sensors Article A Disposable Amperometric Sensor Based on High-Performance PEDOT:PSS/Ionic Liquid Nanocomposite Thin Film-Modified Screen-Printed Electrode for the Analysis of Catechol in Natural Water Samples Francis D. Krampa 1,2 ID , Yaw Aniweh 1, Gordon A. Awandare 1,2 and Prosper Kanyong 1,3,* ID 1 West African Centre for Cell Biology of Infectious Pathogens (WACCBIP), University of Ghana, Legon, Accra, Ghana; fkrampa@gmail.com (F.D.K.); aniweh@gmail.com (Y.A.); gawandare@ug.edu.gh (G.A.A.) 2 Department of Biochemistry, Cell & Molecular Biology, University of Ghana, Legon, Accra, Ghana 3 Nanotechnology & Integrated Bioengineering Centre, Ulster University, Jordanstown BT37 0QB, UK * Correspondence: p.kanyong@waccbip.org Received: 8 June 2017; Accepted: 13 July 2017; Published: 26 July 2017 Abstract: A conducting polymer-based composite material of poly(3,4-ethylenedioxythiophene) (PEDOT): poly(4-styrenesulfonate) (PSS) doped with different percentages of a room temperature ionic liquid (IL), 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), was prepared and a very small amount of the composite (2.0 µL) was drop-coated on the working area of a screen-printed carbon electrode (SPCE). The SPCE, modified with PEDOT:PSS/IL composite thin-film, was characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), profilometry and sessile contact angle measurements. The prepared PEDOT:PSS/IL composite thin-film exhibited a nano-porous microstructure and was found to be highly stable and conductive with enhanced electrocatalytic properties towards catechol, a priority pollutant. The linear working range for catechol was found to be 0.1 µM–330.0 µM with a sensitivity of 18.2 mA·mM·cm−2 and a calculated limit of detection (based on 3× the baseline noise) of 23.7 µM. When the PEDOT:PSS/IL/SPCE sensor was used in conjunction with amperometry in stirred solution for the analysis of natural water samples, the precision values obtained on spiked samples (20.0 µM catechol added) (n = 3) were 0.18% and 0.32%, respectively, with recovery values that were well over 99.0%. Keywords: room temperature ionic liquids; PEDOT:PSS; disposable sensors; cyclic voltammetry; electrochemical impedance spectroscopy; screen-printed electrodes; conducting polymers; nanocomposites; hexacyanoferrate; sessile contact angle measurement 1. Introduction Over the past 20 years, the development of sensitive and real-time analysis of phenolic compounds has received substantial scientific interest due to their high toxicity on the ecosystem, environment as well as human health [1]. Besides this, phenolic compounds as highly toxic organics has been extensively utilized in various industrial products including flavors, pharmaceuticals, antioxidants, agrochemicals, and polymerization inhibitors [1–4]. Among phenolic compounds, catechol, which is an ortho isomer of benzenediols, has been listed as a priority pollutant by both the European Union and the US Environmental Protection Agency [5,6] because it has a poor biodegradability and is extremely toxic to human health and the ecosystem [7,8]. Therefore, there is the need for the development Sensors 2017, 17, 1716; doi:10.3390/s17081716 www.mdpi.com/journal/sensors Sensors 2017, 17, 1716 2 of 13 of analytical tools that allow for simple, rapid, and real-time analysis of trace levels of catechol in environmental samples. Currently, various analytical methods including mass spectrometry, gas and high-performance liquid chromatography, electrochemiluminescence, fluorescence, and electrochemical methods [9–12] have been used to analyze catechol. Even though these methods are sensitive towards catechol, they are usually not only time-consuming, laborious, and require skilled-personnel to operate, but also involve complicated operational procedures that makes them unsuitable for point-of-need applications. Owing to the electroactive nature of catechol, the use of electrochemical techniques, especially at modified electrodes, are most attractive because they give rapid response and are simple, relatively inexpensive, selective, and sensitive [12–15]. Different nanomaterials including carbon nanomaterials, nanoparticles, metals and metal oxides, conducting polymers [16–19] including electrode pre-treatments and/or modifications have been developed for the quantification of catechol [19,20]. The use of conducive polymers in sensor design provides numerous advantages because these materials are relatively inexpensive and environmentally friendly, exhibit good charge storage capacity, and biocompatibility for biomolecules immobilization, wide potential windows and excellent electrical conductivity particularly when doped [21]. Among the different types of conductive polymers, the polythiophene-derived macromolecule species poly(3,4-ethylenedioxythiophene) (PEDOT) is known to be the best in terms of conductivity, stability, and processability [21,22]. Also, colloidal dispersions of PEDOT can be readily made through the addition of poly(4-styrenesulfonate) (PSS) to form the doped compound PEDOT:PSS. This doped version of the polymer has excellent conductivity and exhibits good mechanical properties [23]; thus, it has been applied to the development of various devices and sensors [24–26]. Room temperature ionic liquids (ILs) are organic/inorganic salts that are liquid at room temperature and are usually considered to be ‘green solvents’. They are known to have good chemical stability, high ionic conductivity, negligible vapor pressure, low flammability and have been used in many technological fields [14]. Because of the high affinity of ILs with conductive polymers and their ability of supramolecular ordering, we envisaged the use of ILs as dopants in the conductive polymer PEDOT:PSS to enhance the charge transfer rate of PEDOT:PSS for catechol. Consequently, in this study, we prepared different percentages of the ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) in PEDOT:PSS. Thin films of PEDOT:PSS/ionic liquid were prepared by casting the composite on the working area of a screen-printed carbon electrode (SPCE) and dried at 40 ◦C for about 1 h. Overall, the specific advantages of screen-printed sensors such as miniaturization, disposability, and low-cost, and the synergistic effect of PEDOT:PSS and [EMIM][BF4] are assembled to fabricate a low-cost, disposable, and simple sensor. The applicability of the sensor as a useful analytical tool was demonstrated through analysis of catechol in natural water samples. Details of the sensor fabrication, assembly, and characterization are described and discussed. 