carbon quantum dot

Carbon quantum dots also commonly called carbon nano dots or simply carbon dots (abbreviated as CQDs, C-dots or CDs) are carbon nanoparticles which are less than 10 nm in size and have some form of surface passivation.{{cite journal |doi=10.1039/C4TC00988F |title=Carbon quantum dots: Synthesis, properties and applications |journal=Journal of Materials Chemistry C |volume=2 |issue=34 |pages=6921–39 |year=2014 |last1=Wang |first1=Youfu |last2=Hu |first2=Aiguo |doi-access=free }}{{cite journal |doi=10.1021/acsami.5b00448 |pmid=25845394 |title=Carbon Quantum Dots and Applications in Photocatalytic Energy Conversion |journal=ACS Applied Materials & Interfaces |volume=7 |issue=16 |pages=8363–76 |year=2015 |last1=Fernando |first1=K. A. Shiral |last2=Sahu |first2=Sushant |last3=Liu |first3=Yamin |last4=Lewis |first4=William K. |last5=Guliants |first5=Elena A. |last6=Jafariyan |first6=Amirhossein |last7=Wang |first7=Ping |last8=Bunker |first8=Christopher E. |last9=Sun |first9=Ya-Ping }}{{cite journal |doi=10.1038/nbt994 |pmid=15258594 |title=In vivo cancer targeting and imaging with semiconductor quantum dots |journal=Nature Biotechnology |volume=22 |issue=8 |pages=969–76 |year=2004 |last1=Gao |first1=Xiaohu |last2=Cui |first2=Yuanyuan |last3=Levenson |first3=Richard M |last4=Chung |first4=Leland W K |last5=Nie |first5=Shuming |s2cid=41561027 }}

History

CQDs were first discovered by Xu et al. in 2004 accidentally during the purification of single-walled carbon nanotubes.{{cite journal |doi=10.1021/ja040082h |pmid=15469243 |title=Electrophoretic Analysis and Purification of Fluorescent Single-Walled Carbon Nanotube Fragments |journal=Journal of the American Chemical Society |volume=126 |issue=40 |pages=12736–7 |year=2004 |last1=Xu |first1=Xiaoyou |last2=Ray |first2=Robert |last3=Gu |first3=Yunlong |last4=Ploehn |first4=Harry J. |last5=Gearheart |first5=Latha |last6=Raker |first6=Kyle |last7=Scrivens |first7=Walter A. }} This discovery triggered extensive studies to exploit the fluorescence properties of CQDs.{{Cite journal|last1=Kottam|first1=Nagaraju|last2=S P|first2=Smrithi|date=2021-01-09|title='Luminescent carbon nanodots: Current prospects on synthesis, properties and sensing applications'|url=https://doi.org/10.1088/2050-6120/abc008|journal=Methods and Applications in Fluorescence|volume=9|issue=1|pages=012001|doi=10.1088/2050-6120/abc008|pmid=33043896|bibcode=2021MApFl...9a2001K|s2cid=222301676|issn=2050-6120}}

As a new class of fluorescent carbon nanomaterials, CQDs possess the attractive properties of high stability, good conductivity, low toxicity, environmental friendliness, simple synthetic routes as well as comparable optical properties to quantum dots.{{cite journal |doi=10.1016/S0958-1669(02)00282-3 |pmid=11849956 |title=Luminescent quantum dots for multiplexed biological detection and imaging |journal=Current Opinion in Biotechnology |volume=13 |issue=1 |pages=40–6 |year=2002 |last1=Chan |first1=Warren C.W |last2=Maxwell |first2=Dustin J |last3=Gao |first3=Xiaohu |last4=Bailey |first4=Robert E |last5=Han |first5=Mingyong |last6=Nie |first6=Shuming }} Carbon quantum dots have been extensively investigated especially due to their strong and tunable fluorescence emission properties,{{cite journal |doi=10.1039/C4CS00269E |pmid=25316556 |title=Carbon quantum dots and their applications |journal=Chemical Society Reviews |volume=44 |issue=1 |pages=362–81 |year=2015 |last1=Lim |first1=Shi Ying |last2=Shen |first2=Wei |last3=Gao |first3=Zhiqiang }} which enable their applications in biomedicine, optronics, catalysis, and sensing.{{cite journal |doi=10.1021/ja206030c |pmid=22136359 |title=Nitrogen-Doped Graphene Quantum Dots with Oxygen-Rich Functional Groups |journal=Journal of the American Chemical Society |volume=134 |issue=1 |pages=15–8 |year=2012 |last1=Li |first1=Yan |last2=Zhao |first2=Yang |last3=Cheng |first3=Huhu |last4=Hu |first4=Yue |last5=Shi |first5=Gaoquan |last6=Dai |first6=Liming |last7=Qu |first7=Liangti }} In most cases CQDs emits the light in a band of about several hundred nanometers in visible or near-infrared range, however it was also reported on broadband CQDs covering the spectrum from 800 to 1600 nm.{{cite journal|author=Sinelnik, A.D.|title= Ultra-Broadband Photoluminescent Carbon Dots Synthesized by Laser-Induced Thermal Shock|journal= Laser & Photonics Reviews|pages= 2200295|year=2023|doi=10.1002/lpor.202200295|display-authors=etal|volume=17}}

