Peptide-Directed Hierarchical Mineralized Silver ... - ACS Publications

6 downloads 0 Views 8MB Size Report
May 8, 2018 - Huan Liu,. †. Kang Niu,. †. Weiwei Cao,. † and Dawei Gao*,†,‡. † ... of Process Engineering, Chinese Academy of Sciences, No. 1, Bei er tiao.

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 7574−7588

pubs.acs.org/journal/ascecg

Peptide-Directed Hierarchical Mineralized Silver Nanocages for AntiTumor Photothermal Therapy Kexin Bian,† Xuwu Zhang,†,‡ Kai Liu,§ Tian Yin,† Huan Liu,† Kang Niu,† Weiwei Cao,† and Dawei Gao*,†,‡ †

Applying Chemistry Key Lab of Hebei Province, Department of Bioengineering, Yanshan University, No. 438 Hebei Street, Qinhuangdao, 066004, People’s Republic of China ‡ State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, No. 438 Hebei Street, Qinhuangdao, 066004, People’s Republic of China § State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, No. 1, Bei er tiao Street, Zhongguancun, Beijing 100080, People’s Republic of China S Supporting Information *

ABSTRACT: The size and morphology of metals determine their plasmon resonances. How to elegantly tune their architectures to obtain optical properties as required (e.g., strong absorption in the nearinfrared (NIR) wavelengths) is a bottleneck for phototherapy. Inspired by biomineralization, we develop a simple but robust strategy to fabricate silver nanocages (Ag NCs) based on peptide-directed mineralization of silver. The Ag NCs are organic−inorganic hybrids with octreotide (OCT) templated decoration of Ag shells that are composed of Ag NPs. This hierarchical organization makes Ag NPs get together in close proximity, which facilitates ultrastrong plasmonic coupling to shift the resonant excitation from the visible (420 nm) to the NIR region (810 nm). In addition, the surface plasmon resonance peak of the Ag NCs in the NIR region can be subtly tuned by varying the volume of added silver nitrate (AgNO3) to control the size and morphology of mineralized Ag NCs. The Ag NCs have a light-to-heat conversion efficiency of 46.1%, which is to our knowledge the highest among Ag-based photothermal agents (PTAs). The Ag NCs can selectively induce death of cancer cells in vitro under NIR irradiation at 808 nm and show improved cytocompatibility for normal cells relative to pure Ag NPs. Following intratumor injection into uterine cervix cancer cells (U14) tumor-bearing mice, Ag NCs exert remarkable antitumor performance with tumor killing efficacy up to 82.7% and good biocompatibility in photothermal therapy, suggesting their potential application to work as photothermal nanomedicine for cancer therapy. KEYWORDS: Octreotide, Biomineralization, Controlled synthesis, Organic−inorganic hybrid, Photothermal therapy



ing.11 It is well-established that inorganic nanomaterials are promising candidates for PTAs due to great molar extinction coefficient, high light-to-heat conversion, and tumor selective accumulation.12,13 Unfortunately, inorganic substances inevitably meet a puzzle, that is, potential cytotoxicity, which usually requires surface modifications or polymer envelopment to settle.14,15 Therefore, exploitation of PTAs with features of strong NIR responsiveness, high energy conversion efficiency and photostability, and good biocompatibility is desperately needed for the advance of a biomedical platform. Plasmonic metal (e.g., Ag, Au) nanostructures have attracted wide attention in various applications such as biological and chemical sensing, optoelectronics, and chemical reactions due to their unique optical properties induced by localized surface

INTRODUCTION Photothermal therapy (PTT) is a promising antitumor strategy due to inherent advantages of minimum invasiveness, high spatiotemporal selectivity, and easy operation.1,2 Hyperthermia can cause cell membrane disruption, protein and DNA degeneration, and vessel occlusion, resulting in direct and irreversible damage to tumor cells and tissues.3−5 Light source and PTAs are two essential elements for PTT. NIR light with a range of 700−900 nm can penetrate 10 cm in biological soft tissues and therefore is widely used for phototherapy and medical imaging.6−8 Currently, many types of photoactive nanomaterials, including organic pigments (e.g., melanin, lightabsorbing polymer), semiconductors, graphene, and noble metals (e.g., gold, silver) have been explored as PTAs.9,10 Although great successes have been obtained in these studies, there remains several fundamental problems and technical obstacles. Organic components are usually confronted with low photothermal conversion efficiency and severe photobleach© 2018 American Chemical Society

