Construction of Long Narrow Gaps in Ag Nanoplates

They correspond to 4.1 × 1010 and 5.1 × 108 field enhancement. (E4), respectively. We use 2-naphthelenethiol as a model probe to treat the gapped na...
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Construction of Long Narrow Gaps in Ag Nanoplates Tao Jiang, Gang Chen, Xiaoli Tian, SHIWEI TANG, Jun Zhou, YUHUA FENG, and Hongyu Chen J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06969 • Publication Date (Web): 04 Nov 2018 Downloaded from http://pubs.acs.org on November 5, 2018

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Journal of the American Chemical Society

Construction of Long Narrow Gaps in Ag Nanoplates Tao Jiang,†,‡,§ Gang Chen,‖ Xiaoli Tian,† Shiwei Tang,§ Jun Zhou,§ Yuhua Feng,*,† and Hongyu Chen*,†,‡ †Institute

of Advanced Synthesis, School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 211816, China. ‡Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore. §Institute of Photonics, Department of Microelectronic Science and Engineering, Faculty of Science, Ningbo University, Ningbo 315211, P. R. China. ‖Department

of Chemistry, University of Central Florida, Orlando, FL 32816, USA.

Supporting Information Placeholder ABSTRACT: Hexagonal Ag nanoplates with long and ultranarrow gaps (about 90 nm in length, 2 nm in width) are synthesized via seed-mediated growth method. By growing around the polymer shell on the seed, the Ag domain cannot merge at the meet-up point, leaving a long narrow gap in the resulting plate. These gapped nanoplates exhibit high sensitivity in SERS detection, with limitation of 10-9 M for 2naphthalenethiol.

Surface-enhanced Raman scattering (SERS) as a powerful and versatile analytical tool has attracted tremendous interest in the fields of bioanalysis,1 public security,2 environmental monitoring,3 and food safety.4 It not only provides vibrational fingerprints of molecules but also allows detection at ultrasensitive or even single molecular level.5 It is known that SERS activity depends on the morphology of noble metal structures.6 Great efforts have been made to fabricate metallic nanostructures with nanoscale gaps ( 10 nm), in most cases by using the junction between two aggregated nanoparticles, giving rise to greatly enhanced local electromagnetic field, known as SERS hotspots. Determined by simple geometry, such a gap between two hard spheres has very limited area with non-uniform separation. Indeed, in almost all cases the nanogap is essentially a point, making up only a tiny fraction of the total surface area. Several approaches have been attempted to increase the useful area of hotspots, by making chains,7 core-shell,8 hierarchical,9 and porous nanoparticles.10 Electron-beam lithography can fabricate arrays of specific structures with a large number of uniform nanogaps.11 Thiolated DNA,12 ligand,8a,8c,8d and polymer,8b were found to be excellent modifiers to synthesize core-shell structure with a layer of ultra-narrow interior gap between the core and the shell, greatly enlarging the hotspots. For such an enclosed nanogap, exchange of molecules into the gap is a problem, and the outmost metal layer is expected to suppress the light going in and out of the structure. In comparison, a one-dimensional line of nanogap would avoid these problems and offer a new direction in designing SERS nanostructures. In this work, we report a new strategy for increasing the area of hotspots, more specifically by creating a long line of hotspot with an ultra-narrow gap. Core-shell nanoparticles, where the Ag cores

Scheme 1. Schematics illustrating the growth of triangular and hexagonal Ag nanoplates with nanogaps. are encapsulated in polystyrene-block-poly(acrylic acid) (PSPAA) shells, are used as seeds. Surprisingly, Ag domain could extend from one point of the encapsulated seed and form triangular or hexagonal plate around the polymer shell (Scheme 1). When the two Ag lobes reaching from the two directions meet up, they cannot merge, forcing a line of gap to form in the middle. The longer and more uniform gaps in the hexagonal Ag nanoplates give rise to better SERS sensitivity in detection. In a typical synthesis, Ag nanospheres (20 nm in diameter) were encapsulated by PSPAA shell (10 nm in thickness, Figure S1).13 The resulting Ag@PSPAA core-shell nanoparticles were used as seeds. In their presence, AgNO3 was reduced by ascorbic acid, with the stabilizing ligand sodium citrate and the etchant H2O2, which is intended to remove the single crystalline Ag nuclei.14 To our surprise, nanoplate could form around the Ag@PSPAA core (Figure 1), where the narrow gap and the metal bridge between the internal Ag seed and the external plate are particularly noteworthy. These Ag nanoplates maintain smooth surface and well-defined triangular shape with slightly rounded corners. As shown in SEM images, they are relatively uniform with high yield (90.3%, Figure S2). The obtained edge length is 400 ± 150 nm, and the gap length is 60 ± 40 nm (Figure S3). Measured by Atomic Force Microscope (AFM), their thickness is around 8 nm (Figure S4). Seed-mediated growth is widely used in nanoparticle synthesis. With few exceptions,15 clean seeds were used so that materials can be uniformly deposited on their surfaces. Our synthesis is a pleasant surprise that metal atoms could penetrate the seemingly solid polymer shell and wrap around it. The formation of triangular Ag nanoplates has been extensively studied.16 It is well accepted that such plates originate from a twin

