Sodium alginate-assisted photosynthesis of complex silver microarchitectures

Abstract

Flower-like, complex, polycrystalline silver microarchitectures were synthesized in the presence of sodium alginate, facilitated by the oriental adherence of polysaccharides under natural light. These 3D microarchitectures displayed unique SPR absorption as well as significant Raman scattering activity.

---

Self-organization of complex structures from colloidal particles is a significant synthetic method used to obtain multi-functional materials.^1,2 Dendritic or flower-like Ag microstructures, an intensively studied category of such complex structures, have attracted great attention due to their distinct features.^3–5 Moreover, most of these specific microstructure formations usually obey the underlying principle of diffusion-limited aggregation (DLA) process, or oriented attachment.^5–7 Recently, biomolecules have been utilized to induce the self-organization of metal particles. Biotin was used in the synthesis of fiber-like microstructures from spherical Ag particles, owing to its tunable weak interactions with silver ions.⁸ Similarly, salep, another common biomolecule, was shown to contribute to flower-like Ag microarchitecture assembly from wedge-like particles under sunlight irradiation.³ Such exciting achievements inspired us to develop complex Ag architectures using biomolecules and energy-saving natural light for potential biomedical purposes.

Here we report a new, biomolecule-assisted approach to induce the organization of complex Ag microarchitectures. Sodium alginate (SA), a biocompatible natural polysaccharide with active metal-binding sites arising from carboxylate groups,⁹ was mixed with AgNO₃. The mixture was then irradiated with natural light for 1 h (400–700 lux horizontal illuminance). The color of the solution changed from colorless to crimson (see ESI, Fig. S1†). This solution was directly dropped onto a TEM grid, and typical flower-like particles with ca. 3.8 μm core were observed (Fig. 1a). Branches, forming the corona of the microarchitectures, are 240 nm in diameter (mean) and 4.3 μm in length. The core corona structure is similar to the schematic illustration of self-assembled polymeric micelles.¹⁰ Close inspection of the individual complex architecture (Fig. 1b) revealed that it consists of plate-like particles, which are ca. 460 nm in length and ca. 70 nm in thickness. More interesting is that part of the plates are clearly perpendicular to the substrate, which is in agreement with SEM images (Fig. 1a inset, also see ESI, Fig. S2†). The 3D structure arising from upright plates offers a distinct advantage for SERS detection due to the enhanced active surface for absorbing more molecules than flat counterparts.¹¹ On the other hand, it seemed that these nanoplates were adhered to each other in a plate-by-plate order, as indicated by arrows in Fig. 1b. However, the complex microarchitectures were destroyed after treating the as-synthesized solution by centrifugation. Instead of complex structures, independent plate-like particles were obtained, and the mean size was comparable before and after separation (see ESI, Fig. S3a†). Therefore, considering its low stability, the formation of such complex structure is likely via weak interaction-driven nanoplate organization rather than the traditional crystal growth of dendrites.^6,7

Fig. 1 TEM (a and b), HRTEM (c and d) and SEM (a inset) images of flower-like, complex microarchitectures. (e) XRD patterns of different particles. SA, 0.5%; AgNO₃, 0.1%; natural light irradiation, 1 h.

High-resolution TEM images (Fig. 1c and d) showed that the complex architectures were composed of AgCl and Ag. The lattice spacing of 0.280 is indexed to the (200) plane of AgCl; however, the measured lattice spacing of 0.235 nm and 0.200 nm correspond to the (111) and (200) planes of face-centered cubic (FCC) Ag, respectively. Since the lattice orientation was obviously different between the Ag and AgCl block, the microarchitecture branches must not be epitaxial growth from the stem.^5,7,12 The polycrystalline structure of the architectures was further confirmed by the corresponding selected area electron diffraction pattern (see ESI, Fig. S3b†), which is in accordance with TEM and HRTEM analysis (Fig. 1b–d). The appearance of AgCl was also confirmed by XRD. As shown in Fig. 1e, besides the typical peaks of cubic metal silver (JCPDF no. 04-0783),^5,7 the diffraction peak located at 32.2° was assigned to the (200) crystal plane of AgCl (JCPDF no. 31-1238).^12,13 After centrifugation, the diffraction peak intensity of AgCl became stronger, which could be ascribed to the removal of impurities, including NaAgO (JCPDF no. 01-077-1695) and AgClO₃ (JCPDF no. 01-075-0886), by washing.

