(63g) Novel Microwave Synthesis Carbon Dots from PINE Bark and Its Application for Heavy Metal Sensing

NOVEL
MICROWAVE SYNTHESIS CARBON DOTS FROM PINE BARK AND ITS APPLICATION FOR HEAVY
METAL SENSING

Sanni S.O., ReddyPrasad P.,
Ofomaja A.E.

Biosorption and Waste water Treatment Research
Laboratory, Department of Chemistry,
Faculty of Applied and Computer Sciences, Vaal University of Technology, P. Bag X021, Vanderbijlpark-1900, South Africa.

INTRODUCTION

Microwave pyrolysis for the
production of carbon has the advantages of being cheap, produces high yield of
carbon and is absolutely a green synthetic method [1]. In recent times, the use
of microwave pyrolysis have been extended to the preparation of carbon dots.
Carbon nanodots (C-dots) falls under the class of carbon nanoparticles with
size below 10 nm[2]. C-dots are carbon materials which processes high water
solubility and strong fluorescence depending on their size, edge, shape,
surface ligands, and defects. Since C-dots preparation can be achieved from
difference carbonaceous sources and by varying a range of parameters during
their synthesis, the properties of the final product may vary depending on
these factors. C-dots have sp² characteristics and possess several
functional groups on their surfaces that makes them highly hydrophilic and
easily functionalized with various organic, polymeric, inorganic, or biological
spices[3]. Several synthesis methods that have been explored includes electrochemical synthesis, combustion/thermal/hydrothermal/acidic
oxidation, supported synthesis, microwave/ultrasonic, arc discharge, laser
ablation/passivation, and plasma treatment[4]. C-dots processes excellent such
as water dispersibility, chemical stability and photostability, ease of modification,
excitation-dependent multicolor emission, low toxicity, and good cell
permeability, great efforts have been paid to their potential applications in biosensing,
bioimaging, drug delivery, and other biological related aspects.

METHODS

The carbon dots (C-dots) with green
luminescence was synthesized from microwave assisted pyrolysis of pine bark
waste our carbon source at 800, 1000 1200 W for 1 Hr. The dots produced were
characterized using FTIR, TGA, XRD, UV-Vis and Raman spectroscopy. The dots
were applied for heavy metal sensing.

 

RESULTS
AND DISCUSSION

The functional groups present on the
C-dots (Fig.1) includes -OH
stretching band mode at 3413 cm^-1, C-H
stretching at 2934 and 2845 cm^-1,stretching peaks of
C=O, C=C at 1731, 1604 and 1437 cm^-1. The XRD pattern (Fig 2a &b) of produced C-dots
depicts sharp peak at 20.9^o and 26.8^o, which is highly
crystalline cores and possess amorphous nature of C-dots. This peculiar peak is
ascribed to 002 of the graphitic carbon, which in line with inter-planar
spacing of 3.77 Å with higher spacing than bulk graphite at 3.34 Å. The
thermogravimetric and differential thermal analysis display (Fig 3) that the C-dots possess thermal
stability more than 100 ^oC and mass losses occurs afterwards. The
thermal analysis depicts 3 endothermic peaks at 331, 542 and 735 ^oC,
which are ascribed to oxygenated groups such as carboxyl, hydroxyl, carbonyl
and phenols. The optical properties of the synthesize C-dots were explored
using UV-vis absorption and photoluminescence (PL) spectroscopy. The UV-vis
spectra display (Fig 4)intense
absorption peak in the range of 260 to 290 nm, which is ascribed to π-π^*
transition of the aromatic sp2 domain and also presence of multiple
polyaromatic chromophores. A shoulder peak at 330 nm which is ascribed to
n-π^* transitions of C=O and structural defects of the C-dots.
PL spectrum of the C-dots (Fig 5a)exhibits
an intense maximum emission peak at 432 nm upon excitation at 330 nm, which
also shows characteristic yellowish green color under sunlight and intense blue
coloration UV lamp (365 nm). The PL properties of as prepared C-dots was varied
at (Fig5b) different excitation
wavelength from 300 to 405 nm. There was red-shifting in emission peak as the
excitation peak was increased from 300 to 405 nm and also simultaneously
decrease in fluorescent intensity, which is ascribed to quantum effect and
surface defects. This behavior of the prepared C-dots is dependent on
excitation-fluorescence, which is accordance with other report [13]. The
quantum yield of synthesized C-dots was measured to be 17% at maximum
excitation at 330 nm

Fig.6
shows the Raman spectrum of the PB-GQDs, which has been usually used to confirm
the quality of the prepared PB-GQDs. There is two major Raman peaks at 1370 cm^-1
(D band), 1580 cm-1 (G band). The peak of G band corresponds to the Sp²
bonded carbon atoms in a two-dimensional (2D) hexagonal lattice and the peak of
G band is associated with the vibrations of carbon atoms with dangling bonds in
the termination plane of tangled graphite or glassy carbon. The D/G ratio band
ratio is ID/IG: 0.68 is confirm the formation of graphene like structure from synthesized
pine bark.

