Surface-enhanced Raman scattering from individual au nanoparticles and nanoparticle dimer substrates

Nanogap-Rich TiO2 Film for 2000-Fold Field Enhancement with High Reproducibility

Titanium dioxide (TiO₂) is a crucial semiconductor for photocatalysts, solar cells, hydrogen evolution reactions, and antivirus agents. The properties and performances of these applications can improve significantly if the integrated TiO₂ acts as a light harvester through a large field enhancement. This study investigates the electromagnetic field enhancement of a nanogap-rich TiO₂ film with a large area, prepared by a facile dry process at room temperature. Herein, the loading pressure is applied to the TiO₂ particles for closely packing them in the film. The field enhancement, as a function of the loading pressure, is explored from the fluorescence intensity enhancement of a dye molecule. An average enhancement factor >2000 is achieved, which is a remarkable record for semiconductors. Furthermore, the reproducibility is significant; the relative standard deviation value is small (∼4%). Calculations were performed using the finite-difference-time-domain method. A nanogap of 5 nm yields the highest EF for triangular-prism TiO₂ particles.

Amplifying the excited state chirality through self-assembly and subsequent enhancement via plasmonic silver nanowires

The development of circularly polarized luminescent materials with a large luminescence dissymmetry factor (g_lum) is continuing to be a big challenge. Here, we present a general approach for amplifying circular polarization of circularly polarized luminescence (CPL) through intergrating molecular self-assembly and surface plasmon resonance (SPR). Molecular self-assembly could amplify the CPL performance. Subsequently, the composites built of nanoassemblies and achiral silver nanowires (AgNWs) show intense CPL activity with an amplified g_lum value. By applying an external magnetic field, the CPL activity of the nanoassemblies/AgNWs composites has been significantly enhanced, confirming a plasmon-enhanced circular polarization. Our design strategy based on SPR-enhanced circular polarization of the chiral emissive systems suggests that combining plasmonic nanomaterials with chiral organic materials could aid in the development of novel CPL active nanomaterials.

Selectively enhanced Raman/fluorescence spectra in photonic-plasmonic hybrid structures

The manipulation of the interaction between molecules and photonic-plasmonic hybrid structures is critical for the application of surface-enhanced spectroscopy (SES). Herein, we report a study on the mode coupling mechanism and SES performance in a typical optoplasmonic system constructed with a polystyrene microsphere (PS MS) resonator and gold nanoparticles (Au NPs). The mode coupling mechanism was found to be closely dependent on the relative positions of PS MS, Au NPs, and molecules in the optoplasmonic system, based on which selectively enhanced Raman and fluorescence signals of molecules can be realized via the collaboration of enhancement and quenching channels of the PS MS and Au NPs. We demonstrate two arrangements of the photonic-plasmonic hybrid structure, which can support fluorescence signals with sharp whispering-gallery modes and apparently enhanced Raman signals with relatively low detection limits and good robustness, respectively.

Continuous angular control over anisotropic photoemission from isotropic gold nanoshells

A variety of applications rely on the efficient generation of hot carriers within metal nanoparticles and charge transfer to surrounding molecules or materials. The optimization of such processes requires a detailed understanding of excited carrier spatial, temporal, and momentum distributions, which also leads to opportunities for active optical control over hot carrier dynamics on nanometer and femtosecond scales. Such capabilities are emerging in nanoplasmonic systems and typically rely on tuning optical polarization and/or frequency to selectively excite one or more discrete hot spots defined by the particle geometry. Here, we introduce a unique case in which hot electron excitation and emission distributions can instead be continuously controlled via linear laser polarization in the azimuthal plane of a gold nanoshell supported on a substrate. In this configuration, it is the laser field that breaks the azimuthal symmetry of the supported nanoshell and determines the plasmonic field distribution. Using angle-resolved photoelectron velocity map imaging, we find that the hot electrons are predominantly emitted orthogonal to the nanoshell dipolar surface plasmon resonance axis defined by the laser polarization. Furthermore, such anisotropic emission is only observed for nanoshells, while solid gold nanospheres are found to be isotropic emitters. We show that all of these effects are recapitulated via simulation of the plasmonic electric field distributions within the nanoparticle volume and ballistic Monte Carlo modeling of the hot electron dynamics. These results demonstrate a highly predictive level of understanding of the underlying physics and possibilities for ultrafast spatiotemporal control over hot carrier dynamics.

Biosensing based on surface-enhanced Raman spectroscopy as an emerging/next-generation point-of-care approach for acute myocardial infarction diagnosis

Cardiovascular disease is a major global health issue. In particular, acute myocardial infarction (AMI) requires urgent attention and early diagnosis. The use of point-of-care diagnostics has resulted in the improved management of cardiovascular disease, but a major drawback is that the performance of POC devices does not rival that of central laboratory tests. Recently, many studies and advances have been made in the field of surface-enhanced Raman scattering (SERS), including the development of POC biosensors that utilize this detection method. Here, we present a review of the strengths and limitations of these emerging SERS-based biosensors for AMI diagnosis. The ability of SERS to multiplex sensing against existing POC detection methods are compared and discussed. Furthermore, SERS calibration-free methods that have recently been explored to minimize the inconvenience and eliminate the limitations caused by the limited linear range and interassay differences found in the calibration curves are outlined. In addition, the incorporation of artificial intelligence (AI) in SERS techniques to promote multivariate analysis and enhance diagnostic accuracy are discussed. The future prospects for SERS-based POC devices that include wearable POC SERS devices toward predictive, personalized medicine following the Fourth Industrial Revolution are proposed.

Simulation-directed amplifiable nanoparticle enhanced quantitative scattering assay under low magnification dark field microscopy

Nanoparticle-enhanced assays read by high-magnification dark-field microscopy require time-intensive analysis methods subject to selection bias, which can be resolved by using low magnification dark-field assays (LMDFA), at the cost of reduced sensitivity. We have simulated and experimentally validated a tunable linker-based signal amplification strategy yielding 6-fold enhanced LMDFA sensitivity.

