Detailed investigation of factors affecting the synthesis of SiO2@Au for the enhancement of Raman spectroscopy

Citation: Thao, N.T.P.; Ton-That, L.; Dang, C.-T.; Nedoma, J. Detailed Investigation of Factors Affecting the Synthesis of SiO2@Au for the Enhancement of Raman Spectroscopy. Nanomaterials 2022,12, 3080. https:// doi.org/10.3390/nano12173080 Academic Editor: Maurizio Muniz-Miranda Received: 4 August 2022 Accepted: 1 September 2022 Published: 5 September 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Article Detailed Investigation of Factors Affecting the Synthesis of SiO2@Au for the Enhancement of Raman Spectroscopy Nguyen Thi Phuong Thao 1, Loc Ton-That 2,3, Cong-Thuan Dang 2,3 and Jan Nedoma 1,* 1Department of Telecommunications, VSB Technical University of Ostrava, 708 00 Ostrava, Czech Republic 2 Future Materials & Devices Laboratory, Institute of Fundamental and Applied Sciences, Duy Tan University, Ho Chi Minh City 700000, Vietnam 3Faculty of Natural Sciences, Duy Tan University, Da Nang City 550000, Vietnam *Correspondence: [email protected] Abstract: The reaction time, temperature, ratio of precursors, and concentration of sodium citrate are known as the main factors that affect the direct synthesis process of SiO 2 @Au based on the chemical reaction of HAuCl4 and sodium citrate. Hence, we investigated, in detail, and observed that these factors played a crucial role in determining the shape and size of synthesized nanoparticles. The significant enhancement of the SERS signal corresponding to the fabrication conditions is an existing challenge. Our study results show that the optimal reaction conditions for the fabrication of SiO 2 @Au are a 1:21 ratio of HAuCl 4 to sodium citrate, with an initial concentration of sodium citrate of 4.2 mM, and a reaction time lasting longer than 6 h at a temperature of 80 ◦ C. Under optimal conditions, our synthesis process result is SiO 2 @Au nanoparticles with a diameter of approximately 350 nm. In particular, the considerable enhancement of Raman intensities of SiO 2 @Au compared to SiO 2 particles was examined. Keywords: SiO2@Au; core shell; Raman spectroscopy; SERS 1. Introduction Recently, the surface plasmon resonance properties of metal nanostructures have attracted significant attention from researchers because of their potential applications in various fields, e.g., materials science [ 1 ], biology [ 2 ], chemistry [ 3 ], and environmental analysis [ 4 ]. The plasmonic properties of metal nanoparticles significantly depend on the shape, size, and nature of the nanostructured materials employed [ 5 , 6 ]. Among applications based on the surface plasmon phenomena, surface-enhanced Raman scattering (SERS) has emerged as a strong analytical technique that considerably contributes to the enormous improvement of SERS-based sensing techniques [ 7 , 8 ]. SERS is a novel optical technique that combines nanotechnology with Raman scattering spectroscopy to produce an analytical tool with high sensitivity. It is based on the excitation of surface plasmon resonance to amplify the Raman signal on the metal surface [ 9 ]. Modern SERS techniques have been widely developed in recent years and opened a new step in many scientific fields, such as the environment, food, and biomedicine [ 10 , 11 ]. The SERS sensing technique has become a powerful analytical tool for researchers in the nanotechnology field to identify the component composition of substances and provide information about their molecular structure at low concentrations without destroying them [ 12 – 14 ]. In this technique, the SERS substrate plays a crucial role in determining the correction and effectiveness of the analytical tool. Therefore, developing new materials and optimal nanostructures is necessary to produce stable and efficient substrates for SERS. The nanostructures of noble metals, such as Au [ 15 , 16 ], Ag [ 17 , 18 ], and Cu [ 19 , 20 ], have been studied as excellent plasmonic materials and widely employed in plasmonic applications. Such nobble metals exhibit surface plasmonic resonances in the visible and NIR