Solid-phase extraction fiber with polydimethylsiloxane-reduced graphene oxide coating as well as preparation method and application of solid-phase extraction fiber

Abstract

The invention relates to a polydimethylsiloxane-reduced graphene oxide composite coating solid-phase extraction fiber as well as a preparation method and application thereof, and belongs to the technical field of sample pretreatment and trace analysis. Aiming at the problems of poor conductivity and insufficient coating stability of the traditional solid-phase microextraction fiber, a PDMS-rGO composite coating is constructed by a three-step method, graphene oxide is reduced by ascorbic acid and calcined to form a loose multi-layer structure, PDMS is loaded by a sol-gel method, and the coating fiber with the thickness of 40-60 microns and the conductivity of 30-35 k omega is obtained. In combination with an electrochemical enhanced solid-phase microextraction technology, the fiber is used for efficiently extracting amphetamine stimulants, and in combination with an online derivatization-gas chromatography-nitrogen and phosphorus detector, trace detection is realized. Experiments show that the composite coating has high affinity of PDMS and excellent conductivity of rGO, the thermal stability is improved to 800 DEG C or above, and the mechanical durability reaches 150 times of cyclic extraction. And a high-sensitivity solution is provided for rapid screening of amphetamine substances in a biological matrix.

Description

A polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber, its preparation method and application Technical Field

[0001] This invention belongs to the field of sample pretreatment and trace analysis technology, specifically relating to a polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber, its preparation method and application, which is particularly suitable for rapid and highly sensitive detection of amphetamine stimulants in biological samples. Background Technology

[0002] Amphetamine stimulants are central nervous system stimulants, including compounds such as amphetamine (AMP), methamphetamine (MA), 3,4-methylenedioxyamphetamine (MDA), and 3,4-methylenedioxymethamphetamine (MDMA). In the context of drug abuse, amphetamine stimulants remain a significant global concern. Therefore, developing a sensitive analytical method for the simultaneous determination of amphetamine stimulants in biological matrices such as urine, blood, oral fluid, and hair is crucial. Confirmation of ATS abuse is typically achieved through the analysis of these biological samples. Urine is often preferred due to its large sample volume, non-invasive collection, and the stability of amphetamine stimulants in this matrix. However, the complexity of urine—containing high concentrations of dissolved inorganic salts, proteins, and other endogenous compounds—poses significant challenges to direct instrumental analysis and requires rigorous sample pretreatment.

[0003] Amphetamine stimulants are polar organic amines with pKa values ​​exceeding 8, meaning they exist primarily in positively charged ionic forms in common biological fluids. While this ionic form enhances their solubility, extraction using conventional organic solvents or solid-phase adsorbents complicates the process. To improve extractability, sample pH is often adjusted to an alkaline range to convert ionized amphetamine stimulants to their molecular form. However, this approach compromises pretreatment efficiency. Electro-enhanced solid-phase microextraction (EE-SPME) has emerged as a promising solution, utilizing electricity to guide positively charged ATS ions from a neutral sample to a negatively charged extraction phase, i.e., an SPME fiber coating. The fiber is then introduced into a gas chromatograph injector for instrumental analysis. Due to its simplicity and ease of operation, EE-SPME is widely used in ATS extraction and pre-concentration. Fibers used in EE-SPME require a dual function: strong affinity for ATS molecules and good conductivity. Although some carbon-based materials, such as ordered mesoporous carbon, multi-walled carbon nanotubes, and carbon nanotube / Nafion composites, have been used as fiber coatings for EE-SPME, their applications remain limited due to challenges in achieving uniform and durable coatings. These limitations highlight the need for further research to optimize coating techniques and enhance the performance of carbon-based fibers in microextraction applications.

