Greener saponin induced morphologically controlled various polymorphs of nanostructured iron oxide materials for biosensor applications

Abstract

Presently, the progress of green chemistry in the synthesis of magnetic nanoparticles with the use of plants has engrossed a great attention, that do not employ toxic chemicals in the synthesis protocols to avoid adverse effects in medical applications. Here, it is a report on rapid, energy-efficient, green and economically scalable room temperature protocol for the synthesis of different polymorphs of iron oxide nanostructures such as Fe₃O₄ nanospheres, γ-Fe₂O₃ nanoribbons and α-Fe₂O₃ nanospheres using greener saponin as a bio-surfactant with tunable magnetic properties directed by particle shape. A bio-surfactant directed growth mechanism was proposed for the magnetic anisotropic structure formation and the aspect ratio can be controlled by changing reaction conditions. The electrochemical sensing properties of the as obtained γ-Fe₂O₃ nanoribbon and α-Fe₂O₃ nanospheres modified glassy carbon electrode (GCE) towards dopamine and uric acid with different morphologies were investigated systematically. The electrochemical dynamic results confirmed that the superior electron transport ability of nanostructures exhibited a higher anodic current response with a negative shift in positive potential than the bare GCE. The γ-Fe₂O₃ nanoribbon showed 100% enhanced electrochemical sensing performance compared to the α-Fe₂O₃ nanospheres. These studies suggested that non-spherical iron oxide nanoparticles (NPs) could potentially lead to further advancement in nanomedicine.

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1. Introduction

Acicular iron oxide nanostructures such as needles, nanorods, and nanowires have attracted much attention and research owing to their enhanced mechanical, optical, electrical and electromagnetic properties over spherically shaped nanoparticles.^1–3 Magnetic nanostructures have attracted significant and growing amounts of attention not only because of their fascinating physicochemical properties but also their potential uses in a wide range of applications, including biomedicine, magnetic fluids, magnetic recording, and spin electronics.^4–6 Among the various magnetic materials, the iron oxides (such as hematite, magnetite, and maghemite) represent an important class of magnetic transition metal oxide materials have been widely studied for their excellent physical and chemical properties, but these properties are greatly affected by the synthesis route and nanocrystal size and shape.^7–10

The facile synthesis and fabrication of 1D, 2D, and 3D nanoparticle assemblies with highly controlled structures, uniform morphologies, and novel properties are now emerging as a new field of significance. Techniques that have been used, with varying degrees of success, to produce 1D iron oxide nanoparticles include sol–gel method,¹¹ “wrap_bake_peel” process,¹² temperature-assisted reduction of an akaganeite intermediate,¹³ microwave preparation,¹⁴ ultrasound irradiation technique,¹⁵ laser deposition,¹⁶ and electrochemical precipitation methods.^17,18 All of these methods present some draw backs and include either physical or chemical processes and involve the use of toxic, highly expensive, hazardous, and non-environmental friendly chemicals. Over the past decade, there has been an increased emphasis on the topic of “green” chemistry and chemical processes. Green synthesis provides an advancement over the chemical and physical methods as it is cost effective, environmental friendly, easily scaled up for large scale synthesis and has no need of high pressure, energy, temperature and toxic chemicals.¹⁹

Herein, we report the green chemistry approach for the synthesis of various polymorphs of iron oxide nanostructures such as Fe₃O₄ nanospheres, γ-Fe₂O₃ nanoribbons and α-Fe₂O₃ nanospheres by a simple co-precipitation method through iron ions using aloe vera (AV) extract as an eco-friendly bio-surfactant containing saponin as the main factor. The effect of reaction time and different concentrations of bio-surfactant on the synthesis of iron oxide superstructures have been investigated. In addition to this, electrocatalytic activity and simultaneous determination of uric acid (UA) and dopamine (DA) sensing towards the synthesized materials of γ-Fe₂O₃ nanoribbons and α-Fe₂O₃ nanospheres have been studied. The novelty of the synthesis and different morphological phases of iron oxide nanospheres and nanoribbons are explained with the help of various spectroscopic techniques. To the best of our knowledge, this is the first ever report on facile fabrication of iron oxide nanostructures using AV extract as a bio-surfactant for electrochemical sensing applications.

