Titanium-based Metal-organic Framework Research Articles

Catalytically Active Ti-Based Nanomaterials for Hydroxyl Radical Mediated Clinical X-Ray Enhancement.

Nanoparticle radioenhancement offers a promising strategy for augmenting radiotherapy by locally increasing radiation damage to tumor tissue. While past research has predominantly focused on nanomaterials with high atomic numbers, such as Au and HfO2, recent work has revealed that their radioenhancement efficacy decreases considerably when using clinically relevant megavoltage X-rays as opposed to the orthovoltage X-rays typically employed in research settings. Here, radiocatalytically active Ti-based nanomaterials for clinical X-ray therapy settings are designed. A range of candidate materials, including TiO2 (optionally decorated with Ag or Pt nanoseeds), Ti-containing metal-organic frameworks (MOFs), and 2D Ti-based carbides known as Ti3C2Tx MXenes, is investigated. It is demonstrated that these titanium-based candidates remain consistently performant across a wide energy spectrum, from orthovoltage to megavoltage. This sustained performance is attributed to the catalytic generation of reactive oxygen species, moving beyond the simple physical dose enhancements associated with photoelectric effects. Beyond titania, emergent materials like titanium-based MOFs and MXenes exhibit encouraging results, achieving dose-enhancement factors of up to three in human soft tissue sarcoma cells. Notably, these enhancements are absent in healthy human fibroblast cells under similar conditions of particle uptake, underscoring the selective impact of titanium-based materials in augmenting radiotherapy across the clinically relevant spectral range.

Photocatalytic degradation of methylene blue using a Cu2+-modified bimetallic titanium-based metal organic framework (MIL-125) photocatalyst with enhanced visible light activity.

Cu-modified TiO2 nanoparticles derived from MIL-125 were prepared by solvothermal method for the photocatalytic degradation of methylene blue under visible light illumination. For boosting the photocatalytic performance as well as the physicochemical properties of bare sample, 2 wt % Cu2+ ions were integrated into the nodes of the MIL-125 framework. The results showed that incorporation of 2 wt % Cu2+ ions into the MOF framework had significant effects on the crystallographic structure and morphological and optical properties of photocatalytic samples, as well as catalytic activity for the methylene blue degradation reaction. The high activity profile of Cu-modified TiO2 nanoparticles derived from MIL-125 might be attributed to the increased thermal stability, lower band gap energy, and smaller crystallite size of the sample. Activity tests were carried out at five varying MB initial concentrations and four different pH values. According to the findings, an increase in initial dye concentration resulted in a decrease in degradation efficiency. It was observed that increasing the pH value in the range of 3-11 initially led to higher degradation rates until pH 7, after which the degradation rate began to decline.

Porous Bimetallic Ti-MOFs for Photocatalytic Oxidation of Amines in Air.

A family of microporous titanium-containing metal-organic frameworks (denoted as M2Ti-CPCDC, M = Mn, Co, Ni) has been synthesized by using a bimetallic [M2Ti(μ3-O)(COO)6] cluster and a tritopic carbazole-based organic ligand H3CPCDC. M2Ti-CPCDC are stable and display permanent porosity for N2 and CO2 uptake, ranking among the most porous titanium-based metal-organic frameworks. M2Ti-CPCDC crystals exhibit n-type semiconductor behavior. Further catalytic studies demonstrate that all M2Ti-CPCDC materials are applicable for triggering photo-oxidative reactions of amines in air. More specifically, amines with electron-donating groups afford the aldehydes as the main products, while amines bearing electron-withdrawing groups give rise to imines as the main product. Among them, Mn2Ti-CPCDC exhibit the best photocatalytic activity, with conversion of benzylamine up to 99% and selectivity of 99%. Mn2Ti-CPCDC could be recycled in at least three runs while retaining crystallinity and catalytic activity. The reaction mechanism indicates that photoinduced hole (h+), superoxide radical anion (O2·-), and singlet oxygen (1O2) are the main active species involved in the photo-oxidation process.

