Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Luminogenic materials with aggregation-induced emission (AIE) attributes have attracted much interest since the debut of the AIE concept in 2001. In this critical review, recent progress in the area of AIE research is summarized. Typical examples of AIE systems are discussed, from which their structure–property relationships are derived. Through mechanistic decipherment of the photophysical processes, structural design strategies for generating new AIE luminogens are developed. Technological, especially optoelectronic and biological, applications of the AIE systems are exemplified to illustrate how the novel AIE effect can be utilized for high-tech innovations (183 references).

Aggregation-induced emission

This critical review provides a processing-structure-property perspective on recent advances in cellulose nanoparticles and composites produced from them. It summarizes cellulose nanoparticles in terms of particle morphology, crystal structure, and properties. Also described are the self-assembly and rheological properties of cellulose nanoparticle suspensions. The methodology of composite processing and resulting properties are fully covered, with an emphasis on neat and high fraction cellulose composites. Additionally, advances in predictive modeling from molecular dynamic simulations of crystalline cellulose to the continuum modeling of composites made with such particles are reviewed (392 references).

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Cellulose nanomaterials review: structure, properties and nanocomposites

Cellulose constitutes the most abundant renewable polymer resource available today. As a chemical raw material, it is generally well-known that it has been used in the form of fibers or derivatives for nearly 150 years for a wide spectrum of products and materials in daily life. What has not been known until relatively recently is that when cellulose fibers are subjected to acid hydrolysis, the fibers yield defect-free, rod-like crystalline residues. Cellulose nanocrystals (CNs) have garnered in the materials community a tremendous level of attention that does not appear to be relenting. These biopolymeric assemblies warrant such attention not only because of their unsurpassed quintessential physical and chemical properties (as will become evident in the review) but also because of their inherent renewability and sustainability in addition to their abundance. They have been the subject of a wide array of research efforts as reinforcing agents in nanocomposites due to their low cost, availability, renewability, light weight, nanoscale dimension, and unique morphology. Indeed, CNs are the fundamental constitutive polymeric motifs of macroscopic cellulosic-based fibers whose sheer volume dwarfs any known natural or synthetic biomaterial. Biopolymers such as cellulose and lignin and † North Carolina State University. ‡ Helsinki University of Technology. Dr. Youssef Habibi is a research assistant professor at the Department of Forest Biomaterials at North Carolina State University. He received his Ph.D. in 2004 in organic chemistry from Joseph Fourier University (Grenoble, France) jointly with CERMAV (Centre de Recherche sur les Macromolecules Vegetales) and Cadi Ayyad University (Marrakesh, Morocco). During his Ph.D., he worked on the structural characterization of cell wall polysaccharides and also performed surface chemical modification, mainly TEMPO-mediated oxidation, of crystalline polysaccharides, as well as their nanocrystals. Prior to joining NCSU, he worked as assistant professor at the French Engineering School of Paper, Printing and Biomaterials (PAGORA, Grenoble Institute of Technology, France) on the development of biodegradable nanocomposites based on nanocrystalline polysaccharides. He also spent two years as postdoctoral fellow at the French Institute for Agricultural Research, INRA, where he developed new nanostructured thin films based on cellulose nanowiskers. Dr. Habibi’s research interests include the sustainable production of materials from biomass, development of high performance nanocomposites from lignocellulosic materials, biomass conversion technologies, and the application of novel analytical tools in biomass research. Chem. Rev. 2010, 110, 3479–3500 3479

Cellulose nanocrystals: chemistry, self-assembly, and applications.

Detection of chemical and biological agents plays a fundamental role in biomedical, forensic and environmental sciences1–4 as well as in anti bioterrorism applications.5–7 The development of highly sensitive, cost effective, miniature sensors is therefore in high demand which requires advanced technology coupled with fundamental knowledge in chemistry, biology and material sciences.8–13

In general, sensors feature two functional components: a recognition element to provide selective/specific binding with the target analytes and a transducer component for signaling the binding event. An efficient sensor relies heavily on these two essential components for the recognition process in terms of response time, signal to noise (S/N) ratio, selectivity and limits of detection (LOD).14,15 Therefore, designing sensors with higher efficacy depends on the development of novel materials to improve both the recognition and transduction processes. Nanomaterials feature unique physicochemical properties that can be of great utility in creating new recognition and transduction processes for chemical and biological sensors15–27 as well as improving the S/N ratio by miniaturization of the sensor elements.28

Gold nanoparticles (AuNPs) possess distinct physical and chemical attributes that make them excellent scaffolds for the fabrication of novel chemical and biological sensors (Figure 1).29–36 First, AuNPs can be synthesized in a straightforward manner and can be made highly stable. Second, they possess unique optoelectronic properties. Third, they provide high surface-to-volume ratio with excellent biocompatibility using appropriate ligands.30 Fourth, these properties of AuNPs can be readily tuned varying their size, shape and the surrounding chemical environment. For example, the binding event between recognition element and the analyte can alter physicochemical properties of transducer AuNPs, such as plasmon resonance absorption, conductivity, redox behavior, etc. that in turn can generate a detectable response signal. Finally, AuNPs offer a suitable platform for multi-functionalization with a wide range of organic or biological ligands for the selective binding and detection of small molecules and biological targets.30–32,36 Each of these attributes of AuNPs has allowed researchers to develop novel sensing strategies with improved sensitivity, stability and selectivity. In the last decade of research, the advent of AuNP as a sensory element provided us a broad spectrum of innovative approaches for the detection of metal ions, small molecules, proteins, nucleic acids, malignant cells, etc. in a rapid and efficient manner.37



Figure 1

Physical properties of AuNPs and schematic illustration of an AuNP-based detection system.