[0004] In traditional SPME, polydimethylsiloxane (PDMS) is effectively used for ATS extraction and pre-concentration due to its strong affinity for amphetamine stimulants. However, its poor conductivity limits its applicability in EE-SPME. Over the past few decades, PDMS composites have been developed to improve their physicochemical properties. Examples include PDMS / carbon nanotube composites, PDMS / BaTiO3 composites, PDMS / graphite composites, and PDMS / polyethylene composites. These composites exhibit enhanced flexibility, chemical stability, and tunable properties, making them suitable for a variety of applications such as flexible electronics, biomedical devices, and soft robotics.

[0005] Despite these advances, the preparation of PDMS composites remains a significant challenge. The low mechanical strength and thermal stability of virgin PDMS limit its applicability in harsh environments. Furthermore, achieving uniform dispersion of fillers within the PDMS matrix remains difficult, often resulting in inconsistent properties and poor composite quality. The high viscosity of PDMS further complicates the mixing process, requiring advanced techniques or surface modification to improve compatibility. Summary of the Invention

[0006] To address the aforementioned shortcomings, this invention aims to provide a polydimethylsiloxane-reduced graphene oxide composite coated SPME fiber, which has a simple preparation method, significantly improved coating conductivity, and can achieve efficient extraction and detection of amphetamine stimulants through EE-SPME technology.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: a method for preparing polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber, comprising the following steps: (a): injecting graphene oxide dispersion into a container, inserting a stainless steel wire without contacting the inner wall of the container, adding a reducing agent to the graphene oxide dispersion, and reacting at 80℃-90℃ for 1-2 hours to allow graphene oxide to adhere to the surface of the stainless steel wire and generate reduced graphene oxide, forming a wet coating; (b): air-drying the stainless steel wire with the reduced graphene oxide wet coating obtained in step (a) and then calcining it, and cooling the stainless steel wire and its surface coating to obtain a loose multilayer reduced graphene oxide coated fiber; (c): preparing a polydimethylsiloxane sol solution, then immersing the loose multilayer reduced graphene oxide coated fiber obtained in step (b) into the polydimethylsiloxane sol solution, drying it to obtain a fiber precursor, aging the fiber precursor to obtain polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber.

[0008] Further, in step (a), the container containing the graphite oxide dispersion, reducing agent and stainless steel wire is placed in an autoclave, and the entire autoclave is heated at 80°C for 1 hour.

[0009] Further, the calcination conditions described in step (b) are as follows: the stainless steel wire with the wet coating of reduced graphene oxide is placed in a nitrogen atmosphere for calcination at a temperature of 600℃-800℃ for a duration of 0.5-1 hour.

[0010] Further, the preparation method of the polydimethylsiloxane sol solution in step (c) includes: mixing methyltrimethoxysilane, hydroxyl-terminated polydimethylsiloxane, polymethylhydrosiloxane and trifluoroacetic acid, vortexing at room temperature for 2-3 minutes, then centrifuging the mixture to remove the precipitate, and the resulting supernatant is a clear polydimethylsiloxane sol solution, which is stored at -25°C for later use.

[0011] Further, the drying temperature in step (c) is 120°C and the drying time is 30 minutes; the aging step in step (c) is to insert the dried fiber precursor into the Supelco handle and age it at 200°C, 250°C and 300°C for 30-60 minutes at the gas chromatograph inlet to obtain polydimethylsiloxane-reduced graphene oxide coated solid phase extraction fiber.

[0012] The present invention also provides a polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber, wherein the coating has a loose multilayer structure, the coating thickness is 40-60 μm, and the resistivity is 30-35kΩ.

[0013] This invention also provides an electrochemically enhanced solid-phase microextraction method, utilizing GC-NPD for desorption and quantitative analysis, for the detection of amphetamine stimulants, comprising the following steps: (i) activation and extraction: the sample containing amphetamine stimulants is transferred to a sample vial. A polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber composite coated fiber and a platinum wire are used as the working electrode and counter electrode, respectively, connected to a DC regulated power supply. The sample is extracted under stirring conditions; (ii) online derivatization: during extraction, an internal standard and a derivatization reagent are added to induce a derivatization reaction of the amphetamine stimulants in the sample; (iii) desorption and detection: the polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber is introduced into the gas chromatograph inlet for thermal desorption, and analysis is performed using a nitrogen-phosphorus detector.

