Cyan color-emitting nitrogen-functionalized carbon nanodots (NFCNDs) from Indigofera tinctoria and their catalytic reduction of organic dyes and fluorescent ink applications

Article abstract of DOI:10.1039/d1ra04351j

The present study reports the synthesis of nitrogen-functionalized carbon nanodots (NFCNDs) by a low-cost hydrothermal method using the leaf extract of Indigofera tinctoria as a novel carbon precursor. The synthesized NFCNDs were characterized by diverse spectroscopic techniques. The optical properties of N-CNDs were analyzed by UV-visible and fluorescence spectroscopic studies. The quantum yield (QY) for the prepared NFCNDs was found to be 12.6%. The surface morphology, functional groups, and crystallinity of NFCNDs were evaluated by HR-TEM, FT-IR, XRD and Raman spectroscopic methods, respectively. The Raman results revealed the moderate graphite structure of NFCNDs, and the calculated ID/IG value was 0.49. The spherical appearance of the synthesized NFCNDs was confirmed by HR-TEM, and the calculated size of the NFCNDs was 4 nm. The XRD and SAED pattern results gives an evidence for the amorphous nature of the prepared NFCNDs. The thermal stability of NFCNDs was studied by TGA analysis. The resulting NFCNDs acted as a green nanocatalyst and thus efficiently improved the reducing capability of sodium borohydride (NaBH4) in the catalytic reduction of methylene blue (MB) and methyl orange (MO) dyes. Furthermore, the bright cyan emission characteristics of synthesized NFCNDs were utilized as a labeling agent in anti-counterfeiting applications.

Full text of DOI:10.1039/d1ra04351j

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PAPER
Cyan color-emitting nitrogen-functionalized
carbon nanodots (NFCNDs) from Indigofera
tinctoria and their catalytic reduction of organic
dyes and fluorescent ink applications†
Cite this: RSC Adv., 2021, 11, 27745
a
a
a
b
Jothi Vinoth Kumar, Ganesan Kavitha, Rajaram Arulmozhi,
Velusamy Arul
a
and Natarajan Abirami
*
The present study reports the synthesis of nitrogen-functionalized carbon nanodots (NFCNDs) by a low-
cost hydrothermal method using the leaf extract of Indigofera tinctoria as a novel carbon precursor. The
synthesized NFCNDs were characterized by diverse spectroscopic techniques. The optical properties of
N-CNDs were analyzed by UV-visible and fluorescence spectroscopic studies. The quantum yield (QY)
for the prepared NFCNDs was found to be 12.6%. The surface morphology, functional groups, and
crystallinity of NFCNDs were evaluated by HR-TEM, FT-IR, XRD and Raman spectroscopic methods,
respectively. The Raman results revealed the moderate graphite structure of NFCNDs, and the calculated
D G
I /I value was 0.49. The spherical appearance of the synthesized NFCNDs was confirmed by HR-TEM,
and the calculated size of the NFCNDs was 4 nm. The XRD and SAED pattern results gives an evidence
for the amorphous nature of the prepared NFCNDs. The thermal stability of NFCNDs was studied by TGA
analysis. The resulting NFCNDs acted as a green nanocatalyst and thus efficiently improved the reducing
Received 5th June 2021
Accepted 28th July 2021
4
capability of sodium borohydride (NaBH ) in the catalytic reduction of methylene blue (MB) and methyl
DOI: 10.1039/d1ra04351j
orange (MO) dyes. Furthermore, the bright cyan emission characteristics of synthesized NFCNDs were
utilized as a labeling agent in anti-counterfeiting applications.
rsc.li/rsc-advances
8,9
friendliness, low cost and exibility in surface modication.
