Surface-enhanced Raman scattering on gold quasi-3D nanostructure and 2D nanohole arrays

1-(Pyridin-4-yl)-4-thiopyridine (PTP) in the crystalline state - pure PTP and a cocrystal and salt

The first in situ preparation and single-crystal structure identification of pure 1-(pyridin-4-yl)-4-thiopyridine (PTP), C10H8N2S, a simple and basic derivative of mercaptopyridine, from a crystallization mixture is described. The same PTP was found in two multicomponent crystal forms with 3,5-dinitrobenzoic acid as a classic two-component cocrystal, namely, 1-(pyridin-4-yl)-4-thiopyridine-3,5-dinitrobenzoic acid (1/1), C7H4N2O6·C10H8N2S, and with 2-hydroxy-3,5-dinitrobenzoic acid as a salt formed via proton transfer from the hydroxy group of the acid to the pyridyl N atom of PTP, namely, 4-(4-sulfanylidene-1,4-dihydropyridin-1-yl)pyridin-1-ium 1-carboxy-3,5-dinitrophenolate, C10H9N2S+·C7H3N2O7-. The protonation energy of PTP is 944.64 kJ mol-1, indicating slightly greater N-basicity compared to pyridine, a well characterized and very basic chemical reference. A variety of molecular interactions can be observed in the three new crystal structures of PTP, which are all discussed in detail. Our findings confirm those of previous studies, indicating that PTP and 4-mercaptopyridine may, under suitable conditions, be chemically converted to one another, and that this process can be stimulated by light (UV-Vis).

All-Dielectric Gratings with High-Quality Structural Colors

We present a dual-layer hafnium dioxide (HfO2) grating capable of full-color modulation in the visible spectrum by leveraging the magnetic dipole resonance induced by the lower-layer HfO2 grating, while the upper-layer HfO2 grating serves as a refractive index matching layer to effectively suppress high-order Mie resonances at shorter wavelengths. The HfO2/HfO2 grating exhibits a significantly larger distribution area in the CIE 1931 chromaticity diagram compared to the HfO2 grating. Furthermore, the structural color saturation closely approximates that of monochromatic light. Under varying background refractive index environments, this structure consistently exhibits high-quality structural color. However, the hue of the structural color undergoes alterations. When the polarization angle is below 20°, the saturation of the acquired structural color remains remarkably consistent. However, exceeding 20° results in a significant degradation in the quality of the structural color. This study demonstrates the promising potential for diverse applications, encompassing fields such as imaging and displays.

Layer-by-layer coating of natural diatomite with silver nanoparticles for identification of circulating cancer protein biomarkers using SERS

Surface-enhanced Raman scattering (SERS) is an emerging spectroscopy technique for detecting and characterizing chemical or biological structures in the vicinity of plasmonic nanostructures. Colloidal, solid, and flexible nanostructures are widely used in SERS experiments to enhance the Raman intensity. The nanostructure used in SERS is one of the main influencing parameters and a growing research area. Fabrication of simple and cheap SERS substrates with a high enhancement factor is desired. In this study, we fabricated a reproducible, cheap, and flexible SERS active strip by coating natural diatomite (biosilica) with silver nanoparticles (AgNPs) using the layer-by-layer assembly method and the fabricated strip is used for the label-free identification of circulating cancer protein biomarkers. SERS active strips were fabricated having different numbers of AgNP layers on natural diatomite and comprehensive characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV/vis absorption spectrophotometry were used. SERS activities of the strips depending on the number of layers were evaluated using 4-aminothiophenol (4-ATP) and rhodamine 6G (Rh6G) molecules. We found that the SERS intensity is strongly dependent on the number of AgNP layers, with the maximum SERS intensity obtained from the strip with 5 layers of AgNPs, having a 2.0 × 105 enhancement factor. The strip with the highest SERS activity was used for the label-free identification of circulating cancer protein biomarkers (HER2, CA15-3, PSA, MUC4, and CA27-29). The results demonstrate that the fabricated strip can help in the effective label-free identification of circulating protein biomarkers and open new directions for SERS-based label-free biosensing applications.

