Demonstration pubs.acs.org/jchemeduc
From Coinage Metal to Luminescent Nanodots: The Impact of Size on Silver’s Optical Properties Junhua Yu* Department of Chemistry Education, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, South Korea S Supporting Information *
ABSTRACT: Nanoscience has become a topic covered in undergraduate education. The new properties gained as matter becomes nanoscale are especially useful to illustrate the impact of size. Silver is a good example for such an illustration because it displays completely different properties at different sizes, such as bulk silver, nanoparticles, and few-atom nanodots. Such differences reflect the interaction between photons and the varied silver species. A simple demonstration was designed to illustrate the property change as the silver diminished from bulk phase to silver nanodots. Without any protection, silver ions are reduced directly to large silver particles. The introduction of a protection group, such as poly(acrylic acid), facilitates the formation of silver nanoparticles, which shows strong visible absorption due to surface plasmon resonance. However, subtle adjustment of the protection group by amino-functionalized alkylsilane is required to obtain stable silver nanodots that display not only strong absorption but also bright emission. This demonstration is a general tutorial for upper-level undergraduate students who have an introductory background of photophysics. KEYWORDS: Upper-Division Undergraduate, Demonstrations, Interdisciplinary/Multidisciplinary, Physical Chemistry, Hands-On Learning/Manipulatives, Colloids, Fluorescence Spectroscopy, Nanotechnology, UV−Vis Spectroscopy
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coinage metal due to its excellent stability.12 It has the highest electrical conductivity of all metals. When used in mirror production, the freshly deposited silver film is the best reflector of visible light.13 In modern classrooms, silver has been utilized as part of Tollens’ reagent, producing the “silver mirror”.14 Research on silver nanoparticles has revealed outstanding optical properties resulting from its surface plasmon resonance.15 Several articles on the synthesis and characterization of silver nanoparticles have appeared in this Journal.15−17 However, as far as we know, the newly discovered luminescence of silver nanodots18,19 has not been introduced into undergraduate course. Silver nanodots are encapsulated silver clusters consisting of a few to tens of silver atoms.20 Even though silver nanoparticles can be degraded gradually by oxygen in solution, silver clusters are much more vulnerable in an aerated solution. Stable luminescent silver clusters were recently obtained after massive screening for protection groups of silver clusters.20 To distinguish well-defined, stable clusters from species trapped in matrices or gas-phase clusters, the term silver nanodot was introduced. The synthesis and characterization of luminescent silver nanodots not only helps undergraduate students to understand the effect of size on the properties of a substance, but also introduces the basic concept of photophysics that was mostly experimentally ignored due to the higher cost of fluorometers.21,22
he development of analytic technologies and methodologies has enabled the fast expansion of research on nanoscience and nanotechnology.1 “There’s plenty of room at the bottom”, the title of a lecture given by Feynman decades ago,2 is exactly right, as demonstrated by the giant leap in the ability to assemble components consisting of functional nanosize particles. The “nanosize” of the substance is a key player in this new field.3 Universities have started to develop undergraduate courses on nanoscale science.4−7 The new properties gained as matter becomes nanoscale are especially useful in illustrating the impact of size.8 Silver is a fitting example for such illustrations because it displays completely different properties at different sizes, such as bulk silver, nanoparticles, and few-atom nanodots. There is no gap between the valence and conduction bands of a metal. When the size of the metal decreases enough to nanoscale, however, this gap enlarges and the metal becomes a semiconductor, with confined electronic motion.9 As the coherent oscillation of the free electrons in the conduction band becomes resonant with the frequency of an electromagnetic field, a strong absorption by the metal nanoparticles appears, which is referred to as surface plasmon absorption. Further decrease in the metal particle size to below 2 nm in diameter leads to the formation of molecule-like clusters, among which luminescence can be observed due to the HOMO−LUMO (highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively) electronic transitions of the metal clusters.10,11 Silver has been used as a © 2014 American Chemical Society and Division of Chemical Education, Inc.
Published: April 10, 2014 701
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Demonstration
DEMONSTRATION Before starting the demonstration, some brief background information can be introduced, including luminescence and its detection as well as how to stabilize nanoparticles in aqueous solution (Supporting Information). This demonstration can be especially suitable for a lecture course divided into (at least) two sessions per week. In the first session, a picture of silver bar or a piece of silver jewelry can be shown to students to demonstrate some properties of bulk silver. Subsequently, three 2-dram glass vials can be filled with the following solutions: (1) silver nitrate stock solution (50 μL) and water (5 mL) (silver-alone); (2) silver nitrate stock solution (50 μL), poly(acrylic acid) stock solution (50 μL), and water (5 mL) (silver-polymer); and (3) silver ion-silane complex stock solution (50 μL), poly(acrylic acid) stock solution (50 μL), and water (5 mL) (silver-polymer-silane). These solutions are all colorless and transparent. All solutions can be reduced with 50 μL of freshly prepared sodium borohydride stock solution while being stirred vigorously with a magnetic stirring bar. All of the solutions will become yellow (Figure 1A). A discussion on what is happening in the three vials can be held while this is occurring. The solutions can be stirred continuously in the dark until the following session.
are unstable. The silver-polymer solution and silver-polymersilane solution are still light blue and pink, respectively. The absorption, emission, and excitation spectra can be illustrated as well. Students can discuss why the three solutions display different optical properties.
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HAZARDS
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RESULTS AND DICUSSION
Care must be taken when handling sodium borohydride, which reacts with water to form flammable hydrogen. Concentrated solutions of silver salts may leave dark spots on the skin. Looking directly at the UV lamp for prolonged periods of time is harmful.
