Pure Aqueous Media Research Articles

Detecting Pb2+ in aqueous environment and live cells by amphiphilic pillar[5]arene-assembled supramolecular sensor based on host-guest charge transfer mechanism

Water-soluble fluorescent chemosensors for lead ion are highly desirable in environmental detection and bioimagery. Based on a water-soluble pillar[5]arene WP5 and imidazolium terminal functionalized 2,2ʹ-bibenzimidazole derivative BIHB, we report a host-guest charge transfer assembly BIHB-2WP5 for sensitive and selective detection of Pb2+ in pure aqueous media. As a result of its high electron-rich cavity, WP5 can bind electron-deficiency guest BIHB with various host/guest stoichiometry to easily tune the microtopography of assembly from nanoparticle to nanocube. In view of the good biocompatibility and sensitivity, the supramolecular assembly BIHB-2WP5 was used as a fluorescent probe for the detection of Pb2+ in living cells and a smartphone Pb2+ detection device was constructed for the in situ test.

Recording thermal history of pork meat by self-coloration gel

For real-time dynamic quality evaluation of pork meat, the thermal accumulation monitoring for the entire period is essential. The research aims to develop a novel covalent organic framework (COF) gel capable of recording such information. The COF with self-responsive irreversible color change property was synthesized using benzene-1,3,5-tricarboxaldehyde, 2,5-diamino-1,4-benzenedithiol dihydrochloride and p-phthalaldehyde as monomers. The thermal accumulation was monitored based on the COF over a broad temperature range (−22 °C–55 °C), which covered the storage temperatures of the pork meat. The results were easily readable by series visible color variations (yellow, green, blue and dark blue) displayed by COF during the thermal accumulation monitoring in the temperature range (25 °C–55 °C) of study. At the lower temperature (−22 °C–4 °C), small variation of the thermal information can be effectively represented by quantitative analysis. The color change rates were in the order of k-22 °C < k4°C < k25 °C < k35 °C < k55 °C. The activation energy of reaction was 65.31 kJ mol−1. The indicator gel was prepared by the COF combining with carrageenan in pure aqueous medium. The potential application of the COF gel for real-time monitoring the dynamic quality change of pork samples was investigated. The response signals of the COF gel were well matched with the growth rate of microbe in pork meat. The proposed COF gel based thermal history indicator is promising for real-time monitoring dynamic quality changes of pork meat during the cold supply chain.

A highly selective Ru(II) complex-based phosphorescent probe for the sequential sensing Cu2+ and H2S and its applications

A highly selective Ru(II) complex-based phosphorescent probe Ru-Cyclen for the sequential sensing Cu2+ and H2S in pure aqueous medium was synthesized and characterized. Probe Ru-Cyclen exhibited a turn-off phosphorescence response toward Cu2+ by using a complex formation with a 1:1 binding mode, and the resulting complex enabled to specifically detect H2S based on the displacement approach with a turn-on phosphorescence response. Also, Ru-Cyclen possessed high selectivity and sensitivity with a low detection limit of 0.045 μM and 0.44 μM for Cu2+ and H2S respectively, and worked best in the range of pH 3–11. The kinetics response toward Cu2+ and H2S were very fast, completing within 4 min and 10 s, respectively. Additionally, the sensing mechanism was confirmed by fluorescence Job’s plot analysis, 1H NMR titration, EPR, TOF-mass spectrometry and phosphorescence lifetime analysis. Moreover, Ru-Cyclen showed low cell toxicity and successfully detected Cu2+ and exogenously or endogenously generated H2S in living zebrafish. Finally, combining the unique merits of the probe, including excellent water-solubility, a long emission wavelength of 600 nm, large Stoke shift of 150 nm and rapid response, probe Ru-Cyclen was applied in real water samples, human serum and test strips successfully.

