Advances in localized surface plasmon resonance spectroscopy biosensing

In recent years, localized surface plasmon resonance (LSPR) spectroscopy advancements have made it a sensitive, flexible tool for probing biological interactions. Here, we describe the basic principles of this nanoparticle-based sensing technique, the ways nanoparticles can be tailored to optimize sensing, and examples of novel LSPR spectroscopy applications. These include detecting small molecules via protein conformational changes and resonance LSPR spectroscopy, as well as coupling LSPR with mass spectrometry to identify bound analytes. The last few sections highlight the advantages of single nanoparticle LSPR, in that it lowers limits of detection, allows multiplexing on the nanometer scale, and enables free diffusion of sensors in solution. The cases discussed herein illustrate creative ways that LSPR spectroscopy has been improved to achieve new sensing capabilities.

Surface-Enhanced Raman Spectroelectrochemistry of TTF-Modified Self-Assembled Monolayers

Surface-enhanced Raman spectroscopy (SERS) was used to monitor the response of a self-assembled monolayer (SAM) of a tetrathiafulvalene (TTF) derivative on a gold film-over-nanosphere electrode. The electrochemical response observed was rationalized in terms of the interactions between TTF moieties as the oxidation state was changed. Electrochemical oxidation to form the monocation caused the absorbance of the TTF unit to coincide with both the laser excitation wavelength and the localized surface plasmon resonance (LSPR), resulting in surface-enhanced resonance Raman scattering (SERRS). The vibrational frequency changes that accompany electron transfer afford a high-contrast mechanism that can be used to determine the oxidation state of the TTF unit in an unambiguous manner.

Surface-Enhanced Femtosecond Stimulated Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) and femtosecond stimulated Raman spectroscopy (FSRS) have revolutionized the Raman spectroscopy field. SERS provides spectroscopic detection of single molecules, and FSRS enables the acquisition of Raman spectra on the ultrafast time scale of molecular motion. Here, we present the first successful combination of these two techniques, demonstrating surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS) using gold nanoantennas with embedded reporter molecules. Using a picosecond Raman and femtosecond probe pulse, the time- and ensemble-averaged enhancement factor is estimated to be in the range of 10(4)-10(6). We report the line shapes, power dependence, and magnitude of the SE-FSRS signal and discuss contributions to sample degradation on the minute time scale. With these first successful proof-of-principle experiments, time-resolved SE-FSRS techniques can now be rationally attempted with the goals of investigating the dynamics of plasmonic materials as well as examining the contributions of environmental heterogeneities by probing more homogeneous molecular subsets.

Plasmon-enhanced colorimetric ELISA with single molecule sensitivity

Robust but ultrasensitive biosensors with a capability of detecting low abundance biomarkers could revolutionize clinical diagnostics and enable early detection of cancer, neurological diseases, and infections. We utilized a combination of localized surface plasmon resonance (LSPR) refractive index sensing and the well-known enzyme-linked immunosorbent assay to develop a simple colorimetric biosensing methodology with single molecule sensitivity. The technique is based on spectral imaging of a large number of isolated gold nanoparticles. Each particle binds a variable number of horseradish peroxidase (HRP) enzyme molecules that catalyze a localized precipitation reaction at the particle surface. The enzymatic reaction dramatically amplifies the shift of the LSPR scattering maximum, λ(max), and makes it possible to detect the presence of only one or a few HRP molecules per particle.

Nanosphere Lithography: Synthesis and Application of Nanoparticles with Inherently Anisotropic Structures and Surface Chemistry

Early work with size-tunable periodic particle arrays (PPAs) fabricated by nanospherelithography (NSL) demonstrated that the localized surface plasmon resonance (LSPR) could be tuned throughout the visible region of the spectrum. The LSPR is sensitive to changes in nanoparticle aspect ratio and local dielectric environment. This property has recently been exploited to develop a novel method of measuring surface-enhanced Raman scattering (SERS) excitation profiles. Single layer PPAs consist of size-tunable anisotropic nanoparticles that can be modified to exhibit anisotropic surface chemistry. This work demonstrates multiple schemes for PPA modification using self-assembled monolayers and colloid decoration. Nanoparticle anisotropy can be further exploited with the recent combination of NSL and reactive ion etching (RIE); this extends the two-dimensional PPA structure into the third dimension.

