Optogenetic Modulation of a Catalytic Biofilm for the Biotransformation of Indole into Tryptophan

Article abstract of DOI:10.1002/cssc.201902413

In green chemical synthesis, biofilms as biocatalysts have shown great promise. Efficient biofilm-mediated biocatalysis requires the modulation of biofilm formation. Optogenetic tools are ideal to control biofilms because light is noninvasive, easily controllable, and cost-efficient. In this study, a gene circuit responsive to near-infrared (NIR) light was used to modulate the cellular level of bis-(3′-5′) cyclic dimeric guanosine monophosphate (c-di-GMP), a central regulator of the prokaryote biofilm lifestyle, which allowed the regulation of biofilm formation by using NIR light. The engineered biofilm was applied to catalyze the biotransformation of indole into tryptophan in submerged biofilm reactors and NIR-light-enhanced biofilm formation resulted in an approximately 30 % increase in tryptophan yield, which demonstrates the feasibility of the application of light to modulate the formation and performance of catalytic biofilms for chemical production. The c-di-GMP-targeted optogenetic approach to modulate catalytic biofilms showcases applications for biofilm-mediated biocatalysis.

Full text of DOI:10.1002/cssc.201902413

Article
pubs.acs.org/ac
Cite This: Anal. Chem. 2019, 91, 4860−4867
Ratiometric Cataluminescence for Rapid Recognition of Volatile
Organic Compounds Based on Energy Transfer Process
Jiaxi Hu,† Lichun Zhang,† Hongjie Song,† Jianyu Hu,† and Yi Lv*,†,‡
†Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu,
Sichuan 610064, China
‡Analytical and Testing Center, Sichuan University, Chengdu 610064, China
S
* Supporting Information
ABSTRACT: Recognition of volatile organic compounds (VOCs) is a hot topic
full of challenge from the perspective of environmental protection and human
security. Here, we developed a novel ratiometric cataluminescence (RCTL)
method for fast identification and detection gas compounds at various
concentrations based on the energy transfer process, by the means of introducing
rare earth ions codoped metal oxide into cataluminescence (CTL) sensor system
to work as sensing material. When the prepared stick-like Y2O3:Eu3+,Tb3+ is
exposed to kinds of analytes, different energy transfer process take place to emit
two new signals at the characteristic wavelength of Tb3+ (ITb) and Eu3+ (IEu),
which is available for us to identify miscellaneous gaseous compounds rely on the
ratio of ITb to IEu (ITb/IEu). To confirm the feasibility of the proposed method,
seven kinds of gas compounds, including homologous series and even structural
isomers, were investigated and successfully distinguished in a wide range of concentrations. Besides, further discussion of the
CTL sensing and recognition mechanism in this paper has facilitating effects on exploring reactive intermediates and explaining
the essential principle of catalytic oxidation process.
from excited states back to ground states.9 Specific to diverse
olatile organic compounds (VOCs), chemicals that exist
Vin the air as a form of gaseous states at room temperature, gaseous compounds, the generated active intermediates are
characteristic, which is considered to be the cornerstone of
identifying different VOCs. Put it another way, in contact with
analytes, the CTL sensing systems bring about particular
response pattern toward a certain sample. Many splendid CTL-
based analysis strategies for gaseous recognition were set up on
that premise. For instance, Zhang’s group first developed a
CTL imaging sensor array system6 and further to make it
successfully apply to actual detection.10 Based on these
pathbreaking works, researchers devoted permanent efforts to
establishing novel analysis strategies utilizing less sensing unit
to reach better stabilities and reproducibility, such as closed
reaction cell system,11 cyclic chemiluminescence system,12
transient cataluminescence analysis method,13 and so on.
