Optical Microsensors Research Articles

Design and Fabrication of a Thin and Micro-Optical Sensor for Rapid Prototyping.

Optical sensing offers several advantages owing to its non-invasiveness and high sensitivity. The miniaturization of optical sensors will mitigate spatial and weight constraints, expanding their applications and extending the principal advantages of optical sensing to different fields, such as healthcare, Internet of Things, artificial intelligence, and other aspects of society. In this study, we present the development of a miniature optical sensor for monitoring thrombi in extracorporeal membrane oxygenation (ECMO). The sensor, based on a complementary metal-oxide semiconductor integrated circuit (CMOS-IC), also serves as a photodiode, amplifier, and light-emitting diode (LED)-mounting substrate. It is sized 3.8 × 4.8 × 0.75 mm3 and provides reflectance spectroscopy at three wavelengths. Based on semiconductor and microelectromechanical system (MEMS) processes, the design of the sensor achieves ultra-compact millimeter size, customizability, prototyping, and scalability for mass production, facilitating the development of miniature optical sensors for a variety of applications.

A low-cost discrete Vis-NIR optical sensing method for the determination of pear internal blackheart

In this study, a moldy crown pear core detection system based on a micro-optical sensor was developed. The micro-optical sensor has seven specific wavelengths, 425, 455, 515, 615, 660, 700, and 850 nm, and a cost-effective advantage. For the discrete spectrum, 7 kinds of preprocessing methods were compared. Traditional preprocessing methods, such as the standard normal transform (SNV) and multiple scattering correction (MSC) methods, cannot improve the efficiency of the spectrum. It was verified that the Savitzky − Golay (SG) convolution smoothing preprocessing method could be applied to preprocess discrete spectral data. The correlation of the spectrum after SG preprocessing in the partial least squares regression (PLSR) prediction model was 0.86, and the root mean square error (RMSE) was 0.19. Furthermore, the difference between the nonlinear modeling method without preprocessing and the PLS prediction model after preprocessing was compared. The results showed that the accuracy of the nonlinear modeling method for the discrete spectrum was much higher than that of the PLS linear modeling. The average model accuracy was above 0.9, and the k nearest neighbor (KNN) algorithm had the best effect, reaching an accuracy of 0.96. Finally, a prediction model accuracy of 0.98 was obtained by combining SG + KNN. In summary, the micro-optical sensor system had the advantages of low-cost performance and high precision, which are convenient for popularization and application in practice.

Four-Dimensional Printing of a Fiber-Tip Multimaterial Microcantilever as a Magnetic Field Sensor

“Lab on Fiber” technology integrates different micro- and nanoscale structures or materials onto optical fibers to create additional functionality. With the advanced femtosecond (Fs) laser-induced two-photon polymerization (TPP) technology, polymer microstructures printed on a fiber tip have great potential as micro-optical sensors. However, additional functionalization is usually required, which is costly and complex. Based on the advanced four-dimensional (4D) printing theory, for the first time, a successive multimaterial on-fiber TPP strategy is proposed here, and a fiber-tip polymer microcantilever-based magnetic sensor is prepared in one step. First, the cantilever and the supporting base on the fiber tip are printed by TPP, and then the magnetically responsive cube on the cantilever tip is printed with a self-made magnetic fluid photoresist. The whole process is completed in one step without additional treatment. The prepared sensor shows a minuscule size and a high magnetic sensitivity of 119 pm mT–1 in the range of 0–90 mT, suitable for weak or microspace magnetic field measurement at high spatial resolution. Especially, the proposed 4D successive on-fiber TPP strategy can be extended to other stimulus-responsive materials, providing new guidance for manufacturing various stimulus-responsive microsensors and microactuators on the fiber tip.

A metalens-based optical polarization sensor

An optical microsensor of the laser light polarization state based on a metalens is proposed. Well-known polarization sensors based on metasurfaces usually transmit different types of polarization at different angles to the optical axis. The polarization sensor proposed in this work forms different patterns for different polarization types at the metalens focus. Specifically, the left-handed circular polarization at the focus forms a ring-shaped spot, the right-handed circular polarization - a round focal spot, and linear polarization forms an elliptical spot with two side lobes. Moreover, the angle of inclination of the elliptical focal spot corresponds to the angle of inclination of the initial E-vector of the linearly polarized beam. The simulation is consistent with theoretical predictions. A 30-μm metalens with a low aspect ratio, high numerical aperture, and short focal length equal to the incident wavelength 633 nm in a 120-nm thick thin film of amorphous silicon is designed, fabricated and characterized.