2. Experimental 2.1. Apparatus and Reagents Electrochemical experiments were conducted using PGSTAT204 Autolab Potentiostat/ Galvanostat/EIS FRA32M Module (Metrohm-Autolab, The Netherlands) with Nova 2.1 Software for data acquisition and experimental control. Electrochemical impedance spectroscopy in 5.0 mM potassium hexacyanoferrate ([Fe(CN) ]3−6 /[Fe(CN) 4−6] ) was carried out at open circuit within the frequency range of 100 kHz–0.1 Hz at an applied potential of 0.25 V. The disposable screen-printed carbon electrodes (Ref DS 410) utilized in the sensor design have a carbon working electrode, carbon counter electrode, and silver reference electrode (Scheme 1) and were purchased from DropSens, Asturias, Spain. Scanning electron microscopy (SEM) was performed by JEOL JSM-610PLUS/~LA SEM (JEOL Ltd., Tokyo, Japan). Further surface analysis was performed using Bruker DektakXT® Sensors 2017, 17, 1716 3 of 13 Sensors 2017, 17, 1716 3 of 12 Stylus profilometer (Bruker Optics, Ettlingen, Germany). Sessile contact angle measurements were perfoPromlye(d3,u4s-ienthgyCleAnMed2i0o0xyOtphtioicpahl eCnoen) tac(tPAEnDgOleTM): etepro(KlyS(4V-sItnysrternuemsuenlftosnLattde.), He(PlsSinSk),i , Fi1n-elathnydl)-.3- methPyolilmy(i3d,a4z-eotlhiuymle netedtiroaxflyutohriobpohreante ) ([E(PMEIDMO][TB)F: 4]),p ohleyx(4a-csytaynreonfersrualtfeo n(a[tFee)(CN(P)6S]S3−)/,[Fe1(C-eNth)6y]l4-−3),- pmheotshpyhliamtei dbauzfofelriuedm staeltirnaefl (uPoBrSo)b toarbaletets(,[ aEnMdI cMat]e[BchFo4]l) w, heerxea pcuyracnhoafseerdra ftreo(m[F Se(igCmNa)-6A]3l−d/ri[cFhe,( CStN. L)o]u4−6 is),, MphOo.s pUhSaAte. Abullf foetrheedr scahleinmeic(PalBsS w) tearbe loetfs a,naanldytciacatel cghroaldwe earnedp uusrecdh awseitdhofruotm fuSritghmera p-Aurldifricicahti,oSnt. Louis, MO, USA. All other chemicals were of analytical grade and used without further purification. 2.2. Procedures 2.2. Procedures 2.2.1. Fabrication of PEDOT:PSS/20%IL/SPCE 2.2.1. Fabrication of PEDOT:PSS/20%IL/SPCE The bare Screen-Printed Carbon Electrode (SPCE) was modified by drop-coating 2.0 µL each of PEDOTTh:ePbSaSr aenSdc rieoennic-P lirqinutiedd (ICLa)r (b[EonMEIMlec][tBroFd]e),( aSnPdCE)4 driweda satm 4o0d °iCfi efodrb 1y hd troo pfo-rcmoa tPinEgDO2.0T:µPLSSe/aScPhCoEf PEDOT:PSS and ionic li ◦and IL/SPCE, respectiveqluyi.d D(iIfLfe)r(e[EnMt pIeMrc][eBnFta4]g)e, sa nodf IdLr i(evd/va)t (410.0, C2.f0o, r51.0h, 1to0.0fo, r2m0.0P,E 3D0.O0,T a:PnSdS /50S.P0C%E) iann dPEILD/OSTP:CPESS, rwesepreec tailvseol yp.rDepifafereredn atnpde r2ce.0n tµaLg eosf otfhIeL c(ovm/vp)o(s1it.0e, d2r.0o,p5-.c0o,a1t0e.d0 ,o2n0 .0th, e3 0S.0P,CaEn dto5 0fo.0r%m) PinEDPEODTO:PTS:SP/SILS/SwPeCreE aalnsod pwreeprea raeldlowanedd 2to.0 dµrLy oafs tphreevcoiomupsloys idteesdcrroibpe-dco. aTtheed foanbrtihcaetiSoPnC pErotocefsosr mis iPllEuDstOraTte:PdS Sin/ ISLc/hSePmCeE 1a. nTdhew seurrefaaclleosw oefd atllo tdhrey masodpirfeievdio SuPslCyEdse wscerribee tdh.oTrohuegfhalbyr irciantsioedn pinr oPcBesSs tios rilelmusotvraet eadniyn uSnchbeomuned1 .spTehceiessu. rfOacnecse opf raelpl athreedm, othdei fiseednSsoPrCs Ews ewree resttohroerdo uingh rlyoormin setedminpePraBtSurtoe creomndoivtieoannsy. unbound species. Once prepared, the sensors were stored in room temperature conditions. SScchheemmee 11.. ScShchememataictirce prerepsreensteanttioantioonf thoef Stchree enSc-Preriennt-ePdriCnaterbdo nCEalrebcotrno dEele(SctPrCodEe) (l(eSfPt)CaEn)d (pleroftc)e daunrde pforrocfaebdruicrea tfionrg ftahberiPcEatDinOgT t:hPeS SP/E2D0O%TIL:P/SSSP/C20E%sIeLn/sSoPrC(Eri gshent)s.or (right). 2.2.2. Sessile Contact Angle Measurement 2.2.2. Sessile Contact Angle Measurement The contact angle measurements were carried out by the sessile drop technique; a water droplet The contact angle measurements were carried out by the sessile drop technique; a water droplet was placed onto a flat surface of the bare SPCE and PEDOT:PSS/IL composite modified SPCE, and was placed onto a flat surface of the bare SPCE and PEDOT:PSS/IL composite modified SPCE, and the the contact angle of the droplet with the surface measured. Reported values are the average contact contact angle of the droplet with the surface measured. Reported values are the average contact angle angle (right and left) of 10 droplets. During the measurement time (~50 s), no change in contact angle (right and left) of 10 droplets. During the measurement time (~50 s), no change in contact angle was was observed. A variation of 5° is generally considered to be sufficient to differentiate materials [13]. observed. A variation of 5◦ is generally considered to be sufficient to differentiate materials [13]. 33.. RReessuullttss aanndd DDiissccuussssiioonn 33..11.. Oppttiimiissaattiioon ooff tthhee Peerrcceenttaaggee ooff IIL iin PEDOT::PSS//IIL Coomppoossiittee To aasscceerrttaaiin tthhee aamoouunnt toof fILIL inin PEPDEDOOT:TP:SPSS Srerqeuqiureirde dfofro orpotpimtiummu meleecltercotcraotcaaltyatliyc tricesrpeosnpsoen osef tohfet hmeomdiofideidfi eSdPCSEP,C cEo,mcpomospitoessi twesithw idthiffderieffnetr epnetrcpeenrtcaegnetsa goefs ILof (1IL.0,( 12..00,, 25..00,, 51.00.,0,1 02.00.,0.2 400.0.0. , 4a0n.d0, 5a0n.d0%5)0 .0in% )PiEnDPOETD:POSTS:P SwSerwe erfeorfmorumlauteladt edanadn duusesde dttoo ffaabrriiccaattee PEDOT::PSSSS/IIL/SSPPCCEE sseennssoorrss.. Theerreeaafftteerr,, tthee voollttaammeettrriicc rreesspoonnsseess ffrroomt thhee PPEEDOTT:P:PSSSS//IIL/SSPCE sseenssoorrss weerree meeaassurreed iin PBSS ((pH 77..44)) ccoonttaaiinniinngg 55.0.0m mMMh hexexacaycyananofoefrerrartaete([ F([eF(eC(CNN) )]63]−3−6 /[[Fee((CN))6]]44−6 )− a) nadnd 0.01. 