File:Carbon dots prepared by Paliienko K. Kiev Ukraine.jpg

The fundamental mechanisms responsible of the fluorescence capability of CQDs are very debated. Some authors have provided evidence of size-dependent fluorescence properties, suggesting that the emission arises from electronic transitions with the core of the dots, influenced by quantum confinement effects,{{cite journal |doi=10.1038/ncomms3943 |pmid=24309588 |title=Coal as an abundant source of graphene quantum dots |journal=Nature Communications |volume=4 |pages=2943 |year=2013 |last1=Ye |first1=Ruquan |last2=Xiang |first2=Changsheng |last3=Lin |first3=Jian |last4=Peng |first4=Zhiwei |last5=Huang |first5=Kewei |last6=Yan |first6=Zheng |last7=Cook |first7=Nathan P. |last8=Samuel |first8=Errol L.G. |last9=Hwang |first9=Chih-Chau |last10=Ruan |first10=Gedeng |last11=Ceriotti |first11=Gabriel |last12=Raji |first12=Abdul-Rahman O. |last13=Martí |first13=Angel A. |last14=Tour |first14=James M. |bibcode=2013NatCo...4.2943Y |doi-access=free }}{{cite journal |doi=10.1002/anie.200906154 |pmid=20461744 |title=Water-Soluble Fluorescent Carbon Quantum Dots and Photocatalyst Design |journal=Angewandte Chemie International Edition |volume=49 |issue=26 |pages=4430–4 |year=2010 |last1=Li |first1=Haitao |last2=He |first2=Xiaodie |last3=Kang |first3=Zhenhui |last4=Huang |first4=Hui |last5=Liu |first5=Yang |last6=Liu |first6=Jinglin |last7=Lian |first7=Suoyuan |last8=Tsang |first8=ChiHimA. |last9=Yang |first9=Xiaobao |last10=Lee |first10=Shuit-Tong }} whereas other works, including single particle measurements,{{cite journal |doi=10.1021/acsnano.0c01924 |title= Unraveling the Fluorescence Mechanism of Carbon Dots with Sub-Single-Particle Resolution |journal=ACS Nano |volume=14 |pages=6127-37 |year=2020 }} have rather attributed the fluorescence to recombination of surface-trapped charges,{{cite journal |doi=10.1021/ja062677d |pmid=16771487 |title=Quantum-Sized Carbon Dots for Bright and Colorful Photoluminescence |journal=Journal of the American Chemical Society |volume=128 |issue=24 |pages=7756–7 |year=2006 |last1=Sun |first1=Ya-Ping |last2=Zhou |first2=Bing |last3=Lin |first3=Yi |last4=Wang |first4=Wei |last5=Fernando |first5=K. A. Shiral |last6=Pathak |first6=Pankaj |last7=Meziani |first7=Mohammed Jaouad |last8=Harruff |first8=Barbara A. |last9=Wang |first9=Xin |last10=Wang |first10=Haifang |last11=Luo |first11=Pengju G. |last12=Yang |first12=Hua |last13=Kose |first13=Muhammet Erkan |last14=Chen |first14=Bailin |last15=Veca |first15=L. Monica |last16=Xie |first16=Su-Yuan }}{{cite journal |doi=10.1016/j.jcis.2011.01.065 |pmid=21306724 |title=Synthesis and surface photochemistry of graphitized carbon quantum dots |journal=Journal of Colloid and Interface Science |volume=356 |issue=2 |pages=416–21 |year=2011 |last1=Liu |first1=Yun |last2=Liu |first2=Chun-yan |last3=Zhang |first3=Zhi-Ying |bibcode=2011JCIS..356..416L }} or proposed a form of coupling between core and surface electronic states.{{cite journal |doi=10.1021/acs.jpclett.6b01590 |pmid=27525451 |title=Solvatochromism Unravels the Emission Mechanism of Carbon Nanodots |journal=The Journal of Physical Chemistry Letters |volume=7 |issue=17 |pages=3419–23 |year=2016 |last1=Sciortino |first1=Alice |last2=Marino |first2=Emanuele |last3=Dam |first3=Bart van |last4=Schall |first4=Peter |last5=Cannas |first5=Marco |last6=Messina |first6=Fabrizio }} The excitation-dependent fluorescence of CQDs, leading to their characteristic emission tunability, has been mostly linked to the inhomogeneous distribution of their emission characteristics,{{cite journal |doi=10.1039/C6NR02669A |pmid=27399599 |title=The origin of emissive states of carbon nanoparticles derived from ensemble-averaged and single-molecular studies |journal=Nanoscale |volume=8 |issue=29 |pages=14057–69 |year=2016 |last1=Demchenko |first1=Alexander P. |last2=Dekaliuk |first2=Mariia O. |bibcode=2016Nanos...814057D }} due to polydispersity, although some works have explained it as a violation of Kasha's rule arising from an unusually slow solvent relaxation.{{cite journal |doi=10.1021/acs.nanolett.5b03915 |pmid=26566016 |title=Time-Resolved Emission Reveals Ensemble of Emissive States as the Origin of Multicolor Fluorescence in Carbon Dots |journal=Nano Letters |volume=15 |issue=12 |pages=8300–5 |year=2015 |last1=Khan |first1=Syamantak |last2=Gupta |first2=Abhishek |last3=Verma |first3=Navneet C. |last4=Nandi |first4=Chayan K. |bibcode=2015NanoL..15.8300K }}

