Polymerization of a boronate-functionalized fluorophore by double transesterification: Applications to fluorescence detection of hydrogen peroxide vapor

Article abstract of DOI:10.1039/b809674k

The double transesterification polymerization of 3′,6′- bis(pinacolatoboron)fluoran and pentaerythritol is reported. A model dimeric compound was synthesized to demonstrate the effectiveness of bis-diols to undergo a double transesterification, which is driven by formation of the energetically favored six-membered di-ester ring from a monomer containing a five-membered di-ester ring. This synthetic procedure provides a new route to boronate based polymers, avoiding unstable boronic acid monomers. Formation of poly-3′,6′-bis(1,3,2-dioxaborinane)fluoran, with a molecular weight of 10000, is complete after 48 h at 50 °C. The thermodynamic stability of the six-membered boronic ester rings present in the polymer backbone also improves the stability of the polymer and its resistance to oxidation under ambient and UV light conditions. A surface detection method for the analysis of H₂O₂ vapor by a fluorescence turn-on response was explored. The fluorescent response results from oxidative deprotection of the boronate functionalities forming green luminescent fluorescein. Detection limits as low as 3 ppb were observed for H₂O₂ over an 8 h period. Detection of H₂O₂ in liquids can also be carried out through spot tests at concentrations as low as 1 ppm after 5 min. This new vapor-phase sensor for H₂O₂ provides a robust, low-cost alternative to current technology for potential applications as a self-integrating sensor for the detection of H₂O₂ as well as the direct monitoring of H₂O₂ levels in areas such as cargo shipments, chemical facilities, and pulp bleaching.

Full text of DOI:10.1039/b809674k

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Polymerization of a boronate-functionalized fluorophore by double
transesterification: applications to fluorescence detection of hydrogen
peroxide vapor†
*
Jason C. Sanchez and William C. Trogler
Received 9th June 2008, Accepted 20th August 2008
First published as an Advance Article on the web 29th September 2008
DOI: 10.1039/b809674k
The double transesterification polymerization of 30,60-bis(pinacolatoboron)fluoran and pentaerythritol
is reported. A model dimeric compound was synthesized to demonstrate the effectiveness of bis-diols
to undergo a double transesterification, which is driven by formation of the energetically favored
six-membered di-ester ring from a monomer containing a five-membered di-ester ring. This synthetic
procedure provides a new route to boronate based polymers, avoiding unstable boronic acid
monomers. Formation of poly-30,60-bis(1,3,2-dioxaborinane)fluoran, with a molecular weight of
10 000, is complete after 48 h at 50 ꢀC. The thermodynamic stability of the six-membered boronic ester
rings present in the polymer backbone also improves the stability of the polymer and its resistance to
oxidation under ambient and UV light conditions. A surface detection method for the analysis of H2O2
vapor by a fluorescence turn-on response was explored. The fluorescent response results from oxidative
deprotection of the boronate functionalities forming green luminescent fluorescein. Detection limits as
low as 3 ppb were observed for H2O2 over an 8 h period. Detection of H2O2 in liquids can also be
carried out through spot tests at concentrations as low as 1 ppm after 5 min. This new vapor-phase
sensor for H2O2 provides a robust, low-cost alternative to current technology for potential applications
as a self-integrating sensor for the detection of H2O2 as well as the direct monitoring of H2O2 levels in
areas such as cargo shipments, chemical facilities, and pulp bleaching.
organic and aqueous solvents. Sensor efforts have focused on
Introduction
targeting hydrogen peroxide (H2O2) produced through UV7 or
acid-catalyzed8 decomposition of TATP or HMTD. Bulk TATP
and HMTD may also include residual H2O2 left over from their
synthesis.
