Optofluidic Sensor Research Articles

An optofluidic Bragg fiber sensor for estimating adulterants in a temperature-dependent molar fraction of hydrated mono-alcohol fuels

Since, chemically complex environments, the aroma has been a difficult task so far. Therefore, in the present communication, an optofluidic Bragg fiber artificial nose for perceiving the temperature-functional molar fraction of an adulterated binary composition of hydrated mono-alcohols is optimized and reported. The task is theoretically predicted over an optofluidic Bragg fiber sensor having geometrical defects by creating an asymmetry in mid of periodic cylindrical Bragg reflectors. In a cylindrical coordinate system, Henkel function (HF) and transfer matrix technique (TMT) are used to simulate a multilayer concentric hollow-core Bragg fiber (HCBF). The variation in refractive index (RI) of the adulterated binary mono-alcohol fuel is connected to the temperature-functional molar concentration, which is again anticipated by making use of several models, including the most suitable Dale-Gladstone, Lorentz-Lorenz, etc. A prominent sensing signal of which has the full width at half maximum (FWHM) equal to 0.1 nm is observed in the examined photonic bandgap (PBG). The signal is responsive to fluctuations in optofluidic core RI in the vicinity of a structural defect layer. The suggested sensor's temperature-dependent maximum sensitivity (due to varied weather circumstances) for ethanol fuel rather than methanol fuel is 1057.32 nm/RIU. Furthermore, the surface plasmon-based static temperature sensor is compared. Due to the smallest FWHM of output signal around 0.1 nm, other sensing performance metrics such as detection accuracy and quality parameters are also enhanced in the proposed sensor device.

Liquid Crystal Biosensors: Principles, Structure and Applications.

Liquid crystals (LCs) have been widely used as sensitive elements to construct LC biosensors based on the principle that specific bonding events between biomolecules can affect the orientation of LC molecules. On the basis of the sensing interface of LC molecules, LC biosensors can be classified into three types: LC–solid interface sensing platforms, LC–aqueous interface sensing platforms, and LC–droplet interface sensing platforms. In addition, as a signal amplification method, the combination of LCs and whispering gallery mode (WGM) optical microcavities can provide higher detection sensitivity due to the extremely high quality factor and the small mode volume of the WGM optical microcavity, which enhances the interaction between the light field and biotargets. In this review, we present an overview of the basic principles, the structure, and the applications of LC biosensors. We discuss the important properties of LC and the principle of LC biosensors. The different geometries of LCs in the biosensing systems as well as their applications in the biological detection are then described. The fabrication and the application of the LC-based WGM microcavity optofluidic sensor in the biological detection are also introduced. Finally, challenges and potential research opportunities in the development of LC-based biosensors are discussed.

High sensitivity detection of SARS-CoV-2 by an optofluidic hollow eccentric core fiber.

Since the outbreak of coronavirus disease 2019 (COVID-19), efficient real-time monitoring has become one of the challenges faced in SARS-CoV-2 virus detection. A compact all-fiber Mach-Zehnder interferometer optofluidic sensor based on a hollow eccentric core fiber (HECF) for the detection and real-time monitoring of SARS-CoV-2 spike glycoprotein (SARS-CoV-2 S2) is proposed, analyzed and demonstrated. The sensor is comprised of fusion splicing single mode fiber (SMF), hollow core fiber (HCF) and HECF. After the incident light passes through the HCF from the SMF, it uniformly enters the air hole and the suspended micrometer-scale fiber core of the HECF to form a compact all-fiber Mach-Zehnder interferometer (MZI). HECF is side polished to remove part of the cladding that the suspended fiber core can contact the external environment. Subsequently, the mouse anti SARS-CoV-2 S2 antibody is fixed on the surface of the suspended-core for the sake of achieving high sensitivity and specific sensing of SARS-CoV-2 S2. The limit of detection (LOD) of the sensor is 26.8 pM. The proposed sensor has high sensitivity, satisfactory selectivity, and can be fabricated at low cost making it highly suitable for point-of-care testing and high-throughput detection of early stage of COVID-19 infection.

Caged-Sphere Optofluidic Sensors: Whispering Gallery Resonators in Wicking Microfluidics

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.

