by
dissolving 4 mg in 50 mL PBS. The fluorescent
intensity of 0.1 mM solutions of DFDD was measured
at varying concentrations of Zn2+ (0.025 mM,
0.05 mM, 0.075 mM, 0.1 mM, 0.125 mM, 0.150 mM)
by adding appropriate volume of the test solution.
Another set of experiments was performed by adding
appropriate volume of the standard Zn2+ solution to
0.1 mM DFDD so that Zn2+ ion concentration is the
same as in the test solution. The relative increase in
intensity for these two sets of measurements was
found to be comparable (Table 1), showing the
applicability of DFDD as a sensor for Zn2+ ion.
820
INDIAN J CHEM, SEC A, JUNE 2012
In summary, the present study shows that
2,7-diferrocenyl-3,6-diazaocta-2,6-diene acts as a
fluorescent sensor for Zn2+ ion by “switch on” mode
while it acts as fluorescent sensor for Ag+ by “switch
off” mode. The fluorescence quenching of DFDD by
Ag+ ion is reversible with respect to chloride ion.
DFDD can also recognize Zn2+ ion electrochemically.
Several metal ions, viz., Fe2+, Cu2+, Cd2+, Co2+, Pb2+,
Hg2+, Ca2+, Mn2+, Ni2+ and Mg2+do not interfere with
the fluorescent determination of Zn2+ and Ag+ ion.
This group of metal ions and Ag+ also do not interfere
in the electrochemical determination of Zn2+.
Acknowledgement
Department of Science & Technology, New Delhi,
India and University Grants Commission, New Delhi,
India (SAP Program) are thanked for financial
assistance.
References
1 Li Z, Xiang Y & Tong A, Anal Chim Acta, 619 (2008) 75.
2 Andrews G K, BioMetals, 14 (2001) 223.
3 Truong-Tran A Q, Carter J, Ruffin R E & Zalewski P D,
BioMetals, 14 (2001) 315.
4 Lippard S J & Berg J M, Principles of Bioinorganic Chemistry,
(University Science Books: Mill Valley, CA), 1994.
5 Mondal D P, Das S & Rajput V, Mater Sci Engg: A, 406
(2005) 24.
6 Cuajungco M P & Lees G J, Neurobiol Dis, 4 (1997) 137.
7 Swalie D F & Sepaniak M J, Anal Chem, 63 (1991) 179.
8 Fabrizzi L, Lichelli M, Rabaioli G & Tagliette A, Coord
Chem Rev, 205 (2000) 85.
9 Thompson R B, Curr Opin Chem Biol, 9 (2005) 526.
10 Jiang P & Guo Z, Coord Chem Rev, 248 (2004) 205.
11 Gong Z L, Zhao B X, Liu W Y & Lv H S, Photochem
Photobio A: Chem, 218 (2011) 6.
12 Wu Z, Chen Q, Yang G, Xiao C, Liu J, Yang S & Ma J S,
Sensors Actuators B, 99 (2004) 511.
13 Chen H, Wu Y, Cheng Y, Yang H, Li F, Yang P & Huang C,
Inorg Chem Comm 10 (2007) 1413.
14 Juan Ma Q, Zhang X B, Zhao X H, Gong Y J, Tang J,
Shen G L & Yu R Q, Spectrochim Acta A, 73 (2009) 687.
15 Goswami P & Das D K, J Fluorescence, (2012), DOI
10.1007/s10895-012-1046-0.
16 Kirgoz U A, Marin S, Pumera M, Merkoci A & Algret A,
Electroanalysis, 17 (2005) 881.
17 Adam V, Zehnalek J, Petriova J, Potesil D, Sures B,
Trnkova L, Zelen F, Vitecek J & Kizek R, Sensors, 5 (2005)
70.
18 Stozhko N Y, Lipunova G N, Maslakova T I, Aleshina L V
& Brainina K Z, J Anal Chem, 59 (2004) 179.
19 Das D K, J Surf Sci Tech, 24 (2008) 149.
20 Liu L, Zhang D Q, Zhang G X, Xiang J F & Zhu D B,
Org Lett, 10 (2008) 2271.
21 Zhang B, Sun J, Bi C, Yin G, Pu L, Shi Y & Sheng L,
New J Chem, 35 (2011) 849.
22 Kandaz M, Guney O & Senkal F B, Polyhedron, 28 (2009)
3110.
23 Qu F, Liu J, Yan H, Peng L & Li H, Tetrahedron Lett, 49
(2008) 7438.
24 Li H, Cui Z & Han C, Sensors Actuators B, 143 (2009) 87.
25 Li C Y, Xu F & Li Y F, Spectrochim Acta A, 76 (2010) 197.
26 Park C S, Lee J Y, Kang E J, Lee J E & Lee S S,
Tetrahedron Lett, 50 (2009) 671.
27 Bodenant B, Well T, Pourcel M B, Fages F, Barbe B,
Pianet I & Laguerre M, J Org Chem, 64 (1999) 7034.
28 Unob F, Afsari Z & Vicens J, Tetrahedron Lett, 39 (1998)
2951.
29 Cao Y D, Zheng Q Y, Chen C F & Huang Z T, Tetrahedron
Lett, 44 (2003) 4751.
30 Begatin I A, de Souza E S, Ito A S & Toma H E,
Inorg Chem Commun, 6 (2003) 288.
31 Kim S K, Lee S H, Lee J Y, Bartsch R A & Kim J S,
J Am Chem Soc, 126 (2004) 16499.
32 Kim J S, Kim H J, Kim H M, Kim S H, Lee J W, Kim S K &
Cho B R, J Org Chem, 71 (2006) 8016.
33 Joseph R, Ramanujam B, Pal H & Rao C P, Tetrahedron
Lett, 49 (2008) 6257.
34 Mishra K N, Pandey O P & Sengupta S K, Inorg Metal Org
Chem, 27 (1997) 661
35 Rajbongshi J, Das D K & Mazumdar S, Electrochim Acta, 55
(2010) 4174.
36 Bhattacharjya R & Das D K, Indian J Chem, 46A (2007)
276.