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Submicrometric magnetic nanoporous carbons derived from metal-organic frameworks enabling automated electromagnet-assisted on-line solid-phase extraction Rejane M. Frizzarin, Carlos Palomino Cabello, Maria del Mar Bauzà, Lindomar A. Portugal, Fernando Maya, Víctor Cerdà, José Manuel Estela, and Gemma Turnes Palomino Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b02065 • Publication Date (Web): 23 Jun 2016 Downloaded from http://pubs.acs.org on July 1, 2016

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Analytical Chemistry

Submicrometric magnetic nanoporous carbons derived from metalorganic frameworks enabling automated electromagnet-assisted online solid-phase extraction Rejane M. Frizzarin, Carlos Palomino Cabello, Maria del Mar Bauzà, Lindomar A. Portugal, Fernando Maya*, Víctor Cerdà, José M. Estela, Gemma Turnes Palomino* Department of Chemistry, University of the Balearic Islands, Carretera de Valldemosa km 7.5, 07122, Palma de Mallorca, Spain. ABSTRACT: We present the first application of submicrometric magnetic nanoporous carbons (µMNPC) as sorbents for automated solid-phase extraction (SPE). Small zeolitic imidazolate framework-67 crystals are obtained at room temperature, and directly carbonized under an inert atmosphere to obtain submicrometric nanoporous carbons containing magnetic cobalt nanoparticles. The µMNPCs have a high contact area, high stability, and their preparation is simple and cost-effective. The prepared µMNPC are exploited as sorbent in micro-column format in a sequential injection analysis (SIA) system with on-line spectrophotometric detection, which includes a specially designed 3D-printed holder containing an automatically actuated electromagnet. The combined action of permanent magnets and an automatically actuated electromagnet enabled the movement of the solid bed of particles inside the micro-column, preventing their aggregation, increasing the versatility of the system, and increasing the preconcentration efficiency. The method was optimized using a full factorial design and Doehlert Matrix. The developed system was applied to the determination of anionic surfactants, exploiting the retention of the ion-pairs formed with methylene blue on the µMNPC. Using sodium dodecyl sulphate as a model analyte, quantification was linear from 50-1000 µg L-1, and the detection limit, coefficient of variation (n= 8; 100 µg L-1) and analysis throughput values were 17.5 µg L-1, 2.7% and 13 h-1, respectively. The developed approach was applied to the determination of anionic surfactants in water samples (natural water, groundwater and wastewater), yielding recoveries from 93 to 110% (95% confidence level). Metal-organic frameworks (MOFs) are formed by linking metallic species with organic ligands obtaining materials with large surface area, good thermal stability and uniform nanoscale cavities.1-4 Within the last 5 years, MOFs have been widely used as sorbents for solid phase extraction (SPE).5-7 Despite the great potential of MOFs as solid supports for extraction or separation processes, their applicability as sorbents for SPE can be hindered due to their limited stability in saline or acidic mediums.8,9 To increase MOF stability, new approaches have been developed as for instance encapsulating MOFs in specific polymers.10,11 Additionally, MOFs have been used recently as precursors to obtain nanoporous carbons with enhanced stability in aqueous complex matrices.12 Among the different types of MOFs, the zeolitic imidazole frameworks (ZIF) based on metal coordination to imidazole-based linkers have been extensively studied due to their interesting zeolitelike topologies.13-15 ZIF-67, which is obtained by the coordination of Co(II) ions and 2-methylimidazole ligands, is one of the most studied ZIFs. An interesting feature of ZIF-67 is the possibility of direct carbonization in the absence of additional carbons sources under inert atmosphere, to obtain nanoporous carbon replicas of the ZIF precursor.16,17 ZIF-67 derived carbons have been described as Co-CoO@N-doped carbons. After the carbonization process the 2-methylimidazole linker is converted into N-doped carbon, while the Co(II) ions aggregate as small magnetic Co nanoparticles, which are present as CoO when located on the external surface of the carbon particle. The use porous carbons templated from MOFs has been barely exploited as sorbents for SPE, in despite of the high

