Just What Is It That Makes Silver Nanoprisms so Different, so Appealing?

Laboratory Experiment pubs.acs.org/jchemeduc

Just What Is It That Makes Silver Nanoprisms so Different, so Appealing? Guido Panzarasa* Dipartimento di Scienze ed Innovazione Tecnologica, Università del Piemonte Orientale “Amedeo Avogadro”, Alessandria 15100, Italy S Supporting Information *

ABSTRACT: Thanks to their unique physicochemical properties (e.g., surface plasmon resonance), noble metal nanoparticles are at the cornerstone of nanotechnology. Silver triangular nanoprisms are presented here as an ideal playground to introduce students to nanochemistry concepts such as the formation of shape-controlled nanostructures. Not only a reliable and straightforward protocol for the synthesis of silver nanoprisms is described, but also their formation is explained in terms of basic oxidation−reduction mechanisms; their peculiar optical properties are compared with those displayed by silver nanospheres and discussed in relation to their shape.

KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Interdisciplinary/Multidisciplinary, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Colloids, Materials Science, Nanotechnology, Oxidation/Reduction

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here allows large-scale synthesis of silver nanoprisms with high reproducibility using only four easily available chemicals (silver nitrate, trisodium citrate, hydrogen peroxide, sodium borohydride) and basic instrumentation. The main purpose of this activity is to introduce students to basic concepts of nanotechnology and at the same time strengthen their understanding of redox processes. This procedure has already been successfully carried out by 40 high-school students from two different institutes with excellent results. Complete student handout, notes, and instructions are available in the Supporting Information.

he growing impact of nanosized products in everyday life calls for the introduction of educational programs to make aware the new generations not only of the promises, but also of the potential threats of such a powerful technology. People should be provided with the best tools to understand the scientific revolution that is taking place not only to get direct benefit, but also to have the possibility to make a contribution and advancing global knowledge. One of the results has been the introduction of an “introduction to nanotechnology” teaching modulus in Italian high school.1 In such a context, activities involving the synthesis and characterization of silver nanoparticles are probably the most suitable, thanks to remarkable optical properties and relative ease of production, to introduce students to nanochemistry and related fields.2 Silver nanospheres are receiving great attention in this Journal according to the increasing number of papers published on such subject in the last years,3−9 whereas particles of other shapes are clearly underrepresented despite the importance owned in nanotechnology by shape-control strategies. While the synthesis of silver nanospheres is conceptually simple, synthesis of anisotropic particles requires some additional skill, and to correctly explain their formation, more advanced concepts such as selective etching and adsorption on crystalline facets are involved. Until now, there has been only one report about the didactic possibilities offered by silver nanoprisms;10 however, their potential is far from being completely exploited. Their optical properties are shape-dependent and dramatically different from those displayed by silver nanospheres. The protocol described © XXXX American Chemical Society and Division of Chemical Education, Inc.

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EXPERIMENTAL OVERVIEW

Synthesis of Silver Nanoprisms

One-hundred microliters of 0.1 M silver nitrate, 1.5 mL of 0.1 M trisodium citrate, and 280 μL of 30% hydrogen peroxide are mixed and diluted to 100 mL with water in a flask. The solution is vigorously stirred for 10 min, and then the stirring rate is reduced, and 1 mL of 0.1 M sodium borohydride is rapidly added. After an induction time of 1−2 min, the colorless solution turns yellow and then rapidly darkens until a stable blue color is developed after ∼5 min (Figure 1A). The resulting suspension is stored in the dark at 4 °C. No significant differences were noticed for prisms prepared using mechanical or magnetic stirring. In addition, the quantities can be scaled down to perform the synthesis in vials instead than in flasks.

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DOI: 10.1021/acs.jchemed.5b00320 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 1. (A) Photographic sequence of the color evolution during nanoprisms synthesis. (B) Comparison between the UV−vis absorption spectra of silver nanoprisms and silver nanospheres. (C) HRTEM image of face-to-face stacked silver nanoprisms standing vertically upon their edges.