Detection and analysis of explosive materials and formulations
has become an integral part of national and global security.1 The
lack of robust, low-power, portable detection devices for the
rapid on-site screening of both common and suspicious chem-
icals, materials, cargo, and persons, has driven the need for
improved sensor devices, such as photoluminescent sensors.2
However, improvised peroxide explosives are not detectable by
these technologies.3 The two most common such materials are
triacetone triperoxide (TATP) and hexamethylene triperoxide
diamine (HMTD) (Fig. 1). These chemicals do not contain nitro
or aromatic functionalities but incorporate cyclic peroxides that
are stable enough to be transported, but are moderately shock
sensitive (Table 1).4 Both TATP and HMTD can be synthesized
easily with common chemicals, so starting materials do not need
to be harvested from munition stockpiles or stolen from chemical
factories or institutions.5 Current detection methods for TATP
and HMTD include separation techniques, ion detection, and
UV-vis and fluorescence response to photochemical decompo-
sition.6 These methods either involve complex spectroscopic
evaluation, multiple steps for detection, or a complex matrix of
Fig. 1 Chemical structures of triacetone triperoxide (TATP) and
hexamethylene triperoxide diamine (HMTD).
Table 1 Properties of organic peroxide based explosives compared with
TNT
Melting
Point/ꢀC
Detonation
velocity/m sꢁ1
Explosive
Type
Pvap/Torr
Department of Chemistry and Biochemistry, University of California at
San Diego, 9500 Gilman Drive, La Jolla, California 92093-0358, USA.
E-mail: wtrogler@ucsd.edu
TATP
HMTD
TNTa
Primary
Primary
Secondary
98
148
81
5.3 ꢂ 10ꢁ2
NA
5.8 ꢂ 10ꢁ6
5300
5100
6850
† Electronic supplementary information (ESI) available: The
fluorescence spectra of PolyF-1 (10 mg cmꢁ2 on filter paper) after
exposure to UV light and ambient conditions, and the 1H NMR and
13C NMR for dimer 1. See DOI: 10.1039/b809674k
a
Nitroaromatic based explosive for comparison.
5134 | J. Mater. Chem., 2008, 18, 5134–5141
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While current H2O2 detection methods have advanced in
sensor sensitivity, they typically involve liquid sampling media
and evaluation.9 To overcome these limitations, we have been
interested in sensors that target vapor-phase H2O2.
ability to detect vapor-phase H2O2 by fluorimetric analysis and it
shows promise as a sensitive and selective polymeric sensor film
for the detection of trace quantities of H2O2 in both liquid and
vapor phases.
Use of H2O2 in paper pulp bleaching, speciality chemical
synthesis, and chemical disinfection also require monitoring of
vapor H2O2 levels10 due to the acute toxicity inherent in even
small doses of H2O2 (1 ppm).11 Current methods for the detection
of H2O2 vapor rely on chemiresistive,12 electrochemical,13
colorimetric,14 and vibrational spectroscopy analysis.15
Experimental
General information
All synthetic manipulations were carried out under an atmo-
sphere of dry argon gas using standard Schlenk techniques unless
otherwise noted. Dry solvents were purchased from Aldrich
Chemical Co. Inc. and used after purification with an MBraun
Auto Solvent Purification System. Pentaerythritol (98%) was
purchased from Acros Organics and used as received. For dosing
studies, 30 wt% H2O2 in water (Acros) was purchased, using
a fresh solution for every dosing run. Hydrogen peroxide
solutions were assayed via iodometric titration and the average
peroxide weight percentage was 27.1 ꢃ 2.0%. Di-t-butyl peroxide
(Aldrich, 98%) and benzoyl peꢀroxide (Aldrich, 97%) were used as
purchased and stored at 2–6 C under inert gas. The following
were prepared by literature methods: 4,4,5,5-tetramethyl-
2-p-tolyl-1,3,2-dioxaborolane25 and 30,60-bis(pinacolatoboron)
fluoran.19 NMR data were collected using a Varian Mercury Plus
spectrometer and 9.4 T superconducting magnet (399.911 MHz
Fluorimetric sensing is a potentially promising approach.
Current methods that focus on the use of fluorescence to monitor
the presence of H2O2 include water-soluble FRET-based poly-
electrolytes,16 deprotection of fluorescein derivatives,17 and
benzofurans.18 These methods involve solution-phase determi-
nation of H2O2 for biological assays. One application that
provides very high selectivity for H2O2 detection is the oxidative
deprotection of boronic ester substituted fluoran, xanthanone,
and phenoxazine derivatives, which have been synthesized by
Chang and co-workers.19 These functional fluorophores are
highly specific for the detection of H2O2 in biological systems.
