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Aqueous Synthesis of Nanocrystals

#B-1931


New Approach to Aqueous Synthesis of Nanocrystals with Magnetic and Luminescence Properties

Tech Area / Field

  • CHE-SYN/Basic and Synthetic Chemistry/Chemistry

Status
3 Approved without Funding

Registration date
15.03.2011

Leading Institute
Joint Institute of Energy and Nuclear Research - Sosny, Belarus, Minsk, Sosny

Collaborators

  • Technische Universität Dresden / Institut fur Physikalische Chemie und Elektrochemie, Germany, Dresden\nSwedish University of Agricultural Sciences, Sweden, Uppsala\nPlasmaChem GmbH, Germany, Berlin

Project summary

The aim of the current project is to investigate the speciation at the initial stages of the nanoparticles synthesis and to reveal a correlation between the availability of particular complexes in the synthetic mixture and optimal properties of the final product as applied to the different classes of the NCs synthesized in aqueous media.

Background and Justification. Nanoparticle research is currently an area of intense scientific interest due to a wide variety of potential applications in biomedical, optical and electronic fields. There are wide ranges of nanoparticle compositions and there are several methods for creating nanoparticles, including both chemical and physical techniques. The chemical solution deposition widely used recently in the fields of nanoscience. E.g. colloidal quantum dots (QDs) and ferromagnetic nanoparticles have attracted considerable interest during the last decades. There is a wide range of very efficiently light emitting QDs which can be synthesized by colloidal chemistry approaches. Worth to mention are ZnSe, ZnSe/ZnS and CdS (UV-blue spectral region), CdSe, InP, CdTe (visible region), PbSe, HgTe, and InAs (near-infrared region). The majority of successful syntheses were done in organic media. However, the potential applications of such particles are restricted by their compatibility with water and/or air. This kind of compatibility is especially important in the field of bio-applications. Worth to mention is that the phase transfer of NCs from organics to aqueous solution is a time consuming procedure involving capping-agent exchange, encapsulation in phospholipid micelles, silanization of the inpidual QDs 20 or the preparation of polymeric or dendrimer shells around the particles. Beside the complexity, the main drawbacks of these procedures are occasionally limited stability, low emission quantum yield, large increase of the actual size (critical in the case of conjugation with single bio-molecules or intended penetration as markers through bio-membranes).

At the same time the direct aqueous synthesis of NCs can be a good alternative to the organometallic approach, especially when possible applications demand stability in aqueous media. The simplicity and use of relatively soft conditions, as well as less dangerous materials in comparison with the organometallic approach have made the aqueous synthesis attractive for researches during the last two decades.

Very recently it was shown by Murase et al. that solely the proper choice of the relative concentration of the precursors (Cadmium salt and thioglycolic acid) make it possible to gain strongly emitting (PL QE up to 60%) CdTe NCs without any changes in the preparative approach and avoiding post-preparative treatments. Guo et al. proposed that the formation of appropriate Cd-thiolate complexes might be responsible for the improvement observed. The importance of the metal complexes for a successful aqueous synthesis of the CdS NCs has recently been pointed out by Winter et al.

Wide variety of ferromagnetic materials have been synthesized by chemical solution deposition as well as sol-gel techniques. In application point of view, ferromagnetic nanoparticles exhibit unique properties and have promising technological applications in high-density recording, colour imaging, magnetorheologycal fluids, high- frequency devices, magnetic refrigerators, etc. Nanoparticles of magnetic ferrites are also widely used as contrasting agents in magnetic resonance imaging (MRI), replacement of radioactive materials used as tracers and delivery of drugs to specific areas of the body. The chemical preparation methods for ferromagnetic nanoparticles (such as α-, γ-Fe2O3, CoFe2O4, NiFe2O4, CoxZn1-xFe2O4, NixCu1-xFe2O4 etc) provide to vary in wide ranges of synthesis parameters such as precursors, stabilizer and precipitating agent concentration, temperature and pH. Morphology, structure, size and ferromagnetic properties of final product are strongly depended from synthesis parameters. It is important to note that structurization of metal oxide systems at the stage of formation hydroxocomplexes with participation of stabilizing agents is playing crucial role in forming of structure and properties of final ferromagnetic products.

Thus, the results of the above mentioned studies suggest that further investigation in this direction - complexation at the early stages of NC synthesis - can substantially promote one to development of novel materials with better properties and substantially simplify the synthesis technology. Particularly interesting in this respect is the hydroxo complexation as the hydroxo group is a bridge group that binds the metal ions in polynuclear species which can further grow up to form nano size particles. It is expected that each of the hydroxo complexes forms possess different potential as the precursor for NCs synthesis in aqueous media. However, despite the fact that during the last decades a number of works were dedicated to studying hydrolysis of cations in aqueous solution only the first stages of this process (mononuclear species) are investigated with sufficient reliability and consistency. Much less is known about the formation of polynuclear species in solution. There are still many discrepancies in the literature concerning their composition, stability and structure. Even less is known about the reactions of hydrolytic polymerization of cations in more composite bimetallic systems, or when the processes of hydrolysis and complexation with other ligands proceed simultaneously. At the same time, such data are most interesting not only from scientific point of view but, also, for many practical applications including improvement of the NC synthesis technologies.

