Surface Refractive Index Research Articles

An optical sensor combining surface plasmon resonance, light extinction, and near-critical angle reflection, for thin liquid film biochemical sensing

We present a novel and low-cost optical sensor to obtain at the same time, fast and label-free optical information such as reflectivity, refractive index and extinction coefficient of biochemical samples. The sensor measures in around one minute optical internal reflectivity as a function of the angle of incidence at the interface between a glass prism covered by a gold nanofilm (≈20 nm) in contact with a liquid sample (≈10 µL). The nanofilm is thin enough that a large fraction of the incident light transmits through it at angles of incidence below the critical angle set by the prism and sample refractive indices (≈ 61°), but thick enough to present a Surface Plasmon Resonance dip for angles of incidence beyond that critical angle. The calculated sensitivity of SPR to the surface refractive index is 168.35 deg/RIU. We demonstrate that when a sample contains live bacteria (E. coli) the whole reflectivity curve including the SPR region is clearly affected, and to adjust a theoretical model to the experimental curves, it is necessary to assume a complex refractive index of the nanofilm appreciably different than that of gold. The optical curve of the sensor provides more information than a conventional SPR measurements offering a more complete strategy for the analysis of biochemical substances and bacterial cell suspensions.

Nanoplasmonic Sensing Using Gold Nanostructures

Nanoplasmonic sensing, based on the plasmonic resonance absorption of thin, irregularly-shaped Au nanostructures film, with a starting thickness of about 15 nm (±3 nm) sputtered on a quartz substrate, is used to monitor the CeO2 NPs (with an average diameter of 50 nm) film refractive index variations using different film thicknesses (90 nm, 146 nm, 172 nm, and 196 nm). Increasing the film thickness of solution-processed CeO2 NPs film, with layer-by-layer deposition on top of Au nanostructures, shows a significant redshift in the plasmonic resonance absorption of the plasmonic metal, from 580 nm to 611 nm. Such an increase is related to the change in the building microstructure of the semiconductor’s film which is reflected in changing its refractive index. Plasmonic surface refractive index sensitivity of 437.5 nm/RIU with FOM of 4.2 has been recorded. Such a sensing technique offers a large potential for developing cost-effective plasmonic nanosensing devices for clinical applications. This sensor structure is versatile and can be utilized to sense and monitor a large variety of materials and chemicals.

Direct and Sensitive Detection of Dopamine Using Carbon Quantum Dots Based Refractive Index Surface Plasmon Resonance Sensor.

Abnormality of dopamine (DA), a vital neurotransmitter in the brain’s neuronal pathways, causes several neurological diseases. Rapid and sensitive sensors for DA detection are required for early diagnosis of such disorders. Herein, a carbon quantum dot (CQD)-based refractive index surface plasmon resonance (SPR) sensor was designed. The sensor performance was evaluated for various concentrations of DA. Increasing DA levels yielded blue-shifted SPR dips. The experimental findings revealed an excellent sensitivity response of 0.138°/pM in a linear range from 0.001 to 100 pM and a high binding affinity of 6.234 TM−1. The effects of varied concentrations of DA on the optical characteristics of CQD thin film were further proved theoretically. Increased DA levels decreased the thickness and real part of the refractive index of CQD film, according to fitting results. Furthermore, the observed reduction in surface roughness using AFM demonstrated that DA was bound to the sensor layer. This, in turn, explained the blue shift in SPR reflectance curves. This optical sensor offers great potential as a trustworthy solution for direct measurement due to its simple construction, high sensitivity, and other sensing features.

Integration of a multichannel surface plasmon resonance sensor chip and refractive index matching film array for protein detection in human urine

Most prism-based surface plasmon resonance (SPR) experiments use matching fluid with a similar refractive index to mount the chip on the optical prism. The fluidity of the matching fluid easily affects the transmission of the optical signal. In this paper, an integrated SPR sensor chip comprises a three-layer structure of flow layer, metal layer and refractive index matching layer is demonstrated to address the problems related to consistency and uniformity. The Young's modulus, array spacing, shape and other parameters of the matching film were calculated and optimized. The chip can self-adhere to the optical prism, and effectively avoids the generation of air bubbles. The refractive index detection sensitivity of the integrated SPR sensor chip was 3.4359 × 10−6 RIU (refractive index unit), and the chip stabilization time has been effectively shortened. The integrated SPR sensor chip was also used to detect kappa light chain protein and human serum albumin (HSA) in urine samples. The detection limit of kappa light chain protein was 0.06 μg/mL compared with 18.5 μg/mL by conventional immunoturbidimetry. The integrated SPR sensor chip based on refractive index matching film array has great potential in biomedical detection and other fields, including point-of-care testing (POCT).