[0014] Further, in step (i), the applied voltage is -4 to -8V; the extraction time is 8-12 minutes; the stirring speed is 400-800 rpm; and the pH of the sample solution is 6-8. In step (ii), the derivatization reagent is isobutyl chloroformate, and the amount added is 1-3 μL. The derivatization reaction is stirred at room temperature for 1-3 minutes.

[0015] Furthermore, the polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber can detect amphetamine, methamphetamine, 3,4-methylenedioxyamphetamine, 3,4-methylenedioxymethylamphetamine, ephedrine, and phenylbutanylamine.

[0016] Furthermore, the internal standard is 4-phenylbutylamine, and the concentration added is 50-200 ng / L.

[0017] Compared with existing technologies, the beneficial effects of this invention are mainly reflected in the following aspects: a stable composite of PDMS and rGO is achieved through a three-step process of chemical reduction, calcination, and sol-gel, while retaining the conductive framework of rGO and uniformly dispersing PDMS; the coating combines the high affinity of PDMS for amphetamine stimulants with the excellent conductivity of rGO; and for the first time, electrochemically enhanced solid-phase microextraction is combined with online derivatization technology, significantly improving extraction efficiency. Attached Figure Description

[0018] Figure 1 shows the thermogravimetric curves of (a) pure PDMS and (b) polydimethylsiloxane-reduced graphene oxide; Figure 2 shows scanning electron microscope images of polydimethylsiloxane-reduced graphene oxide coated SPME fibers: cross-sectional views (a) at 100x magnification and (b) at 300x magnification, and (c) at 250x magnification; Figure 3 shows the chromatograms of (a) standard solutions (100 ng L⁻¹): 1. PBA, 2. EPH, 3. AMP, 4. MAP, 5. PBA (IS, 1000 ng L⁻¹), 6. MDMA and 7. MDA; and (b), (c) and (d) three actual urine samples. Detailed Implementation

[0019] Example 1: Using ascorbic acid as a reducing agent, a 5 mg / mL graphene oxide dispersion containing 5% (w / v) ascorbic acid was injected into a 3 cm long glass tube with an inner diameter of 2.0 mm and sealed at one end, forming a 2.1 cm column of dispersion at the bottom of the tube. A 3 cm long stainless steel wire segment, wiped with filter paper, was placed in the center of the graphene oxide dispersion column, avoiding contact with the inner wall of the container to prevent uneven formation of the wet coating. A section of stainless steel wire was placed into each glass tube containing the graphene oxide dispersion. The glass tubes were placed in a 20 mL autoclave containing 10 mL of distilled water to achieve pressure equilibrium. The entire autoclave was placed in an 80°C oven and heated for 1 hour. During heating, the graphene oxide dispersion underwent deoxidation to form reduced graphene oxide, which uniformly coated the end of the immersed stainless steel wire, forming a uniform wet coating.

[0020] After removing the coated wet fibers and air-drying them, they were placed in a tube furnace and calcined at 600°C for 0.5 hours under a nitrogen atmosphere. After cooling to room temperature, the loose, multilayered reduced graphene oxide coated fibers were collected for later use.

