optical sensing schemes for prussian blue/prussian white film system

Analytica Chimica Acta 424 (2000) 27–35

Optical sensing schemes for Prussian Blue/PrussianWhite film system

Robert Koncki∗, Tomasz Lenarczuk, Stanisław Gł˛abDepartment of Chemistry, University of Warsaw, Pasteura 1, PL-02-093 Warsaw, Poland

Received 16 May 2000; received in revised form 3 August 2000; accepted 15 August 2000

Abstract

Optical chemical sensing schemes exploiting redox properties of Prussian Blue/Prusian White film system are reported.Transparent, thin film composed of Prussian Blue andN-substituted polypyrrole was chemically deposited on non-conductingpolyester foil. The process of the film formation is induced and accelerated by UV radiation. The film is robust, crack-free andstrongly adheres to the plastic support. The film was used as a receptor part of a flow-through detector cell. All spectrophoto-metric measurements were performed at the wavelength of 720 nm, i.e. at the absorption maximum of the Prussian Blue film.The Prussian Blue/Prussian White film sensor system exhibits excellent operational stability and long shelf-life. The PrussianBlue film is shown to be suitable for optical determination of selected strong reductants. Also potential application of the filmfor optical determination of alkali cations involved in the redox process is shown. In situ generated Prussian White film ismechanically stable, however, can be easily oxidized to Prussian Blue form. This feature of the Prussian White film can beapplied for optical determination of various oxidants. It was also shown that the presented detector cell is useful for opticaldetermination of redox species in non-transparent, turbid samples. The Prussian Blue/Prussian White film redox system offersa new approach to near-infrared optical sensing applications. © 2000 Elsevier Science B.V. All rights reserved.

Keywords:Prussian Blue; Prussian White; Polypyrrole; Chemosensitive film

1. Introduction

Chemical sensing in optodes is based on changes ofoptical properties of their receptor parts caused by ananalyte. The analytical interactions between receptorphase of a recognizing reagent and an analyte solutionare based on physical processes such as adsorptionor extraction and chemical processes such as acid–base, complexation or redox reactions. A predomi-nant number of chemosensitive optical films is madeof an inert polymer containing a small amount of anorganic reagent (usually dye) which selectively and

∗ Corresponding author. Fax:+48-22-8225-996E-mail address:[email protected] (R. Koncki).

reversibly recognizes and interacts with an appropri-ate analyte using sensing schemes similar to those forion-selective electrodes [1–3]. Recently, application oforganic conducting polymers such as polypyrrole [4],polyaniline [5,6] and its derivatives [6] for optosens-ing purposes has been reported. In such cases, con-trary to dye-based optical films, the whole membranematerial plays a role of a receptor in the process ofanalyte recognition.

Both, molecular and polymer receptors for opticalpurposes are mainly developed by advanced organicchemistry. Inorganic compounds are rather rarelyapplied for fabrication of optical chemosensitivemembranes. In the past years cobalt chloride filmswere intensively studied and successfully developed

0003-2670/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S0003-2670(00)01144-2

28 R. Koncki et al. / Analytica Chimica Acta 424 (2000) 27–35

for optical sensing of moisture [7–9]. Recently, humi-dity-sensitive optical absorption of Co3O4 films [10]and NiO films [11] were reported. Gas sensitivegold-cobalt oxide composite films were used for op-tical recognition of carbon dioxide and hydrogen inair [12].

One of the most intensively studied inorganicsubstances is Prussian Blue (PB, iron(III) hexacyano-ferrate(II), ferric ferrocyanide), the first syntheticmixed-valence coordination compound. The PB filmshave been intensively investigated as components ofrechargeable batteries and fuel cells, electrochromicdevices, electro- and photo-catalysts, sensing elementsof electrochemical sensors and biosensors etc. Onlyrecently two reports on application of composite PBfilms for optical sensing of pH have been published[13,14]. The films enable highly sensitive, selectiveand reversible evaluation of hydrogen ion activity inthe physiological range. The effect of pH on the ma-terials is explained in terms of partial and reversibledecomposition of PB films by hydroxy ions.

