Synthesis of luminescent gold nanoclusters embedded goose feathers

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Synthesis of luminescent gold nanoclusters embedded goose feathers for facile preparation of Au(I) complexes with aggregation-induced emission Tong Shu, Xiaojun Cheng, Jianxing Wang, Xiangfang Lin, Ziping Zhou, Lei Su, and Xueji Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04124 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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ACS Sustainable Chemistry & Engineering

Synthesis of luminescent gold nanoclusters embedded goose feathers for facile preparation of Au(I) complexes with aggregation-induced emission Tong Shu,* Xiaojun Cheng, Jianxing Wang, Xiangfang Lin, Ziping Zhou, Lei Su*, and Xueji Zhang*

Beijing Key Laboratory for Bioengineering and Sensing Technology, Research Center for Bioengineering and Sensing Technology, School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China.

*

Corresponding

authors.

E-mail:

[email protected];

[email protected];

[email protected]. Address: 30 Xueyuan Road, Haidian District, Beijing 100083 P. R.China

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Abstract A new kind of luminescent hybrid materials based on ultra-small nanomaterials, i.e., gold nanoclusters (AuNCs) and goose feathers, precious gifts from nature, has been successfully synthesized. Goose feathers possess high specific surface area due to their hierarchical structure and are mainly composed of thiol-rich proteins, e.g., keratin, which can reduce Au(III) and encapsulate the formed AuNCs in a large scale. The resultant golden feathers exhibit bright red luminescence under UV light, of which the substructures, i.e., barbs and barbules, have been sufficiently stained with red-emitting AuNCs. Furthermore, the AuNC-stained feathers maintain almost unchanged structure (solid and hierarchical) and intact chemical composition. The features of such AuNC/feather hybrids, e.g., bulk, high specific surface, inspired us to develop a facile method to prepare various valuable debris from AuNC-based etching, i.e., Au(I) complexes

with

aggregation-induced

emission

(AIE).

We

selected

tris(2-

carboxyethyl)phosphine and cysteamine as etchants, respectively, which have been previously reported to be able to synthesize AIE-featured Au(I) complexes through etching AuNCs protected by soluble protein. Conveniently, in this approach, the protein-free Au(I) complexes in solution can be attained simply by taking out with bare hand. After adding cation aggregation inducer, e.g., Cd2+, the non-emitting solution can generate bright luminescence, demonstrating the feasibility to synthesize the precious AIE-type Au(I) complexes. This study not only boosts the development of luminescent gold materials through rational use of natural product, enriching the library of gold nanomaterials, but also definitely provides a new view of nanoclusters as reagents to prepare other noble materials.

Keywords: Gold nanoclusters, Goose feathers, Aggregation induced emission, Chemical etching, Gold(I) complexes


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Introduction Noble metal nanoclusters (NCs), typically consisting of a few to several hundreds of atoms, have fascinated the study of ultrasmall nanomaterials for decades due to their intriguing aesthetic structures and exotic photophysical properties.1-7 The ultrasmall size, which is comparable to the Fermi wavelength of electrons, endows metal NCs with molecule-like properties e.g., HOMO-LUMO transitions, and photoluminescence (PL).8-9 However, the quantum yields (QYs) in their PL are quite low (generally < 0.1%). Thus, many efforts have been dedicated to promote QYs of luminescent metal NCs.10-11 Recently, protein-protected gold nanoclusters (AuNCs) with intensive fluorescence (QYs > 6%) stand out due to delocalized electrons of abundant electronrich groups in the protein ligands, resulting in significant enhancement in their luminescence.12-14 Moreover, such AuNCs are featured with easy and green synthesis, excellent biocompatibility and low cytotoxicity, which thus have found their applications in sensing,15 imaging,16 and biomedicine17. A wide variety of proteins, e.g., serum albumin,12, 18 lactalbumin,18 insulin19 and DNAase20, have been used to produce luminescent AuNCs. Besides, protein-based natural materials, e.g., silk,21 egg,22 and pea,23 as both the reductant and the stabilizer, are also been reported to be used in in situ synthesis of AuNCs, greatly enriching the library of AuNCs. Very recently, a new interest in AuNCs has been aroused, due to their potential as chemical reagents for synthesis of various valuable Au(I) complexes via chemical etching reaction.24-26 Chemical etching possesses the ability of sculpturing nanomaterials at atomic level and has become increasingly important in nano-science.27-28 For instance, the etchants, e.g., thiols and phosphines, have been applied to the synthesis of so-called “sizefocused” metal NCs.29-34 These “scalpels” can subdivide large nanoparticles (core sizes > 2 nm) into ultra-small clusters and then refine the relevant raw materials to welldefined NCs. Besides well-known NCs, the other often-ignored debris of “sculpture”, i.e., Au(I) complexes, are actually rather valuable. Due to the strong Au-S or Au-P bonds and specific aurophilic interaction, they can form abundant macromolecules, including self-assembling supermolecules35-36 and metallogels37-38. As metallodrugs,