2. Experimental section

2.1. Chemicals

Iron(III) chloride hexahydrate (FeCl₃·6H₂O), iron(II) sulphate heptahydrate (FeSO₄·7H₂O), sodium hydroxide (NaOH), uric acid (UA) and dopamine (DA) were purchased from E-Merck Speciality Products. All the chemicals are of reagent grade and were used without further purification. Aloe vera leaves were purchased from the local market. Doubly distilled water was used as the solvent.

2.2. Synthesis of iron oxide nanostructures

The iron oxide nanostructures were prepared by an aqueous co-precipitation method using FeCl₃·6H₂O and FeSO₄·7H₂O. In a typical experimental procedure, 30 mL of FeCl₃·6H₂O solution was mixed with 15 mL of FeSO₄·7H₂O solution in a three-necked round bottomed flask. The prepared saponin rich bio-surfactant (AV extract) was added to the above mixture and reacted for 1 h. After that, the NaOH solution was dropped slowly into the solution under vigorous stirring until a pH of 13 was reached and the stirring was continued for another 1 h. The changes of solution color from orange to black could be seen, leading to a reddish brown precipitate. The addition process was performed at ambient atmosphere without the protection of inert gases or vacuum. Finally, the supernatant was discarded by decantation. The resulting solid products were washed with doubly distilled water and dried at room temperature naturally.

2.3. Fabrication of γ-Fe₂O₃ nanoribbons and α-Fe₂O₃ nanospheres modified GCE

Prior to modification, the GCE was polished to a mirror-like surface with 1, 0.3 and 0.05 μm alumina slurry and rinsed thoroughly with doubly distilled water between each polishing step. Then, it was washed successively with doubly distilled water in an ultrasonic bath and dried in air. About 1 mg of iron oxide nanoparticles were dispersed in 3 mL ethanol under ultrasonication. The iron oxide modified GCE were obtained by casting 5 μL of these suspensions on the surface of the electrochemically cleaned GCE, which were dried in air at room temperature. The modified GCE was activated in 7.4 phosphate buffer solution by successive cyclic scans between −0.2 and +0.8 V. Before and after each experiment, the modified GC electrode was washed with distilled water and reactivated by the previously mentioned method.

2.4. Characterization

The nanostructure characteristics were studied using a field emission scanning electron microscope (SEM, Hitachi MODEL S-4800). TEM images were obtained from a JEOL JEM 2010 transition electron microscope operating at 200 kV. X-ray diffraction spectroscopy study [XPERT-PRO with Cu Kα radiation (λ = 0.154060 nm) on a Panalytical X’Pert Pro Diffractometer] was used for studying the structural phase identification. A Fourier transform infrared spectrometer (FT-IR, Nicolet 5700) was used to analyze the surface characteristics of the nanoparticles. X-ray photoemission spectroscopy was recorded on a Kratos ASIS-HS X-ray photoelectron spectroscope equipped with a standard and monochromatic source (Al Kα) operated at 150 W (15 kV, 10 mA). The magnetization curves and hysteresis loop of the nanostructures were characterized with a Lake model 7300 VSM. Cyclic voltammogram (CV) tests were performed by a CHI6131D Electrochemical Impedance Analyzer (USA) using the as-modified electrode and bare GC electrode as working electrode, a platinum wire was the counter electrode, and saturated calomel electrode (SCE) was the reference electrode. CV curves were examined in the voltage range of −0.2 to 0.8 V at a scan rate of 50 mV s⁻¹.

3. Results and discussion

To understand the growth process of the products, the morphology evolution of the iron oxide spherical and ribbon-like superstructures have been studied by FE-SEM and TEM images. The possible formation processes and the FE-SEM images of the samples obtained with different reaction times and concentrations of bio-surfactant are shown in Fig. 1. Three different forms of iron oxides such as Fe₃O₄, γ-Fe₂O₃ and α-Fe₂O₃ nanostructures were synthesized without the use of any complex precursors, templates, or reagents that can induce the directed growth of nanocrystals and the formation of nanoribbon-like structures in a single reaction achieved by a one-step single-pot process. Reaction time is one of the key factors for the growth of crystals of a particular shape. To scrutinize the growth process of the γ-Fe₂O₃ nanoribbons, time dependent evolution of the morphology has been studied at ambient temperature. The FE-SEM observations indicated that, at an earlier stage (Fig. 1a and d) of the co-precipitation reaction in the 2% of bio-surfactant, the product mainly composed of only small Fe₃O₄ iron oxide nanocrystals of 10 nm with monodispersed spherical morphology generated through conventional nucleation and subsequent crystal growth mechanism. As the reaction time is prolonged for 2 h, part of the nanospherical particles begin to combine with each other and the products are totally transformed in to large quantities of γ-Fe₂O₃ nanoribbon shaped morphology (Fig. 1b and e).

