A highly sensitive octopus-like azobenzene fluorescent probe for determination of abamectin B1 in apples

doi:10.21203/rs.3.rs-109482/v1

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A highly sensitive octopus-like azobenzene fluorescent probe for determination of abamectin B₁ in apples

https://doi.org/10.21203/rs.3.rs-109482/v1

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published 25 Feb, 2021

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The development of detecting residual level of abamectin B1 in apples is of great importance to public health. Herein, we synthesized a octopus-like azobenzene fluorescent probe 1,3,5-tris (5'-[(E)-(p-phenoxyazo) diazenyl)] benzene-1,3-dicarboxylic acid) benzene (TPB) for preliminary detection of abamectin B1 in apples. The TPB molecule was characterized by ultraviolet-visible absorption spectrometry, 1H-nuclear magnetic resonance, fourier-transform infrared (FT-IR), electrospray ionization mass spectroscopy (ESI-MS) and fluorescent spectrum. A proper determination condition was optimized, with limit of detection and limit of quantification of 1.3 µg L-1 and 4.4 μg L-1, respectively. The mechanism of this probe to identify abamectin B1 was illustrated in terms of undergoing aromatic nucleophilic substitution, by comparing fluorescence changes, FT-IR and ESI-MS. Furthermore, a facile quantitative detection of the residual abamectin B1 in apples was achieved. Good reproducibility was shown based on relative standard deviation of 2.20%. Six carboxyl recognition sites, three azo groups and unique fluorescence signal towards abamectin B1 of this fluorescent probe decided ideal sensitivity, specificity and selectivity. The results show that the octopus-like azobenzene fluorescent probe may be promising for evaluating abamectin B1 in agricultural foods.

Analytical Biochemistry

Biological Chemistry

azobenzene fluorescent probe

abamectin B1

Avermectins, being one type of macrolide antibiotics, have been widely used as bactericide, insecticide and miticide for plants or animals, and have excellent characteristics of disturbing the target's neurophysiological activities and can easily be decomposed by soil microorganisms¹. Abamectin B₁, is the only avermectin that is widely approved for plants and animals for its efficient antiparasitic activity^2,3. However, wide use of avermectins usually leads to consecutive accumulation in food-producing animals and plants^4,5. To detect avermectins (Abamectin B₁), various analytical approaches were used. The approaches include high-performance liquid chromatography-ultraviolet detection (HPLC-UV)⁴, liquid chromatography-tandem mass spectrometry (LC-MS)⁶, high-performance liquid chromatography-fluorescent detector (HPLC-FLD)⁷, enzyme-linked immunosorbent assay (ELISA)⁸ and liquid chromatography-tandem mass spectrometry/mass spectrometry (HPLC-MS/MS)⁹. However, these approaches either needed a time-consuming process, high-cost accurate instrument, or high professional operators, which makes them difficult to apply in general laboratories¹⁰. Herein, considering the potential harm of abamectin B₁ for public health, a convenient and accurate analysis method for residual abamectin B₁ is necessary^10,11. Therefore, it is necessary to explore a convenient analysis method for detection of residual abamectin B₁ to reduce the potential harm to public health.

Ultilization of fluorescent probe has gained great attention in recent years owing to the advantages of high sensitivity, high selectivity, fast response, low cost, and direct detection¹². Especially, azobenzene fluorescent probe exhibits outstanding fluorescent quantum yield, light stability¹³, and good chemical and thermal stability¹⁴. Fluorescent probe has been applied to analyse organophosphorus pesticides¹⁵, organochlorine pesticides¹⁶ and carbamate pesticides¹⁷. However, few studies on fluorescent probe for monitoring avermectin residual have been reported due to the weaker fluorescence signal of a single chromophore¹⁸ or the difficulty in recognizing their complicated chemical structures containing ketones, aldehydes and hydroxyl groups¹⁹. Therefore, a fluorescent molecule, if rationally designed to contain multiple chromophores and recognition groups, is an ideal probe for monitoring avermectin B₁.

Herein, we report the synthesis of octopus-like 1,3,5-tris (5'-[(E)-(p-phenoxyazo) diazenyl)] benzene-1,3-dicarboxylic acid) benzene (TPB), with six carboxyl groups and three azo chromophores (Figure. 1). Its application as a fluorescent probe was proved to be feasible by evaluating its fluorescence properties. We further establish a sensitive and specific approach for qualitative and quantitative assay of abamectin B₁ and further evaluated its applicability in apple samples, based on a visible fluorescence signal for this probe towards avermectin B₁at 420 nm.