Au(I) complexes also continue to gain credit by applications as drugs for rheumatoid arthritis,39 malaria40-41 and cancer42-45. Specifically, recently, the photophysical properties of Au(I) complexes have been found to conform to well-known aggregationinduced emission (AIE),34, 46-49 thereby enabling their applications as a new class of AIE luminogens (AIEgens) for sensing and light-emitting diodes.25-26, 50-51 Importantly, through chemical etching routes, a series of strong etchant-coordinated novel Au(I) complexes with AIE, which are hard to be synthesized by mixing those etchants with HAuCl4, can be obtained.26 This top-down synthetic strategy considerably compensates for methodology insufficiency of preparation of AIE-featured Au(I) complexes. However, to harvest relatively pure Au(I) complexes, the removal of soluble ligands of AuNCs e.g., bovine serum albumin, is necessary, thus rendering the synthetic process complicated and cumbersome.24, 52 Herein, we report a simple, fast and one-step route to synthesize AuNCs using lowcost and well-shaped large solid protein substrates, i.e., goose feathers. Their hierarchical structure are expected to give a sufficient contact with the solution for AuNC synthesis, thus facilitating to stain feathers with luminescent AuNCs in high density. In addition, features contain abundant thiol-rich proteins, e.g., keratin, which can serve as both the reductant and the stabilizer of AuNCs as evidenced in previous reports.53-54 We showed that the substructures of feathers, barbs and barbules, were sufficiently and uniformly stained with red-emitting AuNCs, without significant changes in structure and chemical composition. Based on high stain density and bulk solid of the luminescent golden feathers (AuNCs/goose feathers hybrids), we then applied them to prepare valuable AIE-feathered Au(I) complexes using wellestablished tris(2-carboxyethyl)phosphine (TCEP)- and cysteamine (CSH)-induced chemical etching approaches, respectively. Handily, after etching reaction, the proteinfree solution of Au(I) complexes could be attained just through manually taking out of the etched feathers. We further showed that upon the addition of cation-type aggregation inducer, e.g., Cd2+, the Au(I) complex-containing solution exhibited strong luminescence. In addition, the etched feathers were shown to be able to be recycled to generate red-emitting AuNCs within them.


