Deposition of Gold Nanotriangles in Large Scale Close-Packed Monolayers for X-ray-Based Temperature Calibration and SERS Monitoring of Plasmon-Driven Catalytic Reactions

Core-Shell Gold Nanoparticles@Pd-Loaded Covalent Organic Framework for In Situ Surface-Enhanced Raman Spectroscopy Monitoring of Catalytic Reactions

A core-shell nanostructure of gold nanoparticles@covalent organic framework (COF) loaded with palladium nanoparticles (AuNPs@COF-PdNPs) was designed for the rapid monitoring of catalytic reactions with surface-enhanced Raman spectroscopy (SERS). The nanostructure was prepared by coating the COF layer on AuNPs and then in situ synthesizing PdNPs within the COF shell. With the respective SERS activity and catalytic performance of the AuNP core and COF-PdNPs shell, the nanostructure can be directly used in the SERS study of the catalytic reaction processes. It was shown that the confinement effect of COF resulted in the high dispersity of PdNPs and outstanding catalytic activity of AuNPs@COF-PdNPs, thus improving the reaction rate constant of the AuNPs@COF-PdNPs-catalyzed hydrogenation reduction by 10 times higher than that obtained with Au/Pd NPs. In addition, the COF layer can serve as a protective shell to make AuNPs@COF-PdNPs possess excellent reusability. Moreover, the loading of PdNPs within the COF layer was found to be in favor of avoiding intermediate products to achieve a high total conversion rate. AuNPs@COF-PdNPs also showed great catalytic activities toward the Suzuki-Miyaura coupling reaction. Taken together, the proposed core-shell nanostructure has great potential in monitoring and exploring catalytic processes and interfacial reactions.

Internal standard optimization advances sensitivity and robustness of ratiometric detection method

We design a p-aminothiophenol (pATP) modified Au/ITO chip to determine nitrite ions in lake water by a ratiometric surface-enhanced Raman scattering (SERS) method based on nitrite ions triggering the transformation of pATP to p,p'-dimercaptoazobenzene (DMAB). Intriguingly, by using the SERS peak (at 1008 cm-1) from benzoic ring deforming as an internal standard instead of the traditional peak at 1080 cm-1, the detection sensitivity of the method was improved 10 times.

Two-Dimensional Superstructures from the Gas Phase: Directed Assembly of Copper-Sulfide Nanoplatelets

We report on a novel plasma-assisted approach for the deposition of free-standing two-dimensional superstructures via directed assembly of copper-sulfide nanoplatelets in the gas phase. For this, the copper-organic complex bis-[bis(N,N-diethyldithiocarbamato)-copper(II)] is thermally evaporated and transported into a capacitively coupled rf plasma to form two-dimensional nanoplatelets upon fragmentation. On a substrate, the highly anisotropic platelets are attached in a directed edge-to-edge configuration. We found that a high substrate temperature of 400 °C is necessary for the 2D vertical growth of copper sulfide. Using plasma reinforces the directional assembly and leads to nanowalls which are several micrometers high with the thickness of a single nanoplatelet. The morphology and crystallographic composition of the emerging superstructures were extensively investigated via scanning and transmission electron microscopy as well as electron diffraction. The data reveal the (010) plane to be the preferred axis for the arrangement of the nanoplatelets.

Plasmon Coupling and Efficient Charge Transfer in Rough-Surfaced Au Nanotriangles/MXene Hybrids as an Ultrasensitive Surface-Enhanced Raman Scattering Platform

The rational design of Raman substrate materials with prominent electromagnetic enhancement and charge transfer is quite important for surface-enhanced Raman scattering (SERS). Herein, an efficient SERS substrate based on two-dimensional ultrathin Ti₃C₂T _x MXene and rough-surfaced Au nanotriangles (NTs) was successfully prepared for efficient detection of organic molecules due to the synthetic effect of an optimized electromagnetic field and charge transfer. Uniform Au NTs with tunable surface roughness were controllably prepared by selectively depositing of Au on the smooth Au NTs. Due to the large surface area, tunable plasmon resonance, and abundant hotspots on the planar surface, the modified Au NTs showed much better SERS performance than initial Au NTs. By combination of the rough-surfaced Au NTs with MXene, the Ti₃C₂T _x /Au NT hybrids exhibited much better SERS performance than initial Au NTs and Au NTs with a rough surface. The detection limit is down to 10^-12 M, and the analytical enhancement factors reach 3.6 × 10⁹ (at 1174 cm^-1) on detecting crystal violet excited at 785 nm. This is because the strong plasmon coupling between the in-plane resonance of Au NTs and transversal plasmon resonance of Ti₃C₂T _x MXene around 785 nm can generate an intense interfacial electromagnetic field for amplifying SERS signals. Additionally, the efficient charge transfer between Au NTs, MXene, and molecules also plays an important role in enhancing the SERS performance. This work presents a new insight to develop high-performance SERS substrates based on plasmon.

