Generation of hot spots with silver nanocubes for single-molecule detection by surface-enhanced Raman scattering

Highly sensitive surface-enhanced Raman scattering based on multi-dimensional plasmonic coupling in Au-graphene-Ag hybrids

We report an efficient surface-enhanced Raman scattering (SERS) substrate by utilizing the multi-dimensional plasmonic coupling in Au nanoparticle (NP)-graphene-Ag NP hybrid structures. An ultrasensitive SERS detection with a limit of down to 10(-13) M has been achieved when the sandwiched hybrid film is fabricated on an Ag substrate.

Structural and optical properties of self-assembled chains of plasmonic nanocubes

Solution-based linear self-assembly of metal nanoparticles offers a powerful strategy for creating plasmonic polymers, which, so far, have been formed from spherical nanoparticles and cylindrical nanorods. Here we report linear solution-based self-assembly of metal nanocubes (NCs), examine the structural characteristics of the NC chains, and demonstrate their advanced optical characteristics. In comparison with chains of nanospheres with similar dimensions, composition, and surface chemistry, predominant face-to-face assembly of large NCs coated with short polymer ligands led to a larger volume of hot spots in the chains, a nearly uniform E-field enhancement in the gaps between colinear NCs, and a new coupling mode for NC chains due to the formation of a Fabry-Perot resonator structure formed by face-to-face bonded NCs. The NC chains exhibited stronger surface-enhanced Raman scattering in comparison with linear assemblies of nanospheres. The experimental results were in agreement with finite difference time domain simulations.

Strong light-matter interactions in sub-nanometer gaps defined by monolayer graphene: toward highly sensitive SERS substrates

The interactions between visible light and sub-nanometer gaps were investigated by sandwiching graphene between two layers of vertically stacked Au nanoparticles. The optical properties of such a hybrid film have been effectively tuned by embedding a monolayer graphene, enabling a suppressed transmission of ∼16% accompanied by a red-shift of the resonant wavelength. Finite element simulations have shown that the strong coupling between two layers of plasmonic Au nanoparticles leads to an electric field enhancement of up to 88 times in graphene defined vertical gaps, in contrast to that of 14 times in the horizontal gaps between Au nanoparticles formed in the fabrication process. In addition, the size of gaps and thus the field enhancement can be readily tuned by the number of graphene layers sandwiched between Au nanoparticles. When being used as surface-enhanced Raman scattering (SERS) substrates, the Au nanoparticle/graphene/Au nanoparticle structures have demonstrated high Raman enhancement factors of up to 1.6 × 10(8) for RhB and 2.5 × 10(8) for R6G, and a detection limit of as low as 0.1 nM for Sudan III and methylene blue molecules.

Alkyne-functionalized superstable graphitic silver nanoparticles for Raman imaging

Noble metals, especially gold, have been widely used in plasmon resonance applications. Although silver has a larger optical cross section and lower cost than gold, it has attracted much less attention because of its easy corrosion, thereby degrading plasmonic signals and limiting its applications. To circumvent this problem, we report the facile synthesis of superstable AgCu@graphene (ACG) nanoparticles (NPs). The growth of several layers of graphene onto the surface of AgCu alloy NPs effectively protects the Ag surface from contamination, even in the presence of hydrogen peroxide, hydrogen sulfide, and nitric acid. The ACG NPs have been utilized to enhance the unique Raman signals from the graphitic shell, making ACG an ideal candidate for cell labeling, rapid Raman imaging, and SERS detection. ACG is further functionalized with alkyne-polyethylene glycol, which has strong Raman vibrations in the Raman-silent region of the cell, leading to more accurate colocalization inside cells. In sum, this work provides a simple approach to fabricate corrosion-resistant, water-soluble, and graphene-protected AgCu NPs having a strong surface plasmon resonance effect suitable for sensing and imaging.

