Autonomous and In Situ Ocean Environmental Monitoring on Optofluidic Platform

Point-of-Care-Testing NO3-N Detection Technology with Selected Transition-Metal-Based Colorimetric Sensor Arrays

Nitrate-nitrogen (NO3-N) is a major contaminant in groundwater and seawater. Significant amounts of ammonia are oxidized to nitrate through nitrification, leading to an imbalance in the nitrogen cycle and causing nitrate pollution in water bodies. Controlling NO3-N levels is a significant challenge for both marine aquaculture and human health. Traditional measurement methods, such as ion chromatography and continuous flow analysis, require pretreatment steps to detect NO3-N in complex matrices, which is time-consuming. However, in this study, we developed a transition-metal-based sensor capable of measuring NO3-N concentrations on-site without the need for pretreatment. We analyzed the color change of transition-metal-based sensors over time and obtained color data by mixing transition metals (Mn, V, Fe, Co, Cr, Cu, and Ni) with solvents and additives at fixed ratios, and combining them with standard solutions of NO3-N at concentrations of 1, 2, 3, 5, 10, 20, 30, 40, 50, 75, and 100 ppm. We selected sensors that exhibited linearly increasing color velocity with increasing NO3-N concentrations and developed an array sensor using the selected sensors. The performance of the array was validated by comparing its results with those of hierarchical cluster analysis (HCA) based on color data and compositional analysis, confirming its ability to detect NO3-N in complex matrices. Additionally, by creating a large data set of color change patterns of the array sensor, we can develop selective array sensors for detecting specific substances, surpassing the capability of merely measuring the NO3-N concentration.

A microfluidic analyzer based on liquid waveguide capillary cells for the high-sensitivity determination of phosphate in seawater and its applications

BACKGROUND

Optical detection is frequently performed on microfluidic chips for colorimetric analysis. Integrating liquid waveguide capillaries with total internal reflection with the microfluidic chip requires less procedures, which is suitable in the optical detection of microfluidic systems and is a practical alternative to increase the optical path length in the colorimetric assay of microfluidic devices for higher sensitivities and lower detection limit. However, this alternative has not been applied to the connection of PMMA chips or the microfluidic devices for the detection of phosphate in seawater.

RESLUTS

Here, a lab-on-a-chip system integrating a microfluidic chip and an external liquid waveguide capillary cell was presented to detect the phosphate in seawater. The detachable total internal reflection capillary made of Teflon AF 2400 connected to the chip transports sample and transmits light, greatly reducing detection limit, eliminating the interference from stray light and widening the dynamic range of the system without specific surface treatment of the microchannel. By utilizing an internal 5-cm absorption cell and an external 20-cm liquid waveguide capillary cell, the system reaches detection limits of 59 nM and 8 nM, respectively, and can detect phosphate concentration from 0 to 23 μM. An online analyzer was developed based on the high-sensitivity system and was applied to shipboard underway analysis for two scientific cruises and to laboratory measurements for seawater samples from Xisha sea area.

SIGNIFICANCE

Correlation analyses between the shipboard and laboratory phosphate measurements and other physical and biochemical elements revealed the marine ecological characteristics of the corresponding areas, demonstrating the high-sensitivity of this method over slight variations and narrow ranges of phosphate and the ability to provide microfluidic systems for high spatiotemporal resolution phosphate determination a practical and cost-effective alternative.

Ppm-level oxygen detection system based on deep-ultraviolet-absorption spectroscopy

Oxygen is a gas essential to human life and industrial processes. Here, we developed a highly sensitive oxygen gas sensor based on deep ultraviolet absorption spectroscopy between 180 and 200 nm. The implemented method relies on differential absorption spectra extracted from the obtained high-resolution absorption spectra. The detection capability was greatly improved (six-fold) by eliminating air from the open optical path, achieved by purging the entire system with pure nitrogen. A linear relationship was obtained between the optical parameter and the oxygen concentration with a slope of 0.107 and determination coefficient of 0.999. A detection limit of 24 ppm per meter was determined with a response time of 25 s. Good repeatability (standarddeviation=16ppm) and stability were confirmed. We demonstrated that this system can detect ppm oxygen levels with high sensitivity and low uncertainty.

Management and Sustainable Exploitation of Marine Environments through Smart Monitoring and Automation

Monitoring of aquatic ecosystems has been historically accomplished by intensive campaigns of direct measurements (by probes and other boat instruments) and indirect extensive methods such as aero-photogrammetry and satellite detection. These measurements characterized the research in the last century, with significant but limited improvements within those technological boundaries. The newest advances in the field of smart devices and increased networking capabilities provided by emerging tools, such as the Internet of Things (IoT), offer increasing opportunities to provide accurate and precise measurements over larger areas. These perspectives also correspond to an increasing need to promptly respond to frequent catastrophic impacts produced by drilling stations and intense transportation activities of dangerous materials over ocean routes. The shape of coastal ecosystems continuously varies due to increasing anthropic activities and climatic changes, aside from touristic activities, industrial impacts, and conservation practices. Smart buoy networks (SBNs), autonomous underwater vehicles (AUVs), and multi-sensor microsystems (MSMs) such as smart cable water (SCW) are able to learn specific patterns of ecological conditions, along with electronic “noses”, permitting them to set innovative low-cost monitoring stations reacting in real time to the signals of marine environments by autonomously adapting their monitoring programs and eventually sending alarm messages to prompt human intervention. These opportunities, according to multimodal scenarios, are dramatically changing both the coastal monitoring operations and the investigations over large oceanic areas by yielding huge amounts of information and partially computing them in order to provide intelligent responses. However, the major effects of these tools on the management of marine environments are still to be realized, and they are likely to become evident in the next decade. In this review, we examined from an ecological perspective the most striking innovations applied by various research groups around the world and analyzed their advantages and limits to depict scenarios of monitoring activities made possible for the next decade.

