New Synthesis Strategy for DNA Functional Gold Nanoparticles

Feb 10, 2011 - New Synthesis Strategy for DNA Functional Gold Nanoparticles. Liguang Xu, Yingyue Zhu, Wei Ma, Wei Chen, Liqiang Liu, Hua Kuang, Libing...
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New Synthesis Strategy for DNA Functional Gold Nanoparticles Liguang Xu, Yingyue Zhu, Wei Ma, Wei Chen, Liqiang Liu, Hua Kuang, Libing Wang,* and Chuanlai Xu* School of Food Science & Technology, State Key Lab of Food Science & Technology, Jiangnan University, Wuxi, 214122, China

bS Supporting Information ABSTRACT: Gold nanoparticles (GNPs) are widely used in many fields due to their compatibility with various biosystems and their ability to be easily functionalized with biomolecules. However, current citrate-reduced GNPs are prone to aggregate and unstable at high ionic strength solution, which greatly limits their applications. Thus, new preparation methods of the GNPs, which can maintain the stability under the extreme conditions, are still urgently needed. Here, we describe a new GNP synthesis strategy by using trisodium citrate to reduce hydrogen tetrachloroaurate (III) (HAuCl4) in the presence of single-strand DNA (ssDNA) at 37 °C. Under the assistance of the ssDNA, the GNPs can be prepared at physiological temperature and still maintain the uniform shape and dispersity. More importantly, the stability of the GNPs against salt-induced aggregation was improved greatly. With their high stability and in situ functionalized ssDNA, these GNPs have great potential in physiological systems and could be used for nanomedicine, nanoapparatus, and other applications.

1. INTRODUCTION Over the past several decades, GNPs have been the subject of intensive research because of their size-dependent optical, chemical, and catalytic properties and surface-enhanced Raman scattering.1 GNPs have been applied in programmed assembly,2-4 selective catalysis,5 crystallization of materials,6,7 and sensitive biosensors8,9 and have been arranged into dimers, trimers, and chain structures on DNA templates.10 Various methods have been used to fabricate GNPs, including the Turkevitch-Frens method,11,12 the Schmid method,13 the Brust-Schiffrin method,14 and seed-mediated growth methods.15,16 The Turkevitch-Frens method is the most widely used. Trisodium citrate serves as both a reducing agent and an anionic stabilizer in this method.11 The Turkevitch-Frens method is relatively convenient, and GNPs obtained exhibit relatively high chemical stability in water but are less stable in biological buffers with high ionic strength.17 Moreover, this method requires a temperature near the boiling point. Thus, novel GNP synthesis strategies using mild conditions that yield GNPs stable against salt-induced aggregation and yet are compatible with downstream processing would be highly desirable. Nucleic acids are increasingly popular as versatile building blocks in nanobiotechnology because of their excellent and predictable self-assembly properties, the high rigidity of their double helices on the nanoscale, and their stabilization of GNPs.18 The stability of GNPs functionalized with thiol-modified DNA has been investigated.17,19-21 DNA on the surface of GNPs could effectively stabilize the particles against salt-induced aggregation. The salt tolerance of ssDNA-modified GNPs was improved at least 6-fold compared to citrate-protected GNPs (GNPs obtained by the classical Turkevitch-Frens method). r 2011 American Chemical Society

