Ce (III) - Porphyrin Sandwich Complex Ce2(TPP)3: A Rod-Like Nanoparticle as a Fluorescence Turn-Off Probe for Detection of Hg (II) and Cu (II)

Article abstract of DOI:10.1007/s10895-015-1761-4

In this study the researcher reports a novel, one step synthesized rod-like nanoparticles of cerium (III)—tetraphenylporphyrin sandwich complex as a spectrofluorometric sensor to measure trace amount of Hg (II) and Cu (II) metal ions. Moreover, the synthesized fluorescent probe was able to detect higher amounts (>10^?4?M) of Hg (II) in aqueous media by changing the color which can also be used as a selective mercury naked-eye sensor. The selectivity and sensitivity of the sensor based on its fluorescence quenching in the presence of Hg (II) and Cu (II) were studied according to the Stern-Volmer equation. The detection limit of the sensor was 16?nM for Hg (II) and about 2.34?μM for Cu (II) ions. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10895-015-1761-4

J Fluoresc
DOI 10.1007/s10895-015-1761-4
ORIGINAL ARTICLE
Ce (III) - Porphyrin Sandwich Complex Ce (TPP) : A Rod-Like
2
3
Nanoparticle as a Fluorescence Turn-Off Probe for Detection
of Hg (II) and Cu (II)
1
Ramin Boroujerdi
Received: 7 December 2015 /Accepted: 28 December 2015
#
Springer Science+Business Media New York 2016
Abstract In this study the researcher reports a novel, one step
synthesized rod-like nanoparticles of cerium (III)—
tetraphenylporphyrin sandwich complex as a spectrofluoro-
metric sensor to measure trace amount of Hg (II) and Cu (II)
metal ions. Moreover, the synthesized fluorescent probe was
able to detect higher amounts (>10 M) of Hg (II) in aqueous
media by changing the color which can also be used as a
selective mercury naked-eye sensor. The selectivity and sen-
sitivity of the sensor based on its fluorescence quenching in
the presence of Hg (II) and Cu (II) were studied according to
the Stern-Volmer equation. The detection limit of the sensor
was 16 nM for Hg (II) and about 2.34 μM for Cu (II) ions.
b) Semiconductor nanoparticles that contains quantum dots,
nanorods, etc. Their bioconjugation ability make them
suitable for synthesizing biosensors [4, 5];
c) Trivalent lanthanide ions, Ln (III), and their complexes are
the third group of fluorescent compounds. Their fluores-
cent emission comes from 4f–5d electron transitions and
metal-to-ligand (MLCT) charge transfer, ligand-to-metal
(LMCT) charge transfer and 4f–4f electrons transition in
complex forms. These type of fluorescent probes beside
their well bioconjugation potential are able to getting
near-infrared (NIR) emission for deeper penetration which
could be used in biology and medicine or sensory studies
with two photon fluorescence light microscopy [1, 6–9].
−4
.
.
Keywords Cerium (III) complex Tetraphenylporphyrin
In this work it is tried to synthesize a fluorescence sensor
based on Ce (III) as trivalent lanthanide ion. For this purpose,
the researcher tries to use tetraphenylporphyrin as ligand. The
first attempt to synthesize Ce-TPP complex was done by
Ching-Ping Wong in 1983; He tried to synthesize lanthanum
.
.
Fluorescent probe Rod-like nanoparticles Mercury
.
detection Copper detection
(
III) - porphyrins with a 4 hours reflux procedure in
Introduction
trichlorobenzene as follows:
H2TPP þ Ln ðacacÞ :xH2O → LnTPPðacacÞ þ 2Hacac
3
Fluorescence sensors fall within three major groups based on
their molecular structures [1]:
But, as Graham Bisset the cheker of the paper argues
that all the lanthanum (III) - porphyrins were synthesized
except Ce-TPP, even after 26 hours refluxing [10]. In
a) Organic luminophores which are mostly large organic
molecules consist of conjugated pi systems[2, 3];
1
986, Johann W. Buchler pointed out that some metals
like Ce (III) could form Ce (TPP) sandwich system
2
3
[
11]. Moreover, Robert J. Donohoe, in 1988, reported
*
Ramin Boroujerdi
boroujerdi.r68@gmail.com
the synthesize of Ce(TPP) by Buchler method, and con-
2
cluded that even after 3 days refluxing the yield of the
reaction was very poor [12]. In addition, in 2011, David
Ecija found a new method of synthesizing Ce (TPP)
1
2
3
Department of Chemistry, Faculty of Sciences, University of Birjand,
Birjand, Iran
sandwich complex nanoparticles; He put an Ag(111)

