Western blotting is a commonly used protein assay that combines the selectivity of electrophoretic separation and immunoassay. The technique is limited by long time, manual operation with mediocre reproducibility, and large sample consumption, typically 10–20 μg per assay. Western blots are also usually used to measure only one protein per assay with an additional housekeeping protein for normalization. Measurement of multiple proteins is possible; however, it requires stripping membranes of antibody and then reprobing with a second antibody. Miniaturized alternatives to Western blot based on microfluidic or capillary electrophoresis have been developed that enable higher-throughput, automation, and greater mass sensitivity. In one approach, proteins are separated by electrophoresis on a microchip that is dragged along a polyvinylidene fluoride membrane so that as proteins exit the chip they are captured on the membrane for immunoassay. In this work, we improve this method to allow multiplexed protein detection. Multiple injections made from the same sample can be deposited in separate tracks so that each is probed with a different antibody. To further enhance multiplexing capability, the electrophoresis channel dimensions were optimized for resolution while keeping separation and blotting times to less than 8 min. Using a 15 μm deep × 50 μm wide × 8.6 cm long channel it is possible to achieve baseline resolution of proteins that differ by 5% in molecular weight, e.g. ERK1 (44 kDa) from ERK2 (42 kDa). This resolution allows similar proteins detected by cross-reactive antibodies in a single track. We demonstrate detection of 11 proteins from 9 injections from a single Jurkat cell lysate sample consisting of 400 ng total protein using this procedure. Thus, multiplexed Western blots are possible without cumbersome stripping and reprobing steps.

Two facile, robust, and universal methods by which various polymeric quaternary ammonium salts (polyquaterniums (PQs)) can be quantified and characterized using simple potentiometric polymeric membrane polyion-sensitive electrodes as detectors are described. The two methods are (a) direct detection with polycation sensitive membrane electrodes based on the sodium salt of dinonylnaphthalenesulfonate (NaDNNS), and (b) indirect detection using polyanion sensors based on tridodecylmethylammonium chloride (TDMAC) and dextran sulfate (DS) as a titrant to complex the various polyquaternary species (four different PQs: PQ-2, PQ-6, PQ-10, and poly(2-methacryloxyethyltrimethylammonium) chloride (PMETAC)). Direct detection yields information regarding the charge density of the polycationic species. For the titration method, a series of polyanion sensors doped with TDMAC are used to follow a potentiometric titration of a PQ species using a syringe pump to deliver the titrant. This indirect detection method is more reliable and yields limits of detection in the ppm range for the four PQs examined. The titration method is further explored for detecting excess levels of PQ-6, a common flocculating agent for municipal water supply systems, within the purified water emitted by the Ann Arbor, MI, drinking water treatment plant.

Droplet microfluidics is an enabling platform for high‐throughput screens, single‐cell studies, low‐volume chemical diagnostics, and microscale material syntheses. Analytical methods for real‐time and in situ detection of chemicals in the droplets will benefit these applications, but they remain limited. Reported herein is a novel heterogeneous chemical sensing strategy based on functionalization of the oil phase with rationally combined sensing reagents. Sub‐nanoliter oil segments containing pH‐sensitive fluorophores, ionophores, and ion‐exchangers enable highly selective and rapid fluorescence detection of physiologically important electrolytes (K+, Na+, and Cl−) and polyions (protamine) in sub‐nanoliter aqueous droplets. Electrolyte analysis in whole blood is demonstrated without suffering from optical interference from the sample matrix. Moreover, an oil phase doped with an aza‐BODIPY dye allows indication of H2O2 in the aqueous droplets, exemplifying sensing of targets beyond ionic species.

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Polymeric quaternary ammonium salts (polyquaterniums) have found increasing use in industrial and cosmetic applications in recent years. More specifically, polyquaternium-10 (PQ-10) is routinely used in cosmetic applications as a conditioner in personal care product formulations. Herein, we demonstrate the use of potentiometric polyion-sensitive polymeric membrane-based electrodes to quantify PQ-10 levels. Mixtures containing both PQ-10 and sodium lauryl sulfate (SLS) are used as model samples to illustrate this new method. SLS is often present in cosmetic samples that contain PQ-10 (e.g., shampoos, etc.) and this surfactant species interferes with the polyion sensor detection chemistry. However, it is shown here that SLS can be readily separated from the PQ-10/SLS mixture by use of an anion-exchange resin and that the PQ-10 can then be titrated with dextran sulphate (DS). This titration is monitored by potentiometric polyanion sensors to provide equivalence points that are directly proportional to PQ-10 concentrations.