Improvement of Thin Film Gas Detectors by
Incorporation of Novel Nanoparticles
A. Farooq, R. Al-Jowder, Dr R. Narayanaswamy and Dr D. Whitehead
Diversion of Chemistry & Materials, Faculty of Science & Engineering,
Manchester Metropolitan University,
Chester Street, Manchester, M1 5GD.
Contact: [email protected]
Introduction
There is vast amount of research carried out on the production of fluorescent sensors. We have produced novel sensors by incorporation of fluorescent dyes
(known as fluorophores) into silica nanoparticles. The sensor works by the detected species quenching the fluorophores luminescence. The sensing
capabilities can be manipulated by selecting the appropriate fluorophore as they bind to specific gases1. The objective of this work is to produce an enhanced
dual sensor that works simultaneously in detecting sulphur dioxide (SO2) and (oxygen) O2 gases. The fluorophore rhodamine B isothiocyanate (RBITC) will
be used to produce a nanosensor that is sensitive to SO2, while ruthenium-tris(4,7-diphenyl-1,10-phenanthroline) dichloride (Ru(dpp)) will be used to sense
O2. The fluorophore itself is by no means sufficient in giving the sensor its action; the matrix it exists in also plays an important role. These nanosensors will
be encapsulated in an organically modified sol-gels (ormosil) matrix. The surface of nanoparticles is modified with carboxylic acid groups to anchor to the
ormosil matrix. The Ru(dpp) and RBITC nanosensor produced a significantly high response to gases along with response recovery. The monodispersed
nanoparticles sizes ranged from 200 nm-400 nm. The carboxylic acid functionalization of dye modified silica nanoparticles was preformed by the ring
opening reaction of succinic anhydride subsequently attaching to the amine group. A thin film was produced onto a glass slide combining different dye
encapsulated in one sol-gel matrix film producing a dual sensor that capable of detecting SO2 and O2 simultaneously using luminescence spectroscopy.
Experimental
Scanning Electron Microscopy (SEM)
Synthesis of dye silica nanoparticles:
Silica nanoparticles were synthesised using Stöber method. The reaction entails the
hydrolysis and condensation of TEOS in aqueous solution of ethanol and water. The
dyes were trapped in the nanoparticles by incorporating it in Stöber method. Silica
nanoparticles were synthesised using the procedure describes by Verhaegh et al2 and
then a seeding technique. NH4OH (2 ml) was added to EtOH (24 ml) and stirred.
The dye (1 mg) and APS (0.01 ml) mixture was placed in the mixture and further
stirred. Finally a solution of TEOS (1.5 ml) and ethanol (6 ml) was added to mixture
and allowed to stir for 24 hr causing the solution to become opaque. A shell was
grown on the seeds to obtain the required diameter.
CH3
H3C
O
+
Si
O
H3C
OH
NH4OH
O
HO
O
Si
OH
+
OH
OH
dye
CH3
OH
HO
Si
Si
O
Si
OH
OH
dye
n
HO
OH
OH
OH
HO
OH
OH
dye
n
n
Si
OH
Si
b)
c)
d)
Fig 1: SEM images of nanosensor SiO2 particles. RBITC containing nanoparticles a) 200
nm and b) 278 nm. Ru(dpp) containing nanoparticles c) 200 nm and d) 390 nm.
OH
O
a)
OH
OH
n
n
RBITC Nanoparticles
Scheme 1: Incorporation of the dye within the silica nanoparicles.
Testing the Sensor using
Luminescence Spectroscopy
Functionalising the SiO2 nanoparticles with carboxylic acid:
10 % SO2
10 % SO2
MeO
OH
OH
OH
+
O
Si
MeO
NH2
O
H3C
MeO
Si
NH2
O
SiO 2-dye
+
O
O
H3C
O
Si
O
O
O
O
NH
OH
O
Scheme 2: Synthesis of the dye modified silica nanoparticles functionalised with carboxylic acid.
