In situ electrochemical method for measuring the freely dissolved methyl parathion based on the negligible depletion micro-extraction

In situ electrochemical method for measuring the freely dissolved methyl parathion based on the negligible depletion micro-extraction | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article In situ electrochemical method for measuring the freely dissolved methyl parathion based on the negligible depletion micro-extraction Long Pang, Zhigao Feng, Xingru Hu, Jiahui Hou, Guangtao Cui, Jingfu Liu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4394035/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Negligible depletion micro-extraction (nd-ME) plays an important role in measuring the freely dissolved concentration of pollutants but is suffered from a long equilibrium time. In this study, a novel method for in situ determination of freely dissolved methyl parathion (MP) by the electrochemical method was developed based on the nd-ME method. The parameters influencing the adsorption kinetic were investigated in the environmentally relevant ranges, including buffer concentration (5-200 mM), salinity (0-500 mM), and pH value (5.0-8.5). The equilibrium time can be achieved within 16 min between the freely dissolved MP and beta-cyclodextrin decorated reduced graphene oxide composites (β-CD/RGO). Under the equilibrium condition, the sorption coefficients (log K DOC ) were 4.56 for Acros humic acid and 4.58 for Suwannee River humic acid, respectively, which were consistent with those by negligible depletion-solid phase microextraction (nd-SPME) with log K DOC = 4.23 for Acros humic acid and log K DOC = 4.27 for Suwannee River humic acid. The freely dissolved MP in water samples ranged from 3.96 to 4.44 µg L − 1 , which were in agreement with those by nd-SPME ( C free = 4.17–4.76 µg L − 1 ). According to the result, a novel method was developed in this study to estimate the freely dissolved concentration of pollutants using the electrochemical method. Negligible depletion micro-extraction Electrochemical method Freely dissolved concentration In situ Sorption coefficients Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction Freely dissolved concentration ( C free ) is an important parameter of environmental chemistry, toxicology, and pharmacology. Meanwhile, C free is also regarded as a driving force of transport, distribution, and bioaccumulation for various pollutants (Fischer et al. 2022 ;Guo et al. 2023 ;Hoess et al. 2023 ). Negligible depletion-solid phase microextraction (nd-SPME) which is developed from the SPME was firstly used by Vaes et al.(Vaes et al. 1996 ) to measure the freely dissolved compounds based on the concentration distributed to the fiber coating from various matrices. Because of the very small volume of the SPME fiber, compounds extracted to the fiber coating is negligible compared to the total amount. Vaes et al.(Vaes et al. 1996 ) proposed that the depletion of the compounds less than 5% during the whole process will not cause the shift in the equilibrium between bound and free fraction, while some other researchers set this limit at 10% (Parkerton et al. 2000 ;Poerschmann et al. 1997 ). In general, measuring the C free of analytes by nd-SPME needs to meet the following conditions: (1) the equilibrium of compounds between the bound and free fraction should be achieved; (2) the depletion of freely dissolved fraction by the extraction is negligible; (3) the uptake kinetics should not be affected by the matrix; (4) the matrix dose not influence the adsorb to the fiber. Hitherto, nd-SPME has been widely applied for measuring the freely dissolved concentration of various pollutants, such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), dichlorodiphenyltrichloroethane (DDT), and other hydrophobic organic chemicals (HOCs) (Allan et al. 2021 ;Jonker 2021 ;Kreutzer et al. 2023 ;Kreutzer et al. 2022 ;Lin et al. 2017 ). Recently, nd-SPME was also applied for evaluating the effect of micriplastics on the availability of PAH, pharmaceuticals, and personal care products in aqueous environment (Li et al. 2020 ;Zhu et al. 2023 ). However, the equilibration time of nd-SPME usually needs days or even tens of days because of the long mass transfer distance, especially for the HOCs (Bartolome et al. 2018 ). Electrochemical analysis is an important method for measuring the trace pollutants in the environmental samples with the advantages of high sensitivity, low cost, and is suitable for in situ monitoring (Goeldner et al. 2023 ;Qu et al. 2024 ;Wu et al. 2023 ). Generally, the modified materials on the surface of electrode are merely nanograms to micrograms, and therefore analytes consumed by the redox reaction should be negligible. Compared with SPME fiber coating, materials decorated on the surface of electrode have a larger specific surface area, which is beneficial to reduce the time to achieve sampling equilibrium. Besides, the sampling process are mainly by the adsorption between the analytes and materials instead of distribution effect, which could largely shorten the time to reach partition equilibrium. Under the equilibrium condition, the freely dissolved concentration of analytes can be measured directly according to the electrochemical response. Therefore, the electrochemical method integrates sampling and determination into one single step, and is promising to be developed as a novel method for in situ determination of the freely dissolved pollutants. In this study, an in situ electrochemical method based on the nd-ME was established to measure the freely dissolved MP by beta-cyclodextrin decorated reduced graphene oxide composite electrode (β-CD/RGO/GCE). The environmental relevant parameters which have significant influence on the equilibrium time and partition coefficient were optimized, including pH value, buffer concentration, and salinity. Acros humic acid and Suwannee River humic acid were used as DOM models to estimate the sorption coefficients. The sorption coefficients of MP to DOM were calculated by fitting the free fraction of analyte at various concentration of humic acid. Besides, nd-SPME was carried out as control experiment to verify the feasibility and accuracy of the proposed method. 2. Materials and methods 2.1. Reagents and Materials Methyl parathion (1000 mg L − 1 , methanol) and graphite powder (99% purity) were purchased from J & K Co., Ltd. (Beijing, China). Sodium nitrate (NaNO 3 ), potassium permanganate (KMnO 4 ), sodium dihydrogen phosphate (NaH₂PO₄), disodium hydrogen phosphate (Na₂HPO₄), concentrated hydrochloric acid (HCl), concentrated sulfuric acid (H 2 SO 4 ), and β-CD were purchased from Sinopharm Co., Ltd. (Beijing, China). Sodium azide (NaN 3 ) was purchased from Xilong Chemical Co., Ltd. (Guangdong, China). Sodium dihydrogen phosphate (NaH₂PO₄), and disodium hydrogen phosphate (Na₂HPO₄) were used to prepare the phosphate buffer solution (PBS). Except for methyl parathion, all reagents and solvents of analytical grades were used without further purification. Acros humic acid (AcHA) and Suwannee River humic acid (SRHA) used as DOM model were purchased from Acros Organics and International Humic Acid Society (IHSS, Denver, Co), respectively. Ultrapure water (18.25 MΩ) was prepared by a Milli-Q water purifier in our laboratory. Polyacrylate (PA) fibers were purchased from Poly Micro Industries (Phoenix, AZ) with coating thickness of 30 µm and core diameter of 110 µm. 2.2 Characterization The surface morphology was performed using a filed-emission scanning electron microscope (JEOL JSM-7001F, Japan). The transmission electron micrographs (TEM) were performed on Hitachi 7500 (Japan) operating at 120 kV. The adsorption of DOM on the electrode was explored by Fourier transform infrared spectroscopy (FT-IR, Bruker TENSOR 27, Germany). The concentration of MP in the water samples were measured using an electrochemical workstation (CHI 630, Chen Hua Co. Ltd, Shanghai, China) with a three-electrode system. 2.3 Fabrication of β-CD/RGO/GCE GO was synthesized as described in the literature with minor modifications (Marcano et al. 2010 ). In brief, three grams of graphite powder and 1.5 g of NaNO 3 were added into a beaker containing 70 mL of concentrated H 2 SO 4 . The mixture was stirred magnetically at room temperature and then was subjected to an ice bath. Nine grams of KMnO 4 was slowly added into the mixture under vigorous stirring with the temperature below 20 o C. After being completely dissolved, the reaction was kept for 30 min at 40 ℃ until a thick paste formed. The mixture was diluted using the ultrapure water to obtain a homogeneous solution. With the addition of H 2 O 2 (30%, v/v), the solution was gradually changed from brown to yellow color. The product separated from the centrifugation was washed with distilled water and 5% of HCl until the pH value was around 7. After the ultrasonic cleaning, the prepared GO powder was re-dispersed in the ultrapure water for further use. The fabrication of β-CD/RGO/GCE was conducted using a three-electrode system, which is composed of glassy carbon electrode (working electrode), saturated AgCl electrode (auxiliary electrode), and Pt wire (counter electrode). Briefly, after being polished by Al 2 O 3 powder, the glassy carbon electrode was washed with deionized water, HNO 3 /H 2 O solution (v/v = 1/1), ethanol solution, and ultrapure water. The cleaned glassy carbon electrode (GCE) was finally dried with pure nitrogen. The β-CD/GO solution was prepared by mixing 0.2 mg mL − 1 of GO and 0.5 mg mL − 1 of β-CD in an ultrasonic bath for 1 h. Then, the β-CD/GO solution was dropped onto the surface of GCE and was dried at the room temperature to obtain the β-CD/GO/GCE. The prepared β-CD/GO/GCE was electrochemically reduced by cyclic voltammetry between − 1.2–1.2 V (scanning speed 100 mV s − 1 ) for 14 cycles to obtain the β-CD/RGO/GCE. Finally, the β-CD/RGO/GCE was washed with deionized water and then dried with nitrogen. For comparison, the RGO/GCE was also prepared with the same method but without the addition of beta-cyclodextrin. 