A fluorescent light-up platform with "aIE + ESIPT" characteristics for multi-target detection both in solution and on paper strip

Article abstract of DOI:10.1039/c4tb01937g

We report a fluorescent light-up platform for multi-target detection in aqueous solution and on paper strip. The platform is based on a salicylaldazine fluorogen with aggregation-induced emission (AIE) and excited state intramolecular proton transfer (ESIPT) characteristics, which shows distinct advantages including ease of chemical modifications, free of self-quenching effect, excellent light-up ratio and large Stokes shift. To demonstrate the versatility of the platform, palladium cation and perborate anion, as well as UV light, were selected as the targets. The three representative probes, AIE-Pd, AIE-perborate and AIE-UV, light up specifically in the presence of the target both in aqueous solution and on paper strip. The immediate naked-eye response makes the probes ideal for instrument-free and power-free detection. This journal is

Full text of DOI:10.1039/c4tb01937g

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A fluorescent light-up platform with “AIE + ESIPT” characteristics for multi-target
detection both in solution and on paper strip
Ruoyu Zhang, ‡a Meng Gao, ‡ b Shiqiang Bai,b Bin Liu*a,b
Abstract
We report a fluorescent lightꢀup platform for multiꢀtarget detection in aqueous solution and on
paper strip. The platform is based on salicylaldazine fluorogen with aggregationꢀinduced
emission (AIE) and excited state intramolecular proton transfer (ESIPT) characteristics, which
shows distinguished advantages including ease of chemical modifications, free of selfꢀquenching
effect, excellent lightꢀup ratio and large Stokes shift. To demonstrate the versatility of the
platform, palladium cation and perborate anion, as well as UV light were selected as the targets.
The three representative probes of AIE-Pd, AIE-Perborate and AIE-UV light up specifically in
the presence of the target both in aqueous solution and on paper strip. The immediate nakedꢀeye
response makes the probes ideal for instrumentꢀfree and powerꢀfree detection.
Key words:
Paperꢀbased detection; Nakedꢀeye sensing; Aggregationꢀinduced emission; Excited state
intramolecular proton transfer; Multiꢀtarget detection; Lightꢀup sensing
[a]
Ms. Ruoyu Zhang, Prof. Bin Liu
Department of Chemical and Biomolecular Engineering
National University of Singapore
4 Engineering Drive 4, Singapore 117585
Eꢀmail: cheliub@nus.edu.sg
[b] Dr. Meng Gao, Dr. Shiqiang Bai, Prof. Bin Liu
Institute of Materials Research and Engineering (A*STAR)
3 Research Link, Singapore 117602
‡
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
1

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Introduction
The prevalence of chemicals in daily life leads to the concerns of subsequent pollutions and
requires convenient methods to monitor their levels. For instance, palladium plays critical roles in
automobile industry, dental crown production, and pharmaceutical manufacture.1 However,
palladium interferes the normal cell metabolism by binding to thiolꢀcontaining proteins and
DNA.2, 3 On the other hand, the perborates have been used extensively in cosmetics, medicinal
formulations and bleach.4, 5 Although they are safer alternatives to hydrogen peroxide,6 mutagenic
effect has been found in in vitro experiments.7 Therefore, close monitoring of their usage and
residues is of high importance. In addition, many useful products such as medication, developing
agents and chemicals are photoꢀsensitive and should be kept away from light. It is ideal to have
an easy and convenient platform, which could be generally used to detect cations, anions and UV
light through simple modification.
