Received date: July 20, 2017; Accepted date: July 26, 2017; Published date: July 31, 2017
Citation: Ponnambalam P, Kumar S, Ramanathan P (2017) Interaction of Fluorescent 2-(1-Methoxynaphthalen-4-Yl)-1-(4-Methoxyphenyl)-4, 5-Diphenyl- 1H-Imidazole with Pristine Zno, Cu-Doped Zno and Ag-Doped Zno Nanoparticles. Mod Chem appl 5:226. doi: 10.4172/2329-6798.1000226
Copyright: © 2017 Ponnambalam P, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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A sensitive 2-(1-methoxynaphthalen-4-yl)-1-(4-methoxyphenyl)-4, 5-diphenyl-1H-imidazole (MNMPI) fluorescent sensor for nanoparticulates like ZnO, Cu-doped ZnO and Ag-doped ZnO has been designed and synthesized. Facile preparation of ZnO, Cu-doped ZnO and Ag-doped ZnO nanoparticles by sol-gel method using PVP K-30 as templating agents is reported and characterised by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), UVVisible spectroscopy and photoluminescence spectroscopy (PL). The synthesized sensor release is enhanced by nanocrystalline pristine ZnO but is suppressed by Cu-doped ZnO and Ag-doped ZnO nanoparticles. The suppression of fluorescence is additional by copper than by silver doping. The LUMO and HOMO energy gap of MNMPI associated with Cu-doped ZnO are lowers compared to those of pristine ZnO and thus red shift compared to that with pristine ZnO. The average crystallite sizes of ZnO, Cu-doped ZnO and Ag-doped ZnO have been deduced as 32 nm, 36 nm and 26 nm and calculated surface area for ZnO, Cu-doped ZnO and Ag-doped ZnO are 30.04 m2/g, 40.66 m2/g and 29.37 m2/g respectively. The observed enhanced absorbance with the distributed semiconductor nanoparticle is due to adsorption of MNMPI on semiconductor surface. This is because of the efficient transfer of electron from the excited state of the MNMPI to the conduction band of the semiconductor nanoparticle.
MNMPI; ZnO; Cu-doped ZnO; Ag-doped ZnO; XRD; SEM; UV; PL
The report as Xia is for Polymer-stabilized nano ZnO with blue emission  and the cell imaging is obtained by tunable photoluminescence with and the [email protected] core-shell nanoparticles [2,3]. Using single crystals or polycrystalline of Co2+: ZnO prepared by pellet sintering many scholars identified the observable photo response of Co-doped ZnO. They establish that the Ni-doped ZnO unfilled spheres exhibited only feeble ferromagnetism at 300 K whereas Co-doped ZnO hollow exhibited ferromagnetism at room temperature. Not due to any cobalt oxide phase formation or any metallic Co isolation the observed nature of ferromagnetism was intrinsic. Heterocyclic imidazole moieties have also attracted significant attention because of their unique optical properties  and for their use in preparing functionalized materials . Nanoparticles can be used as drug carriers because they have enormous surface area and due to their submicron size they can efficiently be taken up by the cells . ZnO is an attractive semiconductor material with wide direct band gap (3.37 eV), large exciton binding energy (60 meV) and a hexagonal structure and have significant applications in optoelectronics, sensors and actuators [7-9]. Hence, ZnO is one of the most attractive platforms for binding enzyme and shows potential material for a wide range of biosensor applications. Au, Ag, or Pt noble metal layered ZnO is significant for photoelectron transfer (PET) in the bulk and interface of ZnO semiconductors . Under clarification of UV light, the exciton absorption bands of ZnO are strongly bleached due to the accumulation of conduction band electrons . Thus, the effectiveness of both the photocatalysis and photoelectric energy conversion can be significantly enhanced by depositing noble metals on the surface of ZnO . The properties and applications of noble metal ZnO nanostructured materials are also determined by its morphology, structure and the organization of nanostructured ZnO architectures [13-16]. ZnO based various ceramics were synthesised by liquid phase sintering of ZnO powder of different sizes and morphologies. The HOMO and LUMO potentials for the considered sensor must match with the conduction and valence band edges of the semiconductor nanocrystals .
Materials and methods
Benzil, 4-methoxyaniline, 4-methoxynaphthaldehyde, ammonium acetate and borontrifluoride ethylethartate were purchased from Sigma Aldrich. Zinc acetate (Sd fine), polyvinylpyrrolidone (PVP K-30, Himedia), ammonia (Qualigens). The solvents used for spectral measurements were of spectroscopic grade and purchased by Hi-media. Distilled ethanol and deionized distilled water were employed for the experiments.