While based on some rare earth ion doped materials, especially
lanthanide ions, there always involves one more energy transfer
process from the original intermediates to the rare earth ions,
in which the luminescence produced by more reason for
excited rare earth ion, leading to the change in the premium
wavelength.14 So that we expect that if double rare earth ions
are doped into the base catalyst to respond toward the
measured gaseous, both of the rare earth ions are tend to
accept the energy released from the original excited
will bring about adverse health effects to human body on
account of their special odor, toxicity, irritation, and
carcinogenicity. More seriously is that immune system,
digestive system, and nervous system function could be
destroyed once severely exposed to VOCs.1 Now, therefore,
detection and recognition of VOCs forced to be a hot issue
demanding prompt solution from a standpoint of environ-
mental protection and human security. During the past
decades, considerable effort has been dedicated to developing
a series of methods for the detection and identification of
gaseous samples, including chromatography/mass spectrome-
try (GC/MS), inductively coupled plasma mass spectrometry
(ICPMS), bioinspired artificial olfactory systems, gas sensors,
and so on. Among others, advantages of rapid response, good
economic benefits, and low background make sensor
technique, especially cataluminescence-based gas sensor, to
be one of the most promising recognition methods. Except for
specific recognition of unknown analyte with superior
selectivity,2−5 cataluminescence (CTL) gaseous sensing
technology is also capable of realizing quantification and
recognition of various simple or complex chemicals.6,7
Cataluminescence, an interesting phenomenon, refers to the
heterogeneous catalytic reaction occurs on the surface of solid
catalytic materials accompanying by luminescence emission.8
During the course of conventional CTL reaction, the emitted
light is derived from the deactivation of active intermediates
Received: February 1, 2019
Accepted: March 14, 2019
Published: March 14, 2019
© 2019 American Chemical Society
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intermediate. Under the circumstance, the premium wave-
length changes to the characteristic wavelength of the rare
earth ions to obtain two new peaks in CTL spectrum, which
magnify the discrepancy of the original active intermediates
versus diverse VOCs, thereby making it possible for us to
identify the analytes using the ratio of the signals under the
characteristic wavelength of the rare earth ions based on the
different energy transfer abilities of the original excited
intermediates. Therefore, a novel ratiometric cataluminescence
(RCTL) analysis method was established for gas discrim-
ination based on a single element of rare earth ions codoped
compounds.
EXPERIMENTAL SECTION
■
Chemicals. All reagents were of analytical grade and no
further purification is required. Y(NO3)3·6H2O, Eu(NO3)3·
6H2O, and Tb4O7 were purchased from Adamas Reagent Co.,
Ltd. NaOH, nitric acid, acetone, butanone, acetaldehyde,
butanal, isopropanol, isobutanol, and ethyl ether were obtained
from Chengdu Kelong Chemical Reagent Company (China).
Deionized water was obtained from a Milli-Q water
purification system (Ulupure Co. Ltd., Chengdu, China).
Sample Preparation of Pure Y2O3 and Eu, Tb Doped
Y2O3. A hydrothermal method was adopted to synthesize a
series of powder samples with the chemical composition of
Sensing material not only play the most critical role to
response toward the tested analytes with regard to CTL-based
sensor system, but also is the foundation of the proposed
method. Rare earth compounds, with premium properties of
reliable optical applications and enhanced catalytic perform-
ances, have broad application value in the fields of electricity,
magnetism, biomedical, optics, and catalytic.15−18 Particularly,
in virtue of the extraordinary spectroscopic properties and
luminescent dynamics deriving from unique 4f electrons,
lanthanide ions are pervasive in the application for
biochemical. Along with introducing rare earth ions into
nanomaterial to act as sensing unit in CTL sensors,
enhancement of luminescence signals and high efficiency
could be obtained, which has been demonstrated by many
researchers.14 It was proved by Zhang’s group19 that among
various ions doped ZrO2 nanomaterials, Eu3+ or Tb3+ could
trigger energy transfer process to produce new peaks. In
addition, Y2O3 with advantages of favorable optical and
electrical properties and superior catalytic performance have
broad application in many fields,17,20 including CTL sensor
systems.21 Thus, rare earth ions Eu3+ and Tb3+ codoped Y2O3
material was synthesized to verify the proposed hypothesis in
our experiments. The strongest emission bands of Tb3+ and
Y
(2−x−y)O3:xEu3+,yTb3+(x = 0∼0.12, y = 0∼0.12), where Y
atoms occupations were considered to be replaced by rare
earth atoms. According to each stoichiometric amount of
Eu2O3, Y2O3, Y1.94−xO3:xEu3+,0.06Tb3+ (x = 0, 0.03, 0.06, 0.09,
0.12), Y1.94−yO3:0.06Eu3+,yTb3+ (x = 0, 0.03, 0.06, 0.09, 0.12),
raw materials of nitrate of Y, Eu, and Tb were added into
proper amount of deionized with continuous stirring until
dissolved completely, respectively. Then, mix the above
solutions together and dropwise add 10% NaOH solution
until pH ∼ 11, accompanied by stirring for 30 min. The
resultant suspension was followed by transferring into Teflon-
lined stainless-steel autoclave and temperature was kept at 140
°C for 12 h. After reacting and cooling naturally to room
temperature, the resulting products were collected by washing
and centrifuging with distilled water and absolute ethanol
several times, and were dried in air at 60 °C for 24 h. Finally,
the required oxidation products were obtained via calcining at
600 °C for 4 h.
