METHOD FOR DETERMINING HYDROGEN SULFIDE BY HEADSPACE SINGLE-DROP LIQUID PHASE MICROEXTRACTION AND INTELLIGENT DEVICE COLORIMETRY

Abstract

The present invention provides a method for determining hydrogen sulfide (H.sub.2S) by headspace single-drop liquid phase microextraction and intelligent device colorimetry, which comprises: taking a silver-gold core-shell triangular nanosheet (Ag@Au TNS) as a nanodetection probe, in combination with an analysis method of headspace single-drop microextraction (HS-SDME), specifically extracting H.sub.2S volatilized from a sample to be detected by the nanodetection probe, and detecting H.sub.2S in the extracted sample with the help of the photographing function of an intelligent device and a color picking software. Compared with the prior art, the present invention adopts intelligent device colorimetry, with the limit of detection of about 65 nM and the linear range of 0.1-100 μM, and the established method can be applied to the determination of H.sub.2S in actual samples such as egg white, milk and other opaque samples, and has the advantages of few procedures, simple operation, high detection efficiency and the like.

Claims

1. A method for determining hydrogen sulfide (H.sub.2S) by headspace single-drop liquid phase microextraction and intelligent device colorimetry, comprising: taking a silver-gold core-shell triangular nanosheet (Ag@Au TNS) as a nanodetection probe, in combination with an analysis method of headspace single-drop microextraction (HS-SDME), specifically extracting H.sub.2S volatilized from a sample to be detected by the nanodetection probe, and detecting H.sub.2S in the extracted sample with the help of the photographing function of an intelligent device and a color picking software.

2. The method according to claim 1, wherein a method for preparing the silver-gold core-shell triangular nanosheet comprises the following steps: step (1) adding silver nitrate, sodium citrate and hydrogen peroxide to deionized water, stirring the solution, and then quickly adding NaBH.sub.4 with the stirring stopped, centrifuging the mixture after a certain period of time, and washing the residue to obtain a silver triangular nanosheet (Ag TNS) for later use; step (2) re-dispersing the Ag TNS in deionized water, adding PVP and ascorbic acid, and then sequentially adding a silver nitrate solution, a sodium citrate solution and a mixed solution of silver nitrate and sodium citrate dropwise, mixing the mixture well, centrifuging the mixture, and washing the residue with deionized water; and step (3) adding PVP, diethylamine, ascorbic acid and an aqueous gold-containing solution to precipitate a gold layer on the surface of the Ag TNS, and finally centrifuging the product, washing the product with deionized water to remove AgNO.sub.3 precipitate and residual PVP to obtain the silver-gold core-shell triangular nanosheet (Ag@Au TNS).

3. The method according to claim 2, wherein in the step (1), the molar ratio of the silver nitrate to the sodium citrate to the hydrogen peroxide is 1:(10-20):(200-360), and the volume ratio of the mixed solution to NaBH.sub.4 is (1.3-2.5):1.

4. The method according to claim 2, wherein in the step (3), the volume ratio of the PVP, the diethylamine and the ascorbic acid is (1-9):(0.25-1.25):1, the aqueous gold-containing solution comprises PVP, KI, HAuCl.sub.4 and ultrapure water, and the total mass ratio of Ag to Au is (10-4):1.

5. The method according to claim 1, wherein a method for specifically extracting by the nanodetection probe comprises the following steps: adding a sample to be detected to a container having a cap, removing the cap, dropping a trace of Ag@Au TNS solution onto the inner surface of the cap, and then placing the cap on the container, leaving the container standing for a period of time to ensure that H.sub.2S is fully extracted by the extraction drop after being volatilized from the sample, and after the extraction is completed, opening the cap, wherein the drop on the inner surface of the cap is the extracted sample.

6. The method according to claim 1, wherein a method for detecting H.sub.2S with the help of the photographing function of the intelligent device and the color picking software comprises the following steps: data measuring: the RGB values are provided directly from the captured image by the color picking software; standard curve plotting: the calculated difference of the R values is taken as the ordinate, and the logarithm of the concentration is taken as the abscissa to obtain a standard curve; and data reading: the limit of detection is obtained from the obtained standard curve according to LOD=3δ black/k, where LOD refers to the limit of detection, δ black is the standard deviation of a blank solution, and k is the slope of the standard curve.

7. The method according to claim 1, wherein the intelligent device comprises a smartphone or a tablet computer.

8. The method according to claim 1, wherein the color picking software is EKColorPicker software, a color picker software or ChemEye.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 is a mechanism diagram of a method for detecting H.sub.2S after the headspace single-drop liquid phase microextraction using a silver-gold core-shell triangular nanosheet.