1MM KKCCl.l .IItt sshhooulld bee meenttiiooneed tthaatt aa ssiimiillaarr prroocceedurree waass usseed ttoo eevaalluaattee tthee PEDOT::PSSSS//SSPCE aand IIL/SSPPCCEE sseennssoorrss. . Figure 1A shows cyclic voltammograms recorded at SPCEs modified with different percentages of IL in PEDOT:PSS while Figure 1B shows a plot of the peak currents vs. the percentage of IL in the composites formulated. It can be seen in Figure 1A,B that the voltammetric peaks increased gradually Sensors 2017, 17, 1716 4 of 13 Figure 1A shows cyclic voltammograms recorded at SPCEs modified with different percentages of ISLenisnorPs 2E0D17O, 1T7,: P17S1S6 while Figure 1B shows a plot of the peak currents vs. the percentage of IL in4 othf 1e2 composites formulated. It can be seen in Figure 1A,B that the voltammetric peaks increased gradually ffrroomm1 1.0.0%%I LILu uppt oto2 200.0.%0%I LIL. S. uSubsbesqeuqeunetnitn icnrceraesaesseisn itnh tehpee prceerncetangtaegoef oILf IdLi ddindo tnsoht oswhoawn yaninyc irneacrseeaisne tihne tvhoel tvamolmtaemtrmicertersicp ornesspeoonfsthe eomf tohdei fimedodseifniesodr ss.eCnsoonrsse.q Cuoenntsleyq, 2u0e.n0t%lyI,L 2w0.a0s%c hILos wenaass cthhoesoepnt iams utmhe aompotiumnutmof aILmroeuqnuti roedf ItLo breeqpureirseedn ttion bPeE DprOeTse:PnSt Si/nI LPEcoDmOpTo:PsiSteS/tIoL gcivoemtphoeshitieg htoes tgeivleec ttrhoec ahtaiglyhteisct reelsepcotrnoscea.tFailgyutirce r1eCspsohnoswe.s Faigcuorme p1aCr isshoonwosf av oclotmampmaroisgorna mofs vreocltoarmdemdougsrianmg st hreecboarrdeeSdP uCsEin, gIL t/hSeP bCaEre, PSEPDCOE,T :PISLS//SPSPCCEE, , aPnEdDPOETD:POSTS:/PSPSSC/E2, 0%aInLd/ SPPCEEDsOenTs:PoSrsS/i2n0%5.I0Lm/SPMCE([ Fes(CenNso)6r]s3 −/i[nF e(C5.N0 ) ]m4−6 M) s(o[Fluet(iCoNn.)6I]n3−/c[oFme(CpaNr)is6]o4−n) stoolbuotitohnP. IEnD cOomT:pPaSrSi/soSnP CtoE baonthd PILE/DSOPTC:EPSsSe/nSsPoCrsE, athned aILno/SdPiCc Ep esaeknscourrsr,e tnhte (aIpnao) danicd pceaatkh ocduircrepneta k(Ipcau) rarnedn tc(aIpthc)oodficth peePaEk DcuOrTr:ePnStS (/Ip2c)0 %ofI Lth/eS PPCEDE OseTn:sPoSrSw/2a0s%mILo/rSePeCnEh asnecnesdorw witahs wmeollr-ed eefinnheadnvcoedlt amwmithe trwiceplle-adkesfi;ntehdis evnohltaanmcemmeetrnitc inpeelaekcstr; octhatisa lyetnichpanrocpemeretinets iisna tetrliebcutrtoecdattoaltyhteic spyrnoepregritsiteics eifsf eactttorifbtuhteedP EtDoO tTh:eP SsSynanerdgIisLt.icC oenffseecqt uoefn ttlhy,et hPeEPDEODTO:PTS:PSS Sa/n2d0 %ILI.L /CSoPnCseEqsueennstolyr ,w tahse uPsEeDd OfoTr:fPuSrSth/2e0r%stIuLd/SiePsC. E sensor was used for further studies. FFiigguurree 11.. ((A)) Cycclliicc vollttaammogrraamss ooff PPEEDOTT::PPSSSS//ILIL/S/PSCPEC Eprpepreapraedre dwiwthi thdifdfiefrfeenret natmaomuonutsn tosf oifoniiocn liiqculiidq u(IiLd) ((1I.L0), 2(.01,. 05,.0,2 1. 0,.0,5 2. 0,.0,1 400.0.0, , a2n0d.0 ,504%0). 0in, PaEnDdO5T0:%PS)S/iInL cPoEmDpOoTsi:tPeS; S(B/)I LPlocto omf paonsoidteic; ((BIp)a)P alontd ocfatahnooddiicc (I(pIcp)a p) eaankd ccuartrhenodtsi cfo(rI p[cF)ep(CeNak)6c]3u−/r[rFeen(tCsNfo)6r]4[−F ves(.C aNm)o6]u3n−t/ o[fF IeL(C (%N)) 6i]n4 −PEvDs.OTam:PoSuS/nILt ocfomILpo(s%it)e; in(CP)E DcOyTcl:iPcS S/vIoLltacmomeptoriseitse ; (C(CV)s)c ycolifc vSPolCtaEm, mIeLt/rSiePsCE(C, VPs)EDoOf TS:PCSSE/,SPILC/ES PCanEd, PPEEDDOOTT:P:PSSS//S2P0C%EILa/nSdPCPE.D OTA:PllS S/2C0V%sI L/SwPeCrEe . AlrleCcoVrsdwede re rienc ord5e.d0 in 5m.0Mm M hexaccyaanooffeerrrraattee (([[FFee((CCNN))6]]33−6 //[F[eF(eC(CNN)6)]4−])4−6 in) pinhposhpohspathea bteubffuefrfeedr esdalsinalei n(PeB(PSB) S(p)H(p H7.47). 4co) ncotanitnaiingin 0g.10 M.1 MKCKl.C l. 33..22.. CChhaarraacctteerriissaattiioonn ooff SSPPCCEE aanndd PPEEDDOOTT:P:PSSSS/2/200%%ILIL/S/SPPCCEES Seennssoorr 33.2.2.1.1.. Cyycclliicc Vooltlatammmmeetrtryy FFiigguurree 22A sshhoowss aa ccoomppaarriissoonn ooff ccyycclliicc vvoollttaammooggrraamss rreeccoorrdeed aatt tthhee bbaarree SSPPCCEE aanndd PPEEDOTT:P:PSSSS//20%IL/SSPPCCEE sseennssoor rinin PPBBSS (p(pHH 7.74.)4 c)ocnotnatianiinnign g5.50. 0mmMM [F[eF(eC(NCN)6])3− 3−6/][Fe/(C[FNe()C6]N4− )an]46 d− 0a.1n Md KCl at a scan rate of 100 mV·s−1. As expected, when compared with what occurred on the bare SPCE (curve a, Figure 2A), the PEDOT:PSS/20%IL/SPCE sensor (curve b, Figure 2A) exhibited a characteristic increase of both the anodic and cathodic peak currents for [Fe(CN)6]3−/[Fe(CN)6]4− redox couple, thus, confirming the successful modification of the SPCE with the composite. Higher peak currents and a smaller peak-to-peak potential separation (ΔEp) were observed at the Sensors 2017, 17, 1716 5 of 13 0.1 M KCl at a scan rate of 100 mV·s−1. As expected, when compared with what occurred on the bare SPCE (curve a, Figure 2A), the PEDOT:PSS/20%IL/SPCE sensor (curve b, Figure 2A) exhibited a characteristic increase of both the anodic and cathodic peak currents for [Fe(CN) ]3−/[Fe(CN) ]4−6 6 rSeednsoorxs 2c0o17u, p17le, 1,71t6h us, confirming the successful modification of the SPCE with the compo5 soift 1e2. Higher peak currents and a smaller peak-to-peak potential separation (∆Ep) were observed at the PEDOT::PSS// 200%IIL//SSPPCCEE sesnesnosro r(I(paI p=a 3=363.386 µ.8Aµ, AIp,c I=p c34=53.94 5µ.A9 ;µ ΔAE;p∆ =E 2p0=2.62 0m2V.6) mwVh)enw choemnpcaormedp awreitdh wthieth btahree bSaPreCESP (CIpEa (=I p3a 2=.43 2µ.A4 ,µ AIpc, I=p c6=9.96 9µ.9Aµ; AΔ;E∆p E=p =34364.6 .6mmVV). )T. Thhisi sisis aatttrriibbuutteedd tto the hiigherr elleeccttrrooccaattaallyyttiiccp prrooppeerrttieiesso offt thheeP PEEDDOOTT:P:PSSS//IL composiitte whiicch lled tto an iincreassee off tthe ttottall acttiive arreea ooff tthhee mmooddiififieedd eeleleccttrrooddee.. TThhee pprreesseenncceeo offt htheeP PEEDDOOTT:P:PSSS//IL ccomposiite prroduced aa nneeggaattiivvee sshiifftt in tthe aannooddicic ppootetenntitaial al nadn da paopsoitsiviteiv