Characterization. The crystal structures of Y2O3, Eu2O3,
and Y2−x−yO3:xEu3+,yTb3+ (x = 0∼0.12, y = 0∼0.12) were
carried out utilizing an X’Pert pro X-ray diffractometer (XRD,
Philips) equipped with Cu Kα1 (λ = 1.5406 Å) radiation. Step
scan mode was employed in the 2θ range of 10∼80° with a
step size of 0.02° and counting time of 0.12 s for each step
(scan rate was 10° per min). The sizes and surface
m o r p h o l o g i e s o f t h e a s - p r e p a r e d m a t e r i a l
5
7
5
7
Eu3+ originated from D4 → F5 (∼545 nm) and D0 → F2
(∼620 nm), respectively, are supposed to be the critical
wavelength,22,23 under which the ratio of their intensities were
used to characterize the different gases. Note that the two
wavelengths are so far apart that two peaks barely overlap. As a
result, the rare earth ions codoped compound of
Y2O3:Eu3+,Tb3+ was used for the mentioned novel RCTL-
based methodology.
Y
1.88O3:0.06Eu3+,0.06Tb3+ were investigated by scanning
electron microscopy (SEM, Hitachi, S3400). Transmission
electron microscopy (TEM), high-resolution transmission
electron microscopy (HRTEM) images was performed on a
transmission electron microscope (Tecnai G2F20 S-Twin, FEI
Co., Hillsboro, OR) at an accelerating voltage of 200 kV.
Shimadzu QP2010 GC/MS instrument was used for the
mechanism research.
RCTL signals were measured based on 0.03 g sensing
material via a BPCL ultraweak luminescence analyzer
(Institute of Biophysics, Academia Sinica, Beijing, China)
equipped with a photomultiplier tube (Hamamatsu, Japan).
And 0.1 s per spectrum was set as the data integration time of
BPCL ultraweak luminescence analyzer, and −800 V served as
the working voltage of photomultiplier tube. Besides, CTL gas
sensor, containing injection system, reaction system and
detection system, were prepared as our previous works.13
In this work, we developed a novel RCTL-based analysis
strategy for the simultaneous detection and recognition of
VOCs upon Eu, Tb codoped Y2O3, which provided new
thoughts for CTL-based gas sensor armed with only single
sensing elements to achieve gas identification. The stick-like
Y2O3:Eu3+,Tb3+ nanomaterial was synthesized by simple
hydrothermal method performing rapid response and extra-
ordinary catalytic activity, and was first introduced into CTL-
based sensing system. As a proof-of-principle work, catalytic
oxidation reactions of different kinds of VOCs, containing
homologous series or even structural isomers, were studied
upon the Y2O3:Eu3+,Tb3+ to verify the hypothesis. In addition,
for the purpose of well understanding, the mechanism of the
established new analysis method, CTL behavior and reaction
process of ethyl ether, acetaldehyde and isobutanol were
further investigated, which also produce the enormous pushing
action to explore and explain the essential principle of
luminescence process and catalytic oxidation reaction.
