14TH EUCHEMS INTERNATIONAL CONFERENCE ON CHEMISTRYAND THE ENVIRONMENT (ICCE 2013, BARCELONA, JUNE 25 - 28, 2013)

Singlet oxygen generation by photoactive polymericmicroparticles with enhanced aqueous compatibility

Víctor Fabregat & M. Isabel Burguete &

Francisco Galindo & Santiago V. Luis

Received: 13 September 2013 /Accepted: 28 October 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Two new photoactive materials compatible withenvironmentally friendly solvents (water and methanol) havebeen synthesized and characterized. They are comprised of aporous matrix of polystyrene and divinylbenzene with boundRose Bengal and additional pendant groups added to increasethe hydrophilicity (ethylenediamine and γ-gluconolactone).The new polymers are efficient photocatalysts capable ofgenerating singlet oxygen after irradiation with visible light.Photochemical oxygenations of 9,10-anthracenedipropionicacid and 2-furoic acid have been carried out. The measuredconversions indicate that the new supported photosensitizersare more effective than the parent hydrophobic polymer.

Keywords Singlet oxygen . Photocatalysis . Photosensitizer

Introduction

Photochemical methodologies for environmental remediationare gaining increasing acceptance (Vasquez et al. 2013;Emeline et al. 2012; Benabbou et al. 2011; Chong et al.2010; Arques et al. 2009; Malato et al. 2007). Treatment ofwastewater with UV light is currently done in order to elim-inate pollutants and pathogens (Guo et al. 2013; Matilainenand Sillanpaa 2010). However, the economic cost associatedto the use of UV light has prompted researchers to look for

alternatives using visible light, taking into consideration that itcan be obtained at no cost from sunlight. In this regard, visiblelight photocatalysts have been reported to degrade a numberof pollutants (Marín et al. 2012; Amat et al. 2007; Mirandaet al. 2000, 2001). The mechanisms of photocatalytic reac-tions are markedly different, ranging from electron transfer toenergy transfer and involving a variety of reactive intermedi-ate species, encompassing from hydroxyl radical (·OH) tosuperoxide radical anion (O2

·−) or singlet oxygen (1O2). Inthe case of 1O2, it can be generated by energy transfer from thetriplet state of the appropriate photosensitizer (Galian andPérez-Prieto 2010; Ogilby 2010).

Apart from the aforementioned environmental application,photochemically generated singlet oxygen is also used insynthetic applications (Clennan and Pace 2005) since it is anexcellent electrophile which adds to unsaturated compoundssuch as, for instance, furane derivatives (Montagnon et al.2008; Corey and Roberts 1997) or terpenes (Lamberts andNeckers 1985), yielding a series of important synthetic inter-mediates or final products of industrial importance. The po-tential use of solar visible light to generate 1O2 is also anecological advantage for these practical applications, as com-pared to the generation of 1O2 by conventional thermalmethods (Wahlen et al. 2004).

Both remediation and synthetic application of singlet oxy-gen using visible light photosensitizers have as their maindrawback the need to remove the dissolved photosensitizerfrom the reaction medium once the chemical transformationhas been carried out. The most typical approach to overcomethis inconvenience is the attachment of the photosensitizer to asolid support yielding an immobilized photocatalyst which iseasily removable from the medium by simple filtration (Zhanget al. 2013; Lacombe and Pigot 2010; Ribeiro et al. 2008;Griesbeck et al. 2004; Wang et al. 2004; Benaglia et al. 2002).However, some loss of photoactivity can be concomitant tothe attachment of the photoactive unit, which can be due, for

Responsible editor: Philippe Garrigues

Electronic supplementary material The online version of this article(doi:10.1007/s11356-013-2311-8) contains supplementary material,which is available to authorized users.

