Metal-organic framework loaded by rhodamine B as a novel

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Metal-organic framework loaded by rhodamine B as a novel chemiluminescence system for the paper-based analytical devices and its application for total phenolic content determination in food samples Javad Hassanzadeh, Haider A. J. Al Lawati, and Iman Al Lawati Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b01862 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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Analytical Chemistry

Metal-organic framework loaded by rhodamine B as a novel chemiluminescence system for the paper-based analytical devices and its application for total phenolic content determination in food samples Javad Hassanzadeh, Haider A.J. Al Lawati*, Iman Al Lawati Department of Chemistry, College of Science, Sultan Qaboos University, Box 36, Al-Khod 123, Oman * E-mail address: [email protected] (H.A.J. Al-Lawati)

ABSTRACT: Herein, a novel paper-based chemiluminescence (CL) device has been reported for the estimation of total phenolic content of food samples. The CL system implemented on the paper was based on hydrogen peroxide (H2O2)-rhodamine b (RhoB)cobalt metal organic framework (CoMOF) reaction. It was found that the reaction of H2O2 with RhoB molecules, loaded into the nano-pores of CoMOF (R@CoMOF), can produce an intensive CL emission. The experiments on the paper indicated that in the presence of CoMOF, the CL emission was greatly increased. In addition to this strong catalyzing effect, application of CoMOF on the paper improved the stability of CL system for several days. As a useful analytical application for the obtained paper-based CL device (PCD), it was examined for the detection of phenolic antioxidants. It was observed that the addition of 5 µL of phenolic compounds (PC) on the paper containing the CL reagents can remarkably decrease the CL intensity. This effect was applied to design a simple analytical assay for PC. After optimization process, the best sensitivity was obtained for gallic acid, quercetin, catechin, kaempferol and caffeic acid with detection limits of 0.98, 1.36, 1.48, 1.81 and 2.55 ng mL-1, respectively. The relative standard deviations (RSD%) were also less than 5%. This study is the first report on the practical application of PCD using a nanomaterial assisted CL reaction. It is simple, portable, low-cost and consumes a very low amounts of reagents and sample solution. The device was successfully applied for the investigation of total antioxidant capacity of molasses and honey samples.

Antioxidants, as the potent free radical scavengers, debilitate destructive effects of reactive oxygen and nitrogen species in human body.1 In spite of the positive effects of free radicals on the cellular responses and immune functions, their high concentration is the major reason for oxidative stress.1,2 Most of degenerative, chronic diseases, and also cancers are potentially generated along of oxidative stress.2 Antioxidants are used as the main key to moderate the concentration of free radicals in body, and improve the immune security.1,3 Therefore, they have a useful role in decreasing the probability of many perilous diseases. Phenolic compounds (PC) have been known as the major participator to the antioxidant capacity of food products of plant origin.3 PC have gained a high scientific interest, due to their preventing effect on lipoprotein oxidation, allergies, inflammation and cancer.4 Consuming a diet rich in PC is of great importance to improve the antioxidant level in blood and neutralize the free radicals.4 So, assessment of PC can be useful for ranking the antioxidant capacity of foods and choosing the finest diet.3 There are several reports on the measurement of PC level and antioxidant capacity of various food products.5-7 However, most of them have some deficiencies, like drastic interferences, which cause serious problems in obtaining reliable results. Also, some of reported methods are timeconsuming and need deep knowledge.8 Simple, rapid, and low-cost optical sensors have been accepted as the useful methods for the on-site determination of PC.9-11 The main attention has recently focused on the chemiluminescence (CL) techniques,12-17 which provide

exceptional advantages, including quite simple instrumentation, broad linear ranges and relatively high selectivity, as well as great sensitivity and excellent detection limits. On the other hand, nanomaterial-assisted CL systems have successfully progressed in recent decays to further improve the classic CL systems and get best detection sensitivities.18 This attractive technique offers outstanding scaffolds for the creation of analytical assays in food analysis, diagnosis, environmental research, etc.18,19 Novel metal-organic frameworks (MOFs) are porous inorganic polymers containing cationic central parts, which connected by certain multi-dentate organic bridges.20 The specific structure of MOFs causes individual properties, including high ordered porosity with tunable pore sizes, great surface areas, high stability and optional functionality. These properties give the MOFs a special place in different fields, especially in catalysis and sensor developments.21-25 Application of MOFs in CL reaction has been also reported in the literature.26, 27 For example, Zhu research group have reported the great catalyzing effect of copper-based MOF (HKUST-1) on the luminol-H2O2 CL reaction, which led to a significant enhancement in the CL intensity up to about 90fold.26 Also, Yang et al. showed that the creation of a peroxide analogous complex between the oxygen-related radicals and the active metal site of the cobalt-MOF, led to an enhanced CL emission from luminol.27 On the other hand, there is a special interest toward the miniaturization of detection systems to obtain portable sensors for on-site screening analysis. Paper-based analytical devices

