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

Fabrication of ordered bi-metallic array with superstructure of gold micro-rings via templated-self-assembly procedure and its SERS application

Bi-metallic patterns with array of Pt discs decorated by Au rings were fabricated onto the substrate by a templated-self-assembly procedure in which multi-step self-assembly processes were involved.

Spontaneous Growth of 3D Silver Mesoflowers on Poly(4-vinylpyridine) Brushes-Grafted-Graphene Oxide Films and Facile Creation of Nanoporosities over their Surface

Fabricating three-dimensional (3D) hierarchical noble-metal particles by spontaneous redox reactions between graphene and noble-metal salts still remains a great challenge. Herein, the fact that graphene oxide (GO) itself acts as both a platform for grafting polymer brushes and a reducing agent to reduce [Ag(NH₃ )₂ ]⁺ ions is taken advantages of. 3D flower-like Ag mesoparticles (Ag mesoflowers, Ag MFs) with tunable size and shapes can spontaneous grow on poly(4-vinylpyridine) brushes-grafted-graphene oxide (P4VP-g-GO) films in Ag(NH₃ )₂ OH solution without the use of any additional reducing agent. The residual Ag(NH₃ )₂ OH on 3D Ag MFs surface can be further reduced by NaBH₄ , causing abundant nanoporosities over the entire Ag MFs. The resulting Ag nanoporous MFs (Ag NMFs) with larger surface-to-volume ratio and higher nanoscale roughness exhibit ultrasensitivity in surface-enhanced Raman spectroscopy (SERS) detection, and the detection limit for 4-aminothiophenol is as low as 10^-13  m.

Manipulating chemistry through nanoparticle morphology

We demonstrate that the protonation chemistry of molecules adsorbed at nanometer distances from the surface of anisotropic gold nanoparticles can be manipulated through the effect of surface morphology on the local proton density of an organic coating. Direct evidence of this remarkable effect was obtained by monitoring surface-enhanced Raman scattering (SERS) from mercaptobenzoic acid and 4-aminobenzenethiol molecules adsorbed on gold nanostars. By smoothing the initially sharp nanostar tips through a mild thermal treatment, changes were induced on protonation of the molecules, which can be observed through changes in the measured SERS spectra. These results shed light on the local chemical environment near anisotropic colloidal nanoparticles and open an alternative avenue to actively control chemistry through surface morphology.

Three-dimensional assembly of silver nanoparticles spatially confined by cellular structure of Spirulina, from nanospheres to nanosheets

Three-dimensional (3D) ordered construction of nanoparticles (NPs) has attracted much attention in wide applications, however, techniques with respect to cost effective nanofabrication of well defined functional architectures is still lacking. To address this specific issue, a bio-interface confinement approach is proposed that precisely replicates the complex cellular structural features of microbes and integrates silver NP (SNP) building blocks into their 3D framework in a precise, low cost and mass production way. Herein, the SNPs with nanospheres and nanosheets structure were synthesized by way of electroless deposition using Spirulina as template. Results showed that SNPs were orderly assembled along the cellular structure, and the spatially confinement of cellular texture induced the transformation of SNPs from sphere to flake morphology during their continuous growth. The silver assembly not only shows good antibacterial activity, but also exhibits excellent surface enhanced Raman scattering (SERS) performance with the enhancement factor as high as 5.95 × 10⁸ and good recuperability towards Rhodamine 6G. The fascinating SERS performance can be ascribed to the combined action of nanosheets morphology of SNPs, hierarchical nanostructure of the cellular structure, and the small interparticle spacing. This strategy provides an effective strategy for controllable and ordered 3D assembly of NPs by using the cellular texture.

Multicomponent Plasmonic Nanoparticles: From Heterostructured Nanoparticles to Colloidal Composite Nanostructures

