Bulk Refractive Index Sensitivity Research Articles

Linear or quadratic plasmon peak sensitivities for individual Au/Ag nanosphere sensors

It is crucial to reveal the bulk (i.e. global) and local refractive index (RI) sensitivity for localized surface plasmon resonance (LSPR) sensors in order to understand their fundamental optical properties and design miniaturized plasmonic devices. In this paper, we investigate the RI sensitivity responses of individual Ag and Au nanosphere (NSP) sensors and disclose that they are featured as quadratic. The underlying reasons were well elucidated in terms of Rayleigh approximation. In addition, we have developed two different effective medium methods to calculate the LSPR peak wavelengths for individual dielectric shell coated metal NSPs. It turns out that they agree well with the results obtained by finite element method calculations for different core radius NSPs. Their sensitivity and differential sensitivity factors are discussed as well.

Polarization dependent Fano resonance in a metallic triangle embedded in split ring plasmonic nanostructures

The extinction properties of a plasmonic nanocavity consisting of a regular triangle embedded in a split ring (RTSR) plasmonic nanostructure have been investigated. Due to symmetry breaking of the RTSR cavity, we find that the extinction properties of the RTSR cavity are highly sensitive to the polarization of the incident light, resulting in strong Fano resonances. The Fano resonances originate from strong coupling between the regular triangle and the split ring. It is found that the size and relative orientation of the regular triangle are crucial parameters for the Fano resonance since these parameters finely control the strength of the electromagnetic coupling and the effective capacitance of the metallic gap region. By using combinations of different symmetrical nanostructures we can actively modulate the Fano resonance spectra in a polarization dependent manner. Calculation results show that bulk refractive index sensitivities exceeding 800 nm/RIU, with a figure of merit exceeding 6.8 in the near-infrared, are obtained with the RTSR cavity. These properties give the RTSR cavity potential applications in plasmonic Fano switches and sensing.

Silicon-on-Glass Graphene-Functionalized Leaky Cavity Mode Nanophotonic Biosensor

Subwavelength silicon nanostructures are known to support highly localized resonant optical modes. These resonances are spectrally narrow with an electromagnetic near-field that extends significantly into the surrounding medium. Here, we demonstrate that leaky cavity mode resonances (LCMR) in periodic silicon nanowire arrays can serve as a platform for low-cost, label-free, and highly sensitive biosensing and establish a theoretical framework for the LCMR phenomenon that is consistent with experimental results. The sensors exhibit bulk refractive index sensitivities up to 213 nm/RIU. Moreover, by functionalizing the surface of silicon nanostructures with a graphene monolayer, such structures can be used to optically detect low-concentration surface adsorption events. The specific label-free detection limit using immunoglobulin G protein (IgG) is found to be on the order of 300 pM with an extracted maximum sensor resonance shift of 5.42 nm. This sensing platform holds significant promise for unraveling pro...

Optimizing the Refractive Index Sensitivity of Plasmonically Coupled Gold Nanoparticles

The possibility to enhance the local refractive index sensitivity using plasmonic coupling between spherical gold nanoparticles (Au-NPs) has been investigated. A strong and distinct optical coupling between Au-NPs of various sizes was achieved by controlling the interparticle separation using a layer-by-layer assembly of polyelectrolytes. The frequency of the coupled plasmon peak could be tuned by varying either the particle size or the interparticle separation, shown both experimentally and by theoretical simulations. The bulk refractive index (RI) sensitivity for the plasmonic coupling modes was investigated and compared to the RI sensitivity of monolayers of well-separated Au-NPs, and the results clearly demonstrates that the RI sensitivity can be significantly enhanced in plasmonically coupled Au-NPs. The proposed approach is simple and scalable and improves the rather modest RI sensitivity of spherical gold nanoparticles with a factor of 3, providing a new route for fabrication of inexpensive sensors based on plasmonic nanostructures.

