1 Introduction

Magnetic nanoparticles (MNPs) have gained much interest in the material science field due to their wide variety of applications, which includes use as heterogeneous catalysts, absorbents, biosensors, and drug delivery system [1, 2]. These nanoparticles have been reported to be easily functionalized and exhibit superparamagnetic behavior [3]. Iron oxide nanoparticles are a subset of MNPs which are frequently utilized due to their non-toxic nature, affordability, and capacity for easy modification as a result of their unique chemical and physical magnetic characteristics. Hematite (α-Fe2O3) is the predominant iron oxide phase, characterized by a hexagonal structure that has ferromagnetic properties. Furthermore, there are two other metastable phases known as maghemite (γ-Fe2O3) and magnetite (Fe3O4), which exhibit a spinel structure and show ferromagnetic characteristics, respectively [4,5,6,7].

Iron oxide nanoparticles can be modified through the addition of other materials, such as organic and inorganic compounds, to produce functional magnetic nano- or microstructures [8, 9]. Iron oxide is frequently coated with an additional outer layer to enhance hydrophilicity and for further surface functionalization [10]. Magnetite (Fe3O4) is widely used in nanoparticles synthesis to create magnetic properties in iron oxides. However, it demonstrates an intense tendency to react strongly in the presence of acidic environmental conditions. Therefore, a various materials are utilized as coatings to fabricate the core-shell structure that envelops the magnetite nanoparticles. The materials include silica, carbon, protein, and polymers.

Considering its non-toxic, inert, varying chemical properties, facile surface modification, and high stability, silica oxide has been frequently utilized as an inorganic compound for iron oxide coating [11,12,13]. The existence of a silica oxide matrix provides 5 between the iron oxide nanoparticles, thoroughly controlling interparticle interactions and preventing agglomeration of the iron oxide nanoparticles. In addition, the silica matrix offers possibilities for surface functionalization with various compounds through employing the silanol groups that exist on the surface [14,15,16,17].

Various studies have shown that the magnetic properties and particle size of iron oxides are significantly affected by the methods and treatments developed. The most frequently utilized and convenient method is coprecipitation [18]. Recently, this method has been developed with a wide variety of modification techniques which have no impact on the primary reaction. As a result of combining Fe2+ and Fe3+ ions, magnetite can be produced and modified to exhibit different sizes and magnetic strengths. A different strategy involves the thermal decomposition of organometallic precursors. Organic iron compounds such as (hydroxylamineferron [Fe(Cup)3], iron pentacarbonyl [Fe(CO)5], ferric acetylacetonate [Fe(acac)3], iron oleate [Fe(oleate)3] are decomposed at high temperature of the non-polar solvent capping agent. However, it is significant to emphasize that this method is hazardous due to the use of toxic chemicals [19].

The methods used to synthesize magnetic silica nanoparticle core–shell structures have been continuously developed. The microemulsion method is frequently used to produce many different systems that possess both isotropic and thermodynamic stability. Furthermore, the controlling of the atomic ratio of water, oil, and surfactants has contributed to the control of the shape and size distribution of the particles [20]. The Stober process, which utilizes hydrolyzed and condensed TEOS (tetraethyl orthosilicate) in an alcohol-water system with ammonia, is widely recognized as the most popular method. Following a long period of stirring, silica will be synthesized and gradually coated onto the particles’ surface by dispersing the original particles in an aqueous alcohol solution, subsequently adding ammonia water and TEOS [21]. Fan et al. reported a study in which they manufactured uniform magnetite using a modified solvothermal approach. They subsequently utilized the Stober process to fabricate monodisperse magnetic silica core-shell nanoparticles [22]. Thermal decomposition that has been defined is also known as an effective method for producing magnetite nanoparticles. The materials were dispersed in cyclohexane with the use of ultrasonic treatment and addition of ammonium hydroxide. Subsequently, TEOS was added, resulting in obtaining of various shell thicknesses for the silica coating [23].

The utilisation of dye-doped silica nanoparticles has been widely studied for various applications including biosensing, bioimaging, forensics, and photocatalysis. These nanoparticles carry unique intrinsic characteristics such as improved photostability and enhanced signal intensity, contributed to a higher concentration of dye molecules per nanoparticle [23, 24]. Since it is unable to absorb visible light, silica allows the presence of photoactive molecules throughout its matrix, either internally or bound on the surface of the nanoparticles. This structure allows the nanoparticles to exhibit photophysical properties that have similarities with those of the chromophores [20].

