Effect of Ag and Au nanoparticles on the SERS of 4-aminobenzenethiol assembled on powdered copper

Ag-Coated Ternary Layered Double Hydroxide as a High-Performance SERS Sensor for Aldehydes

Volatile organic compounds (VOCs) are common environmental pollutants and important biomarkers for early diagnosis of lung cancer. However, aldehydes are difficult to detect directly due to their small Raman scattering cross-section and gaseous phase. Here, a Ag-coated ternary layered double hydroxide (LDH) was designed for the detection and identification of various aldehydes. The specific surface area of CoNi-LDH was increased by doping Fe3+, which provides abundant active sites to capture gas molecules. Furthermore, the energy band gap (Eg) was decreased due to the local amorphous FeCoNi-LDH with an extended band tail, promoting the excitonic transition of Fe0.07(CoNi)0.93-LDH. In addition, the Fermi level of Ag prevented the recombination of electron-hole pairs of Fe0.07(CoNi)0.93-LDH, providing a new bridge for charge transfer between the substrate and the molecule. Ag/Fe0.07(CoNi)0.93-LDH presented excellent surface-enhanced Raman scattering (SERS) performance for aldehyde VOCs by modification with 4-aminothiophenol (4-ATP) to capture aldehydes and realized the detection of benzaldehyde (BZA) at 10 ppb. The enhancement and Raman shift of the b2 mode indicated the contribution of chemical enhancement to the SERS system, so the substrate presented good uniformity. The recycling of the SERS substrate is realized based on the reversibility of the Schiff base reaction. These results manifested that Ag/FeCoNi-LDH has a wide prospect in the application in the trace detection of aldehydes.

Synthesis Methods and Optical Sensing Applications of Plasmonic Metal Nanoparticles Made from Rhodium, Platinum, Gold, or Silver

The purpose of this paper is to provide an in-depth review of plasmonic metal nanoparticles made from rhodium, platinum, gold, or silver. We describe fundamental concepts, synthesis methods, and optical sensing applications of these nanoparticles. Plasmonic metal nanoparticles have received a lot of interest due to various applications, such as optical sensors, single-molecule detection, single-cell detection, pathogen detection, environmental contaminant monitoring, cancer diagnostics, biomedicine, and food and health safety monitoring. They provide a promising platform for highly sensitive detection of various analytes. Due to strongly localized optical fields in the hot-spot region near metal nanoparticles, they have the potential for plasmon-enhanced optical sensing applications, including metal-enhanced fluorescence (MEF), surface-enhanced Raman scattering (SERS), and biomedical imaging. We explain the plasmonic enhancement through electromagnetic theory and confirm it with finite-difference time-domain numerical simulations. Moreover, we examine how the localized surface plasmon resonance effects of gold and silver nanoparticles have been utilized for the detection and biosensing of various analytes. Specifically, we discuss the syntheses and applications of rhodium and platinum nanoparticles for the UV plasmonics such as UV-MEF and UV-SERS. Finally, we provide an overview of chemical, physical, and green methods for synthesizing these nanoparticles. We hope that this paper will promote further interest in the optical sensing applications of plasmonic metal nanoparticles in the UV and visible ranges.

Attomolar Sensitive Magnetic Microparticles and a Surface-Enhanced Raman Scattering-Based Assay for Detecting SARS-CoV-2 Nucleic Acid Targets

Highly sensitive, reliable assays with strong multiplexing capability for detecting nucleic acid targets are significantly important for diagnosing various diseases, particularly severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The nanomaterial-based assay platforms suffer from several critical issues such as non-specific binding and highly false-positive results. In this paper, to overcome such limitations, we reported sensitive and remarkably reproducible magnetic microparticles (MMPs) and a surface-enhanced Raman scattering (SERS)-based assay using stable silver nanoparticle clusters for detecting viral nucleic acids. The MMP-SERS-based assay exhibited a sensitivity of 1.0 fM, which is superior to the MMP-fluorescence-based assay. In addition, in the presence of anisotropic Ag nanostructures (nanostars and triangular nanoplates), the assay exhibited greatly enhanced sensitivity (10 aM) and excellent signal reproducibility. This assay platform intrinsically eliminated the non-specific binding that occurs in the target detection step, and the controlled formation of stable silver nanoparticle clusters in solution enabled the remarkable reproducibility of the results. These findings indicate that this assay can be employed for future practical bioanalytical applications.

