Investigation of free base, Mg, Sn, and Zn substituted porphyrin LB films as gas sensors for organic analytes

Article abstract of DOI:10.1021/jp0626059

The visible absorption spectra of various substituted porphyrin compounds both in chloroform solution and as Langmuir-Blodgett (LB) solid-state films have been investigated. The porphyrin compounds examined were the Zn, Sn, Mg, and free base derivatives of 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,-23H-porphine (EHO). Changes in the absorption spectra of these materials induced by their exposure to various organic compounds are reported with a view toward determining whether this is a useful approach toward an optical gas sensor.

Full text of DOI:10.1021/jp0626059

16646
J. Phys. Chem. B 2006, 110, 16646-16651
Investigation of Free Base, Mg, Sn, and Zn Substituted Porphyrin LB Films as Gas Sensors
for Organic Analytes
Alan D. F. Dunbar,†,* Tim H. Richardson,† Alex J. McNaughton,† Jordan Hutchinson,‡ and
Chris A. Hunter‡
Department of Physics and Astronomy, UniVersity of Sheffield, Hicks Building, Hounsfield Road,
Sheffield S3 7RH, U.K., and Department of Chemistry, UniVersity of Sheffield, Dainton Building,
Brook Hill, Sheffield, S3 7HF, U.K.
ReceiVed: April 28, 2006; In Final Form: June 29, 2006
The visible absorption spectra of various substituted porphyrin compounds both in chloroform solution and
as Langmuir-Blodgett (LB) solid-state films have been investigated. The porphyrin compounds examined
were the Zn, Sn, Mg, and free base derivatives of 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,-
23H-porphine (EHO). Changes in the absorption spectra of these materials induced by their exposure to
various organic compounds are reported with a view toward determining whether this is a useful approach
toward an optical gas sensor.
Introduction
under investigation has a sufficiently strong interaction with the
porphyrin, the energies of the conjugated π-electrons in the
porphyrin are shifted, and hence, the absorption spectra change.
Considerable work has already been performed to develop a
porphyrin-based sensor system for NO2.9-12 Some of the
porphyrin-based sensors can be recovered by gentle heating
(∼90 °C) and are therefore reuseable.10 The concentration
dependence of porphyrin thin films has been found to follow a
Langmuir adsorption model at low concentrations (0.46-4.6
ppm).12 Porphyrin sensors are also being incorporated into single
use colorimetric sensor arrays to detect volatile organic com-
pounds.13
This work is a study of four different porphyrins, EHO in its
free base form and also metalated with Sn, Mg, and Zn. We
compare the reaction of these porphyrins with a range of
different volatile organic compounds; the aim of this work is
to determine the suitability of the four porphyrins studied for
use as optical-based gas sensors for the different volatile organic
analytes. First, each of the four porphyrins was tested in solution
for changes in the optical absorption upon exposure to the full
range of analytes. The results of this preliminary study were
then used to guide the choice of porphyrins to be selected for
fabrication of solid-state LB films, which were subsequently
tested for sensor action with the corresponding analytes. The
materials and experimental procedures used are described in
the next section followed by discussion of the results for the
solution experiments. This is followed by discussion of the
results for the solid-state LB films, and finally, some conclusions
are made.
Concerns about toxins released either maliciously or inadvert-
ently by combustion engines and various industries have
continued to increase. Therefore, the need for accurate and
reliable sensors to detect a broad spectrum of organic com-
pounds has never been greater than it is now. Accurate
information about the impact of these toxins upon the environ-
ment, and the health and safety of those living and working
close to their sources, cannot be collected until affordable,
accurate, and reliable sensors to quantify the presence of such
toxins have been developed. Such concerns have led to more
stringent legislation regarding the production of toxins;1,2
however, for such legislation to make a difference, it is necessary
to be able to accurately monitor the levels of such gases being
produced at a minimum cost. Therefore, there is increasing
motivation to develop more economical gas sensors capable of
monitoring low concentrations of toxic gases.
