Carbon-dot-aerogel sensor for aromatic volatile organic compounds

Sensors and Actuators B 241 (2017) 607–613

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Carbon-dot-aerogel sensor for aromatic volatile organic compounds Susmita Dolai a , Susanta Kumar Bhunia a , Raz Jelinek a,b,∗ a b

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

a r t i c l e

i n f o

Article history: Received 25 August 2016 Received in revised form 23 October 2016 Accepted 26 October 2016 Available online 27 October 2016 Keywords: Carbon dots Aerogel VOCs Gas sensors Fluorescence quenching Phenylenediamines

a b s t r a c t Detection of aromatic volatile organic compounds (VOCs) is important for monitoring occupational hazards, industrial safety, and environmental applications. Here, we present a new in-situ-synthesized carbon dot – aerogel matrix and demonstrate its application for sensing aromatic VOCs. The composite aerogel exhibited high specific surface area and pore diameter, enabling efficient adsorption of the organic vapors. In particular, the excitation-dependent luminescence emission properties of the carbon dots were retained upon embedding within the aerogel host, and provided a sensitive transduction mechanism through both shifts and quenching of the fluorescence emissions. We show that distinct fluorescence shifts and degrees of quenching were induced by different aromatic VOCs. In particular, the C-dot-aerogel sensor could distinguish between isomers of phenylenediamine, an important achievement which has not been previously demonstrated in VOC sensing platforms. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Aromatic volatile organic compounds (VOCs) are harmful to human health and exposure to such vapors is associated with varied pulmonary diseases [1–9]. Aromatic VOC release has been also implicated in ecological damage [3]. Development of sensors for aromatic VOCs is therefore essential for early warning and air quality monitoring applications. Numerous analytical techniques for VOC anslysis are currently in use, including gas chromatographymass spectrometry [10], quartz crystal microbalance [11], surface acoustic wave sensors [12], ion flow-tube mass spectrometry [13], and chemiresistor-based sensing [1]. Despite the versatility of detection techniques, current technologies are limited for practical, easy to apply VOC sensing, specifically elaborate synthesis schemes of the transduction substances, high cost of the devices, and insufficient sensitivity/selectivity. Synthesis of matrixes enabling effective adsorption and detection of volatile substances is a fundamental requisite in gas sensor design. Aerogels, among the lowest density solid materials, have been employed in vapor sensor designs [3,4,14–16]. Varied types of aerogels have been reported, comprising scaffolding of silicon [17], carbon [18], metals [19], metal oxides [20], organic polymers [21], and others. Hydrophobic silica aerogels, in particular, exhibit

∗ Corresponding author at: Department of Chemistry, Ben Gurion University of the Negev, Beer Sheva 84105, Israel. E-mail address: [email protected] (R. Jelinek). http://dx.doi.org/10.1016/j.snb.2016.10.124 0925-4005/© 2016 Elsevier B.V. All rights reserved.

pronounced porous structures with very high internal surface area available for adsorption of guest molecules [4]. Silica aerogels have been employed in diverse applications, including insulation materials in the aerospace industry [22], sorption of miscible organic solvents in water [4], and sensing of air pollutants [3]. Here, we report construction of a sensing platform for aromatic VOCs comprising silica aerogel embedding fluorescent carbon dots (C-dots). C-dots, recently-discovered quasi-spherical carbonaceous nanoparticles, have attracted significant interest due to their unique structural and photophysical properties [23–26]. In particular, C-dots exhibit broad range of excitation-dependent emission spectra that are highly sensitive to the local environments of the dots, thus making possible their use in diverse sensing applications [27,28]. Moreover, C-dots are chemically stable, and are generally produced using inexpensive and readily-available reagents and simple synthesis procedures [29–31]. Recently, a C-dot-aerogel system was reported for NO2 gas sensing [3]. Importantly, the hybrid C-dot-aerogel reported here was formed through a new single-step synthesis scheme which is simple and robust, in which the carbon precursor was encapsulated within the aerogel matrix, and heating of the composite material generated the C-dot-aerogel vapor sensor. The C-dot-aerogel was used for detection of different aromatic VOCs; in particular, we found that different aromatic VOCs induced distinct shifts and quenching of the fluorescence signals associated with the aerogel-embedded C-dots. Overall, the C-dotaerogel matrix could be effectively used as a platform for detection and speciation of aromatic VOCs.

