Substrate-based platform for boosting the surface-enhanced Raman of plasmonic nanoparticles

Phase controlled SERS enhancement

Surface-enhanced Raman spectroscopy (SERS) has attracted increasing interest for chemical and biochemical sensing. Several studies have shown that SERS intensities are significantly increased when an optical interference substrate composed of a dielectric spacer and a reflector is used as a supporting substrate. However, the origin of this additional enhancement has not been systematically studied. In this paper, high sensitivity SERS substrates composed of self-assembled core-satellite nanostructures and silica-coated silicon interference layers have been developed. Their SERS enhancement is shown to be a function of the thickness of silica spacer on a more reflective silicon substrate. Finite difference time domain modeling is presented to show that the SERS enhancement is due to a spacer contribution via a sign change of the reflection coefficients at the interfaces. The magnitude of the local-field enhancement is defined by the interference of light reflected from the silica-air and silica-silicon interfaces, which constructively added at the hot spots providing a possibility to maximize intensity in the nanogaps between the self-assembled nanoparticles by changing the thickness of silica layer. The core-satellite assemblies on a 135 nm silica-coated silicon substrate exhibit a SERS activity of approximately 13 times higher than the glass substrate.

Fluorescence in sub-10 nm channels with an optical enhancement layer

Fluorescence microscopy uniquely enables physical and biological research in micro- and nanofluidic systems. However, in channels with depths below 10 nm, the limited number of fluorophores results in fluorescence intensity below the detection limit of optical microscopes. To overcome this barrier, we applied Fabry-Pérot interference to enhance fluorescence intensity with a silicon nitride layer below the sub-10 nm channel. A silicon nitride layer of suitable thickness can selectively enhance both absorption and emission wavelengths, leading to a fluorescent signal that is enhanced 20-fold and readily imaged with traditional microscopes. To demonstrate this method, we studied the mass transport of a binary solution of ethanol and Rhodamin B in 8 nm nanochannels. The large molecular size of Rhodamin B (∼1.8 nm) relative to the channel depth results in both separation and reduced diffusivity, deviating from behavior at larger scales. This method extends the widely available suite of fluorescence analysis tools and infrastructure to unprecedented sub-10 nm scale with relevance to a wide variety of biomolecular interactions.

Beam steering and impedance matching of plasmonic horn nanoantennas

In this paper, we study a plasmonic horn nanoantenna on a metal-backed substrate. The horn nanoantenna structure consists of a two-wire transmission line (TWTL) flared at the end. We analyze the effect of the substrate thickness on the nanoantenna's radiation pattern, and demonstrate beam steering in a broad range of elevation angles. Furthermore, we analyze the effect of the ground plane on the impedance matching between the antenna and the TWTL, and observe that the ground plane increases the back reflection into the waveguide. To reduce the reflection, we develop a transmission line model to design an impedance matching section which leads to 99.75% power transmission to the nanoantenna.

Probing the acoustic vibrations of complex-shaped metal nanoparticles with four-wave mixing

We probe the acoustic vibrations of silver nanoprisms and gold nano-octahedrons in aqueous solution with four-wave mixing. The nonlinear optical response shows two acoustic vibrational modes: an in-plane mode of nanoprisms with vertexial expansion and contraction; an extensional mode of nano-octahedrons with longitudinal expansion and transverse contraction. The particles were also analyzed with electron microscopy and the acoustic resonance frequencies were then calculated by the finite element analysis, showing good agreement with experimental observations. The experimental mode frequencies also fit with theoretical approximations, which show an inverse dependence of the mode frequency on the edge length, for both nanoprisms and nano-octahedrons. This technique is promising for in situ monitoring of colloidal growth.

High-density ordered Ag@Al₂O₃ nanobowl arrays in applications of surface-enhanced Raman spectroscopy

In this paper, we demonstrate a high-performance surface-enhanced Raman scattering (SERS) substrate based on high-density ordered Ag@Al2O3 nanobowl arrays. By ion beam etching (IBE) the anodized aluminum oxide (AAO) and subsequent Ag coating, ordered Ag@Al2O3 nanobowl arrays were created on the Si substrate. Unlike the 'hot spots' generated between adjacent metallic nanostructures, the Ag@Al2O3 nanobowl introduced 'hot spots' on the metal boundary of its hemispherical cavity. Based on the analysis of SERS signals, the optimized SERS substrate of Ag@Al2O3 nanobowl arrays had both high sensitivity and large-area uniformity. A detection limit as low as 10(-10) M was obtained using chemisorbed p-thiocresol (p-Tc) molecules, and the SERS signal was highly reproducible with a small standard deviation. The method opens up a new way to create highly sensitive SERS sensors with high-density 'hot spots', and it could play an important role in device design and corresponding biological and food safety monitoring applications.