In situ monitoring of silver adsorption on assembled gold nanorods by surface-enhanced Raman scattering

Self-assembly of metal nanocrystals is able to create a gap of sub-nanometer distance for concentrating incoming light by the strong coupling of surface plasmon resonance, known as a 'hot spot'. Although the plasmonic property of silver is better than other metals in the visible range, the superior Raman enhancement of silver compared to gold is still under debate. To provide direct evidence, in this work, we studied the silver adsorption on assembled gold nanorods (AuNRs) using in situ surface-enhanced Raman scattering (SERS) measurements. The self-assembled AuNR multimers were used as the SERS substrate, where the 4-mercaptophenol (MPh) molecules in our experiment played dual roles as both probe molecules for the Raman scattering and linking molecules for the AuNR assembly in a basic environment. Silver atoms were adsorbed on the surface of gold nanorod assemblies by reduction of Ag⁺ anions. The stability of the adsorbed silver was guaranteed by the basic environment. We monitored the SERS signal during the silver adsorption with a home-built in situ Raman spectroscope, which was synchronized by recording the UV-vis absorption spectra of the reaction solution to instantly quantify the plasmonic effect of the silver adsorption. Although a minor change was found in the plasmonic resonance wavelength or intensity, the measured SERS signal at specific modes faced a sudden increase by 2.1 folds during the silver adsorption. The finite element method (FEM) simulation confirmed that the silver adsorption corresponding to the plasmonic resonance variation gave little change to the electric field enhancement. We attributed the mode-specific enhancement mechanism of the adsorption of silver to the chemical enhancement from charge transfer (CT) for targeting molecules with a specific orientation. Our findings provided new insights to construct SERS substrates with higher enhancement factor (EF), which hopefully would encourage new applications in the field of surface-enhanced optical spectroscopies.

Correlation between Near-Field Enhancement and Dephasing Time in Plasmonic Dimers

Near-field enhancement and dephasing time play critical roles in several applications of localized surface plasmon resonance. Here, using an example gold dimer system, we reveal the correlation between the near-field enhancement and dephasing time via time-resolved photoemission electron microscopy. Compared with isolated particles, dimers with small gap sizes show stronger near-field enhancement and shorter dephasing times. These results are well reproduced by numerical simulations and further explained by a coupled dipole approximation model. The roles of near- and far-field coupling and plasmon localization in balancing near-field enhancement and dephasing time are also unveiled.

SERS detection of sodium thiocyanate and benzoic acid preservatives in liquid milk using cysteamine functionalized core-shelled nanoparticles

A cysteamine functionalized core shelled nanoparticles (Au@Ag-CysNPs) was presented for simultaneous and rapid detection of sodium thiocyanate (STC) and benzoic acid (BA) preservatives in liquid milk using surface-enhanced Raman spectroscopy (SERS) technique. A spectrum covering 350-2350 cm^-1 region was selected to detect STC with concentrations ranging from 0.5 to 10 mg/L and BA with concentrations ranging from 15 to 240 mg/L in milk samples. Characterization of nanoparticles using high-resolution TEM confirmed that the successful synthesis of Au@AgNPs with core (gold) size of 28 nm and shell (silver) thickness of about 5 nm was grafted with 120 μL of 0.1 nM cysteamine hydrochloride. Results showed that Au@Ag-CysNPs could be used to detect STC up to 0.03 mg/L with a limit of quantification (LOQ) of 0.039 mg/L and a coefficient of determination (R²) of 0.9833 in the milk sample. For detecting BA, it could be screened up to 9.8 mg/L with LOQ of 10.2 mg/L and R² of 0.9903. The proposed substrate was also highly sensitive and the employed method involved only minor sample pretreatment steps. It is thus hoped that the new substrate could be used in the screening of prohibited chemicals in complex food matrices in future studies.

Regioselective thiocyanation of corroles and the synthesis of gold nanoparticle-corrole assemblies

Herein we demonstrate a synthetic protocol for the regioselective thiocyanation of corroles. To the best of our knowledge, thiocyanato appended corrole has never been reported earlier. The resulting thiocyanato appended corrole turned out to be a good corrole based precursor for the facile synthesis of thiol protected gold nanoparticles (Au NPs). The ligand system acts as a good bidentate framework and passivates the gold surface. A strong electronic interaction between the corrole and the gold nanoparticles is manifested by their unique photo physical properties and it also confirms that the binding through β-substitutions has a more pronounced effect even though the corrole rings are face-off to the gold surface.

Unveiling the effect of electron tunneling on the plasmonic resonance of closely spaced gold particles

Recent experimental techniques enable the nanoparticles' (NPs) ensembles to be made with an angstrom-level of interparticle gap widths. The theoretical description of their optical properties becomes more challenging because of the nonnegligible quantum mechanism (QM) effects such as electron tunneling and nonlocal screening. To demonstrate the microscopic mechanism of QM effects and quantitatively elucidate their impact on a variety of surface plasmon resonance (SPR) models of gold particle oligomers, in this work we performed a theoretical study on closely spaced Au cluster dimers and NP oligomers by the first-principles approach, and the classical or quantum-corrected electromagnetic models (CEM or QCM), respectively. Through the first-principles calculation on a series of Au_N dimers with different interparticle distances d and N, we depicted the variation of the possibility of direct electron tunneling (DET) across the junction constructed by two nearest NPs with d and N, and found that it exactly follows an exponential decay and the decay rate linearly varies with 1/N. The impact of the QM effect on the SPR models excited along the dimer axis is much more profound than those perpendicular to the dimer axis. CEM/QCM calculations on strongly coupled NP dimers and symmetric and asymmetric trimers demonstrated the evolution of their optical properties with variable NP sizes, gap separations and light polarization, as well as the QM effect on the major SPR modes. The side-by-side comparison between the results from time-dependent density functional theory and CEM/QCM models sheds light on understanding the origin of a variety of SPR models of gold NP oligomers and the QM effect on those modes, and makes a connection between the calculations of small cluster and large NP oligomers.