Synthesis 2-(1-methoxynaphthalen-4-yl)-1-(4-methoxyphenyl)- 4, 5-diphenyl-1H-imidazole
The product 2-(1-methoxynaphthalen-4-yl)-1-(4-methoxy phenyl)-4, 5-diphenyl-1H-imidazole was prepared by refluxing benzil (1 mmol), 4-methoxyaniline (1 mmol), 4-methoxynaphthaldehyde (1 mmol) and ammonium acetate (1 mmol) in ethanol (20 mL) for 2 h, borontrifluoride ethylethartate (1 mol%) acting as a catalyst (Scheme-1). The progress of the reaction was followed by TLC. After completion of the reaction, the mixture was cooled, dissolved in acetone and filtered. The product was purified by column chromatography with benzene: ethyl acetate (9:1) as the eluent.
M.P. 296°C. Anal. calcd. for C33H26N2O2: C, 82.13; H, 5.43; N, 5.81. Found: C, 82.11; H, 5.41; N, 5.80. 1H NMR (400 MHz, CDCl3): δ 3.55 (s, 3H), 3.88 (s, 3H), 6.44 (d, J=8.4 Hz, 2H), 6.62 (d, J=8.0 Hz, 1H), 6.73 (d, J=8.4 Hz, 2H), 7.12-7.18 (m, 9H), 7.29-7.44 (m, 4H), 7.82 (dd, J=6.4 Hz, 1H), 8.13 (d, 1H). 13C NMR (400 MHz, CDCl3): δ 54.18, 54.57, 102.98, 112.76, 117.88, 121.03, 124.33, 124.34, 124.46, 125.91, 126.12, 126.55, 127.11, 127.17, 127.32, 127.46, 127.74, 127.78, 129.01, 129.28, 129.97, 132.14, 132.36, 135.40, 145.36, 155.39, 157.70. MS: m/z. 482.57 [M +].
Synthesis of nanocrystalline oxides by Sol-gel method
Zinc nitrate (0.1 g) solution [with or without Cu(NO3)2/AgNO3] in 10 ml 0.01 M PVP K-30, newly prepared solution of 1:1 aq. NH3 was added slowly to reach a pH of 7, under continuous stirring. The stirring was sustained for another 30 min to get a gel. The formed glassy like white gel was allowed to age overnight. It was filtered and washed with water and ethanol several times, dried at 100°C for 12 h and calcinated at 500°C for 3 h to pale grey solid.
The 1H NMR and proton decoupled 13C NMR spectra were recorded to using a Bruker 400 MHz NMR spectrometer operating at 400 MHz and 100 MHz, respectively. The UV-vis absorption and emission spectra were recorded with PerkinElmer Lambda 35 spectrophotometer and PerkinElmer LS55 spectrofluorimeter, respectively. The powder X-ray diffractogram (XRD) was recorded with a PAN analytical X’Pert PRO diffractometer using Cu Kα rays at 1.5406 Ǻ with a tube current of 30 mA at 40 kV. A JEOL JSM 10LV scanning electron microscope (SEM) equipped with a highly sensitive backscattered detector and low vacuum secondary detector was used to get the SEM image of the sample. The UV-vis absorption spectra were recorded with PerkinElmer Lambda 35 spectrophotometer.
XRD analysis of ZnO, Cu-doped ZnO and Ag-doped ZnO nanoparticles
X-ray diffraction patterns (XRD) of pristine ZnO, Cu-doped ZnO and Ag-doped ZnO nanoparticles obtained by sol-gel method (Figure 1). All the diffraction patterns match with the JCPDS pattern of Zincite (89-7102). The crystal structures of pristine ZnO and doped ZnO are primitive hexagonal with crystal constants a and b as 3.249 A° and c as 5.025 A°. In the case of doping with copper, as the radii of Zn2+ and Cu2+ are comparable, Cu2+ can replace Zn2+ in the lattice without change in the lattice parameters. The XRD of Ag-doped ZnO reveals the presence of metallic silver in face centered crystal lattice whereas the Cu-doped ZnO fails to provide any peak other than those of ZnO. The observed peak at 38.2° is characteristic of the 111-peak of face centered cubic phase of metallic silver. The Ag+ ion is better (radius 1.22 A°) than that of Zn2+ (0.72 A°) and hence cannot be included in to the ZnO lattice.
