Two-photon absorption enhancement induced by aggregation with accurate photophysical data: Spontaneous accumulation of dye in silica nanoparticles

Article abstract of DOI:10.1039/b918841j

The accurate photophysical data of dye-concentrated nanoparticles have been measured by the strategy of constructing ethoxysilane hydrolysis-assisted nanomaterials, which give rise to a large enhancement in two-photon absorption.

Full text of DOI:10.1039/b918841j

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Two-photon absorption enhancement induced by aggregation with
accurate photophysical data: spontaneous accumulation of dye
in silica nanoparticlesw
Lin Li,a Yupeng Tian,*ab Jiaxiang Yang,*ab Pingping Sun,a Lin Kong,a Jieying Wu,a
Hongping Zhou,a Shengyi Zhang,a Baokang Jin,a Xutang Taob and Minhua Jiangb
Received (in Cambridge, UK) 11th September 2009, Accepted 11th December 2009
First published as an Advance Article on the web 11th January 2010
DOI: 10.1039/b918841j
The accurate photophysical data of dye-concentrated nano-
particles have been measured by the strategy of constructing
ethoxysilane hydrolysis-assisted nanomaterials, which give rise
to a large enhancement in two-photon absorption.
data of DCNs cannot be exactly measured due to the
complicated components and dubious molecular weight. In
this communication, we devised a novel strategy to construct
ethoxysilane hydrolysis-assisted DCNs, which have determi-
nate photophysical data and aggregation-induced enhance-
ment of TPA (Scheme 1).
In the past decades, researchers have been more interested in
seeking novel two-photon absorption (TPA) materials for
their promising applications in optical limiting, two-photon
laser scanning fluorescence imaging, microfabrication, 3D
optical data storage, photodynamic therapy and so on.1 They
have done substantial research on structure–property relation-
ships. Although it is still under exploration, thanks to the
effort of scientists, some efficient molecular design strategies
were put forward to provide guidelines for the development of
materials with large TPA cross-sections.2 Scaling down these
materials has recently afforded an exciting new class of highly
tailorable nanomaterials known as dye-concentrated nano-
particles (DCNs), which exhibit enhanced fluorescence and
TPA activity by aggregation.
The functional precursor was synthesized by covalently
linking a TPA dye (a pyrimidine derivative) to 3-isocyanato-
propyltriethoxysilane (ICTES) and was characterized by MS,
NMR and IR (see ESIw). Pyrimidine derivatives exhibit
intense single- and two-photon excited fluorescence,6 and
can easily undergo reactions with ICTES. The synthesis of
the DCNs was performed in an inverse micelles solvent
mixture (THF/1-butanol/water) in the presence of a base
(NaOH). The template-directed synthesis followed a procedure
similar to that used by Moreau et al.,7 and differed from that
used by Prasad and Blanchard-Desce et al.8 since tetraethyl-
oxyl orthosilicate (TEOS) or any other co-polymerized silanes
were left out. After hydrolysis, the TEM (Fig. 1(a)) analysis
revealed that the resulting solid particles were relatively
uniform spheres with diameters of B5 mm. The magnified
SEM image exclusively shows that an individual microsphere
is composed of many discrete particles with B100 nm
diameters (Fig. 1(b)). The XRD diffractogram of DCNs is
reproduced in Fig. 1(c). The hybrid materials are totally
amorphous in the range of 101 r n r 801. Broad signals are
observed and it is well known that these broad signals
correspond to the existence of a short-range order in the
material.9 The broad peaks centered at 23.001 were ascribed
to the coherent diffraction of the siliceous backbone of the
hybrids.10 The absence of any crystalline regions in these
samples correlates well with the presence of organic chains
in the host inorganic framework.
Compared with purely inorganic or organic nanomaterials,
new phenomenon can arise in DCNs by the careful selection of
organic building blocks and particle species. Cohanoschi
et al.3a observed strong surface plasmon enhancement of
TPA of chromophores in solution containing gold colloid.
Marder and co-workers3b,c also observed strong enhancement
of the TPA of organic molecules near silver nanoparticle
fractal clusters, and this enhancement effect was manifested
in the composite materials with very strong TPA. Prasad
and co-workers4 reported a novel class of dye-concentrated
composite nanoparticles with enhanced TPA by nanoaggregation.
Our previous work5 found enhanced TPA by combination
of the large organic salt with CdS nanoclusters. This field
has been growing rapidly during the past ten years due to
the realization of the two-photon-based technologies as
mentioned above and improved methods of characterization.
All these interesting results are of scientific significance and
technical potential. However, up till to now, the photophysical
a Department of Chemistry, Anhui University, Hefei, 230039,
P. R. China. E-mail: yptian@ahu.edu.cn; Fax: +(86) 551-5107304;
Tel: +(86) 551-5108151
b State Key Laboratory of Crystal Materials, Shandong University,
250100, Jinan, P. R. China