Direct phase mapping of the light scattered by single plasmonic nanoparticles

In this work, we present a novel technique to directly measure the phase shift of the optical signal scattered by single plasmonic nanoparticles in a diffraction-limited laser focus. We accomplish this by equipping an inverted confocal microscope with a Michelson interferometer and scanning single nanoparticles through the focal volume while recording for each pixel interferograms of the scattered and a reference wave. For the experiments, lithographically prepared gold nanorods were used, since their plasmon resonances can be controlled via their aspect ratio. We have developed a theoretical model for image formation in confocal scattering microscopy for nanoparticles considerably smaller than the diffraction limited focus. We show that the phase shift observed for particles with different longitudinal particle plasmon resonances can be well explained by the harmonic oscillator model. The direct measurement of the phase shift can further improve the understanding of the elastic scattering of individual gold nanoparticles with respect to their plasmonic properties.

Nanoscale plasmonic phase sensor

Using the localized surface plasmon resonance (LSPR) of gold nanoparticles for sensing applications has attracted considerable interest, since it can be very sensitive, even down to a single molecule, and selective for a specific analyte molecule with a suitable surface modification. LSPR sensing is usually based on the wavelength shift of the LSPR or a Fano resonance. Here, we present a new experimental approach based on the phase of the light scattered by a single gold nanoparticle by equipping a confocal microscope with an additional interferometer arm similar to a Michelson interferometer. The detected phase depends on the shape of the nanoparticle and the refractive index of the surrounding medium and can even be detected for off-resonant excitation. This can be used as a new and sensitive detection method in LSPR sensing, allowing the detection of changes to the local refractive index or the binding of molecules to the nanoparticle surface.

Ag@BiOCl super-hydrophobic nanostructure for enhancing SERS detection sensitivity

Surface-enhanced Raman scattering (SERS) has received widespread attention in the rapid detection of trace substances. The super-hydrophobic surface of structures has a significant impact on improving SERS performance. Usually a low concentration of objective molecules is randomly distributed in a large area on a non-hydrophobic SERS substrate, resulting in the Raman signals of the molecules not being easily detected. As a solution, a super-hydrophobic surface can gather a large number of probe molecules around the plasmon hot spots to effectively improve Raman SERS detection sensitivity. In this work, a chloride super-hydrophobic surface is fabricated, for the first time, by a simple and low-cost method of combining surface hydrophobic structures with surface modification. The dispersed and uniform hierarchical Ag@BiOCl nanosheet (Ag@BiOCl NSs) substrate has a higher surface-to-volume ratio and rich nano-gap. Such a chip with a high static contact angle of 157.4° exhibits a Raman signal detection limit of R6G dyes up to 10⁻⁹ M and an enhancement factor up to 10⁷. This SERS chip with a super-hydrophobic surface offers great potential in practical applications owing to its simple fabricating process, low cost, large area, and high sensitivity.

This large-area hierarchical Ag@BiOCl NSs SERS chip with a super-hydrophobic surface offers a great advantage in further enhancing SERS detection sensitivity.

Super Stability of Ag Nanoparticle in Crystalline Lamellar (Lc) Liquid Crystal Matrix at Different pH Environment

The symmetry concept in this paper is related to the natural self-assembly of noble metal nanoparticles in the long range periodic structure of liquid crystal (LC). The current study deliberates the effect of pH on the stability of nanoparticles (NPs) in the lamellar phase of a lyotropic LC environment. The LC was prepared by the mass ratio 0.33:0.22:0.45 for (HDTABr):1-pentanol:water. The LC containing silver nanoparticles (AgNPs) was prepared by replacing the water with Ag solution. The AgNPs were produced by the in situ preparation method in LC. The solution of AgNPs-LC was varied at different pH. The absorption intensities were determined by using ultra-violet spectroscopy (UV-vis). The surface potential and hydrodynamic particle size were determined by using Zeta-potential (measurements). The surface enhanced Raman spectroscopy (SERS) was carried out to enhance the Raman signals of 4-aminobenzenethiol (4-ABT) deposited onto AgNPs as substrate. It is found that all characterizations exhibited super stability for AgNPs dispersed in LC at pH = 3 to 12 with the optimum stability at pH = 5–6. The remarkable stability of NPs is an important indicator of the various applications in nanotechnology and nanoscience fields.

Multicomponent Plasmonic Nanoparticles: From Heterostructured Nanoparticles to Colloidal Composite Nanostructures

Plasmonic nanostructures possessing unique and versatile optoelectronic properties have been vastly investigated over the past decade. However, the full potential of plasmonic nanostructure has not yet been fully exploited, particularly with single-component homogeneous structures with monotonic properties, and the addition of new components for making multicomponent nanoparticles may lead to new-yet-unexpected or improved properties. Here we define the term "multi-component nanoparticles" as hybrid structures composed of two or more condensed nanoscale domains with distinctive material compositions, shapes, or sizes. We reviewed and discussed the designing principles and synthetic strategies to efficiently combine multiple components to form hybrid nanoparticles with a new or improved plasmonic functionality. In particular, it has been quite challenging to precisely synthesize widely diverse multicomponent plasmonic structures, limiting realization of the full potential of plasmonic heterostructures. To address this challenge, several synthetic approaches have been reported to form a variety of different multicomponent plasmonic nanoparticles, mainly based on heterogeneous nucleation, atomic replacements, adsorption on supports, and biomolecule-mediated assemblies. In addition, the unique and synergistic features of multicomponent plasmonic nanoparticles, such as combination of pristine material properties, finely tuned plasmon resonance and coupling, enhanced light-matter interactions, geometry-induced polarization, and plasmon-induced energy and charge transfer across the heterointerface, were reported. In this review, we comprehensively summarize the latest advances on state-of-art synthetic strategies, unique properties, and promising applications of multicomponent plasmonic nanoparticles. These plasmonic nanoparticles including heterostructured nanoparticles and composite nanostructures are prepared by direct synthesis and physical force- or biomolecule-mediated assembly, which hold tremendous potential for plasmon-mediated energy transfer, magnetic plasmonics, metamolecules, and nanobiotechnology.