Fast custom wavelet analysis technique for single molecule detection and identification

Many sensors operate by detecting and identifying individual events in a time-dependent signal which is challenging if signals are weak and background noise is present. We introduce a powerful, fast, and robust signal analysis technique based on a massively parallel continuous wavelet transform (CWT) algorithm. The superiority of this approach is demonstrated with fluorescence signals from a chip-based, optofluidic single particle sensor. The technique is more accurate than simple peak-finding algorithms and several orders of magnitude faster than existing CWT methods, allowing for real-time data analysis during sensing for the first time. Performance is further increased by applying a custom wavelet to multi-peak signals as demonstrated using amplification-free detection of single bacterial DNAs. A 4x increase in detection rate, a 6x improved error rate, and the ability for extraction of experimental parameters are demonstrated. This cluster-based CWT analysis will enable high-performance, real-time sensing when signal-to-noise is hardware limited, for instance with low-cost sensors in point of care environments.

Integrated microfluidic device to monitor unseen Escherichia coli contamination in mammalian cell culture

Microbial contamination in in vitro cell culture technologies is a common problem endangering the reliability of the collected data and biosafety of the biological products. Conventionally, microbial contamination can be detected by monitoring the pH and turbidity of cell culture media when using light microscope and/or molecular probes. These methods have limitations such as incompatibility with some cell lines, requirement for human intervention, and long experimental times. Here we present a new integrated microfluidic device (IMD) for the detection of the presence of Escherichia coli (E. coli) in benign prostatic hyperplasia (BPH-1) cell culture. Cell culture media acidity was monitored using an optofluidic sensor, and the cellular glucose metabolism using an electrochemical sensor having in-line continuous monitoring capability. Using this IMD, detection of a bacterial contamination at stages in which it is undetectable using a light microscope and plate counting methods is achieved. Additionally, since even the short-term (less than 8 h) presence of E. coli can change the cellular metabolism in the long term (at least up to 24 h), the use of non-invasive continuous cell culture monitoring platforms significantly reduce the risk of an undetected contamination, increasing the reliability of the conducted experiments with no interference to the cell culture process.

Perceiving in Chemically Diverse Environments: An Optofluidic Bragg Fiber Sensor for Estimating Adulterants in a Temperature-Dependent Molar Fraction of Hydrated Mono-Alcohol Fuels

Perceiving in Chemically Diverse Environments: An Optofluidic Bragg Fiber Sensor for Estimating Adulterants in a Temperature-Dependent Molar Fraction of Hydrated Mono-Alcohol Fuels

Optical and electrochemical microfluidic sensors for water contaminants: A short review

During the past few years, a large volume of freshwater has been transformed into contaminated water due to anthropogenic activities. For monitoring and tackling the issue of contamination in water, various effective and reasonable methods are available such as titrimetric analysis and chromatography analysis, etc. However, these techniques require a longer time and large sample volumes for the analysis. Therefore, to deal with such problems, a new technology, microfluidic sensing is emerging. Microfluidic sensing is an advanced and cost-effective technique and can perform real-time analysis using a very small quantity of analytes. In this review, different microfluidic technique-based electrochemical and optofluidic sensors have been discussed. Additionally, critical analysis for the implementation of such sensors has been carried out. Among different sensing approaches, the advantages of electrochemical microfluidic devices outweigh that of optical microfluidic sensors.

Photothermal microfluidic-assisted self-cleaning effect for a highly reusable SERS sensor.

The synergistic integration of optofluidic and surface enhanced Raman scattering (SERS) sensing is a new analytical technique that provides a number of unique characteristics for enhancing the sensing performance and simplifying the design of microsystems. Here, we propose a reusable optofluidic SERS sensor by integrating Au nanoisland substrate (AuNIS)-coated fiber into a microfluidic chip. Through both systematic experimental and theoretical analysis, the sensor enables efficient self-cleaning based on its optical-to-heat-hydrodynamic energy conversion property. Besides, the sensor exhibits the instrument detection limit down to 10-13mol/L and enhancement factor of 106 for Rhodamine 6G. Our optofluidic SERS sensor with such a photothermal microfluidic-assisted self-cleaning method has the advantages of portability, simple operation, and high cleaning efficiency, which will provide a new, to the best of our knowledge, concept and approach for cost-effective and reusable sensors.