2.4 Electrochemical measurement The aqueous solution was spiked with 1 mg L − 1 of MP solution and kept the pH value of 7.0 with the addition of 100 mM of PBS solution. The modified working electrode was immersed into the solution and was stirred at 450 rpm until the adsorption equilibrium was achieved. Then the modified working electrode was washed with deionized water and was transferred to a 50 mL electrolytic cell containing 100 mM of PBS (pH = 7.0) for electrochemical testing. Prior to the testing, the electrolyte solution was purged with nitrogen for 10 min. The square wave voltammetry (SWV) scan and the cyclic voltammetry (CV) scan were conducted in the range of -0.4-0.4 V and − 0.5–0.6 V, respectively, with the scan rate of 100 mV s − 1 . The baseline correction was performed using the linear baseline correction mode by the CHI 630 software. 2.5 Estimation of sorption coefficient The sorption coefficients of MP to Acros humic acid and Suwannee River humic acid were calculated according to the following equation as described by Kukkonen et al.(Kukkonen et al. 1989 ). The free fraction of MP at various concentration of humic acid (0, 1, 2.5, 5, 10, and 25 mg L − 1 ) was fitted by Graphpad Prism (ver. 5, GraphPad Software) $$f=\frac{1}{\left(1+{K}_{DOC}\times {C}_{DOM}\right)}$$ 1 where f is the freely dissolved fraction (ratio of the freely dissolved concentration to the total concentration). K DOC is the sorption coefficient of analyte between DOM and aqueous phase. C DOC is the concentration of humic acid in the aqueous phase (normalized to DOC). The adsorption kinetic of MP to the β-CD/RGO/GCE follows the first-order model. The equilibration time was calculated according to the following equation by fitting the electrochemical response of MP at different sampling time. $$\varDelta I\left(t\right)={I}_{0}\left(1-{e}^{-kt}\right)$$ 2 where Δ I is the electrochemical response of MP at the time t (min). I 0 is the electrochemical response of MP at the original concentration. k is a rate constant for estimating the equilibrium time. The equilibrium time can be calculated by the following equation. $${t}_{90\text{\%}}=-\frac{ln\left(0.10\right)}{k} ; {t}_{95\text{\%}}=-\frac{ln\left(0.05\right)}{k}$$ 3 2.6 Nd-SPME method Polyacrylate fiber was used to measure the freely dissolved concentration of MP as described by Pang et al. (Pang et al. 2013 ) with minor modification. Briefly, two polyacrylate fibers were placed into a 250 mL conical flask with 250 mL of sample solution. The conical flask was rotated on a reciprocating shaker under constant temperature to accelerate the partition equilibrium. After reaching equilibrium, the fibers were collected from the flask and then were immersed in the 100 µL of acetonitrile for 24 h. Finally, the quantification of MP was performed by an Agilent 7890B gas chromatography equipped with a flame photometric detector (GC-FPD). The equilibrium time was estimated by fitting the C fiber at different sampling time. The sorption coefficient between MP and humic acid was calculated according to Eq. ( 1 ) by fitting the f at various concentration of humic acid (normalized to DOC). 2.7 Sample collection Three real water samples were collected in this study, including river water, pond water, and tap water. The river water was taken from Kunyu River, the pond water was taken from the campus of Tsinghua University, and the tap water was obtained from our laboratory. All water samples were filtered with 0.45 µm cellulose acetate membrane before use. The parameters of the selected water samples were provided in Table S1 . 3. Results and discussion 3.1 Characterization of β-CD/RGO The morphology of the GO and β-CD/RGO were observed by SEM characterization. As shown in Fig. S1 , the GO (Fig. S1 A and B) displays a typical wrinkled structure. After being electrochemically reduced by cyclic voltammetry, the β-CD/RGO (Fig. S1 C and D) shows a roughness surface, indicating that β-CD have been successfully coated on the surface of RGO and therefore prevent the restacking effect of RGO. FT-IR analysis is utilized to analyze the surface functionalization process. The FT-IR spectra of GO and β-CD/RGO are depicted in Fig. S2. The characteristic peaks at 1047 cm − 1 and 1731 cm − 1 wavelengths were the vibrational peaks of C-OH and C = O, respectively. Additionally, the peaks appeared at 1400 cm − 1 and 3430 cm − 1 were attributed to the stretching vibration of -OH. The characteristic peak at 1630 cm − 1 wavelength was the -OH bending vibration of the water molecule, which was due to the small amount of water in the GO sheets (Singh et al. 2009 ). The presence of oxygen-containing functional groups confirmed that the GO was successfully synthesized. After being reduced by cyclic voltammetry, the intensity of these characteristic peaks significantly decreased, indicating that the GO sheets have been reduced successfully. 3.2 Electrochemical performance of the modified electrodes GCE, GO/GCE, RGO/GCE, β-CD/GO/GCE, and β-CD/RGO/GCE were applied to detect MP in the aqueous solution. The comparison of the electrochemical performance for each electrode toward MP was shown in Fig. 1 . Before the adsorption with MP, the electrode cyclic voltammograms (CVs) of β-CD/RGO/GCE exhibited a pair of redox peaks centered at 0 V due to the generation of quinone redox during the high anodic potential scan. While, a pair of strong redox peaks were clearly observed after the adsorption with 1 mg L − 1 of MP for 10 min (Epa, -0.056 V and Epc, -0.083 V). These peaks were attributed to the reversible redox of the reduction of nitro to hydroxylamine group and hydroxylamine group, respectively (Ferreira Oliveira et al. 2018 ;Liu &Lin 2005 ). However, there were no any redox peaks appeared after the adsorption of MP using the bare GCE, GO/GCE, and β-CD/GO/GCE, respectively. The redox peaks of MP were only detected using RGO/GCE and β-CD/RGO/GCE as the working electrode. The electrochemical response of β-CD/RGO/GCE was twice as high as that of RGO/GCE. This result reveals that β-CD doped in the GO sheets is beneficial to prevent the aggregation of RGO during the electrochemical reduction and therefore improves the specific surface area of RGO/β-CD as well as the electrochemical performance (Ma &Ou 2023 ;Wu et al. 2011 ). 3.3 Effect of the amount of β-CD/RGO The electrochemical response for MP with different amounts β-CD/RGO deposited on the electrode was compared as shown in Fig. 2 . The oxidation signal of MP was monitored after the adsorption of the modified electrode in the MP solution for 10 min. The anodic peak current response increased gradually with the increase of the amount of β-CD/RGO, reaching a maximum value when 1.2 µg of composites was used. However, the electrochemical response kept constant with the further increase of β-CD/RGO, probably because the aggregation of β-CD/RGO increased the electron transfer resistance (Wu et al. 2011 ). Therefore, 1.2 g of β-CD/RGO was utilized in the subsequent experiment. 3.4 Optimization of electrochemical method based on nd-ME The equilibration time is depended on the adsorption kinetic between MP and β-CD/RGO composites in the aqueous solution. The parameters influencing the adsorption kinetic were investigated in the environmentally relevant ranges. Buffer concentration, salinity, and pH value were optimized in the range of 5-200 mM, 0-500 mM, and 5.0-8.5, respectively (Liu et al. 2006 ). As shown in Fig. 3 A, the electrochemical responses gradually increased with the increase of the adsorption time. As a result, the equilibration time ( t 90% ) can be achieved within 19 min. It can be seen from Table. S2, the equilibration time ( t 90% ) was around 16 min with no addition of NaCl but significantly reduced to 7.5 min in the present of 500 mM of NaCl. Similarly, the equilibration time ( t 90% ) was reduced from 19 min to 9.9 min when the PBS concentration increased from 5 mM to 200 mM. The result reveals that ionic strength can accelerate the time of reaching equilibrium between MP and β-CD/RGO composites. While, a negative relationship was found between the pH value and equilibration time. With the increase of pH value from 5 to 8.5, the equilibration time ( t 90% ) increased from 7.5 min to 11 min. Generally, the partition coefficient of analytes between aqueous phase and sorbents are related to the environmental factors, which could have impact on the calculation of sorption coefficients to matrix (Pang et al. 2018 ). Therefore, the effects of some parameters on the partition coefficients were evaluated, including buffer concentration, salinity, and pH value. As shown in Fig. 3 B, the pH value had significant impacts on the electrochemical response. With the increase of pH value from 4.5 to 7.0, the electrical signal was positively correlated with the pH value, with correlation coefficient ( r 2 ) of 0.977. While, the electrical signal showed a negative correlation with the pH value in the range of 7.0-8.5, with r 2 of 0.996. A plausible explanation is that MP is prone to hydrolysis under strong alkaline or acidic condition and therefore performs different relationship with the pH value (Wu et al. 2011 ). For real water samples with large pH range, the sorption coefficients should be calculated according to the linear relationship between the concentration of analyte and the pH value of water samples. It can be seen from Fig. 3 C that the electrochemical response almost kept constant with the increase of buffer concentration. The relative standard deviation of the electrochemical response for freely dissolved MP was 1.5%. Similarly, with the increase of salinity from 0 to 500 mM, there was no significant effect on the electrochemical response, with relative standard deviation of 2.1% (Fig. 3 D). The result indicates that the ionic strength in the environmentally relevant ranges has no important impact on the partition coefficient between analyte and sorbents using the electrochemical method. Considering the buffer and salinity have no obvious influence on the sorption coefficients, the determination of freely dissolved MP in the water samples was carried out in the 100 mM of PBS to reduce the equilibrium time. As a result, the equilibrium time ( t 90% ) was 16 min. 3.5 Sorption coefficients between MP and humic acid Acros humic acid and Suwannee River humic acid were used as DOM model to estimate the sorption coefficients ( K DOC ) between MPs and humic acid. The freely dissolved fraction of MP was calculated under the optimized experimental conditions: 1.2 g of β-CD/RGO, 100 mM of PBS, pH value of 7.00 ± 0.2, and equilibration time of 16 min. The values of K DOC were obtained by fitting the free fraction ( f ) at various