Towards rational design and optimization of near-field enhancement and spectral tunability of hybrid core-shell plasmonic nanoprobes

In biology, sensing is a major driver of discovery. A principal challenge is to create a palette of probes that offer near single-molecule sensitivity and simultaneously enable multiplexed sensing and imaging in the “tissue-transparent” near-infrared region. Surface-enhanced Raman scattering and metal-enhanced fluorescence have shown substantial promise in addressing this need. Here, we theorize a rational design and optimization strategy to generate nanostructured probes that combine distinct plasmonic materials sandwiching a dielectric layer in a multilayer core shell configuration. The lower energy resonance peak in this multi-resonant construct is found to be highly tunable from visible to the near-IR region. Such a configuration also allows substantially higher near-field enhancement, compared to a classical core-shell nanoparticle that possesses a single metallic shell, by exploiting the differential coupling between the two core-shell interfaces. Combining such structures in a dimer configuration, which remains largely unexplored at this time, offers significant opportunities not only for near-field enhancement but also for multiplexed sensing via the (otherwise unavailable) higher order resonance modes. Together, these theoretical calculations open the door for employing such hybrid multi-layered structures, which combine facile spectral tunability with ultrahigh sensitivity, for biomolecular sensing.

Plasmon Enhanced Fluorescence and Raman Scattering by [Au-Ag Alloy NP Cluster]@SiO2 Core-Shell Nanostructure

Silica-shell coated noble metal nanoparticles have shown a good performance in surface enhanced fluorescence and Raman scattering. However, silica-shell coated single noble nanoparticle cannot effectively enhance the optical signal due to the relative weak near-field enhancement. In this paper, [Au-Ag alloy NP cluster]@SiO₂ core-shell nanostructure is employed to achieve the effective electric field enhancement. With the specific structure, simultaneous Raman scattering and fluorescence emission enhancement is obtained, and the enhancement comparison of fluorescence emission with Raman scattering in different type agglomeration of metal NPs is investigated in-situ. With different thickness of SiO₂ shell, the optimized Raman and fluorescence enhancement systems are obtained, respectively, and corresponding study of power dependence are investigated in detail. The selectively enhanced Raman and fluorescence can be realized via controlling the shell thickness and laser power. Our work provides a non-polarization dependent [metal NP cluster]@SiO₂ system, which may have a promising application in portable chemical and biochemistry detecting.

Nanoelectrode-emitter spectral overlap amplifies surface enhanced electrogenerated chemiluminescence

Electrogenerated chemiluminescence (ECL) is a promising technique for low concentration molecular detection. To improve the detection limit, plasmonic nanoparticles have been proposed as signal boosting antennas to amplify ECL. Previous ensemble studies have hinted that spectral overlap between the nanoparticle antenna and the ECL emitter may play a role in signal enhancement. Ensemble spectroscopy, however, cannot resolve heterogeneities arising from colloidal nanoparticle size and shape distributions, leading to an incomplete picture of the impact of spectral overlap. Here, we isolate the effect of nanoparticle-emitter spectral overlap for a model ECL system, coreaction of tris(2,2'-bipyridyl)dichlororuthenium(ii) hexahydrate and tripropylamine, at the single-particle level while minimizing other factors influencing ECL intensities. We found a 10-fold enhancement of ECL among 952 gold nanoparticles. This signal enhancement is attributed exclusively to spectral overlap between the nanoparticle and the emitter. Our study provides new mechanistic insight into plasmonic enhancement of ECL, creating opportunities for low concentration ECL sensing.

DNA Multilayers with Mono-, Hetero-, and Mixed-Type Plasmonic Nanoparticles for Broadband Absorption and Energy Storage

DNA incorporated with functional materials has led to development of hybrids with different functionalities. Among the functional materials, metal nanoparticles such as Au, Ag, and Cu (also known as plasmonic nanoparticles [PNPs]), which can exhibit surface plasmon resonance, are good candidates to fabricate useful optoelectronic devices and sensors. Here, we constructed PNP-assorted DNA (PNP-DNA) layers with mono-, hetero-, and mixed-type PNPs formed by successive spin-coating to obtain the required number of layers. Further, structural analysis of PNP-DNA was performed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The optical evaluation was carried out by Raman, UV-visible, and photoluminescence (PL) spectroscopies followed by measurement of capacitance. Cross-sectional SEM images of DNA single, DNA triple, and PNP-DNA triple layers indicated their thicknesses (i.e., 90, 280, and 395 nm, respectively), while the base pair distance of double helixes (∼0.4 nm) for the PNP-DNA multilayers was measured by XRD. The presence of Ag, Au, and Cu PNPs was confirmed by existence of spin-orbit coupling in the corresponding XPS spectra. The addition of PNPs in DNA multilayers caused significant enhancement in the intensities of Raman bands (especially in the range of 1200-1850 cm^-1) due to Raman resonance. UV-vis absorption and PL demonstrated stacking-order-dependent and layer-dependent light absorption and energy transfer (observed as quenching of fluorescence between PNPs and DNA), respectively. We observed n-type semiconducting behavior with a relatively higher dielectric constant for a PNP-assorted DNA single layer at a low frequency of 5 kHz. The dielectric constants of all samples decreased exponentially with increased frequency. Upon addition of PNPs, enhancement in the dielectric constant as well as capacitance was noted. Consequently, the simple fabrication method used in this study can be adopted to construct various nanomaterial-assorted DNA multilayers whose specific functionalities may be controlled with high efficiency.

