Fiber-optic Singlet Oxygen [1O2 (1Δg)] Generator Device Serving as a Point Selective Sterilizer

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Traditionally, Type II heterogeneous photo-oxidations produce singlet oxygen via external irradiation of a sensitizer and external supply of ground-state oxygen. A potential improvement is reported here. A hollow-core fiber-optic device was developed

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  Fiber-optic Singlet Oxygen [ 1 O 2  ( 1 D g )] Generator Device Serving as a PointSelective Sterilizer David Aebisher 1 , Matibur Zamadar 1 , Adaickapillai Mahendran 1 , Goutam Ghosh 1 , Catherine McEntee 2 and Alexander Greer* 1 1 Department of Chemistry, Graduate Center & The City University of New York (CUNY), Brooklyn College,Brooklyn, NY 2 Department of Biology, The City University of New York (CUNY), Brooklyn College, Brooklyn, NY Received 14 March 2010, accepted 2 April 2010, DOI: 10.1111   ⁄   j.1751-1097.2010.00748.x ABSTRACT Traditionally, Type II heterogeneous photo-oxidations producesinglet oxygen  via  external irradiation of a sensitizer andexternal supply of ground-state oxygen. A potential improve-ment is reported here. A hollow-core fiber-optic device wasdeveloped with an ‘‘internal’’ supply of light and flowing oxygen,and a porous photosensitizer-end capped configuration. Singletoxygen was delivered through the fiber tip. The singlet oxygensteady-state concentration in the immediate vicinity of the probetip was  ca  20 f  MM  by  N  -benzoyl- DLDL -methionine trapping. Thedevice is portable and the singlet oxygen-generating tip ismaneuverable, which opened the door to simple disinfectantstudies. Complete  Escherichia coli   inactivation was observed in2 h when the singlet oxygen sensitizing probe tip was immersedin 0.1 mL aqueous samples of 0.1–4.4  ·  10 7 cells. Photobleach-ing of the probe tip occurred after  ca  12 h of use, requiringbaking and sensitizer reloading steps for reuse. INTRODUCTION Only recently was a fiber-optic singlet oxygen (FOSG) devicedeveloped for the localized ‘‘delivery’’ of singlet oxygen(Scheme 1) (1). The device consisted of a porous Vycor glass(PVG) cap coated with  meso -tetra( N  -methyl-4-pyridyl)por-phine ( 1 ) fixed to the end of a hollow fiber-optic (1). Thehollow fiber flowed O 2  gas and guided 532 nm light from acontinuous-wave or pulsed laser. The lifetime of photoexcitedtriplet-state adsorbed  1  was 57  l s and O 2  quenching at thewater–PVG interface resulted in the formation of   1 O 2  inaqueous solution (2).To date, there are few devices specifically designed to deliversinglet oxygen through space (3–5). Perhaps the best docu-mented example is that of Eisenberg  et al.  (5) who 24 years agoreported a Pyrex tube-bound rose bengal photosensitizerflowing singlet oxygen upon irradiation (Scheme 2). Singletoxygen exited the distal end of the Pyrex tube and wastransported through space   1.5 mm due to a 54 ms lifetime.Despite the novelty of the Eisenberg Pyrex tube method (5)and other gas–solid methods to generate external  1 O 2 , they arenot compatible in aqueous solution because of high oxygen gasflow rates, which would lead to water cooling and evaporation.Thus, interesting questions remain about devices whose distalends generate singlet oxygen in aqueous solution.We wondered whether the FOSG device could be exploitedto inactivate  Escherichia coli   or whether the short diffusiondistance of   1 O 2  in water (  160 nm) (6–8) would prohibit it. Ourinterest in a disinfection application was due to interest in the Scheme 2.  