SiN half-etch horizontal slot waveguides for integrated photonics: numerical modeling, fabrication, and characterization of passive components.

This work presents a "half-etch" horizontal slot waveguide design based on SiN, where only the upper SiN layer is etched to form a strip that confines the mode laterally. The numerical modeling, fabrication, and characterization of passive waveguiding components are described. This novel slot waveguide structure was designed with on-chip light amplification in mind, for example with an Er-doped oxide spacer layer. Proof-of-concept racetrack resonators were fabricated and characterized, showing quality factors up to 50,000 at critical coupling and residual losses of 4 dB/cm at wavelengths away from the N-H bond absorption peak in SiN, demonstrating the high potential of these horizontal slot waveguides for use in active integrated photonics.

Hydrophilic Mechano-Bactericidal Nanopillars Require External Forces to Rapidly Kill Bacteria.

Nanopillars have been shown to mechanically damage bacteria, suggesting a promising strategy for future antibacterial surfaces. However, the mechanisms underlying this phenomena remain unclear, which ultimately limits translational potential toward real-world applications. Using real-time and end-point analysis techniques, we demonstrate that in contrast to initial expectations, bacteria on multiple hydrophilic "mechano-bactericidal" surfaces remained viable unless exposed to a moving air-liquid interface, which caused considerable cell death. Reasoning that normal forces arising from surface tension may underlie this mechano-bactericidal activity, we developed computational and experimental models to estimate, manipulate, and recreate the impact of these forces. Our experiments together demonstrate that a critical level of external force acting on cells attached to nanopillar surfaces can rapidly deform and rupture bacteria. These studies provide fundamental physical insight into how nanopillar surfaces can serve as effective antibacterial materials and suggest use-conditions under which such nanotechnology approaches may provide practical value.

Dielectrophoretic cell trapping for improved surface plasmon resonance imaging sensing.

The performance of conventional surface plasmon resonance (SPR) biosensors can be limited by the diffusion of the target analyte to the sensor surface. This work presents an SPR biosensor that incorporates an active mass-transport mechanism based on dielectrophoresis and electroosmotic flow to enhance analyte transport to the sensor surface and reduce the time required for detection. Both these phenomena rely on the generation of AC electric fields that can be tailored by shaping the electrodes that also serve as the SPR sensing areas. Numerical simulations of electric field distribution and microparticle trajectories were performed to choose an optimal electrode design. The proposed design improves on previous work combining SPR with DEP by using face-to-face electrodes, rather than a planar interdigitated design. Two different top-bottom electrode designs were experimentally tested to concentrate firstly latex beads and secondly biological cells onto the SPR sensing area. SPR measurements were then performed by varying the target concentrations. The electrohydrodynamic flow enabled efficient concentration of small objects (3 µm beads, yeasts) onto the SPR sensing area, which resulted in an order of magnitude increased SPR response. Negative dielectrophoresis was also used to concentrate HEK293 cells onto the metal electrodes surrounded by insulating areas, where the SPR response was improved by one order of magnitude.

Spatial resolution versus contrast trade-off enhancement in high-resolution surface plasmon resonance imaging (SPRI) by metal surface nanostructure design.

Surface plasmon resonance imaging (SPRI) is an optical near-field method used for mapping the spatial distribution of chemical/physical perturbations above a metal surface without exogenous labeling. Currently, the majority of SPRI systems are used in microarray biosensing, requiring only modest spatial resolution. There is increasing interest in applying SPRI for label-free near-field imaging of biological cells to study cell/surface interactions. However, the required resolution (sub-µm) greatly exceeds what current systems can deliver. Indeed, the attenuation length of surface plasmon polaritons (SPP) severely limits resolution along one axis, typically to tens of µm. Strategies to date for improving spatial resolution result in a commensurate deterioration in other imaging parameters. Unlike the smooth metal surfaces used in SPRI that support purely propagating surface modes, nanostructured metal surfaces support "hybrid" SPP modes that share attributes from both propagating and localized modes. We show that these hybrid modes are especially well-suited to high-resolution imaging and demonstrate how the nanostructure geometry can be designed to achieve sub-µm resolution while mitigating the imaging parameter trade-off according to an application-specific optimum.

Monitoring individual cell-signaling activity using combined metal-clad waveguide and surface-enhanced fluorescence imaging.