The precursors for silica oxide coating might be potentially commonly found (such as TEOS) or derived from nature’s products (such as rice husk, bamboo leaves, or geothermal silica) [25]. This study utilizes geothermal silica precipitates as the precursor in order to generate cost-effective, environmentally friendly, and functional nanocomposites using sustainable methods [18]. Geothermal silica precipitate derived from geothermal power plants is known to consist of a large amount of amorphous silica, exceeding 98%, which has the potential to be modified into silica-based nanocomposites [19].

In this study, bifunctional magnetic fluorescent silica core–shell nanocomposites (MFSNC) are synthesized. The outer shell of these nanocomposites is composed of an organic pigment and a geothermal silica-based silica matrix, with iron oxide serving as the core. The geothermal silica-based nanostructures produced by combining magnetic and fluorescent characteristics exhibit the potential to be applied as highly selective biosensing platforms or fluorescent labels.

2 Experimental procedure

2.1 Materials

The silica utilized in this study is derived from the solid residue of the Geodipa Geothermal Power Plant (PLTP). Utilizing this waste is an endeavor towards recycling and repurposing materials which usually appear as byproducts. This research contributes to the development of nanomaterials and supports sustainable practices by utilizing waste from the Geodipa PLTP, thus reducing the environmental impact of industrial waste. Utilizing waste as a raw material may effectively solve sustainability concerns and enhance resource efficiency.

Iron sulfate heptahydrate (FeSO4·7H2O), NaOH, and N-Hydroxysuccinimide (NHS), were purchased from Merck. HCl, rhodamine B, cetyltrimethylammonium bromide (CTAB), undecylenic acid, 1-(3-Dimethylaminopropyl)-3-ethyl carbodiimide (EDC), phosphate-buffered saline (PBS), vancomycin hydrochloride, and sodium broth (NB) were purchased from Sigma-Aldrich. Anhydrous iron (III) chloride (FeCl3) was purchased from Central Drug House (P) Ltd. All chemicals are of analytical grade and used without any further purification. Deionized water sourced from the National Research and Innovation Agency-BRIN was used throughout the whole of work.

2.2 Synthesis of iron oxide

The synthesis of iron oxide, an essential precursor for the MFSNC, was carried out with high accuracy in this precisely conducted experimental approach. A 100 mL solution containing iron (III) chloride (FeCl3) was thoroughly mixed with an equal volume of a solution containing iron sulfate heptahydrate (FeSO4·7H2O). The concentration ratio of the Fe precursors was varied and is shown in Table 1. The systematic mixing of these solutions set the stage for subsequent synthesis processes. Subsequently, 100 mL of sodium hydroxide (NaOH) solution was added dropwise, to attempt to carry out controlled reaction kinetics.

Table 1 Concentration of Fe2+ and Fe3+ for each MFSNC sample
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The stepwise addition of sodium hydroxide led to the formation of magnetite (Fe3O4), as evidenced by the appearance of a dark solution. The solution obtained was given further processing using a sonication procedure at 65 °C for 30 min. This additional process allowed the production of a black precipitate, identified as magnetite (Fe3O4) sample.

Subsequently, the precipitate formed through a filtration process, which was then followed by repeated washing procedures until a neutral pH was achieved. This complete cleansing process effectively eliminated contaminants and unreacted chemicals, contributing to the quality of the final iron oxide product. After the completion of the washing process, the collected precipitate was placed in a controlled drying phase at a temperature range of 80 °C for 5 h. The application of this controlled drying procedure led to the formation of a dry powder, indicating the isolation of iron oxide nanoparticles.

2.3 Synthesis of magnetic fluorescent silica nanocomposites (MFSNC)

In this procedure, 10 g of SiO2 was accurately weighed and dissolved in 400 mL of 1.5 N NaOH. The mixture was stirred for 1 h at 90 °C, and then the Na2SiO3 solution was separated from the impurities through filtration. Subsequently, 5 g of magnetite (Fe3O4) and 2.5 mg/g of rhodamine B were added into the sodium silicate solution and stirred for 5 min. A 2 N HCl solution was added dropwise until a gel formed at pH 7, which was followed by 2% CTAB was added. The resulting gel was allowed to mature for 18 h at room temperature. After maturation, the gel was washed three times with deionized water until it reached a neutral pH. The sol–gel MFSNC was then dried at 100 °C until it reached a stable weight. The formation of MFSNC is shown in Fig. 1, illustrating the sequential steps involved in the synthesis process. This method provided the effective production and stabilization of mesoporous silica nanocomposites (MFSNC) with the desired properties for advanced applications [18, 19].