V. Fabregat :M. I. Burguete : F. Galindo (*) : S. V. Luis (*)Departamento de Química Inorgánica y Orgánica, UniversitatJaume I, Av. Sos Baynat, s/n, 12071 Castellón, Spaine-mail: francisco.galindo@uji.ese-mail: luiss@uji.es

Environ Sci Pollut ResDOI 10.1007/s11356-013-2311-8

instance, to quenching of the excited states by the polymericmatrix or shortening of the lifetime of the reactive speciesclose to the solid support (Bae 2012; Ribeiro et al. 2008). Inaddition to those reasons, there is another cause of inefficientphotochemica l per formance of some suppor tedphotocatalysts, especially in water as a solvent, which isrelated to the poor wettability of the immobilized photosensi-tizer. Supported photocatalysts based on hydrophobic matri-ces are hardly dispersed in aqueous environments and carryout the desired reactions with lower conversions than inorganic solvents like chloroform or dichloromethane. Hence,the development of hydrophilic photocatalysts is an importantcurrent target in order to be able to surpass this problem and tocarry out efficient phototransformations in aqueous media(Urakami et al. 2013).

Herein we report on the synthesis, characterization andphotochemical performance as singlet oxygen generators oftwo new photoactive materials. They are synthesized by mod-ification of a previously reported photoactive porous mono-lithic polymer containing Rose Bengal (RB) as photosensitiz-er (Burguete et al. 2009, 2010b). We have converted a ratherhydrophobic polymer into materials easily dispersible in waterand alcoholic solutions and capable of performing efficientlytwo benchmark photochemical reactions such as the oxidationof 2-furoic acid (FA) and 9,10-anthracenedipropionic acid(ADPA).

Results and discussion

Synthesis and characterization of hydrophilic polymers

In previous papers, we have reported on the synthesis of a newtype of polymeric photosensitizer (P1 ) by grafting RB to thesurface of porous monolithic polymers (Burguete et al. 2009).In order to make these materials more compatible with wateror alcohols, additional groups have been introduced. Thegroup of Fréchet described the efficient hydrophilization of aporous cross-linked polystyrene matrix used for chromato-graphic separations. For this purpose, a two-step modificationprocess involving reaction with ethylenediamine and γ-gluconolactone was developed (Wang et al. 1995).Following this strategy, we synthesized the hydrophobicphotoactive polymer P1 (Scheme 1) and also two hydrophilicderivatives (P2 and P3 ). P1 was modified withethylenediamine to yield P2 , which in turn was reacted withγ-gluconolactone to afford P3 (see details in the“Experimental” section).

The new materials were characterized by means of Fouriertransform infrared (FT-IR), Raman, diffuse reflectance andfluorescence spectroscopies. To carry out the photochemicalreactions in suspension, the polymers were crushed mechan-ically in order to get a powder. Scanning electron microscopy

(SEM) analysis showed that the morphology of the powderedP2 and P3 can be described as a series of aggregates with anaverage size of 24 μm. In Fig. 1, an example of such mor-phology for the case of P3 can be seen.

The presence of the strongly absorbing RB on the surfaceof P1–P3 allows a comparison between the spectroscopicfeatures displayed by each material and to deduce the envi-ronment in which this chromophore is situated. The absorp-tion of P2 (561 nm) and P3 (562 nm) is blue-shifted relativeto that of P1 (571 nm), and all three are broadened in com-parison to the absorption of free RB in solution (Fig. 2a andTable 1). Moreover, there is a shoulder at shorter wavelengths(ca. 530 nm) that can be associated to the formation of aggre-gates, as described in the literature for other RB-containingpolymers (Paczkowski and Neckers 1985). On the other hand,the 10-nm shift in the maxima recorded for P2 and P3 relativeto P1 affords the first indication that the grafting withethylenediamine and γ-gluconolactone altered the surface ofeach matrix. Paczkowski and Neckers (1985) has describedthat Merrifield resins loaded with RB display absorption max-ima between 571 and 578 nm, depending on the load of thephotosensitizer and the format of the sample (methylene chlo-ride solution of the film), whereas RB dianion in methanolicsolution absorbs at 558 nm (Lamberts et al. 1984) and the RBbenzyl ester absorbs at 564 nm also in methanol (Lambertset al. 1984). Hence, our measurement suggests that in P2 andP3 , the photosensitizer is surrounded by a more polar envi-ronment than that found in P1 , as a consequence of thegrafting by ethylenediamine and γ-gluconolactone.Fluorescence measurements corroborate also this idea: as itcan be seen in Fig. 2b, the emission of P1 takes place with amaximum at 602 nm, and the spectra of P2 and P3 havemaxima at 593 and 591 nm, respectively. This result is inaccordance with the data reported for RB and derivatives:polymers containing this photosensitizer are described to emitat 593–595 nm (Paczkowski and Neckers 1985), but free RBdianion emits at 573 nm (MeOH) (Neckers 1989) and RBbenzyl ester at 584 nm (MeOH) (Neckers 1989).

