Push-pull-type purine nucleoside-based fluorescent sensors for the selective detection of Pd2+ in aqueous buffer

Article abstract of DOI:10.1002/ejoc.201301897

Push-pull-type purine nucleoside-based fluorescent sensor L1 bearing a chelating N,N-bis(2-pyridylmethyl)amine group at the C6 position was designed and synthesized. Sensor L1 showed clear and selective detection of Pd ²⁺ in aqueous buffer, and it is the first on-off -type purine nucleoside-based fluorescent sensor for Pd²⁺. Push-pull-type purine nucleoside-based fluorescent sensor L1 bearing a chelating N,N-bis(2-pyridylmethyl)amine group at the C6 position is designed and synthesized. Sensor L1 shows clear and selective detection of Pd²⁺ in aqueous buffer, and it is the first on-off -type purine nucleoside-based fluorescent sensor for Pd²⁺. Copyright

Full text of DOI:10.1002/ejoc.201301897

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201301897
Push–Pull-Type Purine Nucleoside-Based Fluorescent Sensors for the Selective
Detection of Pd2+ in Aqueous Buffer
Jian-Ping Li,[a] Hui-Xuan Wang,[a] Hai-Xia Wang,*[a] Ming-Sheng Xie,[a] Gui-Rong Qu,[a]
Hong-Ying Niu,[b] and Hai-Ming Guo*[a]
Keywords: Fluorescence / Sensors / Nucleosides / Palladium
Push–pull-type purine nucleoside-based fluorescent sensor
L1 bearing a chelating N,N-bis(2-pyridylmethyl)amine group
at the C6 position was designed and synthesized. Sensor L1
showed clear and selective detection of Pd2+ in aqueous
buffer, and it is the first “on–off”-type purine nucleoside-
based fluorescent sensor for Pd2+
.
Introduction
Fluorescence technology is widely used to explore the
structure, dynamics, and recognition of biomolecules,[1] and
this has led many researchers to investigate the design of
various fluorescent sensors. With the aim to apply fluores-
cent sensors in vivo, biological compatibility is important
and favorable for an excellent fluorescent sensor.[2] Purine
nucleosides are ubiquitous in RNA and DNA and possess
outstanding biological compatibility.[3] However, owing to
their weak fluorescent properties, purine nucleosides are
difficult to use as fluorescent sensors.[4] Thus, we developed
an efficient and novel way to enhance their fluorescent
properties by a push–pull strategy.[5] In this new type of
fluorescent sensor, the purine part functioned as the chro-
mophore; an electron-donating amino derivative was di-
rectly incorporated into the C6 position of the purine to
have a push effect, and an electron-withdrawing aryl group
was attached to the C8 position of the purine to increase
the conjugated system and exert a pull influence. Through
this push–pull effect, electrons were able to flow into
the fluorescent sensor, and the fluorescent properties of
the purine nucleosides were simultaneously enhanced
(Scheme 1).
Scheme 1. Push–pull-type purine nucleoside-based fluorescent sen-
sors.
Nowadays, the development of fluorescent sensors for
heavy-metal and transition-metal ions is a hot investigative
topic.[6] Recently, palladium has attracted considerable at-
tention owing to its extensive use in many catalytic reac-
tions and industrial applications.[7] The use of such catalysts
creates ultimate products often containing a high level of
residual palladium, which can lead to the pollution of the
environment, and furthermore, this metal is detrimental to
health.[8] Therefore, searching for an efficient fluorescent
sensor for the selective detection of Pd2+ is highly desir-
able.[9] In our push–pull-type purine nucleoside-based fluo-
rescent sensors, if a chelating group is introduced into the
C6 position of adenosine, this type of fluorescent sensor
may show high selectivity towards metal ions including
Pd2+ (Scheme 1). Herein, we wish to report new push–pull-
type purine nucleoside-based fluorescent sensors for the se-
[a] Collaborative Innovation Center of Henan Province for
Green Manufacturing of Fine Chemicals, Key Laboratory of
Green Chemical Media and Reactions, Ministry of Education,
School of Chemistry and Chemical Engineering, Henan
Normal University,
lective detection of Pd2+
.
Xinxiang, Henan 453007, P. R. China
E-mail: hxwang@htu.cn
guohm518@hotmail.com
http://www.htu.cn/s/57/t/1350/a/28232/info.jspy
[b] Department, School of Chemistry and Chemical Engineering,
Henan Institute of Science and Technology,
Xinxiang 453003, China
Results and Discussion
Initially, we chose the 4-nitrobenzyl group as the elec-
tron-withdrawing group attached to the C8 position of
adenosine to form a push effect and incorporated N,N-di-
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301897.
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H.-X. Wang, H.-M. Guo et al.
SHORT COMMUNICATION
Scheme 2. Synthesis route of L1–L3. Reagents and conditions: (a) 1-iodo-4-nitrobenzene, Pd(OAc)2, CuI, piperidine, DMF, 120 °C, 24 h;
(b) Ac2O, 4-(dimethylamino)pyridine, pyridine, r.t., 4 h; (c) isoamyl nitrite, CHBr3, 65 °C, 3.5 h; (d) RH, EtOH, reflux.