High-sensitivity long-range surface plasmon resonance sensing assisted by gold nanoring cavity arrays and nanocavity coupling

In many of the existing refractive index (RI) sensing works, only the shape and size of plasmonic structures are usually taken into account, while the parameters of spacer layers are ignored. In this publication, we explored the long-range surface plasmon resonance (LRSPR) and Fabry-Pérot resonance coupling effects of our proposed gold nanoring cavity array/spacer layer/Au mirror/glass substrate. Both the RI sensitivity and full width at half-maximum (FWHM) values were superior than those of conventional surface plasmon resonance substrates. We discussed the tunability of the RI sensitivity through changing the RI and thickness of the spacer layer. Then, under the optimized parameter conditions of the spacer layer, the geometry parameters (including size, gap and periodicity) of gold nanoring cavity arrays were tuned to optimize the best RI sensitivity. Finally, we broke the structural symmetry of a nanoring cavity to introduce Fano resonances into our system, and a high RI sensitivity and figure-of-merit (FOM) of 695 nm per RIU (refractive index unit) and 96.5, respectively, were achieved when the breaking angle θ was 30°. This study opens up many possibilities for boosting the FOM of RI sensing by taking into account the hybridization effects of localized surface plasmon resonance, LRSPR, and Fabry-Pérot and Fano resonances.

Surface-enhanced Raman scattering with gold-coated silicon nanopillars arrays: The influence of size and spatial order

Nanopillars have been extensively explored as promising substrates for surface-enhanced Raman scattering (SERS) owing to their high sensitivity and excellent reproducibility. Most of the researches have been focused on the fabrication methods of nanopillars, and the dependences of SERS effects on geometrical size and spatial order are rarely investigated. In this work, SERS properties of nanopillars with different sizes (115-185 nm) and spatial orders (square and rhombus orders) have been studied. The work has shown that the nanopillars not only have high enhancement capability and high signal reproducibility, but also the enhancement is insensitive to the size and spatial orders. The measured enhancement factors (EFs) are 2.3-4.0 × 10⁶ and signal reproducibility (relative standard deviation, RSD) are ∼ 5.2%-6.9%, which are among the best of the similar SERS substrates reported. The variation of SERS intensity was as low as approximately 4.8% with the variation of pillar size from 115, 135, 145, to 160 nm. The insensitiveness and high reproducibility have been ascribed to the combined excitation of localized surface plasmon resonance (LSPR) and propagating surface plasmons (SPPs) of the nanopillars. Optical properties of the nanopillars are studied both experimentally and numerically to understand the physics behind the SERS performance.

Designing dendronic-Raman markers for sensitive detection using surface-enhanced Raman spectroscopy

Surface-Enhanced Raman Spectroscopy (SERS) is well-established as a tool for bio-diagnostics but is often limited by analyte sensitivity and the need for specialized substrates. Signal enhancement can be achieved by attaching multiple Raman markers to a single analyte. Dendronic frameworks with multiple Raman markers attached to the periphery offer an opportunity to examine this idea. In this article, dendrons with thiophenol groups on their periphery were synthesized and tested as a SERS analyte. For this study, simple gold nanoparticles (∼60 nm) were used as a substrate. A 10² fold enhancement in detection was observed upon going from a mono-thiophenol (MT) to a tetra-thiophenol (TT). Dendronic Raman markers increased the probability of SERS occurrence at lower concentrations when compared to a single Raman active molecule. This strategy extends the applicability of SERS, as these analyte molecules can be just mixed or drop-casted on any kind of SERS substrate.

A new approach of tuning SERS enhancement with the aid of coupling chemistry for trace detection. A greater number of Raman-active molecules are constrained in a dendronic framework as an improved SERS analyte.

Combining sonicated cold development and pulsed electrodeposition for high aspect ratio sub-10 nm gap gold dimers for sensing applications in the visible spectrum

Strong interactions between localized surface plasmons and nanoscale objects have led to the development of highly sensitive biochemical sensing in planar metallic nanostructures with sensing performance mainly dependent on the interaction volume and the local electric field. However, the sensitivity and the interaction volume of these planar structures have been limited by the achievable aspect ratios based on the standard lift-off process. We propose a new technique which involves cold sonicated development and pulsed electrodeposition to overcome this limitation, and demonstrate robust gold square dimers with sub-10 nm gaps and a gap aspect ratio of ∼8. We show that smooth gold surfaces can be achieved by growing the gold film directly on a transparent ITO substrate without a gold seed layer, and demonstrate a significant improvement in Q factors and resonance contrast in electrodeposited dimers compared to dimers fabricated by physical vapor deposition. We demonstrate that the electrodeposited dimers exhibit near 50% higher bulk refractive index sensitivity than their planar counterparts. The technique may be used to grow a variety of metals of arbitrary geometries and spatial arrangements.