Unfortunately, there are some drawbacks using thiol-modified DNA stabilized GNPs: (a) it needs an additional procedure to stabilize the GNPs using thiol-modified DNA. The stabilization step commonly requires a long modification time. (b) The thiolmodified DNA requires the deprotection with some special procedure before the modification of citrate protected GNPs. (c) It is difficult to select an optimum buffer and pH value for the GNP modification with thiol-modified DNA to ensure the GNPs, keeping their stability in the entire procedure. (d) Thiol-modified DNA sometimes could bind the surface of GNPs unspecifically, and it is unfavorable for the downstream processing. (e) In the modification of citrate-protected GNPs with thiol-modified ssDNA, it needs salt aging, sonication, and other procedures to prompt the thiol-DNA conjugation with citrateprotected GNPs. ssDNA adsorbs onto citrate-protected GNPs in a sequence-dependent manner.19,22,23 Deoxynucleosides dA, dC, and dG exhibit a much higher binding affinity to gold surfaces than dT.24 In one recently established method, hydroxylamine was used to reduce HAuCl4, and gold metal was selectively deposited on ssDNA adsorbed on the surface of 20 nm citrateprotected GNPs to form flower-like GNPs.25 The authors observed that 30-mer poly A or poly C adsorbed on the GNP surface and acted as a template for gold nanoflower formation. Furthermore, the DNA retained its biorecognition ability. However, it has been reported that hydroxylamine acts solely as a growth reagent under certain conditions.11,26,27 Since citrateprotected GNPs act as seeds for the growth of gold nanoflowers, Received: October 9, 2010 Revised: January 17, 2011 Published: February 10, 2011 3243

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Figure 1. Schematic illustration for the deduced mechanism of highly stable and functional gold nanoparticle formation in the presence of ssDNA at 37 °C.

we wondered what would happen if ssDNA were added in the initial stage of the Turkevitch-Frens method to synthesize GNPs under mild conditions. Here, we present a new synthesis method for preparing GNPs using trisodium citrate to reduce HAuCl4 in the presence of ssDNA at 37 °C. The aims of the present study were: (1) to investigate how different ssDNA lengths and sequences affect the GNP morphology and size distribution; (2) to compare the stability of ssDNAGNPs and citrate-protected GNPs against salt-induced aggregation; and (3) to evaluate the biorecognition of ssDNA on the surface of GNPs for potential applications. We chose three different lengths (1, 6, and 12 mer) and four different DNA types (A, T, C, and G). ssDNA could effectively restrain particle coagulation at 37.0 °C, except for dTMP. GNPs with a uniform morphology and an even size distribution were obtained in the presence of 12-mer polydeoxynucleotides, especially dG12. The stability of ssDNAGNPs was evidently improved compared with citrate-protected GNPs, and the fluorescence of FAM-DNA was enhanced after it was hybridized with ssDNA-GNPs. A mechanism for growth of ssDNA-GNPs is proposed (Figure 1). UV/vis spectroscopy, transmission electron microscopy (TEM), and dynamic light scattering (DLS) measurements were conducted to characterize the morphology and uniformity of the GNPs.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4) (99%) and trisodium citrate were purchased from Sigma-Aldrich. All oligonucleotides used were synthesized and purified using polyacrylamide gel electrophoresis by Sangon Biotechnology (Shanghai, P. R. China). The DNA sequences are shown in Table 1. Milli-Q ultrapure water (18.2 MΩ) was used in all experiments. Tris-HCl and other reagents were all of analytical grade and were used without further purification. All glassware used was cleaned with freshly prepared aqua fortis, rinsed thoroughly in Milli-Q ultrapure water, and then oven-dried prior to use. The sample morphology was examined using a JEOL JEM-2100 transmission electron microscope. Absorbance spectra were acquired on a UNICO 2100 PC UV/vis spectrophotometer. The zeta potential was measured using a Zetasizer Nano ZS system (Malvern). Fluorescence spectra and intensities were measured on an F-7000 FL 220-240 instrument (Hitachi, Tokyo, Japan).

Table 1. Sequences and Length of Synthesized Oligonucleotides Were Used for GNP Synthesisa number

sequence (50 -30 )

length

1

dAMP

1

2

dTMP

1

3

dCMP

1

4 5

dGMP AAAAAA

1 6

6

TTTTTT

6

7

CCCCCC

6

8

GGGGGG

9

AAAAAAAAAAAA

12

10

TTTTTTTTTTTT

12

11

CCCCCCCCCCCC

12

12

GGGGGGGGGGGG

12

6

a

dAMP (dTMP, dGMP, and dCMP) is for the single mer deoxyadenosine (deoxythymine, deoxyguanosine, and deoxycytosine) monophosphate.

