Synthesis of Pyridinic-Rich N, S Co-doped Carbon Quantum Dots as Effective Enzyme Mimics

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1 Liu et l. Nnoscle Reserch Letters (2017) 12:375 DOI /s y NANO EXPRESS Synthesis of Pyridinic-Rich N, S Co-doped Cron Quntum Dots s Effective Enzyme Mimics Teng Liu 1,2, Zhi-wei Cui 1,2, Jun Zhou 1,2, Ying Wng 1,2,3* nd Zhi-gng Zou 2,3* Open Access Astrct N nd S co-doped cron quntum dots (N, S-CQDs) with high N- nd S-doping level were synthesized y microwve solid-phse pyrolysis within 50 s. Owing to the dominnt pyridinic N injection into the conjugted frmework, oth high enzyme mimics ctlytic ctivity nd photoluminescence quntum yield re chieved simultneously. Keywords: Quntum dots, N-doping, Photoluminescence, Enzyme mimics ctlysis Bckground The cron quntum dots (CQDs) which emerged s novel zero-dimensionl cron mteril hve received tremendous ttention ecuse of their high chemicl stility, low cytotoxicity, nd unique electronic nture nd opticl ehviors [1 3]. With ctive surfce groups such s OH nd CO 2 H, CQDs cn e recomined with other orgnics or inorgnics for vrious fntstic pplictions including io-imging [4, 5], optoelectronic devices, nd photoctlysts for degrdtion of orgnic dyes or production of hydrogen from wter splitting [6 8]. Very recently, oth experimentl nd theoreticl results confirmed tht heterotom doping ws n effective method to improve the electronic nd opticl properties of CQDs [9, 10]. Among the novel composites, N-doped CQDs or nitrogen/sulfur co-doped CQDs (N, S-CQDs) demonstrted much high fluorescence quntum efficiency or photoctlytic ctivity thn tht of pristine one [11, 12]. Also, the enhncement in performnce of N-doped CQDs hd shown positive correltion to the nitrogen doping mount [13, 14]. Although these studies convincingly certify tht N-doping strikingly influences the properties of CQDs, however, there re scrce reports on the effective hetero-doping methods for CQDs. Suffered from the high soluility of inorgnic * Correspondence: wngy@nju.edu.cn; zgzou@nju.edu.cn 1 School of Chemistry nd Chemicl Engineering, Eco-mterils nd Renewle Energy Reserch Center (ERERC), Nnjing, Chin 2 Ntionl Lortory of Solid Stte Microstructures, Nnjing, Chin Full list of uthor informtion is ville t the end of the rticle precursor of the dopnt, conventionl hydrotherml croniztion routes would led to lrge mount of dopnts remining in the rection solution nd thus quite low N-doping mount in the finl CQDs. Herein, we reported synthesis of nitrogen-rich N, S co-doped cron quntum dots (N, S-CQDs) y microwve-ssisted pproch within only 50 s. Citric cid (CA) ws chosen s cron source, nd thioure ws used s not only nitrogen nd sulfur source ut lso wek se. The nitrogen nd sulfur concentrtion of N, S-CQDs reches 12.8 nd 7.2 wt%, respectively, which ws out five nd three times higher thn reported for N-CQDs nd N, S co-doped CQDs [11, 14]. Methods The N, S-CQDs were otined in the following wys: the mixture of 0.42 g (2 mmol) of citric cid monohydrte nd 0.46 g of (6 mmol) thioure ws put in porcelin crucile nd heted for 50 s in microwve rector (445 W). The otined rownish yellow product ws dded into 30 ml of deionized wter to form yellow suspension nd centrifuged t 9000 rpm for 20 min. Then, the superntnt ws purified with 0.22-μm filter memrne nd dilyzed with deionized wter through dilysis memrne (retined moleculr weight,1000 D) for 24 h. Finlly, the dilyste ws further freeze-dried under vcuum. The pristine CQDs were synthesized from net citric cid monohydrte, nd the susequent tretment process ws the sme with tht of N, S CQDs. The Author(s) Open Access This rticle is distriuted under the terms of the Cretive Commons Attriution 4.0 Interntionl License ( which permits unrestricted use, distriution, nd reproduction in ny medium, provided you give pproprite credit to the originl uthor(s) nd the source, provide link to the Cretive Commons license, nd indicte if chnges were mde.

