Gadolinium hydrogel, gadolinium hydrogel coated electrode and method for analyzing microplastics using the same

Abstract

A gadolinium hydrogel includes alginate and gadolinium (Gd); a working electrode for detecting microplastics includes an electrode at least partially coated with the gadolinium hydrogel; and a method for analyzing microplastics includes preparing a container including a working electrode coated with gadolinium hydrogel on at least part thereof, a reference electrode, a counter electrode and an electrolyte, and adding an analysis target aqueous solution containing microplastics into the container and determining a relationship between changes in current according to voltage change by an electrochemical method.

Description

GADOLINIUM HYDROGEL, GADOLINIUM HYDROGEL COATED ELECTRODE AND METHOD FOR ANALYZING MICROPLASTICS USING THE SAMEThis application claims benefit of priority to Korean Patent Application No. 10-2025-0049611 filed on April 16, 2025 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.The present disclosure relates to a gadolinium hydrogel, a gadolinium hydrogel coated electrode, and a method for analyzing microplastics using the same, and more particularly, to a technique for selectively detecting and analyzing nano-sized microplastics with high sensitivity by using an electrode coated with a novel gadolinium hydrogel.Due to its direct relation to human survival, the microplastic problem is being raised as a potential hazardous material. Accordingly, despite the causes and harmfulness of microplastics and the establishment of relevant regulations worldwide, microplastics are continuously generated by easily used daily goods, flow into rivers and seas, are absorbed by marine organisms, and then re-absorbed into the human body through diet.In order to address the issue of microplastics, there is a global movement for reducing plastic use in order to reduce the impact of microplastics, but no measurement method and accurate guidelines have been established for measuring the amount and detecting microplastics. The conventional methods for detecting microplastics include optics-based microscopy, spectroscopy, thermal analysis, thermogravimetric analysis (TGA), etc., and in the current method for quantifying microplastics, the process of microscopic analysis is most widely used, where an experimenter classifies microplastics according to size and shape by directly observing with a microscope.However, microscopy has a limitation that cannot be continuously processed from sampling to analysis of samples, is highly dependent on expertise of a researcher, and error rates depending on actual sample characteristics, an expertise of an experimenter, and longtime due to manual work act as obstacles for real-time measurement and economical methods. In addition, since optical based technology requires use of expensive optical parts and optical alignment, an analysis system may become large in size, and it is difficult to directly apply the optical system to existing continuous water treatment systems.More specifically, analytical techniques such as, for example, ATR-FT-IR may be difficult to interpret the absorption spectra of samples smaller than about 20 μm, environmental factors can affect detection accuracy, and Raman spectroscopy is capable of analyzing small particles in a range of 1 to 20 μm but still has difficulty identifying microplastics due to biological organic matter and inorganic impurities that interfere with fluorescence analysis. Meanwhile, a scanning electron microscope (SEM) can examine microparticles by generating high resolution images, but requires sample coating under high vacuum conditions. In addition, although tag technologies such as fluorescence allow rapid screening of microplastics at low cost, there is a risk of overestimating microplastic abundance by staining organic particles. Therefore, there is an urgent need to develop alternative technologies having high sensitivity and selectivity against these contaminants.Therefore, there is a need for an economical and rapid method of detecting and quantifying microplastics that does not affect the subjectivity of researchers or measurers, and ensures measurement accuracy and objectivity of results.An aspect of the present disclosure is to provide a hydrogel that can be used as an electrochemical sensor for detecting microplastics.Another aspect of the present disclosure is to provide an electrode coated with a hydrogel of the present disclosure.Another aspect of the present disclosure is to provide a method for detecting microplastics using an electrode coated with the hydrogel of the present disclosure.According to an aspect of the present disclosure, a gadolinium hydrogel includes alginate and gadolinium (Gd).According to another aspect of the present disclosure, a working electrode for detecting microplastics includes an electrode at least partially coated with the gadolinium hydrogel of the present disclosure.According to another aspect of the present disclosure, a method for analyzing microplastics includes the operations of: preparing a container including a working electrode at least partially coated with the gadolinium hydrogel of the present disclosure, a reference electrode, a counter electrode and an electrolyte; and adding an analysis target aqueous solution containing microplastics into the container and confirming a relationship between changes in current according to voltage change by an electrochemical method.When the electrode coated with the gadolinium crosslinked alginic acid hydrogel (Gd-Alg) according to the present disclosure is applied as a working electrode, it is possible to