Selective sensing of paraquat by simple off-the-shelf compounds: Applications using composite hydrogel beads and smartphone

The massive use of paraquat (PQ) in agricultural practices has posed serious threats to food safety and human health. Real-time monitoring of PQ remains an obstacle. Herein, a fluorometric as well as smartphone-based very cheap sensing assay is discussed for the detection of this non-selective contact herbicide PQ based on simple off-the-shelf commercially available dyes PTSA (1), HPTS (2) and Eosin Y (3) in pure aqueous media. Owing to their sensitivity towards PQ through electrostatic interactions, these dyes undergo a fluorescence quenching process. Dyes (2) and (3) show a rapid colorimetric change in the presence of PQ as shown by UV-Vis absorption spectral studies. 1 exhibits high sensitivity for PQ in comparison to 2 and 3 with the detection limit as low as 182 nM. All three probes exhibit high sensitivity, high selectivity for PQ over other analytes and excellent anti-interference properties. The Stern-Volmer plots and fluorescence lifetime studies confirm the static quenching process of the probes in the presence of PQ. Furthermore, sensor-coated paper-based test strips were successfully used for the ultrafast detection and quantification of PQ in spiked real water, fruit and vegetable samples. For the first time, silica-sodium alginate (SA) polymer hybrid hydrogel beads incorporated with probes 1, 2 and 3 were used for the detection and adsorption of PQ from water samples. For assessing the reliability of the proposed sensing system for the on-site detection of PQ, an easy-to-use portable test cassette kit integrated with a smartphone was used. The signal outputs were captured and analysed via RGB color value to use the probes for the practical application by using a smartphone as a portable analytical device for the rapid point-of-care monitoring of PQ.

Engineered Robust Hydrophobic/Hydrophilic Nanofibrous Scaffolds with Drug-Eluting, Antioxidant, and Antimicrobial Capacity.

Multifunctional nanofibrous architectures have attracted extensive attention for biomedical applications due to their adjustable and versatile properties. Electrospun fabrics stand out as key building blocks for these structures, yet improving their mechanobiological and physicochemical performance is a challenge. Here, we introduce biodegradable engineered hydrophobic/hydrophilic scaffolds consisting of electrospun polylactide nanofibers coated with drug-eluting synthetic (poly(vinyl alcohol)) and natural (starch) polymers. The microstructure of these composite scaffolds was tailored for an increased hydrophilicity, optimized permeability, water retention capacity of up to 5.1 g/g, and enhanced mechanical properties under both dry and wet conditions. Regarding the latter, normalized tensile strengths of up to 32.4 MPa were achieved thanks to the improved fiber interactions and fiber-coating stress transfer. Curcumin was employed as a model drug, and its sustained release in a pure aqueous medium was investigated for 35 days. An in-depth study of the release kinetics revealed the outstanding water solubility and bioavailability of curcumin, owing to its complexation with the hydrophilic polymers and further delineated the role of the hydrophobic nanofibrous network in regulating its release rate. The modified curcumin endowed the composites with antioxidant activities up to 5.7 times higher than that of free curcumin as well as promising anti-inflammatory and bacteriostatic activities. The cytocompatibility and cell proliferation capability on human dermal fibroblasts also evidenced the safe use of the constructs. Finally, the fabrics present pH-responsive color-changing behavior easily distinguishable within the pH range of 5-9. Thus, these designs offer a facile and cost-effective roadmap for the fabrication of smart multifunctional biomaterials, especially for chronic wound healing.

Critical Assessment of Micellar Surface Charge-Dependent Disaggregation and Reaggregation of a Bis-Indole Self-Aggregate: What Should Be Our Case-Dependent Choice?

