Pyrolysis of Helical Coordination Polymers for Metal-Sulfide-Based

Pyrolysis of Helical Coordination Polymers for Metal-Sulfide-Based Helices with Broadband Chiroptical Activity Kenji Hirai,*,† Bongjun Yeom,‡ and Kazuki Sada*,† †

Department of Chemistry, Faculty of Science, Hokkaido University, North-10 West-8, Kita-ku, Sapporo 060-0810, Japan Department of Chemical Engineering, Myongji University, 116 Myongji-ro, Cheoin-gu, Gyeonggi-do 449-728, South Korea

‡

S Supporting Information *

ABSTRACT: Fabrication of chiroptical materials with broadband response in the visible light region is vital to fully realize their potential applications. One way to achieve broadband chiroptical activity is to fabricate chiral nanostructures from materials that exhibit broadband absorption in the visible light region. However, the compounds used for chiroptical materials have predominantly been limited to materials with narrowband spectral response. Here, we synthesize Ag2S-based nanohelices derived from helical coordination polymers. The right- and left-handed coordination helices used as precursors are prepared from L- and Dglutathione with Ag+ and a small amount of Cu2+. The pyrolysis of the coordination helices yields right- and left-handed helices of Cu0.12Ag1.94S/ C, which exhibit chiroptical activity spanning the entire visible light region. Finite element method simulations substantiate that the broadband chiroptical activity is attributed to synergistic broadband light absorption and light scattering. Furthermore, another series of Cu0.10Ag1.90S/C nanohelices are synthesized by choosing the L- or D-Glu-Cys as starting materials. The pitch length of nanohelicies is controlled by changing the peptides, which alters their chiroptical properties. The pyrolysis of coordination helices enables one to fabricate helical Ag2S-based materials that enable broadband chiroptical activity but have not been explored owing to the lack of synthetic routes. KEYWORDS: chiroptical materials, coordination polymers, metal−organic frameworks, nanomaterials, metal sulfides

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superimposed electron transitions in narrow band gap materials allow the strong broadband optical activity. Ag2S is one of the narrow band gap semiconductors25 (∼1.04 eV, smaller than silicon), which is expected to be a promising material for broadband chiroptical activity in the visible light region (Figure S1). However, chiroptical metal sulfides remain unexplored, and methods for their preparation have not been developed. Here, we disclose a synthetic method of Ag2S-based chiral structures that exhibit broadband chiroptical activity in the visible light region but have not been explored due to the lack of synthetic routes. Recently, coordination compounds with infinite structures,26−29 such as metal−organic frameworks (MOFs) or coordination polymers (CPs), have been utilized as precursors for metal oxides.30−34 It can be noted that the pyrolysis of MOFs or CPs yields metal oxides that maintain their original morphology. Occasionally, the self-assembly of chiral organic

he development of chiroptical materials is incentivized by the technological prospects for circular polarizers,1,2 chiroptical sensors,3,4 and negative refractive index materials.5,6 Compared to chiral polymers7−9 or helical supramolecules,10,11 chiroptical inorganic materials have advantages of chemical stability, thermal durability, and synergistic electronic properties.12 In that context, considerable effort has been dedicated in developing approaches to impart nanoscale chirality to achiral inorganic solids. Crystallization assisted by symmetry-breaking molecules13−17 and selfassembly of nanoparticles with chiral configurations18−21 exemplify current bottom-up techniques. By contrast, macroscale twisting,22 lithography,23 and ion beam etching24 represent top-down manufacturing protocols. The majority of chiroptical nanomaterials have been limited to semiconducting metal oxides13−17 and pure metals.1−4,19−24 The chiroptical activity of the inorganic nanomaterials has been mainly derived from plasmon oscillation or light scattering.16 The plasmon-based chiroptical activity has been restricted to the narrow wavelength range; in contrast, light-scattering-based chiroptical activity suffers from weak polarization rotation. The © 2017 American Chemical Society

Received: January 6, 2017 Accepted: April 11, 2017 Published: April 11, 2017 5309

DOI: 10.1021/acsnano.7b00103 ACS Nano 2017, 11, 5309−5317

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Figure 1. Synthetic protocol for preparing CuxAg2−xS/C helices from peptide isomers.

Figure 2. SEM images of (a) coordination fibers and (b) right-handed coordination helices. Magnified SEM images of (c) right-handed and (d) left-handed coordination helices. Insets show schematics of (a) coordination fiber and (c,d) coordination helices.

