Oxygen sensing materials based on mesoporous silica MCM-41 and Pt(II)-porphyrin complexes

Article abstract of DOI:10.1039/b503336e

The preparation and properties of luminescent oxygen sensing materials based on two Pt(II)-porphyrin complexes: platinum meso-tetrakis(4-N-methylpyridyl)porphyrin (PtTMPyP⁴⁺) and platinum meso-tetrakis(4-N-pyridyl)porphyrin (PtTPyP) assembled in mesoporous silica (MCM-41) are described. The luminescence of Pt(II)-porphyrin/MCM-41 assembly materials can be extremely quenched by molecular oxygen with good sensitivity (I₀/I₁₀₀ > 7) and rapid response times (<1 s) suggest that the Pt(II)-porphyrin/MCM-41 system can be used for developing oxygen sensors. PtTMPyP⁴⁺/MCM-41 (20 mg g^-1) exhibits very high sensitivity (I₀/I₀₀ > 50). Even when the concentration of oxygen is 1%, the luminescence intensity of PtTMPyP ⁴⁺/MCM-41 (20 mg g^-1) can be quenched by 83.33%. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b503336e

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Oxygen sensing materials based on mesoporous silica MCM-41 and
Pt(II)–porphyrin complexes{
Huidong Zhang, Yinghui Sun, Kaiqi Ye, Ping Zhang* and Yue Wang*
Received 8th March 2005, Accepted 10th June 2005
First published as an Advance Article on the web 29th June 2005
DOI: 10.1039/b503336e
The preparation and properties of luminescent oxygen sensing materials based on two
Pt(II)–porphyrin complexes: platinum meso-tetrakis(4-N-methylpyridyl)porphyrin (PtTMPyP4+
)
and platinum meso-tetrakis(4-N-pyridyl)porphyrin (PtTPyP) assembled in mesoporous silica
(MCM-41) are described. The luminescence of Pt(II)–porphyrin/MCM-41 assembly materials can
be extremely quenched by molecular oxygen with good sensitivity (I0/I100 . 7) and rapid response
times (,1 s) suggest that the Pt(II)–porphyrin/MCM-41 system can be used for developing oxygen
sensors. PtTMPyP4+/MCM-41 (20 mg g21) exhibits very high sensitivity (I0/I100 . 50). Even
when the concentration of oxygen is 1%, the luminescence intensity of PtTMPyP4+/MCM-41
(20 mg g21) can be quenched by 83.33%.
the mesoporous silica. As the luminescence of molecules
Introduction
incorporated in a mesoporous material could be quenched
by molecules and gases surrounding the mesoporous
material, it is possible to make mesoporous materials that
have optical properties which are sensitive to the presence
of target molecules, i.e. to make mesoporous chemical
sensors.21–23 It is worth noting that the successful synthesis
of transparent and hard mesoporous spheres opens an efficient
avenue to the fabrication of new materials with useful optical
properties.24–26
Over the past decades, luminescence-based optical oxygen
sensors have been greatly developed because the determination
of molecular oxygen in both the gas and liquid phase is
important in many different fields such as analytical chemistry,
medical chemistry and environmental and industrial applica-
tions.1–4 These sensors are based upon the principle that
oxygen is a powerful quencher of the luminescent intensity and
lifetime of luminescent complexes, and the key factors that
play a role include the optical properties of the luminescent
complex and the solubility and diffusion coefficient of oxygen
in the matrix. The most commonly used complexes for
this application are transition metal complexes especially
ruthenium(II) polypyridyl or phenanthroline complexes5–7
and metalloporphyrins8–10 owing to their high quantum yields,
large Stokes’ shifts and long luminescent lifetimes. The host
materials used to encapsulate the luminescent complexes are
sol–gel and polymer films. Some interesting systems based on
sol–gel or polymer immobilized transition metal complexes
have been reported.6,10–13
Recent work carried out by our group27,28 and others29
has demonstrated that [Ru(bpy)3]2+ can be incorporated
into mesoporous silica. In this report, we present the
preparation and properties of two oxygen sensing materials
based on two metalloporphyrins, platinum meso-tetrakis(4-
N-methylpyridyl)porphyrin (PtTMPyP4+
)
and platinum
meso-tetrakis(4-N-pyridyl)porphyrin (PtTPyP) assembled in
mesoporous silica (MCM-41).