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(PADs) are one of the suitable and available techniques, which have gotten more and more interest.27 The low cost, biodegradability, high availability and outstanding chemical compatibility of paper, as well as high simplicity of PADs, are the attractive advantages, which caused their fast progress in the last decade.29 Besides, simple, sensitive and rapid CL technique has been verified to be a potent detection tool for PADs. In recent years, several studies have been reported on the development of paper-based CL devices with exceptional advantages.30-33 These reports are mainly on immunoassays,30, 31 or detection of biomolecules.32,33 The detection process often contains an enzyme-catalyzed reaction. For example, the oxidation of uric acid in the presence of urate oxidase produces H2O2, which has been then utilized for the CL generation.32 The application of nano-assisted CL systems in PADs is rarely reported.34, 35 Also, there is no practical report on the application of paper-based CL devices for the estimation of antioxidant capacity of food products. As a potent analytical platform, paper-based CL analytical device (PCD) provides a simple, portable and low cost tool for food quality control, and can further promote the commercialization of assays. This study reports the application of paper-based CL analytical devices to identify PC in some food products, and estimate their total antioxidant capacity. The CL system was based on the hydrogen peroxide (H2O2)-rhodamine B (RhoB) reaction, as well as the novel catalyst based on cobalt-imidazole MOF (CoMOF). No notable CL emission could be observed as the result of RhoB oxidation by H2O2. However, adding H2O2 on the RhoB molecules loaded into the pores of CoMOF led to an intensive CL emission. The experiments on the paper indicated a great increase in the CL intensity, so it was completely suitable for analytical aims. In addition to the great

catalyzing effect, application of MOF on the paper guaranteed the stability of CL reagents for several days. Moreover, the developed PCD was examined for the determination of PC. It was found that addition of PC in a trace level, could selectively decrease the CL intensity. This effect was linearly related to the concentration of PC, and used as the basis to design a simple and sensitive assay for the measurement of PC (Scheme 1). The method had a good reproducibility and was successfully applied for the investigation of total antioxidant capacity of molasses and honeys in Oman. EXPERIMENTAL SECTION Apparatus and chemicals. The CL intensities were recorded using a photomultiplier tube (PMT, Hamamatsu H7155-2, Japan) with 8 mm detection area. Fluorescence and absorption data were obtained by a spectrofluorometer (PerkinElmer LS 55, USA) and a spectrophotometer (Shimadzu UV-1800, Japan), respectively. The characterization analyses applied for the investigation of morphology and chemical structure of prepared MOF included: scanning electron microscopy (JSM7600F Schottky FE-SEM, JEOL, Tokyo, Japan), Infrared (IR) spectrometry (Cary 630 FTIR, Agilent technologies, USA) and X-ray spectroscopy using PANalytical's X'Pert PRO diffractometer (United Kingdom) with a Cu Kα exciting source ( λ=1.5405 Å). Also, nitrogen adsorption/desorption isotherms at 77 K (using BELSORP-miniII analyzer, Particle Test Pty Ltd, Bel Japan Inc., Japan) were applied for the investigation of the porosity and Brunauer-Emmett-Teller (BET) surface area of prepared MOF.

Scheme 1. Paper-based CL device for the determination of phenolic compounds, and the related CL mechanism


