Characterization of ZnO nanoparticles grown in presence of Folic acid template
© Dutta and Ganguly.; licensee BioMed Central Ltd. 2012
Received: 28 February 2012
Accepted: 2 July 2012
Published: 12 July 2012
ZnO nanoparticles (grown in the template of folic acid) are biologically useful, luminescent material. It can be used for multifunctional purposes, e.g., as biosensor, bioimaging, targeted drug delivery and as growth promoting medicine.
Sol–gel chemical method was used to develop the uniform ZnO nanoparticles, in a folic acid template at room temperature and pH ~ 7.5. Agglomeration of the particles was prevented due to surface charge density of folic acid in the medium. ZnO nanoparticle was further characterized by different physical methods.
Nanocrystalline, wurtzite ZnO particles thus prepared show interesting structural as well as band gap properties due to capping with folic acid.
A rapid, easy and chemical preparative method for the growth of ZnO nanoparticles with important surface physical properties is discussed. Emphatically, after capping with folic acid, its photoluminescence properties are in the visible region. Therefore, the same can be used for monitoring local environmental properties of biosystems.
Nanometer size multifunctional materials are gearing the biological fields in various ways . One of the promising nontoxic and biocompatible semiconductor material is Zinc Oxide (ZnO), which has received extensive application due to its exceptional electrical and optical characteristics  in fabricating nanoscaled electronic and optoelectronic devices. ZnO is a kind of wide band gap (3.37 eV) semiconductor with large exciton binding energy (60 meV) . In comparison to other wide band-gap semiconductors, ZnO possesses higher quantum efficiency  and higher exciton energy [4, 5]. Also, ZnO is a biofriendly oxide semiconductor and an inexpensive luminescent material. Owing to the properties stated above, it is expected to have a wide range of applications in room temperature ultraviolet (UV) lasing , biosensors , bioimaging , drug delivery  and piezoelectric transducers . In general, ZnO is considered “generally recognized as safe” (GRAS)  but ZnO nanoparticle system may be toxic. ZnO nanosystem may be of important relevance in the context of nanomedicine, where targeted treatment of biological systems at molecular level is a necessity .
Recently, there are several physical or chemical synthetic methods of preparing ZnO, such as thermal evaporation , pulsed laser deposition (PLD) , ion implantation , reactive electron beam evaporation , thermal decomposition  and sol–gel technique [18–22]. To obtain ZnO nanoparticle, we choose sol–gel method because of its simplicity, which offers a possibility of large-area yield at low cost.
Materials and methods
Chemical Synthesis of pure Zinc oxide (ZnO)
a) ZnO grown under Folic acid template
Folic acid (M.F.: C19H19N7O6, procured from Sigma. life Science), was dissolved in mildly alkaline TDW  at different percentage concentrations. Folic acid solution of desired dilution was added to zinc acetate solution and the final pH was adjusted to 7.5. The samples are denoted as Z0.2, Z0.5, Z1.0, Z1.3, Z2.0, Z3.0 and Z4.8. The suffix (Zx) represents the percentage concentration (weight/volume) of folic acid solution. After centrifugation, the precipitate was collected and re-dispersed into TDW for removal of excess ions. Finally, the precipitates were recollected and dried at 100°C. The schematic representation of the chemical synthesis is given in Figure 2. The prepared samples have been characterized by various physical techniques as given in the following classified sections.
Physical methods of characterization of the ZnO nanoparticles
X-ray diffraction (XRD) measurements
where, Dhkl is the average grain size, K the shape factor (taken as 0.9), λ is the X-ray wavelength, βP is the full width at half maximum (FWHM) intensity (here 101 peak of the ZnO spectrum fitted with a Gaussian, for precision measurement) and θ is the Bragg angle.
where, ϵ is the micro strain parameter.
Also, an estimation of the lattice parameters has been made by using FullProf program .