concentration of humic acid (normalized to DOC) via Eq. ( 1 ). Figure 4 A shows the profile of f in relation to the concentration of DOC. The f value of MP decreased with the increase of DOC concentration, indicating that the adsorption of analyte to the humic acid decreased the freely dissolved concentration in aqueous phase. The f - C DOC profiles of MP can be well fitted with correlation coefficient of 0.94 for Acros humic acid and 0.91 for Suwannee River humic acid, respectively. As shown in Fig. 4 B, the electrochemical response of MP decreased gradually with the increase of the concentration of Acros humic acid, indicating that MP adsorbed on DOM caused the decrease of free fraction. From Table 1 , the log K DOC values estimated by the electrochemical method were 4.56 ± 0.19 for Acros humic acid and 4.58 ± 0.08 for Suwannee River humic acid, respectively. Table 1 Comparison of log K DOC and equilibrium time measured by the electrochemical method and nd-SPME method Method log K DOC Equilibration time (min) Acros humic acid Suwannee River humic acid t 90% Electrochemical method 4.56 ± 0.19 4.58 ± 0.08 19 nd-SPME method 4.23 ± 0.19 4.27 ± 0.12 360 3.6 Determination of freely dissolved MP The proposed electrochemical method was applied for measuring the the freely dissolved MP from real water samples, including river water, pond water, and tap. To evaluate the feasibility of the proposed method, higher concentrations of MP than environmental levels were spiked in the real water samples. As shown in Table 2 , under the optimized conditions, the freely dissolved concentrations of MP in three water samples were 3.96 µg L − 1 for river water, 4.17 µg L − 1 for pond water, and 4.44 µg L − 1 for tap water, respectively. It can be seen from Table S1 that the DOC concentration of tap water was 0.68 mg L − 1 , which was lower than that of river water (3.05 mg L − 1 ) and pond water (1.36 mg L − 1 ). Therefore, the freely dissolved concentration of MP in the tap water was higher than the other two samples. Table 2 Comparison of the freely dissolved concentration of MP measured by the electrochemical method and nd-SPME method Method C free (µg L − 1 ) River water Pond water Tap water Electrochemical method 3.96 ± 0.04 4.17 ± 0.04 4.44 ± 0.02 nd-SPME method 4.17 ± 0.02 4.48 ± 0.14 4.76 ± 0.05 3.7 Comparison with nd-SPME method As shown in Table 1 , the log K DOC values measured by nd-SPME method were 4.23 ± 0.19 for Acros humic acid and 4.27 ± 0.12 for Suwannee River humic acid, respectively. The equilibrium time can be achieved at 6 h. From Table 2 , the freely dissolved MP measured by nd-SPME were 4.17 µg L − 1 for river water, 4.48 µg L − 1 for pond water, and 4.76 µg L − 1 for tap water, respectively. Therefore, the log K DOC values measured by the proposed electrochemical method were consistent with those of nd-SPME, with the relative standard deviations below 5.6%. Besides, the freely dissolved MP measured by the electrochemical method were also accurate with the results of nd-SPME, with acceptable relative standard deviations ranging from 3.7–5.1%. Compared with 6 h to reach the partition equilibrium by nd-SPME, the equilibrium time was only 16 min by the electrochemical method. Therefore, it proved that the electrochemical method is reliable for in situ measuring the freely dissolved concentration of analyte. However, it should be noted that the log K DOC values measured by the electrochemical method were slightly higher than those of nd-SPME even through the relative standard deviations between these two methods were low than 5.6%. Meanwhile, the C free values measured by the electrochemical method were slightly lower than those of nd-SPME. A plausible explanation is the sorption of DOM on the surface of working electrode decreased the electrochemical response, resulting in the overestimation of the DOM-bound fraction and underestimation of the freely dissolved concentration. Therefore, the adsorption of Acros humic acid on the modified working electrode was further investigated by the FT-IR. As shown in Fig. 5 , the spectrum of humic acid displayed peaks at 2849 and 2928 cm − 1 , which belonged to the asymmetric and symmetric C-H stretching of methyl and methylene groups of aliphatic and non-strained cyclic hydrocarbons. The peak at 1710 cm − 1 was assigned to C = O stretching vibration of carboxylic groups. The strong peak at 1625 cm − 1 was attributed to the aromatic -COO asymmetric stretching of humic acid. The bands in the range of 1000–1200 cm − 1 were attributed to the aromatic ether C-O-C and C-O stretching of polysaccharides. The peaks appeared in the range of 910 − 650 cm − 1 were attributed to the bending vibrations of aromatic C-H of humic acid. Compared with the humic acid and RGO, the spectrum of β-CD/RGO-HA exhibited typical β-CD absorption peaks of CH n stretching vibrations at 2980 cm − 1 . After the adsorption with humic acid, the stretching vibration of aromatic C-H bonds was significantly decreased or even disappeared, indicating that the π-π interaction was formed between β-CD/RGO and humic acid. Besides, the C-O stretching vibration of RGO shifted from 1047 cm − 1 to a higher wavenumber combined with an increase of intensity, indicating that the hydrogen bound was formed between β-CD/RGO and humic acid. Therefore, the adsorption of DOM on the modified working electrode should be a major reason for the minor difference on the sorption coefficients and freely dissolved concentrations between the proposed electrochemical method and nd-SPME method. 4. Conclusion In this study, we developed a novel method for the determination of the sorption coefficient and freely dissolved concentration of MP by the electrochemical method based on nd-ME. The proposed method integrates sampling and determination into one single step, and the equilibrium time can be achieved within a short time. Therefore, the electrochemical method can be applied for in situ measurement of the freely dissolved analytes in the real water samples. DOM in the real water samples may cause an overestimation of the sorption coefficient as well as underestimation of free fraction, the relative standard deviations are proved to be acceptable compared with the results of nd-SPME. More interestingly, owing to the high selectivity of the electrochemical method coupled with different functional materials, the proposed electrochemical method is promising to be used for measuring the freely dissolved chiral enantiomers in the environmental samples, such as chiral pesticides and chiral drugs, providing a convenient method for estimating the bioavailability of chiral pollutants in the further. Declarations Ethical Approval No conflict of ethics. Consent to Participate Not applicable. Consent to Publish Not applicable. Authors Contributions Long Pang: Methodology, Investigation, Writing – review & editing, Funding acquisition. Zhigao Feng: Investigation, Writing – original draft. Xingru Hu: Investigation, Methodology. Jiahui Hou: Data analysis, Data collection. Guangtao Cui: Investigation, Methodology. Jingfu Liu: Conceptualization, Writing – Review & Editing. Funding This study was supported by the National Natural Science Foundation of China (21707124), and Henan Province Scientific and Technological Research Projects (182102311109). Competing Interests The authors declare no competing interests. References Allan IJ, Raffard V, Kringstad A et al (2021) Assessment of marine sediment remediation efficiency with SPME-based passive sampling measurement. Sci Total Environ 756:143854 Bartolome N, Hilber I, Schulin R et al (2018) Comparison of freely dissolved concentrations of PAHs in contaminated pot soils under saturated and unsaturated water conditions. Sci Total Environ 644:835-843. Ferreira Oliveira AE, Bettio GB, Pereira AC (2018) An electrochemical sensor based on electropolymerization of ß-cyclodextrin and reduced graphene oxide on a glassy carbon electrode for determination of neonicotinoids. Electroanalysis 30:1918-1928. Fischer FC, Hiki K, Endo S (2022) Free versus bound concentration: passive dosing from polymer meshes elucidates drivers of toxicity in aquatic tests with benthic invertebrates. Environ Toxicol 75:395-400. Goeldner V, Ulke J, Kirchner B et al (2023) Electrochemistry-mass spectrometry bridging the gap between suspect and target screening of valsartan transformation products in wastewater treatment plant effluent. Water Res 244:120525. Guo R, Zhu D, He J et al (2023) Influence of copper and aging on freely dissolved tetracycline concentration in soil. Environ Sci Pollut R 31:23334-23362. Hoess S, Sanders D, van Egmond R (2023) Determining the toxicity of organic compounds to the nematode Caenorhabditis elegans based on aqueous concentrations. Environ Sci Pollut R 30:96290-96300. Jonker MTO (2021) Effects of sediment manipulation on freely dissolved concentrations of hydrophobic organic chemicals. Chemosphere 265:128694. Kreutzer A, Reininghaus M, Meyer J et al (2023) Application of equilibrium passive sampling to assess the influence of anthropogenic activities and bioturbation on the distribution of hydrophobic organic chemicals in North Sea sediment cores. Environ Pollut 318:120876. Kreutzer A, Schacht S-C, Witt G (2022) Equilibrium passive sampling: A novel approach to determine internal tissue concentrations of hydrophobic organic compounds in biota. Sci Total Environ 824:153764. Kukkonen J, Oikari A, Johnsen S et al (1989) Effects of humus concentrations on benzo[a]pyrene accumulation from water to Daphnia magna: comparison of natural waters and standard preparations. Sci Total Environ 79:197-207. Li Z, Hu X, Qin L et al (2020) Evaluating the effect of different modified microplastics on the availability of polycyclic aromatic hydrocarbons. Water Res 170:115290 Lin K, Lao W, Lu Z et al (2017) Measuring freely dissolved DDT and metabolites in seawater using solid-phase microextraction with performance reference compounds. Sci Total Environ 599:364-371. Liu GD, Lin YH (2005) Electrochemical sensor for organophosphate pesticides and nerve agents using zirconia nanoparticles as selective sorbents. Anal. Chem 77: 5894-5901. Liu JF, Hu XL, Peng JF et al (2006) Equilibrium sampling of freely dissolved alkylphenols into a thin film of 1-octanol supported on a hollow fiber membrane. Anal. Chem 78: 8526-8534. Ma T, Ou G (2023) Fabrication of a highly sensitive electrochemical