We study the role of a strong electron confinement on the surface-enhanced Raman scattering from molecules adsorbed on small noble-metal nanoparticles. We describe a novel enhancement mechanism which originates from the different effect that confining potential has on s-band and d-band electrons. We demonstrate that the interplay between finite-size and screening efects in the nanoparticle surface layer leads to an enhancement of the surface plasmon local field acting on a molecule located in a close proximity to the metal surface. Our calculations, based on time-dependent local density approximation, show that the additional enhancement of the Raman signal is especially strong for small nanometer-sized nanoparticles.

Very large enhancement up to 14 orders of magnitude in the Raman cross section from a molecule adsorbed on a single cluster of a few nano metal particles has been reported recently. The enhancement is believed mainly due to the enhanced electromagnetic (em) field because of the excitation of the localized surface em mode. Using scattering t-matrix approach, we have developed a Green's function theory in the Fourier (wave vector) space to solve the Maxwell equations for the enhanced electric field near the spherical metal particle cluster. The large enhancement in the field is due to the multiple scattering of the localized modes of the individual metal particles that has been included exactly. The advantage of working in the wave vector space is that one does not need the use of complicated translational addition theorem required in the real space as used in earlier calculations. Therefore, our approach can be easily extended to different shape or size of the metal particle cluster. We find the enhancement in the Raman cross section can reach up to 10 orders of magnitude for silver particle cluster. The enhancement is in a broad frequency range and is below the Mie resonance of the single metal sphere. The results for gold particle cluster are also presented.

SERS method for biomolecular analysis has several potentials and advantages over traditional biochemical approaches, including less specimen contact, non-destructive to specimen, and multiple components analysis. Urine is an easily available body fluid for monitoring the metabolites and renal function of human body. We developed surface-enhanced Raman scattering (SERS) technique using 50nm size gold colloidal particles for quantitative human urine creatinine measurements. This paper shows that SERS shifts of creatinine (104mg/dl) in artificial urine is from 1400cm^-1 to 1500cm^-1 which was analyzed for quantitative creatinine measurement. Ten human urine samples were obtained from ten healthy persons and analyzed by the SERS technique. Partial least square cross-validation (PLSCV) method was utilized to obtain the estimated creatinine concentration in clinically relevant (55.9mg/dl to 208mg/dl) concentration range. The root-mean square error of cross validation (RMSECV) is 26.1mg/dl. This research demonstrates the feasibility of using SERS for human subject urine creatinine detection, and establishes the SERS platform technique for bodily fluids measurement.

The interest in optical properties of colloidal metals is driven by their applications to chemistry and optics. Metallic nanoparticles have the ability to enhance the local optical fields and change the spectroscopic properties of organic molecules as well as allow for design new optical devices. Metal-enhanced fluorescence (MEF) is yielding enormous opportunities for enhanced fluorescence sensing and imaging in microfluidics, lab-on-a-chip, clinical diagnostics, microarrays, and cellular applications. MEF is a through-space phenomenon relying on interaction of fluorophores with metallic nano-particles in the presence of excitation light. MEF can be utilized to produce new fluorometric devices with dramatically increased sensitivity. We report on metal-enhanced fluorescence measured on a silicon surface with silver nanoparticles patterned with electron beam lithography. We employ a combinatorial approach, depositing silver particles ranging in size, shape, inter-particle spacing, and nominal thickness. Two nanoparticle shapes were investigated, square and triangular in cross section with side dimensions ranging from 50 nm to 130 nm and spaced at distances ranging from 150 nm to 390 nm center-to-center. The fluorescence enhancement of several fluorophores was measured with excitation sources consisting of an Ar ion laser at 488/514 nm and a HeNe laser at 633 nm. This approach allows an easy and direct comparison of the fluorescence enhancement to the particle size, shape, inter-particle spacing, and excitation wavelength.

We have recently shown that metallic particles or colloids when deposited on the transparent surface can enhance fluorescence properties of nearby fluorophores. We obtained the fluorophore-metal colloid complexes that display significant fluorescence signal enhancement in solution. Silver nanoparticles (about 20-50 nm size) were synthesized as a stable yellow colloidal solution, and coated with proteins labeled with fluorophores. A several-fold amplification of the fluorescence signal in presence of colloid Ag nanoparticles in solution was observed. Such fluorophore-metal complex presents a unique opportunity for developing of new class of contrast agents for optical imaging and fluorescence based sensing. Solution of silver nanoparticles with enhanced fluorescence can be used in various assays such as DNA hybridization or immunoassays for high sensitivity detection.

Luminescent emission from lanthanide complexes on silver island films has been studied. Silver films with and without silica spacer layers have been prepared chemically and by thermal vacuum deposition. Enhancement of emission from lanthanide complexes on silver compared to that on bare glass depends on the optical density of the silver films (silver coverage) and sample coverage. A silica spacer layer reduces the number of molecules in quenching regions of the film and leads to better enhancement. Lanthanides with lower quantum efficiencies are expected to exhibit greater enhancement factors and our preliminary results are consistent with that prediction.