[0030] FIG. 2 shows TEM images of the prepared Ag TNS and Ag@Au TNS with three different thicknesses of gold layers: I (B and C), II (D, E and F), III (G and H); wherein A is a TEM image of Ag TNS; B and C are TEM images of Ag@Au TNS with gold layer I; D, E and F are TEM images of Ag@Au TNS with gold layer II; G and H are TEM images of Ag@Au TNS with gold layer III.

[0031] FIG. 3 shows TEM images of Ag@Au TNS incubated in 100 μM H.sub.2S for 5 min, 10 min, 15 min and 20 min, respectively, and EDX mappings before and after the reaction. wherein 1) shows TEM images of Ag@Au TNS incubated in 100 μM Na.sub.2S for 5 min, 10 min, 15 min and 20 min, respectively; 2) is an EDX mapping of Ag@Au TNS before the reaction with 100 μM H.sub.2S; 3) is an EDX mapping of Ag@Au TNS after the reaction with 100 μM H.sub.2S.

[0032] FIG. 4 is a diagram of changes in UV-vis absorption spectrum of Ag@Au TNS by 100 μM Na.sub.2S HS-SDME for 20 min.

[0033] FIG. 5 is an optimization diagram of Ag@Au TNS incubated in 100 μM Na.sub.2S at temperatures from 15° C. to 35° C. and pH from 5 to 9 for an incubation time from 0 min to 20 min.

[0034] FIG. 6 is a detection diagram of selectivities of Ag@Au TNS in volatile gases such as 10 mM CO2, 10 mM S02, 10 mM NO, 10 mM HBr, 10 mM HCl, 10 mM NH.sub.3 and 100 μM H.sub.2S, respectively.

[0035] FIG. 7 is a diagram of changes in absorption spectra of Ag TNS and Ag@Au TNS I, II and III in 100 μM Na.sub.2S solution (blue) and 100 μM Na.sub.2SO.sub.3 solution (red) by HS-SDME for 20 min.

[0036] FIG. 8 is a calibration curve of the UV detection of H.sub.2S. Conditions are as follows: incubating in 100 μM H.sub.2S at pH 6 and 30° C. for 20 min.

[0037] FIG. 9 is a calibration curve of the smartphone nanocolorimetry of H.sub.2S. Conditions are as follows: incubating in 100 μM H.sub.2S at pH 6 and 30° C. for 20 min.

[0038] FIG. 10 is a diagram of UV spectra corresponding to H.sub.2S at different concentrations.

[0039] FIG. 11 shows change curves of the concentrations of H.sub.2S in the milk stored at 4° C. and 25° C. over 10 days, respectively.

[0040] FIG. 12 shows change curves of the concentrations of H.sub.2S in eggs stored at 4° C. and 25° C. over 10 days, respectively.

DESCRIPTION OF THE EMBODIMENTS

[0041] The method of the present invention is further described in detail below with reference to drawings and specific examples.

Example 1. A Method for Determining Hydrogen Sulfide by Headspace Single-Drop Liquid Phase Microextraction and Intelligent Device Colorimetry

[0042] (1) To a 50 mL beaker were added 40 μL of silver nitrate (0.1 M), 600 μL of sodium citrate (0.1 M), 112 μL of hydrogen peroxide (30%), followed by the addition of deionized water to 39.6 mL. The mixture was stirred rapidly at 30° C. for 10 min using a magnetic stirrer, and then 400 μL of NaBH.sub.4 (0.1 M) was rapidly added with the stirring stopped, at which time a pale yellow solution was formed. After 1-2 min, the solution gradually changed from yellow to red, green and then blue, indicating that the preparation of Ag TNS was completed. The solution was centrifuged, and the residue was washed 3 times with deionized water, and stored in the dark at 4° C. before use.