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Analytical Chemistry

The applied chemicals in all experiments were obtained in analytical grade and used without further purification. Deionized water (DI, Millipore, MilliQ water system) was used in all experiments. Rhodamine B, hydrogen peroxide (H2O2), cobalt(II) nitrate hexahydrate (Co(NO3)2.6H2O), and luminol were purchased from Merck Co. (Germany). Imidazole was obtained from Aldrich (Germany). Sodium hydroxide (NaOH) and Folin-Ciocalteu reagent was bought from Sigma (USA). The phenolic compounds (PC), including gallic acid (GA), kaempferol (KF), caffeic acid (CA), syringic acid (SGA), quercetin (QC), catechin (CT), rutin (RT), alizarin (AZ), salicylic acid (SA) and fisetin (FT) were purchased from BDH (Poole, England). The stock solutions of phenolic compounds (100 mg L-1) were prepared in DI. Preparation of paper device using wax ink printer. The circles on the paper, as the CL reaction zones, were designed by Microsoft Office Word 2010. They were printed on a sheet of filter paper (Whatman, United Kingdom) using a wax ink printer (ColorQube 8580, Xerox, USA). The sheets were then heated for 5 min (120 °C), resulting in the melting of wax. So, it can penetrate the paper and form circles with hydrophobic surroundings. After cooling the papers to room temperature, they were cut in suitable size and put inside the sample cartridges (Figure S1 in supporting information). Synthesis of CoMOF. The synthesis of Co-imidazole MOF was performed in an aqueous solution. First, 7.803 g imidazole was dissolved in 40 mL DI, and sonicated for 5 min. Then, Co2+ solution (prepared by dissolving of 0.578 g of Co(NO3)2.6H2O in 4 mL DI) was added and the sonication was continued for 3 h. Afterwards, the mixture was kept in a stationary state for 24 h. Next, the produced precipitation was separated and washed with deionized water for 3 times. Finally it was dried overnight at 25 °C. For the preparation of rhodamine B loaded CoMOF composite (R@CoMOF), 1 mL of RhoB solution (0.1 mM) was added to the well dispersed CoMOF (1 mL, 0.15 mg mL-1) solution. The mixture was gently stirred for at least 2 h, and reserved for 24 h. Chemiluminescence analysis. A summary of paper-based CL analysis is shown in Scheme 1. After preparation of paper device, 5 µL of R@CoMOF (0.75 mg mL-1, sonicated for 10 min) and 5 µL of NaOH (0.2 M) solution were dropped on the paper, respectively. It was dried for 15 min and followed by addition of PC standard solution (with different concentrations in the range of calibration graph) or sample solution. After 2 min, the paper was transferred in the dark room, and the CL recording was started. Finally, 8 µL H2O2 solution was dropped onto the paper device using a syringe pump (Harvard, PHD 2000, USA, flow rate: 200 µL min-1). The obtained CL data (with a gate time of 2 s) were transferred to Excel software, and the peak area was used as the analytical signal (ICL). Also, ΔI = I0 - ICL, the difference between the signals in the absence (I0) and presence (ICL) of analyte, was related to the analyte concentration. Chemiluminescence spectra. CL spectra were recorded using the spectrofluorometer, while the radiation source was switched off. The process was as follows: 5 µL of R@CoMOF (0.75 mg mL-1, sonicated for 10 min) and 5 µL of NaOH (0.2 M) solution were dropped on the paper, respectively, and dried

for 15 min. Then, the paper was transferred into the sample area of the instrument, in front of the emission monochromator. The maximum CL intensity was recorded after injecting 8 µL H2O2 solution onto the paper device. The experiments were repeated for different wavelengths, adjusted by emission monochromator (with a slit width of 20 nm). The obtained CL data were used to plot CL spectra. Real samples. Some honey samples including Al Shifa (Saudi Arabia), Al Shafi (Saudi Arabia), Diamond (Dubai, UAE) and a local honey, and also five molasses including Samail (Sultanate of Oman), Al Barakah (Dubai, UAE), Amal Al Khair (Saudi Arabia), Golden date (Sultanate of Oman) and Date Crown (Al Ain, UAE) were purchased from local supermarkets in Muscat (Oman). The brown sugar was also locally produced. All samples were analyzed after dilution by DI (3 g dissolved in 100 mL DI, and sonicated for 15 min), and no further preparation process was needed. As required, the further dilutions were also done by DI. Folin-Ciocalteu method. The measurement of total reducing capacity of the selected samples was carried out by FolinCiocalteu (F-C) reagent, according to the Singleton procedure.36 Typically, 0.3 mL of the diluted solution of each real sample was individually mixed with 2.7 mL of 1/10 F-C reagent and incubated for 8 min. Then, 2 mL of sodium carbonate 7.5% solution was added, reserved for 30 min, and finally, the absorbance was recorded at 760 nm. A calibration graph was plotted using the standard solutions of GA in the concentration range of 10-50 mg L-1. RESULTS AND DISCUSSION Characterization of prepared CoMOF. Synthesis of MOFs using Co2+ and imidazolate precursors have been reported previously,37, 38 however, most of them have been done in a relatively harsh condition, such as non-aqueous solvent 37 and high temperatures.38 Herein, cobalt-imidazole MOF was simply synthesized by mixing Co2+ cation and imidazole ligands in aqueous solution. The coordination of Co2+ with nitrogen atoms in the imidazole rings can form stable complexes. Figures 1A and 1B show the SEM images for the prepared CoMOF. As can be seen in SEM images, the rhombic dodecahedron shape of MOFs, with a size in the range of 500-900 nm, was verified. On the other hand, the presence of Co, N and C atoms in the structure of MOF was confirmed by elemental analysis (Figure 1C) using energy-dispersive X-ray spectroscopy (EDX). The weight ratio of Co:N in the prepared MOF was 24.0:26.7, and their molar ratio was about 1:4.6, confirming its accurate synthesis process. A precise elemental analysis for synthesized MOF was performed using CHN and ICP methods. The results are as follow: Co: 29.81%, C: 38.24%, H: 2.11%, N: 29.07% (w/w).



