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. Author manuscript; available in PMC: 2015 Nov 1.
Published in final edited form as: Photochem Photobiol. 2014 Oct 16;90(6):1216–1218. doi: 10.1111/php.12355

Photoactive chitosan: A step towards a green strategy for pollutant degradation

Niluksha Walalawela 1, Alexander Greer 1,*
PMCID: PMC4232976  NIHMSID: NIHMS632185  PMID: 25270888

Abstract

This article is a highlight of the paper by Ferrari et al. in this issue of Photochemistry and Photobiology. It describes the innovative use of rose bengal-conjugated chitosan as a reusable green catalyst that photo-degrades phenolic compounds in aqueous media, and thereby has decontamination potential of polluted waters. Whether a next-generation photoactive polymer that produces singlet oxygen is a solution to pollutant degradation can be argued. It is as yet unclear what polymeric sensitizer would be practical on a large scale. Nonetheless pursuing this goal is worthwhile.

COMMENTARY

The study of organic photoactive polymers for materials to oxidatively decontaminate polluted waters has attracted much interest (1,2). Oxidative methods that use atmospheric oxygen as a green, environmentally friendly route to highly reactive 1O2 (1Δg) are of interest (3,4). However, a challenge is that the polymer’s success for producing 1O2 may lead it to “commit suicide” by self-oxidation. This highlight article of the paper by Ferrari et al. in this issue of Photochemistry and Photobiology (5) describes how this self-oxidation problem can be managed by the incorporation of rose bengal into chitosan.

Figure 1 shows the rose bengal-chitosan conjugate. Chitosan is an excellent polymer for exploring photosensitization phenomena. It is an aminopolysaccharide polymer that is nontoxic and biodegradable, and is available on large scales by chitin deacetylation of shellfish waste or fungal biomass (6). Synthetically, the amino group of chitosan can be functionalized (7).

Figure 1.

Figure 1

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Image of the rose bengal-chitosan conjugate.

Synthesis

Ferrari et al. (5) functionalized chitosan by dehydrative coupling of its amino group and the rose bengal carboxy group, which provided a simple procedure to the corresponding conjugate. The resultant material was precipitated in NaOH solution and then re-dissolved in acetic acid solution, where rose bengal was introduced into the backbone of chitosan in the amount of about 0.3% w/v. The material’s color is light red or pink due two absorptions at 523 and 561 nm in acidic aqueous solution. Rose bengal also fluoresces red albeit only weakly.

The photolysis of the rose bengal-chitosan conjugate was studied with visible light (filtered white light from a halogen lamp) and green Nd:YAG laser light. The light was only absorbed by the rose bengal sites in the polymer, and not by the trihydroxybenzene compounds, 1–3 (Scheme 1), which served as model pollutants as will be discussed later. As a result of this selective irradiation, triplet rose bengal is efficiently quenched by O2 to form 1O2 with traces of superoxide anion (O2·—) (8,9).

Scheme 1.

Scheme 1

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Until now, studies of rose bengal-chitosan conjugates for green chemistry applications have largely neglected analyses of surface properties to reduce problems of self-oxidation. Nevertheless, earlier studies are very instructive. In the 1970s, Neckers et al. developed polymer rose bengal, research using Merrifield chloromethylpolystyrenes, and discovered it as a heterogeneous source of 1O2 (10). Noteworthy properties of the rose bengal polymer included (i) unwanted intramolecular sensitizer-sensitizer interactions were controlled by lower loadings; (ii) monomeric (isolated) rather than aggregated rose bengal sites produced higher yields of 1O2 (11); and (iii) ammonium salts formed at the “phenolic” positions of rose bengal by displacing the phenol proton produced tighter ion pairing and higher 1O2 quantum yields (12). Studies of Ferrari et al. (5) on rose bengal-chitosan conjugate have since been concerned with preserving the photosensitizer sites on the polymer.

