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Fig. 4 The comparative voltammetric responses of the rotaxanated
2
13 C. A. Hunter, J. Am. Chem. Soc., 1992, 114, 5303–5311.
+
6
macrocycle ferrocene (in a SAM of 1/2 PF ) to chloride and dihydrogen
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1
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sensitivity than that shown by the free macrocycle (LOD 0.2 mM
1
+
2
for SAMs of 1/2 PF6 ; the free macrocycle is unresponsive at
these levels). Importantly, this observed selective electrochemical
sensing capability negates the possibility of intermolecular surface
co-operative binding effects between neighbouring adsorbed
molecules being responsible for chloride ion selectivity. The
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2
superior electrochemical response of rotaxane SAMs to Cl over
2
H PO
2 4
(reversed compared to free 1) mirrors the high degree of
2
1 J. A. Wisner, P. D. Beer, M. G. B. Drew and M. R. Sambrook, J. Am.
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chloride anion selectivity of our previous rotaxanes prepared via
chloride anion templation, where the mechanically bonded
assemblies’ unique interlocked binding domain excludes the larger,
2
2
21
more basic, dihydrogen phosphate anion.
2
In summary, we have demonstrated the first application of
rotaxane SAMs in chemical sensing. Novel redox-active bis-
ferrocenyl functionalised rotaxane SAMs can be assembled on to
gold electrode surfaces using an anion-templated surface-stopper-
ing methodology. The selectivity trends observed are markedly
sensitive to axle threading and, with outlined SPR, FTIR and
ellipsometry data, strongly supportive of the anion binding site
being within the confines of the surface interlocked cavity. Such a
potential anion sensing rotaxane system, by virtue of the bespoke
interlocked binding cavity, has been shown to electrochemically
recognise chloride anions selectively. The design and surface
fabrication of interlocked SAM materials for sensory applications
is continuing in our laboratories.
1
2
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3 The association constant for pseudorotaxane formation between 1 and
We thank the EPSRC for postdoctoral fellowships (S.R.B. and
M.J.C.) and a studentship (T.M.G.).
4
3
1
3
Notes and references
+
2
axle SAM of 3 Cl in acetonitrile solution was determined by SPR to
3
21
be 1.13 6 10 M .
1
Molecular Catenanes, Rotaxanes and Knots, ed. J.-P. Sauvage,
C. Dietrich-Buchecker, Wiley-VCH, Germany, 1999.
34 The rotaxane SAM confined macrocycle was found to be electro-
chemically insensitive (DE , 8 mV) to excess hydrogensulfate, acetate,
nitrate, bromide and fluoride in acetonitrile solution.
2
A. R. Pease, J. O. Jeppesen, J. F. Stoddart, Y. Luo, C. P. Collier and
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2
236 | Chem. Commun., 2007, 2234–2236
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