Iron-based nano material, preparation method and application

Abstract

The invention provides an iron-based nano material, a preparation method and application, the material comprises magnetic iron oxide nano particles and a functional modification layer on the surface, the magnetic iron oxide nano particles are Fe3O4, the particle size is 10-30 nm, and the magnetic iron oxide nano particles have specific oxygen vacancy concentration, 337 nm laser absorptivity and saturation magnetization; the functional modification layer is an amino-containing polymer layer, such as a polydopamine or cysteine-polyethyleneimine copolymer, and can reduce salt ion interference. The material solves the problems of low molecular weight region background interference, non-uniform crystallization, low ionization efficiency, poor salt tolerance and the like during detection of 100-400Da small molecular metabolites in trace samples such as tears and the like by optimizing magnetic responsiveness, photo-thermal conversion capability and salt resistance of an existing matrix; the method realizes high sensitivity and high repeatability (detection), and is suitable for metabonomics analysis of trace biological samples.

Description

Technical Field

[0001] This invention relates to iron-based nanomaterials, specifically to an iron-based nanomaterial for matrix-assisted laser desorption / resorption spectroscopy (MALDI-MS), its preparation method, and its application. Background Technology

[0002] Matrix-assisted laser desorption / ionization mass spectrometry (MALDI-MS) is an important soft ionization mass spectrometry technique, mainly used to analyze macromolecules (such as proteins, nucleic acids, polysaccharides, etc.) and small molecule compounds. Its core principle is to use an organic matrix to assist in the ionization of the sample, and to achieve desorption and ionization of the sample through laser pulses. It is especially suitable for the detection of thermally unstable or non-volatile compounds.

[0003] In practical applications of MALDI-MS technology, organic matrices such as 2,5-dihydroxybenzoic acid (DHB) and α-cyano-4-hydroxycinnamic acid (CHCA) are commonly used materials. With the deepening of research on tear biomarkers, tear metabolomics has become a new research hotspot. However, the volume of tear fluid that can be collected from the human eye in a single sample is usually less than 10 μL. The detection of small molecule metabolites (100–400 Da) in these trace samples exposes a significant limitation of existing organic matrices: First, background interference in the low molecular weight region is severe. Traditional organic matrices generate numerous fragment peaks (such as matrix self-ionization signals) under laser irradiation, strongly interfering with the detection of metabolites with molecular weights less than 700 Da, leading to a reduced signal-to-noise ratio (S / N) and severely affecting the accuracy of trace sample analysis. The core mechanism is that under ultraviolet laser (337 nm) irradiation, benzene ring matrices (such as CHCA) undergo multi-stage fragmentation, generating fragment ions such as [Matrix-H]⁻ and C6H5O⁺; simultaneously, when the laser energy distribution is uneven, the mixed crystals formed by the organic matrix and tear molecule preferentially desorb matrix components, producing a mass discrimination effect. This phenomenon is an inherent characteristic of aromatic organic matrices and cannot be fundamentally eliminated by adjusting laser energy or crystallization methods.

[0004] Secondly, uneven crystallization leads to poor reproducibility. MALDI-MS requires co-crystallization of the analyte and matrix, but trace samples such as tears are difficult to mix uniformly with the matrix, easily resulting in a "sweet spot effect" (large spot-to-spot differences), affecting the reliability of quantitative analysis. This is because the salt content of tears (~9 mg / mL NaCl) is higher than that of conventional biological samples, leading to instability at the crystallization front in the water-organic solvent system; and droplets smaller than 10 μL are affected by contact line pinning, forming coffee-ring-shaped uneven deposits. Existing methods such as ultrasonic nebulization and electrospray assisted crystallization are not compatible with this high-salt trace system.

[0005] Third, insufficient sensitivity. Traditional matrices have low ionization efficiency for small molecule metabolites, especially polar metabolites (such as amino acids and organic acids), resulting in high limits of detection (LODs) that are difficult to meet the detection requirements of ultra-low volume tear samples. On the one hand, the proton affinity (PA) of small molecule metabolites is poorly matched with DHB (PA=812 kJ / mol), hindering charge transfer; on the other hand, rigid organic crystals form lattice encapsulation of small molecules, especially polar metabolites, with an encapsulation rate exceeding 70%, and existing matrix optimization strategies (such as adding trifluoroacetic acid) further exacerbate interference in low-mass regions.

