Peroxynitrous-Acid-Induced Chemiluminescence of Fluorescent Carbon Dots for Nitrite Sensing

Published: September 07, 2011

r 2011 American Chemical Society 8245 dx.doi.org/10.1021/ac202039h |Anal. Chem. 2011, 83, 8245–8251

ARTICLE

pubs.acs.org/ac

Peroxynitrous-Acid-Induced Chemiluminescence of FluorescentCarbon Dots for Nitrite SensingZhen Lin, Wei Xue, Hui Chen, and Jin-Ming Lin*

Beijing Key Laboratory of Microanalytical Methods and Instrumentation, Department of Chemistry, Tsinghua University,Beijing 100084, People’s Republic of China

bS Supporting Information

Highly efficient and stable fluorescent nanoprobes are of greatimportance in optoelectronic devices and biomedical re-

search. Intensive research has been focus on the explorationof new efficient fluorescent materials, such as semiconductorquantum dots,1 silicon-dye hybrid nanoparticles,2 and metalnanoclusters.3 Photoluminescent carbon nanoparticles (carbondots) prepared via a laser ablation method were of particularinterest, because of their higher photoactivity, lower toxicity,and less-expensive cost, compared with heavy-metal containedquantum dots.4,5 Carbon dots have been used as benign opticalimaging probes,6,7 because of their excellent luminescenceproperties. TiO2 or gold functionalized carbon dots have beenutilized as photocatalysts for Methyl Blue degradation8 or CO2

conversion,9 because of their upconversion characteristics.Intense research still focused on their further applications andintrinsic photoluminescence mechanism.

Nitrites are widely present in the environment and are usedas preservatives and fertilizing reagents for food. However,nitrites are an essential precursor for the formation of carcino-genic N-nitrosamine, which is very harmful for human beings.Therefore, it is of significant importance to detect nitrites in theenvironment and in food. Many analytical methods based ondifferent principles, including UV�vis absorbance,10,11 electro-chemistry (such as conductimetry and amperometry),12,13

fluorescence,14 and chemiluminescence (CL),15�17 have beendeveloped. UV�vis absorbance determination is usually basedon diazotization of aromatic amine using acidified nitrite. Ahighly colored azo chromophore is formed using a subsequentcoupling reaction. The process requires close control of cou-pling conditions.10 Complex electrodes are necessary for the

electrochemical detection. Fluorescent dyes are always neededfor fluorescent detection, which easily suffer the interferencesfrom background fluorescent compounds in the real samples.CL detection is of great important in many fields, because of itshigh sensitivity, low background interference, and simpleinstrumentation. The CL detection for nitrites can be carriedout either in the gas phase or in the aqueous phase. The gas-phase detection15 is based on the reduction of nitrite to NO by areductant and the subsequent reaction of NOwith ozone to formNO2

*, which is electronically excited and falls to a ground statewith light emission. However, the transformation is tedious andcomplex. In the aqueous phase, nitrites can react with acidifiedH2O2 to form peroxynitrous acid (ONOOH),18 which is un-stable and can be quenched to peroxynitrite, using an alkalinereagent.19 In our previous study, the decomposition of perox-ynitrite with CL emission was observed and was developed asa flow-injection method for nitrite determination.17 Mikuskaet al.16 also reported that the reaction of ONOOH with alkalineluminol could be used for the determination of nitrites. However,both methods were conducted in alkaline media and H2O2

was utilized. The interference from transition metal should beeliminated by a cation-exchange column.

In this work, fluorescent carbon dots were first demonstratedto have the ability of CL in the presence of ONOOH, which wasformed by online mixing of acidifiedH2O2 andNaNO2. A linearrelationship between the nitrite for the formation of ONOOH

Received: August 4, 2011Accepted: September 7, 2011

ABSTRACT: In this work, chemiluminescent (CL) property of thecarbon dots in the presence of peroxynitrous acid was studied.Peroxynitrous acid is formed by online mixing of nitrite and acidifiedhydrogen peroxide. The CL intensity was increased linearly with nitriteconcentration in the range from 1.0 � 10�7 M to 1.0 � 10�5 M, andthe detection limit was 5.3� 10�8M (signal-to-noise ratio of S/N = 3).This method has been successfully applied to the determination ofnitrites in pond water, river water, and pure milk, with recoveries in therange of 98%�108%. The CL mechanism of the peroxynitrousacid�carbon dots system was investigated using the CL, ultraviolet�visible light (UV�vis), and electron paramagnetic resonance (EPR)spectra. The electron-transfer annihilation of hole-injected and electron-injected carbon dots could mainly account for the CLemission, which sheds new light on the optical properties of the carbon dots.

