Open AccessCCS ChemistryCOMMUNICATION1 Apr 2021Solvo-Driven Dimeric Nanoplasmon Coupling Under DNA Direction Yan Hao, Lingling Fang and Zhaoxiang Deng Yan Hao CAS Key Laboratory of Soft Matter Chemistry, Hefei National Research Center for Physical Sciences at the Microscale, Department of Chemistry, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author , Lingling Fang CAS Key Laboratory of Soft Matter Chemistry, Hefei National Research Center for Physical Sciences at the Microscale, Department of Chemistry, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author and Zhaoxiang Deng *Corresponding author: E-mail Address: [email protected] CAS Key Laboratory of Soft Matter Chemistry, Hefei National Research Center for Physical Sciences at the Microscale, Department of Chemistry, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000290 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Strong coupling is a prerequisite for nanoassemblies to deliver functions and applications unrealizable by noninteracting building units. DNA-Directed metamaterials, albeit highly programmable, often show negligible interunit coupling and accordingly limited functions. Herein a simple but highly effective solvent-driven process for dimeric plasmon coupling under DNA’s guidance is reported. The DNA is responsible for a proximity control of plasmonic units to undergo electromagnetic hybridization. This process is inspired by a spontaneous formation of nanoparticle chains in less polar water-miscible solvents. The resulting 1.25 nm surface separation is insensitive to DNA linker length as well as solvent/water ratios above a certain threshold, consistent with a Van der Waals colloidal bonding model. Our strategy avoids surface contamination to the nanoassemblies with the DNA linker in position to ensure coupling selectivity and reversibility. The cooperative but orthogonal roles played by DNA and the organic solvents promise bottom-up materials and devices with prescribed functions. Download figure Download PowerPoint Building metamaterials with designable structures and functions has been a long pursuit in nanotechnology. Solution-based self-assembly of synthetic nanomaterials with distinctive properties is a promising way to this purpose.1,2 In this regard, DNA plays an unrivaled role in aligning the nanoscale building blocks in predefined spatial orders.3–10 So far, various materials have been integrated into DNA nanostructures,3–10 making them ideal to pursue assembly enabled functions.9–16 However, achieving strongly coupled (tightly associated) nanounits under DNA’s direction represents a big challenge.10,17,18 The lack of strong coupling has become a primary obstacle in developing DNA-directed materials and devices with uncompromised functions.10,19,20 Usually, the coupling is distance dependent.17,18,21–23 Therefore, a strategy capable of reducing interparticle spacing down to a sub-2-nm level is especially desirable.23 Localized surface plasmon resonance (LSPR) represents light-driven collective spatial oscillations of conduction electrons confined in a metallic structure. The ability to reshape LSPRs by chemical synthesis and assembly has enabled applications in sensing,24–27 catalysis,28,29 light harvesting,30–32 photonics,33–35 and theranostics.36–38 The LSPRs of closely apposed nanoobjects can hybridize into new resonance modes,20 offering a great chance to engineer their functions. To this end, DNA-programmable plasmonic assemblies are especially welcome.7,39,40 However, the bulky DNA strands and charged colloidal units do not favor a strongly coupled structure.10,12 This dilemma is sometimes circumvented with dried samples or unique DNA linkage with a coupling distance often >2 nm.41–44 Also, the involved processes are either deleterious to DNA structures, uncontrollable, or incompatible with structural DNA nanotechnology. To realize sub-nm coupled, DNA-bonded nanoassemblies in native solutions, Ag ion soldering (AIS) was developed.45–47 AIS offers a facile, scalable, and postsynthetic functionalization of plasmonic assemblies with or without the need of DNA guidance.48 However, it needs Ag+ as a ligand stripper to induce interparticle bonding and extra DNA adsorption to stabilize the coupled products. The residual Ag+ in the products may affect a downstream application. Also, over-soldering can happen under inappropriate conditions, leading to undesired coupling due to a loss of proximity control. In addition, thiols are used to decouple the AIS products and achieve soldering reversibility, but surface passivation is unavoidable.48 The residual Ag+ in the products may affect a downstream application, and substitutes for the Ag cation as a ligand stripper