Excellent Surface Enhanced Raman Spectroscopy Research Articles

Large-scale assembly and pattern transfer of SERS-active nanoparticles for application in drug monitoring of methotrexate in blood serum

Self-assembly of plasmonic gold nanoparticles (AuNPs) in periodic arrays with nanometric gaps results in the optimization of near- and far-field effects, which provides excellent surface-enhanced Raman spectroscopy (SERS) sensitivity. Herein, we report the assembly of ∼30 nm AuNPs on electron beam lithography (EBL) substrates using an inexpensive and simple method of stamping. This method yields a highly regular arrangement of AuNPs in the nanochannels of EBL substrates. An experimental approach reveals that, when using the stamping method, the groove width of the gratings plays an important role in linearly arranging the AuNPs on the EBL substrates and is a result of unidirectional flow of AuNPs due to the capillary effect. For assessment of the substrate’s SERS efficiency, rhodamine 6 G and brilliant blue were tested with a limit of detection (LOD) of 10.2 nM. To practically test the clinical-based translation of SERS substrates, detection of the chemotherapy drug methotrexate (MTX) was performed with an LOD of 13.7 nM in human blood serum to illustrate its potential in therapeutic drug monitoring (TDM) and its translation into clinics.

Ag Nanoparticles Synthesized on Black-Titanium Dioxide by Photocatalytic Method as Reusable Substrates of Surface-Enhanced Raman Spectroscopy

The construction of excellent surface-enhanced Raman spectroscopy (SERS) substrates needs rationally designed architectures of noble metals or semiconductors. In this study, Ag nanoparticles (NPs) are densely and uniformly synthesized on the surfaces of black-titanium dioxide (b-TiO2) NPs through a facile two-step photocatalysis method. The b-TiO2 improved the utilization efficiency of natural sunlight by the extension of light absorption from the ultraviolet (UV) to the visible (Vis) region. First, Ag seeds were densely grown in a short time on the surfaces of b-TiO2 NPs under the irradiation of UV light. Then, Ag NPs were grown slowly and uniformly from the Ag seeds under the irradiation of Vis light. The as-prepared Ag/b-TiO2 with high sensitivity achieved a limit of detection as low as 10−12 M for rhodamine 6G. Meanwhile, the substrate showed reusability due to the high photocatalytic ability of b-TiO2. The Ag/b-TiO2 SERS substrate achieves SERS detections of organic pollutants, such as hydroquinone, p-phenylenediamine, and terephthalic acid, indicating that this substrate possesses potential applications in food safety and environmental monitoring.

Toroidal dipole-modulated dipole-dipole double-resonance in colloidal gold rod-cup nanocrystals for improved SERS and second-harmonic generation.

Colloidal metal nanocrystals (NCs) show great potential in plasmon-enhanced spectroscopy owing to their attractive and structure-depended plasmonic properties. Herein, unique Au rod-cup NCs, where Au nanocups are embedded on the one or two ends of Au nanorods (NRs), are successfully prepared for the first time via a controllable wet-chemistry strategy. The Au rod-cup NCs possess multiple plasmon modes including transverse and longitudinal electric dipole (TED and LED), magnetic dipole (MD), and toroidal dipole (TD) modulated LED resonances, producing large extinction cross-section and huge near-field enhancements for plasmon-enhanced spectroscopy. Particularly, Au rod-cup NCs with two embedded cups show excellent surface-enhanced Raman spectroscopy (SERS) performance than Au NRs (75.6-fold enhancement excited at 633 nm) on detecting crystal violet owing to the strong electromagnetic hotspots synergistically induced by MD, LED, and TED-based plasmon coupling between Au cup and rod. Moreover, the strong TD-modulated dipole-dipole double-resonance and MD modes in Au rod-cup NCs bring a 37.3-fold enhancement of second-harmonic generation intensity compared with bare Au NRs, because they can efficiently harvest photoenergy at fundamental frequency and generate large near-field enhancements at second-harmonic wavelength. These findings provide a strategy for designing optical nanoantennas for plasmon-enhanced applications based on multiple plasmon modes.Electronic Supplementary MaterialSupplementary material (SEM image of Au rod-one-cup NCs; TEM image of Au/PbS hybrids; SEM image of Au rod-two-cup NCs; low-amplification SEM image of Au rod-two-cup NCs; experimental extinction and calculated electric field distributions of Au NR excited at different wavelengths; calculated absorption and scattering spectra of Au rod-one-cup NCs; schematic illustration of the cut plane and the corresponding magnetic field distribution under L3 excitation; Raman spectra of CV (10−6 M) adsorbed on Au rod-cup NCs with different cup sizes; calculated magnetic field distribution of Au rodcup NCs excited at 532 and 633 nm; calculated electric field distributions of Au rod-one-cup NC excited at 600 nm along TE and LE; the models of Au rod-cup NCs used in the simulations) is available in the online version of this article at 10.1007/s12274-022-4562-5.