2.2. GNP Synthesis. GNPs were synthesized with ssDNA by reduction of HAuCl4 using trisodium citrate. The flowchart was shown in Figure 2. In brief, DNA was dissolved in ultrapure water to yield a concentration of 50 μM. HAuCl4 solution (0.1 mL, 5 mM) was added to 1.9 mL of Milli-Q ultrapure water at a constant temperature of 37.0 °C and stirred to blend. Then, 20 μL of 50 μM ssDNAs of different length or sequence was added to the mixture. After stirring for 15 min at 37.0 °C, 40 μL of freshly prepared trisodium citrate (1% by weight) was quickly added to the solution under vigorous stirring. After reaction at 37.0 °C for 12 h under vigorous stirring, the solution color changed from primrose yellow to wine red. The as-synthesized GNPs were stored at 4 °C before use in subsequent experiments. In control experiments, the same volume of ultrapure water replaced the ssDNA added to the HAuCl4 solution. The resulting GNPs are denoted citrate-protected cGNPs. Uniform citrate-protected GNPs with a diameter of 17 nm were prepared by the classical Turkevitch-Frens method. The detailed steps for this method are described in the Supporting Information. These GNPs are denoted citrate-protected GNPs. 2.3. Salt Tolerance of As-Synthesized GNPs. To investigate the salt tolerance of as-synthesized GNPs, we chose NaCl as a 3244

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Figure 2. Scheme showing the steps required for ssDNA-GNPs synthesis in the presence ssDNA under mild conditions.

representative salt and measured aggregation of dGMP-, dG6-, and dG12-GNPs compared to citrate-protected GNPs. A 100 μL aliquot of sample or control GNPs was centrifuged for 15 min at 10 000 rpm. After removal of the supernatant, the GNPs were dispersed in water (100 μL). After one more washing step with water, GNPs were dispersed in 100 μL of NaCl at different concentrations (Figure 4) that were estimated based on the volume fraction of the supernatant removed after centrifugation. All NaCl concentrations were prepared by diluting 5 M NaCl with Milli-Q ultrapure water in triplicate. The color and UV/vis absorption spectrum of the samples were measured after 10 min at room temperature using a digital camera and UV/vis spectrophotometer. 2.4. Characterization of DNA on the Surface of GNPs. Two methods were used to verify the presence of DNA on the surface of GNPs: the fluorescence of ssDNA-GNP-dye hybrid nanostructures was measured, and aggregation of ssDNA-GNPs on addition of cDNA was measured. Complementary single-strand 12-mer polycytosine with a 50 FAM label (HPLC-purified) was purchased from Sangon Biotechnology (Shanghai, P. R. China) (Table S1, Supporting Information). FAM-labeled oligonucleotides were hybridized with GNPs functionalized with polydeoxyguanosine of different length by centrifuging 100 μL of as-synthesized GNPs for 15 min at 10 000 rpm and redispersing the precipitate in 20 μL of hybridization buffer (0.01 M Tris, pH 8.0, 0.13 M NaCl). The dyecontaining complementary strand (4 μL, 10 μM) was immediately added in the dark. The fluorescence response of the resulting mixture was recorded after reaction for 12 h at room temperature and was compared to control experiments in the absence of cDNA. Experiments were repeated at least three times. The procedure for characterizing ssDNA on the surface of GNPs using the other method is described in the Supporting Information. 2.5. Characterization Procedures. TEM images of assynthesized GNPs were acquired using a JEOL JEM-2100 instrument at an acceleration voltage of 200 kV. Typically, 1 mL of solution was centrifuged for 15 min at 10 000 rpm, and the supernatant was discarded. The precipitate was redispersed in 1 mL of Milli-Q ultrapure water and centrifuged again. Finally the precipitate was redispersed in a suitable volume of Milli-Q

Figure 3. Typical TEM images of different homopolydeoxynucleotideassisted GNPs synthesis at 37.0 °C. First row: 1-mer dexoynucletide monophosphate. Second row: 6-mer homopolydeoxynucleotide. Third row: 12-mer homopolydeoxynucleotide. Fourth row: without DNA. Citrate cGNPs: without ssDNA at 37.0 °C. Citrate GNPs: without ssDNA at 100.0 °C. Scale bar: 20 nm.

