An Optical Humidity Sensor Based on CdTe Nanocrystals Modified Porous Silicon

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An optical humidity sensor based on CdTe nanocrystals modi ed porous silicon Jing Hu a, Peng Wu b, Dongyan Deng a, Xiaoming Jiang b, Xiandeng Hou a , b , , Yi Lv a ,

a College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, Chinab Analytical & Testing Center, Sichuan University, Chengdu, Sichuan 610064, China

a b s t r a c ta r t i c l e i n f o

Article history:Received 24 September 2012Accepted 5 October 2012Available online 13 October 2012

Keywords:CdTe nanocrystalsPorous siliconPhotoluminescenceHumidity sensor

In this work, mimicking the preparation of solid catalystin heterogeneous catalysis, a composite for sensing wasconstructed by the loading of mercaptopropionic acid (MPA) capped CdTe nanocrystals (NCs) on the porous sil-icon surface (PSi/CdTe NCs composite). Since water vapor could effectively enhance the photoluminescence of CdTeNCs on PSi,the as-preparedcomposite wasengineered intoan opticalhumidity sensor.Time-resolved uo-rescence measurements showed that the loading of CdTe NCs onto PSi and post-photoradiation led to the gen-eration of surface traps on the surface of CdTe NCs. Upon adsorption of water molecules, the amounts of surface traps were effectively decreased, thus providing PL improvement. By employing the proper modi cationstrategy, this humidity sensor achieved the convenience and repeatability in construction of the sensing layer,good sensitivity and stability. This humidity sensor also shows good selectivity against common volatile organiccompounds. For absolute humidity, the linear range was 2 40 g/mL in N2 or 1.3 26.7 g/mL in dry air, bothwith a limit of detection of 0.3 g/mL. For relative humidity (RH), the linear range could be from 12% to 93% at20 C. This sensor was also successfully applied in the RH determination of real air samples, with results wellagreed with a commercial hygrometer.

2012 Elsevier B.V. All rights reserved.

1. Introduction

Semiconductor nanocrystals (NCs), or quantum dots (QDs) withtheir excellent optical properties, havebeen extensivelyemployedas op-tical labels and probes for biosensing [1 4], and optical probes for metalions [5 9]. These sensing events are typically carried out in solutionmedia, while for gas sensing, solid state NCs are often used and embed-ded into polymer matrix [10 13]. SemiconductorNCs based gas-sensingmaterials mainly feature the following characteristics: (i) sensitive andrapid response to gases, due to the large surface-to-atom ratio and thehigh susceptibility of NCs to the change of surface environment; and(ii) theenhancedselectivityor theexpandeddetection range of analytes,due to the versatile and matured NCs surfacefunctionalization strategies[2,14,15] . However, the utilization of NCs for gas sensing is at the pri-mary stage, mainly because semiconductor NCs are usually cappedwith a dense monolayer of organic ligands which prevents the effec-tive interaction between gas molecules and the surface of NCs [10].Aiming at improving the interaction, a simple and effective strategyof photoactivation was therefore proposed to make the ligandsmonolayer of NCs readily permeable to gas molecules [10]. Another

strategy is the surface-modi cation of NCs for speci c analytes,with improved sensitivity and selectivity [12].In addition to the improvement of NCs themselves, the careful de-

sign of sensing interface between the NCs and gases is also of vital im-portance to the sensing performance. In most cases, NCs areencapsulated into poly(methyl methacrylate) (PMMA) or poly(dimeth-ylsiloxane) (PDMS).Subsequently, themixture is cast on Si or glass sub-strates. However, the use of PMMA or PDMS actually generated anotherbarrier between gas molecules and the surface of NCs, reducing thesensing sensitivity and prolonging the recovery time. In heterogeneouscatalysis, forthe preparation of solid catalysts, theactive components forcatalysis are often dispersed on a suitable supporter. Such design pro-vides the following advantages: (i) improving the effective interactionof catalyst with reactants and accelerating the adsorption desorptionprocess; and (ii) enhancing the catalysis performance through regularlyarrangingthe activecomponentson thesupport with uniform morphol-ogy [16 18]. The construction of a solid catalyst-like NC-based gas sen-sor may bene t enough from the existed knowledge of heterogeneouscatalysis and lead to improved gas sensor performance. However, ele-gant selectionof the supporter for theNCsand subsequent modi cationstrategy is obviously the rst to be considered.

