Optofluidic Sensor Research Articles

Sensitivity enhancement of a fiber-based interferometric optofluidic sensor.

Optofluidic sensors, which tightly bridge photonics and micro/nanofluidics, are superior candidates in point-of-care testing. A fiber-based interferometric optofluidic (FIO) sensor can detect molecular biomarkers by fusing an optical microfiber and a microfluidic tube in parallel. Light from the microfiber side coupled to the microtube leads to lateral localized light-fluid evanescent interaction with analytes, facilitating sensitive detection of biomolecules with good stability and excellent portability. The determination of the sensitivity with respect to the interplay between light and fluidics, however, still needs to be understood quantitatively. Here, we theoretically and experimentally investigate the relationship between refractive index (RI) sensitivity and individual geometrical parameters to determine the lateral localized light-fluid evanescent interaction. Theoretical analysis predicted a sensitive maximum, which could be realized by synergically tuning the fiber diameter d and the tube wall thickness t at an abrupt dispersion transition region. As a result, an extremely high RI sensitivity of 1.6×104 nm/RIU (σ=4074 nm/RIU), an order of magnitude higher than our previous results, with detection limit of 3.0×10-6 RIU, is recorded by precisely governing the transverse geometry of the setup. The scientific findings will guide future exploration of both new light-fluid interaction devices and biomedical sensors.

An optofluidic Young interferometer sensor for real-time imaging of refractive index in μTAS applications

This report describes a Young interferometer (YI) sensor for real-time imaging of refractive index (RI) along the length of microchannels. The YI comprised a unique combination of Fresnel biprism and cylindrical lens to obtain two wedge shaped light beams, which ultimately overlapped to produce the interference fringes. The measured temporal and spatial resolution of the reported YI is 2 s and 295 μm respectively. The RI resolution per mm of the optical pathlength of the YI sensor is 2.04 × 10−6 while providing information on spatial RI distribution in real-time. The ability of the reported YI to image RI in real-time was used to monitor electrokinetic transport of a model protein, bovine serum albumin (BSA), in agarose filled channels of a microfluidic device. This method is particularly suitable to planar μTAS devices as it is free-space, senses through the entire depth of the channel and requires no coupling devices such as gratings or prisms.

Optofluidic in-fiber on-line ethanol sensing based on graphene oxide integrated hollow optical fiber with suspended core

Abstract In this study, a novel in-fiber optofluidic trace ethanol sensor is proposed firstly. The microstructured hollow fiber (MHF) with a suspended core is a key part of the overall device which is integrated with graphene oxide (GO). The GO can be uniformly trapped on the whole surface of the suspended core in the MHF by using evanescent field inducing method. When trace microfluidic ethanol passes through the in-fiber device, the light intensity of the suspended core can be significantly modulated through the interaction between the GO on the core and ethanol. The device presents an excellent linearity on-line response with an average sensitivity of 0.16 dB/% with linear regression equation of y = 0.16x + 25.989. In general, this compact optofluidic in-fiber trace ethanol sensor can be utilized as for on-line detection of trace amounts of ethanol in special environments.

Self-powered microfluidic pump using evaporation from diatom biosilica thin films

In recent years, researchers have successfully applied diatom biosilica to molecular detection platforms including Surface-Enhanced Raman Scattering (SERS) optofluidic sensors that are currently capable of detecting a variety of biological and chemical molecules at concentrations as low as 10−10 M. This study investigates the feasibility of an SERS device that couples the sensing and pumping capabilities of diatom biosilica thin films by determining flow rate limitations and stability. In this paper, we quantify the ability of porous diatom biosilica thin films to continuously pump deionized (DI) water from a reservoir via wicking flow by utilizing the strong capillary forces of the porous film coupled with evaporation. Our microfluidic device is comprised of a narrow horizontal reservoir fixed to a horizontal capillary whose end contacts a diatom biosilica film. Flow rates were controlled by altering the size and/or temperature of the biosilica porous film, determined by tracking the liquid meniscus displacement in the reservoir, and correlated with a modified laminar boundary-layer model. System stability was observed by tracking flow rates over the course of a given experiment, image analysis of the meniscus contacting the film, and a flow duration study. We found that for untreated DI water bubbles begin to form in the capillary tube at temperatures above 40 °C, but degassed water remains stable at temperatures of 90 °C and below. The pumping capabilities of the films ranged from 0.11 to 10.46 µL/min, matched theoretical predictions, demonstrated stable flow trends, and maintained flow for over 48 h.

