Gold Nanorod Coating with Silica Shells Having Controlled Thickness and Oriented Porosity: Tailoring the Shells for Biosensing

Abstract

The coating of gold nanorods with a silica shell (AuNR@SiO2) is an effective way to extend their use in a wide variety of biomedical applications including biosensing, drug delivery and photothermal therapy. A silica shell offers numerous advantages as it provides more stability, frees the surface from toxic cetyltrimethylammonium bromide (CTAB), and preserves the rod shape under photothermal conditions. This shell needs to be very thin for applications such as plasmonic biosensing, while a thicker and porous shell is suited for drug encapsulation and further controlled release. We introduce herein a strategy to perform silica coating based on dissociation of tetraethylorthosilicate (TEOS) hydrolysis and condensation reactions. This dissociation is achieved by a pH modulation of the reaction medium, and, depending on selected pH conditions, AuNR@SiO2 with a thick silica shell having an organized mesoporosity aligned either parallel (AuNR@//m-SiO2) or perpendicular (AuNR@⊥m-SiO2) to the AuNR surface was generated. Moreover, when mercaptopropyltrimethoxysilane (MPTMS) was used as a surface primer prior to TEOS condensation, ultrathin and homogeneous silica shells (AuNR@t-SiO2) of controllable thickness in the range 2–6 nm were produced. While formation, at high TEOS concentration, of core-free silica nanoparticles is evidenced by TEM analysis before the purification procedure, their total elimination during the purification step was achieved by addition of a suitable amount of CTAB to ensure the colloidal stability of the core-free and core–shell nanoparticles. Complete elimination of CTAB from AuNR@SiO2 was demonstrated by XPS, Raman, and ζ-potential measurements. Finally, the efficiency of AuNR@t-SiO2 in label-free plasmonic biosensing of a model target was demonstrated and their refractive index sensitivity factor was improved by 30% compared to CTAB-capped AuNRs