Fabrication Strategies of 3D Plasmonic Structures for SERS

Raman Scattering Enhancement through Pseudo-Cavity Modes

Raman spectroscopy plays a pivotal role in spectroscopic investigations. The small Raman scattering cross-section of numerous analytes, however, requires enhancement of the signal through specific structuring of the electromagnetic and morphological properties of the underlying surface. This enhancement technique is known as surface-enhanced Raman spectroscopy (SERS). Despite the existence of various proposed alternatives, the approach involving Fabry-Pérot cavities, which constitutes a straightforward method to enhance the electromagnetic field around the analyte, has not been extensively utilized. This is because, for the analyte to experience the maximum electric field, it needs to be embedded within the cavity. Consequently, the top mirror of the cavity will eventually shield it from the external laser source. Recently, an open-cavity configuration has been demonstrated to exhibit properties similar to the classic Fabry-Pérot configuration, with the added advantage of maintaining direct accessibility for the laser source. This paper showcases how such a simple yet innovative configuration can be effectively utilized to achieve remarkable Raman enhancement. The simple structure, coupled with its inexpensive nature and versatility in material selection and scalability, makes it an ideal choice for various analytes and integration into diverse Raman apparatus setups.

Utilizing peptide-anchored DNA templates for novel programmable nanoparticle assemblies in biological macromolecules: A review

Biological macromolecules such as proteins and DNA are known to self-assemble into various structural moieties with distinct functions. While nucleic acids are the structural building blocks, peptides exemplify diversity as tailorable biochemical units. Thus, combining the scaffold properties of the biomacromolecule DNA and the functionality of peptides could evolve into a powerful method to obtain tailorable nano assemblies. In this review, we discuss the assembly of non-DNA-coated colloidal NPs on DNA/peptide templates using functional anchors. We begin with strategies for directly attaching metallic NPs to DNA templates to ascertain the functional role of DNA as a scaffold. Followed by methods to assemble peptides onto DNA templates to emphasize the functional versatility of biologically abundant DNA-binding peptides. Next, we focus on studies corroborating peptide self-assembling into macromolecular templates onto which NPs can attach to emphasize the properties of NP-binding peptides. Finally, we discuss the assembly of NPs on a DNA template with a focus on the bifunctional DNA-binding peptides with NP-binding affinity (peptide anchors). This review aims to highlight the immense potential of combining the functional power of DNA scaffolds and tailorable functionalities of peptides for NP assembly and the need to utilize them effectively to obtain tailorable hierarchical NP assemblies.

Alkoxysilane-Mediated Decoration of Si Nanowires Vertical Arrays with Au Nanoparticles as Improved SERS-Active Platforms

The search for improved transducers to fabricate better-performing (bio)sensors is a challenging but rewarding endeavor aiming to better diagnose and treat diseases. In this paper, we report on the decoration of a dense vertical array of ultrathin silicon nanowires (Si NWs), produced by metal-assisted chemical etching, with 20 nm gold nanoparticles (Au NPs) for surface-enhanced Raman scattering (SERS) applications. To optimize the production of a uniform 3D SERS active platform, we tested different Si NW surface functionalizations with various alkoxysilanes before Au decoration. Scanning electron microscopy investigations confirm that Au NPs decorate both bare and (3-glycidiloxypropyl)trimethoxysilane (GPTMS)-modified Si NWs with a high surface coverage uniformity. The SERS response of the decorated NWs was probed using a model dye system (methylene blue; MB) at 633 and 785 nm excitation wavelengths. The GPTMS-modified NWs present the highest enhancements of 2.9 and 2.6 for the 450 cm-1 and 1625 cm-1 peaks under 785 nm excitation and of 10.8 and 5.3 for the 450 cm-1 and 1625 cm-1 peaks under 633 nm excitation. These results demonstrate the perspective role of Si NWs decorated with Au NPs as a low-cost 3D SERS platform.

