Correlation of Dynamic Surface Tension with Sedimentation of PTFE Particles and Water Penetration in Powders.

The dynamic surface tension of aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) [(PEO-PPO-PEO)]-type polymeric surfactant (P103, P105, F108, P123, and F127) solutions were correlated with water penetration in packed Teflon powders, the sedimentation of Teflon suspensions in these solutions, foamability, and contact angle measurements on a Teflon surface. The DST trend with bubble lifetime indicated that the overall slowdown in the diffusion process in aqueous solutions is a function of a higher poly(ethylene oxide) (PEO) molecular weight for a given series of block copolymers containing equal PPO molecular weights, favoring slower diffusion kinetics to the air-water interface caused by preferential partitioning in bulk water. The wettability of poly(tetrafluoroethylene) (PTFE) powder illustrates better water penetration for polymers with low molecular weight and lower HLB values. The wettability of F127 solutions decreases with corresponding increases in concentration resulting from higher viscosity, which restrains the diffusion kinetics at the PTFE-water interface. The foamability decreases drastically with higher PEO molecular weight as attributed by slower diffusion kinetics, leading to a decrease in the effective concentration of molecules at the foam interface. The contact angle on glass and the PTFE surface are in good agreement with assumptions made by other analytical techniques showing a lower value of the contact angle with a lower HLB of the Pluronic, which relates to the higher adsorption of molecules at the interface. It is concluded that the adsorption of molecules at the PTFE-water interface decreases in aqueous Pluronic solutions with corresponding increases in the hydrophilic lipophilic balance (HLB), which is consistent with foaming, water penetration in a packed powder of PTFE, the rate of sedimentation, and DST data. A PTFE dispersion containing P123 showed the maximum wettability and lowest sedimentation among the series of block copolymers introduced, which is attributed to faster diffusion kinetics and a higher PPO contribution fostering faster adsorption at the PTFE surface. The dynamic surface tension of aqueous Pluronic solutions seems to correlate well with the adsorption characteristics at the air-water and PTFE-water interfaces.

Change of boron substitution improves the lasing performance of Bodipy dyes: a mechanistic rationalisation.

Bodipy laser dyes are highly efficient but degrade rapidly in solution by reacting with in situ generated singlet oxygen ((1)O(2)). To increase the lasing lifetimes of these dyes, we have designed and synthesised two different congeners of the widely studied Pyrromethene 567 (PM567) by substitution at the boron centre and/or at both the boron centre and the meso position. The two new dyes showed high lasing efficiencies with increased photostability. The results of theoretical and pulse radiolysis studies revealed that the substitution at the boron centre reduced the (1)O(2) generation capacity of these dyes as well as their rate of reaction with (1)O(2), thereby enhancing their lifetimes even under lasing conditions.

Hydrogen bonding in neutral and cation dimers of H2Se with H2O, H2S, and H2Se.

Structures, hydrogen bonding, and binding energies of H(4)SeA (A = O, S, Se) dimers and their cation radicals have been studied using DFT-B3LYP, MP2, and CCSD methods with 6-31++G(d,p), cc-pVTZ, and aug-cc-pVTZ basis sets. The binding energy (BE) order of the most stable neutral and cationic dimers have been found to be (H(2)Se···HOH) > (H(2)Se···HSH) > (H(2)Se···HSeH), and (H(2)Se···SeH(2))(+) > (H(2)Se···SH(2))(+) > (HSe···HOH(2))(+), respectively, by B3LYP/6-31++G(d,p) and MP2/aug-cc-pVTZ methods. Higher electronegativity of the heteroatom has been found to result in more stability of the neutral dimer but less of the cationic dimer. Among neutral dimers, structure with more electronegative heteroatom acting as proton donor has been found to be more stable. However, the hemibonded structure has been found to be more stable for the dimer cation radical unless the ionization potentials of the involved heteroatoms are very different, e.g., H(4)SeO(+). Vibrational frequency calculation suggests that an increase in electronegativity of A-atom results in a decrease in Se-H bond strength in H(4)SeA and H(4)SeA(+) dimers. The calculated values of Mulliken atomic charge/spin and hydrogen bond lengths of the dimers and their radical cations have also been discussed.

pH and temperature dependent relaxation dynamics of Hoechst-33258: a time resolved fluorescence study.

