Mechanism of the optical response of mesoporous silica impregnated with Reichardt's dye to NH3 and other gases

Article abstract of DOI:10.1021/jp037094u

Neutral amines-templated mesoporous silica systems impregnated with Reichardt's dye are pink in color if previously dehydrated and white if still covered with hydroxyls: the pink coloration is, however, reversibly restored by exposing the latter sample only to NH₃ and amines. The mechanism of color change is discussed, and such systems are proposed as possible ammonia gas sensors.

Full text of DOI:10.1021/jp037094u

J. Phys. Chem. B 2004, 108, 16617-16620
16617
ARTICLES
Mechanism of the Optical Response of Mesoporous Silica Impregnated with Reichardt’s
Dye to NH3 and Other Gases
B. Onida,†,§ S. Fiorilli, L. Borello, G. Viscardi, D. Macquarrie, and E. Garrone*
†
†
‡
¶
,†
Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi
2
4, 10129 Torino, Italy, Dipartimento di Chimica Generale ed Organica Applicata, UniVersit a` di Torino,
Via P. Giuria 7, Torino, Italy, Centre of Clean Technology, Department of Chemistry, UniVersity of York,
Heslington, York, YO10 5DD, England, and INSTM, Unit a` di Ricerca Torino Politecnico
ReceiVed: October 13, 2003; In Final Form: August 9, 2004
Neutral amines-templated mesoporous silica systems impregnated with Reichardt’s dye are pink in color if
previously dehydrated and white if still covered with hydroxyls: the pink coloration is, however, reversibly
3
restored by exposing the latter sample only to NH and amines. The mechanism of color change is discussed,
and such systems are proposed as possible ammonia gas sensors.
Introduction
SCHEME 1: Structure of the Ground and First Excited
State of Reichardt’s Dye
Ordered mesoporous silicas, because of highly uniform
porosity, mechanical stiffness, and thermal stability, are of
interest for several applications, such as separation, sorption,
1
and catalysis. Fairly recently, it is has been recognized that
the porous nature makes them excellent hosts for sensing
molecules, as the analyte can easily diffuse to the active center.
As silica is transparent in the UV-visible range, these materials
are ideal for sensors with an optical response.2
,3
Reichardt’s betaine dye [2,6-diphenyl-4-(2,4,6-triphenyl-N-
pyridinio)-phenolate, represented in Scheme 1 and referred to
4
hereafter as RD] is an excellent solvatochromic compound,
5
,6
which has been already immobilized in polymeric substrates,
7
6
silica, and glasses, to obtain optochemical sensors for various
vapors, including humidity. RD has a charge-transfer transition
in the visible region, which is very medium dependent, because
polar solvents stabilize the molecular electronic ground state
more than the excited state, therefore increasing the energy of
transition.
Experimental Section
The ordered mesoporous silica was prepared using neutral
amines according to the recipe reported by Pinnavaia et al.,14
and it is denoted hereafter as HMS. At room temperature, 2.33
g n-dodecylamine (Aldrich) was added to a solution made of
Several authors have used RD to study the polarity and the
hydrogen-bond donor properties of different surfaces, among
2
5 mL distilled water (Carlo Erba) and 25 mL absolute ethanol
(Aldrich), under stirring. Tetraethyl orthosilicate (9.14 g,
8-13
which are that of silica and MCM-41.
The present paper
Aldrich) was then added. The solution was stirred for 18 h and
yielded a thick white suspension. This was filtered and dried at
deals with the color changes of a RD-containing ordered
mesoporous silica, caused by the contact with gaseous ammonia
or amine vapors, which may be interesting for gas sensing
applications.
The aspects related to hydration/dehydration of the solid and
the presence of polar molecules in the surrounding atmosphere
have also been examined because they interfere with the
phenomena concerning ammonia.
8
0 °C for 1 h. The amine was removed by heating the solid
(ca. 0.5 g) under reflux in absolute ethanol (100 mL) for 6 h:
the treatment was repeated two times. Thermal analysis (DTA-
DTG Setaram92) and FT-IR characterization (Bruker FTIR
Equinox 55 spectrometer, equipped with a MCT cryodetector)
showed that the template was completely removed. The XRD
pattern (Philips X’pert, Cu KR radiation) showed the ordered
structure of silica (d100 ) 3.4 nm), and BET and BJH analysis
of N2 adsorption (Micromeritics ASAP2010) confirmed the high
*
Corresponding author. Fax: (+39) 011-5644699. E-mail:
edoardo.garrone@polito.it.
†
2
Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico
surface area (ca. 1000 m /g) and the mesoporosity (pore diameter
di Torino.
)
2.5 nm).
‡
Universit a` di Torino.
University of York.
INSTM, Unit a` di Ricerca Torino Politecnico.
¶
The sample was dehydrated, either mildly at 300 °C or more
§
severely at 600 °C: corresponding specimens are labeled HMS-
1
0.1021/jp037094u CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/06/2004