"Aggregation-caused quenching" is a deep-seated mechanism and has been widely used by the researchers as the possible basis for new sensor development. Contrast to aggregation, its turn around process, disaggregation, has gained much less consideration so far. Unfortunately, study of the further scope for reaggregation of the disaggregated probe assembly in the same solution, as and when required, is still under the rare category. The central theme of the current study is focused on this aspect. For this purpose, the effects of headgroup charge (cationic, anionic, and nonionic) and polarity of the micelles on the degree of disaggregation and subsequent emission amelioration of a synthesized bis-indole derivative, 3,3'-bisindolyl(phenyl)methane (BIPM), are studied using steady-state and time-resolved spectroscopic techniques. The relative emission yield of BIPM (φf = 0.01) is significantly enhanced in the presence of cetyltrimethylammonium bromide (φf = 0.21) and polyoxyethylene (20) sorbitan monolaurate (φf = 0.24), whereas comparatively less emission enhancement is recorded within the sodium dodecyl sulfate system (φf = 0.07). In contrast, addition of an external biophilic agent, uric acid, causes requenching of the enhanced emission because of the reaggregation of the disaggregated probes. Detailed microscopic and calorimetric studies are also adopted to investigate the disaggregation-reaggregation mechanism of BIPM associations. The study will provide prior insights about the use of suitable micellar systems for the required degree of disaggregation as well as for the modulation of emission efficiency by controlled tuning of the disaggregation-reaggregation equilibrium for similar probe associations in pure aqueous medium avoiding any chemical transformation.

Cavity-Directed Synthesis of Labile Polyoxometalates for Catalysis in Confined Spaces.

The artificial microenvironments inside coordination cages have gained significant attention for performing enzyme-like catalytic reactions by facilitating the formation of labile and complex molecules through a "ship-in-a-bottle" approach. Despite many fascinating examples, this approach remains scarcely explored in the context of synthesizing metallic clusters such as polyoxometalates (POMs). The development of innovative approaches to control and influence the speciation of POMs in aqueous solutions would greatly advance their applicability and could ultimately lead to the formation of elusive clusters that cannot be synthesized by using traditional methods. In this study, we employ host-guest stabilization within a coordination cage to enable a novel cavity-directed synthesis of labile POMs in aqueous solutions under mild conditions. The elusive Lindqvist [M6O19]2- (M=Mo or W) POMs were successfully synthesized at room temperature via the condensation of molybdate or tungstate building blocks within the confined cavity of a robust and water-soluble Pt6L4(NO3)12 coordination cage. Importantly, the encapsulation of these POMs enhances their stability in water, rendering them efficient catalysts for environmentally friendly and selective sulfoxidation reactions using H2O2 as a green oxidant in a pure aqueous medium. The approach developed in this paper offers a means to synthesize and stabilize the otherwise unstable metal-oxo clusters in water, which can broaden the scope of their applications.

Cavity‐Directed Synthesis of Labile Polyoxometalates for Catalysis in Confined Spaces

AbstractThe artificial microenvironments inside coordination cages have gained significant attention for performing enzyme‐like catalytic reactions by facilitating the formation of labile and complex molecules through a “ship‐in‐a‐bottle” approach. Despite many fascinating examples, this approach remains scarcely explored in the context of synthesizing metallic clusters such as polyoxometalates (POMs). The development of innovative approaches to control and influence the speciation of POMs in aqueous solutions would greatly advance their applicability and could ultimately lead to the formation of elusive clusters that cannot be synthesized by using traditional methods. In this study, we employ host–guest stabilization within a coordination cage to enable a novel cavity‐directed synthesis of labile POMs in aqueous solutions under mild conditions. The elusive Lindqvist [M6O19]2− (M=Mo or W) POMs were successfully synthesized at room temperature via the condensation of molybdate or tungstate building blocks within the confined cavity of a robust and water‐soluble Pt6L4(NO3)12 coordination cage. Importantly, the encapsulation of these POMs enhances their stability in water, rendering them efficient catalysts for environmentally friendly and selective sulfoxidation reactions using H2O2 as a green oxidant in a pure aqueous medium. The approach developed in this paper offers a means to synthesize and stabilize the otherwise unstable metal‐oxo clusters in water, which can broaden the scope of their applications.

Salt-induced Fmoc-tripeptide supramolecular hydrogels: a combined experimental and computational study of the self-assembly.