Fluorescence properties of TPB

The fluorescence property of TPB was investigated, with its precursor 5-(4-hydroxyphenylazo)-isophthalic acid dimethyl ester (DDH) for comparison, which only has one azo chromophores. When TPB and DDH were irradiated by ultraviolet light, TPB displayed a maximum emission at 350 nm and excitation at 290 nm (Figure. 2), while no fluorescence signal was visible for DDH (Figure. 3). The difference of fluorescence property, was attributed to the more released energy of TPB than that of DDH when the excited state electrons returned from the excited singlet state (S) to the spin singlet electron (S₀), which is based on the superposition of fluorescence effect of three azo chromophores. Furthermore, improved fluorescence property of TPB is ascribed to enhance resistance to internal rotation of molecule resulting from the introduction of 1,3,5-tris (bromomethyl) benzene and the expansion of space system effect. Moreover, the Stokes shift of TPB was calculated to be 60 nm. The Stokes shift, is considered to effectively decrease detection errors, resulting from the interference from auto-fluorescent of samples²⁴ and the spectral overlap between the fluorescent and excitation light²⁵. Therefore, we think that TPB was suitable for use as a fluorescent probe.

We further investigated the applicability of TPB to abamectin B₁. Upon the addition of abamectin B₁, the emission band at 350 nm and the excitation band at 290 nm of the probe shifted forward to 420 nm and 360 nm respectively (Figure. 3). Generally, different fluorescence molecules have different excitation and emission spectra, which can determine the specificity and selectivity of analysis by using fluorescent probe. The Stokes shift of 60 nm was calculated and reflected ideal anti-interference ability. This result confirms that abamectin B₁ can be identified by probe TPB under the certain conditions, which indeed underpin qualitative analysis of abamectin B₁.

Furthermore, the fluorescence intensity of the reacted product at 420 nm were examined at different concentrations of abamectin B₁. Figure. 4 shows that the fluorescence intensity of the reacted product at 420 nm were enhanced gradually with the increasing volume of abamectin B₁(1.0 mg L^-1). The which was considered as the basis of quantitative analysis. Therefore, TPB can be used as a fluorescence probe for assessing the level of abamectin B₁.

The mechanism of identifying abamectin B₁ by TPB

To investigate the mechanism of identifying abamectin B₁ by TPB, FT-IR and fluorescence spectroscopy, and ESI-MS were employed to compare the difference of the TPB before and after adding abamectin B₁. After adding abamectin B₁, FT-IR spectrum shows additional peaks at 1645 cm^-1and at 1120 cm^-1(Figure. S6), corresponding to the ester group. The appearance verifies the recognition of probe TPB to abamectin B₁ that is achieved by esterification. In addtion, a clear red shift of wavelengths was found, which is assigned to the changed conjugate systems of electron-donating and rearranged internal charges prompted by enhanced ability of TPB to capture abamectin B₁. Moreover, ESI-MS spectrum yields a peak at m/z = 2717.1910 (Figure. S7), which corresponds to a new product that is produced between probe TPB and abamectin B₁, thus underlying the mechanism of detection.

The aforementioned identification is attributed to the excellent activity of hydroxyls at C₅ in bamectin B₁²⁶. Therefore, a recognition mode was tentatively proposed, i.e., the two carboxyl groups of the probe TPB are used as recognition sites to abamectin B₁ with a molar ratio of 1: 2 (Figure. 5). Thus, octopus-like azobenzene fluorescent probe TPB could sensitively capture the abamectin B₁using its two carboxyls undergoing esterification. The azobenzene fluorescent probe TPB can sensitively capture the abamectin B₁using its two carboxyls by undergoing esterification.

Establishment and evaluation of the method

To optimize the determination conditions, effects of different levels of the pH and amount of phosphate buffer were investigated. Figure. S8 shows that the fluorescent intensity of probe TPB at 420 nm was initially increased between pH 5.0 and 6.0, followed by a decrease at pH 6.0-9.0 and eventually by the maximum at pH 6.0. The increase-decrease-maximum trend reflects the proper protonation of carboxyl groups of TPB facilitates the coordination process and enhances the nucleophilicity of hydroxyl of TPB to abamectin B₁. Figure. S9 shows that the fluorescent intensity of probe TPB at 420 nm was initially increased between 0.2 mL and 0.8 mL phosphate buffer (0.2 M, pH 6.0), followed by a stabilization at 0.8-1.2 mL phosphate buffer (0.2 M, pH 6.0). Therefore, the validation of using probe TPB was estimated at 0.8 mL phosphate buffer (0.2 M, pH 6.0), by optimizing the determination conditions, linear equation, correlation coefficients (R²), limit of detection (LOD), limit of quantification (LOQ) precision and linear range. Apparently, the fluorescent intensity of probe TPB mixed with various concentrations of abamectin B₁exhibited a good linear relationship at the linear range from 4.4 to 60 μg L^-1, giving rise to a LOQ of 4.4 μg L^-1(Table S1). In addition, the LOD of the described method was calculated as 1.3 µg L^-1(Table S1).

