A fluorescence turn-on sensor for the detection of palladium ions that operates through in situ generation of palladium nanoparticles

Article abstract of DOI:10.1039/c3cc48364a

A simple and straightforward fluorescence method for the detection of palladium ions at low concentrations has been developed. The mode of operation of the sensor involves in situ generation of palladium nanoparticles (PdNPs), which promote a selective deiodination reaction of iodo-BODIPY that forms highly fluorescent H-BODIPY.

Full text of DOI:10.1039/c3cc48364a

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A fluorescence turn-on sensor for the detection
of palladium ions that operates through in situ
generation of palladium nanoparticles†
Cite this: Chem. Commun., 2014,
50, 1268
Received 1st November 2013,
Accepted 26th November 2013
Dongho Keum, Sungwoo Kim and Youngmi Kim*
DOI: 10.1039/c3cc48364a
www.rsc.org/chemcomm
A simple and straightforward fluorescence method for the detection of
A few fluorescence-based probes have been reported recently for
palladium ions at low concentrations has been developed. The mode of the detection of palladium species.6 One type relies on changes in
operation of the sensor involves in situ generation of palladium nano- photoinduced energy/electron transfer governed emission intensities
particles (PdNPs), which promote a selective deiodination reaction of caused by the formation of Pd2+-specific coordination induced
iodo-BODIPY that forms highly fluorescent H-BODIPY.
complexes.7 The others employ Pd-mediated chemical reactions
(i.e., Tsuji–Trost reaction, oxidative cyclization) to transform none-
Palladium species are versatile and widely used catalysts for numerous missive dyes into highly fluorescent products.8 Although these
chemical transformations employed for the production of fine chemi- sensors are based on conceptually appealing designs, they possess
cals, therapeutic drugs, and polymers.1 Pd is also extensively used in several important drawbacks. In general, the sensing systems exhibit
various commercial materials such as automobile catalytic converters, poor selectivity for palladium, and they require the use of media that
jewellery, dental alloys, fuel cells, medical instruments, and electronics.2 contain a high proportion of organic solvents, as well as extended
However, palladium released by these materials or present in pharma- reaction times at elevated temperatures.
ceuticals has raised great concern because it can adversely affect
In the investigation described below, we have developed a simple
human health and the environment.3 It is known that palladium fluorescence turn-on sensing system for the detection of palladium.
species in body organs disturb a variety of cellular processes and cause In this system, palladium nanoparticles (PdNPs), generated from Pd
serious physiological disorders including memory loss, dizziness, salts, serve as an in situ catalyst for efficient C–I bond cleavage
migraine headaches, allergies, immune system impairment, and facial reactions of a profluorescent iodo-dye. The strategy used to design
paralysis.2 As a result, the threshold level of palladium in pharmaceu- the sensor was stimulated by earlier observations that show that
tical products is strictly limited to be no more than 5–10 ppm and the PdNPs can be readily prepared by reduction of Pd2+ precursors9 and
maximum daily human dietary intake of palladium is restricted to less that they arguably serve as catalysts for C–C coupling reactions.10
than 1.5–15 mg.4 Consequently, efficient methods for monitoring these
low levels of palladium in industrial and pharmaceutical settings, as chromophores to demonstrate this concept. BODIPYs exhibit high
well as in environmental samples, are highly desirable. fluorescence quantum yields, high extinction coefficients, and
Boron dipyrromethane (BODIPY) dyes were selected as the
Conventional methods for palladium detection include atomic excellent photostability.11 In addition, their spectroscopic proper-
absorption spectroscopy (AAS), HPLC coupled with solid-phase ties can be easily tuned by simple synthetic modifications. The
microextraction, and inductively coupled plasma atomic emission singlet excited states of BODIPY derivatives that contain heavy
(ICP-AES/OES), X-ray fluorescence, and inductively coupled plasma atom substituents at C2 and C6 positions are known to undergo
mass spectrometry (ICP-MS).5 Although being the most sensitive and rapid intersystem crossing to generate the corresponding triplets,
reliable techniques, ICP and AAS require costly instrumentation and a process which leads to markedly decreased fluorescence efficien-
time-consuming manipulations. In contrast, a solution fluorescence- cies.12 As a result of this phenomenon, reactions in which heavy
based sensing protocol would be highly advantageous because it iodine atoms are excised from a BODIPY core have been explored
would enable simple, cost-effective, rapid, and sensitive detection of as turn-on transduction schemes for the detection of DNA by
palladium species in a high throughput fashion.
Herrmann et al.,13 and for the detection of gold ions by our
group.14 In the current work, we employed iodo-BODIPY 1 as a
profluorescent probe for the detection of palladium (Scheme 1).