Octupolar Metastructures for a Highly Sensitive, Rapid, and Reproducible Phage-Based Detection of Bacterial Pathogens by Surface-Enhanced Raman Scattering

The development of fast and ultrasensitive methods to detect bacterial pathogens at low concentrations is of high relevance for human and animal health care and diagnostics. In this context, surface-enhanced Raman scattering (SERS) offers the promise of a simplified, rapid, and high-sensitive detection of biomolecular interactions with several advantages over previous assay methodologies. In this work, we have conceived reproducible SERS nanosensors based on tailored multilayer octupolar nanostructures which can combine high enhancement factor and remarkable molecular selectivity. We show that coating novel multilayer octupolar metastructures with proper self-assembled monolayer (SAM) and immobilized phages can provide label-free analysis of pathogenic bacteria via SERS leading to a giant increase in SERS enhancement. The strong relative intensity changes of about 2100% at the maximum scattered SERS wavelength, induced by the Brucella bacterium captured, demonstrate the performance advantages of the bacteriophage sensing scheme. We performed measurements at the single-cell level thus allowing fast identification in less than an hour without any demanding sample preparation process. Our results based on designing well-controlled octupolar coupling platforms open up new opportunities toward the use of bacteriophages as recognition elements for the creation of SERS-based multifunctional biochips for rapid culture and label-free detection of bacteria.

A Low-cost, Highly-stable Surface Enhanced Raman Scattering Substrate by Si Nanowire Arrays Decorated with Au Nanoparticles and Au Backplate

We present a facile and cost-effective manner to fabricate a highly sensitive and stable surface enhanced Raman scattering (SERS) substrate. First, a silicon nanowire array (SiNWA) is tailored by metal-assisted chemical etching (MaCE) method as a scaffold of the desired SERS substrate. Next, with an oblique angle deposition (OAD) method, optimized gold nanoparticles (AuNPs) are successfully decorated on the surface of the SiNWA. These AuNPs enable a strong localized electric field, providing abundant hot spots to intensify the Raman signals from the targeting molecules. By applying a well-established methodology, Taguchi method, which is invented for designing experiments, the optimized combination of parameters is obtained efficiently. The experimental results are also confirmed by finite-difference time-domain (FDTD) simulation calculations. Besides, a gold metal backplate (AuMBP) is applied to further enhancing the Raman signal intensity. Based on this developed SERS substrate, we demonstrated an enhancement factor (EF) of 1.78 × 10⁶ and a coefficient of variation (CV) of 4.2%. Both EF and CV indicate a highly stable property and the optimized SERS substrate substantially outperform the commercial product. In the end, we also demonstrate a quantitative measurement on practical application of detecting malachite green (MG) with concentration from 10 nM to 100 μM.

Bimetallic gold core–silver shell nanorod performance for surface enhanced Raman spectroscopy

Plasmonic gold nanorods (AuNRs) coated with four different thickness silver shells (AuNR\Ags) were synthesized and tested for their efficiency in Surface Enhanced Raman Scattering (SERS) signal enhancement for biomedical applications.

Multi-functional, thiophenol-based surface chemistry for surface-enhanced Raman spectroscopy

This article highlights the recent advances of thiophenol-based surface chemistry for the applications in surface-enhanced Raman spectroscopy (SERS).

Plasmonic nanostructures for surface enhanced spectroscopic methods

The development within the last five years in the field of surface enhanced spectroscopy methods was comprehensively reviewed.

Engineering 3D Nanoplasmonic Assemblies for High Performance Spectroscopic Sensing

We demonstrate the fabrication of plasmonic sensors that comprise gold nanopillar arrays exhibiting high surface areas, and narrow gaps, through self-assembly of amphiphilic diblock copolymer micelles on silicon substrates. Silicon nanopillars with high integrity over arbitrary large areas are obtained using copolymer micelles as lithographic templates. The gaps between metal features are controlled by varying the thickness of the evaporated gold. The resulting gold metal nanopillar arrays exhibit an engineered surface topography, together with uniform and controlled separations down to sub-10 nm suitable for highly sensitive detection of molecular analytes by Surface Enhanced Raman Spectroscopy (SERS). The significance of the approach is demonstrated through the control exercised at each step, including template preparation and pattern-transfer steps. The approach is a promising means to address trade-offs between resolutions, throughput, and performance in the fabrication of nanoplasmonic assemblies for sensing applications.