8246 dx.doi.org/10.1021/ac202039h |Anal. Chem. 2011, 83, 8245–8251

Analytical Chemistry ARTICLE

and the CL signal produced from the carbon dots�NaNO2�H2O2 system was found. This CL system has been developed asa sensitive and convenient method for the determination ofnitrite in pond water, river water, and pure milk.

’EXPERIMENTAL SECTION

Reagents and Materials. Hydrogen peroxide (H2O2, 35%)and quinine sulfate were obtained from Alfa Aesar China, Ltd.Sodium nitrite (NaNO2) was obtained from SinopharmChemical Reagent Co., Ltd. (Shanghai, PRC). Serine andpolyethylene glycol 1500 (PEG 1500) were purchased fromMerck Company (Darmstadt, Germany). 2,2,6,6-Tetramethyl-4-piperidine (TEMP) was obtained from Sigma�AldrichChemical Co. (St. Louis, MO, USA). Sulfuric acid (H2SO4, 98%)and glycerine were purchased from Beijing Chemical Reagent Co.(Beijing, PRC). All of the reagents were analytical-grade and usedas received.Apparatus. The batch experiment was performed with a

BPCL ultraweak CL analyzer (Institute of Biophysics, ChineseAcademy of Science, Beijing, PRC). A Model LumiFlow LF-800 detector (NITI-ON, Funabashi, Japan) was used in theflow injection experiment and the solution was pumped withthree peristaltic pumps (Model SJ-1211, Atto, Tokyo, Japan)into the spiral flow CL cell, which was installed in front of thephotomultiplier tube (PMT). Transmission electron micro-scopy (TEM) image was recorded using a Model JEM 2010electron microscope (JEOL, Tokyo, Japan). The X-ray photo-electron spectrum (XPS) was measured using a PHI ModelQuantera SXM scanning X-ray microprobe, using Al Kα as theexcitation source (1486.6 eV) and binding energy calibrationwas based on C 1s at 284.8 eV. The UV absorption spectra werecollected using a Model UV-3900 spectraphotometer (Hitachi,Japan). The fluorescence spectra were performed using aModelFL-7000 spectrofluorometer (Hitachi, Tokyo, Japan). Fouriertransform infrared (FT-IR) measurements were carried out witha Perkin�ElmerModel 100 FT-IR spectrometer (Waltham,MA,USA). Electron paramagnetic resonance (EPR) spectra weremeasured on a Model JES-FA200 spectrometer (JEOL, Tokyo,Japan).Preparation of Carbon Dots. Fifteen milliliters (15 mL) of

glycerine and 1.0 g of PEG 1500 were mixed and treated bymicrowave to become a homogeneous solution. Then, 1.0 g

serine was injected. The mixture was further heated in a micro-wave oven for 10 min. The color of the solution changed tobrown. In addition, the solution exhibited strong fluorescenceunder UV irradiation. The as-prepared carbon dots were purifiedby dialysis. (The cutoff of the dialysis membrane was equivalenttoMw≈ 2000.) The carbon dots solution was then concentratedusing a rotary evaporator to 10 mL and diluted with water beforeuse in the CL experiment.CL from Carbon Dots. CL kinetic curves were obtained by

batch experiments, which were carried out in the glass cuvette.The CL profiles were displayed and integrated at intervals of0.1 s. Fifty microliters (50 μL) of carbon dots and 50 μL ofnitrites were premixed, and then 50 μL of H2O2 were injectedby a microliter syringe from the upper injection port. Theaddition orders of the reagent were changed to investigate theinteraction of the reagent and design the CL flow injectionanalysis system.The flow injection CL system consists three flow lines. Water

was used as the carrier for nitrites and sample. Carbon dots werediluted by water and mixed with nitrites in the nixing coil. Thecarrier (water) of nitrite, carbon dots ,and H2O2 were pumpedinto the flow cell installed in front of the PMT (see Figure S1 inthe Supporting Information). The nitrite standards or sampleswere injected through a six-valve injector. The CL signal wascollected by the LF-800 detector. The peak height of the signalrecorded was measured relative to CL intensity.