Porous silicon (PSi) is a favorable candidate as the supporter. Sinceaccidentally discovered in the mid 1950s [19], PSi has been extensivelyexplored as chemical/bio-sensors, supporter for chemical applicationsas well as biochemical reactionsand proteomics, due to distinct proper-ties of itsuniquesurface [20]. PSi is rstly characterized of the large sur-face area with controllable and ordered pores, consequently as an

Microchemical Journal 108 (2013) 100 105

Corresponding author at:Collegeof Chemistry, Sichuan University, Chengdu,Sichuan610064, China. Tel.: +86 28 85470818; fax: +86 28 85415695.

Corresponding author.E-mail addresses: [email protected] (X. Hou), [email protected] (Y. Lv).

0026-265X/$ see front matter 2012 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.microc.2012.10.005

Contents lists available at SciVerse ScienceDirect

Microchemical Journal

j o u rn a l h o m ep ag e : w ww . el sev i e r . co m / lo ca t e /m icr o c
http://dx.doi.org/10.1016/j.microc.2012.10.005http://dx.doi.org/10.1016/j.microc.2012.10.005http://dx.doi.org/10.1016/j.microc.2012.10.005mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.microc.2012.10.005http://www.sciencedirect.com/science/journal/0026265Xhttp://www.sciencedirect.com/science/journal/0026265Xhttp://dx.doi.org/10.1016/j.microc.2012.10.005mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.microc.2012.10.005


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excellent supporter. Examples include desorption ionization on PSisurface for mass spectrometry [21], heterogeneous support for catalyticgrowth of highly uniform carbon nanotubes [22], porosi ed enzymemicroreactors with increased activity [23] etc. The versatile and conve-nient surface chemistry is another attracting advantage, since PSi sur-face could achieve any surface termination upon demand, by a varietyof wet chemical functionalization techniques [24 26]. Typically, Sailoret al [27] introduced a series of surface modi cation strategies for load-

ing of analyte-speci c receptors and achieved sensitive protein or DNAdetection. However, to our knowledge, the use of PSi as a support forloading of semiconductor NCs for development of sensing devices hasnever reported before. In this work, we employed a proper modi cationstrategy to decorate mercaptopropionic acid capped CdTe NCs on thesurface of PSi to engineer an optical humidity sensor.

Humidity is one of the most common physical quantities and itsmeasurement is of signi cant importance for a wealth of areas, includ-ing air conditioning, process control and product store in industry aswell as agriculture [28]. Early established hygrometers are con ned bythe limitations suchas slow response, inherent non-linearity, and oper-ation place limit [29]. Electronic humidity sensors [30], on the otherhand, suffer from the temperature dependence and the susceptibilityto some chemical species. The optical humidity sensor in this workwas based on the direct response to the humidity via the PL enhance-ment due to the passivation of NCs surface by adsorbed water mole-cules. Bene ting from the utilization of sensing composite (PSi/CdTeNCs composite), this humidity sensor achieves high sensitivity andgood recovery. Together with thegood selectivity, this sensor is a prom-ising alternative for humidity measurement.

2. Experimental section

2.1. Reagents and materials

Reagents used in this work were of at least analytical grade.CdCl2 2.5H 2O, KBH4 (Kelong Reagent Factory, Chengdu, China), and Tepowder (Sinopharm Chemical Reagent Co. Ltd) were used to prepare

CdTe NCs, and 3-mercaptopropionic acid (MPA) (Alading Reagent Co.,Shanghai, China) was used as the capping agent. P-type, b 100>,boron-doped Si wafers (8 12 cm) were etched by using a mixture of ethanol and HF (40 wt.%) to form the PSi layers. (3-aminopropyl)triethoxysilane (APTES, bought from Alfa Aesar China (Tianjin) Co.,Ltd.), 1-ethyl-3-(3-dimethyllaminopropyl) carbodiimide (EDC HCl) and1-hydroxypyrrolidine-2,5-dione (NHS), both bought from Alading Re-agent Co., were used for the modi cation of PSi. N2 (99.999%) and med-ical O2 (99.5%) were used as the carrier gas because the concentration of water in them were very low ( b 3 ppmv). Dry air was prepared bymixing N 2 and O2 under proper ratio.