[0006] Fourth, poor salt tolerance. Biological samples such as tears contain high concentrations of salt, and traditional organic matrices are easily inhibited by salt, leading to signal attenuation or even complete loss. This is because the carboxyl group of DHB preferentially binds to Na⁺ (binding constant Ka = 10⁻¹⁰). 3 M⁻ 1 The salt content inhibits the formation of [M+H]⁺. At the same time, the salt causes the α-CHCA crystal form to change to the β form, and the UV absorption cross section decreases by 40%. Desalting treatment will result in a recovery rate of less than 300 Da of water-soluble metabolites.

[0007] In recent years, research has attempted to replace organic matrices with inorganic nanomaterials (such as graphene, titanium dioxide, and zinc oxide), but limitations remain: while graphene-based materials possess high specific surface area and UV absorption capacity, they are prone to forming carbon cluster interference peaks (C0). n ⁻), affecting the detection of low-quality regions; metal oxides (such as TiO2, ZnO) have low background interference, but their selective adsorption capacity for some metabolites is insufficient, and nanoparticles are prone to aggregation, affecting reproducibility; existing studies on magnetic nanoparticles (such as Fe3O4) have mostly focused on adsorption and enrichment functions, without optimizing their laser energy transfer efficiency and small molecule desorption / ionization performance as a matrix, and have failed to break through the bottleneck of synergistic optimization of energy transfer and desorption / ionization.

[0008] In summary, existing MALDI-MS matrix materials suffer from problems such as background interference in the low molecular weight region, uneven crystallization, insufficient ionization efficiency, poor salt resistance, and performance defects of existing nanomaterial substitutes in the detection of small molecule metabolites in trace samples such as tears. There is an urgent need to develop new matrix materials to solve the above technical bottlenecks. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the present invention aims to provide an iron-based nanomaterial, its preparation method, and its application for matrix-assisted laser desorption / resorption spectroscopy (MADS).

[0010] The iron-based nanomaterials provided by the present invention include magnetic iron oxide nanoparticles and a functional modification layer disposed on the surface of the magnetic iron oxide nanoparticles. The magnetic iron oxide nanoparticles possess magnetic responsiveness and photothermal conversion capabilities, and are used to assist in the desorption and ionization of the analyte under laser irradiation. The functional modification layer is used to reduce the interference of salt ions on the ionization process.

[0011] Preferably, the magnetic iron oxide nanoparticles are iron(Fe3O4) nanoparticles or iron-based core-shell structured nanoparticles; the iron-based core-shell structured nanoparticles are Fe3O4@Au core-shell structure, wherein the Au layer thickness is less than 3 nm.

[0012] Preferably, the magnetic iron oxide nanoparticles have a particle size of 10-30 nm; The oxygen vacancy concentration of the magnetic iron oxide nanoparticles is 10. 16 -10 18 The spins / g has an absorption rate of over 70% under 337nm laser and a saturation magnetization of not less than 50 emu / g. The adsorption capacity of the functional modified layer for Na⁺ is less than 0.1 mmol / g.

[0013] Preferably, the functional modification layer is an amino-containing polymer layer; The amino-containing polymer layer is a polydopamine layer or a cysteine-polyethyleneimine copolymer layer; the thickness of the polydopamine layer is 1-3 nm.

[0014] Preferably, the -NH2 coverage of the polydopamine layer is greater than 80%; the -NH2 density in the cysteine-polyethyleneimine copolymer layer is not less than 4 groups / nm. The method for preparing iron-based nanomaterials provided by this invention includes the following steps: Step S1: Magnetic iron oxide nanoparticles are prepared by hydrothermal method, and the magnetic properties and photothermal conversion ability of the magnetic iron oxide nanoparticles are controlled by adjusting the reaction parameters. Step S2: Surface modification of the magnetic iron oxide nanoparticles obtained in step S1 is performed to form a functional modification layer to reduce salt ion interference.

[0015] Preferably, in step S1, the reaction temperature of the hydrothermal method is 180-220℃, and the Fe content in the raw material is... 3 The molar ratio of ⁺ to the complexing agent is 3:1-5:1; the complexing agent is citrate.

[0016] Preferably, in step S2, the surface modification is carried out under conditions where the pH is 8.0-9.0; When the functional modification layer is a polydopamine layer, it is formed by the polymerization of dopamine on the surface of magnetic iron oxide nanoparticles.

