High-density Hot Spots Research Articles

Binary Thiol-Capped Gold Nanoparticle Monolayer Films for Quantitative Surface-Enhanced Raman Scattering Analysis.

Surface-enhanced Raman scattering (SERS) can provide fingerprint information of analyte molecules with unparalleled sensitivity. However, quantitative analysis using SERS has remained one of the major challenges owing to the difficulty of obtaining reproducible SERS substrates with high-density hotspots. Here, we report the rational design and fabrication of a binary thiol-capped gold nanoparticle (AuNP) monolayer film (MLF) as a substrate for highly sensitive and quantitative SERS analysis. The two thiol ligands chemically bonded to the AuNPs play different roles: dodecanethiol with a long alkyl chain controls the interparticle gaps and electromagnetic coupling among AuNPs and 4-mercaptopyridine works as a Raman internal standard (IS). The binary thiol-capped AuNPs can self-assemble into an ordered MLF with high-density hotspots anduniformly distributed IS. The as-prepared MLF has been demonstrated as a reliable SERS substrate for quantitative detection of fungicide malachite green in aqueous solution, with a high enhancement factor (up to 3.3 × 107) and a low detection limit (100 pM). Moreover, the MLF SERS substrate is flexible and transparent, which has enabled in situ detection of trace fungicide residues in a shrimp tissue.

Facile fabrication of Ag/graphene oxide/TiO2 nanorod array as a powerful substrate for photocatalytic degradation and surface-enhanced Raman scattering detection

A novel graphene oxide/TiO2 nanorod array (GO/TNR) decorated with Ag nanoparticles (NPs) is successfully prepared as a photocatalyst and a surface-enhanced Raman scattering (SERS) substrate for organic molecule detection. The as-obtained Ag/GO/TNR sample exhibited a large specific surface area and high adsorption capacity toward Rhodamine 6G (R6G) molecules. Moreover, the GO interlayer with suitable content had a significant effect on the absorption intensity of light and Raman signals for the Ag/GO/TNR substrate. For R6G as probe molecules, the Ag/GO/TNR substrate with 5 min of GO-deposition achieved a lower detection limit of 1.0 × 10−12 M and an improved enhancement factor (EF) of 5.86 × 105 as compared to other films. Furthermore, the relative standard deviation (RSD) value of the Raman vibration at 1650 cm−1 cm was approximately 8.71%, and the good uniformity remained. Using Xenon lamp irradiation, the outstanding photocatalytic ability endowed the substrate with good self-cleaning and reusable properties. Based on the above results, a synergistic effect of three components (Ag NPs, GO interlayer, and TNR) is proposed to produce an excellent SERS performance, namely, a large adhesion area, the interaction of excited photons, and high-density hotspots in the active substrate.

10×-Enhanced Heterogeneous Nanocatalysis on a Nanoporous Gold Disk Array with High-Density Hot Spots.

Certain noble metal nanostructures as heterogeneous photocatalysts have drawn significant attention in the recent past because of their unique optical properties which lead to the excitation of localized surface plasmon resonance (LSPR). The LSPR concentrates electromagnetic fields to the surfaces and its relaxation processes can convert photon energy to energetic charge carriers or heat, which can be subsequently harvested to enhance surface catalysis. Here, we report the catalytic performance of a novel plasmonic nanostructure, disk-shaped nanoporous gold (NPG) nanoparticles or simply NPG disks, using a well-tested reduction pathway of resazurin to resorufin. We show that the catalytic reaction rate of NPG disks is enhanced by 10-fold upon external light illumination because of the excitation of LSPR. The plasmon-enhanced catalytic reaction follows a linear-to-superlinear transition in the rate dependence on the input light power. In addition, the light input results in a room temperature reaction rate equivalent to that of an ambient temperature of 70 °C. Together, the results support that hot charge carriers play the dominant role in the enhancement.