Device of Eisenberg  et al.  where singlet oxygen exited thedistal end of the flow tube (5). *Corresponding author email: agreer@brooklyn.cuny.edu (Alexander Greer)  2010TheAuthors.JournalCompilation.TheAmericanSocietyofPhotobiology 0031-8655/10 Scheme 1.  Distal end of fiber-optic device where singlet oxygen exits. Photochemistry and Photobiology, 2010, 86: 890–894890  fundamental factors of singlet oxygen generation water–solidinterfaces, andbecauseofthegreatpotentialthatsingletoxygenphotocatalysts possess for  E. coli   killing under visible lightconditions (9). Fiber optics have been coupled to heterogeneousphotosensitizers for the generation of   1 O 2  (10–14), and used inthe sensing of   3 O 2  or  1 O 2  in biological media (15,16), but nosuch device has been developed for  E. coli   inactivation. MATERIALS AND METHODS Device construction.  A diagram of the fiber-optic apparatus is shown inFig. 1. A similar fiber-optic geometry was chosen because it gave goodresultsinourpreviouspaperonsingletoxygendelivery(1).Inthepresentstudy,adifferentfiberanddifferentilluminationsourcewasusedthaninRef. (1). The hollow core photonic bandgap fiber was Crystal Fibre’sHC-440havinga84  l mdiameter,coatedwitha136  l mlayerofacrylate(attenuationof<2 dB m ) 1 ),whichguided473 nmlightfromablueCWlaser(20 mW,2.0 mmbeamdiameter;DragonLaser),whichwaswithinthe wavelength range covered by the photonic bandgap in the cladding(415–485 nm).Unwantedmeltingoftheacrylatecoatingbytheincident20 mW laser beam required the removal of a 0.5-cm segment of theacrylate,whichwasdonebydippingtheproximalend(beginning)ofthefiber into dichloromethane while flowing oxygen (40 psi). This acrylatestripping method reduced the amount of light that could be transportedthroughthefiber.Theintensityoftheilluminationwas2.2 mW,incident into  the photosensitizing cap hole during the experiment. The immobi-lization of   1  on PVG pieces was conducted as described previously forthis heterogeneous reaction (1). PVG has pore sizes around 40 A ˚andincreases in weight by 30% from its dry weight after soaking in water.Typically,0.015 gPVGcapswereloadedwith4  ·  10 ) 9 mol 1 producing  0.5% sensitizer coverage. Caps were cut and polished into cylindricalshapes,   2 mm o.d.  ·  3–4 mm. The sensitizer-impregnated PVG capwasthenfixedtothedistalendofthehollowfiberandgluedintoplaceattheholeentrancewithethyl-2-cyanoacrylate.Thesensitizer-coatedPVGcap absorbs 473 nm light at the tail of the Soret band ( k max  = 422 nm).Concentrations of oxygen delivered to 0.1 mL water samples  via  theFOSG generator system were determined with a pO 2  micro-oxygenelectrode. E. coli viability.  Singlet oxygen toxicity was judged by  E. coli   K-12cultures grown in tryptic soy broth to an early logarithmic phase( A 590  = 0.2) and following the reduction in the number of colonies intreated  versus  control samples. Standard protocols were used in thegrowth and maintenance of the  E. coli   cultures and the experimentswere carried out at ambient temperature (25  C). Two milliliters of the E. coli   solutions was centrifuged, washed once with distilled water,re-suspended and diluted to 1.1  ·  10 7 to 4.4  ·  10 8 cells mL ) 1 in 100  l Lreaction volumes. In a dark room,  E. coli   cells were exposed to singletoxygen  via  the fiber-optic-based singlet oxygen generator with illumi-nation with the 473 nm laser and introduction of the fiber tip into the100  l L water solution. There was no headspace above the cell culturemedium. The  E. coli   viability was evaluated at hourly intervals, inwhich 10  l L