Structural and electrochemical properties of lutetium bis-octachloro-phthalocyaninate nanostructured films. Application as voltammetric sensors

Article abstract of DOI:10.1166/jnn.2014.9355

Thin films of the bis[2,3,9,10,16,17,23,24-octachlorophthalocyaninate] lutetium(III) complex (LuPc₂Cl₃₂) have been prepared by the Langmuir-Blodgett and the Langmuir-Schaefer (LS) techniques. The influence of the chlorine substituents in the structure of the films and in their spectroscopic, electrochemical and sensing properties has been evaluated. The -A isotherms exhibit a monolayer stability greater than the observed in the unsubstituted analogue (LuPc₂), being easily transferred to solid substrates, also in contrast to LuPc₂. The LB and LS films present a linear growth forming stratified layers, monitored by UV-VIS absorption spectroscopy. The latter also revealed the presence of L LuPc₂Cl₃₂in the form of monomers and aggregates in both films. The FTIR data showed that the L LuPc₂Cl₃₂molecules present a non-preferential arrangement in both films. Monolayers of LB and LS were deposited onto 6 nm Ag island films to record surface-enhanced resonance Raman scattering (SERRS), leading to enhancement factors close to 2×10³Finally, LB and LS films deposited onto ITO glass have been successfully used as voltammetric sensors for the detection of catechol. The improved electroactivity of the LB and LS films has been confirmed by the reduction of the overpotential of the oxidation of catechol. The enhancement of the electrocatalytic effect observed in LB and LS films is the result of the nanostructured arrangement of the surface which increases the number of active sites. The sensors show a limit of detection in the range of 10?5 mol/L.

Full text of DOI:10.1166/jnn.2014.9355

Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 6754–6763, 2014
Copyright © 2014 American Scientific Publishers
All rights reserved
Printed in the United States of America
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Structural and Electrochemical Properties of Lutetium
Bis-Octachloro-Phthalocyaninate Nanostructured Films.
Application as Voltammetric Sensors
P. Alessio1ꢀ2ꢀ3, C. Apetrei , R. J. G. Rubira , C. J. L. Constantino , C. Medina-Plaza ,
1ꢀ4
3
3
1
2
1ꢀ∗
J. A. De Saja , and M. L. Rodríguez-Méndez
1Department of Inorganic Chemistry, Escuela de Ingenierías Industriales, Universidad de Valladolid,
7011 Valladolid, Spain
Faculty of Science, Department of Condensed Matter Physics, Universidad de Valladolid, 47011 Valladolid, Spain
Faculdade de Ciências e Tecnologia, UNESP Univ Estadual Paulista, Presidente Prudente, 19060-900, SP, Brazil
Faculty of Sciences and Environment, Physics and Environment, Department of Chemistry, “Dun a˘ rea de Jos”
4
2
3
4
University of Gala ¸t i, 47 Domneasca Street, 800008 Gala ¸t i, Romania
Thin films of the bis[2,3,9,10,16,17,23,24-octachlorophthalocyaninate] lutetium(III) complex
(
LuPc Cl ) have been prepared by the Langmuir-Blodgett and the Langmuir-Schaefer (LS) tech-
2 32
niques. The influence of the chlorine substituents in the structure of the films and in their spec-
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troscopic, electrochemical and sensing properties has been evaluated. The ꢀ-A isotherms exhibit
IP: 5.62.157.189 On: Tue, 31 May 2016 17:30:03
a monolayer stability greater than the observed in the unsubstituted analogue (LuPc ), being eas-
2
Copyright: American Scientific Publishers
ily transferred to solid substrates, also in contrast to LuPc . The LB and LS films present a lin-
2
ear growth forming stratified layers, monitored by UV-VIS absorption spectroscopy. The latter also
revealed the presence of LuPc Cl in the form of monomers and aggregates in both films. The FTIR
2
32
data showed that the LuPc Cl molecules present a non-preferential arrangement in both films.
2
32
Monolayers of LB and LS were deposited onto 6 nm Ag island films to record surface-enhanced
3
resonance Raman scattering (SERRS), leading to enhancement factors close to 2 × 10 . Finally,
LB and LS films deposited onto ITO glass have been successfully used as voltammetric sensors
for the detection of catechol. The improved electroactivity of the LB and LS films has been con-
firmed by the reduction of the overpotential of the oxidation of catechol. The enhancement of the
electrocatalytic effect observed in LB and LS films is the result of the nanostructured arrangement
of the surface which increases the number of active sites. The sensors show a limit of detection in
−
5
the range of 10 mol/L.
Keywords: Chloro-Substituted
Lutetium
Bisphthalocyanine,
Langmuir-Blodgett
and
Langmuir-Schaefer Films, Sensor, Catechol.
1
. INTRODUCTION
been applied in amperometric, voltammetric or potentio-
metric electrocatalytic determination of several organic
and inorganic compounds.