Synergistic effect of oxygen vacancies and functionalized ligands selectively enhanced the photocatalytic oxidation of methane to carbon monoxide

The titanium-based metal–organic framework MIL-125(Ti) has been widely investigated in photocatalytic carbon dioxide reduction and water splitting, but rarely studied in photocatalytic methane conversion by employing only water as an oxidant under mild conditions. Simultaneously controlling products with high yield and selectivity is highly challenging in methane conversion. Herein, we develop a series of functionalized titanium metal–organic frameworks via a facile organic ligand exchange process. The abundant Ti3+ active sites induced by oxygen vacancies and functionalized ligands synergistically facilitate the adsorption and activation of methane molecules. The carbon monoxide yield over NH2-MIL-125(Ti) increased to 198.6 μmol·gcat−1·h−1 with a high selectivity of 87.1 % under simulated solar illumination. Combined with in-situ characterizations and density functional theoretical calculations, the results confirmed that Ti3+ active sites induced by oxygen vacancies and amino functional groups synergistically enhanced the photocatalytic conversion of methane and elucidate CH4-to-CO conversion mechanism. This study does not only provide insights into the rational design of metal–organic frameworks with copious oxygen vacancies and functional groups, but also provides some significant cognition for photocatalytic methane conversion.

Titanium metal-organic frameworks for photocatalytic CO2 conversion through a cycloaddition reaction.

The elevated levels of CO2 in the atmosphere have been a major concern for environmental scientists. Capturing CO2 gas and its subsequent conversion to useful organic compounds is one of the avenues that have been extensively studied in the last decade. The photocatalytic cycloaddition of CO2 is a promising approach for effective CO2 capture and the production of value-added chemicals such as cyclic carbonates. MOF-901, a titanium-based metal-organic framework with hexagonal layers and imine linkages, was successfully oxidized in this study to MOF-997, incorporating amide linkages using Oxone. Both MOFs displayed remarkable photocatalytic activity in CO2 cycloaddition under mild conditions, including moderate temperatures and visible light exposure. Particularly noteworthy is MOF-997, exhibiting superior performance with donor-acceptor active sites, achieving a 99.9% yield in catalyzing CO2 conversion from styrene epoxide to styrene carbonate under solvent conditions.

An Ester-Functionalized Bidentate Titanium-based Metal-Organic Framework with Visible-Light Enhanced Activity for CO2 Photoreduction.

The limited visible light response is a critical drawback that hampers the photocatalytic efficacy of Ti-MOFs. However, study concerning the enhancement of the visible-light response of Ti-MOFs is still in its nascent stage. In this study, we employ the 'dual-ligand decrystallization strategy' to manipulate the electronic environment of Ti4+, leading to the synthesis of three ester-functionalized bidentate Ti-MOFs with enhanced visible light response. Our findings reveal that this approach not only reduces the bandgap of Ti-MOFs but also enhances their photocatalytic activity for carbon dioxide reduction. Specifically, compared to the bandgap of Ti-BPDC at 2.98 eV, the bandgap of Ti-BPDC-CA 1 : 2 has been reduced to 2.14 eV. Moreover, Ti-BPDC-CA 1 : 2 exhibits extraordinary photocatalytic activity with the formic acid (HCOOH) production rate of 617 μmol g-1 h-1 with over 99.5 % selectivity, which is 3.47 times higher than that of Ti-BPDC. Besides providing a cost-effective strategy for enhancing the visible light response of Ti-MOFs, our study also serves as an illustrative example for establishing the correlation between electronic structure and optical properties.

A Titanium-Based Metal-Organic Framework For Tandem Metallaphotocatalysis.

Metal-organic frameworks (MOFs) have garnered substantial attention for their unique properties, such as high porosity and tunable structures, making them versatile for various applications. This paper constructs photoactive titanium-organic frameworks by combining Ti(IV) clusters and a bipyridine linker. The MOF is synthesized in situ through imine condensation, resulting in NU-2300. Subsequent ex situ nickel salt complexation results in NU-2300-Ni, which is then used for light-mediated carbon-heteroatom cross-couplings. The photophysical properties of the metallaphotocatalyst were investigated by UV-vis and EPR analyses, and both the Ti cluster and the bipyridine linker were found to contribute to successful catalysis, making it a tandem catalyst. The heterogeneous material retained its performance through five cycles of thioetherification. This work contributes not only to MOF synthetic strategies but also to expanding MOF applications as recyclable, tandem metallaphotocatalysts.

