Templated SAMs for metal ion recognition

Article abstract of DOI:10.1039/b102144n

Self assembled monolayers of a polyether derivative selectively detect potassium cations when templated in their presence.

Full text of DOI:10.1039/b102144n

Templated SAMs for metal ion recognition
Barbara Colonna and Luis Echegoyen*
Department of Chemistry and Center for Supramolecular Science, University of Miami, Coral Gables,
Florida 33124, USA. E-mail: echegoyen@miami.edu
Received (in Columbia, MO, USA) 29th January 2001, Accepted 17th April 2001
First published as an Advance Article on the web 25th May 2001
Self assembled monolayers of a polyether derivative se-
lectively detect potassium cations when templated in their
presence.
a Ag/AgCl reference electrode. Experiments were conducted at
the formal redox potential of the [Ru(NH3)6]3+/2+ couple with a
frequency ranging from 100 kHz to 0.1 Hz. The spectra were
analyzed by the ‘EquivalentCircuit’12 software package. Bind-
ing of cations to the receptor moiety of a non-electroactive SAM
can be monitored by impedance spectroscopy using the
positively charged redox couple [Ru(NH3)6]3+/2+ as a probe.13
An increase in the charge transfer resistance (Rct) is observed
upon complexation of the metal cations,13,17 and is due to
electrostatic repulsion between the cation and the positively
charged ruthenium redox couple. These changes were used to
monitor the recognition of alkali metal cations by monolayers of
1.
Self-assembled monolayers (SAMs) that incorporate receptor
molecules can act as molecular recognition interfaces and thus
as selective sensors for specific analytes.1–3 Binding sites can
also be formed within the SAMs during their preparation, if they
are grown in the presence of appropriate analytes that template
the aggregation processes.4 This process could also be de-
scribed as molecular imprinting, although this term is usually
reserved for polymer systems, where binding of functionalized
monomers by a template molecule is followed by polymeriza-
tion and template removal.4–6 Well-defined cavities with
selective binding properties are thus formed in the polymeric
backbone which are able to recognize analytes that are similar
or identical to the template.
The first preliminary report involving imprinted SAMs was
published by Sagiv7 in 1979, but the concept went largely
unexplored for a long time until recently, when the strategy was
employed to generate binding sites for organic substrates at the
surface of modified electrodes.8–11 It seems as if more examples
are appearing more frequently, probably due to the simplicity of
the approach which largely relies on the principle of self-
assembly and requires minimal synthetic effort. Perhaps
surprisingly and to the best of our knowledge, no examples of
SAM formation involving metal ions as templates have been
reported to date. In this communication, we report the
preparation and the electrochemical investigation of potassium-
templated SAMs on gold which selectively detect K+ but not
Na+.
The charge transfer resistances (Rct) of the SAMs A and B
were measured in the presence of increasing concentrations of
KCl. For A [Fig. 1(a)], Rct is 1.35 kW cm2 immediately after
preparing the SAM and in the absence of KCl. Upon KCl
additions, Rct increases slightly until it reaches a limiting value
of 2.14 kW cm2 at a salt concentration of 100 mM. Thus, the
effect of salt addition is reasonably small. For B [Fig. 1(b)], Rct
is 2.20 kW cm2 in the absence of KCl but upon addition of this
salt the resistance increases dramatically and reaches a value of
Receptor 1 was designed to bind metal cations in the
polyether loop, which was purposely fused to the aromatic ring
for rigidity, but allowed to retain enough flexibility for intra- as
well as inter-molecular templating upon self-assembly on the
gold surface. The thiol group provides the anchoring point to the
gold substrate. The polymethylene spacer separating the
polyether recognition site and the anchoring group should
facilitate the formation of the monolayers by van der Waals
forces. The compound was synthesized in three steps. Reaction
of catechol with 2-chloroethoxyethanol afforded a diol 2, which
was monoalkylated with a small excess of 1,6-dibromohexane.
Finally, the resulting bromide 3 was converted into 1 by
treatment with thiourea followed by basic hydrolysis.†
SAMs A and B were prepared by immersing glass-sealed
gold bead electrodes for 24 h in an EtOH solution of 1 (1 mM)
(A) or an EtOH–H2O (1+1) solution of 1 (1 mM) and KCl (0.1
M) (B). The electrodes were then rinsed with H2O, EtOH and
CH2Cl2 and dried under a stream of Ar. Impedance measure-
ments were performed using an aqueous solution of Et4NCl (0.1
M) and [Ru(NH3)6]Cl3 (1 mM), a SAM modified gold bead as
the working electrode, a coiled platinum counter electrode and
Fig. 1 (a) Nyquist electrochemical impedance spectroscopy plot of the
monolayer of 1 grown in the absence of KCl (A), and exposed to different
K+ concentrations. Experiments were conducted in H2O, using Et4NCl
(0.1 M) as supporting electrolyte and [Ru(NH3)6]Cl3 (1 mM) as redox probe
at E = 20.165 V and the frequency range 100 kHz–0.1 Hz. (b) Nyquist
electrochemical impedance spectroscopy plot of the monolayer of 1 grown
in the presence of 0.1 M KCl (B), and exposed to different K+
concentrations. Experiments were conducted in H2O, using Et4NCl (0.1 M)
as supporting electrolyte and [Ru(NH3)6]Cl3 (1 mM) as redox probe at E =
20.165 V in the frequency range 100 kHz–0.1 Hz.
1104
Chem. Commun., 2001, 1104–1105
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b102144n