RESULTS AND DISCUSSION
■
Structure and Morphology of Y2−x−yO3:xEu3+,yTb3+
Catalysts. The identifications of crystal structure and purity
of isomorphic Y2−x−yO3:xEu3+,yTb3+ samples were confirmed
by X-ray powder diffraction analysis with the typical X-ray
diffractograms of the 2θ range 10∼80°. XRD patterns of a
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Figure 1. (a) XRD patterns of nanomaterials of Y1.94−xO3:0.06Eu3+,xTb3+ (x = 0∼0.12). (b) XRD patterns of nanomaterials of
Y
1.94−xO3:xEu3+,0.06Tb3+ (x = 0∼0.12). (c) The linear evolution for the unit cell volume versus the Tb proportion of the Y1.94−xO3:0.06Eu3+,xTb3+
(x = 0∼0.12). (d) The linear evolution of the unit cell volume vs the Eu proportion for the Y1.94−xO3:xEu3+,0.06Tb3+ (x = 0∼0.12).
Figure 2. Comparison of CTL spectra on the surface of (a) pure Y2O3, (b) mixture of Y2O3 and Eu2O3 (Eu:Y = 0.06:1.94), (c) Y1.94O3:0.06Eu3+,
and (d) Y1.88O3:0.06Eu3+,0.06Tb3+. Experiment condition: injection volume, 1 μL; working temperature, 280 °C; flow rate, 300 mL min−1.
series of codoped Y2O3 materials with different doping ratios of
rare earth ions, including Y1.94−yO3:0.06Eu3+,yTb3+ (x = 0, 0.03,
0.06, 0.09, 0.12) and Y1.94−xO3:xEu3+,0.06Tb3+ (x = 0, 0.03,
0.06, 0.09, 0.12), were displayed in Figure 1a,b. Each
diffractogram of the representative sample was indexed and
conformed with that of standard cubic Y2O3 (JCPDS: No. 41-
1105) only accompanied by a slight shift in the peak positions,
demonstrating that the crystal structures were not altered
significantly after doping rare earth ions. It can be deduced that
Eu3+ and Tb3+ ions substitute for Y3+ ions to occupy their sites
in the Y2−x−yO3:xEu3+,yTb3+ samples with the essential
conditions of comparable ionic radii and identical valency.
With the reason that the ionic radii of Eu3+ (0.947 Å) and Tb3+
(0.923 Å) ions were subtle larger than that of Y3+ (0.9 Å),
there will be an extension of lattice and a slight increase in
lattice parameters, resulting in that the diffraction peaks shift to
lower angle in according with Bragg’s Law.24,25 Besides,
specific results of lattice parameters and cell unit density of the
as-synthesized Y2O3:xEu3+,yTb3+ were listed in Table S1. In
our experiment, under the condition of fixing the doping
concentration of Eu3+ (Tb3+) at 6%, the volume of unit cell will
increase linearly as an addition of the Tb3+(Eu3+) concen-
tration with a range of 0∼0.12 in units of 0.03, as presented in
Figure 1c,d. Besides, we also synthesized pure Y2O3 and Eu2O3
for the latter experiments, whose XRD patterns are shown as
Figure S1. EDS pattern of Y1.88O3:0.06Eu3+,0.06Tb3+ is also
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Figure 3. CTL intensities comparison of seven kinds of VOCs on the surface of (a) Y1.94−xO3:0.06Eu3+,xTb3+ (x = 0.03∼0.12) and (b)
1.94−xO3:xEu3+,0.06Tb3+ (x = 0.03∼0.12). Experiment condition: injection volume, 1 μL; working temperature, 280 °C; flow rate, 300 mL min−1.
