NO2 sensing ability of a monolayer of cobalt(II) porphyrin molecules covalently assembled on a engineered silica substrate

Article abstract of DOI:10.1016/j.ica.2008.02.055

The 5,10,15-tri-(p-dodecanoxyphenyl)-20-(p-hydroxyphenyl)- and 5,10,15,20-tetrakis-(p-dodecanoxyphenyl)-cobalt porphyrin complexes were synthesized, purified and characterized. Silica substrates were functionalized with a covalent 4-ClCH₂C₆H₄SiCl₃ monolayer. Additional covalent bonding of the 5,10,15-tri-(p-dodecanoxyphenyl)-20-(p-hydroxyphenyl)-cobalt porphyrin to the silylated substrates was further achieved. The monolayer surface chemical characterization was carried out by X-ray photoelectron measurements. Both the Co 2p and N 1s spectra are evident. The NO₂ sensing capability of the present cobalt porphyrin systems, at ppm levels, has been demonstrated.

Full text of DOI:10.1016/j.ica.2008.02.055

Inorganica Chimica Acta 361 (2008) 3877–3881
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
NO2 sensing ability of a monolayer of cobalt(II) porphyrin molecules
covalently assembled on a engineered silica substrate
*
Antonino Gulino , Placido Mineo, Ignazio Fragalà
Dipartimento di Scienze Chimiche, Università di Catania and I.N.S.T.M. UdR di Catania, Viale Andrea Doria 6, 95125 Catania, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The 5,10,15-tri-(p-dodecanoxyphenyl)-20-(p-hydroxyphenyl)- and 5,10,15,20-tetrakis-(p-dodecanoxy-
phenyl)-cobalt porphyrin complexes were synthesized, purified and characterized. Silica substrates were
functionalized with a covalent 4-ClCH2C6H4SiCl3 monolayer. Additional covalent bonding of the 5,10,15-
tri-(p-dodecanoxyphenyl)-20-(p-hydroxyphenyl)-cobalt porphyrin to the silylated substrates was further
achieved. The monolayer surface chemical characterization was carried out by X-ray photoelectron mea-
surements. Both the Co 2p and N 1s spectra are evident. The NO2 sensing capability of the present cobalt
porphyrin systems, at ppm levels, has been demonstrated.
Received 10 January 2008
Accepted 26 February 2008
Available online 5 March 2008
Dedicated to Professor Dante Gatteschi.
Keywords:
Porphyrin
Cobalt
Ó 2008 Elsevier B.V. All rights reserved.
Assembly
UV–Vis
XPS
NO2
1. Introduction
a challenge for both scientific and technologic reasons [17–20]. In
this perspective, the covalent immobilization of a mono- or sub-
There is an increasing technological interest in the synthesis of
hybrid inorganic/organic nanomaterials. Syntheses based on cova-
lent assembly of appropriate molecules on suitable inorganic sub-
strates represent one of the most powerful approaches in the
perspective of fabrication of devices showing specific single mole-
cule properties and to study functional molecular architectures [1–
12].
Many different interesting molecular properties can be investi-
gated by optical measurements, e.g. molecular switching, lumines-
cence quenching, variation in optical absorbance, non-linear
optical properties, and molecular recognition properties. In these
cases, transparent silica (quartz) substrates are very useful for
covalently assembling of functional molecules. The typical synthe-
sis procedure requires first covalent substrate grafting with a given
bi-functional appropriate coupling layer (CL) that can bond both to
the substrate and to the following functional molecules [11].
Cobalt nanomaterials play a dominant role in numerous bio-
and physico-chemical processes [12]. Cobalt porphyrins exhibit
many interesting properties due to their involvement in electron
transfer reactions [13], catalysis [14], sensing [15], optical behavior
[16], etc. Despite these numerous applications, the knowledge of
their molecular properties at the solid state is crucial and remains
layer of these molecules on a transparent solid substrate probably
represents the most suited prerequisite to accomplish this task. To
this purpose, well-tailored cobalt porphyrin molecules, having
peripheral substituents that increase the steric hindrance, should
1
avoid aggregation phenomena [21] so that the molecular, instead
of bulk, properties should be emphasized. This final goal can be pur-
sued by appropriate chemical synthetic strategies, adopted to obtain
the molecular building block, combined with suitable characteriza-
tion techniques to investigate the engineered substrate.
In this research field, as a part of an on-going study regarding
the engineering of silica substrates by covalent bonding of sensing
molecules [22–29] the surface evolution of transparent SiO2 sub-
strates along the step by step grafting of appropriate coupling layer
groups suited for successive anchoring of functional molecules
[24,30,31] has recently been described. Moreover, we also de-
scribed the fabrication of highly sensitive and selective optical sen-
sors using different porphyrin monolayers [22–25,27–29].
Therefore, there was enough motivation for us to embark on the
synthesis and characterization of a new monolayer of cobalt
1
The presence of only one hydroxyl group in the peripheral position of the cobalt
porphyrin allows a univocal covalent linkage to the substrate. Moreover, the steric
hindrance due to the three long aliphatic groups, covalently bounded to the
remaining peripheral positions of the porphyrin, should prevent aggregation
phenomena.
* Corresponding author. Tel.: +39 095 7385067; fax +39 095 580138.
E-mail address: agulino@dipchi.unict.it (A. Gulino).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.02.055

