Size-selective fractionation of nanoparticles at an application scale using CO2 gas-expanded liquids

Size-based fractionation of nanoparticles remains a non-trivial task for the preparation of well-defined nanomaterials for certain applications and fundamental studies. Typical fractionation techniques prove to be inefficient for large nanoparticle quantities due to several factors including the expense of equipment, throughput constraints, and the amount of organic solvent waste produced. Through the use of the pressure-tunable physico-chemical properties of CO2-expanded liquids, a rapid, precise, and environmentally sustainable size-selective fractionation of ligand-stabilized nanoparticles is possible through simple variations in applied CO2 pressure. An apparatus capable of fractionating large quantities of nanoparticles into distinct fractions with the ability to control mean diameters and size distributions has been developed. This apparatus consists of three vertically mounted pressure vessels connected in series with needle valves. This process, at current design scales, operated at room temperature, and CO2 pressures between 0 and 50 bar, results in a batch size-selective fractionation of a concentrated nanoparticle dispersion. This paper presents this new apparatus and the separation results of various single pass fractionations as well as recursive fractionations.

Precise seed-mediated growth and size-controlled synthesis of palladium nanoparticles using a green chemistry approach

In this paper, we present a "green" and size-controlled seed-mediated growth method by which differently sized palladium (Pd) nanoparticles, spanning from 3.4 to 7.6 nm, with an increment of 1.4 nm, were synthesized. Monodisperse Pd nanoparticles (ca. 3.4 nm, standard deviation = 0.7 nm) were first synthesized and stabilized in an aqueous solution at 95 degrees C using nontoxic ascorbic acid and sodium carboxymethyl cellulose (CMC) as reducing agent and capping agent, respectively. These Pd nanoparticles were subsequently employed as seeds on the surface of which fresh Pd (2+) ions were reduced by the weak reducing agent ascorbic acid. Optimal conditions were determined that favored the homogeneous and sequential accumulation of Pd atoms on the surface of the Pd seeds, rather than the formation of new nucleation sites in the bulk growth solution, thereby achieving atomic-level control over particle sizes. The adsorbed CMC molecules did not inhibit the addition of Pd atoms onto the seeds during the growth but provided stabilization of the Pd nanoparticles in aqueous solution after the growth. Potential mechanisms that underpin this seed-mediated growth process are provided and discussed. One advantage of this seed growth process is that it provides stoichiometric control over the size of the Pd nanoparticles by simply varying Pd(2+) added during the growth stage. Furthermore, the use of ecologically friendly reagents, such as water (solvent), CMC (capping agent), and ascorbic acid (reducing agent), in both the seed synthesis and subsequent seed-mediated growth provides both "green" and economic attributes to this process.

A gas-expanded liquid nanoparticle deposition technique for reducing the adhesion of silicon microstructures

A gas-expanded liquid-based nanoparticle deposition technique was integrated with a critical point drying process to modify the surface of polysilicon microstructures in order to reduce the adhesion that ordinarily occurs due to dominant interfacial surface forces. Dodecanethiol-capped gold nanoparticles (AuNPs) were deposited onto arrays of cantilever beams using gas-expanded liquid technology in an effort to increase the surface roughness, thereby reducing the real contact surface area as well as changing the chemical constituents of the contacting areas. Both AuNP-coated and uncoated (native oxide surface) arrays were actuated electrostatically in order to determine the work of adhesion. The results of this study indicate that while cantilever beams with only their native oxide exhibit apparent adhesion energies of about 700 +/- 100 microJ m(-2), cantilever beam arrays coated with AuNPs exhibit an apparent adhesion energy of about 8 microJ m(-2) or less. These results indicate that metallic nanoparticle coatings can be successfully applied to micromachines and provide a drastic reduction in apparent adhesion energy.

Synaptic integration in hypothalamic gonadotropin releasing hormone (GnRH) neurons

The impact of the A-type GABA (GABA-A) receptor in gonadotropin releasing hormone (GnRH) neurons is controversial. In adult GnRH neurons, the GABA-A receptor conductance has been reported to either hyperpolarize or depolarize GnRH neurons. Regardless of whether GABA is inhibitory or excitatory in GnRH neurons, GABAergic input would be integrated with post-synaptic potentials generated by other synaptic inputs. We used dynamic current clamping and compartmental computer modeling to examine the integration of AMPA-type glutamatergic input and GABA-mediated input in both the hyperpolarizing (inhibitory) and depolarizing (excitatory) modes in GnRH neurons from transgenic mice (Mus Musculus) generated on a C57BL6 background. In both living and model neurons, action potentials were most likely a few ms after a maximum in AMPA conductance coincided with a minimum in inhibitory GABA. Excitatory GABA interacted differently with AMPA, with spikes most likely, in both dynamic clamping of living neurons and in model neurons, when a maximum in AMPA coincided with the decay from peak of a maximum in GABA. Distributing synapses along the dendrite maximized the temporal relationship between AMPA and GABA conductances and therefore, the potential for spiking. Thus, these two dominant neurotransmitters could interact in multiple frames to generate action potentials in GnRH neurons.

