We report the synthesis of new "branched" gold nanocrystals in high yield (over 90%) via a wet-chemical route. The branched nanocrystals exhibit a shape-dependent plasmon resonance that is red-shifted by 130−180 nm from the spherical particle wavelength. Discrete dipole approximation (DDA) calculations qualitatively replicate the observed optical extinction spectra of the nanocrystals, indicating that the surface plasmon resonance is mainly determined by in-plane dipole excitation associated with the sharp tips.

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Recent advances in label-free biosensing techniques have shown the potential to simplify clinical analyses. With this motivation in mind, this paper demonstrates for the first time the use of siliconon-insulator microring optical resonator arrays for the robust and label-free detection of a clinically important protein biomarker in undiluted serum, using carcinoembryonic antigen (CEA) as the test case. We utilize an initial slope-based quantitation method to sensitively detect CEA at clinically relevant levels and determine the CEA concentrations of unknown samples in both buffer and undiluted fetal bovine serum. Comparison with a commercial enzyme-linked immunosorbent assay (ELISA) kit reveals that the label-free microring sensor platform has a comparable limit of detection (2 ng/mL) and superior accuracy in the measurement of CEA concentration across a three order of magnitude dynamic range. Notably, we report the lowest limit of detection to date for a microring resonator sensor applied to a clinically relevant cancer biomarker. Although this report describes the robust biosensing capabilities of silicon photonic microring resonator arrays for a single parameter assay, future work will focus on utilizing the platform for highly multiplexed, label-free bioanalysis.

We present a combination of theory and experiment designed to elucidate the properties of gold nanoshells. Wet chemistry methods are used to prepare the nanoshells, and transmission electron microscopy (TEM) analysis is used to characterize the shell structure, demonstrating the presence of pinholes in the shells. Both Mie theory and the discrete dipole approximation (a numerical method) are used to characterize the electrodynamics of the shell structures, including both perfect and pinhole defected shells. The calculations show that 2-5 nm pinholes have only a small effect on the extinction spectra; however, they lead to local electric fields that are enhanced by a factor of 3-4 close to the plasmon maximum. This makes metal nanoshells (with holes) attractive materials for surface enhanced Raman spectroscopy applications.