Understanding the fate and biological effects of Ag- and TiO₂-nanoparticles in the environment: The quest for advanced analytics and interdisciplinary concepts

Engineered inorganic nanoparticles (EINP) from consumers' products and industrial applications, especially silver and titanium dioxide nanoparticles (NP), are emitted into the aquatic and terrestrial environments in increasing amounts. However, the current knowledge on their environmental fate and biological effects is diverse and renders reliable predictions complicated. This review critically evaluates existing knowledge on colloidal aging mechanisms, biological functioning and transport of Ag NP and TiO2 NP in water and soil and it discusses challenges for concepts, experimental approaches and analytical methods in order to obtain a comprehensive understanding of the processes linking NP fate and effects. Ag NP undergo dissolution and oxidation with Ag2S as a thermodynamically determined endpoint. Nonetheless, Ag NP also undergo colloidal transformations in the nanoparticulate state and may act as carriers for other substances. Ag NP and TiO2 NP can have adverse biological effects on organisms. Whereas Ag NP reveal higher colloidal stability and mobility, the efficiency of NOM as a stabilizing agent is greater towards TiO2 NP than towards Ag NP, and multivalent cations can dominate the colloidal behavior over NOM. Many of the past analytical obstacles have been overcome just recently. Single particle ICP-MS based methods in combination with field flow fractionation techniques and hydrodynamic chromatography have the potential to fill the gaps currently hampering a comprehensive understanding of fate and effects also at a low field relevant concentrations. These analytical developments will allow for mechanistically orientated research and transfer to a larger set of EINP. This includes separating processes driven by NP specific properties and bulk chemical properties, categorization of effect-triggering pathways directing the EINP effects towards specific recipients, and identification of dominant environmental parameters triggering fate and effect of EINP in specific ecosystems (e.g. soil, lake, or riverine systems).

Combination of surface- and interference-enhanced Raman scattering by CuS nanocrystals on nanopatterned Au structures

We present the results of a Raman study of optical phonons in CuS nanocrystals (NCs) with a low areal density fabricated through the Langmuir-Blodgett technology on nanopatterned Au nanocluster arrays using a combination of surface- and interference-enhanced Raman scattering (SERS and IERS, respectively). Micro-Raman spectra of one monolayer of CuS NCs deposited on a bare Si substrate reveal only features corresponding to crystalline Si. However, a new relatively strong peak occurs in the Raman spectrum of CuS NCs on Au nanocluster arrays at 474 cm(-1). This feature is related to the optical phonon mode in CuS NCs and manifests the SERS effect. For CuS NCs deposited on a SiO2 layer this phonon mode is also observed due to the IERS effect. Its intensity changes periodically with increasing SiO2 layer thickness for different laser excitation lines and is enhanced by a factor of about 30. CuS NCs formed on Au nanocluster arrays fabricated on IERS substrates combine the advantages of SERS and IERS and demonstrate stronger SERS enhancement allowing for the observation of Raman signals from CuS NCs with an ultra-low areal density.

Metal double layers with sub-10 nm channels

Double-layer plasmonic nanostructures are fabricated by depositing metal at normal incidence onto various resist masks, forming an antenna layer on top of the resist post and a hole layer on the substrate. Antenna plasmon resonances are found to couple to the hole layer, inducing image charges which enhance the near-field for small layer spacings. For continued evaporation above the resist height, a sub-10 nm gap channel develops due to a self-aligned process and a minimal undercut of the resist sidewall. For such double layers with nanogap channels, the average surface-enhanced Raman scattering intensity is improved by a factor in excess of 60 in comparison to a single-layer antenna with the same dimensions. The proposed design principle is compatible with low-cost fabrication, straightforward to implement, and applicable over large areas. Moreover, it can be applied for any particular antenna shape to improve the signals in surface-enhanced spectroscopy applications.

Effect of surface roughness on self-assembled monolayer plasmonic ruler in nonlocal regime

Recently, self-assembled monolayers (SAMs) have been used for plasmonic rulers to measure the nonlocal influence on the Au nanoparticle - metal film resonance wavelength shift and probe the ultimate field enhancement. Here we examine the influence of surface roughness on this plasmonic ruler in the nonlocal regime by comparing plasmonic resonance shifts for as-deposited and for ultra-flat Au films. It is shown that the resonance shift is larger for ultra-flat films, suggesting that there is not the saturation from nonlocal effects previously reported for the spacer range from 0.7 nm to 1.6 nm. We attribute the previously reported saturation to the planarization of the as-deposited films by thinner SAMs, as measured here by atomic-force microscopy. This work is of interest both in probing the ultimate limits of plasmonic enhancement with SAMs for applications in Raman and nonlinear optics, but also in the study of SAMs planarization as a function surface roughness.