16618 J. Phys. Chem. B, Vol. 108, No. 43, 2004
Onida et al.
Figure 1. DR-UV-visible spectra of HMS (curve 1), R-HMS-600
curve 2), and R-HMS-300 (curve 3).
(
Figure 2. DR-UV-visible spectra related to HMS-300 after prolonged
outgassing at RT (curve 1) and readmission of air (curve 2), ethanol
(curve 3), and acetonitrile (curve 4).
300 and HMS-600, respectively. Both samples were then
contacted with a dichloromethane solution of RD, which was
green in color. The solution became colorless fairly rapidly,
indicating a thorough adsorption of the dye on the mesoporous
silica. After overnight stirring, samples were filtered, washed,
and dried. These samples are referred to in the following as
R-HMS-300 or R-HSM-600.
SCHEME 2: Structure of RD in R-HMS-300 (a) and
R-HMS-600 (b)
Diffuse reflectance UV-visible spectra were recorded on a
Varian Cary 500 instrument. For FT-IR measurements, the
powder was pressed into thin, self-supporting wafers; spectra
-
1
-1
were collected, at a resolution of 2 cm , in the 4000-500 cm
range (128 scan). Adsorption of ammonia (Messer) was carried
out using proper IR and UV-visible cells connected to a
-
4
vacuum frame (residual pressure <10 mbar).
Results and Discussion
Curve 1 in Figure 1 is the visible spectrum of both HMS
and HMS-600, which, not exhibiting, as expected, absorption,
represent the blank for the RD-containing HMS.
estimated to be about 715 and that of HRD 8.6. The closeness
of these values suggests that, although under different circum-
stances, competition for the proton may occur between the RD
+
16
The sample R-HMS-600 has a marked pink coloration: the
related visible spectrum taken in air, reported in the same figure
as curve 2, shows a peak at 490 nm. It compares well with the
values reported for RD adsorbed on silica, which range from
-
and the SiO group. If credit is given to the actual values, proton
transfer to RD is fully accounted for. Protonation of RD by
surface silanols has been already proposed in studies of surface
and the role of chain terminals as most
acidic silanols has been envisaged for protonation of Michler’s
8
-13
477 to 516 nm.
Such values are accounted for by a
perturbation of the RD molecule by the underlying silica through
van der Waals-like interactions and a probable H bond between
8
-12
polarity of silica,
-
an isolated silanol and the O atom in RD.
2
1,22
Curve 3 is the spectrum of R-HMS-300: the sample has no
absorption in the visible region (a rise in the background
suggests absorption in the nearby UV range); accordingly, the
sample is white. The adsorbed RD molecule is in a colorless
form, currently interpreted as arising from protonation to a
ketone adsorbed on silica particles.
The state of the RD molecule interacting with the hexagonal
mesoporous silica wall in R-HMS-300 and R-HMS-600 is
depicted in Scheme 2. Broken lines represent H bonding and
van der Waals interactions.
Prolonged pumping of the sample R-HSM-300 in a vacuum
at room temperature, which only brings about the removal of
adsorbed molecular water, caused coloration of the sample,
described by spectrum 1 in Figure 2 (λmax ) 526 nm). This
effect is depicted in Scheme 3. Atmospheric water, probably
adsorbed and therefore acting as a sort of solvent, stabilizes
+
phenolic species H-RD , not showing the CT band in the
visible typical of RD.8
According to the literature, silica samples outgassed at 300
°
C have a silanol content sizably higher than those outgassed
at 600 °C. We have confirmed by IR spectra (not reported) that
this is also the case with R-HSM-300 and R-HMS-600. It is
inferred that a higher hydration of the silica surface inhibits the
color. Isolated silanols are not able to transfer their proton:
instead, closely packed patches of silanols may provide protons
more acidic than those of isolated silanol.19,20 The pKa value of
silanols in hydrated silica layer in contact with liquid water is
+
the H-RD species, playing a role in the protonation of RD
close to that observed for proton-transfer reactions in solutions.4
Figure 2 shows that readmission of water (even in the form of
atmospheric moisture) led to sample bleaching. The adsorption
of other polar solvents (acetone, methanol, and acetonitrile) on