Investigate on plasma catalytic reaction of 4-nitrobenzenethiol on Ag@SiO2Core-Shell substrate via Surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) is a promising technique to investigate the plasmon-driven catalytic reaction, in which the Raman signal originates from the electromagnetic (EM)enhancement mechanism and the chemical enhancement (CE) mechanism. Here, we designed and synthesized a novel SERS substrate based on SiO₂ wrapped Ag nanoparticles (Ag@SiO₂ core-shell nanoparticles substrate, Ag@SiO₂ CSNS). Meanwhile, the SERS substrate based on Ag nanoparticles (Ag NS) also was prepared for comparison. Then, plasmon-driven catalytic reaction of 4-nitrobenzenethiol (4-NBT) to p,p'-dimercaptoazobenzene (DMAB) were systematically investigated on Ag and Ag@SiO₂, respectively. The result revealed that, the Fermi level of Ag@SiO₂ CSNS is lower than Ag NS, and the catalytic reaction greatly hindered by the Ag@SiO₂ CSNS under the same excitation laser wavelength. With the same condition excitation laser, Raman signal enhancement effects are different when applying Ag NS and Ag@SiO₂ CSNS, which could be attributed to that the inert SiO₂ shell eliminates CE mechanism of the Raman signal. These results provide a simple strategy to figure out the mechanism of the catalytic reaction based on Surface-enhanced Raman scattering.

Preparation and characterization of Ag-nanostars@Au-nanowires hierarchical nanostructures for highly sensitive surface enhanced Raman spectroscopy

In this work we study the surface enhanced Raman scattering (SERS) produced by hierarchical nanostructures obtained by coupling different anisotropic nanomaterial of two SERS active metals, namely Ag nanostars (AgNSs) and Au nanowires (AuNWs). Ag nanostars (AgNSs) are prepared, by a two-step one-pot synthesis by reduction of AgNO3 with hydroxylamine, trisodium citrate and NaOH. AuNWs are obtained by electroless templated synthesis in track-etched polycarbonate membranes with following etching of the template. The two precursors are bound together by bridging with the bifunctional cysteamine molecule, obtaining AgNS@AuNW hierarchical structures. Benzenethiol (BT) is adsorbed on the nanostructured material and used as SERS probe to study the amplification of Raman signals. Experimental results indicate significantly larger Raman enhancement when BT is adsorbed onto the AgNS@AuNW in comparison to AuNWs alone or decorated with quasi-spherical silver nanoparticles obtaining AgNP@AuNW. Digital simulations performed by the boundary element method agree with the experimental findings, showing higher number of hot spots and significantly higher SERS enhancements for AgNS@AuNW versus AuNWs or AgNSs or AgNP@AuNW.

Watching Visible Light-Driven CO2 Reduction on a Plasmonic Nanoparticle Catalyst

Photocatalytic reduction of carbon dioxide (CO₂) by visible light has the potential to mimic plant photosynthesis and facilitate the renewable production of storable fuels. Accomplishing desirable efficiency and selectivity in artificial photosynthesis requires an understanding of light-driven pathways on photocatalyst surfaces. Here, we probe with single-nanoparticle spatial resolution the dynamics of a plasmonic silver (Ag) photocatalyst under conditions of visible light-driven CO₂ reduction. In situ surface-enhanced Raman spectroscopy captures discrete adsorbates and products formed dynamically on single photocatalytic nanoparticles, most prominent among which is a surface-adsorbed hydrocarboxyl (HOCO*) intermediate critical to further reduction of CO₂ to carbon monoxide (CO) and formic acid (HCOOH). Density functional theory simulations of the captured adsorbates reveal the mechanism by which plasmonic excitation activates physisorbed CO₂ leading to the formation of HOCO*, indicating close interplay between photoexcited states and adsorbate/metal interactions.

Development of Fe3O4/Ag core/shell-based multifunctional immunomagnetic nanoparticles for isolation and detection of CD34+ stem cells

Fe₃O₄/Ag core/shell nanoparticles functionalized with the free amino (NH₂) functional groups (Fe₃O₄/Ag-NH₂) were conjugated with fluorescent electron coupled dye (ECD)-antiCD34 antibody using the 1-ethyl-3-(3'-dimethyl-aminopropyl) carbodiimide (EDC) catalyst (ECD - Electron Coupled Dye or R Phycoerythrin-Texas Red is a fluorescent organic dye attached to the antibody). The characteristic fluorescence of ECD in the antibody was investigated and was used as a good indicator for estimating the percentage of the antibodies that were successfully conjugated with the nanoparticles. The conjugation efficiency was found to increase depending on the V_NP:V_AB ratio, where V_NP and V_AB are the volumes of the nanoparticle solution (concentration of 50 ppm) and the as-purchased antibody solution, respectively. The conjugation efficiency rapidly increased from approximately 18% to approximately 70% when V_NP:V_AB was increased from 2:1 to 100:1, and it gradually reached the saturated state at an efficiency of 95%, as the V_NP:V_AB was equal to 300:1. The bioactivity of the abovementioned conjugation product (denoted by Fe₃O₄/Ag-antiCD34) was evaluated in an experiment for the collection of stem cells from bone marrow samples.