Phthalocyanines are a large family of compounds that have
been extensively studied as sensitive materials for electro-
chemical sensors due to their well-known electrocatalytic
2
ꢀ3
Metallophthalocyanine complexes (MPcs) are the most
1
2–5
properties. These metal complexes act as mediators by
widely studied derivatives as electrochemical sensors.
lowering the overpotential of oxidation or reduction of
the target molecules or by increasing the intensity of the
observed peaks. Therefore, phthalocyanine sensors have
In MPcs, the phthalocyanine ring (in oxidation state − 2)
is coordinated with a range of transition metal ions (in
oxidation state + 2). The number of works published
using MPcs as sensitive material is large and includes a
variety of phthalocyanine derivatives (central metal ions,
∗
Author to whom correspondence should be addressed.
6
754
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 9
1533-4880/2014/14/6754/010
doi:10.1166/jnn.2014.9355

Alessio et al.
Structural and Electrochemical Properties of Lutetium Bis-Octachloro-Phthalocyaninate Nanostructured Films
substituents), electrode designs, preparation techniques and
2
–6
target molecules. Other families of phthalocyanines such
as the bisphthalocyanines (LnPc ) have been less studied.
2
LnPc are sandwich type compounds with free radical
2
7
–9
character and ring oxidation state − 1.5. This specific
electronic structure, characterized by the presence of an
unpaired electron and by the intramolecular interaction
between the ligand ꢁ systems, allows sandwich complexes
to find applications in many fields, including the fabrica-
1
0–12
tion of sensors.
LnPc have demonstrated to be particu-
2
larly interesting sensing materials for voltammetric sensors
due to their remarkable electrochemical and electrocatalytic
1
3ꢀ14
properties.
They have been successfully used to detect
phenolic compounds, a complex group of substances of
great interest due to their antioxidant activity.15
The control of the structure at the nanometric level
(size, orientation, alignment, thickness, etc.) is an impor-
tant tool used to modulate the sensor response. Nanostruc-
6
ꢀ16ꢀ17
tured films prepared by the Langmuir–Blodgett (LB),
1
8ꢀ19
20
the Layer-by-Layer (LbL),
self-assembling,
or
1
2
Figure 1. Structure of the LuPc Cl .
2 32
electrodeposition (EDP) techniques exhibit enhanced
surface to volume ratios and well controlled structures that
facilitate the diffusion of ions inside the film. For these rea-
sons, nanostructured sensors show higher sensitivity, faster
kinetic and better reproducibility than non-nanostructured
2.2. Langmuir and LB and LS Films
The LuPc Cl Langmuir, LS and LB films were fabricated
2
32
using a Langmuir trough KSV model 2000. The Lang-
films.16ꢀ21
muir films were produced by spreading 1000 ꢄL of 8ꢂ4×
−5
mol/L solution of LuPc Cl dissolved in THF onto
2 32
10
The sensing properties of bisphthalocyanines can be
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the water subphase. The Langmuir monolayers were char-
modulated by varying the nature of the central rare
earth ion and/or by introducing sCu ob ps tyi tr ui ge hn tt s: A imn e rt ihc ea n Scientific Publishers
IP: 5.62.157.189 On: Tue, 31 May 2016 17:30:03
acterized by surface pressure versus mean molecular area
ꢀ
4
ꢀ22ꢀ23
(ꢁ-A) isotherms at 21 C using the Wilhelmy method. The
monolayer was symmetrically compressed under a con-
stant barrier speed at 10 mm/min.
phthalocyanine rings.
Unlike the tetra-substituted
compounds, which are normally formed as a mix-
ture of isomers and used as such, the octa-substituted
derivatives can be synthesized isomerically pure, a fea-
ture which should encourage molecular ordering in
nanostructured films. In this paper, ultrathin films of
the bis[2,3,9,10,16,17,23,24-octachlorophthalocyaninate]
lutetium(III) complex (LuPc Cl ) were prepared using the
The LB and the LS films were obtained by transfer-
ring the Langmuir films from the air/water interface to
different solid substrates, depending on the characteriza-
tion technique to be applied. The surface pressure was kept
constant at 30 mN/m. For LB films upstroke and down
stroke speeds of 0.5 to 3.0 mm/min were used. Under such
conditions Y-type LB films, with a transfer ratio close to
unity, were obtained. LB and LS films containing up to
2
32
Langmui–Blodgett (LB) and the Langmuir–Schaefer (LS)
techniques, and their structure and electrochemical proper-
ties were studied in detail. The work aims to evaluate the
possibility of using such nanostructured films as voltam-
metric sensors for the detection of catechol in terms of
stability and detection limit.
1
0 layers were deposited onto quartz substrates for UV-Vis
absorption spectroscopy characterization. 20 LB layers and
0 LS layers were deposited onto ZnSe for FTIR analy-
1
ses in transmission mode and 10 LS layers were deposited
onto Ag mirror for the FTIR analyses in reflection mode.
