An optical biosensor based on localized surface plasmon resonance of silver nanostructured films

Label-free biosensing with a multi-box sub-wavelength phase-shifted Bragg grating waveguide

Sub-wavelength grating (SWG) metamaterials have been considered to provide promising solutions in the development of next-generation photonic integrated circuits. In recent years, increasied interest has been paid to silicon photonic planar biosensors based on SWG geometries for performance enhancement. In this work, we demonstrate a highly sensitive label-free phase-shifted Bragg grating (PSBG) sensing configuration, which consists of sub-wavelength block arrays in both propagation and transverse directions. By introducing salt serial dilutions and electrostatic polymers assays, bulk and surface sensitivities of the proposed sensor are characterized, obtaining measured results up to 579.2 nm/RIU and 1914 pm/nm, respectively. Moreover, the proposed multi-box PSBG sensor presents an improved quality factor as high as ∼ 8000 , roughly 3-fold of the microring-based counterpart, which further improves the detection limit. At last, by employing a biotin-streptavidin affinity assay, the capability for small molecule monitoring is exemplified with a minimum detectable concentration of biotin down to 2.28 × 10 - 8 M .

Tip-Specific Functionalization of Gold Nanorods for Plasmonic Biosensing: Effect of Linker Chain Length

Gold nanorods are promising platforms for label-free biosensing. We have functionalized gold nanorods with biotin thiol linkers of increasing chain length and evaluated their ability in the molecular detection of streptavidin. We have found an unexpected effect of the increase in linker length, which resulted in a substantial improvement of the plasmon response at surface saturation. The plasmon peak shift increased from 5 to 14 nm, i.e., more than twice the response, between the short and long biotin linkers. This effect is observed only for site-selective tip functionalization, whereas for a full biotin coating there is no improvement observed with the linker length. The improved plasmon response for tip functionalization is attributed to low biotin coverage but is directed to the most sensitive regions, which, combined with a longer chain linker, reduces the steric hindrance for streptavidin binding on the rod's surface. The model sensors were further characterized by measuring their dose-response curves and binding kinetic assays. Simulations of the discrete dipole approximation give theoretical plasmon shifts that compare well with the experimental ones for the long linker but not with those of the short linker, thus suggesting that steric hindrance affects the latter. Our results highlight the importance of specifically functionalizing the plasmonic hot spots in nanoparticle sensors with the adequate density of receptors in order to maximize their response.

Electrochemical direct writing and erasing of silver nanostructures on phosphate glass using atomic force microscopy

This paper reports a liquid-free, mask-less electrochemical direct-write lithographic technique using an atomic force microscopy (AFM) probe for writing silver nanostructures in minutes on an optically transparent substrate. Under ambient conditions, silver is locally and controllably extracted to the surface of superionic (AgI)_0.25 (AgPO₃)_0.75 glass by bringing a conductive AFM probe tip in contact with it, biasing the probe with a negative voltage, and regulating the resulting current. The growth mechanism of the resulting nanostructure is explored by extracting silver with a stationary AFM tip on the surface of the silver. A moving tip was then used to produce continuous lines, solid films and discrete dots of silver by implementing continuous and pulsed current writing approaches. The line dimensions depend on writing speed and current flowing in the electrochemical circuit, while the size and spacing of the dots depend on the parameters (magnitude, duration and frequency) of the current pulses and the writing speed of the AFM tip. Line-widths in the ∼100 nm range are demonstrated. Our investigation also shows that a threshold potential must be overcome to be able to draw and reduce silver ions on the glass surface. When polarity between the electrodes is reversed, the patterned silver ionizes back into the glass, thus offering the capability to erase and rewrite Ag patterns on the glass surface.

Physical characteristics of localized surface plasmons resulting from nano-scale structured multi-layer thin films deposited on D-shaped optical fiber

Novel surface plasmonic optical fiber sensors have been fabricated using multiple coatings deposited on a lapped section of a single mode fiber. UV laser irradiation processing with a phase mask produces a nano-scaled surface relief grating structure resembling nano-wires. The resulting individual corrugations produced by material compaction are approximately 20 μm long with an average width at half maximum of 100 nm and generate localized surface plasmons. Experimental data are presented that show changes in the spectral characteristics after UV processing, coupled with an overall increase in the sensitivity of the devices to surrounding refractive index. Evidence is presented that there is an optimum UV dosage (48 joules) over which no significant additional optical change is observed. The devices are characterized with regards to change in refractive index, where significantly high spectral sensitivities in the aqueous index regime are found, ranging up to 4000 nm/RIU for wavelength and 800 dB/RIU for intensity.

