Disruption of Model Cell Membranes by Carbon Nanotubes

Carbon nanotubes (CNTs) have one of the highest production volumes among carbonaceous engineered nanoparticles (ENPs) worldwide and are have potential uses in applications including biomedicine, nanocomposites, and energy conversion. However, CNTs possible widespread usage and associated likelihood for biological exposures have driven concerns regarding their nanotoxicity and ecological impact. In this work, we probe the responses of planar suspended lipid bilayer membranes, used as model cell membranes, to functionalized multi-walled carbon nanotubes (MWCNT), CdSe/ZnS quantum dots, and a control organic compound, melittin, using an electrophysiological measurement platform. The electrophysiological measurements show that MWCNTs in a concentration range of 1.6 to 12 ppm disrupt lipid membranes by inducing significant transmembrane current fluxes, which suggest that MWCNTs insert and traverse the lipid bilayer membrane, forming transmembrane carbon nanotubes channels that allow the transport of ions. This paper demonstrates a direct measurement of ion migration across lipid bilayers induced by CNTs. Electrophysiological measurements can provide unique insights into the lipid bilayer-ENPs interactions and have the potential to serve as a preliminary screening tool for nanotoxicity.

Simple replica micromolding of biocompatible styrenic elastomers

In this work, we introduce a simple solvent-assisted micromolding technique for the fabrication of high-fidelity styrene-ethylene/butylene-styrene (SEBS) microfluidic devices with high polystyrene (PS) content (42 wt% PS, SEBS42). SEBS triblock copolymers are styrenic thermoplastic elastomers that exhibit both glassy thermoplastic and elastomeric properties resulting from their respective hard PS and rubbery ethylene/butylene segments. The PS fraction gives SEBS microdevices many of the appealing properties of pure PS devices, while the elastomeric properties simplify fabrication of the devices, similar to PDMS. SEBS42 devices have wettable, stable surfaces (both contact angle and zeta potential) that support cell attachment and proliferation consistent with tissue culture dish substrates, do not adsorb hydrophobic molecules, and have high bond strength to wide range of substrates (glass, PS, SEBS). Furthermore, SEBS42 devices are mechanically robust, thermally stable, as well as exhibit low auto-fluorescence and high transmissivity. We characterize SEBS42 surface properties by contact angle measurements, cell culture studies, zeta potential measurements, and the adsorption of hydrophobic molecules. The PS surface composition of SEBS microdevices cast on different substrates is determined by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The attractive SEBS42 material properties, coupled with the simple fabrication method, make SEBS42 a quality substrate for microfluidic applications where the properties of PS are desired but the ease of PDMS micromolding is favoured.

Diffusive behaviors of circle-swimming motors

Spherical catalytic micromotors fabricated as described in Wheat et al. [Langmuir 26, 13052 (2010)] show fuel concentration dependent translational and rotational velocity. The motors possess short-time and long-time diffusivities that scale with the translational and rotational velocity with respect to fuel concentration. The short-time diffusivities are two to three orders of magnitude larger than the diffusivity of a Brownian sphere of the same size, increase linearly with concentration, and scale as v(2)/2ω. The measured long-time diffusivities are five times lower than the short-time diffusivities, scale as v(2)/{2D(r)[1+(ω/D(r))(2)]}, and exhibit a maximum as a function of concentration. Maximums of effective diffusivity can be achieved when the rotational velocity has a higher order of dependence on the controlling parameter(s), for example fuel concentration, than the translational velocity. A maximum in diffusivity suggests that motors can be separated or concentrated using gradients in fuel concentration. The decrease of diffusivity with time suggests that motors will have a high collision probability in confined spaces and over short times; but will not disperse over relatively long distances and times. The combination of concentration dependent diffusive time scales and nonmonotonic diffusivity of circle-swimming motors suggests that we can expect complex particle responses in confined geometries and in spatially dependent fuel concentration gradients.

