Film-based fluorescence sensing: A "chemical nose" for nicotine

Article abstract of DOI:10.1039/c9cc06771j

A novel series of emissive o-carborane derivatives, which showed multicolor, highly solid-state emission (Φ_F ≥ 43%) and ideal photochemical stability, were synthesized. Inspired by the powerful mammalian olfactory system, we, for the first time, successfully obtained a fluorescent sensor array, which exhibits superior detection capability for nicotine in the gaseous phase (down to 3 ppb). Furthermore, the sensor array can be extended to detect nicotine in aqueous solution at the nano-gram level (～0.1 ng cm^-2) and determine the smoke of cigarette and electronic cigarette.

Full text of DOI:10.1039/c9cc06771j

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Film-based fluorescence sensing: a ‘‘chemical
nose’’ for nicotine†
Cite this: Chem. Commun., 2019,
55, 12679
Ke Liu, Jing Zhang, Ling Xu,
Jing Liu,
Liping Ding,
Taihong Liu * and
Received 31st August 2019,
Accepted 19th September 2019
Yu Fang
*
DOI: 10.1039/c9cc06771j
rsc.li/chemcomm
A novel series of emissive o-carborane derivatives, which showed
Fluorescence provides a highly sensitive and selective
multicolor, highly solid-state emission (UF Z 43%) and ideal photo- method for quantification of a wide variety of analytes. Over
chemical stability, were synthesized. Inspired by the powerful the past decades, fluorescent-based sensors have been increasingly
mammalian olfactory system, we, for the first time, successfully captivating the attention of scientists, and have appeared as one of
obtained a fluorescent sensor array, which exhibits superior detection the most promising candidates for chemical sensing.10 In addition,
capability for nicotine in the gaseous phase (down to 3 ppb). Further- due to their several unique advantages such as great design ability,
more, the sensor array can be extended to detect nicotine in aqueous outstanding sensitivity, re-usability and low cost, film-based
solution at the nano-gram level (B0.1 ng cmÀ2) and determine the fluorescent sensors have achieved great progress during the last
smoke of cigarette and electronic cigarette.
few decades.11 Suitable film-based fluorescent materials have to
fulfill several important requirements: (1) high photochemical
According to the World Health Organization (WHO) report on the stability and high quantum yield in the solid state; (2) a fast
global tobacco epidemic, tobacco smoke killed at least 3.3 million and reversible sensing process; and (3) a simple preparation
people in 2017, and millions more suffer from lung cancer, process and easy to be made into devices. Although a good
tuberculosis, asthma or other chronic lung diseases caused by deal of film-based fluorescent sensors exhibiting high sensing
smoking.1 A common view is that tobacco smoke contains more performances has been developed,12 the photochemical stability
than 7000 chemicals (69 of which are known to cause cancer),2 and and quantum yield in the solid state are often downplayed in
is considered as a dangerous form of indoor air pollution, where practical use, which may explain why few sensors can be actually
nicotine is the most dangerous ingredient and the main reason for used.13 Therefore, the development of fluorescent materials with
addiction. A number of countries have legislated against smoking superior photochemical stability and high quantum yield in the
by restricting cigarette advertising, regulating who can buy and use solid state is of great significance for developing high-performance
tobacco products, and even banning smoking in public areas.3 film-based fluorescent sensors.
