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Understanding the interaction of carbon quantum dots with CuO and Cu2O by fluorescence quenching
Journal of Hazardous Materials 369 (2019) 17–24
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Understanding the interaction of carbon quantum dots with CuO and Cu2O by fluorescence quenching
T
D. Bharathia, R. Hari Krishnab, , B. Siddlingeshwara, , Darshan Devang Divakarc, Abdulaziz Abdullah Alkheraifc ⁎
⁎
a
Department of Physics, M. S. Ramaiah Institute of Technology, Bengaluru, 560054, Karnataka, India Department of Chemistry, M. S. Ramaiah Institute of Technology, Bengaluru, 560054, Karnataka, India c Dental Biomaterials Research Chair, Dental Health Department, College of Applied Medical Sciences, King Saud University, Riyadh, 11433, Saudi Arabia b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Carbon quantum dots Fluorescence quenching CuO nanoparticles Stern-Volmer
In spite copper oxide being one of the essential micronutrient, copper oxide in its nano size is found to be toxic in nature; this instigates for the detection of copper oxides in trace levels. In the present study, we demonstrate simple cost effective detection method for CuO/Cu2O using carbon quantum dots (CQD) by fluorescence quenching technique. CuO/Cu2O nanoparticles are synthesised by mere variation of fuel ratio by solution combustion technique. The resulting oxides are characterized by various analytical and spectroscopic techniques. Powder X- ray diffraction (PXRD) results reveals that samples prepared with oxidizer to fuel (O/F) ratios 1:1, 1:1.5 and 1:2 showed pure nano CuO, major CuO phase (minor Cu2O) and major Cu2O phase (minor CuO) respectively. Further, the samples prepared using 1:1 O/F ratio and calcinated at 700 °C showed highly crystalline CuO phase. In order to study the interaction of CuO/ Cu2O with CQDs the fluorescence quenching method has been employed. The bimolecular quenching rate constants for the samples prepared with different O/F ratios have been measured. The interaction between CQDs and copper oxides, indicates fluorescence quenching greatly depends on the oxidation state of the copper oxide and can be a promising method for detecting CuO/Cu2O through CQDs.
⁎
Corresponding authors. E-mail addresses: [email protected] (R.H. Krishna), [email protected] (B. Siddlingeshwar).
https://doi.org/10.1016/j.jhazmat.2019.02.008 Received 16 November 2018; Received in revised form 25 January 2019; Accepted 3 February 2019 Available online 04 February 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
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1. Introduction
us to investigate the interaction of CQDs with copper oxide nanoparticles prepared by solution combustion synthesis with different O/F ratios. Since CQDs comprises of non-toxic elements, which makes them particularly useful and promising bio-analytical tool. The present study would open up the possible usage of nanosensor for bio-analytical applications. In this paper, we have used both steady state and time resolved methods in aqueous medium to investigate the quenching of CQDs by CuO/Cu2O nanoparticles at room temperature. This study is the first attempt that is dedicated to investigate the interaction between CQDs and CuO/Cu2O nanoparticles.
The oxides of transition metals are an important class of semiconductors that have wider applications in magnetic storage media, solar energy transformation, electronics, and catalysis. Copper oxide has attracted greater attention due to its fascinating properties. Even though copper oxide nano particles have proved their use in various applications; the major disadvantage for their use is due to their potentially toxic effects. Copper is an essential trace element in both plants and animals, including humans. At an elevated concentrated level cupric oxide in nano level is toxic, though it is an essential micronutrient for aquatic organisms [1]. Several studies carried out in recent years have explained that copper in relatively low levels may generate oxidative stress in marine cnidarians [2] and also leads to several brain diseases such as Menkes disease, Parkinson’s disease, Wilson disease, and Alzheimer’s disease [3,4]. The serious neurodegenerative diseases are caused even if there is a little attenuation in the cellular homeostasis of copper that might produce reactive oxygen species [5–8]. Due to accumulation of cupric oxide caused by pollution in the environment, cupric oxide is considered as the major pollutant by the U.S. Environmental Protection Agency (EPA) [9]. Feng et al. [10] have discussed on the functionality, toxicity of the metal oxide nanoparticles on the central nervous system. Karlsson et al. [11,12] have focussed on comparative study of different metal oxide nanoparticles and carbon nanotubes, and found that cupric oxide nanoparticles are more toxic compared to other metal oxide nanoparticles. Chitrada et al. [13] have shown that CuO nano particles are more toxic than their micro sized counterparts at the same Cu concentration, with toxicities approaching those of the ionic Cu species. Margit et al. [14] explored that CuO nano particles agglomerate and settle rapidly in natural water and their toxicity is higher in natural water compared to standard media due to differences in composition. Also, using total reflection Xray fluorescence (TXRF) spectroscopy they have evaluated the effect of test medium on total Cu body burden of CuO nano particles exposed Daphnia Magna [15]. Therefore, there is a great demand for the purpose of controlling CuO nanoparticles induced toxic effect. Hence there is a need for a simple, reliable, green and selective strategy for the sensitive detection of cupric oxide level present in ecosystem and environment. The different methods involved in the detection of cupric oxide level includes electrochemical method [16], atomic absorption spectrometry (AAS) [17], colorimetric method [18], and inductively coupled plasma mass spectrometry (ICPMS) [19]. Compared to these methods detection of CuO by fluorescent method is the most facile and simple method that involves just monitoring the variation in the fluorescence intensity. Therefore, fluorescence quenching is one of the best methods for sensing applications or as a bio analytical tool [20]. Fluorescent Carbon Quantum dots (CQDs) are a fascinating class of zero-dimensional nanomaterials recently discovered which comprises of quasi-spherical carbon nanoparticles with size below 10 nm [21,22] and have drawn great research interest due to their high fluorescence emission in the visible spectral region with strong photostability, biocompatibility, low toxicity, and excellent solubility in aqueous medium [23–27]. Owing to their ease of synthesis and low toxicity, CQDs are used as one of the most efficient and stable nanosensors for detecting various types of analytes compared to other heavy metals contained semiconductor quantum dots and organic dyes [28–31].Quantum dots have remarkable attractive optoelectronic properties including their high emission quantum yields, size tuneable emission profiles. These properties were much different from those of the bulk systems, due to quantum confinement effects [32].Because of these unique properties, CQDs can be employed for many applications such as in drug delivery, bioimaging, biosensing, especially in fluorescent sensors by monitoring the changes of fluorescence intensity under external physical or chemical stimuli [33–37]. The interesting properties associated with CQDs sensitivity and the present need for the detection of harmful nano metal oxides, motivated
