A Fluorescence-Based Chemical Sensor for Detection of Melamine in Aqueous Solutions

Melamine, an industrial chemical, receives wide attention nowadays because of its unethical usage as a nitrogen enhancer in protein-rich foods and dairy products. Since most of the existing melamine detection methods are highly expensive and time-consuming, high sensitivity biosensor-based detection methods have arisen in the scientific literature as promising alternatives. This study reports the design, synthesis, and fluorescent investigations of a carbazole-based sensor (CB) for the detection of melamine in aqueous solutions. The titration studies and microplate experiments on a CB-cyanuric acid mixture (CB-CA) with melamine suggested that the novel sensor could detect melamine even at very low concentrations in both aqueous solutions and dairy samples.

Synthesis and crystal structure of the lanthanum cyanurate complex La[H2N3C3O3]3 · 8.5 H2O

Single crystals of La[H2N3C3O3]3 · 8.5 H2O were obtained from stoichiometric amounts of as-precipitated La(OH)3 with cyanuric acid (CYA) [H3N3C3O3]3 in a boiling aqueous solution, followed by slow cooling and evaporation of water under ambient conditions. According to the X-ray structure analysis of the colorless and transparent crystals, La[H2N3C3O3]3 · 8.5 H2O adopts the triclinic space group P1 (no. 1) and exhibits the unit-cell parameters a = 987.24(7), b = 1110.97(8), c = 1179.81(9) pm, α = 113.716(2), β = 97.053(2), γ = 101.502(2)° for Z = 2. The CYA is singly deprotonated to give monoanions [H2N3C3O3]– which are O,N-coordinated to the La3+ cations. These dihdrogencyanurate anions are assembled in ribbons with two crystallographically different La3+ cations coordinating to either one or two different ligands, respectively. The coordination sphere of the La3+ cations is comprised of water molecules, and interstitial water molecules fill the dead volume of the crystals. The anionic ribbons are stacked to maximize the contact between the six-membered rings, showing distances of about 330 pm.

Thermogravimetric Experiment of Urea at Constant Temperatures

There are still many unsolved mysteries in the thermal decomposition process of urea. This paper studied the thermal decomposition process of urea at constant temperatures by the thermal gravimetric–mass spectrometry analysis method. The results show that there are three obvious stages of mass loss during the thermal decomposition process of urea, which is closely related to the temperature. When the temperature was below 160 °C, urea decomposition almost did not occur, and molten urea evaporated slowly. When the temperature was between 180 and 200 °C, the content of biuret, one of the by-products in the thermal decomposition of urea, reached a maximum. When the temperature was higher than 200 °C, the first stage of mass loss was completed quickly, and urea and biuret rapidly broke down. When the temperature was about 240 °C, there were rarely urea and biuret in residual substance; however, the content of cyanuric acid was still rising. When the temperature was higher than 280°C, there was a second stage of mass loss. In the second stage of mass loss, when the temperature was higher than 330 °C, mass decreased rapidly, which was mainly due to the decomposition of cyanuric acid. When the temperature was higher than 380 °C, the third stage of mass loss occurred. However, when the temperature was higher than 400 °C, and after continuous heating was applied for a sufficiently long time, the residual mass was reduced to almost zero eventually.

Using transient equilibria (TREQ) to measure the thermodynamics of slowly assembling supramolecular systems

Supramolecular chemistry involves the non-covalent assembly of monomers into materials with unique properties and wide-ranging applications. Thermal analysis is a key analytical tool in this field, as it provides quantitative thermodynamic information on both the structural stability and nature of the underlying molecular interactions. However there exist many supramolecular systems whose kinetics are so slow under conditions approaching equilibrium that thermodynamic data are inaccessible. We have developed a simple and rapid spectroscopic method for extracting thermodynamic parameters from these systems. It is based on repeatedly raising and lowering the temperature during assembly and identifying the points of transient equilibrium as they are passed on the up- and down-scans. In a proof-of-principle application to the co-assembly of polydeoxyadenosine containing 15 adenosines (polyA15) and cyanuric acid (CA), we found that roughly 30% of the CA binding sites on the polyA chains were unoccupied, with implications for the assembly of high-valence systems.