and its nanocomposite formed by copper doping
Item Type Article
Authors Khouqeer, Ghada;Alghrably, Mawadda;Madkhali, Nawal;Dhahri, Manel;Jaremko, Mariusz;Emwas, Abdul-Hamid M.
Citation Khouqeer, G., Alghrably, M., Madkhali, N., Dhahri, M., Jaremko, M., & Emwas, A. (2022). Preparation and characterization of natural melanin and its nanocomposite formed by copper doping.
Nano Select. Portico. https://doi.org/10.1002/nano.202200095 Eprint version Publisher's Version/PDF
DOI 10.1002/nano.202200095
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Journal Nano Select
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DOI: 10.1002/nano.202200095
R E S E A R C H A R T I C L E
Preparation and characterization of natural melanin and its nanocomposite formed by copper doping
Ghada Khouqeer
1Mawadda Alghrably
2Nawal Madkhali
1Manel Dhahri
3Mariusz Jaremko
4Abdul-Hamid Emwas
51Department of Physics, College of Science, Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh, Saudi Arabia
2Division of Biological and
Environmental Sciences and Engineering (BESE), King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia
3Biology Department, Faculty of Science Yanbu, Taibah University, Yanbu El Bahr, Saudi Arabia
4Smart-Health Initiative (SHI) and Red Sea Research Center (RSRC), Division of Biological and Environmental Sciences and Engineering (BESE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
5Core Labs, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia
Correspondence
Ghada Khouqeer, Department of Physics, College of Science, Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh, Saudi Arabia.
Email:[email protected]
Mariusz Jaremko, Smart-Health Initiative (SHI) and Red Sea Research Center (RSRC), Division of Biological and Environmental Sciences and Engineering (BESE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia.
Email:[email protected]
Abstract
Natural Melanins have received great interest due to their structural, ther- mostability, electrochemical and paramagnetic properties. Pure melanin (Mel) is extracted from Nigella Sativa seeds and doped with Copper (Cu-Mel) to form nano-composite melanin using wet chemical methods. Several analyt- ics techniques are utilized to evaluate the effects of cop-per doping on the characteristics of melanin. Thermogravimetric analysis (TGA) is employed to measure the thermal stability of Cu-Mel relative to Mel. The morphological characterization is explored using Transmission Electron Microscopy (TEM) while X-Ray Diffraction (XRD) is used to probe the samples’ structure. Vibrat- ing Sample Magnetometer (VSM) is employed to study the magnetic properties of the prepared samples at 10 and 300K. Chemical properties are determined by Spectroscopic techniques such as UV–Vis spectroscopy, RAMAN spectroscopy, Fourier-Transform Infrared Spectroscopy (FTIR), Nuclear Magnetic Resonance (NMR) spectroscopy, and Electron Paramagnetic Resonance (EPR) spectroscopy.
The results show that Mel and Cu-Mel are non-crystalline with different struc- tural, optical, and magnetic properties. The EPR analysis provided evidence of copper coordination with herbal Mel and the copper doping improved the mag- netic properties of Mel from diamagnetic to paramagnetic at low temperature (10K). Overall, this investigation shows that Mel and Cu-Mel are non-crystalline with different structural and magnetic properties.
