CN113109336A

CN113109336A - Method for measuring pH value of aqueous solution

Info

Publication number: CN113109336A
Application number: CN202110418663.5A
Authority: CN
Inventors: 秦卫东; 任航
Original assignee: Beijing Normal University
Current assignee: Beijing Normal University
Priority date: 2021-04-19
Filing date: 2021-04-19
Publication date: 2021-07-13
Anticipated expiration: 2041-04-19
Also published as: CN113109336B

Abstract

The invention relates to a method for measuring the pH value of an aqueous solution based on gold nanorods, belonging to the technical field of nano materials. The method comprises the following steps: (A) adding a gold nanorod solution and an etching solution into a solution to be detected, and then continuously monitoring RGB color space values of the solution, wherein the RGB color space values correspond to red, green and blue color component values respectively until the etching reaction is finished; (B) converting the GRB color space value into a hue value; (C) plotting the hue value to the etching reaction time in a polar coordinate system to obtain a reversal value of the hue within 400 seconds after the etching starts and a maximum forward rotation value in the whole etching process; (D) when the reversion value is less than 38 degrees, the maximum forward rotation value of the hue value is substituted into the correction curve to realize the measurement of the pH value. The method provided by the invention discriminates the quality of the gold nanorods through the reversal value in the measurement process, and rejects unqualified colorimetric probes, thereby obviously improving the measurement precision and accuracy.

Description

Method for measuring pH value of aqueous solution

Technical Field

The invention relates to a method for measuring the pH value of an aqueous solution based on gold nanorods, belonging to the technical field of nano materials.

Background

The traditional method for measuring the pH value of the solution comprises a test paper, a pH meter and other approaches, however, the accuracy and the resolution of the test paper method are poor, the pH meter can complete the measurement only by the solution with a larger volume, and a glass electrode matched with the pH meter is fragile and has complicated maintenance and activation steps.

Gold nanorods (AuNRs) are longitudinally elongated gold nanoparticles with two LSPR absorption peaks corresponding to the transverse and longitudinal absorption bands, respectively. Because of its extremely high molar extinction coefficient, gold nanorods are an ideal probe for colorimetric pH measurement. However, gold nanorods, which are anisotropic nanostructures, have a tendency to decrease the aspect ratio and convert into nanospheres thermodynamically. Thus, after long term storage, the morphology distribution of the AuNRs may become diversified, resulting in a significant proportion of particles of other morphology, referred to as the degradation process. The AuNRs deterioration process seriously affects the accuracy and precision of the AuNRs as colorimetric probes for measuring the pH value of a solution.

Disclosure of Invention

The invention aims to provide a method for accurately measuring the pH value of a water phase. The inventor finds that the hue value of the normal AuNRs is gradually increased in the etching process, and the trend is in a monotonous clockwise direction when the hue value is plotted for the reaction time in a polar coordinate; and after AuNRs are etched in different pH environments, the resolution and linear relation of the hue value are good. Whereas when experiments were performed using degraded AuNRs, the resolution of the hue value and the linear relationship become poor. It is important that the hue values show different trends when plotted in the polar coordinates described above, reversing initially in the early stages of the reaction and then beginning to rotate forward after 400 s. According to the phenomenon, the quality of AuNRs can be identified in the experimental process, and the deteriorated colorimetric probes are removed, so that the accuracy of pH value measurement is improved. In view of the above, the present invention provides a method for measuring a pH value of an aqueous solution, comprising the steps of:

(A) adding a gold nanorod solution and an etching solution into a solution to be detected, and then continuously monitoring RGB color space values of the solution, wherein the RGB color space values correspond to red, green and blue color component values respectively until the etching reaction is finished;

(B) converting the GRB color space value into a hue value;

(C) plotting the hue value to the etching reaction time in a polar coordinate system to obtain the maximum forward rotation value of the hue in the whole etching process;

(D) substituting the maximum forward value of the hue value into the correction curve to realize the measurement of the pH value;

the etching solution consists of 3.3mM KI and 6.6M H₂O₂Composition is carried out;

the hue value (H) is calculated from the RGB color space by:

in the above formula, R, G and B are the red, green and blue component values, max, respectively_channelAnd min_channelR, G, B maximum and minimum values, respectively, of the component value;

the maximum clockwise value refers to the maximum value which changes clockwise in a polar coordinate system of hue value-etching time.