w Electronic supplementary information (ESI) available: Synthesis
procedures and characterization of dye, details of optical spectra.
CCDC 752289. For ESI and crystallographic data in CIF or other
electronic format, see DOI: 10.1039/b918841j
Scheme 1 Schematic representation of the preparation progress of
DCNs.
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Fig. 1 As-synthesized DCNs: (a) TEM image (scale bar = 10 mm); (b) SEM image (scale bar = 2 mm). (c) The XRD diffractogram of the DCNs.
Subsequently, the dye and DCNs were characterized by
optical absorption and emission spectroscopy. Steady-state
absorption and emission spectra of dilute suspensions
(1.0 Â 10À6 mol LÀ1, based on Mol. Wt. of monomer) of
the particles in DMF (Fig. 2(a)) reveal the presence of the
covalently linking TPA dye inside or outside the particles: the
typical dye-related one-photon absorption (lmax E 440 nm)
and emission bands (lmax E 560 nm) can be distinguished.
The extensive extraction applied, ensures the removal of free
dye. No obvious light scattering effect has been observed from
the one-photon absorption spectrum of DCNs, but the
absorption of the DCNs is slightly blue-shifted compared to
the free dye in DMF. More importantly, the emission
bandwidth of DCNs is dramatically narrower than that of
the fluorescence band of free dye. Both the dye and DCNs
show sensitivity to the solvent and exhibit moderate quantum
yields, the detailed linear photophysical data can be found in
the ESIw. All these provide evidence that immobilization of
dyes via ethoxysilane hydrolysis-assisted aggregation is a valid
strategy and it shows only a minor influence on the linear
optical properties of the dye, which is very important for
optical applications.1f
the calculation of life time, the longer decay components of
DCNs are more than those of the free dye. In combination
with the slightly higher fluorescence quantum yields of the free
dye, this reveals that an efficient interaction occurs in DCNs.
Such a feature further confirms the efficient immobilization of
the dyes on the silica mesoporous structures.
As shown in the ESIw, the linear dependence on the square
of the input laser power of the free dye suggests that it is a
two-photon excitation mechanism obviously. The TPA
spectra of the free dye and DCNs were determined in detail
in the wavelength range (from 720 to 860 nm) by investigating
their two-photon-excited fluorescence in DMF with
a
concentration of 1.0 Â 10À3 mol LÀ1 (based on Mol. Wt. of
monomer), and are shown in Fig. 2(b). The TPA cross sections
(d) were determined by comparing their two-photon excited
fluorescence to that of fluorescein in water (pH = 11).11
It is worthwhile noting that at the wavelength range
(720–860 nm) d of the free dye is negligible in the absence of
DCNs but is strongly enhanced in DCNs (d = 284 GM,
1 GM = 10À50 cm4 s photonÀ1), thus offering a 41400-fold
increase, which seems slightly different from Marder’s discussion
of nanoparticles as a carrier for large numbers of dyes.3b It is
worth underlining that this value exceeds that of many
fluorophores widely used in biology, including fluorescein,
BODIPY, DAPI and GFP.12,13 The spectra of DCNs display
one excitation peak, and this feature is similar to that observed
in the linear absorption spectra, except that the optimal
excitation wavelengths (830 nm) are roughly doubled.
In time-correlated single-photon counting measurements,
the fluorescence of the dye and DCNs both show nonmono-
exponential decay. The decay curves can generally be
adequately described by the sum of two exponentials, a shorter
lifetime (1.57 ns for free dye and 0.86 ns for DCNs,
respectively) and a longer lifetime (2.60 ns for free dye and
2.52 ns for DCNs, respectively). Fractional intensities of the
longer decay components are 44.5% for the free dye and
65.5% for DCNs. Also, fractional amplitudes of the longer
decay components are 32.6% for the free dye and 38.7% for
DCNs. Obviously, for either intensity or amplitude based on
In conclusion, we have demonstrated that ethoxysilane
hydrolysis-assisted DCNs result not only in a significant
increase in the TPA cross sections but also a solid holding of
the linear optical properties of the dye, which demonstrates the
utility of using the silica particles as a tool for concentrating
Fig. 2 (a) Normalized UV-vis absorption and PL spectra of dye and DCNs in DMF. The fluorescence was excited at 445 nm for the dye and
431 nm for DCNs, respectively; (b) TPA cross section of dye and DCNs in DMF versus excitation wavelengths from 720 nm to 860 nm in DMF.
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dyes. This approach is not limited to the design of more
efficient TPA materials but should be applicable to a broad
range of modified fluorophores widely used in biology.
This work was supported by a grant for the National
Natural Science Foundation of China (20771001, 50873001,
20875001, 50703001,20775001 and 50532030), the Team for
Scientific Innovation Foundation of Anhui Province
(2006KJ007TD), and the Natural Science Foundation of
Anhui Province (070414188).
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