Detection of arsenic(III) through pulsed laser-induced desorption/ionization of gold nanoparticles on cellulose membranes.

We have developed an assay based on gold nanoparticle-modified mixed cellulose ester membrane (Au NPs-MCEM) coupled with laser-induced desorption/ionization mass spectrometry (LDI-MS)-for the detection of arsenic(III) ions (arsenite, AsO2(-)) in aqueous solution. When the Au NPs reacted with lead ions (Pb(2+)) in alkaline solution (5 mM glycine-NaOH, pH 12), Au-Pb complexes, PbO, and Pb(OH) were formed immediately on the Au NP surfaces. The Pb species reacted rapidly with subsequently added AsO2(-) to form PbOAs2O3, (PbO)2As2O3, and/or (PbO)3As2O3 shells (2-5 nm) on the Au NPs' surfaces. As a result, significant observable aggregation of the Au NPs occurred in the solution. This Pb(2+)/Au NP probe allowed the detection of AsO2(-) at concentrations as low as 0.6 µM with high selectivity (at least 100-fold over other anions and metal ions). To further improve the sensitivity, we prepared Au NPs-MCEM for the LDI-MS-based detection of AsO2(-) ions. The intensity of the signal for the [Pb](+) ions in the mass spectra increased when the Au NPs-MCEM reacted with AsO2(-); in contrast, the intensity of the signal for [Au](+) ions decreased. Accordingly, the [Pb](+)/[Au](+) peak ratio increased upon increasing the AsO2(-) concentration over the range from 10 nM to 10 µM. The limit of detection at a signal-to-noise ratio of 3 was 2.5 nM, far below the action level of As (133 nM, ca. 10 ppb) permitted by the US EPA for drinking water. Relative to other nanoparticle-based arsenic sensors, this approach is rapid, specific, and sensitive; in addition, it can be applied to the detection of AsO2(-) in natural water samples (in this case, streamwater, lake water, tap water, groundwater, and mineral water).

Behavior of water molecules near monolayer-protected clusters with different terminal segments of ligand.

Molecular dynamics simulations are performed to investigate the behavior of water molecules near gold monolayer protected clusters (MPCs) with two different types of surfactant, HS(CH(2))(5)(OCH(2)CH(2))(2)COOH (type1) and HS(CH(2))(11)COOH (type2). The effects of the different moieties of the two ligands on the local structure of the water molecules are quantified by means of the reduced density profiles of oxygen and hydrogen atoms, and the hydrogen bond statistics. The adsorption characteristics of water molecules are evaluated by means of their residence time near the MPCs. The results show that the hydrophilic oligo (ethylene glycol) segment increases the number of water molecules, which penetrate the protective layer of MPC. As a result, the inter-water hydrogen bond network in the protective layer of type1 MPC is stronger than that in the protective layer of the type2 MPC. It is shown that the presence of interfacial hydrogen bonds increases the adsorption of water molecules near the MPCs and therefore constrains the motion of MPCs. As a result, the residence time of the water molecules adjacent to the type1 MPC is longer than that of the molecules adjacent to the type2 MPC.

Influence of alkanethiol self-assembled monolayers with various tail groups on structural and dynamic properties of water films.

Molecular dynamics simulations are performed to investigate the structural and dynamic properties of a water layer lying on a clean Au(111) surface and on alkanethiol self-assembled monolayers (SAMs) with three different tail groups: methyl, carboxyl, and hydroxyl. The effects of these functional groups on the local structure of the water are quantified by analyzing the reduced density profiles of the oxygen and hydrogen atoms, the average number of hydrogen bonds, and the distribution of the O-H bond angle, respectively. Meanwhile, the dynamic properties of the water layer are evaluated by analyzing the diffusion coefficients of the water molecules in the xy-plane and z-direction. The simulation results indicate that in both the hydrophobic and the hydrophilic alkanethiol SAMs, the formation of a two-layer water structure is suppressed. And the water molecules can approach the SAMs composed of hydroxyl tails most closely and SAMs composed of methyl tails furthest. Due to the existence of hydrogen bonds between water molecules and hydrophilic alkanethiol SAMs, the distribution of water molecules is more uniform than that in the hydrophobic interface. Meanwhile, the water-water hydrogen bond network weakens. Furthermore, the mobility of the water molecules in the hydrophilic interface is reduced more significantly than in the hydrophobic interface. The results developed in this study yield detailed insights into the microscopic interfacial phenomena.

Molecular dynamics simulations of structural features and diffusion properties of fullerene-in-water suspensions.

This study performs molecular dynamics (MD) simulations to investigate the structural features and diffusion properties of fullerene-in-water suspensions. The numerical results reveal that an organized structure of liquid water is formed close to the surface of the fullerene molecule, thereby changing the solid/liquid interfacial structure. The organized structure formation becomes more pronounced as the fullerene size is reduced. This observation implies that a transition zone exists between the organized liquid water layers and the random distribution region. Furthermore, the results indicate that the structural stability of fullerene-in-water suspensions improves as the fullerene volume fraction increases, but is insensitive to changes in the fullerene size. Finally, the simulation results reveal that the diffusion coefficient of the water molecules varies as a linear function of the fullerene loading, but is independent of the fullerene size.

