ContentslistsavailableatScienceDirect
Sensors
and
Actuators
B:
Chemical
j ou rn a l h o m ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / s n b
Highly
sensitive
volatile
organic
compounds
vapour
measurements
using
a
long
period
grating
optical
fibre
sensor
coated
with
metal
organic
framework
ZIF-8
Jiri
Hromadka
a,b,
Begum
Tokay
c,
Ricardo
Correia
a,
Stephen
P.
Morgan
a,
Sergiy
Korposh
a,∗aOpticsandPhotonicsGroup,FacultyofEngineering,UniversityofNottingham,UniversityPark,NottinghamNG72RD,UnitedKingdom bInstituteforEnvironmentalStudies,FacultyofSciences,CharlesUniversityinPrague,Benátská2,CZ12843Praha2,CzechRepublic
cChemicalandEnvironmentalEngineeringDepartment,FacultyofEngineering,UniversityofNottingham,UniversityPark,NottinghamNG72RD,United
Kingdom
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received10July2017 Receivedinrevisedform 29December2017 Accepted2January2018 Availableonline3January2018
Keywords:
Longperiodgrating(LPG) Metalorganicframework(MOF) ZIF-8
Volatileorganiccompounds(VOCs)sensor Ethanol
Methanol Acetone
a
b
s
t
r
a
c
t
Anopticalfibrelongperiodgrating(LPG)basedvolatileorganiccompound(VOC)sensorcoatedwith ZIF-8,amaterialfromthezeoliteimidazolateframework(ZIF)family,functionalcoatingispresented. ZIF-8filmwasdepositedontothesurfaceoftheLPGusinganin-situcrystallizationtechniquebymixing freshlyprepared12.5mMzincnitratehexahydrateand25mM2-metyl-imidazolesolutionsinmethanol. Aconcentrationspecificresponsetoacetone,ethanolandmethanolvapourwasobtainedoverthehigh concentrationrangeuptotensthousandsppm.TheLPGbasedsensorworksatphase-matchingturning point(PMTP)andsuchoperatesatthehighestsensitivity.Anovelapproachforthedataanalysisbased onthemeasurementofthebandwidthoftheU-shapedattenuationbandoftheLPGisintroduced.The optimizedsensorshowsasensitivityof0.015±0.001and0.018±0.0015nm/ppmandlimitofdetection (LOD)of6.67and5.56ppmforacetoneandethanolrespectively.
©2018TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).
1. Introduction
Thefabrication oflow-cost,portable, accurateand real-time sensorsfororganicvapourdetectionisofconsiderableinterestin biomedicalfieldsandinfoodqualitymonitoring[1].Ethanoland acetonerepresentthemostcommonexamplesofvolatileorganic compounds(VOCs).Themeasurementofethanolinbreathisofhigh interestintrafficsafety[2],wheretheacceptedalcohollimitsin bloodvaryacrossEuropefrom0.2to0.8‰,whichcorrespondstoa concentrationof30–130ppminbreath[2].Thedetectionofacetone inbreathisimportantinmedicaldiagnostics,wheretheincreaseof acetoneconcentration(singleppmunits)canberelatedtodiabetes [3] and highconcentration(tens-hundredsppm)is observed as symptomofketoacidosis[4].VOCsareusuallymeasuredusinggas chromatography–massspectroscopy(GC–MS),however,the detec-tionisexpensiveandneedsexperiencedpersonnel[5].Currently,
∗Correspondingauthor.
E-mailaddress:[email protected](S.Korposh).
mostethanol sensorsarebasedonmetaloxidesemiconductors suchasSnO2[6],WO3[7],In2O3[8],Fe2O3[9]andZnO[10].
Semi-conductingmaterialbasedsensorsexhibithighsensitivitywhilst theyusuallyoperateathightemperatures(200–400◦C)toachieve chemicalreactivitybetweenthesensingmaterialandthetarget gases.Alsothefabricationof thesensitivelayershouldbe con-ductedathightemperatureandthusisassociatedwithhighenergy consumption[11,12].
Fibre-opticsensingplatformsaresmall,lightweight,immuneto electromagneticinterferenceandassuchcanbeusedinextreme conditions,enablingremoterealtimemonitoringwithnoelectrical powerneededatthesensingpoint[13].Therehavebeenanumber offibre-opticbasedethanolgassensorsproposedrecentlyusingthe depositionofthesensitivelayeronthefibretiptoformFabry-Perot interferometers[14–18]orevanescentwavesensors,wherepartof thecladdingismechanicallyorchemicallyremovedandthenthe functionalcoatingisdepositedoverthestrippedregion[11,19–22]. Acoatingsensitivetoethanolvapourshasbeenalsoappliedonthe surfaceofaphotoniccrystalfibre[23],longperiodgrating(LPG) [24]orquartzcrystalmicrobalance(QCM).Thepropertiesof
pub-https://doi.org/10.1016/j.snb.2018.01.015
Table1
Summaryofethanolgasfibreopticsensors(LOD−limitofdetection).
Sensortype Sensorprinciple Coating Testedrange Sensitivity Resp.time LOD Ref.
