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Article
Novel turn-on fluorescent sensors with mega stoke shifts for dual detection of Al and Zn 3+
2+
Kanokthorn Boonkitpatarakul, Junfeng Wang, Nakorn Niamnont, Bin Liu, Lucas McDonald, Yi Pang, and Mongkol Sukwattanasinitt ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.5b00136 • Publication Date (Web): 23 Nov 2015 Downloaded from http://pubs.acs.org on November 28, 2015
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Novel turn-on fluorescent sensors with mega stoke shifts for dual detection of Al3+ and Zn2+ Kanokthorn Boonkitpatarakul,†,‡ Junfeng Wang,† Nakorn Niamnont,⊥ Bin Liu,† Lucas Mcdonald,† Yi Pang,†* Mongkol Sukwattanasinitt§* †
Department of Chemistry, The University of Akron, Akron, OH 44325, USA
‡
Program of Petrochemistry and Polymer Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
§
Department of Chemistry, Chulalongkorn University, Bangkok, 10330, Thailand
⊥
Department of Chemistry, King Mongkut’s University of Technology Thonburi, Bangkok, 10140, Thailand
ABSTRACT: A series of bishydrazide compounds have been developed for selective turn-on detection of Al3+ cation, based on metal chelation-enhanced fluorescence (CHEF) effects that inhibit the non-radiative PET and ESIPT processes. In aqueous solution, Al3+ selectively induces strong fluorescence of these compounds with large Stokes shifts up to ∼133 nm and emission colors varying from blue to orange. In solid phase, both Al3+ and Zn2+ give apparent fluorescence which is simultaneously detected based on chromatographic separation.
KEYWORDS: aluminum ion, fluorescence sensor, hydrazide, paper chromatography, zinc ion, Development of metal fluorescent sensors is important for chemical and biological analyses that have recently gained increasing research attention evidenced by the exponential growth in publications during the last decade.1,2 Design of new fluorescent metal-binding ligands based on chelation-enhanced fluorescence (CHEF) is one of the most exciting strategies as it can give strong fluorescence turn-on sensing ligand favorable for visualizing and imaging. Ability in optical tuning of the probe is also highly desirable in bioimaging, especially for tuning toward the NIR region.3 A large stoke shift is also beneficial in fluorescence sensing technology to avoid selfabsorption and interference from light source.4 Thus, rational design of ligands with high sensitivity/selectivity and ability in spectroscopic tuning are of great value. In this research work we aimed to develop novel ligand series as Al3+ turn-on fluorescent sensors with large stroke shifts with three emissive colors. Aluminum is one of the most abundant metallic elements in the earth’s crust and it is extensively used in daily life.5 However, high levels of aluminum ion (Al3+) in human body can cause many severe diseases including Alzheimer’s disease, dialysis encephalopathy, and Parkinson’s disease.6 The World Health Organization (WHO) reports that human consume Al3+ about 3–10 mg/day while the weekly tolerable intake is 7 mg/kg of the body weight. The WHO also limits Al3+ concentration in drinking water to 200 µgL-1 (7.41 µM).1, 7-8 Analytical techniques currently available for detecting aluminum ion such as atomic absorption spectrometry,9 inductively coupled plasma mass spectrometry (ICP-MS),10 voltammetry,11 ion
