electroanalytical chemistry with zeolites

Microporous and Mesoporous Materials 33 (1999) 281–289www.elsevier.nl/locate/micmat

Electroanalytical chemistry with zeolites

C. Senaratne 1, J. Zhang, J. Fox, I. Burgess, M.D. Baker *Department of Chemistry and Biochemistry, Guelph-Waterloo Centre for Graduate Work in Chemistry, University of Guelph, Guelph,

Ont. N1G 2W1Canada

Received 10 May 1998; accepted for publication 20 July 1999

Abstract

In this paper we explore how solution phase molecules affect the electrochemistry of silver-cation exchangedzeolite-modified electrodes (ZMEs). Furthermore, we examine the potential utility of ZME response to quantifysolution phase analytes in aqueous and non-aqueous solutions. We give several examples which show that ZMEs areuseful in assessing detection efficiencies and analyte selectivities. However, flow systems are better if one desiresincorporation into a device. A useful method is to place the zeolite in a conventional HPLC column coupled to aconventional thin-layer amperometric electrochemical detector. The detection method described in this paper is basedupon suppressed electroactivity of intra-zeolite silver cations. Indirect analyte detection occurs where the analyteaccelerates the departure of silver ions into the solution phase, whereupon they are electrochemically determined. Weshow examples where promotion of silver in the solution phase occurs, via both analyte–silver interactions, andinteractions between the supporting electrolyte and the analyte. Amperometric determinations of alkali metal cations,benzene, trichloroethylene and water are described. © 1999 Elsevier Science B.V. All rights reserved.

Keywords: Electrochemistry; Sensors; Silver; Zeolite-modified electrodes (ZMEs)

1. Introduction sions and framework charges. Such a diversity ofreadily available materials offers the potential for

Structured zeolitic frameworks are convenient both size and charge selectivity. Furthermore, syn-hosts for use in molecular recognition and sensing. thetic alterations of the framework compositionBesides high crystallinity and thermal stability, one can conveniently adjust the hydrophobicity/can achieve controlled manipulation of the intra- hydrophilicity of molecular sieve zeolites. Severalzeolite environment via post-synthetic modifica- research groups have developed quantitative andtions. Many zeolite structure types have been selective chemical sensors based on zeolites. Forsynthesized, covering a wide range of pore dimen- example, europium-exchanged zeolite Y was used

to detect oxygen via fluorescence quenching [1],and Bein and coworkers [2] have also developed* Corresponding author. Tel.: +1-519-824-4120;zeolite-coated quartz crystal oscillators capable offax: +1-519-766-1499.

E-mail address: [email protected] (M.D. Baker) molecular recognition. The first review of the1 Also corresponding author. Present address: Central electroanalytical implications of zeolites was given

Analytical Laboratories, Research Institute, King Fahd by Rolison et al. [3], and more recently WalcariusUniversity of Petroleum & Minerals, Dhahran 31261, Saudi

[4] has re-reviewed this area.Arabia.E-mail address: [email protected] (C. Senaratne) This paper focuses on amperometric detection

1387-1811/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved.PII: S1387-1811 ( 99 ) 00148-1

