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ELECTROCHEMISTRY OF QUERCETIN ON GOLD ELECTRODES
MODIFIED WITH THIOLATED CYCKLODEXTRINS
MONOLAYERS Anna Huszal b), Sylwester Huszal a),
Andrzej Temeriusz a) , Jerzy Golimowski a) a)
Faculty of Chemistry, Warsaw University Pasteura
1, PL-02-093 Warsaw, Poland ) Email
atemer_at_alfa.chem.uw.edu.pl b) Oil and Gas
Institute, Warsaw Brench Kasprzaka 25, 01-224
Warsaw, Poland Email huszal_at_inig.pl
Cyclodextrins (CDs) are torus-shaped cyclic
oligosaccharides, made up of 6(a), 7(ß), or 8(?)
a(1?4) linked D-glucose units, which have a
relatively hydrophobic cavity for guest binding1.
These compounds are therefore potent hosts for
guest molecules capable of entering the cavity
and thus forming noncovalent host-guest inclusion
complexes. The versatility of CDs as hosts is
related to different cavity sizes and the
possibility of one or many alcohol moieties
functionalization2. Cyclodextrins substituted
with thiol groups have been widely used to modify
the surface of electrodes. Their molecules are
spontaneously chemisorbed on Au surface to give
highly organised self-assembly monolayer3. This
work describes a chemically modified electrodes
(CMEs) based on suitable cyclodextrin
derivatives in the form of self-assembled
monolayer (SAM). Our work regards the synthesis
of thiolated cyclodextrins, the
preparation of CME based on SAM formation on gold
electrode, their electrochemical characterisation
and potent analytical applications. Thiolated
cyclodextrin derivatives were prepared according
to the procedure reported by Stoddart et al.4.
Per-(6-deoxy-6 thio-2,3-di-O-methyl)-a- and
ß-cyclodextrins (SH-aCD and SH-ßCD, respectively
Figure 1) were prepared in four steps from native
species. The synthesis involved the following
sequence of reactions protection of the
primary side by silyl groups, exhaustive
methylation of the secondary rim, removing of the
protection of primary hydroxyl groups with
simultaneously conversion to the bromide and
finally into thiol functionalities. The
monolayer of the particular cyclodextrin
derivative was formed by immersion of a well
cleaned gold electrode into a deoxygenated 1mM
solutions of SH-aCD and SH-ßCD, respectively, in
DMF overnight. The properties of the obtained
SAMs did not significantly improve by changing
the formation time or the solution
concentration. The SAM makes up an array of
ultramicroelectrodes, which capture electroactive
molecules such as those of flavonoids when the
SAM-modified electrode is exposed to their
solutions. Flavonoids are benzo-?-pyrone
derivatives widely distributed in nature. In this
family, polyhydroxyflavones such as quercetin
(Figure 2) are the subgroup of
well-known natural antioxidant molecules5.
Quercetin with the catechol group on the B-ring
(3,4-dihydroxyl) is the most
representative dietary flavonoid and the potent
antioxidant. Due to the 3,4-dihydroxy
substitution patterns it is one of the most
easily oxidised flavonoids. In our studies
quercetin was found to form supramolecular
complexes with both investigated cyclodextrin
derivatives immobilised on the gold surface. The
voltammograms of the cyclodextrin modified
electrodes recorded for appropriate quercetin
solution reveal the presence of its molecules in
the cyclodextrin cavities.
PPh3/Br2 CH2Cl2
(H2N)2CS KOH DMF
Figure 1. Synthesis of thiolated cyclodextrins.
Figure 2. Proposed mechanism of electrochemical
oxidation of quercetin.
Figure 3. Cyclic voltammograms for the Au
electrode coated with a monolayer of SH-aCD
(green line), SH-bCD (blue line), SH-gCD (violet
line) and for a bare Au electrode (red line) in
sodium phosphate buffer (v0.1 V/s).
Figure 4. Calibration curve for quercetin
determination recorded at bare Au electrode
in sodium phosphate buffer (v0.1 V/s).
Figure 5. Cyclic voltammetric peak current ratio
as a function of scan rate in sodium
phosphate buffer.