K E Y W O R D S
copper-melanin nanocomposites, magnetization, melanin
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1 INTRODUCTION
Melanin (Mel) is a group of common natural pigments found in most organisms, consisting of biopolymers with unique physicochemical and biological properties.[1–3]
Mel has various important functions as a protective layer against harmful ultraviolet (UV) light radiation or in a host’s immunological response.[4–6] The chem- ical structure of the pigment allows for effective ther- moregulation of living organisms. Given its various functions, it has been utilized in various biomedical devices, such as bioelectronics, edible electronics, opto- electronics, and sensor devices.[7–10] In addition, recent work on Melanin nanocomposites has shown promising potential in bio-application, including nanocosmetics and nanomedicine.[11]Especially with the use of nanocatalysts, melanin-like nanomaterials have shown great potential in fields like phototherapy by participating in metal-organic frameworks.[12,13] Indeed, several natural and artificial melanin-based nanoparticles have been proposed as a promising addition to different industrial sectors for their remarkable electronic, optical, photophysical, and pho- tochemical properties.[14] This makes sense given how doped-Mel with transition metals improves desirable phys- iochemical properties. For example, the linkage between natural and synthetic melanin plays a key role in the collection and discharge of metal cations in vivo, includ- ing calcium, copper, zinc, manganese, iron, and other metals.[15,16]
Melanin’s biocompatibility, optical absorption, and charge carrier transport characteristics may form the foun- dation of future technologies taking advantage of the inter- faces between melanin and metal oxides. Yet, melanin’s poor processability prevents interaction between the pig- ment and the oxide surface, making these types of interfaces mostly unknown.[16–19]Furthermore, functional groups affiliated with melanin’s molecular structure are fundamental to its metal pairing abilities. Two key points can summarize the roles of melanin in the metabolic pro- cesses. The first is melanin’s ability to serve as a reservoir for metal ions which enables the storage and release and transfer of mineral ions. The second is melanin’s abil- ity to firmly attach and segregate reactive metals, helping mitigate oxidative stress.[20–22]
Melanin has multiple intriguing physical properties, such as broad-band ultraviolet and visible absorption, along with a strong non-radiative relaxation of photoex- cited electronic states.[23]Moreover, it exhibits a variety of chemical properties, such as antioxidant and free radical- scavenging behaviors.[24,25] Recent studies have demon- strated that melanin’s photoconductivity strongly depends on the relative humidity at room temperature.[1,26] All these properties help explain why melanin is useful as a
soft solid in electronic applications, such as photovoltaic, sensor, and photothermal detectors, and other technolog- ical devices.[27–30] However, the mechanism underlying melanin’s conductivity to different physical environment such as hydration remains an open question. This has led to the development of an alternate charge transport model to be able to explain the experimental results.[28]Still, it has already been used in a wide range of industrial applications such as the development of organic electronics, bioelec- tronics, nanoparticles, and bio interfaces.[31–33]Moreover, Mel, along with Mel-like materials, are widely used in the biomedical field in different roles such as imaging, enzyme kinetics, and reactive oxygen species (highly reac- tive molecular oxygen derivatives) scavenging. A recent paper by Cavallini et al. explored the different uses in their review, which highlighted utility in cell attachment as well as skin, nerve, and bone healing.[34]At the nano and microscale level, Mel has a heterogeneous structure, making structural studies challenging.
While Melanin has been heavily studied in the literature, analytical characterizations remain a challenging task.
First, there are various heterogenous sources of melanin, including both animal and plant sources.[35] Second, the usage of different analytical techniques limits equivalency and comparative analysis. Moreover, the number of studies that have looked at Copper-doping of Melanin is lim- ited compared to the overall work in the field. In this study, pure and copper-doped Melanin to investigate the improvement of Melanin’s properties with particular focus on magnetic properties; as recommended by Pralea et al., the samples were fully characterized using Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), electron paramagnetic resonance (EPR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, vibrat- ing sample magnetometer (VSM), ultraviolet (UV), and inferred (IR) spectroscopy.[35]
2 MATERIALS AND METHODS 2.1 Sample preparation
The extraction procedure of herbal melanin from Nigella sativa seeds was performed as described in El-Obeid et al.[5]The seed coats were dissolved for 3 hours in NaOH at a pH of 12.5, producing a dark brown solution.
A stock solution was made through centrifugation in order to extract the Mel by using HCl with a pH of 2. Alkali – acid therapy was performed 2–3 times to produce Mel with higher purity. The precipitate was washed with distilled water and dried at a temperature of 80◦C. Dry powder was stored and subsequently used to prepare certain solutions at pH 7 for the analysis by dissolving the desired amount
of Mel powder in NaOH at pH 12.5 using concentrated HCl to adjust the pH to the desired concentration. The herbal Mel was kept frozen at a temperature of−20◦C to prevent photochemical and photophysical mutations.
