A highly sensitive and selective turn-on fluorescent chemosensor for palladium based on a phosphine-rhodamine conjugate

Article abstract of DOI:10.1039/c2cc37746b

A fluorescent chemosensor with high sensitivity and selectivity for palladium species based on a conjugate of phosphine and rhodamine B has been developed. The chemosensor showed an excellent palladium selectivity with a detection limit down to the 10

Full text of DOI:10.1039/c2cc37746b

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A highly sensitive and selective turn-on fluorescent
chemosensor for palladium based on a
phosphine–rhodamine conjugate†
Songtao Cai,a Yan Lu,a Song He,a Fangfang Wei,a Liancheng Zhaoab and
Xianshun Zeng*a
Cite this: Chem. Commun., 2013,
49, 822
Received 18th October 2012,
Accepted 5th December 2012
DOI: 10.1039/c2cc37746b
www.rsc.org/chemcomm
A fluorescent chemosensor with high sensitivity and selectivity for enhanced chemosensor would be more efficient for Pd2+ detection.
palladium species based on a conjugate of phosphine and rhodamine Recently, Holdt et al. designed a few fluorescence enhanced
B has been developed. The chemosensor showed an excellent palladium chemosensors for Pd2+ ions by using a new concept of photo-
selectivity with a detection limit down to the 10À9 M range, which is induced electron transfer (PET) fluoroionophores in fluorophore–
lower than the WHO limit for palladium content in drug chemicals.
spacer–receptor systems.8 Koide et al. reported a turn-on fluorescent
sensing system for palladium based on the catalytic Tsuji–Trost
The fluorescent chemosensors play pivotal roles in the detection allylic oxidative insertion reaction of fluorescein derivatives.9 Then,
of ionic species and small molecules, especially in monitoring Ahn et al. extended the allylic oxidative insertion reaction in
heavy and transition metal ions.1,2 As a rare transition metal, propargyl ether of fluorescein, which provided a more efficient
palladium plays a significant role in materials chemistry. The way for palladium detection in practice.10 Peng and co-workers
palladium species are widely used as catalysts in chemical developed a novel rhodamine chemosensor for palladium species
transformations in the synthesis of various drugs and fine by taking advantage of both the p-affinity of Pd to allyl groups
chemicals.3 It is also widely used to prepare many dental and the well-known ring-opening process of the spirolactam of
materials, automobile exhaust catalysts, electric products, and rhodamine B.11 More recently, a cyclen-conjugated rhodamine
jewellery. As a thiophilic element, however, the palladium species hydroxamate was also reported with excellent Pd2+ selectivity.12
have an influence on human health and the environment in an Although the above excellent prototype devices have been developed
adverse way as palladium ions can bind to thiol-containing for the detection of palladium species, some of them suffer from the
amino acids, proteins, and other biomacromolecules.4 Recently, disadvantage that the sensing system requires initial conversion of
Bradley et al. reported that palladium could mediate Suzuki– Pd(II) to Pd(0) using a reducing agent,9 high temperature13
Miyaura cross-coupling reaction within cells.5 Thus, palladium or organic solvent.8 Thus, fluorescent chemosensors with better
exposure may disturb a variety of cellular processes.4 As a result, sensitivity and selectivity for palladium species are still needed to be
simple and reliable detection methods for palladium species with developed.
high substrate selectivity are vigorously pursued.
Enlightened by the fact that palladium is a strong phos-
Some conventional methods for palladium detection, such as phorus-affinity element,14 we report herein a palladium specific
atomic absorption/emission spectrophotometry and ion-coupled chemosensor L, a conjugate of phosphine and rhodamine B. It
plasma emission-mass spectrometry, are not only expensive and was observed that L showed excellent selectivity and high
time-consuming in practice, but also require highly trained indivi- sensitivity for Pd2+ ions over relevant competing metal ions
duals.6 In stark contrast, optical detection methods, particularly and anions. A turn on ratio over 154-fold was triggered under
fluorescence methods, show unique potential for the development saturated conditions. Furthermore, L was shown to be a promising
of highly sensitive and relatively simple analysis protocols. As a potential fluorescent chemosensor for the direct qualitative deter-
fluorescence quencher, Pd2+ could be detected by chemosensors mination of residual palladium in drug chemicals.
through fluorescence quenching.7 However,
a
fluorescence-
As shown in Scheme 1, chemosensor L was synthesized by
treatment of rhodamine B with POCl3, and then condensed
with 2-(diphenylphosphino)benzenamine (DPBA), which was
prepared in three steps starting from 2-chlorobenzenamine
and triphenylphosphine according to the known procedure.15
In ethanol–H2O (4 : 1, v/v) solution, L (10 mM) exhibited a very
weak absorption band at 544 nm, which could be attributed to the
a School of Materials Science & Engineering, Tianjin University of Technology,
Tianjin 300384, China. E-mail: xshzeng@tjut.edu.cn; Fax: +86-22-60215226;
Tel: +86-22-60216748
b School of Materials Science and Engineering, Harbin Institute of Technology,
Harbin 150001, China
† Electronic supplementary information (ESI) available: Experimental details,
synthesis of L, spectra and other electronic formats. See DOI: 10.1039/c2cc37746b presence of trace amounts of the ring-opened form of L. Addition of
