DNA-Based Toolkit for Directed Synthesis of Zinc Oxide Nanoparticle

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DNA-Based Toolkit for Directed Synthesis of Zinc Oxide Nanoparticle Chains and Understanding the Quantum Size Effects in ZnO Nanocrystals Na Li,† Yukun Gao,‡ Li Hou,† and Faming Gao*,† † ‡

Department of Applied Chemistry, Yanshan University, Qinhuangdao 066004, P.R. China College of Chemistry, Jilin University, Changchun 130021, P.R. China

bS Supporting Information ABSTRACT: Growth and assembly of inorganic materials with the guidance of biomolecules is a promising route to control over the arrangement of nanoparticles. We present in this article an effective and efficient method for producing zinc oxide (ZnO) nanoparticle chains by directly using DNA as guide. Using extensive experiments over a wide range of synthesis parameters, such as the solvents and the concentrations of reactants, we have obtained high-quality ZnO nanoparticle chains in different sizes. This strategy makes it possible to tailor the optical and structural properties of ZnO nanoparticles aggregated on DNA. We have also studied theoretically the variation of the bandgap energy with the size of the ZnO nanocrystals using a chemical bond theory of quantum size effects. Furthermore, possible mechanisms are discussed in detail.

1. INTRODUCTION Zinc oxide (ZnO) is one of the few oxides that show quantum confinement effects in the experimentally accessible range of sizes (100 nanoparticles were considered for each sample to obtain their size distribution histograms. While for S1 and S2 the average particle sizes are about 5.75 and 6.75 nm with small size dispersion, for S3 and S4 the average particle sizes are about 10.5 and 12 nm with larger size dispersion. It reveals that the size of ZnO nanoparticles could be tuned by facile adjustment of the amount of Zn(II). As the concentrations increased, the larger size of ZnO nanoparticles was observed. It is worth noting that when the concentrations of reagents increased beyond a certain threshold (e.g., [Zn(II)] > 10 mM), higher average particle sizes were obtained. The difference in degree of nanoparticles aggregation on DNA can be accounted for by the excess amount of Zn(II). Because the concentrations of DNA solution for all the samples were almost the same, it provides nearly the same number of active sites that participate in recognition of the Zn(II). The active sites will be in saturation status once there are sufficient Zn(II) to bind to and no further combination of Zn(II) is necessary. Therefore, the excess amount of Zn(II) will cause 25267

dx.doi.org/10.1021/jp2094033 |J. Phys. Chem. C 2011, 115, 25266–25272

The Journal of Physical Chemistry C

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Figure 1. (a d) TEM images of ZnO nanoparticles aggregated on DNA with different concentrations of Zn(II). (a) S1 (5 mM), (b) S2 (10 mM), (c) S3 (20 mM), and (d) S4 (40 mM), respectively. (a’ d’) Particle size distribution histograms of ZnO nanoparticles from TEM images (a d), respectively. (e) Higher magnified TEM image of ZnO nanoparticles aggregated on DNA imperfectly. The inset in (d) shows the EDXS of DNA-based ZnO nanoparticle chains. (f) SAED pattern taken on the ZnO nanoparticle chains shown in (c). Scale bar =100 nm.

Figure 2. X-ray diffraction patterns of ZnO nanoparticles aggregated on DNA. All the XRD patterns can be indexed as the hexagonal wurtzite structure of ZnO. The theoretical XRD pattern was shown as a line pattern (JCPDS file no. 36-1451).

larger agglomeration by the conjugation of primary ZnO nucleus, and it inevitably results in the formation of nanoparticles in a larger size range. Since TEM is the most direct probe to observe the morphology, we use such an appropriate technique to probe the growth of ZnO nanoparticle chains to investigate whether ZnO underwent heterogeneous growth on DNA or homogeneous growth of self-aggregation. Figure 1e shows the higher magnified TEM image of ZnO nanoparticles which were aggregated on DNA imperfectly. We can see the naked DNA strands

