Multifunctional polyoxometalates-modified upconversion nanoparticles: Integration of electrochromic devices and antioxidants detection

Article abstract of DOI:10.1039/c2cc38292j

We report a novel design, based on a combination of lanthanide-doped upconversion nanoparticles and polyoxometalates, for electrically controlled fluorescence switches and sensitive detection of antioxidants in aqueous solution.

Full text of DOI:10.1039/c2cc38292j

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ARTICLE TYPE
Multifunctional Polyoxometalates-Modified Upconversion
Nanoparticles: Integration of Electrochromic Devices and Antioxidants
Detection†
Yanling Zhai, Chengzhou Zhu, Jiangtao Ren, Erkang Wang, and Shaojun Dong*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
or optical sensors.6 Until now, a large number of electricityꢀ
We report a novel design, based on a combination of
dependent luminescenceꢀcontrollable systems have been designed
50 and studied.7 However, no examples of upconversion
luminescence modulations were carried out. To better exert the
advantages of upconversion materials, upconversion fluorescence
stimuliꢀresponsive systems need to be further exploited.
Therefore, the coreꢀshell structured hybrid nanocomposites
55 combining the electricityꢀresponsive POMs and UCNPs into one
unit is predicted to be attractive candidates in fields of electronic
devices, detection, and cell imaging.
lanthanide-doped
polyoxometalates, for electrically controlled fluorescence
10 switches and sensitive detection of antioxidants in aqueous
solution.
upconversion
nanoparticles
and
Recently, lanthanide ion (Ln3+, such as Er3+, Tm3+, Ho3+)ꢀdoped
upconversion nanoparticles (UCNPs), which can emit higherꢀ
energy visible photons after being excited by lowerꢀenergy nearꢀ
15 infrared (NIR) photons via
a twoꢀphoton or multiphoton
mechanism, have attracted a tremendous amount of attention.1
They have become promising alternatives to organic fluorophores
and quantum dots. Compared with conventional downꢀconversion
fluorescent labels, which require ultraviolet or blue excitation
20 wavelength, upconversion fluorescent materials have many
conceivable advantages, including higher photostability and
thermal stability, very weak autoꢀfluorescence background,
greater tissue penetration, and high signalꢀtoꢀnoise ratio. Various
host materials such as fluorides, oxides, chlorides, bromides,
25 oxysulfides, and oxyhalides were selected to synthesize the
lanthanideꢀdoped nanomaterials with high upconversion
efficiency and controllable emission profile.1 Among them,
NaYF4 with low phonon energy has been demonstrated to be the
most efficient host material.2 Therefore, the design and
30 development of new materials functionalized with upconversion
luminescence property is undoubtedly of great importance in
future.
Fig.
1 (A) Schematic illustration of the synthesis and surface
60 modification of OAꢀstabilized upconversion nanoparticles (UCNPs);
TEM (B) and highꢀresolution TEM (C) images of UCNPs. The insets
show the corresponding highꢀresolution TEM images and arrows
highlight d spacing.
