Plasmonic Gold Nanoparticles as Designing Caps

Plasmonic nanomaterials have attracted significant interest because of its wide variety of applications such as sensing, energy conversion, photothermal therapy etc. Among the various plasmonic inorganic metals (Ag, Cu, Al, and Au) Au shows the excellent biocompatibility, chemical stability, and rich surface functionality. It can be seen that some new approaches have include the construction of metal shells on dielectric nanoparticles, and the control of island growth on nanoparticles and nanorods over the design of narrow gaps via nanoplates or nanoshells. Also, the conventional modulation of plasmonics via the control of shape, size and the aspect ratio of nanomaterials. The structural control have also arises from the active surface growth. As a result, the dynamic competition between ligand absorption and metal deposition occurs [1].

The properties of plasmonic nanostructures are highly dependent on its surface morphology, however, there are a very few methods for appending the domain as the “functional group” or modifiers. The fundamental control of the domain appending on a seed nanoparticle is via the interfacial energy and the wetting growth mode leads to the conformal coating on the seed. Also, the modulation of plasmonics will essentially control the size of the seed. Here they have studied a systematic analysis on the growth of Au caps over Au seeds via simultaneously for the use of a hydrophobic thiol ligand and surfactant. Moreover, this combination have achieved a powerful control for the active surface which leads to a dynamic modulation of spreading the vertical growth of the Au caps. By modulating the plasmonic properties, this methodology offers a wide variety of caps and modulate Au hats on Au nanoparticles, which includes of helmets, crowns, antler hats, antenna hat, beret hats and mortar boards [1].

Plasmonic modulations need more synthetic handles with a systematic development from the perspectives of synthetic advance is of a critical importance. The main prerequisite for tuning cap morphology is the stable and monodispersed AuNPs. Strong hydrophobic ligand of 4,5-diphenyl-2-imidazolethiol (DPI) was used with the combination of a cationic surfactant myristyl trimethyl ammonium bromide (MTAB). It can be found that a serious aggregation have caused due to the exchange of direct ligand between DPI and citrate. Therefore, at first we need to replace the citrate with MTAB thus, the resulting mixture will be highly stable and are used as the stock solutions for all the synthesis. DPI was further added to functionalize and activate the Au surface [1].

Figure 1. Schematic representation of the synthesis of mortarboard, helmet and beret hat [1].

At each side of Au seed, a domain growth was observed (Figure 1) when the concentration of NaOH is found to be 0.825 mM, 64% of the domains found to be nanoplate, and the remaining are of polyhedrons. 68% of nanoplate were attached tangent to the spherical seeds and placed via its center and it is named as mortar board for the structural similarity. However, when the concentration of NaOH was increased to 3.96 mM, then the initial color change was occurred after 2 minutes. Here, the emerging Au domain will become more thicker with an obvious curvature, appearing as a helmet on the seed nanoparticle, instead of nanostructures with plate shapes.

To explore the intermediate states, a series of reaction is carried out between the mortar board and the helmet which are distinctively different form of the growth modes. On comparison to the helmets, the thickness of the helmets and beret hats are found to be similar, with the only main differences was the coverage of the seed. The growth of island domain on a seed is essentially found to be a solid–solid wetting phenomenon. As per the literature reports, when a metal domain is directly grown on a metal seed, then it is due to the wetting growth mode. On the other hand, if the metal domain is grown on the ligand covered metal surface, then it is due to the non-wetting growth mode. It can be obviously seen that, the formation of the helmet was not instantaneous, however it gradually grows with its size and coverage [2].

The main critical factor is the dynamic competition between ligand inhibition and the Au deposition. The possible outcome with varying the reduction rates will leads to the different morphologies of the new Au domains. Moreover, 20% of the nanoplates were attached to the side of the seeds which may arise from the independently developed new Au domain of random twinning. Also, the twinning is not occur for all the nuclei, with mainly 36% of the Janus nanostructures did not show any plate morphological structures.

Figure 2. Schematic representation of the active surface growth [1].

As it can be seen that, some of the sites (Figure 2) seems to be new and some may be old, the differentiation can be explained by the non-uniform spreading and the formation of petals. For pushing the limit of spreading growth, they tried to increase the amount of Au growth on each seed. The reactants were also kept at the same concentration to maintain the growth mode.

Figure 3. Schematic representation of the growth of crown, antler hats and antennas [1].

When the concentration of NaOH is further increased to 6.6mM, there is no any structural changes. Moreover, to the increase the rate of reduction, they have tried to increase the concentration of the reductant 2,7-dihydroxynaphthalene (DHN). While, the emerging hat is still on the seed, and its shape has evolved to complete with various different forms. Hence, there is sign of vertical active surface growth in addition to the spreading growth [1].

Our SNB team have emphasize this research article to enrich our viewer’s knowledge about the study on a systematic analysis on the growth of gold caps over Au seeds via simultaneously with a hydrophobic thiol ligand and surfactant for designing the caps (helmets, crowns, antler hats, antenna hat, beret hats and mortarboards). The morphological structures and plasmonic property of a nanomaterials are highly attentive in the research work. The dynamic role and competition between the concern growth sites in contrast to the wetting growth was developed via active nature of surface growth. This will give rise to the encapsulation of shells, while the non-wetting growth gives to appending spheres. Hence, they reported that the active surface growth methodology is an fruitful and effective one to enrich a structural modulation for appending the domain of Au seeds.

References

[1] X. Tian, et al., ChemSci., (2021), DOI: 10.1039/d0sc05780k.

[2] J. Huang, et al., Nano Lett., 16, 617 (2016).

--- Dr. Y. Sasikumar

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