2.2. Synthesis of CdTe NCs

Synthesis of CdTe NCs was based on a previouspublication but withsmall modi cations [31]. Brie y, 1.9 mM KBH4 and 0.8 mM Te powderreacted in 10 mL oxygen free water under 60 C for 60 min to formKHTe. The reaction was protected by N 2 stream, and nally a purplishred solution was obtained. CdCl 2 2.5H 2O (1 mM) and MPA (1.3 mM)were dissolved in 150 mL of water. The mixture was adjusted topH 10.5 with 1 M NaOH and deoxygenized with N 2 stream for20 min. Subsequently, 2 mL of freshly prepared KHTe was quicklyinjected into 150 mLof Cd 2+ precursor solution undervigorous stirringfor20 min, and then theresultant solution was re uxed for 40 min.Theobtained CdTeNCs solution was precipitated with n-propanol, and thencentrifugated (11,000 rpm) to remove the supernatant. The puri edCdTe NCs were redispersed in double distilled water (DDW). The sizeand concentration of the CdTe NCs were estimated based on Peng's

method [32].

2.3. Preparation of PSi

Thesiliconwafers (1 1 cm 2) were etchedat a constantcurrentden-sity of 40 mA/cm 2 in a mixture of HF (40 wt.%): ethanol (v/v=1:1) for30 min. TheformedPSi layers (0.8 0.8 cm)were then rinsed with eth-anol, 1 M KOH (to remove the SiO x), DDW, ethanol and n-pentane inorder. Finally, the PSi layers were dried under N 2 stream. It is worth-while to note that the PSi layer could be reproducibly prepared by this

electrochemical method. However, be careful when using HF, and theexperiment should be carried out under the hood.

2.4. Modi cation of PSi

The schematic procedure was shown in Fig. 1, and the method wasnamed as electrostatic assembly. Brie y, the PSi layer was rstly func-tionalized with APTES according to a previous method [33]. Then, theAPTES-PSi layer was immersed in diluted HCl (concentrated HCl/DDW= 1:9) for 20 s to protonate the amino group and dried under N 2stream [34]. Finally, 50 L 0.025 mM CdTe NCs solution was drippedon the layer and dried in air under room temperature. It was foundthat CdTe NCs solution could easily disperse across the protonated

silanized PSi layer. The other three modi cation methods are for com-parison with electrostatic assembly, and the details are presented inthe Supporting information.

2.5. Characterization of CdTe NCs and sensing layer

The UV visible absorption spectrum of CdTe NCswas obtained usinga Shimadzu 1750 UV vis spectrophotometer, and the spectra of theUV vis absorptionand thePL emission were shown in Fig. 2a. The com-parison of PL spectra of 0.025 mM CdTe NCs solution and PSi/CdTe NCscomposite was shown in Fig. 2b. The morphologies of the sensing layerwere characterized using a JSM-5900LV scanning electron microscope(SEM) operated at an accelerating voltage of 20 kV.

2.6. Sensor fabrication

The PSi/CdTe NCs layer was put into a Te on sensing cell with a gasinlet and an outlet, and a piece of quartz window covered on the cell.The schematic illustration of humidity sensing by the present sensorwas shown in Fig. 3a. The sensing cell was mounted on a solid sampleholder and the PL signals were recorded with a Hitachi F-7000 uores-cence spectrometer (Hitachi Company, Japan). Theexcitation wasset at350 nm throughout the work, with the excitation and emission slitwidth both of 5 nm. The photoluminescence spectra of CdTe NCs andPSi/CdTe NCs layer were recorded in the range of 450 650 nm. For hu-miditymeasurement, the program of time scan wasemployed,withex-citation at 350 nm and emission at the PL peak of the sensing layer. Thepeak area was used for quanti cation throughout the work.

2.7. Absolute humidity measurement

The absolute humidity measurement was carried out in high purityN2 (as a typical inert gas) and dry air. For typical measurement, properamountof water wasdirectly injected into the30 mLheatedbottle (at aconstant temperature of 150 C). After totally vaporized (20 s), the car-rier gaswitha owrateof 400 (N 2 ) or 200 (dry air)mL/min ushed thewater vapor into the sensing cell for detection.

