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Porphyrin-based metal-organic framework and polyvinylchloride composites for fluorescence sensing of divalent cadmium ions in water
Journal Pre-proofs Porphyrin-based metal-organic framework and polyvinylchloride composites for fluorescence sensing of divalent cadmium ions in water Hailey A. J. Hibbard, Michaela J. Burnley, Heather N. Rubin, Jack A. Miera, Melissa M. Reynolds PII: DOI: Reference:
S1387-7003(19)30704-X https://doi.org/10.1016/j.inoche.2020.107861 INOCHE 107861
To appear in:
Inorganic Chemistry Communications
Received Date: Revised Date: Accepted Date:
18 July 2019 11 February 2020 27 February 2020
Please cite this article as: H. A. J. Hibbard, M.J. Burnley, H.N. Rubin, J.A. Miera, M.M. Reynolds, Porphyrinbased metal-organic framework and polyvinylchloride composites for fluorescence sensing of divalent cadmium ions in water, Inorganic Chemistry Communications (2020), doi: https://doi.org/10.1016/j.inoche.2020.107861
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
Porphyrin-based metal-organic framework and polyvinylchloride composites for fluorescence sensing of divalent cadmium ions in water Hailey A. J. Hibbarda, Michaela J. Burnleya, Heather N. Rubina, Jack A. Mieraa, and Melissa M. Reynoldsa,b,* Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA. School of Biomedical Engineering, Colorado State University, Fort Collins, CO 80523, USA *Corresponding author: E-mail: [email protected]
a
b
Keywords: metal-organic framework; porphyrin; cadmium; metal ion sensing; polymer composite Abstract Toxic metal contamination of Earth’s scarce fresh water sources creates a persistent necessity for the development of new materials and techniques to detect metal ions in water. Cadmium in particular is released into the environment by ubiquitous phosphate fertilizers, and chronic cadmium exposure can lead to internal organ damage and bone weakening. The present study shows that NU-902, a water stable zirconium porphyrin-based metal-organic framework (MOF), is fluorescent and undergoes a detectable change in photoluminescent emission properties in the presence of divalent cadmium ions. The MOF displays detectable fluorescence quenching in the presence of cadmium at levels of 0.3 ppb, far below concentrations deemed acceptable by most regulatory agencies, indicating its potential as a fast, reliable chemosensor for cadmium in drinking water. After establishing the fluorescent properties of the powder MOF, it was immobilized within a plasticized polyvinylchloride (PVC) to create a membrane with potential for easy in field use as a sensor. The MOF-PVC composite displayed fluorescence quenching within five minutes of exposure to an aqueous cadmium solution. This study shows the potential use of a water stable porphyrin-based MOF for fast, sensitive detection of metal ion contamination in water in powder form, as well as in a polymer composite.
Wide spread use of heavy metal ions, primarily in anthropological activities including industrial, agricultural, and technological applications, has led to their unwanted prevalence in the environment. This poses a serious threat to the health and well-being of humans and Earth’s ecosystems.[1] Cadmium is present in phosphate rocks and the use of phosphate fertilizers introduces cadmium into the environment and to food sources.[2–4] In addition, anthropogenic cadmium emissions are rapidly growing from fossil fuel combustion, smelting of zinc, lead, and copper ores, pigment manufacture, and from municipal and sewage sludge incinerators.[1–4] The main routes of exposure to cadmium are via ingestion of contaminated food or water and smoking cigarettes.[1,2,4–7] Chronic cadmium exposure leads to various adverse health effects including damage to the kidneys, liver, and lungs.[1,2,4– 8] Cadmium also interferes with bone mineralization and structure, leading to osteoporosis, osteomalacia, and ItaiItai disease, causing severe joint and spine pain.[1–9] The World Health Organization (WHO) guideline for cadmium in drinking water is 3 ug/L (3 ppb).[10] United States Environmental Protection Agency (EPA) guidelines state the maximum contaminant level for cadmium in drinking water is 5 ppb.[11] To detect cadmium contamination in water sources is vital in order to reduce human intake and prevent adverse health effects. Metal-organic frameworks (MOFs) offer a relatively new technological approach towards the development of new chemosensors capable of detecting
and removing target analytes from water, including metal ions.[12,13] MOFs are highly porous crystalline organic-inorganic materials with metal ion or cluster nodes with organic ligand linkers. These materials are highly tunable, offering a unique opportunity to engineer selectivity and reactivity into the porous material for preferential interactions with target analytes, like metal ions, via strategic ligand design or metal node selection. Furthermore, MOFs often contain highly conjugated organic linkers, which are apt to fluoresce upon excitation or be ultra-violet (UV) active. Upon the interaction of the ligands with target-analytes, a detectable optical response such as a change in emission properties (emission intensity or emission wavelength) or colorimetric change (absorbance wavelength) renders these materials attractive for chemosensing with fluorescence and/or UV-Vis detection.[14,15]