Biological accumulation of engineered nanomaterials: a review of current knowledge

Due to the widespread use of engineered nanomaterials (ENMs) in consumer and industrial products, concerns have been raised over their impacts once released into the ecosystems. While there has been a wealth of studies on the short-term acute toxic effects of ENMs over the past decade, work on the chronic endpoints, such as biological accumulation, has just begun to increase in last 2–3 years. Here, we comprehensively review over 65 papers on the biological accumulation of ENMs under a range of ecologically relevant exposure conditions in water, soil or sediment with the focus on quantitative comparison among these existing studies. We found that daphnid, fish, and earthworm are the most commonly studied ecological receptors. Current evidence suggests that ENM accumulation level is generally low in fish and earthworms with logarithmic bioconcentration concentration factor and biota-sediment accumulation factor ranging from 0.85–3.43 (L kg−1) and −2.21–0.4 (kg kg−1), respectively. ENMs accumulated in organisms at the lower trophic level can transfer to higher trophic level animals with the occurrence of biomagnification varying depending on the specific food chain studied. We conclude the review by identifying the challenges and knowledge gaps and propose paths forward.

Multi-vortical flow inducing electrokinetic instability in ion concentration polarization layer

In this work, we investigated multiple vortical flows inside the ion concentration polarization (ICP) layer that forms due to a coupling of applied electric fields and the semipermeable nanoporous junction between microchannels. While only a primary vortex near perm-selective membrane is traditionally known to lead to electrokinetic instability, multiple vortexes induced by the primary vortex were found to play a major role in the electrokinetic instability. The existence of multiple vortexes was directly confirmed by experiments using particle tracers and interdigitated electrodes were used to measure the local concentration profile inside the ICP layer. At larger applied electric fields, we observed aperiodic fluid motion due to electrokinetic instabilities which develop from a coupling of applied electric fields and electrical conductivity gradients induced by the ICP. The electrokinetic instability at micro-nanofluidic interfaces is important in the development of various electro-chemical-mechanical applications such as fuel cells, bio-analytical preconcentration methods, water purification/desalination and the fundamental study of ion electromigration through nanochannels and nonporous perm-selective membranes.

Role of nanoparticle surface functionality in the disruption of model cell membranes

Lipid bilayers are biomembranes common to cellular life and constitute a continuous barrier between cells and their environment. Understanding the interaction of engineered nanomaterials (ENMs) with lipid bilayers is an important step toward predicting subsequent biological effects. In this study, we assess the effect of varying the surface functionality and concentration of 10-nm-diameter gold (Au) and titanium dioxide (TiO(2)) ENMs on the disruption of negatively charged lipid bilayer vesicles (liposomes) using a dye-leakage assay. Our findings show that Au ENMs having both positive and negative surface charge induce leakage that reaches a steady state after several hours. Positively charged particles with identical surface functionality and different core compositions show similar leakage effects and result in faster and greater leakage than negatively charged particles, which suggests that surface functionality, not particle core composition, is a critical factor in determining the interaction between ENMs and lipid bilayers. The results suggest that particles permanently adsorb to bilayers and that only one positively charged particle is required to disrupt a liposome and trigger the leakage of its entire contents in contrast to mellitin molecules, the most widely studied membrane lytic peptide, which requires hundred of molecules to generate leakage.

Distribution of functionalized gold nanoparticles between water and lipid bilayers as model cell membranes

Lipid bilayers are biomembranes common to cellular life and constitute a continuous barrier between cells and their environment. Understanding the interaction of nanoparticles with lipid bilayers is an important step toward predicting subsequent biological effects. In this study, we assessed the affinity of functionalized gold nanoparticles (Au NPs) with sizes from 5 to 100 nm to lipid bilayers by determining the Au NP distribution between aqueous electrolytes and lipid bilayers. The Au NP distribution to lipid bilayers reached an apparent steady state in 24 h with smaller Au NPs distributing onto lipid bilayers more rapidly than larger ones. Au NPs distributed to lipid bilayers to a larger extent at lower pH. Tannic acid-functionalized Au NPs exhibited greater distribution to lipid bilayers than polyvinylpyrrolidone-functionalized Au NPs of the same size. Across the various Au NP sizes, we measure the lipid bilayer-water distribution coefficient (K(lipw) = C(lip)/C(w)) as 450 L/kg lipid, which is independent of dosimetric units. This work suggests that the nanoparticle-cell membrane interaction is dependent on solution chemistry and nanoparticle surface functionality. The K(lipw) value may be used to predict the affinity of spherical Au NPs across a certain size range toward lipid membranes.