Rapid and flexible construction of inverted silicon architectures with nanogaps as high-performance SERS substrates

Three-dimensional (3D) plasmonic nanostructures act as excellent surface-enhanced Raman spectroscopy (SERS) substrates for detecting trace analytes. Nevertheless, an efficient 3D architecture and the optimal fabrication route remain to be uncovered to further raise the performance of 3D SERS substrates and expand application horizons. This work reports a cost-effective and flexible route to fabricate 3D inverted silicon (Si) architectures with nanogaps that are difficult to achieve by conventional lithography towards SERS applications. Finite-difference time-domain simulations demonstrated that the distribution of electrical field depended significantly on the architecture and layout of inverted pyramids. Several parameters including volume ratio of HF/HNO3 mixtures, indentation force and etching time were evaluated to determine optimal fabrication process of inverted Si architecture. Based on an electrochemical dissolution model and molecular dynamic simulations, the rapid selective etching can be ascribed to nanocrystals in amorphous Si (a-Si) regions, and distorted Si beneath a-Si layer. SERS detection illustrated an impressive sensitivity and excellent reproducibility, and the practical applicability of as-prepared SERS substrates was demonstrated by detecting malachite green residues from different water environments as well as fish tissues and scales. The proposed selective etching provides a brand-new route for manufacturing high-performance 3D architecture substrates for SERS applications.

Fast, sensitive and low-cost chemical sensor based on manufacturing nanostructured Co3O4 using Raman Spectroscopy

Nanostructured Co3O4 has novel properties, so it can be manufactured with simple, fast preparation of samples, and a low-cost process. FESEM field emission electron microscope scanning, XRD diffraction, and Raman spectroscopy characterized Co3O4 nanostructured tetrahedral structures with multiple branches. Good sensitivity and selectivity and high stability can be seen for the nanostructured Co3O4. Cationic surfactant (CTAB) and anionic surfactant (PVP) both have a guiding effect on Co3O4 production. The self-assembled Co3O4 particles in Methylene Blue (MB) exhibit excellent Surface Enhanced Raman Spectroscopy efficiency. An average enhancement factor (EF) of 50 × 106 was achieved for the Co3O4 in PVP on 135 gsm printing paper and 41 ×106 for the Co3O4 in CTAB on 240 gsm printing paper.The results showed Co3O4 in PVP produced the highest sensitivity, specificity, and the lowest limit of detection LOD which was 0.52 nM for MB as Raman reporter molecule, and the sensitivity offered almost 98% with selectivity equal to 92%. To the best of the author’s knowledge, this is the first demonstration of Co3O4 in PVP paper-based as a chemical sensor by enhanced Raman spectroscopy signal.

Manipulating the surface-enhanced Raman spectroscopy (SERS) activity and plasmon-driven catalytic efficiency by the control of Ag NP/graphene layers under optical excitation

Abstract Highly efficient plasmon-driven catalysis and excellent surface-enhanced Raman spectroscopy (SERS) performance are proportional to the square of the local electromagnetic field (hot spot). However, a proven way to realize the enhancement in intensity and density of “hot spot” still needs to be investigated. Here, we report on multilayered Ag nanoparticle (Ag NP)/graphene coupled to an underlying Cu film system (MAgNP-CuF) which can be used as an effective SERS substrates realizing ultra-sensitive detection for toxic molecules and in situ monitoring the plasmon-driven reaction for p-nitrothiophenol (PNTP) to p,p′-dimercaptobenzene (DMAB) conversion. The mechanism of ultra-sensitive SERS response and catalytic reaction is investigated via Ag NP/graphene layer-dependent experiments combined with theoretical simulations. The research found that the intensity and density of “hot spot” can be effectively manipulated by the number of plasmonic layers, and the bottom Cu film could also reflect the scattered and excitation beam and would further enhance the Raman signals. Moreover, the MAgNP-CuF exhibits outstanding performance in stability and reproducibility. We believe that this concept of multilayered plasmonic structures would be widely used not only in the field of SERS but also in the wider research in photocatalysis.

