Imaging Active Surface Processes in Barnacle Adhesive Interfaces

Surface plasmon resonance imaging (SPRI) and voltammetry were used simultaneously to monitor Amphibalanus (=Balanus) amphitrite barnacles reattached and grown on gold-coated glass slides in artificial seawater. Upon reattachment, SPRI revealed rapid surface adsorption of material with a higher refractive index than seawater at the barnacle/gold interface. Over longer time periods, SPRI also revealed secretory activity around the perimeter of the barnacle along the seawater/gold interface extending many millimeters beyond the barnacle and varying in shape and region with time. Ex situ experiments using attenuated total reflectance infrared (ATR-IR) spectroscopy confirmed that reattachment of barnacles was accompanied by adsorption of protein to surfaces on similar time scales as those in the SPRI experiments. Barnacles were grown through multiple molting cycles. While the initial reattachment region remained largely unchanged, SPRI revealed the formation of sets of paired concentric rings having alternately darker/lighter appearance (corresponding to lower and higher refractive indices, respectively) at the barnacle/gold interface beneath the region of new growth. Ex situ experiments coupling the SPRI imaging with optical and FTIR microscopy revealed that the paired rings coincide with molt cycles, with the brighter rings associated with regions enriched in amide moieties. The brighter rings were located just beyond orifices of cement ducts, consistent with delivery of amide-rich chemistry from the ducts. The darker rings were associated with newly expanded cuticle. In situ voltammetry using the SPRI gold substrate as the working electrode revealed presence of redox active compounds (oxidation potential approx 0.2 V vs Ag/AgCl) after barnacles were reattached on surfaces. Redox activity persisted during the reattachment period. The results reveal surface adsorption processes coupled to the complex secretory and chemical activity under barnacles as they construct their adhesive interfaces.

Thermally activated long range electron transport in living biofilms

Microbial biofilms grown utilizing electrodes as metabolic electron acceptors or donors are a new class of biomaterials with distinct electronic properties. Here we report that electron transport through living electrode-grown Geobacter sulfurreducens biofilms is a thermally activated process with incoherent redox conductivity. The temperature dependency of this process is consistent with electron-transfer reactions involving hemes of c-type cytochromes known to play important roles in G. sulfurreducens extracellular electron transport. While incoherent redox conductivity is ubiquitous in biological systems at molecular-length scales, it is unprecedented over distances it appears to occur through living G. sulfurreducens biofilms, which can exceed 100 microns in thickness.

Self-Assembly of Protein Nanofibrils Orchestrates Calcite Step Movement through Selective Nonchiral Interactions

The recognition of atomically distinct surface features by adsorbed biomolecules is central to the formation of surface-templated peptide or protein nanostructures. On mineral surfaces such as calcite, biomolecular recognition of, and self-assembly on, distinct atomic kinks and steps could additionally orchestrate changes to the overall shape and symmetry of a bulk crystal. In this work, we show through in situ atomic force microscopy (AFM) experiments that an acidic 20 kDa cement protein from the barnacle Megabalanus rosa (MRCP20) binds specifically to step edge atoms on {101̅4} calcite surfaces, remains bound and further assembles over time to form one-dimensional nanofibrils. Protein nanofibrils are continuous and organized at the nanoscale, exhibiting striations with a period of ca. 45 nm. These fibrils, templated by surface steps of a preferred geometry, in turn selectively dissolve underlying calcite features displaying the same atomic arrangement. To demonstrate this, we expose the protein solution to bare and fibril-associated rhombohedral etch pits to reveal that nanofibrils accelerate only the movement of fibril-forming steps when compared to undecorated steps exposed to the same solution conditions. Calcite mineralized in the presence of MRCP20 results in asymmetric crystals defined by frustrated faces with shared mirror symmetry, suggesting a similar step-selective behavior by MRCP20 in crystal growth. As shown here, selective surface interactions with step edge atoms lead to a cooperative regime of calcite modification, where templated long-range protein nanostructures shape crystals.

Catch and release: integrated system for multiplexed detection of bacteria

An integrated system with automated immunomagnetic separation and processing of fluidic samples was demonstrated for multiplexed optical detection of bacterial targets. Mixtures of target-specific magnetic bead sets were processed in the NRL MagTrap with the aid of rotating magnet arrays that entrapped and moved the beads within the channel during reagent processing. Processing was performed in buffer and human serum matrixes with 10-fold dilutions in the range of 10(2)-10(6) cells/mL of target bacteria. Reversal of magnets' rotation post-processing released the beads back into the flow and moved them into the microflow cytometer for optical interrogation. Identification of the beads and the detection of PE fluorescence were performed simultaneously for multiplexed detection. Multiplexing was performed with specifically targeted bead sets to detect E. coli 0157.H7, Salmonella Common Structural Antigen, Listeria sp., and Shigella sp., dose-response curves were obtained, and limits of detection were calculated for each target in the buffer and clinical matrix. Additional tests demonstrated the potential for using the MagTrap to concentrate target from larger volumes of sample prior to the addition of assay reagents.

