Chiroptical Study of Metal@semiconductorâMolecule Composites

Article pubs.acs.org/JPCC

Chiroptical Study of Metal@semiconductor−Molecule Composites: Interaction between Cysteine and Ag@Ag3PO4 Core−Shell Hybrid Nanorods Guiqing Cheng, Jiancheng Di, and Yu Wang* State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun, 130012, P. R. China

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 15, 2015 | http://pubs.acs.org Publication Date (Web): September 14, 2015 | doi: 10.1021/acs.jpcc.5b06904

S Supporting Information *

ABSTRACT: The coupling between an inorganic nanostructure and a chiral molecule can give rise to a strong chiroptical response in a range from the UV to visible spectrum. In this work, novel plasmonic Ag@Ag3PO4 core−shell hybrid nanorods (CSHNRs) have been prepared via a wet chemical method. The cysteine molecules can adsorb on the surface of the plasmonic hybrid nanorods, leading to the appearance of new circular dichroism (CD) signals in the UV region. The Ag@Ag3PO4−cysteine CSHNRs show a similar CD spectrum to the Ag3PO4−cysteine polyhedral nanocrystals (PNCs), although they have different morphology and structure. The Ag@Ag3PO4−cysteine CSHNRs and Ag3PO4−cysteine PNCs have a stronger chiroptical response than the Ag−cysteine nanorods (NRs) attributed to the effect of phosphate groups. This indicates that the Ag+ ions on the surface of nanostructures can promote the formation of S−S bonds of oxidized cysteine under acidic conditions, rendering the uniformity of molecular conformation on the surface of nanostrucures, thus giving rise to the strong chiroptical response. Furthermore, the Ag@Ag3PO4 CSHNRs can spontaneously form a chiral liquid crystalline phase upon dispersion in the ethylene glycol solution owing to their large aspect ratio and the optimum solvating effect of ethylene glycol.

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INTRODUCTION Chiral inorganic nanostructures have attracted considerable attention because of their intriguing chemical and physical properties as well as tremendous potentials in biosensors, enantioselective catalysis and separation, and optical devices.1−6 Among them, numerous chiral noble metal nanostructures with strong chiroptical response have been reported due to their unique plasmonic properties.7−14 Recently, some helical assemblies with noble metal nanoparticles have been prepared by using DNA and peptide as soft templates.15−25 The selfassembly of silver nanorods with cholesteric-liquid-crystalline (CLC) structure based on van der Waals and electrostatic interactions has been prepared according to Onsager’s theory.26 Small biomolecules such as cysteine and glutathione have been successfully coupled to noble metal nanostructures, resulting in strong chiroptical response,27−36 a notable example being the 1D assembly of cysteine and gold nanorods.37 Although these chiral plasmonic assemblies show obvious chiroptical response due to geometry-related chiral collective excitation and the chiral induction of chiral molecules, the further development of chiral plasmonic nanostructures has been blocked due to their high cost and low wavelength tunability. In contrast to the noble metals, it is inexpensive to synthesize semiconductor nanomaterials, and the wavelength can be tuned by combining semiconductors and noble metals. Recently, © XXXX American Chemical Society

some chiral semiconductor nanostructures have been prepared by using DNA as soft templates via direct crystal growth.38,39 In particular, a kind of novel chiral semiconductor quantum dots has been prepared by coupling biomolecules to quantum dots (QDs). However, such materials show low chiroptical response, thus limiting their applications due to the weak interaction between biomolecules and QDs.40−42 To date, chiral noble metal@semiconductor hybrid nanomaterials have rarely been reported,43 compared to chiral noble metal nanostructures and chiral semiconductor nanomaterials. It is highly desirable to couple biomolecules, such as cysteine molecules, to noble metal@semiconductor core−shell hybrid nanostructures to form chiral noble metal@semiconductor hybrid nanostructures to yield strong CD signals. However, this remains challenging due to the lack of appropriate semiconductor nanostructures. In this contribution, the chiral Ag@Ag3PO4−cysteine CSHNRs have been prepared by coupling cysteine on the surface of Ag@Ag3PO4 CSHNRs. New CD signals different from the cysteine and Ag plasmonic resonance can be observed in the UV region for Ag@Ag3PO4−cysteine CSHNRs, and a similar chiroptical response can also be found in Ag3PO4− Received: July 17, 2015 Revised: August 31, 2015