Large-Area, Reproducible Ag-Film-Functionalized Mixed Polystyrene Spheres on Silicon for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical tool which presents high sensitivity in detection of the low concentration molecules and analytes. Its physical mechanism is based on strong confined electric fields in the tiny gaps between metallic nanostructures. In this work, two different sizes of polystyrene (PS) spheres were mixed to fabricate large-area quasi-random nanoarray on silicon decorated with Ag film for SERS. With the tuning of the volume ratio of two different sizes of spheres and the thickness of Ag film, it will always exhibit higher SERS intensity than periodic nanoarray with only one size. The Finite-Difference Time-Domain (FDTD) simulation shows that this hybrid structure provides high density of hot spots around the hybrid nanostructure due to surface plasmons, and the produced long decay length of the enhanced electric field makes the SERS substrate a zero-gap system for sensitive detection of large biomolecules. And this SERS system achieved an enhancement factor of the order of 105-106 in the detection of R6G molecules. The hybrid nanostructure is reproducible with a relative standard deviation of 10% .And the SERS substrates also produced sensitive Raman signals on long Aβ and hIAPP fibrils, which is potential for sensitive detection of amyloid aggregates that are related to Alzheimer's disease and type 2 diabetes (T2D). Our study demonstrates that the Ag-Film-Functionalized mixed PS spheres substrates may serve as good SERS substrates in the detection for biomedical and chemical analytes.

3D nanoporous plasmonic chips for extremely sensitive NO2 detection.

The detection of toxic gas molecules using the surface-enhanced Raman spectroscopy (SERS) technique is very challenging due to the low affinity of gas molecules. Here, we report extremely sensitive SERS-based NO2 gas sensors based on 3D nanoporous Au nanostructures with a high affinity for NO2 gas molecules and high density of hotspots.

Remarkable surface-enhanced Raman scattering on self-assembled {201} anatase

The self-assembled sea urchin-like {201} TiO2 exhibits Raman enhancement factors at 1.6 × 106, ascribed to shape-dependent electromagnetic enhancement with high-density hotspots in the vicinity of sharp tips and at narrow gaps between spines.

A porous Au-Ag hybrid nanoparticle array with broadband absorption and high-density hotspots for stable SERS analysis.

Constructing high-density hotspots is of crucial importance in surface enhanced Raman scattering (SERS). In this paper, we present a large-area and broadband porous Au-Ag hybrid nanoparticle array which was fabricated by an ultra-thin alumina mask (UTAM) technique incorporated with annealing and galvanic replacement techniques. Experimental results and numerical simulations demonstrated that the porous Au-Ag hybrid nanoparticle array possessed enormous hotspots for high sensitivity, uniformity, and stability in SERS analysis. A large Raman enhancement factor of 2.2 × 107 was achieved with a relative standard deviation (RSD) of 7.7%, leading to excellent reliability for Raman detection. Furthermore, this novel substrate exhibited a long shelf time in an ambient environment and promising practical applications in many SERS-based quantitative analytical and biomedical sensing techniques.

Placing hubs in CO2 pipelines: An application to industrial CO2 emissions in the Iberian Peninsula

Carbon capture and geological storage (CCS) is a key technology for the World deep decarbonization. However, several challenges remain, such as the optimization of the carbon transportation infrastructures. This study proposes a methodology that applies the Kernel Density function in a geographic information system software and uses as input, CO2 emission sources data to identify emission clusters and emission high-density hotspots. The main goal of the proposed methodology is to perform a preliminary screening to identify areas of interest to install hubs when designing an optimized CO2 pipeline network. The methodology includes an estimation of capturable CO2 emissions and a density analysis that was based on Kernel Density function from the ArcGIS Desktop 10. The methodology was applied to the Iberian Peninsula CO2 industrial emission sources such as refineries, coal and natural gas power plants and cement factories (case study) and the results showed that in Portugal, CO2 industrial emissions reduction can reach up to 68% and, in Spain, up to 74% of CO2 industrial emissions, could be avoided. These are called capturable CO2 emissions which means that they are the portion of the total emissions that can be captured from industrial processes before they reach the atmosphere. Moreover, hubs were shown to be more viable when Portugal and Spain are considered together, therefore, carbon routes (pipeline network) in the future may consider an integrated route for the Iberian Peninsula.