Electrochemical devices are under intense study for electrical
energy storage, transduction and information processing, and
35 many
electrochromic
systems
involving
switchable
Herein, we present the first example of reversible
65 upconversion fluorescence switching system, and the strategy
is schematically illustrated in Fig. 1A. Firstly, highly
monodisperse UCNPs (Fig. 1B) in organic solvents were
synthesized according to a reported method.8 The highꢀ
resolution TEM images revealed uniform hexagonalꢀphase
70 UCNPs with size of (20±0.5) nm in diameter (Fig. 1C), and
lattice fringes with a spacing of about 0.52 nm were highly
consistent with (100) phases of the hexagonal NaYbF4
crystalline structures. The oleic acid (OA)ꢀdoped UCNPs were
then transformed to hydrophilic nanoparticles by silica layer
75 capped (Fig. S1A), and the peak at 1088 cmꢀ1 in FTIR spectra
is attributed to the vibration bands of SiꢀOꢀSi (Fig. S1B).5
chromophores included, or grafted on polymers have been
already described.3 In terms of possible components for
electrochromic devices, polyoxometalates (POMs) are promising
candidates due to their ability to act as an electron reservoir,4 and
40 some luminescence devices based on POMs have also been
fabricated because of an extensive range of structures and stable
redox states.5 Spectroelectrochemistry, the combination of
electrochemistry and spectroscopy, has been proved to be an
effective approach for studying the redox chemistry of inorganic,
45 organic, and biological molecules, and has attracted much
scientific and technical interest because of the possible
applications like electrochromic displays, electroꢀoptical switches
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After silica coating, the nanocrystals were dispersible in water
with good chemical and photochemical stability, and a clear
colloidal solution was formed. The SiO2 shell was also used to
process. Because the absorption band of reduced NaꢀPOMs
(500ꢀ800 nm) overlaps greatly with the emission band of
UCNPs (not shown here), fluorescence quenching could
passivate the surfaces of UCNPs.9 Finally, the prepared Naꢀ 50 happen via either inner filter effect (IFE) or energy transfer
5
POMs layers were formed on the surface of the synthesized
UCNPs@SiO2 nanoparticles via electrostatic interaction.10 By
comparison, hydrophilic UCNPs were also synthesized
through a facile hydrothermal method.2 The asꢀsynthesized
effect.13 When UCNPs were excited, the emission travelled
through the hybrids film, and the emitted light located at 523,
541 and 653 nm would be absorbed severely via the
corresponding quenching mechanism12, 14, resulting in the
UCNPs consist of spherical microspheres with an average 55 light reached to the detector rarely. But the above quenching
10 diameter of 150 nm (Fig. S2A, B). However, the luminescence
intensity of hydrophilic UCNPs (Fig. S2C) was much weaker
than that of OAꢀUCNPs (Fig. S2D), which is not favorable for
further use. Therefore, OAꢀUCNPs are employed in our study.
process can be reversed easily under electrical stimulus. Fig.
3B showed the electricalꢀcontrolled “on” and “off”
luminescence switching of UCNPs@NaꢀPOMs nanostructure.
In oxidized state I (black line), UCNPs could emit its
60 characteristic fluorescence. After a potential of ꢀ0.7 V was
applied, the detected emission intensity of the three peaks
from asꢀprepared multilayer structure were all rather weak
(blue line), which was assigned to the upconversion
fluorescence emission was overwhelmingly absorbed by
65 reduced NaꢀPOMs. When an oxidation potential 0 V was
further applied, the fluorescence was recovered to its original
state completely (red line), because the fluorescence
quenching mechanism was insufficient. The contrast
luminescence ratio of the system at ꢀ0.7 and 0 V was
70 calculated to be about 0.8. Moreover, the fluorescence
switching operation was quite reversible, and can be repeated
several times without noticeable fluorescence changes
between “on” and “off” states (inset in Fig. 3B).
15
Fig. 2 Spooled fluorescence spectra of hybrid film during the applied
potential from 0.1 to ꢀ0.9 V.
Electrochemical techniques were used to characterize
UCNPs@NaꢀPOMs hybrids films. The cyclic voltammograms
20 of ITO/PAH/UCNPs@NaꢀPOMs/PDDA electrode at different
scan rates indicated the redox reaction of NaꢀPOMs belongs to
a surface confined process (Fig. S3A),11 and the hybrids films
possess good stability in aqueous solution while no sign of
losing redox current intensity was observed upon performing
25 100 consecutive potential cycles (Fig. S3B). All these results
suggested the great potential of NaꢀPOMs to construct
reversible
and
extremely
stable
switches.
