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The current study reports a facile, single step microwave assisted synthesis of nitrogen, oxygen-doped carbon quantum dots (NCQDs) with high quantum yield (~36%). The synthesized NCQDs with an average size of 4 nm demonstrate many fold increase in the intensity of photoluminescence (PL) over the undoped carbon quantum dots (CQDs). UV–Vis absorption peak exhibits a red shift of 26 nm for NCQDs with respect to pristine CQDs. The spectral shift was further confirmed by density functional theory based calculations. X-ray photoelectron spectroscopy measurements reveal the graphitic nature and significantly high nitrogen doping of NCQDs (38.2%). The NCQDs exhibited excitation dependent PL behaviour in the visible region with maximum emission peak recorded at 415 nm with optimum excitation wavelength of 340 nm. The remarkably enhanced PL properties of NCQDs have been employed as fluorescent probe for sensitive and selective sensing of Cu2+ ions. The Cu2+ quenches the fluorescence intensity of NCQDs due to its high binding affinity towards N and O containing functional groups in quantum dots. The NCQDs render a simple, reliable and sensitive detection of Cu2+ ions with limit of detection as low as 1.8 µM.

The tandem operation of draglines is in use in some of the major Indian opencast coalmines owing to the favourable geo-mining conditions, technical suitability, efficiency and economic viability. In view of the importance of tandem operation, the present study has been undertaken in a large Indian opencast coalmine in order to critically investigate the horizontal and vertical tandem operation of draglines on moderately strong and high sandstone benches (35–42 m), overlying a 15–18 m thick coal seam. The study has revealed that although the preparation of balancing diagram for planning of dragline operations is the first and the most important step, its actual implementation is equally important. Marked discrepancies in the productivity parameters as envisaged by the balancing diagram and as observed in the field studies, have been investigated and reported. The study also propounds the importance of appraisal of dragline productivity parameters, such as, cycle time, swing angle, seating position, availability, utilization, etc., in the field scale. Irrespective of the mode of operation (horizontal or vertical tandem), the study moots the concept of computation of the weighted cycle time and overall cycle time vis-a-vis swing angle variation for the draglines operating in field. The results drawn from the case study have been discussed in terms of cycle time computations, annual output computation and evaluation of earthmoving efficiency for the horizontal and vertical tandem modes of operation.