Adjusting Electric Field Intensity Using Hybridized Dielectric Metamolecule

In this paper, we report on achieving the hybridization effect in a Mie-based dielectric metamolecule and provide its physically intuitive picture. Hybridization results in the splitting of the initial overlapping resonance dips, thus leading to two new collective resonance modes. It was observed via the simulated displacement field distribution that the two modes behave as the in-phase and out-of-phase oscillation of two meta-atoms, thus enhancing and suppressing the intensity of the electric field at the gap between two meta-atoms. Moreover, since the two hybridized modes are caused by the interaction effect, the intensities of the electric field can be adjusted by several external factors, like applied forces and temperature. Taking advantage of this easy-equipped dielectric meta-device, certain zones in it can be applied to receive amplified signals and shielded noises of different frequencies in microwave communication fields. Moreover, due to the function of enhancing electric field intensities, it is also promising in wireless charging technology.

Fano resonances in symmetric plasmonic split-ring/ring dimer nanostructures

The optical properties of symmetric split-ring/ring dimer (SRRD) nanostructures composed of a small nanoring surrounded by an Ag splitting nanoring with a larger diameter are calculated theoretically. The apparent asymmetric Fano line shape in the spectra is related to fast switching of the bonding modes between the split-ring plasmon and ring dipole. The influence of the dimensions of the SRRD nanostructures on the spectral positions and intensity of Fano resonance is studied, and the asymmetric Fano line shape can be flexibly adjusted by varying the geometric parameters. In addition, relatively simple SRRD nanostructures have the same overall sensing figures of merit as conventional nanoparticles, thus rendering them suitable for high-performance optical sensors.

Synthesis of tunable DNA-directed trepang-like Au nanocrystals for imaging application

Multi-branched metal nanomaterials can exhibit precisely controllable plasmonic properties with the precise control of their sizes and morphologies. In this study, trepang-like gold nanocrystals (AuNCs) with tunable plasmonic properties were synthesized via DNA-directed self-assembly technology. The gold precursor was precisely controlled to be reduced and grow along the DNA skeleton of DNA-conjugated gold nanorods to form multi-branched trepang-like nanocrystals. It was investigated in detail and proven that several key factors greatly influenced the precise control of the morphology and plasmonic property of the proposed AuNCs during their synthesis, including the gold precursor, reducing agent, surfactant, loading amount of DNA and DNA structure. The relative finite-difference time-domain calculation results suggested that the change in the plasmonic resonance peak is consistent with the precise change in the size and morphology of the as-synthesized AuNCs. The trepang-like AuNCs exhibited broad absorption bands in the wavelength range of 700-1100 nm with a high photothermal conversion efficiency of 36.2%. Finally, the trepang-like AuNCs with good biocompatibility were applied in photothermal therapy and imaging analysis.

Anisotropic nanomaterials for shape-dependent physicochemical and biomedical applications

This review contributes towards a systematic understanding of the mechanism of shape-dependent effects on nanoparticles (NPs) for elaborating and predicting their properties and applications based on the past two decades of research. Recently, the significance of shape-dependent physical chemistry and biomedicine has drawn ever increasing attention. While there has been a great deal of effort to utilize NPs with different morphologies in these fields, so far research studies are largely localized in particular materials, synthetic methods, or biomedical applications, and have ignored the interactional and interdependent relationships of these areas. This review is a comprehensive description of the NP shapes from theory, synthesis, property to application. We figure out the roles that shape plays in the properties of different kinds of nanomaterials together with physicochemical and biomedical applications. Through systematic elaboration of these shape-dependent impacts, better utilization of nanomaterials with diverse morphologies would be realized and definite strategies would be expected for breakthroughs in these fields. In addition, we have proposed some critical challenges and open problems that need to be addressed in nanotechnology.

Governing factors for preparation of silver nanoparticles using droplet-based microfluidic device

Droplet-based microfluidic devices are now more than ever used for the synthesis of nanoparticles with low polydispersity and well-defined properties suitable for various industrial applications. Very small reaction volumes (microlitre to femtolitre) and short diffusion lengths, provide superior mixing efficiency and heat transport. Both play the dominant role in case of ultra-fast chemical reactions triggered upon reactant mixing, e.g. preparation of colloidal silver by reduction of silver salt. The high sensitivity of these systems to process variables makes otherwise more straightforward batch-wise production prone to suffer from inconsistency and poor reproducibility, which has an adverse effect on the reliability of production and further particle utilisation. This work presents a rigorous description of microfluidic droplet formation, reactant mixing, and nanoparticle synthesis using CFD simulations and experimental methods. The reaction mixture inside of droplets was homogenized in less than 40 milliseconds, which has been confirmed by simulations. Silver nanoparticles produced by droplet-based microfluidic chip showed superior to batch-wise preparation in terms of both particle uniformity and polydispersity.

Plasmonic nanoparticle simulations and inverse design using machine learning

Collective oscillation of quasi-free electrons on the surface of metallic plasmonic nanoparticles (NPs) in the ultraviolet to near-infrared (NIR) region induces a strong electromagnetic enhancement around the NPs, which leads to numerous important applications. These interesting far- and near-field optical characteristics of the plasmonic NPs can be typically obtained from numerical simulations for theoretical guidance of NP design. However, traditional numerical simulations encounter irreconcilable conflicts between the accuracy and speed due to the high demand of computing power. In this work, we utilized the machine learning method, specifically the deep neural network (DNN), to establish mapping between the far-field spectra/near-field distribution and dimensional parameters of three types of plasmonic NPs including nanospheres, nanorods, and dimers. After the training process, both the forward prediction of far-field optical properties and the inverse prediction of on-demand dimensional parameters of NPs can be accomplished accurately and efficiently with the DNN. More importantly, we have achieved for the first time ultrafast and accurate prediction of two-dimensional on-resonance electromagnetic enhancement distributions around NPs by greatly reducing the amount of electromagnetic data via screening and resampling methods. These near-field predictions can be realized typically in less than 10-2 seconds on a laptop, which is 6 orders faster than typical numerical simulations implemented on a server. Therefore, we demonstrate that the DNN is an ultrafast, highly efficient, and computing resource-saving tool to investigate the far- and near-field optical properties of plasmonic NPs, especially for a number of important nano-optical applications such as surface-enhanced Raman spectroscopy, photocatalysis, solar cells, and metamaterials.