Universal surface-enhanced Raman tags: individual nanorods for measurements from the visible to the infrared (514-1064 nm)

Surface-enhanced Raman scattering (SERS) is a promising imaging modality for use in a variety of multiplexed tracking and sensing applications in biological environments. However, the uniform production of SERS nanoparticle tags with high yield and brightness still remains a significant challenge. Here, we describe an approach based on the controlled coadsorption of multiple dye species onto gold nanorods to create tags that can be detected across a much wider range of excitation wavelengths (514-1064 nm) compared to conventional approaches that typically focus on a single wavelength. This was achieved without the added complexity of nanoparticle aggregation or growing surrounding metallic shells to further enhance the surface-enhanced resonance Raman scattering (SERRS) signal. Correlated Raman and scanning electron microscopy mapping measurements of individual tags were used to clearly demonstrate that strong and reproducible SERRS signals at high particle yields (>92%) were readily achievable. The polyelectrolyte-wrapped nanorod-dye conjugates were also found to be highly stable as well as noncytotoxic. To demonstrate the use of these universal tags for the multimodal optical imaging of biological specimens, confocal Raman and fluorescence maps of stained immune cells following nanoparticle uptake were acquired at several excitation wavelengths and compared with dark-field images. The ability to colocalize and track individual optically encoded nanoparticles across a wide range of wavelengths simultaneously will enable the use of SERS alongside other imaging techniques for the real-time monitoring of cell-nanoparticle interactions.

Plasmon Mapping in Au@Ag Nanocube Assemblies

Surface plasmon modes in metallic nanostructures largely determine their optoelectronic properties. Such plasmon modes can be manipulated by changing the morphology of the nanoparticles or by bringing plasmonic nanoparticle building blocks close to each other within organized assemblies. We report the EELS mapping of such plasmon modes in pure Ag nanocubes, Au@Ag core-shell nanocubes, and arrays of Au@Ag nanocubes. We show that these arrays enable the creation of interesting plasmonic structures starting from elementary building blocks. Special attention will be dedicated to the plasmon modes in a triangular array formed by three nanocubes. Because of hybridization, a combination of such nanotriangles is shown to provide an antenna effect, resulting in strong electrical field enhancement at the narrow gap between the nanotriangles.

Morphology and composition controlled synthesis of flower-like silver nanostructures

Flower-like silver nanostructures with controlled morphology and composition were prepared through wet-chemical synthesis. The reaction rate is simply manipulated by the amount of catalyzing agent ammonia added which is the key point to determine the ratio of hexagonal close-packed (HCP) to face-centered cubic (FCC) phase in silver nanostructures. The existence of formic acid that is the oxidation product of aldehyde group is demonstrated to play a crucial role in achieving the metastable HCP crystal structures by replacing ionic surfactants with polyvinylpyrrolidone (PVP). Utilizing flower-like silver nanostructures as surface-enhanced Raman scattering (SERS) substrates, Raman signal of Rhodamine 6G, or 4-aminothiophenol with concentration as low as 10(-7) M was detected. Moreover, it is demonstrated that phase composition has no direct relation to the SERS enhancing factor which is mainly determined by the amount of hot spots.

Galvanic replacement-free deposition of Au on Ag for core-shell nanocubes with enhanced chemical stability and SERS activity

We report a robust synthesis of Ag@Au core-shell nanocubes by directly depositing Au atoms on the surfaces of Ag nanocubes as conformal, ultrathin shells. Our success relies on the introduction of a strong reducing agent to compete with and thereby block the galvanic replacement between Ag and HAuCl4. An ultrathin Au shell of 0.6 nm thick was able to protect the Ag in the core in an oxidative environment. Significantly, the core-shell nanocubes exhibited surface plasmonic properties essentially identical to those of the original Ag nanocubes, while the SERS activity showed a 5.4-fold further enhancement owing to an improvement in chemical enhancement. The combination of excellent SERS activity and chemical stability may enable a variety of new applications.