A Review on Gold Nanotriangles: Synthesis, Self-Assembly and Their Applications

Gold nanoparticles (AuNPs) with interesting optical properties have attracted much attention in recent years. The synthesis and plasmonic properties of AuNPs with a controllable size and shape have been extensively investigated. Among these AuNPs, gold nanotriangles (AuNTs) exhibited unique optical and plasmonic properties due to their special triangular anisotropy. Indeed, AuNTs showed promising applications in optoelectronics, optical sensing, imaging and other fields. However, only few reviews about these applications have been reported. Herein, we comprehensively reviewed the synthesis and self-assembly of AuNTs and their applications in recent years. The preparation protocols of AuNTs are mainly categorized into chemical synthesis, biosynthesis and physical-stimulus-induced synthesis. The comparison between the advantages and disadvantages of various synthetic strategies are discussed. Furthermore, the specific surface modification of AuNTs and their self-assembly into different dimensional nano- or microstructures by various interparticle interactions are introduced. Based on the unique physical properties of AuNTs and their assemblies, the applications towards chemical biology and sensing were developed. Finally, the future development of AuNTs is prospected.

Large-Area Periodic Arrays of Atomically Flat Single-Crystal Gold Nanotriangles Formed Directly on Substrate Surfaces

The advancement of nanoenabled wafer-based devices requires the establishment of core competencies related to the deterministic positioning of nanometric building blocks over large areas. Within this realm, plasmonic single-crystal gold nanotriangles represent one of the most attractive nanoscale components but where the formation of addressable arrays at scale has heretofore proven impracticable. Herein, a benchtop process is presented for the formation of large-area periodic arrays of gold nanotriangles. The devised growth pathway sees the formation of an array of defect-laden seeds using lithographic and vapor-phase assembly processes followed by their placement in a growth solution promoting planar growth and threefold symmetric side-faceting. The nanotriangles formed in this high-yield synthesis distinguish themselves in that they are epitaxially aligned with the underlying substrate, grown to thicknesses that are not readily obtainable in colloidal syntheses, and present atomically flat pristine surfaces exhibiting gold atoms with a close-packed structure. As such, they express crisp and unambiguous plasmonic modes and form photoactive surfaces with highly tunable and readily modeled plasmon resonances. The devised methods, hence, advance the integration of single-crystal gold nanotriangles into device platforms and provide an overall fabrication strategy that is adaptable to other nanomaterials.

In situ SERS monitoring of plasmon-driven catalytic reaction on gap-controlled Ag nanoparticle arrays under 785 nm irradiation

Plasmon-enhanced photocatalysis has attracted considerable attention due to its low energy consumption and high energy throughput. Surface-enhanced Raman scattering (SERS) is a highly sensitive and label-free nondestructive tool to investigate plasmon-driven photocatalytic reactions. Herein, we present a facile method to fabricate gap-controlled Ag nanoparticle (NP) arrays with uniform and high-density distribution of hot spots, which can be employed as both efficient plasmonic photocatalysts and stable SERS platforms. The plasmon-driven catalytic reaction of 4-nitrobenzenethiol (4NBT), which transforms it into p, p'-dimercaptoazobenzene (DMAB), is detected by using an in situ SERS technique at the excited wavelength of 785 nm. According to the temperature and laser power density dependent photocatalytic reaction rates observed on the Ag NP arrays, we quantitatively determined that the reductive coupling of 4NBT is more likely to occur as the gap decreases. The finite-difference time-domain (FDTD) simulation results demonstrate that the plasmonic hot spots are significantly enhanced with a decrease in gap, which in turn reduces activation energy. The gap-controlled Ag NP arrays are efficient for both promotion and detection of plasmon-driven catalytic reactions, and may pave a pathway for implementing efficient plasmonic photocatalytic platforms.