Preservation of the chitosan photosensitizer sites

The chitosan is found to protect the rose bengal sites. A longer service life was found, where the rose bengal-chitosan photodegraded some 5-times slower than free rose bengal in solution. A question could be posed: Why is rose bengal-chitosan more resistant to degradation than free rose bengal? A simple answer relates to a discussion of polymer shielding, which would result in a relatively inaccessible sensitizer site, even though rose bengal is chemically reactive with 1O2, as are many photosensitizers such as bacteriochlorins (13). Singlet oxygen attack can lead to the formation of a hydroperoxide across the quinoid “A-ring” of rose bengal, which is reminiscent of reactions of 1O2 with substituted phenols (14) and subsequent decomposition of the newly formed hydroperoxide (15,16). Depending on the reaction condition and solvent, rose bengal may also decompose by formation of radical cations or radical anions (17). The slower degradation values can result from reduced directions for access of 1O2 to rose bengal in the heterogeneous sample contrary to the greater access of 1O2 to rose bengal in fluid solution.

Efforts to insulate rose bengal sites to reduce 1O2 contact have their limitations. Within chitosan are primary amine sites of glucosamine that can quench 1O2 physically to form triplet dioxygen (1O2 → 3O2) (18), although most sites are positively charged ammonium ions in aqueous solution. In a similar vein, polymers with diazabicyclo[2.2.2]octane (DABCO) covalently attached has been used as a stabilizing agent against 1O2-induced degradation (19).

Photodegradation of phenolic pollutants

To study the polymer, Ferrari et al. irradiated it in the presence of trihydroxybenzene compounds, 1–3, as test organic contaminants that were degraded by 1O2 oxidation. Total quenching rate constants (kt) were determined by time-resolved detection of 1O2 at 1270 nm. The chemical quenching rate constants (kr) of 1–3 were also measured. One tangible outcome was the enhanced chemical quenching efficiency of 1–3 by rose bengal-chitosan compared to free rose bengal, which they attribute to surface association with the polymer. Although not established in the Ferrari study, phenols 1–3 may be oxidized to give o-quinones (20). Nonetheless, phenols quench 1O2 by charge transfer and have quenching rate constants of ~107 M−1 s−1 (21), whereas phenolate anions are even more nucleophilic and reactive with electrophilic 1O2 (22).

It is worth noting that rose bengal and rose bengal-chitosan conjugates have found other uses. Both have had success in tissue bonding and wound closure applications by photocrosslinking proteins (23–25). The bonding and closure effects were accompanied by protein stiffening and reduced wettability. Furthermore, researchers have also shown that rose bengal-chitosan compounds have the ability to photo-induce the killing of bacterial biofilms (26) and cancer cells (27). While rose bengal is also recognized as a cerebral stroke inducer due to closure of local blood flow upon irradiation (28,29), although rose bengal fails to localize well in organelles due to its high water solubility.

Chitosan photocatalyst is recoverable, biodegradable

Ferrari et al. report very promising results. They observe that after the photoreaction, only ~2% of the rose bengal units were lost. The dye content remained high, and provided good evidence for the polymer’s durability. After use, acid workup, precipitation, and simple filtration can be used to recover the catalyst. If not, the material is biodegradable, and when left in water it eventually degrades by hydrolysis.

In conclusion, the results of Ferrari and co-workers’ with their rose bengal-chitosan degradation of trihydroxybenzene compounds will likely inspire the further development of photoactive polymers that are also environmentally friendly.

ACKNOWLEDGMENTS

We acknowledge support from the National Institute of General Medical Sciences (NIH SC1GM093830). We also thank Leda Lee for the graphic arts work.

