Influence of Silver Nanoferrite and Nanochromite on Physical Properties for High-Frequency and Biomedical Applications

In this work, the effects of silver nanoferrite and nanochromite (Ag2Fe2O2, Ag2Cr2O4) were studied using the flash auto-combustion technique. Numerous physical properties were clarified from the different structures determined from the x-ray examination such as lattice parameter the cation dispersion, oxygen parameter, hopping length, and interatomic distance. Among antimicrobial properties, Ag addition to both samples showed high efficacy against both types of tested bacteria (gram-positive and gram-negative); however, they showed moderate effect against Candida albicans fungus. No activity appeared against Aspergillus flavus fungus for both samples. The switching field and high-frequency response of Ag2Fe2O4 and Ag2Cr2O4 nanoparticles were studied from the magnetic properties. Ag2Cr2O4 nanoparticles are expected to be used for high-frequency applications more than Ag2Fe2O4 ones. As a result of all previous analyses, the applications of this study promote the use of Ag2Fe2O4 nanoferrite and Ag2Cr2O4 nanochromite for high-frequency, switching field, and biomedical applications as antimicrobial nanomaterials.


INTRODUCTION
Nano-sized materials, especially magnetic materials, show a significant change in magnetic, physical, and electrical properties due to the small size of the grains compared to the bulk ones. Moreover, these materials are considered to have high efficacy against the antimicrobial activity. Silver nanoparticles are widely used in biomedical applications because of their fascinating ability to kill many bacteria. Ferrite and chromite nanoparticles are magnetic materials that offer potential applications in an assortment of areas such as medical, water purification, and solar energy. [1][2][3][4][5] These applications depend on the preparation method, size, and shape of the nanoparticles. Thus, it is important to choose an effective method to achieve specific performance. The flash auto-combustion method, used in the present study, produces nanoparticles of small size, and it is good to control over stoichiometry. Moreover, it is a low cost and easy method and saves time.
Many previous studies examined the physical properties of nanoferrite and nanochromite; however, to the authors' knowledge, very little research especially in nanochromite systems has been carried out in antimicrobial fields. [6][7][8][9][10][11] Antimicrobial application, especially silver complexes, is gaining importance because of the need for alternative antimicrobial drugs, which have been used badly in the last years. 12,13 Thus, the present antimicrobial study was carried out on three gram-positive strains, three gram-negative strains, and two fungi. The novelty of the present study is evaluating the preparation and physical and antimicrobial properties of Ag nanoferrite and nanochromite.

Synthesis and Characterization of Nanoferrite
The nanosamples Ag 2 Fe 2 O 4 and Ag 2 Cr 2 O 4 were prepared by using the flash auto-combustion method. The starting materials were nitrates (Received August 5, 2021; accepted April 18, 2022; published online May 12, 2022) JOM, Vol. 74, No. 7, 2022 https://doi.org/10.1007/s11837-022-05315-y Ó 2022 The Author(s) (silver, chromium III, or iron III) and urea from Fisher company with 99% purity. Distilled water was added to each material with stirring, and then the components were mixed. The mixture was heated using a hot plate at 250°C till fine powder was produced. By measuring the temperature inside the beaker using a thermocouple, the authors observed that the temperature was around 600°C. This was due to the formation of nanosamples during combustion using nitrate salts, which give extra entire heat when in combination with each other. Therefore, most researchers prefer using nitrate salts during preparation methods of nanosamples other than chloride or sulfide salts. Finally, the investigated samples were collected and ground for half an hour in preparation for all analyses. Thermal decomposition behavior was examined using NETZSCH STA 409 C/CD by DSC (differential scanning calorimetry), TG (thermogravimetry analysis), and DTG (differential thermogravimetry analysis). The magnetic properties were measured using a Lake Shore 7410 vibrating sample magnetometer.

Biological Assays
The in vitro antimicrobial activities of the Ag 2 Fe 2 O 4 nanoferrite were determined against ATCC reference microbial strains, i.e., gram-positive (Bacillus subtilis ATCC 6051, Streptococcus faecalis ATCC 19433, Staphylococcus aureus ATCC 12600), gram-negative (Pseudomonas aeruginosa ATCC 10145, Escherichia coli ATCC 1175, Neisseria gonorrhoeae ATCC 19424), and fungi (Candida albicans ATCC 7102, Aspergillus flavus Link). The investigated sample was solubilized in DMSO, which was used as a negative control. The method in detail was performed by a modified Kirby-Bauer disc diffusion method as was previously reported. 14 Ampicillin and amphotericin B were used as standard discs of antibacterial and antifungal agents, respectively.

