Interaction of water-soluble CdTe quantum dots with octacarboxy metallophthalocyanines: A photophysical and photochemical study Mopelola Idowu, Tebello Nyokong à Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa article info Article history: Received 13 March 2008 Received in revised form 18 October 2008 Accepted 4 November 2008 Available online 24 November 2008 Keywords: Quantum dots L-cysteine Phthalocyanine Triplet quantum yield Singlet oxygen quantum yield Energy transfer abstract Water-soluble CdTe quantum dots capped with L-cysteine (QD-CYS) were found to improve the photophysical and photochemical properties of octacarboxy metallophthalocyanine (MOCPc, M ¼ Zn(II), Al(III)(OH), Ge(IV)(OH) 2 and Si(IV)(OH) 2 ) complexes. Increased diffusional interactions between the molecules and the ground-state molecular oxygen were established resulting from the increase in the triplet-state quantum yield and lifetimes of the MOCPcs in the presence of QD-CYS. Energy transfer occurred from QD-CYS to the MOCPcs upon excitation of QD-CYS. It was found that an efficient energy transfer process, which is not directly related to the amount of spectral overlap between the donor (QD-CYS) and the acceptor (MOCPc) can occur. Singlet oxygen via fluorescence resonance energy transfer (FRET) mechanism was produced in the QD-MPc mixture. & 2008 Elsevier B.V. All rights reserved. 1. Introduction Quantum dots (QDs) with their unique photophysical proper- ties such as narrow, size-dependent tunable emission spectra, a broad absorption band (which both allow simultaneous excitation of particles of different sizes at a single wavelength), a relatively large fluorescence intensity and high photochemical stability, have been shown to be ideal as fluorophores with potential applications in many fields, ranging from energy conversion, photocatalysis, and biological labeling to biomedicine [1–6]. Recently, the potential of QDs as photosensitizers alone or in combination with other known photosensitizers has also been explored [7,8]. Their unique physical properties allow them to be tuned to have spectral overlap with a particular acceptor thereby donating energy to such acceptors. Metal phthalocyanines (MPcs) are a family of promising photo- sensitizers. Their exclusive properties such as intense absorption in the red region of the visible spectrum, effective singlet oxygen generation, coupled with their non-toxicity (in the absence of light) [9–11] have made them a focus of attention and they have been in use as photoreceptors, sensors, sensitizers in photodynamic therapy of cancer (PDT), or in photodynamic inactivation of viruses and bacteria and in photocatalytic reactions [9–17]. Energy transfer from QDs to different photosensitizers (por- phyrins or phthalocyanines) has been demonstrated in a number of studies [18–23]. Indeed the QD sensitization of metal phthalocyanines was first demonstrated by Samia et al. in 2003 [22]. The interactions that occur between the QDs and the photosensitizers leading to energy transfer have been found to occur by electrostatic interaction [18], adsorption [20] or inter- digitization [23]. In this work, we studied photochemical and photophysical behaviour of aluminium, silicon, zinc and germa- nium octacarboxy phthalocyanines ((OH)AlOCPc, (OH) 2 SiOCPc, ZnOCPc and (OH) 2 GeOCPc) in the presence of CdTe QDs capped with L-cysteine (QD-CYS). It is expected that the amino group on the cysteine capping of the QDs and the carboxylic group of the MOCPc will form an amide bond, resulting in a covalent attachment of the MOCPc with the capping molecule L-cysteine on the CdTe QD surface. We also determined the efficiency of energy transfer from QD-CYS to the octacarboxy metallophthalo- cyanines (MOCPc). The effect of the central metal on the efficiency of energy transfer (through Fluorescence Resonance Energy Transfer, (FRET)) from CdTe QDs will be evaluated. MPcs can sufficiently populate the triplet state with long enough lifetime, resulting in the transfer of their energy to ground-state oxygen and production of singlet oxygen which is necessary in many photosensitized reactions. The production of singlet oxygen when QDs and MOCPcs are in the mixture will thus be evaluated. The photochemical behaviour of these MOCPcs in the presence of QD-CYS and the generation of singlet oxygen through indirect activation of the MOCPcs was studied and this can be very valuable in the photodynamic inactivation of viruses. ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence 0022-2313/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.11.005 à Corresponding author. Tel.: +2746 6038260; fax: +2746 6225109. E-mail address: t.nyokong@ru.ac.za (T. Nyokong). Journal of Luminescence 129 (2009) 356–362