Structurally modified 1,10-phenanthroline based fluorophores for specific sensing of Ni2+ and Cu2+ ions

Article abstract of DOI:10.1039/c2dt30525a

We report two fluorophores with open coordination sites for specific sensing of Ni²⁺ and Cu²⁺ ions via a simple synthetic route. The fluorescence activity was completely quenched on coordination of the metal ions with the phenanthroline ring present in the fluorophore as is clearly evident from the photophysical studies.

Full text of DOI:10.1039/c2dt30525a

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Structurally modified 1,10-phenanthroline based fluorophores for specific
sensing of Ni2+ and Cu2+ ions
Selvam Sangeetha,a Gopal Sathyaraj,a Duraisamy Muthamilselvan,a Vaidyanathan Ganesan Vaidyanathanb
and Balachandran Unni Nair*a
Received 5th March 2012, Accepted 13th March 2012
DOI: 10.1039/c2dt30525a
medicine, etc.5 Fluorescence emission-based sensors also have
We report two fluorophores with open coordination sites for
very high sensitivity.6
specific sensing of Ni2+ and Cu2+ ions via a simple synthetic
route. The fluorescence activity was completely quenched on
coordination of the metal ions with the phenanthroline ring
present in the fluorophore as is clearly evident from the
photophysical studies.
In general, the sensing of cations such as Ni2+ and Cu2+ in
aqueous solution requires a strong affinity for cations in the
solvent as well as the ability to convert the cation recognition
into a fluorescent or colorimetric signal.7 Most of the fluorescent
chemosensors for cations are composed of a cation recognition
site (ionophore) with a fluorogenic site (fluorophore) which con-
verts the recognition of the cation by the ionophore into an
optical signal.8
Much attention has been focused in recent years on the develop-
ment of new chemical sensors for the recognition of heavy metal
ions as well as anionic species.1 Metal ions play a vital role in a
wide range of chemical and biological processes, yet some of
them are potentially toxic. It is indeed important to detect metal
ions for applications in waste management and environmental
toxicology.2 Metal ions such as Ni2+ and Cu2+ fall under the cat-
egory of essential trace elements as they are indispensable for
life. However, at higher concentration they damage the central
nervous system and affect blood composition as well as the
kidneys, liver, lungs and other vital parts of the human body.
Hence, there has been an increasing interest in the field of
sensors to monitor these metals selectively in the environment.
The response from these sensors may be either through optical,
electrical or magnetic resonance signals.3
Even though a number of techniques such as atomic absorp-
tion spectroscopy, inductively-coupled plasma mass spec-
trometry and inductively-coupled plasma atomic emission
spectroscopy are available to detect these metal ions in food
samples and industrial effluent, they still involve tedious pro-
cedures for sample treatment and are also expensive.4 Optical
signals based on changes in absorbance or fluorescence is the
most frequently exploited technique because of the simplicity of
the experimental methods.
1,10-Phenanthroline moieties are emerging as fluorescent
sensors for the detection of metal ions.9 Spectral shifts occur in
the phenanthroline moiety after the ligation of the metal ion on
the open coordination site, which in turn changes its color.10
Sensors based on the phenanthroline moiety can be prepared by
reacting with various aryl groups, thus resulting in fluorescent
properties of the compound containing both the ionophore and
fluorophore. The interaction with metal ions may be through
coordinate bonding of the nitrogen atom in the phenanthroline
ring.
In this paper, we report two excellent fluorophores with open
coordination sites for the selective detection of Ni2+ and Cu2+.
The fluorophores 3a and 3b were prepared by employing a very
simple synthetic route of condensation of diamine with a di-
ketone as depicted in Scheme 1.† The reactants were prepared
according to the literature.11,12
Surprisingly, both the compounds had excellent fluorescence
visible to the human eye in solution. The compounds after purifi-
cation from column chromatography were analysed for their
photophysical properties. To evaluate the selectivity of the fluor-
ophores 3a and 3b towards the analyte, their fluorescence behav-
ior upon addition of various transition and heavy metal ions
such as Cr3+, Mn2+, Fe2+, Fe3+, Ni2+, Cu2+, Zn2+, Mg2+, Cd2+,
Hg2+ and Pb2+ have been examined.
