In this paper, foam-structured fluorescent mesoporous silica nanoparticles (FMSNs) are produced in a sol-gel method with the introduction of a phosphonate functional group. It is found that the phosphonate functionalized FMSNs with the foam structure minimizes the aggregation of FMSNs in solution. The average particle size of the FMSNs without and with phosphonate functionalization is 46.3 +/- 5 nm and 60.5 +/- 8 nm in diameter, respectively. The latter one exhibits higher fluorophore loading capacity (similar to 67 +/- 2.5%). The excitation wavelength (lambda(ex)) of FMSNs is observed at 526 nm, approximate 12 nm larger in the Stoke-shift compared to the free organic dye at 494/514 nm. Furthermore, the photostability of the hydrophobic fluorophore is greatly improved by the FMSNs with the foam structure. In addition, the dose-dependent nature of FMSN uptake is assessed for the immune cells, the bone marrow-derived dendritic immune cells (BMDCs). Our results indicate that approximately 42% of BMDCs are able to take up foam-structured FMSNs (>5 mu g/ml) without decreasing the viability of BMDCs. Thus, the phosphonate functionalized FMSNs with the foam structure are suitable to be used for many biomedical applications, especially in cell tracking. (C) 2010 Elsevier Inc. All rights reserved.