Detection of ethylene gas by fluorescence turn-on of a conjugated polymer

Article abstract of DOI:10.1002/anie.201003899

Ripe fruits: The fluorescence of a conjugated polymer is quenched by the presence of copper(I) moieties. Upon exposure to ethylene gas the copper complexes bind to ethylene and no longer quench the polymer fluorescence (see picture). This sensory concept

Full text of DOI:10.1002/anie.201003899

Communications
DOI: 10.1002/anie.201003899
Sensors
Detection of Ethylene Gas by Fluorescence Turn-On of a Conjugated
Polymer**
Birgit Esser and Timothy M. Swager*
The detection and monitoring of ethylene is of great interest
and importance in the food and agricultural industries. As one
of the smallest plant hormones, ethylene is responsible for the
ripening of fruit and plays an important role in many more
developmental plant processes such as seed germination, fruit
ripening, senescence, and abscission. As fruits and vegetables
start ripening, ethylene is produced and emitted, and the
internal ethylene concentration in some fruits is used as a
maturity index to determine the time of harvest. In some
vegetables and fruits, such as bananas, exposure to ethylene
gas results in a continuation of the ripening process after
harvesting, thus the monitoring of ethylene gas in storage
rooms is important to avoid the deterioration of ethylene-
sensitive produce. The ethylene concentration in ripening
rooms is held between 10 and 200 ppm depending on the type
of fruit or vegetable, while a value of less than 1 ppm is
required in storage facilities.[1] Traditionally, gas chromatog-
raphy[2a] and photoacoustic spectroscopy[2b] have been used to
measure ethylene concentrations. Both techniques suffer
from the disadvantage of being operationally impractical
and do not allow for real-time measurements. Other sensing
systems that have been suggested use electrochemical[2c] or
chemoresistive methods,[2d] magnetoelastic sensing,[2e] and
photoluminescence quenching.[2f] However, all of these
systems have drawbacks such as high cost, impracticability,
or insufficient sensitivity towards ethylene.
the detection of ethylene is based on a fluorescence turn-on
mechanism and mimics nature by using a copper(I) complex
to bind to ethylene. The fluorescence of the conjugated
polymer is partially quenched by the presence of copper(I)
moieties that can coordinate to the polymer (Figure 1). Upon
Figure 1. Design of a sensory system for the detection of ethylene gas.
exposure to ethylene gas, the copper complexes bind to the
ethylene molecules and no longer quench the polymer
fluorescence. The advantage of this fluorescence turn-on
over a turn-off mechanism is that it requires a specific binding
event to the copper to create a new signal, whereas
fluorescence quenching can occur in multiple ways. Further-
more, if
a completely dark background (completely
We have designed a sensory system that makes use of the
advantages of fluorescent conjugated polymers as sensory
materials.[3] These compounds readily transform a chemical
signal into an easily measurable optical event and can achieve
large signal amplification compared to small-molecule che-
mosensors. For the recognition of ethylene, we were inspired
by the mechanism of ethylene binding in plants. The reception
of ethylene occurs through the receptor ETR1, which is
studied in Arabidopsis thaliana. It has been found that
copper(I) is an essential cofactor and responsible for the
binding of ethylene, and it is assumed that a cysteine residue
serves as ligand.[4] The sensing scheme we have designed for
quenched) state can be achieved, even a weak turn-on
signal can be readily measured and thereby can allow trace
detection.
Several requirements have to be met by the sensory
system: 1) The copper complex must have the ability to
quench the polymer fluorescence, but 2) the binding of the
copper complex to ethylene must be stronger in order to
provoke a large turn-on response. 3) For a practical applica-
tion of the system in thin films, the polymer matrix must be
porous enough to accommodate the copper moieties.
Conjugated polymers have emerged as an important class
of sensory materials. The signal is amplified through the
migration of excitons throughout the polymer chains or
between different chains in films.[3b] Poly(p-phenylene ethy-
nylene)s (PPEs) have shown impressive performance in
sensing applications. The introduction of shape-persistent
pentiptycenes, such as in polymer P2 (Scheme 1), in thin films
has led to improved photophysical properties compared to
planar polymers[5,6] as the three-dimensional structures create
interstitial space in thin films, thus resulting in higher
luminescence efficiencies.
[*] Dr. B. Esser, Prof. Dr. T. M. Swager
Department of Chemistry, Massachusetts Institute of Technology
Cambridge, MA 02139 (USA)
Fax: (+1)617-253-7929
E-mail: tswager@mit.edu
[**] B.E. is grateful to the German Academy of Sciences Leopoldina for a
postdoctoral fellowship (LPDS 2009-8). This work was supported in
part by the NSF. We thank Prof. S. Buchwald for the use of
computational resources.
In our sensory system, the triple bonds in PPEs can serve
as coordination sites for copper(I) to promote efficient
fluorescence quenching. The complexation of triple bonds
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003899.