The amount of RB loaded in each polymer was estimatedby basic hydrolysis and spectrophotometric measurement ofthe released dye, using the appropriate calibration curve. In allthe three cases, a loading of 2 μmol of RB per gram ofpolymer was calculated, which means that the reaction ofP1 to yield P2 and P3 does not involve displacement of thephotosensitizer out of the matrix.

Finally, the ability of the polymers to be dispersed in waterwas qualitatively tested, prior to conducting the photochemi-cal assays. Samples of 40 mg of P1–P3 were placed in a tubewith 40 mL of water and were stirred magnetically for 10 min,and the dispersions were allowed to stand for 60 s. The resultsrevealed that P1 is much more hydrophobic than P2 and P3since it tends to separate from the aqueous phase very easily.On the other hand, P2 and P3 can be dispersed to a higher

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extent, with no noticeable differences between them.Moreover, as one of the main advantages of supported photo-sensitizers is the possibility of their recovery after the reaction,the samples were centrifuged for 10 min at 10,000 rpm andonly the new supported photosensitizers could be separatedfrom the reaction medium by means of this method.

Photochemical reactions

In order to test the ability of P2 and P3 to generate singletoxygen and, more importantly, to promote sensitized photo-oxidations in water or other polar media, two prototypicaloxygenations of aromatic compounds with 1O2 were used asbenchmark reactions.

The bleaching of the absorption ofADPA has been used totest the ability of a number of photosensitizing systems togenerate 1O2 since ADPA is water soluble and its reactionwith 1O2 is very fast (Scheme 2) (Qin et al. 2011; Tsay et al.2007; Wieder et al. 2006; Moreno et al. 2003). Upon irradia-tion, the decreasing absorption of ADPA in the presence ofthe supported photosensitizers (including P1 for comparison)was monitored.

The absorption changes of ADPA solutions can be safelytransformed into ADPA conversions (to its correspondingendoperoxide) taking into account that the reaction product

does not absorb in the spectral range where the measurementsare made (350–400 nm). Additionally, the first-order rateconstants (k ) for each reaction can be calculated from Eq. 1,as reported in the literature for other assays using ADPA (Qinet al. 2011; Moreno et al. 2003). In Eq. 1, C and C0 are theconcentrations of ADPA at a certain time (t ) and at t = 0,respectively, which can be deduced from the absorption spec-tra recorded during the irradiation. In this way, both rateconstants and conversions can be used as indicators of theefficiency of the polymeric photosensitizers. In Fig. 3, arepresentative example of the reactivity of ADPA with 1O2

can be seen.

lnC

C0

� �¼ −kt ð1Þ

Upon irradiation in pure water and water buffered withPBS (pH 7.4) with visible light, hydrophilic polymers P2and P3 showed higher conversions (93–100 %) than hydro-phobic P1 (84–85 %) after 1 h. In methanol, the differencesare not so marked (84% for P1 and 92–94% for P2–P3 ), andin chloroform, the difference is lower (62 % for P1 and 66–69 % for P2–P3). In terms of reaction rate, the same conclu-sion can be deduced, with higher reaction rates in water forP2–P3 (buffered or not) than for P1 (see Table 2 for details).These rates lie within the range of described values for other

Scheme 1 Schematic representation of the synthesis of P2 and P3

Fig. 1 Scanning electronmicroscopy image of polymer P3

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photosensitizers promoting the photooxygenation of ADPA(Qin et al. 2011; Moreno et al. 2003). Reaction rates andconversions point to the same conclusion in our experimentaldata. It must be noted, however, that this is not always the caseas it could be possible to record high reaction rates (at thebeginning of the reaction) but poor conversions due tophotodegradation of the photosensitizer after long exposureto light, which is not the situation for our system.