A tracking-based nanoimaging method for fast detection of surfaces' inhomogeneities using gold nanoparticles

The localization of surfaces inhomogeneities is central to many areas of technology, chemistry and biology, ranging from surface defects in industry to the identification and screening of early bio-defects inside cells. The development of methods that enable direct, sensitive, and rapid detection of those inhomogeneities is both relevant and timely. To address this challenge, we developed a far-field nanoimaging method to detect the presence of surface's nanodefects that modify the signal emitted by gold nanoparticles (AuNPs) under laser irradiation. Our technique is based on the formation of hot spots due to the confinement of light in the proximity of the AuNP, whose positions depend on the polarization direction of the incident beam. An inhomogeneity is detected as an increase in the intensity collected from the hot spots when a laser beam is orbiting the nanoparticle and the incident polarization direction of the laser beam is changed periodically.

Dark-Field Scattering and Local SERS Mapping from Plasmonic Aluminum Bowtie Antenna Array

On the search for the practical plasmonic materials beyond noble metals, aluminum has been emerging as a favorable candidate as it is abundant and offers the possibility of tailoring the plasmonic resonance spanning from ultra-violet to the infrared range. In this letter, in combination with the numerical electromagnetic simulations, we experimentally study the dark-field scattering spectral mapping of plasmonic resonance from the free-standing Al bowtie antenna arrays and correlate their strong nearfield enhancement with the sensing capability by means of surface-enhanced Raman spectroscopy. The spatial matching of plasmonic and Raman mapping puts another step to realize a very promising application of free-standing Al bowtie antennas for plasmonic sensing.

Bimetallic plasmonic Au@Ag nanocuboids for rapid and sensitive detection of phthalate plasticizers with label-free surface-enhanced Raman spectroscopy

Phthalate plasticizers (PAEs) are posing a serious threat to human health, so it is urgent to develop effective and reliable ways to detect the food additives PAEs sensitively. In this study, we have reported plasmonic bimetallic Au@Ag core-shell nanocuboids for the rapid and sensitive detection of PAEs in liquor samples with a label-free Surface-enhanced Raman Spectroscopy (SERS) strategy. Compared with single-element nanostructures, the bimetallic SERS platform can integrate two distinct functions into a single entity with unprecedented properties. Consequently, we synthesized Au@Ag nanocuboids (Au@Ag NCs) composed of a Au nanorod (Au NR) core and a Ag cuboid shell, which could produce richer and broader plasmonic resonance modes than Au NRs. It is obvious that the SERS signals of crystal violet (CV) and butyl benzyl phthalate (BBP) reached a maximum as the thickness of the Ag coating shell was in a certain threshold and there was a strong dependence of the Raman enhancement on the Ag cuboid shell-thickness. Based on the optimized size, the sensitivity and repeatability of Au@Ag NCs were evaluated with limits of detection (LODs) at around 10^-9 M both for BBP and diethylhexyl phthalate (DEHP). In addition, the SERS active substrate core-shell Au@Ag NCs can be used to detect BBP as low as 1.3 mg kg^-1 spiked into the liquor samples. Thereby, the unique bimetallic Au@Ag NCs showed a huge potential for the rapid and sensitive detection of PAEs in liquor samples.

Revisiting semicontinuous silver films as surface-enhanced Raman spectroscopy substrates

Surface-enhanced Raman spectroscopy (SERS) is a very promising analytical technique for the detection and identification of trace amounts of analytes. Among the many substrates used in SERS of great interest are nanostructures fabricated using physical methods, such as semicontinuous metal films obtained via electron beam physical vapor deposition. In these studies, we investigate the influence of morphology of semicontinuous silver films on their SERS properties. The morphologies studied ranged from isolated particles through percolated films to almost continuous films. We found that films below the percolation threshold (transition from dielectric-like to metal-like) made of isolated silver structures provided the largest SERS enhancement of 4-aminothiophenol (4-ATP) analyte signals. The substrate closest to the percolation threshold has the SERS signal about four times lower than the highest signal sample.

Low-Temperature Vapor-Phase Synthesis of Single-Crystalline Gold Nanostructures: Toward Exceptional Electrocatalytic Activity for Methanol Oxidation Reaction

Au nanostructures (Au NSs) have been considered promising materials for applications in fuel cell catalysis, electrochemistry, and plasmonics. For the fabrication of high-performance Au NS-based electronic or electrochemical devices, Au NSs should have clean surfaces and be directly supported on a substrate without any mediating molecules. Herein, we report the vapor-phase synthesis of Au NSs on a fluorine-doped tin oxide (FTO) substrate at 120 °C and their application to the electrocatalytic methanol oxidation reaction (MOR). By employing AuCl as a precursor, the synthesis temperature for Au NSs was reduced to under 200 °C, enabling the direct synthesis of Au NSs on an FTO substrate in the vapor phase. Considering that previously reported vapor-phase synthesis of Au NSs requires a high temperature over 1000 °C, this proposed synthetic method is remarkably simple and practical. Moreover, we could selectively synthesize Au nanoparticles (NPs) and nanoplates by adjusting the location of the substrate, and the size of the Au NPs was controllable by changing the reaction temperature. The synthesized Au NSs are a single-crystalline material with clean surfaces that achieved a high methanol oxidation current density of 14.65 mA/cm² when intimately supported by an FTO substrate. We anticipate that this novel synthetic method can widen the applicability of vapor-phase synthesized Au NSs for electronic and electrochemical devices.

