Colloidal stability and photophysical characteristics of luminesent silica nanoparticles modified with various nitrogen/oxygen-containing trialkoxysilanes

Article abstract of DOI:10.1134/S1070363216030245

Surface modification of the luminescent silica nanoparticles doped with Tb(III)–p-sulfonatothiacalix[ 4]arene complex was carried out using a series of nitrogen/oxygen-containing trialkoxysilanes. It was found that groups capable of nonspecific interactio

Full text of DOI:10.1134/S1070363216030245

ISSN 1070-3632, Russian Journal of General Chemistry, 2016, Vol. 86, No. 3, pp. 661–667. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © V.A. Burilov, A.T. Latypova, R.A. Safiullin, I.S. Antipin, 2016, published in Zhurnal Obshchei Khimii, 2016, Vol. 86, No. 3,
pp. 523–529.
To the 100th Anniversary of A.N. Pudovik
Colloidal Stability and Photophysical Characteristics
of Luminesent Silica Nanoparticles Modified
with Various Nitrogen/Oxygen-Containing Trialkoxysilanes
V. A. Burilova, A. T. Latypovaa, R. A. Safiullinc, and I. S. Antipina,b
a Kazan (Volga Region) Federal University, ul. Kremlevskaya 18, Kazan, Tatarstan, 420008 Russia
e-mail: ultrav@bk.ru
b Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center,
Russian Academy of Sciences, Kazan, Tatarstan, Russia
c Kazan National Research Technological University, Kazan, Tatarstan, 420015 Russia
Received January 25, 2016
Abstract—Surface modification of the luminescent silica nanoparticles doped with Tb(III)–p-sulfonatothia-
calix[4]arene complex was carried out using a series of nitrogen/oxygen-containing trialkoxysilanes. It was
found that groups capable of nonspecific interactions on the surface of the nanoparticles cause a significant
decrease in their colloidal stability. The chromophore moieties in the modifiers were found to quench the
luminescence of the nanoparticles. The surface modification of the nanoparticles is responsible for the change
in the mechanism of luminescence quenching in the presence of copper(II) ions due to decreased accessibility
of luminophores to the quencher.
Keywords: click reaction, azides, trialkoxysilanes, luminescence, silica nanoparticles, luminescence quenching
DOI: 10.1134/S1070363216030245
Over the past decade researchers have paid
increasing attention to luminescent silica nanoparticles
primarily because they offer great opportunities for use
in bioanalysis [1–3]. Sensor properties of nanoparticles
are manifested in the fluorescence response to the
interaction with substrates of a particular type. As
luminescent labels serve both organic fluorescent dyes
and lanthanide complexes, and the latter are
advantageous in terms of high signal-to-noise ratio
even in the presence of background luminescence
signals from proteins, of large Stokes shift, long
lifetimes of the excited state, narrow emission lines,
and possibility of emission in different regions of the
visible spectrum and in the near infrared region,
depending on the metal center selected [4, 5].
Encapsulation of lanthanide complexes into silica
matrix substantially increases the luminescence
intensity and also significantly mitigates the toxic
effects from lanthanide ions [6]. It is important to note
that silica matrix is fairly loose, which leaves open the
possibility of permeation of small ions and molecules
inside it. Therefore, in the interaction with d-metals as
quenchers both the dynamic (energy transfer) and
static (degradation of the luminescent complex via ion/
ligand exchange) mechanisms of luminescence
quenching can operate [7, 8]. The quenching
mechanism that will dominate depends significantly on
the distribution of the luminophore within the silica
matrix. For example, luminophores located close to the
nanoparticle shell undergo quenching by mixed
dynamic/static mechanism, and those concentrated
deep inside the matrix, predominantly by the static
mechanism [9]. We have previously shown [10] that
the nanoparticles doped by luminescent complex
Tb(III)–p-sulfonatothiacalyx[4]arene (Tb–TCAS) with
the surface modified by amino groups effectively bind
d-metal ions and thus are suitable for fluorometric
determination of copper(II) ions at concentrations
down to 0.1 μM. However, the influence of the
modifier structure on colloidal stability, photophysical
661

662
BURILOV et al.
Scheme 1.