Figure 1. SEM images (A, B) and EDX spectra (C) for synthesized CoMOF.

These values are in a good agreement with a CoC6H4N4 chemical composition, which shows a chemical formula of Co(imidazole)2. The X-ray fluorescence analysis was also performed and the result verified the reported amount for Co element (29.55%). Figure S2A shows the IR spectrum for CoMOF. The absorption peak at about 3125 cm-1 was ascribed as C-H bonds in imidazole moieties. Also, the peaks observed in the range of 750-1550 cm-1 were related to the specific vibration modes of imidazole ring.39 The IR spectrum of R@CoMOF (Figure S2A in supporting information) also showed similar absorption peaks, as well as RhoB specific bands at 2954 and 2862 cm-1 (aliphatic C-H), 3060 cm-1 (aromatic C-H), 3200 cm-1 (COOH), 1050-1350 cm-1 (aromatic ring vibrations), 1123 cm-1 (C-N) and 1405 cm-1 (C=O). The crystal structure of prepared CoMOF was also investigated by XRD analysis (Figure S2B in supporting information). The obtained diffraction spectrum contained some peaks at 2 θ values of 10.3°, 11.9°, 14.5°, 15.7°, 19.8°, 21.5°, 22.9° and 28.8°, respectively. The difference between the obtained spectrum and reported patterns in the literature can be related to the different in synthesis condition.39 Most of reported studies have used methanol as the synthesis solvent. However, CoMOF in this work was synthesized in water, and a different structure is expected.39 On the other hand, the crystalline structure for CoMOF was also confirmed by high intensive diffraction peaks. Finally, N2 sorption/desorption isotherms were utilized to study the porosity and evaluate the specific surface area of prepared CoMOF. A high BET surface area equal to 931.8 m2 g-1, as well as the pore volume of 0.64 cm3 g-1 and median pore width of 0.80 nm were achieved. The great surface area of CoMOF and its high pore volume offer a good catalytic activity. Catalytic activity of CoMOF in chemiluminescence reaction. As a potent CL system, RhoB-H2O2 reaction was utilized for the investigation of the catalytic action of prepared CoMOF. This reaction is a well-known CL system with a weak emission, which can be intensified by application of suitable nano-catalysts.40,41 On the other hand, cobalt-based catalysts have a good potential to improve the speed of H2O2-based CL reactions.42, 43 The great positive effect of Co-based catalysts has been described as the ability of cobalt(II) to react with H2O2 (reaction 1) that leads to the formation of hydroxyl radicals (OH•).42 Subsequently, these radicals can mainly follow two mechanism: 1) they can react with each other and produce the excited double oxygen ((O2)2*) (reactions 2-5), leading to the generation of CL emission at wavelength range of 450-550 nm; 2) due to the high oxidizing behavior of OH•, they can react with other organic or inorganic CL reagents and facilitate their oxidation. Co(II) + H2O2 → Co(III) + OH- + OH• (1) 2 OH• + OH- → HO2- + H2O (2) • HO2 (H2O2) - e → HO2 (3) 2HO2• → H2O2 + O2• (4) • • * O2 + O2 → (O2)2 (5) Herein, it was observed that the CL emission of RhoB-H2O2 system was remarkably enhanced in the presence of CoMOF. The CL-time profiles for this reaction in the presence of