Thickness dependence of the response of metal-dioxide-based micromechanical sensors for sensitive gamma-ray detection

In this study, we investigated the effect of the thickness of metal-dioxide thin films on silicon (Si) micro-electromechanical systems (MEMS) for sensitive, accurate, and reproducible detection of gamma-radiation dose. Silicon wafers and microcantilevers were coated with various thicknesses of titanium dioxide (TiO2) thin films and controlled by the number of cycles in atomic layer deposition (ALD) under 17-mbar pressure and temperature of 200 °C. All samples were exposed to different doses of gamma (0, 10, and 20 kGy) using a60Co source. Before and after gamma irradiation, the optical and mechanical properties of the TiO2 thin films on Si wafers were studied by spectroscopic ellipsometry (SE) and atomic force microscopy (AFM). The resonance frequency shift (RFS) resulting from exposing different thicknesses of TiO2 thin films on MEMS-based cantilevers to gamma-radiation doses were evaluated by AFM. SE results revealed the film thicknesses of 11.91, 21.77, 62.91, and 218.23 nm at 250, 500, 1250, and 2500 coating cycles, respectively. Through SE, other optical constants, such as surface roughness (SR) and refractive index (n), were obtained. The root-mean-square (RMS) SR obtained from AFM images and SE measurements on Si wafers showed the same behavior under gamma radiation according to the TiO2 thin-films thicknesses. The RFS results show that the best film thickness for reproducible and sensitive gamma-radiation detection was 218.23 nm. The frequency shift as the gamma dose increase from 0 to 20 kGy was ∼60 Hz. It is a very palpable and linear shift; hence, it can be used as a dosimeter sensor. The results were verified by the statistical correlation coefficient method to find the correlation between RFS and the gamma dose at different film thicknesses. Correlation results are consistent with other results.

Partially gold-coated tilted FBGs for enhanced surface biosensing.

To date, there is clear experimental evidence that gold-coated tilted fiber Bragg gratings (TFBGs) are highly sensitive plasmonic biosensors that provide temperature-compensated detection of analytes at concentrations in the picomolar range. As most optical biosensors, they bring an evanescent wave in the surrounding medium, which makes them sensitive to both surface refractive index variations (= the useful biosensing signal) and to bulk refractive index changes (= the non-useful signal for biosensing). This dual sensitivity makes them prone to drift. In this work, we study partially gold-coated TFBGs around their cross-section. These gratings present the ability to discriminate both volume and surface refractive index changes, which is interesting in biosensing to enhance the signal-to-noise ratio. The effects induced in the TFBGs transmitted amplitude spectra were analyzed for surrounding refractive index (SRI) changes in the range 1.3360-1.3370. Then, the gold film was biofunctionalized with human epidermal growth factor receptor (HER2) aptamers using thiol chemistry. The detection of HER2 proteins (a relevant cancer biomarker) at 10-9 g/mL, 10-8 g/mL and 10-6 g/mL demonstrated the advantage to identify environmental perturbations through the bare area of the TFBGs, which is left not functionalized. The non-specific drifts that could exist in samples are eliminated and a wavelength shift only related to the surface modification is obtained.

Millimeter wave beam‐tilted antenna using 1D‐phase gradient metasurface

In this article, radiation beam deflection of a mm-Wave antenna is presented using a metasurface (MTS). The beam tilting MTS consists of phase gradient resonators to transform an incoming wavefront to a tilted planar wavefront. The MTS is fed via gain enhanced dipole antenna having a tilted radiation beam. The gain of the dipole antenna is enhanced by printing a high refractive index surface in front of the radiator. The MTS is then loaded vertically in front of the gain enhanced dipole antenna using a 3D printed stand. The prototype antenna covers 27–30 GHz band with a tilted radiation beam. A beam tilt of 20° is achieved using the MTS with a peak gain of 10 dB over the frequency band.

