Advances in localized surface plasmon resonance spectroscopy biosensing

Self-alignment of silver nanoparticles in highly ordered 2D arrays

We have synthesized silver nanoparticles in the non-polar phase of non-aqueous microemulsions. The nanocrystals have been grown by reducing silver ions in the microemulsion cylindrical micelles formed by the reducing agent (ethylene glycol). By a careful deposit of the microemulsion phase on a substrate, the micelles align in a hexagonal geometry, thus forming a 2D array of parallel strings of individual silver nanoparticles on the substrate. The microemulsions are the ternary system of anionic surfactant, non-polar solvent (isooctane), and solvent polar (ethylene glycol); the size of synthesized nanoparticles is about 7 nm and they are monodisperse. The study of the microstructure was realized by transmission electron microscopy, high-resolution technique transmission electron microscopy (HR-TEM), and Fourier processing using the software Digital Micrograph for the determination of the crystalline structure of the HR-TEM images of the nanocrystals; chemical composition was determined using the energy-dispersive X-ray spectroscopy. Addition technique polarizing light microscopy allowed the observation of the hexagonal phase of the system. This method of synthesis and self-alignment could be useful for the preparation of patterned materials at the nanometer scale.

Hybrid magnetic–plasmonic nanocomposite: embedding cobalt clusters in gold nanorods

Preparation of Plasmonic–magnetic hybrid nanorods via modified seed mediated method: using cobalt seeds instead of gold to prepare Au–Co NRs.

Direct glucose sensing in the physiological range through plasmonic nanoparticle formation

An enzyme-free, non-invasive glucose assay is developed involving gold nanoparticle formation and shows glucose sensitivity in the range of 3–50 mM in urine.

Localized surface plasmon resonance as a biosensing platform for developing countries

The discovery of the phenomena known as localized surface plasmon resonance (LSPR) has provided the basis for many research areas, ranging from materials science to biosensing. LSPR has since been viewed as a transduction platform that could yield affordable, portable devices for a multitude of applications. This review aims to outline the potential applications within developing countries and the challenges that are likely to be faced before the technology can be effectively employed.

Dip biosensor based on localized surface plasmon resonance at the tip of an optical fiber

A dip biosensor is realized by depositing metallic nanoparticles onto the tip of a cleaved optical fiber. Light coupled into the fiber interacts with the localized surface plasmons within the nanoparticles at the tip; a portion of the scattered light recouples into the optical fiber and is analyzed by a spectrometer. Characterization of the sensor demonstrates an inverse relationship between the sensitivity and the number of particles deposited onto the surface, with smaller quantities leading to greater sensitivity. The results obtained showed also that by depositing nanoparticles with distinct localized surface plasmon resonance signatures with limited overlap, as for the case of gold and silver nanospheres, a multiplexed dip biosensor can be realized by simply functionalizing the different nanoparticles with different antibodies after the fashion of an immunoassay. In this way different localized surface plasmons resonance bands responsive to different target analytes can be separately monitored, as further presented below, requiring a minimal quantity of reagents both for the functionalization process and for the sample analysis.

Detection and structural characterization of insulin prefibrilar oligomers using surface enhanced Raman spectroscopy

In vitro fibril formation typically exhibits a lag phase followed by a rapid elongation phase. Soluble prefibrilar oligomers form as multiple assembly states occur during the lag phase and, after forming a nucleus, rapidly propagate into amyloid aggregates and fibrils. The structure and morphology of amyloid fibrils have been extensively characterized over the last decades, while little is known about the structural organization of the prefibrilar oligomers or their multiple assembly states. The main difficulty in structural characterization of prefibrilar aggregates is their low concentration (pmolar) and their continual reactive conversion. Herein we overcome these difficulties by utilizing Surface-Enhanced Raman Spectroscopy (SERS) with a model amyloid peptide, insulin. SERS is a powerful analytic tool that is able to provide detection of small molecules down to a single-molecule level. Using SERS we found that during the 3 lag phase before the onset of insulin fibril formation, the amount of insulin oligomers increased more than twice after the first hour of incubation under fibrillation conditions (pH 1.6, 65°C) and then slowly decreased with time. The latter finding is kinetically linked to the conversion of the prefibrilar oligomers into fibril species. This study provides valuable new information about the time-dependent structural organization of insulin oligomers and demonstrates the power and potential of SERS for detection and structural characterization of biological specimens present at low concentrations.