without combining ZnO nanoparticles. It provides the vivid demonstration that ZnO nanoparticles are indeed growing and assembling on DNA chain. The inset in (d) shows the EDXS of DNA-based ZnO nanoparticle chains. The Cu and C peaks originate from the TEM grid, and P signal may arise from DNA. It is confirmed by EDXS that we achieve purely heterogeneous ZnO growth on DNA. The selected area electron diffraction (SAED) pattern recorded from ZnO nanoparticle chains indicates the crystalline nature of the particles (Figure 1f). From the diffraction pattern, we found that the values correspond to (100), (002), (101), (102), (110), (103), and (112) planes with hexagonal wurtzite structure of ZnO, respectively. The results match well with the XRD results . Figure 2 depicts typical powder X-ray diffraction patterns of ZnO nanoparticles aggregated on DNA. All the XRD patterns can be indexed as the hexagonal wurtzite structure of ZnO and in good agreement with the JCPDS file no. of ZnO (JCPDS 361451). No peaks from any other phase of ZnO or impurity were observed in the experimental range. Thus, the phase-pure wurtzite structure of ZnO nanoparticle chains has been successfully synthesized under our preparation route. Presented in Figure 3 are room temperature optical absorption spectra of samples S1 S4. Depending on the concentrations of Zn(II) used to prepare the ZnO nanoparticle chains, the differences can be recognized among the spectra. First, with the increased amount of the Zn(II), the excitonic absorption peak shifts consistently to larger wavelengths. It is likely to be due to the nanoparticle size increase following this order. It coincides exactly with the TEM images. Second, there is another excitonic absorption peak (ca. 260 nm) in the spectra. This absorption peak arises from DNA which has a UV absorption maximum of 25268



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260 nm. It is noteworthy that the absorption intensity decreased gradually with an increasing amount of Zn(II). This result indicates that a certain interaction occurred between Zn(II) and DNA. It is likely to be explained in part by the combinations of ZnO on the DNA surface. DNA served some ligands for nucleation, growth, and capping of ZnO nanoparticles. As the concentrations increased, more and more ZnO nanoparticles would combine with DNA. It will shield the ligands on DNA which could absorb the UV, thus resulting in the reduction intensity of UV absorption. As shown in the inset of Figure 3, common to all the spectra are the significant blue shifts of these features relative to the bulk absorption edge. The bandgap energies determined from absorption spectra show a significant quantum size effect, which can be quantitatively explained using a recent theoretical model,28,29 as discussed below. We have used a chemical bond theory of quantum size effects of semiconductor nanocrystals to study the variation of the bandgap energy with the size of the nanocrystals. According to theory, the energy-gap shift ΔE can be expressed as the sum of the surface effect shift part ΔEsurface and the Kubo effect shift part ΔEkubo: ΔE = ΔEkubo + ΔEsurface, ΔEkubo = (mee4/2p2)(VNe) 1/3, where Ne is the valence electron density of the bonds and V is the volume of nanocrystals. The surface energy shift part may be expressed surface Ebulk is the as ΔEsurface = Fsurface (Esurface g g ), where F and fraction of surface bonds composing the nanocrystal. Esurface g represent the gap of the surface bond of the nanocrystal and Ebulk g the bulk phase, respectively. The energy gap Eg between the highest occupied and the lowest unoccupied energy level may be

given by Eg2 = Eh2 + C2, where Eh is the homopolar gap and C is the heteropolar gap. The ionicity of the chemical bond is fi = C2/Eg2. The detailed results for the spherical ZnO nanocrystals are listed in Table 1. (n is atomic number, nb is the number of bonds, and Nc is the average coordination number; energy-gap shift ΔE, kubo energy shift ΔEkubo, and surface energy shift ΔEsurface are in eV.) From Table 1, it can be seen that the atoms at the surface of the nanocrystals have a lower average coordination number Nc but the valence electron densities at the surface bonds Ne are much higher than that of the bulk. The heteropolar Coulomb gap C and the ionicity fi of the surface bonds decrease by about 3 times relative to the value in bulk materials, leading to negative surface energy shifts ΔEsurface. Therefore, the heteropolar Coulomb gap C plays a key role in the quantum size effects of ZnO nanocrystals. Figure 4 gives the energy-gap shifts calculated for wurtzite ZnO nanocrystals along with the experimental data from this work and Viswanatha’s work.22 Our experimental bandgap shifts of ZnO nanocrystals with the sizes 5.75, 6.75, and 10.5 nm are in good agreement with the calculated values. One of the ZnO nanocrystals with the size of 12 nm is slightly less than the calculated value. Figure 5 shows PL and PLE spectra of samples S0 S4. PLE spectrum shows the optimal excitation wavelength for the emission is ca.350 nm. Correspondingly, the direct photograph of all samples (Figure 5, inset) under UV irradiation demonstrates the strong green emissions. All the above samples evince two emission bands: one is at ca. 411 nm and the other is at ca. 535 nm. The UV emission is usually considered as the characteristic emission of ZnO30,31 and attributed to the band edge transition or the exciton combination.32 For green emissions, a number of different hypotheses have been proposed.33 35 The green emissions are now universally considered to be associated with the intrinsic or extrinsic defects in ZnO.36 39 In our case, these defects or traps are most probably on the nanoparticle

Figure 3. Optical absorption spectra of the samples (S1 S4). The inset is an enlargement of the dashed frame. The absorption edges of the nano ZnO are marked by arrows and correspond to 3.465 eV (S1), 3.452 eV (S2), 3.421 eV (S3), and 3.354 eV (S4). The bulk bandgap is 3.35 eV.