Automated processing integrated with a microflow cytometer for pathogen detection in clinical matrices

A spinning magnetic trap (MagTrap) for automated sample processing was integrated with a microflow cytometer capable of simultaneously detecting multiple targets to provide an automated sample-to-answer diagnosis in 40 min. After target capture on fluorescently coded magnetic microspheres, the magnetic trap automatically concentrated the fluorescently coded microspheres, separated the captured target from the sample matrix, and exposed the bound target sequentially to biotinylated tracer molecules and streptavidin-labeled phycoerythrin. The concentrated microspheres were then hydrodynamically focused in a microflow cytometer capable of 4-color analysis (two wavelengths for microsphere identification, one for light scatter to discriminate single microspheres and one for phycoerythrin bound to the target). A three-fold decrease in sample preparation time and an improved detection limit, independent of target preconcentration, was demonstrated for detection of Escherichia coli 0157:H7 using the MagTrap as compared to manual processing. Simultaneous analysis of positive and negative controls, along with the assay reagents specific for the target, was used to obtain dose-response curves, demonstrating the potential for quantification of pathogen load in buffer and serum.

Hydrodynamic focusing--a versatile tool

The control of hydrodynamic focusing in a microchannel has inspired new approaches for microfluidic mixing, separations, sensors, cell analysis, and microfabrication. Achieving a flat interface between the focusing and focused fluids is dependent on Reynolds number and device geometry, and many hydrodynamic focusing systems can benefit from this understanding. For applications where a specific cross-sectional shape is desired for the focused flow, advection generated by grooved structures in the channel walls can be used to define the shape of the focused flow. Relative flow rates of the focused flow and focusing streams can be manipulated to control the cross-sectional area of the focused flows. This paper discusses the principles for defining the shape of the interface between the focused and focusing fluids and provides examples from our lab that use hydrodynamic focusing for impedance-based sensors, flow cytometry, and microfabrication to illustrate the breadth of opportunities for introducing new capabilities into microfluidic systems. We evaluate each example for the advantages and limitations integral to utilization of hydrodynamic focusing for that particular application.

Optofluidic characterization of marine algae using a microflow cytometer

The effects of global warming, pollution in river effluents, and changing ocean currents can be studied by characterizing variations in phytoplankton populations. We demonstrate the design and fabrication of a Microflow Cytometer for characterization of phytoplankton. Guided by chevron-shaped grooves on the top and bottom of a microfluidic channel, two symmetric sheath streams wrap around a central sample stream and hydrodynamically focus it in the center of the channel. The lasers are carefully chosen to provide excitation light close to the maximum absorbance wavelengths for the intrinsic fluorophores chlorophyll and phycoerythrin, and the excitation light is coupled to the flow cytometer through the use of an optical fiber. Fluorescence and light scatter are collected using two multimode optical fibers placed at 90-degree angles with respect to the excitation fiber. Light emerging from these collection fibers is directed through optical bandpass filters into photomultiplier tubes. The cytometer measured the optical and side scatter properties of Karenia b., Synechococcus sp., Pseudo-Nitzchia, and Alexandrium. The effect of the sheath-to-sample flow-rate ratio on the light scatter and fluorescence of these marine microorganisms was investigated. Reducing the sample flow rate from 200 μL/min to 10 μL/min produced a more tightly focused sample stream and less heterogeneous signals.

Dynamic reversibility of hydrodynamic focusing for recycling sheath fluid

The phenomenon of "unmixing" has been demonstrated in microfluidic mixers, but here we manipulate laminar flow streams back to their original positions in order to extend the operational utility of an analytical device where no mixing is desired. Using grooves in the channel wall, we passively focus a sample stream with two sheath streams to center it in a microchannel for optical analysis. Even though the sample stream is completely surrounded by sheath fluid, reversing the orientation of the grooves in the channel walls returns the sample stream to its original position with respect to the sheath streams. We demonstrate the separation of the sample stream from the contiguous sheath streams and the recycling of the sheath fluid using the reversibility of laminar flow. Polystyrene microspheres and fluorescent dye were used to quantify the performance of the unsheathing process. We found that the maximum numbers of microspheres and all of the fluorescent dye were recaptured at sheath recycling levels <92%. The use of this sheathing technique has previously been demonstrated in a sensitive microflow cytometer; the unsheathing capability now provides the opportunity to recover particles from the sensor with minimal dilution or to recycle the sheath fluid for long-term unattended operation.

A hard microflow cytometer using groove-generated sheath flow for multiplexed bead and cell assays

With a view toward developing a rugged microflow cytometer, a sheath flow system was micromachined in hard plastic (polymethylmethacrylate) for analysis of particles and cells using optical detection. Six optical fibers were incorporated into the interrogation region of the chip, in which hydrodynamic focusing narrowed the core stream to ~35 μm × 40 μm. The use of a relatively large channel at the inlet as well as in the interrogation region (375 μm × 125 μm) successfully minimized the risk of clogging. The device could withstand pressures greater than 100 psi without leaking. Assays using both coded microparticles and cells were demonstrated using the microflow cytometer. Multiplexed immunoassays detected nine different bacteria and toxins using a single mixture of coded microspheres. A549 cancer cells processed with locked nucleic acid probes were evaluated using fluorescence in situ hybridization.

