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Review

Potentiometric Anion Selective Sensors

Martijn M.G. Antonisse,+ and David N. Reinhoudt*

Department of Supramolecular Chemistry and Technology, MESA Research Institute, University of Twente, PO Box 217, 7500 AE Enschede,

The Netherlands�Present address: DSM-Resins, Zwolle, The Netherlands

Received: January 15, 1999

Final version: April 26, 1999

Abstract

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 thetransducer element and the aqueous solution, and the receptor molecule that introduces the selectivity. This review deals with potentiometrictransduction elements that convert membrane potentials into a signal. The structure and properties of membrane materials is discussed. Thenature of the anion receptor ultimately determines the selectivity. Both coordination chemistry and hydrogen bonding have been used todesign anion receptor molecules. The integration of all three elements by covalent linkage of all elements in durable sensorsystem concludesthe review.

Keywords: Potentiometric sensors, Chemical sensors, Selector, Transduction element, Anion complexation, Anion receptors, Ion-selective electrode, Coated

wire electrode, Ion-selective membranes, Plasticizer, MEMFET, CHEMFET, Hofmeister series, Ion-exchange membranes, Heparin sensor, Uranyl salophenes,Porphyrins, Phthalocyanines, Polyacrylate membranes, Polysiloxanes

1. Introduction

The determination of the concentrations of ionic species inaqeous samples is important in many areas of applied analyticalchemistry, e.g., in process control, and in the analysis of clinical,horticultural, or environmental samples. For sensing of charged species potentiometric sensors offer several advantages. The

sensors have generally a large dynamic range because the signalis proportional to the logarithm of the ion activity. Short responsetimes in the order of seconds make the devices very suitable for process control and allow a high sample through-put in, for example, ¯ow injection analysis. Moreover, the potentiometric

sensors can have very small dimensions, and consequently onlysmall sample volumes are required.

Chemical sensors have two elements: the selector and the

transduction element (Fig. 1). In the selector element the chem-ical information is converted into a domain which is measurable by the transduction element. As the transduction element isnonselective the selector element must also introduce the selec-tivity. In potentiometric sensors, chemical interactions of theselector element with charged species in the solution are trans-

formed into an electrochemical potential of an indicator electroderelative to a reference electrode. The selector element of the potentiometric indicator electrode is an ion-selective membrane.This membrane can be either an inorganic salt or an organic polymeric matrix that contains receptor molecules. The receptor molecules introduce the selectivity into the membrane by strongand selective binding of the target analyte. They play a key role in

the development of potentiometric sensors.Anion complexation and the design of anion receptors is far

less developed than the ®eld of cation receptors [1]. This isre¯ected in the large arrears in the development of potentiometricanion sensing. The ®rst publications on cation and anion recep-tors appeared almost at the same time: in 1967 Pedersen [2]reported the complexation of alkali metal cations by crown

ethers, and in 1968 the ®rst synthetic receptor for inorganicanions was reported by Park and Simmons [3]. However, the ®eld of cation recognition developed much more rapidly and there arenow many neutral host molecules for cations [4]. The slowdevelopment of anion recognition can be related to some inherent differences between anions and cations [5]:

i) Anions are relatively large and therefore require (macro-

cyclic) receptors with a much larger binding site. Thesmallest anion, FÀ, has the same ionic radius (1.33 AÊ ) as amoderately sized cation (K �).

ii) Anions have many different shapes, e.g., spherical halides,linear SCNÀ, trigonal planar NOÀ

3 , and tetrahedral H2POÀ4 .

iii) Anions are more strongly hydrated than cations of equal size,whereas the solvation by organic solvents is generally lessfavorable [6].

iv) Several anions are present only in a narrow pH window, e.g.H2POÀ

4 and CO2À3 anions in an acidic and basic environ-

ment, respectively.

Nevertheless, during the past decade several types of poten-tiometric anion sensors have been described in the literature. This

review describes the various aspects of the development of thesenovel sensors. First, the transduction elements of the sensor will be discussed (Section 2). Subsequently, the receptors (Section 3)which determine the selectivity of the sensor, and the membranematrix (Section 4) which has a large in¯uence on the durability of the sensor, will be reviewed.

2. Potentiometric Transduction Elements

In order to measure the electrochemical potential of the ion-selective membrane, the membrane is integrated within thetransduction element. The most widely applied transductionsystem is the two-electrode set-up as depicted in Figure 2. The

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indicator electrode contains the ion-selective membrane and can be either a conventional ion-selective electrode (ISE), a coated-wire electrode (CWE), or a device based on an ion-sensitive ®eld effect transistor (ISFET).

In conventional ISEs the membrane is placed between the

sample solution and the internal reference solution of the ISE(Fig. 3a). The constant composition of the latter solution resultsin a stable potential at both the inner boundary of the membraneand the interface of the internal AgaAgCl electrode. This results

in a high stability of the sensors. Most commercial potentiometricion-selective devices are based on conventional ISEs. However,the presence of the internal reference solution normally requiresrelatively large electrode dimensions of the conventional ISEs.

In CWEs the ion-selective membrane is directly deposited onthe internal electrode (Fig. 3b). This can be either a noble metalwire, a graphite rod, graphite or silver paste. Silver coated with athin layer of AgCl is most often applied. The absence of aninternal electrolyte results in a thermodynamically ill-de®ned

electrode-membrane interface [7]. The function of the internalreference solution container of ISEs can be taken over in CWEs by a (poly(hydroxyethyl methacrylate) or polyvinyl alcohol)

hydrogel soaked with NaCl solution, between the electrode and the membrane [8]. The stability can also be improved by the

addition of a silver complex to the membrane (e.g., a thioether-functionalized calix[4]arene has been used as strong silver

binding ligand [9]). The silver ions will form a redox couple withAg0 of the wire. The main advantage of CWEs over conventionalISEs is the possibility of cheap mass production. The silver electrodes and the membranes can be screen-printed on paper, polyimide (Kapton) or epoxy supports. This procedure is con-

venient to disposable devices and special medical purposes [8b,10, 11]. Furthermore, the small size allows the construction of sensor arrays [11, 12]. A disadvantage of the CWE is that themembrane easily becomes detached from the electrode and that

due to the small membrane volume leaching of membranecomponents, e.g., the plasticizer, quickly deteriorates the sensor characteristics. The leaching can be reduced with the micro-machined silicon sensor depicted in Figure 3c [13]. The membrane is deposited in a pyramidal hole etched in the silicon chip.The reduced membrane-sample contact area improves the dur-ability and protects the membrane from mechanical damage.

