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1. A Paper-Based Device for Ultrasensitive, Colorimetric Phosphate Detection in Seawater

   https://doi.org/10.3390/s20102766  

   Racicot, Joan M.; Mako, Teresa L.; Olivelli, Alexander; Levine, Mindy (May 2020, Sensors)

   High concentrations of certain nutrients, including phosphate, are known to lead to undesired algal growth and low dissolved oxygen levels, creating deadly conditions for organisms in marine ecosystems. The rapid and robust detection of these nutrients using a colorimetric, paper-based system that can be applied on-site is of high interest to individuals monitoring marine environments and others affected by marine ecosystem health. Several techniques for detecting phosphate have been reported previously, yet these techniques often suffer from high detection limits, reagent instability, and the need of the user to handle toxic reagents. In order to develop improved phosphate detection methods, the commonly used molybdenum blue reagents were incorporated into a paper-based, colorimetric detection system. This system benefited from improved stabilization of the molybdenum blue reagent as well as minimal user contact with toxic reagents. The colorimetric readout from the paper-based devices was analyzed and quantified using RGB analyses (via ImageJ), and resulted in the detection of phosphate at detection limits between 1.3 and 2.8 ppm in various aqueous media, including real seawater. 

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2. A Colorimetric Dip Strip Assay for Detection of Low Concentrations of Phosphate in Seawater

   https://doi.org/10.3390/s21093125  

   Heidari-Bafroui, Hojat; Charbaji, Amer; Anagnostopoulos, Constantine; Faghri, Mohammad (May 2021, Sensors)

   Nutrient pollution remains one of the greatest threats to water quality and imposes numerous public health and ecological concerns. Phosphate, the most common form of phosphorus, is one of the key nutrients necessary for plant growth. However, phosphate concentration in water should be carefully monitored for environmental protection requirements. Hence, an easy-to-use, field-deployable, and reliable device is needed to measure phosphate concentrations in the field. In this study, an inexpensive dip strip is developed for the detection of low concentrations of phosphate in water and seawater. In this device, ascorbic acid/antimony reagent was dried on blotting paper, which served as the detection zone, and was followed by a wet chemistry protocol using the molybdenum method. Ammonium molybdate and sulfuric acid were separately stored in liquid form to significantly improve the lifetime of the device and enhance the reproducibility of its performance. The device was tested with deionized water and Sargasso Sea seawater. The limits of detection and quantification for the optimized device using a desktop scanner were 0.134 ppm and 0.472 ppm for phosphate in water and 0.438 ppm and 1.961 ppm in seawater, respectively. The use of the portable infrared lightbox previously developed at our lab improved the limits of detection and quantification by a factor of three and were 0.156 ppm and 0.769 ppm for the Sargasso Sea seawater. The device’s shelf life, storage conditions, and limit of detection are superior to what was previously reported for the paper-based phosphate detection devices. 

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3. Bimetallic Two‐Dimensional Metal‐Organic Frameworks for Chemiresistive Detection of Carbon Monoxide

   https://doi.org/10.1002/anie.202113665  

   Aykanat, Aylin; Meng, Zheng; Stolz, Robert M.; Morrell, Colin T.; Mirica, Katherine A. (November 2021, Angewandte Chemie International Edition)

   This paper describes the demonstration of a series of heterobimetallic, isoreticular 2D conductive metal–organic frameworks (MOFs) with metallophthalocyanine (MPc, M=Co and Ni) units interconnected by Cu nodes towards low-power chemiresistive sensing of ppm-levels of carbon monoxide (CO). Devices achieve sub-part-per-million (ppm) limits of detection (LOD) of 0.53 ppm toward CO at low driving voltages of 0.1 V. MPc-based Cu-linked MOFs can continuously detect CO at 50 ppm, the permissible exposure limit required by Occupational Health and Safety Assessment Series, for multiple exposures and realize CO detection in air and humid environment. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), density functional theory (DFT) calculations, and comparison experiments suggest the contribution of Cu nodes for CO binding and the essential role of MPc units in tuning and amplifying the sensing response. 

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4. Highly Selective Room-Temperature Blue LED-Enhanced NO2 Gas Sensors Based on ZnO-MoS2-TiO2 Heterostructures

   https://doi.org/10.3390/s25061781  

   Flores, Soraya Y; Pacheco, Elluz; Malca, Carlos; Peng, Xiaoyan; Chen, Yihua; Zhou, Badi; Pinero, Dalice M; Diaz-Vazquez, Liz M; Zhou, Andrew F; Feng, Peter X (March 2025, Sensors)

   This study presents the fabrication and characterization of highly selective, room-temperature gas sensors based on ternary zinc oxide–molybdenum disulfide–titanium dioxide (ZnO-MoS2-TiO2) nanoheterostructures. Integrating two-dimensional (2D) MoS2 with oxide nano materials synergistically combines their unique properties, significantly enhancing gas sensing performance. Comprehensive structural and chemical analyses, including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR), confirmed the successful synthesis and composition of the ternary nanoheterostructures. The sensors demonstrated excellent selectivity in detecting low concentrations of nitrogen dioxide (NO2) among target gases such as ammonia (NH3), methane (CH4), and carbon dioxide (CO2) at room temperature, achieving up to 58% sensitivity at 4 ppm and 6% at 0.1 ppm for NO2. The prototypes demonstrated outstanding selectivity and a short response time of approximately 0.51 min. The impact of light-assisted enhancement was examined under 1 mW/cm2 weak ultraviolet (UV), blue, yellow, and red light-emitting diode (LED) illuminations, with the blue LED proving to deliver the highest sensor responsiveness. These results position ternary ZnO-MoS2-TiO2 nanoheterostructures as highly sensitive and selective room-temperature NO2 gas sensors that are suitable for applications in environmental monitoring, public health, and industrial processes. 

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5. Smart-phone, paper-based fluorescent sensor for ultra-low inorganic phosphate detection in environmental samples

   https://doi.org/10.1038/s41378-019-0096-8  

   Sarwar, Mehenur; Leichner, Jared; Naja, Ghinwa M.; Li, Chen-Zhong (October 2019, Microsystems & Nanoengineering)

   Abstract A major goal of environmental agencies today is to conduct point-of-collection monitoring of excess inorganic phosphate (Pi) in environmental water samples for tracking aquatic “dead zones” caused by algae blooms. However, there are no existing commercial devices which have been miniaturized and are suitable for the point-of-need-testing (“PONT”) that is required to fully map a large region, such as the Florida Everglades. To solve this challenge, a reflection-mode fluorescence-sensing apparatus was developed, leveraging an environmentally sensitive fluorophore (MDCC) bound to a bacterial phosphate-binding protein to generate a fluorescent optical signal proportional to the concentration of (Pi) present. The combined end-to-end integrated sensor system had a response time of only 4 s, with minimal effects of common interfering agents and a linear range spanning from 1.1 to 64 ppb. To support ease-of-use during PONT, the platform incorporated disposable wax-printed paper strip sample pads and a smartphone camera detection system. Since the EPA threshold is currently 30 ppb to prevent eutrophication, this system serves as a rapid test of whether a region is compliant. 

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