Nanoparticle research has become important over the past few years mainly because of their potential as drug delivery vehicle. Of metal nanoparticles, noble metal nanoparticles have been studied extensively because many new and important phenomena/properties were discovered in them. Along with this, noble metal nanoparticles gained more importance over other nanoparticles in the biomedical application due to their non-cytotoxicity. In our laboratory we have prepared metal nanoparticles (gold, silica and carbon nanoparticles) in solution via a seed mediated growth method. The nanoparticle size was varied in a controlled manner by varying the concentration of seed nanoparticle and metal precursor. The noble metal nanoparticles have very high nonlinear optical response. The response is enhanced further at the wavelength close to the plasmon resonance band of the nanoparticles. We have used this incoherently scattered resonance enhanced second harmonic response from gold nanoparticles to study the physisorption of proteins on the surface of the nanoparticles. Studying this weak interaction is otherwise not possible with the conventional techniques particularly at concentration where proteins do not aggregate in solution. The size of the gold nanoparticle was varied in the range of 15-60 nm to investigate the effect of size on the energetics of protein adsorption. The binding constant, amount and free energy of adsorption of protein was found to increase with size of the nanoparticles. In some nano-particle protein conjugate the equilibrium adsorption process in entropy driven while in some other combinations enthalpy change drives the adsorption. Currently we have prepared a variety of functionalized nanoparticles when coupled to proteins and DNAs will produce new materials with unique set of properties. Nanoparticle based assays of biologically important species are also looked at.

Nonlinear optical materials have become important with recent advances in optical wave guides and future promises of new photonic device application in telecommunication, signal processing and computer industry. Organic materials offer certain advantages in their possible application in information technology and high-speed data transmission. In our laboratory we measure the second order molecular polarizability (β) of organic and organometallic molecules in solution by the hyper-Rayleigh (two photon Rayleigh) scattering technique. The fundamental of a Nd:YAG laser (1064 nm) and other infrared wavelengths (1540 nm and 1907 nm) generated by stimulated Raman scattering in high pressure gas are used for excitation. Incoherently scattered second harmonic light from the molecules in solution is detected. Apart from measuring β in molecules, hyper - Rayleigh or second harmonic light scattering (HRS or SHLS) can be used to probe the geometry/structure of non covalently bound complexes in solution by using polarisation resolved experiments. By using this, we have measured the geometry of weakly interacting 1:1 molecular complexes such as substituted benzene and chloranil in dichloromethane. We have found that there is a significant tilt and twist between the planes containing the two molecules in the complex. The solution geometry could be altered by changing the temperature of the solution. As the temperature changes from room temperature to lower, one or the other geometry must be stabilized or destabilized. Currently, we are investigating the effect of temperature on solution geometry of different non-covalent complexes which are important in biological and chemical systems e.g., protein-protein or protein-nucleic acid complexes where other spectroscopic techniques do not work.

Molecules which are stable in more than one physical and chemical state can be influenced by external stimuli like pH, solvent and electrochemical potential. These external stimuli can alter the electronic configuration and structure of the molecule. Subsequently, hyperpolarizability of the molecule also varies. Such materials find ideal employability in data storage and optical switching. Organotransition metal complexes hold a promising future in these fields. We study electrochemical and NLO properties of mono and multinuclear Ruthenium complexes, which have two different oxidation states.

Prevention or suppression of protein aggregation is of enormous importance in the context of protein storage, transportation and delivery. Due to aggregation, the protein and peptide based drug formulations lose their efficacy and ability to treat conditions that they are meant for. Traditionally chaperones or other chemically active agents are used to stop or diffuse native protein aggregation. In our laboratory, we have used gold and silica nanoparticles to prevent the aggregation of a few proteins under thermal and chemical denaturation. A second harmonic light scattering assay has been developed to monitor the adsorption of protein on the nanoparticle surface. We have measured the adsorption isotherm using second harmonic light scattering from the Au-NPs in the absence and presence of proteins. The results indicate that the interaction is weak and the free energy change (DG) for the adsorption is of the order of - 55 kJ/mol. We also obtain the number of protein molecules adsorbed per particle from the Langmuir model. Since the interaction is weak, the protein can be detached easily from the nanoparticle surface by applying moderate centrifugal force. By temperature dependent SHLS study we have obtained the changes in enthalpy (DH) and entropy (DS) in the equilibrium adsorption process. The measurements are done at such low concentrations where the calorimetric techniques are not sensitive. In a similar way we also look at protein-protein interaction in two proteins which associate in solution. SHLS study of interaction of spectrin with the three different variants of haemoglobin show that the interactions are different with all the variants and the difference is due to single point mutation in haemoglobin which is picked up by this sensitive nonlinear spectroscopic technique.