Tribological properties of a series of carbon dots modified by ionic liquids with various anion species: experimental findings and density functional theory calculations.

A series of carbon dots modified by ionic liquids with various anion species (CDs-ILs-X) were facilely synthesized by a one-pot pyrolysis method and subsequent anion exchange processes, where X- represented the anions of hexafluorophosphate (PF6-), bis(trifluorosulfonyl) imide (NTf2-), bis(salicylato) borate (BScB-) and oleate (OL-). Their tribological properties as lubricant additives of polyethylene glycol (PEG200) were investigated under the ball-on-plate reciprocating mode and steel/steel contact. The maximum friction coefficient and wear volume reductions of PEG200 triggered by CDs-ILs-PF6, CDs-ILs-NTf2, CDs-ILs-BScB and CDs-ILs-OL were up to 42.5% and 71.8%, 40.5% and 74.0%, 40.5% and 72.8%, and 52.2% and 79.5%, respectively. The excellent friction-reducing and anti-wear properties of CDs-ILs-X could be attributed to the formed boundary lubrication films composed of tribochemical products and CDs on the rubbing surfaces. The density functional theory calculations well explained the specific effects of anion species on the tribological performances of CDs-ILs-X. The strongest absorption stability and lowest steric hindrance of OL- made the CDs-ILs-OL form the densest boundary lubrication films on the sliding interfaces, and hence the CDs-ILs-OL exhibited the best tribological performance. The CDs-ILs-OL are promising lubricant additives to PEG owing to their high-performance, low cost and environmental friendliness.

Red-emission carbon dots-quercetin systems as ratiometric fluorescent nanoprobes towards Zn2+ and adenosine triphosphate.

Carbon dots (CDs) emitting red fluorescence (610 nm) were synthesized by solvent thermal treatment of p-phenylenediamine in toluene. Upon 440 nm excitation, quercetin (QCT) alone endowed slight effects on the red fluorescence of CDs. Once Zn2+ was further introduced, the QCT-Zn2+ complex was quickly formed. This complex absorbs excitation light and emits bright green fluorescence at 480 nm. The red fluorescence of CDs was greatly quenched owing to the inner-filter effect. The ratio of fluorescence intensity at 480 nm and 610 nm (I480/I610) gradually increases with increasing concentration (c) of Zn2+. Al3+ exhibits the same phenomen like Zn2+. Fluoride ions form a more stable complex with Al3+ than QCT-Al3+ complex but have a negligible effect on the QCT-Zn2+ complex. The possible interference of Al3+ on Zn2+ can thus be avoided by adding certain amount of F-. The CD-QCT-F- system was constructed as a ratio-metric fluorescent nanoprobe toward Zn2+ with determination range of 0.14-30 µM and limit of detection (LOD) of 0.14 µM. Due to the stronger affinity of adenosine triphosphate (ATP) to Zn2+ than QCT, the I480/I610 value of CD-QCT-F--Zn2+ system gradually decreases with increasing cATP. The ratiometric fluorescent nanoprobe toward ATP was established with detection ranges of 0.55-10 and 10-35 µM and a LOD of 0.55 µM. The above two probes enable the quantitative determination of Zn2+ and ATP in tap and lake water samples with satisfactory recoveries. Graphical abstract Schematic representation of the ratiometric fluorescent nanoprobes based on the carbon dots (CDs)-quercetin (QCT) system towards Zn2+ and adenosine triphosphate (ATP) with high selectivity and sensitivity.

Hydrothermal synthesis of N-doped carbon dots from an ethanolamine-ionic liquid gel to construct label-free multifunctional fluorescent probes for Hg2+, Cu2+ and S2O32.

N-Doped carbon dots (NCDs) were facilely synthesized by a hydrothermal method using an ethanolamine-ionic liquid (1-carboxyethyl-3-methyl imidazole chloride) gel as a precursor. The NCDs had an average particle size of 3.4 nm and were partially crystalline and abundant in amide and hydroxyl groups and pyridinic/pyrrolic nitrogen atoms on their surfaces. In water, the NCDs showed excitation- and concentration-dependent photoluminescence (PL) properties with a high salt tolerance and a quantum yield of 24.7% under 350 nm excitation. The PL of the aqueous suspension of NCDs remained stable in a pH range of 4-11 and was significantly quenched by Hg2+ and Cu2+ owing to the strong interactions between the metal ions and the surface groups and/or nitrogen atoms of the NCDs. The possible quenching mechanism was determined to be photo-induced electron transfer. By introducing the masking agents of P2O74- and S2O32- into the aqueous suspension of NCDs, two label-free "turn-off" fluorescent probes for Hg2+ and Cu2+ with a high sensitivity and selectivity were built, respectively. Moreover, the quenched NCDs-Hg2+ system could be used as a selective "turn-on" fluorescent probe for S2O32- due to the strong affinity of S2O32- to Hg2+. The above described three probes all illustrated two good linear detection ranges for Hg2+ (0-10 and 10-50 µM), Cu2+ (0-2.5 and 2.5-40 µM) and S2O32- (0-20 and 20-80 µM), and their limits of detection were calculated to be 0.076, 0.125 and 1.17 µM. The recovery tests demonstrated that the above described probes were reliable and capable of detecting corresponding ions in tap water samples with satisfactory results.