Diiminic Schiff bases: An intriguing class of compounds for a copper-nanoparticle-induced fluorescence study

Article abstract of DOI:10.1002/chem.201201242

The condensation products of salicylaldehyde and different diamines constitute an important class of diiminic Schiff bases (DSBs). This class of compounds has been rediscovered as reducing as well as capping agents under UV irradiation. UV irradiation of alkaline DSB solutions in the presence of water-soluble copper salts has been employed to produce copper nanoparticles (CuNPs). Intriguing CuNP-stimulated fluorescence behavior of the solution has been observed. Depending upon the nature of the spacer in between two iminic bonds, fluorescence enhancement or quenching is observed. Such surprising fluorescence contrast has been ascribed to far-field radiation and lossy surface waves. Making light of things: UV irradiation of alkaline diiminic Schiff base (DSB) solutions in the presence of water-soluble copper salts has been employed to produce copper nanoparticles (CuNPs). Fluorescence enhancement of the exposed DSBs was observed in the presence of in situ produced Cu⁰ (see figure). Copyright

Full text of DOI:10.1002/chem.201201242

FULL PAPER
DOI: 10.1002/chem.201201242
Diiminic Schiff Bases: An Intriguing Class of Compounds for a Copper-
Nanoparticle-Induced Fluorescence Study
Mainak Ganguly,[a] Anjali Pal,[b] Yuichi Negishi,[c] and Tarasankar Pal*[a]
Abstract: The condensation products
of salicylaldehyde and different di-
amines constitute an important class of
diiminic Schiff bases (DSBs). This class
of compounds has been rediscovered as
reducing as well as capping agents
under UV irradiation. UV irradiation
of alkaline DSB solutions in the pres-
ence of water-soluble copper salts has
been employed to produce copper
CuNP-stimulated fluorescence behav-
ior of the solution has been observed.
Depending upon the nature of the
spacer in between two iminic bonds,
fluorescence enhancement or quench-
ing is observed. Such surprising fluores-
cence contrast has been ascribed to far-
field radiation and lossy surface waves.
nanoparticles
(CuNPs).
Intriguing
Keywords: copper · diimines · fluo-
rescence
bases
· nanoparticles · Schiff
Introduction
are very commonly introduced for labeling and cell imaging.
Weak fluorescence signals from a low concentration of fluo-
rophores attached to the molecules as well as the photosta-
bility of molecular fluorophores are two major drawbacks of
this technique.[5] Proximal conducting metallic particles, col-
loids, or surfaces are well known to notably control the
emission behavior of luminophores.[6] By exploiting metal-
nanoparticle platforms, the quantum yield and photostability
of weakly fluorescing molecules are increased. This is
caused by the improved emission efficiency, radiative decay
rate, and/or coupling of the emission with far-field through
scattering. Thus, for improved surface immunoassay and
DNA detection, enhanced wavelength ratiometric sensing,
amplified assay detection, and so forth, MEF is very promis-
ing.[7] On the contrary, quenching by metallic nanoparticles
is also fruitfully employed for competitive fluorescence im-
munoassays, hybridization assays, optical detection of DNA
hybridization, and in optoelectronics.[8]
Intense narrow surface plasmon bands (SPBs) have pro-
voked researchers to study silver and gold nanoparticles in
the context of MEF. Gold nanoparticles can enhance or
quench fluorescence depending on the fluorophore–particle
separation distance, molecular dipole orientation with re-
spect to the particle surface, and size of the nanoparticle.[9–11]
It was found that comparatively smaller (<30 nm) gold
nanoparticles quench the fluorescence emission owing to the
nonradiative transfer from the excited states of luminophore
molecules to the gold nanoparticles.[10] The nanoparticle
scattering efficiency is increased for bigger particles, which
results in enhanced fluorescence.[9–12] The proposition of Lu-
komska et al.[13] that larger aggregated colloids enhance the
fluorescence to a larger extent than smaller ones further
supports this.
Recently, metal nanoparticles have attracted immense atten-
tion in the area of fluorescence spectroscopy in which the
particles behave as nanoantennas. The nanoparticles fre-
quently manipulate light and light–matter interactions at the
nanoscale. The radiative decay rate of emitters placed in
their near field[1–3] can easily be influenced owing to the ex-
istence of localized plasmon polaritons of metal nanoparti-
cles. Mohammadi et al. studied nanoantennas from spheroi-
dal metal nanoparticles as a function of aspect ratio,
volume, background index, and metal.[4] Gold and silver
nanostructures have also been established as nanoantennas.
Owing to the cost of generating gold and silver surfaces,
these substrates are unlikely to find widespread metal-en-
hanced fluorescence (MEF) applications. It is an academic
as well as practical challenge to work with stable copper
nanoparticles as they are generally susceptible to oxidation.