At present, the readily available gas sensors include those
that use semiconducting metal oxides such as tin, tungsten, or
chromium titanium oxides.3,4 These materials change conductiv-
ity when they interact with an oxidizing (e.g., NO2) or reducing
(e.g., H2) analyte. The conductivity change is measured, and
therefore, the presence of the oxidizing or reducing gas is
determined. These devices do not respond well to humid
conditions, and they require high operating temperatures,
∼150-600 °C. Other well-known sensor devices include liquid
electrolyte fuel cells and infrared sensors.5 Conducting polymers6
have achieved limited success as “electronic nose” devices, and
phthalocyanines have attracted attention as semiconductometric
sensors.7 However, the majority of these organic sensors suffer
from slow response times and incomplete recovery in the
absence of the analyte gas.
Experimental Section
Materials. The synthesis of 5,10,15,20-tetrakis[3,4-bis(2-
ethylhexyloxy)phenyl]-21H,23H-porphine has been described
elsewhere,10 and its chemical structure is shown in Figure 1a.
Upon metalation, the two hydrogen atoms in the center of the
porphyrin ring are replaced with the corresponding metal ion.
The metal derivatives of 5,10,15,20-tetrakis[3,4-bis(2-ethyl-
hexyloxy)phenyl]-21H,23H-porphine were prepared in ac-
cordance with literature methods.14 Zinc was inserted into the
Porphyrins possess distinctive UV-vis absorption spectra due
to their highly conjugated π-electron systems.8 If the analyte
* To whom correspondence should be addressed. Phone: +44 114 222
4543. Fax: +44 114 272 8079. E-mail: a.dunbar@shef.ac.uk.
† Department of Physics and Astronomy.
‡ Department of Chemistry.
10.1021/jp0626059 CCC: $33.50 © 2006 American Chemical Society
Published on Web 08/01/2006

Porphyrin LB Films as Gas Sensors
J. Phys. Chem. B, Vol. 110, No. 33, 2006 16647
Figure 2. Schematic diagram of the gas sensor testing apparatus.
was carried out. Deposition was performed by dipping the glass
plate vertically at a rate of 10 mm/min. There were 10 excursions
performed for each plate, resulting in the deposition of 20 layers
(one layer for each of the up and down strokes). If necessary,
so as to maintain the surface pressure at 40 mN/m the deposition
was halted while the trough barrier was retracted and an
additional 125 µL of the mixed solution was added to the
subphase before recompressing to 40 mN/m again and continu-
ing the deposition.
Figure 1. Chemical structure of (a) 5,10,15,20-tetrakis[3,4-bis(2-
ethylhexyloxy)phenyl]-21H,23H-porphine and (b) carboxylic acid
substituted calix[8]arene.
porphyrin by using excess zinc acetate in dichloromethane at
room temperature. Similarly, at room temperature and in
dichloromethane, the magnesium porphyrin was prepared using
magnesium bromide ethyl etherate and triethylamine. To insert
tin, the free base porphyrin was refluxed with tin(II) chloride
in pyridine and was then stirred with sodium hydroxide solution.
All three metalloporphyrins upon reaction completion were
purified using grade IV aluminum oxide column chromatogra-
phy. Synthesis of the carboxylic acid substituted calix[8]arene
is described elsewhere,15 and its structure is shown in Figure
1b. The analytes used are 1-octanol, hexylamine, octanal, acetic
acid, 1-hexanethiol, 2-butanone, ethyl acetate, trimethyl phos-
phite, octylamine, and triethylamine. They were all provided
by Sigma Aldrich and used as supplied.
A purpose built sensor testing chamber was used to expose
the solid-state sensor films to the analyte vapors, see Figure 2.
The delivery of clean dry nitrogen to a chamber containing the
analyte was controlled with a Tylan FC-260 mass flow
controller. The temperature of the analyte was maintained at 0
°C by submerging the analyte chamber in iced water, therefore
mixing the N2 gas with the analyte at a known vapor pressure.