The
spectroelectrochemical experiments were all conducted in a
spectroelectrochemical cell shown in Fig. S4. The effect of
30 applied potentials on the upconversion fluorescence
quenching was investigated (Fig. 2). It was found that
fluorescence emission of UCNPs decreased slowly with
potential decreasing from 0.1 to ꢀ0.2 V, and then decreased
rapidly with potential decreasing from ꢀ0.2 to ꢀ0.7. However,
35 as the applied potential became even more negative, the
luminescence intensity basically remained unchanged. The
variations of three characteristic peaks all showed the same
tendency (inset in Fig. 2).
The mechanism of upconversion fluorescence excitation
40 and quenching was given in Fig. 3A. Firstly, under the 980
nm NIR light, double band at 523 and 541 nm, and single
emission band at 653 nm were observed, which can be
2
4
assigned to H11/2→4I15/2
,
4S3/2→4I15/2
,
and F9/2→4I15/2
transitions of Er3+, respectively.12 In the present system,
45 potentialꢀinduced absorbance change of NaꢀPOMs was
considered to be a key factor for the luminescence switching
75 Fig.
3 (A) Schematic energy level diagrams and fluorescence
quenching progress in the UCNPs@NaꢀPOMs systems; (B)
Fluorescence switching of hybrid film from 0 V (black line), to
applied potential of ꢀ0.7 V (blue line), and to applied potential of 0 V
2
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(red line); and (inset) reversible fluorescence (541 nm) on–off cycles
under alternating applied potential with ꢀ0.7 V (bottom lines) and 0 V
(top lines).
973 Project 2010CB933603.
Antioxidants play a central role in cellular defense against
toxins and free radicals, and their levels are implicated in many
diseases.15 Thus, it is important to be able to monitor the change
of antioxidants concentration in real time. Herein, the developed
UCNPs@NaꢀPOMs can also be used for sensing antixidants, such
as BAS, glucose, glutathione (GSH), and so on. We employed
5
45 Notes and references
State Key Laboratory of Electroanalytical Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate
School of the Chinese Academy of Sciences, Changchun 130022, Jilin, PR
China. E-mail: dongsj@ciac.jl.cn
50 †Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/b000000x/
10 GSH as a model molecule and detect it in aqueous solution. GSH
can be oxidized by NaꢀPOMsOx, generating glutathione disulfide
(GSSG) based on the equation,
1
Q. Liu, Y. Sun, T. Yang, W. Feng, C. Li, F. Li, J. Am. Chem. Soc.
2011, 133, 17122.
NaꢀPOMsOx + GSH →NaꢀPOMsRe + GSSG
55
60
65
70
75
80
2
3
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As anticipated, upconversion luminescence was gradually
15 decreased with increasing GSH concentration (Fig. 4). Because of
the low background signal by NIR excitation, this method has a
low detection limit of 0.02 µM, and a good linear relationship
from 5 to 50 µM. In addition, a sensitive detection towards
glucose was also given in Fig. S5, the detection limit was 0.01
20 mM, and a linear relationship from 1 to 20 mM was obtained.
This investigation revealed universal antioxidants can be
sensitively detected in this system.
4
5
6
7
8
9
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Fig.
4
Photoluminescence
response
of
POMꢀmodified
25 NaYF4:Yb/Er@SiO2 as a function of GSH concentration (0, 5, 15, 25, 40,
50, 75, 100 ꢁM) in an aqueous solution.
90
In
summary,
a
UCNPsꢀbased
electricityꢀstimuli
fluorescence switch device has been developed. Under
electricity trigger, NaꢀPOMs transformed between oxidation
30 and reduction states, resulting in reversible and high contrast
upconversion fluorescence switching between “on” and “off”.
Furthermore, by taking advantage of the highly sensitive
redox reaction of NaꢀPOMs and the nonautofluorescent assays
offered by the UCNPs, we have demonstrated the monitoring
35 of antixidants levels in aqueous solution. These findings are
important not only for enabling promising applications of
UCNPs in the photoelectric field but also for providing a
sensor platform useful for detection of antioxidants in living
cells.
40
This work was supported by the National Natural Science
Foundation of China (No. 20935003, 20820102037) and the
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