LB and LS monolayers were deposited on Ag islands
(evaporated film of Ag containing 6 nm in thickness) and
glass for surface-enhanced resonance Raman scattering
2
2
. MATERIALS AND METHODS
.1. Materials
The bis[2,3,9,10,16,17,23,24-octachlorophthalocyaninate]
(SERRS) and resonance Raman scattering (RRS) measure-
lutetium(III) complex (LuPc Cl ), MW = 2302ꢂ4 g/mol,
2
32
ments, respectively. For cyclic voltammetry and sensing
studies 4 and 8 LS layers and 10 and 20 LB layers were
deposited onto ITO electrodes.
was synthetized according to a previously published
procedure24 and the molecular structure is shown in
Figure 1. The solvents (Merck) were of HPLC grade.
The ultrapure water (18.2 Mꢃ · cm) was acquired from
a Milli-Q water purification system model Simplic-
ity. Catechol and other reactants were purchased from
Sigma-Aldrich.
2.3. Film Characterization
The UV-Vis absorption spectroscopy analyses were car-
ried out using a Varian spectrophotometer (model Cary 50)
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Structural and Electrochemical Properties of Lutetium Bis-Octachloro-Phthalocyaninate Nanostructured Films
Alessio et al.
from 190 to 1000 nm. FTIR were performed in a Bruker
spectrometer model tensor 27 with spectral resolution of
same mean molecular area is reached. The latter is con-
sistent with the formation of molecular aggregates at the
air/water interface during the first compression, which is
not reversible with expansion. Besides, the same area val-
ues at the condensed phase found for both compressions
indicate the Langmuir film reaches the same structure,
independent of the compressing/expansion cycles.
−
1
4
cm , 128 scans in the transmission mode. The reflection
ꢀ
spectra were recorded at an incident angle of 80 using an
A118 Bruker accessory. The Raman analyses were carried
out using a micro-Raman Renishaw spectrograph model
in-Via equipped with a Leica microscope, CCD detector,
laser at 633 nm, and 1800 grooves/mm gratings. In this
system a 50× objective allows collecting spectra with ca.
3.2. LB and LS Films-Growth Monitored
by UV-VIS Absorption
2
1
ꢄm spatial resolution.
The cyclic voltammetry was carried out using an
The Langmuir films were transferred to solid substrates
at a pressure of 30 mN/m using both LS and LB tech-
niques. The electronic absorption spectra for the LuPc Cl
EG&G PARC 263A potentiostat/galvanostat (M270 Soft-
ware) with a conventional three-electrode cell. The refer-
ence electrode was an AgꢁAgCl/KCl sat. and the counter
electrode was a platinum plate. Cyclic voltammograms
were registered from − 1.0 up to + 1.0 V at a scan rate
of 100 mV/s (except otherwise indicated), and starting at
2
32
in THF solution, LB film (from 5 up to 10 layers) and
LS film (from 2 up to 10 layers) deposited onto quartz
substrates are given in Figure 3. The UV-Vis spectrum
of the LuPc Cl THF solution shows the characteris-
2
32
0
.0 V.
tic features of macrocyclic compounds. The spectrum
presents strong B and Q bands that have been assigned
∗
to ꢁ → ꢁ transitions. The Q band is centered at 697 nm
3
. RESULTS
and is considerably red-shifted compared to Q band max-
imum (659 nm) observed in the unsubstituted LuPc in
chloroform solution. This bathochromic shift is usually
observed when the aromatic ring is substituted with elec-
3
.1. Langmuir Films
2
The ꢁ-A isotherms were recorded by spreading a THF
solution of LuPc Cl onto an ultrapure water (MilliQ)
subphase kept at a constant temperature at 21 C.
After solvent evaporation (20 minutes), the phthalocya-
nine molecules were compressed at a constant rate of
32
2
32
ꢀ
24ꢀ33
tron withdrawing groups.
In addition, the substitu-
tion with eight acceptor chlorine groups causes a strong
decrease of the charge transfer bands (associated with
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1
0 mm/min until the film pressure rose sharply, indi-
IP: 5.62.157.189 On: Tue, 31 May 2016 17:30:03
cating that a continuous surface film was formed (con-
intramolecular transitions of the unpaired electron) that
Copyright: American S ca ipe pn et ai fri ca tP 4u 5b 0l i sn hm e ras nd 900 nm in unsubstituted analogues,
densed phase). Reproducible ꢁ-A isotherms were recorded
and some of them are given in Figure 2(a). The extrap-
olated area (Aext in Fig. 2(a)) estimated by extrapolat-
but that are almost absent when strong withdrawing chlo-
rine groups are present. By registering spectra at increas-
−5
−5
−5
ing concentrations (8ꢂ4 × 10 , 6ꢂ72 × 10 , 4ꢂ2 × 10 ,
2
ing the condensed phase to ꢁ = 0 is 52 Å , was smaller
−5
and 1ꢂ68 × 10 mol/L), a molar extinction coefficient of
1
6ꢀ25
than the values observed for unsubstituted LuPc
and
4
−1
−1
2
5ꢂ9×10 M cm at 697 nm was obtained (spectra not
2
6–28
for LuPc substituted derivatives.
obtained here for LuPc Cl suggests an edge-on orien-
tation; the face-on orientation (lying flat) would occupy
an area between approximately 160 and 170 Å .
stacking of the LuPc Cl molecules forming molecular
aggregates is another possibility. For instance, an aggregate
with 3 molecules in a face-on orientation would also lead
to an extrapolated area of ca. 53 Å . The monolayer shows
a collapse with a surface pressure of 40 mN/m, which is
close to the collapse observed for the unsubstituted and
The limiting area
2
shown).