Porphyrins are excellent metal ion chelators and are therefore an ideal scaffold to incorporate into MOF ligands for metal ion sensing.[16–18] Porphyrins are a rigid, highly conjugated square-planar class of organic compounds that are prevalent in nature capable of complexing metals by way of 4-center facing nitrogens. Hemoglobin for instance, a biological transporter of oxygen, contains an iron-metallated porphyrin moiety. Porphyrins may also be easily diversified to tune the electronics of the material by using pre-functionalized pyrroles and aldehydes during synthesis. Incorporating such moieties into the ligands of MOFs produces materials strategically engineered for metal ion sensing. The compound tetrakis(4-carboxyphenyl)porphine (TCPP, Figure 1) has been well characterized and is commercially available as a chelating reagent for the selective trace heavy metal detection for Pb2+, Ni2+, Cu2+, Cd2+, and Mn2+, determinable with high performance liquid chromatography (HPLC).[19] One hypothesis for achieving efficient Cd2+ metal ion detection is thus to incorporate TCPP as the ligand of a MOF. Increased surface area within MOFs concentrates analytes to the surface (ligands) of the MOF and often result in enhanced reactivity with little solvent necessary, a result that is not feasible with the corresponding individual homogeneous components.[20–22] This would ultimately lead to faster reactions times and subsequent ion detection. In addition, the interconnected structure of MOFs may lead to enhanced detection signal due to increased mass transport through the network, yielding a highly sensitive material. The ability for the porphyrin to undergo optical changes (fluorescence and/or UV-Vis) is also advantageous for sensing, as these techniques are highly sensitive (ppb). In this study, the porphyrin MOF NU-902 is first used as a powder to determine fluorescence quenching in the presence of divalent cadmium ions, then the powder MOF is encapsulated within a polymer to create a membrane that also displays fluorescence quenching after soaking in an aqueous cadmium solution (Figure 1). NU-902 is a thermodynamically stable MOF, having zirconium (IV)–carboxylate coordination bonds and employs the porphyrin TCPP as the organic ligand. The synthesis of NU-902 was highly reproducible, yielding a dark purple crystalline MOF (>70 mg each, Figure S2). The material has high surface area (1580 m2g-1) and importantly is stable to water.[23] Additionally, preliminary studies reveal the MOF is fluorescent.[24] Accordingly, NU-902 was used for the development of a photoluminescent responsive porphyrin-based MOF metal ion chemosensor. Towards this, the photoluminescent (PL) properties of the powder form of the MOF were investigated. Excitingly, in this study the MOF was found to exhibit fluorescence emission in multiple solvents including water, an environmentally relevant solvent. The powder
Water stable Zr porphyrin MOF
Cd
MOF-polymer membrane
2+
2+
2+
Cd
Cd 2+
Cd
Figure 1. Schematic showing the work performed in this study. Water stable MOF NU-902 was synthesized with TCPP porphyrin ligand and zirconium ions. The MOF powder is incorporated into a PVC matrix to form a composite. The photoluminescent MOF powder and composite display fluorescence quenching in aqueous cadmium Fluorescence solution. sensing of Cd2+
ions inan water material displays fluorescence emission peak at 686 nm when excited at 420 nm in aqueous buffer. The photoluminescent emission of the MOF is also detectable by eye, displaying an emitted bright pink color when placed under a UV lamp (365 nm, Figure S1). As NU-902 displays fluorescent properties and the porphyrin ligand is known to bind metal ions, it was of interest to investigate the MOF for its ability to detect metal ions in solution via fluorescence. Porphyrins are well known to have fluorescent properties, and their incorporation into MOFs as ligands can even increase their fluorescent ability.[24,25] Previous literature has shown that binding of metal ions to the center of a porphyrin at the free nitrogens induces fluorescence quenching via a reverse photoinduced electron transfer process[26,27], although there are other potential
Figure 2. Normalized fluorescence intensity (I/I0) of MOF NU-902 using emission wavelength at 686 nm upon the addition of 5 mM Cd2+ containing aliquots excited at 420 nm. Detectable fluorescence quenching was observed with the addition of 0.5-200 µL of metal solution (0-102 ppb). Inset shows fluorescence quenching from additions of 0.5-20 µL of metal solution (0-10 ppb). All data points are the average of 3 tests, standard deviation bars are omitted for clarity, but can be found in the Supporting Information (Figure S4).