Optical Response of Mesoporous Silica
J. Phys. Chem. B, Vol. 108, No. 43, 2004 16619
SCHEME 3: Effect of the Removal of Water or Polar
Molecules from R-HMS-300a
a
Shadowing represents the presence of atmospheric water (or other
Figure 3. DR-UV-visible spectra related to the exposure of R-HMS-
polar solvents).
300 to NH (broken curve: before exposure to NH ; the arrow indicates
3
3
-
1
NH
3
pressure increasing from 1.0 × 10 to 10 mbar).
SCHEME 4: Effect of Interaction of Ammonia with
R-HMS-300a
a
Shadowing represents the presence of atmospheric water (or other
polar solvents).
the same sample also caused the disappearance of color, whereas
nonpolar solvents (e. g., tetrachloromethane and benzene) did not.
The effect of polar molecules on the state of RD adsorbed
on poorly dehydrated silica has not been reported so far. Since
it is not straightforward to describe the role (solvating?) of polar
molecules on the RD-HMS-300 surface, their presence is
represented in Schemes 2-4 by a mere shadowing.
Figure 4. Difference FTIR spectra related to the adsorption of NH
on R-HMS-300 (curve 1) and HMS-300 (curve 2).
3
at higher pressures, which probably monitors a change in the
environment along the experiment.
Note that the different environment of Reichardt’s dye
By exposing the R-HSM-300 sample to ammonia vapors, the
color turns immediately to a strong pink. The same effect was
obtained with primary amines in the vapor phase, whereas many
other polar substances (alcohols, thiols, and phosphines) were
ineffective. This means that ammonia does not act by virtue of
its polar nature, which would instead favor even more the
protonated form of RD, but because of its basic nature. As a
relatively strong base, it is able to strip a proton from the
(presence/absence of ammonia and/or water) is reflected in the
different wavelength of the absorption maximum in the visible
spectrum, as a consequence of different polarity and H bonding
1
6
interactions.
A behavior with gaseous ammonia similar to that reported
has been observed for Reichardt’s-dye-polymer composites
treated with hydrochloric acid: the acid protonates the dye to
1
7
a white form, as silanols do with R-HMS-300, and ammonia
+
H-RD species, bringing back the charge-transfer transition
brings back the color.
Direct evidence that on R-HMS-300 the reaction occurring
is
in the visible spectrum. The interaction with ammonia is
reversible, and the color readily fades away if the sample is not
in contact with ammonia vapors. Figure 3 illustrates the exposure
of the R-HMS-300 sample to ammonia under controlled
circumstances. After insertion into a UV-visible cell, the sample
is briefly pumped at ambient temperature and exposed to NH3
at increasing pressures. A band at 510 nm appears, the intensity
of which grows steadily with pressure. A slight red shift is seen
+
) RD + NH4+
HRD + NH
3(g)
comes from the infrared spectrum of the sample (Figure 4); in
-
1
the presence of ammonia, a band is seen at 1465 cm , typical