[0017] The application of the iron-based nanomaterials provided by this invention in matrix-assisted laser desorption / resorption spectroscopy (MADS) for the detection of small molecule metabolites in trace biological samples includes: The iron-based nanomaterials were mixed with trace biological samples, and the samples were oriented under the assistance of a magnetic field before matrix-assisted laser desorption / resorption spectroscopy analysis was performed.

[0018] Preferably, the trace biological sample is tears, with a single sample volume of less than 10 μL; the molecular weight of the small molecule metabolite is 100-400 Da; the strength of the magnetic field is 0.3-0.7 T, and the action time is 3-7 seconds.

[0019] Compared with the prior art, the present invention has the following beneficial effects: This invention achieves efficient transfer of laser energy to the target molecule by controlling the magnetic properties and photothermal conversion efficiency of Fe3O4 nanoparticles, overcoming the shortcomings of existing inorganic matrices (such as graphene and TiO2) in terms of energy coupling efficiency and desorption selectivity, and breaking through the energy transfer bottleneck of existing nanomatrices. This invention eliminates background interference in the low molecular weight region and avoids fragment peak interference (m / z <700 Da) generated by traditional organic matrices (such as DHB and CHCA) during laser desorption by utilizing the stable photothermal properties of inorganic nanomatrices, thus significantly improving the signal-to-noise ratio (S / N) of small molecule metabolites in the range of 100–400 Da. This invention improves the crystallization uniformity of trace tear samples by utilizing the magnetic response characteristics of Fe3O4 nanoparticles to achieve uniform dispersion of tear samples (<10 μL) on a target plate, overcoming the "sweet spot effect" of traditional organic matrices and ensuring the repeatability and quantitative reliability of detection results. This invention enhances the ionization efficiency of small molecule metabolites by optimizing the surface structure and energy absorption characteristics of Fe3O4 nanoparticles, thereby improving the proton transfer efficiency for polar small molecule metabolites (such as amino acids and organic acids), reducing the limit of detection (LOD), and meeting the high-sensitivity detection requirements of trace tear samples. This invention improves salt tolerance by utilizing the chemical stability and salt interference resistance of Fe3O4 nanoparticles to avoid the inhibition of ionization by high concentrations of salt (such as Na⁺ and K⁺) in tears, thus ensuring stable detection of metabolite signals. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort. Other features, objects, and advantages of the present invention will become more apparent by reading the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a high-angle annular dark field image (HAADF) and elemental distribution analysis diagram of iron-based nanoparticles in an embodiment of the present invention. Figure 2 This is a mass spectrometry signal intensity coefficient of variation (CV) plot for standard metabolites (Glu, Lys, Arg, Glc, Man) in embodiments of the present invention; Figure 3 This is a comparison of the mass spectrometry signal intensities of iron-based nanomaterials with DHB and CHCA in an embodiment of the present invention. Detailed Implementation

[0021] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0022] The terms “first,” “second,” “third,” “fourth,” etc. (if present) in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a particular order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of the invention described herein can be implemented, for example, in orders other than those illustrated or described herein. Furthermore, the terms “comprising” and “having,” and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0023] The technical solution of the present invention will be described in detail below with reference to specific embodiments. These specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described again in some embodiments.

[0024] The technical solutions of the present invention and how they solve the above-mentioned technical problems will be described in detail below with specific embodiments. These specific embodiments can be combined with each other, and the same or similar concepts or processes may not be repeated in some embodiments. The embodiments of the present invention will now be described with reference to the accompanying drawings.

[0025] In the field of tear metabolomics research, traditional MALDI-MS (matrix-assisted laser desorption / ionization mass spectrometry) technology faces three key technical bottlenecks: (1) Matrix interference problem. The background noise (m / z < 700 Da) generated by the existing organic matrix (DHB / CHCA) during laser desorption completely masks the characteristic peaks of key small molecule metabolites (100-400 Da) in tears. Experiments showed that when using the CHCA matrix, the signal-to-noise ratio of glutamate (m / z 148.06) in tears was only 2.3 ± 0.5, which is far lower than the clinical requirement of S / N ≥ 10.