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DOI: 10.1021/acs.jpcc.5b06904 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) SEM image of the Ag NRs (scale bar = 100 nm). (b) TEM image of the Ag NRs (scale bar = 1 μm). (c) HRTEM image of a single Ag NR (scale bar = 10 nm; distance indicated by arrows = 0.237 nm). (d) Powder XRD pattern of Ag NRs, Ag@Ag3PO4 CSHNRs. (e) SEM image of Ag@Ag3PO4 CSHNRs (scale bar = 1 μm). (f) TEM image of Ag@Ag3PO4 CSHNRs (scale bar = 500 nm).

phase owing to their larger aspect ratio and the optimum solvating effect of ethylene glycol.

cysteine PNCs, confirming their similar structures. The stronger CD signals of Ag3PO4−cysteine PNCs can be attributed to their large geometric curvature compared to the Ag@Ag3PO4−cysteine CSHNRs. The optical activities of the Ag@Ag3PO4−cysteine CSHNRs and Ag3PO4−cysteine PNCs should be associated with the formation and orderly arrangement of S−S bonds of oxidized cysteine. The formation of S−S bonds of oxidized cysteine can be promoted by Ag+ ions on the surface of nanostructures under acidic conditions, leading to the uniformity of molecular conformation on the surface of nanostrucures and thus resulting in a strong chiroptical response. Furthermore, an ethylene glycol solution of Ag@ Ag3PO4 CSHNRs can spontaneously form a liquid crystalline

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EXPERIMENTAL SECTION Materials. All chemicals were obtained from commercial suppliers and used without further purification. Poly(vinylpyrrolidone) (PVP, average molecular weight 40 000), silver nitrate (AgNO3, ≥99.8%), sodium phosphate dibasic (Na2HPO4, ≥99.0%), and iron chloride hexahydrate (FeCl3· 6H2O, ≥98%) were purchased from Sigma-Aldrich. Ethylene glycol (A.R.), ammonia solution (NH3·H2O, 25%), and hydrogen peroxide (H2O2, 30 wt %) were purchased from Beijing Chemical Works. Glycine (≥98%), L-threonine (≥98%), L-methionine (≥98%), L-cysteine (≥98%), and DB


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Figure 2. UV−vis spectra: (a) Ag NRs. (b) Ag@Ag3PO4 CSHNRs. (c) Ag−amino acid NRs (I = glycine, II = L-threonine, III = L-methionine). (d) A series of Ag−L-cysteine NRs (I = 0.05 mM, II = 0.24 mM, III = 0.45 mM, IV = 1.67 mM, V = 3.33 mM, VI = 5 mM). (e) Ag@Ag3PO4−amino acid CSHNRs (I = glycine, II = L-threonine, III = L-methionine). (f) A series of Ag@Ag3PO4−L-cysteine CSHNRs (I = 1.15 mM, II = 1.43 mM, III = 1.67 mM, IV = 3.33 mM, V = 5 mM).

dark at room temperature for 12 h. Then the product was centrifuged and washed with distilled water three times. The assynthesized Ag3PO4 PNCs were dispersed in 6 mL of water for further procedures. Detailed information on the preparation of colloidal suspensions of Ag−amino acid NRs, Ag@Ag3PO4− amino acid CSHNRs, and Ag3PO4−cysteine PNCs can be found in Supporting Information. Characterizations. The XRD patterns were recorded with a Rigaku D/Max 2550 X-ray diffractometer using a monochromatized Cu Kα radiation source (λ = 1.5406 Å). The surface morphologies were analyzed with a JEOL-6700 F field emission scanning electron microscope at an accelerating voltage of 3 kV. The transmission electron micrograph (TEM) and high-resolution transmission electron micrograph (HRTEM) images were recorded on a FEI Tecnai G2S-Twin with a field emission gun operating at 200 kV. The X-ray photoelectron spectra (XPS) were measured with an ESCALab 250 Analytical XPS spectrometer with a monochromatic X-ray source (Al Kα, h = 1486.6 eV). UV−vis absorption spectra were measured with a Shimadzu UV-1800 spectrophotometer. CD spectra were recorded on a Bio-Logic MOS-450 CD spectrometer. Zeta potential was measured on a Malvern Zetasizer Nano-ZS90.

