Characterization and photophysical behavior of phthalocyanines when grafted onto silica nanoparticles Adedayo Fashina, Edith Antunes, Tebello Nyokong ⇑ Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa article info Article history: Received 7 November 2012 Accepted 8 January 2013 Available online 9 February 2013 Keywords: Phthalocyanine Silica nanoparticles Triplet quantum yields Fluorescence quantum yield Carboxyl phenoxy abstract This work reports on the functionalization of silica nanoparticles with zinc phthalocyanine complexes tetra-substituted non-peripherally with 4-carboxyphenoxyl and 3-carboxyphenoxyl groups. The phthalo- cyanine dyes have a free carboxyl group, facilitating the covalent attachment of the dyes on the silica sur- face via amide bond formation. The phthalocyanine functionalized silica nanoparticles showed higher fluorescence and triplet quantum yields as well as longer triplet lifetimes compared to the free phthalocyanines. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Silica nanoparticles (SiNPs) have continued to generate interest from researchers due to their low toxicity, biocompatibility [1–5] and the ease of surface functionalization [6–8]. Silica nanoparticles alone are not fluorescent [9]; however, they can be made lumines- cent by two methods. The first is by doping a dye (a fluoropore) in- side the silica matrix, while the second is by modifying the surface of the silica by coating or grafting with a dye [6,10]. Incorporation of dyes (organic or inorganic) into silica matrices has been reported with the doped dyes showing a higher photosta- bility and increased luminescence [11,12]. Bonacchi et al. [13], re- ported on the positive amplification of the fluorescence response of a chemosensor densely grafted onto silica nanoparticles. SiNPs hold the promise to be highly efficient photodynamic therapy (PDT) drug delivery platforms for photosensitizers such as phthal- ocyanines, owing to attractive features such as large surface area, ease of chemical modification, excellent biocompatibility and avid uptake by the cells. Phthalocyanines (Pcs) are well known as photosensitizers for PDT [14–17], thus it is logical to covalently combine phthalocyanines with SiNPs for potential drug delivery applications. Phthalocyanines (such as tetra-tert-butyl zinc (II) phthalocya- nine) have been encapsulated into silica nanoparticles [18]. How- ever, when phthalocyanines are physically entrapped inside the silica network, they can be prematurely released from the carrier vehicles during applications such as in photodynamic therapy. Tetra-substituted carboxyl aluminium phthalocyanine (AlTCPc) was covalently linked to SiNPs to avoid this problem [19]. It was also reported that covalently linked AlTCPc–SiNPs conjugates exhibited excellent photo-oxidation efficiency. With the above in mind, this work reports on the synthesis and characterization of phthalocyanine-functionalized SiNPs, using Pcs containing carboxy phenoxy groups. A covalent bond was formed between the carboxyl of the Pc and the amino group on the coating on the SiNPs, allowing for a spacer between the phthalocyanine ring and the SiNPs, unlike for AlTCPc, where the COOH group was directly attached to the Pc ring. The effects of the point of attachment of the COOH group (meta versus para) on the phenoxy carboxyl ring substituent will be assessed. It has been reported [20] before that although no effect was ob- served when comparing the spectra of phthalocyanines with meta versus para attachment of the COOH group on the phenoxy group, the triplet and singlet oxygen quantum yields were lower for para compared to meta substitution of the benzene ring. Zn is em- ployed since its use as a central metal for Pcs meant for PDT applications is well known [14]. SiNPs are mainly synthesized by two methods, the Stober meth- od and the reverse microemulsion method [21–23]. We adopted the Stober method due to its high product yield and the porosity of the particles [24]. The SiNPs in this study were functionalized with zinc phthalocyanines tetrasubstituted with 4-carboxyphen- oxyl (1a) and 3-carboxyphenoxyl (1b), Fig. 1. The syntheses of these complexes have been reported before [20,25]. The dyes were chosen due to their absorption in the near infrared (a requirement for PDT applications) region. The carboxyl phenoxy groups are de- signed to allow the covalent grafting of the dyes on the silica 0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2013.01.037 ⇑ Corresponding author. Tel.: +27 46 6038260; fax: +27 46 6225109. E-mail address: t.nyokong@ru.ac.za (T. Nyokong). Polyhedron 53 (2013) 278–285 Contents lists available at SciVerse ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly