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Sensors and Actuators B 241 (2017) 607–613

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo ur nal home page: www.elsev ier .com/ locate /snb

arbon-dot-aerogel sensor for aromatic volatile organic compounds

usmita Dolaia, Susanta Kumar Bhuniaa, Raz Jelineka,b,∗

Department of Chemistry, Ben Gurion University of the Negev, Beer Sheva 84105, IsraelIlse Katz Institute for Nanotechnology, Ben Gurion University of the Negev, Beer Sheva 84105, Israel

r t i c l e i n f o

rticle history:eceived 25 August 2016eceived in revised form 23 October 2016ccepted 26 October 2016vailable online 27 October 2016

a b s t r a c t

Detection of aromatic volatile organic compounds (VOCs) is important for monitoring occupational haz-ards, industrial safety, and environmental applications. Here, we present a new in-situ-synthesized carbondot – aerogel matrix and demonstrate its application for sensing aromatic VOCs. The composite aero-gel exhibited high specific surface area and pore diameter, enabling efficient adsorption of the organicvapors. In particular, the excitation-dependent luminescence emission properties of the carbon dotswere retained upon embedding within the aerogel host, and provided a sensitive transduction mecha-

eywords:arbon dotserogelOCsas sensorsluorescence quenching

nism through both shifts and quenching of the fluorescence emissions. We show that distinct fluorescenceshifts and degrees of quenching were induced by different aromatic VOCs. In particular, the C-dot-aerogelsensor could distinguish between isomers of phenylenediamine, an important achievement which hasnot been previously demonstrated in VOC sensing platforms.

© 2016 Elsevier B.V. All rights reserved.

henylenediamines

. Introduction

Aromatic volatile organic compounds (VOCs) are harmful touman health and exposure to such vapors is associated with var-

ed pulmonary diseases [1–9]. Aromatic VOC release has been alsomplicated in ecological damage [3]. Development of sensors forromatic VOCs is therefore essential for early warning and air qual-ty monitoring applications. Numerous analytical techniques forOC anslysis are currently in use, including gas chromatography-ass spectrometry [10], quartz crystal microbalance [11], surface

coustic wave sensors [12], ion flow-tube mass spectrometry [13],nd chemiresistor-based sensing [1]. Despite the versatility ofetection techniques, current technologies are limited for prac-ical, easy to apply VOC sensing, specifically elaborate synthesischemes of the transduction substances, high cost of the devices,nd insufficient sensitivity/selectivity.

Synthesis of matrixes enabling effective adsorption and detec-ion of volatile substances is a fundamental requisite in gas sensoresign. Aerogels, among the lowest density solid materials, haveeen employed in vapor sensor designs [3,4,14–16]. Varied types

f aerogels have been reported, comprising scaffolding of silicon17], carbon [18], metals [19], metal oxides [20], organic polymers21], and others. Hydrophobic silica aerogels, in particular, exhibit

∗ Corresponding author at: Department of Chemistry, Ben Gurion University ofhe Negev, Beer Sheva 84105, Israel.

E-mail address: razj@bgu.ac.il (R. Jelinek).

ttp://dx.doi.org/10.1016/j.snb.2016.10.124925-4005/© 2016 Elsevier B.V. All rights reserved.

pronounced porous structures with very high internal surface areaavailable for adsorption of guest molecules [4]. Silica aerogels havebeen employed in diverse applications, including insulation mate-rials in the aerospace industry [22], sorption of miscible organicsolvents in water [4], and sensing of air pollutants [3].

