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The rapid development of optofluidic technologies in recent years has seen the need for sensing platforms with ease-of-use, simple sample manipulation, and high performance and sensitivity. Herein, an integrated optofluidic sensor consisting of a pillar array-based open microfluidic chip and caged dye-doped whispering gallery mode microspheres is demonstrated and shown to have potential for simple real-time monitoring of liquids. The open microfluidic chip allows for the wicking of a thin film of liquid across an open surface with subsequent evaporation-driven flow enabling continuous passive flow for sampling. The active dye-doped whispering gallery mode microspheres placed between pillars, avoid the use of cumbersome fibre tapers to couple light to the resonators as is required for passive microspheres. The performance of this integrated sensor is demonstrated using glucose solutions (0.05–0.3 g/mL) and the sensor response is shown to be dynamic and reversible. The sensor achieves a refractive index sensitivity of ~40 nm/RIU, with Q-factors of ~5 × 103 indicating a detection limit of ~3 × 10−3 RIU (~20 mg/mL glucose). Further enhancement of the detection limit is expected by increasing the microsphere Q-factor using high-index materials for the resonators, or alternatively, inducing lasing. The integrated sensors are expected to have significant potential for a host of downstream applications, particularly relating to point-of-care diagnostics. [ABSTRACT FROM AUTHOR]

We propose and demonstrate an improved multimode interference (MMI) structure based on a spider dragline silk (SDS) for relative humidity (RH) sensing. We configure the MMI structure by connecting a section of single-mode fiber, a section of no-core fiber (NCF), and a section of single-mode fiber sequentially, and then obtain the interference spectrum. We spirally wrap the dragline silk on the NCF to improve the MMI and help the interference dips to be significant and identifiable. The refractive index and diameter of the SDS will change due to the change of the RH around the SDS, which will cause the improved interference dips to shift. The experimental results indicate that the average sensitivity of the proposed SDS-based sensor is 1.15 nm/%RH in the range of 33%RH-98%RH and the maximum sensitivity is 2.02 nm/%RH in the range of 83% RH-98%RH. The larger the RH, the larger the sensitivity. The proposed sensor also performs good repeatability with a fluctuation of less than 2.6% and a response time of 558ms. Spider dragline silk has proven itself the right candidate for RH sensing application and promises a way to fabricate environmental-friendly, bio-absorbable and biocompatible protein-based devices for biochemical sensing that uses natural materials as their constituent. [ABSTRACT FROM AUTHOR]

Abstract Biocompatible-sensing materials hold an important role in biomedical applications where there is a need to translate biological responses into electrical signals. Increasing the biocompatibility of these sensing devices generally causes a reduction in the overall conductivity due to the processing techniques. Silicon is becoming a more feasible and available option for use in these applications due to its semiconductor properties and availability. When processed to be porous, it has shown promising biocompatibility; however, a reduction in its conductivity is caused by its oxidization. To overcome this, gold embedding through sputtering techniques are proposed in this research as a means of controlling and further imparting electrical properties to laser induced silicon oxide nanofibers. Single crystalline silicon wafers were laser processed using an Nd:YAG pulsed nanosecond laser system at different laser parameters before undergoing gold sputtering. Controlling the scanning parameters (e.g., smaller line spacings) was found to induce the formation of nanofibrous structures, whose diameters grew with increasing overlaps (number of laser beam scanning through the same path). At larger line spacings, nano and microparticle formation was observed. Overlap (OL) increases led to higher light absorbance’s by the wafers. The gold sputtered samples resulted in greater conductivities at higher gold concentrations, especially in samples with smaller fiber sizes. Overall, these findings show promising results for the future of silicon as a semiconductor and a biocompatible material for its use and development in the improvement of sensing applications.