2. Experimental section 2.1. Preparation of CQDs by microwave method The chemicals used for synthesis of CQDs are of analytical grade and are used without further purification. The precursors used in this study, Ascorbic acid vitamin C, EDTA (Ethylenediaminetetraacetic acid), PEG200 (Poly ethylene glycol), Copper nitrate, diethyl oxalate, and hydrazine hydrate were purchased from Merck. To prepare CQDs by microwave assisted method [38,39], 1 g of ascorbic acid (vitamin C) and 0.06 g of EDTA are dissolved in 10 ml distilled water. To the resulting clear solution 1 ml Poly ethylene glycol (PEG 200) was added, and the solution was homogenised by stirring using magnetic stirrer for 15 min. The beaker containing the solution was introduced to the microwave oven and the reaction is allowed to occur for 1 h. After the reaction the solution is allowed to cool to room temperature and finally addition of 60 ml distilled water resulted in yellow CQDs in liquid form, with no obvious crystal lattice. 2.2. Preparation of copper oxide nanoparticles by solution combustion method To prepare copper oxide nano particles stoichiometric quantities of copper nitrate and the fuel oxalyldihydrazide (ODH) are calculated based on propellant chemistry. The stoichiometric ratio here means the ratio of oxidizer and fuel (O/F) and is set to one. The details of the calculations for taking the reactants are described elsewhere [40]. The petri dish containing aqueous mixture of copper nitrate and ODH is introduced into the pre-heated muffle furnace maintained at 500 °C. The solution first underwent dehydration followed by auto ignition resulting in smouldering type combustion. The whole process got completed within 5 min resulting in black colored porous product. Different samples of copper oxide are prepared using the above procedure. The O/F ratio is varied systematically between 1.0 and 2.0, to tune the oxidation state of the metal oxide. The powders obtained for different O/F ratios are labelled as: CuO-1 for propellant ratio (O/ F = 1.0), CuO-1.5 for fuel rich (O/F = 1.5), CuO-2 for extreme fuel rich (O/F = 2.0) and CuO-1:700 (for O/F = 1) and calcinated at 700 °C for 3 h. 2.3. Characterization of copper oxide nanoparticles The phase purity and crystallinity of nano materials are measured using a powder X-ray diffractometer (PANalytical X′Pert Pro) in the Bragg–Brentano θ-2θ geometry. For PXRD, the powder sample is placed in a zero background holder (ZBH). Flatness of the sample is ensured by pressing the powder against ZBH. Cu Kα (1.541 Å) radiation with a nickel filter is used to obtain the diffraction data. The morphology and structure of the samples are inspected using SEM (JEOL JSM-840 A). For performing SEM, a layer of Au is deposited on the samples using a sputter deposition set up. This is necessary to get SEM images since the samples are insulating. Transmission electron microscopy (TEM) analysis is performed on Hitachi H-8100 (accelerating voltage up to 200 kV, LaB6 filament) equipped with EDS (Kevex Sigma TM Quasar, USA). The sizes of the CQDs are determined by using Malvern 18
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instrument ZETASIZER Nano series. 2.4. Spectroscopic measurements The spectral absorption measurements were recorded using ST S-V IS miniature spectrometer and Analytik Jena Specord200 plus UV Vis spectrophotometer. The spectral emission measurements were carried out using HITACHI F-2700 spectrofluorimeter, the excitation and emission slit widths (each 5 nm) and scan rate (1500 nm/min) and PMT voltage 700 V were kept constant for all the measurements. 2.5. Time resolved fluorescence measurements The fluorescence lifetime measurements were carried out using a Horiba Jobin Yvon TCSPC lifetime instrument in a time correlated single photon counting arrangement. A 370 nm nano-LED was used as the light source. The pulse repetition rate was set to 1 MHz, and the detector response was ∼1.1 ns. The instrument response function was collected using a Scatterer (Ludox AS40 colloidal silica). The decay data were analyzed using IBH software. A value of 2 , 0.99 ≤ 2 ≤ 1.2, was considered as a good fit, which was further analyzed by the symmetrical distribution of the residuals. The average fluorescence lifetime ( avg ) values were obtained by the following equation [20] avg
=
Where
( ( i
n 2 i=1 i i ) n i=1 i i )
is the individual lifetime with corresponding amplitude
(1) i.
3. Results and discussion
Fig. 1. PXRD of CuO prepared with different ratios of fuel and oxidizer (a) CuO1 (b) CuO-1.5 (c) CuO-2 and (d) CuO-1-700 °C sample.
3.1. Powder X-ray diffraction (PXRD)
voids in CuO-2 has significantly increased compared to CuO-1 and CuO1.5. This can be attributed to the higher number of moles of gases liberated in fuel rich synthesis. The formation of voids and pores in the structure is very typical of combustion derived materials. The difference in the morphology of the CuO-2 may also be attributed to formation of Cu2O as major phase rather than CuO. Further, micrographs of CuO1:700 shows that the size of the particle agglomerations has increased and the sample lost its porous nature compared to CuO-1. The increase in the size of particle clusters in heat treated samples can be attributed to sintering effect that aids in the agglomeration of particle clusters thereby affecting the porous structure.
The powder X-ray diffraction patterns of the samples prepared with varying O/F ratios are shown in Fig. 1(a–d). The diffraction pattern for the sample prepared with O/F = 1 shows the diffraction pattern that can be readily matched with the CuO (JCPCDS-80-1917). The peaks are found to be broad and intense suggesting that the prepared CuO is in nanocrystalline form. On the other hand the samples prepared with O/ F = 1.5 & 2 showed the diffraction peaks corresponding to both CuO and Cu2O. It is also interesting to note that the samples prepared with O/F = 1.5 showed major CuO and minor Cu2O composition. However, when O/F = 2, Cu2O is found to be dominating phase with minor CuO phase. Further, the sample prepared by using O/F = 1 and calcinated at 700 °C showed pure highly crystalline CuO phase. From the above XRD results, it can be concluded that fuel quantity determines the fate of the copper oxide phase. With increase in the fuel quantity, Cu2O phase formation is favoured which can be attributed to the reducing atmosphere in fuel rich samples. In fuel rich samples the excess fuel taken creates the reducing environment due to high carbon content that aids in the stabilization of reduced copper oxide, Cu2O. CuO-1:700 °C showed pure CuO phase due to the calcination in open air atmosphere that completely oxidised the CuO phase and resulted in crystalline CuO.