The sensors are utilized in a buffered aqueous media, showing
good stability toward common interferents. The boronic ester
functionalized fluoran shows the greatest fluorescence response
due to the high quantum efficiency of fluorescein produced by the
H2O2 specific oxidation of the two boronic ester functionalities
(Scheme 1). The simplicity, sensitivity, and selectivity of this
system make it an ideal candidate for vapor-phase detection of
H2O2. However, the thin-film stability and processability of the
30,60-bis(pinacolatoboron)fluoran monomer limits its use in
solid-state sensor technology.
1
for H and 100.52 MHz for 13C NMR). A Perkin–Elmer LS 45
luminescence spectrometer was used to recorded fluorescence
emission and excitation spectra. GPC data was obtained with the
use of a Viscotek GPCmax VE 2001 GPC; molecular weights
were recorded relative to polystyrene standards and low molec-
ular weight silole monomers and dimers. The data were fitted
using Origin8.
We report here the synthesis of poly-30,60-bis(1,3,2-dioxabor-
inane)fluoran (PolyF-1) by double transesterification polymeri-
zation of 30,60-bis(pinacolatoboron)fluoran and pentaerythritol.
This is the first synthesis, to the best of our knowledge, which
uses a double transesterification of boronic esters as a polymeri-
zation technique. Similar polymerization routes have used the
condensation polymerization of boronic acids with bis-diols.
This technique has been employed in the synthesis of oligomers,20
polymers,21 and macrocycles22 for use as self-repairing materials
and in thermal dehydration crosslinking applications. The
drawback is that boronic acids are unstable under ambient
conditions and require azeotropic or Dean–Stark removal of
water for the reaction to proceed. The instability and complex
synthetic issues may be avoided through the use of trans-
esterification. In addition, direct conversion of a five-membered
cyclic boronic ester to a six-membered cyclic boronic ester yields
a highly stable polymer structure.23 PolyF-1 was screened for its
Periodic acid test for cis-diols
The periodic acid test can be used to monitor the presence of
1,2-diols. The transesterification between pinacolatoboron and
pentaerythritol produces pinacol as the reaction proceeds. This
1,2-diol can be detected selectively over pentaerythritol through
its oxidative cleavage yielding ketone and iodate products. The
presence of iodate can then be determined by formation of
a white precipitate of AgIO3 in the presence of Ag+. To a periodic
acid reagent (2 mL, 0.1 M) was added 1 drop of nitric acid
(0.1 M) and 1 drop of solution taken from the reaction mixture.
This was stirred for 10 s and 2 drops of AgNO3 was added. The
solution was stirred for 5 s and the resulting white precipitate
confirms the presence of pinacol.
Peroxide detection
Thin films of PolyF-1 were prepared by drop-casting the polymer
from a CHCl3 solution onto thin sheets of Whatman2 filter paper
(4 cm2). The filter paper provides a porous sampling substrate
that maximizes the surface area of the polymer exposed to the
H2O2 analyte. Concentrations of 2.5, 10, and 40 mg cmꢁ2 were
evaluated for PolyF-1. The films are not visible to the naked eye
and show no luminescence over a 5 h period after exposure to UV
light and ambient air. Detection of H2O2 was initially carried out
in the vapor phase using a modified inert gas flow system. Argon
was bubbled through a dilute solution of H2O2 in water, and the
Scheme 1 Selective oxidation of 30,60-bis(pinacolatoboron)fluoran by
H2O2 forming the fluorescein fluorophore.