EtO
OEt
H2N
OEt
Si
OEt
O
OEt
NEt3,
5 h
C6H6
BrCH2CCH, CaH2, THF
PhH2C
Br
N
N
N
O
EtO
OEt
EtO
OEt
PhCH2N3
OEt
EtO
EtO
N
Si
N
O
N
Si
MW 5 min, 100оС
THF, Et3N, CuBr(PPh3)3
OEt
OEt
OEt
N
N N
CH2Ph
3, 71%
1, 96%
2, 94%
O
EtO
OEt
THF, 18 h, 80оС
OEt
OCN
Si
R'
N
Si
EtO
R'NH2, R'OH
H
OEt
OEt
O
OH
O
*
(4, 95%),
(5, 75%).
R' = HN
N
characteristics, and ability of nanoparticles to parti-
cipate in metal ion binding still remains to be
elucidated. Therefore, the aim of this study was to
carry out modification of Tb–TCAS complex-doped
silica nanoparticles by organosilicon modifiers con-
taining donor atoms capable of coordination with d-
metal ions, as well as to examine the colloidal stability
and photophysical characteristics of the resulting
nanoparticles in the presence of some d-metals.
[6]. Covalent modification of the surface silanol
groups of the resulting nanoparticles was carried out
by the previously developed technique involving co-
polycondensation of trialkoxysilane with tetraethoxy-
silane in the presence of ammonia [10].
The modifier fixing on the nanoparticles surface
was confirmed by IR spectroscopy. For example, in the
case of modifier 3 comprising ester moieties the IR
spectrum of the modified particles (Fig. 1, spectra 3
and 4) contains intense bands of the C=O stretching
vibrations of the ester moieties at 1741 cm–1, as well as
intense bands at 2976 cm–1 and 2929 cm–1 associated
with the C–H asymmetric stretching vibrations of the
CH3 and CH2 groups in the ester moieties, respectively,
which were not observed for the unmodified
nanoparticles (Fig. 1, spectrum 1). Sample heating to
180°C improved the resolution due to the elimination
of encapsulated water, so that the bands corresponding
to vibrations of the C–H bond were more clearly
distinguishable (Fig. 1, spectrum 4).
For modification of the nanoparticles we used
derivatives prepared from commercially available 3-
aminopropyltriethoxysilane and 3-isocyanatopropyltri-
ethoxysilane by the atom-economical reaction of
copper-catalyzed azide-alkyne cycloaddition [11, 12]
to give the corresponding triazole 2, addition of
isocyanates to amines and alcohols to produce the
corresponding disubstituted urea 4 and urethane 5, and
the alkylation reaction to afford diester 3 (Scheme 1).
Silica nanoparticles doped with luminescent
complex Tb–TCAS were synthesized by the reverse
microemulsion procedure which has shown good
results in introduction of water-soluble luminophores
The size of the nanoparticles synthesized was
characterized by the dynamic light scattering method.
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COLLOIDAL STABILITY AND PHOTOPHYSICAL CHARACTERISTICS
As seen from the data obtained (see table), all the
663
I
modified nanoparticles, except for SiO2&Tb–
TCAS&Mod3, substantially exceed the unmodified
particles in size and polydispersity index, reaching
micrometer sizes in the case of SiO2&Tb–
TCAS&Mod4. The modification did not cause any
significant reduction in the particle surface charge (see
table), which allows the aggregation of the modified
nanoparticles to be associated with nonspecific
interactions.
The atomic force microscopic (AFM) data (Fig. 2)
agree well with the dynamic light scattering results. In
the case of the particles containing modifiers 2, 4, and
5, large particle sizes correspond to aggregates of
small nanoparticles. Especially highly aggregated are
the nanoparticles coated with modifier 4.
ν, cm–1
Fig. 1. IR spectra of (1) SiO2&Tb–TCAS nanoparticles,
(2) compound 3, and (3, 4) SiO2&Tb–TCAS&Mod3
nanoparticles, taken at (3) 25 and (4) 180°C.