are difficult to find. An inherently different (clean, reversible, robust, and generalizable) process free from these limitations is highly desired to meet the growing demand of functional DNA nanotechnology. Herein we report on a solvent-driven strong coupling strategy for DNA-directed plasmonic nanoassemblies. Such an idea is inspired by a spontaneous formation of chain-like colloidal assemblies in ethanol.49 This phenomenon was attributed to ethanol-enhanced charge separation on the nanoparticles which then function as electric dipoles to achieve a linear assembly.50,51 Another argument asserted that a balance between electrostatic and Van der Waals (VDW) forces plays a big role in determining the one-dimensional (1D) assembly via preferred end attachment of incoming nanoparticles.52 A later study demonstrated that unintentionally introduced salts dictated the chain-like structures,53,54 which adds great controllability to this intriguing phenomenon. Actually, we found that DNA-decorated gold nanoparticles (AuNPs) behaved similarly in forming the chain assemblies ( Supporting Information Figure S1). We are curious to see whether such a self-limited 1D association of colloidal particles could be utilized to address the coupling dilemma in DNA-directed material assembly.10,19,20 The corresponding scheme is depicted in Figure 1, where DNA-assembled AuNP dimers are chosen as an ideal model to explore such a possibility. In a suitably diluted solution, the DNA-tethered AuNPs in a dimer have a very high probability to bond with each other via VDW forces, while their aggregation into longer chains is suppressed due to increased mean free path for interdimer collisions. In this process, Na+ and any cationic species in buffer salts play a key role in neutralizing the colloidal charge toward reduced electrostatic repulsion. Note that DNA-conjugated AuNPs and their hybrids have been explored in various solution environments including organic solvents with a focus on the DNA part.55,56 These previous investigations further increased our confidence in achieving the above goal. Figure 1 | Schematic illustration of strong coupling and decoupling of a DNA-linked AuNP dimer driven by organic and aqueous media. Download figure Download PowerPoint Initially, we chose relatively small (15.6 nm) AuNPs ( Supporting Information Figure S2) to test the aforementioned idea. The AuNP dimer was formed by hybridizing two AuNPs, each conjugated with a single DNA complement. After a gel electrophoretic purification ( Supporting Information Figure S3), the dimeric product was transferred (along with buffer salts) to ethanol. A color transition of the sample from red to purple immediately happened (Figure 2a). Extinction data (Figure 2b) indicated a broadening of the original LSPR peak (522 nm) at the red edge (lower photon energy). This spectral feature originated from two overlapped plasmon modes: transverse and longitudinal LSPRs.57–59 The longitudinal mode is a bonding dipole plasmon (BDP) achieved by plasmon hybridization due to ethanol-driven AuNP coupling. To show the BDP, the spectrum was subtracted from that of uncoupled dimers. The resulting difference spectrum revealed a BDP at 567 nm, with a 45 nm redshift from the transverse peak ( Supporting Information Figure S4). Calculations based on a multiple elastic scattering of multipole expansions (MESME) method revealed a gap size of about 1.25 nm for the coupled dimers ( Supporting Information Figure S5).60,61 Note that the simulation was based on a classical theory without considering any quantum effects and finite atom sizes for a nanoobject. The smallest discernable discrepancy during a spectral peak fitting happened for a gap variation of about 0.05 nm. The gap estimation error should be >0.05 nm, considering that AuNPs were not perfectly spherical as well as other approximations involved. Figure 2 | (a) Photographs of aqueous (left tubes) and ethanol (right tubes) solutions of DNA-bonded 15.6 and 32.9 nm AuNP dimers and unassembled 32.9 nm AuNP monomers. (b and c) Extinction spectra of 15.6 nm (b) and 32.9 nm (c) AuNP dimers in water and ethanol-based solutions. DNA-bearing 32.9 nm AuNP monomers in ethanol are shown as a control (c). The DNA linker has a 50 bp duplex region flanked by two 39b long single-stranded parts (namely LK3, see Figure 3 and Supporting Information for details). Download figure Download PowerPoint The 15.6 nm AuNPs did not give a directly resolvable BDP. To strengthen our finding, AuNPs with a diameter of 32.9 nm were employed ( Supporting Information Figure S2). In this case, it was not convenient to realize a monovalent DNA decoration on the relatively large AuNPs. Alternatively, we minimized