Self-assembled Cyclodextrin Metal-Organic Frameworks on Graphene Oxide as Filter Membrane for Tracelevel Naringin Pre-enrichment before Analysis

LI Yuanyuan ,CHEN Na ,ZHU Nan ,FENG Xiangzhi ,MA Yulong ,JI Wenxin ,SUN Yonggang

(1.State Key Laboratory of High-efficiency Coal Utilization and Green Chemical Engineering,Ningxia University,Yinchuan 750021,China;2.College of Chemistry and Chemical Engineering,Ningxia University,Yinchuan 750021,China)

Abstract: A green,renewable composite was designed and fabricated based on self-assembly of cyclodextrin metal-organic framework (CD-MOF) on graphene oxide(GO).Then,the GO@CD-MOF was embedded in 0.45 μm PTFE membrane to produce a dual-functional membrane which could carry out sample enrichment by capturing naringin molecules.The membrane filter was further improved by investigating the effects of the experimental parameters including amount of GO@CD-MOF,enrichment time and elution solvent on enrichment efficiency of naringin.Further,the present method had been successfully applied to citrus sample and obtained satisfied recovery value (79.7%-100.3%).Moreover,the extraction of naringin can be achieved for 2 min,and GO@CD-MOF loaded membrane can be reused at least for 5 times.The results demonstrate that the fabrication of the novel filter membrane based on GO@CD-MOF is a fast,simple and reliable,and possesses great potential in the determination of naringin from real samples by dual-function of separation and enrichment.

Key words: cyclodextrin metal-organic framework;graphene oxide;membrane filter;naringin;enrichment

1 Introduction

The flavonoids with a common benzo-pirone structure are one of the most important groups of phytonutrients,which may provide beneficial health effects.Naringin as one of flavonoids has been found in many kinds of fruits,and is being used as functional ingredient for several industrial products which are especially based on their antioxidant activity.In the food industry,naringin can not only be used as a natural coloring agent,flavor modifier and bitter agent for food and beverage production,but also as a synthesis of high sweetness,non-toxic,low-energy new sweetener raw materials,being suitable for people with diabetes and obesity.With the development of food,medicine and other industries,naringin will have an increasingly broader role.Most citrus species accumulate substantial quantities of naringin during the development of their different organs.Therefore,the separation and enrichment of naringin from citrus samples play an important role in many areas of science.

Owing to the complexity of sample matrices and the relative low concentration of naringin,it is difficult to directly extract naringin and determine its concentration in real samples.Hence,it is of great importance to develop an effective method for enrichment naringin in real samples.Current techniques for the pre-concentration are maily liquid-liquid extraction(LLE),solid phase extraction (SPE),matrix solid-phase dispersion(MSPD),and membrane filter.Among these methods,it has been found membrane filter as an analytical process has particular application for the preparation and extraction of analytes from various samples,due to some major advantages such as straightforward application,ability to simultaneous performance of extraction and cleanup in a single step,and purification during a single process with good recovery and precision.However,adsorbents in membrane filter have key roles in obtaining high enrichment efficiency for trace analysis in complex matrices.

Recently,MOFs as novel adsorbents have showed great promise in the separation and enrichment of various compounds.Morsali

et al

developed two new three-dimensional porous Cd(II)-based MOFs and these MOFs show high removal efficiency for Congo red solution.Among them,CD-MOF as a green and renewable framework material has been reported in the separation,but further research is greatly needed.Because of weak mechanical strength,CD-MOF can be easily crushed under high pressure.To avoid the abovementioned problem and still maintain the advantageous properties,it is necessary to increase mechanical strength by adding carriers.Graphene oxide (GO)with much more polar moieties has a more polar and hydrophilic character,possesses a large surface area and exhibits fast carrier mobility and excellent optical transparency.Based on the above advantages,it is a good choose that GO is used as a carrier.So we developed a new adsorbent by self-assembling CDMOF to GO sheets.

In this study,a dual-functional membrane based on the self-assembled CD-MOF on GO was designed and fabricated.The obtained GO@CD-MOF loaded membrane was successfully applied to the separation and enrichment of naringin.To investigate the enrichment behavior of naringin on the loaded membrane,a series of optimization experiments such as amount of GO@CD-MOF,enrichment time and elution solvent was carried out.Furthermore,the proposed method was also adopted to analyze naringin from citrus peel.

2 Experimental

2.1 Material and reagents

γ

-cyclodextrin (

γ

-CD,98%) was purchased from Yuanye Bio-Technology Co.,Ltd.(Shanghai,China).Potassium hydroxide(KOH) and cetyltrimethylammonium bromide (CTAB) were obtained from Green Analytical Chemical Technology Co.,Ltd.(Shanghai,China).0.45 μm PTFE membrane was obtained from Tian Jin Hengao Technology Co.,Ltd.GO was purchased from Fenghua Materials Development Co.,Ltd.(Hunan,China).Acetonitrile(ACN) of HPLC grade was from Kermiou Chemical Reagent Co.,Ltd.(Tianjin,China),Anhydrous NaSO,methanol,ethanol and other chemicals were of analytical reagent (Tianjin Chemicals,China).Ultrapure water (>18 MΩ cm) was used throughout the experiments.