sehisfht ifnt tihnet chaethcoadthico pdoictepnotitaeln, tgiaivl,inggiv rinseg troi sae smtoaallesrm paelalekr- ptoea-pke-taok- pseapkarsaetpioanra (tΔioEnp (=∆ 2E0p2=.6 2m02V.6). mThVi)s. mThoirsem thoarne theann-foteldn- ifnocldreianscer einas tehien atnhoedainc opdeiackp ceuarkrecunrt raendt afinvde-fifovled-f oinldcrienacsree ainse tihnet hcaetchaotdhiocd piceapke ackucrurernret nftofro [rF[eF(eC(CNN)6])36−]/3[−Fe/([CFNe()C6]N4−) 6c]a4n− bcae nabtteriabtutrtiebdu teod thtoe tehlecetlreocctarotaclayttailcy etifcfeecftf eocft tohfe tPhEeDPEODTO:PTS:SP/SILS /cIoLmcpoomsiptoe.s ite. Fiigurree 22.. (A(A))C Cyycclilcicv vooltlatammmmooggrraammssr erecocordrdeeddu usisninggS PSPCCEE( c(ucruvrevea )aa) nadndP EPDEODTO:PTS:PSS/S2/02%0%ILIL//SSPPCE sseennssoorr( c(cuurrvveeb b) )a ta1t 0100m0 Vm·Vs−·s1−1s csacnanr artaet;e(;B ()BC) VCsVrse croercdoerdeuds iungsinPgE DPOEDT:OPSTS:P/S20S%/2I0L%/ISLP/CSPECsEen sseonrsaotr1 a0t, 2100,, 3250,,5 305,,7 550, ,1 7050,, 1050, 15705, 210705, 2500,, a2n5d0,3 a0n0dm 3V00·s m−1Vs·csa−1n srcatne sr;a(tCes);P (eCa)k Pceuarkr ecnutrvresn. st qvusa. rsequroaorte orfosocta onf rsactaen; ( Dr)altoeg; Ip(Dv)s . llogg V.IAp llvCs.V slwoge reVr.e coArldl edCiVn s5 .0wmeMre herxeaccoyradneodf erirna te5(.[0F e(mCMN) 3−6]hex/a[cFyea(CnoNf)e6r]r4a−te) i(n[FPeB(CSN(p)H6]3−7/.[4F)ec(ConNta)6i]n4−i)n ign0 P.1BMS (pKHC l7. .4) containing 0.1 M KCl. TThhee eeffffeecctt ooffs sccaannr raattee( v(v))o onnt htheev vooltlatammmmeetrtircicb behehaavvioiorro of ft htheeP EPDEDOOT:TP:SPSS/S2/200%%ILIL//SSPPCCEE sseennssoorr wwaass aallssoo eexxaammiinneedd bbyy CCVV ((FFiigguurree 22BB)).. AAtt tthhee ssccaann rraatteess iinnvveessttiiggaatteedd ((1100.0.0 toto 330000.0.0 mmVV·s·−s1−),1 a), paloptl ootf othf eth sequsqaruea rroeorto ooft tohfet shceansc raanter (avt)e v(sv.) thvse. atnhoedaicn o(Idpai)c a(nIdpa )caatnhdodciact h(Iopcd) ipce(aIkpc c)upreraekntcsu erxrheinbtisteedx hai lbiniteeadr arellianteiaornsrehliapt io(Fnisghuipre( F2iCgu),r ew2hCi)c,hw ihsi cthypisictaylp iocfa lao fdaifdfuifsfuiosnio-cno-ncotrnotlrloeldle dprporcoecsess s[2[277––2299].] . AA lliinneeaarr rreellaattiioonnsshhiippw waassa alslosoo bosbesrevrevdedw whehnenab asbosluotleutvea vluaelus eosf boof tbholtohg lIopga aIpna danlodg lIopgc wIpec rweeprleo tptelodttaegda iangsat ilnosgt vlo(gF ivg u(Freig2uDre) w2Dit)h wsliothp eslvoapleu evsaloufe0s. 7o0f a0n.7d0 0a.n6d4, 0r.e6s4p, ercetsivpeelcyt.ivTehlyes. eTshleospee svloapluee vsaalrueecso amrep caorambplearwaibtlhe twheitthh ethoree ttihceaollryeteixcpalelcyt eedxpvaelcuteedo fv0a.l5ufeo ropf u0r.5el yfodri fpfuusrieolny -cdoifnftursoilolend-ccounrtrreonltlsed[2 7c–u2r9re];ntthsu [s2,7c–o2n9fi];r mthiungs, tchoantftihrme einlegc trthocaht etmheic aellepcrtorcoecshseims diciafflu psiroonc-ecsosn tirso ldleifdfuasnidont-hcaotntthreolsluedrf aacnedo ftthhaet mthoed ifisuerdfaScPeC oEf wtahse nmoot dfoifuieledd S. PCE was not fouled. 3.2.2. Electrochemical Impedance Spectroscopy The interface properties of the bare SPCE and PEDOT:PSS/20%IL/SPCE sensor were further characterized by Faradaic electrochemical impedance spectroscopy (EIS) in the presence of 5.0 mM hexacyanoferrate [Fe(CN)6]3−/[Fe(CN)6]4− (Figure 3). The impedance spectrum associated with the bare SPCE (curve a, Figure 3) consists of a semicircle part in the high frequency region and a linear part in the low frequency region, corresponding to electron transfer and diffusion processes, respectively. The diameter of the semicircle represents the charge-transfer resistance (RCT) at the surface of the electrode [27]. At the bare SPCE (curve a, Figure 3), a semicircle with a larger diameter Sensors 2017, 17, 1716 6 of 13 3.2.2. Electrochemical Impedance Spectroscopy The interface properties of the bare SPCE and PEDOT:PSS/20%IL/SPCE sensor were further characterized by Faradaic electrochemical impedance spectroscopy (EIS) in the presence of 5.0 mM hexacyanoferrate [Fe(CN) 3− 6 4−6] /[Fe(CN) ] (Figure 3). The impedance spectrum associated with the bare SPCE (curve a, Figure 3) consists of a semicircle part in the high frequency region and a linear part in the low frequency region, corresponding to electron transfer and diffusion processes, respectively. TSehnesords i2a0m17e, 1te7,r 1o71f6t he semicircle represents the charge-transfer resistance (RCT) at the surface o6f otfh 1e2 electrode [27]. At the bare SPCE (curve a, Figure 3), a semicircle with a larger diameter was obtained. Hwoaws oebvteari,noendt. hHeoPwEeDvOerT, :PonSS t/h2e0 P%EIDL/OSTP:PCSESs/2e0n%soIrL(/cSuPrCvEe sbe,nFsigoru r(ecu3r)v, eth be, dFiiagmureet e3r),o tfhteh edisaemeicteirc olef wthaes sneemgliicgiirbclle. wThaiss nsieggnliifigicbalnet. cThhainsg seigin iRfiCcTanvta lcuheanisgaet tirnib uRtCeTd vtaoluthee iesn ahtatrnicbeudtecdh atrog eth-tera ennshfearnrcaetde acchraorsgset-htreanmsofedri fireadte inatcerrofsasc ethane dmthoedilfairegde isnutrefrafcaecea raenadp rtohvei dleadrgbey stuhrefaPcEeD aOreTa:P SpSro/vILidceodm bpyo stihte. TPhEiDs OimTp:PeSdSa/nILce croemsuplotssiatge.r eTehwisi tihmtpheedraenscuel tsreosbutlatsin aegdrefreo wmitth ethcye crliecsuvlotlst aombtmaienterdic fmroemas uthrem ceyncltisc; tvhoultsa, mcomnfietrrmici nmgetahseurseumcceenstssf;u tlhmuso,d ciofincfaitrimoninogf tthe sSuPcCcEes.sful modification of the SPCE. FFiigguurree 33.. NyNqyuqisut ipstloptsl ootbsseorbvseedr vfoerd elfeocrtreolcehcetrmocichaelm imicpaeldiamnpcee dspaneccterosspceocptyro (sEcIoSp) yat (SEPICS)E a(ctuSrvPeC aE) (acnudr vePaE)DaOnTd:PPSES/D20O%TI:PLS/SSP/C20E% IsLe/nSsPoCr E(csuenrvseo r (bc)u rvine bP) BiSn P(BpSH (pH7.4)7 .4c)ocnotnaitnaiinngin g55.0.0 mM [[FFee((CCN))66]] 33−/[/F[eF(eC(NCN)6])4−6] a4−nda n0.d1 0M.1 KMCKl. Cl. 33..22..33.. SSccaannnniinngg EElleeccttrroonn SSppeeccttrroossccooppyy aanndd PPrroofifilloommeettrryy AAddddiittiioonnaallllyy,, tthhee mmoorrpphhoollooggiiccaall ffeeaattuurreess ooffb booththt htheeb baraereS PSPCCEEa nadndP EPDEDOOT:TP:SPSS/S2/200%%ILIL//SSPPCCEE