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

2. Experimental section 2.1. Materials Tetraethylorthosilicate (TEOS), d-(+)-glucose, sodium sulfate, pyridine, ammonium hydroxide, aniline, o- & mphenylenediamine, and phenol were purchased from Sigma Aldrich, USA. l-(+)-Tartaric acid and p-phenylenediamine were purchased from Alfa-Aesar, England. Benzene was purchased from Merck. Lauroyl chloride was bought from TCI, Japan. Nitrobenzene was bought from BDH chemicals. Chloroform and n-hexane were purchased from Daejung chemicals, Korea. Ethanolwas purchased from J.T. Baker. Tetrahydrofuran was purchased from Acros Organics, USA. Dimethyl formamide (DMF) and acetone were purchased from Frutarom (Haifa, Israel). Ethyl acetate and concentrated hydrochloric acid (HCl) were purchased from Bio-Lab Ltd (Jerusalem, Israel). 2.2. Preparation of wet silica gel Wet silica gel was prepared according to a previous report [3]. Briefly, 5 mL TEOS, 15 mL anhydrous ethanol (EtOH), 5 mL distilled water and 5 ␮L concentrated hydrochloric acid were mixed in a 100 mL flask and stirred in a 60 ◦ C water bath for 90 min. Subsequently, 25 mL ethanol, 13 mL distilled water and 15 ␮L NH4 OH were added to the solution and stirred for 30 min under the same temperature. The prepared wet silica gel was coated with parafilm before it was further dried. 2.3. Preparation of silica aerogels The aerogel was prepared in a GCF1400 Atmosphere Furnace under N2 gas atmosphere. Specifically, the wet silica gel was transferred carefully into a reaction chamber containing 200 mL anhydrous ethanol. Ultrapure nitrogen (N2 ) gas was passed into the chamber to evacuate all air up to a pressure of 1 MPa. The temperature was then raised quickly from room temperature to 200 ◦ C by applying heating voltage 150 V, then increased slowly to 246 ◦ C, followed by 260 ◦ C for 3 h at 2 MPa N2 gas pressure. A white colored silica aerogel formed in the reaction chamber following the treatment. 2.4. In situ synthesis of C-dot-aerogel The carbon dot precursor 6-O-(O-O -dilauroyl-tartaryl)-dglucose was synthesized according to a published report [25]. Briefly, 166 mL of lauroyl chloride was added to 30 g of finely powdered l-tartaric acid in a 500 mL round bottom flask equipped with a magnetic stirrer bar and an air bubbler. The reaction mixture was heated at 90 ◦ C for 24 h and then cooled to room temperature. In order to remove lauric acid and excess lauroyl chloride, the crude mixture was dissolved in a minimum amount of nhexane and kept at room temperature for 12 h. The product was precipitated in n-hexane and it was filtered, washed thoroughly with n-hexane, and dried under vacuum to obtain (3R,4R)-2,5dioxotetrahydrofuran-3,4-diyl didodecanoate as white powder. To a solution of d-glucose (20 g) in anhydrous DMF (150 mL), (3R,4R)2,5-dioxotetrahydrofuran-3,4-diyl didodecanoate (11 g) was added with stirring under argon and the reaction mixture was allowed to cool down to 0 ◦ C, followed by addition of dry pyridine (1.8 mL). The reaction was continued under an argon atmosphere at 0 ◦ C for 2–3 h, followed by an additional 3 days at room temperature. After completion of the reaction, the mixture was poured into ice-water mixture and then 2 N HCl was added at 0 ◦ C vigorous stirring. The product was extracted with ethyl acetate, washed four times with brine solution, dried over sodium sulphate and the organic solvent