Figure 4. Dependence of the energy shifts of ZnO on the cluster size. The solid line is from our calculations of the spherical clusters. The open circles and open diamonds represent experimental data from this work and ref 22, respectively.

Table 1. Theoretical Parameters for the Spherical Nano ZnO D(Å) 57.5

n 8370

nb inside surface

67.5

13 538

inside surface

105 120

d(Å)

Ne(Å 3)

Nc

C(eV)

Eh(eV)

Eg(eV)

fi

1647

1.988

0.331

4.000

9.145

7.230

11.658

0.615

14 235

1.930

0.544

2.659

3.678

7.781

8.607

0.183

2254

1.988

0.331

4.000

9.145

7.230

11.658

0.615

16 304

1.930

0.547

2.641

3.622

7.781

8.583

0.178

ΔEkubo

ΔEsurface

ΔE

0.422

0.316

0.106

0.359

0.268

0.092

50 963

inside

5603

1.988

0.331

4.000

9.145

7.230

11.658

0.615

0.231

0.172

0.059

76 024

surface inside

23 627 7342

1.930 1.988

0.542 0.331

2.666 4.000

3.701 9.145

7.781 7.230

8.617 11.658

0.185 0.615

0.202

0.151

0.051

surface

140 929

1.930

0.543

2.663

3.693

7.781

8.613

0.184

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Figure 5. PL and PLE spectra of the samples (S0 S4). Samples were excited by ca. 350 nm light, and their PLE spectra were recorded with a relative detection wavelength of ca. 535 nm. The inset shows direct photographs of S0 S4 under irradiation by a UV lamp.

Figure 6. FTIR spectra of pure DNA and DNA-based ZnO nanoparticle chains.

surfaces. The broad green band (ca. 535 nm) appears in the PL spectra of all the samples. Sample S0 was prepared using the same recipe and process as those for S2 except for DNA. On the basis of the PL spectra, the strongest green peak was observed in S0. In contrast to that of S0, the green emission peak of S2 decreased dramatically when the ZnO nanoparticles combined with DNA. The TEM image of sample S0 is presented in Figure S1(a) in the Supporting Information. The ZnO nanoparticles in S0 are seen to be uniform and disperse. The size of nanoparticles in S0 is about 6.5 nm, which is very close to that in S2 (6.75 nm). The size may have little influence on the spectra, but it can be ignored. Therefore, the differences among the PL spectra could not be caused by the size or the shape of S0 particles. This leads us to believe that the green emission is due to the surface defects. DNA molecules are proven to exist on the surface of ZnO nanoparticles and may partially eliminate the defects or traps. It seems DNA has the direct and efficient passivation effect, and the DNA-based ZnO nanoparticle chains therefore have weaker green emissions. As for S1 S4, the green peak intensities decreased with the increasing amounts of Zn(II). The particle size is larger because of excess Zn(II) (Figure 1a d). As the particle size increased, the number, as well as the total surface area, of ZnO particles should be reduced. Correspondingly, the defects or traps on the surface of ZnO nanoparticles decreased dramatically. It inevitably leads to the gradual reduction of green peak

