Understanding the Photoluminescence Mechanism of Nitrogen-Doped Carbon Dots by Selective Interaction with Copper Ions Manjunatha Ganiga and Jobin Cyriac* [a] 1. Introduction The emergence of carbon-based fluorescent nanomaterials, such as carbon dots (CDs), has facilitated the replacement of semiconductor quantum dots (QDs) and organic fluorescent dyes. CDs are recognized with their ease of synthesis, high sta- bility, low toxicity and low cost. Despite the various applica- tions of CDs, the origin of photoluminescence (PL) is still blurry. [1, 2] Plausible reasons could be quantum confinement, size heterogeneity, and the participation of defect/trap states. [3–6] Recently, Lie and co-workers attributed the origin of green luminescence in CDs to special edge effects controlled by edge carbon atoms and C =O functional groups. [7] However, Peng et al. observed size-dependent PL responses from fluo- rescent CDs are also reported. [3] Another group reported the size-dependent PL emission from phenylenediamine derived CDs. [8] Doping provides an opportunity to tune the electronic energy levels for desired applications. Numerous synthetic strategies are available for preparing nitrogen-doped CDs from biological sources such as orange peels, [9] milk, [10] plant leaf, [11] cucumber [12] and so on, as well as from non-biological carbon precursors such as quinolone, [13] ammonium citrate, [14] poly- acrylamide, [15] and so on. Tang et al. showed that there is a sig- nificant change in the energy level structure of CDs doped with nitrogen and thus in their photoluminescence behavior. [16] Furthermore, it is possible to tune the luminescence behavior of nitrogen doped carbon dots by altering N-doping concen- tration. [17] However, the effect of nitrogen on the PL behavior of CDs remains unclear. Thus, a detailed understanding of the PL behavior of CDs and the influence of dopants such as nitro- gen is imperative in extending their scientific and technologi- cal relevance. The electronic energy levels of the fluorophore can be tuned by the interaction with metal atoms/ions, facilitating an indirect insight into its photophysical behavior. For instance, DFT studies shows that a considerable change in the PL behav- ior of anthracene derivatives is possible when it is interacted with metal ions such as Hg II and Zn II due to the alteration in frontier molecular orbital structure. [18] Detailed studies on the interaction of metal ions with CDs are scarce, despite many ex- tensive studies are available with semiconductor QDs counter- part. [19] For example, exciton band of ZnO QDs irradiated with UV light found to vary when in contact with metal islands. [20] Generally, metal-CDs interactions have been widely implement- ed in fluorescent-based sensor applications. For example, FRET- based fluorescence detection of metal ions such as Al 3 + and K + have been separately accomplished using CDs. [21, 22] Copper is an essential element in the human body, present in numerous key enzymes including cytochrome c oxidase, su- peroxide dismutase, and tyrosinase. Similar to other metal ions, a surplus amount of copper in the body causes renal dis- orders and damages in the central nervous system. Moreover, copper is exploited in numerous industrial, environmental, and domestic processes. [23, 24] Therefore, a simple and selective de- tection of Cu 2 + is required in environmental and biological do- mains. Fluorescence sensors are highly sensitive, selective, and easier to operate, compared with other spectroscopic or elec- trochemical sensors. [25, 26] Down and up-conversion fluores- cence based Cu 2 + detection using CDs has demonstrated a high sensitivity towards Cu 2 + but this sensor lacks selectivi- ty. [27] Ye et al. presented the ratiometric fluorescence sensing of Cu 2 + using a CD conjugate of a europium complex. [28] Graphit- ic carbon nitride nanosheets [29, 30] and photoluminescence poly- mer nanodots derived from grass [31] are two other representa- Doped fluorescent carbon dots (CDs) have drawn widespread attention because of their diverse applications and attractive properties. The present report focusses on the origin of photo- luminescence in nitrogen-doped CDs (NCDs), which is unrav- eled by the interaction with Cu 2 + ions. Detailed spectroscopic and microscopic studies reveal that the broad steady-state photoluminescence emission of the NCDs originates from the direct recombination of excitons (high energy) and the involve- ment of defect states (low energy). In addition, highly selective detection of Cu 2 + is achieved, with a detection limit of 10 mm and a dynamic range of 10 mm–0.4 mm. The feasibility of the present sensor for the detection of Cu 2 + in real water samples is also presented. [a] M. Ganiga, Dr. J. Cyriac Department of Chemistry Indian Institute of Space Science and Technology Thiruvananthapuram, 695 547 (India) E-mail : jobincyriac@iist.ac.in Supporting Information for this article can be found under: http:// dx.doi.org/10.1002/cphc.201600294. ChemPhysChem 2016, 17,1–8  2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 & These are not the final page numbers! ÞÞ These are not the final page numbers! ÞÞ Articles DOI: 10.1002/cphc.201600294