Y
supplied in Figure S2, which further proved that Eu3+ and Tb3+
ions were doped into Y2O3 successfully. And the result of the
EDS illustrated that the atomic ratio of Tb to Eu to Y was
around 1.1:1.1:34, which identify with that added in the
synthesis process. In addition, SEM and TEM techniques were
used to reveal the morphology of Y1.88O3:0.06Eu3+,0.06Tb3+. It
can be seen from Figure S3a−c that the uniform stick-like
morphology of Y1.88O3:0.06Eu3+,0.06Tb3+ was obtained by a
new and simple hydrothermal method. Moreover, the HRTEM
image is displayed in Figure S3d, in which the lattice fringe of
sample was clearly observed. The interplanar distance is 0.306
nm, c orresponding to the (222) planes of
Y
2−x−yO3:xEu3+,yTb3+ (x = 0∼0.12, y = 0∼0.12), various
VOCs, including acetone, butanone, butanal, isobutanol,
isopropanol, ethyl ether, and acetaldehyde, were introduced
into the sensor independently by the carrier gas to measure the
CTL intensity, as presented in Figure 3. The results displayed
that each catalyst was able to response to the VOCs with
certain strength, among which the Y1.88O3:0.06Eu3+,0.06Tb3+
showed superior behavior toward most of the VOCs than the
others, so that it was selected to carry out the subsequent
experiments and verify the hypothesis mentioned previous. For
the sake of superior performance of the catalyst toward VOCs,
condition optimization of experiment should be done at the
beginning. As shown in Figure S4, the optimum condition
turned out to be 300 mL min−1 for the air flow rate and 280 °C
for the reaction temperature, which were also used for the
following experiments.
Y
1.88O3:0.06Eu3+,0.06Tb3+.
Energy Transfer Ability of Rare Earth Ions Doped
Y2O3 Catalysts. In order to explore the discrepancy of energy
transfer properties, CTL spectral curves of VOCs, acquired by
several optical filters with wavelength ranged from 400 to 640
nm, were investigated on the surface of different materials,
including pure Y2O3, mixture compound of Y2O3 and Eu2O3
(atomic ratio of Eu:Y = 0.06:1.94), single-doped material of
Y 1 . 9 4 O 3 : 0 . 0 6 E u 3 + a n d c o d o p e d m a t e r i a l o f
Identification of VOCs. On the basis of the assumption
mentioned before, Y2−x−yO3:xEu3+,yTb3+ contains two energy
acceptors to receive the energy released from the original
excited intermediates to generate excited rare earth ions of
Tb3+* and Eu3+*, which are supposed to emit signals at the
wavelength of 545 and 620 nm, respectively,19 during the
deactivation of excited species to the ground states. And the
ratios of ITb/IEu are characteristic for different VOCs. In order
to confirm the feasibility and capacities to differentiate the
compounds based on the proposed novel analysis method,
various concentrations of different VOCs were introduced into
the sensing system individually to conduct further inves-
tigation. The results of the CTL response curves of various
VOCs at five concentrations under 545 and 620 nm were
illustrated in the left side of Figures 4 and S4, in which three
parallel experiment results were offered to reflect the
repeatability and reliability of the proposed method. It could
be calculated to obtain the values of ITb/IEu for various VOCs,
which are different from each other. What counts is that the
ITb/IEu value was not correlated with the concentration for a
given analyte under certain conditions, which makes this
method to be efficient and practical for us to detect and
recognize different compounds at varies concentrations. From
the results of experiment, VOCs with various types (e.g.:
butanone (ITb/IEu = 1.33) and isobutanol (ITb/IEu = 0.75)),
homologous series (e.g.: acetone (ITb/IEu = 1.74) and
butanone (ITb/IEu = 1.33)), or even structural isomers (e.g.:
butanone (ITb/IEu = 1.33) and butanal (ITb/IEu = 0.95)) can be
successfully identified utilizing the analysis method.
Y
1.88O3:0.06Eu3+,0.06Tb3+. From Figure 2a,b, CTL spectra
upon the mixture compound primarily displayed the character-
istic wavelength of their own intermediates and extremely weak
peaks of the Eu3+ characteristic wavelength at around 620 nm,
compared with that on pure Y2O3. While the spectral curves
based on the doped material of Y1.94O3:0.06Eu3+ showed the
dramatically strong peaks at the Eu3+ characteristic wavelength
of 620 nm, accompanied by an almost disappeared original
peak, as seen from Figure 2c. And multipeaks appeared in the
CTL spectra for each VOC on the codoped material of
Y
1.88O3:0.06Eu3+,0.06Tb3+, as employed in Figure 2d. To be
specific, positions of two main new peaks produced in the CTL
spectra were at characteristic wavelength of Tb3+ of 545 nm
and Eu3+ of 620 nm, and another new peak at 490 nm was
assigned to the second characteristic wavelength of Tb3+.
