Intracellular ZnO Nanorods Conjugated with Protoporphyrin for Local Mediated Photochemistry and Efficient Treatment of Single Cancer Cell
© The Author(s) 2010
Received: 22 June 2010
Accepted: 1 July 2010
Published: 15 July 2010
ZnO nanorods (NRs) with high surface area to volume ratio and biocompatibility is used as an efficient photosensitizer carrier system and at the same time providing intrinsic white light needed to achieve cancer cell necrosis. In this letter, ZnO nanorods used for the treatment of breast cancer cell (T47D) are presented. To adjust the sample for intracellular experiments, we have grown the ZnO nanorods on the tip of borosilicate glass capillaries (0.5 μm diameter) by aqueous chemical growth technique. The grown ZnO nanorods were conjugated using protoporphyrin dimethyl ester (PPDME), which absorbs the light emitted by the ZnO nanorods. Mechanism of cytotoxicity appears to involve the generation of singlet oxygen inside the cell. The novel findings of cell-localized toxicity indicate a potential application of PPDME-conjugated ZnO NRs in the necrosis of breast cancer cell within few minutes.
KeywordsZnO nanorods Cancer cell necrosis Photodynamic therapy Protoporphyrin dimethyl ester
Zinc oxide (ZnO) is gaining much research attention due to many advantageous properties like direct band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature and deep level defects emissions that cover the whole visible range. In fact, ZnO is one of the very few materials that can emit ‘intrinsic’ white light. In addition, ZnO nanostructures family is the richest known among all materials so far and the growth of these nanostructures is facilitated by the self organized growth property of this material. It can be grown with good crystalline quality on almost any substrate being crystalline or amorphous . Due to these properties, it is regarded as a very promising material for many photonic devices [1–3] including devices for the treatment of cancer cells [4, 5]. Moreover, ZnO is a bio-safe and bio-compatible material and this makes it an attractive candidate for biomedical applications [6, 7]. Materials when scaled down to the nanoscale realm, unique size-dependent properties of these nanomaterials are manifested.
Moreover, ZnO NRs having a large surface area to volume ratio and can be grown with vertical alignment nanorods makes them also natural wave guiding cavities enhancing the light extraction efficiency in photonic devices . Due to this, ZnO has a potential for a wide range of applications in UV and intrinsic white light–emitting devices. The UV and green emission part of the white light of ZnO can be used to activate some biological process, such as the activation of photosensitizers for photodynamic therapy (PDT) [9–11]. There are few reports dealing with ZnO nanoparticles for bio-medical purposes e.g. PDT [4, 12].
Photodynamic therapy has become an important tool for the treatment of cancer cells [13–18]. As well known photodynamic therapy for cancer cell involves the uptake of photosensitizer by cancer cells followed by exposure to white light. The choice of a suitable irradiation wavelength is one of the basic parameters affecting the efficiency of the PDT. Irradiation with long-wavelength light to protoporphyrin IX dimethyl ester (PPDME), which absorbs 630 nm wavelength light, provides low quantum yields for singlet oxygen . Normally the photosensitizer is typically applied to the tumor for 3–6 h before light exposure so that adequate amount of it can penetrate into the tumor. The molecular oxygen in the cell is essential for the PDT, as the phototoxic reaction involves the formation of singlet oxygen in the PDT . The singlet oxygen is produced by the energy transfer between the molecular oxygen and the triplet state of the excited photosensitizer molecules in the cell . This singlet oxygen will cause initial damage to the mitochondria leading to cell necrosis . The cell necrosis due to this damage of mitochondria is still under discussion. However, the penetration of the drug (aminolevulinic acid (ALA)) is the major limiting factor in the PDT . For this reason, there is a demand for new and improved photosensitizer delivery systems for effective treatment modalities.
In this paper, ZnO NRs are suggested and used as efficient carrier of the photosensitizer for cancerous cell treatment. These ZnO nanorods are grown on the tip of a submicrometer tip glass pipette and the photosensitizer was applied to the surface of the grown ZnO nanorods. By mechanical manipulation, the conjugated ZnO NRs were inserted gently inside the cancer cells. The emission of intrinsic white light which sensitize the cancer cells, when excited by UV light is investigated. For this purpose, an inverted fluorescence microscope (ZEISS) was used. The experiments regarding the florescence (Fl) spectra were performed for configurations with bare ZnO NRs and for those with conjugated photosensitizer using an excitation wavelength of 240 nm.
The main effort has been focused to make the tip geometry small enough. Extremely sharp (sub-micrometer dimensions) and long tips (>10 μm in length) are the basic requirement for the intracellular PDT device in the present work. Such intracellular device should have properties like bending and gentle penetration of the flexible cell membrane, which are provided by ZnO NRs.
We have used a technique of intracellular mechanical manipulation of a bare or functionalized the tip of a sub-micrometer inside single cells [6, 7]. This technique although an invasive of nature, the penetrated cells have shown not to be affected by the invasion. The cancer cells were studied using an inverted fluorescence microscope (ZEISS). Cell necrosis was noted when exposed to the emitted white light.
Results and Discussion
Borosilicate glass capillaries femtotip II coated with silver is shown in the SEM image in Fig. 1a, while Fig. 1b shows the tip with the grown ZnO NRs. As can be seen, the ZnO NRs have a hexagonal shape and a uniform density as shown in SEM image of Fig. 1c.
ZnO nanorods being an intrinsic white light–emitting material have been demonstrated as an efficient light system attached to a photosensitizer for intracellular cell necrosis. ZnO nanorods grown on the femto tip are shown to deliver the photosensitizer to breast cancerous cells and causes the necrosis within few minutes. Topical pain caused by the conventional PDT method can be reduced by this technique. Exceptional care must be taken to avoid the PDT device tip from entering into the lymphatic vessels of the breast to minimize the chance of spread of cancer cells. We present integrated, affordable and improved photosensitizer delivery system which can be used for local cancer cell necrosis. This device opens an era for effective drug delivery for specific localized tumors with reduced pain. We have developed another research tool in which, instead of using many nanorods on the tip, we can produce singlet oxygen in the cell, using single ZnO nanorod with some nanometers accuracy. This will be published in our next paper.
We thank Fredrik Olsson, Genovis, Lund, Sweden, for using their fluorescence microscope. Birgit Olsson and Olle Stål are acknowledged for providing the cells.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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