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Construction of TiO2 nanosheets modified glassy carbon electrode (GCE/TiO2) for the

detection of hydrazine

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2016 Mater. Res. Express 3 074005

(http://iopscience.iop.org/2053-1591/3/7/074005)

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Mater. Res. Express 3 (2016) 074005 doi:10.1088/2053-1591/3/7/074005

PAPER

Construction of TiO2 nanosheets modified glassy carbon electrode(GCE/TiO2) for the detection of hydrazine

KhursheedAhmad1, AkbarMohammad1, RichaRajak1 and ShaikhMMobin1,2,3

1 Discipline of Chemistry, School of Basic Sciences, Indian Institute of Technology Indore, Simrol, Indore 452020,M.P., India2 Centre forMaterial Science and Engineering, Indian Institute of Technology Indore, Simrol, Indore 452020,M.P., India3 Centre for Biosciences and Bio-Medical Engineering, Indian Institute of Technology Indore, Simrol, Indore 452020, India

E-mail: [email protected]

Keywords:TiO2 nanosheets, GCE,Hydrazine, sensor, cyclic and squarewave voltammetry

AbstractTiO2nanosheetswere synthesized via solvothermalmethod and characterizedusingpowder x-raydiffraction (PXRD),UV–vis spectroscopy, scanning electronmicroscopy (SEM) andenergydispersivex-ray (EDX)mapping.Abinder freehydrazine sensorwas fabricatedbymodifying the glassy carbonelectrode (GCE)withTiO2nanosheets, using simpledrop castingmethod (GCE/TiO2). ThemodifiedGCE/TiO2was employed for detectionof hydrazinewhich exhibited a very high sensitivity of70μAmM−1 cm−2with a limit of detection (LOD), 28μMusing cyclic voltammetrywhereas ahighestsensitivity 330μAmM−1 cm−2 andLOD, 150μMwasobtainedby employing squarewave voltammetry.

1. Introduction

Transitionmetal oxides in nano forms have gained considerable attention because of their potential applicationsin various applications includingmaterials, chemistry aswell as for biological purposes [1–7]. Among severalmetal oxides, TiO2 has attracted particular attention due to its exceptional properties, such as stability, highinertness, bio-compatibility, non-toxicity, low cost, etc [8–10]. TiO2 has been examinedwidely as an efficientphotocatalyst for the treatment of water contaminatedwith different harmful dyes, pesticides, herbicides andinsecticides [8–13].

Hydrazine is awell-known toxic and carcinogenic chemical which is heavily used inmany pharmaceuticaland industrial applications including explosives, corrosion inhibitors, oxygen scavenger and herbicides [14].

While there are several applications for hydrazine, its long term exposure to humans, either directly orindirectly from the environment, has hazardous effects. The continuous use of hydrazine can led to varioushealth complications includingDNA and brain damage, as well as deterioration of the central nervous system,while low concentrations cause eye irritation, nose and throat problems [15–17].

Because this is amajor problem, a simple low cost, binder-free sensor for the detection of hydrazine isnecessary. There are various techniques available to detect hydrazine including conductometry, electrochemical,colorimetry and fluorescence [18–21]. Of these, an electrochemicalmethod is found to be the cheapest andsimplest technique for the detection of hydrazine. Thismethod has advantage over others in respect ofsensitivity, low detection limit, wide linear range, reproducibility, stability, simplicity and low cost [22–25]. Avariety of nanometal oxides, nanometal particles and nano polymers has been employed for the detection ofrange of variety of chemicals in the past [26–34].

In general, TiO2 nanoparticles are used for its biocompatibility, high conductivity and low cost [35, 36]. Theuse of TiO2with different characteristic shapes such as nanoparticles, nanoneedles and nanotubes, has emergedas an attractive electrodematerial for electrochemical sensors and biosensors [37]. Owing to its unusualproperty, especially its large surface area, strong absorptive and catalytic ability and electronic properties, TiO2

has been emerged as a benchmark over other reportedmetal oxides.The conductivity changes of thematerials strongly depend on the shape and the size of the nanostructures.