’RESULTS AND DISCUSSION

Characterization of Carbon Dots. Microwave treatmenttime affected the size of the carbon dots. Carbon dots3�4 nm in size were prepared with heating treatment in therange of 5�20min. However, no particles can be observed fromthe transmission electron microscopy (TEM) image with aheating time of <5 min. Figure 1 revealed that the carbon dotswere spherical dots with a diameter of ∼3�4 nm after micro-wave treatment for 10 min. The prepared carbon dots exhibiteda bright green color under ultraviolet radiation (λ = 365 nm)(see Figure 1). The maximum fluorescent emission (∼440 nm)was obtained with an excitation wavelength of 360 nm and theemission shifted with the increased excitation wavelength,which revealed a distribution of the different surface energytraps of the carbon dots.20 The quantum yield (λex = 360 nm)

Figure 1. TEM image of (A) the carbon dots and (B) fluorescent spectra for carbon dots excited at wavelengths of 300�460 nm, with increments of20 nm. The inset in panel A is the photograph of the carbon dots under visible light (left) and under 365-nmUV light (right). Fluorescence experimentalconditions: the photomultiplier tube (PMT) voltage was set at 700 V, and the silt width of excitation and emission was 2.5 nm.



for the carbon dots is as high as 12% with quinine sulfate as thereference.21

XPS results (see Figure S2 in the Supporting Information)indicated that the carbon dots contained carbon, nitrogen,and oxygen in a weight ratio of 67.74:1.26:31.00. The XPSpeak at 397 eV in the N 1s region can be ascribed tochemically bound N-species.22,23 No molecularly absorbedN2, NOx, or NHx species existed at interstitial sites. Fouriertransform infrared (FT-IR) spectroscopy (see Figure S3 in

the Supporting Information) indicated gC�OH (hydroxyl),�CH3, �CH2, gC�O�Ce (ether), and gC�N< groupswere formed on the carbon dots.CL from Carbon Dots�NaNO2�H2O2 System. ONOOH

was formed by the mixing of NaNO2 and acidified H2O2

(reaction 1).17,18 The transform of ONOOH to nitrate via the stageof HOONO* is believed to cause light emission (reaction 2).24,25

HNO2 þ H2O2 f ONOOH þ H2O ð1Þ

Figure 2. CL kinetic curves of (A) the NaNO2�H2O2 system (curve 1) and the carbon dots�H2O2 system (curve 2) and (B) the carbondots�NaNO2�H2O2 systemwith different reagent mixing orders. (Legend: (1) injectingH2O2 into carbon dots andNaNO2, (2) injectingNaNO2 intoH2O2 and carbon dots, and (3) injecting carbon dots intoH2O2 andNaNO2.) Conditions: 0.1MH2O2 in 0.05MH2SO4, 1.0� 10�5MNaNO2, carbondots with a dilution of 1:1000. Voltage of the PMT was set at 1.2 kV; the time interval was set as 0.1 s.

Figure 3. Effects of (A) the concentration of H2SO4, (B) the concentration of carbon dots, (C) the concentration of H2O2, and (D) the flow rate on theflow-injection CL detection system.



ONOOH f ONOOH� f NO�3 þ Hþ ð2Þ

The rapid and weak CL emission resulting from the mixing ofNaNO2 and H2O2 was recorded in Figure 2A. The interaction ofNaNO2 and H2O2 in an acidic medium in the presence of carbondots was accompanied by very strong CL (Figure 2B). The reactionbetween carbon dots andH2O2 only brought in weak and lastingCL(Figure 2A), which indicated that the contribution of H2O2 to thehigh CL was minor. Therefore, the reaction between ONOOHrelated species and carbon dots is the main pathway for the CL.Various mixing orders of reagents were measured by the batchmethod, as shown in Figure 2B. The highest CLwasmonitored withthe injection of H2O2 into the mixture of carbon dots and NaNO2,because of no reaction before the injection.From the batch CL signals, we believe that this system can be

