Contactless conductivity sensor employing moist paper as absorbent for in-situ detection of generated carbon dioxide gas.

This work presents an unconventional use of capacitively coupled contactless conductivity detector (C4D) for detection of gas absorption by moist paper with potential application for chemical analysis. To be suitable for measuring conductivity of moist paper absorbent, the C4D sensor was therefore designed in planar configuration. A layer of dry filter paper, only 20 mm × 25 mm in size, was placed on the C4D sensor and the device installed inside a specifically designed vaporization chamber. A vial (16 mm i.d., 8 mm high) containing a 150-µL solution of sodium bicarbonate was placed alongside. The filter paper was loaded with 110 µL of deionized water through an injection hole in the cover lid. A 100-µL aliquot of 2 M hydrochloric acid solution was directly dispensed into the vial through a second hole in the lid to generate CO2 gas from the bicarbonate solution. It was observed that the C4D sensor gave real-time response that corresponded to the absorption of the gas and subsequent production of H+ and HCO3- in the moist paper. The monitored signal reached a constant value at 160 s after the addition of the acid. Chemistry of the absorption process and equivalent circuit for the C4D are proposed. Direct measurement of cement powder was chosen to demonstrate the potential use of this device for quantifying the CaCO3 content of the cement. The calibration curve for 0.5-3 mg CaCO3 was linear for signals recorded at 160 s: Vdc = (0.172 ± 0.005) · (mg CaCO3) + (0.016 ± 0.009), with coefficient of determination of 0.9965. Linear calibrations were also observed when the signals were monitored at various time less than 160 s. The limit of quantitation (3 SD of intercept/slope) was 0.17 mg CaCO3. The method provided acceptable precision with %RSD of 4.6 (2 mg CaCO3, n = 10).

Contactless conductivity detector from printed circuit board for paper-based analytical systems.

This work presents a capacitively coupled contactless conductivity detector (C4D) etched out from a printed circuit board (PCB) as potential sensor for paper-based analytical systems. Two lines of any desirable pattern forming 35-µm thick planar copper electrodes were produced on a PCB plate (40 mm × 60 mm) by photolithography. The final PCB plate was covered with polypropylene film to serve as the insulating layer for the C4D detector. The film also protected the copper electrodes from corrosion. Electrodes made in this planar geometry make the PCB-C4D suitable as sensor for flat devices such as paper-based analytical devices. For this work, plain paper strips were employed as sample reservoir and as fluidic channel without hydrophobic pattern. A dried paper strip was first placed over the sensor, followed by dispensing a fixed volume of the liquid sample onto the paper. Entrapment of the liquid sample in the paper strip leads to reproducible size and position of the detection zone of the sample liquid for the capacitive coupling effect. High precision was obtained with %RSD ≤1% (n = 18) for standard solutions of KCl. Soil suspensions could be analyzed without prior filtration by placing a drop onto the paper strip extending away from the detector zone. The paper strip filtered out soil particles at the surface of the paper. Therefore, only soil filtrate moved towards the detection zone by lateral flow. The C4D detection using paper strip showed high tolerance to soil suspension with turbidity up to 6657 NTU, offering direct analysis of soil salinity. Cleaning with moist tissue paper between samples is adequate even for dirty samples such as soil suspension. We also monitored conductivity of acid-base reaction in the microfluidic paper channels, which was later applied to the quantification of bicarbonate in water and in antacid tablet ("Soda Mint Tablet").

T-shirt ink for one-step screen-printing of hydrophobic barriers for 2D- and 3D-microfluidic paper-based analytical devices.

This work presents the use of polyvinyl chloride (PVC) fabric ink, commonly employed for screening t-shirts, as new and versatile material for printing hydrophobic barrier on paper substrate for microfluidic paper-based analytical devices (µPADs). Low-cost, screen-printing apparatus (e.g., screen mesh, squeegee, and printing table) and materials (e.g. PVC ink and solvent) were employed to print the PVC ink solution onto Whatman filter paper No. 4. This provides a one-step strategy to print flow barriers without the need of further processing except evaporation for 3-5 min in a fume hood to remove the solvent. The production of the single layer µPADs is reasonably high with up to 77 devices per screening with 100% success rate. This method produces very narrow fluidic channel 486 ±â€¯14 µm in width and hydrophobic barrier of 642 ±â€¯25 µm thickness. Reproducibility of the production of fluidic channels and zones is satisfactory with RSDs of 2.9% (for 486-µm channel, n = 10), 3.7% (for 2-mm channel, n = 50) and 1.5% (for 6-mm diameter circular zone, n = 80). A design of a 2D-µPAD produced by this method was employed for the colorimetric dual-measurements of thiocyanate and nitrite in saliva. A 3D-µPADs with multiple layers of ink-screened paper was designed and constructed to demonstrate the method's versatility. These 3D-µPADs were designed for gas-liquid separation with in-situ colorimetric detection of ethanol vapor on the µPADs. The 3D-µPADs were applied for direct quantification of ethanol in beverages and highly colored pharmaceutical products. The printed barrier was resistant up to 8% (v/v) ethanol without liquid creeping out of the barrier.

Membraneless Gas-Separation Microfluidic Paper-Based Analytical Devices for Direct Quantitation of Volatile and Nonvolatile Compounds.

This work presents new chemical sensing devices called "membraneless gas-separation microfluidic paper-based analytical devices" (MBL-GS µPADs). MBL-GS µPADs were designed to make fabrication of the devices simple and user-friendly. MBL-GS µPADs offer direct quantitative analysis of volatile and nonvolatile compounds. Porous hydrophobic membrane is not needed for gas-separation, which makes fabrication of the device simple, rapid and low-cost. A MBL-GS µPAD consists of three layers: "donor layer", "spacer layer", and "acceptor layer". The donor and acceptor layers are made of filter paper with a printed pattern. The donor and acceptor layers are mounted together with a spacer layer in between. This spacer is a two-sided mounting tape, 0.8 mm thick, with a small disc cut out for the gas from the donor zone to diffuse to the acceptor zone. Photographic image of the color that is formed by the reagent in the acceptor layer is analyzed using the ImageJ program for quantitation. Proof of concept of the MBL-GS µPADs was demonstrated by analyzing standard solutions of ethanol, sulfide, and ammonium. Optimization of the MBL-GS µPADs was carried out for direct determination of ammonium in wastewaters and fertilizers to demonstrate the applicability of the system to real samples.