Two important aspects can be highlighted, then, from theresults obtained with this reaction: (1) introduction ofethylenediamine groups or γ-gluconolactone pendant resi-dues gives rise to an enhanced aqueous compatibility for thisparticular photooxygenation, and (2) the ethylenediaminemoieties seem to be enough to induce such hydrophilicitysince no important differences can be noticed between P2and P3 .

The long-term photostability and recyclability of thesematerials were evaluated in the case of P3 , being the onedisplaying the most complex chemical structure, with func-tional groups that could affect the fluorophore. A sample ofP3 could be used up to ten times for the quantitative conver-sion of ADPA with no signs of RB degradation. After eachirradiation cycle, the photocatalyst and the reaction mediumwere easily separated by simple centrifugation (see Fig. 3b).

ReactionwithADPA is a good comparative test to evaluatethe relative efficiency of several photosensitizers, as it hasbeen shown above, but in order to prove the utility of thenew photoactive polymers, a photooxygenation reaction withpractical applications should be tested. In this regard, we havechosen the reaction of 2-furoic acid with singlet oxygen toyield the γ-hydroxybutenolide 5-hydroxy-5H -furan-2-one(Scheme 3) (White et al. 1982). The reactivity of furan deriv-atives with 1O2 is known since long ago and has been used in agreat number of syntheses involving a butenolide skeleton.However, the vast majority of them require a dissolved mo-lecular photosensitizer that must be removed from themediumby column chromatography (Montagnon et al. 2008; Coreyand Roberts 1997).

Samples of P1–P3 (40 mg in 10mL) and 2-furoic acid (3×10−2 M) were prepared in several media: pure water, bufferedwater with phosphate-buffered saline (PBS; pH 7.4), MeOH/water (1:1), MeOH/water (9:1), MeOH and chloroform. Thesamples were irradiated with visible light and the evolution ofthe reaction was monitored by UV–vis, as previously de-scribed. A representative example can be seen in Fig. 4.

In water (buffered or not), the reaction did not occur evenafter 7 h of irradiation, and in MeOH/water (1:1), the conver-sions were only 11–12%. However, inMeOH/water (9:1), theyield reached 85–90% in the presence ofP2–P3 and 67% for

Fig. 2 a Normalized absorption spectra of solid P1–P3 and RB dianionin methanol (10 μM). Note that the diffuse reflectance spectra of poly-mers were transformed to absorbance units. b Fluorescence spectra ofsolid P1–P3 and RB dianion in methanol (10 μM). Excitation wave-length, 550 nm

Table 1 Spectral properties of photosensitizers P1–P3

Photosensitizer Absorption Emissionλmax (nm)a λmax (nm)

b

P1 571 602

P2 561 593

P3 562 591

a From diffuse reflectance measurements of solid samplesb From fluorescence measurements of solid samples (λexc = 550 nm)

Scheme 2 Reaction of 9,10-antracenedipropionic acid with singletoxygen

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P1 . Remarkably, in pure MeOH, P2 and P3 yielded quanti-tative conversions (100 %), whereas P1 afforded 72 %. Inchloroform, the difference between the new photosentitizers(90–92 %) and P1 (51 %) was even higher (Table 3).

