DOI: 10.1002/cplu.201402105

The Oxygen Reduction Reaction in Ferrofluids: TowardsMembrane-less and Spill-less Gas SensorsJanjira Panchompoo,[a] Mengchen Ge,[a] Chuan Zhao,[a] May Lim,[b] and Leigh Aldous*[a]

Introduction

Iron oxide nanoparticles, including maghemite (g-Fe2O3) andmagnetite (Fe3O4), are an important class of magnetic materialsthat have long been a topic of interest owing to their low tox-icity, easy synthesis, biocompatibility and unique superpara-magnetic properties.[1–6] Typically, iron oxide based nanoparti-cles have been increasingly used in a broad range of applica-tions, for example, as contrast agents in magnetic resonanceimaging (MRI),[6, 7] targeted drug delivery,[6] electrochemical andbiosensors[8–10] and as electrode materials for semiconductorgas sensors.[11–18] In the latter application, iron oxides (g-Fe2O3

and a-Fe2O3) with n-type semiconducting properties havebeen employed as thin-film or thick-film active sensing layersin gas sensors for the detection of H2,[11, 12] CO,[13] NO2,[14] H2S,[15]

O2[16] and ethanol.[17, 18]

Iron oxide (para)magnetic nanoparticles are also widely usedto prepare ferrofluids.[5, 19–21] Ferrofluids, also known as magnet-ic fluids, are stable colloidal dispersions of small, single-domainmagnetic nanosized particles (typically with an average diame-ter of ca. 10–20 nm) suspended in either polar or non-polarliquid carriers, for example, water, benzene and toluene.[1, 2, 20–22]

Characteristically, ferrofluids possess both magnetic and fluidproperties, and their rheological behaviours can be magnetical-ly controlled by means of the magnetic field.[2, 21, 23] Therefore,

ferrofluids can change almost instantaneously from a liquid toa solid-like state and vice versa in the presence and absence ofan applied magnetic field, respectively.[2, 21]

Several synthetic methods have been developed and usedto prepare highly stable and monodisperse iron oxide magnet-ic nanoparticles; however, the chemical co-precipitation ofiron(II) and iron(III) salts generally appears to be the simplestand most efficient technique to synthesise iron oxide nanopar-ticles (typically on a large scale) with reported diameters from2 to 17 nm.[1–3, 21, 24] The surface properties of the iron oxidemagnetic nanoparticles are the key factors that determine thecolloidal stability of ferrofluids.[1, 3] Stabilisation of magneticnanoparticles can be simply achieved by coating the surface ofnanoparticles with polymers or surfactants, which provide re-pulsive forces between particles, keeping them in a stable col-loidal state.

The oxygen reduction reaction is of extensive importance,especially with regard to electrochemical energy systems.[25] Itis also commonly investigated to evaluate novel electrochemi-cal gas-sensor systems. Electrochemical gas sensors make upthe majority of the gas-sensor market,[26] with developmentover the last few decades largely based upon the classic Clarkelectrode design,[27] whereby a working electrode placed incontact with a layer of electrolyte (usually aqueous) is coveredby a gas-permeable membrane. This membrane acts as a barri-er, preventing the liquid electrolyte from spilling out of thesensor, partially reduces electrolyte evaporation and partiallyimpedes gas molecules from reaching the underlying electrodesurface.[28, 29] Once the target gas undergoes electrolysis at thesensing electrode, it generates a current that is proportional tothe gas concentration.[29] These membrane-based gas sensorscan be used effectively (within a narrow range of ambient con-

In this study, a “membrane-less and spill-less” gas-sensingdevice has been evaluated for the electrochemical detection ofoxygen. Iron oxide magnetic nanoparticles were prepared bychemical co-precipitation and used to prepare an aqueous fer-rofluid. The iron oxide nanoparticles were subsequently stabi-lised and passivated with a cationic polymer, namely, poly(dial-lyldimethyl ammonium chloride). The resulting ferrofluid wasevaluated as an electrolyte for the analytical quantification ofoxygen on screen-printed carbon electrodes. An applied mag-netic field immobilised the ferrofluid electrolyte in place to

result in a “membrane-less and spill-less” ferrofluid-based gassensor. The polymer poly(diallyldimethyl ammonium chloride)was found to result in an apparent enhancement in the elec-trocatalysis of the system towards the oxygen reduction reac-tion. Furthermore, as the strength of the applied magneticfield was increased, the oxygen reduction current also in-creased owing to the response of the polymer-coated nano-particles. The oxygen reduction current was linear from 0 to100 % oxygen content.