Fluorescent probes are powerful tools for various detection tasks, which enjoy great promises due
to their high sensitivity and selectivity.8ꢀ10 Last several decades have witnessed the prosperity of
fluorescent probes in solution detection. Moreover, the concept of “labꢀonꢀpaper”, for instance,
lateral flow assays, has been widely reported with advantages such as low cost, portability and
operational simplicity.11ꢀ13 Gold nanoparticles (NPs) and quantum dots (QDs) are typical signal
reporters for colorimetric and fluorescent sensing on paper strip.14 To realize specific analyte
detection, recognition elements, such as nucleic acid or antibody are frequently required for
nanoparticle modification.15, 16 The colorimetric and fluorescence changes are usually induced by
the aggregation of gold NPs or the separation of a donor from an acceptor in the vicinity in a
fluorescence resonance energy transfer (FRET) system. On the other hand, reaction based
fluorescent systems which can simultaneously achieve signal recognition and transduction have
been less reported on solid support.17 For example, although palladium species have been
intensively studied in solution by probes based on rhodamine,18ꢀ20 fluorescein21 and other
23
conventional organic fluorogens,22,
very few of them have been further applied on solid
substrate.20, 23 As compared to Pd detection, fluorescent probes for perborate detection are less
developed, and the reported assays are limited to solution sensing.24,
25
In addition, photoꢀ
sensitive probes in the form of paperꢀstrip are highly desirable as photoꢀexposure indicators, but
remain limited.26 Therefore, a platform that can offer fluorescent multiꢀtarget detection not only
in solution but also on solid substrate is promising for realꢀlife applications.
The development of small molecule based fluorescence lightꢀup paper strip faces a few
challenges. It is well known that traditional fluorogens suffer from aggregationꢀcaused quenching
(ACQ), which greatly affects the detection sensitivity. In addition, due to the intrinsic
fluorescence of most fluorescent probes, additional quencher or fluorescence quenching
mechanism must be included on the substrate prior to analyte sensing in order to yield lightꢀup
signals.27ꢀ29 Bioprobes based on fluorogens with aggregationꢀinduced emission (AIE)
characteristics have attracted increasing attention during the past years, as the AIE fluorogens
generally show high fluorescence in aggregate state.30, 31 This is due to the typical feature AIE
fluorogens: they are nonꢀemissive when moleculary dissolved but become highly emissive in the
aggregate state.32ꢀ34 It is ideal if one could develop a strategy to quench the fluorescence of AIE
fluorogens in solid state, which upon analyteꢀinduced reaction to yield emissive product that
2

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could fully take advantage of AIE characteristics to offer high detection sensitivity. In this regard,
salicylaldazine dye, which shows both AIE and excited state intramolecular proton transfer
(ESIPT) properties, offers a unique opportunity for sensing on solid substrate.35 The
salicylaldazine dye generally shows weak blue emission when the excitedꢀstate proton transfer
from hydroxyl group (proton donor) to nitrogen (proton acceptor) is blocked. In contrast,
deprotection of the hydroxyl groups could revert the fluorescence by restoration of the proton
transfer and formation of intramolecular hydrogen bond. Together with the restriction of
intramolecular motions, this will lead to high fluorescence in the aggregate state.
Herein, we propose a multiꢀtarget lightꢀup platform based on the “AIE + ESIPT” fluorogen of
salicylaldazine. To demonstrate that the design concept is generally applicable to multiple targets,
palladium cation, perborate anion, and UV light are chosen as the representative examples. Three
different functional groups are conjugated onto the structure of salicylaldazine to yield three
probes, namely, AIE-Pd, AIE-Perborate and AIE-UV. The three probes are weakly emissive in
aqueous solution, which light up in the presence of targets as a result of the functional group
deprotection from the hydroxyl groups. Moreover, by dropꢀcasting on paper strip, these probes
are ready for nakedꢀeye detection of analyte with the assistance of a portable UV lamp. The visual
paper assay offers opportunities for use in resourceꢀlimited settings as well as in emergency
situation.
Experimental Section
Materials
2ꢀHydroxybenzaldehyde, 2ꢀ(allyloxy)benzaldehyde, 1ꢀ(bromomethyl)ꢀ2ꢀnitrobenzene, 1,4ꢀ
dibromobutane, cesium carbonate, hydrazine monohydrate, acetic anhydride, morpholine,
ethanol, anhydrous DMF were all purchased from Aldrich. All the other chemicals were
purchased
from
Sigma–Aldrich
or
Merck.
2,2'ꢀ((1E,1'E)ꢀhydrazineꢀ1,2ꢀ
diylidenebis(methanylylidene))diphenol was prepared according to the literature.36
Equipment and Methods
The UVꢀvis absorption spectra were obtained using UVꢀvis spectrometer (Shimadzu, UVꢀ1700,
Japan). PL measurements were carried out on a PerkinꢀElmer LSꢀ55 equipped with a xenon lamp
excitation source and a Hamamatsu (Japan) 928 PMT, using 90° angle detection for solution
samples. 1H and 13C NMR spectra were measured on a Bruker ARX 400 NMR spectrometer. The
molecular mass was acquired using ion trapꢀtimeꢀofꢀflight mass spectrometry (MSꢀITꢀTOF)
(Shimadzu). The pH values of buffers were adjusted using a Sartorius basic pHꢀMeter PBꢀ10.