Self-assembly urchin-like Au-NSs arrays and application as surface-enhanced Raman scattering substrates

A simple and cost-effective approach has been exploited for the synthesis of large-area urchin-like Au nanospheres (Au-NSs) micro-nano arrays in anodic aluminum oxide (AAO) template. By sputtering Au on the surface of highly ordered Fe nanorods arrays, Au nanoparticles (Au-NPs) could self-assembly to form urchin-like micro-nano arrays. The urchin-like micro-nano arrays exhibit strong surface-enhanced Raman scattering (SERS) activity for detecting rhodamine 6G (R6G) because of its regular arrangement and rough surface. According to the analysis of three-dimensional finite-difference-time-domain (FDTD) simulations (Lumerical FDTD Solution, Inc., Canada), the strong SERS activity is mainly due to the high density of hot spots induced by the gaps between the neighboring Au-NSs. The Urchin-like arrays exhibit promising potential as new platforms to realize novel optoelectronic devices.

3D Hybrid Plasmonic Nanostructures with Dense Hot Spots Using Monolayer MoS2 as Sub‐Nanometer Spacer

AbstractHere, 3D hybrid plasmonic nanostructures using monolayer MoS2 as sub‐nanometer spacer with high density hot spots are fabricated. For the 3D hybrid assemblies, the hot spots exist not only on the 0.65 nm thick MoS2 gap regions between large and small nanoparticles (NP) but also between small and small NP. The high‐density hot spots induce prominently enhanced optical absorption, huge surface enhanced Raman scattering effect, excellent reproducibility, and ultralow limit of detection. Besides, the obvious electron transfer between MoS2 and metal NP is also found in the 3D hybrid assemblies indicating the strong interactions between MoS2 and Au or Ag. Combined with the long‐term stability, the 3D hybrid assemblies should perform excellent in plasmon‐enhanced sensor and photocatalysis applications.

Silver nanoparticles/activated carbon composite as a facile SERS substrate for highly sensitive detection of endogenous formaldehyde in human urine by catalytic reaction

Most of efforts have been made to prepare high performance surface-enhanced Raman scattering (SERS) substrate for amplifying Raman signals. It still remains a grand challenging task in building a simple, conventional and low-cost SERS substrate with highly dense hotspots for improved sensitivity of the target analytes. Here, we report a very dexterous strategy to fabricate a distinctive SERS substrate with high density hotspots, using common adsorbent activated carbon (AC) as template to assemble silver nanoparticles (Ag NPs). It can be estimated that the enhancement effect of Ag NPs/AC composite is about 6.5-fold that of bare Ag NPs. Different from the resonant dyes, however, formaldehyde (FA) is a Raman-inactive molecule even though enhanced. Considering that, a novel method for quantitative analysis of FA using the Ag NPs/AC composite as SERS sensor has been developed, based on the catalytic effect of trace FA on the oxidation of malachite green (MG) through bromate under acidic condition. The change of MG from reduced form into oxidized leucomalachite green (LMG) results in the quench of Raman signals of MG, responding to 0.07 ppb FA that is about 2 orders of magnitude lower than the limit defined by the Nash's method as a standard procedure recommended in Europe, Japan and China. Moreover, SERS examinations of endogenous FA in human urine signify that the proposed method has high selectivity, reliability and accuracy. Thus, as-fabricated Ag NPs/AC composite is adequate as inexpensive and versatile SERS sensor utilized in the quantification of trace targets in various complicated matrices.

Hybrid structures of Fe3O4 and Ag nanoparticles on Si nanopillar arrays substrate for SERS applications

Surface-enhanced Raman scattering (SERS)-based sensing is the most promising approach to detect trace amounts of chemical and biological molecules. Three-dimensional (3D) nanostructures have been considered as one of the most promising SERS substrates due to ordered arrangement of high-density hot spots and large surface area for adsorbing analyte. This paper designed a hybrid SERS substrate containing structures of Fe3O4 and Ag nanoparticles on Si nanopillar arrays, which combines the advantages of highly ordered periodic 3D Si nanopillar arrays and Ag coated Fe3O4 core-satellite microspheres, leading to intense light metal and light molecule interactions. The detection limit of the substrate for malachite green (MG) is as low as 10−8 M, meeting the requirements for trace detection of analytes. This work provides a new insight for fabricating SERS substrates with high sensitivity.