Fig. 1 Iron oxide nanostructures synthesized at various concentrations of bio-surfactant for different time intervals at 30 °C (a) FE-SEM images of 2% Fe₃O₄ nanospheres in 30 min (b) 2% γ-Fe₂O₃ nanoribbons in 2 h (c) 10% α-Fe₂O₃ nanospheres in 2 h (d) TEM images of 2% Fe₃O₄ nanospheres in 30 min (e) 2% γ-Fe₂O₃ nanoribbons in 2 h (f) 10% α-Fe₂O₃ nanospheres in 2 h.

We can see that on conversion from Fe₃O₄ to γ-Fe₂O₃, a topotactic phase change occurs through imperfect oxidation of Fe₃O₄. During the oxidation process of Fe₃O₄ nanospheres, there is the possibility to form γ-Fe₂O₃ nanospheres on the surface of the Fe₃O₄ spherical nanoparticles. Since the specific volume of the Fe₂O₃ is slightly larger than that of Fe₃O₄, compressive stresses are generated and accumulated at the interface between Fe₃O₄ and Fe₂O₃ because of the volume shrinkage.²⁰ Under the influence of compressive stress, dipole interactions and the thermodynamic driving force, the neighboring nanodots of γ-Fe₂O₃ have a tendency to join each other through the oriented attachment of initially formed spherical nanodots to share the same crystallographic orientations. Through the outward diffusion of Fe ions along the grain boundaries, iron cations are deposited on to the γ-Fe₂O₃ nanoparticles, where γ-Fe₂O₃ spherical nanoparticles serve as the structural templates for the nucleation of γ-Fe₂O₃ nanoribbons. Together with the subsequent production and deposition of γ-Fe₂O₃ nanoparticles, γ-Fe₂O₃ ribbon-like nanostructures were formed. Moreover, we have also been observed that the reaction time increases after 30 minutes, the color of the solution slowly changes from black (Fe₃O₄) to reddish brown and then the color does not change any more with reaction time. The characteristic reddish brown color of γ-Fe₂O₃ iron oxide solution offered a convenient visual signature to indicate their formation. Consequently, we kept this reaction time as optimum for all other reactions carried out for various concentrations of bio-surfactant and the structure of greener saponin present in the AV extract is given in Fig. 2. The above discussion mainly concerns the evolution of the morphology and phase change occurring over the reaction time; more in-depth investigations are still underway to further understand the detailed influences of other factors on the formation, assembly, and the ripening process of the iron oxide crystals.

Fig. 2 Chemical structure of greener saponin present in the AV extract.