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The phenyl rings in the optimized structure of 2 are
twisted by 688 because of the steric demand of the scorpionate
ligand (Figure 2).[12] The phenyl rings in the diphenylbuta-
Scheme 1. Poly(p-phenylene butadiynylene) (PPB) P1 and poly(p-phen-
ylene ethynylene) (PPE) P2 with pentiptycene units.
in PPEs by transition metals has been shown to reduce their
fluorescence intensity.[7] Copper(I) alkyne complexes are
well-known and well-studied,[8] but, to the best of our
knowledge, fluorescence quenching by copper(I) remains
unreported to date.
Figure 2. Optimized structures (B3LYP/6-31G*, SDD for Cu) of cop-
per(I) alkyne complexes 2 and 3 (hydrogen atoms are omitted for
clarity except for on the boron atoms).
The labile nature of the copper–ethylene interaction has
resulted in only a small number of air-stable copper(I)
ethylene complexes being reported.[9] Compound
1
diyne complex 3 show only a small twist of 318, the coplanarity
of the phenyl rings is better maintained. As a result, butadiyne
units seem more suited to act as coordination sites for copper
scorpionate 1, and thus we chose polymer P1 as a modified
version of P2 (Scheme 1) for the sensory system.
(Scheme 2), which features a fluorinated tris(pyrazolyl)bo-
Calculations using isodesmic equations were performed to
estimate the energy differences between the coordination of
ethylene and an alkyne to copper scorpionate 1.[12] The
coordination of ethylene is more favorable by 13.8 kcalmolÀ1
relative to diphenylacetylene, and by 14.8 kcalmolÀ1 relative
to diphenylbutadiyne. Considering entropic factors, the
equilibrium constants strongly favor ethylene (2 ꢀ 1012 for
diphenylacetylene and 4 ꢀ 1012 for diphenylbutadiyne). Ace-
tonitrile forms a stable complex with 1,[13] thus we used
acetonitrile as a ligand in quantitative fluorescence experi-
ments in solution as a substitute for ethylene because of the
ease of quantitative addition. The energy difference between
the coordination of ethylene or acetonitrile to the copper(I)
center was calculated to be small compared to the alkynes 2
and 3 (À0.6 kcalmolÀ1 or Keq = 10À3).
Scheme 2. Complex of copper(I), a fluorinated tris(pyrazolyl)borate
ligand, and ethylene employed in the sensory system.
rate ligand, demonstrates several of the requisite properties
with regard to stability and synthesis.[10] The copper–ethylene
bond in 1 is sufficiently strong to make the complex stable
under both ambient conditions and high vacuum, and its
electron-poor nature prevents facile oxidation of the copper
center. Furthermore, the fluorinated tris(pyrazolyl)borate
ligand is easily accessible in two steps from nonexpensive
starting materials.[11]
To investigate the coordination of alkynes by copper(I)
scorpionate 1, 1H NMR and 19F NMR experiments and
quantum chemical calculations were performed on complexes
of diphenylacetylene 2, diphenylbutadiyne 3, and bis(2,5-
dimethoxyphenyl)butadiyne, which represent subunits of
polymers P1 and P2. The coordination of the copper complex
is evident by the changes observed in the 1H NMR and
19F NMR spectra. In the 19F NMR spectra, a downfield shift of
approximately d = 1 ppm is observed for the CF3 groups
adjacent to the copper center upon addition of the alkyne.
The proton signals for the pyrazol protons are shifted
downfield by d = 0.04 ppm (see the Supporting Information).
P1 was synthesized by oxidative polymerization of the
terminal diyne 8 (Scheme 3). The synthesis of 8 started with a
single Pd-catalyzed cross-coupling between the diiodo-,
dialkoxy-substituted benzene derivative 4[14] and trimethyl-
silylacetylene (TMSA) to give 5. Coupling with the pentipty-
cene dialkyne 6[5b] followed by basic deprotection of 7 led to 8
in excellent yield. The polymerization was catalyzed by Pd
with p-benzoquinone as oxidant.[15] To maintain complete
solubility, molecular weights of 104 were required for P1, and
thus the oxidative polymerization was performed at 08C with
careful monitoring by gel permeation chromatography
(GPC).
In toluene solution, polymer P1 shows an absorption
maximum at 434 nm and an emission maximum at 462 nm.
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Figure 4. Fluorescence spectra of P1 in toluene during addition of 9
and after purging with ethylene (dashed line; the inset shows the
Stern–Volmer plot (462 nm); concentration of 9 (top to bottom in
10À5 m): 0, 7.2, 14.1, 20.9, 27.4, 33.7, 39.9.
maximum, KSV is the Stern–Volmer constant, and [Q] the
concentration of quencher) leads to a quenching constant of
K
SV = (2.0 Æ 0.3) ꢀ 103 mÀ1. When the solution is purged with
ethylene gas, the polymer fluorescence is restored to 95% of
its original intensity (Figure 4, dashed line). Titration of a
solution of P1 and 9 with micromolar concentrations of
acetonitrile (as a substitute for ethylene) led to a measurable
turn-on of fluorescence (see the Supporting Information).