In biological research, including single-molecule detec-
tion, cellular imaging, gene profiling, proteomics, drug dis-
covery, and disease diagnostics, fluorescence techniques
have increasingly found applications. Fluorescent markers
[a] M. Ganguly, Prof. T. Pal
Department of Chemistry
Indian Institute of Technology, Kharagpur
Kharagpur-721302, West Bengal (India)
Fax : (+91)03222-282252
E-mail: tpal@chem.iitkgp.ernet.in
[b] Dr. A. Pal
Department of Civil Engineering
Indian Institute of Technology, Kharagpur
Kharagpur-721302, West Bengal (India)
[c] Dr. Y. Negishi
Department of Applied Chemistry
Tokyo University of Science
Tokyo-1628601 (Japan)
Copper possesses a very large value of the imaginary
component of dielectric constant (more than twice) than
that of noble metals in the wavelength range of 300 to
600 nm. So, it is expected that in this wavelength range due
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201242.
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to higher ohmic losses, copper nanoparticles will mostly
quench the luminescence at their close proximity.[14] But for
large nanoparticles (>100 nm) with high aspect ratio, the in-
crease of coupling efficiency of the fluorescence emission to
the far field through nanoparticle scattering excels MEF. In
this case, as effect of ohmic losses on MEF is decreased, less
expensive metals such as Al and Cu, which are more lossy
than Ag and Au, can be considered as promising candidates
for enhancing fluorescence.[15]
geometry of the metallic nanostructures. The size and shape
contribute to different surface plasmonic modes with drastic
enhancement of the fluorescence signals at “hotspots”.[31] At
very close proximity (<20 nm), the lifetime of the fluoro-
phore drops dramatically and the emission intensity is
strongly reduced. This quenching effect in the proximity of
metal nanoparticles has been ascribed to lossy surface waves
(LSWs), dissipated losses, ohmic losses, and such similar
phenomena, all of which are examples of nonradiative dissi-
pation of energy within metals.
Salen[16] (the condensation product of salicylaldehyde and
diamines) and salen-like molecules in which two imine
bonds are separated from each other with an aliphatic or ar-
omatic group have been found to act as a potential fluoro-
phore for many applications. Liu et al.[17] reported Mn–salen
as a promising probe molecule to detect traces of DNA in
solution because of the prominent alteration of the fluores-
cence property of the molecule. A remarkable decrease of
fluorescence intensity is noticed for Mn–Schiff bases bound
to DNA with a blueshift of excitation and emission peaks.
Other reports are available in the literature in which the flu-
orescence properties of salen, salen derivatives, and their
metal complexes were exploited for the determination of
trace amounts of hazardous or useful substances, namely,
Mg,[18] H2O2, triacetone organic peroxides,[19] and cyanide.[20]
The determination of aliphatic primary amines by flow-in-
jection fluorometry using beryllium–Schiff base complexes
has been performed by Akoi et al.[21] Very recently, an at-
tempt to make a silver sensor was reported from our labora-
tory.[22] We present herein a very interesting observation for
copper-enhanced fluorescence spectroscopy again by using
salen and salen derivatives. The spacer (the aromatic/ali-
phatic group in between two iminic bonds) determines the
extent of the enhancement/quenching, which is quantified
for the first time with photoproduced copper nanoparticles
(CuNPs) in the solution phase.
Results and Discussion
Six DSBs containing different spacers in between two imine
bonds have been synthesized (Figure 1). It is known that
they form coordination complexes with CuII. In the present
study, we describe the synthesis of CuNPs using a photo-
Figure 1. Different DSBs synthesized from salicylaldehyde and different
diamines.
For the synthesis of CuNPs, many methods can be adopt-
ed: use of supercritical carbon dioxide,[23,24] water-in-oil mi-
croemulsions,[25] high-temperature decomposition of organo-
metallic precursors,[26] a polyol reduction method,[27] and a
photochemical route.[28] Herein, we report the synthesis of
CuNPs by means of a reproducible photoactivation techni-
que using salen and salen-like Schiff bases (DSBs). The evo-
lution strategy for CuNPs is endowed with high fluorescence
enhancement of the exposed reaction mixtures on the one
hand and quenching of the fluorescence on the other. The
contrast of the luminescence behavior depends upon the
spacers (the groups in between the two iminic bonds) of the
DSBs.
