Fluorescent nanoparticles assembled from a poly(ionic liquid) for selective sensing of copper ions

Article abstract of DOI:10.1039/c0cc03900d

Fluorescent nanoparticles were formed from a poly(ionic liquid) through ion interactions. The fluorescent nanoparticles show highly fluorescent intensity and stability to UV light irradiation and were utilized for highly sensitive and selectivity fluoresc

Full text of DOI:10.1039/c0cc03900d

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Fluorescent nanoparticles assembled from a poly(ionic liquid) for
selective sensing of copper ionsw
Kun Cui, Xuemin Lu,* Wei Cui, Jun Wu, Xumeng Chen and Qinghua Lu*
Received 16th September 2010, Accepted 18th October 2010
DOI: 10.1039/c0cc03900d
Fluorescent nanoparticles were formed from a poly(ionic liquid)
through ion interactions. The fluorescent nanoparticles show
highly fluorescent intensity and stability to UV light irradiation
and were utilized for highly sensitive and selectivity fluorescent
sensor of copper ion.
complexes of poly(ionic liquids) (PIL) with organic FL molecules.
There are no nanoparticles formed when pure PIL and FL
molecules form these complexes. However, with the aid of a
second anionic molecule, FNPs were formed due to the
hydrophobic balance between the PIL, FL molecule and the
secondary anion (Fig. 1). The formation of the FNPs not only
leads to enhancement of the FL intensity, but also retards
photo-bleaching. Furthermore, the enhancement of the FL
intensity can be further tuned by changing the structure of the
secondary anion. Due to the formation of the ionic complex
between the FL molecules and the PIL chains, the FNPs show
high thermal stability and durability. Such FNPs show high
sensitivity for different transition metal ions, in particular
showing high selectivity for copper ions.
Transition metal ions are present as micronutrients in most
organs and are essential for sustaining all forms of life.1
However, if unregulated, they also can cause or exacerbate
oxidative stress and damage events connected with aging and
disease. Detecting these transition metal ions becomes an
increasing concern due to their effect on human health and
the environment.2 Molecular fluorescence (FL) technology,
due to its high sensitivity and simplicity, has been widely used
for environmental probes,3 optical sensors4 and biological
images.5 For optimum sensing of transition metal ions, FL
probes should be highly luminescent, water dispersible, stable
in the environment and resistant to photo-bleaching.6 Most
importantly, the probes should be selective for a specific metal
ion. Generally, fluorophores are designed into organic FL
molecules,7,8 conjugated polymers,9 chromphore-functionalized
noble-metal nanoparticles10 and so on. Of these molecules,
organic FL molecules have been considered to be most
suitable because of their high quantum yield. However, most
organic fluorescent molecules are hydrophobic and can be
rapidly photo-bleached in the presence of oxygen,11 which
significantly restricts their application. Introducing polar
groups to the FL molecules can be used effectively to improve
the dispersion of organic FL molecules in aqueous media, but
chemical modification has often resulted in a decrease of the
quantum yield.12 In addition, encapsulation of organic FL
molecules inside silica nanoparticles,13 polymer nano-
particles14 or micelles15 formed from block copolymers have
also received increasing attention due to their high efficiency in
improving the dispersion and emission of fluorescent dyes.
However, complicated synthetic procedures and leakage of FL
molecules in these host systems due to the lack of retaining
force between FL molecules and the nanoparticles or micelles,
still provide many challenges.
A PIL was selected as the polymer backbone because of
their high thermal stability and favourable self-assembly
properties.
A 2-(4-amino-2-hydroxyphenyl) benzothiazole
derivative (AHBTA) was selected as the organic FL molecule
and was ionically attached to PIL backbone. Azo-compounds
bearing different groups were selected as the secondary ions.
The complex, obtained by freeze-drying FNPs in aqueous
solution, shows high thermal stability up to 210 1C according
to thermogravimetric analysis (see ESI, Fig. S2w).