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J. Mater. Chem., 2008, 18, 5134–5141 | 5135

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vapor concentration was calculated based on temperature and
mole fraction of H2O2 in the solution.24 The inert gas flow was
directed into a sealed chamber through a Teflon tube. Cotton
was place inside the chamber to provide a more consistent
saturation of H2O2. The chamber was purged for 10 min before
each film exposure to ensure that an equilibrium vapor concen-
tration wasreached. The films were placed into the sealed chamber
and fluorescence spectra were recorded upon removal of the film
at various time intervals. The film was placed into a solid-support
scaffold on the fluorimeter to ensure repeatability of the site
of photo-excitation on the film. H2O2 vapor concentrations of
91, 29, 3.8, 2.9, and 1.2 ppm were evaluated for both PolyF-1
Scheme 2 Synthesis of model dimer 1 by double transesterification to
form six-membered rings.
and a recent review by Roy and Brown highlights structural
effects of both the boronate and diol on the reaction progress.23
One conclusion drawn from this study was the increased ther-
modynamic stability observed for six-membered ring boronic
esters over five-membered ring boronic esters. This concept
has been used for monomer functionalization but has yet to
be applied as a route to stabilized boronate polymers. To
demonstrate the viability of boronate double transesterification
as a polymerization technique, a model dimer complex (1)
was synthesized according to Scheme 2. 4-(Pinacolboronic
ester)toluene was used to mimic common arylboronates that may
be used as comonomers for polymerization and pentaerythritol
was used as the bis-diol to demonstrate the viability of a double
transesterification of two five-membered ring boronic esters.
The reaction proceeded smoothly at room temperature in
MeOH–H2O over 8 h. The presence of H2O is important for the
solubility of pentaerythritol and its presence as a co-solvent does
not hinder the reaction progress as it does during condensation
polymerization of boronic acids. The percentage of water was
adjusted to ensure solubility of both reactants. Reaction progress
was monitored by both TLC and a periodic acid test for the
presence of cis-diols (see Experimental). The reaction was
complete after 8 h with a 65% yield. Dimer 1 was characterized
by 1H NMR and 13C NMR analysis. The 1H NMR shows
a singlet (d 4.05 ppm) representing the methylene groups of the
pentaerythritol fragment. This singlet integrates with the methyl
resonance of the toluene endgroups (d 2.37 ppm) at a ratio of
4 : 3, respectively. There is no detectable pinacol, confirming the
effectiveness of the purification process. Dimer 1 shows superior
thermal stability (mp ¼ 259–261) and shelf life compared to the
4-(pinacolboronic ester)toluene while maintaining its solubility
in many common organic solvents.
and the fluoran monomer at a film concentration of 10 mg cmꢁ2
.
Solutions of H2O2 were also examined at concentration of
7 ppth, 1 ppth, 300 ppm, and 30 ppm to show application of
PolyF-1 as both a vapor-phase sensor for H2O2 and a qualitative
screening test for the presence of H2O2 in suspicious solutions.
3,9-Di-p-tolyl-2,4,8,10-tetraoxa-3,9-diboraspiro[5.5]undecane
(1)
To a stirring methanol solution of 4,4,5,5-tetramethyl-2-p-tolyl-
1,3,2-dioxaborolane (4 mL, 115 mM) was added an aqueous
solution of pentaerythritol (1 mL, 0.23 mM). The mixture was
stirred at room temperature for 12 h. Reaction progress was
monitored by both TLC and a periodic acid test. After 8 h the
solution became cloudy and a white precipitate formed. The solid
was extracted with methylene chloride, washed with brine and
water, and evaporated to dryness yielding a white powder
(50 mg, 65%). 1H NMR (CDCl3, ppm): d 7.69 (d, 4H, Ph-H), 7.18
(d, 4H, Ph-H), 4.05 (s, 8H, CH2), 2.37 (s, 6H, CH3); 13C{1H}
NMR (CDCl3, ppm): d 141.4, 134.1, 128.6, 65.0, 36.8, 21.9; mp ¼
ꢀ
259–261 C; Calcd for C19H22O4B2: C 67.9, H 6.60; Found: C
68.2, H 6.84.
Poly-30,60-bis(1,3,2-dioxaborinane)fluoran (PolyF-1)
To a stirring methanol solution of 30,60-bis(pinacolatoboron)
fluoran (4 mL, 45 mM) under ambient conditions was added
an aqueous solution of pentaerythritol (1 mL, 0.18 mM). The
mixture was stirred at 50 ꢀC for 48 h. Reaction progress was
monitored by TLC and a periodic acid test. The colorless solu-
tion was extracted with methylene chloride, washed with brine
and water, and the organic extract was evaporated to dryness.
