On the following days however, when exchange sites at the lower part of the stem were assumingly saturated and the amount of radio-Sr in the feed declined due to solution topping and dilution, “wash out” of radio-Sr was demonstrated by negative rates at the lower stem detectors with positive rates at the upper parts indicating continuous accumulation at the flow terminals. Each morning, a sharp increase in both transpiration and translocation rates was seen, which implies coupled transport of water and Sr ions. Within hours however, translocation rates declined although transpiration rate was relatively stable. A possible dilution effect resulting from unloading activity as well as adsorption dynamics to tissue exchange capacity could explain such phenomena. As the day advanced and transpiration rate decreased, translocation rates peaked and subsided suggesting a 2nd balance shift. The 3rd and last daily wave observed after dark can be attributed to the low night transpiration rate and a possible desorption and/or loading of Sr into the sap which needs further studies . A relative high rate daytime inflow of radio-Sr was registered at the fruit detector which contradicts with the assumed night sap filling of xylem-borne ions due to fruit expansion. A possible explanation is that the existing mature fruits having low daily volume change coupled with a high sink term result in day-fill patterns. In regard to the relative high fruit radiation rates as compared with the leaf petiole detection rates, it should be noted that fruit radiation shield geometry was different than all other shields and allowed more radiation interception at the detector as compared to that of the petiole and stem detectors.
It is likely that related normalization would reduce relative fruit radiation levels. Copper is a first-row transition metal and an essential trace mineral that plays key roles in physiological functions,vertical farming equipments such as serving as a cofactor for many enzymes involved in energy production and metabolism. However, as with many essential minerals, excessive amounts can result in toxicity. Copper is common in the environment and contamination of soil and waterways can occur from agricultural sources, where copper is found in pesticides and fertilizers, and from industrial sources, such as mining and manufacturing operations. The World Health Organization has determined the maximum acceptable level of copper in drinking water to be 2 mg/L and the Environmental Protection Agency sets the threshold at 1.3 mg/L. Due to the potential health risks of environmental copper contamination, there is great interest in methods for the analytical detection of Cu2+ ions, particularly for use in field applications. The use of colorimetric sensors offers quick and accurate naked-eye detection without the need for expensive instrumentation, such as inductively coupled plasma mass spectrometry and atomic absorption spectrometry. Several colorimetric and fluorescent sensors with structures ranging from small molecules, large macrocycles, and nanoparticle/quantum dots have been created. The strong interest in copper sensors is highlighted by a recent PubMed search for “colorimetric copper sensor”, which revealed a steady increase in the numbers of copper sensors reported from 2007-2019 . To date, most reviews on copper sensors report fluorescent sensors for copper .
However, there have been fewer reviews addressing small molecules for the colorimetric detection of copper. These reviews were narrowly focused on copper sensors that are carbohydrate-based, pyrene-based or reviewed from the years 2013-2015. Other reviews discuss colorimetric and fluorescent copper sensors in a range of sizes such as small molecules, enzymes, polymers and nanoparticles or organize by the type of optical emission produced from these copper sensors. Further reviews on colorimetric sensing of metals have broadly focused on a number of metals. This review focuses on small molecule copper sensors that offer a colorimetric response in solution, with naked eye detection, published in the years 2010-2022. We felt that researchers developing new copper sensors, or who are interested in using copper sensors, might be most concerned about sensitivity as a starting point. 102 sensors are reviewed and are organized by their reported limits of detection by absorbance or fluorescence spectroscopy. Sensors that did not report colorimetric LOD but only fluorescence LOD are organized into a separate section. Sensors that possessed naked-eye detection but did not report a LOD are included at the end of the review.Upon evaluation of the copper sensors in Table 1-12, metals such as Fe3+, Fe2+, Pb2+ , Hg2+, and Co2+ were commonly found to offer dual detection. According to the hard-soft acid base theory, metals are classified as either hard acids or soft acids . Utilizing Pearson’s absolute hardness values ranging from 3.4-45.8, where the lower the value reflects the softer metal, hardness values for these metal ions were 7.3 , 7.7 , 8.3 , 8.5 , and 13.1. Co2+ was not listed but is considered borderline, displaying intermediate characteristics.