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results show that encircling polymer-coated seeds is possible, thus encircling clean seeds would only be easier. The literature mechanism could be still invoked to explain the formation of triangular plates,16 that any newly grown domain without twin plane would be selectively removed by etching.14,16c,20 It is curious that Ag atoms could penetrate the seemingly solid polymer shell to form the Ag bridge.8b It is known that the polystyrene domain of PSPAA could be swollen by DMF. While in water, the deswelling would kinetically freeze the PS domain,21 and water would replace the space previously occupied by DMF, leaving micropores. To characterize such pores is extremely difficult,22 and the fact that the Ag domain could grow through the

Figure 1. (a) SEM, (b) TEM and (c) HRTEM images, (d) SAED pattern of the triangular Ag nanoplates. The squared, triangled and circled spots are correspond to {220}, {422} and (1/3){422} reflections. (e, f) TEM images of the nanoplates grow from Ag nanocubes. plane defect and end up with a twin plane across the entire plate. Therefore, the obtaining of pure plates suggests that somehow the unfavorable twin plane defect could be introduced in all nuclei. To explain the consistent defect formation, the prevailing hypothesis is that those nuclei/seeds without twin plane are etched by H2O2 in the synthesis.14 We expect our nanoplates to have the same structure as those in the literature, particularly because similar reagents and procedure were followed except the use of the Ag@PSPAA seeds. High resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) studies confirm this expectation. Figure 1c displays the formally forbidden 1/3(422) dspacing of 0.25 nm, consistent with the corresponding SAED pattern and indicating the formation of twin planes.17 The pattern exhibits a six-fold symmetry corresponding to the {220} reflections of FCC Ag orientated in the [111] direction (Figure 1d). Thus, the formation of Ag plate can be attributed to the preferential deposition of Ag at the concave edge along the twin plane and the selective adsorption of citrate on the (111) facets of Ag (Figure S5).17 Our system presents a unique counter example that Ag nanoplates could form without completely etching the seeds. Further control experiments using PSPAA-encapsulated singlecrystalline Ag nanocubes as seeds, the cube shape is maintained after the growth of plate (yield: 95.64%, Figure 1e, f). The coexistence of the seeds without twin plane18 and the plates with twin plane confirms our assumption. However, this does not contradict the literature conclusions, because it would be almost impossible to identify and characterize the original seeds after the plate formation, if anything remained. It is known that the low stacking fault energy of FCC Ag promotes the formation of twin plane.19 Control experiments established that the formation of plates did not have strong correlation with our seeds or PSPAA (Figure S6-7), but with the coexistence of citrate and H2O2.14 HRTEM analysis showed prevailing epitaxial growth of the Ag bridge and plate on seeds (Figure S9-12). Hence, so long as the newly grown Ag domain contains twin plane, it could wrap around the original seed to form a plate. Our

Figure 2. (a) SEM, (b-c) TEM, (d) HRTEM images, and (e) SAED pattern of the hexagonal Ag nanoplates. The squared, triangled and circled spots are correspond to {220}, {422} and (1/3){422} reflections. polymer shell is clear evidence supporting the presence of pores. When additional NH2OH was added, the resulting Ag nanoplates exhibit uniform hexagonal shapes (Figure 2a and S14) with edge length of 300 ± 150 nm (Figure S15a) and thickness of 12 nm (Figure S16). Uniform straight gaps with 90 ± 50 nm in length (Figure S15b) and 2 ± 0.5 nm in width appeared in nearly all of the hexagonal nanoplates (Figure 2b-c). Similar to the triangular nanoplates, such plate structure with (111) surface is originated from twin plane, confirmed by HRTEM and SAED pattern (Figure 2d-e).

Figure 3. (a-f) Growth process of the Ag nanoplates. (g) Calculation of the critical number of atoms needed for filling the gap when the exposed area is equal to the covered area.

