Pedestal subwavelength grating metamaterial waveguide ring resonator for ultra-sensitive label-free biosensing

Dual sensitivity-enhanced microring resonance-based integrated microfluidic biosensor for Aβ42 detection

Sensitive, accurate, and straightforward biosensors are pivotal in the battle against Alzheimer's disease, particularly in light of the escalating patient population. These biosensors enable early adjunctive diagnosis, thereby facilitating prompt intervention, alleviating socioeconomic burdens, and preserving individual well-being. In this study, we introduce the development of a highly sensitive add-drop dual-microring resonant microfluidic sensing chip boasting a sensitivity of 188.11 nm/RIU, marking a significant 20.7% enhancement over single microring systems. Leveraging ultra-thin Parylene C for streamlined antibody immobilization and non-destructive removal, this platform facilitates the precise quantification of the Alzheimer's disease biomarker Aβ42. Employing an immune sensing strategy that amplifies and captures antigen signals using Au-labeled antibodies, we achieve an exceptional limit of detection of 9.02 pg/mL. The designed microring-based microfluidic biosensor chip exhibits outstanding specificity and sensitivity for Aβ42 in serum samples, offering a promising avenue for the early adjunctive diagnosis of Alzheimer's disease.

Sensitivity enhancement of bimodal waveguide interferometric sensor based on regional mode engineering

In this paper, we propose a novel bimodal waveguide based on regional mode engineering (BiMW-RME). Leveraging the orthogonality of the guided modes, the form of patterned SiO2 cladding on the bimodal waveguide can reduce the interaction between the reference mode and the analyte, thereby significantly improving sensitivity. The proposed BiMW-RME sensor experimentally demonstrates a phase sensitivity of 2766 π rad/RIU/cm and a detection limit of 2.44×1-5 RIU. The sensitivity is 2.7 times higher than that of the conventional BiMW sensor on the same SOI platform. The proposed design strategy demonstrates a significant improvement in the sensor's sensitivity, presenting a novel approach to enhancing common-path interferometric sensor performance.

Lab-on-a-chip optical biosensor platform: a micro-ring resonator integrated with a near-infrared Fourier transform spectrometer

In this paper, we demonstrated the design and experimental results of the near-infrared lab-on-a-chip optical biosensor platform that monolithically integrates the MRR and the on-chip spectrometer on the silicon-on-insulator (SOI) wafer, which can eliminate the external optical spectrum analyzer for scanning the wavelength spectrum. The symmetric add-drop MRR biosensor is designed to have a free spectral range (FSR) of ∼19 nm and a bulk sensitivity of ∼73 nm/RIU; then the drop-port output resonance peaks are reconstructed from the integrated spatial-heterodyne Fourier transform spectrometer (SHFTS) with the spectral resolution of ∼3.1 nm and the bandwidth of ∼50 nm, which results in the limit of detection of 0.042 RIU.

Label-free optical biosensing: going beyond the limits

Label-free optical biosensing holds great promise for a variety of applications in biomedical diagnostics, environmental and food safety, and security. It is already used as a key tool in the investigation of biomolecular binding events and reaction constants in real time and offers further potential additional functionalities and low-cost designs. However, the sensitivity of this technology does not match the routinely used but expensive and slow labelling methods. Therefore, label-free optical biosensing remains predominantly a research tool. Here we discuss how one can go beyond the limits of detection provided by standard optical biosensing platforms and achieve a sensitivity of label-free biosensing that is superior to labelling methods. To this end we review newly emerging optical implementations that overcome current sensitivity barriers by employing novel structural architectures, artificial materials (metamaterials and hetero-metastructures) and using phase of light as a sensing parameter. Furthermore, we elucidate the mechanism of plasmonic phase biosensing and review hyper-sensitive transducers, which can achieve detection limits at the single molecule level (less than 1 fg mm-2) and make it possible to detect analytes at several orders of magnitude lower concentrations than so far reported in literature. We finally discuss newly emerging layouts based on dielectric nanomaterials, bound states in continuum, and exceptional points.

