Ru(bpy)2(phen-5-NH2)2+ doped ultrabright and photostable fluorescent silica nanoparticles

Article abstract of DOI:10.1039/c6ra02917e

This work demonstrated that a series of ultrabright fluorescent silica nanoparticles have been synthesized and designed via the reverse microemulsion method to encapsulate (2,2′-bipyridine)₂-(5-aminophenanthroline) Ru bis(hexafluorophosphate) (Rubpy-phen). Transmission electron microscopy (TEM), scanning electron microscopy (SEM), dynamic light scattering (DLS), inductively coupled plasma (ICP) and fluorescence spectra were used to study the morphology and properties of the nanoparticles. By introducing ethanol into the reaction system, the obtained uniform fluorescent nanoparticles showed monodispersity and controllable size. With an increase of nanoparticle size (from 50-180 nm), the relative brightness of the nanoparticles was greatly improved from 534 to 86-972 by comparison with free dye molecules. Due to the protection of the silica matrix, the fluorescence of high concentration dye-doped silica nanoparticles (Rubpy-phen NPs) is not quenched, which results in higher fluorescence intensity and relative brightness. Moreover, the silica matrix can also protect Rubpy-phen from leakage. Due to the high brightness and biocompatibility, Rubpy-phen NPs have potential applications in the biological field.

Full text of DOI:10.1039/c6ra02917e

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PAPER
2
+
Ru(bpy) (phen-5-NH ) doped ultrabright and
2
2
photostable fluorescent silica nanoparticles†
Cite this: RSC Adv., 2016, 6, 51591
a
ab
a
a
a
a
Dongdong Lu, Fangyuan Gai, Zhen-An Qiao, Xue Wang, Tao Wang, Yunling Liu
and Qisheng Huo*a
This work demonstrated that a series of ultrabright fluorescent silica nanoparticles have been synthesized and
0
designed via the reverse microemulsion method to encapsulate (2,2 -bipyridine)
2
-(5-aminophenanthroline)
Ru bis(hexafluorophosphate) (Rubpy-phen). Transmission electron microscopy (TEM), scanning electron
microscopy (SEM), dynamic light scattering (DLS), inductively coupled plasma (ICP) and fluorescence
spectra were used to study the morphology and properties of the nanoparticles. By introducing ethanol
into the reaction system, the obtained uniform fluorescent nanoparticles showed monodispersity and
controllable size. With an increase of nanoparticle size (from 50–180 nm), the relative brightness of the
nanoparticles was greatly improved from 534 to 86 972 by comparison with free dye molecules. Due to the
protection of the silica matrix, the fluorescence of high concentration dye-doped silica nanoparticles
Received 1st February 2016
Accepted 13th May 2016
(
Rubpy-phen NPs) is not quenched, which results in higher fluorescence intensity and relative brightness.
DOI: 10.1039/c6ra02917e
Moreover, the silica matrix can also protect Rubpy-phen from leakage. Due to the high brightness and
www.rsc.org/advances
biocompatibility, Rubpy-phen NPs have potential applications in the biological field.
encapsulated in nanoparticles by using the microemulsion
method through noncovalent interaction between dye mole-
Fluorescent techniques have attracted growing interest due to cules and the silica framework or physical entrapment. The
Introduction
23
their wide application in the elds of chemical analysis, envi- silica shell can protect dyes from quenching by dissolved oxygen
1–3
ronmental monitoring and biological imaging. Fluorescent and assemble single dyes to obtain an enhanced uorescent
4
,5
24–26
nanoparticles such as quantum dots,
dye-doped nano- signal by electron transfer or energy transfer.
6–8
9,10
particles and rare earth-based nanoparticles etc. have been
Besides biocompatibility and water-solubility, the sensitivity
the subjects of widespread study in the past few years. Because and response speed are also important for uorescent silica
27–29
of their chemically stable, water-soluble and nontoxic proper- nanoparticle's application in biological sensing elds.
To
ties as host materials, uorescent silica-based nanoparticles apply uorescent silica nanoparticles in intracellular system,
1
1–13
have been explored and investigated extensively.
example, their high sensitivity and rapid response abilities have could be detected by multiple inuences of biological envi-
For the brightness of particles should be strong so that the signal
14,15
been developed rapidly in many elds.