surface area, magnetic properties and high adsorption capacity. Magnetic nanoporous carbons (MNPC) derived from MOFs have been used for the batch-mode magnetic SPE of neonicotinoid insecticides, or phenylurea herbicides prior to HPLC analysis.18,19 MNPCs have been also used for the enrichment of glycans prior to MALDI-TOF-MS analysis.20 In addition to the direct batch use exploiting their magnetic properties, MNPCs have been incorporated on a stainless steel fiber and used for the solid-phase microextraction of organochloride pesticides prior to GC analysis.21 The automation of analytical sample preparation is convenient to fully exploit the advantages of both liquid-phase,22-24 and solid-phase extraction.7,25-28 Additionally, the use of magnetic sorbents facilitates the automation of SPE procedures using sorbent particles difficult to pack in column format due to their small size or irregular morphology.7,28 Among the different types of automation techniques, the syringe-based flow analysis techniques,29-33 showed excellent performance as platform for the development of analytical methodologies involving automated magnetic SPE,34-36 due to their robustness, moderate operating pressures, reduction of reagents and solvents consumption, and high versatility and precision over the flow control. The aim of this work is to automate for the first time the use of µMNPC derived from ZIFs as sorbents for SPE. µMNPC were immobilized by the action of an external magnetic field in a micro-column format in a sequential injection 1

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analysis (SIA) manifold with online spectrophotometric detection. A dedicated 3D-printed holder was developed for the integration of the magnetic particles, enabling different modes of magnetic SPE due to the combined action of permanent magnets combined with an automatically actuated electromagnet (EM-SPE). With this set-up we have automated the determination of anionic surfactants in natural water samples.37,38 EXPERIMENTAL SECTION Reagents and samples. All solutions were prepared using analytical grade chemicals and deionized water (18 MΩ cm). Stock solutions of sodium dodecyl sulphate (100 mg L-1, SDS, Sigma-Aldrich) and methylene blue (500 mg L-1, SigmaAldrich), were prepared by appropriate dissolution of solid reagents in water. Working solutions (50 to 1000 µg L-1) were daily prepared by stepwise dilution of the SDS and methylene blue stock solutions. Water was used as carrier for the SIA system. Methanol (ACS solvent, Scharlau) was used as eluent. The precursor ZIF-67 sample was prepared using Co(NO3)2.6H2O (Scharlau) and 2-methylimidazole (Sigma). Water samples were collected from different locations in the island of Mallorca, Spain. All samples were stored at 4oC. Natural water and groundwater samples were collected from two different locations of a creek, and two wells nearby an urban solid waste treatment plant. The wastewater sample was collected from a sewage treatment plant. All samples were filtered through 0.45 µm nylon membranes before use. Equipment. The developed SIA system (Figure 1a) incorporates a magnetic micro-column loaded with the µMNPCs (Figure 1b). The SIA system is composed by an eight-port selection valve (Sciware Systems SL, Bunyola, Spain) coupled to an automatic burette with a 5 mL glass syringe (BU 4S, Crison) with a three-way solenoid valve. The SPE microcolumn containing 10.0 mg of the µMNPC was a PTFE tube (40 mm length, 1.6 mm id) connected between the selection valve and an additional solenoid valve connected to the spectrophotometric detector. A dispersion of the µMNPC in methanol was introduced into the PTFE tube. The PTFE tube was placed on a dedicated holder (Figures 1c, d and e) fabricated using 3D printing. This 3D printed device accommodates two permanent magnets at the bottom (5 mm diameter) and a channel to place the PTFE micro-column containing the µMNPC. On top of the microcolumn is placed an electromagnet (Eclipse magnetics, 15 mm diameter) connected to one of the additional ports of the multisyringe pump. Method execution and data acquisition and processing were performed by using the AutoAnalysis 5.0 software (Sciware Systems SL). A CCD spectrophotometer (Ocean Optics, Dunedin, FL, USA; model USB4000) was directly coupled to a 10 mm optical path lenght flow cell. A halogen radiation source (Ocean Optics; model DH-2000-UV-VIS-NIR Light Source) was connected to the flow cell by an optical fiber.