Multiplexed detection of bacteria and toxins using a microflow cytometer

A microfabricated flow cytometer was used to demonstrate multiplexed detection of bacteria and toxins using fluorescent coded microspheres. Antibody-coated microspheres bound biothreat targets in a sandwich immunoassay format. The microfluidic cytometer focused the microspheres in three dimensions within the laser interrogation region using passive groove structures to surround the sample stream with sheath fluid. Optical analysis at four different wavelengths identified the coded microspheres and quantified target bound by the presence of phycoerythrin tracer. The multiplexed assays in the microflow cytometer had performance approaching that of a commercial benchtop flow cytometer. The respective limits of detection for bacteria (Escherichia coli, Listeria, and Salmonella) were found to be 10(3), 10(5), and 10(4) cfu/mL for the microflow cytometer and 10(3), 10(6), and 10(5) cfu/mL for the commercial system. Limits of detection for the toxins (cholera toxin, staphylococcal enterotoxin B, and ricin) were 1.6, 0.064, and 1.6 ng/mL for the microflow cytometer and 1.6, 0.064, and 8.0 ng/mL for the commercial system.

Multi-wavelength microflow cytometer using groove-generated sheath flow

A microflow cytometer was developed that ensheathed the sample (core) fluid on all sides and interrogated each particle in the sample stream at four different wavelengths. Sheathing was achieved by first sandwiching the core fluid with the sheath fluid laterally via fluid focusing. Chevron-shaped groove features fabricated in the top and bottom of the channel directed sheath fluid from the sides to the top and bottom of the channel, completely surrounding the sample stream. Optical fibers inserted into guide channels provided excitation light from diode lasers at 532 and 635 nm and collected the emission wavelengths. Two emission collection fibers were connected to PMTs through a multimode fiber splitter and optical filters for detection at 635 nm (scatter), 665 nm and 700 nm (microsphere identification) and 565 nm (phycoerythrin tracer). The cytometer was capable of discriminating microspheres with different amounts of the fluorophores used for coding and detecting the presence of a phycoerythrin antibody complex on the surface of the microspheres. Assays for Escherichia coli were compared with a commercial Luminex flow cytometer.

Fluoroimmunoassays using the NRL array biosensor

Array-based biosensor technology offers the user the ability to detect and quantify multiple targets in multiple samples simultaneously (Analytical Sciences 23:5-10, 2007). The NRL Array Biosensor has been developed with the aim of creating a system for sensitive, rapid, on-site screening for multiple targets of interest. This system is fluorescence-based, using evanescent illumination of a waveguide, and has demonstrated the use of both sandwich and competitive immunoassays for the detection of both high and low molecular weight targets, respectively. The current portable, automated system has demonstrated detection of a wide variety of analytes ranging from simple chemical compounds to entire bacterial cells, with applications in food safety, disease diagnosis, homeland security and environmental monitoring.

The good, the bad, and the tiny: a review of microflow cytometry

Recent developments in microflow cytometry have concentrated on advancing technology in four main areas: (1) focusing the particles to be analyzed in the microfluidic channel, (2) miniaturization of the fluid-handling components, (3) miniaturization of the optics, and (4) integration and applications development. Strategies for focusing particles in a narrow path as they pass through the detection region include the use of focusing fluids, nozzles, and dielectrophoresis. Strategies for optics range from the use of microscope objectives to polymer waveguides or optical fibers embedded on-chip. While most investigators use off-chip fluidic control, there are a few examples of integrated valves and pumps. To date, demonstrations of applications are primarily used to establish that the microflow systems provide data of the same quality as laboratory systems, but new capabilities-such as automated sample staining-are beginning to emerge. Each of these four areas is discussed in detail in terms of the progress of development, the continuing limitations, and potential future directions for microflow cytometers.

Two simple and rugged designs for creating microfluidic sheath flow

A simple design capable of 2-dimensional hydrodynamic focusing is proposed and successfully demonstrated. In the past, most microfluidic sheath flow systems have often only confined the sample solution on the sides, leaving the top and bottom of the sample stream in contact with the floor and ceiling of the channel. While relatively simple to build, these designs increase the risk of adsorption of sample components to the top and bottom of the channel. A few designs have been successful in completely sheathing the sample stream, but these typically require multiple sheath inputs and several alignment steps. In the designs presented here, full sheathing is accomplished using as few as one sheath input, which eliminates the need to carefully balance the flow of two or more sheath inlets. The design is easily manufactured using current microfabrication techniques. Furthermore, the sample and sheath fluid can be subsequently separated for recapture of the sample fluid or re-use of the sheath fluid. Designs were demonstrated in poly(dimethylsiloxane) (PDMS) using soft lithography and poly(methyl methacrylate) (PMMA) using micromilling and laser ablation.

Target delivery in a microfluidic immunosensor

A study is presented that examines the effect of microfluidic mixing elements on direct and sandwich assays performed in microchannels. Patterned grooves were embossed in the top of microchannels made in PDMS using soft lithography. The grooves redirected the fluid flowing in the channel, enhancing delivery of the target from the bulk fluid to the surface and preventing the formation of a depletion layer at the surface. Comparing assays in grooved and plain channels demonstrated that the mixers improved assay results by 26-46%. A computational flow analysis showed that the grooves caused virtual particles in the bulk flow to come close to the surface ( approximately 11 microm) which is consistent with the signal increase seen experimentally. Direct assays for several concentrations of CY5-labeled biotin were performed in the microchannels. The mixers also improved signal intensity in sandwich assays for botulinum toxin which required mixing of the reagents as well as the direction of the target to the surface.