Recent Advancement in the Surface-Enhanced Raman Spectroscopy-Based Biosensors for Infectious Disease Diagnosis

Diagnosis is the key component in disease elimination to improve global health. However, there is a tremendous need for diagnostic innovation for neglected tropical diseases that largely consist of mosquito-borne infections and bacterial infections. Early diagnosis of these infectious diseases is critical but challenging because the biomarkers are present at low concentrations, demanding bioanalytical techniques that can deliver high sensitivity with ensured specificity. Owing to the plasmonic nanomaterials-enabled high detection sensitivities, even up to single molecules, surface-enhanced Raman spectroscopy (SERS) has gained attention as an optical analytical tool for early disease biomarker detection. In this mini-review, we highlight the SERS-based assay development tailored to detect key types of biomarkers for mosquito-borne and bacterial infections. We discuss in detail the variations of SERS-based techniques that have developed to afford qualitative and quantitative disease biomarker detection in a more accurate, affordable, and field-transferable manner. Current and emerging challenges in the advancement of SERS-based technologies from the proof-of-concept phase to the point-of-care phase are also briefly discussed.

Self-assembly of Janus Dumbbell Nanocrystals and Their Enhanced Surface Plasmon Resonance

Self-assembly is a critical process that can greatly expand the existing structures and lead to new functionality of nanoparticle systems. Multicomponent superstructures self-assembled from nanocrystals have shown promise as multifunctional materials for various applications. Despite recent progress in assembly of homogeneous nanocrystals, synthesis and self-assembly of Janus nanocrystals with contrasting surface chemistry remains a significant challenge. Herein, we designed a novel Janus nanocrystal platform to control the self-assembly of nanoparticles in aqueous solutions by balancing the hydrophobic and hydrophilic moieties. A series of superstructures have been assembled by systematically varying the Janus balance and assembly conditions. Janus Au-Fe₃O₄ dumbbell nanocrystals (<20 nm) were synthesized with the hydrophobic ligands coated on the Au lobe and negatively charged hydrophilic ligands coated on the Fe₃O₄ lobe. We systematically fine-tune the lobe size ratio, surface coating, external conditions, and even additional growth of Au nanocrystal domains on the Au lobe of dumbbell nanoparticles (Au-Au-Fe₃O₄) to harvest self-assembly structures including clusters, chains, vesicles, and capsules. It was discovered that in all these assemblies the hydrophobic Au lobes preferred to stay together. In addition, these superstructures clearly demonstrated different levels of enhanced surface plasmon resonance that is directly correlated with the Au coupling in the assembly structure. The strong interparticle plasmonic coupling displayed a red-shift in surface plasmon resonance, with larger structures formed by Au-Au-Fe₃O₄ assembly extending into the near-infrared region. Self-assembly of Janus dumbbell nanocrystals can also be reversible under different pH values. The biphasic Janus dumbbell nanocrystals offer a platform for studying the novel interparticle coupling and open up opportunities in applications including sensing, disease diagnoses, and therapy.

One-step isothermal detection of multiple KRAS mutations by forming SNP specific hairpins on a gold nanoshell

We developed a one-step isothermal method for typing multiple KRAS mutations using a designed set of primers to form a hairpin on a gold nanoshell upon being ligated by a SNP specific DNA ligase after binding of targets. As a result, we could detect as low as 20 attomoles of KRAS mutations within 1 h.

Two-dimensional SERS encoding method for on-bead peptide sequencing in high-throughput bioanalysis

We developed a ready-to-read on-bead peptide encoding method for high-throughput screening bioassays. With two-dimensional surface-enhanced Raman scattering nano-identifiers (2D-SERS IDs) which are concurrently labelled with two SERS codes (coupling steps and kinds of amino acid), we could possibly generate more than 10 trillion codes with only 30 Raman label compounds.

Gold nanoparticle surface engineering strategies and their applications in biomedicine and diagnostics

Gold nanoparticles (AuNPs) have found a wide range of biomedical and environmental monitoring applications (viz. drug delivery, diagnostics, biosensing, bio-imaging, theranostics, and hazardous chemical sensing) due to their excellent optoelectronic and enhanced physico-chemical properties. The modulation of these properties is done by functionalizing them with the synthesized AuNPs with polymers, surfactants, ligands, drugs, proteins, peptides, or oligonucleotides for attaining the target specificity, selectivity and sensitivity for their various applications in diagnostics, prognostics, and therapeutics. This review intends to highlight the contribution of such AuNPs in state-of-the-art ventures of diverse biomedical applications. Therefore, a brief discussion on the synthesis of AuNPs has been summarized prior to comprehensive detailing of their surface modification strategies and the applications. Here in, we have discussed various ways of AuNPs functionalization including thiol, phosphene, amine, polymer and silica mediated passivation strategies. Thereafter, the implications of these passivated AuNPs in sensing, surface-enhanced Raman spectroscopy (SERS), bioimaging, drug delivery, and theranostics have been extensively discussed with the a number of illustrations.

In Situ Identification of Spherical Ag Monomers and Dimers at Zeptomole Adsorbate Concentrations by Surface-Enhanced Raman Scattering Correlation Spectroscopy

We have developed a bright optical setup, especially optimized for surface-enhanced Raman scattering (SERS) correlation spectroscopy. We show that the brightness of the experiment combined with the correlation approach permits in situ access to the size and shape of the SERS-active aggregates in solution despite a very weak SERS signal of the adsorbed molecules. As a result, we demonstrate that dimers and larger SERS-active aggregates can be identified through the temporal fluctuations of the SERS signal of only a few hundred of adsorbed molecules, that is, at zeptomole adsorbate concentrations. Monomers covered by a monolayer of MBA were also identified. These results open a way for single nanoparticle sensing, for single-molecule SERS-active aggregate characterization, or for quantitative monitoring of functionalization processes on metallic objects.