Synthesis and Characterization
Synthesis of 1,2-bis ((E)-2-(allyloxy)benzylidene)hydrazine (AIE-Pd):
2ꢀ(Allyloxy)benzaldehyde (162 mg, 1.0 mmol) was dissolved in absolute ethanol (10 mL),
followed by the addition of hydrazine monohydrate (25 mg, 0.5 mmol), and the mixture was
refluxed for 4 h. Precipitates were filtrated under vacuum and washed with absolute ethanol for
three times before drying under vacuum. Pure product of 1,2ꢀbis ((E)ꢀ2ꢀ(allyloxy)benzylidene)
1
hydrazine (AIE-Pd) was obtained as a yellow powder solid (135 mg, 84% yield). H NMR
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(CDCl3, 400 MHz):
δ 9.14 (s, 2H), 8.14 (d, J = 7.6 Hz, 2H), 7.39 (t, J = 7.6 Hz, 2H), 7.02 (t, J =
7.6 Hz, 2H), 6.93 (d, J = 8.4 Hz, 2H), 6.12–6.04 (m, 2H), 5.45 (d, J = 17.2 Hz, 2H), 5.31 (d, J =
10.4 Hz, 2H), 4.62 (d, J = 4.8 Hz, 4H); 13C NMR (CDCl3, 100 MHz): 158.1, 157.5, 132.9, 132.3,
127.4, 123.0, 121.0, 117.8, 112.4, 69.2. HRMS (ESI): m/z [M + Na]+ calcd for C20H20N2NaO2:
343.1417; found: 343.1422.
Synthesis of ((1E,1'E)-hydrazine-1,2-diylidene bis(methanylylidene))bis(2,1-phenylene)
diacetate (AIE-Perorate):
2,2'ꢀ((1E,1'E)ꢀhydrazineꢀ1,2ꢀdiylidenebis(methanylylidene))diphenol (240 mg, 1.0 mmol) was
dissolved in DMF (5 mL), followed by addition of Cs2CO3 (652 mg, 2.0 mmol), acetic acid
o
anhydride (408 mg, 4.0 mmol), and the mixture was stirred at 60 C for 8 h. After cooling to
room temperature, the solvent was removed under vacuum, and the residue was extracted with
dichloromethane (40 mL × 3). The extracts were washed with brine, dried over anhydrous
MgSO4, and concentrated under reduced pressure. The residue was further recrystallized from
1
ethanol/hexane to give AIE-perborate as a yellow solid (285 mg, 88% yield). H NMR (CDCl3,
400 MHz):
δ 8.73 (s, 2H), 8.11 (d, J = 7.6 Hz, 2H), 7.49 (t, J = 7.6 Hz, 2H), 7.33 (t, J = 7.6 Hz,
2H), 7.17 (d, J = 8.4 Hz, 2H), 2.39 (s, 6H); 13C NMR (CDCl3, 100 MHz): 169.1, 157.1, 150.3,
132.2, 128.5, 126.3, 126.2, 123.1, 21.0. HRMS (ESI): m/z [M + Na]+ calcd for C18H16N2NaO4:
347.1002; found: 347.1011.
Synthesis of 2-((2-nitrobenzyl)oxy)benzaldehyde:
2ꢀHydroxybenzaldehyde (244 mg, 2.0 mmol) and 1ꢀ(bromomethyl)ꢀ2ꢀnitrobenzene (428 mg, 2.0
mmol) were first dissolved in DMF (10 mL), followed by the addition of Cs2CO3 (652 mg, 2.0
o
mmol). The mixture was stirred at 60 C under nitrogen for 12 h. After cooling to room
temperature, the reaction mixture was extracted by dichloromethane (40 mL × 3). The combined
dichloromethane fractions were dried over anhydrous MgSO4 and concentrated under reduced
pressure. The residue was further separated by column chromatography (silica, petroleum ether:
ethyl acetate = 20 : 1, v/v) to give a light yellow solid (447 mg, 87% yield). 1H NMR (CDCl3, 400
MHz):
δ 9.26 (s, 1H), 8.20 (d, J = 7.6 Hz, 2H), 7.99 (d, J = 7.6 Hz, 1H), 7.73 (t, J = 7.6 Hz, 1H),
7.52 (t, J = 7.6 Hz, 1H), 7.45 (t, J = 7.6 Hz, 1H), 7.10 (t, J = 7.6 Hz, 1H), 7.02 (d, J = 8.4 Hz,
13
1H), 5.61 (s, 2H); C NMR (CDCl3, 100 MHz): 157.7, 157.6, 146.9, 134.2, 133.4, 132.8, 128.6,
128.5, 128.2, 125.1, 123.0, 121.8, 112.8, 67.4.