CoMOF or Co2+ are shown in Figure 2a. The maximum CL intensity for alkaline RhoB-CoMOF (or Co2+) solution was obtained ~2s after H2O2 injection, which was shorter compared with RhoB alone solution. This observation confirms the catalytic effect of CoMOF and Co2+ on the CL reaction of RhoB-H2O2. On the other hand, the increasing effect of Co2+ on the CL intensity was obvious, however, the CoMOF showed a greater effect than Co2+, so CoMOF can be more useful for this system. More control experiments were also done at different concentrations of RhoB and Co2+ (Figure S3A in supporting information), to support our claim. The results showed the more efficiency of CoMOF than Co2+. Similar comparisons were made for RhoB-H2O2 reaction on the paper device (Figure 2B), and the results confirmed the great improving effect of CoMOF on the CL emission, again. As can be seen, two maximum intensities were obtained in the CL profile of RhoB-H2O2CoMOF, respectively at 2s and 15s after addition of H2O2. The second peak is maybe related to the direct reaction of H2O2 with RhoB, which cannot suitably interact with CoMOF catalyst. To investigate this subject, RhoB molecules were loaded into the pores of CoMOF, before its usage for CL reaction. For this aim, RhoB solution was mixed with a well dispersed CoMOF solution, and reserved for 24 h. Then, the CL reaction of RhoBCoMOF composite (R@CoMOF) with H2O2 was studied on the paper device (Figure 2B, curve d). Surprisingly, the CL intensity increased by about 3 times compared with previous experiments (Curve c), and just one maximum was obtained in the CL profile at 2s. In order to get more information about the mechanism of this improving effect, CL spectra were recorded for RhoB-H2O2 and R@CoMOF-H2O2 systems. An emission peak with maximum intensity at about 570 nm was obtained for both states (Figure 2C). This spectrum is very similar to the fluorescence emission spectrum of RhoB (emission wavelength=572 nm, and excitation wavelength=525 nm). Consequently, RhoB molecules probably acted as the emitting species. According to the observed results and also according to the reported mechanisms for cobalt-based CL enhancement, the mechanism of the CL emission for the R@CoMOF-H2O2 system can be considered as follows: the injected H2O2 is decomposed to high reactive OH•, which can react with RhoB molecules and produce the oxidized intermediate species in excited state (RhoB•ox*). This reaction has been confirmed in our previous works.35 Also, the decrease in the fluorescence intensity of RhoB in the presence of H2O2 confirms such reaction. Anyway, the energy of RhoB•ox* is transferred to the RhoB molecules, and the final RhoB excited species (RhoB*) can lead to the CL emission at about 570 nm.37 However, this mechanism is not sufficiently efficient to generate a sensible CL emission. In the presence of CoMOF, a complicated mechanism is probable. First, the reaction of H2O2 with Co(II) in the MOF structure promotes its decomposition to OH• (reaction 1) and the high concentration of OH• can therefore improve the oxidation of RhoB. On the other hand, the energy of produced (O2)2* species (reactions 2-5) can be transferred to the RhoB molecules. Both mechanisms could increase the concentration of RhoB* species, and so, enhance the CL emission. To verify the role of OH• radicals, t-butyl alcohol was added to the CL reaction media. It is a well-known OH• scavenger and should decrease the CL emission intensity of RhoB-H2O2CoMOF system. In the presence of 0.2 mM t-butyl alcohol, the CL intensity decreased remarkably by 83%, confirming the key role of OH• in the CL reaction. On the other hand, the reaction


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Analytical Chemistry of H2O2-CoMOF in the absence of RhoB led to the CL emission at about 490 nm (Figure 2C), which confirms the presence of (O2)2* species (reaction 5).

Also, we found that the synthesis condition played a significant role in the catalytic performance of obtained MOFs; some syntheses were carried out at a same condition using different amounts of precursors.

Figure 3. Comparison between the CL emission intensity for the injection of H2O2 (30 µM) on the paper device containing different CL reagents (RhoB: 0.1 mM, catalyst: 75 mg L-1 and NaOH: 0.2 M), inset shows the relative CL intensities for different systems.