Fabry–Perotinterferometer (FPI)–filmonthefibretip
offibre-filmreflectance Singlewallcarbonnanotubes (SWCNT)
63and73ppm 0.9×10−4/ppm ∼3min* n/a [14]
FPI–filmonthefibretip offibre-filmreflectance SWCNT+cadmiumarachidate 63and73ppm 5×10−4/ppm ∼3min* n/a [14]
FPI–filmonthefibretip influorescence;twosensors tested
vapochromiccomplex [Au2Ag2(C6F5)4(C6H5N)2]
1747ppm(one valueonly)
1dBand3dBper 1747ppm
36s/∼5min n/a [15]
FPI–filmonthefibretip offibre-filmreflectance vapochromiccomplex–2,2
-bipyridine+Liquicoat®
25–127ppm 0.0242dB/ppm 20to30min 2.74ppm [16]
Extrinsicsensor influorescenceof nicotinamideadenine dinucleotide(NADH)
alcoholdehydrogenase(ADH) 0.30–300ppm 17.45a.u./ppm n/a n/a [25]
Longperiodgrating(LPG); periodof407m
inintensityofthe attenuationband
ZnOnanorods ∼50Torr (saturatedEtOH pressure)
∼4.4dB ∼80min n/a [24]
Claddingremoved–poly-methyl methacrylate(PMMA)optical fibre
inintensityat678nm SWCNT 12.5–500ppm 0.26a.u./ppm ∼90min n/a [19]
FPI–filmonthefibretip inluminescencefromthe QDsat750nm
fluorescentsiliconquantum dots(Si-QDs)
533–1305ppmin dryO2
atmosphere
n/a ∼1min ∼380ppm [17]
CladdingremovedPMMA opticalfibre
inintensityatselected wavelengths
Multi-wallcarbonnanotubes (MWCN)
50–500ppm 0.52a.u./ppm (saturationat
∼350ppm)
n/a n/a [11]
CladdingremovedPMMA opticalfibre
inintensityatselected wavelengths
Sm2O3 0to500ppm 76×10−3au/ppm 78min n/a [20]
CladdingremovedPMMA opticalfibre
inintensityatselected wavelengths
Grapheneoxide 100to500ppm 0.26a.u./ppm n/a n/a [21]
Claddingremovedsilicaoptical fibre
inpoweratselected wavelength
SnO2 1000to
20,000ppm
∼0.02W/1000ppm (Saturatedat 15,000ppm)
10–18s n/a [22]
FPI–filmonthefibretip –PMMAfibre
inintensityatselected wavelengths
ZnOnanorods Saturatedvapour 10.53%in reflectedintensity
n/a n/a [18]
Quartzcrystalmicrobalance (QCM)
frequency organicpolymer(pen-tacene) 3074–15374ppm ∼0.0064Hz/ppm ∼30s n/a [26]
Photoniccrystal Braggdiffractionpeak Poly-acrylatetype EtOH/waterp/p0
(0.2-1)
Maxshiftof 59nm/pureEtOH gas
n/a n/a [23]
lishedethanolgasfibreopticbasedsensors(andoneexampleofa QCM)aresummarizedinTable1.Reportedsensorssuffermostly fromahighresponsetimeinarangeof10smin[19,24]andare usu-allytestedoveralimitedconcentrationrange.Forexample,they aretestedatsaturatedvapourconditions[18,24],high(inarange ofthousandsofppm)[22,23,26]orlowlevelconcentrations usu-allybelow500ppm[11,14,19–21].Thecomparisoninsensitivity showninTable1islimitedasonlytwopapersshowthelimitof detectionof2.74[16]and380ppm[17].
Amongthedifferenttypesoffibre-opticsensors,thosebased ongratings,specificallyLPGs,havebeenemployedextensivelyfor refractiveindex measurements [27] and for monitoring associ-atedchemicalprocesses[27],sincetheyofferwavelength-encoded information which overcomesthe referencing issues associated withintensitybasedapproaches.ALPGconsistsofaperiodic per-turbationoftherefractiveindexofthefibrecore,whichcouplesthe coremodetotheco-propagatingcladdingmodesofthefibre.This couplingismanifestedinthetransmissionspectrumoftheoptical fibreasaseriesofresonancebands.Eachresonanceband corre-spondstocouplingtoadifferentcladdingmodeandthusshows differentsensitivitytoenvironmentalchanges[28].Thecoupling wavelengthcanbeobtainedfromthefollowingphasematching equation
x=(ncore−nclad(x)) (1)
where x represents thewavelength at which light is coupled
totheLP0xcladdingmode,ncore is theeffectiverefractiveindex
ofthe modepropagating in thecoreof thefibre, nclad(x) is the
effectiveindex of theLP0x cladding modeand is the period
oftheLPG[28].Thecentralwavelengthoftheresonancebandis sensitivetochangesinthesurroundingenvironmentsuchas tem-peratureorexternalindexofrefraction[28].It hasbeenshown thatthephasematchingconditionforthehigherordercladding modes contains a turning point and that the LPG exhibits the
highestsensitivitywhenphasematchingturningpoint(PMTP)is reached.AtPMTPtheU-shapedattenuationbandgrowthlarger (i.e.transmissiondecreases)andthensplitsintotwobandsthat moveintooppositedirections.Measurementsofthewavelengths separationorwidthoftheband,aswillbeshowninthis work, allowstoachievehighersensitivity.Thiscanbeachievedby choos-inganappropriateLPGperiodandcoatingthickness[29,30].The claddingmodesassociatedwiththeattenuationbandshavebeen calculatedthroughnumericalmodellingusingthephase match-ingequationandweaklyguidedapproximationtodeterminethe effectiverefractiveindicesofthecoreandcladdingmodesandthe knownvaluesofthecentralwavelengthsandthegratingperiod [31].
filmfabricationinvolvesnosubstratemodification,worksatroom temperatureand facilitatescontrol overthethicknesswithone growthcyclethattakeslessthananhour[37,38].Inadditionto beingsensitivetoorganicvapours[36,37],ZIF-8hasbeenselected overotherMOFs (andZIFs)due tosimplicityin fabricationand depositionofthethinfilm.TheprocedureforZIF-8filmfabrication involvesnosubstratemodification[37],worksatroom tempera-tureandfacilitatescontroloverthethicknesswithonegrowthcycle thattakeslessthananhour.Inaddition,sinceZIF-8ishydrophobic no significantresponse to therelative humidity (even at rela-tivehumiditylargerthan70%)isobserved[39].Wehaverecently demonstratedforthefirsttime,tothebestofourknowledge,proof ofconceptdataforthedetectionVOCsusingaLPGcoatedwith ZIF-8[39].ALPGbasedsensorcoatedwithaZIF-8thinfilmexposedto methanol,ethanolandacetonevapoursdemonstratesahigh sen-sitivitytoallanalytestested,suggestingsensitivitytowardslow chainVOCsduetothesmallporeapertures(3.4Å[35]).
