Silver-particle-based surface-enhanced Raman scattering spectroscopy for biomolecular sensing and recognition

Environmental photochemistry with thiol- and silica-modified plasmonic nanocomposites: SERS sensing of municipal solid waste and tannery waste leachate from groundwater

Mismanagement of obsolete solid waste generates a massive deteriorating effect on the environment. There is a high level of open trash disposal contaminating its neighboring water bodies. This despoliation trash causes an endangerment to the living environment. The waste management act is to hinder harmful effects on human beings, animals, plants, and their natural environment through the principles of waste prevention, waste processing, and waste disposal. Surface-enhanced Raman scattering (SERS) enhances the hazardous chemical sensing of environmental pollutants. To vigorously focus on the leaching of a couple of landfills in groundwater and surface water, an unusual combination of SERS-based poly vinyl thiol and silica-modified silver nanocomposites (PVT/SiO2@Ag NCs) was synthesized. The optical, crystalline, and structural properties of PVT/SiO₂@Ag NCs were described with UV-visible spectroscopy (UV-Vis), X-ray diffractometer (XRD), transmission electron microscope (TEM), and energy-dispersive X-ray analysis (EDX). The surface plasmon resonance (SPR) is detected at 403 nm from the PVT/SiO₂@Ag NPs. The average crystallite size of PVT/SiO₂ @ Ag NCs is estimated using the Scherrer formula as 11 nm. The calculated specific surface area (SSA), strains, and dislocation densities demonstrate the improved mechanical properties of the substrate. The well-separated spherical shape of NPs is also observed, and the composition of silica and sulfur element in addition of Ag was confirmed by EDAX. Negatively charged SiO₂ were bound strongly with the SH group and Ag NPs through electrostatic interaction mechanism as S-Ag-O-Si-O-Ag-S. SERS sensitivity is demonstrated by the prepared nanoparticles using an environmentally ignored leachate of municipal solid waste (MSW) and tannery waste (TW) landfill. PVT/SiO₂@Ag NCs has detected the presence of innards of MSW leachate viz., aromatic hydrocarbon, phenols, phthalates, and pesticide from the groundwater. Furthermore, the TW leachate compositions of benzenes, hydrocarbons, amines, and chromium VI were analytically identified. Also, the leaching of TW leachate was confirmed in the water samples referred. Hence, this study provides a novel SERS sensor of PVT/SiO₂@Ag NCs in the tile to detect and analyze environmentally ignored organic and inorganic compounds.

Structural Requirements for Chemoselective Ammonolysis of Ethylene Glycol to Ethanolamine over Supported Cobalt Catalysts

Ethylene glycol is regarded as a promising C2 platform molecule due to the fast development of its production from sustainable biomass. This study inquired the structural requirements of Co-based catalysts for the liquid-phase ammonolysis of ethylene glycol to value-added ethanolamine. We showed that the rate and selectivity of ethylene glycol ammonolysis on γ-Al2O3-supported Co catalysts were strongly affected by the metal particle size within the range of 2–10 nm, among which Co nanoparticles of ~4 nm exhibited both the highest ethanolamine selectivity and the highest ammonolysis rate based on the total Co content. Doping of a moderate amount of Ag further promoted the catalytic activity without affecting the selectivity. Combined kinetic and infrared spectroscopic assessments unveiled that the addition of Ag significantly destabilized the adsorbed NH3 on the Co surface, which would otherwise be strongly bound to the active sites and inhibit the rate-determining dehydrogenation step of ethylene glycol.

Core-Shell Nanoparticle-Enhanced Raman Spectroscopy

Core-shell nanoparticles are at the leading edge of the hot research topics and offer a wide range of applications in optics, biomedicine, environmental science, materials, catalysis, energy, and so forth, due to their excellent properties such as versatility, tunability, and stability. They have attracted enormous interest attributed to their dramatically tunable physicochemical features. Plasmonic core-shell nanomaterials are extensively used in surface-enhanced vibrational spectroscopies, in particular, surface-enhanced Raman spectroscopy (SERS), due to the unique localized surface plasmon resonance (LSPR) property. This review provides a comprehensive overview of core-shell nanoparticles in the context of fundamental and application aspects of SERS and discusses numerous classes of core-shell nanoparticles with their unique strategies and functions. Further, herein we also introduce the concept of shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) in detail because it overcomes the long-standing limitations of material and morphology generality encountered in traditional SERS. We then explain the SERS-enhancement mechanism with core-shell nanoparticles, as well as three generations of SERS hotspots for surface analysis of materials. To provide a clear view for readers, we summarize various approaches for the synthesis of core-shell nanoparticles and their applications in SERS, such as electrochemistry, bioanalysis, food safety, environmental safety, cultural heritage, materials, catalysis, and energy storage and conversion. Finally, we exemplify about the future developments in new core-shell nanomaterials with different functionalities for SERS and other surface-enhanced spectroscopies.

