A Perylene diimide based fluorescent probe for caffeine in aqueous medium

Constructing Mechanochromic Fluorescent Interphase via Core-Shell Microcapsules: A Route for Interface Reinforcing and Damage Self-Reporting of CFRP Composites

Perylenediimide-chitosan/γ-poly (glutamic acid) microcapsules sizing (PDI-CS/γ-PGA) core-shell microcapsule is designed and used to establish a novel interphase in carbon fiber/epoxy (CF/EP) composite, and the interfacial property, as well as the damage self-reporting of the composite, is compared with desized carbon fiber (CF-desized)/EP and commercial carbon fiber (CF-COM)/EP composite. The ruptured PDI-CS/γ-PGA microcapsule exhibits strong "turn-on" green fluorescence from the released PDI upon mechanical stimuli. The anchoring of PDI-CS/γ-PGA microcapsule on carbon fiber with PDI-CS/γ-PGA microcapsules sizing (CF@PDI-CS/γ-PGA) surface results in increased chemical activity and roughness, exhibiting a weak green fluorescence signal instead of non-fluorescence on CF-desized and CF-COM surface. The transverse fiber bundle tensile (TFBT) strength of CF@PDI-CS/γ-PGA composite is 80.97% and 31.09% higher than those of CF-desized/EP and CF-COM/EP composite, which is attributed to the mechanical interlocking and chemical bonding interaction between carbon fiber and epoxy matrix by introducing PDI-CS/γ-PGA microcapsule with spherular structure and active groups. After microdroplet testing, the strong "turn-on" green fluorescence signal of the released PDI from the microcapsules is detected in the interfacial debonding regions, realizing the microscopic damage self-reporting of CF@PDI-CS/γ-PGA composite.

Tricyclic octaurea "Temples" for the recognition of polar molecules in water

Two water-soluble tricyclic "Temple" macrocycles featuring pyrene roof/floor units and bis-urea spacers have been synthesised and studied as receptors for aromatic compounds in aqueous media. The tricycles show good selectivity for methylated purine alkaloids such as caffeine versus unsubstituted heterocycles such as adenine and indole. Binding is signalled by major changes in fluorescence, apparently due to the break-up of intramolecular excimers. The formation of excimers implies cavity collapse in the absence of guests explaining why, unlike an earlier relative, these receptors do not bind carbohydrates. Naphthalenediimides (NDIs) have also been studied as geometrically complementary guests, and indeed bind especially strongly (K_a > 10⁷ M^-1); this powerful and selective association suggests potential applications in supramolecular self-assembly.

Fluorescence modulation of nanodiamond NV- centers for neurochemical detection

Nanodiamond (ND) with nitrogen vacancy (NV-) color centers has emerged as an important material for quantum sensing and imaging. Fluorescent, carboxylated ND (140 nm) is investigated for the detection of dopamine (DA), caffeine (CA), and ascorbic acid (AA). Over a 200 nM range, DA and CA quenched the ND fluorescence by 7.1 and 9.8%, respectively. For AA, fluorescence was quenched (2.9%) at nM concentrations and enhanced at μM concentrations. The quenching fit well to Langmuir adsorption isotherms. For DA-CA mixtures, the CA at nM concentrations did not affect DA quenching but interfered when at μM concentrations. The DA at nM or μM concentrations lessened CA quenching. For DA-AA mixtures with DA at mM concentrations, AA quenched fluorescence throughout the nM and μM range, with increased quenching in the nM range. These studies support ND fluorescence modulation as a possible sensor modality for bioanalyte detection.

Selection of Aptamers for Sensing Caffeine and Discrimination of Its Three Single Demethylated Analogues

With the growing consumption of caffeine-containing beverages, detection of caffeine has become an important biomedical, bioanalytical, and environmental topic. We herein isolated four high-quality aptamers for caffeine with dissociation constants ranging from 2.2 to 14.6 μM as characterized using isothermal titration calorimetry. Different binding patterns were obtained for the three single demethylated analogues: theobromine, theophylline, and paraxanthine, highlighting the effect of the molecular symmetry of the arrangement of the three methyl groups in caffeine. A structure-switching fluorescent sensor was designed showing a detection limit of 1.2 μM caffeine, which reflected the labeled caffeine concentration within 6.1% difference for eight commercial beverages. In 20% human serum, a detection limit of 4.0 μM caffeine was achieved. With the four aptamer sensors forming an array, caffeine and the three analogues were well separated from nine other closely related molecules.