the DNA loading by keeping a low DNA:AuNP ratio at the stage of DNA conjugation, resulting in one to three DNA strands per AuNP for hybridization ( Supporting Information Figure S6). The corresponding DNA–AuNP complexes bearing complementary base sequences formed mixed assemblies upon hybridization due to a lack of valency control ( Supporting Information Figure S3). The dimer products were isolated by electrophoresis, and subjected to ethanol coupling. These large AuNP dimers produced a prominent BDP at 610 nm (Figure 2c), which was well separated from the transverse LSPR. MESME calculations revealed a gap separation of 1.25 nm, which was identical to that of the 15.6 nm dimers ( Supporting Information Figure S5). Such a coincidence supported the same coupling mechanism regardless of particle size. Note that DNA-conjugated AuNP monomers did not exhibit any BDP signals in ethanol (Figure 2c and Supporting Information Figure S7), indicating an excellent coupling selectivity defined by DNA bonding. Another advantage was a good reproducibility of this process such that the coupled dimers in different batches had almost the same LSPRs. The coupled dimers were stable in an ethanol/water mixture as evidenced by their extinction data (Figure 2) which did not show any aggregation-related spectral features. The 1.25 nm gap successfully addressed the coupling issue of DNA-bonded nanoassemblies. Such a small gap size should be determined by the surface capping agent [BSPP, bis(p-sulfonatophenyl) phenyl phosphine] and its counterions responsible for charge neutralization. BSPP has a pyramidal shape about 0.4 nm in height, and the Au-P bond is about 0.2 nm. The 1.25 nm dielectric gap is reasonable considering the existence of two BSPP layers and some counterions along with an interdigitating effect of the tripod-like BSPP ligands under the compressive VDW colloidal forces. Based on this scenario, the coupling strength should be insensitive to the length of DNA linkers. Our experiments verified this hypothesis (Figures 3a and 3b and Supporting Information Figure S8). Although DNA duplexes can adopt different structures transitioning from B to A (tilted base pairing planes), and to P form (base stacking free) with increased ethanol concentration,62 marginal effects were observed in our experiments. While the coupling yield was dependent on the ethanol concentration, the BDP wavelength was almost unvaried for different ethanol fractions above a threshold of about 70% (Figure 3c). As well, AuNPs bearing different DNA densities (mono- vs minimally functionalized) showed almost overlapped spectral profiles (Figure 3d). Figure 3 | (a) Schematic drawing of DNA-bonded AuNP dimers with different DNA linkers. The DNA strands were not drawn to scale: a base in the drawing equals about 10 bases in real DNA strands. (b and c) Extinction spectra of ethanol-coupled 32.9 nm AuNP dimers with different DNA linker lengths (b) and varied ethanol/water ratios (c). A photo showing DNA-linked 32.9 nm AuNP dimers in solutions with increased ethanol content is also given in (c). (d) Extinction spectra of 15.6 nm AuNP dimers (in ethanol and water) built by monovalent and minimally functionalized DNA–AuNP conjugates. LK3 was employed to make the dimers in (c) and (d). Note that, besides an isolated BDP peak, 32.9 nm AuNP dimers are more sensitive to a gap variation than 15.6 nm ones ( Supporting Information Figure S5c). Download figure Download PowerPoint To demonstrate the coupling reversibility, the dimers were isolated from ethanol by centrifugation and redispersed in water. A decoupling process immediately happened as judged from a color recovery of the samples back to red ( Supporting Information Figure S9). This reversibility was expected considering the stable Au–P interaction between BSPP and AuNPs due to π-backbonding.53,54 Rehydration of the surface-bound BSPP and its counterions then caused enlarged interparticle gaps beyond a VDW bonding distance.53,54 The gap expansion was also accompanied by a dissociation of the hydrated counterions from the nanoparticle surface to reintroduce strong electrostatic hindrance. To find out whether the DNA linkers were still connecting the AuNPs after the decoupling, we analyzed the samples by gel electrophoresis. Note that spectral data could not indicate the existence of DNA bonding due to a lack of plasmon coupling (electromagnetic hybridization) in water ( Supporting Information Figure S9). The gel data revealed an unaltered migration speed of the decoupled dimers in comparison with DNA linked ones ( Supporting Information Figure S9), reflecting an intact DNA bond. Addition of formamide to the decoupled samples then caused a complete dissociation