Shape control of inorganic nanoparticles from solution

Inorganic materials with controllable shapes have been an intensely studied subject in nanoscience over the past decades. Control over novel and anisotropic shapes of inorganic nanomaterials differing from those of bulk materials leads to unique and tunable properties for widespread applications such as biomedicine, catalysis, fuels or solar cells and magnetic data storage. This review presents a comprehensive overview of shape-controlled inorganic nanomaterials via nucleation and growth theory and the control of experimental conditions (including supersaturation, temperature, surfactants and secondary nucleation), providing a brief account of the shape control of inorganic nanoparticles during wet-chemistry synthetic processes. Subsequently, typical mechanisms for shape-controlled inorganic nanoparticles and the general shape of the nanoparticles formed by each mechanism are also expounded. Furthermore, the differences between similar mechanisms for the shape control of inorganic nanoparticles are also clearly described. The authors envision that this review will provide valuable guidance on experimental conditions and process control for the synthesis of inorganic nanoparticles with tunable shapes in the solution state.

Morphology-dependent photoelectrochemical properties of multi-scale layered Bi(C2O4)OH

In this work, different morphologies have been synthesized, which can be controlled by simply tuning oxalate source, EG/W volumetric ratio, H₂C₂O₄/Bi(NO₃)₃ molar ratio, hydrothermal temperature and duration.

b2 Peaks in SERS Spectra of 4-Aminobenzenethiol: A Photochemical Artifact or a Real Chemical Enhancement?

Strong b2 peaks (1142, 1391, 1438, and 1583 cm(-1)) in the SERS spectra of 4-aminobenzenethiol (ABT) have been regarded by many as a textbook example of chemically enhanced SERS signals. However, this interpretation is in serious doubt after the recent claim that they arise from 4,4'-dimercaptoazobenzenes (DMAB) photogenerated during the acquisition of SERS, not the genuine chemically enhanced signals of ABT. Subsequent attempts to prove or disprove this claim have failed to provide any decisive verdict. Here we present spectroscopic and mass spectrometric evidence that further support the photogeneration of DMABs from ABTs on an Ag surface. Furthermore, we show that the amount of the DMAB is sufficient to explain the b2 intensities of ABT.

Room Temperature Synthesis and Catalytic Properties of Surfactant-Modified Ag Nanoparticles

Well-dispersed Ag nanoparticles with size of 20–30 nm were synthesized in water at room temperature with a self-made novel imidazoline Gemini surfactant quaternary ammonium salt of di (2-heptadecyl-1-formyl aminoethyl imidazoline) hexanediamine. Transmission electron microscopy, X-ray powder diffraction, ultraviolet-visible absorption spectra, and Fourier transform infrared ray were used to characterize the Ag nanoparticles. Results showed that the micellized aggregation of imidazoline Gemini surfactant in water, the growth of Ag initial particles, and the interaction (adsorption and coordination) between surfactant and Ag+/Ag nanoparticles took place simultaneously to form the well-dispersed Ag nanoparticles. Catalytic results show that the surface-modified Ag product was an active metal catalyst for methyl orange reduction reaction due to the effective adsorption between Ag nanoparticles and methyl orange molecules, which was of promising application in environmental protection.

Spectral analysis of human saliva for detection of lung cancer using surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) has been shown to be able to detect low-concentration biofluids. Saliva SERS readings of 21 lung cancer patients and 20 normal people were measured and differentiated. Most of the Raman peak intensities decrease for lung cancer patients compared with that of normal people. Those peaks were assigned to proteins and nucleic acids, which indicate a corresponding decrease of those substances in saliva. Principal component analysis (PCA) and linear discriminant analysis (LDA) were used to reduce and discriminate between the two groups of data, and the study resulted in accuracy, sensitivity, and specificity being 80%, 78%, and 83%, respectively. In conclusion, SERS of saliva showed the ability to predict lung cancer in our experiment.