2
32
The UV-Vis spectra of the LB and LS films are similar
to that of the THF solution but a red shift of the Q band
is observed in both cases. This band appears centered at
711 nm for the LB film and at 709 nm for the LS one. The
shift observed in relation with maximum absorption of the
solution at 697 nm is characteristic of a face-to-tail stack-
2
29–31
The
2
32
2
3
4
ing of the chromophores (J aggregates), consistent with
ꢁ-A isotherm data. In terms of film growth, the absorbance
increases linearly with the number of monolayers for both
LB and LS films as shown in Figures 3(a) and (b), con-
firming the good quality of the transferred films. Besides,
the absorbance intensity for the Q band, comparing 10 lay-
ers LB and LS films, indicates that a major quantity of
material is transferred to the substrate in the LS film for
each layer, in average. It is important to note that the lay-
ers are deposited only onto one side of the substrate for
LS films and onto both sides of the substrate for LB films.
Finally, focusing on Q band, the greater relative intensity
of the band at higher wavelength for both films indicates
the monomers are dominant in relation to dimers or higher
order of aggregates of LuPc Cl .
2
5
substituted LuPc2, and indicates high molecular packing.
Stability tests (displacement of the barriers to keep con-
stant the surface pressure in a value within the condensed
phase of the film) proved that the LuPc Cl32 monolayer is
2
very stable at the condensed phase (Fig. 2(b)). A decrease
of ca. 10% in the mean molecular area was observed dur-
ing the first 20 minutes, keeping constant after that. Suc-
cessive compression/expansion curves until the condensed
phase (surface pressure at 30 mN/m) reveal a hysteresis
of the thermodynamic process (Fig. 2(c)). It is noted that
the second compression is shifted to lower areas; how-
ever for the first and the second expansion processes, the
2
32
6
756
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Alessio et al.
Structural and Electrochemical Properties of Lutetium Bis-Octachloro-Phthalocyaninate Nanostructured Films
3
3
2
2
1
1
50
00
50
00
50
00
5
4
3
2
1
0
0
0
0
0
0
3
2
(
a)
(b)
Reproducibility → distinct
compressions
28
24
Deposition pressure
2
1
1
8
4
0
0
6
2
→
LB and LS
50
A
ext
0
0
100
200
300
0
1000
2000
3000
Mean molecular area (Å2)
Time (s)
3
3
2
2
5
0
5
0
(c)
1
st cycle
2nd cycle
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15
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Copyright: American Scientific Publishers
1
0
5
0
0
50
100
150
200
250
300
350
Mean molecular area (Å2)
ꢀ
Figure 2. (a) ꢁ-A isotherms registered at 21 C, (b) stability test at 30 mN/m, and (c) compression/expansion cycles for LuPc Cl .
2
32
3
.3. LB and LS Films-Molecular Arrangement
Determined by FTIR
the fundamental vibrations with dynamic dipole perpen-
dicular to the substrate surface have maximum intensity.
3
7
The FTIR is a powerful tool to determine possible molec-
In Figures 4(a) and (b), the FTIR spectra for the LS
film in both transmission and reflection modes present a
similar profile. This indicates a non-preferential molecu-
lar arrangement or an orientation of the phthalocyanine
3
5
ular arrangement in thin films. Therefore, FTIR spec-
tra of LB (transmission mode) and LS (transmission and
reflection modes) films of LuPc Cl were compared in
2
32
ꢀ
Figure 4. Table I shows a list of vibrational modes and
ring forming ca. 45 in relation to the substrate surface
3
6
38
their assignment for LB and LS films. An important
point to be noticed here is that according to the surface
for LS films. A similar profile is also observed com-
paring the spectra in Figures 4(a) and (c), i.e., the FTIR
spectra for LS and LB films in transmission mode, indicat-
ing that both LS and LB present the LuPc Cl structured
3
7
selection rules the fundamental vibrations with dynamic
dipole parallel to the substrate surface have maximum
intensity for the FTIR spectra obtained in transmission
mode (incident electric field parallel to the substrate sur-
face). In reflection mode (incident electric field polarized
perpendicularly to the substrate surface), the intensities of
2
32
in a similar way. Besides, all spectra are similar to that
observed to LuPc Cl in KBr pellet given by Gobernado-
2
32
3
6
Mitre et al. Therefore, because the FTIR spectrum of
the powder is characteristic of randomly structured system,
J. Nanosci. Nanotechnol. 14, 6754–6763, 2014
6757

Structural and Electrochemical Properties of Lutetium Bis-Octachloro-Phthalocyaninate Nanostructured Films
Alessio et al.
