Silver Nanoparticles Mediated by Costus afer Leaf Extract: Synthesis, Antibacterial, Antioxidant and

20 Pages·2017·3.24 MB·English

molecules
Article
Silver Nanoparticles Mediated by Costus afer Leaf
Extract: Synthesis, Antibacterial, Antioxidant and
Electrochemical Properties
EliasE.Elemike1,2,3,OmololaE.Fayemi1,2,AnthonyC.Ekennia4,DamianC.Onwudiwe1,2,*
andEnoE.Ebenso1,2
1 MaterialScienceInnovationandModelling(MaSIM)ResearchFocusArea,FacultyofAgriculture,
ScienceandTechnology,North-WestUniversity,MafikengCampus,PrivateBagX2046,
Mmabatho2735,SouthAfrica;[email protected](E.E.E.);
[email protected](O.E.F.);[email protected](E.E.E.)
2 DepartmentofChemistry,SchoolofMathematicsandPhysicalSciences,FacultyofAgriculture,Scienceand
Technology,North-WestUniversity,MafikengCampus,PrivateBagX2046,Mmabatho2735,SouthAfrica
3 DepartmentofChemistry,CollegeofScience,FederalUniversityofPetroleumResources,
P.M.B1221Effurun,DeltaState,Nigeria
4 DepartmentofChemistry,FederalUniversityNdufu-AlikeIkwo(FUNAI),P.M.B1010Abakiliki,
EbonyiState,Nigeria;[email protected]
* Correspondence:[email protected];Tel.:[phone];Fax:[phone]
AcademicEditor:DiegoMuñoz-Torrero
Received:10March2017;Accepted:20April2017;Published:29April2017
Abstract: Synthesis of metallic and semiconductor nanoparticles through physical and chemical
routeshasbeenextensivelyreported. However,greensynthesizedmetalnanoparticlesarecurrently
in the limelight due to the simplicity, cost-effectiveness and eco-friendliness of their synthesis.
ThisstudyexploredtheuseofaqueousleafextractofCostusaferinthesynthesisofsilvernanoparticles
(CA-AgNPs). The optical and structural properties of the resulting silver nanoparticles were
studiedusingUV-visiblespectroscopy,scanningelectronmicroscopy(SEM),transmissionelectron
microscopy (TEM) and Fourier transform infra–red spectrophotometer (FTIR). TEM images of
the silver nanoparticles confirmed the existence of monodispersed spherical nanoparticles with
ameansizeof20nm.TheFTIRspectraaffirmedthepresenceofphytochemicalsfromtheCostus
afer leaf extract on the surface of the silver nanoparticles. The electrochemical characterization
of a CA-AgNPs/multiwalled carbon nanotubes (MWCNT)-modified electrode was carried out
toconfirmthechargetransferpropertiesofthenanocomposites.Thecomparativestudyshowed
thattheCA-AgNPs/MWCNT-modifiedelectrodedemonstratedfasterchargetransportbehaviour.
TheanodiccurrentdensityoftheelectrodesinFe(CN) ]4−/[Fe(CN) ]3− redoxprobefollowsthe
6 6
order: GCE/CA-Ag/MWCNT(550mA/cm2)>GCE/MWCNT(270mA/cm2)>GCE(80mA/cm2)
> GCE/CA-Ag (7.93 mA/cm2). The silver nanoparticles were evaluated for their antibacterial
propertiesagainstGramnegative(Escherichiacoli,Klebsiellapneumonia,Pseudomonasaeruginosa)and
Grampositive(BacillussubtilisandStaphylococcusaureus)pathogens. Thenanoparticlesexhibited
betterinhibitionofthebacterialstrainscomparedtotheprecursors(leafextractofCostusaferand
silvernitrate). Furthermore,theabilityofthenanoparticlestoscavengeDPPHradicalsatdifferent
concentrationswasstudiedusingtheDPPHradicalscavengingassayandcomparedtothatofthe
leaf extract and ascorbic acid. The nanoparticles were better DPPH scavengers compared to the
leafextractandtheirantioxidantpropertiescomparedfavorablytheantioxidantresultsofascorbic
acid. The green approach to nanoparticles synthesis carried out in this research work is simple,
non-polluting,inexpensiveandnon-hazardous.
Keywords: silvernanoparticles;Costusafer;electrochemical;antioxidant;antibacterial
Molecules2017,22,701;doi:10.3390/molecules22050701 www.mdpi.com/journal/molecules
Molecules2017,22,701 2of20
1. Introduction
Nanoparticleswithcontrolledsize,shapeandstructureareimportantbuildingblocksformany
optical, magnetic, electronic, and biomedical applications [1]. The organization and patterning of
silver NPs (AgNPs) into one and two-dimensional functional templates have recently attracted
muchattentionduetotheirpotentialadvantagesasphotosensitivecomponentsofopticaldevices,
electrocatalysts,chemicalsensorsandantibacterialagents[1–4]. Thesynthesisofsilvernanoparticles
can be achieved by using various methods which involve chemical reduction [5], photochemical
reduction[6], electrochemicalreduction[7], andheatvaporization[8].Theseprocessesallinvolve
different toxic chemicals as reducing agents or stabilization agents for the silver ion. In order to
circumventthecomplicationoftoxicityinthesynthesisofsilvernanoparticlesduetotheirpotential
biologicalapplications,plantsorplantextractshavebeenestablishedtohavealeadingroleinthe
AgNPbiosynthesisprocess[9]. Phytosynthesis(theuseofplantextractsinsynthesisofnanoparticles)
ofmetalnanoparticlesincorporatestheuseofweed,seeds,gums,fruits,roots,bark,sapandflowers[9].
Inusingthisapproachforsynthesis,thepropertiesofthenanoparticlescouldbetunedbychanging
theratiooftheprecursorsaltstothesubstrates;andthemethodissimple,practicable,sustainable
andcosteffective. CurrentlyseveralAgNPshavebeensynthesizedusingdifferentplantextractsas
potentialreducingagents[9,10].
Biosynthesizedsilvernanoparticleshavebeenreportedtoexhibitanumberofbiologicalproperties
thatincludesantimicrobial[10–12],antioxidant[11–14],wouldhealing[9]andanticancereffects[11,15].
Inaddition,silvercatalystshavebeensuccessfullyusedinheterogeneouscatalysisatanindustrialscale
fortheoxidationofmethanoltoformaldehydeandethylenetoethyleneoxide[16].Similarly,Ag-doped
semiconductornanoparticleshavemanyapplicationsinphotocatalysis(i.e.,degradationoforganic
pollutants, hydrogen production, CO photoreduction, disinfection), in order to improve the
2
photo-conversionyieldandallowtheextensionofthelightabsorptionofwidebandgapsemiconductors
tothevisiblelight[17,18].RecentreportssuggestthattheremovaloforganicdyesusingAgnanoparticles
isabetterchoicethanthecommondyeremovaltechniqueslikeredoxtreatment,electrocoagulation,
carbonsorption,andUVphotodegradation[2].
Theadsorptionofchemicalspeciesonthesurfaceofnanotubesisanareaofimmenseinterest
due to its relevance for molecular electronics and because it is a necessary prerequisite for any
surface-mediatedchemicalprocess[19]. Carbonnanotues(CNT)possessinghighthermalcapacityand
abilitytopromoteelectrontransferreactionspresentanenormouspotentialasinorganictemplates[20].
This is explored in the preparation of AgNPs/CNT hybrid materials, which integrate the unique
physical and chemical properties of each component [21] Silver nanoparticles (AgNPs) attached
onto multi-walled carbon nanotubes (MWCNTs) have attracted significant attention due to their
uniqueproperties[22,23]. Themodificationofelectrodesusedinelectrochemicalexperimentsusing
variousmultifunctionalmaterialsinordertoconfernovelelectrochemicalpropertiestotheelectrode
hasbecomeacommonpracticeinelectrochemistrystudies[24–26]. Multi-walledcarbonnanotubes
(MWCNTs)havebeenusedtomodifytheelectrodesurfacesinordertoincreasethesensitivitytowards
theelectrochemicaldeterminationofsomecompoundsandenzymes[25,26].However,literatureonthe
infusionofgreensynthesizedsilvernanoparticlesoncarbonnanotubesisstillscarce. Similarly,there
arenotmanyreportsontheelectrochemistryofthesehybridnanocompositesfromplant-mediated
nanoparticles[27–29]. Hence,inthisworkweutilizedtheactivecomponentsoftheleafextractsof
Costus afer for nanoparticle synthesis and investigated in a comparative study the electrochemical
propertiesofabareglassycarbonelectrodeandanelectrodemodifiedwithCA-AgNPs, MWCNT
and CA-AgNPs/MWCNT nanomatrices as potential electrochemical sensors for biological and
environmentalmolecules.
Costus afer belongs to the family Costaceae, a monocot and a relatively tall, herbaceous,
unbranched tropical plant with creeping rhizomes. It is commonly found in moist or shady and
riverbanksforestofWestandTropicalAfricancountriesincludingSenegal, SouthAfrica, Guinea,
Nigeria, Ghana and Cameroon [30]. Almost all parts of Costus afer have been extensively used in
Molecules2017,22,701 3of20
Mtroaldeciutlieos n20a1l7,m 22e, d70ic1i ne as a remedy for cough, rheumatic pains, sleepiness and as a cardiotonic3 [o3f 120].
The phytochemical composition of extracts of different parts of the plant has been established
panhdytroecphoermteidca[l 3a1n,3a2l]y.sTish eofp thhyet olecahveems icaanlda nstaelmys iasqoufetohuesl eeaxvtreasctasn dshsotwemeda qsoumeoeu psheyxttoraccotmspshoonwenetds
ssuomche pahs ytaolckoamlopidosn, ensatsposuncinhs,a staalnknailnosi,d sf,lasvaopnooniidnss ,taanndn inpsh,eflnaovlso. noAidlksaaloniddsp haerne olass.sAoclkiaatleodi dswaitrhe
aansstio-icniafltaemdmwaitthorayn, atin-itni-flmaimcrmobaiatol reyf,feacnttsi -ams iwcreollb aiasl aenfftei-chtyspaesrwteenlsliavse aangtei-nhty, apnedrt ethnes ihveeaalignegn te,ffaencdt hthaes
ehaeralliienrg beefefenc atthtraisbeuatreldie trob teheen patrtersiebnucteed otfo tathnenipnrse sinen icne tohfet apnlannint.s Tinhein rothoetsp olaf nCt.osTthues arofeort hsaovfeC boseteuns
raefeprohratevde btoe ecnornetpaionr teodxatloicc oancitda,i nlaonxoaslitceraocli,d t,rliagnoogsetneirno,l ,at ringeowg edniions,gaenneinw, dstiiogsmgeanstienr,oslt,i gsmitoasstteerrooll,,
csoitsotsutgeernoiln, caonsdtu ag neenwin satenrdoida nsaepwonsitnearofeidrossiadpeo Ani [n3a2f]e. rLoesaivdees Aof [a3n2o].thLeera svpeescioefs,a Cnoostthues rpiscptuesc,i hesa,vCe obseteuns
rpeicptourst,edh atvoe cobneteaninr eflpaovrotneodidtso scuocnht aaisn kflaaevmopnfoeirdols, s3u,4c-hdi aOs-kMaee-mqupefrecreotli,n 3a,4n-dd iphOe-nMoeli-cq auceirdcse tsiuncha nads
p2,h5-edniohlyicdraocxiydbsesnuzcohic aasci2d,,5 O-d-ichoyudmroaxriycb aecnizdo, ipc-hacyiddr,oOxy-cboeunmzoaicr icacaidci dth,apt- hpyodssreosxsy gboeondzo aicntaiocxididtahnatt,
panotsis-einsfslagmoomdataonrtyio, xhiydpaongt,lyacnetmi-iinc flaanmdm reantoarl yf,uhnycptioognlsy c[e1m4,3ic3]a. nTdhere vnaasltf umnecdtiiocinnsal[ 1a4p,3p3l]i.caTthioenvs aosft
emxetrdaicctins aolfa Cpopsltiucas taiofenr sporfoemxptrtaecdts thofe Caonsttiumsiacfreorbpiraol manpdte adntthioexaindtaimnti cervoabliuaaltaionnd oafn tsiiolvxeidr annatneovpaalurtaitciloens
soyfnstihlveesrizneadn obpy atrhtiec laeqsuseyonuths elseiazfe edxbtryactht eofa qthuieso pulsalneta mfeaxtterraicatl.o Tfhthei srepslualnt twmaast ecroimalp. Tarheedr ewsiuthlt wthaast
ocobmtapinaerded frwomith btahraet aoqbutaeionuesd lefraof mexbtraarcet aoqf uCeoosutussl eaafefre axntrda csttaonfdCaorsdt udsraufgers.a ndstandarddrugs.
22.. RReessuullttss aanndd DDiissccuussssiioonn
2.1. UV-VisibleSpectroscopy
2.1. UV-Visible Spectroscopy
UV-visiblespectroscopyisavaluabletechniqueusedtodetectthecharacteristicsurfaceplasmon
UV-visible spectroscopy is a valuable technique used to detect the characteristic surface plasmon
resonance(SPR)patternofmetalnanoparticles. MetalnanoparticlesexhibitSPRphenomenawhen
resonance (SPR) pattern of metal nanoparticles. Metal nanoparticles exhibit SPR phenomena when
metalelectronsintheconductionbandcollectivelyoscillateinresonancewithcertainwavelengthsof
metal electrons in the conduction band collectively oscillate in resonance with certain wavelengths
incidentlight.ThesynthesiswasmonitoredinatimedependentmannerusingUV-visiblespectroscopy
of incident light. The synthesis was monitored in a time dependent manner using UV-visible
asshowninFigure1.
spectroscopy as shown in Figure 1.

Figure 1. UV-vis spectra showing the surface plasmon resonance exhibited by the nanoparticles at
Figure1. UV-visspectrashowingthesurfaceplasmonresonanceexhibitedbythenanoparticlesat
different time intervals.
differenttimeintervals.
The UV-visible spectra showed an early appearance of a surface plasmon resonance peak (SPR)
TheUV-visiblespectrashowedanearlyappearanceofasurfaceplasmonresonancepeak(SPR)
with high intensity after 10 min. However, as the reaction time increased, the sharpness of the SPR
withhighintensityafter10min.However,asthereactiontimeincreased,thesharpnessoftheSPR
peak increased too. This is often a factor of the size, shape dispersion; nature of biomolecules and
peakincreasedtoo. Thisisoftenafactorofthesize,shapedispersion; natureofbiomoleculesand
surrounding media characteristics of the phytochemical reducing agent, as different extracts give
varying SPR results [34–36]. The range of the wavelengths throughout the incubation time was within
405–411 nm, with continuous increase in intensity which reflects the presence of a considerable
amount of monodispersed and stable AgNPs. AgNPs absorb radiation intensely within the observed
Molecules2017,22,701 4of20
surrounding media characteristics of the phytochemical reducing agent, as different extracts give
varyingSPRresults[34–36].Therangeofthewavelengthsthroughouttheincubationtimewaswithin
405–411 nm, with continuous increase in intensity which reflects the presence of a considerable
amMouolnectuloesf 2m01o7,n 2o2,d 7i0s1p ersedandstableAgNPs. AgNPsabsorbradiationintenselywithintheob4 soef 2r0v ed
wavelengths. Theexactmechanismfortheextracellularbiosynthesisofmetalnanoparticlesisnotwell
wavelengths. The exact mechanism for the extracellular biosynthesis of metal nanoparticles is not
understood,butstudiesareinprogresstounravelthemechanismofthecrystalgrowthandhowto
well understood, but studies are in progress to unravel the mechanism of the crystal growth and
controlthesize,aswellashowtodevelopparticlesofhighermonodispersity[37].
how to control the size, as well as how to develop particles of higher monodispersity [37].
2.2. FTIRAnalysis
2.2. FTIR Analysis
FTIR spectroscopy was used to identify the possible biomolecules present in Costus afer leaf
FTIR spectroscopy was used to identify the possible biomolecules present in Costus afer leaf
powder which are responsible for reducing the Ag+ to Ag0, capping and stabilizing the silver
powder which are responsible for reducing the Ag+ to Ag0, capping and stabilizing the silver
nannoapnaorptaicrlteicsl.esT. hTeheF TFTIRIRs pspeeccttrraa rreeccoorrddeedd ffoorr tthhee leleaaf fanadn dCACA-A-gANgPNsP asrea prerepserensteedn tiend FiinguFrieg 2u. re2.
(b)
(a)

Figure 2. The FTIR spectra of the (a) Costus afer leaf and (b) biosynthesized AgNPs.
Figure2.TheFTIRspectraofthe(a)Costusaferleafand(b)biosynthesizedAgNPs.
The FTIR spectrum of leaf powder showed peaks at 3307, 1625, 1375, and 1029 cm−1 which are
simThilearF tToI Rpesapkesc otrbusmervoefdl eina fCpAo-wAdgeNrPssh. oOwnelyd ap selaigkhsta vta3r3ia0t7io,n1 6a2t5 t,h1e3 l7ig5a,tainndg s1i0te2s9 mcman−if1ewstsh itchhata re
simbiiloamrtoolepcuealek asdosbosreprtvioend oinn tCheA s-iAlvgeNr oPcsc.uOrrendly. Tahsel ivgihbrtavtiaorniaatl ipoenaka tatrhoeunlidg a3t3i0n7g csmit−e1 sism asasnigifneesdts toth at
bioνm(Ool–eHcu) lsetraedtcshoirnpgt iothnato ncotuhled spilovsesribolycc eumrraenda.tTe hfreovmib craartbioonhayldpraetaeks aorro puhnedno3l3ic0s7. cTmhe− 1peisakass sfiogunnedd to
ν(Oa–roHu)nsdt r2e9t1c7h ianngd t2h8a4t9 ccomu−l1d apreo assscibrilbyede mtoa nthaet e–Cfr-oHm stcraetrcbho ohfy tdhrea atelksyol rgprohuepn, oalnicds .thTeh peepake aakrosufnodu n d
aro1u6n2d5 c2m91−17 ias nddue2 8to4 9thcem e−no1laicr eβ-adsickreitboendest oort h-Ce=–OC -sHtrestcthre otcf hcaorfbtohxeylaicl kaycildgsr, owuhpi,lea nthde tchoerrpesepaoknadrinogu nd
162-5Cc-mO −s1triestcdhu eist oobthseerveendo liacroβu-nddik e1t3o7n5e asnodr -1C0=29O csmtr−e1.t cThhoef acsasribgonxmyelinct aocfi dpse,awksh iilse sthimeiclaorr rteos poothnedri ng
-C-Olitesrtarteutrceh riespoorbtsse [r3v8e,3d9]a. rTohuen dap1p3e7a5raannced o1f0 t2h9escem p−e1a.kTs hsuegagsessitgendm theen tproefsepnecaek osf isphsyimtoiclhaermtoicaoltsh er
such as flavonoids, alkaloids, phenolics, organic acids of the Costus afer leaf in the CA-AgNPS, which
literaturereports[38,39].Theappearanceofthesepeakssuggestedthepresenceofphytochemicals
are responsible for stabilizing as well as the capping effects on the nanoparticles. The biomolecules may
suchasflavonoids,alkaloids,phenolics,organicacidsoftheCostusaferleafintheCA-AgNPS,which
have interacted with the Ag ions through their oxygen donor atoms and are adsorbed on the surface
areresponsibleforstabilizingaswellasthecappingeffectsonthenanoparticles. Thebiomolecules
of the metal ions which is manifested by a decrease in the peak intensities of bands observed in the
mayhaveinteractedwiththeAgionsthroughtheiroxygendonoratomsandareadsorbedonthe
AgNPs [34].
surfaceofthemetalionswhichismanifestedbyadecreaseinthepeakintensitiesofbandsobservedin
theAgNPs[34].
2.3. Microscopy/Compositional Analysis
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to
study the surface morphology, shape and size of the nanoparticles and the nanocomposite. The
images are presented in Figure 3a–d. The SEM image showed high density silver nanoparticles with
Molecules2017,22,701 5of20
2.3. Microscopy/CompositionalAnalysis
Scanningelectronmicroscopy(SEM)andtransmissionelectronmicroscopy(TEM)wereusedto
sMtuodleycutlehs e20s1u7r, f2a2,c 7e0m1 orphology,shapeandsizeofthenanoparticlesandthenanocomposite. Thei5m oaf g20e s
arepresentedinFigure3a–d.TheSEMimageshowedhighdensitysilvernanoparticleswithsurface
surface morphology which reveals encapsulation of the biomass in the silver. Bio-capped molecules
morphologywhichrevealsencapsulationofthebiomassinthesilver. Bio-cappedmoleculeshelpto
help to prevent agglomeration of nanoparticles and also enhance antimicrobial activity [40,41].
preventagglomerationofnanoparticlesandalsoenhanceantimicrobialactivity[40,41].Semi-quantitative
Semi-quantitative estimation of the nanoparticles composition was performed by EDS and the
estimation of the nanoparticles composition was performed by EDS and the respective spots of the
respective spots of the phytocapped CA-AgNPs are represented by labels 1 and 2 in Figure 3a.
phytocappedCA-AgNPsarerepresentedbylabels1and2inFigure3a.Quantitativeanalysisprovides
Quantitative analysis provides the weight % of all the elements present in the biosynthesized AgNPs
the weight % of all the elements present in the biosynthesized AgNPs as: C (8.51%), Mg (0.16%),
as: C (8.51%), Mg (0.16%), Cl (3.77%), Ag (60.33%), O (27.24%) and for the nanocomposites: C 61.95%,
Cl(3.77%),Ag(60.33%),O(27.24%)andforthenanocomposites:C61.95%,O9.52%,Mg0.22%,Cl1.73%,
O 9.52%, Mg 0.22%, Cl 1.73%, K 1.07%, Ag 25.51%. The presence of O, Cl, C and Mg peaks along
K1.07%,Ag25.51%.ThepresenceofO,Cl,CandMgpeaksalongwithAgsignals,suggestthatthe
with Ag signals, suggest that the AgNPs are bonded to phytoconstituents of the leaf extract. The Ag
AgNPsarebondedtophytoconstituentsoftheleafextract.TheAgdetectionlimitismuchhigherthanfor
detection limit is much higher than for all other trace elements present in the AgNPs while in the
allothertraceelementspresentintheAgNPswhileintheCA-AgNPs/MWCNTnanocomposite,ahigher
CA-AgNPs/MWCNT nanocomposite, a higher percentage of carbon was recorded. Hence, Ag spots
percentageofcarbonwasrecorded.Hence,Agspotsareprevalentintheelementalmapping.Similar
are prevalent in the elemental mapping. Similar works have reported the synthesis of AgNPs using
workshavereportedthesynthesisofAgNPsusingextractsofA.farnesiana,Dracocephalummoldavica,pine,
extracts of A. farnesiana, Dracoce phalummoldavica, pine, persimmon, magnolia, ginkgo, platanus, and
persimmon,magnolia,ginkgo,platanus,andguavaextractswhichshowedthepresenceofC,Oand
guava extracts which showed the presence of C, O and other minor elements with Ag having higher
otherminorelementswithAghavinghigherpercentagecomposition[42–46].TheTEManalysisshowed
percentage composition [42–46]. The TEM analysis showed well stabilized particles with spherical
wellstabilizedparticleswithsphericalshapeandnegligibleaggregation. AsshowninFigure3b,the
shape and negligible aggregation. As shown in Figure 3b, the size of the AgNPs is about 5–38 nm
sizeoftheAgNPsisabout5–38nmrange.Figure3dshowstheintercalationoftheAgNPswiththe
range. Figure 3d shows the intercalation of the AgNPs with the multiwalled carbon nanotube
multiwalledcarbonnanotubesignifyingthewelldispersednatureofthecompositeandtheinteractive
signifying the well dispersed nature of the composite and the interactive mode leading to enhanced
modeleadingtoenhancedelectrochemicaldetection.
electrochemical detection.

Figure 3. (a) SEM and (b) TEM images of the as-synthesized Costus afer mediated silver nanoparticles
Figure3.(a)SEMand(b)TEMimagesoftheas-synthesizedCostusafermediatedsilvernanoparticles
(c) SEM and (d) TEM images of the CA-AgNPs/MWCNT.
(c)SEMand(d)TEMimagesoftheCA-AgNPs/MWCNT.
2.4. Thermal and EDX Analysis
Figure 4 shows the TGA/DSC/DTG graph of CA-AgNPs. The graph presents a three step
decomposition process. The first step of the trace, which occurred in the 50–115 °C range, was due
to the evaporation of water moisture and residual solvent molecules adsorbed on the surface of
Molecules2017,22,701 6of20
2.4. ThermalandEDXAnalysis
Figure 4 shows the TGA/DSC/DTG graph of CA-AgNPs. The graph presents a three step
Mdeolceocumlesp 2o0s17it, i2o2n, 7p01r ocess. Thefirststepofthetrace,whichoccurredinthe50–115◦Crange,was6 odfu 2e0
to the evaporation of water moisture and residual solvent molecules adsorbed on the surface of
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Tohnet hEeDnXa naonpalayrstiisc lceasrbrieefdo roeutth eornm tahlea nnaanlyospiasrctoicnlefisr mbeefdorteh ethperremseanl caenaolfyCsi,sO co,Cnfli,rMmegda nthde Cparewsehnicche
oafr eCd, Oue, Ctol, tMheg palnadn tCma awtehriicahl .aTreh educoe mtop tohsei tpiolannot fmthateerpialal.n Tthmea cyomdipffoesritbioanse odf othne tphleanetn mviaroyn dmifefenrt
baansdedg eoong rtahpeh eicnavlirreognimonesnt[ 4a8n].dT gheeopgrreaspehniccealo rfeCgiaonnds O[48s]u. pTphoer tpsrtehseenFcTeI Rofr eCsu altnsdw Oh icshupspuogrgtess tthede
FcaTrIbRo hreysdurlatste wsohricphh seungogliecsstaesd ccoamrbpoohnyednrtastoefs tohre pphlaenntoelixctsr aacst scaonmdpaotntaecnhtsm oefn tthoef tphleanptl aenxttreaxcttrsa catntdo
athtteacshilmveernwt oasf tvhiea poxlayngte enx.tract to the silver was via oxygen.
100 4
90 1
3
n)
Weight (%) 7800 Heat Flow (W/g) -10 2 eriv. Weight (%/mi
D
1
60 -2
50 0
0 200 400 600 800
Exo Up Temperature (°C) Universal V4.7A TA Instrum

FFiigguurree 44.. TTGG//DDTTGG aanndd DDSSCC ccuurrvveess ooff tthhee CCAA--AAggNNPPss oobbttaaiinneedd iinn nniittrrooggeenn aattmmoosspphheerree..
22..55.. EElleeccttrroocchheemmiiccaall SSttuuddiieess ooff tthhee AAggNNPPss MMeeddiiaatteedd NNaannooccoommppoossiitteess
AA ccoommppaarraattiivvee ssttuuddyy oonn tthhee cchhaarrggee ttrraannssffeerr pprrooppeerrttiieess ooff tthhee bbaarree aanndd mmooddiiffiieedd ggllaassssyy ccaarrbboonn
eelleeccttrrooddeess wwaass ppeerrffoorrmmeedd wwiitthh ccyycclliicc vvoollttaammmmeettrryy eexxppeerriimmeennttss.. TThhee CCVV wweerree ppeerrffoorrmmeedd iinn 55 mmMM
FFee((CCNN))66]]44−/−[F/e[F(CeN(C)N6]3)−6 ]in3− 0i.1n M0.1 PMBSP (BscSa(ns craanter, a2t5e ,m25Vms−1V) sa−n1d) athned rtehseulrte issu pltriessepnretesden itne dFiignuFreig 5u. rAel5l .
tAhlel tehleecetrleocdteros,d iensc,liundcliungd inthget hbearbea rGeCGEC, Esh,sohwoewde rdedreodxo pxepaekask slalbaeblelellde dAAAA’. ’.TThhisi srreeddooxx ppeeaakk wwaass
aattttrriibbuutteedd ttoo tthhee FFee((CCNN))66]]44−−//[F[eF(eC(CNN)6])36−] 3r−edreodx opxrpocroescse.s sT.hTeh peepaeka kcucrurrernetn trersepspoonnsesse soof ftthhee ggllaassssyy
ccaarrbboonn--mmooddiiffiieedd eelleeccttrrooddeess ffoolllloowweedd tthhee oorrddeerr:: CCAA--AAggNNPPss//MMWWCCNNTT ((339911 µµAA)) >> MMWWCCNNTT ((119922 µµAA))
>> GGCCEE (5(57.71.1µ Aµ)A>) C>A C-AAg-NAPgNs(P5s.6 3(5µ.6A3) .µTAhe).r eTwheerree nwoeorteh enroo bostehrevra bolebspeeravkabalteb apreeaGk CaEt, CbaAr-eA GgNCPEs,
CanAd-AMgWNCPsN aTn,dw hMilWeoCnNthTe, wCAhi-lAe goNn Ptsh/eM CWAC-ANgTNmPso/dMifiWedCNeleTc tmrooddeisfi,etdh eesleecctornoddepsa, irthoef sreedcoonxdp epaakirs
oBfB r’e(dFoigxu preea4k)s iBnBt’h (eFirgeugiroen 4s) oinf 0th.8e6 rmegVioanns dof0 .04.786m mVVw aenrde 0a.t4tr7i bmuVte dwetoret haettrsiiblvueterdn aton othpea rstiilcvleers
nanoparticles redox process. This comparative study shows that the CA-AgNPs/MWCNT-modified
glassy carbon electrode demonstrated faster charge transport behaviour which is evidenced in the
observed anodic current response. The anodic current density of each modified electrode in
Fe(CN)6]4−/[Fe(CN)6]3− redox probe follows the order: CA-AgNPs/MWCNT (550 mA/cm2) >
MWCNT (270 mA/cm2) > GCE (80 mA/cm2) > CA-AgNPs (7.93 mA/cm2). Therefore the glassy
carbon electrode modified with CA-AgNPs/MWCNT remains the best electrode owing to the
Molecules2017,22,701 7of20
redoxprocess. ThiscomparativestudyshowsthattheCA-AgNPs/MWCNT-modifiedglassycarbon
electrodedemonstratedfasterchargetransportbehaviourwhichisevidencedintheobservedanodic
currentresponse. TheanodiccurrentdensityofeachmodifiedelectrodeinFe(CN) ]4−/[Fe(CN) ]3−
6 6
redox probe follows the order: CA-AgNPs/MWCNT (550 mA/cm2) > MWCNT (270 mA/cm2)
>GCE(80mA/cm2)>CA-AgNPs(7.93mA/cm2). Thereforetheglassycarbonelectrodemodified
Molecules 2017, 22, 701 7 of 20
withCA-AgNPs/MWCNTremainsthebestelectrodeowingtotheelectrontransportpropertieswith
currentdensityhigherthanthatofothermodifiedelectrodes. Thestudyfurthershowsthedistribution
electron transport properties with current density higher than that of other modified electrodes. The
ofCA-AgNPsinthecarbonnanotubesasshowninFigure3d.
study further shows the distribution of CA-AgNPs in the carbon nanotubes as shown in Figure 3d.
1.5
(a)
1.0
0.5
A
m 0.0
I /
-0.5
-1.0 GEC
GCE/MWCNT/CA-Ag
GCE/CA-Ag
-1.5 GCE/MWCNT
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
E/V (Ag/AgCl Vs sat'd KCl)

FFiigguurree 55.. CCyycclliicc VVooltlatammmmooggrraammsso offG GCCEE//CCAA--AAgg ((ggrreeeenn)),, bbaarree GGCCEE (b(blalacckk)),,G GCCEE//MMWWCCNNTT ((bblluuee)),,
aanndd GGCCEE//CCAA--AAgg//MMWWCCNNTT ((rreedd)) inin5 5mmMMF eF(eC(CNN)6)]64]−4−//[[FFee((CCNN))66]]33− −sosloulutitoinon pprreeppaarreedd iinn 00..11MM PPBBSS aatt
ssccaann rraattee 2255 mmVVss−−11. .
TThhee ssyynneerrggyy bebtewtweeene nMWMCWNCTN aTnda CndA-CAAgN-APgs NnaPnsocnoamnopcoosmitep ions eitnehainnceinngh tahnec einlegctrthone terlaencstproonrt
tpraroncsepsosr tobptraoicneesds aotb ttahien eGdCaEt mthoedGifCieEd meleocdtirfioeddese lies cwtroeldl epsriosnwouenllcepdro. nTohuen ccoendd.uTchteivceo nnadtuucrtei voef
nmautultriewoafllmedu lctiawrbaollne dncaanrobtounben athnoutsu ibmepthrouvseism tphreo vpersoptheertpierso poef rgtireeseonf pglraenetn epxltarnatcte-xmtreadcita-mteedd siailtveedr
snilavneorpnaarntiocpleasr taicnldes tahned itohneicio ninicteirnatcetriaocnt iobnetbweetwene etnheth eCACA-A-AgNgNPsP snnaannoococommppoosistiete aanndd MMWWCCNNTTss
wwhhiicchh rreessuullttss iinns siiggnniifificcaanntte eleleccttrroonnt rtraannssppoorrtta attt htheeG GCCEE//CCAA--AAggNNPPss//MMWWCCNNTT--mmooddiiffiieedd eelleeccttrrooddeess..
SSiimmiillaarrllyy,, tthhee llaarrggee ssuurrffaaccee aarreeaa ccrreeaatteedd bbyy tthhee ppoorroouuss MMWWCCNNTT oonn tthhee eelleeccttrrooddee ffoorr ffrreeee flflooww ooff
eelleeccttrroollyytteess aanndd cchhaarrggeess bbeettwweeeenn tthhee bbaassee eelleeccttrrooddee aanndd tthhee eelleeccttrrooaaccttiivvee ssppeecciieess aatt tthhee eelleeccttrrooddee
ssuurrffaaccee iiss aalslsoo aannooththeerri mimppoorrtatannttf afactcotorrf oforrt hteheb ebtetettrerp epreforfromrmanacneceo foGf CGEC/EC/CAA-A-AggNNPPs/s/MMWWCCNNTT
eelleeccttrrooddeess.. TThhee ppeeaakk-t-oto-p-peaeka kppotoetnetniatil aslespeapraatriaotnio (nΔE(∆pE) pfo)rf tohret hCeAC-AAg-NAPgsN/MPsW/MCNWTC eNleTctreoledcetsr oisd ≥e s10is0
≥mV10, 0wmhiVch, wish gicrehaitserg trheaante trhteh athneothreetitchael o5r9e.t8i cmalV5 9e.x8pmecVtede xfpore cat efdastf oornae-fealestctoronne -terlaencstrpoonrt.t rAanlssop, othrte.
Aralstioo,st hoef trhaeti oasnoodfitch etoa tnhoed cicatthootdhiec cpaetahko dcuicrrpeenatk recusprroennster (eIsppao/Inpsce) (fIopra C/IAp-cA)gfoNrPCsA/M-AWgNCNPsT/ MeleWctCroNdTe
ealreec tarpopdreoaxriemaapteplryo xuinmitayt,e ilnyduicnaittyiv,ein odfi caa rteivveerosfibalere evleercstirbolceheelmecitcraolc phreomceicsasl. process.
FFuurrtthheerrmmoorree,, tthhee eeffffeecctt ooff cchhaannggeess iinn ssccaann rraatteef rfroomm2 255––550000m mVVss−−11 oonn tthhee aannooddiicc ccuurrrreenntt ((IIppaa))
aanndd ccaatthhooddiicc ccuurrrreenntt ((IIppcc)) aattt thheem mooddifiifeieddC CAA-A-AggNNPPss//MMWWCCNNTT eelleeccttrrooddee wwaass ssttuuddiieedd ffoorr tthhee ssaammee
pprroobbee uussininggc yccylcilcicv ovlotaltmammmeterytryex epxepriemriemnetsn.tsB.o Bthotthh ethane oadniocdciucr rceunrtre(Inpta )(Iapnad) caantdh ocdaitchcoudrirce ncutr(rIpenc)t
i(nIpcrce)a isnecdrewasiethd iwncitrhe aisnecrineasscea inn rsactaens arastsehs oaws nshionwFnig iunr eFi6gau.rTeh 6eap. lTohteo pflaonto odfi canaonddicc aatnhdo dciacthpoedaikc
cpuerarken ctu(Irprean)tv a(Irpieas) lvinaeraiersly liwnietahrltyh ewinitchr etahsee iinncsrceaanser aitne ss(c2a5n– 5r0a0tems V(2s5−–15,0F0i gmurVes6−1b, ),Fwigiuthrer e6gbr)e, swsioitnh
vraeglureesssoiofnR v2a=lu0e.s9 9o8f aRn2 d= 00..999958 faonrda 0n.o9d95ic faonr dancoadthico daincdp cloattshordesicp epcltoivtse lrye,spcoenctfiivrmelyin, gcoandfiirfmfuisnigo na
cdoinffturosilolend cpornotcreosllse.d process.
IInn aaddddiittiioonn,, tthhee ppeeaakk ppootteennttiiaall mmoovveedd ttoo mmoorree ppoossiittiivvee vvaalluueess aass tthhee ssccaann rraattee iinnccrreeaasseedd.. TThhiiss iiss
sshhoowwnn iinn tthhee lliinneeaarr ccoorrrreellaattiioonn ooff tthhee ggrraapphh ooff ppeeaakk ppootteennttiiaall ((EEpp)) aaggaaiinnsstt tthhee llooggaarriitthhmm ooff ssccaann rraattee
((lloogg vv)).. TThhee ssllooppee ooff EEpp vvss.. lloogg vν ffoorr tthhee mmooddiififieedd eelleeccttrrooddee CCAA--AAggNNPPss//MMWWCCNNTT wwaass 00..1100 VV.. FFrroomm tthhee
ssllooppee oobbttaaiinneedd,, tthhee TTaaffeell vvaalluuee ((bb)) wwaass eessttiimmaatteedd bbyy uussiinngg tthhee eexxpprreessssiioonn pprreesseenntteedd inin EEqquuaatitoionn (1(1) )[4[499].] .
(cid:3029)
E ( ) log (v/mVs−1) + constant (1)
(cid:2900) (cid:2880)
(cid:2870)
From the equation, the value of b for this electrode was calculated as 200 mVdec−1. This value is
higher than the theoretical 118 mVdec−1 for a one-electron process involved in the rate-determining
step. Therefore the high Tafel values suggest adsorption of reactants or intermediates on the
electrode surfaces and/or reactions occurring within a porous electrode structure [50]. This process
at the modified CA-AgNPs/MWCNT electrode can be linked with the porous CNT layer [49].
Molecules2017,22,701 8of20
b
E = ( ) log(v/mVs−1)+constant (1)
P
2
Fromtheequation,thevalueofbforthiselectrodewascalculatedas200mVdec−1. Thisvalueis
higherthanthetheoretical118mVdec−1foraone-electronprocessinvolvedintherate-determining
step. ThereforethehighTafelvaluessuggestadsorptionofreactantsorintermediatesontheelectrode
surfaces and/or reactions occurring within a porous electrode structure [50]. This process at the
modifiedCA-AgNPs/MWCNTelectrodecanbelinkedwiththeporousCNTlayer[49].
Molecules 2017, 22, 701 8 of 20
3.0 (a)
1.5
A
m 0.0
I / 25 mVs-1
50 mVs-1
-1.5 100 mVs-1
200 mVs-1
300 mVs-1
-3.0 400 mVs-1
500 mVs-1
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
E/V (Ag/AgCl Vs sat'd KCl)

2 y = 3.628 X - 0.21
R2 = 0.998
1 (b)
0
A
m
I / -1
-2
y = -5.88X - 0.44
R2 = 0.995
-3
-4
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
V 1/2 (mV S-1) 1/2

FFiigguurree 66.. CCyycclliicc vvoollttaammmmooggrraammss oobbttaaiinneedd ffoorr ((aa))G GCCEE//CCAA--AAggNNPPss//MMWWCCNNTT iinn FFEECCNN ssoolluuttiioonn pprreeppaarreedd
iinn 00.1.1M MP PBBSS( s(csacnanra rtae:te2:5 2–55–0500m0 VmsV−s1−;1i;n innenretro toou oteurt)e;r()b; )(bL)i nLeianrepalro ptslootfsI poaf vIpsa. vVs1. /V21a/2n adnIdp cIpvcs v.sV. 1V/12/2 ffoorr
GGCCEE//CCAA--AAggNNPPss//MMWWCCNNTT inin 55 mmMM FFee(C(CNN)6)]64]−4/[−F/e[(FCeN(C)6N]3−) 6s]o3l−utsioolnu ptiroenpparreedpa irne d0.1in M0 .P1BMS.P BS.
22..66.. EElleeccttrroocchheemmiiccaall IImmppeeddaannccee SSppeeccttrroossccooppiicc ((EEIISS)) SSttuuddiieess
FFuurrtthheerr iinnvveessttiiggaattiioonn oonn tthhee eelleeccttrroonn ttrraannssppoorrtt pphheennoommeennoonn aatt tthhee eelleeccttrrooddee--eelleeccttrroollyyttee iinntteerrffaaccee
wwaass ccaarrrriieedd oouutt bbyy uussiinngg eelleeccttrroocchheemmiiccaall iimmppeeddaannccee ssppeeccttrroossccooppiicc ((EEIISS)) tteecchhnniiqquueess [[5511,,5522]] iinn 55 mmMM
[[FFee((CCNN))6]]44−−/[/F[eF(eC(NCN)6])3− ]3s−olusotilounti oant aa tfaixfiexde pdopteontetniatli aolfo 0f.02. 2VV. .TThhee NNyyqquuiisstt pplloottss oobbttaaiinneedd ffrroomm tthhee
6 6
iimmppeeddaannccee eexxppeerrimimeennt t(a(ta tfixfiexde dpoptoentetinatli aolf o0f.20 V.2 vVs. vAsg./AAgg/CAl, gsaCtl’d, sKatC’dl) KbeCtwl)ebeent w10e eKnH1z0 aKndH 0z.1a nHdz
are presented in Figure 7a for the glassy carbon modified electrode CA-AgNPs/MWCNT, CA-AgNPs,
0.1 Hz are presented in Figure 7a for the glassy carbon modified electrode CA-AgNPs/MWCNT,
MWCNT and bare GCE, respectively. The fitting of the impedance data was done by using circuit
CA-AgNPs, MWCNTandbareGCE,respectively. Thefittingoftheimpedancedatawasdoneby
model in the Nova 1.6 Software presented in Figures 7b(i–ii). In this circuit model, Rs is the solution
usingcircuitmodelintheNova1.6SoftwarepresentedinFigure7b(i,ii). Inthiscircuitmodel,Rsis
trheesisstoalnucteio, nCdrl erseipstraensecnet,sC ther depourebsleen latsyethr ecadpoaucbitlaenlcaey, eQr ocar pCaPcEit aisn tchee, QcoonrstCanPtE pihsatshee ecleomnsetnatn, tanpdh aRscet
dl
is the charge transfer resistance. The circuit RQ ([RC]) Figure 7b(i) was used for CA-AgNPs/MWCNT,
element, and Rct is the charge transfer resistance. The circuit RQ ([RC]) Figure 7b(i) was used for
MWCNT and CA-AgNPs, while R(Q[RW]) Figure 7b(ii) was used for the bare GCE electrode. The
CA-AgNPs/MWCNT,MWCNTandCA-AgNPs,whileR(Q[RW])Figure7b(ii)wasusedforthebare
impedance parameters obtained from the fitting of the raw impedance spectra with the circuits are
presented in Table 1. The electron transfer resistance Rct value recorded for the modified
CA-AgNPs/MWCNT electrode had better electron transport with lower Rct values compared with
the bare and CA-AgNps electrodes, as shown in Table 1, and these results agreed with that from
CV where the conductive properties of the MWCNT influence the increase in current response
recorded at the modified CA-AgNPs/MWCNT electrode.
Molecules2017,22,701 9of20
GCEelectrode.Theimpedanceparametersobtainedfromthefittingoftherawimpedancespectrawith
thecircuitsarepresentedinTable1.TheelectrontransferresistanceRctvaluerecordedforthemodified
CA-AgNPs/MWCNTelectrodehadbetterelectrontransportwithlowerRctvaluescomparedwith
thebareandCA-AgNpselectrodes,asshowninTable1,andtheseresultsagreedwiththatfromCV
wheretheconductivepropertiesoftheMWCNTinfluencetheincreaseincurrentresponserecordedat
Molecules 2017, 22, 701 9 of 20
themodifiedCA-AgNPs/MWCNTelectrode.