The array biosensor: portable, automated systems

With recent advances in surface chemistry, microfluidics, and data analysis, there are ever increasing reports of array-based methods for detecting and quantifying multiple targets. However, only a few systems have been described that require minimal preparation of complex samples and possess a means of quantitatively assessing matrix effects. The NRL Array Biosensor has been developed with the goal of rapid and sensitive detection of multiple targets from multiple samples analyzed simultaneously. A key characteristic of this system is its two-dimensional configuration, which allows controls and standards to be analyzed in parallel with unknowns. Although the majority of our work has focused on instrument automation and immunoassay development, we have recently initiated efforts to utilize alternative recognition molecules, such as peptides and sugars, for detection of a wider variety of targets. The array biosensor has demonstrated utility for a variety of applications, including food safety, disease diagnosis, monitoring immune response, and homeland security, and is presently being transitioned to the commercial sector for manufacturing.

Toolbox for the design of optimized microfluidic components

A computational "toolbox" for the a priori design of optimized microfluidic components is presented. These components consist of a microchannel under low-Reynolds number, pressure-driven flow, with an arrangement of grooves cut into the top and bottom to generate a tailored cross-channel flow. An advection map for each feature (i.e., groove of a particular shape and orientation) predicts the lateral transport of fluid within the channel due to that feature. We show that applying these maps in sequence generates an excellent representation of the outflow distribution for complex designs that combine these basic features. The effect of the complex three-dimensional flow field can therefore be predicted without solving the governing flow equations through the composite geometry, and the resulting distribution of fluids in the channel is used to evaluate how well a component performs a specified task. The generation and use of advection maps is described, and the toolbox is applied to determine optimal combinations of features for specified mixer sizes and mixing metrics.

Array biosensor: optical and fluidics systems

Optical and fluidics systems have been developed as central components for an automated array biosensor. Disposable planar waveguides are patterned with immobilized capture antibodies using a physically isolated patterning (PIP) method. The PIP method enables simultaneous deposition of several antibodies and completely circumvents cross-immobilization problems encountered with other array deposition processes. A multi-channel fluidics cell allows numerous assays to be performed on the patterned waveguide. The sensing arrays are optically interrogated using a diode laser with a tailored output to optimize coupling to and maximize excitation uniformity within the waveguide. A patterned cladding is employed to optically isolate the waveguide from perturbations induced by the permanently attached flow cells. Compact optics image the evanescently excited fluorescence onto a large area, cooled CCD array. The image data is processed and automated signal analysis corrects for local background and noise variations.

A microfluidic mixer with grooves placed on the top and bottom of the channel

A new microfluidic mixer is presented consisting of a rectangular channel with grooves placed in the top and bottom. This not only increases the driving force behind the lateral flow, but allows for the formation of advection patterns that cannot be created with structures on the bottom alone. Chevrons, pointing in opposite directions on the top and bottom, are used to create a pair of vortices positioned side by side. Stripes running the width of the channel generate a pair of vertically stacked vortices. Computational fluid dynamics (CFD) simulations are used to model the behavior of the systems and provide velocity maps at cross-sections within the mixer. Experiments demonstrate the mixing that results when two segregated species enter the mixer side-by-side and pass through two cycles of the mixer (i.e., two alternating sets of four stripes and four chevrons).

Design and evaluation of a Dean vortex-based micromixer

A mixer, based on the Dean vortex, is fabricated and tested in an on-chip format. When fluid is directed around a curve under pressure driven flow, the high velocity streams in the center of the channel experience a greater centripetal force and so are deflected outward. This creates a pair of counter-rotating vortices moving fluid toward the inner wall at the top and bottom of the channel and toward the outer wall in the center. For the geometries studied, the vortices were first seen at Reynolds numbers between 1 and 10 and became stronger as the flow velocity is increased. Vortex formation was monitored in channels with depth/width ratios of 0.5, 1.0, and 2.0. The lowest aspect ratio strongly suppressed vortex formation. Increasing the aspect ratio above 1 appeared to provide improved mixing. This design has the advantages of easy fabrication and low surface area.

Surface-immobilized self-assembled protein-based quantum dot nanoassemblies

Luminescent semiconductor quantum dot (QD)-based optical biosensors have the potential to overcome many of the limitations associated with using conventional organic dyes for biodetection. We have previously demonstrated a hybrid QD-protein-based fluorescence resonance energy transfer (FRET) sensor. Although the QD acted as an energy donor and a protein scaffold in the sensor, recognition and specificity were derived from the proteins. Transitioning this hybrid prototype sensor into flow cells and integrated devices will require a surface-immobilization strategy that allows the QD-based sensor to sample the environment and still maintain a distinct protein-covered QD architecture. We demonstrate a self-assembled strategy designed to accomplish this. Using glass slides coated with a monolayer of neutravidin (NA) as the template, QDs with maltose binding protein (MBP) and avidin coordinated to their surface were attached to the glass slides in discrete patterns using an intermediary bridge of biotinylated MBP or antibody linkers. Control of the surface location and concentration of the QD-protein-based structures is demonstrated. The utility of this self-assembly strategy is further demonstrated by assembling a QD-protein structure that allows the QDs to engage in FRET with a dye located on the surface-covering protein.

