Recent advances in microfluidic sensors for nutrients detection in water

Relevant insights and concepts overlooked throughout the development of flow analysis. A tutorial

The conceptual expansion, fast development, and general acceptance of flow analysis are consequence of its adherence to the principles of green and white analytical chemistry, and chemical derivatization plays an essential role in this context. Through the flow analysis development, however, some of its potentialities and limitations have been overlooked. This is more evident when the involved modifications in flow rates, timing and/or manifold architecture deteriorate the analytical signals. These aspects have not always been systematically investigated, and are addressed here in relation to flow analyzers with UV-Vis spectrophotometric detection. Novel strategies for solution handling, guidance for dealing with the aforementioned analytical signal deterioration, and an alternative possibility for exploiting differential aspiration are presented. The concept of blank reagent carrier stream is proposed.

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.

A Simultaneous Calibration and Detection Strategy for Electrochemical Sensing with High Accuracy in Complex Water

The electrochemical sensors loaded with nanomaterials have exhibited a great sensitivity. Nonetheless, the field detection for complex waterbodies can be affected by cross-sensitivity, environmental conditions such as temperature and pH value, as well as the relatively low reproducibility and stability of nanomaterials. In this paper, a simultaneous calibration and detection (SCD) strategy is proposed to introduce a simultaneous and precise calibration during field electrochemical detection, which is composed of a linear regression algorithm and a compact electrochemical sensor containing a series of identical sensing cells. This design can significantly mitigate cross-sensitivity in complex water and the inconsistency of sensing materials. Applied in the NO2- detection for practical waterbodies, the SCD strategy has exhibited a relative error of no more than 9.6% for the measurement compared to the results obtained by the standard Griess method and higher accuracy than the normal electrochemical method. The SCD strategy is independent of sensing materials, indicating that it can be widely applied to various detections by just switching the corresponding sensing material.

Microfluidic sensors for the detection of emerging contaminants in water: A review

In recent years, numerous emerging contaminants have been identified in surface water, groundwater, and drinking water. Developing novel sensing methods for detecting diverse emerging pollutants in water is urgently needed, as even at low concentrations, these pollutants can pose a serious threat to human health and environmental safety. Traditional testing methods are based on laboratory equipment, which is highly sensitive but complex to operate, costly, and not suitable for on-site monitoring. Microfluidic sensors offer several benefits, including rapid evaluation, minimal sample usage, accurate liquid manipulation, compact size, automation, and in-situ detection capabilities. They provide promising and efficient analytical tools for high-performance sensing platforms in monitoring emerging contaminants in water. In this paper, recent research advances in microfluidic sensors for the detection of emerging contaminants in water are reviewed. Initially, a concise overview is provided about the various substrate materials, corresponding microfabrication techniques, different driving forces, and commonly used detection techniques for microfluidic devices. Subsequently, a comprehensive analysis is conducted on microfluidic detection methods for endocrine-disrupting chemicals, pharmaceuticals and personal care products, microplastics, and perfluorinated compounds. Finally, the prospects and future challenges of microfluidic sensors in this field are discussed.

PCB-C4D coupled with paper-based microfluidic sampling for the rapid detection of liquid conductivity

The detection of liquid electrical conductivity has board applications in food safety testing, water quality monitoring, and agricultural soil analysis. Electrodes used in traditional liquid electrical conductivity detection come into direct contact with liquid, leading to electrode contamination and affecting the accuracy of the detection results. The capacitively coupled contactless conductivity detection (C4D) method effectively addresses this issue. However, impurity particles present in the solution can compromise the consistency and repeatability of detection results. This study combines paper-based microfluidic technology with printed circuit board-capacitively coupled contactless conductivity detection (PCB-C4D) to address this issue. Prior to sample detection, in situ rapid filtration is employed to remove impurity particles from the raw solution sample, significantly enhancing detection consistency and reliability. Simultaneously, Optimization of PCB-C4D parameters, channel size, filtration time, and solution drop rate ensures optimal detection conditions. A compact kit design facilitates reliable assembly of the PCB-C4D electrodes and paper-based channel, enhancing practicality. Practical measurements on the conductivity of orange juice, cucumber, and soil solution further validate the method's accuracy, rapidity, and effectiveness in in situ conductivity detection. This work advances the practical application of PCB-C4D technology.

Continuous Flow with Reagent Injection on an Inlaid Microfluidic Platform Applied to Nitrite Determination