[a] Dr. J. Panchompoo, M. Ge, Dr. C. Zhao, Dr. L. AldousSchool of Chemistry, Faculty of ScienceUNSW Australia, Sydney, NSW 2052 (Australia)Fax: (+ 61) (0)-2-9385-6141E-mail : l.aldous@unsw.edu.au

[b] Dr. M. LimSchool of Chemical Engineering, UNSW AustraliaSydney, NSW 2052 (Australia)

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ditions), although they have some drawbacks inherentlycaused by the gas-permeable membrane.[28, 29]

Membrane-free amperometric gas sensors have been report-ed that utilise microlitre volumes of ionic liquid as electro-lyte.[28–31] These systems have also been expanded to dropletson screen-printed electrodes (SPEs).[32, 33] SPEs have beenwidely used in electrochemical sensing applications owing totheir low cost, disposable sensing surface and their regularcombination with simple and portable electrochemical equip-ment. However, at all stages, the above membrane-less sensorsmust be kept perfectly level and protected from rapid gasflows or the liquid droplet will roll off the sensing surface. Ad-ditionally, Compton et al.[29] reported that some ionic liquidlayers were unstable, as they changed shape or split into multi-ple droplets owing to the uptake of atmospheric moisture.

In this study, we have developed a “membrane-less andspill-less” electrochemical gas sensor. Aqueous colloidal disper-sions of iron oxide magnetic nanoparticles were prepared bya co-precipitation method to form an aqueous ferrofluid. Theanalytical performance and utility of this ferrofluid (containing0.1 m HCl as electrolyte) for oxygen gas sensing was then stud-ied on screen-printed electrodes (SPEs). The cationic polymerpoly(diallyldimethyl ammonium chloride) (or poly(ac)) wasadded, whereby it played a threefold role: 1) further stabilisingthe iron oxide nanoparticle colloid, 2) passivating the ironoxide nanoparticles to prevent unwanted electrochemical sig-nals and 3) interacting with the SPE electrode surface to resultin an apparent enhancement in the electrocatalytic reductionof oxygen. An applied magnetic field resulted in immobilisa-tion of the ferrofluid droplet, resulting in a spill-less ferrofluidthat could still be used as a gas sensor for oxygen. Interesting-ly, the applied magnetic field actually resulted in an enhance-ment in the oxygen reduction current, likely owing to thepoly(ac)-coated iron oxide nanoparticles increasing the amountof poly(ac) near the electrode surface and thus enhancing theapparent electrocatalysis of the system with respect to oxygenreduction.

Result and Discussion

Characterisation of the iron oxide nanoparticles

The size and the morphology the iron oxide nanoparticles(Fe2O3 NPs) synthesised and used in this study were character-ised by transmission electron microscopy (TEM). The aqueousferrofluid dispersions were synthesised as described in the Ex-perimental Section, diluted to a concentration of 0.1 % w/w inwater, deposited on a TEM grid and dried under ambient con-ditions. Figure 1a displays a typical TEM image of the as-syn-thesised Fe2O3 NPs. The results demonstrate that the particlesare spherical in shape and moderately polydisperse, which isnot unusual for nanoparticles prepared by the chemical co-pre-cipitation method, as previously reported in the literature.[2, 21]

The primary particle sizes measured from the image werefound to range from 8 to 20 nm, consistent with the observa-tions of others.[2, 21]

Stability of the ferrofluid in an applied magnetic field

The stability of an aqueous suspension of Fe2O3 NPs (the ferro-fluid) in an applied magnetic field was first investigated bycyclic voltammetry (CV). In all cases the ferrofluid was com-posed of 10 % w/w Fe2O3 NPs in water, which was spiked so asto contain 0.1 m HCl to act as electrolyte. Figure 2 shows thecyclic voltammograms of the acidified ferrofluid (50 mL) dropcast on a platinum screen-printed electrode (PtSPE). The CVswere recorded in the potential range of about + 0.1 and+ 0.7 V versus Ag in the absence and presence of the appliedmagnetic field. Also shown are images that highlight the be-haviour of the ferrofluid in the absence and presence of theexternal magnetic field.