2 Liu et l. Nnoscle Reserch Letters (2017) 12:375 Pge 2 of 8 The enzyme-mimic ctivity of N, S-CQDs for the decomposition of H 2 O 2 ws mesured in 30-mL uffer solution of citric cid disodium hydrogen phosphte (ph 3.5, 35 C) contining 1 μg ml 1 of N, S-CQDs nd M of tetrmethylenzidine (TMB) sustrte. After 160 μl ofh 2 O 2 (30%) solution ws dded to the colorless uffer solution, the rection strts nd then tkes the solution to mesure the sornce of the lue oxidtion product of TMB t 652 nm ech 2 min. Finlly, the rection rtes of oxidtion TMB were clculted. The reusility test of N, S-CQDs ws performed in the rection system contining 60-mL uffer solution of citric cid disodium hydrogen phosphte nd 2 μg ml 1 of N, S-CQDs s well s M of TMB sustrte. The rection strted s the ddition of H 2 O 2 solution (0.3%, 320 μl) into the mixed solution nd took smll mount of solution to mesure the sornce t 652 nm fter 1 h nd the first cycle ws finished. Then, 320 μl of fresh H 2 O 2 (0.3%) solution ws dded to the rection system for the next cycle. Other three-time cycle rections were repeted in the sme condition. The corresponding sornce ws clculted y sutrcting the lst sornce. The trnsmission electron microscope (TEM) nd highresolution trnsmission electron microscope (HRTEM) imges were otined on JEM-2100 electron microscope with high voltge (200 kv). The selected-re electron diffrction (SAED) ws mesured y FEITF20 (FEI highresolution field-emission trnsmission electron microscopy) with condition of 200 kv. The UV/vis sorption spectr were crried out with UV-3600 (Shimdzu UV-VIS-NIR Spectrophotometer). The fluorescence spectr were recorded on F-7000 (Hitchi Fluorescence Spectrometer) with the condition of 700 V. The fluorescence lifetime nd FLQY were mesured y the FM-4P-TCSPC (Hori Join Yvon). The excittion nd emission wvelengths re 358 nd 436 nm, respectively. The X-ry powder diffrctometer (XRD) were chrcterized y D8 Advnce (Germny Bruker AXS Ltd.) using Cu Kα with the condition of 40 kv nd 40 ma. The Fourier trnsform infrred (FT-IR) spectr were crried out with Nicolet is10 (Thermo Fisher Infrred Spectrometer). X-ry photoelectron spectrometer (XPS) ws otined on PHI 5000 Vers (UIVAC-PHI). TG-MS (thermogrvimetric-mss spectrometry) is mesured y Netzsch STA 449C with heting rte of 10 K min 1 from 35 C up to finl temperture of 450 C under the N 2 ir (10%, ir) flow. Results nd Discussions It cn e seen from the TEM imges (Fig. 1) tht the s-prepred N, S-CQDs re uniform nd well-dispersed thin nnosheets with n verge size of 2.0 nm in dimeter. The inserted HRTEM imge (Fig. 1) shows very cler lttice fringe spcing of 0.24 nm consistent with the (1120) fcet of grphene, indicting the crystlline cores of N, S-CQDs which might e composed of grphitic sp 2 cron toms [15, 16]. The inserted SAED imge (Fig. 1) shows the N, S-CQDs re crystlline with lttice fringe of nm corresponding to the reported grphic N-CQDs [13]. This d vlue grees well with the c d Fig. 1 TEM imge of N, S-CQDs (the insert is HRTEM imge). SAED imge of N, S-CQDs. c XRD nd d FT-IR spectrum of N, S-CQDs

