N. Scott Lynn

Passive microfluidic pumping using coupled capillary/evaporation effects

Controlled pumping of fluids through microfluidic networks is a critical unit operation ubiquitous to lab-on-a-chip applications. Although there have been a number of studies involving the creation of passive flows within lab-on-a-chip devices, none has shown the ability to create temporally stable flows for periods longer than several minutes. Here a passive pumping approach is presented in which a large pressure differential arising from a small, curved meniscus situated along the bottom corners of an outlet reservoir serves to drive fluid through a microfluidic network. The system quickly reaches steady-state and is able to provide precise volumetric flow rates for periods lasting over an hour. A two-step mathematical model provides accurate predictions of fluid and mass transport dynamics in these devices, as validated by particle tracking in laboratory systems. Precise flow rates spanning an order of magnitude are accomplished via control of the microchannel and outlet reservoir dimensions. This flow mechanism has the potential to be applied to many micro-total analytical system devices that utilize pressure-driven flow; as an illustrative example, the pumping technique is applied for the passive generation of temporally stable chemical gradients.

Low-fouling surface plasmon resonance biosensor for multi-step detection of foodborne bacterial pathogens in complex food samples.

Recent outbreaks of foodborne illnesses have shown that foodborne bacterial pathogens present a significant threat to public health, resulting in an increased need for technologies capable of fast and reliable screening of food commodities. The optimal method of pathogen detection in foods should: (i) be rapid, specific, and sensitive; (ii) require minimum sample preparation; and (iii) be robust and cost-effective, thus enabling use in the field. Here we report the use of a SPR biosensor based on ultra-low fouling and functionalizable poly(carboxybetaine acrylamide) (pCBAA) brushes for the rapid and sensitive detection of bacterial pathogens in crude food samples utilizing a three-step detection assay. We studied both the surface resistance to fouling and the functional capabilities of these brushes with respect to each step of the assay, namely: (I) incubation of the sensor with crude food samples, resulting in the capture of bacteria by antibodies immobilized to the pCBAA coating, (II) binding of secondary biotinylated antibody (Ab2) to previously captured bacteria, and (III) binding of streptavidin-coated gold nanoparticles to the biotinylated Ab2 in order to enhance the sensor response. We also investigated the effects of the brush thickness on the biorecognition capabilities of the gold-grafted functionalized pCBAA coatings. We demonstrate that pCBAA-compared to standard low-fouling OEG-based alkanethiolate self-assemabled monolayers-exhibits superior surface resistance regarding both fouling from complex food samples as well as the non-specific binding of S-AuNPs. We further demonstrate that a SPR biosensor based on a pCBAA brush with a thickness as low as 20 nm was capable of detecting E. coli O157:H7 and Salmonella sp. in complex hamburger and cucumber samples with extraordinary sensitivity and specificity. The limits of detection for the two bacteria in cucumber and hamburger extracts were determined to be 57 CFU/mL and 17 CFU/mL for E. coli and 7.4 × 10(3) CFU/mL and 11.7 × 10(3)CFU/mL for Salmonella sp., respectively. In addition, we demonstrate the simultaneous detection of E. coli and Salmonella sp. in hamburger sample using a multichannel SPR biosensor having appropriate functional coatings.

Evaporation from microreservoirs

As a result of very large surface area to volume ratios, evaporation is of significant importance when dealing with lab-on-a-chip devices that possess open air/liquid interfaces. For devices utilizing a reservoir as a fluid delivery method to a microfluidic network, excessive evaporation can quickly lead to reservoir dry out and overall device failure. Predicting the rates of evaporation from these reservoirs is difficult because the position of the air/liquid interface changes with time as the volume of liquid in the reservoir decreases. Here we present a two-step method to accurately predict the rates of evaporation of such an interface over time. First, a simple method is proposed to determine the shape of an air/liquid meniscus in a reservoir given a specific liquid volume. Second, computational fluid dynamics simulations are used to calculate the instantaneous rate of evaporation for that meniscus shape. It is shown that the rate of evaporation is strongly dependent on the overall geometry of the system, enhanced in expanding reservoirs while suppressed in contracting reservoirs, where the geometry can be easily controlled with simple experimental methods. Using no adjustable parameters, the model accurately predicts the position of the inner moving contact line as a function of time following meniscus rupture in poly(dimethylsiloxane) reservoirs, and predicts the overall time for the persistence of liquid in those reservoirs to within 0.5 minutes. The methods in this study can be used to design holding reservoirs for lab-on-a-chip devices that involve no external control of evaporation, such that evaporation rates can be adjusted as necessary by modification of the reservoir geometry.

Biosensing enhancement using passive mixing structures for microarray-based sensors.

The combination of microarray technologies with microfluidic sample delivery and real-time detection methods has the capability to simultaneously monitor 10-1000 s of biomolecular interactions in a single experiment. Despite the benefits that microfluidic systems provide, they typically operate in the laminar flow regime under mass transfer limitations, where large analyte depletion layers act as a resistance to analyte capture. By locally stirring the fluid and delivering fresh analyte to the capture spot, the use of passive mixing structures in a microarray environment can reduce the negative effects of these depletion layers and enhance the sensor performance. Despite their large potential, little attention has been given to the integration of these mixing structures in microarray sensing environments. In this study, we use passive mixing structures to enhance the mass transfer of analyte to a capture spot within a microfluidic flow cell. Using numerical methods, different structure shapes and heights were evaluated as means to increase local fluid velocities, and in turn, rates of mass transfer to a capture spot. These results were verified experimentally via the real-time detection of 20-mer ssDNA for an array of microspots. Both numerical and experimental results showed that a passive mixing structure situated directly over the capture spot can significantly enhance the binding rate of analyte to the sensing surface. Moreover, we show that these structures can be used to enhance mass transfer in experiments regarding an array of capture spots. The results of this study can be applied to any experimental system using microfluidic sample delivery methods for microarray detection techniques.