ultrapure water, depending on the amount of precipitate. A 7 μL aliquot of the sample was dropped on a Formvar/carbon film Cu grid (200 mesh; 3 mm) and dried under an IR lamp. The absorbance spectrum was measured on a UNICO 2100 PC UV/ vis spectrophotometer. Quartz cuvettes with an optical path length of 1 cm were used for 100 μL of sample diluted with 900 μL of Milli-Q ultrapure water. DLS samples were prepared by diluting 100 μL of sample with 900 μL of Milli-Q ultrapure water in a plastic curvette (1 cm). After equilibration at 20 °C for 1 min, samples were analyzed twice by DLS.28 The photoluminescence intensity of FAM-DNA and hybridization of FAM-DNA and assynthesized GNPs were measured on an F-7000 FL 220-240 3245

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Figure 4. Effect of NaCl on the stability of citrate protected (A) and homopolydeoxyguanosine-GNPs (B-D, dG-, dG6-, dG12-, respectively). The color (left images) and UV/vis spectrum (right images) of GNPs were measured after 10 min at room temperature. The color changes indicate the aggregation of GNPs. The unit of concentration (not marked) of NaCl was mM in the above picture.

instrument with excitation at 490 nm and emission at 519 nm. A 10 μL aliquot of the mixture was diluted with 990 μL of 10 Tris-HCl (pH 8.0) in a quartz cuvette (1 cm). After equilibration at 25 °C for 2 min, samples were analyzed twice.

3. RESULTS AND DISCUSSION 3.1. ssDNA-GNP Synthesis via Hydrogen Tetrachloroaurate Reduction Using Trisodium Citrate. GNPs were chosen

as the synthetic model because of their unique tunable optical and chemical properties and their associated potential for extensive applications.2 The GNP morphology was observed by TEM for samples prepared using oligonucleotides of different length and type (Figure 3). HAuCl4 solution was reduced with trisodium citrate in the absence of oligonucleotides at 37.0 °C as a control (the citrate cGNPs in Figure 3). The color of the mixture darkened, and a purple color was finally obtained. The GNPs obtained were highly irregular with a diameter of ∼20 nm and 40-80 nm length (average for >150 particles in the sample). The results indicate that irregular GNPs were formed by

deposition of gold atoms in straight or bent chains on nucleation and that growth took place during particle coagulation.11 The first row of Figure 3 shows typical 1-mer deoxynucleotideGNPs synthesized using single polydeoxynucleotides. dAMPGNPs, dCMP-GNPs, and dGMP-GNPs predominantly consisted of spherical particles, whereas a high proportion of dTMP-GNPs had an irregular outline. TEM images of ssDNAGNPs synthesized using 6-mer oligonucleotides are shown in the second row of Figure 3. The morphology of these particles was nearly spherical, apart from dT6-GNPs, for which a number of triangular particles were observed. Among all the GNPs prepared, the best morphology and dispersibility were observed for the 12-mer deoxynucleotides (the third row of Figure 3). In particular, dG12-GNPs had a spherical morphology and an almost uniform size distribution. As shown in Figure 3, the quality of dG12-GNPs could match the gold nanoparticle synthesized by the classical trisodium citrate reduction, and moreover, dG12-GNPs functionalized with ssDNA could facilitate the further application. Our results demonstrate that for the same reaction conditions a change in the type and length of 3246