2.2 Instrumentation and chromatographic conditions

Field emission scanning electron microscopy(FE-SEM) images were obtained on a Hitachi S-4800(Hitachi,Japan).X-ray diffraction (XRD) pattern was recorded on a D8 Advance X-ray diffractometer that used graphite-monochromated Cu Kα radiation.IR spectra were obtained on a Nicolet 20 NEXUS 670 FTIR (Madison,USA).Transmission Electron Microscopy(TEM) images were obtained on an H-7500 TEM spectrometer (Hitachi,Japan).

Determination of naringin was performed using an Agilent 1 290 Infinity UHPLC system (Agilent Technologies).An Ultimate AQ-C18 column (250×4.6 mm,5 μm) from Phenomenex (USA) was used.The mobile phase was ACN/water (0.1% acetic acid)(70:30,

V/V

) with a flow rate of 0.8 mL/min at 35 ℃.Prior to use,all mobile phases were passed through a 0.45 μm PTFE membrane and degassed under vacuum.The sample injection volume was 20 μL and analytes were monitored at 254 nm.

2.3 Preparation of GO@CD-MOF

GO@CD-MOF was prepared using a threestep procedure according to seed-mediated growth method.First,50 mg of GO was dispersed in deionized water(50 mL) with ultrasonication for 12 h.Then,0.56 g KOH was added to deionized water(50 mL),and 1.63 g γ-CD was dissolved in KOH aqueous solution.The above solution was filtered through a 0.45 μm PTFE membrane.Finally,the filtering solution is added to well-dispersed GO,and poured into medium beaker.The medium beaker was placed in a large beaker with 19 mL methanol.Methanol was allowed to vapordiffuse into the solution under vigorous stirring at 25℃.Then 5 mL of the above solution was taken and 0.04 g CTAB was added for dissolution,followed by adding 5 mL of methanol solution and stirring for 3 h.The GO@CD-MOF formed was collected by centrifugation,repeated washing with ethanol,and was dried at room temperature.

2.4 Preparation of GO@CD-MOF membrane

40 mg GO@CD-MOF was dispersed in 10 mL of methanol under ultrasound for 30 min.The GO@CD-MOF membrane was prepared by embedding the ultrasonically dispersed GO@CD-MOF material into a 0.45 μm PTFE filter membrane.Schematic procedure of the GO@CD-MOF filter membrane was shown in Fig.1.

Fig.1 Schematic procedure of the GO@CD-MOF loaded membrane

2.5 Sample preparation

The citrus peel bought in the market was cut into strips and dried at 60 ℃ for 36 hours.Then the dried citrus peel was crushed by a crusher and passed through a 400 mesh sieve.20 mL isopropyl alcohol was added to 1 g powdered citrus skin with ultrasonication for 1 h,and supernatant was collected by centrifugation at 4 000 rpm for 5 min,which process is repeated twice.The supernatant was subjected to GO@CDMOF loaded membrane.All sample solutions were determined by UHPLC.

3 Results and discussion

3.1 Preparation and characterization of GO@CD-MOF

Fig.2 (A) TEM image of GO@CD-MOF;(B) SEM image of GO@CD-MOF;(C) XRD pattern of simulated CD-MOF and GO@CD-MOF;(D) FT-IR spectra of GO (a),γ-CD (b) and GO@CD-MOF(c)

The newly synthesized composite was characterized by different techniques.The TEM image of GO@CD-MOF is shown in Fig.2(A).It can be seen that GO with single-layer shows an irregular shape,some wrinkles and a large specific surface area,and provides a large adsorption site.Furthermore,CD-MOF is homogeneously anchored onto the surface of the GO sheets with the lateral size ranging from hundreds of nanometers to several micrometers.It can also be clearly observed in the SEM image (Fig.2(B)) that CDMOF shows uniform cubic crystal and average particle size is around 0.5-1 μm.

Figs.2(C) shows a typical XRD pattern of simulated CD-MOF and GO@CD-MOF,and it was further used to evaluate the preparation of GO@CDMOF.Compared with simulated CD-MOF,GO@CD-MOF presents charactermatic diffraction peak of CD-MOF,indicating that CD-MOF on the GO sheets surfaces was assembled.

The self-assembly of CD-MOF on GO was also confirmed and characterized by FT-IR spectroscopy(Fig.2(D)).By comparison of spectra of GO and GO@CD-MOF,the peak of hydroxyl group at 3 000-3 600 cmwas strengthened after the immobilization of CD-MOF.As we know,the

γ

-CD possesses abundant hydroxyl groups on its external surfaces.Other peaks were 2 940 (

v

),1 100 (

v

)and 900 (

v

)cmin the spectrogram of GO@CDMOF,respectively.This is another evidence of the immobilization of CD-MOF on the GO sheet surfaces.No significant change in the spectra of

γ

-CD and GO@CD-MOF was observed,which indicates that CD-MOF keeps the skeleton structure of

γ

-CD.