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L (1 mM) exhibited a very weak fluorescence (F = 0.003; lex = 530 nm)
at 587 nm. The titration of Pd2+ with L led to a rapid increase of the
emission intensity (F = 0.45; lex = 530 nm) at 587 nm. Over 150-fold
fluorescence enhancement was observed under saturated condi-
tions (ca. 50 equiv.). The increase of the fluorescence intensity at
587 nm followed the sigmoidal curves and the fluorescence turn-on
constant (Kturn-on) was calculated as 32.0 Æ 0.3 mM (with correlation
coefficient R = 0.999) (Fig. S3, ESI†).16 From the changes in Pd2+-
dependent fluorescence intensity at 587 nm (Fig. S4, ESI†), the
detection limit was estimated to be 1.49 Â 10À9 M, indicating that
the limit of detection of L for Pd2+ met the WHO specified threshold
Scheme 1 Structure and the synthesis of the chemosensor L.
limit for palladium content in drug chemicals [4.7 Â 10À5
M
(5 ppm) to 9.4 Â 10À5 M (10 ppm)].17 In fact, over 34-fold and
68-fold fluorescence enhancement was observed in the presence of
5 ppm and 10 ppm of Pd2+ ions, respectively (Fig. S5, ESI†).
Reversible binding of L with Pd2+ was also carried out. The addition
of Na2S to a mixture of L and Pd2+ resulted in diminution of the
fluorescence intensity at 587 nm, which indicated the regeneration
of the free chemosensor L. The fluorescence was recovered by the
addition of Pd2+ again (Fig. S6, ESI†). Such reversibility and
regeneration are important for the fabrication of devices to sense
the Pd2+ ion. In the presence of one equivalent of commonly found
palladium catalysts, such as PdCl2(cod), Pd(PhCN)2Cl2, Pd2(dba)3,
Pd(PPh3)4, K2PdCl4, L showed significant fluorescence enhancement
and prominent colour changes (Fig. S7, ESI†), indicating that L could
be a promising potential selective fluorescent chemosensor for the
direct detection of residual palladium including Pd(II) and Pd(0)
species in drug chemicals. Thus, the fact that L responded not only
to Pd2+ compounds but also to Pd0 species indicated that the
selectivities were determined by ligand exchange between L and
the ligand of palladium species.
In the presence of palladium species, both absorption and
fluorescence spectra present a prominent OFF–ON signal, indicating
that palladium induced the structural conversion of the chemo-
sensor from spirolactam to the ring-opened xanthene form of the
rhodamine moiety. Upon adding 2 equiv. of Pd2+, the maximum
absorbance at 566 nm produced immediately, and then it was
gradually converted to a new one at 550 nm within 30 min. During
the conversion process, the appearance of an isosbestic point at
555 nm indicated that the chelating mode of LÁPd2+ converted from
one to another (Fig. S8, ESI†). In the presence of Pd2+, the time-
dependent fluorescence response was also observed to be maximum
after 20 min (Fig. S9, ESI†). Accordingly we hypothesized that
the interaction of L with Pd2+ to form the final five-membered ring
N4P–Pd2+ complex occurs via a seven-membered intermediate
O4P–Pd2+ as depicted in Scheme 2. High-resolution mass spectra
(HRMS) provided additional evidence for the formation of a 1 : 1
complex of LÁPd2+ (Fig. S10, ESI†).
Fig. 1 UV-vis titration spectra of L (10 mM). Upon titration of Pd2+ (0–2 equiv.) in
ethanol–water (4 : 1, v/v), CPd2+/CL = 0, 0.05, 0.2, 0.4, 0.6, 0.8, 1.0, 1.25, 1.5, 1.75,
2. Inset: the absorbances at 544 nm of L as a function of CPd2+/CL.
1 equiv. of Pd2+ into the solution, however, immediately induced a
significant absorbance enhancement (27-fold) at 544 nm (Fig. S1,
ESI†). Only a very weak increase of absorbance at 544 nm was
observed with the same amount of Cu2+, Fe3+ and Hg2+. No obvious
response could be observed upon the addition of other ions.
Meanwhile, upon the addition of Pd2+, the color of the solution
immediately changed from colorless to pink-red, indicating that L
can serve as a ‘‘naked-eye’’ probe for Pd2+. As shown in Fig. 1, the
absorbance at 544 nm increased smoothly in the presence of
different concentrations of Pd2+, which indicated the formation
of a new complex between L and Pd2+. A linear dependence of
the absorbance at 544 nm as a function of [Pd2+]/[L] was observed
(inset of Fig. 1). Stoichiometry for L and the Pd2+ complex was
evaluated on the basis of the Job’s plot and was found to be 1 : 1
(Fig. S2, ESI†).
Fig. 2 shows the fluorescence spectrum of L and those in the
presence of different concentrations of Pd2+. The free chemosensor
Subsequently, various metal ions were used to evaluate the metal
ion binding properties and the selectivity of L (1 mM) by means of
fluorescence spectra in EtOH–H2O (4 : 1, v/v). Among the metal ions
examined, L showed a selective fluorescence increase only with Pd2+
(Fig. S11, ESI†). The addition of 2 equiv. of Pd2+ resulted in a
significant enhancement of the emission intensity (22-fold)
positioned at 587 nm. However, the addition of other ions has no
obvious effect on the fluorescence emission. Thus, L can function as
a highly selective fluorescent chemosensor for Pd2+ ions.
Fig. 2 Fluorescent titration spectra of L (1 mM) in the presence of different
concentrations of Pd2+ in ethanol–water (4 : 1, v/v), CPd2+/CL = 0, 1, 2.5, 5, 10,
15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 100. lex = 530 nm. Slit:
5.0 nm; 10.0 nm.
c
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This work was sponsored by the NNSFC (21272172, 20972111,
21074093, 21004044), NCET-09-0894, the NSFT (12JCZDJC21000).
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824 Chem. Commun., 2013, 49, 822--824
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