16620 J. Phys. Chem. B, Vol. 108, No. 43, 2004
Onida et al.
of the NH4+ species (curve 1).18 The band at 1635 cm-1 is due
to the asymmetric bending mode of adsorbed molecular am-
monia. Both bands disappear when ammonia is removed. As
well known in the surface chemistry of silica, the same
experiment conducted on a HMS sample without RD does not
lead to any ammonium ions. Indeed, spectrum 2, referring to
HSM-300, only shows the bending mode of molecular ammonia,
so confirming that the formation of NH4 species is due to the
presence of the dye.
A possible structure for the RD-colored species under these
circumstances is given in Scheme 4, in which the ammonium
species is assumed to interact with both the silanolate of the
silica walls and the phenolic oxygen of the dye.
(2) Scott, B. J.; Wirnsberger, G.; Stucky, G. D. Chem. Mater. 2001,
3, 3140.
1
(3) Schulz-Ekloff, G.; W o¨ hrle, D.; van Duffel, B.; Schoonheydt, R.
A. Microp. Mesop. Mater. 2002, 51, 91.
(4) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry;
VCH: Weinheim, 1988.
(
5) Blum, P.; Mohr, G. J.; Matern, K.; Reichert, J.; Spichiger-Keller,
U. E. Anal. Chim. Acta 2001, 432, 269.
6) Dickert, F. L.; Geiger, U.; Lieberzeit, P.; Reutner, U. Sens.
Actuators, B 2000, 70, 263.
(
+
(7) Crowther, D.; Liu, X. J. Chem. Soc., Chem. Commun. 1995, 2445.
(8) Chronister, C. W.; Drago, R. S. J. Am. Chem. Soc. 1993, 115, 4793.
(9) Macquarrie, D. J.; Tavener, S. T.; Gray, G. W.; Heath, P. A.; Rafelt,
J. S.; Saulzet, S. I.; Hardy, J. J. E.; Clark, J. H.; Sutra, P.; Brunel, D.; Di
Renzo, F.; Fajula, F. New J. Chem. 1999, 23, 725.
10) Spange, S.; Zimmermann, Y.; Graeser, A. Chem. Mater. 1999,
1, 3245.
11) Spange, S.; Vilsmeier, E. Colloid Polym Sci. 1999, 277, 687.
12) Spange, S.; Vilsmeier, E.; Zimmermann, Y. J. Phys. Chem. B 2000,
04, 6417.
13) Voigt, I.; Simon, F.; Esteland, K.; Spange, S. Langmuir 2001, 17,
3080.
(
1
1
(
(
Conclusions
In the presence of polar molecules acting as solvents, such
as atmospheric water, methanol, acetonitrile, or acetone, RD
on a highly hydrated HMS surface strips a proton from a SiOH
in a patch of silanols, so becoming colorless. A base stronger
than RD (ammonia or amines) captures the proton, giving rise
to the appearance of color. The process is fully reversible. This
feature, together with the rapidity of the optical response and
its dependence on NH3 pressure (Figure 3), makes this material
promising for optical sensing applications.
(
(
(
(
(
14) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 269, 865.
15) Schindler, P.; Kamber, H. R. HelV. Chim. Acta 1968, 51, 1781.
16) Reichardt, C. Chem. ReV. 1994, 94, 2319.
17) Sadaoka, Y.; Sakai, Y.; Murata, Y. U. Talanta 1992, 39, 1675.
(18) Nakamoto, K. Infrared Spectra of Inorganic and Coordination
Compounds; Wiley: New York, London, 1963.
19) Fubini, B.; Bolis, V.; Cavenago, A.; Garrone, E.; Ugliengo, P.
Langmuir 1993, 9, 2712.
20) Mori, T.; Kuroda, Y.; Yoshikawa, Y.; Nagao, M.; Kittaka, S.
Langmuir 2002, 18, 1595.
21) Zimmermann, Y.; El-Sayed, M.; Prause, S.; Spange, S. Monatsh.
(
(
(
References and Notes
Chem. 2001, 132, 1347.
(
1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck,
(22) Zimmermann, Y.; Anders, S.; Hofmann, K.; Spange, S. Langmuir
2002, 18, 9578.
J. S. Nature (London) 1992, 359, 710.