Development of organically modified silica nanoparticles for monitoring the intracellular level of oxygen using a frequency-domain FLIM platform

Abstract

Monitoring cellular homeostasis is one of the crucial elements in understanding the real causes of the pathological state and in designing more efficient treatments. Fluorescent nanometer sized particles have great potential in the quantitative real-time analysis of important cellular analytes. In this paper we focus on the development of optical chemical nanosensors for probing dissolved oxygen inside living cells. The nanosensor is composed of organically modified silica nanoparticles, doped with a luminescent oxygen-sensitive [Ru(II)-tris(4,7-diphenyl-1,10-phenanthroline)] ([Ru(dpp)₃]²⁺) complex. A monodisperse population of nanoparticles with an average size of 70 nm was obtained based on a modified sol–gel-based Stöber method. The nanoparticles were initially calibrated in water using a phase fluorometry setup. Very good repeatability from cycle to cycle and reversibility in oxygen response was obtained for the nanoparticles. The excellent performance of the nanosensors is reflected in their very low limit of detection (LOD) (0.007 ppm). Transmission electron microscopy images obtained from in vitro studies reveals that the final intracellular location of the nanoparticles is in the lysosomes. The performance of the nanoparticles was tested inside cells using Fluorescence Lifetime Imaging Microscopy (FLIM) instrumentation. Despite a decrease in the oxygen sensitivity of the nanoparticles located in the intracellular milieu (LOD = 0.226 ppm), a significant change in lifetime (∼1.3 μs) is detected within the physiological oxygen concentration range. Moreover, nanoparticles show no cytotoxic effect when incubated with cells for up to 72 h. These results demonstrate therefore the great potential of ([Ru(dpp)₃]²⁺)-doped organically modified silica nanoparticles for monitoring the intracellular oxygen concentration.