4560
Macromolecules 2005, 38, 4560-4562
Sensing of Lead Ions by a
Carboxylate-Substituted PPE: Multivalency
Effects
Ik-Bum Kim, Anna Dunkhorst, James Gilbert, and
Uwe H. F. Bunz*
School of Chemistry and Biochemistry, Georgia Institute of
Technology, 770 State St., Atlanta, Georgia 30332
Received March 21, 2005
Revised Manuscript Received April 14, 2005
We report that the structurally simple polymer 4 is a
potent sensory platform for lead salts in aqueous
solution. Very sensitive fluorescence quenching is ob-
served.
Figure 1. Normalized absorption and emission spectra of 4
in water. λmax abs ) 430 nm; λmax emiss ) 465 nm.
KClO4 solution and PIPES-buffer (piperazine-1,4-bis-
(2-ethanesulfonic acid); 50 mM; pH ) 7.2) led to
quenching of the fluorescence. Table 1 shows the
obtained Stern-Volmer constants for fluorescence
quenching of 4 and its model compound 5. From
Schanze’s investigation it is clear that the fluorescence
of 4 is generally quenched by a static mechanism.8 The
quencher efficiency, as expressed by the Stern-Volmer
constant, is under these conditions equal to the (appar-
ent) complex formation constant of the quencher Q to
the polymer 4.10 When utilzing no buffer, methyl violo-
gen quenches 4 with a similarly high KSV as reported
by Schanze. However, if phosphate or PIPES buffers
were utilized to account for constant ion strength and
pH, the KSV values decreased significantly due to the
diminished electrostatic attraction of 4 and methyl
viologen at the higher ion strength. In biological experi-
ments specimens are utilized either in phosphate or in
PIPES buffers; however, for the sensing of trace metal
ions in cells, PIPES buffer is preferred because phos-
phate buffers can interfere with metal ions.8 And indeed,
we find in most cases much better binding of metal
cations to 4 when performing the quenching experi-
ments in PIPES buffer.9
We find that the model compound 5 is only moder-
ately quenched by metal salts; Stern-Volmer constants
of 10-700 result. When the polymer 4 is examined
under the same condtions, much higher KSV values, up
to 8.8 × 105, for lead(II) salts result. Swager10a has
attributed the enhanced quenching of conjugated poly-
mers when compared to that of monomeric model
compounds to a molecular wire effect: An exciton is
delocalized over many chromophores in the polymeric
receptor, and if only one of the receptors binds to the
analyte, the fluorescence of the whole chain is quenched.
An enhanced signal is observed. Typically enhancement
is 40-70-fold, and for zinc, copper, and manganese as
well as for methyl viologen we find enhancements of
that magnitude. In the case of lead the enhancement is
signifcantly larger, up to 1.5 × 103. The dramatic
enhancement must be attributed to an additional effect.
The crystal structure of lead(II) acetate is instructive:
“...The lead atom is coordinated to eight oxygen atoms
of which two belong to water molecules...”.11 Conse-
quently, lead(II) coordinates three acetate molecules. If
we assume that lead ions will coordinate up to three
carboxylate groups in solution, then the model com-
pound will not bind particularly strongly because the
second carboxylate arm seems to be too far away to bind
to the metal center.
Selective and sensitive determination of heavy metals
such as lead and mercury continues to be significant
due to their role as environmental pollutants. Lead is
found in old water pipes, deteriorating lead-based
paints, and occasionally in residential soil from con-
tamination by tetraethyl lead, a now banned antiknock
additive to gasoline. If small children are exposed to
lead, health effects include anemia and neurological
impairment.1,2 In recent years sophisticated DNA-based
approaches for lead sensing have been developed.3 We
have reported that sugar-substituted PPEs are effective
in the detection of different metal cations in DMF
solution.4-6 Herein we present PPE 4 as a surprisingly
selective and sensitive material to detect lead(II) ions
in water.
Schanze reported the synthesis and the amplified
fluorescence sensing of a protease by 4 and investigated
the photophysics of this fluorescent polyelectrolyte.7,8
We have independently synthesized 4 via a somewhat
different route (Scheme 1): Starting from 2,5-diiodo-
hydroquinone, reaction with 2-bromoethyl acetate in
butanone in the presence of potassium carbonate yields
the ester 1. Alkynylation utilizing trimethylsilylacety-
lene and the (Ph3P)2PdCl2/CuI catalyst system with
triethylamine as solvent furnishes 2 after desilylation
by tetrabutylammonium fluoride in THF.4,5 The ester
groups are not cleaved under these conditions. Coupling
of 1 and 2 utilizing (Ph3P)2PdCl2/CuI in triethylamine
at 50 °C for 72 h affords the polymer 3 in 93% yield
and with an Mn of 12 × 103 and an Mw/Mn of 4.3
according to gel permeation chromatography (GPC in
DMF) with polystyrene standards; 3 is well soluble in
polar organic solvents (methanol, DMF, DMSO) and
fully characterized. It is insoluble in ethyl ether and
hexane, from which it can be precipitated. A solution of
3 in methanol is treated with NaOH to give the
deprotected sodium salt 4. In a similar approach the
model compound 5 is synthesized. In Figure 1 the UV-
vis and the emission spectrum of 4 is displayed. The
polymer shows a strong emission at λmax ) 465 nm,
quite typical for a dialkoxy-PPE. The material seems
to be unaggregated in solution, probably due to the
highly negative charge of 4 which prohibits aggregation
in aqueous solution. The quantum yield of 4 in water is
0.08. Addition of transition metal salts to a solution of
4 either in phosphate buffer (pH ) 7.2) or in 100 mM
* Corresponding author: Fax 01 404 385 1795; Tel 01 404 385
1795; e-mail uwe.bunz@chemistry.gatech.edu.
10.1021/ma050595o CCC: $30.25 © 2005 American Chemical Society
Published on Web 05/03/2005
Kim, Ik-Bum