Synthesis of 1,2-bis((E)-2-((2-nitrobenzyl)oxy) benzylidene)hydrazine (AIE-UV):
2ꢀ((2ꢀNitrobenzyl)oxy)benzaldehyde (257 mg, 1.0 mmol) was dissolved in absolute ethanol (10
mL), followed by addition of hydrazine monohydrate (25 mg, 0.5 mmol), and the mixture was
refluxed for 4 h. Precipitates were filtrated under vacuum and washed with absolute ethanol for
three times, before drying under vacuum. The pure product of 1,2ꢀbis((E)ꢀ2ꢀ((2ꢀ
nitrobenzyl)oxy)benzylidene) hydrazine (AIE-UV) was obtained as a yellow powder solid (237
1
mg, 89% yield). H NMR (CDCl3, 400 MHz):
δ 8.93 (s, 2H), 8.14 (d, J = 8.0 Hz, 2H), 8.01 (d, J
= 7.6 Hz, 2H), 7.86 (d, J = 7.6 Hz, 2H), 7.79 (t, J = 7.6 Hz, 2H), 7.63 (t, J = 7.6 Hz, 2H), 7.50 (t,
J = 7.6 Hz, 2H), 7.22 (d, J = 8.4 Hz, 2H), 7.08 (t, J = 7.6 Hz, 2H), 5.58 (s, 4H); 13C NMR (CDCl3,
100 MHz): 157.3, 156.5, 147.6, 134.0, 133.0, 131.8, 129.5, 129.4, 127.2, 124.8, 122.0, 121.4,
113.3, 67.0. HRMS (ESI): m/z [M + Na]+ calcd for C28H22N4NaO6: 533.1432; found: 533.1447.
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Solution Sensing
The PL spectra of the probes of AIE-Pd and AIE-UV were measured in the mixture of THF/H2O
(v/v = 1/99) at excitation wavelength of 365 nm. For detection of palladium species, 10 mM
NaBH4 was used as a reducing agent to increase the reaction efficiency. The selectivity of the
AIE-Pd was verified through screening various metal ions including Cu2+, Fe3+, Hg2+, Ru3+, Pb2+
and Mn2+. The cation stock solutions were prepared from CuSO4, FeCl3, HgCl2, RuCl3, Pb(OAc)2,
and MnCl2 in aqueous solution with a concentration of 10 mM, respectively. The detection
experiments of AIE-Perborate responsive to perborate ion were conducted in 1 mL of
THF/HEPES (10 mM, pH 5.0) (v/v = 1/99) solution. The selectivity of the AIE-Perborate for
ꢀ
2ꢀ
anions was verified through screening various anions, including Fꢀ, Clꢀ, Brꢀ, Iꢀ, HSO3 and S2O3 .
The anion stock solutions were prepared from KF, NaCl, NaBr, NaI, NaHSO3, and Na2S2O3 in
aqueous solution with a concentration of 100 mM, respectively. The 10 mM stock solution of
AIE-UV was first exposed to UV light for increasing time (0, 10, 20, 30, 40, 50, 60 and 70 min,
respectively), and 1 µL of the stock solution was dilute to 1 mL (10 µM) before PL measurement.
Solid state Sensing
Whatman filter paper (Advantec, qualitative, 70 mm) was used as the solid substrate for all the
solid state sensing. First, 10 µL of THF stock solution of the probe (1.0 mM) was dropꢀcasted
onto the filter paper followed by evaporation of THF. Then analytes of different concentrations
(as specified in the figure captions) were added onto the spots. All the pictures were taken under
UV illumination.