Optical trapping assisted label-free and amplification-free detection of SARS-CoV-2 RNAs with an optofluidic nanopore sensor

Ultrasensitive, versatile sensors for molecular biomarkers are a critical component of disease diagnostics and personalized medicine as the COVID-19 pandemic has revealed in dramatic fashion. Integrated electrical nanopore sensors can fill this need via label-free, direct detection of individual biomolecules, but a fully functional device for clinical sample analysis has yet to be developed. Here, we report amplification-free detection of SARS-CoV-2 RNAs with single molecule sensitivity from clinical nasopharyngeal swab samples on an electro-optofluidic chip. The device relies on optically assisted delivery of target carrying microbeads to the nanopore for single RNA detection after release. A sensing rate enhancement of over 2,000x with favorable scaling towards lower concentrations is demonstrated. The combination of target specificity, chip-scale integration and rapid detection ensures the practicality of this approach for COVID-19 diagnosis over the entire clinically relevant concentration range from 104-109 copies/mL.

Free-Space Excitation of Optofluidic Devices for Pattern-Based Single Particle Detection.

Optofluidic sensors have enabled single molecule sensing using planar, waveguide dependent multi-spot fluorescence excitation. Here, we demonstrate a new approach to single-particle fluorescence sensing using free-space, top-down illumination of liquid-core antiresonant reflecting optical waveguide (ARROW) devices using two different multi-spot excitation techniques. First, the liquid core ARROW waveguide is excited with a focused beam through a slit pattern milled into an opaque aluminum film, showing comparable performance for single bead fluorescence detection as in-plane, multi-mode interference waveguide based excitation. The second top-down illumination technique images the spot pattern from a Y-splitter SiO2 waveguide chip directly onto the detection device for efficient power utilization and circumventing the need for an opaque cover, producing a further 2.7x improvement in signal-to-noise ratio. The two top-down approaches open up new possibilities for chip-based optical particle sensing with relaxed alignment tolerances.

In situ SERS detection of quinolone antibiotic residues in a water environment based on optofluidic in-fiber integrated Ag nanoparticles

In this paper, we present a microstructured optofluidic in-fiber Raman sensor for the detection of quinolone antibiotic residue in a water environment based on Ag surface-enhanced Raman scattering (SERS) substrate grown on the surface of the suspended core of micro-hollow optical fiber (MHF). Here, MHF has a special structure with a suspended core and a microchannel inside, which can become a natural in-fiber optofluidic device. Meanwhile, the self-assembled Ag SERS substrate can be grown on the suspended core's surface through chemical bonds, forming a microstructured optofluidic device with a Raman enhancement effect. Therefore, it can effectively detect the Raman signal of unlabeled trace quinolone antibiotic residue (ciprofloxacin and norfloxacin) inside the optical fiber. The results show that the ciprofloxacin and norfloxacin detection limits (LOD) are 10-10M and 10-11M, respectively. Compared with the maximum residue limit (3.01×10-7mol/L) stipulated by the European Union, the results are much lower, and an ideal quantitative relationship can be obtained within the detection range. Significantly, this study provides an in-fiber microstructured optofluidic Raman sensor for the label-free detection of quinolone antibiotic residue, which will have good development prospects in the field of antibiotic water pollution environmental detection.

Waveguide Mach-Zehnder Interferometer for Temperature and Concentration Sensing

Mach-Zehnder interferometer (MZI) based on optical waveguide is proposed and demonstrated, in which a waveguide core with a core expander and a collector is inscribed in a photoresist SU-8 film by ultrafast laser-induced two-photon polymerization (TPP) technique, whereas the cladding of the waveguide is fabricated by the standard photolithographic technique. Interference occurs in this MZI when the light is scattered into the cladding by the core expander and then collected by the core collector, in which the roles of the core expander and the collector are investigated in details. By integrating with a microchannel, a waveguide-based optofluidic MZI sensor is realized, which exhibits a redshift at a sensitivity of 0.618 nm/°C with the increase of temperature, and blueshifts at sensitivities of -0.102 nm/wt% and -0.092 nm/°Bx for concentration measurement of sodium chloride and sucrose solutions, respectively. Experimental results demonstrate the effectiveness of this sensor for temperature and concentration measurement, manifesting its promising potential for biomedical, chemical, and environmental applications.