Plasmonic hotspots of dynamically assembled nanoparticles in nanocapillaries: Towards a micro ribonucleic acid profiling platform

Plasmonic hot spots, generated by controlled 20-nm Au nanoparticle (NP) assembly, are shown to suppress fluorescent quenching effects of metal NPs, such that hair-pin FRET (Fluorescence resonance energy transfer) probes can achieve label-free ultra-sensitive quantification. The micron-sized assembly is a result of intense induced NP dipoles by focused electric fields through conic nanocapillaries. The efficient NP aggregate antenna and the voltage-tunable NP spacing for optimizing hot spot intensity endow ultra-sensitivity and large dynamic range (fM to pM). The large shear forces during assembly allow high selectivity (2-mismatch discrimination) and rapid detection (15 min) for a DNA mimic of microRNA.

Single gold nanoparticle localized surface plasmon resonance spectral imaging for quantifying binding constant of carbohydrate-protein interaction

Quantifying carbohydrate-protein (ligand-receptor) interactions is important to understand diverse biological processes and to develop new diagnostic and therapeutic methods. We develop an approach to quantitatively study carbohydrate-protein interactions by Au nanoparticle localized surface plasmon resonance (LSPR) peak position shift at the single particles level. Unlike the previous techniques for single particle LSPR spectral imaging, only the first-order streak of an individual nanoparticle is needed to extract a LSPR spectrum, which has great potential to increase throughput to 500 single particle spectra in each frame. LSPR peak shift of protein modified single Au nanoparticles is found to be a function of its ligand concentration, which can be used to fit the binding constants of the interactions. The moderate interactions of Antithrombin III (AT III) and heparins including low molecular weight heparin (LMWH) are determined as well as the strong interaction of transferrin and antitransferrin and the weak interaction of bovine serum album (BSA) and heparin. The measured binding constants of transferrin to antitransferrin, heparin and LMWH to AT III, and BSA to heparin are (3.0 ± 0.6) × 10(9) M(-1), (3.1 ± 0.3) × 10(6) M(-1), (8.0 ± 0.5) × 10(5) M(-1), and (5.1 ± 0.1) × 10(3) M(-1), respectively, which are in good agreement with the reported values.

Nanodots array rapidly fabricated by Dip-Pen nanolithography with temperature and humidity control

This study demonstrates the advantage of Dip-Pen Nanolithography (DPN) as a research and design tool for metal nano-structure fabrications. We design two different gold nano-structures, which are fabricated by DPN etching method with temperature and humidity control. The plasmon resonance frequencies of both structures are measured with dark field scattering spectroscopy. Our results show that with temperature and humidity control, DPN is highly potential in developing photonic circuit, solar cell and biomedical devices due to the rapid fabrication and cost effectiveness.

Direct observation of aminoglycoside-RNA binding by localized surface plasmon resonance spectroscopy

RNA is involved in fundamental biological functions when bacterial pathogens replicate. Identifying and studying small molecules that can interact with bacterial RNA and interrupt cellular activities is a promising path for drug design. Aminoglycoside (AMG) antibiotics, prominent natural products that recognize RNA specifically, exert their biological functions by binding to prokaryotic ribosomal RNA and interfering with protein translation, ultimately resulting in bacterial cell death. The decoding site, a small internal loop within the 16S rRNA, is the molecular target for the AMG antibiotics. The specificity of neomycin B, a highly potent AMG antibiotic, to the ribosomal decoding RNA site, was previously studied by observing AMG-RNA complexes in solution. Here, we study this interaction using localized surface plasmon resonance (LSPR) transducers comprising gold island films prepared by evaporation on glass and annealing. Small molecule AMG receptors were immobilized on the Au islands via polyethylene glycol (PEG)-thiol linkers, and the interaction with target RNA in solution was studied by monitoring the change in the LSPR optical response upon binding. The results show high-affinity binding of neomycin to 27-nucleotide model A-site RNA sequence in the nanomolar range, while no specific binding is observed for synthetic RNA oligomers (e.g., poly-U). The impact of specific base substitutions in the A-site RNA constructs on binding affinity and selectivity is determined quantitatively. It is concluded that LSPR is a powerful tool for providing molecular insight into small molecule-RNA interactions and for the design and screening of selective antimicrobial drugs.

Pitch-dependent resonances and near-field coupling in infrared nanoantenna arrays

We investigate coupling in arrays of nanoparticles resonating as half-wave antennas on both silicon and sapphire, and find a universal behavior when scaled by antenna length and substrate index. Three distinct coupling regimes are identified and characterized by rigorous finite-difference time domain simulations. As interparticle pitch is reduced below the oft-described radiative to evanescent transition, resonances blue shift and narrow and exhibit an asymmetric band consistent with a Fano lineshape. Upon further pitch reduction, a transition to a third regime, termed here as near-field coupling, is observed in which the resonance shifts red, becomes more symmetric, and broadens dramatically. This latter regime occurs when the extension of the resonant mode beyond the physical antenna end overlaps that of its neighbor. Simulations identify a clear rearrangement of field intensity accompanying this regime, illustrating that longitudinal modal fields localize in the air gap rather than in the higher index substrate at a pitch consistent with the experimentally observed transition.