A Portable and Accurate Phosphate Sensor Using a Gradient Fabry-Pérot Array.

Here, a portable and accurate phosphate sensor using a gradient Fabry-Pérot array (FPA) is proposed. It can form a bidirectional gradient concentration (absorbance) distribution in the gradient FPA, simplifying the complex operations to get a standard curve and saving time. The gradient FPA can effectively filter out the interference (bubbles, light intensity, and salinity) while improving the absorbance, achieving a highly accurate and stable detection. Besides, the smartphone simplifies data processing and makes sensors more portable. In this work, the detection errors of standard solutions (100, 50, and 30 μM) are 0.39, 1.48, and 1.84%, respectively, and it has also been demonstrated with errors of 2.46 (sample 1, seawater), 2.08 (sample 2, lake water), and 1.83% (sample 3, sewage) for natural samples detection, which is more accurate than a traditional analyzer. The sensor has a good performance when affected by bubbles, light intensity, and salinity. In addition, the detection time is shortened to 80 s, which is more time saving compared with traditional devices, and the limit of detection (LOD) is 0.4 μM. It can be predicted that the novel optofluidic sensor is conducive to build a smart nutrient monitoring system and will be applied in the field of biochemistry and environmental chemistry.

Structural Stability of Optofluidic Nanostructures in Flow-Through Operation.

Optofluidic sensors based on periodic arrays of subwavelength apertures that support surface plasmon resonance can be employed as both optical sensors and nanofluidic structures. In flow-through operation, the nanoapertures experience pressure differences across the substrate in which they are fabricated, which imposes the risk for structural failure. This work presents an investigation of the deflection and structural stability of nanohole array-based optofluidic sensors operating in flow-through mode. The analysis was approached using experiments, simulations via finite element method, and established theoretical models. The results depict that certain areas of the sensor deflect under pressure, with some regions suffering high mechanical stress. The offset in the deflection values between theoretical models and actual experimental values is overturned when only the effective area of the substrate, of 450 µm, is considered. Experimental, theoretical, and simulation results suggest that the periodic nanostructures can safely operate under trans-membrane pressures of up to 20 psi, which induce deflections of up to ~20 μm.

3D Hydrodynamic Focusing in Microscale Optofluidic Channels Formed with a Single Sacrificial Layer.

Optofluidic devices are capable of detecting single molecules, but greater sensitivity and specificity is desired through hydrodynamic focusing (HDF). Three-dimensional (3D) hydrodynamic focusing was implemented in 10-μm scale microchannel cross-sections made with a single sacrificial layer. HDF is achieved using buffer fluid to sheath the sample fluid, requiring four fluid ports to operate by pressure driven flow. A low-pressure chamber, or pit, formed by etching into a substrate, enables volumetric flow ratio-induced focusing at a low flow velocity. The single layer design simplifies surface micromachining and improves device yield by 1.56 times over previous work. The focusing design was integrated with optical waveguides and used in order to analyze fluorescent signals from beads in fluid flow. The implementation of the focusing scheme was found to narrow the distribution of bead velocity and fluorescent signal, giving rise to 33% more consistent signal. Reservoir effects were observed at low operational vacuum pressures and a balance between optofluidic signal variance and intensity was achieved. The implementation of the design in optofluidic sensors will enable higher detection sensitivity and sample specificity.

Combining whispering gallery mode optofluidic microbubble resonator sensor with GR-5 DNAzyme for ultra-sensitive lead ion detection

Lead ions are deleterious pollutants that often reach drinking water, and can cause significant harm to humans (particularly children). An ultra-sensitive lead ion detection method using a whispering gallery mode (WGM) optofluidic microbubble resonator and the classic GR-5 DNAzyme is proposed in this paper. With the auxiliary piranha and Ploy-l-lysine solution, GR-5 DNAzyme was successfully modified on the inner wall of a microbubble. The mode field distribution of the microbubble was analysed, and the optofluidic sensor with thin wall exhibited a maximum bulk refractive index sensitivity of 265.2 nm/RIU. Lead ions at concentrations ranging from 0.1 pM to 100 pM were tested using the proposed WGM optofluidic sensor. The noise was decreased to 2.43 fM using the self-referenced differential method. Thus, a limit of approximately 15 fM was obtained for the detection of lead ions using the WGM optofluidic biosensor. Eight competing metal ions were also used to evaluate the selectivity of the proposed sensor, with results indicating that it has high selectivity for lead ions. Finally, the sensor performance is verified using real samples.