Periodic Porous Alloyed Au-Ag Nanosphere Arrays and Their Highly Sensitive SERS Performance with Good Reproducibility and High Density of Hotspots.

Periodic porous alloyed Au-Ag nanosphere (NS) arrays with different periodic lengths and tunable composition ratios were prepared on Si substrates on a large scale (∼cm2) using stepwise metal deposition-annealing and subsequent chemical corrosion from a monolayer of colloidal polystyrene (PS) microspheres as the initial template. The porous alloyed Au-Ag NSs possessed a high porosity and bicontinuous morphology composed of hierarchically interconnected ligaments, which were obtained from an optimized dealloying process in nitric acid. Interestingly, when the dealloying time was prolonged, the average size of the porous alloyed NSs slightly decreased, and the width of the ligaments gradually increased. The periodic length of the array could be facilely changed by controlling the initial particle size of the PS template. Moreover, the porous alloyed Au-Ag NS arrays were explored as a platform for the surface-enhanced Raman scattering (SERS) detection of 4-aminothiophenol (4-ATP) and exhibited excellent reproducibility and high sensitivity because of the periodic structure of the arrays and the abundance of inherent "hotspots". After optimization experiments, a low concentration of 10-10 M 4-ATP could be detected on these porous Au-Ag NS array substrates. Such highly reproducible SERS activity is meaningful for improving the practical application of portable Raman detection equipment.

Highly sensitive, reproducible and uniform SERS substrates with a high density of three-dimensionally distributed hotspots: gyroid-structured Au periodic metallic materials

Surface-enhanced Raman spectroscopy (SERS) is an important tool for the analytical, trace detection of many inorganic and organic materials, especially for materials involved in medical care, food safety and environmental pollution. Numerous efforts have been dedicated to exploring periodic metallic materials with a high density of hotspots. However, for most periodic metallic materials fabricated by top-down and bottom-up approaches, the distribution of hotspots is restricted to one or two dimensions. Here, for the first time, we report the successful fabrication of a bio-inspired bicontinuous gyroid-structured Au SERS substrate with a high density of three-dimensionally (3D) distributed hotspots. The as-required gyroid-structured substrates were demonstrated to be highly sensitive, reproducible and uniform, with an enhancement factor of up to 109. Finite-difference time domain (FDTD) simulations were conducted to reveal the mechanism leading to the high enhancement and we found that the interconnected helices in the gyroid structure not only increase the hotspot density but also contribute to increasing the scattering cross-section of the incident laser. The substrate was then adopted for the SERS detection of bis(2-ethylhexyl) phthalate, the most frequently used plasticizer in food, paints, house-hold items, perfumes and so on, and reached a detection limit of 1 fM, which is among the best results ever reported. Moreover, the mechanism deduced here will provide insight into the future design and selection of novel surface plasmonic resonance substrates, as many other bicontinuous interconnected systems are available.

Surface-enhanced Raman scattering effect of silver nanoparticles array

The Raman signal of adsorbed Raman probe molecule can be significantly enhanced by using metallic nanostructures with high-density hot spots as surface enhanced Raman scattering (SERS) substrates. A great effort has been devoted to the improving of the SERS detection sensitivity and reproducibility by preparing ordered metal nanostructure arrays with controlled particle size, shape and hot spot position, which are used as SERS substrates. In this paper, we prepare high-density Ag nanoparticle arrays by electrochemical deposition in anodic aluminum oxide (AAO) templates. The particle size and the nanogap between the adjacent particles can be adjusted by changing the deposition time. The structures and surface plasmons of Ag nanoparticle arrays are characterized by scanning electron microscopy and reflectance spectra. The size of the gap between the particles significantly affects the plasmon resonance and the plasmon coupling between the particles. The SERS properties of Ag nanoparticle arrays are investigated by using 1, 4-benzenedithiol (1, 4-BDT) as Raman probe molecule. The Ag nanoparticle arrays with high SERS detection sensitivity and high reproducibility (uniformity) are prepared by optimizing the deposition time (the nanogap between the adjacent particles), and the detection limit of the 1, 4-BDT can reach 10-13 mol/L. The relative standard deviation of the SERS signal intensity randomly measured from 20 spots on the Ag nanoparticle array substrate is 5.35%. The finite-difference time domain simulations confirm that the plasmon coupling between nanoparticles is strong, and that the coupling between the nanoparticles will increase as the nanogap decreases. Additionally, the local field is enhanced at the bottom of the nanoparticle and the gap between the Ag nanoparticle and the AAO template is larger. These results show that Ag nanoparticle array can be used as a high-efficiency SERS substrate.

