Multichannel Sensors Based on Biphenyl and Cyclohexane Conformational Changes - Designing Receptors for the Next Generation of Biosensors - page 29

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15

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0

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300 350 400 450 500 550

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Wavelength (nm) Wavelength (nm)

Fig. 29 Fluorescent spectrum of ( þ )-27 ( left ) in the presence of TMA L -aspartate and ( right )in

the presence of TMA D -aspartate

)-27 modify the fluorescence spectrum with L -aspartate
showing a more marked effect (Fig. 29 ). With these ligands, the formed excimer
should be intermolecular and in agreement with the 2:1 stoichiometry shown in the
complexes. This stoichiometry may also be responsible for the similar effect that
the same enantiomer has with both ligands because, in this case, the role played by
the stereochemistry of the stereocentre can only relate to steric factors.
The above-described results clearly demonstrate that these cyclohexane derivatives
have sensing properties when faced with different types of carboxylates, where the
sensing mechanism relates directly to changes in ligand conformation. Changes in the
different groups' spatial disposition determine the possible formation of the excimer
responsible for the sensing response. However, it is important to point out that complex
geometry is dependent on complexes stoichiometry, where 1:1 complexes give rise to
more pronounced modifications in the ligand's cyclohexane moiety. It is also important
to bear in mind that the sensing mechanism is possibly due to the rigidity of the
cyclohexane ring and to the correct configuration of substituents.

Both (+)-27 and (

4 Final Remarks

In summary, even though the literature reports a large number of transduction
mechanisms used in designing chemical sensors, many of them are related to an
electronic transmission of the changes induced by the binding event. We herein
describe several examples relating to information transmission through conforma-
tional changes. The sensors designed by following this approach have to be based on
partially rigid structures, which may undergo profound changes after complexation,
and this change in conformation should induce modifications in the photophysical or
electrochemical properties.