Plasmonic nanostructures possessing unique and versatile optoelectronic properties have been vastly investigated over the past decade. However, the full potential of plasmonic nanostructure has not yet been fully exploited, particularly with single-component homogeneous structures with monotonic properties, and the addition of new components for making multicomponent nanoparticles may lead to new-yet-unexpected or improved properties. Here we define the term "multi-component nanoparticles" as hybrid structures composed of two or more condensed nanoscale domains with distinctive material compositions, shapes, or sizes. We reviewed and discussed the designing principles and synthetic strategies to efficiently combine multiple components to form hybrid nanoparticles with a new or improved plasmonic functionality. In particular, it has been quite challenging to precisely synthesize widely diverse multicomponent plasmonic structures, limiting realization of the full potential of plasmonic heterostructures. To address this challenge, several synthetic approaches have been reported to form a variety of different multicomponent plasmonic nanoparticles, mainly based on heterogeneous nucleation, atomic replacements, adsorption on supports, and biomolecule-mediated assemblies. In addition, the unique and synergistic features of multicomponent plasmonic nanoparticles, such as combination of pristine material properties, finely tuned plasmon resonance and coupling, enhanced light-matter interactions, geometry-induced polarization, and plasmon-induced energy and charge transfer across the heterointerface, were reported. In this review, we comprehensively summarize the latest advances on state-of-art synthetic strategies, unique properties, and promising applications of multicomponent plasmonic nanoparticles. These plasmonic nanoparticles including heterostructured nanoparticles and composite nanostructures are prepared by direct synthesis and physical force- or biomolecule-mediated assembly, which hold tremendous potential for plasmon-mediated energy transfer, magnetic plasmonics, metamolecules, and nanobiotechnology.

Hollow Porous Gold Nanoshells with Controlled Nanojunctions for Highly Tunable Plasmon Resonances and Intense Field Enhancements for Surface-Enhanced Raman Scattering

Plasmonic metal nanostructures with nanogaps have attracted great interest owing to their controllable optical properties and intense electromagnetic fields that can be useful for a variety of applications, but precise and reliable control of nanogaps in three-dimensional nanostructures remains a great challenge. Here, we report the control of nanojunctions of hollow porous gold nanoshell (HPAuNS) structures by a facile oxygen plasma-etching process and the influence of changes in nanocrevices of the interparticle junction on the optical and sensing characteristics of HPAuNSs. We demonstrate a high tunability of the localized surface plasmon resonance (LSPR) peaks and surface-enhanced Raman scattering (SERS) detection of rhodamine 6G (R6G) using HPAuNS structures with different nanojunctions by varying the degree of gold sintering. As the neck region of the nanojunction is further sintered, the main LSPR peak shifts from 785 to 1350 nm with broadening because the charge transfer plasmon mode becomes more dominant than the dipolar plasmon mode, resulting from the increase of conductance at the interparticle junctions. In addition, it is demonstrated that an increase in the sharpness of the nanojunction neck can enhance the SERS enhancement factor of the HPAuNS by up to 4.8-fold. This enhancement can be ascribed to the more intense local electromagnetic fields at the sharper nanocrevices of interparticle junctions. The delicate change of nanojunction structures in HPAuNSs can significantly affect their optical spectrum and electromagnetic field intensity, which are critical for their practical use in a SERS-based analytical sensor as well as multiple-wavelength compatible applications.

Nanowire assisted repeatable DEP-SERS detection in microfluidics

Surface enhanced Raman spectroscopy (SERS) detection in microfluidics is an interesting topic for its high sensitivity, miniaturization and online detection. In this work, a SERS detection in microfluidics with the help of the Ag nanowire aggregating based on dielectrophoresis (DEP) is reported. The Raman intensities of molecule in microfluidics is greatly enhanced in the naturally generated nanogaps of Ag nanowire aggregating modulated by DEP. Firstly, the influence of DEP voltage and time on Ag nanowire aggregating is investigated to figure out the optimal condition for SERS. And then, the SERS intensities of methylene blue and rhodamine6G at various concentration with high reproducibility and uniformity are studied. Furthermore, the experiment data demonstrate this DEP-SERS system could be repeated used for different molecule detections. At last, the SERS of melamine is measured to explore its application on food safety. Our work anticipates this nanowire assisted repeatable DEP-SERS detection in microfluidics with high sensitivity could meet the emerging needs in environmental pollution monitoring, food safety evaluation, and so on.

Graphene Oxide-Assisted and DNA-Modulated SERS of AuCu Alloy for the Fabrication of Apurinic/Apyrimidinic Endonuclease 1 Biosensor

Fabrication of high-performance surface-enhanced Raman scattering (SERS) biosensors relies on the coordination of SERS substrates and sensing strategies. Herein, a SERS active AuCu alloy with a starfish-like structure is prepared using a surfactant-free method. By covering the anisotropic AuCu alloy with graphene oxide (GO), enhanced SERS activity is obtained owing to graphene-enhanced Raman scattering and assembly of Raman reporters. Besides, stability of SERS is promoted based on the protection of GO to the AuCu alloy. Meanwhile, it is found that SERS activity of AuCu/GO can be regulated by DNA. The regulation is sequence and length dual-dependent, and short polyT reveals the strongest ability of enhancing the SERS activity. Relying on this phenomenon, a SERS biosensor is designed to quantify apurinic/apyrimidinic endonuclease 1 (APE1). Because of the APE1-induced cycling amplification, the biosensor is able to detect APE1 sensitively and selectively. In addition, APE1 in human serum is analyzed by the SERS biosensor and enzyme-linked immunosorbent assay (ELISA). The data from the SERS method are superior to that from ELISA, indicating great potential of this biosensor in clinical applications.