sseennssoorr wweerreec chhaararactcetreirziezdedb ybysc sacnanninnignegl eecltercotnromn icmroicsrcoospcyop(SyE M(SE) Mas)w aesl lwaeslpl raosfi plormofeiltorym. eFtirgyu.r Fei4gAu,rBe s4hAo,wB sthhoewv tiehwe voiefwth oef SthPeC SEPaCnEd aPnEdD POEDT:OPTSS:P/S2S0/%20I%L/ILS/PSCPECEse snesnosro,rr, ersepspecetcitviveleyl.y.T Thhee mmoorrpphhoollooggyy ooff tthhee babraer eSPSCPEC iEs tyispictyalp fiocar lgrfaoprhigtrea mphaitteeriamlsa wteirtiha lgsrawinisth thagtr aairnes sttahcakteda irne flsatkaceks.e Ads isnhoflwank eisn. AFisgushreo w4Bn, ian uFnigifuorrem4 Bfi,lma uwnaifso fromrmfieldm own atshefo erlmecetdroodne tshuerfealceec,t rionddeicasutirnfga cseu,cicnedsiscfautli ndgepsouscicteiosnsf uolf dtheep osPiEtiDonOTof:PtShSe/IPLE DcoOmT:pPoSsSit/eI.L Wcohmepno sictoem. pWarheedn cwoimthp artehde wbiathre thSePbCaEre S(FPiCguEre(F ig4uAr)e, 4tAh)e, tPhEeDPOETD:OPSTS:P/2S0S%/2IL0%/SIPLC/ES PsCenEssoern (sFoirg(uFrieg u4rBe) 4sBh)oswhoewd ead haihghiglhy lypoprooruosu smmoorprphhoolologgyy, ,ccoonnssiissttiinngg ooff sseevveerraall inintetercroconnnnecetcetdedg ingginegr-elrik-leikdeo tds;otths;i stghriesa tglryeiantclyre ainsecdretahseedsu rthfaec esaurrefaacoef tahreeam oodf ifitheed emleocdtriofidede. Pelreocfitrloomdee.t rPyrmofeilaosmureetrmye nmtsearseuvreeamleedntthsa rtetvheeaalveder athgaets uthrfea caevreorauggeh nseusrsfaocfet hreouSgPhCnEe(sFsi gouf rteh4eC S)PaCndE P(FEiDguOrTe :4PCS)S a/n2d0% PIELD/OSPTC:PESSs/e2n0s%orIL(F/SigPuCrEe s4eDn)sowre (rFeig1u.4r4e µ4Dm) awnedre6 .12.944µ µmm, r aenspde 6c.t2i9v eµlmy;,t rheesspeescutirvfealcye; rtohuesgeh snuersfsavcea lruoeusgahgnreesesw viatlhutehse aSgEreMe i mwaitghe st.he SEM images. Figure 4. (A,B) scanning electron microscopy (SEM) images of SPCE and PEDOT:PSS/20%IL/SPCE sensor and (C,D) their corresponding surface roughness obtained from profilometry. 3.2.4. Sessile Contact Angle Measurements In addition to these, the measurement of the water contact angle for the PEDOT:PSS/20%IL thin film on the surface of the SPCE was performed. The contact angle of water at the surface of the bare Sensors 2017, 17, 1716 6 of 12 was obtained. However, on the PEDOT:PSS/20%IL/SPCE sensor (curve b, Figure 3), the diameter of the semicircle was negligible. This significant change in RCT value is attributed to the enhanced charge-transfer rate across the modified interface and the large surface area provided by the PEDOT:PSS/IL composite. This impedance results agree with the results obtained from the cyclic voltammetric measurements; thus, confirming the successful modification of the SPCE. Figure 3. Nyquist plots observed for electrochemical impedance spectroscopy (EIS) at SPCE (curve a) and PEDOT:PSS/20%IL/SPCE sensor (curve b) in PBS (pH 7.4) containing 5.0 mM [Fe(CN)6]3−/[Fe(CN)6]4− and 0.1 M KCl. 3.2.3. Scanning Electron Spectroscopy and Profilometry Additionally, the morphological features of both the bare SPCE and PEDOT:PSS/20%IL/SPCE sensor were characterized by scanning electron microscopy (SEM) as well as profilometry. Figure 4A,B show the view of the SPCE and PEDOT:PSS/20%IL/SPCE sensor, respectively. The morphology of the bare SPCE is typical for graphite materials with grains that are stacked in flakes. As shown in Figure 4B, a uniform film was formed on the electrode surface, indicating successful deposition of the PEDOT:PSS/IL composite. When compared with the bare SPCE (Figure 4A), the PEDOT:PSS/20%IL/SPCE sensor (Figure 4B) showed a highly porous morphology, consisting of several interconnected ginger-like dots; this greatly increased the surface area of the modified electrode. Profilometry measurements revealed that the average surface roughness of the SPCE (Figure 4C) and PEDOT:PSS/20%IL/SPCE sensor (Figure 4D) were 1.44 µm and 6.29 µm, respectively; thSeensseor ssu20r1f7a,c1e7 ,r1o7u16ghness values agree with the SEM images. 7 of 13 FFigiguurere 44. .((AA,B,B) )ssccaannnniinngg eelleeccttrroonn mmiiccrroossccooppyy ((SSEEMM)) iimmaaggeess ooff SSPPCCEE aanndd PPEEDDOOTT::PPSSSS//2200%%IILL//SSPPCCEE sesennsosor raanndd (C(C,D,D) )ththeeiri rccoorrreressppoonnddiningg ssuurrfafaccee rroouugghhnneessss oobbtatainineedd ffrroomm pprrooffiilloommeettrryy.. 33.2.2.4.4. .SSeesssisliele CCoonntatacct tAAnngglele MMeeaassuureremmeenntsts InIn aadddditiitoionn toto ththeessee, ,ththee mmeeaassuurreemmeenntt ooff tthhee wwaatteerr ccoonnttaacctt aannggllee ffoorr tthhee PPEEDDOOTT::PPSSSS//2200%%ILIL ththinin fifilmlm oonn ththee ssuurrfafaccee oof fththee SSPPCCEE wwaass ppeerrfoforrmmeedd. .TThhee ccoonntatactc taanngglele oof fwwaateter raat tththee susurfrafaccee oof fththee bbaarere SPCE was found to be ~74.3◦. However, it decreased after coating the SPCE with PEDOT:PSS/20%IL composite to ~50.8◦. This increase in the hydrophilicity of the coated electrode means that the properties of the PEDOT:PSS/20%IL composite can be manipulated in buffer solution; thus, making it a suitable surface for the immobilization of biomolecules. This is of considerable relevance for a variety of applications including sensors for biomedical applications, as well as studying biointerfaces [13]. 