cysteine (≥98%) were purchased from Shanghai Huishi Biochemical Co., Ltd. Synthesis of Ag NRs. The AgNRs were synthesized by a modified polyol process.44 Typically, PVP (0.2 g) was mixed with ethylene glycol (25 mL) in a 100 mL flask under gentle stirring at room temperature until PVP was completely dissolved. Next, AgNO3 (0.25 g) was added to the above PVP solution. Subsequently, an ethylene glycol solution of FeCl3 (3.25 g, 600 mM) was added to the aforementioned reaction mixture with stirring for 1 min. Finally, the mixture was immediately transferred into an oil bath, heated for 5 h at 130 °C, and cooled to room temperature in ambient atmosphere. The as-synthesized AgNRs were washed with water several times to remove the residues and dispersed into 10 mL of water for further procedures. Synthesis of Ag@Ag 3 PO 4 CSHNRs. Ag@Ag 3 PO 4 CSHNRs were prepared according to the previous report with a slight modification.45 Typically, a AgNR solution (0.2 mL) was dispersed in PVP aqueous solution (40 mL, 100 mM) and stirred for 20 min at room temperature. Next, a solution of Ag(NH3)2+ complex (5 mL, 0.06 mM) was added to the above mixture with stirring for another 20 min in ambient atmosphere. Subsequently, Na2HPO4 solution (20 mL, 2 mM) was added to the aforementioned reaction solution and stirred for 30 min. Then the mixture was diluted with water. After the mixture was stirred at room temperature for 20 min, the product was centrifuged and washed with water several times. The obtained Ag@Ag3PO4 CSHNRs were dispersed in 6 mL of water for further procedures. Synthesis of Ag3PO4 PNCs. Ag3PO4 PNCs were prepared according to the previous report with little modification.46 A AgNR solution (1.7 mL) was dispersed in 20 mL of saturated Na2HPO4 aqueous solution and stirred for a few seconds in ambient atmosphere. Next, H2O2 solution (1 mL, 3%) was added to the above-mentioned mixture and stirred for 30 min in ambient atmosphere. This process was repeated four times. Subsequently, the reaction mixture was stirred vigorously in the

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RESULTS AND DISCUSSION The AgNRs were synthesized by heating a mixture of silver nitride (AgNO3), poly(vinylpyrrolidone) (PVP), iron chloride (FeCl3), and ethylene glycol at a molar ratio of 1:1.22:1.75 × 10−6:305 in an oil bath according to the previous report with some modification.44 Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that the AgNRs have a rod morphology with an average diameter (D), length (L), and aspect ratio (L/D) of 80 nm, 20 μm, and 250, respectively (Figure 1a,b). The AgNRs are highly crystalline, as confirmed by high-resolution transmission electron microscopy (HRTEM). The average d-spacing of the lattice fringes measured from the HRTEM image is about 0.24 nm, agreeing C


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Figure 3. CD spectra of (a) the colloidal suspensions of Ag@Ag3PO4−cysteine CSHNRs (L- and D-cysteine). (b) A series of Ag@Ag3PO4−Lcysteine CSHNRs (I = 1.15 mM, II = 1.43 mM, III = 1.67 mM, IV = 3.33 mM, V = 5 mM). (c) A series of Ag@Ag3PO4−D-cysteine CSHNRs (I = 1.15 mM, II = 1.43 mM, III = 1.67 mM, IV = 3.33 mM, V = 5 mM).