detect and quantitatively analyze microplastics using an electrochemical method. Thus, the electrode coated with the gadolinium crosslinked alginic acid hydrogel (Gd-Alg) of the present disclosure can be used as an electrochemical sensor for detecting microplastics, thereby selectively and sensitively detecting and quantifying microplastic components.The above and other aspects, features, and advantages of the present disclosure will be more clearly understood from the following detailed description, taken in conjunction with the accompanying drawings, in which:FIG. 1 schematically illustrates the formation of Gd-Alg hydrogel beads and their coordination.In FIG. 2, part(a) is an FT-IR spectrum of Gd-Alg hydrogel beads in a spectral range of 4000 cm-1to 500 cm-1, and part (b) is an FT-IR spectrum of Gd-Alg hydrogel beads in a spectral range of 800 cm-1to 500 cm-1.Parts (a) and (d) of FIG. 3 illustrate the Raman spectra of sodium alginate (Na-Alg) and Gd-Alg hydrogel beads with various spectral ranges.FIG. 4 shows the results of XPS analysis for a Gd-Alg hydrogel bead. In FIG. 4, part (a) shows a survey scan (entire spectrum), part (b) shows an orbital of carbon element 1s (C 1s), part (c) shows an orbital of oxygen element 1s (O 1s), and part (d) shows an orbital of gadolinium element 4d (Gd 4d).FIG. 5 shows the thermogravimetric analysis (TGA) results of sodium alginate (Na-Alg) and Gd-Alg hydrogel.FIG. 6 shows the results of morphological analysis of Gd-Alg hydrogel beads using a field emission scanning electron microscope.FIG. 7 shows the results of FESEM-EDS analysis for sodium alginate (Na-Alg) and Gd-Alg hydrogel beads. Gadolinium (Gd)-alginate (Alg) hydrogel 'Gd-Alg 5' indicates that it contains 6.1% (atomic %) gadolinium element based on the total sum of gadolinium element, carbon element, and oxygen element.FIG. 8 shows the results of TEM analysis of Gd-Alg 5 hydrogel beads, where in FIG. 8, part (a) shows that the hydrogel beads were formed in a spherical shape, parts (b) and (c) are TEM images at magnifications of 400 k and 590 k, respectively, part (d) shows channels between the hydrogel beads connected with each other, parts (e) to (h) which are related to STEM element mapping, show chemical compositions of elements C, O and Gd, respectively, in a Gd-Alg hydrogel network (h), and part (i) shows that the Gd-Alg 5 hydrogel bead included gadolinium element by 3.9 to 5.8% (atomic %) based on total sum of gadolinium element, carbon element and oxygen element.In FIG. 9, part (a) shows linear sweep voltammetry (LSV) for PS (100 nm) detection using a Gd-Alg hydrogel coated glassy carbon electrode (GCE), and part (b) illustrates cyclic voltammetry (CV) plots.FIG. 10 illustrates linear sweep voltammetry (LSV) plots for detection of various sizes of polystyrene microplastics PS (100 nm, 53 μm and 250 μm) using a Gd-Alg hydrogel coated glassy carbon electrode (GCE).FIG. 11 illustrates the current response of a Gd-Alg hydrogel coated electrode to microplastics mixed with polystyrene (PS) and polyethylene (PE), or polypropylene (PP) and polyethylene (PE).FIG. 12 illustrates a comparison of linear sweep voltammetry (LSV) plots for PS (100 nm) detection using Gd-Alg hydrogel coated glassy carbon electrode (GCE) and linear sweep voltammetry (LSV) plots for PS (100 nm) detection using aloe biochar coated glassy carbon electrode (GCE).In FIG. 13, part (a) is a cyclic voltammetry (CV) plot between -1 to 1 V generated during the electrochemical detection of PS (100 nm) using a Gd-Alg hydrogel material at various concentrations of microplastics, part (b) is a voltage-current linear relationship plot generated during linear sweep voltammetry (LSV), and the resulting calibration curve is illustrated in part (c).Hereinafter, a preferred embodiment of the present disclosure will be described with reference to the accompanying drawings. However, the embodiments of the present disclosure may be modified in various other forms and are not limited to the following embodiments. Hereinafter, for example, "part (a) of FIG. 2" may also be referred to as "FIG. 2(a)".According to the present disclosure, a gadolinium hydrogel is provided and when using the gadolinium hydrogel of the present disclosure, detection and quantitative analysis of microplastics can be performed by an electrochemical method.Hereinafter, the term 'gadolinium hydrogel' of the present disclosure is used interchangeably with 'gadolinium-alginate (Gd-Alg) hydrogel'.More specifically, the gadolinium hydrogel of the present disclosure includes alginate and gadolinium (Gd), wherein metal ion crosslinking in hydrogel formation is performed by gadolinium (Gd). The hydrogel of the present disclosure can provide an effective material for electrochemical sensing by applying a combination of alginate and gadolinium to utilize excellent permeability, capillary effect and high porosity of alginic acid hydrogel with electron transfer capability, high electrical conductivity and thermal stability of gadolinium.The method for preparing the gadolinium hydrogel of the present disclosure is not particularly limited, but for example, an aqueous solution of alginate may be mixed with an aqueous solution of a gadolinium salt to obtain a solid product, and more specifically, may be prepared by dropwise adding an aqueous solution containing alginate of 0.1 to 10 wt%, 0.5 to 5 wt%, or 1 to 2 wt% to an aqueous solution containing gadolinium salt of 0.1 to 5 wt%, 0.2 to 3 wt%, or 0.4 to 2.5 wt%.Herein, the gadolinium hydrogel of the present disclosure may include in an amount of 10 to 300 parts by weight, 20 to 250 parts by weight, or 50 to 150 parts by weight, or 60 to 110 