Surface photochemical studies of nano-hybrids of A₃B porphyrins and Fe₃O₄ silica-coated nanoparticles

Highlights

• Porphyrins.

• Silica-coated magnetite nanoparticles.

• Donor-acceptor complex.

• Powdered samples fluorescence quantum yields and lifetimes.

Abstract

This study reports a surface photochemistry behaviour of two tetrapyrrolic structures: 5,10,15,20-tetra-(4-carboxymethylphenyl) porphyrin (P1.1) and 5-(4-hydroxy-3-methoxyphenyl)-10,15,20-tris-(4-carboxymethylphenyl) porphyrin (P1.2), covalently bound to silica-coated magnetite nanoparticles (Fe₃O₄@SiO₂) imbibed into a polyethylene glycol (PEG) matrix and in the form of a fine powder. The initial compounds and the nano-hybrids imbibed in the PEG matrix were characterized using Fourier transformed infrared spectroscopy (FT-IR), diffuse reflectance absorption spectroscopy for ground state molecules (GSDR), X-ray diffaction (XRD), transmission electron electron microscopy (TEM) as well as several luminescence emission spectroscopies. The combined use of these methodologies enabled us to obtain a surface photochemistry characterization of these new nano-hybrid materials.

For the powdered samples of P1.1 or P1.2 and Fe₃O₄@SiO₂ imbibed into the powdered PEG a detailed absorption study and a correlation of fluorescence emission intensity with the magnetite concentration were performed. New covalent bonds porphyrin-Fe₃O₄@SiO₂ were detected using FT-IR transmission spectra. A donor-acceptor interaction with change of the oxidation level of the porphyrins was also observed. Fluorescence emission quantum yields and lifetimes of the two porphyrins were determined for all samples and two populations of the porphyrin molecules exist, free porphyrin (unquenched) and porphyrin with Fe₃O₄@SiO₂ NPs attached, the quenched porphyrin. In this way, the amount of the available excited porphyrin for singlet oxygen formation is reduced, therefore the presence of the Fe₃O₄@SiO₂ NPs may not favor this specific PDT action by the Type II mechanism, but it prevents aggregation of the porphyrins, even when a PEG matrix is used, and Type I mechanisms remain active.

Introduction

Photodynamic therapy (PDT) is now a well-established methodology for cancer treatment and porphyrins or porphyrin-based photosensitizers (PSs) have been the most commonly used compounds up to now [[1], [2], [3], [4]]. Some new families of compounds are being tested for the production of reactive oxygen species (ROS), the most relevant phototoxic agents. Among them, squaraines and squaraine derivatives should be mentioned [[5], [6], [7], [8]].

PDT is a two stage procedure and, following an administration procedure, the photosensitizer should be exposed to the light of a suitable wavelength in the phototherapeutic range (650–850 nm, according to [4]). In this way the PSs are promoted to an excited state that may interact with ground state oxygen, enabling the formation of singlet oxygen. This species is considered to be the main cytotoxic agent in the photodynamic effect [9], causing the cell death of the malignant tissues.

In this study we are particularly interested in topic applications of the porphyrins, following previous studies of cytotoxic agents in films of chitosan (CS) or polyethylene glycol (PEG) [10,11]. In this paper, and following a recent work where the aggregation of two porphyrins in powdered samples of PEG was studied [12], we will pursue the study of two other porphyrins in solid powdered supports, but covalently linked to silica coated magnetite, also for topic applications. PEG is a biocompatible material allowing a topic use of the nano-hybrid material [10,11].

The aggregation of the porphyrin molecules plays a very significant role in their photochemical/photophysical behaviour, especially when adsorbed onto solid substrates, films or powders. Studies performed up to now indicate that fluorescence emission quantum yields and singlet oxygen quantum yields of aggregated porphyrins are lower than those obtained for the monomeric forms of the porphyrins [13,14]. In many cases the monomeric forms of porphyrins exhibit a high quantum yield of ¹O₂* generation (Φ_Δ) and a relatively low cytotoxicity in the dark [2].

Recent studies of our group indicate that important improvements in the phototherapy efficiency can be obtained with the use of asymmetrical porphyrins, namely A₃B forms, which are very promising photosensitizers for PDT [12,[15], [16], [17], [18]]. An effective photosensitizer, should take into account an optimal balance between the hydrophilic and hydrophobic substituents in the tetrapyrrolic structure, thus trying to achieve an optimum uptake in the tumour tissue [8]. Several studies of porphyrins were performed, aiming at the synthesis and photochemical/photocytotoxicity characterization of new compounds to be used in the photodynamic therapy of cancer (PDT) [12,[15], [16], [17], [18]].

Iron oxide magnetic nanoparticles have received considerable attention because of their prospects to be used in biological and medical applications. They exhibit unique magnetic, non-toxic, highly biocompatible properties, with potential application in drug delivery, biological separation and bio-catalysis [19].

In this study, two porphyrins recently synthetized and reported in reference [16], 5,10,15,20-mesotetrakis-(4-carboxymethylphenyl) porphyrin (P1.1), the fully symmetric substituted compound (A₄ type), and 5-(4-hydroxy-3-methoxyphenyl)-10,15,20-tris-(4-carboxymethylphenyl) porphyrin (P1.2), a A₃B type porphyrin (Scheme 1), were covalently bound to silica-coated magnetite nanoparticles (Fe₃O₄@SiO₂) and imbibed into a polyethylene glycol (PEG) matrix, in the form of a fine powder.

Here we present a study of the two porphyrins P1.1 or P1.2@ Fe₃O₄@SiO₂ nano-hybrids imbibed into the biocompatible material PEG, aiming a full surface photochemistry characterization of these new nano-hybrid materials.