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SiO2-coated magnetic clusters for applications in magnetically secured paper

Authors: Izabell Crăciunescu, Rodica Turcu

Keywords: magnetic nanoparticles, magnetic clusters, miniemulsion, magnetically secured paper

Applications

The paper used for official documents, such as banknotes, bank documents, identity cards or study documents, falls into the category of secure paper. In order to eliminate the possibility of falsification, this type of paper needs to contain sophisticated security features.

At national and international level, use securing processes containing wires, chemically reactive paper, ferromagnetic microwires, visible or fluorescent fibers, pigments and others.

Innovative aspects

For specific applications in the field of securitization, magnetically secured paper made by embedding SiO2-coated magnetic cluster with superparamagnetic properties in cellulose is an improved alternative to current securitization systems.

We have optimized the synthesis of a SiO2-coated magnetic clusters in order to obtain a rigorous control of the structure, morphology and physicochemical properties of these nanomaterials for the specific application of secure paper. By embedding magnetic clusters coated with SiO2 layers with controlled thickness in cellulose, a major improvement of the magnetic paper color from reddish-brown to light beige was achieved. It was possible to correlate the final color of the magnetic paper with the thickness of the SiO2 layer and the concentration of magnetic clusters used.

Technology

Self-assembled systems based on of magnetic nanoparticle in the form of magnetic clusters with controlled shape and size (100-300 nm) were prepared by miniemulsion method. Subsequently the magnetic clusters were coated with SiO2 layers whose thickness is strictly controlled from the synthesis parameters. The classical Stober sol-gel method using tetraethoxy orthosilicate as precursor is used to prepare the SiO2 layer.

The synthetic procedure ensures a very good reproducibility of SiO2-coated magnetic clusters, allows a very rigorous control of the physicochemical properties, as well as the possibility of scaling up.

Advantages

  • SiO2-coated magnetic clusters have superparamagnetic properties and saturation magnetization values in the range of 25-60 emu/g depending on the SiO2 layer thickness
  • The properties of SiO2-coated magnetic clusters are tightly controlled from the synthesis parameters
  • SiO2-coated magnetic clusters can be obtained in reproducible large scale
  • Magnetically secured paper based on SiO2-coated magnetic clusters exhibits superior security features than traditional paper