Results and Discussion
Probe design and synthesis. A lightꢀup platform has been designed based on salicylaldazine for
multiꢀtarget sensing. AIE-Pd, AIE-Perborate and AIE-UV were synthesized with allyl, acetyl
and 2ꢀnitrobenzyl group for detection of palladium,37, 38 perborate ions, and UV light (Scheme 1),
respectively. The three probes have unique combination of AIE and ESIPT properties, making
them suitable for detection both in solution and on solid substrate. Initially, the probes is only
weakly blueꢀemissive in solution and on paper strip. Deprotection of hydroxyl groups would light
up the emission at longer wavelength by restoration of the hydrogen bonds allowing the
formation of intramolecular hydrogen bonds. The structures and synthetic routes to the three
probes are shown in Scheme 1. The probe AIE-Pd was synthesized in 84% yield through the
condensation reaction between 2ꢀ(allyloxy)benzaldehyde and hydrazine monohydrate. The probe
AIE-Perborate was synthesized in 88% yield through the reaction between salicylaldazine and
acetyl anhydride in DMF using Cs2CO3 as a base. For the synthesis of the AIE-UV, 2ꢀ((2ꢀ
nitrobenzyl)oxy)benzaldehyde was first prepared from 2ꢀhydroxybenzaldehyde and 1ꢀ
(bromomethyl)ꢀ2ꢀnitrobenzene in 87% yield, which was then reacted with hydrazine
1
monohydrate to afford the AIE-UV product in 89% yield. The H NMR, 13C NMR and HRMS
data confirm their right structures. Detailed spectra are shown in the experimental section and
supporting information.
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Scheme 1 (A) Design principles of fluorescent lightꢀup probes AIE-Pd, AIE-Perborate, and
AIE-UV based on a platform of salicylaldazine with “AIE + ESIPT” characteristics for detection
of palladium cation, perborate anion, and UV light and (B) synthetic routes to the probes.
Optical properties. The UVꢀvis absorption and PL spectra of salicylaldazine were measured in
tetrahydrofuran (THF) and the results are shown in Fig. 1A. In THF, the maximum absorption
wavelength of salicylaldazine is 365 nm, while its emission intensity is very low. The AIE
characteristics of salicylaldazine were studied by measuring its PL intensity changes in a solvent
mixture of THF and water. The PL intensity (I) at 542 nm remains very low even when water
fraction reaches 90% (I/I0 is less than 0.07, I0 is the PL intensity at 542 nm when the water
fraction is 99%), while an intense emission was observed at 542 nm when the water fraction
reaches 99%, which verifies that salicylaldazine has typical AIE properties. Moreover, the large
Stokes shift of 8947 cmꢀ1 derived from ESIPT mechanism is also a desired property for solidꢀstate
sensing.
fw (vol %)
1.0
0.8
0.6
0.4
0.2
0.0
A
B
99
90
80
70
60
50
40
30
20
10
0
300
400
500
600
700
0
20 40 60 80 100
Water fraction (vol %)
Wavelength (nm)
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Fig. 1 (A) UVꢀVis absorption spectrum of salicylaldazine in THF (black dashed line); PL spectra
of the product in THF and THFꢀwater mixture with different water fractions (fw); (B) Plot of the
relative PL intensity at 542 nm (I/I0) versus the solvent with different water fractions (fw). I is PL
intensity at any fw, and I0 is the PL intensity measured at fw = 99%. [salicylaldazine] = 10 µM; λex
= 365 nm.
Pd detection
We designed and synthesized AIE-Pd for detection of palladium based on a Pd0ꢀcatalyzed Tsujiꢀ
Trost reaction which cleaves allyl group and releases the hydroxyl group,21, 39 as shown in Fig.