The fluorescence of PPE polymer P2 is also partially
quenched by the addition of 9, however, the Stern–Volmer
constant for P2 is much lower than for P1, with KS = 3.8 ꢀ
102 mÀ1. This result is consistent with our assumption that the
quenching of P1 by 9 is much more effective than that of P2
because of steric reasons.
Scheme 3. Synthesis of polymer P1 by oxidative polymerization of 8.
a) TMSA, [PdCl2(PPh3)2], CuI, Et3N, RT, 33%; b) [Pd(PPh3)4], CuI,
iPr2NH, toluene, 658C, 16 h, 83%; c) K2CO3, MeOH, THF, RT, 4 h,
92%; d) p-benzoquinone, [Pd(PPh3)4], CuI, iPr2NH, toluene, 08C, 4 h,
73%; PDI=polydispersity index.
The fluorescence quenching of P1 by copper scorpionate 1
and a fluorescence turn-on by ethylene was investigated in a
preliminary visual experiment (Figure 3). A solution of the
in situ prepared “naked” copper scorpionate 9 in toluene was
In a practical application, other fruit metabolites might
interfere with the measurement. The effects of ethanol,
acetaldehyde, and water on the fluorescence of solutions of
P1 and 9 were tested (Figure 5, left). Small increases in
fluorescence could be observed in all three cases; however,
the increases are very small compared to the turn-on observed
with acetonitrile.
The sensory system needs to function in thin films for
many applications. We prepared thin films of mixtures of P1
and 9 by spin-casting solutions in toluene onto glass slides. As
Figure 3. Photographs of a solution of P1 in toluene (left), after
quenching with 9 (center), and after purging with ethylene (right)
illuminated at 365 nm; empty rectangle represents a vacant coordina-
tion site.
employed in all quenching experiments. The fluorescence of
P1 in solution (Figure 3, left) was quenched by the addition of
9 (center). When the resulting solution of P1 and 9 was
purged with ethylene gas, a complete turn-on of polymer
fluorescence was observed (Figure 3, right).
Fluorescence spectra recorded during the quenching of P1
by 9 are shown in Figure 4. A quantitative assessment that
employs the Stern–Volmer equation[16] F0/F = 1 + KSV[Q] (F0
is the (original) fluorescence intensity at the emission
Figure 5. Left: Relative fluorescence of solutions of P1 and 9 at
462 nm during titration with acetonitrile (blue cross), acetaldehyde
(gray triangle), ethanol (yellow square), and water (black asterisk);
concentrations of 9 (in 10À4 m) for acetonitrile: 3.7, acetaldehyde: 2.9,
EtOH: 3.4, H2O: 4.0. Right: Fluorescence spectra of thin films of P1
(a) and P1/9 mixtures (b, c) spin-cast from toluene solutions contain-
ing 0.64 mgmLÀ1 P1 and 0, 2, or 6ꢀ10À5 m 9 (top to bottom); insets:
photographs of films a and c, illuminated at 365 nm.
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shown in Figure 5 (right) the polymer fluorescence in thin
films is effectively quenched in the presence of 9. The degree
of quenching is about ten times larger than in solution.
The fluorescence response of the copper(I)/polymer films
to diluted ethylene gas was tested in real-time fluorescence
response studies using a Fido sensor.[17] In the Fido platform,
thin films are spin-cast on the inside of a glass capillary,
through which the gas flow is directed. The polymer is excited
at 410 nm and its fluorescence above 460 nm is recorded at
the end of the capillary. Figure 6a shows the response of a
Keywords: alkenes · copper · fluorescence · polymers · sensors
.
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Figure 6. Real-time fluorescence responses measured in a Fido
sensor[17] to ethylene/N2 mixtures and to N2 of a) a P1/9 mixed film
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P1/9 film during repeated exposures to 0.6% and 0.1%
ethylene (diluted with dinitrogen). The response to pure
dinitrogen is shown as a control experiment (gray line).
Repeated exposures to ethylene gas mixtures lead to a strong,
stepwise increase in absolute polymer fluorescence (Fig-
ure 6a, black and dashed line), whereas with pure dinitrogen
no absolute increase in fluorescence is observed. Using air
instead of dinitrogen gives the same result (see the Supporting
Information). A film of polymer P1 itself shows no difference
in behavior when exposed to ethylene/dinitrogen mixtures or
to pure dinitrogen, and no increase in absolute fluorescence is
observed (Figure 6b).
In conclusion, we have developed a new sensory concept
for the detection of ethylene gas that is based on the
fluorescence turn-on of a conjugated polymer. The system
can be used in solution and in thin films, thus achieving
sensitivities in the micromolar range in solution and of
1000 ppm in thin films. For a practical application, the
sensitivity of the system has to be improved, and we are
currently investigating the use of different polymers and small
molecule fluorophores.
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[16] J. R. Lakowicz in Principles of Fluorescence Spectroscopy, 3rd
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[17] Fido is
a commercial sensory device available from ICx
Technologies.
Received: June 27, 2010
Revised: August 27, 2010
Published online: October 6, 2010
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