The UV spectrum of the AHBTA molecules shows a high
sensitivity to the polarity of the solution. Beside two intrinsic
absorption peaks at 299 nm (corresponding to the azole
chromophore) and 340 nm (corresponding to the p–p* transition
of the molecular skeleton) in various solutions, a new peak at
389 nm also appeared when it was dissolved in polar solvents
such as DMF and water. The peak comes from the dissociation
of Ar–OH and its intensity increases with the increase of
solvent polarity. Moreover, it is further promoted when
AHBTA formed a complex with PIL (as shown in Fig. 2a)
due to the stronger interaction between the two polar ions. It
should be pointed out here that no particle was formed in this
case, according to the results from scanning electronic
microscopy (SEM) and dynamic light scattering (DLS).
We present in this report a novel and facile strategy for the
preparation of fluorescent nanoparticles (FNPs) based on
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong
University, Shanghai, China 200240. E-mail: qhlu@sjtu.edu.cn,
xueminlu@sjtu.edu.cn; Fax: +86-21-54747535
w Electronic supplementary information (ESI) available: Detail
experimental procedures and characterisation data; IR and TGA
spectra, maximum of FL intensity points of FNPs-CF3 with different
days, SEM of FNPs-CF3 and FL sprectrum of FNPs obtained
by perfluorododecanoic acid as the secondary anion. See DOI:
10.1039/c0cc03900d
Fig. 1 Schematic representation of the formation of the fluorescent
nanoparticles.
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Fig. 2 (a) UV-vis spectra of AHBTA, AHBTA/PIL, FNPs-MeO,
FNPs-CF3 in aqueous solution. Here FNPs-MeO and FNPs-CF3
mean the fluorescent complex nanoparticles formed by PIL, AHBTA
and the azo-component. (b) SEM image of FNPs-MeO (obtained by
freeze-drying) The molar ratio in the complex is PIL/AHBTA/
azo-complex 10 : 1 : 1.
Fig. 3 FL emission spectra (a) AHBTA and its complex with and
without azo-compounds with different bearing groups ([azo-component] =
50 mM), inset: optical fluorescent photo. (b) Complex with different
concentrations of AzoMeO (lex = 389 nm, [AHBTA] = 50 mM,
[PIL] = 500 mM), their diameters were given by DLS.
AHBTA aqueous solution, relatively weak FL emission is
observed when excited with 389 nm light. After the complex
reaction between AHBTA and PIL molecules, the FL emission
at 450 nm becomes stronger. This can be ascribed to two
factors: (1) the increase of the number of Ar–OÀ groups
caused by dissociation of Ar–OH by the positively charged
PIL;17 (2) the formation of a relatively hydrophobic local
environment made up of AHBTA molecules surrounded by
PIL molecular chains. Subsequently, adding the secondary
anion led to a different FL emission behavior depending on the
chemical structure of the bearing group in the secondary
anion. For AzoNMe2, quenching of the FL emission occurred
due to the hydrophilic property of the NMe2 group. In
contrast, for MeO and CF3 as bearing groups, FNPs were
formed with obvious enhancement of the FL emission. With
the increase of the hydrophobic property of the secondary
anions, the enhancement of the FL emission was more
obvious. The FL intensity for FNPs-CF3 increases as much
as 8-fold compared with that of the pure AHBTA molecules.
We also investigated the effect of the main chain of the added
anions on the FL emission of AHBTA molecules. Anions
without an azobenzene group were used and similar results
were found. When the secondary anion was hydrophobic, the
FL intensity became stronger after the FNPs were formed
(see ESI, Fig. S5w).
After the second anion had been added, the peak at 389 nm
became stronger and the complex behaviour was also affected
by the chemical structure of the secondary anions (Fig. 2a).
With MeO or CF3 as the bearing group, the peak at 340 nm
completely disappeared and the peak at 389 nm became very
strong. This change in the UV spectrum of the complex can be
attributed to the variation of the local environment of the
AHBTA molecules in the complex system. After the secondary
anion had been added, nanoparticles were formed due to the
balance between the hydrophobic and hydrophilic interactions.