developed as flow injection analysis for the determination ofnitrites. To establish the optimal conditions for the analysis ofnitrites, the effects of the concentration of H2SO4, carbon dots,and H2O2 on the CL analysis were investigated. It has beenfound that no CL is observed in the absence of H2SO4, becauseONOOH should be formed in an acidic medium. The optimalsignal-to-noise ratio (S/N) was obtained with H2SO4 at aconcentration of 0.025 M (see Figure 3A). Carbon dots witha dilution of 1:1000 provided the highest CL intensity(Figure 3B). The CL signal and the sensitivity of the methodincreased with the concentration of H2O2 in the range from1.0� 10�3M to 0.5M (see Figure 3C). Hence, 0.5MH2O2 andcarbon dots with a dilution of 1:1000 were used as the optimalconditions for nitrite determination. The flow rate of H2O,carbon dots, and H2O2 affected the mixing of the reagent. Thelow flow rate caused a broad CL signal, which decreased the CLintensity. Flow rates that were too high not only increased thepressure in the flow line, but also decreased the reagent passingtime in the spiral CL detection cell, which reduced the CLsignal. As shown in Figure 4C, the most suitable flow rates forH2O, carbon dots, and H2O2 are 2.3, 2.4, and 2.7 mL min�1,respectively.Under the optimal conditions, the CL signals for different

nitrite concentrations are shown in Figure 4A. There was a goodlinear relationship between CL intensity and the nitrite concen-tration in the range from 1.0 � 10�7 M to 1.0 � 10�5 M(Figure 4B) with a correlation coefficient of 0.9998. The relativestandard deviation (RSD) values (n = 10) of the analysis were2.0%, 2.3%, and 2.2% for nitrite concentrations of 5.0 � 10�7,

1.0 � 10�6, and 5.0 � 10�6 M, respectively. The limit ofdetection (S/N = 3) for nitrite was 5.3 � 10�8 M. The flow-injection CL analysis can be applied to detect nitrite in pondwater, river water, and pure milk. The effects of coexistingcompounds in the sample on the CL detection of 1.0 �10�6Mof nitrite were investigated. The tolerance limit was estimatedwith a (5% relative error in peak height. The results weresummarized in Table 1. Most ions caused no interferences atconcentrations of <5.0 � 10�4 M. Some transition metals atconcentrations of >5.0 � 10�6 M have a positive effect on thedetermination. However, the interferences in the acidic carbondots�NaNO2�H2O2 system are much smaller than that in analkaline CL system,17 because hydroxyl radicals could be easilygenerated from the metal-catalyzed breakdown of hydrogenperoxide in alkaline solution. Hydroxyl radicals with very highoxidant potential could interact with the carbon dots andinterfere with the nitrite determination. The acidic mediumcould greatly reduce the interference.The proposed method has been applied to the determination

of the nitrites in pond water, river water, and pure milk. Freshlycollected water samples were filtered through a membrane filterof 0.22-μm pore size and spiked with 1.0 � 10�5 M EDTA as amasking reagent for transition metals before the CL determina-tion. The milk sample was first precipitated by ZnSO4, thencentrifuged and filtered before the determination to eliminate theinterferences from the proteins. The analytical results of theproposed method were evaluated by a national food safetystandard (GB 5009.33-2010).26 As shown in Table 2, the resultsthat obtained with the two methods were in good agreement forthe sample determination. The recoveries for sample determina-tion were in the range of 98%�108%.CLMechanism.The role of nanoparticles in a liquid-phase CL

reaction can be as catalysts27 or emitters.28 In order to identify

Figure 4. Flow injection signals (A) and standard curve (B) for the nitrite concentration in the range from 1.0� 10�7 to 1.0� 10�5 M. Experimentalconditions: 0.5 M H2O2 in 0.025 M H2SO4; carbon dots in a dilution of 1:1000; high voltage (�800 V); a 100-μL sample injector was used.

