Enhancement of the resolution of surface plasmon resonance biosensors by control of the size and distribution of nanoparticles

Review of Polarization Optical Devices Based on Graphene Materials

Graphene has received extensive scholarly attention for its extraordinary optical, electrical, and physicochemical properties, as well as its compatibility with silicon-based semiconductor processes. As a unique two-dimensional atomic crystal material, graphene has excellent mechanical properties, ultra-high carrier mobility, ultra-wide optical response spectrum, and strong polarization dependence effect, which make it have great potential in new optical and polarization devices. A series of new optical devices that are based on graphene have been developed, showing excellent performance and broad application prospects. In this paper, the recent research progress of polarizers, sensors, modulators, and detectors that are based on the polarization characteristics of graphene is reviewed. In particular, the polarization dependence effect and broadband absorption enhancement of graphene under total reflection structure are emphasized, which enhance the interaction between graphene and light and then provide a new direction for research of graphene polarization devices.

Mercaptopyridine-Functionalized Gold Nanoparticles for Fiber-Optic Surface Plasmon Resonance Hg2+ Sensing

As a highly toxic heavy metal ion, divalent mercuric ion (Hg²⁺) is one of the most widely diffused and hazardous environmental pollutants. In this work, a simple, portable, and inexpensive fiber-optic sensor based on surface plasmon resonance (SPR) effect was developed for Hg²⁺ detection, which takes advantage of 4-mercaptopyridine (4-MPY)-functionalized Au nanoparticles (Au NPs/4-MPY) as a signal amplification tag. Based on the coordination between Hg²⁺ and nitrogen in the pyridine moiety, we developed the sensor by self-assembling 4-MPY on Au film surfaces to capture Hg²⁺ and then introducing Au NPs/4-MPY to generate a plasmonic coupling structure with the configuration of nanoparticle-on-mirror. The coupling between localized SPR increased changes in SPR wavelength, which allowed highly sensitive Hg²⁺ sensing in aqueous solution. The sensor exhibited superior selectivity for Hg²⁺ detection compared with other common metal ions in water. The sensor's Hg²⁺ detection limit is 8 nM under optimal conditions. Furthermore, we validated the sensor's practicality for Hg²⁺ detection in tap water samples and demonstrated its potential application for environmental water on-site monitoring.

Enhancing the blinking fluorescence of single-molecule localization imaging by using a surface-plasmon-polariton-enhanced substrate

Super-resolution imaging based on single-molecule localization microscopy combined with the surface plasmon polariton (SPP)-enhanced fluorescence of spontaneously blinking fluorophores was demonstrated to visualize the nanoscale-level positioning information of cell-adhesion-associated proteins. Glass substrates with a deposited silver layer were utilized to induce a SPP-enhanced field on the silver surface and significantly strengthen the fluorescence signals of the fluorophores by more than 300%. The illumination power density for localization imaging at a spatial resolution of 25 ± 11 nm was 31.6 W cm-2. This low illumination power density will facilitate the reduction of phototoxicity of the biospecimens for single-molecule localization imaging. The proposed strategy provides a uniform distribution of the SPP-enhanced field on the silver surface, enabling visualization of the spatial distribution of labeled proteins without interference caused by the enhanced field distribution.

Coarsening of carbon black supported Pt nanoparticles in hydrogen

This study addresses coarsening mechanisms of Pt nanoparticles supported on carbon black in hydrogen. By means of in situ transmission electron microscopy (TEM), Pt nanoparticle coarsening was monitored in 6 mbar 20% H₂/Ar while ramping up the temperature to almost 1000 °C. Time-resolved TEM images directly reveal that separated ca. 3 nm sized Pt nanoparticles in a hydrogen environment are stable up to ca. 800 °C at a heating rate of 10 °C min^-1. The coarsening above this temperature is dominated by the particle migration and coalescence mechanism. However, for agglomerated Pt nanoparticles, coalescence events were observed already above 200 °C. The temperature-dependency of particle sizes and the observed migration distances are described and found to be consistent with simple early models for the migration and coalescence.