[0026] (2) The problem of micro-sample compatibility. The amount of tear sample collected at one time is usually less than 10 μL, while traditional matrix requires at least 1 μL of sample to form effective crystals. Our test data show that when the sample volume is <0.5 μL, the coefficient of variation (CV) of the DHB matrix is ​​as high as 78%, which completely loses its quantitative value.

[0027] (3) Salt interference effect. The presence of 9.2±1.3 mg / mL NaCl in tears will reduce the [M+H]+ signal intensity of traditional matrices by more than 90%. Especially when detecting m / z 89.04 (alanine), the salt effect causes the detection limit to deteriorate from 1 μM to 100 μM.

[0028] In an embodiment of the present invention, the iron-based nanomaterial for matrix-assisted laser desorption / resorption spectroscopy provided by the present invention includes magnetic iron oxide nanoparticles and a functional modification layer disposed on the surface of the magnetic iron oxide nanoparticles.

[0029] Among them, magnetic iron oxide nanoparticles are used to assist in the desorption and ionization of the analyte, and the functional modification layer is used to reduce the interference of salt ions on the ionization process.

[0030] Magnetic iron oxide nanoparticles can be selected from iron(III) oxide (Fe3O4) nanoparticles or Fe3O4@Au core-shell structured nanoparticles, with the Au layer serving as the outer shell. In actual preparation, the particle size of the Fe3O4 nanoparticles is controlled at 20±2 nm, and X-ray diffraction confirms that they have a spinel structure. In the Fe3O4@Au core-shell structure, the thickness of the Au layer must be strictly controlled within 3 nm to avoid affecting the magnetic responsiveness.

[0031] The functional modification layer is an amino-containing polymer layer, specifically a polydopamine layer or a cysteine-polyethyleneimine copolymer layer. In this embodiment, the polydopamine layer has a thickness of 2 nm, and X-ray photoelectron spectroscopy (XPS) analysis shows that its -NH2 coverage is 92%. If a cysteine-polyethyleneimine copolymer layer is used, the -NH2 density must be at least 5 groups / nm. 2 This modified layer reduces salt interference through chelation. Inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the material's adsorption capacity for Na⁺ is less than 0.1 mmol / g, making it particularly suitable for high-salt samples such as tears.

[0032] These nanoparticles must meet the following performance parameters: the oxygen vacancy concentration, as measured by electron paramagnetic resonance spectroscopy (EPR), must be 1 × 10⁻⁶. 17 With spins / g and a 337nm laser, the photothermal conversion efficiency reached 82%, three orders of magnitude higher than that of traditional matrices. This improved the ionization efficiency of glutamate to (3.2±0.4)×10⁻ 3 It is 100 times better than CHCA, ensuring efficient energy transfer under laser action and achieving dispersion in response to magnetic fields.

[0033] like Figure 1 As shown, the compositional distribution of the iron-based nanoparticles is displayed, with iron in red and oxygen in green. The scale bar is 100 nanometers.

[0034] In this embodiment of the invention, the method for preparing iron-based nanomaterials provided by the present invention includes: Step S1: Magnetic iron oxide nanoparticles are prepared by liquid-phase synthesis, and the magnetic properties and photothermal conversion capabilities of the magnetic iron oxide nanoparticles are controlled by adjusting the reaction parameters. Step S2: Surface modification of the magnetic iron oxide nanoparticles obtained in step S1 is performed to form a functional modification layer to reduce salt ion interference.

[0035] Specifically as follows: Synthesis of magnetic iron oxide nanoparticles: A hydrothermal method (a type of liquid-phase synthesis method) was used, based on Fe... 3 ⁺ With a molar ratio of 4:1 to citrate (range 3:1-5:1), weigh 1.35g FeCl3・6H2O and 0.7g trisodium citrate dihydrate, dissolve in 50mL ethylene glycol, and gently shake at 25℃ for 30 minutes until completely dissolved; Add 3.6g of anhydrous sodium acetate and continue stirring for 15 minutes; transfer the mixture to a 100mL high-pressure reactor and hydrothermally treat it at 200℃ (180-220℃ range) for 10 hours; After natural cooling, the particles were washed three times by alternating centrifugation with anhydrous ethanol and deionized water (8000 rpm, 10 min), and then vacuum dried at 60 °C for 6 hours to obtain Fe3O4 nanoparticles. Transmission electron microscopy (TEM) characterization showed good particle size uniformity.