[0021] 300 μL of methyltrimethoxysilane as a precursor, 180 mg of hydroxyl-terminated polydimethylsiloxane as a coating polymer, 30 mg of polymethylhydrosiloxane as a passivating agent, and 200 μL of 95% trifluoroacetic acid as an acid catalyst containing 5% water were placed in a borosilicate culture tube and vortexed for 2 minutes to ensure thorough mixing, thus preparing a sol solution. The resulting mixture was then transferred to an Eppendorf microcentrifuge tube and centrifuged at 10,000 rpm for 5 minutes. Any precipitate formed at the bottom was discarded, and the clear sol solution in the supernatant was used for fiber coating preparation. The prepared sol solution was stored at -25°C. The polydimethylsiloxane sol solution remained in a sol state for one hour, after which it became a hard colloid. The polydimethylsiloxane sol solution was to be used within one hour for preparing polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fibers. Before preparing the target fibers, 1 mL of sol solution was removed from the refrigerator, and the loosely porous multilayered reduced graphene oxide coated fibers were completely immersed in the sol solution for 10 minutes to ensure that the PDMS sol was fully absorbed into the loosely porous multilayered reduced graphene oxide coating structure. Afterwards, the treated fibers were placed in a 120°C oven for 30 minutes. The newly prepared fibers were then inserted into a Supelco handle and aged at 200°C, 250°C, and 300°C for 30 minutes each at the gas chromatograph injection port. Polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fibers were obtained. Aging was performed using standard experimental procedures.

[0022] Example 2 assesses the stability of the polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber prepared in Example 1.

[0023] In the evaluation experiments, polydimethylsiloxane-reduced graphene oxide coated fibers were aged at 300°C for 60 minutes. The extraction efficiency ratio (r) of the fibers after treatment versus before treatment is listed in Table 1. Based on r values ​​ranging from 0.82 to 1.06, the extraction efficiency of the fibers remained almost unchanged after heat treatment, indicating excellent thermal stability. A comparison of the thermal stability of pure PDMS (thickness ~50 μm) and polydimethylsiloxane-reduced graphene oxide was also conducted. During a 30-minute heat test at 260°C, the pure PDMS coating cracked significantly. This result confirms that the thermal stability of PDMS in the polydimethylsiloxane-reduced graphene oxide coating is significantly improved. Notably, the thickness of the PDMS coating significantly affects its thermal stability. For example, the manufacturer recommends an aging temperature of 250°C for 100 μm PDMS fibers, while the recommended temperature for 7 μm PDMS fibers is 270°C. To further evaluate the thermal stability of polydimethylsiloxane-reduced graphene oxide (PDMS), thermogravimetric analysis (TGA) was performed on pure PDMS and PDMS. As shown in Figure 1, when the temperature increased from room temperature to 300°C, the mass percentages of pure PDMS and PDMS decreased from 100% to 95.4% and 97%, respectively. These results indicate that PDMS materials are generally thermally stable within this temperature range. However, as mentioned above, cracking was observed in the 50 μm PDMS coating when the temperature was increased to 260°C and held for 30 minutes. Cracks may lead to coating peeling during subsequent use. This limitation therefore hinders the use of thick PDMS coatings at high temperatures. Pure PDMS completely decomposed at 567.5°C, while PDMS in the PDMS-reduced graphene oxide composite completely decomposed above 800°C. These results further demonstrate that the thermal stability of PDMS is significantly enhanced by incorporating it into a loose, multilayered reduced graphene oxide coating structure. This excellent thermal stability allows the polydimethylsiloxane-reduced graphene oxide coating to be used at high desorption temperatures. To assess durability, the extraction efficiency of the prepared fibers was examined after 150 extractions. As shown in Table 1, the r-values ​​ranged from 0.88 to 1.04, indicating no significant change in extraction efficiency after long-term use. This highlights the fiber's excellent mechanical stability. The strong durability can be attributed to the high mechanical strength of the polydimethylsiloxane-reduced graphene oxide composite coating and the stainless steel wire support material, which together ensure the fiber's long service life, allowing for repeated use more than 150 times.

[0024] Table 1. Stability of polydimethylsiloxane-reduced graphene oxide Example 3: The morphology and structure of the solid-phase extraction fiber with polydimethylsiloxane-reduced graphene oxide coating prepared in Example 1 were analyzed.