Particularly, it is ronment. Increased brightness of uorescence nanoparticles
important to control the size and shape of silica nanoparticles lead to higher signal-to-noise ratio and more reliable identi-
The St ¨o ber cation of biological sensing and imaging. Therefore, ultra-
method and reverse microemulsion method are two typical bright uorescent nanoparticles are fascinating for their high
methods to obtain silica-based nanocomposites with control- sensitivity and rapid respond speed of detection.
lable size and morphology. Compared with the St ¨o ber method, researching the brightness of dye-doped silica nanoparticles
1
6,17
for medical or biological system applications.
18
19
3
0–32
Recently,
33–35
the reverse microemulsion method is more suitable for the has attracted increasing attention.
production of small and uniform size, and variable structure organic and inorganic dye-doped silica nanoparticle in
Tan et al. prepared
26
36
20–22
nanoparticles.
Thousands of dye molecules can be reverse microemulsion and studied the inuence of reactants
37
concentrations on particles size and orescence properties.
Ulrich Wiesner et al. prepared small size nanoparticles with
high uorescence emission by a modied St ¨o ber method.
a
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of
38
Chemistry, Jilin University, Changchun 130012, China. E-mail: huoqisheng@jlu.edu.
cn
Igor Sokolov et al. demonstrated the synthesis and function-
alization of ultrabright uorescence mesoporous silica nano-
b
Department of Petroleum and Chemical Engineering, Dalian University of Technology,
39
2
Dagong Road, New District of Liaodong Bay, Panjin 124221, China
particles with template sol–gel self-assembly method.
†
Electronic supplementary information (ESI) available: The TEM images, SEM However, the method to improve the brightness of nano-
images, DLS gures and IR spectra and synthesis parameter of the samples. See
40,41
particles is still few.
DOI: 10.1039/c6ra02917e
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In this study, we designed and synthesized uorescent silica Synthesis of Rubpy-phen NPs
nanoparticles with ultrabrightness and photostability by
Firstly, 1.876 g of Triton X-100, 1.8 mL of n-hexanol, 7.5 mL of
0
encapsulating
Rubpy-phen
((2,2 -bipyridine) -(5-amino-
ꢂ4
2
cyclohexane, different amount of dye solution (1.3 ꢁ 10 M) and
phenanthroline) Ru bis (hexauorophosphate)) into silica
nanoparticles by a modied reversed microemulsion method.
ꢂ4
ethanol solution (2.5 ꢁ 10 M) was mixed to a glass vial forming
W/O microemulsion. Then 100 mL of TEOS as silica precursor was
added to the microemulsion for additional 30 min stirring. 60 mL
ammonia was added to the system to start the polymerization of
TEOS. Aer stirring for 24 h, using 20 mL of acetone to mix with
microemulsion to separated dye doped NPs from solution by
sonicating and centrifuging. At last, ethanol and water were used
to wash the obtained NPs several times, and the sample was dried
Due to its high brightness and large Stokes Shi, Rubpy-phen
0
(
(2,2 -bipyridine)
2
-(5-aminophenanthroline) Ru bis (hexa-

uorophosphate)) was chosen as the mode uorescent dye to
demonstrate the reverse microemulsion method for the
synthesis of dye doped silica nanoparticles. We introduced
ethanol to reverse microemulsion system to synthesize dye-
doped nanoparticles (NPs) with tunable size from 50 to 180
nm. Meanwhile, by adding different amount of ethanol, the
amount of dyes incorporate with silica matrix also could be
tuned, which resulted in a tunable uorescence. The uores-
cence of Rubpy-phen NPs remains unquenched when the
concentration of Rubpy-phen is higher in the silica matrix,
which results in high uorescence intensity. The silica matrix
facilitates nanoparticles uorescence enhancement and
prevents dyes from leaking out of the matrix. Particularly,
compared with free dye, the relative brightness of nanoparticles
with different sizes regularly increases from 534 to 86 972.
Furthermore, these ultrabright nanoparticles also show good
photobleaching resistance ability in aqueous media.
37
overnight.
Characterization of Rubpy-phen NPs
TEM images were obtained on a FEI Tecnai G2 F20 s-twin D573
transmission electron microscopy operating at 200 kV. SEM
images were obtained on a JEOL JSM-6700F eld emission
scanning electron microscope. Inductively coupled plasma
(ICP) result was obtained on a Perkin-Elmer Optima 3300DV ICP
instrument. The particle size distribution was carried out by
Malvern Zetasizer Nano-S instrument using Dynamic Light
Scattering (DLS) principles with a HeNe laser (633 nm). The

uorescence spectra were measured on an Edinburgh Instru-
ments FLS920 spectrouorimeter equipped with both contin-
uous (450 W) and pulsed xenon lamps. Infrared (IR) spectra
were recorded within a 400–4000 cm region on a Nicolet
Impact 410 FTIR spectrometer with KBr pellets.
