Target dependence of the sensitivity in periodic nanowire-based localized surface plasmon resonance biosensors

Blazed wire-grid polarizer for plasmon-enhanced polarization extinction: design and analysis

We explore plasmon-enhanced wire-gird polarizers (WGPs) to achieve improved polarimetric performance with more relaxed fabrication parameters compared to conventional WGP. A WGP designed with a blazed wire-grid profile was considered for plasmonic enhancement. The results show that a blazed WGP can achieve extremely high polarimetric extinction at a longer wire-grid period (Λ) compared to conventional WGP structure. Under the optimum geometrical parameters, a blazed WGP may attain an extinction ratio of over 40 dB at Λ = 800 nm, which may allow photolithography for fabrication. In contrast, conventional WGPs obtained comparable performance at Λ = 200 nm, requiring more difficult lithographic techniques. The study can therefore be of significant importance for WGPs to be more widely available for diverse applications.

Integrated architecture for the electrical detection of plasmonic resonances based on high electron mobility photo-transistors

We report the design of an integrated platform for on-chip electrical transduction of the surface plasmon resonance supported by a nanostructured metal grating. The latter is fabricated on the active area of a GaAs/AlGaAs photo-HEMT and simultaneously works as the electronic gate of the device. The gold plasmonic crystal has a V-groove profile and has been designed by numerical optical simulations. By showing that the numerical models accurately reproduce the phototransistors experimental response, we demonstrate that the proposed architecture is suitable for the development of a new class of compact and scalable SPR sensors.

Variable incidence angle subwavelegth grating SPR graphene biosensor

This paper investigates the sensitivity enhancement of a variable incidence angle subwavelength grating based multilayer surface plasmon resonance biosensor (SPRB). In the proposed design, a periodic array of subwavelength grating is integrated on top of a layer of graphene sheet in the multilayer SPR biosensor. The performance of the biosensor is investigated through monitoring the biomolecular interactions of cDNA-ssDNA interactions on its surface. The sensitivity improvement is indicated by the shift of the resonance peak angle.

Novel D-shape LSPR fiber sensor based on nano-metal strips

In this study, a novel D-shaped localized surface plasmon resonance (LSPR) fiber sensor was introduced. The construction of this sensor involved etching of a single-mode fiber on the cladding layer and core layer, followed by plating using nano-metal strips. The design and calculations of the entire sensor were based on a numerical simulation method combining the finite element method (FEM) and the eigenmode expansion method (EEM). By using graphical representations of the algorithm results, the excitation of the LSPR was clearly observed. The finished D-shaped LSPR fiber sensor possesses several excellent properties, including a short length (2494.4301 μm), high resolution (approximately 35 dB), and high sensitivity (approximately 20183.333 nm/RIU). In addition, compared with LPG-SPR fiber sensor, the framework provides three advantages, namely, a fabrication process that is compatible with semiconductor fabrication, as well as the low-temperature cross-talk and high-temperature stability of surface grating.

All-thermoplastic nanoplasmonic microfluidic device for transmission SPR biosensing

Early and accurate disease diagnosis still remains a major challenge in clinical settings. Biomarkers could potentially provide useful tools for the detection and monitoring of disease progression, treatment safety and efficacy. Recent years have witnessed prodigious advancement in biosensor development with research directed towards rapid, real-time, label-free and sensitive biomarker detection. Among emerging techniques, nanoplasmonic biosensors pose tremendous potential to accelerate clinical diagnosis with real-time multiplexed analysis, rapid and miniaturized assays, low sample consumption and high sensitivity. In order to translate these technologies from the proof-of-principle concept level to point of care clinical diagnosis, integrated, portable devices having small footprint cartridges that house low-cost disposable consumables are sought. Towards this goal, we developed an all-polymeric nanoplasmonic microfluidic (NMF) transmission surface plasmon resonance (SPR) biosensor. The device was fabricated in thermoplastics using a simple, single step and cost-effective hot embossing technique amenable to mass production. The novel 3D hierarchical mold fabrication process enabled monolithic integration of blazed nanogratings within the detection chambers of a multichannel microfluidic system. Consequently, a single hard thermoplastic bottom substrate comprising plasmonic and fluidic features allowed integration of active fluidic elements, such as pneumatic valves, in the top soft thermoplastic cover, increasing device functionality. A simple and compact transmission-based optical setup was employed with multiplexed end-point or dual-channel kinetic detection capability which did not require stringent angular accuracy. The sensitivity, specificity and reproducibility of the transmission SPR biosensor was demonstrated through label-free immunodetection of soluble cell-surface glycoprotein sCD44 at clinically relevant picomolar to nanomolar concentrations.

