Optofluidic marine phosphate detection with enhanced absorption using a Fabry-Pérot resonator

Measurements and analysis of nitrogen and phosphorus in oceans: Practice, frontiers, and insights

Nitrogen and phosphorus concentrations in oceans have been extensively studied, and advancements in associated disciplines have rapidly progressed, enabling the exploration of novel and previously challenging questions. A keyword analysis was conducted using the Scopus database to examine chronological trends and hotspots, offering comprehensive insights into the evolution of marine nitrogen and phosphorus research. For this purpose, author keyword networks were developed for the periods before 1990, 1990 to 2000, 2001 to 2011, and 2012 to 2022. Furthermore, analytical techniques employed in the recent decade to determine nitrogen and phosphorus concentrations in seawater were assessed for their applicability and limitations through a critical review of more than 50 journal articles. Taxonomy and nitrogen biogeochemistry were the prominent research interests for the first two periods, respectively, while stable isotopic tracking of nitrogen and phosphorus processes emerged as the dominant research focus for the last two decades. The integration of macroeconomic factors in research development and the chronological rise of interdisciplinary research were identified. Conventional analytical techniques such as spectrophotometry, colorimetry, fluorometry, and elemental analysis were noted, along with emerging techniques like remote sensing and microfluidic sensors.

Recent Progress on Microfluidics Integrated with Fiber-Optic Sensors for On-Site Detection

This review introduces a micro-integrated device of microfluidics and fiber-optic sensors for on-site detection, which can detect certain or several specific components or their amounts in different samples within a relatively short time. Fiber-optics with micron core diameters can be easily coated and functionalized, thus allowing sensors to be integrated with microfluidics to separate, enrich, and measure samples in a micro-device. Compared to traditional laboratory equipment, this integrated device exhibits natural advantages in size, speed, cost, portability, and operability, making it more suitable for on-site detection. In this review, the various optical detection methods used in this integrated device are introduced, including Raman, ultraviolet-visible, fluorescence, and surface plasmon resonance detections. It also provides a detailed overview of the on-site detection applications of this integrated device for biological analysis, food safety, and environmental monitoring. Lastly, this review addresses the prospects for the future development of microfluidics integrated with fiber-optic sensors.

Highly sensitive, modification-free, and dynamic real-time stereo-optical immuno-sensor

Modification-free biosensing with high specificity and sensitivity is essential for miniaturized, online, integrated, and rapid, or even real-time molecular analyses. However, most optical biosensors are based on surface pre-modification or fluorescent labeling, and have either low sensitivity or low quality factor (Q). To address these difficulties, in this study, an optical sensor prototype was developed with a microbubble optofluidic channel integrated inside a Fabry-Pérot cavity to three-dimensionally tailor the intra-cavity light field via the intra-cavity lensing (microbubble) configuration. A high Q-factor (∼105), small mode volume, and high light energy density were experimentally achieved with this "stereo-sensor" while maintaining an ultrahigh refractive index (RI) sensitivity (679 nm/RIU) and ultra-small RI resolution (∼10-7 RIU at 950 nm). Moreover, specific detection of very low concentration of biomolecules (5 fg/mL for human IgG and 0.5 pg/mL for human serum albumin (HSA)) and wide range of protein concentrations (e.g., fg/mL-ng/mL for human IgG and pg/mL-ng/mL for HSA) without probe pre-modification were achieved owing to the RI change specifically associated with the probe-target binding and the corresponding bio-macromolecular conformation change. This modification-free stereosensing scenario is applicable to continuous, real-time, and multiplexed operations, thus showing potential for online, integrated, dynamic, biomolecular analyses in vitro or in vivo, such as the dynamic metabolic analysis of single cells or organoids and point-of-care tests.

Reversible and Turn-On Fluorescence Detection of Phosphate in Aqueous Solution and Living Cell Imaging by Supramolecular Metallacycles with AIE-Active Ligands

Luminescent supramolecular metallacycles have attracted great interest as a new promising class of sensing substrates. In this work, two tetraphenylethene (TPE)-based diimidazole and dipyrazole ligands with the aggregation-induced emission (AIE) feature were designed for the construction of supramolecular tetragonal metallacycles 1-4 with two 90° mononuclear [(bpy)M]2+ or dinuclear [(bpy)2M2]4+ acceptors (bpy = 2,2'-dipyridine; M = Pd, Pt), in which the fluorescence can be quenched to an "off" state due to the ligand-to-metal charge transfer (LMCT). Metallacycle 1 was utilized as a fluorescence sensor for phosphate (PO43-) detection in aqueous solution by means of disassembly, leading to the release of the ligand. Additionally, the metallacycle can be regenerated through self-assembly via the introduction of Pd(II) acceptors. PO43- was detected using TPE-based metallacycles over a wide concentration range, with a detection limit as low as 2.1 × 10-8 M. Furthermore, sensor 1 also presented the semiquantitative visual detection ability for PO43- in the test paper mode via fluorescence changes. The aforementioned studies not only enhance the current research on fluorescent materials but also offer a strategy for the creation of stimuli-responsive supramolecular coordination complexes.