Fabrication of the NH2-MIL-125(Ti)@TpPa-NO2 hybrid material and its efficient adsorption and removal of rhodamine B in wastewater

In this work, the hybrid material NH2-MIL-125(Ti)@TpPa-NO2 was synthesized by grafting an amino-functionalized titanium-based metal–organic framework (NH2-MIL-125(Ti)) with 2,4,6-tri(4-pyridyl)-1,3,5-triazine-nitro (TpPa-NO2) moieties. The resulting composite material exhibited remarkable removal efficiency owing to the synergistic effects derived from the combination of the NH2-MIL-125(Ti) and TpPa-NO2 components. The material demonstrated excellent stability and recyclability, making it a promising candidate for the treatment of wastewater containing rhodamine-B (Rhd-B). Some characterization techniques were employed to elucidate the structural features and surface morphology of the hybrid material. The adsorption capacity of NH2-MIL-125(Ti)@TpPa-NO2 for Rhd-B dye was evaluated, and the results revealed an exceptional removal efficiency of 99.96 % and a high adsorption capacity of 25.54 mg/g, highlighting the superior adsorption performance of the material. The adsorption process was found to follow Langmuir isotherm and pseudo-second-order kinetics, indicating that chemisorption is the predominant mechanism governing the uptake of Rhd-B dye onto NH2-MIL-125(Ti)@TpPa-NO2.

A new sensitive layer based on clcinated Zn/Ti-MOF/magnetic molecularly imprinted polypyrrole: Application to preconcentration and electrochemical determination of perfluorooctane sulfonic acid by magnetic carbon paste electrode

In this research, a new approach based on magnetic electrodes has been developed for the determination of per-fluorooctane sulfonic acid (PSOF). Zinc and Titanium-based Metal-organic framework (MOF) was synthesized and used with polypyrrole as a conductive polymer for preparation of the absorbent to achieve the best performance for electrochemical application. The response of the electrode for determination of the PFOS was affected and optimized by different factors such as buffer solution, pH of the solution, amount of absorbent, extraction time of absorbent, accumulation time, as well as the Square Wave Voltammetry (SWV) instrumental parameters including voltage step, pulse amplitude, frequency and resting time. In the optimum condition, the response of the Zn/Ti/C-MOF-magnetic molecular imprinting polymer/carbon paste electrode (MOFMMIP/CPE) has increased logarithmically by increasing the concentration in the range of 0.002–165 μM by the limit of detection (LOD) of 0.7 nM. The obtained percentage of Recovery (96.00–104.14 %), Bias (–4.00 - 4.14 %) and Relative Standard Deviation (RSD) (1.89–3.74 %) for determination of the PFOS in real and spiked tap water, river water and well water samples demonstrates that the proposed method has an acceptable precision. The comparison between the gained data by the presented method and High-performance liquid chromatography (HPLC) showed that the presented method has high accuracy.

Catalytic open metal sites in Ti-MOFs obtained through plasma treatment for oxidative desulfurization

Titanium-based metal–organic frameworks (Ti-MOFs) are known for their redox catalytic potential, inherently limited by coordination saturation. Defect engineering introduces the unsaturated metal sites, but introducing defects for improved performance is challenging due to titanium's hydrolysis tendency and the absence of stable Ti-clusters. This study employs an Ar plasma post-synthesis treatment, adjusting defect quantities by varying duration and intensity. The resulting plasma-induced defective MUV-10, with nearly 50 % missing linkers per cluster, displays heightened catalytic activity in oxidative desulfurization. This activity directly correlates with missing linker numbers, emphasizing the superior activity of defective open sites. Defect formation enhances intrinsic activity around unsaturated metal catalytic sites, rather than increasing active site density. This defect-generation method is adaptable to other Ti-MOFs and potentially to diverse MOFs.

MIL-125-NH2 functionalized chitin scaffold as dual sorbent and photocatalyst filter for degradation of trimethoprim

A porous, photocatalytically active, and water-stable composite filter has been developed based on MIL-125-NH2, a Titanium-based Metal-Organic Framework (MOF) and chitin polymeric scaffold. In order to tune the structure and functional properties of the CH/MOF composites, the loading degree of the MOF within the polymer was systematically varied. The composite material optimal performance was achieved with a 10% wt/wt loading of MIL-125-NH2 into the chitin scaffold. This composite material was able to capture and photodegrade Trimethoprim (TMP) under acid and alkaline conditions with outstanding efficiency (82% within 180 min), as well as in waters collected from different geographical locations nearby Buenos Aires that were spiked with TMP. Furthermore, the chemical stability of MIL-125-NH2 is significantly improved when incorporated into the chitin scaffold, maintaining its structural integrity even after four cycles of reactivation and reusability. The study outlines the experimental procedures for synthesizing MIL-125-NH2, preparing chitin hydrogel and nanostructured composites, and details on the adsorption and photocatalysis experiments. The practical application of our approach can be further expanded to prevent degradation during gas or liquid adsorption processes of hydrolytically unstable MOFs.