CNDs have multipurpose applications in various elds, such as
Carbon-based nanomaterials such as carbon nanotubes, gra- drug delivery, optoelectronic devices, sensing of metal ions,
1
Introduction
1
2
3
4,5
phene oxide, fullerenes, and carbon nanodots have been biological imaging, biomolecules, catalysis, solar cells, and the
10–16
playing vital roles in science and technology in the past two oxygen reduction reaction.
decades. Among these, carbon nanodots (CNDs) are uorescent
CNDs have been synthesized by various top-down methods,
carbon nanomaterials that have been engaged at the forefront including laser ablation, ultrasonic treatment, arc-discharge,
6
of recent research. Carbon nanodots (CNDs) are nanoparticles and electrochemical and chemical oxidation, and bottom-up
1
–10 nm in size, which were unexpectedly discovered by Anbu methods such as hydrothermal carbonization, microwave irra-
17
et al. during the purication of single-walled carbon nanotubes diation, pyrolysis, and plasma treatment. Among these
7
in 2004. The properties of the CNDs mainly depend on two synthesis methods, arc discharge and laser ablation processes
factors: rst, the choice of the carbon source (raw material), and need highly expensive instruments, whereas electrochemical
second, the synthesis method. CNDs are quite divergent from oxidation and chemical oxidation need strong acids. The
other carbon-based nanomaterials due to their exclusive microwave irradiation method demonstrates a simple way to
outstanding properties, such as high water dispersibility, strong synthesize CDs within few minutes; this method can be
chemical inertness, uorescence, excellent photostability, less restricted by uncontrollable reaction conditions. The hydro-
toxic nature, excellent bio-compatibility, environmental thermal method is simplistic, quick, and economical, has
a simple experimental setup, is and eco-friendly when
compared to other synthesis methods. Carbon dots with
tunable degrees of carbonization usually consist of carbon (C),
a
Department of Chemistry, College of Engineering and Technology, SRM Institute of
Science and Technology, Kattankulathur, Tamil Nadu-603 203, India. E-mail:
oxygen (O), and hydrogen (H) decorated with various functional
Department of Science and Humanities, Sri Venkateswaraa College of Technology, groups on their surfaces. Using inexpensive and eco-friendly
abiramin@srmist.edu.in
b
Vadkkal, Sriperumbudur, Chennai-602 105, Tamil Nadu, India
biomass sources such as Phyllanthus emblica, prickly pear
cactus, rice bran, Coccinia indica, and sweet potatoes as carbon
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ra04351j
1
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precursors to harvest CNDs has attracted great attention
18–22
because of their green approach.
The plant extracts con-
taining citric acid, tartaric acid, and ascorbic acid act as efficient
carbon precursors for the synthesis of highly uorescent CNDs.
The uorescence properties of CNDs are inuenced by solvents,
size, pH, and dopants used during the synthesis. Recently,
surface functionalization on CNDs was achieved by doping
heteroatoms (such as boron, sulphur, nitrogen, and phospho-
rous). The functionalization of nitrogen tremendously increases
the optical properties of CNDs when compared to undoped
CNDs. At the same time, other important features of CNDs,
such as the quantum yield (QY) and uorescence lifetime, are
signicantly improved by the functionalization approach.
Further, the heteroatom doping affects the internal sharing of
electrons in CDs and thus can change the bandgap energy,
Fig. 1 Structures of methyl orange and methylene blue.
23,24
which results in escalated uorescence intensity of CNDs.
These functionalized CNDs act as remarkable uorescent
probes in bio-imaging, catalysis, and chemo-sensing that ease the identication of fake currencies, including public
25–28
38
applications.
documents and certicates. CNDs seem to be advantageous as
In this work, Indigofera tinctoria (I. tinctoria), which belongs anti-counterfeiting agents, as they possess good uorescence
to the Fabaceae family, is used as a carbon precursor. This plant properties, great quantum yields, low cytotoxicity, and abun-
with medicinal values is found in various places in India. The dant sources. To our knowledge, the anti-counterfeiting appli-
phytochemical constituents present in the I. tinctoria leaf cations of NFCNDs obtained from I. tinctoria plant extract as
extracts are alkaloids, proteins, free amino acids, phenolic a uorescent material have not been reported yet.
compounds, avonoids, etc. The extracts are very useful for hair
growth, epilepsy, chronic bronchitis, ulcers, skin diseases, and pollutant water to commercial material is reported via the green
In the present research contribution, the conversion of waste
29
asthma, and are also used in cancer treatment. Due to the synthesis of blue-uorescent nitrogen functionalized carbon
availability of rich bioactive components, I. tinctoria leaf nanodots (NFCNDs) using easily available Indigofera tinctoria (I.
extracts are widely used for many biological activities, such as tinctoria) leaf extract as a carbon source with aq. ammonia as
antifungal, antimicrobial, antioxidant, anti-seborrhea, anti- the nitrogen (N) source by a hydrothermal carbonization
30,31
psoriatic, anti-inammatory, and anti-acne.
above merits, I. tinctoria leaf extract was elected as a novel important spectroscopic tools. Furthermore, the catalytic ability
carbon precursor for the synthesis of CNDs.
of the NFCNDs was carefully examined in the reduction of MB
With the rapid development of the dyeing industry, phar- and MO in the presence of NaBH (reducing agent). Also, owing
Besides the process. The prepared NFCNDs were carefully characterized by
4
maceutical industries, and printing technology, untreated to their highly uorescent properties, the NFCNDs may be used
sewage is being mixed into lakes and rivers every day, which is as an anti-counterfeiting agent.
very harmful to humans and also to marine aquatic organisms.
Hence, it is now a critical situation to treat organic dye-
containing industrial wastewater using current technologies.
Various methodologies have been adopted for the removal of 2.1 Chemicals and reagents
2
Materials and methods
32
3
3
harmful pollutants from waste effluents, such as adsorption,
The Indigofera tinctoria leaf was collected from Nathapattu
Village, Cuddalore District of Tamil Nadu, India. Aqueous
ammonia, methylene blue, methyl orange, sodium borohy-
dride, and polyvinyl alcohol (PVA) were purchased from Avra
Synthesis. All laboratory-grade chemicals and reagents were
used exclusive of addistional purication. Double distilled
water was used for solution preparation and other purposes
during the entire study.
34
35
Fenton oxidation, photocatalytic reduction, and adsorbent
membrane separation. Methyl orange (MO) is a sulphonated
36
azo dye; due to its high toxicity, it causes harmful effects to
37
human health and aquatic organisms. MO is widely used in
textiles and in chemical laboratories as a pH indicator (pK
.5). Further, most industries use methylene blue (MB) dye,
a
¼
3
which is a highly carcinogenic thiazine dye, as a coloring agent.
MB creates health problems, such as eye burns, skin irritation,
cancer, and allergic dermatitis, for humans and animals. At
present, it is very necessary to take action to remove these
2.2 Extract preparation
harmful pollutants from the environment using modern tech- Freshly collected Indigofera tinctoria plant leaf (50 g) was care-
nologies. The chemical structures of MO and MB dyes are fully washed with distilled water to remove impurities. The
shown in Fig. 1.
cleaned leaf was homogenically ground in an electric mixer by
Counterfeiting is a major problem owing to its wide-ranging adding 100 mL of double-distilled water. The mixture was
ꢀ
impact on various companies, governments, and customers. continuously heated at 30 C, stirred for 1 h and then ltered,
Therefore, it is mandatory to produce anti-counterfeiting agents rst by using Whatman lter paper and then by centrifugation
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at 10 000 rpm for 15 min; nally, the supernatant was collected operating voltage of 200 kv using a carbon-coated Cu grid. The
and used as a carbon source for the synthesis of NFCNDs.
electronic states and elemental analyses of the NFCNDs were
determined by recording XPS spectra using a Kratos Axis XPES
spectrometer containing Al Ka X-rays as an electron beam
source. Raman spectra were obtained using a Renishaw Raman
scope at 633 nm (lex).
2
.3 Synthesis of NFCNDs
For the green synthesis of NFCNDs, 49 mL of Indigofera tinc-
toria leaf extract was mixed with 1 M solution of aq. NH (1
3
mL), which was transferred to a 100 mL autoclave (Teon-
ꢀ
lined) and placed inside a hot air oven at 180 C for 12
2.5 Quantum yield measurement
hours. Aer 12 h of the hydrothermal process, the autoclave
was allowed to cool to room temperature; it was then carefully
opened, and the formation of NFCNDs was visually conrmed
by the observed color change from light green to dark brown.
The obtained solution containing NFCNDs was ltered by
Whatman 40 lter paper and further subjected to centrifuga-
tion at 10 000 rpm for 40 min to afford pure NFCNDs without
large carbon residues or unreacted organic moieties. The ob-
tained NFCNDs solution was further ltered by a 0.2 mm
membrane to afford the pure NFCNDs, which were collected in
The quantum yield (QY) of the prepared NFCNDs was calculated
using eqn (1). The QY was calculated using the reference of
0
3
.1 M quinine sulfate (QY ¼ 0.54%) dissolved in H
2
SO
4
at
39
70 nm.