Recent Advances in Metallic Nanoparticle Assemblies for Surface-Enhanced Spectroscopy

Robust and versatile strategies for the development of functional nanostructured materials often focus on assemblies of metallic nanoparticles. Research interest in such assemblies arises due to their potential applications in the fields of photonics and sensing. Metallic nanoparticles have received considerable recent attention due to their connection to the widely studied phenomenon of localized surface plasmon resonance. For instance, plasmonic hot spots can be observed within their assemblies. A useful form of spectroscopy is based on surface-enhanced Raman scattering (SERS). This phenomenon is a commonly used in sensing techniques, and it works using the principle that scattered inelastic light can be greatly enhanced at a surface. However, further research is required to enable improvements to the SERS techniques. For example, one question that remains open is how to design uniform, highly reproducible, and efficiently enhancing substrates of metallic nanoparticles with high structural precision. In this review, a general overview on nanoparticle functionalization and the impact on nanoparticle assembly is provided, alongside an examination of their applications in surface-enhanced Raman spectroscopy.

Surface Enhanced Raman Spectroscopy: Applications in Agriculture and Food Safety

Recent global warming has resulted in shifting of weather patterns and led to intensification of natural disasters and upsurges in pests and diseases. As a result, global food systems are under pressure and need adjustments to meet the change—often by pesticides. Unfortunately, such agrochemicals are harmful for humans and the environment, and consequently need to be monitored. Traditional detection methods currently used are time consuming in terms of sample preparation, are high cost, and devices are typically not portable. Recently, Surface Enhanced Raman Scattering (SERS) has emerged as an attractive candidate for rapid, high sensitivity and high selectivity detection of contaminants relevant to the food industry and environmental monitoring. In this review, the principles of SERS as well as recent SERS substrate fabrication methods are first discussed. Following this, their development and applications for agrifood safety is reviewed, with focus on detection of dye molecules, melamine in food products, and the detection of different classes of pesticides such as organophosphate and neonicotinoids.

The time-resolved hard X-ray diffraction endstation KMC-3 XPP at BESSY II

The time-resolved hard X-ray diffraction endstation KMC-3 XPP for optical pump/X-ray probe experiments at the electron storage ring BESSY II is dedicated to investigating the structural response of thin film samples and heterostructures after their excitation with ultrashort laser pulses and/or electric field pulses. It enables experiments with access to symmetric and asymmetric Bragg reflections via a four-circle diffractometer and it is possible to keep the sample in high vacuum and vary the sample temperature between ∼15 K and 350 K. The femtosecond laser system permanently installed at the beamline allows for optical excitation of the sample at 1028 nm. A non-linear optical setup enables the sample excitation also at 514 nm and 343 nm. A time-resolution of 17 ps is achieved with the `low-α' operation mode of the storage ring and an electronic variation of the delay between optical pump and hard X-ray probe pulse conveniently accesses picosecond to microsecond timescales. Direct time-resolved detection of the diffracted hard X-ray synchrotron pulses use a gated area pixel detector or a fast point detector in single photon counting mode. The range of experiments that are reliably conducted at the endstation and that detect structural dynamics of samples excited by laser pulses or electric fields are presented.

Decoding the kinetic limitations of plasmon catalysis: the case of 4-nitrothiophenol dimerization

Plasmon-mediated chemistry presents an intriguing new approach to photocatalysis. However, the reaction enhancement mechanism is not well understood. In particular, the relative importance of plasmon-generated hot charges and photoheating is strongly debated. In this article, we evaluate the influence of microscopic photoheating on the kinetics of a model plasmon-catalyzed reaction: the light-induced 4-nitrothiophenol (4NTP) to 4,4'-dimercaptoazobenzene (DMAB) dimerization. Direct measurement of the reaction temperature by nanoparticle Raman-thermometry demonstrated that the thermal effect plays a dominant role in the kinetic limitations of this multistep reaction. At the same time, no reaction is possible by dark heating to the same temperature. This shows that plasmon nanoparticles have the unique ability to enhance several steps of complex tandem reactions simultaneously. These results provide insight into the role of hot electron and thermal effects in plasmonic catalysis of complex organic reactions, which is highly important for the ongoing development of plasmon based photosynthesis.

