The collaborative oscillation of conductive electrons in metal nanoparticles results in a surface plasmon resonance that makes them useful for various applications including biolabeling. We investigate the coupling between pairs of elliptical metal particles by simulations and experiments. The results demonstrate that the resonant wavelength peak of two interacting particles is red-shifted from that of a single particle because of near-field coupling. It is also found that the shift decays approximately exponentially with increasing particle spacing and become negligible when the gap between the two particles exceeds about 2.5 times the particle short-axis length.

We report experimental and theoretical studies on the plasmon resonances of finite one-dimensional chains of Au nanoparticles excited by evanescent light waves with polarization parallel to the chains. The experimental results show that the plasmon resonance peak wavelengths of these finite 1D chains are significantly red-shifted in comparison to that of single Au nanoparticle. Contrary to previous findings, the peak wavelengths are observed to be a nonmonotonic function of particle numbers in the chain. This phenomenon is reproduced in the theoretical results obtained by using the transfer-matrix method and is shown to occur only for larger particles where phase retardation effects are important in plasmon coupling.

The plasmon resonance of Au∕SiO2 multilayered nanodisks was studied using light scattering spectroscopy and numerical calculations. Compared to single layered Au nanodisks, multilayered nanodisks exhibit several distinctive properties including significantly enhanced plasmon resonances and tunable resonance wavelengths which can be tailored to desired values by simply varying dielectric layer thickness while the particle diameter is kept constant. Numerical calculations show that slicing one metal layer into metal multilayers leads to higher scattering intensity and more “hot spots,” or regions of strong field enhancement. This tunable and augmented plasmon resonance holds a great potential in the applications of surface-enhanced Raman scattering (SERS).

We have developed a novel technique to precisely determine the Raman enhancement factor in single nanoplasmonic resonators (TNPRs). TNPRs are lithographically defined metallodielectric nanoparticles composed of two silver disks stacked vertically, separated by a silica layer. At resonance, the local electromagnetic fields are enhanced at the TNPR surface, making it an ideal surface-enhanced Raman scattering (SERS) active substrate. The ability to control the dimensions of the metallic and dielectric layers offers the unique advantage of fine-tuning the plasmon resonance frequency to maximize the enhancement of the Raman signal. Furthermore, by selective shielding of the outer surface of the metallic structure, the efficiency can be further enhanced by guiding the molecular assembly to the locations that exhibit strong electromagnetic fields. We experimentally demonstrate SERS enhancement factors of (6.1+/-0.3)x10(10), with the highest enhancement factor being achieved by using an individual nanoparticle. By using nanofabrication techniques, we eliminate the issues such as large size variations, cluster aggregation, and interparticle effects common in preparing SERS substrates using conventional chemical synthesis or batch fabrication methods. TNPRs produce very controllable and repeatable SERS signals at the desired locations and, thus, make an ideal candidate for device integration.

Protease activity measurement has broad application in drug screening, diagnosis and disease staging, and molecular profiling. However, conventional immunopeptidemetric assays (IMPA) exhibit low fluorescence signal-to-noise ratios, preventing reliable measurements at lower concentrations in the clinically important pM∼nM range. Here, we demonstrated a highly sensitive measurement of protease activity using nanoplasmonic resonator (NPR). NPRs enhance Raman signals by 6.1×10 10 times in a highly reproducible manner, enabling fast detection of proteolytically active Prostate Specific Antigen (paPSA) activities in real-time, at sensitivity level at 6 pM (0.2 ng/ml) with a dynamic range of 3 orders of magnitude. Experiments on extracellular fluid (ECF) from the paPSA-positive cells demonstrate specific detection in a complex bio-fluid background. This method offers a fast, sensitive, accurate, and one-step approach to detect the proteases activities in very small sample volumes. KeywordsPlasmonic resonator; Surface Enhanced Raman Scattering; Sensing; Prostate Cancer; Protease Originally developed in 1928, Raman spectroscopy has been used extensively to characterize molecular properties 1 . Surface-Enhanced Raman Spectroscopy (SERS) increases the Raman signal significantly 2-5 through enhanced electromagnetic fields in close proximity to a surface. Additional enhancement can be obtained by utilizing molecular resonance Raman (RR) effect when the molecule was excited at its absorption band 6 . SERS measurements performed on dispersed metal nanoparticle aggregates, which is the mostly commonly used SERS substrate 7 , have demonstrated detection sensitivity up to single molecule level [8][9][10] . However, these measurement often suffer from poor reproducibility 11 . To improve the reproducibility, other methods including self-assembly of metallic colloidal nano-particles, 12 nanosphere lithography (NSL) and metal film over nanosphere (MFON) 13 The lack of specificity causes a high false-positive rate and often leads to costly prostate needle biopsies for diagnosis and post-biopsy complications as well as considerable anxiety. 22,28,29 Recent research has identified a family of highly specific peptides that can be cleaved by paPSA isoform in xenografts models 30 and human samples 31, 32 thus, measurement of paPSA protease activity from in vivo samples is possible and would be potentially valuable as a more specific screening agent for prostate cancer and in detection of recurrent disease. However, reported results based on immunopeptidemetric assays (IMPA) exhibit low fluorescence signal-to-noise ratios, preventing reliable measurements at lower concentrations in the clinically important range of 60-300 pM.31 , 32 In addition, there is usually a limited number of prostate cancer cells (<1000) isolated from fine needle biopsy or circulating cell capture. No method exists that can perform a paPSA protease activity assay on a small number of cells for clinical staging. Therefore, a key goal of this work is to devel...