Properties

The structures and components of CQDs determine their diverse properties.{{cite journal |doi=10.1016/j.carbon.2020.11.017 |title= A deep investigation into the structure of carbon dots |journal=Carbon |volume=173 | pages=433–447 |year=2021 |last1= Mintz |first1= Keenan J. |last2=Bartoli |first2=Mattia |last3=Rovere |first3=Massimo |last4= Zhou |first4= Yiqun |last5= Hettiarachchi |first5= Sajini D. |last6= Paudyal |first6= Suraj |last7= Chen |first7= Jiuyan|last8= Domena |first8= Justin B. |last9= Liyanage |first9= Piumi Y. |last10= Sampson |first10= Rachel |last11= Khadka |first11= Durga |last12= Pandey |first12= Raja R. |last13= Huang |first13= Sunxiang |last14= Chusuei |first14= Charles C. |last15= Tagliaferro |first15= Alberto|last16= Leblanc |first16= Roger M.|s2cid= 228855625 |doi-access= free }} Many carboxyl moieties on the CQD surface impart excellent solubility in water and biocompatibility. Such surface moieties enable CQDs to serve as proton conducting nanoparticles.{{cite journal |doi=10.1002/smll.202005526|title=Enhanced Proton Conductivity across Protein Biopolymers Mediated by Doped Carbon Nanoparticles |journal=Small|volume=16 |issue=50 |pages=2005526 |year=2020 |last1=Mondal |first1=Somen |last2=Agam |first2=Yuval |last3=Amdursky |first3=Nadav |pmid=33108059 |s2cid=225083071 }} CQDs are also suitable for chemical modification and surface passivation with various organic, polymeric, inorganic or biological materials. By surface passivation, the fluorescence properties as well as physical properties of CQDs are enhanced. Recently, it has been discovered that amine and hydroxamic acid functionalized CD can produce tricolor (green, yellow and red) emission when introduced with different pH environment and this tricolor emission can be preserved in ORMOSIL film matrix.{{cite journal |doi=10.1021/acs.jpcc.7b08039|title=Carbon Dots from a Single Source Exhibiting Tunable Luminescent Colors through the Modification of Surface Functional Groups in ORMOSIL Films |journal=Journal of Physical Chemistry C|volume=121 |issue=50 |pages=28106–16 |year=2017 |last1=Bhattacharya |first1=Dipsikha |last2=Mishra |first2=Manish K. |last3=De |first3=Goutam |doi-access=free }}

Synthesis

Synthetic methods for CQDs are roughly divided into two categories, "top-down" and "bottom-up" routes. These can be achieved via chemical, electrochemical or physical techniques. The CQDs obtained could be optimized during preparation or post-treatment. Modification of CQDs is also very important to get good surface properties which are essential for solubility and selected applications.

= Synthetic methods =

"Top-down" synthetic route refers to breaking down larger carbon structures such as graphite, carbon nanotubes, and nanodiamonds into CQDs using laser ablation, arc discharge, and electrochemical techniques. For example, Zhou et al. first applied electrochemical method into synthesis of CQDs.{{cite journal |doi=10.1021/ja0669070 |pmid=17243794 |title=An Electrochemical Avenue to Blue Luminescent Nanocrystals from Multiwalled Carbon Nanotubes (MWCNTs) |journal=Journal of the American Chemical Society |volume=129 |issue=4 |pages=744–5 |year=2007 |last1=Zhou |first1=Jigang |last2=Booker |first2=Christina |last3=Li |first3=Ruying |last4=Zhou |first4=Xingtai |last5=Sham |first5=Tsun-Kong |last6=Sun |first6=Xueliang |last7=Ding |first7=Zhifeng |url=https://figshare.com/articles/An_Electrochemical_Avenue_to_Blue_Luminescent_Nanocrystals_from_Multiwalled_Carbon_Nanotubes_MWCNTs_/3030481 }} They grew multi-walled carbon nanotubes on a carbon paper, then they inserted the carbon paper into an electrochemical cell containing supporting electrolyte including degassed acetonitrile and 0.1 M tetrabutyl ammonium perchlorate. Later, they applied this method in cutting CNTs or assembling CNTs into functional patterns which demonstrated the versatile callability of this method in carbon nanostructure manipulations.{{cite journal |doi=10.1016/j.carbon.2008.11.032 |title=Tailoring multi-wall carbon nanotubes for smaller nanostructures |journal=Carbon|volume=47 |issue=3 |pages=829–838 |year=2009 |last1=Zhou |first1=Jigang }}{{cite journal |doi=10.1016/j.carbon.2013.03.001 |title=An electrochemical approach to fabricating honeycomb assemblies from multiwall carbon nanotubes|journal=Carbon|volume=59 |issue=3 |pages=130–139 |year=2013 |last1=Zhou |first1=Jigang }}