The third transducer type, the ISFET, already has been verysuccessful in the development of cation selective sensors, and

recently also anion micro-sensors were reported. The ISFET wasreported for the ®rst time by Bergveld [14], who removed themetal gate electrode from a metal oxide semiconductor ®eld

effect transistor (MOSFET) and exposed the gate insulator oxidedirectly to an aqueous sample. In the ISFET two n-type regions,

the source and the drain, are implanted in the semiconducting p-type bulk silicon (Fig. 4). An external electrical ®eld (VGS)applied to the surface of the ISFET (positive potential at the gateside) will deplete the number of holes in the p-type bulk near thesurface between source and drain [15]. Above a certain threshold voltage (VT) of VGS electrons are attracted which will bridge the potential gap between source and drain. This is the so-called channel formed by the inversion layer. Changes in the gate potential will result in changes in the density of electrons in theinversion layer and consequently a change can be measured in thedrain current through the inversion layer. In practice the drain

current is kept constant because the change in the drain current iscompensated by a change of the potential of the reference elec-trode (feedback mode) [16].

Fig. 2. Two-electrode setup for potentiometric ion-selective electrodes.

Fig. 1. Working mechanism of a chemical sensor.

Fig. 3. Potentiometric transducers. a) conventional ion-selective elec-trode; b) coated-wire electrode; c) micromachined ion-selective elec-trode.

1036 M.M.G. Antonisse, D.N. Reinhoudt

Electroanalysis 1999, 11, No. 14



ISFETs are pH sensitive devices. Depending on the acidity of the solution the SiO2 gate insulator can be protonated or deprotonated, which results in a pH dependent surface potential[15b]. However, for ISFETs with SiO2 gate oxide the sensor

response is nonlinear and sub-Nernstian (20±40mVa pH). The pH sensitivity and the sensor drift are increased, respectivelydecreased by applying a layer of Si3 N4, Al2O3, or Ta2O5 on topof the original SiO2 [17].

Sensitivity for ions other than H�

can be obtained bydeposition of an ion-selective membrane on top of the gateregion, creating a so-called MEMFET (membrane ®eld effect transistor). Janata et al. [18] demonstrated this principle for the®rst time, developing a K � selective MEMFET based on a

plasticized PVC membrane with valinomycin as cation receptor.Like in CWEs, the inner membrane boundary potential inMEMFETs is ill-de®ned due to the absence of an internalreference solution. Furthermore, CO2 or neutral organic acids,

like ascorbic acid and benzoic acid, diffuse through the membrane and cause local pH changes at the hydrated layer betweenthe gate oxide of the ISFET and the membrane [19]. By coveringthe ISFET surface with a thin pH insensitive layer of AgCl [19b]or polyacrylate [20] this effect can be eliminated. The stabilitycan also be improved by the introduction of a poly(2-hydroxy-ethyl methacrylate) (polyHEMA) layer between the ISFET and the ion-selective membrane [21]. In chemically modi®ed ®eld effect transistors (CHEMFETs) [22] the polyHEMA is covalentlyattached to the ISFET by prior treatment of the surface with (3-methacryloxypropyl)trimethoxysilane. Soaking of the layer witha pH buffered solution of the primary ion, results in eliminationof the CO2 interference and an improved stability due to the

constant boundary potential at the inner interface of the membrane. Suggestions in the literature [7, 23] as would this approachfail because ` a small deviation from the osmotic equilibrium

results in transport of water across the polymer which causes drift or a mechanical failure of this structure'' are mere speculations

and conclusively contradicted by the experimental results,showing no drift (additional to the inherent drift of ISFETs) onthe time-scale of the measurements. Furthermore, the stability of CHEMFETs (electrochemical and mechanical) was proven by thegood sensor characteristics even after exposing the sensors to tapwater for over more than half a year [24].

In contrast with more mature ®eld of cation selective sensors,there is still only a limited number of anion microsensors, based

on CWE or ISFET transducers. The majority of the sensors withselectivity deviating from the Hofmeister series is based on theconventional ISE. Recently some anion micro-sensors with

coated wire or ISFET transducer elements were developed,however, are still mainly in combination with solid-state or ion-exchange membranes.

3. Selectivity of Anion-Selective Membranes

3.1. Introduction

The selectivity of potentiometric anion sensors is introduced via an ion-selective membrane deposited on top of the transducer element. Because sensors are often designed for certain appli-

cations and well-de®ned conditions, this membrane does not

necessarily have to be speci®c for the analyte as long as stronginterferents are absent or present only in low concentrations. The®rst anion sensors were based on solid-state inorganic membranes, for example LaF3 or AgCl. For polymeric membranes,anion selectivity is most easily obtained with positively charged ion-exchange membranes. The selectivity of these membranes isgoverned by the partion coef®cients of anions. The relative par-tition is (besides by the solvation of the membrane) determined by the relative dehydration energies which is given by the Hof-meister series [25].

ClOÀ4 b SCNÀ % IÀ b salicylateÀ b NOÀ

3 b Br À b NOÀ2

% ClÀ b HCOÀ3 b H2POÀ

4 % FÀ % SO2À4

This sequence shows that anion sensors based on ion-exchangemembranes can only successfully be applied for the determina-tion of the relatively most lipophilic anions in solution. In practice this means that an anion in the series from ClOÀ

4 to NOÀ3

can be detected if the other ions in this series are absent or areonly present in much lower quantities. Hydrophilic anions may be present in the sample in higher concentrations.

To obtain sensors selective for hydrophilic anions, it is

necessary to add receptor molecules to the membrane that selectively bind the target anion (Fig. 5). The presence of suchselective receptor molecules favors the penetration of the target anion into the membrane, and selectivities can be achieved evenover much more lipophilic anions present in the sample solution.

To obtain a Nernstian sensor response, the receptor should effectively buffer the free primary ion activity. Depending on the

Fig. 4. Schematic representation of ISFET-based chemically modi®ed ®eld effect transistor (CHEMFET).

Fig. 5. Mechanism of an ion-selective membrane with receptor mole-cules.

Potentiometric Anion Selective Sensors 1037




charge of the receptor, this is achieved in different ways. Withneutral anion receptors the constant ion activity is achieved by

the establishment of a constant ratio between the concentration of the anion-receptor complex and the free receptor. According tothe equation for the association constant of the complex forma-tion, this results in a constant concentration of the free anion. Inorder to achieve a nearly constant ratio between free receptor and complex, the amount of lipophilic positive sites, added to the

membrane to compensate the charge of the complexed anions,should be 0.5 equivalent of the total receptor concentration (for monovalent anions). When the anion receptor is positivelycharged there is no need to add cationic sites in order to get asensor response towards anions. The receptor will form a salt with the anion in the membrane. The change in free ion con-centration in the membrane as a consequence to a variation in the

sample composition is effectively buffered by the solubility(product) of the salt. A drawback of a membrane with charged receptors is that the membrane selectivity is lower as would beexpected based on the ratio of the association constants, and therefore lower as would be obtained with a neutral receptor of equal selectivity [26]. However, recently both theoretically and experimentally it was proven that the addition of lipophilic

negative sites in the membrane enhances the selectivity byincreasing the free receptor concentration [27]. In the latter casethe selectivity coef®cient K Pot

iY j is given by Equation 1, whichsimpli®es to Equation 2 for the limiting case of the concentrationanionic sites being almost equal to the concentration of thecharged receptor.