Evanescent field based biosensing systems such as surface plasmon resonance (SPR), diffraction gratings, or metal-clad waveguides (MCWGs) are powerful tools for label-free real-time monitoring of signaling activity of living cells exposed to hormones, pharmacological agents, and toxins. In particular, MCWG-based imaging is well suited for studying relatively thick objects such as cells due to its greater depth of penetration into the sensing medium, compared to SPR. Label-free methods, however, provide only indirect measurements in that the measured signal arises from local changes in material properties rather than from specific biomolecular targets. In the case of cells, the situation is especially complex as the measured label-free signal may result from a combination of very diverse sources: morphological changes, intra-cellular reorganization, cascaded molecular events, protein expression etc. Consequently, deconvolving the contributions of specific sources to a particular cell response profile can be challenging. In the following, we present a cell imaging platform that combines two distinct sensing modalities, namely label-free MCWG imaging and label-based surface enhanced fluorescence (SEF), designed to facilitate the identification of the underlying molecular and structural contributions to the label-free MCWG images. We demonstrate the bimodal capabilities of this imaging platform in experiments designed to visualize actin cytoskeleton organization in vascular smooth muscle cells. We then monitored the real-time response of HEK293 cells expressing the Angiotensin 1 receptor (AT1R), when stimulated by the receptor agonist Angiotensin II (AngII). The analysis of the simultaneous label-free signal obtained by MCWG and the intracellular calcium signal resulting form AT1R activation, measured by SEF, allows relating label-free signal features to specific markers of receptor activation. Our results show that the intracellular calcium levels normally observed following AT1R activation are not required for the initial burst of cellular activity observed in the MCWG signal but rather indicates signaling activity involving the intracellular kinase ROCK.

Metal clad waveguide (MCWG) based imaging using a high numerical aperture microscope objective.

Evanescent-field based methods such as surface plasmon resonance (SPR) have been used very effectively for label-free imaging of microscopic biological material in close proximity to a sensing surface. However, the shallow probing depth of SPR (typically less than ~200 nm) can be problematic when imaging relatively thick biological objects such as cells or bacteria. In this paper, we demonstrate how metal-clad waveguides (MCWG) can be used to achieve deeper probing depth compared to SPR while maintaining good imaging spatial resolution. Comparative numerical simulations of imaging spatial resolution versus probing depth are shown for a number of common SPR, long-range SPR, and MCWG configurations, demonstrating that MCWG offer the best compromise between resolution and depth for imaging thick biological objects. Experimental results of synthetic target and live cell imaging are shown that validate the numerical simulations and demonstrate the capabilities of the method.

Improved resolution in SPR and MCWG microscopy by combining images acquired with distinct mode propagation directions.

In high-resolution surface plasmon (SPR) imaging, lateral resolution is limited along the direction of plasmon propagation by the longitudinal decay length. Though SPR systems can achieve sub-micrometer resolution, the decay length causes a degradation in the images in the direction of plasmon propagation akin to a blurring artifact, with ringing along resonant to nonresonant transition edges. We present a method to significantly reduce this effect based on combining images of a sample acquired with distinct guided-mode propagation directions. As SPR is a special case of the class of optical structures known as metal-clad waveguides (MCWG) that are also affected by the decay length, this work is broadly applicable.

Effects of Surface Topography and Cellular Biomechanics on Nanopillar-Induced Bactericidal Activity.

Bacteria are mechanically resistant biological structures that can sustain physical stress. Experimental data, however, have shown that high-aspect-ratio nanopillars deform bacterial cells upon contact. If the deformation is sufficiently large, it lyses the bacterial cell wall, ultimately leading to cell death. This has prompted a novel strategy, known as mechano-bactericide technology, to fabricate antibacterial surfaces. Although adhesion forces were originally proposed as the driving force for mechano-bactericidal action, it has been recently shown that external forces, such as capillary forces arising from an air-water interface at bacterial surfaces, produce sufficient loads to rapidly kill bacteria on nanopillars. This discovery highlights the need to theoretically examine how bacteria respond to external loads and to ascertain the key factors. In this study, we developed a finite element model approximating bacteria as elastic shells filled with cytoplasmic fluid brought into contact with an individual nanopillar or nanopillar array. This model elucidates that bacterial killing caused by external forces on nanopillars is influenced by surface topography and cell biomechanical variables, including the density and arrangement of nanopillars, in addition to the cell wall thickness and elastic modulus. Considering that surface topography is an important design parameter, we performed experiments using nanopillar arrays with precisely controlled nanopillar diameters and spacing. Consistent with model predictions, these demonstrate that nanopillars with a larger spacing increase bacterial susceptibility to mechanical puncture. The results provide salient insights into mechano-bactericidal activity and identify key design parameters for implementing this technology.