Gold Nanoring Arrays for Near Infrared Plasmonic Biosensing

Gold nanoring array surfaces that exhibit strong localized surface plasmon resonances (LSPR) at near infrared (NIR) wavelengths from 1.1 to 1.6 μm were used as highly sensitive real-time refractive index biosensors. Arrays of gold nanorings with tunable diameter, width, and spacing were created by the nanoscale electrodeposition of gold nanorings onto lithographically patterned nanohole array conductive surfaces over large areas (square centimeters). The bulk refractive index sensitivity of the gold nanoring arrays was determined to be up to 3,780 cm−1/refractive index unit by monitoring shifts in the LSPR peak by FT-NIR transmittance spectroscopy measurements. As a first application, the surface polymerization reaction of dopamine to form polydopamine thin films on the nanoring sensor surface from aqueous solution was monitored with the real-time LSPR peak shift measurements. To demonstrate the utility of the gold nanoring arrays for LSPR biosensing, the hybridization adsorption of DNA-functionalized gold nanoparticles onto complementary DNA-functionalized gold nanoring arrays was monitored. The adsorption of DNA-modified gold nanoparticles onto nanoring arrays modified with mixed DNA monolayers that contained only 0.5 % complementary DNA was also detected; this relative surface coverage corresponds to the detection of DNA by hybridization adsorption from a 50 pM solution.

Refractive-index sensitivities of hybrid surface-plasmon resonances for a core-shell circular silver nanotube sensor

We study the scattering and absorption of an H-polarized plane electromagnetic wave by a circular silver nanotube in the visible range of wavelengths using the separation of variables. The computed spectra of the extinction cross section display several hybrid localized surface-plasmon resonances of the dipole and multipole type. Analytical equations are derived for their resonance wavelengths. Bulk refractive-index sensitivities of nanotube-based sensors are determined, showing higher values for multipole resonances.

A Highly Sensitive Refractometric Sensor Based on Cascaded SiN Microring Resonators

We investigate a highly sensitive optical sensor based on two cascaded microring resonators exploiting the Vernier effect. The architecture consists of two microrings with a slight difference in their free spectral ranges. This allows the generation of the Vernier effect for achieving ultra-high sensitivities. The sensor chip was fabricated using a silicon nitride platform and characterized with isopropanol/ethanol mixtures. A sensitivity of 0.95 nm/% was found for isopropanol concentrations in ethanol ranging from 0% to 10%. Furthermore, a collection of measurements was carried out using aqueous sodium chloride (NaCl) in solutions of different concentrations, confirming a high sensitivity of 10.3 nm/% and a bulk refractive index sensitivity of 6,317 nm/RIU. A limit of detection of 3.16 × 10−6 RIU was determined. These preliminary results show the potential features of cascaded silicon nitride microring resonators for real-time and free-label monitoring of biomolecules for a broad range of applications.

Local Refractive Index Sensing Based on Edge Gold-Coated Silver Nanoprisms

Bulk and surface refractive index sensitivity for localized surface plasmon resonance (LSPR) sensing based on edge gold-coated silver nanoprisms (GSNPs) and gold nanospheres was investigated and compared with conventional surface plasmon resonance (SPR) sensing based on propagating surface plasmons. The hybrid GSNPs benefit from an improved stability since the gold frame protecting the unstable silver facets located at the silver nanoprisms (SNPs) edges and tips prevents truncation or rounding of their sharp tips or edges, maintaining a high refractive index sensitivity even under harsh conditions. By using layer-by-layer deposition of polyelectrolytes and protein adsorption, we found that GSNPs exhibit 4-fold higher local refractive index sensitivity in close proximity (<10 nm) to the surface compared to a flat gold film in the conventional SPR setup. Moreover, the sensitivity was 8-fold higher with GSNPs than with gold nanospheres. This shows that relatively simple plasmonic nanostructures for LSPR-based sensing can be engineered to outperform conventional SPR, which is particularly interesting in the context of detecting low molecular weight compounds where a small sensing volume, reducing bulk signals, is desired.

Surface sensitivity of Rayleigh anomalies in metallic nanogratings

Sensing schemes based on Rayleigh anomalies (RAs) in metal nanogratings exhibit an impressive bulk refractive-index sensitivity determined solely by the grating period. However, the surface sensitivity (which is a key figure of merit for label-free chemical and biological sensing) needs to be carefully investigated to assess the actual applicability of this technological platform. In this paper, we explore the sensitivity of RAs in metal nanogratings when local refractive-index changes are considered. Our studies reveal that the surface sensitivity deteriorates up to two orders of magnitude by comparison with the corresponding bulk value; interestingly, this residual sensitivity is not attributable to the wavelength shift of the RAs, which are completely insensitive to local refractive-index changes, but rather to a strictly connected plasmonic effect. Our analysis for increasing overlay thickness reveals an ultimate surface sensitivity that approaches the RA bulk value, which turns out to be the upper-limit of grating-assisted surface-plasmon-polariton sensitivities.