parts by weight of gadolinium based on 100 parts by weight of alginate.As a result, the gadolinium hydrogel of the present disclosure prepared may contain, for example, from 1 to 10 wt% of gadolinium element based on a total sum of gadolinium element, carbon element and oxygen element.Herein, the alginate may include at least one alginate salt of sodium alginate, potassium alginate and ammonium alginate, but is not limited thereto, and is preferably sodium alginate.In addition, the gadolinium salt may include at least one of anhydrous gadolinium chloride (GdCl3), gadolinium chloride hexahydrate (GdCl3·6H2O), and gadolinium nitrate (Gd(NO3)3), but is not limited thereto, and preferably uses gadolinium chloride hexahydrate.The gadolinium hydrogel of the present disclosure is such that alginate is crosslinked with gadolinium to form a network structure.When the electrode coated with the gadolinium hydrogel of the present disclosure is applied as a working electrode, it is possible to detect and quantitatively analyze microplastics by an electrochemical method.According to another aspect of the present disclosure, a working electrode for detecting microplastics including an electrode at least partially coated with the gadolinium hydrogel of the present disclosure, is provided.The working electrode for detecting microplastics according to the present disclosure may be coated with gadolinium hydrogel of the present disclosure on at least a part of an electrode, for example, 50% or more of a surface area of the electrode, preferably, all over the electrode and there is no particular limitation in coating method.For example, the coating can be performed by applying a slurry of gadolinium hydrogel of the present disclosure and proton conducting polymer solution to an electrode, dipping the electrode in the slurry, spin-coating or the like, followed by drying.Herein, the proton conducting polymer solution may be a solution in which a proton conducting polymer such as Nafion is mixed with an organic solvent such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or N-methyl-2-pyrrolidone (NMP) at a concentration of 1 to 20 w / w%, but is not limited thereto.The drying method is not particularly limited and may be performed by natural drying, hot air drying, oven drying or the like, and for example, it can be dried in an oven at a temperature of 50 to 90°C for 10 minutes to 2 hours.The gadolinium-alginate (Gd-Alg) hydrogel coated electrode of the present disclosure can be used as an electrochemical sensor for detecting microplastics and thus may detect and quantify microplastic components selectively and with high sensitivity.Herein, the electrode material used as the working electrode for detecting microplastics is not particularly limited, and for example, the working electrode may be a glassy carbon electrode, platinum, gold or the like.In addition, according to another aspect of the present disclosure, a method for analyzing microplastics using the working electrode for detecting microplastics is provided.More specifically, the method for analyzing microplastics of the present disclosure includes the operations of: preparing a container including a working electrode at least partially coated with the gadolinium hydrogel of the present disclosure, a reference electrode, a counter electrode and an electrolyte; and adding an analysis target aqueous solution containing microplastics into the container and confirming a relationship between changes in current according to voltage change by an electrochemical method.The reference electrode and the counter electrode are not particularly limited in material, but for example, the reference electrode may be a silver / silver chloride (Ag / AgCl) electrode or a saturated calomel electrode (SCE), and the counter electrode may be a gold or platinum electrode.The container may contain the electrolyte solution, but there is no time limit for adding the electrolyte to the container, and for example, it can be added to the analysis target aqueous solution to be analyzed or after adding the analysis target aqueous solution to be analyzed.Herein, the electrolyte may be, for example, at least one selected from sodium chloride (NaCl), hydrochloric acid (HCl), potassium chloride (KCl), calcium chloride (CaCl2), acetic acid (CH3COOH), a PBS buffered solution, and the like.The electrolyte may be included in an amount of 10 to 30 mM concentration in the aqueous solution.When the container is prepared, an analysis target aqueous solution containing microplastics to be analyzed is added thereto, and in this case, the target detectable and quantifiable by the present disclosure is particularly polystyrene microplastic.More specifically, the microplastics detectable and quantifiable with high sensitivity by the present disclosure may be polystyrene having a particle size of 1 nm to 250 μm, or 5 nm to 200 μm, for example, 10 nm to 150 μm, and microplastics of 250 μm or less, for example 100 μm or less, 1 μm or less, less than 1 μm, or 100 nm or less may be selectively detected with high sensitivity.The operation of adding an analysis target aqueous solution containing microplastics into the container and confirming a relationship between changes in current according to voltage change by an electrochemical method is performed, and such a operation is for confirming changes in current generated from the working electrode while changing voltage, wherein the electrochemical method may be cyclic voltammetry (CV) or linear sweep voltammetry (LSV).In the present disclosure, the electrochemical method may be performed at a potential range of 0 to 2 volts (V), and at a scan rate of 50 (mV / s).