sensor for the rapid detection of nimodipine. Int J Electrochem 18:100277. Marcano DC, Kosynkin DV, Berlin JM et al (2010) Improved synthesis of graphene oxide. ACS Nano 4:4806-4814. Pang L, Liu J, Yin Y et al (2013) Evaluating the sorption of organophosphate esters to different sourced humic acids and its effects on the toxicity to Daphnia magna. Environ Toxicol 32:2755-2761. Pang L, Yang P, Yang H et al (2018) Application of Fe 3 O 4 @MIL-100 (Fe) core-shell magnetic microspheres for evaluating the sorption of organophosphate esters to dissolved organic matter (DOM). Sci Total Environ 626:42-47. Parkerton TF, Stone MA, Letinski DJ (2000) Assessing the aquatic toxicity of complex hydrocarbon mixtures using solid phase microextraction. Toxicol Lett 112-113:273-82. Poerschmann J, Zhang Z, Pawliszyn JB (1997) Solid phase microextraction for determining the distribution of chemicals in aqueous matrices. Anal Chem 69:597-600.. Qu G, Liu G, Zhao C et al (2024) Detection and treatment of mono and polycyclic aromatic hydrocarbon pollutants in aqueous environments based on electrochemical technology: recent advances. Environ Sci Pollut R 31:23334-23362. Singh VK, Patra MK, Manoth M et al (2009) In situ synthesis of graphene oxide and its composites with iron oxide. New Carbon Materials 24:147-152. Vaes WHJ, Urrestarazu-Ramos E, Seinen W et al (1996) Measurement of the free concentration using solid-phase microextraction: binding to protein. Anal Chem 68:4463-4467. Wu D, Karimi-Maleh H, Liu X et al (2023) Bibliometrics analysis of research progress of electrochemical detection of tetracycline antibiotics. J Anal Methods Chem 2023. Wu S, Lan X, Cui L et al (2011) Application of graphene for preconcentration and highly sensitive stripping voltammetric analysis of organophosphate pesticide. Anal Chim Acta 699:170-176. Zhu SH, Qin LX, Li ZW, et al. (2023) Effects of nanoplastics and microplastics on the availability of pharmaceuticals and personal care products in aqueous environment. J Hazard 458:131999. Supplementary Files SupportingInformation.doc Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4394035","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":308188319,"identity":"39ba0dd4-f87a-455f-a0a8-eddd5cca260d","order_by":0,"name":"Long Pang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA0UlEQVRIiWNgGAWjYBACPjBZAcTsjA0MDGxEaIGoOQPEzCRpYWwDaUGYQECLdPOzh1/n1dkbHGZuYPhQdpiBf3YDAS0yx8yNZbexMRscZmxgnHHuMIPEnQMEtEgkmElLbuNhA2lh5m07zGAgkUBIS/o3ack5EjxgLX+J05JjJvmxwUACrIWRKC0yZ8qkGY4lGEgCtRzsOZfOI3GDgBZ+6fZtkj9q6uz5jrc/fPCjzFqOfwYBLQwSwBjhgbIPADEPHrUILYw/CCsbBaNgFIyCkQwAZxY5r+YDpHwAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-5726-0175","institution":"Zhengzhou University of Light Industry","correspondingAuthor":true,"prefix":"","firstName":"Long","middleName":"","lastName":"Pang","suffix":""},{"id":308188320,"identity":"3630173a-5b64-46b0-82bf-3d02b28f8878","order_by":1,"name":"Zhigao Feng","email":"","orcid":"","institution":"Zhengzhou University of Light Industry","correspondingAuthor":false,"prefix":"","firstName":"Zhigao","middleName":"","lastName":"Feng","suffix":""},{"id":308188321,"identity":"6a609a99-1b78-442a-88be-9f4de90c1574","order_by":2,"name":"Xingru Hu","email":"","orcid":"","institution":"Zhengzhou University of Light Industry","correspondingAuthor":false,"prefix":"","firstName":"Xingru","middleName":"","lastName":"Hu","suffix":""},{"id":308188322,"identity":"f509dbaa-ccc9-44bd-bd31-9146e0bebe36","order_by":3,"name":"Jiahui Hou","email":"","orcid":"","institution":"Zhengzhou University of Light Industry","correspondingAuthor":false,"prefix":"","firstName":"Jiahui","middleName":"","lastName":"Hou","suffix":""},{"id":308188323,"identity":"87de1f26-7553-43b2-88d4-586d2e2f8ba1","order_by":4,"name":"Guangtao Cui","email":"","orcid":"","institution":"Zhengzhou University of Light Industry","correspondingAuthor":false,"prefix":"","firstName":"Guangtao","middleName":"","lastName":"Cui","suffix":""},{"id":308188324,"identity":"714352fb-dbd9-46a8-98a0-285a13dd065f","order_by":5,"name":"Jingfu Liu","email":"","orcid":"","institution":"Southern University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Jingfu","middleName":"","lastName":"Liu","suffix":""}],"badges":[],"createdAt":"2024-05-09 09:19:04","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4394035/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4394035/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":58114323,"identity":"d0c7b7c2-aad3-4922-a704-8ee95a00a283","added_by":"auto","created_at":"2024-06-11 10:13:06","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":636624,"visible":true,"origin":"","legend":"\u003cp\u003eCyclic voltammetry responses of different electrodes in 1 mg L\u003csup\u003e-1\u003c/sup\u003e of MP\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-4394035/v1/515310c7060949d917e712d9.png"},{"id":58114320,"identity":"111fee5f-b078-4386-8ab4-9bf2432ad738","added_by":"auto","created_at":"2024-06-11 10:13:06","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":155600,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of β-CD/RGO dosage on the electrochemical response of MP\u003c/p\u003e","description":"","filename":"Fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-4394035/v1/673673ef65bada8011699371.png"},{"id":58114319,"identity":"bfd10c47-a3a4-42c5-b452-dab7d5f7b04d","added_by":"auto","created_at":"2024-06-11 10:13:06","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":333471,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of environmental parameters on the equilibration time (A). Effect of environmental parameters on the electrochemical response: pH value (B), PBS buffer (C), and salinity (D)\u003c/p\u003e","description":"","filename":"Fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-4394035/v1/2fed756a5e3c62fcbe7b9161.png"},{"id":58114832,"identity":"b9c04e10-6184-4606-b0ac-a85d8df3f8de","added_by":"auto","created_at":"2024-06-11 10:21:06","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":467796,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of humic acid concentration on the free fraction of MP measured by electrochemical method and nd-SPME (A). Effect of Acros humic acid concentration on the electrochemical response (B)\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-4394035/v1/3e0ddfe22eba689a845c8f38.png"},{"id":58114321,"identity":"c7b53871-d5fb-43e9-ad01-902c15bcbfa8","added_by":"auto","created_at":"2024-06-11 10:13:06","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":443336,"visible":true,"origin":"","legend":"\u003cp\u003eThe FT-IR spectrum of RGO, HA, and β-CD/RGO-HA\u003c/p\u003e","description":"","filename":"Fig5.png","url":"https://assets-eu.researchsquare.com/files/rs-4394035/v1/a983b16b55edda44ea330eb4.png"},{"id":59076264,"identity":"301b862c-3a31-4470-9d26-4607f149954b","added_by":"auto","created_at":"2024-06-26 06:10:56","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2700466,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4394035/v1/18ab61be-788b-4994-b30c-31c3c7cd03cc.pdf"},{"id":58114324,"identity":"7c4a3ce9-c8ee-477a-9abf-f8a804436001","added_by":"auto","created_at":"2024-06-11 10:13:06","extension":"doc","order_by":11,"title":"","display":"","copyAsset":false,"role":"supplement","size":1860096,"visible":true,"origin":"","legend":"","description":"","filename":"SupportingInformation.doc","url":"https://assets-eu.researchsquare.com/files/rs-4394035/v1/fbb884a200cb27f7dd928057.doc"}],"financialInterests":"","formattedTitle":"In situ electrochemical method for measuring the freely dissolved methyl parathion based on the negligible depletion micro-extraction","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eFreely dissolved concentration (\u003cem\u003eC\u003c/em\u003e\u003csub\u003efree\u003c/sub\u003e) is an important parameter of environmental chemistry, toxicology, and pharmacology. Meanwhile, \u003cem\u003eC\u003c/em\u003e\u003csub\u003efree\u003c/sub\u003e is also regarded as a driving force of transport, distribution, and bioaccumulation for various pollutants (Fischer et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2022\u003c/span\u003e;Guo et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2023\u003c/span\u003e;Hoess et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Negligible depletion-solid phase microextraction (nd-SPME) which is developed from the SPME was firstly used by Vaes et al.(Vaes et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) to measure the freely dissolved compounds based on the concentration distributed to the fiber coating from various matrices. Because of the very small volume of the SPME fiber, compounds extracted to the fiber coating is negligible compared to the total amount. Vaes et al.(Vaes et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) proposed that the depletion of the compounds less than 5% during the whole process will not cause the shift in the equilibrium between bound and free fraction, while some other researchers set this limit at 10% (Parkerton et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2000\u003c/span\u003e;Poerschmann et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e1997\u003c/span\u003e). In general, measuring the \u003cem\u003eC\u003c/em\u003e\u003csub\u003efree\u003c/sub\u003e of analytes by nd-SPME needs to meet the following conditions: (1) the equilibrium of compounds between the bound and free fraction should be achieved; (2) the depletion of freely dissolved fraction by the extraction is negligible; (3) the uptake kinetics should not be affected by the matrix; (4) the matrix dose not influence the adsorb to the fiber. Hitherto, nd-SPME has been widely applied for measuring the freely dissolved concentration of various pollutants, such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), dichlorodiphenyltrichloroethane (DDT), and other hydrophobic organic chemicals (HOCs) (Allan et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2021\u003c/span\u003e;Jonker \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2021\u003c/span\u003e;Kreutzer et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2023\u003c/span\u003e;Kreutzer et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2022\u003c/span\u003e;Lin et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Recently, nd-SPME was also applied for evaluating the effect of micriplastics on the availability