[0025] Figures 2a and 2b show low-magnification surface SEM images, revealing a uniform coating distribution around the SSW. The 1mPDMS / rGO coating surface is rough, and the typical bark-band morphology of rGO has disappeared. This rough morphology is advantageous as it increases the surface area, potentially enhancing the extraction capacity of the coating. The cross-sectional image in Figure 2c shows that the 1mPDMS / rGO material is well encapsulated on the SSW, with a coating thickness of approximately 50 μm. Furthermore, PDMS is uniformly dispersed within the rGO layer, while preserving the original loose multilayer structure of the rGO coating. These results indicate that high-temperature treatment promotes the formation of a unique loose multilayer structure, enabling successful integration of PDMS into the rGO composite.

[0026] Example 4 describes a method for detecting amphetamine stimulants using polydimethylsiloxane-reduced graphene oxide-coated solid-phase extraction fibers, comprising the following steps: Actual urine samples were collected from suspected ATS abusers. All urine samples were stored at -20°C prior to analysis. Prior to analysis, samples were thawed at room temperature and filtered through a polyvinylidene fluoride (PVDF) membrane filter (0.2 μm pore size, Fisher Scientific). The filtered urine samples were diluted as follows: 1.00 mL each of samples 1, 2, and 3 were transferred to 10 mL volumetric flasks using an Eppendorf pipette, diluted to the mark with deionized water, and mixed thoroughly. Drug-free urine samples were obtained from volunteer laboratory personnel with no history of drug use. These samples were used immediately after collection and filtration through the PVDF membrane. For matrix matching, drug-free urine was diluted 10-fold using the same dilution procedure described above.

[0027] Transfer 10 mL of standard solution or diluted sample solution to a 12 mL sample vial. Use polydimethylsiloxane-reduced graphene oxide composite coated fiber and platinum wire as the working electrode and counter electrode, respectively, connected to a DC regulated power supply. Inject 2 μL of isobutyl chloroformate into 10 mL of diluted sample solution, and simultaneously add 4-phenylbutylamine as an internal standard to the diluted sample solution, and disperse thoroughly by vigorous stirring for 2 minutes. Perform EE-SPME for 10 minutes under stirring within a potential range of 0 to -8 V using conventional stirring equipment. During EE-extraction, the positively ionized analyte, the internal standard in the vial, and neutral isobutyl chloroformate are simultaneously adsorbed onto the polydimethylsiloxane-reduced graphene oxide coating. After extraction, retract the fiber into the sleeve and introduce it into the gas chromatograph injector for thermal desorption. After extraction, retract the fiber into the needle and immediately introduce it into the gas chromatograph injector for thermal desorption at 280°C. Simultaneously, the analyte and internal standard were derivatized by reacting with isobutyl chloroformate at 280°C.

[0028] Figure 3 shows typical chromatograms of a 100 ng L⁻¹ standard solution and three actual urine samples. Analysis detected ephedrine, amphetamine, and methamphetamine in urine sample 1, at concentrations of 49, 60, and 39 ng L⁻¹, respectively. In urine sample 2, 3,4-methylenedioxymethamphetamine and 3,4-methylenedioxyamphetamine were detected at concentrations of 120 and 170 ng L⁻¹, respectively. Furthermore, phenbutylamine was detected only in urine sample 3, at a concentration of 87 ng L⁻¹. These findings confirm the effectiveness of the developed method in forensic toxicology analysis, providing a reliable and accurate detection of amphetamine-type stimulants in biological samples.

Claims

1. A method for preparing polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fibers, characterized in that, Includes the following steps: (a): Inject graphene oxide dispersion into a container, insert a stainless steel wire without contacting the inner wall of the container, add a reducing agent to the graphene oxide dispersion, and react at 80℃-90℃ for 1-2 hours to allow graphene oxide to adhere to the surface of the stainless steel wire and generate reduced graphene oxide to form a wet coating; (b): After air drying the stainless steel wire with the reduced graphene oxide wet coating obtained in step (a), calcine it. After cooling the stainless steel wire and the coating on its surface, a loose multilayer reduced graphene oxide coated fiber is obtained. (c): Prepare a polydimethylsiloxane sol solution, then immerse the loose multilayer reduced graphene oxide coated fiber obtained in step (b) into the polydimethylsiloxane sol solution, dry it to obtain a fiber precursor, and age the fiber precursor to obtain polydimethylsiloxane-reduced graphene oxide coated solid phase extraction fiber.