region [21]. Nanomaterials 2022,12, 3080. https://doi.org/10.3390/nano12173080 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2022,12, 3080 2 of 16 The core-shell nanostructures have been discovered as efficient SERS substrates to amplify the Raman signal in which the cores are SiO 2 spherical particles, and their shell components are composed of noble metal nanoparticles. The position of the surface plasmon band of core-shell nanostructures can be tuned according to the size, shape, and surface morphology [ 22 , 23 ]. Among these substrates, SiO 2 @Au nanoparticles have been extensively studied in many chemical and biomedical research fields over the past year due to their unique optoelectronic and physicochemical properties [24,25]. However, synthesizing a complete metal shell with uniform and high coverage around the silica core is challenging. Therefore, a series of strategies for the synthesis of SiO 2 @Au nanoparticles on SiO 2 have been developed, such as gold electroless plating [ 26 ], the selfassembled monolayer method [ 27 ], the layer-by-layer cross-linking method [ 28 ], controlled seed growth [ 29 ], the ultrasound-assisted Stober method [ 30 ], and the sol-gel method [ 31 ]. The most common method for synthesizing SiO 2 @Au core-shell structure nanoparticles is through a two-step process that involves decorating a functionalized SiO 2 surface with small gold seeds and the growth of a gold shell. Additionally, some studies have produced SiO 2 @Au nanoparticles through the direct growth of gold shells on SiO 2 spheres without gold seeds. Michael et al. proposed coating a thin and uniform gold layer on bare silica nanoparticles by reducing a gold (I) chloride solution dissolved in acetonitrile with ascorbic acid [ 32 ]. Zhang et al. reported a facile method with a one-pot, one-step process for preparation SiO 2 @Au by heating a solution containing chloroauric acid (HAuCl 4 ), 2-methylaminoethanol (2-MAE), CTAB, and TEOS at 80◦[33]. In some approaches related to the deposition of gold seeds on SiO 2 spheres, the functionalization of nanoparticles is essential to increase the absorption of Au seeds on the SiO 2 surface and maintain the stability of the nanostructure during the synthesis process. The coverage level of the gold nanoparticles on the silica core is affected by the strength of attraction between the gold particles and the core, as well as by the force balance between the particles in the shell region [ 34 , 35 ]. Therefore, the surface of SiO 2 spheres is commonly functionalized by organic agents with high adhesion with gold, such as (3-aminopropyl) trimethoxysilane (APTMS), polyethyleneimine (PEI), and 3-mercaptopropyltrimethoxysilane (MPTMS) [36,37]. An approach involving an amine-terminated coupling agent, which acts as an adhesive agent to attach gold nucleation to silica nanoparticles, was first proposed by Halas et al. [ 38 ]. The controlled shell growth method involving small gold seeds (diameters of 1 to 2 nm) acting as nucleation center on the surface of SiO 2 spheres was proposed by Yukhymchuk et al. In this study, SiO 2 spheres with a diameter of 160 to 210 nm were functionalized with positively charged amino groups before decoration with gold nanoparticles. A complete gold shell was formed by reducing gold salt with reducing agents of hydroxyl-amine muriatic NH 2 OH.HCl and sodium borohydride NaBH 4 [ 39 ]. Moreover, Kandpal et al. synthesized SiO 2 @Au nanoparticles with a multistep process involving the combination of the citrate and borohydride methods. The authors started with the citrate method, reducing the HAuCl 4 gold salt with trisodium citrate Na 3 (C 6 H 5 O 7 ) at 80 for 30 min for deposition of gold nucleation on functionalized silica spheres with 3-aminopropyltriethoxysilane (APS). Then, the borohydride method was applied to develop gold nucleation for formation a gold shell, which reduces HAuCl 4 /K 2 CO 3 solution with sodium borohydride (NaBH 4 ). The obtained results demonstrate that the combination of the two methods resulted in the significant enhancement of the SERS signal compared