Dielectrophoresis-Based Protein Enrichment for a Highly Sensitive Immunoassay Using Ag/SiO2 Nanorod Arrays.

A nanoscale insulator-based dielectrophoresis (iDEP) technique is developed for rapid enrichment of proteins and highly sensitive immunoassays. Dense arrays of nanorods (NDs) by oblique angle deposition create a super high electric field gradient of 2.6 × 1024 V2 m-3 and the concomitant strong dielectrophoresis force successfully traps small proteins at a bias as low as 5 V. 1800-fold enrichment of bovine serum albumin protein at a remarkable rate of up to 180-fold s-1 is achieved using oxide coated Ag nanorod arrays with pre-patterned sawtooth electrodes. Based on this system, an ultrasensitive immunoassay of mouse immunoglobulin G is demonstrated with a reduction in the limit of detection from 5.8 ng mL-1 (37.6 pM) down to 275.3 fg mL-1 (1.8 f M), compared with identical assays performed on glass plates. This methodology is also applied to detect a cancer biomarker prostate-specific antigen spiked in human serum with a detection limit of 2.6 ng mL-1 . This high sensitivity results from rapid biomarker enrichment and metal enhanced fluorescence through the integration of nanostructures. The concentrated proteins also accelerate binding kinetics and enable signal saturation within 1 min. Given the easy fabrication process, this nanoscale iDEP system provides a highly sensitive detection platform for point-of-care diagnostics.

Two chiroptical modes of silver nanospirals.

As an emerging three-dimensional chiral metamaterial, plasmonic nanospirals (NSs) possess inherent chiroptical activity that has attracted increasing attention for developing potential photonic applications. However, the study of chiroptical activity of plasmonic NSs is still in its infancy, especially for NSs made of silver, which is a typical plasmonic material with high plasmonic quality. Herein, we present a systematic study of circular dichroism (CD) of silver NSs (AgNSs) that are fabricated and engineered in helical lengths by glancing-angle deposition (GLAD) and dispersed in ethanol. The CD spectrum is composed of a bisignated mode of two peaks, one in the UV regime and the other in the visible. The UV mode has a resonance wavelength saturating at ∼375 nm and a linewidth decoupled from the helical elongation, while the visible mode tends to have a redshift and its linewidth broadens linearly with the elongation of AgNS. Helical elongation generally amplifies the chiroptical activity of both modes. Finite-element simulation shows good agreement with the experimental CD results, and accounts for the wavelength-related chiroptical distinction in terms of the resonance wavelength. This work contributes to understanding the bisignated chiroptical responses of plasmonic nanospirals, and introduces a simple method to tailor the visible chiroptical activity that is strongly desired to explore a wide range of chirality-related bio-applications.

Microfluidic-based metal enhanced fluorescence for capillary electrophoresis by Ag nanorod arrays.

As metal nanorods show much higher metal enhanced fluorescence (MEF) than metal nanospheres, microfluidic-based MEF is first explored with Ag nanorod (ND) arrays made by oblique angle deposition. By measuring the fluorescein isothiocyanate (FITC) solution sandwiched between the Ag NDs and a piece of cover slip, the enhancement factors (EFs) are found as 3.7 ± 0.64 and 6.74 ± 2.04, for a solution thickness at 20.8 µm and 10 µm, respectively. Because of the strong plasmonic coupling between the adjacent Ag NDs, only the emission of the fluorophores present in the three-dimensional NDs array gets enhanced. Thus, the corresponding effective enhancement factors (EEFs) are revealed to be relatively close, 259 ± 92 and 340 ± 102, respectively. To demonstrate the application of MEF in microfluidic systems, a multilayer of SiO2 NDs/Ag NDs is integrated with a capillary electrophoresis device. At a microchannel depth of 10 µm, an enhancement of 6.5 fold is obtained for amino acids separation detection. These results are very encouraging and open the possibility of MEF applications for the Ag ND arrays decorated microchannels. With the miniaturization of microfluidic devices, microfluidic-based MEF by Ag ND arrays will likely find more applications with further enhancement.