mechanisms of quenching.[28,29] MOF sensors are also known to have multiple interaction sites, any number of which could lead to fluorescence quenching.[13,30,31] The fluorescence intensity of the NU-902 MOFcontaining solution was determined with no metal present, then varying aliquots of a 5 mM Cd2+ solution were doped into the solution (Figure 2). The fluorescence intensity (I) with metal solution aliquots added was compared to the initial intensity (Io) without metal present. Fluorescence was measured after less than 2 minutes, at which time quenching was observed, demonstrating the potential of NU-902 as a fast chemosensor. Fluorescence quenching was observed after the addition of only 0.5 µL of a 5 mM Cd2+ solution, which is equivalent to only 0.3 ppb of cadmium. This is far below the EPA guidelines of 5 ppb (9 µL) and WHO guidelines of 3 ppb (6 µL), at which amount, the MOF displays clear fluorescence quenching as can be seen in the inset in Figure 2. Previous literature investigating porphyrin ligands and a similar zirconium-based porphyrin MOF to selectively detect heavy metal ions showed that the presence of group 1 and 2 cations and other transition metals do not affect the sensing ability of these sensors.[26,27] To highlight the selectivity of MOF NU-902 toward Cd2+ ions, other M2+ ions were investigated for fluorescence quenching abilities but were not observed to the degree that cadmium ions quench fluorescence (Figure S6). As the MOF is sensitive well below acceptable regulatory levels, it could be useful as a chemosensor to determine if drinking water is contaminated by cadmium. Due to the negative effects of even low levels of cadmium exposure, it is critical to have fast, reliable chemosensors to indicate when drinking water is contaminated, which is demonstrated here.
Carboxylate C=O stretch
Aromatic N-H stretch
Figure 4. FTIR spectra of PVC and soaking NU-902inMOF-PVC composite, Figure 3. PXRD of MOF NU-902 powder before and after a inset cm-1 showing Cd2+ solution (0.01 M) forfrom 120 1700-1300 minutes. Crystallinity of incorporation the MOF is of MOF into PVC basedtoon C=O and aromatic N-H stretches, which are characteristic of maintained after exposure aqueous cadmium solution for an extended ligand the MOF. period, demonstratingthe theporphyrin robustness of theinchemosensor.
Powder x-ray diffraction (PXRD) analysis was performed to determine the stability of the MOF to experimental conditions. Figure 3 shows that crystallinity in the MOF is maintained after soaking in a cadmium solution. The MOF is stable in the cadmium solution and does not decompose, indicating that the chemosensor is robust. To use the MOF in real-world applications (in-field detection), a necessary first step is to develop a useful material. One route is to synthetically modify and grow a MOF on the surface (SURMOF) of a material such as cotton, aluminum, or silicon. Alternatively, MOFpolymer blended composites are promising materials for such applications.[12] FeBTC-polydopamine for instance, shows recent precedence for the utility of MOF-polymer composites to selectively and efficiently remove heavy metals from water.[32]
Figure 5. (a) Normalized fluorescence intensity (I/I0) of MOF NU-902 PVC membrane upon soaking in 0.01 M Cd2+ solution using emission wavelength at 720 nm when excited at 467 nm. Detectable fluorescence quenching was observed after 5 minutes of soaking in metal solution, up to 60 minutes. Data points are the average of at least 3 scans of 3 different films, standard deviation bars are omitted for clarity, but this data may be found in the Supporting Information (Figure S5). (b) Representative plots of fluorescence counts per second (CPS) from 600-800 nm at time points from 0-60 minutes showing decreasing fluorescent intensity of MOF-PVC composite.