Manipulating "Hot Spots" from Nanometer to Angstrom: Toward Understanding Integrated Contributions of Molecule Number and Gap Size for Ultrasensitive Surface-Enhanced Raman Scattering Detection

Narrower gaps between metal nanoparticles (so-called "hot spots") in surface-enhanced Raman scattering (SERS) substrates contribute to stronger electromagnetic (EM) enhancement; however, the accompanying steric effect hinders analyte molecules entering hot spots to access the benefit. To comprehensively understand integrated contributions of the gap size and molecule number accommodated in hot spots and then optimize design of SERS substrates, the thermal shrinking method was employed to manipulate hot spots and the "hottest zone" was defined to evaluate the integrated contributions to SERS intensity of the two factors. In the conventional shrink-adsorption mode, the contributions of the molecule number and gap size are competitive when the gap width is comparable with the target molecule size, which leads to oscillating behavior of SERS intensity versus gap size, and it is analyte molecule size dependent. This result suggests that engineering hot spots should be target molecule directed to achieve ultrasensitive detection. In the proposed adsorption-shrink mode, the contributions of the molecule number and gap size are synergistic, which makes the detection ability of the adsorption-shrink mode attains a single-molecule (SM) level. Excellent performance of the adsorption-shrink SERS strategy benefits detection of trace level pollutants in complex environments. Detection ranges for contaminants with different metal affinity, such as thiram, malachite green (MG), and formaldehyde, are as low as parts per billion, even down to parts per trillion.

One Spot-Two Sensors: Porous Silicon Interferometers in Combination With Gold Nanostructures Showing Localized Surface Plasmon Resonance

Sensors composed of a porous silicon monolayer covered with a film of nanostructured gold layer, which provide two optical signal transduction methods, are fabricated and thoroughly characterized concerning their sensing performance. For this purpose, silicon substrates were electrochemically etched in order to obtain porous silicon monolayers, which were subsequently immersed in gold salt solution facilitating the formation of a porous gold nanoparticle layer on top of the porous silicon. The deposition process was monitored by reflectance spectroscopy, and the appearance of a dip in the interference pattern of the porous silicon layer was observed. This dip can be assigned to the absorption of light by the deposited gold nanostructures leading to localized surface plasmon resonance. The bulk sensitivity of these sensors was determined by recording reflectance spectra in media having different refractive indices and compared to sensors exclusively based on porous silicon or gold nanostructures. A thorough analysis of resulting shifts of the different optical signals in the reflectance spectra on the wavelength scale indicated that the optical response of the porous silicon sensor is not influenced by the presence of a gold nanostructure on top. Moreover, the adsorption of thiol-terminated polystyrene to the sensor surface was solely detected by changes in the position of the dip in the reflectance spectrum, which is assigned to localized surface plasmon resonance in the gold nanostructures. The interference pattern resulting from the porous silicon layer is not shifted to longer wavelengths by the adsorption indicating the independence of the optical response of the two nanostructures, namely porous silicon and nanostructured gold layer, to refractive index changes and pointing to the successful realization of two sensors in one spot.

SERS study of Ag/FeS/4-MBA interface based on the SPR effect

In this work, an ordered metal-semiconductor molecular system was introduced, and 4-mercaptobenzoic acid (4-MBA) was employed to study the charge transfer (CT) at the metal-semiconductor interface based on surface-enhanced Raman scattering (SERS) spectra. The thickness of the sputtered FeS was controlled so that the surface plasmon resonance (SPR) of Ag underwent a displacement change, and the contribution of the SPR to the CT was studied through surface plasmon (SP) absorption. Furthermore, SERS spectra obtained at different excitation wavelengths were used to calculate the degree of CT in the layer-by-layer sputtering system. When Ag was irradiated with incident light, the strong SPR of Ag was excited, generating an increased electromagnetic field (EM). This amplified EM generated hot electrons at the interface between the FeS and Ag, and then the hot electrons were rearranged. Therefore, we established a simple and effective method for studying the impact of SPR on interfacial CT and analyzed the SERS spectra in accordance with Lombardi's basic theory and the physical effects associated with SPR. This theory is in good agreement with the experimental results. On this basis, we also proposed a mechanism by which SPR impacts the CT, which is beneficial for studying interfacial CT and obtaining an in-depth understanding of the CT mechanism in SERS. This work also enables the expansion of the applications of the SERS technique in the field of nanomaterials.