Search of Extremely Sensitive Near-Infrared Plasmonic Interfaces: A Theoretical Study

The sensitivity of the wavelength position of localized surface plasmon resonance (LSPR) in metal nanostructures to local changes in the refractive index has been widely used for label-free detection strategies. Tuning the optical properties of the nanostructures from the visible to the infrared region is expected to have a drastic effect on the refractive index sensitivity. Here, we theoretically investigate the optical response of a newly designed plasmonic interface to changes in the bulk refractive index by the finite difference time domain method. It consists of a structured interface, where the planar interface is superposed with dielectric pillars 30 nm in height and 125 nm in length with a separation distance of 15 nm. The pillars are covered with U-shaped gold nanostructures of 50 nm in height, 125 nm in length, and 5 nm of gold base thickness. The whole structure is finally covered with a 5-nm thick dielectric layer of n2 = 2.63. This plasmonic structure shows bulk refractive index sensitivities up to 1750 nm/RIU (RIU : refractive index unit) in the near infrared (λ = 2621 nm). The enhanced sensitivity is a consequence of the extremely enhanced electrical field between the gold nanopillars of the plasmonic interface.

Cytotoxic and genotoxic assessment of Euterpe oleracea fruit oil and Pentaclethra macroloba oil in human peripheral lymphocytes

Rapid and sensitive detections of the growth of bacteria are essential assay procedures in the food industry, water and environment control and clinical diagnosis. Here, we present a plasmonic nanohole sensor for rapid, sensitive and quantitative monitoring of the bacterial growth and antibiotic susceptibility test. The concept is based on extraordinary optical transmission (EOT) phenomenon in plasmonic nanohole and employs Escherichia coli specific antibody for bacteria capturing. The challenge lies in detecting the much larger dimension of E. coli with the smaller penetration depths of the electromagnetic field. The plasmonic nanohole array was fabricated using mask-based deep ultraviolet (DUV) lithography method and characterized for bulk refractive index sensitivity and surface mass sensitivity. After gold-specific surface modification and antibody immobilization, bacteria were effectively captured and grown on the sensor surface. The sensor platform with temperature-controlled environment was successfully tested for monitoring the growth of E. coli. The plasmonic nanohole sensor has immense potential for clinical diagnosis such as rapid determination of pathogen susceptibility test; with bulk refractive index sensitivity of 406.4 nm/RIU and surface mass sensitivity of ~1.802 ng/mm2. We explored the capability of the sensor to provide sensitive monitoring of low bacterial quantity (<100 cells) and rapid detection within 2 h. Finally, the rapid antibiotic susceptibility test (AST) of E. coli was demonstrated with the plasmonic nanohole sensor.

Highly sensitive Mach–Zehnder interferometer biosensor based on silicon nitride slot waveguide

We demonstrate a highly sensitive label-free Mach–Zehnder interferometer (MZI) biosensor based on silicon nitride slot waveguide. Unlike the conventional MZI sensors, the sensing arm of the sensor consists of a slot waveguide while the reference arm consists of a strip waveguide. Thanks to the slot waveguide's property to provide high optical intensity in a subwavelength-size low refractive index region (slot region), which allows high light–analyte interaction, higher sensitivity can be obtained as compared to conventional waveguides using the slot waveguide as sensing region. The bulk refractive index sensitivity of the slot waveguide MZI sensor was found to be 1864π/RIU (refractive index unit) with 7mm long slot waveguide sensing arm, which shows higher sensitivity compared to the conventional MZI device based on silicon nitride. The biosensing capability of the developed slot waveguide MZI was investigated using biotin–streptavidin binding as a model system. The sensitivity of the system was demonstrated down to 18.9fM or 1pg/ml of streptavidin solution and to the best of our knowledge, it is the best reported experimental value for the limit of detection of a MZI sensor. Furthermore, we investigated the specific detection and quantification of the methylation of DAPK (Death-associated protein kinase) gene, which is a widely used biomarker for human cancers. We have shown that methylation sequences of DAPK gene of various methylation densities (100%, 50%, and 0% of methylation sites) can be quantified and discriminated even at a concentration as low as 1fmol/μl or 1nM.