Experimental section
Materials
ꢂ1
Triton X-100, tetraethylorthosilicate (TEOS, 99.95%), LiCl and
0
2
,2 -bipyridine were obtained from Xilong Chemical Works. n-
Results and discussion
Hexanol ($99%), cyclohenane ($99.5%), aqueous ammonia
(
NH OH, 25–28%), acetone (CH COCH , 99.5%), dimethyl sulf- Synthesis of Rubpy-phen NPs
4 3 3
oxide (DMSO), ether and ethanol (C H OH, 99.7%) were
2
5
By our modied reverse microemulsion method, dye-doped
silica nanoparticles have been prepared by Triton X-100 as
surfactant, n-hexnol as co-surfactant, cyclohexnane as oil, TEOS
as silica precursor, and aqueous ammonia as catalyst. The dye
encapsulated nanoparticles are formed in three steps, rstly we
dissolved the dye, then the condensation of silicate precursor
facilitated the shell of nanoparticles, and nally the surfactant
have been removed (Scheme 1). Through the electrostatic
interaction between the positively charged dyes(Scheme 2,
Fig. S1†) and the negatively charged silica, the dyes can be
purchased from Beijing Chemical Works. Ruthenium(III) chloride
hydrate (RuCl $3H O, 99.9%) were purchased from Shanghai
3
2
Junyue Chemical Works. 1,10-phenanthrolin-5-amine (99.7%)
were purchased from Aldrich. Ammonium hexauorophosphate
4 6
(NH PF , 98%) were purchased from Aladdin. Deionised water
was used in all experiment. All reagents were used without further
purication.
Synthesis of Rubpy-phen
41
effectively entrapped inside silica particles.
Because Rubpy-phen is hard to dissolve in water, we use
ethanol to enhance the solubility of Rubpy-phen in silica matrix.
4
2,43
Dyes were prepared according to literature
with some modi-
0

cations. 1 g of RuCl $3H O, 1.2 g of 2,2 -bipyridine and 1.1 g of
LiCl dissolved in 25 mL DMF and reuxed at 170 C. Aer 8 h,
distil the solution to 10 mL. Aer cooling to room temperature,
3
2
ꢀ
8
0
5
0 mL acetone was added to the solution and le it overnight at
ꢀ
C. Then ltered the solution and washed with 5 mL water and
0
mL ether. Ru(2,2 -bipyridine)
6
2
Cl
2
0
was obtained.
20 mg (1.28 mmol) of Ru(2,2 -bipyridine) Cl was reuxed
2 2
with 285 mg (1.26 mmol) of 1,10-phenanthrolin-5-amine in 40
ꢀ
mL water and 40 mL ethanol at 110 C with the protection of N .
2
Aer 8 h, distil the ethanol out of the solution. Orange-red solid
was obtained with the addition of 5 mL NH PF . Then the solu-
4
6
2+
tion was ltered and washed with water and ether. The obtained
Scheme 1 The synthetic process of Ru(bpy)
silica nanoparticles.
2
(phen-5-NH
2
)
doped
ꢀ
Ru(bpy) (phen-5-NH )(PF )
2 2 6 2
was dried at 70 C.
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0
2
2 Molecule structure of (2,2 -bipyridine) -(1,10-phenan-
Scheme
throlin-5-amine) Ru bis (hexafluorophosphate).
Compared with classical reverse microemulsion method, our
nanoparticles have been prepared by adding a certain amount
ethanol for increasing solubility of dye thus increased the uo-
rescent intensity of dye-doped silica nanoparticles. By tuning the
amount of ethanol in reaction system, the amount of the dye in
silica core could be tuned easily so that the uorescent intensity
could be controlled. Furthermore, by tuning the volume ratio of
water and ethanol, we prepared a series of dye-doped particles with
Fig. 1 TEM images of synthesized dye doped NPs-1 (a), NPs-3 (c),
different size in the range of 50 nm to 180 nm. By analysis of NPs-5 (e) and SEM images of NPs-1 (b), NPs-3 (d), NPs-5 (f).
nanoparticles suspension, we could get the concentration of dye in
silica core (Table 1).