3 Liu et l. Nnoscle Reserch Letters (2017) 12:375 Pge 3 of 8 interplnr spcing of (002) diffrction fcets of reported N, S co-doping CQDs with grphitic structure [11]. The XRD pttern of N, S-CQDs illustrtes single rod pek centered t 2θ vlue of round 25.5 tht ws ssigned to the diffrction pek of grphene (Fig. 1c), corresponding to the interlyer spcing of 0.33 nm [17]. However, g-cnqds nd β-c 3 N 4 synthesized y ure with sodium citrte or citric cid hve een reported [18, 19]. Different with our smples, the g-cnqds hve two of the chrcteristic peks t 27.4 nd 13.1 in the XRD. The strong pek t 27.4 represents the chrcteristic interplnr stcking of romtic systems, indexed for grphitic cron nitride s the (002) pek, nd the wek diffrction pek t 13.1 corresponds to n interplnr structurl pcking motif indexed s the (100) pek. The g-cnqds were synthesized with high molr rtio (6:1) of N/C precursor (ure to sodium citrte) [18]. In ddition, more N injected into the core to form cron nitride dots with longer therml tretment time rech to 60 min. But the heting time for microwve solid-phse pyrolysis of our N, S-CQD smple is only 50 s. Different with our smples, the volume of the mixture of ure nd citric cid solution is oiled up to point of 100 C nd otined β-cron nitride nnocrystlline. In conclusion, we speculte tht low mole rtio of N/C precursor, short rection time, nd reltively higher temperture my led to grphitic cron structure. Compred to tht of free-doping CQDs [20], the (002) diffrction pek of our N, S-CQD smple shifts from 23 to higher ngle of 25.5, implying decrese of interlyer spcing. Strong interplnr electronic stcking interctions etween the grphene-like lyers of N, S-CQDs. Possessing stronger electronegtivity thn cron tom, the hetero-doping of lrge mount of nitrogen nd sulfur toms in the conjugted cron frmework would cusenincreseintheelectronicdensityofthewhole conjugted cron frmework, nd thus, the interplnr distnce shortened [21, 22]. The FT-IR spectrum (Fig. 1d) confirms vrious surfce groups of N, S-CQDs. The nds t 3163cm 1 with shoulder t 3416 cm 1 in cm 1 rnge represent N H nd O H stretching virtions, respectively [11]. These considerle mino, hydroxyl hydrophilic groups could enle N, S-CQDs superior hydrophilicity [23]. The triple peks tht pper t round 1582, 1656, nd 1704 cm 1 cn e ssigned to the different chrcteristic virtion onds, respectively. The peks t round 1704 cm 1 re stretch virtion of C=O croxylic groups nd C=N onds [24], nd the other two peks t 1656 nd 1582 cm 1 re the chrcteristic virtion of mide groups stretching C=O (mide I) nd in-plne ending of N H ond (mide II) [24, 25]. The peks t 1405 nd 1345 cm 1 cn e ssigned to the virtion of C S ndc N, respectively [17], while the nds t 1177 nd 1084 cm 1 further confirm the existence of C O nd S=O onds on N, S- CQDs [17, 23]. UV/vis sorption spectrum of N, S-CQDs depicts two cler sorption nds (Fig. 2). The strong sorption nd t 234 nm is scried to the π-π* electronic trnsition of romtic conjugted system sp 2 domins [17], while the wek sorption pek t 340 nm is ttriuted to the n-π* trnsition of C=O ond [26]. It ws noticed tht the reltive intensity of sorption pek t 234 nm is much stronger thn tht of the N, S-CQD smples synthesized y hydrotherml method [17, 26], suggesting the formtion of more romtic sp 2 domins with N-doping into conjugted core system s pyridinic N. Besides, rod shoulder t round 430 nm overlpped y the pek t 340 nm stems from mny kinds of surfce stte trnsitions [26]. The photoluminescence (PL) spectr (Fig. 2) illustrte tht the N, S-CQDs hve rod distriution of excittions. The mximum excittion wvelength is t 358 (emission wvelength 436 nm) owing to the 340 nm of sorption pek. It cn e seen from the inset imge in Fig. 2 tht the