Iodinated molecularly imprinted polymer for room temperature phosphorescence optosensing of fluoranthene

Article abstract of DOI:10.1039/b502706c

A novel molecularly imprinted polymer (MIP) of high interest for room temperature phosphorescence (RTP) sensing systems is described; the synthesized MIP contains iodine as internal heavy atom in the polymeric structure and its applicability for RTP sensing of fluoranthene at μg L^-1 levels is demonstrated. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b502706c

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Iodinated molecularly imprinted polymer for room temperature
phosphorescence optosensing of fluoranthene
a
A. Salinas-Castillo, I. S a´ nchez-Barrag a´ n, J. M. Costa-Fern a´ ndez, R. Pereiro, A. Ballesteros,
b
b
b
c
c
J. M. Gonz a´ lez, A. Segura-Carretero, A. Fern a´ ndez-Guti e´ rrez and A. Sanz-Medel*
a
a
b
Received (in Cambridge, UK) 24th February 2005, Accepted 25th April 2005
First published as an Advance Article on the web 19th May 2005
DOI: 10.1039/b502706c
A novel molecularly imprinted polymer (MIP) of high interest
for room temperature phosphorescence (RTP) sensing systems
is described; the synthesized MIP contains iodine as internal
heavy atom in the polymeric structure and its applicability for
signals, an adequate heavy atom (e.g. Br, I, Tl) must be present to
populate the excited triplet state of the phosphor by increasing the
1
6
rate of intersystem crossing. In many cases, the heavy atom is
added to the samples using a high concentration of salt in aqueous
solution. However, such methodology lacks general use and has
some inconveniences, including impurities in the reagents, cost of
21
RTP sensing of fluoranthene at mg L levels is demonstrated.
17
Polycyclic aromatic hydrocarbons (PAHs) are well-known hazar-
dous chemicals with carcinogenic and mutagenic character,
originated from incomplete combustion of organic matter. In line
with this, the European Parliament and the Council of the
European Union have included fluoranthene (as an indicator of
other more dangerous PAHs) in a list of 33 priority substances in
the heavy atom salts, etc. In this work, we have investigated the
use of an iodinated monomer in the polymerization (so ensuring
that the heavy atom is already present in the polymeric structure of
the MIP). The potential applicability of this novel iodinated
polymer for RTP fluoranthene optosensing has been evaluated
and compared to that of the corresponding traditional non-
iodinated polymer.
1
the field of water policy.
Identification and quantification of PAHs are usually performed
by means of liquid and gas chromatography. On the other hand,
PAHs’ native luminescent properties have been exploited in the
design of sensors for in-situ analysis and several optosensors for
PAHs control have been described so far, with different degrees of
success, based on on-line pre-concentration of the analytes onto
The iodination of bisphenol A to obtain the monomer
tetraiodobisphenol
A (4,49-isopropylidenedi(2,6-diiodophenol))
was carried out using IPy BF (bis(pyridine)iodinium(I) tetrafluor-
2
4
18
oborate) as iodinating agent{.
Then, an iodinated polyurethane was prepared through bulk
polymerization at room temperature starting from fluoranthene
(as the template molecule), tetraiodobisphenol A and diphenyl-
methane-4,49-diisocyanate (MDI) as functional monomers,
phloroglucinol as an additional cross-linker and tetrahydrofuran
as solvent{ (see Fig. 1)§. Control (non-imprinted) polymer was
prepared and treated in exactly the same way, except that no print
molecule was used in the polymerization stage.
2–8
different solid supports. In recent years, molecular imprinting
techniques have turned out to be an attractive tool to enhance the
sensitivity and selectivity of luminescence optosensors through the
development of robust tailor-made molecularly imprinted poly-
mers (MIPs) as active phases for recognition.
Recently, some approaches for the development of MIPs-based
9–14
fluorescent sensing systems have been described.
However, no
The analogous non-iodinated polyurethane MIP was prepared
exactly as the iodinated polymer, except that tetraiodobisphenol
A was replaced by the appropriate equimolar amount of
bisphenol A.
organic MIPs-based room temperature phosphorescence (RTP)
sensing systems have been reported to date, even though
phosphorescent detection provides a series of important advan-
tages, including wide separation between the excitation and
emission spectra as well as low background emission. Besides,
since phosphorescence is less common than fluorescence the
selectivity of the detection is highly increased. In addition,
interferences from any short-lived fluorescence and scattered light
can be easily avoided using an appropriate delay time.