3.3. High resolution transmission electron microscope measurements Transmission electron microscope (HRTEM) is employed to understand the influence of O/F ratio on particle size of CuO nano materials and the images are presented in Fig. 3(a–d). From the TEM images it is evident that with varying the fuel quantity there is significant variation in the particle size. CuO-1 shows agglomerated particles with varying sizes of particle clusters and the average size of the particles are found to be ≈20–30 nm. However, TEM images of CuO-1.5 and CuO-2.0 shows relatively bigger and irregular particles. The extent of agglomeration in these samples increased and the average size of the particles in CuO-1.5 and CuO-2.0 are found to be ≈60–80 nm. The increase in the particle size in samples prepared with fuel rich samples might be due to the increased exothermicity of combustion reaction due to larger extent of gaseous by products released. Further, the sample CuO-1:700 show that particle size has grown significantly and can be attributed to sintering effect during high temperature calcination.
3.2. Field emission scanning electron microscope measurements The morphology of the samples was investigated by field emission scanning electron microscopy and the micrographs are presented in Fig. 2(a–d). Micrograph of CuO-1 shows that the surface morphology exhibits agglomerated particles having fluffy and porous nature. The agglomerated clusters form interconnected masses and numerous voids of varying sizes. The surface morphology of CuO-1.5 also shows similar morphology as of CuO-1 and no significant changes are observed. However, it is worth noticing that the micrograph of CuO-2 shows highly porous structure and spongy like morphology. The number of
3.4. Relative quantum yield of CQDs Relative quantum yield ( 19
f)
for CQDs was determined using Eq. (2)
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Fig. 2. Scanning electron micrographs of (a) CuO-1 (b) CuO-1.5 (c) CuO-2 and (d) CuO-1-700 °C.
[41,42] f
=
r (I f
intensity, and ‘A’ is the area under the emission curve. The refractive index of the solvents used is represented by ‘n’. The measured quantum yield of CQDs is 42%, using the refractive index nf = 1.33(water) and nr = 1.497(Toluene). (Z)-N,N-diethyl-N’-(7-oxo-7H-benzo[de]anthracen-3-yl)acetamidine in Toluene ( r = 0.56) is used as the reference molecule [41].
Ar nf2 )
(Ir Af nr2 )
(2)
Where the subscript f and r refers to the CQDs and the reference molecule respectively, ‘∅’represents the quantum yield, ‘I’ is the integral
Fig. 3. Transmission electron microscope images of (a) CuO-1 (b) CuO-1.5 (c) CuO-2 and (d) CuO-1-700 °C. 20
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Fig. 6. S-V plot for the steady state and time resolved fluorescence quenching of CQDs by CuO nanoparticles (CuO-1.5, CuO-2, and CuO-1:700 °C). Table 1 S-V constant (Ksv), and bimolecular quenching rate parameter (kq) for CQDs with different CuO nano particles as quenchers. Excited state life time for CQDs 0 is 2 ns. Quencher M CuO-1 CuO-1.5 CuO-2 CuO-1:7000C
Fig. 4. (a) the absorption and (b) the emission spectrum of CQDs.
Ksv (M−1)
kq(x1010 M−1s−1)
150 27.5 15 25
7.5 1.37 0.75 1.25
Fig. 5. (a–d):Fluorescence quenching of CQDs in the presence of CuO nanoparticles (a): CuO-1, (b): CuO-1.5, (c): CuO-2 and (d) CuO-1:700 °C. 21
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presence of CuO nano particles as quencher. CuO nano particles of different ratiometric O/F ratios viz. CuO-1, CuO-1.5, CuO-2, and CuO1:700 °C were used in the range of 0–20 mM concentration. It was observed that fluorescence intensity of CQDs decreased by the addition of quencher as shown in Fig. 5(a–d). The fluorescence quenching mechanism is described by SternVolmer relation (S-V) [20]:
I0 = 1 + KSV [Q] = 1 + kq I
I0
I I
(3)
Where I0 and I are the emission intensities of CQDs in the absence and presence of CuO nano particles (CuO-1, CuO-1.5, CuO-2, and CuO1:700 °C), respectively. KSV is the Stern-Volmer constant related to the bimolecular quenching rate constant (kq ) by KSV= kq 0 , and 0 is the excited state lifetime of CQDs and [Q] is the concentration of CuO nano particles. Eq. (3) is applicable as long as the experimental results show linear variation. The S-V plot of CQDs for quenchers CuO-1.5, CuO-2, and CuO-1:700 °C NPs is shown in Fig. 6. We observe the linearity in SV plots in the concentration range 0–20 mM. The linear S-V plots in steady state measurements alone does not prove the dynamic quenching process but sometimes static process also results in linear S-V plot. In general, time resolved measurement is the most definitive method for differentiating static and dynamic quenching processes [43,44]. The inset of Fig. 6 shows the S-V plot for time resolved measurements. The phenomenon of quenching by the time resolved method also follows the S-V relation. Also, from the S-V plot it is observed that Cu2O is less accessible for CQDs for quenching compared to CuO-1.5 and CuO-1:700 °C. The values of kq (Table 1) and the linearity observed in both steady state and time resolved measurements for CuO-1.5, CuO2, and CuO-1:700 °C indicates the quenching of CQDs by these CuO nanoparticles is diffusion limited. Further, the experiments are performed in triplicates in order to check the repeatability of the interactions and statistical analyses was done for the obtained Ksv values (ESI Fig. S2). Form the figure it is evident that the standard deviation obtained for the triplicates are well within the acceptable error and proves the validity of the process.
Fig. 7. S-V plot for the steady state and time resolved fluorescence quenching of CQDs by CuO-1 nanoparticles.
Fig. 8. Plot of log
0 [Q]
versus log[Q].
3.6. Binding constant and number of binding sites The interesting thing to be noted is behaviour of CuO-1 in CQDs. From Fig. 7 it is observed that S-V plot with steady state measurements is found to be non-linear, showing positive deviation. This non linearity indicates the presence of static quenching. In case of time resolved measurements the S-V plot is parallel to abscissa with increase in concentration of CuO-1, which again confirms the presence of static quenching. Normally, values of kq that are apparently larger than the diffusion controlled limit usually indicate some type of binding interaction. In case of CuO-1 for concentration range 0–20 mM the quenching rate constant value is ≈7.5 × 1010M−1s−1 (Table 1), which implies there is a formation of complex between CuO-1 and CQDs that leads to static quenching. Therefore the binding constant (K) is calculated by the following method [45–47]. The relationship between fluorescence intensity and the quencher medium can be deduced from the Eq. (4), if it is assumed that there are independent and similar binding sites in the CQDs.