This cooling of the gas also helped to reduce contamination
problems due to condensation of the analytes onto the walls of
the delivery pipes which were at room temperature and therefore
warmer then the analyte vapors. The resultant mixture was then
further mixed with a second supply of N2 controlled with a
second Tylan FC-260 mass flow controller and then fed into
the test chamber. The solid-state film to be tested was mounted
in the test chamber on a Peltier heating stage capable of
controlling the film temperature in the range 5-52 °C. The
temperature stage has a hole through the center such that there
is a clear optical path between the two optical fibers used to
deliver and collect the light for the optical measurements. The
fibers were connected to a World Precision Instruments Spec-
tromate spectrophotometer which incorporates a tungsten white
light source and a multichannel photodiode detector allowing
the absorption spectra of the film under test to be recorded in
situ every 3 s over the spectral range 350-850 nm. The exhaust
gas passes through an activated charcoal filter to absorb the
toxic vapor before being vented to the atmosphere. The presence
of analyte vapors in the test chamber was found to cause
increased scattering and therefore increase the optical absorbance
at all wavelengths. In these experiments, such effects were
accounted for by subtracting a reference absorbance (measured
at 800 nm where all the samples investigated are transparent)
from the recorded experimental data.
Experimental Methods. Solutions of the free base and
metalated EHO were prepared at concentrations of 4.0 × 10-6
,
3.2 × 10-6, 2.6 × 10-6, and 4.0 × 10-6 M of the free base,
Mg, Sn, and Zn EHO, respectively, using chloroform as the
solvent. These EHO solutions (2 mL portions) were placed in
quartz cuvettes, and initial absorption spectra of the solutions
were recorded using an Ocean Optics USB2000 spectrometer
with a Mikropack Mini D2 UV-vis-IR light source. The
absorbance spectra for each were calculated in the range 350-
850 nm with respect to a reference taken of chloroform. The
analyte to be investigated was then added 20 µL at a time, and
the mixture was then stirred. Analyte was added until no further
spectral changes were noticed, and then, the exposed spectrum
was recorded. This process was repeated for the four EHOs with
each of the 10 different analytes described in the Materials
subsection.
Absorbance spectra of each of the analytes on their own were
also recorded, with respect to an empty quartz cuvette reference,
to confirm the range of optical transparency of each analyte.
Langmuir-Blodgett films of each of the EHOs were prepared
from the chloroform solutions. For the initial experiments,
solutions of just the EHO were used. However, for later
experiments the EHO solutions were mixed with a solution of
calix[8]arene in an EHO/calix[8]arene 1/2 molar ratio. A 150
µL portion of the EHO or EHO/calixarene solution (∼0.3 mg/
mL) was spread on a clean water (Elga PURELab Option >15
MΩcm) subphase surface in a NIMA model 611 Langmuir
Blodgett trough. After evaporation of the solvent, the film was
compressed to and then held at a surface pressure of 40 mN/m
while deposition onto a glass plate (rendered hydrophobic by
exposure to 1,1,1,3,3,3-hexamethyldisilazane vapors for 12 h)
Results and Discussion
The initial absorption spectra for the four different EHO
solutions are shown in Figure 3. For the free base EHO, the
distinctive Soret absorption band at 426 nm is much more
intense than the four weak Q-bands at 519, 556, 592, and 650
nm. For the Mg EHO, the Soret band occurs at 430 nm with
only two weak Q-bands at 565 and 610 nm. There are only
two Q-bands at longer wavelengths for Mg EHO and the other

16648 J. Phys. Chem. B, Vol. 110, No. 33, 2006
Dunbar et al.
Figure 3. Absorbance spectra for (a) free base (solid black), (b) Mg
(dashed black), (c) Sn (solid white), and (c) Zn EHO (dashed white)
solutions in CHCl3 (the spectra from 475 to 850 nm have been
multiplied by 10).
Figure 5. Average of the square of the difference between the before
exposure spectrum and the after exposure spectrum for the free base,
Mg, Sn, and Zn EHOs and 10 different analytes.
nm for hexylamine) and also increase in intensity (by ∼24%
for hexylamine). Meanwhile, the two Q-bands shift to longer
wavelengths, and often more than two Q-bands appear. The
magnitude of the Soret band shift and the intensity change
appear to be dependent upon the amine used during the
exposure. This effect is the focus of further investigations.
