A new fluorescent sensor for the detection of pyrophosphate based on a tetraphenylethylene moiety

Article abstract of DOI:10.1016/j.tetlet.2010.02.009

We have developed a new fluorescent sensor 1-2Zn based on a tetraphenylethylene (TPE) moiety for the detection of PPi. This TPE-based chemosensor showed 'turn-on' fluorescence emission according to the concentration of PPi. The fluorescence enhancement upon binding of PPi to 1-2Zn resulted from the restriction of intramolecular rotation of phenyl rings in 1-2Zn.

Full text of DOI:10.1016/j.tetlet.2010.02.009

Tetrahedron Letters 51 (2010) 1960–1962
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A new fluorescent sensor for the detection of pyrophosphate based on a
tetraphenylethylene moiety
Chuneung Park a,b, Jong-In Hong a,
*
a Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul 151-747, Republic of Korea
b SK Holdings, Life Science Business Division, Drug Development Center, 140-1, Wonchon-dong, Yuseong-gu, Daejeon 305-712, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We have developed a new fluorescent sensor 1–2Zn based on a tetraphenylethylene (TPE) moiety for the
detection of PPi. This TPE-based chemosensor showed ‘turn-on’ fluorescence emission according to the
concentration of PPi. The fluorescence enhancement upon binding of PPi to 1–2Zn resulted from the
restriction of intramolecular rotation of phenyl rings in 1–2Zn.
Received 9 December 2009
Revised 1 February 2010
Accepted 4 February 2010
Available online 8 February 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Biological anions such as pyrophosphate (P2O7 4ꢀ, PPi), adeno-
sine 50-monophosphate (AMP), and adenosine 50-triphosphate
(ATP) play important roles in a wide range of biological processes.
In particular, PPi is known to be involved in several biochemical
reactions, such as hydrolysis of ATP, DNA polymerization, and
other metabolic processes.1 Therefore, the selective detection of
PPi has been a major research focus. Although traditional methods
of anion sensing such as the use of ion-selective electrodes have al-
ready been discovered, there is an increasing need to find alterna-
tive means of analysis, including the use of selective fluorescent
chemosensors.2 Because the fluorescent chemosensors present
many advantages such as high sensitivity, low cost, easy detection,
and versatility, many research groups have made an effort to dis-
cover selective fluorescent probes for PPi.3
Usually, fluorescent probes are composed of a signaling subunit
known as a fluorophore, and a binding site which is covalently
linked to a fluorophore. Mechanisms which regulate the response
of a fluorophore to the substrate binding include photoinduced
electron transfer (PET), fluorescence resonance energy transfer
(FRET), excimer/exciplex formation or extinction, and photoin-
duced charge transfer (PCT).4 In addition to these mechanisms, a
new photoluminescence (PL) process has been identified, known
as ‘aggregation-induced emission (AIE)’.5 In this case, a non-emis-
sive material is induced to emit fluorescence by aggregation. It is
assumed that the AIE effect is mainly caused by the restriction of
intramolecular rotation of the phenyl rings. For example, the com-
pounds which have a tetraphenylethylene (TPE) moiety show weak
fluorescence in solution but they show strong emission upon
aggregation because of the restriction of intramolecular rotation
of the TPE moiety. In fact, a chemosensor having a TPE moiety
has been described recently as having the ability to detect Ag+
and Hg2+ ions selectively.6
Herein, we report a fluorescence ‘turn-on’ chemosensor (1–2Zn)
which can be used for PPi sensing by means of combining a TPE
moiety showing unique aggregation-induced emission and a dipic-
olylamine(dpa)–Zn(II) complex exhibiting high sensitivity and
selectivity for PPi ions compared to other anions. It was reported
that dpa could form a complex with Zn2+ ions and that this dpa–
Zn(II) complex could be used as a binding module for PPi.7 We ex-
pected that when dpa–Zn(II) moieties bind PPi, the rotational
restriction of the phenyl rings of the TPE moiety would lead to fluo-
rescence emission.
The synthesis of 1–2Zn is described in Scheme 1. Compounds 2
and 3 were synthesized according to the reported methods.6 Com-
pound 1 was prepared by the SN2 reaction of 3 with 2,20-dipicolyl-
amine in the presence of K2CO3 and KI in acetonitrile at reflux
temperature. Sensor 1–2Zn, dinuclear Zn2+ complex of compound
1, is easily formed by the addition of a methanolic solution of 1
to an aqueous solution of 2 equiv of Zn(NO3)2.
First, we investigated the fluorescence emission change of com-
pound 1 according to Zn2+ concentration. Figure 1 shows the fluo-
rescence emission changes (kex = 320 nm) of 1 (100 l
M) as Zn2+
(nitrate salt) concentration increases in H2O/DMSO (10:1, v/v) at
25 °C. In the absence of Zn2+ ions, compound 1 showed strong
emission at 472 nm. Because of the hydrophobic feature of a TPE
moiety, compound 1 seems to be aggregated in an aqueous solvent
system, which results in fluorescence emission due to the ‘aggrega-
tion-induced emission (AIE)’. The higher the fraction of water in a
mixed solvent system (water–DMSO), the stronger the fluores-
cence emission becomes, as
a result of higher aggregation
(Fig. S1). However, upon the addition of Zn2+ ions, the emission
intensity of 1 gradually decreases as displayed in Figure 1. When
about 3 equiv of Zn2+ ions was added, the emission was almost
quenched. The decrease in fluorescence emission seems to be
* Corresponding author. Tel.: +82 28806682; fax: +82 28891568.
E-mail address: jihong@snu.ac.kr (J.-I. Hong).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.02.009