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A. Gulino et al. / Inorganica Chimica Acta 361 (2008) 3877–3881
porphyrin molecules covalently assembled on a functionalized sil-
ica surface.
an electrostatic lens system Omni Focus III. Samples were mounted
on Mo stubs. Spectra were excited with Al Ka radiation. The struc-
ture due to the Ka2 satellite radiation has been subtracted from the
spectra before the data processing. The XPS peak intensities were
obtained after Shirley background removal [35]. Procedures to ac-
count for steady state charging effect have been described else-
where [33,34]. Experimental uncertainties in binding energies lie
within 0.45 eV.
2. Experimental details
2.1. Synthesis and characterization of porphyrin cobalt complexes
Aldrich grade reagents and solvents, some of them packed un-
der nitrogen, were used throughout all the present syntheses.
The dodecanoxyphenyl porphyrin derivatives namely 5,10,15-tri-
(p-dodecanoxyphenyl)-20-(p-hydroxyphenyl) porphyrin (R3PH2)
and 5,10,15,20-tetrakis-(p-dodecanoxyphenyl) porphyrin (R4PH2)
were synthesized, purified and characterized as already reported
[32]. Cobalt complexes of R3PH2 and R4PH2 (hereafter R3PCo and
R4PCo, respectively) were obtained by reacting appropriate por-
phyrin derivatives for 4 hours with an excess (1:30 mol/mol) of co-
balt(II) acetate tetrahydrate, in pyridine at 100°C, in nitrogen
atmosphere. After evaporation of the solvent, the pure product
was isolated by column chromatography separation, using silica
UV–Vis measurements were performed using a UV–Vis-1601
Shimadzu spectrophotometer. Experimental uncertainty in wave-
length lies within 0.5 nm.
3. Results and discussion
The MALDI-TOF mass spectra of R3PCo and R4PCo (Fig. 1) show
only a relevant peak at m/z = 1240 and 1408, respectively, for
molecular ions detected as MH+ species. These values strongly con-
firm both R3PCo and R4PCo structures.
Fig. 2 (black solid line) shows the UV–Vis spectrum of a
3.5 Â 10À7 M THF solution of R4PCo. Two features are evident: a
Soret band at 419.7 nm and a broad band at 533.2 nm. The most
evident difference between the present cobalt porphyrin UV–Vis
spectrum and that of the free porphyrin [28] is represented by
gel as
a stationary phase and a mixture of CHCl3/C2H5OH/
N(C2H5)3 (97.5/2.0/0.5) as eluant.
The chemical characterization of R3PCo and R4PCo was per-
formed by MALDI-TOF mass spectrometry. To this purpose, sam-
ples were dissolved in THF, mixed with a 0.4 M THF solution of
trans-3-indoleacrylic acid (used as a matrix), and loaded on the
sample plate. Mass spectra were acquired by a Voyager DE instru-
ment (PerSeptive Biosystem) using a delay extraction procedure
R3PCo
(25 kV applied after 2600 ns with
a potential gradient of
454.54 V/mm and a wire voltage of 25 V) and a detection in linear
mode. The instrument was equipped with a nitrogen laser (emis-
sion at 337 nm for 3 ns, 50 Hz) and a flash AD converter (time base
2 ns).
R4PCo
1250
1350
m/z
2.2. Synthesis and characterization of the covalently assembled
monolayer
Fig. 1. The MALDI-TOF mass spectra of R3PCo and R4PCo.
Fused silica (quartz) substrates were cleaned by immersion in
‘‘piranha” solution (concd. H2SO4:30% H2O2 70:30 v/v) at 80 °C
for 1 h and then left to cool to room temperature. Subsequently,
substrates were repeatedly rinsed with double distilled water
and immersed in a H2O:30% H2O2:NH3 5:1:1 v/v/v mixture at room
temperature for 40 min [24,28]. Then, these were washed with
double distilled water and dried under vacuum immediately before
coupling agent deposition. All the successive sample manipula-
tions were performed in a glove box under N2 atmosphere. In
particular, freshly cleaned substrates were immersed, at room
temperature for 30 min, in a 1:100 (v/v) n-heptane solution of
the chemisorptive siloxane, trichloro[4-(chloromethyl)phenyl]-
silane (4-ClCH2C6H4SiCl3) to afford a monolayer of the coupling
agent (CA) [24,28]. Then, these were washed with copious amounts
of pentane, sonicated 2 min in acetone to remove any physisorbed
CA, immersed in a toluene 1.0 Â 10À4 M solution of the R3PCo
and heated up to 80 °C under stirring for 48 h. The substrates bear-
ing the covalently self-assembled cobalt porphyrin monolayer
(R3PCo-SAM) were cooled to room temperature and repeatedly
washed and sonicated with toluene, THF and CH2Cl2 to remove
any residual unreacted porphyrin.
0.10
0.05
X-ray photoelectron spectra (XPS) were measured at 45°, rela-
tive to the surface plane with a PHI 5600 Multi Technique System
which offers a good control of the electron take-off angle (base
pressure of the main chamber 2 Â 10À10 Torr) [33,34]. The accep-
tance angle of the analyzer and the precision of the sample holder
concerning the take-off angle are 3° and 1°, respectively. The
spectrometer is equipped with a dual anode X-ray source; a spher-
ical capacitor analyzer (SCA) with a mean diameter of 279.4 mm;
0.00
400
600
Wavelength / nm
Fig. 2. UV–Vis spectra of a 3.5 Â 10À7 M THF solution of the R4PCo detected after 5,
10, 30, 60, 120 and 150 s bubbling 10 ppm NO2 in N2. The dashed line refers to the
recovered starting spectrum.