Fast Response PDMS/GaN Optical Vacuum Microsensors

In this letter, we demonstrate a PDMS/GaN-based optical microsensor for measuring vacuum pressure. The submillimeter-sized GaN device responsible for optical emission and detection is fabricated through the wafer-scale process. The integration of low-cost porous PDMS film can provide significant reflection changes under atmospheric pressure and vacuum conditions, enabling the GaN device to sense pressure variations in 22–760 Torr. The attractive features of the sensor such as compactness, ease of operation, fast response of 1.0 s, small hysteresis of 1.3 %, and high stability make it suitable for wide applications of vacuum pressure sensing.

Optical Polarization Sensor Based on a Metalens.

We investigated an optical microsensor of the polarization state of a laser light based on a metalens. In contrast to known polarization sensors based on metasurfaces that deflect different polarization types using various angles to the optical axis, the studied polarization sensor generated different patterns in the metalens focus to realize varied polarization states: left circular polarization generated a light ring in the focus, right circular polarization generated a circular focal spot, and linear polarization generated an elliptic spot with two sidelobes. Moreover, the tilt angle of the linear polarization matched the tilt angle of the elliptic focal spot. The simulation results were consistent with the theoretical predictions. A metalens with a diameter of several tens of microns was designed and fabricated in a thin amorphous silicon film with a thickness of 120 μm and a low aspect ratio, high numerical aperture, and short focal distance equal to a wavelength of 633 nm.

Heterodyne detection of backscattering for whispering-gallery-mode sensors.

Whispering-gallery-mode (WGM) microcavities have shown significant applications in nanoparticle sensing for environmental monitoring and biological analysis. However, the enhancement of detection resolution often calls for active cavities or elaborate structural designs, leading to an increase of fabrication complexity and cost. Herein, heterodyne amplification is implemented in WGM microsensors based on backscattering detection mechanism. By interfering with an exotic reference laser, the reflecting light backscattered by perturbation targets can be strongly enlarged, yielding an easy-to-resolve and consequently sensitive microsensor. The dependence of detection laser frequency has also been characterized with the assistance of optothermal dynamics. We show that exploiting heterodyne interferometry boosts the detection of weak signals in microresonator systems and provides a fertile ground for optical microsensor development.

Green fluorescent protein-like pigments optimise the internal light environment in symbiotic reef-building corals.

Pigments homologous to the green fluorescent protein (GFP) have been proposed to fine-tune the internal light microclimate of corals, facilitating photoacclimation of photosynthetic coral symbionts (Symbiodiniaceae) to life in different reef habitats and environmental conditions. However, direct measurements of the in vivo light conditions inside the coral tissue supporting this conclusion are lacking. Here, we quantified the intra-tissue spectral light environment of corals expressing GFP-like proteins from widely different light regimes. We focus on: (1) photoconvertible red fluorescent proteins (pcRFPs), thought to enhance photosynthesis in mesophotic habitats via wavelength conversion, and (2) chromoproteins (CPs), which provide photoprotection to the symbionts in shallow water via light absorption. Optical microsensor measurements indicated that both pigment groups strongly alter the coral intra-tissue light environment. Estimates derived from light spectra measured in pcRFP-containing corals showed that fluorescence emission can contribute to >50% of orange-red light available to the photosynthetic symbionts at mesophotic depths. We further show that upregulation of pink CPs in shallow-water corals during bleaching leads to a reduction of orange light by 10-20% compared to low-CP tissue. Thus, screening by CPs has an important role in mitigating the light-enhancing effect of coral tissue scattering and skeletal reflection during bleaching. Our results provide the first experimental quantification of the importance of GFP-like proteins in fine-tuning the light microclimate of corals during photoacclimation.

Upconversion-luminescent optical fiber probe for in situ tyrosinase monitoring

Tyrosinase (TYR) is related to a variety of skin diseases, melanoma and Parkinson's disease, and it is essential for the quantitative detection of TYR. Although the traditional TYR sensors in solution have extremely high sensitivity, and it is expected to develop a biosensor for point of care (POC) detection due to its poor portability, nanoparticle aggregation, reusability, and complexity of use. Here, we report a simple and compact interrogation setup of an optical fiber sensor that facilitates signal transmission and can identify and quantify TYR. The sensor consists of multimode optical fibers (MOF) modified by up-conversion nanoparticles (UCNPs) based on chemical bonds, and the photoconductive MOF can both excite UCNPs and collect the upconversion fluorescence of UCNPs. TYR is detected based on photo-induced electron transfer (PET) between UCNPs and dopaquinone (DQ). The fiber optic sensor has an optical reading, a linear response of up to 0.6 U mL−1, and a detection limit as low as 0.028 U mL−1. Our optical microsensor has great potential to be used as a portable and in situ detection probe to monitor TYR.