Preferably, the volume of the solution to be tested is 800. mu.L.

Preferably, the volume of the gold nanorod solution is 50-100 mu L, and the volume of the etching solution is 100-150 mu L.

Preferably, the temperature of the etching reaction is 20-40 ℃.

Preferably, the etching reaction time is 1000-2000 s.

Alternatively, the change in the polar coordinate system of the tone value versus the etching reaction time within 400 seconds after the start of etching may be a counterclockwise change, and the inverse value of the tone value may be read from the graph.

Preferably, when the reversion value is less than 38 degrees in the measurement process, the maximum forward rotation value of the hue value is used for calculating the pH value, and if the reversion value is more than 38 degrees, the measurement is invalid and needs to be measured again.

The reversal value is used as an index for judging whether the quality of the AuNRs is good or not, and whether the reversal value is larger than 38 degrees or not is used as a basis, so that unqualified AuNRs colorimetric probes are effectively removed in the measuring process, and the measuring accuracy and the linear relation are improved. The examples in this specification show the correlation coefficient (R) of the standard curve of pH measurements obtained using this method²) Above 0.95, the maximum deviation of the experimental and theoretical values is less than 0.5pH units. The method realizes effective discrimination of AuNRs quality without the help of large-scale equipment such as TEM and the like, satisfactorily solves the problem of inaccurate measurement result caused by the quality degradation of the gold nanorods, and expands the application range of AuNRs in the measurement field.

Drawings

FIG. 1 is a TEM image of gold nanorods synthesized in example 1.

Fig. 2 is a polar plot of tone value versus etch time for etches at different pH for freshly prepared AuNRs as colorimetric probes in example 2. Curves 1-7 in the figure correspond to the response of pH 1-7, respectively.

FIG. 3 is a calibration curve of maximum smooth-transition of hue values versus pH values for freshly prepared AuNRs as colorimetric probes in example 2 (error bars are standard deviations for 3 replicates).

Figure 4 polar coordinate corresponding curves of tone value versus etch time for AuNRs as colorimetric probes etched at different pH after 25 days of aging in example 3. Curves 1-7 in the figure correspond to the response of pH 1-7, respectively.

FIG. 5 is a calibration curve of maximum smooth-transition of hue values versus pH values for freshly prepared AuNRs as colorimetric probes in example 2 (error bars are standard deviations for 3 replicates).

FIG. 6 is a TEM image of 25-day-old gold nanorods in example 3.

Detailed Description

The patent provides a method for measuring the pH value of an aqueous solution, which comprises the following steps:

adding a gold nanorod solution and an etching solution into a solution to be detected, and then continuously monitoring RGB color space values of the solution, wherein the RGB color space values correspond to red, green and blue color component values respectively until the etching reaction is finished; the volume of the solution to be detected is 800 mu L; the etching solution consists of 3.3mM KI and 6.6M H₂O₂Composition is carried out; the volume of the gold nanorod solution is 50-100 mu L, and the volume of the etching solution is 100-150 mu L; the temperature of the etching reaction is 20-40 ℃; the etching reaction time is 1000-2000 s.

The GRB color space value is converted to a hue value (H) according to

Where R, G and B are the red, green and blue component values, max, respectively_channelAnd min_channelR, G, B maximum and minimum values, respectively, of the component value;

plotting the hue value to the etching reaction time in a polar coordinate system to obtain the reverse value of the hue within 400 seconds after the value etching begins and the maximum forward value in the whole etching process; the reverse rotation value is a value (°) that changes counterclockwise in a polar coordinate system of tone value-etching time, and the maximum forward rotation value is a maximum value (°) that changes clockwise in a polar coordinate system of tone value-etching time.

And when the reverse value is less than 38 degrees in the measurement process, substituting the maximum forward value of the hue value into the correction curve for calculating the pH value, and if the reverse value is more than 38 degrees, the measurement is invalid and needs to be measured again.

