Calcium Phosphate and Calcium Phosphosilicate Mediated Drug Delivery and Imaging - Intracellular Delivery: Fundamentals and Applications - page 721

circulation while smaller particles usually remain in circulation for longer time
periods (Kumar 2006 ; Burns 2009 ). Agglomerated particles (and large particles)
are more likely to be sequestered by organs or membranes. Therefore, appropriate
synthetic and dispersion schemes lead to better bioavailability at the tumor site
while minimizing local off-target cytotoxic effects.
The three main classes of dispersion schemes are electrostatic, electrosteric and
steric stabilization. For example, particles can be electrostatically dispersed with
citrate (Li 2010 ; Morgan 2008 ; Kester 2008 ) to yield a negative zeta potential (ZP)
or with aminopropyltriethoxysilane (APTES) to yield a positive ZP or sterically
stabilized with a polymer such as polyethylene glycol (PEG) as shown in Fig. 5
(Morgan 2008 ). Other steric stabilizing molecules commonly used include dex-
trans, poly(vinylpyrrolidone) and polyacrylates (Kumar 2006 ). In addition to stabi-
lization with synthetic molecules, a necessary criterion for successful bio-conjugation
of a target molecule, such as an antigen or a polypeptide for an over-expressed
receptor site on target cells, is to maintain colloidal dispersion during bio-conjugation
and during the subsequent preparation of the particles for in vitro or in vivo delivery
in solutions such as phosphate buffered saline (PBS) (Adair 2010 ; Barth 2010 ;
Morgan 2008 ).
Fig. 5 The effect of the surface functionalization of calcium phosphosilicate nanocomposite
particle (CPSNP) in 70:30 EtOH:H 2 O on zeta potential. The COO - - terminated CPSNPs ( red
line ) have a mean zeta potential of −30 ± 3 mV. The -NH 3 + terminated CPSNPs ( green line ) have
a mean zeta potential of +24 ± 6 mV. The PEG-terminated CPSNPs ( blue line ) have a mean zeta
potential of 3 ± 2 mV (Reproduced from Morgan ( 2008 ). With permission)