Alternating Plasmonic Nanoparticle Heterochains Made by

Letter pubs.acs.org/JPCL

Alternating Plasmonic Nanoparticle Heterochains Made by Polymerase Chain Reaction and Their Optical Properties Yuan Zhao,†,# Liguang Xu,†,# Luis M. Liz-Marzán,‡,§ Hua Kuang,† Wei Ma,† Ana Asenjo-Garcıa,́ ∥ F. Javier García de Abajo,∥ Nicholas A. Kotov,⊥ Libing Wang,*,† and Chuanlai Xu*,† †

State Key Lab of Food Science & Technology, School of Food Science & Technology, Jiangnan University, Wuxi 214122, China BioNanoPlasmonics Laboratory, CIC biomaGUNE, Paseo de Miramón 182, Donostia - San Sebastian 20009, Spain § Ikerbasque, Basque Foundation for Science, Bilbao 48011, Spain ∥ IQFR-CSIC, Serrano 119, 28006 Madrid, Spain ⊥ Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan, 48109, United States ‡

S Supporting Information *

ABSTRACT: Organization of nanoparticles (NPs) of different materials into superstructures of higher complexity represents a key challenge in nanotechnology. Polymerase chain reaction (PCR) was used in this study to fabricate chains consisting of plasmonic NPs of different sizes, thus denoted heterochains. The NPs in such chains are connected by DNA oligomers, alternating in a sequence big−small−big−small−... and spanning lengths in the range of 40−300 nm by varying the number of PCR cycles. They display strong plasmonic chirality at 500−600 nm, the chiral activity revealing nonmonotonous dependence on the length of heterochains. We find the strength of surface-enhanced Raman scattering (SERS) to increase with chain length, while the chiral response initially increased and then decreased with the number of PCR cycles. The relationship between the optical properties of the heterochains and their structure/length is discussed. The length-dependent intense optical response of the plasmonic NP heterochains holds great potential for biosensing applications. SECTION: Plasmonics, Optical Materials, and Hard Matter

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have a hardly predictable influence on both the structural morphology of the chains and the resulting photonic properties.28 The ability to scale up the production of complex NP systems also represents an important challenge.29 Polymerase chain reaction (PCR) is a common technique in molecular biology for DNA replication. The exponential amplification of the target nucleotide sequence enables its rapid production and greatly magnifies the target signal. Although PCR is generally performed in solution, it has been extended to solid surfaces, including nylon, glass, microtiter wells, and even inorganic NPs. DNA possesses intrinsic programmability, structural plasticity, and coordination interactions with NPs, so that it has a great potential as a powerful molecular tool for large scale NP assembly.30−32 We show in this Letter the first example of application of the PCR technique toward the assembly of chains consisting of Au NPs of different sizes with highly controlled sequences. Specifically, we demonstrate the production of alternating NP chains with big−small−big− small−... sequences, which we denote heterochains. These multiparticle superstructures are unique among other reported

ight-matter interaction can be substantially enhanced in the presence of strongly localized plasmons and their associated near-fields. Plasmonic gap modes in assembled superstructures have received considerable attention because of their multiple applications, among which chiral metamaterials and biosensors are prominent examples.1−5 In contrast to conventional homogeneous materials, metamaterials, including chiral artificial media, formed by assembling nanoscale building blocks display interesting collective properties that result from the extremely high local-field enhancements, thus revealing selfassembly as a potential tool to fabricate superstructures for metamaterial optics.6−12 Nanoparticles (NPs) have often been considered as structural and functional building-blocks in the fabrication of one-, two-, and three-dimensional superstructures.13−18 Chainlike assemblies are of tremendous interest for optoelectronics and biosensor devices. A suite of techniques based upon electron-beam lithography,19 the intrinsic ability of NPs to selfassemble,20 as well as templating driven by biomolecules21−25 and other molecular linkers,26,27 have been used to fabricate plasmonic NP chains. However, the precise control of interparticle spacing and NP spatial arrangement still poses a major challenge, in particular when highly isotropic NPs are used as building blocks. Additionally, the postsynthetic physical or chemical treatments required for template-directed strategies © 2013 American Chemical Society