to each of the methods alone [ 40 ]. The technique of grafting Au nanoparticles on trimethoxysilane (APTMS)-functionalized SiO 2 spheres and then priming these nanoparticles with gold colloids of 2 nm and 10 nm before chemically reducing the gold salt with NaBH 4 (5.3 mM) was investigated by Saini et al. The thickness of the gold shell is controlled by the ratio between the precursor particles and the gold salt solution. This method was proven to improv the SERS signal with a small amount of gold [ 41 ]. Wang et al. reported a fabrication process of SiO 2 @Au nanoparticles using the isotropic growth method of gold seeds (3–5 nm) on PEI-functionalized SiO 2 spheres (diameter of 190 nm) by reducing gold salt with hydroxylamine hydrochloride Nanomaterials 2022,12, 3080 3 of 16 (NH2OH.HCl) under ultrasound assistance instead of traditional methods. This approach successfully reduced the fabrication time and the time for completing the gold shell within 5 min. Moreover, the attained SiO 2 @Au nanoparticles were highly homogeneous and welldefined in size and shape [ 42 ]. Khurana et al. proposed synthesizing freckled SiO 2 @Au nanocomposites (NCs) by gold-seed-mediated growth on MPTMS-functionalized silica spheres. Freckled SiO 2 @Au NCs with SiO 2 core sizes of 880 nm and 430 nm and a shell thickness in the range of 12–50 nm exhibited high surface consistency of the shell and excellent enhancement of the SERS signal [ 14 ]. Table 1presents a review of the methods and related conditions for the fabrication of SiO 2 @Au. Although reports have presented the preparation process of Au@SiO 2 using different methods, as well as the strong points of each method, no reports have been published showing factors affecting the synthesis of SiO2@Au nanomaterial in detail. Table 1. A review of the methods and related conditions for the fabrication of SiO2@Au. Method Reducing Agents Functionalization Agents Reducing Reaction Temperature Reducing Reaction Time Ratio of Precures Concentration of Reducing Agent Ref Electroless plating NH2O. HCl APTMS - 14 min - 0.4 mM [26] Self-assembly CH2O APTMS - - - 0.36 mM [27] Layer by layer NaBH4MPTMS - 6 h - - [28] Seed-mediated growth CH2O APTES - 10 min - - [29] Ultrasound-assisted Stober method NaBH420 ◦C 2 h - 10 mM [30] Direct growth of gold shell Ascorbic acid - - - - 3 mM [32] Direct growth of gold shell 2-methylaminoethanol - 80 ◦C. 30 min - - [33] Seed-mediated growth NH2OH·HCl APTES - - 8:1 (Na3Cit/HAuCl4)40 mM [37] Seed-mediated growth NH2OH·HCl + NaBH4 APES - - - - [39] Seed-mediated growth Na3(C6H5O7) + NaBH4APS - 60 min 0.002 M:0.01 M (HAuCl4/ NaBH4) 0.01 M [40] Seed-mediated growth with grafting, priming technique NaBH4APTMS - 12 h - 5.3 mM [41] Ultrasound-assisted seed growth NH2O. HCl PEI - 5 min - - [42] Seed-mediated growth for synthesis of freckled SiO2@Au NCs NaBH4MPTMS - 10 min 8:1 (K-gold/SiO2)5.3 mM [43] In this study, we present a deep view of factors that affect the synthesis of SiO 2 @Au material, including temperature, reaction time, the ratio of precursors, and the concentration of sodium citrate. The characteristics of the obtained SiO 2 @Au nanoparticles will be investigated using UV-Vis spectra, SEM, and DLS. The Raman spectroscopy of SiO 2 spheres before and after coating Au nanoparticles will also be examined. 2. Materials and Methods 2.1. Materials Non-functionalized Silica microspheres approximately 0.27 µ m in the dry form with 100% solids were purchased from (Polyscience Asia Pacific Inc., Taipei, Taiwan). The surface group of non-functionalized SiO 2 is normally silanol (SiOH), and the refractive index is around 1.43–1.46 (589 nm). Gold (III) chloride solution (HAuCl 4 , 99.9%), polyethylenimine (-[-CH 2 -CH 2 -NH-]-), and sodium citrate dihydrate (HOC(COONa)(CH 2 COONa) 2 2H 2 O) were purchased from of Sigma-Aldrich, St. Louis, MO, USA commercial suppliers. The gold (III) chloride solution Nanomaterials 2022,12, 3080 4 of 16 has a composition Au of 17 wt.% and concentration of 30 wt.% in dilute HCl. Sodium citrate dihydrate is the trisodium salt of citric acid, which takes in citrate (3 − ). All chemicals were of analytical grade and used without further purification. 