Received: January 26, 2018 Revised: April 20, 2018 Published: May 8, 2018 7574

DOI: 10.1021/acssuschemeng.8b00415 ACS Sustainable Chem. Eng. 2018, 6, 7574−7588

Research Article

ACS Sustainable Chemistry & Engineering plasmon resonance (LSPR).16−18 The LSPR of plasmonic metal can be tuned via adjusting particle size, morphology, and architectural organization (e.g., hollow structure and aggregation of particles).19 In general, the absorption is red-shifted when just increasing the size of Ag/Au NPs but still limited in the visible region.20 This limitation can be broken through changing particle morphology into nanocubes, nanorods, or nanoplates via localized oxidative etching, the sulfide-mediated polyol method, and the photoinduced method;21−24 however, the toxicity (e.g., cetyltrimethylammonium bromide (CTAB) surfactant,25,26 N,N-dimethylformamide (DMF) dissolvent27) reagents used in their preparation severely limited their biomedical applications. A hollow structure (e.g., Au-based nanocages) has been fabricated via galvanic replacement reaction from Ag nanocubes.28 Another strategy is to make plasmonic NPs assemble into high order structures (e.g., Au nanovesicles29), which may lead to the near-field coupling of meta-surface plasmons between adjacent particles to shift plasmonic peaks, and the generation of “hot spots” to enhance Raman scattering.30 Although the plasmonic effect can be tuned to obtain NIR absorbance, these approaches heavily rely on multistep skillful manipulation, which requires the prepreparation of a metal nanostructure followed by polymer-induced selfassembly. Therefore, there are still great challenges in developing simple and green strategies to design and fabricate plasmonic metal-based PTAs. Biomineralization is a green, simple, and precise method in the synthesis of organic−inorganic hybrids.31−33 Proteins widely participate in biomineralization (e.g., ferritin34) via mediating nucleation and growth. Inspired by natural phenomena, peptides are becoming popular to the bottom-up synthesis of the inorganic components for their tailored structural modularity and accompanying molecular recognition and programmable self-assembled structure.35,36 Also, peptides can selectively combine with the crystal plane of metal NPs to control their size and crystal form.37,38 Peptide-based hybrids are widely used as functional materials with a feature of high spatial precision and tunable physicochemical performance.39,40 Peptides have been used as structure-defined scaffolds to control the synthesis of various inorganic nanomaterials (e.g., Ag, Au, Pd NPs) to work as antibacterial agents, catalysts, imaging agents, and signal sensing probes.41−45 However, to the best of our knowledge, the peptide-mineralized metal architecture capable of ultrastrong plasma response has not been reported. It is possible to rationally control the architectural organization of plasmonic metal by designing the structure of peptides and regulating the process of corresponding mineralizing. OCT (Figure S1) has been proved to be a template to mediate the growth of inorganic substances, because it possesses multiple coordination sites (e.g., amino, hydroxy and imidazole groups) to strongly bind metal surfaces and control their growth.46−48 Meanwhile, OCT shows good biocompatibility and a long half-life period and has been used as the targeting bullet to anchor cancer cells.49 In this work, we describe a structure controllable Ag NC, which can be facilely prepared by a biomineralization method and applied to high-efficiency photothermal therapy. Ag NCs exhibit a remarkably enhanced surface plasmon response and display NIR absorption up to 900 nm, because of the hollow nanoshell structure with strongly coupled Ag NPs. We systematically investigate the growth kinetics of the control factors, including the duration of incubation and the dosage of AgNO3, which have significant effects on the size and

morphology of the Ag NCs, resulting in tunable optical properties. In addition, Ag NCs are endowed with superior photothermal performance, including high light-to-heat conversion efficiency and excellent photostability under 808 nm laser irradiation. The in vitro cancer cell damage and in vivo tumor regression indicate that Ag NCs are promising PTAs for PTT with excellent antitumor efficacy accompanied by good biocompatibility.



