Analytica Chimica Acta 1079 (2019) 212e219

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Analytica Chimica Acta

journal homepage: www.elsevier.com/locate/aca

Blue emitting nitrogen-doped carbon dots as a fluorescent probe for

nitrite ion sensing and cell-imaging
Jayasmita Jana a, Hye Jin Lee a, Jin Suk Chung a, Mun Ho Kim b, Seung Hyun Hur a, *
a
School of Chemical Engineering, University of Ulsan, Daehak-ro 93, Nam-gu, Ulsan, 44610, Republic of Korea
b
Department of Polymer Engineering, Pukyong National University, 45 Yongso-ro, Nam-gu, Busan, 48513, Republic of Korea

h i g h l i g h t s g r a p h i c a l a b s t r a c t

Fluorescent nitrogen-doped carbon

nanodots were synthesized using a
facile green chemical pathway.

The as-synthesized carbon dots
showed selective and sensitive nitrite
ion detection.

The as-synthesized carbon dots
exhibited high efficiency for cell
imaging.

a r t i c l e i n f o a b s t r a c t

Article history: This study examined the efficiency of pH-dependent, fluorescent carbon dots for the sensing of haz-
Received 23 May 2019 ardous anions in aqueous media and cell imaging. The nitrite anion, an important water-soluble element
Received in revised form for environmental and biological systems, requires continuous monitoring because a high concentration
28 June 2019
can affect the systems severely. The as-synthesized carbon dots efficiently detected the nitrite anion in
Accepted 29 June 2019
Available online 3 July 2019
aqueous solution through a fluorescent ‘Turn Off’ phenomenon. The quenching mechanism was inves-
tigated through proper microscopic and spectroscopic studies. The limit of detection and linear detection
range were 7.9 nM and 2.3mM-7.7 mM, respectively. The sensitivity was tested with different water
Keywords:
Carbon dot
samples. In a parallel experiment, the as-synthesized carbon dots were used as a cell-imaging probe for
Fluorescence HeLa cells, highlighting their potential in different biological studies.
‘Turn Off’ phenomenon © 2019 Elsevier B.V. All rights reserved.
Nitrite ion
Sensing
Cell-imaging

1. Introduction of nitrite can cause hazardous environmental problems and health

risks, such as methemoglobinemia due to interference in blood
The nitrite ion is an important nitrogen-containing ingredient oxygen transport, stomach cancer etc. [3e5]. The World Health
used in fertilizers for the healthy growth of plants [1]. Nitrites play Organization has set the fatal dose of nitrite as 1.0 mg/L [6].
important roles in food preservatives, biological nitrogen cycle, Henceforth, the detection of nitrite ion in biological and environ-
environment system etc. [2]. On the other hand, an excess amount mental systems has become necessary. Nitrite detection
methods include spectrophotometry [7], electrochemistry [8],
chromatography [9], chemiluminescence [10], photoluminescence
* Corresponding author. [2] and surface-enhanced Raman scattering [11] methods. The
E-mail address: shhur@ulsan.ac.kr (S.H. Hur).

https://doi.org/10.1016/j.aca.2019.06.064
0003-2670/© 2019 Elsevier B.V. All rights reserved.
 J. Jana et al. / Analytica Chimica Acta 1079 (2019) 212e219 213