Millimeter-Sized Suspended Plasmonic Nanohole Arrays for Surface-Tension-Driven Flow-Through SERS

We present metallic nanohole arrays fabricated on suspended membranes as an optofluidic substrate. Millimeter-sized suspended nanohole arrays were fabricated using nanoimprint lithography. We demonstrate refractive-index-based tuning of the optical spectra using a sucrose solution for the optimization of SERS signal intensity, leading to a Raman enhancement factor of 107. Furthermore, compared to dead-ended nanohole arrays, suspended nanohole arrays capable of flow-through detection increased the measured SERS signal intensity by 50 times. For directed transport of analytes, we present a novel methodology utilizing surface tension to generate spontaneous flow through the nanoholes with flow rates of 1 μL/min, obviating the need for external pumps or microfluidic interconnects. Using this method for SERS, we obtained a 50 times higher signal as compared to diffusion-limited transport and could detect 100 pM 4-mercaptopyridine. The suspended nanohole substrates presented herein possess a uniform and reproducible geometry and show the potential for improved analyte transport and SERS detection.

Fabrication and characterization of flexible and tunable plasmonic nanostructures

We present a novel method to fabricate flexible and tunable plasmonic nanostructures based on combination of soft lithography and nanosphere lithography, and perform a comprehensive structural and optical characterization of these structures. Spherical latex particles are uniformly deposited on glass slides and used as molds for polydimethylsiloxane to obtain nanovoid structures. The diameter and depth of the nanostructures are controlled by the size of the latex particles. These surfaces are coated with a thin Ag layer for fabrication of uniform plasmonic nanostructures. Structural characterization of these surfaces is performed by SEM and AFM. Optical properties of these plasmonic nanostructures are evaluated via UV/Vis absorption spectroscopy, dark field microscopy, and surface–enhanced Raman spectroscopy (SERS). Position of the surface plasmon absorption depends on the diameter and depth of the nanostructures. SERS enhancement factor (measured up to 1.4 × 10⁶) is dependent on the plasmon absorption wavelength and laser wavelength used in these experiments.

Tunable and highly reproducible surface-enhanced Raman scattering substrates made from large-scale nanoparticle arrays based on periodically poled LiNbO3 templates

This work describes novel surface-enhanced Raman scattering (SERS) substrates based on ferroelectric periodically poled LiNbO₃ templates. The templates comprise silver nanoparticles (AgNPs), the size and position of which are tailored by ferroelectric lithography. The substrate has uniform and large sampling areas that show SERS effective with excellent signal reproducibility, for which the fabrication protocol is advantageous in its simplicity. We demonstrate ferroelectric-based SERS substrates with particle sizes ranging from 30 to 70 nm and present tunable SERS effect from Raman active 4-mercaptopyridine molecules attached to AgNPs when excited by a laser source at 514 nm.

Nanopore-induced spontaneous concentration for optofluidic sensing and particle assembly

Metallic nanopore arrays have emerged as optofluidic platforms with multifarious sensing and analytical capabilities such as label-free surface plasmon resonance (SPR) sensing of molecular binding interactions and surface-enhanced Raman spectroscopy (SERS). However, directed delivery of analytes through open nanopores using traditional methods such as external electric fields or pressure gradients still remains difficult. We demonstrate that nanopore arrays have an intrinsic ability to promote flow through them via capillary flow and evaporation. This passive "nano-drain" mechanism is utilized to concentrate biomolecules on the surface of nanopores for improved detection sensitivity or create ordered nanoscale arrays of beads and liposomes. Without using any external pump or fluidic interconnects, we can concentrate and detect the presence of less than a femtomole of streptavidin in 10 μL of sample using fluorescence imaging. Liposome nanoarrays are also prepared in less than 5 min and used to detect lipid-protein interactions. We also demonstrate label-free SPR detection of analytes using metallic nanopore arrays. This method provides a fast, simple, transportable, and small-volume platform for labeled as well as label-free plasmonic analysis while improving the detection time and sensitivity.