LSP-SPP Coupling Structure Based on Three-Dimensional Patterned Sapphire Substrate for Surface Enhanced Raman Scattering Sensing

Although the fabrication of controllable three-dimensional (3D) microstructures on substrates has been proposed as an effective solution for SERS, there remains a gap in the detection and manufacturability of 3D substrates with high performance. In this study, photolithography is adopted to obtain a pyramid-like array on a patterned sapphire substrate (PSS), with Al₂O₃ as the dielectric layer. In addition, silver nanoparticles (AgNPs) are used to decorate Au films to obtain mass-producible 3D SRES substrates. In the case of low fluorescence, the substrate realizes the coupling of localized surface plasmon polaritons (LSPs) and surface plasmon polaritons (SPPs), which is consistent with the simulation results obtained using the finite element method. The performance of the SERS substrate is evaluated using rhodamine 6G (R6G) and toluidine blue (TB) as probe molecules with detection limits of 10^-11 M and 10^-9 M, respectively. The substrate exhibits high hydrophobicity and excellent light-capturing capability. Moreover, it shows self-cleaning ability and long-term stability in practical applications. Allowing for the consistency of the composite substrate in the preparation process and the high reproducibility of the test results, it is considered to be promising for mass production.

Fabry-Perot Cavity Control for Tunable Raman Scattering

The Fabry-Perot (FP) resonator is an intuitive and versatile optical structure owing to its uniqueness in light-matter interactions, yielding resonance with a wide range of wavelengths as it couples with photonic materials encapsulated in a dielectric cavity. Leveraging the FP resonator for molecular detection, a simple geometry of the metal-dielectric-metal structure is demonstrated to allow tuning of the enhancement factors (EFs) of surface-enhanced Raman scattering (SERS). The optimum near-field EF from randomly dispersed gold nano-gaps and dynamic modulation of the far-field SERS EF by varying the optical resonance of the FP etalon are systematically investigated by performing computational and experimental analyses. The proposed strategy of combining plasmonic nanostructures with FP etalons clearly reveals wavelength matching of FP resonance to excitation and scattering wavelengths plays a key role in determining the magnitude of the SERS EF. Finally, the optimum near-field generating optical structure with controlled dielectric cavity is suggested for a tunable SERS platform, and its dynamic SERS switching performance is confirmed by demonstrating information encryption through liquid immersion.

Combining thin-film microextraction and surface enhanced Raman spectroscopy to sensitively detect thiram based on 3D silver nanonetworks

By coupling thin-film microextraction (TFME) with surface enhanced Raman scattering (SERS), a facile method was developed for the determination of thiram in the complex matrix (orange juice or grape peel). The substrate of TFME was made by self-assembling silver sol on the silicon wafer to form a three-dimensional (3D) silver nanonetwork structure, without adding any template, which was used for TFME and SERS detection, respectively. The substrate exhibits high reproducibility with a relative standard deviation of about 7.32 % in spot and spot SERS intensity. The SERS signal intensity at a shift of 1384 cm^-1 and the thiram concentration showed good linearity in the range of 0.01-5 µg/L and the linear correlation coefficient was 0.9912. The detection limit for thiram was found to be 0.01 µg/L. The TFME-SERS method was applied for the determination of thiram in fruit juice and the results were obtained very well. Therefore, this method is expected to play a role in the detection of trace pollutants.

Therapeutic significance of nano- and biosensor technology in combating SARS-CoV-2: a review

The diagnosis of novel coronavirus (COVID-19) has gained the spotlight of the world's scientific community since December 2019 and it remains an important issue due to the emergence of novel variants around the globe. Early diagnosis of coronavirus is captious to prevent and hard to control. This pandemic can be eradicated by implementing suppressing strategies which can lead to better outcomes and more lives being saved. Therefore, the analysis showed that COVID-19 can only be managed by adopting public health measures, such as testing, isolation and social distancing. Much work has been done to diagnose coronavirus. Various testing technologies have been developed, opted and modified for rapid and accurate detection. The advanced molecular diagnosis relies on the detection of SARS-CoV-2 as it has been considered the main causative agent of this pandemic. Studies have shown that several molecular tests are considered essential for the confirmation of coronavirus infection. Various serology-based tests are also used in the detection and diagnosis of coronavirus including point-of-care assays and high-throughput enzyme immunoassays that aid in the diagnosis of COVID-19. Both these assays are time-consuming and have less diagnostic accuracy. Nanotechnology has the potential to develop new strategies to combat COVID-19 by developing diagnostics and therapeutics. In this review, we have focused on the nanotechnology-based detection techniques including nanoparticles and biosensors to obstruct the spread of SARS-CoV-2.

