No abstract

Microfluidic paper-based analytical devices (μPADs) are a versatile and inexpensive point-of-care (POC) technology, but their widespread adoption has been limited by slow flow rates and the inability to carry out complex in field analytical measurements. In the present work, we investigate multilayer μPADs as a means to generate enhanced flow rates within self-pumping paper devices. Through optical and electrochemical measurements, the fluid dynamics are investigated and compared to established flow theories within μPADs. We demonstrate a ∼145-fold increase in flow rate (velocity = 1.56 cm s, volumetric flow rate = 1.65 mL min, over 5.5 cm) through precise control of the channel height in a 2 layer paper device, as compared to archetypical 1 layer μPAD designs. These design considerations are then applied to a self-pumping sequential injection device format, known as a three-dimensional paper network (3DPN). These 3DPN devices are characterized through flow injection analysis of a ferrocene complex and anodic stripping detection of cadmium, exhibiting a 5× enhancement in signal compared to stationary measurements.

Three-dimensional numerical solutions have been obtained for steady, linear shear flow past a fixed, heated spherical particle over a wide range of Reynolds number (0.1 [les ] R [les ] 100) and dimensionless shear rates (0.005 [les ] α [les ] 0.4). The results indicate that at a fixed shear rate, the dimensionless lift coefficient is approximately constant over a wide range of intermediate Reynolds numbers, and the drag coefficient also remains constant when normalized by the known values of drag for a sphere in uniform flow. At lower values of the Reynolds number, the lift and drag coefficients increase sharply with decreasing R, with the lift coefficient being directly proportional to R−½. For the range of shear rates studied here, the rate of heat transfer to the particle surface was found to depend only on the Reynolds number, that is, it was insensitive to the shear rate. The dimensionless rate of heat transfer, the Nussel number Nu, was seen to increase monotonically with R.

This paper presents an updated and systematic overview of the recent development in studies on nucleation process in diamond CVD. The nucleation mechanisms are discussed, and the nucleation enhancement methods developed to date are summarized. The effects of surface conditions and deposition parameters on the surface nucleation are described. Finally, a brief description of theoretical and modeling studies on the surface nucleation is given.

Analyte affinity capture by surface-immobilized diagnostic agents is a routinely used assay format for profiling numerous medically and technologically important target analytes. These assays suffer from numerous performance limitations, including sensitivity and rapidity. Assay miniaturization is advocated to improve surfacecapture performance, specifically exploiting the inverse relationship between analyte flux and capture feature size under mass transfer-limiting capture conditions that characterize many such assay formats. Reduced capture feature sizes, e.g., microarrays, are proposed to overcome mass transfer limitations, yet this is difficult to achieve across several size scales. This study validates certain advantages advocated for capture spot miniaturization using a rationale to understand surface capture miniaturization strategies. Experimentally derived immobilized ligand and target capture densities as a function of microspot size for DNA oligomers immobilized on model gold substrates are compared directly with theoretical analysis, validating the hypothesis that miniaturization yields many practical assay advantages. Specifically, results show that transitions from assay mass transfer limiting to kinetically limiting conditions as feature size decreases identify an optimal microspot size range for a specific bioassay system. Analytical advantages realized from such assay miniaturization are more uniform target-spot coverage and substantially increased rate of capture (hybridization), increasing assay signal and rapidity.bioassay ͉ mass transport ͉ miniaturization N ew strategies to improve bioanalytical methods, clinical assay designs, diagnostic devices, and rapid screening tools for disease biomarkers, biosecurity threats, and food pathogens have nearly universally emphasized miniaturization as a route to improve performance, cost, convenience, speed-to-answer, and portability. Reducing size scales for these applications has many practical implications to the measurement of biological analytes and such assay designs. Optimal device sizing is a key design feature for assays that commonly involve affinity binding of analytes to surfaces. Surface capture microassays employ diverse affinity reagents (e.g., antibodies, aptamers, and DNA) to capture broad varieties of analytes (e.g., small molecules, peptides, proteins, nucleic acids, and pathogens). Without active transport (e.g., stirring or field-induced), all current microassay platforms suffer from severe mass transfer limitations, that is, rates of analyte transport to the assay capture surface significantly lag rates of analyte binding. This problem is particularly important in producing rapid results in DNA microassays, where resulting DNA-DNA charge-charge interactions produce complications. A long-standing yet experimentally tentative assertion is that surface capture assays benefit significantly from reduced capture feature (i.e., microarray spot) size, specifically, that these assay systems capitalize on the inverse relationship between an...