Metal positioning on silicon surfaces using the etching of buried dislocation arrays

Large-area Si(001) nanopatterned surfaces obtained by etching dislocation line arrays have been used to drive the positioning of metallic islands. A method combining wafer bonding of (001) silicon on insulator layers and preferential chemical etching allows controlling the periodicity of square trench arrays in the 20-50 nm lateral periodicity range with an accuracy of less than 1 nm and a depth of about 4-5 nm. The interfacial area containing the dislocation line plane can be removed and a single crystal maintaining the morphological patterning can be obtained. It is shown that oxidized or deoxidized silicon nanopatterned surfaces can drive the positioning of Ni, Au and Ag islands for a 20 nm lateral periodicity and that a lateral long range order, directly transferred from the dislocation network, can be obtained in the Ni and Au cases.

Silver-coated dye-embedded silica beads: a core material of dual tagging sensors based on fluorescence and Raman scattering

We have developed a new type of dual-tag sensor for immunoassays, operating via both fluorescence and surface-enhanced Raman scattering (SERS). A one-shot fluorescence image over the whole specimen allows us to save considerable time because any unnecessary time-consuming SERS measurements can be avoided from the signature of the fluorescence. Dye-embedded silica beads are prepared initially, and then SERS-active silver is coated onto them via a very simple electroless-plating method. The Raman markers are subsequently assembled onto the Ag-coated silica beads, after which they are stabilized by silanization via a biomimetic process in which a poly(allylamine hydrochloride) layer formed on the Raman markers by a layer-by-layer deposition method acting as a scaffold for guiding silicification. In the final stage, specific antibodies are attached to the silica surface in order to detect target antigens. The fluorescence signal of the embedded dye can be used as a fast readout system of molecular recognition, whereas the SERS signals are subsequently used as the signature of specific molecular interactions. In this way, the antibody-grafted particles were found to recognize antigens down to 1 × 10(-10) g mL(-1) solely by the SERS peaks of the Raman markers.

Visualizing chromatographic separation of metal ions on a surface-enhanced Raman active medium

Metal ion carboxylato complexes possess ion-specific carboxylate Raman bands. Using this attribute we follow the chromatographic separation of a microliter aliquot of an initially equimolar solution of Pb(2+) and Hg(2+) using the surface-enhanced Raman spectroscopy spectra of their carboxylato complexes as unique identifiers. A glass capillary whose interior is lined with a dense layer of gold nanoparticles treated with 4-mercaptobenzoic acid simultaneously acts as a separation medium and an efficient SERS reporter of the step-by-step separation process. The observed adsorption-desorption equilibrium along the capillary is shown to conform with theory. Although Hg(2+) complexes with COO(-) much more strongly than Pb(2+), it is the Pb(2+) that survives the separation process as the sole surface species. We show that this is because so much mercury is taken out of solution during early separation steps that the surface equilibrium is ultimately driven toward adsorbed Pb(2+).

Surface-enhanced Raman scattering for protein detection

Proteins are essential components of organisms and they participate in every process within cells. The key characteristic of proteins that allows their diverse functions is their ability to bind other molecules specifically and tightly. With the development of proteomics, exploring high-efficiency detection methods for large-scale proteins is increasingly important. In recent years, rapid development of surface-enhanced Raman scattering (SERS)-based biosensors leads to the SERS realm of applications from chemical analysis to nanostructure characterization and biomedical applications. For proteins, early studies focused on investigating SERS spectra of individual proteins, and the successful design of nanoparticle probes has promoted great progress of SERS-based immunoassays. In this review we outline the development of SERS-based methods for proteins with particular focus on our proposed protein-mediated SERS-active substrates and their applications in label-free and Raman dye-labeled protein detection.

Time resolved emission studies of Ag-adenine-templated CdS (Ag/CdS) nanohybrids

Ag-adenine-templated CdS (Ag/CdS) nanohybrids have been synthesized and characterized by transmission electron microscopy, selected area electron diffraction, x-ray diffraction, and optical, fluorescence and time resolved emission spectroscopy. Adenine serves as an effective matrix for the stabilization of Ag/CdS through interaction of N(1), N(3) and -NH(2) with Ag. The amount of Ag in the nanohybrid is observed to influence the organization of the Ag and CdS phase in the composite and also modifies the nature of electronic transition in CdS. For the nanohybrid containing a molar ratio of 0.1 of Ag/ CdS, CdS nanoparticles (2.5 nm) surround the Ag (6.5 nm) core. The excitation of these particles by 340 nm light, where the absorption due to the Ag phase in the nanohybrid is negligibly small, results in the enhancement of fluorescence by a factor of 7 compared to that of bare CdS. For the particles containing a molar ratio of Ag/CdS of unity, bigger clusters (14 nm) are produced causing the quenching of emission of CdS. In time resolved emission spectroscopy the spectral shift from 415 nm (3.0 eV) to 550 nm (2.26 eV) monitored over a period of 1-220 ns is understood by the relaxation of charge within the surface states of varied energy from 180 to 370 eV. The observed changes in fluorescence behavior in terms of intensity, lifetime and spectral shift are understood in terms of electronic interaction between Ag and CdS phases. The manipulation of electronic and fluorescence properties in these nanohybrids could be exploited for optoelectronic, molecular-recognition and sensing applications.