To prepare Copper-melanin Nanocomposites (Cu-Mel), 1 м sodium hydroxide (Sigma Aldrich) was added to five grams of the extracted organic Mel powder and mixed for an hour at 300◦C using an ultrasonic device. One mole of hydrochloric acid with 36% concentration was used to normalize the solution pH. Next, CuCl2 metal salt solvent was added with a ratio of 1:20 to the Mel solution with a 30% concentration. After isolating the mixture, the solu- tion changed color and gradually dissolved within a couple of weeks due to precipitation of the solid granules. The final sample was washed with deionized water (DIW), then filtered and dried in the oven at 50◦C over a period of 3 days. Dried Cu-Mel powder nanocomposites were pressed at 1000 MPa into 0.5 mm thick pellets one inch in diameter by employing KBr.
2.2 Thermogravimetric analysis (TGA)
Experiments were performed using a thermogravimet- ric analyzer (NETZSCH TG 209F1 Iris) which measured changes in sample mass as a function of temperature with high levels of sensitivity. Temperature was set from 25◦C to 600◦C using aluminum oxide crucibles in an atmosphere of nitrogen gas at a flow rate of 20 mL min–1 and a heat- ing rate of 10 K/min. The accuracy of the sample mass measurement was 1×10−7 g.
2.3 Elemental analysis
The percentage of carbon, hydrogen, nitrogen, and sulphur in the samples was estimated using an Organic Elemental Analyzer OEA Flash 2000 (Thermo scientific).
2.4 X-ray diffraction
Structural study of the Mel and Cu-Mel samples was per- formed using X-ray diffraction (XRD) using a Bruker D8 Discover diffractometer with Cu-Kα radiation (λ=1.5406 Å).
2.5 Transmission electron microscopy
The morphology of the samples was investigated utilizing high-resolution Transmission Electron Microscopy (TEM) (FE-TEM, JEM-2100 F JEOL, Japan). The specimens for
the TEM were prepared by dissolving the samples in NaOH (0.1м) and immersing them in an ultrasonic bath for 15 minutes, then dropping a few drops of the resulting suspension onto the TEM grid.
2.6 RAMAN spectroscopy
A Bruker dispersive Raman microscope (Senterra model) was used to record Raman spectra for powder samples.
The wavenumbers ranged from 500 to 3500 cm–1 in this study. The specimen was excited using a He/Ne laser with a wavelength of 473 nm and a power of 5 mW.[36]
2.7 Fourier-transform infrared spectroscopy (FTIR)
Mel powder was crushed along with spectrometry grade KBr to prepare the sample for FTIR measurement. After that, the homogeneous mixture was compacted into tablets and scanned with an FTIR spectrophotometer. Although the IR spectra of pure Mel and Mel containing copper are slightly different, there are numerous distinct bands that can be used to identify the Mel macromolecule major func- tional groups. Non-destructive analysis was performed on the Mel-KBr disks using an FT-IR Vertex 70 Spectrometer with a Raman RAM II module and an IR probe mod- ule. The FTIR spectra were taken in transmission mode, with absorbance conversion and vector normalization, on the spectral domain 400–4000 cm–1, spectral resolution of 4 cm–1and 64 scans/sample.
2.8 Nuclear Magnetic Resonance (NMR) spectroscopy
A Bruker 400 MHz AVANACIII NMR spectrometer equipped with 4 mm Bruker double resonance MAS probe (BrukerBioSpin, Rheinstetten, Germany) was used to record the CPMAS NMR spectrum of Mel samples at room temperature. The 1D 13C NMR spectrum was recorded using a cross polarization CP pulse program using reported parameters.[37]The 13C signals were referenced to the methylene signal of adamantine at 37.78 ppm. The Two-dimensional heteronuclear correlation (HETCOR) spectrum was recoded using published parameters.[38,39]
Bruker Topspin 3.5pl7 software (Bruker BioSpin, Rhein- stetten, Germany) was used for data collection and for data analysis. The EPR spectra were recorded using an x-band continuous wave Bruker EMX PLUS spectrometer equipped with Q (ER 4122 SHQ) resonator using recently reported parameters.[40,41]Bruker Xenon software (Bruker
BioSpin, Rheinstetten, Germany) was used to collect data and for post processing.