EXPERIMENTAL SECTION

Materials. OCT was purchased from GL Biochem Co., Ltd. (Shanghai, China); for the molecular structure, see the Supporting Information. AgNO3 was obtained from Beijing North Fine Chemicals Co., Ltd. (Beijing, China). Sodium borohydride (NaBH4) was purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Ascorbic acid (H2Asc) was obtained from Beijing Biodee Biotechnology Co., Ltd. (Beijing China). Trisodium citrate dihydrated (NaCit) was purchased from Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd. Hydrogen chloride (HCl) was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). A standard solution of elemental silver (1000 μg mL−1) was obtained from the National Steel Materials Testing Center (Beijing Iron and Steel Research Institute). All chemicals and reagents employed in the experiments were analytical grade reagents, and Milli-Q ultrapure water was used in all assays. Preparation Procedures of Ag NPs. Ag NPs were synthesized by the wet-chemical method according to a previous study with slight modifications.50 In short, 10 mL of 1% NaCit solution was heated at 70 °C, while stirring for 15 min. Then, 500 μL of 10 mM Ag NO3 and 1 mL of NaBH4 solution were injected successively under heating and stirring. Preparation of Ag NCs. To synthesize Ag NCs, a sample of 200 μL of 0.25 mM OCT HCl aqueous solution at pH 2.2 was heat treated at 80 °C for 20 min. The cooled solution was mixed with 63 μL of 7 mg mL−1 H2Asc and 327 μL of 1−4.6 mM AgNO3 solution and incubated at room temperature for 0−48 h. Finally, the freshly prepared 100 μL 15 mM NaBH4 solution was quickly added into the above incubated solution. Ag NCs was collected by centrifugation and washed with ultrapure water three times. Characterization. Mean particle size and morphology of the asprepared Ag NCs were monitored by dynamic light scattering (Zetasizer Nano-ZS90; Malvern Instruments, UK) and transmission electron microscopy (TEM, HT 7700, Japan) operated at an accelerating voltage of 100 kV, respectively. The surface morphology and element distribution of Ag NCs are determined by a field emission scanning electron microscope (FESEM, SUPRA 55). FTIR spectra of samples were recorded with a Nicolet Is10 (USA) spectrometer. The phase and crystallographic structure of Ag NCs were obtained using Xray power diffraction (XRD, Rigaku SmartLab). The elemental composition and chemical valence states of Ag NCs were detected by X-ray photoelectron spectroscopy (XPS) recorded on an Escalab 250Xi (Thermo Fisher Scientific) with radiation from an Al Kα (1486.6 eV) X-ray source. Optical properties of Ag NCs were carried out on a UV−vis spectrometer (UV2550, Shimadzu, Japan). The ultimate concentration of the as-prepared Ag NCs solution was determined using atomic absorption spectroscopy (AAS, AAnalyst 400, USA). Photothermal Effects of Ag NCs. To elucidate the photothermal conversion performance, aqueous suspensions (200 μL) of Ag NCs with different concentrations (6, 12, 24, 48, 100 μg mL−1) were placed in a 1.5 mL Eppendorf tube and exposed to an 808 nm laser (MDL-N808-10W-12120445, China) at various power densities (1, 1.25, 1.5, 1.75 W cm−2) for 5 min. Meanwhile, in order to further examine the NIR photostability of Ag NCs, 200 μL of Ag NC (48 μg mL−1) aqueous solution was irradiated with an 808 nm NIR laser at a 1.5 W cm−2 power density for 120 s, and then the NIR laser was turned off. This procedure was reiterated 10 times. After treatment, the changes of UV−vis absorption and morphologies of the Ag NCs were examined to evaluate the photothermal stability. A thermal imaging 7575