Metal-enhanced fluorescence of rhodamine B isothiocyanate from micrometer-sized silver powders

We report here the use of micrometer-sized silver (microAg) powders to enhance the fluorescence emission of rhodamine B isothiocyanate (RhBITC). Our findings clearly show that the RhBITC displays up to a 36-fold increase in fluorescence emission intensity when RhBITC-labeled poly(allylamine hydrochloride) (RhBITC-PAH) is assembled onto a polyelectrolyte layer (about 30 nm thick) above microAg, when compared to that measured for RhBITC-PAH assembled directly onto a glass slide. A similar experiment conducted using a vacuum-evaporated Ag film displayed only a 5-fold increase. The enhanced fluorescence observed for microAg powders is obviously due to their ability to concentrate the electric fields. This suggests that metal-enhanced fluorescence (MEF) can potentially be employed to increase the sensitivity and detection limit of various bioassays that employ RhBITC as a fluorescence label. Since microAg powders are also an efficient substrate for surface-enhanced Raman scattering (SERS), molecular sensors operating via both SERS and MEF may then be able to be fabricated using microAg powders.

Silanization of polyelectrolyte-coated particles: an effective route to stabilize Raman tagging molecules adsorbed on micrometer-sized silver particles

Micrometer-sized Ag (microAg) powders are very efficient surface-enhanced Raman scattering (SERS) substrates. To use microAg powders as a core material for molecular sensors operating via SERS, it is necessary to stabilize the tagging (i.e., SERS-marker) molecules adsorbed onto them. We demonstrate in this work that once the tagging molecules are coated with aliphatic polyelectrolytes such as poly(allylamine hydrochloride), the base-catalyzed silanization can be readily carried out to form stable silica shells around the polyelectrolyte layers by a biomimetic process; any particle can therefore be coated with silica since polyelectrolytes can be deposited beforehand via a layer-by-layer deposition method. Even after silanization, the SERS peaks of marker molecules on microAg particles are the only observable peaks since aliphatic polyelectrolytes, as well as silica shells, are intrinsically weak Raman scatterers, and more importantly, the SERS signals must be derived mostly from the first layer of the adsorbates (i.e., the marker molecules) in direct contact with the microAg particles. Silica shells, once fabricated, can further be derivatized to possess biofunctional groups; therefore, the modified microAg particles can be used as platforms of highly stable SERS-based biological sensors, as well as barcoding materials.

Environmentally friendly synthesis of highly monodisperse biocompatible gold nanoparticles with urchin-like shape

We report a facile and environmentally friendly strategy for high-yield synthesis of highly monodisperse gold nanoparticles with urchin-like shape. A simple protein, gelatin, was first used for the control over shape and orientation of the gold nanoparticles. These nanoparticles, ready to use for biological systems, are promising in the optical imaging-based disease diagnostics and therapy because of their tunable surface plasmon resonance (SPR) and excellent surface-enhanced Raman scattering (SERS) activity.

Expanding generality of surface-enhanced Raman spectroscopy with borrowing SERS activity strategy