A. Gulino et al. / Inorganica Chimica Acta 361 (2008) 3877–3881
3879
the decreased number of Q-bands. In fact, the free porphyrin solu-
tion shows four Q-bands in the 515–660 nm range. The decreased
Q-band number can be interpreted on the basis of the already re-
ported the so-called four-orbital model that describes the low-ly-
ing (p,p*) excited states of porphyrins in terms of electronic
transitions between the two topmost filled molecular orbitals
(HOMO’s), a2u (p) and a1u (p), to two degenerate lowest empty
molecular orbitals (LUMO’s), eg (p*) [36]. On the basis of this model,
metallo-porphyrins should show two visible Q-bands whilst, the
free base shows four Q-bands. Taking into account all these obser-
vations, the broad envelope at 533.2 nm, observed for the present
R4PCo complex, accounts for two Q-bands [29c].
The NO2 sensing capability of the present cobalt porphyrin sys-
tem was first tested in solution. The UV–Vis spectra (Fig. 2) of the
THF porphyrin 3.5 Â 10À7 M solution, during bubbling of 10 ppm
NO2 in a N2 gas stream (flow rates = 100 sccm), show an evident
red shift of both Soret and Q-band that now appear at 432 and
548 nm, respectively. In particular, UV–Vis spectra were acquired
after 5, 10, 30, 60, 120 and 150 s bubbling 10 ppm NO2 in N2. In
addition, the Soret band suffers some intensity decrease whilst,
the broad band at 548 nm shows an intensity increase and, as a con-
sequence, the second Q-band at 591 nm it is now more evident. The
starting spectrum was totally recovered by removing the solvent in
vacuum and re-dissolving the solid in THF. This result confirms the
reversible sensing capability of the present system towards NO2 at
ppm level. On the basis of this observation, a sensing mechanism
consistent with cobalt coordination chemistry can be inferred.
The R3PCo-SAM (Fig. 3) was synthesized by covalent grafting of
the cobalt porphyrin to a silica substrate that was previously
cleaned, hydroxylated and silylated. In particular, the silylation
was performed rigorously under inert atmosphere, with the Cl3–
Si–C6H4–CH2Cl siloxane, a bi-functional coupling agent that bonds
both the substrate and the cobalt porphyrin molecules. This por-
phyrin siloxane-based monolayer strongly adhere to the substrate,
is robust since insoluble in common organic solvents, and cannot
be removed by the ‘‘Scotch-tape decohesion” test as evidenced
by UV–Vis measurements. Comparison between the UV–Vis spec-
trum for R3PCo-SAM (Fig. 4, solid line) and that of the R4PCo com-
plex in solution (Fig. 2) is straightforward once a 16 nm red shift
for the monolayer is taken into account.
Using the Beer–Lambert law (A = elc, where A is the absorbance
and e, l and c are the extinction coefficient, the thickness and the
density of the porphyrin in the layer, respectively), one can calcu-
late the surface coverage, dsurf = AeÀ1 (number of cobalt porphyrin
molecules/cm2 of the R3PCo-SAM) [37,38]. Taking into account
the e value of 210000 in THF, the calculated density value is
6.31 Â 1012 molecules/cm2. This value is lower than that already
observed for the free porphyrin SAM (1.1 Â 1013) and can be
understood in terms of increased interactions between porphyrin
molecules due to the presence of Co(II) ions in the porphyrin core.
The molecular monolayer characterization of the R3PCo-SAM
was carried out with X-ray photoelectron spectroscopy. This tech-
nique is ideal since it allows high vertical resolution, gives informa-
tion on the bonding states of the grafted molecules [22–31], and
allows to estimate the surface elemental composition, making
due allowance for the relevant atomic sensitivity factors [39].
In particular, the Si 2p, at 103.1 eV and the O 1s, at 533.2 eV sig-
nals are consistent with the presence of the SiO2 substrate [40–42].
The C 1s spectrum with band at 285.0 and 286.5 eV is typical for
porphyrin systems [43–47].
The cobalt porphyrin complex possesses a D4h symmetry.
Therefore, all nitrogen atoms are equivalent and should show a
symmetric N 1s XPS peak. Indeed, this behavior has been reported
for CuTPC and CuTPP porphyrin systems that show the N 1s signal
at 398.3 and 398.2 eV, respectively [44]. In addition, a broad there-
fore uninformative N 1s band in the 402–395 eV range has been re-
vealed for a zinc protoporphyrin monolayer linked to amino-
terminated silanized glass surfaces [48]. Besides, some zinc por-
phyrin polymers display two N 1s peaks (ꢀ400 eV) due to multiple
chemical environments at the porphyrin nitrogen atoms that, in
turn, cause different intermolecular interactions [43]. The N 1s
spectrum of the present R3PCo-SAM (Fig. 5) shows a peak at
O(CH2)11CH3
0.002
0.001
0.000
N
N
N
N
O(CH2)11CH3
Co
CH3(CH2)11
O
O
CH2
Si
O
O
400
450
500
550
600
O
Wavelength / nm
Fig. 4. UV–Vis spectra of a representative R3PCo-SAM in air (solid line) and upon
Fig. 3. R3PCo-SAM.
10 ppm NO2 in N2 (dashed line).