of PAH, pharmaceuticals, and personal care products in aqueous environment (Li et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e;Zhu et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). However, the equilibration time of nd-SPME usually needs days or even tens of days because of the long mass transfer distance, especially for the HOCs (Bartolome et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eElectrochemical analysis is an important method for measuring the trace pollutants in the environmental samples with the advantages of high sensitivity, low cost, and is suitable for in situ monitoring (Goeldner et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2023\u003c/span\u003e;Qu et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2024\u003c/span\u003e;Wu et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Generally, the modified materials on the surface of electrode are merely nanograms to micrograms, and therefore analytes consumed by the redox reaction should be negligible. Compared with SPME fiber coating, materials decorated on the surface of electrode have a larger specific surface area, which is beneficial to reduce the time to achieve sampling equilibrium. Besides, the sampling process are mainly by the adsorption between the analytes and materials instead of distribution effect, which could largely shorten the time to reach partition equilibrium. Under the equilibrium condition, the freely dissolved concentration of analytes can be measured directly according to the electrochemical response. Therefore, the electrochemical method integrates sampling and determination into one single step, and is promising to be developed as a novel method for in situ determination of the freely dissolved pollutants.\u003c/p\u003e \u003cp\u003eIn this study, an in situ electrochemical method based on the nd-ME was established to measure the freely dissolved MP by beta-cyclodextrin decorated reduced graphene oxide composite electrode (β-CD/RGO/GCE). The environmental relevant parameters which have significant influence on the equilibrium time and partition coefficient were optimized, including pH value, buffer concentration, and salinity. Acros humic acid and Suwannee River humic acid were used as DOM models to estimate the sorption coefficients. The sorption coefficients of MP to DOM were calculated by fitting the free fraction of analyte at various concentration of humic acid. Besides, nd-SPME was carried out as control experiment to verify the feasibility and accuracy of the proposed method.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Reagents and Materials\u003c/h2\u003e \u003cp\u003eMethyl parathion (1000 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, methanol) and graphite powder (99% purity) were purchased from J \u0026amp; K Co., Ltd. (Beijing, China). Sodium nitrate (NaNO\u003csub\u003e3\u003c/sub\u003e), potassium permanganate (KMnO\u003csub\u003e4\u003c/sub\u003e), sodium dihydrogen phosphate (NaH₂PO₄), disodium hydrogen phosphate (Na₂HPO₄), concentrated hydrochloric acid (HCl), concentrated sulfuric acid (H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e), and β-CD were purchased from Sinopharm Co., Ltd. (Beijing, China). Sodium azide (NaN\u003csub\u003e3\u003c/sub\u003e) was purchased from Xilong Chemical Co., Ltd. (Guangdong, China). Sodium dihydrogen phosphate (NaH₂PO₄), and disodium hydrogen phosphate (Na₂HPO₄) were used to prepare the phosphate buffer solution (PBS). Except for methyl parathion, all reagents and solvents of analytical grades were used without further purification. Acros humic acid (AcHA) and Suwannee River humic acid (SRHA) used as DOM model were purchased from Acros Organics and International Humic Acid Society (IHSS, Denver, Co), respectively. Ultrapure water (18.25 MΩ) was prepared by a Milli-Q water purifier in our laboratory. Polyacrylate (PA) fibers were purchased from Poly Micro Industries (Phoenix, AZ) with coating thickness of 30 \u0026micro;m and core diameter of 110 \u0026micro;m.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Characterization\u003c/h2\u003e \u003cp\u003eThe surface morphology was performed using a filed-emission scanning electron microscope (JEOL JSM-7001F, Japan). The transmission electron micrographs (TEM) were performed on Hitachi 7500 (Japan) operating at 120 kV. The adsorption of DOM on the electrode was explored by Fourier transform infrared spectroscopy (FT-IR, Bruker TENSOR 27, Germany). The concentration of MP in the water samples were measured using an electrochemical workstation (CHI 630, Chen Hua Co. Ltd, Shanghai, China) with a three-electrode system.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Fabrication of β-CD/RGO/GCE\u003c/h2\u003e \u003cp\u003eGO was synthesized as described in the literature with minor modifications (Marcano et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). In brief, three grams of graphite powder and 1.5 g of NaNO\u003csub\u003e3\u003c/sub\u003e were added into a beaker containing 70 mL of concentrated H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e. The mixture was stirred magnetically at room temperature and then was subjected to an ice bath. Nine grams of KMnO\u003csub\u003e4\u003c/sub\u003e was slowly added into the mixture under vigorous stirring with the temperature below 20 \u003csup\u003eo\u003c/sup\u003eC. After being completely dissolved, the reaction was kept for 30 min at 40 ℃ until a thick paste formed. The mixture was diluted using the ultrapure water to obtain a homogeneous solution. With the addition of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (30%, v/v), the solution was gradually changed from brown to yellow color. The product separated from the centrifugation was washed with distilled water and 5% of HCl until the pH value was around 7. After the ultrasonic cleaning, the prepared GO powder was re-dispersed in the ultrapure water for further use.\u003c/p\u003e \u003cp\u003eThe fabrication of β-CD/RGO/GCE was conducted using a three-electrode system, which is composed of glassy carbon electrode (working electrode), saturated AgCl electrode (auxiliary electrode), and Pt wire (counter electrode). Briefly, after being polished by Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powder, the glassy carbon electrode was washed with deionized water, HNO\u003csub\u003e3\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO solution (v/v\u0026thinsp;=\u0026thinsp;1/1), ethanol solution, and ultrapure water. The cleaned glassy carbon electrode (GCE) was finally dried with pure nitrogen. The β-CD/GO solution was prepared by mixing 0.2 mg mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of GO and 0.5 mg mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of β-CD in an ultrasonic bath for 1 h. Then, the β-CD/GO solution was dropped onto the surface of GCE and was dried at the room temperature to obtain the β-CD/GO/GCE. The prepared β-CD/GO/GCE was electrochemically reduced by cyclic voltammetry between \u0026minus;\u0026thinsp;1.2\u0026ndash;1.2 V (scanning speed 100 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) for 14 cycles to obtain the β-CD/RGO/GCE. Finally, the β-CD/RGO/GCE was washed with deionized water and then dried with nitrogen. For comparison, the RGO/GCE was also prepared with the same method but without the addition of beta-cyclodextrin.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Electrochemical measurement\u003c/h2\u003e \u003cp\u003eThe aqueous solution was spiked with 1 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of MP solution and kept the pH value of 7.0 with the addition of 100 mM of PBS solution. The modified working electrode was immersed into the solution and was stirred at 450 rpm until the adsorption equilibrium was achieved. Then the modified working electrode was washed with deionized water and was transferred to a 50 mL electrolytic cell containing 100 mM of PBS (pH\u0026thinsp;=\u0026thinsp;7.0) for electrochemical testing. Prior to the testing, the electrolyte solution was purged with nitrogen for 10 min. The square wave voltammetry (SWV) scan and the cyclic voltammetry (CV) scan were conducted in the range of -0.4-0.4 V and \u0026minus;\u0026thinsp;0.5\u0026ndash;0.6 V, respectively, with the scan rate of 100 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The baseline correction was performed using the linear baseline correction mode by the CHI 630 software.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Estimation of sorption coefficient\u003c/h2\u003e \u003cp\u003eThe sorption coefficients of MP to Acros humic acid and Suwannee River humic acid were calculated according to the following equation as described by Kukkonen et al.(Kukkonen et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1989\u003c/span\u003e). The free fraction of MP at various concentration of humic acid (0, 1, 2.5, 5, 10, and 25 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) was fitted by Graphpad Prism (ver. 5, GraphPad Software)\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$f=\\frac{1}{\\left(1+{K}_{DOC}\\times {C}_{DOM}\\right)}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003ef\u003c/em\u003e is the freely dissolved fraction (ratio of the freely dissolved concentration to the total concentration). \u003cem\u003eK\u003c/em\u003e\u003csub\u003eDOC\u003c/sub\u003e is the sorption coefficient of analyte between DOM and aqueous phase. \u003cem\u003eC\u003c/em\u003e\u003csub\u003eDOC\u003c/sub\u003e is the concentration of humic acid in the aqueous phase (normalized to DOC).\u003c/p\u003e \u003cp\u003eThe adsorption kinetic of MP to the β-CD/RGO/GCE follows the first-order model. The equilibration time was calculated according to the following equation by fitting the electrochemical response of MP at different sampling time.\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\varDelta I\\left(t\\right)={I}_{0}\\left(1-{e}^{-kt}\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere Δ\u003cem\u003eI\u003c/em\u003e is the electrochemical response of MP at the time \u003cem\u003et\u003c/em\u003e (min). \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e is the electrochemical response of MP at the original concentration. \u003cem\u003ek\u003c/em\u003e is a rate constant for estimating the equilibrium time. The equilibrium time can be calculated by the following equation.