aliquots were removed and serial dilutions made rangingfrom 10 ) 4 to 10 ) 6 . Each 100  l L serial dilution was then added to 3 mLmolten tryptic soy top agar (TSA), briefly vortexed and overlaid ontothe TSA plates. Dilutions were made in duplicate. When the overlaysolidified, the plates were inverted and incubated for 48 h. Temper-atures were kept at 25  C throughout the course of the experiments.The number of colony-forming units was determined by direct countand the final concentration of   E. coli   is reported as the number of viable cells mL ) 1 . Photo-oxidation.  Reports of   N  -benzoyl- DLDL -methionine anion ( 2 ),9,10-anthracenedipropionic acid ( 4 ) and  trans -2-methyl-2-pentenoateanion ( 6 ) as chemical probes for the evaluation of singlet oxygen inaqueous solution are available in the literature (1,17,18). The exper-iments in this manuscript were conducted at room temperature withthe FOSG device with 40 psi oxygen gas and blue CW laser lightpassing through a hollow core bandgap fiber. Steady-state concentra-tions of singlet oxygen were determined as described previously (19,20), but with the use of methionine anion  2  rather than furfuryl alcoholdue to adsorption of the latter onto the PVG surface. Chemicals.  Deionized H 2 O was obtained from a U.S. FilterCorporation deionization system. PVG (Corning 7930) was purchasedfrom Advanced Glass and Ceramics.  N  -benzoyl- DLDL -methionine sodiumsalt (Aldrich), 9,10-anthracenedipropionic acid (Aldrich), sodiumazide (Aldrich),  meso -tetra( N  -methyl-4-pyridyl)porphine tetratosylate(FrontierScientific)andadipicacid(MonsantoChemicalCo.)wereusedas received. Deuterium oxide- d  2  (Aldrich), chloroform- d  1  (Aldrich) andacetonitrile- d  3  (Isotec, Inc.) were of spectrophotometric grade. RESULTS AND DISCUSSION A mechanism that involves the generation of singlet oxygen asoutlined in Scheme 3 is consistent with our results. As can beseen, the FOSG generator is essentially a three-phase system:thegasphaseisthehollowcoreofthefiber,thesolidphaseisthefiber cap and the aqueous phase is the bulk solution. Blue lightand ground-state triplet O 2  were delivered through a hollowfiber to the porous cap (Eq. [1]) and sensitized to  1 O 2  at thewater–solid interface (Eq. [2]). The heterogeneous sensitizerwasthe probetipcoated with porphyrin 1  (2.5  ·  10 ) 7 mol  1  g ) 1 PVG). Subsequent physical and chemical quenching reactionsof singlet oxygen can take place by processes involving solvent,substrates or biological materials (Eqs. [3 – 5]). Figure 1.  Schematic of the fiber-optic-based singlet oxygen generator(FOSG): (1) continuous-wave laser; (2) microscope objective; (3)cuvette; (4) cuvette top with tube coupled to compressed oxygen tank;(5) laser-to-fiber coupler with micrometer  x–y  (a) and  y–z  (b)translation resolution; (6) hollow core fiber-optic transporting oxygengas and 473 nm light; (7) aqueous reaction solution containing thedistal end of the fiber capped with the singlet oxygen sensitizing porousVycor glass tip. gas phasemembrane phaseaqueous phase 3 sens*-adssens-ads 1 O 2 (hollow optical fiber)(PVG cap)(bulk solution) 3 O 23 O 2 (1)h  ν (2)(3)k d k r Aphysical quenching by Achemical reaction with AAk q physical quenching by water(4)(5)potential surface interactions with 1 O 2  or E. coli   not shownA: oxygen-acceptor compound or E. coli  Scheme 3.  