Recently, TiO2 nanostructures with different shapes have received extensive attention for gas-sensing

RECEIVED

31March 2016

REVISED

11 June 2016

ACCEPTED FOR PUBLICATION

21 June 2016

PUBLISHED

15 July 2016

© 2016 IOPPublishing Ltd

applications due to their unique physical and chemical properties. The nanostructured titania with tubularshape has been considered one of themost promisingmaterials for the fabrication of gas sensing devices [38].

Herein, due to the abovementioned properties of titaniumdioxide (TiO2), we have synthesized TiO2

nanosheets via solvothermal approach. Further, TiO2 nanosheets have been explored for the fabrication of ahydrazine sensor.

2.Materials andmethod

2.1. Chemicals and reagentsTitanium isobutoxidewas purchased fromAlfa Aesar. Ethanol and sulphuric acidwere purchased fromMerckand usedwithout any further purification.

2.2. InstrumentationThe powder x-ray diffraction patterns (PXRD)were recorded on aRigaku, Japan, RINT2500 V x-raydiffractometer withCuKα irradiation (λ=1.5406 Å) to confirm the formation of TiO2 nanosheets. UV-visible analysis was conducted usingVarianCary 100 BioUV-Visible Spectrophotometer. Themorphologicalfeatures of the sample and energy dispersive x-raymapping of elements was analyzed by field emission scanningelectronmicroscopy (FE-SEM, Supra55 Zeiss Field-Emission Scanning ElectronMicroscope) and EDX attachedwith it. All electrochemicalmeasurements were performed using a computer controlled conventional three

Scheme 1. Schematic representation for TiO2 nanosheets preparation.

Figure 1.XRDpattern of TiO2 nanosheets.

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Mater. Res. Express 3 (2016) 074005 KAhmad et al

Figure 2.UV–vis spectra of TiO2 nanosheets.

Figure 3. FE-SEM image of TiO2 nanosheets.

Figure 4.EDXmapping of TiO2 nanosheets.

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electrode systemMetrohmAuto lab PGSTAT204 usingNOVA software version 1.1 including a glassy carbon asworking electrode, platinumwire as counter electrode andAg/AgCl as reference electrodewere used for allelectrochemicalmeasurements in phosphate buffer solution (PBS) at room temperature.

2.3. Synthesis of TiO2 nanosheetsTitanium isobutoxide (5.1 g)was dissolved in 40 ml of ethanol (EtOH) at vigorous stirring and 0.33 ml ofsulphuric acid (H2SO4) in 4 ml ofH2Owas added to the above stirred solution. Further, the solutionwas stirredfor 1 h at room temperature and poured into the stainless steel Teflon cup and heated at 200 °C for 4 h. TheobtainedTiO2was recovered by centrifugation andwashed repeatedlywith ethanol andwater and dried at 70 °Candfinally the obtained dried powderwas calcined at 600 °C to remove organic volatiles and to obtain the phase.Further the phase andmorphological characteristics was confirmed by different techniques.

2.4. Fabrication of TiO2 nanosheetsmodifiedGCE (GCE/TiO2)The glassy carbon electrode (GCE)wasfirst washed properly by distilledwater and polished using aluminaslurry. After polishing, the electrode is cleaned by sonication inwater and ethanol for 5 min respectively. TiO2

nanosheets were dispersed in distilled water by sonication for 15–30 min to form a homogenous dispersion.

Figure 5.CVof (a)GCE, (b)TiO2 nanosheetsmodifiedGCE/TiO2 in the presence of PBS (pH7.0).

Figure 6.CVof TiO2 nanosheetsmodifiedGCE/TiO2 in the presence of PBS of different pH.

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Then, suspension of TiO2 nanosheets was drop casted onto theGCE surface and kept at room temperature for1 h tomake it dry, smooth and stable and a uniform surfacemodification of theGCEwas obtained.