K Pot iY j �

k j b jL

k ibiL

Â�R À�biL � 1 À ��R À �biL � 1�2 � 4biL� LT À �R À��1a2

�R À�b jL � 1 À ��R À �b jL � 1�2 � 4b jL� LT À �R À��1a2

�1�

lim��R À� 3 LT�Log K Pot iY j � Log

k j

k i� Log

b jL

biL�2�

with k i the partion coef®cient, biL the stability constant of the ionreceptor complex, [R À] the concentration of negative sites, LT thetotal concentration of receptor molecules, and the subscripts i

and j indicating the primary and interfering anionic species,respectively.

For the anion binding by the receptors applied in the anion-selective membranes several binding principles have beenexplored. Charged receptors, like bisquaternary ammonium sitesor charged metal complexes, attract anions via Coulomb inter-actions. With neutral receptors, the anion can be bound by Lewis

acidic coordination of metal centers, or by hydrogen bond for-mation. These latter interactions are weaker than Coulombinteractions and depend strongly on the directionality and on the

electron density of the anion. Nevertheless, combination and preorganization of several binding sites can lead to strong anion binding receptors. Furthermore CO2À

3 and carboxylate selective

sensors have been developed that are based on reversible covalent bond formation.

3.2. Sensors with Inorganic Salts as Selector Element

In 1966 Frant and Ross [28] discovered that a single crystal of LaF

3contacted to an electrode could be used for FÀ sensing. The

mechanism of this ®rst anion-selective sensor is based on themobility of the ¯uoride ions in the LaF3 crystal lattice. Upon

contact with the aqueous solution, the anions distribute over thecrystal-water interface leaving positively charged vacancies in thelattice. This charge separation results in a boundary potentialwhich depends on the activity of FÀ in the sample. By chemicalvapor deposition (CVD) of LaF3 on a Si3 N4-insulator layer of ISFETs also with these transducers high FÀ selectivity was

obtained (Log K Pot

FY j ` À3.7) over more lipophilic anions like ClÀ

and NOÀ3 [29].

In a similar fashion, deposition of AgBr [30] and AgCl [30b,31] yield Br À and ClÀ selective ISFETs, respectively. The silver salts can also be suspended in an inert polymeric membranematrix (elastomeric poly¯uorinated polyphosphazene) on top of the gate yielding ISFETs with almost Nernstian responses for

ClÀ, IÀ, and CNÀ [32]. ISEs selective for Br À or IÀ were con-structed with heterogeneous membranes made of silicone rubber [33] or poly(methyl methacrylate) [34], respectively. Bakker [35]developed ClÀ sensitive electrodes by using membranes that

contain strong silver complexes with organic ligands. In theabsence of silver ions in the sample, these cations will be released slowly from the membrane, forming an AgCl precipitate at the

membrane interface.ISEs for NOÀ

3 detection have been developed based on solid,crystalline membranes of silver diethyldithiocarbamate and silver sul®de [36]. Although the NOÀ

3 selectivity over ClOÀ4 (Log

K Pot NO3YClO4

� 0) is improved compared with common ion-exchange membranes, the selectivity coef®cient is lower in the presence of FÀ and SO2À

4 as interfering ions. Coating of the gatearea of ISFETs with a ®lm of alkali metal-free lead phosphateglass results in anion sensitive sensors with selectivities that follow the Hofmeister series (SCNÀb IÀbBr Àb NOÀ

2 ) but

without sensitivity for NOÀ3 [37]. Electrodes of hydroxyapatite

(Ca10(PO4)6(OH)2) are sensitive towards phosphate and highselectivity over NOÀ

3 and ClOÀ4 is reported [38]. At pH 5 a

response of À34mVadecade is observed, suggesting sensitivitytowards bivalent HPO2À4 although this is not the predominant

species at pH 5. Electrodes of hydroxyapatite with graphite inepoxy resin show at the same pH a response of À55mVadecade.Cobalt wire electrodes are responding to H2POÀ

4 in a sub- Nernstian manner (À47mVadecade) with Log K Pot

H2PO4Y j between

À2 and À3 vs. halides and NOÀ3 [39]. The response is probably

not due to a special host-guest interaction at the CoO surface of the electrode as was ®rst proposed, but originates from slowoxidation of Co0 followed by the formation of Co3(PO4)2 [40].

Di Natale et al. [41] developed an array of 20 sensors of different composition including solid state ClÀ and FÀ selectiveelectrodes and several chalcogenide glass electrodes. The mea-

surements of about 180 different solutions were used as the input

for an arti®cial neural network. Although no sensor was present that is speci®c for SO2À

4 , it was possible to detect this anion onthe basis of cross-selectivities.

3.3. Sensors with Quaternary Ammonium or

Guanidinium Sites

As mentioned in Section 3.1, sensors with polymeric ion-exchange membranes with cationic sites that do not selectively bind any speci®c anion, follow a selectivity pattern according to

the Hofmeister series. Lipophilic anions, like ClOÀ4 and NOÀ

3 , arefavored over the more hydrophilic anions, like H2POÀ

4 and ClÀ.There is much interest in the development of sensors for NOÀ

3





because of the importance of this anion in environmental and agrocultural samples. Lipophilic tetraalkylammonium ions as

ion-exchange sites are used for nitrate selective sensors based onISE-, [42] CWE-, [43] or ISFET-transducers [44]. According tothe Hofmeister series, selectivity is obtained over other, morehydrophilic natural occurring anions like ClÀ and NOÀ

2 (Logk Pot

NO3Y j � À2X3 and À1.2, respectively). Sensors with a selectivity

for lipophilic organic anions like anionic surfactants [45] or

carboxylate anions [46] are based on the same principle, viz. theincorporation of tetraalkylammonium or bis(triphenylphos- phoranylidene)ammonium salts in the ion-selective membrane.Brzozka [47] investigated ion-exchange membranes sensitive for Au(CN)À2 as an analytical method for gold. Increased lipophili-city of the tetraalkylammonium ion-exchange sites improved theselectivity for Au(CN)À2 vs. hydrophilic anions, like ClÀ and

IOÀ3 . Similar results were reported for ClÀ selective ion-exchange

membranes [48].Meyerhoff et al. [49] developed heparin sensors based on ion-

exchange membranes with methyltridodecylammonium chloride

sites. For poly-ions like heparin the response is small accordingto the Nernst equation, because the slope is proportional to thereciprocal of the charge. However, when a heparin-free mem-

brane is exposed to a solution with a low concentration of heparin, ClÀ ions from the membrane are exchanged by heparinions. The inward ¯ux of heparin creates a nonequilibrium,steady-state potential, which is given by Equation 3.

D EMF � RT

F ln 1 À

z

Rt

Dadm

Dmda

chepY bulk

�3�

D EMF is the nonequilibrium potential change at sample heparinconcentration chepY bulk Y Rt refers to the concentration of ion-exchange sites in the membrane, and D and d are the diffusion

coef®cient and diffusion layer thickness in the membrane (m) and aqueous sample (a).

