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Sample digital technology is a powerful method for absolute quantification of target molecules such as nucleic acids and proteins. The excellent sample stability and mass production capability has enabled the development of microwell array-based sample digitizing methods. However, in current microwell array chips, samples are loaded by the liquid scraping method, which requires complex manual operation and results in a low filling rate and limited hole filling uniformity. Here, we perform sample loading of a through-hole array chip by a microfluidics-driven method and design a double independent S-shaped flow channels sandwiched through-hole array chip. Because of the capillary force and capillary burst pressure, the sample flowing in the channel can be trapped into through-holes, but cannot flow through the other side. Via air flow and displacement of the remaining sample in the channel, the sample can be partitioned consistently, with zero surplus sample residue in the channel. We evaluated the actual performance of the sample-loading process: the chip enables 99.10% filling rate of 18 500 through-holes, with a grayscale coefficient of variation value of 6.03% determined from fluorescence images. In performing digital polymerase chain reaction on chip, the chip demonstrates good performance for the absolute quantification of target DNA. The simple and robust design of our chip, with excellent filling rate and microsample uniformity, indicates potential for use in a variety of sample digitization applications. [ABSTRACT FROM AUTHOR]

Electrostatic interactions are important in non-contact atomic force microscopy (AFM) measurement. Previous reports had focused on the calculation of electrostatic interactions in AFM with metal and dielectric samples, and the present work extended the discussion to semiconductor samples based on Green's function theory and Debye-Hückel theory, considering sample dielectric polarization and free carriers at the same time. In order to enhance the calculation efficiency, an equivalent charge method was implemented and developed with a linear algebra-based algorithm. The calculation results of two limiting cases, metal and dielectric limit with infinite and zero carrier concentrations respectively, were in good agreement with the boundary element method. For a finite carrier concentration, it is found that the electrostatic force on the tip cone is quickly saturated whereas that on the tip apex slowly increases as the carrier concentration increases. On the other hand, the interaction radius on the sample surface is found independent of the sample free carriers, but it linearly increases as the tip-sample distance. Our work can be useful for the carrier concentration detection of semiconductor samples using non-contact electrical AFM modes such as Kelvin probe force microscopy and electrostatic force microscopy. [ABSTRACT FROM AUTHOR]