The Effect of Surface Modification of Gold Nanotriangles for Surface-Enhanced Raman Scattering Performance

A surface modification of ultraflat gold nanotriangles (AuNTs) with different shaped nanoparticles is of special relevance for surface-enhanced Raman scattering (SERS) and the photo-catalytic activity of plasmonic substrates. Therefore, different approaches are used to verify the flat platelet morphology of the AuNTs by oriented overgrowth with metal nanoparticles. The most important part for the morphological transformation of the AuNTs is the coating layer, containing surfactants or polymers. By using well established AuNTs stabilized by a dioctyl sodium sulfosuccinate (AOT) bilayer, different strategies of surface modification with noble metal nanoparticles are possible. On the one hand undulated superstructures were synthesized by in situ growth of hemispherical gold nanoparticles in the polyethyleneimine (PEI)-coated AOT bilayer of the AuNTs. On the other hand spiked AuNTs were obtained by a direct reduction of Au3+ ions in the AOT double layer in presence of silver ions and ascorbic acid as reducing agent. Additionally, crumble topping of the smooth AuNTs can be realized after an exchange of the AOT bilayer by hyaluronic acid, followed by a silver-ion mediated reduction with ascorbic acid. Furthermore, a decoration with silver nanoparticles after coating the AOT bilayer with the cationic surfactant benzylhexadecyldimethylammonium chloride (BDAC) can be realized. In that case the ultraviolet (UV)-absorption of the undulated Au@Ag nanoplatelets can be tuned depending on the degree of decoration with silver nanoparticles. Comparing the Raman scattering data for the plasmon driven dimerization of 4-nitrothiophenol (4-NTP) to 4,4'-dimercaptoazobenzene (DMAB) one can conclude that the most important effect of surface modification with a 75 times higher enhancement factor in SERS experiments becomes available by decoration with gold spikes.

Gold Nanotriangles with Crumble Topping and their Influence on Catalysis and Surface-Enhanced Raman Spectroscopy

By adding hyaluronic acid (HA) to dioctyl sodium sulfosuccinate (AOT)-stabilized gold nanotriangles (AuNTs) with an average thickness of 7.5±1 nm and an edge length of about 175±17 nm, the AOT bilayer is replaced by a polymeric HA-layer leading to biocompatible nanoplatelets. The subsequent reduction process of tetrachloroauric acid in the HA-shell surrounding the AuNTs leads to the formation of spherical gold nanoparticles on the platelet surface. With increasing tetrachloroauric acid concentration, the decoration with gold nanoparticles can be tuned. SAXS measurements reveal an increase of the platelet thickness up to around 14.5 nm, twice the initial value of bare AuNTs. HRTEM micrographs show welding phenomena between densely packed particles on the platelet surface, leading to a crumble formation while preserving the original crystal structure. Crumbles crystallized on top of the platelets enhance the Raman signal by a factor of around 20, and intensify the plasmon-driven dimerization of 4-nitrothiophenol (4-NTP) to 4,4'-dimercaptoazobenzene in a yield of up to 50 %. The resulting crumbled nanotriangles, with a biopolymer shell and the absorption maximum in the second window for in vivo imaging, are promising candidates for biomedical sensing.

Spiked gold nanotriangles: formation, characterization and applications in surface-enhanced Raman spectroscopy and plasmon-enhanced catalysis

We show the formation of metallic spikes on the surface of gold nanotriangles (AuNTs) by using the same reduction process which has been used for the synthesis of gold nanostars. We confirm that silver nitrate operates as a shape-directing agent in combination with ascorbic acid as the reducing agent and investigate the mechanism by dissecting the contribution of each component, i.e., anionic surfactant dioctyl sodium sulfosuccinate (AOT), ascorbic acid (AA), and AgNO₃. Molecular dynamics (MD) simulations show that AA attaches to the AOT bilayer of nanotriangles, and covers the surface of gold clusters, which is of special relevance for the spike formation process at the AuNT surface. The surface modification goes hand in hand with a change of the optical properties. The increased thickness of the triangles and a sizeable fraction of silver atoms covering the spikes lead to a blue-shift of the intense near infrared absorption of the AuNTs. The sponge-like spiky surface increases both the surface enhanced Raman scattering (SERS) cross section of the particles and the photo-catalytic activity in comparison with the unmodified triangles, which is exemplified by the plasmon-driven dimerization of 4-nitrothiophenol (4-NTP) to 4,4′-dimercaptoazobenzene (DMAB).

We show the formation of metallic spikes on the surface of gold nanotriangles (AuNTs) by using the same reduction process which has been used for the synthesis of gold nanostars leading to a higher SERS enhancement by a factor of 75.