A point-of-care biosensor for rapid detection and differentiation of COVID-19 virus (SARS-CoV-2) and influenza virus using subwavelength grating micro-ring resonator

In the context of continued spread of coronavirus disease 2019 (COVID-19) caused by SARS-CoV-2 and the emergence of new variants, the demand for rapid, accurate, and frequent detection is increasing. Moreover, the new predominant strain, Omicron variant, manifests more similar clinical features to those of other common respiratory infections. The concurrent detection of multiple potential pathogens helps distinguish SARS-CoV-2 infection from other diseases with overlapping symptoms, which is significant for providing tailored treatment to patients and containing the outbreak. Here, we report a lab-on-a-chip biosensing platform for SARS-CoV-2 detection based on the subwavelength grating micro-ring resonator. The sensing surface is functionalized by specific antibody against SARS-CoV-2 spike protein, which could produce redshifts of resonant peaks by antigen-antibody combination, thus achieving quantitative detection. Additionally, the sensor chip is integrated with a microfluidic chip featuring an anti-backflow Y-shaped structure that enables the concurrent detection of two analytes. In this study, we realized the detection and differentiation of COVID-19 and influenza A H1N1. Experimental results indicate that the limit of detection of our device reaches 100 fg/ml (1.31 fM) within 15 min detecting time, and cross-reactivity tests manifest the specificity of the optical diagnostic assay. Furthermore, the integrated packaging and streamlined workflow facilitate its use for clinical applications. Thus, the biosensing platform presents a promising approach for attaining highly sensitive, selective, multiplexed, and quantitative point-of-care diagnosis and distinction between COVID-19 and influenza.

The Design of a Glycerol Concentration Sensor Based on an LRSPP Hybrid Photonic Biosensor

A refractive index sensor based on an on-chip silicon nitride (Si₃N₄) ridge waveguide long-range surface plasmon polariton (LRSPP) is theoretically designed. The waveguide sensor consists of a gold film to enable the plasmonic resonance on top of a Cytop polymer layer. A proper finite element method was used to design and optimize the geometric parameters at the optical wavelength of 633 nm. In addition, the spectral performance was evaluated using the transfer matrix method from 580 to 680 nm. The redshifted interference spectrum results from an increasing analyte refractive index. The sensitivities of 6313 dB/cm/RIU and 251.82 nm/RIU can be obtained with a 400 nm wide and 25 nm thick Au layer. The proposed sensor has the potential for point-of-care applications considering its compactness and simplicity of construction.

Optimization of bulk sensitivity for strip, slot, and subwavelength grating-based waveguides for dual-polarization operation

We propose a dual-polarization optimization method for the bulk sensitivity of silicon-on-insulator (SOI) waveguides by defining a multi-objective function that accounts for the substrate leakage losses. The proposed optimization method was used to design micro-ring resonator bulk sensors with strip, slot, subwavelength grating, and subwavelength grating slot waveguides. The subwavelength grating slot waveguide has a bulk sensitivity of 520 nm/RIU and 325 nm/RIU for the TE and TM modes, respectively, both of which are higher than the bulk sensitivities of strip, slot, and subwavelength grating waveguides. Moreover, our Monte Carlo analysis shows that the subwavelength grating slot waveguide has the highest immunity to fabrication errors.

An Optimization Framework for Silicon Photonic Evanescent-Field Biosensors Using Sub-Wavelength Gratings