2.9 Vibrating Sample Magnetometer (VSM)
The samples were mounted in a plastic holder. Experi- ments were carried out in a SQUID-VSM magnetometer (Quantum Design, USA) at 10K and at 300K in the field range of±7 Tesla for magnetization measurements.
2.10 UV visible light spectroscopy
The absorption of Mel and Cu doped Mel pellets with thickness 0.001 m were measured at room temperature using a Shimadzu UV Probe UV-1650 PC double beam with a wavelength range between 200 and 900 nm in absorbance mode. The wavelength accuracy is ± 0.5 nm, and the p accuracy of absorbance is±0.2.
2.11 Electron Paramagnetic Resonance (EPR) spectroscopy
The paramagnetic properties of Mel samples were deter- mined using a Bruker EMX PLUS spectrometer equipped with standard resonator for high sensitivity CW-EPR (Bruker BioSpin, Rheinstetten, Germany). Bruker Xenon software (Bruker BioSpin) was used to collect data and for processing.[41]
3 RESULTS AND DISCUSSION
3.1 Thermogravimetric analysis (TGA)
Thermogravimetric curves of both Mel and Cu-Mel at a heating rate of 10K min–1at 25◦C to 600◦C are illustrated in Figure 1. The result shows that the two samples have one large weight drop of approximately 70% in the region between 200◦C and 500◦C. The entire thermal decompo- sition process of both samples begins at the same heating point,∼170◦C. The mass loss of the Mel sample is slightly higher than Cu-Mel.
3.2 Elemental analysis
Elemental analysis of Mel and Cu-Mel shows a minor dif- ference between each compound in terms of organic con- tent. Mel had the following composition: 52.93% carbon,
F I G U R E 1 TGA curves of Mel (A), and Cu-Mel (B)
T A B L E 1 Elemental analysis of Mel samples
N [%] C [%] H [%] S [%] O [%]
Mel 6.76 52.93 7.20 0.015 24.78
MelCuCl2 7.36 52.12 7.42 0.17 26.41
7.20% hydrogen, 24.78% oxygen, 0.01% sulphur, and 6.75%
nitrogen. Cu-Mel gave the following molar ratio: 52.11%
carbon, 7.42% hydrogen, 26.41% oxygen, 0.17% sulphur, and 7.35% nitrogen (Table 1). The elemental composition of melanin samples are comparable to those isolated from Cryptococcus neoformans.[42]
3.3 X-ray diffraction
The X-ray diffraction pattern of natural Mel and (Cu-Mel) is shown in Figure2. The XRD pattern is characterized by a broad peak indicating a lack of definite structure. In fact, pure melanin is known to not have a definite structure.[43]
As for Cu-Mel, the pattern also shows a broad peak, high- lighting that the the predominant phase is melanin with copper only giving rise to small peaks to the pattern.
3.4 Transmission electron microscopy
The TEM micrograph of pure Mel and Cu- Mel are shown in Figure3. The result of pure Mel microscopy (Figure3A)
F I G U R E 2 XRD pattern of both Mel and Cu-Mel
shows that it is formed by many aggregates agglomerated together. This is due to a small spherical granule with the contribution of varying size distributions.[44]The Cu ions, when added, moved and diffused inside the Mel as indi- cated by the dark spots in Figure3B. The micrographs of Pure Mel and Cu-Mel confirmed the XRD results above as our samples did not have a definite structure. This is related to the predominant phase of Mel in all samples, which is characterized by a noncrystalline structure (4).
3.5 RAMAN spectroscopy
RAMAN spectroscopy data is informative for determining different vibrational, rotational, and low-frequency modes.