Fig. 1
figure 1

Schematic of the formation of MFSNC

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2.4 Characterizations

X-ray diffractometry (XRD) was employed to examine the crystalline phases of the magnetic fluorescent silica nanocomposites (MFSNC) and iron oxide. The XRD patterns were recorded on a Rigaku Miniflex 600 diffractometer with Cu Kα-radiation. The data were collected over the 2θ range of 5–90°, with the instrument operated at 45 kV and 40 mA. The investigation of saturation magnetization was performed using a Vibrating Sample Magnetometer manufactured by Dexing Magnet Ltd, which has a measuring capability of 250 Oersted. The optimized MFSNC sample was imaged using a field emission gun-transmission electron microscope (FEG-TEM), Talos F200X (Thermo Fisher Scientific), and an acceleration voltage of 200 kV. The equipment utilized in this study included an Energy Dispersive X-ray (EDX) detector of Super-X type, a High-Angle Annular Dark-Field detector Fishione CL 98 mm, a Scanning Transmission Electron Microscopy Bright-Field and Dark-Field (STEM BF-DF), and a Panther detector with CL 160 mm.

2.5 Fluorescence spectrometer

The solution was prepared by dissolving 10 mg of MFSNC in 10 mL of deionized water, resulting in a sample concentration of 1 mg/mL. The fluorescence intensity of the MFSNC samples was measured using a fluorescence spectrophotometer (Agilent, Singapore) with an excitation wavelength of 545 nm. Emission was observed within the spectral range spanning from 550 to 750 nm. The slit widths of the excitation and emission were both at 5 nm. There was no evidence of filters.

3 Result and discussion

The efficient synthesis of magnetite, maghemite, and cobalt ferrite can be achieved by varying certain chemical composition of the nanoparticles and using the iconic molecule separation coprecipitation approach. The synthesis of iron oxide nanoparticles was carried out via the coprecipitation method, which is a facile and effective method to obtain an appropriate yield in a brief amount of time [26]. This method involves the simultaneous precipitation of Fe2+ and Fe3+ in solution under alkaline conditions (Eq. 1) [27].

$${{{\mathrm{Fe}}}}^2 + _{\left( {{{{\mathrm{aq}}}}} \right)} + 2{{{\mathrm{Fe}}}}^{3 + }_{\left( {{{{\mathrm{aq}}}}} \right)} + 8{{{\mathrm{OH}}}}^ - _{\left( {{{{\mathrm{aq}}}}} \right)} \to {{{\mathrm{Fe}}}}_3{{{\mathrm{O}}}}_{4\left( {{{\mathrm{s}}}} \right)} + 4{{{\mathrm{H}}}}_2{{{\mathrm{O}}}}$$
(1)

The iron oxide core was then coated with silica, which was then further functionalized to have an optimum magnetic and fluorescence properties, forming the MFSNC nanocomposites. These bifunctional nanomaterials have been found to be useful as biosensing platforms due to their unique features. The sol–gel method was utilized to synthesize the MFSNC. This method, frequently known as Stober’s method, is a conventional approach used for synthesizing nanoparticles. The convenience of this process results from the ease of operation, as well as the fact that it may be carried out at ambient temperature and pressure, resulting in a higher production yield [28].

Prior to the dispersion of magnetite (Fe3O4) in a sodium silicate solution, such solution was prepared through the reaction of SiO2 and NaOH (Eq. 2). A mixture of sodium silicate, organic dye, and iron oxide undergoes a chemical reaction to produce MFSNC gel. This reaction is catalyzed by hydrochloric acid, as shown by Eq. (3). The organic dye was introduced into silica matrices, while the iron oxide was coated by silica. The final result generated the MFSNC materials [19, 23, 29].

$${{{\mathrm{SiO}}}}_{2\left( {{{\mathrm{s}}}} \right)} + 2{{{\mathrm{NaOH}}}}_{\left( {{{{\mathrm{aq}}}}} \right)} \to {{{\mathrm{Na}}}}_2{{{\mathrm{SiO}}}}_{3\left( {{{{\mathrm{aq}}}}} \right)}$$
(2)
$$\begin{array}{l}{{{\mathrm{Na}}}}_2{{{\mathrm{SiO}}}}_{3\left( {{{{\mathrm{aq}}}}} \right)} + {{{\mathrm{Rh}}}}\,{{{\mathrm{B}}}}_{\left( {{{{\mathrm{aq}}}}} \right)} + {{{\mathrm{Fe}}}}_3{{{\mathrm{O}}}}_{4\left( {{{\mathrm{s}}}} \right)} + 2{{{\mathrm{HCl}}}}_{\left( {{{{\mathrm{aq}}}}} \right)} \to {{{\mathrm{SiO}}}}_2\\ - \,{{{\mathrm{Fe}}}}_3{{{\mathrm{O}}}}_4/{{{\mathrm{Rh}}}}{{{\mathrm{B}}}}_{\left( {{{\mathrm{s}}}} \right)} + 2{{{\mathrm{NaCl}}}}_{\left( {{{{\mathrm{aq}}}}} \right)}\end{array}$$
(3)