a collective, periodic oscillation of the conduction band electrons arises, which is called “surface plasmon resonance” (SPR), the “plasmon” being a quantum of electron oscillation. Noble metals such as copper, silver, and gold display this property, which is also the origin of the brilliant colors displayed by their colloidal suspensions in such a great extent that are typically referred to as “plasmonic metals”. This means that noble metal nanoparticles efficiently absorb light of specific frequencies, depending on the size and shape of the nanoparticles themselves and the refractive index of the surrounding medium, as revealed by UV−vis spectroscopy.11 The UV−vis spectrum of the obtained blue nanoprism suspension features a sharp peak at 333 nm, a shoulder at around 450 nm, and an intense band at around 725 nm (see also Figure S1 in the Supporting Information). It is instructive to compare such spectrum with that displayed by silver nanospheres (Figure 1B). In the case of isotropic silver nanospheres, electrons can only oscillate forward and backward giving rise to a single absorption peak, corresponding to the dipole plasmon resonance peak, which for 10 nm nanospheres is centered at about 400 nm. The anisotropy of triangular nanoprisms makes it possible for the electrons to accumulate in correspondence of the tips generating quadrupole oscillations, also to make both in-plane and out-of-plane oscillations. The three absorption features observed for nanoprisms are, according to theoretical calculations,12 assigned, respectively,

This procedure allows silver nanoprisms to be easily synthesized on a large scale with high reproducibility and stability (Supporting Information, Figure S2). The experience can be completed in less than 1 h if the solutions have been prepared in advance. To ensure reproducibility, the sodium borohydride solution should be used immediately after preparation.

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HAZARDS Silver nitrate is toxic and an irritant. Trisodium citrate is potentially an irritant. Sodium borohydride is flammable and an irritant; its aqueous solutions should only be stored at +4 °C for limited time and in loosely capped vessels to avoid pressure buildup due to hydrogen generation. Concentrated (30 wt %) hydrogen peroxide is toxic and corrosive and can cause severe skin burns and eye damage. Its strong oxidizing properties may cause fire or explosion when mixed with organic compounds. Protective gloves and goggles must be worn all the time when preparing solutions and performing the experiments.

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RESULTS AND DISCUSSION Much of the interest about silver nanoprisms derives from their optical properties. When an electromagnetic wave (i.e., light) impinges on a metal nanoparticle, its electric field interacts with the electrons of the conduction band. If the frequency of the incident wave is equal to the bulk metal plasma frequency, then B




Figure 2. (A) Phase transfer of silver nanoprisms from an aqueous to a hexane phase and (B) the corresponding UV−vis spectra (normalized).

cause red-shift of the SPR band, an increase in electron density on the particles would blue-shift the band.14 The synthesis of silver nanoprisms involves many intriguing aspects, and the color changes accompanying their formation are among these. The observed induction time previous to the development of color series is a behavior very similar to that of a clock reaction. In this context, it is interesting also to note that the color development is accompanied by the evolution of gas bubbles, presumably a mixture of oxygen and hydrogen.15,16 The main process involved is the reduction of silver cations to metallic silver operated by sodium borohydride in the presence of sodium citrate, which is the basic procedure to obtain silver nanospheres. The crucial reagent here is hydrogen peroxide, which is known to be a powerful oxidizing agent. The standard potential of the peroxide−water couple in alkaline solution is 0.867 V, a value that is higher than that of Ag+/Ag0 (E0 = 0.7996 V), thus making hydrogen peroxide an effective etchant to dissolve metallic silver. The observed induction time suggests that hydrogen peroxide acts as an oxidant from the very beginning of the reaction and that a dynamic equilibrium between the reduction of silver ions by borohydride and oxidative dissolution of metallic silver by peroxide is established. On the basis of these considerations and the aforementioned experimental observations, a plausible mechanism to explain the formation of silver nanoprisms could be delined:17 upon the addition of sodium borohydride, silver ions are partially reduced to form very small silver nanoparticles or nuclei, which are stabilized by the adsorption of borohydride and citrate ions. At the same time, the growth of such small particles is hindered because of continuous etching by hydrogen peroxide. The dynamic equilibrium between reduction and oxidation is altered when the stabilizing borohydride ions begin to be decomposed, allowing the growth of nuclei. Because of the coordinative interaction of citrate ions, which bind preferentially to (111) facets, nuclei would contain many defects including twinned defects that favor the planar growth into plates. The fast growth of (100) side facets enhances their expansion along the twin plan into nanoprisms. However, proper mixing of the reaction mixture and exposure to ambient light both have been found to have

