Synthesis of Densely Immobilized Gold-Assembled Silica Nanostructures

Efficient Analysis of Small Molecules via Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (LDI-TOF MS) Using Gold Nanoshells with Nanogaps

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is a commonly used technique for analyzing large biomolecules. However, the utilization of organic matrices limits the small-molecule analysis because of the interferences in the low-mass region and the reproducibility issues. To overcome these limitations, a surface-assisted laser desorption/ionization (SALDI), which utilizes nanostructured metallic surfaces, has been developed. Herein, a novel approach for SALDI-MS was proposed using silica@gold core-shell hybrid materials with a nanogap-rich shell (SiO2@Au NGS), which is an emerging material due to its excellent heat-generating capabilities. The gold shell thickness was controlled by adjusting the concentration of gold precursor for the growth of gold nanoparticles. SALDI-MS measurements were performed on a layer formed by drop-casting a mixture of SiO2@Au NGS and analytes. At the optimized process, the gold shell thickness was observed to be 17.2 nm, which showed the highest absorbance. Based on the enhanced SALDI capability, SiO2@Au NGS was utilized to detect various small molecules, including amino acids, sugars, and flavonoids, and the ionization softness was confirmed with a survival yield upon fragmentation. The limits of detection, reproducibility, and salt tolerance of SiO2@Au NGS demonstrate its potential as an effective and reliable SALDI material for small-molecule analyses.

Silica Encapsulation of Hydrophobic Optical NP-Embedded Silica Particles with Trimethoxy(2-Phenylethyl)silane

Nanoparticles (NP) with optical properties embedded silica particles have been widely used in various fields because of their unique properties. The surfaces of optical NPs have been modified with various organic ligands to maintain their unique optical properties and colloidal stability. Among the surface modification methods, silica encapsulation of optical NPs is widely used to enhance their biocompatibility and stability. However, in the case of NPs with hydrophobic ligands on the surface, the ligands that determine the optical properties of the NPs may detach from the NPs, thereby changing the optical properties during silica encapsulation. Herein, we report a generally applicable silica encapsulation method using trimethoxy(2-phenylethyl)silane (TMPS) for non-hydrophilic optical NPs, such as quantum dots (QDs) and gold NPs. This silica encapsulation method was applied to fabricate multiple silica-encapsulated QD-embedded silica NPs (SiO₂@QD@SiO₂ NPs; QD²) and multiple silica-encapsulated gold NP-embedded silica NPs labeled with 2-naphthalene thiol (SiO₂@Au_2-NT@SiO₂). The fabricated silica-encapsulated NPs exhibited optical properties without significant changes in the quantum yield or Raman signal intensity.

Synthesis of Gold-Platinum Core-Shell Nanoparticles Assembled on a Silica Template and Their Peroxidase Nanozyme Properties

Bimetallic nanoparticles are important materials for synthesizing multifunctional nanozymes. A technique for preparing gold-platinum nanoparticles (NPs) on a silica core template (SiO₂@Au@Pt) using seed-mediated growth is reported in this study. The SiO₂@Au@Pt exhibits peroxidase-like nanozyme activity has several advantages over gold assembled silica core templates (SiO₂@Au@Au), such as stability and catalytic performance. The maximum reaction velocity (V_max) and the Michaelis–Menten constants (K_m) were and 2.1 × 10⁻¹⁰ M⁻¹∙s⁻¹ and 417 µM, respectively. Factors affecting the peroxidase activity, including the quantity of NPs, solution pH, reaction time, and concentration of tetramethyl benzidine, are also investigated in this study. The optimization of SiO₂@Au@Pt NPs for H₂O₂ detection obtained in 0.5 mM TMB; using 5 µg SiO₂@Au@Pt, at pH 4.0 for 15 min incubation. H₂O₂ can be detected in the dynamic liner range of 1.0 to 100 mM with the detection limit of 1.0 mM. This study presents a novel method for controlling the properties of bimetallic NPs assembled on a silica template and increases the understanding of the activity and potential applications of highly efficient multifunctional NP-based nanozymes.

Highly sensitive near-infrared SERS nanoprobes for in vivo imaging using gold-assembled silica nanoparticles with controllable nanogaps

Background

To take advantages, such as multiplex capacity, non-photobleaching property, and high sensitivity, of surface-enhanced Raman scattering (SERS)-based in vivo imaging, development of highly enhanced SERS nanoprobes in near-infrared (NIR) region is needed. A well-controlled morphology and biocompatibility are essential features of NIR SERS nanoprobes. Gold (Au)-assembled nanostructures with controllable nanogaps with highly enhanced SERS signals within multiple hotspots could be a breakthrough.

Results

Au-assembled silica (SiO₂) nanoparticles (NPs) (SiO₂@Au@Au NPs) as NIR SERS nanoprobes are synthesized using the seed-mediated growth method. SiO₂@Au@Au NPs using six different sizes of Au NPs (SiO₂@Au@Au₅₀–SiO₂@Au@Au₅₀₀) were prepared by controlling the concentration of Au precursor in the growth step. The nanogaps between Au NPs on the SiO₂ surface could be controlled from 4.16 to 0.98 nm by adjusting the concentration of Au precursor (hence increasing Au NP sizes), which resulted in the formation of effective SERS hotspots. SiO₂@Au@Au₅₀₀ NPs with a 0.98-nm gap showed a high SERS enhancement factor of approximately 3.8 × 10⁶ under 785-nm photoexcitation. SiO₂@Au@Au₅₀₀ nanoprobes showed detectable in vivo SERS signals at a concentration of 16 μg/mL in animal tissue specimen at a depth of 7 mm. SiO₂@Au@Au₅₀₀ NPs with 14 different Raman label compounds exhibited distinct SERS signals upon subcutaneous injection into nude mice.

Conclusions

SiO₂@Au@Au NPs showed high potential for in vivo applications as multiplex nanoprobes with high SERS sensitivity in the NIR region.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01327-7.

Synthesis of Finely Controllable Sizes of Au Nanoparticles on a Silica Template and Their Nanozyme Properties

The precise synthesis of fine-sized nanoparticles is critical for realizing the advantages of nanoparticles for various applications. We developed a technique for preparing finely controllable sizes of gold nanoparticles (Au NPs) on a silica template, using the seed-mediated growth and interval dropping methods. These Au NPs, embedded on silica nanospheres (SiO₂@Au NPs), possess peroxidase-like activity as nanozymes and have several advantages over other nanoparticle-based nanozymes. We confirmed their peroxidase activity; in addition, factors affecting the activity were investigated by varying the reaction conditions, such as concentrations of tetramethyl benzidine and H₂O₂, pH, particle amount, reaction time, and termination time. We found that SiO₂@Au NPs are highly stable under long-term storage and reusable for five cycles. Our study, therefore, provides a novel method for controlling the properties of nanoparticles and for developing nanoparticle-based nanozymes.

Metal Nano/Microparticles for Bioapplications

Nano/micro particles are considered to be the most valuable and important functional materials in the field of materials science and engineering [...].