RAMAN spectroscopy provides comparative structural fin- gerprints for herbal Mel and Cu-Mel. Mel typically had two major peaks ranging between 1300 cm–1and 1600 cm–1, as shown in Figure4. The peaks at 1362–1and 1383 cm–1were assigned to indole C–N stretching. The peaks at 1578 cm–1 and 1591 cm–1were attributed to aromatic C=C stretch-
ing of the indole structure and C = N stretching/N–H bending.[45]
3.6 Magnetic properties
Figure 5A shows magnetization versus an applied mag- netic field for Mel samples at 10 and 300K. At room tem- perature (300K) Mel has a diamagnetic behavior, while it has paramagnetic properties at low temperature (10K) with weak ferromagnetic behavior. Figure5Bshows the effect of Cu doping on the magnetic properties of Mel. Figure5C overlaps the results from the previous two figures. At room temperature, the diamagnetic behavior of Mel is weak- ened and tends towards paramagnetic character. However, the paramagnetic behavior becomes clear at 10K. These results highlight how copper doping improved the param- agnetic properties. The obtained values of coercivity (Hc) and remanence magnetization (Mr) of undoped and Cu- Mel are listed in Table2. It shows that the coercivity (Hc) of undoped Mel increased with decreasing the temperature.
However, the presence of copper had an inverse influence on the coercivity when the temperature decreased.
3.7 UV visible light absorption spectrum
The UV-visible absorption spectrum of Mel was used pri- marily to characterize and describe the extracted melanin pigment. Complex conjugated molecules in Mel highly absorb UV light. The optical absorption spectra of pure Mel and Copper doped Mel (Cu-Mel) were measured at room temperature in the range of 200 to 900 nm, as shown in Figure 6A. The absorption of the pure Mel and Cu-Mel samples decreased exponentially. Both spectra have sharp
F I G U R E 3 TEM micrography of (A) Mel and (B) Cu-Mel
F I G U R E 4 RAMAN spectra of Mel (A) and Cu-Mel (B) obtained with 473 nm laser with 5 mW, using 300 g mm–1, resolution rel. 6 cm−1. Inset is a high-resolution RAMAN at 1800 g mm−1cm−1
absorption peaks in the UV region. Absorption is increased in the 200 to 350 nm region due to incorporation of Cu ions into Mel nanoparticles in the Cu-Mel sample. The Cu-Mel sample produced a monotone broad band, which could be attributed to copper metal’s connection with the functional groups in Mel. The direct band gap was determined using the Tauc’s equation:[46]
𝛼ℎ𝜈 = 𝐴(ℎ𝜈 − 𝐸𝑔)𝑛
where α is the absorption coefficient, h is Planck’s con- stant, A is a constant,νis the photon frequency,Egis the energy band gap and n is equal to½for a direct band gap semiconductor.
Figure6Bshows how Mel’s band gap energy decreased from 3.83 to 3.64 eV after it was doped with Cu++ions. This is due to a connection between the Cu atom’s 3d orbital and the p orbital of the O atom in the Mel monomers’ hydroxyl group, which reduces the direct band gap.[47]
3.8 Fourier-transform infrared spectroscopy (FTIR)
FTIR is a spectroscopic technique that is widely used to investigate and characterize organic materials.[6,48]
Figure7presents the comparative FTIR spectra of pure Mel and copper doped Mel. The wavelength and intensity of mid-infrared light absorption by Mel was measured using FTIR spectroscopy. Mid-infrared light (between 4000 and 200 cm–1) has enough energy to excite molecular vibrations to higher energy states. IR absorption bands have wave- lengths that are specific to certain types of chemical bonds, hence IR spectroscopy is most useful for qualitative inves- tigation of organic and organometallic compounds..[48]
Figure5shows that the FTIR spectra of herbal melanin extracted from Nigella Sativa seeds presented structural properties in compliance with natural melanin reported