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The Journal of Physical Chemistry C oligonucleotides could change the shape and size distribution of GNPs. The zeta potential of as-synthesized GNPs is shown in Figure S1 (Supporting Information). The zeta potential of all GNPs was negative and was higher for all ssDNA-GNPs than for citrateprotected cGNPs. This increasing zeta potential indicates that the surface of ssDNA-GNPs was covered with a certain amount of ssDNA. As shown in Figure S1 (Supporting Information), the zeta potentials of different length polydeoxynucleotide modified GNPs were almost similar due to the homopolydeoxynucleotides with the similar charge. 3.2. Mechanism of GNP formation. In the classical Turkevitch-Frens method, GNP synthesis involves nucleation and growth, and the shape and size distribution depend largely on the relative rates of these two separate processes.11 Trisodium citrate causes both nucleation and growth.29 Researchers have explored the mechanism of GNP formation.30-34 Polte et al. studied the mechanism of GNP formation in the classical Turkevitch-Frens method via in situ SAXS and XANES using synchrotron radiation. The reaction should involve fast initial formation of small nuclei, coalescence of nuclei to form larger particles, slow particle growth sustained by ongoing reduction of precursor species, and finally fast reduction of residual precursor species. Here, we synthesized GNPs using oligonucleotides of different length and type at 37.0 °C. A simple schematic of the mechanism of GNP formation in the presence of DNA is shown in Figure 1. Deoxynucleosides dA, dC, and dG have a high binding affinity for citrate-protected gold surfaces. After addition to HAuCl4, trisodium citrate gradually binds gold ions into large macromolecules, which can undergo molecular rearrangement to produce nuclei. The surface of these nuclei is covered by a layer of DNA due to dipolar interactions between ssDNA and the surface of citrateprotected nuclei. Gold atoms reduced by trisodium citrate or autocatalytic reduction on the surface of nuclei30 could further deposit on the ssDNA on the surface of nuclei to form an additional layer, and then free ssDNA in solution could adsorb onto this additional layer of newly deposited gold atoms. This process of ssDNA adsorption and gold atom deposition could continue in a cycle. GNPs would gradually increase in size until complete conversion of HAuCl4. A layer of ssDNA on the surface of GNPs could effectively inhibit particle aggregation at 37.0 °C to yield regular spherical GNPs of a relatively uniform size distribution. 3.3. Enhanced Colloidal Stability of ssDNA-GNPs. To investigate the stability of ssDNA-GNPs, 17 nm citrate-protected GNPs and ssDNA-GNPs prepared with dGMP, dG6, or dG12 were dispersed in solutions of different NaCl concentrations. The stability of the solutions was evaluated using UV/vis absorbance measurements and colorimetric observations. NaCl triggers particle aggregation, which leads to a red shift and broadening of the UV/vis absorbance peak and a color change that can be visualized with the naked eye. As shown in Figure 4A, GNPs were not stable in solution, and the color change to blue and the red shift of the UV/vis peak occurred at all concentrations g50 mM NaCl. These changes are indicative of classical citrate-protected GNP aggregation in water. For dGMP-, dG6-, and dG12-GNPs, red shifts of the UV/vis peak and color changes only occurred at >400 mM NaCl (Figure 4B). Thus, guanine oligonucleotides on the GNP surface improve the particle stability in salt solution. The NaCl concentration that induced particle aggregation increased with the number of guanine mers used to synthesize GNPs. For example, the changes typical for

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Figure 5. Fluorescence emission spectra of FAM-labeled 12-mer polydeoxycytosine under different experiment conditions. The emission spectra of FAM-labeled polydeoxycytosine are excited at the wavelength of 490 nm.