For further understanding of the present invention, the following examples are provided to illustrate the method for measuring pH of an aqueous solution according to the present invention, and the scope of the present invention is not limited by the following examples.

The experimental methods described in the following examples are all conventional methods unless otherwise specified; the reagents and materials are commercially available, unless otherwise specified.

Example 1

1.1 preparation of gold nanorods

The AuNRs used in this specification were prepared by seed synthesis.

The seed solution was prepared by the following steps: mix 25. mu.L of HAuCl in a 25mL round bottom flask₄(50mM) and 5mL CTAB (0.1M). After stirring slowly for 5 minutes, freshly prepared 300. mu.L NaBH was added₄Ice solution (0.01M) and then stirred vigorously for 30 seconds. Finally, the seed solution is incubated in a water bath at 30 ℃ for 1h to obtain the seed solution.

The growth solution was prepared as follows: 200 μ L HCl (1M), 100 μ L HAuCl₄(50mM) and 10mL CTAB (0.1M) were added to a 50mL glass round bottom flask with gentle stirring. Finally, 120. mu.L of AgNO was added₃(10mM) and 100. mu.L ascorbic acid (100mM) to obtain a seed growth solution.

In preparing AuNRs, 24. mu.L of the seed solution was poured into the above-mentioned seed growth solution and vigorously stirred for 30s, followed by incubation in a water bath at 30 ℃ for 24 hours to wait for completion of growth. 10mL of AuNRs were centrifuged (10000 rpm, 15 min), washed twice with ultrapure water and finally dispersed in 2mL of CTAB (10 mM).

1.2 characterization of gold nanorods

The AuNRs morphology was characterized using TF20 transmission electron microscopy. FIG. 1 shows that the synthesized gold nanorods have a length of about 40nm and a diameter of about 10nm, and the newly synthesized AuNRs have regular and uniform shapes.

Example 2

In this example, AuNRs synthesized in example 1 were used for pH measurement. The AuNRs solution is prepared freshly, the solutions to be detected are standard solutions with pH values of 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 and 7.0 respectively, and the etching solution is prepared from 3.3mM KI and 6.6M H₂O₂And (4) forming.

Adding 100 μ L AuNRs solution and 100 μ L etching solution into 800 μ L solution with different pH values, shaking vigorously and mixing, transferring into a cuvette, etching at 20 deg.C for 1500s, monitoring with TCS3200 color sensor (TAOS corporation, USA), continuously collecting RGB signals, and converting RGB values into hue value H according to the following formula

R, G and B in equation (1) are component values of red, green and blue, Max_channelAnd min_channelAre the maximum and minimum intensities in the red, green and blue channels.

When newly prepared AuNRs are used as colorimetric probes, and tone value-reaction time in the etching process is plotted in a polar coordinate, the trend of the colorimetric probes is found to be clockwise (clockwise rotation), the curve is smooth, the resolution ratio is good, and the process of anticlockwise trend does not exist in the middle (figure 2). Plotting the maximum hue against the pH, it was found that there is good linearity in the pH range 1-7 (FIG. 3), the correlation coefficient R²0.987, and the linear equation is-38.22 x +427.34 (where x is pH and y is hue).

The measured pH values in fig. 3 were compared with the calculated values of the calibration curve and the deviations obtained are listed in table 1, and the results show that the maximum deviation from the linear calibration curve is 0.281pH units and the compliance with the calibration curve is high when the pH values are measured using freshly prepared AuNRs as colorimetric probes.

TABLE 1 Experimental data for freshly prepared AuNRs colorimetric probes

Example 3

This example uses AuNRs after long-term storage (aging) at room temperature as a colorimetric probe for pH measurement.

After the AuNRs synthesized in example 1 were left at room temperature for 25 days, the pH of the aqueous solution was measured using the same conditions as in example 2, using the AuNRs as a colorimetric probe. It was found that AuNRs after being left for 25 days showed different response patterns when subjected to the experiment (fig. 4), the response of which was first rotated counterclockwise at the beginning of the reaction and then started to rotate clockwise after 400 s. This phenomenon is consistent at each pH value tested. Overall, the resolution between the profiles is poor.

