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were measured by the ion X-7 series inductively coupled plasma-mass spectrometer (ICP-MS). (Thermo Elemental, Cheshire, UK). (The contents of In. 3+...
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Applied Chemistry

Luminescent hybrid of Tb3+ functionalized metal-organic frameworks act as food preservative sensor and water scavenger for NO2Jing-Xing Wu, and Bing Yan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00762 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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Luminescent hybrid of Tb3+ functionalized metal-organic frameworks act as food preservative sensor and water scavenger for NO2Jing-Xing Wu, Bing Yan* School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China

* Corresponding author at: [email protected] (Bing Yan) ACS Paragon Plus Environment

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ABSTRACT

Luminescent Tb3+ functionalized metal-organic frameworks (MOFs) are prepared and act as food preservatives sensor and water scavenger for NO2-. Classical metal-organic frameworks with uncoordinated N atoms in pores are elected as carrier to encapsulate Tb3+ ions. This Tb3+-incorporated material reveals excellent characteristic green luminescence of Tb3+ and good fluorescence stability in water. Subsequently, we choose this probe for sensing NO2- among several food preservative compounds, showing a highly sensitive capability for detection of NO2-; it is then proved that the Dexter energy transfer (DET) causes the luminescent quenching between Tb3+ and NO2-, achieving the detection of NO2-. This probe is also employed to detect the NO2- in real water samples and act as water scavenger to remove the NO2- in drinking water, showing a good removal capacity 3.45 mg (75.0 µmol) of NO2- per gram of particles. KEYWORDS

Nitrite ions, hybrid material, terbium ions, indium-based metal-organic frameworks, fluorescence probe.

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INTRODUCTION

Nowadays, food preservatives are widely used in food, among which nitrite is an important member with a long history. Since nitrite can prevent the production of clostridium botulinum - a kind of toxic microorganism, it truly improves the safety of meat products.[1-4] Besides, nitrite, combining with myoglobin can be more stable, so it is often used to make meat good-looking and fresh.[5-7] However, some unscrupulous merchants will use high content nitrite in order to seek high profits, which easily lead to poisoning and cause intestinal cancer or gastric carcinoma.[8-9] So, it is of great practical significance to measure the existing content of NO2- in drinking water or daily foods. [10-12] Though now there are plenty of methods to detect nitrite, including the classic chromatography, electrochemical detecting, colorimetric detection and so forth, [13-16] the methods of nitrite detection are still very limited. For they usually require professional personnel, sophisticated instruments, tedious and timecausing operation. Recently some reporting methods have the advantages that are more practical and effective. For instances, self-assembly of gold nanoparticles are performed to apply in nitrite detection; the up-conversion of nanoparticles are employed in fluorescence test for the nitrite. [17-19] However, the visual and highly selective probes are still worthy of research and development, simple and rapid preparation of these probes are still basic requirement. In recent decades, metal organic frameworks (MOFs) with diverse and adjustable structures as well as rich pores, composed of metal ions and organic ligands, [20-22] are extremely popular materials receiving continuing and up-to-date attentions. It is highly free to choose appropriate metal ions and proper ligands to assemble into MOFs. This high degree of autonomy and creativity makes MOF gradually develop into an extended family of materials, thus they are employed in various applications such as drug transport [23-25], gas storage [21, 26-27] and sensing [28-30]. There are kinds of ways for MOFs to generate luminescence, which are explained as “antenna effect”: organic ligands excitation; charge-transfer as well as metal-centered emission.[31-32] As a special luminescent MOF, Ln-MOF [33-35] refers to the encapsulation of lanthanide ions through ions exchange, in-situ synthesis [36-37] or post-synthesis modification (PSM)[38-40]. This type of MOF is favored in the field of solid-state light, ACS Paragon Plus Environment