The resulting off-white solid was washed with cold methanol,
yielding a white powder (61 mg, 75%). 1H NMR (400.053 MHz,
CDCl3): d 8.03 (m, 1H, Ph-H), 7.73 (br d, 2H, Ph-H), 7.60
(m, 2H, Ph-H), 7.43 (m, 2H, Ph-H), 7.06 (m, 1H, Ph-H), 6.85
(br d, 2H, Ph-H), 4.07 (br s, 8H, CH2); 13C{1H} NMR (100.59
MHz, CDCl3): d 163.4, 135.4, 130.0, 129.6, 127.1, 125.4, 123.9,
84.5, 65.1, 45.9, 30.1; Calcd for C25H18O7B2: C 66.4, H 4.01;
Found: C 66.5, H 4.4.
The promising results seen by the preceding synthesis of
dimer 1 led us to consider this technique for polymerization of
30,60-bis(pinacolatoboron)fluoran. The double transesterification
polymerization of 30,60-bis(pinacolatoboron)fluoran and pen-
taerythritol was carried out in MeOH–H2O at 50 ꢀC for 48 h
(Scheme 3). Again, a periodic acid test was used to monitor the
formation of pinacol. The polymer (PolyF-1) was extracted with
methylene chloride and washed with H2O. The organic solvent
was removed under vacuum and the resulting light yellow powder
Results and discussion
Synthesis and characterization
Transesterification of heterocyclic boronic esters is traditionally
applied for the functionalization or protection of complex
organic frameworks. There is much known about this process
Scheme 3 Synthesis of PolyF-1 by double transesterification to form
six-membered boronic ester rings.
5136 | J. Mater. Chem., 2008, 18, 5134–5141
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was purified by cold MeOH washings to produce a white solid in
good yield (75%). Several washing are required to fully remove
the excess pinacol. This was the optimum result obtained after
varying the reaction temperature, water content, and diol–
monomer ratio. The molecular weight of PolyF-1 was determined
to be 10 000 by GPC with a polydispersity index (PDI) of 1.5. This
polymer is thermally stable and non-luminescent. The 1H NMR
spectrum shows the presence of the pentaerythritol and the
absence of the pinacol protecting group. Both 1H and 13C NMR
show the presence of the fluoran comonomer in the polymer.
Detection of hydrogen peroxide
Detection of vapor-phase H2O2 was evaluated using thin-films of
PolyF-1 drop-cast onto a Whatman2 porous sampling substrate
to increase the surface area for polymer–analyte interactions.
Polymer films were maintained at 10 mg cmꢁ2 throughout the
study. Both 2.5 mg cmꢁ2 and 40 mg cmꢁ2 films of PolyF-1 were
tested for their sensor effectiveness; however, the 10 mg cmꢁ2 film
shows the best results when considering sensor stability and
response while limiting the quantity of polymer used. A modified
flow system using an inert carrier gas was used to achieve
constant equilibrium vapor concentrations of H2O2 (see Exper-
imental). Vapor concentrations of H2O2 were calculated using
published data.24 Time-dependent fluorescence spectra were
taken on exposure of the PolyF-1 film to H2O2 at concentrations
of 91, 29, 3.8, 2.9, and 1.2 ppm. A representative fluorescence
response plot for exposure of PolyF-1 to 2.9 ppm H2O2 is seen in
Fig. 2. An 8-fold increase in fluorescence intensity (510 nm) is
observed over a 3.5 h period for 2.9 ppm H2O2. The fluorescence
spectrum observed is nearly identical to the fluorescence emission
of a thin film of fluorescein deposited on the porous substrate.
The decomposition of the PolyF-1 upon exposure to H2O2 was
Fig. 3 Fluorescence trace of 10 mg cmꢁ2 PolyF-1 exposed to various
vapor concentrations of H2O2.
