Corrole and corrole functionalized silica nanoparticles as new metal ion chemosensors: A case of silver satellite nanoparticles formation

Article abstract of DOI:10.1021/ic4006295

Corrole macrocycles are very appealing dyes for incorporation into light harvesting devices. This work shows the sensorial ability of 5,10,15-tris(pentafluorophenyl)corrole 1 and its monoanionic species toward Na⁺, Ca²⁺, Cu²⁺, Cd²⁺, Pb ²⁺, Hg²⁺, Ag⁺, and Al³⁺ metal ions in toluene and acetonitrile. The photophysical studies toward metal ions were carried out by absorption and emission spectroscopy. From all metal ions studied, corrole 1 shows to be colorimetric for Hg²⁺ allowing a naked-eye detection of Hg²⁺ through a change of color from purple to blue in acetonitrile and from green to yellow in toluene. In addition a new β-imine corrole 4 was successful synthesized and further functionalized with 3-isocyanatopropyl-trimethoxysilane resulting in an alkoxysilane derivative 5. The grafting of alkoxysilane derivative 5 with optically transparent silica nanoparticles (SiNPs) was achieved succesfully. The new-coated silica nanoparticles with corrole 5 were studied in the presence of Cu²⁺, Hg²⁺, and Ag⁺ as metal ion probes. Interestingly, upon addition of Ag⁺, groups of satellite AgNPs were formed around the SiNPs and were checked by transmission electron microscopy (TEM). At same time, a change of color from green to yellow was observed.

Full text of DOI:10.1021/ic4006295

Article
pubs.acs.org/IC
Corrole and Corrole Functionalized Silica Nanoparticles as New Metal
Ion Chemosensors: A Case of Silver Satellite Nanoparticles Formation
Carla I. M. Santos,†,‡ Elisabete Oliveira,*,‡,§ Javier Fernandez-Lodeiro,‡ Joana F. B. Barata,†,∥
́
Sergio M. Santos,∥ M. Amparo F. Faustino,† Jose A. S. Cavaleiro,† M. Graca
̧
P. M. S. Neves,*,†
́
́
and Carlos Lodeiro*,‡
∥
†Chemistry Department and QOPNA and Chemistry Department and CICECO, University of Aveiro, Campus Universitar
́
io de
Santiago, 3810-193 Aveiro, Portugal
‡BIOSCOPE Group, REQUIMTE-CQFB, Chemistry Department, Faculty of Science and Technology, University NOVA of Lisbon,
2829-516 Monte de Caparica, Portugal
§Veterinary Science Department, CECAV, University of Tras
́
-os-Montes and Alto Douro, 5001-801 Vila Real, Portugal
S
* Supporting Information
ABSTRACT: Corrole macrocycles are very appealing dyes for incorporation into light harvesting devices. This work shows the
sensorial ability of 5,10,15-tris(pentafluorophenyl)corrole 1 and its monoanionic species toward Na+, Ca2+, Cu2+, Cd2+, Pb2+,
Hg2+, Ag+, and Al3+ metal ions in toluene and acetonitrile. The photophysical studies toward metal ions were carried out by
absorption and emission spectroscopy. From all metal ions studied, corrole 1 shows to be colorimetric for Hg2+ allowing a naked-
eye detection of Hg2+ through a change of color from purple to blue in acetonitrile and from green to yellow in toluene. In
addition a new β-imine corrole 4 was successful synthesized and further functionalized with 3-isocyanatopropyl-trimethoxysilane
resulting in an alkoxysilane derivative 5. The grafting of alkoxysilane derivative 5 with optically transparent silica nanoparticles
(SiNPs) was achieved succesfully. The new-coated silica nanoparticles with corrole 5 were studied in the presence of Cu2+, Hg2+,
and Ag+ as metal ion probes. Interestingly, upon addition of Ag+, groups of satellite AgNPs were formed around the SiNPs and
were checked by transmission electron microscopy (TEM). At same time, a change of color from green to yellow was observed.
emit light in the visible region, and show high excitation
coefficients, fluorescent quantum yields, and photostability.
Corroles also display inner nitrogen donor atoms available for
metal recognition.7 However, in contrast to porphyrins, the
sensorial ability of corroles has been under study. Up to date
there are few reports about the use of corroles as sensing
materials in analytical chemistry.8,9
5,10,15-tris(pentafluorophenyl)corrole 1 is one of the most
studied macrocycles of the corrole family, and one of our major
interests is concerned with its functionalization and further
applications.10
INTRODUCTION
■
The design, synthesis, and application of fluorescent molecular
probes that selectively recognize the presence of biologically
important analytes, such as metal ions, is an area in constant
growth.1 In the past few years the easy detection and
quantification of pollutant metallic species by fluorescent
molecular probes have attracted interest in fields such as
waste management, environmental chemistry, and clinical
toxicology.2
Corroles are tetrapyrrolic macrocycles with one meso carbon
less than porphyrin analogues, providing a trianionic coordina-
tion sphere that is able to stabilize metal ions in high valent
oxidation state.3 Considering the synthetic approaches
developed for corroles, their chemical functionalization can
afford new compounds with potential applications as catalysts,
dyes for solar energy conversion and in Medicine.4,5
Among the metal ions, we are particularly interested in
metals with biological and environmental impact.1,6,11−17
In this way we decided to evaluate the sensorial ability of
5,10,15-tris(pentafluorophenyl)corrole 1 in the absence and in
These macrocyles also have useful properties to be used as
Received: March 14, 2013
Published: July 15, 2013
fluorescent probes for molecular recognition.6 They absorb and
© 2013 American Chemical Society
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the presence of OH− anion toward Na+, Ag+, Ca2+, Cu2+, Cd2+,
Pb2+, Hg2+, Ni2+, Zn2+, Al3+, Cr3+, Fe3+, and Ga3+ metal ions in
the aprotic solvents, as toluene and acetonitrile (see Figure 1).
for real metal ion detection and for analytical applications. For these
measurements, ten different analyses for the selected receptor were
performed to obtain the LOQ. The LOD was obtained by applying the
formula:
y = yblank + 3std
dl
where ydl = signal detection limit and std = standard deviation.