J Fluoresc
isotope, contained a layer of TPP, under ultra-high-
vacuum conditions and a beam of atomic Ce by a temper-
ature programed reaction all of which led to the creation
of Ce-TPP sandwich complex [13]. Lately in 2014, Qiu-
Unlike the toxic mercury, some other metals like cop-
per are necessary for human body and play an important
role in various biological processes. However, exposure
to a high level of even these metals could lead to health
damages. In particular, high amounts of copper even for
a short time can cause gastrointestinal disturbance,
while long-term exposure can cause liver and kidney
damages [33–39]. Consequently, over the past decades,
various analytical methods such as spectroscopic and
electrochemical methods have been developed to detect
the amount of this metal ion [40–44].
xia Xu reported the synthesize of Ce(TPP)NO which was
3
not reported before [14]. These were all the studies and
researches about Ce-TPP complexes which have been
done so far similar to this study. The most important fac-
tors which make studying this complex out of scholars’
interest might be the lack of a simple, cheap and better
yield method for synthesizing Ce-TPP complex. This pa-
per tries to introduce a one-step cheap method which
yields better results than the old methods of synthesizing
Ce-TPP. The rod-like nanoparticles of the Ce (TPP) has
Here, selective fluorescence quenching ability of the
Ce (TPP) nano-complex in the presence of mercury and cop-
2
3
per metal ions is reported for measuring the amount of these
toxic metals in real samples as one of its applications.
2
3
been synthesized for the first time.
Mercury can accumulate in cells and cause damages
to the central nervous system. It constitutes a serious
threat to natural environment which, due to these toxic-
ity effects, makes it one of the most concerned elements
Materials and Methods
Reagents
[
15–18]. Atomic absorption spectrometry (AAS) [19],
inductively coupled plasma atomic emission spectrome-
try (ICP-AES) [20], inductively coupled plasma mass
spectrometry (ICP-MS) [21], surface-enhanced Raman
scattering (SERS) [22, 23] and electrochemical analysis
All chemicals were purchased from E. Merck, Darmstadt,
Germany, and belonged to analytical reagent grade. All the
aqueous solutions were prepared using double distilled water
(DDW). Sigma-Aldrich HEPES used as buffer. The metal ions
stock solutions were made up of their nitrate and chloride
salts. The Hg (II) ions were of its acetate salt.
[
24, 25] are well known methods which conventionally
use for detecting mercury. These methods suffer from
several problems like being time-consuming pretreat-
ments, sophistications, complicated detection processes,
and costliness. Thus, development of a simple, rapid,
sensitive and selective method for mercury detection
like spectrophotometric and spectrofluorometric is still
desirable. The design of selective and sensitive fluores-
cence sensors based on organic conjugated molecules
that are capable of detecting these metal ions has been
the subject of strong interest, due to its serious threat to
human health and natural environment [26–32].
Apparatus
The fluorescence measurements were carried out on a
Shimadzu RF-5301PC spectrofluorometer with a 1 cm
quartz fluorescence cell at room temperature (298±2 K).
UV-visible spectra were taken with double beam Analytik
Jena Specord Plus spectrophotometer. AA-6300 Shimadzu
atomic absorption spectrometer (AAS) employed to
Fig. 1 UV–vis spectrum of
synthesized TPP ligand

