Turn-on detection of pesticides: Via reversible fluorescence enhancement of conjugated polymer nanoparticles and thin films

Article abstract of DOI:10.1039/c6nj00690f

Reported herein is the significant fluorescence enhancement of conjugated polymer nanoparticles in the presence of aromatic organochlorine pesticides. This pesticide-mediated fluorescence enhancement leads to reversible pesticide detection systems with hi

Full text of DOI:10.1039/c6nj00690f

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Turn-on detection of pesticides via reversible
fluorescence enhancement of conjugated polymer
Cite this:DOI: 10.1039/c6nj00690f
nanoparticles and thin films†
Received (in Montpellier, France)
3rd March 2016,
William Talbert, Daniel Jones, Joshua Morimoto and Mindy Levine*
Accepted 5th July 2016
DOI: 10.1039/c6nj00690f
www.rsc.org/njc
Reported herein is the significant fluorescence enhancement of
Techniques for the detection of organic pesticides generally
conjugated polymer nanoparticles in the presence of aromatic rely on chromatography followed by mass spectrometry.4 These
organochlorine pesticides. This pesticide-mediated fluorescence methods offer good sensitivity and resolving power, but suffer
enhancement leads to reversible pesticide detection systems with from the high cost of operation and tedious and time-consuming
high sensitivity (as low as 5 lM), as well as significant generality and sample preparations,5 which limits the ability to conduct
straightforward reversibility.
high throughput assays. Newer techniques for pesticide
detection include molecularly imprinted polymer systems,6
The widespread use of pesticides has been highly effective in nanoparticle-based immunoassays,7 and gold nanoparticle-
increasing the harvested yields of many crops worldwide through based Raman spectroscopy.8 A variety of fluorescence-based
eliminating the threat of common pests, but their use has also methods for pesticide detection have also been reported,9
been of concern due to their known and suspected toxicity to although in many cases these methods require derivatization
humans and other species, as well as their long term environ- steps,10 chromatographic purification,11 and/or are substan-
mental persistence.1 One class of pesticides that is of continuing tially limited in terms of the range of pesticides that can be
concern is organochlorine pesticides (OCPs), the most common detected.12
of which is dichlorodiphenyltrichloroethane (DDT), sold com-
One method of detection that has shown a lot of promise
mercially as a mixture of the para, para- (compound 1, Fig. 1) and in the detection of multiple classes of analytes with extremely
ortho, para- (compound 4) isomers.2 Dichlorodiphenyldichloro- high sensitivity and selectivity is the use of conjugated fluor-
ethane (DDD, compound 2) and dichlorodiphenyldichloro- escent polymer sensors.13 Typically, detection efficiencies are
ethylene (DDE, compound 3) are some of the metabolites of optimal in polymer aggregates such as thin films14 or conju-
DDT, also with known toxicities.3
gated nanoparticles,15 which enable inter-polymer as well as
intra-polymer exciton migration.16 Formation of conjugated
polymer-derived nanoparticles can occur through a variety of
methods,17 including reprecipitation,18 in which the hydropho-
bic polymer collapses upon its introduction into aqueous
solution, resulting in the formation of well-defined spherical
nanoparticles.
Reported herein is the detection of DDT and its metabolites
(compounds 1–4) in aqueous solutions via the fluorescence
enhancement of nanoparticles derived from conjugated organic
polymers. These particles were fabricated via the reprecipitation
of 2,1,3-benzooxadiazole-alt-fluorene (PFBO, polymer 7), synthe-
sized following literature-reported procedures.19 This polymer
was fully characterized by spectroscopic techniques, with a
Mn = 3.8 Â 103 g molÀ1 and Mw = 7.3 Â 103 g molÀ1. The
polymer-derived nanoparticles were characterized by dynamic
light scattering experiments, with an average particle diameter
of 139 nm (see ESI† for details).
Fig. 1 Pesticides (1–4), control analytes 5–6, and conjugated polymer 7.