2.3. TEM Measurements. To obtain the transmission electron
microscopy (TEM) images of the samples, 1 μL of the colloidal
suspension was dropped on a copper grid coated with a continuous
carbon film, and the solvent was allowed to evaporate. TEM images
were obtained through a JEOL JEM 1010F transmission electron
microscope operating at 100 kV. Data for size distribution histograms
were made by measuring for the smaller nanoparticles in more than
200 particles per sample in several TEM images.20
2.4. Spectroscopic Characterization of Ligands. 1H NMR and
Figure 1. Structure of compounds 1 and 4.
19F NMR spectra were recorded on Bruker Avance 300 (at 300 and
282 MHz, respectively) spectrometer. CDCl3 and pyridine-d5 were
used as solvents with tetramethylsilane (TMS) as the internal
reference; the chemical shifts are expressed in δ (ppm) and the
coupling constants (J) in hertz (Hz).
Taking into account that the incorporation of a highly
emissive corrole unit in silica nanoparticles18 could improve the
affinity, versatility, and sensitivity of the chromophore to the
guest, we decide to coat silica nanoparticles with corrole 5 via
an imine linkage (see Figure 1). The sensorial ability of these
nanoparticles was also evaluated toward Hg2+, Ag+, and Cu2+.
High-resolution mass spectra analysis (HRMS-ESI+) was performed
on a microTOF (focus) mass spectrometer. Ions were generated using
an Apollo II (ESI) source. Ionization was achieved by electrospray,
using a voltage of 4500 V applied to the needle, and a counter voltage
between 100 and 150 V applied to the capillary.
2. EXPERIMENTAL SECTION
IR spectra were obtained for KBr pellets, in the range 400−4000
2.1. Chemicals and Starting Materials. AgCH3SO3, NaCH3SO3,
Ca(CH3SO3)2, Cd(CH3SO3)2, Cu(BF4)2·6H2O, Pb(BF4)2·6H2O,
Hg(CH3SO3)2, Zn(CH3SO3)2, Ni(CH3SO3)2, Ga(NO3)3·xH2O, Cr-
(NO3)3·9H2O, FeCl3·6H2O, and Al(NO3)3·9H2O salts, and
(CH3)4NOH (tetramethylammonium hydroxide), have been pur-
chased from Strem Chemicals, Sigma Aldrich and TCI. Ethylenedi-
amine, triethylamine, anhydrous sodium sulfate acetic acid and silica
gel for thin layer chromatography (TLC) were purchase from Merck.
Ludox AS-30 silica nanoparticles by Sigma-Aldrich (30 nm). Water
was always used purified and deionized (Milli-Q grade, Millipore).
All chemicals were used without further purification with the
exception of triethylamine which was dried under standard procedures.
cm−1, with a Shimadzu FTIR 8400S instrument.
2.5. Synthesis of Ligands. The 5,10,15-tris(pentafluorophenyl)-
corrole 1 was synthesized by condensation of pentafluorobenzalde-
hyde with pyrrole at room temperature, according to the literature,21
and the 5,10,15-tris(pentafluorophenyl)corrolatogallium(III)-
(pyridine) 2 was obtained by refluxing the free base 1 in pyridine
with GaCl3.22 The gallium complex of 5,10,15-tris(pentafluorophenyl)-
corrole-3-carbaldehyde 3 was obtained by Vilsmeier−Haack formyla-
tion of corrole 2 using POCl3 and DMF.23
2.5.1. Synthesis of 3-(2-Aminoethyl)imino)methyl)-5,10,15-tris-
(pentafluorophenyl)corrolate Gallium(III) 4. Ethylenediamine (3.4
mg, 0.056 mmol) was added to a solution of 3 (25 mg, 0.028 mmol) in
ethanol absolute (5 mL). The resulting solution was gently refluxed
with magnetic stirring for about 4 h. After this period, the solvent was
evaporated under reduced pressure, and the resultant product
crystallized from a mixture of dichloromethane−hexane (18.2 mg,
69% yield).
The solvents were obtained from Panreac and Riedel-de-Haen with
̈
HPLC grade. Absolute ethanol, dichloromethane, and pentane were
distilled and dried under standard procedures.
2.2. Spectrophotometric and Spectrofluorimetric Measure-
ments. Absorption spectra were recorded on a JASCO V-650
spectrophotometer and the fluorescence emission on a spectro-
fluorimeter HORIBA-JY Scientific Fluoromax-4. The linearity of the
fluorescence emission vs the concentration was checked out in the
concentration used (10−4−10−6 M). A correction for the absorbed
light was performed when necessary. The spectrometric character-
izations and titrations were performed as follows: the stock solutions
of compounds 1 and 4 (ca. 10−3 M) were prepared by dissolving an
appropriate amount of each compound in a 10 mL volumetric flask
and diluting it to the mark with toluene or acetonitrile. The solutions
were prepared by appropriate dilution of the stock solutions up to
10−5−10−6 M. Titrations of compounds 1 and 4 were carried out by
adding microliter amounts of standard solutions of the ions in DMSO
or acetonitrile, when the ligand was in toluene or acetonitrile,
respectively. All studies have been performed in standard quartz cells
with 1 cm pathway.