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intensities and suggests that the green emission is a surfacerelated behavior. The choice of DNA ligands used in synthesis provides a powerful means of rational assembly of nanoparticles. A number of moieties on nucleotides, including backbone phosphate groups and base functionalities, are candidates for terminating growth and serving as a capping layer. However, the salient chemical features of polynucleotides essential for ZnO nanoparticle chains synthesis have yet to be identified. Here we investigate systematically how nucleotide functionalities influence nanoparticle growth. As seen from the FTIR spectrum in Figure 6, the two peaks in the range of 980 1335 cm 1 are attributed to the stretching vibrations of PdO groups in DNA molecules. The peaks at ca. 1335 1820 cm 1 are assigned to ν(C O X) mode. Moreover, the stretching vibrations of C H (ca. 2820 3000 cm 1) and OH groups (ca. 3200 3600 cm 1) are also detected. The measured intensity directly indicates the changes in the character of a particular bond. As seen from the FTIR, the remarkable changes can be distinguished at ν(PdO) and ν(C O X) mode. The vibration of surface-free ZnO was reported to be located only at 450 cm 1.40 Therefore, these changes discovered here are mainly attributed to the interaction between ZnO and DNA molecules. For pure DNA, the ν(PdO) stretch appears as a strong and sharp peak at 1235 cm 1 but this is much weaker in sample “DNA+ZnO”. It indicates ZnO is actually bonded to the phosphate groups of DNA. These phosphate groups guide the interactions between DNA and Zn(II) by electrostatic forces, helping control the growth of ZnO nanoparticles on DNA molecules. Hence, the phosphate groups are the functionalities present on DNA, which are a prerequisite for the production of DNA-based ZnO nanoparticle chains. The backbone of the DNA strand is made from alternating phosphate and sugar residues. The sugars are joined together by phosphate groups that form phosphodiester bonds. Conjugation of ZnO nanoparticles with phosphate groups leads to enhanced polarity of nearby groups (C O X), thus inducing the stronger absorption intensity of ν(C O X) mode. Infrared spectroscopy exploits the fact that phosphate groups presented on DNA molecules serve as useful ligands to bind Zn(II) and control the growth for nanoparticle synthesis. Based on the above, we propose a general mechanistic framework describing how DNA promotes and controls ZnO nanoparticles growth (see Scheme 1). The above results showed that DNA offered negatively charged phosphate group from its backbone as the attaching site to localize inorganic metal complex. This novel combination has yielded stimuli-responsive nanomaterial assemblies. It appears Zn(II) is directly bonded to the phosphate groups present on DNA electrostatically when Zn(II) source is added. After addition of NaOH solution, Zn(OH)2 forms in an alkaline condition and subsequently begins to decompose by slight heating. Finally, in the solution with adequate Zn(II), Zn(OH)2 are bonded together through a dehydration reaction forming the ZnO nuclei. As the size reaches the critical value required for the formation of ZnO nanoparticles, the ZnO nanoparticles forms along the backbones of DNA. Then the DNA-based ZnO nanoparticle chains structure is expected to form. It is worth noting that the concentrations of DNA solution for all the samples were almost the same; it provides nearly the same number of active sites that participate in recognition of the Zn(II). Therefore, when the concentrations of reagents increased beyond a certain threshold (e.g., [Zn(II)] > 10 mM 25270


The Journal of Physical Chemistry C Scheme 1. Schematic Illustration of the Formation of DNABased ZnO Nanoparticle Chains (Drawing Not to Scale)

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: 86 335 8387552. Fax: 86 335 8061569.

’ ACKNOWLEDGMENT Financial support from the National Natural Science Foundation of China (Grants 21071122, 21101134) and Research Fund for the Doctoral Program of Higher Education of China (Grant 20091333110009) and the Natural Science Foundation of Hebei (Grant E2010001169, ZD2010112) and the Science Foundation of Yanshan University for the Excellent Ph.D. Students ’ REFERENCES

in our experiment), only a part of the Zn(II) is attached to DNA, and the rest is then used for crystal growth. Excess amount of Zn(II) will cause larger agglomeration by the conjugation of primary ZnO nucleus, and results in a larger particle size (Figure 1c). However, continued increase of the amount of Zn(II) does not have much effect on the size of the nanoparticles (Figure 1d). This is probably due to the steric hindrance effect of ZnO nanoparticles saturated coverage on the DNA ligands. Although the formulation mechanism is premature at this stage, it might be viewed as the extremely important guide for future elaboration of the methodology for the preparation of DNA-based ZnO nanoparticle chains.

4. CONCLUSION We have coupled the natural properties of DNA with its recognition capacity to guide ZnO nanoparticle chains synthesis. DNA molecules are proven to partially eliminate the defects or traps on the surface of ZnO nanoparticles, and leads to the reduction of green peak intensities. This suggests that the green emission is a surface-related behavior. Meanwhile, we have also used a chemical bond theory of quantum size effects of semiconductor nanocrystals to calculate the bandgap as a function of the size of ZnO nanocrystals. The results are in good agreement in the case established by our approach. Furthermore, a probable mechanism of DNA-based ZnO nanoparticle chains has also been described. FTIR spectra confirm the phosphate groups on DNA are the favorable targets to feed nanoparticle growth. Our method provides a novel way to fabricate ZnO nanoparticle chains by virtue of its simplicity and environmentally benign, which makes them promising candidates for further processing, assembly, or practical applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. TEM image and particle size distribution histograms of sample S0. This material is available free of charge via the Internet at http://pubs.acs.org.

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DNA-Based Toolkit for Directed Synthesis of Zinc Oxide Nanoparticle

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