Here, we report construction of a sensing platform for aromaticVOCs comprising silica aerogel embedding fluorescent carbondots (C-dots). C-dots, recently-discovered quasi-spherical carbona-ceous nanoparticles, have attracted significant interest due to theirunique structural and photophysical properties [23–26]. In partic-ular, C-dots exhibit broad range of excitation-dependent emissionspectra that are highly sensitive to the local environments of thedots, thus making possible their use in diverse sensing applications[27,28]. Moreover, C-dots are chemically stable, and are gener-ally produced using inexpensive and readily-available reagents andsimple synthesis procedures [29–31]. Recently, a C-dot-aerogel sys-tem was reported for NO2 gas sensing [3]. Importantly, the hybridC-dot-aerogel reported here was formed through a new single-stepsynthesis scheme which is simple and robust, in which the carbonprecursor was encapsulated within the aerogel matrix, and heat-ing of the composite material generated the C-dot-aerogel vaporsensor. The C-dot-aerogel was used for detection of different aro-matic VOCs; in particular, we found that different aromatic VOCsinduced distinct shifts and quenching of the fluorescence signals

associated with the aerogel-embedded C-dots. Overall, the C-dot-aerogel matrix could be effectively used as a platform for detectionand speciation of aromatic VOCs.

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. Experimental section

.1. Materials

Tetraethylorthosilicate (TEOS), d-(+)-glucose, sodiumulfate, pyridine, ammonium hydroxide, aniline, o- & m-henylenediamine, and phenol were purchased from Sigmaldrich, USA. l-(+)-Tartaric acid and p-phenylenediamine wereurchased from Alfa-Aesar, England. Benzene was purchased fromerck. Lauroyl chloride was bought from TCI, Japan. Nitroben-

ene was bought from BDH chemicals. Chloroform and n-hexaneere purchased from Daejung chemicals, Korea. Ethanolwasurchased from J.T. Baker. Tetrahydrofuran was purchased fromcros Organics, USA. Dimethyl formamide (DMF) and acetoneere purchased from Frutarom (Haifa, Israel). Ethyl acetate and

oncentrated hydrochloric acid (HCl) were purchased from Bio-Labtd (Jerusalem, Israel).

.2. Preparation of wet silica gel

Wet silica gel was prepared according to a previous report [3].riefly, 5 mL TEOS, 15 mL anhydrous ethanol (EtOH), 5 mL distilledater and 5 �L concentrated hydrochloric acid were mixed in a

00 mL flask and stirred in a 60 ◦C water bath for 90 min. Subse-uently, 25 mL ethanol, 13 mL distilled water and 15 �L NH4OHere added to the solution and stirred for 30 min under the same

emperature. The prepared wet silica gel was coated with parafilmefore it was further dried.

.3. Preparation of silica aerogels

The aerogel was prepared in a GCF1400 Atmosphere Furnacender N2 gas atmosphere. Specifically, the wet silica gel wasransferred carefully into a reaction chamber containing 200 mLnhydrous ethanol. Ultrapure nitrogen (N2) gas was passed intohe chamber to evacuate all air up to a pressure of 1 MPa. The tem-erature was then raised quickly from room temperature to 200 ◦Cy applying heating voltage 150 V, then increased slowly to 246 ◦C,ollowed by 260 ◦C for 3 h at 2 MPa N2 gas pressure. A white col-red silica aerogel formed in the reaction chamber following thereatment.

.4. In situ synthesis of C-dot-aerogel

The carbon dot precursor 6-O-(O-O′-dilauroyl-tartaryl)-d-lucose was synthesized according to a published report [25].riefly, 166 mL of lauroyl chloride was added to 30 g of finely pow-ered l-tartaric acid in a 500 mL round bottom flask equipped with

magnetic stirrer bar and an air bubbler. The reaction mixtureas heated at 90 ◦C for 24 h and then cooled to room tempera-

ure. In order to remove lauric acid and excess lauroyl chloride,he crude mixture was dissolved in a minimum amount of n-exane and kept at room temperature for 12 h. The product wasrecipitated in n-hexane and it was filtered, washed thoroughlyith n-hexane, and dried under vacuum to obtain (3R,4R)-2,5-ioxotetrahydrofuran-3,4-diyl didodecanoate as white powder. To

solution of d-glucose (20 g) in anhydrous DMF (150 mL), (3R,4R)-,5-dioxotetrahydrofuran-3,4-diyl didodecanoate (11 g) was addedith stirring under argon and the reaction mixture was allowed to

ool down to 0 ◦C, followed by addition of dry pyridine (1.8 mL).he reaction was continued under an argon atmosphere at 0 ◦C for–3 h, followed by an additional 3 days at room temperature. After