282 C. Senaratne et al. / Microporous and Mesoporous Materials 33 (1999) 281–289

methods, based upon an understanding of how the lated vitreous carbon, Electrosynthesis Co. Inc.,nature of the solution phase affects zeolite-modi- East Amherst, NY ) containing 80 pores per inch.fied electrode (ZME) redox chemistry. In the The latter were formed by loading 20 mg of zeoliteexamples given, indirect quantitative analyses are in THF onto a 1 cm2×1 mm thick piece of RVC.highlighted. Here the redox currents of an All electrochemical experiments employed a three-electroactive moiety placed within the zeolite are electrode cell. Cyclic voltammetry and anodicused to signify the presence of the target analyte. stripping were recorded using a PAR 273A EG&GDevelopment of a rational strategy is based on potentiostat. Indirect analyte determinations at Agsystematic studies of silver redox in various zeo- ZMEs used the peak current of the silver anodiclites, in particular zeolites A and Y. We have stripping wave (see later) unless otherwise stated.shown that the electrochemical reduction of silver Flow-through experiments used a Waterscations, originally placed within the zeolite, occur Associates 6000A water solvent delivery systemat the electrode–solution interface and not in the coupled to an amperometric controller (LC-4) andzeolite bulk [5]. Moreover, the site location of the flowcell (Bioanalytical Systems). Injections weresilver cation, the nature of the supporting electro- effected using a rheodyne injector with a looplyte and the electrolyte solution critically influence volume of 100 ml. The working electrode was glassythe electrochemistry. For example, at low silver carbon (1 mm diameter), and the working andloadings, silver cations in zeolite Y show site reference electrodes were stainless steel and SCE,specificity for the hexagonal prisms and sodalite respectively. Silver reduction was achieved at acages. Ag+/0 redox is prevented when the electro- bias potential of −200 mV vs. SCE. Columns withlyte cation is too large to enter the zeolite cages, lengths between 5 and 15 cm and 0.5 mm ID werestopping silver cations gaining an ingress to the packed with large particle zeolite X or A. Theseexternal electrolyte solution. In effect this places were synthesized using Charnell’s method. Sodiumthe ZME in an ‘off ’ state. When solution phase aluminate (40 g for zeolite X or 80 g for zeolitespecies lift this condition (i.e. turn the ZME ‘on’),

A) was added to 100 ml of triethanolamine insilver is released into solution. Redox at the700 ml of deionized water. A silicate solution waselectrode–solution interface can then occur.prepared by dissolving 100 g of sodium metasili-Quantitation follows if the amount of silver pro-cate nonahydrate and 100 ml of triethanolamineduced scales with the concentration of the analyte.in 700 ml of deionized water in a polypropyleneWe examine the utility of this approach in thisjar. The solutions were mixed and digested in apaper.water bath at 75 to 85°C. Crystallization wascomplete in three to five weeks for zeolite X, andin two to three weeks for zeolite A. Crystals as2. Experimentallarge as 140 mm (zeolite X ) and 100 mm (zeoliteA) resulted. Columns packed with large crystalZeolites were from UOP ( Whistler, AL) andzeolites operated at reasonable pressures (1000–Engelhard. Large particle size zeolites were synthe-3000 psi). Benzene solutions prepared in deionizedsized separately (see below). Before ion exchange,water were injected either into the electrochemicalthe powdered samples were stirred overnight incell through a septum using a high precision0.1 M NaCl and then washed with a minimumsyringe or into the columns via the rheodyneamount of distilled and deionized water. Silver ioninjector. Water determinations were performed inexchange was achieved (in the absence of light)a Vacuum Atmospheres dry-box. Anhydrous N,N-using silver nitrate solutions of the desired concen-dimethylformamide (DMF from Aldrich) was fur-tration to attain the loading required. Elementalther dried over activated 4 A molecular sieves andanalyses were performed at Galbraith LaboratoriesLiClO4 was dried at 140°C. Although this may( Kentucky), by ICP or at Guelph using atomiclead to alkali metal ions in the solution, theabsorption spectroscopy of the filtrates for silver.concentration is negligible compared with that ofZMEs were cast on ITO (indium tin oxide on

glass) as previously described [5] or RVC (reticu- the supporting electrolyte.

283C. Senaratne et al. / Microporous and Mesoporous Materials 33 (1999) 281–289

2.1. Amperometric electrochemical detection with In Fig. 1(b), bulk intra-zeolite silver cations areredox silent (see Introduction) within the insulatingzeoliteshost but are reducible in solution. Quantitation ofthe solution phase is possible when the targetAmperometric detection at ZMEs can be

accomplished in two general ways as illustrated in analyte enhances the concentration of Ag+ at theelectrode–solution interface. This occurs for exam-Fig. 1. In the first, Fig. 1(a), electroactive intra-

zeolite silver cations interact with the analyte ple when the analyte promotes ion exchange. Size-excluded cations, or those which exchange slowly(shown as A in the figure). Besides a shift in redox

potential, one would expect that analyte complex- on the electrochemical time scale, are ideal toinitially lock silver cations within the zeolite host.ation or adsorption within the zeolite pores would