REFERENCES

  • 1.Marin ML, Santos-Juanes L, Arques A, Amat AM, Miranda MA. Organic photocatalysts for the oxidation of pollutants and model compounds. Chem. Rev. 2011;112:1710–1750. doi: 10.1021/cr2000543. [DOI] [PubMed] [Google Scholar]
  • 2.Lacombe S, Pigot T. New materials for sensitized photo-oxygenation. In: Albini A, editor. Photochemistry. Vol. 38. The Royal Society of Chemistry; 2010. pp. 307–329. [Google Scholar]
  • 3.Lévesque S, Seeberger FPH. Highly efficient continuous flow reactions using singlet oxygen as a “green” reagent. Org. Lett. 2011;13:5008–5011. doi: 10.1021/ol2017643. [DOI] [PubMed] [Google Scholar]
  • 4.Dichiarante V, Protti S. Photochemistry in ecosustainable syntheses: Recent advances. In: Griesbeck A, Oelgemöller M, Ghetti F, editors. CRC Handbook of Organic Photochemistry and Photobiology. 3rd Edition. Vol. 1. 2012. pp. 213–235. [Google Scholar]
  • 5.Ferrari G, Andrada ME, Natera J, Muñoz V, Montaña MP, Gambetta C, Boiero MI, Montenegro MA, Massad WA, García NA. The employment of a removable chitosan-derivatized polymeric sensitizer in the photooxidation of polyhydroxylated water-pollutants. Photochem. Photobiol. 2014 doi: 10.1111/php.12350. (in press). [DOI] [PubMed] [Google Scholar]
  • 6.Muzzarelli RA, Boudrant J, Meyer D, Manno N, DeMarchis M, Paoletti MG. Current views on fungal chitin/chitosan, human chitinases, food preservation, glucans, pectins and inulin: A tribute to Henri Braconnot, precursor of the carbohydrate polymers science, on the chitin bicentennial. Carbohydr. Polym. 2012;87:995–1012. [Google Scholar]
  • 7.Moczek L, Nowakowska M. Novel water-soluble photosensitizers from chitosan. Biomacromolecules. 2007;8:433–438. doi: 10.1021/bm060454+. [DOI] [PubMed] [Google Scholar]
  • 8.Lambert CR, Kochevar IE. Does rose bengal triplet generate superoxide anion? J. Am. Chem. Soc. 1996;118:3297–3298. [Google Scholar]
  • 9.Srinivasan VS, Podolski D, Westrick NJ, Neckers DC. Photochemical generation of superoxide ion (O2−) by rose bengal and Ru(bpy)32+ J. Am. Chem. Soc. 1978;100:6513–6515. [Google Scholar]
  • 10.Blossey EC, Neckers DC, Thayer AL, Schaap AP. Polymer-based sensitizers for photooxidations. J. Am. Chem. Soc. 1973;95:5820–5822. [Google Scholar]
  • 11.Neckers DC, Valdes-Aguilera OM. Photochemistry of the xanthene dyes. Adv. Photochem. 1993;18:315–394. [Google Scholar]
  • 12.Paczkowska B, Paczkowski J, Neckers D. Properties of rose bengal. X. New derivatives of polymer-rose bengal. Photochem. Photobiol. 1985;42:603–604. [Google Scholar]
  • 13.Brückner C, Samankumara L, Ogikubo J. Synthesis of Bacteriochlorins and Isobacteriochlorins. In: Kadish KM, Smith KM, Guilard R, editors. Handbook of Porphyrin Science. Vol. 17. River Edge, NY: World Scientific; 2012. pp. 1–112. [Google Scholar]
  • 14.Thomas MJ, Foote CS. Chemistry of singlet oxygen. XXVI. Photooxygenation of phenols. Photochem. Photobiol. 1978;27(6):683–693. [Google Scholar]
  • 15.Carlsson DJ, Wiles DM. Photostabilization of polypropylene. II. Stabilizers and hydroperoxides. J. Polym. Sci. 1974;12(10):2217–33. [Google Scholar]
  • 16.Gollnick K. In: Singlet Oxygen: Reactions with Organic Compounds and Polymers. Rånby B, Rabek JF, editors. New York: John Wiley and Sons; 1978. p. 111. [Google Scholar]