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Experiments and Methods Materials Goose feathers were cropped from shuttlecocks, which were purchased from LI-NING, China. HAuCl4, TCEP and CSH were purchased from Sigma-Aldrich. Other chemicals at least analytical purity were purchased from Sigma-Aldrich and used as received. Water used in all experiments was deionized by the Millipore purification system (resistivity > 18.2 MΩ·cm). Synthesis of the golden feathers 50 mg of feather was soaked in a 4 mL of freshly prepared aqueous solution of HAuCl4 (1.5 mM) under vigorous stirring at 80 oC for 10 min. Then, 60 μL of 1 M NaOH solution was introduced and the mixed solution was allowed to react for 45 min under stirring at 80 oC. The resultant feather was taken out and washed with water and alcohol, and finally dried with nitrogen gas for storage at room temperature. TCEP-induced chemical etching of the golden feathers Gold feathers (~ 50 mg for each) were immersed in 3 mL of tris-HCl buffer (50 mM, pH 8.5) solutions containing varied concentrations of TCEP for 1 h. Then, the etched feathers were removed, washed with water and alcohol, and dried with nitrogen gas. The resulting solutions were collected and cooled to room temperature. Evaluation of AIE phenomenon Gold feathers (~ 50 mg for each) were immersed in 3 mL of tris-HCl buffer (50 mM, pH 8.5) solutions containing 13 mM TCEP aqueous solution for 1 h. Then, the etched feathers were removed. After cooling to room temperature, the resultant solutions were incubated with a series of final concentrations of Cd2+ from 0 to 15 mM for 1 h and the final product was collected. Characterization Emission and excitation spectra were recorded on a F-4500 spectrometer (HITACHI) at an excitation and an emission wavelength at 365 nm and 625 nm, respectively. Xray photoelectron spectroscopy (XPS) spectra were recorded on a VG Scientific (United



Kingdom) X-ray photoelectron spectrometer (Model ESCALab220i-XL). The morphology of the feathers was observed using a field emission scanning electron microscopy (SEM, Supra 55, Zeiss, Oberkochen, Germany). Fluorescence photographs of feather structure were obtained on a confocal laser scanning fluorescence microscope (LSM710META, Zeiss, Germany). Fourier transform infrared spectra (FTIR) were recorded on Nicolet 400 Fourier transform infrared spectrometer (Madison, WI). All measurements were carried out at room temperature.

Results and discussion Goose feathers, the precious gifts from nature, are composed of thiol-rich mainly keratins, keratins, which are assembled and crosslinked to form large solid matrix with hierarchical structures. Importantly, due to popular the famous sport, badminton, standard goose feathers are, nowadays, commercially available. Those goose feathers are selected with strict quality control (similar size, shape, density and mass) and then bundled onto a rooster, to produce commercial shuttlecocks. As for this study, those similar feathers fully contribute to operation and, most importantly, reproducibility of experiments. As shown in Figure 1A, a typical pennaceous feather, e.g., goose feather, is structured with a hard rachis, where abundant plane-symmetrical branches, i.g., barbs, are fused. Those barbs are further branched and form their secondary structure, i.e., barbules. In this study, we used a pseudo-biomineralization approach to prepare AuNCs-modified goose feathers, also called the golden feathers. To realize reproducibility of the synthesis, standard goose feathers with similar shape and quality were obtained from commercial shuttlecocks A desired weight (~ 50 mg) of as-cropped feathers were soaked in an alkaline aqueous solution containing HAuCl4 (0.143 mM) for 1 h. As can be seen in Figure 1A, the as-prepared golden feathers showed a lightbrown color under visible light, compared to the pristine feathers. Under UV light, a distinct red emission (verse blue in the pristine feathers) appeared. It has been revealed in the luminescent spectra that the emission of the gold feathers reached maximum in intensity at 625 nm, while excitation peak was located at 380 nm, with a shoulder peak


