Photoluminescent carbon quantum dots (CQDs) were synthesized from lemon juice precursors via single-step, hydrothermal techniques under different temperatures to control the optical properties. The synthesized CQDs were characterized by PL, illuminated UV analyzer chamber, UV–vis spectroscopy, X-ray diffractometer, Fourier-transformed infrared spectrophotometry, and zeta potential techniques. The results show that the synthesized CQDs had an excellent blue–green emission extending up to the infrared region with high quantum yield (Φ) in the range of 14%–41%. The effect of reaction temperature and the aging of CQDs on the emission spectra of CQDs are also investigated. Furthermore, the adsorption effect of the synthesized CQDs was evaluated on methylene blue (MB) dyes. The result indicated the synthesized CQDs have excellent adsorbent properties with a removal efficiency of 60%–82% and an extremely fast adsorption rate of 6 × 10−2 min−1 for MB dyes.

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