2.8. Relative humidity calibration

Six over-saturation solutions of speci c salts, i.e. LiCl, MgCl2 ,Mg(NO3 )2 , NaCl, KCl and KNO3 were put insix 100 mLconical asks,re-spectively.Sixplastic syringes containing60 ml clean airwere mountedon those asks and sealed at 20 C for at least 24 h. After that, the rela-

tive humidity (RH) values of the air in the corresponding syringes were

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12, 33, 55, 75.7, 85 and 93%, respectively [35]. For RH calibration, 10 mL airwith thespeci c RHvalue was injected and ushed bythedryairat aspeed of 100 mL/min to reach the sensing cell for determination.

2.9. Air samples

Four air samples (20 C) were collected from a bathroom, an of cebuilding, an instrument room and a laboratory for organic synthesisrespectively. For each measurement, 10 mL sample was injected and

ushed by the dry air also with a speed of 100 mL/min to reach thesensing cell. A commercial hygrometer (Beijing Yaguang InstrumentCorporation, supervised by the National Research Center for Certi edReference Materials) was used for validation.

3. Results and discussion

3.1. Design and characterization of the PSi/CdTe NCs composite

Thephysical/chemicalpropertiesof PSi surface and CdTeNCstogeth-er with the interaction between them dictate the convenience and re-peatability in construction of the sensing layer. The native PSi washydrophobic with ordered pores of 1 3 m spreading over the etchedregion of Si wafer ( Fig. 4a), while CdTe NCs used in this work werewater soluble. Therefore, in order to render the PSi surface hydrophilic,amino groups were mounted onto the PSi surface via silanized withAPTES and then protonated ( Fig. 1). Consequently, CdTe NCs were easilyspread over the PSi region, due to the electrostatic interaction betweenprotonated amino groups on PSi and negatively charged MPA of CdTeNCs. Branch-like strips of NCs covered the surface of PSi pore skeletondensely ( Fig. 4b), indicating the facile construction of sensing layer.Three otherCdTeNCsloading methods werealso investigated, includingamide bonding modi cation, physical adsorption and gel coating (seethe Supporting information). However, possibly due to the poor com-patibility of these methods, PSi surface morphology characterizationsshowed that CdTe NCs in these cases could not spread as evenly as

that of electrostatic assembly (see Fig. S2). Accordingly, electrostatic as-sembly was employed for loading of CdTe NCs onto the surface of PSi.

Interestingly and importantly, post-photoradiation of PSi/CdTe NCscomposite could re-regulate the loaded CdTe NCs on the surface of PSi,resulting in further dispersing of CdTe NCs over the ordered pores of PSi. As shown in Fig. 4b and c, after the as-prepared PSi/CdTe NCs com-posite (sealed in N 2) was subjected to photoradiation for 10 min, theloadedCdTe NCsover thesurfaceof PSichanged from randomscatteringto the well-dispersing along the PSi pore skeleton. Obviously, electro-static assembly of CdTe NCs with PSi plus post-photoradiation providea barrier-less sensing layer for water vapor to well contact with CdTeNCs, which is advantageous over the conventional PMMA or PDMS en-capsulating strategies. The reason for such photo re-arrangement wasnot very clear at the present stage. However, the weak assembly of CdTeNCs on thesurface of PSithrough electrostatic interaction providedfurther probability for such re-arrangement.

3.2. Brief discussion of sensing mechanism

The change in the surface state of NCs would signi cantly affect theirPL properties [36] , which is the fundamental for thedevelopment of theNCs-based gas sensors [10]. Time-resolved PL experiments showed thatthe lifetime of CdTe NCs deposited on PSi was greatly shortened com-pared with that of CdTe NCs solution ( Fig. 5), indicating that surface de-fects were created after CdTe NCs were transferred from the solution tothe solid state. Furthermore, post-photoradiation of the sensing layer(PSi/CdTe NCs composite) not only provides increased contact area be-tween analyte and CdTe NCs, but also creates defects on the surface of CdTe NCs[37] . It hasbeenreportedthat theelectron-donating molecules(Lewisbasis) adsorbed on thesurface of CdSe NCsare prone to passivatetheir surface traps, resulting in PL enhancement [38,39] . Based on thesimilarity of CdTe and CdSe NCs, we thus speculated that PL enhance-ment caused by gaseous water (a well-known Lewis basis) in thiswork could also be attributed to the elimination of surface traps. Uponthe adsorption of water molecules, the lifetime of CdTe NCs deposited

Fig. 1. Schematic diagram of silanization of PSi and electrostatic assembly.