The photophysical behavior of Hoechst 33258 (H33258) in aqueous solution has been studied by steady-state and time-resolved fluorescence measurements. The intriguing intramolecular geometrical orientations of the dye bring out major modulation on its photophysical behavior, especially in the fluorescence emission characteristics with pH. It has been seen that a change in the solution pH from 7 to 4.5 enhances the emission yield by ~20 fold and this change is ~80-fold on changing the pH from 1.5 to 4.5. While a fast flipping motion among the two benzimidazole rings is considered to be one of the most probable mechanisms for the fast fluorescence decay, a more planar structure of the dicationic form at pH 4.5 having a double bond character between the two benzimidazolium groups is suggested to be the most likely fluorescent species. A similar planar structure is in fact considered to be the fluorescent emitting species of H33258 on minor groove binding to DNA. On the basis of temperature dependent fluorescence decay dynamics explored for the dye in solutions at pH 7 and 4.5, it is understood that a nearly isoenergetic double-well excited state potential is possibly involved in the excited state relaxation dynamics of the dye at pH 7. On increasing the temperature, the conversion to the planar structure is facilitated from the non-planar LE state, enhancing the emission probability of the dye.

Identifying the bond responsible for the fluorescence modulation in an amyloid fibril sensor.

An ultrafast intramolecular bond twisting process is known to be the responsible mechanism for the sensing activity of the extensively used amyloid fibril sensor thioflavin T (ThT). However, it is not yet known which one of the two possible single bonds in ThT is actually involved in the twisting process. To resolve this fundamental issue, two derivatives of ThT have been designed and synthesized and subsequently their photophysical properties have been studied in different solvents. It is understood from the present study that the rotation around the central C-C single bond, and not that around the C-N single bond, is primarily responsible for the sensor activity of ThT. Detailed viscosity-dependent fluorescence studies revealed that the ThT derivative with restricted C-N bond rotation acts as a better sensor than the derivative with free C-N bond rotation. The better sensory activity is directly correlated with a shorter excited-state lifetime. Results obtained from the photophysical studies of the ThT derivatives have also been supported by the results obtained from quantum chemical calculations.

Global minimum-energy structure and spectroscopic properties of I2(*-) x n H2O clusters: a Monte Carlo simulated annealing study.

The vibrational (IR and Raman) and photoelectron spectral properties of hydrated iodine-dimer radical-anion clusters, I(2)(*-) x n H(2)O (n=1-10), are presented. Several initial guess structures are considered for each size of cluster to locate the global minimum-energy structure by applying a Monte Carlo simulated annealing procedure including spin-orbit interaction. In the Raman spectrum, hydration reduces the intensity of the I-I stretching band but enhances the intensity of the O-H stretching band of water. Raman spectra of more highly hydrated clusters appear to be simpler than the corresponding IR spectra. Vibrational bands due to simultaneous stretching vibrations of O-H bonds in a cyclic water network are observed for I(2)(*-) x n H(2)O clusters with n > or = 3. The vertical detachment energy (VDE) profile shows stepwise saturation that indicates closing of the geometrical shell in the hydrated clusters on addition of every four water molecules. The calculated VDE of finite-size small hydrated clusters is extrapolated to evaluate the bulk VDE value of I(2)(*-) in aqueous solution as 7.6 eV at the CCSD(T) level of theory. Structure and spectroscopic properties of these hydrated clusters are compared with those of hydrated clusters of Cl(2)(*-) and Br(2)(*-).

Three component model of cylindrical electric double layers containing mixed electrolytes: A systematic study by Monte Carlo simulations and density functional theory.

The structure of electric double layer around a hard rigid impenetrable cylindrical polyion is studied using density functional theory as well as Monte Carlo simulations. The three component model, presented here, is an extension of solvent primitive model where the solvent molecules are treated as the neutral hard spheres, counterions and coions as the charged hard spheres, all of equal diameters, and in addition the mixture of mono- and multivalent counterions are also considered. The theory is partially perturbative where the hard sphere interactions are treated within the weighted density approach and the corresponding ionic interactions have been evaluated through second-order functional Taylor expansion with respect to the bulk electrolyte. The theoretical predictions in terms of the density profiles and the mean electrostatic potential profiles are found to be in good agreement with the simulation results. The presence of neutral hard spheres incorporate the effects of exclude volume interactions (ionic size correlations) while the mixture of mono- and multivalent counterions enhance the ionic charge correlation effects. Thus, this model study shows clear manipulations of ionic size and charge correlations in dictating the ionic density profiles as well as mean electrostatic potential profiles of the diffuse layer. The behavior of diffused double layer has been characterized at varying ionic concentrations, at different concentration ratios of mono- and multivalent counterions of mixed electrolytes, at different diameters of hard spheres, and at varying polyion surface charge density.

