Design of a Novel Nanosensors Based on Green Synthesized CoFe2O4/Ca-Alginate Nanocomposite-Coated QCM for Rapid Detection of Pb(II) Ions

Pb(II) is a significant contaminant that is known to have negative effects on both humans and animals. Recent industrial operations have exacerbated these consequences, and their release of several contaminants, including lead ions, has drawn attention to the potential effects on human health. Therefore, there is a lot of interest in the rapid, accurate, and selective detection of lead ions in various environmental samples. Sensors-based nanomaterials are a significant class among the many tools and methods developed and applied for such purposes. Therefore, a novel green synthesized cobalt ferrite (CoFe2O4) nanoparticles and functionalized CoFe2O4/Ca-alginate nanocomposite was designed and successfully synthesized for the fabrication of nanoparticles and nanocomposite-coated quartz crystal microbalance (QCM) nanosensors to detect the low concentrations of Pb(II) ions in the aqueous solutions at different temperatures. The structural and morphological properties of synthesized nanoparticles and nanocomposite were characterized using different tools such as X-ray diffraction (XRD), N2 adsorption–desorption isotherm, dynamic light scattering (DLS), zeta potential analyzer (ζ-potential), atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX). The QCM results revealed that the green synthesized CoFe2O4 nanoparticles and functionalized CoFe2O4/Ca-alginate nanocomposite-coated QCM nanosensors exhibited high sensitivity, stability, and rapid detection of Pb(II) ions in the aqueous solutions at different temperature. The lowest detection limit for Pb(II) ions in the aqueous solutions could reach 125 ng, which resulted in a frequency shift of 27.49 ± 0.81, 23.63 ± 0.90, and 19.57 ± 0.86 Hz (Δf) for the QCM detector coated with green synthesized CoFe2O4 nanoparticles thin films, and 25.85 ± 0.85, 33.87 ± 0.73, and 6.87 ± 0.08 Hz (Δf) for the QCM detector coated with CoFe2O4/Ca-Alg nanocomposite thin films in a real-time of about 11, 13, and 13 min at 25 °C, 35 °C, and 45 °C, respectively. In addition, the resonance frequency change results showed the superiority of functionalized CoFe2O4/Ca-alginate nanocomposite coated QCM nanosensor over CoFe2O4 nanoparticles towards Pb(II) ions detecting, which attributed to the beneficial properties of alginate biopolymer.


Preparation of the Clove Leaves Extract
After being bought from a local market, the clove (Syzygium aromaticum) was initially washed with tap water. After that, they were rinsed with double distilled to remove waste and impurities. They were then allowed to dry naturally for five days. The leaves extract Nanomaterials 2022, 12, 3620 4 of 16 was made by mixing 6 g of dried leaves with 100 mL of double-distilled water. It was then heated at 60 • C for 30 min. Afterward, the clove leaf extract was filtered and stored for future investigation [43].

Green Synthesis of CoFe 2 O 4 Nanoparticles
With a few minor modifications, the co-precipitation method was used to synthesize spinel CoFe 2 O 4 nanoparticles. Double-distilled water was used to dissolve 5 g of FeCl 3 ·6H 2 O and 3 g of CoCl 2 ·6H 2 O. The mixture was then heated on a hot plate for about 15 min at 50 • C. Then, 10 mL of the clove leaves extract was added to the chloride solution while stirring vigorously. By adding drops of a 0.5 M NaOH solution to the mixture, the pH was raised to 10. The mixture was then stirred for 2 h at 60 • C. The resulting nanoparticles were calcined for 2 h at 600 • C after being washed with double-distilled water.

Preparation of CoFe 2 O 4 /Ca-Alg Nanocomposite
CoFe 2 O 4 /Ca-Alg nanocomposite was prepared via the ionotropic gelation mechanism. An amount of 0.025 g of CoFe 2 O 4 was sonicated in 50 mL of distilled water for 20 min. Subsequently, 1 g of sodium alginate was added to the CoFe 2 O 4 nanoparticles solution and stirred for 1 h followed by sonicating for 20 min. Afterward, calcium chloride solution (2M) was added to the mixture and stirred for 1 h. The mixture was left in the refrigerator for 2 days. The formed nanocomposite was filtrated and washed 3 times with distilled water. The prepared CoFe 2 O 4 /Ca-Alg nanocomposite was then dried in an oven at 60 • C for 2 days.