Two mechanisms have been anticipated for fluorescence
enhancement: 1) localized surface plasmon resonance
(LSPR) at the surface of the metal NPs causing enhance-
ment of the electromagnetic field and 2) coupling between
the surface plasmon field of the metal and the molecular
dipole of probe molecules.[29] The radiating plasmon (RP)
model suggests that the far-field radiation originated from
scattering for it to be responsible for the observed fluores-
cence intensity.[30] The extent of the enhancement lies in the
chemical route for the first time. As a result of the synthetic
procedure, we have found interesting fluorescence contrast
behavior from CuNPs bearing exposed DSBs with variable
spacers.
Nine hours of irradiation of an alkaline reaction mixture
of a copper salt in the presence of salen or salen-like Schiff
bases, that is, DSBs, causes the formation of CuNPs under
UV light. The in situ generated CuNPs exhibit strikingly dif-
ferent fluorescence behavior out of the exposed reaction
mixture. An irradiation time of more than nine hours shows
a constant fluorescence intensity of the reaction mixture,
that is, the fluorescence or absorption property does not
change further. Figure 2 represents the formation kinetics of
CuNPs (from fluorescence studies).
DSB acts as both a reducing and a stabilizing agent since
no other reducing agent and/or stabilizer is introduced into
the reaction mixture. We presume semiquinone formation
from the phenolic OH group, through the same mechanism
of nanoparticle synthesis in the presence of DSB as pro-
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Diiminic Schiff Bases
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Figure 2. Formation kinetics of CuNPs under UV irradiation (ꢀ365 nm)
in the presence of C2 observed from fluorescence measurements. Condi-
tions: C2 concentration=1.66ꢁ10À4 m; CuSO4 concentration=83ꢁ10À6 m;
lex =290 nm.
posed by Selvakannan et al.[32] Phenolate ions lose electrons,
which in turn reduce CuII to Cu0. UV light provides the nec-
essary activation energy for the productive photoactivation
process. Without UV irradiation, quenching of fluorescence
due to the formation of cupric hydroxide in the alkaline
medium occurs. Visible light of variable wavelengths, for ex-
ample, 450, 550, and 650 nm, shows a similar type of
quenching. However, it has been found that only UV irradi-
ation (ꢀ365 nm) causes the drastic change in fluorescence
(Figure 3).
A new peak in the UV-visible spectrum at approximately
300 nm (see the Supporting Information, Figure S1) and an
approximately 52 nm blueshifted fluorescence spectrum
(lem =449 nm) of the alkaline DSB solution after UV expo-
sure are due to the formation of semiquinone. The blueshift
of 20 nm of the emission maxima of the exposed C2 in the
presence of photoproduced CuNPs has been observed as
proposed by Geddess and Lakowicz due to polarity consid-
erations.[33] From the mass spectra (Figure S2 in the Support-
ing Information), we observed the molecular-ion peak of C2
at m/z 282 before and after UV-light irradiation in the pres-
ence and absence of copper salts. The observed molecular-
ion peak speaks against the formation of a dimer (molecular
weightꢁ2) or quinone (molecular weight+32).
Figure 3. Fluorescence spectra of a C2 solution recorded under different
experimental conditions. A) An unexposed C2 solution (1), a C2 solution
after UV exposure (2), a mixture of C2 and CuSO4 solution after UV ex-
posure (3), a mixture of C2 and CuSO4 solution without UV exposure (4).
Inset: fluorescence microscopic images of solutions (2) and (3) drop-
casted on glass slides. B) Comparative fluorescence spectra of the mix-
ture of C2 and CuSO4 solution after UV (Y) and visible light (X) expo-
sure. Conditions: C2 concentration=1.66ꢁ10À4 m; CuSO4 concentration=
83ꢁ10À6 m; lex =290 nm.
The oxidation state of the photoproduced CuNPs has
been confirmed by X-ray photoelectron spectroscopy (XPS;
Figure 4) studies. The peaks at 932.66 and 952.72 eV are as-
signed to 2p3/2 and 2p1/2, respectively (and no shake-up
peak) when copper is in a zero oxidation state. TEM images
of the photoproduced CuNPs capped with exposed C2
reveal that the particles are spherical with diameters of 3–
6 nm. The lattice fringe of 0.208 nm for the [111] plane and
the selected-area electron diffraction (SAED) image shown
in Figure 5 further support the zero oxidation state of Cu0.
The TEM images of CuNPs that are capped with other cap-
ping agents (C1, C3, C4, C5, and C6) are shown in Figure S3
in the Supporting Information.
light exposure, this peak is greatly decreased and formation
of a new peak at 449 nm is observed. Formation of semiqui-
none as a result of UV irradiation causes a loss of resonance
in the fluorophore system, which results in a higher energy
emission (blueshift of the emission maxima) but with lower
intensity.[34] If UV irradiation of the DSB is performed in
the presence of the copper salt, the in situ generated Cu0
produces a dramatic enhancement of the fluorescence (lem
ꢀ429 nm) for the individual exposed reaction mixtures for
the cases of C1, C2, C3, and C4 (Scheme 1).