Fig. 2. (a)Photoluminescenceemission andUV absorption spectraof MPA-CdTeNCs, and(b) photoluminescenceemissionspectra of MPA-CdTe NCsof 0.025 mM andPSi/CdTe NCscomposite.

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on PSiwasobviously increased (stillmuch shorter than that of solution),which proved the passivation of partial surface defects by water mole-cules and consequently, enhanced PL.

3.3. The sensitivity and stability of the sensing layer

Surface chemistry has been reported to have a pronounced effecton analyte response in PSi based sensors, in terms of sensitivity, selec-tivity and stability [40]. Sensitivity investigations showed that elec-trostatic assembly gave the best sensitivity ( Fig. 6), which could beascribed to the ever-increased contact area between analyte withCdTe NCs. Note that the poor sensitivity of surface by NCs gel coating(similar with PMMA or PDMS encapsulating strategies), which wasattributed to the block from the dense siloxane network aroundCdTe NCs. This further proves that the barrier-less sensing interfaceis a better choice.

The stability of the sensing layer was investigated in a period of 22 days. As shown in Fig. 6, the stability of PSi/CdTe NCs was the bestcompared with those by other three modi cation methods,probably be-cause of the unique environment around the NCs and morphology bythis electrostatic assembly. The stability of the sensing layer was accept-able for relative long-timeuse, since theexposure to airduring humiditysensingonly caused weak oxidationand PLdecrease, andit wassealed inN2 when not used. Note that there was an increase in response on the5th day, possibly due to the increase of surface traps on the CdTe NCsduring the 5-day aging process. In this case, the CdTe NCs would beeven more accessible to water molecules, thus the PL enhancementwasenlarged. In contrast, thesensing layer provided by physical adsorp-tionexhibited an obviously decreased sensing response. In suchphysicaladsorption, the spreading of CdTe NCs over PSi was orderless andcouldn't spread orientally over theorderedporesof PSi. Thus PSicouldn'tprovide a protective environment forCdTe NCs andover oxidationof theCdTe NCs was generated during the preparation and utilization.

Fig. 3. (a) Schematic illustration of humidity sensing by the present sensor, and (b) the responses of PSi/CdTe NCs composite to 13.3 g/mL water vapor.

Fig. 4. SEM images of (a) porous silicon; (b) PSi/CdTe NCs composite; and (c) PSi/CdTe

NCs composite after photoradiation.

Fig. 5. PL decay curves of CdTe NCs solution, PSi/CdTe NCs and PSi/CdTe NCs with the

adsorption of water molecules.

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3.4. Effect of carrier ow rate on recovery and sensitivity

Previous research results showed that the PL enhancement by theadsorption of water was a semi-reversible behavior due to the strongbonding of water on NCs surface [38,39] . Therefore, the present sensorwas used under ow mode: the carrier gas brought the gaseous waterinto the sensing cell for detection and ushed it away from the sensinglayer. Together with the small sampling amount, this sampling modecould effectively reduce the strong bonding effect of water molecule,bene ting the recovery, repeatability and the lifetime of this sensor.As demonstrated in Fig. 3b, the reproducible PL enhancement with re-covery time about 2 min was achieved. For the absolute humidity de-termination in high purity N 2 atmosphere, the signal decreased as theincrease of ow rate in the range of 160 to 600 mL min 1 (Fig. S3).However, low ow rate would be detrimental to sensor recovery.Therefore, 400 mL/min was nally selected for use in consideration of both sensitivity and recovery. In the case of humidity determinationin air, the high ow rate of air would bring more O 2 each time, whichquenched the PL effectively to depress the PL enhancement by watermolecules. A low ow rate of 200 mL/min was therefore employed forthe determination of the absolute humidity in air, while an evenlower oneof 100 mL/min wasused forthe relative humidity calibration.

3.5. Selectivity of the sensor

The selectivity of the present humidity sensor was evaluated using aseries of common volatile organic compounds (VOCs) of 26.4 g/mL, in-cluding alcohol (methanol, ethanol and n-propanol), acetone, ethyl ace-tate, n-hexane,cyclohexane, chloralkanes (dichloromethane, chloroform

and carbon tetrachloride), acetonitrile, tert-butylamine and NH 3 . Of these VOCs, no detectable or very weak (for NH 3) signals were obtained,indicating good selectivity of the proposed humidity sensor. The reasonfor the selectivity was not very clear at the present stage. Possibly, thephysical/chemical property of the PSi (support), the capping agent andthe surrounding environment for NCs, contributed greatly to the selec-tivity of this sensor. The present sensor was not suitable for atmospherewith NH 3 of high concentration, but could be widelyapplied in commoncases.

