Journal of Environmental Chemical Engineering 9 (2021) 104803 Available online 26 November 2020 2213-3437/© 2020 Elsevier Ltd. All rights reserved. Fluorescent graphitic carbon nitride and graphene oxide quantum dots as effcient nanozymes: Colorimetric detection of fuoride ion in water by graphitic carbon nitride quantum dots Mamta Devi a , Punamshree Das a, b , Purna K. Boruah a, b , Manash J. Deka a , Rituparna Duarah a , Abinash Gogoi a , Dipankar Neog c , Hemant S. Dutta d , Manash R. Das a, b, * a Advanced Materials Group, Materials Sciences and Technology Division, CSIR-North East Institute of Science and Technology (CSIR-NEIST), Jorhat 785006, Assam, India b Academy of Scientifc and Innovative Research (AcSIR), Ghaziabad 201002, India c General Engineering Group, Engineering Sciences and Technology Division, CSIR-NEIST, Jorhat 785006, Assam, India d Analytical Chemistry Group, Materials Sciences and Technology Division, CSIR-NEIST, Jorhat 785006, Assam, India A R T I C L E INFO Editor: Zhang Xiwang Keywords: Graphitic carbon nitride quantum dots Graphene oxide quantum dots Enzyme mimics Colorimetric sensing Fluoride detection 3, 3 ′ ,5,5 ′ Tetramethylbenzidine (TMB) ABSTRACT Development of strategies based on artifcial nanozymes over natural enzymes for sensing of toxic ions in water has drawn tremendous interest among researchers. Herein, we have reported a facile synthesis of fuorescent graphitic carbon nitride quantum dots (g-CNQDs) and graphene oxide quantum dots (GQDs) using simple, cost- effective and low-temperature methods from readily available precursors such as urea, trisodium citrate, and citric acid. The peroxidase mimic activity of prepared g-CNQDs and GQDs (nanozymes) were established without functionalizing or doping with other nanoparticles. These nanozymes acted as effcient catalysts, facilitating the simultaneous reduction of H 2 O 2 and oxidation of colorless peroxidase substrate 3,3 ′ ,5,5 ′ - tetramethylbenzidine (TMB) to produce a blue color oxidized TMB in presence of H 2 O 2 . Thus, a simple colorimetric system is estab- lished based on the enzyme mimic activity of the synthesized g-CNQDs for detection of fuoride ions in aqueous solution. The g-CNQDs showed high sensitivity and selectivity towards the detection of fuoride ions in water with a detection limit 4.06 μM in a wide detection range upto 120 μM. This study provides the evidence that the reported colorimetric technique is green, economic, fast responsive, sensitive, and highly selective which can be further utilized for the detection of fuoride ion in naturally occurring water samples. 1. Introduction Natural enzymes are effcient biocatalysts that participate in com- plex metabolic processes in living organisms. They have found huge applications in domains such as food, agriculture, pharmaceutical and chemical industries, sensing, biomedicine, environmental engineering, etc. [1]. The natural enzyme, horseradish peroxidase (HRP) helps in catalyzing peroxide decomposition and protects the cells from toxic materials. H 2 O 2 can be activated by peroxidases to participate in oxidation processes in numerous biological and biochemical systems [2]. However, there are many challenges associated with natural en- zymes that include costly purifcation, high rate of denaturation, reus- ability, and preservation processes [3,4]. Thus, it is necessary to develop an artifcial enzyme mimic exhibiting peroxidase-like activity like HRP enzyme to counteract the problems associated with natural enzymes [5]. Previous reports cite the development of numerous artifcial enzymes with remarkable advantages over the natural enzymes due to their facile synthesis processes, reusability, stability in harsh conditions, and eco- nomic viability [2,6]. In 2007, Yan and co-workers demonstrated the peroxidase-like activity of Fe 3 O 4 nanoparticles (NPs) and proposed the term “nanozymes” for nanomaterials with an enzyme like characteristics [7]. Since then, tremendous growth of research interest in this feld was noticed and several nanomaterials such as CeO 2 NPs [7,8], Au NPs [9], Co 3 O 4 NPs [10], and various carbon nanomaterials [11–14] etc. have been reported in the literature as peroxidase enzyme mimic. The peroxidase-like activity of the desired nanomaterial can be analyzed by a redox reaction caused by reduction of H 2 O 2 and oxidation of a number of the chromogenic substrates including hydroquinone, 1,2,3-trihydroxy * Corresponding author at: Materials Sciences & Technology Division, CSIR-North East Institute of Science and Technology, Jorhat 785006, Assam, India. E-mail addresses: mnshrdas@yahoo.com, mrdas@neist.res.in (M.R. Das). Contents lists available at ScienceDirect Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece https://doi.org/10.1016/j.jece.2020.104803 Received 1 September 2020; Received in revised form 28 October 2020; Accepted 12 November 2020