2. The method for preparing a polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber according to claim 1, characterized in that, In step (a), the container containing the graphite oxide dispersion, reducing agent and stainless steel wire is placed in an autoclave, and the entire autoclave is heated at 80°C for 1 hour.

3. The method for preparing a polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber according to claim 2, characterized in that, The specific calcination conditions described in step (b) are as follows: the stainless steel wire with the wet coating of reduced graphene oxide is placed in a nitrogen atmosphere for calcination at a temperature of 600℃-800℃ for a duration of 0.5-1 hour.

4. The method for preparing a polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber according to claim 3, characterized in that, The preparation method of the polydimethylsiloxane sol solution in step (c) includes: mixing methyltrimethoxysilane, hydroxyl-terminated polydimethylsiloxane, polymethylhydrosiloxane and trifluoroacetic acid, vortexing at room temperature for 2-3 minutes, then centrifuging the mixture to remove the precipitate, and the resulting supernatant is a clear polydimethylsiloxane sol solution, which is stored at -25℃ for later use.

5. The method for preparing a polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber according to claim 4, characterized in that, The drying temperature in step (c) is 120°C and the drying time is 30 minutes; the aging step in step (c) is to insert the dried fiber precursor into the Supelco handle and age it at 200°C, 250°C and 300°C for 30-60 minutes at the gas chromatograph inlet to obtain polydimethylsiloxane-reduced graphene oxide coated solid phase extraction fiber.

6. A polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber, characterized in that, The fiber is prepared by any one of claims 1-5, the polydimethylsiloxane-reduced graphene oxide coating has a loose multilayer structure, the polydimethylsiloxane-reduced graphene oxide coating is located 2 cm from the insertion end, the coating thickness is 40-60 μm, and the resistance value is 30-35 kΩ.

7. An electrochemically enhanced solid-phase microextraction method, characterized in that, The detection method employs the polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber as described in claim 6, and utilizes GC-NPD for desorption and quantitative analysis to detect amphetamine stimulants. The method includes the following steps: (i) activation and extraction: the sample containing amphetamine stimulants is transferred to a sample vial; the polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber composite coated fiber and platinum wire are used as the working electrode and counter electrode, respectively, connected to a DC regulated power supply, and the sample is extracted under stirring conditions; (ii) online derivatization: during the extraction process, an internal standard and derivatization reagent are added to induce a derivatization reaction of the amphetamine stimulants in the sample; (iii) desorption and detection: the polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber is introduced into the gas chromatograph inlet for thermal desorption, and analyzed using a nitrogen-phosphorus detector.

8. The electrochemically enhanced solid-phase microextraction method according to claim 7, characterized in that, In step (i), the applied voltage is -4 to -8V; the extraction time is 8-12 minutes; the stirring speed is 400-800 rpm; and the pH of the sample solution is 6-8. In step (ii), the derivatization reagent is isobutyl chloroformate, and the amount added is 1-3 μL. The derivatization reaction is stirred at room temperature for 1-3 minutes.

9. The electrochemically enhanced solid-phase microextraction method according to claim 8, characterized in that, The polydimethylsiloxane-reduced graphene oxide coated solid-phase extraction fiber can detect amphetamine, methamphetamine, 3,4-methylenedioxyamphetamine, 3,4-methylenedioxymethylamphetamine, ephedrine and phenylbutanylamine.

10. The electrochemically enhanced solid-phase microextraction method according to claim 9, characterized in that, The internal standard is 4-phenylbutylamine, and the concentration added is 50-200 ng / L.

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