selective membrane,12 liquid chromatography-mass spectrometry,13 are not easily applied for on-site and real time monitoring of environments and biological systems. The development of selective fluorescent ligands for detection of Al3+ in aqueous system has to overcome the strong hydration nature of Al3+ without having to sacrifice its selectivity is extremely challenging. To achieve a high and selective binding, most fluorescent Al3+ sensors contain rather complicated ligand structures that are difficult to synthesize and poorly soluble in water.14 It is thus highly desirable to develop an aqueous soluble Al3+ sensor that can be readily synthesized and exhibit high selectivity to detect Al3+ ion in aqueous environments even at minimal concentration. Hydrazides have been extensively used in several research fields due to their facile syntheses, tunable electronic properties and good chelating capability.15 We have previously reported a simple N-salicylidenehydrazide (1) in which the hydrazide works synergistically with adjacent phenol group to selectively bind Al3+ cation in aqueous solution.16 Upon binding Al3+, the sensor exhibited high fluorescence turn-on in aqueous solution, giving strong blue emission (441 nm) based on suppression of non-radiative pathways i.e. excited state intramolecular proton transfer (ESIPT) and photoinduced electron transfer (PET). Bo et al. reported that the aluminum complex of furan-2-carbohydrazide (2) showed significant redshifts in both absorption λmax (by ~17 nm) and emission (by ~24 nm) spectra in comparison with 1 that attributed to the electron delocalization from the furyl ring via π-
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conjugation with the deprotonated form of the hydrazide complex (form B).17
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Scheme 1. Synthesis routes of fluorophores O
O
OH N NH OH CHO 2, 84% yield
O
OH
O
NHNH2
N NH
O
Figure 1. Complexation of 1 and 2 with Al
3+ EtOH/ reflux
OH
Since the N-salicylidenehydrazide ligands (such as 1 and 2) exhibit both good selectivity toward Al3+ and large spectral response, it is desirable use 2 as a core structure for further design in shifting the blue emission to a longer wavelength. In this manuscript, we report a strategy that could effectively tune the emission peak to longer wavelengths, while retaining the chromophore’s valuable features i.e. high Al3+ selectivity, large fluorescence turn-on and large Stokes’ shift. The inclusion of an additional hydroxyl substituent on the phenol ring to raise the HOMO level is exploited in the molecular design of compounds 3. Addition of the second hydrazide group on the phenol ring will further extends the π-conjugation to give 4. We expected the Al3+ complexes of 3 and 4 to give strong emission with notable red-shift useful for color tuning of the sensors. O O
O
O
OH
CHO
O
HO 3, 76% yield O
OH
O
OH CHO
N NH HN N
OHC
HO
OH O
O 4, 73% yield
Table 1. Photophysical properties of sensor 2-4 in aqueous solution (∼ ∼0.5% DMSO/H2O) Sensor
λabs
Log ε
(nm)
λem
Φ
(nm)
2
320, 328
4.40, 4.28
457
0.020
3
297, 350
4.46, 4.10
531
0.001
4
309, 396
4.01, 3.87
615
0.003
OH
OH
N NH N NH HN N HO
HO
O 3
O 4
Figure 2. Structures of compounds 3 and 4.
RESULTS AND DISCUSSION The synthesis of the designed fluorophores was accomplished by condensation between 2-furoic hydrazine and the benzaldehyde derivative (salicylaldehyde, 2,5dihydroxybenzaldehyde and 2,5dihydroxyterephthalaldehyde),18 which afforded compounds 2-4 in high yield. The compounds were fully characterized by 1H, 13C NMR and ESI mass spectrometry. The photophysical properties of 2-4 are summarized in Table 1. The UV-vis absorption spectrum of each compound in aqueous solution (∼0.5% DMSO) exhibited two major absorption bands corresponding to the π-π* and nπ* transitions. The absorption and emission spectra of 3 and 4 showed notable red-shift in comparison with that of 2, as the results of the additional hydroxyl group (3) and increasing π conjugation (4). In aqueous solution, all of the compounds gave very weak fluorescence owing to the PET effect from the amine and the ESIPT process which was also evidenced by the extraordinary large Strokes’ shift (>100 nm).