[0029] In particular, when the method for analyzing microplastics of the present disclosure is performed using linear sweep voltammetry (LSV), quantitative analysis of microplastics in an analysis target to be analyzed can also be performed.For example, in the case of polystyrene microplastics, linear sweep voltammetry may be performed at concentrations ranging from 1 nM to 1,000 nM and the content which is concentration of microplastics in an analysis target can be estimated by Equation (1):Y = 160.88 + 167.83 X ... Equation (1)where Y is a current (μAmp), and X means the concentration of microplastics (log M).Herein, the operating environment conditions may be set to a range of concentration of microplastics such as polystyrene from 1 nM to 1,000 nM, a pH of neutral, for example, pH 6 to 8, preferably pH 7, an electrolyte concentration of 20 mM NaCl, and a voltage from 0 to 2 V with a scan rate of 50 mV / s.Thus, according to the present disclosure, a gadolinium crosslinked alginic acid hydrogel is provided wherein gadolinium provides electron transfer capability, high electrical conductivity and thermal stability while alginic acid provides good permeability, capillary effect and porosity so that it can be used as an electrochemical sensor for detecting microplastics.The electrode coated with gadolinium crosslinked alginate hydrogel illustrates selectivity for polystyrene microplastics.Hereinafter, the present disclosure will be described in more detail with reference to specific examples. The following examples are only for illustrative purposes and do not limit the scope of the present disclosure.Embodiment1. Preparation of hydrogel and hydrogel coated electrodes(1) MaterialsSodium alginate (Na-Alg) extracted from brown algae was purchased from Sigma Aldrich in China. Gadolinium chloride (III) hexahydrate (GdCl3·6H2O) was purchased from Sigma Aldrich in the United States of America. Xylenol orange disodium salt (C31H30N2Na2O13S) for metal ion detection and Nafion binder were purchased from Sigma Aldrich in Japan. Sodium chloride (NaCl) was purchased from JUNSEI in Japan. Hydrochloric acid (HCl, 35%) was purchased from Daejung in Korea. Polystyrene (PS, 100 nm) microplastics were purchased from Sigma Aldrich in Germany. Highly purified deionized water (DI water) was used to make reaction solutions, and all materials were used without further purification.(2) Preparation of Gadolinium-Alginate HydrogelEmbodiment 1For the preparation of gadolinium (Gd)-alginate (Alg) hydrogel, a gelling solution was prepared by dissolving 1 g of GdCl3·6H2O in 100 ml of deionized water. This is referred to as a '1 w / w% Gd' gelling solution and various concentrations of Gd gelling solutions were made in the same manner, a solution prepared using 3 g of GdCl3·6H2O is referred to as 3 w / w% Gd, and using 5 g is referred to as 5 w / w% Gd.About a 2% (w / w) Na-Alg aqueous solution was prepared by dissolving 2 g of Na-Alg in 100 ml of deionized water (DI). The Na-Alg aqueous solution was stirred at 300 rpm with heating to 70°C temperature to ensure complete dissolution and removal of air bubbles.Na-Alg aqueous solution prepared as described above was put into a syringe and added dropwise to 1 w / w% Gd gelling solution to prepare gadolinium (Gd)-alginate (Alg) hydrogel beads (hereinafter interchangeably referred to as 'Gd-Alg 1'). The Na-Alg aqueous solution prepared in the same manner was put into a syringe and added dropwise to 3 w / w% Gd gelling solution and 5 w / w% Gd gelling solution to prepare various gadolinium (Gd)-alginate (Alg) hydrogel beads (hereinafter interchangeably referred to as 'Gd-Alg 3', and 'Gd-Alg 5' respectively).When the Na-Alg solution was added dropwise to the Gd gel solution, spherical solid beads were formed and settled at the bottom of the beaker. The resulting beads were collected and placed in a dialyzing bag (MW cut off 10k) for dialysis in deionized water for three days with monitoring by Xylenol Orange disodium salt. Finally, the beads were freeze-dried for further analysis.The resulting product was dried and is referred to as 'Gd-Alg hydrogel beads' hereinafter. FIG. 1 schematically illustrates the formation of Gd-Alg hydrogel beads and their coordination.