Dual-ligand two-dimensional terbium-organic frameworks nanosheets for ratiometric fluorescence detection of phosphate

Here, we reported a ratiometric fluorescence strategy for the detection of phosphate (Pi) in artificial wetland water. The strategy was based on dual-ligand two-dimensional terbium-organic frameworks nanosheets (2D Tb-NB MOFs). 2D Tb-NB MOFs were prepared through blending 5-boronoisophthalic acid (5-bop), 2-aminoterephthalic acid (NH₂-BDC) and Tb³⁺ ions at room temperature in the presence of triethylamine (TEA). The dual-ligand strategy realized dual emission originated from ligand NH₂-BDC and Tb³⁺ ions at 424 and 544 nm, respectively. Pi could compete with ligands to coordinate Tb³⁺ due to the strong binding ability between Pi and Tb³⁺, resulting in structural destruction of 2D Tb-NB MOFs, so static quenching and antenna effect between ligands and metal ions were interrupted, and emission at 424 nm was enhanced and emission at 544 nm was weakened. This novel probe had excellent linearity with Pi concentrations from 1 to 50 μmol/L; the detection limit was 0.16 μmol/L. This work revealed that mixed ligands improved sensing efficiency of MOFs by enhancing the sensitivity of the coordination between the analyte and MOFs.

Microfluidic Chip with Fiber-Tip Sensors for Synchronously Monitoring Concentration and Temperature of Glucose Solutions

Monitoring the properties of fluids in microfluidic chips often requires complex open-space optics technology and expensive equipment. In this work, we introduce dual-parameter optical sensors with fiber tips into the microfluidic chip. Multiple sensors were distributed in each channel of the chip, which enabled the real-time monitoring of the concentration and temperature of the microfluidics. The temperature sensitivity and glucose concentration sensitivity could reach 314 pm/°C and -0.678 dB/(g/L), respectively. The hemispherical probe hardly affected the microfluidic flow field. The integrated technology combined the optical fiber sensor with the microfluidic chip and was low cost with high performance. Therefore, we believe that the proposed microfluidic chip integrated with the optical sensor is beneficial for drug discovery, pathological research and material science investigation. The integrated technology has great application potential for micro total analysis systems (μ-TAS).

Novel method in emerging environmental contaminants detection: Fiber optic sensors based on microfluidic chips

Recently, human industrial practices and certain activities have caused the widespread spread of emerging contaminants throughout the environmental matrix, even in trace amounts, which constitute a serious threat to human health and environmental ecology, and have therefore attracted the attention of research scholars. Different traditional techniques are used to monitor water pollutants, However, they still have some disadvantages such as high costs, ecological problems and treatment times, and require technicians and researchers to operate them effectively. There is therefore an urgent need to develop simple, inexpensive and highly sensitive methods to sense and detect these toxic environmental contaminants. Optical fiber microfluidic coupled sensors offer different advantages over other detection technologies, allowing manipulation of light through controlled microfluidics, precise detection results and good stability, and have therefore become a logical device for screening and identifying environmental contaminants. This paper reviews the application of fiber optic microfluidic sensors in emerging environmental contaminant detection, focusing on the characteristics of different emerging contaminant types, different types of fiber optic microfluidic sensors, methodological principles of detection, and specific emerging contaminant detection applications. The optical detection methods in fiber optic microfluidic chips and their respective advantages and disadvantages are analyzed in the discussion. The applications of fiber optic biochemical sensors in microfluidic chips, especially for the detection of emerging contaminants in the aqueous environment, such as personal care products, endocrine disruptors, and perfluorinated compounds, are reviewed. Finally, the prospects of fiber optic microfluidic coupled sensors in environmental detection and related fields are foreseen.

Space-time-regulated imaging analyzer for smart coagulation diagnosis

The development of intelligent blood coagulation diagnoses is awaited to meet the current need for large clinical time-sensitive caseloads due to its efficient and automated diagnoses. Herein, a method is reported and validated to realize it through artificial intelligence (AI)-assisted optical clotting biophysics (OCB) properties identification. The image differential calculation is used for precise acquisition of OCB properties with elimination of initial differences, and the strategy of space-time regulation allows on-demand space time OCB properties identification and enables diverse blood function diagnoses. The integrated applications of smartphones and cloud computing offer a user-friendly automated analysis for accurate and convenient diagnoses. The prospective assays of clinical cases (n = 41) show that the system realizes 97.6%, 95.1%, and 100% accuracy for coagulation factors, fibrinogen function, and comprehensive blood coagulation diagnoses, respectively. This method should enable more low-cost and convenient diagnoses and provide a path for potential diagnostic-markers finding.

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An ultraportable optofluidic analyzer empowers convenient coagulation diagnoses

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The system enables optical clotting biophysics (OCB) properties acquisition and process

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Coagulation function diagnoses uses intelligent OCB properties identification

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Space-time regulation of OCB properties endow it capability to diverse diagnoses

Photothermal Localization in an Optofluidic Microreactor for Rapid Pretreatment toward Online Pollutant Analysis

The realization of high-efficient digestion in a microfluidic reactor is considered to be advantageous for pretreatment toward online pollutant detection. However, it is difficult to achieve satisfactory device performance due to the gap between the low digestion reaction efficiency and the demand for rapid pretreatment for online detection. Herein, we design and manufacture an optofluidic microreactor combined with a MnO₂ nanofilm localizing the heat inside the reaction chamber under solar irradiation, which contributes a lot to the on-chip nutrient digestion efficiency enhancement. The overall temperature of the water sample in the reactor chamber can be dramatically increased in a fleeting time of less than 1 s and maintained at 78 °C. The digestion rate constant of the microreactor is improved by about 100 times compared with that obtained by the traditional method in the national standard, which is attributed to temperature enhancement and various oxidation reactions in the heated reaction chamber. Notably, when pretreating the actual total phosphorus water samples, the digestion efficiency is demonstrated to be higher than 95% within 12 s under solar light irradiation. The optofluidic platform brings many benefits to accelerate the various photochemically enhanced reactions using solar light and is extremely adapted for rapid pretreatment of biochemical samples to further develop their online analysis.