Figure 1. a) SIA system for online SPE of anionic surfactants. Micro-column connected through a solenoid valve to the flowcell of a spectrophotometric detector (670 nm); eluent (methanol 87.5%, v/v). b) Schematic representation of the carbonization process of ZIF-67. c) Schematic depiction of the magnetic micro-column device. d) 3D design of the holder for the magnetic micro-column. e) Picture of the holder of the magnetic micro-column coupled with an automatically actuated electromagnet connected to the syringe pump. Powder X-ray diffraction data were collected using CuKα (λ = 1.54056 Å) radiation on a Siemens D5000 diffractometer. Particle morphology was studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) using a Hitachi S-3400N microscope operated at 15 kV and a Hitachi ABS microscope operated at 100 kV, respectively. Nitrogen adsorption isotherms were obtained at 77 K using a TriStar 3020 (Micromeritics) gas adsorption analyzer, up to atmospheric pressure. The samples were previously outgassed under dynamic vacuum (10-6 kPa) at 393 K overnight. Data were analyzed using the Brunauer–Emmett–Teller (BET) model to determine the specific surface area. Synthesis of submicrometric magnetic nanoporous carbon (µMNPC). Two solutions, one of 4 mmol of Co(NO3)2.6H2O in 100 mL of methanol and another one of 16 mmol of 2methylimidazole in 100 mL of methanol, were prepared. The solution containing the organic ligand (2-methylimidazole) was poured into the Co(II) solution and the resulting mixture was kept under stirring at room temperature for 2 hours. The obtained bright purple solid was collected by centrifugation, washed 4 times with fresh ethanol and dried at 60ºC. µMNPC were obtained from ZIF-67 crystals by carbonization under a N2 flow at 800 oC for 6 hours (heating rate: 1 oC min-1). Flow procedure. The SIA system for the determination of anionic surfactants (Figure 1) was operated using the automated procedure described in Table 1. The developed method starts with the syringe module in the empty position. This module is coupled to a selection valve (SV) connecting the syringe pump to the sample and eluent reservoirs, as well as to the micro-column containing the µMNPC and the spectrophotometric detector. For the sampling step (step 1), 1.5 mL of sample volume (SDS standard or sample containing methylene blue as ion-pair) is loaded at 5.0 mL min-1 into the holding coil 2


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(port 2 of the SV). In step number 2 the volume of the syringe is adjusted by loading an appropriate amount of carrier. After that, the SV is connected to port 1 to dispense a volume of 2.5 mL of the sample towards the micro-column (1.0 mL min-1) to carry out the SPE. For the elution step, the SV was first connected to port 3 to aspirate 150 µL of eluent (1.0 mL min-1), and subsequently changed to port 1 for dispensing 1.0 mL (eluent followed by carrier) to elute the analytes retained in the micro-column and subsequently measure the absorbance of the eluted ion-pair at 670 nm. This step was also used for the cleaning of the column with the water carrier, in order to avoid carry-over. Measurements were carried out by triplicate based on the peak height. Table 1. Methodology for online SPE of anionic surfactants

1

Load sample

2

Pick up

Electromagnet On

2

Load carrier

2

Pick up

Off

3

1

4

Inject to column Load eluent

3

Pick up

5

Start measure

-

-

6

Elution step

1

Dispense

7

Stop measure

-

-

Step Instruction

SV

Pump

Dispense Off/On* Off

Flow rate (mL min-1)

Volume (mL)

5.0

1.5

10.0

3.5

1.0

2.5

1.0

0.15

-

-

Off

1.0

1.0

-

-

-

*Electromagnet is switched On and Off on depending of the followed procedure. See sections below.