Distribution of fullerene nanomaterials between water and model biological membranes

Biological membranes are one of the important interfaces between cells and pollutants. Many polar and hydrophobic chemicals can accumulate within these membranes. For this reason, artificial biological membranes are appealing surrogates to complex organisms for assessing the bioaccumulation potential of engineered nanomaterials (ENMs). To our knowledge, this work presents the first quantitative study on the distribution of fullerene ENMs between lipid bilayers, used as model biological membranes, and water. We evaluated the lipid bilayer-water association coefficients (K(lipw)) of aqueous fullerene aggregates (nC(60)) and fullerol (C(60)(ONa)(x)(OH)(y), x + y = 24). Kinetic studies indicated that fullerol reached apparent equilibrium more rapidly than nC(60) (2 h versus >9 h). Nonlinear isotherms can describe the distribution behavior of nC(60) and fullerol. The lipid bilayer-water distributions of both nC(60) and fullerol were pH-dependent with the accumulation in lipid bilayers increasing systematically as the pH decreased from 8.6 (natural water pH) to 3 (the low end of physiologically relevant pH). This pH dependency varies with the zeta potentials of the ENMs and leads to patterns similar to those previously observed for the lipid bilayer-water distribution behavior of ionizable organic pollutants. The K(lipw) value for nC(60) was larger than that of fullerol at a given pH, indicating a greater propensity for nC(60) to interact with lipid bilayers. For example, at pH 7.4 and an aqueous concentration of 10 mg/L, K(lipw) was 3.5 times greater for nC(60) (log K(lipw) = 2.99) relative to fullerol (log K(lipw) = 2.45). Comparisons with existing aquatic organism bioaccumulation studies suggested that the lipid bilayer-water distribution is a potential method for assessing the bioaccumulation potentials of ENMs.

Octanol-water distribution of engineered nanomaterials

The goal of this study was to examine the effects of pH and ionic strength on octanol-water distribution of five model engineered nanomaterials. Distribution experiments resulted in a spectrum of three broadly classified scenarios: distribution in the aqueous phase, distribution in the octanol, and distribution into the octanol-water interface. Two distribution coefficients were derived to describe the distribution of nanoparticles among octanol, water and their interface. The results show that particle surface charge, surface functionalization, and composition, as well as the solvent ionic strength and presence of natural organic matter, dramatically impact this distribution. Distributions of nanoparticles into the interface were significant for nanomaterials that exhibit low surface charge in natural pH ranges. Increased ionic strengths also contributed to increased distributions of nanoparticle into the interface. Similarly to the octanol-water distribution coefficients, which represent a starting point in predicting the environmental fate, bioavailability and transport of organic pollutants, distribution coefficients such as the ones described in this study could help to easily predict the fate, bioavailability, and transport of engineered nanomaterials in the environment.

The release of nanosilver from consumer products used in the home

Nanosilver has become one of the most widely used nanomaterials in consumer products because of its antimicrobial properties. Public concern over the potential adverse effects of nanosilver's environmental release has prompted discussion of federal regulation. In this paper, we assess several classes of consumer products for their silver content and potential to release nanosilver into water, air, or soil. Silver was quantified in a shirt, a medical mask and cloth, toothpaste, shampoo, detergent, a towel, a toy teddy bear, and two humidifiers. Silver concentrations ranged from 1.4 to 270,000 microg Ag g product(-1). Products were washed in 500 mL of tap water to assess the potential release of silver into aqueous environmental matrices (wastewater, surface water, saliva, etc.). Silver was released in quantities up to 45 microg Ag g product(-1), and size fractions were both larger and smaller than 100 nm. Scanning electron microscopy confirmed the presence of nanoparticle silver in most products as well as in the wash water samples. Four products were subjected to a toxicity characterization leaching procedure to assess the release of silver in a landfill. The medical cloth released an amount of silver comparable to the toxicity characterization limit. This paper presents methodologies that can be used to quantify and characterize silver and other nanomaterials in consumer products. The quantities of silver in consumer products can in turn be used to estimate real-world human and environmental exposure levels.

Rapid fabrication of bimetallic spherical motors

Catalytic bimetallic nanomotors can swim at 100 body lengths per second as well as pick up, haul, and release micrometer-scale cargo. The electrokinetic locomotion of bimetallic nanomotors is driven by the electrocatalytic decomposition of hydrogen peroxide. The motors are typically fabricated by electrodeposition-based template synthesis techniques that result in heterogeneous samples and require specialized knowledge of electrochemistry, a three-electrode potentiostat setup, cyanide-based chemistry, and porous membranes. This paper presents a rapid and facile method for fabrication of spherical bimetallic motors that only requires access to metal deposition equipment and commercially available microspheres. The resulting spherical motors swim at speeds comparable to rod-shaped motors with the same dimensions and composition. The spherical motors' velocity increases with fuel concentration and decreasing diameter.