C. Park, J.-I. Hong / Tetrahedron Letters 51 (2010) 1960–1962
1961
O
O
Diphenyl ketone
BrCH2CH2CH2Br
K2CO3, DMF, N2
O
O
McMurry Reaction
O
O
HO
OH
Br
Br
Br
Br
2
3
4NO3
Zn(NO3)2
H2O, MeOH
2,2'-Dipicolylamine
K2CO3, KI, CH3CN
O
O
O
O
N
N
N
N
N
N
N
N
N
N
N
N
1-2Zn
1
Scheme 1. Synthesis of chemosensor 1–2Zn.
Figure 1. The fluorescence emission changes of 1 (100
addition of Zn2+ (nitrate salt) in H2O/DMSO (10:1, v/v) at 25 °C: [Zn2+] = 0, 47.6,
90.9, 130, 167, 200, 231, 259, 286, 310 M.
lM, kex = 320 nm) upon the
l
attributable to the degree of aggregation of compound 1. As Zn2+
ions are added, the dpa–Zn(II) complex (1–2Zn) is formed and in-
duces the destruction of aggregation which leads to the decrease in
the emission.
Next, we monitored the fluorescence emission change of sensor
1–2Zn according to the PPi (sodium salt) concentration. In contrast
to the Zn2+ addition, the addition of PPi to 1–2Zn resulted in a
gradual increase in the emission band at 472 nm as shown in
Figure 2. This fluorescence enhancement was almost saturated
with the addition of 1 equiv of PPi. The fluorescence intensity of
1–2Zn at 472 nm increases linearly with the concentration of PPi
Figure 2. The fluorescence emission changes of sensor 1–2Zn (100 lM,
kex = 320 nm) upon the addition of PPi (sodium salt) in H2O/DMSO (10:1, v/v) at
25 °C: [PPi] = 0, 9.90, 19.6, 29.1, 38.5, 47.6, 56.6, 65.4, 74.1, 82.6, 90.9, 99.1, 107, 115,
123, 130, 138, 145, 153, 167, 200 lM.
phenyl rings, which results in fluorescence enhancement. It is pos-
sible that two ways of coordination between 1–2Zn and PPi can oc-
cur as expected in the previous report: intramolecular
coordination and intermolecular coordination.6 However, in any
cases, the intramolecular rotations would be also restricted, lead-
ing to fluorescence enhancement.
in the range of 0ꢀ60
(I472nm = 5.62[PPi] + 32.9, r = 0.993, n = 7). The detection limit
turned out to be 0.90 M (Z = 2).
lM as shown in the inset of Figure S4
Another possible sensing mechanism would be induced charge
transfer via the conjugation between a phenolic oxygen atom and a
TPE moiety. Weakening the bond between the phenolic oxygen
and Zn2+ at the binding of PPi induces more negative charge char-
acter on the phenolic oxygen atom and thus the fluorescent
l
A putative sensing mechanism for this fluorescence OFF–ON is
shown in Scheme 2. A presumed binding mode for PPi and 1–
2Zn is based on our previous work involving a structurally similar
sensor and its X-ray crystal structure.7 The proposed complex
shows that the two sets of oxygen anions on each P atom of PPi
bind to the dinuclear Zn complex by bridging the two metal ions
to give rise to two hexacoordinated Zn2+ ions in 1–2Zn. Therefore,
when 1–2Zn binds to PPi, two dpa–Zn(II) moieties are fixed by PPi.
This fixation induces the restriction of intramolecular rotation of
enhancement occurs due to a more extended
p
-conjugation.7
Although the significant spectral shift is not shown in Figure 2,
we can find the gradual formation of the shoulder peak at around
510 nm with increasing PPi concentration. Therefore, we think that
the induced charge transfer may have a partial influence on the
sensing event.