ompletion of the reaction, the mixture was poured into ice-waterixture and then 2 N HCl was added at 0 ◦C vigorous stirring. The

roduct was extracted with ethyl acetate, washed four times withrine solution, dried over sodium sulphate and the organic solvent

tors B 241 (2017) 607–613

was removed under reduced pressure to obtain the crude product.Then the crude mixture was dissolved in a minimum amount of n-hexane under reflux and a half volume of acetone was added. Thesolution was cooled to 0 ◦C in an ice-water bath and then kept 12 hat room temperature. The compound was precipitated from themixture, filtered and dried to obtain 6-O-(O-O′-dilauroyl-tartaryl)-d-glucose.

10 mg of the C-dot precursor were mixed with 100 mg aerogel ina glass vial and 300 �L distilled water was added to the mixture. Thesuspension was then sonicated and heated at 125 ◦C for 2.5 h. Thesynthesized C-dots-aerogel was purified by CHCl3 several times toremove unbound C-dots.

2.5. VOC sensing experiments

Predetermined quantities of organic compounds were placedin 5 mL closed glass vials and vaporised at ∼80–300 ◦C, depend-ing upon the vaporization temperatures of the respective VOCs.5 mL of each VOC were extracted and transferred to sealed 5 mLglass vials which already contained 10 mg C-dot-aerogel, and incu-bated for 2 h at room temperature prior to fluorescence analysis.5 mL of each VOC were separately extracted from the containersand transferred to a closed 500 mL container to obtain accurateconcentration values using a MiniRAE Lite (PID) system.

2.6. Instrumentation and characterization

Transmission electron microscopy (TEM) experiments utilized a C-dot-aerogel that was dissolved in toluene for extraction of carbondots from the aerogel matrix. High resolution TEM (HRTEM) sampleswere prepared by placing a drop of solution on a graphene-coatedcopper grid and observed with a 200 kV JEOL JEM-2100F micro-scope (Japan). Scanning electron microscopy (SEM) experimentswere conducted using a JEOL (Tokyo, Japan) model JSM-7400Fscanning electron microscope. X-ray photoelectron spectroscopy(XPS) was performed using an X-ray photoelectron spectrometerESCALAB 250 ultrahigh vacuum (1*10−9 bar) apparatus with anAlK� X-ray source and a monochromator. The X-ray beam sizewas 500 �m and survey spectra was recorded with pass energy(PE) 150 eV and high energy resolution spectra were recorded withpass energy (PE) 20 eV. Processing of the XPS results was car-ried out using AVANTGE program. Fluorescence emission spectra ofthe C-dot-aerogel film using different excitation wavelengths wererecorded on a Varioskan plate reader. Confocal microscopy images ofC-dot-aerogel were acquired on an UltraVIEW system (PerkinElmerLife Sciences, Waltham, MA) equipped with an Axiovert-200 Mmicroscope (Zeiss, Oberkochen, Germany) and a Plan-Neofluar63x/1.4 oil objective. Excitation wavelengths of 405 nm, 440 nm,and 568 nm were produced by an argon/krypton laser. Surface area,pore volume and pore diameters of the C-dot-aerogel were measuredby a BET instrument (Quantachrome-HIGH SPEEDGAS SORPTIONANALYZER- NOVA-1200e). Degassing of the C-dot-aerogel was car-ried out for 21 h in order to evaporate all traces of solvents andmoisture followed by N2 adsorption-desorption in liquid nitrogen.Relative humidity (RH) conditions were produced by different sat-urated salt solutions in their equilibrium states including LiCl for11% RH, KCH3COO for 22%, MgCl2 for 33% RH, Mg(NO3)2 for 53% RH,KI for 68% RH and K2SO4 for 97% RH, respectively, at 25 ◦C [32].

3. Results and discussion

The C-dot-aerogel composite was prepared through a newin-situ synthetic route depicted in Fig. 1. The porous aerogelframework was first generated through high temperature sil-ica annealing in the presence of pressurized nitrogen gas [33].