Interestingly, for C5 and C6 the quenching phenomenon is
observed. When the two iminic bonds are separated by ali-
phatic groups (C1–C3), a huge enhancement of the emission
intensity is observed. On the contrary, an aromatic group
between the two iminic bonds exhibits a low enhancement
The alkaline DSB solutions exhibit fluorescence spectra
with an emission maximum at 501 nm. After 365 nm UV-
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T. Pal et al.
Figure 5. A) TEM image, B) SAED pattern, and C) HRTEM image of
the photoproduced CuNPs prepared from C2.
Figure 4. XPS spectra of C2-capped CuNPs: A) wide angle and B) high
resolution.
of the fluorescence (C4) and sometimes quenching (C5 and
C6) in the presence of photoproduced Cu0. So, the spacer in
between the iminic bonds dramatically tunes the fluores-
cence behavior of the exposed DSBs in the presence of
CuNPs produced in situ in the reaction mixture as a conse-
quence of UV-light irradiation. The bar diagram (Figure 6)
shows the extent of different fluorescence intensities in the
presence of different spacers when CuSO4 is employed. The
C2-capped CuNP shows the maximum enhancement of fluo-
rescence and the C6-capped CuNP exhibits the most effi-
cient quenching capability of the DSBs.
It was observed that the fluorophore emission is strongly
quenched by metallic surfaces when the metal to fluoro-
phore proximity falls by approximately 50 ꢂ.[33] The opti-
mum distance for the probe molecules from the metallic sur-
face to exhibit maximum fluorescence enhancement is ap-
proximately 100 ꢂ, whereas Sokolov et al.[35] claimed the
distance to be approximately 600 ꢂ. The DSBs containing
an aromatic group in between two iminic bonds (C4, C5, and
C6) are more rigid than those with aliphatic spacers (C1, C2,
and C3). Aliphatic spacers can acquire different rotameric
conformations that hinder a closer approach of the DSBs to-
wards the metal-nanoparticle surface, thereby showing a
higher degree of enhancement; whereas the rigid capping
agents entangle the metal nanoparticles, that is, enable a
closer approach, which thereby produces quenching (or less
Scheme 1. Formation of CuNPs under UV-light exposure from an alka-
line DSB solution in the presence of a copper salt (DSB/CuII =2:1).
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Diiminic Schiff Bases
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Figure 6. Fluorescence spectra (A–F) showing enhancement/quenching of exposed DSBs in the presence of photoproduced CuNPs. The bar diagram rep-
resents different degrees of enhancement from the exposed DSBs in the presence of photoproduced CuNPs (I=fluorescence intensity of exposed C2 in
the presence of photogenerated CuNPs; I0 =fluorescence intensity of exposed C2 solution without copper). Conditions: C2 concentration=1.66ꢁ10À4 m;
CuSO4 concentration=83ꢁ10À6 m; lex =290 nm.
enhancement). The oscillations created in the CuNPs at the
close proximity of C5 and C6 are not capable of radiating
owing to optical constraints at the metal–sample interface.
Induced plasma at short metal–fluorophore distances are
trapped and decay as heat.[30] The phenomena of fluores-
cence enhancement and quenching are dependent on the
scattering cross section and absorption cross section. When
the scattering cross section dominates, the induced plasmons
radiate, which results in enhancement of the fluorescence as
in the cases of C1, C2, C3, and C4.
Not only CuSO4, but also other common water-soluble
copper salts have been tested to check the efficacy of anions
for the fluorescence enhancement of exposed C2 (Figure 7;
Figure S4 in the Supporting Information). The extent of the
fluorescence enhancements was comparable for all the
copper salts except for the chloride salts. This indicates that
in situ produced Cu0 is mainly responsible for the drastic en-
hancement of fluorescence, not the counter anions. Howev-
er, if CuCl2 is employed, the degree of enhancement is com-
paratively lower. This is not a very unusual occurrence. The
fluorescence quenching capability of ClÀ reported by Garcia
et al.[36] and several other groups is observed in our case also
(ClÀ causes spin–orbit coupling effects).[37]
We have also performed a fluorescence study of exposed
C2, unexposed C2, and exposed C2 in the presence of CuSO4
with temperature variation (Figure 8). In all three cases, the
fluorescence intensity decreases with the increase of temper-
ature.[38] The increase of temperature assists important
Brownian motions that facilitate energy loss through dynam-
ic quenching by the solvent molecule. Consequently, a de-
crease of fluorescence intensity as well as fluorescence
quantum yield is observed with the rise of temperature. The
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Figure 7. Effect of different counter anions of CuII on the fluorescence
enhancement of the exposed C2 solutions. Conditions: C2 concentration=
1.66ꢁ10À4 m; CuIIX concentration=83ꢁ10À6 m (X=counter anion); lex
=
290 nm.
