Attoliter Volumes Research Articles

Molecular Engineering: Engineering Molecular Organization of Naphthalenediimides: Large Nanosheets with Metallic Conductivity and Attoliter Containers (Adv. Funct. Mater. 20/2011)

On page 3875, M. B. Avinash and T. Govindaraju report the molecularly engineered self-assembly of phenylalanine methylester-appended naphtha-lenediimide, an organic n-type semiconductor, into large free-floating nanosheets and attoliter containers. Nanosheets with high-level molecular ordering exhibit metallic conductivity in the undoped state. Chlorinated co-solvents promote molecular organization into attoliter volume con-tainers, such as nanocups, mesocups, and bowl-like novel and complex architectures, through halogen bonding.

Femtoliter and Attoliter Electrochemical Cells on Chips

Chip electrochemistry is one of the top ambitions of today's electrochemistry. Here, a study for manufacturing electrochemical microcells on chips in a cost-effective, facile, and mass-producible way is presented. The ultrasmall, planar electrochemical cells, ranging from 140 femtoliter to 14 attoliter, can work independently as electroanalytical devices with embedded functional microelectrodes. Electrochemical responses of the miniaturized cells have been characterized by cyclic voltammetry. Ideal steady-state voltammograms were recorded with femtoliter volume cells, indicating the domination of a radical diffusion regime and a greatly improved signal/background ratio. Quasi-thin-layer behavior was observed for attoliter volume cells, which exhibited a special capability of offering accurately confined domains for redox processes. Positive feedback effect of the cells indicated that interactions between the close-by working and reference/counter microelectrodes can be well developed and potentially utilized for trace level electroanalysis. This study vividly offers i) a new protocol of electrochemical chip for applications, ii) a new tool for trace electroanalysis, and iii) a more approachable insight for single molecule electrochemistry in the near future.

Cylindrical Nanopore Electrode and Its Application to the Study of Electrochemical Reaction in Several Hundred Attoliter Volume

A method to fabricate cylindrical nanopore electrodes is presented. The volume of the cavity formed in the cylindrical nanopore electrode can be as small as several hundred attoliters. It has been characterized by using scanning electron microscopy and electrochemical methods. Our results show that the radius of the cavity can affect the diffusion coefficient of a redox species in the cavity. The cylindrical nanopore electrode has also been used to study charge transfer across the interface between an aqueous phase of several hundred attoliters in volume and a bulk chloroform phase. Compared with the same charge-transfer reaction across the interface between a bulk aqueous phase and a bulk chloroform phase, the potential of the charge-transfer reaction has a negative shift. The effect of the phase ratio on the distribution of the supporting electrolyte in the aqueous and organic phases has been used to explain the shift.

Localized Generation of Attoliter Protein Solution Droplets by Electrofocused Liquid−Liquid Separation

We develop a simple technique for the generation of droplets of a high-concentration protein solution of attoliter volume and a diameter of >or=500 nm, containing several thousand protein molecules, and their accurate deposition on micron-size electrodes, standalone or in an array. The technique is based on liquid-liquid phase separation in the homogeneous region of the phase diagram, induced by a nonuniform weak electric field, which also accurately directs the generated droplets toward the electrode. We show that the protein molecules are not chemically modified and retain their integrity, conformation, and activity post deposition. The technique is applicable to the manufacture of various types of protein/microelectrode arrays and is tunable, scalable, and applicable to a broad range of proteins.