Previously, most of the reported flower-like Ag structures were attributed to DLA or oriented attachment, and few of them involved Cl⁻.^5,7 In our system, the residual Cl in commercially available SA of ca. 2.5 g kg⁻¹ was detected using ion chromatography. To verify the role of Cl ions in the formation of these complex composite microarchitectures, a control experiment was conducted using Cl⁻ removed SA by dialysis. In comparison with commercial SA, Cl⁻ removed SA led to changes in both color (from colorless transparent to greenish-gray, see ESI, Fig. S4†) and time consumption (8 h). Further TEM characterization showed that most particles are ca. 23 nm and spherical in shape (Fig. 2a). The XRD diffraction peaks of the spherical particles could be indexed to the corresponding (111), (200), (220) and (311) crystal planes for FCC phase silver with negligible AgCl signal (Fig. 1e).^5,7 Previously, it was proposed that Cl ions can not only guide anisotropic growth, but also transform the shape of silver nanoparticles.^14,15 Moreover, AgCl could be photoconverted to functional silver nanoparticles.¹⁶ Therefore, we consider that Cl ions, in the form of AgCl, play a pivotal role in inducing flower-like complex architecture formation.

Fig. 2 TEM images of photosynthesized Ag nanoparticles employing different polysaccharides (0.5%), AgNO₃ (0.1%): (a) Purified SA with Cl⁻ removal, 8 h; (b) chitosan, 1 h; (c) SA, 30 min.

Besides Cl⁻, SA is likely to be another key factor affecting the formation of complex architectures. Using other commercially available natural polysaccharides, including chitosan, carboxymethylcellulose and soluble starch, to replace SA in the system yields only dispersive particles, without any complex assemblies (Fig. 2b, also see ESI, Fig. S5†). Moreover, we also tracked the shape transition from spherical particles to nanoplates after irradiation for 30 min. (Fig. 2c), which indicated particle evolution under natural light over time.¹⁷

Based on the above results, the formation mechanism of complex microarchitectures is clarified in Scheme 1. First, trace Cl⁻ reacted with Ag⁺ to produce the AgCl crystal nucleus, which absorbed photons and released electrons to reduce Ag⁺ to Ag seeds.^13,16 Then, under irradiation with natural light, such Ag seeds continued growth and further converted into anisotropic nanoplates because of the long-wavelength excitation effect.¹⁷ The (111) plane of as-converted AgCl/Ag nanoplates is known to absorb SA. The absorbed SA on the (111) plane not only accelerated the growth of nanoplates via chelating silver ions by carboxyl groups, but also induced nanoplate adherence, one by one, erectly, which contributed to the final complex architecture formation via branch self-organization.

Scheme 1 Photosynthesis of complex Ag microarchitectures through order adherence of nanoplates using sodium alginate (SA) followed by self-organization of branches. LWL, long-wavelength light.

Finally, the optical properties of as-synthesized complex architectures were characterized for their potential biomedical applications. As shown in Fig. 3a, the single UV absorption band of spherical particles was located at ca. 410 nm, which is in agreement with previous literature for Ag particles.^17,18 However, the spectrum for the flower-like architectures showed an additional absorption band at ca. 969 nm compared to that of mixed particles after centrifugation, which can be ascribed to the aggregation of metal particles.¹⁷ The localized surface plasmon resonance adsorption in the NIR region suggested that the complex architectures are likely to contribute Raman scattering enhancement.¹⁸ In fact, flower-like silver architectures displayed significant Raman scattering activity in comparison with quartz or silver nanosphere substrates using crystal violet (2 × 10⁻⁶ M) as Raman tag (Fig. 3b). We considered that both the size and 3D geometry of flower-like, complex particles played an important role in the enhancement, as the localized surface plasmon coupling and the enhanced electromagnetic field intensity provided useful features for SERS detection.^4,11,17

Fig. 3 (a) UV-vis spectra of flower-like microarchitectures, particles separated from flower-like architectures by centrifugation, and Ag nanospheres (from top to bottom). (b) Raman spectra of crystal violet (2 × 10⁻⁶ M) on different substrates: 1, quartz; 2, silver nanospheres; 3, flower-like microarchitectures.