3.3. Application of PEDOT:PSS/20%IL/SPCE to Catechol Analysis 3.3.1. Cyclic Voltammetry Figure 5A shows cyclic voltammograms for catechol at the bare SPCE and PEDOT:PSS/20%IL/SPCE sensor, respectively. During the forward scan, two prominent oxidation peaks at 0.27 V (a1) and 0.50 V (a2) were observed on the PEDOT:PSS/20%IL/SPCE sensor. The anodic peak (a1) can be attributed to the formation of o-semiquinone intermediates while the second another peak (a2) pertains to the oxidation of the catechol to o-quinone [30]. Previous studies identified the formation of the o-semiquinone and found the redox potential of catechol/o-semiquinone pair to be 0.53 V [30], which agrees with this finding. On the reverse scans, a cathodic peak (c1 = 0.01 V) on the PEDOT:PSS/20%IL/SPCE sensor was observed. This cathodic peak (c1) corresponds to the reduction of the o-quinone [31]. A peak current ratio (Ipc1/Ipa2) for the repetitive recycling of potential was found to be near unity, which is a criterion for the stability of o-quinone produced at the surface of the electrode [32,33]. These findings agree with the oxidation of catechol at similar surfaces [31–33]. The two oxidation peaks (a1 and a2) and one cathodic peak (c1) broadened and shifted to more positive potentials (a1 = 0.44 V, a2 = 0.63 V, c1 = 0.04 V) with a significant decrease in the peak currents at the bare SPCE. In comparison to what occurred at the bare SPCE, the PEDOT:PSS/20%IL/SPCE sensor exhibited a characteristic increase of both the anodic and cathodic peak currents for catechol. The enhanced prominence and the shifts in peak potentials to less positive values, and the more than four-fold increase in peak currents are attributed to the electrocatalytic properties of the PEDOT:PSS/20%IL composite. Sensors 2017, 17, 1716 7 of 12 SPCE was found to be ~74.3°. However, it decreased after coating the SPCE with PEDOT:PSS/20%IL composite to ~50.8°. This increase in the hydrophilicity of the coated electrode means that the properties of the PEDOT:PSS/20%IL composite can be manipulated in buffer solution; thus, making it a suitable surface for the immobilization of biomolecules. This is of considerable relevance for a variety of applications including sensors for biomedical applications, as well as studying biointerfaces [13]. 3.3. Application of PEDOT:PSS/20%IL/SPCE to Catechol Analysis 3.3.1. Cyclic Voltammetry Figure 5A shows cyclic voltammograms for catechol at the bare SPCE and PEDOT:PSS/20%IL/SPCE sensor, respectively. During the forward scan, two prominent oxidation peaks at 0.27 V (a1) and 0.50 V (a2) were observed on the PEDOT:PSS/20%IL/SPCE sensor. The anodic peak (a1) can be attributed to the formation of o-semiquinone intermediates while the second another peak (a2) pertains to the oxidation of the catechol to o-quinone [30]. Previous studies identified the formation of the o-semiquinone and found the redox potential of catechol/o-semiquinone pair to be 0.53 V [30], which agrees with this finding. On the reverse scans, a cathodic peak (c1 = 0.01 V) on the PEDOT:PSS/20%IL/SPCE sensor was observed. This cathodic peak (c1) corresponds to the reduction of the o-quinone [31]. A peak current ratio (Ipc1/Ipa2) for the repetitive recycling of potential was found to be near unity, which is a criterion for the stability of o-quinone produced at the surface of the electrode [32,33]. These findings agree with the oxidation of catechol at similar surfaces [31–33]. The two oxidation peaks (a1 and a2) and one cathodic peak (c1) broadened and shifted to more positive potentials (a1 = 0.44 V, a2 = 0.63 V, c1 = 0.04 V) with a significant decrease in the peak currents at the bare SPCE. In comparison to what occurred at the bare SPCE, the PEDOT:PSS/20%IL/SPCE sensor exhibited a characteristic increase of both the anodic and cathodic peak currents for catechol. The enhanced prominence and the shifts in peak potentials to less positive values, and the more than fSoenusro-rfso2l0d17 , i1n7,c1r7e1a6se in peak currents are attributed to the electrocatalytic properties of8 otfh1e3 PEDOT:PSS/20%IL composite. Fiigurre 5.. ((A)) Cycclliicc vollttammogrramss rreccorrded ussiing tthhee bbaarreeS SPPCCEEa nanddP PEEDDOOTT:P:PSS/S/2200%IILL//SPCE ssensorr inin5 .05m.0M mcaMte chcaotlescohluolt iosnoilnutPioBnS (pinH 7P.4B)Sa t (apsHca n7r.a4t)e oaft 10a0 mscVa·ns −1ra; (tBe ) Cofh ro1n00o a mpVe·rso−1g; ra(mB)s CobhtraoinoeadmatpPerEoDgOraTm:PsS oSb/t2a0i%neIdL /aSt PCEEDsOeTn:sPoSrSi/n20th%eIpLr/SesPeCnEce soenf s(ao)r 0i;n( bth)e1 .p0r;e(sce) n3c.0e; o(df )(a5). 00;;( (eb))6 1.0.0; ;a (ncd) 3(f.0) ;1 0(d.0) m5.M0; (cea) 6.0; an 1 techol indP (Bf)S 1(0p.H0 m7.M4). cIantseecrht;oilc aitn/ iPPBBSS v(sp.Ht 2 7p.4lo).t Idnesreirvte; dicaftr/oiPmBS cvhsr. otn½ opalmotp dereormiverdi cfrdoamta cfhoroPnBoSam(ap)earnodm1e.r0icm dMatac afoter cPhBoSl (ba) ;a(nCd) 1L.i0n meaMr s ceagtmecehnotls (bo)f; p(Clo)t Liivnse.ar 1 t −se2gfmoren(at)s 1o.f0 p; l(obt) i2 v.0s;. (tc−½) f5o.0r ; (ad)) 16..0;; (abn)d 2(.0e); 1(c0). 05.m0;M (dc)a 6te.0c;h aonl dan (de); (1D0.)0p mloMt o fcathtecshlolp aesndfr;o (mD)g rpalpoth oCf vthse. csolonpcens tfrraotimon goraf pcaht eCc hvosl.. concentration of catechol. The effect of scan rate on the voltammetric behavior of catechol at the PEDOT:PSS/20%IL/SPCE sensor was examined by CV and the two oxidation peaks and one reduction peak currents increased linearly with increasing scan rate; thus, suggesting a behavior consistent with surface confined voltammetry and corresponding ‘thin-layer’ type voltammetry [13]. To further evaluate the electrochemical behavior of the PEDOT:PSS/20%IL/SPCE sensor, the influence of scan rate on both the anodic peak potentials and cathodic peak potential of catechol were analyzed. With an increase in scan rate, the anodic peak potential shifted towards a positive value and a linear relationship was observed in the range of 10 to 300 mV·s−1. The equation of this behavior for Epa2 can be expressed as: ( ) Epa2(V) = 0.183 log v V.s−1 + 0.709; R2 = 0.9995. (1) According to Laviron’s expression for an electro(chem)ical process [34,35], Ep is governed by:0 Ep = E0′ (2.303RT) RTk (2.303RT) + ′ log + (2)(∝ n F) (α n′ F) (∝ n′ F) where v is the scan rate, n′ is the number of electrons transferred before the rate-determining step, α is the transfer coefficient, E0′ is the formal standard redox potential, and k0 is the standard heterogeneous rate constant of the reaction, and the other symbols have their usual meaning. The value of αn′ can be calculated using the slope of Epa2 vs. log v plot (here slope = 0.183). Taking R = 8.314 J·K−1·mol−1, T = 298 K, and F = 96480 C·mol−1, the value of αn′ was calculated to be 0.32. According to Bard and Faulkner [36], (47.7) α = mV (3) (Ep− Ep/2) where Ep − Ep/2 is