However,beforesystemsbasedonthetechnologycanbewidely deployed,itisessentialthatthesensoroperatesunderoptimized conditionsintermsofbothsensitivityandsignalextraction.The researchdescribedhereaddressesthesechallenges.The sensitiv-ityofthesensorisimprovedsignificantlybyadjustingthegrating periodinsuchawaythatthesensoroperatespreciselyatthePMTP. Thisoperationalsofacilitatesthedevelopmentofanovelsignal processingmethodappliedtothemeasuredLPGspectrum.Higher sensitivityisobtainedwhen thesensoroperates inthe proxim-ityofthePMTPassociatedwiththeoccurrenceoftwoattenuation bandsin thespectrum, bothof themassociated withthesame claddingmode.TherelationshipbetweentheLPGspectrumand analyteconcentrationisusuallytrackedviathechangeinthe cen-tralwavelengthassociated withdistinctcladdingmodes. When twoattenuationbandsarepresent,theshiftoftheindividualbands canbetrackedorthedifferencebetweenthecentralwavelengths relatedtothoseattenuationbandscanbeusedfortheevaluation ofthesensorperformance.Thelatteronegivesthehigher sensi-tivity[39].WhenthesensorisoperatingpreciselyatthePMTPa U-shapedbandispresent.Inthiscasechangesinthetransmission atthecentralwavelengthcanbeusedforthemeasurementof tar-getanalytes[42].Anovelalternativeproposedhereisthatchanges inthebandwidthoftheU-shapedbandcanalsoberelatedtothe targetanalyte.Thisapproachiscomparedwithalternativesbased onmonitoringtheintensityandcentralwavelengthchanges.
Thenextsectionpresentsthemethodofmanufactureof ZIF-8filmsonLPGswith2differentperiods,oneintheproximityof andonepreciselyatthePMTP;thetestmethodologyforthe sen-sors;and thenovelsignalprocessingmethodproposed.Section
3showsresultsdemonstratingtheperformanceoverawide con-centrationrangefordifferentVOCs.DiscussionsandConclusions followinSections4and5respectively.
2. Methodology
2.1. Materials
Zinc nitrate hexahydrate, 2-metyl-imidazole, methanol, ethanol, 2-propanol and acetone were purchased from Sigma-Aldrich.Allofthechemicalswereanalyticalgradereagentsand usedwithoutfurtherpurification.
2.2. Longperiodgratingfabrication
LPGswithgratingperiodsof109.0and109.5mandlengthof 30mm(furtherreferredtoasSensorAandSensorB,respectively) were fabricated in photosensitive boron–germanium co-doped optical fibre (Fibercore PS750) using custom made amplitude
Fig.1. TransmissionspectrumofaLPGwithperiodofa)109.0m–SensorAand b)109.5m–SensorBmeasuredinair:uncoated(black)andcoatedwith5growth cyclesofZIF-8(blue),GC,growthcycle.SensorAisclosetothePMTPandSensorB preciselyatthePMTP.(Forinterpretationofthereferencestocolourinthisfigure legend,thereaderisreferredtothewebversionofthisarticle.)
2.3. Sensorfunctionalization
TheLPGwascoatedwithZIF-8followingtheprocessdescribed in[37,38]anddemonstratedonthesurfaceofanopticalfibreLPG inourpreviouswork[39].Briefly,theLPGwasfixedinacustom designedholdertokeeptheLPGtautandstraight.TheLPGwas placedinsideaPetridishandcoveredbythefilmformingsolution (15mlof12.5mMzincnitratehexahydrateand15mlof25mM 2-metyl-imidazoleinmethanol)foraperiodof30min.Thenthe LPGwaswashedbymethanolanddriedundernitrogenflow.The processwasrepeatedtoobtainthickerfilmsconsistingof5growth cycles.ThetransmissionspectrumoftheLPGwasmonitored dur-ingeachdepositionstep.Therewasnoindicationthatthefilmmay beremovedduetospeedoftheimmersionascontinuous build-ingofthefilmwasconfirmedviathechangesinthetransmission spectrum.
2.4. Organicvapourtesting
TheresponseoftheLPGtomethanol,ethanol,2-propanoland acetonewas investigated withthe fibrepositionedin an envi-ronmentalchambercomposedofaclosedpolytetrafluoroethylene (PTFE)box(15×15×15cm witha totalvolume of3.375l).The LPGwasfixed5cmabovethebase.Thechemicalofinterest (vol-umeof10and 50lor0.5and 2lforexperimentsconducted withSensorAandSensorB,respectively)wasinjectedfromthe topoftheboxbyapipetteandthetransmissionspectrumwas monitoredandrecorded.Theconcentrationoftheanalytewas cal-culatedaccordingtotheamountofthesolutioninjectedandthe volumeofthebox.SensorA wasexposed tomethanol,ethanol andacetoneconcentrationsrangingfrom1790to27900,1240to 24800and987to19700ppm,respectively.SensorBoperatesmore closelytothePMTPandthushighersensitivitywasexpected.This hypothesiswastestedbyexposingSensorBtolower concentra-tionof acetoneand ethanol vapoursin a range of 49–543 and 62–666ppm,respectively.Thetemperatureandrelativehumidity levelsweresimultaneouslyrecordedusingiButton® commercial datalogger(iButton® HygrochronTemperature/HumidityLogger DS1923,MaximIntegratedTM).Thetemperaturelevelwasstable
duringtheexperimentswithminorfluctuationsupto0.5◦C.All experimentshavebeenconductedinatemperatureandhumidity controlledlaboratory.