Average hydrodynamic diameters (d), zeta potentials, and polydispersity indices of the nanoparticles in the presence of
Fe(III) and Cu(II) ionsa
FeCl3·6H2O
PDI
CuSO4·5H2O
PDI
System
d, nm
80±1
ζ, mV
–33.3
–32.7
–16.4
36.9
d, nm
ζ, mV
SiO2&Tb-TCAS
0.154
0.164
0.881
0.960
0.489
0.683
0.306
0.937
0.945
1.000
1.000
0.857
0.122
0.190
0.893
0.595
0.672
0.537
0.790
0.497
с(M) = 1 × 10–6 M
с(M) = 1 × 10–4 M
с(M) = 5 × 10–4 M
SiO2&Tb–TCAS&Mod2
с(M) = 1 × 10–6 M
с(M) = 1 × 10–4 M
с(M) = 5 × 10–4 M
SiO2&Tb-TCAS&Mod4
с(M) = 1 × 10–6 M
с(M) = 1 × 10–4 M
с(M) = 5 × 10–4 M
SiO2&Tb–TCAS&Mod3
с(M) = 1 × 10–6 M
с(M) = 1 × 10–4 M
с(M) = 5 × 10–4 M
SiO2&Tb–TCAS&Mod5
с(M) = 1 × 10–6 M
с(M) = 1 × 10–4 M
с(M) = 5 × 10–4 M
83±1
95±5
0.276
0.509
0.470
–20.8
1743±396
713±189
809±17
915±17
1424±111
843±164
1715±584
3536±1435
2527±1041
699±494
88±1
110±19
209±12
–19.0
–13.4
–30.6
–29.4
–2.1
1005±22
1998±150
1342±86
0.474
0.484
0.325
–22.4
–15.0
–12.3
35.3
–23.2
–23.3
–3.1
787±196
1541±561
623±251
0.847
0.975
0.543
–18.0
–13.5
–0.3
37.9
–24.9
–18.3
–2.7
71±1
74±1
80±4
0.226
0.268
0.575
–15.0
–9.3
–6.5
853±669
147±31
622±52
653±23
1420±665
103±50
36.5
715±26
–29.6
–28.8
–17.0
38.1
586±92
753±90
693±83
0.544
0.730
0.617
–26.6
–20.1
–12.8
a cparticles = 0.028 g/L, c(M) = 0.001–0.5 mM, pH = 7.4 (Tris).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 3 2016

664
BURILOV et al.
(a)
(bб)
б
1.0 μm
200 nm
1.0 μm
200 nm
в
(c)
1.0 μm
200 nm
г
(d)
200 nm
1.0 μm
д
(e)
400 nm
1.0 μm
Fig. 2. AFM images of (a) SiO2&Tb–TCAS, (b) SiO2&Tb–TCAS&Mod2, (c) SiO2&Tb–TCAS&Mod3, (d) SiO2&Tb–TCAS&Mod4,
and (e) SiO2&Tb–TCAS&Mod5 samples.
The modification significantly affects the photo-
physical properties of the nanoparticles (Fig. 3). For all
samples, excluding the SiO2&Tb–TCAS&Mod3 dis-
persions, no less than twofold decrease in the
luminescence intensity was observed. This is due both
to aggregation of the nanoparticles (the most highly
aggregated SiO2&Tb–TCAS&Mod4 and SiO2&Tb–
TCAS&Mod2 nanoparticles exhibited the lowest
luminescence intensity) and absorption of aromatic
chromophores of the modifiers near 330 nm, which is
the wavelength of excitation of the Tb–TCAS
complex.
which induce the fluorescence response in nano-
particles via metal binding [10]. To study in more
detail the luminescence quenching mechanism for
functionalized nanoparticles in the presence of Fe(III)
and Cu(II) ions, we carried out fluorescence titration
and measured the lifetimes of excited states.