2
2
Q ¼ Q (I /I )(h /h )(Astd/Ax)
(1)
x
std
x
std
x
std
where Q
while I
and Astd are the optical densities, and h
x
and Qstd are the quantum yields of NFCNDs and QS,
x
and Istd are the uorescence emission intensities, A
x
2
2
x
and hstd are the
ꢀ
an amber glass bottle and stored at 4 C for further charac-
terization and applications.
refractive indices of the reference and NFCNDs, respectively.
2.4 Instrumentation
2.6 Evaluation of the photocatalytic reduction of NFCNDs
The synthesized NFCNDs were examined and conrmed by The reduction of MB and MO dyes using sodium borohydride
various spectroscopic studies. The characteristics of the (NaBH ) with the help of the prepared NFCNDs catalyst was
NFCNDs were analyzed by UV-visible spectra (Shimadzu UV- performed under the irradiation of visible light produced by
600 spectrophotometer) and a HORIBA JOBIN YVON F- a 300 W xenon lamp. For conducting the photocatalytic reac-
4
2
luromax-4 spectrouorometer, respectively. The recorded tion, 1 mL of freshly prepared ice-cold NaBH (0.05 M) was
4
absorption and emission data were plotted using “UVPROBE’’ separately added to 14 mL (10 ppm) of newly prepared MB and
soware. The various functionalities present on the synthesized MO dye solution, and 15 mL of DD water was also added to the
NFCNDs were explored by an Agilent Resolution FTIR spec- mixture. To start the catalytic reduction reaction, the identical
ꢁ1
trometer in the operating range between 400 and 4000 cm . A mixture of the initial reaction solution was taken, and 60 mL of
PAN analytical X'Pert power diffractometer was used to examine catalyst (NFCNDs) was added to the MO and MB reaction
the amorphous nature of the NFCNDs using a Cu Ka radiation mixtures. The solution was placed at 80 mm away from the
ꢀ
source with a range of 10 to 80 (2q). The structural morphology visible light source (xenon lamp) and irradiated with slow stir-
of the NFCNDs was analyzed by a JOEL/JEM 2100 high resolu- ring until dye decoloration occured. A 2.5 mL sample was taken
tion transmission electron microscope (HR-TEM) at an at linear time intervals, and the reduction dynamics (absorption
Scheme 1 Schematic of the synthesis and application of NFCNDs.
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Fig. 2 (a) UV-vis absorbance spectra of the plant extract and prepared NFCNDs; (b) fluorescence excitation spectrum of the prepared NFCNDs.
intensity) were observed using UV-vis spectroscopy (Shimadzu was performed; then, the ink was allowed to dry in ambient air.
UV-2600).
Aer 2 h, the dried paper was examined under UV light at
65 nm and the obtained visible images were captured using
a camera.
3
2.7 Preparation of NFCNDs uorescent ink
For the preparation of the anti-counterfeiting uorescent ink,
a 5 mL NFCNDs sample was dissolved in 10 mL of 5% polyvinyl
alcohol under ultrasonic treatment for 10 min. This mixture was
3
Results and discussion
magnetically stirred for 2 h until a high-viscosity ink formed. The leaf extract of I. tinctoria and aq. NH were employed as the
3
Next, the prepared ink was transferred into an empty rell tube, carbon and nitrogen source, respectively. The formation of
and hand-writing or drawing of images on normal lter paper NFCNDs was preliminarily conrmed thorough visual
Fig. 3 Fluorescence spectra of the prepared NFCNDs at different excitation wavelengths from (a) 270–310 nm and (b) 310–400 nm and (c) anti-
Stokes shift of the NFCNDs.
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observation of the color change. The reaction time and of the synthesized NFCNDs indicated emission of cyan color
temperature play crucial roles in the formation of highly uo- (blue luminescence) under UV light at 365 nm.
40
rescent and stable NFCNDs with desired properties. The
graphical representation of the hydrothermal synthesis of of the synthesized NFCNDs at various excitation wavelengths.