The optical pH-sensitivity of PB films is based onprotolytic properties of the materials. In this paperwe demonstrate the optical sensing exploiting theredox properties of Prussian Blue/Prussian Whitefilm system. The chemosensitive film incorporatedinto flow-injection analytical system as a receptorpart of optical flow-through cell detector can beused for detection of selected reductants, oxidantsand non-electroactive species involved in recognizingchemical redox processes.

The choice of method of sensing film preparation iscrucial in this study. The PB films obtained by directchemical deposition, usually from two-componentsolutions are not robust. Either electrochemically de-posited films are usually not strongly adherent andconsequently show lack of mechanical stability. More-over, electrochemical methods allow film depositiononly on conductive or semiconductive materials. Foroptical purposes usually non-conducting materialssuch as plastics and glasses are used. Finally, purePB films exhibit high tendency for cracking. In thepresent work, composite film consisting of PB andN-substituted polypyrrole has been utilized. Stable,thin, homogeneous and optically transparent filmswere chemically deposited on a plastic support. Fortheir fabrication, a slightly modified method recentlydeveloped by Koncki and Wolfbeis [13] was applied.

The proposed modification significantly shortens theprocedure. Moreover, the obtained experimental re-sults are useful for an explanation of mechanism ofthe film deposition.

2. Experimental section

2.1. Apparatus and reagents

All optical measurements were carried out at thewavelength of 720 nm with Shimadzu 2401/PC spec-trophotometer. The measurements were performed us-ing simple single-channel flow injection system con-sisting of peristaltic pump Minipuls 3 (Gilson MedicalElectronics), rotary injection valve and flow-throughoptical cell (both laboratory-made). All organic andinorganic reagents were of analytical grade. All exper-iments were carried out with solutions prepared im-mediately before use with doubly distilled water.

2.2. Deposition of the film

The composite PB film was deposited from 0.1 Msolution of K3Fe(CN)6 in 1.0 M hydrochloric acid sat-urated with 4-(pyrrol-1-yl)-benzoic acid (Pyr-Bac). Adust-free polyester was used as the support. The foilwas immersed in the reaction mixture exposed to ultra-violet radiation from conventional UV lamp (Philips,15 W). The films having absorbance about 1 unit (mea-sured at 720 nm) were obtained after 1 day of the de-position process. The foil with the deposited film wasintensively washed with 1.0 M hydrochloric acid, wa-ter and finally with 0.2 M potassium phosphate bufferof pH 6.9.

3. Results and discussion

3.1. Deposition of the film

Usually PB films are deposited chemically or elec-trochemically from solutions containing two compo-nents (iron(II) or (III) salt and hexacyanoferrate(II) or(III)). In the previous investigation [13], it was shownthat the films of PB can be chemically depositedfrom an acidic solution containing only hexacyano-ferrate(III). Also recently, electrochemical deposition

R. Koncki et al. / Analytica Chimica Acta 424 (2000) 27–35 29

of PB film from a single ferricyanide solution hasbeen reported [15]. In this study, the compositefilm predominantly made of PB was deposited on apolyester support from the solution of K3Fe(CN)6in hydrochloric acid saturated with Pyr-BAc. Undersuch conditions, i.e. in the presence of oxidant instrongly acidic solution the oxidative polymerizationof Pyr-BAc is initiated. Since the formations of bothPB and N-substituted polypyrrole (PPyr-BAc) pro-ceed in parallel and at comparable rates, a mixed or-ganic/inorganic composite film is deposited (Fig. 1).The amount of the polypyrrole is very low and notdeterminable in the course of elemental analysis ofthe composite. The presence of the organic polymerin the resulting material is proved by the differentmorphology of the composite and the pure PB filmsas well as by the results of the infrared analysis [13].The presence of the organic component considerablyimproves the homogeneity and prevents mechanicalcracking of the composite film.