3.6. Analytical gures of merit

The linear dynamic range of the PL enhancement versus water con-centration was found to be 2 40 g/mL in high purity N 2 and 1.3

26.7 g/mL in dry air, both with a limit of detectionof 0.3 g/mL.Typical

calibration curves using the optimized parameters could be character-ized by the following calibration functions: y= 29.6x+ 9.3 ( R=0.996) in N 2 , and y=41.3x+10.2 ( R=0.998) in dry air, where y isthe improved PL intensity, x is the concentration of water, and R is theregression coef cient. Relative standard deviations (RSDs, n =3) were0.7%, 0.6%, and 1.9% for 2 g/mL, 13.3 g/mL, and 40 g/mL water inN2 ; and 4.7%, 1.9%, and 0.4% for 1.3 g/mL, 13.3 g/mL, and 26.7 g/mL water in dry air, respectively. For relative humidity, six relative hu-midity concentrations of 12%, 33%, 55%, 75.7%,85% and93%were tested,and thelinearregressionequation is describedby y= 250.6x+8.6 ( R=0.998). RSDs (n=3) were 4.2%, 2.4%, 1.5% for 33% RH, 75.7% RH and93%RH, respectively. Thegood analyticalperformance indicates the pro-posed sensoris suitable forhumidity measurement either in inert gasorin the air.

3.7. Relative humidity determination of air samples

Theproposed PSi/CdTe NCscomposite sensorwasapplied to the rel-ative humidity determination of real air samples, including three sam-ples with different RH range in common places, and a sample from anorganic synthesis laboratory in which contains some ethyl acetate,dichloromethane and n-hexane. As shown in Table 1 , the determinedRH valuesagreed well with thatmeasured by a commercial hygrometer,even for the VOCs-containing sample. These results demonstrated thepotential of this sensor in real world sample determination.

4. Conclusion

An optical humidity sensor using the CdTe NCs modi ed porous sil-icon composite was developed. Porous silicon was demonstrated as afavorable supporting material for loading of semiconductor NCs. Moreimportantly, the CdTe NCs could be regulated along the PSi pore skele-ton under UV irradiation to form reliable and well-performed sensinglayers. Furthermore,its various surface chemistrycould providea versa-tile platform available for different ligand-capped NCsor functionalizedNCs for target analyte. For the modi cation strategy, electrostatic as-sembly shows the best compatibility with the surface of porous silicon,thus forming the most uniform, sensitive and stable sensing layers. The

ow sampling mode is also important for good recovery and stability of the present sensor. This optical humidity sensor, which features directdetection of water vapor with high sensitivity and good selectivity,should be promising in on-site monitoring humidity of environment,warehouse areas and more.

Acknowledgment

The authors gratefully acknowledge the nancial support for thisproject from the National Natural Science Foundation of China (no.20835003). The authors also thank the Analytical & Testing Centerof Sichuan University for obtaining SEM data, and Professor HUANGChengzhi in Southwest University for his assistance in carrying out

uorescence time-resolved experiments.

Fig. 6. Sensingsensitivityands tability comparisonwith otherthree modi cationmethods.Water vapor concentration, 13.3 g/mL for PSi/CdTe NCs composite, the surface by amidebonding modi cation, and the surface by physical adsorption; 33.3 g/mL for the surfaceby gel coating. Air ow rate, 200 mL/min. The PL quenching response from the surfaceby gel coating was called for further investigation.

Table 1Analytical results of relative humidity determination of air samples.

Sample Found by this method(average , n=3)/%RH

Found by acommercial hygrometer(average , n=3)/%RH

Air in a bathroom 79.70.6 81.00.8Air in an of ce building 63.81.4 62.01.0Air in an instrument room 50.6 0.6 50.0 1.2Air in an organic synthesis

laboratory

49.20.8 50.00.9

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Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.microc.2012.10.005 .

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An Optical Humidity Sensor Based on CdTe Nanocrystals Modified Porous Silicon