NFCNDs is given in Scheme 1. With increasing excitation wavelengths (lex) from 270 nm to
Fig. 3 displays the uorescence emission spectral responses
4
4
00 nm, the highest intensity of emission (lmax) was found at
00 nm. The uorescence intensity (FL) was progressively
3.1 UV-visible and uorescence spectra of the synthesized
increased by increasing lex from 270 to 310 nm (Fig. 3a) owing
to the pi–pi* transitions of the sp2 carbon cores. Furthermore
NFCNDs
Fig. 2(a) displays the UV-vis spectrum of the carbon source (I.
tinctoria leaf extract) and prepared NFCNDs in aqueous media.
The absorption peak for I. tinctoria leaf extract appeared at
(Fig. 3b), the emission intensity gradually decreased with
increasing lex from 310 nm to 400 nm, and the lmax of 400 nm
was observed at 310 nm. This specic uorescence property of
the NFCNDs may arise due to the anti-Stokes uorescence
279 nm owing to the pi–pi* transition; it was obtained due to
presence of polyphenols in the leaf extract and the aq. ammonia
added to the reaction mixture, and then carbonization took
place via condensation reactions to nally afford the NFCNDs.
(l
max(ext) > lmax(em)), where two or more photons are equally
absorbed by carbon particles, resulting in the emission of
41
44
light. The uorescence quantum yield (QY) for NFCNDs was
found to be 12.6%. In our previous work, we had reported the
preparation of self-passivated CDs using rice bran as a carbon
source, and the obtained QY was 7.4%. It is conrmed that
nitrogen is an effective functional moiety for the functionali-
zation of CNDs, which would be helpful to alter their optical
properties. For clear understanding, a comparison of the QYs
and uorescence nature of CDs prepared from different
precursors with those of the synthesized NFCNDs is given in
Table 1 (ESI).† Moreover, the highly uorescent nature of the
NFCNDs indicates that they are the most suitable probe for
The two main characteristic peaks appeared for NFCNDs at
71 nm and 304 nm, which correspond to the pi–pi* transition
for the sp2 carbon (C]C bonds) and n-pi* transitions of the
2
carbonyl and amine functionalization on the NFCNDs surface,
42
respectively. The UV-vis absorption values of the NFCNDs
spectrum showed a red shi due to the introduction of
ammonium functionalities by the addition of ammonia solu-
tion during the synthesis, and this could have efficiently con-
verted the phenolic (–OH) groups into ammonium phenolate
43
ions. The uorescence excitation spectra of the NFCNDs
Fig. 2b) exhibited peaks at 310 nm, respectively, which
evidently conrms the n–pi* transition. The UV-visible spectra
(
Fig. 4 (a) TEM images of the formed NFCNDs at 20 nm, (b) histogram of the NFCNDs, (c) high-resolution image of the lattice fringes, (d) SAED
outline of the NFCNDs.
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3.3 X-ray diffraction
Fig. 5 shows the X-ray diffraction (XRD) pattern of the formed
NFCNDs. The XRD spectrum provides a wide peak at 2q values
ꢀ
ꢀ
ꢀ
of 22.73 , 43.6 and 47.7 for the C (002), C (100) and C (101)
47,48
planes of graphitized carbon core, respectively.
The amor-
phous phase of the NFCNDs is conrmed by the appearance of
ꢀ
a broad peak at 2q ¼ 22.73 . Bragg's equation (d ¼ nl/2 sin q)
was used to calculate the d-spacing (interlayer distance) value
for the formed NFCNDs. Using Bragg's equation, the d-spacing
values were found to be 0.38 nm and 0.21 nm, which corre-
spond to the C (002) and C (100) planes, respectively. These
results could be associated with the calculated-spacing values
49
from the HR-TEM lattice fringe images (Fig. 4(c)). These
observations clearly conrmed that the formed NFCNDs
generally have an amorphous structure due to the presence of
well-dened graphitic carbon cores.
Fig. 5 XRD spectrum of the NFCNDs.
3.4 FTIR spectroscopy
The FT-IR spectral responses of I. tinctoria extract and the
formed NFCNDs are depicted in Fig. 6. Two absorption bands
appeared in the IR spectrum of I. tinctoria leaf extract at 3336
sensing, multi-color bio-imaging, and drug delivery
applications.