RESULTS AND DISCUSSIONS Thermal Analyses
Thermal analysis is a branch of science where the properties of the nanomaterial are studied by changing the temperature. Figures 1 and 2 illustrate the TG, DTG, and DSC analysis (thermogravimetry, differential thermogravimetry, and differential scanning calorimetry respectively) for Ag 2  This mass loss might be due to the removal of the chemically bound water or the combustion of nitrates. Thermogravimetry (TG) is used to evaluate the thermal stability of a material. If there is no mass change or a very small amount of mass change in the desired temperature range, the material will have thermal stability. However, the temperatures > 350°C for Ag 2 Fe 2 O 4 nanoferrite and 500°C for Ag 2 Cr 2 O 4 nanochromite showed thermal stability because the weight loss behavior for both samples was around 300-500°C for Ag 2 Fe 2 O 4 and Ag 2 Cr 2 O 4 nanoferrite, respectively. The weight loss of both samples may be due to the release of water from the initial ingredients (AgNO 3 , Cr (NO 3 ) 3 , and Fe (NO 3 ) 3 ) and the formation of Ag 2 Fe 2 O 4 nanoferrite and Ag 2 Cr 2 O 4 nanochromite in more stable forms. A closer look at the preparation temperature of both samples, owing to the starting material's nature, which is nitrate, shows the combustion of nitrates differs from the combustion of other salts such as chloride, oxalate, or sulfate. This is because the nitrates make the reaction more rapid, and the internal temperature differs from the reading temperature on the heater. In the present study, the samples were prepared at 250°C but when the thermocouple was put inside the beaker during the combustion the entire temperature was 600°C. Therefore, the thermal analysis TGA curve shows that at 600°C the samples are thermally stable, and nanoferrite and nanochromite are prepared. Figures 1 and 2 show the differential scanning calorimetry (DSC) of Ag 2 Fe 2 O 4 nanoferrite and Ag 2 Cr 2 O 4 nanochromite. Figure 1 shows that two endothermic peaks appear at around 200-230°C and 300-350°C. However, Fig. 2 shows two exothermic peaks at around 200-250°C and 450-500°C. The first endothermic peak of Ag 2 Fe 2 O 4 (200-230°C) was due to the chemically bound water, and the exothermic one of Ag 2 Cr 2 O 4 (200-250°C) was due to the combustion of nitrates. The other endothermic peak of Ag 2 Fe 2 O 4 (300-350°C) was due to the cation redistribution, and the exothermic one of Ag 2 Cr 2 O 4 (450-500°C) was due to the recrystallization. 15 Above 350°C and 500°C for Ag 2 Fe 2 O 4 and Ag 2 Cr 2 O 4 , respectively, the nanosamples were thermally stable as deduced from TG analysis. As a result, according to thermal analyses, the Ag nanoferrite and nanochromite were formed as prepared at 250°C. Table I summarizes the kinetic parameters using the Coats-Redfern equation. 16 These parameters are D E, D S, D H, and D G (activation energy, activation entropy, activation enthalpy, and free energy change of decomposition, respectively). From a plot deduced from the Coats-Redfern equation, the DE was calculated from the slope, and the preexponential factor A, which is the intercept, was also calculated as reported in Table I. The activation entropy DS, activation enthalpy D H, and Gibbs free energy D G were calculated from the following relations:  Influence of Silver Nanoferrite and Nanochromite on Physical Properties for High-Frequency and Biomedical Applications where h is Plank's constant, R is the gas constant, k B is Boltzmann constant, T s is DTG peak temperature, and A is the pre-exponential factor. From Table I, one can obtain that Gibbs free energy (DG) had a positive sign for both samples, and this revealed that the final residue of (DG) in each sample was larger than that of the initial compound. Moreover, the decomposition steps for each nano-sample were non-spontaneous processes.