1962
C. Park, J.-I. Hong / Tetrahedron Letters 51 (2010) 1960–1962
Scheme 2. The putative sensing mechanism.
Acknowledgments
This work was supported by the NRF grant funded by the MEST
(Grant No. 2009-0080734).
Supplementary data
Supplementary data (synthesis, 1H, 13C NMR and HRMS data of
compound 1, 1–2Zn, the fluorescence emission change of 1–2Zn
with different water fractions, and the comparison of the emission
intensity of 1 and 1–2Zn with other metal ions and anions) associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2010.02.009.
Figure 3. The comparison of the fluorescence emission intensity (I) at 472 nm of 1–
2Zn after the addition of 1 equiv of each anion with the emission intensity (Io) at
472 nm of 1–2Zn (100 lM, kex = 320 nm) before the addition of each anion.
References and notes
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Unlike the strong fluorescence enhancement observed upon the
addition of PPi, relatively small fluorescence changes are seen with
other anions. The fluorescence spectrum of sensor 1–2Zn was mea-
sured in the presence of other anions including Brꢀ, Clꢀ, Fꢀ,
ꢀ
ꢀ
ꢀ
H2PO4ꢀ, HCO3ꢀ, I , N3ꢀ, NO3ꢀ, CH3CO2 and SO4 under identical
conditions. As shown in Figure 3, enhancement of the fluorescence
intensity of 1–2Zn at 472 nm was rather small compared to that
upon the addition of PPi after the addition of 4 equiv of these an-
ions. Competition experiments between PPi and these anions also
indicate that 1–2Zn shows good selectivity toward PPi (Fig. S6).
However, the addition of AMP and ATP also enhanced the fluores-
cence intensity although the intensity was relatively small com-
pared to that after the addition of PPi.
In summary, we have developed a TPE-based fluorescent
chemosensor 1–2Zn which exhibits fluorescence ‘turn-on’ upon
binding with PPi anions. This study demonstrates that chemosen-
sor 1–2Zn can be used as a fluorescence turn-on sensor for PPi.
Fluorescence enhancement upon the addition of PPi to 1–2Zn re-
sults from the restriction of intramolecular rotation and/or induced
charge transfer. Further studies will include the design of new ana-
logues of 1–2Zn with enhanced sensitivity and selectivity for PPi
over AMP and ATP to enable the practical application of this type
of sensor to the monitoring of enzyme reactions.
6. Liu, L.; Zhang, G.; Xiang, J.; Zhang, D.; Zhu, D. Org. Lett. 2008, 10, 4581–4584.
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