Furthermore, to understand more about the formation of γ-Fe₂O₃ nanoribbons, the influence of bio-surfactant at various concentrations was also investigated (Fig. 5). At first, the experiments were conducted with a fixed mass of ferric and ferrous ions but various concentrations or quantities of AV extract (bio-surfactant) and a series of iron oxide nanostructures including γ-Fe₂O₃ nanoribbons and α-Fe₂O₃ nanospheres were synthesized. For the experiments carried out at lower concentrations (0.5%, 1%, 2% and 3%) of bio-surfactant, only nanoribbons were formed with a low aspect ratio (10). The aspect ratio of the nanoribbon increases (10, 12, 15, and 18) with increasing concentration of bio-surfactant (Fig. 5a–d). The higher aspect ratio may be due to the increased length and slight increment in diameter of the nanoribbon. However, the reaction undergoes morphological changes of nanospherical particles in to nanoribbon formation at 2% concentration for various time intervals. It seems to be caused by the irreversible binding of strongly binding bio-surfactant on the central region of the growing nanoparticles.²¹ The whole process includes nucleation, surface regularity, growth and oriented attachment. Initially, the crystal would nucleate from the precursor solution, simultaneously the growth habit of the nanoribbon emerged. The –OH groups of the saponin molecules present in the bio-surfactant (Fig. 2) would easily attached to the surface of crystal faces and slows down the growth rate of the crystal at a given crystallographic orientation.²² After the fusion of two nanospheres, the third nanosphere will be bound on the edge (instead of the central region where the bio-surfactant was strongly bound), generating a catenated structure. The continued growth of nanospheres on the edge of the growing nanoparticles would generate unidirectional nanoribbons. Under the capping effect of greener saponin, crystals preferentially grew along the long chain of saponin and finally ribbon like morphology formed via oriented attachment. At this juncture, the bio-surfactant acts as a structure directing agent for the growth of the γ-Fe₂O₃ nanocrystals mainly along the direction (311 plane) by absorbing on the surface of the iron oxide as seen from the XRD (Fig. 3a). Conversely, the higher concentration (10%) of the bio-surfactant strongly competes with the iron oxide nanostructures, does not help in growing the nanoribbons. Only α-Fe₂O₃ nanospherical morphology with an average particle size of 12 nm is formed and the particle distribution is relatively homogeneous (Fig. 1c and f). At this time, increased amounts of bio-surfactant may form micelles on the surface of the particles after nucleation, thereby prohibiting further growth in such a way. The ability of the micelles to arrest the growth of the particles becomes stronger in response to the increase in surfactant, so the average particle size decreases, finally leading to the homogeneous dispersal of the spherical nanoparticles. Additionally, more surfactant can provide increased opportunity to contact with the surface at any direction of the spherical α-Fe₂O₃ nanoparticles. It also means that the distribution of the surfactant in various directions of the surface of the α-Fe₂O₃ nanoparticles is highly isotropic, so it can be found that the better size distribution in the process of the crystal growth could be obtained with the more befitting amount of the surfactant used in the reaction.²³ This idiosyncratic property of the saponin rich bio-surfactant not only plays a vital role in the design of the morphology of Fe₃O₄ and γ-Fe₂O₃, but also plays a key role in forming the α-Fe₂O₃ phase of iron oxide.

Fig. 3 Iron oxide nanostructures synthesized at various concentrations of bio-surfactant for different time intervals at 30 °C. (a) XRD patterns of different polymorphs of iron oxide (b) XPS spectrum of different polymorphs of iron oxide.

The highly crystalline Fe₃O₄, γ-Fe₂O₃ and α-Fe₂O₃ phase transformation for the obtained iron oxide products have been confirmed with the X-ray diffraction method and the corresponding XRD patterns of the samples are shown in the Fig. 3a. All the reflections can be indexed to a cubic spinel phase of Fe₃O₄ (magnetite, JCPDS no. 89-0951), γ-Fe₂O₃ (maghemite, JCPDS no. 89-5892) and pure rhombohedral phase of α-Fe₂O₃ (hematite, JCPDS no. 33-0664). The XRD spectra of the spherical nanoparticles formed at 30 min indicate the formation of Fe₃O₄ phase, but the nanoribbon grown at 2% bio-surfactant in 2 h exhibit the pure form of γ-Fe₂O₃ with cubic spinel structure with a very small intensity peak matching with the Fe₃O₄ phase in the initial stage of crystal growth. The γ-Fe₂O₃ nanoribbon grows through the oriented attachment particularly along the 〈311〉 direction, which gives rise to relatively intense diffraction peak of the (311) plane. However, the iron oxide samples synthesized from increased bio-surfactant (10%), the samples show pure α-Fe₂O₃ phase. Additionally, the appearance of two dominant peaks for (110) and (104) planes also confirm the presence of α-Fe₂O₃ phase. Whereas, the bio-surfactant varied from 0.5% to 3%, only pure phase of γ-Fe₂O₃ nanoribbons are formed without any discrepancies (Fig. S1†). The increased intensity of XRD peaks for the Fe₃O₄ and α-Fe₂O₃ spherical nanoparticles are noted and compared to the γ-Fe₂O₃ nanoribbon as can be visualized in the FE-SEM images (Fig. 1). At the same time, the intensity of the diffraction peaks is a size dependent behavior, the aspect ratio of the nanoribbon increases with increasing the concentration of bio-surfactant (Fig. S1†) and the diffraction pattern shrinks.²⁴ However, the XRD patterns of magnetite and maghemite just differ in a few low intensity reflections and very difficult to differentiate them. Further investigations are carried out to examine the Fe₃O₄, γ-Fe₂O₃ and α-Fe₂O₃ phases. First, the colors of the synthesized samples are observed and the characteristic black color of Fe₃O₄ which is significantly different from that of γ-Fe₂O₃ (reddish-brown) and α-Fe₂O₃ (red). Secondly, a sensitive XPS measurement have been consulted to unambiguously assigned the crystal phase of Fe²⁺ and Fe³⁺ cations.²⁵