Plasmon-Mediated Synthesis of Periodic Arrays of Gold Nanoplates Using Substrate-Immobilized Seeds Lined with Planar Defects

The seed-mediated growth of noble metal nanostructures with planar geometries requires the use of seeds lined with parallel stacking faults so as to provide a break in symmetry in an otherwise isotropic metal. Although such seeds are now routinely synthesized using colloidal pathways, equivalent pathways have not yet been reported for the fabrication of substrate-based seeds with the same internal defect structures. The challenge is not merely to form seeds with planar defects but to do so in a deterministic manner so as to have stacking faults that only run parallel to the substrate surface while still allowing for the lithographic processes needed to regulate the placement of seeds. Here, we demonstrate substrate-imposed epitaxy as a viable synthetic control able to induce planar defects in Au seeds while simultaneously dictating nanostructure in-plane alignment and crystallographic orientation. The seeds, which are formed in periodic arrays using nanoimprint lithography in combination with a vapor-phase assembly process, are subjected to a liquid-phase plasmon-mediated synthesis that uses light as an external stimuli to drive a reaction yielding periodic arrays of hexagonal Au nanoplates. These achievements not only represent the first of their kind demonstrations but also advance the possibility of integrating wafer-based technologies with a rich and exciting nanoplate colloidal chemistry.

A Journey Through the Landscapes of Small Particles in Binary Colloidal Assemblies: Unveiling Structural Transitions from Isolated Particles to Clusters upon Variation in Composition

Two-dimensional (2D) amorphous binary colloidal assemblies composed of particles of two different sizes are characterized by the loss of hexagonal close-packing for larger particles, occurring when the size ratio between small (S) and large (L) particles dSdL exceeds a certain threshold value. For moderately low particle number ratios NSNL large particles still retain a denser arrangement with transitions from hexagonal symmetry to the coexistence of different types of symmetries as NSNL progressively departs from 0 to higher values. On the other hand, small particles reveal sparser arrangements: shape identification and quantification of structural transitions in small particle arrangements appear particularly challenging. In this article, we investigate their shapes and transitions for amorphous binary colloidal particles assembled at the air/water interface. For the quantitative characterization of the evolution in particle arrangements for NSNL variable between 0.5 and 2, we develop an innovative procedure for morphological analysis, combining Minkowski functionals, Voronoi diagrams and ad hoc techniques to recognize and classify specific features. Such a powerful approach has revealed a wide variety of landscapes featuring isolated particles, dimers, chains, small clusters evolving with the colloidal suspension composition. Our method can be applied to the analysis of spatial configurations of sparse colloidal patterns obtained in different conditions.

The importance of plasmonic heating for the plasmon-driven photodimerization of 4-nitrothiophenol

Metal nanoparticles form potent nanoreactors, driven by the optical generation of energetic electrons and nanoscale heat. The relative influence of these two factors on nanoscale chemistry is strongly debated. This article discusses the temperature dependence of the dimerization of 4-nitrothiophenol (4-NTP) into 4,4′-dimercaptoazobenzene (DMAB) adsorbed on gold nanoflowers by Surface-Enhanced Raman Scattering (SERS). Raman thermometry shows a significant optical heating of the particles. The ratio of the Stokes and the anti-Stokes Raman signal moreover demonstrates that the molecular temperature during the reaction rises beyond the average crystal lattice temperature of the plasmonic particles. The product bands have an even higher temperature than reactant bands, which suggests that the reaction proceeds preferentially at thermal hot spots. In addition, kinetic measurements of the reaction during external heating of the reaction environment yield a considerable rise of the reaction rate with temperature. Despite this significant heating effects, a comparison of SERS spectra recorded after heating the sample by an external heater to spectra recorded after prolonged illumination shows that the reaction is strictly photo-driven. While in both cases the temperature increase is comparable, the dimerization occurs only in the presence of light. Intensity dependent measurements at fixed temperatures confirm this finding.