Tip-Selective Growth of Silver on Gold Nanostars for Surface-Enhanced Raman Scattering

Nanogaps as "hot spots" with highly localized surface plasmon can generate ultrastrong electromagnetic fields. Superior to the exterior nanogaps obtained via aggregation and self-assembly, interior nanogaps within Au and Ag nanostructures give stable and reproducible surface-enhanced Raman scattering (SERS) signals. However, the synthesis of nanostructures with interior hot spots is still challenging because of the lack of high-yield strategies and clear design principles. Herein, gold-silver nanoclusters (Au-Ag NCs) with multiple interior hot spots were fabricated as SERS platforms via selective growth of Ag nanoparticles on the tips of Au nanostars (Au NSs). Furthermore, the interior gap sizes of Au-Ag NCs can be facilely tuned by changing the amount of AgNO₃ used. Upon 785 nm excitation, single Au-Ag NC₃₅₀ exhibits 43-fold larger SERS enhancement factor and the optimal signal reproducibility relative to single Au NS. The SERS enhancement factors and signal reproducibility of Au-Ag NCs increase with the decrease of gap sizes. Collectively, the Au-Ag NCs could serve as a flexible, reproducible, and active platform for SERS investigation.

SERS effect of selectively adsorbed dyes by hydrothermally-produced MoS2 nanosheets

The electro-magnetic mechanism and chemical transfer mechanism coexist in the surface-enhanced Raman scattering (SERS) procedure.

SERRS and absorption spectra of pyridine on Au m Ag n (m + n = 6) bimetallic nanoclusters: substrate composition and applied electric field effects

Surface-enhanced Raman scattering (SERS) and absorption spectra of the pyridine molecule adsorbed on Au _m Ag _n (m + n = 6) bimetallic clusters are theoretically investigated by time-dependent density functional theory. The contributions of static chemical enhancement to the ground-state system are analyzed, and the static Raman intensity of Py-Au _m Ag _n complexes are enhanced by an order of 10. A method of visualization on charge transfer is used to distinguish the contributions of charge-transfer enhancement and electromagnetic enhancement. The intensity of surface-enhanced resonance Raman scattering (SERRS) spectroscopy of Py-Au _m Ag _n is strongly enhanced by an order of 10³-10⁵, compared to the static Raman intensity of pyridine. The influence of the static external electric field on SERS is investigated by calculating the optical properties of the Py-Au₃Ag₃ complex. The intensity of SERRS spectra and normal Raman spectra can be significantly enhanced by the positive electric fields, and the intensities of specific Raman vibrational modes could be selectively enhanced or weakened by tuning the direction and strength of the static electric field applied on Py-Au₃Ag₃.

Copper–gold sandwich structures on PE and PET and their SERS enhancement effect

In this paper we have investigated the SERS effect of gold–copper sandwich structures i.e. the coupling between surface plasmon polaritons supported by the gold grating and localized surface plasmons excited on the grafted copper nanoparticles.

Highly-dispersed TiO2 nanoparticles with abundant active sites induced by surfactants as a prominent substrate for SERS: charge transfer contribution

Highly-dispersed TiO₂ with abundant surface oxygen vacancies was presented as an effective substrate for charge-transfer-induced SERS.

Component conversion from pure Au nanorods to multiblock Ag-Au-Ag nanorods assisted by Pt nanoframe templates

We developed a new method for synthesizing multiblock Ag-Au-Ag nanorods using Pt nanoframes that had been deposited on the edges of Au nanorod seeds. As a function of Au etching time, the length of the Au nanorod decreased symmetrically starting from the two ends, leading to the formation of empty inner space at the ends. Subsequent reduction of Ag ions could be selectively performed in the inner space confined by Pt nanoframes and the resulting Ag-Au-Ag nanorods exhibited characteristic LSPR modes originating from each block component (in a transverse direction) and SPR coupling (in a longitudinal direction). The high quality of the resulting multiblock nanorods enabled observation of the longitudinal quadrupole mode that was induced by Ag-Au SPR coupling in a long axis. The mode exhibited high sensitivity in accordance with the change in the surrounding media, demonstrating great potential for sensor applications.