S. Dolai et al. / Sensors and Actuators B 241 (2017) 607–613 609

Fig. 1. Synthesis of the carbon-dot-aerogel and its fluorescence properties. A. Schematic illustration of C-dots-aerogel fabrication. The digital photographs depict regular( . B. Cow ecorde H (iv

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left) and fluorescence (excitation 365 nm, right) images of the material producedere adsorbed within the aerogel. (i) Bright-field image; fluorescent images were r

mission filter EM 477/45 (iii); and excitation at 568 nm, emission filter EM 640/120

ollowing aerogel formation, the carbonaceous precursor 6-O-O-O′-dilauroyl-tartaryl)-d-glucose [25] and aerogel were mixedn water by mild sonication, and a hydrothermal treatment125 ◦C) was carried out to generate C-dots embedded withinhe aerogel pores (Fig. 1). Thorough washing of the C-dot-erogel hybrid with chloroform was subsequently carried out toemove unbound C-dots. Importantly, the choice of the 6-O-(O-′-dilauroyl-tartaryl)-d-glucose as precursor for C-dot fabricationithin the aerogel matrix was based upon the amphiphilicity of

he compound, expected to facilitate its efficient adsorption andmmobilization within the aerogel pores which exhibit amphiphilicurface domains [34]. It should be noted that the synthesis schemeepicted in Fig. 1A is simple and robust, different than a previoustudy reporting C-dot-aerogel formation, which utilized separatereparation of C-dots in solution, purification, and subsequent

nsertion into the aerogel host [3].The regular and fluorescence photographs in Fig. 1A visu-

lly depict the reaction products. The as-synthesized aerogelppeared as a whitish powder exhibiting slight blue fluorescenceFig. 1A, bottom left images). However, the C-dot-aerogel com-osite acquired a yellow fluorescence upon excitation at 365 nmue to the C-dots embedded within the aerogel matrix. Theonfocal microscopy images recorded using different excitationavelengths/emission filters (Fig. 1B) further confirm the associ-

tion of the C-dots within the aerogel particles. Specifically, theicroscopy images in Fig. 1B reveal that the aerogel particulates

ppear in different colors upon excitation using distinct wave-engths. The specific colors correspond to the aerogel-associated-dots, reflecting the well-known excitation-dependent emissionavelengths of C-dots [34].

Figs. 2 and 3 present spectroscopic and microscopic charac-erization of the C-dot-aerogel hybrid. The excitation-dependentmission spectra in Fig. 2A underscore the distinct chemical envi-onment of the C-dots within the aerogel pores. Specifically, the

nfocal fluorescence images of C-dot-aerogel particles confirming that the C-dotsed upon excitation at 405 nm, emission filter EM 445/60 (ii); excitation at 440 nm,) for blue, green and red fluorescence respectively. Scale bar corresponds to 10 �m.

excitation-dependent emission spectra of the aerogel-embeddedC-dots (Fig. 2A,ii) exhibit different peak positions and emission inten-sities (e.g. peak heights) compared to the corresponding spectrarecorded for water-solubilized C-dots (Fig. 2A,i). The spectral differ-ences are due to the pronounced sensitivity of C-dots’ fluorescenceto their local molecular environments, confirming that the aerogel-associated C-dots were adsorbed onto the internal pore surfacewithin the aerogel and were less exposed to the aqueous solution.

The x ray photoelectron spectroscopy (XPS) analysis in Fig. 2Bcorroborates the interpretation of the fluorescence spectroscopydata, providing evidence for immobilization of the C-dots upon theaerogel surface. Specifically, the C 1S peak at approximately 286 eV(Fig. 2B,ii) corresponds to the C-dots [24,25] and indicates the pres-ence of abundant C-dots attached to the aerogel scaffold. The C 1Ssignal is absent in the XPS of the parent aerogel material (Fig. 2B,i)as it comprises of only a silica framework. Transmission electronmicroscopy (TEM) experiments of the C-dots extracted from theaerogel matrix reveal quite a uniform size distribution of the car-bon nanoparticles (Fig. 2C); statistical analysis based upon the TEMimages indicate diameters of 2.4 ± 0.5 nm (Fig. S1, Supporting infor-mation). The representative high resolution TEM (HR-TEM) imageFig. 2C (right) illuminates the graphitic crystal planes within theC-dots’ cores [25,26].