Theoretical study on the spectroscopic properties of CO3(*-).nH2O clusters: extrapolation to bulk.

Vertical detachment energies (VDE) and UV/Vis absorption spectra of hydrated carbonate radical anion clusters, CO(3)(*-).nH(2)O (n=1-8), are determined by means of ab initio electronic structure theory. The VDE values of the hydrated clusters are calculated with second-order Moller-Plesset perturbation (MP2) and coupled cluster theory using the 6-311++G(d,p) set of basis functions. The bulk VDE value of an aqueous carbonate radical anion solution is predicted to be 10.6 eV from the calculated weighted average VDE values of the CO(3)(*-).nH(2)O clusters. UV/Vis absorption spectra of the hydrated clusters are calculated by means of time-dependent density functional theory using the Becke three-parameter nonlocal exchange and the Lee-Yang-Parr nonlocal correlation functional (B3LYP). The simulated UV/Vis spectrum of the CO(3)(*-).8H(2)O cluster is in excellent agreement with the reported experimental spectrum for CO(3)(*-) (aq), obtained based on pulse radiolysis experiments.

Excess entropy scaling of transport properties of Lennard-Jones chains.

Excess-entropy scaling relationships for diffusivity and viscosity of Lennard-Jones chain fluids are tested using molecular dynamics simulations for chain sizes that are sufficiently small that chain entanglement effects are insignificant. The thermodynamic excess entropy S(e) is estimated using self-associating fluid theory (SAFT). A structural measure of the entropy S(2) is also computed from the monomer-monomer pair correlation function, g(m)(r). The thermodynamic and structural estimators for the excess entropy are shown to be very strongly correlated. The dimensionless center-of-mass diffusivities, D(cm) (*), obtained by dividing the diffusivities by suitable macroscopic reduction parameters, are shown to conform to the excess entropy scaling relationship, D(cm) (*)=A(n) exp(alpha(n)S(e)), where the scaling parameters depend on the chain length n. The exponential parameter alpha(n) varies as -(1n) while A(n) varies approximately as n(-0.5). The scaled viscosities obey a similar relationship with scaling parameters B(n) and beta(n) where beta(n) varies as 1n and B(n) shows an approximate n(0.6) dependence. In accordance with the Stokes-Einstein law, for a given chain length, alpha(n)=-beta(n) within statistical error. The excess entropy scaling parameters associated with the transport properties therefore display a simple dependence on chain length.

Molecular solvent model of cylindrical electric double layers: a systematic study by Monte Carlo simulations and density functional theory.

We present the Monte Carlo simulation and density functional study of structure of cylindrical double layers considering solvent as the third component. We have chosen molecular solvent model, where ions and solvent molecules are considered as charged and neutral hard spheres, respectively, having equal diameter. The polyionic cylinder is modeled as an infinite, rigid, and impenetrable charged hard cylinder surrounded by the electrolyte and the solvent spheres. The theory is partially perturbative where the hard-sphere interactions are treated within the weighted density approach, the corresponding ionic interactions have been evaluated through second-order functional Taylor expansion with respect to the bulk electrolyte. The Monte Carlo simulations have been performed in canonical ensemble. The system is studied at varying concentrations of electrolyte ions and the solvent molecules, at different valences of the electrolyte, at different sizes of hard spheres, and at varying surface charge density. The theory and the simulation results are found to be in good agreement at different parametric conditions. The hard-sphere exclusion effects due to molecular nature of the solvent are shown to have special implications in characterizing diffuse layer phenomena such as layering and charge inversion.

Structure of cylindrical electric double layers: a systematic study by Monte Carlo simulations and density functional theory.