A simple one-step procedure to synthesise gold nanostars in concentrated aqueous surfactant solutions

Due to the enhanced electromagnetic field at the tips of metal nanoparticles, the spiked structure of gold nanostars (AuNSs) is promising for surface-enhanced Raman scattering (SERS). Therefore, the challenge is the synthesis of well designed particles with sharp tips. The influence of different surfactants, i.e., dioctyl sodium sulfosuccinate (AOT), sodium dodecyl sulfate (SDS), and benzylhexadecyldimethylammonium chloride (BDAC), as well as the combination of surfactant mixtures on the formation of nanostars in the presence of Ag⁺ ions and ascorbic acid was investigated. By varying the amount of BDAC in mixed micelles the core/spike-shell morphology of the resulting AuNSs can be tuned from small cores to large ones with sharp and large spikes. The concomitant red-shift in the absorption toward the NIR region without losing the SERS enhancement enables their use for biological applications and for time-resolved spectroscopic studies of chemical reactions, which require a permanent supply with a fresh and homogeneous solution. HRTEM micrographs and energy-dispersive X-ray (EDX) experiments allow us to verify the mechanism of nanostar formation according to the silver underpotential deposition on the spike surface in combination with micelle adsorption.

Due to the enhanced electromagnetic field at the tips of metal nanoparticles, the spiked structure of gold nanostars (AuNSs) is promising for surface-enhanced Raman scattering (SERS).

Binary "island" shaped arrays with high-density hot spots for surface-enhanced Raman scattering substrates

We have demonstrated a facile and low-cost approach for the fabrication of binary "island" shaped arrays (BISA) with high-density hot spots as reproducible surface-enhanced Raman scattering (SERS) substrates by depositing a self-assembled monolayer Au nanoparticle (AuNP) film with small gaps onto a two-dimensional (2D) silica microsphere opal structure. By varying the size of silica spheres, the SERS performance of the BISA substrate with an enhancement factor (EF) of 3.74 × 10¹⁰ magnitude and the corresponding signal intensity deviation of below 8% using 770 nm silica sphere arrays were achieved. Compared with the assembled monolayer AuNP film on a planar substrate, the BISA enabled the installation of more AuNPs as a source of hot spots due to the undulation of morphology on the nanoscale within the designated laser-illumination area. In addition, a finite-difference time-domain (FDTD) simulation suggested that the BISA structure provided geometric conditions for increasing the intensity of the formed hot spots, and the strong periodic electric fields on the BISA are located not only in the gap between adjacent AuNPs, but also along the boundary of the neighboring island of silica spheres. Surface plasmon-decayed hot carriers (hot electrons and hot holes) from AuNPs can be applied in the field of energy conversion (i.e., photocatalysis), integrated with the SERS as a sensitive optical indicator to accurately monitor the catalytic reaction process. Furthermore, we examined the catalytic reaction process of the dimerization of 4-ATP into DMAB and found that photocatalytic activity could be tuned by changing the size of silica spheres. This study provides a new design route for the fabrication of the SERS platform with high sensitivity and reproducibility to detect molecules or improve catalyst efficiency.

Plasmon-Mediated Drilling in Thin Metallic Nanostructures

Thin and ultraflat conductive surfaces are of particular interest to use as substrates for tip-enhanced spectroscopy applications. Tip-enhanced spectroscopy exploits the excitation of a localized surface plasmon resonance mode at the apex of a metallized atomic force microscope tip, confining and enhancing the local electromagnetic field by several orders of magnitude. This allows for nanoscale mapping of the surface with high spatial resolution and surface sensitivity, as demonstrated when coupled to local Raman measurements. In gap-mode tip-enhanced spectroscopy, the specimen of interest is deposited onto a flat metallic surface and probed by a metallic tip, allowing for further electromagnetic confinement and subsequent enhancement. We investigate here a geometry where a gold tip is used in conjunction with a silver nanoplate, thus forming a heterometallic platform for local enhancement. When irradiated, a plasmon-mediated reaction is triggered at the tip-substrate junction due to the enhanced electric field and the transfer of hot electrons from the tip to the nanoplate. This resulting nanoscale reaction appears to be sufficient to ablate the thin silver plates even under weak laser intensity. Such an approach may be further exploited for patterning metallic nanostructures or photoinduced chemical reactions at metal surfaces.

Plasmonic Nanowires for Wide Wavelength Range Molecular Sensing

In this paper, we propose the use of a standing nanowires array, constituted by plasmonic active gold wires grown on iron disks, and partially immersed in a supporting alumina matrix, for surface-enhanced Raman spectroscopy applications. The galvanic process was used to fabricate nanowires in pores of anodized alumina template, making this device cost-effective. This fabrication method allows for the selection of size, diameter, and spatial arrangement of nanowires. The proposed device, thanks to a detailed design analysis, demonstrates a broadband plasmonic enhancement effect useful for many standard excitation wavelengths in the visible and NIR. The trigonal pores arrangement gives an efficiency weakly dependent on polarization. The devices, tested with 633 and 830 nm laser lines, show a significant Raman enhancement factor, up to around 6 × 10⁴, with respect to the flat gold surface, used as a reference for the measurements of the investigated molecules.