Colloidal Multiscale Assembly via Photothermally Driven Convective Flow for Sensitive In-Solution Plasmonic Detections

The assembly of metal nanoparticles and targets to be detected in a small light probe volume is essential for achieving sensitive in-solution surface-enhanced Raman spectroscopy (SERS). Such assemblies generally require either chemical linkers or templates to overcome the random diffusion of the colloids unless the aqueous sample is dried. Here, a facile method is reported to produce 3D multiscale assemblies of various colloids ranging from molecules and nanoparticles to microparticles for sensitive in-solution SERS detection without chemical linkers and templates by exploiting photothermally driven convective flow. The simulations suggest that colloids sub 100 nm in diameter can be assembled by photothermally driven convective flow regardless of density; the assembly of larger colloids up to several micrometers by convective flow is significant only if their density is close to that of water. Consistent with the simulation results, the authors confirm that the photothermally driven convective flow is mainly responsible for the observed coassembly of plasmonic gold nanorods with either smaller molecules or larger microparticles. It is further found that the coassembly with the plasmonic nanoantennae leads to dramatic Raman enhancements of molecules, microplastics, and microbes by up to fivefold of magnitude compared to those measured in solution without the coassembly.

Covalently Attaching Hollow Silica Nanoparticles on a COC Surface for the Fabrication of a Three-Dimensional Protein Microarray

Compared to traditional two-dimensional (2D) biochips, three-dimensional (3D) biochips exhibit the advantages of higher probe density and detection sensitivity due to their designable surface microstructure as well as enlarged surface area. In the study, we proposed an approach to prepare a 3D protein chip by deposition of a monolayer of functionalized hollow silica nanoparticles (HSNs) on an activated cyclic olefin copolymer (COC) substrate. First, the COC substrate was chemically modified through the photografting technique to tether poly[3-(trimethoxysilyl) propyl methacrylate] (PTMSPMA) brushes on it. Then, a monolayer of HSNs was deposited on the modified COC and covalently attached via a condensation reaction between the hydrolyzed pendant siloxane groups of PTMSPMA and the Si-OH groups of HSNs. The roughness of the COC substrate significantly increased to 50.3 nm after depositing a monolayer of HSNs (ranging from 100 to 700 nm), while it only caused a negligible reduction in the light transmittance of COC. The HSN-modified COC was further functionalized with epoxide groups by a silane coupling agent for binding proteins. Immunoglobulin G could be effectively immobilized on this substrate with the highest immobilization efficiency of 75.2% and a maximum immobilization density of 1.236 μg/cm², while the highest immobilization efficiency on a 2D epoxide group-modified glass slide was only 57.4%. Moreover, immunoassay results confirmed a competitive limit of detection (LOD) (1.06 ng/mL) and a linear detection range (1-100 ng/mL) of the 3D protein chip. This facile and effective approach for fabricating nanoparticle-based 3D protein microarrays has great potential in the field of biorelated detection.

Development of Gold Nanoparticle-Based SERS Substrates on TiO2-Coating to Reduce the Coffee Ring Effect

Hydrophilic surface-enhanced Raman spectroscopy (SERS) substrates were prepared by a combination of TiO₂-coatings of aluminium plates through a direct titanium tetraisopropoxide (TTIP) coating and drop coated by synthesised gold nanoparticles (AuNPs). Differences between the wettability of the untreated substrates, the slowly dried Ti(OH)₄ substrates and calcinated as well as plasma treated TiO₂ substrates were analysed by water contact angle (WCA) measurements. The hydrophilic behaviour of the developed substrates helped to improve the distribution of the AuNPs, which reflects in overall higher lateral SERS enhancement. Surface enhancement of the substrates was tested with target molecule rhodamine 6G (R6G) and a fibre-coupled 638 nm Raman spectrometer. Additionally, the morphology of the substrates was characterised using scanning electron microscopy (SEM) and Raman microscopy. The studies showed a reduced influence of the coffee ring effect on the particle distribution, resulting in a more broadly distributed edge region, which increased the spatial reproducibility of the measured SERS signal in the surface-enhanced Raman mapping measurements on mm scale.