Fig. 3 presents microscopic and gas adsorption analysis of theaerogel, indicating that the porous structure of the host matrixwas retained following the in-situ synthesis of the embeddedC-dots. The scanning electron microscopy (SEM) image of the C-dot-aerogel in Fig. 3A shows the typical hierarchical high-surfacearea organization of the aerogel [3]. The Brunauer–Emmett–Teller(BET) experiments of the C-dot-aerogel summarized in Fig. 3B indi-

2

cate a relatively high specific surface area (325 m /g) suitable forvapor adsorption. Specifically, the isotherms in Fig. 3B,i exhib-ited type IV adsorption displaying a distinct hysteresis loop [35].Fig. 3B,ii reveals that the pore volume and average pore diameter of

610 S. Dolai et al. / Sensors and Actuators B 241 (2017) 607–613

Fig. 2. Characterization of the carbon-dot-aerogel. A. Excitation-dependent emission spectra of the C-dots in aqueous solution (i) and in the C-dot-aerogel (ii);B. X-rayphotoelectron spectra (XPS) of the silica aerogel host (without embedded C-dots) (i) and C-dots-aerogel (ii). The assignment of peaks to specific atomic species is indicated.C. Transmission electron microscopy (TEM) image (left) and high resolution TEM image (right) of C-dots extracted from the aerogel. The crystal planes of the C-dot graphiticcore are apparent. Scale bars correspond to 10 nm (left image) and 2 nm (right image).

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ig. 3. Properties of the aerogel host matrix. A. Scanning electron microscopy (SEMore size distribution curve indicates average pore size of 4.35 nm.

he C-dot-aerogel were 0.17 cc/g and 4.35 nm, respectively. Overall,he BET analysis underscores the high porosity of the C-dot-aerogelnd its applicability for efficient vapor adsorption.

Figs. 4 and 5 illustrate applications of the C-dot-aerogel com-osite for sensing aromatic volatile organic compounds (VOCs).ig. 4A shows the modulation of the fluorescence emission spectrapon exposure of the C-dot-aerogel to aniline vapor at a concen-ration of 90 ppm. Two effects can be discerned in Fig. 4A. Anilinelearly quenched the emission spectra (in all excitation wavelengthsxamined, Fig. 4A). In addition, shifts of the emission peaks werepparent in the presence of the aniline vapor (Numerical informa-

ion is provided in Table S1, Supporting information). Modulationf the fluorescence signals in Fig. 4A is ascribed to adsorption of theniline molecules within the aerogel pores and proximity betweenhe unpaired electrons of aniline and surface moieties of the C-dots.

ge of C-dot-aerogel; B. i. N2 adsorption-desorption isotherms of C-dot-aerogel. ii.

XPS data confirmed adsorption of aniline upon the aerogel poresurface (Fig. S2, Supporting information). Quenching of C-dots’ flu-orescence and spectral shifts were previously reported when C-dotswere localized in close distance to electron donors [36], and in somecases also electron acceptors [3,28]. Amine-containing compoundsin particular were shown to induce C-dot fluorescence quenchingin varied systems [36]. Such fluorescence changes are believed toreflect perturbations to the C-dots’ excitons induced by reactiveresidues in close proximity [3,28,36].

Fig. 4B depicts the relationship between the concentrationof aniline vapor and extent of quenching of the fluorescence

signal at 540 nm (excitation 450 nm). The graph in Fig. 4B demon-strates direct correlation between aniline concentration (i.e. vaporpressure) and inhibition of C-dots’ fluorescence, confirming thataniline adsorption was the likely factor responsible for fluorescence

S. Dolai et al. / Sensors and Actuators B 241 (2017) 607–613 611

Fig. 4. Effect of aniline vapor upon the fluorescence properties of the C-dot-aerogel. A. Excitation dependent emission spectra of C-dot-aerogel without (i) and in thepresence of 90 ppm aniline vapor at room temperature (ii); B. Concentration-dependent fluorescence quenching of C-dots (�ex 450 nm/�em 540 nm) in presence of anilinevapor. Experiments were carried out at 25 ◦C.