3. Results and discussion

3.1. Synthesis of TiO2 nanosheetsTiO2 nanosheets were synthesized by facile solvothermal by dissolving titanium isobutoxide (5.1 g) in 40 ml ofethanol (EtOH) at vigorous stirring and 0.33 ml of sulphuric acid in 4 ml ofH2Owas added to the above stirredsolution. Further, the solutionwas stirred for 1 h at room temperature and poured into the stainless steel Tefloncup and heated at 200 °C for 4 hwhich yielded TiO2 nanosheets (scheme 1). The as obtained product waswashed and dried. To confirm the purity and properties, TiO2 nanosheets was further characterized by PXRD,UV–vis, FESEMandEDXmapping. The synthesized TiO2 nanosheets were applied as amodifier to the glassycarbon electrode by simple drop castmethod. Asmodified electrode (GCE/TiO2)was employed as aworkingelectrode in the electrochemical setup to study the efficient catalytic activity of TiO2 nanosheets for hydrazineoxidation by usingCyclic voltammetry (CV), differential pulse voltammetry (DPV) and linear sweepvoltammetry (LSV).

Figure 7.CVof (a) bare, (b)TiO2 nanosheetsmodifiedGCE/TiO2 in the presence of 1 mMhydrazine in PBS (pH7.0).

Figure 8.DPVof TiO2 nanosheetsmodifiedGCE/TiO2 in the presence of hydrazine in PBS (pH7.0).

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3.2. Characterization of TiO2 nanosheetsThe crystallinity and phase purity of TiO2 nanosheets was analyzed by using a powder x-ray diffractometer(XRD) equippedwithCuKα radiation (1.54 Å) in the range of 20–80° as shown infigure 1. TheXRDpattern ofTiO2 nanosheets reveal that sheets have the crystalline nature prepared under solvothermal condition. Theobserved peaks of TiO2 nanosheets werematched to anatase phase (JCPDSNo.21-1272). The diffraction peakswere corresponding to the (101), (004), (200), (105), (211), (204), (220) and (215)planes of the sheet like TiO2

nanoparticles (figure 1). The optical property of TiO2 nanosheets were studied usingUV–vis spectroscopy and astrong absorption peakwas observedwhich is in accordancewith the formation of TiO2 nanosheets (figure 2).The textural features of TiO2 nanosheets were analyzed by the scanning electronmicroscopy (SEM) at differentmagnifications (figure 3). TiO2 sheets are having the dimensions ranging from30–60 nm inwidth and100–180 nm in length. Themorphology of TiO2 nanosheets appeared uniform throughout the analyzed area(10 μm). This uniform sheet would probably have the advantage of good sensing ability ofmodified electrode.To confirm the elemental constituents of synthesized TiO2 nanosheets, EDXmapping have been performed.EDX results revealed the presence of Ti andO elements which again confirmed the synthesized sheets havingtheir original constituents (figure 4).

3.3. Electrochemical performance of the hydrazine sensorThe electrochemical behavior of the bareGCE andmodifiedGCE/TiO2were investigated in the in PBS (pH7.0)with a scan potential of 100 mV S−1. The cyclic voltammograms of the bareGCE andmodifiedGCE/TiO2were

Figure 9.TiO2 nanosheetsmodifiedGCE/TiO2 in the presence of different concentration of hydrazine (0.1–1 mM) in PBS (pH7.0).

Figure 10.Peak current versus concentration of the hydrazine.

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shown infigure 5. The obtained results revealed that the higher current was observed formodified electrodeGCE/TiO2 as compared to the bareGCEwhich suggested the successful surfacemodification of the bareGCEbyTiO2 nanosheets and gave the idea for the detection of analyte due to higher current obtained forGCE/TiO2.

The effect of the pHon the oxidation peak of hydrazinewas investigated in different pH range (pH4.0, 7.0,8.0, 9.0, and 10.0) using cyclic voltammetry. Figure 6 revealed that the oxidation peak potential of hydrazineshifted to less positive values with increasing the pH values. Themaximumoxidation peak current was obtainedfor hydrazine at pH7.0 and this pH valuewas thus employed for further studies.