Experimental
Mesoporous materials have attracted considerable attention
recently for their synthesis and functionalization.14–20 The
highly controllable and monodisperse nature of the large
accessible pore size, high surface area and periodic nano-scale
pore spacing make mesoporous materials attractive media for
applications such as heterogeneous catalytic, environmental,
sensory or electronic media. Mesoporous materials are able to
physically encapsulate and immobilize the functional mole-
cules in the pores, while at the same time the existence
of channels in mesoporous silica allows the transportation of
solvent and other small molecules or ions into the interior of
The free base meso-tetrakis(4-N-pyridyl)porphyrin (H2TPyP)
was prepared using a variation of a method in the literature.30
H2TPyP (50 mg) was refluxed with 50 mg PtCl2 in 700 ml of
anhydrous degassed benzonitrile solution in an inert atmo-
sphere for more than 30 h until no free base was left as
revealed by TLC. The mixture was cooled to room tempera-
ture and the solvent was removed by vacuum. The crude
product was purified by column chromatography to obtain
43 mg pure PtTPyP. PtTMPyP4+ was synthesized according
to the literature procedures.31 The synthesis scheme of Pt(II)–
porphyrins applied in the study is given in the electronic
supplementary information (ESI).{
Mesoporous silica (MCM-41) was prepared from cetyl-
trimethylammonium bromide (CTAB), tetraethyl orthosilicate
(TEOS), and NaOH according to the literature.32 Then the
template was removed from the mesoporous silicate by
calcination at 560 uC for 8 h.
Key Laboratory for Supramolecular Structure and Materials of Ministry
of Education, College of Chemistry, Jilin University, Changchun,
130012, P. R. China. E-mail: yuewang@jlu.edu.cn;
Fax: +86-431-5193421
{ Electronic supplementary information (ESI) available: The synthesis
scheme of Pt(II)–porphyrins. See http://dx.doi.org/10.1039/b503336e
This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 3181–3186 | 3181

View Article Online
The Pt(II)–porphyrin/MCM-41 assembly materials were
prepared by the following procedure. MCM-41 was
added into the aqueous solution of PtTMPyP4+ or chloroform
solution of PtTPyP. After stirring, filtration, washing with the
corresponding solution, and drying in air, PtTMPyP4+/MCM-
41 or PtTPyP/MCM-41 was obtained. For example, 1 mg
PtTMPyP4+ was dissolved in 8 ml water to form a red
transparent aqueous solution, then 50 mg MCM-41 was added
and the mixture was stirred for 1 h. The mixture was filtered
to give
a purplish powder. The purplish powder was
washed repeatedly with water until the filtered water was
colorless, and after drying in air 20 mg PtTMPyP4+/g MCM-
41 was obtained. Samples with different loading levels of
PtTMPyP4+ (10, 20 and 40 mg PtTMPyP4+/g MCM-41) or
PtTPyP (20, 40 and 80 mg PtTPyP/g MCM-41) were prepared
by changing the concentration of the starting solution of
Pt(II)–porphyrin. UV-vis spectrophotometric analysis of
the filtrate showed the quantity of remaining PtTPyP or
PtTMPyP4+, which was unincorporated into MCM-41.27,28
The Pt(II)–porphyrin contents of PtTPyP/MCM-41 and
PtTMPyP4+/MCM-41 samples are listed in Table 1.
Fig. 1 Powder X-ray spectra of MCM-41 (a) and PtTMPyP4+
MCM-41 (b). Insert: the molecular structure of PtTMPyP4+
/
.
MCM-41. This indicates that after the incorporation of
Pt(II)–porphyrin, the hexagonal arrangement of channels in
MCM-41 remains unchanged. The UV-vis absorption spec-
truma of PtTMPyP4+/MCM-41 shows a similar profile to that
of PtTMPyP4+ in water (Fig. 2). The peak at 404 nm is due
to the Soret band of PtTMPyP4+, and peaks due to the Q
bands are at 512 nm and 546 nm. The above experiments
indicate that after the Pt(II)–porphyrin is incorporated into
MCM-41, the structure of MCM-41 is not destroyed and
Pt(II)–porphyrin/MCM-41 exhibits the optical properties of
the Pt(II)–porphyrin.
Each Pt(II)–porphyrin/MCM-41 sample was fixed inside a
laboratory-made cell that was equipped with two quartz
windows and an air-tight stopper which had inlet and outlet
lines to allow gas flow. To investigate the sensing properties of
Pt(II)–porphyrins to different oxygen concentrations, dry
nitrogen and dry oxygen were mixed to obtain gas mixtures.
All luminescence measurements were performed in the dark.
