Applications of AuNRs@SiO2 Matrix Nanocomposites

Gold nanorods (AuNRs/GNR) have unique, controllable and anisotropic local surface plasmon resonance (SPR) properties, which have been widely used in biochemistry, electronics and catalysis. It is of great value and significance to study the synthesis, properties, surface modification and structural regulation of AuNRs. This paper introduces AuNRs and AuNRs@SiO2. The synthesis of AuNRs@SiO2 composite nanomaterials with quantum dots, graphene, rare earth materials and magnetic materials is described. The applications of nanocomposites in optics, biomedicine and biosensors are also discussed. The future development of nanocomposites is proposed.


Introduction
This, in recent years, nanotechnology has rapidly developed due to its unique characteristics and range of applications. Nanoparticles (NPs) typically smaller than 100 nanometers in size have been widely used in various products and fields, including environmental, energy, consumer goods, medicine, and life science applications. Gold nanoparticles are stable and have been widely used in biomedical fields for drug delivery [1], biosensors and imaging [2], and cancer diagnosis and treatment [3].

Synthetic Method
The definition of seed growth method was proposed by Jana et al. [17]. Wesner and Wokaum found that when there were small gold nuclei in the solution in 1989, adding hydrogen peroxide and other weak reducing agents into the solution could transform HAuCl 4 into gold, and slowly deposit gold nanorods. The gold nanoparticles can be prepared by changing the size of gold nanoparticles and the ratio of gold ions to gold nanorods. Under the constraint of template, the AuNRs with corresponding aspect ratio are grown directionally [18]. Since then, according to this principle, researchers have done a lot of work to improve and perfect the seed growth method. At present, there are three kinds of seed growth methods: Silver ion-mediated "Seedless" growth method, silver ion-mediated seed growth method, and three-step seed growth method. The seed growth method has low requirements on equipment, mild reaction and can expand production. It is the most successful method for preparing gold nanorods at present. The specific flow chart is shown in Figure 1 [19], the seed solution was prepared first, and then the aspect ratio of AuNRs was controlled by controlling Ag + content. Most preparation methods in research are based on this method [20] [21] [22] [23].
In addition to the above methods for preparing and synthesizing GNRs, there is also electron beam lithography. However, among the many methods to prepare AuNRs, the seed growth method can successfully prepare GNRs with an axial diameter ratio of 1.5 -20, and obtain polarization peaks of different absorption wavelengths, moreover, the seed method has the advantages of low requirements for reaction conditions and equipment, simple preparation steps, high yield and easy regulation of aspect ratio.

Property of AuNRs
Among the numerous nanomaterials, precious metal nanomaterials, as a new type of research field, have been widely concerned by researchers in the past decade. This is due to the special physical and chemical properties of noble metal Figure 1. Process diagram of preparing AuNRs by seed growth method [19].  [28]. Noble metal nanomaterials have unique local surface plasmon resonance (SPR) peaks and surface enhanced Raman scattering (SERS) as two important properties of gold nanomaterials, these characteristics can be controlled by controlling the size and shape of metal nanostructures [29] [30]. Among different gold nanostructures, gold nanorods are the most widely used, mainly because they have two resonance peaks, LSPR peak and TSPR peak [31] [32], which correspond to the longitudinal and transverse SPR respectively. The position of LSPR is very sensitive to the aspect ratio of gold nanorods. According to the needs, we can change the aspect ratio (AR) of gold nanorods, The SPRL peak is modulated in a large range near the nearinfrared region [33], while the position of TSPR is not affected by the aspect ratio of gold nanorods, and is still near the wavelength range of visible light [34].
Moreover, the SERS activity of metal nanoparticles is obviously related to the morphology, size and size of the particles. The larger the size of the nanoparticles, the higher the SERS activity [35]. For gold nanorods, the closer the longitudinal vibration peak is to the laser excitation wavelength, the better the enhancement effect is.