RESULTS AND DISCUSSION

ligands with metal ions forms coordination compounds with nanoscale helical structures.35 One could notice that the pyrolysis of a coordination helix is a powerful method for fabricating helical inorganic structures using a variety of compounds. Unfortunately, pyrolysis of MOFs and CPs has also been limited to the preparation of metal oxides30−34 because MOFs and CPs mostly comprise coordination bonds of metal ions and oxygens.36 Using thiolate-based coordination compounds as precursors would extend this calcination method to the fabrication of metal sulfides. Thus, our synthetic approach is based on the calcination of coordination helices composed of Ag and chiral peptides containing cysteine moieties. Calcination of coordination helices comprising Ag−sulfur bonds would enable one to yield Ag2S-based helical structures. Owing to a variety of peptides, this approach will be a versatile method to fabricate broadband chiroptical materials. Furthermore, this work will give an insight into CPs as precursors for chiroptical inorganic solids. These fundamental and practical considerations inspired us to exploit helical CPs as precursors for a Ag2S-based nanohelix that would exhibit exceptionally broadband chiroptical activity spanning the entire visible light region (Figure 1).

Synthesis of Metal-Sulfide-Based Helices. A first step for the fabrication of metal-sulfide-based helical structures is to synthesize helical coordination polymers with transition metal ions. Although some pioneering studies demonstrate the synthesis of coordination hydrogels37,38 composed of Ag+ and L-glutathione (L-GSH: L-γ-glutamyl-L-cysteinyl-glycine), these coordination compounds cannot be utilized as precursors for Ag2S-based helices due to their inappropriate morphology or chemical compositions. We developed a synthetic route for helical coordination structures that are composed of transition metal ions and enantiomeric isomers of peptides. Ag(ClO4) and L-glutathione (L-GSH: L-γ-glutamyl-L-cysteinyl-glycine) were dissolved in a mixture of water and dimethylsulfoxide (DMSO), and the solution’s pH was adjusted to 7.0. Note that Ag+ is known to form coordination bonds with thiolate groups. After 5 days of incubation at 37.0 °C, the resulting gel was washed with DMSO and collected by centrifugation. Scanning electron microscopy (SEM) was used to visualize nanofibers with widths of 37.7 ± 8.9 nm, which are expected to be coordination complexes comprising Ag+ and L-GSH (Figure 5310


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ACS Nano 2a). The helicity of the fibers could not be clearly observed by SEM at this stage. Energy-dispersive X-ray spectroscopy (EDX; Figure S2) showed that the nanofibers contained Ag, S, N, C, O, and a small amount of Na atoms. Infrared (IR) spectroscopy of LGSH revealed an IR peak at 2550 cm−1 corresponding to the stretching vibration of the thiol group of L-GSH. This thiol group’s IR peak disappeared after reaction with Ag(ClO4) (Figure S3a,b), suggesting the formation of coordination bonds between thiolate groups and Ag+. The coordination bonds between Ag+ and thiolate groups were also observed by Raman spectroscopy39 (Figure S4a). Although a weak IR peak of neutral carboxylic acid (1700 cm−1) was observed, an antisymmetric (1620 cm−1) vibration of carboxylate anions was also observed (Figure S3a,b). As the fibers contained a small amount of Na atoms, these carboxylate groups most likely formed sodium salt. Furthermore, an IR peak of hydrogenbonded OH groups of carboxylic acid was observed (Figure S3b). The fibers gave several peaks in X-ray diffraction (XRD) measurements (Figure S5a). The chemical formula of the fibers was estimated to be [Na0.1Ag(L-glutathione)]n by inductively coupled plasma (ICP) mass spectrometry and elemental analysis (EA), as shown in Table S1. Although the coordination structures of these soft materials cannot be clearly unveiled, the series of measurements suggested that the coordination fibers were composed of lamellar stacks of quasi-hexagonal Ag−S sheets which are commonly observed in the coordination compounds of Ag and thiolate groups with a 1:1 ratio40,41 (Figure 3). Chiral fibers are often assembled into the helical structures by coordination bonding with divalent cations.42 To fabricate the helical coordination structures, the coordination fibers were soaked in a solution of Cu(NO3)2 and stirred for 1 day. After the monovalent Na+ was replaced with Cu2+ (EDX; Figure S6), the fibers were assembled to form right-handed coordination helices with widths of 181.4 ± 30.6 nm and pitch length of 408.2 ± 82.2 nm (Figure 2b,c). The original fibers with widths of 40 nm were observed in the helical structures (Figure 2c). In this cation exchange, no significant change was observed in the XRD patterns (Figure S5) or in Raman spectra of the Ag−S bond region (Figure S4b), suggesting that the transformation from fibers to helices was attributable not to recrystallization but to the assembly of fibers. This helical assembly of fibers was also observed when immersing the fibers in an aqueous solution of divalent cations, such as Zn(NO3)2 and Cd(NO3)2 (Figure S7). However, the fibers were dissolved by immersion in an aqueous solution of monovalent cation, namely, K(NO3), indicating that the fibers were assembled by coordination bonding with divalent cations. Indeed, IR spectra showed that carboxylate anions were present in the helices that can form coordination bonds with Cu2+ (Figure S3c). Circular dichroism (CD) measurements showed that the weak signals originated from coordination complexes of Cu2+ with peptides (Figure S8a). The positive signal over 700 nm can be assigned to B1g → A1g in the d−d transition of Cu(II). The broad negative signals in the 500−650 nm range were attributed to the superposition of B1g → B2g and B1g → Eg transitions. The CD signals observed in coordination helices matched well with previous reports on coordination complexes of Cu(II) with peptides.43 The chemical formula of the fibers was estimated to be [Cu0.12Ag1.94(L-glutathione)2]n by ICP and EA, as shown in Table S1. Note that the ratio of Cu2+ is only 6 mol % against Ag+. The weak CD signals of the Cu(II) d−d