For the free base EHO, which has a strong interaction with
acetic acid and trimethyl phosphite, the Soret peak is replaced
by a broader, lower intensity peak at longer wavelength (∼465
nm), and the four Q-bands initially present are replaced with a
single broader but more intense absorbance band at much longer
wavelength (∼700 nm), as is also the case for free base EHO
upon exposure to NO2.17
It was noted that, upon exposure of MgEHO to acetic acid,
there was a slow interaction during which the absorbance
spectrum changed from that which is characteristic of MgEHO
to that which is typical for the free base EHO before then
partially undergoing a further change similar to that observed
when free base EHO is exposed to acetic acid. It has been well
documented that Mg(II) porphyrins are demetalated by acids
to form the corresponding free base porphyrin.18 It is therefore
likely that exposure to acetic acid causes the conversion of
MgEHO back to the free base EHO. There are also weak
responses noted for MgEHO with hexylamine and octanal.
Magnesium(II) porphyrins have the tendency to axially bind a
fifth and sixth ligand sequentially. It is clear in the literature
that hard ligands (such as oxygen and nitrogen) are greatly
favored over soft and that the binding of nitrogenous bases is
stronger than that of oxygen containing ligands.19
Sn(IV) porphyrins are known to bind strongly to oxygen
containing ligands such as alcohols to form 6-coordinate
complexes.20 Although in our experiments no response is
observed for SnEHO exposed to any of the analytes, there may
still be some interaction occurring with the alcohol. The SnEHO
when synthesized already has two axial hydroxy ligands to
satisfy the +4 oxidation state of the Sn metal atom; exchanging
these hydroxy ligands for other groups may not affect the
absorption spectrum significantly, and hence, no appreciable
optical response would be observed.
Figure 4. Absorbance spectra for a solution of ZnEHO before (gray)
and after (black) exposure to hexylamine (the spectra from 475 to 750
nm have been multiplied by 10).
metalated EHOs because they have increased symmetry,
therefore making the energy levels involved in the Q-band
absorption degenerate and thus reducing the number of visible
absorption bands from four to two. The Sn EHO solution has
its Soret absorption at 440 nm with Q-bands at 564 and 608
nm, and finally, the Zn EHO solution has its Soret absorption
peak at 427 nm with Q-bands at 550 and 592 nm.
The absorption spectra of the different analytes were mea-
sured, and it was found that all the analytes are highly
transparent from 350 to >850 nm. Short wavelength (<350 nm)
absorption features which were observed in some of the
interaction experiments (data not shown) containing octanal,
butanone, and triethylamine are simply due to absorption by
unreacted analyte in the solution rather than due to any
interaction with the EHO.
The spectra for Zn EHO before and after exposure to
hexylamine are shown in Figure 4 as an example of a strongly
interacting EHO/analyte pair. To be useful as an optical sensor,
the absorption spectrum of the sensor material must change upon
exposure, as is the case in Figure 4. Zinc(II) porphyrins are
well-known to coordinate nitrogen ligands to form 5-coordinate
complexes16 with an associated change in the Soret absorption
spectrum because the metal is pulled slightly out of the plane
of the porphyrin in the complex that is formed. The trend for
ZnEHO exposure to amines is for the Soret absorbance band
to both shift slightly to longer wavelength (from 427 to 433
The interaction upon exposure of 2 mL of the four EHO
solutions to the different analytes is summarized in Figure 5.
The difference between the unexposed and exposed spectra was
calculated and then squared (such that all differences are

Porphyrin LB Films as Gas Sensors
J. Phys. Chem. B, Vol. 110, No. 33, 2006 16649
base EHO/calixarene films little evidence of the original EHO
absorption spectra features remains after NO2 exposure, indicat-
ing a much more complete switching process for the EHO/calix-
[8]arene mixture. This improvement in the film quality was most
marked for the free base EHO, and similar improvements in
the LB film quality were found for the Mg, Sn, and Zn EHO/
calix[8]arene LB films. Therefore, all four EHO materials used
in this study were mixed with calix[8]arene when being
transferred to solid-state films.