The emission properties of 1 and its corresponding deiodinated
derivative 2 were elucidated to demonstrate that they are suitable
for this approach (Table S1, ESI†). As expected, the fluorescence
Department of Chemistry, Dankook University, 126 Jukjeon-dong, Yongin-si,
Gyeonggi-do, 448-701, Korea. E-mail: youngmi@dankook.ac.kr;
Fax: +82 31-8005-3148; Tel: +82 31-8005-3156
† Electronic supplementary information (ESI) available: Formation of PdNPs under
various conditions and fluorescence responses. See DOI: 10.1039/c3cc48364a
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Fig. 1 (a) Time-dependent emission spectra of iodo-BODIPY 1 (5 mM)
upon treatment with PdCl2 (20 mM) in ethanol–water solution (1 : 4, v/v) at
25 1C. lex = 465 nm. Inset contains photographs of solutions of probe 1 in
the absence (left) and presence (right) of PdCl2 after incubation for 30 min
under UV light (365 nm) illumination. (b) Relative fluorescence intensities at
510 nm as a function of incubation time.
Scheme 1 (a) Overview of the sensing method using iodo-BODIPY 1 to
detect Pd2+. In situ generated PdNPs from Pd salts in C2H5OH–water
trigger the conversion of iodo-BODIPY 1 to H-BODIPY 2, which results in
turn-on fluorescence. (b) Absorption and emission spectra of iodo-
BODIPY 1 and H-BODIPY 2 in C2H5OH–water (1 : 4, v/v).
quantum yield of iodo-BODIPY 1 (FPL = 0.017) is greatly reduced
compared to that of the reduced analog 2 (FPL = 0.55).
To demonstrate the feasibility of the new in situ generated
NP-based palladium sensing system, we first investigated the genera-
tion of PdNPs from Pd salts in aqueous media under mild reducing
conditions. To probe this feature, PdCl2 was selected as a representa-
tive palladium species. Among several possible reducing agents,
aqueous ethanol was selected for its rapid promotion of the Pd2+ to
Pd0 conversion, leading to aggregation and generation of PdNPs.
Accordingly, using UV-Vis spectroscopy we showed that PdNPs are
readily generated by mixing PdCl2 in aqueous ethanol at room
temperature under an air atmosphere (Fig. S1, ESI†). The conversion
of Pd2+ to PdNPs resulted in generation of a dark colloidal solution,
accompanied by a smooth increase in the shorter wavelength absorp-
tion band in a manner that is similar to the palladium monolayer
Fig. 2 (a) Fluorescence titration spectra of iodo-BODIPY 1 (5 mM) in the
presence of different concentrations of Pd2+ in ethanol–water (1:4, v/v) at
25 1C. [Pd2+] = 0, 0.5, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90,
100 mM. lex = 465 nm. Each spectrum was obtained 30 min after addition of Pd2+
.
Inset shows a plot of the relative fluorescence intensity at 510 nm as a function of
[Pd2+]. (b) HPLC chromatograms of 1 without Pd2+ (top); with Pd2+ treatment in
ethanol–water (1 : 4, v/v) for 30 min at 25 1C (middle); 2 only (bottom).
protected clusters reported by Murray and co-workers.15 Transmission 30 min. As expected, the fluorescence intensities at 510 nm increase
electron microscopy (TEM) analysis further showed that the PdNPs, with increasing PdCl2 concentrations in this range. Subsequent data
formed by rapid reduction of the Pd salts, have a nearly spherical analysis shows that a linear relationship (R2 = 0.98) exists between the
shape with an average diameter of ca. 4.2 nm (Fig. S3, ESI†).
The sensing behavior of the system, promoted by in situ generation tions in the 10–50 mM (1–5 ppm Pd content) range (Fig. 2a, inset).
of PdNPs, was investigated by simply adding PdCl2 to a solution of LC-MS analysis showed that H-BODIPY 2 is the sole BODIPY-
normalized fluorescence intensities at 510 nm and PdCl2 concentra-
iodo-BODIPY 1 in ethanol–H2O (1:4, v/v) at 25 1C. Upon addition of derived product generated in the assay solution upon addition
PdCl2, the absorption and emission bands of 1 at lmax,abs = 535 nm of PdCl2 (Fig. 2b), thus confirming that the formation of PdNPs
and lmax,em = 555 nm, respectively, undergo a rapid and large decrease promote C–I bond cleavage of iodo-BODIPY 1. Whether PdNPs
in intensities. In addition, concomitant increases take place in the can directly react with 1 to result in deiodinated 2, or merely act
intensities of absorption and emission bands at lmax,abs = 501 nm and as a reservoir for an active soluble Pd(0) species is not currently
lmax,em = 512 nm, respectively, which are characteristic of the known.10 However, under the assay conditions, deiodination
deiodinated product, H-BODIPY 2 (Fig. 1 and see ESI†). Importantly, always coincides with the formation of PdNPs.