Figure 3. (a) Quasi-hexagonal Ag−S sheet comprising Ag+ and S atoms of L-GSH. L-GSH substituents alternately extend point-up and -down from the Ag−S sheet. The distances between the atoms in the Ag−S sheet (9.72 and 8.43 Å) correspond to the peaks observed in XRD (Figure S5). (b) L-GSH binding to Ag−S sheet. Silver, yellow, red, blue, gray, and white spheres indicate Ag, S, O, N, C, and H atoms, respectively. The local geometries of molecules are optimized by molecular mechanics calculations. Only one LGSH is shown for clarity. The chiral coordination sites are exposed on the surface of Ag−S sheets, which can induce the chiral assembly of the fibers by coordination bonding. (c) Proposed molecular structures of coordination fibers ([Ag(L-GSH)]n and [Ag(D-GSH)]n). The hydrogen bonding between the Ag−S sheets results in lamellar packing. The distance between the sheets can be approximately estimated to be 17 Å, which corresponds to the strongest peak in XRD (Figure S5).

transitions were ascribed to the very small amount of Cu2+. Considering the small amount of Cu2+, coordination bridging via Cu2+ was formed mostly at the outer surfaces of the coordination fibers. As predicted in Figure 3b,c, the spatial configuration of carboxylate groups in L-GSH can induce the coordination bridging in a right-handed helical direction. The chiral coordination sites on Ag−S sheets would lead to the formation of right-handed coordination helices. To ensure the origin of the helical configurations of the coordination helices, a chiral isomer of the peptide (D-γglutamyl-D-cysteinyl-glycine (D-GSH)) was used. As was the case for L-GSH, Ag(ClO4) and D-GSH were mixed in water and DMSO, and the solution’s pH was adjusted to 7.0. Coordination fibers were formed after 5 days of incubation. The fibers fabricated from D-GSH were soaked in a Cu(NO3)2 solution and stirred for 1 day, giving rise to left-handed coordination helices (Figure 2d). Compared with the helices with L-GSH, mirror image CD signals were observed, indicating the opposite chirality of Cu(II) coordination geometries (Figure S8b). These results further supported the prediction that coordination bonding of Cu-carboxylate induced the helical assembly of fibers into the helices, and that the helical 5311


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permanent porosity, indicating that the original CPs are relatively dense compared to the porous materials of MOFs. (ii) The organic components are not completely removed by calcination at 300 °C (Figure S17). The XRD patterns of the calcined helices corresponded to Ag2S; no XRD peaks characteristic of other materials, such as CuS and CuO, were observed (Figure S11). By applying the Scherrer equation, the mean size of crystal domains is estimated to be 316.8 Å, which is not small enough to show quantum size effect. As shown by EDX, the resulting helices contained Cu, C, Ag, and S (Figure S12). Ag and small amounts of Cu and C were homogeneously distributed in the helices (Figure S13). X-ray photoelectron spectroscopy (XPS) showed that both Cu+ and Cu2+ were involved in the nanohelices (Figure S14a), indicating that Cu2+ was partially reduced in the calcination process under a nitrogen atmosphere.34 XPS of Ag showed a peak shift to a lower binding energy compared with pure Ag2S (Figure S14b). Mixing the divalent cation in Ag2S weakened the binding energy of Ag+ and resulted in the peak shift to a lower binding energy, indicating the formation of solid−solution Cu0.12Ag1.94S.46 The lattice fringes of Cu0.12Ag1.94S and amorphous carbon were observed by transmission electron microscopy (TEM; Figure 4d). The helical nanostructures were retained throughout this calcination process, thereby giving rise to right- and left-handed helices comprising Cu0.12Ag1.94S and carbon, denoted as (Right)- and (Left)-Nanohelix-pitch400. Temperature to pyrolyze the coordination helices is vital to fabricate Ag2S-based nanohelices. We carried out the pyrolysis of left- and right-handed coordination helices by keeping the temperature at 200 and 500 °C for 3 h. By the pyrolysis of coordination helices at 200 °C, the fibers in the coordination helices were partially decomposed (Figure S15a,b). XRD showed that a weak peak around 2θ = 6° still remained, and Ag2S was not formed at 200 °C, suggesting that the coordination structures still remained (Figure S16g,h). The helical structures were completely decomposed by the pyrolysis at 500 °C (Figure S15c,d). After pyrolysis at 500 °C, the XRD pattern corresponded to pure silver (Figure S16a,b), indicating that Ag2S was converted into Ag along with the removal of sulfur. These results matched the thermogravimetric analyses (Figure S17). The coordination helices showed a large weight loss derived from the decomposition of glutathione around 240 °C. In other words, Ag2S was not formed at 200 °C, which was also evidenced by the results of XRD and SEM (Figures S15 and S16). The weight loss was observed even over 300 °C because of the removal of sulfur, leading to the formation of Ag. Chiroptical Properties of Metal-Sulfide-Based Helices. The absorption spectra of (Right)- and (Left)-Nanohelixpitch400 in the visible light region exhibited a broadband absorption in the 400−1500 nm wavelength range, which was attributed to electronic transition in the narrow band gap semiconductor Cu0.12Ag1.94S/C47 (Figure 5a). The band gaps of (Right)- and (Left)-Nanohelix-pitch400 were estimated to be 0.96 and 0.94 eV, respectively (Figure 5b). The doping of divalent Cu(II) into Ag2S slightly increased the band gap compared with that of bulk Ag2S24,47 (∼1.05 eV). There were no significant differences in the absorption spectra of (Right)and (Left)-Nanohelix-pitch400, suggesting that the oppositehanded nanohelices of the same compound were obtained. The CD measurements on solid-state samples require careful analysis as the apparent CD signals can contain other artifact signals originating from coherence of linear birefringence and linear dichroism.48,49 Diffuse reflectance CD measurement is