Silicon photonic (SiP) evanescent-field biosensors aim to combine the information-rich readouts offered by lab-scale diagnostics, at a significantly lower cost, and with the portability and rapid time to result offered by paper-based assays. While SiP biosensors fabricated with conventional strip waveguides can offer good sensitivity for label-free detection in some applications, there is still opportunity for improvement. Efforts have been made to design higher-sensitivity SiP sensors with alternative waveguide geometries, including sub-wavelength gratings (SWGs). However, SWG-based devices are fragile and prone to damage, limiting their suitability for scalable and portable sensing. Here, we investigate SiP microring resonator sensors designed with SWG waveguides that contain a "fishbone" and highlight the improved robustness offered by this design. We present a framework for optimizing fishbone-style SWG waveguide geometries based on numerical simulations, then experimentally measure the performance of ring resonator sensors fabricated with the optimized waveguides, targeting operation in the O-band and C-band. For the O-band and C-band devices, we report bulk sensitivities up to 349 nm/RIU and 438 nm/RIU, respectively, and intrinsic limits of detection as low as 5.1 × 10^-4 RIU and 7.1 × 10^-4 RIU, respectively. This performance is comparable to the state of the art in SWG-based sensors, positioning fishbone SWG resonators as an attractive, more robust, alternative to conventional SWG designs.

Highly Sensitive Plasmonic Waveguide Biosensor Based on Phase Singularity-Enhanced Goos-Hänchen Shift

The detection for small molecules with low concentrations is known to be challenging for current chemical and biological sensors. In this work, we designed a highly sensitive plasmonic biosensor based on the symmetric metal cladding plasmonic waveguide (SMCW) structure for the detection of biomolecules. By precisely designing the configuration and tuning the thickness of the guiding layer, ultra-high order modes can be excited, which generates a steep phase change and a large position shift from the Goos−Hänchen effect (with respect to refractive index changes). This position shift is related to the sharpness of the optical phase change from the reflected signal of the SPR sensing substrate and can be directly measured by a position sensor. Based on our knowledge, this is the first experimental study done using this configuration. Experimental results showed a lateral position signal change > 90 µm for glycerol with a sensitivity figure-of-merit of 2.33 × 104 µm/RIU and more than 15 µm for 10−4 M biotin, which is a low molecular weight biomolecule (less than 400 Da) and difficult to be detected with traditional SPR sensing techniques. Through integrating the waveguide with a guiding layer, a strong improvement in the electric field, as well as sensitivity have been achieved. The lateral position shift has been further improved from 14.17 µm to 284 µm compared with conventional SPR substrate with 50 nm gold on single side. The as-reported sensing technique allows for the detection of ultra-small biological molecules and will play an important role in biomedical and clinical diagnostics.

Fast, accurate, point-of-care COVID-19 pandemic diagnosis enabled through advanced lab-on-chip optical biosensors: Opportunities and challenges

The sudden rise of the worldwide severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic in early 2020 has called into drastic action measures to perform instant detection and reduce the rate of spread. Common clinical and nonclinical diagnostic testing methods have been partially effective in satisfying the increasing demand for fast detection point-of-care (POC) methods to slow down further spread. However, accurate point-of-risk diagnosis of this emerging viral infection is paramount as the need for simultaneous standard operating procedures and symptom management of SARS-CoV-2 will be the norm for years to come. A sensitive, cost-effective biosensor with mass production capability is crucial until a universal vaccination becomes available. Optical biosensors can provide a noninvasive, extremely sensitive rapid detection platform with sensitivity down to ∼67 fg/ml (1 fM) concentration in a few minutes. These biosensors can be manufactured on a mass scale (millions) to detect the COVID-19 viral load in nasal, saliva, urine, and serological samples, even if the infected person is asymptotic. Methods investigated here are the most advanced available platforms for biosensing optical devices that have resulted from the integration of state-of-the-art designs and materials. These approaches include, but are not limited to, integrated optical devices, plasmonic resonance, and emerging nanomaterial biosensors. The lab-on-chip platforms examined here are suitable not only for SARS-CoV-2 spike protein detection but also for other contagious virions such as influenza and Middle East respiratory syndrome (MERS).