Colour and SERS patterning using core-satellite nanoassemblies

We explore spatial control of the formation of core-satellite nanoassemblies on glass substrates. UV irradiation leads to the photooxidative desorption of thiol linkers from gold nanoparticles deposited on the substrates, thereby prohibiting further assembly in the irradiated region. The distribution of assemblies and monomers yields a pattern with stark contrasts in colour and Raman enhancement. Our findings can be utilised in the fabrication of microfluidic SERS sensors, colour displays, photonic devices, and metamaterials.

Plasmons of hollow nanobar oligomers

Assembling metal nano-objects into well-defined configurations is an effective way to create hybrid plasmonic structures with unusual functionalities.

Size-controllable and uniform gold bumpy nanocubes for single-particle-level surface-enhanced Raman scattering sensitivity

Gold nanocubes modified to form roughened structures with very strong and uniform single-particle surface-enhanced Raman scattering intensity were developed.

Waveguide-Integrated Compact Plasmonic Resonators for On-Chip Mid-Infrared Laser Spectroscopy

The integration of nanoplasmonic devices with a silicon photonic platform affords a new approach for efficient light delivery by combining the high field enhancement of plasmonics and the ultralow propagation loss of dielectric waveguides. Such a hybrid integration obviates the need for a bulky free-space optics setup and can lead to fully integrated, on-chip optical sensing systems. Here, we demonstrate ultracompact plasmonic resonators directly patterned atop a silicon waveguide for mid-infrared spectroscopic chemical sensing. The footprint of the plasmonic nanorod resonators is as small as 2 μm², yet they can couple with the mid-infrared waveguide mode efficiently. The plasmonic resonance is directly measured through the transmission spectrum of the waveguide with a coupling efficiency greater than 70% and a field intensity enhancement factor of over 3600 relative to the evanescent waveguide field intensity. Using this hybrid device and a tunable mid-infrared laser source, surface-enhanced infrared absorption spectroscopy of both a thin poly(methyl methacrylate) film and an octadecanethiol monolayer is successfully demonstrated.

Morphology dependent nonlinear optical and photocatalytic activity of anisotropic plasmonic silver

Anisotropic nanoparticles are ideal building blocks for a number of functional materials due to their exceptional and anisotropic optical, electronic, magnetic and mechanical properties. In this work we present systematic studies on morphology dependent ultra-sensitive thermal diffusivity and photodegradation capability of anisotropic plasmonic silver for the first time. Hydrogen peroxide centered synthesis was performed to prepare anisotropic silver nanosystems spherical (14 nm), quasi-spherical (17 nm), elliptical (18 m), rods (aspect ratio 2.1), hexagonal (22 nm) and prisms (19 nm). The synthesized nanosystems were characterized using UV-VIS spectroscopy, high resolution transmission electron microscopy (HRTEM) and band gap analysis. A dual beam mode matched thermal lensing method was adopted for evaluating the thermal diffusivity of the anisotropic system. The present anisotropic nanoparticle system exhibited strong morphology based thermal diffusivity. An increase of 140% in the thermal diffusivity value points to the nonlinear optical application potential of the anisotropic systems. Sunlight mediated photodegradation of methylene blue showed a promising increase in the degradation rate for anisotropic systems compared to other similar systems reported in the literature.

Probing the SERS brightness of individual Au nanoparticles, hollow Au/Ag nanoshells, Au nanostars and Au core/Au satellite particles: single-particle experiments and computer simulations

Different classes of plasmonic nanoparticles functionalized with the non-resonant Raman reporter molecule 4-MBA are tested for their SERS signal brightness at the single-particle level: gold nanoparticles, hollow gold/silver nanoshells, gold nanostars, and gold core/gold satellite particles. Correlative SERS/SEM experiments on a set of particles from each class enable the unambiguous identification of single particles by electron microscopy as well as the characterization of both their elastic (LSPR) and inelastic (SERS) scattering spectra. Experimental observations are compared with predictions from FEM computer simulations based on 3D models derived from representative TEM/SEM images. Single gold nanostars and single gold core/gold satellite particles exhibit a detectable SERS signal under the given experimental conditions, while single gold nanoparticles and single hollow gold/silver nanoshells are not detectable.

Influence of gold nanoholes and nanoslits arrays on Raman spectra and optical reflectance of graphene oxide

We report the effect of gold nanostructured substrates, fabricated by interference lithography technique (IL), on the Raman spectra and optical reflectance of graphene oxide (GO) layers. For purposes of comparison two gold nanostructured substrates, nanoslits (AuNSs) and circular nanoholes (AuNHs) were compared with a non-nanostructured gold substrate. Effects induced by the gold nanostructured substrates are discussed in terms of the I_D/I_G ratio and the FWHM of the G band (FWHM(G)) as a function of the G band intensity (I_G), showing that both I_D/I_G and FWHM(G) parameters are highly sensitive to the number of GO layers (n_GO), which would allow to identify the number of GO layers in a reliable way. Optical reflectance spectra (R(λ)) reveal that plasmons are generated on the surface of nanostructured substrates by the incident radiation. Dips in R(λ) are ascribed as coupling by surface plasmon polaritons described by Bloch waves (BW-SPP). A peak in R(λ) is also observed and it is ascribed to visible radiation produced by Förster resonance energy transfer and Purcell effect. The relevance of these results lies in the possibility of designing colorimetric plasmonic sensors, based on few layers of GO with an excellent control of n_GO and with potential in detection of molecules by fluorescent absorption.