When the EHOs from solution were transferred to a solid-
state LB film, the wavelengths of the Soret absorption feature
are slightly red-shifted and bandwidths of all absorption features
are slightly increased, indicating that the local environments of
the EHO molecules have changed slightly, see Figure 6 in
comparison with Figure 3. It is evident that the Soret absorption
bands have broadened and shifted to the longer wavelengths.
The Soret peaks in the solid-state films are at 438, 436, 442,
and 435 nm for free base EHO, MgEHO, SnEHO, and ZnEHO,
respectively. These red shifts are thought to be partly due to
the effect of removing the solvent and partly due to aggregation
effects upon transfer to an LB film, despite having mixed the
porphyrins with calix[8]arene to minimize any aggregation
effects.
Figure 6. Absorbance spectra for (a) free base EHO/calix[8]arene
(solid black), (b) Mg EHO/calix[8]arene (dashed black), (c) Sn EHO/
calix[8]arene (solid white), and (d) Zn EHO/calix[8]arene (dashed
white) LB films on glass substrates (the spectra from 475 to 750 nm
have been multiplied by 10).
positive). The data plotted in Figure 5 is the average of the
squared difference in the range 350-750 nm. It is evident that
there is a strong interaction between the free base EHO with
acetic acid and trimethyl phosphite. ZnEHO also reacts strongly
with the amines hexylamine, octylamine, and to a lesser degree
triethylamine. Therefore, these interacting sensor/analyte pairs
were investigated further when transferred to solid-state LB
films.
Previous work17 has shown that upon transferring EHO from
a solution to an LB film the absorption bands broaden, implying
that there is a slight increase in the inhomogeneity of the local
environment of the EHO molecules in the LB films compared
to the solution. This is most likely due to the occurrence of J
(or edge-to-edge) aggregate formation in the EHO films.21-23
The presence of such J aggregates is thought to reduce the
sensitivity of the EHO as a sensor material. To reduce any
aggregate formation in the solid-state LB films used in this study
the EHOs were mixed with calix[8]arene which is optically inert
in the wavelength range of interest and makes high quality LB
films. This process is known to improve the quality of the films
formed for free base EHO, and it dramatically improves the
sensor dynamics of this material as an NO2 sensor.17 The solid-
state film of pure free base EHO does not switch completely
upon exposure to NO2, whereas in contrast for the mixed free
The results from the solution experiments summarized in
Figure 5 were used to guide the choice of sensor/analyte pairs
that were chosen for further investigation as solid-state films.
Sensor/analyte pairs that displayed wavelength red shifts greater
than 2 nm and absorbance intensity changes greater than (10%
were selected for further investigation. Therefore, the interac-
tions of the sensor films with the amine analytes (looking at
ZnEHO in particular), trimethyl phosphite (looking at free base
EHO in particular), and acetic acid were studied.
Figure 7a compares the spectra measured for the ZnEHO film
before and after exposure to hexylamine at 48 °C, and Figure
7b shows the absorbance at 435 nm (the Soret absorption peak
for ZnEHO/calix[8]arene films) for a ZnEHO/calix[8]arene LB
film as a function of time upon exposure to hexylamine. The
N2 flow is switched between direct supplying 500 sccm clean
N2 gas or 500 sccm N2 which has passed through the analyte
chamber (maintained at 0 °C) and therefore contains analyte
vapors. Exposure to the analyte for 300 s at 48 °C was followed
by 300 s recovery period also at 48 °C. The concentration of
hexylamine vapor during the exposure cycle is estimated by
considering the vapor pressure of hexylamine at 0 °C (all analyte
Figure 7. (a) Absorption spectra for a ZnEHO/calix[8]arene LB film before (gray) and after (black) exposure to hexylamine (estimated concentration
∼2000 ppm, the spectra from 475 to 750 nm have been multiplied by 10) and (b) the absorbance change at the ZnEHO Soret wavelength of 435
nm during three exposure (∼2000 ppm hexylamine for 300 s) and recovery (N2 for 300 s) cycles at 48 °C for the solid-state ZnEHO/calix[8]arene
film.