2.2. Preparation of SiO2@Au SiO 2 @Au material was synthesized via chemical reduction pursuant to Scheme 1. A mixture of 0.01-g SiO 2 spheres (approximately 270 nm in size) and PEI was dispersed in DI water under sonication for 20 min. Next, an HAuCl 4 solution was gradually dropped into the mixture. The obtained mixture was then shaken for 60 min before adding sodium citrate at the investigated temperature. Factors affecting the preparation were examined, such as temperature, reaction time, the ratio of precursors, and the concentration of sodium citrate. Nanomaterials 2022, 12, x FOR PEER REVIEW 4 of 17 Seed-mediated growth for synthesis of freckled SiO2@Au NCs NaBH 4 MPTMS - 10 min 8:1 (K-gold/ SiO 2 ) 5.3 mM [43] 2. Materials and Methods 2.1. Materials Non-functionalized Silica microspheres approximately 0.27 µm in the dry form with 100% solids were purchased from (Polyscience Asia Pacific Inc., Taipei, Taiwan). The surface group of non-functionalized SiO 2 is normally silanol (SiOH), and the refractive index is around 1.43–1.46 (589 nm). Gold (III) chloride solution (HAuCl 4 , 99.9%), polyethylenimine (−[−CH 2 -CH 2 -NH-]-), and sodium citrate dihydrate (HOC(COONa)(CH 2 COONa) 2 2H 2 O) were purchased from of Sigma-Aldrich, St. Louis, MO, USA commercial suppliers. The gold (III) chloride solution has a composition Au of 17 wt.% and concentration of 30 wt.% in dilute HCl. Sodium citrate dihydrate is the trisodium salt of citric acid, which takes in citrate (3−). All chemicals were of analytical grade and used without further purification. 2.2. Preparation of SiO 2 @Au SiO 2 @Au material was synthesized via chemical reduction pursuant to Diagram 1. A mixture of 0.01-g SiO 2 spheres (approximately 270 nm in size) and PEI was dispersed in DI water under sonication for 20 min. Next, an HAuCl 4 solution was gradually dropped into the mixture. The obtained mixture was then shaken for 60 min before adding sodium citrate at the investigated temperature. Factors affecting the preparation were examined, such as temperature, reaction time, the ratio of precursors, and the concentration of sodium citrate. Diagram 1. Process of preparing SiO 2 @Au material. 2.3. Characterization The extinction spectra of the synthesized SiO 2 @Au nanoparticles were determined using a UV-Vis spectrometer V630 (V630, Jasco, Japan). This instrument operates with high speeds of up to 8000 nm/min and can measure wavelength ranges from 190 to 1100 nm. The images of surface morphologies of SiO 2 , SiO 2 @PEI, and SiO 2 @Au microspheres were observed and evaluated by a scanning electron microscope (S-4800 SEM, Hitachi, Japan) with an accelerating voltage range the electron beam in the range of 0.5 to 30kV. A DLS analytical instrument (Horiba, SZ-100Z2, Kyoto, Japan) was used to determine the size of SiO 2 and SiO 2 @Au by measuring the intensity of the dynamic light scattering of these particles. The Raman scattering intensity was measured on a Raman spectrometer (Horiba Ihr 550, Kyoto, Japan) using a laser source with an excitation light wavelength of 532 nm and methylene blue analyte. Scheme 1. Process of preparing SiO2@Au material. 2.3. Characterization The extinction spectra of the synthesized SiO 2 @Au nanoparticles were determined using a UV-Vis spectrometer V630 (V630, Jasco, Japan). This instrument operates with high speeds of up to 8000 nm/min and can measure wavelength ranges from 190 to 1100 nm. The images of surface morphologies of SiO 2 , SiO 2 @PEI, and SiO 2 @Au microspheres were observed and evaluated by a scanning electron microscope (S-4800 SEM, Hitachi, Japan) with an accelerating voltage range the electron beam in the range of 0.5 to 30kV. A DLS analytical instrument (Horiba, SZ-100Z2, Kyoto, Japan) was used to determine the size of SiO 2 and SiO 2 @Au by measuring the intensity of the dynamic light scattering of these particles. The Raman scattering intensity was measured on a Raman spectrometer (Horiba Ihr 550, Kyoto, Japan) using a laser source with an excitation light wavelength of 532 nm and methylene blue analyte. 