2.5. Signalprocessing
AsdescribedinSection1,threemethodsfortheevaluationof thechanges intheLPGtransmissionspectrumarecompared in thispaper:viai)changesinthecentralwavelengthdifference,ii) changesinthetransmissionatthecentralwavelengthofthe U-shapedattenuationbandandiii)changesinthebandwidthofthe U-shapedattenuationband.
LODshavebeencalculatedusingthefollowingequation
LOD=Sd
m (2)
whereLODindicateslimitofdetection,Sdistheaveragestandard deviationobtainedoverthevaluesmeasuredinstableconditions (ateachconcentrationlevel)overthe2minperiodandmisthe slopeofthecalibrationcurve(alinearapproximationwasusedfor therangeupto10,000ppm).
Fig.2.SensorA:calibrationcurvesformethanol(black),ethanol(blue)andacetone (red).Errorbarsrepresentthestandarddeviationofthevaluestakenoverthethree separateexperimentsb)showstheinsetofa)wherethedataupto10,000ppmare shownforclarity.(Forinterpretationofthereferencestocolourinthisfigurelegend, thereaderisreferredtothewebversionofthisarticle.)
3. Results
3.1. Sensorfabrication
Fig.1ashowsthetransmissionspectraofabareLPGandaLPG coatedwith5layersofZIF-8foragratingperiodof109.0m (Sen-sorA).The“U”shapedbandoftheLP019claddingmodesplitsinto
twoattenuationbandsafterthedepositionofthefilm.Fig.1bshows agratingperiodof109.5m,wherenoattenuationband corre-spondingtotheLP019claddingmodeisobservedbeforethecoating
processandwhichisdevelopedonlyafterthedepositionofZIF-8 (SensorB).AsdiscussedinSection2.2,thisispreciselyatthePMTP andislikelytoresultinthehighestsensitivity.
3.2. SensorA:volatileorganiccompoundssensing
to5000ppm,thesensorshowsthehighestsensitivitytoethanol, inducingacentralwavelengthshiftof12.34±1.97nmat4963ppm. Highersensitivitytoalltestedanalyteswasobservedin com-parisontotheresultspresentedin[39]measuredusingthecentral wavelengthsdifferencecorrespondingtotheLPGcoatedwith5 growthcyclesofZIF-8.Forexample,thechangeinthecentral wave-lengthdifferenceinducedby∼9000ppmofmethanolvapourwas measuredtobe8.37nmincomparisonto2.17nmobtainedinour preliminarywork[39].Thehighersensitivityofthecurrentsensors isduetothesmallerseparationoftheattenuationbandsinairas thesensoroperatesclosertothePMTP.
Thedifferencesinthesensitivitycanbeassociatedwiththesize ofthemolecule,wherethesmallermethanolmoleculeshavelower capturerateinthecavitiesatlowerconcentrations.Thefunctional groupcanalsoplayaroleandcanexplainthebiggerresponseof thesensortoethanolincomparisontoacetoneatlowlevelsupto 5000ppm.Theshapeofthecurveindicatesthesaturationofthe sensorathighconcentrationsover20,000ppm.Forsimplicity,a linearapproximationisassumedintherangeupto10,000ppm inordertocalculatetheLODusingEq.[2].LODsof164.4,421.7 and162.8ppmwereobtainedforacetone,methanolandethanol respectivelyusingthedatapresentedinFig.2.
3.3. SensorB:volatileorganiccompoundssensing
ThehighsensitivityofSensorAtoethanolandacetonevapours leadstothefurtherinvestigationofimprovingtheLODbyoperating theLPGpreciselyatthePMTPusingagratingperiodof109.5m.As describedinSection2.4,ashighersensitivitywasanticipated,lower concentrationsofethanolandacetone(62–666and49–543ppm, respectively)wereused.DuetothepresenceoftheU-shapedband (Fig.2b,blueline)itisnowpossibletotrackchangesinthespectrum duetoachangeinthetransmissionatthecentralwavelengthand alsothechangeinbandwidthoftheattenuationband.Duetothe absenceofthedualattenuationbands,itisnotpossibletousethe centralwavelengthdifferencemethodusedinSection3.2.
TheperformanceofSensorBhasbeenevaluatedviathechanges in transmission at the central wavelength. For instance, con-centration of 543ppm of acetone vapour induced a change of approximately2%intransmissionatthecentralwavelengthof Sen-sorB,Fig.S1a.Similarly,a concentrationof666ppmof ethanol caused approximately3% change in transmission,Fig. S1b.The responseof SensorBtoacetonevapoursin arange from49to 543ppmbasedonthechangesinthetransmissionatthecentral
wavelengthareshowninFig.3a.Similarly,theresponseof Sen-sorBtoethanolvapoursinrangefrom62to666ppmisshown inFig.2b.Thesevalueswereusedtocalculatethestandard the standarddeviationoftheintensitychanges10mVandforthis rea-sontheadequatechangeof0.1%intransmissionhasbeenused forthecalculationoftheLOD.LODsof25.64and21.28ppmhave beencalculatedforacetoneandethanolrespectivelyforSensorB basedonthechangeintransmissionatthecentralwavelengthof theattenuationbandcorrespondingtoLP019claddingmode.
