Effects of Surface Functional Groups on the Adhesion of SiO2 Nanospheres to Bio-Based Materials

The interactions between nanoparticles and materials must be considered when preparing functional materials. Although researchers have studied the interactions between nanoparticles and inorganic materials, little attention has been paid to those between nanoparticles and bio-based protein materials, like leather. In this study, organically modified silica nanospheres (SiO2 nanospheres) loaded with rose fragrance were prepared using (3-aminopropyl) triethoxysilane (APTES), (3-mercaptopropyl) triethoxysilane (MPTES), or 3-(2, 3-epoxypropyloxy) propyl triethoxysilane (GPTES) using the sol-gel method. To study the interactions between the modified SiO2 nanospheres and leather, a non-cross-linking adsorption experiment was conducted. According to the Dubinin–Radushkevich isotherm calculation, we found that the adsorption process of leather fiber and organically modified silica nanospheres is physical. The average adhesion energies of APTES-, MPTES-, and GPTES-modified SiO2 nanospheres on the leather are 1.34016, 0.97289, and 2.09326 kJ/mol, respectively. The weight gain, adsorption capacity, and average adhesion energy show that the modified SiO2 nanospheres can be adsorbed on leather in large quantities. The sensory evaluation confirmed that GPTES-modified SiO2 nanospheres endowed the leather with an obvious rose aroma.


Introduction
The adsorption behavior of nanoparticles can be divided into two categories: adsorbents adsorbing substances, or being adsorbed on the surfaces of materials [1]. Many reports have been published about the former. For example, the adsorption process of Fe 3 O 4 magnetic nanoparticles to remove Ni (II) is a spontaneous and endothermic process [2]. Maghemite nanoparticles (Fe 2 O 3 ) were used to adsorb As(V), and the high adsorption capacity was 50 mg/g [3]. The maximum adsorption capacity of magnetic Ni x Cu (1−x) Fe 2 O 4 (x = 0.1-0.9) nanoparticles for methyl blue was 78.3 mg/g at pH 5 [4]. For the latter category, nanoparticles can be adsorbed on the surfaces of both organic and inorganic materials. Therefore, studying the interaction between nanoparticles and substrate is important for the preparation of functional materials. Modifying the surface of silica particles with hydrophilic -NH 2 and -SH groups is beneficial to the adsorption of gold nanoparticles, whereas modification with hydrophobic -CH 3 and -PPh 2 groups is not conducive to the adsorption of gold nanoparticles [5]. Studying the adsorption behavior of a surfactant onto sandstone rock in the presence of nano-particles was important to improving the performance of chemical stimulations in conventional oil reservoirs [6]. Examination of metal nanoparticles showed that palladium, platinum, and titanium particles strongly chemisorb onto carbon nanotube (CNT) surfaces [7]. The adsorption of gold hydrosol nanoparticles onto the surfaces of polystyrene and poly(2-vinyl pyridine) was found to be irreversible [8]. Although

Agent
Chemical Construction Relative Molecular Mass TEOS relative molecular mass and chemical structure of silane precursor and silane coupling agent are shown in Table 1.

Preparation of Organically Modified Silica Rose Fragrance Nanospheres
The organically modified silica rose fragrance nanospheres were prepared according to the improved lavender nanospheres preparation method [21]. The specific operation methods are as follows. First, a mixture of rose fragrance (1.0 g), the silane precursor TEOS (1.0 mL), and silane coupling agents APTES, MPTES, or GPTES (0.5 mL) were fully mixed to form a clear solution. Second, certain concentrations of CTAB, deionized water (28.5 mL), and anhydrous ethanol (14.2 mL) were added in turn, and magnetically stirred for 30 min under at 1500 rpm and 35 °C. A stable microemulsion was formed using an ultrasonic cell breaker for 10 min. An ammonia solution (1.04 mL) was added to catalyze the interfacial hydrolysis condensation reaction between the precursor and the silane coupling agent. The solution was stirred at a speed of 300 rpm for 16 h, and the temperature of the stirring process was 35 ℃. Finally, the reaction liquid was cooled to room temperature (25 ℃), filtered, and then washed with anhydrous ethanol and deionized water. After freeze-drying at -58 ℃ for 24 h, organically modified silica rose fragrance nanospheres were obtained.