Regarding the absence of reactivity in media with highcontent of water, it could be considered that furan derivatives

react notably slower than other substrates. For instance, 9,10-dialkylanthracenes are described to react to 1O2 with overallbimolecular reaction rates ranging from 107 to 108 M−1 s−1

(depending on the solvent) whereas the reactivity of furanderivatives can be up to several orders of magnitude slower,with rates ranging from 104 to 107 M−1 s−1 (Wilkinson andBrummer 1981). Additionally, the physical deactivation of1O2 in water is at least 1 order of magnitude faster than thatin MeOH (5.0×105 s−1 vs. 3.1×104 s−1, respectively)(Wilkinson and Brummer 1981) which disfavours even morethe reactivity of furan derivatives in aqueous media.Nevertheless, it must also be taken into account that the initialconcentration of 2-furoic acid employed is low (30 mM). As arecent example of the importance of these factors, it can bementioned that Zhang et al. (2013) and Urakami et al. (2013)have reported ca. 90 % conversion of 100 mM 2-furoic acid inwater after 22 h of irradiation.

Examination of yields inMeOH and chloroform affords thefollowing conclusions. Firstly, the quantitative conversionsattained with modified polymers P2 and P3 in MeOH dem-onstrate the effect iveness of the graf t ing usingethylenediamine and γ-gluconolactone for enhancing the wet-tability of the polymeric matrices in this polar medium, incomparison with the parent apolar polymer P1 . More strik-ingly, the enhanced polarity of the surface of polymers P2 andP3 does not hamper their effective use even in a less polarsolvent like chloroform. Yields in this medium reached up to92% and could be rationalized considering that the introducedgroups, specially the amines from ethylenediamine, could actas basic centres capable of inducing the approach of the acidsubstrate by ion pairing (carboxylate–ammonium), hence in-creasing the local concentration of this reactant around thepolymeric matrix where 1O2 is generated. Similar local con-centration effect has been described for other supported pho-tosensitizers and substrates (Burguete et al. 2010a; Suzukiet al. 2000; Neckers and Paczkowski 1986). Moreover, thelonger lifetime of 1O2 in chloroformwould favour reactivity inthis medium (reported deactivation rate for 1O2 in CHCl3 is1.7×104 s−1) (Wilkinson and Brummer 1981).

Table 2 Photosensitized oxygenation of ADPA in several media

Photosensitizera Water PBS (pH 7.4) MeOH Chloroform

Conversion (%)b k (10−4 s−1)c Conversion (%)b k (10−4 s−1)c Conversion (%)b k (10−4 s−1)c Conversion (%)b k (10−4 s−1)c

P1 85 7.3 85 5.1 84 5.2 62 2.8

P2 100 10.8 100 10.1 94 7.2 66 3.0

P3 100 10.1 93 6.9 92 6.7 69 3.3

a Polymer (40 mg) dispersed in 10 mL of solvent containing 1.2×10−4 M of ADPAb From UV–vis measurement at 398 nmc From Eq. 1

Fig. 3 Illustrative examples of ADPA reactivity with singlet oxygenpromoted by P1–P3. a ADPA (1.2×10−4 M) + P2 (40 mg in 10 mL)in Milli-Q water as a solvent. b Conversions of ADPA using P1–P3 asphotosensitizers in methanol as a solvent

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In summary, a poorly dispersible polymer (in polar sol-vents) like P1 , comprised of a matrix of highly cross-linkedpolystyrene and divinylbenzene with attached Rose Bengal,has been converted into two polymeric derivatives with en-hanced compatibility with environmentally friendly solventslike water and methanol. The new polymers are obtained bygrafting ethylenediamine and γ-gluconolactone and are activefor the photocatalytic oxygenation of ADPA and 2-furiocacid. The use of both new materials allows attaining higherphotochemical conversions with both substrates, not only inwater and methanol but also in non-polar solvents like chlo-roform. Further work will be oriented towards the use of the

systems herein described for specific phototransformations ofenvironmental value (degradation of contaminants using solarlight).