\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$${t}_{90\\text{\\%}}=-\\frac{ln\\left(0.10\\right)}{k} ; {t}_{95\\text{\\%}}=-\\frac{ln\\left(0.05\\right)}{k}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.6 Nd-SPME method\u003c/h2\u003e \u003cp\u003ePolyacrylate fiber was used to measure the freely dissolved concentration of MP as described by Pang et al. (Pang et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) with minor modification. Briefly, two polyacrylate fibers were placed into a 250 mL conical flask with 250 mL of sample solution. The conical flask was rotated on a reciprocating shaker under constant temperature to accelerate the partition equilibrium. After reaching equilibrium, the fibers were collected from the flask and then were immersed in the 100 \u0026micro;L of acetonitrile for 24 h. Finally, the quantification of MP was performed by an Agilent 7890B gas chromatography equipped with a flame photometric detector (GC-FPD). The equilibrium time was estimated by fitting the \u003cem\u003eC\u003c/em\u003e\u003csub\u003efiber\u003c/sub\u003e at different sampling time. The sorption coefficient between MP and humic acid was calculated according to Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) by fitting the \u003cem\u003ef\u003c/em\u003e at various concentration of humic acid (normalized to DOC).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e2.7 Sample collection\u003c/h2\u003e \u003cp\u003eThree real water samples were collected in this study, including river water, pond water, and tap water. The river water was taken from Kunyu River, the pond water was taken from the campus of Tsinghua University, and the tap water was obtained from our laboratory. All water samples were filtered with 0.45 \u0026micro;m cellulose acetate membrane before use. The parameters of the selected water samples were provided in Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Characterization of β-CD/RGO\u003c/h2\u003e \u003cp\u003eThe morphology of the GO and β-CD/RGO were observed by SEM characterization. As shown in Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, the GO (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003eA and B) displays a typical wrinkled structure. After being electrochemically reduced by cyclic voltammetry, the β-CD/RGO (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003eC and D) shows a roughness surface, indicating that β-CD have been successfully coated on the surface of RGO and therefore prevent the restacking effect of RGO.\u003c/p\u003e \u003cp\u003eFT-IR analysis is utilized to analyze the surface functionalization process. The FT-IR spectra of GO and β-CD/RGO are depicted in Fig. S2. The characteristic peaks at 1047 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1731 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e wavelengths were the vibrational peaks of C-OH and C\u0026thinsp;=\u0026thinsp;O, respectively. Additionally, the peaks appeared at 1400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 3430 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were attributed to the stretching vibration of -OH. The characteristic peak at 1630 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e wavelength was the -OH bending vibration of the water molecule, which was due to the small amount of water in the GO sheets (Singh et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). The presence of oxygen-containing functional groups confirmed that the GO was successfully synthesized. After being reduced by cyclic voltammetry, the intensity of these characteristic peaks significantly decreased, indicating that the GO sheets have been reduced successfully.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Electrochemical performance of the modified electrodes\u003c/h2\u003e \u003cp\u003eGCE, GO/GCE, RGO/GCE, β-CD/GO/GCE, and β-CD/RGO/GCE were applied to detect MP in the aqueous solution. The comparison of the electrochemical performance for each electrode toward MP was shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Before the adsorption with MP, the electrode cyclic voltammograms (CVs) of β-CD/RGO/GCE exhibited a pair of redox peaks centered at 0 V due to the generation of quinone redox during the high anodic potential scan. While, a pair of strong redox peaks were clearly observed after the adsorption with 1 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of MP for 10 min (Epa, -0.056 V and Epc, -0.083 V). These peaks were attributed to the reversible redox of the reduction of nitro to hydroxylamine group and hydroxylamine group, respectively (Ferreira Oliveira et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2018\u003c/span\u003e;Liu \u0026amp;Lin \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). However, there were no any redox peaks appeared after the adsorption of MP using the bare GCE, GO/GCE, and β-CD/GO/GCE, respectively. The redox peaks of MP were only detected using RGO/GCE and β-CD/RGO/GCE as the working electrode. The electrochemical response of β-CD/RGO/GCE was twice as high as that of RGO/GCE. This result reveals that β-CD doped in the GO sheets is beneficial to prevent the aggregation of RGO during the electrochemical reduction and therefore improves the specific surface area of RGO/β-CD as well as the electrochemical performance (Ma \u0026amp;Ou \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2023\u003c/span\u003e;Wu et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Effect of the amount of β-CD/RGO\u003c/h2\u003e \u003cp\u003eThe electrochemical response for MP with different amounts β-CD/RGO deposited on the electrode was compared as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The oxidation signal of MP was monitored after the adsorption of the modified electrode in the MP solution for 10 min. The anodic peak current response increased gradually with the increase of the amount of β-CD/RGO, reaching a maximum value when 1.2 \u0026micro;g of composites was used. However, the electrochemical response kept constant with the further increase of β-CD/RGO, probably because the aggregation of β-CD/RGO increased the electron transfer resistance (Wu et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Therefore, 1.2 g of β-CD/RGO was utilized in the subsequent experiment.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Optimization of electrochemical method based on nd-ME\u003c/h2\u003e \u003cp\u003eThe equilibration time is depended on the adsorption kinetic between MP and β-CD/RGO composites in the aqueous solution. The parameters influencing the adsorption kinetic were investigated in the environmentally relevant ranges. Buffer concentration, salinity, and pH value were optimized in the range of 5-200 mM, 0-500 mM, and 5.0-8.5, respectively (Liu et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eA, the electrochemical responses gradually increased with the increase of the adsorption time. As a result, the equilibration time (\u003cem\u003et\u003c/em\u003e\u003csub\u003e90%\u003c/sub\u003e) can be achieved within 19 min. It can be seen from Table. S2, the equilibration time (\u003cem\u003et\u003c/em\u003e\u003csub\u003e90%\u003c/sub\u003e) was around 16 min with no addition of NaCl but significantly reduced to 7.5 min in the present of 500 mM of NaCl. Similarly, the equilibration time (\u003cem\u003et\u003c/em\u003e\u003csub\u003e90%\u003c/sub\u003e) was reduced from 19 min to 9.9 min when the PBS concentration increased from 5 mM to 200 mM. The result reveals that ionic strength can accelerate the time of reaching equilibrium between MP and β-CD/RGO composites. While, a negative relationship was found between the pH value and equilibration time. With the increase of pH value from 5 to 8.5, the equilibration time (\u003cem\u003et\u003c/em\u003e\u003csub\u003e90%\u003c/sub\u003e) increased from 7.5 min to 11 min. Generally, the partition coefficient of analytes between aqueous phase and sorbents are related to the environmental factors, which could have impact on the calculation of sorption coefficients to matrix (Pang et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Therefore, the effects of some parameters on the partition coefficients were evaluated, including buffer concentration, salinity, and pH value. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eB, the pH value had significant impacts on the electrochemical response. With the increase of pH value from 4.5 to 7.0, the electrical signal was positively correlated with the pH value, with correlation coefficient (\u003cem\u003er\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e) of 0.977. While, the electrical signal showed a negative correlation with the pH value in the range of 7.0-8.5, with \u003cem\u003er\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e of 0.996. A plausible explanation is that MP is prone to hydrolysis under strong alkaline or acidic condition and therefore performs different relationship with the pH value (Wu et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). For real water samples with large pH range, the sorption coefficients should be calculated according to the linear relationship between the concentration of analyte and the pH value of water samples. It can be seen from Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eC that the electrochemical response almost kept constant with the increase of buffer concentration. The relative standard deviation of the electrochemical response for freely dissolved MP was 1.5%. Similarly, with the increase of salinity from 0 to 500 mM, there was no significant effect on the electrochemical response, with relative standard deviation of 2.1% (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eD). The result indicates that the ionic strength in the environmentally relevant ranges has no important impact on the partition coefficient between analyte and sorbents using the electrochemical method. Considering the buffer and salinity have no obvious influence on the sorption coefficients, the determination of freely dissolved MP in the water samples was carried out in the 100 mM of PBS to reduce the equilibrium time. As a result, the equilibrium time (\u003cem\u003et\u003c/em\u003e\u003csub\u003e90%\u003c/sub\u003e) was 16 min.