Abstract: A method for imaging leverages the fluorescence lifetime of a fluorescent Si-containing particle to distinguish from background fluorescence. A particle is introduced into tissue. An excitation light pulse is applied to excite luminescence from the fluorescent Si-containing particle. Time-gated measuring of a responsive luminescence signal identifies the particle. In preferred embodiments the particle is coated or encapsulated with an organic material. The fluorescence lifetime of particles can be controlled during manufacture, such as by oxidation levels, quenching treatments, or by aging. This permits introducing and using groups of particles in imaging that have unique lifetimes and multiple time gating can be used to identify different particles or to monitor the change in lifetime of a single set of particles as they respond to a biochemical stimulus. The particles can also be functionalized for affinity to particular tissues and can be loaded with treatment molecules.