Spin-Orbital Conversion of a Strongly Focused Light Wave with High-Order Cylindrical-Circular Polarization.

We discuss interesting effects that occur when strongly focusing light with mth-order cylindrical–circular polarization. This type of hybrid polarization combines properties of the mth-order cylindrical polarization and circular polarization. Reluing on the Richards-Wolf formalism, we deduce analytical expressions that describe E- and H-vector components, intensity patterns, and projections of the Poynting vector and spin angular momentum (SAM) vector at the strong focus. The intensity of light in the strong focus is theoretically and numerically shown to have an even number of local maxima located along a closed contour centered at an on-axis point of zero intensity. We show that light generates 4m vortices of a transverse energy flow, with their centers located between the local intensity maxima. The transverse energy flow is also shown to change its handedness an even number of times proportional to the order of the optical vortex via a full circle around the optical axis. It is interesting that the longitudinal SAM projection changes its sign at the focus 4m times. The longitudinal SAM component is found to be positive, and the polarization vector is shown to rotate anticlockwise in the focal spot regions where the transverse energy flow rotates anticlockwise, and vice versa—the longitudinal SAM component is negative and the polarization vector rotates clockwise in the focal spot regions where the transverse energy flow rotates clockwise. This spatial separation at the focus of left and right circularly polarized light is a manifestation of the optical spin Hall effect. The results obtained in terms of controlling the intensity maxima allow the transverse mode analysis of laser beams in sensorial applications. For a demonstration of the proposed application, the metalens is calculated, which can be a prototype for an optical microsensor based on sharp focusing for measuring roughness.

Integrated sensor with a whispering-gallery mode and surface plasmonic resonance for the enhanced detection of viruses

Surface plasmonic resonance (SPR) is integrated into a whispering-gallery mode (WGM) optical microsensor to augment sensitivity in this study. The performance of such WGM silica ring sensors of 20 µm in size with an Ag or Au metal core was evaluated for detection of small respiratory viruses such as COVID-19 via the finite-element modeling. Compared with pure WGM sensors, the integration with SPR enhances sensitivity by 3–5 times and facilitates combination with the polymerase chain reaction method to achieve fast, accurate, and specific virus detection. The presence of a single respiratory virus of 70 to 100 nm in diameter in air environment could shift the sensor resonance wavelength by 36 to 64 pm in the case of an Au metal core or by 34 to 63 pm of an Ag metal core. With use of a general-purpose optical analyzer of 10 pm resolution, a single airborne virus of 20 nm in size is detectable using the proposed hybrid sensor. This corresponds to about 0.005 vol. %. For viruses in aqueous solution, the detection limit rises to about 0.2 vol. %. A fundamental enhancement factor based on relative electric energy ratio is introduced and defined to analyze and quantify sensitivity enhancement for the first time, to the best of our knowledge.

Interplay between H1N1 influenza a virus infection, extracellular and intracellular respiratory tract pH, and host responses in a mouse model.

During influenza A virus (IAV) entry, the hemagglutinin (HA) protein is triggered by endosomal low pH to undergo irreversible structural changes that mediate membrane fusion. HA proteins from different isolates vary in the pH at which they become activated in endosomes or become irreversible inactivated if exposed to extracellular acid. Little is known about extracellular pH in the upper respiratory tracts of mammals, how pH may shift during IAV infection, and its impact on replication of viruses that vary in HA activation pH. Here, we inoculated DBA/2J mice intranasally with A/TN/1-560/2009 (H1N1) (activation pH 5.5) or a mutant containing the destabilizing mutation HA1-Y17H (pH 6.0). We measured the kinetics of extracellular pH during infection using an optical pH-sensitive microsensor probe placed in the naris, nasal sinus, soft palate, and trachea. We also measured intracellular pH of single-cell suspensions of live, primary lung epithelial cells with various wavelength pH-sensitive dyes localized to cell membranes, cytosol, endosomes, secretory vesicles, microtubules, and lysosomes. Infection with either virus decreased extracellular pH and increased intracellular pH. Peak host immune responses were observed at 2 days post infection (DPI) and peak pH changes at 5 DPI. Extracellular and intracellular pH returned to baseline by 7 DPI in mice infected with HA1-Y17H and was restored later in wildtype-infected. Overall, IAV infection altered respiratory tract pH, which in turn modulated replication efficiency. This suggests a virus-host pH feedback loop that may select for IAV strains containing HA proteins of optimal pH stability, which may be approximately pH 5.5 in mice but may differ in other species.

