Simple Flow-Based System with an In-Line Membrane Gas–Liquid Separation Unit and a Contactless Conductivity Detector for the Direct Determination of Sulfite in Clear and Turbid Food Samples

This study presents a simple flow-based system for the determination of the preservative agent sulfite in food and beverages. The standard method of conversion of sulfite ions into SO2 gas by acidification is employed to separate the sulfite from sample matrices. The sample is aspirated into a donor stream of sulfuric acid. A membrane gas–liquid separation unit, also called a ‘gas-diffusion (GD)’ unit, incorporating a polytetrafluoroethylene (PTFE) hydrophobic membrane allows the generated gas to diffuse into a stream of deionized water in the acceptor line. The dissolution of the SO2 gas leads to a change in the conductivity of water which is monitored by an in-line capacitively coupled contactless conductivity detector (C4D). The conductivity change is proportional to the concentration of sulfite in the sample. In this work, both clear (wine) and turbid (fruit juice and extracts of dried fruit) were selected to demonstrate the versatility of the developed method. The method can tolerate turbidity up to 60 Nephelometric Turbidity Units (NTUs). The linear range is 5–25 mg L−1 SO32− with precision <2% RSD. The flow system employs a peristaltic pump for propelling all liquid lines. Quantitative results of sulfite were statistically comparable to those obtained from iodimetric titration for the wine samples

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 x 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-mu L solution of sodium bicarbonate was placed alongside. The filter paper was loaded with 110 mu L of deionized water through an injection hole in the cover lid. A 100-mu L 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: V-dc = (0.172 +/- 0.005) center dot (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). (C) 2020 Elsevier B.V. All rights reserved

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

This work presents a capacitively coupled contactiess conductivity detector ((CD)-D-4) etched out from a printed circuit board (PCB) as potential sensor for paper-based analytical systems. Two lines of any desirable pattern forming 35-mu m thick planar copper electrodes were produced on a PCB plate (40 mm x 60 mm) by photolithography. The final PCB plate was covered with polypropylene film to serve as the insulating layer for the (CD)-D-4 detector. The film also protected the copper electrodes from corrosion. Electrodes made in this planar geometry make the PCB-(CD)-D-4 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 (CD)-D-4 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")

Non‐Invasive Sweat‐Based Tracking of L‐Dopa Pharmacokinetic Profiles Following an Oral Tablet Administration

Levodopa (L-Dopa) is the "gold-standard" medication for symptomatic therapy of Parkinson disease (PD). However, L-Dopa long-term use is associated with the development of motor and non-motor complications, primarily due to its fluctuating plasma levels in combination with the disease progression. Herein, we present the first example of individualized therapeutic drug monitoring for subjects upon intake of standard L-Dopa oral pill, centered on dynamic tracking of the drug concentration in naturally secreted fingertip sweat. The touch-based non-invasive detection method relies on instantaneous collection of fingertip sweat on a highly permeable hydrogel that transports the sweat to a biocatalytic tyrosinase-modified electrode, where sweat L-Dopa is measured by reduction of the dopaquinone enzymatic product. Personalized dose-response relationship is demonstrated within a group of human subjects, along with close pharmacokinetic correlation between the finger touch-based fingertip sweat and capillary blood samples

A self-powered ingestible wireless biosensing system for real-time in situ monitoring of gastrointestinal tract metabolites

Information related to the diverse and dynamic metabolite composition of the small intestine is crucial for the diagnosis and treatment of various diseases. However, our current understanding of the physiochemical dynamics of metabolic processes within the small intestine is limited due to the lack of in&nbsp;situ access to the intestinal environment. Here, we report a demonstration of a battery-free ingestible biosensing system for monitoring metabolites in the small intestine. As a proof of concept, we monitor the intestinal glucose dynamics on a porcine model. Battery-free operation is achieved through a self-powered glucose biofuel cell/biosensor integrated into a circuit that performs energy harvesting, biosensing, and wireless telemetry via a power-to-frequency conversion scheme using magnetic human body communication. Such long-term biochemical analysis could potentially provide critical information regarding the complex and dynamic small intestine metabolic profiles