Received: January 8, 2013 Accepted: January 30, 2013 Published: January 30, 2013 641

dx.doi.org/10.1021/jz400045s | J. Phys. Chem. Lett. 2013, 4, 641−647

The Journal of Physical Chemistry Letters

Letter

conjugates, prior to their use in the PCR process. For 10 nm Au NPs-R50-primer conjugates (Au10 NPs-R50) and 25 nm Au NPsF50-primer conjugates (Au25 NPs-F50), the hydrodynamic diameters increased upon surface modification with PEG from 15.4 to 36.6 nm and from 27.1 to 50.8 nm, together with a substantial increase in the ζ potential to −18.9 and −24.6 mV, respectively (Supporting Information, Figure S3). We prepared the alternating big−small−big−small−... heterochains (Figures 1 and 2), by carrying out the DNA polymerization reaction directly on the surface of Au NPs. TEM was used to assess the structure and length of each assembled superstructure, as shown in Figure 2. We show representative TEM images that clearly demonstrate that the heterochains are formed by alternating 25 and 10 nm Au NPs. With an increasing number of PCR cycles, the number of NPs in the assembled heterochains was found to increase from 2 up to 12 (Supporting Information, Figure S4). The total elongation of the chain was found to be in the 40−300 nm range. The continuous growth of heterochains during the PCR process is made possible by the tight control over the number of primers per Au NP, so that it is limited to only two per each big or small NP. For two cycles, the assembled superstructures are predominantly dimers, with a high yield of 72.4% (Figure 2a). By increasing the number of cycles from two to five, the proportion of heterodimers progressively decreased, while heterogeneous trimers, tetramers, and short heterochains were formed. After three cycles, the percentage of dimers decreased to 52.1%, while trimers appeared with a yield of about 33.7% (Figure 2b, Supporting Information Figure S5a). Almost no dimers were present after five cycles, and the proportion of short chains comprising four, five, or six Au NPs reached a yield of 70.6% (Figure 2c−e, Figure S5b-c). This phenomenon can be attributed to the slower rate of NP chain elongation, as compared to the dimerization step.53 Therefore, the entire NP dimers could be regarded as the basic units in the follow-up assembly of longer chains. Two additional primers around the dimers were continuously extended and amplified to form longer chains at higher cycles. In addition, amplified dsDNA linked dimers would also be regarded as a repeating template to undergo the usual denaturation, annealing, and extension PCR steps. Comparing this process to the amplification of dsDNA on solid surfaces, the dissociative templates in the PCR solution competitively perform the PCR process. Such prolongation process and competitive amplification phenomenon played crucial roles at higher cycles, enabling the scaled-up production of heterochains. The length of the chains extended from 7 to 12 NPs. The yield of the chains with Au NP numbers between seven and nine after 10 cycles was as high as 62.3% (Figure 2f−h, Figure S5d−f). After 20 cycles, the percentage of chains with a number of Au NPs ranging from 10 to 12 was about 53.7% (Figure 2i−l, Figure S5g−i). At higher cycle numbers between 25 and 40, the heterochains would cross each other, so that bifurcated motif heterochains and interconnected heterochain networks were formed (Figure S5j−l). The properties of the assembled superstructures are predicted to depend not only on the size, shape, and composition of the NPs, but largely upon the surface orientation and even spatial addressability.54 TEM images could provide sufficient information about the size and morphological dispersion of the heterochains. However, during TEM sample preparation, solvent evaporation is likely to drive the NPs away from their positions in solution, due to the

NP assemblies and can be of various lengths, which are controlled by the number of PCR cycles. Au NPs with two different sizes were selectively capped with two oligonucleotide primers (Figure 1). Using the PCR

Figure 1. Schematic illustration of heterochains assembled based on PCR.

process, heterochains of controlled length were successfully assembled, formed by alternating 25 and 10 nm Au NPs. By increasing the number of PCR cycles from 2 to 20, the number of NPs in each chain was also increased, while the gaps between adjacent NPs in the chains were still well-controlled due to the specific length of the DNA linkers. Even though a number of reports are available on the scattering and absorption of light by NP chains,33−38 we are still far from understanding all possible collective properties of such superstructures. In this respect, our heterochains are expected to display collective plasmonic chirality, as well as surface-enhanced Raman scattering (SERS) activity similarly to other DNA-bridged nanoassemblies that we previously reported.31,39,40 The placement of chiral dsDNA between NP pairs within the heterostructures is a potential source of chirality for these superstructures.40−45 SERS activity due to plasmon coupling between the NPs within the chains is also expected.39,46−49 Au NPs of two different sizes, 10 and 25 nm, were synthesized by reduction of HAuCl4 using the sodium citrate− tannin and sodium citrate methods, respectively (see Supporting Information for experimental details, transmission electron microscopy (TEM) images, and hydrodynamic radii, Figures S1 and S3a).31 The resulting 10 and 25 nm Au NPs were then surface modified with thiolated reverse and forward primers (R50, F50), respectively (Supporting Information, Table S1). Using the fluorescence-based method,50,51 the average number of primers grafted onto the surface of each NP was estimated to be 2.3. The ability to control the precise number of primers on each NP enabled the assembly of NP heterochains with different particle sizes. Additionally, the successful NP surface functionalization with the primers was confirmed by comparing the electrophoretic mobility between individual NPs and NP−primer conjugates (Supporting Information, Figure S2).52 Thiolated polyethylene glycol (PEG-SH) was then used to stabilize the Au NP−primer 642