Minimizing Intracochlear Pressure: Influence of the Insertion Sheath

Objective: Atraumatic cochlear implantation (CI) and insertion of the electrode in particular are major goals of recent CI surgery. Perimodiolar electrode arrays need a stylet or exosheath for insertion. The sheath can influence the intracochlear pressure changes during insertion of the electrode. The aim of this study was to modify the insertion sheath to optimize intracochlear pressure changes. Methods: In an artifical cochlear model, 7 different modified insertion sheaths were used. The intracochlear pressure was measured with a micro-optical sensor in the apical part of the model cochlea. Results: Significant lower intracochlear pressure changes were observed when the apical part of the insertion sheath was either shortened or tapered. Modification of the stopper does influence the intracochlear pressure significantly. Conclusion: Modification of the insertion sheath leads to lower intracochlear pressure gain. The differences and impact on intracochlear pressure changes found in this study underline the importance of even subtle modifications of the electrode insertion technique.

Real-time, non-invasive thrombus detection in an extracorporeal circuit using micro-optical thrombus sensors

Introduction:Real-time, non-invasive monitoring of thrombus formation in extracorporeal circuits has yet to be achieved. To address the challenges of conventional optical thrombus detection methods requiring large devices that limit detection capacity, we developed a micro-optical thrombus sensor.Methods:The proposed micro-optical thrombus sensor can detect the intensity of light scattered by blood at wavelengths of 660 and 855 nm. Two thrombus sensors were installed on in vitro circuit: one at the rotary blood pump and one at a flow channel. To evaluate the variation in the ratio of incident light intensity at each wavelength of the two sensors, Rfluct (for 660 nm) and Ifluct (for 855 nm) were defined. Using fresh porcine blood as a working fluid, we performed in vitro tests of haematocrit (Hct) and oxygen saturation (SaO2) variation and thrombus detection. Thrombus tests were terminated after Rfluct or Ifluct showed a larger change than the maximum range of those in the Hct and SaO2 variation test.Results:In all three thrombus detection tests, Ifluct showed a larger change than the maximum range of those in the Hct and SaO2 variation test. After the tests, thrombus formation was confirmed in the pump, and there was no thrombus in the flow channel. The results indicate that Ifluct is an effective parameter for identifying the presence of a thrombus.Conclusion:Thrombus detection in an extracorporeal circuit using the developed micro-optical sensors was successfully demonstrated in an in vitro test.

Real time monitoring of oxygen uptake of hepatocytes in a microreactor using optical microsensors

Most in vitro test systems for the assessment of toxicity are based on endpoint measurements and cannot contribute much to the establishment of mechanistic models, which are crucially important for further progress in this field. Hence, in recent years, much effort has been put into the development of methods that generate kinetic data. Real time measurements of the metabolic activity of cells based on the use of oxygen sensitive microsensor beads have been shown to provide access to the mode of action of compounds in hepatocytes. However, for fully exploiting this approach a detailed knowledge of the microenvironment of the cells is required. In this work, we investigate the cellular behaviour of three types of hepatocytes, HepG2 cells, HepG2-3A4 cells and primary mouse hepatocytes, towards their exposure to acetaminophen when the availability of oxygen for the cell is systematically varied. We show that the relative emergence of two modes of action, one NAPQI dependent and the other one transient and NAPQI independent, scale with expression level of CYP3A4. The transient cellular response associated to mitochondrial respiration is used to characterise the influence of the initial oxygen concentration in the wells before exposure to acetaminophen on the cell behaviour. A simple model is presented to describe the behaviour of the cells in this scenario. It demonstrates the level of control over the role of oxygen supply in these experiments. This is crucial for establishing this approach into a reliable and powerful method for the assessment of toxicity.

Coupling Waveguide-Based Micro-Sensors and Spectral Multivariate Analysis to Improve Spray Deposit Characterization in Agriculture.

The leaf coverage surface is a key measurement of the spraying process to maximize spray efficiency. To determine leaf coverage surface, the development of optical micro-sensors that, coupled with a multivariate spectral analysis, will be able to measure the volume of the droplets deposited on their surface is proposed. Rib optical waveguides based on Ge-Se-Te chalcogenide films were manufactured and their light transmission was studied as a response to the deposition of demineralized water droplets on their surface. The measurements were performed using a dedicated spectrophotometric bench to record the transmission spectra at the output of the waveguides, before (reference) and after drop deposition, in the wavelength range between 1200 and 2000 nm. The presence of a hollow at 1450 nm in the relative transmission spectra has been recorded. This corresponds to the first overtone of the O–H stretching vibration in water. This result tends to show that the optical intensity decrease observed after droplet deposition is partly due to absorption by water of the light energy carried by the guided mode evanescent field. The probe based on Ge-Se-Te rib optical waveguides is thus sensitive throughout the whole range of volumes studied, i.e., from 0.1 to 2.5 μL. Principal Component Analysis and Partial Least Square as multivariate techniques then allowed the analysis of the statistics of the measurements and the predictive character of the transmission spectra. It confirmed the sensitivity of the measurement system to the water absorption, and the predictive model allowed the prediction of droplet volumes on an independent set of measurements, with a correlation of 66.5% and a precision of 0.39 μL.