The linear regression equation was thus determined to be Y=3987X – 0.7143 (Figure. 6), where Y is the fluorescent intensity of probe TPB at 420 nm, and X represents the concentration of abamectin B₁. Correlation coefficients (R²) was determined to be 0.9997, which manifests the satisfactory precision of the method. The enhancement clarifies the suitability of this convenient and sensitive method for determination of the abamectin B₁with the advantages of tiny LOD, miniature LOQ, excellent precision and epic linear range.

Further comparison was conducted between our fluorescent probe method and those described previously. Table S2 shows that the recovery, linear range and LOQ of our fluorescent probe method are comparable to or exceed those of the other methods reported previously.

A good reproducibility is represent by a relative standard deviation of 2.20 % between eleven parallel experiments shown in Table S1. The final product has an unique fluorescence spectra at EX of 360 nm and EM of 420 nm, which can be considered to embody the specificity and selectivity of the method generally, as mentioned above. Sensitivity of the method is benefit from the six carboxyl recognition sites and three azo response sites of TPB.

Preliminary detection of abamectin B₁ in apple samples

Table 1 shows 2.0 mg mL^-1, 3.0 mg mL^-1 and 1.6 mg mL^-1abamectin B₁ in Malus pumila mill, Qinguan and Huangxiangjiao, respectively. The recoveries of abamectin B₁ spiked to three apple samples are separately calculated as 98.3%, 101.8% and 104.3% (Table 1), which indicate the suitability of this detection method in apples.

Table 1. Determination of abamectin B₁ concentrations in apple samples and recovery rate of the approach

Sample	Found level (mg mL^-1)	Added level (mg mL^-1)	Found level after adding (mg mL^-1)	Average value (mg mL^-1)	RSD (%) (n = 3)
Malus pumila mill	2.0	4.0	6.0	5.93	98.3
	2.0	4.0	5.8
	2.0	4.0	6.0
Qinguan	3.0	4.0	7.10	7.07	101.8
	3.0	4.0	7.10
	3.0	4.0	7.00
Huangxiangjiao	1.6	4.0	5.90	5.77	104.3
	1.6	4.0	5.70
	1.6	4.0	5.70

In summary, we have demonstrated the probe TPB synthesized for facile quantitative detection of the residual abamectin B₁ in apples. The octopus-like TPB molecule was characterized using UV-Vis, ¹H NMR, FT-IR, ESI-MS and fluorescent spectra. The determination conditions were tuned by varying different pH value and concentration, and a proper condition was achieved at pH 6.0 with LOD and LOQ of 1.3 µg L^-1and 4.4 μg L^-1, respectively. The mechanism of the probe to identify abamectin B₁was tentatively proposed in a aromatic nucleophilic substitution through a combined mode of TPB and abamectin B₁ with a molar ratio of 1: 2. In particular, the facile quantitative detection of the residual abamectin B₁ in apples was achieved. Our results show that the novel approach of quantitative assay based on fluorescent probe can be helpful to evaluate the abamectin B₁ in agricultural foods.

Reagents and materials

Dimethyl 5-aminoisophthalate, 1,3,5-tris (bromomethyl) benzene, phenol, sodium acetate anhydrous, sodium nitrite, sodium hydroxide, N, N-dimethylformamide (DMF), ethanol anhydrous, methanol, magnesium sulfate anhydrous, potassium carbonate, sodium chloride, acetonitrile, tetrahydrofuran, hydrochloric acid, sulphuric acid, pH 4.0 phosphate buffer (0.2 M), pH 7.0 phosphate buffer (0.2 M), pH 8.0 phosphate buffer (0.2 M), pH 9.0 phosphate buffer (0.2 M), sodium dihydrogen phosphate anhydrous, sodium phosphate dibasic, potassium bromide, deuterated chloroform, octadecylsilane chemically bonded silica (C₁₈) were purchased from Shanghai Macklin Biochemical Co., Ltd (Shanghai, China) and were of analytical grade. pH 6.0 phosphate buffer (0.2 M) was purchased from Beijing Lanyi chemical products Co., Ltd. (Beijing, China) and were of analytical grade. Quinine sulfate fluorescent standard substance (98.6%) and abamectin B₁ were purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Demonized water (18 MΩ cm) was produced by using a water purification system (Water Purification System, Casccada 1, Pall, Beijing, China).