with the d111 lattice plane of the metallic Ag face-centered cubic (f.c.c) phase (Figure 1c). All of the X-ray diffraction peaks can be indexed to the metallic Ag f.c.c phase (JCPDS no.: 04-0783), and no impure phase can be found (Figure 1d). The Ag@ Ag3PO4 CSHNRs were prepared by coating the Ag3PO4 shell on the surface of AgNRs according to the previous report with some modification.45 Ag@Ag3PO4 CSHNRs also have a rod morphology with an average diameter (D), length (L), and aspect ratio (L/D) of 400 nm, 5 μm, and 12, respectively (Figure 1e,f). The size of core and shell are 80 and 160 nm, respectively. The Ag@Ag3PO4 CSHNRs are crystalline, and all diffraction peaks in the XRD pattern can be indexed to the body-centered cubic (b.c.c) phase of Ag3PO4 (JCPDS no. 060505) and metallic Ag f.c.c phase (JCPDS no. 04-0783) with the Ag3PO4 phase being dominant (Figure 1d). The UV−vis absorption spectrum of the AgNRs shows two strong plasmon bands at approximately 350 and 382 nm (Figure 2a), corresponding to several plasmonic resonance modes of silver NRs.26 Figure 2b shows that Ag@Ag3PO4 CSHNRs have a broad band around 406 nm, corresponding to the superposition of the plasmonic resonance of AgNRs and the adsorption of the Ag3PO4 nanoshells. Furthermore, the Fano dip appears at 330 nm for AgNRs and Ag@Ag3PO4 CSHNRs, indicating the destructive interference of several Ag plasmonic resonance modes.47−50 Figure 2c shows that the UV−vis spectra of AgNR−amino acid composites, including Ag−I NR (I = glycine), Ag−II NR (II = L-threonine), and Ag−III NR (III = L-methionine), have UV−vis absorption spectra similar to those of the AgNRs, confirming that the original plasmonic resonance modes cannot be affected by coupling them with amino acids.26 However, the UV−vis absorption spectra of a series of Ag−cysteine NRs show two bands at 254 and 281 nm besides the Ag plasmonic resonance, unrelated to the original AgNR and cysteine species, indicating the emergence of new species (Figure 2d). The Ag@ Ag3PO4−cysteine CSHNRs have similar UV−vis spectra to those of the Ag−cysteine NRs with more obvious peaks at 254 and 281 nm (Figure 2b,f). The above results indicate that the amino acids with the exception of cysteine have no obvious effect on the UV−vis adsorption of Ag@Ag3PO4 CSHNRs. However, cysteine can couple with Ag@Ag3PO4 CSHNRs to form new species. The CD spectroscopic analysis was conducted to characterize the chiroptical responses of the Ag@Ag3PO 4−cysteine CSHNRs. The coupled Ag@Ag3PO4−(L,D)-cysteine CSHNRs show the characteristic mirrored CD signals, expressed as a pair of enantiomers (Figure 3a).