"Bottom-up" synthetic route involves synthesizing CQDs from small precursors such as carbohydrates, citrate, and polymer-silica nanocomposites through hydrothermal/solvothermal treatment, supported synthetic, and microwave synthetic routes.{{cite journal |doi=10.1021/cm901593y |title=Simple Aqueous Solution Route to Luminescent Carbogenic Dots from Carbohydrates |journal=Chemistry of Materials |volume=21 |issue=23 |pages=5563–5 |year=2009 |last1=Peng |first1=Hui |last2=Travas-Sejdic |first2=Jadranka }} For instance, Zhu et al. described a simple method of preparing CQDs by heating a solution of poly(ethylene glycol) (PEG) and saccharide in 500 W microwave oven for 2 to 10 min.{{cite journal |doi=10.1039/B907612C |pmid=20448965 |title=Microwave synthesis of fluorescent carbon nanoparticles with electrochemiluminescence properties |journal=Chemical Communications |issue=34 |pages=5118–20 |year=2009 |last1=Zhu |first1=Hui |last2=Wang |first2=Xiaolei |last3=Li |first3=Yali |last4=Wang |first4=Zhongjun |last5=Yang |first5=Fan |last6=Yang |first6=Xiurong |s2cid=205730336 }} By varying the molar ratio of citric acid and urea (two common precursor molecules) of the mixture that is subjected to pyrolysis, a number of distinct fluorescent materials in both liquid and solid state can be synthesised, predominantly comprising Carbon dots with embedded fluorophores.{{cite journal |last1=Stachowska |first1=Joanna D. |last2=Murphy |first2=Andrew |last3=Mellor |first3=Claire |last4=Fernandes |first4=Diogo |last5=Gibbons |first5=Ella N. |last6=Krysmann |first6=Marta J. |last7=Kelarakis |first7=Antonios |last8=Burgaz |first8=Engin |last9=Moore |first9=Joshua |last10=Yeates |first10=Stephen G. |title=A rich gallery of carbon dots based photoluminescent suspensions and powders derived by citric acid/urea |journal=Scientific Reports |date=18 May 2021 |volume=11 |issue=1 |doi=10.1038/s41598-021-89984-w|pmc=8131706 }} Also a laser-induced thermal shock method is exploited for synthesis ultra-broadband QCDs. Recently, green synthetic approaches have also been employed for fabrication of CQDs.{{cite journal |doi=10.1007/s10895-015-1598-x |pmid=26123675 |title=Biogenic Synthesis of Fluorescent Carbon Dots at Ambient Temperature Using Azadirachta indica (Neem) gum |journal=Journal of Fluorescence |volume=25 |issue=4 |pages=1103–7 |year=2015 |last1=Phadke |first1=Chinmay |last2=Mewada |first2=Ashmi |last3=Dharmatti |first3=Roopa |last4=Thakur |first4=Mukeshchand |last5=Pandey |first5=Sunil |last6=Sharon |first6=Madhuri |s2cid=17521709 }}{{cite journal |doi=10.1007/s10895-014-1477-x |pmid=25367312 |title=A Green Route Towards Highly Photoluminescent and Cytocompatible Carbon dot Synthesis and its Separation Using Sucrose Density Gradient Centrifugation |journal=Journal of Fluorescence |volume=25 |issue=1 |pages=9–14 |year=2014 |last1=Oza |first1=Goldie |last2=Oza |first2=Kusum |last3=Pandey |first3=Sunil |last4=Shinde |first4=Sachin |last5=Mewada |first5=Ashmi |last6=Thakur |first6=Mukeshchand |last7=Sharon |first7=Maheshwar |last8=Sharon |first8=Madhuri |s2cid=13623073 }}{{cite journal |doi=10.1016/j.msec.2013.03.018 |pmid=23623114 |title=Green synthesis of biocompatible carbon dots using aqueous extract of Trapa bispinosa peel |journal=Materials Science and Engineering: C |volume=33 |issue=5 |pages=2914–7 |year=2013 |last1=Mewada |first1=Ashmi |last2=Pandey |first2=Sunil |last3=Shinde |first3=Sachin |last4=Mishra |first4=Neeraj |last5=Oza |first5=Goldie |last6=Thakur |first6=Mukeshchand |last7=Sharon |first7=Maheshwar |last8=Sharon |first8=Madhuri }}{{cite journal |doi=10.1155/2014/282193 |pmid=24744921 |pmc=3976943 |title=Antibiotic Conjugated Fluorescent Carbon Dots as a Theranostic Agent for Controlled Drug Release, Bioimaging, and Enhanced Antimicrobial Activity |journal=Journal of Drug Delivery |volume=2014 |pages=282193 |year=2014 |last1=Thakur |first1=Mukeshchand |last2=Pandey |first2=Sunil |last3=Mewada |first3=Ashmi |last4=Patil |first4=Vaibhav |last5=Khade |first5=Monika |last6=Goshi |first6=Ekta |last7=Sharon |first7=Madhuri |doi-access=free }}{{cite journal |doi=10.1016/j.msec.2016.05.007 |pmid=27287144 |title=Milk-derived multi-fluorescent graphene quantum dot-based cancer theranostic system |journal=Materials Science and Engineering: C |volume=67 |pages=468–77 |year=2016 |last1=Thakur |first1=Mukeshchand |last2=Mewada |first2=Ashmi |last3=Pandey |first3=Sunil |last4=Bhori |first4=Mustansir |last5=Singh |first5=Kanchanlata |last6=Sharon |first6=Maheshwar |last7=Sharon |first7=Madhuri }} Care must be taken to separate the "bottom-up" carbon dots from fluorescent byproducts such as small molecules or polyester condensates by using multiple dialysis and chromatography separation methods.{{cite journal |doi=10.1021/acsnano.3c07486 |pmid=37970787 |title=Properties of Carbon Dots versus Small Molecules from "Bottom-up" Synthesis |journal=ACS Nano |volume=17 |issue=22 |pages=22788-22799 |year=2023 |last1=Bian |first1=Zhengyi |last2=Wallum |first2=Allison |last3=Mehmood |first3=Arshad |last4=Gomez |first4=Eric |last5=Wang |first5=Ziwen |last6=Pandit |first6=Subhendu |last7=Nie |first7=Shuming |last8=Link |first8=Stephan |last9=Levine |first9=Benjamin |last10=Gruebele |first10=Martin }}

= Size control =

In addition to post-treatment, controlling the size of CQDs during the preparing process is also widely used. For instance, Zhu et al. reported hydrophilic CQDs through impregnation of citric acid precursor. After pyrolyzing CQDs at 300 °C for 2 hours in air, then removing silica, followed by dialysis, they prepared CQDs with a uniform size of 1.5–2.5 nm which showed low toxicity, excellent luminescence, good photostability, and up-conversion properties.

= Modification =

Being a new type of fluorescent nanoparticles, applications of CQD lie in the field of bioimaging and biosensing due to their biological and environmental friendly composition and excellent biocompatibility. In order to survive the competition with conventional semiconductor quantum dots, a high quantum yield should be achieved. Although a good example of CQDs with ~80% quantum yield was synthesized,{{cite journal |doi=10.1002/anie.201300519 |pmid=23450679 |title=Highly Photoluminescent Carbon Dots for Multicolor Patterning, Sensors, and Bioimaging |journal=Angewandte Chemie International Edition |volume=52 |issue=14 |pages=3953–7 |year=2013 |last1=Zhu |first1=Shoujun |last2=Meng |first2=Qingnan |last3=Wang |first3=Lei |last4=Zhang |first4=Junhu |last5=Song |first5=Yubin |last6=Jin |first6=Han |last7=Zhang |first7=Kai |last8=Sun |first8=Hongchen |last9=Wang |first9=Haiyu |last10=Yang |first10=Bai }} most of the quantum dots synthesized have a quantum yield below 10% so far. Surface-passivation and doping methods for modifications are usually applied for improving quantum yield.