[0043] (2) The prepared Ag TNS (20 mL) was washed by centrifugation, re-dispersed in 4.5 mL of deionized water, and grown laterally and vertically by the following steps. 500 μL of PVP (17.5 mM vinyl pyrrole monomer) and 18.7 μL of ascorbic acid (0.5 M) were added to aqueous Ag TNS solution, and 300 μL of silver nitrate (0.6 mM) was added to the Ag TNS solution at 0.1 mL/min using a 1 mL disposable syringe. Then 150 μL of sodium citrate (0.1 M) was added to the Ag TNS solution at 0.1 mL/min using a 1 mL disposable syringe, and after 15 min, 1.5 mL of a mixed solution of silver nitrate and sodium citrate was added to the Ag TNS solution at 0.1 mL/min using a 2 mL disposable syringe. The solution was mixed well without further purification and centrifuged, and the residue was washed with deionized water. Then 500 μL of PVP, 75 μL of diethylamine, 100 mL of ascorbic acid (0.5 M) and 500 μL of gold-containing solution were added to precipitate a gold layer on the Ag TNS surface. Finally, the product (Ag@Au TNS) was centrifuged and washed several times with deionized water at room temperature to remove AgNO.sub.3 precipitate and residual PVP. Ag@Au TNS was stored in the dark at 4° C. before use. TEM images of the prepared Ag TNS and Ag@Au TNS with three different thicknesses of gold layers are shown in FIG. 2. It can be seen from FIG. 2A that the average side length of Ag TNS was about 55 nm. The three different thicknesses of Au layers of Ag@Au TNS were 0.8 nm, 1.58 nm, and 2.7 nm, respectively, as shown in FIGS. 2B-2H.

[0044] (3) Sample solutions were prepared, and their aqueous solutions were freshly prepared and stored at 4° C. in the dark due to the instability of Na.sub.2S. Fresh milk and eggs were purchased directly from a supermarket without further processing. Each actual sample type was stored in two groups at ambient temperature (25° C.) and 4° C. TEM images of the extraction of H.sub.2S by Ag@Au TNS for 5 min, 10 min, 15 min and 20 min, respectively, and EDX mappings of Ag@Au TNS before and after the extraction are shown in FIG. 3. The overall shape of Ag@Au TNS remained unchanged after contact with H.sub.2S, although the edges were shrunk and deformed. The EDX results show that there is no significant difference in sulfur distribution between edges and corners.

[0045] (4) A mechanism diagram of a method for detecting H.sub.2S after the headspace single-drop liquid phase microextraction using a silver-gold core-shell triangular nanosheet is shown in FIG. 1. Firstly, 1.0 mL of Na.sub.2S solutions at various concentrations or actual samples were added to a 1.5 mL capped polypropylene centrifuge tube, and then 3.0 μL of the Ag@Au TNS solution was dropped onto the inner surface of the cap. With the help of the surface tension of water and intermolecular forces, the drop does not easily fall from the top unless the centrifuge tube is shaken vigorously. It is therefore feasible to perform SDME in this way. The cap was quickly and gently placed on the centrifuge tube and the tube was left standing for 20 min to ensure that H.sub.2S was fully extracted by the extraction drop after being volatilized from the sample. After SDME, the cap of the centrifuge tube was carefully opened and 3.0 μL of the extractant was transferred to a NanoPhotometer UV-vis spectrophotometer using a pipette for analysis of its UV-vis signal. For SNC, a picture of the drop was taken by the smartphone camera. The RGB (red, green, blue) colors of the image were analyzed by EKColorPicker. The R (red) value of the image was provided directly by EKColorPicker software to indicate the intensity of the color.

[0046] The addition of PVP, ascorbic acid and sodium citrate needs to be carried out at intervals of 10 min.

[0047] In the mixed solution of silver nitrate and sodium citrate, the concentration of silver nitrate was 0.75 mM, and the concentration of sodium citrate was 1.13 mM.

[0048] The gold-containing solution comprises 400 μL of PVP (0.5 M vinylpyrrolidone monomer), 80 μL of KI (0.2 M), 20 μL of HAuCl.sub.4 (0.25 M) and 3 mL of ultrapure water, and was added at a rate of 0.05 mL/min.

[0049] The NaBH.sub.4 (0.1 M) was prepared in ice water and prepared freshly just before use.

[0050] (5) Data processing

[0051] Data measuring: the RGB values were provided directly from the captured image by the color picking software, and a standard curve was plotted by measuring the R (red) values.

[0052] Standard curve plotting: the calculated difference of the R values was taken as the ordinate, and the logarithm of the concentration of 0.01-100 μM was taken as the abscissa to obtain a standard curve.

[0053] Data reading: the limit of detection was calculated from the obtained standard curve according to LOD=3δ black/k, where LOD refers to the limit of detection, δ black is the standard deviation of unreacted Ag@Au TNS, and k is the slope of the standard curve. FIG. 5 is a diagram of optimization experiments, with the optimal experimental conditions of pH 6, 30° C. and 20 min of the extraction. FIGS. 8 and 9 are calibration curves of the UV detection of H.sub.2S and the smartphone nanocolorimetry of H.sub.2S, respectively. The LODs are about 7 nM and about 65 nM, respectively, and the linear ranges are 10 nM-10 μM and 0.1 μM-100 μM, respectively.

[0054] FIG. 10 is a diagram of UV spectra corresponding to H.sub.2S at different concentrations.