Further, the performance of the bareGCE andGCE/TiO2modified electrode in presence and absence ofhydrazinewere examined using cyclic voltammetry. Figure 7 shows the cyclic voltammograms of the bareGCEandGCE/TiO2 in the presence of 0.1 mMhydrazine in PBS (pH7.0)with a scan potential of 100 mV S−1.Addition of hydrazine induces the current, similar trendswas foundwith an additional strong oxidation peak ofthe hydrazine at 0.65 V in the potential range between−0.2 and 1.0 V. Therewas no other reverse current wavewas observedwhich suggested that the oxidation of hydrazine hydrate is an irreversible process. The presence ofpeakwith large oxidation current with the addition of hydrazine clearly indicates that theGCE/TiO2 showedgood electrocatalytic oxidation property towards hydrazine.

Differential pulse voltammetry (DPV) is a significant electrochemical tool to investigate the oxidation andreduction of the analyte using electrochemicalmeasurement. Therefore, differential pulse voltammetry wasemployed to investigate the electrochemical behavior of the bareGCE andGCE/TiO2modified electrode in

Figure 11.TiO2 nanosheetsmodifiedGCE (GCE/TiO2) in the presence of different scan rate of hydrazine in PBS (pH7.0).

Figure 12.Peak current versus square root of the scan rate.

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presence of 0.1 mMhydrazine in PBS (pH7.0)with a scan potential of 100 mV s−1 as shown infigure 8. Fromthe obtained results it can be clearly seen that the higher current for GCE/TiO2was obtainedwith strongoxidation peak following the similar enhanced trend as for cyclic voltammograms. Interestingly; differentialpulse voltammograms forGCE/TiO2 also shows the enhanced peak current as compared to bare electrodewhich is due to the electrocatalytic behavior of TiO2 towards the oxidation of hydrazine.

Further, the performance ofmodified electrode, GCE/TiO2was also tested in different concentrations of thehydrazine (0.1–1 mM) using cyclic voltammetry as shown infigure 9. It was observedwith the addition ofdifferent concentrations of hydrazine, the peak current increases and the current was found to be increases insimilar trend up to 0.1 mMwhich further suggested that themodifiedGCE/TiO2 have excellent electrocatalyticproperty for detection of hydrazine. The linear calibration plot of the peak current versus concentration of thehydrazinewas plotted (figure 10)which shows the as concentration of hydrazine increases, the peak current alsoincrease linearly. This linear behavior for electrocatalytic oxidation of hydrazine was confirmed that themodifiedGCE/TiO2 shown electrocatalytic oxidation of hydrazine in controlledmanner.

The scan rate dependence of themodifiedGCE/TiO2was also investigated towards oxidation of 1 mMofhydrazine in PBS (pH7.0)with a scan potential of 100 mV s−1 and the results are shown in figure 11. Theoxidation peak current from cyclic voltammograms, for hydrazine at different scan rates shows, as increasing thescan rates the oxidation peak current increases correspondingly suggesting that the oxidation process occurringat the surface of TiO2 nanosheetsmodified electrode is controlled process. The linear plot of the peak current

Figure 13. LSV of TiO2 nanosheetsmodifiedGCE (GCE/TiO2) in the presence of different concentration of hydrazine (0.1–1 mM) inPBS (pH7.0).

Figure 14.Peak current versus the concentration of the hydrazine.

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obtained from cyclic voltammograms of different scan ratemeasured versus the square root of the scan rate isplotted infigure 12. The obtained plot shows the linear behavior in between twomeasured parameters.

The linear sweep voltammetry (LSV)which is a derivative of differential pulse voltammetry was alsoemployed in the detection of hydrazine using bare GCE andGCE/TiO2modified electrodes in PBS (pH7.0)with a scan rate of 100 mV s−1 and it was found thatwith increasing the concentration of the hydrazine, thecurrent was found to be increases linearly as shown infigure 13 and a linear plot of the peak current versusconcentration of the hydrazine was plotted in figure 14which shows the linearity of the fabricatedGCE/TiO2

over the detection of hydrazine.Further, square wave voltammetry (SWV)was employed for the detection of hydrazine, as shown in

figure 15which revealed that the higher current was obtained formodified electrodeGCE/TiO2 as compare tothe current obtained for bare electrode in presence of 0.1 mMhydrazine.