45
ꢁ1
and 1634 cm , which belong to the –OH str and –C]O str,
respectively. The prepared NFCNDs showed absorption peaks at
ꢁ
1
3
387, 1652, 1456, and 1261 cm , corresponding to the –OH/–
3.2 Morphology and size of the NFCNDs
NH, –C]C (sp2 carbon), –C–N (N functionalization), and –C–O–
C functionalities; it was clearly observed that the functional
moieties of I. tinctoria leaf extract were transferred due to the
hydrothermal reaction, and the nitrogen was also functional-
The morphological structure and particle size of the formed
NFCNDs were inspected by HR-TEM. Fig. 4a displays the
spherical morphology of the formed NFCNDs, which clearly
indicates that the carbon particles were evenly spread on the Cu
grid. Fig. 4b shows the size distribution chart (ranging from 1.0
to 10 nm) and depicts that the formed NFCNDs have an average
size of around 4 nm. The higher magnication TEM image
50,51
ized in the NFCNDs.
These FT-IR results revealed that acid
functionalities, carbonyl groups, and amide, amine, and
hydroxyl moieties were present in the aqueous soluble NFCNDs.
(
Fig. 4c) of the NFCNDs clearly displays the lattice fringes, and
3.5 Raman spectra
the calculated interlayer distance (d-spacing) is about 0.21 nm
46
parallel to the (100) planes of graphite. Furthermore, Fig. 4d Fig. 7 presents the Raman spectrum of the prepared NFCNDs;
shows the selected area electron diffraction (SAED) outline of this is the most valuable technique for the examination and
the NFCNDs, which conrmed the amorphous phase of carbon conrmation of carbon-related nanomaterials. The Raman
33,34,40
ꢁ1
ꢁ1
by the formation of clear circles.
spectrum depicts two peaks at 1337 cm and 1519 cm , which
Fig. 6 FTIR spectra of the I. tinctoria extract (red line) and NFCNDs (black line).
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ꢀ
a consequent weight loss was acquired from 300–400 C, which
was ascribed to the desorption of the hydroxyl and carboxyl
ꢀ
groups. When the temperature was raised to 700 C, a 60%
weight loss was observed, which could be due to the elimination
of functional groups on the carbon skeleton of the prepared
54
NFCNDs.
3.7 X-ray photoelectron spectroscopy
The diverse surface electronic states and elemental composition
of the NFCNDs were explored by X-ray photoelectron spectros-
copy (XPS). Fig. 9a depicts the survey spectrum of the NFCNDs,
which displayed the C 1s (carbon), N 1s (nitrogen), and O 1s
(
oxygen) peaks at 285, 399, and 530 eV, respectively. The C 1s
spectra were deconvoluted into three distinct peaks at 284.5,
85.7, and 288.1 eV, as shown in Fig. 9b, which were ascribed to
2
52
C–H, C]C (sp2), and C–N bonds, respectively. Fig. 9c
Fig. 7 Raman spectra of the NFCNDs.
demonstrates the deconvoluted N 1s spectra, which show two
diverse peaks at 398.3 and 399.8 eV owing to the amine (C–NH
2
)
and amide (O]C–NH ) groups existing in the formed NFCNDs.
2
belong to the D band (defect region) and G band (graphitic The high resolution (O 1s) spectrum (Fig. 9d) provides two
region) in the NFCNDs, respectively. The D band arises due to disparate peaks at 531.5 and 532.1 eV, which can be attributed
sp3 defects, and the appearance of the G band indicates that the to the C]O and C–OH/C–O–C functionalities in the NFCNDs,
52
55
NFCNDs are mostly composed of sp2 carbon cores. The respectively. Finally, the XPS spectrum suggests that the
graphitic nature of the formed NFCNDs was determined by the atomic percentages of the elements in the NFCNDs were C
degree of graphitization. The intensity ratio between the D and (74.6%), N (8.2%), and O (17.2%). From the overall discussion,
53
G bands (I
D
/I
G
) is less than 1 for ideal graphite. Here, the I
D
/I
G
it is evident that amine, hydroxyl, carboxyl and amide func-
value was found to be 0.49, which depicts the moderate graphite tional groups are present in the formed NFCNDs.
structure of the prepared NFCNDs and also conrms the exis-
tence of minimal surface defects in the graphitic structure of
the NFCNDs.