Delving into the mechanism behind the molecular interactions at the atomic level of short-sequence peptides plays a key role in the development of nanomaterials with specific structure-property-function relationships from a bottom-up perspective. Due to their poor water solubility, the self-assembly of Fmoc-bearing peptides is usually induced by dissolution in an organic solvent, followed by a dilution step in water, pH changes, and/or a heating-cooling process. Herein, we report a straightforward methodology for the gelation of Fmoc-FFpY (F: phenylalanine; Y: tyrosine; and p: PO42-), a negatively charged tripeptide, in NaCl solution. The electrostatic interactions between Fmoc-FFpY and Na+ ions give rise to different nanofibrillar hydrogels with rheological properties and nanofiber sizes modulated by the NaCl concentration in pure aqueous media. Initiated by the electrostatic interactions between the peptide phosphate groups and the Na+ ions, the peptide self-assembly is stabilized thanks to hydrogen bonds between the peptide backbones and the π-π stacking of aromatic Fmoc and phenyl units. The hydrogels showed self-healing and thermo-responsive properties for potential biomedical applications. Molecular dynamics simulations from systems devoid of prior training not only confirm the aggregation of peptides at a critical salt concentration and the different interactions involved, but also corroborate the secondary structure of the hydrogels at the microsecond timescale. It is worth highlighting the remarkable achievement of reproducing the morphological behavior of the hydrogels using atomistic simulations. To our knowledge, this study is the first to report such a correspondence.

AIE active fluorescent organic nanoparticles based optical detection of Cu2+ ions in pure water: a case of aggregation-disaggregation reversibility.

An AIE-active pyrene-terpyridine derivative, (4'-(pyren-1-yl)-2,2':6',2''-terpyridine) (1) was found to form nanoaggregate in an aqueous medium. The probe involved hydrogen bonding with solvent molecules that modulated the charge transfer behavior and consequently resulted in different spectroscopic behavior due to the formation of fluorescent organic nanoparticles (FONs). In the presence of Cu2+ ions, FONs displayed a ratiometric red shift of the absorption band (360 to 420 nm) accompanied by a prominent naked-eye color change from colorless to light yellow. With a gradual increase in water content, 1 displayed a huge red shift of the emission band (430 to 475 nm) denoting its switching from monomer to FONs. In the presence of Cu2+, the 475 nm emission band of FONs gradually diminished, facilitating the micromolar scale detection of Cu2+ (LOD = 8.57 μM) in a 100% aqueous medium with a fluorogenic color change from cyan to dark. The SEM and DLS data indicated the cation-induced disaggregation of FONs, which was further confirmed by mass spectral analysis and electron paramagnetic resonance measurement. In addition, the high selectivity of FONs towards Cu2+ ions over other potential cations and the 2 : 1 (1-Cu2+) binding stoichiometry were also determined. Moreover, the spectroscopic behavior of the monomeric amphiphilic probe was well supported by extensive DFT study. Such detection of Cu2+ ions in pure aqueous medium denoting an aggregation-disaggregation event is very rare in the literature.

Expanding the Enzymatic Polymerization Landscape by Lipid Mesophase Soft Nanoconfinement.

Soft nanoconfinement can increase chemical reactivity in nature and has therefore led to considerable interest in transferring this universal feature to artificial biological systems. However, little is known about the underlying principles of soft nanoconfinement responsible for the enhancement of biochemical reactions. Herein we demonstrate how enzymatic polymerization can be expanded, optimized, and engineered when carried out under soft nanoconfinement mediated by lipidic mesophases. By systematically varying the water content in the mesophase and thus the diameter of the confined water nanochannels, we show higher efficiency, turnover rate, and degrees of polymerization as compared to the bulk aqueous solution, all controlled by soft nanoconfinement effects. Furthermore, we exploit the unique properties of unfreezing soft nanoconfined water to perform the first enzymatic polymerization at -20 °C in pure aqueous media. These results underpin lipidic mesophases as a versatile host system for chemical reactions and promote them as an original and unexplored platform for enzymatic polymerization.

