David Sinton

Nanoholes as nanochannels: flow-through plasmonic sensing.

We combine nanofluidics and nanoplasmonics for surface-plasmon resonance (SPR) sensing using flow-through nanohole arrays. The role of surface plasmons on resonant transmission motivates the application of nanohole arrays as surface-based biosensors. Research to date, however, has focused on dead-ended holes, and therefore failed to harness the benefits of nanoconfined transport combined with SPR sensing. The flow-through format enables rapid transport of reactants to the active surface inside the nanoholes, with potential for significantly improved time of analysis and biomarker yield through nanohole sieving. We apply the flow-through method to monitor the formation of a monolayer and the immobilization of an ovarian cancer biomarker specific antibody on the sensing surface in real-time. The flow-through method resulted in a 6-fold improvement in response time as compared to the established flow-over method.

A microfluidic fuel cell with flow-through porous electrodes.

A microfluidic fuel cell architecture incorporating flow-through porous electrodes is demonstrated. The design is based on cross-flow of aqueous vanadium redox species through the electrodes into an orthogonally arranged co-laminar exit channel, where the waste solutions provide ionic charge transfer in a membraneless configuration. This flow-through architecture enables improved utilization of the three-dimensional active area inside the porous electrodes and provides enhanced rates of convective/diffusive transport without increasing the parasitic loss required to drive the flow. Prototype fuel cells are fabricated by rapid prototyping with total material cost estimated at 2 USD/unit. Improved performance as compared to previous microfluidic fuel cells is demonstrated, including power densities at room temperature up to 131 mW cm-2. In addition, high overall energy conversion efficiency is obtained through a combination of relatively high levels of fuel utilization and cell voltage. When operated at 1 microL min-1 flow rate, the fuel cell produced 20 mW cm-2 at 0.8 V combined with an active fuel utilization of 94%. Finally, we demonstrate in situ fuel and oxidant regeneration by running the flow-through architecture fuel cell in reverse.

Attomolar Protein Detection Using in-Hole Surface Plasmon Resonance

An in-hole nanohole surface plasmon resonance sensing scheme is demonstrated. Arrays of periodic nanoholes milled through thin layers of SiO(x) and gold were used to detect the binding of organic and biological molecules inside the nanoholes, while blocking the gold surfaces outside the holes. This new approach is more efficient than the previous nanohole array method, where the response was related to binding events taking place inside of the holes and on the top gold surface. The improved sensitivity to binding events and lower detection limit are related to resonant surface plasmon enhanced transmission through the arrays of nanoholes. The sensitivity was found to be 650 nm/RIU and the detection of three attomoles of proteins was estimated from this scheme.

Hydrogen Peroxide as an Oxidant for Microfluidic Fuel Cells

We demonstrate a microfluidic fuel cell incorporating hydrogen peroxide oxidant. Hydrogen peroxide (H 2 O 2 ) is available at high concentrations, is highly soluble and exhibits a high standard reduction potential. It also enables fuel cell operation where natural convection of air is limited or anaerobic conditions prevail, as in submersible and space applications. As fuel cell performance critically depends on both electrode and channel architecture, several different prototype cells are developed and results are compared. High-surface area electrodeposited platinum and palladium electrodes are evaluated both ex situ and in situ for the combination of direct H 2 O 2 reduction and oxygen reduction via the decomposition reaction. Oxygen gas bubbles produced at the fuel cell cathode introduce an unsteady two-phase flow component that, if not controlled, can perturb the co-laminar flow interface and reduce fuel cell performance. A grooved channel design is developed here that restricts gas bubble growth and transport to the vicinity of the cathodic active sites, enhancing the rate of oxygen reduction, and limiting crossover effects. The proof-of-concept microfluidic fuel cell produced power densities up to 30 mW cm -2 and a maximum current density of 150 mA cm -2 , when operated on 2 M H 2 O 2 oxidant together with formic acid-based fuel at room temperature.

Flow-Through vs Flow-Over: Analysis of Transport and Binding in Nanohole Array Plasmonic Biosensors

We quantify the efficacy of flow-through nanohole sensing, as compared to the established flow-over format, through scaling analysis and numerical simulation. Nanohole arrays represent a growing niche within surface plasmon resonance-based sensing methods, and employing the nanoholes as nanochannels can enhance transport and analytical response. The additional benefit offered by flow-through operation is, however, a complex function of operating parameters and application-specific binding chemistry. Compared here are flow-over sensors and flow-through nanohole array sensors with equivalent sensing area, where the nanohole array sensing area is taken as the inner-walls of the nanoholes. The footprints of the sensors are similar (e.g., a square 20 μm wide flow-over sensor has an equivalent sensing area as a square 30 μm wide array of 300 nm diameter nanoholes with 450 nm periodicity in a 100 nm thick gold film). Considering transport alone, an analysis here shows that given equivalent sensing area and flow rate the flow-through nanohole format enables greatly increased flux of analytes to the sensing surface (e.g., 40-fold for the case of Q = 10 nL/min). Including both transport and binding kinetics, a computational model, validated by experimental data, provides guidelines for performance as a function of binding time constant, analyte diffusivity, and running parameters. For common binding kinetics and analytes, flow-through nanohole arrays offer ∼10-fold improvement in response time, with a maximum of 20-fold improvement for small biomolecules with rapid kinetics.

