Fluorescent sensing and selective Pb(II) extraction by a dansylamide ion-exchanger

Article abstract of DOI:10.1021/ja050296e

The (bis)dansylated sulfonamide 1,2-C₆H₄(NHSO₂C₁₀H₆-5-N(CH₃)₂)₂ (1) extracted Pb(II) selectively from water into 1,2-dichloroethane via an ion-exchange mechanism and showed fluorescence quenching upon Pb(II) extraction. The distribution ratios for metal extraction (determined by ICP-MS) for Pb(II) were 133-1410 times higher than those for other metal cations [Co(II), Ni(II), Cu(II), Zn(II), and Cd(II)] under identical conditions. Fluorescence quenching was observed upon Pb(II) extraction, which was dependent on Pb(II) concentration. The monodansylated control, C₆H₅NHSO₂C₁₀H₆-5-N(CH₃)₂(2), showed neither extraction nor quenching, indicating that the fluorescence effects are a direct result of Pb coordination to 1. The observed selectivity for Pb(II) is ascribed to the formation of a low-coordinate binary Pb(II)-Sulfonamido complex in the organic phase. Copyright

Full text of DOI:10.1021/ja050296e

Published on Web 04/15/2005
Fluorescent Sensing and Selective Pb(II) Extraction by a Dansylamide
Ion-Exchanger
Konstantinos Kavallieratos,*,† Jay M. Rosenberg,† Wei-Zhong Chen,‡ and Tong Ren‡
Department of Chemistry and Biochemistry, Florida International UniVersity, 11200 SW 8th Street,
Miami, Florida 33199, and Department of Chemistry, UniVersity of Miami, 1301 Memorial DriVe,
Coral Gables, Florida 33146
Received January 17, 2005; E-mail: kavallie@fiu.edu
Lead poisoning, which is especially prevalent in children, is still
the most common environmentally caused disease nationally and
worldwide.1 Development of extractants and fluorescent chemosen-
sors2 for Pb(II) is of importance for monitoring EPA mandated
levels and for a variety of applications related to lead toxicity,
biodistribution, and removal. For such applications, it is essential
to achieve high selectivity for Pb(II) against both alkaline earth
and other common metal ions present in biological systems, such
as Cu(II) and Zn(II).3 Exploiting the unique coordination properties
of Pb(II)3,4 provides opportunities for the design of selective sensors5
and extractants.6 Successful fluorescent Pb(II) sensor designs are
based on complexation by peptides,7 ionizable chelates,8 DNAzymes,9
or macrocycles10 with covalently linked fluorophores. Macrocycles
can be tedious to synthesize and typically exhibit a wide variety of
polydentate Pb(II) coordination patterns. On the other hand, ion
exchange extraction from water to an organic solvent by simpler
chelates can lead to improved selectivity by taking advantage of
the unique preference of Pb(II) for low-coordinate hemidirected3
geometries with a stereochemically expressed lone pair. Moreover,
simpler chelates tend to be easier to synthesize, an advantage that
could potentially be exploited for sensor discovery by using rapid
screening methods.11 As part of our effort to design low-coordinate
Pb(II) extractants and sensors, we now wish to report efficient and
selective ion-exchange extraction of Pb(II) from water into 1,2-
dichloroethane (DCE) with concurrent fluorescence quenching using
as an ion-exchanger, the sulfonamide fluorophore 1. This simple
system does not require a secondary co-ligand in order to extract
Pb and shows remarkable extraction selectivity against other metals
with DPb > 130 DCu and DPb > 1400 DZn.
Reinhoudt et al.8b have reported quenching of the dansyl fluores-
cence by Pb(II) on a self-assembled monolayer (SAM)-modified
glass surface. The X-ray crystal structure of 1 shows the SdO
groups located in an anti conformation with self-association via
N-H‚‚‚OdS hydrogen bonds, giving dimers, instead of the 1D
chains observed before for analogous disulfonamides.14
Ligand 1 was shown to extract Pb(II) from water into DCE via
ion-exchange when used together with NH(i-Pr)2. One clear
advantage of 1 over the analogous sulfonamide extractants, 1,2-
C6H4(NH2SO2C6H5)2 and 1,2-C6H4(NH2SO2C6H4-p-But)215, is that
the formed binary complex, Pb[1,2-C6H4(NSO2C10H6-5-N(CH3)2)2]
(3), is soluble in DCE, and therefore, there is no need for the use
1
of a co-ligand. H NMR of the organic phases after extraction
indicated the presence of 3. The negative mode electrospray