The spectrometric characterization of silica nanoparticles function-
alized with corrole 5 was performed in dichloromethane. Titrations
involving silica nanoparticles were carried out by the addition of
microliter amounts of solutions of metals in dimethylsulfoxide
(DMSO) or acetonitrile. All the luminescence measurements were
performed at 298 K.
Fluorescence quantum yield of compound 4 was measured using a
solution of cresyl violet perchlorate in absolute ethanol as a standard
([ϕ] = 0.54)19 and was corrected for different refraction indexes of
solvents. The limit of detection (LOD) and the limit of quantification
(LOQ) for the metal ions were performed, having in mind their use
1H NMR (300.13 Hz, CDCl3, a few drops of pyridine- d5): 9.28 (s,
1H, H-2), 9.18 (d, 1H, J 4.1, H-18), 8.80−8.74 (m, 4H, H-β), 8.66 (s,
1H, H-1′), 8.54 (d, 1H, J 4.1, H-17), 4.13 (s, 2H, N−H), 3.25−3.23
(m, 2H,CH2), 3.01−2.97(m, 2H,CH2).
19F NMR (282.37, CDCl3, a few drops of pyridine-d5): δ −185.78
to −185.75 (m; 1 F; Fmeta); −185.62 to −185.59 (m; 1 F; Fmeta);
−185.51 to −185.35 (m; 1 F; Fmeta); −185.12 to −185.05 (m; 2 F;
Fmeta); −184.98 to −184.96 (m; 1 F; Fmeta); −176. 90 (t; 1 F; J 21.2
Hz; Fpara); −176.59 (t; 1 F; J 21.2 Hz; Fpara); −175,19 (t; 1 F; J 21.2
Hz; Fpara), −163.38 (dd; 1 F; J 24.0 Hz and J 7.1 Hz; Fortho); −160.
94 (dd; 1 F; J 25.4 Hz e J 8.5 Hz; Fortho), −160.91 to −160.82 (m; 3
F; Fortho), −159.16 (dd; 1 F; J 24.0 Hz and J 7. One Hz; Fortho).
IR (cm−1): 3484 (N−H, Amine), 1657 (CN, Imine), 1589 and
1492 (CC, Ar).
HRMS [ESI+]: m/z ([M+H]+) calcd for C40H14F15GaN6: 933.1352,
found: 933.1316 λmax (log ε) CH2Cl2 410 (4.61), 432 (5.21), 601
(4.35), 623 (4.48) nm.
2.5.2. Synthesis of Silane Corrole 5. Corrole 4 (16 mg, 0.017
mmol) was dissolved in 5 mL of dichloromethane, followed by the
addition of triethylamine (3 μL, 0.017 mmol) and of the linker 3-
isocyanatopropyltrimethoxysilane (3.6 mg, 0.017 mmol). The reaction
mixture was kept under stirring at room temperature for 2 days. After
this period the solvent was evaporated, and the resulting solid was
washed under vacuum. Compound 5 was used in the subsequent
reaction without further purification (12.6 mg, 64% yield).
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Scheme 1. General Synthetic Pathway of Compounds 4 and 5
1H NMR (300.13 Hz, CDCl3, a few drops of pyridine- d5): 9.63 (s,
1H, H-2), 9.10 (d, 1H, J 4.0, H-18), 8.76−8.72 (m, 3H, 2H-β + H-1′),
8.67(d, 1H, H-β, J 4.9 Hz), 8.51 (d, 1H, H-β, J 4.7 Hz), 8.47 (d, 1H,
H-β, J 4.5 Hz), 5.52 (bs, 1H, NH) 5.04 (bs, 1H, NH), 3.84−3.74
(3×CH3 under solvent), 3.28−3.30 (m, 4H, 2× CH2), 3.17−3.12 (m,
2H, CH2), 1.63−1.60 (m, 2H, CH2), 0.65−0.60 (m, 2H, CH2).
HRMS [ESI+]: [M+H]+ calcd for C47H29F15GaN7O4Si: m/z
1138.1048, found: m/z 1138.1036.
Elem Anal. Calc. for C47H29F15GaN7O4Si. 2 CH2Cl2. 2 C5H12: C,
48.78; H, 3.95; N, 6.75. Found: C, 48.43; H, 3.81; N, 6.46.
λmax (log ε) CH2Cl2 413 (4.53), 436 (5.25), 603 (4.31), 626 (4.47)
nm.
2.5.3. Synthesis of Silica Nanoparticles Decorated with
Compound 5. Compound 5 (12 mg, 0.011 mmol) was dissolved in
a mixture of 0.5 mL of ethanol, 0.5 mL of water, 0.5 mL of
CH3COOH, and 0.680 mL of a water solution of commercial Ludox
AS-30 silica nanoparticles (30 nm). The resultant mixture was stirred
at 80 °C for 2 days, then 200 μL were withdrawn, dissolved in 1 mL of
dichloromethane, and centrifuged at 6000 rpm, at 21 °C, for 10 min.
The pellet was washed three times with ethanol (3 × 1 mL), and
finally dissolved in 0.5 mL of dichloromethane.
The synthesis of the new β- imine corrole 4 was carried out
by adding ethylenediamine to an absolute ethanol solution of β-
formyl corrole 3. The reaction was performed at reflux, for 4 h.