Hence silver preferentially desire to segregate around the ZnO grain boundaries. The average crystallite sizes (L) of the sol-gel synthesized ZnO, Cu-doped ZnO and Ag-doped ZnO have been deduced as 32 nm, 36 nm and 26 nm, respectively. They have been obtained from the full width at half maximum (FWHM) of the most intense peaks of the individual crystals using the Scherrer equation, L=0.9 λ/βcosθ, where λ is the wavelength of the X-rays used, θ is the diffraction angle and β is the full width at half maximum of the peak. The calculated surface area for ZnO, Cu-doped ZnO and Ag-doped ZnO are 30.04 m2/g, 40.66 m2/g and 29.37 m2/g, respectively.
SEM and EDS analysis of ZnO, Cu-doped ZnO, Ag-doped ZnO nanoparticles and imidazole-ZnO complex
The SEM images of pristine ZnO, Cu-doped ZnO and Ag-doped ZnO nanoparticles are displayed in Figure 2. The particles are flower like use of PVP as templating agent provides finite morphology. The EDS spectra are shown in Figure 3 confirm the existence of zinc, oxygen, copper and silver signals which implies the purity of the synthesized ZnO and Cu-doped ZnO nanoparticles.
Absorption and emission behaviours of imidazole with ZnO, Cu-doped ZnO, Ag-doped ZnO nanoparticles
The photoluminescence spectra of pristine ZnO, Cu-doped ZnO and Ag-doped ZnO nanoparticles have been recorded at room temperature. They are shown in Figure 4. The pristine ZnO, Cu-doped ZnO and Ag-doped ZnO nanoparticles exhibit near band gap emission (NBE) and deep level emission (DLE). The DLE arises due to different intrinsic and extrinsic structural defects in all the nanoparticles . The NBE originates from the recombination of free photogenerated electrons and holes. These UV emissions concur with the absorption edges deduced from the Kubelka-Munk plots. Pristine ZnO display the emission around 418 nm was observed [19,20]. The emission originates from the electron transition from the superficial donor level of oxygen vacancies to the valence band (VB) and electron transition from the superficial donor level of zinc interstitials to the VB . This emission energy corresponds to the electron change from deep-level donor of the ionized oxygen vacancies to the VB. The absorption spectra of MNMPI in the existence of pristine ZnO, Cu-doped ZnO and Ag-doped ZnO nanoparticles distributed at different loading and also in their absence are displayed in Figure 5. The nanoparticles enhance the absorbance of MNMPI remarkably. The observed enhanced absorbance with the distributed semiconductor nanoparticle is due to adsorption of MNMPI on semiconductor surface. This is because of the efficient transfer of electron from the excited state of the MNMPI to the conduction band of the semiconductor nanoparticle. The emission spectra of MNMPI in the existence of pristine ZnO, Cu-doped ZnO and Ag-doped ZnO nanoparticles distributed at different loading and also in their absence are displayed in Figure 4. The pristine ZnO nanoparticle enhances the emission of MNMPI. The enhanced emission with the dispersed semiconductor nanoparticles is due to the adsorption of MNMPI on semiconductor surface. Fluorescence enhancement is due to the formation of complex [MNMPI-nanoparticulate ZnO]. Doping of ZnO by Ag and Cu shows that the dopants inhibit the fluorescence enhancement by ZnO. Figure 6 shows the linear variation of log [F0- F/F0] vs. [nanoparticles] and the calculated binding constant (K) is given in Table 1. The order of binding constant (K) is Cu-doped ZnO>ZnO>Ag-doped ZnO. The binding constant (K) of the imidazole with ZnO and Cu-doped ZnO are in the order of 107 whereas that with Ag-doped ZnO is far less. This is because in Ag-doped ZnO, Ag0 is deposited on the surface of the crystal thereby inhibiting the binding of the imidazole with ZnO. In Cu-doped ZnO, Cu2+ is likely to be present in the cationic sites or the interstitial positions thereby not influence the binding of imidazole with ZnO. Both the dopants suppress the enhancement of fluorescence and the inhibition is more by copper than by silver doping. The possible reason is Cu2+ in Cu-doped ZnO may bind with the imidazole and this binding could be much stronger than that by Zn2+. The binding of Ag with imidazole is not as strong as that of Cu2+ or Zn2+ and thus less binding constant.
|MNMPI...ZnO||3.33||0.90||2.1||2.98 × 108||0.97|
|MNMPI...Cu-ZnO||3.21||0.69||2.6||9.89 × 109||0.98|
|MNMPI...Ag-ZnO||3.42||0.79||2.2||9.02 × 107||0.91|
Table 1: Photoluminescence (τ, ns), radiative (kr, 108 s-1), non-radiative (knr, 108 s-1), binding constant (K), binding sites (n).
