Highly selective optical monitoring of O2via multiple-channels

Article abstract of DOI:10.1039/c2ra22292b

An optical probe for the detection of O₂ is outlined. The multi-responsive, redox-based sensor system is highly selective and response can be monitored optically either with conventional spectroscopic techniques, such as UV-Vis, IR or fluorescence, or by eye as a result of contrast colour changes from light yellow to dark red.

Full text of DOI:10.1039/c2ra22292b

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Highly selective optical monitoring of O2 via multiple-
channels3
Cite this: RSC Advances, 2013, 3, 390
Anup Kumar,a Rinkoo D. Guptab and Tarkeshwar Gupta*a
Received 25th September 2012,
Accepted 13th November 2012
DOI: 10.1039/c2ra22292b
www.rsc.org/advances
An optical probe for the detection of O2 is outlined. The multi-
responsive, redox-based sensor system is highly selective and
response can be monitored optically either with conventional
spectroscopic techniques, such as UV-Vis, IR or fluorescence, or by
eye as a result of contrast colour changes from light yellow to
dark red.
Therefore, there is continuous interest to formulate an efficient
oxygen detection probe. To the best of our knowledge, multiple-
channel analyses for O2 using single molecules has not been
reported so far.11 Here, we aimed to construct a simple diamine-
based optical probe to specifically detect the presence of molecular
oxygen over wide range of other gases. Our strategy takes
advantage of multiple-channel analyses for O2 using a single
organic molecule.
The rigorous catalytic hydrogenation reaction of 2-methoxy-4-
nitro-pheneylamine (1) under a H2 atmosphere with added PtO2 as
the catalyst affords compound 2 in good yield (85%). Reaction of 2
with acetic anhydride in pyridine yields 3 that can be again easily
Molecular recognition1 at low concentration is one of the most
promising research areas in supramolecular chemistry2 and
sensor industries.3 Various host–guest interactions based mole-
cular receptors4 have been developed for the detection of different
analytes in solid, liquid or gas phases. Detection of gaseous
analytes such as N2, CO, CO2, NO, NO2, O2 etc. have gained
tremendous interest5 in environmental monitoring, green house
effect monitoring and in scientific laboratories. A range of
techniques have been reported for the detection of such analytes
including titrometric, electrochemical, microscopic and optical.
Optical detection is reported to be an ideal method, as it is simple
to use, does not suffer from electrical interference and can be
integrated in remote sensing devices.6 Recent, state-of-the-art
optical detection of gaseous molecules is based on quenching of
either emission or absorption intensity.7 However, ‘‘signal-on’’
detection of gases is relatively rare.8
Monitoring of molecular oxygen (O2) is mandatory in various
fields such as deep-sea environment, clinical analysis, ecosystem,
atmosphere and diagonostic.9 Additionally, detection of O2 levels
in airtight containers such as glove boxes is crucial to carry out
several O2/air sensitive reactions, where presently fume/flame-
based O2 detectors, such as dialkyzinc, are utilized. The existing O2
detection approaches are principally based on traditional methods
i.e., the single signalling technique for a single sensory unit that
too often suffers from selectivity and storage/stability problems.10
converted to target compound
2 under acidic conditions
(Scheme 1). Interestingly, 2 quickly transforms to 4 when exposed
1
to air/O2, whereas 1 and 3 do not respond to O2 as judged by H
NMR and UV-Vis spectroscopy (Fig. S1, S2 ).
3
3
The 1H NMR spectrum of 2 displays peaks at d = 3.83 and 4.25
ppm due to the NH2 proton; upon exposure to O2 (300 ppm) it
transforms to d = 4.05 ppm, which could be assigned to the NH
proton of 4 (Fig. 1a). Similarly, the peaks observed at n = 3352 and
3476 cm21, due to the NH2 group of 2, transformed to n = 3385
cm21 in the FTIR spectrum of 4 (Fig. 1b). In addition, a new peak
appears at n = 3186 cm21, which could be due to the formation of
H2O as a result of consecutive redox reactions. Interestingly, the
successive intensity changes (decrease of NH2 and increase of NH,
H2O peaks) as a function of O2 concentration can be monitored
with infrared and NMR spectroscopy (Fig. S3 ). These data specify
3
aDepartment of Chemistry, University of Delhi, New Delhi-110 007, India.