Characterization of the Morphology and Chemical Structure of Modified SiO2 Nanospheres
The SiO2 nanospheres prepared with different silane coupling agents were bonded to the conducting resins, which were located on the metal stubs. Before observation, the samples were coated with gold using a gold sputter coater in a high-vacuum evaporator (E-1010 ION SPUTTER, Hitachi, Ltd., Tokyo, Japan). After the pretreatment, a Hitachi S-3400N scanning electron microscope (SEM, Hitachi High-Technologies, Tokyo, Japan) was used to observe the morphology of the nanospheres at an acceleration voltage of 10 kV. The particle size of the modified SiO2 nanospheres was analyzed using Nano Measurer 1.2 software [22]. The structure of the modified SiO2 nanospheres was characterized using a transmission electron microscope (TEM; Talos F200S, ThermoFisher, Waltham, MA, USA). 208 APTES relative molecular mass and chemical structure of silane precursor and silane coupling agent are shown in Table 1.

Preparation of Organically Modified Silica Rose Fragrance Nanospheres
The organically modified silica rose fragrance nanospheres were prepared according to the improved lavender nanospheres preparation method [21]. The specific operation methods are as follows. First, a mixture of rose fragrance (1.0 g), the silane precursor TEOS (1.0 mL), and silane coupling agents APTES, MPTES, or GPTES (0.5 mL) were fully mixed to form a clear solution. Second, certain concentrations of CTAB, deionized water (28.5 mL), and anhydrous ethanol (14.2 mL) were added in turn, and magnetically stirred for 30 min under at 1500 rpm and 35 °C. A stable microemulsion was formed using an ultrasonic cell breaker for 10 min. An ammonia solution (1.04 mL) was added to catalyze the interfacial hydrolysis condensation reaction between the precursor and the silane coupling agent. The solution was stirred at a speed of 300 rpm for 16 h, and the temperature of the stirring process was 35 ℃. Finally, the reaction liquid was cooled to room temperature (25 ℃), filtered, and then washed with anhydrous ethanol and deionized water. After freeze-drying at -58 ℃ for 24 h, organically modified silica rose fragrance nanospheres were obtained.

Characterization of the Morphology and Chemical Structure of Modified SiO2 Nanospheres
The SiO2 nanospheres prepared with different silane coupling agents were bonded to the conducting resins, which were located on the metal stubs. Before observation, the samples were coated with gold using a gold sputter coater in a high-vacuum evaporator (E-1010 ION SPUTTER, Hitachi, Ltd., Tokyo, Japan). After the pretreatment, a Hitachi S-3400N scanning electron microscope (SEM, Hitachi High-Technologies, Tokyo, Japan) was used to observe the morphology of the nanospheres at an acceleration voltage of 10 kV. The particle size of the modified SiO2 nanospheres was analyzed using Nano Measurer 1.2 software [22]. The structure of the modified SiO2 nanospheres was characterized using a transmission electron microscope (TEM; Talos F200S, ThermoFisher, Waltham, MA, USA).

221
MPTES relative molecular mass and chemical structure of silane precursor and silane coupling agent are shown in Table 1.

Preparation of Organically Modified Silica Rose Fragrance Nanospheres
The organically modified silica rose fragrance nanospheres were prepared according to the improved lavender nanospheres preparation method [21]. The specific operation methods are as follows. First, a mixture of rose fragrance (1.0 g), the silane precursor TEOS (1.0 mL), and silane coupling agents APTES, MPTES, or GPTES (0.5 mL) were fully mixed to form a clear solution. Second, certain concentrations of CTAB, deionized water (28.5 mL), and anhydrous ethanol (14.2 mL) were added in turn, and magnetically stirred for 30 min under at 1500 rpm and 35 °C. A stable microemulsion was formed using an ultrasonic cell breaker for 10 min. An ammonia solution (1.04 mL) was added to catalyze the interfacial hydrolysis condensation reaction between the precursor and the silane coupling agent. The solution was stirred at a speed of 300 rpm for 16 h, and the temperature of the stirring process was 35 ℃. Finally, the reaction liquid was cooled to room temperature (25 ℃), filtered, and then washed with anhydrous ethanol and deionized water. After freeze-drying at -58 ℃ for 24 h, organically modified silica rose fragrance nanospheres were obtained.

Characterization of the Morphology and Chemical Structure of Modified SiO2 Nanospheres
The SiO2 nanospheres prepared with different silane coupling agents were bonded to the conducting resins, which were located on the metal stubs. Before observation, the samples were coated with gold using a gold sputter coater in a high-vacuum evaporator (E-1010 ION SPUTTER, Hitachi, Ltd., Tokyo, Japan). After the pretreatment, a Hitachi S-3400N scanning electron microscope (SEM, Hitachi High-Technologies, Tokyo, Japan) was used to observe the morphology of the nanospheres at an acceleration voltage of 10 kV. The particle size of the modified SiO2 nanospheres was analyzed using Nano Measurer 1.2 software [22]. The structure of the modified SiO2 nanospheres was characterized using a transmission electron microscope (TEM; Talos F200S, ThermoFisher, Waltham, MA, USA).