Three-phase system. Photochemistry and Photobiology, 2010, 86 891  Flowing oxygen through the probe tip Gasdiffusionthroughhollowcorefibershasbeenstudiedbefore(21), but not with the aim of generating singlet oxygen. TheFOSGgeneratorwasoperatedat40 psiO 2 pressureasthepiecesglued at the cuvette   ⁄   fiber and fiber   ⁄   cap junctions otherwisecome apart. The transmission rate of O 2  through the porous tipwas   1.0  l L min ) 1 (  0.1 ppm h ) 1 ). After 4 h at 40 psi, anincrease in oxygen concentration of 4–12% (  2  ·  10 ) 5 MM ) canbe obtained beyond the starting air-saturated concentration(2.6  ·  10 ) 4 MM ;4.68 ppm).Thevariationinthepercentofoxygentransported across the PVG tip can be explained by thedifferences in the cap shape, its length and outer diameter, andthe hole inner diameter (Fig. 2). When illuminated, oxygenrapidly quenches the triplet PVG adsorbed  1 ; its lifetime inN 2 -purged water ( s 0  = 57 ± 1  l s) was reduced by oxygenin air-saturated solution ( s  = 7 ± 1  l s) (2). Photo-oxidation and kinetics Ene and [4+2]-cycloaddition reactions are often diagnosticand verify the presence of singlet oxygen (22–24) in solution.Here, we found the formation of 9,10-anthracene-9,10-endo-peroxide dipropionate dianion ( 5 ) took place  via  a [4+2]cycloaddition of singlet oxygen with 9,10-anthracene dipropi-onate dianion (0.1–0.2  MM , pH = 10,  4 ) (Scheme 4). Controlreactions demonstrated that >99% of   4  was photo-oxidizedby the device tip, and was not self-sensitized under the reactionconditions.  N  -benzoyl- DLDL -methionine  2  was also photo-oxi-dized by the FOSG giving predominantly  N  -benzoyl- DLDL -methionine S-oxide ( 3 ). Products  3  and  5  were stable inaqueous solution at room temperature and identified by  1 HNMR and by LCMS. We next wanted to learn if the FOSGproduction could be quantified with the chemical trap  2 .Control experiments showed that anions, such as methio-nine anion  2 , do not penetrate through nor associate with PVG(Scheme 5), suggesting  2  could be used as a trapping agent forthe FOSG and its concentration followed in the surroundingaqueous solution. The chemical quenching ( k r ), and solvent( k d ) and substrate ( k q )-induced physical quenching reactions of singlet oxygen are shown in Eqs. (3–5) (Scheme 3). Thequantum yield of singlet oxygen production is  F , and the rateof absorption of 473 nm light by the PVG-sensitizer  1  is  I  a (Eq. [6]). As a 2 mW output of the CW laser through the fiberwas constant to ±0.1 mW over the course of the experiments,we applied a steady-state approximation for singlet oxygen(Eqs. [7] and [8]) in a similar fashion as Haag and Hoigne´ (19). Protic solvents suppress the physical quenching (k q ) of singletoxygen by organic sulfides (typically 0% physical quenching)(25) thus permitting the rate of formation of singlet oxygen inthe solution around the probe tip to be estimated by thereduction in the concentration of   2 . The rate constant of thereaction between  1 O 2  and DL-methionine ( k T ) has beenexamined previously (  3  ·  10 7 MM ) 1 s ) 1 in water at pH 6–11)(26) and we assumed a similar or identical rate constantbetween  1 O 2  and  N  -benzoyl- DLDL -methionine  2 . The initialconcentration of   2  was 0.1 m MM  so that k d  was seven timesgreater than  k T [ 2 ] and the loss of   2  was first-order in  2 . The  k d  of singlet oxygen in water is 2.5  ·  10 5 s ) 1 (27). Under constant2-mW light intensity, pseudo-first order plots with the FOSGgenerator were obtained by Eq. (8), which suggested a [ 1 O 2 ] ss of    1–4  ·  10 ) 14 MM  in the immediate vicinity of the probe tip.  d ½ 2  d t  ¼  I  a U k d   k r ½ 2  ð 6 Þ d ½ 2  d t  ¼  k r ½ 1 O 2  ss ½ 2  ¼  k obs ½ 2  ð 7 Þ (ss = steady state) k obs k r ¼ ½ 1 O 2  ss  ð 8 Þ Inactivation of   E. coli  Figure 3 shows the results of   E. coli   inactivation throughsinglet oxygen sensitized damage together with control data.The reduced number of cells was compared against blanks Figure 2.  