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at 520 nm (Figure 1B), which are in agreement with previous protein-protected AuNCs.12, 14 Of note, the blue fluorescence of the pristine feather is attributed to some aromatic amino acids in feather proteins, including tryptophan, tyrosine and phenylalanine.21 Observation of luminescent staining details in feathers can be realized using confocal fluorescent microscopy. As shown in Figure 1C-H, after AuNC formation, the feather substructures, e.g., barbs, and barbules, can be still clearly identified and almost all of them have been uniformly stained with red-emitting AuNCs, indicating that such natural hierarchical structure is conducive to achieving high staining efficiency and density of AuNCs in feathers. A series of detailed characterizations were next conducted to unearth physicochemical properties of the golden feathers. Strong peaks of O 1s (530.9 eV), N 1s (399.8 eV), C 1s (284.8 eV) were observed in wide-scan XPS spectra of both the pristine and golden feathers, indicating that chemical compositions of feathers almost maintain unchanged after AuNC incorporation (Figure S1). It is worthy to note that the characteristic peaks of Na 1s (1071.1 eV) derived from the introduction of NaOH were not shown in the XPS spectrum of the gold feathers, suggesting ease of removing unbound species in comparison with other protein-based natural materials, e.g., silk.21 Upon AuNC formation, a new pair of peaks located at 87.9 eV and 84.3 eV occurred, as shown in Figure S1 and 2A, which were denoted to the featured Au 4f5/2 and Au 4f3/2, respectively. The bands of Au accords with the binding energy of Au(0)/Au(I) and indicate the formation of AuNCs.12 FTIR spectra can offer information of secondary structure change of acylamino in the proteins of the native feathers, as a result of high sensitivity of their amide bands to environment, including amide I band (-C=O) at 1651 cm-1, which can be assigned to α-helix, amide II band at 1531 cm-1 (-N-H bending coupled with –C-N stretching), and amide III band at 1242 cm-1, an integration of C–N stretching, C=O in plane bending, and C–C stretching and -C-N stretching.14, 55 As can be seen in Figure 2B, those characteristic bands still sustained after AuNC incorporation, indicating a rather low damage of this approach to proteins in feathers. Figure 2C showed the morphology of the feathers via SEM. It was observed that the barbules were slightly separated from each other after AuNC encapsulation, which may be attributable



to a weak hydrolyzing process of feathers in alkaline solution. In addition, we tried to attain soluble samples to investigate the size or composition of AuNCs via transmission electron microscopy (TEM) or electrospray ionization (ESI) mass spectrometry. 1, 56-58 Strong alkaline hydrolysis is report to be a common method to digest feathers to soluble protein solution.59 However, as shown in Figure S2, as alkalinity increased, the golden feather was degraded to a brown solution and only blue emission could be observed upon exposure to UV light. The quenching of the red emission of the AuNCs is attributable to the hydrolysis-assisted destruction of their structure, rendering difficulty or impossibility to conduct accurate size or composition analysis of the AuNCs. With understanding of structural and chemical details of the golden feathers, we next optimized the synthetic route to achieve desirable golden feathers. We first confirmed the necessity of reagents. As shown in Figure 3A, only the reaction with both HAuCl4 and NaOH can generate luminescent golden feathers, while no red emission was found by the introduction of NaOH or HAuCl4 alone. Of note, it could be observed that the barbs in the NaOH-treated sample were bundled, while in the HAuCl4-treated sample they still maintained standing status. The slight morphology changes of the golden feathers are thus attributed to alkali rather than HAuCl4 or heating treatment. Then, the amounts of NaOH for AuNC formation were investigated. As can be seen in Figure 3B, neither less nor more NaOH can produce bright red-emissive golden feathers. At low alkalinity, large Au particles are formed, rendering the features dark purple, possibly due to less protection of feather protein from aggregation and growth of AuNCs.12 At high alkalinity, the week luminescence may be associated with strong alkali-induced structure damage of AuNCs as mentioned before. Therefore, Au(I) luminescent components, particularly staple-like Au(I) motifs which are recognized to be responsible for luminescence of AuNCs, are not formed, leading to weak even no emission generated from such golden feathers.60 Finally, with established alkalinity, the amounts of HAuCl4 were optimized. As can be seen in Figure 3C, the increased concentration of HAuCl4 deepened the color of feathers, while the red emission of the golden feathers was correspondingly enhanced. But when the concentration reached 3 mM, the feather turned to be black and no luminescence was observed. In short, with


