Because of their low polarity and
polarizability, fluorous sensing
membranes are both hydrophobic and lipophobic and exhibit very high
ion selectivities. Here, we report on a new fluorous-membrane ion-selective
electrode (ISE) with a wide sensing range centered around physiologically
relevant pH values. The fluorophilic tris[perfluoro(octyl)butyl]amine
(N[(CH2)4Rf8]3) was synthesized
and tested as a new H+ ionophore using a redesigned electrode
body that provides excellent mechanical sealing and much improved
measurement reliability. In a challenging 1 M KCl background, these
fluorous-phase ISEs exhibit a sensing range from pH 2.2 to 11.2, which
is one of the widest working ranges reported to date for ionophore-based
H+ ISEs. High selectivities against common interfering
ions such as K+, Na+, and Ca2+ were
determined (selectivity coefficients: logK
H, K
pot = –
11.6; logK
H, Na
pot = – 12.4; logK
H, Ca
pot <
– 10.2). The use of the N[(CH2)4Rf8]3 ionophore with its −(CH2)4– spacers separating the amino group from the strongly
electron-withdrawing perfluorooctyl groups improved the potentiometric
selectivity as compared to the less basic tris[perfluoro(octyl)propyl]amine
ionophore. The use of N[(CH2)4Rf8]3 also made the ISE less prone to counter anion failure
(i.e., Donnan failure) at low pH than the use of tris[perfluoro(octyl)pentyl]amine
with its longer −(CH2)5– spacers,
which more effectively shield the amino center from the perfluorooctyl
groups. In addition, we exposed both conventional plasticized PVC-phase
pH ISEs and fluorous-phase pH ISEs to 10% serum for 5 days. Results
show that the PVC-phase ISEs lost selectivity while their fluorous-phase
counterparts did not.