Unified Etching and Protection of Faceted Silver Nanostructures by

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C: Physical Processes in Nanomaterials and Nanostructures

Unified Etching and Protection of Faceted Silver Nanostructures by DNA Oligonucleotides Shengqiang Hu, Jianxiu Wang, and Juewen Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02653 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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The Journal of Physical Chemistry

Unified Etching and Protection of Faceted Silver Nanostructures by DNA Oligonucleotides

Shengqiang Hu,†,‡ Jianxiu Wang,*,† Juewen Liu*,‡

†College

of Chemistry and Chemical Engineering, Central South University, Changsha

410083, China ‡Department

of Chemistry, Waterloo Institute for Nanotechnology, University of Waterloo,

Waterloo, Ontario, N2L 3G1, Canada

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ACS Paragon Plus Environment

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ABSTRACT: Single-stranded DNA oligonucleotides have been widely used to functionalize metal nanoparticles, and the nanoparticles are often assumed to be stable. We herein communicate that DNA can both protect and etch silver nanomaterials, such as triangular plates (AgTPs). For DNA with a high affinity to Ag such as poly-cytosine (poly-C) and poly-guanine (poly-G), they display concentration-dependent etching, and DNA length was vital. Etching was less effective when DNA is folded into more compact structures. Poly-thymine (poly-T) DNA is adsorbed very weakly on silver and it cannot etch the AgTPs. Instead, poly-T DNA effectively protects AgTPs from oxidation or dissolution against various etching conditions including Br-, Hg2+, H2O2 and heat. Compared to other types of synthetic polymers, poly-T DNA shows a much stronger protection effect. The change from protection to etching can be rationalized based on the interaction strength with silver. Adsorbed DNA can protect the AgTPs, while a high concentration of strongly binding DNA can increase its solubility and appear an etching effect. This understanding is critical for rational design of biosensors and controlled growth of nanomaterials.

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INTRODUCTION Various metallic silver nanostructures have been synthesized ranging from spheres, cubes, triangles to rods.1-3 Because of unique plasmonic and chemical properties, they have found interesting applications in biosensor development, materials synthesis, drug delivery, and anti-microbial therapy.4-7 Compared to highly stable gold, silver is more easily oxidized and some silver nanostructures are quite unstable.8 For example, silver nanoplates can transform to spherical nanoparticles even under ambient conditions after storage for a long time.9 Therefore, stabilizing these structures is important for reproducible results. At the same time, for certain applications, such as device fabrication, killing bacteria and biosensing, it is also useful to dissolve silver in a controlled manner.10-12 Taken together, it would be highly desirable to have a molecule that can both protect and etch silver-based nanomaterials. Single-stranded DNA oligonucleotides can be chemically synthesized with tunable sequence and length.13-15 Extensive studies have been performed on the adsorption of DNA on gold surfaces,16-17 and it was concluded that poly-adenine (poly-A) DNA has the highest affinity, while poly-thymine (poly-T) DNA has the lowest affinity.18-19 DNA-directed synthesis of various gold nanostructures have also been reported.20-23 The amount of work performed on silver was much less. DNA oligonucleotides can also adsorb on silver surfaces via its bases, and cytosine was known to strongly bind Ag+.24-25 Cytosine and guanine rich DNAs were used to stabilize fluorescent silver nanoclusters.26-27 We recently showed that poly-cytosine (poly-C) DNA could etch spherical silver nanoparticles, while the other three types of DNA homopolymers were not effective for this reaction.28-29 Compared to many other etching/protecting agents, DNA has the advantages of being highly biocompatible, programmable, and binding to a diverse range of molecules.30-32 In this work, we demonstrate that depending on the length, sequence and concentration of DNA, DNA can both stabilize and etch faceted silver. These processes have been rationalized based on the interaction strength between 3



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DNA and silver. High-affinity DNAs (e.g. poly-C and poly-G) displayed concentration-dependent etching, while low-affinity DNAs (e.g. poly-A and poly-T) protected faceted silver. The protecting effect of poly-T DNA was quite strong, and the resultant AgTPs resisted oxidation or dissolution. Since the conformation of DNA can be controlled via molecular recognition (e.g. by using aptamers), we also demonstrated chemically-directed etching.