was removed under reduced pressure to obtain the crude product. Then the crude mixture was dissolved in a minimum amount of nhexane under reflux and a half volume of acetone was added. The solution was cooled to 0 ◦ C in an ice-water bath and then kept 12 h at room temperature. The compound was precipitated from the mixture, 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 in a glass vial and 300 ␮L distilled water was added to the mixture. The suspension was then sonicated and heated at 125 ◦ C for 2.5 h. The synthesized C-dots-aerogel was purified by CHCl3 several times to remove unbound C-dots. 2.5. VOC sensing experiments Predetermined quantities of organic compounds were placed in 5 mL closed glass vials and vaporised at ∼80–300 ◦ C, depending upon the vaporization temperatures of the respective VOCs. 5 mL of each VOC were extracted and transferred to sealed 5 mL glass vials which already contained 10 mg C-dot-aerogel, and incubated for 2 h at room temperature prior to fluorescence analysis. 5 mL of each VOC were separately extracted from the containers and transferred to a closed 500 mL container to obtain accurate concentration values using a MiniRAE Lite (PID) system. 2.6. Instrumentation and characterization Transmission electron microscopy (TEM) experiments utilized a Cdot-aerogel that was dissolved in toluene for extraction of carbon dots from the aerogel matrix. High resolution TEM (HRTEM) samples were prepared by placing a drop of solution on a graphene-coated copper grid and observed with a 200 kV JEOL JEM-2100F microscope (Japan). Scanning electron microscopy (SEM) experiments were conducted using a JEOL (Tokyo, Japan) model JSM-7400F scanning electron microscope. X-ray photoelectron spectroscopy (XPS) was performed using an X-ray photoelectron spectrometer ESCALAB 250 ultrahigh vacuum (1*10−9 bar) apparatus with an AlK␣ X-ray source and a monochromator. The X-ray beam size was 500 ␮m and survey spectra was recorded with pass energy (PE) 150 eV and high energy resolution spectra were recorded with pass energy (PE) 20 eV. Processing of the XPS results was carried out using AVANTGE program. Fluorescence emission spectra of the C-dot-aerogel film using different excitation wavelengths were recorded on a Varioskan plate reader. Confocal microscopy images of C-dot-aerogel were acquired on an UltraVIEW system (PerkinElmer Life Sciences, Waltham, MA) equipped with an Axiovert-200 M microscope (Zeiss, Oberkochen, Germany) and a Plan-Neofluar 63x/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 measured by a BET instrument (Quantachrome-HIGH SPEEDGAS SORPTION ANALYZER- NOVA-1200e). Degassing of the C-dot-aerogel was carried out for 21 h in order to evaporate all traces of solvents and moisture followed by N2 adsorption-desorption in liquid nitrogen. Relative humidity (RH) conditions were produced by different saturated salt solutions in their equilibrium states including LiCl for 11% RH, KCH3 COO for 22%, MgCl2 for 33% RH, Mg(NO3 )2 for 53% RH, KI for 68% RH and K2 SO4 for 97% RH, respectively, at 25 ◦ C [32]. 3. Results and discussion The C-dot-aerogel composite was prepared through a new in-situ synthetic route depicted in Fig. 1. The porous aerogel framework was first generated through high temperature silica annealing in the presence of pressurized nitrogen gas [33].


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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 (left) and fluorescence (excitation 365 nm, right) images of the material produced. B. Confocal fluorescence images of C-dot-aerogel particles confirming that the C-dots were adsorbed within the aerogel. (i) Bright-field image; fluorescent images were recorded upon excitation at 405 nm, emission filter EM 445/60 (ii); excitation at 440 nm, emission filter EM 477/45 (iii); and excitation at 568 nm, emission filter EM 640/120H (iv) for blue, green and red fluorescence respectively. Scale bar corresponds to 10 ␮m.

Following aerogel formation, the carbonaceous precursor 6-O(O-O -dilauroyl-tartaryl)-d-glucose [25] and aerogel were mixed in water by mild sonication, and a hydrothermal treatment (125 ◦ C) was carried out to generate C-dots embedded within the aerogel pores (Fig. 1). Thorough washing of the C-dotaerogel hybrid with chloroform was subsequently carried out to remove unbound C-dots. Importantly, the choice of the 6-O-(OO -dilauroyl-tartaryl)-d-glucose as precursor for C-dot fabrication within the aerogel matrix was based upon the amphiphilicity of the compound, expected to facilitate its efficient adsorption and immobilization within the aerogel pores which exhibit amphiphilic surface domains [34]. It should be noted that the synthesis scheme depicted in Fig. 1A is simple and robust, different than a previous study reporting C-dot-aerogel formation, which utilized separate preparation of C-dots in solution, purification, and subsequent insertion into the aerogel host [3]. The regular and fluorescence photographs in Fig. 1A visually depict the reaction products. The as-synthesized aerogel appeared as a whitish powder exhibiting slight blue fluorescence (Fig. 1A, bottom left images). However, the C-dot-aerogel composite acquired a yellow fluorescence upon excitation at 365 nm due to the C-dots embedded within the aerogel matrix. The confocal microscopy images recorded using different excitation wavelengths/emission filters (Fig. 1B) further confirm the association of the C-dots within the aerogel particles. Specifically, the microscopy images in Fig. 1B reveal that the aerogel particulates appear in different colors upon excitation using distinct wavelengths. The specific colors correspond to the aerogel-associated C-dots, reflecting the well-known excitation-dependent emission wavelengths of C-dots [34]. Figs. 2 and 3 present spectroscopic and microscopic characterization of the C-dot-aerogel hybrid. The excitation-dependent emission spectra in Fig. 2A underscore the distinct chemical environment of the C-dots within the aerogel pores. Specifically, the