[0036] Take 0.1 g of dried Fe3O4 nanoparticles, disperse them in 100 mL of Tris-HCl buffer (0.1 M), adjust the pH to 8.5 (range 8.0-9.0), and sonicate for 10 minutes to prepare a 1 mg / mL suspension; Add 0.01g of dopamine and stir magnetically at 30℃ for 24 hours to polymerize dopamine on the particle surface; after centrifugation (8000rpm, 10 minutes), wash three times with deionized water and vacuum dry at 60℃ for 4 hours to obtain iron-based nanomaterials with surface-modified polydopamine.

[0037] The reagents used in the preparation process are as follows: The reagents included ferric chloride hexahydrate (FeCl3・6H2O, purity 99%), trisodium citrate dihydrate (Na3C6H5O7・2H2O, purity 99.5%, as a complexing agent), anhydrous sodium acetate (purity 99%), ethylene glycol (purity 99.5%), and anhydrous ethanol (purity 99.7%), all purchased from Sinopharm Chemical Reagents Beijing Co., Ltd.

[0038] The following are material characterization techniques: (1) Transmission electron microscopy analysis: A JEOL JEM-2100F transmission electron microscope (JEOL Ltd.) was used. 5 μL of material suspension was dropped onto a 200-mesh copper grid and detected by conventional TEM imaging, high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) modes, respectively. (2) X-ray diffraction analysis: The powder samples were tested directly using a D8 Advanced X-ray diffractometer (Bruker, Germany); (3) Scanning electron microscopy analysis: An S-4800 scanning electron microscope (Hitachi, Japan) was used. The sample preparation method was to take 2 μL of suspension and drop it onto the silicon wafer. (4) Observation of crystal morphology: The observation was performed using an Eclipse Ti optical microscope (Nikon, Japan). 1 μL of the suspension was dropped onto a glass slide for observation. (5) Microarray imaging: Images were acquired using a P40 Pro smartphone (Huawei, China), with the substrate being a polished chip surface.

[0039] This material is mainly used for matrix-assisted laser desorption / resorption spectroscopy (MELDS) detection of small molecule metabolites in trace biological samples, with tear samples as an example to illustrate the application process: Take 10 nL of tear fluid (single sample volume less than 10 μL) and mix it with 1 μL of 5 mg / mL iron-based nanomaterial suspension (solvent is 0.1% formic acid water), vortex for 10 seconds; place the mixture in a 0.5T magnetic field (range 0.3-0.7T) for 5 seconds (range 3-7 seconds), and use magnetic responsiveness to achieve directional alignment of nanoparticles; Take 0.5 μL of the mixture and spot it onto a MALDI target plate. After air drying, detect it using a MALDI-TOF mass spectrometer.

[0040] After testing tear and aqueous humor samples from 168 patients, the metabolite characteristics of tear and aqueous humor samples from the experimental and control groups were analyzed within the m / z range of 100-400, verifying the following technical effects: (1) Baseline noise < 500 counts (traditional method > 2000 counts) (2) The half width at half maximum (WWHM) of the characteristic peak is less than 0.2 Da (5 times improvement in resolution) (3) Repeated tests to verify system stability (median CV < 15%) (4) Through the surface plasmon effect of Fe3O4 nanomatrix, the signal-to-noise ratio of the characteristic peak reaches 152:1 (compared to only 18:1 for traditional CHCA matrix). (5) Correlation between tear fluid and aqueous humor test results r 2=0.93 (proving the reliability of cross-sample type detection) In this embodiment of the invention, MALDI-MS technology was used to perform eight independent tests on standard metabolites such as glutamate (Glu), lysine (Lys), arginine (Arg), glucose (Glc), and mannitol (Man). The coefficients of variation (CVs) of the obtained mass spectrometry signal intensity are as follows: Figure 2 As shown below, the diagram demonstrates that the present invention has good stability and uniformity. Figure 3 As shown, comparative analysis of the mass spectrometry signal intensity of iron-based nanoparticles, DHB, and CHCA against standard metabolites demonstrates that the present invention can increase the mass spectrometry signal intensity by 10 to 100 times compared to traditional organic matrices.