Optofluidic Concentration: Plasmonic Nanostructure as Concentrator and Sensor

The integration of fluidics and optics, as in flow-through nanohole arrays, has enabled increased transport of analytes to sensing surfaces. Limits of detection, however, are fundamentally limited by local analyte concentration. We employ the nanohole array geometry and the conducting nature of the film to actively concentrate analyte within the sensor. We achieve 180-fold enrichment of a dye, and 100-fold enrichment and simultaneous sensing of a protein in less than 1 min. The method presents opportunities for an order of magnitude increase in sensing speed and 2 orders of magnitude improvement in limit of detection.

Flow-Directed Block Copolymer Micelle Morphologies via Microfluidic Self-Assembly

The self-assembly of amphiphilic block copolymers in a gas-liquid microfluidic reactor produces variable, flow-directed micellar morphologies entirely different from off-chip equilibrium structures. A polystyrene-block-poly(acrylic acid) copolymer, which forms exclusively spheres off-chip, generates kinetic cylinders, Y-junctions, bilayers, and networks by a mechanism of collision-coalescence enabled by strong and localized on-chip shear fields. Variation in the size and relative amount of flow-directed nanostructures is achieved by changing the water content and flow rate. These results demonstrate on-chip processing routes to specific functional colloidal nanostructures.

Lab-on-chip methodologies for the study of transport in porous media: energy applications

We present a lab-on-chip approach to the study of multiphase transport in porous media. The applicability of microfluidics to biological and chemical analysis has motivated much development in lab-on-chip methodologies. Several of these methodologies are also well suited to the study of transport in porous media. We demonstrate the application of rapid prototyping of microfluidic networks with approximately 5000 channels, controllable wettability, and fluorescence-based analysis to the study of multiphase transport phenomena in porous media. The method is applied to measure the influence of wettability relative to network regularity, and to differentiate initial percolation patterns from active flow paths. Transport phenomena in porous media are of critical importance to many fields and particularly in many energy-related applications including liquid water transport in fuel cells, oil recovery, and CO(2) sequestration.

Natural Convection in an Enclosure with Distributed Heat Sources

ABSTRACT Presented in this article is a computational analysis of the heat transfer due to an array of distributed heat sources on the bottom wall of a horizontal enclosure. The heat sources are modeled as flush-mounted sources with prescribed heat flux boundary conditions. Optimum heat transfer rates and the onset of thermal instability triggering various regimes are found to be governed by the length and spacing of the sources and the width-to-height aspect ratio of the enclosure. With respect to source spacing, we found that spacing equal to that of the source length provides effective convective heat transfer, and increasing the source spacing further does not result in significant improvements. The transition from a conduction-dominated regime to a convection-dominated regime is found to be characterized by a range of Rayleigh numbers, in contrast to the classical bottom wall heating problem. The range of Rayleigh numbers at which transition takes place decreases as the source length increases. At the transition region for very small source lengths, the Rayleigh-Bénard cell structure grows significantly to form fewer and larger cells, which accounts for higher heat transfer rates compared to configurations with longer heat sources where the cell structure remains the same throughout transition. Following the transition to a convection-dominated regime, bifurcations in the Rayleigh-Bénard cell structures as well as further regime changes are observed, reflecting the instabilities in the physical system.

Direct DNA Analysis with Paper-Based Ion Concentration Polarization

DNA analysis is essential for diagnosis and monitoring of many diseases. Conventional DNA testing is generally limited to the laboratory. Increasing access to relevant technologies can improve patient care and outcomes in both developed and developing regions. Here, we demonstrate direct DNA analysis in paper-based devices, uniquely enabled by ion concentration polarization at the interface of patterned nanoporous membranes in paper (paper-based ICP). Hepatitis B virus DNA targets in human serum are simultaneously preconcentrated, separated, and detected in a single 10 min operation. A limit of detection of 150 copies/mL is achieved without prior viral load amplification, sufficient for early diagnosis of hepatitis B. We clinically assess the DNA integrity of sperm cells in raw human semen samples. The percent DNA fragmentation results from the paper-based ICP devices strongly correlate (R(2) = 0.98) with the sperm chromatin structure assay. In all cases, agreement was 100% with respect to the clinical decision. Paper-based ICP can provide inexpensive and accessible advanced molecular diagnostics.