Upon addition of 10 equiv. Al3+, the fluorescent intensity of each sensing compound showed remarkable enhancement, but in different emission wavelengths and apparent colors: blue for 2, green for 3 and orange red for 4 as shown in Figure 3. The strong fluorescence turn-on was associated with the formation of the complex formation between Al3+ and each sensing compound which suppressed both the PET and ESIPT in the sensors and induced rigidity in the complexes resulting in CHEF (chelation enhanced fluorescence)19 (Scheme 2). The solution of 2-Al3+ complex has strong blue emission with the maximum emission wavelength (λem ≈ 455 nm, Stokes shift = 89 nm) with high quantum yield (Φ = 0.49). As designed, the 3-Al3+ complex showed green fluorescence at λem ≈ 524 nm (Φ = 0.16) which was red-shifted by 70 nm from 2Al3+ and the Stokes shift was 114 nm as a result of an extra hydroxyl substituent. Impressively, the emission peak of 4-Al3+ complex moved even further to 601 nm (Φ = 0.18) which also gave the largest Stokes shift of 133 nm (∆λem-ex = 601−468 nm). The presence of the additional hydroxyl and hydrazine substituents thus has intriguing impact on the radiative decay process besides lowering the HOMOLUMO band gap. The fluorescence sensing selectivity of 2-4 was also investigated with other common metal ions (Na+, K+, Ag+, Mg2+, Ca2+, Hg2+, Ba2+, Pb2+, Cd2+, Mn2+, Ni2+, Co2+, Cu2+, Fe2+, Zn2+, Cr3+, Fe3+, Al3+) at 10 equiv in aqueous solution (0.1%DMSO/HEPES). As shown in Figure 3, the fluorescence signals of all sensing compounds did not change significantly with other metal ions. Therefore,
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these hydrazide compounds are highly selective for Al3+ detection
The association constant (Ka) of the Al3+ complexation with each ligand was determined on the basis of Benesi Hildebrand plot20 as shown in Figure S12-S13. The Ka values for the complexation with 2-4 are 1.6 x 105, 5.4 x 105 and 2.0 x 1010 M-1, respectively. The particularly large Ka value for the complex of 4 is the product of two association constants supporting the formation of 2:1 complex. The Al3+ complexation with each ligand was reversible as the fluorescence turn on signal could be nullified with the addition of 10 equiv EDTA (Figure S14-16).
Figure 3. Fluorescence spectra of 2-4 (10 µM in 0.1% DMSO/HEPES) before and after addition 10 equiv of + + + 2+ 2+ 2+ 2+ 2+ 2+ metal ion: Na , K , Ag , Mg , Ca , Hg , Ba , Pb , Cd , 2+ 2+ 2+ 2+ 2+ 2+ 3+ 3+ 3+ Mn , Ni , Co , Cu , Fe , Zn , Cr , Fe , Al . The spectra were obtained after 10 minute mixing for 2 and 3 and 30 minute mixing for 4 with λex = 369, 410 and 468 nm, 3+ respectively. Only Al gives significant enhancement.
Scheme 2. Fluorescence enhancement mechanism of the 2-Al3+ complex.
All the experiments were carried out at pH 5.5 that the fluorescence of all sensors showed remarkable signal turn-on ratios after addition Al3+ ion. At a pH below 5.0, the N atoms in –C=N– groups were protonated and the abstraction of phenolic protons was suppressed that inhibited the chelation of Al3+ with the sensors. At high pH values, the competition of OH- ions with the sensors for Al3+ coordination might cause the decrease of the turn-on ratios (Figure S11). The optimal pH range for sensing application of 2 and 4 are significantly wider than that of 3 that may be attributed to the intramolecular hydrogen bonding decreasing the acidity of their phenolic protons and thus the quenching effects of their aluminum complexes. For quantitative analysis of Al3+, the fluorescence signals of the solutions of 2-4 were measured in the presence of Al(NO3)3 at various concentrations and the results are shown in Figure 4. The intensity of fluorescence signal increased almost linearly with the increasing Al3+ concentration up to 1 equiv. On the basis of Job plot, the complexation ratios of Al3+ and the ligand 2 and 3 were 1:1 (Figure S7). The assumption was supported by ESI-MS that showed m/z 381.3 and 366.7 corresponding to the mass of [AlNO3•(2−H) •2(MeOH)]+ and [AlNO3• (3−H) •MeOH]+ (Figure S8 and S9). Meanwhile, the job plot of the Al3+ complexed with 4 gave 2:1 stoichiometric metal:ligand ratio. The ESI-MS spectrum showed both 1:1 and 2:1 ion peaks at 502.9 for [AlNO3•(4−H)•MeOH]+ and 562.9 for [Al2•(4−4H)NO3•MeOH•2H2O]+ (Figure S10).
Figure 4. Fluorescence spectra of 2 (a), 3 (b) and 4 (c) (10 µM), respectively, upon addition of different concentra3+ tions of Al in 0.1% DMSO/HEPES. Plots of intensity at 3+ each λem versus the amount of Al added. The spectra were obtained after mixing for 10 minutes (sensor 2 (d) and 3 (e)) and for 30 minutes (sensor 4 (f)) using λex = 369, 410 and 468 nm, respectively.