(Fig. S2†). The reducing of water makes the less formation of
monomers and Si–O–Si, and the bigger rigid at the surface of
surfactant and water. These conditions enhance the particles
growth rate, so the terminal particles size is larger. We added
ethanol in the reaction aimed to dissolve more dye molecules.
Ethanol as by produced can be produced when TEOS hydrolysis;
it can promote the intermicellar exchange and the generation of
nuclear and smaller particles. In classical method, the droplet of
microemulsion tend to form uniform silica nanoparticles.
Therefore, the modication in reverse microemulsion method
would retain the uniformity of particles.
Size and morphology of Rubpy-phen NPs
According to Tan's work, the particles size is affected by the molar
ratio of water and surfactant in reverse microemulsion method:
particle size decreased gradually when of the addition of water
37
increased. Our results are similar with Tan's previous work, as
showed in Table 1 and Fig. 1, keeping reactants parameter and
the volume of solvent constant, by varying water-to-ethanol
volume ratio, with the volume of water from 480 to 200 mL,
while the volume of ethanol in the range of 0 to 280 mL, we ob-
tained different sized nanoparticles from 50 nm to 180 nm. As is
shown in Fig. 1a, the uniform dye-doped silica nanoparticles with
the diameter around 50 nm were obtained. Similar result of
particles size could also be observed from SEM image (Fig. 1b). By
varying the water-to-ethanol volume ratio, we could get bigger
particles around 100 nm (Fig. 1c and d) and 150 nm (Fig. 1e and
f). Therefore, by varying water-to-ethanol volume ratio in the
synthesis process, we got nanoparticles with different sizes
37
Dynamic light scattering (DLS) was used to study the homo-
geneity of particles. We tested the diameter of all kinds of uo-
rescent nanoparticles by DLS method, as is shown in Fig. 2. Each
sample suggested narrow peak in Fig. 2, which indicated good
uniform of the particles in different range. It can be seen that the
trend of particles size is agreed with the TEM results. The poly-
dispersity (PDI) of each sample showed good monodispersity of
44
nanoparticles (Table S1†).
Table 1 Synthesis parameter and dye concentrations of dye doped silica nanoparticles. Dye concentration 1: the original concentration of dye
molecules in 480 mL water and ethanol mixed solvents before the reaction; dye concentration 2: the concentration of dye molecules in 2.5 mg
nanoparticles dispersed in 10 mL H O
2
Sample
Triton X-100
Hexanol
Cyclohexane
TEOS
NH
4
OH
H
2
O
EtOH
Dye concentration 1
Dye concentration 2
ꢂ4
ꢂ7
NPs-1
NPs-2
NPs-3
NPs-4
NPs-5
NPs-6
1.876 g
1.876 g
1.876 g
1.876 g
1.876 g
1.876 g
1.8 mL
1.8 mL
1.8 mL
1.8 mL
1.8 mL
1.8 mL
7.5 mL
7.5 mL
7.5 mL
7.5 mL
7.5 mL
7.5 mL
100 mL
100 mL
100 mL
100 mL
100 mL
100 mL
60 mL
60 mL
60 mL
60 mL
60 mL
60 mL
480 mL
400 mL
320 mL
280 mL
240 mL
200 mL
0 mL
80 mL
160 mL
200 mL
240 mL
280 mL
1.35 ꢁ 10
1.54 ꢁ 10
1.72 ꢁ 10
1.81 ꢁ 10
1.90 ꢁ 10
1.99 ꢁ 10
M
M
M
M
M
M
5.10 ꢁ 10
5.17 ꢁ 10
5.47 ꢁ 10
5.94 ꢁ 10
6.26 ꢁ 10
6.63 ꢁ 10
M
M
M
M
M
M
ꢂ4
ꢂ4
ꢂ4
ꢂ4
ꢂ4
ꢂ7
ꢂ7
ꢂ7
ꢂ7
ꢂ7
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concentration 2), we prepared same concentration of dye solu-
tion, respectively. Compared with uorescence spectra of free
dye aqueous solution peak at 609 nm (Fig. 3b), the uorescence
emission peak of NPs has a 17 nm blue shi located at 592 nm
(Fig. 3a). The blue shi is caused by different environments of
dyes in aqueous solutions and NPs. Fig. 3c–e are photos of
liquid, solid, membrane materials of nanoparticles under 365
nm UV lamp and sun light, respectively. It is obvious that dye
doped nanoparticles have very high brightness as liquid, solid
and membrane materials.
