Fluorescent Sulfonate-Based Inorganic-Organic Hybrid Nanoparticles for Staining and Imaging

Sulfonate-based inorganic-organic hybrid nanoparticles (IOH-NPs) with the general saline composition [Gd(OH)]²⁺_n/2[ R_dye(SO₃) _n] ^n- showing optical absorption and emission in the blue to red spectral regime are presented for the first time. All IOH-NPs are prepared via straightforward aqueous synthesis and instantaneously result in colloidally highly stable suspensions with mean particle diameters of 40-50 nm and high zeta potentials (-20 to -40 mV at pH 7.0). Specifically, the IOH-NPs comprise [Gd(OH)]²⁺₂[CSB]^4-, [Gd(OH)]²⁺₂[DB71]^4-, [Gd(OH)]²⁺[NFR]^2-, [Gd(OH)]²⁺[AR97]^2-, and [Gd(OH)]²⁺₂[EB]^4- showing blue, orange, red, and infrared absorption and emission ([CSB]: Chicago Sky Blue; [DB71]: Direct Blue 71; [NFR]: Nuclear Fast Red; [AR97]: Acid Red 97; [EB]: Evans Blue). The novel IOH-NPs are characterized by electron microscopy, dynamic light scattering, infrared spectroscopy, energy-dispersive X-ray analysis, thermogravimetry, elemental analysis, and fluorescence spectroscopy. In vitro studies based on HeLa and HUVEC cells were exemplarily performed with [Gd(OH)]²⁺₂[EB]^4- IOH-NPs and show intense fluorescence and only moderate toxicity at concentrations of 1 to 10 μg/mL. Based on aqueous synthesis, good colloidal stability, absence of severely toxic metals (e.g., Cd²⁺, Pb²⁺), use of molecular dyes that are already known for staining in cell biology and histology, extremely high dye load per nanoparticle (70-80 wt %), and blue to red absorption and fluorescence, the sulfonate-based IOH-NPs can be highly interesting for staining, fluorescence microscopy, and optical imaging.