Facile synthesis of Ag@C@Ag hybrid nanoparticles as SERS substrate

Ag@C core-shell nanoparticles (NPs) were first prepared by a low-temperature heating-stirring method and subsequently modified with polyethyleneimine (PEI) at different concentrations. Finally, Ag@C@Ag hybrid NPs were prepared by a simple self-assembly procedure, and 24-nm Ag NPs were attached onto the surface of the initially fabricated PEI-modified Ag@C NPs via interaction between the NH₂ groups of PEI and Ag. The results demonstrated that rhodamine 6G (R6G) could be detected at a concentration as low as 10^-10 M using the Ag@C@Ag NPs as a substrate. To further understand the signal enhancement mechanism, finite-difference time-domain (FDTD) simulations were performed to calculate the electromagnetic field distributions and illustrate the generated Raman hot spots. The FDTD indicated that this enhancement was attributed to the surface plasmon resonance effects of the core Ag NPs in the Ag@C NPs, hot spots between the Ag@C NPs, and external assembly of the 24-nm Ag NPs, as well as between the massive outlayer 24-nm Ag NPs themselves. These fabricated materials were further applied for the detection of folic acid as an actual sample. The outstanding performance of the Ag@C@Ag NPs can be attributed to both the excellent properties of this hybrid substrate and the absorption capability of the carbon layer. Thus, this Ag@C@Ag NP material demonstrates excellent and stable optical properties, and can be used as a surface-enhanced Raman scattering (SERS) substrate in the field of ultrasensitive spectral analysis. Graphical abstract Ag@C@Ag hybrid nanoparticles are prepared by a simple self-assembly method. Then the synthesized Ag@C@Ag hybrid nanoparticles are used as SERS substrate for folic acid detection. To further understand the signal enhancement mechanism, finite-difference time-domain simulations are performed to calculate the electromagnetic field distributions and illustrate the generated SERS hot spots.

Raman Spectroscopy-Based 3D Analysis of Odontogenic Differentiation of Human Dental Pulp Stem Cell Spheroids

Numerous efforts have been made to establish three-dimensional (3D) cell cultures that mimic the structure, cell composition, and functions of actual tissues and organs in vitro; however, owing to its intrinsic complexity, precise characterization of 3D differentiation remains challenging and often results in high variations in the resulting differentiated spheroids. This study reports a 3D Raman mapping-based analytical method that helps us to identify the crucial factors responsible for inducing variability in differentiated stem cell spheroids. Human dental pulp stem cell spheroids were generated at various cell densities using the hanging drop (HD) and molded parafilm-based (MP) methods and were then further subjected to odontogenic differentiation. Thereafter, the 3D cellular differentiation in these spheroids was analyzed based on three different Raman peaks, namely, 960, 1156/1528, and 2935 cm^-1, which correspond to hydroxyapatite (HA, odontogenic differentiation marker), β-carotene (precursor of HA), and proteins/cellular components (cell reference). By correlating such cell differentiation-related peaks and water/medium peaks (3400 cm^-1), we discovered that the diffusion of the medium containing various nutrients and differentiation factors was crucial in determining the variations in 3D differentiation of stem cell spheroids. Odontogenic differentiation was majorly induced at the outer shell of HD spheroids (up to ∼20 μm), whereas odontogenic differentiation was markedly induced in MP spheroids (up to 40-50 μm). Considering the challenges associated with high variations in spheroid and organoid differentiation, we conclude that the proposed Raman-based 3D analysis plays a pivotal role in stem cell-based regenerative therapy and drug screening.

