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- Carbon dots synthesis from coffee grounds, and sensing of nitroanilinesPublication . Moraes, Bianca; Costa, Alexandra I.; Barata, Patrícia; Prata, José V.Fluorescent carbon dots (C-dots) were directly synthesized by a sustainable and eco friendly one-pot microwave-assisted hydrothermal carbonization method from coffee grounds waste. The coffee grounds obtained from automatic coffee machines, after being heated at 190 °C for 1–4 h in the presence of nitrogen additives, furnished the desired carbon nanomaterials. Struc tural and photophysical properties of the as-synthesized nanomaterials were evaluated by FTIR, 1H NMR, UV-Vis, and fluorescence spectroscopies. The ability of the C-dots to behave as probes for isomeric nitroanilines (ortho-, meta- and para-nitroaniline) was explored through fluorimetric titra tion experiments. High sensitivities and selectivities were obtained for the detection of nitroanilines in aqueous media.
- Synthesis, structure, and optical properties of an alternating calix[4]arene-based meta-linked phenylene ethynylene copolymerPublication . D. Barata, Patrícia; Costa, Alexandra; Ferreira, Luis F. V.; Prata, José VirgílioNovel alternating copolymers comprising biscalix[4]arene-p-phenylene ethynylene and m-phenylene ethynylene units (CALIX-m-PPE) were synthesized using the Sonogashira-Hagihara cross-coupling polymerization. Good isolated yields (60-80%) were achieved for the polymers that show M-n ranging from 1.4 x 10(4) to 5.1 x 10(4) gmol(-1) (gel permeation chromatography analysis), depending on specific polymerization conditions. The structural analysis of CALIX-m-PPE was performed by H-1, C-13, C-13-H-1 heteronuclear single quantum correlation (HSQC), C-13-H-1 heteronuclear multiple bond correlation (HMBC), correlation spectroscopy (COSY), and nuclear overhauser effect spectroscopy (NOESY) in addition to Fourier transform-Infrared spectroscopy and microanalysis allowing its full characterization. Depending on the reaction setup, variable amounts (16-45%) of diyne units were found in polymers although their photophysical properties are essentially the same. It is demonstrated that CALIX-m-PPE does not form ground-or excited-state interchain interactions owing to the highly crowded environment of the main-chain imparted by both calix[4]arene side units which behave as insulators inhibiting main-chain pi-pi staking. It was also found that the luminescent properties of CALIX-m-PPE are markedly different from those of an all-p-linked phenylene ethynylene copolymer (CALIX-p-PPE) previously reported. The unexpected appearance of a low-energy emission band at 426 nm, in addition to the locally excited-state emission (365 nm), together with a quite low fluorescence quantum yield (Phi = 0.02) and a double-exponential decay dynamics led to the formulation of an intramolecular exciplex as the new emissive species.
- Carbon dots from coffee grounds: synthesis, characterization, and detection of noxious nitroanilinesPublication . Costa, Alexandra I.; Barata, Patrícia; Moraes, Bianca; Prata, José V.Coffee ground (CG) waste is generated in huge amounts all over the world, constituting a serious environmental issue owing to its low biodegradability. Therefore, processes that simultaneously aim for its valorization while reducing its environmental impact are in great demand. In the current approach, blue luminescent carbon dots (C-dots) were produced in good chemical yields from CGs following hydrothermal carbonization methods under an extended set of reaction parameters. The remarkable fluorescent properties of the synthesized C-dots (quantum yields up to 0.18) allied to their excellent water dispersibility and photostability prompted their use for the first time as sensing elements for detection of noxious nitroanilines (NAs) in aqueous media. Very high levels of NA detection were achieved (e.g., limit of detection of 68 ppb for p-nitroaniline), being the regioisomeric selectivity attributed to its higher hyperpolarizability and dipole moment. Through ground-state and time-resolved fluorescence assays, a static fluorescence quenching mechanism was established. H-1 NMR titration data also strongly suggested the formation of ground-state complexes between C-dots and NAs.
- Fluorescent bis-calix[4]arene-carbazole conjugates: synthesis and inclusion complexation studies with fullerenes C-60 and C-70Publication . Barata, Patrícia; Costa, Alexandra I.; Costa, Sérgio; Prata, José V.Supramolecular chemistry has become a central theme in chemical and biological sciences over the last decades. Supramolecular structures are being increasingly used in biomedical applications, particularly in devices requiring specific stimuli-responsiveness. Fullerenes, and supramolecular assemblies thereof, have gained great visibility in biomedical sciences and engineering. Sensitive and selective methods are required for the study of their inclusion in complexes in various application fields. With this in mind, two new fluorescent bis-calix[4]arene-carbazole conjugates (4 and 5) have been designed. Herein, their synthesis and ability to behave as specific hosts for fullerenes C-60 and C-70 is described. The optical properties of the novel compounds and their complexes with C-60 and C-70 were thoroughly studied by UV-Vis and steady-state and time-resolved fluorescence spectroscopies. The association constants (K-a) for the complexation of C-60 and C-70 by 4 and 5 were determined by fluorescence techniques. A higher stability was found for the C-70@4 supramolecule (K-a = 5.6 x 10(4) M-1; Delta G = -6.48 kcal/mol). Evidence for the formation of true inclusion complexes between the host 4 and C-60/C-70 was obtained from NMR spectroscopy performed at low temperatures. The experimental findings were fully corroborated by density functional theory (DFT) models performed on the host-guest assemblies (C-60@4 and C-70@4).