to the plasmon resonance of out-of-plane quadrupoles, in-plane quadrupoles, and in-plane dipoles. The absence of a peak around 400 nm demonstrates the absence of silver nanospheres. High-resolution transmission electron microscopy (HRTEM, Figure 1C) and scanning electron microscopy (SEM, Figure S3 in the Supporting Information) confirmed the success of the synthesis by showing planar particles with triangular shape, mean edge length of 34 nm, and mean thickness of 4 nm. The presence of lattice fringes demonstrated also the crystalline nature of the silver nanoprisms. The present method has many advantages: first, its reliability. Second, it can be easily scaled-up or down without loss of reproducibility. In addition, the obtained suspension, properly stored, is stable for more than one month (Figure S2). Silver nanoprisms can be concentrated by centrifugation (typically 30 min at 10 000 rpm) and redispersion in water or ethanol without losing their colloidal stability. Transfer in an organic phase (Figure 2A) can also be easily achieved by vigorously stirring 10 mL of the nanoprism aqueous suspension with 2 mL of hexane in the presence of 100 μL of oleylamine as the phase-transfer agent. Phase exchange of silver nanoprisms into nonaqueous solvents opens up the possibility to make composites with hydrophobic polymers such as poly(styrene) and poly(methyl methacrylate) to obtain composites and coatings for, for example, fundamental optical studies or for the realization of sensors. The UV−vis spectrum of the nanoprisms in hexane (Figure 2B) shows a strong blue-shift of the main absorption peak, which becomes also sharper and more intense, whereas the 333 nm peak and the 450 nm shoulder are less affected. The interpretation of this phenomenon is not trivial and could be ascribed to the electron-donating (nucleophile) effect of the amine group of oleylamine, in analogy to what has been reported in the case of phosphine-stabilized nanoparticles.13 The surface oscillation of the electron gas in the silver particles can be influenced by adsorbed molecules and especially if the adsorbates have direct interactions with the silver atoms (as for amine groups), it is expected that they will not only change the refractive index in the vicinity of the silver particles, but also affect the density of free electrons. It has been demonstrated that while a decrease in electron density could C




Figure 3. Photographic sequence showing the influence of stirring on the color evolution during the synthesis of nanoprisms. (A) Stirring was stopped after complete mixing of sodium borohydride solution. (B) Stirring was stopped before complete mixing of sodium borohydride solution.

Figure 4. Comparison of (A) the physical aspect and (B) the UV−vis spectrum of silver nanoprisms synthesized without stirring in different conditions: (1) without stirring after borohydride addition, ambient light; (2) without stirring, ambient light; (3) without stirring, dark.

Effect of Silver Nanoprisms on the Chemiluminescence of Luminol−H2O2

profound effects on the kinetics of formation of silver nanoprisms, as judged by the evolution of the colors (Figures 3 and 4). The effects of stirring and exposition to ambient light are more evident when considering the UV−vis spectra of the resulting suspensions. The combination of dark and no stirring seems to be the most unfavorable to obtain nanoprisms because for the suspension obtained with these conditions, the 333 nm peak was barely visible, a peak at 400 nm indicated possible presence of nanospheres, and no peak was present at around 700 nm.

As an example of potential applications of silver nanoprisms, their effect on the chemiluminescence of the luminol−H2O2 system was investigated according to a recently published procedure.2 To ensure reproducibility, nanoprisms suspensions were aged for 16 h in the dark at room temperature before use to decompose excess hydrogen peroxide and sodium borohydride. The emission of light observed in the presence of silver nanoprisms was found to be weaker, almost the half, than that observed in the presence of silver nanospheres, but it was longer-lasting (30 min instead than 15 min). Nanoprisms dissolved during the reaction, as did nanospheres (Figure 5A), D




Figure 5. UV−vis spectra showing the evolution of silver nanoprisms (A) before and after the luminol−H2O2 reaction, (B) in the presence of tiron (superoxide scavenger), and (C) in the presence of ascorbate (generic scavenger). “Control” means a 0.03 mM silver nanoprism aqueous suspension.

treatment. Students were rapidly able to relate typical concepts of colloidal science such as emulsions and dispersions to formation of nanoparticles and were amazed by realizing the ubiquitous distribution of natural (e.g., those making up opals and those, nastier, present in smog) and artificial nanoparticles (e.g., those found in sunscreens and silver-treated antimicrobial textiles). Students found the procedure easy enough to be completed without significant aid from the instructor. They collected experimental data and were asked to complete a laboratory report (see the Supporting Information for details) from which the comprehension level achieved could be estimated. Students correctly solved the redox equation for borohydride reduction of silver cations, identified the etching role of hydrogen peroxide and that of citrate as shaping agents, and were able to relate the color appearance to formation of nanoparticles through the observation of the Tyndall effect. In general, students expressed a strong appreciation for a reaction capable of directly introducing them, without the need of special reagents and instrumentation, to the beautiful mysteries of the nanoscale world.