XRD Study
The investigated samples showed single-phase cubic spinel structure from the x-ray diffraction pattern (XRD) with crystallite size of 73.2 nm for Ag 2 Fe 2 O 4 nanoferrite and 80.6 nm for Ag 2 Cr 2 O 4 nanoferrite as reported in our previous studies. 17,18 Moreover, the surface morphology scanning electron microscopy (SEM), energy-dispersive x-ray analysis (EDX), and atomic force microscopy (AFM) of the investigated samples were discussed in detail in our previous studies 5,17,18 and assured that the samples in the nanoscale had the presence of clusters in the morphology due to the absence of the surfactant during the preparation technique. To understand the structure of both samples, it is important to illustrate the calculations estimated from XRD analysis. Thus, in the present study, imported calculations are described in detail based on the cation distribution of Ag 2 Fe 2 O 4 and Ag 2 Cr 2 O 4 nanoferrite. The physical properties of distinctive ferrites were sensitive to the valence state and cation dispersion of tetrahedral A-site and octahedral B-site of the spinel lattice. 19 Hence, it was vital to ponder the cation dispersion to understand the numerous physical properties of spinel ferrites as appearing in Table II.
From Table II, the change in the lattice parameters ''a'' of Ag nanoparticles by substitution of Fe by Cr ion was shown. One can determine that there was a relationship between the ionic radius and lattice parameter. This means that the ionic radius of Fe 3+ ion is higher than that of Cr ion; this reflects that the theoretical and experimental lattice parameters of Ag 2 Fe 2 O 4 nanoferrite are larger than those of Ag 2 Cr 2 O 4 nanochromite. The lattice parameter could be calculated theoretically (a th ) from the following relation: 20 The increase of the lattice parameter by increasing ionic radius of Fe 3+ ion obeyed Vegard's law. 21 The variation between the experimental and theoretical lattice parameters could be due to the defects of the lattice of the polycrystalline material. 22  Table III shows the mean ionic radius per molecule r A and r B for the tetrahedral A-site and the octahedral B-site, respectively, utilizing the cation distribution as follows: 23,24 where r is the ionic radius and C is the concentration. The r A was decreased and r B increased by replacing Fe 3+ ion instead of Cr 3+ as shown in The value of the lattice parameter is strongly affected by the substitution of the octahedral Bsite. 24 Also, the r (average ionic radius) was slightly decreased in case of the replacement of Fe 3+ ion, and its equation is given as follows: 25 Also, Table III shows the tolerance factor T for spinel ferrite, which was suggested by Steinfink et al. 26,27 as follows: where r A is the ionic radius of tetrahedral A-site, r B is the ionic radius of octahedral B-site, and R o = 1.38 Å is the ionic radius of oxygen. 28 The value of the tolerance factor was close to unity for an ideal spinel ferrite structure. 29 In the present study, the values of the tolerance factor for Ag 2 Cr 2 O 4 nanochromites and Ag 2 Fe 2 O 4 nanoferrite were closed to unity. Thus, there was no defect formation of the ferrite structure.
The values of the following equations are reported in Table III: where u 3m is the oxygen parameter (1/4 center of symmetry) for which the ideal value of the oxygen parameter when the origin is at the octahedral B-site is 0.25, u 43m is the oxygen parameter (1/3 center of symmetry) for which the ideal value of the oxygen parameter when the origin is at the tetrahedral Asite is 0.375. It is known that the oxygen parameter depends on the preparation condition, heating treatment, and chemical composition. The study of the oxygen parameter is important to determine the magnetic interactions between metal-oxygen ions. Table IV shows that the experimental oxygen parameter u exp is larger than that of the theoretical one u th , and this is in good agreement with previous literature. 30,31 This could be attributed to the oxygen ions' displacement of tetrahedral-octahedral interaction (A-B); thus, a decrease occurred between the distances of the tetrahedral oxygen ions (A-O) and the octahedral oxygen ions (B-O), in turn increasing the B-B and A-A interactions. The increment of the experimental oxygen parameter u exp compared to that of the theoretical one u th may be due to the moving of the anions along the tetrahedral A-site through<111> direction. As a result, the volume of the A-site interstice increased with a decrease happening in turn in the octahedral site (B-site). 32 It is important to know the hopping length L A and L B for the tetrahedral and octahedral sites, respectively, based on the experimental lattice parameter to determine the distance between the magnetic ions, calculated as follows: 33 Table IV shows that there is a correlation between the lattice parameter and the hopping length. As a result, the hopping length of tetrahedral ion L A and hopping length of octahedral site L B values By adding a larger ionic radius Fe 3+ ion, the d AL (length of the shared octahedral edge) and the d AE (length of the tetrahedral edge) decreased; however, the d BL (the bond length of the octahedral site) increased. These might be attributed to the replacement of the Cr 3+ ion by the larger ionic radius of the Fe 3+ ion on the octahedral site. Also, the unshared octahedral edge length (d BEu ) and the shared tetrahedral bond length (d BE ) increased by increasing large Fe 3+ ion. Table IV shows the magnetic interaction distances and angles of the investigated samples, which were calculated from the following equations: [35][36][37] where a is the experimental lattice parameter; b, c, d, e, and f were the lengths of the bond between cations (M-M). Also, p, q, r, and s were the lengths of the bonds between cations and anions (M-O).
It is important to know the bond lengths and angles between cation-anion and cations to determine the magnetic interaction strength of A-B, A-A, and B-B interactions. It is known that the bond length was directly proportional to the magnetic interaction strength; however, the bond angle is inversely proportional with it. 35 One can observe that the p-value (cation-anion interatomic distance) and the b, c, d, e, and f values (cation-cation interatomic distance) were increased by replacing the large ionic radius Fe 3+ ion as reported in Table IV. However, the q, r, and s (cation-anion interatomic distances) were decreased by increasing the Fe 3+ ion. Moreover, the calculation of the values of h 1 , h 2 , h 3 , h 4 , and h 5 (bond angles) was from the following relations and reported in Table IV: One can observe that the weakness of A-B and A-A interactions is the main reason for the decrease in h 3 and h 4 ; however, the strength of the B-B interaction is the main reason for the increase in h 1 , h 2 , and h 5 by replacing the large ionic radius of Fe 3+ ion.
Finally, the addition of different ionic radii to Ag nanoparticles was vital in numerous innovative applications, and knowing the cation dispersion and different physical properties helped in choosing the appropriate applications.