We have observed that the process of the film de-position is significantly accelerated when the reactionmixture is exposed on UV radiation. On the otherhand, the formation of the film in darkness is consid-

Fig. 1. Scheme of formation of the composite film.

erably slower. Moreover, the films deposited in dark-ness are green. Such films are easily converted intoblue form using weak reductants such as iodide orhydrogen peroxide. The photocatalytic effect of lighton the kinetics of the film deposition is presented inthe Fig. 2. Instead of hydrochloric acid in the reactionmixture, sulfuric acid can be used. In both the cases,deposited films have the same chemical and physicalproperties. Since the acidity of both the reaction mix-tures is similar, the differences in kinetics of the filmsdeposition are negligible. Finally, under such condi-tions but in the absence of Pyr-Bac, the pure PB filmis formed. It was confirmed by energy-dispersive an-alytical X-ray spectroscopy that the compositions ofpure PB films deposited from reaction mixtures acidi-fied with hydrochloric acid and with sulfuric acid areidentical.

Although the process of the film deposition hasbeen discussed previously [13], the results presentedhere are useful for a more detailed consideration ofthe mechanism of non-electrochemical deposition ofPB from the single component solution. High acidityof the reaction mixture causes partial decompositionof hexacyanoferrate. The iron(III) cations liberated


Fig. 2. Kinetics of the deposition of the film measured by theincrease of absorbance at 720 nm in time. In the course of de-position, the reaction mixture was exposed, to UV light radiation(15 W lamp) (A), to white light (40 W lamp) (B), or was storedin darkness (C).

in the course of the reaction and the hexacyanofer-rate(III) anions present in the solution can form ahighly reactive complex (ferric ferricyanide) which isreduced to PB film through intermediate Berlin Green(BG) form (Fig. 1). Similar explanation was con-cluded on the basis of electrochemical investigations[15]. For conversion of BG into PB, the presence ofa reducing agent is necessary (Fig. 1). It has beenreported earlier that BG may oxidize solution impuri-ties or possibly solvent [16–20]. Taking into accountour observations, we suppose that under the givenconditions the role of the main reducing agent indis-pensable for the process of PB formation is playedby the simultaneously liberated cyanides rather thanchloride ions, water (solvent) or Pyr-BAc (organicimpurities). Moreover, we suppose that the reductionprocess is induced by ultraviolet radiation. Compli-cated processes of photodegradation of hexacyano-ferrates in aqueous solutions are not clear, althoughthere are some reports on this subject [21–23]. Thephotoproducts after UV exposure show degradationof hexacyanoferrate(III) through the formation of in-termediate mixed and probably binuclear complexes.

The photolysis involves primary photoaquation. Lightpromotes the release of cyanide ions, i.e. aquasub-stitution reaction. Appearance of aquapentacyanofer-rate was confirmed by capillary electrophoresis [23].However, solutions of aquapentacyanoferrate are sta-ble in dark [21]. Cyanide ions liberated by photodis-sociation from thermodynamically stable iron-cyanospecies can be photocatalytically oxidized by variousiron(III)-containing species via cyanate and nitrite allthe way to nitrate [22]. The final iron products ofthe photolytic degradation depend on the pH of thesolutions, producing PB in acidic media [21,23].

For the further investigations presented in this con-tribution, the composite films made of PB doped withPPyr-Bac obtained after 1 day of UV-induced chemi-cal deposition have been used.

3.2. Schemes of sensing

Analyte recognition processes applied in opticalsensing can be classified as ‘indicating’ or ‘stoichio-metric’ [1]. Recently, reported [13,14], application ofPB films for reversible evaluation of pH in physio-logical range is a typical example of an ‘indicating’process for optosensing purposes. However, for higherpHs, the process of the analyte recognition becomes‘stoichiometric’ and irreversible [13]. Optical deter-mination of reductants with the use of PB film isalso an example of ‘stoichiometric’ kinetic process.The reducing analyte reacts with the reagent phase(PB film) and irreversibly converts it into a colorlessproduct in the form of Prussian White (PW) film.Such ‘stoichiometric’ processes are mostly useful insingle-use optical devices like disposable cuvettesand strips. The ‘stoichiometric’ mode of analyterecognition has been used recently for developmentof a disposable test for vitamin C determination [24]based on integrated chemosensing layer of PB.