We analyze the response of surface plasmon (SP) sensors using a transmission line model. We illustrate this analysis with particular reference to a layered structure in which plasmon hybridization occurs. By applying the appropriate resonant condition to the system, we derive a circuit model which predicts the responsivity of different modes. This gives new physical insight into the sensing process. We discuss how the change in the sample region may be modeled as a change in the reactance in the equivalent circuit and from this, it follows that a single parameter can determine the change in resonance position with reactance. This approach is used to predict the response of a generic sensor to binding of an analyte and the bulk change of refractive index. This parameter arises naturally from the circuit representation in a way not readily accessible with the transfer matrix approach. The parameters can be expressed in terms of the ${Q}$ of a resonant circuit and confirms the intuition that a high ${Q}$ is associated with poor responsivity, however, we demonstrate that there is another circuit parameter, the resistance at resonance, that can mitigate this effect, providing a route for optimization of the sensor properties. [ABSTRACT FROM AUTHOR]

In this paper, we report how tube lattice hollow-core fibers can be successfully used to build sensors for molecule detection. The inner silica surface of the fiber is functionalized and coated with a probe layer, which permits to bond only with a particular molecule (the target). When the fiber is infiltrated with a solution containing the target, an additional layer is created on the silica surface, causing a redshift of the fiber transmission spectrum. The technique does not require any additional transducer component, such as Bragg gratings, amplifying techniques, such as nano-particles, nor coherent sources. It simply consists of the measurement of the transmission spectrum of a piece of fiber some tens of centimeters long. The principle is validated with experimental results showing the detection of the streptavidin protein. A solution containing streptavidin was flowed through the hollow core of the fiber coated with biotin. The measurement of the transmitted spectrum before and after the infiltration showed the presence of a few nanometer thick bio-layer. [ABSTRACT FROM AUTHOR]

A phenylboronic-acid-derivatized poly (acrylic acid) (PAA-PBA)/poly(vinyl alcohol) (PVA) functionalized thin-core fiber (TCF) optical sensor is proposed for glucose detection. The unique Michelson-type fiber interferometer sensor was fabricated by splicing a short segment of TCF with a single-mode fiber, where the tip of TCF was melted into a rounded shape. PAA-PBA was synthesized by grafting PBA onto the PAA chain using N-hydroxysuccinimide and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride as coupling reagent. Multilayer films of PAA-PBA and PVA were subsequently functionalized onto the fiber-optic Michelson interferometer structure through layer-by-layer assembly. Experiments showed that the PAA-PBA/PVA functionalized fiber-optic probe exhibits glucose-sensitive behavior under physiological pH. The proposed sensor offers benefits such as simple fabrication, compact size, and glucose selectivity and has potential applications, such as insulin release system, in a biodevice. [ABSTRACT FROM AUTHOR]

Surface Plasmon Resonance (SPR) biosensors are widely used for real-time label-free detection in medical diagnostics, pharmaceutical researches and food safety. Although there is a growing interest in miniaturization of biosensors for self-detection and diagnostics at out of laboratory, the performance of conventional metal SPR sensors is still limited. In this paper, we propose graphene–MoS 2 – metal hybrid structures based plasmon sensors under the best minimum light intensity approach, which represents the performance analysis in case of the lowest reflected light strength. It is demonstrated that the metal thickness can be reduced from 55 nm to 32 nm and 37 nm meanwhile the performance of the background sensor can be improved by 87% and 13% with the 4 additional MoS 2 and graphene layers, respectively. We show that MoS 2 based SPR devices provide much better sensitivity performance than graphene based devices. Our results reveal the another promising property of MoS 2 : The sensitivity of SPR sensors can be greatly increased with a few number of MoS 2 within the angular SPR system while reducing the size of the device, especially for particular applications such as detecting a single molecule and biosensing at low biomolecule concentration. Furthermore, we show that the equivalent optical properties of multilayered nanostructures also depend on the layer thickness which is a novel knowledge for the next studies on 2D material based SPR plasmonic devices. [ABSTRACT FROM AUTHOR]

We proposed a compact and cost-effective red-green dual-color fiber optic surface plasmon resonance (SPR) sensor based on the smartphone. Inherent color selectivity of phone cameras was utilized for realtime monitoring of red and green color channels simultaneously, which can reduce the chance of false detection and improve the sensitivity. Because there are no external prisms, complex optical lenses, or diffraction grating, simple optical configuration is realized. It has a linear response in a refractive index range of 1.326 to 1.351 (R2 = 0.991) with a resolution of 2.3 × 10-4 RIU. We apply it for immunoglobulin G (IgG) concentration measurement. Experimental results demonstrate that a linear SPR response was achieved for IgG concentrations varying from 0.02 to 0.30 mg/ml with good repeatability. It may find promising applications in the fields of public health and environment monitoring owing to its simple optics design and applicability in real-time, label-free biodetection. [ABSTRACT FROM AUTHOR]