Fluorescence detection of
Cu²⁺ ion

The
detection of Cu²⁺ was performed at room temperature in phosphate
buffer (PBS) solution pH 4.3. 1.0 mL of PB-CQDs solution was added into 0.5 mL
of PBS buffer, followed by the addition of 1.0 mL different concentrations of
Cu²⁺. The fluorescence emission spectra were recorded after reaction
for 5 min at room temperature (~28 ^oC).

Selectivity and sensitivity studies

Moreover sensitivity and selectivity is additional significant
parameter to assess the performance of the sensing system. Consequently, we
examined the fluorescence intensity changes in the presence of representative
metal ions such as including Cu²⁺, Hg²⁺, Ca²⁺,
Al³⁺, Ba²⁺, Mn²⁺, Ni²⁺, Pb²⁺,
Co²⁺, Zn²⁺, Fe³⁺, Cs²⁺and Mg²⁺,
of each at a concentration of 15.0 µg mL^-1 were reacted with a PB-CQDs
solution. Fig. 7 A displays the relative change in PL intensity of PB-CQDs
in the presence of various metal ions. Fe²⁺,
Mn²⁺ and Hg²⁺ ions cause the slight PL changes (defined
as the relative change of PL intensity in 60–80% as compared to blank) which
can be attributed to the nonspecific interactions between the carboxylic and/or
amine groups and the metal ions. Among these metal ions, Cu²⁺ displays
the strongest PL quenching effect on PB-CQDs,
assigning to the special coordination between the Cu²⁺ ions and the
phenolic hydroxyl and/or amine groups of PB-CQDs which has been widely used for the detection of Cu²⁺ ions or colored
reactions in traditional organic  chemistry.
Such a specific fluorescence quenching effect may originate from the strong
interactions between Cu²⁺ ions and the surface groups of PB-CQDs
which transfer the photoelectrons from PB-CQDs
to Cu²⁺ ions. These results clearly
demonstrate that the PB-CQDs-based Cu²⁺
sensor is highly selective to Cu²⁺ over the other metal ions. Fig.8.
Fluorescence responses of CCDs to the addition of different metal ions under
acetate buffer (pH 4.3). The concentration of each metal ion is 15 µg mL^-1. F₀ and F correspond
to the fluorescence intensities of PB-CQDs at emission wavelength of 440 nm
excited at 320 nm in the absence and presence of metal ions, respectively.

As
shown in Fig. 3, the fluorescence intensity of PB-CQDs gradually decreased with
the Cu²⁺ concentration. The
fluorescence intensity displayed liner responses with the Cu²⁺ concentrations, with two linear
ranges of 0.0‒14.0 μg mL^-1 and 15.0‒25.0 μg mL^-1
(Fig.3B). The linear equations are F/F0 =0.9827‒0.8485C_Cu2+ (R = 0.993) and F/F0 = 0.7565–0.0033
C_u2+ (R = 0.996),
respectively.

REFERENCES

[1]   
W. B. Lu, X. Y. Qin, S. Liu, G. H. Chang, Y.
W. Zhang, Y. L. Luo, A. M. Asiri, A. O. Al-Youbi, X. P. Sun, Economical, green
synthesis of fluorescent carbon nanoparticles and their use as probes for
sensitive and selective detection of mercury(II) ions, Anal. Chem. 2012, 84, 5351-5357.

[2]   
J. Jiang, Y. He, S. Li, H. Cui, Amino acids as the source for producing
carbon nanodots: microwave assisted one-step synthesis, intrinsic
photoluminescence property and intense chemiluminescence enhancement. Chem
Commun 2012, 48, 9634–6.

[3]   
Y. Wang, A. Hu, Carbon quantum dots: synthesis, properties and
applications, J. Mater. Chem. C 2014,
2, 6921–6939.

[4]     C. Liu, P.
Zhang, X. Zhai, F. Tian, W. Li, J. Yang, et al., Nano-carrier for gene delivery
and bioimaging based on carbon dots with PEI-passivation enhanced fluorescence,
Biomater. 2012, 33, 3604-13.

Figures:

Fig 1. FTIR spectra of
as-prepared C-dots from Pine Bark

(a)
 (b)

Fig. 2a & b. XRD pattern of C-dots.

Fig 3. Thermal
curves for C-dots under N₂ atmosphere.

Fig 4. UV-vis absorption of C-dots.

 (a)
 (b)

Fig 5a. PL
excitation and emission wavelength (b) PL emission spectra at different
excitation wavelength from 300 to 405 nm in 15 nm increments.

Fig. 6. Raman Spectra of PB-CQDs

Fig.7. Fluorescence responses of CCDs to
the addition of different metal ions under acetate buffer (pH 4.3).

 

Figure
8. Fluorescence titrations of the y-CDs with Cu²⁺ from 0 to 25 μg mL^-1.
(Influence of Cu²⁺ concentration on the fluorescence intensity of PB-CQDs
(top to bottom: 0, 1, 2, 3, 4, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23 and 25.0
μg mL^-1)