Hence, it can be concluded that high-efficiency energy transfer
processes from the original intermediates to rare earth ions are
able to occur only based on rare earth ion doping materials.
Furthermore, different intensities at the wavelength of 545 and
620 nm or even disparate ratios of the two intensities were
acquired for diverse VOCs on Y1.88O3:0.06Eu3+,0.06Tb3+,
which lay a foundation for recognizing various VOCs.
CTL Properties of VOCs upon Rare Earth Doped Y2O3
Catalysts. Owing to a positive correlation between catalytic
activity of materials and conversion of reaction,26 total CTL
signals were measured to select premium sensing material.
Based on various codoped materials of as-synthesized
Simultaneously, quantitative analysis of these VOCs also can
be achieved by this sensing system as presented in the right
side of Figures 4 and S4, in which either correlation curves of
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Figure 4. CTL response curves of actone, butanoe, butanal and isobutanol at 545 and 620 nm vs five concentrations on the basis of
Y1.88O3:0.06Eu3+,0.06Tb3+ material. (left) Calibration curves for ITb intensity at 545 nm and IEu intensity at 620 nm of various gas compounds
based on the Y1.88O3:0.06Eu3+,0.06Tb3+ material. (right) Experiment condition: working temperature, 280 °C; flow rate, 300 mL min−1.
545 and 620 nm was employed to quantify the initial
concentration of the measured analytes. By the way, as
shown in Figure S5, total intensity detected without optical
filters also can be used to realize quantitative analysis, which
exhibited higher sensitivity and wider linear range. And the
specific value of ITb/IEu, linear range, and the limit of detection
of seven kinds of VOCs were listed in Table 1.
Possible Mechanism. When referring to the mechanism
of distinguishing VOCs by the ratios of ITb/IEu, take ethyl
ether, acetaldehyde, and isobutanol as model examples to
specifically explore. GC-MS experiments were carried out to
prove and speculate the possible intermediates in the catalytic
oxidation reaction for the three analytes. Accompanied by
carrier gas with the flow rate as low as possible to prevent the
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for isobutanol were considered to be CHC(CH3)−CHO*
and CO2* with the reason of the high yields and more general
forms of intermediates.
Table 1. Data of Confidence Interval (α = 0.05) for ITb/IEu,
Linear Range, and the Detection Limit of Seven Kinds of
VOCs
As mentioned above, various intermediates could generate
during a course of the catalytic oxidation of a compound,30
which could be the sufficient condition of the following
formula: aA → bB* + cC* (B* and C* stand for the
intermediates generated from the catalytic reaction of
compound A). It is widely known that the chemiluminescence
(CL) efficiency (φcl) is equal to the product of chemical
reaction efficiency (φr) and luminescence efficiency (φf),
which represent the percentages of excited molecules and the
molecules in the reaction, as well as luminescent molecules and
excited molecules, respectively.31 And the intensity of CL (Icl)
is positively correlated with the reaction rate and CL efficiency,
which can be expressed as Icl = φcl·dc/dt. In this work, energy
transfer procedure introduces a parameter of energy transfer
efficiency (φET), meaning the percentage of excited rare earth
ions and excited molecules, so that the expression of CL
intensity could be regarded as Icl = φcl·φET·dcA/dt. In detail, the
whole procedure can be divided into three steps as follow.
First step is catalytic oxidation process, which can be
expressed as
confidence interval of
linear range
(μg mL−1
detection limit
VOCs
acetone
butanone
I
Tb/IEu (α = 0.05)
)
(μg mL−1
)
1.74 0.03
1.33 0.07
0.95 0.06
0.75 0.03
2.08 0.08
0.59 0.04
1.58 0.15
5.98−203.50
3.12−128.86
4.38−140.40
3.58−229.14
2.62−78.54
0.094
0.050
0.059
0.18
0.63
0.53
butanal
isobutanol
isopropanol
ethyl ether
acetaldehyde
4.76−428.10
28.49−522.27
0.21
air bubbles from taking the exhaust away, the gaseous products
of the measured VOCs were collected by DMF solution for a
certain time to ensure that the samples were sufficient for
detection. Then the collected residual compounds were
introduced into the Shimadzu QP2010 GC/MS instrument
to analyze. It has already been proved that the intermediates of
the ethyl ether and acetaldehyde were CH3CHO* and
CH3CHCHCHO*, respectively, by many researchers,27,28
which is also in accord with our results of GC-MS experiment
as shown in Figure 5a,b. Aimed at the products of the catalytic
oxidation of ethyl ether and acetaldehyde, except for the
undecomposed reactants, CH3CHO and CH3CHCHCHO
were detected and found at 1.641 and 3.178 min, respectively.