The TLC control revealed the absence of the starting corrole 3
and the presence of a new product. After workup, this
compound was identified by NMR and HRMS as the novel
1
corrole derivative 4, obtained in 69% yield. The H NMR
spectrum showed at lower field, a singlet at δ 9.28
corresponding to the resonance of β-pyrrolic H-2 and a
doublet at δ 9.18 due to the resonance of proton H-18. Besides
the resonances of the β-pyrrolic protons H-17 (δ 8.54 doublet)
and H-7,8,12,13 (δ 8.80−8.74 multiplets), the 1H NMR
spectrum showed a singlet at δ 8.66 identified as the resonance
of proton H-1′, and a broad signal at δ 4.13 as the resonance of
the NH2 protons. In the aliphatic region, the spectrum
presented two multiplets between δ 3.25 and δ 3.23 and δ
3.01 and δ 2.97 corresponding to the resonances of the protons
of the ethyl group.
The functionalization reaction of the imine-corrole 4 with 3-
isocyanatopropyl-trimethoxysilane was performed in dichloro-
methane in the presence of triethylamine. The success of the
coupling was confirmed by HRMS; the mass spectrum of
intermediate 5 showed the expected molecular ion [M + H]+ at
m/z 1138.1036. The grafting of alkoxysilane derivative 5 with
LUDOX was achieved by magnetic stirring, at 80 °C for two
days. The resulting nanoparticles were further characterized by
UV−vis, fluorescence emission spectroscopy, and TEM.
3.2. Spectrophotometric and Spectrofluorimetric
Titrations and Metal Ion Sensing Effects in the Presence
of Corrole 1. To further understand how 5,10,15-tris-
(pentafluorophenyl)corrole 1, the corrole precursor, behaves
in the presence of cations, its sensorial ability was evaluated.
The first studies concerning the sensorial ability of
compound 1 toward the different metals, Ag+, Na+, Ca2+
Cd2+, Pb2+, Zn2+, Ni2+,Cu2+, Hg2+, Fe3+, Cr3+, Ga3+, Al3+, were
performed in toluene, where the decomposition of the corrole
macrocyle is minimized.24
The commercial silica nanoparticles (LUDOX) were measured by
TEM and dynamic light scattering (DLS), and a diameter of 30
5
nm (TEM) and a hydrodynamic diameter of 40 3 nm (DLS) were
determined, respectively. TEM measurements of the new material
SiNPs@5 revealed a diameter of 80
30 nm in 98%. A smaller
population of bigger nanoparticles about 2% was observed with 600
200 nm.
UV−vis absorption band (dichloromethane) = 601 nm; emission
band (dichloromethane) = 624 nm; fluorescent quantum yield = 0.03.
3. RESULTS AND DISCUSSION
3.1. Synthesis. The synthetic route used to prepare the
precursor 5 for further coating with commercial silica
nanoparticles is outlined in Scheme 1. The 5,10,15-tris-
(pentafluorophenyl)corrole 1,21 the corresponding gallium-
(III)(pyridine) complex 222 and the gallium(III) complex of
5,10,15-tris(pentafluorophenyl)corrole-3-carbaldehyde 3 were
synthesized according to the procedures described previously.23
The protection of the corrole 1 with gallium(III) was
fundamental to obtain efficiently the 5,10,15-tris-
(pentafluorophenyl)corrole-3-carbaldehyde 3 under Vilsme-
ier−Haack conditions.
The absorption spectrum of compound 1 in toluene shows
the Soret-type band at 415 nm and the Q-type band centered at
565 nm, while the emission spectrum shows a band at 645 nm,
with a fluorescence quantum yield of 0.05.9 To avoid the
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reabsorption phenomena, the emission spectrum was collected
on the Q-bands. However, no significant changes were detected
in the ground and excited state of this corrole after titration in
the presence of the metals selected.
Knowing that compound 1 presents a high N−H acidity
being really sensitive to anions prompts us to evaluate if its
metal sensing ability is modified after deprotonation with
OH−.25
The addition of this anion to corrole 1 promotes in the
absorption spectrum a decrease accompanied by a red shift in
the Soret band from 415 to 440 nm (Δλ = 25 nm) and in the
Q-band from 565 to 606 nm (Δλ = 41 nm) (see Figure 2). At
Figure 3. Spectrophotometric (A, C) and spectrofluorimetric (B, D)
titration of 1− with the addition of Cd2+ (A, B) and Cu2+ (C, D) in
toluene. The inset represents the absorption at 440 nm, 575 nm, and
620 nm for A, at 589 and 620 nm for C; and the emission intensity at
626 nm (B, D) as a function of [Cd2+]/[1−], and [Cu2+]/[1−],
Figure 2. Spectrophotometric (A) and spectrofluorimetric (B)
titration of compound 1 with the addition of OH− in toluene. The
inset represents the absorption (A) and the emission intensity (B) as a
function of [OH−]/[1] at 415 nm, 440 nm, 565 nm, and 620 nm for
(A), 626 and 680 nm for (B). ([1] = 1 × 10−5 M, λexc = 565 nm, T =
298 K).
respectively. ([1−] = 1 × 10−5 M for [Cd2+] and [1−] = 1.2 × 10−5
for [Cu2+], λexc = 565 nm, T = 298 K).
M
same time, a naked-eye change of color from purple to green is
visualized upon addition of OH−. In the fluorescence spectra a
strong enhancement in the emission intensity at 644 nm was
observed, as well as a blue shift from 644 to 626 nm (see Figure
2 and picture in Figure 2). In addition in the emission spectra
the appearance of a new band was detected at 670 nm. These
features are similar to the ones described for corroles in the
presence of organic bases,26 confirming that after titration we
are in the presence of the monoanionic form of a corrole
[(Cor)H2]−. A similar spectroscopic profile was also reported
by Gross and co-workers,25 where a pH study was carried out,
allowing to conclude that the pH range at which neutral
corroles exist is extremely limited.
