This paper describes 96- and 384-microzone plates fabricated in paper as alternatives to conventional multiwell plates fabricated in molded polymers. Paper-based plates are functionally related to plastic well plates, but they offer new capabilities. For example, paper-microzone plates are thin (approximately 180 microm), require small volumes of sample (5 microL per zone), and can be manufactured from inexpensive materials ($0.05 per plate). The paper-based plates are fabricated by patterning sheets of paper, using photolithography, into hydrophilic zones surrounded by hydrophobic polymeric barriers. This photolithography used an inexpensive formulation photoresist that allows rapid (approximately 15 min) prototyping of paper-based plates. These plates are compatible with conventional microplate readers for quantitative absorbance and fluorescence measurements. The limit of detection per zone loaded for fluorescence was 125 fmol for fluorescein isothiocyanate-labeled bovine serum albumin, and this level corresponds to 0.02 the quantity of analyte per well used to achieve comparable signal-to-noise in a 96-well plastic plate (using a solution of 25 nM labeled protein). The limits of detection for absorbance on paper was approximately 50 pmol per zone for both Coomassie Brilliant Blue and Amaranth dyes; these values were 0.4 that required for the plastic plate. Demonstration of quantitative colorimetric correlations using a scanner or camera to image the zones and to measure the intensity of color, makes it possible to conduct assays without a microplate reader.

This paper describes a paper-based microfluidic device that measures two enzymatic markers of liver function (alkaline phosphatase ALP, and aspartate aminotransferase AST) and total serum protein. A device consists of four components: i) a top plastic sheet, ii) a filter membrane, iii) a patterned paper chip containing the reagents necessary for analysis, and iv) a bottom plastic sheet. The device performs both the sample preparation (separating blood plasma from erythrocytes) and the assays; it also enables both qualitative and quantitative analysis of data. The data obtained from the paper-microfluidic devices show standard deviations in calibration runs and “spiked” standards that are acceptable for routine clinical use. This device illustrates a type of test useable for a range of assays in resource-poor settings.

Linear 1,2-bis(pyridinium)ethane 'axles' and macrocyclic 24-membered crown ether 'wheels' (, and ) combine to form [2]pseudorotaxanes. These interpenetrated adducts are held together by N+...O ion-dipole interactions, a series of C-H...O hydrogen bonds and pi-stacking between electron-poor pyridinium rings of the axle and electron-rich catechol rings of the wheel. 1H NMR spectroscopy was used to identify the structural details of the interaction and to determine the thermodynamics of the binding process in solution. Analysis of nine of these adducts by single crystal X-ray crystallography allowed a detailed study of the non-covalent interactions in the solid state. A wide variety of structural changes could be made to the system. The versatility and potential of the template for the construction of permanently interlocked structures such as rotaxanes and catenanes is discussed.

Here we describe a study of the charging and discharging of solids in a system comprising a metal sphere that rolls across an electrically insulating plate. [1][2][3][4] There are two kinetically distinct processes: 1) charging at a constant rate; 2) abrupt discharging, when the potential difference between sphere and surface reaches a critical value determined by the dielectric strength of air. This work has two objectives: 1) to develop a procedure for examining the rate of charging and discharging as a function of a range of relevant variables; 2) to use this information to test the hypotheses that charge separation involved ions and that discharge of the potential produced involved a breakdown of air. In published work, we have described this system; [2] this study demonstrates the wealth of quantitative information it can provide as a tool for studying the atomic/molecular mechanisms of contact electrification. These mechanisms are relevant to processes ranging from lightning [5] to xerography, and are a subject of active controversy. [6][7][8][9][10] Three mechanisms appear to contribute to contact electrification:[1] 1) ion transfer between surfaces having mobile ions, 2) partitioning of ions from adsorbed water onto the surfaces of non-ionic insulators, [11] and 3) electron transfer between conductors and semiconductors (materials with mobile electrons and well-defined Fermi surfaces). We have concluded [1] -in agreement with a hypothesis by Diaz [12,13] -that the transfer of ions between the contacting surfaces is the most common mechanism for charge separation when organic materials are involved. The data we present here are consistent with contact charging by the slow transfer of ions, interrupted by episodic, rapid discharge events involving ionized plasmas when the difference in electrical potential between the surfaces exceeds the breakdown limit of air.These experiments used the rolling sphere tool (RST, Figure 1) developed by Grzybowski et al. [2][3][4] We investigated contact electrification between stainless steel spheres (d = 3.2 mm) and three different surfaces (relative humidity, RH = 20-25 %, T % 22 8C, w = 80 rpm). The Supporting Information contains the experimental procedures we followed for preparing the insulating surfaces: 1) glass (a 1.0 mm thick, 76 mm diameter wafer of low-alkali glass); 2) glass silanized with N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride; 3) glass silanized with 3-(trihydroxysilyl)-1-propane-sulfonic acid.When the sphere was far (more than ca. 2.5 cm) from the electrode (width 5 mm, 0.2 radians), the electrometer reported only the charge on the portion of the insulator (the glass plate) to which the electrode coupled (Q w ). When the sphere passed over the electrode, the electrometer registered a peak in the charge, the height of which was the sum (Q s+w ) of the charges that the electrode sensed on the sphere (Q s ) and Q w . Figure 2 a shows the charge the electrometer recorded for one revolution of the sphere. The fullwidth at half-maximum of the pea...

This communication describes a new approach for controlling static charging (contact electrification), and resulting electrical discharging, that occurs when two contacting materials separate. The prevention of contact electrification is an important problem; unwanted adhesion between oppositely charged materials, spark-initiated explosions, and damage to microelectronic circuitry are some of the deleterious effects of static charging. Current strategies for controlling contact electrification rely upon dissipating an accumulated charge by making contacting surfaces conductive and, therefore, can be difficult to implement with electrically insulating materials. Specifically, using our understanding of the ion-transfer mechanism of contact electrification, we patterned glass slides with negatively charging areas (clean glass) and positively charging areas (glass silanized with a cationic siloxane terminated with a quaternary ammonium group). The rate of charge separation due to a steel sphere rolling on the patterned glass surface correlated linearly with the percentage of the glass surface that was silanized; the rate of charge transfer was minimal when 50% of the glass surface area was silanized. Patterned surfaces also prevented electrical discharges between electrically conducting (bare steel) or insulating (acrylate-coated steel) spheres rolling on the glass, because the rate of charging was sufficiently slow to prevent electric fields greater than the dielectric strength of air to develop. This strategy for preventing static charging therefore does not require one of the two contacting surfaces to be electrically conductive. More generally, these results show that our enhanced understanding of the ion-transfer mechanism of contact electrification enables the rational design of chemically tailored surfaces for functional electrets.