aggregation were observed at 1 M NaCl for dG6-GNPs (Figure 4C), whereas even 2 M NaCl did not induce an evident color change and UV/vis peak red shift for dG12-GNPs (Figure 4D). The high salt tolerance of ssDNA-GNPs suggests that they could be used in biological experiments without any further stabilization. 3.4. Fluorescence of hybridized FAM-DNA and ssDNAGNPs. We confirmed that ssDNA was present on the surface of as-synthesized GNPs and explored potential applications of ssDNA-GNPs by measuring their effect on the fluorescence of fluorophore-labeled oligonucleotides.35,36 The change in fluorescence of 5-carboxyfluorescein (FAM) was investigated on hybridization of ssDNA-GNPs and FAM-labeled oligonucleotides. Emission spectra for the samples are shown in Figure 5. The intensity of FAM-DNA fluorescence emission increased approximately 5-, 7- and 10-fold after hybridization with dGMP-, dG6- and dG12-GNPs, respectively, compared with control FAM-DNA. The only spectral difference is a change in intensity, suggesting that the emission spectral properties of FAM-DNA are retained and the fluorescence intensity is enhanced on the surface of dGMP-, dG6- and dG12-GNPs. Normalized emission spectra confirming the emission spectral properties of FAM-labeled oligonucleotides are shown in Figure 5. The fluorescence intensity of dC12-GNP hybridized with FAM-dC12 was only slightly improved compared to FAM-dC12 alone. The fluorescence intensity of citrate-protected GNPs mixed with FAM-DNAs decreased, suggesting that binding of FAM-DNA to the GNP surface causes quenching of the fluorophore. The similarity of the energies of the ssDNP-GNPs plasmon (529 nm) and the FAM exciton (518 nm) results in resonance conditions in the hybrid complex of GNPs and FAM. The exciton energy is very close to the plasmon resonance, which could induce a plasmon enhancement of emission.37 Subwavelength-sized ssDNA-GNPs exhibit strong light scattering of visible light.38 Thus, light scattering plays another role in the observed effect of fluorescence enhancement. As we known, the length of single nucleotide to 12-mer poly G was about 0.33 nm 4 nm. As shown in Figure 3, we see no evidence of clustering of dG12-GNPs in the TEM images, which also supports the suggested theoretical model. 3.5. Aggregation of ssDNA-GNPs with cDNA. To confirm the presence of oligonucleotides on the surface of GNPs, we 3247

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The Journal of Physical Chemistry C carried out aggregation experiments in which complementary oligonucleotides were added to the system. TEM images and UV/vis spectra were then recorded to evaluate GNP aggregation for the different oligonucleotides. As expected, ssDNAGNPs aggregated, and the absorbance peak was red-shifted and broadened after addition of complementary oligonucleotides (Figure S3 and Table S1 ,Supporting Information). As shown in Table S1 (Supporting Information), the size of GNPs in the aggregated solution was increased. It displayed that DNAs were integrated on the surface of GNPs again. This result confirms that oligonucleotides were integrated on the surface of GNPs. From the above observations, we conclude that DNA of the chainlike structure in situ attached to GNPs during reduction could be partially buried in the GNPs.

4. CONCLUSIONS We described a strategy for the synthesis of highly stable, dispersible, and functional GNPs of almost uniform size with ssDNAs of three different lengths and four different types using trisodium citrate to reduce HAuCl4 at 37 °C. The morphology and size distribution of the as-synthesized homopolydeoxynucleotides-GNPs are tunable by varying the ssDNA length and type. Polydeoxynucleotides adsorption on the surface of GNPs effectively inhibited particle coagulation at 37 °C and enhanced their stability against salt-induced aggregation. Because of the light scattering of ssDNA-GNPs, the fluorescence intensity of FAM-DNA was enhanced on hybridization with ssDNA-GNPs. The uniform morphology, the high stability in biological buffers, and ssDNA functionalization on the surface of GNPs open up new horizons for nanomaterial synthesis, studies on the mechanism of GNP formation, and exploration of the biological modification and interactions of nanomaterials. We believe that our ssDNA-GNPs could be used to create films with good mechanical properties and for gene knockdown to control gene expression under in vivo conditions. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional details on the procedures of citrate-protected synthesis by the classical Turkevitch-Frens method and the procedure of the aggregation of the as-synthesized GNPs to character the DNA on the surface of GNPs as well as recording and TEM images of GNP and the zeta potential of the as-synthesized GNPs are shown. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected].

’ ACKNOWLEDGMENT This work is financially supported by the National Natural Science Foundation of China (21071066, 20835006, 91027038), the 11th Five Years Key Programs for Science and Technology Development of China (2008BAK41B03, 2009BAK61B04, 2008ZX08012-001, 2010GB2C100167), and grants from Natural Science Foundation of Jiangsu Province, MOF and MOE (BK2010001, BK2010141, 2010DFB3047, JUSRP11019,

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201110060, 201110016, 201110061, 201010078, 201010216, 200910013, 200910083, 200910011, 200910277).

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dx.doi.org/10.1021/jp109688q |J. Phys. Chem. C 2011, 115, 3243–3249