CdS quantum dots sensitized Ti-MOFs as high-performance photocathodic materials for immunoassay of alpha-fetoprotein based on Ag+ exchange strategy

Cathodic photoelectrochemical (PEC) analysis has drawn much attention because of its excellent anti-interference capability toward reductive substances in samples, but the development high-performance photocathodic materials is challenging. We report here CdS quantum dots (QDs) sensitized titanium-based metal–organic frameworks (Ti-MOFs) as high-performance photocathodic materials for ultrasensitive immunoassay of alpha-fetoprotein (AFP) based on Ag+ exchange strategy. Due to the excellent sensitization effect of CdS QDs, the photocathodic current of CdS QDs decorated MIL-125-NH2(Ti) is 25-fold that of MIL-125-NH2(Ti) alone. The photocathodic current of CdS/MIL-125-NH2(Ti) can be quenched after Ag+ exchange, because CdS is partially converted to Ag2S that serves as a charge-recombination center. Based on the PEC response of CdS/MIL-125-NH2(Ti) to Ag+, a cathodic PEC immunoassay platform is constructed for AFP detection by using CdS/MIL-125-NH2(Ti) as the photoactive material and Ag nanoparticles modified SiO2 nanospheres as labels. The linear range for AFP detection is from 1.0 × 10−4 to 1.0 × 102 ng mL−1, and the detection limit is 20 fg mL−1. The constructed PEC immunosensor is successfully used for detecting AFP in human serum samples. In this work, a novel and high-performance photoactive material is developed for cathodic PEC analysis, and a simple and effective PEC immunoassay platform for AFP detection is constructed based on Ag+ exchange strategy.

Titanium-based metal-organic frameworks as pH-responsive drug delivery carriers of 5-Fluorouracil

In this study, we synthesized two titanium-based metal-organic frameworks (Ti-MOFs), namely MIL-125 and MIL-125-NH2, known for their excellent biocompatibility, as potential carriers for the anti-cancer drug 5-Fluorouracil (5-FU). We conducted comprehensive examinations of their morphologies, particle size distributions using SEM, stability, permanent porosity, and in vitro drug loading and release behaviors. Both materials demonstrated significant advantages due to their excellent stability, large surface area, and nanoscale sizes, which resulted in a high capacity for loading 5-FU. Significantly, both MIL-125 and MIL-125-NH2 consistently displayed pH-responsive release of 5-FU. This result may not only help to enhance the therapeutic efficacy of 5-FU but is also expected to improve its targeting potential. Our current research suggests that Ti-MOFs hold great promise as carriers for 5-FU in cancer treatment, offering guidance for the design and development of new customized and responsive drug delivery system.

Hydrophobic Porphyrin Titanium-Based MOFs for Visible-Light-Driven CO2 Reduction to Formate.

Three hydrophobic porphyrin titanium-based metal-organic frameworks (MOFs) (HPA/DGIST-1, DPA/DGIST-1, and OPA/DGIST-1) were synthesized through a postsynthetic coordination reaction by using alkylphosphonic acid of different lengths (HPA, hexylphosphonic acid; DPA, dodecylphosphonic acid; OPA, octadecylphosphonic acid). Compared with the hydrophilic DGIST-1, modified DGIST-1 exhibits excellent hydrophobicity and presents good stability in humid atmospheres. Due to the introduction of porphyrin ligands, HPA/DGIST-1, DPA/DGIST-1, and OPA/DGIST-1 showed good visible-light absorption (380-700 nm) and sensitive photogenerated charge responses. When acted as catalysts, these hydrophobic Ti-MOFs can selectively reduce CO2 to HCOO- under visible-light irradiation with average reaction rates of 150.9, 178.5, and 228.3 μmol·h-1·g-1, where these values are 1.3-2.0 times higher than the system mediated by the initial porphyrin Ti-MOF catalyst. 13C NMR spectroscopy demonstrates that the catalytic product HCOO- anion originates from the reactant CO2. The photocatalytic experiments, electron paramagnetic resonance, and photoluminescence spectra tests showed that porphyrin ligands and Ti-O units can act as catalytic activity centers to realize the conversion of CO2 to HCOO-. This work demonstrated that the combination of porphyrin titanium-based MOF and alkyl hydrophobic groups is an effective way to enhance the stability of titanium-based MOFs and maintain their high photocatalytic performance.