A gradual red shift of the CD peaks can be observed with an increase in the concentration of L- and D-cysteine (Figure 3b,c), indicating that the chiroptical response can be tuned by varying the cysteine concentration. The Ag@Ag3PO4 CSHNRs and Ag@Ag3PO4−amino acid CSHNRs do not exhibit a chiral response, confirming that the amino acids except for cysteine cannot couple with the Ag@Ag3PO4 CSHNRs and transfer their chirality to the Ag@Ag3PO4 CSHNRs (Figure S1, Supporting Information). This suggests that the chiroptical activity of Ag@Ag3PO4−cysteine CSHNRs should be attributed to other factors. Some possible mechanisms have been recently reported to explain the chiroptical activity of metal− amino acid with thiol group nanostructures:51−53 (i) the CD bands originate from the formation of a chiral metal cluster, (ii) ligands can be chirally arranged on the surface of metal nanostructures and chirality originates from the interaction between their electrons, (iii) the vicinal effects can be induced by the linkage of chemical bonds between the dissymmetric metal@ligand composites. Especially, Huang et al. deeply investigated the interaction of glutathione (GSH) with Ag nanoparticles with the surface-enhanced Raman scattering technique.54 It is reported that the interaction between GSH and the surface of noble metal nanostructures can make the GSH molecules adopt a different orientation with respect to the surface and enhance the uniformity of the molecular conformation on the surface owing to the formation of disulfide bonds under acidic conditions.54,55 We speculate that a similar interaction takes place between the cysteine and the surface of Ag@Ag3PO4 CSHNRs. Experimentally, the CD peak obviously decreased when the NaBH4 solution was added to the system, confirming the disulfide bond reduction, which is consistent with the previous report.56 In the current work, Ag@ Ag3PO4 CSHNRs do not present any chiral characteristics. The chirality of Ag@Ag3PO4−cysteine CSHNRs emerges when the cysteine concentration approaches the critical concentration of 1.43 mM, which is higher than the critical concentration for AgNRs. Moreover, the CD signals are at positions similar to those of the UV−vis peaks, confirming the exciton coupling between the cysteine and Ag@Ag3PO4 CSHNRs. Thus, it is speculated that mechanisms 2 and 3 may work in this system. It is worth noting that the Ag@Ag3PO4−cysteine CSHNRs show a chiroptical response stronger than that of the Ag−cysteine NRs although their CD signals are at similar positions (Figure 4). It is well-known that the self-arranged cysteine on the surface of AgNRs and Ag@Ag3PO4 CSHNRs strongly depends on the S−S bond dihedral angles.57 The two S−S bonds should have different dihedral angle values when the disulfide is D


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Figure 4. CD spectra of Ag@Ag3PO4−L-cysteine CSHNRs and Ag−Lcysteine NRs (c = 3.33 mM).

coupled to the surface of the AgNRs and Ag@Ag3PO4 CSHNRs, which is the requirement of the minimum energy conformation of the unconstrained disulfide. The difference in S−S bond dihedral angles between AgNRs and Ag@Ag3PO4 CSHNRs may originate from different chemical constitutions. The phosphate groups on the surface of Ag@Ag3PO4 CSHNRs can change the orientation of cysteine molecules and improve the relative chiroptical response. To further verify this hypothesis, X-ray photoelectron spectra (XPS) characterization was performed. Figure S2 (Supporting Information) displays the XPS of Ag 3d of AgNRs and Ag@ Ag3PO4 CSHNRs. The XPS of Ag 3d peaks exhibit a doublet with binding energies of 367.74 eV (3d3/2), 373.74 eV (3d5/2) and 368.05 eV (3d3/2), 374.06 eV (3d5/2), for AgNRs and Ag@ Ag3PO4 CSHNRs, respectively. However, the XPS of Ag 3d peaks show a quadruplet and a doublet with bonding energies of 367.81 eV (3d3/2), 368.78 eV (3d3/2), 373.81 eV (3d5/2), and 374.76 eV (3d5/2), and a doublet with bonding energies of 368.50 eV (3d3/2) and 374.51 eV (3d5/2) for Ag−cysteine NRs and Ag@Ag3PO4−cysteine CSHNRs (Figure 5a,b), respectively. This suggests that the cysteine molecules are coupled to the surface of Ag@Ag3PO4 CSHNRs, and the obvious increase in CD intensity of Ag@Ag3PO4−cysteine CSHNRs compared to Ag−cysteine NRs can be attributed to the effect of the phosphate groups. To further understand the effect of morphology and structure on the Ag3PO4 nanostructures, the Ag3PO4 PNCs were synthesized by using the AgNRs, H2O2, and Na2HPO4 as precursors according to the previous report with some modification.46 The detailed synthesis procedure and characterization of Ag3PO4 PNCs and Ag3PO4−cysteine PNCs are described in the Supporting Information (Figure S3). The UV−vis spectrum of Ag3PO4 PNCs (Figure S4a, Supporting Information) exhibits a broad band around 473 nm, which can be attributed to the adsorption of the Ag3PO4. No Fano dip appears at 330 nm, indicating that there is no plasmonic effect in this nanostructure. Figure S4b also shows that the Ag3PO4− cysteine PNCs strongly exhibit two bands at 254 and 281 nm, as observed for Ag−cysteine NRs and Ag@Ag3PO4−cysteine CSHNRs, confirming the formation of disulfite in the surface of Ag3PO4 PNCs once more. The chiroptical activity of the Ag3PO4−cysteine PNCs is also experimentally detected by using CD spectroscopy. The assembled nanoarchitectures exhibit anticipated characteristic mirrored CD signals by