Fig. 9. Fluorescence quenching of CQDs in the presence of CuO-1 nanoparticles at 20 mM concentration at different temperatures.
nQ + F ⟶Qn……F
3.5. Interaction of CQDs with CuO nanoparticles (CuO-1, CuO-1.5, CuO-2, and CuO-1-700°C)
(4)
Where n is the number of molecules of the quencher, F is the fluorophore, Q is the quencher, and nQ + F is the postulated complex between a fluorophore and the quencher molecules. The constant K is given by;
To investigate the optical properties of CQDs, the absorption and fluorescence spectra were recorded. Fig. 4(a) shows the absorption peak of CQDs at 370 nm and the maximal emission peak of CQDs at 450 nm is shown in Fig. 4(b).To study the interaction between CQDs and CuO nano particles the fluorescence spectra of CQDs is recorded in the
K=
Qn……F [Q]n [F ]
(5)
If the overall amount of biomolecules bound or unbound with the quencher is F0, then it is considered that [F0] = [Q…….F] + [F]. Here 22
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[F] represents the unbound biomolecules concentration, then the relationship between fluorescence intensity and the unbound biomolecule F F0
log
=
I I0
I0
is given by,
I I
specific fluorescent CQDs nanosensor can be applied for rapid detection of CuO/Cu2O nanoparticles in aqueous medium, suggesting its potential and significance in bioanalytical and biomedical detection in future.
= log [K] + n log[Q]
Acknowledgement
(6)
The authors are grateful to the Deanship of Scientific Research, King Saud University for funding through Vice Deanship of Scientific Research Chairs.
Where, K is the binding constant of CuO-1 nanoparticles with CQDs, I I that can be determined from the intercept of log 0 I versus log[Q] as shown in Fig. 8. Thus we obtained the following value for binding constant; K = 311 M−1 and number of binding sites (n) = 1.2. The value of “n” approximately equals to 1 indicates the existence of one binding site in CQDs for CuO-1 nano particles.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.02.008.
3.7. Effect of pH and temperature on Fluorescence quenching of CQDs by CuO-1 nano particles
References
In our recent studies we have showed that fluorescence emission of CQDs greatly depends on the pH and temperature [38]. Therefore, we have investigated the effect of pH and temperature on fluorescence quenching of CQDs by CuO nano particles. In order to evaluate the effect of pH, the quenching studies are carried at pH 3 and 10 at room temperature. It is interesting to note that when compared to acidic pH (pH = 3) the emission peaks recorded in basic pH (pH = 10) are found to show red shift. However, S-V plots in both the cases showed positive deviation (ESI Fig. S1) and the quenching constants (Ksv and kq) obtained are the same (ESI Table S1). At both pH, with increase in the temperature it is observed that there is decrease in the Ksv and kq values. This shows that at pH 3 and 10 interaction of CuO-1 nanoparticles results in static quenching. The influence of temperature at constant pH on fluorescence quenching of CQDs with the interaction of CuO nanoparticles was investigated. Emission spectra of CQDs with 20 mM concentrations CuO1 nanoparticles at different temperatures is shown in Fig. 9. It is found that with increase in temperature the S–V plots shift towards abscissa (ESI Fig. S3). Further, it can also be noticed that with raise in temperature, there is decrease in the Ksv and kq values (ESI Table S2) this further supports the fact that quenching is static in nature. The decrease in Ksv and kq values with increase in temperature is due to the fact that, higher temperature disfavours the binding interaction responsible for static quenching. This is confirmed with the decrease in the value for binding constant; K = 34.2 M−1 and number of binding sites (n) = 0.9 (≈1) at 338 K in case of CQDs interacting with CuO-1 nano particles. When the quenching of CQDs with CuO nanoparticles was conducted by exciting at 350 nm we could notice that Ksv and kq values reduced compared to values when excited at 370 nm (ESI Table S3). This shows that quenching of CQDs fluorescence by CuO nanoparticles is more efficient when excitation wavelength is 370 nm.
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4. Conclusion The water dispersible CQDs of particle size less than 10 nm prepared by microwave approach were employed for detecting CuO/Cu2O nanoparticles with four different ratiometric oxidizer to fuel ratios (CuO1, CuO-1.5, CuO-2, and CuO-1,700 °C) as quencher in concentration range of 0–20 mM.The interaction of fluorescent CQDs with quencher CuO NPs resulted in the fluorescence quenching of CQDs in concentration-dependent manner. The value of kq for CuO-1.5, CuO-2, and CuO-1:700 °C indicates the quenching of CQDs by CuO nanoparticles is diffusion limited, and it is observed that the quenching ability of CuO nanoparticles increases with decrease in O/F ratio. Whereas, CuO-1 acts as an efficient quencher with kq≈7.5 × 1010M−1s−1. Further, with the use of steady state and time resolved measurements, it is concluded that the bimolecular reactions are due to the presence of static quenching process for CuO-1 and dynamic (diffusion limited) for CuO-1.5, CuO-2 and CuO-1:700 °C nano particles. Therefore, a reliable and highly 23