2A. To demonstrate the potential of the probe AIE-Pd for quantitative palladium detection, the
fluorescence spectra were measured upon addition of increasing concentrations of Pd(PPh3)4 in
the mixture of THF/H2O (v/v = 1/99). NaBH4 was chosen as a reducing agent because of its
ability to reduce Pd(II) to Pd(0), which could prompt the Pdꢀcatalyzed deallylation.37 As shown in
Figs. 2B and 2C, the fluorescence of AIE-Pd shows good linear increase with Pd concentration
with a correlation coefficient of R2 = 0.9925 in the range of 0.0 – 8.0 µM. The detection limit was
calculated to be 4.97 nM (0.5 ppb Pd content) by (3σ/k), where σ is the standard deviation of the
blank measurement and k is the slope of fluorescence intensity over Pd concentration. The results
indicate that the probe AIE-Pd has high sensitivity with linear response to Pd, which is suitable
for detection of trace amount of palladium sample. Filter paper was then used as a substrate to
investigate the potential of AIE-Pd for solidꢀstate detection. As described in the Experimental
Section, the probe AIE-Pd was stained onto the filter paper by dropꢀcasting 10 µL of 1.0 mM
probe in THF stock solution. As Pd(PPh3)4 samples (1 µL) with increasing concentrations from
0.1 µM to 1.0 mM were added onto the probe spot, clear fluorescent color change was visualized.
Fig. 2D shows photographs taken under UV illumination. The AIE-Pd on paper strip gives a very
sensitive response to Pd solution even at 1 µM (0.1 ppm Pd content), which clearly satisfies the
requirement of governmental restriction limit of Pd in drugs (5ꢀ10 ppm).40 To the best of our
knowledge, this probe has the best detection limit for visual sensing of palladium on paper strip
reported so far.
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Fig. 2 (A) Detection mechanism of Pd catalyzed deallylation. (B) Changes of the PL spectra of
the probe AIE-Pd in the presence of increasing concentrations of Pd(PPh3)4. (C) Plot of the PL
peak intensity at 540 nm versus the concentrations of Pd(PPh3)4 in the mixture of THF/H2O (v/v
= 1/99, with 10 mM NaBH4). [AIE-Pd] = 10 µM, λex = 365 nm. (D) Solidꢀstate fluorescence
response of the AIE-Pd with different concentrations of Pd under UV illumination of 365 nm.
The response of AIE-Pd toward other metal ions was then explored. In aqueous solution, the
fluorescence of the probe hardly shows any change in the presence of other metal ions, including
Cu2+, Fe3+, Hg2+, Ru3+, Pb2+ and Mn2+ even at the concentration of 160 µM (20 equiv. of Pd
concentration). In contrast, upon addition of 8 µM Pd(PPh3)4, the probe lights up significantly as
shown in Figs. 3A and 3B. The selectivity experiment was also carried out on the paper strip and
the result is shown as the inset of Fig. 3B, demonstrating that other metal ions have negligible
response to AIE-Pd. The excellent selectivity is due to the specificity of Pdꢀcatalyzed allyl
deprotection. Next, the reactivity of probe AIE-Pd toward other palladium species including
PdCl2, Pd(OAc)2, PdCl2(dppf)2, and Pd(PPh3)2Cl2 was also examined respectively and the
results are shown in Fig. 3C. The probe was found to be useful for all the tested sources because
Pd (II) can be efficiently reduced to Pd(0) by NaBH4.
Fig. 3 (A) PL spectra of AIE-Pd in the absence and presence of different metal (ions) Pd , Cu2+,
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Fe3+, Hg2+, Ru3+, Pb2+ and Mn2+ (as their Clꢀ, COOHꢀ and SO4 salts) in THF/H2O (v/v = 1/99,
with 10 mM NaBH4) solution. (B) Bar chart of PL peak intensity at 540 nm responsive to
different metal ions. ([AIE-Pd] = 10 µM, [Pd] = 8 µM, [Mm+] = 160 µM, λex = 365 nm). Inset:
Fluorescence response of AIE-Pd to various metal cations on paper strip under UV illumination
(365 nm). [Pd(PPh3)4] = 10 µM, [Mn+] = 200 µM. (C) Comparison of the fluorescent responses of
AIEꢀPd toward different palladium species (4 µM) in THF/H2O (v/v = 1/99, with 10 mM
2ꢀ
NaBH4), A. Pd(PPh3)4; B. PdCl2; C. Pd(OAc)2; D. PdCl2(dppf)2; E. Pd(PPh3)2Cl2
.