J Fluoresc
were used for Elemental Analysis. For centrifugation,
Hettich zentrifugen model EBA 200 and for magnetic stir-
ring of the solutions Wisettir model MSH-20D were used.
Synthesize of TPP
The meso-tetraphenylporphyrin (TPP) ligand were pre-
pared based on D. Adler’s reported method [45]. Put
it briefly, freshly distilled pyrrole (18.7 mL, 279 mmol)
and 32.8 mL (345 mmol) pure benzaldehyde were
added to 200 mL refluxing propionic acid. After
refluxing for 30 minutes, the solution cooled to room
temperature and filtered, and filter cake washed with
methanol and then with hot water till purple crystals
results. Air dried crystals finally dried in vacuum. For
more purification, the outcome crystals applied to a
chromatographic column filled with Merck alumina 90
as absorbant TPP eluted with dichloromethane/hexane
(
see Fig. 1).
Synthesize of Ce (TPP) Sandwich Complex
2
3
The center of the TPP ring is its main reaction cite which make
metal-porphyrin reaction possible, i.e., pyrrolic N-H needs to
Fig. 2 Outcome synthesize sediments (up), green solution of separated
metalloporphyrin complex (right) and purple solution of recovered,
purified, unreacted TPP (left)
3
+
be deprotonated to make Ce capable of react with the ligand.
Due to its multi-functionality, Pyridine selected as the solvent
for this reaction. Using pyridine as solvent makes us needless
to add base. Pyridine was able to solve both cerium nitrate and
TPP at the same time and there will be no need to use the
mixture of the solvents to solve these two and it can act as
reducing agent [46, 47], increasing the yield of Ce (TPP)
measure the amount of mercury and copper in the com-
plex matrix. Fourier transform infrared (FTIR) spectra
were obtained by Bruker model VEATOR 22. The rod-
like morphology of the synthesized nanomaterials was
probed by Zeiss model EM10C-100KV transmission elec-
tron microscopy (TEM). Elementar CHN analyzer model
Vario EL III and Varian inductively coupled plasma opti-
cal emission spectrometry (ICP-OES) model OES-730
2
3
complexation by prohibiting Ce (III) to form Ce (IV). It is
worthy to note that the toxicity of pyridine is much lower than
solvents used in old methods like trichlorobenzene [10].
Synthesized TPP (0.30 g) and Ce(NO ) .6H O (0.12 g)
3
3
2
were dissolved in 15 mL pyridine. Then, the solution was
2
1
0
.5
2
Fig. 3 UV–vis specta of TPP (a),
Ce (TPP) (b) and recovered TPP
c)
2
3
Recovered TPP
Ce-TPP complex
TPP
(
.5
1
.5
0
1
90
290
390
490
Wavelength
590
690
790

J Fluoresc
Table 1 Elemental analysis of Cd–Cysteine complex nanorods
Results and Discussion
CHN
ICP
Ce
UV–Vis Spectra
Content %
According to the previous studies, each porphyrin-based com-
pound, like metalloporphyrin complexes, mostly has its
unique Soret band and Q band wavelength [48–55], i.e., any
changes in the structure and any complexation procedure can
be monitored by studying the absorption bands of the porphy-
rin reagents [10].
N
C
H
Experimental
Calculated
8.71
7.93
73.10
74.84
4.90
4.00
13.48
13.23
stirred magnetically (1200 RPM) and heated to reflux
393.15 ±1 K). After that, the balloon which contained
As shown in Fig. 3, the Soret band and Q band of
the synthesized complex is red-shifted, and its Q band
is broader as compared with the TPP spectrum. The
UV–vis spectrum of the recovered TPP, after the reac-
tion, was compared to TPP spectrum before the reaction
and shows well overlapping.
(
resulting solution placed in a 273.15 K water and ice
cooling bath for about 10 minutes to stop the reaction.
Next, mixture of 50 mL acetone and 10 mL toluene added
to the balloon to help the evaporation of the pyridine and,
then, all the mixture moved to a crystallizer plate. The
remaining dried sediments moved to dichloromethane-
methanol (1:6) solution. The synthesized complex solved
in the mixed solvents and form a green solution (Fig. 2)
which can be separated from the impurities using a filter
paper. After all, the synthesized complex dried at room
temperature. The reaction yield average was about 21 %.
The unreacted TPP could be separated from filtered sedi-
ments and reused from the impurities using a chromato-
graphic column filled with Merck alumina 90 as absorbant
eluted with toluene.
Elemental Analyses
There was chances of forming low amounts of Ce(TPP)2
[12] or forming other complexes between cerium and pyr-
idine [56, 57] other than the target complex. For this
purpose, the researcher uses ICP and CHN analyses to
study the molecular structure of the synthesized complex,
which further proves the formation of the Ce (TPP) sand-
2
3
wich complex as shown in Table 1.
Fig. 4 TEM images of the rod-
like nanoparticles of the
Ce (TPP) complex
2 3