EXPERIMENTAL SECTION Chemicals. All of the DNA samples were obtained from Integrated DNA Technologies (IDT, Coralville, IA, USA) and their sequences are shown in Table S1. Silver nitrate (AgNO3), trisodium citrate, sodium borohydride (NaBH4), hydrogen peroxide (H2O2, 30 wt%), mercury acetate, polyvinyl pyrrolidone (PVP, Mw = 40,000), polyacrylic acid (PAA), polyethylene glycol (PEG, Mw = 8000), polyvinylsulfonic acid (PVSA, 25 wt%), and poly(sodium 4-styrenesulfonate) (PSS, 30 wt%) were purchased from Sigma-Aldrich (St Louis, MO). Lithium bromide (LiBr), lithium nitrate (LiNO3), and potassium nitrate (KNO3) were purchased from Mandel Scientific (Guelph, ON, Canada). The citratecapped AgNPs (20 nm diameter) were purchased from Nanocomposix (San Diego, CA, USA). Instrumentation. UV-vis absorption spectra were recorded on a spectrometer (Agilent 8453A). The morphology of triangular plates (AgTPs) was examined by a transmission electron microscope (TEM, Philips CM10). Preparation of AgTPs. The AgTPs were synthesized using a previously reported method with some modifications.33 Briefly, a 24.75 mL aqueous solution containing AgNO3 (50 mM, 50 μL), trisodium citrate (75 mM, 0.5 mL), PVP (17.5 mM, 0.1 mL) and H2O2 (30 wt%, 60 μL) was vigorously stirred at 37 C. Then, a freshly prepared NaBH4 (100 mM, 250 μL) was rapidly injected to initiate the reduction reaction. After about 30 min, the AgTPs were obtained. The synthesis of PVP-free AgTPs was similar to 4


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the above-mentioned process except that PVP was replaced with H2O. The concentration of AgTPs was calculated according to the edge length (~53.2 nm) and thickness (~3.7 nm). DNA-Mediated Etching of AgTPs. Typically, 30 μM G18 DNA was mixed with an equal volume of the as-prepared AgTPs, followed by incubation at 37 °C for 1.5 h. The final concentration of G18 DNA was 15 μM. For some experiments, DNA length, concentration, and sequence were varied. Protection of AgTPs by T5. First, the as-prepared AgTPs were incubated with 108 μM T5 at 37 °C for 1.5 h. Then, 70 μL of the T5-treated AgTPs (T5/AgTPs) were incubated with 3.5 μL of 0.1 mM Br-, 1 mM Hg2+ or 0.1 mM H2O2 at 37 °C for 5 min. The final concentrations of Br-, Hg2+ and H2O2 were 5 μM, 50 μM and 5 μM, respectively. For the high temperature treatment, 70 μL of T5/AgTPs was placed in a water bath at 80 °C for 3 min. To compare the protecting ability of T5 with other polymers, 0.1 mM PVP, PAA, PEG, PVSA, and PSS, or 108 μM T5 were incubated with an equal volume of the AgTPs at 37 °C for 1.5 h. Structure-Dependent Etching. 8 μL of K+ of different concentrations (1, 10, 25, 50, 100, 200, 400, 600 mM) was first respectively mixed with 35 μL of 30 μM G4 DNA, followed by heating to 95 °C for 5 min and cooled slowly to room temperature. Then, 35 μL of the as-prepared AgTPs were added and incubated at 37 °C for another 1.5 h.

