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Water‐dispersible and fluorescence‐stable carbon quantum dots (CQDs) have been synthesized at gram scale using pyrene as carbon precursor by employing the bottom‐up approach. X‐ray diffraction, FTIR, Raman spectroscopy and UV visible spectroscopy confirm the formation of CQDs. TEM images shows that size of the CQDs ranges from 2 to 5 nm. A detailed study of their optical and fluorescence characteristics was performed. Their aqueous dispersion radiates bright yellow light under UV lamp (365 nm). The research work includes derivation of their excitation wavelength, observation of influence of dilution on fluorescence and monitoring of emission intensity for about 4 months. The CQDs can effectively recognize the presence of heavy metal ions in aqueous medium at neutral pH via fluorescence quenching. [ABSTRACT FROM AUTHOR]

Graphene quantum dots (GQDs) have been prepared from graphene oxide (GO) and characterized by standard analytical techniques. The size of the prepared GQDs ranges from 2-10 nm. Aqueous dispersion of GQDs exhibited excitation-dependent emission behavior. Emission intensity of the aqueous dispersion found stable for the examined duration of about four months. GQDs exhibited selective recognition of Fe3+ and Cr3+ out of various common ions such as alkali, alkaline-earth and transition metal ions in aqueous medium through fluorescence quenching. The lower limit of detection of Fe3+ is 1 μM and that of Cr3+ is 4 μM. [ABSTRACT FROM AUTHOR]