The data obtained at varying concentrations of H2O2 is
summarized in Fig. 3. At high vapor concentrations of H2O2
(>3.8 ppm) there is a rapid initial fluorescence response of the
PolyF-1 film on exposure to H2O2. After ꢄ1 h, a maximum
fluorescence intensity is reached for each H2O2 concentration. At
low H2O2 vapor concentrations (<2.9 ppm), there is a slow initial
fluorescence response followed by a gradual increase in the time
required to reach a maximum fluorescence intensity. However,
the threshold for the maximum change in fluorescence intensity
increases as the concentration of H2O2 decreases (Table 2). This
inverse response is unique to this system. Typical turn-on fluo-
rescence response scales proportionally with the concentration of
analyte. In this case, there is an inverse relationship between the
maximum fluorescence response and H2O2 concentration, which
may be explained by the bleaching effect that H2O2 has on
organic materials, especially at high concentrations. At high
vapor concentrations, pseudo first order kinetics in H2O2 are
expected (Fig. 4). However, there is also apparent competitive
decomposition of the organic fluorophore with the excess H2O2.
This competing degradation prevents the maximum fluorescence
response from being reached at high concentrations of H2O2.
While the kinetics at high concentrations of H2O2 are pseudo first
monitored by GPC, showing the presence of oligomers (Mw
¼
700, PDI ¼ 1.8) after 30 min of H2O2 exposure. The sensor
reaction proceeds by an oxidative deprotection of the boronic
ester functionalities, forming fluorescein from fluoran (Scheme 1).
Table 2 Summary of PolyF-1 (10 mg cmꢁ2) fluorescence responses on
exposure to H2O2 vapor
Flu. intensity
at N (IN
Time to
reach IN/103 sc
b
[H2O2]/ppm
ka/10ꢁ4 sꢁ1
)
91
29
3.8
2.9
1.2
12
12
10
7.0
3.0
18.9
22.2
28.3
32.4
49.6
3.0
3.6
4.2
13
24
a
b
Derived from first order kinetic plots seen in Fig. 4. Calculated from
the exponential growth fit (eqn 2) of the time-dependent fluorescence
trace of PolyF-1 exposed to various concentrations of H2O2 (Fig. 3).
Fig. 2 Fluorescence response of a 10 mg cmꢁ2 film of PolyF-1 to 2.9 ppm
of H2O2 vapor after 220 min. Solid line at 0 min represents the baseline
fluorescence intensity of the PolyF-1 film. The dashed line represents the
fluorescence emission of 100 mg cmꢁ2 of fluorescein.
c
Time to reach within 3s of IN
.
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Fig. 4 First order kinetics plot of the fluorescence response of PolyF-1
exposed to 91 ppm H2O2 (-, R2 ¼ 0.98), 29 ppm H2O2 (:, R2 ¼ 0.98),
3.8 ppm H2O2 (C, R2 ¼ 0.97), 2.9 ppm H2O2 (,, R2 ¼ 0.94), and 1.2 ppm
H2O2 (D, R2 ¼ 0.94). The apparent rate constant (k) is derived from the
slope of the linear regression fit. At low concentrations of H2O2, the
reaction deviates from first order kinetics in H2O2.
Fig. 6 Fluorescence responses of a 10 mg cmꢁ2 film of PolyF-1 and the
same mass of 30,60-bis(pinacolatoboron)fluoran (fluoran monomer) to
2.9 ppm H2O2, UV light, and ambient conditions over a 5 h period.
PolyF-1 shows a greater fluorescence response to H2O2 and is more stable
as a thin film than the fluoran monomer.
The monomer material 30,60-bis(pinacolatoboron)fluoran was
also screened for its ability to detect H2O2. This experiment was
performed to determine whether there is an advantage to using
the polymeric version of the sensor as PolyF-1. The fluoran
monomer shows decreased stability when exposed to both
ambient conditions (7.4% of the maximum fluorescence inten-
sity) and UV light (3.4%), as compared with PolyF-1. The
response to H2O2 is also weaker, accounting for only a 4-fold
increase in fluorescence intensity on exposure to 2.9 ppm H2O2 as
compared to an 8-fold increase for PolyF-1 (Fig. 6). The stability
of the six-membered boronic ester functionalities in PolyF-1 may
assist in directing the peroxide oxidation to deprotection of the
boronic ester rather than decomposing the organic framework.
In addition, processability of the monomer material for thin-film
application is much more difficult than for PolyF-1.