colorless nd trnsprent N, S-CQDs queous solution ecomes right lue under 365 nm UV irrdition. The solution of N, S-CQDs remined cler for 10 months without precipittions; this high stility of N, S-CQD prticles is scrie to their much smll nd uniform size s well s hydrophilic groups on the surfce. The photoluminescence quntum yield (PLQY) of N, S-CQDs is clculted to e 23.6% under excittion t 358 nm, which is three times higher thn tht the reported for N-doped or N, S-CQDs [20, 23]. In contrst, the PLQY of pristine CQDs is only 1.15% tht is fr less thn tht of N, S-CQDs. It hs een reported tht the PLQY of CQDs ws relted to the N-doping mount in CQDs [17, 27], nd thus, mny works hve ttempted to increse the N-doping mount in CQDs y prolonging the rection time even up to 19 h or rising the rection temperture to 260 C [11, 23]. However, the N-doping mount in the finl solid smples ws still less thn 6%. In our work, high N-doping level of 12.5% is chieved on N, S-CQDs y n efficient solid-phse microwvessisted wy, wherein citric cid nd thioure molecules rpidly rect nd void sulimtion. In ddition, thioure works s wek se to ccelerte the polymeriztion rte. Intriguingly, chnging the rtio of thioure nd CA from 3:1 to 1:3 nd 1:1, the PLQY of N, S-CQDs ws slightly reduced to 7 nd 2.1%, respectively. Moreover, incresing the rection time up to 2 min merely otined the ulk cron. To further clrify the rection mechnism, we cquired the thermogrvimetric-differentil therml nlyzer (TG-DTA) nd TG-mss curves of net CA nd the rectnt mixture of CA nd thioure. As displyed in Fig. 3, there re three exothermic peks tht cn e oserved in the TG-DTA curves of net CA; the first pek corresponds to the soring wter including crystl wter. Thesecondshrppekt154 Cssignstothemelting

4 Liu et l. Nnoscle Reserch Letters (2017) 12:375 Pge 4 of 8 c d Fig. 2 The UV/vis of N, S-CQDs. The PL of N, S-CQDs nd the inset is the imges of N, S-CQDs under mient light nd 365 nm irrdition. c The PL spectr of N, S-CQDs with different excittion wvelengths. d Photoluminescence intensity decy curves t the excittion light of 358 nm c Fig. 3 TG nd DTA curves of net citric cid, rectnt mixture of citric cid nd thioure. c TG-Mss curves of net CA nd rectnt mixture of citric cid nd thioure

5 Liu et l. Nnoscle Reserch Letters (2017) 12:375 Pge 5 of 8 het relese of CA crystlline, nd the rod one centered t 214 C reltes to the intermoleculr dehydrtion nd croniztion. Wheres for the mixture of CA nd thioure, much chnges on the ltter two exothermic peks cn e oserved on the TG-DTA spectrum (Fig. 3), the second exothermic pek ppers t low temperture of 118 C, indicting tht the cid se interction etween CA nd thioure results in drmtic drop of 36 C in the melting het relese step. In ddition, the esides third exothermic pek tht corresponded to the dehydrtion nd croniztion derives two peks t 214 nd 236 C, nd wek pek t 170 C cn e oserved, implying tht the ddition of thioure cn dvnce the dehydrtion nd croniztion process. Compring the TG-mss spectr of the net CA nd the mixture of CA nd thioure (Fig. 3c), it cn e found tht the temperture mximum of H 2 O relese pek decreses from 215 C for the net CA to 180 C for the mixture of CA nd thioure. Similrly, the mximum of the CO 2 relese pek is t 227 C for CA, ut it shifts to low temperture nd ecomes two-step relese t 179 nd 198 C for the rectnt mixture of CA nd thioure, respectively. This temperture