Figure 2. Kinetic profiles of CL emission for different systems: A) H2O2 (30 µM) was injected into the CL reagents solutions (batch experiments, RhoB: 0.1 mM, catalyst: 75 mg L-1, NaOH: 0.2 M and total volume 10 µL) and B) H2O2 (30 µM) was injected on the paper device (RhoB: 0.1 mM, catalyst: 75 mg L-1, NaOH: 0.2 M and total volume loaded on the paper: 10 µL); C) CL spectra for different systems, obtained by paper device (Condition is similar to B).

Finally, the blank experiments were done to investigate the effects of precursors (Co2+ and imidazole) used for the synthesis of CoMOF (Figure 3). Adding NaOH on the mixture of Co2+ and RhoB resulted in a blue solid on the paper, and a very low CL emission was occurred by the addition of H2O2. Different concentrations of RhoB and Co2+ were examined again, but no enhancement was observed in the CL intensity (Figure S3B in supporting information). About imidazole, it improved the CL emission of RhoB-H2O2 reaction by about 2 times, due to the reaction of imidazole with H2O2. However, simultaneous adding of Co2+ and imidazole notably increased the CL intensity by 4 times. This effect is due to the formation of initial Co2+imidazole complexes, which can act as catalyst for CL reaction.

Figure S4A (in supporting information) shows the relative catalytic activity of products in the RhoB-H2O2 CL reaction. The products could well intensify the generated CL intensity, however, the best efficiency was obtained using 2 mmol Co2+ and 116 mmol imidazole. The change in the activity of synthesized products can be ascribed to their different size or morphologies.44 On the other hand, various imidazole-based linkers (containing imidazole, 2-methylimidazol and 2ethylimidazole) were also applied for the synthesis of CoMOF. As it can be seen in Figure S4B, the CL intensity of mentioned system was more enhanced in the presence of Co2+-imidazol MOF than others. The catalytic performance of MOF decreased by increasing the size of substituted group. Effect of phenolic compounds. The phenolic compounds are well-known as hydroxyl radical scavengers, and so, they can affect the CL intensity of developed system. The initial experiments were performed using GA, which showed a remarkable quenching effect on the CL emission of R@CoMOF-H2O2. Based on this observation, a useful analytical application can be developed for the novel CL system. To more simplify the determination process and also, to decrease the amount of required CL reagents, paper-based CL device was designed as shown in scheme 1. The common filter papers with a circle reaction zone, made by wax printer were applied for this aim. After dropping a very small volume of CL reagents, it was dried for a short time and then, the CL intensity was recorded after H2O2 injection. To obtain the maximum sensitivity, the effects of some key factors including the concentration and volumes of CL reagents, as well as the size of reaction zone on the paper were investigated (Figure S5 in supporting information). The best sensitivity in the detection of gallic acid (0.5 µg mL-1) was obtained using 0.2 M NaOH and 0.75 mg L-1 R@CoMOF prepared by 0.1 mM RhoB (Figures S5A and S5B). High concentrations of RhoB applied for the preparation of R@CoMOF decreased the CL signal (Δ I), probably due to its self-absorbing effect on the CL emission. On the other hand, higher amounts of R@CoMOF than 0.75 mg L-1 also decreased both CL intensity and ΔI, maybe because of