development of a simple, low-cost, and non-toxic sensing platform solution was mixed with 4 mL of a 0.1 M citric acid solution and
has become important because some of the techniques use transferred to a 20 mL Teflon-lined stainless-steel autoclave. The
expensive probes, toxic solvents, complicated pathways. reaction temperature was maintained at 180  C for 6 h. After the
Fluorescence detection techniques have become quite success- reaction was complete, a pale pink colored solution was obtained.
ful because of their high accuracy and high signal to noise ratio [12]. The as-obtained solution was subjected to centrifugation to remove
Among the different fluorescent probes, carbon dots have become the unwanted solid non-fluorescent particles. The supernatant
very popular because of their unique physicochemical behavior. showed emission and was stored at room temperature for further
Carbon dots are non-toxic, chemically resistant, water-soluble, and characterization and experiments. The quantum yield was calcu-
have tunable emission, wide absorption range, and photostability lated using a quinine sulfate solution prepared in 0.1 M H2SO4 using
[13e15]. These properties of carbon dots have broadened their the equation,
applications in the region of electrochemistry [16], catalysis [17],
nanomedicine [18], drug delivery [19], photovoltaics [20e22], bio- Fsl ¼ Fst (Isl/Ist)(Ast/Asl)(hsl/ hst)2 (1)
imaging [23], and sensing of different hazardous moieties including
cations, anions, organic molecules [24e27]. The principle of carbon where h, I, A, and F represent the refractive index, fluorescence
dot-based sensors depends on the fluorescence “Turn Off or Turn intensity, absorbance, and quantum yield, respectively. The sub-
On’ phenomenon of carbon dots in the presence of analytes. Sen- scripts “sl” and “st” refer to the sample (carbon dots) and standard
sors, whose operations are based on signal ‘Turn Off’, may follow (quinine sulfate), respectively.
different quenching mechanisms, including static quenching, dy-
namic quenching, Fo €rster resonance energy transfer (FRET), 2.3. Characterization
photoinduced electron transfer (PET), and inner filter effect (IFE)
[28]. These mechanisms depend on the nature of the analyte and Fluorescence of the samples was recorded using a Cary Eclipse
the way it interacts with carbon dots. fluorescence spectrophotometer (Agilent Technologies, USA) with a
This paper reports a carbon dot-based fluorescent sensing 1.0 cm quartz cuvette. The lifetime values of the moieties were
platform for nitrite ions in aqueous media. The sensing is based on analyzed using a FS5 spectrophotometer (Edinburgh Instruments,
quenching of the intriguing fluorescence of as-synthesized carbon EPL-375 ps pulsed diode laser the light source) through time-
dots. The carbon dots were synthesized using citric acid and 3- correlated single-photon counting (TCSPC) mode. Confocal images
aminophenyl boronic acid under hydrothermal conditions. Citric for cell imaging were acquired in sequential mode using a
acid is an easily available chemical that can form an electron-rich 60  Plan-Apochromat (1.4 NA) oil objective and the appropriate
carbon core under hydrothermal reaction condition. On the other filter combination on an Olympus 2000 laser-scanning confocal
hand, 3-aminophenyl boronic acid contains both N and B atoms, fluorescence microscope. The absorption spectrum of the samples
which provide a heteroatom to the core and surface functional was recorded using a UVeVis spectrometer (UVevis, SPECORD 210
group. Doped carbon dots have modified optoelectronic property, PLUS, Analytik Jena, German). The functional groups were analyzed
surface and local chemical reactivity [13]. The as-synthesized N- by Fourier transform infrared (FTIR, Nicolet iS5, Thermo Fisher
doped carbon dots contain boronic acid, carboxylic, and hydroxyl Scientific, USA) spectroscopy. The oxidation states of the constitu-
groups on the surface. In the present case, which was supported by ent elements were examined by X-ray photoelectron spectroscopy
microscopic analyses, it was assumed that there would be a direct (XPS, Thermo ESCALAB 250 Xi, Thermo Fished Scientific, USA) using
interaction between the boron center and nitrite group that Al Ka X-ray radiation (1486.6 eV). The size and morphology of the
quenches the inherent fluorescence of the as-synthesized carbon as-synthesized particles were observed using a high-resolution
dots. The fluorescence can be tuned by changing the medium pH. transmission electron microscopy (HReTEM, JEMd2100 F, JEOL,
These blue emitting carbon dots have been used for cell imaging to Japan) at an operating voltage of 200 kV.
ensure their further application in bio-systems.
2.4. Preparation of sensing platform
2. Experimental section
The sensing was performed in an aqueous solution at room
2.1. Reagents and instruments temperature. The anions were added individually to the as-
prepared carbon dot solution and the spectroscopy study was
3-Aminophenylboronic acid (3-APBA), citric acid (CA), all then conducted. The excitation wavelength was maintained at
cationic and anionic salts, hydrochloric acid (HCl), and sodium 360 nm. A time dependent quenching study showed that the most
hydroxide (NaOH) were purchased from Sigma-Aldrich Co. significant result was obtained after 12 h. Nitrite ions showed the
(USA). Dulbecco's modified eagle media (DMEM) was acquired concentration dependent quenching of the fluorescence of these
from Welgene Company (South Korea) and 4,6-diamidino-2- carbon dots. The interference study was performed by mixing other
phenylindole (DAPI) was obtained from Vector Laboratories anions with nitrite ions at a 1:1 ratio. The interference was removed
(USA). Human epithelial carcinoma (HeLa) cells (ATCC® CCLd2™) by tuning the pH of the reaction medium.
were supplied by American Type Culture Collection (USA). Water
samples for real sample analysis were collected from a local area. 2.5. Real sample analysis
All chemical reagents were used as received. Deionized (DI) water
was used for the preparation of the samples in all experiments. The water samples were collected from a nearby tap and
drinking water reservoir. The samples were filtered and centrifuged
2.2. Synthesis of carbon dots prior to use. The samples were prepared by spiking the nitrite ion
solution using a standard addition method. The spectral measure-
Water-soluble carbon dots were synthesized by the hydrother- ments were taken by fluorescence spectroscopy.
mal process of a mixture of an aqueous alkaline solution of 3-
aminophenyboronic acid (3-APBA) and aqueous citric acid. A 2.6. Cell culture and fluorescence imaging
0.1 M aqueous alkaline solution of 3-APBA was prepared using
water and 0.1 M NaOH at a 3:2 ratio. First, 2 mL of a 0.1 M 3-APBA HeLa cells (approximately 8  104 cells/well) were used for the
214 J. Jana et al. / Analytica Chimica Acta 1079 (2019) 212e219