Perborate detection
The sensing of perborate is designed based on selective deprotection of aryl acetates under mild
conditions as shown in Fig. 4A. AIE-Perborate shows very weak fluorescence at 444 nm in
THF/HEPES (10 mM, pH = 5.0) buffer (v/v = 1/99), (Fig. 4B). However, after treatment of the
probe with sodium perborate with concentrations ranging from 0 to 50 µM, intense fluorescence
turnꢀon is observed at both 444 and 550 nm. Fig. 4C was obtained by plotting the peak PL
intensities of the probe to the concentrations of perborate. The detection limit was calculated to be
0.10 µM, with a correlation coefficient R2 = 0.998 at the range of 0 – 50 µM, which is similar to
that reported by Zheng24 and better than that reported by Chang.25 The sensing of perborate on
filter paper was subsequently conducted after dropꢀcasting 10 µL of 1.0 mM probe AIE-
Perborate stock solution in THF and drying. Sodium perborate from 10 µM to 1 mM was added
onto the stained filter paper, and pictures were taken under UV illumination, which are shown in
Fig. 4D. It is obvious that the AIE-Perborate on paper strip gives concentrationꢀdependent turnꢀ
on response to perborate, and 50 µM perborate anion can be readily recognized by naked eyes.
This represents the first probe which can offer visual detection of perborate on paper strip.
ꢀ
Fig. 4 (A) Detection mechanism of AIE-Perborate for BO3 through specific deprotection of
acetate group. (B) Changes of PL spectra of AIE-Perborate in the presence of increasing
ꢀ
concentration of BO3 . (C) Plot of the PL peak intensity at 550 nm versus the concentration of
ꢀ
BO3 in the mixture of THF/HEPES (v/v = 1/99). [AIE-Perborate] = 20 µM, λex = 365 nm. (D)
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Solidꢀstate fluorescence response of the AIE-Perborate with different concentrations of
perborate under UV illumination at 365 nm.
The selectivity of AIE-Perborate to perborate against other anions was also examined in the
same solvent medium. The concentrations of other anions are 20ꢀfold as that of perborate. As
ꢀ
shown in Figs. 5A and 5B, AIE-Perborate can only be lit up by BO3 . Other anions including Fꢀ,
Clꢀ, Brꢀ, Iꢀ, HSO3 and S2O3 , at a concentration of 20ꢀfold as that of BO3 , do not result in any
ꢀ
2ꢀ
ꢀ
apparent fluorescent changes of AIE-Perborate. The fluorescence changes of the probe stained
ꢀ
on paper strip toward BO3 and other anions were also studied. The inset of Fig. 5B shows that the
probe AIE-Perborate is highly specific to perborate, which validates its high sensitivity and
selectivity for sensing perborate both in aqueous solution and on paper strip.
Fig. 5 (A) PL spectra of AIE-Perborate in the absence and presence of various anions including
ꢀ
ꢀ
2ꢀ
BO3 , Fꢀ, Clꢀ, Brꢀ, Iꢀ, HSO3 and S2O3 (as their K+ and Na+ salts) in THF and 10 mM pH = 5.0
HEPES buffer solution (v/v = 1/99). (B) Bar chart of PL peak intensity at 550 nm responsive to
–
different anions. ([AIE-Perborate] = 20 µM, [BO3 ] = 50 µM, [An–] = 1 mM. λex = 365 nm).
ꢀ
Inset: Fluorescent response of AIE-Perborate to BO3 and various anions on paper strip under
UV illumination (365 nm).
UV light detection
Nitrobenzyl group has been well established in the literature as a photocleavable protecting
reagent for alcohols and amines.41 The UV responsive probe AIE-UV is designed by integration
of this reaction and “AIE + ESIPT” mechanism through salicylaldazine core. Based on a similar
mechanism as AIE-Pd and AIE-Perborate, AIE-UV should be turned on by deprotection of
nitrobenzyl groups as shown in Fig. 6A. To validate the potential of AIE-UV for photoꢀcleavage
application, the changes in the PL spectrum were measured after exposure to UV light. From Fig.
6B, as the illumination time increases from 0 to 60 min, the fluorescence intensities of AIE-UV
show an increasing trend. The peak PL intensity at 554 nm is saturated when exposure time
reaches 60 min, providing a 12ꢀfold turnꢀon compared with the probe without any UV
illumination. The sensing on the filter paper strip was also examined. 10 µL of AIE-UV (1.0 mM)
in THF was used to stain the filter paper. From Fig. 6C, AIE-UV shows a quick response to UV
light. Even illumination for four minutes is able to yield distinguished fluorescence for nakedꢀeye
detection.