Nanosphere Lithography: Self-Assembled Photonic and Magnetic Materials

Early work with size-tunable periodic particle arrays (PPAs) fabricated by nanosphere lithography (NSL) demonstrated that the localized surface plasmon resonance (LSPR) could be tuned throughout the visible region of the spectrum. Further developments of the NSL technique have produced a myriad of nanoparticle configurations. Presented in this paper are several array types and examples of their utility in current applications. Both the sensitivity and tunability of the LSPR have been firmly established using single layer PPAs. Magnetic force microscopy (MFM) has been used to show that double layer PPAs act as single domain magnets and give strong MFM contrast. Angle-resolved NSL has produced nanogap and nano-overlap structures with manipulation resolution of one nanometer. Nanowell structures extend the original twodimensional structure into the third dimension. Exploitation of this flexible, materials-general NSL technique allows for investigation of the catalytic, electrochemical, magnetic, optical and thermodynamic properties of nanoparticles.

A conformation- and ion-sensitive plasmonic biosensor

The versatile optical and biological properties of a localized surface plasmon resonance (LSPR) sensor that responds to protein conformational changes are illustrated. The sensor detects conformational changes in a surface-bound construct of the calcium-sensitive protein calmodulin. Increases in calcium concentration induce a 0.96 nm red shift in the spectral position of the LSPR extinction maximum (λ(max)). Addition of a calcium chelating agent forces the protein to return to its original conformation and is detected as a reversal of the λ(max) shift. As opposed to previous work, this work demonstrates that these conformational changes produce a detectable shift in λ(max) even in the absence of a protein label, with a signal:noise ratio near 500. In addition, the protein conformational changes reversibly switch both the wavelength and intensity of the resonance peak, representing an example of a bimodal plasmonic component that simultaneously relays two distinct forms of optical information. This highly versatile plasmonic device acts as a biological sensor, enabling the detection of calcium ions with a biologically relevant limit of detection of 23 μM, as well as the detection of calmodulin-specific protein ligands.

Optimized Silver Film over Nanosphere Surfaces for the Biowarfare Agent Detection Based on Surface-Enhanced Raman Spectroscopy

This work presents the rapid detection of Bacillus subtilis spores, harmless simulants for Bacillus anthracis, using surface-enhanced Raman spectroscopy (SERS) on silver film over nanosphere (AgFON) substrates. Calcium dipicolinate (CaDPA), a biomarker for bacillus spores, can be extracted effectively from spores with nitric acid and successfully detected by SERS. The highly tunable nature of AgFON optical properties was exploited to establish general optimization conditions. AgFON surfaces optimized for 750-nm laser excitation have been characterized by UV-vis diffuse reflectance spectroscopy. The SERS signal from extracted CaDPA was evaluated over the spore concentration range 10-15-10-12M to determine the adsorption capacity of the AgFON surface and the limit of detection (LOD). These sensing capabilities have been successfully transitioned to an inexpensive, portable Raman spectrometer. Using the extraction method and this field-portable instrument, the anthrax infectious dose of 104spores were detected with only a 5-second collection period on a one-month-old prefabricated AgFON substrate.

LSPR Biosensor Signal Enhancement Using Nanoparticle-Antibody Conjugates

A method to amplify the wavelength shift observed from localized surface plasmon resonance (LSPR) bioassays is developed using gold nanoparticle-labeled antibodies. The technique, which involves detecting surface-bound analytes using gold nanoparticle conjugated antibodies, provides a way to enhance LSPR shifts for more sensitive detection of low-concentration analytes. Using the biotin and antibiotin binding pair as a model, we demonstrate up to a 400% amplification of the shift upon antibody binding to analyte. In addition, the antibody-nanoparticle conjugate improves the observed binding constant by 2 orders of magnitude, and the limit of detection by nearly 3 orders of magnitude. This amplification strategy provides a way to improve the sensitivity of plasmon-based bioassays, paving the way for single molecule-based detection and clinically relevant diagnostics.