Fig. 5. Modulation of the fluorescence response of the C-dot-aerogel to different aromatic VOCs. A. Chemical structures of compounds tested. i, benzene; ii, aniline; iii,o nzenep 540 nr

qwed

-phenylenediamine; iv, m-phenylenediamine; v, p-phenylenediamine; vi, nitroberesence of the aromatic VOCs i–vii. C. Shifts the fluorescence peak (�ex 450 nm/�em

elative humidity (RH) of 53%.

uenching. Similar aniline concentration-dependent quenchingas apparent for other emission peaks (i.e. using different

xcitation/emission pairs, data not shown). The concentration-ependent fluorescence quenching graph in Fig. 4B essentially

; vii, phenol. B. Intensity of the fluorescence peak (�ex 450 nm/�em 540 nm) in them) in the presence of vapors i–vii. All measurements were carried out at 25 ◦C and

constitutes a calibration curve, pointing to potential use of theC-dot-aerogel hybrid as a quantitative sensor for aniline vapor.Moreover, the detection threshold attained by the C-dot-aerogelhybrid, which is less than 5 ppm, is low and comparable to other

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eported aniline vapor sensors [37–39]. Notably, this concentra-ion value is the recommended exposure limit for aniline and itsomologs by the U.S. National Institute for Occupational Safety andealth (NIOSH) [40].

To further explore the applicability of the C-dot-aerogel as alatform for VOC sensing and investigate the molecular parametersffecting modulation of the fluorescence properties, we recordedhe relative quenching and shifts of the fluorescence signals uponxposure to different vapor compounds (Fig. 5). The bar diagramsn Fig. 5 outline the relative intensities (Fig. 5A) and peak posi-ions (Fig. 5B) of the fluorescence emission signal at around 540 nmexcitation 450 nm) upon exposure to different VOCs, at concentra-ion of ∼90 ppm. Notably, the selection of VOCs in the experimentsummarized in Fig. 5 was designed to examine the contributionso fluorescence modulation induced by the phenyl unit as well asunctional substituents linked to the aromatic ring. Accordingly, inddition to aniline (also referred to as phenylamine) which is a mildlectron donor, we tested the three isomers of phenylenediamines,henol (electron donor), and nitrobenzene (exhibiting electron with-rawing properties).

Fig. 5 displays significant differences among the VOCs examined,oth in the extent of fluorescence quenching (in comparison withhe control C-dot-aerogel sample not exposed to vapors) as well asn the induced shifts of the emission peak (in relation to the control).pecifically, both aniline and p-phenylenediamine gave rise to sig-ificant fluorescence quenching (Fig. 5A) and pronounced positivehifts of the fluorescence signal (Fig. 5B). In contrast, lesser quench-ng, and negligible or small negative shifts were apparent when the-dot-aerogel was treated with vapors of the other aromatic VOCs.

The fluorescence results presented in Fig. 5 likely reflect theistinct structural and electronic properties of the aromatic VOCsested and their interactions with the aerogel-embedded C-dots.oth aniline and p-phenylenediamine possess basic properties dueo the unpaired electrons at the outer shell of nitrogen. Theuter electrons of the amine moieties strongly interact withhe C-dots, giving rise to the observed fluorescence quenchingFig. 5A) and signal shifts (Fig. 5B). In particular, the two aminesn the p-phenylenediamine isomer at the opposite ends of the

olecule do not sterically hinder each other and probably enablefficient electronic interactions with the aerogel-embedded C-ots. In comparison, the amine residues in o-phenylenediaminend m-phenylenediamine are more sterically restricted therebyxerting lesser interference with the electronic properties ofhe co-adsorbed C-dots. This interpretation probably accountsor the significantly less fluorescence quenching induced by o-henylenediamine and m-phenylenediamine (Fig. 5A,iii,iv) andmall shifts of the emission peak (Fig. 5B,iii,iv). Indeed, the dif-erences in electron donating properties and reactivity between-phenylenediamine and the two other isomers are well known41].