For the unmodified particles in the presence of the
Fe(III) ions (Fig. 4a, spectra 5 and 5') the static
mechanism of the luminescence quenching dominates,
since the addition of excess Fe(III) ions cause only
insignificant changes in the lifetime of the excited
state. However, the dynamic mechanism also makes a
small contribution. Coating the nanoparticles with the
modifiers does not produce substantial changes, except
for a slight reduction of quenching in the case of
modifiers 2, 3, and 4. Study of the nanoparticles size
and surface charge in the presence of Fe(III) ions (see
As mentioned above, the addition of Cu(II) and
Fe(III) ions to aqueous dispersions of the Tb–TCAS
complex-doped nanoparticles results in luminescence
quenching, and this may be useful in fluorometric
determination of the metals as well as of substrates
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 3 2016

COLLOIDAL STABILITY AND PHOTOPHYSICAL CHARACTERISTICS
table) showed that the addition of Fe(III) ions
665
I
substantially affects both the colloidal stability and
surface charge of the nanoparticles. For example, in
the case of unmodified particles the addition of Fe(III)
ions at c 10–4 M leads to a significant decrease in the
surface charge of the nanoparticles and to a formation
of micron-sized aggregates, with an increase in the
amount of Fe(III) ions to c 5 × 10–4 M resulting in
surface recharging accompanied by a decrease in the
aggregate size. Thus, for the unmodified nanoparticles
both the ion exchange and aggregation of nanoparticles
are the key mechanisms of the luminescence quenching.
A similar trend is exhibited by the modified
nanoparticles. Specifically, the addition of Fe(III) ions
causes surface recharging, and when the isoelectric
point is attained the size of the aggregates is at a
maximum. In the case of the nanoparticles coated with
modifiers 2, 4, and 5 the aggregate size after surface
recharging is substantially smaller than that observed
initially.
450
500
550
600
650
λ, nm
Fig. 3. Emission spectra of the solutions of (1) SiO2&Tb–
TCAS, (2) SiO2&Tb–TCAS&Mod3, (3) SiO2&Tb–TCAS&Mod5,
(4) SiO2&Tb–TCAS&Mod4, and (5) SiO2&Tb–
TCAS&Mod2. cparticles = 0.028 g/L, pH = 7.4 (0.001 M
Tris), λexc = 330 nm.
In the presence of copper(II) ions the situation
significantly differs from that for the systems with
iron(III) ions. For example, for the unmodified
nanoparticles the dynamic quenching mechanism
dominates, with the titration curves for the lumine-
scence intensity and lifetime exhibiting similar trends
(Fig. 4b, curves 5 and 5'). This is possibly due to the
luminophore distribution both inside the nanoparticles
and close to the shell. Specifically, luminophores
located near the nanoparticle shell undergo quenching
primarily via energy transfer due to proximity to the
quencher, and luminophores located deeper inside the
nanoparticles, only via ion exchange by the static
mechanism at a quencher ion concentration sufficiently
high for diffusion. The modified particles exhibit
higher I/I0 ratios (Fig. 4b, curves 1–4), while the t/t0
ratios change insignificantly, which indicates a large
contribution from the static mechanism. A good reason
thereto is an increase in the shell thickness via
modification of the nanoparticles, whereby the latter
take the form of a core with the luminophore
(a)
(b)
I/I0, t/t0
I/I0, t/t0
с(Fe3+), mM
с(Cu2+), mM
Fig. 4. Variation in the I/I0 and t/t0 (apostrophized) ratios at 547 nm for the dispersions of (1) SiO2&Tb–TCAS&Mod2,
(2) SiO2&Tb–TCAS&Mod4, (3) SiO2&Tb–TCAS&Mod3, (4) SiO2&Tb–TCAS&Mod5, and (5) SiO2&Tb–TCAS nanoparticles
with (a) Cu(II) and (b) Fe(III) ion concentration.
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BURILOV et al.
concentrated deep inside the nanoparticles, surrounded
by a thick outer shell which contains no luminophore.
As a result, quenching occurs only via permeation of
the copper ions which replace the terbium ions, and the
energy transfer to the quencher ions is not so
significant.
The atomic force microscopy measurements were
conducted on a Multimode V (Veeco, USA) micro-
scope in the contact mode with an RTESP (Veeco)
cantilever. Mica surface was coated by 20 μL of an
aqueous dispersion of the nanoparticles with the
concentration of 0.0028 g/L and then dried at 80°C for
3 h. The AFM images were obtained in the Laboratory
of Spectroscopy, Microscopy, and Thermal Analysis,
Kazan National Research Technological University.