Enhanced Ultrasensitive Photoelectrochemical Probe for Phosphate Detection in Water Based on a Zirconium-Porphyrin Framework

Excessive phosphate poses a serious ecological and human health risk, and thereby, monitoring its trace concentration is of great significance to environmental protection and human health. In this work, a zirconium-porphyrin framework (PCN-222) with excellent stability and unique luminescence properties was designed to modify the surface of the indium tin oxide electrode, which was first used as a photoelectrochemical (PEC) probe for phosphate detection. The PCN-222-modified PEC probe demonstrated an excellent selectivity and stability and indicated a linear response to phosphate in the range of 0-10⁶ nM with a limit of detection (LOD) as low as 1.964 nM. To the best of our knowledge, this is the phosphate probe with the lowest LOD, and this is also the first signal-on PEC probe toward phosphate based on PCN-222. More importantly, the PEC probe can be validated for the good applicability of trace phosphate detection in real water samples, indicating a good application prospect. Finally, a series of electrochemical and spectroscopic studies have proved that phosphate can bind to the indium tin oxide (ITO)/PCN-222 electrode, which shortens the distance of the space charge region while reducing the bandwidth and thus facilitates the transfer of photogenerated electrons across the energy band barrier to reduce O₂ in the electrolyte, producing an enhanced cathodic photocurrent signal. The proposed strategy of the highly sensitive PEC probe provides a promising platform for more effective label-free phosphate monitoring in the environment and organisms.

Optofluidic Platform for Rapid On-Chip Analysis of Total Phosphorus in Surface Water Using Absorption Spectrometry

Optofluidic devices are of high interest for online monitoring and analyzing biochemical targets in water by integrating the complex on-chip pretreatment of target analytes and spectral analysis. Compared with the traditional bulk equipment, versatile optical detection and biochemical analysis are more easily integrated on an optofluidic chip, which promotes the development of on-chip real-time rapid detection and monitoring. Here, we report an optofluidic platform for online monitoring total phosphorous in water by absorption spectrometry, which naturally combines the merits of both the photo-Fenton effect and microfluidics to realize the rapid on-chip digestion of phosphate at room temperature and normal pressure. The functional cells for chromogenic reaction and optical absorption detection are, respectively, fabricated on the platform to analyze the content of total phosphorus in surface water. In the experiment, the on-chip digestion time of phosphate is dramatically declined to 8.6 sec, and thus, the detection time is greatly shortened to a few minutes. The detection range of total phosphorus is demonstrated as 0.005-1.00 mg L^-1, which satisfies the detection requirements of most environmental water samples. Its availability for measuring the total phosphorous in real water samples is also verified. Predictably, this platform is adapted to on-chip analysis of many other biochemical targets in water.

High-temperature-sensitive and spectrum-contrast-enhanced sensor using a bullet-shaped fiber cavity filled with PDMS

Low temperature sensitivity and low spectral contrast are serious but common issues for most Fabry Perot (FP) sensors with an air cavity. In this paper, a high-temperature-sensitive and spectrum-contrast-enhanced Fabry Perot interferometer (FPI) is proposed and experimentally demonstrated. The device is composed of a hollow cylindrical waveguide (HCW) filled with polydimethylsiloxane (PDMS) and a semi-elliptic PDMS end face. The semi-elliptic PDMS end face increases the spectral contrast significantly due to the focusing effect. Experimentally, the spectral contrast is 11.97 dB, which is two times higher than the sensor without semi-elliptic PDMS end face. Ultra-high temperature sensitivity of 3.1501 nm/°C was demonstrated. The proposed sensor exhibits excellent structural stability, high spectral contrast and high temperature sensitivity, showing great potential in biomedicine, industrial manufacturing, agricultural production and other applications.

A submersible phosphate analyzer for marine environments based on inlaid microfluidics

In situ sensors are needed to further our understanding of phosphate flux dynamics in marine environments during short term events such as tidal cycles, algae blooms and runoff periods. Here, we present a fully automated in situ phosphate analyzer based on an inlaid microfluidic absorbance cell technology. The microfluidic device employs colorimetric absorbance spectrophotometry, using the phosphomolybdenum blue (PMB) assay modified by the addition of polyvinylpyrrolidone (PVP), to measure phosphate concentrations in seawater. Bench top calibrations were performed with both copper(II) sulfate dye and the PMB assay, as well as temperature sensitivity studies to characterize the sensor's performance in a range of conditions. It achieves a limit of detection of 15.2 nM, a limit of quantification of 50.8 nM, and a high in situ precision with a relative standard deviation of less than 1.5% across three consecutive measurements. Two consecutive field deployments are conducted as assessments for its intended in situ applications. The sensor is first deployed from a pier at a depth of 6 m, with simultaneous bottle samples taken to perform cross-validation. It is next deployed on the Stella Maris testbed, a multi-sensor seabed platform (MSSP), 100 m offshore and 9 m deep in the inlet to the Bedford Basin in Nova Scotia, Canada. Over 300 successful phosphate measurements were acquired, showing the influence of the tidal cycle, and confirming the sensor's viability in observing nutrient flux dynamics with nanomolar variations.

