Method to synthesize complex nanoframes for SERS biosensing

Although three-dimensional (3D) nanoframe structures are desirable for optical sensing, it is difficult to achieve nanoframes with a complex architecture due to the lack of a suitable synthetic protocol that could help achieve high yields.

Study: Double-rimmed three-dimensional nanoframes as nanoprobes for biosensing. Image Credit: schlyx/Shutterstock.com

An article published in the journal Nature Communication, demonstrated a multi-step synthetic approach to complex 3D nanoframes with complex architecture. Each facet of the octahedral nanoframe was etched with two-dimensional (2D) double-rim nanostructures.

Mono-rim octahedral platinum (Pt) nanoframes were prepared by edge-selective Pt deposition using gold (Au) nanoparticles as a sacrificial matrix. The growth of Au adlayers on Pt skeletons was based on the Frank-van der Merwe mode, forming well-developed and sharp ridges.

Selective deposition of Pt on the boundaries (inner/outer) can help to tune the geometry of the Au patterns. Ultimately, selective Au etching led to octahedral double-rim Pt nanoframes with homogeneous shapes and sizes. Additionally, the Au coating around the Pt frameworks endowed the nanoframes with plasmonic characteristics.

Thus, the fabricated plasmonic double-rim etched nanoframes possessed light-trapping ability through surface-enhanced Raman scattering (SERS) on their surface and served as nanoprobes in biosensing.

Figure 1. Schematic illustration of the synthetic strategy using multistep chemical reaction conditions. Multiple synthetic routes for octahedral 3D NFs etched on 2D nanoframes (NFs) by stepwise chemical toolkits. © Hilal, H. et al. (2022)

Fabrication of Plasmonic Nanoframes

Nanocrystals with complex structures have different physicochemical properties and are applied in nanoscience. Among different types of nanocrystals, nanoframes have found their applications in sensing and imaging due to their high exposed surface area and the light-matter interactions of their ridges and internal void, promoting the ability of analyte detection. .

The synthesis of nanoframes reported to date was limited to single-rim 3D nanoframes, which also limited the use of their structural functionality. On the other hand, the synthesis of nanoframes with a complex architecture is difficult.

Moreover, it is difficult to control the surface structure of nanoframes when their edges are reduced to a few nanometers in thickness. Only a few previous studies have attempted to resolve specific facets exposed on the ridges of nanoframes. To this end, it is of crucial importance to extend the capability of a synthetic protocol to precisely control the atomic composition and arrangement on the outermost surface of a nanoframe.

Although lithography methods or galvanic replacement reactions were adopted to realize plasmonic nanoframes, the galvanic replacement reaction controlled the geometric parameters of the nanoframe due to the simultaneous occurrence of reduction and oxidation of the noble metal .

Despite the feasibility of producing nanostructures with high precision and fidelity using the top-down lithographic method, this method lacks control over the physical dimensions of the nanoframe in a 3D frame. Moreover, the fabrication of such structures by a bottom-up lithographic process is impractical.

Morphological evolution of double-rim octahedral NFs using multistep chemical reaction conditions.  FE-SEM images of skeletons a mono-rim Pt, b NF mono-rim Au, c NF Au@Pt and d octahedral double-rim Pt NFs. -60°, -35°, 44°, 65°, 72° and 90°.

Figure 2. Morphological evolution of double-rim octahedral NFs using multistep chemical reaction conditions. FE-SEM images of a Single Rim Pt Skeletons, b On the NFs mono-rim, vs [email protected] NF, and D NF double octahedral rim in Pt. e Rotation snapshots and cartoons of Pt double rim NF at various angles of −60°, −35°, 44°, 65°, 72° and 90°. © Hilal, H. et al. (2022)

3D nanoframes with double rims as nanoprobes

Previous reports mentioned the controlled synthesis of 2D nanorings with dual rims and an electromagnetic field between the outer and inner rims. The synthesis of 3D Au double-rim octahedral nanoframes has also been reported with truncated flat terraces on (100) facets.

Thus, stereoscopic nanoframes with double rims on each facet could amplify the interaction of particles with light due to gap-induced coupling in 3D nanostructures. Additionally, the intragap distance would help tune the light trapping ability of the nanostructure.

The present work demonstrated the synthesis of 3D metallic nanoframes via multistep wet chemistry. The fabricated nanoframes consisted of a 2D double rim with a 3D etched Pt skeleton. Here, several intra-nanogaps of homogeneous shape and size have been developed, improving near-field electrical confinement.