in the literature. Furthermore, the addition of cop- per did not have a noticeable impact on Mel chemi- cal structure. Table 3 contain some characteristic peaks from the spectra and their assignments. The wide band in the region from 3100 to 3300 cm−1 is attributed to OH group.[48]Additionally, the broad absorption band observed at 3296.83 cm–1 has the characteristic of O–H or N–H stretching vibration modes. Centeno et al. com- mented on a broad absorption between 3600 and 3200 cm–1 spectral regions, resulting from O–H and N–H stretching vibrations of the carboxylic acid, and phenolic, in addi- tion to aromatic amino functions present in the indolic and pyrrolic systems.[48] The same peak was reported by Apte et al. at 3438 cm–1.[49] The stretching vibration of the aliphatic C–H group was assigned to a peak at 2925 cm−1.[50]The O–H and N–H stretching vibrations of the aromatic amino, amide, amine, carboxylic acid, pheno- lic groups found in the indolic and pyrrolic structures are assigned to the signals in the range of 3600–2800 cm–1.[51]
The peaks in the region between 1654 and 1540 cm–1 are associated to aromatic C = C and C = N and to the C =O bond of carboxylic function that are characteris- tic of the biological origin of melanin.[52] Furthermore C=O stretching is responsible for the band at∼1710 cm–1. Melanin pigments are characterized by the occurrence of bands between 1400 and 1500 cm–1 that are correlated to aliphatic C–H groups.[53] CH in-plane of aliphatic struc- ture, that is a hallmark of melanin pigment, is indicated by the peak at 1045 cm–1. The signals detected in the range of 950 to 750 cm–1 is correlated to the aromatic C–H out-of- plane bend. The bands under 700 cm–1are characteristic band in the melanin pigment and are attributed to Alkene C–H substitution.[51]
In this study the absorption peak at 2925.5 cm–1had a higher intensity in the herbal Mel sample than that in the Cu-Mel sample, and this could be assigned to the stretch- ing vibration of aliphatic C–H group, as similar peaks have been observed in literature.[44,45,51,54,55]
F I G U R E 5 Magnetic hysteresis loops of (A) Mel, (B) Cu-Mel, and (C) both at 10oK and 300oK
T A B L E 2 Magnetic properties of Mel and Cu-Mel
Sample Mr[emu g–1] Hc[Oe]
Mel (10k0) 1.29×10−3 118.5
Mel (300k0) 3×10−4 40.08
Cu-Mel (300k0) 2.27×10−4 86.3
F I G U R E 6 (A) The UV-VIS spectra of Mel (gray) and Cu-Mel (red), (B) The relation between (αhv)[2]and photon energy (hv)
3.9 Solid-State NMR spectroscopy
NMR spectroscopy is a versatile analytical tool that has been developed and widely used for molecular identi- fication and for structural elucidation.[57–60] The main advantage of NMR spectroscopy is its ability to study molecules at the atomic level. Information on the envi- ronment of each atom and neighboring nuclei can be evaluated to allow researchers to differentiate the unique NMR signal of the same nuclei in different positions of a single molecule.[61,62]Due to its insolubility, and amor- phous and heterogeneous nature, specific methods like solid state NMR are necessary to investigate the chemical structure of Mel.[42]
The NMR spectrum of Mel (Figure8A) showed three common spectral regions as previously reported for Mel of different origin.[63–65] Resonances in the regions 10–
95 ppm, 95–145 ppm, and 165–200 ppm was respectively correlated to aliphatic carbons, aromatic carbons, and car- bonyl carbons.[64]A peak at 172 ppm is typical of quinine
F I G U R E 7 FTIR spectra of herbal Mel (A) and herbal Cu-Mel (B)
T A B L E 3 Spectral position of FTIR peaks, respective center positions, and their corresponding assignments, Figure7
Peaks Center [cm−1] Assignments References
1 3296.83 O–H or N–H stretching vibration modes [54]
2 3008.36 O–H or N–H stretching vibration modes [54]
3 2925.54 stretching vibration of aliphatic C–H group [54], [55]
4 2853.17 stretching vibration of aliphatic C–H group [54], [55]
5 1654.83 Bending vibrations modes of aromatic ring C=C and C=N bond of aromatic [49], [55]
6 1540.81 Bending vibrations modes of aromatic ring C=C and C=N bond of aromatic [49], [55]