emission peak at 501 nm for the unexposed C2 rapidly de-
creases with the increase of temperature. On the contrary,
the fluorescence intensity of the emission peak is slowly re-
duced for the exposed C2 and exposed C2 containing in situ
generated CuNPs. But, the extent of the lowering of the flu-
orescence for the exposed C2 and for the CuNP-induced en-
hanced fluorescence of exposed C2 is exactly same; this im-
plies that the enhancement by CuNPs is not very sensitive
to temperature variation. Figure 8C shows that the I/I0 value
(I=fluorescence intensity of exposed C2 in the presence of
Cu0; I0 =fluorescence intensity of exposed C2 in the absence
of Cu0) remains unaltered with an increase of temperature.
If we add ethylenediaminetetraacetic acid disodium salt
(Na2–EDTA) along with the copper salts in the aqueous al-
kaline solution of the DSB and then expose it to UV light,
instead of enhancement of fluorescence, damping of the flu-
orescence takes place. Again, very insignificant enhance-
ment is noticed when we irradiate a cuproammonium com-
plex in the presence of a DSB. Both Na2–EDTA and ammo-
nia form highly stable complexes with CuII (see Figure S5 in
the Supporting Information). At room temperature, the sta-
bility constant of the former complex is 5ꢁ1018 and for the
latter it is 1.1ꢁ1013. DSBs could not at all reduce CuII to Cu0
from such stable complexes. The phenomenon of quenching
is due to the effect (spin–orbit coupling effect) of CuII,
which is very commonly reported in the literature.[39] Inter-
estingly, the introduction of an individual complexing agent,
Na2–EDTA or ammonia, to the already exposed reaction
mixtures, that is, the reaction mixtures with preformed
CuNPs, had no effect on the fluorescence behavior.
Figure 8. A) Temperature-dependent fluorescence intensity study. B) Plot
of IT/IRT versus the temperature of exposed C2 in the presence of in situ
generated CuNPs and unexposed and exposed C2 solutions in the ab-
sence of copper (IT =fluorescence intensity at temperature T; IRT =fluo-
rescence intensity at room temperature (308C)). C) Plot of I/I0 versus
temperature (I=fluorescence intensity of exposed C2 in the presence of
photogenerated CuNPs; I0 =fluorescence intensity of exposed C2 solution
without copper). Conditions: C2 concentration=2.5ꢁ10À4 m; CuSO4 con-
centration=83ꢁ10À6 m; lex =290 nm.
The fluorescence-lifetime measurement (Figure 9) has
been performed with exposed C2 in the presence of photo-
produced CuNPs. It is observed that 31% of molecules have
a lifetime of 1.5 ns and 69% of molecules possess a lifetime
of 3.6 ns. The average lifetime of exposed C2 in presence of
CuNPs is 2.9 ns. Owing to insignificant fluorescence intensi-
ty, the time-correlated single-photon counting (TCSPC) in-
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Figure 9. Fluorescence-lifetime measurement of the exposed C2 solution
(gray color) in the presence of in situ generated CuNPs. Conditions: C2
concentration=1.66ꢁ10À4 m; CuSO4 concentration=83ꢁ10À6 m; lex
=
290 nm.
strument cannot measure the fluorescence lifetime of the
exposed C2 in the absence of copper salts.
Excitation energy is an important parameter for observing
fluorescence enhancement (Figure 10). We chose 290 nm as
the excitation wavelength for all the cases mentioned above
and observed the highest I/I0 value. Above and below this
particular excitation wavelength, the I/I0 ratio becomes less.
It is worth mentioning that just by changing the excitation
wavelength, I/I0 can be tuned many times larger than 1 to a
fraction (<1). Figure 10 shows that in between 260 and
350 nm, I/I0 >1, that is, there is enhancement. Above and
below the range, quenching occurs, that is, I/I0 becomes <1.
The concentration of the DSB used in the experiment is
1.66ꢁ10À4 m and with this concentration, a 83ꢁ10À6 m cop-
per(II) concentration shows maximum fluorescence en-
hancement. The phenomenon of enhancement starts to
appear when the concentration of CuII becomes 11ꢁ10À6 m.
A gradual increase of the CuII concentration causes the in-
crease of the fluorescence emission peak. Above 83ꢁ10À6 m,
a further increase of the copper concentration causes a grad-
ual decrease of fluorescence. Consequently, when the DSB/
CuII ratio is 2:1, the highest fluorescence intensity is noticed.