Binary "island" shaped arrays with high-density hot spots for surface-enhanced Raman scattering substrates.

We have demonstrated a facile and low-cost approach for the fabrication of binary "island" shaped arrays (BISA) with high-density hot spots as reproducible surface-enhanced Raman scattering (SERS) substrates by depositing a self-assembled monolayer Au nanoparticle (AuNP) film with small gaps onto a two-dimensional (2D) silica microsphere opal structure. By varying the size of silica spheres, the SERS performance of the BISA substrate with an enhancement factor (EF) of 3.74 × 1010 magnitude and the corresponding signal intensity deviation of below 8% using 770 nm silica sphere arrays were achieved. Compared with the assembled monolayer AuNP film on a planar substrate, the BISA enabled the installation of more AuNPs as a source of hot spots due to the undulation of morphology on the nanoscale within the designated laser-illumination area. In addition, a finite-difference time-domain (FDTD) simulation suggested that the BISA structure provided geometric conditions for increasing the intensity of the formed hot spots, and the strong periodic electric fields on the BISA are located not only in the gap between adjacent AuNPs, but also along the boundary of the neighboring island of silica spheres. Surface plasmon-decayed hot carriers (hot electrons and hot holes) from AuNPs can be applied in the field of energy conversion (i.e., photocatalysis), integrated with the SERS as a sensitive optical indicator to accurately monitor the catalytic reaction process. Furthermore, we examined the catalytic reaction process of the dimerization of 4-ATP into DMAB and found that photocatalytic activity could be tuned by changing the size of silica spheres. This study provides a new design route for the fabrication of the SERS platform with high sensitivity and reproducibility to detect molecules or improve catalyst efficiency.

Facile fabrication of silver nanoparticle decorated α-Fe2O3 nanoflakes as ultrasensitive surface-enhanced Raman spectroscopy substrates

Although great progress has been made on designing noble metal nanoparticle aggregates/assemblies as surface-enhanced Raman spectroscopy (SERS) substrates, an ever increasing research interest has focused on fabrication of hierarchical nanostructures for superior SERS performance. Here, we report effective decoration of silver nanoparticles (AgNPs) onto vertically and densely grown α-Fe2O3 nanoflakes (NFs) as SERS active substrates. The SERS substrate was prepared by thermally annealing Fe foil at 450 °C to grow α-Fe2O3 NFs and electroless deposition of AgNPs onto α-Fe2O3 NFs, with excellent ability to control the particle size and density. The AgNP-decorated α-Fe2O3 NFs offer high density hot spots and exhibited high SERS performance with enhancement factor of 8.1 × 106. Integration of α-Fe2O3 nanostructure functions as intrinsic internal standard and selective binder for specific analytes like the toxic arsenic. The substrate was successfully used for quantitative detection of arsenic with spatial uniformity (relative standard deviation, RSD, 8.8%) and good reproducibility (RSD, 9.7%). The Raman signal of α-Fe2O3 integrated to the SERS substrate was utilized as internal standard which significantly improves reproducibility. The substrate exhibited a high sensitivity for arsenic detection with a limit of detection down to 1.5 μg L−1, which is much lower than permissible limit (10 μg L−1) set by the WHO. Moreover, the SERS substrates showed excellent practicability and selectivity for determination of arsenic in environmental waters which is valuable for environmental and human health protection.

