In the present study, we have tried to establish the correlation between changes in Zeta potential with that of cell surface permeability using bacteria (Escherichia coli and Staphylococcus aureus). An effort has been made to establish Zeta potential as a possible marker for the assessment of membrane damage, with a scope for predicting alteration of cell viability. Cationic agents like, cetyl trimethyl ammonium bromide and polymyxin B were used for inducing alteration of Zeta potential, and the changes occurring in the membrane permeability were studied. In addition, assessment of poly-dispersity index (PDI), cell viability along with confocal microscopic analysis were performed. Based on our results, it can be suggested that alteration of Zeta potential may be correlated to the enhancement of membrane permeability and PDI, and it was observed that beyond a critical point, it leads to cell death (both Gram-positive and Gram-negative bacteria). The present findings can not only be used for studying membrane active molecules but also for understanding the surface potential versus permeability relationship.

In the present study, Au nanoparticle based surface energy transfer (SET) has been used to measure conformational changes in proteins. A significant photoluminescence (PL) quenching (91-97%) of tryptophan intensities of bovine serum albumin (BSA) protein is observed in the presence of Au nanoparticles, and the measured distances (r) between the donor (tryptophan) and the acceptor (Au nanoparticle) are 27.0, 22.9, and 25.7 Å for E, N, and B forms of BSA protein, respectively. Results indicate that Au nanoparticle quenches BSA fluorescence mainly through a static quenching mechanism. Analysis suggests that binding constant and bound/unbound ratio varies with changing the conformation of protein. The PL quenching of dye varies from 47.2 to 86.6% with changing the conformation of protein without changing the radiative rate of dye. The measured distances (d) between the donor (dye) and the acceptor (Au nanoparticle) are 116.5, 76.1, and 86.4 Å for E, N, and B forms of BSA protein, respectively, using the efficiency of surface energy transfer (SET) which follows 1/d 4 distance dependence. The estimated radii of different conformations of the protein nicely match with the reported values of hydrodynamic radii of different conformations of BSA protein. Therefore, such bioconjugated Au nanoparticle based surface energy transfer should have great potentials for optical-based molecular ruler.

We demonstrate the synthesis of Au nanostar dimers with tunable interparticle gap and controlled stoichiometry assembled on DNA origami. Au nanostars with uniform and sharp tips were immobilized on rectangular DNA origami dimerized structures to create nanoantennas containing monomeric and dimeric Au nanostars. Single Texas red (TR) dye was specifically attached in the junction of the dimerized origami to act as a Raman reporter molecule. The SERS enhancement factors of single TR dye molecules located in the conjunction region in dimer structures having interparticle gaps of 7 and 13 nm are 2 × 10 and 8 × 10, respectively, which are strong enough for single analyte detection. The highly enhanced electromagnetic field generated by the plasmon coupling between sharp tips and cores of two Au nanostars in the wide conjunction region allows the accommodation and specific detection of large biomolecules. Such DNA-directed assembled nanoantennas with controlled interparticle separation distance and stoichiometry, and well-defined geometry, can be used as excellent substrates in single-molecule SERS spectroscopy and will have potential applications as a reproducible platform in single-molecule sensing.

Here, the authors report the energy transfer from rhodamine 6G dyes to gold nanoparticles. There is a pronounced effect on the photoluminescence and a shortening of the lifetime of the dye when interacting with the Au nanoparticles. The calculated distance (d) between the donor and acceptor varies from 86.06to102.47Å with changing the concentrations of Au and dye. Analysis suggests that the energy transfer from dye to the Au nanoparticles is a surface energy transfer process and follows a 1∕d4 distance dependence.