Although optical sensing schemes based on‘stoichiometric’ reactions are predominantly dedi-cated for disposable probes [1], there are some ex-ceptions to this general statement. In this paper, wedemonstrate that the regeneration of the PB/PW-basedreceptor phase is possible, and consequently, the samesensing film can be used many times as a receptorpart of the optical sensor for some redox species.The changes of the film absorbance are related to thetotal amount of analyte that has contacted with the


receptor phase. Therefore, controlled conditions withrespect to mass transport of an analyte to the receptorfilm and to time of contact between an analyte andchemosensitive film are required. Also the volumeof sample should be known in order to determinethe analyte concentration because the optical signalis related to the amount of the reacting receptor film(not to the concentration as in cases of ‘indicating’schemes of sensing). The required conditions areusually offered by conventional systems developedfor flow-injection analysis. For the present investiga-tion simple single-channel flow-injection system withoptical flow-through cell based on PB film has beenconstructed.

A strong reductant, ascorbic acid, has been appliedas a model analyte recognizing by the PB film basedoptical detector. The analyte causes discoloration ofthe PB film. The reduced form of the film does notabsorb the light in the wide range of wavelength from400 to 900 nm. The formed PW film can be easilyre-oxidized to PB using various oxidants. In thisstudy K3Fe(CN)6 was used as a regeneration agent.The measurements were performed in 0.2 M phos-phate buffer made of equimolar mixture of KH2PO4

Fig. 3. Optical response of PB/PW film system on reductant (A–C) and oxidant (D–F) in different modes of measurements (details in thetext). Vertical and slant arrows indicate injections of analyte and regeneration agent, respectively. In the cases C and F, regeneration agentis added to the carrier buffer. Concentrations of analyte (in mM) are given in the figure.

and K2HPO4. Two modes of measurements wereapplied. In the first one, the regeneration agent wasinjected after each injection of analyte (Fig. 3A) orafter a series of analyte injections (Fig. 3B). In thesecond mode, the analyte was injected into streamof carrier buffer additionally containing regenerationagent (Fig. 3C). In such measurements, the carrierwas spiked with 5.0 mM K3Fe(CN)6. Evidently, thelatter method is much simpler from the measurementpoint of view, however, also less sensitive as in thefirst mode a larger amount of analyte reacts withoptosensing layer. In the latter mode, the reductantis partially consumed in the course of reaction withregeneration reagent (oxidant) present in the carrierstream. An increase of concentration of the regenera-tion agent in the carrier stream shortens the recoverytime of the sensing film but also has a dumping effecton the sensitivity of the system on the analyte.

The presented analytical system can also be appliedfor determination of oxidants with the use of reducedPB film as a sensor. In such case the roles of oxidantand reductant are reversed. In this study, hydrogenperoxide and ascorbic acid were used as model ana-lyte recognized by PW film and regeneration agent,


Fig. 4. Effect of potassium (A) and sodium (B) ions on sensitivityof the PB film to the reductant. Measurements performed in 20 mMpotassium phosphate buffer. Samples contain 2.0 mM ascorbic acidand additives of KCl (A) or NaCl (B) in concentrations (mM)given in the figure.

respectively. Similarly to sensing schemes showed forreductants (Fig. 3A–C) in case of oxidant determina-tion, two modes of measurements are also possible asit can be seen in the Fig. 3D–F. The same phosphatebuffer was used as a carrier solution as previouslyused. If ascorbic acid (regeneration agent) was presentin the carrier, its concentration would be 0.5 mM.

The optical sensitivity of PB film on reductants isaffected by alkaline cations (Fig. 4). In the absenceof certain cations when TRIS-HCl buffer as a car-rier was used, the PB film did not display definedand reproducible redox response. It is well-known[18,19,25], that during electroreduction and subse-quent re-oxidation of PB, cations are transported intoand out of the film. These non-electroactive cationsare necessary as counter-ions for electroneutralityof the PB/PW film system. In the course of opticaldetections of reductants with the PB film, the levelof these interfering cations should be controlled. Asthe sensitivity to reductants is limited by cation con-centration, it is recommended to keep a high level ofthe ions. The potassium phosphate buffer used as acarrier fulfils such requirement.

