CdSe/ZnS/CdS/ZnS QDs with advanced two-photon properties for tumor diagnosis and treatment
The current strategy for development of advanced methods of tumor treatment focuses on targeted drug delivery to tumor cells. Linking an imaging (fluorescence) agent to a biomarker recognizing molecule conjugated with the pharmacological agent ensures real-time tracking of the delivery of the active substance. Quantum dots (QDs) are semiconductor photoluminescent nanocrystals1 with unique fluorescent characteristics: size-tunable light emission, a high signal brightness, and values of two-photon absorption cross section orders of magnitude higher than those of organic molecules. Thus, water-soluble QDs can be used as efficient biomedical fluorescent labels operating in the two-photon excitation mode, with fluorescence in the visible spectral range excited by irradiation of the QDs in the biological tissue transparency window2.
Here, we have used an advanced procedure of the synthesis of highly luminescent QDs based on charge carrier confinement engineering developed earlier to synthesize core–multishell QDs (CdSe/ZnS/CdS/ZnS) with different sizes3. The QD cores were obtained by the hot-injection method1 using cadmium n-hexadecylphosphonate and selenium tri-n-octylphosphine complex as precursors, tri-n-octylamine as a stabilizing additive, and 1-octadecene as a solvent3. Then, purified CdSe cores were coated with a ZnS/CdS/ZnS shell three monolayers in thickness with the use of the SILAR (Successive Ionic Layer Adsorption and Reaction) approach. This shell structure provides a high potential barrier for effective protection of excited charge carriers from the environment. As a result, we obtained series of highly luminescent CdSe/ZnS/CdS/ZnS QDs with emission wavelength maxima at 540, 575, and 610 nm and PL quantum yields of 90, 97, and 80%, respectively.
We also studied the two-photon properties of three series of the synthesized QDs in chloroform4. The two-photon excitation action cross-sections of QD-540, QD-570, and QD-610 were found to be 11,000, 16,000, and 13,000 GM, respectively (1 GM = 1050 cm4 s/photon)5. These values are comparable with those of the best samples of CdSe/ZnS QDs and are three orders of magnitude higher than those of conventional organic fluorophores.
The large two-photon excitation action cross sections make the synthesized nanocrystals excellent photoluminescent materials for engineering of bright nanoprobes for multiphoton microscopy. Thus, the core/multishell QDs with a bright PL in the one- and two-photon modes can be used for engineering of small-sized fluorescence labels for tumor diagnosis and treatment systems capable of penetrating into live cells.
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- P. Samokhvalov, P. Linkov, J. Michel, M. Molinari, I. Nabiev, Photoluminescence quantum yield of CdSe-ZnS/CdS/ZnS core-multishell quantum dots approaches 100% due to enhancement of charge carrier confinement, Proc. SPIE 8955, Colloidal Nanoparticles for Biomedical Applications IX, 89550S (2014).
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