accumulation process, which decrease its catalyzing effect. Investigation of the effects of reagents volume showed that 5 µL of NaOH and R@CoMOF caused the maximum ΔI. Very high or low volumes led to the poor reproducibility. Besides, circles with three different diameter (4, 6 and 8 mm) were printed as the reaction zone on the paper, and its effect was investigated on the CL signal. The smaller circles produced higher signals, however the best reproducibility was obtained by a diameter of 6 mm (Figure S5C). Figure S5D shows the effect of H2O2 concentration on the CL intensity in the absence and presence of GA. The CL intensity linearly increased by increasing the H2O2 concentration up to about 10 µM, and a constant intensity was obtained for H2O2 concentrations higher than 30 µM. The linear relationship between the CL intensity and lower H2O2 concentrations offers a sensitive assay for H2O2. Also, the best signal for GA was obtained by 30 µM H2O2. The excess H2O2 can decrease the sensitivity of system toward the PC. The volume of injected H2O2 solution was adjusted at 8 µL; using lower amounts, the size of created drop on the tip of injection capillary was not large enough to fall down on the paper. On the other hand, the higher volumes also led to low repeatability. Finally, as can be seen from the results in Figure S5E, the drying time of CL reagents had a remarkable effect on both CL intensity and ΔI, in first 15 min after their loading on the paper. But, after 15 min, the CL intensity and Δ I both reached to a constant value and were stable for even 10 days (Figure S6 in supporting information). This means that the developed PCD is practically applicable with a good long-time stability. The application of high stable MOF and use of paper as the support maybe the reason for this high stability. Analytical figures of merit. The developed paper-based CL device was first applied for GA measurement. The calibration graph, in Figure 4A, indicates a linear relationship between the ΔI and the concentration of GA in the range of 2-300 ng mL-1 (ΔI = 62177 (CGA) + 567654, R2 =0.9997). Then, the system was examined for the determination of some other PC. The calibration graphs for five PC (including GA, QC, CT, KF and CA) are shown in Figure 4B, and the figure of merits are summarized in Table 1. The results indicate the best sensitivity of developed CL system for GA, QC, CT, KF and CA, respectively. These results are almost similar to the reported studies, and can be described according to the chemical structure of PC (Figure S7 in supporting information). For example, the structure of QC and CT are similar and they both have 5 hydroxyl groups on the aromatic rings. So, they almost produced similar effects on the CL intensity. KF has a similar structure to QC, however, it has 4 hydroxyl groups. So, KF caused a lower sensitivity in the CL system. Also, GA and CA both contain one aromatic ring, but CA has lower hydroxyl groups. Therefore, the sensitivity for GA is more than CA. As a result, the developed PCD provides a simple, rapid and high sensitive assay for the measurement of PC. The comparison between the features of developed method and some other reported methods for the determination of PC is summarized in Table S1. It is clear that novel paper-based assay has more advantages compared with other methods. It does not need complex flow-based or microfluidic systems and need just simple filter papers. Also, the developed system has more potential to be used as a portable method in on-site screening analysis. Besides, using a high efficient catalyst, a high sensitivity was caused for the CL system.

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To study the precision and reproducibility of developed PCD, it was applied for the analysis of five similar solutions containing a constant concentration of GA. The calculated relative standard deviations (RSD%) were 3.74 and 3.99 %, respectively for 25 and 100 ng mL-1 GA. Table 1. Figures of merit for the determination of some phenolic compounds using the paper-based CL device Phenolic compound

LOD Linear range (ng mL-1) (ng mL-1)

Regression equation

Gallic acid

0.98

2-300

ΔI= 62177C + 567654

Quercetin

1.36

5-400

ΔI = 47016C + 596085

Catechin

1.48

10-400

ΔI = 45050C + 267999

Kaempferol

1.81

10-500

ΔI = 37179C + 450857

Caffeic acid

2.55

20-600

ΔI = 28504C + 362217

Figure 4. Calibration graph for the determination of (A) gallic acid and (B) five different phenolic compounds using developed paperbased CL device [in optimal condition: 5 µL of R@CoMOF (0.75 mg mL-1) and 5 µL of NaOH (0.2 M) solution were dropped on the paper, respectively, dried for 15 min and followed by addition of PC standard solution (with different concentrations). After 2 min, 8 µL H2O2 solution was injected onto the paper and the CL intensity was recorded].

H2O2 determination. There was a good linear relationship between the CL intensity of R@CoMOF-H2O2 system and H2O2 concentration. This effect was a persuading effect to plan the calibration graph for H2O2 determination. A great sensitivity was obtained for this measurement (Figure S8 in supporting information), with a wide dynamic linear range of 1.0 nM-10 µM (R2 =0.9990) and detection limit of 0.5 nM. This result is comparable with reported works and is very interesting, because H2O2 is of great importance for biological applications, and developing a simple and sensitive method for the


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Analytical Chemistry determination of H2O2 is a challenging field for researchers. This work can be considered as the next applications of developed paper-based CL device. Selectivity. The paper-based CL system was performed in the presence of 30 compound, including 10 PC, sugars, amino acids, vitamins etc. The results in Figure 5A demonstrate the high specificity of method toward the determination of PC. No remarkable signals were obtained for sugars, as the main constituents in real samples. Also, the interfering effects of some compounds were investigated on the response of PCD for GA, QC and CT (Figure 5B). The results confirmed the acceptable selectivity of the method for PC measurement. Determination of total phenolic content in real samples. The reliability of the developed assay was examined by its application for the analysis of total PC in some molasses and honey samples (Table 2). The reported concentration for the total PC in real samples were calculated using the calibration curve of GA, and reported as mg GA equivalent per 1 kg sample. Also, the spiked samples were applied for the validation experiments. Certain amounts of GA standard solution was added into the real samples before their dilution process, and then, the samples were analyzed by developed PCD. The recoveries were in the range of 96.28-104.39% (Table 2). Also, the real samples were analyzed by official F-C method (Table 2). The results verified the good agreement between the results of two methods. Some incongruities in the results can be related to the specificity of F-C method. The F-C method can detect not only PC, but also other reducing agents present in the samples. But, the offered method is able to measure total phenolic compounds rather than reducing agents.