  • 17.Neckers DC. Rose bengal. J. Photochem. Photobiol. A. 1988;47(1):1–29. doi: 10.1111/j.1751-1097.1988.tb08842.x. [DOI] [PubMed] [Google Scholar]
  • 18.Tanielian C, Wolff C. Mechanism of physical quenching of singlet molecular oxygen by chlorophylls and related compounds of biological interest. Photochem. Photobiol. 1988;48(3):277–280. [Google Scholar]
  • 19.Enko B, Borisov SM, Regensburger J, Bäumler W, Gescheidt G, Klimant I. Singlet oxygen-induced photodegradation of the polymers and dyes in optical sensing materials and the effect of stabilizers on these processes. J. Phys. Chem. A. 2013;117(36):8873–8882. doi: 10.1021/jp4046462. [DOI] [PubMed] [Google Scholar]
  • 20.Kaneko A, Ishii Y, Kirimura K. High-yield production of cis, cis-muconic acid from catechol in aqueous solution by biocatalyst. Chem. Lett. 2011;40:381–383. [Google Scholar]
  • 21.Wilkinson F, Helman WP, Ross AB. Quantum yields for the photosensitized formation of the lowest electronically excited singlet state of molecular oxygen in solution. J. Phys. Chem. Ref. Data. 1995;24:663–1021. [Google Scholar]
  • 22.Lee J, Hong S, Mackeyev Y, Lee C, Chung E, Wilson LJ, Kim J-H, Alvarez PJJ. Photosensitized oxidation of emerging organic pollutants by tetrakis C60 aminofullerene-derivatized silica under visible light irradiation. Environ. Sci. Technol. 2011;45(24):10598–10604. doi: 10.1021/es2029944. [DOI] [PubMed] [Google Scholar]
  • 23.Yao M, Yaroslavsky A, Henry FP, Redmond RW, Kochevar IE. Phototoxicity is not associated with photochemical tissue bonding of skin. Lasers Surg. Med. 2010;42:123–131. doi: 10.1002/lsm.20869. [DOI] [PubMed] [Google Scholar]
  • 24.Tsao S, Yao M, Tsao H, Henry F, Zhao Y, Kochevar J, Redmond R, Kochevar I. Light-activated tissue bonding for excisional wound closure: a split-lesion clinical trial. Br. J. Dermatol. 2012;166:555–563. doi: 10.1111/j.1365-2133.2011.10710.x. [DOI] [PubMed] [Google Scholar]
  • 25.Barton M, Piller SC, Mahns DA, Morley JW, Mawad D, Longo L, Lauto A. In vitro cell compatibility study of rose bengal–chitosan adhesives. Lasers Surg. Med. 2012;44:762–768. doi: 10.1002/lsm.22076. [DOI] [PubMed] [Google Scholar]
  • 26.Shrestha A, Hamblin MR, Kishen A. Photoactivated rose bengal functionalized chitosan nanoparticles produce antibacterial/biofilm activity and stabilize dentin-collagen. Nanomed. 2014;10:491–501. doi: 10.1016/j.nano.2013.10.010. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 27.Wang XL, Zeng Y, Zheng YZ, Chen JF, Tao X, Wang LX, Teng Y. Rose bengal-grafted biodegradable microcapsules: Singlet-oxygen generation and cancer-cell incapacitation. Chem. Eur. J. 2011;17:11223–11229. doi: 10.1002/chem.201100975. [DOI] [PubMed] [Google Scholar]
  • 28.Watson BD, Dietrich WD, Busto R, Wachtel MS, Ginsberg MD. Induction of reproducible brain infarction by photochemically initiated thrombosis. Ann. Neurol. 1985;17:497–504. doi: 10.1002/ana.410170513. [DOI] [PubMed] [Google Scholar]
  • 29.Saiki R, Nishimura K, Ishii I, Omura T, Okuyama S, Kashiwagi K, Igarashi K. Intense correlation between brain infarction and protein-conjugated acrolein. Stroke. 2009;40:3356–3361. doi: 10.1161/STROKEAHA.109.553248. [DOI] [PubMed] [Google Scholar]

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