Switching Field and High-Frequency Applications
The switching field and high-frequency response of Ag 2 Fe 2 O 4 and Ag 2 Cr 2 O 4 nanoparticles were depicted from the magnetic properties. The magnetic properties were illustrated in detail in our previous work. 17,18 The switching field distribution was of great interest because of its importance in data storage technology and recording media. 36 Moreover, it is known as a property of interphase exchange. Figure 3a shows the switching field distribution [first derivative of the magnetization (dM/dH) versus the field (H 2 )] of the investigated samples. The switching field distribution of Ag 2 Fe 2 O 4 nanoparticles gave a 3.8-fold higher value than that of Ag 2 Cr 2 O 4 nanoparticles because of the high magnetic interaction of the cations of Ag 2 Fe 2 O 4 nanoparticles. Figure 3b shows the operating frequency measurements of Ag 2 Fe 2 O 4 and Ag 2 Cr 2 O 4 nanoparticles. It is an important study to avoid the noise and interference of the frequency. 37 The frequency is classified according to the ranges of the operating frequency. 38 Many factors affected these ranges such as the geometry of the devices and magnetization of the nanoparticles. These parameters are important for industrialists to check before performing any devices. Thus, it is important to calculate the operating frequency before using any device, and it can be calculated from the following equation: 39 where c is the gyromagnetic ratio (c = 2.8 MHz/G) and M is the magnetization of the investigated samples. The

Biological Activity
The antimicrobial properties of the investigated samples were illustrated in vitro. In our previous work 5 the surface roughness was studied from atomic force microscopy analysis and showed that Ag 2 Fe 2 O 4 nanoferrite (1.623 lm) had a slightly higher value than Ag 2 Cr 2 O 4 nanoferrite (1.602 lm). This indicated that Ag 2 Fe 2 O 4 had slightly higher surface activity than Ag 2 Cr 2 O 4 nanoferrite. There is a correlation between the surface roughness and surface activity of the nanosamples, which in turn influence the antimicrobial study. 40 The high surface activity of the nanosamples provided a strong antimicrobial property as shown in the investigated samples. Figure 3c, d shows the in vitro antimicrobial properties of Ag 2 Fe 2 O 4 nanoferrite against gram-positive, gram-negative bacteria and fungi. These figures and Table 5 reveal a strong antimicrobial activity against gram-positive and gramnegative bacteria. However, they had a moderate effect against Candida albicans fungus compared with ampicillin and amphotericin B. The strong effect of killing the tested bacteria and fungus may be attributed to the strong toxicity of Fe and Cr ion added to Ag ion; thus, the cell membrane of the bacteria was damaged. [41][42][43][44][45][46][47][48][49][50] Moreover, no activity appeared against Aspergillus flavus fungus. Our previous study 18

CONCLUSIONS
Ag nanoferrite and Ag nanochromite are interesting nanomaterials that allow various applications, especially in antimicrobial studies. Thermal analyses were studied and assured that the investigated samples were formed at room temperature. Moreover, many physical properties were clarified from the different structures determined from XRD investigation such as the lattice parameter, cation dispersion, oxygen parameter, hopping length, and interatomic distances. Also, the antimicrobial activity was studied, and the investigated samples showed a strong antimicrobial effect against the tested gram-positive and gram-negative strains; however, they show moderate effect against Candida albicans fungus. Considering all previous results, the effects of Ag nanoferrite and Ag nanochromite were studied, and the authors hardly recommend the use of Ag 2 Fe 2 O 4 nanoferrite and