16650 J. Phys. Chem. B, Vol. 110, No. 33, 2006
Dunbar et al.
concentrations were estimated using data found in ref 24; no
data was available for octylamine so an extrapolation between
the values for pentylamine and nonylamine was used), the
estimated concentration for hexylamine at 0 °C is ∼2000 ppm.
The rise time for the absorption change observed at 435 nm
for a ZnEHO/calix[8]arene LB film upon exposure to hexy-
lamine as shown in Figure 7 is very fast. The time taken to
reach 50% of the complete change is ∼7 s, and the time taken
to reach 90% of complete change is ∼11 s. These values are
comparable for the 50% response rates for free base EHO upon
exposure to NO2 reported in ref 17, and considerably faster than
the 90% response rate. The ZnEHO/calix[8]arene sensor is seen
to recover when the hexylamine vapor flow is stopped. The
recovery takes ∼150 s at 48 °C. It is very promising to note
that after recovery the absorbance returns to approximately the
same value for every cycle indicating that this system is readily
reset for reuse as a sensor. It should be noted that when ZnEHO
is in solution exposure to hexylamine causes a shift in the Soret
absorption wavelength from 427 to 433 nm and an increase in
the absorption peak. In contrast, the ZnEHO/calix[8]arene LB
film experiments showed no shift in the Soret peak position;
however, there is still an increase in absorbance intensity at the
LB film Soret wavelength of 438 nm. Upon transfer of the
ZnEHO from solution to an LB film, the Soret absorption peak
is shifted from 427 to 438 nm.
Figure 8. Comparison of the absorbance change at 435 nm during
three exposure (analyte for 300 s) and recovery (N2 for 300 s) cycles
for the solid-state ZnEHO/calix[8]arene film upon exposure to the
triethylamine (black), hexylamine (gray), and octylamine (light gray)
at 48 °C.
As can be seen in Figure 4, the exposed solution absorption
at 433 nm is stronger by ∼24% compared to the unexposed
solution at 427 nm. This compares reasonably with the solid-
state film which undergoes ∼8% increase (at 438 nm without
any wavelength shift) upon exposure to hexylamine at 48 °C,
the difference in intensity of the increase presumably being due
to the larger quantity of analyte used in the solution exposure
compared to the solid-state exposure and the fact that the
absorbance peaks are somewhat broadened in the solid-state
films. Temperature dependence of the response magnitude, as
discussed later, also implies that the accessibility of the active
sites of the ZnEHO molecules in the LB films plays an important
role in determining the magnitude of the sensor response. The
solid film response is also similar to the solution response in
that the Q-band absorption peaks are slightly red shifted upon
exposure and the intensity of the longer wavelength band of
the two Q-bands increases significantly and a third band at even
longer wavelength appears upon exposure.
Figure 9. ZnEHO/calix[8]arene LB film response to hexylamine
(∼2000 ppm) at 48 (black), 40 (dark gray), 31 (gray), and 20 (light
gray) °C.
influence of these parameters on the ZnEHO sensor response
to amines is subject to further investigations
As was expected from the solution results, there were no
significant absorption changes when the free base EHO/calix-
[8]arene LB film or the SnEHO/calix[8]arene LB film was
exposed to any of the amine analytes. There was some evidence
of a very weak response for MgEHO/calix[8]arene LB film
exposed to hexylamine; however, it was comparable to the noise
levels in measurement. The responses measured for these films
displayed changes of less than 3% of the Soret intensity, which
are within the margins of experimental error.
As the temperature of the ZnEHO/calix[8]arene LB film was
increased from 20 up to 48 °C, the response to amine was found
to increase. Figure 9 shows the variation of the ZnEHO/calix-
[8]arene LB sensor response as a function of temperature upon
exposure to hexylamine (estimated concentration ∼2000 ppm).