Investigation on an application of silver substrates for sensitive surface plasmon resonance imaging detection

A surface plasmon resonance (SPR) imaging biosensor based on silver substrates was investigated to demonstrate that silver could be used as a substrate material for sensitive detection of biomolecular interactions, despite its poor chemical stability. The calculation results showed that oxidation of silver film may lead to a decrease in the sensitivity due to a variation in SPR characteristics such as a broader curve width and shallower minimum reflectance at resonance. The effect of a change in the refractive index of target analytes on the sensitivity was also explored. In particular, it is noteworthy that Ag/Au bimetallic substrates with a thin gold protection layer to prevent oxidation of a silver film can provide a significant amplification of SPR imaging signals in comparison with conventional gold substrates.

Nanowire-based sensors

Nanowires are important potential candidates for the realization of the next generation of sensors. They offer many advantages such as high surface-to-volume ratios, Debye lengths comparable to the target molecule, minimum power consumption, and they can be relatively easily incorporated into microelectronic devices. Accordingly, there has been an intensified search for novel nanowire materials and corresponding platforms for realizing single-molecule detection with superior sensing performance. In this work, progress made towards the use of nanowires for achieving better sensing performance is critically reviewed. In particular, various nanowires types (metallic, semiconducting, and insulating) and their employment either as a sensor material or as a template material are discussed. Major obstacles and future steps towards the ultimate nanosensors based on nanowires are addressed.

Angulo-spectral surface plasmon resonance imaging of nanofabricated grating surfaces

We present a surface plasmon resonance imaging (SPRI) setup, based on the Kretschmann configuration, capable of simultaneously acquiring the complete spectral and angular plasmonic reflectivity response on all points of the sensing area. Several line poly(methyl methacrylate) grating regions were fabricated on a thin-film gold surface and characterized with this SPRI system. Reflectivity maps of the corrugated regions showing plasmon bandgaps were obtained to illustrate the capability of the setup.

Nanoimprinted plastic substrates for enhanced surface plasmon resonance imaging detection

Periodic nanostructures fabricated by Nanoimprint Litography (NIL) in low-cost plastic substrates and coated with thin gold film were explored for enhanced surface plasmon resonance imaging (SPRi) detection. Rigorous coupled-wave analysis was used to model the SPRi response of these nanostructured surfaces. Two-dimensional nanogratings and nanogrooves were fabricated on Zeonor 1060R(TM) by NIL and followed by metal deposition. The detection of refractive index changes in the dielectric layer due to bulk medium change, DNA immobilization and DNA hybridization events were monitored using SPRi to assess the corresponding signal amplification. The results indicate target-dependent sensitivity enhancement which is maximized for the detection of biomolecular binding events. The 500 nm period nanogrooves provided a 4 times SPR signal amplification compared to the conventional uniform gold film on SF-11 glass for DNA hybridization detection. Our work demonstrates that the use of nanoimprinted plastic substrates provides a low-cost solution for the SPR-based detection with sensitivity that meets the requirements in practical diagnostic applications.

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.

Localized surface plasmon resonance detection of layered biointeractions on metallic subwavelength nanogratings

Enhanced detection of multiple targets such as self-assembled monolayer (SAM) formation, DNA hybridization, and ethanol ambient changes was explored using localized surface plasmon resonance (LSPR) excited by metallic surface nanogratings. The sensitivity enhancement depends on the target as well as the nanostructure with a maximum at 242% over a conventional structure when detecting an 11-mercaptoundecanoic acid SAM with an LSPR structure of 200 nm period. The measured enhancement shows smaller target-dependent variance when detecting various layered biointeractions, while structure-dependent variance was much larger. The result suggests the feasibility of the efficient detection of multiple biointeractions at enhanced sensitivity and extends the applicability of a nanostructured LSPR biosensor for diverse biomolecular events.

Effect of target localization on the sensitivity of a localized surface plasmon resonance biosensor based on subwavelength metallic nanostructures

A localized surface plasmon resonance (LSPR) biosensor using surface relief nanostructures was investigated to evaluate the importance of target localization on the sensitivity enhancement. The LSPR device was modeled as periodic metallic nanowires with a square profile on a gold film and the target as a self-assembled monolayer in buffer solution. The numerical results using rigorous coupled-wave analysis and the finite-difference time domain method demonstrated localized plasmonic fields induced by the surface nanostructure from which the effect of target localization on the sensitivity was quantitatively analyzed. Interestingly, it was found that target localization on nanowire sidewalls improves sensitivity significantly because of strong overlap with localized plasmonic fields. An LSPR structure optimized for a localized target on sidewalls provides sensitivity enhancement per unit target volume by more than 20 times in water ambience.