Figure 4. Spectrophotometric (A) and spectrofluorimetric (B)
titration of compound 1− with the addition of Hg2+ in toluene. The
inset represents the absorption (A) and the emission intensity (B) as a
function of [Hg2+]/[1−] at 420 nm, 440 nm, 575 nm, and 620 nm for
(A), and 626 and 650 nm for (B). ([1−] = 9.0 × 10−6 M, λexc = 565
nm, T = 298 K).
Corrole 1 does not present any spectral changes in the
ground and excited states after addition of the metal ions
mentioned above.
However, the addition of Cu2+, Cd2+, Pb2+, Hg2+, Ga3+, Cr 3+
,
Fe 3+, Ag+, or Al3+ to compound 1, after its previous
deprotonation with 0.5 equiv of OH−, is responsible for
significant alterations in the absorption and emission spectra.
Only the titrations with Na+, Ca2+, Ni2+, Zn2+ did not affect the
ground and the excited state of the monoanionic form of
corrole 1 that will be referred here as 1−.
concomitant appearance of blue-shifted bands at 606 nm (Δλ =
14 nm) and at 415 nm (Δλ = 25 nm). An isosbestic point at
426 nm was observed.
Considering the fluorescence spectra, the addition of these
metal ions promotes a quenching in the emission intensity at
626 nm, accompanied by a red shift from 626 to 645 nm (Δλ =
19 nm).
Figure 3 shows the modifications observed in the absorption
and emission spectra of 1− in the presence of Cd2+, Cu2+ and in
Figure 4 in the presence of Hg2+. The titrations including the
other metal ions (Ga3+, Fe3+, Cr3+, Pb2+, Al3+, and Ag+), since
they presented similar behavior, are shown in the Supporting
Information, Figures SN1 and SN2. The addition of Cd2+
produces alterations in the absorption spectrum, such as, a
decrease in the Q-type absorption band at 620 nm and in the
Soret-type band at 440 nm that is accompanied with the
The titration with Cu2+ produces only alterations in the Q-
band region in the absorption spectra where a decrease of the
band at 620 nm with the concomitant appearance of a blue shift
band at 589 nm is observed. In the emission spectra a
quenching was only observed in the emission intensity at 626
nm without any shift (Figure 3D).
The most interesting results arise from the interaction of the
monoanionic form of corrole 1 with Hg2+, where a colorimetric
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effect was visualized, with a change of color from dark green to
pale green (see Figure 4). The addition of Hg2+ to 1− results in
a blue shift of the Soret band from 440 to 420 nm (Δλ = 20
nm) and in the Q-type band from 620 to 575 nm (Δλ = 45
nm) with a decrease in the absorbance. The appearance of a
charge transfer band (LMCT, ligand to metal charge transfer)
at 478 nm and at 720 nm was observed as well. The blue shift
in the ICT (intramolecular charge transfer) transitions can be
attributed to Hg2+ metal ion binding to free nitrogen atoms in
the inner of the macrocycle. In the emission spectra, the
addition of Hg2+ is responsible for the total quenching in the
fluorescence intensity at 626 nm accompanied by a red shift
from 626 to 650 nm. This behavior is not unexpected because
of the Hg2+ “heavy” metal ion nature, which increases the
probability for nonradiative deactivation processes to occur,
such as intersystem crossing, responsible for chelation enhance-
ment of quenching (CHEQ) in the emission intensity.27
The association constants for the interaction of compound
1− with the different metal ions were calculated using the
HypSpec28 program and are summarized in Table 1. Using the
Figure 5. Normalized fluorescence at 626 nm of compound 1 with the
first addition of OH− (species 1−), and after addition of 0.5 equiv of
[Mn+ = Na+, Ca2+, Ni2+, Zn2+, Cd2+, Cu2+, Pb2+, Hg2+, Ga3+, Cr3+, Fe3+,
Ag+, and Al3+] = Mn+ metal ions in toluene.
Ca2+, Ni2+, Zn2+, Cd2+, Cu2+, Pb2+, Hg2+, Ga3+, Cr3+, Fe3+, Ag+,
and Al3+ ions), after deprotonating corrole 1 with OH− anion.
Taking into account the results of Figure 5, we can see the
evident CHEQ effect of the specie 1− in the presence of 0.5
equiv of the heavy pollutants and toxic Pb2+, Hg2+, Cd2+, and
Ag+ metal ions, while a strong quenching between 50% and
80% was observed.
The detection (LOD) and quantification (LOQ) limits for
the metal ions studied were also determined in the presence of
the monoanionic form 1−, using the emission band centered at
626 nm (Table 1). The values obtained show that the lowest
values of LOD and LOQ were found for Ag+; with this
macrocycle the minimum amount of Ag+ detectable was 0.02,
and the minimum amount quantified was 0.03 ppm.
Table 1. Association Constants (Emission Data), Detection
(LOD), and Quantification (LOQ) Limits of 1− in Toluene
metal
Log Kassoc (L:M)
LOD (ppm)
LOQ (ppm)
Al3+
14.32 0.25 (2:1)
10.99 0.01 (2:1)
11.32 0.01 (2:1)
8.43 0.02 (2:1)
12.33 0.07 (2:1)
12.61 0.02 (2:1)
7.66 0.02 (1:1)
6.49 0.02 (1:1)
4.23 0.02 (1:1)
0.03
0.04
0.04
0.02
0.02
0.09
0.04
0.02
0.03
0.06
0.08
0.07
0.03
0.03
0.12
0.08
0.04
0.06
Cd2+
Pb2+
Hg2+
Ag+
Cr3+
Fe3+
Ga3+
Cu2+
Since all the metal salts used show good solubility in
acetonitrile, it was decided to perform several titrations toward
the same metal ions, in this solvent with and without the
previous addition of OH− anion. When the titrations were
carried after deprotonation with OH−, no spectral alterations
were detected in the absorption and emission spectra.
