Very few sensors exist that display high selectivity and reliability for biologically important anions such as phosphate, pyrophosphate, ATP, AMP, and function in aqueous media without interference from endogenous substrates such as chloride anion. That is because selective and reliable sensing of anions is generally difficult to accomplish. Compared to isoelectric cations, anions often display high energy of hydration, tautomerism, possess low surface-charge density, features that make the binding of anions less effective.
The increase in receptor-anion affinity may be achieved by utilizing positively charged moieties as a part of receptor. Unfortunately, electrostatic interactions are nondirectional and, as a result, all anions present in the medium are attracted to such receptor. It is, therefore, desirable to also include weak directional interactions such as hydrogen bonding to improve the selectivity in receptor-anion association.
We have designed a conductive polymer with incorporated receptors capable of hydrogen bonding, and at the same time can be p-doped. P-doping means that a conductive material is subjected to electric potential and as a result becomes positively charged. The adjustable degree of p-doping (introduction of a positive charge) could provide an inexpensive alternative to the synthesis of sensors with incorporated positively charged-moieties (e.g. metal ions). The positive charge in a conductive polymer (e.g. polythiophene) can be adjusted by externally applied potential. In the design of our conductive polymers, we have also incorporated receptors capable of binding target anions via hydrogen bonding, while displaying both change in color as well as polymer conductivity.
Left: A schematic drawing of synergy in a p-doped conductive polymer with integrated hydrogen-bonding receptors. Center: structure of the conductive polymer capable of hydrogen-bonding to the anions. Right: Anion is bound to the material via hydrogen bonding as well as coulombic attraction with the positively charged polymer backbone.
The conductive polymers are electrodeposited on the prefabricated interdigitated transparent electrodes (doubleclick on the electrode image to see the structure of the interdigitated electrode ). The interdigitation was biased in such a way that the polymer grew from digit A until it reached bare digit B causing closing of the circuit as observed by the change in conductivity (Figure below).
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The sensor chips with deposited conductive polymers are then tested for anion sensing in custom-made spectroelectrochemical cells connected to a fiber optic spectrometer and bipotentiostat. The changes in color as well as conductivity are then registered by the computer. The following figure shows the changes in absorption spectrum of the conductive polymer (Left panel) and the decrease in the material conductivity (Right panel) induced by the addition of aqueous pyrophosphate solution.
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We believe that this method utilizing the synergy between low-level p-doping in a polythiophene polymer and hydrogen bonding to increase anion-sensor affinity will yield more reliable anion sensors in the near future.