The PB and some of its transition metal analoguesare ion-selective in the sense that certain group Ications can freely migrate into or out of the film dur-ing electrochemical reduction or re-oxidation whereasothers are excluded. This exclusion is correlated withthe size of hydrated ion. Cations with hydrated radiimuch greater than the channel radius of PB should notbe able to enter the rather rigid lattice of the material.

For this reason both the type and the concentrationof cations present in solution affect the electrochem-istry of PB films. Thus, voltammetric response of PBmodified electrodes can be useful for cations detec-tion [26,27]. To increase selectivity of the detectionscheme, the mass of the cations in the film during thevoltammetric response can be measured. Such type ofexperiments is possible with PB coated quartz crystalmicrobalance [28,29]. As can be seen from the Fig. 4similar indirect detection of alkaline cations exploitingoptical properties of the PB/PW film is also possible.The system exhibits the highest and nearly the samesensitivity to potassium and ammonium ions. Slightlylower sensitivity on rubidium and sodium ions wasobserved and the lowest for lithium and caesium. Thesensitivity sequence is in agreement with the resultsof electrochemical investigations [19,20,25–29]. Thepresented indirect optical sensitivity of the PB film toalkaline cations constitutes an interesting alternativefor ionophore-based optosensitive membranes [2,3].However, it is apparent that from the practical pointof view, the optical PB/PW film system could bedeveloped for the application in limited cases of realsamples where interfering cations are absent.

3.3. Selectivity, interferences and turbid samples

The optical sensitivity of the PB film to reductantsis highly selective. The following tested inorganicreductants give no analytical response: chloride, bro-mide, iodide, oxalate, sulfite, arsenite, cyanide andthiocyanate. Only hydrogen sulfide reduces the sensorfilm to give PW. Typical organic reductants such asalcohols, aldehydes and reducing sugars do not causediscoloration of the PB film. We have found thatonly ascorbic acid and selected mercapto-compoundsreduce the film (Fig. 5). The values of analytical sig-nal correlate well with the reducing reactivity of thetested thiols connected with some kind of substituentspresent in their structures (given in the Fig. 5). Nu-cleophilic, alkaline substituents increase the abilityof the compounds to discolorate the PB film. Thesubstituent effects cause high reactivity of cysteamineand negligible reactivity of mesna, lower sensitivityof the system toN-acetylcysteine than to cysteine, etc.

The PW film is useful for determination of variousoxidants. We have observed optical response of thefilm to hydrogen peroxide, chromate and dichromate,


Fig. 5. Optical response of the PB film on 2.0 mM solutions ofascorbic acid (AA), cystamine (A), mercaptoethanol (B), cysteine(C), homocysteine (D), penicylamine (E), mesna (F), acetylcysteine(G). The structures of the mercaptocompounds (A–G) are shownin the insert.

permanganate, chlorine, bromine, iodine, hexacyano-ferrate(III), iron(III), etc. The film is also sensitive tooxygen, however, the oxidation of PW film is veryslow, being observed only as a drift of the baseline(lower than 0.1 AU/h for carrier saturated with oxy-gen). Until the carrier was degassed (as usually inflow-measurements) or contained regeneration agent(reductant), the measurements were not influenced byoxygen dissolved in analyzed samples. Effects of al-kaline cations on the response were not observed butobviously high level of potassium ions in the course offilm regeneration (reduction) is necessary. It is worthto note that none of the tested oxidants also at highconcentrations cause oxidation of the film to the greenform (BG). Only in case of an excess of hydrogen per-oxide when the film was totally oxidized to the PBform, gas bubbles formed at the film surface disturbedthe optical measurements. This observation confirmsthe ability of PB to catalyze the hydrogen perox-ide degradation and indicates the uselessness of theoxidant as a potential regeneration agent in sensingschemes for reductants.