Understanding the effects of dielectric medium, substrate, and depth on electric fields and SERS of quasi-3D plasmonic nanostructures

The local electric field distribution and the effect of surface-enhanced Raman spectroscopy (SERS) were investigated on the quasi-3D (Q3D) plasmonic nanostructures formed by gold nanohole and nanodisc array layers physically separated by a dielectric medium. The local electric fields at the top gold nanoholes and bottom gold nanodiscs as a function of the dielectric medium, substrate, and depth of Q3D plasmonic nanostructures upon the irradiation of a 785 nm laser were calculated using the three-dimensional finite-difference time-domain (3D-FDTD) method. The intensity of the maximum local electric fields was shown to oscillate with the depth and the stronger local electric fields occurring at the top or bottom gold layer strongly depend on the dielectric medium, substrate, and depth of the nanostructure. This phenomenon was determined to be related to the Fabry-Pérot interference effect and the interaction of localized surface plasmons (LSPs). The enhancement factors (EFs) of SERS obtained from the 3D-FDTD simulations were compared to those calculated from the SERS experiments conducted on the Q3D plasmonic nanostructures fabricated on silicon and ITO coated glass substrates with different depths. The same trend was obtained from both methods. The capabilities of tuning not only the intensity but also the location of the maximum local electric fields by varying the depth, dielectric medium, and substrate make Q3D plasmonic nanostructures well suited for highly sensitive and reproducible SERS detection and analysis.

Inclined nanoimprinting lithography for 3D nanopatterning

We report a non-conventional shear-force-driven nanofabrication approach, inclined nanoimprint lithography (INIL), for producing 3D nanostructures of varying heights on planar substrates in a single imprinting step. Such 3D nanostructures are fabricated by exploiting polymer anisotropic dewetting where the degree of anisotropy can be controlled by the magnitude of the inclination angle. The feature size is reduced from micron scale of the template to a resultant nanoscale pattern. The underlying INIL mechanism is investigated both experimentally and theoretically. The results indicate that the shear force generated at a non-zero inclination angle induced by the INIL apparatus essentially leads to asymmetry in the polymer flow direction ultimately resulting in 3D nanopatterns with different heights. INIL removes the requirements in conventional nanolithography of either utilizing 3D templates or using multiple lithographic steps. This technique enables various 3D nanoscale devices including angle-resolved photonic and plasmonic crystals to be fabricated.

Plasmonic nanopillar arrays for large-area, high-enhancement surface-enhanced Raman scattering sensors

Efforts to create reproducible surface-enhanced Raman scattering (SERS)-based chemical and biological sensors has been hindered by difficulties in fabricating large-area SERS-active substrates with a uniform, reproducible SERS response that still provides sufficient enhancement for easy detection. Here we report on periodic arrays of Au-capped, vertically aligned silicon nanopillars that are embedded in a Au plane upon a Si substrate. We illustrate that these arrays are ideal for use as SERS sensor templates, in that they provide large, uniform and reproducible average enhancement factors up to ∼1.2 × 10(8) over the structure surface area. We discuss the impact of the overall geometry of the structures upon the SERS response at 532, 633, and 785 nm incident laser wavelengths. Calculations of the electromagnetic field distributions and intensities within such structures were performed and both the wavelength dependence of the predicted SERS response and the field distribution within the nanopillar structure are discussed and support the experimental results we report.

Tailoring plasmonic nanostructures for optimal SERS sensing of small molecules and large microorganisms

Local electric fields can be tuned dramatically by varying the diameter of quasi-3D gold plasmonic nanostructure arrays, as indicated by 3D finite-difference time-domain calculations. Utilizing quasi-3D arrays that exhibit a maximum electric field intensity (i.e., a "hot" spot) either at the bottom (gold nanodisks) or on the top (gold film patterned with nanoholes), the optimal surface-enhanced Raman scattering (SERS) sensitivity for the detection of small molecules or large microorganisms can be achieved. The precisely fabricated and optimized SERS-active quasi-3D nanostructure arrays make it possible to quantitatively and reproducibly detect chemical and biological species using SERS, leading to a new sensing platform with molecular specificity based on SERS for many important applications.