Surface Tension and Refractive Index Measurement Based on Tapered Microfiber

Surface Tension and Refractive Index Measurement Based on Tapered Microfiber

Synthesis of low viscosity of polymethylhydrosiloxane using monomer of dichloromethylsilane

Polydimethylsiloxane (PDMS) is a vitreous humour substitutes in vitreoretinal surgery to treat retinal detachment. PDMS was synthesized from the monomer octamethylcyclotetrasiloxane (D4). However, D4 is difficult to obtain in Indonesia. So, we need an alternative to replace PDMS. One type of siloxane polymer that can be a potential substitute as PDMS is polymethylhydrosiloxane (PMHS). In this study, PMHS was obtained from the hydrolysis-condensation process of dichloromethylsilane (DCHS). The hydrolysis process produces monomer that is ready for the condensation process to produce PMHS. Monomer and polymer samples were characterized to determine viscosity, surface tension, refractive index, and functional groups. The monomer sample has a viscosity in the range of 28.7-36.4 mPa.s. The surface tension and refractive index are in the range of 17-18 mN/m and 1.3954-1.3958. The polymer sample has a viscosity of 560-1130 mPa.s with a surface tension of 19 mN/m and a refractive index in the range of 1.3966-1.3970. From the FTIR measurements, all major functional groups in both monomers and polymers of PDMS were observed with the addition of Si-H functional groups which characterize PMHS. These results indicate that PMHS with low viscosity has been successfully obtained by hydrolysis- condensation method of DCHS.

Measuring the refractive index and sub-nanometre surface functionalisation of nanoparticles in suspension.

Direct measurements to determine the degree of surface coverage of nanoparticles by functional moieties are rare, with current strategies requiring a high level of expertise and expensive equipment. Here, a practical method to determine the ratio of the volume of the functionalisation layer to the particle volume based on measuring the refractive index of nanoparticles in suspension is proposed. As a proof of concept, this technique is applied to poly(methyl methacrylate) (PMMA) nanoparticles and semicrystalline carbon dots functionalised with different surface moieties, yielding refractive indices that are commensurate to those from previous literature and Mie theory. In doing so, it is demonstrated that this technique is able to optically detect differences in surface functionalisation or composition of nanometre-sized particles. This non-destructive and rapid method is well-suited for in situ industrial particle characterisation and biological applications.

Research advances on surface plasmon resonance biosensors.

The surface plasmon resonance (SPR) phenomenon is of wide interest due to its sensitivity to changes in surface refractive index for the label-free, highly sensitive and rapid detection of biomarkers. This paper reviews research progress on SPR biosensors modified with different substrate structures and surface materials, surface plasmon resonance imaging (SPRI), and SPR-enhanced electrochemiluminescent (ECL) biosensors for applications in biosensing in the last five years. This paper focuses on the research on the application of the SPR phenomenon in the field of bio-detection, reviews the sensing characteristics of SPR biosensors with substrate structures of prisms, gratings, and optical fibers, and summarizes and analyzes the sensitivity and interference resistance of SPR sensors with surface modification of different materials (high-refractive index dielectric films, metallic micro- and nanostructures, and surface antifouling materials). Considering that imaging is an important tool for biomedical detection, this paper reviews the research progress on SPRI technology in the field of biomedical detection. In addition, this paper also reviews the research progress on SPR-enhanced ECL biosensors in the field of biosensing. Finally, this paper provides an outlook on the development trends of biosensing technology in terms of portable high-precision SPR sensors, reduction of self-loss of thin film materials, optimization of image processing techniques and simplification of electrode modification for ECL sensors.

Maximizing the Surface Sensitivity of LSPR Biosensors through Plasmon Coupling-Interparticle Gap Optimization for Dimers Using Computational Simulations.

The bulk and surface refractive index sensitivities of LSPR biosensors, consisting of coupled plasmonic nanosphere and nano-ellipsoid dimers, were investigated by simulations using the boundary element method (BEM). The enhancement factor, defined as the ratio of plasmon extinction peak shift of multi-particle and single-particle arrangements caused by changes in the refractive index of the environment, was used to quantify the effect of coupling on the increased sensitivity of the dimers. The bulk refractive index sensitivity (RIS) was obtained by changing the dielectric medium surrounding the nanoparticles, while the surface sensitivity was modeled by depositing dielectric layers on the nanoparticle in an increasing thickness. The results show that by optimizing the interparticle gaps for a given layer thickness, up to ~80% of the optical response range of the nanoparticles can be utilized by confining the plasmon field between the particles, which translates into an enhancement of ~3–4 times compared to uncoupled, single particles with the same shape and size. The results also show that in these cases, the surface sensitivity enhancement is significantly higher than the bulk RI sensitivity enhancement (e.g., 3.2 times vs. 1.8 times for nanospheres with a 70 nm diameter), and thus the sensors’ response for molecular interactions is higher than their RIS would indicate. These results underline the importance of plasmonic coupling in the optimization of nanoparticle arrangements for biosensor applications. The interparticle gap should be tailored with respect to the size of the used receptor/target molecules to maximize the molecular sensitivity, and the presented methodology can effectively aid the optimization of fabrication technologies.