General instrumentation

Fourier-transform infrared spectroscopy (FT-IR) was recorded on a Thermo 330 spectrometer at 500-4000 cm^-1 wavelengths and a resolution of 3 cm^-1 over 32 scans. The ultraviolet-visible absorption spectrometry (UV-Vis) was measured with a TECHCOMP UV2600 Spectrometer (TECHCOMP, Shanghai, China) at 200-800 cm^-1wavelengths. ¹H-nuclear magnetic resonance (NMR) spectrum was recorded on a JNM-ECA 600 MHz-NMR using tetramethylsilane (TMS) (JEOL, Japan). Electrospray ionization mass spectroscopy (ESI-MS) data of TPB and the product were obtained by using a Bruker New ultrafleXtreme MALDI-TOF mass spectrometer at NL: 7,990,000, RT: 4.58-4.66, AV: 34, T: FTMS-p ESI and NL: 41,300, RT: 2.13-2.22, AV: 36, T: FTMS-p ESI. Fluorescent spectrum was performed using a Hitachi F-7000 spectrometer at Sampling Interval: 5 nm, Scan speed: 12000 nm min^-1, EX Slit: 5 nm, EM Slit: 5 nm, PMT Voltage: 700V, Contour interval: 10 nm, Temperature: room temperature.

Preparation of probe TPB

TPB was synthesized by the scheme showed in Figure. S1 based on a previously reported method²⁰, which was confirmed by FT-IR, UV-Vis, ¹H NMR and ESI-MS. FT-IR: characteristic absorption peaks at 3432 cm^-1(C-H of aryl), 1699 cm^-1(C=O of carboxyl), 1597 cm^-1(N=N), 1499 cm^-1(aryl), 1253 cm^-1(aryl-N). UV-Vis: λmax 355 nm (azobenzene). ¹H NMR (600 MHz, CDCl₃) δ (ppm): 8.71 (s, 3 H), 8.65-8.68 (s, 6 H), 7.94-7.95 (d, 6 H), 7.25 (s, 3 H), 7.0-7.1 (d, 6 H), 5.22 (s, 6 H). HRMS (m/z, ESI): [M]⁻ calcd. Found 971.2133 (TPB), 703.1660 (TPB-C₁₄N₂O₄H₈), 485.1024 (TPB-C₂₂N₄O₉H₂₂). All supplementary data used for confirmation can be found in Figures S2-S5.

General procedure for fluorescent spectra measurement

Tetrahydrofuran, as a solvent, was used to prepare the probe TPB solution (0.06 mmol L^-1) and abamectin B₁ solution (1.0 mg L^-1). 0.5832 g TPB was added to a volumetric flask and mixed with tetrahydrofuran solution to 10 mL to get probe TPB solution (0.06 mmol L^-1). 1.0 mg abamectin B₁ was added to a volumetric flask and mixed with tetrahydrofuran solution to 1 L to get abamectin B₁ solution (1.0 mg L^-1).

Fluorescent spectra of probe TPB were analyzed before and after adding abamectin B₁ to ascertain the appropriate excitation wavelength and emission wavelength. One 10 mL colorimetric tube was filled with probe TPB (1.0 mL, 0.06 mmol L^-1) and avermectin B₁ (0.2 mL, 1.0 mg L^-1). The other was only filled with probe TPB (1.0 mL, 0.06 mmol L^-1). Then, they all fixed with tetrahydrofuran to 10 mL. For each sample in colorimetric tubes, test condition of the fluorescence intensity was as follows: Sample mixed time: 10 s, Sampling interval: 5 nm, Scan speed: 12000 nm min^-1, EX Slit: 5 nm, EM Slit: 5 nm, PMT Voltage: 700V, Contour interval: 10 nm, Temperature: room temperature, EX WL: 200-600 nm, EM WL: 200-600 nm.

Establishment and validation of the analysis method

To obtain the ideal pH condition of test, considering the emergence of esterification at pH below 5.0 and salt forming at pH above 5.0 for avermectin B₁, the effect of pH 5.0-9.0 on the analysis result was investigated. Five 10 mL colorimetric tubes were filled with probe TPB (1.0 mL, 0.06 mmol L^-1) and avermectin B₁ (0.2 mL, 1.0 mg L^-1), and fixed to 10 mL with pH 5.0-9.0 phosphate buffer (0.2 M) at room temperature, respectively. For each sample in colorimetric tubes, test condition of the fluorescence intensity was same as that of probe TPB, except EX WL: 360 nm and EM WL: 420 nm. pH 6.0 phosphate buffer (0.2 M) was found as an ideal pH condition and used in following tests.