E-mail: tgupta@chemistry.du.ac.in; Fax: +91-11-27662642; Tel: +91-11-27662642
bFaculty of Life Sciences and Biotechnology, South Asian University, New Delhi-110
021, India
Electronic supplementary information (ESI) available: Experimental details,
3
Scheme 1 Synthesis of optical probe (2), acylated product (3) and oxidative
product (4).
including characterization data for 2, 3 and 4, details of photophysical properties
and sensing experiments. See DOI: 10.1039/c2ra22292b
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dependent response may make it possible to quantify the amount
of O2 present in a medium (Fig. 2, inset). The observed absorption
intensity is proportional to the concentration of O2 up to 300 ppm
for 10 min exposure time (Fig. 2, inset) and shows a linear
correlation (R2 = 0.99), which is an important characteristic of
sensors.
Notably, fluorometric detection of O2 is also possible with 2. In
fact, 2 display an emission peak at l = 456 nm (excitation l = 320
nm) that shows interesting successive ‘‘turn-on’’ (wrt intensity)
coupled with ‘‘turn-off’’ (wrt wavelength) optical responses upon
exposure to O2 in the 50–300 ppm range (Fig. 3). The emission
intensity of 2 increases (2.9 fold) along with a large blue shift (Dl =
45 nm) upon exposure to 300 ppm of O2 for 10 min. It is
noteworthy that generally O2 acts as a quencher for the
electronically excited state of the luminescence molecules.14 In
contrast, our sensor shows enhancement of emission properties
after interaction with O2 that could be due to positive inductive
effects of the methoxy group along with increased conjugation in
the oxidative product. Further, the variation of emission intensity
or wavelength display linear behaviour upon stepwise increases of
the concentration of O2 in the detection range of 50–300 ppm
(Fig. 3, inset) that is common for fluorescent probes.15 The
detection limit was found at 15 ppm using the detection limit
equation16 3S. D./s, where S. D. is standard deviation of ten blank
measurements and s is the slope of emission intensity.
In order to have a practical application, the sensor should
exhibit some sort of selectivity, which is normally based on host–
guest interactions, ligand coordination, chemical reactivity, con-
formation change, etc. In the present case, the gas sensor 2
exhibits selectivity based on redox properties. Indeed, 2 displays a
high degree of selectivity for O2 since no considerable response
was observed in both absorption (Fig. 4a) and emission (Fig. 4b)
spectra upon exposure to pure CO, CO2, Ar, N2, H2, CH4 and C2H4
for 30 min each. Alternatively, a mixture of any of the above test
Fig. 1 Comparative 1H-NMR (a) and FTIR (b) spectra of 2 (black color) and 4 (red
color), showing changes after exposure to 300 ppm of O2 for 10 min.
transformation of 2 (benzonoid form) into 4 (quinoid form)12
upon exposure to O2 (Scheme 1). This transformation was
estimated at y60% based on NMR data.
The UV-Vis spectrum of 2 in dry acetonitrile solution displays
an intense peak in the UV region at l = 315 nm (e = 30 245 M21
cm21), which can be assigned as a p–p* transition of the
‘‘benzonoid’’ ring.13 Exposing the acetonitrile solution of 2
(degassed with N2 for 15 min) with N2 containing 100 ppm of
O2 for 10 min resulted in an increase of absorption intensity along
with a slight red shift (Dl = 5 nm) at l = 315 nm. In addition, a
new band appeared in the visible region at l = 502 nm (Fig. 2).
This optical behaviour can be rationalized by the formation of a
‘‘quinoid’’ ring promoted through oxidation of 2 by O2.13
The response time of the sensor relies on the concentration of
O2. For instance, the saturation plateau for 100 ppm of O2 was
observed after y90 min (Fig. 2, inset). However, a pronounced
increase in absorption intensity was already marked after only 10
min (35% with respect to saturation values), which can be used as
a response time of the sensor for this concentration of O2. Further,
precise determination of absorption intensity as a function of time
or concentration is possible by integrating the entire absorption
window (l = 275–600 nm). Apparently, the observed concentration
Fig. 2 Absorption intensity changes of 2 (1 6 1025 M in CH3CN) as a function of
time upon exposure to O2 (100 ppm in N2). Saturation was reached after y90 min.
Inset shows: the enhancement of the peak area in the visible region (l = 275–600
nm) as a function of time at 100 ppm of O2 (black dots) and as a function of
concentration of O2 at 10 min exposure time (blue dots).