decrese in the dehydrtion nd croniztion well grees with the TG-DTA results, implying different rection pproch for these two systems. For the net CA, the intermoleculr dehydrtion nd croniztion simultneously occurs t high temperture. While for the mixture of CA nd thioure, the intermoleculr dehydrtion rects firstly etween the croxyl groups of CA nd the mino groups of thioure, nd then, the stepwise croniztion hppens to form the cron core of N, S-CQDs. Compred to the wek hydrogen ond interction etween the CA molecules, strong cid se interction etween croxyl groups nd mino groups results in the significnt decrese of dehydrtion temperture. Intriguingly, s displyed in Fig. 3c, the residul mss for the net CA nd the mixed rectnt of CA nd thioure re 1 nd 21 wt%, respectively, which indictes tht the dded thioure cn ply role s wek se to lower the rection temperture nd void sulimtion, thus enhnces the N- nd S-doping content in N, S-CQDs with high yield. Figure 2c illustrtes the emission spectr of N, S- CQDs with different excittion wvelengths. When the excittion wvelength chnges from 290 to 370 nm, the peks of emission t 440 nm show nerly no shift. The emission components re firly constnt in energy nd most proly originte from sorption of n-π* trnsition t 340 nm. The excittion-independent emission property of CQDs hs een studied y fitting the complex emission peks to multiple Gussin functions nd deduced similr conclusion [26]. While when the excittion wvelength is vried from 390 to 490 nm, the PL emission spectrum exhiited redshift s the increse of excittion wvelength, chrcterizing n excittion wvelengthdependent property. This cn e scried to vrious surfce sttes of C=O or mide group s rolesdiscrete exciton trpping centers to ffect the emission energy in PL process [11, 19, 28]. Polydispersity nd surfce heterogeneity is the origin of excittion wvelengthdependent PL ehvior [28, 29]. The rod sorption pek t round 430 nm is n ensemle of vrious surfce sttes, including croxyl nd mide, which enles excittion wvelength-dependent PL ehvior of N, S-CQDs. The fluorescence lifetime of N, S-CQDs ws determined to evlute its opticl property (Fig. 2d). The PL decy curves of N, S-CQD smple cn e fitted y douleexponentil formul, where τ 1 is 3.48 ns, τ 2 is ns, nd the verge lifetime is 6.72 ns. Compred to the verge lifetime of 2.42 ns of pristine CQDs [30], drmtic longer fluorescence lifetimes of oth τ 1 nd τ 2 were otined on our smple. It hs een reported tht the τ 2 proportion nd verge lifetime ecome longer with the N-doping mount incresing nd concluded tht the longer τ 2 stemmed from the surfce sttes [11, 31]. The formtion of N, S-CQDs ws corroorted y XPS. As displyed in Fig. 4, five distinct peks t 530, 399, 284, 222, nd 164 ev present of O 1s, N1s, C1s,S 2s nd S 2p signls, respectively, indicting N nd S were indeed injected into the frmework of CQDs [17]. Highresolution C 1s XPS spectrum (Fig. 4) indictes three chrcteristics of C structure, including romtic conjugted sp 2 C (C=C) t ev, sp 3 C(C N, C O, C S) t ev, nd C=O/C=N t ev [11]. The N 1s XPS spectrum (Fig. 4c) of the N, S-CQDs shows three peks t 399.5, 400.3, nd ev, which represent pyridinic N, pyrrolic N, nd midic N, respectively [17, 24]. In the g-cnqd, the tomic rtio Ncore/Ccore, s derived from the experimentl XPS intensity rtio, is equls to 1.40, which is close to the expected vlue of 1.33 expected for C 3 N 4 [19]. We conducted