To obtain the ideal mount of phosphate buffer, the effect of 0.2-1.2 mL phosphate buffer (0.2 M, pH 6.0) was investigated. Six 10 mL colorimetric tubes were filled with probe TPB (1.0 mL, 0.06 mmol L^-1) and avermectin B₁ (0.2 mL, 1.0 mg L^-1), and fixed with 0.2, 0.4, 0.6, 0.8, 1.0 and 1.2 mL phosphate buffer (0.2 M, pH 6.0) at room temperature, respectively. For each sample in colorimetric tubes, test condition of the fluorescence intensity was same as that of probe TPB, except EX WL: 360 nm and EM WL: 420 nm. 0.8 mL phosphate buffer (0.2 M, pH 6.0) was found as an ideal mount and further applied in followed tests.

To establish the relationship between probe TPB and abamectin B₁, at room temperature, 0.0 mL, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6 mL abamectin B₁(1.0 mg L^-1) were added to probe TPB (1.0 mL, 0.06 mmol L^-1), mixed with 0.8 mL phosphate buffer (0.2 M, pH 6.0) and fixed with tetrahydrofuran to 10 mL, respectively. For each sample, test condition of the fluorescence intensity was same as that of part 2.4, except EX WL: 360 nm, EM WL: 200-700 nm.

To validate the method under analytical control, the limit of detection (LOD), limit of quantification (LOQ), precision and linear range were implemented according to the previous methods^21,22. A 10 mL colorimetric tubes was mixed with abamectin B₁(0.20 mL, 1.0 mg L^-1) and probe TPB (1.0 mL, 0.06 mmol L^-1), mixed with 0.8 mL phosphate buffer (0.2 M, pH 6.0 ), fixed with tetrahydrofuran to 10 mL and fluorescently detected according to that of probe TPB, except EX WL: 360 nm and EM WL: 420 nm. Eleven groups of parallel experiments were conducted. LOD and LOQ were calculated using the formulas shown in Eq. (1-2). Precision was evaluated using relative standard deviation (RSD). The linear range was from LOQ to the maximum measured value.

LOD = 3 δ/k (1)

(δ: standard deviation of the experiments, k: slope for the range of the linearity)

LOQ = 10×δ (2)

(δ: standard deviation of the experiments)

Preparation and analysis of apple samples

Malus pumila mill, Qinguan and Huangxiangjiao were purchased from local Xingfu Supermarket (Beijing, China) and analyzed. Apple samples were prepared according to a published method²³. Each sample (20.0 g) was respectively added with acetonitrile (10.0 mL) and vigorously shaken for 2.0 min by a vortex mixer. Next, each mixture was mixed with 4.0 g anhydrous magnesium sulfate anhydrous and 1.0 g sodium chloride and shook for another 1.0 min. Following centrifugation at 4000 rpm for 5.0 min, 2.0 mL of the upper layer was transferred to a 20 mL volumetric flask and filled with tetrahydrofuran to obtain tested Malus pumila mill, Qinguan and Huangxiangjiao samples. These samples were fluorescently detected according to that of probe TPB, except EX WL: 360 nm and EM WL: 420 nm, respectively. Value of abamectin B₁was calculated according to the linear regression equation of this work.

To perform the recovery rate, two 10 mL colorimetric tubes were mixed with 2.0 mL apple sample and probe TPB (1.0 mL, 0.06 mmol L^-1), mixed with 0.8 mL phosphate buffer (0.2 M, pH 6.0 ), fixed with tetrahydrofuran to 10 mL and marked as A and B, respectively. Afterwards, A was filled by the standard abamectin B₁(0.05 mL, 1.0 mg L^-1). They all were fluorescently detected according to that of probe TPB, except EX WL: 360 nm and EM WL: 420 nm. Level of abamectin B₁was calculated according to the linear regression equation of this work. And the recovery rate was calculated using the formulas shown in Eq. (3).

Recovery rate (%) = 100 × (C_A- C_B)/C_S (3)

(C_A: level of abamectin B₁of A filled by the standard abamectin B₁, C_B: level of abamectin B₁of B, C_S: level of added standard abamectin B₁)

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21671021) and the Fundamental Research Funds for the Central Universities (No. 2015ZCQ-LY-03).

Compliance with ethical standards

Conflicts of interest

The authors declared that there are no conflicts of interest.

Supplementary information

Supplementary data related to this article can be found at Supplementary material.

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A highly sensitive octopus-like azobenzene fluorescent probe for determination of abamectin B₁ in apples

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