Carbon based dots capped tin oxide nanosheets hybridizing with silver nanoparticles for ultra-sensitive surface enhanced raman scattering substrate

Carbon based dots capped tin oxide nanosheets (SnO2/CDs) were synthesized by liquid exfoliation of tin oxide (SnO2) nanoparticles in the presence of single-layer carbon based (CDs). The obtained SnO2/CDs have lateral sizes of about 20–50 nm and heights of about 3–4 nm. A thin layer of CDs of about 1–2 nm in thickness are presence on the surface of the SnO2 nanosheets. The SnO2/CDs present excellent surface enhanced Raman spectroscopy (SERS) activity due to the chemical enhancement (CM) led by the charge transfer between SnO2/CDs and the target molecules. The enhancement factor (EF) of the SnO2/CDs is 1.42 × 105, taking rhodamine 6G as a model target molecule. The SnO2/CDs are further hybridized with silver nanoparticles (AgNPs) using an in situ synthesis method. The obtained homogeneous nano-hybrids (SnO2/CDs@AgNPs) have a lot of nanogaps. The nanogaps among the AgNPs/CDs create strong electromagnetic enhancement (EM) due to the “hot spots” effect. The nanogaps among the SnO2/CDs and AgNPs/CDs allow the target molecules to be embedded, and thus gained both the CM effect of SnO2/CDs and the EM effect of AgNPs/CDs. As a result, the as-prepared SnO2/CDs@AgNPs exhibits excellent SERS activities, with an ultrahigh enhancement factor of 3.27 × 1011.

Morphology-Related Functionality in Nanoarchitectured GaN

Integrating silicon and III-nitride technologies for high-speed and large bandwidth communication demands optically interconnected active components that detect, process, and emit photons and electrons. It is imperative that multifunctional materials can enhance the performance and simplify fabrication of such devices. Spontaneously grown GaN in the nanowall network (NwN) architecture simultaneously displays unprecedented optical and electrical properties. A two-order increase in band-edge emission makes it suitable for high-brightness light-emitting diodes and laser applications. Decorating this NwN with silver nanoparticles further enhances emission through plasmonic interactions and renders it an excellent surface-enhanced Raman spectroscopy substrate for biomolecular detection. The observation of very high electron mobility (approximately 104 cm2/Vs) and large phase-coherence length (60 μm) is a consequence of two-dimensional (2D) electron gas formation applicable for high electron mobility transistors. Detecting ballistic transport in the nanowalls confirms proximity-induced superconductivity (<5 K and 8 T). Charge separation properties render it a device material for UV photodetectors, photoanodes for water splitting, and thermionic field emitters.

Optimizing the SERS Performance of 3D Substrates through Tunable 3D Plasmonic Coupling toward Label-Free Liver Cancer Cell Classification

Three-dimensional (3D) plasmonic nanostructures are emerging as excellent surface-enhanced Raman spectroscopy (SERS) substrates for chemical and biomedical applications. However, the correlation of 3D (including both in-plane and out-of-plane) plasmonic coupling with the SERS properties to deepen the understanding of 3D SERS substrates remains a challenge. Here, we perform correlation studies of 3D plasmonic coupling and SERS properties of the 3D hierarchical SERS substrates by tuning the multiscale structural elements. The effects of zero-dimensional (0D; the size of the building blocks), one-dimensional (1D; the thickness of the 3D substrates), and two-dimensional (2D; the composition of individual monolayers) structural elements on 3D plasmonic coupling are studied by performing UV-vis-near-infrared (NIR) spectroscopy and measuring SERS performance. It shows that both the extinction spectra and SERS enhancement are tuned at the 3D structural level. It is demonstrated that the plasmonic resonance wavelength (PRW) stemming from the 3D plasmonic coupling correlates with the SERS averaged surface enhancement factor (ASEF) and is improved by more than tenfold at the optimum 3D nanostructure. The optimized substrate is used to quantitatively analyze two small biological molecules. Moreover, as a proof-of-concept study, the substrate is first applied to differentiate between living liver normal and cancer cells with a high prediction accuracy through the spectral features of the cell membranes and the metabolites secreted outside the cells. We expect that the tuning of plasmonic coupling at the 3D level can open up new routes to design high-performance SERS substrates for wide applications.