Figure 4. FTIR spectra in transmission mode for both (a) 10-layer LS
film and (c) 20-layer LB film, deposited on ZnSe and in reflection mode
for (b) 10-layer LS film deposited on Ag mirror.
LB and LS monolayers are physisorbed onto the Ag
islands.
The enhancement factors (EF) can be estimated for
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−
1
Copyright: American Scientific Publishers
SERRS/RRS of the band at 1525 cm . For instance, for
the LB film (Fig. 5(b)) the intensity of the SERRS band
−
1
at 1525 cm is ca. 32,500 counts and in the RRS the
band intensity is ca. 3,500 counts (the intensity is the
height from the top to the bottom of the band). Consid-
ering the intensity ratio SERRS/RRS for this band, and
that the RRS spectrum was collected with a laser power
Figure 3. UV-Vis absorption spectra for (a) LB film and (b) LS film of
LuPc Cl up to 10 layers deposited onto quartz plates. The dashed line in
2
32
(
a) correspond to the THF solution spectrum of LuPc Cl (out-of-scale).
2 32
2
00 times higher than the SERRS spectrum, the enhance-
3
ment factor is estimated to be ca. 2×10 . A similar EF was
it can be concluded that, despite LB and LS grow in a well
stratified and packed layers, they present a non-preferential
molecular arrangement.
observed for the LS film. These EF observed are in agree-
4
0ꢀ41
ment with those found for phthalocyanine molecules
and in agreement with the model considering the electro-
magnetic mechanism.
3
5ꢀ42
3
.4. Surface-Enhanced Resonance
Raman Scattering (SERRS)
Table I. Infrared active vibrations and their assignment in the LB and
LS films.
The SERRS spectra for both LB and LS monolayers onto
Ag island films (6 nm of Ag evaporated onto glass slides,
leading to Ag nanoparticles) are shown in Figure 5(a).
It is observed that the SERRS spectra for both LB and
LS monolayers present the same profile. This SERRS pro-
file is the same shown by the LuPc Cl powder (result
Wavenumber (cm−1)
Assignment
7
59
C
Cl stretching
Benzene ring
932
091
135
205
275
305
1
1
1
1
1
Phthalocyanine ring breathing
Pyrrole ring breathing
Isoindole stretching
Isoindole stretching
Isoindole stretching
Isoindole stretching
Isoindole stretching
Benzene stretching
2
32
not shown) and similar to that presented in Ref. [36].
The simply enhancement of the RRS spectra without
any change in the spectra profile, as the relative inten-
sity of the bands, is a common observation for SERRS
1370
1385
3
9
of phthalocyanines. This absence of changes in the
SERRS spectra indicates that there is no chemical inter-
action between molecule-metal (LuPc Cl -Ag) and both
1
1
653
733
C
O stretching (from acetate)
2
32
6
758
J. Nanosci. Nanotechnol. 14, 6754–6763, 2014

Alessio et al.
Structural and Electrochemical Properties of Lutetium Bis-Octachloro-Phthalocyaninate Nanostructured Films
(a) SERRS
LBmonolayer
on Ag islands
LSmonolayer
on Ag islands
Figure 6. CV of LuPc Cl LB (10 monolayers) and LS (8 monolayers)
films immersed in 0.1 mol/L KCl. Solid line corresponds to LB film and
dashed line corresponds to LS film.
2
32
2
00
400
600
800 1000 1200 1400 1600
Wavenumber (cm–1
)
3
3
2
2
1
1
5000
0000
5000
0000
5000
0000
1525
(
b) RRSandSERRS
remarkable aspect of the CVs is the absence of the peaks
attributed to the quasi-reversible processes correspond-
+
ing to one electron oxidation Ln(III)Pc /Ln(III)Pc and
2
2
−
the one electron reduction Ln(III)Pc /Ln(III)Pc of the
LBmonolayeron Ag islands
2
2
phthalocyanine ring, which are usually observed in unsub-
1
1ꢀ22ꢀ28ꢀ44
stituted LnPc2.
This absence can be explained tak-
ing into account the computed ionization potential and the
electro-affinity published for asymmetrically substituted
5000
45
chlorine bisphthalocyanines. According to these data, the
halogen electro-attracting inductive effect tends to stabilize
the phthalocyanine frontier orbital. The presence of elec-
0
LBmonolayeron glass
800 1000 1200 1400 1600
2
00
400
600
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–1
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reduction of the phthalocyanine ring. For this reason, the
Copyright: American Scientific Publishers
peaks corresponding to these processes are not observed
in the studied range.
Figure 5. (a) SERRS spectra for both LB and LS monolayers of
LuPc Cl onto Ag island films (Ag nanoparticles). (b) RRS and SERRS
2
32
spectra for LB monolayers on glass and Ag islands, respectively.