Upon the addition of Al3+, the absorption band of each compound showed the large spectral bathochromic shift inferring deprotonation of the phenolic proton to enable its binding with Al3+. The λmax of 2 and 3 exhibited a single shift from 325 to 369 nm and 352 to 397 nm, respectively (Figure 5a and b). On the other hand, 4 exhibited a twostepped spectral shift in methanol. The first step was from 408 nm to 457 with the addition of Al3+ up to 1 equiv, and the second step was from 457 nm to 504 nm with the addition of 1 to 2 equiv that corresponded to the stepwise deprotonation of the two phenolic protons upon the complexation (Figure 5c). It should be noted that the additional hydroxyl group present in 3-Al3+ would increase the electron density on the phenol ring, which could play an important role in the formation of the complex. In order to shed some light on the impact of the second hydroxyl group, the molecular orbitals of 3-Al3+ was calculated by using Gaussian 09 with 6-31G basis set. Comparison the molecular orbital shapes
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and sizes revealed that the phenol−Al bond in 3−Al3+ (labeled as O1−Al bond in Figure 6) was clearly stronger than that in 2−Al3+. Therefore, the addition of the second hydroxyl group at the para-position enhanced the ligand interaction with Al3+ cation, which is consistent with the experimental observation (association constant Ka for 3 is larger than Ka for 2).
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120 nM Al3+ (R2 = 0.9969) giving the detection limit (LOD = 3 std/K) of 3.1 nM (Figure S17). Herein, std is the standard deviation of the blank sample (4 in the absence of Al3+), and K is the slope of the calibration curve. The detection limit is much lower than the permissible limit of 7.41 µM for Al3+ in drinking water established by WHO. To evaluate the possible interference from other metal ions in Al3+ detection, we measured the fluorescence responses of 4 to Al3+ in the presence of another metal ion in HEPES. At the same concentration, most metal ions did not interfere with the detection of Al3+ by 4 (Figure 7). However, Fe3+, Fe2+ and Cr3+ partially reduced the fluorescence intensity of the 4-Al3+ complex. Only Cu2+completely quenched the fluorescence signal. Therefore, positive turn-on fluorescence signal of this sensor is a good indication for the predominant presence of Al3+. In quantitative determination of Al3+, samples however should be free from these interfering metal ions.
Figure 7. Relative fluorescence of 4 (10 µM) in 0.1% 3+ DMSO/HEPES pH 5.5 in the presence of Al (100 µM) plus another interfering metal ion (100 µM) tested. Figure 5. Absorption spectra of (a) 2 (10 µM) (b) 3 (10 µM) 3+ (c) 4 (20 µM) after 10 minute mixing with Al in 0.2% 3+ DMSO/HEPES for 2 and 3 and 3hours mixing with Al in CH3OH for 4.
Figure 6. HOMO and LUMO orbitals for aluminum 3+ 3+ complexes 2-Al and 3-Al , calculated with DFT at the B31LYP/6-31G level using Gaussian 09. For clarity, the aluminum atoms are labeled as shown.