The effect of concentration on the SWVwas also studied in different concentration of (0.1–1 mM) ofhydrazine as shown infigure 16. It was observed that current peak increases linearly with increasing theconcentration of the hydrazine and the calibration curve of the current peak verses concentration of thehydrazine is potted infigure 17.

Figure 15. SWVof TiO2 nanosheetsmodifiedGCE/TiO2 in the presence of hydrazine in PBS (pH7.0).

Figure 16. SWVof TiO2 nanosheetsmodifiedGCE (GCE/TiO2) in the presence of different concentration of hydrazine (0.1–1 mM)in PBS (pH7.0).

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3.4. Interference studyThe competitive effect of some other electro-active species on the sensing property of GCE/TiO2 towardshydrazinewas also studied by employing SWVmeasurements. Figure 18 shows the squarewavevoltammograms ofGCE/TiO2 in presence of 0.1 mM (figure 18(a)) and 0.5 mMhydrazine (figure 18(g)) andother electro-active species (figure 18(b)) glucose, (figure 18(c)) fructose, (figure 18(d)) citric acid, (figure 18(e))sucrose and (figure 18(f)) dopamine.

From the observed results it was found that the addition of electro-active species does not affect the overpotential of the hydrazine, except a negligible slightly current response was observed. The concentrations of theelectro-active species were 10 times higher than hydrazinewhereas, a higher current response was observedwhen 0.5 mMof hydrazinewas added. These results suggested that dopamine, glucose, fructose, citric acid anduric acid have no obvious interferencewith the detection of hydrazine.

3.5. Reproducibility, repeatability and stability studiesTo check the reproducibility and repeatability of GCE/TiO2 electrode, the cyclic voltammograms curves of a0.5 mMhydrazinewere recorded in PBS of pH7.0. TheGCE/TiO2 electrode exhibits appreciable repeatabilityand the relative standard deviationwas found to be∼2.22% for 20 repetitive cycles using single GCE/TiO2

electrode.

Figure 17.Peak current versus the concentration of the hydrazine.

Figure 18. Square wave voltammograms of hydrazine with different interfering species in PBS (pH7.0).

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The six freshlyGCE/TiO2 electrodes were prepared and their current response towards 0.5 mMhydrazinein PBS of pH7.0was testedwith a scan potential of 100 mV S−1. It was observed that all the sixGCE/TiO2

electrodes have shownnegligible variation in current response, which revealed that GCE/TiO2 electrodes can berepeated and shows satisfactory reproducibility of 2.89%. Further, to check the stability, the electrodes were alsostored in air at 4 °C (when not in use) and the current responsewas checked thrice a day for 10 days and nosignificant variationwas observed in current response which revealed its high stability and could be used for longterm applications. The fabricated sensor using uniformTiO2 nanosheets has achieved the considerable goodsensing response for themeasured concentration of hydrazine. The parameters have been calculated fordetection of hydrazine using cyclic voltammetry and found to be as limit of detection (LOD) and sensitivity28 μMand 70 μAmM−1 cm−2, respectively whereas, SWV techniquewas found to bemore sensitive for thedetection of hydrazine and a higher sensitivity; 330 μAmM−1 cm−2 and LOD; 150 μMwas calculated using theequationwhich has been discussed in our previous report [43, 44, 50].

3.6. Comparison of hydrazine sensorwith previous reportsThere are several reports on hydrazine sensor based on differentlymodified electrodes. Since the performance ofsensor is strongly dependent on various conditions such as pH, electrodematrix and also electrodes (gold, ITO,FTO and glassy carbon etc). To compare the performance of theGCE/TiO2 electrode, towards the detection ofhydrazinewith the previously reported electrodes w.r.t. to their LOD, sensitivity and linear range aresummarized in table 1.

Table 1.Comparison studywith previously reported hydrazine sensor.