Voltammograms also show a cathodic wave at − 0.90 V
that is caused by the decomposition of water at the elec-
trode surface. This peak is observed at − 1.1 V when using
neat ITO electrodes. The shift of the potential at which
decomposition of water occurs, is due to the electrocat-
alytic effect of the phthalocyanine layer that facilitates the
reduction process. The intensity of the cathodic wave was
dependent on the pH of the solution: it decreases when the
pH increases (results not shown), confirming the assign-
ment of this peak. The results obtained from LB and LS
were similar and only a difference in the decomposition of
water (favored at LS electrodes) was observed.
Basically, according to the EM mechanism, the enhance-
ment of the Raman signal is achieved by the localized sur-
face plasmon resonances (LSPR), which are sustained by
the Ag nanoparticles and lead to the enhancement of the
3
5ꢀ42
electric field surrounding the Ag nanoparticles.
There-
fore, the laser line (633 nm) must be in resonance with the
Ag plasmon absorption (necessary condition) to achieve
the surface-enhanced Raman scattering (SERS). In our
case, because the 633 nm laser line is also in resonance
with the LuPc Cl absorption (Fig. 3(a)), the resonance
2
32
Raman scattering (RRS) is achieved. Therefore, when the
LuPc Cl monolayer is deposited onto the Ag island films
3.6. Electrochemical Detection of Catechol
2
32
and excited with the 633 nm laser line, a double resonance
is established and the SERRS phenomenon is achoeved.
As stated before, unsubstited or octa-terbutyl bisph-
thalocyanines immobilized on inert electrode surfaces,
exhibit electrocatalytic activity towards phenolic
4
3
1
4ꢀ15ꢀ24
3
.5. Electrochemical Properties
compounds.
The presence of bisphthalocyanine
In order to evaluate the electrochemical behavior of the
LuPc Cl sensors, the electrodes were immersed in a
results in a decrease in the overpotential of oxidation or
reduction of the target molecules and/or an increase in the
intensity of the peaks observed.
2
32
KCl solution and cyclic voltammograms were registered
from − 1.0 to + 1.0 V at a scan rate of 100 mV/min.
Figure 6 shows the cyclic voltammograms (CV) of LB
and LS films of LuPc Cl deposited onto ITO glass in
In order to evaluate the electrocatalytic activity of
the LuPc Cl films towards phenols, the electrochemi-
2
32
−
3
cal response of 10 mol/L catechol in 0.1 mol/L KCl
at LuPc Cl modified ITO electrodes was tested at a
2
32
contact with 0.1 mol/L KCl aqueous solution. The most
2
32
J. Nanosci. Nanotechnol. 14, 6754–6763, 2014
6759

Structural and Electrochemical Properties of Lutetium Bis-Octachloro-Phthalocyaninate Nanostructured Films
Alessio et al.
of the irreversible reduction of catechol catalyzed by the
phthalocyanine.
It is also important to notice that the redox peak currents
of catechol increase about 25% with respect to the inten-
sity observed for a bare ITO glass (the increase in intensity
observed in unsubstituted LuPc is tenfold the observed
2
when using bare ITO glass). In summary, the excellent
electrocatalytic activity of LuPc Cl is responsible for the
2
32
significant improved electrochemical behavior of catechol
for the LuPc Cl cast film.
2
32
It is well known that the structure of the sensing
layer plays a key role in the electrochemical behavior of
modified electrodes. The voltammograms obtained using
LuPc Cl nanostructured films immersed in a catechol
Figure 7. Cyclic voltammograms of 10 3 mol/L catechol in the pres-
ence of 0.1 mol/L KCl at a bare ITO electrode (solid line) and a cast
film of LuPc Cl (dashed line). Scan rate 100 mV/s.
−
2
32
2
32
solution differ significantly from voltammograms recorded
using a bare ITO glass or an ITO electrode modified with
a cast film. This is illustrated in Figure 8 where the elec-
trochemical responses of LB and LS films immersed in
scan rate of 100 mV/s. For comparison purposes, the
cyclic voltammogram of catechol for a bare ITO elec-
trode was also recorded (Fig. 7). In good acordance with
the literature, when a bare ITO glass is used as elec-
trode, the voltammogram shows an anodic peak (at 0.55 V)
in the positive-going scan and a cathodic counterpart
peak (at 0.22 V) in the negative-going scan. These
peaks correspond to the transformation of catechol to o-
benzoquinone and vice-versa in a quasi-reversible two-
−
3
a solution of 10 mol/L catechol (in 0.1 mol/L KCl)
are shown. When LuPc Cl LB or LS electrodes are
2
32
immersed in catechol, a set of anodic (positive) and
cathodic (negative) peaks are observed. For more clarity,
peaks have been numbered.
The oxidation of the catechol to a quinone gives rise
to a broad anodic peak on the forward scan of the cyclic
voltammograms (peak I), while reduction of the quinone
back to the original polyphenol produces the correspond-
IP: 5.62.157.189 On: Tue, 3i n1 g Mc aa tyh o2 d0 i 1c 6p e1 a7 k: 3 o0 n: 0t 3h e reverse scan (peak III). Due to
4
6–48
electron process.
Under these conditions, ꢅE is 0.33 V
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and peak current ratio (I /I ) is close to unity.