affect the rate of intra-zeolite electron transport. The background current is recorded when ionexchange is slow or absent. In the best case, noWhile this idea is attractive in proffering size

selectivity, it suffers from several flaws. First, intra- faradaic current flows when the analyte is absentfrom the solution, and the blank yields a nullzeolite electron transfer has not been convincingly

demonstrated in any ZME. Second, it is not trivial response.The principal difference between these methodsto gauge the effect of an intruding analyte on

electron hopping and percolation phenomena asso- is the locus of electron transfer to the silver cation,based on the mechanism of electron transfer atciated with intra-zeolite redox [5]. Rational design

is only feasible when the modification of the elec- ZMEs, which was first formalized by Shaw et al.[6 ]. Eqs. (1) and (2) below give the two plausibletrochemical signal occurs in a predictable and

proportional way. routes for the reduction of silver cations initiallylocated within the zeolite:

Ag+(z)+C+(s)PAg+(s)+C+(z)

Ag+(s)+e−�Ag0 (1)

Ag+(z)+C+(s)+e−�Ag0(z)+C+(z) (2)

where C is an electrolyte cation and z and s referto the zeolite and solution phases, respectively.Eqs. (1) and (2) describe extra-zeolite and intra-zeolite redox, respectively. Bessel and Rolison [7]extended this model to cover various scenariosinvolving topological isomers of a zeolite and aredox probe. Four silver species exist, each withthe potential to provide a redox sensitive sensor.

(Ag–Z)o: Silver is outside the zeolite (i.e. in thesolution phase).(Ag–Z)B: Silver is bound to the external boundary(B) of the zeolite.(Ag–Z)i,G: Silver is intra-zeolite but is mobileenough to sample the global (G) lattice withinFig. 1. A schematic representation of amperometric detectionthe experimental time scale.at zeolite-modified electrodes. Note that both cases require

electrolyte cations (not shown) to enter the zeolite to maintain (Ag–Z)i,L: Silver is intra-zeolite and bound to ancharge balance. (a) Intra-zeolite electron transfer. Adsorption extra-framework site that provides its local (L)of target analyte influences the electrochemistry of intra-zeolite environment during the experiment.silver. (b) Extra-zeolite electron transfer. The target analyte

(Ag–Z)o and (Ag–Z)B are extra-zeolite in natureinduces intra-zeolite silver to enter the solution phase.and do not sample the bulk of the zeolite. TheReduction subsequently occurs at the electrode–solution

interface. (Ag–Z)B category can be further subdivided into


two components, namely internal (Ag–Z)B,i andexternal (Ag–Z)B,e. The former involves species atthe particle boundary that reside in incompletecages. The latter retain electroactivity and arebound at the surface of the zeolite particles, closeto the conductive substrate of the ZME. Withoutelectrochemistry related to (Ag–Z)i,L and(Ag–Z)i,G [5,7], placement of silver cations withinthe electrically insulating zeolite support conve-niently masks electroactivity. However, voltamme-try requires a supporting electrolyte, whichproduces (Ag–Z)o via ion exchange. A decreasein (Ag–Z)o through retardation of ion exchange isnevertheless achievable with size-excluded electro-lyte cations (vide supra). Analyte-assisted pro-duction of (Ag–Z)o or (Ag–Z)B then affordsquantitative determinations. We term maskingelectroactivity of ZMEs and subsequent indirectanalyte quantification via reduction of (Ag–Z)o as‘suppressed electrochemical detection’. Severalexamples are given in the remainder of this paper.

