RU2610942C1 - Method for optical measurement of calculating concentration of dispersed particles in liquid environments and device for its implementation - Google Patents

Abstract

FIELD: measurement technology.

SUBSTANCE: method of optical measuring of counting of concentration parties in liquid environments comprises measuring of average hydrodynamic diameter of particles by dynamic light scattering, the calculation of the measured value of the extinction efficiency of the particles, measuring of the optical density at one of wavelengths of the visible range and calculation data obtained by counting the particle concentration. Device for optical measurement of the counting concentration of dispersed particles in liquid environments comprises a laser, LED light source, the direction of emission of which coincides with the direction of the laser, rotating mirror, the guide on sample light one of these sources, located along the laser beam aperture focusing lens, a cell with sample, photodetector measuring intensity of the transmitted radiation and disposed at an angle to the laser beam scattered light collection system, including the diaphragm, the collecting lens and the photodetector, which measures the dependence on time of the intensity of the scattered radiation.

EFFECT: possibility of measuring the absolute concentrations of particles, the expansion of the range of particle diameters, for which we apply the method, improve of the accuracy of determining the concentration.

2 cl, 1 dwg

Description

The invention relates to measuring equipment, more specifically to optical methods for measuring the concentration of dispersed particles suspended in a liquid. It can be used to determine the concentration of dispersed particles (for example, nanoparticles) in the process and according to the results of their synthesis, as well as in liquid media containing dispersed particles and used in various technologies, for example, biomedical ones. In these cases, knowledge of the particle concentration is necessary either to control the efficiency of their synthesis, or to select the optimal dispersed composition of the liquid medium for a particular technology. Moreover, in many cases, the counted concentration of particles, i.e. the number of particles per unit volume, the unit of measurement of the concentration can be, for example, cm ^-3 . Along with the counting, the mass concentration of the particle material in the sample is also measured. The calculated concentration n and mass concentration C are interconnected by a simple ratio:

Here V is the unit volume of the liquid (for example, 1 cm ³ ), v is the volume of one particle, ρ is the density of the particle material, For spherical particles of diameter d, formula (1) takes the form:

Known methods for measuring the mass concentration of dispersed particles in liquid media, based on measuring the volume of the investigated liquid sample, drying (evaporation), weighing the dry residue and calculating the mass concentration according to the data. The disadvantage of this approach is the need for a relatively large volume of liquid for evaporation (at least 100-200 ml), a certain complexity and duration of the entire procedure, and the inability to track the change in concentration in real time.

Measurement of the mass concentration of individual chemical elements that make up dispersed particles suspended in a liquid can be performed on atomic absorption or atomic emission spectrometers. Such measurements also require destruction of the sample and, accordingly, exclude tracking changes in concentration in real time. In addition, both of the above methods allow measuring only the mass concentration of a substance - a nanoparticle material. To calculate the calculated concentration, as can be seen from formulas (1-2), the values of the geometric parameters of the particles are needed, which require additional, often laborious measurements, for example, using an electron or atomic force microscope.

Known methods for measuring the mass or counting concentration of nanoparticles using optical absorption spectrophotometry. These methods include the construction of a calibration graph (the dependence of the optical density of the sample at the selected wavelength on the concentration of particles) using a series of samples with known values of the concentration of particles (comparison samples). Then measure the optical density of the analyzed sample at the same wavelength and using the calibration graph to determine the corresponding concentration value. To implement any of these methods, a series of comparison samples with previously known concentration values is required.

