RESUMEN
Photoluminescent carbon dots (PL CDs) have drawn tremendous attention from researchers owing to their admirable properties and wide range of applications. Herein, highly PL nitrogen and sulfur doped carbon dots (N,S-CDs) were synthesized through a facile, green and rapid one-step microwave assisted method using goat hooves, a bio-waste and a green precursor. The structural and photophysical properties of as obtained N,S-CDs were thoroughly investigated. From the investigation, it is revealed that the N,S-CDs possess a spherical morphology with an average particle size of about 2 nm, highly amorphous nature, high functionality, negative zeta potential (-32 mV), good water-solubility, excitation dependant PL, high PL quantum yield (23.8%), nanosecond lifetime (τ avg = 3.38 ns) and excellent storage stability for 180 days without any agglomeration. In addition, the N,S-CDs exhibit high PL stability under diverse pH conditions, wide ionic strength and resistance towards photobleaching, which are very important properties for practical applications. The N,S-CDs selectively sense Au3+ ions and also reduce the Au3+ ions to metallic gold. Hence, the N,S-CDs were successfully applied as a potential candidate for sensing of Au3+ and simultaneous extraction of metallic gold in aqueous media without any further reducing agents. It is a significant green way for the recovery of gold in aqueous media.
RESUMEN
Light induced multisite electron proton transfer in two different phenol (simple and phenol carrying an intramolecularly hydrogen bonded base) pendants on acridinedione dye (ADD) and an NADH analogue was studied by following fluorescence quenching dynamics in an ultrafast timescale. In a simple phenol derivative (ADDOH), photo-excited acridinedione acquires an electron from phenol intramolecularly, coupled with the transfer of a proton to solvent water. But in a phenol carrying hydrogen bonded base (ADDDP), both electron and proton transfer occur completely intramolecularly. The sequence of this electron and proton transfer process was validated by discerning the pH dependency of the reaction kinetics. Since photo-excited ADDs are stronger oxidants, the sequential electron first proton transfer mechanism (ETPT) was observed in ADDOH and hence there is no change in the PCET reaction kinetics kETPT â¼ 6.57 × 109 s-1 in the entire pH range (pH 2-12). But the phenol carrying hydrogen bonded base (ADDDP) unleashes concerted electron proton transfer where the PCET reaction rate decreases upon decreasing the pH below its pKa. Noticeably, the concerted EPT process in ADDDP mimics the donor side of photosystem II and it occurs by two distinct pathways: (i) through direct intramolecular hydrogen bonding between the phenol and amine, kDEPT â¼ 12.5 × 1010 s-1 and (ii) through the bidirectional hydrogen bond extended by the water molecule trapped in between the proton donor and acceptor, which mediates the proton transfer and serves as a proton wire, kWMEPT â¼ 2.85 × 1010 s-1. These results unravel the incognito role played by water in mediating the proton transfer process when the structural elements do not favor direct hydrogen bonding between the proton donor and acceptor in a concerted PCET reaction.
RESUMEN
Highly selective fluorogenic detection of the fluoride ion becomes viable due to its propensity towards cleaving Si-O and Si-C bonds, the key reactive elements in fluoride selective chemodosimeters. Herein, acridinedione derived, two novel fluorescent probes bearing tertiarybutyldiphenylsilyloxy (TBDPS) and tertiarybutyldimethylsilyloxy (TBDMS) groups were synthesized and their fluoride selective dosimetric action in organic solvents and in mixed aqueous medium was established through steady state and time resolved fluorescence techniques. Unusually, these molecular probes maintain their sensitivity down to 10 ppb in both organic and mixed aqueous medium; hence they can be considered as highly selective and sensitive fluorescent probes for the fluoride anion. By following the kinetics of the desilylation process it is established that the reaction follows second order kinetics with respect to fluoride ion concentration in acetonitrile whereas it becomes first order in mixed aqueous medium owing to its high degree of hydration. Also, the hydrophobic and sterically crowded substitution on the silyl receptor hampers the reaction kinetics only in organic solvents whereas its influence in mixed aqueous medium is relatively very less. However, common inorganic cations (Na+) effectively hinder the reaction kinetics through strong ion pair interaction and prolong the response time. Therefore, the indigenous influences of three different factors which encumber the desilylation process were quantitatively enumerated and the prospective application of these fluorescent probes in detecting and validating fluoride ions in various environmental samples is demonstrated.