Figure 5. XPS spectra of Ag 3d for (a) Ag−L-cysteine NRs, and (b) Ag@Ag3PO4−L-cysteine CSHNRs.

coupling L- and D-cysteine to Ag3PO4 PNCs, shown as a pair of enantiomers (Figure 6a). A gradual red shift of the CD peaks is observed when increasing the cysteine concentration (Figure 6b). Compared to the Ag@Ag3PO4−cysteine CSHNRs and Ag−cysteine NRs, an obvious increase in the CD intensity is observed for the Ag3PO4−cysteine PNCs (Figure 7). This may be attributed to different geometrical morphology and chemical constitution among the AgNRs, Ag@Ag3PO4 CSHNRs, and Ag3PO4 PNCs. The Ag3PO4 PNCs have surficial curvature larger than that of the Ag@Ag3PO4 CSHNRs, which can optimize the orientation of cysteine molecules and improve the relative chiroptical activity (Figure 8). Furthermore, the XPS characterization shows that not only do the Ag3PO4 PNCs exhibit a similar Ag 3d doublet to the Ag@Ag3PO4 CSHNRs, but also the Ag 3d and S 2p peaks exhibit a similar doublet with binding energies of 368.50 eV (3d3/2), 374.51 eV (3d5/2) and 162.26 eV (2p3/2), 163.33 eV (2p3/2) for Ag@Ag3PO4−cysteine CSHNRs and 368.55 eV (3d3/2), 374.55 eV (3d5/2) and 162.25 eV (2p3/2), 163.23 eV (2p3/2) for Ag3PO4−cysteine PNCs, respectively (Figure S5, Supporting Information), which is consistent with the above-mentioned results. The anisotropic factor (g factor) of the CD peaks increases in the order Ag3PO4−cysteine PNCs > Ag@Ag3PO4−cysteine CSHNRs > Ag−cysteine NRs as shown in Figure 9. The g factor of Ag3PO4−L-cysteine PNCs is about 12 times as much as that of Ag−L-cysteine NRs and about 5 times that of the Ag@ Ag3PO4−L-cysteine CSHNRs. This result is consistent with the above-mentioned discussion that the surficial curvature and phosphate groups have a strong effect on the chiroptical response. Table 1 lists the g factors of the CD peaks for the E


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Figure 6. CD spectra of (a) Ag3PO4−cysteine PNCs (L- and D-cysteine, c = 3.33 mM). (b) A series of Ag3PO4−L-cysteine PNCs (I = 1.67 mM, II = 2.73 mM, III = 3 mM, IV = 3.33 mM, V = 5 mM).

Figure 7. CD spectra of Ag−L-cysteine NRs, Ag@Ag3PO4−L-cysteine CSHNRs, and Ag3PO4−L-cysteine PNCs (c = 3.33 mM).

Figure 9. Anisotropy factor (g factor) of Ag−L-cysteine NRs, Ag@ Ag3PO4−L-cysteine CSHNRs, and Ag3PO4−L-cysteine PNCs (c = 3.33 mM).