Asdescribed in Section2.5,the changeinthebandwidthof theU-shapedbandrepresentsanovelapproachfortheevaluation ofLPGbasedsensors.Trackingthewidthoftheattenuationband overcomesanyintensitybasedissuesasthereisnoneedtotrack theabsoluteintensityatreferencewavelength.Theevolutionof thedifferencesinthewidthoftheattenuationband correspond-ingtoLP019claddingmodehasbeendemonstratedinresponseto
ethanolvapoursintherangeof62–666ppm,Fig.4a.Thebandwidth wasmeasuredatthewavelengthscorrespondingto85%of trans-mission,asshownindetailfortheleftsectionoftheattenuation band(intheregionof820nm)inFig.4b.Asanexample,the band-widthincreasedfrom54.68to66.27nminresponseto666ppmof ethanol,Fig.4.
The calibrationcurves (Fig.5)werecalculated based onthe changeofbandwidthviatheresultsofthreerepeated measure-ments. The slope of 0.018±0.0015nm/ppm was obtained for ethanol, Fig.5. Theerrorbar representsthe standard deviation ofthecalculatedslopevalue.Similarly,theresponseofSensorB hasbeenevaluatedforacetoneovertherangeof49–543ppmwith aslopeofwidthdifferenceof0.015±0.001nm/ppm.TheLODof thesensorhasbeencalculatedinrelationtoastandarddeviation of 0.1nmand following Eq.(2).LODof 5.56and6.67ppm was obtainedforethanolandacetone,respectively.Thesensor’s per-formanceanddynamicchangeintimecanalsobetrackedviathe shiftofthecentralwavelengthoftheLP018claddingmode.Asan
example,SensorBrespondedeventothelowestconcentrationof 61ppmofethanolandshowedafurthershiftofthecentral wave-lengthuptohighestconcentrationof666ppmofethanol,inducing theshiftofthecentralwavelengthof0.60nmat666ppm,Fig.S2.
4. Discussion
ThesensitivityofSensorAandSensor Busingdifferentdata evaluationtechniquesovertherangeofanalytesiscomparedin
Table2.Theobtainedsensitivityfortheethanolvapour
Table2
ComparisonoftheperformanceofSensorAandSensorBtowardsorganicvapoursacrossthedifferentevaluationtechniques.
Sensor Analyte Conc.range[ppm] Dataevaluationtechnique Sensitivity[nm/1000ppm] LOD[ppm]
SensorA MetOH 1790to27,900 CWshift 0.83±0.08 421.7
SensorA EtOH 1240to24,800 CWshift 2.58±0.28 162.8
SensorA Acetone 987to19,700 CWshift 1.46±0.06 164.4
SensorB EtOH 61to666 Bandwidth 18±1.5 5.56
SensorB EtOH 61to666 Transmission 4.5±0.2** 21.28
SensorB Acetone 49to543 Bandwidth 15±1 6.67
SensorB Acetone 49to543 Transmission 3.9±0.2** 25.64
*Thelinearresponseapproximationwasassumedforthesensitivitycalculation. **Theunitsaretransmissionpercentlossper1000ppm.
Fig.4.SensorB:a)Transmissionspectraandb)leftsectionoftheattenuationband correspondingtoLP019claddingmodeindetail,measuredinair(black),inresponse
to62(red),121(green),181(blue),424(cyan)and666ppmofethanol(magenta). (Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderis referredtothewebversionofthisarticle.)
mentfulfilsthecriteriaforethanollimitvaluesfordriversinthe rangeof0.2–0.8%inblood(thevaluesdifferacrossthecountries. Theconversionofthoselimitsintotheamountofethanolingas phasewillgivetherangeof30–130ppm[2].
Theobtainedsensitivityforthedetectionofacetoneissufficient tomonitoroccupationalexposureintheworkplace.TheHealthand SafetyExecutiveintheUKstatestheexposurelimitsof500and 1500ppmofacetoneasaweightedaverageover8hand15min respectively[43].Electrochemicalbasedsensorrepresentthe
com-petingdevelopingtechnologyinafieldofnewacetonegassensors, theLODsvaryfrom0.4to500ppm,however,theysufferfromhigh operatingtemperature(200–500◦C)andsometimesweretested onlyinN2atmosphere[44].Theconcentrationrangesfrom1.7ppm
to3.7ppmcouldbedetectedinbreathofpeoplewithdiagnosed diabeticconditionsincomparisonto0.8ppmmeasuredinbreath ofhealthypeople[3].Furthermore,theLPGbasedsensorswith cur-rentdesigncanbeusedformeasurementofacetoneinbreathfor themonitoringofketogenicdietandassociatedweightloss(range of2–40ppm)ordiabeticketoacidosis(75–1250ppm)[4].
TheobtainedLODinarangeofunitsofppmforethanoland acetonesuggeststhepotentialofthesensorforrealworld appli-cations.Furtherimprovementcouldbemadebyusinga higher resolutionspectrometerincombinationwithanewapproachof dataanalysis(includingthetransmission,centralwavelengthand widthchange).Thesubppmlevelwillbehighlydesiredinhealth monitoringapplications,e.g.breathanalysis.Thehighersensitivity couldbealsoachievedusingthethickerfilmandthishypothesis shouldbetestedin futurework.Althoughthere isaclearcross sensitivitytodifferentVOCsmanyofthetestscanbedesignedto reducethisproblem.Itisexpectedhoweverthatoneconcentration willbesignificantlyhigherthantheothersformanyapplications. Forexample,inbreathtestsforalcoholconsumption,ethanol lev-elswillbetwoordersofmagnitudehigherinbreath(whenthey reachthelevelsthatbreakthelaw)incomparisontotheotherVOCs presentinthebreath.Similarly,patientssufferingfrom ketoacido-sisareexpectedtohavemuchhigherconcentrationofacetonein theirbreathincomparisontootherVOCs.Theimplementationof thesensorintoanarraywillenablein-situmeasurement,e.g.for theoccupationalexposure,oranyindustrialmonitoring,wherea referencegratingisnecessarytosubtractanytemperatureeffect [45].Thecross-sensitivitytowatervapourinhighlyhumid envi-ronmentshouldalsobeincludedtotestthepossibilityofusingthis sensorinbreathanalysis[46].