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D. Bharathi et al. precipitation, Anal. Chem. 367 (1998) 183–191. [20] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd ed., Springer, New York, NY, USA, 2006. [21] R. Liu, D. Wu, S. Liu, K. Koynov, W. Knoll, Q. Li, An aqueous route to multicolour photoluminescent carbon dots using silica spheres as carriers, Angew. Chem., Int. Ed. 48 (2009) 4598–4601. [22] X.M. Lin, G.M. Gao, L.Y. Zheng, Y.W. Chi, G.N. Chen, Encapsulation of strongly fluorescent carbon quantum dots in metal-organic frameworks for enhancing chemical sensing, Anal. Chem. 86 (2014) 1223–1228. [23] W.B. Lu, X.Y. Qin, S. Liu, G.H. Chang, Y.W. Zhang, Y.L. Luo, A.M. Asiri, A.O. AlYoubi, 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. 84 (2012) 5351–5357. [24] A. Jaiswal, S.S. Ghosh, A. Chattopadhyay, One step synthesis of C-dots by microwave mediated caramelization of poly(ethylene glycol), Chem. Commun. 48 (2012) 407–409. [25] X.Y. Qin, W.B. Lu, A.M. Asiri, A.O. Al-Youbi, X.P. Sun, Microwave-assisted rapid green synthesis of photoluminescent carbon nanodots from flour and their applications for sensitive and selective detection of mercury(II) ions, Sens. Actuators B Chem. 184 (2013) 156–162. [26] X. Ji, J. Zheng, J. Xu, V.K. Rastoqi, T.C. Cheng, J.J. DeFrank, R.M. Leblanc, (CdSe) ZnS quantum dots and organophosphorus hydrolase bioconjugate as biosensors for detection of paraoxon, J. Phys. Chem. B 109 (2005) 3793–3799. [27] L. Zhou, Y.H. Lin, Z.Z. Huang, J.S. Ren, X.G. Qu, Carbon nanodots as fluorescence probes for rapid, sensitive, and label-free detection of Hg2+ and biothiols in complex matrices, Chem. Commun. 48 (2012) 1147–1149. [28] Y.S. Xia, C.Q. Zhu, Use of surface-modified CdTe quantum dots as fluorescent probes in sensing mercury (II), Talanta 75 (2007) 215–221. [29] H.M. Goncalves, A.J. Duarte, J.C.G. Esteves da Silva, Optical fiber sensor for Hg(II) based on carbon dots, Biosens. Bioelectron. 26 (2010) 1302–1306. [30] H.M. Goncalves, P.A.S. Jorge, J.R.A. Fernandes, J.C.G. Esteves da Silva, Hg(II) sensing based on functionalized carbon dots obtained by direct laser ablation, Sens. Actuators B: Chem. 145 (2010) 702–707. [31] A.B. Bourlinos, A. Bakandritsos, A. Kouloumpis, D. Gournis, M. Krysmann, E.P. Giannelis, K. Polakova, K. Safarova, K. Hola, R. Zboril, Gd(III)-doped carbon dots as a dual fluorescent-MRI probe, J. Mater. Chem. 22 (2012) 23327–23330. [32] J.M.C. Fernández, R. Pereiro, A.S. Medel, The use of luminescent quantum dots for optical sensing, Trends Analyt. Chem. 25 (2006) 207–218. [33] H.M.E. Azzazy, M.M. Mansour, S.C. Kazmierczak, From diagnostics to therapy: prospects of quantum dots, Clin. Biochem. 40 (2007) 917–927.
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Understanding the interaction of carbon quantum dots with CuO and Cu2O by fluorescence quenching
T
D. Bharathia, R. Hari Krishnab, , B. Siddlingeshwara, , Darshan Devang Divakarc, Abdulaziz Abdullah Alkheraifc ⁎
⁎
a
Department of Physics, M. S. Ramaiah Institute of Technology, Bengaluru, 560054, Karnataka, India Department of Chemistry, M. S. Ramaiah Institute of Technology, Bengaluru, 560054, Karnataka, India c Dental Biomaterials Research Chair, Dental Health Department, College of Applied Medical Sciences, King Saud University, Riyadh, 11433, Saudi Arabia b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Carbon quantum dots Fluorescence quenching CuO nanoparticles Stern-Volmer
In spite copper oxide being one of the essential micronutrient, copper oxide in its nano size is found to be toxic in nature; this instigates for the detection of copper oxides in trace levels. In the present study, we demonstrate simple cost effective detection method for CuO/Cu2O using carbon quantum dots (CQD) by fluorescence quenching technique. CuO/Cu2O nanoparticles are synthesised by mere variation of fuel ratio by solution combustion technique. The resulting oxides are characterized by various analytical and spectroscopic techniques. Powder X- ray diffraction (PXRD) results reveals that samples prepared with oxidizer to fuel (O/F) ratios 1:1, 1:1.5 and 1:2 showed pure nano CuO, major CuO phase (minor Cu2O) and major Cu2O phase (minor CuO) respectively. Further, the samples prepared using 1:1 O/F ratio and calcinated at 700 °C showed highly crystalline CuO phase. In order to study the interaction of CuO/ Cu2O with CQDs the fluorescence quenching method has been employed. The bimolecular quenching rate constants for the samples prepared with different O/F ratios have been measured. The interaction between CQDs and copper oxides, indicates fluorescence quenching greatly depends on the oxidation state of the copper oxide and can be a promising method for detecting CuO/Cu2O through CQDs.
⁎
Corresponding authors. E-mail addresses: [email protected] (R.H. Krishna), [email protected] (B. Siddlingeshwar).
https://doi.org/10.1016/j.jhazmat.2019.02.008 Received 16 November 2018; Received in revised form 25 January 2019; Accepted 3 February 2019 Available online 04 February 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
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1. Introduction
us to investigate the interaction of CQDs with copper oxide nanoparticles prepared by solution combustion synthesis with different O/F ratios. Since CQDs comprises of non-toxic elements, which makes them particularly useful and promising bio-analytical tool. The present study would open up the possible usage of nanosensor for bio-analytical applications. In this paper, we have used both steady state and time resolved methods in aqueous medium to investigate the quenching of CQDs by CuO/Cu2O nanoparticles at room temperature. This study is the first attempt that is dedicated to investigate the interaction between CQDs and CuO/Cu2O nanoparticles.
