Sensors and Actuators B 241 (2017) 607–613
Contents lists available at ScienceDirect
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].
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
(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
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-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
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 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
612
S. Dolai et al. / Sensors and Actuators B 241 (2017) 607–613
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.
S. Dolai et al. / Sensors and Actuators B 241 (2017) 607–613
[4] D. Wang, E. McLaughlin, R. Pfeffer, Y.S. Lin, Adsorption of organic compounds
in vapor liquid, and aqueous solution phases on hydrophobic aerogels, Ind.
Eng. Chem. Res. 50 (2011) 12177–12185.
[5] G. Konvalina, H. Haick, Sensors for breath testing: from nanomaterials to
comprehensive disease detection, Acc. Chem. Res. 47 (2014) 66–76.
[6] U. Tisch, H. Haick, Chemical sensors for breath gas analysis: the latest
developments at the Breath Analysis Summit 2013, J. Breath Res. 8 (2014)
027103.
[7] N. Shehada, G. Bronstrup, K. Funka, S. Christiansen, M. Leja, H. Haick,
Ultrasensitive silicon nanowire for real-world gas sensing: noninvasive
diagnosis of cancer from breath volatolome, Nano Lett. 15 (2015) 1288–1295.
[8] B. Wang, J.C. Cancilla, J.S. Torrecilla, H. Haick, Artificial sensing intelligence
with silicon nanowires for ultraselective detection in the gas phase, Nano
Lett. 14 (2014) 933–938.
[9] B. Wang, T.P. Huynh, W.W. Wu, N. Hayek, T.T. Do, J.C. Cancilla, J.S. Torrecilla,
M.M. Nahid, J.M. Colwell, O.M. Gazit, S.R. Puniredd, C.R. McNeill, P. Sonar, H.
Haick, A highly sensitive diketopyrrolopyrrole-based ambipolar transistor for
selective detection and discrimination of xylene isomers, Adv. Mater. 28
(2016) 4012–4018.
[10] B. Buszewski, A. Ulanowska, T. Ligor, N. Denderz, A. Amann, Analysis of
exhaled breath from smokers, passive smokers and non-smokers by solid
phase microextraction gas chromatography/mass spectrometry, Biomed.
Chromatogr. 23 (2009) 551–556.
[11] C.D. Natale, A. Macagnano, E. Martinelli, R. Paolesse, G. D’Arcangelo, C.
Roscioni, A. Finazzi-Agro, A. D’Amico, Lung cancer identification by the
analysis of breath by means of an array of non-selective gas sensors, Biosens.
Bioelectron. 18 (2003) 1209–1218.
[12] J.W. Grate, S.L. Rose-Pehrsson, D.L. Venezky, Smart sensor system for trace
organophosphorus and organosulfur vapor detection employing a
temperature-controlled array of surface acoustic wave sensors automated
sample preconcentration, and pattern recognition, Anal. Chem. 65 (1993)
1868–1881.
[13] C. Turner, P. Spanel, D. Smith, A longitudinal study of ethanol and
acetaldehyde in the exhaled breath of healthy volunteers using selected-ion
flow-tube mass spectrometry, Rapid Commun. Mass Spectrom. 20 (2006)
61–68.
[14] L.Z. Zuo, Y.F. Zhang, L.S. Zhang, Y.E. Miao, W. Fan, T.X. Liu,
Polymer/carbon-based hybrid aerogels: preparation, properties and
applications, Materials 8 (2015) 6806–6848.
[15] H.S. Qi, J.W. Liu, J. Pionteck, P. Potschke, E. Madera, Carbon nanotube-cellulose
composite aerogels for vapour sensing, Sens. Actuators B 213 (2015) 20–26.
[16] U. Thubsuang, D. Sukanan, S. Sahasithiwat, S. Wongkasemjit, T. Chaisuwan,
Highly sensitive room temperature organic vapor sensor based on
polybenzoxazine-derived carbon aerogel thin film composite, Mater. Sci. Eng.
B 200 (2015) 67–77.
[17] L.C. Li, B. Yalcin, B.C.N. Nguyen, M.A.B. Meador, M. Cakmak, Flexible
nanofiber-reinforced aerogel (xerogel) synthesis manufacture, and
characterization, ACS Appl. Mater. Interfaces 1 (2009) 2491–2501.
[18] J.H. Zou, J.H. Liu, A.S. Karakoti, A. Kumar, D. Joung, Q. Li, S.I. Khondaker, S. Seal,
L. Zhai, Ultralight multiwalled carbon nanotube aerogel, ACS Nano 4 (2010)
7293–7302.
[19] T. Kristiansen, K. Mathisen, M.A. Einarsrud, M. Bjorgen, D.G. Nicholson,
Single-site copper by incorporation in ambient pressure dried silica aerogel
and xerogel systems: an X-ray absorption spectroscopy study, J. Phys. Chem.
C 115 (2011) 19260–19268.
[20] C.N. Chervin, B.J. Clapsaddle, H.W. Chiu, A.E. Gash, J.H. Satcher, S.M.
Kauzlarich, Aerogel synthesis of yttria-stabilized zirconia by a non-alkoxide
sol-gel route, Chem. Mater. 17 (2005) 3345–3351.
[21] D.J. Boday, R.J. Stover, B. Muriithi, M.W. Keller, M.W. Werta, K.A.D. Obrey, D.A.
Loy, Strong, low-density nanocomposites by chemical vapor deposition and
polymerization of cyanoacrylates on aminated silica aerogels, ACS Appl.