Plasmon Scanned Surface-Enhanced Raman Scattering Excitation Profiles

Since the discovery of surface-enhanced Raman spectroscopy (SERS) in 1977, scientists have come to understand the enhancement mechanism, but have been unable to consistently optimize the weak signals inherent in Raman experiments. Surface-enhanced Raman signals originate from excitation of the localized surface plasmon resonance (LSPR) of a nanostructured metal surface, thus producing concentrated electromagnetic fields at the surface of the nanostructure. Design of the nanostructured metal substrate plays an important role in understanding and optimizing SERS experiments. In this research, the size-dependent optical properties accessible by nanosphere lithography (NSL) are exploited to fabricate topographically predictable SERS-active substrates with systematically varying LSPRs. Correlated microextinction and micro-Raman measurements, as well as quantitative implementation of a Raman standard, allow significant improvements over the current method used to optimize SERS experiments. The knowledge gained in the novel plasmon scanned SERS excitation profiles clearly indicates the substrate parameters necessary for experimental optimization and promotes further understanding of the SERS enhancement mechanism.

Alkanethiol Mediated Release of Surface Bound Nanoparticles Fabricated by Nanosphere Lithography

This work presents an innovative approach to produce monodisperse solution-phase triangular silver nanoparticles with well-controlled geometry. Ag nanotriangles are fabricated by nanosphere lithography (NSL), functionalized with alkanethiol molecules and then released from the substrate into solution. The resulting single isolated nanoparticles are subsequently asymmetrically functionalized with alkanedithiol molecules to form dimer pairs. The optical properties of the Ag nanoparticles have been measured using UV-Vis spectroscopy while their structural properties have been characterized using atomic force microscopy (AFM) and transmission electron microscopy (TEM). Theoretical calculations based on Mie theory and the Discrete Dipole Approximation (DDA) method have been done to interpret the optical properties of the released Ag nanoparticles.

A Highly Sensitive and Selective Surface-Enhanced Nanobiosensor

Nanosphere lithography (NSL) derived triangular Ag nanoparticles were used to create an extremely sensitive and specific optical biological and chemical nanosensor. Using simple UV-vis spectroscopy, biotinylated surface-confined Ag nanoparticles were used to detect streptavidin down to one picomolar concentrations. The system was tested for nonspecific binding interactions with bovine serum albumin and was found to display virtually no adverse results. The extremely sensitive and selective response of the Ag nanoparticle sensor indicates an exciting use for biological and chemical sensing.

Effect of Size, Shape, Composition, and Support Film on Localized Surface Plasmon Resonance Frequency: A Single Particle Approach Applied to Silver Bipyramids and Gold and Silver Nanocubes

Localized surface plasmon resonances (LSPR), collective electron oscillations in nanoparticles, are being heavily scrutinized for applications in chemical and biological sensing, as well as in prototype nanophotonic devices. This phenomenon exhibits an acute dependence on the particle’s size, shape, composition, and environment. The detailed characterization of the structure-function relationship of nanoparticles is obscured by ensemble averaging. Consequently, single-particle data must be obtained to extract useful information from polydisperse reaction mixtures. Recently, a correlated high resolution transmission electron microscopy (HRTEM) LSPR technique has been developed and applied to silver nanocubes. We report here a second generation of experiments using this correlation technique, in which statistical analysis is performed on a large number of single particles. The LSPR dependence on size, shape, material, and environment was probed using silver right bipyramids, silver cubes, and gold cubes. It was found that the slope of the dependence of LSPR peak on size for silver bipyramids increases as the edges become sharper. Also, a plasmon shift of 96 nm was observed between similar silver and gold cubes, while a shift of 26 nm was observed, for gold cubes, between substrates of refractive index (RI) of 1.5 and 2.05.

Structure-activity relationships in gold nanoparticle dimers and trimers for surface-enhanced Raman spectroscopy

Understanding the detailed relationship between nanoparticle structure and activity remains a significant challenge for the field of surface-enhanced Raman spectroscopy. To this end, the structural and optical properties of individual plasmonic nanoantennas comprised of Au nanoparticle assemblies that are coated with organic reporter molecules and encapsulated by a SiO(2) shell have been determined using correlated transmission electron microscopy (TEM), dark-field Rayleigh scattering microscopy, surface-enhanced Raman scattering (SERS) microscopy, and finite element method (FEM) calculations. The distribution of SERS enhancement factors (EFs) for a structurally and optically diverse set of nanoantennas is remarkably narrow. For a collection of 30 individual nanoantennas ranging from dimers to heptamers, the EFs vary by less than 2 orders of magnitude. Furthermore, the EFs for the hot-spot-containing nanoparticles are uncorrelated to aggregation state and localized surface plasmon resonance (LSPR) wavelength but are crucially dependent on the size of the interparticle gap. This study demonstrates that the creation of hot spots, where two particles are in subnanometer proximity or have coalesced to form crevices, is paramount to achieving maximum SERS enhancements.