5. Conclusions
Fig.5.SensorB:a)acetoneandb)ethanolcalibrationcurvebasedonthechangesinbandwidth(theerrorbarsrepresentedbythestandarddeviationofthevalueatthe stableconditionsateachconcentrationsteparesmallerthanthedatapoints).
Acknowledgment
This work was supported by the Engineering and Physi-cal Sciences Research Council [grant numbers EP/N026985/1, EP/N025725/1].
References
[1]E.Wongrat,N.Chanlek,C.Chueaiarrom,B.Samransuksamer,N.Hongsith,S. Choopun,LowtemperatureethanolresponseenhancementofZnO nanostructuressensordecoratedwithgoldnanoparticlesexposedtoUV illumination,Sens.ActuatorsAPhys.251(2016)188–197,http://dx.doi.org/ 10.1016/j.sna.2016.10.022([Internet]Availablefrom).
[2]G.Domènech-Gil,J.Samà,P.Pellegrino,S.Barth,I.Gràcia,C.Cané,etal.,Gas nanosensorsbasedonindividualindiumoxidenanostructures,ProcediaEng. 120(2015)795–798,http://dx.doi.org/10.1016/j.snb.2016.07.084([Internet] Availablefrom).
[3]C.Deng,J.Zhang,X.Yu,W.Zhang,X.Zhang,Determinationofacetonein humanbreathbygaschromatography-massspectrometryandsolid-phase microextractionwithon-fiberderivatization,J.Chromatogr.BAnal.Technol. Biomed.LifeSci.810(2)(2004)269–275.
[4]J.C.Anderson,Measuringbreathacetoneformonitoringfatloss:review, Obesity23(12)(2015)2327–2334.
[5]Federal-provincialadvisorycommitteonenvironmentalandoccupational health,in:IndoorAirQualityinOfficeBuildings:ATechnicalGuide,1995,pp. 1–57.
[6]M.Parthibavarman,B.Renganathan,D.Sastikumar,Developmentofhigh sensitivityethanolgassensorbasedonCo-dopedSnO2nanoparticlesby microwaveirradiationtechnique,Curr.Appl.Phys.13(7)(2013)1537–1544,
http://dx.doi.org/10.1016/j.cap.2013.05.016([Internet]Availablefrom). [7]M.Z.Ahmad,A.Z.Sadek,J.Z.Ou,M.H.Yaacob,K.Latham,W.Wlodarski,Facile
synthesisofnanostructuredWO3thinfilmsandtheircharacterizationfor ethanolsensing,Mater.Chem.Phys.141(2-3)(2013)912–919,http://dx.doi. org/10.1016/j.matchemphys.2013.06.022([Internet]Availablefrom). [8]H.Ko,S.Park,S.An,C.Lee,EnhancedethanolsensingpropertiesofTiO2/ZnO
core-shellnanorodsensors,Curr.Appl.Phys.13(2013)919–924,http://dx. doi.org/10.1016/j.cap.2013.01.020([Internet]Availablefrom).
[9]L.Wang,T.Fei,Z.Lou,T.Zhang,Three-dimensionalhierarchicalflowerlike
␣-Fe2O3nanostructures:synthesisandethanol-sensingproperties,Appl MaterInterfaces.(2011)4689–9694.
[10]E.Wongrat,P.Pimpang,S.Choopun,Comparativestudyofethanolsensor basedongoldnanoparticles:ZnOnanostructureandgold:ZnOnanostructure, Appl.Surf.Sci.256(4)(2009)968–971.
[11]L.R.Shobin,B.Renganathan,D.Sastikumar,K.C.Park,S.Manivannan,Pureand iso-butylmethylketonetreatedmulti-walledcarbonnanotubesforethanol andmethanolvaporsensing,IEEESens.J.14(4)(2014)1238–1243.
[12]M.NaderiNasrabadi,Y.Mortazavi,A.A.Khodadadi,Highlysensitiveand selectiveGd2O3-dopedSnO2ethanolsensorssynthesizedbyahigh temperatureandpressuresolvothermalmethodinamicroreactor,Sens. ActuatorsBChem.230(2016)130–139,http://dx.doi.org/10.1016/j.snb.2016. 02.045([Internet]Availablefrom).
[13]A.D.Kersey,Areviewofrecentdevelopmentsinfiberopticsensortechnology, Opt.FiberTechnol.2(1996)291–317([Internet]Availablefromhttp://www. sciencedirect.com/science/article/pii/S106852009690036X).
[14]A.Cusano,M.Consales,A.Crescitelli,M.Penza,P.Aversa,P.D.Veneri,etal., Chargetransfereffectsonthesensingpropertiesoffiberopticchemical nano-sensorsbasedonsingle-walledcarbonnanotubes,CarbonN.Y.47(3) (2009)782–788,http://dx.doi.org/10.1016/j.carbon.2008.11.014([Internet] Availablefrom).
[15]C.Elosúa,C.Bariáin,I.R.Matías,F.J.Arregui,A.Luquin,E.Vergara,etal., IndicatorimmobilizationonFabry-Perotnanocavitiestowardsdevelopment offiberopticsensors,Sens.ActuatorsBChem.130(1)(2008)158–163.