RESULTS AND DISCUSSION Based on the APHA recommended method for the determination of methylene blue active substances (MBAS),38 a novel automated method exploring the features of the µMNPC for the extraction and determination of MBAS, using sodium dodecylsulphate (SDS) as model analyte, has been developed. An ion-pair is formed by electrostatic attraction between the MBAS and the cationic dye methylene blue (1:1). The neutral species obtained as products are potential candidates to be retained by π-π interactions between methylene blue and the graphitic (sp2) carbon of the prepared µMNPC, thus avoiding the use of large quantities of chloroform, as required in the APHA method. Preparation and characterization of the µMNPCs. In a first step, micro sized crystals of the ZIF-67 were obtained at room

temperature,39 followed by combustion at 800 ºC under N2 atmosphere. Figure 2a, shows the X-ray diffraction (XRD) pattern of the prepared ZIF-67. Peak positions of ZIF-67 are in agreement with those of the XRD previously reported.39 After carbonization, the characteristic peaks of ZIF-67 at low diffraction angles (40 degree) angles are observed, which are assigned to a crystalline face-centered cubic (fcc) Co phase. Furthermore, the weak and broad diffraction peak also present at a lower diffraction angle (around 2θ = 25°) is attributed to the formation of a graphitic carbon structure. The measured nitrogen adsorption-desorption isotherms at 77 K, showed that µMNPC is a microporous material, exhibiting reversible type I isotherms with no hysteresis (Figure 2b). ZIF-67 is a highly porous material with a surface area ranging from 1500-1700 m2 g-1 on depending of the selected synthesis conditions.16,39 In the carbonization process the crystal structure of ZIF-67 partially collapses losing part of the initial porosity, but still retaining a high surface area.16 Concretely, the BET specific surface area of the prepared µMNPC was 282 m2 g-1 (BET surface area plot, Figure S1). The measured total pore volume for the µMNPC was 0.15 cm3 g-1. Figure 2c shows the rhombic dodecahedral micrometric of the ZIF-67 precursor crystals. After carbonization, the obtained µMNPCs retained the shape of the original ZIF-67 crystals although the particle surface is distorted and bumpy (Figures 2d and 2e). A more detailed image shows the presence of cobalt nanoparticles (of approximately 15 nm) dispersed in the carbon matrix (Figures 2f). Optimization of the automated online magnetic SPE using µMNPC. The concentration of methylene blue (ion pair agent) and methanol (eluent) were studied as independent factors, while the absorbance signal was chosen as the dependent factor (response). As a first step, a two level full factorial design (Table S1) and three assays at central point were performed. Based on preliminary experiments, the experimental domain was established, ranging from 1.0 to 10.0 mg L-1 and 10 to 90% (v/v) for methylene blue and methanol concentrations, respectively. An analysis of the results using a statistic software indicate that both factors (methylene blue and methanol) have significant effect in the experimental domain with 95% confidence level (p= 0.05), as presented in the Pareto chart (Figure S2), while the interaction between them is negligible. The analysis of the results indicated that methylene blue and methanol had a positive effect, indicating that a higher response can be obtained increasing their concentrations. .

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Figure 2. a) Powder X-ray diffraction patterns of the ZIF-67 precursor crystals and the µMNPC obtained from the direct carbonization of ZIF-67. b) Nitrogen adsorption/desorption isotherms of the µMNPC. SEM micrographs of c) the precursor ZIF-67 crystals and d) the µMNPC particles. e) and f) TEM images with different magnifications of the µMNPC. The ANOVA presented a model with a significant lack of fit (Table S2). In this sense, a response surface model could be employed in a modified experimental domain to investigate the quadratic terms and the presence of maximum critical values of response. Thus, a Doehlert Matrix was exploited using methylene blue and methanol as independent factors and the absorbance signal as dependent factor. Table S3 shows the experimental domain, the matrix and the responses for the exploited design. Maximum critical values were obtained in the optimization and the response surface is shown in Figure 3a. The high correlation (0.9209) and adjust (0.8221) coefficients, low pure error value (0.0361) without lack of fit effect prove the efficiency of the experimental design exploited in the optimization. The critical value for methylene blue concentration was 11.3 mg L-1, while for the methanol concentration a 87.5% (v/v) was selected, which is the median value between the central point and the highest level for methanol in the experimental domain. Once the optimum conditions were established for methylene blue and methanol, the effect of the eluent volume on the extraction process was studied in a univariate study from 50 µL to 200 µL to identify the minimum volume required in the elution step, which also ensured the column conditioning for the next preconcentration. Results are shown in Figure 3b, where it is observed that 150 µL of eluent were sufficient to obtain the highest response. The influence of the sample volume on the measured absorbance was also evaluated from 0.3 mL to 4.5 mL (Figure 3c). For the selected analyte concentration, the preconcentration was linear up to 4.5 mL of sample volume, which is the maximum sample volume loading allowed by the developed SIA manifold. To come to a compromise between sensitivity and sampling rate, a sample volume of 1.5 mL was selected for further experiments. However, up