3. Results 3.1. Factors Affecting the Synthesis of SiO2@Au 3.1.1. Effect of Temperature Figure 1shows pictures of samples at different temperatures and their UV-Vis spectra. As shown in Figure 1a, gold nanoparticles (AuNPs) are formed with a characteristic color range from light pink to dark purple, corresponding to the surface plasmon resonance (SPR) peaks between 520 and 535 nm shown in Figure 1b,c. When the temperature increases from 40 ◦ C to 80 ◦ C, the sample color changes from pale pink to red, and the maximum extinction wavelength ( λmax ) increases from 520 to 525 nm (Figure 1b). In this study, our goal was to synthesize AuNPs 40 nm in size related to the wavelength of 525 nm, so the chosen optimal temperature was 80 ◦C. Nanomaterials 2022,12, 3080 5 of 16 Nanomaterials 2022, 12, x FOR PEER REVIEW 5 of 17 3. Results 3.1. Factors Affecting the Synthesis of SiO2@Au 3.1.1. Effect of Temperature Figure 1 shows pictures of samples at different temperatures and their UV-Vis spectra. As shown in Figure 1a, gold nanoparticles (AuNPs) are formed with a characteristic color range from light pink to dark purple, corresponding to the surface plasmon resonance (SPR) peaks between 520 and 535 nm shown in Figure 1b,c. When the temperature increases from 40 °C to 80 °C, the sample color changes from pale pink to red, and the maximum extinction wavelength (λmax) increases from 520 to 525 nm (Figure 1b). In this study, our goal was to synthesize AuNPs 40 nm in size related to the wavelength of 525 nm, so the chosen optimal temperature was 80 °C. Nanomaterials 2022, 12, x FOR PEER REVIEW 6 of 17 Figure 1. Pictures of samples at different temperatures (a). UV-Vis spectra of samples at different temperatures (b) and their UV-Vis baseline spectra (c). 3.1.2. Effect of Reaction Time Figure 2 shows the formation of SiO2@Au at different reaction times from 3 to 7 h. Clearly, the color of the synthesized solution changes from light red to dark red, and the maximum extinction wavelength is within the range of 524 nm to 525 nm. Additionally, the intensity of peaks increases from 3 to 6h and remains stable after 6h. Figure 1. Pictures of samples at different temperatures ( a ). UV-Vis spectra of samples at different temperatures (b) and their UV-Vis baseline spectra (c). 3.1.2. Effect of Reaction Time Figure 2shows the formation of SiO 2 @Au at different reaction times from 3 to 7 h. Clearly, the color of the synthesized solution changes from light red to dark red, and the maximum extinction wavelength is within the range of 524 nm to 525 nm. Additionally, the intensity of peaks increases from 3 to 6 h and remains stable after 6 h. Nanomaterials 2022,12, 3080 6 of 16 Nanomaterials 2022, 12, x FOR PEER REVIEW 6 of 17 Figure 1. Pictures of samples at different temperatures (a). UV-Vis spectra of samples at different temperatures (b) and their UV-Vis baseline spectra (c). 3.1.2. Effect of Reaction Time Figure 2 shows the formation of SiO2@Au at different reaction times from 3 to 7 h. Clearly, the color of the synthesized solution changes from light red to dark red, and the maximum extinction wavelength is within the range of 524 nm to 525 nm. Additionally, the intensity of peaks increases from 3 to 6h and remains stable after 6h. Nanomaterials 2022, 12, x FOR PEER REVIEW 7 of 17 Figure 2. Pictures of samples at different reaction times (a). UV-Vis spectra of samples at different reaction times (b) and their UV-Vis baseline spectra (c). 3.1.3. Effect of the Ratio of HAuCl4 to Sodium Citrate The pictures and UV-Vis spectra of SiO2@Au shown in Figure 3 demonstrate that the color of SiO2@Au solution changes from pink to burgundy when the ratio of HAuCl4 to sodium citrate changes 1:104 to 1:15, corresponding to the maximum extinction wavelength within the range of 523–536 nm. Figure 2. Pictures of samples at different reaction times ( a ). UV-Vis spectra of samples at different reaction times (b) and their UV-Vis baseline spectra (c). 