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combining the structural advantages of traditional MOFs with the advantages of lanthanide ionizing light.[35, 41-43] For instance, MIL-121 combined with Eu3+ can perform as sensitive luminescent probe to detect Ag+ effectively, and the detection time is as low as 5 min and detection limit reaches 0.1 µM (µmmol/L);[44] and high sensitivity of Fe3+ in aqueous solutions is also exhibited by the fluorescence probe based on Eu3+-MIL-53-COOH.[45] However, the application of Ln-MOFs to detect food preservatives, especially NO2- in aqueous solutions, is still very limited. Although Qi etc. have reported the U-Tb-OBBA, [46] which can be used to sense the NO2- in aqueous solutions, they do not test further whether it can detect the NO2- among food preservatives. Therefore, more efforts about studying and developing Ln-MOFs are still needed to pay for detecting NO2- among food preservatives. In light of the above consideration, a kind of typical MOF [33, 47-48] is employed to perform as a carrier and "antenna" to encapsulate lanthanide ions (Tb3+).[49-51] The ligand of this MOF contains structural units of bipyridine, whose above N atoms have extra coordination ability to support combination with other rare earth ions, which makes PSM achievable to obtain Ln-MOF. This luminescence probe is proved that it is suitable for the detection of NO2- in aqueous solution because its luminescence and structure can remain stable in water for several days. Besides, a kind of fast and effective sensing plate is designed to detect NO2-, and the ability of adsorbing and removing NO2- of the probe is also testified in the water.

EXPERIMENTAL SECTION

Chemicals The chemicals involved in this work were commercially available and used directly without any treatment or purification. In(NO3)3·6H2O and the ligand H2bpydc were purchased from Aldrich. Tb2O3 was mixed with concentrated HNO3 and H2O2, the mixture was then evaporated and crystallized to synthesize the salts of Tb(NO3)3·6H2O. Physical characterization

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The powder X ray diffraction (PXRD) images of MOF powder were collected from Bruker D8 diffractometer under CuKa radiation, the recording was taken at 2 theta within 5-50° range at 40 mA and 40 kV, the scanning speed was 0.10 seconds, and the step size was 0.02°. Fourier Transform Infrared Spectroscopy (FTIR) was recorded in the KBr tablet using a Nexus 912 AO446 infrared spectrometer in the 3500-500 cm-1 range. Thermogravimetric analysis (TGA) was carried out in the Netzsch 449C STA system at a temperature rate of 5° C min−1, which was heated from 40 °C to 800 °C in an alumina crucible within nitrogen atmosphere. Scanning electron microscopy (SEM) images of HitachiS-4800 using field emission gun were collected at 10 µA and 2 kV. The surface areas of materials were recorded by the classical method of Brunauer-Emmett-Teller (BET). The contents of In3+ and Tb3+ were measured by the ion X-7 series inductively coupled plasma-mass spectrometer (ICP-MS) (Thermo Elemental, Cheshire, UK). (The contents of In3+ and Tb3+ were measured by standard solution method, firstly, a series of solutions containing deuterium ions and indium ions were prepared respectively. Then samples were dissolved in concentrated nitric acid and diluted with water. After the samples were prepared, the concentration of ions was determined by using a standard curve.) Edinburgh Analytical Instrument FLS920 was employed to obtain fluorescence spectra. Using a microsecond (100 mW) lamp as excitation source, lifetime measurements were determined on an Edinburgh Instruments FLS920 fluorescence spectrometer and lifetime data was collected from fitting the experiment luminescent decay. The luminescent quantum efficiency was measured using an integrating sphere (150 mm diameter, BaSO4 coating) from Edinburgh FLS920 phosphorimeter. Preparation of In(OH)bpydc. In(OH)bpydc was synthesized in light of the previously reported work. [48] At beginning, 409 mg In(NO3)3·6H2O and 244 mg H2bpydc are dissolved adequately in 20 mL Dimethyl formamide (DMF), and the mixture was transferred to a Pyrex tube container (50 mL). After adequate stirring, the container was gradually heated to 160 °C and kept stable for 48 h. Then, the crude product was cooled down naturally and centrifuged, following by washing up with DMF and acetonitrile for several times, then