A Frequency-domain optofluidic dissolved oxygen sensor with total internal reflection design for in situ monitoring

Continuous measurement of dissolved oxygen (DO) variation is important in water monitoring and biomedical applications, which require low-cost and low-maintenance sensors capable of automated operation. A frequency-domain optofluidic DO sensor with total internal reflection (TIR) design has been developed based on fluorescence quenching of Ruthenium complex (Ru(dpp) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Cl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). To minimize artifacts causing drift in fluorescence measurements such as background autofluorescence, photobleaching, optical alignment variation, a low-cost frequency-domain approach is implemented in an optofluidic platform to measure the phase shift between the excitation and emission light. We show that the frequency domain optofluidic DO sensor provides absolute DO concentrations in repeated measurements. TIR design can enhance fluorescence signal in the integrated device and minimize background autofluorescence in the sample, which can subsequently improve overall sensitivity. Furthermore, photobleaching in the samples would be mitigated as the incident light does not enter the microfluidic channel. Our results demonstrate a measurement resolution of 0.2 ppm and response times of less than one minute. In accelerated photobleaching conditions, the long-term drift is shown to be less than ±0.4 ppm. These results suggest the potential of this optofluidic DO sensor as an in situ platform for water monitoring and biomedical applications.

Packaged microbubble resonator optofluidic flow rate sensor based on Bernoulli Effect.

A novel flow sensor based on dynamic fluid pressure changing in a packaged microbubble resonator without additional modification on its structure has been proposed and experimentally demonstrated. The results of sensing performance under both tunable laser source and broadband light source are presented. The flow rate sensitivity can reach up to 0.0196 pm / (µL/min). The fluid pressure variation caused by Bernoulli Effect is also analyzed theoretically.

Optofluidic Dissolved Oxygen Sensing With Sensitivity Enhancement Through Multiple Reflections

The development of compact and low-cost dissolved oxygen (DO) sensors is essential for the continuous in situ monitoring of environmental water quality and wastewater treatment processes. The optical detection of dynamic and reversible quenching of fluorescent dyes by oxygen has been used for DO sensing. In this paper, we have optimized a multilayer optofluidic device based on the measurement of fluorescence quenching in a Ruthenium-based oxygen sensitive dye by employing total internal reflection (TIR) of the excitation light to achieve sensitivity enhancement for the detection of 0-20-ppm DO in water. The incident angles of light and sensitive layer thickness are optimized experimentally in order to increase the path length of light in the sensitive layer of the device through multiple reflections. A model is developed to demonstrate how light propagates through different layers of the device at varying angles of excitation and to describe the mechanism of fluorescence generation for each of the types of TIR observed. The design principles identified in this paper may be applied to the development and optimization of new multilayered optofluidic sensors by employing TIR for sensitivity enhancement.

Optofluidic in-fiber integrated surface-enhanced Raman spectroscopy detection based on a hollow optical fiber with a suspended core.

In this Letter, we propose, to the best of our knowledge, the first in-fiber optofluidic Raman surface-enhanced Raman spectroscopy (SERS) sensor based on a microstructured hollow fiber (MHF) with a suspended core. Taking advantage of the unique internal structure, we immobilize silver nanoparticles with an SERS effect in the MHF by chemical bonding. The Raman signal of the microfluidic sample is excited by the excitation light in the suspended core through an evanescent field. Then the online SERS signal can be coupled back into the core and detected. To demonstrate the feasibility of the device, rhodamine 6G is chosen as the analyte, and high-quality SERS spectra are detected with the limit of detection of 1×10-14 M. Furthermore, an online optofluidic test is conducted on ceftriaxone (C18H18N8O7S3) to examine its capabilities in antibiotic sensing. The results show that the LOD of the samples is 10-6 M. Significantly, this Letter provides an integrated optofluidic in-fiber SERS sensor with a microchannel that can be integrated with chip devices without spatial optical coupling, which has a broad application in medicine and food safety, as well as various aspects of biological in-fiber sensing.