our previous report.26 We expect that the similar CLC phase should be observed from the Ag@Ag3PO4 CSHNRs solution due to their large aspect ratio. Experimentlly, such the LC phase cannot be observed from the Ag@Ag3PO4 CSHNRs aqueous solution even though they have a large enough aspect ratio (Figure 10a). However, it can be obtained from the Ag@ Ag3PO4 CSHNRs ethylene glycol solution (Figure 10b). This result indicates that the aspect ratio is not the sole factor for the formation of LC phase with respect to the Ag@Ag3PO4 CSHNRs. It is worthwhile to investigate the reason why the Ag@Ag3PO4 CSHNRs cannot form a liquid crystalline phase in water but can give rise to a liquid crystalline phase in ethylene glycol. It is well-known that the electrostatic interactions among the nanorods play an important role in the formation of the LC phase. Experimental results show that the Ag@Ag3 PO4 CSHNRs have a zeta potential (−9.7 mV) smaller than that of the AgNRs (−14.7 mV), not favoring the formation of a chiral liquid crystalline phase for Ag@Ag3PO4 CSHNRs. In addition, the selection of solvent is also important for the formation the LC phase due to different solvation free energies for charged Ag@Ag3PO4 CSHNRs.59,60 The Ag@Ag3PO4 CSHNRs with a small surficial potential are so weighty that they cannot homogeneously disperse in water, whereas they can homogeneously disperse in ethylene glycol due to the larger viscosity of ethylene glycol. Therefore, for the formation of the Ag@Ag3PO4 CSHNRs LC phase in water, it is necessary

Figure 8. Schematic illustration for the evolution of Ag@Ag3PO4 CSHNRs and Ag3PO4 PNCs and assembly of Ag−cysteine, Ag@ Ag3PO4−cysteine CSHNRs, and Ag3PO4−cysteine PNCs.

Ag3PO4−cysteine PNCs, Ag@Ag3PO4−cysteine CSHNRs, and Ag−cysteine NRs. In general, the liquid crystalline phase can be formed when the aspect ratio of rod-like particles is large enough according to Onsager’s theory.58 The cholesteric liquid crystalline (CLC) phase has been observed from the AgNRs solution according to F


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Table 1. Anisotropy Factor (g factor) of Ag−Cysteine NRs, Ag@Ag3PO4−Cysteine CSHNRs, and Ag3PO4−Cysteine PNCs L-cysteine

CD peak

5 mM


Ag−Cys NRs Ag@Ag3PO4−Cys CSHNRs Ag3PO4−Cys PNCs

D-cysteine

3.33 mM

CD peak

5 mM

3.33 mM

λ/nm

AF

λ/nm

AF

λ/nm

AF

λ/nm

AF

279 283 281

0.002 0.010 0.026

279 279 279

0.003 0.014 0.034

279 283 284

−0.002 −0.013 −0.024

278 282 282

−0.003 −0.013 −0.035

Figure 10. CD spectra of Ag@Ag3PO4 CSHNRs liquid crystalline phase: (a) in water; (b) in ethylene glycol.

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to reduce the thickness of the coating shell and increase the surficial potential of the Ag@Ag3PO4 CSHNRs by surface modification.

Corresponding Author

*Tel: +86 431 85167482. E-mail: [email protected].

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Notes

CONCLUSION The chiral Ag@Ag3PO4−cysteine CSHNRs have been prepared by coupling cysteine on the core−shell hybrid surface, leading to the emergence of new CD signals in the UV region. The Ag3PO4−cysteine PNCs with larger surficial curvature show CD spectra similar to those of the Ag@Ag3PO4−cysteine CSHNRs with higher CD intensity, confirming the dependence of the variation of surficial curvature. The optical activities of the Ag@Ag3PO4−cysteine CSHNRs and Ag3PO4−cysteine PNCs can be associated with the formation and orderly arrangement of S−S bonds of oxidized cysteine, which are promoted by Ag+ ions on the surface of the nanostructures. Furthermore, the Ag@Ag3PO4 CSHNRs can spontaneously form a liquid crystalline phase in ethylene glycol due to the large aspect ratio of Ag@Ag3PO4 CSHNRs and the optimum solvating effect of ethylene glycol. This work contributes insights into the structure−property relationships and provides guidance for exploring other chiral inorganic hierarchical structures.

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AUTHOR INFORMATION

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (20971051).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06904. Experimental details, CD spectra of Ag@Ag 3 PO 4 CSHNRs and Ag@Ag3PO4−amino acid CSHNRs, TEM images, XRD, and UV−vis spectra of Ag3PO4 PNCs and Ag3PO4−cysteine PNCs and XPS of Ag NRs, Ag@Ag3PO4 CSHNRs, Ag@Ag3PO4−cysteine CSHNRs, and Ag3PO4−cysteine PNCs (PDF) G


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