Structurally Engineered Silica Shells on Gold Nanorods for Biomedical Applications

Abstract

Owing to their unique optical and chemical properties, gold nanorods (AuNRs) are among the most frequently used nanomaterials for biomedical applications, including cancer therapy, imaging, and drug delivery. In particular, the longitudinal dipole plasmon wavelength of AuNRs can be verified from the visible to the near‐infrared (NIR) region, allowing AuNRs to be used as photodynamic/photothermal and imaging contrast agents. At the same time, the silica shell is important as it enhances stability and facilitates the functionalization and biocompatibility of AuNRs, offering numerous advantages in biomedical applications. In this review, silica‐coated AuNRs from a bioapplication perspective are focused. First, the importance of AuNRs for biomedical applications is explained and the purpose of silica coating on AuNRs is discussed. Then, recent studies on the development of silica‐coated AuNRs from a biomedical perspective are reviewed. Subsequently, various strategies for engineering silica coatings and their properties in the biomedical field are reviewed. This review is expected to promote further research on next‐generation silica‐coated AuNRs for biomedical applications.

Effective Distribution of Gold Nanorods in Ordered Thick Mesoporous Silica: A Choice of Noninvasive Theranostics

“…For example, at low temperature (<65 °C) and stirring, a large network is formed due to the slow rate of hydrolysis and condensation for silica formation, whereas higher temperature (60–80 °C) and vigorous stirring result in mesostructured particles in the nanometer range. − It is reported that aggregated and large clumps of silica particles are formed beyond 80 °C of reaction temperature. ,, On the other hand, the slow rate of silicification at low acidity and high ionic strength is well documented in the literature. , Cationic surfactant CTAB assembled in a hexagonal array, which helps in creating 2D pores/channels with an interchannel distance of about 4 nm followed by base-catalyzed hydrolysis–condensation of an orthosilicate precursor (source of silica particle formation) for mesoporous silica network formation, which are characteristics of MCM-41 type silica. − Silica nanoparticles are also prepared by using TEA as a sterically bulky chelating agent (pH stabilizer and as an additive) that controls the particle diameter . Later, surface modified mesoporous silica nanoparticles with various small molecules have been reported for biomedical applications, either imaging or therapeutics of cancer. , Silica is a well understood biomaterial and claimed as a safe material for preclinical and clinical studies. , Obviously, silica, especially MS, is inferior as a contrast agent. , It should be noted that silica and gold based hybrid structures have been recognized as translational materials for clinical trials. − Although an integration of these two components seems logically promising for exceptional theranostics, several hurdles make it a challenge. , Silica coating is a versatile approach for maintaining the thermal and chemical stability, optical property, and aspect ratio of metal nanoparticles, especially gold nanorods, which also makes them applicable for drug delivery, photothermal therapy, photoacoustic imaging, sensing, etc. − Although using a nonporous silica coating to maintain t...…”

“…45,50−56 Several attempts including Gorelikov and Matsuura's method require 10 h to 3 days; 31−47,50−56 however, they are limited with poor product yield (∼10 mg) 40 and moderate surface area (∼641 and 934 m 2 /g). 43,44,47,57 Disordered pores and thin silica shells (13−30 nm) 47 significantly reduce the drug loading efficacy of GMS nanoparticles, which is about 25 ± 1.5%. 58 Further, the scalability has always been a bottleneck for such hybrid structures.…”

“…Later, surface modified mesoporous silica nanoparticles with various small molecules have been reported for biomedical applications, either imaging or therapeutics of cancer. , Silica is a well understood biomaterial and claimed as a safe material for preclinical and clinical studies. , Obviously, silica, especially MS, is inferior as a contrast agent. , It should be noted that silica and gold based hybrid structures have been recognized as translational materials for clinical trials. − Although an integration of these two components seems logically promising for exceptional theranostics, several hurdles make it a challenge. , Silica coating is a versatile approach for maintaining the thermal and chemical stability, optical property, and aspect ratio of metal nanoparticles, especially gold nanorods, which also makes them applicable for drug delivery, photothermal therapy, photoacoustic imaging, sensing, etc. − Although using a nonporous silica coating to maintain the aspect ratio is simple, direct deposition of a well-ordered MS having a higher cargo capacity and surface area with a better theranostics response at a large scale has yet to be achieved. − For achieving MCM-41 types of mesoporous silica nanoparticles, a base-catalyzed (sodium hydroxide, 2 M, 3.5 mL) sol–gel route at high temperature (70–80 °C) using CTAB surfactant (1 g in 480 mL of aqueous medium) has been widely tried. , Similar conditions of the MCM-41 protocol have been adopted for effective distribution of gold nanorods in ordered thick mesoporous silica, as discussed below in detail. Reports suggest that the silica layer thickness over gold nanorods can be controlled (13–30 nm) with respect to CTAB concentration. − ...…”

“…Previous attempts reported diverse hurdles in achieving this. The primary bottleneck is in achieving an effective distribution of a single GNR in the deep core of each MS shell through direct deposition without compromising its synergistic theranostics performance. − ,− Second, controlling this heavily reduces the effective yield per batch. The largest scale reported so far is 190 mg/batch by J.…”

Dynamically modulating the mid-infrared localized surface plasmon resonance of Al-doped ZnO nanocrystals

“…The localized surface plasmon resonance (LSPR) of metal nanoparticles has been used in biomedical applications, sensors, catalysis, and surface-enhanced spectroscopy [1][2][3][4][5]. Unlike metal nanoparticles, the LSPR of doped semiconductor nanocrystals (NCs) can be modulated by changing the doping content.…”