HOMO-LUMO energy levels of imidazole with ZnO, Cudoped ZnO, and Ag-doped ZnO nanoparticles
From the onset oxidation potential (Eox) and the onset reduction potential (Ered) of MNMPI derivative, HOMO and LUMO energy levels have been calculated using the equations , HOMO=-e (Eox+4.71) (eV); LUMO=-e (Ered+4.71) (eV). On the origin of the relative HOMO and LUMO energy levels of an inaccessible MNMPI molecule along with the conduction band and valence band edges of ZnO nanoparticles as shown in Figure 7, the electron injection would be thermodynamically allowed from the excited singlet of the MNMPI derivative to the conduction band of ZnO. The energy levels presented in Figure 7 reveals the enhancement of fluorescence of MNMPI by ZnO nanocrystals. Electron transfer from the excited imidazole to the nanocrystals is also possible because the electron in the LUMO of the excited imidazole is of higher energy compared to that in the CB of ZnO nanocrystals . This should lead to quenching of fluorescence of MNMPI. However, in differing to the expectations, enhancement of fluorescence is observed in the presence of ZnO nanocrystals. This may be because of lowering of the HOMO and LUMO energy levels of MNMPI due to adsorption on ZnO nanoparticles. The polar ZnO covering enhances the delocalisation of the π electrons and lowers the HOMO and LUMO energy levels due to adsorption. The chemical affinity between the nitrogen atom of the imidazole and zinc ion on the surface of the nano-oxide may be a reason for strong adsorption of the imidazole on nanoparticle causes the enhancement.
Binding interaction of imidazole with ZnO, Cu-doped ZnO, Ag-doped ZnO nanoparticles
The binding strength of MNMPI through its azomethine nitrogen with Cu2+ in the doped ZnO is likely to be stronger than that with Zn2+ in pristine ZnO (Figure 8). The LUMO and HOMO energy gap of MNMPI associated with Cu-doped ZnO are lowers compared to those of pristine ZnO. This inference stems from the observed red shift of the fluorescence of MNMPI on binding with Cu2+-doped ZnO compared to that with pristine ZnO. The emission of MNMPI is blue shifted on association with Ag-doped ZnO. The doped silver is likely to be present as metallic silver nano deposit on the surface of ZnO nanoparticles. The metallic silver on the surface of ZnO nanoparticles is likely to interrelate with MNMPI through its azomethine nitrogen. The interaction is likely to be weak as Ag0 is involved instead of its ionic form. The deposited silver on the surface of ZnO nanocrystals act as a shield making the MNMPI-Zn2+ of ZnO interaction less possible. The energy gap between HOMO and LUMO of the MNMPI-Ag-doped ZnO becomes larger compared to that with complex MNMPI-ZnO.
In conclusion, a sensitive MNMPI fluorescent sensor for nanoparticulate ZnO has been designed and synthesized. Facile preparation of ZnO, Cu-doped ZnO and Ag-doped ZnO nanoparticles by sol-gel method using PVP K-30 as templating agents is reported and characterised by X-ray diffraction, energy dispersive X-ray, UV-visible diffuse reflectance and photoluminescence spectra. MNMPI is adsorbed on the surface of semiconductor nanoparticle during azomethine nitrogen. The polar ZnO surface enhances the delocalisation of the π electrons and lowers the HOMO and LUMO energy levels due to adsorption. The LUMO and HOMO energy gap of MNMPI associated with Cu-doped ZnO are lowers compared to those of pristine ZnO and thus red shift compared to that with pristine ZnO. The energy gap between HOMO and LUMO of the complex MNMPI-Ag-doped ZnO becomes larger compared to that with complex MNMPI-ZnO and the emission compared to those of pristine ZnO. The conduction band energy position determines the electron transfer from excited state MNMPI to the ZnO, Cu-doped ZnO and Ag-doped ZnO nanoparticles.
The authors are very thankful to guide Dr. S. Kumar, Professor and Head, Department of Chemistry, Thiruvalluvar College of Engineering and Technology, Vandavasi for moral support in my studies. Instrumentation facilities are will be provided by Department of Chemistry, Annamalai University, Annamalainagar, Chidambaram.