Department of Chemistry, University of Rhode Island, 140 Flagg Road, Kingston,
RI 02881, USA. E-mail: mlevine@chm.uri.edu; Tel: +1 401-874-4243
† Electronic supplementary information (ESI) available: Syntheses of polymers 7 and 9,
experimental details for fluorescence experiments, dynamic light scattering experi-
ments, nanoparticle and thin film fabrications. See DOI: 10.1039/c6nj00690f
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The degree of fluorescence changes observed with the intro-
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duction of aromatic pesticides to the aqueous nanoparticle (or
free polymer) solution was calculated according to eqn (1):
Fluorescence modulation = PFBO70mM/PFBO0mM (1)
where PFBO70mM is the integrated polymer fluorescence in the
presence of 70 mM analyte in acetonitrile, and PFBO0mM is the
integrated polymer fluorescence in the presence of 0 mM analyte in
acetonitrile. Little to no fluorescence interference from the pesti-
cides themselves is expected due to the fact that these analytes
show absorption and emission maxima primarily in the ultraviolet
region of the UV-Vis spectra,20 well removed from the absorption
and emission of the donor–acceptor polymer (lmax absorption:
polymer = 413 nm; nanoparticles = 411 nm; lmax emission:
polymer = 507 nm; particles = 534 nm).21 The concentration of
7 was varied (see ESI† for more details), and optimal fluorescence
responses were obtained with a 1.25 Â 10À3 mg mLÀ1 polymer
solution.
Results of the fluorescence modification experiments are
shown in Table 1 and Fig. 2, and key trends are discussed in
further detail below.
Fluorescence enhancements of the PFBO nanoparticles were
observed in the presence of compounds 1–4. In contrast to the
strong fluorescence responses observed in the case of the
conjugated polymer-derived nanoparticles, the conjugated polymer
itself displayed a marked insensitivity to the presence of any of the
pesticides investigated (Table 1 and Fig. 3). The strong dependence
of the PFBO fluorescence responses on its aggregation state
Fig. 3 Fluorescence changes of PFBO polymer in the presence of pes-
ticides 1–4. [PFBO] = 1.25 Â 10À3 mg mLÀ1
.
indicates the necessity of inter-chain polymer communication to
enable efficient fluorescence enhancement behaviors, a result that
has been demonstrated previously in the literature for the detec-
tion of other analytes, although not for the detection of pesticides
to date.22 Additionally, the differential responses of compound 1
and compound 4 are particularly noteworthy as these compounds
are structural isomers (with identical masses) and would be
difficult to differentiate using standard mass-spectral techniques.
This phenomenon was shown to be specific for organochlor-
ine pesticides by measuring the fluorescence responses of the
nanoparticles to aromatic compounds 5 and 6, which have
been found in a variety of food products.23 Neither analyte was
found to effect significant fluorescence changes (a fluorescence
modulation value of 1.02 with 70 mM of analyte 5 in acetonitrile;
a value of 0.99 with 70 mM of analyte 6 in acetonitrile).
Substantially higher concentrations of the control analytes led
to limited fluorescence decreases of the nanoparticle solution
(Fig. 4), highlighting the selectivity of the fluorescence-based
detection system.
The sensitivity of the fluorescence enhancement-based detec-
tion for analytes 1–4 is shown through the low limits of detection
(Table 2),8 which approach current levels of concern for these
pesticides24 and highlight the practicality of this fluorescence-
based detection system. Other literature-reported detection systems
for these compounds have also been reported, with somewhat
more sensitive detection limits (8 mg LÀ1 for a custom-made C18
column;20 50 ppt for a molecularly imprinted polymer),25 although
many of these systems may have other operational disadvantages.
Table 1 Average% change fluorescence of PFBO 7 and particle size with
added pesticide
Fluorescence
Fluorescence
Sizeb
(nm)
Analyte
modulation particlea
modulation polymera
1
2
3
4
5
6
2.47
1.17
3.48
3.08
1.02
0.99
1.02
1.03
0.96
1.01
220
164
190
205
a
Fluorescence modulation calculated according to eqn (1); [PFBO] =
1.25 Â 10À3 mg mLÀ1
.
Particle size with 70 mM analyte in acetonitrile
b
as measured by dynamic light scattering experiments.
Fig. 4 Fluorescence changes of PFBO nanoparticle solutions in the
presence of (A) analyte 5 and (B) analyte 6. The black line represents emission
in the presence of 0 mM analyte, the red line represents emission in the
presence of 70 mM analyte, and the blue line represents emission in the
presence of 1 mM analyte.
Fig. 2 Fluorescence changes of PFBO nanoparticles in the presence of
pesticides 1–4. [PFBO] = 1.25 Â 10À3 mg mLÀ1
.
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Table 2 Limits of detection for pesticides 1–4 and literature-reported
levels of concern
Literature-reported levels of
concern (ppm)
Analyte
LOD (ppm)
1
2
3
4
1.6
33.8
27.9
26.2
0.05–526
0.05–526
0.05–526
0.05–526
Fig. 6 Dynamic light scattering experiments of polymer 7-derived nano-
particles with (A) pesticide 1 and (B) pesticide 2, indicating no significant
changes in particle size in the presence of the pesticides.