structures are attributable to the chiral coordination geometries of Cu2+ (Figure 3c). Prior to the fabrication of metal-sulfide-based nanohelices, we carried out the CD measurements on coordination helices and their components (Figure S9). L-GSH and D-GSH showed negative and positive peaks around 220 nm, respectively. In contrast, coordination fibers synthesized from L-GSH showed the bisignate CD signals with negative and positive bands centered at 360 and 340 nm and multiple Cotton effects with two negative peaks and one positive peak from 260 to 310 nm. A mirror image of CD signals was observed with coordination fibers synthesized from D-GSH. After the cation exchange of Na+ with Cu2+, the positive and negative signals at 290 nm were observed for right- and left-handed coordination helices, respectively. The emergence of the signal at 290 nm region is most likely due to the conformational change of peptides induced by helical stacking. These bisignate CD signals epitomize the CD signals of coordination compounds comprising Ag(I) and cysteine.44,45 At the low concentration, the CD signals derived from the d−d transition of Cu(II) were not clearly observed due to their weak signals compared with strong signals of chiral peptides. The coordination helices did not show the strong chiroptical activity in the visible light region. Since both left- and right-handed coordination helices involved coordination bonding of thiolate groups with Ag+, these coordination helices are promising precursors for helical Ag2S/C composites. The left- and right-handed coordination helices were calcined at 300 °C under a nitrogen atmosphere for 3 h. SEM observations revealed that the morphologies of the left- and right-handed nanohelices were maintained after calcination (Figure 4a−c). The significant shrinkage of helical

Figure 4. SEM images of (a,b) (Right)-Nanohelix-pitch400 and (c) (Left)-Nanohelix-pitch400. The red insets in (b,c) show schematic illustrations of (Right)- and (Left)-Nanohelix-pitch400, respectively. (d) TEM observation of (Right)-Nanohelix-pitch400. Lattice fringes (d spacing = 0.270 nm) corresponding to the (120) plane of Cu-doped Ag2S were observed.

structures was not observed by calcination of coordination helices (right- and left-handed coordination helices: 181.4 ± 30.6 nm and 182.8 ± 39.1 nm; right- and left-handed coordination helices after calcination: 152.7 ± 46.6 nm and 159.2 ± 48.4 nm shown in Figure S10). As in previous work on the pyrolysis of MOFs or CPs,30−34 MOFs and CPs shrank slightly but not significantly. Two additional reasons can be considered for the retention of helical structures. (i) Coordination helices synthesized in this work do not possess 5312