Electrokinetic vortices and traveling waves in nondilute colloidal dispersions

We report a diverse set of coherent fluid flow instabilities, particle patterns, and traveling waves that develop when an ac electric field is applied to nondilute colloidal dispersions with volume fractions that span 3 orders of magnitude. Novel observed phenomena include the following: vortices with steady and unsteady axes of rotation, unsteady time evolution of vortices formation and vortex merging, as well as traveling waves that propagate through the dispersion. Vortical flows are influenced by strong interactions between particle electrical dipoles as well as electric body forces due to electric fields coupled with gradients in particle volume fraction. We use a 1D Burgers equation to predict the existence of traveling waves in colloidal dispersions.

Locomotion of electrocatalytic nanomotors due to reaction induced charge autoelectrophoresis

Bimetallic rod-shaped nanomotors swim autonomously in hydrogen peroxide solutions. Here, we present a scaling analysis, computational simulations, and experimental data that show that the nanomotor locomotion is driven by fluid slip around the nanomotor surface due to electrical body forces. The body forces are generated by a coupling of charge density and electric fields induced by electrochemical reactions occurring on the nanomotor surface. We describe the dependence of nanomotor motion on the nanomotor surface potential and reaction-driven flux.

Submicron scale patterning in sintered silica colloidal crystal films using a focused ion beam

Focused ion beam milling is used to fabricate micron and submicron scale patterns in sintered silica colloidal crystal films. Rectangular cavities with both solid and porous boundaries, fluidic channels, and isolation of a small number of packed spheres are patterned. The ion beam can pattern sintered films of individual submicron size spheres and create patterns that cover up to 40 mum in less than 15 min. The experiments in this work indicate that the amount of redeposited material on the surface of a milled cavity determines whether the surface will be porous or solid. FIB direct patterning has applications in colloidal crystal based lithography, integrated photonic devices, optofluidic devices, and micrototal-analytical systems.

Synthetic nanomotors in microchannel networks: directional microchip motion and controlled manipulation of cargo

We illustrate the use of catalytic nanowire motors for directional motion and microscale transport of cargo within microfluidic channel networks. The CNT-based synthetic nanomotor can propel a large cargo load at high speeds through predetermined paths and junctions of the microchannel network. The magnetic properties of the nickel-containing nanomotors offer controlled cargo manipulations, including en-route load, drag, and release. Such use of synthetic nanomachines can lead to chemically powered versatile laboratory-on-a-chip devices performing a series of tasks simultaneously or sequentially.

Particle dispersion and separation resolution of pinched flow fractionation

This paper investigates a hydrodynamic particle separation technique that employs pinching of particles to a narrow microchannel. The particles are subject to a sudden expansion which results in a size-based particle separation transverse to the flow direction. The separation resolution and particle dispersion are measured using epifluorescence microscopy. The resolution and dispersion are predicted using a compact theoretical model. Devices are fabricated using conventional soft lithography of polydimethylsiloxane. The results show that the separation resolution is a function of the microchannel aspect ratio, particle size difference, and the microchannel sidewall roughness. A separation resolution as large as 3.8 is obtained in this work. This work shows that particles with diameters on the order of the sidewall roughness cannot be separated using pinched flow fractionation.

Review of electrophoresis and BioMEMS in 2007: American Electrophoresis Society 24th Annual Meeting

Researchers came together for the 24th Annual Meeting of the American Electrophoresis Society (AES), which was held November 4-9, 2007, at the Salt Palace Convention Center in Salt Lake City, UT, U.S.A. The Annual AES meeting is held in conjunction with the annual meeting of the American Institute of Chemical Engineers (AIChE). This year's meeting had a significant emphasis on theoretical and experimental advances in Biological Micro Electro Mechanical Systems (BioMEMS), electrokinetics, and proteomics technologies. A total of 15 sessions were held, within which 71 presentations and 18 posters were discussed. This review provides a brief sampling of the exciting research presented at the conference.