C
A
Voltammograms obtained using electrodes modified
with LuPc Cl films immersed in a catechol solution,
Copyright: American S ct hi ee n et li fe i cc t rPo uc ab tl ai sl yh t ei cr sa ctivity of the LuPc Cl , peak I is dis-
2
32
2
32
placed to 0.27 V for both LB and LS films (0.55 V when
using a bare ITO glass electrode and 0.4 V when using a
cast film). In turn, peak III is also shifted to lower poten-
tials (at 0.1 V for LB, − 0.05 V for LS and 0.2 V for
cast film). The anodic peak I associated with the oxidation
of catechol is quite broad and by similarity with the cast
film, it can be assumed that this broad peak is the wave
resulting from the overlapping of redox processes at two
different active sites.
differ significantly from voltammograms recorded using
a bare ITO glass electrode. For instance, when a cast
film is exposed to catechol (Fig. 7), voltammograms
show the expected redox pair associated with the oxida-
4
8
tion/reduction of catechol. The cathodic peak appears at
.22 V (the same potential observed when using a bare
0
ITO glass), whereas the anodic peak is shifted to lower
potentials and appears at 0.4 V (0.55 V at ITO electrode).
The peak potential difference (ꢅEꢆ between anodic and
cathodic peak (ꢅE = 0ꢂ18 V) is smaller to the observed
for bare ITO glass (ꢅE = 0ꢂ33 V), indicating that the
reversibility of electrochemical reaction is improved in the
presence of LuPc Cl . This electrocatalytic activity has
2
32
been already observed in electrodes modified with unsub-
stituted LuPc where the oxidation of catechol occurs at
2
1
6ꢀ32
0
.47 V with ꢅE = 0ꢂ30 V.
In our case, when the
phthalocyanine ring is substituted with chlorine atoms, the
shift to lower potentials is more pronounced.
We would like to remark that the anodic peak at 0.4 V
is accompanied by a shoulder at 0.25 V. This splitting
might be due to the existence of two types of binding sites
for immobilization, giving rise to active sites with distinct
electrocatalytic efficiencies. Further proofs of the existence
of two types of active sites will be shown in the dynamic
study of the electrodes at a different scan rates. In addition,
a cathodic wave at − 0.5 V is also observed as a result
−3
Figure 8. Cyclic voltammograms of 10 mol/L catechol in the pres-
ence of 0.1 mol/L KCl at 20 monolayers LB film (solid line); 10 mono-
layers LB (dotted line) and a 8 monolayers LS film (dot-dashed line) of
LuPc Cl . Scan rate 100 mV/s.
2
32
6
760
J. Nanosci. Nanotechnol. 14, 6754–6763, 2014

Alessio et al.
Structural and Electrochemical Properties of Lutetium Bis-Octachloro-Phthalocyaninate Nanostructured Films
An important difference between LB (or LS) and cast
films is that in nanostructured films a new intense anodic
wave is observed at 0.75 V (peak II). According to the
literature, this new redox process can be associated with
4
4–49
the polymerization of the previously oxidized catechol.
This peak was not observed when using a LuPc Cl cast
2
32
film, indicating that the structure of the surface and the
homogeneous molecular packaging of the nanostructured
films facilitate the second oxidation. The polymeric form is
reduced at potentials close to 0.2 V. That is, in the cathodic
sweep, the peak observed at 0.2 V (peak III) corresponds
to the overlapping of the reduction of both the quinoid
form and polymeric form of catechol, justifying thus the
increase in intensity and in broadness with respect to the
cast film. The cathodic wave at − 0.5 V is also observed
Figure 9. Cyclic voltammograms of a LS sensor (8 layers) registered
at different scan rates from 50 to 500 mV/s. The inset shows the plot of
−
3
Ia versus ꢇ (calculated for peak I). Electrolyte solution was 10 mol/L
(peak IV).
catechol (in KCl 0.1 mol/L).
The enhanced electrocatalytic effect observed in LB and
LS films can be attributed to the LuPc Cl molecular
2
32
rate in the range 50–500 mV/s (Table II). As shown in
Figure 9, the intensity of peak I increases linearly with the
scan rate, indicating the dominance of the surface confined
processes. Under these conditions the surface coverage (ꢈ)
can be calculated using the Laviron equation:
arrangement/packing. The homogeneous layered structure
provides well-defined surfaces with an increased number
of active sites. The small structural difference between LB
and LS films described in Sections 3.1 to 3.4. justifies
some discrepancies in their electrochemical behavior. For
instance, the oxidation and the polymerization of catechol
2
I = nF vAꢈ/4RT
(1)
(
peaks I and II) are favored at the surface of LS films.
An important difference between LB and LS films is their
stability. It is well known that in Dc he el mi v ei cr ae l dl y bmy oI dn i gfi ee dn t ea l et co -: Purdue University Libraries
where n is the number of electrons, F the Faraday con-
stant, v the rate, A the area of the sensor, ꢈ the surface
coverage, R the gas constant and T the temperature. The
trodes, the first cycles differ fro mI P:s 5u b. 6s 2e q. 1u 5e n7 t. 1 o8 n 9e sO. nF :o Tr ue, 31 May 2016 17:30:03
this reason, before using the electrodes for sensing pur-
Copyright: American Scientific Publishers
surface coverage of the LuPc Cl films calculated using
2
32
poses, it is necessary to run several stabilization cycles.