After investigation into polymers to form membranes with the MOF chemosensor, plasticized polyvinylchloride (PVC) was chosen for its ubiquity in commercial use and its non-fluorescent properties.[33– 35] The MOF NU-902 was blended with plasticized PVC and cast to make films that could be placed in water and removed for potential repeated use. Characterization of the MOF-PVC membranes showed that the MOF is fully incorporated into the PVC composite and that the MOF structure is maintained. The MOF is well distributed within the polymer composite based on scanning electron microscopy images (SEM, Figure S3). Fourier-transform infrared (FTIR) spectra of PVC and the MOF-blended PVC (Figure 4) shows the appearance of peaks indicating that the porphyrin ligand of NU-902 is incorporated into the PVC composite. Peaks at 1599 and 1545 cm-1 represent the carboxylate C=O stretch of the ligand, and peaks at 1425 and 1379 cm-1 indicate the aromatic N-H stretch of the porphyrin aromatic core. PXRD of the MOF-PVC composite (Figure S7) shows that crystallinity of the MOF is maintained after incorporation into the PVC membrane. To ensure that the stucture of the MOF is not affected by soaking in the cadmium solution, the MOF-PVC composite was soaked in a 0.01 M Cd2+ solution, then elemental analysis was examined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). ICP-AES determined that there was a no detectable amount of zirconium observed in the solutions (n ≥ 3), indicating that the MOF does not decompose in the aqueous cadmium solution and that the MOF is stable in solution. After confirmation of successful incorporation of the MOF into PVC without decomposition of the MOF structure, the fluorescent capabilities of the MOFpolymer composite were investigated. In the PVC b composite, the MOF displays an emission peak at 720 nm when excited at 467 nm. For these experiments, the MOF was placed in a 0.01 M Cd2+ solution for varying amounts of time. At specific time intervals, the MOFPVC film was removed from the solution and a fluorescence reading of the MOF membrane was taken using a fiber optic attachment to the fluorometer. Figure 5a shows that fluorescence quenching occurs over time similarly to the powder form of the MOF. As the exposure time of the MOF-PVC composite to the Cd2+ solution increases, the fluorescent intensity of the film decreases. This observation is further demonstrated in Figure 5b, as over time, the counts per second (CPS), or the intensity of the fluorescence, decreases over time while the MOF film soaks in a cadmium solution. These results show that as the powder form of the MOF can act as a chemosensor, when blended into a PVC polymer composite, it continues to display selective sensing capabilities. The MOF-PVC polymer film can be easily applied in real-world situations to sense cadmium contamination in drinking water.
In summary, water-stable porphyrin-based MOF NU-902 was synthesized for the detection of cadmium ions in aqueous environments. The MOF is water-stable and shows fluorescence quenching in the presence of cadmium solutions, at levels below EPA guidelines, indicating its potential usefulness as a chemosensor for water contamination. The MOF was blended to make a PVC composite that also displayed fluorescence quenching with cadmium, making a real-world applicable form for the sensor to be easily used. The next step currently underway is to perform a full fluorescence study to assess the efficacy of the MOF for other metal ion sensing beyond cadmium. Future outcomes from this study include efforts to optimize metal ion detection, which may require tuning of the MOFs electronics, achievable by modifying the porphyrin backbone. Alternatively, this may include investigating other solid support platforms for MOFs to tune the uptake kinetics of the material and overall increase the rate of ion accessibility to the blended MOF. Acknowledgments We acknowledge financial support from a grant from the Monfort Foundation. We thank Dr. Patrick McCurdy (Colorado State University) for assistance using the scanning electron microscope. References [1]
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• Water stable porphyrin MOF displays fluorescence as powder and in PVC composite • Detectable fluorescence quenching with Cd2+ ions in water below regulatory levels • MOF in PVC composite displays fluorescence quenching with Cd2+ in five minutes
Hailey Hibbard: Methodology, Validation, Investigation, Writing – Original Draft, Writing – Review & Editing, Visualization Michaela Burnley: Methodology, Validation, Investigation, Writing – Review & Editing Heather Rubin: Conceptualization, Methodology, Validation, Investigation, Writing – Original Draft, Writing – Review & Editing Jack Miera: Methodology, Investigation Melissa Reynolds: Supervision, Project administration, Funding acquisition