J Fluoresc
Fig. 5 FTIR spectra of TPP and
Ce (TPP)
2 3
TEM Image
spectrum. Absence of this peak in the FTIR spectrum of
synthesized complex proves the interaction of cerium with
the center on the porphyrin ring.
Figure 4 illustrates TEM image of the Ce (TPP)3
2
nanocomplex in 300 and 120 nm scales. The rod-shaped
structure of the nanocomplex can be clearly distinguished
from the TEM images. As the images explain, the average
diameter and range of the nanoparticles are about 472.40
and 1755.13 nm, respectively.
Fluorescence Measurements
Concentration of about 10− M of TPP and Ce (TPP)
3
2
3
prepared in DMF solvent in different containers by 2 hours
magnetic stirring (2000 RPM, 308.15 K) as stock solu-
tions. The choosing of DMF as solvent was due to its well
miscibility in other solvents which makes analyzing sam-
ples in both aqueous and organic media possible. HEPES
buffer used to set the pH to neutral (~7.2).
FTIR Spectra
The main reaction cite of the TPP was its central pyrrolic
nitrogens, and to prove that pyridine is able to deprotonate
these nitrogens and make it suitable reaction cite for ceri-
um, the FTIR spectra of the TPP and Ce (TPP) compared
The fluorescence emission spectra of the solution were
scanned from 300 to 750 nm (λ = 420 nm) in fast scan
2
3
ex
to each other (see Fig. 5). The C–N stretching (aromatic
amine) band can be clearly seen at 1636 cm [58] of TPP
mode, while the excitation and emission slits were 10 and
5 nm opened, respectively.
−
1
2 3
Fig. 6 The fluorescence emission spectra of the TPP and Ce (TPP)