Similar trends were observed for the ZnEHO upon exposure to
octylamine and triethylamine. It is thought that the increasing
temperature improves the sensor response by improving the
accessibility of the active site of the ZnEHO, either by improving
the diffusion of the analyte species through the film or by
removing other gaseous species that may occupy the sites
without causing a spectral change.
In Figure 7b, the first exposure/recovery cycle has not been
shown because during this cycle the absorbance level undergoes
an initial increase, which is thought to be due to both stabilizing
of the temperature and the removal of contaminants, such as
water or oxygen, which evaporate off the film during the first
cycle. In the latter cycles, i.e., those shown in Figure 7b, the
absorbance returns to the same level for each of the exposed
and recovered halves of the cycle.
Similar results were obtained for the interaction of ZnEHO/
calix[8]arene LB film with octylamine and triethylamine. The
Soret absorbance at 438 nm increased by ∼7% for the
octylamine, and it increased by ∼10% for the triethylamine
compared to ∼8% for hexylamine as shown in Figure 8. The
different response intensities observed for the different amines
are due to differences in the vapor pressure of the different
amines at 0 °C (resulting in different concentrations of each
amine), diffusion rates for the amines into the sensor film, and
the accessibility of the nitrogen atom in the amine to form
ligands with the ZnEHO. The concentrations for triethylamine,
hexylamine, and octylamine estimated from their vapor pressure
data are ∼14000, ∼2000, and ∼200 ppm, respectively. The
There was no significant response measured for any of the
four EHO/calix[8]arene LB films upon exposure to either acetic
acid or trimethyl phosphite. In all cases, the exposure to acetic

Porphyrin LB Films as Gas Sensors
J. Phys. Chem. B, Vol. 110, No. 33, 2006 16651
(2) http://www.environment-agency.gov.uk/subjects/airquality/
?lang)_e, May 2006.
(3) Mandelis; Christofides, C. Physics, Chemistry, and Technology of
Solid State Gas Sensor DeVices; Wiley-Interscience Publications: New
York, 1993.
(4) Albert, K. J.; Lewis, N. S.; Schauer, C. L.; Sotzing, G. A.; Stitzel,
S. E.; Vaid, T. P.; Walt, D. R. Chem. ReV. 2000, 100, 2595.
(5) Moseley, P. T.; Norris, J. O. W.; Williams, D. E. Techniques and
Mechanisms in Gas Sensing; Adam Hilger: Bristol, U.K., 1991.
(6) Lonergan, M. C.; Severin, E. J.; Doleman, B. J.; Beaber, S. A.;
Grubb, R. H.; Lewis, N. S. Chem. Mater. 1996, 8 (9), 2298.
(7) Capone, S.; Mongelli, S.; Rella, R.; Siciliano, P.; Valli, L. Langmuir
1999, 15, 5 (5), 1748.
acid or trimethyl phosphite resulted in changes of less than 3%
of the Soret intensity (the estimated experimental margin of
error). This was surprising since the solution experiments
showed a very strong response for acetic acid interacting with
the free base EHO and a weaker interaction for the acetic acid
and MgEHO. It would appear that the reaction can only occur
when the acid can freely dissociate in solution, whereas in
gaseous form the acetic acid cannot dissociate and therefore
cannot protonate the free base EHO. Similarly, the strong
interaction between trimethyl phosphite and free base EHO only
occurs in the solution experiments; the exact nature of this
interaction is uncertain.
(8) George, C. D.; Richardson, T.; Hofton, M. E.; Vale, C. M.; Neves,
M. G. M.; Cavaleiro, J. A. S. Mater. Sci. Eng., C 1999, 8-9, 559.
Conclusions
(9) Richardson, T.; Smith, V. C.; Johnstone, R. A. W.; Sobral A. J. F.
N.; Rocha-Gonsalves, A. M. d’A. Thin Solid Films 1998, 327-329, 3151.