excitation-dependent emission spectra of the aerogel-embedded C-dots (Fig. 2A,ii) exhibit different peak positions and emission intensities (e.g. peak heights) compared to the corresponding spectra recorded for water-solubilized C-dots (Fig. 2A,i). The spectral differences are due to the pronounced sensitivity of C-dots’ fluorescence to their local molecular environments, confirming that the aerogelassociated C-dots were adsorbed onto the internal pore surface within the aerogel and were less exposed to the aqueous solution. The x ray photoelectron spectroscopy (XPS) analysis in Fig. 2B corroborates the interpretation of the fluorescence spectroscopy data, providing evidence for immobilization of the C-dots upon the aerogel surface. Specifically, the C 1S peak at approximately 286 eV (Fig. 2B,ii) corresponds to the C-dots [24,25] and indicates the presence of abundant C-dots attached to the aerogel scaffold. The C 1S signal is absent in the XPS of the parent aerogel material (Fig. 2B,i) as it comprises of only a silica framework. Transmission electron microscopy (TEM) experiments of the C-dots extracted from the aerogel matrix reveal quite a uniform size distribution of the carbon nanoparticles (Fig. 2C); statistical analysis based upon the TEM images indicate diameters of 2.4 ± 0.5 nm (Fig. S1, Supporting information). The representative high resolution TEM (HR-TEM) image Fig. 2C (right) illuminates the graphitic crystal planes within the C-dots’ cores [25,26]. Fig. 3 presents microscopic and gas adsorption analysis of the aerogel, indicating that the porous structure of the host matrix was retained following the in-situ synthesis of the embedded C-dots. The scanning electron microscopy (SEM) image of the Cdot-aerogel in Fig. 3A shows the typical hierarchical high-surface area organization of the aerogel [3]. The Brunauer–Emmett–Teller (BET) experiments of the C-dot-aerogel summarized in Fig. 3B indicate a relatively high specific surface area (325 m2 /g) suitable for vapor adsorption. Specifically, the isotherms in Fig. 3B,i exhibited type IV adsorption displaying a distinct hysteresis loop [35]. Fig. 3B,ii reveals that the pore volume and average pore diameter of

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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-ray photoelectron 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 graphitic core are apparent. Scale bars correspond to 10 nm (left image) and 2 nm (right image).

Fig. 3. Properties of the aerogel host matrix. A. Scanning electron microscopy (SEM) image of C-dot-aerogel; B. i. N2 adsorption-desorption isotherms of C-dot-aerogel. ii. Pore size distribution curve indicates average pore size of 4.35 nm.

the C-dot-aerogel were 0.17 cc/g and 4.35 nm, respectively. Overall, the BET analysis underscores the high porosity of the C-dot-aerogel and its applicability for efficient vapor adsorption. Figs. 4 and 5 illustrate applications of the C-dot-aerogel composite for sensing aromatic volatile organic compounds (VOCs). Fig. 4A shows the modulation of the fluorescence emission spectra upon exposure of the C-dot-aerogel to aniline vapor at a concentration of 90 ppm. Two effects can be discerned in Fig. 4A. Aniline clearly quenched the emission spectra (in all excitation wavelengths examined, Fig. 4A). In addition, shifts of the emission peaks were apparent in the presence of the aniline vapor (Numerical information is provided in Table S1, Supporting information). Modulation of the fluorescence signals in Fig. 4A is ascribed to adsorption of the aniline molecules within the aerogel pores and proximity between the unpaired electrons of aniline and surface moieties of the C-dots.