Biodiversity contributes significantly towards human livelihood and development and thus plays a predominant role in the well being of the global population. According to WHO reports, around 80 % of the global population still relies on botanical drugs; today several medicines owe their origin to medicinal plants. Natural substances have long served as sources of therapeutic drugs, where drugs including digitalis (from foxglove), ergotamine (from contaminated rye), quinine (from cinchona), and salicylates (willow bark) can be cited as some classical examples.Drug discovery from natural sources involve a multifaceted approach combining botanical, phytochemical, biological, and molecular techniques. Accordingly, medicinal-plant-based drug discovery still remains an important area, hitherto unexplored, where a systematic search may definitely provide important leads against various pharmacological targets.Ironically, the potential benefits of plant-based medicines have led to unscientific exploitation of the natural resources, a phenomenon that is being observed globally. This decline in biodiversity is largely the result of the rise in the global population, rapid and sometimes unplanned industrialization, indiscriminate deforestation, overexploitation of natural resources, pollution, and finally global climate change.Therefore, it is of utmost importance that plant biodiversity be preserved, to provide future structural diversity and lead compounds for the sustainable development of human civilization at large. This becomes even more important for developing nations, where well-planned bioprospecting coupled with nondestructive commercialization could help in the conservation of biodiversity, ultimately benefiting mankind in the long run.Based on these findings, the present review is an attempt to update our knowledge about the diverse therapeutic application of different plant products against various pharmacological targets including cancer, human brain, cardiovascular function, microbial infection, inflammation, pain, and many more.

The interaction of dyes and metallic nanostructures strongly affects the fluorescence and can lead to significant fluorescence enhancement at plasmonic hot spots, but also to quenching. Here we present a method to distinguish the individual contributions to the changes of the excitation, radiative and non-radiative rate and use this information to determine the quantum yields for single molecules. The method is validated by precisely placing single fluorescent dyes with respect to gold nanoparticles as well as with respect to the excitation polarization using DNA origami nanostructures. Following validation, measurements in zeromode waveguides reveal that suppression of the radiative rate and enhancement of the non-radiative rate lead to a reduced quantum yield. Because the method exploits the intrinsic blinking of dyes, it can generally be applied to fluorescence measurements in arbitrary nanophotonic environments.

Here, we study the human serum albumin (HSA) protein–Au nanoparticle interaction to identify the specific binding site of protein with nanoparticles by using the surface energy transfer (SET) method among tryptophan (Trp) of HSA, ANS-dye-labeled HSA protein, and Au nanoparticles. Here, ANS dye is used as a probe located at domain IIIA of HSA. In particular, absorbance, fluorescence quenching, decay time, circular dichroism, dynamic light scattering, and TEM measurements are performed to understand the physical properties of protein-conjugated Au nanoparticles. Using the SET method, the measured distances between the Trp residue of HSA and the binding site of HSA interacting with Au nanoparticles are 42.5, 41.9, and 48.1 Å for 1.5, 2.0, and 2.9 nm HSA-conjugated Au nanoparticles, respectively. The measured distances between the binding site of ANS dye (located at domain IIIA) in HSA to the binding site of HSA interacting with Au nanoparticles are 51, 51.5, and 54.7 Å for 1.5, 2.0, and 2.9 nm HSA-conjugated Au nanoparticles, respectively. From the protein structural data (using PyMol software), the distances from the center of domain IIIA to Cys53–Cys62 disulfide bond and Trp to Cys53–Cys62 disulfide bond are obtained to be 51.5 and 39.1 Å, respectively. Thus, the distances calculated by using SET equation (Trp to Au binding site distance and ANS to Au binding site distance) nicely match with the distances obtained from protein structural data by using PyMol software. Analysis suggests that the Au nanoparticle is attached to HSA by linkage through Cys53–Cys62 disulfide bond which is located at subdomain IA of HSA.

This feature article highlights the recent developments of energy transfer processes of Au-nanoparticle-based assemblies. Many recent studies reveal that the energy transfer from dye to Au nanoparticle is a surface energy transfer process as established from 1/d4 distance dependence. Such distance dependent energy transfer phenomenon serves as spectroscopic ruler for long distance measurement. Recently, energy transfer processes in Au nanoparticle assemblies have been used to understand specific binding site and conformational changes of protein, DNA hybridization, RNA folding/unfolding, metal ion detection, and designing of new optical-based materials using porous materials. Here we highlight various aspects of energy transfer between dye molecule and Au nanoparticle, particularly focusing on the size- and shape-dependent energy transfer, understanding the interactions between biomolecules (protein, DNA, and RNA) and Au nanoparticle and the energy transfer between confined dye and Au nanoparticle. The designing of nanostructures materials with efficient energy transfer between confined dye in porous materials (mesoporous silica, zeolites, and cyclodextrin) and Au nanoparticle for developing new photonic devices has also been highlighted. Interesting findings reveal that Au-nanoparticle-based energy transfer offers an exciting opportunity to overcome many obstacles and this will help to solve the challenging problems for future applications. Finally, a tentative outlook on future developments of this research field is given.