Nanoscale PAPER Cite this: Nanoscale, 2020, 12, 602 Received 9th October 2019, Accepted 28th November 2019 DOI: 10.1039/c9nr08663c rsc.li/nanoscale Influence of the solvent environment on luminescent centers within carbon dots† Evgeniia A. Stepanidenko, a Irina A. Arefina, a Pavel D. Khavlyuk, a Aliaksei Dubavik, a Kirill V. Bogdanov, a Daniil P. Bondarenko, a Sergei A. Cherevkov, a Evgeny V. Kundelev, a Anatoly V. Fedorov, a Alexander V. Baranov, a Vladimir G. Maslov, a Elena V. Ushakova * a,b and Andrey L. Rogach a,b Carbon dots (CDs) are luminescent nanomaterials, with potential use in bioimaging and sensorics. Here, the influence of the surrounding solvent media on the optical properties of CDs synthesized from the most commonly employed precursors, namely citric acid and ethylenediamine, is investigated. The posi- tion of optical transitions of CDs can be tuned by the change of pH and solvent polarity. The most striking observation is related to the interaction of CDs with chlorine containing solvents, which results in resol- ving a set of narrow peaks within both the absorption and PL bands, similar to those observed for poly- cyclic aromatic hydrocarbons or organic dyes. We assume that the chlorine containing molecules pene- trate the surface layers of CDs, which results in an increase of the distance between the luminescent centers; this correlates well with an enhanced D-band in their Raman spectra. A model of CDs composed of a matrix of hydrogenated amorphous carbon with the inclusions of sp 2 -domains formed by polycyclic aromatic hydrocarbons and their derivatives is suggested; the latterare stacked ensembles of the lumino- phores and are considered as the origin of the emission of CDs. Introduction Carbon nanomaterials continue to attract attention from mul- tiple research groups around the world; one of the most recent additions to their family is strongly luminescent carbon dots (CDs). 1,2 Simple and cost-effective synthesis, biocompatibility, and ease of incorporation into various matrices are among the advantages of CDs. 3,4 CDs emitting over a wide spectral range have been developed, 5–7 opening broad application prospects, 8 such as for bioimaging, 7,9–11 as probes for analyte detection, 12 data encryption, 13,14 and fabrication of light emitting devices (LEDs). 15,16 There have been a number of studies focused on revealing the nature of the electron transitions in CDs, and how they depend on their structural characteristics. 1,5,17–22 Even the origin of such widespread property reported for plenty of CDs as their excitation dependent photoluminescence (PL) is still under discussion in literature, studies of optical properties of CDs in different environments (different solvents, pH, and temperature) were thought to offer additional information in this respect. Lin and co-workers reported on the solvent dependence of the emission of m-phenylenediamine based CDs. 13 Reckmeier et al. showed that PL bands of CDs related to different emission states experience opposite solvatochro- mic shifts in protic and aprotic solvents. 23 Kozák et al. studied the influence of solvent polarity on the optical properties of CDs and showed a significant PL band shift from the blue to yellow spectral region. 24 Papaioannou et al. observed a slight red shift of the PL band of the sugar derived CDs with the increase in solvent polarity, 25 similar to what has been reported in some other studies. 16,26,27 In particular, in ref. 16, a red shift of the absorption and PL bands has been observed upon transfer of the CDs from an aprotic to a protic solvent. In contrast, a blue shift of the absorption and PL bands has been detected for CDs dispersed in a protic solvent. 28 We note that these studies addressed the optical properties of CDs syn- thesized by a number of techniques, showing either excitation dependent 25,27,28 or independent behavior. 16,26 In addition, the CD energy structure depends on the synthesis procedures, including the type of precursor, 10,23 temperature, 17,20,23 and pressure regimes, hence, affecting the degree of carbonization and type of luminescent center formed during CD synthesis. † Electronic supplementary information (ESI) available: Spectral parameters of CDs in different solvents with various polarities; PLE and PL spectra of CDs in Cl-containing solvents; Raman spectra; and calculated D and G band positions and their area ratios. See DOI: 10.1039/c9nr08663c a Center of Information Optical Technologies, ITMO University, Saint Petersburg 197101, Russia. E-mail: elena.ushakoca@itmo.ru b Department of Materials Science and Engineering, and Centre for Functional Photonics (CFP), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong SAR 602 | Nanoscale, 2020, 12, 602–609 This journal is © The Royal Society of Chemistry 2020 Published on 28 November 2019. Downloaded by University of Reading on 1/2/2020 8:31:45 PM. View Article Online View Journal | View Issue