Mesoporous nano/micro noble metal particles: synthesis and applications

The morphology, size and composition often govern the physical and chemical properties of noble metal units with a size in the nano or micro scale. Thus, the controlled growth of noble metal crystals would help to tailor their unique properties and this would be followed by their practical application. Mesoporous nano/micro noble metal units are types of nanostructured material that have fascinating properties that can generate great potential for various applications. This review presents a general view on the growth mechanisms of porous noble metal units and is focused on recent progresses in their synthetic approaches. Then, their potential applications in the field of drug delivery, cell imaging and SERS substrates, as well as fuel cell catalysts are overviewed.

Multipole plasmon resonances in self-assembled metal hollow-nanospheres

Recently, multipole plasmonic mode resonances in metal hollow structures, such as dipole, quadrupole, and octupole modes, have been widely investigated by researchers with the aim for potential applications in bio-sensing, fluorescence, nanolasers or nonlinear nano-photonics. Here, in this work, the multipole plasmon resonances in self-assembled metal hollow-nanospheres (HNSs) are theoretically and experimentally demonstrated and the hot spots originating from the higher order mode plasmonic resonance and interparticle coupling effect are proposed to be used for Raman scattering enhancements. Dipole, quadrupole, octupole and hexadecapole mode plasmonic resonances were clearly resolved in the extinction spectra of these Ag HNS arrays showing good agreement with the theoretical simulation results. Strong regular hot spots were obtained around the surface and in the gaps of the Ag HNSs through the higher order mode plasmonic resonances and corresponding interparticle coupling effect between the HNSs. Maximum local field intensity was accomplished by optimizing the size of as well as the coupling distance between the HNSs and then it was applied to SERS sensing. Raman mapping also demonstrated these self-assembled plasmonic cavity arrays to be a stable and uniform SERS-active substrate.

Transformation of Ag nanocubes into Ag-Au hollow nanostructures with enriched Ag contents to improve SERS activity and chemical stability

We report a strategy to complement the galvanic replacement reaction between Ag nanocubes and HAuCl4 with co-reduction by ascorbic acid (AA) for the formation of Ag-Au hollow nanostructures with greatly enhanced SERS activity. Specifically, in the early stage of synthesis, the Ag nanocubes are sharpened at corners and edges because of the selective deposition of Au and Ag atoms at these sites. In the following steps, the pure Ag in the nanocubes is constantly converted into Ag(+) ions to generate voids owing to the galvanic reaction with HAuCl4, but these released Ag(+) ions are immediately reduced back to Ag atoms and are co-deposited with Au atoms onto the nanocube templates. We observe distinctive SERS properties for the Ag-Au hollow nanostructures at visible and near-infrared excitation wavelengths. When plasmon damping is eliminated by using an excitation wavelength of 785 nm, the SERS activity of the Ag-Au hollow nanostructures is 15- and 33-fold stronger than those of the original Ag nanocubes and the Ag-Au nanocages prepared by galvanic replacement without co-reduction, respectively. Additionally, Ag-Au hollow nanostructures embrace considerably improved stability in an oxidizing environment such as aqueous H2O2 solution. Collectively, our work suggests that the Ag-Au hollow nanostructures will find applications in SERS detection and imaging.

The effect of dielectric constants on noble metal/semiconductor SERS enhancement: FDTD simulation and experiment validation of Ag/Ge and Ag/Si substrates