Nanocomposite biosensors for point-of-care—evaluation of food quality and safety

Nanosensors have wide applications in the food industry. Nanosensors based on quantum dots for heavy metal and organophosphate pesticides detection, and nanocomposites as indicators for shelf life of fish/meat products, have served as important tools for food quality and safety assessment. Luminescent labels consisting of NPs conjugated to aptamers have been popular for rapid detection of infectious and foodborne pathogens. Various detection technologies, including microelectromechanical systems for gas analytes, microarrays for genetically modified foods, and label-free nanosensors using nanowires, microcantilevers, and resonators are being applied extensively in the food industry. An interesting aspect of nanosensors has also been in the development of the electronic nose and electronic tongue for assessing organoleptic qualities, such as, odor and taste of food products. Real-time monitoring of food products for rapid screening, counterfeiting, and tracking has boosted ingenious, intelligent, and innovative packaging of food products. This chapter will give an overview of the contribution of nanotechnology-based biosensors in the food industry, ongoing research, technology advancements, regulatory guidelines, future challenges, and industrial outlook.

Plasmonic ruler on field-effect devices for kinase drug discovery applications

Protein kinases are cellular switches that mediate phosphorylation of proteins. Abnormal phosphorylation of proteins is associated with lethal diseases such as cancer. In the pharmaceutical industry, protein kinases have become an important class of drug targets. This study reports a versatile approach for the detection of protein phosphorylation. The change in charge of the myelin basic protein upon phosphorylation by the protein kinase C-alpha (PKC-α) in the presence of adenosine 5'-[γ-thio] triphosphate (ATP-S) was detected on gold metal-insulator-semiconductor (Au-MIS) capacitor structures. Gold nanoparticles (AuNPs) can then be attached to the thio-phosphorylated proteins, forming a Au-film/AuNP plasmonic couple. This was detected by a localized surface plasmon resonance (LSPR) technique alongside MIS capacitance. All reactions were validated using surface plasmon resonance technique and the interaction of AuNPs with the thio-phosphorylated proteins quantified by quartz crystal microbalance. The plasmonic coupling was also visualized by simulations using finite element analysis. The use of this approach in drug discovery applications was demonstrated by evaluating the response in the presence of a known inhibitor of PKC-α kinase. LSPR and MIS on a single platform act as a cross check mechanism for validating kinase activity and make the system robust to test novel inhibitors.

Plasmonic biosensors

The unique optical properties of plasmon resonant nanostructures enable exploration of nanoscale environments using relatively simple optical characterization techniques. For this reason, the field of plasmonics continues to garner the attention of the biosensing community. Biosensors based on propagating surface plasmon resonances (SPRs) in films are the most well-recognized plasmonic biosensors, but there is great potential for the new, developing technologies to surpass the robustness and popularity of film-based SPR sensing. This review surveys the current plasmonic biosensor landscape with emphasis on the basic operating principles of each plasmonic sensing technique and the practical considerations when developing a sensing platform with the various techniques. The 'gold standard' film SPR technique is reviewed briefly, but special emphasis is devoted to the up-and-coming localized surface plasmon resonance and plasmonically coupled sensor technology.

Highly tunable self-assembled plasmonic lattices through nanosphere lithography

This Letter reports a method to produce two-dimensional self-assembled plasmonic nanopillar (NP) arrays with independent control of the diameter (d), spacing (s), and height (h) of the NPs. A plasmonic lattice was designed and optimized for maximum plasmonic activity at 980 nm using three-dimensional finite-difference time-domain simulations. The optimized lattice with d=365 nm, s=410 nm, and h=70 nm was fabricated utilizing a self-assembled nanosphere lithography approach. Outstanding agreement between the observed and predicted results confirms the validity of the design process and the controllability and repeatability of the fabrication process. The excellent short-range order in the lattice structure suggests that this method can replace the electron-beam lithography approach in a scalable and cost-effective manner.

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.