The oxides of transition metals are an important class of semiconductors that have wider applications in magnetic storage media, solar energy transformation, electronics, and catalysis. Copper oxide has attracted greater attention due to its fascinating properties. Even though copper oxide nano particles have proved their use in various applications; the major disadvantage for their use is due to their potentially toxic effects. Copper is an essential trace element in both plants and animals, including humans. At an elevated concentrated level cupric oxide in nano level is toxic, though it is an essential micronutrient for aquatic organisms [1]. Several studies carried out in recent years have explained that copper in relatively low levels may generate oxidative stress in marine cnidarians [2] and also leads to several brain diseases such as Menkes disease, Parkinson’s disease, Wilson disease, and Alzheimer’s disease [3,4]. The serious neurodegenerative diseases are caused even if there is a little attenuation in the cellular homeostasis of copper that might produce reactive oxygen species [5–8]. Due to accumulation of cupric oxide caused by pollution in the environment, cupric oxide is considered as the major pollutant by the U.S. Environmental Protection Agency (EPA) [9]. Feng et al. [10] have discussed on the functionality, toxicity of the metal oxide nanoparticles on the central nervous system. Karlsson et al. [11,12] have focussed on comparative study of different metal oxide nanoparticles and carbon nanotubes, and found that cupric oxide nanoparticles are more toxic compared to other metal oxide nanoparticles. Chitrada et al. [13] have shown that CuO nano particles are more toxic than their micro sized counterparts at the same Cu concentration, with toxicities approaching those of the ionic Cu species. Margit et al. [14] explored that CuO nano particles agglomerate and settle rapidly in natural water and their toxicity is higher in natural water compared to standard media due to differences in composition. Also, using total reflection Xray fluorescence (TXRF) spectroscopy they have evaluated the effect of test medium on total Cu body burden of CuO nano particles exposed Daphnia Magna [15]. Therefore, there is a great demand for the purpose of controlling CuO nanoparticles induced toxic effect. Hence there is a need for a simple, reliable, green and selective strategy for the sensitive detection of cupric oxide level present in ecosystem and environment. The different methods involved in the detection of cupric oxide level includes electrochemical method [16], atomic absorption spectrometry (AAS) [17], colorimetric method [18], and inductively coupled plasma mass spectrometry (ICPMS) [19]. Compared to these methods detection of CuO by fluorescent method is the most facile and simple method that involves just monitoring the variation in the fluorescence intensity. Therefore, fluorescence quenching is one of the best methods for sensing applications or as a bio analytical tool [20]. Fluorescent Carbon Quantum dots (CQDs) are a fascinating class of zero-dimensional nanomaterials recently discovered which comprises of quasi-spherical carbon nanoparticles with size below 10 nm [21,22] and have drawn great research interest due to their high fluorescence emission in the visible spectral region with strong photostability, biocompatibility, low toxicity, and excellent solubility in aqueous medium [23–27]. Owing to their ease of synthesis and low toxicity, CQDs are used as one of the most efficient and stable nanosensors for detecting various types of analytes compared to other heavy metals contained semiconductor quantum dots and organic dyes [28–31].Quantum dots have remarkable attractive optoelectronic properties including their high emission quantum yields, size tuneable emission profiles. These properties were much different from those of the bulk systems, due to quantum confinement effects [32].Because of these unique properties, CQDs can be employed for many applications such as in drug delivery, bioimaging, biosensing, especially in fluorescent sensors by monitoring the changes of fluorescence intensity under external physical or chemical stimuli [33–37]. The interesting properties associated with CQDs sensitivity and the present need for the detection of harmful nano metal oxides, motivated
2. Experimental section 2.1. Preparation of CQDs by microwave method The chemicals used for synthesis of CQDs are of analytical grade and are used without further purification. The precursors used in this study, Ascorbic acid vitamin C, EDTA (Ethylenediaminetetraacetic acid), PEG200 (Poly ethylene glycol), Copper nitrate, diethyl oxalate, and hydrazine hydrate were purchased from Merck. To prepare CQDs by microwave assisted method [38,39], 1 g of ascorbic acid (vitamin C) and 0.06 g of EDTA are dissolved in 10 ml distilled water. To the resulting clear solution 1 ml Poly ethylene glycol (PEG 200) was added, and the solution was homogenised by stirring using magnetic stirrer for 15 min. The beaker containing the solution was introduced to the microwave oven and the reaction is allowed to occur for 1 h. After the reaction the solution is allowed to cool to room temperature and finally addition of 60 ml distilled water resulted in yellow CQDs in liquid form, with no obvious crystal lattice. 2.2. Preparation of copper oxide nanoparticles by solution combustion method To prepare copper oxide nano particles stoichiometric quantities of copper nitrate and the fuel oxalyldihydrazide (ODH) are calculated based on propellant chemistry. The stoichiometric ratio here means the ratio of oxidizer and fuel (O/F) and is set to one. The details of the calculations for taking the reactants are described elsewhere [40]. The petri dish containing aqueous mixture of copper nitrate and ODH is introduced into the pre-heated muffle furnace maintained at 500 °C. The solution first underwent dehydration followed by auto ignition resulting in smouldering type combustion. The whole process got completed within 5 min resulting in black colored porous product. Different samples of copper oxide are prepared using the above procedure. The O/F ratio is varied systematically between 1.0 and 2.0, to tune the oxidation state of the metal oxide. The powders obtained for different O/F ratios are labelled as: CuO-1 for propellant ratio (O/ F = 1.0), CuO-1.5 for fuel rich (O/F = 1.5), CuO-2 for extreme fuel rich (O/F = 2.0) and CuO-1:700 (for O/F = 1) and calcinated at 700 °C for 3 h. 2.3. Characterization of copper oxide nanoparticles The phase purity and crystallinity of nano materials are measured using a powder X-ray diffractometer (PANalytical X′Pert Pro) in the Bragg–Brentano θ-2θ geometry. For PXRD, the powder sample is placed in a zero background holder (ZBH). Flatness of the sample is ensured by pressing the powder against ZBH. Cu Kα (1.541 Å) radiation with a nickel filter is used to obtain the diffraction data. The morphology and structure of the samples are inspected using SEM (JEOL JSM-840 A). For performing SEM, a layer of Au is deposited on the samples using a sputter deposition set up. This is necessary to get SEM images since the samples are insulating. Transmission electron microscopy (TEM) analysis is performed on Hitachi H-8100 (accelerating voltage up to 200 kV, LaB6 filament) equipped with EDS (Kevex Sigma TM Quasar, USA). The sizes of the CQDs are determined by using Malvern 18
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instrument ZETASIZER Nano series. 2.4. Spectroscopic measurements The spectral absorption measurements were recorded using ST S-V IS miniature spectrometer and Analytik Jena Specord200 plus UV Vis spectrophotometer. The spectral emission measurements were carried out using HITACHI F-2700 spectrofluorimeter, the excitation and emission slit widths (each 5 nm) and scan rate (1500 nm/min) and PMT voltage 700 V were kept constant for all the measurements. 2.5. Time resolved fluorescence measurements The fluorescence lifetime measurements were carried out using a Horiba Jobin Yvon TCSPC lifetime instrument in a time correlated single photon counting arrangement. A 370 nm nano-LED was used as the light source. The pulse repetition rate was set to 1 MHz, and the detector response was ∼1.1 ns. The instrument response function was collected using a Scatterer (Ludox AS40 colloidal silica). The decay data were analyzed using IBH software. A value of 2 , 0.99 ≤ 2 ≤ 1.2, was considered as a good fit, which was further analyzed by the symmetrical distribution of the residuals. The average fluorescence lifetime ( avg ) values were obtained by the following equation [20] avg
=
Where
( ( i
n 2 i=1 i i ) n i=1 i i )
is the individual lifetime with corresponding amplitude
(1) i.