To prevent surfaces of CQDs from being polluted by their environment, surface passivation is performed to alleviate the detrimental influence of surface contamination on their optical properties.{{cite journal |doi=10.1116/1.1316388 |title=Surface Passivation of Semiconductors |journal=Journal of Vacuum Science and Technology |volume=8 |issue=5 |pages=S39–S49 |year=1971 |last1=Nicollian |first1=E. H. |bibcode=1971JVST....8S..39N }} A thin insulating layer is formed to achieve surface passivation via the attachment of polymeric materials on CQDs surface treated by acid.

In addition to surface passivation, doping is also a common method used to tune the properties of CQDs. Various doping methods with elements such as N,{{cite journal |doi=10.1002/chem.201203641 |pmid=23322649 |title=Nitrogen-Doped Carbon Dots: A Facile and General Preparation Method, Photoluminescence Investigation, and Imaging Applications |journal=Chemistry - A European Journal |volume=19 |issue=7 |pages=2276–83 |year=2013 |last1=Xu |first1=Yang |last2=Wu |first2=Ming |last3=Liu |first3=Yang |last4=Feng |first4=Xi-Zeng |last5=Yin |first5=Xue-Bo |last6=He |first6=Xi-Wen |last7=Zhang |first7=Yu-Kui }} S,{{cite journal |doi=10.1016/j.carbon.2013.07.095 |title=Hair fiber as a precursor for synthesizing of sulfur- and nitrogen-co-doped carbon dots with tunable luminescence properties |journal=Carbon |volume=64 |pages=424–34 |year=2013 |last1=Sun |first1=Dong |last2=Ban |first2=Rui |last3=Zhang |first3=Peng-Hui |last4=Wu |first4=Ge-Hui |last5=Zhang |first5=Jian-Rong |last6=Zhu |first6=Jun-Jie }} P{{cite journal |doi=10.1002/ppsc.201300020 |title=Microwave-Assisted One-Pot Synthesis of Metal-Free Nitrogen and Phosphorus Dual-Doped Nanocarbon for Electrocatalysis and Cell Imaging |journal=Particle & Particle Systems Characterization |volume=30 |issue=6 |pages=557–64 |year=2013 |last1=Prasad |first1=K. Sudhakara |last2=Pallela |first2=Ramjee |last3=Kim |first3=Dong-Min |last4=Shim |first4=Yoon-Bo |s2cid=93569150 }} have been demonstrated for tuning the properties of CQDs, among which N doping is the most common way due to its great ability in improving the photo luminescence emissions.{{cite journal|last1=Ayala|first1=Paola|last2=Arenal|first2=Raul|last3=Loiseau|first3=Annick|author-link3=Annick Loiseau|last4=Rubio|first4=Angel|last5=Pichler|first5=Thomas|year=2010|title=The physical and chemical properties of heteronanotubes|journal=Reviews of Modern Physics|volume=82|issue=2|pages=1843|bibcode=2010RvMP...82.1843A|doi=10.1103/RevModPhys.82.1843|hdl-access=free|hdl=10261/44279}} The mechanisms by which Nitrogen doping enhances the fluorescence quantum yield of CQDs, as well as the structure of heavily N-doped CDs, are very debated issues in the literature.{{cite journal |doi=10.1039/C5TC04096E |title=Fluorescent nitrogen-rich carbon nanodots with an unexpected β-C3N4nanocrystalline structure |journal=Journal of Materials Chemistry C |volume=4 |issue=13 |pages=2598–605 |year=2016 |last1=Messina |first1=F. |last2=Sciortino |first2=L. |last3=Popescu |first3=R. |last4=Venezia |first4=A. M. |last5=Sciortino |first5=A. |last6=Buscarino |first6=G. |last7=Agnello |first7=S. |last8=Schneider |first8=R. |last9=Gerthsen |first9=D. |last10=Cannas |first10=M. |last11=Gelardi |first11=F. M. |hdl=10447/179373 |hdl-access=free }}{{cite journal |doi=10.1039/C3CC42266F |pmid=23749222 |title=A low-temperature solid-phase method to synthesize highly fluorescent carbon nitride dots with tunable emission |journal=Chemical Communications |volume=49 |issue=77 |pages=8605–7 |year=2013 |last1=Zhou |first1=Juan |last2=Yang |first2=Yong |last3=Zhang |first3=Chun-Yang }} Zhou et al. applied XANES and XEOL in investigating the electronic structure and luminescence mechanism in their electrochemically produced carbon QDS and found that N doping is almost certainly responsible for the blue luminescence.{{cite journal |title=Electronic structure and luminescence center of blue luminescent carbon nanocrystals |journal=Chemical Physics Letters |volume=474 |issue=4–6 |pages=320–324 |doi=10.1016/j.cplett.2009.04.075|year=2009 |last1=Zhou |first1=Jigang |last2=Zhou |first2=Xingtai |last3=Li |first3=Ruying |last4=Sun |first4=Xueliang |last5=Ding |first5=Zhifeng |last6=Cutler |first6=Jeffrey |last7=Sham |first7=Tsun-Kong |bibcode=2009CPL...474..320Z }} Synthesis of new nanocomposites based on CDs have been reported with unusual properties. For example, a nanocomposite has been designed by using of CDs and magnetic {{chem2|Fe3O4}} nanoparticles as precursors with nanozyme activity.{{cite journal |doi= 10.1016/j.snb.2017.02.145 |title= Design of C-dots/Fe₃O₄ magnetic nanocomposite as an efficient new nanozyme and its application for determination of H₂O₂ in nanomolar level |journal= Sensors and Actuators B: Chemical |volume=247 |issue=August |pages=691–6 |year=2017 |last1= Yousefinejad |first1= Saeed |last2= Rasti |first2= Hamid |last3= Hajebi |first3= Mehdi |last4= Kowsari |first4= Masoud |last5= Sadravi |first5= Shima |last6= Honarasa |first6= Fatemeh }}

Post synthesis electrochemical etching results in dramatic changes in GQDs size and fluorescence intensity.