In solution, there are strong responses upon exposure for
ZnEHO and all three amines investigated. Both trimethyl
phosphite and acetic acid produced strong interactions with the
free base EHO, and MgEHO and acetic acid produced a weak
interaction which is thought to be due to the conversion of the
MgEHO back into free base EHO by action of the acid followed
buy a free base EHO interaction with acetic acid.
(10) Dooling, C. M.; Worsfold, O.; Richardson, T. H.; Tregonning, R.;
Vysotsky, M. O.; Hunter, C. A.; Kato, K.; Shinbo K.; Kaneko, F. J. Mater.
Chem. 2001, 11, 392.
(11) Worsfold, O.; Dooling, C. M.; Richardson, T. H.; Vysotsky, M.
O.; Tregonning, R.; Hunter, C. A.; Malins, C. J. Mater. Chem. 2001, 11,
399.
(12) Pedrosa, J. M.; Dooling, C. M.; Richardson, T. H.; Hyde, R. K.;
Hunter, C. A.; Mart´ın, M. T.; Camacho, L. J. Mater. Chem. 2002, 12, 2659.
When transferred to solid-state LB films, aided by mixing
with calix[8]arene, only some of the interactions observed in
the solution experiments continue to occur. There is no optical
change observed upon exposure of the free base EHO/calix[8]-
arene LB films to vapors of either acetic acid or trimethyl
phosphite, and similarly there is no response when MgEHO/
calix[8]arene LB films are exposed to acetic acid. The ZnEHO/
calix[8]arene LB films, however, do continue to respond to the
three amines investigated. Therefore, ZnEHO/calix[8]arene LB
films have been demonstrated to be effective solid-state amine
sensors. The response for the ZnEHO/calix[8]arene LB films
to amines increases with increasing temperature in the range
20-48 °C and also depends on the analyte concentration and
the amine structure.
(13) Suslick, K. S.; Rakow, N. A.; Sen, A. Tetrahedron 2004, 60, 11133.
(14) Sanders, J. K. M.; Bampos, N.; Clyde-Watson, Z.; Darling, S. L.;
Hawley, J. C.; Kim, H.-J.; Mak, C. C.; Webb, S. J. The axial coordination
chemistry of metalloporphyrins. The Porphyrin Handbook; Academic
Press: New York, 1999; Vol. 3, Chapter 15.
(15) Gutsche, C. D. Calixarenes, Monographs in Supramolecular
Chemistry; Royal Society of Chemistry: London, 1989.
(16) Taylor, P. N.; Anderson, H. L. J. Am. Chem. Soc. 1999, 121,
11538-11545.
(17) Dunbar, A.; Richardson, T. H.; McNaughton, A. J.; Barford, W.;
Hutchinson, J.; Hunter, C. A. Colloids Surf., A 2006, 284-285, 339-344.
(18) Dolphin, D. The Porphyrins: Structure and Synthesis, Part A, Vol.
1; Academic Press: London, 1978.
(19) Kadish, K. M.; Shuie, L. R. Inorg. Chem. 1982, 21, 1112.
(20) Holloway, C. E.; Melnik, M. Main Group Met. Chem. 1998, 21,
371.
(21) Pedrosa, J. M.; Dooling, C. M.; Richardson, T. H.; Hyde, R. K.;
Hunter, C. A.; Mart´ın, M. T.; Camacho, L. Mater. Sci. Eng. 2002, C22,
433.
(22) Chou, H.; Chen, C.-T.; Stork, K. F.; Bohn, P. W.; Suslick, K. J.
Phys. Chem. 1994, 98, 383.
Acknowledgment. Financial support of this work by the
EPSRC (UK) Grant GR/596845/01 is gratefully acknowledged,
as is the advice of Dr. W. Barford.
(23) Kroon, J. M.; Sudho¨lter, E. J. R. Langmuir 1995, 11, 214.
(24) CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC
References and Notes
(1) http://europa.eu.int/comm/environment/air_en.htm, May 2006.
Press: Boca Raton, FL, 2005.