A portable array biosensor for detecting multiple analytes in complex samples

The Multi-Analyte Array Biosensor (MAAB) has been developed at the Naval Research Laboratory (NRL) with the goal of simultaneously detecting and identifying multiple target agents in complex samples with minimal user manipulation. This paper will focus on recent improvements in the biochemical and engineering aspects of this instrument. These improvements have enabled the expansion of the repertoire of analytes detected to include Salmonella typhimurium and Listeria monocytogenes, and also expanded the different sample matrices tested. Furthermore, all components of the biochemical assays could be prepared well in advance of sample testing, resulting in a "plug-and-play" methodology. Simultaneous detection of three toxins (ricin, staphylococcal enterotoxin B, and cholera toxin) was demonstrated using a novel fluidics cube module that limits the number of manipulations to only the initial sample loading. This work demonstrates the utility of the MAAB for rapid analysis of complex samples with multianalyte capability, with a minimum of operator manipulations required for either sample preparation or final analysis.

Array biosensor for detection of toxins

The array biosensor is capable of detecting multiple targets rapidly and simultaneously on the surface of a single waveguide. Sandwich and competitive fluoroimmunoassays have been developed to detect high and low molecular weight toxins, respectively, in complex samples. Recognition molecules (usually antibodies) were first immobilized in specific locations on the waveguide and the resultant patterned array was used to interrogate up to 12 different samples for the presence of multiple different analytes. Upon binding of a fluorescent analyte or fluorescent immunocomplex, the pattern of fluorescent spots was detected using a CCD camera. Automated image analysis was used to determine a mean fluorescence value for each assay spot and to subtract the local background signal. The location of the spot and its mean fluorescence value were used to determine the toxin identity and concentration. Toxins were measured in clinical fluids, environmental samples and foods, with minimal sample preparation. Results are shown for rapid analyses of staphylococcal enterotoxin B, ricin, cholera toxin, botulinum toxoids, trinitrotoluene, and the mycotoxin fumonisin. Toxins were detected at levels as low as 0.5 ng mL(-1).

A comparison of imaging methods for use in an array biosensor

An array biosensor has been developed which uses an actively-cooled, charge-coupled device (CCD) imager. In an effort to save money and space, a complementary metal-oxide semiconductor (CMOS) camera and photodiode were tested as replacements for the cooled CCD imager. Different concentrations of CY5 fluorescent dye in glycerol were imaged using the three different detection systems with the same imaging optics. Signal discrimination above noise was compared for each of the three systems.

Voltage-induced inhibition of antigen-antibody binding at conducting optical waveguides

Optical waveguides coated with electrically conducting indium-tin oxide (ITO) are demonstrated here as a new class of substrate for fluorescent immunosensors. These waveguides combine electrochemical control with evanescent excitation and image-based detection. Presented here are preliminary results utilizing these waveguides that demonstrate influence of waveguide voltage on antigen binding. Specifically, waveguide surfaces were bisected into electrically addressable halves, anti-ovalbumin immobilized in patterns on their surfaces, and a 1.3 V bias applied between waveguide halves in the presence of Cy5-labeled ovalbumin in 10 mM phosphate buffer (pH 7.4) containing 150 mM NaCl and 0.05% Tween-20. Fluorescence imaging indicated that binding of the antigen to positively biased waveguide halves was inhibited nearly 10-fold compared with negatively biased waveguide halves and unbiased controls. Furthermore, it is shown that ovalbumin binding to positively biased waveguide regions is regenerated after removal of applied voltage. These results suggest that electrochemical control of immunosensor substrates can be used as a possible strategy toward minimizing cross-reactive binding and/or nonspecific adsorption, immunosensor regeneration, and controlled binding.

Integrating waveguide biosensor

A capillary biosensor is demonstrated which uses the waveguiding properties of the capillary to integrate the signal over an increased surface area without simultaneously increasing the background noise from the detector. This biosensor achieves limits of detection of 30-50 pg/mL in immunoassays using a diode laser for excitation and a PMT for detection. This is approximately 2 orders of magnitude greater sensitivity than was achieved using the same immunoassay reagents in a fiber optic biosensor or a planar array biosensor. Two different approaches to using the capillaries as immunosensors are described, either of which could be adapted for multianalyte sensing.

Attachment of plastic fluidic components to glass sensing surfaces

Two types of flow manifolds have been developed in order to provide a reliable method for attaching plastic microfluidic systems to glass sensing surfaces. A permanently mounted flow manifold has grooves in the inter-channel barriers to confine the epoxy used for attachment. The reusable flow manifold has inter-channel barriers composed of both plexiglass for rigidity and polydimethysiloxane for leak-free attachment.

Fluidics cube for biosensor miniaturization

To create a small, portable, fully automated biosensor, a compact means of fluid handling is required. We designed, manufactured, and tested a "fluidics cube" for such a purpose. This cube, made of thermoplastic, contains reservoirs and channels for liquid samples and reagents and operates without the use of any internal valves or meters; it is a passive fluid circuit that relies on pressure relief vents to control fluid movement. We demonstrate the ability of pressure relief vents to control fluid movement and show how to simply manufacture or modify the cube. Combined with the planar array biosensor developed at the Naval Research Laboratory, it brings us one step closer to realizing our goal of a handheld biosensor capable of analyzing multiple samples for multiple analytes.

Simultaneous detection of six biohazardous agents using a planar waveguide array biosensor

Recently, we demonstrated that an array biosensor could be used with cocktails of fluorescent antibodies to perform three assays simultaneously on a single substrate, and that multiple samples could be analyzed in parallel. We extend this technology to demonstrate the simultaneous analysis of six samples for six different hazardous analytes, including both bacteria and protein toxins. The level of antibody cross-reactivity is explored, revealing a possible common epitope in two of the toxins. A panel of environmental interferents was added to the samples; these interferents neither prevented the detection of the analytes nor caused false-positive responses.