3.1.3. Effect of the Ratio of HAuCl4to Sodium Citrate The pictures and UV-Vis spectra of SiO 2 @Au shown in Figure 3demonstrate that the color of SiO 2 @Au solution changes from pink to burgundy when the ratio of HAuCl 4 to sodium citrate changes 1:104 to 1:15, corresponding to the maximum extinction wavelength within the range of 523–536 nm. Nanomaterials 2022,12, 3080 7 of 16 Nanomaterials 2022, 12, x FOR PEER REVIEW 7 of 17 Figure 2. Pictures of samples at different reaction times (a). UV-Vis spectra of samples at different reaction times (b) and their UV-Vis baseline spectra (c). 3.1.3. Effect of the Ratio of HAuCl4 to Sodium Citrate The pictures and UV-Vis spectra of SiO2@Au shown in Figure 3 demonstrate that the color of SiO2@Au solution changes from pink to burgundy when the ratio of HAuCl4 to sodium citrate changes 1:104 to 1:15, corresponding to the maximum extinction wavelength within the range of 523–536 nm. Nanomaterials 2022, 12, x FOR PEER REVIEW 8 of 17 Figure 3. Pictures of samples at different ratios of HAuCl4 to sodium citrate (a). UV-Vis spectra of samples at different ratios of HAuCl4 to sodium citrate (b) and their UV-Vis baseline spectra (c). 3.1.4. Effect of the Concentration of Sodium Citrate Sodium citrate was chosen as a reducing agent, so it directly affects the reaction rate and amount of synthesized AuNPs. As shown in Figure 4, when the concentration of sodium citrate is increased, the color of the obtained solution changes from light pink to dark red, and the intensity of the SPR peaks increases. Figure 3. Pictures of samples at different ratios of HAuCl 4 to sodium citrate ( a ). UV-Vis spectra of samples at different ratios of HAuCl4to sodium citrate (b) and their UV-Vis baseline spectra (c). 3.1.4. Effect of the Concentration of Sodium Citrate Sodium citrate was chosen as a reducing agent, so it directly affects the reaction rate and amount of synthesized AuNPs. As shown in Figure 4, when the concentration of Nanomaterials 2022,12, 3080 8 of 16 sodium citrate is increased, the color of the obtained solution changes from light pink to dark red, and the intensity of the SPR peaks increases. Nanomaterials 2022, 12, x FOR PEER REVIEW 9 of 17 Figure 4. Pictures of samples at different concentrations of sodium citrate (a). UV-Vis spectra of samples at different concentrations of sodium citrate (b) and their UV-Vis baseline spectra (c). N1: 0.21 mM; N2: 0.42 mM; N3: 0.84 mM; N4: 1.5 mM; N5: 4.2 mM. 3.2. Characterizations of Obtained SiO2@Au Based on the obtained results reported above, the optimal conditions for the synthesis of SiO2@Au are a ratio of HAuCl4 to sodium citrate of 1:21, an initial concentration of sodium citrate of 4.2 mM, and a temperature of 80 °C for 6h. Characteristics of SiO2@Au are determined using UV-Vis spectra, SEM images, and DLS. Figure 4. Pictures of samples at different concentrations of sodium citrate ( a ). UV-Vis spectra of samples at different concentrations of sodium citrate ( b ) and their UV-Vis baseline spectra ( c ). N1: 0.21 mM; N2: 0.42 mM; N3: 0.84 mM; N4: 1.5 mM; N5: 4.2 mM. Nanomaterials 2022,12, 3080 9 of 16 3.2. Characterizations of Obtained SiO2@Au Based on the obtained results reported above, the optimal conditions for the synthesis of SiO 2 @Au are a ratio of HAuCl 4 to sodium citrate of 1:21, an initial concentration of sodium citrate of 4.2 mM, and a temperature of 80 ◦ C for 6 h. Characteristics of SiO 2 @Au are determined using UV-Vis spectra, SEM images, and DLS. Figure 5presents pictures of SiO 2 , SiO 2 @PEI, AuNPs, and SiO 2 @Au synthesized under optimal conditions and their UV-Vis spectra. Notably, there is no SPR peak of SiO 2 and SiO 2 @PEI, whereas the maximum extinction wavelengths of AuNPs and SiO 2 @Au are at 529 nm and 525 nm, respectively. Nanomaterials 2022, 12, x FOR PEER REVIEW 10 of 17 Figure 5 presents pictures of SiO2, SiO2@PEI, AuNPs, and SiO2@Au synthesized under optimal conditions and their UV-Vis spectra. Notably, there is no SPR peak of SiO2 and SiO2@PEI, whereas the maximum extinction wavelengths of AuNPs and SiO2@Au are at 529 nm and 525 nm, respectively. 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