Schematic of the singlet oxygen sensitizing porous Vycorglass tip. 5 OO 4 1 O 21 O 2  + 3 O 2 O 2 CO 2 CSNHBzCO 2 SNHBzCO 2 O  32 1 O 2 pH = 8pH = 10CO 2 O 2 C Scheme 4.  Photo-oxidation of   N  -benzoyl- DLDL -methionine anion and9,10-anthracene dipropionate dianion. Scheme 5.  Inability of anions to penetrate through or adsorb ontoPVG. 892 David Aebisher  et al.  containing from 1.1  ·  10 7 to 4.4  ·  10 8 cells mL ) 1 (entry a). Adecrease in the number of viable cells was observed over a 1 hperiod with the FOSG (cf. control [2.1  ·  10 8 cells mL ) 1 ] withphoto-oxidation sample [9.7  ·  10 7 cells mL ) 1 ]) (entry b).  Esc-herichia coli   was completely killed after 2 h (entry c). Addi-tional data with higher oxygen concentrations (O 2 -saturatedsolution [1.4 m MM ]  versus  air-saturated solution [0.26 m MM ])revealed enhanced  E. coli   killing by a factor of    2. Higheroxygen concentrations are known to enhance photodynamicbacterial inactivation (28). After a 4 h irradiation period the E. coli   samples turned yellow, which we attribute to photo-chemical oxygen uptake.  Escherichia coli   was completely killedafter 1 h when a 0.02 g piece of heterogeneous PVG sensitizerwas irradiated externally with 473 nm light (entry d). Ahomogeneous solution of 5.0  ·  10 ) 8 MM  1  in the presence of 473 nm light was also lethal concentration to  E. coli   growthafter 2 h (entry e), which was expected due to previous reportsof the phototoxicity of   1  toward  E. coli   in homogeneoussolution (29). Singlet oxygen inactivation of   E. coli   with anumber of homogeneous photosensitizers has been established(30,31). The viable number of   E. coli   significantly increased inthe presence of blue light and/or oxygen without sensitizer.Twenty percent  E. coli   inactivation was observed in thepresence of sparging oxygen in the dark (entry f), and in thepresence of 473 nm light and sparging oxygen withoutsensitizer (entry g). The sensitizer  1 -coated PVG showednegligible dark toxicity toward  E. coli   (entry h), which wassimilar to the low dark toxicity previously reported forhomogeneous  1  (32) . Sensitizer leaching and  E. coli   adhesion onto the PVG surface Control experiments suggested that  E. coli   inactivation wasnot due to desorption of the sensitizer off the PVG andfunctioning as a homogeneous photosensitizer catalyst.Absorption spectra of   E. coli   cell solutions were identical inthe 300–1100 nm range prior to and after removal of thePVG   ⁄   adsorbed sensitizer  1 . Sensitizer  1  desorption was notdetected when PVG (1.1  ·  10 ) 8 mol  1  adsorbed onto 0.04 gPVG) was placed in 2 mL of H 2 O solution at pH = 7, andstirred for 8 h in the presence of   E. coli   in the dark. Figure 4shows that even 1.6% desorption of   1  would be readilydetected in the presence of   E. coli   M (Soret band, k max  = 422 nm) .  Previous results also showed that porphyrin 1  remained immobilized on PVG when sensitizer-coated PVGwas placed in H 2 O solution at pH = 3, 7 and 10 (1), and thatcations and metals tend to adsorb strongly onto PVG (2,33).The possible adhesion of   E. coli   to the PVG surface was thenexamined. A 0.3 g PVG sample coated with 8.0  ·  10 ) 8 mol  1 wassubmergedin 1.0 mL E. coli  suspension(2.1  ·  10 ) 8 cells mL ) 1 )for 2 h. The PVG sample was washed with sterilized water andthen plated onto TSA. An aliquot of the sterilized water fromthe last rinse was also plated onto TSA. The two platedsamples showed no difference in colony growth, suggestingthat  E. coli   adhesion did not take place onto the PVGphotocatalyst surface. Electrostatic repulsion may be expectedbetween  E. coli   and the negatively charged PVG surface, E. coli   adhesiveness is known to correlate inversely withsurface electro-negativity (34).  