In both cases, the reduction and oxidation peaks of the FeII/FeIII redox couple could be clearly observed at about + 0.31and + 0.45 V versus Ag, respectively. The direct electrochemicalinterrogation of nanoparticles in solution is an area of increas-ing significance with regard to fundamental investigation, envi-ronmental monitoring and so forth.[34, 35] The electrochemicalresponse of the Fe2O3 NPs were then monitored as a functionof time for up to 2 h in the absence and presence of the exter-nal magnetic field. Figure 2a shows that when there was noapplied magnetic field, the reduction and oxidation peaks thatcorrespond to the FeII/FeIII redox process remained largely un-changed (no significant shift in either peak potentials or peakcurrents observed) for at least 2 h. In addition, we could ob-serve that with no magnetic field applied, the water-basedFe2O3 NPs indeed remained extremely stable under acidic con-ditions (at pH�2.75) for at least several months (Figure 2c).Generally, water- and oil-based ferrofluids are reported to bestable at pH values below 5 (acidic ferrofluids) or above 8 (al-kaline ferrofluids).[1, 20, 24, 36]

Once the external magnetic field was present, these magnet-ic Fe2O3 NPs were strongly attracted towards the applied mag-netic field and could be easily magnetically manipulated (Fig-ure 2d). After an hour under the applied magnetic field, itcould be clearly observed that the magnetic nanoparticlesstarted to settle and became increasingly unstable (Figure 2e).The voltammetric behaviour of the FeII/FeIII redox couple wasmonitored periodically when the external magnetic field wasapplied continuously for 2 h, and the resulting CVs are shownin Figure 2b. As the length of time of exposure to the magnet-ic field increased, both cathodic and anodic peak currents at-tributed to the FeII/FeIII redox couple increased constantly (ca.

Figure 1. Typical TEM images of (a) bare and (b) poly(ac)-coated Fe2O3 NPs,about 0.1 % w/w suspended in water. Sale bar: 100 nm.

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27 mA per hour). This is likely due to the fact that more andmore Fe2O3 NPs aggregated and precipitated over time ontothe screen-printed electrode where the magnetic field exist-ed,[1, 3] resulting in the increase in the peak currents of the FeII/FeIII redox couple. Alternatively, an increase in the local concen-tration of Fe2O3 NPs in the vicinity of the electrode would in-crease the probability of nanoparticles interacting with theelectrode surface. A plot of the oxidative peak current of theFeII/FeIII redox couple against the time of the magnetic field ex-posure also shows a good linear response (shown as an insetin Figure 2b), indicating the moderate instability of thesewater-based Fe2O3 NPs in the presence of an external magneticfield.

Stabilisation of the Fe2O3 NP colloid with polymer coating

The stability of colloidal dispersions (against aggregation) issignificantly influenced by the equilibrium between the attrac-tive and repulsive forces that exist between particles.[1, 3, 5, 20, 37, 38]

In general, the repulsive interactions (both electrostatic andsteric repulsion) between particles play an important role inimproving the stability of the colloidal ferrofluids.[1, 3] Stabilisa-tion of these magnetic nanoparticles could therefore be ach-ieved by either controlling the surface charge of the parti-cles[1, 24] or coating the surface of the nanoparticles with poly-mers and surfactants.[1, 3, 4, 19, 39]

Recently, several coating methods have been developed (in-cluding in situ coatings and post-synthesis coatings) and vari-ous coating materials have been employed to coat Fe2O3 mag-netic nanoparticles to yield stable ferrofluids.[1–4, 19, 21, 39] General-ly, polymers and surfactants could be either chemically teth-ered to or physically adsorbed onto the magnetic nanoparti-cles, forming a single or double layer in which the repulsiveforces (with steric repulsion being predominant) were generat-ed to balance the magnetic and van der Waals attractive forcesthat acted on the nanoparticles, maintaining the stability ofthese nanoparticles under the influence of gravity and an ex-ternal magnetic field.[1, 40] It has also been reported in the litera-ture that the suitable polymers used for coating usually con-tain hydroxyl, amino, phosphate, sulfate and carboxylic acidfunctional groups that can actively bind to the surface of ironoxide magnetic nanoparticles.[1, 4]

In this study, the synthesised Fe2O3 NPs were deliberatelycoated with a range of polymeric stabilisers, including anionic(poly(ss) and poly(ss-Na)) and cationic (poly(ac)) polymers(Figure 3, with average molecular weights of about 75 000–200 000 g mol�1 to obtain the colloidal dispersion of magneticnanoparticles that are highly stable against the applied mag-netic field. For the modification, a 20 % w/w solution of thepolymers in water was added to a 20 % w/w aqueous suspen-sion of Fe2O3 NPs (to result in 10 % w/w of each). The mixturewas then sonicated for 30 min at room temperature to obtainthe polymer-coated ferrofluid, which appeared more viscousrelative to the uncoated one. The stability of the resultingpolymer-coated Fe2O3 NPs under the applied magnetic fieldwas investigated next.