S1387-7003(19)30704-X https://doi.org/10.1016/j.inoche.2020.107861 INOCHE 107861
To appear in:
Inorganic Chemistry Communications
Received Date: Revised Date: Accepted Date:
18 July 2019 11 February 2020 27 February 2020
Please cite this article as: H. A. J. Hibbard, M.J. Burnley, H.N. Rubin, J.A. Miera, M.M. Reynolds, Porphyrinbased metal-organic framework and polyvinylchloride composites for fluorescence sensing of divalent cadmium ions in water, Inorganic Chemistry Communications (2020), doi: https://doi.org/10.1016/j.inoche.2020.107861
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
Porphyrin-based metal-organic framework and polyvinylchloride composites for fluorescence sensing of divalent cadmium ions in water Hailey A. J. Hibbarda, Michaela J. Burnleya, Heather N. Rubina, Jack A. Mieraa, and Melissa M. Reynoldsa,b,* Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA. School of Biomedical Engineering, Colorado State University, Fort Collins, CO 80523, USA *Corresponding author: E-mail: [email protected]
a
b
Keywords: metal-organic framework; porphyrin; cadmium; metal ion sensing; polymer composite Abstract Toxic metal contamination of Earth’s scarce fresh water sources creates a persistent necessity for the development of new materials and techniques to detect metal ions in water. Cadmium in particular is released into the environment by ubiquitous phosphate fertilizers, and chronic cadmium exposure can lead to internal organ damage and bone weakening. The present study shows that NU-902, a water stable zirconium porphyrin-based metal-organic framework (MOF), is fluorescent and undergoes a detectable change in photoluminescent emission properties in the presence of divalent cadmium ions. The MOF displays detectable fluorescence quenching in the presence of cadmium at levels of 0.3 ppb, far below concentrations deemed acceptable by most regulatory agencies, indicating its potential as a fast, reliable chemosensor for cadmium in drinking water. After establishing the fluorescent properties of the powder MOF, it was immobilized within a plasticized polyvinylchloride (PVC) to create a membrane with potential for easy in field use as a sensor. The MOF-PVC composite displayed fluorescence quenching within five minutes of exposure to an aqueous cadmium solution. This study shows the potential use of a water stable porphyrin-based MOF for fast, sensitive detection of metal ion contamination in water in powder form, as well as in a polymer composite.
Wide spread use of heavy metal ions, primarily in anthropological activities including industrial, agricultural, and technological applications, has led to their unwanted prevalence in the environment. This poses a serious threat to the health and well-being of humans and Earth’s ecosystems.[1] Cadmium is present in phosphate rocks and the use of phosphate fertilizers introduces cadmium into the environment and to food sources.[2–4] In addition, anthropogenic cadmium emissions are rapidly growing from fossil fuel combustion, smelting of zinc, lead, and copper ores, pigment manufacture, and from municipal and sewage sludge incinerators.[1–4] The main routes of exposure to cadmium are via ingestion of contaminated food or water and smoking cigarettes.[1,2,4–7] Chronic cadmium exposure leads to various adverse health effects including damage to the kidneys, liver, and lungs.[1,2,4– 8] Cadmium also interferes with bone mineralization and structure, leading to osteoporosis, osteomalacia, and ItaiItai disease, causing severe joint and spine pain.[1–9] The World Health Organization (WHO) guideline for cadmium in drinking water is 3 ug/L (3 ppb).[10] United States Environmental Protection Agency (EPA) guidelines state the maximum contaminant level for cadmium in drinking water is 5 ppb.[11] To detect cadmium contamination in water sources is vital in order to reduce human intake and prevent adverse health effects. Metal-organic frameworks (MOFs) offer a relatively new technological approach towards the development of new chemosensors capable of detecting
and removing target analytes from water, including metal ions.[12,13] MOFs are highly porous crystalline organic-inorganic materials with metal ion or cluster nodes with organic ligand linkers. These materials are highly tunable, offering a unique opportunity to engineer selectivity and reactivity into the porous material for preferential interactions with target analytes, like metal ions, via strategic ligand design or metal node selection. Furthermore, MOFs often contain highly conjugated organic linkers, which are apt to fluoresce upon excitation or be ultra-violet (UV) active. Upon the interaction of the ligands with target-analytes, a detectable optical response such as a change in emission properties (emission intensity or emission wavelength) or colorimetric change (absorbance wavelength) renders these materials attractive for chemosensing with fluorescence and/or UV-Vis detection.[14,15]