No. Electrode

Limit of detection

(LOD)μMLinear

range (μM)Sensitivity

(μAmM−1 cm−2) References

1. GC–CoII-OEP) 52 — — [39]2. CoPc/carbon paste 0.5 20–200 0.2 [40]3. NiHCFmodified electrode 96 400–4000 — [41]4. Cobalt Phthalocyanine-modified

Electrode

73.5 125–980 — [42]

5. CoHCF@TNTModifiedGCE 1000 500–2500 72.8 [43]6. ZnOnanonails 0.20 0.1–1.2 — [30]7. TiO2modified titanium electrode 30 300–1600 — [44]8. Au/WO3 144.73 — 184.71 [45]9. CoOOHnanosheets 20 — 155 [46]10. BiHCF 3 7–1100 4.2 [47]11. GC/dihydroxysalophen derivatives 1.6 10–400 17.04 [48]12. Flower shapeCuO 2400 5–10 000 7.145 [49]13. GCE/TiO2 sheet (Cyclic Voltammetry) 28 200–700 70 ThisWork

14. GCE/TiO2 sheet (SquarewaveVoltammetry)

150 100–700 330 ThisWork

Scheme 2. Schematic representation for oxidation of the hydrazine onGCE/TiO2.

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The differentmodified electrodes such asGC–CoII-OEP) (Camila et al 2016), CoPc/carbon paste (Weenaet al 2005), NiHCF (Shankaran et al 2000), Cobalt Phthalocyanine (Cleone et al 2008), CoHCF@TNT (Sophiaet al 2012), ZnOnanonails (Umar et al 2007), TiO2modified titanium electrode (Reza et al 2014), Au/WO3

(Sheifali et al 2014), CoOOHnanosheets (Kian et al 2013), BiHCF (Jianbin et al 2007), GC/dihydroxysalophenderivatives (M´onica et al 2005) and Flower shapeCuO (Sher et al 2013) have been used for hydrazine sensingusing electrochemicalmethod.

The results obtained forGCE/TiO2 electrodewas found to be satisfactory and comparable with the reportedhydrazine sensors in terms of LODand sensitivity (table 1).

The proposedmechanism for the sensing of hydrazine is shown in scheme 2.In this study, the oxidation of hydrazine hydrate was found an irreversible oxidation process inwhich

hydrazine hydrate oxidized to produceN2 and hydronium ion (H3O+) alongwith 4 electrons. The reaction

mechanism can be represented as given below:

+ + ++ - ( )N H H O N H H O e 12 4 2 2 3 3

+ + ++ - ( )N H 3H O N 3H O 3e 22 3 2 2 3

Here, in equation (1) the rate determining step is slowwhich involve the 1 electron transfer process whereas thestep (2) is fast and involve 3 electron transfer process therefore the oxidation of hydrazinewas found to beirreversible oxidation process and complete hydrazine oxidation reaction could be represented as in scheme 2.

4. Conclusion

TiO2 nanosheets were obtained under the facile condition of solvothermal. A binder free hydrazine sensor(GCE/TiO2)was fabricated using TiO2 nanosheets by simple drop castingmethod. The developedGCE/TiO2

shows excellent electrochemical response towards the detection of hydrazine. The TiO2 nanosheets wereemerged as an activematerial for the electrochemical detection of hydrazine. The limit of detection (LOD) andsensitivity of the developed sensor were found to be 28 μMand 70 μAmM−1 cm−2, respectively using cyclicvoltammetry and higher sensitivity; 330 μAmM−1 cm−2 alongwith LOD; 150 μMwas calculated by employingSWV. The presentedworkwould be efficient way to develop the binder free sensors to reduce the hazardouseffect of hydrazine andmany of other derivatives for industrial and environmental concerns.

Acknowledgments

The authors would like to thankDiscipline of Chemistry and Sophisticated InstrumentCenter (SIC), IndianInstitute of Technology Indore, for providing research facilities. The authors are also grateful toDiscipline ofChemistry, IIT Indore andCSIR,NewDelhi, India to provide the research grant. KAwould like to acknowledgeUniversity Grant Commission (UGC)Govt. of India, NewDelhi, India for providing fellowship RGNFDandAM is thankful toMHRD,Govt. of India, NewDelhi, India.

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