Salen-like compounds form copper complexes in which the
ligand/copper ratio is 2:1.[40] So, it is thought that the
copper–salen complex is formed first. Then, owing to UV ir-
radiation, it is converted to Cu0. The CuNPs maintain an
exact distance from the exposed DSBs as a result of the syn-
thetic strategy, which is responsible for the huge fluores-
cence intensity enhancement from the DSB in the reaction
mixture. If we do not use UV exposure, such CuII to Cu0 re-
duction does not take place at all. The CuII ion is prone to
coordination and is weakly bonded to DSB and therefore
easily forms CuNPs in solution upon UV irradiation. As a
matter of fact, without UV exposure, no enhancement of the
fluorescence is observed from the unexposed solution.
When a higher copper concentration is employed, reduction
Figure 10. A) A selection of optimum excitation wavelengths (lex) to indi-
cate maximum fluorescence enhancement of exposed C2 in the presence
of in situ produced CuNPs with respect to exposed C2 (without copper).
B) A pie diagram of I/I0 at different excitation wavelengths (lex). I=fluo-
rescence intensity of exposed C2 in presence of in situ generated CuNPs,
I0 =fluorescence intensity of exposed C2 solution. Conditions: C2 concen-
tration=1.66ꢁ10À4 m; CuSO4 concentration=83ꢁ10À6 m.
of CuII is not quantitative and quenching is observed due to
the spin–orbit coupling effect.[37] It may also be possible that
Cu0 comes closer to the DSBs, that is, some DSBs may be
present at the quenching zone of Cu0. A copper concentra-
tion of less than 83ꢁ10À6 m supports the idea that some
DSBs are deprived of the effect of Cu0, which is responsible
for the fluorescence enhancement (Figure 11).
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T. Pal et al.
Figure 11. Fluorescence intensity of the exposed C2 solutions with differ-
ent concentrations of CuNPs. The CuNP concentration varies because of
the use of different concentrations of aqueous copper salt solution. Con-
ditions: C2 concentration=1.66ꢁ10À4 m; lex =290 nm.
Copper, a base metal, is prone to corrosion. Metallic
copper is easily oxidized in air and the oxidation reaction is
facilitated in alkaline solution. Copper hydroxide is formed
under ambient conditions owing to the interaction of alkali
and copper metal. With this idea in mind, we have designed
an experiment to monitor the corrosion of a piece of copper
wire 51 cm long and 126 mm wide (measured optically) in
0.1m NaOH. The submerged commercial copper wire cor-
roded in alkaline solution. The copper ions from the alka-
line solution were estimated fluorometrically regularly for
seven days after a time interval of one day by using our own
protocol with DSB as described above. Alkaline C2 solution
was added to an aliquot of the corrosion-produced cop-
ACHTUNGTRENNUNGper(II) solutions and exposed under 365 nm UV light for
nine hours. Then, from the fluorescence studies, copper dis-
solution and quantification were performed (Figure 12).
Copper is generally susceptible to oxidation and it is even
more so at the nanoscale. The CuNPs prepared by the pro-
posed UV irradiation routes are exceptionally stable and
robust as revealed from fluorescence measurements. No per-
ceptible change of the fluorescence intensity was observed
even after keeping DSB-bound CuNPs in solution for more
than one month at ambient temperature. An unaltered fluo-
rescence intensity proves that no aggregation of the CuNPs
or decomposition of the fluorophore takes place (Figure 13).
Thus, CuNPs enjoy exceptional stability even in alkaline sol-
ution. This may be due to the presence of a large excess
amount of exposed DSBs that act as capping agents and
protect the particles for a longer time. Diamines,[41] alkane-
thiols, oleic acid,[42] and so forth are well-known stabilizers
for CuNPs in solution. In the present case, exposed DSB-
capped CuNPs do not undergo oxidation, presumably as a
Figure 12. A) Experimental setup showing CuII leaching from the copper
wire in NaOH (0.1m) solution. B) Time-dependent CuII leaching, quanti-
fied from a fluorescence study after UV irradiation. Conditions: C2 con-
centration=1.66ꢁ10À4 m; lex =290 nm. C) A plot of CuII concentration
leached in NaOH (0.1m) solution at different time intervals.