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Figure 5. (a) Diffuse reflectance UV−vis spectra of solid-state (Right)-Nanohelix-pitch400 (red line) and (Left)-Nanohelix-pitch400 (black line). (b) Tauc plots of (Right)-Nanohelix-pitch400 (red line) and (Left)-Nanohelix-pitch400 (black line). The band gap energy is estimated by the value where the tangent lines cross the horizontal axis. (c) (i) Absorption spectra of (Right)-Nanohelix-pitch400 (red line) and (Left)Nanohelix-pitch400 (black line) measured by CD spectroscopy. (ii) Experimental CD spectra of (Right)-Nanohelix-pitch400 (red line) and (Left)-Nanohelix-pitch400 (black line) and simulated CD spectra of (Right)-Nanohelix-pitch400 (brown dots) and (Left)-Nanohelix-pitch400 (blue dots). The intensity of simulated CD spectra is in arbitrary units. (iii) g-Factors of (Right)-Nanohelix-pitch400 (red line) and (Left)Nanohelix-pitch400 (black line). (d) Calculated absorption and scattering cross section (σabs and σsc) of RCPL and LCPL for (Right)- and (Left)- Nanohelix-pitch400. The green dots, red triangles, yellow dots, and brown triangles indicate σabs of RCPL, σabs of LCPL, σsc of RCPL, and σsc of LCPL, respectively. (e) Calculated CD signals derived from absorption (blue dots), scattering (red dots), and sum of absorption and scattering (black dots).

tion helices pyrolyzed at 200 and 500 °C for 3 h did not show the broadband chiroptical activity (Figure S18) because Ag2S was not formed at 200 °C, and helical structures decomposed at 500 °C (Figures S15 and S16). These results suggest that the optical properties of Ag2S and its nanoscale morphology are vital for achieving the broadband chiroptical activity. To understand the chiroptical distinction between the (Right)- and (Left)-Nanohelix-pitch400, the CD signals were numerically simulated using the finite-element method (FEM).18,22,51 The dispersed (Right)- and (Left)-Nanohelixpitch400 were randomly oriented in the solvent, so the orientationally averaged CD of a single (Right)- and (Left)Nanohelix-pitch400 was calculated by performing an integration over all polar and azimuthal angles in a polar coordinate system. The geometrical model used in the simulations was a nanoscale helix with a pitch length of 400 nm and a width of 180 nm. To reduce the computational complexity, the small amount of Cu in Ag2S was excluded from the structural model. As shown in Figure 5d, FEM simulations gave the absorption cross section (σabs) and scattering cross section (σscat) for rightand left-handed circularly polarized light (RCPL and LCPL, respectively). CD signals derived from absorption (CDabs, Figure 5e, blue dots) were calculated by subtracting σabs of RCPL (Figure 5d, black circles) from σabs of LCPL (Figure 5d, red triangles). In the same manner, CD signals derived from scattering (CDscat, Figure 5e, red dots) were calculated by

one of the best methods to evaluate the intrinsic chiroptical properties of solid-state samples.50 On the other hand, CD measurements on solid-state samples dispersed in liquid paraffin also give the spectra without artifact signals. The solid-state samples dispersed in liquid paraffin often contain the effect of solvent interacting with the solid samples. However, chiroptical activity of inorganic solids is not affected by organic solvents. Indeed, the measurements on nanomaterials dispersed in liquid is a common protocol to perform CD measurements, as reported in recent papers.22,48,49 This measurement method can eliminate the linear effects by agitation generating random orientation of nanomaterials. Therefore, the CD signals measured in liquid are solely based on the chiroptical activity derived from the ensemble average of individual nanostructures and do not include other artifact signals. Since we are focusing on the chiroptical properties rather than other properties such as the linear effects, we performed CD measurements for (Right)- and (Left)-Nanohelix-pitch400 dispersed in liquid paraffin (Figure 5c). (Right)- and (Left)-Nanohelix-pitch400 exhibited mirror image CD signals over the entire visible light region (Figure 5c(i,ii)). Spectra were not collected above 900 nm owing to the detection limit of the CD instrument. The intensity of CD signals gradually increased with an increase of the wavelength. The g-factors of (Right)- and (Left)-Nanohelixpitch400 were estimated to be 1.32 × 10−3 and −1.31 × 10−3 at 900 nm, respectively (Figure 5c(iii)). Additionally, coordina5313


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Figure 6. SEM images of (a) coordination fibers of [Ag(L-Glu-Cys)]n, (b) right-handed helix of [Cu0.10Ag1.90(L-Glu-Cys)2]n, (c) coordination fibers of [Ag(D-Glu-Cys)]n, (d) left-handed helix of [Cu0.10Ag1.90(D-Glu-Cys)2]n, (e) (Right)-Nanohelix-pitch750, (f) (Left)-Nanohelixpitch750. (g) Experimental CD spectra of (Right)-Nanohelix-pitch750 (red line) and (Left)-Nanohelix-pitch750 (black line) and simulated CD spectra of (Right)-Nanohelix-pitch750 (brown dots) and (Left)-Nanohelix-pitch750 (blue dots). The intensity of simulated CD spectra is in arbitrary units. (ii) g-Factors of (Right)-Nanohelix-pitch750 (red line) and (Left)-Nanohelix-pitch750 (black line).