However, without the previous addition of OH− anion, the
increase of the dipole moment of the solvent from 0.36 D
(toluene) to 3.92 D (acetonitrile), leaded to a better sensorial
and selectivity capability of corrole 1 for Hg2+. In Figure 6 are
presented the modifications observed in the absorption and
emission spectra of compound 1 with the increasing addition of
Hg2+ metal ion. In these conditions, no changes were detected
in the ground and excited state of corrole 1 upon addition of
the other metal ions studied.
In this case, the Soret band of compound 1 upon addition of
Hg2+ was practically not affected, whereas the Q-band region
showed significant changes. Looking at Figure 6A, it can be
observed that the addition of the metal ion is responsible by a
decrease in the absorbance band at 565 nm that is accompanied
by the appearance of two bands at 635 and 680 nm. In these
conditions a colorimetric effect (a change of color from purple
to green-blue) was also observed, which allows again a naked-
eye detection of Hg2+ (see Figure 6C). In the emission spectra
a quenching was seen in the emission intensity at 645 nm
(Figure 6B). The association constant was also determined by
use of the HypSpec program, and the value obtained for 1 in
the presence of Hg2+ (acetonitrile) was log Kassoc Hg2+ = 9.04
0.01, with a stoichiometry of two ligands per one metal ion,
which is slightly higher than the one obtained for 1− in the
Job’s plot method (see Supporting Information, Figure SN3),29
the stoichiometry of two ligands per metal ion (L2M) could be
postulated for Cd2+, Pb2+, Ag+, Cr3+, or Al3+ and one ligand per
metal in the presence of Cu2+, Ga3+, and Fe3+; these results are
in accordance with the ones obtained from the HypSpec
program.
Looking at Table 1 the most stable complexes were formed
with Al3+, Cr3+, and Ag+, with log Kassoc = 14.30 0.25, log
Kassoc = 12.61
0.02, and log Kassoc = 12.33
0.07,
respectively. From the other association constant values: log
Kassoc Pb2+ = 11.32 0.01 (L2M) > log Kassoc Cd2+ = 10.99 0.01
(L2M) > log Kassoc Hg2+ 8.43 0.02 (L2M) > log Kassoc Fe3+ 7.66
2+
0.02 (LM) > log Kassoc Ga3+ 6.49 0.02 (LM) > log Kassoc Cu
= 4.23 0.02 (LM) we can conclude that the monoanion of
corrole 1 coordinated better with Pb2+ followed by Cd2+, Hg2+,
and finally Cu2+ metal ions.
Apart from the type of donor atoms present in the inner of
the corrole, where the metal complexation occurs, other
parameters in the macrocycle such as the cavity size, shape,
conformation, topology, and rigidity and the radius of the metal
ion are important because they influence the thermodynamic
and kinetics properties of the corresponding final metal
complexes. Moreover, in macrocyclic ligands the chelate and
macrocycle effects must be taken into account.30,31
Figure 5 shows the normalized fluorescence intensity of
corrole 1 after being deprotonated with OH− and after the
addition of 0.5 equiv of Mn+ metal ions in toluene, (Mn+ = Na+,
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loss obtained upon formation of the complex from its individual
components.
The complexation of Al3+ with 1− occurs preferentially in a
2:1 stoichiometry, as shown in Figure 7. The coordination of
the cation occurs with four nitrogens from one corrole unit and
with one nitrogen from a second corrole, with Al3+···N
distances ranging from 1.91 to 2.12 Å, in what resembles a
square pyramid coordination environment. Our DFT calcu-
lations revealed that this 2:1 binding arrangement is more
stable than the 1:1 stoichiometry in about −0.127 Ha (−332.8
kJ mol−1; see Figure 8 for the analogous 1:1 complex with
Cu2+). The lack of stability of the 1:1 complex with Al3+ seems
to arise from the fact that the cation, to efficiently coordinate
with all four nitrogens, needs to fully fit inside the corrole
cavity, which then leads to electrostatic repulsion between the
cation and the two positively charged interior protons.
However, when considering the 1:1 binding arrangement of
1− with Cu2+, as shown in Figure 7, the larger size of the Cu2+
cation requires that coordination occurs slightly above the
corrole plane, thus minimizing the electrostatic hindrance
between the cation and the interior protons. In this case, the
coordination of the cation occurs with the four nitrogens of the
corrole and one oxygen from a solvent molecule (in this case
DMSO). The incorporation of two solvent molecules was
required to provide the necessary stabilization of the overall
complex. In fact, when solvent molecules were not incorporated
at all, the 2:1 stoichiometry was slightly more stable than the
“unsolvated” 1:1 model in about −0.11 Ha (−307.7 kJ mol−1)
which suggested that the completion of the coordination sphere
of Cu2+ requires at least one solvent molecule, whereas the
solvated model is more stable than the 2:1 one in about 0.07
Ha (−185.4 kJ mol−1), thus supporting the 1:1 stoichiometry
found experimentally. The coordination environment of the
cation is a slightly distorted square pyramid involving the four
corrole nitrogens and a nitrogen from a solvent molecule,
having Cu2+···X (X = Ncorrole or ODMSO) distances ranging from
2.01 to 2.57 Å. As also shown in Figure 8, a second DMSO
molecule is able to establish two simultaneous hydrogen bonds
with the two interior corrole protons, with ODMSO···Hcorrole
distances of 1.779 and 1.928 Å. Finally, the coordination of Ag+
with 1− (Figure 9) occurs, similarly to Al3+, in a 2:1
Figure 6. Spectrophotometric (A) and spectrofluorimetric (B)
titration of 1 with the addition of Hg2+ in acetonitrile. The inset
represents the absorption (A) and the emission intensity (B) as a
function of [Hg2+]/[1] at 565 and 635 nm for (A), 645 nm for (B).