À
result of a strong Cu N bond affinity. Again, zeta-potential
measurements for representative DSB-capped CuNPs (C2-
hydrosol stable for a longer time The particle size and mor-
phology are not changed even when the solution becomes
aged (>30 days). It speaks in favor of the excellent capping
capability of exposed DSBs for CuNPs.
capped CuNPs) indicate
(À30.3 mV), which is also responsible for making the copper
a high surface-charge density
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Diiminic Schiff Bases
FULL PAPER
for nine hours. We have repeated the experiment for other
metal ions, such as, Co, Ni, Zn, Hg, Mn, Ag, Au, Mg, K, and
Na. The fluorescence enhancement was not observed from
these metal ions under the proposed experimental condi-
tions. As a consequence, copper is an exceptional candidate
in the family of coinage metals for displaying such metal-en-
hanced fluorescence unlike gold and silver. This observation
clearly indicates that DSBs become a simple potential fluo-
rometric sensor for copper. We have also studied the
copper-enhanced fluorescence intensity of the exposed
DSBs in presence of other metal ions. The enhanced fluores-
cence is slightly dampened[37] in the presence of other metal
ions. Nevertheless, the copper-enhanced fluorescence (in the
presence of other metal ions) is much higher than the other-
metal-induced fluorescence. So this strategy is reliable and
convenient for generating a copper sensor (Figure 14).
Figure 13. Fluorescence intensity of exposed C2 CuNPs after ageing.
The high fluorescence enhancement of the fluorophore in
the presence of metal nanoparticles is due to an increased
rate of excitation and intrinsic radiative decay rate. Metal
nanoparticles concentrate the local field on the fluorophore,
which increases the rate of excitation (a phenomenon
The uniqueness of Cu0 is very important. A drastic en-
hancement of fluorescence is observed when any copper salt
solution is added to the alkaline C2 solution and is irradiated
Figure 14. A) Bar diagram indicating different I/I0 values in the presence of different metal ions. A1) Fluorescence intensity of exposed C2 in the pres-
ence of different metal ions. B) Bar diagram indicating different I/I0 values in the presence of copper and other metal ions. B1) Fluorescence intensity of
exposed C2 in the presence of copper and other metal ions. C2 concentration=1.66ꢁ10À4 m; CuSO4 concentration=83ꢁ10À6 m; lex =290 nm, Other
metal-ion concentration=83ꢁ10À6 m.
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15853

T. Pal et al.
known as the “lightening rod effect”).[34,43–45] Metal nanopar-
ticles can also modify the rate at which a fluorophore emits
a photon.[46] According to the radiating plasmon (RP)
model, instead of the fluorophore it is the metal that radi-
ates after energy transfer from fluorophore. Far-field radia-
tion from fluorophore-induced plasmons and trapped plas-
mons are accountable for enhancement and quenching of
fluorescence.
Smaller CuNPs are able to have a stronger interaction with
the metal, which facilitates energy transfer from the fluoro-
phore. So, we have found a new type of fluorophore show-
ing an intriguing fluorescence behavior. The virtue of our
synthetic procedure keeps the fluorophore, that is, capping
agent (the exposed DSBs), at an interesting position from
the CuNPs thereby sometimes causing quenching (due to
lossy surface waves) or enhancement (far-field radiation by
means of the nanoantenna) of fluorescence.
In our earlier paper, a silver- and gold-nanoparticle-in-
duced fluorescence contrast phenomenon was reported.[22]
AgNPs or AuNPs were produced upon UV irradiation from
alkaline DSB solutions as has been reported herein for the
copper hydrosol evolution. The irradiation time was 3 h for
silver and 11 h for gold systems. AgNPs always induce fluo-
rescence enhancement (except for C5), whereas the present
CuNPs exhibit enhancement for C1 to C4. On the contrary, a
stable gold hydrosol is obtained after prolonged photoacti-
vation only from C2 and C5. The gold particles quench the
fluorescence of these DSB solutions. But all the DSBs
(except C5) show an enhancement of fluorescence in the
presence of photoproduced AgNPs. In this context it is im-
portant to mention that with the increase of irradiation time
(>3 h), the stability of the silver hydrosol decreased, and as
a result, so did the extent of the fluorescence enhancement.
It is imperative to use silver in a twofold excess over DSBs,
that is, the metal/DSB ratio remains at 2:1 for photoactiva-
tion, to observe maximum fluorescence enhancement (ꢀ9-
fold for C2). Using this ratio and 3 h of irradiation time did
not produce any significant fluorescence enhancement for
the copper system. More precisely, for the copper system,
the copper/DSB ratio needs to be 1:2 and the required irra-
diation time is 9 h for maximum enhancement (ꢀ10-fold for
C2). So under the present experimental conditions silver
does not show any significant fluorescence enhancement
whereas copper does (i.e., no interference from silver). Thus
by tuning the experimental conditions, a silver(I) and cop-
per(II) sensor can be designed with a proper DSB.