[0041] In this embodiment of the invention, the magnetic response characteristics of Fe3O4 nanoparticles (particle size 20±2 nm) were utilized to achieve uniform dispersion (CV<8%) of a 10 nL tear sample under magnetic field assistance, thus solving the "sweet spot effect" problem. TEM characterization showed that these nanoparticles could form monodisperse nanoscale seed crystals in the tear environment. By controlling the oxygen vacancy concentration of Fe3O4 (measured by EPR ~10), the problem was further solved. 17 The photothermal conversion efficiency of glutamate under 337 nm laser light (spins / g) reaches 82%, which is three orders of magnitude higher than that of traditional matrices. This improves the ionization efficiency of glutamate to (3.2±0.4)×10⁻ 3 This represents a 100-fold improvement over CHCA. A polydopamine-modified layer (~2 nm thick) was constructed on the surface of the nanoparticles, selectively excluding Na⁺ interference through its chelating effect. XPS analysis confirmed that this design improves signal stability by 15 times under salt conditions.

[0042] The various embodiments described in this specification are presented in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0043] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. An iron-based nanomaterial, characterized in that, It includes magnetic iron oxide nanoparticles and a functional modification layer disposed on the surface of the magnetic iron oxide nanoparticles; The magnetic iron oxide nanoparticles possess magnetic responsiveness and photothermal conversion capabilities, and are used to assist in the desorption and ionization of the analyte under laser irradiation. The functional modification layer is used to reduce the interference of salt ions on the ionization process.

2. The iron-based nanomaterial according to claim 1, characterized in that, The magnetic iron oxide nanoparticles are iron(III) oxide (Fe3O4) nanoparticles or iron-based core-shell structured nanoparticles; the iron-based core-shell structured nanoparticles are Fe3O4@Au core-shell structure, wherein the Au layer thickness is less than 3 nm.

3. The iron-based nanomaterial according to claim 1 or 2, characterized in that, The magnetic iron oxide nanoparticles have a particle size of 10-30 nm. The oxygen vacancy concentration of the magnetic iron oxide nanoparticles is 10. 16 -10 18 The spins / g has an absorption rate of over 70% under 337nm laser and a saturation magnetization of not less than 50 emu / g. The adsorption capacity of the functional modified layer for Na⁺ is less than 0.1 mmol / g.

4. The iron-based nanomaterial according to claim 1, characterized in that, The functional modification layer is an amino-containing polymer layer; The amino-containing polymer layer is a polydopamine layer or a cysteine-polyethyleneimine copolymer layer; the thickness of the polydopamine layer is 1-3 nm.

5. The iron-based nanomaterial according to claim 4, characterized in that, The polydopamine layer has a -NH2 coverage of greater than 80%; the cysteine-polyethyleneimine copolymer layer has a -NH2 density of not less than 4 groups / nm.

6. A method for preparing the iron-based nanomaterial as described in claim 1, characterized in that, Includes the following steps: Step S1: Magnetic iron oxide nanoparticles are prepared by hydrothermal method, and the magnetic properties and photothermal conversion ability of the magnetic iron oxide nanoparticles are controlled by adjusting the reaction parameters. Step S2: Surface modification of the magnetic iron oxide nanoparticles obtained in step S1 is performed to form a functional modification layer to reduce salt ion interference.

7. The preparation method according to claim 6, characterized in that, In step S1, the reaction temperature of the hydrothermal method is 180-220℃, and the Fe in the raw material... 3 The molar ratio of ⁺ to the complexing agent is 3:1-5:1; the complexing agent is citrate.

8. The preparation method according to claim 6, characterized in that, In step S2, the surface modification is carried out under conditions of pH 8.0-9.0; When the functional modification layer is a polydopamine layer, it is formed by the polymerization of dopamine on the surface of magnetic iron oxide nanoparticles.

9. An application of the iron-based nanomaterial as described in claim 1 in matrix-assisted laser desorption / resorption spectroscopy (MADS), characterized in that, For the detection of small molecule metabolites in trace biological samples; including: The iron-based nanomaterials were mixed with trace biological samples, and the samples were oriented under the assistance of a magnetic field before matrix-assisted laser desorption / resorption spectroscopy analysis was performed.

10. The application according to claim 9, characterized in that, The trace biological sample is tears, with a single sample volume of less than 10 μL; the molecular weight of the small molecule metabolite is 100-400 Da; the strength of the magnetic field is 0.3-0.7 T, and the action time is 3-7 seconds.

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