Luminescent chemosensors for recognizing environmentally
significant ionic species are currently a challenging research
topic. Among the various metal ion sensor molecules, fluor-
escent sensors have the additional advantage of finding appli-
cation in trace ion analysis and imaging, since change in color
immediately reveal the presence of a given analyte, which makes
it valuable for application in the field of chemistry, biology,
aChemical Laboratory, Central Leather Research Institute, CSIR, Adyar,
Chennai 600020, India. E-mail: bunair@clri.res.in
bDepartment of Biomedical and Pharmaceutical Sciences, University of
Rhode Island, Kingston, RI 02881, USA
Scheme 1
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Fig. 1 Absorption spectra of 3a with Ni2+ (top left), 3a with Cu2+ (top right), 3b with Ni2+ (bottom left) and 3b with Cu2+ (bottom right).
Fig. 2 Fluorescence response of 3a to various metal ions in methanol. Bars represent fluorescence quenching ratio (I0 − I)/I0 of various metal ions.
Fluorescence studies were carried out for compounds 3a and
3b and both the emission spectra were mirror images of the
absorption spectra of the respective compounds. The fluor-
escence spectra of 3a and 3b show strong emission of the phe-
nanthroline moiety at 513 nm and 503 nm respectively.
On addition of metal ions, the phenanthroline moiety is
expected to bind to a certain analyte, in turn altering the photo-
physical properties. Fluorescence of the fluorophores in the pres-
ence of various metal ions was initially studied in a 1 : 1
stoichiometric ratio. The fluorescence intensity was found to
decrease on addition of different metal ions. The extent of
quenching depended upon the binding interaction of the metal
ion with the receptor molecule. Addition of metal ions other
than Cu2+ and Ni2+ led to very little change in the intensity of
emission of compound 3a at 513 nm.
The excitation energy was found to be 365 nm and 295 nm
for 3a and 3b respectively. The excitation energy of the two
fluorophores did not show any change in the presence of the two
metal ions. These results are also in line with the absorption
spectra of the fluorophores in the presence of the two metal ions
(Fig. 1).
The absorption spectra of 3a and 3b with Ni2+ and Cu2+ is
given in Fig. 1. In both the cases, addition of Cu2+ or Ni2+ did
not lead to any shift in the position of the absorption peaks of
5770 | Dalton Trans., 2012, 41, 5769–5773
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Fig. 3 Fluorescence response of 3b to various metal ions in methanol. Bars represent fluorescence quenching ratio (I0 − I)/I0 of various metal ions.
fluorescence emission of both the fluorophores, the quenching
effect of Ni2+ and Cu2+ was much more than that observed with
Zn2+, Fe3+ and Cd2+ as can be seen from the bar graph shown in
Fig. 3. Pictorial representation of the response of fluorophores
with metal ions is shown in Fig. 4.
Fluorescence titration experiments were carried out to investi-
gate the complexation of Ni2+/Cu2+ with the fluorophores 3a and
3b (Fig. 5 and 6). Both the fluorophores showed a considerable
decrease in fluorescence intensity on increasing the metal ion
concentration and a complete quenching of fluorescence band
was observed at 1 : 1 equivalents of the fluorophore to metal ion.
From the Job’s plot, 1 : 1 stoichiometry of the metal ion to
fluorophore was confirmed. The fluorophore 3a was able to
detect 0.4 ppm of Ni2+ and 0.35 ppm of Cu2+ with a quenching
efficiency of 80% and in the case of 3b, the detection limits
were 0.2 ppm and 0.4 ppm for Cu2+ and Ni2+ respectively. From
the Stern–Volmer relationship, the binding constant of fluoro-
phores with the metal ions were calculated and the values were
found to be 8.96 × 106 M and 2.51 × 107 M for 3a with Cu2+
and Ni2+ respectively. The values for 3b with Cu2+ and Ni2+
were 8.13 × 106 M and 3.75 × 106 M respectively.