to 4.5 ml volumes may be used if a higher sensitivity is required. Study of the effect of magnetic field on SPE (EM-SPE). Usually, when very small magnetic particles are packed in a micro-column format using permanent magnets tend to aggregate in detriment of the extraction performance of the material. To circumvent this drawback, an automatically actuated electromagnet was placed on top of the micro-column to enable the development of different SPE modes involving a dynamic bed of a magnetic material, using µMNPCs as proof of concept. The electromagnet is directly connected to an auxiliary port of the syringe pump and it can be automatically activated or deactivated as desired, using the software AutoAnalysis 5.0. With this approach, it is possible to move the bed of magnetic particles between each extraction thus preventing their aggregation. Different possibilities for online magnetic SPE were tested and the results for the extraction of SDS are summarized in Figure 3d. In this figure, is shown how the use of two permanent magnets is sufficient to hold the magnetic particles and obtain an adequate preconcentration of the SDSmethylene blue ion pair. Activating/deactivating the electromagnet between extractions produced and increase of a 30% on the measured analytical signals, by decreasing the aggregation of the µMNPC particles within the micro-column. Additionally, the use of activation/deactivation cycles during the SPE process, led to a further improvement of the preconcentration efficiency. In this case, the activated electromagnet was deactivated and immediately activated each 0.5 mL of sample flowed through the µMNPC micro-column. Using this dynamic online magnetic SPE configuration, a further 16% improvement was obtained achieving a total increase of a 50% on the preconcentration efficiency due to the possibility to 4


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automatically actuate the electromagnet for the developed magnetic online SPE procedure. Analytical features of the automated online magnetic SPE using µMNPC and sample analysis. After the optimization step, the developed procedure using a 1.5 mL of SDS standard solution showed a linear dynamic range for the determination of SDS from 50 to 1000 µg L-1, as described by the equation A = 0.633 + 0.003 CSDS (R2 = 0.998). The detection limit, coefficient of variation (n= 8; 100 µg L-1) and sampling rate were 17.5 µg L-1, 2.7%, and 13 determinations h-1, respectively. µMNPC micro-column packings are robust, enabling approximately 300 extractions in consecutive days without replacing the sorbent. The procedure did not suffer from matrix effects since only the ion-pairs were retained on the µMNPC by the effect of π-π interactions between methylene blue and the graphitic carbon. So, the use of µMNPCs particles as solid sorbent ensured the effective retention and elution of the ionpairs providing high selectivity in the presence of other matrix components.

cedures for the determination of SDS.40-43 Furthermore, the proposed method just requires of 150 µL methanol as eluent, so using an organic solvent which is less toxic than the CHCl3 used in the reference method and requiring an organic solvent volume per analysis approximately 300-fold lower.38 The organic solvent volume used in the developed method is also much smaller than the required in other reported batch procedures (80 and 30-fold less volume, respectively),44,45 Furthermore, in front of the 1.5 ml of sample used in the developed procedure, the official standard method requires up to 400 mL of sample to achieve similar levels of sensitivity, which makes more difficult the handling and storage of a large number of samples. Besides that, the combined use of advanced automated techniques, such as sequential injection analysis incorporating automatically actuated electromagnets allow safer handling and additional control over the use for SPE applications of small particles of complex magnetic materials, such µMNPC, allowing also further improvements in the extraction efficiency. Table 2. Recoveries of SDS spiked to natural water, groundwater and wastewater samples (100 and 500 µg L-1). Mean values and uncertainties refer to triplicate measurements Sample Natural water 1