3.2 Optimization of the enrichment of naringin with GO@CD-MOF membrane

3.2.1 Amount of GO@CD-MOF

The amount of GO@CD-MOF is a crucial factor influencing the enrichment performance,since it can increase the interface area between the sorbent and target compounds,and allow complete adsorption of the sample components to promote their transfer into sorbent.In this study,the effect of the sorbent dosage was investigated,ranging from 10 to 60 mg.GO@CD-MOF composite material with different mass was embedded in 0.45 μm PTFE membrane.The experimental results (Fig.3) showed that the adsorption quantity was significantly increased when the amount of GO@CD-MOF was changed from 10 to 40 mg.However,there was a great decrease of adsorption quantity by adding over 40 mg.The possible reason was that due to the limited size and area of the loaded membrane,the sorbent dosage over 40 mg led to filter membrane too thick,which made the filter pressure increase and solution did not flowed smoothly.There was less contact of naringin with the active site on the GO@CD-MOF.Therefore,the amount of GO@CD-MOF (40 mg) was selected for the following experiments.

Fig.3 Effect of the amount of GO@CD-MOF

3.2.2 Type of elution solvent

The selection of an appropriate elution solvent to the enrichment process plays an important role to bring about an efficient and selective desorption of naringin from the GO@CD-MOF membrane.Therefore,four eluting solvents were used in the desorption process of naringin,including methanol,ethanol,ACN and methanol/ACN(1:1;

V/V

).From the exhibited results in Fig.4,it is obvious that the recovery is the highest with methanol as elution solvent and can reach 101.3%compared with the other three eluents.The recovery of naringin using ethanol,ACN and methanol/ACN was less than 80%.Such results may be related to the high polarity of methanol and its ability to interrupt the interactions between target molecules and the sorbent.Thus,methanol was selected as the best eluting solvent in this study.

Fig.4 Effect of the elution solvent for naringin

3.2.3 Effect of enrichment times

Due to the rapid filtration of GO@CD-MOF membrane,it is possible that the naringin in the citrus is not sufficient interaction with GO@CD-MOF.In order to obtain the maximum enrichment effect,enrichment times from 1-6 times were investigated.As can be seen from Fig.5,the content of enriched naringin has been increasing when enrichment times was increased from 1-4 times.The highest enrichment amount of naringin from citrus peel (381.36 μg naringin/g citrus peel) was obtained after four times.However,the enrichment amount of naringin decreased when adsorption times were five and six.A possible reason may be that a small amount of naringin was resolved because of the solubility of organic solvents as the increase of adsorption times.Based on the above results,four times was applied as the optimum enrichment times in the subsequent experiment.

Fig.5 Effect of enrichment times

3.2.4 Regeneration study of GO@CD-MOF membrane

During the enrichment process,the number of membrane cycle is an important parameter for obtaining good recoveries.Since sorption of naringin on GO@CD-MOF membrane is a reversible process,it is possible to regenerate.In order to investigate the reusability of the GO@CD-MOF membrane,it was washed three times with 3 mL of methanol after desorption of the analytes,and then reused for the next enrichment.The results (Fig.6) show that GO@CD-MOF membrane can be reused at least for 5 times without a significant loss of enrichment capacity.The recovery of naringin was more than 90% after three cycles.In contrast,the recovery of naringin was still above 80% after five cycles of use.Therefore,the GO@CD-MOF loaded membrane can be effectively regenerated for further use and has good recycling ability.Then the recognition site of GO@CDMOF loaded membrane is stable.It saves on sample pretreatment costs,which meets the principles of green analytical chemistry.

Fig.6 Recovery of naringin onto the GO@CD-MOF loaded membrane after six reuse cycles

3.3 Validation of the developed method

Linearity of the calibration standards was investigated at eight concentration levels in the range of 15-600 μg/mL.Good linearity between the concentration of the naringin and the peak area was obtained throughout the concentration range(

R

=0.994 2).

Fig.7 Typical chromatograms of the fresh citrus peel samples before(a) and after(b) being treated by the GO@CD-MOF loaded membrane

Fig.7(a) and 7(b) illustrated the chromatograms of the fresh citrus peel samples before and after being treated by the GO@CD-MOF loaded membrane.Some peaks were found at the retention positions of compounds including naringin(retention time 4.5 min)before loaded membrane filter because of the complexity sample.After loaded membrane filter,other impurity peaks except for the peak of naringin were obviously weakened or disappeared and naringin peak area was increased,which indicates that the GO@CD-MOF loaded membrane achieved dual-function of separation and enrichment for the naringin.

4 Conclusions

In summary,the dual-functional GO@CD-MOF loaded membrane for separation and enrichment of naringin was designed and naringin was detected by UHPLC.Naringin could be well enriched by the GO@CD-MOF loaded membrane.The enrichment process can be achieved for 2 min.It was also successfully applied to the analysis of citrus peel samples with satisfactory recoveries.The developed method was sensitive,rapid,and highly repeatable for the determination of trace naringin in environmental samples.Moreover,the GO@CD-MOF loaded membrane required less volume of elution solvent,and lower consumption of adsorbent,which met the principles of green analytical chemistry.