subtracting σscat of RCPL (Figure 5d, blue circles) from σscat of LCPL (Figure 5d, brown triangles). The CD signals of the nanomaterials were derived from the difference in absorption and scattering between LCPL and RCPL. Thus, the total CD signals (CDtotal, Figure 5d, black dots) were calculated as the sum of CDabs and CDscat. The total CD signals simulated by FEM showed that (Right)- and (Left)-Nanohelix-pitch400 gave positive and negative values of CD, respectively, in the 400−900 nm range (Figure 5e, black dots). The chiroptical activity in the 450−900 nm range was mainly attributed to CDabs of Ag2S/C (Figure 5e, blue dots). The light absorption of Cu-doped Ag2S originates from the electronic transitions where electrons are excited from the ground state to the excited states. The chiral arrangement of individual crystallites of Cu-doped Ag2S in the nanohelix affords the chiral light absorption (CDabs) based on electronic transition.16 By contrast, scattering by the chiral Ag2S/C also contributed to the CD signals, especially in the 650−900 nm range (Figure 5d, red dots). Light scattering in the 650−900 nm range was derived from the relatively large refractive index of Ag2S and amorphous carbon.52,53 Broad shoulders were observed at around 560 and 760 nm in the simulated spectra but not in the experimental data. The positions of the shoulders were influenced by the structural parameters of the nanohelix (for example, width and pitch length). The morphologies of (Right)- and (Left)-Nanohelixpitch400 were not uniform; thus, the shoulders were presumably not observed in the experiments owing to an averaging of the spectra with various shoulder positions. Additionally, the small amount of Cu contained in Ag2S may

have led to the mismatch between the simulation and the experimental data. However, the FEM simulation captured the chiroptical essence of (Right)- and (Left)-Nanohelix-pitch400 (Figure 5c(ii)). It can be concluded that the synergistic combination of light absorption and scattering of Cu0.12Ag1.94S/ C nanohelices afforded chiroptical activity over the entire visible light region. To demonstrate the versatility of our approach, we synthesized nanohelices from dipeptides. Ag(ClO4) and LGlu-Cys (γ-L-glutamyl-L-cysteine) were dissolved in a mixture of water and DMSO. After the pH was adjusted to 7.0, the solution was incubated at 37.0 °C for 3 days. Nanofibers with widths of 39.8 ± 7.2 nm were observed by SEM (Figure 6a). The immersion of the nanofibers into Cu(NO3)2 solutions induced the helical assembly of the fibers to form right-handed coordination helices with a width of 188.2 ± 52.3 nm and pitch length of 753.5 ± 98.2 nm (Figure 6b). The chemical formula was estimated to be [Cu0.10Ag1.90(L-Glu-Cys)2]n by ICP. The self-assembly of a chiral isomer of D-Glu-Cys (γ-D-glutamyl-Dcysteine) with Ag+ yielded the coordination fibers (Figure 6c), followed by helical assembly into the left-handed coordination helices (Figure 6d). The right- and left-handed coordination helices of [Cu0.10Ag1.90(Glu-Cys)2]n were synthesized through protocols identical to those used in the case of glutathione. However, the pitch length of the coordination helices was larger than those prepared from Ag+ with GSH (pitch length, [Cu 0 . 1 2 Ag 1 . 9 4 (glutathione) 2 ] n = 408.2 ± 82.2 nm, [Cu0.10Ag1.90(Glu-Cys)2]n = 753.5 ± 98.2 nm). This was proposed to be because the tripeptides (glutathione) are structurally more flexible than dipeptides (Glu-Cys). The 5314