RESULTS AND DISCUSSION Sequence-Dependent Etching of Silver Triangular Plates. Before testing the effect of DNA oligonucleotides, we first compared the stability of two types of representative silver nanomaterials: spherical nanoparticles (AgNPs) and triangular plates (AgTPs). The color of 20 nm citrate-capped AgNPs solutions was yellow with a strong plasmonic peak at 395 nm (Figure 1A). Our freshly prepared bluecolored AgTPs were capped with citrate and PVP with a main peak at 677 nm (the black spectrum and inset in Figure 1B). 5



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When incubated at 37 C, these AgTPs were unstable and their extinction peak gradually blue-shifted to 630 nm in 90 min (Figure 1B). This is consistent with the literature.34 Under the same condition, the AgNPs were much more stable and their peak remained constant (Figure 1A). The lower stability of the AgTPs can be attributed to its triangular structure with regions of high surface energy, leading to the dissolution of Ag from those regions.35 Similar instability was also observed for the citrate-capped AgTPs (no PVP, Figure S1), and this confirmed that the instability was not due to difference in surface ligands but due to the shape. With the intrinsic lower stability of the AgTPs, we may better compare the effect of DNA sequence, and this study was focused on AgTPs. In this work, to gain fundamental insights into the effect of DNA sequence, we chose to study singlestranded homopolymeric oligonucleotide. We respectively mixed the four kinds of 18-mer DNA homopolymers with the AgTPs and the blue color turned to faint pink with both C18 and G18 after 90 min at 37 C, while the A18 and T18 samples remained blue (the top row in Figure 1C). From their UV-vis spectra, the peaks of the C18 and G18 samples blue shifted and significantly dropped (Figure 1D), suggesting changes in the morphology of the AgTPs. To confirm this, these samples were observed under TEM (Figure 1G and 1H), and indeed the AgTPs changed to ultrasmall spherical AgNPs. Without DNA, the spectral change (Figure 1B) and the morphology change (Figure 1F) due to the intrinsic instability of the AgTPs were much smaller. Therefore, the C18 and G18 oligonucleotides promoted etching of the AgTPs. In comparison, AgNPs were etched only by the C18 DNA (the bottom row in Figure 1C), indicating its higher stability. We reason that AgTPs and AgNPs are representative of silver-based nanomaterials with high-energy faceted surface and low-energy smooth surface, respectively. After incubation at 37 C for 90 min, the peaks of the A18 and T18 added samples were located at 673 nm and 663 nm, respectively, while without DNA, the peak of the AgTPs shifted to 630 nm after the same incubation (Figure 1D). Therefore, these two DNAs reduced the amount of blue shift of the peak. Although compared to the freshly prepared AgTPs (before incubation at 37 C, the 0 min spectrum in 6


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Figure 1B), their peaks still slightly blue shifted (e.g. 4 nm for A18 and 14 nm for T18). Since the shifts with A18 and T18 were much smaller compared to those observed with C18 and G18, we need to discuss them in more detail. First, partial dissolution of the AgTPs before the adsorption of A18 and T18 may contribute to the slight blue shift. In addition, the localized surface plasmon resonance (LSPR) effect often shows a few nanometer of red shift upon DNA adsorption.36-37 We also measured the kinetics of the peak shift of AgTPs with A18 and T18 (Figure S2), and no time-dependent changes were observed. Therefore, the observed shifts here must be due to the very fast dissolution process before the equilibrium of DNA adsorption