Even so, our group have demonstrated that multifarious
intermediates were possible to produce in the catalytic
oxidation process of ethyl ether by theoretical calculation.29
As for isobutanol, from the Figure 6a, double peaks of the CTL
kinetic curve on pure Y2O3 might be generated from the
producing of two or more intermediates during the course of
the catalytic oxidation reaction, which emitted two signals with
the energy release during the deactivation of two excited
species to the ground states. Moreover, CTL signals at
different wavelength were recorded with optical filters in
Figure 3a. It could be speculated that multiple peaks of the
CTL spectrum correspond to the presence of multiple excited
species at different wavelength. In addition, the concrete
products were further verified by GC-MS experiment as
presented in Figure 5c. Except for the undecomposed reactant
isobutanol (2.703 min), another five products of the catalytic
oxidation of isobutanol were found to be CO2 (1.595 min),
CH2C(CH3)2 (1.694 min), CH3COCH3 (1.883 min),
epoxy isobutane (2.157 min), and CH2C(CH3)CHO
(2.220 min). Based on the fundamental organic chemistry
knowledge, the catalytic reaction pathway might be carried out
as Figure 6b. In consequence, the most probable intermediates
[O]
(1)
aA ⎯⎯→ bB* + cC*
c
B* = αB*cA0
(2)
(3)
cC* = αC*cA0
(a, b and c are the stoichiometric numbers of reactant A,
product B*, and product C*, respectively. αB* and αC*
represent conversion rate of the reactant. cB* and cC* represent
the concentration of intermediates B* and C* generated at
some point, and cA0 represents the initial concentration of
compound A.)
Second step is energy transfer process,32 which can be
expressed as
B* + Eu3+ → B + Eu3+*
φ
)
B1
(
(4)
(5)
(6)
(7)
3+
3+*
B* + Tb → B + Tb
C* + Eu3+ → C + Eu3+*
φ
)
B2
(
φ
(
)
C1
3+
3+*
C* + Tb → C + Tb
φ
)
C2
(
or
B* + Eu3+ + Tb → B + Eu3+*(φB1) + Tb3+*(φB2)
3+
(8)
Figure 5. GC chromatogram of the reaction products generated from the catalytic oxidation of (a) ethyl ether, (b) acetaldehyde, and (c) isobutanol
upon the Y1.88O3:0.06Eu3+,0.06Tb3+ material.
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Analytical Chemistry
Article
Figure 6. (a) CTL response kinetic curve of isobutanol upon the Y1.88O3:0.06Eu3+,0.06Tb3+ material. (b) Possible catalytic oxidation process of
isobutanol.
3+
C* + Eu3+ + Tb → C + Eu3+*(φC1) + Tb3+*(φC2)
stick-like rare earth ions (Eu, Tb) codoped Y2O3 that was the
first time to be introduced into CTL sensors. On the basis of
(9)
Y2O3:Eu3+,Tb3+, the energy transfer process was involved in
(φB1, φB2, φC1, and φC2 represent the energy transfer efficiency
the whole catalytic reaction to produce the excited species of
Tb3+* and Eu3+* that will emit two signals at 545 and 620 nm,
respectively, to return back to ground states, which make it
possible for us to determine different compounds with
homologous series and even structural isomers on the grounds
of the ratio of ITb/IEu. The proposed strategy provides
researchers diverse CTL behavior from single sensing material
by taking advantage of energy transfer mechanism. It should be
expected that the application of rare earth ions codoped
compounds in CTL area could be a new trend to realize the
identification of more complex analytes or even gas mixture via
introducing more rare earth ions into the host nanomaterial or
establishing sensor array with analogous codoped compounds
containing different types and ratios of rare earth ions.