Previous reports mentioned that flat terraces facilitate tight compaction in single-layer arrays contributing to highly sensitive SERS signals. Moreover, the SERS activity of an individual particle depends on its polarization and the orientation of the particles, which hinders the generation of uniform SERS signals. However, the Au coating on the nanoframes in the present work served as a plasmonic component and helped improve near-field focusing ability due to intra-nanogaps in a single entity, enabling single-particle SERS.

For the first time, the present work reported the construction of frame-frame structures with octahedral nanoparticles in a 3D configuration etched with 2D double-rim patterns. Furthermore, the SERS-based biosensing immunoassay demonstrated that the fabricated Au double-rim 3D nanoframes exhibited higher biosensing activity for the detection of human chorionic gonadotropin (HCG) than their 2D counterparts, indicating the superior functionality of the nanoframes. 3D complexes.

Structural characterization of Au octahedral double-rim NFs.  a A low-resolution FE-SEM image of double-rim etched Au NFs.  b A HAADF-STEM image of Au double-rim NFs, elemental mapping by energy dispersive spectroscopy (EDS) and c EDS line mapping for Au, Ag and Pt components. Scale bar indicates 50 nm.  d–f FE-SEM images and cartoons seen in the directions <100>, <110> and <111>, respectively.  g Extinction spectra of Au single-rim NF, Au@Pt NF, Pt double-rim NF and Au double-rim NF.  Numbers indicate localized surface plasmon peaks for each particle.  h Calculated average optical extinction, absorption and scattering spectra of Au octahedral double-rim NFs.” class=”enlarge-image-child”  data-src=”https://d1otjdv2bf0507.cloudfront.net/images/news/ ImageForNews_39581_16612437369438516.png”  data-srcset=”https://d1otjdv2bf0507.cloudfront.net/image-handler/ts/20220823043639/ri/685/src/images/news/ImageForNews_39581_16612437369438516.png 685w, https://d1otjdv2bf0507.cloudfront.net/ image-handler/ts/20220823043639/ri/650/src/images/news/ImageForNews_39581_16612437369438516.png 650w, https://d1otjdv2bf0507.cloudfront.net/image-handler/ts/20220823043639/ri/450/src/images/news /ImageForNews_39581_16612437369438516.png 450w” sizes=”(min-width: 1200px) 673px, (min-width: 1090px) 667px, (min-width: 992px) calc(66.6vw – 60px), (min-width: 725px) 685px , (min-width: 480px) calc(100vw – 40px), calc(100vw – 30px)” style=”width: 685px;  height: 802px;” width=”685″ height=”802″/></p>
<p style=Figure 3. Structural characterization of Au octahedral double-rim NFs. a A low-resolution FE-SEM image of double-rim etched NF Au. b A HAADF-STEM image of Au double-rim NFs, energy dispersive spectroscopy (EDS) elemental mapping and vs EDS line mapping for Au, Ag and Pt components. Scale bar indicates 50 nm. d–f FE-SEM images and cartoons seen from the directions , and respectively. g Extinction spectra of mono-rim Au NFs, [email protected] NF, NF double rim Pt and NF double rim Au. Numbers indicate localized surface plasmon peaks for each particle. h Calculated average optical extinction, absorption and scattering spectra of Au octahedral double-rim NFs. © Hilal, H. et al. (2022)

Conclusion

To summarize, a multi-step synthesis method was adopted to fabricate complex 3D nanoframes with double rims etched on each facet of the octahedral nanoframes. Well-faceted growth was essential to obtain double-rim nanoframes. The layer-by-layer Au growth formed sharp edges, facilitating the deposition of Pt on the mono-rim Au nanoframes on the inner and outer boundaries.

The current synthetic scheme resulted in complex double-rim nanoframes with intra-nanogaps between the outer and inner rims, improving near-field electrical confinement, confirmed by single-particle SERS analysis. Moreover, the strong near-field focusing ability of the 3D nanoframe was in agreement with the electromagnetic field distribution calculations and the surface charge density distribution.

Double-rim 3D Au nanoframes have been applied for SERS-based biosensing immunoassay, proving their potential as nanoprobes in biosensors. Thus, the colloidal chemistry of double-rim 3D nanoframes can contribute to the fabrication of complex nanoparticles with advanced physicochemical and optical properties.

Reference

Hilal, H. et al. (2022) Three-dimensional double-rim nanoframes as nanoprobes for biosensing. Nature Communication. https://doi.org/10.1038/s41467-022-32549-w.

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