Self‐assembled organic nanotubes: Toward attoliter chemistry

AbstractDiverse chemical functionalization of the inner and outer surfaces of the nanotubes enables us to sense and visualize the encapsulation and transport behavior of biomacromolecular guests. The event occurs specifically in attoliter volume nanospace inside the hollow cylinder of the nanotubes. Comparison of the organic nanotube history with that of well‐known carbon nanotubes and a variety of molecular building blocks as tube‐forming compounds were first introduced. Asymmetric organic nanotubes with different inner and outer surfaces were discussed in terms of molecular design, immobilization of functional moieties, and molecular packing. Finally, the practical examples of the organic nanotubes as a nanocontainer, nanochannel, and nanopipette were also described to feature the concept of “attoliter chemistry.” © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2601–2611, 2008

Field enhancement in a circular aperture surrounded by a single channel groove

Numerical analysis of diffraction by a single aperture surrounded by a circular shallow channel in a metallic screen shows the possibility of a 50-fold increase of the electric field intensity inside the central aperture, when compared to the incident field. Detailed analysis of cavity modes and their coupling through surface plasmon wave determine the parameters leading to maximum field enhancement. This effect can be used in high-efficiency single-molecule fluorescence analysis in attoliter volumes.

Molecular imaging of epidermal growth factor receptor in live cells with refractive index sensitivity using dark-field microspectroscopy and immunotargeted nanoparticles

We present a study using plasmonic nanoparticles (NPs) to image epidermal growth factor receptor (EGFR) in live cells. Through detailed analysis of the NP scattering spectra, we determine the intracellular refractive index (RI) within attoliter volumes inside of the living cells. Molecular imaging is demonstrated using anti-EGFR labeled gold nanospheres delivered to cancer cells that overexpress EGFR, with targeted binding confirmed by appropriate control experiments. RI determination is achieved by measurement of the bound NPs' scattering spectra, acquired using a precision dark-field microspectroscopy system and through careful characterization of the NP properties throughout the immuno-labeling procedure. To demonstrate the effect of receptor-mediated uptake, the data are compared to similar spectral measurements using antibody-free NPs, taken up by the cells through nonspecific mechanisms. In these experiments, NP aggregation introduces interparticle effects in the scattering spectra, suggesting that EGFR-mediated internalization of NPs may provide an advantage for maintaining NP isolation upon uptake. The results of this study show the potential utility of dark-field microspectroscopy and labeled NPs for live cell imaging. By demonstrating RI sensitivity over nanometer length scales, this study also presents a potential new avenue for assessing the structure and dynamics of live cells.

Three-dimensional integrated microfluidic architectures enabled through electrically switchable nanocapillary array membranes.

The extension of microfluidic devices to three dimensions requires innovative methods to interface fluidic layers. Externally controllable interconnects employing nanocapillary array membranes (NCAMs) have been exploited to produce hybrid three-dimensional fluidic architectures capable of performing linked sequential chemical manipulations of great power and utility. Because the solution Debye length, kappa(-1), is of the order of the channel diameter, a, in the nanopores, fluidic transfer is controlled through applied bias, polarity and density of the immobile nanopore surface charge, solution ionic strength and the impedance of the nanopore relative to the microfluidic channels. Analyte transport between vertically separated microchannels can be saturated at two stable transfer levels, corresponding to reverse and forward bias. These NCAM-mediated integrated microfluidic architectures have been used to achieve highly reproducible and tunable injections down to attoliter volumes, sample stacking for preconcentration, preparative analyte band collection from an electrophoretic separation, and an actively-tunable size-dependent transport in hybrid structures with grafted polymers displaying thermally-regulated swelling behavior. The synthetic elaboration of the nanopore interior has also been used to great effect to realize molecular separations of high efficiency. All of these manipulations depend critically on the transport properties of individual nanocapillaries, and the study of transport in single nanopores has recently attracted significant attention. Both computation and experimental studies have utilized single nanopores as test beds to understand the fundamental chemical and physical properties of chemistry and fluid flow at nanometer length scales.