The finite-difference time-domain (FDTD) method was employed to simulate the electric field distribution for noble metal (Au or Ag)/semiconductor (Ge or Si) substrates. The simulation showed that noble metal/Ge had stronger SERS enhancement than noble metal/Si, which was mainly attributed to the different dielectric constants of semiconductors. In order to verify the simulation, Ag nanoparticles with the diameter of ca. 40 nm were grown on Ge or Si wafer (Ag/Ge or Ag/Si) and employed as surface-enhanced Raman scattering substrates to detect analytes in solution. The experiment demonstrated that both the two substrates exhibited excellent performance in the low concentration detection of Rhodamine 6G. Besides, the enhancement factor (1.3 × 10⁹) and relative standard deviation values (less than 11%) of Ag/Ge substrate were both better than those of Ag/Si (2.9 × 10⁷ and less than 15%, respectively), which was consistent with the FDTD simulation. Moreover, Ag nanoparticles were grown in-situ on Ge substrate, which kept the nanoparticles from aggregation in the detection. To data, Ag/Ge substrates showed the best performance for their sensitivity and uniformity among the noble metal/semiconductor ones.

Probing the effect of charge transfer enhancement in off resonance mode SERS via conjugation of the probe dye between silver nanoparticles and metal substrates

The charge transfer-mediated surface enhanced Raman scattering (SERS) of crystal violet (CV) molecules that were chemically conjugated between partially polarized silver nanoparticles and optically smooth gold and silver substrates has been studied under off-resonant conditions. Tyrosine molecules were used as a reducing agent to convert silver ions into silver nanoparticles where oxidised tyrosine caps the silver nanoparticle surface with its semiquinone group. This binding through the quinone group facilitates charge transfer and results in partially oxidised silver. This establishes a chemical link between the silver nanoparticles and the CV molecules, where the positively charged central carbon of CV molecules can bind to the terminal carboxylate anion of the oxidised tyrosine molecules. After drop casting Ag nanoparticles bound with CV molecules it was found that the free terminal amine groups tend to bind with the underlying substrates. Significantly, only those CV molecules that were chemically conjugated between the partially polarised silver nanoparticles and the underlying gold or silver substrates were found to show SERS under off-resonant conditions. The importance of partial charge transfer at the nanoparticle/capping agent interface and the resultant conjugation of CV molecules to off resonant SERS effects was confirmed by using gold nanoparticles prepared in a similar manner. In this case the capping agent binds to the nanoparticle through the amine group which does not facilitate charge transfer from the gold nanoparticle and under these conditions SERS enhancement in the sandwich configuration was not observed.

Superhydrophobic Surface Enhanced Raman Scattering Sensing using Janus Particle Arrays Realized by Site-Specific Electrochemical Growth

Site-specific electrochemical deposition is used to prepare polystyrene (PS)-Ag Janus particle arrays with superhydrophobic properties. The analyte molecules can be significantly enriched using the superhydrophobic property of the PS-Ag Janus particle array before SERS detections, enabling an extremely sensitive detection of molecules in a highly diluted solution (e.g., femtomolar level). This superhydrophobic surface enhanced Raman scattering sensing concept described here is of critical significance in biosensing and bioanalysis. Most importantly, the site-specific electrochemical growth method we developed here is a versatile approach that can be used to prepare Janus particle arrays with different properties for various applications.

Silica-coated dimers of silver nanospheres as surface-enhanced Raman scattering tags for imaging cancer cells

Surface-enhanced Raman scattering (SERS) tags have been actively explored as a multiplexing platform for sensitive detection of biomolecules. Here, we report a new type of SERS tags that was fabricated by sequentially functionalizing dimers made of 50 nm Ag nanospheres with 4-mercaptobenzoic acid as the Raman reporter molecule, silica coating as a protective shell and antibody as a targeting ligand. These dimer-based tags give highly enhanced and reproducible Raman signals owing to the presence of a well-defined SERS hot spot at the junction between two Ag nanospheres in the dimer. The SERS enhancement factor (EF) of an individual dimer tag supported on a glass slide can reach a level as high as 4.3 × 10(6). In comparison, the EFs dropped to 2.8 × 10(5) and 8.7 × 10(5), respectively, when Ag nanospheres and nanocubes with sizes similar to the spheres in the dimer were used to fabricate the tags using similar procedures. The SERS signals from aqueous suspensions of the dimer-based tags also showed high intensity and good stability. Potential use of the dimer-based tags was demonstrated by imaging cancer cells overexpressing HER2 receptors with good specificity and high sensitivity.