Dendrites determine the contribution of after depolarization potentials (ADPs) to generation of repetitive action potentials in hypothalamic gonadotropin releasing-hormone (GnRH) neurons

The impact of structure in modulating synaptic signals originating in dendrites is widely recognized. In this study, we focused on the impact of dendrite morphology on a local spike generating mechanism which has been implicated in hormone secretion, the after depolarization potential (ADP). Using multi-compartmental models of hypothalamic GnRH neurons, we systematically truncated dendrite length and determined the consequence on ADP amplitude and repetitive firing. Decreasing the length of the dendrite significantly increased the amplitude of the ADP and increased repetitive firing. These effects were observed in dendrites both with and without active conductances suggesting they largely reflect passive characteristics of the dendrite. In order to test the findings of the model, we performed whole-cell recordings in GnRH neurons and elicited ADPs using current injection. During recordings, neurons were filled with biocytin so that we could determine dendritic and total projection (dendrite plus axon) length. Neurons exhibited ADPs and increasing ADP amplitude was associated with decreasing dendrite length, in keeping with the predictions of the models. Thus, despite the relatively simple morphology of the GnRH neuron's dendrite, it can still exert a substantial impact on the final neuronal output.

Polysugar-stabilized Pd nanoparticles exhibiting high catalytic activities for hydrodechlorination of environmentally deleterious trichloroethylene

In this paper, we present a straightforward and environmentally friendly aqueous-phase synthesis of small Pd nanoparticles (approximately 2.4 nm under the best stabilization) by employing a "green", inexpensive, and biodegradable/biocompatible polysugar, sodium carboxymethylcellulose (CMC), as a capping agent. The Pd nanoparticles exhibited rather high catalytic activity (observed pseudo-first-order reaction kinetic rate constant, k(obs), is up to 828 L g(-1) min(-1)) for the hydrodechlorination of environmentally deleterious trichloroethene (TCE) in water. Fourier transform IR (FT-IR) spectra indicate that CMC molecules interact with the Pd nanoparticles via both carboxyl (-COO-) and hydroxyl (-OH) groups, thereby functioning to passivate the surface and suppress the growth of the Pd nanoparticles. Hydrodechlorination of TCE using differently sized CMC-capped Pd nanoparticles as catalyst was systematically investigated in this work. Both the catalytic activity (k(obs)) and the surface catalytic activity (turnover frequency, TOF) of these CMC-capped Pd nanoparticles for TCE degradation are highly size-dependent. This point was further verified by a comparison of the catalytic activities and surface catalytic activities of CMC-capped Pd nanoparticles with those of beta-D-glucose-capped Pd and neat Pd nanoparticles for TCE degradation.

Finely controlled size-selective precipitation and separation of CdSe/ZnS semiconductor nanocrystals using CO2-gas-expanded liquids

A technique was developed to size-selectively separate polydisperse dispersions of CdSe/ZnS nanocrystals into distinct color fractions using only the tunable solvent properties of CO2-expanded hexane. This size-selective precipitation of semiconductor nanoparticles is achieved by finely tuning the solvent strength of the CO2/hexane medium by simply adjusting the applied CO2 pressure. These subtle changes affect the balance between osmotic repulsive and van der Waals attractive forces, thereby allowing fractionation of the nanocrystals into multiple narrow size populations.

Solvent Effects on AOT Reverse Micelles in Liquid and Compressed Alkanes Investigated by Neutron Spin−Echo Spectroscopy

Neutron Spin-Echo (NSE) spectroscopy has been employed to study the interfacial properties of reverse micelles formed with the common surfactant sodium bis-2-ethylhexyl-sulfosuccinate (AOT) in liquid alkane solvents and compressed propane. NSE spectroscopy provides a means to measure small energy transfers for incident neutrons that correspond to thermal fluctuations on the nanosecond time scale and has been applied to the study of colloidal systems. NSE offers the unique ability to perform dynamic measurements of thermally induced shape fluctuation in the AOT surfactant monolayer. This study investigates the effects of the bulk solvent properties, water content, and the addition of octanol cosurfactant on the bending elasticity of AOT reverse micelles and the reverse micelle dynamics. By altering these solvent properties, specific trends in the bending elasticity constant, k, are observed where increasing k corresponds to an increase in micelle rigidity and a decrease in intermicellar exchange rate, k(ex). The observed corresponding trends in k and k(ex) are significant in relating the dynamics of microemulsions and their application as a reaction media. Compressed propane was also examined for the first time with a high-pressure, compressible bulk solvent where variations in temperature and pressure are used to tune the properties of the bulk phase. A decrease in the bending elasticity is observed for the d-propane/AOT/W = 8 reverse micelle system by simultaneously increasing the temperature and pressure, maintaining constant density. With isopycnic conditions, a constant translational diffusion of the reverse micelles through the bulk phase is observed, conforming to the Stokes-Einstein relationship.