Applications

File:Applications.png

= Bioimaging =

CQDs can be used for bioimaging due to their fluorescence emissions and biocompatibility.{{cite journal |doi=10.1038/srep21286 |pmid=26905737 |pmc=4764906 |title=Camphor-mediated synthesis of carbon nanoparticles, graphitic shell encapsulated carbon nanocubes and carbon dots for bioimaging |journal=Scientific Reports |volume=6 |pages=21286 |year=2016 |last1=Oza |first1=Goldie |last2=Ravichandran |first2=M. |last3=Merupo |first3=Victor-Ishrayelu |last4=Shinde |first4=Sachin |last5=Mewada |first5=Ashmi |last6=Ramirez |first6=Jose Tapia |last7=Velumani |first7=S. |last8=Sharon |first8=Madhuri |last9=Sharon |first9=Maheshwar |bibcode=2016NatSR...621286O }} By injecting solvents containing CQDs into a living body, images in vivo can be obtained for detection or diagnosis purposes. One example is that organic dye-conjugated CQDs could be used as an effective fluorescent probes for {{chem2|H2S}}. The presence of {{chem2|H2S}} could tune the blue emission of the organic dye-conjugated CQDs to green. So by using a fluorescence microscope, the organic dye-conjugated CQDs were able to visualize changes in physiologically relevant levels of {{chem2|H2S}}. Another example can be dual-mode bioimaging using their highly accessible surface functional groups to conjugate them via EDC-NHS chemistry.{{Cite book |last=Kilic |first=Nüzhet Inci |url=http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-298302 |title=Graphene Quantum Dots as Fluorescent and Passivation Agents for Multimodal Bioimaging |date=2021}} Saladino et al. demonstrated the concept using MW-assisted synthesized nitrogen-doped excitation-independent CQDs. These were conjugated with rhodium nanoparticles – X-ray fluorescence contrast agents – leading to dual-mode nanohybrids with both optical and X-ray fluorescent properties. Moreover, the conjugation process not only accounts for dual-mode bioimaging but also passivates the rhodium nanoparticle surface, resulting in reduced cytotoxicity.{{Cite journal |last1=Saladino |first1=Giovanni M. |last2=Kilic |first2=Nuzhet I. |last3=Brodin |first3=Bertha |last4=Hamawandi |first4=Bejan |last5=Yazgan |first5=Idris |last6=Hertz |first6=Hans M. |last7=Toprak |first7=Muhammet S. |date=September 2021 |title=Carbon Quantum Dots Conjugated Rhodium Nanoparticles as Hybrid Multimodal Contrast Agents |journal=Nanomaterials |language=en |volume=11 |issue=9 |pages=2165 |doi=10.3390/nano11092165 |issn=2079-4991 |pmc=8470909 |pmid=34578481|doi-access=free }}

= Sensing =

CQDs were also applied in biosensing as biosensor carriers for their flexibility in modification, high solubility in water, nontoxicity, good photostability, and excellent biocompatibility. The biosensors based on CQD and CQs-based materials could be used for visual monitoring of cellular copper,{{cite journal |doi=10.1002/anie.201109089 |pmid=22407813 |title=Carbon-Dot-Based Dual-Emission Nanohybrid Produces a Ratiometric Fluorescent Sensor for InVivo Imaging of Cellular Copper Ions |journal=Angewandte Chemie International Edition |volume=51 |issue=29 |pages=7185–9 |year=2012 |last1=Zhu |first1=Anwei |last2=Qu |first2=Qiang |last3=Shao |first3=Xiangling |last4=Kong |first4=Biao |last5=Tian |first5=Yang }} glucose,{{cite journal |doi=10.1039/C1CC11943E |pmid=21562663 |title=Carbon nanodots as peroxidase mimetics and their applications to glucose detection |journal=Chemical Communications |volume=47 |issue=23 |pages=6695–7 |year=2011 |last1=Shi |first1=Wenbing |last2=Wang |first2=Qinlong |last3=Long |first3=Yijuan |last4=Cheng |first4=Zhiliang |last5=Chen |first5=Shihong |last6=Zheng |first6=Huzhi |last7=Huang |first7=Yuming |s2cid=23383050 }} pH,{{cite journal |doi=10.1002/anie.201202533 |pmid=22644672 |title=A Tunable Ratiometric pH Sensor Based on Carbon Nanodots for the Quantitative Measurement of the Intracellular pH of Whole Cells |journal=Angewandte Chemie International Edition |volume=51 |issue=26 |pages=6432–5 |year=2012 |last1=Shi |first1=Wen |last2=Li |first2=Xiaohua |last3=Ma |first3=Huimin }} trace levels of {{chem2|H2O2}} and nucleic acid.{{cite journal |doi=10.1039/C0CC04326E |pmid=21079843 |title=Nucleic acid detection using carbon nanoparticles as a fluorescent sensing platform |journal=Chemical Communications |volume=47 |issue=3 |pages=961–3 |year=2011 |last1=Li |first1=Hailong |last2=Zhang |first2=Yingwei |last3=Wang |first3=Lei |last4=Tian |first4=Jingqi |last5=Sun |first5=Xuping |s2cid=11066086 }} A general example is about nucleic acid lateral flow assays. The discriminating tags on the amplicons are recognized by their respective antibodies and fluorescence signals provided by the attached CQDs. More generally, the fluorescence of CQDs efficiently responds to pH,{{cite journal |doi=10.1016/j.jlumin.2013.12.007 |title=Optical properties of pH-sensitive carbon-dots with different modifications |journal=Journal of Luminescence |volume=148 |pages=238–42 |year=2014 |last1=Kong |first1=Weiguang |last2=Wu |first2=Huizhen |last3=Ye |first3=Zhenyu |last4=Li |first4=Ruifeng |last5=Xu |first5=Tianning |last6=Zhang |first6=Bingpo |bibcode=2014JLum..148..238K }} local polarity, and to the presence of metal ions in solution,{{cite journal |doi=10.1039/C6RA15691F |title=Potential prospects for carbon dots as a fluorescence sensing probe for metal ions |journal=RSC Advances |volume=6 |issue=93 |pages=90526–36 |year=2016 |last1=Chaudhary |first1=Savita |last2=Kumar |first2=Sandeep |last3=Kaur |first3=Bhawandeep |last4=Mehta |first4=S. K. |bibcode=2016RSCAd...690526C }} which further expands their potential for nanosensing applications,{{cite journal |doi=10.1021/acsomega.9b00858 |pmid=31460168 |pmc=6648105 |title=Nitrogen-Doped Graphene Oxide Dots-Based "Turn-OFF" H₂O₂, Au(III), and "Turn-OFF–ON" Hg(II) Sensors as Logic Gates and Molecular Keypad Locks |journal=ACS Omega |volume=4 |issue=6 |pages=10702–10713 |year=2019 |last1=Bogireddy |first1=Naveen Kumar Reddy |last2=Barba |first2=Victor |last3=Agarwal |first3=Vivechana }} for instance in the analysis of pollutants.{{cite journal |doi=10.1016/j.aca.2013.10.031 |pmid=24267089 |title=Strong luminescence of Carbon Dots induced by acetone passivation: Efficient sensor for a rapid analysis of two different pollutants |journal=Analytica Chimica Acta |volume=804 |pages=246–51 |year=2013 |last1=Cayuela |first1=Angelina |last2=Laura Soriano |first2=M. |last3=Valcárcel |first3=Miguel }}