DOI: 10.1021/acssuschemeng.8b00415 ACS Sustainable Chem. Eng. 2018, 6, 7574−7588

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Schematic Illustration of the Synthesis of Ag NCs Using OCT as the Biotemplate and Their Application as a PTA against Tumor

camera (InfraTec VarioCAM, Germany) is simultaneously used to record the temperature response of the solution over time and corresponding thermal images. In Vitro Cytotoxicity and Photothermal Therapy. Viability of HeLa cells and 293T cells after Ag NC exposure was assessed by MTT assays. Cells were seeded into 96-well plates at a density of 5 × 103 cells per 100 μL of culture medium and cultured for 24 h. Subsequently, the cell culture medium was replaced, and cells were incubated with fresh cell medium containing different concentrations of Ag NCs or Ag NPs for 24 h. To determine toxicity, the cells were rinsed once with PBS to remove the residual Ag NCs or Ag NPs on the cell surface; then 200 μL of MTT solution was added and cultured for an additional 4 h. After incubation, the medium was removed, and the cells were lysed by 150 μL of dimethyl sulfoxide (DMSO). After shaking, the absorbance at 490 nm was determined with an ELISA reader (MK3, Thermo Co., USA). For the photothermal ablation efficiency of Ag NCs in vitro, the HeLa cells were exposed to a certain concentration of Ag NCs (48 μg mL−1) and irradiated with an 808 nm laser with different output power densities (1.25, 1.5, 2 W cm−2) for various times (2, 4, 6 min). Under the same conditions, the treated cells nontreated with Ag NCs were served as a control. Afterward, the cell viabilities after photothermal treatment were analyzed by the standard MTT assay. In Vivo Photothermal Tumor Ablation and Safety Evaluation. Kunming mice (5−7 weeks old, female) were inoculated subcutaneously with U14 cells in the right armpit. After the volume of the tumors grew to 150 mm3, the mice were randomly divided into four groups consisting of six mice in each group. The mice were intratumorally administered 100 μL of the following formulations: (a) saline only, (b) saline + NIR, (c) saline solution of Ag NCs only (48 μg mL−1), and (d) saline solution of Ag NCs + NIR (48 μg mL−1). For the NIR-treated groups, the mice were subjected to irradiation with an 808 nm laser for 2 min at a power density of 1.5 W cm−2. The tumor temperature changes of mice were monitored by thermal imaging camera. During the therapy, the tumor size and body weight was measured every other day. Tumor volumes (V) were calculated as

V = (tumor length) × (tumor width)2 /2

RTV = V /V0

(V0 means initial tumor volume)

(2)

TGI was calculated as follows:

TGI = (1 − RTV of treated group/RTV of control group) × 100%

(3)

Mice were sacrificed at day 14, and the serum separated from the ocular blood was used to assess biochemical parameters comprised of lactate dehydrogenase (LDH), alanine aminotransferase (ALT), aspartate aminotransferase (AST), and blood urea nitrogen (BUN). At the same time, the tumor and the main organs (heart, liver, spleen, lung, and kidney) of mice were dissected for histological analysis. All animal procedures were conducted in accordance with the guidelines of the Institutional Animal Care and Use Committee. Statistical Analysis. Data were presented as the mean ± SD and analyzed using Student’s t-test.



RESULTS AND DISCUSSION To synthesize Ag NCs, AgNO3 was used as the precursor; NaBH4 and H2Asc were used as strong and mild reducing agents, respectively, and OCT was used as a peptide template, where a substoichiometric amount of NaBH4 was added to the mixed solution of precursor, peptide, and ascorbic acid. In a typical process, AgNO3 (3.4 mM) was first incubated with OCT (0.25 mM) and H2Asc (7 mg mL−1) for 24 h followed by the addition of NaBH4 (15 mM). In order to find out the growth mechanism of Ag NCs, the sample without NaBH4 was analyzed by TEM. As shown in Figure S2, as the basis for the formation of silver NCs, the dispersed AgCl nanoparticles were discovered, which can be rapidly reduced under a high-energy electron beam of TEM (i.e., Ag+ + e− → Ag0).51,52 In order to display the detailed process, an in-situ replacement reaction of gold atoms with silver atoms was carried out, as shown in Figure S3. Gold nanoparticles gather together through the support of the OCT template, so OCT exists in the interior of the silver nanoshells. It has been reported that amine or carbonyl groups can chelate Ag and control the growth of Ag

(1)

Relative tumor volume (RTV) was computed as 7576

DOI: 10.1021/acssuschemeng.8b00415 ACS Sustainable Chem. Eng. 2018, 6, 7574−7588

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) SEM image, (b) TEM image with inset magnified version, (c) EDS spectrum, (d) XRD pattern of Ag NCs.