Experimental section

Materials and methods

All commercially available reagents and solvents were used asreceived: p -chloromethylstyrene (Aldrich, 90 %),divinylbenzene (DVB; Fluka, ~80 % mixture of isomers; theresidual is composed mainly of 1,3- and 1,4-ethylstyreneisomers), 2,2′-azobis(isobutyronitrile) (AIBN; Fluka,≥98 . 0 %) , Rose Benga l sod i um sa l t ( F l uka ) ,tetrabutylammonium hydroxide solution ~25 % in MeOH(~0.8 M) (TBAOH solution; Fluka), ethylenediamine(Sigma-Aldrich, ≥99 %), γ-gluconolactone (Sigma-Aldrich,≥99 %), 9,10-anthracenedipropionic acid (ADPA; Aldrich,≥98.0 %), 2-furoic acid (Merck, ≥99 %), 1-dodecanol(Aldrich, 98 %), tetrahydrofuran (Scharlab, synthesis grade),ethyl acetate (Scharlab, synthesis grade), ethanol (Scharlab,96 %), methanol (Scharlab, synthesis grade), methanol(Scharlab, spectroscopy grade), 1,4-dioxane (Scharlab, spec-troscopy grade), toluene (Scharlab, synthesis grade) and N ,N ′-dimethylformamide (treated previously with anhydrousMgSO4).

To characterize the polymeric materials, the followingtechniques were used: FT-IR spectra were acquired using aFT-IR-6200 type A JASCO spectrometer, with 4-cm−1 reso-lution and 50 scan accumulation. Fourier transform Raman(FT-Raman) spectra were recorded using a JASCO laserRaman spectrophotometer with 4-cm−1 resolution and 100scan accumulation (λex = 632 nm). Thermogravimetric anal-ysis (TGA) was carried out using a Mettler Toledo TG-STDAinstrument (30–1,000 °C at a heating rate of 5 °Cmin−1). UV–vis absorption spectra were recorded in a Hewlett-Packard8453 apparatus. Steady-state fluorescence spectra were

Fig. 4 Illustrative examples of furoic acid reactivity with singlet oxygenpromoted byP1–P3. a Furoic acid (3×10−2M) + P3 (40mg in 10mL) inmethanol as a solvent. b Conversions of furoic acid using P1–P3 asphotosensitizers in methanol as a solvent

Table 3 Photosensitized oxygenation of 2-furoic acid in several media

Photosensitizera Conversion (%)b

MeOH/waterc MeOH CHCl3

P1 67 72 51

P2 85 100 92

P3 90 100 90

a Polymer (40mg) dispersed in 10mL of solvent containing 3×10−2 M of2-furoic acidb From UV–vis measurements at 247 nmcMeOH/water = 9:1 (vol/vol)

Scheme 3 Reaction of 2-furoic acid with singlet oxygen

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recorded in a Spex Fluorog 3–11 equipped with a 450-Wxenon lamp. Scanning electron micrographs were taken on aLEO 440I microscope equipped with a digital camera. Thesamples were placed on top of a tin plate and sputtered withAu/Pd in a Polaron SC7610 Sputter Coater from FisonsInstruments. The particle size distribution of the synthesizedpolymer was determined by means a MASTERSIZER 2000(MALVERN) laser diffraction instrument. To perform themeasurements, the sample was suspended in MeOH. The datawere analyzed with the software supplied with the instrument.

Experimental procedure

Synthesis of P1

Polymer P1 was prepared in a similar procedure to thatdescribed in the literature (Burguete et al. 2009) by thermalfree radical-initiated polymerization of the monomers in thepresence of a porogenic mixture and using a glass tube as themould (1-cm diameter). AIBN (80 mg, 1 wt% with respect tomonomers) was dissolved in p -chloromethylstyrene (90 %,1.84 g) and divinylbenzene (80 % grade, 6.16 g). Then, theporogenic mixture, consisting of 1-dodecanol (10 g) andtoluene (2 g), was added. The homogenized polymerizationmixture was transferred to several glass tubes and purged withnitrogen in order to remove the dissolved oxygen. Then, thetubes were sealed with rubber septums and placed in a verticalposition in a silicon bath heated at 80 °C. The polymerizationwas allowed to proceed at this temperature for 24 h. The glasstubes were carefully crushed and the polymer was then disag-gregated mechanically and washed with tetrahydrofuran for24 h in a Soxhlet apparatus in order to remove the porogenicmixture and any other soluble compounds remaining withinthe polymer; finally, the polymer was dried in a vacuum oven.

