Lysozyme adsorption on carbonaceous nanoparticles probed by second harmonic light scattering.

The first hyperpolarizability (ß) of two different sizes (15 and 35 nm) of carbonaceous nanoparticle (CNP) is reported for the first time using second harmonic light scattering (SHLS). The ß values of the CNPs were found to be larger than those of organic molecules like pNA but lower than those of plasmonic nanoparticles like gold and silver. SHLS was further used to investigate the adsorption of a model protein Lysozyme (Lyz) on these CNPs, which is crucial for the design of safe and effective CNP-based therapeutics. The change in SH intensity from the CNPs on the addition of Lyz was recorded and fitted to the modified Langmuir adsorption model (MLM). The binding constant, free energy changes and surface coverage values show that Lyz is physisorbed on the CNPs forming less than a monolayer. The temperature dependent SH intensity measurements enabled direct determination of enthalpy and entropy changes for Lyz adsorption. The enthalpy and entropy changes reveal that Lyz adsorption is endothermic and entropically driven.

Thermodynamics of multilayer protein adsorption on a gold nanoparticle surface.

We report the thermodynamics of protein adsorption on negatively charged colloidal gold nanoparticles (GNPs) of 16 nm to 69 nm at pH 7.0. Three biologically important proteins of varying size, namely tetrameric alcohol dehydrogenase (ADH) from Saccharomyces cerevisiae, bovine serum albumin (BSA), and insulin from the porcine pancreas were taken and their adsorption on the GNP surface was investigated by dynamic light scattering (DLS), absorption spectroscopy and zeta potential measurements. The hydrodynamic size of the GNPs was found to increase with protein addition. At very low protein concentrations, the adsorption of these proteins was earlier described by Langmuir-type adsorption isotherms. However, when the protein concentration is raised, the adsorption data are found to fit the BET (Brunauer, Emmett, and Teller) adsorption isotherm well, indicating multilayer formation on the GNP surface. The equilibrium binding constants: KS (monolayer) ∼108-109 M-1, KL (subsequent layers) ∼105-106 M-1, and monolayer thickness are obtained from our measurements. KS is found to increase with the size of the GNPs, while KL does not change significantly with either the size of the GNPs or that of the protein. KL, which originates from protein-protein interaction, compares very well with the protein dimerization constant in solution, indicating that the interaction beyond the first layer is not significantly influenced by the nanoparticles. The first layer forms the soft corona on the GNP surface since this layer, although stable, is removable by centrifugation. But the subsequent layers are weakly bound and we call it an ultra-soft corona in order to distinguish it qualitatively from the strongly bound first layer. The modest magnitudes of Gibbs free energy changes of 43-55 kJ mol-1 for the first layer and 31-35 kJ mol-1 for the subsequent layers indicate that ADH, BSA, and insulin are all physiosorbed on the GNP surface. The temperature dependent DLS data point out that the adsorption of these proteins on GNPs is endothermic but is accompanied by a large increase in entropy. The nature of the BET fits to the adsorption data shows that multilayer adsorption starts even before the monolayer formation is complete, and it does not occur in a layer-by-layer sequence.

Probing Polymer Chain Folding in Solution Using Second Harmonic Light Scattering.

Periodically grafted amphiphilic copolymers (PGACs) were earlier shown by us to adopt a zigzag folded conformation in the solid state, which enabled the backbone and pendant segments to segregate and occupy alternate layers in a lamellar structure. The conformational transition from a random coil to a zigzag folded chain in solution is an interesting problem, which is largely unexplored. To examine this, an orthogonally clickable parent polyester was sequentially clicked with two types of poly(ethylene glycol) (PEG) segments: one is a simple PEG and the other is a PEG that carries a dipolar chromophore. These two hydrophilic PEG segments, installed in a periodic and alternating fashion along the hydrocarbon-rich (HC) polyester backbone, ensure that the Janus folded chains are formed upon folding and carry chromophoric dipoles oriented along the same direction, thereby generating a large net dipole. The folding-induced alignment of chromophores in solution was followed using second harmonic light scattering (SHLS), wherein the intensity of the frequency-doubled scattered light (I2ω) is measured. Folding was induced by adding a polar solvent, like methanol, to a chloroform solution of the polymer; methanol desolvates the HC backbone but solubilizes the pendant PEG segments, thus inducing folding. The second harmonic intensity (I2ω) increased initially with methanol concentration and then saturated; in contrast, I2ω remained invariant with the solvent composition in the case of an analogous model chromophore. Furthermore, in a model PGAC carrying chromophore-bearing PEG segments on every repeat unit, I2ω decreased with increasing methanol composition, revealing the formation of a centrosymmetric folded chain, wherein the chromophoric dipoles on either side cancel each other. Thus, this study clearly reveals that the zigzag chain folding of PGACs can be induced by a segment-selective solvent, resulting in the rather elusive directional ordering of chromophoric dipoles in solution.

Linear and Nonlinear Optical Properties of Tricyanopropylidene-Based Merocyanine Dyes: Synergistic Experimental and Theoretical Investigations.

New merocyanines dyes with tricyanopropylidene-based acceptor units connected to dihexylaminophenyl or dihexylaminothiophenyl donor moieties through polyenic bridges of different lengths have been designed. All derivatives exhibited a strong dipolar character and showed a typical intramolecular charge transfer (ICT) transition. NMR spectroscopy experiments combined with DFT calculations demonstrated that both the nature of the donor-acceptor pair and the length of the conjugated linker strongly impact the electronic structure of the dyes and induce alteration in the bond-length alternation (BLA) and marked shifts in the ICT absorption bands. Hyper-Rayleigh scattering experiments revealed an exponential increase in the second-harmonic generation response as the polyenic chain length was increased. Strikingly, the largest chromophores with the strongest donor-acceptor pair exhibited a very high first hyperpolarizabilty together with a cyanine-like electronic structure, which apparently contradicts the paradigm of optimal BLA predicted by the two-state model. Although it decreased as the polyenic chain length increased, all dyes also exhibited high thermal stability, which demonstrates their potential for applications in nonlinear optical devices.

Combination of Cyanine Behaviour and Giant Hyperpolarisability in Novel Merocyanine Dyes: Beyond the Bond Length Alternation (BLA) Paradigm.

Merocyanine dyes that exhibit antithetic cyanine-like behaviour and giant first-order hyperpolarisability (ß) values have been designed. These cyanine-type dyes open up an intriguing route towards molecular-based electro-optic materials as well as new second-harmonic generation dyes for imaging.

Chemical applications of hyper-Rayleigh scattering in solution.

Hyper-Rayleigh or second harmonic Rayleigh scattering (HRS or SHLS) has been employed to measure equilibrium physical properties of molecules in solution. It has been utilized in our laboratory to obtain the dissociation constants of weak organic acids such as substituted benzoic and cinnamic acids in solution. Partition coefficient, supramolecular structure formation and stoichiometry, critical micelle concentration, and binding or association constant of a molecule with another molecule have all been measured by the HRS technique by us. We find that both the symmetry of the molecule and its concentration in the medium play an important role in finding new chemical applications of the technique.