A new route to gold nanoflowers

Catanionic vesicles spontaneously formed by mixing the anionic surfactant bis(2-ethylhexyl) sulfosuccinate sodium salt with the cationic surfactant cetyltrimethylammonium bromide were used as a reducing medium to produce gold clusters, which are embedded and well-ordered into the template phase. The gold clusters can be used as seeds in the growth process that follows by adding ascorbic acid as a mild reducing component. When the ascorbic acid was added very slowly in an ice bath round-edged gold nanoflowers were produced. When the same experiments were performed at room temperature in the presence of Ag⁺ ions, sharp-edged nanoflowers could be synthesized. The mechanism of nanoparticle formation can be understood to be a non-diffusion-limited Ostwald ripening process of preordered gold nanoparticles embedded in catanionic vesicle fragments. Surface-enhanced Raman scattering experiments show an excellent enhancement factor of 1.7 · 10⁵ for the nanoflowers deposited on a silicon wafer.

Toxicological investigations of "naked" and polymer-entrapped AOT-based gold nanotriangles

Negatively charged ultrathin gold nanotriangles (AuNTs) were synthesized in a vesicular dioctyl sodium sulfosuccinate (AOT)/phospholipid-based template phase. These "naked" AuNTs with localized surface plasmon resonances in the NIR region at about 1300 nm and special photothermal properties are of particular interest for imaging and hyperthermia of cancerous tissues. For these kinds of applications the toxicity and the cellular uptake of the AuNTs is of outstanding importance. Therefore, this study focuses on the toxicity of "naked" AOT-stabilized AuNTs compared to polymer-coated AuNTs. Polymeric coating consisted of non-modified hyperbranched poly(ethyleneimine) (PEI), maltose-modified poly(ethyleneimine) (PEI-Mal) and heparin. The toxicological experiments were carried out with two different cell lines (embryonic kidney carcinoma cell line HEK293T and NK-cell leukemia cell line YTS). This study revealed that the heparin-coating of AuNTs improved biocompatibility by a factor of 50 when compared to naked AuNTs. Of note, the highest nontoxic concentration of the AuNTs coated with PEI and PEI-Mal is drastically decreased. Overall, this is mainly triggered by the different surface charges of polymeric coatings. Therefore, AuNTs coated with heparin were selected to carry out uptake studies. Their promising high biocompatibility and cellular uptake may open future studies in the field of biomedical applications.

Vapor-Phase Nanopatterning of Aminosilanes with Electron Beam Lithography: Understanding and Minimizing Background Functionalization

Electron beam lithography (EBL) is a highly precise, serial method for patterning surfaces. Positive tone EBL resists enable patterned exposure of the underlying surface, which can be subsequently functionalized for the application of interest. In the case of widely used native oxide-capped silicon surfaces, coupling an activated silane with electron beam lithography would enable nanoscale chemical patterning of the exposed regions. Aminoalkoxysilanes are extremely useful due to their reactive amino functionality but have seen little attention for nanopatterning silicon surfaces with an EBL resist due to background contamination. In this work, we investigated three commercial positive tone EBL resists, PMMA (950k and 495k) and ZEP520A (57k), as templates for vapor-phase patterning of two commonly used aminoalkoxysilanes, 3-aminopropyltrimethoxysilane (APTMS) and 3-aminopropyldiisopropylethoxysilane (APDIPES). The PMMA resists were susceptible to significant background reaction within unpatterned areas, a problem that was particularly acute with APTMS. On the other hand, with both APTMS and APDIPES exposure, unpatterned regions of silicon covered by the ZEP520A resist emerged pristine, as shown both with SEM images of the surfaces of the underlying silicon and through the lack of electrostatically driven binding of negatively charged gold nanoparticles. The ZEP520A resist allowed for the highly selective deposition of these alkoxyaminosilanes in the exposed areas, leaving the unpatterned areas clean, a claim also supported by contact angle measurements with four probe liquids and X-ray photoelectron spectroscopy (XPS). We investigated the mechanistic reasons for the stark contrast between the PMMA resists and ZEP520A, and it was found that the efficacy of resist removal appeared to be the critical factor in reducing the background functionalization. Differences in the molecular weight of the PMMA resists and the resulting influence on APTMS diffusion through the resist films are unlikely to have a significant impact. Area-selective nanopatterning of 15 nm gold nanoparticles using the ZEP520A resist was demonstrated, with no observable background conjugation noted in the unexposed areas on the silicon surface by SEM.