The potential for using plasmon-resonant gold nanorods as targeted contrast agents for in vivo coherent optical imaging is investigated. Separation of the relative strengths of light scattering and absorption of plasmon-resonant nanorods are measured with a double-integrating sphere system at 774 and 1304nm. The maximum likelihood ratio is then used to test the statistical significance of optical changes observed after application of contrast agents to tissue phantoms. Gold plasmon-resonant nanorods with a longitudinal resonance near 800nm are imaged within varying concentrations of intralipid using a 101dB sensitivity, 800nm optical coherence tomography (OCT) system. We estimate the minimum OCT detectible concentration of these nanorods (ca. 15 by 45nm) within 1.1% intralipid to be 25microg/mL of gold.

It is well known that the use of noble metal nanoparticles can considerably enhance the sensitivity of conventional surface plasmon resonance (SPR) biosensors. In our study, we theoretically investigate this sensitivity enhancement effect using rigorous coupled-wave analysis. It is based on the assumption that the enhancement of localized plasmons can be demonstrated by the coupling phenomenon between the periodic noble metal structures and the incident light with an appropriate polarization. It is shown that the rigorous coupled-wave method can be applied to calculating a SPR structure that includes metallic nanoparticles of rectangular-like geometry, where the presence of nanoparticles induces significant changes in the position of reflectivity minimum. The influence of the nanoparticle period on the sensitivity enhancement is also confirmed. In the calculation, Au nanoparticles deposited on an Au film or adsorbed on a SAM layer are modified to regularly patterned one-dimensional nanowires. When the period is less than 300 nm, the calculated sensitivity enhancement of the nanoparticle-based SPR structure is more than ten-fold compared with that of a conventional SPR biosensors configuration.

We propose a new approach to improving the resolution of a Surface Plasmon Resonance (SPR) based biosensing system through the use of the statistical technique of hypothesis testing. Simulated SPR reflectance curves at the wavelength of 632.8 nm in the Kretschmann configuration were generated. Detector specifications were taken from Ames Photonics CMOS Larry-CCD 1024 array detector with a total integration time of 100s and a noise amplitude relative to signal σ = 3 × 10^-6. We were able to achieve the detection limit of the refractive index difference with respect to water of Δn = 4 × 10^-8 with 95 % significance level.

Adaptive silver films (ASFs) have been studied as a substrate for protein microarrays. Vacuum evaporated silver films fabricated at certain range of evaporation parameters allow fine rearrangement of the silver nanostructure under protein depositions in buffer solution. Proteins restructure and stabilize the ASF to increase the surface-enhanced Raman scattering (SERS) signal from a monolayer of molecules. Preliminary evidence indicates that the adaptive property of the substrates make them appropriate for protein microarray assays. Head-to-head comparisons with two commercial substrates have been performed. Protein binding was quantified on the microarray using the streptavidinCy3/biotinylated goat IgG protein pair. With fluorescence detection, the performance of ASF substrates was comparable with SuperAldehyde and SuperEpoxy substrates. Additionally, the ASF is also a SERS substrate and this provides an additional tool for analysis. It is found that the SERS spectra of the streptavidinCy5 fluorescence reporter bound to true and bound to false sites show distinct difference.

We study coherent oscillations of radial breathing modes in metal nanoparticles with a dielectric core. Vibrational modes are impulsively excited by a rapid heating of the particle lattice that occurs after laser excitation, while the energy transfer to a surrounding dielectric leads to a damping of the oscillations. In nanoshells, the presence of two metal surfaces leads to a substantially different energy spectrum of acoustic vibrations. The lowest and first excited modes correspond to in-phase (n=0) and out-of-phase (n=1) contractions of shell-core and shell-matrix interfaces respectively. We calculated the energy spectrum as well as the damping of nanoshell vibrational modes in the presence of surrounding medium, and found that the size-dependences of in-phase and anti-phase modes are different. At the same time, the oscillator strength of the symmetric mode is larger than that in solid nanoparticles leading to stronger oscillations in thin nanoshells.

A gold nanowire array that we call nanorainbow SPR sensor array can be chemically functionalized and used to capture biomolecules. The localized plasmon resonance wavelength of the gold nanowires shifts on the biomolecule binding and reaction sites. The plasmon resonance shift of the gold nanorainbow is sensitive to the biomolecule immobilization in sub-nM concentration. As an application example, label-free oligonucleotide hybridizations are detected on the nanorainbow sensor in a multiplexed microfluidic chip.

We study enhancement of fluorescence of molecular species bound within metallic nanocavities. These nanostructures possess a number of desirable properties for real-time microarrays, such as localization of excitation light within the nanocavities, strong isolation from fluorescence produced by unbound species,and an apparent increase in fluorescence yield for bound species. Experimental measurements show a nearly a factor of 2 increase in excitation intensity within the nanocavities, and factor of 6 increase in yield. A simple electromangetic model of a dipole within a nanocavity shows an increase in radiative output consistent with our yield estimates and also verifies the strong fluorescence isolation from species lying outside the nanocavity.