This work was supported by the National Science Founda-
tion, grant CHE-9816503.
Notes and references
† Experimental conditions: Preparation of 2-(2-{2-[2-(2-hydroxyethoxy)-
ethoxy]phenoxy}ethoxy)ethanol 2: a mixture of catechol (5.5 g, 50 mmol)
and K2CO3 (35 g, 250 mmol) in anhydrous MeCN, was heated for 30 min
under a stream of Ar. 2-Chloroethoxyethanol (14.9 g, 120 mmol) was added
and reflux was maintained for 40 h. The suspension was then filtered and the
solids washed with CH2Cl2 (150 mL). The combined organic phases were
evaporated under reduced pressure and the residue was re-dissolved in
CH2Cl2 (150 mL) and washed with water (3 3 50 mL). Purification by
column chromatography [SiO2, CH2Cl2–EtOAc (1+1) to EtOAc–EtOH
(9+1)] afforded 2 in 44% yield.
Preparation of 2-[2-(2-{2-[2-(6-bromohexyloxy)ethoxy]ethoxy}phen-
oxy)ethoxy]ethanol 3: a mixture of 2 (0.9 g, 3.15 mmol) and NaH (60% in
mineral oil) (0.36 g, 9 mmol) in anhydrous THF (200 mL) was stirred at
room temperature under a stream of Ar. After 30 min 1,6-dibromohexane
(4.76 g, 19.5 mmol) was added and the mixture was left stirring for 16 h.
After addition of MeOH the solvent was evaporated under reduced pressure.
The residue was dissolved in CH2Cl2 (200 mL) and washed with H2O (3 3
100 mL). Purification by column chromatography [SiO2, hexane–EtOAc
(1+1)] afforded 3 in 76% yield.
Fig. 2 Cartoon representation of the binding of metal cations by SAM of 1
grown: (a) in the absence of KCl (A), K+ titration; (b) in the presence of 0.1
M KCl (B), K+ titration; (c) in the absence of KCl (A), Na+ titration; (d) in
the presence of 0.1 M KCl (B), Na+ titration.
Preparation of 2-[2-(2-{2-[2-(6-mercaptohexyloxy)ethoxy]ethoxy}-
phenoxy)ethoxy]ethanol 1: a solution of 2 (1.2 g, 2.7 mmol) and thiourea
(0.813 g, 10.7 mmol) in EtOH (80 mL) was left refluxing under a stream of
Ar for 16 h. The solvent was evaporated under reduced pressure and the
residue suspended in an aqueous solution of KOH (60 mL, 0.84 g, 15
mmol), and stirred for 2 h. The reaction mixture was acidified with HCl and
CH2Cl2 (150 mL) was added. Washing with H2O (3 3 100 mL) and
7.65 kW cm2 at a salt concentration of 100 mM. The area
coverage for monolayers A (fA = 1.96 3 1029 mol cm22) and
B (fB = 2.43 3 1029 mol cm22) were determined by reductive
desorption in a 0.5 M KOH solution,18 and are in accordance
with the relative Rct values of A and B. The association
constants (K) for potassium binding by A and B were
determined by fitting the experimental Rct values and concentra-
tions of KCl using a Langmuir isotherm.13 The value of K was
found to be 124 ± 20 M21 for A and 353 ± 30 M21 for B). The
difference between these two values demonstrates that the
templated monolayer B has a higher binding affinity for
potassium cations than A. In the case of B, the self-assembly of
1 on gold is assisted by the cations which induce the formation
of optimal binding domains [Fig. 2(b)]. The resulting pre-
organized recognition sites favor cation recognition at the
interface. The template effect is also reflected in the response to
changes in cation concentration. The addition of 5 mM KCl