Plasmonic cavities derived from silver nanoparticles atop a massed silver surface for surface enhancement Raman scattering

Various plasmonic cavities (PC) are formed by positioning silver nanocubes or nanospheres on a massed silver surface, being magnificently useful for surface enhancement Raman scattering (SERS) application.

Scalable preparation of ultrathin silica-coated Ag nanoparticles for SERS application

Silica-coated Ag nanoparticles (Ag@SiO2 NPs) have been successfully prepared by a scalable flame spray pyrolysis (FSP) technique with production rate up to 4 g/h in laboratory-scale. The ultrathin SiO2 shell, with a thickness 1 nm, not only effectively avoids the intersintering of Ag nanoparticles core at the high temperature, but also serves as a protective layer of the SERS-active nanostructure. The silica-coated Ag nanoparticles form agglomerates in the large temperature gradient zone, which with several nanometers gaps from each other but not contact. Such an intriguing feature can result in more Raman hot-spots generated at the gaps among Ag core active sites, which will beneficial for the whole SERS substrate enhancement. The results demonstrate that a maximum enhancement factor can reach ~10(5) with a detectable concentration as low as 10(-10) M for rhodamine 6G (R6G) molecules, indicating that the as-obtained unique nanostructure will be a promising candidate for SERS applications.

Silver nanocube on gold microplate as a well-defined and highly active substrate for SERS detection

Strong enhancement and good reproducibility in Raman signals are two major requirements for a surface-enhanced Raman scattering (SERS) substrate to be used for sensitive detection of an analyte. Here we report a new type of SERS substrate that was fabricated by depositing a Ag nanocube (AgNC) on the surface of a Au microplate (AuMP). Owing to the strong and reproducible hot spots formed at corner sites of the AgNC in proximity with the AuMP surface, the new substrate showed high sensitivity and reproducibility. Using 1,4-benzenedithiol as a probe, the SERS enhancement factor of a typical "AgNC on AuMP" substrate could reach a level as high as 4.7×10⁷. In addition to the high sensitivity and reproducibility, the "AgNC on AuMP" substrate also displayed very good stability. Potential use of the "AgNC on AuMP" substrate was demonstrated by detecting crystal violet with high sensitivity.

Single molecule SERS and detection of biomolecules with a single gold nanoparticle on a mirror junction

Using the bianalyte method we unambiguously demonstrate that a single gold nanosphere on a metal film junction, in the so-called nanoparticle on a mirror configuration, is capable of single molecule detection with surface-enhanced Raman spectroscopy (SERS). Also this configuration serves as a convenient and highly sensitive SERS sensor for detection of biomolecules. Such simple nano-junction based systems are ideal for chemical and biomedical analysis.

Simple synthesis of monodisperse, quasi-spherical, citrate-stabilized silver nanocrystals in water

Monodisperse, quasi-spherical silver nanocrystals (Ag NCs) have been produced directly in water via adding the aqueous solution of a mixture of AgNO3, sodium citrate, and KI into the boiling aqueous solutions of ascorbic acid (AA). The AA is used to significantly accelerate reduction of AgNO3 in order to promote a very fast nucleation, and the KI is used to tailor the growth of the Ag NCs into a quasi-spherical shape via its preferential adsorption on the NC {111} facets. The major role of citrate is to stabilize the newly formed NCs, whereas it has a minor contribution to reduction of AgNO3. The synergy of the effects of AA, citrate, and KI can significantly narrow the size distributions of the Ag NCs obtained so and transform the NC shapes to be truly quasi-spherical.