[16]S.C.Terrones,C.E.Aguado,C.Baria´ıin,A.S.Carretero,I.R.Mati´ıasMaestro,A.F. Gutie´ırrez,etal.,Volatile-organic-compoundopticfibersensorusinga gold-silvervapochromiccomplex,Opt.Eng.45(4)(2006)44401.
[17]Z.H.Zhang,R.Lockwood,J.G.C.Veinot,A.Meldrum,Detectionofethanoland watervaporwithsiliconquantumdotscoupledtoanopticalfiber,Sens. ActuatorsBChem.181(2013)523–528,http://dx.doi.org/10.1016/j.snb.2013. 01.070([Internet]Availablefrom).
[18]M.Q.Lokman,H.R.BinAbdulRahim,S.W.Harun,G.L.Hornyak,W.S. Mohammed,Lightbackscattering(e.g.reflectance)byZnOnanorodsontipsof plasticopticalfibreswithapplicationforhumidityandalcoholvapour sensing,Micro.NanoLett.11(12)(2016)832–836([Internet]Availablefrom
http://digital-library.theiet.org/content/journals/10.1049/mnl.2016.0321). [19]S.Manivannan,A.M.Saranya,B.Renganathan,D.Sastikumar,G.Gobi,K.C.
Park,Single-walledcarbonnanotubeswrappedpoly-methylmethacrylate fiberopticsensorforammonia,ethanolandmethanolvaporsatroom temperature,Sens.ActuatorsBChem.171–172(2012)634–638,http://dx.doi. org/10.1016/j.snb.2012.05.045([Internet]Availablefrom).
[20]B.Renganathan,D.Sastikumar,R.Srinivasan,A.R.Ganesan,Nanocrystalline samariumoxidecoatedfiberopticgassensor,Mater.Sci.Eng.BSolid-State Mater.Adv.Technol.186(1)(2014)122–127,http://dx.doi.org/10.1016/j. mseb.2014.03.018([Internet]Availablefrom).
[21]T.Kavinkumar,D.Sastikumar,S.Manivannan,Effectoffunctionalgroupson dielectric,opticalgassensingpropertiesofgrapheneoxideandreduced grapheneoxideatroomtemperature,RSCAdv.5(14)(2015)10816–10825 ([Internet]Availablefrom)http://xlink.rsc.org/?DOI=C4RA12766H. [22]S.Sharifpour-Boushehri,S.M.Hosseini-Golgoo,M.H.Sheikhi,Alowcostand
reliablefiberopticethanolsensorbasedonnano-sizedSnO2,Opt.Fiber
Technol.24(2015)93–99,http://dx.doi.org/10.1016/j.yofte.2015.05.002
([Internet]Availablefrom).
[23]Y.Ko,H.Tsai,K.Lin,Y.Chen,H.Yang,Reusablemacroporousphotonic crystal-basedethanolvapordetectorsbydoctorbladecoating,J.Colloid InterfaceSci.487(2017)360–369,http://dx.doi.org/10.1016/j.jcis.2016.10. 061([Internet]Availablefrom).
[24]M.Konstantaki,A.Klini,D.Anglos,S.Pissadakis,Anethanolvapordetection probebasedonaZnOnanorodcoatedopticalfiberlongperiodgrating,Opt. Express20(8)(2012)8472.
[25]H.Kudo,M.Sawai,Y.Suzuki,X.Wang,T.Gessei,D.Takahashi,etal., Fiber-opticbio-sniffer(biochemicalgassensor)forhigh-selectivemonitoring ofethanolvaporusing335nmUV-LED,Sens.ActuatorsBChem.147(2) (2010)676–680,http://dx.doi.org/10.1016/j.snb.2010.03.066([Internet] Availablefrom).
[26]A.Bearzotti,A.Macagnano,P.Papa,I.Venditti,E.Zampetti,AstudyofaQCM sensorbasedonpentaceneforthedetectionofBTXvaporsinair,Sens. ActuatorsBChem.240(2017)1160–1164,http://dx.doi.org/10.1016/j.snb. 2016.09.097([Internet]Availablefrom).
[27]G.Rego,Areviewofrefractometricsensorsbasedonlongperiodfibre gratings,Sci.WorldJ.2013(2013).
[29]S.M.Topliss,S.W.James,F.Davis,S.P.J.Higson,R.P.Tatam,Opticalfibrelong periodgratingbasedselectivevapoursensingofvolatileorganiccompounds, Sens.ActuatorsBChem.143(2)(2010)629–634.
[30]X.Shu,L.Zhang,I.Bennion,Sensitivitycharacteristicsoflong-periodfiber gratings,J.LightTechnol.20(2)(2002)255–266.
[31]H.J.Patrick,A.D.Kersey,F.Bucholtz,Analysisoftheresponseoflongperiod fibergratingstoexternalindexofrefraction,J.LightTechnol.16(9)(1998) 1606–1612.
[32]P.Kumar,A.Deep,K.-H.Kim,Metalorganicframeworksforsensing applications,TrACTrendsAnal.Chem.73(2015)39–53([Internet]Available fromhttp://www.sciencedirect.com/science/article/pii/
S0165993615001090).
[33]L.E.Kreno,K.Leong,O.K.Farha,M.Allendorf,R.P.VanDuyne,J.T.Hupp,Metal ÀOrganicFrameworkMaterialsasChemicalSensors,Chem.Rev.112(2012) 1105–1125.
[34]A.Betard,R.A.Fischer,MetalÀOrganicFrameworkThinFilms:From Fundamentalsto,2012,pp.1055–1083.
[35]K.S.Park,Z.Ni,A.P.Côté,J.Y.Choi,R.Huang,F.J.Uribe-Romo,etal., Exceptionalchemicalandthermalstabilityofzeoliticimidazolate frameworks,Proc.Natl.Acad.Sci.U.S.A.103(27)(2006)10186–10191.
