Single Nanoparticle SERS Probes of Ion Intercalation in Metal-Oxide Electrodes

Probing ion-intercalating processes in electrodes is hugely important for batteries, supercapacitors, and photovoltaic devices. In this work we use single-nanoparticle (NP) probes to see real-time molecular changes correlated to electrochemically modulated ion-intercalation in metal-oxide electrodes. Using surface-enhanced Raman spectroscopy (SERS) transduced by single NP probes, we observe that the Raman frequencies and spectral intensities of the adsorbed molecules vary on cycling the electrochemical potential on a vanadium-oxide electrode. The potential-dependent frequency shifts in SERS from an electrochemically inert molecule are attributed to a Stark effect induced by chemical and structural changes as a result of ion-intercalation processes in vanadium oxide. Our study opens up a unique strategy to explore adsorbates and molecular reaction pathways on ion-intercalating materials and semiconducting interfaces

SERS-Melting: A New Method for Discriminating Mutations in DNA Sequences

The reliable discrimination of mutations, single nucleotide polymorphisms (SNPs), and other differences in genomic sequence is an essential part of DNA diagnostics and forensics. It is commonly achieved using fluorescently labeled DNA probes and thermal gradients to distinguish between the matched and mismatched DNA. Here, we describe a novel method that uses surface enhanced (resonance) Raman spectroscopy (SER(R)S) to follow denaturation of dsDNA attached to a structured gold surface. This denaturation is driven either electrochemically or thermally on SERS active sphere segment void (SSV) gold substrates. Using this method, we can distinguish between wild type, a single point mutation (1653C/T), and a triple deletion (ΔF 508) in the CFTR gene at the 0.02 attomole level, and the method can be used to differentiate the unpurified PCR products of the wild type and ΔF 508 mutation. Our method has the potential to provide small, rapid, sensitive, reproducible platforms for detecting genetic variations and sequencing genes

Surface Enhanced Coherent Anti-Stokes Raman Scattering on Nanostructured Gold Surfaces

Coherent anti-Stokes Raman spectroscopy (CARS) is a well-known tool in multiphoton imaging and nonlinear spectroscopy. In this work we combine CARS with plasmonic surface enhancement on reproducible nanostructured surfaces. We demonstrate strong correlation between plasmon resonances and surface-enhanced CARS (SECARS) intensities on our nanostructured surfaces and show that an enhancement of ∼105 can be obtained over standard CARS. Furthermore, we find SECARS to be >103 times more sensitive than surface-enhanced Raman Spectroscopy (SERS). We also demonstrate SECARS imaging of molecular monolayers. Our work paves the way for reliable single molecule Raman spectroscopy and fast molecular imaging on plasmonic surfaces

Tunable Microstructured Surface-Enhanced Raman Scattering Substrates via Electrohydrodynamic Lithography

Readily fine-tuned structures are an important requirement for the optimization of surface-enhanced Raman scattering (SERS) to obtain the highest enhancements. Here, a lateral modulation of an electric field applied to a dielectric interface enables the rapid replication of nearly any topographic morphology with micrometer resolution by electrohydrodynamic lithography (EHL). Gold-covered periodic EHL-generated arrays yielded the reproducible enhancement of adsorbed SERS-active molecules. Periodic arrays of micropillars with square and circular cross sections give rise to the effective coupling of surface plasmon modes, which generate enhanced SERS signals. The overall enhancement factors depend on the geometry of the gold-coated structures, and intriguingly, a strong correlation is found with the gap-to-width ratio of the square pillar morphology. A numerical simulation of the EHL-based SERS substrates is consistent with this dependence. The EHL surface architectures can be easily tailored at micrometer-to-submicrometer dimensions, allowing the fabrication of reliably engineered and cost-effective highly sensitive SERS substrates

Near-Field Plasmonics of an Individual Dielectric Nanoparticle above a Metallic Substrate

We simulate and discuss the local electric-field enhancement in a system of a dielectric nanoparticle placed very near to a metallic substrate. We use finite-element numerical simulations in order to understand the field-enhancement mechanism in this dielectric NP-on-mirror system. Under appropriate excitation conditions, the gap between the particle and the substrate becomes a “hot spot”, i.e., a region of intense electromagnetic field. We also show how the optical properties of the dielectric NP placed on a metallic substrate affect the plasmonic field enhancement in the nanogap and characterize the confinement in the gap. Our study helps to understand and design systems with dielectric NPs on metallic substrates which can be equally as effective for SERS, fluorescence, and nonlinear phenomena as conventional all-metal plasmonic structures