Enhanced refractive index sensitivity of elevated short-range ordered nanohole arrays in optically thin plasmonic Au films

A simple development of the colloidal lithography technique is demonstrated for fabrication of perforated plasmonic metal films elevated above the substrate surface. The bulk refractive index sensitivity of short-range ordered nanohole arrays in 20 nm thick Au films exhibits an increase of up to 37% due to reduction of substrate effect caused by lifting with a 40 nm silica layer. Analysis of the local electric field distribution suggests that the sensitivity increase is due to revealing of the enhanced field near the holes.

Plasmonic metal nanostructure array by glancing angle deposition for biosensing application

This paper investigates the application of gold nanostructure array fabricated on a layer of closely packed polystyrene (PS) nanospheres as a plasmonic chemical or biological sensor. The sensing chip was fabricated by first functionalizing a layer of closely packed PS nanospheres as a colloidal template, and then depositing the gold at an oblique angle, where the nanospheres shaded each other and formed a gold nanostructure on the top of the PS nanospheres. The surface profiles of the gold nanostructures on PS nanospheres were investigated by Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM). A chip fabricated at the deposition angle of 30° was found to have a high consistency for keeping the shape of the nanostructures on a large fabrication area. The sensor's bulk refractive index sensitivity was detected to be 255nm per RIU. In the study of biotin–streptavidin binding, the limit of detection (LOD) for streptavidin was 10nM, and the surface-confined thermodynamic binding constant is 3×109M−1. The plasmonic signal is amplified by using biotinylated Au colloids as labels. To further demonstrate the sensing capability, biotin–antibiotin IgG binding with a lower affinity constant was tested. The obtained LSPR peak shift caused by IgG (molecular weight 150kDa) was about 2 times larger than that by streptavidin (molecular weight 60kDa). The correlation of peak shift to the molecular weight further confirms the sensing capability of the chip for quantitative detection of proteins.

Plasmonic nanohole arrays for monitoring growth of bacteria and antibiotic susceptibility test

Rapid and sensitive detections of the growth of bacteria are essential assay procedures in the food industry, water and environment control and clinical diagnosis. Here, we present a plasmonic nanohole sensor for rapid, sensitive and quantitative monitoring of the bacterial growth and antibiotic susceptibility test. The concept is based on extraordinary optical transmission (EOT) phenomenon in plasmonic nanohole and employs Escherichia coli specific antibody for bacteria capturing. The challenge lies in detecting the much larger dimension of E. coli with the smaller penetration depths of the electromagnetic field. The plasmonic nanohole array was fabricated using mask-based deep ultraviolet (DUV) lithography method and characterized for bulk refractive index sensitivity and surface mass sensitivity. After gold-specific surface modification and antibody immobilization, bacteria were effectively captured and grown on the sensor surface. The sensor platform with temperature-controlled environment was successfully tested for monitoring the growth of E. coli. The plasmonic nanohole sensor has immense potential for clinical diagnosis such as rapid determination of pathogen susceptibility test; with bulk refractive index sensitivity of 406.4nm/RIU and surface mass sensitivity of ~1.802ng/mm2. We explored the capability of the sensor to provide sensitive monitoring of low bacterial quantity (<100cells) and rapid detection within 2h. Finally, the rapid antibiotic susceptibility test (AST) of E. coli was demonstrated with the plasmonic nanohole sensor.

Polarization analysis of an asymmetrically etched rib waveguide coupler for sensing applications

Deeply etched rib waveguides on silicon on insulator platform were not addressed well in research publications. We have analyzed single mode condition and polarization independence of a deeply etched rib waveguide (DE-RW) structure from biosensing perspective. With this rib structure, an asymmetrically etched integrated optic directional coupler has been numerically modeled to have the same coupling length for quasi- TE and TM modes. The coupling coefficients with the glucose solution as an upper cladding were calculated using a full vector mode solver, and the bulk refractive index sensitivity of the sensor was found as 28.305×10 -2 /RIU for a fundamental quasi-TE mode.