Receptor molecules with charged nitrogen atoms that pre-

ferentially bind a speci®c anion, due to complementarity in sizeor in geometrical positioning of the charged binding sites, canexhibit a sensor selectivity that deviates from the Hofmeister series. Incorporation of diquaternary ammonium salt 1 in an ion-selective membrane results in IÀ selectivity over ClOÀ

4 and SCNÀ

[50]. Apparently the IÀ ion can interact simultaneously with bothquaternary nitrogen atoms. The distance between these qua-ternary nitrogen atoms plays an important role, because thenormal Hofmeister selectivity is obtained when these sites areseparated by a propylene spacer. Several mono-, di- and tri-

alkylguanidines have been tested for possible speci®c anioninteractions [51]. However, these sensors show only Hofmeister selectivity. Bachas et al. [52] have shown that guanidinium

derivative 2 exhibits selectivity for HSO

À

3 . Receptor 3 has adifferent structure and this results in loss of the HSOÀ3 sensitivity.

Instead selectivity for salicylate is observed with selectivitycoef®cients (Log K Pot

SalY j ) over the most interfering anions benzo-ate, acetate, and ClOÀ

4 of À1.7, À2.4 and À2.6, respectively [53]

Protonated cyclic aza diamide 4 (n � 1) selectively bindsHPO2À

4

[54]. At pH 7.2 the slope for this divalent anion is almost

Nernstian with selectivity over thiocyanate (Log K Pot

HPO4YSCN � À2X3). Sensors with protonated cyclic triaza dia-mide 5 are capable to distinguish planar anionic metal cyanocomplexes like Ni(CN)2À

4 and Pt(CN)2À4 and complexes with

octahedral structures (e.g., Fe(CN)3À6 and Fe(CN)4À

6 ) [55] Thecorresponding pentaaza derivative 6 shows the same sensor selectivity for both types of anions. Umezawa et al. [56] reported sensors for nucleotides. ATP4À sensitive sensors were obtained

with protonated pentaazamacrocycle 6. Although the high chargeof ATP4À results in a Nernstian slope of only À15mVadecade,the selectivity over the polyphosphates ADP3À and AMP2À and even over the lipophilic ClOÀ

4 anion is a factor of 30. Receptor 6

does not discriminate between different nucleotide bases like

ATP and GTP. Ditopic receptor 7 has, besides a (protonated)triamine moiety for phosphate binding, a cytosine moiety whichis complementary to guanine [57]. The additional selectiveinteraction between the guanidine moiety of GTP4À and thecytosine moiety is re ected in a selectivity for GTP4À over ATP4À.

Electropolymerized, oxidized polypyrrole membranes are positively charged and can act as ion-exchange membranes [58].

Depending on the salt present during the polymerization of theaqueous pyrrole solution and during the conditioning, the sensorsare selective for ClÀ [59] or NOÀ

3 [60]. These sensors reveal a

selectivity deviating from the Hofmeister series, probably because the size of the larger anion prohibits exchange with thesmall anion included during the polymerization; even withselectivity of ClÀ or NOÀ

3 over the highly lipophilic ClOÀ4 .

However, the sensor characteristics are strongly dependent on themembrane preparation procedure, like polymerization and con-ditioning times, and the polymer ®lm deteriorates when exposed to light [60]. Besides the instantaneous Nernstian response,sensors based on polypyrrole membranes show drift towards along term equilibrium potential when exposed to a new analyte,

which is due to changes in the oxidation level of the ®lm [61].

3.4. Sensors with Organometallic Receptors

Receptors with Lewis acidic metal centers as anion bindingsites are often applied for anion sensors with selectivities

deviating from the Hofmeister series. The metal center can beeither ionic (complexed in an organic ligand), or can be cova-lently bound as part of an organometallic structure. Dependingon the organic ligand and the metal center this class of receptorsapplied in ISEs contains both charged and neutral receptors. Asdescribed in Section 2.2, with positively charged receptors lipophilic anionic sites should be added to the membrane to optimizethe sensor selectivity, whereas in the case of neutral receptorscationic sites should be added.

The type of ligand and metal center in¯uence the sensor selectivity due to differences in the electron-accepting character of the complex. For example, with Au3�-triisobutylphosphine





sul®de sites, SCNÀ selective electrodes are obtained [62]. Theselectivity over ClOÀ

4 and Br À (Log K Pot SCNY j � À1X8 and À0.4,

respectively) is strongly deviating from the Hofmeister series.Membranes containing a N -thiocarbamoylimine-dithioether sil-ver complex show selectivity for IÀ over ClOÀ

4 , SCNÀ, or other halides [63]. HPO2À

4 sensitive electrodes were obtained with plasticized PVC membranes that contain a Ni2� mixed ligand complex with one diketonate and one N-alkylated ethylenedia-

mine ligand [64]. ISEs that are highly selective for IÀ

, even withrespect to lipophilic anions like ClOÀ4 and SCNÀ, are obtained

with the tetraazaannulene Ni2� complex 8 [65]. Due to the saddleshape of 8, the two axial positions of the metal center are not equivalent and a pentagonal coordination is preferred.

Sometimes ligands are used that have also been applied asreceptor in cation-selective sensors, like for example crown

ethers or calixarenes. Electrodes with electropolymerized binaphthyl-20-crown-6 potassium complex respond to HSÀ

with a slope (À110 mVadecade) strongly deviating from thetheoretically expected Nernstian slope of 59 mVadecade [66].

Bis(thioamido)- and tetrakis(thioamido)-functionalized calix[4]-arenes strongly bind heavy metal ions, and have been applied asreceptors for Pb2� selective CHEMFETs [67]. These calix[4]-

arene derivatives in membranes without the addition of lipophilicionic sites or with a small amount of positive sites give anionsensitive sensors with a selectivity generally following the Hof-meister series except with an increased sensitivity for IÀ [68].

Schiff's base ligands are frequently used to complex anion binding metal ions. Cammann et al. [169] reported CNÀ selectiveelectrodes using biscopper complex 9. CNÀ binding is favored because this anion ®ts well into the cavity and can coordinate to both copper centers. The sensors show selectivity of one order of magnitude over ClOÀ

4 and SCNÀ. The Schiff's base complexes of Ba2�, Cu2�, or Pb2� were used as ion-exchange sites in ClOÀ

4

selective CWEs [702]. Sensors with the neutral complex of Co2�

and a salophene are selective for IÀ over ClOÀ4 and SCNÀ (Log

K Pot iY j � À2X4 and À2.2, respectively) [71].

In our group anion-selective CHEMFETs were developed withneutral anion receptors based on an uranyl cation immobilized ina salophene ligand. Bound by this ligand the uranyl cation prefersa pentagonal, bipyramidal coordination, with the two oxygenatoms at the apical positions and four of the equatorial positionsoccupied by the salophene ligand. The ®fth equatorial positioncan coordinate to nucleophilic guest species. This can be anucleophilic moiety (e.g., C�O or S�O) of a neutral molecule[72], or an anionic species [73, 74]. The salophene building block can be modi®ed with lipophilic docyl substituents for

improved solubility of the uranyl salophenes in the polymericmembrane matrix. The hard Lewis acidic uranyl cation favorsinteraction with hard Lewis basic anions, and consequently the

parent salophene 10 gives H2POÀ

4

selective CHEMFETs. The

sensors have an almost Nernstian response slope of À56mVadecade, and show selectivity over more lipophilic

anions, like Br À and NOÀ3 (Log K Pot

H2PO4Y j � À1X7 and À1.3,respectively). By variation of the ratio between the concentrationsof receptor and lipophilic cations in the membrane the stoichio-metry of the anion receptor complex was studied and appeared to be 2 : 1 receptor to anion [75].

The advantage of the uranyl salophene building block is that

additional hydrogen bond binding sites can be introduced closeto the uranyl center and this made it possible to improve or change the selectivity in anion binding. For example, the meth-oxy substituents in receptor 11 can accept hydrogen bonds from aH2POÀ

4 anion coordinated at the uranyl center. CHEMFETs withthis receptor show improved H2POÀ

4 detection limits and selec-tivity over halide anions compared to the sensors with the parent salophene 10 [76]. Also hydrogen bond donating amido sub-stituents can be introduced (receptor 12). Due to the reduced size

of the binding cleft and the capability to donate hydrogen bonds,this receptor is a stong binder of FÀ, which is a hard Lewis baseand stong hydrogen bond acceptor. With this receptor it is pos-sible to detect FÀ at the (sub)millimolar level, even in the pre-

sence of a large excess of the very lipophilic perchlorate (Log K Pot FYClO4

� À1X7). Uranyl salophene derivative 13 gives CHEM-FETs selective for acetate. In spite of the high hydrophilicity of acetate, they are selective for acetate in the presence of ClÀ, Br À,and NOÀ

3 (Log M K Pot AcOY j � À1X2, À1.2, and À0.3, respectively).

The selectivity over H2POÀ4 is a factor of 250. Apparently, the

phenyl substituents of 13 which give a favorable interaction withthe methyl group of the acetate anion, unfavorably interact withthe tetrahedral H2POÀ

4 anion and consequently reduce the bind-ing strength of the H2POÀ

4 anion with the uranyl center.Co3� cobyrate derivatives, structurally related to vitamin B12

(14), give well-functioning anion sensors. In vitamin B12 thecobalt cation is bound at the four equatorial coordination sites by

the nitrogen atoms of the corrin cycle. At the axial positions

cyanide and the dimethylbenzimidazole ribonucleotide part (the proximal base) of the corrin ring are coordinated. This results in amonovalent complex. For sensor applications more lipophilicderivatives like 15 have been synthesized [77]. Derivative 15

lacks the proximal base and this leaves one of the axial coordi-nation sites free for anion complexation [77b]. These sensorshave high NOÀ

2 selectivity over more lipophilic anions like NOÀ3

and ClOÀ4 (Log K Pot

NO2Y j � À3X6 and À2.4, respectively). Theoptimal sensor characteristics with this positively charged receptor are observed with about 60 mol % (vs. receptor) of lipophilic borate anions in the membrane [78]. With corrin 16, inwhich an imidazole ring is present as proximal base, sensors

become IÀ selective, because this ion might interact with both thecobalt center and the (protonated) imidazole ring [79].





In porphyrins the Co3� ion is also coordinated via the equa-torial sites. The axial positions of the monovalent complex are

available for anion binding. ISEs with Co3� porphyrin 17 aresensitive towards NOÀ

2 with two orders of magnitude selectivityover NOÀ

3 [80]. After conditioning of the sensor in NOÀ2 solu-

tions, one of the axial coordination sites strongly binds a NOÀ2

anion. This results in a neutral anion receptor in which a second anion can coordinate to the other axial position [81]. The maininterfering ions in the NOÀ

2 response are OHÀ and SCNÀ

[81b,82]. HodinaÂr and Jyo [83] used this receptor for SCNÀ

sensors with selectivity over ClOÀ4 . One of the advantages of

metalloporphyrins is that various types of metal cations can be

incorporated [84]. Due to the differences in the electron-accept-ing character of the metal cores, metalloporphyrins can inducesensor selectivity for several anions (Table 1). Charged Mn3�

porphyrins induce a less pronounced deviation from the Hof-meister series than Co3� porphyrins, and exhibit small preferencefor ClÀ over NOÀ

3 and selectivity for SCNÀ over ClOÀ4 [80, 85].

The oxo-bridged manganese porphyrin (MnTPP)2O exhibits lessOHÀ interference of SCNÀ sensors [86]. The selectivity for SCNÀ over ClOÀ

4 is improved with (FeTPP)2O as anion receptor (Log K Pot

SCNYClO4� À2X3) [87]. Sn4� porphyrins are charged anion

receptors [81a] and they are strongly selective for salicylate [88].PVC membranes with 1 wt.% of Sn(IV)-TPP show a Nernstianresponse, but with larger amounts the slopes are super-Nernstian.

Sensors based on the Sn4� complex of octaethylporphyrin (OEP)did not exhibit this high salicylate selectivity and sensitivity. Thesuper-Nernstian response is attributed to a complicated salicylate

binding mechanism in which ®rst water is coordinated to Sn4�

and salicylate anions are held in a second sphere bound byhydrogen bonds in a 1 : 3 porphyrin-anion stoichiometry [89]. Inthe second step the water ligands are exchanged by salicylate and one salicylate anion is released. ISEs with Sn(IV)-TPP have also been used for the detection of 2-hydroxybenzhydroxamate, adrug for the prevention of kidney stones [90]. The In3� complexof TPP and OEP used in ClÀ selective electrodes show super-

Nernstian slopes of À80mVadecade [91]. The interference of lipophilic anions to the sensor response of this charged receptor

could be reduced by the addition of borate salts to the membrane,although the response slopes remained super-Nernstian [81a].Gao et al. [92] applied the In(III)-TPP complex for the NOÀ

2

selective sensors. Sensors with MoO3� TPP are responding toOHÀ with an almost Nernstian slope in the range 10À11 ± 10À1 MÀ1 [93]. The selectivity over lipophilic ClOÀ

4 (Log

K Pot

OHYClO4 � À4X7) is high. Another way to in¯uence the selec-tivity of metalloporphyrins is the introduction of either electron-donating or -withdrawing substituents at the porphyrin ringwhich change the electron density of the metal center [94], or substituents that have a steric interaction with the anion coordi-nating to the metal [95].