Induction of BIM, a proapoptotic BH3-only BCL-2 family member, is critical for neuronal apoptosis

Sympathetic neuronal death induced by nerve growth factor (NGF) deprivation requires the macromolecular synthesis-dependent translocation of BAX from the cytosol to mitochondria and its subsequent integration into the mitochondrial outer membrane, followed by BAX-mediated cytochrome c (cyt c) release. The gene products triggering this process remain unknown. Here, we report that BIM, a member of the BH3-only proapoptotic subfamily of the BCL-2 protein family, is one such molecule. NGF withdrawal induced expression of BIM(EL), an integral mitochondrial membrane protein that functions upstream of (or in parallel with) the BAX/BCL-2 and caspase checkpoints. Bim deletion conferred protection against developmental and induced neuronal apoptosis in both central and peripheral populations, but only transiently, suggesting that BIM--and perhaps other BH3-only proteins--serve partially redundant functions upstream of BAX-mediated cyt c release.

Development of cranial parasympathetic ganglia requires sequential actions of GDNF and neurturin

The neurotrophic factors that influence the development and function of the parasympathetic branch of the autonomic nervous system are obscure. Recently, neurturin has been found to provide trophic support to neurons of the cranial parasympathetic ganglion. Here we show that GDNF signaling via the RET/GFR(alpha)1 complex is crucial for the development of cranial parasympathetic ganglia including the submandibular, sphenopalatine and otic ganglia. GDNF is required early for proliferation and/or migration of the neuronal precursors for the sphenopalatine and otic ganglia. Neurturin exerts its effect later and is required for further development and maintenance of these neurons. This switch in ligand dependency during development is at least partly governed by the altered expression of GFR(α) receptors, as evidenced by the predominant expression of GFR(α)2 in these neurons after ganglion formation.

A ganglioside-based assay for cholera toxin using an array biosensor

A rapid assay for cholera toxin (CT) has been developed using a fluorescence-based biosensor. This sensor was capable of analyzing six samples simultaneously for CT in 20 min with few manipulations required by the operator. The biochemical assays utilized a ganglioside-"capture" format: ganglioside GM1, utilized for capture of analyte, was immobilized in discrete locations on the surface of the optical waveguide. Binding of CT to immobilized GM1 was demonstrated with direct assays (using fluorescently labeled CT) and "sandwich" immunoassays (using fluorescently labeled tracer antibodies). Limits of detection for CT were 200 ng/ml in direct assays and 40 ng/ml and 1 microg/ml in sandwich-type assays performed using rabbit and goat tracer antibodies. Binding of CT to other glycolipid capture reagents was also observed. While significant CT binding was observed to loci patterned with GD1b, Gb3, and Gb4, CT did not bind significantly to immobilized GT1b at the concentrations tested. This is the first description of such a non-antibody-based recognition system in a multi-specific planar array sensor.

Array biosensor for detection of biohazards

A fluorescence-based biosensor has been developed for simultaneous analysis of multiple samples for multiple biohazardous agents. A patterned array of antibodies immobilized on the surface of a planar waveguide is used to capture antigen present in samples; bound analyte is then quantified by means of fluorescent tracer antibodies. Upon excitation of the fluorophore by a small diode laser, a CCD camera detects the pattern of fluorescent antibody:antigen complexes on the waveguide surface. Image analysis software correlates the position of fluorescent signals with the identity of the analyte. This array biosensor has been used to detect toxins, toxoids, and killed or non-pathogenic (vaccine) strains of pathogenic bacteria. Limits of detection in the mid-ng/ml range (toxins and toxoids) and in the 10(3)-10(6) cfu/ml range (bacterial analytes) were achieved with a facile 14-min off-line assay. In addition, a fluidics and imaging system has been developed which allows automated detection of staphylococcal enterotoxin B (SEB) in the low ng/ml range.

Analysis of the retrograde transport of glial cell line-derived neurotrophic factor (GDNF), neurturin, and persephin suggests that in vivo signaling for the GDNF family is GFRalpha coreceptor-specific

Neurturin (NRTN) and glial cell line-derived neurotrophic factor (GDNF) are members of a family of trophic factors with similar actions in vitro on certain neuronal classes. Retrograde transport of GDNF and NRTN was compared in peripheral sensory, sympathetic, and motor neurons to determine whether in vivo these factors are transported selectively by different neuronal populations. After sciatic nerve injections, NRTN was transported by sensory neurons of the dorsal root ganglion (DRG). Competition studies demonstrated only limited cross-competition between NRTN and GDNF, indicating selective receptor-mediated transport of these factors. By using immunohistochemistry, we identified two populations of NRTN-transporting DRG neurons: a major population of small, RET-positive, IB4-positive, non-TrkA-expressing neurons that also show the ability to transport GDNF and a minor population of calretinin-expressing neurons that fail to transport GDNF. Spinal motor neurons in the adult showed relatively less ability to transport NRTN than to transport GDNF, although NRTN prevented the cell death of neonatal motor neurons in a manner very similar to GDNF (Yan et al., 1995) and persephin (PSPN) (Milbrandt et al., 1998). Last, NRTN, like GDNF, was not transported to sympathetic neurons of the adult superior cervical ganglion (SCG) after injection into the anterior eye chamber. These data reveal a high degree of functional selectivity of GDNF family receptor-alpha (GFRalpha) coreceptor subtypes for NRTN and GDNF in vivo.

Array biosensor for simultaneous identification of bacterial, viral, and protein analytes

The array biosensor was fabricated to analyze multiple samples simultaneously for multiple analytes. The sensor utilized a standard sandwich immunoassay format: Antigen-specific "capture" antibodies were immobilized in a patterned array on the surface of a planar waveguide and bound analyte was subsequently detected using fluorescent tracer antibodies. This study describes the analysis of 126 blind samples for the presence of three distinct classes of analytes. To address potential complications arising from using a mixture of tracer antibodies in the multianalyte assay, three single-analyte assays were run in parallel with a multianalyte assay. Mixtures of analytes were also assayed to demonstrate the sensor's ability to detect more than a single species at a time. The array sensor was capable of detecting viral, bacterial, and protein analytes using a facile 14-min assay with sensitivity levels approaching those of standard ELISA methods. Limits of detection for Bacillus globigii, MS2 bacteriophage, and staphylococcal enterotoxin B (SEB) were 10(5) cfu/mL, 10(7) pfu/mL, and 10 ng/mL, respectively. The array biosensor also analyzed multiple samples simultaneously and detected mixtures of the different types of analytes in the multianalyte format.

Expression of neurturin, GDNF, and GDNF family-receptor mRNA in the developing and mature mouse

The GDNF family of neurotrophic factors currently has four members: neurturin (NRTN), glial cell line-derived neurotrophic factor (GDNF), persephin, and artemin. These proteins are potent survival factors for several populations of central and peripheral neurons. The receptors for these factors are complexes that include the Ret tyrosine kinase receptor and a GPI-linked, ligand-binding component called GDNF family receptor alpha 1-4 (GFRalpha1-4). We have used in situ hybridization to study the mRNA expression of NRTN, GDNF, Ret, GFRalpha1, and GFRalpha2 during embryonic development and in the adult mouse. GDNF receptors were prominently expressed during embryonic development in the nervous system, the urogenital system, the digestive system, the respiratory system, and in developing skin, bone, muscle, and endocrine glands. In some regions, incomplete receptor complexes were expressed suggesting that other, as yet unidentified, receptor components exist or that receptor complexes are formed in trans. NRTN and GDNF were expressed in many trigeminal targets during embryonic development including the nasal epithelium, the teeth, and the whisker follicles. NRTN and GDNF were also expressed in the developing limbs and urogenital system. In the embryo, GDNF factors and receptors were expressed at several sites of mesenchyme/epithelial induction, including the kidney, tooth, and submandibular gland. This expression pattern is consistent with the possibility that the GDNF factors function in inductive processes during embryonic development and with the recently discovered role of NRTN as a necessary trophic factor for the development of some parasympathetic neurons. In the mature animal, receptor expression was more limited than in the embryo. In the adult mouse, NRTN was most prominently expressed in the gut, prostate testicle, and oviduct; GDNF was most prominently expressed in the ovary.

Gene targeting reveals a critical role for neurturin in the development and maintenance of enteric, sensory, and parasympathetic neurons

Neurturin (NTN) is a neuronal survival factor that activates the Ret tyrosine kinase in the presence of a GPI-linked coreceptor (either GFR alpha1 or GFR alpha2). Neurturin-deficient (NTN-/-) mice generated by homologous recombination are viable and fertile but have defects in the enteric nervous system, including reduced myenteric plexus innervation density and reduced gastrointestinal motility. Parasympathetic innervation of the lacrimal and submandibular salivary gland is dramatically reduced in NTN-/- mice, indicating that Neurturin is a neurotrophic factor for parasympathetic neurons. GFR alpha2-expressing cells in the trigeminal and dorsal root ganglia are also depleted in NTN-/- mice. The loss of GFR alpha2-expressing neurons, in conjunction with earlier studies, provides strong support for GFR alpha2/Ret receptor complexes as the critical mediators of NTN function in vivo.

An array immunosensor for simultaneous detection of clinical analytes

A fluorescence-based immunosensor has been developed for simultaneous analysis of multiple samples. A patterned array of recognition elements immobilized on the surface of a planar waveguide is used to "capture" analyte present in samples; bound analyte is then quantified by means of fluorescent detector molecules. Upon excitation of the fluorescent label by a small diode laser, a CCD camera detects the pattern of fluorescent antigen:antibody complexes on the sensor surface. Image analysis software correlates the position of fluorescent signals with the identity of the analyte. This immunosensor was used to detect physiologically relevant concentrations of staphylococcal enterotoxin B (SEB), F1 antigen from Yersinia pestis, and D-dimer, a marker of sepsis and thrombotic disorders, in spiked clinical samples.