(3) Preparation of hydrogel coated electrodesPreparation Example 1To prepare the Gd-Alg hydrogel coated electrode that can be used as a working electrode, Nafion was mixed in DMF to make 5 w / w % Nafion solution and the Nafion solution was mixed with the Gd-Alg hydrogel beads at 0.1 w / w % to prepare slurry, which was coated on glassy carbon electrode and the coated electrode was put into an oven for 1 hour at 80°C to prepare the Gd-Alg hydrogel coated electrode.Comparative Example 1 - Preparation of an electrode coated with aloe biocharA biocarbon coated electrode was prepared in the same manner as Preparation Example 1, except that aloe biochar was used instead of a Gd-Alg hydrogel, was used.To prepare the aloe biochar coated electrode that can be used as a working electrode, Nafion was mixed in DMF to prepare a 5 w / w % Nafion solution and aloe biochar was mixed with the Nafion solution so that it becomes 0.1 w / w %, thereby preparing a slurry, coating the slurry on a glassy carbon electrode, and placing the coated electrode in an oven at 80°C for one hour to prepare the aloe biochar coated electrode.2. Analysis of physicochemical properties of the hydrogel(1) FT-IR analysis.The structure of the Gd-Alg hydrogel beads was confirmed using ATR-FT-IR (Vertex70 spectrometer (Bruker AXS Corp., Germany)). The scan range, scan rate and resolution were set to 4000 cm-1to 500 cm-1, 64 scans / s, and 4 cm-1, respectively. FT-IR technique is a powerful tool for identification and characterization of polymeric and biopolymeric materials including hydrogels.The results are illustrated in FIG. 2 which illustrates the FT-IR spectra of Na-Alg and Gd-Alg hydrogel beads. In FT-IR spectrum of Gd-Alg hydrogel bead, the broad peak range at 3238 cm-1is assigned to -OH of alginate and absorption band at 2953 cm-1corresponds to -CH stretching vibration of alkane. Absorption peaks at 1593 cm-1and 1415 cm-1are due to COO- asymmetric vibration (asym) and COO- symmetric (sym) vibration of alginate anion. Sharp and strong peak at 1026 cm-1is attributed to C-O-C bond of alginate polymer. Decrease in intensity of -OH peak in FT-IR spectrum of Gd-Alg hydrogel bead indicates that this functional group participates in formation of Gd-Alg hydrogel. Furthermore, shift of COO- asym from 1594 cm-1to 1578 cm-1suggests formation of coordination complex between Gd and COO- leading to formation of Gd-Alg hydrogel. Appearance of a new peak at 557 cm-1due to Gd-O is another indication of successful coordination of Gd with alginate anion polymer.(2) Raman AnalysisRaman analysis of the sodium alginate (Na-Alg) and Gd-Alg hydrogel beads was performed using a Raman microscope (HoribaXploRa). The Raman shift range was applied from 100 to 4000 cm-1with a filter 10% and a laser 532 nm edge. Raman spectroscopy is considered one of the useful techniques to obtain structural information of composite materials.The results are illustrated in FIG. 3 which illustrates the Raman spectra of sodium alginate (Na-Alg) and Gd-Alg hydrogel beads measured at various spectral ranges. The peak observed between 3400 to 3500 cm-1for Na-Alg is assigned to the -OH group of alginic acid. The peaks at 1550 to 1600 cm-1and 1450 to 1500 cm-1are attributed to symmetric stretching and asymmetric stretching of COO- respectively. Several other prominent peaks were also seen near 1320 cm-1, 1240 cm-1, 1190 cm-1and 1085 cm-1corresponding to C-H, C-O, C-OH and C-O-C bonds of alginic acid polymer, respectively. Formation of Gd-Alg hydrogel can be confirmed by the appearance of two new peaks near 490 cm-1and 205 cm-1in the Raman spectrum of Gd-Alg. All other peaks in the Gd-Alg spectrum are similar to those of Na-Alg.(3) XPS analysisX-ray photoelectron spectroscopy (Thermo Fisher Scientific (K-Alpha)) was performed to analyze the elemental composition and chemical state of the surface of the Gd-Alg hydrogel beads. The results obtained by evaluating the elemental composition and chemical state of the surface of the Gd-Alg hydrogel beads using photoelectron peak area and binding energy are illustrated in FIG. 4. FIG. 4(a) illustrates a survey, FIG. 4(b) illustrates carbon element 1s orbital (C 1s), FIG. 4(c) illustrates oxygen element 1s orbital (O 1s), and FIG. 4(d) illustrates gadolinium element 4d orbital (Gd 4d).