Giant Vesicles with Anchored Tiny Gold Nanowires: Fabrication and Surface-Enhanced Raman Scattering.

Sensitivity and reproducibility are two major concerns to improve the performance and extend the range of practical applications of surface-enhanced Raman scattering (SERS). A theoretical report reveals that hot spots formed by gold nanoparticles with a tip-to-tip configuration would generate the maximum electric field enhancement because of the lightning rod effect. In our present study, we constructed a giant vesicle consisting of anchored tiny gold nanowires to provide a high density of sharp tip-to-tip nanogaps for SERS application. The tiny gold nanowires were directly grown and anchored onto the surfaces of polystyrene (PS) microspheres by a seed-mediated method. Then, the removal of PS microspheres by tetrahydrofuran led to the formation of the giant gold vesicles with hierarchical cage structures, providing the sharp tips and high density of hot spots for improving SERS performance. Compared with the nonwire structure (island and inhibited nanoparticle), giant gold vesicles with tiny wires showed a higher SERS enhancement factor (9.90 × 107) and quantitative SERS analysis in the range of 10-4 to 10-7 M. In addition, the large-scale giant gold vesicle array on the silica substrate resulted in a high reproducibility of SERS signals with the variation of intensities less than 7.6%.

\u201cOptical and Surface Enhanced Raman Scattering properties of Ag modified silicon double nanocone array\u201d

Surface enhanced Raman scattering (SERS) systems with large number of active sites exhibit superior capability in detection of low concentration analytes. In this paper, we present theoretical as well as experimental studies on the optical properties of a unique hybrid nanostructure, Ag NPs decorated silicon double nanocones (Si-DNCs) array, which provide high density of hot spots. The Si-DNC array is fabricated by employing electron beam lithography together with plasma etching process. Multipole analysis of the scattering spectra, based on the multipole expansion theory, confirms that the toroidal dipole moment dominates over other electric and magnetic multipole moments in the Si-DNCs array. This response occurs as a result of generating current densities flowing in opposite directions and consequently generating H-field vortexes inside the nanocones. Moreover, SERS applicability of this type of nanostructure is examined. For this purpose, the Si-DNCs array is decorated with Ag nanoparticles (NPs) by means of electroless deposition method. Simulation results indicate that combination of multiple resonances, including LSPR resonance of Ag NPs, longitudinal standing wave resonance of Ag layer and inter-particle interaction in the gap region, result in a significant SERS enhancement. Our experimental results demonstrate that Si-DNC/Ag NPs array substrate provides excellent reproducibility and ultrahigh sensitivity.

Three-dimensional hierarchical anatase@rutile TiO2 nanotree array films decorated by silver nanoparticles as ultrasensitive recyclable surface-enhanced Raman scattering substrates

In this paper, we report a three-dimensional (3D) hierarchical nanostructure as a surface-enhanced Raman scattering (SERS) sensor by decorating Ag nanoparticles on a TiO2 nanotree (TNT) array film via a simple two-step hydrothermal treatment and a magnetron sputtering technique for the first time. The sensitivity, recyclability, and uniformity of the SERS-active substrates were investigated using Rhodamine 6G (R6G) as the probe molecule. A detection limit of 1.0 × 10−12 M R6G molecules and an enhancement factor of 5.29 × 105 were achieved on the 3D hierarchical TNT array film with 60 s Ag-sputtering. More importantly, the optimized TNT/Ag also exhibits excellent recyclability and uniformity. The relative standard deviation in the intensity of Raman vibration mode (1506 cm−1) was about 1.2%. Furthermore, the 3D hierarchical nanostructure was more efficient than the 1D nanostructure for SERS measurements. The excellent SERS performance is attributed to the formation of high-density hot spots for 3D nanostructure as well as the synergistic effects of electromagnetic and charge transfer. Moreover, the anatase@rutile mixed phase provides an additional charge transfer effect, which can lead to further enhancement of the Raman signal. The results show that the TNT/Ag nanostructure can be a promising candidate for SERS sensors.