Fade and quench-resistant emission in calcium phosphate nanoreactors

The fluorescence emission and photodegradation properties of fluorescein dye inside fluid-filled spherical nanoreactors ～ 150 nm in diameter and surrounded by a few nanometres thick layer of calcium phosphate are considered in detail. Steady state, stopped flow, and laser pulsed fluorescence spectroscopies, absorption spectroscopy, dynamic light scattering and electron microscopy were used to characterize the materials as a function of encapsulated dye concentration, particle concentration, illumination time, and pH. Fluorescein tends to form stable J-aggregates inside the nanoreactors. The molecular collision rate constants between the dye aggregates and between the dyes and soluble quenchers are greatly reduced inside the nanoreactors and are responsible for the observed resistance to photodegradation and reduced emission quenching. A model for dye behaviour in nanoreactors is suggested. Nanoreactors can be concentrated to a high suspension concentration, yielding exceptionally strong luminescence affected only by inner filter effects absent particle-particle crosstalk. These and similar nanoreactors can be utilized as building blocks for three-dimensional photo-optical devices, and as versatile and resilient supramolecular chromophores or tracers in complex fluids, cells and microfluidic systems where high resolution visualization is needed.