7 1456.83 aliphatic C–H group [49], [55]
8 1412.02 aliphatic C–H group [49], [55]
13 1045.83 C–H in plane/C–H out of plane deformation [56]
17 668.15 Out-of-plane bending of the aromatic C–H bond [56]
groups of the carbonyl group of Mel samples of different origin.[45]
Based on what we know about natural or synthetic Mel, the peaks detected in the aromatic regions are related to aromatic carbons including indole or pyrrole form of carbon.[64] The existence of the alkyl group, methyl, or methylene might explain the aliphatic carbon in the 10–
40 ppm range. The peaks in the 40–60 ppm range are attributable to the presence of -carbon or carbon in the CH- N/CH-S.[45]In addition, two-dimensional HETCOR NMR spectra was employed to correlate directly between proton and carbon, results of which are shown in Figures8Band 8C. The spectrum provides a clear support of the 1D 13C NMR spectrum where the proton peaks at 2 and 4 ppm are associated with aliphatic protons and the cross peaks around 7–8 ppm are typical aromatic peaks
3.10 Electron Paramagnetic Resonance (EPR) Spectroscopy
EPR is a potent technique to study species with unpaired electrons such as free radicals[66–68] and paramagnetic
transition metals.[41,69–71]In this study we employed ERP spectroscopy to study copper complexation with Mel (Figure 9). The EPR data reaffirmed that copper coordi- nates with Mel as reported previously.[72] Najder et al.
concluded that the paramagnetic centered concentration in Mel decreases because of Cu (II) cations.[72] EPR has filled an important gap in the characterization of Mel pig- ments, making it an important and effective technique.[73]
In 2019, Z ˙ądło used EPR to conclude that Mel-bound iron could contribute to retinal pathology.[74] We utilized X- band continuous EPR spectroscopy to examine the copper interaction with herbal Mel. The EPR results present evi- dence of copper coordination with herbal Mel, as shown in Figure9.
4 CONCLUSION
In this study, we have prepared and characterized pure herbal Mel from Nigella sativa seeds and their nanocom- posite formed by copper doping (Cu-Mel) by several techniques. TEM and XRD showed that Mel and Cu-Mel are non-crystalline with no definite structure. The absorp-
F I G U R E 8 Analysis of Mel by solid state 13C NMR (A) and HETCOR NMR (B, C) techniques
F I G U R E 9 X-band EPR spectra of herbal Cu-Mel (blue), and the pure copper (red)
tion improvement of the Cu-Mel sample is attributed to the association between the 3d orbital of the Cu atom and the p orbital of the O atom in the hydroxyl group of the Mel monomers, which contracts the direct band gap. VSM analysis of Mel and Cu-Mel samples at 300 and 10K temperatures showed that the magnetic proper- ties of Mel increased with decreasing temperature. Doping of metal Copper into Mel improved the magnetic prop- erties of Mel from diamagnetic to paramagnetic at low temperature (10K). Moreover, the EPR analysis provides
evidence of copper coordination with herbal Mel. Finally, the UV results show that the band gap of Mel decreased with Cu doping, proposing possible optical properties improvement.
A C K N O W L E D G E M E N T S
G. K. and N. M. would like to thank Imam Mohammed Ibn Saud Islamic University (IMSIU) for the continuous encour-agement to conduct research. M.J., M. A., and A.- H. E. would like to thank the King Abdullah University of Science and Technology (KAUST) for financial support.
C O N F L I C T O F I N T E R E S T The authors declare no conflict of interest.
D A T A AVA I L A B I L I T Y S T A T E M E N T The data that support the findings of this study are available from the corresponding author upon reasonable request.
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How to cite this article: G. Khouqeer, M.
Alghrably, N. Madkhali, M. Dhahri, M. Jaremko, A-H Emwas4,Nano Select.2022, 1.
https://doi.org/10.1002/nano.202200095