Literature precedent by Swager and co-workers demonstrated
that fluorescent polymer systems underwent reversible fluores-
cence enhancements as a result of analyte-mediated reduction of
the polymer chain,27 an effect that was easily reversed by introduc-
tion of iodine vapor.28 Although similar reversibility was observed
in this system, with the fluorescence increases demonstrated by
solutions of polymer 7-derived nanoparticles in the presence of
analyte 1 nearly completely reversed with the addition of iodine
(Fig. 5A), compound 1 is highly unlikely to act as an effective
reductant of the polymer chain.29 Rather, the reversibility in our
system is likely a result of the formation of reversible charge-
transfer complexes between the conjugated polymer chain and
iodine vapor, which is disrupted with the addition of aromatic
organochlorine pesticides that are able to pi-stack efficiently
with the conjugated polymer chain. Selectivity for compounds
1–4 compared to control analytes 5 and 6, in turn, is likely due
to the electron deficient nature of analytes 1–4 and the resul-
tant electronic complementarity with the conjugated polymer.
Other examples of iodine doping of conjugated polymer systems
have also been reported,30 although to the best of our knowl-
edge, this phenomenon has not been used for reversible
fluorescence-based detection to date. The fact that this fluores-
cence switching was reversible over several cycles (Fig. 5B) is
highly significant for the development of practical fluorescence
detection systems.
onto glass slides. These films were briefly exposed to the vapor
from a solution of compound 1 in tetrahydrofuran. The measurable
response of these films to compound 1 vapor (Fig. 7A) is remark-
able considering the low vapor pressure of compound 1,32 and
indicates high levels of sensitivity in these fluorescent polymer-
derived detection systems. Moreover, control experiments indicated
that the tetrahydrofuran itself had negligible effects on the photo-
physical properties of polymer 7-derived thin films. These fluores-
cence changes were also reversible with exposure of the thin film to
iodine vapor, leading to a nearly complete return to the initial thin
film fluorescence state (1.27-fold increase followed by 1.20-fold
decrease, Fig. 7).
Finally, the fluorescence responses of other conjugated polymers
(Fig. 8) in the presence of 70 mM of compound 1 were measured,
and the results are summarized in Table 3 and Fig. 9. These
polymers were either commercially available (compounds 8, 10,
and 11) or easily synthesized using a synthetic procedure devel-
oped for the undergraduate teaching laboratory (compound 9).33
For most of the polymers, analogous fluorescence enhancements
in the presence of compound 1 were observed, highlighting the
general applicability of the pesticide-mediated fluorescence
Oftentimes fluorescence enhancements of conjugated polymer-
derived nanoparticles involve macroscopic changes in the particle
architecture that translate into measurable fluorescence response
changes;31 however, in this case the addition of pesticides 1–4
effected little to no change in the average particle size and size
distribution (Fig. 6).
An extension of this fluorescence-based detection to polymer
7-derived thin films was conducted by fabricating fluorescent thin
films from the spin casting of a polymer 7 solution in chloroform
Fig. 7 Fluorescence changes of thin films of polymer 7 with exposure to
vapors from compound 1.
Fig. 5 (A) Illustration of reversibility of fluorescence changes of polymer
7-derived nanoparticles (polymer treated with I2 prior to addition of
compound 1). (B) Switching behavior of polymer 7-derived nanoparticles
with alternating additions of I2 and compound 1 over 11 cycles.
Fig. 8 Structures of other fluorescent conjugated polymers investigated.
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Table 3 Average% change fluorescence of polymers 8–11 with 70 mM
analyte 1a
Notes and references
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Fluorescence modulation
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Fluorescence modulation
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Polymer
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enhancement of PFBO-derived nanoparticles and thin films
in the presence of aromatic organochlorine pesticides, and
marked class-specific fluorescence changes of PFBO-derived
nanoparticles in the presence of a variety of other small
molecule pesticides. These fluorescence responses have a
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polymer chain aggregation to enable efficient inter-polymer
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introduction of iodine vapor; (c) a ‘turn-on’ rather than ‘turn-
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improved sensitivity in practical detection schemes; (d) low
limits of detection, which approach practical levels of concern;
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synthesized polymers. Efforts towards developing practical
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Acknowledgements
Funding for this research was provided by the University of Rhode
Island Chemistry Department start-up funds.
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