= Drug delivery =

The nontoxicity and biocompatibility of CQDs enable them with broad applications in biomedicine as drug carriers, fluorescent tracers as well as controlling drug release.{{cite journal |doi=10.1039/C3TB21436B |title=Swarming carbon dots for folic acid mediated delivery of doxorubicin and biological imaging |journal=Journal of Materials Chemistry B |volume=2 |issue=6 |pages=698–705 |year=2014 |last1=Mewada |first1=Ashmi |last2=Pandey |first2=Sunil |last3=Thakur |first3=Mukeshchand |last4=Jadhav |first4=Dhanashree |last5=Sharon |first5=Madhuri |pmid=32261288 }}{{cite journal |doi=10.1039/C3RA42139B |title=Cysteamine hydrochloride protected carbon dots as a vehicle for the efficient release of the anti-schizophrenic drug haloperidol |journal=RSC Advances |volume=3 |issue=48 |pages=26290–6 |year=2013 |last1=Pandey |first1=Sunil |last2=Mewada |first2=Ashmi |last3=Thakur |first3=Mukeshchand |last4=Tank |first4=Arun |last5=Sharon |first5=Madhuri |bibcode=2013RSCAd...326290P }}{{cite journal |doi=10.1039/C3TB20761G |title=Carbon dots functionalized gold nanorod mediated delivery of doxorubicin: Tri-functional nano-worms for drug delivery, photothermal therapy and bioimaging |journal=Journal of Materials Chemistry B |volume=1 |issue=38 |pages=4972–82 |year=2013 |last1=Pandey |first1=Sunil |last2=Thakur |first2=Mukeshchand |last3=Mewada |first3=Ashmi |last4=Anjarlekar |first4=Dhanashree |last5=Mishra |first5=Neeraj |last6=Sharon |first6=Madhuri |pmid=32261087 }} This is exemplified by the use of CQDs as photosensitizers in photodynamic therapy to destroy cancer cells.{{cite journal |doi=10.1063/1.4817787 |title=Photoactivatable carbon nanodots for cancer therapy |journal=Applied Physics Letters |volume=103 |issue=6 |pages=063701 |year=2013 |last1=Juzenas |first1=Petras |last2=Kleinauskas |first2=Andrius |last3=George Luo |first3=Pengju |last4=Sun |first4=Ya-Ping |bibcode=2013ApPhL.103f3701J }}