Expression of neurturin, GDNF, and their receptors in the adult mouse CNS

Neurturin (NTN) and glial cell line-derived neurotrophic factor (GDNF) are the first two members of the GDNF family (GF) of neurotrophic factors. These two proteins are potent survival factors for several populations of central and peripheral neurons in mature and developing rodents. The receptor for these factors is a multicomponent complex that includes the RET (rearranged during transfection) tyrosine kinase receptor and one of two glycosyl phosphatidylinositol (GPI)-linked ligand-binding components called GDNF family receptor alphas (GFRalpha-1 and GFRalpha-2). We have used in situ hybridization to study the mRNA expression of NTN, GDNF, RET, GFRalpha-1, and GFRalpha-2 in the central nervous system (CNS) of adult mice. GF receptors are expressed in several areas in which neuronal populations known to respond to NTN and GDNF are located, including the ventral horn of the spinal cord and the compacta region of the substantia nigra. In addition, we have demonstrated receptor expression in other areas of the brain including the thalamus and hypothalamus. Neurons in these areas express GF receptors, and therefore, may respond to NTN or GDNF. NTN and GDNF are expressed in targets of neurons that express GF receptors. The pattern of GF factor and receptor expression in the adult brain suggests a role for these factors in maintaining neuronal circuits in the mature CNS.

GFRalpha3 is an orphan member of the GDNF/neurturin/persephin receptor family

GDNF, neurturin, and persephin are transforming growth factor beta-related neurotrophic factors known collectively as the GDNF family (GF). GDNF and neurturin signal through a multicomponent receptor complex containing a signaling component (the Ret receptor tyrosine kinase) and either of two glycosyl-phosphatidylinositol-linked binding components (GDNF family receptor alpha components 1 and 2, GFRalpha1 or GFRalpha2), whereas the receptor for persephin is unknown. Herein we describe a third member of the GF coreceptor family called GFRalpha3 that is encoded by a gene located on human chromosome 5q31.2-32. GFRalpha3 is not expressed in the central nervous system of the developing or adult animal but is highly expressed in several developing and adult sensory and sympathetic ganglia of the peripheral nervous system. GFRalpha3 is also expressed at high levels in developing, but not adult, peripheral nerve. GFRalpha3 is a glycoprotein that is glycosyl-phosphatidylinositol-linked to the cell surface like GFRalpha1 and GFRalpha2. Fibroblasts expressing Ret and GFRalpha3 do not respond to any of the known members of the GDNF family, suggesting that GFRalpha3 interacts with an unknown ligand or requires a different or additional signaling protein to function.

Detection of multiple toxic agents using a planar array immunosensor

A planar array immunosensor, equipped with a charge-coupled device (CCD) as a detector, was used to simultaneously detect 3 toxic analytes. Wells approximately 2 mm in diameter were formed on glass slides using a photoactivated optical adhesive. Antibodies against staphylococcal enterotoxin B (SEB), ricin, and Yersinia pestis were covalently attached to the bottoms of the circular wells to form the sensing surface. Rectangular wells containing chicken immunoglobulin were used as alignment markers and to generate control signals. After removing the optical adhesive, the slides were mounted over a scientific grade CCD operating at ambient temperature in inverted (multipin phasing) mode. A two-dimensional graded index of refraction lens array was used to focus the sensing surface onto the CCD. Solutions of toxins were then placed on the slide. After rinsing, Cy5-labeled antibodies were introduced. The identity and amount of toxin bound at each location on the slide were determined by quantitative image analysis. Concentrations as low as 25 ng/mL of ricin, 15 ng/mL of pestis F1 antigen, and 5 ng/mL of SEB could be routinely measured.

Persephin, a novel neurotrophic factor related to GDNF and neurturin

A novel neurotrophic factor named Persephin that is approximately 40% identical to glial cell line-derived neurotrophic factor (GDNF) and neurturin (NTN) has been identified using degenerate PCR. Persephin, like GDNF and NTN, promotes the survival of ventral midbrain dopaminergic neurons in culture and prevents their degeneration after 6-hydroxydopamine treatment in vivo. Persephin also supports the survival of motor neurons in culture and in vivo after sciatic nerve axotomy and, like GDNF, promotes ureteric bud branching. However, in contrast to GDNF and NTN, persephin does not support any of the peripheral neurons that were examined. Fibroblasts transfected with Ret and one of the coreceptors GFRalpha-1 or GFRalpha-2 do not respond to persephin, suggesting that persephin utilizes additional, or different, receptor components than GDNF and NTN.

Postnatal development of terminals and synapses in laminae I and II of the rat medullary dorsal horn

To better understand developing orofacial nociceptive circuits and to provide a baseline for evaluating injury-induced plasticity, the ultrastructure of the superficial laminae in the rat medullary dorsal horn was examined at birth and at postnatal days 1, 4, 17, and 90. Quantitative features of terminals and synapses were studied with stereological methods. In laminae I and II: 1) Axon terminal density increased significantly from birth to day 4 and again from day 4 to day 90. 2) The density of degenerating profiles increased significantly from birth to day 1 and from birth to day 4 and then decreased from day 4 to day 90. 3) Degenerating profiles were most dense on day 1 and declined steadily thereafter; by day 90, such profiles were rare. 4) Cavitation was by far the most common form of degeneration seen at early postnatal ages. 5) Growth cone-like profiles were most dense at birth and declined steadily during the first 2 postnatal weeks; by day 90, such profiles were absent. 6) Terminals with flat synaptic vesicles were rarely seen before day 90, when they accounted for 7% of the terminal population. 7) The density of synapses increased continuously from birth until day 90. These data suggest that, as in the spinal cord, medullary dorsal horn circuits are very immature at birth. Adult-like quantitative features are not attained until after day 17. Moreover, whereas degenerating profiles are prevalent during early postnatal development, and they have features that resemble naturally occurring degeneration, the total numbers of terminals and synapses continue to increase dramatically and gradually during a protracted postnatal period (to postnatal day 17).