Comparison of free-solution and surface-immobilized molecular interactions using a single platform

While it is generally accepted that surface immobilization affects the binding properties of proteins, it has been difficult to quantify these effects due to the lack of technology capable of making affinity measurements with species tethered and in free solution on a single platform. Further, quantifying the interaction of binding pairs with widely differing masses has also been challenging, particularly when it is desirable to tether the high molecular weight protein. Here we describe the use of backscattering interferometry (BSI) to quantify the binding affinity of mannose and glucose to concanavalin A (ConA), a 106 KDa homotetramer protein, in free solution using picomoles of the protein. Using the same platform, BSI, we then studied the effect on the binding constants of the ConA-carbohydrate interactions upon chemically immobilizing ConA on the sensor surface. By varying the distances (0, 7.17, and 20.35 nm) of the ConA tether and comparing these results to the free-solution measurements, it has been possible to quantify the effect that protein immobilization has on binding. Our results indicate that the apparent binding affinity of the sugar-lectin pair increases as the distance between ConA and the surface decreases. These observations could lend insight as to why the affinity values reported in the literature sometimes vary significantly from one measurement technique to another.

Molecular plasmonics for biology and nanomedicine

The optical excitation of surface plasmons in metal nanoparticles leads to nanoscale spatial confinement of electromagnetic fields. The confined electromagnetic fields can generate intense, localized thermal energy and large near-field optical forces. The interaction between these effects and nearby molecules has led to the emerging field known as molecular plasmonics. Recent advances in molecular plasmonics have enabled novel optical materials and devices with applications in biology and nanomedicine. In this article, we categorize three main types of interactions between molecules and surface plasmons: optical, thermal and mechanical. Within the scope of each type of interaction, we will review applications of molecular plasmonics in biology and nanomedicine. We include a wide range of applications that involve sensing, spectral analysis, imaging, delivery, manipulation and heating of molecules, biomolecules or cells using plasmonic effects. We also briefly describe the physical principles of molecular plasmonics and progress in the nanofabrication, surface functionalization and bioconjugation of metal nanoparticles.

Single plasmonic nanoparticle tracking studies of solid supported bilayers with ganglioside lipids

Single-particle tracking experiments were carried out with gold nanoparticle-labeled solid supported lipid bilayers (SLBs) containing increasing concentrations of ganglioside (GM(1)). The negatively charged nanoparticles electrostatically associate with a small percentage of positively charged lipids (ethyl phosphatidylcholine) in the bilayers. The samples containing no GM(1) show random diffusion in 92% of the particles examined with a diffusion constant of 4.3(±4.5) × 10(-9) cm(2)/s. In contrast, samples containing 14% GM(1) showed a mixture of particles displaying both random and confined diffusion, with the majority of particles, 62%, showing confined diffusion. Control experiments support the notion that the nanoparticles are not associating with the GM(1) moieties but instead most likely confined to regions in between the GM(1) clusters. Analysis of the root-mean-squared displacement plots for all of the data reveals decreasing trends in the confined diffusion constant and diameter of the confining region versus increasing GM(1) concentration. In addition, a linearly decreasing trend is observed for the percentage of randomly diffusing particles versus GM(1) concentration, which offers a simple, direct way to measure the percolation threshold for this system, which has not previously been measured. The percolation threshold is found to be 22% GM(1) and the confining diameter at the percolation threshold only ∼50 nm.

High performance surface-enhanced Raman scattering substrates of Si-based Au film developed by focused ion beam nanofabrication

A novel method with high flexibility and efficiency for developing SERS substrates is proposed by patterning nanostructures on Si substrates using focused ion beam direct writing (FIBDW) technology following with precise thermal evaporation of gold film on the substrate. The effect of SERS on the substrate was systematically investigated by optimizing the processing parameters and the gold film thickness. The results proved that small dwell time could improve the machining accuracy and obtain smaller nanogap. The Raman-enhanced performance of the substrate was investigated with 10-6mol/L Rhodamine 6 G solution. It was indicated that the elliptic nanostructures with 15-nm spacing on Si substrates, coated with approximately 15-nm thick gold film, have exhibited a high-enhanced performance, but dramatic performance degradation was found as the gold film thickness further increased, which most probably resulted from changes of the nanostructures' morphology such as elliptical tip and spacing. To avoid the morphological changes effectively after depositing gold film, optimization design of the nanostructures for FIBDW on Si substrates was proposed. Besides, a similar phenomenon was found when the gold film was less than 15nm because there was little gold remaining on the substrate. The method proposed in this paper shows a great potential for the higher performance SERS substrates development, which can further reduce the spacing between hot spots.