Immobilization of Antibodies on Polyaniline Films and Its Application in a Piezoelectric Immunosensor

Conducting polymers, especially polyaniline (PAni), have been extensively used in biosensor applications. A protocol for covalent immobilization of human IgG on polyaniline using glutaraldehyde as the cross-linker is described in this report and utilized in development of a piezoelectric immunosensor. Here, PAni was used as the substrate for immobilization. The electropolymerization parameters were optimized to get suitable thickness and surface morphology of the PAni for obtaining high density and uniformity of immobilized antibodies on the surface of our films. Possible reaction between PAni thin films and glutaraldehyde was explored using FT-IR characterization in grazing angle mode and XPS. The protocol has been characterized with the help of quartz crystal microbalance analysis. An antibody surface density of 4.86 ng/mm2 was obtained. A piezoelectric biosensor developed for detection of IgG with the proposed protocol was capable of differentiating the target analyte concentrations between 500 ng/mL and 25 μg/mL with nonspecific binding of ∼10%

Reproducible Deep-UV SERRS on Aluminum Nanovoids

Surface-enhanced Raman scattering (SERS) with deep-UV excitation is of particular interest because a large variety of biomolecules such as amino acids exhibit electronic transitions in the UV spectral range and resonant excitation dramatically increases the cross section of the associated vibrational modes. Despite its potential, UV-SERS is still little-explored. We present a novel straightforward scalable route to fabricate aluminum nanovoids for reproducible SERS in the deep-UV without the need of expensive lithographic techniques. We adopt a modified template stripping method utilizing a soluble template and self-assembled polymer spheres to create nanopatterned aluminum films. We observe high surface enhancement of approximately 6 orders of magnitude, with excitation in the deep-UV (244 nm) on structures optimized for this wavelength. This work thus enables sensitive detection of organics and biomolecules, normally nonresonant at visible wavelengths, with deep-UV surface-enhanced resonant Raman scattering on reproducible and scalable substrates

Intracellular SERS Nanoprobes For Distinction Of Different Neuronal Cell Types

Distinction between closely related and morphologically similar cells is difficult by conventional methods especially without labeling. Using nuclear-targeted gold nanoparticles (AuNPs) as intracellular probes we demonstrate the ability to distinguish between progenitor and differentiated cell types in a human neuroblastoma cell line using surface-enhanced Raman spectroscopy (SERS). SERS spectra from the whole cell area as well as only the nucleus were analyzed using principal component analysis that allowed unambiguous distinction of the different cell types. SERS spectra from the nuclear region showed the developments during cellular differentiation by identifying an increase in DNA/RNA ratio and proteins transcribed. Our approach using nuclear-targeted AuNPs and SERS imaging provides label-free and noninvasive characterization that can play a vital role in identifying cell types in biomedical stem cell research

Quantitative SERS Using the Sequestration of Small Molecules Inside Precise Plasmonic Nanoconstructs

We show how the macrocyclic host, cucurbit[8]­uril (CB[8]), creates precise subnanometer junctions between gold nanoparticles while its cavity simultaneously traps small molecules; this enables their reproducible surface-enhanced Raman spectroscopy (SERS) detection. Explicit shifts in the SERS frequencies of CB[8] on complexation with guest molecules provides a direct strategy for absolute quantification of a range of molecules down to 10^–11 M levels. This provides a new analytical paradigm for quantitative SERS of small molecules

Optimized Vertical Carbon Nanotube Forests for Multiplex Surface-Enhanced Raman Scattering Detection

The highly sensitive and molecule-specific technique of surface-enhanced Raman spectroscopy (SERS) generates high signal enhancements via localized optical fields on nanoscale metallic materials, which can be tuned by manipulation of the surface roughness and architecture on the submicrometer level. We investigate gold-functionalized vertically aligned carbon nanotube forests (VACNTs) as low-cost straightforward SERS nanoplatforms. We find that their SERS enhancements depend on their diameter and density, which are systematically optimized for their performance. Modeling of the VACNT-based SERS substrates confirms consistent dependence on structural parameters as observed experimentally. The created nanostructures span over large substrate areas, are readily configurable, and yield uniform and reproducible SERS enhancement factors. Further fabricated micropatterned VACNTs platforms are shown to deliver <i>multiplexed</i> SERS detection. The unique properties of CNTs, which can be synergistically utilized in VACNT-based substrates and patterned arrays, can thus provide new generation platforms for SERS detection