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Sorption coefficients between MP and humic acid\u003c/h2\u003e \u003cp\u003eAcros humic acid and Suwannee River humic acid were used as DOM model to estimate the sorption coefficients (\u003cem\u003eK\u003c/em\u003e\u003csub\u003eDOC\u003c/sub\u003e) between MPs and humic acid. The freely dissolved fraction of MP was calculated under the optimized experimental conditions: 1.2 g of β-CD/RGO, 100 mM of PBS, pH value of 7.00 \u0026plusmn; 0.2, and equilibration time of 16 min. The values of \u003cem\u003eK\u003c/em\u003e\u003csub\u003eDOC\u003c/sub\u003e were obtained by fitting the free fraction (\u003cem\u003ef\u003c/em\u003e) at various concentration of humic acid (normalized to DOC) via Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA shows the profile of \u003cem\u003ef\u003c/em\u003e in relation to the concentration of DOC. The \u003cem\u003ef\u003c/em\u003e value of MP decreased with the increase of DOC concentration, indicating that the adsorption of analyte to the humic acid decreased the freely dissolved concentration in aqueous phase. The \u003cem\u003ef\u003c/em\u003e-\u003cem\u003eC\u003c/em\u003e\u003csub\u003eDOC\u003c/sub\u003e profiles of MP can be well fitted with correlation coefficient of 0.94 for Acros humic acid and 0.91 for Suwannee River humic acid, respectively. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eB, the electrochemical response of MP decreased gradually with the increase of the concentration of Acros humic acid, indicating that MP adsorbed on DOM caused the decrease of free fraction. From Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the log \u003cem\u003eK\u003c/em\u003e\u003csub\u003eDOC\u003c/sub\u003e values estimated by the electrochemical method were 4.56 \u0026plusmn; 0.19 for Acros humic acid and 4.58 \u0026plusmn; 0.08 for Suwannee River humic acid, respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComparison of log\u003cem\u003eK\u003c/em\u003e\u003csub\u003eDOC\u003c/sub\u003e and equilibrium time measured by the electrochemical method and nd-SPME method\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eMethod\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003elog\u003cem\u003eK\u003c/em\u003e\u003csub\u003eDOC\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eEquilibration time (min)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAcros humic acid\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSuwannee River humic acid\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003et\u003c/em\u003e\u003csub\u003e90%\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eElectrochemical method\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e4.56\u0026thinsp;\u0026plusmn;\u0026thinsp;0.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e4.58\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e19\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003end-SPME method\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e4.23\u0026thinsp;\u0026plusmn;\u0026thinsp;0.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e4.27\u0026thinsp;\u0026plusmn;\u0026thinsp;0.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e360\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e3.6 Determination of freely dissolved MP\u003c/h2\u003e \u003cp\u003eThe proposed electrochemical method was applied for measuring the the freely dissolved MP from real water samples, including river water, pond water, and tap. To evaluate the feasibility of the proposed method, higher concentrations of MP than environmental levels were spiked in the real water samples. As shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, under the optimized conditions, the freely dissolved concentrations of MP in three water samples were 3.96 \u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for river water, 4.17 \u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for pond water, and 4.44 \u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for tap water, respectively. It can be seen from Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e that the DOC concentration of tap water was 0.68 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which was lower than that of river water (3.05 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and pond water (1.36 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Therefore, the freely dissolved concentration of MP in the tap water was higher than the other two samples.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComparison of the freely dissolved concentration of MP measured by the electrochemical method and nd-SPME method\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eMethod\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e \u003cp\u003e\u003cem\u003eC\u003c/em\u003e\u003csub\u003efree\u003c/sub\u003e (\u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRiver water\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePond water\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTap water\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eElectrochemical method\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e3.96\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e4.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e4.44\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003end-SPME method\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e4.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e4.48\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e4.76\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003e3.7 Comparison with nd-SPME method\u003c/h2\u003e \u003cp\u003eAs shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the log \u003cem\u003eK\u003c/em\u003e\u003csub\u003eDOC\u003c/sub\u003e values measured by nd-SPME method were 4.23\u0026thinsp;\u0026plusmn;\u0026thinsp;0.19 for Acros humic acid and 4.27\u0026thinsp;\u0026plusmn;\u0026thinsp;0.12 for Suwannee River humic acid, respectively. The equilibrium time can be achieved at 6 h. From Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the freely dissolved MP measured by nd-SPME were 4.17 \u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for river water, 4.48 \u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for pond water, and 4.76 \u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for tap water, respectively. Therefore, the log \u003cem\u003eK\u003c/em\u003e\u003csub\u003eDOC\u003c/sub\u003e values measured by the proposed electrochemical method were consistent with those of nd-SPME, with the relative standard deviations below 5.6%. Besides, the freely dissolved MP measured by the electrochemical method were also accurate with the results of nd-SPME, with acceptable relative standard deviations ranging from 3.7\u0026ndash;5.1%. Compared with 6 h to reach the partition equilibrium by nd-SPME, the equilibrium time was only 16 min by the electrochemical method. Therefore, it proved that the electrochemical method is reliable for in situ measuring the freely dissolved concentration of analyte.\u003c/p\u003e \u003cp\u003eHowever, it should be noted that the log\u003cem\u003eK\u003c/em\u003e\u003csub\u003eDOC\u003c/sub\u003e values measured by the electrochemical method were slightly higher than those of nd-SPME even through the relative standard deviations between these two methods were low than 5.6%. Meanwhile, the \u003cem\u003eC\u003c/em\u003e\u003csub\u003efree\u003c/sub\u003e values measured by the electrochemical method were slightly lower than those of nd-SPME. A plausible explanation is the sorption of DOM on the surface of working electrode decreased the electrochemical response, resulting in the overestimation of the DOM-bound fraction and underestimation of the freely dissolved concentration. Therefore, the adsorption of Acros humic acid on the modified working electrode was further investigated by the FT-IR.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, the spectrum of humic acid displayed peaks at 2849 and 2928 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which belonged to the asymmetric and symmetric C-H stretching of methyl and methylene groups of aliphatic and non-strained cyclic hydrocarbons. The peak at 1710 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was assigned to C\u0026thinsp;=\u0026thinsp;O stretching vibration of carboxylic groups. The strong peak at 1625 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was attributed to the aromatic -COO asymmetric stretching of humic acid. The bands in the range of 1000\u0026ndash;1200 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were attributed to the aromatic ether C-O-C and C-O stretching of polysaccharides. The peaks appeared in the range of 910\u0026thinsp;\u0026minus;\u0026thinsp;650 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were attributed to the bending vibrations of aromatic C-H of humic acid. Compared with the humic acid and RGO, the spectrum of β-CD/RGO-HA exhibited typical β-CD absorption peaks of CH\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e stretching vibrations at 2980 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. After the adsorption with humic acid, the stretching vibration of aromatic C-H bonds was significantly decreased or even disappeared, indicating that the π-π interaction was formed between β-CD/RGO and humic acid. Besides, the C-O stretching vibration of RGO shifted from 1047 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to a higher wavenumber combined with an increase of intensity, indicating that the hydrogen bound was formed between β-CD/RGO and humic acid. Therefore, the adsorption of DOM on the modified working electrode should be a major reason for the minor difference on the sorption coefficients and freely dissolved concentrations between the proposed electrochemical method and nd-SPME method.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn this study, we developed a novel method for the determination of the sorption coefficient and freely dissolved concentration of MP by the electrochemical method based on nd-ME. The proposed method integrates sampling and determination into one single step, and the equilibrium time can be achieved within a short time. Therefore, the electrochemical method can be applied for in situ measurement of the freely dissolved analytes in the real water samples. DOM in the real water samples may cause an overestimation of the sorption coefficient as well as underestimation of free fraction, the relative standard deviations are proved to be acceptable compared with the results of nd-SPME. More interestingly, owing to the high selectivity of the electrochemical method coupled with different functional materials, the proposed electrochemical method is promising to be used for measuring the freely dissolved chiral enantiomers in the environmental samples, such as chiral pesticides and chiral drugs, providing a convenient method for estimating the bioavailability of chiral pollutants in the further.