Copper-enhanced fluorescence in the solution phase is
rarely reported. Although gold and copper possess similar
optical constants, copper exhibits normally larger losses than
gold, as evidenced from its broader plasmon resonance.[4]
Surprisingly, we have observed efficient quenching phenom-
ena for in situ generated AuNPs (Figure 14), whereas both
enhancement and quenching are obtained for in situ pro-
duced CuNPs depending on the spacer in between two
iminic bonds of the DSB. Usually, the scattering efficiency is
smaller for copper, and CuNPs having an especially smaller
size are known for quenching of fluorescence. For larger
nanoparticles, copper-enhanced fluorescence is found in the
literature.[15] The synthetic protocol disclosed herein gener-
ates CuNPs of a smaller size (3–6 nm) showing fluorescence
enhancement. The concept of the RP model can adequately
explain the enhancement phenomena. Our synthetic strategy
can eliminate the interband absorption for C1, C2, C3, and C4
responsible for quenching. Low-quantum-yield fluorophores
(exposed DSBs) readily transfer energy to CuNPs, which ra-
diate more efficiently than the fluorophore in free space.
Conclusion
The dramatic fluorescence contrast (enhancement and
quenching) of exposed DSBs in the solution phase has been
reported in the presence of in situ generated CuNPs. The
spacers in between the two iminic bonds have been found to
be responsible for such dramatic contrast phenomena. The
nature of the synthetic route and that of the spacer increase
the fluorescence enhancement in the solution phase, which
may prove useful in metal-enhanced fluorescence studies in
the solution phase. The unique fluorescence enhancement
under these experimental conditions may provide promising
information for the design of copper sensors.
Experimental Section
Materials and instruments: All the reagents were of AR grade. Through-
out the experiments, triple-distilled water was used. Copper salts, sali-
AHCTUNGTERGcNNUN ylaldehyde, all the diamines, and ethylenediaminetetraacetic acid disodi-
um salt were obtained from Sigma–Aldrich. NaOH was purchased from
HiMedia Laboratories Pvt. Ltd. Ammonia was purchased from Merck.
All glassware was cleaned with freshly prepared aqua regia, subsequently
rinsed with copious amounts of distilled water and dried well before use.
The sample solution was irradiated with a TUV 15W/G 15 T8 ultraviolet
light (Philips India) source. All UV/Vis absorption spectra were recorded
in a SPECTRASCAN UV 2600 digital spectrophotometer (Chemito,
India). FTIR spectra were recorded in a FTIR Nexus spectrophotometer
(Thermo Nicolet). 1H NMR spectrum was obtained with a 400 MHz
Bruker NMR instrument. XPS analysis was carried out with a VG Scien-
tific ESCALAB MK II spectrometer (UK) equipped with a MgKa excita-
tion source (1253.6 eV) and a five-channeltron detection system. The flu-
orescence measurements were carried out at room temperature using a
LS55 fluorescence spectrometer (Perkin–Elmer, USA). TEM analysis
was performed with a H-9000 NAR instrument (Hitachi) using an accel-
erating voltage of 300 kV.
Preparation of DSBs: Methanolic solutions of ethylenediamine (10À2 m)
and salicylaldehyde (2ꢁ10À2 m) were mixed with constant stirring. Then
the mixture was heated at reflux for approximately 4 h and a yellow
product (C1) was obtained after cooling. The product was recrystallized
from methanol. In a similar procedure, C2 (salprn), C3 (salben), C4, C5,
and C6 were synthesized by using 1,3-propylenediamine, 1,4-butanedia-
mine, o-phenylenediamine, m-phenylenediamine, and p-phenylenedi-
AHCTUNGERTGaNNUN mine, respectively. The only difference is that in place of ethylenedi-
AHCTUNGERTGaNNUN mine different amines were used. Figure 1 indicates the structures of all
the six DSBs.[16]
The melting points (see the Supporting Information), 1H NMR spectra
(see Figure S6 in the Supporting Information), and IR spectra (see Fig-
ure S7 in the Supporting Information) confirm the synthesis of the DSBs.
Synthesis of the CuNPs: DSBs are insoluble in water. So, we dissolved
them in an alkaline medium. A stock solution of 2.5ꢁ10À3 m DSB was
15854
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Chem. Eur. J. 2012, 18, 15845 – 15855

Diiminic Schiff Bases
FULL PAPER
prepared by dissolving an appropriate amount of DSB in 0.1m aqueous
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solution (0.2 mL), and 10À2 m CuSO4 solution (0.025 mL) were mixed to-
gether. The final concentration ratio of DSB to CuSO4 was maintained at
2:1. Then the well-stoppered cuvette was irradiated under a UV lamp
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Received: April 12, 2012
Revised: August 15, 2012
Published online: October 12, 2012
Chem. Eur. J. 2012, 18, 15845 – 15855
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15855