FT-IR (cm−1): 3,018, 2,917, 1,629, 1,425, 1,265, 906, 795,707. FT-Raman (cm−1): 3,058, 2,906, 1,631, 1,408, 1,267, 1,181, 1,001. Decomposition temperature (°C): 500–510.

The resulting polymeric material (5.77 g, 7.5 mmol of -CH2Cl) and Rose Bengal sodium salt (8.81 g, 8.66 mmol)were mixed in a 500-mL round-bottom flask and stirred in400mL of dimethylformamide (DMF; treated previously withanhydrous MgSO4) at 80 °C for 8 h in a nitrogen atmosphere.Then, the reaction mixture was cooled to ambient temperatureand filtered through a sintered glass funnel. The obtainedresin, P1 , was washed with 250-mL portions of the followingsolvents: DMF, ethyl acetate, ethanol, ethanol/water (1:1),water, methanol/water (1:1) and methanol. Next, the polymerwas extracted with methanol in a Soxhlet apparatus until novisible colour appeared in the solvent. Finally, the light pinkpolymeric particles were dried in a vacuum oven.

FT-IR (cm−1): 3,020, 2,920, 1,625, 1,453, 1,266, 902, 794,709. FT-Raman (cm−1): 3,054, 2,907, 1,627, 1,411, 1,266, 1,180, 1,003. Decomposition temperature (°C): 500–510. UV–

vis absorption spectroscopy (λmax): 571 nm. Fluorescenceemission spectroscopy (λmax): 602 nm (λex = 572 nm). RoseBengal loading: 2 μmol RB g−1 resin.

Synthesis of P2

For the synthesis of polymer P2 , 4.66 g of P1 was introducedwith 15 mL of ethylenediamine (excess) in a 250-mL two-neck round-bottom flask. The mixture was dispersed in100 mL of predried tetrahydrofuran (THF) and the flask wassealed and purged with nitrogen for 30min. Then, the reactionmixture was refluxed for 8 h at 70 °C. After this time, thereaction mixture was cooled to ambient temperature and fil-tered through a sintered glass funnel. The obtained resin, P2 ,was washed with 200-mL portions of the following solvents:THF, dioxane, dioxane/water (1:1), water, dioxane/water(1:1), dioxane and ethanol. Finally, the polymer was dried ina vacuum oven at 55 °C.

FT-IR (cm−1): 3,358, 3,046, 2,921, 1,619, 1,447, 990, 906,798, 713. FT-Raman (cm−1): 3,055, 2,903, 1,632, 1,407, 1,315, 1,179, 1,105, 1,003, 803. Decomposition temperature(°C): 500–510. UV–vis absorption spectroscopy (λmax):561 nm. Fluorescence emission spectroscopy (λmax): 593 nm(λex = 564 nm). Rose Bengal loading: 2 μmol RB g−1 resin.

Synthesis of P3

In a 250-mL two-neck round-bottom flask, 2.87 g of polymerP2 and 1.335 g of γ-gluconolactone (excess amount) wereintroduced. The mixture was dispersed in 100 mL of absoluteethanol and purged with nitrogen for 30min. The reaction wascarried out under reflux for 18 h at 80 °C. Then, the reactionmixture was cooled to ambient temperature and filteredthrough a sintered glass funnel. The obtained resin, P3 , waswashed with 250-mL portions of the following solvents: eth-anol, methanol/water (1:1), water, methanol/water (1:1) andmethanol. Finally, the obtained polymer was dried in a vacu-um oven at 55 °C.

FT-IR (cm−1): 3,367, 3,056, 2,928, 1,628, 1,451, 1,081,902, 804, 704. FT-Raman (cm−1): 3,055, 2,903, 1,634, 1,408,1,307, 1,188, 1,086, 1,001, 930, 803. Decomposition temper-ature (°C): 500–510. UV–vis absorption spectroscopy (λmax):562 nm. Fluorescence emission spectroscopy (λmax): 591 nm(λex = 564 nm). Rose Bengal loading: 2 μmol RB g−1 resin.Average particle diameter: 26.3 μm.

