Silica/CdTe/silica fluorescent composite nanoparticles via electrostatic assembly as a pH ratiometer

Abstract

In this report, we present a ratiometric pH nanoprobe based on stable fluorescent colloidal silica composite nanoparticles encapsulating hydrophilic CdTe quantum dots. Quantum dots, owing to their fascinating optical properties, have been widely used for sensors and bioimaging. To avoid the inherent chemical instability and serious photoluminescence quenching of quantum dots, a facile electrostatic assembly method is developed to prepare sandwich-like silica/CdTe quantum dots/silica composite nanoparticles stabilized by mercaptopropyl trimethoxysilane (SQMS). This approach is of high efficiency, e.g., 98.9% of quantum dots are instantly adsorbed on the surface of silica nanospheres, and nearly 80% of the original fluorescence of the quantum dots is retained for SQMS while traditional silica coating processes caused dramatic quenching. Finally, the bright SQMS with remarkable stability is modified with pH-sensitive fluorescein isothiocyanate to fabricate a high-resolution pH ratiometric nanoprobe. Above all, SQMS shows a uniform sandwich-like structure, narrow size distribution, and stable fluorescence in strongly acidic and highly salted solutions, and the remarkable stability favours quantitative analyses and nanoprobes.