cell-imaging study. The cells were seeded in 6-well plates and identify their different states. An analysis of C1s showed peaks for
cultured for 24 h in DMEM at 37  C in a humidified atmosphere C-B, C-C, C-O, and N-C¼O at 283.8, 284.9, 286.9, and 288.7 eV,
containing 5% CO2 for 24 h to adhere the cells to the surface. The respectively. An analysis of N1s showed peaks of C-N-C and N-H at
culture medium was then replaced with 0.5 mL of fresh medium 399.9 and 401.1 eV, respectively. An examination of B1s revealed
containing 1 mg/mL CD1, and then incubated for 6 h. Before the peaks for B-C and B-O bonds at 192.06 and 193.15 eV respectively,
confocal imaging experiments, the cells were washed three times and O1s showed peaks of C¼O, C-O, and B-O at 530.06, 531.6, and
with PBS buffer (pH ¼ 7.2) to eliminate the excess CD1, followed by 532.7 eV, respectively (Fig. 2) [30].
fixation with a 4% paraformaldehyde (PFA) solution for 15 min and Under these experimental conditions, a condensation reaction
washing twice using PBS buffers. The cells were then dyed by DAPI was assumed to occur between CA and 3-APBA, where the car-
to stain the nucleus for the detection of the fluorescence signal. The boxylic acid group of CA and amine group of 3-APBA are involved
fixed cells were then observed under 405 and 488 nm laser exci- (Scheme 1). The boronic acid groups remained at the surface along
tation using a confocal fluorescence system. with carboxylate and hydroxyl groups. The benzene ring of 3-APBA
with an enriched carbon content became a significant carbon
3. Results and discussion source at the core [31].

3.1. Carbon dot synthesis and characterization 3.2. Photo-physical property of CD1

The water soluble, blue emitting heteroatom doped carbon dots The optical properties of the CD1 particles were analyzed by
(CD1) were synthesized using simple hydrothermal method with absorbance and fluorescence spectroscopy. In UVevis spectroscopy,
3-APBA and CA as the precursor compounds (Scheme 1). After CD1 particles exhibit two characteristics peaks at 254 nm and
some trials, the temperature and time of reaction were determined 320 nm. These peaks were assigned to p-p* transitions related to
to be 180  C and 6 h, respectively (Fig. S1). The experimental pH the sp2 hybridized carbon core [32] and n-p* transitions along with
was measured to be 4.0 and the maximum emissive product was surface moieties, respectively [33] (Fig. 3).
obtained at this pH (Fig. S2). The CD1 showed significant stability in The CD1 particles were found to be enormously fluorescent. The
terms of emission peak and emission intensity for more than 2 calculated quantum yield was 57.8% compared to quinine sulfate
months (Fig. S3). CD1 was found to be pH responsive. While (Fig. S5). By varying the excitation wavelength within a range of
decreasing the pH, the emission was slightly decreased with 320e380 nm, the fluorescence peak remained at ~420 nm with
decreasing pH, but the decrease was quite significant when the pH varying intensities, but upon an increase in the excitation wave-
was increased (Fig. S4). Previously it was reported that the hydro- length, the peak shifted towards a longer wavelength but with a
philicity and dispersion of carbon dot particles increase at low pH significantly decreased intensity (Fig. S2). This shows that during
[29]. 320e389 nm excitation, the emission was predominantly from a
The morphology and size of the as-synthesized CD1 particles single fluorophore. Upon excitation at 360 nm, the fluorescence
were obtained from TEM and HR-TEM images (Fig. 1). The particle spectrum showed a maximum intense peak at 420 nm with a
size distribution showed a uniform dispersion of the particles with Stokes shift of 60 nm (Fig. 3). The fluorescence of the carbon dot
a relatively narrow size distribution, ranging from 1.14 to 4 nm with particles may originate from the carbon core as well as surface
a mean diameter of 2.27 nm. A lattice fringe of 0.25 nm is obtained functional groups [34]. In the present case, doping and hybridiza-
from the HR-TEM image. FTIR spectroscopy of CD1 revealed the tion of the carbon core may be the controlling factor for the fluo-
presence of peaks at 1068 cm1, 1180 cm1, 1250 cm1, 1402 cm1, rescence of CD1 via modification of the core electronic
1731 cm1, and 3210 cm1, which were assigned to the stretching environment. Fluorescence decay analysis revealed an average
vibration of B-C, bending vibration of B-O-H, stretching vibration of lifetime of 4.60 ns (Fig. 3).
C-O, stretching vibration of B-O, stretching vibration of carboxylic
C¼O, and stretching vibration of O-H, respectively (Fig. 1). XPS 3.3. Nitrite ion sensing
elemental analyses revealed the presence of C1s, N1s, B1s, and O1s.
Further deconvolution of the individual elements was done to The effects of different anions on the emission of the as-