High adsorption of ammonia in a titanium-based metal-organic framework.

We report the high adsorption of NH3 in a titanium-based metal-organic framework, MFM-300(Ti), comprising extended [TiO6]∞ chains linked by biphenyl-3,3',5,5'-tetracarboxylate ligands. At 273 K and 1 bar, MFM-300(Ti) shows an exceptional NH3 uptake of 23.4 mmol g-1 with a record-high packing density of 0.84 g cm-3. Dynamic breakthrough experiments confirm the excellent uptake and separation of NH3 at low concentration (1000 ppm). The combination of in situ neutron powder diffraction and spectroscopic studies reveal strong, yet reversible binding interactions of NH3 to the framework oxygen sites.

Synthesis and Analysis of Titanium Based Metal Organic Frameworks for Photocatalysis

Metal-Organic Frameworks (MOFs) are porous inorganic networks that have recently shown potential in energy applications as photocatalysts. The porosity of MOFs make them excellent for guest-host interactions, however, most MOFs are slightly insulating due to metal oxide nodes, resulting in larger band gaps and poor conductivity. In order to combat these issues, post-synthetic modifications (PSM) can be used to slightly alter MOF properties. In this study, PSM’s were introduced to Titanium-based MOFs with MIL topology, such as MIL-125 and MIL-125-NH2, to improve their ability to photocatalytically degrade organic dyes through the addition of polyaniline nanofibers and additional metal interactions. The performance of these modified MOFs were tested and compared to the unmodified MOFs. In addition, titanium-based MOFs, were investigated because their nodes that are more semiconducting than other MOF types. Each MOF was characterized using BET surface area analysis, infrared spectroscopy, and powder X-ray diffraction. The conductivity of different MOFs were compared using cyclic voltammetry. The relative photocatalytic properties of each MOF was determined by monitoring their efficiency for photocatalytic degradation of rhodamine B and phenol red dyes in solution under controlled environments.

Silver Nanoparticle-Loaded Titanium-Based Metal-Organic Framework for Promoting Antibacterial Performance by Synergistic Chemical-Photodynamic Therapy.

The abuse of antibiotics leads to an increasing emergence of drug-resistant bacteria, which not only causes a waste of medical resources but also seriously endangers people's health and life safety. Therefore, it is highly desirable to develop an efficient antibacterial strategy to reduce the reliance on traditional antibiotics. Antibacterial photodynamic therapy (aPDT) is regarded as an intriguing antimicrobial method that is less likely to generate drug resistance, but its efficiency still needs to be further improved. Herein, a robust titanium-based metal-organic framework ACM-1 was adopted to support Ag nanoparticles (NPs) to obtain Ag NPs@ACM-1 for boosting antibacterial efficiency via synergistic chemical-photodynamic therapy. Apart from the intrinsic antibacterial nature, Ag NPs largely boost ROS production and thus improve aPDT efficacy. As a consequence, Ag NPs@ACM-1 shows excellent antibacterial activity under visible light illumination, and its minimum bactericidal concentrations (MBCs) against E. coli, S. aureus, and MRSA are as low as 39.1, 39.1, and 62.5 μg mL-1, respectively. Moreover, to expand the practicability of Ag NPs@ACM-1, two (a dense and a loose) Ag NPs@ACM-1 films were readily fabricated by simply dispersing Ag NPs@ACM-1 into heated aqueous solutions of edible agar and sequentially cooling through heating or freeze-drying, respectively. Notably, these two films are mechanically flexible and exhibit excellent antibacterial activities, and their antimicrobial performances can be well retained in their recyclable and remade films. As agar is nontoxic, degradable, inexpensive, and ecosustainable, the dense and loose Ag NPs@ACM-1 films are potent to serve as recyclable and degradable antibacterial plastics and antibacterial dressings, respectively.