Plotting the maximum hue against the pH value, it was found that the linear relationship was poor in the range of pH 1-7 (fig. 5), and the calibration curve possessed a low linear correlation coefficient (R)²0.927, less than 0.95), and the linear equation is-23.96 x + 368.72. Compared with freshly prepared AuNRs in example 2, the absolute value of the slope of the correction curve is low when the aged AuNRs are used as a probe to measure the pH value of an aqueous solution, which indicates that the sensitivity is reduced.

The deviation values obtained by comparing the actually measured pH values in fig. 5 with the values of the calibration curve are shown in table 2, and the results show that the maximum deviation from the linear calibration curve is 1.00pH unit and the difference in conformity with the calibration curve occurs when the AuNRs, which is left as a colorimetric probe after being left at room temperature for 25 days, measures the pH value. The inversion values within 400s in fig. 4 are tabulated and found to be between 156.9 deg. and 168.2 deg. in the polar coordinate system.

TABLE 2 Experimental data for 25-day old AuNRs colorimetric probes

TEM characterization of the AuNRs in this example revealed that these AuNRs degraded after 25 days of standing, with only 52.9% of the nanoparticles being rod-shaped, 30.2% converted to nanocubes, and the remaining 16.9% being irregular nanostructures. Due to the significant difference in optical properties of nanocube and irregular nanostructures from nanorods, the AuNRs have not been suitable as colorimetric sensors for measuring pH.

Example 4

This example explores the following: (1) the effect of the extent of degradation of the AuNRs on the aqueous pH measurements; (2) the influence of the degree of deterioration of AuNRs on the inversion value; (3) the feasibility of selecting a proper AuNRs colorimetric probe to accurately measure the pH value of the aqueous solution by taking the reverse value as an index; (4) and (4) solving the threshold value of the reversion value of the qualified AuNRs colorimetric probe through an experimental result.

The AuNRs solutions are respectively freshly prepared AuNRs solutions which are placed at room temperature for 5, 10, 15, 20 and 25 days, the solutions to be detected are respectively standard solutions with pH values of 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 and 7.0, and the etching solution is composed of 3.3mM KI and 6.6MH₂O₂And (4) forming.

Each of the above AuNRs solutions was used as a colorimetric probe in turn, and the response of the ph1.0-ph7.0 series of standard solutions was measured by etching reaction. The experimental conditions were as follows:

adding 50 mul of AuNRs solution and 150 mul of etching solution into a series of 800 mul of solutions with different pH values (pH value is 1.0-pH value is 7.0), shaking violently and mixing uniformly, transferring into a cuvette, monitoring by using a TCS3200 color sensor, continuously acquiring RGB signals, and converting the RGB values into hue values H according to a formula (1), wherein the etching temperature is 40 ℃, the etching time is 1000 s. For each pH of the test solution, the measured hue value H is plotted against the reaction time in polar coordinates, and the reversed value and the maximum reversed value (maximum hue value) are read. The maximum value of the reversal values of H in the etching process was the maximum reversal value by comparing 7 solutions of pH1.0 to pH7.0, and is shown in Table 3. The maximum hue values of 7 solutions to be measured are regressed to corresponding pH values to obtain a regression equation and a linear correlation coefficient (R)²) R is to be²Are reported in Table 3. Calculating the actually measured pH value and the correction curve p of each solution to be measuredAnd (4) comparing the H values to obtain the experimental deviation values of the solution, comparing the experimental deviation values of 7 solutions to be tested, and taking the maximum experimental deviation value to be recorded in a table 3.

TABLE 3 influence of AuNRs deterioration on pH measurement

The experimental results in table 3 show that the maximum reversal values gradually increase with increasing days of storage of the AuNRs at room temperature, indicating that the deterioration is gradually severe. Meanwhile, with the increase of the number of days for placing, the linear relation of the nanorod as a colorimetric probe for measuring the pH value of the aqueous solution is worse and worse, and the linear relation is expressed as a correlation coefficient (R)²) The maximum experimental deviation (in pH units) is also increasing with decreasing.