Any significant interferences from typical anionswere not observed. Besides cross-sensitivity to alka-line cations discussed above (Fig. 4), the optical PBfilms shown pH-sensitivity as it has been reportedpreviously [13]. In redox sensing schemes, a constantpH defines the baseline absorbance in the case of re-ductants determination. Moreover, acidity should becontrolled because reactivity of many reductants in-creases with an increase of pH (pH-dependent redox

potential). Finally, as it was reported earlier, the PBfilms are damaged in alkaline solutions, producingyellow film composed of hydrated iron oxides [13].

The PB/PW film system in any redox state is stableuntil it does not stay in contact with redox species.For this reason after the contact of any analyte withthe sensor film the use of regeneration agents is nec-essary until repeated measurements are carried out.This drawback of the PB/PW film system, typicalfor ‘stoichiometric’ schemes of sensing can be anadvantage in the case of spectrophotometric measure-ments performed with turbid samples. The conceptof optical determinations in turbid samples is basedon measurements of analyte effect (i.e. change of ab-sorbance of PB/PW film) after releasing the samplefrom through-flow cell but before the film regener-ation. Utility of this method is demonstrated in theFig. 6 for both reductant and oxidant determination.

Fig. 6. Optical response of the PB/PW film system on, transparent(A) and turbid samples (B) of reductant (top — 1.0 mM ascorbicacid) and oxidant (bottom — 0.4 mM hydrogen peroxide). Verticaland slant arrows indicate injections of the analyte and the regener-ation agent, respectively. For PB and PW film regeneration 5.0 mMK3Fe(CN)6 and 5.0 mM ascorbic acid were applied, respectively.


In this experiment, both turbid and transparent sam-ples having the same level of analyte were injected byturns. The turbidity of the samples was caused by ad-dition of 5.0 mM solution of CaCl2 to samples that incontact with the carrier forms a suspension of calciumphosphate. The recordings (Fig. 6) clearly show thatthe changes of the absorbance of the sensing film forboth kinds of samples are nearly the same and repro-ducible. When similar measurements were performedwith continuous regeneration (i.e. with regenerationagent added to carrier buffer) the recorded peaks forturbid samples were poorly reproducible and differedfrom peaks for transparent samples.

The method seems to be useful for analysis ofreal samples originally existing in non-homogenouosforms or forming precipitates in the course of analyt-ical procedure (as in the case illustrated in the Fig. 6)or showing significant self-absorption at the givenwavelength. In all these cases where conventionalspectrophotometric methods are useless, the opti-cal determination based on ‘stoichiometric’ opticalsensing offers great advantage.

4. Conclusions

We have described the preparation and attractiveperformance characteristics of optical sensing systembased on mixed inorganic/organic polymer film com-posed of PB and polypyrrole network. The film waschemically deposited on non-conducting substrateaccording to very simple and inexpensive procedureinvolving UV-induced parallel process of hexacyano-ferrate decomposition and oxidative polymerizationof N-substituted polypyrrole.

The resulting films exhibit a high operational andstorage stability. No changes of physical and chem-ical properties of the films were observed after 10months of measurements or after 3 years of storage inair. Contrary to chromoionophore-based optical mem-branes, in case of the crystalline and practically insol-uble PB film, problems such as membrane swelling orleaching of membrane components do not exist. Animportant feature of the developed film is its instru-mental compatibility with inexpensive near-infraredsemiconductor light sources and detectors.

The optical sensing schemes based on redoxproperties of the film system are useful for spectropho-

tometric detection of selected reductants, oxidants aswell as non-electroactive cations. The species can beoptically determined also in turbid, non-transparentsamples.

In all schemes and modes of sensing presented inthis paper the analytical characteristics of the system(i.e. sensitivity, concentration range of determination,detection limits, response and recovery times) dependon the measurement conditions such as flow rate, in-jection volume, geometry of optical cell, method offilm regeneration, kind of regeneration agent etc. Theparameters could be optimized when the system isdedicated to the analysis of given type of real samples.Investigations of the practical utility of the presentedsystem are in progress [30].

Acknowledgements

The authors thank the KBN grant 3T09A 04715 forfinancial support of this work.

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optical sensing schemes for prussian blue/prussian white film system

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