Noble Metal Nanomaterial-Based Biosensors for Electrochemical and Optical Detection of Viruses Causing Respiratory Illnesses

Noble metal nanomaterials, such as gold, silver, and platinum, have been studied extensively in broad scientific fields because of their unique properties, including superior conductivity, plasmonic property, and biocompatibility. Due to their unique properties, researchers have used them to fabricate biosensors. Recently, biosensors for detecting respiratory illness-inducing viruses have gained attention after the global outbreak of coronavirus disease (COVID-19). In this mini-review, we discuss noble metal nanomaterials and associated biosensors for detecting respiratory illness-causing viruses, including SARS-CoV-2, using electrochemical and optical detection techniques. this review will provide interdisciplinary knowledge about the application of noble metal nanomaterials to the biomedical field.

Extra- and Intracellular Monitoring of TGF-β Using Single Immunoplasmonic Nanoprobes

Transforming growth factor-β (TGF-β) is a well-known disease-related biomarker associated with fibrotic diseases, and initiation and progression of cancer in many organs. Therefore, quantitative and sensitive detection of TGF-β and similar biomarkers is crucial for patient treatment in the early stages of diagnosis. In many studies, the detection of TGF-β, an important profibrotic and cancer promoting cytokine, has been generally conducted by fluorescence or absorbance-based immunoassays. However, conventional methods for detecting TGF-β have problems including use of time-consuming sample pretreatment steps and multiple reagents for signal amplification and difficulty in real-time detection from living cells. Herein, we present a plasmon-based immunoassay for TGF-β using antibody-conjugated single gold nanoparticles that act as optically excellent intracellular and extracellular detection probes that do not require additional signal amplification. To detect TGF-β sensitively and selectively, we exploited the localized surface plasmon resonance (LSPR) property of antibody-conjugated plasmonic gold nanoparticles at a single particle level. By measuring the LSPR spectral shifts of the single plasmonic nanoprobes, TGF-β can be detected down to the picomolar level, which is comparable with the conventional methods but without significant interference from other proteins. The optimized plasmonic nanoprobes were applied to quantify and monitor the extracellular TGF-β level secreted from the cells under stress conditions, such as cancer, and exposure to toxic environments. Owing to the ease of cellular internalization of the nanoprobes, we directly image and detect increases in intracellular TGF-β levels in living cells under the given stress conditions without cell lysis. We envision that this strategy of using individual nanoparticles as sensors to monitor protein biomarkers in living cells could be applied for various biological assays and diagnosis.

Controlling the Multiscale Topography of Anodized Aluminum Oxide Nanowire Structures for Surface-Enhanced Raman Scattering and Perfect Absorbers

In this study, a plasmonically active substrate is developed with the aim of controlling the perfect absorption and manipulating its optical properties for application in SERS (in NIR regime) and colorimetry. Based on modified fabrication method of anodized aluminum oxide (AAO), the cost-effective self-aggregation technique is presented to fabricate unique topography of bone-fire-like funnel-shaped collapsed and vertically aligned nanowire structures. The length of the nanowire and the modification of surface topography induced by capillary force inside the nanowire are set to structural parameters, and the effect of their changes is closely studied. After deposition of 40 nm gold (Au) film on numerous AAO nanowire structures with different wire lengths and unique topography, the localized surface plasmon resonance excitation is generated, and also its application on reflection and SERS spectra have been shown quantitatively. The length of the wire and surface topography modification are identified as suitable parameters to tune the reflection/absorption (from <40 to >90%) as well as colorimetric effect. Finally, an optimized wire length of Au-coated AAO substrate in SERS sensing application with 3.92 × 10⁵ order of enhancement of rhodamine 6G (R6G) Raman signal is demonstrated.

Surface Display Technology for Biosensor Applications: A Review

Surface display is a recombinant technology that expresses target proteins on cell membranes and can be applied to almost all types of biological entities from viruses to mammalian cells. This technique has been used for various biotechnical and biomedical applications such as drug screening, biocatalysts, library screening, quantitative assays, and biosensors. In this review, the use of surface display technology in biosensor applications is discussed. In detail, phage display, bacterial surface display of Gram-negative and Gram-positive bacteria, and eukaryotic yeast cell surface display systems are presented. The review describes the advantages of surface display systems for biosensor applications and summarizes the applications of surface displays to biosensors.