When the external magnetic field was applied, only theaqueous dispersion of Fe2O3 NPs coated with cationic polymer(poly(ac)) was observed to be very stable for at least 24 h,

Figure 2. Cyclic voltammograms of an acidified ferrofluid (50 mL; ca. 10 % w/w Fe2O3 NPs in water containing 0.1 m HCl) drop cast onto a PtSPE (a) beforeand (b) after a magnetic field was applied and recorded as a function oftime (for up to 2 h) at a scan rate of 50 mV s�1. Also shown are pictures illus-trating the behaviours of the ferrofluid in the (c) absence and (d and e) pres-ence of an applied magnetic field for 1 h. Inset : Plot of oxidative peak cur-rent for the ferrofluid under an applied magnetic field versus time.

Figure 3. Chemical structures of the three polymers, poly(diallyldimethyl am-monium chloride), poly(4-styrenesulfonic acid) and poly(sodium 4-styrene-sulfonate), used in this work.

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demonstrating that the nanoparticles could be favourably sta-bilised by this positively charged polymer through the electro-statically charged layer between particles and the steric repul-sion between the polymer chains, as previously described.[1, 3]

The Fe2O3 NPs coated with anionic polymers (both poly(ss) andpoly(ss-Na)) ; however, settled very quickly within a few mi-nutes in the presence of the applied magnetic field. This is dueat least in part to the fact that when the anionic polymerswere added to the inherently acidic ferrofluid (with pH�2.75),protonation of the negatively charged sulfonate group onthese anionic polymers would have occurred extensively in thesystem, resulting in neutral polymeric molecules that could nolonger electrostatically stabilise or balance the magnetic attrac-tive force applied to the ferrofluids. Steric repulsion appears toplay a negligible role in these systems.

The resulting poly(ac)-coated Fe2O3 NPs were subsequentlycharacterised by means of TEM and CV. Figure 1b displaysa typical TEM image of about 0.1 % w/w Fe2O3 NPs coated withabout 0.1 % w/w poly(ac). In comparison with Figure 1a, nosignificant change in the primary particle size or shape couldbe observed for the polymer-coated Fe2O3 NPs, suggestingthat the morphology of Fe2O3 NPs remained largely unaffectedby the polymeric stabilisation. However, aggregation of thepolymer-coated Fe2O3 NPs could be seen on the TEM grid,which was not significant in the actual ferrofluid (see the elec-trochemical characterisation of the system below), and couldhave resulted from capillary action during the drying processon the grid.[2]

Next, the electrochemical behaviour of the poly(ac)-contain-ing ferrofluid (10 % w/w Fe2O3 NPs, 10 % w/w poly(ac), 0.1 m

HCl) was examined by CV. Figure 4 shows the overlaid CVs ob-tained from bare Fe2O3 NPs in the acidified ferrofluid (dottedline) and poly(ac)-coated Fe2O3 NPs in the acidified ferrofluid(solid line). As seen in Figure 4, no reduction or oxidation fea-tures could be observed for the poly(ac)-coated Fe2O3 NPs,whereas for the uncoated Fe2O3 NPs both cathodic and anodicpeaks that correspond to the FeII/FeIII redox couple could be

clearly seen. Further voltammetric measurements of the bareand the poly(ac)-coated Fe2O3 NPs (in the ferrofluid) were alsoconducted over a wider electrochemical potential window andthe resulting CVs are shown as an inset in Figure 4. The ab-sence of electrochemical activity over the whole electrochemi-cal window demonstrates that the polymer-coated Fe2O3 NPswere no longer electrochemically active as a result of surfacepassivation by the coating process.

The oxygen reduction reaction should be clearly observed atplatinum surfaces under acidic conditions.[32] However, in thisstudy only small and ill-defined oxygen reduction featurescould be observed at the PtSPEs, regardless of the presence orabsence of both polymer and ferrofluid. In addition, these re-duction features disappeared in consecutive scans, demon-strating further electrode passivation. Silvester et al.[32] investi-gated oxygen reduction using DropSens PtSPEs in ionic liquidsand found that oxygen reduction features on the PtSPEs weresignificantly different from what was observed on the conven-tional Pt macroelectrodes. This was attributed to reaction ofthe electrogenerated superoxide with the compounds in thePtSPE ink/paste, resulting in a change in the PtSPE surface.[32]

