Ratiometric and reusable fluorescent nanoparticles for Zn²⁺ and H₂PO₄^- detection in aqueous solution and living cells

In this work, three kinds of core-shell silica nanoparticle-based fluorescent materials were prepared based on a modified Stöber-Van Blaaderen method. They were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), dynamic light scattering (DLS), FT-IR, and several other spectroscopic methods. Firstly, The silica@sensor-1 nanoparticle (SSN) showed high selectivity toward Zn²⁺, which can detect Zn²⁺ in aqueous solution and living cells. It also can be reused to detect Zn²⁺ for at least four cycles after a simple regeneration. Secondly, to create a ratiometric measurement platform, the dye-2@silica nanoparticles (DSN), a new class of core-shell fluorescent silica nanoparticles were prepared with an acenaphtho[1, 2-b]pyrrol-9-carbonitrile chromophore derivative as the inner reference. It showed negligible sensing properties towards Zn²⁺, and the fluorescent intensity was not subjected to interference induced by pH change. Thirdly, the dye-2@silica@sensor-1 nanoparticles (DSSN), with the above reference dye buried inside the silica matrix and a layer of chemosensors anchored onto the surface of silica nanoparticles were prepared. DSSN showed not only the same sensing ability as SSN, but also a clear ratiometric fluorescent signal toward Zn ²⁺ in aqueous solutions and living cells. On the other hand, H ₂PO₄^- is a well-known Zn²⁺ binder, so the [DSSN@Zn²⁺] complex was found to ratiometrically detect H ₂PO₄^-. It responded to H₂PO ₄^- at a neutral aqueous solution with a detection limit lower than 6 × 10^-6 M. Moreover, the ratio of fluorescence intensity was linearly increased in the range 6∼500 μM of H ₂PO₄^-, which implies a potential application for the quantitation of H₂PO₄^- in aqueous solution. To the best of our knowledge, this is the first example of core-shell silica nanoparticle-based fluorescent materials that can be repeatedly used to ratiometrically detect Zn²⁺ and H₂PO₄ ^- in 100% neutral aqueous solutions.