Preparation of Organically Modified Silica Rose Fragrance Nanospheres
The organically modified silica rose fragrance nanospheres were prepared according to the improved lavender nanospheres preparation method [21]. The specific operation methods are as follows. First, a mixture of rose fragrance (1.0 g), the silane precursor TEOS (1.0 mL), and silane coupling agents APTES, MPTES, or GPTES (0.5 mL) were fully mixed to form a clear solution. Second, certain concentrations of CTAB, deionized water (28.5 mL), and anhydrous ethanol (14.2 mL) were added in turn, and magnetically stirred for 30 min under at 1500 rpm and 35 °C. A stable microemulsion was formed using an ultrasonic cell breaker for 10 min. An ammonia solution (1.04 mL) was added to catalyze the interfacial hydrolysis condensation reaction between the precursor and the silane coupling agent. The solution was stirred at a speed of 300 rpm for 16 h, and the temperature of the stirring process was 35 ℃. Finally, the reaction liquid was cooled to room temperature (25 ℃), filtered, and then washed with anhydrous ethanol and deionized water. After freeze-drying at -58 ℃ for 24 h, organically modified silica rose fragrance nanospheres were obtained.

Characterization of the Morphology and Chemical Structure of Modified SiO2 Nanospheres
The SiO2 nanospheres prepared with different silane coupling agents were bonded to the conducting resins, which were located on the metal stubs. Before observation, the samples were coated with gold using a gold sputter coater in a high-vacuum evaporator (E-1010 ION SPUTTER, Hitachi, Ltd., Tokyo, Japan). After the pretreatment, a Hitachi S-3400N scanning electron microscope (SEM, Hitachi High-Technologies, Tokyo, Japan) was used to observe the morphology of the nanospheres at an acceleration voltage of 10 kV. The particle size of the modified SiO2 nanospheres was analyzed using Nano Measurer 1.2 software [22]. The structure of the modified SiO2 nanospheres was characterized using a transmission electron microscope (TEM; Talos F200S, ThermoFisher, Waltham, MA, USA). 278

Preparation of Organically Modified Silica Rose Fragrance Nanospheres
The organically modified silica rose fragrance nanospheres were prepared according to the improved lavender nanospheres preparation method [21]. The specific operation methods are as follows. First, a mixture of rose fragrance (1.0 g), the silane precursor TEOS (1.0 mL), and silane coupling agents APTES, MPTES, or GPTES (0.5 mL) were fully mixed to form a clear solution. Second, certain concentrations of CTAB, deionized water (28.5 mL), and anhydrous ethanol (14.2 mL) were added in turn, and magnetically stirred for 30 min under at 1500 rpm and 35 • C. A stable microemulsion was formed using an ultrasonic cell breaker for 10 min. An ammonia solution (1.04 mL) was added to catalyze the interfacial hydrolysis condensation reaction between the precursor and the silane coupling agent. The solution was stirred at a speed of 300 rpm for 16 h, and the temperature of the stirring process was 35 • C. Finally, the reaction liquid was cooled to room temperature (25 • C), filtered, and then washed with anhydrous ethanol and deionized water. After freeze-drying at −58 • C for 24 h, organically modified silica rose fragrance nanospheres were obtained.

Characterization of the Morphology and Chemical Structure of Modified SiO 2 Nanospheres
The SiO 2 nanospheres prepared with different silane coupling agents were bonded to the conducting resins, which were located on the metal stubs. Before observation, the samples were coated with gold using a gold sputter coater in a high-vacuum evaporator (E-1010 ION SPUTTER, Hitachi, Ltd., Tokyo, Japan). After the pretreatment, a Hitachi S-3400N scanning electron microscope (SEM, Hitachi High-Technologies, Tokyo, Japan) was used to observe the morphology of the nanospheres at an acceleration voltage of 10 kV. The particle size of the modified SiO 2 nanospheres was analyzed using Nano Measurer 1.2 software [22]. The structure of the modified SiO 2 nanospheres was characterized using a transmission electron microscope (TEM; Talos F200S, ThermoFisher, Waltham, MA, USA).
The SiO 2 nanospheres that were modified using different coupling agents and rose fragrance were scanned at full waveband using Fourier transform infrared spectroscopy (Bruker, Billerica, MA, USA) in the range of 500 to 4000 cm −1 . The obtained infrared spectrograms were used to investigate the chemical structures of the modified SiO 2 nanospheres. X-ray powder diffraction (XRD; X'Pert PRO MPD, Nalytical, Netherlands) was used to characterize the crystal types of modified SiO 2 nanospheres.