4
Applications of the NFCNDs
4.1 Photocatalytic activity of the NFCNDs
3
.6 Thermal stability of the NFCNDs
The photocatalytic activity of the formed NFCNDs was evaluated
through catalytic reduction of MB and MO dyes. The absorption
intensity of blue MB dye was at 664 nm and that of orange MO
was at 466 nm. The prepared NFCNDs served as a green catalyst
in this environmental remediation process. Photocatalytic
reduction of MB and MO dyes performed under light irradiation
Thermogravimetric analysis (TGA) was employed to determine
the thermal stability of the synthesized NFCNDs, as shown in
Fig. 8. In the beginning, a 12% weight loss occurred owing to
the decomposition of the water molecules. Further,
with an ice-cooled conditioned reducing agent (NaBH ) and in
4
the presence and absence of NFCNDs (green catalyst) was
investigated in this work. Fig. 10a shows the UV-vis absorption
spectra of MB and MO aer UV light irradiation using NaBH
4
without prepared NFCNDs for a regular period. The intensity of
MB and MO (Fig. 10a and d) was found to decrease to the
minimum extent aer 90 min and 135 min, respectively, which
displays no signicant change in intensity; this indicates that
complete reduction does not occur. On the other hand, when
the prepared NFCNDs (60 microliters) were added to the
mixtures of MB and MO containing sodium borohydride,
a signicant reduction was observed in the intensity of the dyes
(Fig. 10b and e). Finally, the reduction of MB was completed
within 10.5 min, and that of MO was completed in 12 min by
addition of the efficient green catalyst NFCNDs. Further, the
photocatalytic degradation of MB and MO dyes was investigated
using undoped CNDs, and the results are given in ESI Fig. 1.†
Even aer 120 min, no sigcant reduction was attained while
using undoped CDs as a catalyst. In the case of using NFCNDs
Fig. 8 TGA plot of the NFCNDs.
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Fig. 9 (a) Full survey spectrum of the synthesized NFCNDs (inset: elemental percentage graph); (b) C 1s, (c) N 1s, (d) O 1s.
Fig. 10 UV-vis spectra of the MB/MO reduction using sodium borohydride (a and d) without NFCNDs and (b and e) with NFCNDs; (c and f)
pseudo first order kinetics plots for the reduction of MB/MO with NFCNDs.
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as a catalyst, the photocatalytic degradation of MB and MO was reaction of MB and MO dyes. During this reaction, electrons
ꢁ
completed in a very short period of time compared to the from BH4 ions transfer from the VB to the CB in the NFCNDs.
undoped CNDs catalyst. The results conrmed that the catalytic Due to this electron transfer, a hole is created in the VB, and this
activity of the NFCNDs effectively improved the rate of the hole reacts with H O and O to produce hydroxide (OHc) and
2
2
4 2 2
reduction of the MB and MO dyes using NaBH . This could be oxygen (O c) radicals, simultaneously. If only O c radicals are
achieved by decreasing the activation energy of the reduction formed, the reduction reaction takes place at a comparatively
reaction owing to the incorporation of NFCNDs into the slow rate. The formation of both radicals led to the reduction
56
reduction process.
reactions of MB and MO, which were thus completed in a short
The determination of the rate of the reaction (Kcat) for the dye period. NFCNDs efficiently served as a mediator for these elec-
reduction of MB/MO by the prepared NFCNDs (catalyst) could tron transformations, and this suggests reduction of the energy
be calculated from the pseudo-rst-order kinetics. Fig. 10c and f obstacles among the starting materials (reactants) and nal
57
show the pseudo-rst-order kinetics plots for the reduction of products. Hence, the wide surface to volume ratio and effec-
MB and MO using NFCNDs. The rate constants (Kcat) and tive electron withdrawing potential of the synthesized NFCNDs
2
coefficients (R ) could be obtained from the slopes of the linear are the major reasons for this remarkable catalytic photo-
2
40
plots; the Kcat and R values of MB and MO reduction were catalytic reduction.