Colorimetric Phosphate Detection Using Organic DFB Laser Based Absorption Spectroscopy

A novel compact laser absorption spectrometer is developed for colorimetric detection. We demonstrate the realization of the system as well as example measurements of phosphate in water samples based on the malachite green (MG) method. A phosphate concentration range of 1 mg/L to 31.25 μg/L (which corresponds to a molar concentration range of 10.5 μmol/L to 329 nmol/L) is investigated. This photometer demonstrates the ease of integration of organic distributed feedback (DFB) lasers and their miniaturizability, leading the way toward optofluidic on-chip absorption spectrometers. We constructed an optically pumped organic second-order DFB laser on a transparent substrate, including a transparent encapsulation layer, to have access to both emission directions of the surface-emitting laser. Using the two different surface emission directions of the laser resonator allows monitoring of the emitted light intensity without using additional optical elements. Based on these advances, it is possible to miniaturize the measurement setup of a laser absorption spectrometer and to measure analytes, such as phosphate.

The Rise of the OM-LoC: Opto-Microfluidic Enabled Lab-on-Chip

The integration of optical circuits with microfluidic lab-on-chip (LoC) devices has resulted in a new era of potential in terms of both sample manipulation and detection at the micro-scale. On-chip optical components increase both control and analytical capabilities while reducing reliance on expensive laboratory photonic equipment that has limited microfluidic development. Notably, in-situ LoC devices for bio-chemical applications such as diagnostics and environmental monitoring could provide great value as low-cost, portable and highly sensitive systems. Multiple challenges remain however due to the complexity involved with combining photonics with micro-fabricated systems. Here, we aim to highlight the progress that optical on-chip systems have made in recent years regarding the main LoC applications: (1) sample manipulation and (2) detection. At the same time, we aim to address the constraints that limit industrial scaling of this technology. Through evaluating various fabrication methods, material choices and novel approaches of optic and fluidic integration, we aim to illustrate how optic-enabled LoC approaches are providing new possibilities for both sample analysis and manipulation.

Optical-Switch-Enabled Microfluidics for Sensitive Multichannel Colorimetric Analysis

The implementation of colorimetric analysis within microfluidic environments engenders significant benefits with respect to reduced sample and reagent consumption, system miniaturization, and real-time measurement of flowing samples. That said, conventional approaches to colorimetric analysis within microfluidic channels are hampered by short optical pathlengths and single-channel configurations, which lead to poor detection sensitivities and low analytical throughputs. Although the use of multiplexed light source/photodetector modules allows for multichannel analysis, such configurations significantly increase both instrument complexity and cost. To address these issues, we present a four-channel colorimetric measurement scheme within an optical-switch-enabled microfluidic chip (OSEMC) fabricated by two-photon stereolithography. The integration of optical switches enables sequential signal readout from each detection channel, and thus, only a single light source and a photodetector are required for operation. Optical switches can be controlled in a bespoke manner by changing the medium in the switch channel between a "light-transmitting" fluid and a "light-blocking" fluid using pneumatic microvalves. Such optical switches are characterized by fast response times (approximately 200 ms), tunable switching frequencies (between 0.1 and 1.0 Hz studied), and excellent stability. Operational performance demonstrates both good sensitivity and reproducibility through the colorimetric analysis of nitrite and ammonium samples using four detection channels. Furthermore, the use of OSEMC for parallel and real-time analysis of flowing samples is investigated via characterization of the adsorption kinetics of tartrazine on activated charcoal and the catalytic reaction kinetics of horseradish peroxidase (HRP).

Ratiometric fluorescence for sensitive detection of phosphate species based on mixed lanthanide metal organic framework

Phosphate (PO₄^3-) plays a major role in aquatic ecosystems and biosystems. Developing a highly sensitive and selective ratiometric fluorescence probe for detection of PO₄^3- is of great significance to the ecological environment and human health. In this work, a novel dual lanthanide metal organic framework was synthesized via hydrothermal reaction based on Tb³⁺ and Ce³⁺ as the center metal ions and terephthalic acid as the organic ligand (designated as Tb-Ce-MOFs). The fluorescence of Tb-Ce-MOFs shows emission at 375 nm. In the presence of PO₄^3-, with increased concentration of PO₄^3-, the fluorescence intensity of Tb-Ce-MOFs at 500 nm and 550 nm increased, while the intensity at 375 nm was reduced. Hence, ratiometric fluorescence detecting of PO₄^3- can be achieved by measuring the ratio of fluorescence at 550 nm (FL₅₅₀) to 375 nm (FL₃₇₅) in the fluorescent spectra of the Tb-Ce-MOFs. In this sensing approach, the Tb-Ce-MOFs probe exhibits highly sensitive and selective for detection of PO₄^3-. The limit of detection is calculated to be 28 nM and the detection range is 0.1 to 10 μM. In addition, the Tb-Ce-MOFs were used in the detection of PO₄^3- in real samples. We design and synthesize a mixed lanthanide metal organic framework fluorescence probe (Tb-Ce-MOFs) for ratiometric fluorescence for the detection of PO₄^3- based on Tb³⁺ and Ce³⁺ as the center metal ions and terephthalic acid as the organic ligand.