Fig. 5 Images of the fluorescence response of 10 mg cmꢁ2 PolyF-1 to
various concentrations of H2O2 vapor over a 5 h period. An increase in
fluorescence intensity is observed at lower concentrations of H2O2
providing a highly sensitive sensor response.
order, a change in kinetics is observed at around 2.9 ppm H2O2.
At lower concentrations the reaction begins to slow based on the
decrease in the molar ratio of H2O2 to PolyF-1. At the same time,
H2O2 is more readily consumed in the more energetically favored
oxidative deprotection reaction and oxidative decomposition
of the organic fluorophore begins to decrease. This causes
a continual increase in the maximum fluorescence intensity (IN)
at decreasing concentrations of H2O2 (Fig. 5). The reaction is
also time-dependent, showing much longer exposure times to
reach IN at lower concentrations of H2O2, so there is no problem
distinguishing between high and low concentrations of H2O2.
The films of PolyF-1 were also screened for their stability
under ambient UV light. This is relevant for real world inter-
ferents and film stability over time. PolyF-1 shows a minimal
fluorescence response under ambient conditions (0.06%) or UV
light exposure (0.5%) over a period of 5 h. This demonstrates the
good photo-stability of the films, as well as their stability to
atmospheric oxidizers that may be created in the presence of UV
light. The data collected for the UV light control experiment was
used to calculate the standard deviation (s ¼ 0.24) for the film
stability at 0 ppm H2O2 (Fig. S1, ESI†). This standard deviation
was used to calculate the time at which a detectable signal is
achieved for a given concentration of H2O2.
Quantifying the detection results of a vapor phase turn-on
fluorescence sensor based on the chemical modification of
a polymer thin film presents a unique challenge. The sensor
response is proportional to time but inversely proportional to
H2O2 concentration. The increase in signal response with
decreasing concentrations of H2O2 prevents typical analysis of
the detection response. In order to better quantify this system,
the fluorescence intensity at infinity (IN), calculated from a single
exponential growth (eqn 1), was fitted to the time-dependent
fluorescence response data (Fig. 3). Using IN, a correlation
between the time required to reach within 3s of IN and H2O2
concentration can be made (Fig. 7). The data was fit to an
exponential decay with an R2 of 0.999 (eqn 2). From this fit
a detection limit of 0.7 ppm (at time ¼ N) can be determined.
This limit is based on the maximum fluorescence response from
PolyF-1 at the film thickness used. However, there are measur-
able fluorescence responses above the noise limit (3s) of the
spectrophotometer that better quantify the useful detection limit
of the sensor. When this response is placed in a time domain,
a correlation can be made between the time and H2O2 detection
limits (Fig. 8). This plot correlates the proportional response
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Fig. 7 Correlation between the concentration of H2O2 and the time to
reach within 3s of IN. IN was calculated from the exponential growth fits
of the time-dependent fluorescence traces at various concentrations of
H2O2 observed in Fig. 3. The plot was fit to an exponential decay (eqn 2).
A detection limit of 0.7 ppm H2O2 is calculated from the threshold
reached by eqn 2. This detection limit is based on a maximum fluores-
cence response of PolyF-1 to H2O2.
Fig. 8 Correlation between the concentration of H2O2 and the time to
reach 3s of the fluorescence response noise. The noise was calculated
from the fluorescence response of PolyF-1 to UV light over a 5 h period.
The data was fit to a power function (eqn 3) providing the ability to
calculate the time required to detect a desired concentration of H2O2
using a 10 mg cmꢁ2 film of PolyF-1.