Organic dye concentration monitoring through an optical microfiber enabled by multiwalled carbon nanotubes

A tapered microfiber sensor based on multiwalled carbon nanotubes (MWCNTs) for detecting the concentration of alizarin red S dye is proposed and verified by experiments. MWCNTs fixed on the surface of optical fiber can adsorb the alizarin red S dye molecule due to its porous structure. The surface refractive index (RI) change caused by MWCNTs coated with the fiber surface adsorbing of red dye can be captured by optical microfiber, which converted into macroscopic wavelength drift in the interference spectrum. The sensor has a high RI sensitivity of 1942.25 nm/RIU and a temperature sensitivity of − 0.03 n m / ∘ C , which has a low temperature cross-sensitivity. The sensor provides rapid detection of alizarin red S dye in the concentration range of 0–1000 mg/L, and the sensitivity is 0.412 nm/(mg/L) in the low concentration range of 0–25 mg/L. The sensor is expected to be a high-quality platform for safety considerations in environmental protection.

Determination of the refractive index profile and surface topography of optically smooth objects using interference of optical vortices

In this paper, we present the results of the propagational dynamics of vortex beams in the scope of their possible applications for interferometric non-contact robust and precision optical surface profilometry with nanoscale longitudinal resolution. The result of coaxial superposition of the reference plane wave with singly charged vortex beams represents a dynamically changing intensity distribution. The nature of this changes, namely, rotational effects of intensity zeros, allows to determine directly the optical path difference which is introduced by the surfaces and internal structure of test object. We have proposed the experimental setup for examination of reflecting and transmitting objects.

Surface refractive index sensor based on titanium dioxide composite thin film for detection of cadmium ions

In this work, a refractive index sensor using TiO2 extracted from moringa leaves-based nanocomposite thin film was developed to detect cadmium ions in an aqueous solution. The experimental results showed a good sensitivity response of 0.4825° ppm−1, a strong binding affinity of 1.4362 ppm−1, and a low signal-to-noise ratio of 0.0755. The proposed sensor also showed good selectivity towards cadmium ions compared to zinc and mercury ions. The changes in the optical properties of titanium dioxide-based nanocomposite thin film towards different concentrations of cadmium ions solution were further demonstrated theoretically. Also, the observed increase in the surface roughness using atomic force microscopy proved the binding of Cd ions to the sensor film. This study is proposing a new potential of titanium dioxide extracted from moringa leaves-based nanocomposite thin film as a surface refractive index sensor for the detection of cadmium ions.

Vector Magnetometer Based On Localized Scattering Between Optical Fiber Spectral Combs and Magnetic Nanoparticles

The apparent increase in magnetic field measurement has led to a growing demand for new sensing technologies. However, a long-lasting challenge unaddressed is how to achieve ultrahigh sensitive magnetic field measurement in both amplitude and orientation. Here, we demonstrate a compact-in-size and simple-to-implement fiber-optic magnetometer for highly sensitive vector magnetic field measurement. The sensor is based on magnetic nanoparticles coated tilted fiber Bragg grating inscribed in a commercial single mode fiber. The sensing mechanism stems from the backward coupled cladding mode resonances with narrow bandwidth (Q factor > 10⁴), which is highly sensitive to slight surface refractive index perturbation. The switching of the magnetic field direction on the surface of fiber induces redistribution of coated magnetic nanoparticles with spatial anisotropy density and scattering by formation of magnetic chains hereafter the effective index is changed, which in turn modulates the cladding mode resonance with wavelength shift and amplitude attenuation. Through calculating the peak-to-peak intensity of target cladding modes of TFBG, the external magnetic field was precisely measured with an intensity sensitivity of 0.39 dB/mT and a direction sensitivity of 0.038 dB/rad. The inherent core mode can be used as a reference to get rid of temperature crosstalk and light source disturbance. The proposed sensor is compact in size and easy to be used in hard-to-reach place, showing good potentials for industrial applications.