Characterization
The composition and phase identification of both green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite was obtained using X-ray diffraction (EQUINOX 1000, Thermo Scientific CO., Lafayette, CO, USA). The employed X-ray source was Cu Kα radiation with a current of 31 mA and an applied voltage of 33 kV. The 2θ angles ranged from 5 • to 80 • with a scan speed of 0.1 • /min. N 2 adsorption-desorption analyzer (Nova Touch 4L, Quanta Chrome, Boynton Beach, FL, USA) was used to determine the surface area and pore size of the green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite according to the BET and DA methods, respectively. Prior to performing the BET test, the green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite was degassed at 75 • C for 2 h to dispose of any moisture or gas molecules on the surface of the tested materials. In addition, the zeta seizer instrument (NanoSight NS500, Malvern Panalytical, Malvern, UK) was used to determine the particle size (DLS method) and surface charge of the green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite. Moreover, in order to identify the shape and morphology of green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite, the topographic properties of the prepared CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite were investigated using AFM, SEM, and TEM instruments. The AFM equipment (5600LS, Agilent technology firm, Santa Clara, CA, USA) was used to provide 2D and 3D topographic images for the synthesized materials. Before the AFM analyses, the samples were first introduced to ultrasonic waves for two hours using an ultrasonic probe sonicator (UP400S, Hielscher, Oderstraße, Teltow, Germany) for 20 min at 59 kHz, 83% amplitude, and 0.79 cycles. Finally, a thin film of the samples was created under a vacuum using a spin coater instrument (WS-650Sz, Laurell, North Wales, PA, USA) at 600 rpm. Additionally, Gwyddion software (supported by the department of nanometrology and technical length, Czech Metrology Institute, Okružní, Czech Republic) was utilized to evaluate the AFM outcomes. The AFM images and data profiles were obtained at 100 nm × 67 nm using tapping mode imaging (Al tap, 0.4 In/S speed, I. gain 0.4, and P. gain 20). An SEM instrument (JEOL, JSM-6701F Plus, Peabody, MA, USA) equipped with energy dispersive X-ray spectrometry (EDX) for elemental analysis and TEM (JEOL, JEM-2100 high-resolution, Peabody, MA, USA) were devoted to giving information on the size, shape, and surface morphology of the green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite. The SEM images were taken at an acceleration voltage of 10 kV and magnification of 3000 Kx. Before TEM investigation, the CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite were mixed with double-distilled water and were sonicated for 20 min using an ultrasonic probe sonicator at a frequency of 55 kHz, an amplitude of 55%, and a cycle of 0.55. Then drops with 5 to 10 microns of the dispersed mixture were dropped over a carbon-coated copper grid, which was subsequently submitted to the TEM test.