Expanding the Enzymatic Polymerization Landscape by Lipid Mesophase Soft Nanoconfinement

AbstractSoft nanoconfinement can increase chemical reactivity in nature and has therefore led to considerable interest in transferring this universal feature to artificial biological systems. However, little is known about the underlying principles of soft nanoconfinement responsible for the enhancement of biochemical reactions. Herein we demonstrate how enzymatic polymerization can be expanded, optimized, and engineered when carried out under soft nanoconfinement mediated by lipidic mesophases. By systematically varying the water content in the mesophase and thus the diameter of the confined water nanochannels, we show higher efficiency, turnover rate, and degrees of polymerization as compared to the bulk aqueous solution, all controlled by soft nanoconfinement effects. Furthermore, we exploit the unique properties of unfreezing soft nanoconfined water to perform the first enzymatic polymerization at −20 °C in pure aqueous media. These results underpin lipidic mesophases as a versatile host system for chemical reactions and promote them as an original and unexplored platform for enzymatic polymerization.

Solubility and dissolution thermodynamics of L-histidine and L-tryptophan in aqueous ethanol solution at five equidistant temperatures

This study highlights the saturated solubilities and dissolution thermodynamical properties of two very important amino acids viz. L-histidine and L-tryptophan having various roles in many field in pure aqueous and aqua-ethanol medium at particular as well as range of temperature employing static gravimetric method which is very systematic and error free. Finding confirms that L-histidine shows more solubility in water rich region and L-tryptophan shows more solubility in organic rich region. L-tryptophan has an extra benzene ring which makes L-tryptophan more covalent than L-histidine moiety. Consequently, solubility of former in water rich region becomes low. In connection with those mentioned properties other various physiochemical parameters like co-solvent diameter, apparent dipole moment, apparent molar volume, solvent diameter etc. are also estimated. All observations are explained clearly on the ground of several solution related interactions namely cavity forming, solvent-solvent, solvent-solute interactions etc. Both total and chemical transfer Gibbs energies as well as entropies are calculated and explained to give support to stability and variation of solubility in both aqueous and aqua-ethanol media.

Counteraction Effects of Ammonium-Based Ionic Liquids on Urea-Induced Denaturation of α-Lactalbumin: A Comprehensive Molecular Simulation Study.

Ionic liquids (ILs) are known to stabilize protein conformations in aqueous medium. Importantly, ILs can also act as refolding additives in urea-driven denaturation of proteins. However, despite the importance of the problem, detailed microscopic understanding of the counteraction effects of ILs on urea-induced protein denaturation remains elusive. In this work, atomistic molecular dynamics (MD) simulations of the protein α-lactalbumin have been carried out in pure aqueous medium, in 8 M binary urea-water solution and in ternary urea-IL-water solutions containing ammonium-based ethyl ammonium acetate (EAA) as the IL at different concentrations (1-4 M). Attempts have been made to quantify detailed molecular-level understanding of the origin behind the counteraction effects of the IL on urea-induced partial unfolding of the protein. The calculations revealed significant conformational changes of the protein with multiple free energy minima due to its partial unfolding in binary urea-water solution. The counteraction effect of the IL was evident from the enhanced structural rigidity of the protein with propensity to transform into a single native free energy minimum state in ternary urea-IL-water solutions. Such an effect has been found to be associated with preferential direct binding of the IL components with the protein and simultaneous expulsion of urea from the interface, thereby providing additional stabilization of the protein in ternary solutions. Most importantly, modified rearrangement of the hydrogen bond network at the interface due to the formation of stronger protein-cation (PC) and protein-anion (PA) hydrogen bonds by breaking relatively weaker protein-urea (PU) and protein-water (PW) hydrogen bonds has been recognized as the microscopic origin behind the counteraction effects of EAA on urea-induced partial unfolding of the protein.

