A new class of anion selective receptors is based on the neutral
uranylsalophene building block as
Lewis acidic binding site. Additional hydrogen bond accepting or
donating moieties near the anion
binding site offer the possibility of varying the binding selectivity.
Field effect transistors chemically
modified with such receptors exhibit anion selectivities that strongly
deviate from the classical
Hofmeister series favoring phosphate or fluoride anions, depending on
the structure of the
uranylsalophenes. The phosphate selective chemically modified
field effect transistors (CHEMFETs)
detect phosphate with high selectivity over much more lipophilic
anions, such as nitrate (log
= −1.3), at [H2PO4
-] ≥ 6.3
× 10-4 M. CHEMFETs modified with
salophenes with amido
substituents result in a high fluoride selectivity; even in the
presence of 0.1 M chloride, fluoride
can be detected at [F-] ≥ 6 × 10-4 M
(log
= −2.0).
In comparison with selective receptors (and sensors) for cationic species, work on the selective complexation and detection of anions is of more recent date. There are three important components for a sensor, a transducer element, a membrane material that separates the transducer element and the aqueous solution, and the receptor molecule that introduces the selectivity. This review deals with potentiometric transduction elements that convert membrane potentials into a signal. The structure and properties of membrane materials is discussed. The nature of the anion receptor ultimately determines the selectivity. Both coordination chemistry and hydrogen bonding have been used to design anion receptor molecules. The integration of all three elements by covalent linkage of all elements in durable sensorsystem concludes the review.
Impedance spectroscopy can be used to determine the influence of several membrane parameters on the membrane resistance of anion selective CHEMFETs. The concentration of the ammonium sites in the membrane, the anion-receptor complex stoichiometry, and the polarity of the membrane matrix are of particular importance. In general the resistance of polysiloxane membranes is higher than that of PVC membranes. However, in polysiloxane membranes the membrane polarity can be influenced by the type or concentration of polar substituents on the polysiloxane chain. Polysiloxane ion-exchange membranes with 25 mol% of polar sulfone substituents exhibit the same conductance as NPOE plasticized PVC membranes. Remarkably, the membrane resistance of cation-selective polysiloxane membranes is much lower and is much less dependent on the substituents.
In PVC/NPOE ion-selective membranes of potentiometric sensors, the guest−host stoichiometry
of the anion complex of H2PO4
- and F- selective uranyl salophene derivatives is 2:1. This
stoichiometry is different from the stoichiometry observed in DMSO solution (1H NMR) or solid
state (X-ray crystal structure). However, the 2:1 stoichiometry like that in the PVC/NPOE
membrane matrix is also observed by 1H NMR spectroscopy in non
polar solvents such as chloroform.
In contrast with the relatively hard H2PO4
- and F- anions, the softer Cl- is bound in a 1:1
stoichiometry by the uranyl salophenes in chloroform.
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