Complexometric determination of metal ions by microscopic diffusional titration

Novel cyclic azobenzene-containing vesicles: photo/reductant responsiveness and potential applications in colon disease treatment

The vesicles formed by an amphiphilic copolymer with cyclic azobenzene pendants revealed higher drug loading content and better photo/reductant responsiveness than an analogue of the amphiphilic copolymer with linear azobenzene units.

Fabricating large arrays of microwells with arbitrary dimensions and filling them using discontinuous dewetting

This paper describes the fabrication of large (up to 45 cm(2)) arrays of microwells, with volumes as small as ∼3 fL/well and densities as high as 10(7) wells/cm(2). These arrays of microwells are formed by casting an elastomer, poly(dimethylsiloxane) (PDMS), against "masters" prepared by photolithography; arrays of microwells in other polymers can be formed by using a master consisting of posts in PDMS. A straightforward technique, discontinuous dewetting, allows wells to be filled rapidly (typically on the order of 10(4) wells/s) and uniformly with a wide range of liquids. Several rudimentary strategies for addressing microwells are investigated, including electroosmotic pumping and gaseous diffusion.

Continuous and quantitative delivery of molecules into individual cells with a diffusional microburet

Direct delivery of molecules into the cytosol of live cells is required in many areas of biology and clinical research. Molecules of interest include indicator dyes, biomolecules, and pharmacological agents. In this work we describe continuous delivery of molecules into single cells using a diffusional microburet, DMB. The DMB is a pulled glass micropipette with a fine tip that contains a microscopic plug made of a hydrogel such as agar or polyacrylamide. This plug prevents flow but allows diffusive delivery of the molecule of interest from the DMB body into the cytosol, driven by its concentration gradient. This leads to a scheme of sustained intracellular dosing that is highly reproducible and quantifiable yet does not require the addition of solution volume to the cell. Potential loss of biomolecules from the cytosol through the plug of the DMB can be greatly reduced by proper choice of the pore size and tortuosity of the hydrogel in the DMB tip. The intracellular concentration of fluorescent molecules during delivery can be obtained calibration free. In this work we demonstrate dosing of Lucifer Yellow CH, LY, a charged fluorescent dye, into individual a7r5 vascular smooth muscle cells with a DMB. New types of quantitative analytical experiments on single live cells that the DMB technology enables are titration of intracellular ions and ligands, binding sites, and efflux pathways such as those that are involved in drug resistance.

Direct sensing of total acidity by chronopotentiometric flash titrations at polymer membrane ion-selective electrodes

Polymer membrane ion-selective electrodes containing lipophilic ionophores are traditionally interrogated by zero current potentiometry, which, ideally, gives information on the sample activity of ionic species. It is shown here that a discrete cathodic current pulse across an H (+)-selective polymeric membrane doped with the ionophore ETH 5294 may be used for the chronopotentiometric detection of pH in well-buffered samples. However, a reduction in the buffer capacity leads to large deviations from the expected Nernstian response slope. This is explained by the local depletion of hydrogen ions at the sample-membrane interface as a result of the galvanostatically imposed ion flux in direction of the membrane. This depletion is found to be a function of the total acidity of the sample and can be directly monitored chronopotentiometrically in a flash titration experiment. The subsequent application of a baseline potential pulse reverses the extraction process of the current pulse, allowing one to interrogate the sample with minimal perturbation. In one protocol, total acidity is found to be proportional to the magnitude of applied current at the flash titration end point. More conveniently, the square root of the flash titration end point time observed at a fixed applied current is a linear function of the total acid concentration. This suggests that it is possible to perform rapid localized pH titrations at ion-selective electrodes without the need for volumetric titrimetry. The technique is explored here for acetic acid, MES and citric acid with promising results. Polymeric membrane electrodes based on poly(vinyl chloride) plasticized with o-nitrophenyl octyl ether in a 1:2 mass ratio may be used for the detection of acids of up to ca. 1 mM concentration, with flash titration times on the order of a few seconds. Possible limitations of the technique are discussed, including variations of the acid diffusion coefficients and influence of electrical migration.

A glass capillary ultramicroelectrode with an electrokinetic sampling ability

A glass capillary ultramicroelectrode (tip diameter approximately 1.2 microm) having an electrokinetic sampling ability is described. It is composed of a pulled glass capillary filled with an inner solution and three internal electrodes (Pt working and counter electrodes and an Ag/AgCl reference electrode). The voltammetric response of the capillary electrode is based on electrokinetic transport of analyte ions from the sample solution into the inner solution across the conical tip. It was found that the electrophoretic migration of analytes at the conical tip is faster than electroosmotic flow, enabling electrokinetic transport of analyte ions into the inner solution of the electrode. By using [Fe(CN)6]4- and (ferrocenylmethyl)trimethylammonium (FcTMA+) ions as model analytes, differential pulse voltammetric responses of the capillary electrode were investigated in terms of tip diameter of the capillary, sampling voltage, sampling time, detection limit and selectivity. The magnitude of the response depends on the size and charge of analyte ions. With a capillary electrode having a approximately 1.2-microm tip diameter, which minimizes non-selective diffusional entry of analytes, the response after 1 h sampling at +1.7 V is linearly related to [Fe(CN)6]4- concentration in the range of 0.50-5.0 mM with the detection limit of 30 microM. Application of a potential of the same sign as that of the analyte ion forces the analyte to move out from the electrode to the solution, enabling reuse of the same capillary electrode. The charge-selective detection of analytes with the capillary electrode is demonstrated for [Fe(CN)6]4- in the presence of FcTMA+.

Optical detection in microscopic domains. 1. Monitoring chemical manipulations with absorption microspectrometry

In this research, we test whether optical detection techniques, such as absorption microspectrometry, fluorescence detection, and inner filter effects, show different characteristics in microscopic domains (nano-, pico- and femtoliter range) with respect to usual solution volumes. In this part 1, characterization of absorption microspectrometry is facilitated by the use of a novel microscopic tool, the diffusional microburet (DMB), suitable for fine chemical manipulations of microscopic liquid samples. Since diffusional delivery of substances will not induce appreciable changes in volume and shape of the sample, a DMB makes it possible to obtain spectral recordings and a reference spectrum from the same microscopic droplet. Thus, good quantitative spectra of microscopic domains can be assessed. With this approach, despite the curvature of the sample boundaries, the large surface-to-volume ratios, and microscopic optical path lengths, both Beer-Lambert's law and the law of superposition were found to be directly applicable in microscopic domains without any corrections for absorption at sample boundaries. Use of a combination of microspectrometry and chemical manipulations by DMBs made it possible for the first time to record in real time, in truly microscopic domains, spectral evolution upon addition or subtraction of chemicals, as well as monitor the progress of chemical reactions. This approach is expected to contribute to the optical exploration of microchemistry and microanalysis.