CMOS buried quad p-n junction photodetector for multi-wavelength analysis.

This paper presents a buried quad p-n junction (BQJ) photodetector fabricated with a HV (high-voltage) CMOS process. Multiple buried junction photodetectors are wavelength-sensitive devices developed for spectral analysis applications where a compact integrated solution is preferred over systems involving bulk optics or a spectrometer due to physical size limitations. The BQJ device presented here is designed for chip-based biochemical analyses using simultaneous fluorescence labeling of multiple analytes such as with advanced labs-on-chip or miniaturized photonics-based biosensors. Modeling and experimental measurements of the spectral response of the device are presented. A matrix-based method for estimating individual spectral components in a compound spectrum is described. The device and analysis method are validated via a test setup using individually modulated LEDs to simulate light from 4-component fluorescence emission.

The ultra high sensitivity blood counter: a compact, MRI-compatible, radioactivity counter for pharmacokinetic studies inµl volumes.

Quantification of physiological parameters in preclinical pharmacokinetic studies based on nuclear imaging requires the monitoring of arterial radioactivity over time, known as the arterial input function (AIF). Continuous derivation of the AIF in rodent models is very challenging because of the limited blood volume available for sampling. To address this challenge, an Ultra High Sensitivity Blood Counter (UHS-BC) was developed. The device detects beta particles in real-time using silicon photodiodes, custom low-noise electronics, and 3D-printed plastic cartridges to hold standard catheters. Two prototypes were built and characterized in two facilities. Sensitivities up to 39% for18F and 58% for11C-based positron emission tomography (PET) tracers were demonstrated.99mTc and125I based Single Photon Emission Computed Tomography (SPECT) tracers were detected with greater than 3% and 10% sensitivity, respectively, opening new applications in nuclear imaging and fundamental biology research. Measured energy spectra show all relevant peaks down to a minimum detectable energy of 20 keV. The UHS-BC was shown to be highly reliable, robust towards parasitic background radiation and electromagnetic interference in the PET or MRI environment. The UHS-BC provides reproducible results under various experimental conditions and was demonstrated to be stable over days of continuous operation. Animal experiments showed that the UHS-BC performs accurate AIF measurements using low detection volumes suitable for small animal models in PET, SPECT and PET/MRI investigations. This tool will help to reduce the time and number of animals required for pharmacokinetic studies, thus increasing the throughput of new drug development.

Love Wave Sensor with High Penetration Depth for Potential Application in Cell Monitoring.

Love wave (L-SAW) sensors have been used to probe cell monolayers, but their application to detect changes beyond the focal adhesion points on cell monolayers, as viscosity changes on the cytoskeleton, has not been explored. In this work we present for the first time a Love wave sensor with tuned penetration depth and sensitivity to potentially detect mechanical changes beyond focal adhesion points of cell monolayers. We designed and fabricated a Love wave sensor operating at 30 MHz with sensitivity to detect viscous changes between 0.89 and 3.3 cP. The Love wave sensor was modeled using an acoustic transmission line model, whereas the response of interdigital transducers (IDTs) was modeled with the Campbell's cross-field circuit model. Our design uses a substrate with a high electromechanical coupling coefficient (LiNbO3 36Y-X), and an 8-µm polymeric guiding layer (SU-8). The design aims to overcome the high insertion losses of viscous liquid environments, and the loss of sensitivity due to the low frequency. The fabricated sensor was tested in a fluidic chamber glued directly to the SU-8 guiding layer. Our experiments with liquids of viscosity similar to those expected in cell monolayers showed a measurable sensor response. In addition, experimentation with SaOs-2 cells within a culture medium showed measurable responses. These results can be of interest for the development of novel cell-based biosensors, and novel characterization tools for cell monolayers.