Temperature-independent ultra-sensitive refractive index sensor based on hollow-core silica tubes and tapers

We demonstrate a simple and ultra-sensitive refractive index (RI) sensor using a hollow-core silica tube (HCST) sandwiched between an up-taper and a down-taper in single mode fibers (SMF). According to our theoretical analysis, the interference spectrum comes from a combination of a three-beam multi-mode interference and anti-resonance effects. RI sensing will affect the mode interference. By demodulating the fringe contrast of the interference spectra, an ultrahigh sensitivity of -120.18 dB/RIU is achieved, implying a RI resolution of ∼ 8×10^-6 in the RI range from 1.35 to 1.43. What's more, the sensor has great temperature insensitivity of -0.0085 dB/°C, indicating an extremely low cross sensitivity of 7×10^-5 RIU/°C, which further benefits its practical application. The proposed configuration does not require special fiber or fabrication technique. In addition, the sensor's other merits such as simple and compact structure and ease offabrication offer the potential in biochemical sensing applications.

Rapid nitrate determination with a portable lab-on-chip device based on double microstructured assisted reactors

Determining the nitrate levels is critical for water quality monitoring, and traditional methods are limited by high toxicity and low detection efficiency. Here, rapid nitrate determination was realized using a portable device based on innovative three-dimensional double microstructured assisted reactors (DMARs). On-chip nitrate reduction and chromogenic reaction were conducted in the DMARs, and the reaction products then flowed into a PMMA optical detection chip for absorbance measurement. A significant enhancement of reaction rate and efficiency was observed in the DMARs due to their sizeable surface-area-to-volume ratios and hydrodynamics in the microchannels. The highest reduction ratio of 94.8% was realized by optimizing experimental parameters, which is greatly improved compared to conventional zinc-cadmium based approaches. Besides, modular optical detection improves the reliability of the portable device, and a smartphone was used to achieve a portable and convenient nitrate analysis. Different water samples were successfully analysed using the portable device based on DMARs. The results demonstrated that the device features fast detection (115 s per sample), low reagent consumptions (26.8 μL per sample), particularly low consumptions of toxic reagents (0.38 μL per sample), good reproducibility and low relative standard deviations (RSDs, 0.5-1.38%). Predictably, the portable lab-on-chip device based on microstructured assisted reactors will find more applications in the field of water quality monitoring in the near future.

A fast phosphate colorimetric sensor based on MoS2/UiO-66 (Fe/Zr) nanocomposites as oxidase-/peroxidase-like nanoenzymes

According to the excellent peroxidase activity of MoS2/UiO-66(Fe/Zr) as well as specific recognition towards phosphate (Pi), a fast cheap colorimetric sensing platform has been established to determine phosphate.

Turn-on detection of assorted phosphates by luminescent chemosensors

This review illustrates a variety of luminescent chemosensors for the selective detection of assorted phosphates via the “Turn-On” emission mechanism with focus on their design aspects, chemical structures and sensing mechanism.

Amplitude Holographic Interference-Based Microfluidic Colorimetry at the Micrometer Scale

Quantitative molecular analysis is usually based on spectrophotometric methods using colorimetric assay. Conventional methods, however, rely on the direct uniform absorption of the sample under test, and the detection sensitivity is strictly limited by the length of the absorption cell at the millimeter scale. Here, we report a new methodology for colorimetric assay based on the amplitude holographic interference (AHI) caused by nonuniform absorption of light, with detection sensitivity at the micrometer scale. In our method, the curved surface of the microfluidics results in a phase profile with a high diffraction efficiency, and the nonuniform absorption of samples exactly matches with the amplitude modulation in the holographic interference. The signal intensity is affected by not only direct sample absorption but also the sequential optical interference behind the liquid level. Both single- and multiple-wavelength colorimetric analyses of the Griess-Saltzman dye (GSD) were carried out using this method, and we found that the sensitivity can be improved by approximately 2-fold in comparison to the conventional method. This interference-based method would be a useful tool for the colorimetric assay of chemical samples in highly integrated systems with better performance.

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.

Using the interfacial barrier effects of p-n junction on electrochemistry for detection of phosphate

A novel type of electrochemical sensor for detection of phosphate in water environment was developed by combining the interfacial barrier of p-n junction with the adsorption of phosphate. The electrochemical response was produced by the induced change of the barrier height, which was only caused by the specific adsorption of phosphate. Two linear concentration ranges (0-0.045 mg L^-1 and 0.045-0.090 mg L^-1) with two sensitivities (4.98 μA (μg L^-1)^-1 and 1.28 μA (μg L^-1)^-1) were found. The good performance made the sensor meet the requirements of the World Health Organization for drinking water (1 mg L^-1 of phosphate). It is an approach to develop electrochemical sensors by employing the interfacial barrier effects on electrochemistry.

Silver Nanoprism Enhanced Colorimetry for Precise Detection of Dissolved Oxygen

Dissolved oxygen (DO) content is an essential indicator for evaluating the quality of the water body and the main parameter for water quality monitoring. The development of high-precision DO detection methods is of great significance. This paper reports an integrated optofluidic device for the high precision measurement of dissolved oxygen based on the characteristics of silver nanoprisms. Metal nanoparticles, especially silver nanoprisms, are extremely sensitive to their surroundings. In glucose and glucose oxidase systems, dissolved oxygen will be transformed into H₂O₂, which affects the oxidation and erosion process of nanoprisms, then influences the optical properties of nanoparticles. By detecting the shift in the plasma resonance peak of the silver nanoparticles, the dissolved oxygen (DO) content can be determined accurately. Great reconfigurability is one of the most significant advantages of the optofluidic device. By simply adjusting the flow rate ratio between the silver nanoprisms flow and the water sample flow, real-time continuous adjustment of the detection ranges of DO from 0 to 16 mg/L can be realized dynamically. The detection limit of this device is as low as 0.11 µM (3.52 µg/L) for DO measurement. Thus, the present optofluidic system has a wide range of potential applications in fields of biomedical analyses and water sensing.