Application of Modified SiO 2 Nanospheres on Leather
The leather was cut into small pieces (10 × 10 cm) with scissors. The solution was prepared using ammonia, anhydrous ethanol, and deionized water in a weight ratio of 1:2:17. After the solution was Nanomaterials 2019, 9, 1411 4 of 13 mixed evenly, dry gauze was dipped into the solution. The gauze with the solution was used to gently wipe the leather surface to remove the dust and dirt, and to clean the leather surface. After, the wet leather was placed in a dry and ventilated place, and then was placed into a pp bag after the leather surface was completely dry.
The modified SiO 2 nanospheres were added to 100 mL deionized water and then vibrated using a rotary shaking table (ZHWY-304, Zhicheng analytical instrument manufacturing co. LTD, Shanghai, China) at 110 rpm and 25 • C for 30 min to form a dispersed solution. The leather with a clean surface was immersed in the solution, and the duration of adsorption was 4 h. Then, the wet leather was placed in a dry and ventilated place at 25 • C for 24 h to completely dry the leather. Finally, the nano-encapsulated aromatic leather was obtained.

Characterization of Aromatic Leather
The leather that was soaked in the solution of SiO 2 nanospheres modified with different coupling agents was bonded to the conductive resin on the metal post. Before observation, the samples were plated with a gold sputter coating machine in a high vacuum evaporator (E-1010 ION SPUTTER, Hitachi, Ltd., Tokyo, Japan). After pretreatment, the morphology of the aromatic leather was observed using a Hitachi S-3400N scanning electron microscope (Hitachi High Technology, Tokyo, Japan) under a 10 kV accelerated voltage.
The dried weights of the leather before and after soaking were accurately weighed using electronic scales (AL204, Mettler Toledo, Shanghai, China) to calculate the weight gain. A thermogravimetric analyzer (Q5000IR; TA Instruments, New Castle, DE, USA) was used to explore the thermal stability of the modified SiO 2 nanospheres. The sample injection volume was 3 to 5 mg. The whole process was conducted with the protection of N 2 . The test temperature programming was controlled ranging from 30 to 600 • C, and the heating rate was 10 • C·min −1 . On the basis of the above thermogravimetric analysis results, the capacity of leather to adsorb modified SiO 2 nanospheres was calculated [23].
We invited 10 evaluators (5 men, 5 women; 20-30 years old) who had received sensory evaluation training to evaluate the aroma quality of the leather. The aroma intensity of the aromatic leather was scored on a scale ranging from 0 to 9, with 0 indicating the weakest rose aroma intensity and 9 indicating the strongest rose aroma intensity.

Interaction Between Modified SiO 2 Nanospheres and Leather
The SiO 2 nanospheres modified using different coupling agents were dispersed into a deionized aqueous solution, and the linear Beer-Lambert standard curve was determined using UV-vis spectrophotometer (Alpha-1860, Shanghai Element, Shanghai, China). We conducted an experiment using leather and modified SiO 2 nanospheres without cross-linking agent adsorption. Simultaneously, the concentration of the modified SiO 2 nanospheres, which were adsorbed by leather in the initial solution, was analyzed using a UV-vis spectrophotometer at a wavelength of 200 nm. After adsorption equilibrium, the concentration of the modified SiO 2 nanospheres in the residual solution was measured by UV-vis spectrophotometry at a wavelength of 200 nm. All the above experiments were performed three times. The concentration analysis was based on the linear Beer-Lambert standard curve established previously. The adsorption capacity of modified SiO 2 nanospheres on leather was determined as follows [24]: where q e (mg/g) is the capacity of leather to adsorb modified SiO 2 nanospheres, C 0 (mg/L) is the initial concentration of modified SiO 2 nanospheres in dispersion, C e (mg/L) is the concentration of modified SiO 2 nanospheres in the residual solution, V (L) is the volume of dispersion, and M (g) is the quality of the leather used for adsorption. To determine the adsorption capacity of SiO 2 nanospheres on the leather, the equilibrium experimental data were analyzed with Langmuir, Freundlich, and Dubinin-Radushkevich isotherm models. The linearized equation of the Langmuir isotherm model can be expressed as: where C e is the concentration of modified SiO 2 nanospheres in residual solution (mg/L), q e (mg/g) is the capacity of leather to adsorb modified SiO 2 nanospheres, Q 0 is the maximum adsorption capacity of the modified SiO 2 nanospheres (mg/g), and K L is the adsorption equilibrium constant (L/mg). The Freundlich isotherm is an empirical equation based on the adsorption on a heterogeneous surface and assumes that the adsorption occurs at sites with different adsorption energies. The equation is commonly expressed as: ln(q e ) = ln(K F ) where K F (mg/g) and n are the Freundlich constants characteristics. The expression of the Dubinin-Radushkevich isotherm adsorption model is as follows [2,24]: where q m is the maximum adsorption capacity of the modified SiO 2 nanospheres (mg/g), R is the ideal gas constant (8.314 kJ·mol −1 K −1 ), T is the adsorption temperature (K), and K D is the Dubinin-Radushkevich isotherm adsorption constant (mol 2 ·kJ −2 ). According to K D , the average adhesion energy of the adsorption process can be calculated. According to the fitting parameters of the Dubinin-Radushkevich equation, the average adhesion energy E (kJ/mol) of leather adsorption modified SiO 2 nanospheres was calculated as follows: where K D is the Dubinin-Radushkevich isotherm adsorption constant (mol 2 kJ −2 ).