Although GC-mass,4 electrochemical methods,5 surface enhanced
To construct fluorescent materials that possess superior
Raman spectrometry,6 chemiresistors,7 and fiber optic sensors8 photochemical stability in the solid state, we focused on a
have been developed to detect nicotine, there are no methods globular substituent called o-carborane. Unique o-carborane
which can be used for the in situ detection or monitoring of bears multiple properties such as a three-dimensional cage
nicotine vapor in public places since they must be robust, structure and intrinsic electron-deficient properties, and has
inexpensive, and have a rapid response, especially when there been proven to be a promising building block for photo-
are large numbers of samples to process. The desire to detect functional materials.14 In particular, its electron-withdrawing
and identify trace amounts of airborne analytes related to ability can lead to considerable charge redistribution in the
tobacco smoke with ever increasing sensitivity and selectivity luminescent core, which can further result in a large emission
continues to be one of the main drivers in sensor research.9
shift and better sensitivity to its chemical surroundings.15
Recent studies showed that most of the o-carborane derivatives
exhibited aggregation-induced emission (AIE) properties, when
a p-conjugated aryl unit is connected to the C1 (and/or C2)
position of o-carborane,16 which means that the derivatives
are fluorescence silent in the solution state, whereas they are
recovered in the aggregated state owing to freezing the C1–C2
bond vibrations.17 Therefore, o-carborane-based materials could
Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education),
School of Chemistry and Chemical Engineering, Shaanxi Normal University,
Xi’an 710119, P. R. China. E-mail: liuth121@snnu.edu.cn, yfang@snnu.edu.cn
† Electronic supplementary information (ESI) available: Experimental procedures,
characterization, crystallographic data and the sensing response. CCDC 1917767–
1917770. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c9cc06771j
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be one of the promising candidates for developing high- substituents from hydrogen to dimethylamine, the absorption
performance fluorescent film sensors.18
maxima bathochromically shifted from 300 nm to 307 nm, then
Meanwhile, by using the simultaneous interaction of multi- to 312 nm, and finally to 334 nm for the films from 1 to 4. In
ple analytes with different sensing materials, the ‘‘chemical contrast to the ground state absorption, the fluorescence emission
nose’’ is able to recognize the overall changes in the makeup maxima covered a wide range from 500 nm to 648 nm. In
of complex mixtures, rather than just identifying specific addition, Stokes’ shift of the fluorophores F1–F4 increased from
elements.19 The data-rich outputs of array-based sensing methods 200 nm of F1 to 314 nm of F4 with the same trend of the electron
are becoming much powerful and widely adopted by the analytical donating ability of the substituents. In addition, the four fluoro-
community, due to the improved capabilities with statistical and phores as obtained showed high fluorescence quantum yields
cheminformatic methods in the analysis process, as well as the in the solid state with B0.43 for F1, B0.59 for F2, B0.92 for F3,
recognition that many complex sensing challenges cannot be and B0.58 for F4, respectively.
solved with conventional analytical tools. To get an o-carborane-
To provide an additional understanding of the optical
based fluorescent sensor, herein, a new series of multicolor properties, the highest occupied molecular orbitals (HOMOs)
o-carborane-based fluorophores were designed, synthesized and and the lowest unoccupied molecular orbitals (LUMOs) of the
further used for fabricating a film-based fluorescent sensor array. four fluorophores were calculated by using density functional
Four o-carborane derivatives demonstrated high fluorescence theory (DFT) at the B3LYP/6-31G(d) level with the Gaussian 09
quantum yields and superior photochemical stability in the solid suit program. As provided in Table S1 (ESI†), the calculated
state. Interestingly, the array-based sensing platform is super- energy levels of the HOMOs of the compounds gradually
sensitive to nicotine and tobacco smoke. This communication increased from À7.82 eV (F1) to À6.46 eV (F4), and the LUMOs
reports the findings.
increased from À0.262 eV (F1) to À0.049 eV (F4), confirming
The four o-carborane-derived fluorophores (F1, F2, F3, and F4) that the electron-donating ability of the substituent is capable
were synthesized with high yields via a modified Pd-catalyzed of modifying the energy levels of the compounds. The large
Suzuki cross-coupling reaction (Scheme S1, ESI†). The details of Stokes’ shifts and strong emissions as demonstrated, no doubt,
the synthesis and characterization are provided in the ESI.† The are in favour of sensing and device-making, owing to the
crystal structures of the o-carborane derivatives were determined, reducing requirements for hard-wares and signal processing.
and the results are shown in Fig. 1a. As seen from Fig. 1b, the
Since photochemical stability is a necessary requirement for
compounds as created showed bright, different colors under UV practical applications of organic photo-materials, such as
light, varying from blue to red that covers almost the entire visible organic light emitting diodes (OLEDs), semiconductor lasers,
light region, which are probably due to the different donating fluorescent sensors, etc.20 Therefore, we evaluated such properties
abilities of the substituents. Casting the CHCl3 solutions of the of the compounds as created in the film state and the results
relevant compounds in routine filter papers (Whatmans, are shown in Fig. S1b (ESI†). As observed, the fluorescence
No GB/T1914-2007) resulted in four fluorescent films, which intensity of the films showed less than 4% decrease in 10 hours
are respectively named Film 1, Film 2, Film 3 and Film 4, in the illumination, indicating that the compounds possess good
same order of the fluorophores (F1–F4). Again, the films are stability towards air and light under ambient conditions, laying
highly fluorescent as depicted in the figure. The observation the foundation for long-term use. This property can be ascribed
may be ascribed to the hindrance from the rigid caging structure to the unique nonplanar structure of the o-carborane-based
of the o-carborane unit, which is able to reduce concentration- compounds as revealed by others.16
related self-quenching owing to the prevention of intermolecular
interactions.
