Abstract
This work provides a new perception toward the application of the graphenic-biopolymeric composites as a solid-bed for separation and purification of bioactive compounds. Graphene oxide nanocomposites with functionalized sheets by soluble and insoluble nanocomplexes of chitosan and Arabic gum, were successfully synthesized and employed for the adsorption and purification of crocin, a nutraceutical from saffron. The composites exhibited a nanostructured scaffold with a particle size of 10 nm and experienced an unprecedented increase in the surface area by about 300% and improved d-spacing sheets by 17%. The optimum conditions for crocin separation were temperature = 318 K, stirring rate = 300 rpm, initial concentration = 100 mg L−1 and pH = 6. Under these conditions, the nanocomposites separated 99.1% of crocin in an equilibrium time of 30 min. The adsorption data were best represented by Freundlich isotherm and pseudo-second-order kinetic models. The thermodynamic studies indicated that the crocin adsorption on nanocomposites was an endothermic, spontaneous and physisorption process. The high-performance liquid chromatography (HPLC) analysis revealed that produced nanocomposites adsorbed crocin efficiently from saffron extract with a purity similar to the standard sample. The possible interaction mechanisms between crocin and nanocomposites were electrostatic interactions and hydrogen bonding.
Graphical abstract
Introduction
During the last decade, nano‑carbons owing to their potential toward meeting human needs have been stood in the center of research in different fields. Graphene (G) as a layered honeycomb crystal structure of 2D layers of sp2-bonded carbon, has sparked much interest in the consequence of its unique properties including ultrahigh specific surface area, flat structure, high Young’s modulus, and fracture strength [1], [2], [3]. The controlled oxidation of G sheets leads to producing a heavily oxygenated derivative, that its basal plane is decorated by functional groups of hydroxyl and epoxide while bearing carbonyl and carboxyl groups on the edge of the sheet, named graphene oxide (GO) [4], [5]. The surface modification of GO could improve its physicochemical properties [6], which can be achieved through the interaction of different molecules of interest with these oxygenated functional groups (OFGs) [7]. For instance, Rajabi et al. [6] synthesized a 3D nanocomposite of GO with decorated sheets by nanoparticles of complexed Arabic gum (AG)/chitosan (CS). On the other hand, the ability of GO to form colloidal suspensions [8] makes it possible to shape different dimensional structures as an excellent adsorbent for a great amount of different molecules [9], [10].
Saffron as the world’s most expensive spice has gained popularity not only due to its unique color, taste, and aroma, but also because of its majestic pharmaceutical properties such as memory enhancing [11], antidepressant and anxiolytic [12], aphrodisiac [13], and neuroprotective [14] properties. The main bioactive compound of saffron, responsible for its color and pharmaceutical features, is crocin(s), an exceptional water soluble carotenoid. Crocin refers to a group of crocetin esters that are categorized based on their sugar moieties and crocin-1 is the main member of this group [15], as depicted in Fig. 1.
Different kinds of drugs with different therapeutic goals have been produced from crocin such as Crocina (Iran), Exir (USA), Caruso’s (Australia), Saffronia (Australia), and Depromin (Finland). Accordingly, many attempts have been made to purify crocin [16], [17], but none of them can be commercialized and the need for a comprehensive approach remains tangible. On the other hand, the use of GO until now has been practiced as a filter to remove unpleasant compounds and pollutants such as heavy metals, synthetic colors, and residual drugs.
So, our approach was to introduce a scalable-feasible crocin purification method from the saffron extract using a 3D nanocomposite of GO-CS/AG. The biopolymers of CS and AG were combined with GO dispersion, to improve its physicochemical properties toward crocin adsorption from the saffron extract. Moreover, given the need to eliminate the prolonged ultrahigh centrifugation process for precipitating the dispersed GO, its 3D structure was prepared. The physicochemical properties of nanostructures were analyzed using field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) and Barrett–Joyner–Halenda (BJH) methods. The purity of crocin was also evaluated through high-performance liquid chromatography (HPLC).
Section snippets
Materials and methods
Fresh saffron was picked from a farm around Kashmar (Khorasan-Razavi province, Iran) followed by stigma separation. Sample preparation was done as follows [18]: (i) stigma drying through microwave (1000 W, 3.5 min), (ii) crashing and sieving (0.421 mm mesh), and (iii) keeping saffron powder (SP) in air-tight plastic bags within a desiccator. The following chemicals were obtained from Sigma-Aldrich (USA): Natural graphite powder (>99.9% purity), low molecular weight chitosan (~120 kDa,
Characterization of biopolymeric nanoparticles
The results of analyzing the NPs showed that the mean diameter, polydispersity index (PDI), and zeta potential of complexed biopolymeric NPs were 148 nm, 0.22, and 39.80 mV, respectively. Fig. 2 represents the TEM and FE-SEM images of CS/AG NPs. The spherical shapes along with a smooth surface are the characteristics of prevailing NPs, revealing the successful bonding of BFGs to create uniform nano-sized complexes [6].
Characterization of nanocomposites
Fig. 3A represented the FTIR spectra for graphite (G), GO and nanocomposites.
Conclusion
The ability of ternary nanocomposites was explored toward adsorption of crocin from the saffron extract. The ternary nanocomposites were prepared through the combination of CS/AG solution containing both free and bonded complexes of two biopolymers with GO dispersion followed by freeze-drying. The nanocomposites were successfully synthesized and showed unprecedented structural properties than GO as confirmed by TEM, FE-SEM, XRD, FTIR, and BET analysis. These nanocomposites adsorbed >90% of
CRediT authorship contribution statement
Hamid Rajabi did the experiments and wrote the 1st draft of manuscript.
Seid Mahdi Jafari designed the framework of this study, finalized the manuscript and take responsibility of submissions and publishing stages.
Javad Feizy was the advisor for chemical instrumental analysis.
Mohammad Ghorbani contributed to discussion of instrumental data.
Seyed Ahmad Mohajeri was the advisor for the synthesis of GO and complexed biopolymers.
Declaration of competing interest
Authors declare no conflicts of interest regarding this paper.
Acknowledgements
This study was financially supported by the Iran National Science Foundation (INSF), Grant No. 96011404.
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