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Figure 4. (a-d) Cross-sectional views of calculated E-field distributions on gapped nanoplates (gap size: 2nm; red arrow: polarization). SERS detection of 2-naphthalenethiol molecules: (e) in solution, (f) on substrate. (g-h) single particle SERS of the normal and gapped nanoplates with 785 (red) and 532 (blue) nm laser (inset: SEM images of the selected nanoparticle). As shown in Figure 1 and 2, the external Ag nanoplates grow closely around the PSPAA shell. Such affinity is likely due to the negative charged PAA surface,8b,13 to which Ag could form coordination bonds. Figure 3a-f, S18-19 show the trapped growth intermediates. As the bridgehead grows larger, it starts to take hexagonal form and wraps around the PSPAA surface. When the Ag@PSPAA seed is finally enclosed, the two lobes reaching from two directions meet at roughly the opposite side of the Ag bridge, indicating their similar growth rate. Without exception, the plates do not merge at the meet-up point, forming lines of narrow gaps. The observation that the two lobes cannot merge is intriguing, with at least two issues: why the gap is stuck at 2 nm and why it cannot merge. It is likely that polymer or ligand is responsible. Obviously, the gap did grow from a large width to 2 nm, indicating that the ligand binding is not permanent, but dynamically on and off.23 It is likely that the ligand effects are small when the gap is large, but becomes inhibitive once the gap is reduced to its limit. It is difficult to speculate why 2 nm is the limit. Double-layer of 2-naphthalenethiol gives a 2 nm gap,24 but

the PSPAA shells are usually much thicker.25 It is possible that a double-layer of citrate may fill the gap due to steric hindrance and charge repulsion. For the second issue, the key question is why merging cannot happen at such a narrow gap. The reason may be the instability of an initial junction. From the point of surface energy, a junction between the lobes would be favorable if the area covered by the junction exceeds the newly exposed surface area. For a line of Ag atoms, the exposed surface is too large to be stable. The surface energy would always go uphill until the junction reaches a certain critical size. Among all shapes, a junction with cylindrical shape would have the smallest exposed surface. Simple geometry gives a critical cylinder of 2 nm thick and 2 nm diameter, which would contain 200 atoms (Figure 3g and S21). Since random addition of Ag atoms can hardly reach such a large size, the nanogap cannot be closed during Ag deposition. We carried out finite-difference time-domain (FDTD) simulation to investigate the dependence of electromagnetic field on the gap size (0, 2 and 8 nm) and the shape of nanoplate. The polarization of the 785 nm incident light (along z-direction) is set as perpendicular (x-direction) or parallel (y-direction) to the nanogap. The field strength is much stronger in the gap (Figure 4a-d) and decreases with increasing gap size (Figure S22). For the triangular nanoplate, the field strength is stronger when the polarization is perpendicular to the nanogap, whereas the opposite is true for the hexagonal nanoplate. For clarification, we set 50, 25 and 20, 150 for triangular and hexagonal nanoplates as the maximum field strength. While in the insets of Figure 4a-d, we use 150, 50, 40 and 450 as the maximum field strength for the enlarged gap area to clearly show the field distribution. The largest field strength of the hexagonal nanoplate at y-direction (450) is higher than that of the triangular one at x-direction (150). They correspond to 4.1 × 1010 and 5.1 × 108 field enhancement (E4), respectively. We use 2-naphthelenethiol as a model probe to treat the gapped nanoplates. Using 785 nm laser excitation, strong characteristic SERS signals were observed (Figure 4e-h),26 indicating that the probe has entered the open nanogaps. The UV-Vis-NIR absorption spectra show 2 broad absorptions typical of the outand in-plane peaks of nanoplates (Figure S23). In solution (Figure 4e), ensemble-averaged SERS show that the plates with gaps are 2-4 times stronger than those without gaps, which is further confirmed by single particle SERS measurements (Figure 4g-h). For nanoplates anchored on a substrate, as the probe concentration decreased, SERS signals gradually declined for both the triangular and hexagonal nanoplates, reaching detection limit of 10−6 and 10−9 M, respectively (Figure 4f). The signal is consistently larger for the hexagonal nanoplates, with a ratio of 3.26 at 10−6 M. Ag nanoplates of similar size but without nanogaps gave a detection limit of 10−4 M (Figure S25-26). The enhancement by several orders of magnitude demonstrates the effectiveness of the nanogaps. In summary, we explore a new synthetic strategy to give long lines of ultra-narrow gaps in Ag nanoplates. The new growth phenomena emerged in the process are of great interest for future synthetic endeavor, including the growth of Ag bridge from the encapsulated seeds; the wrapping of nanoplate around the polymer shell; and the inability of merging the nanogaps. The open nanogaps allow easy access of probe molecules and exhibit excellent sensitivity in SERS detection.

ASSOCIATED CONTENT Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website. Details of the synthesis and measurements; Figure S1-S30 with detailed discussions; TEM, SEM, AFM images and SERS spectra of the nanoplates. (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected] and [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 21673117), recruitment Program of Global Experts, Jiangsu Provincial Foundation for Specially-Appointed Professor, start-up fund at Nanjing Tech University (39837102, 39837140), and SICAM Fellowship from Jiangsu National Synergetic Innovation Center for Advanced Materials.

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