In our case we have observed that LB films are stabilized
after three cycles, whereas, LS films are not stabilized until
−11
2
Eq. (1), was ꢈ = 7ꢀ77449×10
mol/cm for the LB film
−10
2
and ꢈ = 1ꢀ27552 × 10
mol/cm for the LS film. The
inherent irreproducibility of cast films does not allow cal-
culating the surface coverage accurately. These values are
higher than those obtained using a bare ITO glass electrode
mol/cm ), indicating that the presence of the
nanostructured layer of LuPc Cl increases the number of
1
5 cycles are run (all the voltammograms shown in the
above figures correspond to stabilized signals). After stabi-
lization, Standard Deviation (SD) values (calculated from
peak I intensity values) were lower than 5% for both LB
and LS films.
−11
2
(2ꢂ37×10
2
32
active sites, thus facilitating the oxidation of the catechol.
The intensity of the peak I is linearly dependent on the
The thickness also influences the electrochemical
response of LB or LS films exposed to catechol (Fig. 8).
In both types of films, peaks associated with the poly-
merization of the catechol (peaks II and III) increase in
intensity when increasing the number of monolayers. This
effect can be explained by assuming that the inner layers
facilitate the transfer of electrons to the ITO glass. In con-
trast, by increasing the number of monolayers the intensity
of the peaks I and IV, associated exclusively with the oxi-
dation/reduction of catechol, does not change. This means
that these redox processes occur exclusively at the active
sites located at the electrode surface.
−
5
concentration of catechol in the range from 6 × 10
M
−
4
to 5 × 10 M, as shown in Figure 10. The correspond-
ing detection limits were calculated according to the 3s /m
b
criterion, where m is the slope of the calibration curve,
and s was estimated as the standard deviation (seven rep-
b
etitions) of the voltammetric signal of the electrode at the
concentration level corresponding to the lowest concentra-
tion of the calibration plot. The detection limit calculated
−
5
−5
was 7ꢂ5 × 10 mol/L for LB and 8ꢂ4 × 10 mol/L for
2
Table II. Slope (m), coefficient of correlation (R ) and surface coverage
The dynamic character of the electrode was exam-
ined by registering voltammograms at different scan rates
(ꢈ) obtained by representing the intensity of the anodic peak I and the
scan rate (ꢇ).
(Fig. 9). At high scan rates, the splitting of peak I become
Electrode
m (slope)
R2
ꢈ (mol/cm2)
more evident, confirming the existence of two active sites
at the film surface. The inset in Figure 9 shows the linear
relationship between the peak current (peak I) and the scan
LuPc Cl /20LB
7×10−
1×10−
5
0ꢂ9954
0ꢂ996
7ꢂ77449×10
1ꢂ2755×10
−11
−10
2
32
4
LuPc Cl /8LS
2
32
J. Nanosci. Nanotechnol. 14, 6754–6763, 2014
6761

Structural and Electrochemical Properties of Lutetium Bis-Octachloro-Phthalocyaninate Nanostructured Films
Alessio et al.
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2
32
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octachlorophthalocyaninate] lutetium(III) complex have
been successfully prepared forming well stratified layers.
The molecular packing induced by the presence of the
substituents differs from that shown by other bisphthalo-
cyanine compounds and strongly affects the electrochem-
ical properties of the LB and LS films when compared
to LuPc Cl cast film and bare ITO glass electrode. The
electrocatalytic activity of the LuPc Cl and the abun-
dant binding sites for the immobilization provided by
the nanostructured films are responsible for the signifi-
cant improved electrochemical behavior of catechol at the
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32
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1
2
LuPc Cl films.
2
32
The small structural difference between LB and LS films
justifies some divergence in their sensing properties. For
instance, the oxidation and the polymerization of catechol
are favored at the surface of LS films, whereas LB films
are more stable.
In summary, the electrochemical results indicate that the
LuPc Cl LB and LS films modified ITO glass electrodes
can be used as catechol sensing units, with good stability
2
2
2
3. E. G. R. Fernandes, L. C. Brazaca, M. L. Rodríguez-Méndez, J. A.
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Acknowledgments: Financial support of the CICYT
Spanish Ministry of Science (Grant AGL2012-33535) and
from the Programa Hispano-Brasileño de Cooperación
Interuniversitaria con Brasil (PHB2011-0004PC) is grate-
fully acknowledged. One of us CMP wants to thanks
the University of Valladolid for a PIF grant. The brazil-
ian agencies FAPESP, CNPq and CAPES (CAPES/DGU
2
(
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Received: 30 April 2013. Accepted: 7 June 2013.
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Copyright: American Scientific Publishers
J. Nanosci. Nanotechnol. 14, 6754–6763, 2014
6763