Au nanoparticle based localized surface plasmon resonance substrates fabricated by dynamic shadowing growth.

Au nanoparticle (NP) substrates, Au NP/TiO(2)/Au NP sandwich structures, and Ti coated Au NP substrates are fabricated by glancing angle deposition (GLAD) and oblique angle deposition (OAD) methods. Under the same deposition condition, the Au NP substrates produced by GLAD are more uniform and reproducible compared to those fabricated by OAD. The localized surface plasmon resonance (LSPR) wavelength of Au NP substrates can be easily tuned by changing the film thickness, the deposition angle, and the coating of the dielectric layer (TiO(2)) and metallic layer (Ti). In addition, the thickness and the deposition angle of the Ti coating on Au NP also affect the LSPR wavelength. Our results demonstrate that GLAD is a very versatile fabrication technique to produce reproducible and fine-tuned LSPR substrates.

Nanorod-mediated surface plasmon resonance sensor based on effective medium theory.

We investigate a nanorod-mediated surface plasmon resonance (SPR) sensor for sensitivity enhancement. The theoretical model containing an anisotropic layer of nanorod is investigated using four-layer Fresnel equations and the effective medium theory. The properties of the nanorod-mediated SPR curves versus the metal thin film thickness d(f), length l, and diameter D of the nanorod are studied in the environment with refractive indices of 1.00 and 1.33. Compared to the conventional thin metal film SPR configuration, the nanorod-mediated SPR sensor presents a larger resonance angle shift and the sensitivity increases with increasing refractive index of the target analyte. Besides the theoretical analysis, we fabricate different Ag nanorod array/Ag film substrates by oblique angle deposition and characterize their SPR responses using a laboratory-made SPR setup in air and in deionized (DI) water. Compared with the Ag film sample, the SPR angles observed for Ag nanorods/Ag film samples shift to larger angles in air (for shorter nanorods), while it is hard to observe the SPR angle in DI water, which is qualitatively consistent with theoretical results. We believe that the nanorod-mediated SPR sensor is able to improve the sensitivity and the theoretical discussion is helpful for sensor fabrication.

High-resolution separation of DNA/proteins through nanorod sieving matrix.

In this paper, we report on an integrated nanorod array as a novel sieving matrix for high-resolution separation of biomolecules. A wide size range of DNA (100 bp-166 kbp) and proteins (11.4 kDa-205 kDa) are resolved into sharp peaks within several minutes using devices with various pore apertures, either in a reptation mode or Ogston mechanism. The device delivers notable performance with the minimum resolvable size difference 10 bp for 100 bp and 150 bp DNA fragments, 15.9 kbp for 48 kbp and 166 kbp DNA chains, and 7.9 kDa for 11.4 and 68 kDa proteins, where the separation efficiency is higher than or comparable to other micro/nanofabricated artificial sieving structures. The process utilizes localized oblique angle deposition (LOAD) to simply integrate densely packed nanorods onto steep sidewalls of microfluidic channels, which eliminates the requirement of advanced lithography tools routinely used in conventional artificial sieving structures. We further demonstrate the modulation of pore aperture that employs the sculptured sidewalls by a custom deep reactive-ion etching (DRIE) recipe. Given the easy fabrication, excellent engineering control and outstanding sieving capability, this nanorod sieve device has great potentials to provide a simple and effective separation technique for biomolecule analysis.

Multilayered Si/Ni nanosprings and their magnetic properties.

A two-turn, eight-armed, rectangular Si/Ni heterogeneous nanospring structure on Si(100) has been fabricated using a multilayer glancing-angle deposition technique. The multilayered nanosprings with a height of approximately 1.98 mum were composed of alternating layers of amorphous Si nanorods approximately 580 nm in length and face-centered cubic Ni nanorods approximately 420 nm in length, both with a diameter of approximately 35 nm. The magnetic anisotropy of the nanosprings showed that the in-plane easy and hard axes were parallel and perpendicular to the Ni nanorod plane, respectively. The out-of-plane magnetic hysteresis loop was very sensitive to the applied magnetic field direction when rotating the nanosprings about their in-plane hard axis, and the magnetization measurement revealed that the nanosprings tilted at approximately 7.5 degrees toward the plane of the Si nanorods. The magnetic anisotropy of the nanosprings is determined by their structure, and the experimental results can be interpreted by the shape anisotropy energy.