An optical method is also known, which does not require comparison samples, but uses extinction spectra to determine the countable concentration, i.e. dependence of optical density on wavelength in the spectral range in the vicinity of the peak of plasmon resonance. The method involves calculating the concentration of particles according to the value of their extinction at a wavelength located at a sufficient distance from the peak of plasmon resonance. For such a calculation, it is necessary to know the particle size, which is determined by the position of the plasmon resonance peak on the wavelength scale. This method is described in [W. Haiss et al. Determination of Size and Concentration of Gold Nanoparticles from UV-Vis Spectra, Analytical Chemistry, 2007, V. 79, P.P. 4215-4221; N.G. Khlebtsov Determination of Size and Concentration of Gold Nanoparticles from Extinction Spectra, Analytical Chemistry, 2008, V. 80, P.P. 6620-6625] for gold and [D. Parmelle et al. A rapid method to estimate the concentration of citrate capped silver nanoparticles from UV-visable light spectra, Analyst, 2014, V. 139, P.P. 4855-4861] for silver nanoparticles. Its disadvantage is limited applicability - it can be used only for particles having a plasmon resonance peak in the observable region, i.e. only for particles of gold and silver. Moreover, even for particles of these materials, the method is applicable only to monodisperse systems, i.e. having only one peak in the particle size distribution. For polydisperse systems in which there are particles of two or more characteristic sizes that are very different from each other, the considered method is unsuitable. In addition, the position of the plasmon resonance peak depends not only on the size of the particles, but also on the properties of the liquid in which they are suspended and the state of the surface of the particle. The influence of these factors limits the accuracy of determining the particle size and, accordingly, the concentration of particles, and the relative error in determining the concentration due to this factor can significantly exceed the error in measuring particle size. According to recent work [T. Hendel et al In Situ Determination of Colloidal Gold Concentrations with UV-Vis Spectroscopy: Limitations and Perspectives, Analytical Chemistry, 2014 V. 86, P.P. 11114-11124], this error may exceed 30%. This disadvantage manifests itself even in the rather narrow region in which the method works (in this case, for monodisperse gold particles).

Closest to the claimed invention is a method for determining the concentration of nanoparticles by dynamic light scattering, described in [V.V. Vysotsky, O.Ya. Uryupina, A.V. Guselnikova, V.I. Roldugin. On the possibility of determining particle concentration by dynamic light scattering, Colloid Journal, 2009, Volume 71, No. 6, p. 728-733]. This method includes:

- measurement on the instrument of dynamic light scattering of the average hydrodynamic diameter of the nanoparticles d _H for two samples;

- measurement on the same device of an additional optical characteristic - the counting rate of scattered light photons for each of the samples;

- calculation of the relative concentration of nanoparticles in one of the samples with respect to the other, under the assumption that the scattering intensity is directly proportional to the sixth power of the particle diameter, i.e. under the assumption that the scattering intensity I obeys the Rayleigh law

This method has the following limitations:

- the method allows to determine only the relative concentration of nanoparticles in the sample relative to some other sample. For most tasks, it is the absolute concentrations that are of interest, i.e. number of particles per unit volume. In order for the method adopted for the prototype to be used for measuring absolute concentrations, it is necessary to have at least one reference sample — a colloidal solution of nanoparticles of this type with a known concentration;

- the method is applicable only to spherical nanoparticles of a sufficiently small size (particle diameter should not exceed 30-40 nm). This limitation is due to the fact that when calculating the concentration, the Rayleigh law is used, which is valid only for spherical particles whose diameter is less than the wavelength of the scattered radiation by at least 20 times;

- the accuracy of the method is limited due to the very sharp dependence of the calculated concentration value C on the particle diameter d _H (C ~ d ⁶ ). Therefore, even a slight error in measuring the particle diameter leads to a significantly larger error for the concentration.

An object of the present invention is to significantly increase the capabilities of the method, i.e. ensuring the measurement of absolute concentration values, a significant expansion of the range of particle sizes for which the method is applicable, and increasing its accuracy.

The technical result obtained from the implementation of the proposed method is the ability to measure absolute particle concentrations without the need to use any comparison samples, as well as expanding the upper boundary of the range of particle diameters from 30-40 nm to 6000 nm, i.e. to the limit of applicability of the dynamic light scattering method, as well as increasing the accuracy of determining the concentration.

, где k - постоянная Больцмана, Т - абсолютная температура, η - динамическая вязкость растворителя,

- волновой вектор рассеянного излучения, n - показатель преломления растворителя, затем измеряют интенсивности излучения, прошедшего через кювету с коллоидным раствором I₁ и кювету с фоном I₀, вычисляют оптическую плотность частиц относительно фона по формуле:

и по полученным значениям оптической плотности, гидродинамического диаметра частиц и, вычисленной по формулам теории светорассеяния, например, Ми, эффективности экстинкции частиц Q_экст.(d_H), определяют счетную концентрацию частиц по формуле:

, где h - длина оптического пути в кювете.This technical result is achieved by the proposed method of optical measurement of the counted concentration of dispersed particles in liquid media, namely, that a cuvette with a colloidal solution containing the particles to be studied is illuminated by laser radiation with a wavelength of λ ₀ , and the time dependence of the radiation intensity scattered at a certain angle is measured θ - I _diff. (τ), calculate the characteristic time of fluctuations of this intensity τ ₀ ,, the value of τ ₀ determine the hydrodynamic particle diameter d _H by the formula:

where k is the Boltzmann constant, T is the absolute temperature, η is the dynamic viscosity of the solvent,

- wave vector of scattered radiation, n is the refractive index of the solvent, then measure the intensity of radiation passing through a cell with a colloidal solution I ₁ and a cell with a background I ₀ , calculate the optical density of the particles relative to the background by the formula:

and according to the obtained values of optical density, hydrodynamic diameter of the particles and calculated by the formulas of the theory of light scattering, for example, Mie, the extinction efficiency of particles Q _ext. (d _H ), determine the calculated concentration of particles according to the formula:

where h is the length of the optical path in the cell.