Temperature field measurements of small, nonpremixed flames with use of an Abel inversion of holographic interferograms

Interferometry has been used for many years as a semi-quantitative image-based diagnostic for combustion research. In this paper, we use image-plane, double-pulse holographic interferograms of axisymmetric flames to infer their radial temperature distribution. An Abel inversion is performed on the fringe data to account for line-of-sight integration through the flame. The sensitivity of nonresonant refractive diagnostics decreases inversely with temperature, and the accuracy of the technique is discussed in this context. A small, nonpremixed capillary flame is investigated, and the temperatures inferred from interferometry are compared with those obtained with N2 coherent anti-Stokes Raman spectroscopy thermometry. Additionally, the thermal field of a burning monodisperse methanol droplet stream is investigated interferometrically. Because of their small size, both of these flames challenge the performance limit of temperature interferometery.

Crossed two-beam coherent anti-stokes Raman spectroscopy in dispersive media

Coherent anti-Stokes Raman spectroscopy (CARS) is a nonlinear optical wave mixing process that is used in gas-phase systems to determine the energy distribution of the probed species (usually N2) and, through a fitting procedure, the temperature giving rise to it. CARS signal strengths are maximized when the phase matching condition is met. Because gases are generally non-dispersive, this phase matching condition can be found geometrically as a function of the crossing angles between the CARS beams and their wavelengths. In addition, perfect phase matching in non-dispersive media occurs automatically for collinear beams. To improve spatial resolution, however, intersecting the laser beams is desirable. Being a third-order process, phase matching for CARS in gases typically requires three input laser beams. This paper discusses and demonstrates the issues of phase matching for CARS when the medium is dispersive, and the ability for CARS phase matching to occur with only two crossed laser beams (one pump and one probe). This two-beam X-CARS in dispersive media can be used as an alignment tool for gas-phase CARS and may be relevant as a simpler diagnostic in high-pressure environments. The paper also discusses the effects of non-ideal phase matching in dispersive and non-dispersive media.

Pain in the elderly hospice patient

Pain in the elderly hospice patient presents a complex clinical problem that requires special attention because of the multiple physiologic changes in the older patient. Pain in the elderly is often under-treated despite the fact that this pain can be managed effectively and safely. A basic knowledge of the physiologic changes of aging, as well as the changes in pharmacokinetics, can help the clinician and the hospice team treat pain effectively while minimizing side effects for the patient and the family.

Identification of peripheral vascular disease in elderly subjects using optical spectroscopy

The purpose of this study was to determine the usefulness of near-infrared spectroscopy (NIRS) measurements to identify peripheral vascular disease (PVD). Usefulness was determined by the frequency of a successful test, as well as comparison with standard clinical assessments. Study subjects (N = 117, mean age = 67.8 +/- 8.1 yrs) responded to a free screening for PVD. NIRS was used to measure the relative O2 saturation of hemoglobin in the soleus muscle. The time to 1/2 recovery of O2 saturation (O2T1/2) was measured after 1 minute of repeated plantar flexions using a Cybex Eagle seated calf machine. O2T1/2 was used as many subjects had recovery curves that did not have an exponential line shape. The test was done on both legs and the worst leg was used for analysis. For comparative purposes, a clinical history and physical examination were performed by a physician or nurse practitioner, which included questions on intermittent claudication, examination of peripheral pulses, and questions to identify cardiovascular risk factors. NIRS signals were obtained on 105 of 117 subjects (89% success rate). Subjects with body mass index (BMI) values above 32 appeared to have NIRS O2T1/2 values that were less reliable than subjects with BMI values < or = 32 (77% success rate). The O2T1/2 was longer in subjects with claudication and reduced pulses than in subjects without these conditions. Sensitivity comparing O2T1/2 to claudication and reduced pulse varied from 51-76% and specificity from 65-80%, depending on the cutoff value for O2T1/2 that was used (normal value plus 1 or 2 SD). A longer O2T1/2 was significantly associated with incidence of diabetes, smoking, hypercholesterol, and coronary bypass surgery. In summary, successful NIRS O2T1/2 measurements were made in 77% of the subjects, with failure primarily occurring in obese subjects. NIRS O2T1/2 measurements showed reasonable although not strong agreements with clinical assessment of PVD, and with some risk factors for cardiovascular disease.