Large-area patterning of full-color quantum dot arrays beyond 1000 pixels per inch by selective electrophoretic deposition

Colloidal quantum dot (QD) emitters show great promise in the development of next-generation displays. Although various solution-processed techniques have been developed for nanomaterials, high-resolution and uniform patterning technology amicable to manufacturing is still missing. Here, we present large-area, high-resolution, full-color QD patterning utilizing a selective electrophoretic deposition (SEPD) technique. This technique utilizes photolithography combined with SEPD to achieve uniform and fast fabrication, low-cost QD patterning in large-area beyond 1,000 pixels-per-inch. The QD patterns only deposited on selective electrodes with precisely controlled thickness in a large range, which could cater for various optoelectronic devices. The adjustable surface morphology, packing density and refractive index of QD films enable higher efficiency compared to conventional solution-processed methods. We further demonstrate the versatility of our approach to integrate various QDs into large-area arrays of full-color emitting pixels and QLEDs with good performance. The results suggest a manufacture-viable technology for commercialization of QD-based displays.

Discrimination of Bulk and Surface Refractive Index Change in Plasmonic Sensors with Narrow Bandwidth Resonance Combs.

A method to enable surface plasmon resonance (SPR) sensors to discriminate between bulk and surface-localized refractive index changes is demonstrated with modified gold-coated tilted fiber Bragg grating SPR sensors (TFBG-SPR). Without this capability, all high-resolution SPR sensors should be using reference channels and strict temperature control to prevent the contamination of the desired detection of surface-localized chemical or binding events by drift of the refractive index of the medium, in which the experiment is carried out. The very fine comb of high-quality-factor resonances of a TFBG-SPR device coupled to the large differential sensitivity of some of the resonances to various perturbations is used to measure unambiguously the refractive index changes within a surface layer thinner than 25 nm from those of the bulk surrounding. The enabling modification of the conventional TFBG-SPR is a reduction of the gold coating from its optimum value near 50-30 nm: at this lower thickness, a surface plasmon wave can still be excited by a limited number of cladding mode resonances, but at the same time, the metal is thin enough to allow modes away from the SPR to tunnel across the metal and probe the bulk RI value. Measurements and simulations of the deposition of a self-assembled monolayer of 1-dodecanethiol in ethanol show that the bulk refractive index changes as small as 0.0004 can be distinguished from the formation of a 1 nm thick coating on the surface of the fiber.

Critical angle reflection imaging for quantification of molecular interactions on glass surface

Quantification of molecular interactions on a surface is typically achieved via label-free techniques such as surface plasmon resonance (SPR). The sensitivity of SPR originates from the characteristic that the SPR angle is sensitive to the surface refractive index change. Analogously, in another interfacial optical phenomenon, total internal reflection, the critical angle is also refractive index dependent. Therefore, surface refractive index change can also be quantified by measuring the reflectivity near the critical angle. Based on this concept, we develop a method called critical angle reflection (CAR) imaging to quantify molecular interactions on glass surface. CAR imaging can be performed on SPR imaging setups. Through a side-by-side comparison, we show that CAR is capable of most molecular interaction measurements that SPR performs, including proteins, nucleic acids and cell-based detections. In addition, we show that CAR can detect small molecule bindings and intracellular signals beyond SPR sensing range. CAR exhibits several distinct characteristics, including tunable sensitivity and dynamic range, deeper vertical sensing range, fluorescence compatibility, broader wavelength and polarization of light selection, and glass surface chemistry. We anticipate CAR can expand SPR′s capability in small molecule detection, whole cell-based detection, simultaneous fluorescence imaging, and broader conjugation chemistry.

Prediction of thermal conductivity of micro/nano porous dielectric materials: Theoretical model and impact factors

With the stupendous latent of microscale and nanoscale technologies in energy conversion and utilization, the design and analysis of porous dielectric materials with open cells have required a more accurate calculation of the radiative thermal conductivity. This work introduces a mathematical model to accurately calculate the radiative thermal conductivity of micro/nanoscale porous open cell structures. Due to the limitations of the existing radiative thermal conductivity models, a full-scale method based on the Rosseland diffusion equation is proposed. Combining this full-scale Rosseland diffusion equation and fractal thermal conduction methods, the predicted total thermal conductivity values were well matched with the experimental results for various microscale and nanoscale porous open cell dielectric materials, with less than 15% error. Besides, seven influential factors on the thermal conductivity including cell size, porosity, cellular pore shape, volume specific surface area, temperature, refractive index, and extinction index were extensively investigated. The results show that the thermal conductivity of porous open cell materials mainly decreased with an increase in extinction index and/or the porous structure's volume specific surface area but increased with increase in temperature. This certainly indicated the potential of the full-scale Rosseland diffusion method for use in the design of specific micro/nanoscale porous dielectric structures like polymer foam in the personal energy management device or the silica aerogel in radiative cooling system.