(Fig. 2b) here the hydrophobic part arises mainly from the
secondary anions and the hydrophilic part mainly from the
non-complex part of PIL. The AHBTA molecules still appear
somewhat hydrophobic though they were unable to form
nanoparticles with PIL. However, the hydrophobic nature of
the AHBTA molecules is very weak so that it cannot be mixed
completely with the secondary anion to form a hydrophobic
core. This means that the AHBTA molecules were most
probably assembled between the hydrophobic core and
hydrophilic shell, as shown in Fig. 1. The formation of FNPs
leads to a decrease in the absorption at 340 nm because of the
consumption of free AHBTA molecules and the formation of
intermolecular interaction/aggregation.16 The increase of the
absorption at 389 nm resulted from the high concentration of
phenol anions around the FL molecules. The fluorescent
molecules mixed with the azo-compound and formed a sandwich
layer in the nanoparticle. With the increase of the hydrophobic
property of the secondary anion, a more compact hydrophobic
core was formed and the AHBTA molecules were pushed aside
to the middle shell from the core of the nanoparticle. This
increases the peak intensity at 389 nm.
The FL intensity of the FNPs can be further tuned by
changing its diameter (Fig. 3b). By increasing the amount of
AzoMeO from 25 mM to 100 mM, the diameter of the
FNPs-MeO increased from 149 nm to 167 nm and the FL
emission was enhanced 8 times due to the more hydrophobic
local environment.
In general, organic FL molecules have a high intensity of FL
emission, but they are often rapidly photo-bleached in the
presence of oxygen. Fig. 4 shows the FL intensity of the FNPs
in aqueous solution irradiated by a UV lamp (365 nm) for
different times. For the pure AHBTA molecules, quenching
occurred very rapidly and the FL intensity was almost completely
quenched after 1 h. This is a result of the intermolecular
collision between AHBTA and water. However, if FNPs were
formed, such as FNPs-MeO and FNPs-CF3, the FL intensity
of the complex was dramatically enhanced and was more
stable to UV light irradiation, although slight photo-quenching
occurred initially. However, the duration of stability of the
FNPs was also evaluated and this shows only a modest change
after one week (see ESI, Fig. S3w).
The diameters of the FNPs were 158 nm and 162 nm for
FNP-MeO and FNP-CF3, when the molar ratio of PIL, AHBTA
and the azo-compound was fixed at 10 : 1 : 1. With the increase of
the amount of the secondary anion, the diameters of the nano-
particles became a little larger. Such FNPs show excellent
dispersion in aqueous media. It can be concluded that the
formation of the FNPs was mainly due to the self-assembly
between the hydrophobic secondary anions, the weakly hydro-
phobic FL molecules and the hydrophilic PIL chains. If the
hydrophobic secondary anions become hydrophilic, e.g. with
NMe2 as the bearing group, no particle is formed.
Fig. 3a shows the relationship between the FL intensity and
secondary anions with a variety of bearing groups. For pure
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FNPs show high stability to photo-bleaching and good
durability. Their most important property is that the FNPs
show high sensitivity and selectivity for copper ions. Therefore,
our study may provide useful information for the design of
functional nanoparticles in the field of biological imaging and
environmental testing.
The authors gratefully acknowledge financial support from
National Science Fund for Distinguished Young Scholar
(50925310), National Science Foundation of China
(50902094 and 20874059), 973 project (2009CB93043) and
the Shanghai Leading Academic Discipline Project (No: B202)
Fig. 4 Plots of the maximum of the FL intensity (lem = 450 nm) vs.
UV irradiation time (B365 nm) ([PIL]/[AHBTA]/[azo-complex] =
10 : 1 : 1).
Notes and references
We have subjected these FNPs to an investigation into the
detection of transition metal ions. Fig. 5a shows how the FL
intensity changes with the addition of a metal salt PBS buffer
solution (pH = 7.4). The emission of FNPs-CF3 shows very
high sensitivity to the transition metal ions Cu2+ and Sn4+
due to the coordination between the AHBTA and metal ions.
If we define the detection limit as the Cu2+ concentration at
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Fig. 5 Relative FL intensity of FNPs-CF3 as a sensor (I and I0
represent the FL intensity of FNPs-CF3 in the presence and absence of
metal ions, respectively) (a) with the addition of different concentrations
of metal ions (b) addition of [Cu2+] (25 mM) (red bar), followed by the
addition of 5 (blue bars), 50 (pink bars), 500 (orange bars) equiv. of
M2+ (M2+ = Ca2+, Mg2+, and Zn2+), respectively. (lex = 389 nm,
lem = 450 nm, [FNPs-CF3] = 50 mM (showed by [AHBTA]),
PBS buffer pH = 7.4).
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