Synthesis and Steric Stabilization of Silver Nanoparticles in Neat Carbon Dioxide Solvent Using Fluorine-Free Compounds

The adjustable solvent properties, vanishingly low surface tensions, and environmentally green characteristics of supercritical carbon dioxide present certain advantages in nanoparticles synthesis and processing. Unfortunately, most current techniques employed to synthesize and disperse nanoparticles in carbon dioxide use environmentally persistent fluorinated compounds as metal precursors and/or stabilizing ligands. This paper illustrates a one-step process for synthesis and stabilization of silver nanoparticles in carbon dioxide using only fluorine-free compounds. Isostearic acid coated silver nanoaparticles were formed and stably dispersed through arrested precipitation. Silver bis(3,5,5-trimethyl-1-hexyl)sulfosuccinate (Ag-AOT-TMH) was reduced in the presence of isostearic acid as a capping ligand in carbon dioxide solvent to form silver nanoparticles. The addition of cyclohexane as cosolvent or an increase in carbon dioxide solvent density enhances the dispersibility of the particles due to an increase in solvent strength. The dispersibility of the isostearic acid capped silver nanoparticles diminished with time until a stable dispersion was achieved due to the precipitation of a fraction of particle sizes too large to be stabilized by the solvent medium, thereby leaving a smaller size fraction of nanoparticles stably dispersed in the CO2 mixtures. This paper presents the one-step synthesis and stabilization of metallic nanoparticles in neat carbon dioxide without the aid of any fluorinated compounds.

Synthesis and extraction of beta-D-glucose-stabilized Au nanoparticles processed into low-defect, wide-area thin films and ordered arrays using CO2-expanded liquids

This letter presents a straightforward and economic strategy to create aqueous Au nanoparticle dispersions using small amounts of beta-D-glucose as a "green" alternative to the conventional phase-transfer catalyst approach. Furthermore, this new process provides for efficient extraction of these monodisperse Au nanoparticles into an organic phase that was successfully processed into wide-area, locally ordered nanoparticle arrays and thin films using a precisely controlled CO2-expanded liquid particle deposition technique. This CO2-based technique allows for pressure-tunable particle deposition while eliminating the detrimental surface tension and dewetting effects common to normal solvent-evaporation techniques.

Dendritic processing of excitatory synaptic input in hypothalamic gonadotropin releasing-hormone neurons

The activity of hypothalamic GnRH neurons results in the intermittent release of GnRH required for reproductive function. This intermittent neurosecretory activity has been proposed to reflect integration of intrinsic properties of and synaptic input to GnRH neurons. Determining the relative impact of synaptic inputs at different locations on the GnRH neuron is difficult, if not impossible, using only experimental approaches. Thus, we used electrophysiological recordings and neuronal reconstructions to generate computer models of GnRH neurons to examine the effects of synaptic inputs at varying distances from the soma along dendrites. The parameters of the models were adjusted to duplicate measured passive and active electrophysiology of cells from mouse brain slices. Our morphological findings reinforce the emerging picture of a complex dendritic structure of GnRH neurons. Furthermore, analysis of reduced morphology models indicated that this population of cells is unlikely to exhibit low-frequency tonic spiking in the absence of synaptic input. Finally, applying realistic patterns of synaptic input to modeled GnRH neurons indicates that synapses located more than about 30% of the average dendrite length from the soma cannot drive firing at frequencies consistent with neuropeptide release. Thus, processing of synaptic input to dendrites of GnRH neurons is probably more complex than simple summation.

Stable dispersions of silver nanoparticles in carbon dioxide with fluorine-free ligands

Iso-stearic acid, a short, stubby compound with branched, methylated tails has been shown to have high solubility in carbon dioxide. Tail solvation by carbon dioxide makes iso-stearic acid a good choice for use as a ligand to sterically stabilize metallic nanoparticles. Iso-stearic acid coated silver nanoparticles have been stably dispersed in carbon dioxide with hexane cosolvent. Neat carbon dioxide has successfully dispersed iso-stearic acid coated silver nanoparticles that had been deposited on either quartz or polystyrene surfaces. These results are the first reports of sterically stabilized nanoparticles in carbon dioxide without the use of any fluorinated compounds.