The single-crystals of the o-carborane derivatives as synthesized
were obtained via controlled evaporation in their solutions in
To verify the statement, UV-vis absorption and photo- CH2Cl2/n-hexane. The crystal structures and the relevant crystallo-
luminescence spectra of the films were measured (Fig. S1a, ESI†). graphic data are provided in Fig. 1a and Tables S2–S5 (ESI†).
It is observed that with increasing electron-donating ability of the As seen, the benzene rings of the diphenyl units are nonplanar
ascribing to the rotation of the C–C single bond in between,
and the bond length of C1–C2 in the o-carborane unit of
the compounds under examination changed from 1.721 Å to
1.729 Å, then to 1.731 Å, and finally to 1.732 Å for the four
fluorophores from F1 to F4. This is a reasonable result as they are
significantly longer than that of the unsubstituted o-carborane,
1.62 Å, as substitution will increase the length of the bond due to
steric and electronic reasons.21
Based on the discoveries, it is anticipated that intermolecular
interactions could be weakened in the crystal state due to the bulky
icosahedral o-carborane structure. As seen from the structures
depicted in Fig. 1a, there should not be any strong noncovalent
Fig. 1 Structure and fluorescent pictures of the o-carborane-based fluoro-
phores. (a) Crystal structures of fluorophores F1–F4, carbon (gray), boron
(yellow), oxygen (red), nitrogen (blue) and hydrogen (white) atoms are shown
interaction within the crystals of the o-carborane derivatives except
F2 and F3, and the driving force for the crystallization could only
as ellipsoids at a 50% probability level. (b) Relevant fluorescent photographs
of the crystals and films under UV light.
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come from the dispersion force, orientation force, induction force,
etc. As seen from Fig. S2a (ESI†), the hydrogen bond does exist in
the crystal of F2. As for F3, stronger Bcage–H...p (2.568 Å), C–H...p
(3.390 Å) and p...p (2.838 Å) interactions are observed (Fig. S2c,
ESI†). It is the weak intermolecular interactions that make the
compounds pack into the crystals as depicted in Fig. S2b and d
(ESI†). With comparison of the structures of the two crystals, it is
seen that F2 exists in a more regular structure and possesses rich
channels, which is beneficial to chemical sensing due to a mass
transfer reason.
Based on the aforementioned properties, a conceptual
multicolor sensor array was constructed using the o-carborane-
based fluorophores as sensing elements. All sensing tests related
Fig. 3 Sensing performances of Film 2 to nicotine and tobacco smoke.
(a) Net responses of different concentrations of nicotine vapor with the
film-made device. (b) Photograph of the fluorescence quenching of Film 2
to evaluating the sensing performance of the individual by nicotine on the contact mode (2 mL of the nicotine with a spot area of
B0.2 cm2) when viewed under 254 nm UV illumination. (c) Different
response of nicotine, tobacco smoke and electronic cigarette.
compound-based films were conducted on a home-made, one-
film device-based sensing platform.22 In detail, a photodiode
with maximum luminescence at B310 nm was chosen as the
light source for all the film-based devices. The measurements for more pronounced responses to the analyte (Fig. S3, ESI†),
different compound-based films were conducted by simply which is possibly a result of its unique adlayer structure related
changing the photosensitive diodes, of which 500 nm for Film 1, to the packing of the sensing fluorophore in the crystal state as
530 nm for Film 2, 545 nm for Film 3 and 650 nm for Film 4, depicted in Fig. S2a and b (ESI†). Furthermore, the presence of
respectively. The results are depicted in Fig. 2.
the hydroxyl groups in the sensing fluorophore may endow the
As seen, 9 chemicals with different properties were selected film with an ability to enrich nicotine from the air through
as representatives to conduct the tests, and the compound- hydrogen bonding. To examine the performance, the detection
based films exhibited different responses to the chemicals limit (DL) of Film 2 to nicotine in the vapor phase was
under examination in the vapor phase. Hot plots of the determined by collecting the response intensities in the
response intensities of the film-based devices to the chemicals presence of different concentrations of the chemical. As shown
resulted in easily distinguishable patterns as shown in Fig. 2a. in Fig. 3a, the response intensity increased significantly upon
To verify the reliability of the discrimination, a pattern recognition increasing the concentration of the analyte. Even 3 ppb of
algorithm (PCA) was used to conduct additional analysis. The nicotine vapor could result in a significant response to the
results are depicted in Fig. 2b. Clearly, the PCA score plot as film, suggesting that the DL is lower than the value.