the potential at which the current is at half its peak value. From this, the value of α was calculated to be 0.15. Consequently, the number of electrons (n) involved in the electrochemical process was calculated to be ~2.0; which indicates that the reaction is a two-electron transfer process. Sensors 2017, 17, 1716 9 of 13 3.3.2. Chronoamperometry The catalytic rate constant (Kcat) and diffusion coefficient (D) of catechol at the PEDOT:PSS/20%IL/SPCE sensor were estimated by chronoamperometry. Chronoamperometric measurements were carried out in PBS (pH 7.4) containing various concentrations of catechol (1.0, 2.0, 5.0, 6.0, and 10.0 mM) at an applied potential of +0.5 V (Figure 5B). The catalytic rate constant Kcat, was calculated using the equation [37]: i ( cat ) = π1/2(Kcat.C.t) 1/2 (4) iPBS where icat and iBRB are the currents obtained at the PEDOT:PSS/20%IL/SPCE sensor for catechol and PBS solution, respectively, C is the concentration of catechol, and t is time in seconds. The catalytic rate constant was calculated from the slope of the plot of icat/iPBS vs. t1/2 (insert of Figure 5B) for 1.0 mM catechol concentration. A value of ~6.99 × 104 M−1·s−1 was calculated for the PEDOT:PSS/20%IL/SPCE sensor, which is satisfactory for the analysis of catechol [33]. The slope of the linear parts of i vs. t1/2 plots (Figure 5C) for the different concentrations of catechol (1.0, 2.0, 5.0, 6.0, and 10 mM) were selected and used to construct the i·t1/2 vs. Ccatechol plot (Figure 5D). The slope of i·t1/2 vs. Ccatechol plot was used in conjunction with the Cottrel expression [37]: nFAD1/2C i = ( π1/2t1/2 ) (5) where i is current (in A), n is the number of electrons (here n = 2), F is Faraday’s constant, A is the electrode area (A = 0.12566 cm2), C is the concentration (1.0 × 10−6 mol·cm−3), D is the diffusion coefficient (cm2 s−1), and t is time (s), to estimate the diffusion coefficient (D) for catechol and was calculated to be ~1.17 × 10−6 cm2·ps−1. Sensors 2017, 17, 1716 9 of 12 3.3.3. Amperometry in Stirred Solution 3.3.3. Amperometry in Stirred Solution The amperometric response of catechol in PBS (pH 7.4) was measured on the The amperometric response of catechol in PBS (pH 7.4) was measured on the PEPDEODTO:PTS:PSS/S2/02%0%ILIL//SSPPCCEE sseennssoorr aatt ccoonnssttaanntt ppootteenntitaial loof f00.5. 5VV, ,wwhhicihch wwasa sthteh eoxoixdiadtaiotino pnopteontetinatli aolf of catceactheochl (oal 2()a2(F) i(gFuigruer6e) .6)A. As ssh sohwownnin inF Figiguurere6 6a annddt thhee iinnsseerrtt 66AA,, tthhee aammppeerroommeettrriicc ccuurrrreenntt vvss. .titmimee (i(-i-t) curtv) ceuorfvcea otfe ccahtoelchshool swheodwtehda tthtahte thPeE PDEODTO:PTS:PSS/S2/02%0%ILIL//SSPPCCEE sseennssoorr hhaadd aa rarappidid rersepsopnosnes teot voavryairnygin g concocenncetrnattriaotniosnos focfa ctaetcehcohloli nins tsitrirrereddb buuffffeerrs soolluuttiioonn.. TThhee eessttaabblliisshhmmeennt toof fwwelell-ld-defeinfiende dstestaedayd-syt-asttea te curcruernretnrte srpesopnosnessesto tos tsatnadndaradrda daddditiitoionnss ooff ccaatteecchhooll iinnddiiccaatteess tthhaatt tthhee sseennsosor risi ssesnensistiivtiev. eA. Alinleinare ar ranrganegwe awsarse rceocrodrdededf rforomm 00..11 µµM ttoo 333300.0.0 µµMM (F(iFgiugruer 6eB6)B w) iwthi tah seansseitnivsiittyiv oitfy 18o.f2 1m8A.2· mMA··cmM−2 ·acnmd −2 anda acaclcaulclautleadte dlimliimt iotf odf edteectteicotnio n(ba(bseadse dono n3×3 ×thet hbeabsealsinelei nneonisoei)s eo)f o2f32.73 .µ7MµM; t;htehsee seanaanlyatliyctailc al perpfeorfmoramnacen ceh cahrarcatecrteisrtiisctsicas raerec oconnsisdideerereddt otob bee satisfactory fforr rroouuttiinnee aannaalylysissi sofo cfactaetcehcohlo inl inantuartaulr al wawteartesar msapmlepsle[s3 2[3,323,3]3. ]. FigFuigreur6e. 6A. mApmepreormometertircicr ersepspoonnsesesso offt htheeP PEEDDOOTT:P:PSSSS//2200%%IILL/S/PSCPEC Esesnesnosr oirn instisrtrierdre PdBPSB (SpH(p H7.47) .4) solsuotliuotnioant aant aanp paplipedliepdo pteontetinatliaolf o0f. 50V.5 tVo tvoa rvyairnygincgo ncocenncetrnattriaotniosnosf ocfa cteactehcohloflr ofrmom0. 10.µ1 MµMto t3o3 303.00.µ0 M; insµerMt ;( Ain)szeorto m(A)a tztohoemfi rastt tsheev efinrsstt asenvdeanr dstaadnddiatridon asdodfitciaotnesc hoof lcaantedch(Bol) panlodt (oBf)s tpelaodt yofs tsatteeadcuyr rsetantte vs. catceucrhroelnct ovns.c ecanttercahtiooln c.oncentration. 3.3.4. Stability of PEDOT:PSS/20%IL/SPCE sensor The stability of the conducting polymer composite is crucial for any practical applications. In order to investigate the stability and durability of the electrocatalytic activity of the PEDOT:PSS/20%IL/SPCE sensor, several voltammograms were recorded in catechol solution. In general, unstable electrodes have unstable voltammograms. Figure 7, shows 40 repetitive voltammograms recorded for 5.0 mM catechol and their corresponding anodic (Ipa1, Ipa2) and cathodic (Ipc) peak currents for selected cycles are shown in Figure 7 (insert). The standard deviation values for Ipa1, Ipa2, and Ipc1 were found to be 1.84%, 0.59%, and 1.27%, respectively. These standard deviation values indicate that the procedure for the sensor fabrication is highly reproducible. Figure 7. Repetitive cyclic voltammograms (40 scans) recorded at PEDOT:PSS/20%IL/SPCE sensor; insert is peak current vs. cycle number. Voltammograms were recorded in 5.0 mM catechol in PBS (pH 7.4) containing 0.1 M KCl and at a scan rate of 100 mV·s−1. Sensors 2017, 17, 1716 9 of 12 3.3.3. Amperometry in Stirred Solution The amperometric response of catechol in PBS (pH 7.4) was measured on the PEDOT:PSS/20%IL/SPCE sensor at constant potential of 0.5 V, which was the oxidation potential of catechol (a2) (Figure 6). As shown in Figure 6 and the insert 6A, the amperometric current vs. time (i- t) curve of catechol showed that the PEDOT:PSS/20%IL/SPCE sensor had a rapid response to varying concentrations of catechol in stirred buffer solution. The establishment of well-defined steady-state current responses to standard additions of catechol indicates that the sensor is sensitive. A linear range was recorded from 0.1 µM to 330.0 µM (Figure 6B) with a sensitivity of 18.2 mA·mM·cm−2 and a calculated limit of detection (based on 3× the baseline noise) of 23.7 µM; these analytical performance characteristics are considered to be satisfactory for routine analysis of catechol in natural water samples [32,33]. Figure 6. Amperometric responses of the PEDOT:PSS/20%IL/SPCE sensor in stirred PBS (pH 7.4) solution at an applied potential of 0.5 V to varying concentrations of catechol from 0.1 µM to 330.0 Sensorsµ2M01; 7i,n1s7e, r1t7 1(6A) zoom at the first seven standard additions of catechol and (B) plot of steady stat1e0 of 13 current vs. catechol concentration. 