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the product was then moved in vacuum environment for drying at 80 °C under around 12 h. At last, white product In-MOF was collected. Preparation of Tb3+@In-MOF. Tb3+ ions were loaded to In-MOF via post-synthetic method. [48] The compounds 100 mg In-MOF (0.26 mmol) and 100 mg Tb(NO3)3·6H2O (0.26 mmol) were mixed in 20 mL methanol, and they were transferred to a 40 mL tube and then heated at the stable temperature of 60 °C for around 24 h. After that, the product was then centrifuged several times to collect products completely. After 24 h, acetonitrile was used to wash the solids about two times, followed by soaking in acetonitrile for 4 d. Finally, the white solids were centrifuged and dried in vacuum environment at the temperature of 80 °C. Luminescence sensing experiments of NO2- ions. The fluorescent detection of NO2- was carried out at room temperature in de-ionized water. Some mainstream food preservative ingredients were dissolved into DI-water and prepared into a concentration of 0.5 mmol/L aqueous solutions (sodium benzoate (BA), potassium sorbate (PC), sodium diacetate (SDA), calcium propionate (CP), sodium salicylate (SS), sodium deacetate (SA)), NO2- and H2O were selected as the sensing object and blank experiment, respectively. Subsequently, 3 mg of Tb3+@In-MOF and 3 mL of 0.5 mmol/L above aqueous solutions were added into a 4 mL centrifuge tube, and then the mixture was equilibrated thoroughly for 5 min by ultrasound processing. After that, the luminescent experiments were performed. Removal of NO2- in aqueous solution. Nitrite was severally dissolved in water to obtain the ultimate concentration (0, 0.5, 1, 1.5, 2, 2.5, 4, 5, 8, 10, 12, 15, 20, 25, 30, 40, 50 and 60 mM) and each total volume was 5 mL. These above solutions were mixed with Tb3+@In-MOF (1 mg) and reacted for approximately 15 min, after that, the mixture was centrifuged and removed the precipitate, supernatant was collected to determine the concentration of NO2-.

RESULTS AND DISCUSSION ACS Paragon Plus Environment

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Characterization of In-MOF and Tb3+@In-MOF. In this work, we choose the MOFs with uncoordinated points as the carrier for loading Tb3+ due to several main reasons. Firstly, the preparation of In-MOF resembled by In(NO3)3 and H2bpydc is easy and time-saving, whose satisfactory structural and fluorescent stability in aqueous solutions are proved later. Secondly, pores of this MOFs are sufficient and they can absorb kinds of small molecules and metals ions, Tb3+ ions. Thirdly, the extra charges of uncoordinated N atoms of bipyridine can coordinate Tb3+ easily, allowing itself to act as a carrier. Finally, thanks to the “antenna effect”, Tb3+ can be sensitized effectively by MOFs and produce pure green emission. To demonstrate our strategy, we prepare this MOFs through solvothermal method and determine its PXRD, as shown in Fig. S1 (ESI), suggesting that it is highly consistent with the simulated one of report. [48] Fig. S2 (ESI) tells the molecular structure model of In-MOF and reveals that octahedra of InO6 are inter-linked by ligands bpydc2- to form thousands of pores. After that, we further prepare the luminescent Tb3+ loaded In-MOF via postsynthetic method (PSM). The PXRD (Fig. S1, ESI) of Tb3+@In-MOF matches well with that of In-MOF, illustrating that the structure of In-MOF can remain stable after loading Tb3+ by PSM. The SEM images (Fig. S3, ESI) reveals the morphology of In-MOF and Tb3+@In-MOF, they are observed as cuboid crystals with crystal sizes in the range of 10–30 µm. Elemental mapping of Tb3+@In-MOF is shown in Fig. S4a-b, ESI, also suggesting that Tb3+ is encapsulated into MOFs successfully. Loading level of Tb3+ into MOFs is tested by ICP-MS analysis, and the data is measured in Table S1, ESI, revealing the atomic% of In3+/Tb3+ is 1:0.267. N2 adsorption-desorption isotherms determine their BET surface, as depicted in Fig. S5, ESI, the BET surface of Tb3+@In-MOF decreases from 586 m2/g (In-MOF) to 371 m2/g, some -COOH in the channel are coordinated with the Tb3+ ions and a certain degree of special volume is occupied and the BET surface decrease. The FTIR spectra are similar to each other among the In-MOF and Tb3+@In-MOF, as given in Fig. S6, ESI. The peaks at 785 and 1049 cm-1 can be found due to the deformation vibrations of the C-H in benzene of the FTIR spectra, and the band at 1670 cm-1 are attributed to the C=O asymmetric