Porphyrins are excellent metal ion chelators and are therefore an ideal scaffold to incorporate into MOF ligands for metal ion sensing.[16–18] Porphyrins are a rigid, highly conjugated square-planar class of organic compounds that are prevalent in nature capable of complexing metals by way of 4-center facing nitrogens. Hemoglobin for instance, a biological transporter of oxygen, contains an iron-metallated porphyrin moiety. Porphyrins may also be easily diversified to tune the electronics of the material by using pre-functionalized pyrroles and aldehydes during synthesis. Incorporating such moieties into the ligands of MOFs produces materials strategically engineered for metal ion sensing. The compound tetrakis(4-carboxyphenyl)porphine (TCPP, Figure 1) has been well characterized and is commercially available as a chelating reagent for the selective trace heavy metal detection for Pb2+, Ni2+, Cu2+, Cd2+, and Mn2+, determinable with high performance liquid chromatography (HPLC).[19] One hypothesis for achieving efficient Cd2+ metal ion detection is thus to incorporate TCPP as the ligand of a MOF. Increased surface area within MOFs concentrates analytes to the surface (ligands) of the MOF and often result in enhanced reactivity with little solvent necessary, a result that is not feasible with the corresponding individual homogeneous components.[20–22] This would ultimately lead to faster reactions times and subsequent ion detection. In addition, the interconnected structure of MOFs may lead to enhanced detection signal due to increased mass transport through the network, yielding a highly sensitive material. The ability for the porphyrin to undergo optical changes (fluorescence and/or UV-Vis) is also advantageous for sensing, as these techniques are highly sensitive (ppb). In this study, the porphyrin MOF NU-902 is first used as a powder to determine fluorescence quenching in the presence of divalent cadmium ions, then the powder MOF is encapsulated within a polymer to create a membrane that also displays fluorescence quenching after soaking in an aqueous cadmium solution (Figure 1). NU-902 is a thermodynamically stable MOF, having zirconium (IV)–carboxylate coordination bonds and employs the porphyrin TCPP as the organic ligand. The synthesis of NU-902 was highly reproducible, yielding a dark purple crystalline MOF (>70 mg each, Figure S2). The material has high surface area (1580 m2g-1) and importantly is stable to water.[23] Additionally, preliminary studies reveal the MOF is fluorescent.[24] Accordingly, NU-902 was used for the development of a photoluminescent responsive porphyrin-based MOF metal ion chemosensor. Towards this, the photoluminescent (PL) properties of the powder form of the MOF were investigated. Excitingly, in this study the MOF was found to exhibit fluorescence emission in multiple solvents including water, an environmentally relevant solvent. The powder
Water stable Zr porphyrin MOF
Cd
MOF-polymer membrane
2+
2+
2+
Cd
Cd 2+
Cd
Figure 1. Schematic showing the work performed in this study. Water stable MOF NU-902 was synthesized with TCPP porphyrin ligand and zirconium ions. The MOF powder is incorporated into a PVC matrix to form a composite. The photoluminescent MOF powder and composite display fluorescence quenching in aqueous cadmium Fluorescence solution. sensing of Cd2+
ions inan water material displays fluorescence emission peak at 686 nm when excited at 420 nm in aqueous buffer. The photoluminescent emission of the MOF is also detectable by eye, displaying an emitted bright pink color when placed under a UV lamp (365 nm, Figure S1). As NU-902 displays fluorescent properties and the porphyrin ligand is known to bind metal ions, it was of interest to investigate the MOF for its ability to detect metal ions in solution via fluorescence. Porphyrins are well known to have fluorescent properties, and their incorporation into MOFs as ligands can even increase their fluorescent ability.[24,25] Previous literature has shown that binding of metal ions to the center of a porphyrin at the free nitrogens induces fluorescence quenching via a reverse photoinduced electron transfer process[26,27], although there are other potential
Figure 2. Normalized fluorescence intensity (I/I0) of MOF NU-902 using emission wavelength at 686 nm upon the addition of 5 mM Cd2+ containing aliquots excited at 420 nm. Detectable fluorescence quenching was observed with the addition of 0.5-200 µL of metal solution (0-102 ppb). Inset shows fluorescence quenching from additions of 0.5-20 µL of metal solution (0-10 ppb). All data points are the average of 3 tests, standard deviation bars are omitted for clarity, but can be found in the Supporting Information (Figure S4).