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Figure 1. Time-dependent UV-vis spectra of the (A) AgNPs and (B) AgTPs over 1.5 h at 37 C. Insets: photographs of the (A) AgNPs and (B) as-synthesized AgTPs. (C) Photographs of the AgTPs and AgNPs after mixing with 15 μM 18-mer DNA for 1.5 h at 37 C. (D) UV-vis spectra of the AgTPs after mixing with 15 μM 18-mer DNA for 1.5 h at 37 C. TEM micrographs of the (E) freshly prepared AgTPs and AgTPs after mixing with (F) H2O, 15 μM of (G) C18, (H) G18, (I) A18 and (J) T18 DNA for 1.5 h at 37 C, respectively. Scale bar, 100 nm. Overall, A18 and T18 were able to inhibit the blue shift of the AgTPs (thus less etching). We speculate that instead of etching, the A18 and T18 even protected the AgTPs from its background dissolution (e.g. no time-dependent changes observed after the initial shifts). The triangular shape remained for the samples with A18 (Figure 1I) and T18 (Figure 1J), which further confirmed the protection function of A18 and T18. On the other hand, C18 and G18 can fully etch the AgTPs. Unified Protection and Etching Effects. The above studies indicated that homopolymeric oligonucleotide might both protect the AgTPs and enhance their etching, simply by using different DNA sequences. So far, we have only examined one DNA concentration with the 18-mer DNAs. We then systematically varied the length of DNA from 5-mer to 30-mer, but the total base concentration was kept the same (e.g. the concentration of C5 was six times of C30). All of the poly-A (Figure 2A) and poly-T (Figure 2B) DNA protected the AgTPs as judged from the slightly red shifted peaks compared to the sample without any DNA added (the black spectrum in each figure). The peak positions were measured and plotted, and most of the poly-A and poly-T DNAs showed around 30 nm red shifts (black and red solid lines in Figure 2E). Adsorption of DNA on the AgTPs can decrease the surface energy and therefore improve their stability. Additionally, DNA capping the surface can also sterically prevent etching chemicals such as dissolved oxygen to reach the surface. Therefore, even weak poly-T can prevent the 8


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background dissolution. If a DNA sequence interacts with silver too strongly (e.g. poly-C and poly-G), it then can etch the AgTPs. Interestingly, relative to the freshly prepared AgTPs, the main plasmon peak of A5 or A10 treated samples even red-shifted several nanometers and the red-shifting decreased with the increase of poly-A DNA length. With A18, the resulting peak was within 4 nm blue-shift from the fresh AgTPs. This was attributed to the difference in adsorption rate of DNA of different lengths.38 Even though DNA adsorption always caused a few nanometer of red shift due to the LSPR effect,36-37 short DNA such as A5 can quickly adsorb on the AgTPs and start the protection effect, while it took longer for the A18 DNA to adsorb. Therefore, the AgTPs had a longer background etching time for the A18 sample. These experiments were performed with a relatively low concentration of DNA (e.g. 9 μM for A30 or 0.27 mM total adenine base). A higher concentration of poly-A and poly-T DNA (e.g. 25 μM for A30) was further tested. In such cases, the longer the poly-A DNA length, the smaller the red shift (the red dashed lines in Figure 2E). A30 could even slightly etched the AgTPs as indicated by its slightly blueshifted peak. Since a longer DNA is expected to adsorb more strongly on the silver surface,18, 39-40 one would expect a stronger LSPR effect or a larger red shift. The blue shift thus could only be attributed to etching by the longer DNA. With a higher concentration of DNA added, they can bind more dissolved silver species (e.g. Ag+ or small dissolved silver clusters), which effectively increased the solubility of the AgTPs and led to its etching. Since poly-T DNA has the lowest affinity for Ag+, in spite of increasing length to 30-mer with a DNA concentration of 25 μM (total 0.75 mM thymine), no etching was observed, confirming the importance of silver binding affinity. Silver binding can be achieved by high-affinity sequences or high DNA concentrations. Poly-T DNA is too weak, and we likely have not reached the critical concentration, but with 25 μM of the other three DNAs, all were sufficient for etching. It is still possible that etching of the AgTPs might take place by using even longer poly-T DNA at even higher concentrations, but those conditions are unlikely to be of practical relevance. 9