One pending topic related to the synthesis of MIPs is the
undesirable formation of unspecific cavities, leading to non-specific
1
5
recognition. Moreover, undesirable non-specific adsorption may
occur on a polymer surface depending on the hydrophobicity of
the MIP surface and the presence of monomer residues randomly
distributed to the surface. With the aim of increasing specificity for
sensing purposes, here we investigated the possibility of combining
the important advantages of MIPs with those provided by
phosphorescent detection. In order to achieve RTP analytical
Fig. 1 Chemical structures of the selected template and monomers for
*asm@uniovi.es
the synthesis of the iodinated molecularly imprinted polymer.
3
224 | Chem. Commun., 2005, 3224–3226
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The performance of the non-iodinated MIP was initially tested.
First, the template molecule (fluoranthene) was removed from the
polymer by passing a continuous flow of acetonitrile until the
template in the extraction solvent could not be detected by
fluorescence. Then, the washed polymer was packed in a
conventional luminescence flow-through quartz cell and this
flow-cell was placed inside the sample holder of a Varian Cary
Eclipse luminescence spectrometer. Since molecular oxygen is a
very efficient quencher of phosphorescence, it was removed from
19
the samples by its reaction with sodium sulfite.
fluoranthene aqueous solutions containing 2% of acetonitrile, KI
as an external heavy atom and Na SO as oxygen scavenger were
pumped through the polymer using a peristaltic pump (see Fig. 2)
Thus,
2
3
2
1
at a constant flow rate of 1 mL min . Although fluorescence
emission from fluoranthene retained in the MIP could be
observed, no RTP emission was detected when working with the
non-iodinated polymer.
Fig. 3 RTP emission spectrum (lex 5 365 nm) of fluoranthene (template
molecule) in the iodinated imprinted polymer (dashed line). RTP spectrum
of the polymer after template removal via a microwave treatment (solid
line).
Then, the iodinated MIP was thoroughly tested. The polymer
was packed in the flow cell and its RTP emission spectrum, while
passing a distilled water flow through the flow cell, was recorded.
Two very intense fluoranthene phosphorescence emission peaks (at
To verify whether this iodinated polymer retained recognition
properties after the microwave treatment and to evaluate its
2
1
potential as sensing material, a fluoranthene solution (10 mg L
)
550 and 593 nm) were observed, even in the absence of a
was prepared and pumped through the packed MIP (see Fig. 2).
As expected, after the microwave-assisted template extraction,
subsequent fluoranthene rebindings seemed to be weaker, since
deoxygenation of the samples with sodium sulfite was necessary in
order to achieve a detectable fluoranthene RTP emission. Fig. 4
shows the response profile of the MIP for three successive
injections of the fluoranthene aqueous samples (2 mL sample).
Now, the removal of the retained fluoranthene after each injection
is straightforward and can be accomplished by a 2 mL acetonitrile
flow (thus proving that the MIP can be easily regenerated).
In order to verify that the observed RTP emissions derive from
fluoranthene retained in the MIP selective cavities, rather than
from simple adsorptions on the material, the RTP emission from a
deoxygenating agent (see Fig. 3). This fact suggests that
fluoranthene imprinting molecules rest within such a rigid
environment that their phosphorescence emission is protected
even from dissolved oxygen quenching effects.
The tight confinement of the template molecules within the
polymer structure was responsible for the need for an aggressive
treatment to accomplish the polymer washing step. In fact, the
difficulty of completely removing the template molecule from
2
0
MIPs is a well known issue in molecular imprinting. The
imprinting molecule was removed by means of microwave-assisted
extraction". Afterwards, the MIP phosphorescence emission at
550 nm was checked once again. As can be seen in Fig. 3, as a
result of the microwave treatment the fluoranthene imprinting
molecule was almost completely removed from the polymer,
since the observed phosphorescence emission due to residual
fluoranthene molecules can be considered negligible.
21
20 mg L fluoranthene aqueous solution injected through the
MIP was compared to that obtained after injecting the same
Fig. 4 RTP response profile (lex 5 365 nm, lem 5 550 nm) of the
iodinated MIP (solid line) and the non-imprinted polymer (NIP,
dashed line) for three successive fluoranthene aqueous samples injection
2
1
Fig. 2 Optosensing manifold for RTP monitoring of fluoranthene after
(10 mg L ). For polymer regeneration, acetonitrile (ACN) is injected after
its retention by the imprinted polymer.
each sample injection.
