A colorimetric agarose gel for formaldehyde measurement based on nanotechnology involving

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Showcasing research from Jing-bin Zeng’s Laboratory, State Key Laboratory of Heavy Oil Processing and College of Science, China University of Petroleum (East China)

A colorimetric agarose gel for formaldehyde measurement based on nanotechnology involving Tollens reaction

Au NPs along with Tollens reagent were immobilized into agarose gels for the colorimetric measurement of formaldehyde. Au@Ag core/shell NPs were produced, and the thickness of the silver shell was strongly dependent on the concentration of HCHO, which further generated signi? cant color changes for visual http://wendang.chazidian.com/chemcomm

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Citethis:http://wendang.chazidian.commun.,2014,50,8121

Received4thFebruary2014,Accepted6thMay2014DOI:10.1039/c4cc00914b

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Acolorimetricagarosegelforformaldehyde

measurementbasedonnanotechnologyinvolvingTollensreaction?

Jing-binZeng,*aShi-guangFan,aCui-yingZhao,aQian-ruWang,aTing-yaoZhou,bXiChen,bZi-fengYan,*aYan-pengLi,aWeiXingaandXu-dongWang*cd

Goldnanoparticles(AuNPs)coupledwithTollensreagentwereusedformeasuringformaldehyde.Au@Agcore–shellNPswereformedalongwithdistinctcolorchangesfrompinktodeepyellow.Thiscolorimetricsystemwasfurtherimmobilizedintoanagarosegel,whichwasusedformonitoringofgaseousformaldehyde.

Formaldehyde(HCHO)iswidelyusedinindustryasabondingagent,especiallyforbuildingmaterials,suchaswoodfurnitureandpaint.However,theuseofthesematerialshascausedtheremainingHCHOconcentrationindoorstoexceedcertainthreshold,whichputspeople’shealthatpotentialrisk.Forexample,HCHOhasbeenfoundtobehighlyassociatedwiththesickbuildingsyndrome,suchaseyesandnoseirritation,fatigue,headacheandcoughing.1Inaddition,ithasbeenclassifiedasoneofthehumancarcinogensbytheworldhealthorganization(WHO).2ThemeasurementofindoorHCHOiscloselyrelatedtothesafetyofliving,whichemphasizestheneedforasimple,sensitiveandreliablemethodformeasuringHCHO.Severalapproachesincludingchromatography,3a,belectrophoresis,3cfluorimetry,3denzyme-basedbiosensors3e,fandgassensorsbasedonmetaloxide3g–ihavebeendeveloped.Theseapproachesaresensitiveandreliable,butrelyontheuseofscientificinstrumentswhicharenoteasilyaccessiblebythepublic.Atthesametime,specialtechniquesarerequiredforinstrumentoperationanddataassessment.

ColorimetricmethodsarealternativesimpleapproachesformeasuringHCHO,butextraneousorganicchromophoreswith

delocalizedconjugatedfunctionalgroups(e.g.,acetylacetone,pararosaniline,chromotropicacid)aregenerallyrequiredasrecognitionunits.4Moreover,theseapproachesarenotsensi-tiveenough,aresusceptibletointerference,requirestrongacidorbase,and/oraretime-consuming.5Recently,acolorimetricmethodformeasuringHCHOhasbeenproposedbyusingaNafionfilmsaturatedwith[Ag(NH3)2]+andATP.However,themethodrequiredalongreactiontime(1h)andsu?eredfrominadequatesensitivity.6Suslickandco-workersfabricatedasensitiveHCHOcolorimetricsensorarraybasedonthereactionbetweenprimaryamineandHCHO.Thesensorarraywasfullyreversible,butreliedonaseriesofpHindicatorsandstatisticaldata-processing.7Thus,areliable,easy-to-operate,sensitiveandlow-costHCHOdetectionmethodisstillhighlydesiredfordailyuse.

ItiswelldocumentedintextbooksthatHCHOcanreactwithTollensreagent(thekeycomponentis[Ag(NH3)2]OH)topro-duceasilvermirror.Similartoapreviousreport,8wealsohaverevealedthatsilvernanoparticles(AgNPs)wereproducedwhentheconcentrationofHCHOwasrelativelylow(Scheme1,downrowandFig.S1a–cintheESI?).Therewasanindistinguishablecolorchangefromcolorlesstoslightlyyellowalongwiththeappearanceofasurfaceplasmonresonance(SPR)bandat416

nm

a

StateKeyLaboratoryofHeavyOilProcessingandCollegeofScience,ChinaUniversityofPetroleum(EastChina),Qingdao,266555,China.E-mail:xmuzjb@http://wendang.chazidian.com,zfyancat@http://wendang.chazidian.comb