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50 mg of goose feathers, 1.5 mM HAuCl4 and 30 mM NaOH can be applied to prepare the desired red-emitting golden feathers. In addition, we further expanded this method to other feathers, e.g., chicken feathers. As shown in Figure 3D, the golden chicken feather can emit strong red luminescence under UV light. It could be thus concluded that animal feathers are suitable substrates for preparing AuNC-embedded bulk hybrid materials with bright luminescence. The novel luminescent hybrid materials are expected to possess various potential applications, such as solid fluorescent sensor, powders of light-emitting diode.61-62 AIE-active materials have exhibited their unique values in the research fields of chem/biosensing, bioimaging, biomedical applications, and optoelectronic devices.63-67 We thus were inspired to explore the possibility to use such bulk materials with high density of incorporated AuNCs to prepare AIE-active Au(I) complexes. To the best of our knowledge, there are only two AuNCs etchants, TCEP and CSH,24-26, 52, 68capable of etching AuNCs and finally forming AIE-active compounds. We then firstly evaluated the synthesis of AIE-active TECP-Au(I) complexes using the golden feathers. We immersed the feathers in TCEP-containing buffers (pH 8.5) for 1 h. As shown in Figure 4A, the brown feathers were almost bleached. After exposure to TCEP, the red luminescence of the golden feathers completely disappeared and, instead, the blue emission of feather proteins occurred under UV light, which was in agreement with the etching reaction reported previously.24 In addition, the effects of TCEP-stimulated bleaching and quenching were positively associated with the concentration of TCEP in solution (Figure 3S). Conveniently, the protein-free solution containing TCEP-Au(I) complexes was collected merely by picking the feathers out. Some organometallic compounds, e.g., here TCEP-Au(I) complexes, have emerged as a novel type of AIEactive materials, which can harvest luminescence, through solvent- or cation-induced aggregation49, 69. The generation of AIE phenomenon of Au(I) complexes is based on the formation of Au(I)…Au(I) interaction, i.g., aurophilic bonds, which can give rise to luminescence through ligand-to-metal charge transfer (LMCT) or ligand-to-metalmetal charge transfer (LMMCT). Previous studies have demonstrated that nonluminescent TCEP-Au(I) complexes could give bright yellow emission by adding



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Cd2+.25-26 We thus used Cd2+ as the representative cation to trigger AIE of the complexes. A series of varied amounts of Cd2+ were introduced in the solutions and, as can be seen in Figure 4B, a row of the solutions produced luminescence with increased brightness.49 The luminescent spectrum in Figure 4C showed that the maximum emission was located at 513 nm in accordance with previous report.25 Interestingly, as the concentration of Cd2+ increased, both the wavelength and the intensity of emission peak were changed (Figure 4C). The wavelength was red-shifted to yellow in accompany with the increasing intensity. This phenomenon should be attributed to the aggregation-induced change of Au(I)…Au(I) distance in the TCEP-Au(I) complexes, giving rise to the wavelength shift of emission.26, 70 Subsequently, we examined the feasibility of CSH-induced etching of the golden feathers and the following formation of CSH-Au(I) complexes with AIE. As shown in Figure S4, CSH was also able to bleach the color of and quench the red emission of the golden feathers and such changes were enhanced with the increase of CSH concentration (Figure S5). Similarly, upon introduction of Cd2+, the solution showed red emission under UV light and a novel peak around 600 nm appeared in its emission spectrum. In addition, the etched feather could been recycled and carried out a second in situ formation of AuNCs. In the case of TCEP-etched feathers, they underwent treatment in the same synthetic approach with the pristine one. A red-emitting golden feather was retrieved (Figure S6). But the barbs in the feather was softened and hard to stand on the rachis. With respect to the CSHetched feathers, a much less addition of HAuCl4 (3 mM) could regenerated bright golden feathers (Figure S7). In short, in comparison with previously-reported methods for specific AIE-active Au(I) complexes synthesis,24-26,

52

this method has several

remarkable advantages. First, the synthetic process is significantly simplified and no cumbersome operations, e.g., protein precipitation, ultrafiltration, or dialysis, are required. Then, the goose feathers are cheap and easy to form AuNCs of high density within them, resulting in large-scale synthesis of the AIE-active complexes possible. Finally, the synthesis of the Au(I) complexes is time-saving. A total of 2 h (versus 1 day in the previous method25-26) is enough to obtain AIE-active materials.