Meditating metal coenhanced fluorescence and SERS around gold nanoaggregates in nanosphere as bifunctional biosensor for multiple DNA targets

Gold nanoparticles (Au NPs) are very attractive candidate nanoparticles in biological assay because of their high chemical stabilities, high homogeneities, good biocompatibilities, and low toxicities. However, molecular beacon assays via encapsulating the combined fluorescence or surface-enhanced Raman scattering (SERS) signals of reporters and Au NPs in nanobarcodes particles usually suffer from fluorescence quenching or weak Raman enhancement when Au NPs are employed (especially with size smaller than 15 nm). Herein, we present a new design of simultaneously realizing metal-enhanced fluorescence and coenhanced surface-enhanced Raman scattering by facilely embedding Ag nanoparticle into the shell of two kinds of Au nanoaggregate (5 and 10 nm), meanwhile, fluorophore is located between the silver core and gold nanoparticle layers and the distance among them is adjusted by SiO2 spacer (Ag@first SiO2 spacer@FiTC+SiO2@second SiO2 spacer@Au nanoaggregate). In this architecture, Ag nanoparticle not only is utilized as an efficient fluorescence enhancer to overcome the common fluorescence quenching around Au nanoaggregates but also behaves like a mirror. Thus, incident light that passes through the SERS-active Au nanoaggregate and the intervening dielectric layer of SiO2 could be reflected multiply from the surface of Ag nanoparticle and coupled with the light at the nanogap between the Au nanoaggregates to further amplify Raman intensity. This results in enhancement factors for fluorescence and SERS ~1.6-fold and more than 300-fold higher than the control samples without silver core under identical experimental conditions, respectively. Moreover, fluorophore and SERS reporters are assembled onto different layers of the concentric hybrid microsphere, resulting in a feasible fabrication protocol when a large number of agents need to be involved into the dual-mode nanobarcodes. A proof-of-concept chip-based DNA sandwich hybridization assay using genetically modified organisms as a model system has been investigated based on the concentric hybrid microsphere. The high specificity and sensitivity of the assays suggest that the new architecture has a potential for various bioanalytical applications and provides opportunities for other similar metal nanoparticles to realize coenhancement effect.

Plasmon hybridization for enhanced nonlinear optical response

We report the plasmon hybridization between silver nanoprisms and a thin gold film as a means to tune the plasmon resonance and to achieve enhanced optical second harmonic generation. The hybridization enhances the second harmonic counts by nearly three orders of magnitude when varying the spacer layer between the nanoprisms and the gold film. Finite-difference time-domain calculations agree within a factor of 2 with the experimental findings in terms of the predicted enhancement factor. This plasmon hybridization approach is promising for future applications, including multi-photon lithography and nonlinear sensing using metal nanoparticles.

Single molecule directivity enhanced Raman scattering using nanoantennas

Single molecule detection by directivity enhanced Raman scattering is demonstrated using nanoantennas. Bianalyte Raman scattering is used to confirm the detection of single molecules of Rhodamine 6G and Nile Blue A in aqueous solution. Calculations show that Raman enhancement factors of 10(13) can be achieved by combined optimization of the local field enhancement (hotspot with 10(11) enhancement) and antenna directionality (with 10(2) enhancement).

Radiation engineering of optical antennas for maximum field enhancement

Optical antennas have generated much interest in recent years due to their ability to focus optical energy beyond the diffraction limit, benefiting a broad range of applications such as sensitive photodetection, magnetic storage, and surface-enhanced Raman spectroscopy. To achieve the maximum field enhancement for an optical antenna, parameters such as the antenna dimensions, loading conditions, and coupling efficiency have been previously studied. Here, we present a framework, based on coupled-mode theory, to achieve maximum field enhancement in optical antennas through optimization of optical antennas' radiation characteristics. We demonstrate that the optimum condition is achieved when the radiation quality factor (Q(rad)) of optical antennas is matched to their absorption quality factor (Q(abs)). We achieve this condition experimentally by fabricating the optical antennas on a dielectric (SiO(2)) coated ground plane (metal substrate) and controlling the antenna radiation through optimizing the dielectric thickness. The dielectric thickness at which the matching condition occurs is approximately half of the quarter-wavelength thickness, typically used to achieve constructive interference, and leads to ∼20% higher field enhancement relative to a quarter-wavelength thick dielectric layer.

Directivity enhanced Raman spectroscopy using nanoantennas

Directing the emission from optical emitters is highly desired for efficient detection and, by reciprocity, efficient excitation as well. As a scattering process, Raman benefits from directivity enhancements in both excitation and emission. Here we demonstrate directivity enhanced Raman scattering (DERS) using a nanoantenna fabricated by focused ion beam milling. The nanoantenna uses a resonant ring-reflector to shape the emitted beam and achieve DERS-this configuration is most similar to a waveguide antenna. The ring reflector boosts the measured Raman signal by a factor of 5.5 (as compared to the ground plane alone), and these findings are in quantitative agreement with comprehensive numerical simulations. The present design is nearly optimal in the sense that almost all the beam power is coupled into the numerical aperture of the microscope. Furthermore, the emission is directed out of the plane, so that this design can be used to achieve DERS using conventional Raman microscopes, which has yet to be achieved with Yagi-Uda and traveling wave antenna designs.