Apart from bimetallic systems we will also investigate the influence of hydrolysis and formation of complexes on the synthesis of relatively less known nanocrystals (e.g. chalcogenides and phosphides of Cu (II), In(III), Ag (I) and several other metals) with the aim to achieve reproducibility, monodispersity, stable colloidal solutions, size and shape control.

The project’ influence on progress in this area. The implementation of this Project will lead to more explicit understanding of the NCs nucleation and growth under different pH and ligand conditions. The results of the Project will allow to develop new approaches to the NCs synthesis in aqueous media with controlled dispersion, size, shape and photoluminescent and magnetic properties. Also, the data obtained as a result of the proposed study will allow one to better understand the regularities of metal ions hydrolytic polymerisation in solution, and as a result - the behaviour and role of polynuclear species in different physiochemical processes proceeding in aqueous systems such as sorption, extraction, catalysis, migration of metals in natural ecosystems and processes that involve metal ions in biological systems. The results of the research will be published in internationally recognised scientific journals and, also, presented at international conferences. It is expected that at least 5 publications will be a result of the current research. It is also probable that some of the results will be patented.

The participants’ expertise. The group of Radiochemical laboratory of Joint Institute for Power and Nuclear Research – Sosny had studied the hydrolysis of metal ions in aqueous solution since 1969. More than 200 scientific works and 20 patents, including 1 book “The state of radionuclides in solution”.

Expected Results and Their Application.

  • dominating forms of Cd, Zn, Cu, Hg hydroxocomplexes under different pH and complexing agents conditions will be established and the structure of the polynuclear complexes will be determined;
  • effect of hydroxocomplex dominating form on the photoluminescent nanocrystals formations and properties will be established;
  • condition of the formation and structure of Cd-Zn, Cd-Hg, Fe-Co, Fe-Ni mixed polynuclear hydroxocomplexes will be determined;
  • effect of Cd-Zn, Cd-Hg, Fe-Co, Fe-Ni mixed polynuclear hydroxocomplexes structure on structure and properties of nanocrystals synthesized will be established;
  • influence of phosphides and chalcogenides on polynuclear hydroxocomplexes structure and properties will be determined;

Results of the research will allow to propose ways of the optimization of aqueous synthesis of the nanocrystals with different properties (optical, ferromagnetic etc).

Meeting the ISTC goals and objectives. The current project allows realisation of the following principal ISTC aims:

  • to redirect activity of weapon scientists – project participants for solution of peaceful problems – participation in the work on fundamental problems of modern physical and co-ordination chemistry;
  • to closer integrate project participants into international scientific community – as a result of co-operation with project’s collaborators;
  • to support basic and applied research for peaceful purposes – the results of proposed research, apart from their significance for development of novel inorganic materials, will be also useful for development of technologies connected with removal of heavy metals from solution (waste treatment), catalysis and some other practical applications.

Role of Foreign Collaborators/Partners

The collaboration between the participating teams and collaborators will be maintained through regular exchange of information, preparation of together publications, and together experiments. Also, the collaborators will participate in discussing the final report. SLU group (Sweden) will also conduct structural studies (one of the world leading experts in structural studies of metal complexes in solution applying EXAFS and LAXS methods); Dresden technical university (Germany) – the group has all the necessary expertise and more than 20 years of experience in the field of NCs synthesis and their physical chemical characterization.

Technical Approach and Methodology

Our approach involves application of a set of physiochemical methods: potentiometric titration, dialysis, centrifugation, ultrafiltration spectrophotometry, radioactive indicators, EXAFS and LAXS.

Conditions of polynuclear hydroxocomplexes formation in solution will be determined applying the methods of dialysis, centrifugation, and spectrophotometry. A method developed in our laboratory, which involves the use of dialysis, will be applied for determination of the polymerization degree. Determination of polynuclear hydroxo complex charge will be carried out also applying a method developed in our laboratory. This method is based on application of dialysis and radioactive anions. The effect of anions on hydrolysis of cations with formation of polynuclear hydroxocomplexes will be investigated applying a complex of physico-chemical methods: dialysis, centrifugation, рН-metric titration, method of radioactive indicators, spectrophotometry. The structure of different polynuclear hydroxo species will be studied applying EXAFS (extended X-ray absorption fine structure) and LAXS (large angle X-ray scattering) techniques during the visit to Swedish University of Agricultural Sciences).


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