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For poly-guanine (poly-G) and poly-C DNA, their 5-mer samples protected the AgTPs, while they started to etch when the DNA length increased to 10-mer or more (Figure 2C, 2D, and the blue and pink lines in Figure 2E), demonstrating the importance of polyvalent binding. It is interesting to note that with a total base concentration of 0.75 mM, poly-A DNA transitioned from protection to etching between 18mer and 30-mer. For poly-C and poly-G DNA with a total base concentration of only 0.27 mM, their transition occurred between 5-mer and 10-mer. This difference again is a reflection of the base affinity to silver. Compared to the concentration of the AgTPs (~0.4 nM), the DNA was in great excess in the above experiments. Therefore, a large fraction of the DNA oligonucleotides were free in solution. In general, adsorption of ligands on a crystal surface can decrease the surface energy and have a protection effect. This has been used to control the growth of various nanomaterials.20-23 On the other hand, the free DNA molecules in solution were important for the etching process. With a longer DNA in solution, it was able to better stabilize the dissolved silver species to make the AgTPs thermodynamically less stable (e.g. increased solubility). It is likely that a longer DNA was better at binding dissolved silver species. Since poly-C DNAs induced a larger blue shift compared to poly-G DNAs of the same length, poly-C DNA is the most potent for etching the AgTPs among the four types of DNA homopolymers (Figure 2E). Finally, the color of the AgTPs was recorded (Figure 2F), also proving the possibility of tuning the protection and etching process by DNA length and sequence. Based on these data, we concluded the ability to etch AgTPs followed the order of poly-C > poly-G > poly-A > poly-T DNA. DNA can strongly adsorb on AgTPs and also bind Ag+ ions. At relatively low DNA concentrations, DNA covers the AgTPs, preventing the background dissolution/oxidation of the AgTPs. When the DNA concentration is sufficiently high, the excessed free DNA could coordinate with the free Ag+ and such binding may drive dissolution/etching of the AgTPs. This can explain that high concentrations of A30 etched the AgTPs, which was opposite from its low concentration case (e.g. protection). According to the 10


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Gibbs-Thomson effect,41 the sharp corners of the AgTPs might be most easily targeted by foreign molecules due to a high surface energy. As a result, the triangular morphology of AgTPs switched into sphere after the DNA treatment.

Figure 2. UV-vis spectra of the AgTPs after mixing with various lengths of (A) poly-A, (B) poly-T, (C) poly-G, and (D) poly-C DNA. (E) Wavelength shift and (F) photographs of the AgTPs after mixing with DNA homopolymers of various lengths with a total bases concentration of 0.27 mM. A higher concentration (0.75 mM base) of poly-A and poly-T DNA was also tested (the dashed lines in (E)). The dotted line cross zero wavelength shift separates the protection effect (red shift) and etching (blue shift). The comparison of wavelength shift was made with the AgTPs without DNA added. Poly-T DNA as a Stabilizing Agent. It is a general problem that AgTPs have low stability. Since we did not observe etching even with a high concentration of poly-T DNA, we reason that poly-T DNA might be useful for protecting the AgTPs. Here, we used the T5 DNA as a model stabilizing agent. We first compared the relative stability of the AgTPs without and with T5 DNA under some extreme conditions, such as adding etching agents (anionic Br-, cationic Hg2+, and oxidizing H2O2), and high temperature (80 11



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C). Without the T5 DNA, various colors were observed indicating transformation of the AgTPs to other morphologies, while its blue color was retained in the presence of T5 DNA under all these conditions (Figure 3A). When measured by UV-vis spectra, a large blue shift up to 250 nm was observed without DNA, while the T5 DNA added samples shifted much less (Figure 3B). Therefore, T5 DNA improved the stability of the AgTPs.