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stretching vibrations, and because of the existences of carboxylic group, the peak (1355 cm-1) is caused by the C-N stretching, which proving the existence of bipyridine. Luminescence properties of In-MOF and Tb3+@In-MOF. As given in Fig. S7a, ESI, we determine the luminescent property of In-MOF at room environment. Under the emission at 480 nm, the excitation spectra of In-MOF is firstly recorded, which exhibits a wide peak located at around 390 nm. The blue luminescent color can be observed by CIE chromaticity diagram of Fig. S7b (ESI), whose value is X = 0.24, Y = 0.36, which also can be confirmed by the inset photograph of Fig. S7b, ESI. This luminescent color can be explained by the transfer of ligand-to-metal charge. We further detect the luminescence of Tb3+@In-MOF, as depicted in the Fig. S8a (ESI), we can easily find the four intense peaks of Tb3+ at 488, 545, 582, 625 nm under excitation at 320 nm due to transitions of 5D4→7FJ (J = 6, 5, 4, 3), leading to the marked green emission under the UV light when the excitation wavelength is 320 nm, which is shown in Fig. S8b (ESI). The green luminescence is proved vividly by the chromaticity diagram of CIE (X = 0.30, Y = 0.58) as well as the inset photograph. This phenomenon can be attributed to that the lanthanides of Tb3+@In-MOF are sensitized efficiently by energy transfer, the so called “antenna effect”. This luminescent exam also confirms the successful uploading of Tb3+ into MOFs. Lifetimes of luminescence (t) and quantum yields (ψ) of Tb3+@In-MOF are 0.315 ms and 34.14%, as shown in Table. S1, ESI. For the study of stability of Tb3+@In-MOF, the compound is evenly immersed into the aqueous solution for several days whose luminescent intensity and PXRD are displayed in the Fig. S9a-b (ESI). In the luminescent curve, the relative intensity of Tb3+ peaks remain well for 7 days, and the PXRD of Tb3+@In-MOF illustrates that the framework structure remains almost stable. Both of above considerations suggest that the Tb3+@In-MOF can meet the requirement of being probe -being stable in targeted solution. Detection of NO2- ions.

Due to the above evidence, it can be proved that the structural stability of the Tb3+@In-MOF is excellent and the luminescent property is satisfactory. We further examine the potential sensing capacity of Tb3+@In-MOF to detect NO2-. The research of NO2--sensing function is conducted by immersing 8 ACS Paragon Plus Environment