Quartz Crystal Microbalance (QCM)
The sensitivity, selectivity, and stability of the produced CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite-coated QCM nanosensors for the detection of Pb(II) ions in an aqueous solution were assessed using a quartz crystal microbalance with dissipation monitoring (QCM, Q-senses, Biolin Scientific, Linthicum Heights, MD, USA). Gold-coated AT-cut quartz crystals (Q-Sense) were utilized with a fundamental frequency (f 0 ) of 5 MHz. Before measurement and spin coating, the quartz crystals were rinsed with double-distilled water and dried.
In the experiment, the flow cell of the QCM-D was fitted with a recently cleaned quartz crystal. The quartz crystal's frequency shift was measured in order to compare them to the manufacturer's calibration standards. The QCM-D flow cell was flushed with double-distilled water until a steady baseline was attained. Afterward, 50 µg/L of green synthesized  nanoparticles, which is present in a facecentered cubic lattice structure. The high crystallinity of green synthesized CoFe 2 O 4 was delivered in sharp and narrow diffraction peaks with good intensities [44,45]. In addition, some impurities peaks were displayed in the diffractogram, which may be attributed to the clove leaf extract compounds that coated the prepared CoFe 2 O 4 nanoparticles during the synthesis process. For CoFe 2 O 4 /Ca-Alg nanocomposite, it could be observed the presence of a few peaks differ from that of the CoFe 2 O 4 nanoparticles pattern and the disappearance of the featured peaks of CoFe 2 O 4 nanoparticles. These results demonstrate the successful  Figure 2c,d shows the pore size distribution of both green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite, respectively. According to the IUPAC classification of adsorption isotherms, the isotherm curve of CoFe 2 O 4 nanoparticles showed a typical type three (III) class, which does not have the "sharp knee" shape indicating that stronger adsorbate-adsorbate interactions than adsorbate-adsorbent interactions [46]. The green synthesized CoFe 2 O 4 nanoparticles displayed an H3 hysteresis type, in which the pores have a wedge-shaped pore, according to de Boer's classification of hysteresis loops [47]. The isotherm curve of CoFe 2 O 4 /Ca-Alg nanocomposite has exhibited type four (IV) of adsorption isotherm, which reveals non-porous or microporous adsorbents with unlimited monolayer-multilayer adsorption. In this isotherm, the adsorption volume quickly increases at low relative pressures due to contact of the adsorbate molecules with the higher energetic section followed by the interaction with the less energetic section. Following the completion of the monolayer formation of the adsorbed molecules, multilayer formation begins to occur in accordance with the "sharp knee" of the isotherm. In contrast, a sudden rise signal indicates the bulk condensation of adsorbate gas to liquid as the relative pressure approaches unity. Moreover, the synthesized CoFe 2 O 4 /Ca-Alg nanocomposite displayed an H2 hysteresis type, in which the pores have an inkbottle-shaped pore and are associated with capillary condensation phenomena in mesoporous structures. This hysteresis loops type indicates the presence of complex pore networks. A summary of area, volume, and pore size results is tabulated in Table 1.

DLS and Zeta Potential
DLS measurements were used to determine the particle size of green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite. The average size measurements were found to be 75 and 90 nm for green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite. All the suspensions were comparatively monodispersed and induced good colloidal stability; the analysis found a unimodal size distribution with polydispersity indices. The observed increase in the average size of the CoFe 2 O 4 /Ca-Alg nanocomposite demonstrates the effective stabilization of the CoFe 2 O 4 nanoparticle with alginate biopolymer. Additionally, the average size is a measure of the hydrodynamic size; as such, its value will consider both the existence of nanoparticles and any solvent molecules connected to the tumbling particle.
In order to investigate the stability of green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite in the aqueous mediums, the ζ-potentials of green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite were studied at different applied voltage values. The ζ-values were recorded as −12 and −20 mV for green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite, respectively.
It could observe the ζ-value negatively increased for CoFe 2 O 4 /Ca-Alg nanocomposite, which may be attributed to the carboxylic groups contributed by the alginate structure and indicates the good linking of CoFe 2 O 4 nanoparticles with Ca-Alg binder. The coating of CoFe 2 O 4 nanoparticles with Ca-Alg biopolymer participated effectively in decreasing aggregation and deposition of NPs.