Undulated Gold Nanoplatelet Superstructures: In Situ Growth of Hemispherical Gold Nanoparticles onto the Surface of Gold Nanotriangles

Negatively charged flat gold nanotriangles, formed in a vesicular template phase and separated by an AOT-micelle-based depletion flocculation, were reloaded by adding a cationic polyelectrolyte, that is, a hyperbranched polyethylenimine (PEI). Heating the system to 100 °C in the presence of a gold chloride solution, the reduction process leads to the formation of gold nanoparticles inside the polymer shell surrounding the nanoplatelets. The gold nanoparticle formation is investigated by UV-vis spectroscopy, small-angle X-ray scattering, and dynamic light scattering measurements in combination with transmission electron microscopy. Spontaneously formed gold clusters in the hyperbranched PEI shell with an absorption maximum at 350 nm grow on the surface of the nanotriangles as hemispherical particles with diameters of ∼6 nm. High-resolution micrographs show that the hemispherical gold particles are crystallized onto the {111} facets on the bottom and top of the platelet as well as on the edges without a grain boundary. Undulated gold nanoplatelet superstructures with special properties become available, which show a significantly modified performance in SERS-detected photocatalysis regarding both reactivity and enhancement factor.

Seeded Growth Synthesis of Gold Nanotriangles: Size Control, SAXS Analysis, and SERS Performance

We studied the controlled growth of triangular prismatic Au nanoparticles with different beveled sides for surface-enhanced Raman spectroscopy (SERS) applications. First, in a seedless synthesis using 3-butenoic acid (3BA) and benzyldimethylammonium chloride (BDAC), gold nanotriangles (AuNTs) were synthesized in a mixture with gold nanooctahedra (AuNOCs) and separated by depletion-induced flocculation. Here, the influence of temperature, pH, and reducing agent on the reaction kinetics was initially investigated by UV-vis and correlated to the size and yield of AuNT seeds. In a second step, the AuNT size was increased by seed-mediated overgrowth with Au. We show for the first time that preformed 3BA-synthesized AuNT seeds can be overgrown up to a final edge length of 175 nm and a thickness of 80 nm while maintaining their triangular shape and tip sharpness. The NT morphology, including edge length, thickness, and tip rounding, was precisely characterized in dispersion by small-angle X-ray scattering and in dry state by transmission electron microscopy and field-emission scanning electron microscopy. For sensor purposes, we studied the size-dependent SERS performance of AuNTs yielding analytical enhancement factors between 0.9 × 10⁴ and 5.6 × 10⁴ and nanomolar limit of detection (10^-8-10^-9 M) for 4-mercaptobenzoic acid and BDAC. These results confirm that the 3BA approach allows the fabrication of AuNTs in a whole range of sizes maintaining the NT morphology. This enables tailoring of localized surface plasmon resonances between 590 and 740 nm, even in the near-infrared window of a biological tissue, for use as colloidal SERS sensing agents or for optoelectronic applications.

Nanoporous Au-Ag shell with fast kinetics: integrating chemical and plasmonic catalysis

Nanoporous noble metals and alloys are widely utilized as efficient catalysts, because they have high surface-to-volume ratios for sufficient active sites and induce molecule polarization through plasmon excitation as well. Herein, we demonstrate one approach to fabricate nanoporous Au-Ag shell. Such material represents the dual functions serving as efficient catalysts and high-performance surface-enhanced Raman scattering substrate. In situ spectrum acquisition can track the conversion of p-nitrothiophenol to 4, 4'-dimercapto-azobenzene at ambient temperature. In particular, as a result of chemical catalysis of Ag elements and strong plasmon-molecule coupling, catalytic kinetics of nanoporous Au-Ag shell is 79.2-123.8 times faster than Au nanoparticles (NPs), and 2.2-3.3 times faster than Ag NPs. This investigation offers a route to design superior catalysts to integrate chemical and plasmonic catalysis.