Fabry-Pérot optical sensor and portable detector for monitoring high-resolution ocular hemodynamics.

Our understanding of ocular hemodynamics and its role in ophthalmic disease progression remains unclear due to the shortcomings of precise and on-demand biomedical sensing technologies. Here, we report high-resolution in vivo assessment of ocular hemodynamics using a Fabry-Pérot cavity-based micro-optical sensor and a portable optical detector. The designed optical system is capable of measuring both static intraocular pressure and dynamic ocular pulsation profiles in parallel. Through a dynamic intensity variation analysis method which improves sensing resolution by 3-4 folds, our system is able to extract systolic/diastolic phases from a single ocular pulsation profile. Using a portable detector, we performed in vivo studies on rabbits and verified that ophthalmic parameters obtained from our optical system closely match with traditional techniques such as tonometry, electrocardiography, and photo-plethysmography.

Chalcogenide rib waveguides for the characterization of spray deposits

With the aim to develop optical micro-sensors for "on-line" measurement of spray deposits on plant leaves, and thus to optimize the use of phytosanitary inputs in agriculture, straight waveguides based on GeSeTe chalcogenide layers were elaborated and their sensitivity to water droplet deposition was tested. As expected, water, the main constituent of pesticides, absorbed part of the evanescent wave of the guided light, which led to a decrease in the intensity transmitted at the output of the waveguides. Both experimental and simulation results proved that the position of the drop on the waveguide had no impact on the intensity measured at the guide outlet. At the opposite, they highlighted the correlation between the light intensity at the outlet and the volume and number of deposited droplets: the greater the volume of the drop and the greater the number of drops, the greater the decrease in intensity.

Surface-Doped Polystyrene Microsensors Containing Lipophilic Solvatochromic Dye Transducers.

Ion-selective optical microsensors based on surface-modified polystyrene (PS) beads with positively charged lipophilic solvatochromic dye (SD) on the surface were studied with K+ as model ion. Water-soluble SDs are expelled into the aqueous phase by ion exchange with the cationic analyte of interest, resulting in a detectable color change. In contrast, lipophilic SDs are more attractive, as they appear to remain anchored to the surface after expulsion from the sensing phase. This transfers just the ionic chromophore functionality into the aqueous phase and allows the system to act as a reversible, truly self-contained sensor. In this work this mechanism was evaluated with ζ-potential measurements on microsensor suspensions. It indeed provides experimental evidence for the mechanism of SD transfer, as a reversal of the ζ-potential of the PS microsensors was observed for higher potassium concentrations with valinomycin-doped microspheres. For a discriminated ion such as sodium, the ζ-potential change occurs at much higher electrolyte concentrations, in agreement with sensor selectivity. Undoped microspheres showed no apparent dependence of ζ-potential on electrolyte concentration. The study also shows that the effective range of microsensor surface charge is tunable and depends on the concentration of the SD on the coating phase.

High‐Performance Volatile Organic Compounds Microsensor Based on Few‐Layer MoS2‐Coated Photonic Crystal Cavity

AbstractDetection of volatile organic compounds (VOCs) is extremely important for industrial and human safety. Here, a high‐performance optical microsensor is demonstrated for VOCs detection using a photonic crystal (PC) cavity integrated with a few‐layer molybdenum disulfide (MoS2) flake. Because of the strong interactions between the cavity's evanescent field and the coated VOCs‐sensitive MoS2 layer, the resonant wavelength of the PC cavity exhibits distinct responses toward VOCs. The MoS2‐PC cavity microsensor presents good capability to identify different vapor concentrations and has a good repeatability. In particular, the compact structure, thin MoS2 flake and possible presence of chemisorption with oxygen‐containing VOCs species promise a fast response time of 300 ms. The sensor's high selectivity for specific VOCs is validated as well. Comparing to the electrically driven VOCs‐sensors based on 2D layered materials, the proposed sensor has advantages of compact size, easy fabrication, mass production, and simply optical access.