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Figure 2. Representative TEM images of heterochains (a−l) and statistical analysis of Au NPs number in heterochains for different numbers of PCR cycles (m). (a) 2 cycles, (b) 3 cycles, (c−e) 5 cycles, (f−h) 10 cycles, (i−l) 20 cycles.

increasing length, obtained after 5−20 cycles, by threedimensional (3D) electron tomography reconstruction. As a general observation, the NPs were not kept in a straight line within the assembled chains, but rather usually presented a curved morphology in solution. Considering the presence of chiral dsDNA molecules and the curved configuration of the heterochains in solution, we further analyzed the chirality of the scaled-up heterochains obtained after 2−20 cycles.55,56 As shown in Figure 4a,b, chirality in the UV range at around 260 nm was observed, which is consistent with the spectral range of the intrinsic chiral response of dsDNA molecules. Compared to dsDNA amplified by naked primers or a mixture of the Au NPs-primer-PEG without PCR, the circular dichroism (CD) signals of dsDNA in the NPs assembled heterochains at about 260 nm increased by up to about 10-fold. One of the possible reasons for the emergence of CD signals was suggested to be the near-field enhancement,42 although the spectral position of the observed CD bands and their dependence on the number of NPs in the chain (i.e., the number of PCR cycles) do not support this attribution at least for this specific case.

structural plasticity of amplified dsDNA, thereby changing the relative position of the NPs in the chains. To gather more accurate information about the actual spatial conformation of the heterochains in solution, cryo-electron tomography was carried out from a variety of specimen orientations. Figure 3 shows representative tomography images of heterochains with

Figure 3. Tomography 3D reconstruction of heterochains with different lengths. (a−c) 5 cycles, (d,e) 10 cycles, (f−h) 20 cycles. 643



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Figure 4. (a−c) CD spectra of heterochains from 2 to 5 cycles (a), from 5 to 20 cycles (b), and from 20 to 40 cycles (c). Control group (0 cycles): mixtures of Au25 NPs-F50-PEG and Au10 NPs-R50-PEG in PCR buffer solution without PCR. (d) The corresponding UV−visible spectra. Inset: enlarged UV−visible spectra from 510 to 540 nm. The absorption spectra in panel d have been normalized at 525 nm.

Figure 5. (a) Raman spectrum of 4-ATP solution and SERS spectra of nonassembled NPs carrying 4-ATP and heterochains assemblies of Au NPs carrying 4-ATP after 2−20 cycles. (b) Dependence of the Raman intensity at 1085 cm−1 on the number of NPs in the chain. (c) Calculated extinction cross-section of heterochains for different numbers of particles, as indicated by insets. (d) Estimated increase in SERS signal associated with the heterochains for different incident light wavelengths and a Raman shift of 1085 cm−1. In the calculations, the particles have diameters of 10 and 25 nm, with a center-to-center distance of 34 nm and a trimer bond angle of 100 degrees; the gold is described by tabulated dielectric data.65

scattering and/or formation of new plasmon modes in Au NP chains (Figure 4d).58,59 Chirality was also observed in the heterochains at about 525 nm, with no obvious wavelength shifts. Interestingly, the chirality decreased from 2 to 5 cycles but increased from 5 to 20 cycles and decreased from 20 to 40 cycles (Figure 4a−c). After two cycles, the chiral signal was 48.37 mdeg. Although we do not rule out an amplification of the intrinsic chiral signal of the molecules surrounding the particles (Supporting Information Figure S6), this is an unlikely

No CD signal is evident in the visible range after 2 to 20 cycles for the dsDNA or the mixture of Au NPs-primer-PEG without PCR. It has been reported that spatial disorder of the NPs in the heterochains and the Au NPs heterogeneity have little effect on the plasmonic properties of the chains.57 As compared to the individual Au NPs, the UV−visible spectra of the chains showed a small red-shift of the maximum position from 523 to 526 nm. An increased absorption around 650 nm was observed after heterochain formation, due to the increased 644



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preparation of a variety of NP chains. The heterochains were found to exhibit strong chirality, which was nonmonotonic in respect to the number of cycles. NP heterochains were also found to greatly enhance the SERS signal of 4-ATP due to the interaction between Au NPs in the assembled chains. It is prospective to engineer and construct highly sophisticated superstructures having unusual properties that depend upon the multicomponent building blocks and the spatial configuration of the assemblies and further implementing applications in plasmonics, electronics, and biosensing.