XPS data confirmed adsorption of aniline upon the aerogel pore surface (Fig. S2, Supporting information). Quenching of C-dots’ fluorescence and spectral shifts were previously reported when C-dots were localized in close distance to electron donors [36], and in some cases also electron acceptors [3,28]. Amine-containing compounds in particular were shown to induce C-dot fluorescence quenching in varied systems [36]. Such fluorescence changes are believed to reflect perturbations to the C-dots’ excitons induced by reactive residues in close proximity [3,28,36]. Fig. 4B depicts the relationship between the concentration of aniline vapor and extent of quenching of the fluorescence signal at 540 nm (excitation 450 nm). The graph in Fig. 4B demonstrates direct correlation between aniline concentration (i.e. vapor pressure) and inhibition of C-dots’ fluorescence, confirming that aniline adsorption was the likely factor responsible for fluorescence


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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 the presence 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 aniline vapor. 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-phenylenediamine; iv, m-phenylenediamine; v, p-phenylenediamine; vi, nitrobenzene; vii, phenol. B. Intensity of the fluorescence peak (␭ex 450 nm/␭em 540 nm) in the presence of the aromatic VOCs i–vii. C. Shifts the fluorescence peak (␭ex 450 nm/␭em 540 nm) in the presence of vapors i–vii. All measurements were carried out at 25 ◦ C and relative humidity (RH) of 53%.

quenching. Similar aniline concentration-dependent quenching was apparent for other emission peaks (i.e. using different excitation/emission pairs, data not shown). The concentrationdependent fluorescence quenching graph in Fig. 4B essentially

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

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reported aniline vapor sensors [37–39]. Notably, this concentration value is the recommended exposure limit for aniline and its homologs by the U.S. National Institute for Occupational Safety and Health (NIOSH) [40]. To further explore the applicability of the C-dot-aerogel as a platform for VOC sensing and investigate the molecular parameters affecting modulation of the fluorescence properties, we recorded the relative quenching and shifts of the fluorescence signals upon exposure to different vapor compounds (Fig. 5). The bar diagrams in Fig. 5 outline the relative intensities (Fig. 5A) and peak positions (Fig. 5B) of the fluorescence emission signal at around 540 nm (excitation 450 nm) upon exposure to different VOCs, at concentration of ∼90 ppm. Notably, the selection of VOCs in the experiments summarized in Fig. 5 was designed to examine the contributions to fluorescence modulation induced by the phenyl unit as well as functional substituents linked to the aromatic ring. Accordingly, in addition to aniline (also referred to as phenylamine) which is a mild electron donor, we tested the three isomers of phenylenediamines, phenol (electron donor), and nitrobenzene (exhibiting electron withdrawing properties). Fig. 5 displays significant differences among the VOCs examined, both in the extent of fluorescence quenching (in comparison with the control C-dot-aerogel sample not exposed to vapors) as well as in the induced shifts of the emission peak (in relation to the control). Specifically, both aniline and p-phenylenediamine gave rise to significant fluorescence quenching (Fig. 5A) and pronounced positive shifts of the fluorescence signal (Fig. 5B). In contrast, lesser quenching, and negligible or small negative shifts were apparent when the C-dot-aerogel was treated with vapors of the other aromatic VOCs. The fluorescence results presented in Fig. 5 likely reflect the distinct structural and electronic properties of the aromatic VOCs tested and their interactions with the aerogel-embedded C-dots. Both aniline and p-phenylenediamine possess basic properties due to the unpaired electrons at the outer shell of nitrogen. The outer electrons of the amine moieties strongly interact with the C-dots, giving rise to the observed fluorescence quenching (Fig. 5A) and signal shifts (Fig. 5B). In particular, the two amines in the p-phenylenediamine isomer at the opposite ends of the molecule do not sterically hinder each other and probably enable efficient electronic interactions with the aerogel-embedded Cdots. In comparison, the amine residues in o-phenylenediamine and m-phenylenediamine are more sterically restricted thereby exerting lesser interference with the electronic properties of the co-adsorbed C-dots. This interpretation probably accounts for the significantly less fluorescence quenching induced by ophenylenediamine and m-phenylenediamine (Fig. 5A,iii,iv) and small shifts of the emission peak (Fig. 5B,iii,iv). Indeed, the differences in electron donating properties and reactivity between p-phenylenediamine and the two other isomers are well known [41]. We also assessed the sensitivity of the new C-dot-aerogel sensor to water vapor. The emission spectra of the C-dots-aerogel were measured in different relative humidity (RH) values and exhibited insignificant changes of intensity in a broad range of humidity values (Fig. S3, Supporting information). This result indicates that the C-dots’ fluorescence was not affected by water vapor. In particular, similar extents of gas-induced fluorescence quenching were induced in considerably different humidity conditions (Fig. S4, Supporting information), confirming that water vapor did not affect the sensor response. The recorded modulations of the fluorescence spectra by nitrobenzene and phenol, respectively, are consistent with the VOC sensing mechanism outlined above. Nitrobenzene has strong electron withdrawing properties, accordingly the effects of nitrobenzene vapor upon the fluorescence signals of the C-dots are small (very low quenching (Fig. 5A,vi) and small shift in emis-