Greatly Enhanced Single Particle Fluorescence Detection Using High Refractive Index Liquid-Core Waveguides

High sensitivity and easy integration with microfabrication techniques has made silicon photonics one of the leading technologies used to build biosensors for diagnostic applications. Here we introduce a new silicon dioxide based optofluidic platform having a planar solid-core (SC) waveguide orthogonally intersecting a liquid-core (LC) waveguide with high refractive index ZnI2 salt solution as core. This enables both more uniform collection of particle fluorescence by the core mode and its propagation to an off-chip detector. This approach results in ultra-high sensitivity performance, demonstrated by achieving 8X enhancement in signal-to-noise ratio, a 45x increase in detection efficiency, and a 100x lower detection limit of 80 aM of fluorescent nanobeads. This represents a key step towards an ultrasensitive biosensor system for analyzing pathogens at clinical concentrations.

Narrowband Bragg filters based on subwavelength grating waveguides for silicon photonic sensing

Subwavelength grating (SWG) waveguides have been shown to provide enhanced light-matter interaction resulting in superior sensitivity in integrated photonics sensors. Narrowband integrated optical filters can be made by combining SWG waveguides with evanescently coupled Bragg gratings. In this paper, we assess the sensing capabilities of this novel filtering component with rigorous electromagnetic simulations. Our design is optimized for an operating wavelength of 1310 nm to benefit from lower water absorption and achieve narrower bandwidths than at the conventional wavelength of 1550 nm. Results show that the sensor achieves a sensitivity of 507 nm/RIU and a quality factor of 4.9 × 10⁴, over a large dynamic range circumventing the free spectral range limit of conventional devices. Furthermore, the intrinsic limit of detection, 5.1 × 10^-5 RIU constitutes a 10-fold enhancement compared to state-of-the-art resonant waveguide sensors.

Portable Automatic Microring Resonator System Using a Subwavelength Grating Metamaterial Waveguide for High-Sensitivity Real-Time Optical-Biosensing Applications

The slow light sensor techniques have been applied to bio-related detection in the past decades. However, similar testing-systems are too large to carry to a remote area for diagnosis or point-of-care testing. This study demonstrated a fully automatic portable biosensing system based on the microring resonator. An optical-fiber array mounted on a controller based micro-positioning system, which can be interfaced with MATLAB to locate a tentative position for light source and waveguide coupling alignment. Chip adapter and microfluidic channel could be packaged as a product such that it is cheap to be manufactured and can be disposed of after every test conducted. Thus, the platform can be more easily operated via an ordinary user without expertise in photonics. It is designed based on conventional optical communication wavelength range. The C-band superluminescent-light-emitting-diode light source couples in/out the microring sensor to obtain quasi-TE mode by grating coupler techniques. For keeping a stable chemical binding reaction, the cost-effective microfluidic pump was developed to offer a specific flow rate of 20 μL/min by using a servo-motor, an Arduino board, and a motor driver. The subwavelength grating metamaterial ring resonator shows highly sensitive sensing performance via surface index changes due to biomarker adhered on the sensor. The real-time peak-shift monitoring shows 10 μg/mL streptavidin detection of limit based on the biotin-streptavidin binding reaction. Through the different specific receptors immobilized on the sensor surface, the system can be utilized on the open applications such as heavy metal detection, gas sensing, virus examination, and cancer marker diagnosis.

High-performance sub-wavelength grating-based resonator sensors with substrate overetch

We propose a strategy to improve sensing performance of sub-wavelength-grating (SWG) waveguide-based sensors by introducing a substrate-overetch (SOE) geometry. The proposed SOE-SWG waveguide shows enhanced analyte interaction and a reduced group index, which improves the sensitivity of resonator-based sensors. The SiO₂ overetch process was realized in Ar/C₄F₈/O₂ plasma for 40 sec with a SiO₂/Si selectivity of 10:1, obtaining a 285-nm anisotropic overetch in the SiO₂ layer. Sensor performance of the SOE-SWG architecture is characterized by using isopropyl alcohol solutions, indicating an enhanced bulk sensitivity up to 575 nm/RIU (479 nm/RIU before the SOE), and a maximum waveguide mode sensitivity larger than one.