Nonresonant chemical mechanism in surface-enhanced Raman scattering of pyridine on M@Au12 clusters

By employing density functional theory (DFT), this study presents a detailed analysis of nonresonant surface-enhanced Raman scattering (SERS) of pyridine on M@Au12 (M = V(-), Nb(-), Ta(-), Cr, Mo, W, Mn(+), Tc(+), and Re(+))-the stable 13-atom neutral and charged gold buckyball clusters. Changing the core atom in M@Au12 enabled us to modulate the direct chemical interactions between pyridine and the metal cluster. The results of our calculations indicate that the ground-state chemical enhancement does not increase as the binding interaction strengthens or the transfer charge increases between pyridine and the cluster. Instead, the magnitude of the chemical enhancement is governed, to a large extent, by the charged properties of the metal clusters. Pyridine on M@Au12 anion clusters exhibits strong chemical enhancement of a factor of about 10(2), but the equivalent increase for pyridine adsorbed on M@Au12 neutral and cation clusters is no more than 10. Polarizability and deformation density analyses clearly show that compared with the neutral and cation clusters, the anion clusters have more delocalized electrons and occupy higher energy levels in the pyridine-metal complex. Accordingly, they produce larger polarizability, leading to a stronger nonresonant enhancement effect.

Simple electrochemical synthesis of an Au–Ag–Cu trimetallic nanodendrite and its use as a SERS substrate

An Au–Ag–Cu trimetallic nanodendrite was constructed by simple electrochemical methods and was evaluated as a surface enhanced Raman scattering (SERS) substrate.

Silver nanoparticle functionalized glass fibers for combined surface-enhanced Raman scattering spectroscopy (SERS)/surface-assisted laser desorption/ionization (SALDI) mass spectrometry via plasmonic/thermal hot spots

Closely-packed silver nanoparticles with a size of 20–50 nm and an inter-particle nanoscale gap of less than 10 nm were effective for a simultaneously enhanced SERS/SALDI substrate via plasmonic/thermal “hot spots”.

Intense Raman scattering on hybrid Au/Ag nanoplatforms for the distinction of MMP-9-digested collagen type-I fiber detection

Well-ordered Au-nanorod arrays were fabricated using the focused ion beam method (denoted as fibAu_NR). Au or Ag nanoclusters (NCs) of various sizes and dimensions were then deposited on the fibAu_NR arrays using electron beam deposition to improve the surface-enhanced Raman scattering (SERS) effect, which was verified using a low concentration of crystal violet (10(-)(5)M) as the probe molecule. An enhancement factor of 6.92 × 10(8) was obtained for NCsfibAu_NR, which is attributed to the combination of intra-NC and NR localized surface plasmon resonance. When 4-aminobenzenethiol (4-ABT)-coated Au or Ag nanoparticles (NPs) were attached to NCsfibAu_NR, the small gaps between 4-ABT-coated NPs and intra-NCs allowed detection at the single-molecule level. Hotspots formed at the interfaces of NCs/NRs and NPs/NCs at a high density, producing a strong local electromagnetic effect. Raman spectra from as-prepared type I collagen (Col-I) and Ag-NP-coated Col-I fibers on NCsfibAu_NR were compared to determine the quantity of amino acids in their triple helix structure. Various concentrations of matrix-metalloproteinase-9-digested Col-I fibers on NCsfibAu_NR were qualitatively examined at a Raman laser wavelength of 785nm to determine the changes of amino acids in the Col-I fiber structure. The results can be used to monitor the growth of healing Col-I fibers in a micro-environment.

Tuning interior nanogaps of double-shelled Au/Ag nanoboxes for surface-enhanced Raman scattering

Double-shelled Au/Ag hollow nanoboxes with precisely controlled interior nanogaps (1 to 16 nm) were synthesized for gap-tunable surface-enhanced Raman scattering (SERS). The double-shelled nanoboxes were prepared via a two-step galvanic replacement reaction approach using Ag nanocubes as the templates, while 4-aminothiolphenol (4-ATP) as SERS probe molecules were loaded between the two shells. More than 10-fold enhancement of SERS is observed from the double-shelled nanoboxes than Ag nanocubes. In addition, the SERS of the double-shelled nanoboxes increase significantly with the decrease of gap size, consistent with the theoretical prediction that smaller gap size induces larger localized electromagnetic enhancement.