Scheme 1. Schematic illustration of CD1 synthesis.

 J. Jana et al. / Analytica Chimica Acta 1079 (2019) 212e219 215

Fig. 1. (A) TEM image of CD1. Inset: (a) Size distribution diagram, (b) HRTEM image to show lattice fringes. (B) FTIR spectrum of CD1.

Fig. 2. (A) Broad range XPS spectra of CD1. Elemental analyses of (B) C1s, (C) N1s, (D) B1s, and (E) O1s. XPS analysis was done using the vacuum dried solid.

synthesized CD1 particles were evaluated individually in an increase in absorption intensity (Fig. S7). On the other hand, the
aqueous solution. Fig. 4 shows the spectral profile of CD1 in the position of CD1 emission peak was not significantly changed in the
presence of different anions. The emission was quenched in the presence of NO 
2 . It is assumed that, in the presence of NO2 , the
presence of borate and nitrite (NO 2 ) anions. On the other hand, non-radiative electron or energy transfer becomes predominant.
NO2 mediated quenching was quite significant compared to borate. Because nitrite is a bidentate ligand, it is possible to interact with
NO2 is a strong anion that interacts easily with the CD1 particles at either O or N center. In the XPS spectra, however, the absence of a B-
the surface. Maximum quenching was achieved after 12 h (Fig. S6). N peak (~397 eV) [35] indicates that nitrite interacts with the O
The lifetime measurements showed that the average lifetime of the center. NO2 -mediated quenching was found to be non-recoverable
CD1 NO 2 system is 4.4 ns (Fig. 3). The similar lifetime of CD1 in the in the presence of different cations (Fig. S8), indicating a strong
presence and absence of NO 2 indicates a ground state interaction CD1 NO 2 interaction. The quenching kinetics of fluorescence of
between NO 2 and CD1 particles. The interaction is supported by CD1 particles in the presence of the NO 2 ion was obtained from the
XPS analysis (Fig. S7). The UVevis spectra show the bathochromic Stern-Volmer equation as follows:
shift of the CD1 peaks in the presence of NO 2 along with the
216 J. Jana et al. / Analytica Chimica Acta 1079 (2019) 212e219

Fig. 3. (A) Spectral profile of CD1; (a) absorption spectrum, (b) excitation spectrum, and (c) fluorescence spectrum. Inset digital image of CD1 under visible and UV light. (B)
Fluorescence decay profile of CD1 and CD1-NO-2.

Fig. 4. (A) Bar diagram showing the relative fluorescence intensity (I0/I) CD1 in the presence of different anions, [anion] ¼ 0.9 mM, [CD1] ¼ 0.01 g/mL, lex ¼ 360 nm. (B) Contour
diagram of CD1 and CD1-NO-2, emission was obtained at 420 nm. (inset: emission spectra of CD1 and CD1-NO-2), lex ¼ 360 nm. (C) Schematic mechanism showing the quenching
phenomenon.

