Leonardo Scarabelli

Organizing metallic nanoparticles (NPs) into well‐defined arrays enables the design of complex, collective plasmon responses. In periodic lattices with controlled spacing, the plasmon modes of individual NPs can couple with the diffraction modes of the arrays, giving rise to lattice‐plasmon resonances. By controlling the geometry of the NP arrangement, the optical, electromagnetic, and chemical properties of the system can be precisely engineered, opening pathways to advanced technologies in electronics, optics, sensing, and catalysis. However, achieving such architectures over large areas while maintaining precise control over composition and morphology remains challenging. Electrochemically fabricated metasurfaces offer a complementary bottom‐up alternative to top‐down patterning, providing direct control over material composition and morphology through low‐cost processing. This Perspective discusses how techniques such as templated electrodeposition, in situ redox patterning, and electrochemical growth and dissolution enable the formation of subwavelength architectures with tunable functionalities, typically associated with more complex fabrication routes. Despite notable advances, the link between electrochemical growth mechanisms and the design of functional optical and plasmonic arrays remains fragmented across the literature. This Perspective synthesizes these developments, clarifies the advantages and current limitations of electrochemical approaches, and outlines emerging opportunities for scalable, electrochemistry‐enabled design of functional metasurfaces.

Chiral gold nanoparticles exhibit unique chiroptical properties, but their integration into plasmonic devices requires an understanding of the formation of chiral features and controlling their growth on substrates. Here, we combine microfluidic synthesis and liquid-phase transmission electron microscopy (LP-TEM) to study the growth of chiral wrinkled gold nanorods (AuNRs). Microfluidic growth from AuNR seeds produced g-factors of up to 0.01, approaching values reported for colloidal synthesis. Complementary LP-TEM experiments in a nanofluidic reactor generated gradients of gold precursor and reducing agent concentrations, enabling the direct observation of wrinkle formation under different conditions within a single experiment. Both methods showed that a higher gold precursor availability promotes longer wrinkles and stronger chiroptical activity. Higher flow rates in microfluidics produced larger nanoparticles with enhanced chiroptical responses, while LP-TEM revealed that increased reducing agent concentrations accelerate wrinkle growth. Reagent depletion ultimately limits the growth. Overall, wrinkle development is governed by precursor availability and reduction kinetics, determining the chiroptical performance.

Patterned in situ growth of plasmonic nanostructures is emerging as a rational bottom-up route to fabricating functional plasmonic metamaterials. In this family of approaches, the combination of simple wet-chemistry principles with tailored surface-chemistry modifications unlocks new, scalable, and versatile strategies to produce ordered nanoparticle arrays showing strong electromagnetic responses. Herein, the self-polymerization of catecholamine monomers is exploited to achieve region-specific chemical contrast via a one-step polymer patterning approach that is compatible with different materials and can be completed in a few minutes. To highlight the robustness and versatility of this methodology, the confined polymerization protocol was successfully applied across multiple length scales, from micrometer-sized patterns to nanometric arrays with feature sizes down to a few tens of nanometers, enabling the simultaneous fabrication of multiple patterns on a single substrate within the same preparation. The obtained chemically patterned substrates were exploited to control the gold nanoparticle nucleation point with nanometric precision, leading to the growth of plasmonic arrays. The size of the gold nanoparticles and the number of particles per polymer-patterned feature can be controlled by tuning the growth conditions, underscoring the power of bottom-up wet-chemical in situ approaches for the scalable fabrication of tunable plasmonic architectures showing single-particle resolution (i.e., single nucleation for each active area). The obtained plasmonic metasurfaces exhibit lattice plasmon resonances in the visible and near-infrared spectral regions, reaching quality factors as high as 130. Overall, these results pave the way for advanced sensing, catalytic, and other nanophotonic applications requiring large-area, highly ordered arrays.

Plasmonic gold nanoparticle size and shape can be engineered via chemical synthesis to achieve strong light-matter interactions at specific wavelengths. However, incorporation of tailored nanoparticles into functional devices is often limited by multistep, time-consuming processes specific to the type of substrate and nanoparticles. In situ growth is an unconventional bottom-up approach where nanoparticles are grown directly on the target substrate, thereby circumventing colloidal synthesis and surface attachment. Despite these advantages, there remains a considerable gap in synthetic control and product quality between particles grown in situ and their counterparts prepared in colloidal dispersion. Herein, we present optimized in situ strategies for the growth of anisotropic gold nanoparticles directly on the internal walls of microfluidic channels, with no need for any top-down lithography or colloidal synthesis steps. Careful optimization enabled us to obtain shape-yields for gold nanorods around 90% (with size dispersion down to 15%)─a significant improvement compared to previous approaches, which have reported a maximum yield in situ growth as a viable alternative to colloidal synthesis, with high control over the synthesis of anisotropic products directly on a substrate surface. We propose that microfluidic systems containing in situ prepared nanostructures are promising for sensing applications, offering full control over nanoparticle crystallography, morphology, and surface chemistry.