In this current review, we have highlighted the several synthetic routes and properties of AuNPs that make them excellent probes for different sensing strategies. Furthermore, we will discuss various sensing strategies and major advances in the last two decades of research utilizing AuNPs in the detection of variety of target analytes including metal ions, organic molecules, proteins, nucleic acids, and microorganisms.

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Gold nanoparticles in chemical and biological sensing.

The volatile nature of olfactory compounds has led to the development of pro-fragrances, which slowly release the active fragrance molecules upon cleavage of a chemical bond to a substrate. Based on the hypothesis that monosaccharide motifs could serve to effectively anchor pro-fragrances on cotton, which is an important requirement for use in laundry products, we investigated new galactose-based pro-fragrances. A retro 1,4-Michael-type reaction was employed as the release mechanism. Thus, δ-damascone was reacted in a 1,4-addition with mercaptoacetic acid, and the product was coupled with 1,2:3,4-di-O-isopropylidene-α-D-galactopyranose. To explore the influence of the molecules' polarity on the deposition and release kinetics, both the isopropylidene-protected hydrophobic as well as the deprotected hydrophilic pro-fragrance were studied. The fragrance release was investigated in aqueous solution by 1H-NMR spectroscopy as a function of pH; the data show that both pro-fragrances are stable under acidic conditions, but release the δ-damascone under basic conditions. The release kinetics are well described by a first-order process, and observed to be much faster in the case of the isopropylidene-protected hydrophobic pro-fragrance. The fragrance release from washed and dried cotton tissue was investigated via dynamic headspace analysis followed by gas chromatography. The data show that the deposition from solution is much better for the hydrophobic pro-fragrance, that the δ-damascone is slowly released in both cases, and that the amount of δ-damascone that can be released is increased by over two orders of magnitude higher than in the case of tissue washed with the neat fragrance under identical conditions.

Controlled fragrance release from galactose-based pro-fragrances

Simultaneous detection of single molecules by absorption and fluorescence is demonstrated using confocal microscopy at cryogenic temperature. Dynamical processes such as blinking and spectral jumping of single emitters are observed in both detection channels. The relative magnitude of fluorescence and absorption varies between molecules. In particular, we observe molecules that do not emit detectable Stokes-shifted fluorescence but show a strong absorption signal. The fact that coherent resonant scattering underlies the absorption process is demonstrated by a correlation between small linewidth and large absorption amplitude.

Absorption and fluorescence of single molecules.

Light-reflecting multilayer polymer films that irreversibly change their optical characteristics upon exposure to suitable, high-intensity irradiation

Photo-patternable nanomaterials

Cellulose nanocrystals as a tunable nanomaterial for pervaporation membranes with asymmetric transport properties

Polyamides based on 2',5'-diamino-4-(dimethylamino)-4-nitro-stilben (DDANS) and aliphatic diacids represent a well-established class of polymers for nonlinear optical (NLO) applications, in which the NLO units are fixed in the polymer backbone with their dipole moments oriented transversly to the polymer main chain. The introduction of a hole transport (HT) agent, [4((diphenylhydrazono)-methyl)phenyl]diethylamin (DEH) or 4-(bis(2-hydroxyethyl)amino)benzaldehyde 1,1-diphenylhydrazone (BBDH), was found to generate or enhance photorefractivity in these materials. Using appropriate chemical pathways, the NLO and HT moieties were connected in various ways, and polymer systems with dramatically different properties were obtained. Thus, (i) polyamides based on DDANS and aliphatic diacids, (ii) polyamide esters based on DDANS, BBDH, and adipic acid, (iii) polymer blends of the DDANS polyamides and a polyester based on BBDH and adipic acid, and (iv) DDANS polyamides doped with DEH have been prepared. All types of polymer systems were processed into transparent thin films by spin coating and could be oriented at elevated temperatures, applying a corona discharge poling technique. Second-order NLO coefficients (d 33 ) of up to 45 pm/V at a fundamental wavelength of 1542 nm have been measured. The poling efficiency was found to depend strongly on the content of the hydrazone moiety and the polymer system, decreasing with increasing content of hydrazone. Preliminary temperature-dependent two-beam coupling experiments demonstrated photorefractivity for DDANS-based systems, even at temperatures below the glass transition.

Christoph Weder

Papers

Controlled fragrance release from galactose-based pro-fragrances

Absorption and fluorescence of single molecules.

Photo-patternable nanomaterials

Cellulose nanocrystals as a tunable nanomaterial for pervaporation membranes with asymmetric transport properties

Synthesis and Characterization of New Photorefractive Polymers with High Glass Transition Temperatures