The powder X-ray diffraction (XRD) patterns were recorded
on a Siemens D5005 diffractometer with Cu-Ka radiation, step
size 0.02u and step count 1 s. UV-vis absorption spectra were
obtained using a Variancary 500 UV-vis spectrophotometer.
The luminescence spectra and response curves were obtained
using a PTI time-resolved emission spectrophotometer.
The emission spectra recorded for PtTPyP/MCM-41 and
PtTMPyP4+/MCM-41 with different loading levels are pre-
sented in Figs. 3 and 4, respectively. For PtTPyP/MCM-41, the
emission spectra exhibit a red shift from 640 nm to 645 nm and
peak broadening with increasing porphyrin loading level. A
similar tendency was observed for the PtTMPyP4+/MCM-41
system. The molecular arrangement and aggregated forms
should vary with the increase of loading level of PtTPyP or
Results and discussion
In order to examine whether Pt(II)–porphyrin molecules were
incorporated inside the channels of MCM-41 or only absorbed
by the interior surface of MCM-41, uncalcined MCM-41 was
put into the Pt(II)–porphyrin solution to give
a non-
luminescent product. However, calcined MCM-41, which has
unoccupied channels, in Pt(II)–porphyrin solution resulted in
the formation of a red luminescent solid under excitation by
UV light (lexc = 404 nm). The luminescence of the solid
remained unchanged even after the solid was washed
repeatedly with solvent. It is demonstrated that the Pt(II)–
porphyrin molecules are incorporated inside the channels of
MCM-41. The powder X-ray diffraction (XRD) spectra (in
Fig. 1) of PtTMPyP4+/MCM-41 show identical patterns with
Table 1 Pt(II)–porphyrin contents of PtTPyP/MCM-41 and
PtTMPyP4+/MCM-41
(PtTPyP/MCM-41)/
mg g21
(PtTMPyP4+/MCM-41)/
mg g21
20
40
80
10
20
40
Pt(II)–porphyrin 2.0
content (%)
4.0
7.3
1.0
2.0
4.0
Fig. 2 UV-vis absorption spectra of PtTMPyP4+ in water (a) and
PtTMPyP4+/MCM-41 (b).
3182 | J. Mater. Chem., 2005, 15, 3181–3186
This journal is ß The Royal Society of Chemistry 2005

View Article Online
Fig. 3 Emission spectra of PtTPyP/MCM-41: 20 mg g21 (a),
40 mg g21 (b) and 80 mg g21 (c). Insert: the molecular structure of
PtTPyP.
Fig. 5 Emission spectra of PtTPyP/MCM-41 (20 mg g21) under
different oxygen concentrations.
concentration. The variation of the emission spectra of
PtTPyP/MCM-41 (40 mg g21) displays similar trends to that
of PtTPyP/MCM-41 (20 mg g21). The relative luminescent
intensities of PtTPyP/MCM-41 (20 mg g21) and PtTPyP/
MCM-41 (40 mg g21) decrease by 86.4% and 85.7%, respec-
tively, upon changing from pure nitrogen to pure oxygen
conditions. The oxygen concentration dependent emission
spectra of PtTMPyP4+/MCM-41 (20 mg g21) are presented in
Fig. 6. Upon increasing oxygen concentration, the emission
peak of the sample remains at 668 nm and the emission inten-
sity drops quickly. For PtTMPyP4+/MCM-41 (40 mg g21
a similar result was obtained. The relative luminescent
intensities of PtTMPyP4+/MCM-41 (20 mg g21
and
)
)
PtTMPyP4+/MCM-41 (40 mg g21) decrease by 98.2% and
95.6%, respectively, upon changing from pure nitrogen to pure
oxygen conditions. The PtTMPyP4+/MCM-41 system is more
sensitive compared with the PtTPyP/MCM-41 system. This
can be attributed to the fact that the excited state lifetime of
PtTMPyP4+ is longer than that of PtTPyP and the quenching
Fig. 4 Emission spectra of PtTMPyP4+/MCM-41: 10 mg g21 (a),
20 mg g21 (b) and 40 mg g21 (c).