To determine the detection limit of 4, the fluorescence response to Al3+ concentration in nanomolar range was investigated in HEPES buffered aqueous solution pH 5.5. A good linear response was obtained in the range of 20-
For convenient use in an on-site analysis, we developed paper-based fluorescent sensors for Al3+ detection. A series of 3.0 µL of 0.5 mM 4 solution in EtOH were dropped on to filter paper strips and then dried to get light blue fluorescence spots (Figure 8). Each spot was dropped with 1 µL containing 1.0 nmol of various metal ions. Al3+ gave a yellow while Zn2+ gave an orange emission area within the blue spot. Co2+, Ni2+, Cu2+ and Fe2+ generated a dark quenching area within the blue spots while other metal ion tested did not change the appearance of the spots. Interestingly, this detection test in solid state not only Al3+ but also Zn2+ showed the turn-on fluorescence responses, despite the fact that Zn2+ did not give significant change in aqueous media. In aqueous solution, both phenolic and hydrazidic protons are readily deprotonated to form the hard-base donor sites, phenoxide (Ar−O-) and enolate (N=C−O-), that prefers Al3+ coordination (form B in Figure 1). In an aprotic medium, the acidic protons are presumably less labile that both ArOH and NHC=O remain in neutral form (starting form in Figure 1) providing soft-base binding sites. To support this hypothesis, the fluorescence sensing study of 4 was carried out in CH3CN and found that Zn2+ gave significantly higher fluorescence
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turn-on signal in CH3CN than that of Al3+ (Figure S-18). On cellulosic material such as paper, 4 was probably in intermediate environment between water and CH3CN that allows the formation of fluorescent complexes with both Al3+ and Zn2+. The result prompted us to explore for the application of 4 in dual detection of Al3+ and Zn2+ with the aid of paper chromatography separation. The 0.5 µL of solution of Al3+, Zn2+ and their mixture containing the total ions of 5 nmol were spotted on a filter paper strip (Whatman No. 1, 8 x 2.5 cm2). The solution of 4 (0.5 µL of 2 mM) was then spotted on the metal spots as shown in Figure 9a. After drying, the strips were placed in a closed chamber containing 5:20 (v/v) of Et2NH: DMF as mobile phase21 and developed to achieve the solvent front distance of 5.5 cm. After developing, the strip clearly showed well separated orange fluorescent spot of Al3+ and Zn2+ complexes as shown in Figure 9b. To the best of our knowledge, this is the first simultaneous fluorescence detection of Al3+ and Zn2+. These results demonstrate a convenient application of 4 for dual detection of Al3+ and Zn2+.
Figure 8. Photographic image of metal ion (1 nmol) detection by 4 on filter paper tested by simple drop and dry from solutions of 4 and metal ions, consecutively. The sample was irradiated by UV-light (wavelength 365±50 nm)
3+
Figure 9. Photographic image for dual detection of Al 2+ and Zn by 4 using paper chromatography for separation: (a) before and (b) after elution with Et2NH: DMF (5: 20 v/v). The sample was irradiated by UV-light (wavelength 365±50 nm).
CONCLUSION In conclusion, we have developed a series of hydrazide derivatives, which exhibit large fluorescence turn-on for selective detection of Al3+ in aqueous media. The study illustrates that addition of the second hydroxyl group at the para-position of a phenol could have a large impact on the response of the probe, which not only tunes the emission to a longer wavelength but also enhances the ligand interaction with Al3+ cation. This para-substitution effect is further enhanced in the new double hydrazide
compound 4, which also exhibits strong fluorescence response selectively to Al3+ in a highly aqueous environment. The high Al3+ chelation-enhanced fluorescence (CHEF) is probably due to multiple mechanisms including inhibition of PET and ESIPT. The developed 4-Al3+ complex thus integrates many attractive features into a single probe molecule, which includes emission at long wavelength, remarkably large stroke shifts (133 nm), and large fluorescence turn-on. Therefore, the current aluminum sensors could find wide use for aluminum detection, since they are easier to synthesize and exhibit excellent response to Al3+ cation. In paper-based chromatography, compound 4 can also be used for fluorescent detection of both Al3+ and Zn2+ simultaneously.