([1] = 1 × 10−5 M, λexc = 565 nm, T = 298 K). (C) Normalized
fluorescence at 645 nm of compound 1 with addition of 0.5 equiv of
metal ions in acetonitrile [(Na+, Ca2+, Cd2+, Cu2+, Pb2+, Hg2+, Ag+, and
Al3+) = Mn+]; and picture with a color change from purple to green-
blue with the addition Hg2+ in acetonitrile.
2+
presence of Hg2+ in toluene [log Kassoc Hg = 8.43
0.02,
(L2M)].
As a conclusion, corrole 1 in acetonitrile shows a higher
affinity and an unprecedented selectivity for Hg2+ metal, as can
be seen in Figure 6C, where a normalized intensity of
fluorescence toward the addition of 0.5 equiv of metal ions
studied is shown.
3.2.1. Theoretical Studies. To understand better the
complex formation, the preferential stoichiometries and binding
modes of corrole 1− to Al3+, Cu2+, and Ag+ were assessed
through a series of electronic calculations32 performed at the
density functional theory (DFT) level, using Gaussian09.33 The
energetics of the 1:1 and 2:1 (receptor:ligand) binding
stoichiometries was evaluated by calculating the energy gain/
Figure 7. Lowest energy gas-phase complex of 1− with Al3+, calculated at the B3LYP level. The inset provides details about the coordination
environment of the cation, in the same orientation shown for the complex (only key particles were kept for sake of clarity). Coordinating interactions
shown as red dashes; distances in Ångstrom. Color code: hydrogen−white; carbon−green (different shades for distinct macrocyles); aluminum−
light pink; fluorine−teal, nitrogen−blue.
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Figure 8. Lowest energy gas-phase complex of 1− with Cu2+. Details as in Figure 7; Cu2+ shown as orange sphere.
Figure 9. Lowest energy gas-phase complex of 1− with Ag+. Details as in Figure 7; Ag+ cation shown as purple sphere.
stoichiometry, this stoichiometry being more stable than the
1:1 one by about −0.08 Ha (−251.15 kJ mol−1). Coordination
of the cation occurs mainly with three nitrogens from one
corrole unit and three nitrogens from a second unit, in what
resembles a distorted trigonal prismatic coordination environ-
ment. The Ag+···N distances can be subdivided into a set of two
shorter distances involving the nonprotonated nitrogens (2.217
and 2.238 Å) and four longer ones involving the protonated
nitrogens (ranging from 3.30 to 3.43 Å). Because of the larger
cationic radius of Ag+ relative to that of Al3+, the efficient
accommodation of Ag+ into the binding pocket formed by the
two corrole receptors does not impose large distortions on the
planarity of the corrolic core, has happens with Al3+.
compound 5 was studied and compared with that of the free
precursor 4.
The main photophysical data of compound 4 in dichloro-
methane and in toluene is gathered in Table 2. The Q
absorption bands of corrole 4 were centered between 599 and
601 nm, in toluene and CH2Cl2, the emission band at 626 and
623 nm, respectively.
The study of the sensorial ability of compound 4 toward
metal ions Cu2+, Hg2+, and Ag+ was carried out by ligand
titrations with the addition of small amounts of the adequate
metallic salts.
The titrations were followed by UV−vis and fluorescence
measurements in toluene. The most significant alterations in
It is possible that both complexes with Al3+ and Ag+ gain
further stabilization in solution from π−π interactions that can
be established as result of the momentary alignment of
aromatic fragments from adjacent units. However, evaluation
of such hypothesis requires molecular dynamics simulations,
which fall beyond the scope of the current analyses.
3.3. Spectrophotometric and Spectrofluorimetric
Titrations and Metal Ion Sensing Effects in the Presence
of Nanoparticles Functionalized with Silane Derivative
5. To develop new corrole fluorescent materials to be used as
new sophisticated metal ion chemosensors in environmental
fields, the sensorial ability of the coated silica nanoparticles with
Table 2. Photophysical Data of Corrole 4
Stokes
shift
λmax (nm)
(nm)
ϕ (sd
Q-
λem
=
compound
solvents
Soret bands (nm) Q-bands 0.01)
4
dichloromethane
toluene
432
431
430
431
601
599
603
601
623
624
626
624
22
25
23
23
0.03
0.03
0.03
0.03
5
dichloromethane
dichloromethane
SiNPs@5
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Figure 10. Top: Absorption (A) and emission spectra (B) of SiNPs@5 with the increasing amount of Ag+ in dichloromethane; and picture of color
change of the nanoparticles’ formation. The inset B represents the emission at 626 nm as function of [Ag+]/[ SiNPs@5]. ([SiNPs@5] = ca. 5.86 ×
10−6 M, λexc = 410 nm, T = 298 K). Bottom: Silica nanoparticles coated with silane derivative 5 (SiNPs@5) and with Ag+ (AgSiNPs@5). TEM
image of SiNPs@5 and AgSiNPs@5.
the ground and excited states were observed in the presence of
the ion Ag+. No changes were detected in the presence of the
nanostructures was detected. These macro nanostructures are
due to self-aggregation after interaction with the corrole 5 and
silver ions.
