A facile approach for preparation of photoluminescent (PL) carbon dots (CDs) is reported. The three resulting CDs emit bright and stable red, green and blue (RGB) colors of luminescence, under a single ultraviolet-light excitation. Alterations of PL emission of these CDs are tentatively proposed to result from the difference in their particle size and nitrogen content. Interestingly, up-conversion (UC)PL of these CDs is also observed. Moreover, flexible full-color emissive PVA films can be achieved through mixing two or three CDs in the appropriate ratios. These CDs also show low cytotoxicity and excellent cellular imaging capability. The facile preparation and unique optical features make these CDs potentially useful in numerous applications such as light-emitting diodes, full-color displays, and multiplexed (UC)PL bioimaging.

Truly fluorescent excitation-dependent carbon dots are prepared, and the relationship between their chemical composition and fluorescent emission is discussed. Furthermore, potential applications of the as-prepared carbon dots to multicolor bio-labeling and multidimodal sensing are demonstrated.

Long-lifetime room-temperature phosphorescence (RTP) materials are important for many applications, but they are highly challenging materials owing to the spin-forbidden nature of triplet exciton transitions. Herein, a facile, quick and gram-scale method for the preparation of ultralong RTP (URTP) carbon dots (CDs) was developed via microwave-assisted heating of ethanolamine and phosphoric acid aqueous solution. The CDs exhibit the longest RTP lifetime, 1.46 s (more than 10 s to naked eye) for CDs-based materials to date. The doping of N and P elements is critical for the URTP which is considered to be favored by a n→π* transition facilitating intersystem crossing (ISC) for effectively populating triplet excitons. In addition, possibilities of formation of hydrogen bonds in the interior of the CDs may also play a significant role in producing RTP. Potential applications of the URTP CDs in the fields of anti-counterfeiting and information protection are proposed and demonstrated.

Stimuli-responsive optical materials have received tremendous interest in the last several decades due to their numerous promising applications. Here, fluorescence emissive polymer carbon dots (F-CDs), prepared with a simple heating treatment from ethylenediamine and phosphoric acid, are found to produce unexpected ultralong room-temperature phosphorescence (URTP), which lasts for about 10 s with a lifetime of 1.39 s. This is the first example to achieve the conversion of a fluorescence material to URTP by means of an external heating stimulus. Further investigations reveal that the doping of N and P elements and self-immobilization of the excited triplet species are likely mainly responsible for the observed URTP after the heating treatment, due to the facilitation of the intersystem crossing and formation of more compact cores for effective intraparticle hydrogen bonds, respectively. Importantly, this study also demonstrates the potential for aqueous dispersion of the F-CDs as an advanced security ink for information encryption and anticounterfeiting; this is a feature that has not been reported before. This study is believed to open possibilities to extend stimuli-responsive optical materials to rarely exploited phosphorescence-relevant systems and applications, and also to provide a novel strategy to easily prepare URTP materials.

The achievement of high-efficient pure red emissive carbon dots (CDs) is still a great challenge as well as one of the most critical issues that hinders widespread applications of CDs. Herein, a facile approach for the preparation of high-efficient red emissive CDs (R-CDs) is reported, and they exhibit numerous unique features including pure red emission (λmax ≈ 640 nm), respectable quantum yield (22.9%), low cytotoxicity, two-photon excited fluorescence (TPEF), and high photothermal conversion efficiency (43.9% under irradiation of 671 nm laser). Moreover, the chemical composition and photophysical properties of the R-CDs are detailed characterized and analyzed, and from which their photoluminescence mechanism is proposed. Interestingly, the R-CDs are found to particularly light up RNA-rich nucleolus both in one-photon and two-photon modes as well as show excellent counterstain compatibilities with other classical subcellular dyes. The localization of the R-CDs in nucleolus is supported by ribonuclease digestion testing, and the stronger emission is further verified to be due to an accumulation process. In addition, the R-CDs are confirmed to be facilely conjugated with fluorescein isothiocyanate (FITC) and then bring it into living cells, which reveals their potentials to perform as carriers for delivery of drugs that cannot (or hardly) enter into living cells directly. Finally, the R-CDs are shown to be excellent in photothermal cancer therapy in vitro due to their high photothermal conversion efficiency. This study represents not only a facile method for the preparation of high-efficient R-CDs, but also opens many possibilities for applications, such as in biomedicine (multifunctional theranostic agents) and emitting/display devices, thanks to their unique and superior properties.

The achievement of long afterglow in aqueous solution and as well as with small effects of dissolved oxygen is critical for its applications, but this is still a highly difficult and challenging task. Herein, a novel strategy for facilely preparing room temperature long afterglow material is reported via covalently fixing carbon dots (CDs) onto colloidal nanosilica (nSiO 2 ). The as-obtained materials (named m-CDs@nSiO 2 ) show not only an unexpected long afterglow emission in water dispersion (lifetime as high as 0.703 s) but also with small effects of the dissolved oxygen. Further studies revealed that the observed long afterglow of m-CDs@nSiO 2 possesses a predominant delayed fluorescence nature and mixed with a portion of phosphorescence. Some key knowledge that can be concluded from this study are (i) covalent interaction could be employed as an option to fix and rigidify triplet emission species; (ii) covalent bonds fixation strategy could behave as a better alternative than that of the frequently used hydrogen/halogen bonds for stabilizing triplets, because this benefits in extending the occurrence of long afterglow from only solid to solution/dispersion forms; and (iii) the containing unsaturated bonds (e.g., CC) on the surface of CDs make them to be self-protection agents from the usual quenching effects of oxygen to the triplets due to their capabilities of reaction with oxygen during the irradiation process. On the basis of the unique long afterglow features of m-CDs@nSiO 2 in water dispersion and oxygen insensitivity, a moisture-related strategy for high-level information protection is proposed and demonstrated.

Fluorescent carbon dots (CDs) have attracted much attention in recent years because of their superior optical and chemical properties, thus demonstrating many potential applications. However, the previously reported CDs mostly show strong emission only in the blue-light region, and the long-wavelength (i.e., yellow- to red-light) emissions are usually very weak. Such a drawback restricts their further applications, particularly in the biology-relevant fields. Herein, a rare example of N-doped CDs that emit bright-yellow fluorescence (i.e., y-CDs) is reported using 1,2,4-triaminobenzene as carbon precursor. The as-prepared y-CDs exhibit not only respectable emission quantum yield and highly optical stabilities but superior biocompatibility and biolabeling potentials. In addition, the y-CDs are found to show an interesting "ON-OFF-ON" three-state emission with the stepwise addition of Ag(+) and cysteine (Cys), indicating potential applications as a bifunctional sensing platform. Thanks to the highly intense emission of y-CDs, the gradual quenching and restoration of their fluorescence with the addition of Ag(+) and further Cys could also be observed with the naked eye. More importantly, the ensemble of the y-CDs and Ag(+) demonstrates practicability for the highly selective and sensitive detection of Cys in human plasma samples with satisfactory results.

A disposable pre-oxidation technique is reported that dramatically improves the detection and identification of volatile organic compounds (VOCs) by a colorimetric sensor array. By passing a vapor stream through a tube packed with chromic acid on silica immediately before the colorimetric sensor array, the sensitivity to less reactive VOCs is substantially increased and limits of detection (LODs) are improved ~300-fold, permitting the detection, identification, and discrimination of 20 commonly found indoor VOC pollutants at both their immediately dangerous to life or health (IDLH) and at permissible exposure limits (PEL) concentrations. LODs of these pollutants were on average 1.4% of their respective PELs.