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DOI: 10.1039/C4TB01937G
Fig. 6 (A) Schematic illustration of the photoꢀresponsive AIE-UV upon UV illumination; (B) The
PL spectra of 10 µM probe AIE-UV with UV illumination (365 nm) time of 0, 10, 20, 30, 40, 50,
60 and 70 min. (C) Solidꢀstate fluorescence response of the AIE-UV with different exposure time
to UV illumination at 365 nm.
Because of the ESIPT characteristics of salicylaldazine, it has both enol (E*) (at ~440 nm) and
keto (K*) emission (at ~540 nm) (Fig. 7), and the K* emission is formed through excitedꢀstate
intramolecular proton transfer (ESIPT).42 According to the literature reports,43 the ratio of K*/E*
emission will decrease because of the disruption on intramolecular hydrogen bonds by polar
solvents, which will inhibit the excitedꢀstate intramolecular proton transfer process and decrease
the K* emission. Because of the aggregationꢀinduced emission (AIE) characteristics of
salicylaldazine, at aggregation state, the intramolecular hydrogen bonds of salicylaldazine can be
protected from the disruption of polar solvents, and the K*/E* emission ratio will be increased via
smooth ESIPT process. However, at nonꢀaggregated state, its intramolecular hydrogen bonds can
be disrupted by the polar solvents, and the K*/E* emission ratio will be decreased. To verify that
the aggregation extend will affect its emission spectra shape, we have measured the emission
spectra of salicylaldazine at different concentrations in THFꢀwater mixture (v/v, 1/99) (Fig. S9). It
shows that with decreasing concentration of salicylaldazine (from 10 ꢁM to 1 ꢁM), it shows
decreasing K*/E* emission ratio, because the lower concentration of salicylaldazine will lead to
lower aggregation extent and the intramolecular hydrogen bonds can be disrupted by the protic
water molecules. The reactions between AIE-Pd, AIE-perborate and AIE-UV toward their
targets will lead to the same product of salycylaldazine, but their different reaction efficiency will
lead to different concentrations and aggregation extent of salycylaldazine product, and the
corresponding fluorescence spectra of salicylaldazine with different K*/E* emission ratio have
been observed.
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DOI: 10.1039/C4TB01937G
Fig. 7 The emission mechanism of salicylaldazine with both AIE and ESIPT characteristics.
Conclusion
We have designed a fluorescent lightꢀup platform based on salicylaldazine fluorogen with both
AIE and ESIPT characteristics. The combination of the two emission mechanisms offers unique
optical properties that are highly desirable for sensing in aqueous solution and on solid substrate.
Three probes AIE-Pd, AIE-Perborate and AIE-UV have been synthesized to demonstrate the
versatility of the platform. Experimental results have shown that the probes can light up linearly
by target analytes with increasing concentration or increasing exposure time. To the best of our
knowledge, AIE-Pd based paper strip offers the best detection limit for visual sensing of
palladium. A similar lightꢀup response is also observed for perborate anion and UV light sensing
both in solution and on paper strip. This versatile platform has advantages including free of selfꢀ
quenching effect, excellent lightꢀup ratio and large Stokes shift, which offers new opportunities
for convenient visual sensing. Further adjusting the functionalities on the hydroxyl groups could
easily yield new probes for different analytes with the same operation mechanism.
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Journal of Materials Chemistry B
Page 14 of 14
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DOI: 10.1039/C4TB01937G
Abstract
We report a fluorescent light-up platform for multi-target detection in aqueous solution and on
paper strip. The platform is based on salicylaldazine fluorogen with aggregation-induced
emission (AIE) and excited state intramolecular proton transfer (ESIPT) characteristics,
which shows distinguished advantages including ease of chemical modifications, free of self-
quenching effect, excellent light-up ratio and large Stokes shift. To demonstrate the versatility
of the platform, palladium cation and perborate anion, as well as UV light were selected as
the targets. The three representative probes of AIE-Pd, AIE-Perborate and AIE-UV light up
specifically in the presence of the target both in aqueous solution and on paper strip. The
immediate naked-eye response makes the probes ideal for instrument-free and power-free
detection.
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