In our case, the levels of Fe2p_3/2 and Fe2p_1/2 for γ-Fe₂O₃ and α-Fe₂O₃ are appeared at 710.8 and 724.4 eV; 710.5 and 724.2 eV respectively. However, the levels are emerged at higher binding energy of 711.1 and 724.9 eV for Fe₃O₄ and a satellite peak at around 719.4 and 719.2 eV is observed for γ-Fe₂O₃ nanoribbon and α-Fe₂O₃ nanospheres respectively (Fig. 3b). It is in good agreement with the literature that the peaks shift to high binding energy with broadening for Fe₃O₄ due to the appearance of Fe²⁺(2p_3/2) and Fe²⁺(2p_1/2). A shoulder around 711 eV seems to appear in Fig. 3b, which further evidences that the initially formed spherical nanoparticles are Fe₃O₄ iron oxide.^26,27 In addition, the presence of the satellite peak at 719.4 eV is the characteristic of γ-Fe₂O₃, while the satellite peak appeared in Fe₃O₄ is covered and becomes less resolved.²⁸ The XPS patterns are well in agreement with the XRD data and revealed the complete phase transformation of Fe₃O₄ in to γ-Fe₂O₃ and morphological changes could be achieved by carefully controlling the reaction conditions (Fig. 3a and b).

Thermal analysis further supports the iron oxide phase transformation from Fe₃O₄ in to γ-Fe₂O₃. The Fe₃O₄ nanospheres show 27.4% weight loss below 110 °C resulting from the removal of adsorbed water which is only 11.3% for the 120 minutes grown γ-Fe₂O₃ nanoribbon. This peculiar interesting behavior may be due to the surface availability (finite size effect) to anchoring the water molecule (–OH) (Fig. 1a and d). A notable weight gain has been observed for Fe₃O₄ nanospherical particles at 200 °C, which is due to the oxidation behavior of Fe₃O₄ to γ-Fe₂O₃, which is overlapped with the decomposition of the bio-surfactant (Fig. S2†). Above 330 °C, stable residue could reasonably be ascribed to the pure γ-Fe₂O₃ phase. The result can also be confirmed by the XRD and XPS analysis (Fig. 3a and b). The FT-IR spectra of Fe₃O₄ nanospheres, γ-Fe₂O₃ nanoribbons and α-Fe₂O₃ nanospheres and the different % (0.5%, 1%, 2%, 3% and 10%) of bio-surfactant have been illustrated in the Fig. S3a and b.† The intense and broad band appearance in the region 3200–3600 cm⁻¹ can be noted, where the iron oxide surfaces are readily bound with hydroxyl groups in an aqueous environment. The absorption band appeared below 700 cm⁻¹ is attributed to Fe–O stretching vibration, indicates the formation of iron oxide nanostructures.²⁹