The proposed method for optical measurement of the calculated concentration of dispersed particles in liquid media, including measuring the average hydrodynamic diameter by dynamic light scattering and measuring the optical density at one of the visible wavelengths, has the following distinctive features:

- use as an additional optical characteristic of the sample optical density (extinction) at one of the characteristic wavelengths, and not the counting rate of scattered light photons, as in the prototype;

- calculation of the absolute concentration value for the test sample, and not the relative calculated concentration of one sample with respect to another;

- the use for calculating the calculated concentration of formulas for the extinction cross section of the general Mie scattering theory, valid for diameters up to 10,000 nm, and not the Rayleigh law (3), valid only for nanoparticles whose diameter does not exceed 30-40 nm.

The proposed method is implemented as follows:

1. Measure the hydrodynamic particle diameter d _H in the studied colloidal solution by dynamic light scattering. [ISO 22412-2008 Particle sizes analysis - Dynamic light scattering].

2. Measure the optical density of the sample A _λ relative to the background (ie, the liquid in which the test particles are suspended) at one of the characteristic wavelengths λ.

3. The extinction cross section Q (d _H , m, λ) is calculated by the formulas of the Mie scattering theory.

4. By reference or literature data, the values of the real and imaginary parts of the complex refractive index of the material of suspended particles and background liquid are determined, respectively, n _part. and k _frequent , n _background. and k _background. . The relative complex refractive index of particles is calculated

- Формулы для вычисления эффективности экстинкции Q_экст. при заданных значениях m и x приведены в различных источниках, например, [К. Борен, Д. Хафмен Поглощение и рассеяние света малыми частицами, пер. с англ, М, «Мир», 1986]. Имеются также коды компьютерных программ, приведенные в [С. Matzler Matlab codes for Mie scattering and absorption, 2002; S. Prahl Mie Scattering (Version 2-3-3); интернет - ресурс http://omlc.org/software/mie/mie_src.pdf5. Using the formulas of the Mie scattering theory, the extinction efficiency Q _ext (m, x) is calculated, where x is a dimensionless parameter,

- Formulas for calculating extinction efficiency Q _ext. for given values of m and x are given in various sources, for example, [K. Boren, D. Huffman Absorption and scattering of light by small particles, trans. with English, M, Mir, 1986]. There are also codes of computer programs given in [S. Matzler Matlab codes for Mie scattering and absorption, 2002; S. Prahl Mie Scattering (Version 2-3-3); Internet resource http://omlc.org/software/mie/mie_src.pdf

According to the calculated value of Q _ext. and the measured values of d _h and A _λ calculate the calculated concentration of nanoparticles N by the formula

, где

where

h is the optical path length in the cell.

To implement the method, a device for optical measurement of the counted concentration of dispersed particles in liquid media is proposed, comprising a laser, an LED source, the direction of radiation of which coincides with the direction of laser radiation, a rotary mirror directing one of these sources to the sample, located along the diaphragm along the laser beam, a focusing lens, a sample cell, a photodetector measuring the intensity of transmitted radiation, and a scattering system located at an angle to the laser beam th radiation, comprising a diaphragm, a collecting lens and a photodetector measuring the time dependence of the intensity of scattered radiation.

Distinctive features of the proposed device is that to the photodetector measuring the intensity of the scattered radiation, an additional photodetector is installed in the path of the radiation, measuring the intensity of the radiation transmitted through the cell, the value of this intensity is used to determine the optical density of the sample, and in addition to the laser source, an additional emitter is installed for example, an LED whose radiation direction coincides with the direction of the laser radiation.

The presence of these elements is absent in the known particle size analyzers by the method of dynamic light scattering.

The technical result obtained from the implementation of the proposed device is the ability, along with measuring the hydrodynamic diameter of the particles present in the test sample, to measure the optical density of these particles relative to the background. The measurement of optical density can be performed at a wavelength of either a laser or LED radiation source, which are part of the proposed device.

The proposed device is an advanced particle analyzer that implements the method of dynamic light scattering.