Table 3 Time-dependent detection limits of H2O2 vapor by PolyF-1 at
various exposure times
observed for the fluorescence intensity at 3s above the noise of
the spectrophotometer with the concentration of H2O2. The
response is no longer an inverse response and therefore can
provide more useful information on the detection capabilities of
PolyF-1. The noise was calculated to be 0.08 from the fluores-
cence measurements taken of the PolyF-1 film on exposure to UV
light and ambient conditions (Fig. S1, ESI†). The data was fitted
to a power function (R2 ¼ 0.999, eqn 3) as opposed to an
exponential decay (R2 ¼ 0.966, eqn 2) to prevent a threshold limit
from constraining the analysis. Using this equation, the time
required to reach a desired detection limit can be calculated
(Table 3). This plot is not limited by a maximum fluorescence
intensity, revealing the low levels of detection that can be ach-
ieved with PolyF-1. For example, 9 ppb H2O2 can be detected
after 3 h of exposure according to this analysis. A detection limit
of 3 ppb is estimated to be possible after 8 h of exposure, which is
two orders of magnitude below the permissible exposure limit
(1 ppm) over an 8 h period established by OSHA.22 It is impor-
tant to note, that these sensor films are effectively acting as
integrating sensors for low levels of H2O2. This application may
be useful in estimating average exposures over, for example, an
H2O2 detection
limit/ppb
Time/min
10
60
180
300
30
9
8 h working shift or for monitoring the contents of shipping
cargo containers for H2O2.
y ¼ y0 + A1e(x/t)
y ¼ y0 + Ae(ꢁx/t)
y ¼ xA
(1)
(2)
(3)
The high vapor pressure of H2O2 and the specificity that
boronic esters show toward H2O2 oxidation19 make PolyF-1
a highly sensitive and selective sensor for H2O2. The films show
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little response to ambient conditions as well as UV light above
the noise limit of the spectrophotometer over a 5 h period,
indicating that radical oxygen species (ROS) and other oxidants
found in the atmosphere as well as those that may be generated
under a UV lamp (l ¼ 302 nm) are not interferents. The vapor
pressures of most organic peroxides are much lower than for
H2O2, and thus are not significant interferents during vapor-
phase detection. Many of these interferents, including various
ROS and anionic species, have previously been tested in solution-
phase studies and show little to no response.19 Besides its use as
a vapor sensor, PolyF-1 can also be used to screen suspicious
liquids. This may be applicable to high throughput screening
areas where concealed liquids would not produce a measurable
vapor concentration of H2O2. To test this application, solutions
of H2O2 in water at various concentrations were spotted onto the
same 10 mg cmꢁ2 films of PolyF-1 used during the vapor-phase
detection studies. Several common organic peroxides, including
di-t-butylperoxide and benzoyl peroxide, were also spotted to
confirm that the oxidative depolymerization of PolyF-1 is
insensitive to these interferents in spot tests (Fig. 9). PolyF-1
easily detects 30 ppm of H2O2 during solution spot tests after
30 s. After 5 min, this signal increased further. Benzoyl peroxide
(100 ppm) and di-t-butylperoxide (98%) show no detectable
response after 5 min when spotted onto the PolyF-1 film.
Conclusions
A new method of polymerization was developed using the double
transesterification of arylboronates to synthesize PolyF-1. The
chemical principle used to favor the polymer structure relies on
the formation of six-membered boronic ester rings throughout
the backbone from a monomer containing a five-membered
boronic ester ring. This method of polymerization proved facile
for the polymerization of fluoran and may be generalized to the
polymerization of complex systems that are not compatible with
boronic acid monomers. PolyF-1 was designed and synthesized
as a self-integrating sensor for the selective detection of vapor
phase H2O2 by a fluorescence turn-on mechanism. H2O2 is an
important by-product produced through thermal and UV
degradation of the peroxide-based primary high explosives
TATP and HMTD and its levels are also an occupational health
and safety concern for its increasing use as a disinfectant and
bleaching agent. Detection limits on the order of 3 ppb over
an 8 h period can be achieved using thin films of PolyF-1 on
a porous sampling substrate. The detection process is insensitive
to common interferents including organic peroxides and many
ROS species. This technology can also be used to screen for H2O2
in suspicious liquids. A spot test of 30 ppm H2O2 shows visual
detection within 30 s, with an increasing fluorescence response
over time. The synthetic ease, stability, and processability of
PolyF-1 make it an ideal candidate for H2O2 detection applica-
tions that require a robust, low cost, sensitive, and selective
sensor with rapid qualitative and semi-quantitative signal
analysis.
Acknowledgements
This work was supported by the Air Force Office of Scientific
Research (AFOSR-MURI #F49620-02-1-0288), RedXDefense
(DHS subcontract 2006-0740R2), and the National Science
Foundation (NSF-GRFP).
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