sion peak (Fig. 5B,vi)). In case of phenol, the lower pKa value of phenol compared to aniline means that the unpaired electrons of phenol are likely less prone to interactions with the aerogelencapsulated C-dots, giving rise to less significant fluorescence modulation 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, metaphenylenediamine, or nitrobenzene (Fig. 5A), consistent with the prominent role of the electron donating profile of the aromatic VOCs in affecting the fluorescence response of the C-dot-aerogel hybrid. 4. Conclusions We constructed an aromatic VOC sensor comprising fluorescent C-dots prepared in-situ within the porous framework of a silica aerogel. The structural and physical properties of both the aerogel matrix and embedded C-dots were retained following the synthesis procedure. The C-dot-aerogel hybrid was used as a sensitive sensor for aromatic VOCs, exploiting the porosity and high surface area for adsorption of the volatile compounds and their effects upon both the quenching and shifts of the C-dots’ fluorescence. Importantly, the observed fluorescence modulation was dependent upon the electronic and structural features of the VOCs, specifically the presence and size of electron donating residues linked to the phenyl ring. Accordingly, the C-dot-aerogel vapor sensor could distinguish among different aromatic VOCs. Modulation of the C-dot-aerogel fluorescence was particularly apparent in case of strongly electrondonating molecules such as aniline and para-phenylenediamine. It should be noted that the C-dot-aerogel system has been applied here for analysis of the individual gases rather than gas mixtures. Experiments examining multicomponent analysis of VOC mixtures using the new hybrid aerogel are currently pursued. The C-dot-aerogel sensor exhibits notable practical advantages. Preparation of the hybrid material is straightforward, using inexpensive and readily-available reagents. The actual sensing experiments are easy to perform and carried out through fluorescence analysis of the C-dot-aerogel powder after exposure to the VOCs. The sensor material is resilient and can be kept at ambient conditions for long time periods (months) without adversely affecting the sensing properties. The C-dot-aerogel construct was applied in our laboratory for detection of other, non-aromatic, VOCs, underscoring its broad sensing capabilities. Acknowledgment Dr. Susanta Kumar Bhunia is grateful to the Planning and Budgeting Committee (PBC) of the Israeli Council for Higher Education for an Outstanding Post-doctoral Fellowship. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2016.10.124. References [1] J.S. Kim, H.W. Yoo, H.O. Choi, H.T. Jung, Tunable volatile organic compounds sensor by using thiolated ligand conjugation on MoS2 , Nano Lett. 14 (2014) 5941–5947. [2] J. Pauluhn, Subacute inhalation toxicity of aniline in rats: analysis of time-dependence and concentration-dependence of hematotoxic and splenic effects, Toxicol. Sci. 81 (2004) 198–215. [3] R.X. Wang, G.L. Li, Y.Q. Dong, Y.W. Chi, G.N. Chen, Carbon quantum dot-functionalized aerogels for NO2 gas sensing, Anal. Chem. 85 (2013) 8065–8069.

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Biographies Susmita Dolai is a visiting student at the Department of Chemistry, Ben Gurion University, 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 at the Department of Chemistry and the Ilse Katz Institute for Nanoscle Science and Technology, Ben Gurion University, Beer Sheva, Israel.