Calcium Phosphate and Calcium Phosphosilicate Mediated Drug Delivery and Imaging - Intracellular Delivery: Fundamentals and Applications - page 737

2010 ; Doat 2004 ; Lebugle 2006 ; Mondejar 2007 ). Europium ions have an atomic
radii similar to calcium ions and will substitute for calcium in a calcium phosphate
lattice (Doat 2004 ). Eu-doped calcium deficient apatites form when the particles
precipitate in the presence of Eu (Mondejar 2007 ; Doat 2004 ; Lebugle 2006 ;
Al-Kattan 2010 ). Unfortunately, efficient fluorescent behavior requires well crys-
tallized apatites and with increased crystallinity the size of the europium doped
particles are greater than 100 nm in diameter; sacrificing the small size required for
efficient endocytosis (Mondejar 2007 ; Al-Kattan 2010 ).
The europium ion content in the lanthanide-doped apatite particles is cause for
some concern in terms of toxicity for use in biological systems (Doat 2004 ;
Mondejar 2007 ). In vitro studies show cells experience morphological changes
when exposed to the particles, however no toxicity is reported (Mondejar 2007 ). It
should be noted that these particles were developed due to the relative low toxicity
as compared to semiconductor nanoparticles and the Eu 3+ ion has an LD50 dosage
of 5,000 mg/kg in rats (Doat 2003 ).
Calcium phosphosilicate nanoparticles have been shown to be biocompatible
both in vitro and in vivo (Kester 2008 ; Morgan 2008 ; Altınoğlu 2008 ; Barth 2010 ).
The calcium phosphosilicate particles additionally have been shown to not interfere
with calcium channels in even very sensitive cells such as adult rat stellate ganglion
neurons in cell culture (Kester 2008 ).
Association of organic fluorescent molecules with calcium phosphate nanopar-
ticles is accomplished through both surface adsorption and encapsulation during
synthesis. Some synthetic schemes use the fluorophore or DNA as the dispersant
by adsorbing it onto the surface of the particle (Sokolova 2007b ; Ganesan 2008 ).
Calcium phosphates that are dispersed with a fluorescent porphyrin are well dis-
persed spheres but have a large diameter of 250 nm (Ganesan 2008 ). Using DNA
as the dispersant permits transfection of fluorescent nanoparticles into cells but the
particles are poorly dispersed with sizes of agglomerates ranging from tens to hun-
dreds of nanometers (Sokolova 2007a ).
In contrast, fluorescent calcium phosphosilicate nanoparticles restrain the fluo-
rophore within the rigid, inorganic matrix giving the molecule improved optical
performance. The encapsulated molecules show increased quantum efficiency and
resistance to solvatochromic effects (Altınoğlu 2008 ; Morgan 2008 ; Russin 2010 ).
The increase in quantum efficiency is due to the matrix shielding effect, preventing
conformational changes which lead to non-radiative decay (Altınoğlu 2008 ).
Calcium phosphosilicate particles containing Cy3 are used to investigate the route
by which particles are taken up into cells (Morgan 2008 ). As shown in Fig. 10
particles enter the cell through endocytosis followed by particle dissolution in the
late endosome (Morgan 2008 ).
Using fluorescent molecules to image living animals requires the penetration
of tissue by excited and emitted photons. At near infrared wavelengths, absorption
and scattering from tissue components is minimal (Altınoğlu 2010 b) presenting a
window of opportunity for enhanced imaging. Calcium phosphosilicate encapsu-
lated indocyanine green (ICG-CPSNPs) nanoparticles excite and emit within this