Improving correlated SERS measurements with scanning electron microscopy: an assessment of the problem arising from the deposition of amorphous carbon

For surface-enhanced Raman scattering (SERS) substrates with nonspherical symmetry, it is critical to correlate spectroscopy measurements with imaging by scanning electron microscopy (SEM). However, the deposition of carbon resulting from e-beam exposure during SEM imaging contaminates the surface of nanoparticles, potentially preventing their further functionalization with Raman probe molecules. In addition, the deposited carbon leads to unwanted background SERS signals. In this study, we systematically investigated the deposition of carbon during SEM imaging and examined how it affects the functionalization of nanoparticles with probe molecules and impacts the detection of SERS signals. Significantly, we found that the carbon could be removed or replaced from the surface of Ag nanoparticles through chemical or physical means, rendering the nanoparticles the capability for correlated SEM/SERS studies.

Molecularly-mediated assemblies of plasmonic nanoparticles for Surface-Enhanced Raman Spectroscopy applications

In recent years, Surface-Enhanced Raman Spectroscopy (SERS) has experienced a tremendous increase of attention in the scientific community, expanding to a continuously wider range of diverse applications in nanoscience, which can mostly be attributed to significant improvements in nanofabrication techniques that paved the way for the controlled design of reliable and effective SERS nanostructures. In particular, the plasmon coupling properties of interacting nanoparticles are extremely intriguing due to the concentration of enormous electromagnetic enhancements at the interparticle gaps. Recently, great efforts have been devoted to develop new nanoparticle assembly strategies in suspension with improved control over hot-spot architecture and cluster structure, laying the foundation for the full exploitation of their exceptional potential as SERS materials in a wealth of chemical and biological sensing. In this review we summarize in an exhaustive and systematic way the state-of-art of plasmonic nanoparticle assembly in suspension specifically developed for SERS applications in the last 5 years, focusing in particular on those strategies which exploited molecular linkers to engineer interparticle gaps in a controlled manner. Importantly, the novel advances in this rather new field of nanoscience are organized into a coherent overview aimed to rationally describe the different strategies and improvements in the exploitation of colloidal nanoparticle assembly for SERS application to real problems.

Surface-enhanced Raman spectroscopy at single-molecule scale and its implications in biology

Single-molecule (SM) spectroscopy has been an exciting area of research offering significant promise and hope in the field of sensor development to detect targets at ultra-low levels down to SM resolution. To the experts and developers in the field of surface-enhanced Raman spectroscopy (SERS), this has often been a challenge and a significant opportunity for exploration. Needless to say, the opportunities and excitement of this multidisciplinary area impacts span the fields of physics, chemistry and engineering, along with a significant thrust in applications constituting areas in medicine, biology, environment and agriculture among others. In this review, we will attempt to provide a quick snapshot of the basics of SM-SERS, nanostructures and devices that can enable SM Raman measurement. We will conclude with a discussion on SERS implications in biomedical sciences.

Fabricating a reversible and regenerable Raman-active substrate with a biomolecule-controlled DNA nanomachine

A DNA configuration switch is designed to fabricate a reversible and regenerable Raman-active substrate. The substrate is composed of a Au film and a hairpin-shaped DNA strand (hot-spot-generation probes, HSGPs) labeled with dye-functionalized silver nanoparticles (AgNPs). Another ssDNA that recognizes a specific trigger is used as an antenna. The HSGPs are immobilized on the Au film to draw the dye-functionalized AgNPs close to the Au surface and create an intense electromagnetic field. Hybridization of HSGP with the two arm segments of the antenna forms a triplex-stem structure to separate the dye-functionalized AgNPs from the Au surface, quenching the Raman signal. Interaction with its trigger releases the antenna from the triplex-stem structure, and the hairpin structure of the HSGP is restored, creating an effective "off-on" Raman signal switch. Nucleic acid sequences associated with the HIV-1 U5 long terminal repeat sequences and ATP are used as the triggers. The substrate shows excellent reversibility, reproducibility, and controllability of surface-enhanced Raman scattering (SERS) effects, which are significant requirements for practical SERS sensor applications.