- Evaluation of the molecular architecture of fluorescence calix[4]arene-based sensors in detection of toxic metalsPublication . Costa, Alexandra; Barata, Patrícia; Fialho, Carina B.; Prata, José VirgílioChemical sensors have been playing a crucial role in analytical chemistry, bio-medicinal science and environmental chemistry. Chemosensors offer na accurate and low-cost finding of anions, cations, enzymes and toxic metal ions with high selectivity and sensitivity. In this regard, many organic compounds have been synthesized and are being used as successful chemosensors, however calixarenes offer distinct advantage in term of selectivity and the easy incorporation of a fluorophore into the structure. Cation complexing ligands containing calix[4]arene have been used to obtain more selective metal ions receptors. Herein, we report fluorescente calix[4]arene-based sensors with diferente molecular architecture [3] and their potentialities to address the detection of toxic metals. Using fluorescence as the signal transduction technique, experiments have shown that CALIX-OCP-2-CBZs are the most sensitiveto Cu.
- Fluorescent calix[4]arene-carbazole-containing polymers as sensors for nitroaromatic explosivesPublication . Barata, Patrícia; Prata, José VirgílioTwo highly fluorescent calix[4]arene-containing phenylene-alt-ethynylene-carbazolylene polymers (Calix-PPE-CBZs) were used in the detection of explosives from the nitroaromatic compounds (NACs) family, in solution and in vapour phases. Both fluorophores exhibit high sensitivity and selectivity towards NACs detection. The quenching efficiencies in solution, assessed through static Stern-Volmer constants (K-SV), follow the order picric acid (PA) >> 2,4,6-trinitrotoluene (TNT) > 2,4-dinitrotoluene > (2,4-DNT) > nitrobenzene (NB). These correlate very well with the NACs electron affinities, as evaluated from their lowest unoccupied molecular orbitals (LUMOs) energies, indicating a photo-induced electron transfer as the dominant mechanism in fluorescence quenching. Moreover, and most interesting, detection of TNT, 2,4-DNT and NB vapours via thin-films of Calix-PPE-CBZs revealed a remarkably sensitive response to these analytes, comparable to state-of-the-art chemosensors. The study also analyses and compares the current results to previous disclosed data on the detection of NACs by several calix[4]arene-based conjugated polymers and non-polymeric calix[4]arenes-carbazole conjugates, overall highlighting the superior role of calixarene and carbazole structural motifs in NACs' detection performance. Density functional theory (DFT) calculations performed on polymer models were used to support some of the experimental findings.
- Inherently chiral calix[4]arenes with planar chirality: two new entries to the familyPublication . Prata, José Virgílio; D. Barata, Patrícia; Pescitelli, GennaroThe synthesis of two new inherently chiral calix[4]arenes (ICCs, 1 and 2), endowed with electron-rich concave surfaces, has been achieved through the desymmetrization of a lower rim distal-bridged oxacyclophane (OCP) macrocycle. The new highly emissive ICCs were resolved by chiral HPLC, and the enantiomeric nature of the isolated antipodes proved by electronic circular dichroism (CD). Using time-dependent density functional calculations of CD spectra, their absolute configurations were established. NMR studies with (S)-Pirkle's alcohol unequivocally showed that the host-guest interactions occur in the chiral pocket comprehending the calix-OCP exo cavities and the carbazole moieties.
- Calix[4]arene-carbazole-containing polymers: synthesis and propertiesPublication . D. Barata, Patrícia; Costa, Alexandra; Prata, José VirgílioNew highly fluorescent calix[4]arene-containing phenylene-alt-ethynylene-3,6- and 2,7-carbazolylene polymers (CALIX-PPE-CBZs) have been synthesized for the first time and their photophysical properties evaluated. Both polymers were obtained in good isolated yields (70-84%), having M-w ranging from 7660-26,700 g mol(-1). It was found that the diethynyl substitution (3,6- or 2,7-) pattern on the carbazole monomers markedly influences the degree of polymerization. The amorphous yellow polymers are freely soluble in several nonprotic organic solvents and have excellent film forming abilities. TG/DSC analysis evidences similar thermal behaviors for both polymers despite their quite different molecular weight distributions and main-chain connectivities (T-g, in the range 83-95 degrees C and decomposition onsets around 270 degrees C). The different conjugation lengths attained by the two polymers dictates much of their photophysical properties. Thus, whereas the fully conjugated CALIX-PPE-2,7-CBZ has its emission maximum at 430 nm (E-g = 2.84 eV; Phi(F) = 0.62, CHCl3), the 3,6-linked counterpart (CALIX-PPE-3,6-CBZ) fluoresces at 403 nm with a significant lower quantum yield (E-g = 3.06 eV; Phi(F) = 0.31, CHCl3). The optical properties of both polymers are predominantly governed by the intrachain electronic properties of the conjugated backbones owing to the presence of calix[4]arenes along the polymer chain which disfavor significant interchain interactions, either in fluid- or solid-state.