We present a systematic study of the structure of cylindrical double layers to envisage the distribution of small ions around a cylindrical polyion through canonical Monte Carlo simulation and density functional theory. The polyion is modeled as an infinite, rigid, and impenetrable charged cylinder surrounded by charged hard spheres of equal diameter modeled for small ions of the electrolyte. The solvent is considered as dielectric continuum. The theory is partially perturbative where the hard sphere contribution to the total excess free energy is evaluated using weighted density approximation, and the ionic interactions are calculated using quadratic Taylor expansion with respect to a uniform fluid. The system is studied over a wide range of parameters, viz., ionic concentrations, valences, and ionic sizes as well as for varying axial charge densities of the polyion. The theoretical predictions are observed to be in good agreement with that of simulation results. Some interesting phenomena relating to the width of the diffuse layer, mean electrostatic potential, and charge inversion have been observed to be dependent on different parametric conditions.

Studies on adsorption of mono- and multi-chromophoric hemicyanine dyes on silver nanoparticles by surface-enhanced resonance Raman and theoretical calculations.

Structural and vibrational properties of mono- and multichromophoric hemicyanine (HC) dyes in solution and adsorbed on silver-coated films have been investigated using optical absorption and resonance Raman scattering techniques, with interpretations aided by theoretical calculations. This is the first report on the Raman spectroscopic studies of multichromophoric HC derivatives. The structure of the monomer, N-propyl-4-(p-N,N-dimethylamino styryl)pyridinium bromide (HC3), and its charged and neutral silver complexes (HC3-Ag) in the ground electronic (S(0)) state were optimized using density functional calculations with the B3LYP method using the 6-31G(*) and LANL2DZ basis sets. The ground state structure of N-hexyl-4-(p-N,N-dimethylamino styryl)pyridinium bromide (HC6) and multichromophoric HC dyes were computed using the HF6-31G(*) method. The negligible shift or broadening observed in the electronic absorption and resonance Raman spectra in solution with increasing size of the HC chromophore suggests that the excitations are localized within individual monomer units in bis and tetra chromophores. However, in the tris chromophore, considerable redshift and broadening were observed, indicating a significant electronic interaction between the nonbonded electrons of the N atom and the aromatic pi-system that is supported by the calculated excitation energies using the time-dependent density functional theory method. The effect of HC dye concentration on the electronic absorption spectra of the silver-coated film showed significant broadening, which was attributed to the formation of H- and J-aggregates in addition to the formation of a metal-molecule complex. A considerable redshift along various vibrations observed in the surface-enhanced resonance Raman scattering (SERRS) spectra of the HC derivatives indicates that adsorption on the silver surface leads to a considerable interaction of the electron rich moiety of HC derivatives with the silver surface. The enhancement of various in-plane and out-of-plane vibrations, along with slight broadening and redshifts observed in the SERRS spectra, suggests that binding of the HC dyes to the silver surface occurs through the aromatic pi-system attached to the electron rich dimethylamino group, with the ring lying almost parallel to the silver surface. Theoretical results have further indicated adsorption via chemisorption for the charged HC3-Ag complexes and by physisorption for the neutral HC3-Ag complexes.

Borosilicate glasses modified with organic ligands: a new selective approach for the removal of uranyl ion.

Barium borosilicate glass was found to have high uptake capacity for many cations. To improve its selectivity, surface modification was carried out. In order to make the glass selective towards uranyl ion, organic ligands like tri-n-octylphosphine oxide (TOPO) and 8-hydroxy quinoline (Oxine) were used. It was observed that the surface modification resulted in the change in uptake property of the glass. The uptake process was faster and within 5 h, 90% of the uranyl ion could be taken up from a 0.01 mM solution. With use of the modified barium borosilicate glass and EDTA as masking agent, uranyl ion could be selectively removed from mixtures of cations.

Enhanced Photocatalytic Efficiency of a Least Active Ag-TiO2 by Amine Adsorption.

An Ag-TiO2 photocatalyst with 3.5 atom % silver content, synthesized by a single step sol-gel method, possessed silver nanoparticles (AgNPs; 1-5 nm) on the TiO2 surface, but owing to the nonplasmonic nature of AgNPs and the wide band gap of TiO2, this material exhibited poor activity in a photocatalytic degradation reaction. However, this least active Ag-TiO2 catalyst showed a sudden increase in activity during a photocatalytic amine self-coupling reaction showing the highest activity, which was interpreted as amine (reactant) adsorption-driven activity enhancement. We found that amine adsorption occurred over AgNPs converting into plasmonic AgNPs as well as on the TiO2 surface reducing the band gap and therefore facilitated the visible light excitation and the electron-transfer process efficiently, resulting into overall enhancement in the photocatalytic activity. Thus, a very efficient, stable, and visible light active photocatalyst (amine-adsorbed Ag-TiO2) was developed by simply adsorbing an amine in the least active Ag-TiO2 photocatalyst.