Figure 2. (a) FTIR spectra; (b) XPS survey spectrum; (c) high-resolution XPS spectrum; (d) UV−vis absorption spectra of Ag NCs. 7577

DOI: 10.1021/acssuschemeng.8b00415 ACS Sustainable Chem. Eng. 2018, 6, 7574−7588

Research Article

ACS Sustainable Chemistry & Engineering

Figure 3. TEM images of Ag NCs prepared with same concentration of AgNO3 (3.4 mM) after incubation for different times [(a) 0, (b) 12, (c) 24, (d) 48 h]. (e) Hydrodynamic sizes of Ag NCs as a function of the durations of incubation. (f) UV−vis absorption spectra of Ag NCs influenced by the durations of incubation.

nanoparticles.53−56 In the experiment, OCT not only combined with AgCl to control the crystal growth and structures but also chelated silver ions on their outer surface. After the addition of sodium borohydride, the Ag+ of [email protected]/Ag+ was reduced to Ag0 and attached to the surface of complexes to form Ag seeds. The AgCl nanocubes were then reduced to metallic silver to the surface of the seeds and gradually dissolved,57,58 eventually forming a hollow cage (Scheme 1). The scanning electron microscope (SEM) image (Figure 1a) reveals monodispersed Ag NCs with mean sizes of about 120 nm. As shown in TEM, there is a group of Ag NPs dispersed on the nanocages’ surface with the formation of Ag shells. The cores of the Ag NCs are transparent (Figure 1b, Figure S4a), indicating the peptide components due to their high electron

transmission. Energy dispersive spectroscopy (EDS) analysis and corresponding element mapping images confirm that the Ag NCs are the silver/octreotide hybrids (Figure 1c, Figure S4a). The X-ray diffraction (XRD) pattern exhibits five peaks centered at 38.1°, 44.3°, 64.6°, 77.7°, and 81.9° (Figure 1d), which correspond to the (111), (200), (220), (311), and (222) planes of the cubic structure of Ag NCs (JCPDS 99-0094), respectively. The intensity of the (200) diffraction peak is the strongest in the pattern (Figure 1d), suggesting that the (200) facet is enriched in the Ag NCs. A quick single-shot injection of NaBH4 can reduce the silver ion precursor and produce a burst of nucleation. Ascorbic acid can then gradually reduce the residual metal ion precursor to produce metal atoms that are preferentially added onto the higher-energy facets on which the 7578

DOI: 10.1021/acssuschemeng.8b00415 ACS Sustainable Chem. Eng. 2018, 6, 7574−7588

Research Article

ACS Sustainable Chemistry & Engineering

Figure 4. TEM images of Ag NCs at constant 24 h durations of incubation and different concentration of AgNO3 [(a) 1.6, (b) 2.2, (c) 2.8, (d) 3.4, (e) 4.0, (f) 4.6 mM]. (g) Hydrodynamic sizes of Ag NCs as a function of the concentrations of added AgNO3. (h) UV−vis absorption spectra of Ag NCs influenced by the concentration of AgNO3.

peptide shows no selective binding.45 Therefore, OCT may favor the growth of the AgNP-seeds along the direction via passivation of {100} facets. To further confirm the conjugation of OCT to Ag NPs, Fourier-transform infrared spectroscopy (FTIR) is used to measure Ag NCs after three cycles of purification to exclude the influence of the OCT remaining in solution. As shown in Figure 2a, the intense broad absorption bands at about 3438 cm−1 can be assigned to the superposition of the stretching vibration of O−H bands of OCT. A slight red shift of the band from 3402 to 3448 cm−1 is attributed to the binding of −OH groups with Ag NPs.59 The sharpness of the peak is due to the rupture of H bonds.60 The characteristic peaks located at 1645, 1538, and 1400 cm−1 are all due to (NH)CO groups. When the hybrid structure is formed, the three peaks disappear, while new absorption peaks at 1624 and 1384 cm−1 appear, which is believed to be due to involvement of (NH)CO groups in the mineralization process.61,62 It is well-known that biomass can bind to silver via either the carbonyl oxygen of the carboxyl and amide groups or free amine groups; therefore it can be used as a molecular framework to control the nanostructure of Ag.63,64 The absorption band around 2926 cm−1 is attributed to C−H stretching vibrations. In the XPS spectra, the elements of C, N,