Transmission Electron Microscopic (TEM) study
The morphology of the synthesized product were characterized by transmission electron microscopy, TEM (Tecnai S-twin, FEI) using an accelerating voltage of 200 kV, having a resolution of ~ 1 Å. For this analysis, the ZnO sample has been dispersed in TDW through a probe sonicator; a drop of the same was placed onto a carbon coated copper grid and dried at room temperature. Furthermore, selected area electron diffraction (SAED) patterns are recorded to determine the growth orientation of the synthesized ZnO.
i) Fourier transmission infrared (FT-IR) spectra
Fourier transmission infrared (FT-IR) spectra of the powders (as pellets in KBr, without moisture) were recorded using a Fourier transform infrared spectrometer (Perkin Elmer FTIR system; Spectrum GX) in the range of 400–6000 cm-1 with a resolution of 0.2 cm-1.
ii) UV –Vis Spectroscopic measurements
The optical absorption spectra were measured in the range of 250–800 nm using a UV–VIS-NIR scanning spectrometer (Lamda 750, Perkin Elmer).
iii) Room temperature Photoluminescence (PL) Spectroscopy
Room temperature Photoluminescence (PL) measurement was carried out by a laser induced luminescence spectrometer (model IK3102R-G), the excitation source at room temperature being 325 nm line from a He-Cd laser.
Results and discussions
X-ray Diffraction (XRD) study
Lattice constant calculated from Fullprof programming
Miller indices (hkl) and corresponding peak position for Z 4.8 sample
h k l
1 0 0
0 0 2
1 0 1
1 0 2
1 1 0
1 0 3
2 0 0
1 1 2
2 0 1
0 0 4
2 0 2
Average grain size, agglomeration number (n), surface to volume ratio of the crystallites and the molecular organization of ZnO crystallites grown without and with folic acid template
Avg. grain size
No. of molecules in the surface
40 nm (pure ZnO)
(with 3% folic acid concentration)
Morphological Investigation by TEM
However, the size estimate of nanoparticles from Scherrer method differs considerably, but it may be a rough estimate from the Dhkl values in the Scherrer’s formula. Both TEM and XRD method justifies the crystalline nature of the nanoparticles.
IR frequency shift of Zn-O stretching frequency under the influence of folic acid
IR frequency (cm-1)
FTIR absorption frequencies for residual groups of folic acid
Wave number (cm-1)
Symmetric C = O stretching mode
asymmetric C = O stretching mode
UV–vis spectrum analysis
Photoluminescence spectroscopy (PL)
In pure ZnO spectrum, a weak emission peak at 440 nm (blue emission) has been observed due to surface defect in ZnO, mainly due to Zn vacancy and broad green emission band (~ 550 nm), known as a deep level emission, relates to the deep-level defect states . Singly ionized oxygen vacancy is responsible for this green emission in the ZnO . It results from the recombination of the photo-generated hole with an electron, occupying the oxygen vacancy and interstitials of zinc.
In all samples, green light emission is most prominent. With increase in folic acid concentration in the medium (above 1.3%) a dramatic change in emission spectrum is observed. The spectrum now shows the emission peak only ~ 464 nm (for Z2.0) to 472 nm (Z3.0 and Z4.8) (with the blue shift of single emission peak), which is a signature of charge transfer reaction . This clear transition is in corroborative confirmation of the effect of folic acid concentration on ZnO particle size and band gap properties shown by Figures 5 and 11. ZnO is now virtually ensconced structure with folic acid and the effect is drastic. The effect is also evident from FTIR (Tables 4 and 5) and TEM (Figure 7) study. The surface defects of ZnO are in the proximity of the functional groups of folic acid. Therefore, charge transfer effect becomes prominent and viable.
However, the physical mechanism behind visible light emission in ZnO is claimed by different authors in different ways and is still under controversy [49–52]. Therefore, it is important to investigate the luminescent mechanism caused by the defects of ZnO thin films, since they are the key factors for obtaining the visible luminescence. In our case, we find that the ZnO nanoparticle size decreases under influence of folic acid, there is a structural transition and finally the nano rod like structure is formed under the strong influence of folic acid. As a consequence to this the emission spectrum has shown the pronounced green light emission, which is conferred to photon induced charge transfer transition state.
Influence of folic acid in controlling the structural effects of ZnO nanoparticle under physiological conditions of temperature and pH has been studied as a novel method. The physical investigations with XRD, TEM and spectroscopic tools have been carried out in order to understand the interesting structural changes involved in the system which may find important biomedical applications. Photo induced charge transfer due to folic acid ensconced ZnO nanosystem is particularly a noticeable effect as seen from our results.
The author acknowledge the technical help in measurements received from Solid State Physics Division, Indian Association for the Cultivation of Science for XRD data, Biophysics Division, Saha Institute of Nuclear Physics (SINP) (Mr. Pulak Ray) for TEM measurement, Surface Physics Division, SINP (Smita Mukherjee, Dr. Satyaban Bhunia) for FTIR and PL measurements. The author also thanks Prof. P. M. G. Nambissan for UV–vis spectroscopy and Soma Roy for all kinds of technical help.
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