Linear cavity tapered fiber sensor using amplified phase-shift cavity ring-down spectroscopy

Phase-shift cavity ring-down spectroscopy (PS-CRDS) enables the measurement of minute fluctuations in optical loss encountered by an intensity modulated laser beam via lock-in demodulation of the accumulated signal phase as the laser beam propagates through the cavity. A linear fiber cavity containing two highly reflective fiber Bragg gratings (FBGs) and a tapered fiber is a favorable cavity geometry for sensor applications due to its compact nature and wide availability of its components at telecom wavelengths. However, due to the optical absorption of water, usage of telecom wavelengths for sensing of aqueous solutions degrades the sensitivity. This problem can be worked around via an optical amplifier placed inside the fiber cavity, compensating for intrinsic cavity losses. In this work, we demonstrate amplified PS-CRDS using such a linear cavity tapered fiber sensor utilizing an optical amplifier that enables the use of thinner tapered fibers, thus achieving higher sensitivities. We also employ continuous laser wavelength sweeps and analyze peak-to-peak PSs of individual cavity modes instead of using laser-cavity mode locking with the Pound–Drever–Hall technique. This enables further simplification of the experimental arrangement without compromising sensor performance. To test the performance of the reported sensor, solutions of varying sucrose concentrations in deionized water were measured systematically by tracking the average peak-to-peak PS ⟨ ϕ pp ⟩ of the cavity modes. Tapered fibers with waist diameters ( D t a p e r ) around 2.2 µm were used during the experiment and limit of detection (LOD) values were measured down to 2.7 µM, corresponding to 1.01 × 10 − 7 R I U . The reported LOD values can be further improved by mechanical stabilization and thermal control of the device by the use of FBGs with higher reflectivity, by applying automatic optical gain control, and by spectral filtering to remove errors caused by amplified spontaneous emission. The presented concentration sensing device can be suitable for developing compact and highly sensitive, label-free optofluidic sensors.

In-fiber optofluidic online SERS detection of trace uremia toxin.

In this Letter, we propose a microstructured in-fiber optofluidic surface-enhanced Raman spectroscopy (SERS) sensor for the initial inspection of uremia through the detection of unlabeled urea and creatinine. As a natural microfluidic device, microstructured hollow fiber has a special structure inside. Through chemical bonds, the SERS substrate can be modified and grown on the surface of the suspended core. Here, the silver nanoparticles (Ag NPs) are embedded on the poly diallyl dimethyl ammonium chloride-modified graphene oxide sheet to achieve the self-assembled SERS substrate. The reduced distance between Ag NPs can increase the strong hot spots that generate enhanced Raman signals. Therefore, it can effectively detect the Raman signal of unlabeled trace uremic toxin analytes (urea, creatinine) inside the optical fiber. The results show that under simulated biophysical conditions, the limit detection (LOD) for urea is 10-4M and the linearity is good, especially at the clinical conventional concentration range (2.5-6.5×10-3M). In addition, the online Raman detection of creatinine aqueous solution LOD is 10-6M, which also has good linearity. Significantly, this Letter provides a microstructured optofluidic in-fiber Raman sensor for the preliminary detection of uremia, which will have good development prospects in the field of clinical biomedicine.

Analysis of integrated silicon nitride lab-on-a-chip optofluidic sensor at visible wavelength for absorbance based biosensing applications

Analysis of integrated silicon nitride lab-on-a-chip optofluidic sensor at visible wavelength for absorbance based biosensing applications

A 3D-cascade-microlens optofluidic chip for refractometry with adjustable sensitivity.