Since Cu2+ is considered a borderline soft acid, it is reasonable to suggest interference from Fe2+, Pb2+, Hg2+, and Co2+ are due to HSAB theory. Although Fe3+ is regarded as a hard acid, it is plausible that HSAB does not apply in this case. Recognition of Fe3+ was primarily in the form of fluorescence “turn-on” detection. Interestingly, all sensors utilized a Schiff-base unit in the sensing mechanism. It is well known that various metal ions preferentially bind a Schiff-base imine due to the non-bonded electrons on nitrogen in the C=N unit. Depending on several factors such as pH, coordinating ability of the counter anions, the amine or aldehyde fragment regenerated, etc., two possible mechanisms could explain this phenomenon. Coordination of Fe3+ in the binding pocket containing a Schiff-base unit induces hydrolytic cleavage of the C=N and formation of an amine and carbonyl. This results in partial decomposition of the sensor and generation of a fluorophore enabling fluorescent enhancement. The second possible sensing mechanism involves the coordination of Fe3+ in the binding pocket containing a Schiff-base unit but instead of undergoing hydrolysis, the Fe3+-sensor complex is stabilized by the donation of the electrons from nitrogen on C=N imine. Uponemission of this complex, PET is inhibited due to the Fe3+-sensor stabilization, allowing for full relaxation of the electrons back to the ground state, resulting in fluorescence. As for Cu2+, it has been often used as a fluorescent “turn-off” sensor due to its paramagnetism. Upon emission of a Cu2+-fluorophore complex, PET is possible when an excited electron relaxes to the dx2 -y2 orbital, resulting in fluorescence quenching. Common anions that interfered with copper sensing, and offered dual detection, were S2- , CN- , and F- . Further expanding on HSAB theory, hard acids preferentially react with hard bases and analogously,vertical farm tower soft acids preferentially react with soft bases. Therefore, the HSAB theory could account for interference by sulfur and cyanide acting as soft bases. The high affinity of copper for these ligands can displace the metal from the sensor to form CuS or Cu2. Since fluoride is considered a hard base, the possible mechanism for detection of F- could be due to its electronegativity and high propensity to intermolecular hydrogen bond. Of the sensors that detected F- , this is particularly seen with hydrogens covalently bound to either an amine or phenol. The lone pair electrons on nitrogen and oxygen induce a dipole creating a partial positive charge on hydrogen, making it susceptible to intermolecular hydrogen bonding with fluoride. Overall, the ideal copper sensor used for in-field analysis would be able to detect copper only, even in the presence of competing metal ions, and be able to do so in a 100% aqueous medium, whether it be free in solution or fixed to a test strip. Even though there are 102 sensors reported in this review paper, only 60 sensors detect solely copper.
From these 60 sensors, 51 of the reports performed competition studies to rule out interference from other metal ions. 39 sensors were able to selectively detect copper exclusively, over other competing metal ions. After inspecting the number of sensors that were selective for copper detection with no interference, it is clear that there is a necessity to analyze beyond 1:1 Cu2+: Mn+ for competition studies. Only 11 sensors analyzed selectivity at higher ratios of competing metals; yet this is a very important aspect of developing an in-field sensor. Assessing the selectivity of Cu2+ with excess metal ions can reveal if the sensor renders a false positive or false negative. If so, pretreatment methods will need to be administered. Another important feature in developing an in-field sensor for detecting Cu2+ contamination in soil and water is the ability of the sensor to be applied to aqueous solutions. In this review, 9 sensors achieved solubility in 100% aqueous medium. A common workaround to adapt a sensor that was soluble in an organic or mixed-organic solvent, was to fix them to paper and make test strips. This is a practical option as long as competition studies are performed to confirm that Cu2+ selectivity remains. However, this was not fulfilled in the papers discussing paper-based copper sensors that are reviewed here. Interference studies, especially with excess competing metal ions and solubility in water, should be a priority that is addressed for future advancement of sensors being developed for copper detection. Plants adapt to their below ground environment by root morphological and metabolic plasticity. In turn, they influence soil physiochemical properties and root‐associated organisms by creating the rhizosphere, an environmental niche formed by the physical structure of roots and the release of metabolites . These complex root–environment interactions are challenging to study in general, and even more so in a manner that is reproducible across laboratories.Nutrient availability of soils, for example, can profoundly affect root morphology and provoke changes in root metabolism. Phosphate limitation typically results in elongated lateral roots and root hairs in a context‐dependent manner and in increased exudation of organic acids that solubilize phosphate . Root morphology and metabolism are further affected by microbes and microbial compounds . The presence of plant growth‐promoting bacteria can stimulate lateral root and root hair growth of Arabidopsis . Plant responses to abiotic and biotic factors are likely intertwined, as illustrated recently by a study that linked phosphate stress in plants with the structure of root‐associated microbial communities . Thus, plant phenotypes in soil are a result of a complex response to abiotic and biotic factors, and an integrated view of root morphology and metabolism is necessary to gain a holistic understanding of plant–environment interactions. Characterization of plant phenotypes in response to abiotic and biotic stresses in soil can have a profound impact on agriculture, especially as many resources, such as phosphate‐based fertilizers, are limited , and global food demand is projected to have to increase by 60% by the year 2050 due to an ever‐growing population . Grasses are central to bio-fuel production and provide 70% of human calories . Thus, research on model grasses such as Setaria viridis and Brachypodium distachyon can inform growth strategies for many crops . B. distachyon is gaining popularity as a model grass because of its small genome, short generation time, genetic tractability, and the availability of extensive germplasm and mutant collections . Additionally, sinceit uses C carbon fixation, it is a good laboratory model plant relevant to cereal crops such as barley , rice , and wheat . It has recently been utilized to investigate plant developmental processes, abiotic stresses, biotic interactions, and root morphology . The relationship between plants and their environment is ideally studied in an agriculturally relevant field setting. Environmental factors, especially the type of soil in which plants are grown, are major determinants of root‐associated microbial communities , and of root morphology . However, investigation of root morphology in soil is challenging due to its opacity, and investigation of exudation in soil is challenging due to soils physiochemical complexity . Specialized imaging techniques, such as magnetic resonance imaging, computed tomography , or the use of labeled plants , have been developed, but they are not widely accessible or amenable to high‐throughput experimentation . Similarly, approaches for the investigation of root exudation in soils include the use of in situ soil drainage systems in fields , which are low throughput and require complex installations, or of laboratory‐based extraction methods that are based on flushing the soil with large volumes of liquids .