Gas sensing with high-resolution localized surface plasmon resonance spectroscopy

We report the first inert gas sensing and characterization studies based on high-resolution localized surface plasmon resonance (HR-LSPR) spectroscopy. HR-LSPR was used to detect the extremely small changes (<3 × 10(-4)) in bulk refractive index when the gas was switched between He(g) and Ar(g) or He(g) and N2(g). We also demonstrate submonolayer sensitivity to adsorbed water from exposure of the sensor to air (40% humidity) versus dry N2(g). These measurements significantly expand the applications space and characterization tools for plasmonic nanosensors.

Transcutaneous glucose sensing by surface-enhanced spatially offset Raman spectroscopy in a rat model

This letter presents the first quantitative, in vivo, transcutaneous glucose measurements using surface enhanced Raman spectroscopy (SERS). Silver film over nanosphere (AgFON) surfaces were functionalized with a mixed self-assembled monolayer (SAM) and implanted subcutaneously in a Sprague-Dawley rat. The glucose concentration was monitored in the interstitial fluid. SER spectra were collected from the sensor chip through the skin using spatially offset Raman spectroscopy (SORS). The combination of SERS and SORS is a powerful new approach to the challenging problem of in vivo metabolite and drug sensing.

Surface-enhanced Raman spectroscopy of dyes: from single molecules to the artists' canvas

This perspective presents recent surface-enhanced Raman spectroscopy (SERS) studies of dyes, with applications to the fields of single-molecule spectroscopy and art conservation. First we describe the development and outlook of single-molecule SERS (SMSERS). Rather than providing an exhaustive review of the literature, SMSERS experiments that we consider essential for its future development are emphasized. Shifting from single-molecule to ensemble-averaged experiments, we describe recent efforts toward SERS analysis of colorants in precious artworks. Our intention is to illustrate through these examples that the forward development of SERS is dependent upon both fundamental (e.g., SMSERS) and applied (e.g., on-the-specimen SERS of historical art objects) investigations and that the future of SERS is very bright indeed.

Screening of type I and II drug binding to human cytochrome P450-3A4 in nanodiscs by localized surface plasmon resonance spectroscopy

A prototype nanoparticle biosensor based on localized surface plasmon resonance (LSPR) spectroscopy was developed to detect drug binding to human membrane-bound cytochrome P450 3A4 (CYP3A4). CYP3A4 is one of the most important enzymes in drug and xenobiotic metabolism in the human body. Because of the inherent propensity of CYP3A4 to aggregate, it is difficult to study drug binding to this protein in solution and on surfaces. In this paper, we use a soluble nanometer scale membrane bilayer disk (Nanodisk) to functionally stabilize monomeric CYP3A4 on Ag nanoparticle surfaces fabricated by nanosphere lithography. CYP3A4-Nanodiscs have absorption bands in the visible wavelength region, which upon binding certain drugs shift to either shorter (type I) or longer wavelengths (type II). On the basis of the coupling between the LSPR of the Ag nanoparticles and the electronic resonances of the heme chromophore in CYP3A4-Nanodiscs, LSPR spectroscopy is used to detect drug binding with high sensitivity. This paper combines LSPR and Nanodisc techniques to optically sense drug binding to a functionally stable membrane protein, with the goal of integrating this with microfluidics and expanding it into a multiarray format, enabling high-throughput screening.

Detection and Identification of Bioanalytes with High Resolution LSPR Spectroscopy and MALDI Mass Spectrometry

High resolution localized surface plasmon resonance (HR-LSPR) sensors were combined with matrix assisted laser desorption ionization mass spectrometry (MALDI-MS) for the first time. LSPR sensors provide real-time label-free detection of molecular adsorption. Subsequent MALDI-MS analysis enables identification of the adsorbed molecules. This synergistic LSPR-MS approach was applied to the detection and identification of amyloid beta oligomers which play an important role in the molecular pathogenesis of Alzheimer's Disease.