Suppression of non-specific adsorption using sheath flow

The use of a confining sheath fluid within a microfluidic channel in order prevent non-specific adsorption of analytes to the walls of microchannels is demonstrated. A sheath-flow channel fabricated using laser cutting of Mylar films is developed. Numerical simulations of convective and diffusive mass transport within the channel are presented. The device is characterized experimentally using epifluorescence microscopy. It is demonstrated that the device is capable of preventing the adsorption of Rhodamine B to the walls of the channel for a period that would allow for adsorption-free T-sensor measurements to be made within the core of the flow channel. Generalized scaling rules based on the diffusion coefficient, sheath thickness and affinity of the potential adsorbant for the surface material are discussed. The controlled adsorption of the protein bovine serum albumin (BSA) to a gold surface is also demonstrated using SPR microscopy.     

Vibrational corrections to the charge and momentum densities of H2

Vibrational corrections to the charge and momentum densities of H₂ are calculated using the Wang wavefunction and various expressions for the dependence of the effective nuclear charge on the interatomic distance. Both harmonic oscillator and Morse potentials are employed for the vibrational wavefunctions, and excited as well as ground vibrational states are considered. The corrections to the momentum density, though considerably smaller than the charge density corrections, appear to be somewhat more sensitive qualitatively to the choice of both nuclear and electronic wavefunctions.     

Two-dimensional paper networks: programmable fluidic disconnects for multi-step processes in shaped paper

Most laboratory assays take advantage of multi-step protocols to achieve high performance, but conventional paper-based tests (e.g., lateral flow tests) are generally limited to assays that can be carried out in a single fluidic step. We have developed two-dimensional paper networks (2DPNs) that use materials from lateral flow tests but reconfigure them to enable programming of multi-step reagent delivery sequences. The 2DPN uses multiple converging fluid inlets to control the arrival time of each fluid to a detection zone or reaction zone, and it requires a method to disconnect each fluid source in a corresponding timed sequence. Here, we present a method that allows programmed disconnection of fluid sources required for multi-step delivery. A 2DPN with legs of different lengths is inserted into a shared buffer well, and the dropping fluid surface disconnects each leg at in a programmable sequence. This approach could enable multi-step laboratory assays to be converted into simple point-of-care devices that have high performance yet remain easy to use.     

Experimental and model investigation of the time-dependent 2-dimensional distribution of binding in a herringbone microchannel

A microfluidic device known to mix bulk solutions, the herringbone microchannel, was incorporated into a surface-binding assay to determine if the recirculation of solution altered the binding of a model protein (streptavidin) to the surface. Streptavidin solutions were pumped over surfaces functionalized with its ligand, biotin, and the binding of streptavidin to those surfaces was monitored using surface plasmon resonance imaging. Surface binding was compared between a straight microchannel and herringbone microchannels in which the chevrons were oriented with and against the flow direction. A 3-dimensional finite-element model of the surface binding reaction was developed for each of the geometries and showed strong qualitative agreement with the experimental results. Experimental and model results indicated that the forward and reverse herringbone microchannels substantially altered the distribution of protein binding (2-dimensional binding profile) as a function of time when compared to a straight microchannel. Over short distances (less than 1.5 mm) down the length of the microchannel, the model predicted no additional protein binding in the herringbone microchannel compared to the straight microchannel, consistent with previous findings in the literature.     

Conditioning saliva for use in a microfluidic biosensor

This report details an approach to saliva conditioning for compatibility of raw patient samples with microfluidic immunoassay components, principally biosensor surfaces susceptible to fouling. Stimulated whole human saliva spiked with a small molecule analyte (phenytoin, 252 Da) was first depleted of cells, debris and high molecular weight glycoproteins (mucins) using membrane filtration. This process significantly reduced but did not eliminate fouling of biosensor surfaces exposed to the sample. An H-filter, which separates solutes from mixed samples based on their diffusion in laminar flow, was used to extract the analyte from the remaining large molecular weight species in the filtered saliva sample. Patient samples treated in this way retained 23% of the analyte with 97% and 92% reduction in glycoproteins and proteins, respectively, and resulted in 3.6 times less surface fouling than either untreated or filtered saliva alone. These sample conditioning steps will enable the use of fouling-sensitive detection techniques in future studies using clinical saliva samples.     

A method for characterizing adsorption of flowing solutes to microfluidic device surfaces

We present a method for characterizing the adsorption of solutes in microfluidic devices that is sensitive to both long-lived and transient adsorption and can be applied to a variety of realistic device materials, designs, fabrication methods, and operational parameters. We have characterized the adsorption of two highly adsorbing molecules (FITC-labeled bovine serum albumin (BSA) and rhodamine B) and compared these results to two low adsorbing species of similar molecular weights (FITC-labeled dextran and fluorescein). We have also validated our method by demonstrating that two well-known non-fouling strategies [deposition of the polyethylene oxide (PEO)-like surface coating created by radio-frequency glow discharge plasma deposition (RF-GDPD) of tetraethylene glycol dimethyl ether (tetraglyme, CH(3)O(CH(2)CH(2)O)(4)CH(3)), and blocking with unlabeled BSA] eliminate the characteristic BSA adsorption behavior observed otherwise.     

Investigation of heterogeneous electrochemical processes using multi-stream laminar flow in a microchannel

A multi-component microfluidic electrochemical cell is shown to be a useful analytical tool for probing complex coupled processes in electrolytic systems. We recently reported an enzymatic signal amplification phenomenon that may provide increased sensitivity when detecting bio-analytes (M. S. Hasenbank, E. Fu and P. Yager, Langmuir, 2006, 22, 7451-7453), but to fully harness this method requires an improved understanding of the underlying electrochemical and chemical processes. We use spatial control of electrolyte streams on patterned conductive substrates in a microfluidic platform to elucidate the coupling of homogeneous chemical steps to heterogeneous electrochemical charge transfer processes. Because the gold surface was observable using SPR imaging, electrochemical phenomena could be monitored optically in real time. Based on these and additional results, we propose a mechanism for the novel amplification phenomenon that involves direct electron transfer between surface-immobilized enzyme molecules and the gold surface. This improved understanding of the underlying mechanism should enable the future implementation of this phenomenon in signal amplification schemes for highly sensitive lab-on-a-chip biosensors.     

Lateral spread of an amplification signal using an enzymatic system on a conductive surface

In this letter, we report a novel signal amplification phenomenon that rapidly and dramatically increases both the magnitude and the lateral extent of the original signal. This phenomenon utilizes an enzyme immobilized on a conductive surface to generate amplified signals at locations remote from the original site of enzyme activity. The result is demonstrated on a microfluidic platform using the established precipitating enzyme-substrate system of horseradish peroxidase (HRP) and 3,3',5,5'-tetramethylbenzidine (TMB) on a surface plasmon resonance (SPR) imaging system.     

Regioselective aziridine ring expansions

1-Phenylmethyl-2-hydroxymethylaziridine ( 1 ) undergoes regioselective reactions with carbon disulfide to form thiazolidinethiones. Deuterium labeling experiments suggest that the reaction proceeds exclusively via aziridinium ring expansion. The regiospecificity appears to be electronically controlled.