Several fabrication techniques are recently used to produce a nanopattern for sensing, as focused ion beam milling (FIB), e-beam lithography (EBL), nanoimprinting, and soft lithography. Here, interference lithography is explored for the fabrication of large area nanohole arrays in metal films as an efficient, flexible, and scalable production method. The transmission spectra in air of the 1 cm2 substrate were evaluated to study the substrate behavior when hole-size, periodicity, and film thickness are varied, in order to elucidate the best sample for the most effective sensing performance. The efficiency of the nanohole array was tested for bulk sensing and compared with other platforms found in the literature. The sensitivity of ~1000 nm/RIU, achieved with an array periodicity in the visible range, exceeds near infrared (NIR) performances previously reported, and demonstrates that interference lithography is one of the best alternative to other expensive and time-consuming nanofabrication methods.

We performed numerical analysis and design parameter optimization of a silicon-based grating waveguide refractive index (RI) sensor. The performance of the grating waveguide RI sensor was determined by the full-width at half-maximum (FWHM) and the shift in the resonance wavelength in the transmission spectrum. The transmission extinction, a major figure-of-merit of an RI sensor that reflects both FWHM and resonance shift performance, could be significantly improved by the proper determination of three major grating waveguide parameters: duty ratio, grating period, and etching depth. We analyzed the transmission characteristics of the grating waveguide under various design parameter conditions using a finite-difference time domain method. We achieved a transmission extinction improvement of >26 dB under a given bioenvironmental target change by the proper choice of the design procedure and parameters. This design procedure and choice of appropriate parameters would enable the widespread application of silicon-based grating waveguide in high-performance RI biochemical sensor. [ABSTRACT FROM AUTHOR]

Biocompatible-sensing materials hold an important role in biomedical applications where there is a need to translate biological responses into electrical signals. Increasing the biocompatibility of these sensing devices generally causes a reduction in the overall conductivity due to the processing techniques. Silicon is becoming a more feasible and available option for use in these applications due to its semiconductor properties and availability. When processed to be porous, it has shown promising biocompatibility; however, a reduction in its conductivity is caused by its oxidization. To overcome this, gold embedding through sputtering techniques are proposed in this research as a means of controlling and further imparting electrical properties to laser induced silicon oxide nanofibers. Single crystalline silicon wafers were laser processed using an Nd:YAG pulsed nanosecond laser system at different laser parameters before undergoing gold sputtering. Controlling the scanning parameters (e.g., smaller line spacings) was found to induce the formation of nanofibrous structures, whose diameters grew with increasing overlaps (number of laser beam scanning through the same path). At larger line spacings, nano and microparticle formation was observed. Overlap (OL) increases led to higher light absorbance's by the wafers. The gold sputtered samples resulted in greater conductivities at higher gold concentrations, especially in samples with smaller fiber sizes. Overall, these findings show promising results for the future of silicon as a semiconductor and a biocompatible material for its use and development in the improvement of sensing applications. [ABSTRACT FROM AUTHOR]

Although, using cut-out I-II and H-II structures, it has been proved that symmetry broken is necessitous to have a plasmonically induced reflectance (PIR) but it is also possible to create PIR effect in a symmetric cut-out H-II structure. In this paper, in addition to reaffirming the possibility of creating PIR effect in symmetric structure using I-II structure, it is also proved, for both the I-II and H-II structures, that the created PIR effect in symmetric case can be vanished by breaking the symmetry. The created PIR effect in the two I-II and H-II structures will be compared in different situations. [ABSTRACT FROM AUTHOR]

In this paper, we report a novel wavelength interrogation-based surface plasmon resonance (SPR) system, in which a film of three Ag layers and three Au layers are alternately deposited on a Kretschmann configuration as sensing element. This multilayer film shows higher sensitivity for refractive index (RI) measurement by comparing with single Au layer structure, which is consistent with its theoretical calculation. A sensitivity range of 2056-5893 nm/RIU can be achieved, which is comparable to RI sensitivities of other wavelength-modulated SPR sensors. Compared with Ag film, this Ag/Au multilayer arrangement offers anti-oxidant protection. This SPR biosensor based on a cost-effective Ag/Au multilayer structure is applicable to the real-time detection of specific interactions and dissociation of low protein concentrations. To extend the application of this highly-sensitive metal film device, we integrated this concept on an optical fiber. The range of RI sensitivities with Ag/Au multilayer was 1847-3309 nm/RIU. This miniaturized Ag/Au multilayer-based fiber optic sensor has a broad application in chemical and biological sensing. [ABSTRACT FROM AUTHOR]

