Plasmonic and Photothermal Properties of Silica-Capped Gold Nanoparticle Aggregates

Abstract

Owing to their biocompatibility, gold nanoparticles have many applications in healthcare, notably for targeted drug delivery and the photothermal therapy of tumors. The addition of a silica shell to the nanoparticles can help to minimize the aggregation of the nanoparticles upon exposure to harsh environments and protect any Raman reporters adsorbed onto the metal surface. Here, we report the effects of the addition of a silica shell on the photothermal properties of a series of gold nanostructures, including gold nanoparticle aggregates. The presence of a Raman reporter at the surface of the gold nanoparticles also allows the structures to be evaluated by surface-enhanced Raman scattering (SERS). In this work, we explore the relationship between the degree of aggregation and the position and the extinction of the near-infrared plasmon on the observed SERS intensity and in the increase in bulk temperature upon near-infrared excitation. By tailoring the concentration of the silane and the thickness of the silica shell, it is possible to improve the photothermal heating capabilities of the structures without sacrificing the SERS intensity or changing the optical properties of the gold nanoparticle aggregates.

Figure 1

SEM images of a variety of (A) SHINs functionalized with BPE, and (B) silica-capped BPE-functionalized AuNP aggregates. (C) DLS measurements for the AuNP, SHINs, and aggregates. (D) Extinction spectra for the three nanostructures, collected using a Cary60 UV–vis spectrophotometer scanning from 300 to 1100 nm at a medium scanning rate of 600 nm/min. (E) SERS spectra of SHINs and aggregates, collected using a hand-held Snowy Range Instruments CBEx spectrometer at an excitation wavelength of 785 nm, a laser power of 10 mW at the sample, and an acquisition time of 0.1 s. Following collection, spectra were baseline corrected using MATLAB (Version 2022b) and plotted in Excel. For characterization with extinction spectroscopy and SERS, samples were adjusted to an optical density of 1 for a volume of 500 μL. DLS measurements were performed using the samples as prepared.

Figure 2

(A) Overview of in-house photothermal heating setup; the 785 nm laser beam is directed toward the sample using a mirror and lens. (B) Schematic representation of the sample in the cuvette holder in the path of the laser beam, with the temperature probe positioned so as not to interfere with the laser beam. (C) Change in temperature for AuNPs, SHINs, and aggregates when irradiated for 45 min at 785 nm, adjusted to an optical density of 1 based on their plasmon band at ∼540 nm. The temperature was digitally recorded at 1 s intervals from a starting temperature of 20.8 °C, using a thermocouple probe connected to PicoLog software. Comparison of (D) extinction, and (E) SERS spectra before and after heating the aggregates. Extinction spectra were collected using a Cary60 UV–vis spectrophotometer scanning from 300 to 1100 nm at a medium scanning rate of 600 nm/min. SERS spectra were collected using a hand-held Snowy Range Instruments CBEx spectrometer with an excitation wavelength of 785 nm, a laser power of 10 mW at the sample, and an acquisition time of 0.1 s. Following collection, spectra were baseline corrected using MATLAB (Version 2022b) and plotted in Excel. For characterization with extinction spectroscopy and SERS, samples were adjusted to an optical density of 1 for a volume of 500 μL as this was the sample volume used for heating experiments. All heating and characterization measurements were carried out in triplicate and averaged.

Figure 3

(A) Change in temperature (°C) for heating and cooling cycles for AuNP 500 nM BPE SiO₂ aggregates at 45 min intervals, adjusted to an optical density of 1 based on the plasmon at ∼540 nm. The temperature was digitally recorded at 1 s intervals from a starting temperature of 20.8 °C, using a thermocouple probe connected to PicoLog software. (B) Extinction and (C) SERS spectra for AuNP 500 nM BPE SiO₂ aggregates before and after cycling experiments. Extinction measurements were collected using a Cary60 UV–vis spectrophotometer scanning from 300 to 1100 nm at a medium scanning rate of 600 nm/min, and SERS spectra were collected using a hand-held Snowy Range Instruments CBEx spectrometer at an excitation wavelength of 785 nm, a laser power of 10 mW at the sample, and an acquisition time of 0.1 s. Following collection, spectra were baseline corrected using MATLAB (Version 2022b) and plotted in Excel. For characterization with extinction spectroscopy, and SERS samples were adjusted to an optical density of 1 for a volume of 500 μL.

Figure 4

(A) Extinction spectra of AuNP BPE SiO₂ aggregates with increasing APTES concentrations, collected using a Cary60 UV–vis spectrophotometer scanning from 300 to 1100 nm at a medium scanning rate of 600 nm/min. The excitation wavelength (785 nm) is indicated by a dashed line. (B) SERS spectra of aggregates with the increasing APTES concentration. SERS spectra were collected using a hand-held Snowy Range Instruments CBEx spectrometer at an excitation wavelength of 785 nm, a laser power of 10 mW at the sample, and an acquisition time of 0.1 s. Following collection, spectra were baseline corrected using MATLAB (Version 2022b) and plotted in Excel. (C) Change in temperature for the samples of aggregates when irradiated for 45 min at 785 nm, adjusted to an optical density of 1 based on their shared peak at ∼540 nm. The temperature was digitally recorded at 1 s intervals using a thermocouple probe connected to PicoLog software. (D) 3D scatter plot showing the change in temperature in relation to extinction and λ_max of the NIR plasmon.

Figure 5

TEM images of SHINs prepared (A) without and (B) with TEOS, and silica-capped aggregates prepared (C) without and (D) with TEOS. (E) Extinction spectra of SHINs and aggregates prepared with and without TEOS, collected by using a Cary60 UV–vis spectrophotometer scanning from 300 to 1100 nm at a medium scanning rate of 600 nm/min. (F) SERS spectra of SHINs and aggregates prepared with and without TEOS. SERS spectra were collected using a hand-held Snowy Range Instruments CBEx spectrometer at an excitation wavelength of 785 nm, a laser power of 10 mW at the sample, and an acquisition time of 0.1 s. Following collection, spectra were baseline corrected using MATLAB (Version 2022b) and plotted in Excel. (G) Change in temperature for the samples of SHINs and aggregates when irradiated for 45 min at 785 nm, adjusted to an optical density of 1 based on their shared peak at ∼540 nm. The temperature was digitally recorded at 1 s intervals using a thermocouple probe connected to PicoLog software. (H) 3D scatter plot showing the change in temperature in relation to extinction and λ_max of the NIR plasmon.