Compartmentalization of reactants in different regions of sodium 1,4-bis(2-ethylhexyl)sulfosuccinate/heptane/water reverse micelles and its influence on bimolecular electron-transfer kinetics.

Sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micellar medium has been used to study the photoinduced electron-transfer (ET) reactions between some coumarin derivatives and amines, namely, aniline (AN) and N,N-dimethylaniline (DMAN) at different w(0) (w(0) = [water]/[AOT]) values, to explore the appearance of Marcus inversion and also the possible role of w(0), if any, on the Marcus correlation curves. The coumarin derivatives are found to partition between the heptane-like and the water-like phases of the reverse micelles, and their locations have been confirmed by time-resolved anisotropy measurements. Fluorescence quenching is found to depend both on the location of the coumarin molecules and on the hydrophobicity of the amine donors. Various aspects such as the effect of differential partitioning of the quenchers, the location of the probes in the two phases, the diffusion of the reactants in the micellar phase, etc. have been considered to rationalize the fluorescence quenching rates in reverse micelles. Rotational relaxation times and the diffusion parameters estimated from the anisotropy results do not show good correlation with the observed quenching rates indicating that the diffusion of reactants has no role in the quenching kinetics in reverse micelles. Marcus inversion behavior has been observed for the coumarin-amine systems in the water-like phase at a relatively high exergonicity of approximately 1.2 eV suggesting that the solvent reorganization energy contributes fully to the free energy of activation for the ET reactions in the present systems. This is in accordance with the fast solvent relaxation dynamics reported in reverse micelles. Quenching rates in the water-like phase are found to decrease or increase marginally with increasing w(0) for the coumarin-DMAN and coumarin-AN systems, respectively. This is explained on the basis of the changing solubility of these amines in the water-like phase with changing w(0) values of the reverse micelles. In the heptane-like phase, no clear inversion in the quenching rate versus free energy plot could be observed because the study could not be extended to higher exergonicity due to nonsolubility of the dye C151 in this phase. Present results, especially in the water-like phase, suggest that the confinement of reactants in micellar media can effectively remove the influence of reactant diffusion on bimolecular ET rates and thus make the systems more conducive for the observation of the Marcus inverted region.

Free radical scavenging reactions and antioxidant activity of embelin: biochemical and pulse radiolytic studies.

Embelin (from Embelia ribes) is a component of herbal drugs and possess wide range of medicinal properties. These properties may be, in part, due to scavenging of oxidizing free radicals. In this context, free radical scavenging reactions and antioxidant activity of embelin (2,5-dihydroxy-3-undecyl-1,4-benzoquinone) have been studied. It has been found to scavenge DPPH radical and inhibit hydroxyl radical induced deoxyribose degradation. It has been also found to inhibit lipid peroxidation and restore impaired Mn-superoxide dismutase in rat liver mitochondria. Further, kinetics and mechanism of the reactions of embelin with hydroxyl, one-electron oxidizing, organo-haloperoxyl and thiyl radicals have been studied using nanosecond pulse radiolysis technique. Its redox potential has been also evaluated with cyclic voltammetry. These studies suggest that embelin can act as a competitive antioxidant in physiological conditions.

Ascorbic acid monoglucoside as antioxidant and radioprotector.

Ascorbic acid monoglucoside (AsAG), a glucoside derivative of ascorbic acid, has been examined for its antioxidant and radioprotective abilities. AsAG neutralized 1, 1 diphenyl -2-picryl-hydrazyl (DPPH), a stable free radical in a concentration dependent manner thus indicating its antioxidant ability. AsAG protected mice liver tissues in vitro from peroxidative damage in lipids (measured as TBARS) resulting from 25Gy gamma irradiation. It also protected plasmid pBR322 DNA from gamma-radiation induced strand breaks as evidenced from studies on agarose gel electrophoresis of the plasmid DNA after radiation exposure. Oral administration of AsAG to mice prior to whole body gamma radiation exposure (4Gy) resulted in a reduction of radiation induced lipid peroxides in the liver tissue indicating in vivo radiation protection of membranes. Pulse radiolysis studies indicated that AsAG offered radioprotection by scavenging free radicals. The rate constants for the reactions OH and N(3) radicals with AsAG were determined to be 6.4 x 10(9) dm(3) mol(-1) s(-1) and 2.3 x 10(9) dm(3) mol(-1) s(-1), respectively at pH 7. It was observed that AsAG radicals undergo conjugation as the pH of the solution is raised to 11 in the case of a one-electron oxidation reaction. As the OH(*) radical adds to the ring, the conjugation effect starts appearing at pH 10.