Waveguide biosensor with integrated detector array for tuberculosis testing

A label-free immunoassay using a local evanescent array coupled (LEAC) biosensor is reported. Complementary metal oxide semiconductor chips with integrated photoconductor arrays are used to detect an antibody to a M. tuberculosis protein antigen, HspX. The metrology limits of the LEAC sensor using dc and ac measurement systems correspond to average film thicknesses of 28 and 14 pm, respectively. Limits of detection are 87 and 108 pm, respectively, for mouse immunoglobulin G antibody patterning and antigen detection.

Biosensor Enhancement Using Grooved Micromixers: Part I, Numerical Studies

In this study we examine the use of the staggered herringbone mixer (SHM) to increase the efficiency of analyte delivery to a planar biosensor surface. Although there has been an extensive amount of research regarding the optimization of the SHM for mixing purposes, there has been very little work regarding the use of said micromixers for sensing purposes. Here, we use numerical methods to examine the effect of the SHM geometry on the efficiency of analyte delivery to a biosensor surface. We show the level of sensing enhancement of an SHM-based sensing chamber over that of an unmixed chamber has a strong dependence on the SHM geometry, the Péclet number, and the overall sensor length. The results presented herein are applicable to a very wide range of biosensor transduction mechanisms and target analytes.

Mapping spatiotemporal molecular distributions using a microfluidic array.

The spatial and temporal distributions of an extensive number of diffusible molecules drive a variety of complex functions. These molecular distributions often possess length scales on the order of a millimeter or less; therefore, microfluidic devices have become a powerful tool to study the effects of these molecular distributions in both chemical and biological systems. Although there exist a number of studies utilizing microdevices for the creation of molecular gradients, there are few, if any, studies focusing on the measurement of spatial and temporal distributions of molecular species created within the study system itself. Here we present a microfluidic device capable of sampling multiple chemical messengers in a spatiotemporally resolved manner. This device operates through spatial segregation of nanoliter-sized volumes of liquid from a primary sample reservoir into a series of analysis microchannels, where fluid pumping is accomplished via a system of passive microfluidic pumps. Subsequent chemical analysis within each microchannel, achieved via optical or bioanalytical methods, yields quantitative data on the spatial and temporal information for any analytes of interest existing within the sample reservoir. These techniques provide a simple, cost-effective route to measure the spatiotemporal distributions of molecular analytes, where the system can be tailored to study both chemical and biological systems.

Detection of virus-like nanoparticles via scattering using a chip-scale optical biosensor

A local evanescent array coupled biosensor is used to detect spherical polystyrene nanoparticles with diameters of 40 nm and 200 nm, whose sizes and refractive index are similar to virus particles. The sensitivity is ∼1%/particle for 200 nm particles and 0.04%/particle for 40 nm particles. Mie scattering in an evanescent field theory is used to model the scattered light intensity for both sizes of nanoparticles.

(Bio)Sensing Using Nanoparticle Arrays: On the Effect of Analyte Transport on Sensitivity

There has recently been an extensive amount of work regarding the development of optical, electrical, and mechanical (bio)sensors employing planar arrays of surface-bound nanoparticles. The sensor output for these systems is dependent on the rate at which analyte is transported to, and interacts with, each nanoparticle in the array. There has so far been little discussion on the relationship between the design parameters of an array and the interplay of convection, diffusion, and reaction. Moreover, current methods providing such information require extensive computational simulation. Here we demonstrate that the rate of analyte transport to a nanoparticle array can be quantified analytically. We show that such rates are bound by both the rate to a single NP and that to a planar surface (having equivalent size as the array), with the specific rate determined by the fill fraction: the ratio between the total surface area used for biomolecular capture with respect to the entire sensing area. We characterize analyte transport to arrays with respect to changes in numerous parameters relevant to experiment, including variation of the nanoparticle shape and size, packing density, flow conditions, and analyte diffusivity. We also explore how analyte capture is dependent on the kinetic parameters related to an affinity-based biosensor, and furthermore, we classify the conditions under which the array might be diffusion- or reaction-limited. The results obtained herein are applicable toward the design and optimization of all (bio)sensors based on nanoparticle arrays.

Consideration of photonic and mass-transfer aspects on the performance of a biosensor based on localized surface plasmons on an array of gold cylinders

Here we present a theoretical study of the sensing capability of a biosensor based on localized surface plasmons (LSP). The sensing structure consists of an array of gold cylinders which support coupling of LSP with Rayleigh anomalies, resulting in a narrow line spectrum. We utilize numerical simulations pertaining to both the photonic and masstransfer aspects of this sensor, where the sensor performance as a function of various fabrication parameters is investigated and compared with a conventional surface plasmon resonance sensor. The simulations predict an improvement in the limit of detection (LOD) of such a device.