Tip-enhanced Raman spectroscopy (TERS) has been developed by using atomic force microscopy and scanning tunneling microscopy since its birth in the early 2000s. Because, in principle, any scanning probe microscopy (SPM) can be combined with TERS, there is an increasing interest in extending TERS to other SPMs, especially the liquid-phase ones, targeting applications in biological or catalytic systems. We present the design, construction, and characterization of a laser-coupled scanning ion-conductance microscope (SICM) integrated with TERS for near-field spectroscopy under liquid environments. Key design elements include a full sample-scanning SICM, optics for TERS, and noise-isolating methods. The SICM-TERS system demonstrates direct-current and alternating-current SICM imaging modes on polydimethylsiloxane (PDMS) structures. Furthermore, the TERS capability was tested using silica-nanosphere-grafted PDMS. This work illustrates the feasibility and performance of SICM-TERS as a versatile tool for label-free nanoscale spectroscopy, opening new possibilities for real-time, in situ analysis of soft matter, electrochemical, and biological systems.

Colloidal plasmonic metasurfaces are emerging as versatile platforms for advancing nanophotonic applications, whose rational design of increasing complexity allows for tailoring and integrating advanced plasmonic and photonic optical properties. Traditional lithographic techniques limit crystallographic and chemical adaptability, hindering the dynamic tuning of optical properties. In this work, a template‐assisted self‐assembly approach for achieving plasmonic metasurfaces using colloidal silver nanoparticle arrays and a thin film of core–shell CdSe/ZnS quantum dots are demonstrated. This system leverages plasmonic surface lattice resonances to create open nanophotonic cavities that enhance the two‐photon excited emission process. The near‐field interactions within these nanophotonic structures offer spectral tunability and efficient light–matter coupling. Moreover, using synthetic nanoparticles enables the integration of a rich library in nanoparticles’ size, shape, and composition, positioning them as promising candidates for compact and scalable photonic devices targeting nonlinear optical processes, advanced sensing, and dynamic light manipulation.

Colloidal metasurfaces are emerging as promising candidates for the development of functional chemical metamaterials, combining the undisputed control over crystallography and surface chemistry achieved by synthetic nanochemistry with the scalability and versatility of colloidal self‐assembly strategies. In light of recent reports of colloidal plasmonic materials displaying high‐performing optical cavities, this Minireview discusses the use of this type of metamaterials in the specific context of non‐linear optical phenomena and non‐linear molecular spectroscopies. Our attention is focused on the opportunities and advantages that colloidal nanoparticles and self‐assembled plasmonic metasurfaces can bring to the table compared to more traditional nanofabrication strategies. Specifically, we believe that future work in this direction will express the full potential of non‐linear molecular spectroscopies to explore the chemical space, with a deeper understanding of plasmon‐molecule dynamics, plasmon‐mediated processes, and surface‐enhanced chemistry.

The characteristic narrow spectral features of surface lattice resonances emerge as great candidates for the rational design of optical nanocavities targeting enhanced light‐matter interaction, ultrasensitive detection, or efficient light‐energy conversion. Traditional fabrication of metal arrays involves thermal evaporation and annealing steps, limiting scalability and adaptability. In contrast, template‐assisted self‐assembly provides a high‐throughput all‐around approach for implementing colloidal plasmonic metasurfaces on a variety of different materials. Here, the use of pre‐synthesized silver nanoparticles is designed and tested for the construction of versatile lasing architectures. Plasmonic arrays are prepared directly on top of the gain media (a photoresist thin film doped with Rhodamine B), creating optical nanocavities with quality factors as high as 85. The proposed architecture circumvents the need for an index‐matching superstrate to promote the generation of collective resonances, leaving the plasmonic surface accessible for post‐assembly modification. Additionally, the angular dispersion of the metasurfaces is used to modify the angle of the lasing emission, achieving both normal and off‐normal lasing upon modification of the lattice parameter of the array. The results demonstrate how state‐of‐the‐art colloidal self‐assembly techniques offer a scalable and versatile alternative for the fabrication of plasmonic and photonic devices targeting advanced and non‐linear optical phenomena.

Plasmonic gold nanoparticles have been used increasingly in solid-state systems because of their applicability in fabricating novel sensors, heterogeneous catalysts, metamaterials, and thermoplasmonic substrates. While bottom-up colloidal syntheses take advantage of the chemical environment to control size, shape, composition, surface chemistry, and crystallography of the nanostructures precisely, it can be challenging to assemble nanoparticles rationally from suspension onto solid supports or within devices. In this Review, we discuss a powerful recent synthetic methodology, bottom-up in situ substrate growth, which circumvents time-consuming batch presynthesis, ligand exchange, and self-assembly steps by applying wet-chemical synthesis to form morphologically controlled nanostructures on supporting materials. First, we briefly introduce the properties of plasmonic nanostructures. Then we comprehensively summarize recent work that adds to the synthetic understanding of in situ geometrical and spatial control (patterning). Next, we briefly discuss applications of plasmonic hybrid materials prepared by in situ growth. Overall, despite the vast potential advantages of in situ growth, the mechanistic understanding of these methodologies remains far from established, providing opportunities and challenges for future research.