I0/I ¼ 1þKsv[Q] (2) concentration ratio of 1:1. Fig. 5 shows that the quenching of NO
2 is
observed, even in presence of other anions. The quenching was
where I0 and I denote the fluorescence intensities in the absence greater in the presence of borate due to the simultaneous effects of
and presence of NO 2 at an excitation wavelength of 360 nm, borate and nitrite. On the other hand, upon an increase in medium
respectively. [Q] represents the NO 2 concentration and KSV is the acidity, a very small increase in fluorescence was observed for
SterneVolmer quenching constant. borate-nitrite-mediated quenching that was not observed for
Fig. 5 illustrates the NO 2 ion concentration dependent nitrite-mediated quenching. Again, the borate mediated quenching
quenching process. The data show that the fluorescence intensity was recovered easily by increasing the Hþ concentration in the
decreases monotonously with the gradual addition of NO 2 . At a medium (Fig. S9). Under experimental condition pH ¼ 4 was kept
concentration of 0.77 mM, 70.1% quenching was obtained. The for nitrite sensing. As with the increase in medium pH, the fluo-
relative fluorescence intensity (I0/I) vs. concentration of NO 2 plot rescence intensity of CD1 decreases, the NO 2 induced quenching
was obtained within a concentration range of 0e0.9 mM, but a could be interfered. The reproducibility of NO 2 detection was
linear relationship was observed within a range of 2.3 mMe7.7 mM checked and it showed consistency (Fig. S10).
with a R2 value of 0.97 (Fig. 5). Another linear dynamic range was
obtained at very low concentration, 4.68  107 M-6.96  109 M, 3.4. Real sample analysis
with a R2 value of 0.82. Based on the experimental results, the limit
of detection [LOD ¼ (3  standard deviation of the regression line)/ Further applicability of the CD1-based NO2 sensing platform
slope] was calculated to be 7.9 nM. was examined using tap water and drinking water collected from a
The selectivity of the prescribed method of nitrite ion sensing local area. The water sample had been spiked with different con-
was studied by observing the effects of different anionic salts at a centrations of NO
2 and the fluorescence measurements were done
 J. Jana et al. / Analytica Chimica Acta 1079 (2019) 212e219 217

Fig. 5. (A) NO - - 

2 concentration dependent fluorescence of the CD1-NO2 system. (B) Relative fluorescence intensity of the CD1-NO2 system with the concentration of NO2 . (C) Linear
dynamic range. (D) Anion interference study at a 1:1 ratio, [anion] ¼ 0.9 mM, [CD1] ¼ 0.01 g/mL, lex ¼ 360 nm.

under experimental conditions. The recovery results, which are cells in the CD solutions, the Hela cells were brightly luminated
listed in Table 1, show that the recovery is much better at higher when imaged on a fluorescence microscope at 405 nm and later at
concentrations. Table S2 compared the result of nitrite ion sensing 488 nm excitation. The micrograph under illumination at 488 nm
using the prescribed method with other methods to indicate the (green/without interference of DAPI) showed that the CD1 particles
significance of the study. were mostly distributed in cytoplasm without accumulating on the
cell membrane and without nucleus area (Fig. 6). Therefore, CD1
3.5. Cell imaging by CD1 particles are potential candidates for cell-imaging and other bio-
applications.
Previous research data has indicated the non-toxic nature of the
carbon dot particles [36,37]. The as-synthesized CD1 particles were 4. Conclusion
utilized for in vitro cell imaging. CD1 uptake and imaging of the cells
were conducted by fluorescence microscopy. The CD1 particles A carbon dot-based sensing platform for the fluorescence ‘Turn
were allowed to penetrate the cell membrane for 6 h followed by Off’ detection of nitrite ions was established based on the in-
imaging. For this experiment, no external transfection agent or teractions between the as-synthesized carbon dot and nitrite ion in
specific ligand was used. The intake of the tiny CD1 particles aqueous solutions. The as-synthesized CD1 revealed pH dependent
(~2.27 nm diameter) into the cells may depend on the electric emission that could be tuned reversibly. The detection procedure
charge gradient or passive diffusion through the ion channels and was independent of the medium pH and could not be recovered
aquaporins of the cell membrane [38,39]. Upon incubation of the under the experimental conditions. CD1 was also used successfully

Table 1
Determination of the NO
2 concentration in water obtained from different sources using the proposed strategy.

Sample Nitrite ion added (ml 103 M) Nitrite ion recovered (ml 103 M) Recovery % RSD %

Drinking water 50 56.4 ± 2.6 112.8 ± 5.2 4.6

100 89.7 ± 1.5 89.7 ± 1.5 1.7
200 189.0 ± 3.6 94.5 ± 1.8 2.4
Tap water 50 55.8 ± 2.2 111.6 ± 4.4 3.9
100 109.3 ± 1.3 109.3 ± 1.3 1.2
200 180.6 ± 4.5 90.3 ± 2.25 2.5
218 J. Jana et al. / Analytica Chimica Acta 1079 (2019) 212e219

Fig. 6. Micrographs of HeLa cells illuminated at (A) 405 nm and (B) 488 nm. (C) Differential interference contrast microscopy image of HeLa cells. Merged image of (D) A and C
(illuminated at 405 nm) and (E) B and C (illuminated at 488 nm). Scale bar 30 mm. (F) Micrograph at 488 nm without DAPI.

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