The effects of dye concentration and solvent ratio on particle
sizes and emission properties were studied. Nanoparticles a1 to
a6 were prepared with different dye concentrations under the
same water-to-ethanol volume ratio (Table S2†). As shown in
Fig. S3,† all the nanoparticles have uniform particle sizes, with
diameters of around 120 nm. Increasing the concentration of
dye, the sizes of nanoparticles did not change. The emission
intensity increased gradually from NPs-a6 to NPs-a1, with
increasing the concentration of dye under the same solvent
ratio (Fig. S4†). Therefore, the dye concentration is the key
factor for the emission spectra, and the dye concentration
almost has no effects on particle sizes.
Fig. 2 Dynamic light scattering result of nanoparticles.
Fluorescence properties
Compared with traditional reverse microemulsion method, we
added ethanol in the system to improve the solubility of dye. The
inuence of ethanol on the spectral characteristics of dye can be
ignored. As showed in Fig. 3a, the samples made by addition of
ethanol (NPs-2 to NPs-6) did not perform shi or shape change
of uorescence emission spectra compared with NPs-1, but the
uorescence intensity increased gradually from NPs-2 to NPs-6.
Furthermore, keeping dye concentration constant, we
synthesized NPs-b1 to b6, and investigated the effects of water-
to-ethanol volume ratio on the particles sizes and emission
spectra (Table S3†). As shown in Fig. S5,† with the water-to-
ethanol volume ratio increasing, the particle sizes increased
accordingly, which is agreed with Fig. 1. But the emission
intensity of NPs-b1 to NPs-b6 is almost identical, and the
emission peaks are located at 592 nm (Fig. S6†). Therefore, the
solvent ratio in synthesis system almost has no effects on
This uorescence enhancement resulted from the increasing
ethanol in the synthesis processes. The uorescence intensity is
proportional to the dye concentration in nanoparticles diluted
solution, thus the emission intensity is enhanced when the
concentration of dye molecules is increased. As is shown in
Table 1, dye concentration 2 also performed good correlation
with uorescent spectra, which suggested that the more ethanol
used in the synthesis process, the more enhancement of uo-
rescence could be detected by different samples. According to
the dye concentration in the nanoparticles suspension (dye
45

uorescence properties of nanoparticles.
Fig. 3 Fluorescence emission spectra of dye doped silica nanoparticles (a), dye aqueous solution (b). (c)–(e) are photos of liquid, solid,
membrane materials of nanoparticles under 365 nm UV lamp and sun light respectively, from left to right: silica nanoparticles without dye,
sample 1–6 dye doped silica nanoparticles.
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We have investigated the effect from solvents on the emis-
RSC Advances
The relative brightness of synthesized dye doped silica nano-
sion properties of as-prepared nanoparticles. The Fig. S7† particle was researched by compared the integral area of uo-
shows the emission spectrum of the NPs-1 dispersed in H O, rescence emission spectrum of a single particle with a single dye
2
4
0,41
DMSO, ethanol and CH Cl respectively, the different solvents molecule.
With NPs-1 and free dye solution 1, for example, the
2
2
ꢂ3 45
almost have no effects on emission properties of our samples. density of silica nanosphere material is 2.2 g cm
, the most
Due to dye molecules are surrounded by silica matrix, the abundant particles diameter of NPs-1 as statistical results is 51
inuence of solvents on photobleaching of our products can be nm. The sample for testing uorescence emission is 0.25 mg
ꢂ1
ignored.