Optofluidic detection setup for multi-parametric analysis of microbiological samples in droplets

High-throughput microbiological experimentation using droplet microfluidics is limited due to the complexity and restricted versatility of the available detection techniques. Current detection setups are bulky, complicated, expensive, and require tedious optical alignment procedures while still mostly limited to fluorescence. In this work, we demonstrate an optofluidic detection setup for multi-parametric analyses of droplet samples by easily integrating micro-lenses and embedding optical fibers for guiding light in and out of the microfluidic chip. The optofluidic setup was validated for detection of absorbance, fluorescence, and scattered light. The developed platform was used for simultaneous detection of multiple parameters in different microbiological applications like cell density determination, growth kinetics, and antibiotic inhibition assays. Combining the high-throughput potential of droplet microfluidics with the ease, flexibility, and simplicity of optical fibers results in a powerful platform for microbiological experiments.

Microfluidic Mixer with Automated Electrode Switching for Sensing Applications

An electronic tongue (e-tongue) is a multisensory system usually applied to complex liquid media that uses computational/statistical tools to group information generated by sensing units into recognition patterns, which allow the identification/distinction of samples. Different types of e-tongues have been previously reported, including microfluidic devices. In this context, the integration of passive mixers inside microchannels is of great interest for the study of suppression/enhancement of sensorial/chemical effects in the pharmaceutical, food, and beverage industries. In this study, we present developments using a stereolithography technique to fabricate microfluidic devices using 3D-printed molds for elastomers exploring the staggered herringbone passive mixer geometry. The fabricated devices (microchannels plus mixer) are then integrated into an e-tongue system composed of four sensing units assembled on a single printed circuit board (PCB). Gold-plated electrodes are designed as an integral part of the PCB electronic circuitry for a highly automated platform by enabling faster analysis and increasing the potential for future use in commercial applications. Following previous work, the e-tongue sensing units are built functionalizing gold electrodes with layer-by-layer (LbL) films. Our results show that the system is capable of (i) covering basic tastes below the human gustative perception and (ii) distinguishing different suppression effects coming from the mixture of both strong and weak electrolytes. This setup allows for triplicate measurements in 12 electrodes, which represents four complete sensing units, by automatically switching all electrodes without any physical interaction with the sensor. The result is a fast and reliable data acquisition system, which comprises a suitable solution for monitoring, sequential measurements, and database formation, being less susceptible to human errors.

Linker-Eliminated Nano Metal-Organic Framework Fluorescent Probe for Highly Selective and Sensitive Phosphate Ratiometric Detection in Water and Body Fluids

Phosphate is an important anion in both the aquatic environment and biological systems. The search for a selective and sensitive phosphate ratiometric fluorescent probe to quantify the phosphate level in water samples and body fluids is of great significance for the protection of the ecological environment and human health. Here, a porphyrin-based nano metal-organic framework (NMOF), PCN-224, was successfully exploited as a simple but highly sensitive and selective single-component ratiometric fluorescent probe with accurate composition and measurable structure for the quantitative determination of phosphate, based on the interesting double-emission fluorescence of the porphyrin ligand itself. Compared with other zirconium-based NMOF probes for phosphate, the reduced number of connections for ZrO clusters with the ligand in PCN-224 obtained by a linker-elimination strategy simultaneously provides more active recognition sites for phosphate, which effectively improves the sensitivity of the zirconium-based NMOF probes. The detection limit of the probe is only 54 nM. Additionally, the accuracy of the ratiometric detection based on this probe was further proved by the detection of phosphate in human serum and drinking water.

Autonomous and In Situ Ocean Environmental Monitoring on Optofluidic Platform

Determining the distributions and variations of chemical elements in oceans has significant meanings for understanding the biogeochemical cycles, evaluating seawater pollution, and forecasting the occurrence of marine disasters. The primary chemical parameters of ocean monitoring include nutrients, pH, dissolved oxygen (DO), and heavy metals. At present, ocean monitoring mainly relies on laboratory analysis, which is hindered in applications due to its large size, high power consumption, and low representative and time-sensitive detection results. By integrating photonics and microfluidics into one chip, optofluidics brings new opportunities to develop portable microsystems for ocean monitoring. Optofluidic platforms have advantages in respect of size, cost, timeliness, and parallel processing of samples compared with traditional instruments. This review describes the applications of optofluidic platforms on autonomous and in situ ocean environmental monitoring, with an emphasis on their principles, sensing properties, advantages, and disadvantages. Predictably, autonomous and in situ systems based on optofluidic platforms will have important applications in ocean environmental monitoring.

Easily fabricated monolithic fluoropolymer chips for sensitive long-term absorbance measurement in droplet microfluidics

Maintaining a hydrophobic channel surface is critical to ensuring long-term stable flow in droplet microfluidics. Monolithic fluoropolymer chips ensure robust and reliable droplet flow as their native fluorous surfaces naturally preferentially wet fluorocarbon oils and do not deteriorate over time. Their fabrication, however, typically requires expensive heated hydraulic presses that make them inaccessible to many laboratories. Here we describe a method for micropatterning and bonding monolithic fluoropolymer flow cells from a commercially available melt-processable fluoropolymer, Dyneon THV 500GZ, that only requires a standard laboratory oven. Using this technique, we demonstrate the formation of complex microstructures, specifically the fabrication of sensitive absorbance flow cells for probing droplets in flow, featuring path lengths up to 10 mm. The native fluorous channel surface means the flow cells can be operated over extended periods, demonstrated by running droplets continuously through a chip for 16 weeks.

