Dynamic pH mapping in microfluidic devices by integrating adaptive coatings based on polyaniline with colorimetric imaging techniques

Abstract

In this paper we present a microfluidic device that has integrated pH optical sensing capabilities based on polyaniline. The optical properties of polyaniline coatings change in response to the pH of the solution that is flushed inside the microchannel offering the possibility of monitoring pH in continuous flow over a wide pH range throughout the entire channel length. This work also features an innovative detection 10 system for spatial localisation of chemical pH gradients along microfluidic channels through the use of a low cost optical device. Specifically, the use of a microfluidic channel coated with polyaniline is shown to respond colorimetrically to pH and that effect is detected by the detection system, even when pH gradients are induced within the channel. This study explores the capability of detecting this gradient by means of imaging techniques and the mapping of the camera's response to its corresponding pH after a 15 successful calibration process. The provision of an inherently responsive channel means that changes in the pH of a sample moving through the system can be detected dynamically using digital imaging along the entire channel length in real time, without the need to add reagents to the sample. This approach is generic and can be applied to other chemically responsive coatings immobilised on microchannels. Introduction 20Conventional glass-type electrodes have been widely used for pH measures for many years in both industry and academic areas. However, in terms of specific applications (e.g. in vivo, food industry, or for clinical applications), they posses several disadvantages due to their size constraints, rigidity, and the 25 inflexibility of the glass electrode. In recent years, a wide number of pH sensors have been developed to overcome these limitations, including ion sensitive field-effect transistor (iSFET) pH sensors 1-4 , optical pH sensors based on pH responsive dyes 5-8 , hydrogel film pH sensors 9, 10 , and solid-state metal oxides pH sensors 11-13 . 30 In particular, optical pH sensors present several advantages over the traditional pH electrodes as such their low costs, immunity from electromagnetic field, absence of electric contacts, possibility of reference electrode removal and a high degree of miniaturisation 14 . Optical fiber-based pH sensors have been 35 particularly popular, as the fibre allows the optical signal to be transported over long distances, which can facilitate applications in remote sensing 15 .Usually, these optical pH sensors (or optrodes) employ a dye or an indicator that requires immobilisation onto a solid support 40 material. There are several critical issues related to this approach: firstly, the dye should retain its optical properties after the immobilisation process 16 and secondly, it should not leach into the solution 17. A third issue of practical importance is their inherently narrow dynamic response range which is usually 45 around 3-4 pH-units centred on the dye´s pKa 18 .Therefore, the further improvement of such sensors focuses...

An Optical pH Sensor Based on Diazocine

“…Increasing the pH from 1.5 to 3.2 leads to a shift in the absorption λ max of diazocine in visible region from 300 nm (at pH 1.5) to 350 nm (at pH 4). This shift can be explained by the different degree of protonation of the nitrogen atoms . More specifically, in a low pH environment, diazocine exhibits strong absorbance at approximately 300 nm and 350 nm assigned to polar and bipolar on transitions.…”

“…The wavelength shift gradually decreased until pH 2 and then rapidly decreased under the pH 2 to 4. In the inset picture of the Figure C, the characteristic diazocine curve (r 2 =0.983) was obtained for the wavelength shift versus pH, similar to the previously reported results in the case of polyaniline . All these changes are fully reversible.…”

Precipitation of hydroxyapatite at 37 °C in a meso oscillatory flow reactor operated in batch at constant power density

“…24 Concerning pH monitoring, conventional glass-type electrodes have been widely used. However, they possess several constraints when implemented in scaled-down reactors due to their size, rigidity, inflexibility, 26 and response time. The methodology used in this work for pH monitoring attempts to address part of these limitations.…”

Development of microscopic time‐domain dual lifetime referencing luminescence detection for pH monitoring in microfluidic free‐flow isoelectric focusing

“…These are favorable tools due to their sensitivity, selectivity, and capability of spatially resolved analyte and parameter monitoring. Fluorescent pH sensor layers were incorporated into microfluidic platforms using various methods such as photolithography, inkjet printing, dip, blade spin coating and used for chemical reaction and bioprocess engineering [7][8][9][10][11][12][13][14][15][16][17][18]. Because of its simplicity and sensitivity, fluorescence intensity detection is widely used in miniaturized systems.…”