Research Profile: Reliable dispensing of attoliter volumes of fluid

Research Profile: Reliable dispensing of attoliter volumes of fluid

Theoretical and experimental investigation of aqueous liquids contained in carbon nanotubes

The dynamic response—as caused by different means of thermal stimulation or pressurization—of aqueous liquid attoliter volumes contained inside carbon nanotubes is investigated theoretically and experimentally. The experiments indicate an energetically driven mechanism responsible for the dynamic multiphase fluid behavior visualized in real time with high spatial resolution using electron microscopy. The theoretical model is formulated using a continuum approach, which combines temperature-dependent mass diffusion with intermolecular interactions in the fluid bulk, as well as in the vicinity of the carbon walls. Intermolecular forces are modeled by Lennard-Jones potentials. Several one-dimensional and axisymmetric cases are considered. These include situations which physically represent liquid volume pinchoff, jetting, or fluid relocation due to thermal stimulation by a steady or modulated electron beam, as well as liquid precipitation (condensation) from vapor due to overcooling or pressurization. Comparisons between theoretical predictions and experimental data demonstrate the ability of the model to describe the characteristic trends observed in the experiments.

Thermal stimulation of aqueous volumes contained in carbon nanotubes: Experiment and modeling

The dynamic response, as caused by thermal stimulation, of aqueous liquid attoliter volumes contained inside multiwall carbon nanotubes is investigated theoretically and experimentally. The experiments indicate an energetically driven mechanism responsible for the dynamic multiphase fluid behavior visualized under high resolution in the transmission electron microscope. The theoretical model is formulated using a continuum approach, which combines temperature-dependent diffusion with intermolecular interactions in the fluid bulk, as well as in the vicinity of the carbon wall. Intermolecular van der Waals forces are modeled by Lennard-Jones 12-6 potentials. Comparisons between theoretical predictions and experimental data demonstrate the ability of the model to describe the major trends observed in the experiments.

Nanodispenser for attoliter volume deposition using atomic force microscopy probes modified by focused-ion-beam milling

In this letter, we describe the on-demand dispensing of single liquid droplets with volumes down to a few attoliters and submicrometric spacing. This dispensing is achieved using a standard atomic force microscope probe, with a 200nm aperture at the tip apex, opened by focused ion beam milling. The inside of the tip is used as reservoir for the liquid. This maskless dispensing, realized in ambient environment, permits the direct creation of droplet arrays. Nanoparticles, suspended in the liquid, were organized on a surface.

Self-assembled microarrays of attoliter molecular vessels.

High-density microarrays of attoliter volume elements can be created (see figure) within minutes in a parallel and effortless manner by using self-assembly of nanometer-size components based on biological recognition. These ultrasmall volumes allow localization of single molecules, screening for (bio)chemical properties, and assessing the performance of confined chemical reactions.

Density fluctuations under confinement: when is a fluid not a fluid?

Knowing the behavior of a fluid in small volumes is essential for the understanding of a vast array of common problems in science, such as biological interactions, fracture propagation, and molecular tribology and adhesion, as well as pressure solvation and other geophysical processes. When a fluid is confined, its phase behavior is altered and excluded-volume effects become apparent. Pioneering measurements performed with the surface forces apparatus have revealed so-called structural or oscillatory solvation forces as well as the occurrence of a finite shear stress, which was interpreted as a solidification transition. Here, we report measurements obtained with an extended surface forces apparatus, which makes use of fast spectral correlation to gain insight into the behavior of a thin film of cyclohexane confined within attoliter volumes, with simultaneous measurement of film thickness and refractive index. With decreasing pore width, cyclohexane is found to undergo a drastic transition from a three-dimensional bulk fluid to a two-dimensional adsorbate with strikingly different properties. Long-range density fluctuations of unexpected magnitude are observed.

Probing Phase-Separation Behavior in Polymer-Blend Microparticles: Effects of Particle Size and Polymer Mobility

We describe a new method for probing phase separation of polymer-blend systems in spherical microparticles generated from microdroplets of dilute polymer solution. Two-dimensional optical diffractiona technique sensitive to material inhomogeneity on a length scale of ≈30 nmis used to probe phase-separation behavior of bulk-immiscible polymers in attoliter and femtoliter volumes. Under conditions of rapid solvent evaporation (≈2−10 ms) and relatively low polymer mobility, homogeneous composite particles can be formed using different polymers that ordinarily undergo phase separation in bulk preparations. We show that, for homogeneous particles, the refractive index (related to material dielectric constant) can be tuned by adjusting the relative weight fractions of polymers in the blend. Molecular dynamics simulations of polymer-blend microparticles are presented to illustrate at a molecular level effects of polymer interactions and boundary conditions on phase separation in polymer-blend nanoparticles. Fina...