produces a small change in the Rct of A but a large increase in
the Rct of B [Fig. 2(a) and (b)].
purification by column chromatography afforded
1 in 33% yield.
dH(CDCl3) 1.28–1.39 (5H, m), 1.54–1.63 (4H, m), 2.48 (2H, t, J 7.3 Hz),
3.45 (2H, t, J 6.7 Hz), 3.59 (2H, t, J 5.2 Hz), 3.64–3.76 (6H, m), 4.12–4.20
(4H, m), 6.88–6.90 (4H, m); m/z (FAB+): 402 (M+,70%), 403 (M+ + 1,
100%).
1 S. Flink, F. C. J. M van Veggel and D. N. Reinhoudt, Adv. Mater., 2000,
12, 1315.
2 R. M. Crooks and A. J. Ricco, Acc. Chem. Res., 1998, 31, 219.
3 A. E. Kaifer, Isr. J. Chem., 1996, 36, 389.
4 K. Haupt and K. Mosbach, Chem. Rev., 2000, 100, 2495.
5 H. Asanuma, T. Hishiya and M. Komiyama, Adv. Mater., 2000, 12,
1019.
6 G. Wulff, Angew. Chem., Int. Ed. Engl., 1995, 34, 1812.
7 J. Sagiv, Isr. J. Chem., 1979, 18, 346.
8 V. M. Mirsky, T. Hirsch, S. Piletsky and O. S. Wolfbeis, Angew. Chem.,
Int. Ed., 1999, 38, 1108.
9 M. Lahav, E. Katz, A. Doron, F. Patolsky and I. Willner, J. Am. Chem.
Soc., 1999, 121, 862.
10 O. Chailapakul, R. M. Crooks, C. B. Ross, L. Sun and J. Schoer,
Interfacial Design and Chemical Sensing, ed. T. E. Mallouk and D. J.
Harrison, ACS Symp. ser. 561, Washington, DC, 1994.
11 S. A. Piletsky, E. V. Piletskaya, T. A. Sergeyava, T. L. Panasyuk and
A. V. El’skaya, Sens. Actuators B, 1999, 60, 216.
12 B. A. Boukamp, Equivalent Circuit version 4.55, University of Twente,
Department of Chemical Technology, Enschede, The Netherlands,
1996.
13 S. Flink, B. A. Boukamp, A. van der Berg, F. G. J. M. van Veggel and
D. N. Reinhoudt, J. Am. Chem. Soc., 1998, 120, 4652.
14 S. Flink, F. G. J. M. van Veggel and D. N. Reinhoudt, J. Phys. Chem. B,
1999, 103, 6515.
15 K. Bandyopadhyay, H. Liu, S.-G. Liu and L. Echegoyen, Chem.
Commun., 2000, 141.
16 K. Bandyopadhyay, H. Liu, S.-G. Liu and L. Echegoyen, Chem. Eur. J.,
2000, 6, 1176.
17 K. Bandyopadhyay, H. Liu, S.-G. Liu and L. Echegoyen, Langmuir,
2000, 16, 2706.
18 Y. Wang and A. E. Kaifer, J. Phys. Chem. B, 1998, 102, 9922.
The ability of A and B to recognize sodium cations was also
tested. In both instances, however, small changes in the Rct
values were observed [Fig. 2(c) and (d)] upon addition of NaCl,
suggesting that recognition of Na+ ions by the monolayer of 1 is
very modest. Interestingly, the Rct of the templated monolayer B
is only 2.76 kW cm2 in the presence of 100 mM NaCl. This
monolayer has instead a Rct of 7.65 kW cm2 in the presence of
100 mM KCl. The different behavior indicates that B has a
pronounced selectivity for potassium cations.
Summarizing, we have demonstrated for the first time that
binding sites for potassium cations can be imprinted into SAMs
when the monolayer is assembled on gold in the presence of this
metal cation. The resulting modified gold electrodes detect
potassium cations in water with good selectivity over sodium
cations. This approach to chemical sensors is simple and
efficient and can be easily extended to the realization of SAMs
for the recognition of analytes other than potassium cations.
Chem. Commun., 2001, 1104–1105
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