Table 1. Assessment of Interferences on the Determinationof 1.0 � 10�6 M Nitrite

tolerance (M) species added

5.0 � 10�4 Na+, K+, Ca2+, F�, Cl�, H2PO4�, HPO4

2�, CO32�,

NO3�, SO4

2�, Mg2+, NH4+, citric acid, glucose,

sucrose, malic acid, tartaric acid

5.0 � 10�5 Zn2+, Cu2+, Co2+, Fe2+, Fe3+

5.0 � 10�6 I�, ascorbic acid



the roles of the carbon dots in the carbon dots�NaNO2�H2O2

CL system, CL spectrum was measured by a fluorescencespectrometer with the xenon lamp turned off. As shown inFigure 5A, themaximumCL spectrum for carbon dots�NaNO2�H2O2 system located in the wide range of 400�600 nm and wascentered at 520 nm. The wide range is similar to the fluorescentemission wavelength of the carbon dots. Hence, it is reasonablethat CL could be attributed to the various surface energy trapsthat existed on the carbon dots.4,6,29 Note that the CL spectrumis red-shifted in comparison to the most intense photolumines-cence (PL), which mainly occurs through excitation and emis-sion within the core of the nanoparticles.30,31 The red-shift mostlikely results from the smaller energy separations of the carbondots surface states (for CL emission), compared with the energyfor the most intense PL.32

NaNO2 has an absorption peak at 354 nm (see Figure 5B).The peak became lower and another absorption peak at 301 nm,ascribed to nitrates, appeared with the addition of acidified H2O2

into NaNO2 solution. This change was due to the isomerizationof ONOOH, which was formed through the mixing of acidifiedH2O2 andNaNO2 (reaction 2). Besides nitrates, no new absorptionpeaks appeared when carbon dots were present with NaNO2�H2O2.Singlet oxygen (1O2) as the CL emitter was reported to exist

in the NO2��H2O2 system (see reactions 3�5).33 Although

the CL of the system was enhanced greatly by carbon dots, theproduction of 1O2 was not increased greatly, which wasconfirmed by EPR spectra using 2,2,6,6-tetramethyl-4-piper-idine (TEMP) as the specific detection reagent for 1O2 (see

Figure 6A). Furthermore, 1.0� 10�3 M sodium azide (NaN3),which is a scavenger for 1O2,

34 had no inhibition of the CL inthe system. The result indicated that 1O2 was not the main CLemitting species responsible for the increased CL in the carbondots�NaNO2�H2O2 system. Excited carbon dots should actas the emitting species, rather than catalysts in the CL system.

ONOOH f 3NO2 þ 3OH ð3Þ

ONOOH þ H2O2 f O2 3� þ 3NO2 þ Hþ þ H2O

ð4Þ

O2 3� þ 3OH f 1O2 þ OH� ð5Þ

EPR was also utilized to investigate the ground-state proper-ties of luminescent species in the carbon dots. The carbon dotsshow an EPR signal at g = 1.9903 (see Figure 6B), which revealssingly occupied orbital in ground-state carbon dots.35 The singleoccupied orbital indicated that the carbon dots could be electrondonors or electron acceptors during reaction.36 EPR spectra ofthe carbon dots after their reaction with H2O2 or ONOOH arepresent in Figure 6. The g-value for the carbon dots was reducedfrom 1.9903 to 1.9896 after oxidation by ONOOH, whichindicated the change of single occupied orbital in carbon dotsafter the CL. Electron transfer from the carbon dots to ONOOHhas been suggested.The g-value of the carbon dots after reaction with H2O2

showed little shift, compared to that of the fresh carbon dots(see Figure 6B), which could explain why a rather low CLintensity was obtained from the interaction between carbon dotsand H2O2. The fact indicates only oxidants that strongly interactwith the singlet orbital of the carbon dots could cause high CLemission.Effects of different radical scavengers on the CL system were

investigated to confirm the oxidant species in the CL system.Ascorbic acid with a concentration of >5.0 � 10�6 M had anegative effect on the signal, which indicated that the generationof free radical appeared to be critical in the CL reaction.Thiourea, which is an effective hydroxyl radical scavenger,37 witha concentration of >5.0 � 10�5 M, had a negative effect on theCL. The result revealed that hydroxyl radical played an importantpart in the CL reaction.

Table 2. Determination of Nitrites in Pond Water, RiverWater, and Milk Samplesa

sample

proposed

methodb

(� 10�6 M)

comparison

methodc

(� 10�6 M)

nitrite spiked

(� 10�6 M)

foundb

(� 10�6 M)

recovery

(%)

pond water 2.46 ( 0.01 2.30 1.00 3.53 ( 0.04 107

river water 1.90 ( 0.07 2.10 1.00 2.98 ( 0.01 108

milk 0.12 ( 0.01 0.14 1.00 1.10 ( 0.03 98aOptimum conditions: 0.5 M H2O2 in 0.025 M H2SO4 with a flow rateof 2.7 mL min�1; carbon dots with a dilution ratio of 1:1000 with a flowrate 2.4 mL min�1; and sample carrier (H2O) with a flow rate of 2.3 mLmin�1. bValues include mean values of three determinations and thestandard deviation. cData taken from ref 26.