Phthalocyanines (18) related to porphyrins also form com-

plexes with various metal ions. Cobalt phthalocyanine in plasti-cized PVC membranes of ISEs exhibit selectivity for NOÀ

2 [96]and H2POÀ

4 [97]. CWEs with electropolymerized cobalt phtha-locyanine detect NOÀ

2 [98], H2POÀ4 [99], or S2À [100]. Electro-

des modi®ed with poly(acrylate) membranes with covalentlylinked cobalt phthalocyanine detect FÀ and CNÀ in acetonitrile[101]. Although the cobalt phthalocyanines are often referred to

as neutral receptors, the Co2� center is easily oxidized and theactive receptor is the charged Co3� phthalocyanine [96]. Theredox couple Co2�aCo3� is the origin of the ascorbic acid (Vitamin C) response of cobalt phthalocyanine pressed pellet solid electrodes [102]. Several other phthalocyanines with mag-nesium, lead, or copper centers are sensitive for NOÀ

3 , although inthe latter two cases the reproducibility is low [103].

Organotin derivatives have been extensively investigated asanion receptor for sensors. ISEs with trialkyltin chlorides or acetates (alkyl� octyl or butyl) respond to ClÀ with selectivityover NO3

À and ClOÀ4 [104]. The neutral tetracoordinated tin

center converts into the pentacoordinated complex upon the binding of ClÀ [105]. This can lead to exchange of the initialligands. Addition of lipophilic nucleophiles to the membraneenhances this process and results in improved response times and slopes [106]. Although the initial anionic ligand does not in¯u-ence the selectivity, derivatives without such ligands, e.g., 1,2-

(trimethylstannyl)benzene or hexabutyldistannane, do not induceany sensor sensitivity [107]. The selectivity can be modi®ed bythe type of the organic ligand. While bis( p-chlorobenzyl)tin or bis(p-¯uorobenzyl)tin dichloride were applied in HPO2À

4 selec-tive electrodes [108], tris(benzyltin)chloride was used to intro-

duce salicylate selectivity [109]. Binuclear bis(tribenzyltin)oxidewas used to introduce HPO2À

4 selectivity [110]. Chaniotakis et al.[111] have developed several di- and tristannyl derivatives (e.g., bis(phenyldibromostannyl)methane and tris(3-(chlorodimethyl-stannyl)propyl)tin chloride) which are selective for H2POÀ

4 . Insensor membranes the response slopes are sub-Nernstian (À40 to

À45mVadecade), but there is selectivity over SCNÀ and ClOÀ4 .

Mercury is the Lewis acidic site in ClÀ selective receptor 19.The ortho-positioning of the two mercury substituents improves

Table 1. In¯uence of the metal center of porphyrins on the sensor selectivity

Metal center Detected anion Reference

Mn3� ClÀ [80]SCN- [85]

Fe3� SCNÀ [87]Co3� NOÀ

2 [80]SCNÀ [83]

MoO3� OHÀ [93]In3� ClÀ [91]

NOÀ2 [92]

Sn4� SalicylateÀ [88]





the selectivity compared to mono or meta-derivatives [112]. For an almost Nernstian slope of À57mVadecade for ClÀ the addi-

tion of a small amount (1 mol %) of lipophilic ammonium sites isnecessary, although this also results in a somewhat lower selec-tivity over more lipophilic anions like IÀ and SCNÀ. Variousmercury(II) complexes with organic ligands have been applied inseveral anion-selective sensors. The Hg2� complex of triisobu-tylphosphine sul®de is applied in IÀ selective ISEs [113]. Bis(4-

methylpiperidinedithiocarbamato)- or bis(alkyldithiocabamato)-mercury(II) are used for SO2À3 aHSOÀ

3 sensors [114]. The potentiometric response of these sensors is induced by thereduction of Hg2� to Hg� by SO2À

3 or HSOÀ3 , which results in

free organic ligand as negatively charged sites in the membrane[114b]. Prolonged exposure to reducing agents deteriorates theresponse due to conversion of all Hg2�. Main interferents of the

sensors are other reductive anions like Br À, NOÀ2 , and NÀ

3 .Although the metallocenes have been investigated thoroughly

and show strong anion complexation in solution [115], ISEs withthese receptors give generally a poor anion response [116].

However, hafnocene and zirconocene dichloride show selectivitytowards salicylate with benzoate as the main interferent.

3.5. Sensors with Anion Binding Based on Hydrogen

Bonding or Ion-Dipole Interactions

Compared to the receptors that bind anions by Lewis acidiccenters, the application of receptors that bind anions via hydro-gen bonding or ion-dipole interactions is far less developed.Recently Umezawa et al. [117] reported the application of bis(urea) derivative 20 (R � hexyl) as receptor in ISE membranes. Remarkably H2POÀ

4 could not be detected as would beexpected on the basis of the high association constants for thision in solution; and instead a moderate ClÀ sensor was obtained.The potentiometric selectivity coef®cient vs. Br À (Log K Pot

ClYBr � 0X4) re¯ects the almost equal sensitivity for ClÀ and

Br À. Receptor 21, in which two thiourea moieties are linked via am-xylene spacer, introduces SO2À

4 sensitivity [118]. The slope is Nernstian from 10À6 to 10À2 M Na2SO4 (À28.1 mVadecade) and compared to the Hofmeister series the interference of ClÀ, Br À,

or NOÀ3 is strongly reduced (Log K Pot

SO4Y j � À0X1, 1.1, and 1.6,respectively). Lipophilic derivatives of cytosine (22) and thymine(23) were synthesized as receptor sites for potentiometric dis-crimination between AMP2À and GMP2À [119]. The hydrogen bond donating and accepting atoms of 22 are at complementary positions compared to GMP2À. This results in preferential bind-ing of GMP2À, and there is a small selectivity for this anionwhen the receptor is applied in ISE membranes (Log K Pot

GMPYAMP � À0X4). However, with thymine derivative 23 there is

no selectivity for AMP

2À

over GMP

2À

.

The phosphadithia macrocycle 24 in membranes of potentio-metric sensors becomes easily oxidized to the corresponding

phosphine oxide disulfoxide [120]. The resulting macrocycle binds anions via hydrogen bond formation or ion-dipole inter-

actions and gives sensors sensitive towards ClOÀ4 with selectivity

according to the Hofmeister series. In receptor 25 the methylenehydrogen atoms interact with anions due to their electron de®-cient character caused by the electron-withdrawing effect of theadjacent CF2 moieties [121]. However, sensors with 25 show a

selectivity that follows the Hofmeister series, although the sen-sitivity for FÀ is somewhat increased.