PtTMPyP4+, which may lead to the emission red shift and
peak broadening.33,34 The other possible explanation is that
the increase of Pt(II)–porphyrin concentration can enhance
the intermolecular interactions between Pt(II)–porphyrin
molecules in the channels. This may lead to the formation of
excimers between adjacent lumophores in the channels that
results in the red shift and peak broadening of the emission
spectra.27,28
The luminescence of most Pt(II)–porphyrin complexes could
be quenched effectively by molecular oxygen. The mechanism
of the quenching process consists of exchange energy transfer
from the lowest triplet excited state of metalloporphyrin to
molecular oxygen, which is accompanied by the formation of
singlet oxygen. The room temperature emission spectra, which
are recorded for PtTPyP/MCM-41 (20 mg g21) under different
concentrations of oxygen, are presented in Fig. 5. The emission
peak of PtTPyP/MCM-41 (20 mg g21) is at 640 nm, and
constant under different oxygen concentrations. However, the
relative intensity decreased markedly with increasing oxygen
Fig. 6 Emission spectra of PtTMPyP4+/MCM-41 (20 mg g21) under
different oxygen concentrations.
This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 3181–3186 | 3183

View Article Online
Fig. 7 Stern–Volmer plot for PtTPyP/MCM-41 (20 mg g21) at
different concentrations of oxygen. (I0 and I are the luminescent
intensities in the absence and in the presence of oxygen). Inset: Stern–
Volmer plot at low oxygen concentration.
Fig. 9 Response time and relative intensity change of PtTPyP/MCM-
41 (20 mg g21) to alternate environments of 100% nitrogen and 100%
oxygen.
The plots indicate that high sensitivity is obtained in the
oxygen concentration range of 0–5%. Even when the concentra-
sensitivity increases with the unquenched lifetime of the
luminescent complex.23,35,36 On the other hand, the distributed
and aggregated forms of PtTPyP and PtTMPyP4+ in MCM-41
should be different, which can also influence the sensitivity.37
Figs. 7 and 8 present the Stern–Volmer plots for PtTPyP/
MCM-41 (20 mg g21) and PtTMPyP4+/MCM-41 (20 mg g21),
respectively. The plots are nonlinear within a wide range of
oxygen concentration. It has been demonstrated that for
heterogeneous systems, Stern–Volmer plots for sensor systems
based on luminescence quenching often display non-linear
features within a wide range of oxygen concentration,38–41
which is attributed to the simultaneous presence of static
and dynamic quenching, and the inequality of the micro-
environment of the immobilized lumophore molecules in the
matrix.42,43 Close investigation reveals that the Stern–Volmer
plots exhibit perfect linear characteristics when the concentra-
tion of oxygen varied from 0 to 5% (insets of Figs. 7 and 8).
tion of oxygen is only 1%, the quenching of PtTMPyP4+
/
MCM-41 and PtTPyP/MCM-41 can reach 83.33% (I0/I = 6)
and 33.33% (I0/I = 1.33), respectively. It is well know that
the measurement of oxygen at low concentration is more
important, so Pt(II)–porphyrin/MCM-41 has great potential
for application in oxygen sensors.
For oxygen sensors the response time is also very important.
Generally, 95% response time, i.e., tQ(95%, N2 A O2), is
defined as the time required for the luminescent intensity to
decrease by 95% on changing from 100% nitrogen to 100%
oxygen. Similarly, 95% recovery time, i.e., tq(95%, O2 A N2),
means the time required for the luminescent intensity to reach
the 95% of the initial value recorded under 100% nitrogen on
changing from 100% oxygen to 100% nitrogen. Figs. 9 and 10
show the response properties of PtTPyP/MCM-41 (20 mg g21
)
Fig. 8 Stern–Volmer plot for PtTMPyP4+/MCM-41 (20 mg g21) at
different concentrations of oxygen. Inset: Stern–Volmer plot at low
oxygen concentration.
Fig. 10 Response time and relative intensity change of PtTMPyP4+
/
MCM-41 (20 mg g21) to alternate environments of 100% nitrogen and
100% oxygen.
3184 | J. Mater. Chem., 2005, 15, 3181–3186
This journal is ß The Royal Society of Chemistry 2005

View Article Online
Table 2 Oxygen sensing properties of PtTPyP/MCM-41 and
PtTMPyP4+/MCM-41
Acknowledgements
This work was supported by the National Natural Science
Foundation of China, the Major State Basic Research
Development Program (2002CB613401) and the Program
for Changjiang Scholars and Innovative Research Team in
University (IRT0422).
(PtTPyP/MCM-41)/
mg g21
(PtTMPyP4+/MCM-41)/
mg g21
20
40
20
40
I0/I100
tQ/s
tq/s
7.34
0.75
7.00
0.50
55.50
0.33
26.62
23.53
0.36
29.58
204.00
220.12
References
1 Y. Amao, T. Miyashita and I. Okura, Analyst, 2000, 125, 871.
2 E. Singer, G. L. Duveneck, M. Ehrat and H. M. Widmer, Sens.
Actuators A, 1994, 542, 41.