AFM
The surface topography of the green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 / Ca-Alg nanocomposite was measured using an atomic force microscope. The two-and three-dimensional images of green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite are shown in Figure 3a-d, respectively. Although some particles slightly varied in size and shape with many overlaps but can be clearly observed the homogeneity and uniformity of CoFe 2 O 4 nanoparticles. The green synthesized CoFe 2 O 4 nanoparticles exhibited rhombus bipyramid shapes and formed in good crystallinity with sharp edges. In addition, the nanoparticles fall within the 100 nm scale, with the height of the surface of the particles reaching 61.2 nm. This causes the granular boundary to move more freely, leading to the growth of granule sizes and decreasing the internal and surface defects in the structural texture. This also causes a strong cohesiveness between the granular boundaries. Additionally, the mechanical, electrical, and magnetic properties are improved. Whereas the images of CoFe 2 O 4 /Ca-Alg nanocomposite revealed the surface coating of CoFe 2 O 4 nanoparticles by Ca-alginate gel, and the alginate network effectively surrounded the majority of CoFe 2 O 4 nanoparticles with a prominence of some particles on the surface of alginate. Additionally, the coating with alginate further resulted in the appearance of needle shapes and increased the height of the surface of the nanocomposite to 92.7 nm. These results are consistent with the porosity findings from the BET analysis. The average grain size of the samples obtained from AFM images is larger than the particle sizes observed using SEM and TEM measurements, which indicates that each grain is formed by aggregation of a number of nanocrystals.   It is obvious that, in addition to particle size and shape, stoichiometry and cation distribution are among the factors that have the greatest impact on the properties of ferrites. Therefore, understanding a material's chemical composition, structure, and properties is essential to designing it for a particular purpose. In the spinel structure, metal cations fill one of the eight tetrahedral interstices (usually designated as Td or with round brackets) and half of the octahedral interstices (Oh or square brackets), which are closely packed together in a cubic arrangement. In a direct or normal spinel (M II )[M III ]2O4, divalent cations occupy the tetrahedral positions, while in an inverse spinel (M III )[M II ; M III ]2O4, trivalent cations replace them. The composition and structure have a direct relationship with the chemical and physical properties. In terms of inversion degree or the proportion of divalent cations in octahedral sites, the bulk cobalt ferrite exhibits an inverse spinel structure, with all Co II cations occupying the octahedral sites while the Fe III cations are equally distributed in the Td and Oh sites (γ = 1, γ is the inversion degree or the fraction of divalent cations in octahedral sites). Therefore, the ferrimagnetic behavior of the cobalt ferrite below 860 K is explained by the coupling of the magnetic moments linked to the ions in the It is obvious that, in addition to particle size and shape, stoichiometry and cation distribution are among the factors that have the greatest impact on the properties of ferrites. Therefore, understanding a material's chemical composition, structure, and properties is essential to designing it for a particular purpose. In the spinel structure, metal cations fill one of the eight tetrahedral interstices (usually designated as T d or with round brackets) and half of the octahedral interstices (O h  with the chemical and physical properties. In terms of inversion degree or the proportion of divalent cations in octahedral sites, the bulk cobalt ferrite exhibits an inverse spinel structure, with all Co II cations occupying the octahedral sites while the Fe III cations are equally distributed in the T d and O h sites (γ = 1, γ is the inversion degree or the fraction of divalent cations in octahedral sites). Therefore, the ferrimagnetic behavior of the cobalt ferrite below 860 K is explained by the coupling of the magnetic moments linked to the ions in the Td and Oh sites. In contrast, Co II and Fe III are randomly dispersed when the material is synthesized as a nanostructured material (γ = 0.66). In the literature, the inversion degree of nanostructured CoFe 2 O 4 prepared using various methods was measured using different techniques ( 57 Fe-Mössbauer spectroscopy, EXAFS, neutron diffraction), and the values ranged from 0.68 to 0.76 [48][49][50][51][52].
The elemental composition of green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 / Ca-Alg nanocomposite was presented in Figure 5a,b as well in the corresponding inset tables. It could be observed that the green synthesized CoFe 2

Green Synthesized CoFe2O4 Nanoparticles and CoFe2O4/Ca-Alg Nanocomposite-Coated QCM Nanosensors for Detecting Pb(II) Ions in the Aqueous Solutions
Quartz crystal microbalance (QCM)-based heavy metals sensing techniques enable real-time monitoring of the mechanical response of green synthesized CoFe2O4 nanoparticles and CoFe2O4/Ca-Alg nanocomposite to Pb(II) ions on the QCM chip surface. However, green synthesized CoFe2O4 nanoparticles and CoFe2O4/Ca-Alg nanocomposite precipitate on the QCM chip surface allowed the sense of nano-gram of Pb(II) ions loaded from an aqueous solution in real-time under the controlling of solution temperature using the QCM method. QCM techniques depend on the piezoelectric phenomena of quartz crystal, where the gold electrodes conduct electrical signals to detectors when nano-gram