similr dt nlysis of our smple, tking ev of C 1s pek s Ccore while oth ev (pyrrolic) nd ev (pyridinic) of N 1s s Ncore (ecuse the inding energy vlues re similr with ev of NCore in the [19]), nd the clculted Ncore/Ccore is 0.43, much smller thn 1.33 for C 3 N 4. Moreover, the reltive rtio of pyridinic N to pyrrolic N in our N, S-CQDs ws found to e very different from tht of N- or N, S co-doped CQDs synthesized y hydrotherml method [17, 21]. Pyridinic N is the dominnt dopnt in our N, S-CQD smple, which is 1.5 times s pyrrolic N, ut it is usully less thn 1.0 for mny therml synthesized smples. Such high pyridinic N my endow N, S-CQDs superior property for further ctlysis ppliction due to they cn ct s ctlytic ctive sites [32]. Moreover, the pyrrolic N on the edge is n importnt composition of surfce defects nd cn ct s photoluminescence center [17, 27]. The S 2p XPS spectrum (Fig. 4d) displys two

6 Liu et l. Nnoscle Reserch Letters (2017) 12:375 Pge 6 of 8 c d Fig. 4 The full scn XPS of N, S-CQDs. The high resolution XPS of C 1s N1s c nd d S 2p spectr of N, S-CQDs typicl signls t nd ev, which correspond to S2p 3/2 nd S 2p 1/2 of thiophenic S, respectively [16]. Comining with the FT-IR spectrum, we speculte tht sulfur toms successfully dope into the frmework of N, S-CQDs s thiophenic S nd exist t the edge of N, S-CQDs to improve their PLQY. Enzyme ctlysis is expecttive owing to its high specificity nd ctivity. Horserdish peroxidse (HRP) is the most studied plnt enzyme which contins the ctive center of porphyrin cycle in heme group to ctlyze effectively the one electron oxidtion, wide vriety of orgnic nd inorgnic sustrte y hydrogen peroxide [33, 34]. The mimicking properties of pyridinic-rich N, S-CQDs were tested for the oxidtion of the peroxidse sustrtes of 3, 3, 5, 5 -tetrmethylenzidine (TMB) in the presence of H 2 O 2 y mesuring the sorption of lue oxidtion product of TMB t 652 nm. The UV/vis sorption of N, S-CQDs peks t 234 nd 340 nm. Figure 5 illustrtes the fitting lines of the concentrtion of TMB-derived oxidtion products (μmol/l)vitimeinthepresenceof N, S-CQDs nd pristine CQDs. The rection rte (r) for the decomposition of H 2 O 2 on N, S-CQDs s enzyme mimics is μmol 1 L 1 S 1, which is two times higher thn tht of pristine CQDs nd previous reported doped-free CQDs under the sme conditions [35, 36]. The excellent ctivity of pyridinic-rich N, S-CQDs cn e ttriuted to the high doping content of N tht possesses lrge electronegtivity thn cron tom to increse the electron density of N, S-CQDs nd, especilly, the dominnt pyridinic N owning one lone pir of electrons tht Fig. 5 The initil rection rte of N, S-CQDs nd free doped CQDs. Reusility test of N, S- CQDs

7 Liu et l. Nnoscle Reserch Letters (2017) 12:375 Pge 7 of 8 leds to the enhncement in the electron density nd moility in the π-conjugted frmework of N, S-CQDs, thus ccelertes the rection. This is the first report on the drmtic improving of ctlse-mimic property of CQDs dependence on the dominnt doping of pyridinic N in cron frmework. The reusility of N, S-CQDs ws investigted y consecutive four times usge for ctlse-mimic rection (Fig. 5). On the four-cycle usge, no ovious decrese in the ctivity of N, S-CQDs ws oserved. The high stility of the intrinsic ctlysis ctivity of N, S-CQDs is scried to the dominnt pyridinic N-doping in the C=C frmework ecuse the pyridinic N cn ply role s effective enzyme mimic