3 M. A. Chan, S. K. Lam and D. Lo, J. Fluoresc., 2002, 12, 327.
4 F. G. Gao, A. S. Jeevarajan and M. M. Anderson, Biotechnol.
Bioeng., 2004, 86, 425.
5 M. T. Murtagh and M. R. Shahriari, Chem. Mater., 1998, 10, 3862.
6 P. Payra and P. K. Dutta, Microporous Mesoporous Mater., 2003,
64, 109.
7 E. Yavin, L. Weiner, R. A. Yellin and A. Shanzer, J. Phys. Chem.
A, 2004, 108, 9274.
8 D. B. Papkovsky, G. V. Ponomarev, W. Trettnak and P. O’Leary,
Anal. Chem., 1995, 67, 4112.
9 Y. Amao, K. Asai and I. Okura, J. Porphyrins Phthalocyanines,
2000, 4, 179.
10 R. N. Gillanders, M. C. Tedford, P. J. Crilly and R. T. Bailey,
Anal. Chim. Acta, 2004, 502, 1.
and PtTMPyP4+/MCM-41 (20 mg g21), respectively. Upon
increasing oxygen concentration, the emission intensity drops
very quickly, while upon decreasing oxygen concentration, the
emission intensity increases and recovers to the initial level
under 100% nitrogen again. This cycle is repeated and the
emission intensity changes are monitored when the sample is
exposed to an alternating atmosphere of nitrogen and oxygen,
indicating that the emission intensity changes are reversible.
The values of response time (tQ), recovery time (tq) and
sensitivity (I0/I100, where I0 and I100 represent the luminescent
intensities in 100% nitrogen and 100% oxygen, respectively)
are listed in Table 2. All oxygen sensing materials based on
Pt–porphyrins and MCM-41 have short response times (,1 s)
11 S. -K. Lee and I. Okura, Analyst, 1997, 122, 81.
12 S. -K. Lee and I. Okura, Anal. Commun., 1997, 34, 185.
13 S. M. Borisov and V. V. Vasil’ev, J. Anal. Chem., 2004, 59, 155.
14 A. M. Liu, K. Hidajat, S. Kawi and D. Y. Zhao, Chem. Commun.,
2000, 1145.
and good sensitivity (I0/I100 . 7), especially PtTMPyP4+
/
MCM-41 (20 mg g21) with a very fast response time (,0.5 s),
recovery time (,30 s) and excellent sensitivity (I0/I100 . 50).
The oxygen sensing property of this kind of material is much
better than that of other Pt–porphyrin based sensing materials
reported previously.20–23 The results show that the recovery
time is significantly longer than the response time, similar to
results obtained for Pt-OEP in polymer medium.44 Because
small molecules such as oxygen are well known to be adsorbed
strongly on silica surfaces, the longer recovery time can be
attributed to slow desorption of oxygen from the silica surface
in the support.45 PtTMPyP4+/MCM-41 and PtTPyP/MCM-41
show similar response times, but their recovery times are
obviously different. This phenomenon may be because the O2
desorption rate of PtTMPyP4+/MCM-41 is faster than that of
PtTPyP/MCM-41.
15 X. Feng, G. E. Fryxell, L. Q. Wang, A. Y. Kim, J. Liu and
K. M. Kemner, Science, 1997, 276, 923.
16 Z. Luan, J. Y. Bae and L. Kevan, Chem. Mater., 2000, 12, 3202.
17 M. Kaneda, T. Tsubakiyama, A. Carlsson, Y. Sakamoto,
T. Ohsuna and O. Terasaki, J. Phys. Chem. B, 2002, 106, 1256.
18 M. H. Lim, C. F. Blanford and A. Stein, J. Am. Chem. Soc., 1997,
119, 4090.
19 I. Honma and H. S. Zhou, Adv. Mater., 1998, 10, 1532.
20 Y. Ueno, A. Tate, O. Niwa, H. S. Zhou, T. Yamada and I. Honma,
Chem. Commun., 2004, 746.
21 H. Fau, Y. Lu, A. Stump, S. T. Reed, T. Baer, R. Schunk,
V. Perezlund, G. P. Lopez and C. J. Brinker, Nature, 2000, 405, 56.
22 G. Wirnsberger, B. J. Scott and G. D. Stucky, Chem. Commun.,
2001, 119.