Synthesis of Monolayer Gold Nanorings Sandwich Film and Its Higher Surface-Enhanced Raman Scattering Intensity

Most previous studies relating to surface-enhanced Raman spectroscopy (SERS) signal enhancement were focused on the interaction between the light and the substrate in the x-y axis. 3D SERS substrates reported in the most of previous papers could contribute partial SERS enhancement via z axis, but the increases of the surface area were the main target for those reports. However, the z axis is also useful in achieving improved SERS intensity. In this work, hot spots along the z axis were specifically created in a sandwich nanofilm. Sandwich nanofilms were prepared with self-assembly and Langmuir-Blodgett techniques, and comprised of monolayer Au nanorings sandwiched between bottom Ag mirror and top Ag cover films. Monolayer Au nanorings were formed by self-assembly at the interface of water and hexane, followed by Langmuir-Blodgett transfer to a substrate with sputtered Ag mirror film. Their hollow property allows the light transmitted through a cover film. The use of a Ag cover layer of tens nanometers in thickness was critical, which allowed light access to the middle Au nanorings and the bottom Ag mirror, resulting in more plasmonic resonance and coupling along perpendicular interfaces (z-axis). The as-designed sandwich nanofilms could achieve an overall ~8 times SERS signals amplification compared to only the Au nanorings layer, which was principally attributed to enhanced electromagnetic fields along the created z-axis. Theoretical simulations based on finite-difference time-domain (FDTD) method showed consistent results with the experimental ones. This study points out a new direction to enhance the SERS intensity by involving more hot spots in z-axis in a designer nanostructure for high-performance molecular recognition and detection.

A TiO2 nanowire photocatalyst for dual-ion production in laser desorption/ionization (LDI) mass spectrometry

It has been challenging to detect small analytes in both positive and negative ion modes in MALDI-MS. Herein, TiO₂ nanowires are presented as a solid matrix to produce dual-ion of any analytes and to demonstrate the versatile applicability in LDI-MS.

Charge-transfer plasmons with narrow conductive molecular bridges: A quantum-classical theory

We analyze a new type of plasmon system arising from small metal nanoparticles linked by narrow conductive molecular bridges. In contrast to the well-known charge-transfer plasmons, the bridge in these systems consists only of a narrow conductive molecule or polymer in which the electrons move in a ballistic mode, showing quantum effects. The plasmonic system is studied by an original hybrid quantum-classical model accounting for the quantum effects, with the main parameters obtained from first-principles density functional theory simulations. We have derived a general analytical expression for the modified frequency of the plasmons and have shown that its frequency lies in the near-infrared (IR) region and strongly depends on the conductivity of the molecule, on the nanoparticle-molecule interface, and on the size of the system. As illustrated, we explored the plasmons in a system consisting of two small gold nanoparticles linked by a conjugated polyacetylene molecule terminated by sulfur atoms. It is argued that applications of this novel type of plasmon may have wide ramifications in the areas of chemical sensing and IR deep tissue imaging.

A review on recent advances in the applications of surface-enhanced Raman scattering in analytical chemistry

This review is focused on recent developments of surface-enhanced Raman scattering (SERS) applications in Analytical Chemistry. The work covers advances in the fabrication methods of SERS substrates, including nanoparticles immobilization techniques and advanced nanopatterning with metallic features. Recent insights in quantitative and sampling methods for SERS implementation and the development of new SERS-based approaches for both qualitative and quantitative analysis are discussed. The advent of methods for pre-concentration and new approaches for single-molecule SERS quantification, such as the digital SERS procedure, has provided additional improvements in the analytical figures-of-merit for analysis and assays based on SERS. The use of metal nanostructures as SERS detection elements integrated in devices, such as microfluidic systems and optical fibers, provided new tools for SERS applications that expand beyond the laboratory environment, bringing new opportunities for real-time field tests and process monitoring based on SERS. Finally, selected examples of SERS applications in analytical and bioanalytical chemistry are discussed. The breadth of this work reflects the vast diversity of subjects and approaches that are inherent to the SERS field. The state of the field indicates the potential for a variety of new SERS-based methods and technologies that can be routinely applied in analytical laboratories.