In situ coarsening study of inverse micelle-prepared Pt nanoparticles supported on γ-Al2O3: pretreatment and environmental effects

The thermal stability of inverse micelle prepared Pt nanoparticles (NPs) supported on nanocrystalline γ-Al(2)O(3) was monitored in situ under different chemical environments (H(2), O(2), H(2)O) via extended X-ray absorption fine-structure spectroscopy (EXAFS) and ex situ via scanning transmission electron microscopy (STEM). Drastic differences in the stability of identically synthesized NP samples were observed upon exposure to two different pre-treatments. In particular, exposure to O(2) at 400 °C before high temperature annealing in H(2) (800 °C) was found to result in the stabilization of the inverse micelle prepared Pt NPs, reaching a maximum overall size after moderate coarsening of ∼1 nm. Interestingly, when an analogous sample was pre-treated in H(2) at ∼400 °C, a final size of ∼5 nm was reached at 800 °C. The beneficial role of oxygen in the stabilization of small Pt NPs was also observed in situ during annealing treatments in O(2) at 450 °C for several hours. In particular, while NPs of 0.5 ± 0.1 nm initial average size did not display any significant sintering (0.6 ± 0.2 nm final size), an analogous thermal treatment in hydrogen leads to NP coarsening (1.2 ± 0.3 nm). The same sample pre-dosed and annealed in an atmosphere containing water only displayed moderate sintering (0.8 ± 0.3 nm). Our data suggest that PtO(x) species, possibly modifying the NP/support interface, play a role in the stabilization of small Pt NPs. Our study reveals the enhanced thermal stability of inverse micelle prepared Pt NPs and the importance of the sample pre-treatment and annealing environment in the minimization of undesired sintering processes affecting the catalytic performance of nanosized particles.

Surface plasmon resonance biosensors incorporating gold nanoparticles

SPR biosensing is increasingly popular for the detection of a multitude of biomolecules. It offers label-free detection and study of proteins, nucleic acids, and other biomolecules in real time. A recent trend involves incorporation of AuNPs, either within the sensing surface itself or as signal enhancing tagging molecules. The importance of AuNP and detecting agent spacing is described and techniques using macromolecular spacing aids are highlighted. Recent methods to enhance SPR detection capabilities using gold nanoparticles are reviewed, as well as device fabrication and the results of incorporation. SPR detection is a highly versatile method for the detection of biomolecules and, with the incorporation of AuNPs, shows promise in extending it to a number of new applications.

Study of optical phase transduction on localized surface plasmon resonance for ultrasensitive detection

Localized surface plasmon resonance (LSPR) has shown its remarkable applications in biosensing, bioimaging, and nanophotonics. Unlike surface plasmon polariton (SPP), the current studies regarding LSPR as biosensor were restricted in probing the extinction spectra, and thus limit the performance in biosensing and bioimaging. Here, we reveal that optical phase of LSPR provides an acute change at resonance beyond extinction spectra, which permits an ultra-high sensitivity in phase interrogation. We found that optical phases of LSPR show two orders of magnitude higher sensing resolution than extinction spectra among the same nanostructures. For the first time, we demonstrated the feasibility of probing optical phase transduction in LSPR for biosensing, and the sensitivity is superior to not only the extinction spectra among the same metallic nanostructures, but also the LSPR sensors among the current literatures. In summary, the exploitation of LSPR by phase interrogation essentially complements the sensitivity insufficiency of LSPR, and provides new access to understanding and using the rich physics of LSPR.

Enhancement of localized surface plasmon resonance detection by incorporating metal-dielectric double-layered subwavelength gratings

In this study, we investigated the enhanced sensing performance of a localized surface plasmon resonance (LSPR) biosensor by employing metal-dielectric double-layered subwavelength grating structures. The numerical results showed that the LSPR substrate with a dielectric spacer can provide not only a better sensitivity but also a significantly improved reflectance characteristic. While the presence of metallic gratings leads to a broad and shallow reflectance curve inevitably, the dielectric spacer can prevent the propagating surface plasmons from being interfered by the locally enhanced fields excited at the gold gratings, finally resulting in a strong and deep absorption band at resonance. Therefore, the proposed structure could potentially open a new possibility of the enhanced LSPR detection for monitoring biomolecular interactions of low molecular weights.