The device diagram is shown in the drawing. It includes the main radiation source - laser 1, a rotary mirror 2, which can occupy two working positions, an additional radiation source - LED 3, the radiation direction of which coincides with the direction of laser radiation 1, aperture 4, focusing lens 5, holders of the cell 6, in which cuvettes with the analyzed sample and with the background, a photodetector 7 measuring the intensity of the radiation transmitted through the cuvette, a system for collecting scattered radiation, consisting of a diaphragm 8, collecting lenses 9 and phot receiver 10.

To implement the proposed method, it is necessary to sequentially implement two modes of operation of the device.

The optical density measurement mode, depending on the wavelength of the optical density, requires one of two light sources to turn on, either laser 1 or LED 3. Using the photodetector 7, the radiation intensity transmitted through successively installed cuvettes 6 with sample (I) and with background (I ₀ ). Using these values, the computer program calculates the optical density A using the formula A = log (I ₀ / I). In known particle size analyzers by the method of dynamic light scattering, the possibility of measuring the optical density is not provided.

The measurement mode of the hydrodynamic particle diameter

This is the usual mode that implements the method of dynamic light scattering. Laser radiation 1 is directed by a rotary mirror 2 to aperture 4, passes through this aperture and focusing lens 5, and is focused in the center of the cell with the sample placed in holder 6. Radiation scattered at a certain angle (in the diagram shown in the figure, this angle is 90 °), is collected by the diaphragm 8 and the lens 9 and sent to the photodetector 10. Using the photodetector 10, the time dependence of the intensity of the scattered radiation I _{pacc is measured.} (t). According to this dependence, the hydrodynamic particle diameter d _{H is} determined by a computer program using the dynamic light scattering algorithm.

Although the present invention has been described with specific embodiments, it will be apparent to those skilled in the art that numerous modifications of the invention will be possible without departing from the scope of its legal protection as defined by the appended claims.

Experimental data

To verify the proposed method, it was used to measure the calculated concentration of nanoparticles in five samples of liquid dispersions - four dispersions based on silicon dioxide nanoparticles and one based on silver nanoparticles. According to the values of the calculated concentration determined by the proposed method, the mass concentration was calculated, the values of which were compared with the initial data. The initial data for particles of silicon dioxide were the concentration values in weight percent reported by the manufacturer, and for silver particles, the mass concentration values determined from the plasmon resonance spectra were taken. The values of the hydrodynamic diameter were measured on an ARN-2 particle size analyzer, and the optical density was measured on an SFF-2 Fluoran spectrophotometer-fluorimeter (both devices were developed by FSUE VNIIOFI). The data obtained are shown in Table 1.

When calculating Q _ext. according to the formulas of the Mie theory, we used the values for the real (n) and imaginary (k) parts of the refractive index given in Table 2. Moreover, corrections were introduced for SiO ₂ nanoparticles taking into account the dependence of n and k on the particle sizes.

Claims (2)

1. The method of optical measurement of the counted concentration of dispersed particles in liquid media, namely, that a cell with a colloidal solution containing the particles to be studied is illuminated by laser radiation with a wavelength of λ ₀ , and the time dependence of the radiation intensity scattered at a certain angle θ - I is measured _rass. (τ), calculating a characteristic time τ of the intensity fluctuations of _0, the value of τ ₀ is determined the hydrodynamic diameter of the particle d _H according to the formula:

where k is the Boltzmann constant, T is the absolute temperature, η is the dynamic viscosity of the solvent,

- wave vector of scattered radiation, n is the refractive index of the solvent, then measure the intensity of radiation passing through a cell with a colloidal solution I ₁ and a cell with a background I ₀ , calculate the optical density of the particles relative to the background by the formula:

and according to the obtained values of optical density, hydrodynamic diameter of the particles and the extinction efficiency of particles calculated by the formulas of the theory of light scattering Q _ext. (d _H ) determine the calculated concentration of particles according to the formula:

where h is the length of the optical path in the cell. 2. Device for optical measurement of the counted concentration of dispersed particles in liquid media, containing a laser, an LED source, the direction of radiation of which coincides with the direction of laser radiation, a rotary mirror directing the radiation of one of these sources to the sample, located along the laser beam, aperture, focusing lens , a sample cuvette, a photodetector measuring the intensity of transmitted radiation, and a system for collecting scattered radiation at an angle to the laser beam, including a diaphragm a PMU collecting a lens and a photodetector measuring the time dependence of the intensity of the scattered radiation.

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