Plasmonic near-electric field enhancement effects in ultrafast photoelectron emission: correlated spatial and laser polarization microscopy studies of individual Ag nanocubes

Electron emission from single, supported Ag nanocubes excited with ultrafast laser pulses (λ = 800 nm) is studied via spatial and polarization correlated (i) dark field scattering microscopy (DFM), (ii) scanning photoionization microscopy (SPIM), and (iii) high-resolution transmission electron microscopy (HRTEM). Laser-induced electron emission is found to peak for laser polarization aligned with cube diagonals, suggesting the critical influence of plasmonic near-field enhancement of the incident electric field on the overall electron yield. For laser pulses with photon energy below the metal work function, coherent multiphoton photoelectron emission (MPPE) is identified as the most probable mechanism responsible for electron emission from Ag nanocubes and likely metal nanoparticles/surfaces in general.

Metal oxide nanoparticle mediated enhanced Raman scattering and its use in direct monitoring of interfacial chemical reactions

Metal oxide nanoparticles (MONPs) have widespread usage across many disciplines, but monitoring molecular processes at their surfaces in situ has not been possible. Here we demonstrate that MONPs give highly enhanced (×10(4)) Raman scattering signals from molecules at the interface permitting direct monitoring of their reactions, when placed on top of flat metallic surfaces. Experiments with different metal oxide materials and molecules indicate that the enhancement is generic and operates at the single nanoparticle level. Simulations confirm that the amplification is principally electromagnetic and is a result of optical modulation of the underlying plasmonic metallic surface by MONPs, which act as scattering antennae and couple light into the confined region sandwiched by the underlying surface. Because of additional functionalities of metal oxides as magnetic, photoelectrochemical and catalytic materials, enhanced Raman scattering mediated by MONPs opens up significant opportunities in fundamental science, allowing direct tracking and understanding of application-specific transformations at such interfaces. We show a first example by monitoring the MONP-assisted photocatalytic decomposition reaction of an organic dye by individual nanoparticles.

Individual nanostructured materials: fabrication and surface-enhanced Raman scattering

The progress of surface-enhanced Raman scattering (SERS) microscopy and spectroscopy on individual nanostructured materials has been reviewed in this feature article. After a brief introduction on individual nanomaterial SERS, we provide a systematic overview on the fabrication and SERS studies of individual nanoparticulates, nano-junctions and hierarchical nano-aggregate. These SERS-active nanomaterials have great potential in designing novel highly sensitive SERS substrates for the development of SERS-based sensing devices with a broad range of applications.

Quantitative analysis of the role played by poly(vinylpyrrolidone) in seed-mediated growth of Ag nanocrystals

This article presents a quantitative analysis of the role played by poly(vinylpyrrolidone) (PVP) in seed-mediated growth of Ag nanocrystals. Starting from Ag nanocubes encased by {100} facets as the seeds, the resultant nanocrystals could take different shapes depending on the concentration of PVP in the solution. If the concentration was above a critical value, the seeds simply grew into larger cubes still enclosed by {100} facets. When the concentration fell below a critical value, the seeds would evolve into cuboctahedrons enclosed by a mix of {100} and {111} facets and eventually octahedrons completely covered by {111} facets. We derived the coverage density of PVP on Ag(100) surface by combining the results from two measurements: (i) cubic seeds were followed to grow at a fixed initial concentration of PVP to find out when {111} facets started to appear on the surface, and (ii) cubic seeds were allowed to grow at reduced initial concentrations of PVP to see at which concentration {111} facets started to appear from the very beginning. We could calculate the coverage density of PVP from the differences in PVP concentration and the total surface area of Ag nanocubes between these two samples. The coverage density was found to be 140 and 30 repeating units per nm(2) for PVP of 55,000 and 10,000 g/mol in molecular weight, respectively, for cubic seeds of 40 nm in edge length. These values dropped slightly to 100 and 20 repeating units per nm(2), respectively, when 100 nm Ag cubes were used as the seeds.