Preparation of AuNRs@SiO2
Silicon dioxide as a shell to coat gold nanorods is the mainstream of current application. Because it is the most chemically inert material and is optically transparent, it has the characteristics of adjustable size, easy surface structure modification, mature coating process, etc., and has attracted wide attention in biomedicine, sensing, energy storage and other aspects [36] [37] [38]. The preparation method of silica coated gold nanorods is based on the traditional Stöber method. The structure of Au NRs coated with silica has better controllability and better colloidal stability. Gorelikov et al. [39] strongly located CTAB molecules on single nanoparticles were used as templates for the three-dimensional polymerization of tetraethyl orthosilicate (TEOS), the precursor of alkoxy silicon, so as to obtain high yield mesoporous silica coating on nanoparticles, and developed a widely used direct coating method. Chiang et al. [40] studied the effects of single factors on the particle size, pore structure and morphology of monodisperse silica nanospheres. Liu et al. [41] used the acoustic assisted synthesis method to directly coat PVP activated AuNRs onto silica shell, which was more simple and efficient. Meanwhile, ultrasonic promoted the uniform silicification reaction, which made the silicon shell compact and the coating yield was high. Meng et al. [42] prepared gold nanorods by seed growth method, and hydrolyzed tetraethyl orthosilicate in alkaline solution to obtain nano gold structure coated with silica. By coating silicon dioxide on gold nanorods and adjusting the aspect ratio of gold nanorods with different silver ion concentrations in the growth solution, the gold nanoparticles that need to be absorbed in specific UV region can be synthesized. The specific flow chart is shown in Figure 2. World Journal of Engineering and Technology

Property of AuNRs@SiO2
The special surface plasmon resonance (SPR) spectra of gold nanorods make their optical applications more and more extensive. Due to the large surface energy of the metal nanorods, the dispersion system formed is very unstable and easy to aggregate. However, the stability of the nanoparticles is significantly improved after the surface of gold nanorods is wrapped with silicon dioxide, forming a silica shell. Meanwhile, the existence of silica shell does not change the surface plasmon resonance properties of gold nanorods. Therefore, the preparation of core-shell structure of gold nanoparticles and silica, and the study of plasma resonance optical properties related to this core-shell structure, and because of the loading characteristics of this structure, it is not only operable, but also of great significance.
The dimensions of QDs in three dimensions are no more than twice the Bohr radius of excitons in the corresponding semiconductor material. QDs are generally spherical or quasi-spherical, with a diameter of 2 -20 nm with applications in both life sciences [43] and energy storage [44]. Zhou et al. [45] synthesized gold nanorods@silica CdTe QD hybrid nanostructures and studied the effect of gold nanorods on the fluorescence of CdTe QDs. The quenching effect was reduced through the coating of silicon dioxide onto the surface of the gold nanorods. Han et al. [46] synthesized a novel hierarchical silica coating AuNR@CdSeTe core/shell structures of QDs. Gold nanoparticles were mediated by an initial mesoporous silica layer, covered with a dense silica layer, and then directly as-

AuNRs@SiO2@Graphene
Reduced graphene oxide (rGO) is a derivative of graphene with a 2D carbon honeycomb lattice. As a rising star in material science, rGO has potential applications in energy, catalysis, electronics and molecular sensing. Due to their high biocompatibility, low cytotoxicity and high thermal conductivity, RgO-biomolecular conjugates and RgO-nanoparticle hybrids now offer new directions for electrode materials [47], drug delivery [48] and sensing detection [49]. Zhang et al. [50] prepared near-infrared responsive core-shell gold nanorods/mesoporous silica/reduced graphene oxide NPs through electrostatic interactions, which synergistically enhanced both the photo thermal stability and transition effect. These novel near-infrared responsive core-shell hybrid NPs with enhanced photo thermal stability and conversion effects were suitable for photo thermal therapy, bioimaging and drug delivery. Qi et al. [51] prepared near-infrared (NIR) responsive core-shell gold nanorods/mesoporous silica NPs through electrostatic and physical absorption, and coated them with PEGylated graphene oxide. Due to the introduction of biocompatible GO-PEG, the hybrid NPs showed unique photo thermal stability under physiological and acidic conditions. The performance of composite graphene AuNRs@SiO 2 has been improved through these methods. The synthetic route is shown in Figure 3 [51].
Through adjustment of the thickness of the SiO 2 shell and Au@SiO 2 concentration, plasma-enhanced upconversion (UC) and quantum cutting (QC) were obtained and optimized in these nanocomposites. When the thickness of SiO 2 shell was 35 nm, the enhancement factors of UC and QC increased by a maximum of 1.91-and 3.47-fold respectively. These nanocomposites can be applied to biofluorescent labeling, super-resolution imaging and solar cells.