Pushing the heterometal doping limit while preserving long-lived charge separation in a Ti-based MOF photocatalyst

This study explores the nature, dynamics, and reactivity of the photo-induced charge separated excited state in a Fe3+-doped titanium-based metal organic framework (MOF), xFeMIL125-NH2, as a function of iron concentration. The MOF is synthesized with doping levels x = 0.5, 1 and 2 Fe node sites per octameric Ti-oxo cluster and characterized by powder x-ray diffraction, UV-vis diffuse reflectance, atomic absorption, and steady state Fe K-edge X-ray absorption spectroscopy. For each doping level, time-resolved X-ray transient absorption spectroscopy studies confirm the electron trap site role of the Fe sites in the excited state. Time scan data reveal multiexponential decay kinetics for the charge recombination processes which extend into the microsecond range for all three concentrations. A series of dye photodegradation studies, based on the oxidative decomposition of Rhodamine B, demonstrates the reactivity of the charge separated excited state and the photocatalytic capacity of these MOF materials compared to traditional heterometal-doped semiconductor photocatalysts.

A novel nanogold sol SERS/RRS dual-mode assay for trace imidacloprid with Ti3C2Tx@Ti-MOFs/peptide bifunctional nanoprobe

The development of rapid, simple, and highly sensitive analytical methods for detecting pesticide residues is crucial for safeguarding the environment and human health. In this study, MXeneTi3C2Tx nanosheets are used to load a titanium-based metal-organic framework (Ti-MOFs) and imidacloprid peptide (PTIMI), and a novel bifunctional nanoprobe (TN@Ti-MOFs/PT) is prepared by irradiation in a domestic microwave oven and stirring at room temperature. Powder X-ray diffraction, transmission electron microscope and Fourier transform infrared spectrometer are used to characterize the prepared nanomaterials. It is found that TN@Ti-MOFs/PT not only specifically recognizes imidacloprid (IMI), but also catalyzes the nanogold indicator reaction to generate gold nanoparticles (AuNP) in an 80 ℃ constant temperature water bath. The generated AuNP can be directly used as an optical probe for resonance Rayleigh scattering (RRS) at wavelength of 370 nm. In the presence of the Victoria blue 4 R molecule probe, it shows a robust surface enhanced Raman scattering (SERS) signal at wavelength number of 1617 cm−1. When the target molecule IMI is added, it can interact with the PTIMI specific base sequence, resulting in PT chain shedding from the TN@Ti-MOFs/PT nanoprobe surface, exposing abundant redox sites and accelerating the redox charge transfer, resulting in AuNP increase and SERS/RRS signal enhancement. As a result, a highly sensitive and specific PT-based dual-mode biosensing platform is developed for the detection of trace IMI, with a linear range of 0.01–0.25 ng/mL and a detection limit of 0.0011 and 0.0039 ng/mL by SERS and RRS assays, respectively. To demonstrate its practical application, the platform is used to detect IMI in vegetable samples, with relative deviations of 1.36–5.86% and recoveries of 95.0–107%. Additionally, the mechanism of nanocatalysis enhancement is also investigated.

A novel sandwich-type electrochemical immunosensor for sensitive detection of zearalenone using NG/PDDA/HNTs and Ti-MOF-KB composites for signal amplification

In this work, a novel sandwich-type electrochemical immunosensor based on signal amplification was developed for the ultrasensitive detection of zearalenone (ZEA). The composite consisting of poly(diallyldimethylammonium chloride) functionalized nitrogen doped graphene, halloysite nanotubes and toluidine blue (Tb/NG/PDDA/HNTs), was synthesized for the first time. Then it was modified with gold nanoparticles (AuNPs) to bind the secondary antibody (Ab2, 10 μg mL−1) and form the tracer label. In addition, ketjen black (KB) was doped into titanium-based metal–organic framework (Ti-MOF), which provided large specific surface area and employed as the sensing platform to increase the immobilization of the primary antibody (Ab1, 10 μg mL−1). This immunosensor showed a wide linear range for ZEA from 10 fg mL−1 to 100 ng mL−1 with a limit of detection (LOD) as low as 0.57 fg mL−1, which was below the maximum tolerable levels (50∼100 µg kg−1) set by the United Nations Food and Agriculture Organization (FAO).