The synthesized MFSNC, with varied Fe2+ and Fe3+ ratio, were characterized by XRD and the results showed the structure of magnetite for all variations (Fig. 2). The XRD patterns exhibited prominent peaks at 2θ angles of 30.20°; 35.6°; 43.30°; 53.33°; 57.37 and 62.87° corresponding to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) planes of Fe3O4 (magnetite), respectively, which are in good agreement with the references values (JCPDS 19–0629) [30,31,32]. The strong and sharp peaks indicate that all the MFSNC synthesized are crystalline and structurally ordered in a long range. The shape of the iron oxide peak was narrow and sharp, without interference from irregular diffraction peaks, indicating that the iron oxide MFSNC had a higher purity [33].

Fig. 2
figure 2

XRD pattern of MFSNC sample MFSNC0.5 (A), MFSNC1.0 (B), MFSNC1.5 (C), and MFSNC2.0 (D)

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Figure 2 also showed that the silica peak was shown at an angle of 2θ, precisely between 20° and 30°. Furthermore, the broadening of the peak indicated that the silica of MFSNC is in its amorphous phase. Thus, a certain peak corresponds to the presence of amorphous silica, while other peaks correspond to the presence of crystalline iron oxide. These diffraction peak positions confirmed that the MFSNC nanomaterials containing SiO2 and Fe3O4 had been successfully synthesized [13, 34].

The XRD pattern of MFSNC had a relatively high peak intensity at 2θ, an angle of 35.50° precisely corresponding to the (3 1 1) peak, which indicates the products have an abundance of crystallinity. The average size of the crystallites can be estimated by quantitatively calculating the (3 1 1) peak using the Debye-Scherrer formula (Eq. 4).

$$D = \frac{{k \times \lambda }}{{\beta \times cos\theta }}$$
(4)

Where λ represents the wavelength of the Cu Kα line, and β is the full width at one-half the maximum (FWHM) of the most intense diffraction peak of the crystallographic plane (3 1 1). In the X-ray diffraction (XRD) analysis, variations in the particle size of the (3 1 1) plane can be observed, and these variations are attributed to the crucial role played by ferric chloride (FeCl3) in the synthesis of iron oxide nanoparticles, specifically magnetite (Fe3O4). The concentration of FeCl3 can influence the size of Fe3O4 crystallites during the synthesis process as shown in Table 2.

Table 2 XRD parameters
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By altering the concentration ratio, particle growth can be controlled during the synthesis process, affecting the final crystallite size. The (3 1 1) plane refers to a specific crystallographic plane in the crystal lattice of Fe3O4. The XRD analysis provides insights into the arrangement of atoms within the crystal lattice and allows researchers to characterize the material’s structure. The observed variations in the (3 1 1) plane suggest that different concentrations of FeCl3 impact the growth kinetics of the particles during synthesis, leading to variations in the final crystallite size. This phenomenon underscores the sensitivity of nanoparticle synthesis to the reaction conditions, emphasizing the need for precise control over parameters such as FeCl3 concentration to achieve desired particle characteristics [35].

The MFSNC was designed to exhibit both magnetic and fluorescence properties. For the latter, the MFSNC samples were analyzed by a fluorescent spectrometer. Figure 3 showed that the MFSNC could be examined for its fluorescence emission within the wavelength range of 550–750 nm, with an excitation wavelength of 545 nm. The maximum fluorescence intensity was observed at 578 nm, where MFSNC0.1 exhibited the greatest value of 852 a.u., whilst MFSNC0.5 had the lowest value of 280 a.u.

Fig. 3
figure 3

Fluorescence spectra of MFSNC sample MFSNC0.5 (A), MFSNC1.5 (B), and MFSNC2.0 (C) and MFSNC1.0 (D) at an excitation wavelength of 545 nm

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This study further investigates the fluorescence stability at the maximum emission wavelength of 578 nm for each sample variation over the interval of 120 min. Figure 4 illustrates that the MFSNC0.5 was more stable than other samples. The MFSNC0.1 sample had the maximum fluorescence intensity compared to the other samples. However, it demonstrated less photostability over time, with a decrease in intensity of around 27% [36]. Considering the photostability of the sample, MFSNC0.5 was determined as the optimum sample.