Tailorable chiroptical activity of metallic nanospiral arrays.

The engineering of the chiroptical activity of the emerging chiral metamaterial, metallic nanospirals, is in its infancy. We utilize glancing angle deposition (GLAD) to facilely sculpture the helical structure of silver nanospirals (AgNSs), so that the scope of chiroptical engineering factors is broadened to include the spiral growth of homochiral AgNSs, the combination of left- and right-handed helical chirality to create heterochiral AgNSs, and the coil-axis alignment of the heterochiral AgNSs. It leads to flexible control over the chiroptical activity of AgNS arrays with respect to the sign, resonance wavelength and amplitude of circular dichroism (CD) in the UV and visible regime. The UV chiroptical mode has a distinct response from the visible mode. Finite element simulation together with LC circuit theory illustrates that the UV irradiation is mainly adsorbed in the metal and the visible is preferentially scattered by the AgNSs, accounting for the wavelength-related chiroptical distinction. This work contributes to broadening the horizons in understanding and engineering chiroptical responses, primarily desired for developing a wide range of potential chiroplasmonic applications.

Metal enhanced fluorescence improved protein and DNA detection by zigzag Ag nanorod arrays.

As metal nano-arrays show great potential on metal enhanced fluorescence (MEF) than random nanostructures, MEF of Ag zigzag nanorod (ZNR) arrays made by oblique angle deposition has been studied for biomolecule-protein interaction and DNA hybridization. By changing the folding number and the deposition substrate temperature, a 14-fold enhancement factor (EF) is obtained for biotin-neutravidin detection. The optimal folding number is decided as Z=7, owing to the high scattering intensity of Ag ZNRs. The substrate temperature T=25°C and 0°C slightly alters the morphology of Ag ZNRs but has no big difference in EF. Further, Ag ZNRs deposited on a layer of Ag film have been introduced to the DNA hybridization and a significant signal enhancement has been observed through the fluorescence microscope. Through a detailed quantitative EF analysis, which excludes the enhancing effect from the increased surface area of ZNRs and only considers the contribution of MEF, an EF of 28 is achieved for the hybridization of two single-stranded oligonucleotides with 33 bases. Furthermore, a limit of detection is determined as 0.01pM. We believe that the Ag ZNR arrays can serve as a universal and sensitive bio-detection platform.

Porosification-reduced optical trapping of silicon nanostructures.

Metal-assisted chemical etching (MACE) was carried out to fabricate solid silicon nanowires (s-SiNWs) and mesoporous silicon nanowires (mp-SiNWs). Total reflection and transmission were measured using an integrated sphere to study optical properties of the MACE-generated silicon nanostructures. Without NW aggregation, mp-SiNWs vertically standing on a mesoporous silicon layer trap less light than s-SiNWs over a wavelength range of 400-800 nm, owing to porosification-enhanced optical scattering from the rough inner surfaces of the mesoporous silicon skeletons. Porosification substantially weakens the NW mechanical strength; hence the elongated mp-SiNWs aggregate after 30 min etching and deteriorate optical trapping.

An Au/Si hetero-nanorod-based biosensor for Salmonella detection.

We present a novel and effective food-borne bacteria detection method. A hetero-structured silicon/gold nanorod array fabricated by the glancing angle deposition method is functionalized with anti-Salmonella antibodies and organic dye molecules. Due to the high aspect ratio nature of the Si nanorods, dye molecules attached to the Si nanorods produce an enhanced fluorescence upon capture and detection of Salmonella. This bio-functional hetero-nanorod detection method has great potential in the food safety industry as well as in biomedical diagnostics.