According to the experimental results, the pH value of the aqueous solution should be measured by using freshly prepared AuNRs as colorimetric probes theoretically, but the synthesis process of the AuNRs is long; furthermore, HAuCl₄The reagent is expensive, and if only the freshly prepared AuNRs are used, great waste is caused; furthermore, due to the difference in technical proficiency of different operators, the quality of synthesized AuNRs also varies, and it is not guaranteed that all newly synthesized AuNRs have good quality. Therefore, finding out a suitable quality evaluation index of AuNRs is an important and meaningful task.

Since the maximum inversion value has better correlation with the degradation degree of AuNRs, the method is used as a basis for judging the quality of AuNRs. In the actual measurement referred to in this application, R is required according to the requirements associated with the pH measurement²Not less than 0.95 and not more than 0.5 pH.

According to the above requirements, the experimental data in Table 3 show that the reversion value of a qualified AuNRs colorimetric probe should be less than or equal to 38 degrees.

Example 5

This example shows the results of using an inversion value of 38 ° or less as the basis for selecting a qualified AuNRs colorimetric probe for measuring the pH of an aqueous solution.

The AuNRs solution is respectively placed at room temperature for 5,10. 15, 20 and 25 days of AuNRs solution, the solution to be detected is standard solution with pH values of 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 and 7.0 respectively, and the etching solution is composed of 3.3mM KI and 6.6M H₂O₂And (4) forming.

The experimental conditions were as follows: respectively adding 50 mul of AuNRs solution and 150 mul of etching solution into 800 mul of solution to be detected, violently shaking and uniformly mixing, and transferring into a cuvette, wherein the etching temperature is 20 ℃, and the etching time is 2000 s. The same as in example 4.

The experiment was performed by both the first and second people simultaneously, and 5 AuNRs solutions were numbered during the measurement of 7 pH standard solutions, with information relating to their quality hidden, and were taken randomly during the experiment.

Although the inversion value is measured in the measurement process, the quality of AuNRs is not discriminated through the inversion value, and the experimental result is directly subjected to regression calculation.

Randomly taking an AuNRs solution in the measurement process, screening the quality of the AuNRs through a reversal value, if the reversal value of a certain AuNRs serving as a colorimetric probe in the etching process is less than 38 degrees, judging that the quality of the AuNRs is qualified, and continuing the experiment; otherwise, another AuNRs solution is taken for measurement again.

TABLE 4 influence of AuNRs random access (A) and quality discrimination (B) on the results of the experiment

The results of the experiments in both the second and third people are shown in Table 4. The data shows that AuNRs with different qualities are randomly taken, if discrimination is not carried out, the correlation coefficient of a correction curve is 0.937, which is smaller than the requirement of 0.95 in the industry; the maximum experimental deviation was 0.850pH units, greater than the set deviation value (0.50pH units). And if the inversion value is less than or equal to 38 degrees, the quality of AuNRs is screened in the measurement process, unqualified colorimetric probes are removed, the measurement precision and accuracy can be obviously improved, the correlation coefficient of the measured correction curve is improved to 0.975, the maximum experimental deviation is reduced to 0.324pH unit, and the requirement set by the industry is met.

The experimental results in the above examples illustrate the effectiveness of the method of the present invention in measuring the pH of aqueous solutions using AuNRs as colorimetric probes.

Claims

1. A method of measuring pH of an aqueous solution comprising the steps of:

(B) converting the GRB color space value into a hue value;

the hue value H is calculated from the RGB color space by:

2. The method of claim 1, wherein the volume of the solution to be tested is 800 μ L.

3. The method as claimed in claim 1, wherein the volume of the gold nanorod solution is 50-100 μ L, and the volume of the etching solution is 100-150 μ L.

4. The method according to claim 1, wherein the temperature of the etching reaction is 20-40 ℃.

5. The method according to claim 1, wherein the etching reaction time is 1000-2000 s.

6. A method according to claim 1, characterized in that the change in the polar coordinate system of tone value versus etch reaction time within 400 seconds after the start of etching is possibly a counterclockwise change, and the inverse of the tone value can be read from the graph.

7. The method according to claim 1, wherein the maximum forward rotation value of the hue value is used for the calculation of the pH value when the inversion value is less than 38 ° during the measurement, and is re-determined if the inversion value is greater than 38 °.