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Developement of robust and quantitative biosensors based on near-infrared two-dyed silicate nanoparticles

Final Report Summary - NIRNANOBIOSENS (Development of robust and quantitative biosensors based on near-infrared two-dyed silicate nanoparticles)

The aim of this project was to prepare near-infrared (NIR) ratiometric sensors based on silicate nanoparticles (NPs). The NIR optical properties provide a better resolution in biosamples, as well as a deeper penetration of the incident light and limit photodamages. Silicate nanoparticles offer highly engineerable platforms and enable a non-invasive probing of the targeted analyte. Ratiometric measurements allow the real-time monitoring of the analyte with a good spatial resolution. These nanosensors (NSs) are developed based on core-shell concept designs. The core is made from a spherical silicate (amorphous silica, zeolite) in which is embedded a reference NIR dye inert towards the targeted analyte as well as any potential interfering species. Then, a sensing shell is built up around this core either through surface immobilisation of an indicator dye or by secondary growth of a silicate shell which embeds the indicator dye.

Four NIR reference NPs have been successfully prepared. In particular, two different dyes supplied by Dyomics GmbH (Jena, Germany), DY680 and DY 751, have been covalently bound to the silica NP network via co-condensation with silica precursors during a modified Stober process. A squaraine dye was successfully synthesised in three steps within the pores of zeolite beta NPs. Another system, mitoxantrone@beta, was also prepared by sorption of the dye (mitoxantrone) to the zeolite NPs in the liquid phase. The yielded host-guest system was found to be stable. All the prepared NPs have been characterised in terms of morphological, structural and optical properties.

A pH-sensitive BODIPY dye was synthesised which gave promising results as a sensor in solution. Unfortunately, we were not able to obtain the compound in a satisfactory purity and attempts to prepare core-shell pH-NSs with the crude product were unsuccessful. Indeed, fluorescence of the indicator dye was largely quenched after immobilisation and no sensitivity to pH was observed. In parallel, NIR cyanine dye derivative bearing a receptor unit selective to Zn(II) was synthesised. Again, product purification was difficult to perform properly and the compound could not be recovered as pure as expected. Nevertheless, studies in solution showed a turn-off luminescence upon addition of Zn(II). The indicator dye, in its most pure form, was then immobilised onto the surface of amino-functionalised DY680-doped silica NPs. Only a preliminary study could have been carried out on time, the resulting NSs showing the expected response upon addition of Zn(II).

The optimisation of the NIR ratiometric NSs also consists of any further modification that can be done - typically at the interface - to improve the stability, the biocompatibility, the cell uptake or the cytosolic delivery. The surface modification with small organic ligands or polymer brushes as well as peptides and biomolecules have been investigated on model NPs. The resulting NPs were characterised in terms of size and surface charge. In particular, it has been evidenced that the grafting of aminosilane provides positive charge to the system at physiological pH. However, preliminary studies on living cells showed no significant difference between the uptake of modified and unmodified NPs. Some investigations in biosamples are still in progress. On another hand, the system mitoxantrone@beta was studied as a potential targetable nanocarrier of a cytostatic (mitoxantrone) on cancer cells showing a comparable activity with the free drug.

Even not accomplished to the best expectations, this project was carried out seriously and the results obtained provide a proof-of-concept towards NIR NSs. Complementary studies might be relevant to reach the initially planned scientific objectives, and beyond, to harvest the fruits of the resulting socio-economic impact.
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