Enzyme Method-Based Microfluidic Chip for the Rapid Detection of Copper Ions

Metal ions in high concentrations can pollute the marine environment. Human activities and industrial pollution are the causes of Cu²⁺ contamination. Here, we report our discovery of an enzyme method-based microfluidic that can be used to rapidly detect Cu²⁺ in seawater. In this method, Cu²⁺ is reduced to Cu⁺ to inhibit horseradish peroxidase (HRP) activity, which then results in the color distortion of the reaction solution. The chip provides both naked eye and spectrophotometer modalities. Cu²⁺ concentrations have an ideal linear relationship, with absorbance values ranging from 3.91 nM to 256 μM. The proposed enzyme method-based microfluidic chip detects Cu²⁺ with a limit of detection (LOD) of 0.87 nM. Other common metal ions do not affect the operation of the chip. The successful detection of Cu²⁺ was achieved using three real seawater samples, verifying the ability of the chip in practical applications. Furthermore, the chip realizes the functions of two AND gates in series and has potential practical implementations in biochemical detection and biological computing.

Portable analyser using two-dimensional ion chromatography with ultra-violet light-emitting diode-based absorbance detection for nitrate monitoring within both saline and freshwaters

A portable and automated IC system with a dual-capability for the analysis of both fresh and saline environmental waters has been developed. Detection of nitrate in complex matrices such as seawater was achieved by the employment of an automated two-dimensional (heart-cut) IC method utilised in tandem with on-column matrix elimination, using a sodium chloride eluent. The system also demonstrated the capability to switch to a second mode of analysis, whereby direct one-dimensional IC analysis was employed to rapidly detect nitrite and nitrate in freshwater, with direct UV LED based absorption detection in under 3 minutes. Calibration curves using a 195 µL sample loop were generated for both freshwater and artificial seawater samples. For marine analysis, an analytical range of 0.1 mg L^-1 - 40 mg L^-1 NO₃^- was possible, while an analytical range (0.1 mg L^-1 - 15 mg L^-1 NO₂^-, 0.2 - 30 mg L^-1 NO₃^-) appropriate for freshwater analysis was also achieved. Chromatographic repeatability for both marine and freshwater analysis was verified over 40 sequential runs with RSD values of < 1% demonstrated for both peak area and retention times for each mode of analysis. The selectivity of both methods was demonstrated with interference tests with common anions present in environmental waters. Recovery analysis was carried out on marine samples from Tramore Bay, Co. Waterford, Ireland, and the systems analytical performance was compared with that of an accredited IC following environmental sample analysis.

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 Phosphorescence Quenching-Based Intelligent Dissolved Oxygen Sensor on an Optofluidic Platform

Continuous measurement of dissolved oxygen (DO) is essential for water quality monitoring and biomedical applications. Here, a phosphorescence quenching-based intelligent dissolved oxygen sensor on an optofluidic platform for continuous measurement of dissolved oxygen is presented. A high sensitivity dissolved oxygen-sensing membrane was prepared by coating the phosphorescence indicator of platinum(II) meso-tetrakis(pentafluorophenyl)porphyrin (PtTFPP) on the surface of the microfluidic channels composed of polydimethylsiloxane (PDMS) microstructure arrays. Then, oxygen could be determined by its quenching effect on the phosphorescence, according to Stern–Volmer model. The intelligent sensor abandons complicated optical or electrical design and uses a photomultiplier (PMT) counter in cooperation with a mobile phone application program to measure phosphorescence intensity, so as to realize continuous, intelligent and real-time dissolved oxygen analysis. Owing to the combination of the microfluidic-based highly sensitive oxygen sensing membrane with a reliable phosphorescent intensity detection module, the intelligent sensor achieves a low limit of detection (LOD) of 0.01 mg/L, a high sensitivity of 16.9 and a short response time (22 s). Different natural water samples were successfully analyzed using the intelligent sensor, and results demonstrated that the sensor features a high accuracy. The sensor combines the oxygen sensing mechanism with optofluidics and electronics, providing a miniaturized and intelligent detection platform for practical oxygen analysis in different application fields.

Lab-on-a-chip technology for in situ combined observations in oceanography

The oceans sustain the global environment and diverse ecosystems through a variety of biogeochemical processes and their complex interactions. In order to understand the dynamism of the local or global marine environments, multimodal combined observations must be carried out in situ. On the other hand, instrumentation of in situ measurement techniques enabling biological and/or biochemical combined observations is challenging in aquatic environments, including the ocean, because biochemical flow analyses require a more complex configuration than physicochemical electrode sensors. Despite this technical hurdle, in situ analyzers have been developed to measure the concentrations of seawater contents such as nutrients, trace metals, and biological components. These technologies have been used for cutting-edge ocean observations to elucidate the biogeochemical properties of water mass with a high spatiotemporal resolution. In this context, the contribution of lab-on-a-chip (LoC) technology toward the miniaturization and functional integration of in situ analyzers has been gaining momentum. Due to their mountability, in situ LoC technologies provide ideal instrumentation for underwater analyzers, especially for miniaturized underwater observation platforms. Consequently, the appropriate combination of reliable LoC and underwater technologies is essential to realize practical in situ LoC analyzers suitable for underwater environments, including the deep sea. Moreover, the development of fundamental LoC technologies for underwater analyzers, which operate stably in extreme environments, should also contribute to in situ measurements for public or industrial purposes in harsh environments as well as the exploration of the extraterrestrial frontier.

Editorial for the Special Issue on Optofluidic Devices and Applications

Optofluidic devices are of high scientific and industrial interest in chemistry, biology, material science, pharmacy, and medicine [...].