StateKeyLaboratoryofMarineEnvironmentalScience,XiamenUniversity,Xiamen361005,Chinac

InstituteofAnalyticalChemistry,Chemo-andBiosensors,UniversityofRegensburg,93040Regensburg,Germany.E-mail:xmuwxd@http://wendang.chazidian.comd

KarlsruheInstituteofTechnology(KIT),InstituteforBiologicalInterfaces(IBG-1),Hermann-von-Helmholtz-Platz,76344Eggenstein-Leopoldshafen,Germany

?Electronicsupplementaryinformation(ESI)available:ExperimentalproceduresandFig.S1–S13.SeeDOI:10.1039/c4cc00914b

Scheme1SchematicdiagramsoftheTollensreactionwith(upperrow)andwithout(downrow)AuNPs.

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(Fig.S1d,ESI?).AsshownintheupperrowofScheme1,whenAuNPswereintroducedinthisclassicreaction,thegeneratedsilvernanoshellswerecoatedontoAuNPs,leadingtotheformationofAu@Agcore–shellnanostructures.TheshellthicknessoftheproducedNPswasstronglydependentontheconcentrationofHCHO.Thechangeofshellthicknesswasaccomplishedwithadistinctcolorchangefrompinktodeepyellow,whichcanserveasacolorimetricapproachformeasuringHCHO.

TheAuNPsweresynthesizedbyreducingtheHAuCl4withtrisodiumcitrate,andtheas-synthesizedAuNPsexhibitedsphericalshapesanduniformsizes(13.0Æ1.5nmdiameter,Fig.S2intheESI?).ThesoleadditionofTollensreagentorHCHOintothesuspensionofAuNPsdidnotleadtoobviousspectralorcolorchanges(Fig.S3inESI?).Incontrast,theadditionofincrementalamountsofHCHOintothemixtureofAuNPsandTollensreagentledtoapparentcolorchangesfrompink,toorange,andfinallytodeepyellow(Fig.1a).Thesedistinctcolorchangeswereobtainedinseveralminutes(Fig.S4,ESI?)atroomtemperature(251C),whichprovidesarapidandsimplewaytomeasureHCHOwithoutusinginstrumentationandchromo-phores.Thelowesteye-distinguishableconcentrationcanbeaslowas100nM(3ngmLÀ1).

AUV-VisspectrometerwasusedtomonitorthespectraduringtheadditionofHCHOintothemixtureofAuNPsandTollensreagent.Fig.1bshowsthattheSPRbandofAuNPspeakedat520nmblue-shifted,alongwiththeemergenceofasecondSPRbandataround410nm,whichimpliedthefor-mationofcore–shellNPs.9Theabsorbanceat410nmincreasedwiththeincrementalconcentrationofHCHO(Fig.1b).TherewasagoodlinearrelationshipbetweentheA410/A520ratioandHCHOconcentrationsintherangeof0.1–40mM(Fig.1b,inset),whichalsoenablesaquantitativereadoutusingUV-Visspectro-scopy.Thelowestdetectableconcentrationwasfoundtobe50nM(1.5ppb).Tothebestofourknowledge,thisisthesmallestLOQforHCHObasedoncolorimetricmethods.1b,4h,5,6,10

Fig.1(a)Photographsand(b)UV-visspectraofthecolorimetricassayformeasuringHCHOatvariousconcentrations.Insetinfigure(b)showsthelinearrelationshipbetweentheA410/A520ratioandHCHO

concentration.

8122|http://wendang.chazidian.commun.,2014,50,8121--8123Fig.2Representative(a)TEM,(b)HR-TEMandelementalmapsforthecore(c)andtheshell(d)oftheobtained

nanoparticles.

Therelativestandarddeviationoftheassayislessthan1.6%(Fig.S5,ESI?).

ThestructureandchemicalcompositionoftheproducedNPswerecharacterizedusingtransmissionelectronmicroscopy(TEM).Fig.2ashowsthatmostoftheNPsweresphericalandexhibitedinhomogenouselectronicdensitywithadarkercentralpartandalighterouterpart.Ahigh-resolutionTEM(HR-TEM)image(Fig.2b)clearlydisplaysasphericalNPwithapparentdi?erentopticalcon-trastinthecoreandintheshell,respectively.Energy-dispersiveX-rayelementalanalysiswasusedtomaptheelementdistributioninatypicalnanoparticle.AsdepictedinFig.2candd,theelementalcompositionofthecorewasgold,andthatoftheshellwhichsurroundedthegoldcorewassilver,verifyingtheformationofAu@Agcore–shellstructures.ThepossibilityofformingalloyswasexcludedbythefactthatnitricaciddissolvedtheAgshellalonebutbarelyaffectedtheAucores(Fig.S6inESI?).

