Tailoring plasmonic properties of gold nanohole arrays for surface-enhanced Raman scattering.

The wide plasmonic tuning range of nanotriangle and nanohole array patterns fabricated by nanosphere lithography makes them promising in surface-enhanced Raman scattering (SERS) sensors. Unfortunately, it is challenging to optimize these patterns for SERS sensing because their optical response is a complex mixture of localized surface plasmon resonance (SPR) and propagating surface plasmon polariton (SPP). In this paper, transmission and reflection measurements are combined with finite difference time domain simulations to identify and separate each plasmonic mode, discerning which resonance leads to the electromagnetic field enhancement. The SERS enhancement is found to be dominated by the absorption, which is shifted from the transmission and reflection dips usually used as tuning points, and by the 'gap' defects formed within the pattern. These effects have different spectral and geometric dependences, forming two optimization curves which can be used to predict the best performance for a given excitation wavelength. The developed model is verified with experimental SERS measurements for several nanohole sizes and periodicities, and then used to give optimal fabrication parameters for a range of measurement conditions. The results will promote the application of two-dimensional plasmonic nanoarrays in SERS sensors.

Dual detection of cancer biomarker CA125 using absorbance and electrochemical methods.

An enzyme-linked immunoassay based on dual signal transduction mechanisms has been developed for detection of ovarian cancer biomarker CA125. The immunoassay uses a nanoelectrode array (NEA) chip and absorbance methods for the dual detection. The NEA is used to confirm the optical detection of CA125 that is carried out in a high-binding 96-well plate. An alkaline phosphatase (AP) enzyme was used to label the detection antibody to allow for both the optical and electrochemical detection of CA125. Two kinds of substrates were catalyzed by the AP enzyme. para-Nitrophenylphosphate (PNPP) produces chromogenic para-nitrophenol (PNP), which can be optically detected at 405 nm. para-Aminophenylphosphate (PAPP) produces electroactive para-aminophenol (PAP), which can be detected amperometrically between -0.1 and 0.3 V. The linear ranges have been determined to be 5-1000 U mL(-1) and 5-1000 U mL(-1) for the optical and electrochemical immunoassays, respectively. The limit of detection of the optical immunoassay is 1.3 U mL(-1) and 40 U mL(-1) for the optical and electrochemical methods, respectively.

Three-dimensional hierarchical plasmonic nano-architecture enhanced surface-enhanced Raman scattering immunosensor for cancer biomarker detection in blood plasma.

A three-dimensional (3D) hierarchical plasmonic nano-architecture has been designed for a sensitive surface-enhanced Raman scattering (SERS) immunosensor for protein biomarker detection. The capture antibody molecules are immobilized on a plasmonic gold triangle nanoarray pattern. On the other hand, the detection antibody molecules are linked to the gold nanostar@Raman reporter@silica sandwich nanoparticles. When protein biomarkers are present, the sandwich nanoparticles are captured over the gold triangle nanoarray, forming a confined 3D plasmonic field, leading to the enhanced electromagnetic field in intensity and in 3D space. As a result, the Raman reporter molecules are exposed to a high density of "hot spots", which amplifies the Raman signal remarkably, improving the sensitivity of the SERS immunosensor. This SERS immunosensor exhibits a wide linear range (0.1 pg/mL to 10 ng/mL) and a low limit of detection (7 fg/mL) toward human immunoglobulin G protein in the buffer solution. This biosensor has been successfully used for detection of the vascular endothelial growth factor in the human blood plasma from clinical breast cancer patient samples.

Plasmonic nanorice antenna on triangle nanoarray for surface-enhanced Raman scattering detection of hepatitis B virus DNA.

The sensitivity and the limit of detection of Raman sensors are limited by the extremely small scattering cross section of Raman labels. Silver nanorice antennae are coupled with a patterned gold triangle nanoarray chip to create spatially broadened plasmonic "hot spots", which enables a large density of Raman labels to experience strong local electromagnetic field. Finite difference time domain simulations have confirmed that the quasi-periodic structure increases the intensity and the area of the surface plasmon resonance (SPR), which enhances the surface-enhanced Raman scattering (SERS) signal significantly. The SERS signal of the nanorice/DNA/nanoarray chip is compared with that of the nanorice/DNA/film chip. The SERS signal is greatly enhanced when the Ag nanorices are coupled to the periodic Au nanoarray instead of the planar film chip. The resulting spatially broadened SPR field enables the SERS biosensor with a limit of detection of 50 aM toward hepatitis B virus DNA with the capability of discriminating a single-base mutant of DNA. This sensing platform can be extended to detect other chemical species and biomolecules such as proteins and small molecules.

Photocatalytic activity enhanced by plasmonic resonant energy transfer from metal to semiconductor.

Plasmonic metal nanostructures have been incorporated into semiconductors to enhance the solar-light harvesting and the energy-conversion efficiency. So far the mechanism of energy transfer from the plasmonic metal to semiconductors remains unclear. Herein the underlying plasmonic energy-transfer mechanism is unambiguously determined in Au@SiO(2)@Cu(2)O sandwich nanostructures by transient-absorption and photocatalysis action spectrum measurement. The gold core converts the energy of incident photons into localized surface plasmon resonance oscillations and transfers the plasmonic energy to the Cu(2)O semiconductor shell via resonant energy transfer (RET). RET generates electron-hole pairs in the semiconductor by the dipole-dipole interaction between the plasmonic metal (donor) and semiconductor (acceptor), which greatly enhances the visible-light photocatalytic activity as compared to the semiconductor alone. RET from a plasmonic metal to a semiconductor is a viable and efficient mechanism that can be used to guide the design of photocatalysts, photovoltaics, and other optoelectronic devices.

Photoelectrochemical performance enhanced by a nickel oxide-hematite p-n junction photoanode.

A p-n junction photoanode has been fabricated by depositing p-type NiO nanoparticles on the n-type hematite thin film. Such a photoanode is employed for a photoelectrochemical cell. NiO not only facilitates the extraction of accumulated holes from hematite via the p-n junction, but also lowers the barrier for oxygen evolution reaction.

Detection of adenosine triphosphate with an aptamer biosensor based on surface-enhanced Raman scattering.

A simple, ultrasensitive, highly selective, and reagent-free aptamer-based biosensor has been developed for quantitative detection of adenosine triphosphate (ATP) using surface-enhanced Raman scattering (SERS). The sensor contains a SERS probe made of gold nanostar@Raman label@SiO(2) core-shell nanoparticles in which the Raman label (malachite green isothiocyanate, MGITC) molecules are sandwiched between a gold nanostar core and a thin silica shell. Such a SERS probe brings enhanced signal and low background fluorescence, shows good water-solubility and stability, and exhibits no sign of photobleaching. The aptamer labeled with the SERS probe is designed to hybridize with the cDNA on a gold film to form a rigid duplex DNA. In the presence of ATP, the interaction between ATP and the aptamer results in the dissociation of the duplex DNA structure and thereby removal of the SERS probe from the gold film, reducing the Raman signal. The response of the SERS biosensor varies linearly with the logarithmic ATP concentration up to 2.0 nM with a limit of detection of 12.4 pM. Our work has provided an effective method for detection of small molecules with SERS.