Detection of 6-benzylaminopurineplant growth regulator in bean sprouts using OFRR biosensor and QuEChERS method

We identified 6-benzylaminopurine (6-BAP) plant growth regulator using an opto-fluidic ring resonator (OFRR) biosensor and a Quick, Easy, Cheap, Effective, Rugged and Safe (QuEChERS) method. In this study, the OFRR biosensor with a bulk refractive index sensitivity (BRIS) of 14.5 nm per RIU was fabricated through the optimization process of the OFRR capillary and tapered fiber optic cable. The detection limit of the OFRR biosensor using the 6-BAP standard solution and 6-BAP sample solution extracted from bean sprouts by the QuEChERS method is 0.01 mg kg−1. This result is similar to the experimental results of HPLC and LC-MS/MS. To apply the OFRR biosensor and the QuEChERS method on the spot, improvement in the accuracy of the OFRR biosensor and the recovery of the QuEChERS method is needed.

Quantum dot–metallic nanorod sensors via exciton–plasmon interaction

We investigate quantum nanosensors based on hybrid systems consisting of semiconductor quantum dots and metallic nanorods in the near-infrared regime. These sensors can detect biological and chemical substances based on their impact on the coherent exciton–plasmon coupling and molecular resonances supported by such systems when they interact with a laser field. We demonstrate that the ultrahigh sensitivity of such molecular resonances on environmental conditions allows dramatic and nearly instantaneous changes in the total field experienced by the semiconductor quantum dot via minuscule variations of the local refractive indices of the quantum dot or nanorod. The proposed nanosensors can utilize quantum effects to control the sense (or direction) of the changes in the quantum dot emission, allowing us to have bistable switching from dark to bright states or vice versa via adsorption (or detachment) of biomolecules. These sensors can also offer detection of ultra-small variations in the local dielectric constant of the quantum dots or metallic nanorods via coherent induction of time delays in the effective field experienced by the quantum dots when the hybrid systems interact with time-dependent laser fields. This leads to unprecedented bulk refractive index sensitivities. Our results show that one can utilize quantum phase to control the coherent exciton–plasmon dynamics in these sensors such that introduction of a biomolecule can increase or decrease the time delay. These results offer novel ways to detect single biomolecules via application of quantum coherence to convert their impact into spectacular optical events.

Designing Efficient Localized Surface Plasmon Resonance-Based Sensing Platforms: Optimization of Sensor Response by Controlling the Edge Length of Gold Nanoprisms

Over the past few years, the unique localized surface plasmon resonance properties of plasmonic nanostructures have been used to design label-free biosensors. In this article, we demonstrate that it is the difference in edge length of gold nanoprisms that significantly influences their bulk refractive index sensitivity and local sensing efficiency. Nanoprisms with edge lengths in the range of 28–51 nm were synthesized by the chemical-reduction method and sensing platforms were fabricated by chemisorption of these nanoprisms onto silanized glass substrates. The plasmonic nanosensors fabricated from 28 nm edge length nanoprisms exhibited the largest sensitivity to change in bulk refractive index with a value of 647 nm/RIU. The refractive index sensitivity decreased with increasing edge length, with nanoprisms of 51 nm edge length, displaying a sensitivity of 384 nm/RIU. In contrast, we found that the biosensing efficiency of sensing platforms modified with biotin increased with increasing edge length, with sensing platforms fabricated from 51 nm edge length nanoprisms displaying the highest local sensing efficiency. The lowest concentration of streptavidin that could be measured reliably with this edge length nanoprism was 1.0 pM and the limit of detection was 0.5 pM, which is much lower than found with gold bipyramids, nanostars, and nanorods. In addition, the electromagnetic-field decay length of the sensing platforms was substantially influenced by the edge-length of the nanoprisms.

Direct laser writing for nanoporous liquid core laser sensors

We report the fabrication of nanoporous liquid core lasers via direct laser writing based on two-photon absorption in combination with thiolene-chemistry. As gain medium Rhodamine 6G was embedded in the nanoporous polybutadiene matrix. The lasing devices with thresholds of 19 µJ/mm(2) were measured to have bulk refractive index sensitivities of 169 nm/RIU at a laser wavelength of 600 nm, demonstrating strongly increased overlap of the modes with the analyte in comparison to solid state evanescent wave sensors.