3. Results and discussion
Fig. 1. PXRD of CuO prepared with different ratios of fuel and oxidizer (a) CuO1 (b) CuO-1.5 (c) CuO-2 and (d) CuO-1-700 °C sample.
3.1. Powder X-ray diffraction (PXRD)
voids in CuO-2 has significantly increased compared to CuO-1 and CuO1.5. This can be attributed to the higher number of moles of gases liberated in fuel rich synthesis. The formation of voids and pores in the structure is very typical of combustion derived materials. The difference in the morphology of the CuO-2 may also be attributed to formation of Cu2O as major phase rather than CuO. Further, micrographs of CuO1:700 shows that the size of the particle agglomerations has increased and the sample lost its porous nature compared to CuO-1. The increase in the size of particle clusters in heat treated samples can be attributed to sintering effect that aids in the agglomeration of particle clusters thereby affecting the porous structure.
The powder X-ray diffraction patterns of the samples prepared with varying O/F ratios are shown in Fig. 1(a–d). The diffraction pattern for the sample prepared with O/F = 1 shows the diffraction pattern that can be readily matched with the CuO (JCPCDS-80-1917). The peaks are found to be broad and intense suggesting that the prepared CuO is in nanocrystalline form. On the other hand the samples prepared with O/ F = 1.5 & 2 showed the diffraction peaks corresponding to both CuO and Cu2O. It is also interesting to note that the samples prepared with O/F = 1.5 showed major CuO and minor Cu2O composition. However, when O/F = 2, Cu2O is found to be dominating phase with minor CuO phase. Further, the sample prepared by using O/F = 1 and calcinated at 700 °C showed pure highly crystalline CuO phase. From the above XRD results, it can be concluded that fuel quantity determines the fate of the copper oxide phase. With increase in the fuel quantity, Cu2O phase formation is favoured which can be attributed to the reducing atmosphere in fuel rich samples. In fuel rich samples the excess fuel taken creates the reducing environment due to high carbon content that aids in the stabilization of reduced copper oxide, Cu2O. CuO-1:700 °C showed pure CuO phase due to the calcination in open air atmosphere that completely oxidised the CuO phase and resulted in crystalline CuO.
3.3. High resolution transmission electron microscope measurements Transmission electron microscope (HRTEM) is employed to understand the influence of O/F ratio on particle size of CuO nano materials and the images are presented in Fig. 3(a–d). From the TEM images it is evident that with varying the fuel quantity there is significant variation in the particle size. CuO-1 shows agglomerated particles with varying sizes of particle clusters and the average size of the particles are found to be ≈20–30 nm. However, TEM images of CuO-1.5 and CuO-2.0 shows relatively bigger and irregular particles. The extent of agglomeration in these samples increased and the average size of the particles in CuO-1.5 and CuO-2.0 are found to be ≈60–80 nm. The increase in the particle size in samples prepared with fuel rich samples might be due to the increased exothermicity of combustion reaction due to larger extent of gaseous by products released. Further, the sample CuO-1:700 show that particle size has grown significantly and can be attributed to sintering effect during high temperature calcination.
3.2. Field emission scanning electron microscope measurements The morphology of the samples was investigated by field emission scanning electron microscopy and the micrographs are presented in Fig. 2(a–d). Micrograph of CuO-1 shows that the surface morphology exhibits agglomerated particles having fluffy and porous nature. The agglomerated clusters form interconnected masses and numerous voids of varying sizes. The surface morphology of CuO-1.5 also shows similar morphology as of CuO-1 and no significant changes are observed. However, it is worth noticing that the micrograph of CuO-2 shows highly porous structure and spongy like morphology. The number of
3.4. Relative quantum yield of CQDs Relative quantum yield ( 19
f)
for CQDs was determined using Eq. (2)
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Fig. 2. Scanning electron micrographs of (a) CuO-1 (b) CuO-1.5 (c) CuO-2 and (d) CuO-1-700 °C.
[41,42] f
=
r (I f
intensity, and ‘A’ is the area under the emission curve. The refractive index of the solvents used is represented by ‘n’. The measured quantum yield of CQDs is 42%, using the refractive index nf = 1.33(water) and nr = 1.497(Toluene). (Z)-N,N-diethyl-N’-(7-oxo-7H-benzo[de]anthracen-3-yl)acetamidine in Toluene ( r = 0.56) is used as the reference molecule [41].
Ar nf2 )
(Ir Af nr2 )
(2)
Where the subscript f and r refers to the CQDs and the reference molecule respectively, ‘∅’represents the quantum yield, ‘I’ is the integral
Fig. 3. Transmission electron microscope images of (a) CuO-1 (b) CuO-1.5 (c) CuO-2 and (d) CuO-1-700 °C. 20
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Fig. 6. S-V plot for the steady state and time resolved fluorescence quenching of CQDs by CuO nanoparticles (CuO-1.5, CuO-2, and CuO-1:700 °C). Table 1 S-V constant (Ksv), and bimolecular quenching rate parameter (kq) for CQDs with different CuO nano particles as quenchers. Excited state life time for CQDs 0 is 2 ns. Quencher M CuO-1 CuO-1.5 CuO-2 CuO-1:7000C
Fig. 4. (a) the absorption and (b) the emission spectrum of CQDs.