3.6. Sensors with Tri¯uoroacetophenone Receptors

In contrast to the receptors described in the previous sections,

where selectivity results from noncovalent interactions with theanion, tri¯uoroacetophenone derivatives (26) form a covalent bond with CO2À

3 (Scheme 1) [122]. The electrophilicity of the

carbonyl group of tri¯uoroacetophenone is enhanced by the presence of the adjacent tri¯uoromethyl group, and 2:1 receptor-anion adducts are formed with CO2À

3 . Several sensors have beendeveloped with near-Nernstian slopes towards CO2À

3 and aselectivity over ClÀ, Br À, and NOÀ

3 [123]. Because derivatives 26

act as neutral receptors, addition of lipophilic ammonium sitesimproves the sensor characteristics [122, 123d]. The selectivity isfurther enhanced by the introduction of electron-withdrawingsubstituents at the benzene ring para to the tri¯uoroacetyl sub-stituent [124]. However, this also results in increased suscept-ibility for hydration. The main interfering species for thesesensors are lipophilic organic carboxylate anions. Consequently,the tri uoroacetophenone derivatives are also suitable for the

development of salicylate [125] and benzoate [126] selectiveISEs. Hexyl-4-tri¯uoroacetylbenzoate has been applied in SO2À

4

sensitive ISEs [127]. However, the selectivity is low and super- Nernstian slopes of À87mVadecade are obtained.

4. Materials for Polymeric Ion-SelectiveMembranes

In the ion-selective sensors described in the previous section,the receptor molecules are dissolved in a polymeric membrane

Scheme 1





matrix (Table 2) to immobilize these selector molecules in close

proximity of the transducer. Generally, the membranes consist of high molecular weight poly(vinyl chloride) (PVC) mixed with plasticizing high boiling organic liquids like o-nitrophenyl octylether (NPOE) or bis(2-ethylhexyl) sebacate (DOS). The plasti-cizer lowers the glass transition temperature and increases the polarity of the membrane to facilitate the partition of ions.Because of the large in¯uence that the plasticizer can have on the partitioning of the ions, several types of plasticizer have beendeveloped [128] and used to optimize the selectivity of the sensor [129].

The use of plasticized PVC membranes has also some distinct disadvantages. The gradual leaching of the plasticizer and theelectroactive components from the membrane into the sample

solution deteriorates the sensor characteristics like slope and signal-to-noise ratio, thus limiting the durability of these membranes [130]. This is mainly the case for microsensors like CWEsand CHEMFETs. The conventional ISEs have a much larger membrane volume and consequently the effects of leaching of the plasticizer are observed after a longer period. Moreover, when plasticized membranes are used on solid-state transducers, theweak physical interaction between the membrane and the trans-ducer surface is another disadvantage for long-term durability.For ISFETs some improvement in the adhesion could be obtained

by covering the ISFET surface with a polyimide mesh [131] or ahydrophobic polyacrylate ®lm [132]. Carboxylated PVC [133,134,] and copolymers of PVC, poly(vinyl alcohol) (PVA), and

poly(vinyl acetate) (PVAc) [135] show improved adhesion tosolid-state transducers. However, the former polymer is lesssuitable for anion sensors since interactions of the carboxylatemoieties with anion receptors can be expected [136]. The adhe-

sion of PVA can be further enhanced by pretreatment of theISFET wafer with SiCl4 [133, 137]. This results in covalent bond formation between the polymer and the silicon surface. Urushi is

a naturally occurring lacquer which has a strong adhesion to theISFET surface. When a plasticizer is added, this matrix can bewell used for sensor membranes [138]. However, a disadvantageof this material is the long curing time up to 10 days. The

addition of formaldehyde as cross-linking agent or increased temperature reduces the curing time to 2±3 days [139].

Plasticizer-free membranes are preferred for the development of sensors with long lifetimes. The latter type of membrane hasalso advantages for large scale production of sensors whichrequires the possibility that the membrane can be deposited on

wafer scale via photolithographic techniques. The use of plasti-cizers has been circumvented by using Langmuir-Blodgett ®lmsof polyglutamate mixed with a receptor that were transfered onto

an ISFET to obtain Na� sensitivity [140]. However, the leachingof receptor results in a rapid loss of sensitivity within three days.Covering the ®lm with a phthalocyaninato-substituted poly-siloxane membrane improves the durability and a 10 mVadecadeloss in sensitivity was observed over 180 days. Thin membranes

made of the polyion complex of dioctadecyldimethylammonium poly(styrene sulphonate) without the addition of plasticizers were

also applied for ClÀ selective ISFETs [141]. Grafting of theISFET surface with cationic quaternary ammonium moieties[142], anionic phosphonate moieties [143], or crown-ether-functionalized phthalocyanine molecules [144] was applied tointroduce NOÀ

3 , Ca2�, and Na� sensitivity. Since complete cov-erage of the ISFET surface cannot be obtained by this method a

large residual sensitivity towards pH changes is observed. Anovel approach is the application of membranes obtained withsol-gel methods [145, 146,]. These membranes are formed byreaction of a mixture of tetraethoxysilane, diethoxy-dimethylsilane and the receptor yielding a viscous solution.Deposition of this solution on top of an ISFET and heating for two days results in a glassy membrane which is very well adhered

to the ISFET surface. Membranes with selectivities according tothe Hofmeister series are obtained with hibrid organic-inorganicsol-gel matrices based on a biscarbamate with two triethoxysilanefunctional groups, doped with lipophilic ammonium sites [146].

Although the adhesion of the above-mentioned membranematerials to the ISFET surface is improved compared to plasti-cized PVC membranes, the photolithographic deposition and

patterning of these membranes on wafer scale are still not possible. Therefore photopolymerizable (meth)acrylates, like bisphenol A epoxydiacrylates, hexanedioldiacrylate, and alkyl-methacrylates, have often been investigated as membrane mate-rial [147, 148,]. Also polyurethanes have been applied as sensor membranes because of the good adhesive properties to solid-statetransducers and their biocompatibility [149]. Urethane oligomers

with acrylate moieties have been used to make these membrane polymers photopolymerizable [147d, 150]. However, all poly-acrylate membranes require a plasticizer, and besides the limiting

effect on the sensor durability, also the photopolymerization isnegatively in¯uenced by these additives [147c].

The siloxane polymers are very promising materials for the

development of durable ion-selective membranes. The intrinsicelastomeric properties (T g 4 À 100 C) of these polymers makethe addition of plasticizers super¯uous [151]. Moreover, the biocompatibility and the adhesion to silicon surfaces are better than that of PVC membranes [149a]. Several commercial roomtemperature vulcanizing (RTV) silicone rubbers, like Bayer'sSilopren and Dow Corning 3140 RTV, have been investigated for this purpose. However, the polarity of the parent poly(dimethylsiloxane) is very low and this restricts a suf®ciently high partitionof ions into the membrane phase. The result is a high membrane

resistance [152]. The addition of more lipophilic ionic additivesresults in a slight decrease in membrane resistance [153]. How-ever, the solubility of the common electroactive species is limited [148, 154]. Homogeneous membranes could be obtained by

modi®cation of borate salts and receptor molecules with trisdi-methylsiloxane or polymerizable trisethoxysilane substituents[154, 155]. Another way to increase the polarity of the membraneand thereby improve the solubility of the receptor is by theaddition of a plasticizer [153, 156, 157]. However, this greatlyeliminates the advantage of polysiloxanes with respect to dur-ability, and indeed it was reported that the durability of suchmembranes is only moderate due to the gradual leaching of the plasticizer [156].