Escherichia coli   adhesion isgenerally favored on positively rather than negatively chargedsurfaces (35). Photocatalyst bleaching and reuse An important matter concerned the photostability of the probetip. After 12 h of use, the probe tip began to photobleach andthe rate of   E. coli   killing declined. We analyzed the photo-bleaching process with blue-light irradiation of a PVG sample(2.2  ·  10 ) 8 mol  1  adsorbed onto 0.084 g PVG) in D 2 O. Adichloromethane solvent extract of the D 2 O solution afterremoval of the PVG catalyst revealed organics presumablyfrom fragmented   ⁄   oxidized sensitizer molecules by GCMS inthe  m   ⁄   z  range 206.0–246.1. Baking the PVG catalyst at 500  Cremoved the residual bleached and unbleached material fromthe surface, and an active photocatalyst tip was regenerated bybaking of the PVG and following the procedure of Ref. (1) toreload the tip with sensitizer  1 . Such baking and reloadingsteps could be done more than 10 times. Figure 3.  Percent survival of   Escherichia coli   cells (initial concentra-tion = 1.1  ·  10 7 to 4.4  ·  10 8 cells mL ) 1 , 0.1 mL, pH = 7) in aqueousmedia. Each bar represents an average of two to three runs. (a) Thevalues are shown as the increase in cell numbers relative to blankscontaining the same initial  E. coli   cell concentration without introduc-tion of the singlet oxygen generator tip.  Escherichia coli   inactivation inthe presenceof (b)the FOSGdelivering473 nmlightand40 psi O 2  overa 1 h period; (c) the FOSG delivering 473 nm light and 40 psi O 2  over a2 h period; (d) the heterogeneous PVG sensitizer irradiated externallywith 473 nm light over a 1 h period; (e) the homogeneous sensitizer  1  inthe presence of oxygen and 473 nm light over a 2 h period; (f) sparging20  l L min ) 1 oxygen in the dark over a 2 h period; (g) sparging20  l L min ) 1 oxygen with 473 nm laser light over a 2 h period; and(h) the heterogeneous PVG sensitizer in the dark over a 2 h period. 00.10.20.30.40.50.6300 400 500 600 700 800 900 1000 Wavelength, nm       A      b    s    o    r      b    a    n    c    e ab Figure 4.  (a) 1.0  ·  10 8 Escherichia coli   cells mL ) 1 in aqueous solutionwith 6.6  ·  10 ) 7 MM  sensitizer  1 . (b) 1.0  ·  10 8 E. coli   cells mL ) 1 inaqueous solution. Photochemistry and Photobiology, 2010, 86 893  CONCLUSION Heterogeneous 1 O 2 -photosensitizerstypicallyrequireexternallysupplied oxygen and light, and in some cases cannot berecovered for reuse. One way of improving heterogeneousphotosensitizer performance would be if a hollow-core fiber-optic system flowing oxygen were coupled to a  porous  Type IIheterogeneous photosensitizer tip, which is the subject of thispaper. In this paper, it was shown that singlet oxygen can bedeliveredthroughthefibertipintoaqueoussolution. Escherichiacoli   inactivation was analyzed to help establish conditions thatcan generate singlet oxygen, which extends our previous study(1) and provides a potential application in water disinfection.The porous fiber tip can be reused with baking and sensitizerreloading steps to enhance the photodynamic  E. coli   killing. 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