Similar experiments were also performed at DropSenscarbon SPEs pre-modified with bismuth oxide and nickel oxideto evaluate their suitability as an electrode substrate. Both dis-played stable oxygen reduction features and identical resultswere observed at both SPE materials, indicating that oxygenreduction was likely occurring at the carbon electrode surface;the incorporated metals result in significantly lowered back-ground currents relative to unmodified carbon SPEs.[33] Thenickel oxide modified (NiO-modified) electrodes were usedthroughout, and the resulting cyclic voltammograms areshown in Figure 5. In comparison to the voltammetric re-sponse of ferrofluid (dashed line), no comparable voltammetricsignal could be observed for the poly(ac)-containing ferrofluid

Figure 4. Cyclic voltammograms of the acidified ferrofluid (50 mL) drop castonto a PtSPE in the absence (dotted line) and presence (solid line) of about10 % w/w poly(ac), scanned over the redox potential of FeIII/FeII couple and(inset) over a wider potential range of + 1.0 to �1.0 V versus Ag, both ata scan rate of 50 mV s�1.

Figure 5. Cyclic voltammograms recorded in 50 mL of the acidified ferrofluiddrop cast onto an NiO-modified SPE in the absence (dashed line) and pres-ence (solid line) of about 10 % w/w poly(ac), scanned over a wide potentialrange of �1.2 to + 1.0 V versus Ag at a scan rate of 50 mV s�1. Inset : Cyclicvoltammograms of the acidified ferrofluid containing about 10 % w/wpoly(ac) (black solid line) and for 0.1 m HCl containing 10 % w/w poly(ac)itself (blue dotted line), scanned in the oxygen reduction potential region ata scan rate of 50 mV s�1.

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(solid line), highlighting the electrochemically inactive behav-iour of the poly(ac)-coated Fe2O3 NPs, consistent with that ob-served earlier at the PtSPE. However, a newly formed irreversi-ble reduction peak with a peak potential at about �0.5 Vversus Ag (solid line) could be clearly seen for the polymer-coated Fe2O3 NPs at the NiO-modified SPE that correspondedto oxygen reduction (see below).

The oxygen reduction feature was observed at about �0.5 Vversus Ag in both 10 wt % poly(ac)-containing acidified ferro-fluid (inset in Figure 5, solid line) and also for 0.1 m HCl thatcontained 10 wt % poly(ac) (blue dotted line in inset). As nearidentical electrochemical behaviour could be observed forboth the 10 % w/w poly(ac)-containing acidified ferrofluid andjust 10 % w/w poly(ac) in 0.1 m HCl (no Fe2O3 NPs), only datafor one system is presented here. Figure 6 displays the cyclic

voltammograms of 10 % w/w poly(ac) solution in 0.1 m HClunder an ambient (black dotted line), an oxygen-free (bluesolid line) and an oxygen-saturated (red solid line) environ-ment. These experiments demonstrate that the large irreversi-ble reduction peak at about �0.5 V versus Ag corresponds tothe electrochemical reduction of oxygen, likely to H2O2 owingto the non-platinum electrode.[41]

Interestingly, the reduction of oxygen was independent ofthe presence of Fe2O3 NPs but was clearly altered by the addi-tion of the poly(ac). The reduction of oxygen in 0.1 m HCl atthe NiO-modified SPE (in the absence of both Fe2O3 NPs andpoly(ac)) is shown as an inset in Figure 6 (orange solid line)and compared against the same system that contained 10 %w/w poly(ac) (black dashed line). Addition of the polymer in-creased both the peak current and shifted the peak potentialtowards more positive values, which indicated an apparentelectrocatalytic effect upon the oxygen reduction reaction. Al-though the peak shift is not absolutely quantitative owing to

the use of the quasi-reference electrode of the SPE, the in-crease in peak current was reproducible and observed repeat-edly. Apparent electrocatalysis has recently been discussed indetail[25] and can occur when a surface-modifying layer such asa polymer alters the local conditions at the electrode surface(such as pH or ionic strength), and such layers can also possessoxygen solubility higher than that of the bulk solution.[42]

Effect of the applied magnetic field on the reduction ofoxygen in the poly(ac)–ferrofluid

The effects of an applied magnetic field on various electro-chemical processes have been comprehensively studied anddiscussed in the literature.[43–46] Generally, the effects an exter-nal magnetic field can have on the electrochemical reactionscan be divided into three categories: those that relate to elec-trolyte properties, mass transport and electrode kinetics.[43–46]