Fig. 4
figure 4

Stability graph of MFSNC sample MFSNC0.5 (A), MFSNC1.0 (B), MFSNC1.5 (C), and MFSNC2.0 (D)

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The conventional magnetic hysteresis loop measurement provided an analysis of the magnetic properties of the optimized MFSNC sample, i.e., MFSNC0.5. The hysteresis loop is a magnetization curve representing a material of magnetic behavior of the multiple domains. It occurs because all the domains can not return to their original orientations after the magnetic saturation (Ms) is obtained and the field is decreased. There is remnant magnetization (MR) which can be removed by applying a magnetic field opposite the initially used field, defined as the coercive field (Hc). The mass magnetization at 300 °K was measured because it is assumed that the magnetic of materials is used near room temperature. At the same time, the magnetic response of the bare silica was minimal [9, 37]. Hence, Fig. 5 represented that the resulting MFSNC0.5 materials had a magnetization saturation of 14.57 emu/g and a loop area value of 0.7 kOe confirming the type of magnetization was ferromagnetic.

Fig. 5
figure 5

Hysteresis loops of MFSNC

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Ferromagnetism is the most intense magnetism, which has a spontaneous magnetic phenomenon and is produced by the self-alignment of the unpaired (same spin) forming electronic configurations of compounds (e.g., Fe, Co, Ni, Cr, Mn, and some rare earth). Ferromagnetic materials have magnetic properties highly dependent on anisotropy, including structure, shape, and surface anisotropy and generally recognized as either hard (permanent) magnets or soft magnets (i.e., rapidly/easily demagnetize) [38, 39]. It was reported that the iron oxide with coating agents will decrease magnetic saturation. The low magnetic saturation was presented by several phenomena relevant to the size of the synthesized particles, including finite size effect and change in cation distribution, shape effect or spin pinning, spin canting, crystal defect (amorphous), etc. [32]. The MFSNC sample had shown low coercivity arising from their soft-magnetic properties. In this study, the prepared MFSNC gave a relatively large diameter without superparamagnetic behavior, such as using temperature. Temperature influences the ferromagnetic, determined above a critical temperature value (i.e., Curie point, Tc) [38].

The FE-TEM images and its corresponding EDX mapping of the optimized MFSNC0.5 as shown in Fig. 6. The samples were clearly in their nanocomposite form with around 60 nm in diameter with the Fe3O4 size of around 40 nm (in red). The result further confirms the XRD crystallite results were the average size was calculated using the Scherrer equation as shown in Table 2 [11, 40].

Fig. 6
figure 6

TEM micrographs of the optimized MFSNC sample (a) and the corresponding EDX spectroscopy mapping: Si-Fe-K (b) N-O-Si-Fe-K (c) O-K (d) Si-K (e) Fe-K (f) and N-K (g), The incident electron beam was at 90 nA

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Table 3 encapsulates a comprehensive exploration of diverse synthesis methods, magnetization types, and saturation magnetization (Ms) values for magnetic nanoparticles or nanocomposites derived from different precursors. The optimized MFSNC generated form this work was compared with previous studies and showed comparable magnetic properties to other nanomaterials derived from commercial precursors.

Table 3 Comparison between different synthesis methods of silica-coated iron oxide with different precursors
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4 Conclusion

In conclusion, this study successfully synthesized MFSNC from natural geothermal silica, leading to materials that exhibit both magnetic and fluorescent characteristics. The XRD patterns, particularly the (3 1 1) peak, provided insights into the crystallite size variations influenced by different concentrations of ferric chloride during synthesis. The fluorescence spectra analysis highlighted the varying fluorescence intensities among MFSNC samples, with MFSNC0.5 showing the most stable fluorescence over time. The ferromagnetic behavior of MFSNC0.5, demonstrated by the hysteresis loop, indicated its suitability for applications requiring magnetic properties. The photostability graph further confirmed MFSNC0.5 as the optimized sample with dual magnetite and fluorescence properties. The TEM micrographs and EDX mapping provided a nanoscale view of the MFSNC sample, reinforcing the findings from XRD and offering valuable insights into the material’s microscopic structure and elemental distribution. Comparative analysis with other studies using different precursors and synthesis methods underscored the uniqueness of the synthesized MFSNC materials, particularly in exhibiting ferromagnetic properties. This research contributes valuable information on the synthesis and characterization of MFSNC materials, emphasizing their potential applications in diverse fields, such as biomedical imaging and sensing. The combination of magnetic and fluorescent properties opens up avenues for multifunctional applications, highlighting the importance of understanding how different synthesis parameters influence the properties of nanocomposites.