Template growth of Au/Ag nanocomposites on phosphorene for sensitive SERS detection of pesticides

We report template growth and dense packing of noble metallic nanoparticles (NPs) on few-layer phosphorene for sensitive surface-enhanced Raman scattering (SERS) detection. Phosphorene obtained by electrochemical exfoliation serves as both the template and reductant in the fabrication of noble metallic NPs which are dispersed on phosphorene without aggregation or pile-up. The BP/Ag/Au and BP/Au/Ag nanocomposites with a nanogap structure exhibit excellent SERS sensitivity and reproducibility with respect to Rhodamine 6G. The BP/Au/Ag nanocomposite is further utilized as the SERS substrate for the detection of two fungicides, thiabendazole and thiram, and detection limits of 10^-7 and 10^-8 M are achieved. Template growth of Au/Ag nanocomposites is demonstrated to be a green strategy for the fabrication of SERS substrates and has large potential in applications such as pesticide detection.

Synthesis and characterization of size controlled bimetallic nanosponges

Metallic and bimetallic nanosponges with well-defined size and form have attracted increasing attention due to their unique structural properties and their potential for many applications. In this chapter, the recently developed methods for the synthesis and preparation of metallic and bimetallic nanosponges are presented. These methods can be mainly cataloged in two groups: dealloying-based methods and reduction reaction-based methods. Different topographical reconstruction methods for the investigation of their structural properties are then reviewed briefly. The optical properties of the metallic nanosponges are clearly different from those of the solid counterparts due to the tailored disordered structure. The recent advances in the exploration of the distinct linear and non-linear optical properties of the nanosponges are summarized.

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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.

Size Control Synthesis of Monodisperse, Quasi-Spherical Silver Nanoparticles To Realize Surface-Enhanced Raman Scattering Uniformity and Reproducibility

In this work, we reported the synthesis of monodisperse, quasispherical Ag nanoparticles (NPs) with sizes of 40-300 nm by using ascorbic acid reduction of a silver-ammonia complex onto preformed, 23 nm Ag-NP seeds in the aqueous solution with an optimal pH of about 9.6 at room temperature. The as-prepared Ag NPs with such a large size span (from 40 to 300 nm) and high quality by one-pot seeded growth method are reported for the first time to the best of our knowledge. It is found that the key in the present seed-mediated growth method is to introduce a proper amount of ammonia water for the formation of a stable complex with a silver precursor (silver-ammonia complex) while maintaining the pH value of the growth solution simultaneously. By using rhodamine 6G molecules as probes, the surface-enhanced Raman scattering (SERS) activities of the as-prepared Ag NPs in ethanol solution are highly dependent on the sizes of Ag NPs at the fixed 633 nm laser and at the fixed particle number, which show a volcano-like curve. Moreover, 125 nm Ag NPs bear the largest SERS activity among them. Furthermore, Ag NPs with narrow distributions in shape and size (say, less than 10%) can achieve the uniformity and reproducibility of their SERS signals in solution; their relative standard deviations can be as low as 5% in space and temporal scale.

SERS based detection of multiple analytes from dye/explosive mixtures using picosecond laser fabricated gold nanoparticles and nanostructures

Surface enhanced Raman spectroscopy (SERS) is a cutting edge analytical tool for trace analyte detection due to its highly sensitive, non-destructive and fingerprinting capability. Herein, we report the detection of multiple analytes from various mixtures using gold nanoparticles (NPs) and nanostructures (NSs) as SERS platforms. NPs and NSs were achieved through the simple approach of laser ablation in liquids (LAL) and their morphological studies were conducted with a UV-Visible absorption spectrometer, a high resolution transmission electron microscope (HRTEM) and a field emission scanning electron microscope (FESEM). The fabricated NPs/NSs allowed the sensitive and selective detection of different mixed compounds containing (i) rhodamine 6G (Rh6G) and methylene blue (MB), (ii) crystal violet (CV) and malachite green (MG), (iii) picric acid (explosive) and MB (dye), (iv) picric acid and 3-nitro-1,2,4- triazol-5-one (explosive, NTO) and (v) picric acid and 2,4-dinitrotoluene (explosive, DNT) using a portable Raman spectrometer. Thus, the obtained results demonstrate the capability of fabricated SERS substrates in identifying explosives and dyes from various mixtures. This could pave a new way for simultaneous detection of multiple analytes in real field applications.