possibility because this type of intrinsic chiral response is extremely weak above 400 nm. Additionally, the variation of the chiral signal with increasing number of NPs cannot be easily explained from the near-field enhancement argument, as the net effect of this enhancement is simply proportional to the number of gaps (Supporting Information Figure S6). As an intriguing possibility, the optical interaction between chiral molecules and NP assemblies has been suggested to boost the chiral signal near the plasmonic region.36,60−62 It should be noted that the synthesized Au NPs were irregular ellipsoids, which were characterized through tomography images in Figure 3. Elongation factors for 25 ± 3 nm Au NPs and 10 ± 2 nm Au NPs were statistically calculated to be 1.18 and 1.11. The nonsphericity of the Au NPs could be one of the reasons that render dimers or trimers chirality. From 5 to 20 cycles, the chirality of the heterochains was directly proportional to the number of Au NPs and the length of heterochains (Figure 4b). The strong collective chirality of the long chains could be affected not only by the unique spatial conformation of the heterochains but also by the relative orientation among the heterogeneous Au NPs. To further understand the field enhancement in the big− small−big−small−... chains, we additionally examined SERS in the chains, where hot spots at the spaces between adjacent NPs are expected to arise from the coupling of surface plasmon resonances in the Au NP chains. This is actually confirmed both from experiment (Figure 5a,b) and multiple-scattering electromagnetic simulations63−65 (Figure 5c,d). Thus, Au NP heterochains assembled in the course of 2−20 cycles provided an effective model to explore the role of the structure in the NP heterochains by measuring the SERS intensity. 4-Aminothiophenol (4-ATP), which is commonly used as a model analyte in the SERS evaluation of plasmonic NP superstructures, was attached to the Au NPs surface via a strong Au− S bond. The characteristic SERS peaks of 4-ATP, such as δ(C− S) at 395 cm−1, ν(C−S) at 1085 cm−1, and ν(C−C) at 1594 cm−1,39,46,66 could be clearly observed in the SERS spectra for heterochains of different lengths, in the liquid phase. 4-ATP showed distinct intensities for the heterochains with different lengths, from 40 to 300 nm (Figure 5a). For the NP heterochains, the Raman intensity at the main SERS peak of 4-ATP (1085 cm−1) increased monotonically with the number of NPs involved in the corresponding NP heterochains due to strong plasmon coupling between the NPs (Figure 5b).28,41,45 The Au NP heterochains with high SERS intensity demonstrated the potential of using such structures as efficient SERSactive substrates for sensing applications in DNA detection. This conclusion is supported by the estimate of the magnitude of the SERS signal obtained from the product of the electric near-field enhancement at the incident and scattered light frequencies, summed over the gaps of each structure (Figure 5d). The theoretical prediction is approximately proportional to the number of gaps, in contrast to the experiment. The latter shows a nonmonotonic increase with a shoulder at around 5 NPs, which could be ascribed to conformational changes in the structure of large chains, such as bending away from the perfect zigzag chain configuration. In summary, plasmonic heterogeneous NP chains were assembled using the PCR technique for controlling the density of primers per Au NP and modulating the PCR cycle number. From 2 to 20 cycles, the number of NPs in the heterochains and their length correspondingly increased. The PCR technique can thus become a versatile and convenient tool for the

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ASSOCIATED CONTENT

S Supporting Information *

Materials and experimental details for the synthesis of 25 ± 3 nm and 10 ± 2 nm Au NPs, primer modified Au NPs, the performed PCR process, and the characterization of assembled structures, as well as calculation of electric field enhancement are included. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected] (L.W.); [email protected]. cn (C.X.). Author Contributions #

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (21071066, 91027038, 21101079, 21175034), the Key Programs from MOST (2012BAC01B07, 2012BAD29B05, 2012AA06A303, 2012BAD29B04, 2011BAK10B07, 2011BAK10B05, 2011BAK10B01, 2010AA06Z302, 2010DFB3047, 2011ZX08012-001, 2012BAK17B10, 2012BAK08B01), and grants from the Natural Science Foundation of Jiangsu Province, MOF and MOE (BE2011626, BK2010001, BK2010141, 201210036, 311002). L.M.L.-M. acknowledges funding from the ERC (Advanced Grant # 267867, PLASMAQUO). N.A.K. is grateful for support from the Center for Photonic and Multiscale Nanomaterials (C-PHOM) funded by the National Science Foundation Materials Research Science and Engineering Center program DMR 1120923 and by AFOSR MURI under Grant # AFOSR MURI 444286-P061716. A.A.-G. acknowledges financial support through FPU from the Spanish ME.

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