We demonstrate a tapered optical fiber interferometer integrated electrophoretic trace mixture separating and on-line detecting device. In this design, we first introduce tapered optical fiber interferometer into electrophoresis system to dynamically on-line monitor the separation of the proteins. The tapered optical fiber interferometer is embedded into the separating medium of the electrophoresis system. The mixture of the proteins with negative charge driven by electric field in the separation gel can be separated and contact with the embedded fiber taper. When the separated components pass through the taper, the refractive index (RI) of the gel around the taper was dynamically adjusted and the interference spectra will present a reversible shift. To visually display the on-line separation and detection process, two kinds of simple proteins of Bovine serum albumin (BSA) and Chicken egg albumin (CEA) labeled with fluorescence indicator are adopted as the analytes in the device. Results show that a trace amount of the proteins (5 μl) can be completely separated in the separation gel. Simultaneously, when each component contacts with the tapered region of the optical fiber, interference spectra immediately presents an obvious change because of the dynamic change of the refractive index of the transparent polymer gel. In addition, the complex fixing and developing steps in traditional electrophoresis analysis can be replaced by on-line detection through interference spectra. Then, simultaneous separating treatment and dynamic detection of the proteins are first realized in this experiment. Significantly, this fiber taper integrated separating and on-line detecting device can find great potentials in various analysis fields involves composite samples such as DNA separation and even the virus and cell research. [ABSTRACT FROM AUTHOR]

The blue OLED emission from a mobile phone was characterised, revealing a sharp emission band centred at λ = 445 nm with a 3dB bandwidth Δλ ~ 20 nm. It was used to excite Rhodamine 123 doped within a “giant” mesostructured silica sphere during fabrication through evaporative self-assembly of silica nanoparticles. Fluorescence was able to be detected using a standard optical microscope fitted with a green transmission pass filter and cooled CCD and with 1 ms exposure time demonstrating the potential of mobile platforms as the basis for portable diagnostics in the field.

Plasmonic metamaterials support the localized surface plasmon resonance (LSPR), which is sensitive to the change in the dielectric environment and highly desirable for ultrasensitive biochemical sensing. In this work, a novel design of supercell metamaterials of four mutually rotating split ring resonators (SRRs) is proposed, where simultaneous excitations of odd ( N = 1 and N = 3) and even ( N = 2) resonance modes are realized due to additional asymmetry from the rotation and show insensitivity to two orthogonal polarizations. The full utilization of these three resonance dips show bright prospects for multispectral application. As a refractive index (RI) sensor, ultrahigh sensitivities ∼1000 nm/RIU for LC mode ( N = 1) and ∼500 nm/RIU for plasmon mode ( N = 2) are obtained in the near infrared (NIR) spectrum. [ABSTRACT FROM AUTHOR]

We investigate an intensity and wavelength modulation combined plasmon resonance-based fiberoptic sensor technology. Composed of gold nanoparticles (GNPs) and sandwich configuration of Au/indium tin oxide (ITO)/Au film, two sensing regions are fabricated separately along with unclad portions of the fiber-optic probe. It can simultaneously monitor both the light intensity from the Au NP channel and the wavelength from the Au/ITO/Au film channel with a single detector. As the refractive index (RI) of the external environment changes, the transmission intensity and resonance wavelength in the two channels are modified, which provides an interrogation of intensity and wavelength modulation. The sandwich film structure is formed using magnetron sputtering technology, and the GNPs functioning as localized surface plasmon resonators are coated on a multimode optical fiber via the layer-by-layer method. The experimental results reveal that the RI sensitivities of the two sensing channels are 334.1% RIU-1 and 1963.2 nm/RIU, respectively. Based on the above sensing design, we conduct real-time and label-free monitoring of IgG/anti-IgG and Con A/RNase B biomolecular interaction. The resonant dips excited by different sensing modes make it more attractive as a multichannel surface plasmon resonance analysis technology, which is valuable in biological and life sciences research and rapid diagnostics. [ABSTRACT FROM AUTHOR]