Ksv (M−1)
kq(x1010 M−1s−1)
150 27.5 15 25
7.5 1.37 0.75 1.25
Fig. 5. (a–d):Fluorescence quenching of CQDs in the presence of CuO nanoparticles (a): CuO-1, (b): CuO-1.5, (c): CuO-2 and (d) CuO-1:700 °C. 21
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presence of CuO nano particles as quencher. CuO nano particles of different ratiometric O/F ratios viz. CuO-1, CuO-1.5, CuO-2, and CuO1:700 °C were used in the range of 0–20 mM concentration. It was observed that fluorescence intensity of CQDs decreased by the addition of quencher as shown in Fig. 5(a–d). The fluorescence quenching mechanism is described by SternVolmer relation (S-V) [20]:
I0 = 1 + KSV [Q] = 1 + kq I
I0
I I
(3)
Where I0 and I are the emission intensities of CQDs in the absence and presence of CuO nano particles (CuO-1, CuO-1.5, CuO-2, and CuO1:700 °C), respectively. KSV is the Stern-Volmer constant related to the bimolecular quenching rate constant (kq ) by KSV= kq 0 , and 0 is the excited state lifetime of CQDs and [Q] is the concentration of CuO nano particles. Eq. (3) is applicable as long as the experimental results show linear variation. The S-V plot of CQDs for quenchers CuO-1.5, CuO-2, and CuO-1:700 °C NPs is shown in Fig. 6. We observe the linearity in SV plots in the concentration range 0–20 mM. The linear S-V plots in steady state measurements alone does not prove the dynamic quenching process but sometimes static process also results in linear S-V plot. In general, time resolved measurement is the most definitive method for differentiating static and dynamic quenching processes [43,44]. The inset of Fig. 6 shows the S-V plot for time resolved measurements. The phenomenon of quenching by the time resolved method also follows the S-V relation. Also, from the S-V plot it is observed that Cu2O is less accessible for CQDs for quenching compared to CuO-1.5 and CuO-1:700 °C. The values of kq (Table 1) and the linearity observed in both steady state and time resolved measurements for CuO-1.5, CuO2, and CuO-1:700 °C indicates the quenching of CQDs by these CuO nanoparticles is diffusion limited. Further, the experiments are performed in triplicates in order to check the repeatability of the interactions and statistical analyses was done for the obtained Ksv values (ESI Fig. S2). Form the figure it is evident that the standard deviation obtained for the triplicates are well within the acceptable error and proves the validity of the process.
Fig. 7. S-V plot for the steady state and time resolved fluorescence quenching of CQDs by CuO-1 nanoparticles.
Fig. 8. Plot of log
0 [Q]
versus log[Q].
3.6. Binding constant and number of binding sites The interesting thing to be noted is behaviour of CuO-1 in CQDs. From Fig. 7 it is observed that S-V plot with steady state measurements is found to be non-linear, showing positive deviation. This non linearity indicates the presence of static quenching. In case of time resolved measurements the S-V plot is parallel to abscissa with increase in concentration of CuO-1, which again confirms the presence of static quenching. Normally, values of kq that are apparently larger than the diffusion controlled limit usually indicate some type of binding interaction. In case of CuO-1 for concentration range 0–20 mM the quenching rate constant value is ≈7.5 × 1010M−1s−1 (Table 1), which implies there is a formation of complex between CuO-1 and CQDs that leads to static quenching. Therefore the binding constant (K) is calculated by the following method [45–47]. The relationship between fluorescence intensity and the quencher medium can be deduced from the Eq. (4), if it is assumed that there are independent and similar binding sites in the CQDs.
Fig. 9. Fluorescence quenching of CQDs in the presence of CuO-1 nanoparticles at 20 mM concentration at different temperatures.
nQ + F ⟶Qn……F
3.5. Interaction of CQDs with CuO nanoparticles (CuO-1, CuO-1.5, CuO-2, and CuO-1-700°C)
(4)
Where n is the number of molecules of the quencher, F is the fluorophore, Q is the quencher, and nQ + F is the postulated complex between a fluorophore and the quencher molecules. The constant K is given by;
To investigate the optical properties of CQDs, the absorption and fluorescence spectra were recorded. Fig. 4(a) shows the absorption peak of CQDs at 370 nm and the maximal emission peak of CQDs at 450 nm is shown in Fig. 4(b).To study the interaction between CQDs and CuO nano particles the fluorescence spectra of CQDs is recorded in the
K=
Qn……F [Q]n [F ]
(5)
If the overall amount of biomolecules bound or unbound with the quencher is F0, then it is considered that [F0] = [Q…….F] + [F]. Here 22
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[F] represents the unbound biomolecules concentration, then the relationship between fluorescence intensity and the unbound biomolecule F F0
log
=
I I0
I0
is given by,
I I
specific fluorescent CQDs nanosensor can be applied for rapid detection of CuO/Cu2O nanoparticles in aqueous medium, suggesting its potential and significance in bioanalytical and biomedical detection in future.
= log [K] + n log[Q]
Acknowledgement
(6)
The authors are grateful to the Deanship of Scientific Research, King Saud University for funding through Vice Deanship of Scientific Research Chairs.
Where, K is the binding constant of CuO-1 nanoparticles with CQDs, I I that can be determined from the intercept of log 0 I versus log[Q] as shown in Fig. 8. Thus we obtained the following value for binding constant; K = 311 M−1 and number of binding sites (n) = 1.2. The value of “n” approximately equals to 1 indicates the existence of one binding site in CQDs for CuO-1 nano particles.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.02.008.
3.7. Effect of pH and temperature on Fluorescence quenching of CQDs by CuO-1 nano particles
References
In our recent studies we have showed that fluorescence emission of CQDs greatly depends on the pH and temperature [38]. Therefore, we have investigated the effect of pH and temperature on fluorescence quenching of CQDs by CuO nano particles. In order to evaluate the effect of pH, the quenching studies are carried at pH 3 and 10 at room temperature. It is interesting to note that when compared to acidic pH (pH = 3) the emission peaks recorded in basic pH (pH = 10) are found to show red shift. However, S-V plots in both the cases showed positive deviation (ESI Fig. S1) and the quenching constants (Ksv and kq) obtained are the same (ESI Table S1). At both pH, with increase in the temperature it is observed that there is decrease in the Ksv and kq values. This shows that at pH 3 and 10 interaction of CuO-1 nanoparticles results in static quenching. The influence of temperature at constant pH on fluorescence quenching of CQDs with the interaction of CuO nanoparticles was investigated. Emission spectra of CQDs with 20 mM concentrations CuO1 nanoparticles at different temperatures is shown in Fig. 9. It is found that with increase in temperature the S–V plots shift towards abscissa (ESI Fig. S3). Further, it can also be noticed that with raise in temperature, there is decrease in the Ksv and kq values (ESI Table S2) this further supports the fact that quenching is static in nature. The decrease in Ksv and kq values with increase in temperature is due to the fact that, higher temperature disfavours the binding interaction responsible for static quenching. This is confirmed with the decrease in the value for binding constant; K = 34.2 M−1 and number of binding sites (n) = 0.9 (≈1) at 338 K in case of CQDs interacting with CuO-1 nano particles. When the quenching of CQDs with CuO nanoparticles was conducted by exciting at 350 nm we could notice that Ksv and kq values reduced compared to values when excited at 370 nm (ESI Table S3). This shows that quenching of CQDs fluorescence by CuO nanoparticles is more efficient when excitation wavelength is 370 nm.
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