Optical Field
Gold nanorods have attracted attention due to their unique surface plasmon resonance (SPR) and transverse (SPR-T) and longitudinal (SPR-L) collective oscillations of quasi-free electrons [58]. The GNRs are coupled together due to the  Luna et al. [67] discussed the photocatalytic performance of Au-TiO 2 /SiO 2 coating on two different building stones. During the experiment, a sol-gel method was used to synthesize Au-TiO 2 /SiO 2 photocatalysts for building materials with self-cleaning and decontamination properties. The Au-TiO 2 -NPs were integrated into the silica by spraying, so that they were sprayed in the building Material. The sol gelled spontaneously to produce a long-term Au-TiO 2 /SiO 2 coating with photocatalytic properties, the integration of TiO 2 in the silica matrix enhances its adhesion to the substrate and subsequent coating durability.
AuNPs can effectively improve the photocatalytic activity, self-cleaning and decontamination properties of coated building materials. Bai et al. [68] will be doped with SiO 2 @TiO 2 double shell (AuNR@SiO 2 @TiO 2 ) A novel composite photoanode was formed by mixing graphene and gold nanorods into dye-sensitized solar cells (DSSCs). The maximum short-circuit current density is 16.26 macm −2 and the power conversion efficiency is 8.08%, which is 37.7% and 32.9% higher than that of conventional DSSC. These significant improvements in short-circuit current density and power conversion efficiency are attributed to the synergistic effect of LSPR of graphene and AuNRs, as well as the TiO 2 shell of AST, which will also increase the specific surface area and dye adsorption to a certain extent.
The paper [69] [70] also carried out the corresponding work, and further explored AuNR@SiO 2 For the application in the field of photosensitive battery, Figure 6 is the schematic diagram of battery structure.

Biomedical Fields
Gold nanorods are useful in a range of biomedical applications, including cell imaging, drug or gene delivery, and cancer therapy, due to their strong and tunable plasmon resonance properties [71] [72] [73] [74]. Photothermal therapy is an emerging anti-tumor therapeutic that involves the artificial injection of photo thermal conversion materials into the tumor using targeted recognition technology. Light energy is then converted into heat energy under the irradiation of near-infrared light, generating local high heat in the tumor tissue, leading to cancer cell death. The ability to specifically target tumor cells is critical to the diagnosis and treatment of cancer [75]. Photothermal therapy uses a range of materials with high photo thermal conversion efficiency. The photo thermal effects of gold nanorods can increase the local temperature to 1000˚C in the nanosecond range, highlighting its potential application in tumor radiotherapy [76].
Zhou et al. [77] fabricated gold nanorods coated with mesoporous silica by  imaging-guided photo thermal therapeutic for biomedical applications. Xuan et al. [80] formed AuNS on the surface of mesoporous silica NPs (MSNs) using the seed-mediated growth method to which the NIR dye cyanine 7 (Cy7) was conju- Zhang et al. [81] have grown multifunctional nanocomposites with up to the third generation (G3) grafted polyamide type dendrimers on the surface of gold nanorods coated with mesoporous silica by divergent technology. The AuNRs@SiO 2 -G3 nanocomposites have uniform size and excellent stability, which can not only be used as a targeted contrast agent for photothermal cancer treatment, but also be used as a scaffold for the intracellular delivery of anticancer drugs and small interfering RNA (siRNA) to improve the efficiency of cancer treatment, Figure 9 is the preparation flow chart of the material. Ma et al. [82] prepared Au nanorods/mesoporous silica/hydroxyapatite (HAP) nanocarriers with thermal responsive polymer caps for remote drug delivery. In vitro drug delivery results showed that the hybrid nanoparticles exhibited excellent drug loading efficiency and unique near-infrared (NIR), heat and pH responsive drug delivery characteristics. The results of cell viability also showed that the P (NIPAM-co-AAC) capped AuNRs/SiO 2 /HAP nanoparticles had excellent biocompatibility.