In this study, the influence of the external magnetic field onthe electrochemical reduction of oxygen in the poly(ac)–ferro-fluid was further investigated with regard to the strength ofthe magnetic field applied. Once the magnetic field was ap-plied to the ferrofluid, the Fe2O3 NPs were actively controlled,resulting in an immobilised droplet of ferrofluid, which was at-tached strongly to the surface of the SPE. Figure 7 displays (a)an image of a magnetic-field-immobilised ferrofluid droplet ona NiO-modified SPE with magnets held underneath (both drop-let and magnets immobilised purely by their interaction witheach other with no other adhesives or similar in place) ; and (b)overlaid cyclic voltammograms of the poly(ac)–ferrofluid, ex-amined as a function of magnetic field strength at an NiO-modified SPE in an oxygen-saturated system. Note that themagnetic field strength was increased by varying the numberof the magnets applied to the ferrofluid, with each magnetquoted to have a magnetic flux density of about 2229 gauss.When magnets were applied, measurements could be made atany inclination or angle; without the magnets in place theelectrode had to be kept level at all times or the dropletwould move away from the electrodes.

As seen in Figure 7b, the cathodic peak current for oxygenreduction increased with increasing magnetic field strength.This is believed to be due to the fact that when the magneticfield strength increased, more magnetic nanoparticles were lo-cated close to the surface of the electrode and therefore morepoly(ac) was present, further enhancing the apparent electro-catalytic effect of poly(ac). A plot of the cathodic peak current(Ipc) versus the strength of the applied magnetic field (inset inFigure 7b) also shows a linear relationship, confirming the sig-nificant positive effect of the external magnetic field on theelectrocatalytic reduction of oxygen.

Magnetic nanoparticles have been dispersed to collect ana-lytes and magnetic fields have typically been used to bringthem to an electrode surface.[10] Polymer, liquid and other sur-face modifiers have also been used to increase the local con-centration of analytes at the electrode surface and induce ap-parent electrocatalysis.[25, 42] This is believed to be the firstknown application of magnetic nanoparticles that can increase

Figure 6. Cyclic voltammograms of 0.1 m HCl containing 10 % w/w poly(ac)(50 mL) drop cast onto an NiO-modified SPE under ambient conditions (blackdotted line), under 100 % nitrogen (blue solid line) and 100 % oxygen (redsolid line) at a scan rate of 50 mV s�1. Inset : Cyclic voltammograms foroxygen reduction in the 0.1 m HCl blank solution (orange solid line) in com-parison to 0.1 m HCl containing 10 % w/w poly(ac) (black dotted line), re-corded at an NiO-modified SPE under ambient conditions.

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the quantity of a surface modifier proportional to an appliedmagnetic field, and thus induce apparent electrocatalysis.

A scan-rate study in the poly(ac)–ferrofluid (10 to200 mV s�1) in the presence of the applied magnetic field dem-onstrated that the current increased with scan rate (notshown). A plot of the log of cathodic peak current (Ipc) versusthe log of scan rate also shows a linear response (R2 = 0.996)with a slope of 0.51, indicating a diffusion-controlled ratherthan adsorption-controlled process.[47]

As the magnetic strength of the field increased, a secondaryoxygen reduction peak could be increasingly observed atabout �0.6 V. For the two-electron reduction of oxygen to hy-drogen peroxide, either one 2 e� or two 1 e� reduction peakscan be observed, depending upon the pH, electrolyte and therelative rates of the first and second electron-transfer con-stants.[41, 48, 49] In this study, this feature is tentatively attributedto localised pH changes in the polymer layer (larger amountsof oxygen reduction in the thin polymer layer result in moresignificant pH changes and proton depletion).

The reproducibility of the poly(ac)–ferrofluid in the presenceof the magnetic field was found to be good. When the experi-ment demonstrated in Figure 7b was performed sequentiallythree times (on the same SPE) the relative standard deviationin the oxygen reduction peak heights was only 4.4 %.

Quantitative electrochemical determination of oxygen inthe poly(ac)–ferrofluid in an applied magnetic field

The electrocatalytic detection of oxygen in the poly(ac)–ferro-fluid was next examined quantitatively (with and without anapplied magnetic field) by varying the concentration ofoxygen from 0 to 100 % (calculated on the basis of the relativeflow rates of oxygen and nitrogen gases being mixed in a gas-mixing system). Although an initial response was observedwithin seconds (as expected for an aqueous-based sensor com-posed of a small droplet) all samples were purged for 10 minto ensure equilibration. No significant difference was observedin response and equilibration time as an effect of the magneticfield strength.

Figure 8 displays CVs recorded in poly(ac)–ferrofluid dropcast on an NiO-modified SPE, which was then exposed to dif-ferent concentrations of oxygen (0, 33, 50, 67 and 100 %)before being analysed in the absence and presence of the ap-plied magnetic field with increasing strength by using 4 and6 magnets, respectively.