Microfabrication and optical properties of highly ordered silver nanostructures

Using thermal evaporation, we fabricated five uniform and regular arrays of Ag nanostructures with different shapes that were based on an anodized aluminum oxide template and analyzed their optical properties. Round-top-shaped structures are obtained readily, whereas to obtain needle-on-round-top-shaped and needle-shaped structures, control of the directionality of evaporation, pore size, length, temperature of the substrate, etc., was required. We then observed optical sensitivity of the nanostructures by using surface-enhanced Raman scattering, and we preliminarily investigated the dependency of Raman signal to the roughness and shape of the nanostructures.

Electron beam lithography designed silver nano-disks used as label free nano-biosensors based on localized surface plasmon resonance

We present a label-free, optical nano-biosensor based on the Localized Surface Plasmon Resonance (LSPR) that is observed at the metal-dielectric interface of silver nano-disk arrays located periodically on a sapphire substrate by Electron-Beam Lithography (EBL). The nano-disk array was designed by finite-difference and time-domain (FDTD) algorithm-based simulations. Refractive index sensitivity was calculated experimentally as 221-354 nm/RIU for different sized arrays. The sensing mechanism was first tested with a biotin-avidin pair, and then a preliminary trial for sensing heat-killed Escherichia coli (E. coli) O157:H7 bacteria was done. Although the study is at an early stage, the results indicate that such a plasmonic structure can be applied to bio-sensing applications and then extended to the detection of specific bacteria species as a fast and low cost alternative.

A new optical label-free biosensing platform based on a metal-insulator-metal structure

A new label-free biosensing platform is proposed based on a metal-insulator-metal (MIM) structure. This platform allows us to perform biosensing by a simple reflectivity measurement at normal incidence illumination without using any bulky optical setup. Theoretical calculation was made to find optimized MIM structural parameters, and experimental results on the label-free biosensing using the MIM platform are presented. This platform has greater sensitivity and mass resolution with respect to the anomalous reflection method, which is a label-free biosensing platform previously proposed by us. Hence, it is suitable for high throughput analysis of biomolecular detection in a microarray format.

Poly(amidoamine)-dendrimer-modified gold surfaces for anomalous reflection of gold to detect biomolecular interactions

Label-free protein detecting chip technology has encouraged a number of discoveries, as it is a powerful analytical tool in the postgenomic era. In particular, we have focused on a unique characteristic of anomalous reflection of gold (AR) as a new class of label-free detection method for a protein chip system. In this paper, in order to improve the sensitivity of detection of biomolecular interactions by the AR method, we have constructed three-dimensional (3D) nanostructures on gold surfaces with a series of well-defined structures of poly(amidoamine) dendrimers (PAMAMs) from generation 2 to 4 (G2, G3, and G4) tethering biotin moieties as capturing agents for avidin and antibiotin IgG. Comparison of features of such 3D nanostructured surfaces with a diamine-modified flat-like surface revealed a 2-fold increase in the amount of avidin for 3D surfaces relative to the flat surface, and surface-assisted nonspecific interactions were significantly suppressed. We thus obtained 91% coverage for avidin detection on the PAMAM G4-modified surface, indicating a theoretically maximum attainable absorption considering a hexagonal-packed arrangement as a saturated monomolecular layer. In the antibiotin IgG assay, the PAMAM G4-modified surface clearly improved the amount of proteins captured compared to that for the flat surface, indicating that an appropriate density of capturing agents played a more important role in the interaction of larger molecular-sized proteins such as antibiotin IgG, which requires more space for interaction than the medium-sized avidin. These findings should assist in the development of a simple and practical tool for high-throughput protein detection, particularly with the AR method.

A label-free immunoassay based upon localized surface plasmon resonance of gold nanorods

Robust gold nanorod substrates were fabricated for refractive index sensing based on localized surface plasmon resonance (LSPR). The substrate sensitivity was 170 nm/RIU with a figure of merit of 1.3. To monitor biomolecular interactions, the nanorod surfaces were covered with a self-assembled monolayer and conjugated to antibodies by carbodiimide cross-linking. Interactions with a specific secondary antibody were monitored through shifts in the LSPR spectral extinction peak. The resulting binding rates and equilibrium constant were in good agreement with literature values for an antibody-antigen system. The nanorod LSPR sensors were also shown to be sensitive and specific. These results demonstrate that given a sufficiently stable nanoparticle substrate with a well defined chemical interface, LSPR sensing yields similar results to the surface plasmon resonance technique, yet with much simpler instrumentation.