Ultrasensitive Optofluidic Interferometer for Online Monitoring of Photocatalytic Reactions

Ultrasensitive sensing devices contribute to the accurate detection and quantitation of chemical and biochemical reaction processes such as photocatalysis. This study proposes a simplified type of on-chip optofluidic sensor constructed by combining a fiber optic interferometer with a microfluidic chip to obtain an integrated and ultrasensitive device for the quantitative measurement of fluids. In the sensing region, the microchip splits the optical beam in half, forcing the transmission of one part through the polydimethylsiloxane material and that of the other part through the fluids in the microfluidic channel. This leads to the formation of a Mach–Zehnder interferometer that shows a strong response to the refractive index (RI) variation of fluids (22,331 nm/RIU). As an example, the photocatalytic degradation of rhodamine 6G was considered. Results indicate that the proposed optofluidic sensor has a low limit of detection (0.3125 μM) as well as a capacity of dynamic monitoring of photochemical reaction. With a simplified and compact structure, this mass-producible optofluidic sensor can potentially be applied in the fields of energy and environment.

Piezophototronic gated optofluidic logic computations empowering intrinsic reconfigurable switches

Optofluidic nano/microsystems have advanced the realization of Boolean circuits, with drastic progression to achieve extensive scale integration of desirable optoelectronics to investigate multiple logic switches. In this context, we demonstrate the optofluidic logic operations with interfacial piezophototronic effect to promote multiple operations of electronic analogues. We report an optofluidic Y-channeled logic device with tunable metal-semiconductor-metal interfaces through mechanically induced strain elements. We investigate the configuration of an OR gate in a semiconductor-piezoelectric zinc oxide nanorod-manipulated optofluidic sensor, and its direct reconfiguration to logic AND through compressive strain-induced (−1%) piezoelectric negative polarizations. The exhibited strategy in optofluidic systems implemented with piezophototronic concept enables direct-on chip working of OR and AND logic with switchable photocurrent under identical analyte. Featured smart intrinsic switching between the Boolean optoelectronic gates (OR↔AND) ultimately reduces the need for cascaded logic circuits to operate multiple logic switches on-a-chip.

Optofluidic laser explosive sensor with ultralow detection limit and large dynamic range using donor-acceptor-donor organic dye

Due to the significance of explosive detection for homeland security and environmental protection, the research of new materials and new methodologies for sensing electron-deficient explosive nitryl aromatics is urgently imperative. In this paper we demonstrate an optofluidic laser sensor for explosive detection by using a highly electron-rich dye (dye-1) with donor-acceptor-donor conjugated structure. The design and facile synthesis process of dye-1 are described. Theoretical calculations, UV–vis absorption and fluorescence spectra are investigated to understand the structure-property relations. Exceptionally high quantum yield of emission in solution is achieved. As a proof of concept, electron-deficient explosive molecule 2, 4-dinitrotoluene (DNT) is used as a model analyte to evaluate the performance of the optofluidic laser sensor. Experimental results indicate that the optofluidic dye laser sensor achieves a large dynamic range with 4 orders of magnitude and an ultralow limit of detection (10 nM), much better than fluorescent sensor. This work provides a route to expand the applications of optofluidic laser for sensitive biochemical detection through specific luminescent materials.

Optofluidic refractive index sensor based on asymmetric diffraction.

A novel optofluidic refracrtive index (RI) sensor was proposed based on asymmetric Fraunhofer diffraction. In-plane optofluidic lens, light source, slit, diffraction pattern visualization zone and optical path were integrated into the microfluidic networks to avoid the manual alignment of the optical components as well as to reduce the cost of external bulky components. Unlike the conventional RI sensor, this device visualizes the bulk refractive index change of the liquid through a diffraction image, which is readily read-out for clinical diagnosis right at the point-of-care or on-site security check. In the experiment, the device can measure a RI change of as low as ~10-5 RIU. A low noise-equivalent detection limit (NEDL) of ~10-6 refractive index unit (RIU) and high sensitivity of ~1.1 × 104/RIU were achieved. The new device is practical and suitable to be extended for high throughput applications by simultaneously reading multiple chips with an 2D-array image sensor.