mechanisms of quenching.[28,29] MOF sensors are also known to have multiple interaction sites, any number of which could lead to fluorescence quenching.[13,30,31] The fluorescence intensity of the NU-902 MOFcontaining solution was determined with no metal present, then varying aliquots of a 5 mM Cd2+ solution were doped into the solution (Figure 2). The fluorescence intensity (I) with metal solution aliquots added was compared to the initial intensity (Io) without metal present. Fluorescence was measured after less than 2 minutes, at which time quenching was observed, demonstrating the potential of NU-902 as a fast chemosensor. Fluorescence quenching was observed after the addition of only 0.5 µL of a 5 mM Cd2+ solution, which is equivalent to only 0.3 ppb of cadmium. This is far below the EPA guidelines of 5 ppb (9 µL) and WHO guidelines of 3 ppb (6 µL), at which amount, the MOF displays clear fluorescence quenching as can be seen in the inset in Figure 2. Previous literature investigating porphyrin ligands and a similar zirconium-based porphyrin MOF to selectively detect heavy metal ions showed that the presence of group 1 and 2 cations and other transition metals do not affect the sensing ability of these sensors.[26,27] To highlight the selectivity of MOF NU-902 toward Cd2+ ions, other M2+ ions were investigated for fluorescence quenching abilities but were not observed to the degree that cadmium ions quench fluorescence (Figure S6). As the MOF is sensitive well below acceptable regulatory levels, it could be useful as a chemosensor to determine if drinking water is contaminated by cadmium. Due to the negative effects of even low levels of cadmium exposure, it is critical to have fast, reliable chemosensors to indicate when drinking water is contaminated, which is demonstrated here.
Carboxylate C=O stretch
Aromatic N-H stretch
Figure 4. FTIR spectra of PVC and soaking NU-902inMOF-PVC composite, Figure 3. PXRD of MOF NU-902 powder before and after a inset cm-1 showing Cd2+ solution (0.01 M) forfrom 120 1700-1300 minutes. Crystallinity of incorporation the MOF is of MOF into PVC basedtoon C=O and aromatic N-H stretches, which are characteristic of maintained after exposure aqueous cadmium solution for an extended ligand the MOF. period, demonstratingthe theporphyrin robustness of theinchemosensor.
Powder x-ray diffraction (PXRD) analysis was performed to determine the stability of the MOF to experimental conditions. Figure 3 shows that crystallinity in the MOF is maintained after soaking in a cadmium solution. The MOF is stable in the cadmium solution and does not decompose, indicating that the chemosensor is robust. To use the MOF in real-world applications (in-field detection), a necessary first step is to develop a useful material. One route is to synthetically modify and grow a MOF on the surface (SURMOF) of a material such as cotton, aluminum, or silicon. Alternatively, MOFpolymer blended composites are promising materials for such applications.[12] FeBTC-polydopamine for instance, shows recent precedence for the utility of MOF-polymer composites to selectively and efficiently remove heavy metals from water.[32]
Figure 5. (a) Normalized fluorescence intensity (I/I0) of MOF NU-902 PVC membrane upon soaking in 0.01 M Cd2+ solution using emission wavelength at 720 nm when excited at 467 nm. Detectable fluorescence quenching was observed after 5 minutes of soaking in metal solution, up to 60 minutes. Data points are the average of at least 3 scans of 3 different films, standard deviation bars are omitted for clarity, but this data may be found in the Supporting Information (Figure S5). (b) Representative plots of fluorescence counts per second (CPS) from 600-800 nm at time points from 0-60 minutes showing decreasing fluorescent intensity of MOF-PVC composite.