ctlytic sites for H 2 O 2 decomposition. Conclusions In summry, we synthesized pyridinic-rich N, S-CQDs with high N- nd S-doping level y microwve solid polymeriztion method within mere 50 s. Thioure roles not only s S source ut lso s wek se to ccelerte the intermoleculr dehydrtion t low temperture nd multistep croniztion, which enles the high N- nd S-doping level in N, S-CQDs nd dominnt pyridinic N to inject into the conjugted frmework s the enzyme mimic ctive sites. Our work provides n effective method to synthesize pyridinic-rich N, S-CQDs possessing oth high PLQY nd enzyme mimics ctivity. Acknowledgements This work ws supported y the Ntionl Nturl Science Foundtion of Chin (Grnt Nos , , nd ) nd the Jingsu Provincil Nturl Science Foundtion (No.BK , BK ).The uthors thnk the Anlysis Center nd High Performnce Computing Center of Nnjing University for the smple chrcteriztion nd theoreticl clcultions. Authors Contriutions TL contriuted to the experiment nd mnuscript preprtion. ZC contriuted to the mnuscript preprtion. JZ contriuted to the mnuscript preprtion. YW contriuted to the ide of the study nd mnuscript preprtion. ZZ contriuted to the mnuscript preprtion. All uthors red nd pproved the finl mnuscript. Competing Interests The uthors declre tht they hve no competing interests. Ethics Approvl nd Consent to Prticipte We dmit tht ethicl identity is not involved. Pulisher s Note Springer Nture remins neutrl with regrd to jurisdictionl clims in pulished mps nd institutionl ffilitions. Author detils 1 School of Chemistry nd Chemicl Engineering, Eco-mterils nd Renewle Energy Reserch Center (ERERC), Nnjing, Chin. 2 Ntionl Lortory of Solid Stte Microstructures, Nnjing, Chin. 3 Jingsu Key Lortory for Nnotechnology, Kunshn Innovtion Institute of Nnjing University, Nnjing , Chin. Received: 14 Mrch 2017 Accepted: 17 My 2017 References 1. Liu H, Ye T, Mo C (2007) Fluorescent cron nnoprticles derived from cndle soot. Angew Chem Int Ed 46: Bourlinos AB, Stssinopoulos A, Anglos D et l (2008) Surfce functionlized crogenic quntum dots. Smll 4: Ji X, Li J, Wng E (2012) One-pot green synthesis of opticlly ph-sensitive cron dots with upconversion luminescence. Nnoscle 4: Co L, Wng X, Mezini MJ (2007) Cron dots for multiphoton ioimging. J Am Chem Soc 129: Liu Q, Guo B, Ro Z et l (2013) Strong two-photon-induced fluorescence from photostle, iocomptile nitrogen-doped grphene quntum dots for cellulr nd deep-tissue imging. Nno Lett 13: Sekiy R, Uemur Y, Murkmi H et l (2014) White-light-emitting edgefunctionlized grphene quntum dot. Angew Chem Int Ed 126: Liu J, Liu Y, Liu N, Hn Y (2015) Metl-free efficient photoctlyst for stle visile wter splitting vi two-electron pthwy. Science 347: Guo Y, Yo P, Zhu D, Gu C (2015) A novel method for the development of cron quntum dot/cron nitride hyrid photoctlyst tht responds to infrred light irrdition. J Mter Chem A 3: Hn Y, Tng D, Yng Y, Li C et l (2015) Non-metl single/dul doped cron quntum dots: generl flme synthetic method nd electro-ctlytic properties. Nnoscle 7: Jing S, Sun L, Zhng Y et l (2015) Red, green, nd lue luminescence y cron dots: full-color emission tuning nd multicolor cellulr imging. Angew Chem Int Ed 54: Ding H, Wei JS, Xiong HM (2014) Nitrogen nd sulfur co-doped cron dots with strong lue luminescence. Nnoscle 6: M Z, Ming H, Hung H (2012) One-step ultrsonic synthesis of fluorescent N-doped cron dots from glucose nd their