Physico-chemical studies on the evaluation of the antioxidant activity of herbal extracts and active principles of some Indian medicinal plants.

Understanding of the efficacy and mechanism for the reaction of the biologically important radicals with natural and/or synthetic antioxidants is the first step towards the development of future therapeutic agents. The kinetic parameters e.g., formation and decay rate constants predict the efficacy of an antioxidant and its fate after reaction. These parameters also dictate the ease with which competing reactions would occur in a bio-environment. The spectroscopic parameters provide the clue to the site of free radical attack to these antioxidants. Here, in this article an attempt has been made to show the use of physico-chemical methods in the evaluation of antioxidant activity of some important medicinal plants commonly used in India and the subcontinent. The systems chosen here for discussions are herbal extracts as such, curcumin from turmeric, methoxy phenols from Indian spices, dehydrogingerdione from ginger and bakuchiol from Psoralea corylifolia. All the examples shown in this article illustrate the potential of the pulse radiolysis coupled with kinetic spectroscopy and other physicochemical techniques for the study of antioxidants either in the form of mixture as in herbal extract or as an isolated compound.

Microenvironment in the corona region of triblock copolymer micelles: temperature dependent solvation and rotational relaxation dynamics of coumarin dyes.

Dynamic Stokes' shift and fluorescence anisotropy measurements using coumarin-153 (C153) and coumarin-151 (C151) as the fluorescence probes have been carried out in aqueous poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 (P123) and poly(ethylene oxide)100-poly(propylene oxide)70-poly(ethylene oxide)100 (F127) block copolymer micelles with an aim to understand the water structures and dynamics in the micellar corona region. It has been established that the probes reside in the micellar corona region. It is indicated that the corona regions of P123 and F127 micelles are relatively less hydrated than the Palisade layers of neutral micelles like Triton-X-100 and Brij-35. From the appraisal of total Stokes' shift values for the probes in the two block copolymer micelles, it is inferred that the F127 micelle is more hydrated than the P123 micelle. It is observed that the dynamic Stokes' shift values for both of the probes remain more or less similar at all the temperatures studied in the P123 micelle. For C153 in F127, however, the observed Stokes' shift is seen to decrease quite sharply with temperature, though it remains quite similar for C151. Moreover, the fraction of the unobserved initial dynamic Stokes' shift is appreciably higher for both the probes in the F127 micelle compared to that in P123. Over the studied temperature range of 293-313 K, the spectral shift correlation function is described adequately by a bi-exponential function. Rotational relaxation times for C153 in both the micelles show a kind of transition at around 303 K. These results have been rationalized assuming collapse of the poly(ethylene oxide) (PEO) blocks and formation of water clusters in the corona region due to dehydration of poly(ethylene oxide) blocks with an increase in temperature. A dissimilar probe location has been inferred for the differences in the results with C153 and C151 probes in F127. Comparison of the microviscosity and the hydration of the block copolymer micelles has also been made with those of the other commonly used neutral micelles, for a better understanding of the results in the block copolymer micelles.

Possible binding sites for biotin stabilized water soluble Ag nanoparticles: an experimental and theoretical study.

The evolution of Ag nanoparticles by photochemical reduction method and the effect of biotin on their UV-Visible absorption spectrum were studied. Surface modification studies were carried out on chemically reduced Ag nanoparticles. ATR-FTIR studies showed that the biotin molecules bind with the surface of Ag nanoparticles through the oxygen of the carboxylate group. Theoretical calculations were carried out on the structure of the biotin and the silver complex of biotin (biotin(-)-Ag+) by optimizing their structures using density functional calculations with the B3LYP method using the LANL2DZ basis set. Theoretical calculations and experimental evidence favors a preferential binding of biotin molecule to Ag nanoparticles through the carboxylate group.