Natural water 2

Groundwater 1

Groundwater 2

Figure 3. a) Response surface for methanol vs methylene blue obtained with Doehlert Matrix (Table S3) optimization (R = 0.9209; Adj.: 0.8221; Pure error = 0.0361). Effect of b) the eluent volume (methanol) and c) the sample volume on the measured absorbance of a 500 µg L-1 SDS solution (blank subtracted). d) Comparison of different µMNPC microcolumn magnetic configurations for the extraction of SDS. Direct SDS analysis; SPE using the electromagnet disabled; SPE using the electromagnet enabled (SPE+EM); SPE procedure commutating the electromagnet during the SPE process (On/Off) each 0.50 mL of injected sample through the microcolumn (SPE+ComEM). As a proof of concept of real applicability of the developed EM-SPE procedure using µMNPCs, was applied to the determination of anionic surfactants in water samples (natural water, groundwater and wastewater) (Table 2). In two of the samples, anionic surfactants were detected at concentration levels of 61 and 96 µg L-1, which cannot be reached by direct analysis. Additionally, all samples were spiked with SDS at two different concentration levels (100 and 500 µg L-1). Satisfactorily recovery factors ranging between 93 - 110% (95% confidence level) were obtained. The obtained detection limits are comparable with those obtained using other reported pro-

Wastewater

Spiked (µg L-1) 0 100 500 0 100 500 0 100 500 0 100 500 0 100 500

SDS found (µg L-1) -110 ± 10 505 ± 28 61 ± 7 156 ± 4 577 ± 29 -93 ± 13 505 ± 12 96 ± 8 192 ± 13 610 ± 26 -95 ± 3 492 ± 28

Recovery (%) -110 101 -95 103 -93 101 -96 103 -95 98

The main advantages of the EM-SPE developed technique using µMNPCs are (1) low cost and facile preparation of the µMNPCs, (2) high contact area due to small particle size and high surface area, (3) enhanced interaction with analytes due to the graphitic carbon structure of the µMNPC, (4) easy sorbent handling due to the presence of strongly magnetic nanoparticles well dispersed in the carbon matrix, (6) safe and reproducible use of micro- nanomaterials due to automation using the SIA technique, (7) and improved extraction due to the action of an automatically actuated electromagnet, preventing irreproducibility produced by agglomeration of small sorbent particles in a micro-column format. CONCLUSIONS In summary, a simple, fast, robust, and environmentally friendly automated SIA system with on-line SPE has been developed for the determination of anionic surfactants in water samples. The proposed approach is based on the use for the first time, of µMNPCs derived from ZIFs as sorbents for online magnetic SPE. µMNPCs are prepared from cost5



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effective commercially available reagents and no specialized synthetic skills are needed for their preparation. µMNPCs showed interesting features as sorbents for on-line SPE, such as magnetic properties, graphitic carbon structure, and high contact area and stability. Furthermore, exploiting the implementation of an automatically actuated electromagnet in the flow manifold, we have achieved an advanced control over the µMNPCs packed in micro-column format, minimizing particle aggregation and consequently increasing the extraction efficiency. The proposed approach proved to be selective and efficient for the extraction of SDS after ion pair formation with methylene blue, allowing a 300-fold reduction of the use of organic solvents in comparison with the official procedure. The applicability of the method to complex matrices has been demonstrated, obtaining good recoveries for spiked samples. Future work in the field will be directed towards the use of µMNPCs for the automation of other SPE modes and combination with separation techniques (GC-MS), as well as the study of different µMNPCs, and their modifications for the development of novel automated analytical methodologies.

ASSOCIATED CONTENT Supporting Information Optimization tables and figures, and BET surface area plot are available in the Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION

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Corresponding Author *E-mail: [email protected]. *E-mail: [email protected] Fax: (+34) 971 173426. Phone: (+34) 971 173260.

ACKNOWLEDGMENT The Spanish Ministerio de Economía y Competitividad (MINECO) and the European Funds for Regional Development (FEDER) are gratefully acknowledged for financial support through Project CTQ2013-47461-R. R.M.F. acknowledges the CNPq Brazilian Agency (Process 201514/2014-1).

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