Thin and transparent films of graphene/silver nanoparticles obtained at liquid-liquid interfaces: preparation, characterization and application as SERS substrates

We report here the synthesis and characterization of transparent and homogeneous thin films of reduced graphene oxide/silver nanoparticles (rGO/AgNPs) nanocomposites, starting from graphene oxide (GO) or reduced graphene oxide (rGO), directly obtained at a water/toluene liquid-liquid interface. Different films (obtained by varying the Ag/rGO or Ag/GO ratio) were prepared, deposited over glass or plastic substrates, and characterized by X-ray diffraction, UV-Vis and Raman spectroscopy, thermal analysis, transmission and scanning electron microscopy. Samples were evaluated as substrates for surface-enhanced Raman spectroscopy (SERS), using dilute solutions (1×10(-7) mol L(-1)) of a common probe molecule, 4-aminothiophenol (4-ATP). These materials exhibit significant high-quality SERS activity, and enhanced modes could be observed for 4-ATP, which suggested that charge transfer occurred between the Ag nanoparticles and 4-ATP molecules.

SERS biosensor using metallic nano-sculptured thin films for the detection of endocrine disrupting compound biomarker vitellogenin

A biosensor chip is developed for the detection of a protein biomarker of endocrine disrupting compounds, vitellogenin (Vg) in aquatic environment. The sensor chip is fabricated by immobilizing anti-Vg antibody on 4-Aminothiophenol (4-ATP) coated nanosculptured thin films (nSTFs) of silver on Si substrates. The biosensor is based on the SERS of 4-ATP, enhanced by the Ag nSTFs. Before the fabrication of the sensor, the performance of the enhancement is optimized with respect to the porosity of nSTFs. Further, the biosensor is developed on the nSTF with optimized enhancement. The SERS signals are recorded from the sensor chip for varying concentrations of Vg. A control experiment is performed on another similar protein Fetuin to confirm the specificity of the sensor. The repeatability and reusability of the sensor, along with its shelf life are also checked. The limit of detection of the sensor is found to be 5 pg mL −1 of Vg in PBS within our experimental window. Apart from high sensitivity, specificity and reusability, the present sensor provides additional advantages of miniaturization, requirement of very small volumes of the analyte solution (15 μL) and fast response as compared to conventional techniques e.g., ELISA, as its response time is less than 3 minutes.

An in situ approach for facile fabrication of robust and scalable SERS substrates

The widespread implementation of surface enhanced Raman scattering (SERS) techniques for chemical and biological detection requires an inexpensive, yet robust SERS substrate with high sensitivity and reproducibility. To that end, we present a facile method to fabricate plasmonic SERS substrates with well-distributed SERS "hot spots" on a large scale with reproducible SERS enhancement factors of ∼10(8) for the Raman probe molecule 4-aminobenzenethiol (4-ABT). The SERS enhancement is attributed to the synergistic interactions between the strong plasmonic coupling among the assembled Au NPs and the structure-associated tip enhancement. Additionally, these mechanically-flexible substrates exhibit remarkably reproducible SERS signals, demonstrating the merits of our methodology. Our approach illustrates the potential opportunities for fabricating robust, commercially-viable SERS substrates with well-distributed "hot spots" on a large scale while avoiding costly vacuum deposition technologies.

Gold-decorated titania nanotube arrays as dual-functional platform for surface-enhanced Raman spectroscopy and surface-assisted laser desorption/ionization mass spectrometry

In this report, we demonstrate gold-decorated titania nanotube arrays (Au-TNA substrate) as a dual-functional platform for surface-enhanced Raman spectroscopy (SERS) and surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). The Au nanoparticles are grown on the substrate using vapor deposition of Au. The resulting substrates perform better than Au colloids in terms of the reproducibility of the SERS measurements, long-term stability of the fabricated structures, and clean surface of the Au. The nanostructure of the Au-TNA substrate was designed to optimize the SALDI-MS and SERS performance. Excellent reproducibility of the SERS measurements using the Au-TNA substrate was obtained, with a standard error less than 6 %. SALDI activity was also demonstrated for the same Au-TNA substrates. Finally, the Au-TNA substrate was used for combined SERS and SALDI-MS analysis (i) to discriminate the structural isomers of pyridine compounds (para-, meta-, and ortho-pyridinecarboxylic acid) and (ii) to detect polycarbamate, a dithiocarbamate fungicide. These results are difficult to obtain using either approach alone.