ꢁ
1
ꢁ1
found to be 0.2483 min and 0.9136 and 0.2392 min and
Fig. 11a displays the possible mechanisms of the reduction
0
.9074, respectively. The acquired Kcat values for the reduction of MB and MO. Finally, photographs of the decolorized dye
of MB and MO by the NFCNDs were higher than those in earlier solutions are shown in Fig. 11b, and schematics of the reduc-
reports. tion reactions of the dyes are also shown in Fig. 11c. The toxic
This photocatalytic catalytic reduction of MB and MO could MB and MO dyes were converted into eco-friendly products
be explored using the well-known Langmuir Hinshelwood through the green catalyst. The results conclude that the
56
mechanism. The MB and MO dyes function as electrophiles, prepared NFCNDs were proved to be an effective alternate for
ꢁ
and the borohydride (BH
4
) ions act as a nucleophile. Concur- metal nanocatalysts in the removal of environmental
58
rent adsorption takes place above the surface of the NFCNDs, pollution.
and the transformation of electrons occurs from BH
ꢁ
4
ions to
MB and MO through the green catalyst. The synthesized
NFCNDs act as an electron transfer mediator between dye
molecules and BH4 ions. This only leads to the reduction
4
.2 NFCNDs as a uorescent ink
ꢁ
Finding an innovative application which is suitable for devel-
oping materials such CNDs is economically important.
Fig. 11 (a) Possible reduction mechanism. (b) Photographs of the initial dyes and treated water. (c) Reaction reduction mechanism of MB/MO.
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Fig. 12 Images of fluorescent ink-written paper: (a) naked eye view and (b) under UV light (365 nm).
4
Currently, a wide range of the dyes, coloring agents, and poly- NaBH . Complete reduction was attained within 10.5 min for
mers that are used for various industrial and forensic applica- MB and 12 min for MO using the NFCNDs green catalyst.
tions are extremely toxic and also have high cost. Here, the Moreover, the NFCNDs were utilized as a biocompatible uo-
synthesized NFCNDs possess unique properties, high biocom- rescent ink and also emitted cyan color under UV light. Overall,
patibility, strong uorescence, a low/nontoxic nature, and the study enlightened the uses of a novel green catalyst for
transparency in the visible region, which encourages the use of environmental remediation and also identied a promising
this carbon nanomaterial as an alternate candidate for various probe for anti-counterfeiting applications.
important applications. In this recent scenario, the solubilized

uorescent NFCNDs were utilized as a uorescent ink in anti-
Data availability
counterfeiting applications. The prepared uorescent ink con-
taining NFCNDs was lled in an empty ink pen; some letters All data generated or analyzed during this study are included in
were written on white paper, allowed to dry at room temperature this published article.
and placed under UV light (365 nm). Fig. 12a displays the image
of the white paper in naked eye view, and no visible letters can
be observed; however, Fig. 12b depicts the image of the same
Author contributions
white paper under 365 nm UV light, which shows clearly visible
blue uorescent patterns of the written letters when subjected
to ultra-violet light irradiation, and these letters can be easily
removed by water. The visible patterns evidently indicate the
cyan uorescence emission of the NFCNDs and are also helpful
to extend the applications of the material to the forensic and
anti-counterfeiting elds.
Jothi Vinoth Kumar: conceptualization, methodology, writing –
original dra. Ganesan Kavitha: data curation, formal analysis.
Rajaram Arulmozhi: visualization. Velusamy Arul: data cura-
tion. Natarajan Abirami: supervision, investigation.
Conflicts of interest
Authors declared that there are no conicts of interest.
5
Conclusion
Acknowledgements
Here, we reported the synthesis of highly uorescent nitrogen
functionalized CNDs using Indigofera tinctoria leaf extract as The authors wish to acknowledge DST-FIST (fund for improve-
a novel carbon source and aq. ammonia as a nitrogen source by ment of S&T) for the nancial assistance at the Department of
a cost-effective, uncomplicated, and eco-friendly single-step Chemistry, SRM IST (grant no SR/FST/CST-266/2015(c)).
hydrothermal process. The obtained NFCNDs exhibits good
water solubility, greater QY, and strong uorescence. The
optical properties were studied by UV-visible and uorescence
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evidently conrmed the functionalization of nitrogen on the
CNDs surface. From the environmental point of view, the study
describes the catalytic efficiency of NFCNDs explored for the
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