Surface-enhanced Raman scattering (SERS) was discovered three decades ago and has gone through a tortuous pathway to develop into a powerful diagnostic technique. Recently, the lack of substrate, surface and molecular generalities of SERS has been circumvented to a large extent by devising and utilizing various nanostructures by many groups including ours. This article aims to present our recent approaches of utilizing the borrowing SERS activity strategy mainly through constructing two types of nanostructures. The first nanostructure is chemically synthesized Au nanoparticles coated with ultra-thin shells (ca. one to ten atomic layers) of various transition metals, e.g., Pt, Pd, Ni and Co, respectively. Boosted by the long-range effect of the enhanced electromagnetic (EM) field generated by the highly SERS-active Au core, the originally low surface enhancement of the transition metal can be substantially improved giving total enhancement factors up to 10(4)-10(5). It allows us to obtain the Raman spectra of surface water, having small Raman cross-section, on several transition metals for the first time. To expand the surface generality of SERS, tip-enhanced Raman spectroscopy (TERS) has been employed. With TERS, a nanogap can be formed controllably between an atomically flat metal surface and the tip with an optimized shape, within which the enhanced EM field from the tip can be coupled (borrowed) effectively. Therefore, one can obtain surface Raman signals (TERS signals) from adsorbed species at Au(110), Au(111) and, more importantly, Pt(l10) surfaces. The enhancement factor achieved on these single crystal surfaces can be up to 106, especially with a very high spatial resolution down to about 14 nm. To fully accomplish the borrowing strategy from different nanostructures and to explain the experimental observations, a three-dimensional finite-difference time-domain method was used to calculate and evaluate the local EM field on the core-shell nanoparticle surfaces and the TERS tips. Finally, prospects and further developments of this valuable strategy are briefly discussed with emphasis on the emerging experimental methodologies.

Bi2WO6 nano- and microstructures: shape control and associated visible-light-driven photocatalytic activities

The shape-controlled synthesis of nano- and microstructured materials has opened up new possibilities to improve their physical and chemical properties. In this work, new types of Bi(2)WO(6) with complex morphologies, namely, flowerlike, tyre- and helixlike, and platelike shapes, have been controllably synthesized by a facile hydrothermal process. The benefits of the present work also stem from the first report on the transformation of Bi(2)WO(6) from three-dimensional (3D) flowerlike superstructures to 2D platelike structures, and on the formation of tyre- and helixlike Bi(2)WO(6) superstructures. UV/Vis absorption spectra show that the optical properties of Bi(2)WO(6) samples are relevant to their size and shape. More importantly, the photocatalytic activities of Bi(2)WO(6) nano- and microstructures are strongly dependent on their shape, size, and structure for the degradation of Rhodamine B (RhB) under visible-light irradiation. The reasons for the differences in the photocatalytic activities of these Bi(2)WO(6) nano- and microstructures are further investigated.

Silver-particle-based surface-enhanced resonance Raman scattering spectroscopy for biomolecular sensing and recognition

We demonstrate in this work that 2-microm-sized Ag (microAg) powders can be used as a core material for constructing biomolecular sensing/recognition units operating via surface-enhanced resonance Raman scattering (SERRS). This is possible because microAg powders are very efficient substrates for both the diffuse reflectance IR and the surface-enhanced Raman scattering-SERRS spectroscopic characterization of molecular adsorbates prepared in a similar manner on silver surfaces. Besides, the agglomeration of microAg particles in a buffer solution can be prevented by the layer-by-layer deposition of cationic and anionic polyelectrolytes such as poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). In this particular study, we used rhodamine B isothiocyanate (RhBITC) as a SERRS marker molecule, and microAg powders adsorbed consecutively with RhBITC and PAH-PAA bilayers were finally derivatized with biotinylated poly(L-lysine). On the basis of the nature of the SERRS peaks of RhBITC, those microAg powders were confirmed to selectively recognize streptavidin molecules down to concentrations of 10(-10) g mL-1. Since a number of different molecules can be used as SERS-SERRS marker molecules, the present method proves to be an invaluable tool for multiplex biomolecular sensing/recognition via SERS and SERRS.

Stabilization of hydroxyl-group-terminated SERS-marker molecules on μAg particles by silanization

Micrometer-sized Ag (microAg) powders are very efficient substrates for both the infrared and Raman spectroscopic characterization of molecular adsorbates assembled on silver. In particular, the Raman spectrum of organic monolayers on microAg powders is a surface-enhanced Raman scattering (SERS) spectrum. To use microAg powders as a core material for constructing molecular sensing/recognition units operating via SERS, it is first necessary to stabilize the SERS-marker molecules that are directly in contact with the microAg powders. One promising strategy is the fabrication of silica shells onto SERS-marker molecules, and herein we demonstrate its feasibility by choosing 4-mercaptophenol (4-MPH) as a model SERS-marker molecule. Due to the presence of the hydroxyl group of 4-MPH, silica was readily deposited onto microAg particles by the base-catalyzed hydrolysis of tetraethyl orthosilicate, and its subsequent condensation, to form a cagelike structure. The formation of silica shells was confirmed with infrared, Raman, and X-ray photoelectron spectroscopy, coupled with field emission scanning electron microscopy. We were able to tune the thickness of silica shells simply by varying the silanization reaction time.