We present a widely accessible method for fabricating monolithic fluoropolymer microfluidic chips, which allows droplet absorbance measurement over multi-month periods.

An Integrated Optofluidic Platform Enabling Total Phosphorus On-Chip Digestion and Online Real-Time Detection

This paper presents a total phosphorus online real-time monitoring system integrated with on-chip digestion based on the merits of optofluidic technology. The integrated optofluidic device contains a hollow optical fiber employed for pretreatment and digestion of phosphorus solution samples, a polydimethylsiloxane (PDMS)-based micromixer with convergent-divergent walls designed to enable sufficient mixing and chromogenic reaction, and a couple of optical fiber collimators attached with a Z-shaped flow cell for optical detection. Details of system design and fabrication are introduced in this paper. In the experiment, on-chip digestion of four typical phosphates in aqueous solution including organophosphorus and inorganic phosphorus is investigated under different reaction conditions, such as digestion temperature, concentration of oxidant and pH value, and the optimal reaction parameters are explored under different conditions. Meanwhile, we demonstrate the online real-time monitoring function of the optofluidic device, and the digestion mechanisms of four different phosphates are analyzed and discussed. Compared with the national standard method, we find that the measurement accuracy and sensitivity are acceptable when the concentration of total phosphorus is between 0.005-0.9 mg/L (by weight of P) in aqueous solution, which covers the range defined in the national standard. The traditional digestion time of several hours is greatly reduced to less than 10 s, and the content of total phosphorus can be obtained in a few minutes. The integrated optofluidic device can significantly shorten the test time and reduce the sample amount, and also provides a versatile platform for the real-time detection and analysis of many biochemical samples.

Coiled Optical Nanofiber for Optofluidic Absorbance Detection

A challenge for optofluidic absorbance detection is the high concentration limit of detection due to the short optical path length. Herein, we introduce a concept of utilizing the coiled optical nanofiber for highly sensitive and robust optofluidic absorbance detection. Investigated by measuring the absorbance of FeCl₃ solutions, the sensor shows a detection limit down to 10 μM with excellent reversibility in a concentration range of 0-5 mM. The sensitivity is 10-fold higher than that of standard absorbance measurement by using a 1 cm cuvette. Also, highly sensitive chloramphenicol sensing was demonstrated by using the enzyme-linked immunosorbent assay (ELISA) method, achieving a detection limit below 0.5 ng/L. The higher sensitivity and lower detection limit are caused by the large fractional power of evanescent field outside the nanofiber and the long detection length, which can effectively improve the absorption of the evanescent field, while the excellent reversibility is caused by the support of a polydimethylsiloxane (PDMS) pillar rather than by suspending the nanofiber in the microchannel. We envision that the present work may open up new opportunities for ultrasensitive chemical and biological sensing.

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.

Effects and improvements of different reagents preservation methods on the determination of phosphate in seawater by phosphomolybdenum blue spectrophotometric method

This work attempted to prolong the validity of the molybdate mixed solution and ascorbic acid solution used in the phosphomolybdenum blue spectrophotometric method by improving their preservation according to the influence factors. The results showed that the molybdate mixed solution can be directly preserved in darkness with validity over half a year. The ascorbic acid solution is influenced by light, temperature, pH, metal ions, oxygen, and bacteria. The validity of ascorbic acid is shortened as the temperature rises. Through keeping in darkness, adding complexing agents, adjusting pH, removing oxygen and sterilization, the validity of ascorbic acid solution was prolonged to over 2.7 times under 4 °C and over 5 times under 25 °C. At the same time, the hybrid solution of ascorbic acid solution and molybdate mixed solution should be preserved separately, otherwise the using effect is poor.

Detection of phosphorus species in water: technology and strategies

This review highlights recent advances in methods of detection of total phosphorus in water, including photoelectric strategies, spectroscopy techniques, and modeling algorithms.

Optofluidic differential colorimetry for rapid nitrite determination

Nitrite detection plays a very important role in environmental monitoring and for industrial purposes. The commonly used colorimetric analysis requires the measurement of a system's calibration curve by asynchronously preparing and detecting a dozen standard samples, leading to time-consuming, slow and cumbersome procedures. Here, we present a differential colorimetry method that determines the nitrite level based on the paired chromaticity gradient, formed by coupling the colour reaction into the microfluidic network. The two gradients reshape each other and contain enough information for the quantitative analysis of the sample being tested, without the need for a calibration curve. The independence of the two gradients of the absorbance change caused by the detecting system and water quality results in a high stability and anti-interference performance, with the assistance of its self-correcting ability. This differential colorimetry method requires little time and energy consumption as only one sample is needed. Standard nitrite solutions of 0.50 mM and 0.33 mM have been determined with an error of 1.16% and 0.50%, respectively. These measurements are advantageous in terms of greater stability by up to 10 times and accuracy by 6 times, compared with the calibration curve approaches. It is foreseeable that this differential colorimetry method will find a wide range of applications in the field of chemical detection.