3..3..4.. Sttabiilliitty off PEDOT::PSS//2200%IIL//SSPPCCEE seSnensosro r The ssttaabbiilliittyyo of ft htehec ocnodnudcuticntignpgo plyomlyemr ceor mcopmospitoesiistec rius cciraul cfoiarla fnoyr parnayc tpicraalcatpicpalli caaptipolnicsa.tIinonosr.d Ienr toordinevre sttiog atientvheestsitgaabtieli tythane d dstuarbaibliitlyit y aonf tdh e deluercatrboicliattya lyotifc atchtiev iteyleocfttrhoecPatEaDlyOtiTc :PSaSct/iv20it%y ILo/fS PtChEe sPeEnDsoOrT, s:PevSSer/2a0l %voILlt/aSmPCmEo gsreanmsosrw, seerveerreaclo rvdoeltdaminmcaotgercahmosl swoleurtei orne.coInrdgeedn eirna lc,autnecshtaobl lesoelluetcitorno.d eIns hgaenveruanl, stuabnlsetavbollet amelemctorgordaems s.hFaivgeu reun7s, tsahbolwe sv4o0ltraempemtiotigvreamvosl.t aFmigmuorge ra7m, ssrheocworsd e4d0 forre5p.e0timtivMe cvaotletcahmoml aongdratmhesi rreccoorrredsepdo fnodri 5n.g0 amnMod cicat(eIpcah1o, lI paan2)da tnhdeicra ctohrordesicp(oInpcd)ipnega akncoudrirce n(Itpsa1,f oIpra2s)e alencdte cdatchyocdleisc a(Irpec) spheoawk ncuinrrFenigtsu rfoer7 s(eilnescetertd) .cTyhcleess taarned sahrodwdne vinia Ftiigounrvea 7l u(ienssfeorrt)I. pTa1h,eI psat2a,nadnadrdIp dc1evwiaetrieonfo vuanlduetso fboer 1Ip.a814, %Ipa,20, .a5n9d% ,Ipacn1 dw1e.r2e7 %fo,urnedsp teoc tbivee 1ly.8. 4T%he, s0e.5s9ta%n,d anrd d1e.2v7ia%ti, ornesvpaelucteisveinlyd.i cTahtesteh asttathnedaprrdo cdeedvuiraetifonr tvhaelusesn sinodr ifcaabtrei ctahtaito nthies phriogchelydurerpe rfodr uthceib sle.nsor fabrication is highly reproducible. Fiigurre 7.. Reepettiittiive ccyycclliicc vvoollttaammooggrraamss ((4400 ssccaannss))r reeccoorrddeedda at tP PEEDDOOTT:P:PSSS//2200%IIL/SPCE ssenssorr;; iinsertt iis peak currentt vs.. cyclle number.. Vollttammograms were recorded iin 5..0 mM cattecholl iin PBS (pH 7..4) contaiiniing 0..1 M KCll and att a ssccan rrattee off 110000 mV··s−11. . 3 .3.5. Analysis of Natural Water Samples To demonstrate the feasibility of the PEDOT:PSS/20%IL/SCPE sensor for routine analysis, the sensor was used to analyze natural water samples. Prior to this analysis, the water samples were analyzed for the presence (or otherwise) of endogenous catechol; this analysis indicated no detectable catechol in the water sampled. After verifying the absence of endogenous catechol in the water samples, amperometry, in conjunction with the method of standard additions [38–41], was employed to determine the recovery of catechol spiked into the water samples. The analytical performance data for three repeated measurements are summarized in Table 1. Table 1. Recovery of spiked catechol from natural water samples. Sample [Catechol]/µM Mean Recovery (%) Amount Added Amount Found Tap Water Repeat 1 20 19.98 Repeat 2 20 19.96 Repeat 3 20 19.89 %Recovery (19.94= )20.0 × 100 = 99.7. Mean - 19.943 SD - 0.035 CV (%) - 0.177 River Water Repeat 1 20 19.76 Repeat 2 20 19.88 Repeat 3 20 19.79 %Recovery = (19.81)20.0 × 100 = 99.1 Mean - 19.81 SD - 0.062 CV (%) - 0.315 NB: SD—Standard Deviation; CV—Coefficient of Variation. Sensors 2017, 17, 1716 11 of 13 The recoveries were found to be well over 99.0% with coefficient of variations of 0.04 and 0.32. Clearly, the presence of interfering species in the water samples did not have any significant interference with the analysis of the compound; thus, the sensor can be used for routine quantification of catechol in the natural water samples. 4. Conclusions A stable, high-performance composite combining the synergistic effects of the conducting polymer PEDOT:PSS and the room temperature ionic liquid, [EMIM][BF4], was formulated and utilized to fabricate a disposable screen-printed sensor. The formulated PEDOT:PSS/IL composite exhibited a highly nano-porous microstructure, excellent stability, and enhanced electrocatalytic properties towards catechol, a priority pollutant. When the sensor was used to analyze catechol, satisfying selectivity and sensitivity data were found. Potential applicability of the sensor in the analysis of catechol in natural water samples was demonstrated with stable, accurate results obtained; which demonstrates that the sensor holds a great promise for routine application in the analysis of this priority pollutant. In the future, sensors based on the transduction capabilities of PEDOT:PSS/IL composite would be developed for biomedical diagnostic applications. Acknowledgments: This work was supported by funds from a World Bank African Centres of Excellence grant (ACE02-WACCBIP: Awandare) and a DELTAS Africa grant (DEL-15-007: Awandare). Francis Krampa was supported by a WACCBIP-World Bank ACE PhD fellowship and Yaw Aniweh was supported by a DELTAS Africa postdoctoral fellowship. The DELTAS Africa Initiative is an independent funding scheme of the African Academy of Sciences (AAS)’s Alliance for Accelerating Excellence in Science in Africa (AESA) and supported by the New Partnership for Africa’s Development Planning and Coordinating (NEPAD) Agency with funding from the Wellcome Trust (107755/Z/15/Z: Awandare) and the UK government. The views expressed in this publication are those of the authors and not necessarily those of AAS, NEPAD Agency, Wellcome Trust or the UK government. 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