Poly(acrylic acid) has been utilized to stabilize silver clusters.23 Its carboxyl groups bind silver ions and silver nanoparticles to form a charged polymer layer that prevents nanoparticles from agglomeration. Such a phenomenon is demonstrated in Figure 1. When solutions of silver nitrate (145 μM) in the absence of the polymer (silver-alone) and in the presence of the polymer (silver-polymer) are reduced with sodium borohydride, both solutions become yellow instantly (Figure 1A, left and middle vials). The color of the silver in the absence of the polymer becomes darker and some flocs are obvious after 72 h (Figure 1C, left vial), indicating that unprotected silver nanoparticles are unstable. However, the solution in the presence of the polymer (middle vial) gradually changes to light blue due to the absorption of low concentration silver nanoparticles. This solution can be stable for weeks. However, the protection provided by such polymers alone may not produce luminescent silver nanodots. The silver nanodots consist of a few to tens of silver atoms, which are very vulnerable to oxidation. The unstable silver clusters may either be oxidized to silver ions, or undergo agglomeration to form larger and more stable silver nanoparticles. Therefore, the successful generation of silver nanodots requires not only protection groups to encapsulate individual silver clusters but also an adequate protective ligand to increase the reduction potential of the resulting silver cluster-protection group complex. To implement such a strategy, an amino silane is introduced, in which the amino groups coordinate silver atoms to stabilize the resulting silver clusters. Silver nitrate and 3-(2aminoethylamino)propyltrimethoxysilane are stirred in anhydrous methanol, yielding complexes with amino-coordinated silver ions. In the above experiment, methanol of at least commercially available anhydrous quality must be used because too much water in methanol induces the polymerization of alkyl silane to silica particles that afford no protection to the silver clusters. The silver-silane complexes are then mixed with the polymer, followed by sodium borohydride reduction. Immediately after the reduction, the solution shows yellow color similar to the other two vials that produced silver nanoparticles. However, the solution changes to pink after 18 h (Figure 1A to C, right vial), indicating the formation of silver nanodots. In addition to the significant color change, the difference between silver nanoparticles and silver nanodots can also be characterized by spectroscopy. As shown in Figure 2, the unprotected silver nanoparticles exhibits strong typical surface plasmon resonance absorption at 390 nm (NPs without polymer). However, the polymer prevents the formation of large spherical nanoparticles, and consequently the surface
Figure 1. Images of silver nanoparticles and silver nanodots in aqueous solutions: (left vial) Ag-alone, (middle vial) Ag-polymer, and (right vial) Ag-polymer-silane. (A) Solutions after mixing with NaBH4. (B) Solutions shown in (A) after stirred continuously in the dark for 18 h. (C) Solutions shown in (A) after 72 h. (D) Vials in (B) are excited with UV lamp (365 nm) and their luminescence recorded with a color camera.
Eighteen hours later, the silver-alone solution will still be yellow, whereas the silver-polymer solution is light blue. The color of silver-polymer-silane solution changes to pink (Figure 1B). Outside of the lecture, some volunteers in the class can obtain the optical spectra of the three solutions together with the instructor. The scanning region for the absorption spectrum is from 900 to 200 nm in a quartz cuvette. The emission spectrum can be obtained at 520 nm excitation and the excitation spectrum can be monitored at 625 nm. If a fluorometer is not available, a picture of the solutions under UV lamp excitation (365 nm) in the darkroom can be taken, in which only the silver-polymer-silane solution will show red emission. Students can also analyze and present the spectra together with the instructor. In the following session, the three vials can be shown to the students. The silver-alone solution is dark yellow with obvious flocs, indicating that silver nanoparticles without any protection 702
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pumps an electron from its HOMO to LUMO, resulting in its excited states. One of the subsequent decays from such highenergy states releases another photon, accomplishing a photoluminescence cycle.24 Students can easily conduct the experiment following the demonstration procedure.
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ASSOCIATED CONTENT
S Supporting Information *
Instruments and a brief background introduction. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
Figure 2. Absorption spectra of silver nanoparticles and silver nanodots in aqueous solutions.
*E-mail:
[email protected]. Notes
plasmon resonance absorption is weak. Instead, strong absorption shows up at 610 nm in silver polymer solution, which can be ascribed to the absorption from polymer-coated nonspherical silver nanoparticles. In the silver nanodot solution, however, a peak at 525 nm is observed, in line with the excitation peak of silver nanodots (Figure 3). This suggests that the 525 nm peak is due to the silver nanodot absorption.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Research Foundation (NRF) grant (2011-0013865), NRF-NSFC Cooperative Program (2012K1A2B1A03000558).
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REFERENCES
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Figure 3. Emission and excitation spectra of silver nanoparticles and silver nanodots in aqueous solutions. “Ex” stands for excitation spectrum that is monitored at 625 nm and “Em” for emission spectrum excited at 520 nm.
The emission spectra are significantly different as well. The unprotected silver nanoparticles show no emission. The highest level of its emission spectrum at 530 nm is owing to light scattering by large silver nanoparticles. The silver polymer solution exhibits very weak emission at 625 nm. As expected, the addition of silane dramatically enhances the emission. Because not every teaching lab is equipped with a fluorometer, the strong emission can be seen by the naked eye in this solution when excited by a UV lamp (365 nm), but the other two solutions are dark (Figure 1D). Be noted that the blue is from the excitation light and the red is due to nanodot emission. Because a silver nanodot exhibits molecular characteristics, its photoluminescence can be explained by the typical photophysics of molecules, that is, the absorption of a photon 703
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