O, S, and Ag for the Ag NCs can be observed (Figure 2b). As shown in Figure S4c, the peak at 285.08 eV is assigned to the binding energy of C 1s, which can be assigned to four peaks at 284.6, 285.8, 286.6, and 287.6 eV, corresponding to the C−C, C−N, and CO groups in OCT, respectively.65 The spin orbit doublet with an S 2p peak at 161.8 eV can be associated with the S−Ag chemical state (Figure S 4d); the main reason is that the reduction agent reduces the disulfide bond to thiol that forms strong bonds to metal surfaces.66,67 The characteristics of the O 1s spectrum consist of peaks at 531.5 and 533.3 eV (Figure S4e), corresponding to CO and C−O, respectively.68 The peaks at 399.6 eV can be assigned to the binding energy of N 1s, further revealing the existence of OCT in Ag NCs. It is observed from Figure 2c, the binding energies of Ag 3d can be resolved into two peaks at 368.0 and 374.0 eV, which are ascribed to the Ag 3d5/2 and Ag 3d3/2, respectively, suggesting the zerovalent state of Ag in the Ag NCs.69 UV−vis absorption of Ag NCs is an important parameter for photothermal applications. The typical LSPR peak of Ag NPs at 422 nm is observed (Figure 2d), which is attributed to the surface plasmon resonance of Ag NPs. After the Ag NCs synthesized, the absorption peak was found to redshift to the NIR region (centered at ∼750 nm) with a large cross section, 7579

DOI: 10.1021/acssuschemeng.8b00415 ACS Sustainable Chem. Eng. 2018, 6, 7574−7588

Research Article

ACS Sustainable Chemistry & Engineering which is a good therapeutic window for PTT. Due to the finite thickness of the shell layer of the Ag NCs, the plasmons on the inner and outer surfaces of the shell can interact with each other, resulting in the red-shift of the absorption band.70−72 This is a simple method to prepare for peptide-based silver nanostructures with tailored LSPR to obtain NIR absorbance. When the reaction takes place without OCT, the solution appears milky and turbid within a few seconds, indicating the rapid formation of AgCl (Figure S5a).73 When the reaction added sodium borohydride, only solid irregular Ag NPs with a size of ∼150 nm were obtained (Figure S5b), and the corresponding absorbance was limited to the visible region centered at ∼420 nm (Figure S5c). These results indicate that OCT can work as a structure directing agent to not only control the growth of AgCl into a specific morphology and size but also stabilize the AgCl nanocubes to avoid aggregation. Actually, the chelation between OCT and Ag+ controls the growth rate of AgCl crystals. The color of the mixed solution containing OCT, silver ions, and H2Asc gradually becomes turbid after a few hours (Figure S6), which indicates the formation of AgCl nanocubes stabilized by OCT. When solution of OCT containing H2Asc and AgNO3 is directly mixed with NaBH4 without preincubation, the color of the resulting solution is converted from colorless and transparent to grayish blue, and obvious precipitations are observed after 8 h (Figure S7). This phenomenon was caused by a large number of aggregated Ag NPs and some smaller box-like structures (Figure 3a); the corresponding LSPR peaks are located in the range of 400−540 nm (Figure 3f). As the incubation time increases, free Ag NPs disappear, and more Ag NCs are formed with an increased size from 102 to 136 nm (Figure 3b−d, Figure S8). The corresponding plasma absorption band shifts from 569 to 790 nm (Figure 3e), which is due to the enhanced couple between Ag NPs in the Ag NCs. At the initial stage of solution mixing, no AgCl was formed, which was attributed to the fact that the formation constant of OCT/Ag+ complexes was larger than that of AgCl.74 Subsequently, the Cl+ gradually contacts the Ag+ reaction site on OCT/Ag+ complexes to form AgCl. The chelation of OCT with Ag+ reduces the diffusion coefficient of Ag+ and controls the growth rate of AgCl.73 We suggest that the binding of OCT to silver ions or AgCl is a time-dependent dynamic process. Enough incubation time can guide the morphology of AgCl into cubes and shift the chemical equilibrium to the OCT/Ag+ complex, which may facilitate OCT-tuned mineralization of Ag due to adjacent distance. After preincubation of OCT and Ag ions, the size of the obtained Ag NPs in the form of building blocks for the Ag NCs is smaller than that of free Ag NPs (7.5 nm vs 14.2 nm; Figure 3a,b), suggesting that OCT can control the nucleation and growth of Ag seeds into confined size. In addition, regular Ag NCs with obvious edges and random Ag NP aggregation are simultaneously observed after 12 h of preincubation (Figure 3b), indicating that Ag NCs are evolved from the high-order organization of Ag NPs by using a “soft” biological peptide as a mediating template at the organic−inorganic interface. To control the architectural aggregation, it is necessary to temporarily stabilize particles long enough after formation to find their position in a controlled manner.75 It is remarkable that nearly spherical primary Ag NPs can give rise to square aggregation to form cubic shaped aggregates, and the mutual alignment of nanocrystals can be realized with the help of the [email protected]/Ag+. As proven by TEM, this aggregate of peptide-inorganic hybrid nanocrystallites exhibits hollow shell