PEPPSI-Type Pd(II)—NHC Complexes on the Base of p-tert-Butylthiacalix[4]arene: Synthesis and Catalytic Activities

The creation of effective catalytic systems for cross-coupling reactions, reduction, etc., capable of working in water-organic or pure aqueous media is in great demand. The article presents the synthesis of NHC-palladium complexes of the PEPPSI type based on monoimidazolium derivatives of thiacalix[4]arene. The structure of the imidazolium precursors, obtained in 81–88% yields and the complexes themselves, obtained in 40–50% yields, is established using modern methods, including X-ray structural analysis and high-resolution mass spectrometry. It is shown that the obtained complex with bulk substituents near the palladium atom is not inferior to the well-known PEPPSI-type Organ’s catalyst in the catalysis of Suzuki-Miyaura coupling and is four times superior to the latter in the p-nitrophenol reduction reaction. Given the presence of free phenolic hydroxyl groups in the macrocycle, the obtained complexes are of interest for further post-modification or for immobilization on a carrier.

Green removal of low and high levels of Cu(II) and Cr(III) cations from concentrated saline chloride medium achieved by a mixture of N,N′-bis(salicylidene)-thiocarbohydrazide-TritonX-100 micellar system via cloud point extraction process

Abstract In this work, we performed the cloud point extraction with low and high concentrations of Cu(II) and Cr(III), two particularly toxic metallic pollutants, that have been shown to have adverse effects on human health and the environment. Removal of the metal ions was achieved in the presence of Triton X-100 as a non-ionic surfactant and N,N′-bis(salicylidene)-thiocarbohydrazide as a chelating ligand in concentrated chloride medium. The behavior of the Schiff base was initially investigated in homogeneous DMSO-water and micellar Triton X-100-water media. The obtained values of the acid constants show that the extractant is more acidic in micellar media than in pure aqueous medium. The recorded cloud point temperatures of Triton X-100 are significantly impacted by the sodium chloride concentration, the surfactant concentration, and the Schiff base ligand. Under the optimised experimental conditions, a quantitative extraction of Cu(II) and a maximum extraction rate of 74 % of Cr(III) were obtained. The reduction of the metal concentration from 10−3 mol L−1 to 10−6 mol L−1 resulted in a reduction of the temperature by 10 °C and a reduction of the contact time from 10 h to 3 h.

Excited-state properties of 6-methoxyflavone in the presence of halide ions in aqueous media

The present work investigated the influence of different halides on the excited state dynamics of 6-methoxyflavone (6MF) in an aqueous solution with steady-state and time-resolved techniques. On successive addition of I− and Br− ions, the fluorescence of 6MF quenched significantly, whereas the respective ions do not change the maximum fluorescence band. Fluorescence of 6MF was quenched 66% by I− ions and 34% by Br− ions. In a pure aqueous medium, both the H-bonded: CT and protonated species of 6MF participate in the quenching of fluorescence. The quenching process was categorized by Stern–Volmer (S–V) and Lehrer equations. Quenching parameters such as KSV, KSV-L and kq were higher for I− ions than Br− ions. The decrease in fluorescence intensity and a reduction in fluorescence lifetime suggested the dynamic nature of quenching by I− ions following the electron transfer mechanism. Fluorescence quenching of 6MF has also been observed in the acidic medium in the presence of different halides. Thus, the study reveals that 6MF is responsive towards I− ions in a wide range of pH, specifically in a purely aqueous environment (pH∼7), hence important for sensing/detection applications.

Exploring the Heterogeneous Dynamical Environment at the Interface of Aβ42 Peptide in Aqueous Ionic Liquid Solution.

