• Universitat de València
• INNCEINNMAT S.L.
• Spanish National Research Council

• Ángel M López Buendía
• Mar Urquiola Casas
• José Francisco Fernández Lozano
• Esther Enríquez Pérez

In recent decades, research in nanochemistry has driven the development of innovative materials for application in various technological areas. Of particular interest are upconversion nanoparticles (UCNPs), which are composed of inorganic nanocrystals doped with lanthanide ions, which can emit in the ultraviolet, visible and/or infrared after excitation in the near-infrared using low-cost laser diodes. UCNPs are generally prepared with a layer of organic material anchored to their surface by ionisable carboxylate-type groups. However, these groups protonate at pHs below 4 and are removed from the nanoparticle surface, which can cause deterioration of the UCNPs and thus their luminescence. This lack of particle passivation stability at pH < 4 prevents the use of UCNPs in applications requiring acidic media, such as the synthesis of ceramic composites or paint synthesis.

Researchers from the Universitat de València, INNCEINNMAT S.L. and Spanish National Research Council have developed a novel method for the fabrication of upconversion nanoparticles (UCNP) with a stable organic passivate in extremely acidic media, even close to two, thus preserving their luminescent efficiency.

These UCNPs remain dispersed and stable over a wide range of pHs, both basic and acidic, which is of great interest for their technological applications. Thus, a composite of such nanoparticles can be prepared by a sol-gel process, so that the composite material possesses high luminescence, high resistance to chemicals and thermal stability, preferably at temperatures ≥ 400°C.

The present invention enables the use of upconversion nanoparticles in the manufacture of composites that require the presence of acidic media to give rise to a wide range of luminescent materials responsive to infrared light. This invention may be of great interest in the paper, plastic, packaging, textile, automotive, paints, medical-pharmaceutical, document security, tracers, ceramic pigments, optoelectronic or photovoltaic sectors.

Interestingly, the luminescent composite material is colourless, which allows its application as an authentication or quality marker in sectors such as paper, packaging or textiles.

The advantages provided by the invention are summarised as follows:

• Stability of UCNPs in acidic media up to pH 2, allowing their implementation in the preparation of various materials of technological interest.
• High luminescence of UCNPs, allowing the use of low concentrations of the material.
• Transparency of nanoparticles in the visible.
• Excitation of the material using low-cost infrared laser diodes.

• Patent granted