Detection
Glycoproteins regulate cell growth, proliferation and signaling. Accordingly, the aberrant expression of glycoproteins leads to disease occurrence, with many Figure 9. Preparation of multifunctional AuNR@SiO 2 -G3 nanocomposites [81].
H. Q. Li et al. employed as disease biomarkers for clinical diagnosis. Due to the low detection efficiency of glycoproteins in biological samples, improving the selectivity and sensitivity of glycoprotein detection is an important requirement for clinical diagnosis and treatment. Molecular imprinting is an effective technique to simulate the molecular recognition of natural receptors. Molecularly imprinted polymers (MIPs) show high recognition ability and chemical stability, are easy to synthesize, and have been used for chromatographic separation, sensors, drug delivery and other fields. MIP based on the affinity of borate shows high specificity, affinity and anti-interference, and can be used as an alternative to natural antibodies for the detection of target substances in complex samples.
Sun [83] used in situ growth methods and immobilized AuNR layers on the surface of PAD, and introduced MPBA to the electrode surface through Au-S bonds to capture target glycoproteins and prepared borate affinity-based MIP and SiO 2 @Au/dsDNA/CeO 2 composites. The formation of hybrid chains and the large surface area of SiO 2 @Au enhanced the sensitivity of the platform permitting rapid and accurate glycoprotein detection. The schematic diagram of the detection method is shown in Figure 10. Gan [84] et al. showed that multilayered AuNR@Ag@SiO 2 doped into the MIP matrix of molecularly imprinted polymers can significantly accelerate electron transfer and enhance the electrochemical signals. AuNR@Ag@SiO 2 @MIP/GCEAs were prepared as a novel electrochemical sensing platform and theobromine THb was quantified with a detection limit of 8.0 nM at a range of 10 -100 nM, The flowchart is shown in Figure 11. Tribrominated biphenyls in food and environmental samples were extracted and determined. The recovery rates ranged from 92.20% to 107.1%, with a relative standard deviation of <4%. This sensor provides a reliable means for the detection of alkaloids in complex environments and lays the foundation for the development of reusable, sensitive and selective electrode materials.
Chen et al. [85] used AuNR@SiO 2 to detect temperature jumps through the photoexcitation of gold nanorods coated with SiO 2 using a confocal fluorescence thermometer and 1064 nm pulsed laser to accurately monitor calorific energy. A spatially and temporally resolved method was established to perform these experiments, permitting the study of protein dynamics as a whole without segmenting the target protein ( Figure 12).
With continuous economic developments, a large number of new organic substances continue to be artificially synthesized, including drugs, personal care products, and flame retardants. As traditional wastewater treatment and drinking water treatment processes are generally unable to effectively remove such pollutants, these types of organic matter usually exist in both natural and tap water at trace concentrations, collectively termed organic micro pollutants.
Wei et al. [86] designed double-sided gold nanorod/silica composite structure (AuJNRs) in which the presence of silica guaranteed the colloidal stability of the gold nanorods. The uncovered side of the gold nanorods provided an active site Figure 11. Schematic illustration of the preparation procedure of AuNR@Ag@SiO 2 @MIP [84].  for the catalysis of the advanced oxidation reaction of persulfate. The experimental results showed that using local surface plasmon resonance for energy conversion, photons in the visible and near infrared bands of sunlight were converted into heat and hot electrons, driving the degradation of organic micro pollutants through the advanced oxidation of persulfate. This not only provides an ideal platform for studying plasmon-based photochemistry in aqueous phases, but also creates new opportunities for the development of solar-based water treatment technologies. Zhang et al. [87] combined aunr and Rhodamine B (RB) as the core and silica as the shell to construct a novel core-shell hybrid nanostructure, Poly(sodium 4-phenylenesulfonate) (PSS) uses a negatively charged polyelectrolyte to change the surface charge of AuNRs and acts as a linker to capture RB on AuNRs in an electrostatic manner. Due to the overlapping of the maximum fluorescence emission peak of Rb at 580 nm and the fluorescence spectra of AuNRs at 560 nm, the strong green fluorescence of compound nanostructures excited at 488 nm was observed. Therefore, the red fluorescence and enhanced green fluorescence of hybrid nanostructures were obviously observed, so as to explore the potential application of hybrid nanostructures to replace traditional fluorescent dyes in cell two-color fluorescence labeling.
Cao et al. [88] proposed a new platform based on go Cys AuNRs hybrid nanostructure system to absorb R6G dye molecules for SERS detection. Go Cys AuNRs composites can be easily prepared by combining go sheet with AuNRs by using Cys molecules as functional bonds. Raman measurement shows that the signal intensity of R6G molecule is very high and the detection limit is as low as 5 × 10 −6 M. Luo et al. [89] proposed an plasmon near-infrared photodetector for detecting 980 nm illumination by coating plasma on SLG/InP Schottky junction diode AuNR@SiO 2 The specific flow chart is shown in Figure 13. In use AuNR@SiO 2 After surface modification, some key indicators including responsivity, detection rate and response time were significantly improved. In addition, the linear dynamic range and noise equivalent power are much better than other devices with similar geometry.

Conclusion
This article reviews the related development history, preparation methods and properties of AuNRs and AuNRs@SiO 2 , and elaborates on the preparation processes of AuNRs@SiO 2 and the composite forms of other materials, discussing their application in the fields of optics, medicine and detection. The interest in Au nanocomposites continues to rise as does their application in biomedical fields. However, many challenges remain, including the control of the content of the reagent added during large-scale synthesis which dictates the morphology and properties of the final product. Safety remains the top priority in biomedical applications with further studies on the toxicity of AuNRs@SiO 2 now required.
We also need to further try in the AuNRs@SiO 2 The substrate is modified by doping other materials such as nanotubes to broaden the field of vision, so as to explore more applications.