The aim of this work was to prepare and
investigate the electrochemical behaviour and
application of the gold electrode modified with
monolayers of thiolated cyclodextrins. In a-
and ß-cyclodextrin molecules, the six and seven
primary hydroxyl groups respectively have been
replaced by thiol groups. The electrochemical
data presented here indicate that these receptors
chemisorb strongly and readily on the gold
surface through the formation of thiolate-gold
bonds. The resulting monolayers of a- and
ß-cyclodextrins exhibit excellent binding ability
toward appropriate species in the contacting
solution. Our voltammetric data demonstrate that
quercetin molecules are included in the cavities
of the interfacial receptors. Modified electrodes
showed effective binding properties when immersed
in aqueous solution containing low concentrations
(10-6 M for SH-aCD and 510-7 M for SH-ßCD) of
quercetin. Their voltammetric response exhibited
the waves anticipated for the reversible
oxidation of the surface-confined
(cyclodextrin-bound) quercetin molecules. Hendric
kson and co-workers5 reported the
electrochemistry of quercetin at a glassy carbon
electrode. According to them the response of 3,
4-adjacent hydroxyl groups is a two-electron and
two-proton electrode reaction. We obtained
similar results using gold electrodes. This
oxidation is a chemically reversible process. The
oxidation peak (Epa 0.140 V) corresponds to
the oxidation of the 3, 4-dihydroxy
substituents on the B-ring. A reduction peak
(Epc 0.100 V) appeared on the negative scan,
indicating that the electrochemical oxidation
product was easily reduced. The reversibility is
highly scan rate dependent, indicating
an electrochemical-chemical reaction
mechanism. The effect of scan rate on the
reversibility of the Ia/Ic couple observed by
cyclic voltammetry for quercetin is shown in
Figure 5. The peak currents of quercetin are
linearly dependent on the scan rate. This
suggests that the electrode reaction of this
molecule is a reversible surface electrochemical
reaction, with both the reactant and product
strongly adsorbed on the electrode surface. We
observed the electrochemical oxidation
of quercetin for gold electrodes modified with
monolayers of thiolated
cyclodextrins, too. It allowed us to
quantitatively determine very low amounts of
the electroactive compounds in its methanolic
solutions.
Figure 6. Cyclic voltammograms for the Au
electrode coated by a monolayer of SH-aCD
recorded in the presence of 10-6, 210-6, 410-6,
10-5, 210-5, 410-5, 10-4 M of quercetin in
sodium phosphate buffer (v0.1 V/s).
Figure 7. Calibration curve for quercetin
determination recorded at Au electrode coated by
a monolayer of SH-aCD in sodium phosphate buffer
(v0.1 V/s).
Cyclic voltammetry measurements were performed
in a three-electrode arrangement with Ag/AgCl
electrode as the reference, platinum wire as the
counter, and gold electrode as the working
electrode. A supporting electrolyte used in all
experiments was 0.05 M sodium phosphate buffer
(pH7) with 0.5 M potassium nitrate in 50
methanol.
References 1) J.Szejtli, Chem. Rev. 98, 1743
(1998). 2) e.g. G. Wenz, Angew. Chem. Int. Ed.
Engl. 33, 803 (1994). 3) A. E. Kaifer, M.
Gómez-Kaifer, Supramolecular Electrochemistry,
Wiley-VCH, Weinheim (1999). 4) M. T. Rojas, R.
Königer, J. F. Stoddart, A. E. Kaifer, J. Am.
Chem. Soc. 117, 336 (1995). 5) H. P.
Hendrickson, A. D. Kaufman, C. E. Lunte, J.
Pharm. Biomed. Anal. 12, 325 (1994).
Figure 8. Cyclic voltammograms for the Au
electrode coated by a monolayer of SH-bCD
recorded in the presence of 10-6,
510-6, 10-5, 210-5, 510-5, 10-4 M of
quercetin in sodium phosphate buffer (v0.1 V/s).
Figure 9. Calibration curve for quercetin
determination recorded at Au electrode coated by
a monolayer of SH-bCD in sodium phosphate buffer
(v0.1 V/s).
Acknowledgment Financial support from the Warsaw
University is gratefully acknowledged.