(2-pyridylmethyl)amine (DPA) and similar amines[10] as the of sensor L1 was 8.83ϫ103 Lmol–1 cm–1 and the quantum
chelating groups in our purine nucleoside-based fluorescent yield of sensor L1 was 0.011; the fluorescence quantum
sensors (Scheme 2, L1–L3). It was expected that the elec- yield was determined relative to known standards such as
tron density of the conjugating system would be redistrib- tryptophan in H2O (λex = 286 nm, λem = 355 nm, Ø = 0.13).
uted upon coordination to specific metal ions, which would
give rise to a clear change in the emission spectra.
To evaluate the selectivity of L1–L3, the fluorescence
spectra of L1–L3 (6.0 μm) in the presence of different metal
Subsequently, the photophysical properties of L1–L3 ions (4.0 equiv.) were recorded in aqueous buffer (10%
were studied (Figure 1). According to the equation of fluores- DMSO, v/v, pH 7.3). As shown in Figure 2 (a), the fluores-
cence quantum efficiency,[5d,11] the emission intensity of cence of L1 was strongly quenched upon the addition of
sensor L1 is more than 3 times stronger than that of sensor Pd2+, and the presence of other metal ions including Zn2+
,
L2 and about 1.5 times stronger than that of sensor L3. Pb2+, Cd2+, Cr3+, Co2+, Cu2+, Fe3+, Ni2+, Mn2+, Hg2+, and
With respect to the luminous properties, the structural dif- Ag+ resulted in negligible fluorescence intensity changes
ference of the electron-donating groups at the C6 position under identical conditions. However, for L2 there were no
resulted in a distinction in the fluorescence intensity, and distinctive fluorescence changes upon the addition of dif-
modification by the DPA group was clearly favored over ferent metal ions (Figure S1b, Supporting Information),
the other amine groups. Moreover, the molar absorptivity and the fluorescence of L3 increased to different extents,
Figure 1. Absorption spectra and fluorescence spectra of L1–L3 (6.0 μm) in 50.0 mm aqueous Tris-HCl buffer (10% DMSO, v/v, pH 7.3).
Excitation wavelength of L1 and L2 was 310 nm. Excitation wavelength of L3 was 300 nm.
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Push–Pull-Type Purine Nucleoside-Based Fluorescent Sensors
Figure 2. (a) Fluorescence responses of L1 toward various metal ions (4.0 equiv.). (b) Fluorescence response of L1 (6.0 μm) upon the
addition of different metal cations (4.0 equiv.), and the bars are relative fluorescence intensities (I0/I). All the experiments were performed
in aqueous Tris-HCl buffer (10% DMSO, v/v, pH 7.3, λex = 310 nm).
except for Pd2+ (Figure S1c, Supporting Information). The quenched completely upon the addition of 6.0 equiv. Pd2+
.
effect of metal ions on the fluorescence intensity of L1 is Moreover, the fluorescence intensity of L1 is linearly pro-
shown in Figure 2 (b), which suggests that the selectivity of portional to the concentration of Pd2+ in the range from 0
L1 toward Pd2+ over other metal ions is extremely high. To to 1.8 equiv. (Figure 3, b, c). The detection limit of sensor
our delight, this high selectivity of the fluorescence probe L1 for Pd2+ was estimated to be 6.47ϫ10–7 m; conse-
towards Pd2+ has not been found in other probes with a quently, L1 can be used for Pd2+-polluted analysis in drugs
DPA receptor.
according to the World Health Organization specified
To understand the coordination effect of sensor L1 with threshold limit for palladium content in drug chemicals
Pd2+ in detail, fluorescence titration experiments of L1 with [4.7ϫ10–5 m (5.0 ppm) to 9.4ϫ10–5 m (10.0 ppm)].[8e,12] Be-
Pd2+ were performed at room temperature in aqueous Tris- sides Pd(OAc)2, other Pd2+ sources such as PdCl2 could
HCl buffer (10% DMSO, v/v, pH 7.3). As shown in Fig- also be detected, which demonstrates the potential applica-
ure 3 (a), with an increase in the amount of Pd2+, a clear tions of Pd2+ sensor L1 (Figure S2, Supporting Infor-
change in the fluorescence spectrum of L1 took place. The mation).
intensity of the fluorescent band of L1 centered at 400 nm
Then, competition experiments were conducted to evalu-
decreased progressively, and the fluorescence was almost ate the selectivity of the binding event between L1 and Pd2+
Figure 3. (a) Fluorescence spectra of L1 (10.0 μm) upon the addition of various concentrations of Pd2+ (0, 0.2, 0.5, 0.8, 1.0, 1.2, 1.8, 2.2,
2.6, 3.0, 3.6, 4.0, 5.0, 6.0, 7.0, 8.0 equiv.) in aqueous buffer (10% DMSO, v/v, pH 7.3). (a) Relative fluorescence intensities (I/I0) as a
function of the concentration of Pd2+ in aqueous buffer (10% DMSO, v/v, pH 7.3). (c) Fluorescence intensities of L1 versus Pd2+ concen-
tration. Excitation wavelength was 310 nm.