visile-light sensitive photoctlytic ility. New J Chem 36: Briscoe J, Mrinovic A, Sevill M et l (2015) Biomss-derived cron quntum dot sensitizers for solid-stte nnostructured solr cells. Angew Chem Int Ed 54: Yng Z, Xu M, Liu Y et l (2014) Nitrogen-doped, cron-rich, highly photoluminescent cron dots from mmonium citrte. Nnoscle 6: Peng J, Go W, Gupt BK, Liu Z et l (2012) Grphene quntum dots derived from cron fiers. Nno Lett 12: Qu D, Zheng M, Du P et l (2013) Highly luminescent S, N co-doped grphene quntum dots with rod visile sorption nds for visile light photoctlysts. Nnoscle 5: Dong Y, Png H, Yng HB et l (2013) Cron-sed dots co-doped with nitrogen nd sulfur for high quntum yield nd excittion-independent emission. Angew Chem Int Ed 52: Jun Z, Yong Y, Chun-yng Z (2013) A low-temperture solid-phse method to synthesize highly fluorescent cron nitride dots with tunle emission. Chem Commun 49: Messin F, Sciortino L et l (2016) Fluorescent nitrogen-rich cron nnodots with n unexpected β-c 3 N 4 nnocrystlline structure. J Mter Chem C 4: Zhng J, Shen W, Pn D et l (2010) Controlled synthesis of green nd lue luminescent cron nnoprticles with high yields y the croniztion of sucrose. New J Chem 34: Hung H, Li C, Zhu S et l (2014) Histidine-derived nontoxic nitrogendoped cron dots for sensing nd ioimging pplictions. Lngmuir 30: Li J, Shen B, Hong Z et l (2012) A fcile pproch to synthesize novel oxygen-doped g-c 3 N 4 with superior visile-light photorectivity. Chen Chem Commun 48: Sun Y, Shen C, Wng J, Lu Y (2015) Fcile synthesis of iocomptile N, S-doped cron dots for cell imging nd ion detecting. RSC Adv 5: Songnn Q, Xioyun W, Qipeng L et l (2012) A iocomptile fluorescent ink sed on wter-solule luminescent cron nnodots. Angew Chem 124: Mrt JK, Antonios K, Pngiotis D et l (2012) Formtion mechnism of crogenic nnoprticles with dul photoluminescence emission. J Am Chem Soc 134: Yu W, Sergii K, Yu Z et l (2014) Thickness-dependent full-color emission tunility in flexile cron dot ionogel. J Phys Chem Lett 5:

8 Liu et l. Nnoscle Reserch Letters (2017) 12:375 Pge 8 of Qu D, Zheng M, Zhng L, Sun Z et l (2014) Formtion mechnism nd optimiztion of highly luminescent N-doped grphene quntum dots. Sci Rep 4: Alice S, Emnuele M, Peter S et l (2016) Solvtochromism unrvels the emission mechnism of cron nnodots. J Phys Chem Lett 7: Alexnder P, Demchenko MO (2016) The origin of emissive sttes of cron nnoprticles derived from ensemle-verged nd single-moleculr studies. Nnoscle 8: Ruqun Y, Chngsheng X, Jin L et l (2013) Col s n undnt source of grphene quntum dots. Nt Commun 4: Wng X, Qu L, Zhng J, Peng X, Xio M (2003) Surfce-relted emission in highly luminescent CdSe quntum dots. Nno Lett 3: Surmnin NP, Li X, Nllthmi V (2009) Nitrogen-modified cron-sed ctlysts for oxygen reduction rection in polymer electrolyte memrne fuel cells. J Power Sources 188: Sun D, Ci C, Li X, Xing W, Lu T (2004) Direct electrochemistry nd ioelectroctlysis of horserdish peroxidse immoilized on ctive cron. J Electronl Chem 566: Guo S, Zhou Q, Lu T, Ding X, Hung X (2008) Spectroscopic studies of interctions involving horserdish peroxidse nd T 3+. Spectrochimi Act Prt A 70: Shi W, Wng Q, Hung Y (2011) Cron nnodots s peroxidse mimetics nd their pplictions to glucose detection. Chem Commun 47: Song C, Pehrsson PE, Zho W (2005) Recoverle solution rection of hipco cron nnotues with hydrogen peroxide. J Phys Chem B 109: