Atom Lithography

We review the two kinds of forces that near-resonant light exerts on atoms the spontaneous force that is used for laser cooling, and the stimulated force that is used for coherent manipulation of atoms. We will discuss an experiment where laser cooling is used to collimate an atomic beam of sodium atoms, and the stimulated force within one period of a one-dimensional standing wave is used as a lens to focus the atoms to a narrow line about 20 nm wide. This kind of atom lithography is an example of the general field of atom optics in which light is used to manipulate atoms.

How does contrasting dependence of impurity-atom diffusivity on the density of host disordered medium arise?

We report results of molecular dynamics investigations into neutral impurity diffusing within an amorphous solid as a function of the size of the diffusant and density of the host amorphous matrix. We find that self diffusivity exhibits an anomalous maximum as a function of the size of the impurity species. An analysis of properties of the impurity atom with maximum diffusivity shows that it is associated with lower mean square force, reduced backscattering of velocity autocorrelation function, near-exponential decay of the intermediate scattering function (as compared to stretched-exponential decay for other sizes of the impurity species) and lower activation energy. These results demonstrate the existence of size-dependent diffusivity maximum in disordered solids. Further, we show that the diffusivity maximum is observed at lower impurity diameters with increase in density. This is explained in terms of the Levitation parameter and the void structure of the amorphous solid. We demonstrate that these results imply contrasting dependence of self diffusivity (D) on the density of the amorphous matrix, p. D increases with p for small sizes of the impurity but shows an increase followed by a decrease for intermediate sizes of the impurity atom. For large sizes of the impurity atom, D decreases with increase in p. These contrasting dependence arises naturally from the existence of Levitation Effect.

Dependence of pH and surfactant effect in the synthesis of magnetite(Fe3O4) nanoparticles and its properties

Nanoparticles of Fe3O4 were synthesized by co-precipitation in an aqueous solution containing ferrous and ferric salts (1:2) at varying pH with ammonia as a base. It was found that the value of pH influences the reaction mechanism for the formation of Fe3O4. Furthermore, the addition of mercaptoethanol significantly reduced the crystalline size of Fe3O4 nanoparticles from 15.03 to 8.02 nm. X-ray diffraction (XRD) spectra revealed that the synthesized nanoparticles were epsilon-Fe2O3 or Fe3O4 phase. To further prove the composition of the product, as-prepared Fe3O4 were examined by X-rayphotoelectron spectroscopy (XPS). Magnetic properties of the obtained particles were determined by vibrating sample magnetometer (VSM). Further analysis of the X-ray studies shows that while maintaining a pH value of 6 and 9 in a solution containing iron salts II and III ions produces epsilon-Fe2O3. Whereas a pH value of 11 produces magnetite (Fe3O4) phase. All of these results show that the pH has a major role in the observed phase formation of (Fe3O4) nanoparticles.

Diffusion of flexible, charged, nanoscopic molecules in solution: Size and pH dependence for PAMAM dendrimer

In order to understand self-diffusion (D) of a charged, flexible, and porous nanoscopic molecule in water, we carry out very long, fully atomistic molecular dynamics simulation of PAMAM dendrimer up to eight generations in explicit salt water under varying pH. We find that while the radius of gyration (R-g) varies as N-1/3, the self-diffusion constant (D) scales, surprisingly, as N-alpha, with alpha=0.39 at high pH and 0.5 at neutral pH, indicating a dramatic breakdown of Stokes-Einstein relation for diffusion of charged nanoscopic molecules. The variation in D as a function of radius of gyration demonstrates the importance of treating water and ions explicitly in the diffusion process of a flexible nanoscopic molecule. In agreement with recent experiments, the self-diffusion constant increases with pH, revealing the importance of dielectric friction in the diffusion process. The shape of a dendrimer is found to fluctuate on a nanosecond time scale. We argue that this flexibility (and also the porosity) of the dendrimer may play an important role in determining the mean square displacement of the dendrimer and the breakdown of the Stokes-Einstein relation between diffusion constant and the radius.

Expanding the Peptide beta-Turn in alpha gamma Hybrid Sequences: 12 Atom Hydrogen Bonded Helical and Hairpin Turns

Hybrid peptide segments containing contiguous alpha and gamma amino acid residues can form C-12 hydrogen bonded turns which may be considered as backbone expanded analogues of C-10 beta-turns) found in alpha alpha segments. Exploration of the regular hydrogen bonded conformations accessible for hybrid alpha gamma sequences is facilitated by the use of a stereochemically constrained gamma amino acid residue gabapentin (1-aminomethylcyclohexaneacetic acid, Gpn), in which the two torsion angles about C-gamma-C-beta (theta(1)) and C-beta-C-alpha (theta(2)) are predominantly restricted to gauche conformations. The crystal structures of the octapeptides Boc-Gpn-Aib-Gpn-Aib-Gpn-Aib-Gpn-Aib-OMe (1) and Boc-Leu-Phe-Val-Aib-Gpn-Leu-Phe-Val-OMe (2) reveal two distinct conformations for the Aib-Gpn segment. Peptide 1 forms a continuous helix over the Aib(2)-Aib(6) segment, while the peptide 2 forms beta-hairpin structure stabilized by four cross-strand hydrogen bonds with the Aib-Gpn segment forming a nonhelical C-12 turn. The robustness of the helix in peptide 1 in solution is demonstrated by NMR methods. Peptide 2 is conformationally fragile in solution with evidence of beta-hairpin conformations being obtained in methanol. Theoretical calculations permit delineation of the various C-12 hydrogen bonded structures which are energetically feasible in alpha gamma and gamma alpha sequences.

Photodissociation of isomeric dichloroethylenes in the ultraviolet: Effect of the second chlorine atom substitution on the dynamics

We report the Cl-*(P-2(1/2)) production dynamics in the near-UV dissociation of three isomers (cis-, gem-, and trans-) of dichloroethylene using the conventional resonance enhanced multiphoton ionization technique. Substantial amounts of Cl-* are produced in the wavelength range 222-304 nm. The Cl-* quantum yield (phi(*)) i maximum at 304 nm for all the isomers and phi(*)(cis) is markedly higher than phi(*)(gem) and phi(*)(trans) except at 222 nm. Existence of both direct and indirect dissociation pathways at these wavelengths complicates the Cl* production dynamics. The higher value of phi(*)(cis) originates from a large contribution from direct dissociation via the (n, sigma(*)) state.

Interaction of single-walled carbon nanotubes with poly(propyl ether imine) dendrimers

We study the complexation of nontoxic, native poly(propyl ether imine) dendrimers with single-walled carbon nanotubes (SWNTs). The interaction was monitored by measuring the quenching of inherent fluorescence of the dendrimer. The dendrimer-nanotube binding also resulted in the increased electrical resistance of the hole doped SWNT, due to charge-transfer interaction between dendrimer and nanotube. This charge-transfer interaction was further corroborated by observing a shift in frequency of the tangential Raman modes of SWNT. We also report the effect of acidic and neutral pH conditions on the binding affinities. Experimental studies were supplemented by all atom molecular dynamics simulations to provide a microscopic picture of the dendrimer-nanotube complex. The complexation was achieved through charge transfer and hydrophobic interactions, aided by multitude of oxygen, nitrogen, and n-propyl moieties of the dendrimer. (C) 2011 American Institute of Physics. doi:10.1063/1.3561308]

PAMAM Dendrimer-Drug Interactions: Effect of pH on the Binding and Release Pattern

Understanding the dendrimer-drug interaction is of great importance to design and optimize the dendrimer-based drug delivery system. Using atomistic molecular dynamics (MD) simulations, we have analyzed the release pattern of four ligands (two soluble drugs, namely, salicylic acid (Sal), L-alanine (Ala), and two insoluble drugs, namely, phenylbutazone (Pbz) and primidone (Prim)), which were initially encapsulated inside the ethylenediamine (EDA) cored polyamidoamine (PAMAM) dendrimer using the docking method. We have computed the potential of mean force (PMF) variation with generation 5 (G5)-PAMAM dendrimer complexed with drug molecules using umbrella sampling. From our calculated PMF values, we observe that soluble drugs (Sal and Ala) have lower energy barriers than insoluble drugs (Pbz and Prim). The order of ease of release pattern for these drugs from G5 protonated PAMAM dendrimer was found to be Ala > Sal > Prim > Pbz. In the case of insoluble drugs (Prim and Pbz), because of larger size, we observe much nonpolar contribution, and thus, their larger energy barriers can be reasoned to van der Waals contribution. From the hydrogen bonding analysis of the four PAMAM drug complexes under study, we found intermolecular hydrogen bonding to show less significant contribution to the free energy barrier. Another interesting feature appears while calculating the PMF profile of G5NP (nonprotonated)-PAMAM Pbz and G5NP (nonprotonated)-PAMAM-Sal complex. The PMF was found to be less when the drug is bound to nonprotonated dendrimer compared to the protonated dendrimer. Our results suggest that encapsulation of the drug molecule into the host PAMAM dendrimer should be carried out at higher pH values (near pH 10). When such complex enters the human body, the pH is around 7.4 and at that physiological pH, the dendrimer holds the drug tightly. Hence the release of drug can occur at a controlled rate into the bloodstream. Thus, our findings provide a microscopic picture of the encapsulation and controlled release of drugs in the case of dendrimer-based host-guest systems.

Theoretical analysis of photoinduced H-atom elimination in thiophenol

The photoinduced hydrogen elimination reaction in thiophenol via the conical intersections of the dissociative (1)pi sigma* excited state with the bound (1)pi pi* excited state and the electronic ground state has been investigated with ab initio electronic-structure calculations and time-dependent quantum wave-packet calculations. A screening of the coupling constants of the symmetry-allowed coupling modes at the (1)pi pi*-(1)pi sigma* and (1)pi sigma*-S-0 conical intersection shows that the SH torsional mode is by far the most important coupling mode at both conical intersections. A model including three intersecting potential-energy surfaces (S-0, (1)pi pi*, (1)pi sigma*) and two nuclear degrees of freedom (SH stretch and SH torsion) has been constructed on the basis of ab initio complete-active-space self-consistent field and multireference second-order perturbation theory calculations. The nonadiabatic quantum wave-packet dynamics initiated by optical excitation of the (1)pi pi* and (1)pi sigma* states has been explored for this three-state two-coordinate model. The photodissociation dynamics is characterized in terms of snapshots of time-dependent wave packets, time-dependent electronic population probabilities, and the branching ratio of the (2)sigma/(2)pi electronic states of the thiophenoxyl radical. The dependence of the timescale of the photodissociation process and the branching ratio on the initial excitation of the SH stretching and SH torsional vibrations has been analyzed. It is shown that the node structure, which is imposed on the nuclear wave packets by the initial vibrational preparation as well as by the transitions through the conical intersections, has a profound effect on the photodissociation dynamics. The effect of additional weak coupling modes of CC twist (nu(16a)) and ring-distortion (nu(16b)) character has been investigated with three-dimensional and four-dimensional time-dependent wave-packet calculations, and has been found to be minor. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.4709608]

An experiment that shaped the physics of the century

In this article it is pointed out how the different layers of substructure of matter were revealed to us by experiments which were essentially very similar to the famous α-particle scattering experiment performed by Rutherford. This experiment, which revealed the nuclear structure of an atom, paved the way towards our current understanding of the fundamental constituents of matter and shaped the course of physics for the 20th century.

Ultrafast photoinduced enhancement of nonlinear optical response in 15-atom gold clusters on indium tin oxide conducting film

We show that the third order optical nonlinearity of 15-atom gold clusters is significantly enhanced when in contact with indium tin oxide (ITO) conducting film. Open and close aperture z-scan experiments together with non-degenerate pump-probe differential transmission experiments were done using 80 fs laser pulses centered at 395 nm and 790 nm on gold clusters encased inside cyclodextrin cavities. We show that two photon absorption coefficient is enhanced by an order of magnitude as compared to that when the clusters are on pristine glass plate. The enhancement for the nonlinear optical refraction coefficient is similar to 3 times. The photo-induced excited state absorption using pump-probe experiments at pump wavelength of 395 nm and probe at 790 nm also show an enhancement by an order of magnitude. These results attributed to the excited state energy transfer in the coupled gold cluster-ITO system are different from the enhancement seen so far in charge donor-acceptor complexes and nanoparticle-conjugate polymer composites.

pH Induced switching in hydrogel coated fiber bragg grating sensor

In this paper we report a novel hydrogel functionalized optical Fiber Bragg Grating (FBG) sensor based on chemo-mechanical-optical sensing, and demonstrate its specific application in pH activated process monitoring. The sensing mechanism is based on the stress due to ion diffusion and polymer phase transition which produce strain in the FBG. This results in shift in the Bragg wavelength which is detected by an interrogator system. A simple dip coating method to coat a thin layer of hydrogel on the FBG has been established. The gel consists of sodium alginate and calcium chloride. Gel formation is observed in real-time by continuously monitoring the Bragg wavelength shift. We have demonstrated pH sensing in the range of pH of 2 to 10. Another interesting phenomenon is observed by swelling and deswelling of FBG functionalized with hydrogel by a sequence of alternate dipping between acidic and base solutions. It is observed that the Bragg wavelength undergoes reversible and repeatable pH dependent switching.

pH sensing by single and multi-layer hydrogel coated Fiber Bragg Grating

In this paper we show a novel chemo-mechanical-optical sensing mechanism in single and multi-layer hydrogel coated Fiber Bragg Grating (FBG) and demonstrate specific application in pH activated processes. The sensing device is based on the ionizable monomers inside the hydrogel which reversibly dissociates as a function of the pH and consequently resulting in osmotic pressure difference between the gel and the solution. This pressure gradient causes the hydrogel to deform which in turn induces secondary strain on the FBG sensor resulting in shift in the Bragg wavelength. We also report on the sensitivity factor of single and multilayer hydrogel coated FBG at various different pH.