Real-time detection and monitoring of the drug resistance of single myeloid leukemia cells by diffused total internal reflection

Real-time detection and monitoring of the drug resistance of single cells have important significance in clinical diagnosis and therapy. Traditional methods operate a number of times for each individual concentration, and innovation is required for the design of more simple and efficient manipulation platforms with necessary higher sensitivity. Here, we have developed a novel diffused total internal reflection (TIR) method to perform drug metabolism and cytotoxicity analysis of trapped myeloid leukemia cells. Molm-13 cells, a type of acute myeloid leukemia cell, were chosen and injected into the device and fittingly captured by cell traps. Differing from previous studies, a series of different concentrations of azelaic acid (AZA) drug could be used from 0 mM to 50 mM through convection and diffusion processes in a single chip, with each concentration region featuring 50 cells, with a total of 549 cell trapping units. Thanks to the high sensitivity of the TIR method, only cells with the same drug concentration could be illuminated in the detection process. By adjusting the incident angle, we could exactly detect and monitor the drug resistance of the cells using different drug concentrations and the experimental resolution of the drug concentration was as small as 5 mM. Images of the membrane integrity and morphology of the cells in the bright field were measured and we also monitored the cell viabilities in the dark field over 2 hours. The effects of AZA on the Molm-13 cells were explored in different concentrations at the single cell level. Compared with the results of the traditional MTT assay method, the experimental results are more simple and accurate. A cell death of 5% at an AZA concentration of 5 mM was observed after 30 minutes, while a concentration of 40 mM corresponded to a 98% cell death. The designed method in this study provides a novel toolkit to control and monitor drug resistance at the single cell level more easily with higher sensitivity and we believe it has significant potential application in single cell quality assessment and medicine analysis in clinical practice.

Light Manipulation in Inhomogeneous Liquid Flow and Its Application in Biochemical Sensing

Light manipulation has always been the fundamental subject in the field of optics since centuries ago. Traditional optical devices are usually designed using glasses and other materials, such as semiconductors and metals. Optofluidics is the combination of microfluidics and optics, which brings a host of new advantages to conventional solid systems. The capabilities of light manipulation and biochemical sensing are inherent alongside the emergence of optofluidics. This new research area promotes advancements in optics, biology, and chemistry. The development of fast, accurate, low-cost, and small-sized biochemical micro-sensors is an urgent demand for real-time monitoring. However, the fluid flow in the on-chip sensor is usually non-uniformed, which is a new and emerging challenge for the accuracy of optical detection. It is significant to reveal the principle of light propagation in an inhomogeneous liquid flow and the interaction between biochemical samples and light in flowing liquids. In this review, we summarize the current state of optofluidic lab-on-a-chip techniques from the perspective of light modulation by the unique dynamic properties of fluid in heterogeneous media, such as diffusion, heat transfer, and centrifugation etc. Furthermore, this review introduces several novel photonic phenomena in an inhomogeneous liquid flow and demonstrates their application in biochemical sensing.

Optofluidic Technology for Water Quality Monitoring

Water quality-related incidents are attracting attention globally as they cause serious diseases and even threaten human lives. The current detection and monitoring methods are inadequate because of their long operation time, high cost, and complex process. In this context, there is an increasing demand for low-cost, multiparameter, real-time, and continuous-monitoring methods at a higher temporal and spatial resolution. Optofluidic water quality sensors have great potential to satisfy this requirement due to their distinctive features including high throughput, small footprint, and low power consumption. This paper reviews the current development of these sensors for heavy metal, organic, and microbial pollution monitoring, which will breed new research ideas and broaden their applications.

Optofluidic gutter oil discrimination based on a hybrid-waveguide coupler in fibre

Discriminating edible oils from gutter oils has significance in food safety, as illegal gutter oils cannot meet a variety of criteria such as the acid value, peroxide value and quality. To discriminate these illegal cooking oils, we propose an ultrasensitive optofluidic detection method based on a hybrid-waveguide coupler. Prior to the straight waveguide inscription in the cladding of the silica tube using a femtosecond laser, a section of coreless fibre is firstly spliced with the ST to supply a platform for the inscription of an S-band waveguide. Then a pair of microfluidic channels are ablated on the ST using the fs laser to enable liquid analytes to flow in and out of the air channel. In the transmission spectrum, a unique resonant loss dip can be observed, which is produced by coupling the light from the laser inscribed waveguide to the liquid core when the phase-matching condition is met. This hybrid-waveguide coupler with a simplified structure realizes dynamic optofluidic refractive index sensing with an ultrahigh sensitivity of -112 743 nm RIU^-1, a detection limit of 2.08 × 10^-5 RIU and a refractive index detection range from 1.4591 to 1.4622. This novel method can be used for food safety detection, specifically, for the discrimination of gutter oils.

Ultra-sensitive chemical and biological analysis via specialty fibers with built-in microstructured optofluidic channels

All-in-fiber optofluidics is an analytical tool that provides enhanced sensing performance with simplified analyzing system design. Currently, its advance is limited either by complicated liquid manipulation and light injection configuration or by low sensitivity resulting from inadequate light-matter interaction. In this work, we design and fabricate a side-channel photonic crystal fiber (SC-PCF) and exploit its versatile sensing capabilities in in-line optofluidic configurations. The built-in microfluidic channel of the SC-PCF enables strong light-matter interaction and easy lateral access of liquid samples in these analytical systems. In addition, the sensing performance of the SC-PCF is demonstrated with methylene blue for absorptive molecular detection and with human cardiac troponin T protein by utilizing a Sagnac interferometry configuration for ultra-sensitive and specific biomolecular specimen detection. Owing to the features of great flexibility and compactness, high-sensitivity to the analyte variation, and efficient liquid manipulation/replacement, the demonstrated SC-PCF offers a generic solution to be adapted to various fiber-waveguide sensors to detect a wide range of analytes in real time, especially for applications from environmental monitoring to biological diagnosis.