generated shows clear clustering of the data using only the first
Detection of extremely small amounts of nicotine in aqueous
two principle components, PC1 and PC2, where PC1 carries about solution is also very important. To test this possibility, nicotine
47.26% of the variance and PC2 carries 15.28%, demonstrating solutions of different concentrations were prepared and 2 mL of
the strong discriminating capability of the o-carborane-based film each solution was piped on a routine test paper strip to receive a
devices. Fortunately, nicotine, one of the 9 chemicals under spot area of B0.2 cm2. The visual fluorescence responses are
examination, can be identified with no difficulty from either of shown in Fig. 3b. As seen, the minimum detectable amount by the
the two analysis methods.
naked eye is lower than 2 mL of 6.0 Â 10À8 mol LÀ1 solution, which
The sensing performances of the sensing devices to nicotine is B0.1 ng cmÀ2, a value much lower than the results previously
were specially estimated owing to carcinogen reasons. Film 2 achieved using other detection methods (Table S6, ESI†).23
was chosen for conducting the specific analysis as it shows
In order to mimic the real scenario, four kinds of tobacco
smoke and electronic cigarette have also been tested using
the same film-based device. The obtained results, depicted in
Fig. 3c, show that 10 times dilution of their saturated vapors
with air could still induce high response, suggesting the
existence of gaseous nicotine. This is in support of the state-
ment from WHO that electronic smoke could still lead to health
problems and it cannot be taken as an effective method to quit
smoking.3 Humidity may also be an affecting factor to the tests,
and thereby its effect on the sensing film’s performance was
investigated, and the results are shown in Fig. S4 (ESI†).
As seen, the film’s emission is hardly affected by water vapor.
In addition, the film recovers much faster than that after detection
of nicotine.
Fig. 2 Sensing performances of the multicolor sensor array. (a) Hot plot
of the response intensity of the films on exposure to various analytes.
(b) Two-dimensional PCA score plot for discriminating saturated vapor of
the tested chemicals at 293 K via utilization of the response intensity of the
sensor array.
Reusability is an unavoidable factor to be considered if an
analytical technique is to be used practically. For this reason,
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be sensitively and selectively detected. We strongly believe that
the Film 2-based device as obtained has the potential to be
developed into a portable fluorescent nicotine detector.
This work was supported by the Natural Science Foundation
of China (21527802, 21673133, and 21820102005), the 111
project (B14041), the Program for Changjiang Scholars and
Innovative Research Team in University (IRT-14R33), the Natural
Science Basic Research Program of Shaanxi (2019JM-404) and
the Fundamental Research Funds for the Central Universities
(GK201803024).
Fig. 4 Schematic representation of the sensing mechanism and dynamic
responses of Film 2 to different analytes. (a) Schematic representation of
typical fluorophore-receptor sensors for analytes; (b) probable interaction
of nicotine with fluorophore F2; and (c) dynamic responses of Film 2 to the
presence of different analytes in the vapor phase.
Conflicts of interest
the reusability of the Film 2-based device to tobacco smoke was
examined. Before testing, the saturated vapor from the smoke
was diluted 10 times with air, and then tested with the device.
As depicted in Fig. S5 (ESI†), the measurement lasted 5 days,
and each day contained 10 independent tests. The result
demonstrated that the sensing is fully reversible, and the device
showed no distinct change in the sensing behaviour within
5 days, indicating that the film-based device has the potential
to be developed into a portable fluorescent nicotine detector.
To understand the sensing performance of the film-based
device, a possible sensing mechanism and some real time-
dependent response traces to different chemicals are shown in
Fig. 4. As aforementioned, o-carborane-based fluorophores
F1–F4 are all aggregation-induced emitters (AIEgens) with
bright emission in the solid state. As seen, the sensitive response
of the Film 2-based device to nicotine could be understood by
considering the possible hydrogen bond formation as shown,
The authors declare no conflict of interest.
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12682 | Chem. Commun., 2019, 55, 12679--12682
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