Togetmoreinsightintothemechanismoftheapproach,TEMwasusedtomeasurethedimensionofAu@Agcore–shellNPsproducedbythereactionwithdifferentconcentrationsofHCHO.Fig.3indicatesthatthedimensionalsizeofthe

Au@Ag

Fig.3TEMimagesandsizedistributionhistogramsoftheAu@Agcore–shellNPsgeneratedbytheTollensreactioninthepresenceofAuNPswithHCHOconcentrationof(a,d)5mM;(b,e)25mM;and(c,f)100mM.Morethan130NPsweremeasuredforeachhistogram.

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Fig.4PhotographsoftheagarosegelteststripsforthedetectionofHCHOwithvariousconcentrations.

core–shellNPsincreasedastheconcentrationofHCHOincreased,whichcorrespondedwellwiththeenergydispersiveX-rayandX-raydiffractionanalysisresults(Fig.S7andS8inESI?).Inaddition,aspresentedinFig.S9(ESI?),theplasmaresonanceofAu@Agcore–shellNPswashighlydependentonAuNPsintotheclassicTollensreaction,Au@Agcore–shellNPswereproducedwhenexposedtoHCHO.ThethicknessofthesilvershellwasstronglydependentontheconcentrationofHCHO,whichfurthergeneratedsignificantcolorchangesforvisualreadout.TheapproachwasfurthertransformedintoasolidmatrixbyusingagarosegelstoimmobilizetheAuNPsandTollensreagent.TheproducedagarosegelscanmeasureHCHOquickly,whichisbeneficialforimmediateassessmentofpotentialriskanddanger.

ThisworkwasfinanciallysupportedbytheNationalNatureScientificFoundationsofChina(No.21105123,21106185)andtheShandongYoungScientistAwards(BS2012CL037)whichthethicknessratioofthesilvershelltothegoldcore,anditsslightdifferencecanleadtoanobservablechangeinabsorp-tionspectraandapparentcolor.ThisleadstothedevelopmentofahighlysensitiveapproachforHCHOmeasurement.

Theselectivityofthisassaywasstudiedagainstcommonindoororoutdoorgases,especiallythosewithchemicalreduci-bility,suchasalcohol,ketone,anilineandphenol.Thecon-centrationsofinterferencesweresetatleast100timesthatofHCHO.AsshowninFig.S10(ESI?),thereisnoobviousinter-ferenceobserved.Acetaldehyde,benzaldehydeandglucoseweretestedtofurtherevaluatetheselectivityofourmethod.Fig.S11(ESI?)showsthatthesealdehydesonlyproducedlittleinter-ference.Asimilarapproachhasbeendevelopedforcolorimetricdetectionofglucose,11butourresultsindicatethatglucoseexhibitedlimitedresponseatroomtemperatureevenafterthereactionfor40min.SuchagoodselectivityisattributedtothefactthatHCHOisthesimplestmoleculewithvirtuallytwoaldehydegroups,12andsoitcanreactwiththeTollensreagentquicklyandleadstodepositionofathickersilvershellontothegoldcore.Itshouldbenotedthatifallthetestedaldehydesarecollectedfromgasphaseusingadi?usionbasedcollector,thediscriminationwouldbeevengreater.13

TheAuNPsalongwithTollensreagentcanbefurtherimmobilizedintothesolidmatrixforpracticaluse.AnagarosegelwasemployedtoembedtheAuNPsandTollensreagent,sinceitistransparent,porous,andcontainsalargeamountofwatertoallowtheoccurrenceoftheTollensreaction.TheobtainedagarosegelswereusedforcolorimetricdetectionofaqueousHCHO.AsshowninFig.4,thecoloroftheagarosegelschangedfrompinktoyellowastheHCHOconcentrationincreased.Theresults(Fig.S12inESI?)alsoshowedthattheagarosegelswereabletomeasuretheconcentrationofgaseousHCHOaslowas80ppb,whichisthesafetylimitregulatedbyWHO.7Themeasurementrangecanreachupto20ppmwhichisregulatedbytheOccupationalSafetyandHealthAdministrationasanimmediatelydangerouslimittolifeorhealth.ThetimerequiredforthevisualmeasurementofHCHOat20ppmwaslessthan1min(Fig.S13,ESI?),whichisbeneficialforimmediateassessmentofpotentialriskanddanger.

Insummary,wehavedevelopedarapid,low-costandhighlysensitivecolorimetricassayforHCHOmeasurement.Byintroducing

Thisjournalis©TheRoyalSocietyofChemistry2014aregratefullyacknowledged.

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