Energy Transfer at Nano-Bio Interfaces
Many technologies rely on the properties of "pure" semiconductors materials for operation. It has been proposed that future technologies will be based on hybrid materials, developed as a result of multidisciplinary studies combining the expertise in Physics, Chemistry and Biology. The interface between nano- and bio-technology, for example, has enormous potential to supply such hybrid materials. In this project, we have studied energy transfer processes in three systems, all containing colloidal semiconductor quantum dots (QDs) as one of the components. Energy transfer processes with methylene blue (MB), bacteriorhodopsin protein (bR) and photosynthetic reaction centres (RC) extracted from bacterial membranes were studied.
Although we rarely realise this, energy conversion and transfer processes play a massive role in human life. Alkaline batteries and solar cells use energy conversion processes to produce voltage. Our bodies convert chemical energy to thermal and mechanical energy, whereas green plants convert solar energy to chemical energy via photosynthesis, thus producing new molecules and supramolecular structures. Very often the energy receptors and energy convertors are spatially distant species and practically all processes of energy conversion are accompanied by the transfers of energy on the more or less significant scales. For some species, energy transfer and conversion processes are essential for their survival. For example, bacteria Halobacterium salinarum is able to survive under extreme pH, ionic strengths, high pressures and temperatures due to its energy transfer and conversion capabilities. It is worth mentioning that millions of years of evolution have often solved problems of a nature similar to those that man attempts to solve in harnessing organic compounds through perfection of the built-in functionalities of many biological systems. Moreover, self-assembly and genetic engineering of such proteins and bR, for example, provide sophisticated control and manipulation of large molecules or ensembles, thereby attesting to the considerable promise of molecular electronics and bioelectronics areas as the basis for the development of future materials.
On the other hand, nanoparticle research, as a part of extended studies of nanoscale materials, is one of the fields of Nanotechnology, which today is a highly dynamic and already strongly interdisciplinary area. The immense and varied interest in nanoscience arises from the finding that many properties of nanoscale materials (optical, catalytic, magnetic or electronic) depend not only on their chemical composition, but also on their size. The basic physical mechanisms of the energy transfer and energy conversion phenomena in all of the above mentioned systems have been thoroughly investigated. Nonetheless, an appearance of interfaces between the biological and nanosized non-organic moieties may produce very interesting effects. So, the multidisciplinary biological studies involving nanotechnological approaches should certainly lead to the novel breakthrough results in a near future.
PS system for photodynamic therapy
The aim of this study was to attempt to increase the efficiency of a model photosensitizer by utilizing energy transfer from QDs. We found that QDs luminescence was primarily quenched by charge transfer, but energy transfer also occurred if there was sufficient spectral overlap. Both of these processes, however, result in an increased excitation of the PS molecules, suggesting an increased production of singlet oxygen as confirmed by NIR PL measurements in D2O. In vitro cell growth studies (HeLa cells) revealed increased cell kill efficiency for QD-PS complexes, implying that semiconductor QDs can be employed to improve the efficiency of any photodynamic therapy photosensitizer.
Bacteriorhodopsin-QD hybrid material
The interactions between colloidal CdTe quantum dots (QDs) and bacteriorhodopsin (bR) protein were studied by a variety of spectroscopic techniques, including integrated and time-resolved fluorescence spectroscopies, zeta potential and size measurement, and fluorescence correlation spectroscopy. QDs’ luminescence was found to be strongly modulated by bacteriorhodopsin via Förster resonance energy transfer. Concave Stern-Volmer plots and sigmoidal photoluminescence quenching curves imply that the self-assembling of NCs and bR occurred, and the number of nanocrystals (NCs) per bR contributing to energy transfer was found to be highly dependent not only on NCs’ size, but also on its surface charge. Observed interactions between CdTe nanocrystals and bacteriorhodopsin can provide the basis for the development of novel functional materials with unique photonic properties and applications in areas such as all-optical switching, photovoltaics and data storage.
Figure 2. Energy transfer between a Quantum dot (red sphere) and bacteriorhodopsin protein. bR is a transmembrane protein and it consists of seven alpha helices. Retinal, the choromophore element to which the energy is transferred, is highlighted in red. bR forms trimers within the membrane, which are arranged in 2-D hexagonal array. This accounts for extraordinary stability of this protein.
Energy transfer in QD-RC complexes
The development of artificial photosynthetic systems that utilize solar energy is one of the most challenging goals of material sciences. In naturally photosynthetic organisms, light is initially absorbed by antenna protein-pigment complexes and transferred to specialized reaction centers (RCs) in which the captured light energy is converted into chemical energy for synthesis of high-energy molecules that fuel the organism. Mimicry of this process for artificial solar energy conversion should include, among other components, an efficient light-harvesting antenna capable of transferring the excitation energy to RCs. The use of organic chromophores is rather limited due to their narrow light absorption and their lack of photostability. However, inorganic nanocrystals, may achieve significantly greater absorption than natural photosystems, thus enhancing the light-harvesting process. Simultaneously, these nanocrystals are also very efficient in excitation energy transfer. So far, a three-fold increase in the rate of generation of excitons in the RC was demonstrated by us in specifically designed QD-RC complexes, and QD-theoretical estimates predict even stronger enhancements.
Figure 2. Organization and functionality of a complex composed from reaction center and QD. Active (A) and inactive (B) parts in the electron transfer cofactor branches are shown. The positions of absorption/fluorescence maxima for BChl special pair (P), BChl monomer (B), bacteriopheophytine (H) and quinone (Q) are indicated for the active branch (A) only. Photons are absorbed by both RC and QD. An exciton from the QD is transferred to the RC via FRET. Excitons inside RC relax to the lowest energy level located at the special pair P. In nature, excitons trapped at the special pair P are dissociated (i.e. photo-induced charge separation occurs) and an optically-excited electron is transferred along the active branch, leaving a positively-charged hole at P. In our experimental conditions, charge separation does not occur; instead excitons at the special pair recombine via both radiative and non-radiative channels
References:1. "Optical Studies of the Methylene Blue-Semiconductor Nanocrystals Hybrid System", Aliaksandra Rakovich, Yury P. Rakovich and John F. Donegan, e-Journal of Surface Science and Nanotechnology, Vol. 7, No. 0, pp.349-353 (2009)
2. "Photosensitizer Methylene Blue-Semiconductor Nanocrystals Hybrid System for Photodynamic Therapy", Aliaksandra Rakovich, Tatsiana Rakovich, Vincent Kelly, Vladimir Lesnyak, Alexander Eychmüller, Yury P. Rakovich, and John F. Donegan Journal of Nanoscience and Nanotechnology, Vol. 10, pp. 2656–2662 (2010)
3. "Energy transfer processes in semiconductor quantum dots: bacteriorhodopsin hybrid system", Aliaksandra Rakovich, Alyona Sukhanova, Nicolas Bouchonville, Michael Molinari, Michel Troyon, Jacques H. M. Cohen, Yury Rakovich, John F. Donegan, and Igor Nabiev, Proc. SPIE, Vol. 7366, p. 736620 (2009)