Spatially-Localized Functionalization on Nanostructured Surfaces for Enhanced Plasmonic Sensing Efficacy.

This work demonstrates the enhancement in plasmonic sensing efficacy resulting from spatially-localized functionalization on nanostructured surfaces, whereby probe molecules are concentrated in areas of high field concentration. Comparison between SERS measurements on nanostructured surfaces (arrays of nanodisks 110 and 220 nm in diameter) with homogeneous and spatially-localized functionalization with thiophenol demonstrates that the Raman signal originates mainly from areas with high field concentration. TERS measurements with 10 nm spatial resolution confirm the field distribution profiles predicted by the numerical modeling. Though this enhancement in plasmonic sensing efficacy is demonstrated with SERS, results apply equally well to any type of optical/plasmonic sensing on functionalized surfaces with nanostructuring.

Integrated active mixing and biosensing using surface acoustic waves (SAW) and surface plasmon resonance (SPR) on a common substrate.

This article presents a device incorporating surface plasmon resonance (SPR) sensing and surface acoustic wave (SAW) actuation integrated onto a common LiNbO(3) piezoelectric substrate. The device uses Rayleigh-type SAW to provide active microfluidic mixing in the fluid above the SPR sensor. Validation experiments show that SAW-induced microfluidic mixing results in accelerated binding kinetics of an avidin-biotin assay. Results also show that, though SAW action causes a parasitic SPR response due to heat injection into the fluid, a relatively brief relaxation time following the SAW pulses allows the effect to dissipate, without affecting the overall assay response. Since both SPR sensors and SAW transducers can be fabricated simultaneously using low-cost microfabrication methods on a single substrate, the proposed design is well-suited to lab-on-chip applications.

In vivo intravital endoscopic confocal fluorescence microscopy of normal and acutely injured rat lungs.

We present a new lung imaging technique based on endoscopic confocal fluorescence microscopy (ECFM), which is a new method that is able to provide cellular and structural assessment of living tissue using a small confocal probe in direct contact with the visceral pleura. To observe distal airspace structure and cellular condition in normal and injured lungs (hyperoxic and bleomycin challenged), we used fluorescent-specific marker contrast and ECFM. Alveolar space ECFM with spectral analyses were performed at 488-nm excitation using FITC-labeled markers or naturally fluorescent dyes. The normal lung was compared with the sick lung, where our in vivo imaging experiments correlated well with results obtained with corresponding ex vivo conventional assays. Four main elements pertaining to the acute lung injury/acute respiratory distress syndrome (ALI/ARDS) pathophysiology and established early key events were specifically studied: alveolar epithelial membrane phenotype, lung cell apoptosis, neutrophil recruitment, and edema. ECFM allowed visualization of (i) fine-tuned ultrastructural lectin (RCA-1) and sialoglycoprotein (RTI40) epithelial cell membrane expression, (ii) YO-PRO-1-related DNA linking of lung cell apoptosis, (iii) PKH2 green fluorescent cell linker-labeled neutrophil tracking in lung microcirculatory network and airspaces, (iv) FITC-dextran plasma contrast and extravasation with edema formation. ECFM provides reliable results to corresponding ex vivo fluorescent methods. ECFM, using the minimally invasive Five-1(R) optical instrument and specific fluorescent markers, is able to provide real-time potentially useful imaging of live unfixed normal and injured lung tissue with promising developments for improving bedside diagnostic and decision-making therapeutic strategy in patients with ALI.

An integrated hybrid interference and absorption filter for fluorescence detection in lab-on-a-chip devices.

We present a hybrid optical filter design that combines interference and absorbing components for enhanced fluorescence detection in miniaturized highly-integrated lab-on-a-chip devices. The filter is designed in such a way that the advantages of each technology are used to offset the disadvantages of the other. The filter is fabricated with microfabrication compatible processes and materials for monolithic integration with microelectronics and microfluidics devices. The particular embodiment of the filter described herein is designed to discriminate fluorescence emission at 650 nm from excitation at 532 nm. The 9-layer interference filter component is fabricated with alternating TiO(2) and SiO(2) thin-film layers and has an attenuation of -12.6 dB at 532 nm and -0.76 dB at 650 nm. The absorbing filter component is fabricated using a dyed photopolymer (KMPR + Orasol Red) having an attenuation of -32.6 dB at 532 nm and -1.28 dB at 650 nm. The total rejection ratio of the hybrid filter is 43 dB. The filter exhibits very low autofluorescence and performs equally well at off-axis incidence angles.