Gold Nanopopcorn Attached Single-Walled Carbon Nanotube Hybrid for Rapid Detection and Killing of Bacteria

We report a strategy to fabricate a rapid and stable surface-enhanced Raman scattering (SERS)-based hybrid nanomaterial using gold nanopopcorns attached single-walled carbon nanotubes (AuNP@f₃-SWCNTs) for label-free detection and photothermal killing of bacteria. Herein, previously ester-functionalized single-walled carbon nanotubes (f₁-SWCNTs) undergo 1,3-dipolar cycloaddition reaction with in-situ generated nitrile imine under Microwave (MW) irradiation to form a doubly ester terminated SWCNTs cycloadduct (f₂-SWCNTs). The ester terminals are further modified with 4-aminothiophenol (4-ATP) under MW-irradiation to form thiol-terminated SWCNTs templates (f₃-SWCNTs) that allow gold nanopopcorns (AuNPs) to covalently and uniformly attach at a minimum inter-particle distance thus yielding a hybrid nanomaterial (AuNP@f₃-SWCNT) with good aqueous stability and abundant 'hotspots'. Consequently, monoclonal E. coli antibody-conjugated bioassays fabricated with our AuNP@f₃-SWCNT substrates (mAb-AuNP@f₃-SWCNT) rapidly detect E. coli in water with good selectivity and impressive SERS sensitivity. The detection limit of E. coli 49979, selected as a model to establish proof of principle, was found to be 1.0×10² CFU/mL. Furthermore, the AuNP@f₃-SWCNT hybrid nanomaterial offers impressive photothermal pathogen killing effects. The synergy-type enhancement effect arising from the inherent noble properties of the respective components of the hybrid nanomaterial indicate that our AuNP@f₃-SWCNT has the potential for further application in multiplex detection in samples.

Bio-inspired in situ growth of monolayer silver nanoparticles on graphene oxide paper as multifunctional substrate

In this study, we report a facile bio-inspired method for large-scale preparation of highly dispersed Ag nanoparticles (NPs) on the surface of flexible reduced graphene oxide (rGO) paper with using dopamine (DA) both as a reductant and a surface modifier. Through the self-polymerization of dopamine, free-standing GO paper can be simultaneously reduced and modified with following in situ growth of monolayer Ag NPs on such a substrate. The spherical Ag NPs with an average diameter of 80 nm have a narrow size distribution and tunable cover density. Such a flexible rGO/Ag hybrid paper presents enhanced antibacterial activity against E. coli and a high active and sensitive SERS response toward Rhodamine 6G (R6G) molecules. The detection signals can be obtained while the R6G concentration is as low as to 10(-8) M. This work provides a simple strategy for large-scale fabrication of monolayer Ag NPs on flexible rGO paper as a portable antibacterial substrate and a potential SERS substrate for molecular detection applications.

Facile fabrication of truncated octahedral Au nanoparticles and its application for ultrasensitive surface enhanced Raman scattering immunosensing

Monodispersed truncated octahedral (TOH) Au nanoparticles (NPs) with an average edge-length of about 16 nm were synthesized using poly(diallyldimethylammonium chloride) (PDDA) both as a stabilizing and reducing agent via a one-step reaction. Remarkably, no seeds, surfactants or additional reductant were used in this reaction. In addition, the PDDA molecules on the surface of the TOH AuNPs make them convenient for use in layer-by-layer assembly by electrostatic interactions. Importantly, the TOH AuNPs show a significant surface enhanced Raman scattering (SERS) activity, and can be directly used for building SERS-active substrates and tags. Based on these promising properties, an ultrasensitive SERS-based immunosensing platform was developed. Using human immunoglobulin (h-IgG) as a model target analyte, a detection limit of 36.56 fg ml(-1) was reached.

Meditating metal coenhanced fluorescence and SERS around gold nanoaggregates in nanosphere as bifunctional biosensor for multiple DNA targets

Gold nanoparticles (Au NPs) are very attractive candidate nanoparticles in biological assay because of their high chemical stabilities, high homogeneities, good biocompatibilities, and low toxicities. However, molecular beacon assays via encapsulating the combined fluorescence or surface-enhanced Raman scattering (SERS) signals of reporters and Au NPs in nanobarcodes particles usually suffer from fluorescence quenching or weak Raman enhancement when Au NPs are employed (especially with size smaller than 15 nm). Herein, we present a new design of simultaneously realizing metal-enhanced fluorescence and coenhanced surface-enhanced Raman scattering by facilely embedding Ag nanoparticle into the shell of two kinds of Au nanoaggregate (5 and 10 nm), meanwhile, fluorophore is located between the silver core and gold nanoparticle layers and the distance among them is adjusted by SiO2 spacer (Ag@first SiO2 spacer@FiTC+SiO2@second SiO2 spacer@Au nanoaggregate). In this architecture, Ag nanoparticle not only is utilized as an efficient fluorescence enhancer to overcome the common fluorescence quenching around Au nanoaggregates but also behaves like a mirror. Thus, incident light that passes through the SERS-active Au nanoaggregate and the intervening dielectric layer of SiO2 could be reflected multiply from the surface of Ag nanoparticle and coupled with the light at the nanogap between the Au nanoaggregates to further amplify Raman intensity. This results in enhancement factors for fluorescence and SERS ~1.6-fold and more than 300-fold higher than the control samples without silver core under identical experimental conditions, respectively. Moreover, fluorophore and SERS reporters are assembled onto different layers of the concentric hybrid microsphere, resulting in a feasible fabrication protocol when a large number of agents need to be involved into the dual-mode nanobarcodes. A proof-of-concept chip-based DNA sandwich hybridization assay using genetically modified organisms as a model system has been investigated based on the concentric hybrid microsphere. The high specificity and sensitivity of the assays suggest that the new architecture has a potential for various bioanalytical applications and provides opportunities for other similar metal nanoparticles to realize coenhancement effect.