Tunable Solvation Effects on the Size-Selective Fractionation of Metal Nanoparticles in CO2 Gas-Expanded Solvents

This paper presents an environmentally friendly, inexpensive, rapid, and efficient process for size-selective fractionation of polydisperse metal nanoparticle dispersions into multiple narrow size populations. The dispersibility of ligand-stabilized silver and gold nanoparticles is controlled by altering the ligand tails-solvent interaction (solvation) by the addition of carbon dioxide (CO2) gas as an antisolvent, thereby tailoring the bulk solvent strength. This is accomplished by adjusting the CO2 pressure over the liquid, resulting in a simple means to tune the nanoparticle precipitation by size. This study also details the influence of various factors on the size-separation process, such as the types of metal, ligand, and solvent, as well as the use of recursive fractionation and the time allowed for settling during each fractionation step. The pressure range required for the precipitation process is the same for both the silver and gold particles capped with dodecanethiol ligands. A change in ligand or solvent length has an effect on the interaction between the solvent and the ligand tails and therefore the pressure range required for precipitation. Stronger interactions between solvent and ligand tails require greater CO2 pressure to precipitate the particles. Temperature is another variable that impacts the dispersibility of the nanoparticles through changes in the density and the mole fraction of CO2 in the gas-expanded liquids. Recursive fractionation for a given system within a particular pressure range (solvent strength) further reduces the polydispersity of the fraction obtained within that pressure range. Specifically, this work utilizes the highly tunable solvent properties of organic/CO2 solvent mixtures to selectively size-separate dispersions of polydisperse nanoparticles (2 to 12 nm) into more monodisperse fractions (+/-2 nm). In addition to providing efficient separation of the particles, this process also allows all of the solvent and antisolvent to be recovered, thereby rendering it a green solvent process.

Oxygenated Hydrocarbon Ionic Surfactants Exhibit CO2 Solubility

Several oxygenated hydrocarbons, including acetylated sugars, poly(propylene glycol), and oligo(vinyl acetate), have been used to generate CO2-soluble ionic surfactants. Surfactants with vinyl acetate tails yielded the most promising results, exhibiting levels of CO2 solubility comparable to those associated with fluorinated ionic surfactants. For example, a sodium sulfate with single, oligomeric vinyl acetate (VAc) tails consisting of 10 VAc repeat units was 7 wt % soluble in CO2 at 25 degrees C and 48 MPa. Upon introduction of water to these systems, only surfactants with the oligomeric vinyl acetate tails exhibited spectroscopic evidence of a polar environment that was capable of solubilizing the methyl orange into the CO2-rich phase. For example, a single-phase solution of CO2, 0.15 wt % sodium bis(vinyl acetate)8 sulfosuccinate, and water, at water loading (W) values ranging from 10 to 40 at 25 degrees C and 34.5 MPa, exhibited a methyl orange peak at 423 nm. This result indicated that the core of a reverse micelle provided a microenvironment with a polarity similar to that of methanol. Quantum chemical calculations indicate that the acetylated sugars may be too hydrophilic to readily form reverse micelles, whereas the VAc-based surfactants appear to have the correct balance of hydrophilic and hydrophobic forces necessary to form reverse micelles.

Chloride ion effects on synthesis and directed assembly of copper nanoparticles in liquid and compressed alkane microemulsions

Microemulsions are effective media for solution-based synthesis of metallic nanoparticles where surfactants and other ionic species influence the directed assembly of the nanomaterials with specific sizes, geometries, and compositions. This study demonstrates the effects of chloride ion on the synthesis of copper nanoparticles within the sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelle system utilizing both liquid isooctane and compressed propane as the bulk solvent. Copper nanoparticle synthesis can be achieved in the presence of HCl in the micelle core, taking advantage of the buffering action of the AOT surfactant. The concentration of chloride ions influence the particle growth rate and dispersion in liquid isooctane. The presence of chloride ions during particle synthesis in compressed propane has a significant effect on the geometry and structure of the copper nanomaterials produced. Chloride ion addition to the compressed propane/Cu(AOT)(2)-AOT/water reverse micelle system at 20 degrees C and 310 bar results in the formation of diamond-shaped copper nanoparticle assemblies. The copper nanoparticle assemblies exhibit unique structure and retain this structure through repeated solvent processing steps, allowing separation and recovery of the assembled diamond-shaped copper nanoparticle structures.

CO2-expanded liquid deposition of ligand-stabilized nanoparticles as uniform, wide-area nanoparticle films

Deposition of nanoparticles into uniform, wide-area thin films using CO(2) as an antisolvent is presented. Ligand-stabilized silver particles are controllably precipitated from organic solvents by pressurizing and expanding the solution with carbon dioxide. Subsequent addition of carbon dioxide as a dense supercritical fluid provides for removal of the organic solvent while avoiding the surface tensions common to evaporating solvents that are detrimental to nanoscale assemblies and structures. This brand new CO(2)-expanded liquid particle deposition technique allows for the targeted deposition of particles and results in more uniform and lower defect metal nanoparticle thin films than are provided by conventional solvent evaporation techniques.