[0055] To evaluate the selectivity of Ag@Au TNS, 10 mM CO2, 10 mM SO.sub.2, 10 mM NO, 10 mM HBr, 10 mM HCl, 10 mM NH.sub.3, and 100 μM H.sub.2S were extracted for the study, respectively. As shown in FIG. 6, Ag@Au TNS has the best selectivity to H.sub.2S. Since the thickness of the Au layer plays a key role in the successful detection of H.sub.2S, the effect of this parameter on sensitivity and selectivity was also evaluated. Herein, SO.sub.2 was used as interference factor. As shown in FIG. 7, the sensitivity is slightly reduced but the selectivity is enhanced with the increase in the thickness of the Au layer. As shown in FIG. 7, Ag@Au II TNS is the best extractant of the four materials for the extraction under the conditions of H.sub.2S and SO.sub.2.

Example 2. Detection of H.SUB.2.S in Milk

[0056] To detect the trend of H.sub.2S content in milk, H.sub.2S in fresh milk samples was detected and continuously monitored for 10 days. Fresh milk purchased directly from a supermarket was stored in two beakers, which were kept in a refrigerator at 4° C. and at room temperature (25° C.), respectively. The experimental data were measured using a UV-vis spectrophotometer to obtain a trend diagram of H.sub.2S in milk over 10 days, as shown in FIG. 11. To evaluate the matrix effect, the samples were added with 0.02 μM, 0.2 μM and 2 μM H.sub.2S, respectively, and detected by a UV-vis spectrophotometer; added with 0.2 μM, 2 μM and 20 μM H.sub.2S, respectively, and detected by the smartphone nanocolorimetry. The results are shown in Tables 1 and 2, respectively, and good recovery rates are obtained (97%-105%). In HS-SDME, the matrix interference problem is completely avoided, and the data measuring and calculating are similar to those of the determination of H.sub.2S in aqueous solution.

TABLE-US-00001 TABLE 1 Determination of the content of acid labile sulfides added to fresh milk samples by the HS-SDME-UV-vis method Concentration of Concentration of Relative added sulfide sulfide (μM) recovery.sup.a Sample (μM) (RSD %, n = 3) (%) Milk — 1.31 (5).sup.  — 0.02 1.33 (4.5) 101.10 0.2  1.52 (3.21) 104.50 2 3.32 (6.3) 100.70 .sup.aRelative recovery = (total concentration − blank concentration)/incorporation concentration

TABLE-US-00002 TABLE 2 Determination of the content of acid labile sulfides added to fresh milk samples by the HS-SDME-SNC method Concentration of Concentration of Relative added sulfide sulfide (μM) recovery Sample (μM) (RSD %, n = 3) (%) Milk — 0.27 (5.45) — 0.2 0.46 (1.33) 97.10 2 2.22 (3.86) 97.60 20 20.0 (1.79) 99.80

Example 3. Detection of H.SUB.2.S in Eggs

[0057] To detect the trend of H.sub.2S content in eggs, H.sub.2S in egg samples was detected and continuously monitored for 10 days. Eggs purchased directly from a supermarket were stored in a refrigerator at 4° C. and at room temperature (25° C.), respectively. An opening of 5 mm in diameter is made at one end of the egg shell. The experimental data were measured using a UV-vis spectrophotometer to obtain a trend diagram of H.sub.2S in eggs over 10 days, as shown in FIG. 12. To evaluate the matrix effect, the samples were added with 0.02 μM, 0.2 μM and 2 μM H.sub.2S, respectively, and detected by a UV-vis spectrophotometer; added with 0.2 μM, 2 μM and 20 μM H.sub.2S, respectively, and detected by the smartphone nanocolorimetry. The results are shown in Tables 3 and 4, respectively, and good recovery rates are obtained (95%-104%).

TABLE-US-00003 TABLE 3 Determination of the content of acid labile sulfides added to egg samples by the HS-SDME-UV-vis method Concentration of Concentration of Relative added sulfide sulfide (μM) recovery Sample (μM) (RSD %, n = 3) (%) Egg — 3.61 (2.62) — 0.02 3.62 (5.4)  95.20 0.2 3.81 (2.17) 104.00 2 5.61 (3.9)  100.13

TABLE-US-00004 TABLE 4 Determination of the content of acid labile sulfides added to egg samples by the HS-SDME-SNC method Concentration of Concentration of Relative added sulfide sulfide (μM) recovery Sample (μM) (RSD %, n = 3) (%) Egg — 2.15 (6.2) — 0.2 2.35 (5.3) 100.20 2  4.19 (1.82) 102.32 20 21.8 (1.6) 98.34