We know that the iron-based materials often have intriguing magnetic properties influenced by the structure, shape anisotropy, and crystallinity of the nanostructured materials.³⁰ Therefore, we examined the magnetic properties of Fe₃O₄ nanospheres, γ-Fe₂O₃ nanoribbon and α-Fe₂O₃ spherical nanoparticles which are presented in Fig. 4. The room-temperature hysteresis curves for a series of samples showed neither remanence nor coercivity, regardless of the nanocrystal size and shape, and therefore indicate a superparamagnetic behavior. The saturation magnetization (M_s) of Fe₃O₄ nanospheres, γ-Fe₂O₃ nanoribbon and α-Fe₂O₃ spherical nanoparticles is 40, 31 and 1.06 emu g⁻¹, the decrease in M_s value of the iron oxide nanoparticles compared to the bulk materials is most likely attributed to the decrease in particle size,^31–34 the spin disorder on the surface and surface oxidation would significantly reduce the total magnetic moment. From this fact, here also the Fe₃O₄ spherical nanoparticles formed at 30 min undergoes oxidation and forms γ-Fe₂O₃ nanoribbons at 120 min in 2% bio-surfactant. The observed magnetic property of α-Fe₂O₃ spherical nanoparticles synthesized at 10% bio-surfactant is a result of the high surface to-volume ratio (S/V) of the formed nanostructures.³⁵ Compared to γ-Fe₂O₃ nanoribbons, α-Fe₂O₃ nanospheres have a much higher S/V, therefore the higher percentage of surface atoms is linked to the bio-surfactant and the surface effects generate a magnetic dead layer on the particle surface. Further support to this interpretation is provided by the recognition of a clear paramagnetic component (i.e., linear dependence of M_s vs. H) which prevents the hysteresis curves from becoming flat at saturation, especially in the sample with the lowest M_s value. We have also explored the magnetic properties of iron oxide nanostructures synthesized at various concentrations (0.5% to 10%) of bio-surfactant (Fig. S4†). Interestingly, in γ-Fe₂O₃ nanoribbons, the M_s drops become even more pronounced as the length of the ribbon increases and/or their diameter narrows. Therefore, the more anisotropically the ribbons grow, the more M_s is depressed, despite the actual increase in the nanocrystal volume. Such results clearly indicate that the magnetic states of the nanocrystals evolve gradually as a function of both the particle size and shape.

Fig. 4 Magnetization curves of different polymorphs of iron oxide nanostructures synthesized at various concentrations of bio-surfactant for different time intervals at 30 °C.

Fig. 5 FE-SEM images of iron oxide nanostructures synthesized at various concentrations of bio-surfactant at 30 °C in 2 h (a) 0.5% nanoribbons (b) 1% nanoribbons (c) 2% nanoribbons (d) 3% nanoribbons (e) 10% nanospheres.

The electrocatalytic oxidation of DA and UA on γ-Fe₂O₃ nanoribbons and α-Fe₂O₃ nanospheres modified GCE in 7.4 PBS in the potential range of −0.2 to +0.8 V at a 50 mV s⁻¹ scan rate have been performed by cyclic voltammetry. Fig. 6 and 7 show the voltammetric responses of the DA and UA at bare GCE and γ-Fe₂O₃ nanoribbons and α-Fe₂O₃ nanospheres modified GCE, respectively. The iron oxide nanostructures modified GCE reduces the anodic over potentials of DA & UA and exhibits well-defined anodic peaks compared with the bare GCE. At the bare GCE, UA shows an oxidation peak at 0.52 V. It is well known that the oxidation of UA at the bare GC electrode is generally believed to be totally irreversible and requires a high over potential. However, the UA voltammogram obtained on γ-Fe₂O₃ nanoribbons and α-Fe₂O₃ nanospheres modified GC electrode showed oxidation waves at 0.33 V and 0.37 V potentials, respectively with increased current response (100% for γ-Fe₂O₃ nanoribbon) (Fig. 6). DA demonstrates an oxidation peak at 0.30 V at the bare GCE. Conversely, the DA voltammogram obtained on γ-Fe₂O₃ nanoribbons and α-Fe₂O₃ nanospheres modified GC electrode showed an oxidation wave at reduced potentials of 0.13 and 0.22 V, respectively (Fig. 7) with the enhanced peak current, attributed to the excellent absorption ability of γ-Fe₂O₃ nanoribbons and α-Fe₂O₃ nanospheres.

Fig. 6 Cyclic voltammetric response of 0.5 × 10⁻³ M dopamine in 7.4 PBS at 50 mV s⁻¹ (a) bare GCE (b) α-Fe₂O₃ nanospheres and (c) γ-Fe₂O₃ nanoribbon modified GCE.

Fig. 7 Cyclic voltammetric response of 0.5 × 10⁻³ M uric acid in 7.4 PBS at 50 mV s⁻¹ (a) bare GCE (b) α-Fe₂O₃ nanospheres and (c) γ-Fe₂O₃ nanoribbon modified GCE.