and the dissolution was accompanied by the evolution of some gas bubbles as well. Scavengers such as sodium ascorbate, tiron, and tert-butanol were also tried: each was added to a concentration of 10 mM. While tert-butanol had no effect on chemiluminescence, both ascorbate and tiron completely quenched it. Also, in their presence, no gas bubbles developed. The effect of tiron seemingly confirmed the involvement of the superoxide anion radical already noticed for nanospheres. It is interesting to comment about the observed changes in the optical properties, revealed by UV−vis spectroscopy. Addition of luminol caused a 30 nm red-shift of the main band, which revealed that silver nanoprisms are more sensitive than silver nanospheres to nucleophiles. Tert-butanol had no effect on the optical properties of silver nanoprisms. Sodium ascorbate produced a blue-shift of the main absorption band, but the shoulder at 450 nm was not affected, while in the presence of tiron, the color changed from blue to violet. Such changes in the optical properties can be assigned to absorption of the molecules on the surface of nanoprisms, as for the case of oleylaminemediated phase transfer. After the chemiluminescent reaction, induced by addition of hydrogen peroxide, in the presence of tiron, a greenish−yellow color is indicative of formation of smaller nanospheres, confirmed by the presence of a sharp band at 440 nm in the UV−vis spectrum (Figure 5B), while a broad band at 940 nm could be assigned to larger particles. In presence of ascorbate, the shoulder at 450 nm disappeared; the intensity of the main peak decreased and experienced a more pronounced blue-shift (Figure 5C).

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CONCLUDING REMARKS Silver nanoprisms are very easy to synthesize and display remarkable optical properties. The initial induction time and the rapid color change that follows make it a visually attractive experience that capture the students’ attention and drives them to find a plausible explanation based on the chemistry involved in the process. Silver nanoprisms can be regarded not only as a versatile tool to introduce students to the main topics of nanochemistry, but also as an endless source of undergraduate research projects. It is the author’s hope that this work will spur others to exploit the possibilities offered by the exploration of this fascinating topic.

Assessment of Students’ Understanding

Before entering the laboratory, the students were engaged to balance model redox equations and to discuss their vision of nanotechnology. It turned out that there was a tendency to apply rigid mathematical rules to solve redox equations without paying enough attention to the overall meaning of the reactions themselves. The instructors verified, by postlab examinations, that this behavior was positively changed: for many students, this visually appealing experience helped them to understand the relevance of redox processes otherwise considered as mere algebraic exercises. The students’ general conception of nanotechnology was only slightly influenced by fictional elements. The most frequently asked questions regarded the possibility to use nanoparticles for medical purposes, for example, for cancer

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.5b00320. Additional figures, complete student handout, and notes for the instructor (PDF, DOC)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. E




Notes

Nanoparticles, Nanorods, Cubes, or Bipyramids to Triangular Prisms of Silver with PVP, Citrate, and H2O2. Langmuir 2012, 28 (24), 8845− 8861. (16) Yu, H.; Zhang, Q.; Liu, H.; Dahl, M.; Joo, J. B.; Li, N.; Wang, L.; Yin, Y. Thermal Synthesis of Silver Nanoplates Revisited: A Modified Photochemical Process. ACS Nano 2014, 8 (10), 10252−10261. (17) Zhang, Q.; Li, N.; Goebl, J.; Lu, Z.; Yin, Y. A Systematic Study of the Synthesis of Silver Nanoplates: Is Citrate a “Magic” Reagent? J. Am. Chem. Soc. 2011, 133 (46), 18931−18939.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author wants to express personal gratitude to Gianluigi Marra (ENI Research Center for Renewable Energies and Environment, Istituto ENI Donegani, Novara) for kindly providing the HRTEM and SEM images. Nadia Semino (Istituto Superiore “G. Marconi”, Tortona), Elisabetta Gaita (Istituto Superiore “Ascanio Sobrero”, Liceo Scientifico opzione Scienze Applicate, Casale Monferrato) and students, especially Chiara Figazzolo, Filippo Cotta Ramusino, and Stefano Sesia, are kindly acknowledged for testing the experiments and providing feedback. The anonymous reviewers are acknowledged for their contributions.

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REFERENCES

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Just What Is It That Makes Silver Nanoprisms so Different, so Appealing?

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