OVERVIEW OF METHODS AND TECHNOLOGIES OF PRODUCING OXYGEN-SENSITIVE ELEMENTS FOR MEASURING DISSOLVED OXYGEN IN A RESERVOIR

Research overview on this topic

When studying the concentration of dissolved oxygen in water, various methods are used, one of them is the titration method [1]. This method was one of the first, and although it is considered one of the oldest methods, it is still relevant. The essence of the method is the reaction of dissolved oxygen in a sample with freshly precipitated Mn (II) hydroxide, which is formed by the addition of sodium or potassium hydroxide to manganese sulfate.

Acidification and oxidation of iodide by a higher-valence manganese compound results in the release of iodine in oxygen-equivalent amounts. When choosing a method, you should take into account an important factor-the environment where the study will be conducted. The Winkler Method, when used in natural waters, showed the presence of numerous interferences.

The next proposed method for determining the oxygen concentration was the pyrophosphate measurement method [2]. It uses the same reaction of oxidation of Mn (II) with dissolved oxygen to Mn(III) in an alkaline medium, which served as the basis for the Winkler method. However, due to the presence of sodium pyrophosphate in the solution, the precipitate is dissolved, since the pyrophosphate complexes Mn(II) and Mn(III) are soluble in water.

The main advantage of this method is that it can be used in the presence of many substances that react with iodine or iodide ions, in particular-in the presence of nitrites, and thus interfere with the determination of oxygen by Winkler method.

A photometric method based on the oxidation of a monovalent copper ion to a divalent copper ion with oxygen contained in the sample is recommended for determining the concentration of molecular oxygen in the certification of calibration solutions [3]. Photometric methods for determining the concentration of molecular oxygen in mixtures have the following advantages: high sensitivity and selectivity; the ability to create universal designs of analyzers with several indicator solutions for simultaneous determination of a number of micro-mixtures. Disadvantages of the photometric method for determining the concentration of molecular oxygen are bulky hardware design and low reliability.

At the moment, one of the best research methods is the method of fluorescence. This method is based on the interaction of oxygen molecules with luminescent indicators. It is characterized by a particularly high speed and sensitivity, which in turn leads to the most widespread use of it in research in recent times. One of the main advantages of this method is that its sensitivity is so high that it allows you to detect several billionths of a gram of a luminescent substance in the desired sample, which in its results is many times higher than the sensitivity of other methods.

The fluorescence method for determining dissolved oxygen in water is the most practical both in terms of the quality of measurements made and in terms of its maintenance. It makes it possible to reduce both the period of control and the sample collection time, which in turn can become a determining factor for a quick solution of the problem. Sensors based on the fluorescent method work in difficult conditions. Since any contamination affects the accuracy of measurements and has serious consequences [4].

The sensor for measuring the concentration of oxygen dissolved in water consists of two main components:

1) Sensor cover with a layer of phosphor applied to a transparent substrate;

2) Sensor housing with red and blue light-emitting diode, photodiode and signal Converter.

When measured, a blue light-emitting diode emits a pulse of light that passes through a transparent substrate and is partially absorbed by the phosphor. As mentioned earlier, this transparent substrate contains a fluorescent film in which oxygen atoms interact with the phosphor.

The process of obtaining such a film that is the process of obtaining a luminescent oxygen sensitive element has two stages:

1) Obtaining a porous matrix;

2) Immobilization of the indicator in the matrix.

There are cases when these processes are combined. Each step of the process affects the properties of the sensor element as a whole. The sensor element matrices into which the luminescent indicator is adsorbed can be either thin films (for ash and ash processes), or volume-porous elements.

At the moment, the most promising direction in the development of luminescent sensitive elements of oxygen sensors is the use of ash technologies [5]. In comparison with existing technologies, the resulting matrix with the help of ash technology has a number of advantages. Such as: large surface area, the predetermined surface roughness, high chemical, photochemical and temperature stability.

The possibility of using the sol gel synthesis process at normal (low) temperatures allows creating favorable conditions for the immobilization of organic molecules in an inorganic glass. Also, using the solgel process, it facilitates obtaining the specified coating properties that determine the critical parameters of the sensor-sensitivity and response time [6]. Not the least of the features of the solgel technology is the possibility of miniaturization of the sensor's sensitive element up to micro-dimensions. This process can be used to create films up to 0.2 microns thick that provide short sensor response times.

One of the biggest advantages of this film manufacturing technology is its ability to miniaturize the sensor's sensitive element to micro-dimensions. An important characteristic of the oxygen-sensitive element is its ability to stably transfer the process of operation and storage [7]. However, it is the ability of fluorescent sensors to preserve the metrological characteristics of the sensor element, during long storage and operation that helped them become popular and compete with previously widely used electrochemical detectors.

The peculiarity of the location of the sensitive element relative to the photo detector and the source of excitation is no less important than the choice of technology and method for analysis. To some extent, this determines the choice of the method and scheme for registering fluorescence quenching. The choice of circuit depends on the sensor and its operating conditions. Most often, the signal at which the fluorescence is measured is registered in the transmitted and reflected light, at an angle of 90 degrees to the exciting radiation.

Currently, the following main directions of development of fluorescent sensors can be distinguished:

1) synthesis of new stable indicators with high quantum outputs;

2) creating Sensitive Elements with weakly temperature – dependent (ideally-independent) parameters;

3) obtaining Sensitive Elements with hydrophobic properties;

4) creating materials with stable luminescence characteristics that are independent of environmental influences, which can be used to produce support elements;

5) development of new methods for sustainable implementation of indicators in the matrix in order to improve the temporary stability of the sensor;

6) introduction of components that increase the sensitivity and response time of the sensor in the operating range of oxygen concentrations;

7) creation of highly selective Sensitive Elements.

In addition, the development of lines of Sensitive Elements is promising. Such sensors can be created using:

1) luminescent indicators with different properties for different analytes immobilized in a single matrix;

2) coatings with different permeability for different analytes;

3) a set of selective sensors (sensor line) based on modern Microtechnologies.

The direction of creating luminescent oxygen sensors is undoubtedly promising, since with all the accompanying problems it reflects the trend of development of modern instrumentation aimed at creating microsensors and sensor multisystems based on advanced science-intensive technologies.