Results and Discussion
Verhaegh et al found that when RBITC dye was not modified with APS and used in
an alcohol solution it did not incorporate into the silica. This is because APS couples
to the dye and silica preventing any loss of the dye when it further reacts with
ammonia. The dye APS mixture was stirred for 4 hr under N2 atmosphere in a dark
room the colour changed from an intense purple to orange as self-quenching has
occurred. Therefore we coupled RBITC and Ru(dpp) dye with APS, and found both
dyes incorporated onto the silica nanoparticles. When TEOS is hydrolysis by
ammonia, shown in scheme 1, it causing ethoxy groups to be substituted for hydroxyl
groups. Therefore it become hydrophilic like the dyes and causing it to be trapped in
the nanoparticles.
Monodispersed carboxylic functionalised RBITC-silica
nanoparticles with a diameter of a) 200 nm, b) 278 nm and Ru(dpp) functionalisedsilica nanoparticles with a diameter of c) 200 nm and d) 390 nm were successfully
synthesised and SEM images are shown in figure 1.
A thin film was prepared by drying the
nanosensor particles onto a glass slide.
Ormosil was spin coated onto the
nanoparticles to produce a continuous
film. The film was placed into a test
chamber
under
vacuum
and
luminescence was obtained whilst
exposing to the test gasses. The results
for exposure of RBITC to SO2 are
shown in figures 2. The luminescence
was quenched when exposed to 10 %
and 20 % test gas. The results for
exposure of Ru(dpp) to O2 are shown in
figures 3. The luminescence was
quenched when exposed to 5 % and 10
% test gas. In figure 4 the results of a
duel sensor thin film are shown. The
Ru(dpp) and RBITC nanosensor
produced a significantly fast and
sensitive response to the test gases.
The response recovery time was also
quick proving the viability of the
sensor.
20 % SO2
Fig 2: Luminescence of RBITC film response to SO2
Ru(dpp) Nanoparticles
10 % O2
5 % O2
atm
Fig 3: Luminescence of Ru(dpp) film response to O2
RBITC & Ru(dpp) Dual Sensor
40
Intensity ( a.u.)
The carboxylic acid functionalised silica nanoparticles were prepared using Yanqing
An et al method3. APS (0.4 ml) was added to the SiO2@dye nanoparticles and stirred
for 20 h. The nanoparticles were cleaned by centrifugation to remove any un-reacted
reactants. DMF (25 ml) was mixed with the nanoparticles and added to a mixture of
DMF (25 ml) and succinic anhydride (0.25 g). The solution was allowed to stir for
24 hr and further cleaned by centrifugation.
420 nm ex
350 nm ex
35
10 % SO2
30
25
10 % O2
20
15
0
2000
4000
6000
8000
10000
Time(s)
Fig 4: Response to SO2 and O2 of dual film
Conclusions
A RBITC-SiO2 and Ru(dpp)-SiO2 dual sensor was successfully synthesized. These
nanosensors produced a significantly fast and sensitive response to the test gases.
The response recovery time was also quick proving the viability of the sensor.
References
1. P. J. R. Roche, R. Al-Jowder, R. Narayanaswamy, J. Young and P. Scully: ‘A novel luminescent lifetime-based optrode for the detection of gaseous and dissolved oxygen utilising a mixed ormosil
matrix containing ruthenium (4, 7-diphenyl-1, 10-phenanthroline)3Cl2 (Ru.dpp)’, Anal Bioanal Chem, 2006, 386, 1245-1257.
2. N. A. M. Verhaegh and A. van Blaaderen: ‘Dispersions of Rhodamine-Labeled Silica Spheres: Synthesis, Characterization, and Fluorescence Confocal Scanning Laser Microscopy’, Langmuir, 1994,
10, 1427-1438.
3. Y. An, M. Chen, Q. Xue and W. Liu: ‘Preparation and self-assembly of carboxylic acid-functionalized silica’, Journal of Colloid and Interface Science, 2007, 311, 507-513.