In summary, flower-like, complex, polycrystalline Ag microarchitectures with distinct optical features are reported. The Cl⁻ and SA served as unique assistant reagents to induce plate-by-plate adherence. This simple methodology could be potentially applied to other systems involving different nanoparticles. Also, the 3D microstructures have promising applications for biomedical detection and photocatalysis.

Acknowledgements

This work was supported by the Major Research Plan of NSFC (91323104) & NSFC (31271023, 31170917), the PUMC Youth Fund (33320140054) and the Fundamental Research Funds for the Central Universities, and the Basic Research Fund of State Institutes for Public Career.

Notes and references

1. S. C. Glotzer and M. J. Solomon, Nat. Mater., 2007, 6, 557–562.
2. M. Grzelczak, J. Vermant, E. M. Furst and L. M. Liz-Marzán, ACS Nano, 2010, 4, 3591–3605.
3. A. Pourjavadi and R. Soleyman, Mater. Res. Bull., 2011, 46, 1860–1865.
4. K. A. Homan, J. Chen, A. Schiano, M. Mohamed, K. A. Willets, S. Murugesan, K. J. Stevenson and S. Emelianov, Adv. Funct. Mater., 2011, 21, 1673–1680.
5. X. Wen, Y. T. Xie, M. W. Mak, K. Y. Cheung, X. Y. Li, R. Renneberg and S. Yang, Langmuir, 2006, 22, 4836–4842.
6. J. A. Corno, J. Stout, R. Yang and J. L. Gole, J. Phys. Chem. C, 2008, 112, 5439–5446.
7. L. Lu, A. Kobayashi, Y. Kikkawa, K. Tawa and Y. Ozaki, J. Phys. Chem. B, 2006, 110, 23234–23241.
8. S. Hegde, S. Kapoor, S. Joshi and T. Mukherjee, J. Colloid Interface Sci., 2006, 297, 637–643.
9. L. Meng, W. Xia, L. Liu, L. Niu and Q. Lu, ACS Appl. Mater. Interfaces, 2014, 6, 4989–4996.
10. J. Rodríguez-Hernández, F. Chécot, Y. Gnanou and S. Lecommandoux, Prog. Polym. Sci., 2005, 30, 691–724.
11. B. B. Xu, L. Wang, Z. C. Ma, R. Zhang, Q. D. Chen, C. Lv, B. Han, X. Z. Xiao, X. L. Zhang, Y. L. Zhang, K. Ueno, H. Misawa and H. B. Sun, ACS Nano, 2014, 8, 6682–6692.
12. M. Choi, K.-H. Shin and J. Jang, J. Colloid Interface Sci., 2010, 341, 83–87.
13. P. Wang, B. Huang, X. Qin, X. Zhang, Y. Dai, J. Wei and M. H. Whangbo, Angew. Chem., Int. Ed., 2008, 47, 7931–7933.
14. B. Wiley, T. Herricks, Y. Sun and Y. Xia, Nano Lett., 2004, 4, 1733–1739.
15. J. An, B. Tang, X. Zheng, J. Zhou, F. Dong, S. Xu, Y. Wang, B. Zhao and W. Xu, J. Phys. Chem. C, 2008, 112, 15176–15182.
16. G. Wang, T. Nishio, M. Sato, A. Ishikawa, K. Nambara, K. Nagakawa, Y. Matsuo, K. Niikura and K. Ijiro, Chem. Commun., 2011, 47, 9426–9428.
17. K. G. Stamplecoskie and J. C. Scaiano, J. Am. Chem. Soc., 2010, 132, 1825–1827.
18. X. Cui, C. M. Li, H. Bao, X. Zheng, J. Zang, C. P. Ooi and J. Guo, J. Phys. Chem. C, 2008, 112, 10730–10734.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra01441g

---