Green Synthesized CoFe 2 O 4 Nanoparticles and CoFe 2 O 4 /Ca-Alg Nanocomposite-Coated QCM Nanosensors for Detecting Pb(II) Ions in the Aqueous Solutions
Quartz crystal microbalance (QCM)-based heavy metals sensing techniques enable real-time monitoring of the mechanical response of green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite to Pb(II) ions on the QCM chip surface. However, green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite precipitate on the QCM chip surface allowed the sense of nano-gram of Pb(II) ions loaded from an aqueous solution in real-time under the controlling of solution temperature using the QCM method. QCM techniques depend on the piezoelectric phenomena of quartz crystal, where the gold electrodes conduct electrical signals to detectors when nano-gram of materials are loaded on the QCM chip surface. The quartz crystals can be eager to the resonance frequency, which is related to the mass (thickness) of the QCM chip. If the mass changes, the resonance frequency (f ) changes. By real-time monitoring resonance frequency (∆f ) changes, detecting small changes in the QCM chip mass (thickness) can be possible. The QCM measurement data enable us to detect nano-scale mass changes such as molecules binding or adsorbing to the surface, which is detected as mass (thickness) increases.
As discussed before, the green synthesized Note that the resonance frequency of the green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite in double-distilled water were taken as the reference state for calculating the frequency shift in response to the Pb(II) ions. It can be seen that the resonance frequency decreased when exposing the green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite-coated QCM quartz crystal to the Pb(II) aqueous solution, indicating the adsorption or complexation of Pb(II) onto the nanosensors surfaces. It was found that the 125 ng Pb(II) in the aqueous solutions can lead to frequency shifts of 27.49 ± 0.81, 23.63 ± 0.90, and 19.57 ± 0.86 Hz (∆f ) for the quartz crystal coated with green synthesized CoFe 2 O 4 nanoparticles thin films and 25.85 ± 0.85, 33.87 ± 0.73, and 6.87 ± 0.08 Hz (∆f ) for the quartz crystal coated with CoFe 2 O 4 /Ca-Alg nanocomposite thin films in a realtime of about 11, 13, and 13 min at 25 • C, 35 • C, and 45 • C, respectively. Increasing the Pb(II) solution temperature resulted in more adsorption of Pb(II) ions, leading to larger frequency shifts. Thus, the nanosensors had a higher frequency response in Pb(II) aqueous solution with higher temperatures due to increasing the loaded masses of adsorbed Pb(II) ions. Accordingly, the sensing efficiency of Pb(II) increased gradually from 25 • C to 45 • C. From the obtained results, temperature increase enhances the mobility of Pb(II) ions and decreases the retarding force that acts on the diffusing ions. This results in the enhancement of the sorptive capacity of the adsorbent, an increase in chemical interaction between adsorbate and adsorbent, and the generation of active surface centers on an enhanced rate of intraparticle diffusion of Pb(II) ions into the pores of adsorbent at the higher temperatures. For the temperature of 45 °C in the case of CoFe2O4/Ca-Alg nanocomposite, when the equilibrium in adsorption and complexation was clearly reached, partial desorption was observed. It was understandable because parts of Pb(II) ions were absorbed via the chemical complexation between CoFe2O4/Ca-Alg nanocomposite units and Pb(II) ions, where parts of Pb(II) ions may be only physically adsorbed. The physically adsorbed Pb(II) will be easily released away via the running water during a temperature increase. On the other hand, the process of chemical complexation was also in a dynamic equilibrium state. The continuous flushing of Pb(II) solution at high temperatures may also flush away parts of the Pb(II) complex that formed with CoFe2O4/Ca-Alg nanocomposite units. Additionally, this phenomenon may occur due to the overlapping of adsorption sites as a result of overcrowding of adsorbent particles and decreased adsorption, which was the major cause of the reduction in the contact surface of CoFe2O4/Ca-Alg nanocomposite for the removal of metal ions. By considering these results, the frequency shift of the adsorption step was selected to present the frequency response of green synthesized CoFe2O4 nanoparticles and CoFe2O4/Ca-Alg nanocomposite-coated QCM nanosensors in the aqueous solutions of Pb(II) ions. The schematic curves of Pb(II) detection by CoFe2O4 nanoparticles and CoFe2O4/Ca-Alg nanocomposite-QCM nanosensors included in the supplementary materials provide more information (Figures S1-S3).   For the temperature of 45 • C in the case of CoFe 2 O 4 /Ca-Alg nanocomposite, when the equilibrium in adsorption and complexation was clearly reached, partial desorption was observed. It was understandable because parts of Pb(II) ions were absorbed via the chemical complexation between CoFe 2 O 4 /Ca-Alg nanocomposite units and Pb(II) ions, where parts of Pb(II) ions may be only physically adsorbed. The physically adsorbed Pb(II) will be easily released away via the running water during a temperature increase. On the other hand, the process of chemical complexation was also in a dynamic equilibrium state. The continuous flushing of Pb(II) solution at high temperatures may also flush away parts of the Pb(II) complex that formed with CoFe 2 O 4 /Ca-Alg nanocomposite units. Additionally, this phenomenon may occur due to the overlapping of adsorption sites as a result of overcrowding of adsorbent particles and decreased adsorption, which was the major cause of the reduction in the contact surface of CoFe 2 O 4 /Ca-Alg nanocomposite for the removal of metal ions. By considering these results, the frequency shift of the adsorption step was selected to present the frequency response of green synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite-coated QCM nanosensors in the aqueous solutions of Pb(II) ions. The schematic curves of Pb(II) detection by CoFe 2 O 4 nanoparticles and CoFe 2 O 4 /Ca-Alg nanocomposite-QCM nanosensors included in the Supplementary Materials provide more information (Figures S1-S3). Figure 8 shows the frequency shifts response of green synthesized /Ca-Alg nanocomposite towards low limit concentration of Pb(II) ions may be attributed to its higher surface area, more available active sits, porosity/diffusion nature, swelling capacity, and presence of carboxylic groups in the side groups as electron donors, which can easily form complexes with Pb(II) ions, and finally, its negative charge density that higher than of CoFe 2 O 4 nanoparticles as estimated by the ζ-potential experiment, which enables it to attract Pb(II) ions by electrostatic interaction.