Mater. Interfaces 1 (2009) 1364–1369.
[22] J.P. Randall, M.A.B. Meador, S.C. Jana, Tailoring mechanical properties of
aerogels for aerospace applications, ACS Appl. Mater. Interfaces 3 (2011)
613–626.
[23] S.N. Baker, G.A. Baker, Luminescent carbon nanodots: emergent nanolights,
Angew. Chem. Int. Ed. 49 (2010) 6726–6744.
[24] S.K. Bhunia, A. Saha, A.R. Maity, S.C. Ray, N.R. Jana, Carbon nanoparticle-based
fluorescent bioimaging probes, Sci. Rep. 3 (2013), No. 1473.
613
[25] S.K. Bhunia, S. Nandi, R. Shikler, R. Jelinek, Tuneable light-emitting
carbon-dot/polymer flexible films prepared through one-pot synthesis,
Nanoscale 8 (2016) 3400–3406.
[26] S.K. Bhunia, A.R. Maity, S. Nandi, D. Stepensky, R. Jelinek, Imaging cancer cells
expressing the folate receptor with carbon dots produced from folic acid,
Chembiochem 17 (2016) 614–619.
[27] C.X. Wang, H.H. Lin, Z.Z. Xu, Y.J. Huang, M.G. Humphrey, C. Zhang, Tunable
carbon-dot-based dual-emission fluorescent nanohybrids for ratiometric
optical thermometry in living cells, ACS Appl. Mater. Interfaces 8 (2016)
6621–6628.
[28] A. Cayuela, C. Carrillo-Carrion, M.L. Soriano, W.J. Parak, M. Valcarcel, One-step
synthesis and characterization of N-doped carbon nanodots for sensing in
organic media, Anal. Chem. 88 (2016) 3178–3185.
[29] S.K. Bhunia, N. Pradhan, N.R. Jana, Vitamin B1 derived blue and green
fluorescent carbon nanoparticles for cell-imaging application, ACS Appl.
Mater. Interfaces 6 (2014) 7672–7679.
[30] Y.P. Sun, B. Zhou, Y. Lin, W. Wang, K.A.S. Fernando, P. Pathak, M.J. Meziani, B.A.
Harruff, X. Wang, H.F. Wang, P.J.G. Luo, H. Yang, M.E. Kose, B.L. Chen, L.M.
Veca, S.Y. Xie, Quantum-sized carbon dots for bright and colorful
photoluminescence, J. Am. Chem. Soc. 128 (2006) 7756–7757.
[31] H. Ali, S.K. Bhunia, C. Dalal, N.R. Jana, Red fluorescent carbon
nanoparticle-based cell imaging probe, ACS Appl. Mater. Interfaces 8 (2016)
9305–9313.
[32] L. Greenspan, Humidity fixed points of binary saturated aqueous solutions, J.
Res. Nat. Bur. Stand. Sect. 81A (1977) 89–96.
[33] N. Bheekhun, A.R. Abu Talib, M.R. Hassan, Aerogels in aerospace: an overview,
Adv. Mater. Sci. Eng. 406065 (2013).
[34] S. Nandi, R. Malishev, K.P. Kootery, Y. Mirsky, S. Kolusheva, R. Jelinek,
Membrane analysis with amphiphilic carbon dots, Chem. Commun. 50 (2014)
10299–10302.
[35] D.R. do Carmo, L.L. Paim, Investigation about the copper adsorption on the
chloropropylsilica gel surface modified with a nanostructured dendrimer
DAB-Am-16: an analytical application for determination of copper in different
samples, Mat. Res. 16 (2013) 164–172.
[36] X. Wang, L. Cao, F.S. Lu, M.J. Meziani, H. Li, G. Qi, B. Zhou, B.A. Harruff, F.
Kermarrec, Y.P. Sun, Photoinduced electron transfers with carbon dots, Chem.
Commun. 377 (2009) 4–3776.
[37] Y. Liu, K.R. Wang, D.S. Guo, B.P. Jiang, Supramolecular assembly of perylene
bisimide with ␤-cyclodextrin grafts as a solid-state fluorescence sensor for
vapor detection, Adv. Funct. Mater. 19 (2009) 2230–2235.
[38] B.-P. Jiang, D.-S. Guo, Y. Liu, Self-assembly of amphiphilic
perylene-cyclodextrin conjugate and vapor sensing for organic amines, J. Org.
Chem. 75 (2010) 7258–7264.
[39] L.Q. Shi, C. He, D.F. Zhu, Q.G. He, Y. Li, Y. Chen, Y.X. Sun, Y.Y. Fu, D. Wen, H.M.
Cao, J.G. Cheng, High performance aniline vapor detection based on
multi-branched fluorescent triphenylamine-benzothiadiazole derivatives:
branch effect and aggregation control of the sensing performance, J. Mater.
Chem. 22 (2012) 11629–11635.
[40] The National Institute for Occupational Safety and Health, NIOSH Pocket
Guide to Chemical Hazards (accessed: April 2016) https://www.cdc.gov/
niosh/npg/npgd0033.html.
[41] S. Dong, L. Chi, Z. Yang, P. He, Q. Wang, Y. Fang, Simultaneous determination
of dihydroxybenzene and phenylenediamine positional isomers using
capillary zone electrophoresis coupled with amperometric detection, J. Sep.
Sci. 32 (2009) 3232–3238.
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.