We present the discrete dipole approximation simulation of light scattering from bead enhanced diffraction biosensor and report the effect of bead material, number of beads forming the grating and spatial randomness on the diffraction intensities of 1st and 0th orders. The dipole models of gratings are formed by volume slicing and image processing while the spatial locations of the beads on the substrate surface are randomly computed using discrete probability distribution. The effect of beads reduction on far-field scattering of 632.8 nm incident field, from fully occupied gratings to very coarse gratings, is studied for various bead materials. Our findings give insight into many difficult or experimentally impossible aspects of this genre of biosensors and establish that bead enhanced grating may be used for rapid and precise detection of small amounts of biomolecules. The results of simulations also show excellent qualitative similarities with experimental observations. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate a nanostructure sensor based on hexagonal arrays of annular aperture operating in the near-infrared wavelength range. The strong coupling interaction between propagating surface plasmons (PSP) mode and localized surface plasmons (LSP) mode in the designed structure generates two sharp spectral features under normal incidence. The mode coupling strongly enhances the electromagnetic fields and increases the interaction volume of the analyte and optical field. A high refractive index sensitivity of 623 nm/RIU is demonstrated in a wide refractive index range of 1.33 to 1.40. Due to the excitation of sharp spectral feature, as narrow as 7 nm, high figure of merits of 93 was obtained in the refractive index range, which is nearly 10 times larger than that from hole arrays and disk arrays. Furthermore, sharp spectral feature in the designed structure provides more error margin for structure parameters, which is advantageous for experimental realization of systems without requiring challenging fabrication resolution. The sensor is promising for biosensing applications with high sensitivity and low limit of detection. [ABSTRACT FROM AUTHOR]

We have developed an efficient numerical method to investigate light scattering from plasmonic nanospheres on a substrate covered by a shell, based on the half-space Green's function approach. We use this method to study optical scattering from DNA molecules attached to metallic nanoparticles on a substrate and compare with the experiment. We obtain fairly good agreement between theoretical predictions and the measured ellipsometric spectra. The metallic nanoparticles were used to detect the binding with DNA molecules in a microfluidic setup via spectroscopic ellipsometry (SE), and a detectable change in ellipsometric spectra was found when DNA molecules are captured on a Au nanoparticle surface. Our theoretical simulation indicates that the coverage of the Au nanosphere by a submonolayer of DNA molecules, which is modeled by a thin layer of dielectric material, can indeed lead to a small but detectable change in ellipsometric spectra. Our studies demonstrated the ultrasensitive capability of SE for sensing the submonolayer coverage of DNA molecules on Au nanospheres. [ABSTRACT FROM AUTHOR]

We numerically demonstrated the characteristics of a functional hydrogel layer on a silicon-based grating waveguide for a simple, cost-effective refractive index (RI) biochemical sensor. The RI of the functional hydrogel layer changes when a specific biochemical interaction occurs between the hydrogel-linked receptors and injected ligand molecules. The transmission spectral profile of the grating waveguide shifts depends on the amount of RI change caused by the functional layer. Our characterization includes the effective RI change caused by the thickness, functional volume ratio, and functional strength of the hydrogel layer. The results confirm the feasibility of, and set design rules for, hydrogel-assisted silicon-based grating waveguides.

In this paper, the coupling interaction is investigated between a metallic nanowire array and a metal film under the Kretschmann condition. The plasmonic multilayer is composed of a metallic nanowire array embedded in a polymer layer positioned above a metal film, exploiting the classical surface plasmon resonance (SPR) configuration. We analyze the influence of various structural parameters of the metallic nanowire array on the SPR spectrum of thin metal film. The results show that the coupling interactions of nanowires with the metal film can greatly affect SPR resonance wavelength and increase SPR sensitivity. The coupling strength of metallic nanowire array and metal film also impacts resonance wavelength, which can be used to adjust SPR range but have little effect on its sensitivity. The results are confirmed using a dipole coupling resonance model of metallic nanowire. We demonstrated that this nanostructured hybrid structure can be used for high sensitivity SPR monitoring in a large spectral range, which is important for advanced SPR measurement including fiber-optic SPR sensing technology. [ABSTRACT FROM AUTHOR]