We also assessed the sensitivity of the new C-dot-aerogel sen-or to water vapor. The emission spectra of the C-dots-aerogel wereeasured in different relative humidity (RH) values and exhibited

nsignificant changes of intensity in a broad range of humidity val-es (Fig. S3, Supporting information). This result indicates that the-dots’ fluorescence was not affected by water vapor. In partic-lar, similar extents of gas-induced fluorescence quenching were

nduced in considerably different humidity conditions (Fig. S4, Sup-orting information), confirming that water vapor did not affect theensor response.

The recorded modulations of the fluorescence spectra byitrobenzene and phenol, respectively, are consistent with the

OC sensing mechanism outlined above. Nitrobenzene has stronglectron withdrawing properties, accordingly the effects ofitrobenzene vapor upon the fluorescence signals of the C-dotsre small (very low quenching (Fig. 5A,vi) and small shift in emis-

tors B 241 (2017) 607–613

sion peak (Fig. 5B,vi)). In case of phenol, the lower pKa value ofphenol compared to aniline means that the unpaired electronsof phenol are likely less prone to interactions with the aerogel-encapsulated C-dots, giving rise to less significant fluorescencemodulation compared to aniline (Fig. 5A,vii and Fig. 5B,vii). Note,however, that the quenching induced by phenol vapor (Fig. 5A,vii)is more pronounced compared to ortho-phenylenediamine, meta-phenylenediamine, or nitrobenzene (Fig. 5A), consistent with theprominent role of the electron donating profile of the aromaticVOCs in affecting the fluorescence response of the C-dot-aerogelhybrid.

4. Conclusions

We constructed an aromatic VOC sensor comprising fluorescentC-dots prepared in-situ within the porous framework of a silicaaerogel. The structural and physical properties of both the aerogelmatrix and embedded C-dots were retained following the synthesisprocedure. The C-dot-aerogel hybrid was used as a sensitive sensorfor aromatic VOCs, exploiting the porosity and high surface areafor adsorption of the volatile compounds and their effects uponboth the quenching and shifts of the C-dots’ fluorescence. Impor-tantly, the observed fluorescence modulation was dependent uponthe electronic and structural features of the VOCs, specifically thepresence and size of electron donating residues linked to the phenylring. Accordingly, the C-dot-aerogel vapor sensor could distinguishamong different aromatic VOCs. Modulation of the C-dot-aerogelfluorescence was particularly apparent in case of strongly electron-donating molecules such as aniline and para-phenylenediamine. Itshould be noted that the C-dot-aerogel system has been appliedhere for analysis of the individual gases rather than gas mixtures.Experiments examining multicomponent analysis of VOC mixturesusing the new hybrid aerogel are currently pursued.

The C-dot-aerogel sensor exhibits notable practical advan-tages. Preparation of the hybrid material is straightforward, usinginexpensive and readily-available reagents. The actual sensingexperiments are easy to perform and carried out through fluores-cence analysis of the C-dot-aerogel powder after exposure to theVOCs. The sensor material is resilient and can be kept at ambientconditions for long time periods (months) without adversely affect-ing the sensing properties. The C-dot-aerogel construct was appliedin our laboratory for detection of other, non-aromatic, VOCs, under-scoring its broad sensing capabilities.

Acknowledgment

Dr. Susanta Kumar Bhunia is grateful to the Planning and Bud-geting Committee (PBC) of the Israeli Council for Higher Educationfor an Outstanding Post-doctoral Fellowship.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.snb.2016.10.124.

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Biographies

Susmita Dolai is a visiting student at the Department of Chemistry, Ben GurionUniversity, Beer Sheva, Israel.

Dr. Susanta Kumar Bhunia is a post-doctoral fellow at the Department of Chemistry,

Ben Gurion University, Beer Sheva, Israel.

Professor Raz Jelinek holds the Carole and Barry Kaye Chair in Applied Science atthe Department of Chemistry and the Ilse Katz Institute for Nanoscle Science andTechnology, Ben Gurion University, Beer Sheva, Israel.