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and used without further purification. D-Glutathione and γ-D-glutamylwere synthesized by GenScript Japan Inc. Synthesis of Right- and Left-Handed Coordination Helices. Anhydrous silver(I) perchlorate (5.60 mg, 2.70 × 10−2 mmol) and Lglutathione or D-glutathione (13.8 mg, 4.49 × 10−2 mmol) were dissolved in a mixture of water (540 μL) and DMSO (360 μL). The pH was adjusted to 7.0 by addition of sodium hydroxide aqueous solution (200 mmol/L). The mixtures were left in an incubator at 37.0 °C. After incubation for 5 days, a yellowish-orange gel was formed. The gel was washed with DMSO three times and collected by centrifugation. The resulting gel was soaked in a water/DMSO solution of copper nitride (20 mmol/L) and vigorously stirred for 1 day. The right- and left-handed helices of [Cu0.12Ag1.94(glutathione)2]n were collected by centrifugation and washed with deionized water. The right- and left-handed helices of [Cu0.12Ag1.94(Glu-Cys)2]n were synthesized when L- or D-Glu-Cys were used as starting materials. Inductively Plasma Coupled Measurement. The coordination compounds were dried at 100 °C under vacuum to remove the moisture. After being dried, the weight of the coordination compounds was measured. The coordination compounds were decomposed by 0.1 M HCl. The obtained solution was analyzed by ICP (ICPE-9000, Shimadzu Corporation) to estimate the amount of Ag, Cu, and Na contained in the coordination compounds. Elemental Analysis. The coordination compounds were dried at 100 °C under vacuum to remove the moisture. After being dried, the weight of the coordination compounds was measured. The amount of Ag, Cu, and Na contained in the coordination compounds was analyzed by a CHN analyzer (CE440, External Analytical, Inc.). To support the combustion of the analyte, the coordination compounds were mixed with WO3 as an oxidation catalyst. Calcination of Right- and Left-Handed Coordination Helices. Calcination of right- and left-handed coordination helices was carried out in a ceramic tube furnace (TMF-500N, As One Corporation, Japan). The resulting white powder was placed in crucibles and kept at 300 °C for 3 h under a nitrogen flow. CD Spectroscopy. Nanohelices were dispersed in liquid paraffin and stirred for several hours to disperse and randomly orient the helices. CD spectra for nanohelices were measured using J-820 and J720 instruments (JASCO Corporation, Japan). The background spectra of liquid paraffin were subtracted from the CD spectra of nanohelices dispersed in liquids. Scanning Electron Microscopy. DMSO solution containing the coordination fibers or aqueous solution containing the coordination helices was casted on a silicon wafer and dried in nitrogen flow. The samples were observed using JEOL JIB-4600F/HKD and JEOL JSM7100F. X-ray Photoelectron Spectroscopy. Powders of nanohelices were placed on a carbon conductive tape to avoid the powders from swirling in the chamber. XPS data were collected by JEOL Ltd. JPS9200. FEM Simulation of CD Spectra. Simulations of the CD spectra were carried out using COMSOL Multiphysics version 5.2, which adopts FEM to solve the electromagnetic scattering problem. Other Apparatus. X-ray diffraction patterns were collected by Bruker D8 Advance ECO. UV−vis absorption was measured by JASCO V-570.

flexible glutathione enables formation of helices with a large coverture, leading to the formation of nanohelices with shorter pitch length. We have also tested another peptide (cysteinylglycine) to fabricate the coordination helices. The coordination fibers were obtained by Cys-Gly and Ag+ (Figure S19); however, the coordination helices were not formed by immersing the fibers in Cu2+ (Figure S19b). This result was in agreement with the structural model proposed in Figure 3. A key to form the helical assembly of the fibers is the chiral configuration of two carboxylates in the peptides. Dipeptides of Cys-Gly, which has only one free carboxylate, cannot induce the helical assembly of the fibers. The pyrolysis of right- and left-handed coordination helices of ([Cu0.10Ag1.90(Glu-Cys)2]n) gave the right- and left-handed nanohelices of Cu0.1Ag1.9S/C, denoted as (Right)- and (Left)Nanohelix-pitch750 (Figure 6e,f). As indicated by the mirror image CD signals from 400 to 900 nm, (Right)- and (Left)Nanohelix-pitch750 exhibited chiroptical activity over the entire visible light region (Figure 6g(i)). In the region of 450−500 nm, (Right)- and (Left)-Nanohelix-pitch750 showed a CD signal that was inverted from negative to positive or from positive to negative, respectively. The characteristic features of CD signals were also simulated by FEM (Figure 6g(i), brown and blue dots). FEM simulations for the right- and left-handed nanohelices with a pitch of 750 nm showed a sign inversion at 500 nm. The g-factors of (Right)- and (Left)-Nanohelixpitch750 were estimated to be 2.96 × 10−3 and −3.53 × 10−3 at 900 nm (Figure 6g(ii)), respectively, which is relatively high among chiroptical nanomaterials.51 The helical structures and chiroptical properties of nanohelices were altered by changing the peptides from glutathione to Glu-Cys. Because of the designability of peptides, our approach would be a versatile methodology for the fabrication of Ag2S-based broadband chiroptical materials (Table S2). The optically active metal sulfides have not been explored owing to the synthetic difficulties in the conventional approaches. The characteristic broadband absorption of Ag2S results in a broadband chiroptical activity, which has not been observed in the chiroptical materials of pure metals and metal oxides.

D-cysteine

CONCLUSIONS In summary, calcination of coordination helices represents a simple yet powerful method for fabricating Ag2S-based composite helices with broadband chiroptical activity. The composite material synergizes chiral light absorption and light scattering, resulting in chiroptical activity spanning the entire visible light region. Helical organic templates coupled with sol− gel method13−17 have been excellent ways to synthesize metal oxide helix but cannot be applied to the synthesis of metal sulfides. This synthetic method enables one to fabricate a series of chiroptical Ag2S-based materials that would otherwise be incompatible with conventional fabrication methods. This work will demonstrate a way to utilize coordination polymers as precursors for broadband chiroptical materials. As a result, the research fields of coordination polymers and chiral nanomaterials are expected to be bridged, leading to the formation of an interdisciplinary area between the two.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b00103. Spectral data, electron microscopic images, X-ray diffraction data, and detailed description of finite element method simulations (PDF)

METHODS Chemicals. L-Glutathione, dimethylsulfoxide, diluted water, sodium hydroxide, and copper(II) nitrate tetrahydrate were purchased from Wako Chemical, Ltd. Silver(I)chloride, anhydrous, γ-L-glutamyl-Lcysteine, and L-cysteinyl-glycine were purchased from Sigma-Aldrich

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. 5315


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ACS Nano *E-mail: [email protected].