Figure 5. (A) CL spectrum of the carbon dots�NaNO2�H2O2 system. (Experimental conditions: the PMT voltage was set at 900 V, and the silt widthof excitation and emission was 20 nm.) (B) UV�vis absorption of the reagent in the CL reaction. (Experimental conditions: 0.03 M NaNO2, 0.03 MH2O2 in 0.05 MH2SO4, carbon dots in a dilution of 1:1000.)



Based on the discussion above, it can be concluded that carbondots are themain emitters in the CL system. TheCLmechanism ofthe carbon dots�NaNO2�H2O2 system can be mainly explainedby radiative electron�hole annihilation between hole-injectedand electron-injected carbon dots (R 3 + and R 3 �).31,32,38,39

(See Figure 7.) Oxidant, ONOOH, and OH 3 that was producedduring themixing of nitrites and acidifiedH2O2 (reactions 1 and 3),

40

could serve as the hole injector and convert carbon dots to R 3 +

(reactions 6 and 7). O23 � was formed through the reaction

between ONOOH and H2O2 in acidic solution (reaction 4).33

The electron donation fromO23 � to carbon dots produced carbon

dots with a negative charge (reaction 8).35,38 Electron�holeannihilation in or between the R 3 + and R 3 � resulted in the energyrelease with CL emission (reactions 9 and 10).

R þ ONOOH þ H þ f R 3 þ þ 3NO2 þ H2O ð6Þ

R þ 3OH þ Hþ f R 3 þ þ H2O ð7Þ

R þ O2 3� f R 3 � þ O2 ð8Þ

R 3 þ þ R 3 � f R� þ R ð9Þ

R� f R þ hv ð10Þ

The transform of ONOOH to nitrate via the HOONO* stageis believed to cause light emission in a wavelength region of

350�450 nm,24,25 which overlaps the excitation wavelength forcarbon dots. Hence, the energy from the transformation couldtransfer to carbon dots and bring in the CL emission. However,the maximum of the CL for the transform was attained in thepH region of 6.5�7.0,24 the maximum CL intensity in thecarbon dots�NaNO2�H2O2 system was observed in a severeacidic medium. Hence, the energy transformation only partiallyaccounted for the CL emission in the carbon dots�NaNO2�H2O2 system.

’CONCLUSIONS

In conclusion, carbon dots were found to have chemilumi-nescent (CL) property in the presence of ONOOH that wasformed by the online mixing of acidified H2O2 and NaNO2. TheCL property of the carbon dots was suggested to be due to theradiative recombination of hole-injected and electron-injectedcarbon dots, which gave us new insight into the characteristic ofcarbon dots. With the advantage of the CL of carbon dots in thepresence of ONOOH, we have developed a sensitive, simple, andstraightforward flow-injection CL method for nitrite. The estab-lishedmethod has been successfully applied to the determinationof nitrite in pond water, river water, and milk with good recoveryand high reproducibility.

’ASSOCIATED CONTENT

bS Supporting Information. Schematic diagram of the flow-injection manifolds, the characterization of the carbon dots byXPS and FT-IR. This material is available free of charge via theInternet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Tel./Fax: +86-10-62792343. E-mail: [email protected].

’ACKNOWLEDGMENT

This work was supported by National Natural Science Foun-dation of China (Nos. 20935002) and 973 Program (No.2007CB714507).

Figure 7. Schematic illustration of the CL mechanism of carbondots�NaNO2�H2O2 system.

Figure 6. EPR spectra of (A) nitroxide radicals generated via the reaction of 2,2,6,6-tetramethyl-4-piperidine (TEMP) probe in the NaNO2�H2O2 andcarbon dots�NaNO2�H2O2 systems, and (B) the carbon dots and the carbon dots oxidized by H2O2 and ONOOH. Experimental conditions: 0.05 MTEMP in 0.05 M deoxygenized phosphate buffer (pH 7.4), 1 � 10�3 M NaNO2, 0.1 M H2O2 in 0.05 M H2SO4, carbon dots in a dilution of 1:100.



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