600 (a)
450
Ω z ' / 300
GCE/MWCNT
150 GCE/CA-Ag
GCE
GCE/CA-Ag/MWCNT
0
0 400 800 1200 1600 2000
z '' / Ω
(bi)
(bii)
(b)
FigureF7ig.uTrey p7i.c TalypNicyaql uNisytqupilsot tspl(oat)s o(ab)t aoibnteaidnefdo rfothr etheel eecletrcotrdoedsesi nin5 5m mMM FFee((CCNN))6]]44−−/[/F[eF(Ce(NC)N6]3)− ]3−
solution prepared in 0.1 M PBS (pH 7) at a fixed potential of 0.2 V (vs. Ag|AgCl,6 saturated KCl6)
solutionpreparedin0.1MPBS(pH7)atafixedpotentialof0.2V(vs. Ag|AgCl, saturatedKCl)
(b i,ii) represent the circuits used in the fitting of the EIS data.
(b(i,ii))representthecircuitsusedinthefittingoftheEISdata.
Table 1. Impedance data obtained for the bare GCE and the modified electrodes in 5 mM
Table F1e.(CNIm)6]p4−e/d[Faen(CceN)d6]a3−t saoloubtitoani naet d0.2f oVr (vtsh. eAgb|aAregCGl sCaEturaantedd KthCel). mAoll dviafilueeds weleercet roobdtaeisneidn fr5omm M
Fe(CNt)h6e]4 f−it/te[dF eim(CpNed)a6n]3c−e sspoelcutrtaio. Tnhaet v0a.l2ueVs i(nv sp.arAengt|hAesgesC alrsea ptuerrcaetnetd erKroCrsl) o.fA dlaltav afiltutiensg.w ereobtained
fromthefittedimpedancespectra.Thevaluesinparenthesesarepercenterrorsofdatafitting.
Electrochemical Impedance Spectroscopy Data
Electrodes
R (Ω) Q (×106 nF) R (Ω) W × 102
EslectrochemicalImpedanceScptectroscopyData
ElectrodGesCE 116.10 (1.04) 4.53 (4.05) 1572.00 (1.14) 0.14 (10.97)
Rs(ΩE)lectrocQhe(m×i1c0a6l InmF)pedanceR Sctp(eΩct)roscopy DWat×a 102
Electrodes
GCE 116.10(R1.0(4Ω)) 4Q.5 3(×(140.065 n)F) 157R2.0(0Ω()1.14) C0.14 (u(1F0). 97)
s ct dl
GCE/MWCNT 82.E30le (c0t.r5o9c)h em0i.c3a6l (I3m.3p3e) dan1c5e.0S0p (e2c7t.r0o5s)c op3y02D.1a0ta (23.24)
Electrodes
GCE/CA-AgNPs Rs72(Ω.20) (0.50) Q(0×.6160 (62.n5F2)) 76.R50c t(4(Ω.36)) 399.C00d l(4(u.6F5))
GCE/CA-AgNPs/MWCNT 76.10 (2.37) 0.21 (5.46) 74.70 (11.74) 100.30 (9.79)
GCE/MWCNT 82.30(0.59) 0.36(3.33) 15.00(27.05) 302.10(23.24)
GCE/CA-AgNPs 72.20(0.50) 0.66(2.52) 76.50(4.36) 399.00(4.65)
The Bodes plots (Figure 8a,b) show the capacitive (charge storage) or resistance to charge
GCE/CA-AgNPs/MWCNT 76.10(2.37) 0.21(5.46) 74.70(11.74) 100.30(9.79)
transfer behaviour of some of the modified electrodes. The CA-AgNPs/MWCNT, CA-Ag and
MWCNT-modified electrodes have lower phase angles, 4.3°, 4.6° and 1.3° respectively compared
ThweitBho tdhee sbparloe tGsC(FEi g(5u0r.e4°8)a. ,Tbh)es hreoswulttsh aelscoa paagcreiteidv ew(icthh atrhgee Rsctto vraagluee)so wrhreicshis itmanpcley tao fcahstaerrg cehtarragne sfer
behavitoruanrsoffers opmroeceosfst hate tmheo dCiAfi-eAdgeNlePcst/rModWesC.NTTh-emCoAdi-fAiegdN gPlass/sMy cWarCboNnT e,lCecAtr-oAdge.a TnhduMs, WthCe NloTw-emr othdeif ied
electropdheasseh aavneglelo, wtheer lopwhears ethae ncgalpeasc,it4iv.3e◦ b,e4h.6av◦ioaunrd a1n.d3 ◦three shpigehcetirv tehley ccoonmdupcatirveed owr icthhartghee trbaanrsefeGr CE
properties of the sensor [53].
Molecules2017,22,701 10of20
(50.4◦).TheresultsalsoagreedwiththeRctvalueswhichimplyafasterchargetransferprocessatthe
CA-AgNPs/MWCNT-modifiedglassycarbonelectrode. Thus,thelowerthephaseangle,thelowerthe
Molecules 2017, 22, 701 10 of 20
capacitivebehaviourandthehighertheconductiveorchargetransferpropertiesofthesensor[53].
70
GCE/MWCNT/CA-Ag
GCE/CA-Ag
60 GCE (a)
GCE/MWCNT
50
e
e
gr
e 40
d
e /
gl 30
n
a
se 20
a
h
P
- 10
0
0 1 2 3 4
log (f/Hz)
GCE/MWCNT/CA-Ag
3.2 GCE/CA-Ag
(b) GCE
GCE/MWCNT
2.8
Ω|
Z /
g |
lo 2.4
2.0
0 1 2 3 4
log (f/Hz))

Figure8.Bodeplotsobtainedforsomeoftheelectrodes,showing(a)theplotsofphaseangle/deg.vs.
loFgig(fu/rHe z8).) Baondde (pbl)otths eopbltoaitnoefdl ofogr| sZo/mΩe |ofv tsh.el oegle(cft/roHdze)s., showing (a) the plots of phase angle/deg.
vs. log (f/Hz)) and (b) the plot of log |Z/Ω| vs. log (f/Hz).
Thechargetransferresistance(Rct)valuescalculatedafterfittingtheEISdatatotheRandle's
The charge transfer resistance (Rct) values calculated after fitting the EIS data to the Randle's
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in the past few decades has become a serious healthcare problem. A growing number of
immuno-compromised patients (with immune responses attenuated by the administration of
immunosuppressive drugs) as a result of cancer chemotherapy, organ transplantation and HIV
infection are the major factors contributing to this increase [55]. Hence there will always be a vital