In all cases, as the concentration of oxygen increased, theoxygen reduction peak current increased with no shift in peakpotential observed. Moreover, when the applied magnetic fieldstrength was increased with 0, 4 and 6 magnets applied to thepoly(ac)–ferrofluid (from Figure 8a, b and c, respectively),a larger cathodic peak current that corresponded to the reduc-tion of oxygen could be observed (at the same percentageconcentration of oxygen), consistent with the previous investi-gations described above. In addition, plots of oxygen-reduc-tion-peak current versus concentration of oxygen in the ab-sence and presence of the applied magnetic field with increas-ing strength were found to be linear. The highest magneticfield strength applied here was relatively more sensitive to thereduction of oxygen, highlighting the ability of the appliedmagnetic field to not only immobilise the ferrofluid dropletonto the SPE (to create a membrane-less and spill-less gassensor), but also enhance the (apparent) electrocatalytic reduc-tion of oxygen in the poly(ac)–ferrofluid. The system also dis-plays linearity from 0 to 100 % oxygen content. Therefore, itcan be concluded that a colloid of poly(ac)-coated Fe2O3 NPs(as a poly(ac)–ferrofluid) represents a simple “membrane-less,spill-less” gas-sensor assembly, which is both achieved and en-hanced by the presence of an applied magnetic field.

Conclusion

An aqueous colloidal suspension of Fe2O3 nanoparticles (a fer-rofluid) was prepared by a chemical co-precipitation methodand subsequently coated with poly(diallyldimethyl ammoniumchloride) (poly(ac)) to enhance the stability of these nanoparti-cles against agglomeration and precipitation. The resulting

Figure 7. Photograph highlighting the mutually immobilised droplet of fer-rofluid and magnets, inside the test cell with a NiO-modified SPE in betwee-n (a), and overlaid cyclic voltammograms recorded in the poly(ac)–ferrofluidas a function of magnetic field strength at a NiO-modified SPE with 100 %oxygen concentration, at a scan rate of 50 mV s�1 (b). Inset: Plot showing theeffect of the magnetic field strength on the oxygen reduction in thepoly(ac)–ferrofluid.

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poly(ac)-containing ferrofluid was drop cast as a 50 mL dropletonto a disposable NiO-modified screen-printed electrode.When used in the presence of an applied magnetic field, itacted as a “membrane-less and spill-less” electrolyte for thequantification of oxygen content. The results surprisingly re-vealed that the poly(ac) not only acted as a polymeric stabiliserto stabilise and passivate the surface of the Fe2O3 nanoparti-cles, but also resulted in an apparent increase in electrocatalyt-ic activity towards the reduction of oxygen, making thepoly(ac)–ferrofluid ideal for the electrochemical detection ofoxygen.

Experimental Section

Reagents and equipments

Iron(III) chloride hexahydrate (FeCl3·6 H2O, �98 %), iron(II) chlo-ride tetrahydrate (FeCl2·4 H2O, �99 %) and iron(III) nitrate nona-hydrate (Fe(NO3)3·9 H2O, �98 %) were purchased from Sigma–Aldrich (Castle Hill, NSW, Australia). Ammonium hydroxide(28 % w/w NH3 in water) was supplied by Ajax Finechem(Seven Hills, NSW, Australia). Poly(diallyldimethyl ammoniumchloride) solution (poly(ac) ; 20 wt % in water, average Mw

�100 000–200 000), poly(4-styrenesulfonic acid) solution(poly(ss) ; 18 wt % in water, average Mw�75 000), poly(sodium4-styrenesulfonate) solution (poly(ss-Na) ; 30 wt % in water,average Mw�200 000) and all other chemicals were purchasedfrom Sigma–Aldrich (Castle Hill, NSW, Australia). All the re-agents were used without further purification. All solutionswere prepared using deionised water of a resistivity not lessthan 18.2 MW cm at (298�2) K (Millipore Pty Ltd. , North Ryde,NSW, Australia). High-purity compressed oxygen and nitrogengases were obtained from Air Liquide (Fairfield, NSW, Australia).Neodymium–iron–boron (NdFeB) disc magnets with a size of6 mm diameter and 1 mm height used in this study were pur-chased from Frenergy Magnets. Each disc magnet was typicallymagnetised through the height/thickness with a theoreticalholding force (pull strength) of about 0.43 kg and a reportedsurface gauss (which represented the magnetic flux density orthe magnetic induction) of about 2229 gauss.