structures because AgCl, acting as a core, is reduced and converted into single crystal silver nanoparticles during the reaction process. In chemical control, physical and chemical factors (e.g., solubility, supersaturation, nucleation, crystal growth) can be regulated by controlling the composition of ionic compounds.76 Herein, we investigate the effects of the concentration of a silver precursor on the formation of Ag NCs (Figure S9). When the added AgNO3 is 1−1.6 mM, the color of the solution changes from pale milky white to light blue after adding NaBH4 (Figure S10a), and subrotund particles accompanied by Ag NPs are observed (Figure 4a). The surface of the particles is smooth due to the fusing of Ag NPs to a shell, which is an Ostwald ripening process where smaller crystals are sacrificed to further grow larger ones by the diffusion of atoms within an ensemble of crystalline materials.77 When the concentration of AgNO3 increases to 2.2−3.4 mM, the obtained particles show clear outlines for cubic morphology and the surface become rough (Figure 4b−d, Figure S10b). With the continuous increase of AgNO3 up to 4.0−4.6 mM, the resulting Ag NCs exhibited a hierarchical structure with a square core and protruding branches (Figure 4e,f, Figure S10c) and an obviously increased particle size (Figure 4g, Figure S11). These results suggest that the concentration of Ag+ can influence the morphology of AgCl. Only a small amount of AgCl is generated at a low concentration of Ag+, and OCT was overcoated on the surface of silver chloride, resulting in the formation of spherical morphology with a smaller particle size. In addition, the recognition sites of the peptide tend toward saturation at a high concentration of AgNO3; therefore the increasing mineralized Ag NPs organize into larger aggregates, and some of them may lack the guidance of a peptide to form an epitaxial dendrite on the surface. Aggregation of nanocrystals will cause the formation of a complex and varied morphologies; however, it is generally hard to accurately control the aggregation process, i.e., morphology and size of the final architecture.75 For this system, it is realized that the morphologies and superstructure of particles are systematically regulated in a wide range by simply adjusting the concentration of reactants. Interestingly, the corresponding plasmonic NIR absorptions of these are redshifted in the range of 710−800 nm as the AgNO3 increases (Figure 4h). It is well-established that the surface plasmon resonance of metallic shells can be readily tailored by varying the ratio of shell thickness to core radius.78,79 The thickness of the Ag shell may have negligible change at a low concentration of AgNO3 (

Suggest Documents