J Fluoresc
2
1
1
1
1
1
00
80
60
40
20
00
in the same concentrations. Varying the temperature from 283
to 338 K did not affected the intensity of the Ce (TPP) emis-
2
3
sion peaks, as shown in Fig. 7.
The fluorescence emission spectra of Ce (TPP) sandwich
2
3
2
+
2+
complex upon addition of various metal ions (Pb , Zn ,
Fe , Cr , K , Ag , Ca , Ni , Co , Mg , Na , Mn ,
Ce , Cu , Hg ) in DDW have been illustrated in Fig. 8.
Developed nano-sensor represented selective behavior by in-
dicating a significant fluorescence quenching in the presence
of mercury and copper metal ions.
2
+
2+
+
+
2+
2+
2+
2+
+
2+
8
6
4
2
0
0
0
0
0
3
+
2+
2+
0
15
30
45
60
75
Temperature
2
+
2+
2 3
Fig. 7 Effect of temperature on the Ce (TPP) fluorescence emission
peak intensity
Selectivity of Ce (TPP) Nano-Sensor for Cu and Hg
2
3
In fluorescence quenching studies, the intensity of both
peaks of the nano-sensor was decreased by addition of
copper or mercury in the matrix. But, the decrease in
the 651 nm peak represented more linear behavior than
the 530 nm peak, so, the sharper peak changes used to
study the sensor response.
The Ce (TPP) nanocomplex fluorescence response was
examined using the well-known Stern-Volmer equation and
their corresponding plots:
2
3
2+
F0
Figure 9 illustrates that increasing the amounts of Cu ions
¼
1 þ KSV fQgðSretn−Volmer equationÞ
F
in presence of the concentration of about 1 μM of the nano-
−
5
sensor in the aqueous solution from 1.00 × 10 M to
−3
F0 and F are Fluorescence intensity in the absence
1.00×10 M showed a linear correlation between the ratios
2+
and presence of the quencher, KSV is the Stern–Volmer
quenching constant, and {Q} is the concentration of the
quencher.
of fluorescence intensity and the concentration of Cu . The
2
+
detection limit of the sensor for Cu was found to be
−6
2.34×10 M. Furthermore, the nano-sensor put forward a lin-
Solutions with 10− M concentration of TPP and
6
ear relationship in the range of 1.00×10 M to 1.00×10
between the fluorescence intensity and the concentration of
−7
−5
M
Ce (TPP) were made in 5 mL volumetric flask from their
2
3
2+
stock solutions while DDW used as dilution solvent. As it is
clear from comparing the two spectra in Fig. 6, the synthesized
complex shows two peaks comparing to TPP emission spec-
trum. The broad peak at 530 nm only belongs to the
metalloporphyrin complex, but the emission peak at 651 nm
can be seen for both compounds. The emission peak of
Ce (TPP) at 651 nm is almost two times greater than TPP
Hg ions, and the detection limit for of the purposed fluores-
2+
−8
cent probe for Hg was about 1.62 × 10 M. (RSD = 0.0108)
2
+
Naked Eye Detection of Hg
By increasing the amount of mercury ions to the higher
concentrations, a sudden changing of color in the
2
3
Fig. 8 Fluorescence response of
30
−6
2 3
the Ce (TPP) nano-sensor (10
−5
M) to different metal ions (10
M) in DDW
2
2
1
1
5
0
5
0
5
0
Cu2+Hg2+Ce3+Mn2+Mg2+Co2+Pb2+ Ag+ Ca2+Ni2+ Na+ K+ Cr2+ Fe2+ Zn2+
Metals

J Fluoresc
2+
2+
2+
Fig. 9 Calibration curve of the nano-sensor for Cu (a) and Hg (b). The decrease in the fluorescence intensity by increasing the amounts of Hg (c)
and Cu (d)
2+
solution into light green happened (Fig. 10). This phe-
Mechanism of Sensing Reaction
2
+
nomena occurs when the concentration of the Hg in-
−
4
creases more than 10 M, which can be used for qual-
itative analyzes of the aqueous samples.
Due to the fact that the main center of reaction be-
tween porphyrin rings and metals are the center of

J Fluoresc
Conclusions
The sandwich complex of the Ce-TPP successfully synthesized
by a novel, cheap, one-step reflux reaction, which puts forward a
higher yield more than the previous methods. The synthesized
Ce-TPP complex forms rod-like nanoparticles which act perfect-
2
+
ly as a fluorescent probe for measuring trace amounts of Hg
2+
and Cu metal ions. The developed nano-sensor could detect
2+
higher concentrations of Hg by changing the color that makes
the qualitative mercury analyzing possible. Finally, the response
of the method compared with AAS in measuring the amounts of
Hg and Cu in complex matrix of a real sample and it repre-
sents the well-functioning of the nano-sensor.
2+
2+
Fig. 10 Changing in the color of the aqueous solution by increasing the
2
+
amount of Hg in the solution
Acknowledgments I wish to express my warmest thanks to Dr.
Eugenia Fagadar-Cosma from Institute of Chemistry Timisoara of Roma-
nian Academy in Romania and Dr. Mikhail A Filatov from Max Planck
Institute for Polymer Research in Germany for their helps and guidance in
synthesizing procedure of Ce-TPP complex. And a special thanks to Dr.
Rouhollah Khani and Dr. Ebrahim Ghiamati from University of Birjand
for their kindness and financial support.
the ring, the quenching is only possible by formation a
new complex by replacement of cerium ions with Cu
II) and Hg (II) in the structure of the nano-sesnsor,
(
which all lead to decreasing the fluorescence emission
intensity due to destroying the fluorescent dye and for-
mation of a dark complex.
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