The above results demonstrate that the iron oxide nanostructures modified GC electrode not only accelerate the oxidation of DA and UA, they also significantly enlarge the peak separation among DA and UA. The enlarged anodic peak potential separation coupled with the increased sensitivity renders the simultaneous determination of DA and UA feasibly. Here, the γ-Fe₂O₃ nanoribbons and α-Fe₂O₃ nanospheres modified GCEs exhibit well-defined two separate anodic peaks for the oxidation of DA and UA simultaneously with the enhanced current response using CV (Fig. 8). The presence of functional groups on iron oxide nanostructures modified GCE resolved the mixed voltammetric response of these species (DA and UA) into two well-defined voltammetric peaks at the potentials of 0.19 and 0.48 V (0.29 V peak separation) for γ-Fe₂O₃ nanoribbons and 0.23 and 0.50 mV (0.27 V peak separation) for α-Fe₂O₃ nanospheres modified GCE, respectively. Further the peak separation between the DA and UA on γ-Fe₂O₃ nanoribbons and α-Fe₂O₃ nanospheres is sufficiently enough to oxidize them as two well-defined separate peaks for easy determination, where γ-Fe₂O₃ nanoribbon modified GCE is best as it exhibits 0.29 V wide separations between the DA and UA voltammetric responses with increased peak current. The reason for this improved catalytic activity of γ-Fe₂O₃ nanoribbon is the 1D nanostructure acts as an electron wire, where the electron diffusion takes place at a faster rate. The α-Fe₂O₃ nanospheres also exhibit better performance than that of the flat electrode (bare GCE). The inferiority of the α-Fe₂O₃ nanospheres compared to γ-Fe₂O₃ nanoribbon is due to the increased intercontacts among the distributed ions. The outcome of the voltammetric responses indicate that the electrocatalytic reaction on the iron oxide nanostructures facilitates the electron transfer between the electrode and the analyte molecules, as a result the electrochemical oxidation of DA and UA becomes easier. The iron oxide nanostructures can act as a promoter to increase the rate of electron transfer and lowering the over potential of the DA and UA at the modified electrode, it is clear that the iron oxide nanostructures modified GC electrodes can be successfully used for the determination of biomolecules and bioelectronics/biosensor applications.

Fig. 8 CVs of simultaneous determination of DA and UA using (a) bare (b) α-Fe₂O₃ nanospheres (c) γ-Fe₂O₃ nanoribbons modified GCE in 0.5 × 10⁻³ M in 7.4 PBS at 50 mV s⁻¹.

4. Conclusions

In summary, a facile environmentally benign shape controlled synthesis of anisotropic crystalline iron oxide nanostructures using saponin rich aloe vera extract as a bio-surfactant and shape directing agent has been explored. By carefully monitoring the reaction parameters, the aspect ratio of the γ-Fe₂O₃ nanoribbon and morphology of the nanostructures could be designed. The saponin molecules present in the AV extract played a key role in the formation of γ-Fe₂O₃ nanoribbon and it was a kinetically controlled process. The obtained γ-Fe₂O₃ nanoribbon and α-Fe₂O₃ nanospheres were exhibited a high electrocatalytic activity towards the oxidation of DA and UA by significantly decreasing their oxidation over potentials with the enhanced peak currents. Large peak separation between DA and UA was obtained, indicating that the modified GCE facilitated their simultaneous determination. With excellent selectivity and high sensitivity, 1D γ-Fe₂O₃ nanoribbon showed better electrochemical response towards DA and UA than the α-Fe₂O₃ nanospheres. The present simple synthetic greener method provides high-yield nanosized materials with good magnetic properties, and this method can be used to prepare nanocrystalline oxides of other interesting materials. Therefore, we strongly believe that the use of saponin rich bio-surfactant offers a large scale production of biocompatible iron oxide nanostructures that can be used as a vehicle for biomedical/sensing applications. However, the role of AV extract is not yet specified and the complexation between bio-surfactant and iron ions in the solution is still unknown, and it is under investigation.

Acknowledgements

The authors wish to thank the Department of Physics, Alagappa University, Karaikudi, India for providing XRD analysis and National Taipei University of Technology, Taiwan for providing TEM facility.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15166j

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