After investigation into polymers to form membranes with the MOF chemosensor, plasticized polyvinylchloride (PVC) was chosen for its ubiquity in commercial use and its non-fluorescent properties.[33– 35] The MOF NU-902 was blended with plasticized PVC and cast to make films that could be placed in water and removed for potential repeated use. Characterization of the MOF-PVC membranes showed that the MOF is fully incorporated into the PVC composite and that the MOF structure is maintained. The MOF is well distributed within the polymer composite based on scanning electron microscopy images (SEM, Figure S3). Fourier-transform infrared (FTIR) spectra of PVC and the MOF-blended PVC (Figure 4) shows the appearance of peaks indicating that the porphyrin ligand of NU-902 is incorporated into the PVC composite. Peaks at 1599 and 1545 cm-1 represent the carboxylate C=O stretch of the ligand, and peaks at 1425 and 1379 cm-1 indicate the aromatic N-H stretch of the porphyrin aromatic core. PXRD of the MOF-PVC composite (Figure S7) shows that crystallinity of the MOF is maintained after incorporation into the PVC membrane. To ensure that the stucture of the MOF is not affected by soaking in the cadmium solution, the MOF-PVC composite was soaked in a 0.01 M Cd2+ solution, then elemental analysis was examined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). ICP-AES determined that there was a no detectable amount of zirconium observed in the solutions (n ≥ 3), indicating that the MOF does not decompose in the aqueous cadmium solution and that the MOF is stable in solution. After confirmation of successful incorporation of the MOF into PVC without decomposition of the MOF structure, the fluorescent capabilities of the MOFpolymer composite were investigated. In the PVC b composite, the MOF displays an emission peak at 720 nm when excited at 467 nm. For these experiments, the MOF was placed in a 0.01 M Cd2+ solution for varying amounts of time. At specific time intervals, the MOFPVC film was removed from the solution and a fluorescence reading of the MOF membrane was taken using a fiber optic attachment to the fluorometer. Figure 5a shows that fluorescence quenching occurs over time similarly to the powder form of the MOF. As the exposure time of the MOF-PVC composite to the Cd2+ solution increases, the fluorescent intensity of the film decreases. This observation is further demonstrated in Figure 5b, as over time, the counts per second (CPS), or the intensity of the fluorescence, decreases over time while the MOF film soaks in a cadmium solution. These results show that as the powder form of the MOF can act as a chemosensor, when blended into a PVC polymer composite, it continues to display selective sensing capabilities. The MOF-PVC polymer film can be easily applied in real-world situations to sense cadmium contamination in drinking water.
In summary, water-stable porphyrin-based MOF NU-902 was synthesized for the detection of cadmium ions in aqueous environments. The MOF is water-stable and shows fluorescence quenching in the presence of cadmium solutions, at levels below EPA guidelines, indicating its potential usefulness as a chemosensor for water contamination. The MOF was blended to make a PVC composite that also displayed fluorescence quenching with cadmium, making a real-world applicable form for the sensor to be easily used. The next step currently underway is to perform a full fluorescence study to assess the efficacy of the MOF for other metal ion sensing beyond cadmium. Future outcomes from this study include efforts to optimize metal ion detection, which may require tuning of the MOFs electronics, achievable by modifying the porphyrin backbone. Alternatively, this may include investigating other solid support platforms for MOFs to tune the uptake kinetics of the material and overall increase the rate of ion accessibility to the blended MOF. Acknowledgments We acknowledge financial support from a grant from the Monfort Foundation. We thank Dr. Patrick McCurdy (Colorado State University) for assistance using the scanning electron microscope. References [1]
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• Water stable porphyrin MOF displays fluorescence as powder and in PVC composite • Detectable fluorescence quenching with Cd2+ ions in water below regulatory levels • MOF in PVC composite displays fluorescence quenching with Cd2+ in five minutes
Hailey Hibbard: Methodology, Validation, Investigation, Writing – Original Draft, Writing – Review & Editing, Visualization Michaela Burnley: Methodology, Validation, Investigation, Writing – Review & Editing Heather Rubin: Conceptualization, Methodology, Validation, Investigation, Writing – Original Draft, Writing – Review & Editing Jack Miera: Methodology, Investigation Melissa Reynolds: Supervision, Project administration, Funding acquisition