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A. Gulino et al. / Inorganica Chimica Acta 361 (2008) 3877–3881
Finally, the NO2 molecular recognition capability of the R3PCo-
SAM was investigated by UV–Vis measurements. In particular,
the monolayer-bearing substrate was fixed in a cuvette sealed with
a septa rubber lid. UV–Vis spectra were recorded before and upon
10 ppm NO2 in N2. Fig. 4 (dashed line) shows evidence of a ꢀ15 nm
higher wavelength shifted spectrum, in tune with the behavior al-
ready observed in the solution, thus confirming the ability of the
system to detect NO2.
4. Conclusions
In the present study, two new cobalt porphyrin systems have
been obtained and their sensing characteristics towards NO2
exploited. Therefore, a new monolayer of one of these cobalt por-
phyrin molecules, covalently assembled to a silylated substrate,
was synthesized. This robust monolayer system was characterized
using both UV–Vis and X-ray photoelectron measurements. A well-
resolved XPS Co 2p3/2 signal confirms the cobalt porphyrin assem-
bling. The absorbance spectrum of the monolayer closely resem-
bles that of the R4PCo solution and provides evidence of NO2
molecular recognition ability. Further efforts concerning the selec-
tivity towards NO2 of the present R3PCo-SAM are in progress. Since
the ultimate goal of molecular bottom-up approaches is to employ
functional building blocks to construct nanometer-scale devices
that address specific applications, such a monolayer is a good can-
didate for the development of sensor, switches, displays and mag-
netic devices.
405
395
400
Binding Energy / eV
Fig. 5. XPS of the R3PCo-SAM in the N 1s energy region.
398.0 eV with an intense shoulder at 400.1 eV [11,43,44,48–50].
According to these reported studies on similar porphyrin systems,
the presence of the shoulder is consistent with the lift of the D4h
symmetry that, in turn, cause the lift of the nitrogen–cobalt inter-
action degeneracy because of the cobalt porphyrin axial ligation to
the silylated substrate [43].
The XPS signal at 778.2 eV (Fig. 6) accounts for the Co 2p3/2
spin–orbit component thus confirming the presence of the R3PCo
on the surface. This value is almost coincident with already re-
ported XPS data for parent systems [13a].
Acknowledgements
The authors thank the NATO (SfP project 981964) and the Min-
istero Universitaè Ricerca (MUR, Roma) for financial supports
(PRIN 2005 and FIRB 2003).
Further experiments were made to rule out either any possible
physisorption or alternative Si–O–porphyrin surface grafting. A
hydrophilic SiO2 terminated surface was exposed to a 1 Â 10À4
M
R3PCo solution and then rinsed. Neither UV–Vis nor XPS evidences
of cobalt porphyrin were found.
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