Conclusions
In this study, a novel green synthesis of CoFe2O4 nanoparticles using clove (Syzygium aromaticum) leaf extract was achieved. Then the synthesized nanoparticles were functionalized with Ca-alginate biopolymer to yield the CoFe2O4/Ca-Alg nanocomposite. The synthesized nanoparticles and nanocomposite underwent characterization of crystallinity,

Conclusions
In this study, a novel green synthesis of CoFe 2 O 4 nanoparticles using clove (Syzygium aromaticum) leaf extract was achieved. Then the synthesized nanoparticles were functionalized with Ca-alginate biopolymer to yield the CoFe 2 O 4 /Ca-Alg nanocomposite. The synthesized nanoparticles and nanocomposite underwent characterization of crystallinity, size, and shape using several tools such as XRD, N 2 adsorption-desorption, DLS, ζ-potential, AFM, SEM, TEM, and EDX. Subsequently, the prepared nanoparticles and nanocomposite were used to fabricate nanosensors based on the QCM technique for reliable and rapid detection of low concentrations of Pb(II) ions in the aqueous solutions at different temperatures. The results exhibited frequency shift responses as follows; 27.49 ± 0.81, 23.63 ± 0.90, and 19.57 ± 0.86 Hz (∆f ) and 25.85 ± 0.85, 33.87 ± 0.73, and 6.87 ± 0.08 Hz (∆f ) for the quartz crystal coated with green synthesized CoFe 2 O 4 nanoparticles thin films and CoFe 2 O 4 /Ca-Alg nanocomposite thin films in a real-time of 11, 13, and 13 min at 25 • C, 35 • C, and 45 • C, respectively, for a solution with a concentration of 125 ng Pb(II) ions. Furthermore, the CoFe 2 O 4 /Ca-Alg nanocomposite-coated QCM nanosensor displayed more advanced sensing for Pb(II) ions than the CoFe 2 O 4 nanoparticles-coated QCM nanosensor.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano12203620/s1, Figure S1: Illustrates resonance frequency changes in the curve of air, water, and CoFe 2 O 4 nanoparticles in which resonance frequency decrease divided into three parts; the first one is the stable baseline frequency for empty QCM due to flow of air, the second step is due to the flow of water, and the third step is due to the deposition of CoFe 2 O 4 nanoparticles on QCM chip with different temperature; Figure S2: Illustrates the resonance frequency changes curve which is divided into two parts; the left one is the stable baseline resonance frequency for CoFe 2 O 4 nanoparticles precipitation on the QCM chip, and the gently slop part illustrates the beginning of Pb(II) adsorbing on the surface of CoFe 2 O 4 nanoparticles and right part represents the complete deposition of Pb(II) ions on the CoFe 2 O 4 nanoparticles surface at different temperature; Figure