From the photophysical studies, it is observed that most of the
metal ions can bind to the phenanthroline moiety in the ligand,
yet a complete quenching was observed only in the case of Ni2+
and Cu2+ in a 1 : 1 ratio of metal to ligand. In order to further
understand the behaviour of these metal ions with the fluoro-
phores, DFT calculations have been carried out. As Ni2+ and
Fig. 4 Image under UV light of 3a/3b to various metal ions in
methanol.
fluorophores. Addition of these two metal ions also did not lead
to appearance of any new peak in the visible region of the
absorption spectra. Addition of metal ions other than Cu2+ or
Ni2+ also did not result in any change in the energy of the fluoro-
phore absorption. The intensity of the fluorophore absorption
peak however, increased in the presence of Cu2+/Ni2+.
On the other hand, a significant change was observed in the
intensity of emission of compound 3a in the presence of Ni2+
and Cu2+. A complete quenching of the emission band at
513 nm, which is characteristic of the phenanthroline ring was
observed in the presence of Ni2+/Cu2+ (Fig. 2).
In a similar manner, the emission band of 3b at 503 nm was
completely quenched only on addition of Ni2+ and Cu2+;
whereas no marked changes were seen in the emission band of
3b in the presence of other metal ions, except for a slight change
in intensity. Even though Zn2+, Fe3+ and Cd2+ ions quenched the
Fig. 5 Fluorescence titration experiment of 3a with various concentrations of Ni2+ and Cu2+ ions. Inset represents the respective Job’s plot.
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Fig. 6 Fluorescence titration experiment of 3b with various concentrations of Ni2+ and Cu2+ ions. Inset represents the respective Job’s plot.
Fig. 7 Optimised structures of 3a with Cu2+ (top left), 3a with Ni2+
(top right), 3b with Cu2+ (bottom left) and 3b with Ni2+ (bottom right).
Cu2+ are known for their 4-coordination complexes, the input
structures were designed in a way to have bidentate coordination
with the fluorophores, whereas the other two coordination sites
are occupied by the solvent molecule (Fig. 7). The structural
optimisation was done using the Gaussian 03 package adopting
the B3LYP method with 6-31G as the basis set.13,14
It can be seen from the optimised structures that Ni2+ and
Cu2+ adopts distorted square planar symmetry and tetrahedral
symmetry respectively, which is due to the angle strain around
the five membered ring structure formed by the coordination of
the metal ion with the ligand.
In order to understand the absorption spectral behaviour of the
fluorophores on addition of Ni2+/Cu2+ ions, TD-DFT calcu-
lations15,16 were carried out for the first 35 excitations lying in
the region of 350–800 nm. Electron density of the HOMO of
both fluorophores 3a and 3b lies on the electron-donating part of
the ligand and for the LUMO, the electron density lies on the
electron-withdrawing part. From the energy level diagram
(Fig. 8), it can be seen that addition of the metal ions leads to
the stabilization of the HOMO and LUMO of the fluorophores.
The contour plots of the HOMO after the addition of metal ions
shows that the electron density is over the electron-donating part
of the fluorophore as in the case of free fluorophore. However,
Fig. 8 Energy levels of various HOMO and LUMO’s of 3a (top) and
3b (bottom) in the absence and the presence of Cu2+ and Ni2+ ions.
Contour plots of the HOMO and LUMO are shown (isosurface cut-off
value: 0.02).
the electron density of the LUMO was situated over the metal
ion, leading to the quenching of the fluorescence.
In conclusion, we have reported two fluorophores which are
prepared via condensation without involving tedious procedures.
Both compounds involve only slight structural modification in
the fluorogenic site and both of them specifically recognize Ni2+
and Cu2+.
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Notes and references
†Synthesis of 3a (2,3-bis((E)-2-(furan-2-yl)vinyl)pyrazino[2,3-f]-
[1,10]phenanthroline): To
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