The surface charge of semiconductor nanoparticles, Q, is an important parameter which determines their electrokinetic behavior, stability in water and polar solvents, functions of optical and electronic devices, self-assembly properties, and interactions with cell membranes. We have developed a simple method for quantitative determination of Q in their native aqueous environment. The method does not require the knowledge of exact atomic structure or make assumptions about effects of drying on charge distribution. The method is based on titration of nanoparticle dispersion with a solution of oppositely charged polyelectrolyte. The point of complete neutralization is recognized as an inflection point on the dependence of fluorescence intensity on the amount of polyelectrolyte added. Thioglycolic acid-stabilized CdTe nanoparticles 2 nm in diameter were found to carry an average Q from -2.6 to -5.5 for pH 7.5 to 10, respectively. This charge is found to be smaller than that calculated theoretically for an analogous structure (i.e., Q = -8), presumably due to adsorption of Cd(2+) ions on the stabilizer shell and on Te atoms with unsaturated valence located on the side planes of CdTe tetrahedrons.
This paper involves small unilamellar liposomes (SULs) of phosphatidylcholine (PC) imparted with a
negative charge via the incorporation of cardiolipin (CL2-
), a lipid with a double-negative charge. Such
liposomes in water strongly adsorb polycations such as CP(2) (a 93/7 copolymer of N-ethyl-4-vinylpyridinium
bromide and 4-vinylpyridine) or CP(2,16) (a 83/3/14 copolymer of N-ethyl-4-vinylpyridinium bromide,
N
-cetyl-4-vinylpyridinium bromide, and 4-vinylpyridine). Neither CP(2) nor CP(2,16) disrupts the integrity
of the PC−CL2- liposome despite the tight binding. Addition of NaCl is able to totally dissociate CP(2)
from the negative SULs, whereas CP(2,16) is only partially dissociated even at high salt concentrations.
Thus, only 3% long-chain pendant groups on the polycation is sufficient to immobilize the polycation onto
the negative surface of SULs. Similarly, poly(acrylic acid) (PAA) in its anionic state will remove CP(2)
from the liposome surface to form a CP(2)−PAA complex. CP(2,16), on the other hand, is more resistant
to removal. Evidence is provided that the CP(2,16) associates with PAA but nonetheless remains on the
surface of the liposome. Thus, a liposome−CP(2,16)−PAA ternary complex is created. This work makes
heavy use of photon correlation spectroscopy, conductometry, electrophoretic mobility, and fluorescent
labeling of the liposomes.
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