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Anal Chem ; 2020 Jun 18.
Artigo em Inglês | MEDLINE | ID: mdl-32484335


The simultaneous optical readout of a potentiometric sensor array of ion-selective electrodes (ISEs) based on PVC membranes is described here for the first time. The optical array consists of electrochromic Prussian Blue (PB) films in multiple closed ion-selective bipolar electrodes (BPEs), which gives a physical separation between the optical detection and sample compartments. The potential-dependent turnover of PB generates Prussian White (PW). A near-Nernstian response of the PB film is confirmed by colorimetric absorbance experiments as a function of applied potential. In the combined bipolar electrode cell, the overall potential is kept constant with a single potentiostat over the entire array where each PB spot indicates the potential change of an individual connected potentiometric probe. For cation-selective electrodes, the absorbance or blue intensity of the connected PB film is enhanced with increasing target cation activity. The colorimetric absorbance changes are simultaneously followed by a digital camera and analyzed by Mathematica software. A multiple cation-BPE array allows one to achieve simultaneous quantitative analysis of potassium, sodium, and calcium ions, demonstrated here in highly colored fruit juices. Mass transport at the PB thin film is shown not to be rate-limiting. The measuring ranges can be tuned in a wide range by potential control. The PB film exhibits greatly improved reproducibility and stability as compared to previous work with a ferroin redox probe confined in a thin solution layer.

Lab Chip ; 20(8): 1441-1448, 2020 Apr 21.
Artigo em Inglês | MEDLINE | ID: mdl-32211667


A wide range of microfluidic paper-based analytical devices (µPADs) have been developed in the last decade. Despite this, the quality of colorimetric analysis has not substantially improved as the data is vulnerable to heterogeneous color distribution (e.g., coffee ring effects), non-uniform shapes of colored detection area, and noise from the underlying paper structure. These limitations are here addressed by a colorimetric method to quantify freely discharged dye on paper substrate, without the need for a defined channel or hydrophobic barrier. For accurate quantification, colorimetric absorbance values are calculated for each pixel based on the recorded RGB values and noise from the paper structure eliminated, to extract accurate absorbance information at the pixel level. Total analyte quantity is then calculated through the conversion of absorbance values into quantity values for each pixel followed by integration across the entire image. The resulting quantity is shown to be independent of the shape of the applied colored dye spot, with a cross, circle or rod shape all giving the same quantity information. The approach is applied to a capillary-based potassium-selective sensor, where the sample solution is loaded with the dye thioflavin T (ThT) obtained by quantitative exchange with K+ in a sensing capillary, which is discharged onto a bare paper substrate without any channels. The resulting dye quantity is successfully obtained by flatbed scanner and smartphone. The successful automated computation of colorimetric data on µPADs will help realize simpler paper-based assay and reaction systems that should be more applicable to addressing real world analytical problems.

ACS Sens ; 5(3): 650-654, 2020 03 27.
Artigo em Inglês | MEDLINE | ID: mdl-32106675


Potentiometric pH probes remain the gold standard for the detection of pH but are not sufficiently sensitive to reliably detect ocean acidification at adequate frequency. Here, potentiometric probes are made dramatically more sensitive by placing a capacitive electronic component in series to the pH probe while imposing a constant potential over the measurement circuit. Each sample change now triggers a capacitive current transient that is easily identified between the two equilibrium states, and is integrated to reveal the accumulated charge. This affords dramatically higher precision than with traditional potentiometric probes. pH changes down to 0.001 pH units are easily distinguished in buffer and seawater samples, at a precision (standard deviation) of 28 µpH and 67 µpH, respectively, orders of magnitude better than what is possible with potentiometric pH probes.

Chimia (Aarau) ; 73(11): 944, 2019 Nov 01.
Artigo em Inglês | MEDLINE | ID: mdl-31753077
ACS Sens ; 4(12): 3093-3101, 2019 12 27.
Artigo em Inglês | MEDLINE | ID: mdl-31744290


Colorimetric measurements by image analysis, giving RGB or HSV data, have become commonplace with optical indicator-based assays and as a readout for paper-based analytical devices (PADs). Yet, most works on PADs tend to ignore the quantitative relationship between color data and concentration, which may hamper their establishment as analytical devices and make it difficult to properly understand chemical or biological reactions on the paper substrate. This Perspective Article discusses how image color data are computed into colorimetric absorbance values that correlate linearly to dye concentration and compare well to traditional spectrophotometry. Thioflavin T (ThT), Neutral Red (NR), and Orange IV are used here as model systems. Absorbance measurements in solution correlate well to image data (and Beer's law) from the color channel of relevance if the gamma correction normally used to render the picture more natural to the human eye is removed. This approach also allows one to correct for color cast and variable background color, which may otherwise limit quantitation in field measurements. Reflectance measurements on paper color spots are equally found to correlate quantitatively between spectroscopy and imaging devices. In this way, deviations from Beer's law are identified that are explained with dye interactions on the paper substrate.

ACS Sens ; 4(3): 670-677, 2019 03 22.
Artigo em Inglês | MEDLINE | ID: mdl-30702271


A distance-based analysis of potassium ion (K+) is introduced that is performed on a microfluidic paper-based analytical device (µPAD) coupled to an ion-selective capillary sensor. The concept is based on two sequential steps, the selective replacement of analyte ion with an ionic dye, and the detection of this dye in a distance-based readout on paper. To achieve the first step, the capillary sensor holds a poly(vinyl chloride) (PVC) membrane film layer plasticized by dioctyl sebacate (DOS) that contains the potassium ionophore valinomycin, a lipophilic cation-exchanger and the ionic indicator Thioflavin T (ThT) on its inner wall. Upon introduction of the sample, K+ in the aqueous sample solution is quantitatively extracted into the film membrane and replaced with ThT. To convert the ion exchange signal into a distance-based analysis, this solution was dropped onto the inlet area of a µPAD to flow the ThT along a channel defined by wax printing, resulting in the electrostatic binding of ThT to the cellulose carboxylic groups. The initial amount of K+ determines the amount of ThT in the aqueous solution after ion-exchange, and consequently the distance of ThT-colored area reflects the sample K+ concentration. The ion exchange reaction was operated in a so-called "exhaustive sensing mode" and gave a distinct response in a narrow range of K+ concentration (1-6 mM) that cannot be achieved by the classical optode sensing mode. The absence of hydrogen ions from the equilibrium competition of the capillary sensor contributed to a complete pH-independence, unlike conventional optodes that contain a pH sensitive indicator. A very high selectivity for K+ over Na+ and Ca2+ has been confirmed in separate solutions and mixed solutions tests. K+ measurements in pooled serum samples at concentrations between 2 and 6 mM are successfully demonstrated on a temperature controlled support.

Técnicas de Química Analítica/instrumentação , Corantes/química , Dispositivos Lab-On-A-Chip , Papel , Potássio/análise , Concentração de Íons de Hidrogênio , Limite de Detecção , Potássio/sangue , Potássio/química