The interaction between tiny-molecular-weight analytes and receptors can be detected directly using advanced plasmonic biosensors. This paper proposes a plasmonic biosensor with Au nanoclusters embedded in a dielectric film. The sensor uses the attenuated total reflection (ATR) method to excite both the surface plasmons (SPs) and the particle plasmons (PPs). The local electro-magnetic (EM) field is enhanced by controlling the size and volume fraction of the embedded Au nanoparticles. The developed biosensor demonstrates a 10-fold improvement in resolution compared to a conventional surface plasmon resonance (SPR) biosensor. Using the proposed ultra-sensitive plasmonic biosensor, a direct detection approach is adopted to analyze the interactions of tiny molecules (< 200 Da) without the need for high molecular weight competitors or explicit labeling. The plasmonic biosensor developed in this paper provides a rapid diagnostic capability and has considerable potential for use in pharmaceutical research.

When light passes through a hole smaller than the wavelength of the light, the transmission is very low and the light is diffracted. This however changes if holes are arranged in a periodic array on metal. In that case the light couples to surface plasmons; this results in enhanced transmission, spectral selection and a small angular diffraction. We develop a novel microscopic method based on a periodic hole-array, which will be used as a multiple-apertures near-field source for illuminating a biological sample while the light is collected in far-field. The measurement speed is high, due to the use of an array instead of a single source. The main advantage of this microscope originates from the low diffraction of light through a relatively thick sample with enhanced transmission. It results in the ability to measure the samples interior and 3D reconstruction can be made by semi-confocal techniques. This overcomes the major limitation of near-field methods for which only a shallow layer of the surface (~20 nm) is detectable. For our measurements we use glass coated devices. The holes are processed with a focused ion beam. The photon-plasmon coupling process is characterized as a function of the wavelength. Our experiments aim on gaining a better understanding of the transmission process. We tested the dependence of the transmitted spectrum on angle of incidence was tested as well as far-field spectral imaging measurements of the transmission in both Koehler and collimated light illumination. The results as well as the description of the microscope that we are constructing are presented.

In vivo fluorescent spectroscopy and imaging using endogenous and exogenous sources of contrast can provide new approaches for enhanced demarcation of brain tumor margins and infiltration. Quantum dots (QDs), nanometer-size fluorescent probes, represent excellent contrast agents for biomedical imaging due to their broader excitation spectrum, narrower emission spectra, and higher sensitivity and stability. The epidermal growth factor receptor (EGFR) is implicated in the development and progression of a number of human solid tumors including brain tumors and thus a potential target for brain tumor diagnosis. In this study, we investigate the up-take of ODs by brain tumor cells and the potential use of EGFR-targeted QDs for enhanced optical imaging of brain tumors. We conducted fluorescence microscopy studies of the up-take mechanism of the anti-EGFR-ODs complexes by Human U87, and SKMG-3 glioblastoma cells. Our preliminary results show that QDs can enter into glioma cells through anti-EGFR mediated endocytosis, suggesting that these nano-size particles can tag brain tumor cells.

Transient and substantial elevation of postsynaptic calcium was important for hippocampal long-term potentiation (LTP), so detection of calcium changes in spine was necessary to understand the mechanisms underlying synaptic plasticity. Unfortunately most recent calcium fluorescence indicators severely perturbed calcium transients, and traditional cameleons’ poor dynamic ranges prevented detection of changes of calcium. We presented a new method to monitor quantificationally free calcium concentration in dendritic spines with a new yellow cameleon (YC3.60) basing on culture of hippocampal neurons and calcium phosphate transfection technique and confocal microscopy with 458nm laser. In transiently transfected hippocampal neurons, the ratio of YFP to CFP was detected as FRET level. In our study, we got the parameters of YC3.60 excited with 458nm laser. Under control conditions, FRET levels in different dendritic spines of cultured hippocampal neurons were diverse but showed robust increases upon treatment with potassium chloride. FRET levels in different parts of hippocampal neurons were also different, the calcium concentration decreased with the distance from soma. These results suggested that the FRET methodology with YC3.60 could monitor calcium concentration in spines and it might be useful in analyzing mechanisms underlying synaptic plasticity.

A biosensing imaging system is proposed based on the integration of surface plasmon resonance (SPR) and common-path phase-shift interferometry (PSI) techniques to measure the two-dimensional spatial phase variation caused by biomolecular interactions upon a sensing chip. The SPR phase imaging system can offer high resolution and high-throughout screening capabilities to analyze microarray biomolecular interaction without the need for additional labeling. With the long-term stability advantage of the common-path PSI technique even with external disturbances such as mechanical vibration, buffer flow noise, and laser unstable issue, the system can match the demand of real-time kinetic study for biomolecular interaction analysis (BIA). The SPR-PSI imaging system has achieved a detection limit of 2×10^-7 refraction index change, a long-term phase stability of 2.5x10^-4π rms over four hours, and a spatial phase resolution of 10^-3 π with a lateral resolution of 100μm.