Electrochemical experiments

Electrochemical measurements were recorded with a mAutolabPGSTAT 101 computer-controlled potentiostat (EcoChemie,Utrecht, The Netherlands) with Nova 1.10 software. CV was

Figure 8. Cyclic voltammograms of the poly(ac)–ferrofluid (50 mL) drop-cast on a NiO-modified SPE at 0, 33, 50, 67 and 100 % O2 concentrations at a scan rateof 50 mV s�1, in the (a) absence and (b and c) presence of an applied magnetic field with increasing magnetic field strength by using 4 and 6 magnets, respec-tively. Also shown are (d) plots of O2 reduction peak current versus percentage concentration of O2 in the (a) absence (~, R2 = 0.944) and in the presence ofthe external magnetic field with increasing magnetic field strength of (b) 4 magnets, &, R2 = 0.993 and (c) 6 magnets, *, R2 = 0.996).

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performed at room temperature at a scan rate of 50 mV s�1

(unless stated otherwise) with a standard three-electrode con-figuration that employed the working, counter and referenceelectrodes of the SPEs, which were connected with a cableconnector to the potentiostat. The SPEs obtained from Drop-Sens (Oviedo, Spain) used in this study consisted of a Pt (DPR-550) or C (DPR-110Ni) working electrode (4 mm diameter)modified with NiO, an Ag quasi-reference electrode and a Pt orC counter electrode. The sample solution (ca. 50 mL) was dropcast onto the SPE, entirely covering the three electrode surfa-ces. The SPE was then placed into an airtight cell with the inletand outlet gas lines, and the cell (with the SPE inside) was setunder a controlled atmosphere of either oxygen, nitrogen ora mixture of the two. Basically, the anhydrous gas (eitheroxygen or nitrogen from the compressed gas cylinders) wasfirst humidified by passing through a water bubbler beforebeing introduced into the cell in which the SPE with the drop-cast aqueous sample was placed. The different concentrationsof oxygen were obtained by mixing oxygen and nitrogengases together with a gas-mixing system that consisted of twoflow meters with a floating ball (Cole–Parmer, Chatswood,NSW, Australia) and a glass gas-mixing segment. The relativeflow rates of the two flow meters were then used to calculatethe percentage concentration of oxygen being introduced intothe cell.

Synthesis of iron oxide magnetic nanoparticles in aqueousmedium

Iron oxide magnetic nanoparticles were synthesised by a chem-ical co-precipitation method, which has been previously re-ported to result in maghemite nanoparticles (g-Fe2O3) thatranged from 2 to 17 nm.[1–3, 21, 23, 24, 50] A solution of 2 m

FeCl3·6 H2O/1 m HCl (80 mL) and FeCl2·4 H2O/1 m HCl (40 mL)were mixed, and the mixture was diluted to 1.2 L with water.Then NH4OH (250 mL, 28 % w/w NH3 in water) was quicklyadded into the iron(II) and iron(III) solution, and the mixturewas vigorously stirred at room temperature for 30 min. Afterthe addition of NH4OH, the colour of the mixture changedfrom orange to black owing to the precipitation of magnetite(Fe3O4).[3, 23, 50]

Next, Fe(NO3)3 was added, and the mixture (in acidicmedium) was subsequently heated to 90 8C with stirring for1 h to oxidise the precipitate Fe3O4 to maghemite (g-Fe2O3).[3]

The synthesised iron oxide magnetic nanoparticles (henceforthreferred to as Fe2O3 NPs) were then magnetically decanted,washed extensively with water and redispersed in water, yield-ing a very stable colloidal dispersion with about (7.96�0.02) %w/w nanoparticle concentration before being pre-concentratedto a concentration of about 10 % w/w by removing somewater from this water-based ferrofluid in the oven at 60 8C. Theresulting pre-concentrated suspension of Fe2O3 NPs was alsofound to be, to the naked eye, very stable for at least severalmonths. The pH of this water-based ferrofluid was found to beabout 2.75, indicating that the surfaces of the nanoparticleswere positively charged, preventing them from agglomeratingand resulting in a stable ferrofluid.[22]

Characterisation of iron oxide magnetic nanoparticles

The synthesised iron oxide magnetic nanoparticles were exam-ined with an FEI Tecnai G2 20 TEM operating at 2 keV and0.2 mA. The aqueous ferrofluid dispersions were first diluted toa concentration of 0.1 % w/w in water. A single drop of theparticle suspension was deposited onto a Formvar coatedcopper grid and dried over a period of 3 days prior to imaging.

Acknowledgements

The Australian Research Council (ARC DECRA DE130100770) is ac-knowledged for funding.

Keywords: electroanalysis · iron · magnetic properties ·nanoparticles · sensors

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Received: April 11, 2014

Revised: June 5, 2014

Published online on July 30, 2014

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