Design analysis of doped-silicon surface plasmon resonance immunosensors in mid-infrared range

This paper reports the design analysis of a microfabricatable mid-infrared (mid-IR) surface plasmon resonance (SPR) sensor platform. The proposed platform has periodic heavily doped profiles implanted into intrinsic silicon and a thin gold layer deposited on top, making a physically flat grating SPR coupler. A rigorous coupled-wave analysis was conducted to prove the design feasibility, characterize the sensor's performance, and determine geometric parameters of the heavily doped profiles. Finite element analysis (FEA) was also employed to compute the electromagnetic field distributions at the plasmon resonance. Obtained results reveal that the proposed structure can excite the SPR on the normal incidence of mid-IR light, resulting in a large probing depth that will facilitate the study of larger analytes. Furthermore, the whole structure can be microfabricated with well-established batch protocols, providing tunability in the SPR excitation wavelength for specific biosensing needs with a low manufacturing cost. When the SPR sensor is to be used in a Fourier-transform infrared (FTIR) spectroscopy platform, its detection sensitivity and limit of detection are estimated to be 3022 nm/RIU and ~70 pg/mm(2), respectively, at a sample layer thickness of 100 nm. The design analysis performed in the present study will allow the fabrication of a tunable, disposable mid-IR SPR sensor that combines advantages of conventional prism and metallic grating SPR sensors.

Surface plasmon-enhanced two-photon fluorescence microscopy for live cell membrane imaging

A surface plasmon-enhanced two-photon total-internal-reflection fluorescence (TIRF) microscope has been developed to provide fluorescent images of living cell membranes. The proposed microscope with the help of surface plasmons (SPs) not only provides brighter fluorescent images based on the mechanism of local electromagnetic field enhancement, but also reduces photobleaching due to having a shorter fluorophore lifetime. In comparison with a one-photon TIRF, the two-photon TIRF can achieve higher signal-to-noise ratio cell membrane imaging due its smaller excitation volume and lower scattering. By combining the SP enhancement and two-photon excitation TIRF, the microscope has demonstrated it's capability for brighter and more contrasted fluorescence membrane images of living monkey kidney COS-7 fibroblasts transfected with an EYFP-MEM or EGFP-WOX1 construct.

Development of mid-infrared surface plasmon resonance-based sensors with highly-doped silicon for biomedical and chemical applications

Biomedical and chemical sensors utilizing surface plasmon resonance (SPR) in the mid-infrared range were developed with the aid of highly doped silicon owing to its tailored optical constants. SPR may be excited by light incident on a periodic doping profile embedded in an intrinsic silicon film without constraints on the flow of chemical solutions or activities of biomedical samples. General guidance for tuning SPR wavelengths based on dispersion curves to catch different target materials in free space or water was also provided. The feasibility of sensors was demonstrated with a sharp spectral-directional reflectance dip, which shifted with optical constants variation. The effects of doping concentration, doping profile, and angle of incidence on sensor performance were numerically studied with a rigorous coupled-wave analysis algorithm. Developed sensors could work well for a real target and show superiority in sensitivity over existing sensors.

Perspective on optical biosensors and integrated sensor systems

Optical biosensors have begun to move from the laboratory to the point of use. This trend will be accelerated by new concepts for molecular recognition, integration of microfluidics and optics, simplified fabrication technologies, improved approaches to biosensor system integration, and dramatically increased awareness of the applicability of sensor technology to improve public health and environmental monitoring. Examples of innovations are identified that will lead to smaller, faster, cheaper optical biosensor systems with capacity to provide effective and actionable information.