(4) Thermogravimetric analysis (TGA)Thermal stability of the Gd-Alg hydrogel beads was confirmed using a TGA instrument (SDT model Q50, New Castle, DE). For this analysis, temperature and scanning range were maintained at room temperature to 800°C and 10°C / min, respectively. The sample was run with a nitrogen gas flow rate of 60 mL / min.To confirm the change in thermal stability, sodium alginate (Na-Alg) and Gd-Alg hydrogel beads were analyzed by thermogravimetric analysis (TGA), and the results are illustrated in FIG. 5. According to the results of the TGA analysis, both compounds have three mass loss stages. The first mass loss stage is a region between 25°C and 250°C due to water loss, and in the second stage, Na-alginate illustrates rapid mass loss between 250°C and 280°C due to polymer decomposition, whereas Gd-Alg has an extended temperature range for secondary mass loss (250°C to 350°C), indicating that bonding between alginate and Gd is formed and thermal stability is improved. In the last stage, mass loss occurs due to complete combustion of remaining organic material. The last stage starts at 280°C in the case of Na-alginate, while the last stage starts at 350°C in the case of Gd-Alg hydrogel. The remaining mass of sodium alginate (Na-Alg) and Gd-Alg hydrogel was 30% and 40%, respectively. Overall, it could be confirmed that the formation of hydrogel due to gelling ion (Gd-III) improves the thermal stability of alginate.(5) FESEM analysisThe porous morphology of the Gd-Alg hydrogel beads was confirmed using a field emission scanning electron microscope (Carl Zeiss, SUPRA40VP). The sample was gold sputtered for several seconds to minimize charging. Introduction of Gd(III) into the alginate solution results in rapid formation of hydrogel beads with heterogeneous structure and non-uniform crosslinking density, which is generally higher at the outer core of the hydrogel bead.As a result, referring to FIG. 6, the Gd-Alg hydrogel beads have a "brain like" shape with interconnected channels. The wide pore size distribution is characteristic of M(III)-alginate hydrogels due to three dimensional crosslinking.(6) FESEM-EDX analysisEnergy dispersive x-ray spectroscopy (EDS) was utilized for elemental composition analysis of sodium alginate (Na-Alg) and Gd-Alg hydrogel beads. SEM-EDS analysis was performed to determine the elemental composition of Gd-Alg hydrogel, and results are illustrated in FIG. 7.Referring to the results of SEM-EDS for alginate and Gd-Alg in FIG. 7, it can be confirmed that gadolinium is involved in forming a hydrogel bead. Alginate consists of elements such as carbon, oxygen, and sodium without gadolinium, whereas Gd-Alg includes elements such as C, O, and Gd.(7) TEM analysisA multifunctional transmission electron microscope (FEI(Tecnai-F20 Super twin)) operating at 200 kv was used for transmission electron microscopy (TEM) studies. To understand and predict physical properties, it is necessary to investigate the network structure of polymer materials from macroscopic scale down to nanoscale. For this purpose, transmission electron microscopy (TEM) is one of the promising techniques that directly observe material in real space with nanometer resolution.FIG. 8 illustrates a TEM image of the Gd-Alg for observing nano scale network structure. The TEM image of FIG. 8(a) illustrates that the hydrogel beads are formed into spherical shapes, and FIGS. 8(b) and 8(c) are high magnification TEM images at 400 k (400,000 times) and 590 k (590,000 times) respectively, illustrating the network structure of the Gd-Alg hydrogel formed by three-dimensional crosslinking. FIG. 8(d) illustrates channels between the connected hydrogel beads, and FIGS. 8(e) to (h) associated with STEM elemental mapping illustrate chemical composition of elements such as C, O, and Gd in the Gd-Alg hydrogel network (h), respectively, and FIG. 8(i) illustrates percentages thereof.3. Detection of microplastics(1) Preparation of microplastic contaminantTwo models which are polystyrene (PS), which is microplastics (MPs), and polyethylene terephthalate (PET), were obtained by grinding from food containers and water bottles, respectively, followed by sieving with a 53 μm mesh to obtain uniform-sized microplastic particles. The sieved microplastics were washed in 1M HCl solution at 50°C for 10 h to remove impurities. Then both microplastics were rinsed in deionized water and filtered using vacuum filtration. Finally, the obtained microplastics were dried in a vacuum oven at 80°C for 12 h for further analysis.(2) Experimental Example 1: Use of a Gd-Alg hydrogel coated electrode1) PS (100 nm) nano plastic detection.Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were applied to detect the PS (100 nm) nanoplastics in an aquatic environment.For this electrochemical sensing, the glassy carbon electrode (GCE) coated with Gd-Alg hydrogel prepared in Preparation Example 1 was used as a working electrode, and three electrodes were arranged together with a silver / silver chloride (Ag / AgCl) electrode as a reference electrode and a counter electrode (gold wire), and current generation was measured.As a result, the voltage-current linear relationship generated during linear sweep voltammetry (LSV) is illustrated in FIG. 9(a). The significant current generation indicates the detection capability of the Gd-Alg sensing material. As the concentration of PS (100 nm) increases, the current generation also increases, illustrating that the sensing phenomenon varies with the concentration.FIG. 9(b) illustrates the cyclic voltammetry (CV) plot between -1 and 2 V, which was generated during electrochemical sensing of PS (100 nm) using a Gd-Alg hydrogel material with various concentrations of microplastics. The significant increase in current as the concentration of PS increases from 0.001 μg to 0.003 μg indicates good detection capability. As the concentration of PS increases, the oxidation current and oxidation peak increase as illustrated in FIG. 9(b).2) Confirmation of an effect of the size of microplasticsElectrochemical sensing of Gd-Alg hydrogel coated electrodes was performed for polystyrene (PS) microplastic particles with average particle diameters of 100 nm, 53 μm, and 250 μm to evaluate the effect of size of PS microplastic particles on current-voltage (I-V) graphs of linear sweep voltammetry (LSV).FIG. 10 illustrates current-voltage (I-V) graphs obtained through electrochemical sensing of the Gd-Alg hydrogel coated electrode for polystyrene (PS) microplastics of various sizes. As the size of the polystyrene (PS) microplastic particles decreases, the electrochemical reaction increases, suggesting high sensitivity to smaller sized polystyrene (PS) microplastics.3) Selectivity analysisTo analyze the selectivity of the electrode coated with a Gd-Alg hydrogel for polystyrene (PS) microplastic sensing, electrochemical sensing of microplastics including polypropylene (PP) and polyethylene (PE) was performed by linear sweep voltammetry (LSV). Herein, microplastics having an average particle size of 250 μm were used.FIG. 11 illustrates the current response of a Gd-Alg hydrogel coated electrode to microplastics mixed with polystyrene (PS) and polyethylene (PE), or polypropylene (PP) and polyethylene (PE). As a result, after adding polyethylene (PE) or polypropylene (PP) and polyethylene (PE), the electrochemical reaction was significantly reduced, confirming high selectivity of the Gd-Alg hydrogel coated electrode for detecting polystyrene (PS).(3) Comparative Experimental Example 1: Detection of PS (100 nm) nanoplastics using an electrode coated with aloe biocharAn electrode of Comparative Preparation Example 1, which is a biochar coated electrode prepared in the same manner as Preparation Example 1 except that aloe biochar was used instead of Gd-Alg hydrogel.Linear sweep voltammetry (LSV) was applied to detect the PS nanoplastics (100 nm).As a result, for the electrochemical sensing, a glassy carbon electrode (GCE) coated with Gd-aloe biochar prepared in Comparative Example 1 was used as working electrode and three electrodes were arranged including reference electrode (Ag / AgCl) and counter electrode (gold wire), and current generation was measured.The resulting linear voltage-current relationship generated during the linear sweep voltammetry (LSV) is illustrated in FIG. 12.The Gd-Alg hydrogel coated electrode suggests higher sensitivity for polystyrene (PS) microplastics than the Gd-aloe biochar coated electrode.4. Quantitative analysis of polystyrene microplasticsIn order to prepare a calibration curve for detecting the PS (100 nm) nanoplastics, linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were applied to detect the PS nanoplastics (100 nm).For the electrochemical sensing, a glassy carbon electrode (GCE) coated with Gd-Alg hydrogel prepared in Preparation Example 1 was used as working electrode and three electrodes were arranged including reference electrode (Ag / AgCl) and counter electrode (gold wire), and current generation was measured.FIG. 13(a) illustrates cyclic voltammetry (CV) between -1 and +1 V, which was generated during the electrochemical sensing of PS (100 nm) using the Gd-Alg hydrogel material with various concentrations of microplastics (1 nM, 10 nM, 100 nM, 1 μM, 10 μM, and 100 μM). The significant increase in current as the concentration of