Thus, silica nanoparticles modified with nitrogen/
oxygen-containing moieties were obtained. It was
shown that the introduction of modifiers comprising
chromophore groups, in particular triazole moieties,
absorbing in the near UV region results in substantial
reduction of the luminescence intensity of the nano-
particles. At covalent modification of the nano-
particles, due to an increase in the outer shell thick-
ness, the luminescence quenching in the presence of
Cu(II) ions occurs by predominantly static mechanism.
Silica nanoparticles doped with Tb(III)–p-sulfo-
natothiacalyx[4]arene complex were synthesized by
the method of reverse microemulsion [6]. Covalent
modification of the nanoparticles surface was per-
formed by the known procedure from [10].
N,N-Bis(2-ethoxy-2-oxoethyl)-N-[3-(triethoxysilyl)-
propyl]amine (3). To a solution of 25.7 g (154 mmol)
of ethyl bromoacetate in 35 mL of benzene a mixture
of 9.5 g (43 mmol) of 3-aminopropyltriethoxysilane
and 15.6 g (154 mmol) of triethylamine in 50 mL of
benzene was added dropwise at stirring. The reaction
mixture was refluxed using a Dimroth condenser for
5 h under an inert atmosphere. After cooling the
precipitated ammonium salt was filtered off on a frit
glass filter and washed with benzene. The resulting
filtrate was concentrated. Yield 11.9 g (71%), yellow
viscous oily substance. IR spectrum, ν, cm–1: 1080 s
(Si–O), 1741 s (C=O), 2929 m (CH2), 2976 s (CH3).
1H NMR spectrum (400 MHz, 25°C), δ, ppm: 0.55–
0.64 m [2H, (SiCH2CH2CH2NH)], 1.18 t (9H,
EXPERIMENTAL
The solvents were purified before use by the
procedures described in [13]. Known techniques were
employed for the syntheses of compounds 1 [14], 2
[14], 4 [15], 5 [16], and p-sulfonatothiacalix[4]arene
[17]. The chemicals used were commercially available
from “Alfa Aesar,” “Acros,” and “Lancaster.” All
physicochemical measurements were performed using
bidistilled water. All samples were preliminarily
maintained for 30 min in an ultrasonic bath at 25°C.
Elemental analysis was carried out on a Perkin
Elmer PE 2400 series II CHNS/O analyzer. The NMR
spectra were recorded on a Bruker Avance 400
Nanobay spectrometer in CDCl3. Mass spectra were
obtained on a Shimadzu GCMS 2010 Ultra gas
chromatograph-mass spectrometer with an HP-5MS
column. The IR spectra of the samples (suspensions in
mineral oil or KBr pellets) were recorded on a Bruker
Vector-22 spectrometer. Dynamic light scattering and
zeta potential measurements were performed on a
Malvern Zetasizer instrument at 25°C using poly-
styrene cells (l = 10 mm). Luminescence spectra were
recorded on a Fluorolog FL-221 (HORIBA Jobin
Yvon) spectrofluorometer in the 450–650 nm range at
the excitation wavelength of 330 nm.
3
OCH2CH3, JHH = 7.0 Hz), 1.21–1.26 m (6H,
COOCH2CH3), 1.51–1.62 m (2H, SiCH2CH2CH2NH),
2.65–2.70 m (2H, SiCH2CH2CH2NH), 3.51 s (4H,
NHCH2), 3.74–3.81 m (6H, OCH2CH3), 4.09–4.17 m
(4H, COOCH2CH3). Mass-spectrum, m/z 394 [M]+.
Found, %: C 50.51; H 8.85; N 3.38; Si 7.33. C17H35NO7Si.
Calculated, %: C 50.64; H 8.76; N 3.69; Si 7.40.
ACKNOWLEDGMENTS
This study was financially supported by the Russian
Foundation for Basic Research (project no. 14-03-
31235).
The excited-state lifetimes were measured on an
FL-1042 accessory for the spectrofluorometer using a
xenon flash lamp with the following parameters: time
per flash 50.00 ms, flash count 200 ms, initial delay
0.05 ms, and sample window 2 ms. Excitation of the
sample was performed at 330 nm, and emission was
detected at 546 nm.
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