We present a systematic analytic and numerical study of the detection limit of a refractive index sensor employing a directional coupler architecture within a photonic crystal fiber (PCF). The device is based on the coupling between the core mode and a copropagating mode of a satellite waveguide formed by a single hole of the PCF infiltrated by a high-index analyte. Using coupled mode theory as well as full simulations, we investigate the influence of changes in the geometrical parameters of the PCF and the analyte's refractive index on sensor performance, including sensitivity, resonance width, and detection limit. We show that regardless of the details of the sensor's implementation, the smallest detectable refractive index change is inversely proportional to the coupling length and the overlap integral of the satellite mode with the analyte, so that best performance comes at the cost of long analyte infiltration lengths. This is experimentally confirmed in our dip sensor configuration, where the lowest detection limit achievable for realistic implementation is estimated to 7 × 10 ^-8 refractive index units (RIU) based on realistic signal to noise ratios in a commercially available PCF. [ABSTRACT FROM PUBLISHER]

We investigate an optically resonant cavity which is created between a reflecting micro-cantilever and a diffraction grating etched into a silicon waveguide. Changes in cavity resonance, induced by small deflections of the micro-cantilever result in large changes in an optical signal transmitted through the waveguide. An analytical model can predict the cantilever position for maximum and minimum transmission and is confirmed by three-dimensional finite difference time domain (FDTD) simulations. This approach can be used to accurately determine the position of a micro-cantilever with a predicted optimal shot noise limited deflection noise density of 4.1 fm/ Hz. [ABSTRACT FROM PUBLISHER]

In this work we consider a refractometer based on a Michelson interferometer (MI) integrated with a Fabry-Perot interferometer (FPI). In one arm of the MI there is a moving mirror and in the second arm there is an FPI. As the mirror is scanned, we get the interferogram of the first harmonic and the peak position is extracted for the calculation the refractive index. Then we study the characteristics of this refractometer. We discuss different parameters of such a refractometer. We consider peak position and resolution as a function of refractive index, center wavelength, bandwidth and reflection coefficient of the plates of the FPI. [ABSTRACT FROM AUTHOR]

Heavy metals, such as lead ions, are regarded as the main environmental contaminants and have a negative impact on human bodies, making detection technologies of lead ions critical. However, most existing detection methods suffer from time consumption, complicated sample pretreatment, and expensive equipment, which hinder their broad use in real-time detection. Herein, we show a new fluorescence sensor for detecting lead ions derived from liquid crystals doped with an aggregation-induced emission luminogen. The mechanism is based on the variation of fluorescence intensity caused by the disturbance of an ordered liquid crystal configuration in the presence of Pb 2+ , induced by DNAzyme and its catalytic cleavage. The proposed fluorescence sensor exhibits a low detection limit of 0.65 nM, which is 2 orders of magnitude lower than that previously reported in an optical sensor based on liquid crystals. The detection range of the Pb 2+ fluorescence sensor is broad, from 20 nM to 100 μM, and it also selects lead ions from numerous metal ions exactly, resulting in a highly sensitive, highly selective, simple, and low-cost detection strategy of Pb 2+ with potential applications in chemical and biological fields. This approach to designing a liquid crystal fluorescence sensor offers an inspiring stage for detecting biomacromolecules or other heavy metal ions by varying decorated molecules.

Several fabrication techniques are recently used to produce a nanopattern for sensing, as focused ion beam milling (FIB), e-beam lithography (EBL), nanoimprinting, and soft lithography. Here, interference lithography is explored for the fabrication of large area nanohole arrays in metal films as an efficient, flexible, and scalable production method. The transmission spectra in air of the 1 cm2 substrate were evaluated to study the substrate behavior when hole-size, periodicity, and film thickness are varied, in order to elucidate the best sample for the most effective sensing performance. The efficiency of the nanohole array was tested for bulk sensing and compared with other platforms found in the literature. The sensitivity of ~1000 nm/RIU, achieved with an array periodicity in the visible range, exceeds near infrared (NIR) performances previously reported, and demonstrates that interference lithography is one of the best alternative to other expensive and time-consuming nanofabrication methods. [ABSTRACT FROM AUTHOR]