mg mL-1) and 5 µL of NaOH (0.2 M) solution were dropped on the paper, respectively, dried for 15 min and followed by addition of interfering species (500 ng mL-1 with or without 200 ng mL-1 of PC compound). After 2 min, 8 µL H2O2 solution was injected onto the paper and the CL intensity was recorded].

CONCLUSION In summary, a paper-based CL device was designed for the first time to measure the total PC content of food samples. It was indicated that cobalt-imidazole frameworks remarkably improve the emission intensity of RhoB-H2O2 CL reaction. The mechanism of this effect was discussed based on Co cations which can facilitate the decomposition of H2O2 to hydroxyl radicals. Also, it was shown that the enhancing effect of CoMOF is greater than only Co2+. Furthermore, the application of CoMOF catalyst for the paper-based CL reaction assured the stability of detection system for several days. The developed PCD was applied for the determination of PC, and the sensitivity was comparable with previous reported methods. Detection limits were in the range of 1-25 ng mL-1, and the reproducibility was acceptable. Also, the effect of probable interfering effect of some species, especially sugars, were investigated and the results indicate the high specificity of method toward phenolic compounds. Finally, the method showed good recoveries for the determination of PC in molasses and honey samples. The developed PCD can be considered as a new research field, which has a good potential to be extended for other analytical purposes.

ASSOCIATED CONTENT Supporting Information FTIR and XRD spectra for synthesized MOF, results for blank experiments, optimization graphs, results for stability during the time, chemical structures of phenolic compounds, calibration graph for H2O2 and comparing Table are supplied as Supporting Information. The material is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author * E-mail address: [email protected] (H.A.J. Al-Lawati).

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors would like to acknowledge His Majesty Trust Funds (SR/SCI/CHEM/16/01) and Sultan Qaboos University for financial support.

Figure 5. A) Response (ΔI) of R@CoMOF-H2O2 CL system on the paper for different compounds; B) CL intensity of R@CoMOFH2O2 CL system on the paper for different interfering compounds in the absence and presence of gallic acid (GA), quercetin (QC) or Catechin (CT), [in optimal condition: 5 µL of R@CoMOF (0.75



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Table 2. Analysis of total phenolic content in some real samples by developed paper-based CL device Sample

Added (mg GA/kg)

Found a

Honey (Al Shifa)

25.0 50.0 25.0 50.0 25.0 50.0 25.0 50.0 100.0 100.0 100.0 100.0 100.0 100.0

128.03±1.54 152.56±0.79 177.39±0.57 233.13±2.07 248.30±1.06 272.61±0.99 168.30±6.01 194.36±1.01 218.00±1.32 392.97±1.92 417.35±0.58 441.72±0.48 1954.46±22.95 2053.61±4.08 1761.68±22.40 1858.96±1.85 1202.69±6.57 1299.98±3.19 2347.30±28.70 2451.69±2.39 2950.96±34.92 3048.05±1.31 936.24±17.90 1032.52±2.67

Honey (Diamond)

Honey (Local)

Honey (Al Shafi)

Molasses (Samail) Molasses (Al Barakah) Molasses (Date Crown) Molasses Golden date) Molasses (Amal Al Khair) Brown sugar

Recovery % Found a ± RSD (F-C method) 98.1±3.2 98.7±1.2 101±4 99.0±2.0 104±4 99.4±2.6 97.5±2.4 97.5±1.0 99.15±4.11 97.28±1.90 97.29±3.27 104.4±2.3 97.09±1.35 96.28±2.78

176.0±6.1 246.3±4.1 176.0±5.0 397.9±4.9 2022.3±20.4 1804.7±26.3 1231.6±15.1 2359.1±33.3 2978.4±36.7 1032.4±14.9 -

Mean of three determinations (as mg gallic acid equivalent per 1 kg sample) ± standard deviation a

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