in the absence of Pd2+, iodo-BODIPY 1 in aqueous ethanol (1 : 4, v/v) at
25 1C is stable for more than 2.5 h under continuous visible aqueous alcohols (i.e., isopropyl alcohol, 1-propanol) were employed,
irradiation (lex = 465 nm) (Fig. S11, ESI†). but negligible fluorescence intensity changes were observed when a
Similar fluorescence turn-on responses were observed when other
Notably, the fluorescence turn-on process, associated with the reducing alcohol was not present (i.e., acetone, acetonitrile, or DMSO in
production of bright green fluorescence, occurs in less than 2 min, water), or when the alcohol contains amine groups susceptible to
and it reaches saturation within 30 min at room temperature (Fig. 1). coordinate to and stabilize mononuclear Pd species (i.e., 3-amino-1-
As large as a 560-fold enhancement in the fluorescence intensity at propanol in water) (Fig. S8, ESI†). The results of these control experi-
lmax,em = 510 nm is observed upon addition of 4 equiv. Pd2+ to a ments suggest that the conversion of iodo-BODIPY 1 to H-BODIPY 2
solution of 1. This new fluorescence emission is easily discernible to the upon addition of Pd2+ salts does not occur without formation of PdNPs.
naked eye under 365 nm irradiation (Fig. 1a inset). In Fig. 2a is shown
The sensing response of the system comprised of 1 toward
the fluorescence sensing response of 1 (5 mM) to different concentra- other metal ions was explored using the conditions established
tions of PdCl2 in the range of 0–100 mM after incubation times of above (ethanol–H2O (1 : 4, v/v), 25 1C). Spectroscopic analyses,
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simple sample preparation ensures reliable detection regardless of
palladium speciation.
Finally, an application of the new sensor to the determination of
residual Pd content in synthetic compounds prepared by palladium-
catalyzed reactions was explored. Specifically, the amount of Pd in a
biphenyl derivative 3, synthesized by using a Suzuki–Miyaura cross
coupling reaction and purified by column chromatography, was
assayed (Scheme S1, ESI†). Our fluorometric analysis showed that
the purified 3 (38 mg) contains 2.0 Æ 0.1 ppm of Pd (Fig. S21, ESI†).
The results demonstrate the practical utility of the new sensing
system for the quantitative analysis of Pd in the synthetic inter-
mediate prepared by palladium-catalyzed reactions.
In conclusion, the study described above has led to the develop-
ment of a highly sensitive fluorescence sensing system for the
detection and quantitation of palladium species. The sensor relies
on the reductive deiodination reaction of iodo-BODIPY promoted by
PdNPs, which are generated in situ by the reduction of palladium
Fig. 3 Fluorescence emission spectra of iodo-BODIPY 1 (5 mM) in ethanol–
water (1 : 4, v/v) containing various metal ions (as their chloride salts except for
AgNO3), measured 30 min after addition of each metal ion at 25 1C. [metal ion] =
20 mM for Pd2+ and 50 mM for all other metal ions. lex = 465 nm.
carried out 30 min after addition of 4.0 equiv. of each of the metal salts in ethanol–water mixtures. The new method for generating a
ions, show that large fluorescence turn-on response occurs only in the strong (>560-fold) and specific turn-on response to Pd species has
presence of Pd2+. Importantly, none of the other metal species, potential practical applications for the quantitative and qualitative
including Al3+, Ca2+, Cd2+, Cu2+, Co2+, Cr2+, Fe2+, Fe3+, Pb2+, Zn2+, assay of Pd species in environmental and chemical samples.
Mg2+, Hg2+, Ni2+, Mn2+, K+, Ag+, Na+, Au+, Au3+, and Pt2+, causes
This research was supported by the research fund of Dankook
changes in the emission profile of probe 1 (Fig. 3). The excellent University in 2011.
selectivity displayed by the sensor is attributed to the highly specific
formation of the PdNPs, which mediate the C–I bond cleavage
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palladium is interfered with by other ions that have higher standard
reduction potentials (E0) than that of Pd2+ to Pd (E0 = +0.915 V). For
example, the addition of Pd2+ to a solution of probe 1 containing
either Au3+, Ag+, Pt2+, or Hg2+ leads to negligible increases in
fluorescence intensities at 510 nm.16 This phenomenon is likely a
consequence of the ability of these metal ions either to promote
oxidation of Pd0 to form Pd2+, hence preventing the growth of PdNPs,
or to generate catalytic inactive alloyed NPs.
´
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determined using initial rate data, was found to be kobs = 3.15
(Æ0.26) Â 103 MÀ1 minÀ1 (see ESI†). The rate of this process is
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Pd(NO3)2 E Pd(OAc)2 E PdCl2 E Na2PdCl4 > Pd(PPh3)4
>
16 Addition of an external reducing agent such as NaBH4 does not
affect the interference of these metal ions.
PdCl2(PPh3)2 E PdCl2(dppf)2 (Fig. S20, ESI†). As shown below,
1270 | Chem. Commun., 2014, 50, 1268--1270
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