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Chem. Commun., 2005, 3224–3226 | 3225

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with ether (2 6 10 mL) to give a pure product in 85% yield. Spectral data
(
fluoranthene solution through the control (non imprinted)
polymer similarly packed in the flow-cell. The RTP signal obtained
from the control polymer was negligible. We have also carried out
fluoranthene retention studies in the NIP, using fluorescence
detection, showing that fluorescent fluoranthene molecules are
partially retained on the NIP surface. However, since phos-
phorescence is a luminescence phenomenon typical of molecules in
a ‘‘highly rigid’’ environment (tailor-made cavities of MIPs in our
particular case), any simple adsorption process does not ensure
enough rigidity to produce RTP emissions. Moreover, a ‘‘heavy
atom effect’’ by the iodine atoms is also needed. Therefore we can
assume that the RTP recognition results from host–guest
interactions between the iodinated MIP cavity matrix and the
target molecule.
1 13
H and C) were in full agreement with the assigned structure.
{
(
Fluoranthene (20 mg), phloroglucinol (70 mg) and tetraiodobisphenol A
612 mg) were placed in a glass vial and 5 mL of an MDI solution (70 mM)
in THF were added. The mixture was stirred and stored uncapped in the
absence of light for 4 days until complete evaporation of the organic
solvent. The solid obtained was ground in an agate mortar, washed with
acetonitrile (3 6 5 mL) and dried at 30–35 uC. The ground polymer was
sieved and particle sizes of diameters between 80 and 160 mm were selected.
§ CAUTION Special caution was taken in the handling of fluoranthene
and in the preparation of the MIPs. Nitrile globes, protective clothing and
safety glasses were worn all the time. A respirator equipped with two
organic vapour/particulate filters was used. Hazardous wastes were
collected, stored in glass containers, labeled and sent to a waste
management company.
"
100 mg of the polymer were placed in a PTFE vessel and 4 mL of a
MeOH/hexane (1 : 1) mixture were added. The vessel was tightly capped,
placed in the microwave oven and treated at 30 W for 5 minutes. Then the
polymer was finally cleaned with ethanol and left to dry at room
temperature.
Finally, a selectivity study showed that the combination of the
RTP detection with the recognition by the iodinated MIP is highly
selective to the fluoranthene molecule when exposed to mixtures of
21
21
1 Off. J. Eur. Communities, 15.12.2001. L331.
this compound (10 mg L ) and other PAHs (e.g. 40 mg L of
naphthalene, benzo(a)pyrene and benzo(b)fluoranthene). In sum-
mary, molecular imprinting using a novel iodinated monomer,
tetraiodobisphenol A, for copolymerization with MDI, is pro-
posed here for chemical recognition of photoluminescent analytes
by means of RTP detection. The occurrence of a ‘‘heavy atom
effect’’ when the imprinted polymer contains iodine in its structure
has been shown to be responsible for inducing the highly intense
RTP emission observed. Thus, this novel iodinated MIP has
shown a great potential for the development of selective and simple
and easy to use RTP optosensors, opening new paths for the
development of high performance MIP-RTP optosensors for
different applications.
2
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This work was supported by the projects MCT-03-BQU-04671
and MAT2003-09074 (Feder Programme and Ministerio de
Ciencia y Tecnolog ´ı a, Spain).
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R. Pereiro, A. Ballesteros, J. M. Gonz a´ lez, A. Segura-Carretero,
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A. Fern a´ ndez-Guti e´ rrez and A. Sanz-Medel*
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a
Department of Analytical Chemistry, University of Granada,
Fuentenueva s/n, 18071, Granada, Spain. E-mail: albertof@ugr.es
Department of Physical and Analytical Chemistry, University of
b
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1
E-mail: asm@uniovi.es; Fax: +34 985 10 31 25; Tel: +34 985 10 34 74
Instituto Universitario de Qu ´ı mica Organomet a´ lica ‘‘Enrique Moles’’,
Unidad Asociada al C. S. I. C., Juli a´ n Claver ´ı a, 8, 33006, Oviedo, Spain
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Notes and references
{
(
Bisphenol A (5 g, 21.90 mmol) was suspended in dichloromethane
200 mL), iodination reagent IPy BF (33.48 g, 90 mmol) was added and
2
4
the mixture was stirred at room temperature overnight. Distilled water and
HCl (1%) were added up to acid pH. The organic phase was separated
and washed with water (3 6 100 mL), thiosulfate (5%, 100 mL) and again
19 M. E. D ´ı az-Garc ´ı a and A. Sanz-Medel, Anal. Chem., 1986, 58, 1436.
20 A. Ellwanger, C. Bergren, S. Bayoudh, C. Crecenzi, L. Karlsson,
P. K. Owens, P. Cormack, D. Sherrington and B. Sellergren, Analyst,
2001, 126, 784.
2 4
water (2 6 100 mL) and then was dried over Na SO . Finally, the organic
solvent was removed by rotary evaporation and the crude solid was washed
3
226 | Chem. Commun., 2005, 3224–3226
This journal is ß The Royal Society of Chemistry 2005