PS increases from 1 nM to 100 μM indicates excellent sensitivity. As the concentration of PS increases, the oxidation current and oxidation peak also increase as illustrated in FIG. 13(a).Meanwhile, FIG. 13(b) illustrates a voltage-current linear relationship plot generated during the linear sweep voltammetry (LSV), indicating that as the concentration of PS (100 nm) increases, current generation also increases and thus the sensing phenomenon varies with the concentration. As a result, a calibration curve drawn at voltage from 0 to 2 V is illustrated in FIG. 13(c).When the method for analyzing microplastics of the present disclosure is performed by using such a calibration curve, quantitative analysis of microplastics in an analysis target (a sample to be analyzed) can be achieved by applying linear sweep voltammetry (LSV).More specifically, in the condition of polystyrene microplastic concentration range of 1 nM to 1,000 nM, a linear sweep voltammetry is performed and then the amount of microplastics in an analysis target can be estimated by the following Equation (1)Y = 160.88 + 167.83 X ... Equation (1)where Y is a current (μAmp), and X means the concentration of microplastics (log M). The operating environment conditions of linear sweep voltammetry were set as follows:Polystyrene microplastic concentration range: 1 nM to 1,000 nM.pH: Neutral (pH 7).Electrolyte: 20 mM NaCl.Voltage: 0 to 2 V.Scan rate: 50 mV / s.By substituting the current measured at voltage of 0 to 2 V in Equation (1) with Y, it is possible to obtain the concentration of microplastics X (log M).While example embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.

Claims

1. A gadolinium hydrogel comprising alginate and gadolinium (Gd).2.The gadolinium hydrogel according to claim 1, wherein the alginate and gadolinium (Gd) are included in an amount of 10 to 300 parts by weight of gadolinium, based on 100 parts by weight of alginate.3.The gadolinium hydrogel according to claim 1, wherein the alginate comprises at least one of sodium alginate, potassium alginate, and ammonium alginate.4.The gadolinium hydrogel according to claim 1, wherein alginate is crosslinked with gadolinium to form a network structure.5.A working electrode for microplastic detection, comprising an electrode at least partially coated with the gadolinium hydrogel according to any one of claims 1 to 4.6.The working electrode for microplastic detection according to claim 5 wherein the working electrode is a glassy carbon electrode.7.A method for analyzing microplastics comprising the operations of:preparing a container including a working electrode at least partially coated with the gadolinium hydrogel according to any one of claims 1 to 4, a reference electrode, a counter electrode, and an electrolyte; andadding an analysis target aqueous solution containing microplastics into the container and confirming a relationship between changes in current according to voltage change by an electrochemical method.8.The method for analyzing microplastics according to claim 7 wherein the reference electrode is a silver / silver chloride (Ag / AgCl) electrode, and the counter electrode is a gold or platinum electrode.9.The method for analyzing microplastics according to claim 7, wherein the microplastic is polystyrene having a particle size from 1 nm to 250 μm.10.The method for analyzing microplastics according to claim 7, wherein the electrolyte is included in an amount to be a concentration of 10 mM to 30 mM in an aqueous solution.11.The method for analyzing microplastics according to claim 7 wherein the electrochemical method is cyclic voltammetry (CV) or linear sweep voltammetry (LSV).12.The method for analyzing microplastics according to claim 11, wherein the electrochemical method is performed at a potential range of 0 to 2 volts (V), and at a scan rate between 40 and 60 (mV / s).

1. The method for analyzing microplastics according to claim 7, further comprising calculating the content of microplastics in an analysis target aqueous solution by using Equation (1):Y = 160.88 + 167.83 X ... Equation (1)wherein Y is a current (μAmp), and X is a concentration (log M) of microplastics.14.The method for analyzing microplastics according to claim 13 wherein the operation of calculating a content of microplastics in an aqueous solution is performed at conditions of pH 7, electrolyte concentration of 20 mM NaCl and voltage from 0 to 2 V.

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