Preparation of Modified SiO 2 Nanospheres Using the Sol-Gel Method
The sol-gel method was used to prepare rose fragrance nanospheres coated with organically modified silica. Hydrolysis and polycondensation of the precursor and coupling agent at the two-phase interface of the microemulsion were completed, and the reaction mechanism is shown in Figure 1. Rose fragrance, organosilane precursor (TEOS), silane coupling agents (APTES, MPTES, and GPTES), CTAB, deionized water, and anhydrous ethanol were mixed and emulsified at 1500 rpm to form stable oil-in-water (O-W) microemulsion. When the ammonia hydroxide solution was added to the microemulsion system, the silane precursor and the coupling agent reacted via hydrolysis and condensation to form a negatively charged organically modified silica oligomer and silicone alcohol. The negatively charged products were adsorbed by positively charged CTAB on the surface of oil droplets. With the reaction progression, the precursor and the coupling agent continuously diffused to the water/oil interface and formed a complete organically modified silica wall material on the surface of the rose fragrance.
to the water/oil interface and formed a complete organically modified silica wall material on the surface of the rose fragrance.  Figure 2 shows the SEM, TEM, and particle size distribution of modified SiO2 nanospheres. The particle size distribution graph shows that the average particle size of the modified SiO2 nanospheres was mainly distributed in the range of 600 to 700 nm (± 50 nm). The average particle size of SiO2 nanospheres modified by APTES, MPTES, and GPTES was 551 ± 50, 581 ± 50, and 688 ± 50 nm, respectively. Their particle size distribution graphs show a normal distribution, indicating that the size of the modified SiO2 nanospheres was uniform. By comparing Figure 2a-c, the SiO2 nanospheres modified by MPTES and GPTES have good dispersibility. The TEM results in Figure 2e,f also demonstrate the core-shell structure. The shell thicknesses of MPTES nanospheres and GPTES nanospheres are 108.21 and 154.34 nm, respectively. The SiO2 nanospheres modified by APTES were the most agglomerated, potentially due to the introduction of -NH2, which endowed the strong silicon interaction with -OH among the nanospheres.   Figure 2 shows the SEM, TEM, and particle size distribution of modified SiO 2 nanospheres. The particle size distribution graph shows that the average particle size of the modified SiO 2 nanospheres was mainly distributed in the range of 600 to 700 nm (± 50 nm). The average particle size of SiO 2 nanospheres modified by APTES, MPTES, and GPTES was 551 ± 50, 581 ± 50, and 688 ± 50 nm, respectively. Their particle size distribution graphs show a normal distribution, indicating that the size of the modified SiO 2 nanospheres was uniform. By comparing Figure 2a-c, the SiO 2 nanospheres modified by MPTES and GPTES have good dispersibility. The TEM results in Figure 2e,f also demonstrate the core-shell structure. The shell thicknesses of MPTES nanospheres and GPTES nanospheres are 108.21 and 154.34 nm, respectively. The SiO 2 nanospheres modified by APTES were the most agglomerated, potentially due to the introduction of -NH 2 , which endowed the strong silicon interaction with -OH among the nanospheres.

Morphology Analysis of Modified SiO 2 Nanospheres
to the water/oil interface and formed a complete organically modified silica wall material on the surface of the rose fragrance.  Figure 2 shows the SEM, TEM, and particle size distribution of modified SiO2 nanospheres. The particle size distribution graph shows that the average particle size of the modified SiO2 nanospheres was mainly distributed in the range of 600 to 700 nm (± 50 nm). The average particle size of SiO2 nanospheres modified by APTES, MPTES, and GPTES was 551 ± 50, 581 ± 50, and 688 ± 50 nm, respectively. Their particle size distribution graphs show a normal distribution, indicating that the size of the modified SiO2 nanospheres was uniform. By comparing Figure 2a-c, the SiO2 nanospheres modified by MPTES and GPTES have good dispersibility. The TEM results in Figure 2e,f also demonstrate the core-shell structure. The shell thicknesses of MPTES nanospheres and GPTES nanospheres are 108.21 and 154.34 nm, respectively. The SiO2 nanospheres modified by APTES were the most agglomerated, potentially due to the introduction of -NH2, which endowed the strong silicon interaction with -OH among the nanospheres.