EXPERIMENTAL SECTION A typical synthesis of sensors was performed by refluxing a mixture of o-hydroxybenzaldehyde (1.0 mmol) and 2-furoic hydrazide (1.05 mmol or 2.1 mmol in case of 2,5dihydroxyterephthalaldehyde) in ethanol 5 ml for 3h. The resulting mixture was cool down. The obtained solid was filter off and washed with cold methanol. Sensor 2: The precipitate was collected in 84% yield as needle-like crystal. 1H NMR (400 MHz, DMSO): δ (ppm) 12.11 (1H, s), 11.15 (1H, s), 8.64 (1H, s), 7.97 (1H, s), 7.54 (1H, d, J = 7.4 Hz), 7.34-7.25 (2H, m), 6.95-6.89 (2H, m), 6.72 (1H, dd, J = 3.5, 1.7 Hz) ppm. 13C NMR (400 MHz, DMSO): 157.3, 153.9, 148.2, 146.0, 131.4, 129.3, 119.3, 118.7, 116.4, 115.2 and 112.1 ppm. HRMS Cal. C12H11N2O3 231.0770: found M+H+ 231.1103. Anal. Calcd for C12H10 N2O3: C, 62.60; H, 4.38; N, 12.17. Found: C, 62.24; H, 4.13; N, 11.68. Sensor 3: The precipitate was collected in 76% yield as a pale yellow solid. 1H NMR (400 MHz, DMSO): δ (ppm) 11.99 (1H, s), 10.26 (1H, s), 8.98 (1H, s), 8.57 (1H, s), 7.96 (1H, s), 7.30 (1H, s), 6.97 (1H, s) and 6.75-6.70 (3H, m) ppm. 13C NMR (400 MHz, DMSO): 153.9, 150.1, 149.9, 147.6, 146.4, 145.9, 118.9, 117.1, 115.0, 113.7 and 112.1 ppm. HRMS Cal. C12H11N2O4 247.0719: found M+H+ 247.0635; C12H10N2NaO4 269.0538: found M+Na+ 269.0179. Anal. Calcd for C12H10 N2O4: C, 58.54; H, 4.09; N, 11.38. Found: C, 58.94; H, 3.64; N, 10.90. Sensor 4: The precipitate was collected in 73% yield as a yellow solid. 1H NMR (500 MHz, DMSO): δ (ppm) 12.06 (1H, s), 10.25 (1H, s), 8.62 (1H, s), 7.94 (1H, s), 7.33 (1H, s), 7.22 (1H, s) and 6.72 (1H, s) ppm. 13C NMR (500 MHz, DMSO): 154.5, 150.2, 146.8, 146.5, 146.3, 122.6, 115.6, 114.5 and 112.6 ppm. HRMS Cal. C18H15N4O6 383.0986: found M+H+ 383.1396; M+Na+ C18H14N4O6Na Cal. 405.0811, found 405.1244. Anal. Calcd for C18H14 N4O6: C, 56.55; H, 3.69; N, 14.65. Found: C, 56.18; H, 3.71; N, 13.87. Fluorescence and UV-Vis titrations For fluorescence: The stock solution of the sensing compound in DMSO (10 mM, 1 μL) was diluted with HEPES buffer pH 5.5 (10 mM, 900 μL) in a 1 mL quartz cuvette. Designated volumes (0-100 µL) of the Al3+ stock solution (10 mM) in the HEPES buffer was added into the sensor solution. The final volumes were adjusted to 1 mL by adding the solution of HEPES buffer. The final concen-
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tration of each fluorophore is 10 μM in 0.1% DMSO/HEPES aqueous solution. The spectra were recorded after mixing for 10 minutes (sensor 2 and 3) and for 30 minutes (sensor 4) using λex = 369, 410 and 468 nm, respectively. For UV-Vis: The procedures were similar to those applied for the fluorescence titration. Except for sensor 4 that methanol was used in place of HEPES buffer solution and the Al3+ solution was also prepared in methanol. The final concentration of the 4 was also higher at 20 μM in 0.2% DMSO/MeOH by using 2 μL of the stock solution of 4 (1oo mL). The UV-Visible absorption spectra were recorded from 250 nm to 700 nm at ambient temperature.
ASSOCIATED CONTENT Supporting Information The Supporting Information Available: The following files are available free of charge. 1 13 Supporting Information (PDF). H, C MNR, MS spectra of 3+ the sensors and their Al complexes, Job’s plots, pH response of the sensors, Benesi-Hildebrand plots, LOD plot and Re3+ versibility of the Al complexes have been presented here.
AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected],
[email protected]
Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. / ‡These authors contributed equally.
Funding Sources The Thailand Research Fund (Grant No. PHD/0234/2552 and TRF-MRG5680031). Nanotechnology Center (NANOTEC), though its program of Center of Excellence Network Ratchadaphiseksomphot Endowment Fund (AM1006A) University of Akron and the Coleman endowment for financial support and student scholarships. Thailand Research Fund and KMUTT Research Fund.
ACKNOWLEDGMENT We would like to thank The Thailand Research Fund through the Royal Golden Jubilee Ph. D. Program (Grant No. PHD/0234/2552), Nanotechnology Center (NANOTEC), though its program of Center of Excellence Network, National Research University of CHE and the Ratchadaphiseksomphot Endowment Fund (AM1006A), the University of Akron and the Coleman endowment for financial support and student scholarships. N. N. would also like to acknowledge the financial supports from the Thailand Research Fund (TRFMRG5680031) and KMUTT Research Fund.
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