Furthermore, the discussion and exploration of the mechanism
in this paper contribute to investigating reactive intermediates
and explaining catalytic oxidation process.
from B* to Eu3+ and Tb3+, as well as C* to Eu3+ and Tb3+,
respectively).
Third step is luminescence process, which can be expressed
as
Eu3+* → Eu3+
K
)
Eu
(10)
(
3+*
3+
Tb → Tb
K
)
Tb
(11)
(
(KEu and KTb represent the luminescent efficiency of Eu3+* and
Tb3+*).
Since the characteristic wavelength of Tb3+ and Eu3+ are 545
and 620 nm, respectively,33−35 the CL intensity under 545 and
620 nm represent the luminescent signal generated by the
energy transferred from intermediates to Tb3+ and Eu3+,
respectively.
under 545 nm: ITb = KTb(φB2dcB*/dt + φC2dcC*/dt)
= KTb(φB2dαB*cA0/dt + φC2dαC*cA0/dt)
ASSOCIATED CONTENT
■
S
* Supporting Information
= KTb(φB2αB* + φC2αC*)dcA0/dt
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.anal-
chem.9b00592.
(12)
under 620 nm: IEu = KEu(φB1dcB*/dt + φC1dcC*/dt)
= KEu(φB1dαB*cA0/dt + φC1dαC*cA0/dt)
Lattice parameters of the prepared Y2−xO3:xEu3+,yTb3+
(x = 0∼0.12, y = 0∼0.12) (Table S1); XRD patterns of
pure Y2O3 and Eu2O3 (Figure S1); EDS pattern of
= KEu(φB1αB* + φC1αC)dcA0/dt
Y
1.88O3:0.06Eu3+,0.06Tb3+ (Figure S2); SEM, TEM, and
HRTEM patterns of Y1.88O3:0.06Eu3+,0.06Tb3+ (Figure
S3); optimization of flow rate and temperature (Figure
S4); CTL response curves and calibration curves of
isopropanol, ethyl ether, and acetaldehyde under 545
and 620 nm (Figure S5); calibration curves of acetone,
butanone, butanal, isobutanol, isopropanol, ethyl ether,
and acetaldehyde (Figure S6) (PDF)
(13)
I
Tb/IEu = KTb(φB2
αB* + φC2αC*)dcA0/dt/KEu(φB1αB* + φC1αC*)dcA0/dt
= KTb(φB2
α
B* + φC2αC*)/KEu(φB1αB* + φC1αC*
)
(14)
Under certain conditions, α, φ, and K are constant, so that
the value of ITb/IEu should also be constant, which is consistent
with the results of our experiment that ITb/IEu is independent
with the initial concentration of compound A. It also can be
inferred from the eq 14 that the ratio of ITb/IEu depends on the
types and proportion of the intermediates (B* and C*)
produced in the catalytic oxidation reaction.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: lvy@scu.edu.cn. Tel./Fax: +86-28-8541-2798.
ORCID
CONCLUSIONS
■
Yi Lv: 0000-0002-7104-2414
Broadly speaking, we established a novel RCTL analysis
method based on energy transfer to determine and distinguish
various VOCs at different concentrations on the surface of
Notes
The authors declare no competing financial interest.
4866
DOI: 10.1021/acs.analchem.9b00592
Anal. Chem. 2019, 91, 4860−4867

Analytical Chemistry
Article
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ACKNOWLEDGMENTS
■
The National Natural Science Foundation of China is
gratefully acknowledged (Nos. 21874094 and 21675113).
This work is also supported by the Science and Technology
Department of Sichuan Province (19YYJC2423) and the
Fundamental Research Funds for the Central Universities.
Prof. Yingying Su and Dr. Shuguang Yan from Analytical and
Testing Center, Sichuan University, are gratefully thanked for
technical assistance.
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4867
DOI: 10.1021/acs.analchem.9b00592
Anal. Chem. 2019, 91, 4860−4867