Fourier Infrared Spectrum and XRD Study of Modified SiO 2 Nanospheres
The Fourier transform infrared (FTIR) results of the modified SiO 2 nanospheres are shown in Figure 3a. We analyzed the infrared spectra of the SiO 2 nanospheres that were modified by different coupling agents and rose fragrance. The wavenumber near 3400 cm −1 is the stretching vibration absorption peak of Si-OH, 1060 cm −1 and 776 cm −1 are the antisymmetric stretching vibration peak of Si-O-Si and the symmetrical stretching vibration peak of Si-O, respectively. The bending vibration peak of Si-O is located at 467 cm −1 . These spectral bands are the functional groups of silica, and they indicate that the condensation and polymerization between TEOS and APTES, MPTES, and GPTES were successful. From the infrared spectra of the modified SiO 2 nanospheres, we observed that in addition to the vibration peak of Si-O in the structure, the vibration absorption peaks at 2929 cm −1 and 2881 cm −1 belonged to the antisymmetric and symmetric telescopic vibration of C-H in -CH 3 and -CH 2 , respectively. The characteristic peaks of the three different types of silane coupling agents also appeared. For example, the characteristic absorption peaks of -NH 2 [25], -SH, and epoxy groups occur at 1504, 2553, and 913 cm −1 , respectively. These results showed that different functional groups had been successfully grafted onto silica nano-fragrance using APTES, MPTES, and GPTES. The characteristic absorption peak of rose fragrance was observed near 1753 cm −1 , indicating that rose fragrance was encapsulated in nanospheres with modified silica as a shell. From Figure 3b, the XRD of nanospheres showed a wide and dispersive peak at 2θ ≈ 22.5 • [26], indicating that the modified SiO 2 nanospheres existed in amorphous form.
nanospheres; the insets in the SEM images are the particle size distributions of SiO2 nanospheres modified by different functional groups.

Fourier Infrared Spectrum and XRD Study of Modified SiO2 Nanospheres
The Fourier transform infrared (FTIR) results of the modified SiO2 nanospheres are shown in Figure 3a. We analyzed the infrared spectra of the SiO2 nanospheres that were modified by different coupling agents and rose fragrance. The wavenumber near 3400 cm −1 is the stretching vibration absorption peak of Si-OH, 1060 cm −1 and 776 cm −1 are the antisymmetric stretching vibration peak of Si-O-Si and the symmetrical stretching vibration peak of Si-O, respectively. The bending vibration peak of Si-O is located at 467 cm −1 . These spectral bands are the functional groups of silica, and they indicate that the condensation and polymerization between TEOS and APTES, MPTES, and GPTES were successful. From the infrared spectra of the modified SiO2 nanospheres, we observed that in addition to the vibration peak of Si-O in the structure, the vibration absorption peaks at 2929 cm −1 and 2881 cm −1 belonged to the antisymmetric and symmetric telescopic vibration of C-H in -CH3 and -CH2, respectively. The characteristic peaks of the three different types of silane coupling agents also appeared. For example, the characteristic absorption peaks of -NH2 [25], -SH, and epoxy groups occur at 1504, 2553, and 913 cm −1 , respectively. These results showed that different functional groups had been successfully grafted onto silica nano-fragrance using APTES, MPTES, and GPTES. The characteristic absorption peak of rose fragrance was observed near 1753 cm −1 , indicating that rose fragrance was encapsulated in nanospheres with modified silica as a shell. From Figure 3b, the XRD of nanospheres showed a wide and dispersive peak at 2θ ≈ 22.5° [26], indicating that the modified SiO2 nanospheres existed in amorphous form.  Figure 4a shows that the untreated leather had many collagen fibers with a smooth surface and uniform thickness. These observations are similar to those reported in the literature [27]. Many cracks were observed between the leather collagen fibers [28]. The leather treated with the modified SiO2 nanospheres exhibited good adhesion to the leather surface, as shown in Figure 4b-d. The modified SiO2 nanospheres can be absorbed into the collagen fibers. Compared with the leather treated with the other two modified SiO2 nanospheres, more nanospheres were observed on the sample surface treated with epoxy-group-modified nanospheres. This may be due to the epoxy group reacting with the active groups such as the amino group, carboxyl group, and amide group  Figure 4a shows that the untreated leather had many collagen fibers with a smooth surface and uniform thickness. These observations are similar to those reported in the literature [27]. Many cracks were observed between the leather collagen fibers [28]. The leather treated with the modified SiO 2 nanospheres exhibited good adhesion to the leather surface, as shown in Figure 4b-d. The modified SiO 2 nanospheres can be absorbed into the collagen fibers. Compared with the leather treated with the other two modified SiO 2 nanospheres, more nanospheres were observed on the sample surface treated with epoxy-group-modified nanospheres. This may be due to the epoxy group reacting with the active groups such as the amino group, carboxyl group, and amide group on the collagen fiber [29]. This interaction may form hydrogen bonds [30], van der Waals forces, and electrostatically interact to promote more modified SiO 2 nanospheres to bind to the collagen fiber.