A Novel SERS Substrate Platform: Spatially Stacking Plasmonic Hotspots Films

Surface-enhanced Raman scattering (SERS) technique has presented great potential in medical diagnosis, environment monitoring, and food detection due to its high sensitivity, rapid response, and fingerprint effect. Many efforts have been concentrated on all kinds of strategies to produce efficient SERS platforms. Here, we report a simple and controllable method to produce large-area efficient SERS platforms with spatially stacked plasmonic hotspots. The SERS platforms consist of double-layer metal porous films and are easily fabricated by magnetron sputtering and annealing, assisted by the evaporation of hydrofluoric acid. The stacked dual-layer metal porous films show prominent Raman enhancement and ultrasensitive SERS sensing capability for different target molecules. The detection limit is demonstrated down to 10^-13 M by detecting rhodamine 6G molecules. These superior Raman properties can be mainly ascribed to the highly dense spatially stacked plasmonic hotspots formed in the dual-layer metal porous films. The simple, controllable, and scalable fabrication strategy and superior Raman performance make these platforms promising candidates for the development of inexpensive, efficient, and mass-produced SERS substrates.

Effect of Alkylamines on Morphology Control of Silver Nanoshells for Highly Enhanced Raman Scattering

Morphology control of the surface of a nanostructure is a key issue in modulating its surface plasmon resonance and scattering properties. Here, we studied the effect of alkylamines on morphology control during the one-step fabrication of silver nanoshells (NSs) for highly enhanced Raman scattering. Various types of alkylamines were used to study the effects of chain length, existence of hydroxyl groups, and degree of alkyl chains on the surface morphology of silver NSs. The alkylamines influenced the silver ion reduction and the growth of silver domains, resulting in distinctive morphology changes. The optical properties of the silver NSs of different surface morphologies were characterized by surface-enhanced Raman spectra. Especially, when long alkylamines were used, intense and uniform surface-enhanced Raman scattering signals were obtained at the visible and near-infrared (NIR) region, and their enhancement factor was ∼10⁷. To detect cancer biomarkers in vivo, as a feasibility test, silver NSs were modified to highly NIR-active nanoprobes and successfully applied to detect colon cancer without causing nonspecific interactions. Our one-step fabrication method of silver NSs is simple and can overcome various hurdles of morphology control and can be extended to other metal nanostructures of controlled surface morphologies or shape.

Controllable Preparation of SERS-Active Ag-FeS Substrates by a Cosputtering Technique

In this work, we introduced an ordered metal-semiconductor molecular system and studied the resulting surface-enhanced Raman scattering (SERS) effect. Ag-FeS nanocaps with sputtered films of different thicknesses were obtained by changing the sputtering power of FeS while the sputtering power of Ag and the deposition time remained constant. When metallic Ag and the semiconductor FeS are cosputtered, the Ag film separates into Ag islands partially covered by FeS and strong coupling occurs among the Ag islands isolated by FeS, which contributes to the SERS phenomenon. We also investigated the SERS enhancement mechanism by decorating the nanocap arrays produced with different FeS sputtering powers with methylene blue (MB) probe molecules. As the FeS sputtering power increased, the SERS signal first increased and then decreased. The experimental results show that the SERS enhancement can mainly be attributed to the surface plasmon resonance (SPR) of the Ag nanoparticles. The coupling between FeS and Ag and the SPR displacement of Ag vary with different sputtering powers, resulting in changes in the intensity of the SERS spectra. These results demonstrate the high sensitivity of SERS substrates consisting of Ag-FeS nanocap arrays.

Porous Silicon Photonic Crystals Coated with Ag Nanoparticles as Efficient Substrates for Detecting Trace Explosives Using SERS

Picric acid (PA) is an organic substance widely used in industry and military, which poses a great threat to the environment and security due to its unstable, toxic, and explosive properties. Trace detection of PA is also a challenging task because of its highly acidic and anionic character. In this work, silver nanoparticles (AgNPs)-decorated porous silicon photonic crystals (PS PCs) were controllably prepared as surface-enhanced Raman scattering (SERS) substrates using the immersion plating solution. PA and Rhodamine 6G dye (R6G) were used as the analyte to explore the detection performance. As compared with single layer porous silicon, the enhancement factor of PS PCs substrates is increased to 3.58 times at the concentration of 10-6 mol/L (R6G). This additional enhancement was greatly beneficial to the trace-amount-detection of target molecules. Under the optimized assay condition, the platform shows a distinguished sensitivity with the limit of detection of PA as low as 10-8 mol/L, the linear range from 10-4 to 10-7 mol/L, and a decent reproducibility with a relative standard deviation (RSD) of ca. 8%. These results show that the AgNPs-modified PS PCs substrates could also find further applications in biomedical and environmental sensing.