Surface-enhanced Raman scattering-active photonic crystal fiber probe: Towards next generation liquid biopsy sensor with ultra high sensitivity.

The tremendous enhancement factors that surface-enhanced Raman scattering (SERS) possesses coupled with the flexibility of photonic crystal fibers (PCFs) pave the way to a new generation of ultrasensitive biosensors. Thanks to the unique structure of PCFs, which allows direct incorporation of an analyte into the axially aligned air channels, interaction between the analyte and excitation light could be increased many folds leading to flexible, reliable and sensitive probes that can be used in preclinical or clinical biosensing. SERS-active PCF probes provide unique opportunity to develop an opto-fluidic liquid biopsy needle sensor that enables one-step integrated sample collection and testing for disease diagnosis. Specificity being a key parameter to biosensors, the PCF inside the biopsy needle could be functionalized with targeting moieties to detect specific biomarkers. In this review article, we present some of the most promising recent biosensors based on PCFs including hollow-core PCFs, suspended-core PCFs and side-channel PCFs. We provide a wide range of applications of such platform using Raman spectroscopy, label free SERS or labeled SERS detection and analyze some of the main challenges to be addressed for translating it to a clinically viable next generation sensitive biopsy needle sensing probe.

Mode-splitting based optofluidic sensing at exceptional points in tubular microcavities

Optical microcavities have been experimentally demonstrated as an ultrasensitive device for optofluidic sensing, and monitoring of spectral shifts has been widely used in these microcavity-based optofluidic sensors. Here, spectral splitting around exceptional points (EPs) is proposed for optofluidic applications, which is based on tubular microcavities with a subwavelength-thin wall in a liquid-in-tube configuration. Both possibilities of a highly integrated optofluidic refractometer and an extinction coefficient sensor of liquid are discussed. Discussions on the sensitivity of these mode-splitting based optofluidic devices are presented to reveal their great potential in the detection of ultra-small refractive indices changes.

On-Channel Integrated Optofluidic Pressure Sensor with Optically Boosted Sensitivity.

A novel optofluidic sensor that measures the local pressure of the fluid inside a microfluidic channel is presented. It can be integrated directly on-channel and requires no additional layers in fabrication. The detection can be accomplished at a single wavelength; and thereby, only a single laser diode and a single photodetector are required. This renders the sensor to be compact, cheap and easy to fabricate. Basically, the sensor consisted of a Fabry–Pérot microresonator enclosing the fluidic channel. A novel structure of the Fabry–Pérot was employed to achieve high-quality factor, that was essential to facilitate the single wavelength detection. The enhanced performance was attributed to the curved mirrors and cylindrical lenses used to avoid light diffraction loss. The presented sensor was fabricated and tested with deionized water liquid and shown to exhibit a sensitivity up to 12.46 dBm/bar, and a detection limit of 8.2 mbar. Numerical simulations are also presented to evaluate the mechanical–fluidic performance of the device.

Design of an optofluidic sensor for rapid detection of hemolysis

Blood tests are a vital source of information for disease diagnosis and monitoring. Blood sample contamination due to hemolysis (the rupturing of red blood cells) is a significant problem that can result in incorrect diagnoses, delayed treatment, and inappropriate prescriptions. Therefore, a simple, rapid test is needed to detect the occurrence of hemolysis. Here we propose a new miniature spectroscopic device, integrated onto an optical fiber platform, which can quickly and reliably determine the amount of free hemoglobin in a small droplet of blood plasma. Optimal design parameters for the device were first calculated theoretically, and then the device was fabricated and tested. Experimental tests demonstrated that the device can quickly measure free plasma hemoglobin over a wide range of concentration (0–500 mg/dL) and with a low detection limit (<4 mg/dL), sufficient to detect both low- and high-grade hemolysis in a droplet of blood plasma. This device can therefore be useful for rapid quality control in blood-testing laboratories, hospitals, and dialysis settings.