Morphology Analysis of Modified SiO 2 Nanospheres Leather
on the collagen fiber [29]. This interaction may form hydrogen bonds [30], van der Waals forces, and electrostatically interact to promote more modified SiO2 nanospheres to bind to the collagen fiber.

Thermogravimetric Analysis and Sensory Evaluation
The thermogravimetric (TG) analysis of modified SiO2 nanospheres is shown in Figure 5. The weight of modified SiO2 nanospheres decreased with increasing temperature. The thermal degradation process of modified SiO2 nanospheres can be divided into three steps. In the first stage, the thermal decomposition of 30-200 °C was due to the weight loss of water and fragrance. In the second stage, the weight loss of 200-350 °C can be explained by the rapid release of the fragrance, which was the core material, and the decomposition of the wall material. In the third stage, the weight loss of 350-600 °C is the decomposition of the wall material. The TG curve also showed that 50% of the modified SiO2 nanospheres still had mass residues even at 600 °C, which proved that the organically modified silica had good thermal stability as a wall material.
The thermogravimetric analysis curve showed that the weight loss rate of blank leather without adsorbed modified SiO2 nanospheres was 67.45% after heating to 600 °C. The weight loss rates of leather adsorption of the APTES, MPTES, GPTES modified SiO2 nanospheres were 64.58%, 65.59%, and 64.63% at 30-600 °C, respectively. This was due to the residue of modified SiO2 nanospheres, which confirmed that leather adsorbed modified SiO2 nanospheres.
The dried weights of the leather before and after soaking were accurately weighed using

Thermogravimetric Analysis and Sensory Evaluation
The thermogravimetric (TG) analysis of modified SiO 2 nanospheres is shown in Figure 5. The weight of modified SiO 2 nanospheres decreased with increasing temperature. The thermal degradation process of modified SiO 2 nanospheres can be divided into three steps. In the first stage, the thermal decomposition of 30-200 • C was due to the weight loss of water and fragrance. In the second stage, the weight loss of 200-350 • C can be explained by the rapid release of the fragrance, which was the core material, and the decomposition of the wall material. In the third stage, the weight loss of 350-600 • C is the decomposition of the wall material. The TG curve also showed that 50% of the modified SiO 2 nanospheres still had mass residues even at 600 • C, which proved that the organically modified silica had good thermal stability as a wall material. statistical calculation, the leather treatment with SiO2 nanospheres modified by GPTES received the highest score. This may be due to the leather having the strongest ability to adsorb GPTES-modified SiO2 nanospheres.   The thermogravimetric analysis curve showed that the weight loss rate of blank leather without adsorbed modified SiO 2 nanospheres was 67.45% after heating to 600 • C. The weight loss rates of leather adsorption of the APTES, MPTES, GPTES modified SiO 2 nanospheres were 64.58%, 65.59%, and 64.63% at 30-600 • C, respectively. This was due to the residue of modified SiO 2 nanospheres, which confirmed that leather adsorbed modified SiO 2 nanospheres.
The dried weights of the leather before and after soaking were accurately weighed using electronic scales, and all the dried weights of leather increased. Among the leather with increased weight, that which was soaked in GPTES-modified SiO 2 nanospheres dispersion solution increased the most. The capacity of leather to adsorb modified SiO 2 nanospheres were calculated from the thermogravimetric curve, and the results are shown in Table 2. The adsorption capacity calculation showed that the modified SiO 2 nanospheres were adsorbed on leather in the order of GPTES, APTES, and MPTES. The weight gain and the adsorption capacity together indicate that the capacity of leather adsorb GPTES-modified SiO 2 nanospheres was the strongest, and the ability to adsorb MPTES-modified SiO 2 nanospheres was relatively weak. This may be due to the different functional groups on the surface of SiO 2 nanospheres. These functional groups had different reactive activities, and they can form hydrogen bonds or other intermolecular interactions with amino groups, carboxyl groups, and other functional groups on leather collagen fibers. Due to the interaction, the modified SiO 2 nanospheres can be adsorbed on leather without a cross-linking agent. In addition, the epoxy group of SiO 2 nanospheres modified by GPTES had relatively strong activity, so more easily reacted with the active functional groups on collagen fibers to form strong interactions between the modified SiO 2 nanospheres and the leather. This strong interaction may promote the leather adsorbing more GPTES-modified SiO 2 nanospheres. We invited 10 evaluators to evaluate the aroma quality of leather soaked in SiO 2 nanospheres that were modified with different coupling agents. The results are shown in Table 3. Through statistical calculation, the leather treatment with SiO 2 nanospheres modified by GPTES received the highest score. This may be due to the leather having the strongest ability to adsorb GPTES-modified SiO 2 nanospheres. Table 3. Sensory evaluation of leather absorbing the modified SiO 2 nanospheres.

