Gold Raspberry Shell Grown onto Nonspherical Lithium Niobate Nanoparticles for Second Harmonic Generation and Photothermal Applications

Abstract

They should also have a high photothermal conversion efficiency (percentage of absorbed light converted to heat), good photothermal stability, be nontoxic and allow for easy surface modifications. [2] The examples of PTAs found in the literature can be classified as noble metal-based nanoparticles (NPs), semiconductor nanocrystals, carbon-based NPs, and organic semiconducting polymeric NPs. [3] The noble metal PTAs, particularly the Au-based nanostructures are very attractive as Au has been extensively studied for its biocompatibility and ease of surface functionalization. Additionally, their shape and morphology also enable excitation wavelength tunability. The Au-based nanostructures that are interesting for PTT include nanorods, nanoshells, and nanostars. Among them, the Au nanoshell structure is particularly appealing as it is inherently a hybrid NP combining another core material to provide multifunctional and multimodal applications (for example, a gold shell encapsulating a material with contrast agent capabilities). Their light-to-heat conversion properties are due to their high absorption cross-section because of their localized surface plasmon resonances that can be tuned in a broad range of wavelength from 700 to 1000 nm depending on the shell thickness to core diameter ratio. [4,5] Recently, gold Nanoparticles (NPs) containing a lithium niobate (LiNbO 3 ) core and a gold shell are prepared using a combination of seeded-growth and layer-by-layer approaches. The method includes first the surface charge reversal of lithium niobate with branched polyethyleimine, second, the electrostatic binding of gold seeds, and third, successive reduction steps of gold chloride onto the gold-seeded LiNbO 3 NPs for the progressive and surface-directed growth of the gold shell. The influence of three synthesis parameters, namely, pH, initial density of gold seeds covering the lithium niobate core, and gold chloride concentration, on the NPs' characteristics (structural properties, plasmon band, surface charge, hydrodynamic diameter) is studied. The progress of the gold shell growth is investigated by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. In addition, it is shown that LN@Au core-shell NPs can emit a second harmonic generation signal when excited by a femtosecond laser. Finally, photothermal properties are studied, showing an increase of temperature of 8.6 °C upon infrared excitation, with an estimated light-to-heat conversion efficiency of 40% and a specific absorption rate of 8000 W g −1 .

Black Gold Plasmon Response of a Raspberry Shell Grown on Lithium Niobate Nonlinear Nanoparticles

“…Au seeds were then grown, and the as-obtained raspberry-like Au nanoshells grown on LN NPs were thoroughly optimized and characterized. 25 As expected, broad plasmonic responses were demonstrated from the extinction spectra.…”

“…The first hyperpolarizability was shown to slightly decrease at high surface densities of Au nanoseeds for an almost constant depolarization ratio corresponding to the one of bare LN, thus evidencing the predominance of the core volume contribution. Au seeds were then grown, and the as-obtained raspberry-like Au nanoshells grown on LN NPs were thoroughly optimized and characterized . As expected, broad plasmonic responses were demonstrated from the extinction spectra.…”

“…LN@ Au NPs were synthesized using an LbL process involving a first step of Au seeding on BPEI-modified LN NPs followed by three Au reducing steps to grow and optimize the Au shell as already detailed. 25 TEM images of LN@Au NPs obtained after each reduction step are presented in Figure 1a−c. The Au shell has a raspberry shape composed of almost spherical Au NP drupelets, with the diameter increasing after each reduction step.…”

Synergetic Peroxidase-Mimic Catalytic Activity of Noble-Metal-Decorated Lithium Niobate Nanozymes for Potential Biological Applications

“…The gold loading on LN NPs was also verified by EDS (Figure S2A), which confirmed increasing gold loading by varying the volume of AuSeeds suspension added to LN@BPEI (Figure S2A, inset). This was also supported by previously published XPS analysis . This work showed that upon BPEI coating followed by gold decoration, the presence of Au is observed in addition to an increase of C and N percentages, while Nb and O percentages decreased.…”

“…This was also supported by previously published XPS analysis. 25 This work showed that upon BPEI coating followed by gold decoration, the presence of Au is observed in addition to an increase of C and N percentages, while Nb and O percentages decreased. It was also noticed that the Au to Nb atomic ratio increased when the gold seeds to LN ratio increased, as expected.…”