ionization mass spectrum of a methanol solution of 3 showed the
formation of the 3‚CH3O- ion at m/z ) 811.
The distribution ratios (DPb ) [Pb]o/[Pb]aq) were determined by
inductively coupled plasma mass spectrometry (ICP-MS) of the
aqueous phases after extraction (for [Pb]aq) and after stripping the
organic phases with 0.1 M HNO3 (for [Pb]o). Double stripping of
the organic phases showed no increase in [Pb]o. Specifically, when
1 (3.5 mM) was used together with 2.2 equiv of NH(i-Pr)2, it
extracted 99.5% of Pb from a 3.5 mM Pb(NO3)2 solution. A plot
of [Pb]o versus [1]t showed saturation for [1]t > 3.5 mM, indicating
1:1 complexation. Control experiments, using 1 or NH(i-Pr)2 only,
showed that the presence of both components is essential for ex-
tracted. The monodansylamide 216 did not extract any Pb(II) under
the same conditions. A comparison of 1 with the ion-pair extractant
18-crown-6 under identical conditions gave a DPb ) 0.950 for
extraction by 1 versus DPb ) 1.26 × 10-3 by the crown ether, which
is also higher than the extraction by synergistic combinations of
2,2′-bipy and analogous disulfonamide ion-exchangers.15 Ligand
1 was also shown to be effective in the extraction of micromolar
Pb(II) concentrations, with [1]t ) 2.97 mM, [NH(i-Pr)2]t ) 6.53
mM, and [Pb(II)]t between 3.75 and 284 µM ([1]t/[Pb(II)]t ranging
from 10.5 to 792); ligand 1 extracted 100% of Pb(II) ([Pb]aq
<
0.01 µM).
The extraction of various dicationic metal nitrates was investi-
gated at constant total metal concentration ([M(II)]t) and varying
ligand concentrations (0.60-2.99 mM). Ligand 1 was found to
extract Pb(II) selectively over Zn(II), Co(II), Cd(II), Ni(II), and
Cu(II) with DPb(II)/DM(II) ) 1410 (for Zn), 1380 (for Co), 829 (for
Cd), 794 (for Ni), and 133 (for Cu) (Figure 1). Neither Ca(II) nor
Na(I) were extracted to any appreciable extent. We ascribe the
observed selectivity to the formation of the low-coordinate complex
3, which is apparently disfavored for other competing metals. The
crystal structure of the binary Pb complex of the analogous bis-
(phenylsulfonyl) derivative of phenylenediamine, which is insoluble
in DCE, has showed the formation of a coordination polymer via
Sulfonamide 1 is available in good yields from o-phenylenedi-
amine and 1-(dimethylamino)-5-naphthalene sulfonyl chloride (dan-
syl chloride).12 The dansyl group is a well-known fluorophore that
is sensitive to its chemical environment.13 Godwin et al.7 and Leray
et al.10a have reported elegant examples of Pb(II) fluorescence
detection in aqueous matrices based on the dansyl fluorophore.
† Florida International University.
‡ University of Miami.
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C O M M U N I C A T I O N S
by 2, and indicates that bidentate coordination is necessary for Pb(II)
extraction, and that the observed quenching is a direct result of
Pb(II) complexation by 1. Overall, the Pb selectivity for this
remarkably simple extraction-based system could be considered
comparable, or even superior to previously reported designs.7,10a
However, in our case, Pb sensing is based on quenching and not
on ratiometric fluorescence enhancement.
In conclusion, it has been demonstrated that a simple (bis)-
dansylamide ion-exchanger selectively extracts Pb(II) from water
into DCE, with concurrent fluorescence quenching. We are planning
extensive structural and spectroscopic studies with 1 and analogous
chelates, as well as application of the ion-exchange extraction
methodology for rapid screening of potential extractants and sensors
for Pb(II) and other toxic metals.
Figure 1. Plot of log [M(II)]t versus [1]t: [Pb]t ) 2.72 mM, [Ni]t ) 2.72
mM, [Co]t ) 2.67 mM, [Cu]t ) 2.80 mM, [Zn]t ) 2.53 mM, [Cd]t ) 2.59
mM.
Acknowledgment. We thank Drs. R. Alvarado and W. Zhang
for experimental help. This project was supported in part by NIH-
NIEHS/ARCH (S11 ES11181), NIH-MBRS/RISE (R25 GM061347),
and the University of Miami (diffractometer fund).
Supporting Information Available: Synthesis, procedures, extrac-
tion plots, and X-ray crystallographic details of 1. This material is
available free of charge via the Internet at http://pubs.acs.org.
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