A continuous flow with reagent injection method on a novel inlaid microfluidic platform for nitrite determination has been successfully developed. The significance of the high-frequency monitoring of nutrient fluctuations in marine environments is crucial for understanding our impacts on the ecosystem. Many in-situ systems face limitations in high-frequency data collection and have restricted deployment times due to high reagent consumption. The proposed microfluidic device employs automatic colorimetric absorbance spectrophotometry, using the Griess assay for nitrite determination, with minimal reagent usage. The sensor incorporates 10 solenoid valves, four syringes, two LEDs, four photodiodes, and an inlaid microfluidic technique to facilitate optical measurements of fluid volumes. In this flow system, Taylor-Aris dispersion was simulated for different injection volumes at a constant flow rate, and the results have been experimentally confirmed using red food dye injection into a carrier stream. A series of tests were conducted to determine a suitable injection frequency for the reagent. Following the initial system characterization, seven different standard concentrations ranging from 0.125 to 10 µM nitrite were run through the microfluidic device to acquire a calibration curve. Three different calibrations were performed to optimize plug length, with reagent injection volumes of 4, 20, and 50 µL. A straightforward signal processing method was implemented to mitigate the Schlieren effect caused by differences in refractive indexes between the reagent and standards. The results demonstrate that a sampling frequency of at least 10 samples per hour is achievable using this system. The obtained attenuation coefficients exhibited good agreement with the literature, while the reagent consumption was significantly reduced. The limit of detection for a 20 µL injection volume was determined to be 94 nM from the sample intake, and the limit of quantification was 312 nM. Going forward, the demonstrated system will be packaged in a submersible enclosure to facilitate in-situ colorimetric measurements in marine environments.

Microfluidics in environmental analysis: advancements, challenges, and future prospects for rapid and efficient monitoring

Microfluidic devices have emerged as advantageous tools for detecting environmental contaminants due to their portability, ease of use, cost-effectiveness, and rapid response capabilities. These devices have wide-ranging applications in environmental monitoring of air, water, and soil matrices, and have also been applied to agricultural monitoring. Although several previous reviews have explored microfluidic devices' utility, this paper presents an up-to-date account of the latest advancements in this field for environmental monitoring, looking back at the past five years. In this review, we discuss devices for prominent contaminants such as heavy metals, pesticides, nutrients, microorganisms, per- and polyfluoroalkyl substances (PFAS), etc. We cover numerous detection methods (electrochemical, colorimetric, fluorescent, etc.) and critically assess the current state of microfluidic devices for environmental monitoring, highlighting both their successes and limitations. Moreover, we propose potential strategies to mitigate these limitations and offer valuable insights into future research and development directions.

Simple microfluidic devices for in situ detection of water contamination: a state-of-art review

Water security is an important global issue that is pivotal in the pursuit of sustainable resources for future generations. It is a multifaceted concept that combines water availability with the quality of the water's chemical, biological, and physical characteristics to ensure its suitability and safety. Water quality is a focal aspect of water security. Quality index data are determined and provided via laboratory testing using expensive instrumentation with high maintenance costs and expertise. Due to increased practices in this sector that can compromise water quality, innovative technologies such as microfluidics are necessary to accelerate the timeline of test procedures. Microfluidic technology demonstrates sophisticated functionality in various applications due to the chip's miniaturization system that can control the movement of fluids in tiny amounts and be used for onsite testing when integrated with smart applications. This review aims to highlight the basics of microfluidic technology starting from the component system to the properties of the chip's fabricated materials. The published research on developing microfluidic sensor devices for monitoring chemical and biological contaminants in water is summarized to understand the obstacles and challenges and explore future opportunities for advancement in water quality monitoring.

Deployable Lab-on-a-Chip Sensor for Colorimetric Measurements

The infield measurement of nutrients, heavy metals, and other contaminants in water is still a needed tool in environmental sciences. The Lab-on-a-chip approach can develop deployable instruments that use the standardized analytical assay in a miniaturized manner in the field. This paper presents a Lab-on-a-chip platform for colorimetric measurements that can be deployed for nutrient monitoring in open water (oceans, rivers, lakes, etc.). Nitrite was selected as an analyte. Change to other analytes is possible by changing the reagents and the detection wavelength. In this paper, the principle of the sensor, technical realization, setup of the sensor, and test deployment are described. The sensor prototype was deployed at the Jade Bay (German Bight) for 9 h, measuring the nitrite value every 20 min. Reference samples were taken and processed in the lab. The work presented here shows that an infield measurement using a colorimetric assay is possible by applying Lab-on-a-chip principles.

Microfluidic Platform Integrated with Carbon Nanofibers-Decorated Gold Nanoporous Sensing Device for Serum PSA Quantification

Prostate cancer is a disease with a high incidence and mortality rate in men worldwide. Serum prostate-specific antigens (PSA) are the main circulating biomarker for this disease in clinical practices. In this work, we present a portable and reusable microfluidic device for PSA quantification. This device comprises a polymethyl methacrylate microfluidic platform coupled with electrochemical detection. The platinum working microelectrode was positioned in the outflow region of the microchannel and was modified with carbon nanofibers (CNF)-decorated gold nanoporous (GNP) structures by the dynamic hydrogen bubble template method, through the simultaneous electrodeposition of metal precursors in the presence of CNF. CNF/GNP structures exhibit attractive properties, such as a large surface to volume ratio, which increases the antibody's immobilization capacity and the electroactive area. CNFs/GNP structures were characterized by scanning electron microscopy, energy dispersive spectrometry, and cyclic voltammetry. Anti-PSA antibodies and HRP were employed for the immune-electrochemical reaction. The detection limit for the device was 5 pg mL^-1, with a linear range from 0.01 to 50 ng mL^-1. The coefficients of variation within and between assays were lower than 4.40%, and 6.15%, respectively. Additionally, its clinical performance was tested in serum from 30 prostate cancer patients. This novel device was a sensitive, selective, portable, and reusable tool for the serological diagnosis and monitoring of prostate cancer.