Surface Enhanced Raman Scattering (SERS) Studies of Gold and Silver Nanoparticles Prepared by Laser Ablation

Gold and silver nanoparticles (NPs) were prepared in water, acetonitrile and isopropanol by laser ablation methodologies. The average characteristic (longer) size of the NPs obtained ranged from 3 to 70 nm. 4-Aminobenzebethiol (4-ABT) was chosen as the surface enhanced Raman scattering (SERS) probe molecule to determine the optimum irradiation time and the pH of aqueous synthesis of the laser ablation-based synthesis of metallic NPs. The synthesized NPs were used to evaluate their capacity as substrates for developing more analytical applications based on SERS measurements. A highly energetic material, TNT, was used as the target compound in the SERS experiments. The Raman spectra were measured with a Raman microspectrometer. The results demonstrate that gold and silver NP substrates fabricated by the methods developed show promising results for SERS-based studies and could lead to the development of micro sensors.

A novel application of plasmonics: plasmon-driven surface-catalyzed reactions

The first experimental and theoretical evidence of the surface-catalyzed reaction of p,p'-dimercaptoazobenzene (DMAB) produced from para-aminothiophenol (PATP) by local surface plasmons was reported in 2010, and since that time a series of investigations have supported these findings using different experimental and theoretical methods. Recent work has also found that local plasmons can drive a surface-catalyzed reaction of DMAB converted from 4-nitrobenzenethiol (4NBT), assisted by local surface plasmons. There are at least three important discoveries in these investigations: 1) in the field of surface-enhanced Raman scattering (SERS) the widely accepted misinterpretation (since 1994) that the chemical mechanism resulting in three additional Raman peaks of PATP in Ag or Au solutions has been corrected with a new mechanism; 2) it is confirmed that SERS is not always a noninvasive technique, and under certain conditions cannot always obtain the vibrational fingerprint information of the original surface species; 3) a novel method to synthesize new molecules, induced by local surface plasmons or plasmon waveguides on the nanoscale, has been found. This Review considers recent novel applications of plasmonics to chemical reactions, especially to plasmon-driven surface-catalyzed reactions.

Surface-enhanced Raman scattering of 4,4'-dimercaptoazobenzene trapped in Au nanogaps

The surface-enhanced Raman scattering (SERS) of 4,4'-dimercaptoazobenzene (4,4'-DMAB), an alpha, omega-dithiol possessing also an azo moiety, has seen a surge of interest recently, since 4,4'-DMAB might be able to form from 4-aminobenzenethiol (4-ABT) via a surface-induced photoreaction. An understanding of the intrinsic SERS characteristics of 4,4'-DMAB is thus very important to evaluate the possibility of such a photoreaction. We found in this work that 4,4'-DMAB should adsorb on a flame-annealed Au substrate via one of its two thiol groups such that Au nanoparticles could adsorb further on the pendent thiol group, forming a SERS hot site. The most distinctive feature in the SERS of 4,4'-DMAB was the appearance of a(g) bands, which were quite similar to the b(2)-type bands occurring in the SERS of 4-ABT. In an electrochemical environment, the a(g) bands of 4,4'-DMAB at 1431, 1387, and 1138 cm(-1) became weakened at lower potentials, completely disappearing at -1.0 V, but the bands were restored upon increasing the electrode potential, implying that neither electro- nor photo-chemical reaction to break the azo group took place, in agreement with data from a cyclic voltammogram. The appearance and disappearance of these a(g) bands are thus concluded to be associated with the charge transfer phenomenon: 4,4'-DMAB must then be one of a unique group of compounds exhibiting chemical enhancement when subjected to a SERS environment.