= Catalysis =

The flexibility of functionalization with various groups CQDs makes them possible to absorb lights of different wavelengths, which offers good opportunities for applications in photocatalysis.{{cite journal |last1=Kim |first1=Jinhyun |last2=Lee |first2=Sahng Ha |last3=Tieves |first3=Florian |last4=Choi |first4=Da Som |last5=Hollmann |first5=Frank |last6=Paul |first6=Caroline E. |last7=Park |first7=Chan Beum |title=Biocatalytic C=C Bond Reduction through Carbon Nanodot‐Sensitized Regeneration of NADH Analogues |journal=Angewandte Chemie International Edition |date=15 October 2018 |volume=57 |issue=42 |pages=13825–13828 |doi=10.1002/anie.201804409|pmid=30062834 |s2cid=51870319 |url=http://resolver.tudelft.nl/uuid:240f014a-a6f2-4225-866e-79582f481bca }} CQDs-modified P25 TiO₂ composites exhibited improved photocatalytic H2 evolution under irradiation with UV-Vis. The CQDs serve as a reservoir for electrons to improve the efficiency of separating of the electron-hole pairs of P25.{{cite journal |doi=10.1166/jbn.2011.1344 |pmid=22416585 |title=Rapid Detection of Bacteria by Carbon Quantum Dots |journal=Journal of Biomedical Nanotechnology |volume=7 |issue=6 |pages=846–8 |year=2011 |last1=Mandal |first1=Tapas K. |last2=Parvin |first2=Nargish }} In the recent times, metal-free CQDs have been found to improve the kinetics of hydrogen evolution reaction (HER), making CQDs a sustainable choice for catalysis.{{Cite journal |last1=Rimal |first1=Vishal |last2=Mahapatra |first2=Susanta Sinha |last3=Srivastava |first3=Prem Kumar |date=2022-10-15 |title=Metal-free oleic acid-derived carbon dots as efficient catalysts for hydrogen evolution reaction |url=https://link.springer.com/10.1007/s10800-022-01780-0 |journal=Journal of Applied Electrochemistry |volume=53 |issue=2 |pages=285–295 |language=en |doi=10.1007/s10800-022-01780-0 |s2cid=252950678 |issn=0021-891X}}

= Optronics =

CQDs possess the potential in serving as materials for dye-sensitized solar cells,{{cite journal |doi=10.1039/C4TA03203A |title=Remarkable photoelectrochemical performance of carbon dots sensitized TiO₂ under visible light irradiation |journal=Journal of Materials Chemistry A |volume=2 |issue=39 |pages=16365–8 |year=2014 |last1=Xie |first1=Shilei |last2=Su |first2=Hua |last3=Wei |first3=Wenjie |last4=Li |first4=Mingyang |last5=Tong |first5=Yexiang |last6=Mao |first6=Zongwan }} organic solar cells, supercapacitor,{{cite journal |doi=10.1039/C3EE41776J |title=A carbon quantum dot decorated RuO₂ network: Outstanding supercapacitances under ultrafast charge and discharge |journal=Energy & Environmental Science |volume=6 |issue=12 |pages=3665–75 |year=2013 |last1=Zhu |first1=Yirong |last2=Ji |first2=Xiaobo |last3=Pan |first3=Chenchi |last4=Sun |first4=Qingqing |last5=Song |first5=Weixin |last6=Fang |first6=Laibing |last7=Chen |first7=Qiyuan |last8=Banks |first8=Craig E. }} and light emitting devices.{{cite journal |doi=10.1021/nn405017q |pmid=24246067 |title=Color-Switchable Electroluminescence of Carbon Dot Light-Emitting Diodes |journal=ACS Nano |volume=7 |issue=12 |pages=11234–41 |year=2013 |last1=Zhang |first1=Xiaoyu |last2=Zhang |first2=Yu |last3=Wang |first3=Yu |last4=Kalytchuk |first4=Sergii |last5=Kershaw |first5=Stephen V. |last6=Wang |first6=Yinghui |last7=Wang |first7=Peng |last8=Zhang |first8=Tieqiang |last9=Zhao |first9=Yi |last10=Zhang |first10=Hanzhuang |last11=Cui |first11=Tian |last12=Wang |first12=Yiding |last13=Zhao |first13=Jun |last14=Yu |first14=William W. |last15=Rogach |first15=Andrey L. }} CQDs can be used as photosensitizer in dye-sensitized solar cells and the photoelectric conversion efficiency is significantly enhanced.{{cite journal |doi=10.1021/am400930h |pmid=23668995 |title=Bioinspired Photoelectric Conversion System Based on Carbon-Quantum-Dot-Doped Dye–Semiconductor Complex |journal=ACS Applied Materials & Interfaces |volume=5 |issue=11 |pages=5080–4 |year=2013 |last1=Ma |first1=Zheng |last2=Zhang |first2=Yong-Lai |last3=Wang |first3=Lei |last4=Ming |first4=Hai |last5=Li |first5=Haitao |last6=Zhang |first6=Xing |last7=Wang |first7=Fang |last8=Liu |first8=Yang |last9=Kang |first9=Zhenhui |last10=Lee |first10=Shuit-Tong }} CQD incorporated hybrid silica based sol can be used as transparent Fluorescent paint,{{cite journal |doi=10.1039/C4TC02140A |title=Carbon nanodot–ORMOSIL fluorescent paint and films |journal=Journal of Materials Chemistry C |volume=3 |issue=4 |pages=714–9 |year=2015 |last1=Mishra |first1=Manish Kr |last2=Chakravarty |first2=Amrita |last3=Bhowmik |first3=Koushik |last4=De |first4=Goutam |s2cid=54851790 }}

= Rocket fuels =

Recently, CQDs have been employed in hybrid rocket fuels.{{cite journal |last1=Oztan |first1=Cagri |last2=Ginzburg |first2=Eric |last3=Akin |first3=Mert |last4=Zhou |first4=Yiqun |last5=Leblanc |first5=Roger M. |last6=Coverstone |first6=Victoria|author6-link=Victoria Coverstone |title=3D printed ABS/paraffin hybrid rocket fuels with carbon dots for superior combustion performance |journal=Combustion and Flame |year=2021 |volume=225 |pages=428–434 |doi=10.1016/j.combustflame.2020.11.024 |s2cid=229419770 |url=https://www.researchgate.net/publication/346433516|doi-access=free }}

= Fingerprint recovery=

CQDs are used for the enhancement of latent fingerprints.{{cite journal |last1=Fernandes |first1=Diogo |last2=Krysmann |first2=Marta J. |last3=Kelarakis |first3=Antonios |date=2015 |title=Carbon dot based nanopowders and their application for fingerprint recovery |journal=Chemical Communications |volume=51 |issue=23 |pages=4902–4905 |doi=10.1039/C5CC00468C |pmid=25704392 }}