Adsorption Isotherm
The Beer-Lambert standard curve of modified SiO 2 nanospheres was determined using the UV-vis absorbance method as shown in Figure 6a. The coefficient of determination (R 2 ) > 0.99 of the standard curve showed that the standard curve had an excellent linear relationship. Figure 6d shows that the Dubinin-Radushkevich isotherm adsorption model was suitable for application to leather adsorption of modified SiO 2 nanospheres. The R 2 > 0.99 indicated that the fitting curve had good linearity. The Langmuir [2,24,31] and Freundlich [2] isotherm adsorption models of leather adsorption modified SiO 2 nanospheres are provided in Figure 6b,c. The R 2 was less than 0.99, and the fitting curve did not have a good linear relationship. The results show that the leather adsorption modified SiO 2 nanospheres do not conform to the Langmuir and Freundlich isotherm adsorption models. The results of K D , which were obtained from the adsorption isotherms, are shown in Table 4. The average adhesion energies of the SiO 2 nanospheres with modified by APTES, MPTES, and GPTES are 1.34016, 0.97289, and 2.09326 kJ/mol, respectively. Since the values average adhesion energy E were all less than 8 kJ/mol, the adsorption process is physical [32]. In other words, hydrogen bonds or van der Waals forces between modified SiO 2 nanospheres and leather collagen fibers may play roles to make the nanospheres adsorbed on the surface of the leather. By comparing the average adhesion energies E, we found that the average adhesion energy of leather that adsorbed the GPTES-modified SiO 2 nanospheres was the largest. The second was the APTES-modified SiO 2 nanospheres that were adsorbed on the leather, and the average adhesion energy of the MPTES-modified SiO 2 nanospheres was the lowest. The result showed that leather had a strong adsorption capacity for GPTES-modified SiO 2 nanospheres. This result is consistent with the leather weight gain of modified SiO 2 nanospheres and the capacity of leather to adsorb modified SiO 2 nanospheres.

Conclusion
In this study, to investigate the interaction between modified SiO2 nanospheres and leather, three kinds of modified SiO2 nanospheres loaded with rose fragrance were prepared with the sol-gel method using APTES, MPTES, or GPTES. The weight gain, adsorption capacity, and average adhesion energy calculation showed that modified SiO2 nanospheres can be physically adsorbed on leather in the order of GPTES, APTES, MPTES. The sensory evaluation confirmed that GPTESmodified SiO2 nanospheres endowed the leather with an obvious rose aroma. These results indicate that the surface functional groups play roles in the adhesive capacity of SiO2 nanospheres on the leather surface. Among the three tested active groups, the epoxy group endowed SiO2 nanospheres with the best adhesive capacity. This paper not only outlined a promising method for fragrance adhesion to leather surfaces without cross-linking agents but also provided insight into the interaction between nanospheres and bio-based materials.
Author Contributions: All the authors contributed equally to this work.

Conclusions
In this study, to investigate the interaction between modified SiO 2 nanospheres and leather, three kinds of modified SiO 2 nanospheres loaded with rose fragrance were prepared with the sol-gel method using APTES, MPTES, or GPTES. The weight gain, adsorption capacity, and average adhesion energy calculation showed that modified SiO 2 nanospheres can be physically adsorbed on leather in the order of GPTES, APTES, MPTES. The sensory evaluation confirmed that GPTES-modified SiO 2 nanospheres endowed the leather with an obvious rose aroma. These results indicate that the surface functional groups play roles in the adhesive capacity of SiO 2 nanospheres on the leather surface. Among the three tested active groups, the epoxy group endowed SiO 2 nanospheres with the best adhesive capacity. This paper not only outlined a promising method for fragrance adhesion to leather surfaces without cross-linking agents but also provided insight into the interaction between nanospheres and bio-based materials.
Author Contributions: Z.X. and X.K. conceived and designed the experiments; S.W. performed the experiments; S.W. and X.K. analyzed the data; Y.N. and G.Z. contributed reagents/materials/analysis tools; S.W. and X.K. wrote the paper.