Gold nano-popcorn attached SWCNT hybrid nanomaterial for targeted diagnosis and photothermal therapy of human breast cancer cells

Breast cancer presents greatest challenge in health care in today's world. The key to ultimately successful treatment of breast cancer disease is an early and accurate diagnosis. Current breast cancer treatments are often associated with severe side effects. Driven by the need, we report the design of novel hybrid nanomaterial using gold nano popcorn-attached single wall carbon nanotube for targeted diagnosis and selective photothermal treatment. Targeted SK-BR-3 human breast cancer cell sensing have been performed in 10 cancer cells/mL level, using surface enhanced Raman scattering of single walls carbon nanotube's D and G bands. Our data show that S6 aptamer attached hybrid nanomaterial based SERS assay is highly sensitive to targeted human breast cancer SK-BR-3 cell line and it will be able to distinguish it from other non targeted MDA-MB breast cancer cell line and HaCaT normal skin cell line. Our results also show that 10 min of photothermal therapy treatment by 1.5 W/cm(2) power, 785 nm laser is enough to kill cancer cells very effectively using S6 aptamer attached hybrid nanomaterials. Possible mechanisms for targeted sensing and operating principle for highly efficient photothermal therapy have been discussed. Our experimental results reported here open up a new possibility for using aptamers modified hybrid nanomaterial for reliable diagnosis and targeted therapy of cancer cell lines quickly.

A SERS DNAzyme biosensor for lead ion detection

SERS biosensor for sensitive and selective detection of lead ions (Pb(2+)) based on DNAzyme was developed by taking advantage of the specific catalytic reaction of DNAzyme upon binding to Pb(2+) ions. Detection was accomplished by SERS nanoprobe labeled with DNA and Raman reporters for signal amplification.

Nanocomposites of size-controlled gold nanoparticles and graphene oxide: formation and applications in SERS and catalysis

In this paper, we describe the formation of Au nanoparticle-graphene oxide (Au-GO) and -reduced GO (Au-rGO) composites by noncovalent attachment of Au nanoparticles premodified with 2-mercaptopyridine to GO and rGO sheets, respectively, viaπ-π stacking and other molecular interactions. Compared with in situ reduction of HAuCl4 on the surface of graphene sheets that are widely used to prepare Au-GO composites, the approach developed by us offers well controlled size, size distribution, and morphology of the metal nanoparticles in the metal-GO nanohybrids. Moreover, we investigated surface enhanced Raman scattering (SERS) and catalysis properties of the Au-graphene composites. We have demonstrated that the Au-GO composites are superior SERS substrates to the Au NPs. Similarly, a comparative study on the catalytic activities of the Au, Au-GO, and Au-rGO composites in the reduction of o-nitroaniline to 1,2-benzenediamine by NaBH4 indicates that both Au-GO and Au-rGO composites exhibit significantly higher catalytic activities than the corresponding Au nanoparticles.

Monodisperse, micrometer-scale, highly crystalline, nanotextured Ag dendrites: rapid, large-scale, wet-chemical synthesis and their application as SERS substrates

In this letter, we report on our interesting finding that the direct mixing of aqueous AgNO(3) and NH(2)OH solutions at room temperature leads to rapid, high-yield production of monodisperse, micrometer-scale, highly crystalline, nanotextured Ag dendrites. The surface-enhanced Raman scattering (SERS) effect of these Ag dendrites was evaluated by using 4-aminothiophenol (p-ATP) as the Raman probe and the results demonstrate that they exhibit strong SERS effects.

Au nanoparticle monolayers: preparation, structural conversion and their surface-enhanced Raman scattering effects

An environment-friendly method is developed to fabricate close-packed Au nanoparticle (AuNP) monolayers with sub-10 nm interparticle spacing simply by covering n-butanol on the surface of an Au aqueous colloid. The close-packed nanostructure can further transform into two-dimensional (2D) aggregates with different aggregation degrees upon aging for several days. This structural evolution process was disclosed by transition electron microscopy (TEM) and UV-vis spectroscopy and its influence on the ensemble optical properties was further demonstrated by surface-enhanced Raman scattering (SERS). It was revealed that creating sub-10 nm interparticle spacing and particle dimers are highly desirable for engendering strong SERS activity under a 632.8 nm excitation. Further aging the film leads to the formation of larger aggregates, which moves the surface plasmon resonance of the aggregates gradually 'off-resonance' from the 632.8 nm excitation line and costs some numbers of sub-10 nm interparticle spacings. The two parameters together decrease the SERS activity of the close-packed AuNP monolayers. The present strategy thus provides an easy way to finely tune the SERS properties of thin nanoparticle films and other ensemble properties, which can easily be realized by creating sub-10 interparticle spacing, controlling the particle aggregation degree and by adopting suitable particle sizes and shapes.