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Conclusions In this work, brightly red-emitting AuNCs have been synthesized and encapsulated by goose feathers, which acted as both the reducing and the stabilizing reagents. Luminescent AuNCs uniformly distribute within barbs and barbules, demonstrating that the hierarchical structure of goose feathers is conducive to staining of AuNCs with high density. Furthermore, the incorporation of AuNCs almost has no harm to the structure and the chemical compositions of feathers. Such method also can be applied to AuNC synthesis with other features. Using this bulk luminescent hybrids, a facile method to synthesize TCEP-or CSH-Au(I) complexes with AIE was established. The etchants, TCEP and CSH, can bleach the brown golden feathers and quench the red emission through chemical etching pathway. Notably, collection of protein-free solution containing AIE-featured Au(I) complexes was rather simple and only need to remove the etched feathers by hand. The non-emissive solution exhibited strong luminescence after supplementing Cd2+. In addition, the etched feathers could be reused to generated red-emitting AuNCs-incorporated feathers. This study not only builds a new bridge between natural products and functional nanomaterials, but also provides a novel view of AuNCs as reagents for other functional material synthesis, which boosts the development of chemistry based on AuNCs.

Supporting Information Wide-scan XPS spectra of the pristine feathers and the golden feathers. Images of gold feathers reacting with 4 M NaOH. Images of the gold feathers after adding varied concentrations of TCEP and CSH. Images and corresponding luminescent spectra of Cd2+-CSH-Au(I) complexes. Images of the golden feathers generated from the pristine and the TCEP-and CSH-etched (right) feathers.

Notes The authors declare no conflicts of interest.



ACKNOWLEDGMENTS We acknowledges support from Beijing Natural Science Foundation (2184107), the Beijing Advanced Innovation Center for Food Nutrition and Human Health, Beijing Technology and Business University (20181035) and the National Natural Science Foundation of China (21727815).


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Figure captions Figure 1. A) Structure of a piece of goose feather (left) and schematically illustration of the synthetic process of golden feathers (right). The pictures also show the pristine and golden commercial shuttlecocks under both visible and UV light; B) PL excitation (black) and emission (red) spectra of the gold feathers; C-E) confocal fluorescent, DIC, and their overlay images of several barbs that bind to rachis in an individual pristine feather and an individual golden feather F-G). Scale bars are 200 μm in all of these images.

Figure 2. A) XPS spectra of the pristine feathers (black) and the gold feathers (red), respectively; B) FTIR spectrum of the pristine feathers (black) and the gold feathers (red), respectively; C) SEM images of the pristine feathers (left) and the gold feathers (right), respectively.

Figure 3. A) images of feathers treated by reagents of only NaOH, both HAuCl4 and NaOH, only HAuCl4 (from left to right), under visible (top row) and UV light (bottom row), respectively; B) images of feathers treated with varied concentration of NaOH (from left to right: 5, 10, 20, 30, 40, 50 mM), under visible (top row) and UV light (bottom row), respectively; C) images of feathers treated with varied concentration of HAuCl4 (from left to right: 0.7 mM, 1.5 mM, 2.1 mM, 3 mM), under visible (top row) and UV light (bottom row), respectively; D) images of the AuNC-embedded chicken feather hybrids, under visible (top) and UV light (bottom), respectively.

Figure 4. A) images of the gold feathers before (left) and after (right) etching reaction in the presence of 13 mM TCEP, under visible (top row) and UV light (bottom row), respectively; B) images and C) corresponding luminescent spectra of TCEP-etched solutions after adding different concentrations of Cd2+. (from 0 to 15 mM along the arrow)



Figure 1


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Figure 2



Figure 3


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Figure 4



Table of Contents

Green preparation of luminescent gold nanoclusters using bulk nature product, goose feathers, for AIE-active gold(I) complex synthesis


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Synthesis of luminescent gold nanoclusters embedded goose feathers

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