Refractive index (RI) sensing as a label-free and non-invasive method has been playing an important role in industrial metrology, biochemical detection, and environmental analysis. Due to the combined advantages of microoptics and microfluidics, optofluidic RI sensors have attracted growing interest. Despite a variety of prototypes of optofluidic RI sensors, comprehensive improvement in sensitivity, detection range, fabrication procedures and cost can still bring substantial benefits to the field. In this work, we fabricated a 3D-cascade-microlens optofluidic chip (3DCMOC) for RI sensing. Two-photon stereolithography was employed to fabricate the chip mold, with which the 3DCMOC could be easily manufactured via mold replication. By virtue of integrating four detection channels configured with different numbers (1, 3, 5, and 7) of cascaded microlenses within the 3DCMOC, adjustable sensitivity for RI sensing has been demonstrated through measuring standard sucrose solutions. It was found that the seven-microlens configuration achieved an excellent sensitivity (mean: 21 ± 5 AU·RIU (refractive index unit)-1) and resolution (mean: 3.8 × 10-5 ± 0.9 × 10-5 RIU) at a cost of a narrow linear dynamic range (LDR, 1.3326-1.3548). In contrast, the single-microlens configuration led to an extended LDR (1.3326-1.5120 tested) despite the lower sensitivity (mean: 2.6 ± 0.2 AU·RIU-1) and resolution (mean: 1.5 × 10-4 ± 0.1 × 10-4 RIU). Furthermore, the use of the 3DCMOC was investigated via real-time salinity sensing and analysis of urine specific gravity.

Highly stable and reproducible Au nanorod arrays for near-infrared optofluidic SERS sensor

Surface-enhanced Raman spectroscopy (SERS) is a sensitive vibrational spectroscopy technique that can enable fast and non-destructive detection of trace molecules. SERS substrates are critical for the advancement of the SERS application. By incorporating SERS substrates into microfluidic devices, the function of microfluidic devices can be extended, and an efficient on-site trace analysis platform with powerful sensing capabilities can be realized. In this paper, we report the fabrication of a rapid and sensitive optofluidic SERS device using a unique Au nanorod array (AuNRA) with a plasmon resonance frequency in the near IR region. The highly stable and reproducible AuNRA were fabricated by a facile dynamic oblique angle deposition technique. A typical spectrum of 4,4-bipyridine (BPY) with enhanced peaks was observed within a few seconds after the injection of an aqueous solution BPY. Time-course measurements revealed an outstandingly quick response of SERS in this system. Using the AuNRA microfluidic device, approximately 2 × 10−12mol molecules were enough to produce detectable SERS signals, and BPY was detected down to a concentration of 15.6 × 10−9 g/mL. This work demonstrates rapid and sensitive chemical sensing using an optofluidic device equipped with a unique noble metal nanorod array.

Low-cost planar waveguide-based optofluidic sensor for real-time refractive index sensing.

We report on the design, fabrication, and characterization of mass-producible, sensitive, intensity-detection-based planar waveguide sensors for rapid refractive index (RI) sensing; the sensors comprise suspended glass planar waveguides on glass substrates, and are integrated with microfluidic channels. They are facilely and cost-effectively constructed via vacuum-less processes. They yield a high throughput, enabling mass production. The sensors respond to solutions with different RIs via variations in the transmitted optical power due to coupling loss in the sensing region, facilitating real-time and simple RI detection. Experiments yield a good resolution of 5.65 × 10-4 RIU. This work has major implications for several RI-sensing-based applications.

Highly sensitive optofluidic refractive index sensor based on a seven-liquid-core Teflon-cladding fiber.

We propose and theoretically demonstrate a highly sensitive optofluidic refractive index (RI) sensor based on a spectral filter formed by a segment of liquid-filled seven-hole Teflon-cladding fiber sandwiched by two standard single mode fibers (SMFs). When liquid flows through the air hole channels of the seven-hole Teflon-cladding fiber, it forms a seven-liquid-core fiber (SLCF) and the lightwaves are well guided by the liquid cores owing to total inner reflection. When the input SMF is aligned to the central core of the SLCF, the light excited in the central core will couple to outer cores periodically along the length of the SCLF. At the detection port, the output SMF is also aligned to the central core of the SLCF. Since the coupling coefficient depends on wavelength, the coupling efficiency is also wavelength dependent, leading to a filter spectrum for a given length of the SLCF. The spectral response of the filter to the change in RI of the liquid cores is numerically simulated based on the coupled-mode theory through finite-element method. The dependence of the RI sensitivity on the diameter and pitch of air holes of the SLCF are studied, respectively. Finally, a very high sensitivity of 25,300 nm/RIU for RI around 1.333 is achieved.