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eEthical Approval\u003c/p\u003e\n\u003cp\u003eNo conflict of ethics.\u003c/p\u003e\n\u003cp\u003eConsent to Participate\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003eConsent to Publish\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003eAuthors Contributions\u003c/p\u003e\n\u003cp\u003eLong Pang: Methodology, Investigation, Writing \u0026ndash; review \u0026amp; editing, Funding acquisition. Zhigao Feng: Investigation, Writing \u0026ndash; original draft. Xingru Hu: Investigation, Methodology. Jiahui Hou: Data analysis, Data collection. Guangtao Cui: Investigation, Methodology. Jingfu Liu: Conceptualization, Writing \u0026ndash; Review \u0026amp; Editing.\u003c/p\u003e\n\u003cp\u003eFunding\u003c/p\u003e\n\u003cp\u003eThis study was supported by the National Natural Science Foundation of\u0026nbsp;China\u0026nbsp;(21707124),\u0026nbsp;and\u0026nbsp;Henan\u0026nbsp;Province\u0026nbsp;Scientific\u0026nbsp;and\u0026nbsp;Technological\u0026nbsp;Research\u0026nbsp;Projects\u0026nbsp;(182102311109).\u003c/p\u003e\n\u003cp\u003eCompeting Interests\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAllan IJ, Raffard V, Kringstad A et al (2021) Assessment of marine sediment remediation efficiency with SPME-based passive sampling measurement. Sci Total Environ 756:143854\u003c/li\u003e\n \u003cli\u003eBartolome N, Hilber I, Schulin R et al (2018) Comparison of freely dissolved concentrations of PAHs in contaminated pot soils under saturated and unsaturated water conditions. Sci Total Environ 644:835-843. \u0026nbsp;\u003c/li\u003e\n \u003cli\u003eFerreira Oliveira AE, Bettio GB, Pereira AC (2018) An electrochemical sensor based on electropolymerization of \u0026szlig;-cyclodextrin and reduced graphene oxide on a glassy carbon electrode for determination of neonicotinoids. Electroanalysis 30:1918-1928.\u003c/li\u003e\n \u003cli\u003eFischer FC, Hiki K, Endo S (2022) Free versus bound concentration: passive dosing from polymer meshes elucidates drivers of toxicity in aquatic tests with benthic invertebrates. Environ Toxicol 75:395-400.\u003c/li\u003e\n \u003cli\u003eGoeldner V, Ulke J, Kirchner B et al (2023) Electrochemistry-mass spectrometry bridging the gap between suspect and target screening of valsartan transformation products in wastewater treatment plant effluent. Water Res 244:120525.\u003c/li\u003e\n \u003cli\u003eGuo R, Zhu D, He J et al (2023) Influence of copper and aging on freely dissolved tetracycline concentration in soil. Environ Sci Pollut R 31:23334-23362. \u0026nbsp;\u003c/li\u003e\n \u003cli\u003eHoess S, Sanders D, van Egmond R (2023) Determining the toxicity of organic compounds to the nematode Caenorhabditis elegans based on aqueous concentrations. Environ Sci Pollut R 30:96290-96300. \u0026nbsp;\u003c/li\u003e\n \u003cli\u003eJonker MTO (2021) Effects of sediment manipulation on freely dissolved concentrations of hydrophobic organic chemicals. Chemosphere 265:128694.\u003c/li\u003e\n \u003cli\u003eKreutzer A, Reininghaus M, Meyer J et al (2023) Application of equilibrium passive sampling to assess the influence of anthropogenic activities and bioturbation on the distribution of hydrophobic organic chemicals in North Sea sediment cores. Environ Pollut 318:120876.\u003c/li\u003e\n \u003cli\u003eKreutzer A, Schacht S-C, Witt G (2022) Equilibrium passive sampling: A novel approach to determine internal tissue concentrations of hydrophobic organic compounds in biota. Sci Total Environ 824:153764.\u003c/li\u003e\n \u003cli\u003eKukkonen J, Oikari A, Johnsen S et al (1989) Effects of humus concentrations on benzo[a]pyrene accumulation from water to Daphnia magna: comparison of natural waters and standard preparations. Sci Total Environ 79:197-207.\u003c/li\u003e\n \u003cli\u003eLi Z, Hu X, Qin L et al (2020) Evaluating the effect of different modified microplastics on the availability of polycyclic aromatic hydrocarbons. Water Res 170:115290\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eLin K, Lao W, Lu Z et al (2017) Measuring freely dissolved DDT and metabolites in seawater using solid-phase microextraction with performance reference compounds. Sci Total Environ 599:364-371.\u003c/li\u003e\n \u003cli\u003eLiu GD, Lin YH (2005) Electrochemical sensor for organophosphate pesticides and nerve agents using zirconia nanoparticles as selective sorbents. Anal. Chem 77: 5894-5901. \u0026nbsp;\u003c/li\u003e\n \u003cli\u003eLiu JF, Hu XL, Peng JF et al (2006) Equilibrium sampling of freely dissolved alkylphenols into a thin film of 1-octanol supported on a hollow fiber membrane. Anal. Chem 78: 8526-8534.\u003c/li\u003e\n \u003cli\u003eMa T, Ou G (2023) Fabrication of a highly sensitive electrochemical sensor for the rapid detection of nimodipine. Int J Electrochem 18:100277.\u003c/li\u003e\n \u003cli\u003eMarcano DC, Kosynkin DV, Berlin JM et al (2010) Improved synthesis of graphene oxide. ACS Nano 4:4806-4814.\u003c/li\u003e\n \u003cli\u003ePang L, Liu J, Yin Y et al (2013) Evaluating the sorption of organophosphate esters to different sourced humic acids and its effects on the toxicity to Daphnia magna. Environ Toxicol 32:2755-2761. \u0026nbsp;\u003c/li\u003e\n \u003cli\u003ePang L, Yang P, Yang H et al (2018) Application of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@MIL-100 (Fe) core-shell magnetic microspheres for evaluating the sorption of organophosphate esters to dissolved organic matter (DOM). Sci Total Environ 626:42-47.\u003c/li\u003e\n \u003cli\u003eParkerton TF, Stone MA, Letinski DJ (2000) Assessing the aquatic toxicity of complex hydrocarbon mixtures using solid phase microextraction. Toxicol Lett 112-113:273-82.\u003c/li\u003e\n \u003cli\u003ePoerschmann J, Zhang Z, Pawliszyn JB (1997) Solid phase microextraction for determining the distribution of chemicals in aqueous matrices. Anal Chem 69:597-600..\u003c/li\u003e\n \u003cli\u003eQu G, Liu G, Zhao C et al (2024) Detection and treatment of mono and polycyclic aromatic hydrocarbon pollutants in aqueous environments based on electrochemical technology: recent advances. Environ Sci Pollut R 31:23334-23362.\u003c/li\u003e\n \u003cli\u003eSingh VK, Patra MK, Manoth M et al (2009) In situ synthesis of graphene oxide and its composites with iron oxide. New Carbon Materials 24:147-152. \u0026nbsp;\u003c/li\u003e\n \u003cli\u003eVaes WHJ, Urrestarazu-Ramos E, Seinen W et al (1996) Measurement of the free concentration using solid-phase microextraction:\u0026thinsp; binding to protein. Anal Chem 68:4463-4467.\u003c/li\u003e\n \u003cli\u003eWu D, Karimi-Maleh H, Liu X et al (2023) Bibliometrics analysis of research progress of electrochemical detection of tetracycline antibiotics. J Anal Methods Chem 2023.\u003c/li\u003e\n \u003cli\u003eWu S, Lan X, Cui L et al (2011) Application of graphene for preconcentration and highly sensitive stripping voltammetric analysis of organophosphate pesticide. Anal Chim Acta 699:170-176. \u0026nbsp;\u003c/li\u003e\n \u003cli\u003eZhu SH, Qin LX, Li ZW, et al. (2023) Effects of nanoplastics and microplastics on the availability of pharmaceuticals and personal care products in aqueous environment. J Hazard 458:131999.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Negligible depletion micro-extraction, Electrochemical method, Freely dissolved concentration, In situ, Sorption coefficients","lastPublishedDoi":"10.21203/rs.3.rs-4394035/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4394035/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eNegligible depletion micro-extraction (nd-ME) plays an important role in measuring the freely dissolved concentration of pollutants but is suffered from a long equilibrium time. In this study, a novel method for in situ determination of freely dissolved methyl parathion (MP) by the electrochemical method was developed based on the nd-ME method. The parameters influencing the adsorption kinetic were investigated in the environmentally relevant ranges, including buffer concentration (5-200 mM), salinity (0-500 mM), and pH value (5.0-8.5). The equilibrium time can be achieved within 16 min between the freely dissolved MP and beta-cyclodextrin decorated reduced graphene oxide composites (β-CD/RGO). Under the equilibrium condition, the sorption coefficients (log \u003cem\u003eK\u003c/em\u003e\u003csub\u003eDOC\u003c/sub\u003e) were 4.56 for Acros humic acid and 4.58 for Suwannee River humic acid, respectively, which were consistent with those by negligible depletion-solid phase microextraction (nd-SPME) with log \u003cem\u003eK\u003c/em\u003e\u003csub\u003eDOC\u003c/sub\u003e = 4.23 for Acros humic acid and log \u003cem\u003eK\u003c/em\u003e\u003csub\u003eDOC\u003c/sub\u003e = 4.27 for Suwannee River humic acid. The freely dissolved MP in water samples ranged from 3.96 to 4.44 \u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which were in agreement with those by nd-SPME (\u003cem\u003eC\u003c/em\u003e\u003csub\u003efree\u003c/sub\u003e = 4.17\u0026ndash;4.76 \u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). According to the result, a novel method was developed in this study to estimate the freely dissolved concentration of pollutants using the electrochemical method.\u003c/p\u003e","manuscriptTitle":"In situ electrochemical method for measuring the freely dissolved methyl parathion based on the negligible depletion micro-extraction","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-06-11 10:13:02","doi":"10.21203/rs.3.rs-4394035/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"2fbfb684-e986-4877-82d5-a48953b3d8c2","owner":[],"postedDate":"June 11th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-06-26T06:02:47+00:00","versionOfRecord":[],"versionCreatedAt":"2024-06-11 10:13:02","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4394035","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4394035","identity":"rs-4394035","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}