Voltammetric and pharmacological characterization of dopamine release from single exocytotic events at rat pheochromocytoma (PC12) cells.

Although rat pheochromocytoma (PC12) neurotransmitter storage vesicles are known to contain a variety of neurotransmitters including catecholamines, there is little evidence that the molecular species detected during amperometric monitoring of exocytosis is a catecholamine. Rather, as these are catecholamine-containing cells, one assumes catecholamines are released. Additionally, although the total amount of transmitter released can be quantified, it has been extremely difficult to evaluate the concentration at the point of release for each exocytosis event. Interpreting voltammograms obtained in the attoliter volume affected between the electrode and the cell and defined by the size of the exocytosis pore during exocytosis is an extreme analytical challenge. Here we use voltammetry of approximately 10(-19) mol released from individual exocytosis events to identify, along with pharmacological evidence, the released compound at PC12 cells as a catecholamine, most likely dopamine. The area of the electrode at which oxidation occurs following an exocytosis event is proportional to the temporal delay prior to acquisition of a voltammogram. This model allows determination of relative concentrations from individual release events and has been used to examine events at control cells and cells incubated with the dopamine precursor, L-3,4-dihydroxyphenylalanine (L-DOPA). Exposure to L-DOPA (100 microM for 1 h) results in 145 detectable events for 11 cells compared to 77 events for 29 control cells, clearly indicating that vesicles can be "loaded" with dopamine. However, the concentrations measured at the electrode surface provide similar distributions for both L-DOPA-treated and control cells. Cyclic voltammetric measurements of relative concentration for zeptomole levels of transmitter in attoliter volumes provide evidence that loading vesicles by increased transmitter synthesis does not lead to elevated concentrations at individual release sites.

Confinement and Manipulation of Individual Molecules in Attoliter Volumes

We report observation of fluorescence from individual rhodamine 6G molecules in streams of charged 1-μm-diameter water droplets. With this approach, illumination volumes comparable to diffraction-limited fluorescence microscopy techniques (≤500 aL) are achieved, resulting in similarly high contrast between single-molecule fluorescence signals and nonfluorescent background. However, since the fluorescent molecules are confined to electrically charged droplets, in situ electrodynamic manipulation (e.g., focusing, switching, or merging) can be accomplished in a straightforward manner, allowing experimental control over both the delivery of molecules of interest to the observation region and the laser-molecule interaction time. As illustrated by photocount statistics that are independent of molecular diffusion and spatial characteristics of the excitation field, individual rhodamine 6G molecules in 1-μm droplets are reproducibly delivered to a target a few micrometers in diameter at a rate of between 10 and 100 Hz, with laser beam transit times more than 1 order of magnitude longer than diffusion-limited laser-molecule interaction times in equivalent volumes of free solution.

Direct Measurement of Single-Molecule Diffusion and Photodecomposition in Free Solution

Continuous monitoring of submillisecond free-solution dynamics of individual rhodamine-6G molecules and 30-base single-stranded DNA tagged with rhodamine was achieved. Fluorescence images were recorded from the same set of isolated molecules excited either through the evanescent field at the quartz-liquid interface or as a thin layer of solution defined by micron-sized wires, giving diffraction-limited resolution of inter-connected attoliter volume elements. The single-molecule diffusion coefficients were smaller and the unimolecular photodecomposition lifetimes were longer for the dye-DNA covalent complex as compared with those of the dye molecule itself. Unlike bulk studies, stochastic behavior was found for individual molecules of each type, and smaller diffusion coefficients were observed.