Adsorption of a Cationic Dye Molecule on Polystyrene Microspheres in Colloids:  Effect of Surface Charge and Composition Probed by Second Harmonic Generation

Abstract

Nonlinear optical probe, second harmonic generation (SHG), of the adsorption of the dye molecule malachite green (MG), in cationic form at pH < or = 5, on polystyrene microspheres in aqueous solution is used to study the effect of surface charge and composition on molecular adsorption. Three types of polystyrene microspheres with different surface composition are investigated: (1) a sulfate terminated, anionic surface, (2) a neutral surface without any functional group termination, and (3) an amine terminated, cationic surface. The cationic dye was found to adsorb at all three surfaces, regardless of surface charge. The adsorption free energies, DeltaG's, measured for the three surfaces are -12.67, -12.39, and -10.46 kcal/mol, respectively, with the trend as expected from the charge interactions. The adsorption density on the anionic surface, where attractive charge-charge interaction dominates, is determined by the surface negative charge density. The adsorption densities on the neutral and cationic surfaces are on the other hand higher, perhaps as a result of a balance between minimizing repulsive charge interaction and maximizing attractive molecule-substrate and intermolecular interactions. The relative strength of the SH intensity per molecule, in combination of a model calculation, reveals that the C(2) axis of the MG molecule is nearly perpendicular to the surface on the anionic surface and tilts away from the surface norm when the surface is neutral and further away when cationic. Changing the pH of the solution may alter the surface charge and subsequently affect the adsorption configuration and SH intensity.

Adsorption and transport of charged vs. neutral hydrophobic molecules at the membrane of murine erythroleukemia (MEL) cells

“…This ordering suggests that the electrostatic binding of MG with sulfate groups (SO 3− ) on the PSS surface yields the strongest interaction (−12.7 ± 0.2 kcal mol −1 ), followed next by the carboxyl groups on the MEL cell and PSC surface (−10.9 ± 0.1 kcal mol −1 and −10.7 ± 0.1 kcal mol −1 ), and finally the phosphate head groups (PO 4− ) of the liposome surfaces (−8.6 ± 0.2 kcal mol −1 ). Overall, these results are in agreement with the general understanding of the associated strengths of charge-charge interactions[49][50][51].…”

Probing the colloidal gold nanoparticle/aqueous interface with second harmonic generation

“…Previous SHG studies on the free adsorption of the triphenyl methane dye, malachite green (MG), to the surface of colloidal polystyrene microparticles has allowed the free energy of adsorption as well as the maximum number of adsorbate sites to be determined [29,30]. This SHG research has since been extended to study the adsorption of molecules to a variety of types and sizes of colloidal polymer microparticles and nanoparticles [31][32][33][34][35], clay [36], TiO 2 [37], and carbon black [38] nanoparticles, oil droplets in water [30], as well as phospholipid liposomes bilayer membranes [39,40]. Related work using SHG to measure the surface electrostatic potential [41] and the surface acidity pK a [42] of nanoparticles has also been achieved.…”

Label-free probe of HIV-1 TAT peptide binding to mimetic membranes

“…Second harmonic generation (SHG), as an interface-selective technique (18)(19)(20)(21)(22)(23)(24), does not require a label, and because SHG is sensitive to the interface potential, it is an attractive method to selectively probe the binding of the highly charged (+8) TAT peptide to liposome surfaces. Although coherent SHG is forbidden in centrosymmetric and isotropic bulk media for reasons of symmetry, it can be generated by a centrosymmetric structure, e.g., a sphere, provided that the object is centrosymmetric over roughly the length scale of the optical coherence, which is a function of the particle size, the wavelength of the incident light, and the refractive indexes at ω and 2ω (25)(26)(27)(28)(29)(30). As a secondorder nonlinear optical technique SHG has symmetry restrictions such that coherent SHG is not generated by the randomly oriented molecules in the bulk liquid, but can be generated coherently by the much smaller population of oriented interfacial species bound to a particle or planar surfaces.…”