Early detection of bacteria using SPR imaging and event counting: experiments with Listeria monocytogenes and Listeria innocua.

Foodborne pathogens are of significant concern in the agrifood industry and the development of associated rapid detection and identification methods are of major importance. This paper describes the novel use of resolution-optimized prism-based surface plasmon resonance imaging (RO-SPRI) and data processing for the detection of the foodborne pathogens Listeria monocytogenes and Listeria innocua. With an imaging spatial resolution on the order of individual bacteria (2.7 ± 0.5 µm × 7.9 ± 0.6 µm) over a field of view 1.5 mm2, the RO-SPRI system enabled accurate counting of individual bacteria on the sensor surface. Using this system, we demonstrate the detection of two species of Listeria at an initial concentration of 2 × 102 CFU mL-1 in less than 7 hours. The surface density of bacteria at the point of positive detection was 15 ± 4 bacteria per mm2. Our approach offers great potential for the development of fast specific detection systems based on affinity monitoring.

Nanoplasmonics-enhanced label-free imaging of endothelial cell monolayer integrity.

Surface plasmon resonance imaging (SPRI) is a powerful label-free imaging modality for the analysis of morphological dynamics in cell monolayers. However, classical plasmonic imaging systems have relatively poor spatial resolution along one axis due to the plasmon mode attenuation distance (tens of µm, typically), which significantly limits their ability to resolve subcellular structures. We address this limitation by adding an array of nanostructures onto the metal sensing surface (25 nm thick, 200 nm width, 400 nm period grating) to couple localized plasmons with propagating plasmons, thereby reducing attenuation length and commensurately increasing spatial imaging resolution, without significant loss of sensitivity or image contrast. In this work, experimental results obtained with both conventional unstructured and nanostructured gold film SPRI sensor chips show a clear gain in spatial resolution achieved with surface nanostructuring. The work demonstrates the ability of the nanostructured SPRI chips to resolve fine morphological detail (intercellular gaps) in experiments monitoring changes in endothelial cell monolayer integrity following the activation of the cell surface protease-activated receptor 1 (PAR1) by thrombin. In particular, the nanostructured chips reveal the persistence of small intercellular gaps (<5 µm2) well after apparent recovery of cell monolayer integrity as determined by conventional unstructured surface based SPRI. This new high spatial resolution plasmonic imaging technique uses low-cost and reusable patterned substrates and is likely to find applications in cell biology and pharmacology by allowing label-free quantification of minute cell morphological activities associated with receptor dependent intracellular signaling activity.

Numerical method for high accuracy index of refraction estimation for spectro-angular surface plasmon resonance systems.

An eigenvector analysis based algorithm is presented for estimating refractive index changes from 2-D reflectance/dispersion images obtained with spectro-angular surface plasmon resonance systems. High resolution over a large dynamic range can be achieved simultaneously. The method performs well in simulations with noisy data maintaining an error of less than 10(-8) refractive index units with up to six bits of noise on 16 bit quantized image data. Experimental measurements show that the method results in a much higher signal to noise ratio than the standard 1-D weighted centroid dip finding algorithm.

Label-free visualization and quantification of single cell signaling activity using metal-clad waveguide (MCWG)-based microscopy.

Label-free biosensing methods are very effective for studying cell signaling cascade activation induced by external stimuli. Assays generally involve a large number of cells and rely on the underlying assumption that cell response is homogeneous within a cell population. However, there is an increasing body of evidence showing that cell behavior may vary significantly even among genetically identical cells. In this paper, we demonstrate the use of metal-clad waveguide (MCWG)-based microscopy for label-free real-time monitoring of signaling activity and morphology changes in a small population of cells, with the ability to resolve individual cells. We demonstrate the potential of this approach by quantifying apoptosis-induced intracellular activity in individual cells following exposure to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and by visualizing and quantifying extracellular changes in endothelial cell layer integrity following the activation of the proteinase-activated receptor 1 (PAR1) by thrombin. Results show that averaged signals obtained from a cell population may incorrectly reflect the actual distribution of morphology and kinetics parameters across a cell population by a significant margin.