Enhanced surface plasmon resonance imaging detection of DNA hybridization on periodic gold nanoposts

We explore periodic gold nanoposts as substrates for the enhanced surface plasmon resonance imaging (SPRi) detection of DNA hybridization. Rigorous coupled-wave analysis was used to model and design the nanopost-based SPRi biosensor. Arrayed gold nanoposts on gold-coated glass substrate, with various widths and periodicity, were fabricated using electron-beam lithography and characterized with scanning electron and atomic force microscopy. A scanning-angle SPRi apparatus was used to conduct the kinetic analysis of DNA hybridization on nanopost-based sensor surface and assess the corresponding SPR signal amplification. Experimental results showed that both the nanostructure size and period influenced the SPR signal enhancement; the optimized 30 nm height, 50 nm size, and 110 nm period nanoposts provided a fivefold SPR signal amplification compared with the plain 50 nm thick gold film used as control.

Nanopillar array structures for enhancing biosensing performance

Fabrication of metallic nanopillar array structures and their application as electrodes in electrochemical-based biosensors are discussed in this report. Vertically standing nanopillar array structures were fabricated using an electrodeposition technique and their electrochemical characteristics were evaluated. For possible use in biosensing applications, these standing nanopillars should have sufficient mechanical stability to sustain the capillary forces caused by the nanopillar – liquid interactions in aqueous environment and should provide increased signal response in an electrochemical process. Our results showed that the developed nanopillar arrays were mechanically stable in aqueous environments and the nanostructured electrodes exhibited increased electrochemical response compared with flat electrodes.

Enhanced live cell membrane imaging using surface plasmon-enhanced total internal reflection fluorescence microscopy

Using a total internal reflection fluorescence microscopy (TIRFM) technique to image live cells on a biosurface not only provides an enhanced understanding of cellular functions, but also improves the signal-to-noise ratio of the images. However, the intensity of the fluorescence signal must be increased if a more dynamic biomolecular imaging capability is required. Accordingly, this study presents a surface plasmon-enhanced TIRFM technique in which the fluorescence signals are enhanced via surface plasmons offered by a silver nanolayer. The developed microscopy technique is successfully applied to the real-time observation of the thrombomodulin proteins of live cell membranes. The experimental results and the simulation results demonstrate that the live cell membrane images obtained in the proposed surface plasmon-enhanced TIRFM technique are brighter by approximately one order of magnitude than those provided by conventional TIRFM.

Effect of resonant localized plasmon coupling on the sensitivity enhancement of nanowire-based surface plasmon resonance biosensors

A nanowire-based surface plasmon resonance (SPR) is investigated as a structure that offers improved sensor performance. The results calculated by rigorous coupled-wave analysis on a model using a hexanedithiol self-assembled monolayer (SAM) indicate that the resonant coupling between localized surface plasmons (LSPs) of nanowires affects the sensitivity enhancement substantially, while the LSP resonance in a single nanowire also contributes. SPR characteristics change significantly by applying a SAM, which can give rise to zero sensitivity for a given SAM. The results suggest that a properly designed nanowire-based SPR biosensor can enhance sensitivity by an order of magnitude with reasonable detection properties.

Angular-interrogation attenuated total reflection metrology system for plasmonic sensors

We develop an angular-interrogation attenuated total reflection (ATR) metrology system for three different plasmonic sensors, namely, a conventional surface plasmon resonance (SPR) device, a coupled-waveguide SPR device, and a nanoparticle-enhanced SPR device. The proposed metrology system is capable of measuring the reflectivity spectra of the transverse magnetic mode and the transverse electric mode simultaneously. Through the optimal control of the fabrication process and use of sophisticated system instrumentation, the experimental results confirm that the developed ATR system is capable of measuring the resonant angle with an angular accuracy of 10(-4) deg.

Enhanced readout signal of superresolution near-field structure disks by control of the size and distribution of metal nanoclusters

We present a study in which we enhance the carrier-to-noise ratio of a superresolution near-field structure (super-RENS) disk to read below 100-nm marks by implementing a mask layer comprising a Au nanocluster-embedded dielectric film. Various Au nanocluster-embedded mask layers are fabricated by a radio-frequency cosputtering process, and the size and distribution of Au nanoclusters are controlled. To verify the enhancement of the various films for super-RENS disk applications, the sensitivity enhancement of plasmonic gas sensing is demonstrated.