Fig. 2 presents new data for silver ion exchangedmordenites, Ag0.88Na7.1M and Ag7.7Na0.33M in0.1 M TBAClO4 in 3:1 (v/v) methanol/water. Herewe plot the silver anodic stripping peak currentagainst the concentration of alkali metal cationsin solution. As the alkali metal cations are addedto the solution, the redox suppression of Ag+/0 islifted. It is interesting that the currents for bothsamples follow the trend Cs+>Na+>Li+. Sincethe currents reflect the ion exchange rate [and thus Fig. 2. Anodic stripping peak currents for Ag+-mordenite

ZMEs as a function of alkali metal cation concentration in 3:1the concentration of (Ag–Z)o], the hydrated radiiv/v methanol/water containing 0.1 M TBAClO4: (a)of the alkali cations are important [see inset toAg0.88Na7.1Mor; (b) Ag7.7Na 0.3Mor. & Li+, × Na+, +Fig. 2(a)]. The non-linearity of the response is Cs+. Scan rate 10 mV/s. Preconcentration time 30 s, 600 mV

likely due to the loss of silver from the zeolite. cathodic of the anodic stripping peak potential. Inset to (a)This zeolite is composed of large pores of dimen- shows the hydrated and ionic radii of relevant alkali metal

cations.sions 6.7×7.0 A2 and smaller side-pockets ofdimensions 2.9×5.7 A2 [8]. Partial cation desolva-tion must occur before entry into these smaller First a water sensor based on the critical influence

of trace water on the non-aqueous electrochemistrycavities, so one can conclude that silver cationsapparently do not locate in the small side-pockets of AgA ZMEs is described. Following this, we

examine the potential for detection of benzene andof the mordenite structure. We discuss site selectiv-ity of silver in ZMEs elsewhere [9]. trichloroethylene in water.

Quantitative analysis of neutral molecules isalso possible when the analyte promotes 2.2. Quantitation of water(Ag–Z)o. This occurs when the analyte interactswith: (i) the electrolyte cation or (ii) the intra- Selective and quantitative determinations of

trace water in polar organic media are importantzeolite silver cation. Examples are given below.


in several applications [10], resulting in manydevices and methods [11–16 ]. For example, nearIR [15] and surface plasmon [16 ] methods givereported detection limits of 20 ppm and 0.3%,respectively. In off-line measurements, the KarlFischer titrator is a popular method with detectionlimits close to 1 ppm. Understanding the effect oftrace water on the non-aqueous electrochemistryof AgA ZMEs provides an opportunity to form auseful on-line electrochemical sensor for flowingstreams as we now describe.

The electrolyte solvent strongly influences thecyclic voltammetry of Ag12A [17]. For example,in pure DMF, cathodic and anodic peak currentsare 500 and 200 times smaller than in water (seefigs. 1 and 2 of Ref. [17]). Recall, from Eqs. (1)and (2) that charge balancing cations must enterthe zeolite for either mechanism involving Ag+/0redox. When this step is rate determining, electro-lyte ion incorporation controls the cathodic cur-rents regardless of the mechanism and regulationof the redox rate via cation sieving occurs [18,19].Formation of (Ag–Z)o depends on the ionexchange rate, so the size of the cation–solventcomplex is important. Non-aqueous ion exchangeoccurs slowly, contrasting rapid ion exchange inwater. Note that the primary solvation number ofthe silver cation is four, rendering it very mobilewhen hydrated. Trace water in organic solventscontaining alkali metal electrolytes promotes(Ag–Z)o. So detection is based on an interactionbetween the analyte and the electrolyte cations(vide supra). Cyclic voltammetry of Ag12A [17]confirms this notion, since addition of water tothe electrochemical cell in controlled quantitiesproduced a gradual increase in the redox waveswith a detection limit close to 20 ppb. Detectionat this concentration is two orders of magnitudesuperior to the Karl Fischer method. Calibration

Fig. 3. Calibration plots for Ag12A ZME to trace water at:plots are shown in Fig. 3(a) and (b). The response(a) ppm and (b) ppb levels. The point at zero concentrationis reasonably linear at ppm levels, but severe non- corresponds to the background current.

linearity occurs in the ppb regime [see Fig. 3(b)].The data strongly resemble an adsorption isotherm

16 h. Non-linearity of the calibration plots in thesuggesting that the response is not a reflection ofppb regime is therefore likely due to partial dehy-ion exchange. This is not surprising since partialdration. Peak current reproducibility to water indehydration occurs from the pump down in thethe ppm regime was variable (±10%), but eachdry box ante chamber. Darkening of the zeoliteelectrode gave a linear response. Together withdue to silver ion reduction which occurs in the

presence of light and water was absent even after variable hydration levels, each electrode is of