In this study, we have investigated the heterogeneous dynamical environment around an ensemble of full-length amyloid-β (Aβ42) peptide monomers in binary aqueous solution containing the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] as a co-solvent. Atomistic molecular dynamics (MD) simulations have been employed with the aim of understanding the effect of the IL on the distribution of water molecules and IL components around distinct segments of the peptide. As compared to pure aqueous medium, locally heterogeneous restricted water motions at the interface have been spotted in the presence of the IL. Our calculations revealed faster diffusion of water molecules hydrating hydrophilic segments (N-term and turn) as opposed to that around hydrophobic segments (hp1, hp2, and C-term). The extent of non-uniform restriction on the center-of-mass motions as well as the reorientation of water molecules and IL ions have been similarly affected in the binary IL-water solution. The effects of IL on the formation of hydrogen bond networks have been evident from the longer hydrogen bond relaxation time scales of peptide-water, with only a small fraction of peptide-anion hydrogen bonds contributing to the structural relaxation. Due to the size and shape factors, the increasingly sluggish dynamics of the IL components in the solvation shell can be attributed to a longer time scale for the onset of maximum dynamic heterogeneity. Interestingly, the water molecules around the polar segments of the peptide take longer to attain dynamic heterogeneity, which intensifies in the presence of IL. These calculations clearly suggest that electrostatic interaction plays a crucial role in water-mediated peptide-IL interaction, thereby shielding the surface from hydrophobic collapse and preventing possible further growth of the monomers into fibrils at higher peptide concentrations.

Fabrication of FRET based nano sensor from biomass-derived fluorescent carbon quantum dots and naphthalimide for ratiometric detection of nitric oxide: To examine nitrite levels in meat samples

Nitric oxide (NO) is a ubiquitous, gaseous, free radical signaling molecule which plays a key role in physiological and pathological processes. Literature reports revealed that the conventional methods such as colorimetry, electron paramagnetic resonance (EPR), electrochemical etc. to detect NO are costly, time consuming and lack resolution, particularly in aqueous or biological system. Thus, in this context, herein we have developed covalently linked biomass derived carbon quantum dots (CQDs) and naphthalimide based nano sensor system for FRET based ratiometric detection of nitric oxide (NO) in pure aqueous media. The CQDs derived from orange peels were characterized using UV–visible absorption, fluorescence spectroscopy, PXRD, TEM, FT-IR and zeta potential studies. Further, the obtained CQDs were functionalized with amine functionality, and subsequently linked with naphthalimide derivative (5) using terephthaldehyde through covalent bond formation. The conjugation of naphthalimide (5) and functionalized CQDs was studied using DLS, zeta potential, FT-IR and time resolved fluorescence spectroscopy. The excitation of developed nano sensor system at λex 360 nm results in fluorescence emission at λem 530 nm which establishes the FRET pair between the CQDs and naphthalimide unit. However, in the presence of NO, the observed FRET pair abolishes due to the cleavage of NO susceptible imine bond. The developed sensor demonstrates high selectivity towards NO with limit of detection (LOD) and limit of quantification (LOQ) of 15 nM and 50 nM respectively. Further, the developed sensor system was also utilized for indirect detection of nitrite (NO2−) in food samples for food safety and monitoring.

Water-Assisted Cold Sintering of Alumina Ceramics in SPS Conditions

A developing energy-saving approach of cold sintering in a pure aqueous medium was applied to the preparation of α-Al2O3 ceramics and performed on spark plasma sintering equipment. The initial γ-Al(OH)3 and γ-AlOOH powders and the cold-sintered ceramics were studied by X-ray diffraction analysis, infrared spectroscopy, thermal analysis, and scanning electron microscopy to reveal the chemical and structural transformations they experienced during the cold sintering. At 450 °C and 70 MPa, initially γ-AlOOH transformed into a fragile α-Al2O3 material. Porous α-Al2O3 ceramics with about 60% porosity were obtained after cold sintering of γ-Al(OH)3 in the same conditions combined with subsequent annealing at 1250 °C for 3 h. The role of water molecules in the studied processes was considered as the enhancement of structural mobility in the cold-sintered material due to its reversible hydroxylation similar to earlier investigated supercritical water actions on the precursors during α-Al2O3 formation. Further improvement of the cold sintering setup and regimens would open prospects in α-Al2O3 ceramics manufacturing by an ecologically benign route.