of the dimers due to DNA denaturation ( Supporting Information Figure S9). We further relied on transmission electron microscopy (TEM) to visualize the AuNP dimers before and after ethanol coupling. TEM images revealed very small interparticle gaps for the 15.6 nm dimers deposited from an ethanol solution, in sharp contrast to water-dispersed ones (Figures 4a and 4b and Supporting Information Figure S10). Due to drying-induced capillary forces,63–65 the 32.9 nm dimers from an aqueous sample showed some “coupling” which did not exist in their solution (Figure 2). Nevertheless, the conformational difference between coupled and uncoupled dimers were still distinguishable (Figures 4d and 4e). The conformation change was also evidenced by dynamic light scattering ( Supporting Information Figure S11). Apart from ethanol, acetonitrile, methanol, acetone, and isopropanol were also tested for their coupling ability encouraged by the formation of similar AuNP chains in these water-miscible solvents ( Supporting Information Figure S12). Interestingly, the solvents (acetone and isopropanol) with a dielectric constant (ɛ) lower than that of ethanol all acted well in making strongly coupled dimers ( Supporting Information Figure S13). Other solvents with a higher ɛ led to weaker and unstable coupling ( Supporting Information Figure S13). These results meant that a lowered ɛ could enhance counterion binding in the double layer and thus favor VDW interactions. Besides dimers, a micron-long AuNP linear array was assembled on a rolling-circle polymerized DNA template ( Supporting Information Figure S14).66 Upon ethanol coupling, the mechanically flexible nanoarrays turned into tightly coiled (intra-cross-linked) globules ( Supporting Information Figure S15). This phenomenon indicated undesirable coupling beyond nearest AuNP neighbors in a linear assembly, consistent with a surface-to-surface coupling mechanism (also see Supporting Information Figures S16–S18). Theoretically, VDW coupling should also be realizable by increasing the strength of counterions in an aqueous solution to achieve effective charge neutralization of AuNPs. It was found that divalent Mg2+ was able to induce intradimer coupling of AuNPs in a narrow concentration range, with a gap size of 2 nm as estimated by MESME calculations ( Supporting Information Figure S19). The increased (2 vs 1.25 nm) surface separation could be rationalized by the presence of hydration layers for the ionic species on the AuNP surface. Figure 4 | TEM images of LK3-linked 15.6 nm (a–c) and 32.9 nm (d–f) AuNP dimers prepared from water (a and d) and ethanol (b and e) solutions, and corresponding AIS-fixed products precoupled in ethanol (c and f). Insets are magnified views of the dimers. Download figure Download PowerPoint While coupling reversibility is desired for an active plasmonic control,67 it is necessary to find a way to fix the coupled structures for other applications. We developed two strategies for this purpose. The first one relied on AIS, which had been used to capture spontaneous colloidal assemblies in water.49 We previously found that the soldering solution does not require a high ethanol/water ratio to maintain the coupling, a benefit from the speedy AIS reaction.49 The ethanol-coupled dimers permanently retained their closely associated conformations after the AIS fixation (Figures 4c and 4f). To exclude an ambiguity that the DNA-assembled dimers may be recoupled by AIS after losing their in-ethanol coupling, we compared DNA-linked dimers directly soldered by AIS and those postfixed by AIS after precoupling in ethanol. Very different results were obtained for these two processes ( Supporting Information Figures S20–S23). The latter required an Ag+ concentration two to three orders of magnitude lower than the former process (Figures 5a and 5b), implying negligible Ag+ contamination of the products. Accordingly, the ethanol and AIS couplings can be used in a combinative way to minimize Ag+ usage and improve coupling selectivity. The reduced Ag+ usage is not a surprise, as strongly coupled dimers could increase an AIS efficiency. In addition to AIS, silica coating was also useful to fix the coupled dimers. In fact, the first instance in which we observed the ethanol-driven plasmon coupling was in an earlier attempt to coat DNA-bonded AuNP dimers with silica via a Stöber reaction in ethanol toward a sintering protection.68,69 This experience provided us another way to fix ethanol-coupled nanodimers. Extinction and TEM data verified a successful silica fixation with a fully retained BDP peak (Figures 5c and 5d). Note that all the fixed dimers were stable in water. Also, the silica-covered nanogaps may be