Fig. 3 Monitoring of fluorescence intensity along with wavelength changes of 2 (1
6 1025 M in CH3CN) after exposure to various concentrations of O2 in N2 for 10
min, each ranging from 50 ppm (light grey), 100 ppm (magenta), 150 ppm (cyan),
200 ppm (yellow), 250 ppm (orange), 300 ppm (dark gray). Inset shows ‘‘turn-on’’
coupled with ‘‘turn-off’’ changes in intensity and wavelength.
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Fig. 4 Representative bar chart showing % change in area under the peak for
absorption (a) and emission (b) intensities of 2 (1 6 1025 M in CH3CN) after
exposure to pure gas analytes (blue bar) and 300 ppm of O2 in each gas analyte
(black bar) for 10 min. 1, CO; 2, CO2; 3, C2H4; 4, CH4; 5, Ar; 6, N2; 7, H2; and 8, air.
Fig. 6 Monitoring the absorption intensity changes in the area under the peak of 2
(1 6 1025 M in CH3CN) as a function of pH of the solution upon exposure to 300
ppm of O2 for 10 min each.
stored in an air-stable acetylated form, 3, which can be converted
gases with 300 ppm of O2 show full response in both absorption
and emission spectra (Fig. 4) and the colour change of the solution
from pale yellow to dark red can be seen by eye.
to the active form, 2, by refluxing in an acid (see SI ).
3
Notably, the pH of the solution considerably affects the sensing
performance of 2. As shown in Fig. 6, the optimum response was
observed in the pH range 5.0–7.0. The failure of the sensor
response at a pH lower than 5.0 could be due the non-availability
of the lone pair on nitrogen as a result of quaternization and at a
pH higher than 7.0 could be due to the non-participation of lone
pairs.18
More importantly, the O2 detection and the simultaneous
absorption/emission intensity changes of 2 were clearly followed
by a gradual change in colour from pale yellow to pink and finally
dark red that could be clearly seen by eye (Fig. 5). Other test gases
did not induce any significant colour change of the solution. The
‘‘by-eye’’ detection of O2 with contrast colour change renders this
sensor suitable for practical application in scientific laboratories
for the detection of O2 level in conventional glove boxes or in
organic solvents. Interestingly, the change in colour after exposure
of O2 (100–300 ppm) can also be monitored on test paper (see Fig.
Conclusions
In conclusion, we have demonstrated a simple approach for ‘‘turn-
on’’ detection of O2 via multiple-channel analyses. The optically-
rich (both chromogenically and fluorogenically), photo-stable (see
S4 ).
3
Reversibility of a sensor is much sought-after in sensor
Fig. S5 ) and redox-active (see Fig. S6, S7 ) diamine-based organic
3
3
3
engineering, but it is not necessarily a requirement. Our sensor
shows partial recovery (up to 65%) upon addition of a suitable
reducing agent such as Zn/NaOH (Scheme 1) under a N2
atmosphere as judged by H NMR and UV-Vis spectroscopy. The
lack of full recovery could be due to the slow autopolymerisation of
4 as a function of time.17 However, the sensing assets can be
probe is highly selective toward O2 since no reactivity was observed
with a variety of other test gases. A highly O2-selective fluorescence
enhancing property (2.9 fold) in conjunction with a visible
colorimetric change from pale yellow to dark red was observed.
Most importantly, our results state that the detection of O2 can be
1
1
monitored by H NMR, FTIR, absorption and emission spectro-
scopy or by eye using a single molecule. Further, the sensing assets
have been stored in the acetylated form of the sensor that can be
converted to the active form by maintaining the pH of the
solution. The single input coupled with multiple output behaviour
of this multi-responsive molecule opens the door for the
development of molecular logic gates19 and the principle may
also be extended to other optical probes in order to improve
sensitivity, response time and reusability.
Financial assistance from the Department of Science and
Technology (DST), Department of Science and Technology-Nano
Mission (DST-Nano Mission) and the University of Delhi is
gratefully acknowledged. A. K. thanks the Department of
Chemistry, University of Delhi, New Delhi for use of the NMR
facility and UGC for providing fellowship.
Fig. 5 Colorimetric changes of 2 upon exposure with various concentrations of O2.
(a) no O2, (b) 50, (c) 100, (d) 150, (e) 200, (f) 250, and (g) 300 ppm for 10 min
exposure time.
392 | RSC Adv., 2013, 3, 390–393
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