23 A. B. Descalzo, D. Jimenez, M. D. Marcos, R. M-Ma´n˜ez, J. Soto,
J. E. Haskouri, C. Guille´m, D. Beltra´n, P. Amoro´s and
M. V. Borrachero, Adv. Mater., 2002, 14, 966.
24 Q. Huo, J. Feng, F. Schu¨th and G. D. Stucky, Chem. Mater., 1997,
9, 14.
25 M. Gru¨n, I. Lauer and K. K. Unger, Adv. Mater., 1997, 9, 254.
26 S. Han and T. Hyeon, Chem. Commun., 1999, 1955.
27 M. Fang, Y. Wang, P. Zhang, S. G. Li and R. R. Xu, J. Luminesc.,
2000, 91, 67.
28 P. Zhang, J. H. Guo, Y. Wang and W. Q. Pang, Mater. Lett., 2002,
53, 400.
29 M. Ogawa, T. Nakamura, J. I. Mori and K. Kuroda, J. Phys.
Chem., 2000, 104, 8554.
30 F. R. Longo, M. G. Finarelli and J. B. Kim, Heterocycl. Chem.,
1969, 6, 927.
31 R. F. Pasternack, P. R. Huber, P. Boyd and G. Engasser, J. Am.
Chem. Soc., 1972, 4511.
32 B. Onida, B. Bonelli, L. Flora, F. Geobaldo, C. O. Arean and
E. Garrone, Chem. Commun., 2001, 2216.
33 K. Kano, K. Fukuda, H. Wakami, R. Nishiyabu and
R. F. Pasternack, J. Am. Chem. Soc., 2000, 122, 7494.
34 Y. H. Kim, D. H. Jeong, D. Kim, S. C. Jeoung, H. S. Cho,
S. K. Kim, N. Aratani and A. Osuka, J. Am. Chem. Soc., 2002,
123, 76.
35 V. V. Vasil’ev and S. M. Borisov, Sens. Actuators B., 2002, 82, 272.
36 B. H. Han, I. Manners and M. A. Winnik, Chem. Mater., 2005, in
press.
Conclusions
New oxygen sensing materials based on Pt(II)–porphyrin
assembled in MCM-41 were prepared. Oxygen sensing studies
demonstrate that the luminescence of the materials exhibits
strongly oxygen concentration dependent characteristics and
is easily quenched by oxygen. The oxygen sensing property of
PtTMPyP4+/MCM-41 is obviously better than that of PtTPyP/
MCM-41. The reasons for these results remain unclear and
highlight the fact that PtTMPyP4+/MCM-41 system is a good
candidate for developing high performance oxygen sensing
materials. In this paper our interests are focused on the
functions of PtTPyP/MCM-41 and PtTMPyP4+/MCM-41;
investigation of the detailed photophysics and operation
mechanism of Pt(II)–porphyrin/MCM-41 and further optimi-
zation of the Pt–porphyrin/mesoporous silica system by
using other mesoporous materials such as MCM-48 are
under way.
This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 3181–3186 | 3185

View Article Online
37 I. Klimant and O. S. Wolfbeis, Anal. Chem., 1995, 67, 3160.
38 B. D. MacCraith, C. M. McDonagh, G. O’Keeffe, E. T. Keyes,
J. G. Vos, B. O’Kelly and J. F. McGilp, Analyst, 1993, 118,
385.
41 J. R. Lakowicz, Principle of Fluorescence Spectroscopy, Plenum,
New York, 1986, p. 266.
42 P. Hartmann, M. J. P. Leiner and M. E. Lippitsch, Anal. Chem.,
1995, 67, 88.
39 W. Xu, R. Schmidt, M. Whaley, J. N. Demas, B. A. DeGraff,
E. K. Karikari and B. L. Farmer, Anal. Chem., 1995, 67, 3172.
40 E. R. Carraway, J. N. Demas, B. A. DeGraff and J. R. Bacon,
Anal. Chem., 1991, 63, 337.
43 M. E. Lippitsch and S. Draxler, Sens. Actuators B, 1993, 11, 97.
44 A. Mills and A. Lepre, Anal. Chem., 1997, 69, 4653.
45 A. Yekta, Z. Masoumi and M. A. Winnik, Can. J. Chem., 1995, 73,
202.
3186 | J. Mater. Chem., 2005, 15, 3181–3186
This journal is ß The Royal Society of Chemistry 2005