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H.-X. Wang, H.-M. Guo et al.
SHORT COMMUNICATION
(Figure 4). The results embodied that the Pd2+-induced
fluorescence was unaffected in the presence of competitive
metal ions, which could be ascribed to the tight coordina-
tion of the DPA chelator to Pd2+. Thus, L1 can function as
a fluorescence sensor for the selective detection of Pd2+
.
Figure 5. Fluorescence spectral changes of L1–Pd2+ mixed solution
(6 μm for L1, [Pd2+]/[L1] = 6) upon increasing the concentration of
S2– in aqueous buffer. Excitation wavelength is 310 nm.
Figure 4. Fluorescence responses of L1 (6.0 μm) toward Pd2+
(30.0 μm) in 50.0 mm aqueous Tris-HCl buffer (10% DMSO, v/v,
pH 7.3) after the addition of other competitive metal ions
(30.0 μm). Red bar represents the emission intensity of L1 (6.0 μm).
Yellow bar represents the emission intensity of L1 (6.0 μm) in the
presence of Pd2+ (30.0 μm). Blue bars represent the emission inten-
sities that occurred upon the subsequent addition of Pd2+ (30.0 μm)
to the mixed solutions of L1 (6.0 μm) toward 30.0 μm other compet-
itive metal ions (Na+, K+, Mg2+, Ca2+, Ba2+, Hg2+, Cr3+, Mn2+
,
Cd2+, Fe3+, Pb2+, Zn2+, Cu2+, Ni2+, Co2+, and Ag+). Emission in-
tensities were recorded at 400 nm, and the excitation wavelength
was 310 nm.
Subsequently, the mechanism of the fluorescent sensor
for the selective detection of Pd2+ was investigated.[13] As
mentioned above, after adding various metal ions, the
fluorescence intensity of L3 increased to different extents
(Figure S1c, Supporting Information). However, the ad-
dition of Pd2+ to the solution of L3 demonstrated a negligi-
ble effect on the fluorescence. Thus, we confirmed that
(1) Pd2+ was not an inherent fluorescent quencher and
(2) the N atom of the purine skeleton probably did not in-
teract with Pd2+ to quench the fluorescence of L1. More
importantly, the clearly comparative results indicated that
the DPA moiety of L1 played a key role in the selective
binding with Pd2+
.
Figure 6. Partial 1H NMR (400 MHz) spectra of L1 in [D6]DMSO.
(a) [Pd2+]/[L1] = 0 equiv., (b) [Pd2+]/[L1] = 0.3 equiv., (c) [Pd2+]/[L1]
= 0.5 equiv., (d) [Pd2+]/[L1] = 0.7 equiv., (e) [Pd2+]/[L1] = 1.0 equiv.,
and (f) [Pd2+]/[L1] = 2.0 equiv.
Next, S2– titration experiments of the L1-Pd2+ system
were used to test the coordination between L1 and Pd2+. If
an excess amount of S2– was added to a solution of L1-
Pd2+, the fluorescence of L1 recovered gradually (Figure 5).
This suggested that the binding of Pd2+ with L1 was the
reason for the fluorescence quenching. To our delight, the
binding process was reversible, and thus, L1 is the first “on–
off”-type purine nucleoside-based fluorescent sensor for
Pd2+ (Figure 6).
Conclusions
In conclusion, push–pull-type purine nucleoside-based
Moreover, this hypothesis was supported by HRMS ex- fluorescent sensor L1 possessing a DPA moiety as the bind-
periments. The appearance of a clear peak at m/z = ing unit was designed and synthesized. Sensor L1 displays
837.1113 was assigned to [L1 + Pd2+ + Cl–]+ (Figure 7, a). specific selectivity and sensitivity toward Pd2+ with a low
On the basis of the overall consideration of all the various detection limit in aqueous buffer, and it therefore has po-
evidence, a clear binding mode of Pd2+ with L1 was pro- tential application in vivo and industrial analyses. To the
posed as shown in Figure 7 (b).
best of our knowledge, this is the first “on–off”-type purine
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Push–Pull-Type Purine Nucleoside-Based Fluorescent Sensors
Figure 7. (a) HRMS spectra of L1 in the presence of PdCl2 (2.0 equiv.) in CH3CN. (b) Proposed interaction model between L1 and Pd2+
.
nucleoside-based fluorometric sensor for Pd2+ that exhibits
remarkable fluorescent quenching. Further application of
fluorescent sensor L1 is currently underway.
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Acknowledgments
The authors are grateful for financial support from the National
Natural Science Foundation of China (NSFC) (grant numbers
21072047, 21172059, 21272059, 21202039, 21372066, and
21372064), the Excellent Youth Foundation of Henan Scientific
Committee (grant number 114100510012), the Program for Innov-
ative Research Team from the University of Henan Province
(2012IRTSTHN006), the Program for Changjiang Scholars and In-
novative Research Team in the University (IRT1061), the Research
Fund for the Doctoral Program of Higher Education of China
(grant number 20124104110006), and the Program for Science &
Technology Innovation Talents in Universities of Henan Province
(grant number 13HASTIT013).
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