Fig. 4. Cyclic voltammetry of Ag12A ZME in dry DMF contain-ing 0.1 M LiClO4. # Blank, $ 1%, ( 2%, , 3% and % 4%

Fig. 5. Schematic of the flow-through detector. Silver ions(v/v) methanol. Each addition was made sequentially and there-

released from the column are detected by a thin-layer electro-fore the data shown are for a single electrode. Scans were initi-

chemical cell (enlarged). As the zeolite saturates with waterated at the anodic limit of the scan program. Scan rate 20 mV/s.

(and loses silver ions) ion exchange with silver ions from anOnly the first cycles are shown. Subsequent scans show little

external solution regenerates the column. Heating and evacua-change.

tion follow to remove water. A standard HPLC pumping stationprovides flow-through (not shown).

different mass. Better performance would resultfrom more reproducible coating methods. enter the pores of zeolite A [23], the cation–solvent

complexes exchange immeasurably slowly (on theCalzaferri and coworkers [20] have prepared highquality monograin ZMEs and the in situ growth time scale of the electrochemical experiment) due

to steric [23] and electrostatic interactions [24]of zeolite films on electrodes [21] is an interestingalternative. with the zeolite lattice.

Development of the Ag ZME into a workingA problem often encountered with water sensorsis interference by polar organics. For example, device presents several obstacles. Apart from non-

linearity at ppb concentrations, the capacity of thehydrophilic polymer films such as Nafion andother perfluorosulfonate ionomers respond to both modified electrode is limited since each electrode

contains only about 1 mg of zeolite. Exposure ofwater and methanol [22]. Fig. 4 shows the effectof gradual addition of dry methanol to Ag12A a totally dehydrated ZME to 100 ppm aliquots of

water once per minute results in saturation inZME in DMF. In the presence of methanol at ppmconcentrations no measurable change in current about 20 min. The amount of zeolite can be use-

fully increased by using a packed zeolite columnoccurred, and up to 4 wt% the Ag12A ZME anodicstripping peaks remained constant, although there and an amperometric flow cell. (Ag–Z)o released

from the zeolite in the presence of water is thenwere small changes in the cathodic currents. Wedid not investigate the source of this behavior. detected downstream. For gram quantities of zeo-

lite the useful lifetime of the detector is extendedSimilar results were obtained with acetonitrileadditions. Although acetonitrile and methanol may by three orders of magnitude. The flow-through


cheaper) alternative to conventional sampling elim-inating chain-of-custody concerns. Severalapproaches have surfaced for the BTEX family.These include fibre optic laser induced fluorescence[28] and liquid optical waveguide sensors [29].

We now describe the electrochemical responseof Ag ZMEs in aqueous solution containing tracebenzene. In zeolites X and Y there are two channelsystems distinguishable by their cation exchangecharacteristics. Large channel cations (those ininterconnecting supercages) exchange rapidly andsmall channel cations (those in the sodalite cagesand hexagonal prisms) exchange less rapidly.Cations that are size-excluded from the smallchannels will exchange very slowly with smallchannel cations. The combination of small channelsilver cations and electrolyte cations that ingressthe small channels slowly is therefore an excellentway to achieve suppressed electroactivity. In zeo-Fig. 6. Response of zeolite–electrochemical water sensor (seelites Y and X, silver ions locate preferentially inFig. 5) to injection of 100 ppm of water. The mobile phase was

dry DMF containing 0.1 M LiClO4. A positive spike marks the the sodalite cavities and hexagonal prisms belowinjection. Sample volume 100 ml. Flow rate 2 ml/min. 4.2 silver ions per unit cell [30,31].