One-step synthesis of biofunctional carbon quantum dots for bacterial labeling.

In this study, we used a simple one-step dry heating method to synthesize mannose-modified fluorescent carbon quantum dots (Man-CQDs) from solid ammonium citrate and mannose, and successfully applied for labeling Escherichia coli. The highly soluble Man-CQDs had an average particle diameter of 3.1±1.2 nm and exhibited a quantum yield of 9.8% at excitation and emission wavelengths of 365 and 450 nm, respectively. The fluorescent Man-CQDs could selectively bind to the FimH lectin unit in the flagella of the wild-type 1 E. coli K12 strain. We optimized the labeling efficiency of the Man-CQDs by controlling the ratio of ammonium citrate to mannose during their synthesis. The specific binding of the mannose units to E. coli allowed quantitative detection of the bacteria at levels down to 450 colony forming units mL(-1) in lab samples, and facilitate the application of the Man-CQDs for bacterial analyses of real samples (tap water, apple juice, human urine). The synthesis of our Man-CQDs, their labeling, and their use in the detection of bacteria were all simple, inexpensive and efficient processes.

Temperature sensing using individual single-walled carbon nanotubes.

Molecular dynamics simulations and quantum transport theory are employed to study the temperature-dependent electrical properties of individual (12,0) zigzag and (5,5) armchair carbon nanotubes deposited on silicon substrates. The results demonstrate that the magnitude of the leakage current depends on the length of the nanotube. Furthermore, the leakage current is generated periodically along the length of the nanotube. Finally, the results indicate that given an appropriate value of the applied bias voltage, the induced current varies linearly with the temperature over specific temperature ranges. As a result, the temperature can be inversely derived from the measured current signal. Overall, the results show that the (12,0) zigzag and (5,5) armchair carbon nanotubes are suitable for temperature sensing applications over temperature ranges of 200-420 K and 300-440 K, respectively.

Molecular dynamics investigation into the structural features and transport properties of C60 in liquid argon.

Molecular dynamics (MD) simulations were performed to investigate the structural features and transport properties of C60 in liquid argon. The results reveal that an organized structure shell of liquid argon is formed close to the surface of a C60 fullerene molecule, thereby changing the solid/liquid interfacial structure. Furthermore, the simulation indicates that the C60-liquid argon fluid becomes structurally more stable as the C60 molecule volume fraction and the temperature increase. The viscosity of the fluid increases significantly as the C60 molecule loading is increased, particularly at a lower temperature. The thermal conductivity enhancement of the fluid in the present simulations is anomalously an order of magnitude higher than the theoretical predictions from either the Maxwell or the Lu and Liu models, and is found to vary approximately linearly with the C60 molecule volume fraction. The increased thermal conductivity is attributed to the nature of heat conduction in C60 molecule suspensions and an organized structure at the solid/liquid interface.

The effect of added oversized elements on the microstructure of binary alloy nanoparticles.

Molecular dynamics simulations are used to investigate the microstructures of Cu-Ni nanoparticles with different concentrations of oversized atoms added to them. A many body second moment tight binding approximation potential is adopted to model the interatomic interactions. The Honeycutt-Anderson (HA) pair analysis technique is adopted to analyse in detail the transformation between local structures at different temperatures. From the simulation results, at temperatures higher than the melting point, the nanoparticles are in a liquid state and an icosahedral local structure is most frequently found inside the nanoparticles. At temperatures beneath the melting point, the fraction of FCC local structure increases with decreasing concentrations of the larger size atoms, whereas a larger fraction of amorphous structure still remains in the solid state for higher concentrations of oversized atoms. This is because the effects of distortion and misfit are more significant for a nanoparticle having a higher concentration of oversized atoms.

An investigation into the melting of silicon nanoclusters using molecular dynamics simulations.

Using the Stillinger-Weber (SW) potential model, we have performed molecular dynamics (MD) simulations to investigate the melting of silicon nanoclusters comprising a maximum of 9041 atoms. This study investigates the size, surface energy and root mean square displacement (RMSD) characteristics of the silicon nanoclusters as they undergo a heating process. The numerical results reveal that an intermediate nanocrystal regime exists for clusters with more than 357 atoms. Within this regime, a linear relationship exists between the cluster size and its melting temperature. It is found that melting of the silicon nanoclusters commences at the surface and that T(m,N) = T(m,Bulk)-αN(-1/3). Therefore, the extrapolated melting temperature of the bulk with a surface decreases from T(m,Bulk) = 1821 K to a value of T(m,357) = 1380 K at the lower limit of the intermediate nanocrystal regime.