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ORCID

Kenji Hirai: 0000-0003-3307-3970 Bongjun Yeom: 0000-0001-8914-0947 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was funded by a Grant-in-Aid for Young Scientists (B) KAKENHI 16K17875 to K.H. Partial support was also provided by ICHIJYU Industrial Science and Technology Promotion Foundation to K.H. and National Research F o u n d a t i o n o f K o r e a ( N R F ) g r a n t (N o . NR F 2015R1D1A1A01058029) to B.Y. We thank Prof. Masako Kato and Dr. Atsushi Kobayashi for assistance with the use of powder XRD. We thank the open facility at Hokkaido University for the use of SEM, TEM, Raman spectroscopy, XPS, and CD. REFERENCES (1) Gansel, J. K.; Thiel, M.; Rill, M. S.; Decker, M.; Bade, K.; Saile, V.; von Freymann, G.; Linden, S.; Wegener, M. Gold Helix Photonic Metamaterial as Broadband Circular Polarizer. Science 2009, 325, 1513−1515. (2) Zhao, Y.; Belkin, M. A.; Alù, A. Twisted Optical Metamaterials for Planarized Ultrathin Broadband Circular Polarizers. Nat. Commun. 2012, 3, 870. (3) Zhao, Y.; Yang, Y.; Zhao, J.; Weng, P.; Pang, Q.; Song, Q. Dynamic Chiral Nanoparticle Assemblies and Specific Chiroplasmonic Analysis of Cancer Cells. Adv. Mater. 2016, 28, 4877−4883. (4) Ma, W.; Kuang, H.; Xu, L.; Ding, L.; Xu, C.; Wang, L.; Kotov, N. A. Attomolar DNA Detection with Chiral Nanorod Assemblies. Nat. Commun. 2013, 4, 2689. (5) Pendry, J. B. A Chiral Route to Negative Refraction. Science 2004, 306, 1353−1355. (6) Soukoulis, C. M.; Linden, S.; Wegener, M. Negative Refractive Index at Optical Wavelengths. Science 2007, 315, 47−49. (7) Yashima, E.; Maeda, K.; Okamoto, Y. Memory of Macromolecular Helicity Assisted by Interaction with Achiral Small Molecules. Nature 1999, 399, 449−451. (8) Akagi, K.; Piao, G.; Kaneko, S.; Sakamaki, K.; Shirakawa, H.; Kyotani, M. Helical Polyacetylene Synthesized with a Chiral Nematic Reaction Field. Science 1998, 282, 1683−1686. (9) Nagata, Y.; Takagi, K.; Suginome, M. Solid Polymer Films Exhibiting Handedness-Switchable, Full-Color-Tunable Selective Reflection of Circularly Polarized Light. J. Am. Chem. Soc. 2014, 136, 9858−9861. (10) Kumar, J.; Tsumatori, H.; Yuasa, J.; Kawai, T.; Nakashima, T. Self-Discriminating Termination of Chiral Supramolecular Polymerization: Tuning the Length of Nanofiber. Angew. Chem., Int. Ed. 2015, 54, 5943−5947. (11) Ziegler, M.; Davis, A. V.; Johnson, D. W.; Raymond, K. N. Supramolecular Chirality: A Reporter of Structural Memory. Angew. Chem., Int. Ed. 2003, 42, 665−668. (12) Valev, V. K.; Baumberg, J. J.; Sibilia, C.; Verbiest, T. Chirality and Chiroptical Effects in Plasmonic Nanostructures: Fundamentals, Recent Progress, and Outlook. Adv. Mater. 2013, 25, 2517−2534. (13) Che, S.; Liu, Z.; Ohsuna, T.; Sakamoto, K.; Terasaki, O.; Tatsumi, T. Synthesis and Characterization of Chiral Mesoporous Silica. Nature 2004, 429, 281−284. (14) Duan, Y.; Han, L.; Zhang, J.; Asahina, S.; Huang, Z.; Shi, L.; Wang, B.; Cao, Y.; Yao, Y.; Ma, L.; Wang, C.; Dukor, R. K.; Sun, L.; Jiang, C.; Tang, Z.; Nafie, L. A.; Che, S. Optically Active Nanostructured ZnO Films. Angew. Chem., Int. Ed. 2015, 54, 15170−15175. 5316


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Pyrolysis of Helical Coordination Polymers for Metal-Sulfide-Based

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