Figure 3. (A) Photographs and (B) UV-vis based stability comparison between AgTPs and T5 DNA (54 µM) protected AgTPs under different conditions. (C) Br- concentration-dependent stability of AgTPs and T5 protected AgTPs. (D) Protection of the AgTPs from the background dissolution by T5 DNA and other polymers. TEM micrographs of (E) AgTPs and (F) T5 protected AgTPs mixing with 2.9 µM Br-. Scale bar, 100 nm. We then quantitatively compared the effect of T5 DNA on the stability of the AgTPs by adding various concentrations of Br- (Figure 3C). The AgTPs were particularly sensitive to Br-, and saturated etching was observed with just 2.9 µM Br-. Interestingly, the peak shift of the AgTPs was much smaller (by 12


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around 73-fold) with in the presence of T5 DNA. To further confirm the protection effect of T5 DNA, the following experiments were performed. First, the TEM image of the Br--treated AgTPs showed mainly spherical AgNPs of different sizes (Figure 3E), while the triangular shape was retained in the presence of T5 DNA (Figure 3F) which was similar to the freshly prepared AgTPs (Figure 1E). In addition, the color of the T5-treated samples (Figure 3A) was the same as that of the fresh AgTPs (the inset in Figure 1B), while without T5 DNA, the color changed to yellow (e.g. AgNPs). All these indicated the protection effect of T5 DNA. Finally, we tested whether this protection effect was unique to DNA oligonucleotides, or other polymers can also achieve similar effects (Figure 3D). The tested polymers including nonionic PVP and PSS, anionic PAA and PEG, and cationic PVSA. Interestingly, none of them protected the AgTPs and all these polymers showed over 40 nm shift in the peak. In comparison, T5 DNA had only around 10 nm shift. This might be attributable to their lower affinity for silver. Controllable DNA Dependent Etching. After understanding the stabilization effect, we then turned our attention to the other aspect: etching. We have already demonstrated that etching can be achieved using strongly silver binding DNA sequences with a high concentration, and long DNA sequence. Here, we studied the kinetics of etching. We compared the reaction kinetics of C18- and G18-mediated AgTPs etching (Figure 4A and 4B). For both samples, we observed fast time-dependent spectral changes. To better compare C18 and G18 DNA, we plotted their wavelength shift (Figure 4C) and drop in the peak intensity (Figure 4D). In the initial 10 min, a sharp wavelength blue shift of the AgTPs was observed for both C18 and G18 DNA, indicating their strong etching efficacy (Figure 4C). After that, stronger etching was observed for C18 DNA from both wavelength shift and peak intensity. The above studies all used DNA homopolymers. To further understand the effect of DNA sequence, we then inserted different numbers of thymine into G18 to obtain different DNA sequences. The number

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of consecutive guanine was gradually reduced and the etching effect was also gradually inhibited (Figure 4E and 4F). Therefore, it is critical to have more consecutive guanines for stronger etching. It is interesting to note that the (GT)17G DNA has almost doubled base concentration compared to that of G18 (the extra base from thymine), but G18 DNA had much stronger etching effect. Since our above studies indicated that binding of dissolved silver species is critical for etching, consecutive guanines may have an optimal polyvalent effect,42 and thus the increased overall binding affinity of the DNA to the dissolved silver species. Adsorption of the DNA on the surface of the AgTPs might be less important here.43 The molecular level explanation of higher affinity of consecutive guanines will be a topic of further studies by methods such as computer simulation. The G18 DNA could not induce such an optical response for spherical AgNPs (Figure S3), indicating that the AgTPs are a more sensitive system for such studies. Of a side note, the freshly prepared AgTPs had a shoulder at ~450 nm in addition to the main peak at 677 nm. The 677 nm peak is from the in-plane dipole plasmon resonance, while the peak at ~450 nm is associated with the in-plane quadrupole resonance. According to theoretical calculations,1 truncation of the corners of AgTPs results in a blue-shifted and decreased of the 677 nm peak, while the peaks from other resonances remain intact. Interestingly, the 450 nm peak almost disappeared in most of our poly-G and poly-C etched samples, indicating etching also took place in other sites beyond the sharp corners of the AgTPs. This global etching was further supported by the spherical morphology of the etched AgTPs (Figure 1G and 1H).