AgxNa56−xY (x<4.2) ZMEs are suppressed in

the presence of electrolyte cations that are slow towater detector is depicted in Fig. 5 [25]. Ag12A ispacked into a standard HPLC column connected enter the small channel system [14–16 ]. For exam-

ple NH+4 and Cs+ enter the supercages readily butto a thin-layer electrochemical cell (see Section 2).The organic mobile phase contains the electrolyte not the sodalite cages. The highest level of suppres-

sion is achieved by ‘back-ion exchange’. Furthercation (typically LiClO4). When the organicsolvent is dry, little or no silver is released from exchange of Ag

xNa56−xY (x<4.2) is implemented

with a cation that enters the small channels slowlythe column. Injection of water into the columnreleases electroactive (Ag–Z)o into the amperomet- but readily exchanges with supercage silver cations.

This method produces a sample with silver cationsric detector. We give an example of the utility ofthis approach in Fig. 6 for an injection of 100 ppm in the small channels, and ammonium ions in the

large channels [17], effectively removing tracewater into a DMF mobile phase. Convenient insitu regeneration is possible by flowing silver supercage silver. Molecules that complex with

silver in solution produce (Ag–Z)o [18]. This is annitrate solution through the column.example of suppressed electrochemical detection,where the analyte interacts directly with intra-2.3. Quantitation of benzene and trichloroethylenezeolite silver (vide supra).

The detection of benzene in solution can inPollution of fresh water is a serious environmen-tal problem. A ubiquitous contaminant is trace principle be facilitated using redox suppressed

ZMEs, since it forms a weak complex with silverorganic matter including the BTEX family (ben-zene, toluene, ethyl benzene, and xylene). State- [32,33]. Fig. 7(a) shows the response of an ITO

electrode modified with a back-exchanged Agof-the-art off-line analysis involving GCMS,SPME (solid phase microextraction) and LCMS NH4Y to 400 ppb benzene. The detection limit is

lower with a RVC substrate [Fig. 7(b)]. The back-was recently reviewed [26,27]. Current detectionlimits for benzene by GCMS are 10 pptr. In situ ground peak is due to (Ag–Z)o likely caused by

ion exchange with solution phase protons. Note(on-line) analysis is however still an attractive (and


Fig. 8. Response of flow-through detector to benzene injectionsat indicated concentrations. Mobile phase TBAF/H2O. Flowrate 0.8 ml/min. Bias potential −0.4 V vs. SCE. Samplevolume 100 ml.

Once again the response to benzene is non-linear.The source of this was not investigated.

Another example, where a strong interactionbetween solution phase molecules and intra-zeolite

Fig. 7. Differential pulse anodic stripping voltammetry ofAgNH4Y ZMEs in water upon addition of 400 ppb benzene.(a) ZME on ITO substrate. Electrolyte was 0.1 M TBAF.Anodic stripping peak of silver was collected after 1 min depos-ition time at −0.8 V. Scan rate 20 mV/s. Pulse width 50 ms,pulse height 50 mV. (b) ZME on RVC substrate. Electro-chemical scan parameters as above. Note that the apparent peakshift is due to the Pt wire QRE shifting potential. Two replicatemeasurements on the same electrode are shown. The lower scansin both cases are the background traces.

that the currents observed for the RVC electrodeare larger due to the increased mass of zeolite.Fig. 8 shows data collected at ppm concentrationsusing the flow-through detector with a back-exchanged AgNH4X column packing [25]. Theeluent is TBA+ rather than NH+4 used for ZMEs(see Fig. 7) to further suppress (Ag–Z)o in the

Fig. 9. Response of Ag0.6Na6.8(NH4)48.6Y on ITO to 100 ppmabsence of benzene. Note that the blank also showsof aqueous trichloroethylene. Linear sweep voltammetry atsilver released by the column. This is reasonable20 mV/s in 0.1 M Ba(NO3)2. Here the first and twentieth cycles

on two counts: TBA is not strictly size-excluded are shown. The signal denoting the presence of TCE grows withfrom zeolite X, and the eluent was slightly acidic time (see also Ref. [19]). Scans were initiated at the anodic

potential limit. Lower trace is the background.(pH 5.7) promoting hydrolysis and ion exchange.


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[17] C. Senaratne, M.D. Baker, J. Electroanal. Chem. 332can be effectively produced by decelerating ion(1992) 357.exchange between silver cations and solution phase

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electroanalytical chemistry with zeolites

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