An alternative approach to enhance the polarity of poly-siloxane membranes is the introduction of polar substituents tothe polysiloxane backbone. Attachment of the relatively polar

cyanopropyl moieties to the polysiloxanes has been successfullyapplied for this purpose [158]. Impedance measurements showed

Table 2. Membrane materials used for potentiometric anion-selectivesensors

Polymer Detected anion Reference

PVCaPVA NOÀ3 [129d, 135]

Urushi ClÀ [138]Acrylate NOÀ

3 [147c]Polysiloxane NOÀ

3 [156a, 160, 162, 166]Siloxane sol-gel ClÀ [146]

Monolayer ClÀ, NOÀ3 [141, 143]





that a polysiloxane with only 3 mol % of cyanopropyl sub-stituents has a 10±20 times lower membrane resistance than

membranes made of commercial poly(dimethylsiloxane) [159].Recently, also commercially available tri¯uoropropyl-substituted polysiloxanes have been used as membranes on CHEMFETswith selectivity towards Na�, K �, or NOÀ

3 [160]. However, for the detection of Mg2�, Cd 2�, Pb2�, or Ag� ions these membranes are inferior in terms of sensitivity and durability compared

with their PVC counterparts.To obtain suf®cient mechanical stability of the polysiloxanemembranes, the siloxane chains have to be cross-linked. For silanol end-capped polymers, cross-linking is possible by acondensation reaction of the end groups in the presence of acatalyst [148, 161, 162]. However, this method does not allow patterning of the membranes by photolithographic techniques.

Polysiloxanes with methacrylate substituents can be cross-linked via photo-initiated radical polymerization [158a], and via photolithographic patterning multi-ion sensing CHEMFET arrays become possible [162, 163].

Previously, in our group photopolymerizable siloxane ter- polymers were developed with methacroyl and cyanopropylsubstituents by cationic emulsion polymerization of octa-

methylcyclotetrasiloxane, a mixture of the cyclotetramer and cyclotrimer of (3-cyanopropyl)methylcyclosiloxane, and 3-methacroylpropyldichloromethylsilaan [161]. However, due tothe different reactivities of the monomers in the polymerization(decreasing in the order dichlorosilaneb cyclotrisiloxanebcyclotetrasiloxane) and impurities in the commercially availablestarting compounds, this reaction was not very reproducible and

batches of polymers were obtained that were inhomogeneous. Wehave found that the polysiloxane production can be improved interms of quality and reproducability by synthesis via anionic

polymerization of a mixture of hexamethylcyclotrisiloxane, 3-methacroylpropylpentamethylcyclotrisiloxane, and a R-propylsubstituted pentamethylcyclotrisiloxane initated by CsOH

(Scheme 2) [164]. The obtained polymers have a narrow mole-cular weight distribution and there is good agreement betweenthe relative amount of siloxane moieties in the polymer and in themonomer mixture.

Besides for cross-linking of the polymer backbone, themethacroyl substituents are used for the covalent attachment of the electroactive species [165]. For this purpose tetra-

phenylborate and several cation receptors have been modi®ed with a methacroyl substituent. For durable anion sensors photopolymerizable ammonium sites were synthesized [166]. The

NOÀ3 selective CHEMFETs obtained with these polysiloxane

ion-exchange membranes have an improved durabilty (b190days) upon continuous exposure of the sensors to water compared

to sensors in which the ammonium sites are not immobilized.

The polarity of the polysiloxane membranes can be modi®ed by changing the type or number of polar substituents at the

polysiloxane back-bone. Impedance spectroscopy has shown that the benzoylaminopropyl or phenylsulfonylpropyl moieties result in a lower membrane resistance than obtained with similar amount of cyanopropyl, acetylphenoxypropyl, or acetylpropyl

substituents [167]. Moreover, the membrane resistance can belowered by increasing the degree of substitution, e.g., a mem-

brane with 25 mol% of cyanopropyl substituents has a similar membrane resistance as the polysiloxane membranes with 10mol% of phenylsulfonylpropyl moieties.

The variation in polar substituents can be used to optimize thesensor characteristics like sensitivity and selectivity. First, thesubstituents can in¯uence the solvation of the ions in the membrane phase. This is re¯ected in differences in the selectivity

when NOÀ3 selective ion-exchange membranes of polysiloxanes

with different polar moieties are compared. For membranes with benzoylaminopropyl substituents more interference of ClÀ and Br À is observed than with cyanopropyl substituted polysiloxanes

(Log K Pot NO3YCl � À1X6 and À2.3, respectively) [166]. The halide

anions have a favorable hydrogen bond interaction with theamido substituents and consequently the partitioning from water

to the membrane is increased. Secondly, the polar substituentscan in¯uence the functioning of receptor molecules in sensorswith selectivity deviating from the Hofmeister series. Highlyaromatic receptors like cobalt porphyrins or uranyl salophenederivatives are only soluble in polysiloxanes with aromatic polar substituents [168]. However, in the case of polysiloxane membranes with aromatic benzoylaminopropyl substituents, the car-

bonyl oxygen atoms of amido substituents can interact with themetal centers of these receptors and consequently inferior sensor characteristics are observed compared to PVC membranes.

Sensors with the optimal sensitivity and selectivity for hydrophilic anions like NOÀ

2 or FÀ (based on receptor 17 and 12,respectively) have membranes of polysiloxane with phe-

nylsulfonylpropyl subsituents. These moieties allowing easy partitioning of the anions due to the high polarity, result in good solubility of the receptor, and do not have any interactions withthese receptors.

5. Concluding Remarks

The examples in this review illustrate that although thedevelopment of anion selective sensors lays behind the counterparts for cationic species, several potentiometric sensors have been developed for anionic analytes. The progress in potentio-

Scheme 2





metric anion sensing is generally driven by the development of novel anion receptors which introduce the selectivity in the

sensor. Most successful until now appear to be the receptors based on Lewis acidic organo-metallic anion binding sites.However, the introduction of organo-metallic binding sites isdif®cult and can not easily be combined with geometric restric-tions of the receptor to enhance the selectivity as is used inseveral cation receptors. Moreover, the metal centers are often

labile in aqueous environment. Regarding these limitations, anionrecognition exclusively based on hydrogen bonding is very promising. Hydrogen bonding moieties, e.g., amido or ureamoieties, are easily synthesized and the high directionalityof the interactions can be used to improve the selectivity.

For the applicability of the anion sensors durability of themembranes is required, in particular when the sensors are in

contact for prolonged time with aqueous samples. Several novelmembrane materials which can substitute plasticized PVC asmembrane matrix can improve the sensor durability. They have better adhesion when solid state transducers are used, and theylack membrane components (e.g., plasticizers) which easilyleach. However, most of the materials have only been investi-gated for cation selective sensors or anion sensors with selectivity

according to the Hofmeister series. The low partitioning of anions and limited solubility of several anion receptors might restrict the applicability of these materials. However, the siloxane polymers with several types of polar substituents that we havedeveloped, are also successful for the development of anionsensors.

6. References

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anion review

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