[36]A.Demessence,C.Boissière,D.Grosso,P.Horcajada,C.Serre,G.Férey,etal., AdsorptionpropertiesinhighopticalqualitynanoZIF-8thinfilmswith tunablethickness,J.Mater.Chem.20(36)(2010)7676.
[37]G.Lu,J.T.Hupp,MetalOrganicFraneworksasSensorsAZIF(BasedFabryPerot DeviceasaSelectiveSensorforChemicalVapoursandGases.pdf,2010,pp. 7832–7833.
[38]G.Lu,O.K.Farha,W.Zhang,F.Huo,J.T.Hupp,EngineeringZIF-8thinfilmsfor hybridMOF-baseddevices,Adv.Mater.24(2012)3970–3974.
[39]J.Hromadka,B.Tokay,S.James,R.P.Tatam,S.Korposh,Opticalfibrelong periodgratinggassensormodifiedwithmetalorganicframeworkthinfilms, Sens.ActuatorsBChem.221(2015)891–899.
[40]J.Hromadka,R.Correia,S.Korposh,Fabricationoffiberopticlongperiod gratingsoperatingatthephasematchingturningpointusinganamplitude mask,ProceedingsofSPIE–TheInternationalSocietyforOpticalEngineering (2016).
[42]S.Korposh,R.Selyanchyn,W.Yasukochi,S.W.Lee,S.W.James,R.P.Tatam, Opticalfibrelongperiodgratingwithananoporouscoatingformedfrom silicananoparticlesforammoniasensinginwater,Mater.Chem.Phys.133 (2-3)(2012)784–792,http://dx.doi.org/10.1016/j.matchemphys.2012.01.094
([Internet]Availablefrom).
[43]EH4O/2005WorkplaceExposureLimitsEH4O/2005WorkplaceExposure Limits.Eh40/2005,HealthandSafetyExecutive,2011,pp.1–74(March2013).
[44]K.W.Kao,M.C.Hsu,Y.H.Chang,S.Gwo,J.AndrewYeh,ASub-ppmacetonegas sensorfordiabetesdetectionusing10nmthickultrathinInNFETs,Sens. (Switzerland)12(6)(2012)7157–7168.
[45]J.Hromadka,S.Korposh,M.C.Partridge,S.W.James,F.Davis,D.Crump,etal., Multi-parametermeasurementsusingopticalfibrelongperiodgratingsfor indoorairqualitymonitoring,Sens.ActuatorsBChem.244(2017).
[46]F.U.Hernandez,S.P.Morgan,B.R.Hayes-gill,D.Harvey,W.Kinnear,A.Norris, etal.,CharacterizationanduseofafiberopticsensorbasedonPAH/SiO2film forhumiditysensinginventilatorcareequipment,IEEETrans.Biomed.Eng. 63(9)(2016)1985–1992.
Biographies
Dr.JiriHromadkareceivedbothhisbachelorandmasterdegreesin2011and2013
respectivelyinenvironmentalsciencesfromCharlesUniversityinPrague(Czech Republic)andanMScinHealthandtheEnvironmentfromCranfieldUniversityin 2013andPh.D.degreefromtheUniversityofNottinghamin2017.Hisresearch focusesonadevelopmentoffibreopticchemicalsensorswithaspecialinterestin environmentalapplications.
DrBegumTokayreceivedherbachelor(1999)andmaster(2001)degreesin
Chemi-calEngineeringfromIstanbulTechnicalUniversity(ITU),Istanbul(Turkey).Shewas awardedaPhDinChemicalEngineering(ITU,2007)forworkonnanosizezeolite crystallizationandgrowthmechanismduringsynthesisfromclearsolutions.She workedattheUniversityofColorado,Boulder(USA)asaresearchassociateon devel-opmentofzeolitemembranesforseparationsofgas,liquidorvapourmixturesinthe DepartmentofChemicalandBiologicalEngineeringfrom2008to2011.Sheworked asalecturerattheMiddleEastTechnicalUniversity(NorthernCyprusCampus)in theChemicalEngineeringProgramfrom2011to2012.Sheiscurrentlyassistant professorattheUniversityofNottingham.Herresearchinterestsfocuson appli-cationanddevelopmentofmembranes,thinfilmsandmicroporous&mesoporous materials
DrRicardoGoncalvesCorreiagraduatedin2004witha5yeardegreein
Instrumen-tationandIndustrialQualityEngineeringfromtheSuperiorInstituteofEngineering ofPorto.In2005hejoinedtheEngineeringPhotonicsgroupatCranfield Univer-sitytopursuitaPhDinthedevelopmentofafibreopticporepressuresensor employingFibreBraggGratings.Asapost-doctoralfellowhehasfurtherdeveloped interferometricandBragggratingbasedfibreopticsensorsforcivilandaerospace engineeringapplications.In2015hejoinedtheUoNwhereheiscurrentlyworking onthedevelopmentoffibreopticsensorsforhealthcareapplications.
ProfessorStephenP.MorganisProfessorofBiomedicalEngineeringatUoN.Since
1992,hehasinvestigatednovelopticaltechniquesforimagingandspectroscopy oftissueusinglaserDopplerflowmetery,acousto-opticimagingandhyperspectral imaging.Hisresearchinvolvesthedevelopmentofdevicestomonitorthe micro-circulationspecificallyintissuebreakdownandwoundhealing.Forexample,he currentlydevelopinganovelendotrachealtubethatcanmonitorthe microcircu-lationatthecuff/tracheainterface.Recentworkinvolvestheintegrationofoptical fibresensorsintotextiles.Thesesensingsystems,incorporatedintogarmentscan monitorpressure,temperatureandthemicrocirculation.
DrSerhiyKorposhreceivedbothhisbachelorandmasterdegreesin2001and