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Tb3+@In-MOF in various food preservatives aqueous solutions (sodium benzoate (BA), potassium sorbate (PC), sodium diacetate (SDA). calcium propionate (CP), sodium salicylate (SS), sodium deacetate (SA)). The luminescent properties recorded and compared in Fig. 1a, indicate that the difference of compounds has variant influence on the luminescence intensity of Tb3+@In-MOF. Compared with these results, a striking phenomenon is that only NO2- has the distinct quenching effect on the fluorescence and leads to a dark emission, while luminescent emission of others can be observed clearly. Considering the Fig. 1b, the photograph suggests that only NO2- causes quenching effect and leads to the dark luminescent. To make clear observation and comparison, we compare the Tb3+ characteristic transitions at 545 nm with each other, as shown in Fig. 1c. The relative luminescent intensity of probe in NO2- aqueous solution can be ignored virtually, whereas fluorescent intensity of the larger one (SA) is about 32 times greater than that of NO2-. Furthermore, a temporal scan of the luminescent intensity of Tb3+@In-MOF at 545 nm after adding NO2- is performed to examine the response time of Tb3+@In-MOF. As given in Fig. S9, (ESI) a sharp reduction occurs in the luminescent emission after NO2- addition and the intensity levels off after 10 min, showing that the probe responds to the target fast. The quantum yield and luminescence stability of the probe in different compounds is then detected, the luminescence of probe remains stably after 3 days, as shown in the Table S2. To make NO2- sensing simple and practical, we achieve it by our designed test device based on filter paper and cover of centrifuge tube, as shown in Fig. S10 (ESI). Paper is firstly put into the cover of tube, then it is dissolved in suspensions with the mixture of Tb3+@In-MOF and different various food preservatives for 5 min, and later they are allowed to dry at room temperature so that fluorescent test paper can be obtained. Only the test paper in the NO2- solution can be discriminated well due to its dark color under the light of UV at 320 nm. According to the results described previously, our naked eye can distinguish the NO2-, allowing the NO2- test much easier and more convenient. Sulfite, also a familiar oxyacid salt, is once used as a food preservative, yet is harmful to health and prohibited by law now. In order to compare the different effect between NO2- and SO32-, the fluorescent probe Tb3+@In-MOF is immersed into SO32- solution. As a result, the luminescent record is shown in ACS Paragon Plus Environment

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Fig. S11a (ESI). SO32- has no significant effect on the fluorescence of the probe, and there is a distinct emission of Tb3+ characteristic peaks. From the graph Fig. S11b, the fluorescence intensity at 545 nm is 70.5 times than that of NO2-, which indicates that SO32- does not affect the NO2- sensing test. Simultaneously, anti-interference is tested via adding extra NO2- ions (1 mL, 0.5 mmol/L) into these compound solutions, respectively. As shown in Fig. S12, the luminescence of various solutions all quench greatly, thus leading to a conclusion that this Tb3+@In-MOF probe has great selectivity and sensitivity to NO2-. To better understand the luminescence quenching that caused by NO2-, concentration-dependent researches in the luminescence properties of Tb3+@In-MOF in NO2- solutions are carried out, as described in Fig. 2a. With the existence of NO2- ions, the intensity of Tb3+@In-MOF decreases gradually with the increase of NO2- content from 0 µM to 500 µM. The peak intensity of 545 nm declines swiftly and linearly from 0 µM to 70 µM, followed by a gradual decrease to 150 µM, it tends to be stable eventually. It can be observed that there is a clear linear relationship between the luminescence intensity of Tb3+@In-MOF and the content of NO2-. This quenching effect can be rationalized quantitatively within the range of NO2- concentration from 0 µM to 70 µM, and we can draw a fitting linear equation with the slope Ksv = -5.3×104 and the linear correlation coefficient R = 0.993, which indicates a marked quenching effect to the luminescence of Tb3+@In-MOF. Effect and possible mechanism of NO2- ions on the luminescence of this probe. To date, the mechanism about quenching effect of luminescent Ln-MOF is not clearly understandable, but whether the original structure is intact or not should be considered firstly because the energy transfer will not occur due to the destructive structure. [52-55] PXRD of Tb3+@In-MOF collected from NO2solution is tested, as shown in Fig. S13. The Tb3+@In-MOF does not collapse and the structure is not damaged by NO2-. So, we attribute the luminescent quenching effect of Tb3+@In-MOF by NO2- to the interaction of Tb3+ with NO2-, which leads to energy transfer. There are several reasons. Nitrite ion is a rich electron system, with electron-donating ability (a nucleophile), while the Tb3+ has empty orbit, and both will form coordination bond easily, as shown in Fig. 3. And earlier literatures have reported that ACS Paragon Plus Environment

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D4 energy level of Tb3+ (20500 cm−1) matches well with the T1 energy of NO2- (18958 cm−1), which

can make the energy transfer from Tb3+ to NO2- achievable.[56-57] Emission lifetime of Tb3+@In-MOF in the presence of NO2- is 0.272 ms, which is less than that in absence of NO2- (0.315 ms), suggesting that the interaction between excited-state NO2- and Tb3+ leads to the decrease of fluorescent lifetime, and consequently decreases the radiative transition of Tb3+. Apart from this, previously, it is reported that the existence of Dexter energy transfer between Tb3+ and NO2- makes the NO2- often serve as a luminescent quencher for the Tb3+. [58] Detection and removal of NO2- in several water samples. As shown in Table 1, natural samples (water collected from pond, tap and lake) are used to analyze and confirm the capability of the luminescent sensor probe for NO2- test in practical application. The outcomes illustrate that the fluorescent probe Tb3+@In-MOF can be employed effectively to analyze NO2- in natural samples. Apart from this, the scavenging capacity of Tb3+@In-MOF to NO2- is further examined, as given in Fig. 4. When the added NO2- concentration is less than 15 mM, no NO2- in supernatant is found, indicating that NO2- is completely removed by Tb3+@In-MOF. The surplus of NO2- in supernatant increases with the added NO2- when its concentration is more than 15 mM. Thus, the removal capacity of Tb3+@In-MOF is approximately 3.45 mg (75.0 µmol) of NO2- per gram of particles. This value exceeds the standard limits of NO2- in drinking water set by the WHO and China (65.1 µmol, 21.7 µmol) respectively. [59-60] Therefore, the scavenging capacity of Tb3+@In-MOF is enough to clear NO2- in drinking water.

CONCLUSIONS

In summary, we design a fluorescent probe using the MOFs photo-functionalized with green Tb3+ and employ this probe to sense NO2- ions among several food preservatives. Briefly, we obtain MOF by hydrothermal synthesis high temperature and high-pressure conditions and Tb3+ ions are then loaded to MOF via post-synthetic method. And a kind of fast and effective sensing plate is designed to detect NO2-, and this probe can also act as adsorbent to adsorb and remove NO2- in the water, whose removal ACS Paragon Plus Environment

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capacity is approximately 3.45 mg (75.0 µmol) of NO2- per gram of particles. According to the satisfactory results of the actual sample test, we believe that this fluorescent probe has real application value and can be put into entrepreneurial production with the future development.

ACKNOWLEDGMENT

This work is supported by the National Natural Science Foundation of China (21571142), and the Developing Science Fund of Tongji University.

ASSOCIATED CONTENT

Supporting Information Available: Luminescent hybrid of Tb3+ functionalized metal-organic frameworks act as food preservative sensor and water scavenger for NO2-.

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Figure legends Figure 1 (a) Luminescence intensity of the Tb3+@In-MOF dispersed into different food preservatives aqueous solutions (excited monitored at 320 nm). (b) Corresponding photographs under UV-light irradiation at 320 nm. (c) Luminescence intensity of the 545 nm of Tb3+@In-MOF dispersed into different food preservatives aqueous solutions. Figure 2 (a) Emission spectra and (b) Ksv curve of Tb3+@In-MOF in aqueous solutions in the presence of various concentrations of NO2- under excitation at 320 nm. Figure 3 Scheme of Tb3+@In-MOF coordinates with NO2-, leading to energy transfer and fluorescent quenching. Figure 4 Removal capacity of Tb3+@In-MOF to NO2- solution.

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Figure 1

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Table 1 Detection of NO2- in several natural samples using the standard addition method Added concentration of NO2- (µM) 5 50

Detected concentration of NO2-a (µM) 5.09± 0.21 50.58 ± 1.29

RSDb (%)

Lake water

5 50

5.21 ± 0.18 50.81 ± 1.37

3.45 2.70

Pond water

5 50

5.18 ± 0.23 50.85 ± 1.36

4.44 2.67

Water samples

Tap water

4.13 2.55

a The result is expressed as the mean of four measurements ± standard deviation (SD). b The relative standard deviation (RSD) is defined as (SD/mean) ×100%.

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