+
ions Hg2+ and Cu2+. The addition of Ag to compound 4
promoted a quenching of about 60% at 626 nm in the emission
spectrum. Spectral alterations were not observed in the ground
state (Supporting Information, Figure SN4).
The profiles of the absorption and emission spectra of the
derivatized nanoparticles were very similar to the ones of the
ligand 4. This result is expected since those silica nanoparticles
are totally transparent to the visible light and inert to electron-
transfer processes.34 TEM measurements of the new-coated
The polydispersed distribution of AgSiNPs@5 can be due to
the stepwise addition of Ag+. The decrease observed in the
emission intensity in the fluorescent spectra is probably due to
the proximity between silver and the nanoparticle monolayer
increasing the accessibility to nonradiative processes.
4. CONCLUSIONS
Corrole 1 was successfully studied in the presence Ag+, Na+,
Ca2+, Zn2+, Cd2+, Cu2+, Pb2+, Hg2+, Zn2+, Ni2+, Cr3+, Ga3+, Fe3+,
and Al3+ metal ions in toluene and acetonitrile. Metal
complexes with formula L2M, were obtained in the interaction
of 1− with Cd2+, Pb2+, Hg2+, Ag+, Cr3+, and Al3+, while LM
species were identified with Cu2+, Ga3+, and Fe3+, in toluene.
The higher association constants were observed for Ag+, Cr3+,
and Al3+. In toluene, the species corrole 1− shows to be a naked
eye sensor in the presence of Hg2+; a change of color from dark
green to green was vizualized. Also, corrole 1 reveals to be
selective for Hg2+ in acetonitrile, where a change of color from
purple to green-blue was observed.
A new sophisticated material based on silica nanoparticles
coated with compound 5 was successfully synthesized and
studied in the presence Cu2+, Hg2+, and Ag+ metal ions. Upon
addition of Ag+ new satellite AgNPs were formed around the
silica nanoparticles, followed by a change of color from green to
yellow.
material revealed a diameter of 80
30 nm (98%)
demonstrating that the derivatization of Ludox with silane
derivative 5 induced nanoparticle growth via aggregation.
Moreover, a smaller sample of bigger nanostructures with a 2%
of population was observed with sizes of about 600 200 nm.
The interaction of the silica nanoparticles coated with the new
silane 5 (SiNPs@5) was also explored in the presence of Ag+,
Cu2+, and Hg2+ metal ions.
The SiNPs@5 nanoparticles did not show any spectral
changes in the presence of Cu2+ and Hg2+ metal ions, whereas,
upon addition of Ag+ new satellite AgNPs were formed around
the silica nanoparticles. Figure 10 shows the silica nanoparticles
coated with silane derivative 5 (SiNPs@5) and with AgNPs
(AgSiNPs@5); the TEM images, the spectrophotometric and
spectrofluorimetric titrations of SiNPs@5 with the increasing
addition of Ag+ resulted on the SiNPs@5 with a layer of AgNPs
inducing a strong aggregation.
As can be seen in the absorption spectra of SiNPs@5, the
increase of Ag+ amount induced the formation of satellite
AgNPs around the silica nanoparticles coated with compound
5. Its formation is proven by a change of color from green to
yellow, as well as by the appearance of a resonance plasmon
band centered at 410 nm. Through TEM images it is possible
to visualize the formation of the satellite AgNPs around the
SiNPs@5, affording AgSiNPs@5 with sizes around 600 200
nm, and a small population between 2 and 5% of bigger
All these promising results support the use of chemosensor
derivatives based on corroles in environmental control, biology,
chemistry, or medicine. It is also important to point out the
efficiency of species 1− against the heavy pollutants and toxic
Pb2+, Hg2+, and Ag+ metal ions, whereas a strong and very
effective quenching was observed only with the addition of 0.5
equiv of metal ion.
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Escriche, L.; Casabo,
́
J.; Capelo, J. L.; Covelo, B.; Kivekas, R.;
ASSOCIATED CONTENT
* Supporting Information
Further details are give in Figures SN1−SN6. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
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Y.; Kang, E.-J.; Lee, J.-E.; Lee, S. S. Tetrahedron Lett. 2009, 50, 671.
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: cle@fct.unl.pt (C.L.), gneves@ua.pt (M.G.P.M.S.N.),
ejo14209@fct.unl.pt (E.O.).
■
(d) Tamayo, A.; Oliveira, E.; Covelo, B.; Casabo,
Lodeiro, C. Z. Anorg. Allg. Chem. 2007, 633, 1809.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Authors are grateful to the Scientific PROTEOMASS
Association (Portugal) for financial support. Thanks are due
■
to the Fundaca̧ o para a Ciencia e Tecnologia (FCT, Portugal),
̂
̃
(20) San
Rodríguez-Gonzal
Mater. 2006, 18, 2529.
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chez-Iglesias, A.; Pastoriza-Santos, I.; Per
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, L. Adv.
European Union, QREN, FEDER, and COMPETE for funding
the QOPNA research unit (project Pest-C/QUI/UI0062/
2011) and the Portuguese National NMR Network, also
supported by funds from FCT. C.S., E.O, J.F.B.B, and S.M.S.
also thank FCT-MEC (Portugal) for their doctoral and Post-
Doctoral grants, SFRH/BD/64155/2009 and SFRH/BPD/
72557/2010, SFRH/BPD/63237/2009, and SFRH/BPD/
64752/2009, respectively. J.F.L. thanks Scientific PROTEO-
MASS Association (Portugal) for financial Postdoctoral
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