Analytical procedure to estimate the loading of Rose Bengalin P1–P3

The po l yme r i c pho t o s en s i t i z e r s ( 20 mg ) andtetrabutylammonium hydroxide solution ~0.8 M in MeOH(3 mL) were mixed in a 25-mL round-bottom flask containing10 mL of 1,4-dioxane. The flask was sealed and the mixture

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was stirred for 24 h at room temperature. The reaction mixturewas then filtered through a sintered glass funnel, and the resinwas washed with MeOH until no visible colour appeared inthe solvent. The filtrate was transferred into a 100-mL volu-metric flask and diluted to 100 mL with MeOH. The finalsolvent ratio of the solution was 87:10:3 (MeOH/1,4-dioxane/TBAOH solution). From the UV–vis absorption spectrum ofthe solution, the amount of free Rose Bengal was determined,using ε = 78,028±1,291 L mol−1 cm−1 at 556 nm from aprevious calibration (Burguete et al. 2009) of Rose Bengal inMeOH/1,4-dioxane/TBAOH (87:10:3).

Photochemical experiments: photooxidation of ADPA

The rate of photooxidation of ADPA (1.2×10−4 M) wasdetermined in several reaction media: water, PBS, MeOHand chloroform, for the polymeric photosensitizers P1 , P2and P3 . In each of the experiments, 40 mg of the photosen-sitizer was added to 10 mL of solution of ADPA (1.2×10−4 M) in a test tube. The heterogeneous mixture was keptunder stirring and in equilibrium with air. The test tubes wereirradiated at room temperature with a halogen lamp of 50 W,placed at a distance of 2 cm. The same experimental condi-tions were performed for the polymers in the dark in order toevaluate a possible adsorption of the photocatalyst into thepolymeric substrate. Moreover, other control experimentswere carried out with the absence of polymer and irradiationof ADPA with free Rose Bengal in solution (5 μM) asphotosensitizer. The decreasing absorbance of ADPA wasmonitored by means of UV–visible spectroscopy at 398 nmfor 60 min. Before every measurement, 3 mL of the reactionmixture was filtered (nylon syringe filter, 0.2 mm) to anothercuvette in order to remove the polymeric particles insuspension.

Photochemical experiments: synthesisof 5-hydroxy-5H -furan-2-one

The oxidation of 2-furoic acid (3×10−2 M) to 5-hydroxy-5H-furan-2-one was studied for photosensitizers P1–P3 in sever-al reaction media: methanol/water (9:1), methanol and chlo-roform. The photosensitizer (40 mg) was added to 10 mL ofsolution of 2-furoic acid (3×10−2 M) in a test tube. Theheterogeneous mixture was kept under stirring and in equilib-rium with air. Such solutions were irradiated with a 125-Wmedium-pressure Hg vapour lamp for 6 h surrounded by anaqueous solution of 0.1 M FeCl3 used as a filter for wave-lengths under 450 nm, and the tubes were placed at a distanceof 2 cm from the solution. The same experimental conditionswere used for the polymers in the dark in order to evaluate apossible adsorption of the photocatalyst into the polymericsubstrate. Moreover, other control experiments were carriedout: (a) irradiation in the absence of polymer and (b)

irradiation of 2-furoic acid with free Rose Bengal in solution(5 μM) as photosensitizer. The decreasing absorbance of 2-furoic acid was monitored by means of UV–vis spectroscopyat 246 nm for 420 min. For every measurement, aliquots of80 μL were removed from the reaction mixture and diluted in25 mL of the appropriate solvent.

Acknowledgments Financial support from the Spanish MINECO(CTQ2009-14366-C02-01) and Fundació Caixa Castelló-UJI (projectP1 1B-2009-59, P1 1B2009-58, P1 1B2012-41) are acknowledged. V.F. thanks the financial support from UJI (predoctoral fellowship). Wethank J. Javier Gómez (SCIC) for the technical assistance in SEMmeasurements.

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