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A novel Eu(tta)3([4,4′-(t-bu)2-2,2′-bpy)] complex (tta-thenoyltrifluoroacetone), a ratiometric luminescent-based optical sensor for the quantitative determination of aluminum ion, is synthesized and characterized using XRD and 1H NMR. The XRD data reveal the slightly distorted octahedral structure. The complex displays a bright red emission at 613 nm in methanol which is characteristic of europium (III) complexes. Upon the addition of Al3+ ions, the red emission disappears, and a new blue emission at 398 nm emerges, manifesting the ratiometric nature of the complex. The turn-off of the red emission and turn-on of the blue emission are attributed to Eu-Al trans-metalation, as supported by Raman data that show the emergence of Al-O vibrations at 418, 495, and 608 cm−1 concomitant with the disappearance of Eu-O and Eu-N bond vibrations. Most aluminum sensors are known to suffer from interferences from other metals including Cu2+, Co2+, and Cd2+. However, the sensor reported here is tested for 11 common cations and shows no interference on sensitivity. To the best of our knowledge, this is the first known Eu-based luminescence sensor that successfully exhibited the ability to detect aluminum ions in ppb levels in aqueous environments. The calculated Al3+ binding constant is 2.496 × 103 ± 172. The complex shows a linear relationship in the range of 0–47.6 ppb (1.76 × 10−6 M) Al3+ and the limit of detection (LOD) is 4.79 ppb (1.77 × 10−7 M) in MeOH. ICP-OES is used for validation of the sensor complex in water and then it was used for quantitative detection of Al3+ ions in water as a real-life application. The complex can accurately detect Al3+ ions in the range of 4.97–24.9 ppb (1.84 × 10−7 M–9.2 × 10−7 M) with an LOD of 8.11 ppb (2.99 × 10−7 M). Considering that the aluminum ion serves no recognized function within the human body, its accumulation can lead to severe neurological disorders, including Parkinson's and Alzheimer's diseases. With the LOD value significantly lower than the WHO-recommended maximum permissible level of 200 ppb for aluminum in drinking water, even without high-power laser-aided signal enhancement, the sensor shows promise for detecting trace amounts of aluminum contamination in water. Therefore, it can significantly aid in the monitoring of even the smallest aluminum ion contamination in drinking water, industrial effluents, and natural water bodies. [ABSTRACT FROM AUTHOR]

A new europium-based complex, K[Eu(hfa)4] with hfa = hexafluoroacetylacetonate is synthesized and its structure confirmed via X-ray crystallography. The structure unravels an anionic octa-coordinate complex, K[Eu(hfa)4], as opposed to the neutral hexacoordinate complex Eu(hfa)3 routinely/ubiquitously presumed to be the case in the literature. The complex displayed pH-dependent, "on–off" emission changes in solution and exhibited a pKa of 6.13 ± 0.06 in ethylene glycol. In solution, the sensor complex exhibited drastic variation in emission intensity corresponding to changes in the concentration of CO2 gas purged. Based on multiple purge cycles of N2 and CO2, the emission intensity changes can be correlated to the concentration of CO2 in the solution. The sensor's ability to quantify the CO2 presence is based on emission variations of the 5D0 → 7F2 line in the Eu(III) complex at 618 nm. The sensor exhibits a linear response to CO2 concentrations in the range of 0–25% (0–8.50 mM or 0–189.95 mmHg). Based on calibration data, the limit of detection (LOD) is determined to be 0.57% (0.19 mM or 4.33 mmHg) in solution. The I100/I0 ratio is determined to be 80.29 ± 3.79. The percent change in intensity from purging N2 to 100% CO2 is 7911.16%. Over the course of seven cycles of purging different concentrations of CO2, there is essentially no deviation in the emission intensity of the sensor in solution, indicating stability and reversibility. In addition to the analytical characterization of the sensor, the mechanism of CO2 sensing is investigated using cyclic voltammetry, IR, and Raman spectroscopy. These data indicate the reduction of europium(III) to europium(II) in an alkaline medium and suggest changes in the hfa ligand chemistry (association/dissociation and protonation) due to CO2 purging. The potential use of the sensor complex for real-life applications is herein evaluated via a well-known fermentation reaction. The CO2 generated during yeast's anaerobic respiration in sucrose media is quantified using the sensor complex and a calibrated, commercial CO2 probe; both exhibit similar CO2 concentration values, validating the calibration curve and the viability of the complex as a bona fide sensor. Based on the data collected, a highly stable, brightly red-emissive Eu(III) complex with the ability to differentiate concentrations of CO2 in solution is hereby developed and characterized with benefits for various CO2 sensing applications. [ABSTRACT FROM AUTHOR]

Neutral Red (NR), a eurhodin dye, is often used for staining living cells, but we demonstrated for the first time that NR can also serve as a CO2 sensor, because of NR's unique optical properties, which change with dissolved carbon dioxide (dCO2) concentrations. In the present study, optical sensitivity of NR was quantified as a function of changes in absorption and emission spectra to dCO2 in a pH 7.3 buffer medium at eight dCO2 concentrations. NR exhibited a response time of two minutes for equilibration in pure N2 to 100% CO2 with an ~200% percent change (%∆) in emission intensity and >400%∆ in absorbance, with full reversibility. Important to its application to biological systems, NR exhibited zero sensitivity to dissolved oxygen, which has routinely caused interference for CO2 measurements. NR exhibited pH sensitive emission and excitation energies with dual excitation wavelengths at 455 nm and 540 nm, and a single emission at 640 nm. The CO2 sensing properties of NR were benchmarked by a comparison to pyranine (8-hydroxypyrene-1, 3,6-trisulfonic acid trisodium salt) (HPTS). Future studies will evaluate the feasibility of NR as an intracellular in vivo pCO2 sensor in aquatic organisms critically impacted by increasing global CO2 levels. [ABSTRACT FROM AUTHOR]

Chitosan (CS) is a natural polymer derived from chitin that has found its usage both in research and commercial applications due to its unique solubility and chemical and biological attributes. The biocompatibility and biodegradability of CS have helped researchers identify its utility in the delivery of therapeutic agents, tissue engineering, wound healing, and more. Industrial applications include cosmetic and personal care products, wastewater treatment, and corrosion protection, to name a few. Many researchers have published numerous reviews outlining the physical and chemical properties of CS, as well as its use for many of the above-mentioned applications. Recently, the cationic polyelectrolyte nature of CS was found to be advantageous for stabilizing fascinating photonic materials including plasmonic nanoparticles (e.g., gold and silver), semiconductor nanoparticles (e.g., zinc oxide, cadmium sulfide), fluorescent organic dyes (e.g., fluorescein isothiocyanate (FITC)), luminescent transitional and lanthanide complexes (e.g., Au(I) and Ru(II), and Eu(III)). These photonic systems have been extensively investigated for their usage in antimicrobial, wound healing, diagnostics, sensing, and imaging applications. Highlighted in this review are the different works involving some of the above-mentioned molecular-nano systems that are prepared or stabilized using the CS polymer. The advantages and the role of the CS for synthesizing and stabilizing the above-mentioned optically active materials have been illustrated. [ABSTRACT FROM AUTHOR]