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Figure 4. UV-vis spectra showing the time-dependent etching of the AgTPs by the (A) C18 and (B) G18 DNA. Kinetics of (C) wavelength shift and (D) decreased extinction peak intensity of C18- and G18-treated AgTPs. DNA length-dependent (E) photographs and (F) UV-vis spectra of the AgTPs treated with various G-rich DNA containing thymine insertions. Single-stranded DNA oligonucleotides can fold into various secondary structures, and this may allow us to understand the effect of DNA conformation on etching. For example, poly-G DNA can form quadruplex structures at neutral pH. We can also deduce that typical G-quadruplex (G4) forming DNAs with other base insertions are likely to etch the AgTPs.44-45 K+ is known to stabilize G-quadruplex 15



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structures.46 Thus, we want to use this property to test the effect of DNA conformation. We mixed different concentrations of K+ with the G4 DNA, followed by adding the AgTPs. Etching by the G4 DNA was dependent on the K+ concentration; the red shifted peak relative to that of the G4 DNA/AgTPs sample without K+ (the black spectrum) was directly proportional to the added K+ concentration (Figure 5D). Compared to the Li+ control (Figure 5C), the wavelength shift was much larger in the presence of K+ (Figure 5B). This can be explained by the formation of G4-quadruplex structured DNA. The linear G4 DNA could provide more flexible binding sites for etching AgTPs while the rigid G-quadruplex structure of G4 DNA forbids its optimal binding of dissolved silver species. Therefore, we can adjust DNA etching activity by altering its conformation.

Figure 5. (A) DNA structure-dependent etching of the AgTPs by forming G4 DNA structure. (B) The effect of Li+ and K+ on etching the AgTPs by 15 μM of the G4 DNA. UV-vis spectra of 15 μM G4 DNA

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etching the AgTPs as a function of (C) Li+ and (D) K+ concentration. The UV-vis spectrum of the intact AgTPs (no G4 DNA) was in bold with olive color.

CONCLUSIONS In summary, we discovered a simple strategy to etch and to stabilize AgTPs through designing DNA length, sequence, and structure or by changing DNA concentration. Etching and protection can be unified based on surface adsorption and affinity to the dissolved silver species. Surface adsorption protect the AgTPs, while an excess of free DNA in solution can etch/solubilize the AgTPs. The etching ability ranking was consistent with their binding affinity with Ag: poly-C > poly-G > poly-A > poly-T DNA. For the same sequence, DNA concentration played a vital role, and the same DNA can be both protective and etching. Chemically controlled etching was achieved by adjusting DNA conformation. Without etching effect, poly-T DNA could provide effective protection of AgTPs even under some extreme conditions. These findings deepened our insights into DNA and silver, and will boost the design of DNAfunctionalized nanomaterials, biosensors and biotherapy.

ASSOCIATED CONTENT Supporting information Supporting Information is available free of charge on the ACS Publications website at DOI The DNAs sequences and some additional characterizations. (PDF)

AUTHOR INFORMATION *E-mail: [email protected] (J.W.). *E-mail: [email protected] (J.L.). Notes 17



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The authors declare no competing financial interest.

ACKNOWLEDGMENTS Funding for this work is from the Natural Sciences and Engineering Research Council of Canada (NSERC). S.H. was supported by the China Scholarship Council (CSC) (201706370185) to visit the University of Waterloo. We also thank the National Natural Science Foundation of China (21575166, 21876208).

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17, 3809-3814. (4) Xu, L.; Yan, W.; Ma, W.; Kuang, H.; Wu, X.; Liu, L.; Zhao, Y.; Wang, L.; Xu, C. SERS Encoded Silver Pyramids for Attomolar Detection of Multiplexed Disease Biomarkers. Adv.

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Unified Etching and Protection of Faceted Silver Nanostructures by

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