Multistimulus-Responsive Supramolecular Hydrogels Derived by in situ Coating of Ag Nanoparticles on 5'-CMP-Capped β-FeOOH Binary Nanohybrids with Multifunctional Features and Applications.

The present manuscript reports the synthesis of multistimulus-responsive smart supramolecular hydrogels derived by in situ coating of silver nanoparticles (Ag NPs) on colloidal cytidine-5′-monophosphate-capped β-FeOOH nanohybrids (β-FeOOH@5′-CMP) under physiological conditions forming a polycrystalline building block (Ag-coated β-FeOOH@5′-CMP). The presence of Ag in the binary nanohybrids induces the puckering of ribose sugar, bringing a change in its conformation from C2′-endo to C3′-endo, which enhanced the supramolecular interactions among different moieties of other building blocks to construct a porous network of hydrogels in the self-assembly via the formation of a micellar structure. Such a supramolecular network in hydrogel is also evidenced by the reversible sol⇌gel transformation under multistimulus-responsiveness in a narrow range of pH, temperature, and sonication, as well as by the manifestation of rapid self-healing and injectability features. As-synthesized hydrogels exhibiting shear-thinning behavior under higher strain and converting back into the sol under low strain, suggests their potential for localized drug delivery. The presence of Ag NPs in the hydrogel enhanced its viscoelastic properties, % swelling (580) and loading capabilities (590 mg g–1) for methylene blue (MB), and its controlled release over days (∼2–30) as a function of pH. It displayed excellent surface-enhanced Raman spectroscopy activity allowing to detect MB-like drug molecules at ≤10–12 M. Thus, the as-synthesized hydrogels represent a unique superparamagnetic nanosystem consisting of all greener components (5′-CMP/β-FeOOH/Ag) with superior viscoelastic, sensing, and antimicrobial properties, displaying multistimulus-responsiveness (pH/temperature/sonication), thereby suggesting their vast potential for biomedical and environmental applications.

Tunable plasmonics of hollow raspberry-like nanogold for the robust Raman scattering detection of antibiotics on a portable Raman spectrometer.

It is important to develop novel sensors for the simple, rapid, and quantitative detection of residual antibiotics in food, considering their potential threats to human health. Herein, we report the successful fabrication of raspberry-like nanogold (R-like Au) structures with rough surfaces and partially hollow structures for the rapid and sensitive detection of antibiotics in duck meats on a portable Raman spectrometer. The R-like Au with tunable plasmonic wavelengths was fabricated by a two-step method. Ag particles as seeds were substituted by Au particles through a replacement reaction; moreover, the surface plasmon resonances (SPRs) of R-like Au red-shifted with the increase in the Au ion concentration. The R-like Au with a diameter of about 120 nm that matched well with the 785 nm laser on the portable Raman spectrometer was used as an excellent surface-enhanced Raman spectroscopy (SERS) active platform for detecting two kinds of antibiotics, namely, nitrofurantoin (NFT) and nitrofurazone (NFZ), in spiked duck meats. Both of them have sufficient sensitivity (0.05-10 mg L-1), good linear relationship (R2 > 0.99) and high recovery in quantitative SERS analysis. The platform is simple without the need for complex pretreatment of the food samples or use of large scale Raman spectrometers, promising easy implementation for the on-site analysis of residuals of antibiotics in food.

In Situ Decoration of Gold Nanoparticles on Graphene Oxide via Nanosecond Laser Ablation for Remarkable Chemical Sensing and Catalysis.

Gold decorated graphene-based nano-hybrids find extensive research interest due to their enhanced chemical catalytic performance and biochemical sensing. The unique physicochemical properties and the very large surface area makes them propitious platform for the rapid buildouts of science and technology. Graphene serves as an outstanding matrix for anchoring numerous nanomaterials because of its atomically thin 2D morphological features. Herein, we have designed a metal-graphene nano-hybrid through pulsed laser ablation. Commercially available graphite powder was employed for the preparation of graphene oxide (GO) using modified Hummers’ method. A solid, thin gold (Au) foil was ablated in an aqueous suspension of GO using second harmonic wavelength (532 nm) of the Nd:YAG laser for immediate generation of the Au-GO nano-hybrid. The synthesis strategy employed here does not entail any detrimental chemical reagents and hence avoids the inclusion of reagent byproducts to the reaction mixture, toxicity, and environmental or chemical contamination. Optical and morphological characterizations were performed to substantiate the successful anchoring of Au nanoparticles (Au NPs) on the GO sheets. Remarkably, these photon-generated nano-hybrids can act as an excellent surface enhanced Raman spectroscopy (SERS) platform for the sensing/detection of the 4-mercaptobenzoic acid (4-MBA) with a very low detection limit of 1 × 10−12 M and preserves better reproducibility also. In addition, these hybrid materials were found to act as an effective catalyst for the reduction of 4-nitrophenol (4-NP). Thus, this is a rapid, mild, efficient and green synthesis approach for the fabrication of active organometallic sensors and catalysts.

Ag@Au Core⁻Shell Porous Nanocages with Outstanding SERS Activity for Highly Sensitive SERS Immunoassay.

A highly sensitive immunoassay of biomarkers has been achieved using 4-mercaptobenzoic acid-labeled Ag@Au core–shell porous nanocage tags and α-fetoprotein immuno-sensing chips. The Ag@Au porous nanocages were uniquely synthesized by using an Ag core as a self-sacrificial template and reducing agent, where the slow reaction process led to the formation of a porous Au layer. The size of the remaining Ag core and surface roughness of the Au shell were controlled by adjusting the chloroauric acid concentration. The porous cage exhibited excellent surface-enhanced Raman spectroscopy (SERS) activity, presumably due to a synergetic interaction between newly generated hot spots in the rough Au shell and the retained SERS activity of the Ag core. Using α-fetoprotein as a model analyte for immunoassay, the SERS signal had a wide linear range of 0.20 ng mL−1 to 500.0 ng mL−1 with a detection limit of 0.12 ng mL−1. Without the need of further signal amplification, the as-prepared Ag@Au bimetallic nanocages can be directly used for highly sensitive SERS assays of other biomarkers in biomedical research, diagnostics, etc.

Femtosecond Photon‐Mediated Plasma Enhances Photosynthesis of Plasmonic Nanostructures and Their SERS Applications

Laser ablation in liquid has proven to be a universal and green method to synthesize nanocrystals and fabricate functional nanostructures. This study demonstrates the superiority of femtosecond laser-mediated plasma in enhancing photoredox of metal cations for controllable fabrication of plasmonic nanostructures in liquid. Through employing upstream high energetic plasma during laser-induced microexplosions, single/three-electron photoreduction of metallic cations can readily occur without chemical reductants or capping agents. Experimental evidences demonstrate that this process exhibits higher photon utilization efficiency in yield of colloidal metal nanoparticles than direct irradiation of metallic precursors. Photogenerated hydrated electrons derived from strong ionization of silicon and water are responsible for this enhanced consequences. Furthermore, these metallic nanoparticles are accessible to self-assemble into nanoplates for silver and nanospheres for gold, favored by surface-tension gradients between laser irradiated and unirradiated regions. These metallic nanostructures exhibit excellent surface-enhanced Raman spectroscopy performance in trace detection of Rhodamine 6G (R6G), 4-mercaptobenzoic acid (4-MBA), and mercapto-5-nitrobenzimidazole molecules with high sensitivity (down to 10-12 mol L-1 , 30 × 10-15 m for R6G), good reproducibility (relative standard deviation < 7%), and good dual-analyte detection ability with mixture ratios of R6G to 4-MBA ranging from 20 to 0.025. The conceptual importance of this plasma-enhanced-photochemical process may provide exciting opportunities in photochemical reactions, plasmofluidics, and material synthesis.

Protein-aided formation of triangular silver nanoprisms with enhanced SERS performance.

In this work, we present a modified seed-mediated synthetic strategy for the growth of silver nanoprisms with low shape polydispersity, narrow size distribution and tailored plasmonic absorbance. During the seed nucleation step, consensus sequence tetratricopeptide repeat (CTPR) proteins are utilized as potent stabilizers to facilitate the formation of planar-twinned Ag seeds. Ag nanoprisms were produced in high yield in a growth solution containing ascorbic acid and CTPR-stabilized Ag seeds. From the time-course UV-Vis and transmission electron microscopy (TEM) studies, we postulate that the growth mechanism is the combination of facet selective lateral growth and thermodynamically driven Ostwald ripening. The resultant Ag nanotriangles (NTs) exhibit excellent surface enhanced Raman spectroscopy (SERS) performance. The enhancement factor (EF) measured for the 4-mercapto benzoic acid (4-MBA) reporter is estimated to be 3.37 × 105 in solution and 2.8 × 106 for the SERS substrate.

Rapid Au recovery from aqueous solution by a microorganism-mediated, surfactant-directed approach: Effect of surfactants and SERS of bio-Au

Gold recovery is of practical significance while functional Au nanostructures are of great interest in modern nanotechnology. In this work, highly and hierarchically branched Au nanowires (AuNWs) were produced to obtain bio-AuNW composites while the Au was effectively recovered from aqueous HAuCl4 solution by a microorganism-mediated, surfactant-directed (MSD) approach with Lactobacillus spp. cells (LSCs). Ternary coordination of the cell-surfactant-AA played an important role in recovering Au and producing the bio-AuNW composites in the cases of tetradecyl trimethyl ammonium bromide (TTAB), hexadecyltrimethylammonium bromide (CTAB) and octadecyl trimethyl ammonium bromide (OTAB). In contrast, Au recovery could not be achieved with hexadecyltrimethylammonium chloride (CTAC). Strong interaction between [AuCl4]− and the LSCs prior to addition of surfactant (except CTAC) and ascorbic acid (AA) was vital for the highly and hierarchically branched nanostructures. Furthermore, the bio-AuNW composites could serve as excellent surface-enhanced Raman spectroscopy (SERS) substrate. the SERS detection limits of toxic rhodamine 6G (R6G) and 4-mercaptobenzoic acid (MBA) with the bio-AuNW composites are down to 10−9 and 10−6M, respectively. This work therefore exemplifies combining recovery of Au and fabrication of functional Au nanomaterials, which was environmentally benign and application-oriented.

Capillary force-induced glue-free printing of Ag nanoparticle arrays for highly sensitive SERS substrates.

The fabrication of well-ordered metal nanoparticle structures onto a desired substrate can be effectively applied to several applications. In this work, well-ordered Ag nanoparticle line arrays were printed on the desired substrate without the use of glue materials. The success of the method relies on the assembly of Ag nanoparticles on the anisotropic buckling templates and a special transfer process where a small amount of water rather than glue materials is employed. The anisotropic buckling templates can be made to have various wavelengths by changing the degree of prestrain in the fabrication step. Ag nanoparticles assembled in the trough of the templates via dip coating were successfully transferred to a flat substrate which has hydrophilic surface due to capillary forces of water. The widths of the fabricated Ag nanoparticle line arrays were modulated according to the wavelengths of the templates. As a potential application, the Ag nanoparticle line arrays were used as SERS substrates for various probing molecules, and an excellent surface-enhanced Raman spectroscopy (SERS) performance was achieved with a detection limit of 10(-12) M for Rhodamine 6G.

Structured Metallic Films for Optical and Spectroscopic Applications via Colloidal Crystal Templating

Two new methods of preparing thin porous metallic films, based on controlled co-crystallization of latex microspheres and gold nanoparticles, have been developed by these authors. The new materials show interesting optical properties and excellent surface-enhanced Raman spectroscopy (SERS) activity. The Figure shows a square pore array obtained at a specific film thickness.