accessed by introducing porosity in the silica or simply allowing molecules to interact with the dimers before fixation.70 In addition, the reversibility and nonstripping features of the solvo-driven process would facilitate an access to the very small interparticle gaps by preadsorption of target molecules on the nanoparticles ahead of the coupling (superior to AIS structures). Figure 5 | (a and b) An agarose gel electropherogram (a) and extinction spectra (b) of LK3-linked 32.9 nm AuNP dimers soldered by different concentrations of Ag+ with (eAIS) or without (AIS) ethanol precoupling. Note: The AIS-fixed samples were measured in water. (c and d) Extinction spectra (c) and a TEM image (d) showing LK3-linked 32.9 nm AuNP dimers upon ethanol coupling and silica fixation. The redshift of the BDP peak was caused by the highly refractive silica coating. Download figure Download PowerPoint To prove that the 1.25 nm gap of the coupled/fixed dimers was not passivated by DNA, we tested their use as substrates for surface-enhanced Raman scattering (SERS). We chose 4-nitrothiophenol (4-NTP) and 4-mercaptobenzonitrile (4-MBN) as nonresonant SERS dyes for the measurements ( Supporting Information Figures S24–S26). Raman vibrations of 4-NTP and 4-MBN were clearly visible for both ethanol-coupled dimers and the fixed products, indicating their unpassivated surface. In addition, SERS signals were resolved for the BSPP ligands in the nanogaps (hotspots) of the ethanol-coupled dimers and the silicified ones ( Supporting Information Figures S24–S27). These BSPP peaks completely disappeared after an AIS treatment ( Supporting Information Figure S27), supporting the inherently different mechanism of ethanol coupling in comparison with AIS. The latter requires a stripping of BSPP by Ag+ to achieve sub-nm interparticle coupling.45–47 In conclusion, we have developed an especially simple, clean, reversible, and highly controllable strategy to achieve strong plasmon coupling down to 1.25 nm for DNA-programmable nanodimers. Such a process is highly efficient and rapid, which takes a couple of seconds to finish. The simple rules we follow to select a suitable solvent include: (1) it is polar enough to be miscible with aqueous colloidal solutions, (2) nanoparticles tend to form chain-like assemblies (instead of bulky aggregates) in a salted solvent, and (3) it does not denature DNA. The coupling strength is insensitive to DNA linker size and ethanol concentrations (above a threshold), which supports a direct surface-to-surface coupling mechanism conceptually distinct from ethanol-induced DNA compaction.62,71–73 The VDW forces responsible for the coupling are enhanced in ethanol due to lowered solvent polarity and accordingly suppressed hydration and ionization ( Supporting Information Figures S28 and S29).74 The coupled structures can be fixed by Ag+ soldering or silica coating. The reversibility of the coupling, and the ability to fix the dimer, offer an unprecedented chance for dynamic/static plasmonics toward optical switches, sensors, and various smart materials and devices.67 Our strategy avoids surface contamination and passivation of the coupled products, which is desirable for downstream applications. This study provides a new toolkit for functional colloidal nanotechnology toward versatile coupling-enabled physicochemical properties. The very small 1.25 nm plasmonic gap benefits applications including SERS (and infrared absorption), photocatalysis, light harvesting, quantum plasmonics, and nonlinear optics. The reversible coupling could then facilitate target loading in a nanosized gap, which is otherwise difficult. In the future, structures containing more than two components should be considered (using branched DNA linkers or DNA origami templates) once it is possible to control their conformational isomerism during the coupling. To this end, the coupling reversibility would allow nanounits to readjust their positions toward a desired configuration. Besides gold, other materials should also be involved due to their similar assembly behaviors in ethanol.49 The good compatibility of the solvo-driven coupling with AIS and DNA nanotechnology opens a new avenue toward metamaterials with great structural complexity and functional tunability. Supporting Information Supporting Information is available. Conflict of Interest There is no conflict of interest to report. Acknowledgments This study was supported by the National Key Research and Development Program of China (grant nos. 2016YFA0201300 and 2018YFA0702001), the National Science Fund for Distinguished Young Scholars (grant no. 21425521), and the National Natural Science Foundation of China (grant nos. 21972130 and 21521001).