mL . So the density of synthesised NPs in tested spectra solu-
1
2
To investigate the uorescent properties of dye-doped nano- tion is 1.736 ꢁ 10 particles per mL water. As for the free dye in
ꢂ7
particles, we further analysis the excitation uorescent spectra of concentration of 5.10 ꢁ 10 M, which concentration is the same
1
4
different samples. Fig. 4 a shows the normalized excitation as NPs-1. The dye concentration is 3.0702 ꢁ 10 dye molecules
ꢂ4
spectra of dimerization dye aqueous solution (1.3 ꢁ 10 M), per mL water. The uorescence spectra integral area of NPs-1 and
ꢂ5
7
6
monodisperse dye aqueous solution (1.3 ꢁ 10 M) and dye- free dye solution is 1.069 ꢁ 10 and 5.590 ꢁ 10 , respectively. So
doped NPs-6. From the observation of the spectra, we could the relative brightness of the rst group samples is 534 by
40

nd that when the concentration of the dye increased largely, the calculating with the equation below (eqn (1))
dimerization of the uorescent spectra could be detected. The
dimerization of dye cause different excitation peaks and spectra
shape compared with the diluted dye sample or dye-doped silica
nanoparticles. Fig. 4b–d shows the 3D uorescent spectra of the
solution just mentioned. It is obvious that the dimerization of
FLNPs=CNPs
NPs relative brightness ¼
ꢀ
(1)
FLfree dye
C
free dye
Based on this method, the relative brightness of the other group
the dye has the different luminescent centre by comparison with samples were calculated, they are 3185, 5420, 14 411, 41 529, and
diluted dye solution and silica nanoparticles. Fig. 4b and c both 86 972 respectively. It can be seen that the uorescence intensity
had only one uorescent centre, and they showed the location of and the relative brightness are improved by using our modied
centre as EX: 466 and EM: 592, EX: 466 and EM: 609 respectively. reversed microemulsion method to introduce ethanol in synthesis
While Fig. 4d had three centres, they were EX: 397 and EM: 609, process.
EX: 470 and EM: 609, EX: 480 and EM: 609. This result is
The increase of the relative brightness is more obvious accord
correlated with Fig. 4a, which further conrm that the dye in with the increase of dye concentration and particles size. Several
nanoparticle haven't been aggregated and dimerized. In other samples synthesized with the same amount of TEOS and different
words, the silica nanoparticles prevented dye from quenching by concentration of dye, as the particles become larger, the number of
aggregation or dimerization.
particles become less, while the amount of dye incorporated in
Fig. 4 (a) Normalized excitation spectra of NPs-6, free dye solution-6 and dimerization dyes. 3D spectra of NPs-6 (b), free dye solution 6 (c) and
dimerization dyes (d).
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introducing ethanol to reverse microemulsion system. The
relative uorescence brightness of nanoparticles obtained is
improved greatly (86 972) compared with free dye molecules in
aqueous solution. The nanoparticles have very small dimension
distribution and good dispersion. Moreover, their size is
controllable via changeable amount of ethanol. The photo-
stability of dyes is also improved when they are incorporated in
silica matrix.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant No. 21171064, 21371067, and
Fig. 5 The photobleaching of NPs and free dye solution.
21373095).
Table 2 The relative brightness and photobleaching comparison with
other materials
Notes and references
Sample
NPs-6
FMSNP
NanoOrigami
NP
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86 972
7%
670
5%
230
10%
72 431
20%
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3
4
5
each particle increase. Nevertheless, the excitation spectra and 3D
spectra of the dye did not quench in silica matrix. So the uores-
cence intensity and the relative brightness are increased.
We also studied the inuence of the silica matrix on the pho-
tostability of the dyes. As Fig. 5 shows above, we recorded the
emission spectra of free dyes and dye-doped NPs every 5 min
interval for a period of 75 min. The free dye solution-1 uores-
cence intensity decreases very sharply. The initial luminescence
intensity was decreased only by 5–10% aer 15 excitations for NPs-
4
6
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8
9
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to NPs-6. But the total decrease intensity of free dye solution-1 is
2
2%. Table 2 shows the comparison of relative brightness and
40,46,47
photobleaching data with other similar materials.
We also studied the effects of different solvent ratio on the
photobleaching under the same concentration of dye. The
photobleaching data of NPs-b6 is shown in Fig. S8.† The results
show that NPs-6b has similar photo beaching trend with NPs-1
1
1
1
(Fig. 5). Therefore, the inuence of solvent ratio on photo-
bleaching of our products can be ignored. Normally, photo-
bleaching is usually caused by the oxygen molecules both in air
and aqueous solution. The result of photobleaching is the
decrease of the uorescence intensity. Silica as a protective layer
can reduce the penetration of oxygen molecules. The dye
molecules are prevented from contacting with chemicals in the
surrounding environments aer encapsulated in the silica core,
so they are prevented from photobleaching. Thus the photo-
stability of dye will increase compared with free dye in solution.
Thus, we can conclude that dyes are more stable in matrix than
aqueous solution.
1
1
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